Colorants and Auxiliaries: Vol 2 Organic Chemistry and Application Properties

Colorants and auxiliaries ORGANIC CHEMISTRY AND APPLICATION PROPERTIES Second Edition

Volume 2 – Auxiliaries 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 78 3

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Contributors John Shore Formerly of BTTG/Shirley and ICI Dyes (now DyStar), Manchester, UK Terence M Baldwinson Formerly Dye & Information Service Manager, Yorkshire Chemicals plc, Leeds, UK

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Contents Preface

ix

CHAPTER 8

Functions and properties of dyeing and printing auxiliaries

8.1 8.2

The need for auxiliaries 471 The general types and characteristics of auxiliaries 474 References 476

CHAPTER 9

The chemistry and properties of surfactants

9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8

Introduction 477 Hydrophiles 477 Hydrophobes 477 Anionic surfactants 479 Cationic surfactants 485 Nonionic surfactants 486 Amphoteric surfactants 489 The general properties of surfactants 490 References 496

CHAPTER 10

Classification of dyeing and printing auxiliaries by function

10.1 10.2 10.3 10.4 10.5 10.6 10.7 10.8

Electrolytes and pH control 497 Sequestering agents 505 Macromolecular complexing agents 519 Enzymes 539 Preparation of substrates 553 Dispersing and solubilising agents 636 Levelling and retarding agents 642 Thickening agents, migration inhibitors and hydrotropic agents used in printing and continuous dyeing 645 Treatments to alter dyeing properties or enhance fastness 664 Agents for fibre lubrication, softening, antistatic effects, soil release, soil repellency and bactericidal activity 705 Foaming and defoaming agents 744 References 750

10.9 10.10 10.11

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471

477

497

CHAPTER 11

Fluorescent brightening agents

760

11.1 11.2 11.3 11.4 11.5 11.6 11.7 11.8 11.9 11.10 11.11 11.12 11.13

Introduction 760 Mode of action of a fluorescent brightener 761 Evaluation of FBAs: measurement of whiteness 765 General factors influencing FBA performance 768 Chemistry and applications of FBAs 770 Brighteners for cellulosic substrates 770 Brighteners for cellulose acetate and triacetate fibres 781 Brighteners for nylon 784 Brighteners for wool 788 Brighteners for polyester fibres 790 Brighteners for acrylic fibres 799 Brighteners in detergent formulations 803 Analysis of FBAs 809 References 811

CHAPTER 12

Auxiliaries associated with main dye classes

12.1 12.2 12.3 12.4 12.5 12.6 12.7 12.8 12.9

Introduction 813 Acid dyes 813 Azoic components 820 Basic dyes 824 Direct dyes 832 Disperse dyes 837 Reactive dyes 856 Sulphur dyes 882 Vat dyes 893 References 913

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813

Preface to Volume 2 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 second volume of the book collects together a remarkable quantity and variety of factual information linking the application properties of auxiliary products in textile coloration and related processes to as much as is known of the chemical structure of these agents. The environmental impact of auxiliary products has become of major importance and developments during the 1990s have necessitated substantial modification and expansion of the text of this volume. The opportunity has also been taken to highlight novel chemical types of auxiliaries that are under evaluation to overcome or avoid many of the drawbacks shown by traditional products. Thus the two volumes of this Second Edition are now approximately equal in size, whereas in the 1990 edition Volume 2 was only about half as big as its sibling. Virtually all of this development and improvement of Volume 2, especially in the much expanded Chapters 10 and 12, is thanks to the thorough and painstaking work of Terry Baldwinson, who has carefully sifted through an extensive yet scattered range of primary sources. 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 1 Chapter 1

Classification and general properties of colorants

Chapter 2

Organic and inorganic pigments; solvent dyes

Chapter 3

Dye structure and application properties

Chapter 4

Chemistry of azo colorants

Chapter 5

Chemistry and properties of metal-complex and mordant dyes

Chapter 6

Chemistry of anthraquinonoid, polycyclic and miscellaneous colorants

Chapter 7

Chemistry of reactive dyes

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471

CHAPTER 8

Functions and properties of dyeing and printing auxiliaries Terence M Baldwinson

8.1 THE NEED FOR AUXILIARIES There is hardly a dyeing or printing process of commercial importance that can be adequately operated by the use of dyes and water alone. Practically every colorant–substrate system requires the use of additional products, known as auxiliaries, to ensure its reliable functioning and control. This was the case even centuries ago, when the use of natural vat and mordant dyes depended entirely on the proper, albeit rule of thumb, use of additives. These controlled pH, reduction, oxidation and mordanting to enable the dyes to be applied to the natural fibres of those days. Many of the auxiliaries, like the dyes and fibres, were of natural origin. Dung and urine [1] were among the agents used, and soap was clearly the first surfactant to be employed. Indeed, from the standpoint of today, one can only wonder at the degree of purely empirical expertise so successfully developed and applied by the ancient dyers and printers. Our current level of understanding is clearly a phenomenal advance on the ancient arts, yet our need for auxiliaries remains. For example, even before dyeing or printing the substrate must be cleaned and wetted. Products are needed to convert non-substantive vat and sulphur dyes to substantive forms, to help stabilise the conditions that bring about the substantivity, and then to reconvert the dyes to their insoluble forms in the substrate (sections 1.6.1 and 1.6.2). Mordant dyes still require the appropriate chelating agents, as well as other agents to create and maintain the optimum chelating conditions (section 5.8). Printers still need thickening agents to facilitate the localised application of dyes. Inevitably, however, the present-day plethora of dyes, fibres and coloration processes has created additional reasons for the use of auxiliaries, whilst the concurrent evolution of the chemical industry has satisfied these needs as they arose. Moreover, a vastly more comprehensive understanding of the physico–chemical processes involved has enabled auxiliaries to be precisely engineered for specific purposes. Hand in hand with this theoretical knowledge, practical evaluation has become increasingly sophisticated. Nevertheless, it is often difficult to differentiate between auxiliaries promoted purely for commercial reasons and those that serve a definite technical need. Dye manufacturers are acutely aware of the positive part played by auxiliaries in helping to sell dyes, and dyers today are under constant pressure to use more of them. Some additives offer cost savings by improving reproducibility and minimising reprocessing; nevertheless it is all too tempting to incorporate too many products without critically evaluating their efficacy, thus inevitably and unnecessarily increasing processing costs. Consequently it is more important than ever that the dyer or printer understands the 471

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FUNCTIONS AND PROPERTIES OF DYEING AND PRINTING AUXILIARIES

functions of auxiliary products and is equipped to evaluate their use realistically and to monitor it continually. As has been implied already, functional demands for auxiliaries continue to grow, with each dye–fibre system and dyeing or printing process having particular needs. The primary functions of auxiliaries are: (a) to prepare or improve the substrate in readiness for coloration by – scouring, bleaching and desizing – wetting – enhancing the whiteness by a fluorescent brightening effect (b) to modify the sorption characteristics of colorants by – acceleration – retardation – creating a blocking or resist effect – providing sites for sorption – unifying otherwise divergent rates of sorption – improving or resisting the migration of dyes (c) to stabilise the application medium by – improving dye solubility – stabilising a dispersion or solution – thickening a print paste or pad liquor – inhibiting or promoting foaming – forming an emulsion – scavenging or minimising the effects of impurities – preventing or promoting oxidation or reduction (d) to protect or modify the substrate by – creating or resisting dyeability – lubricating the substrate – protecting against the effects of temperature and other processing conditions (e) to improve the fastness of dyeings, as in – the aftertreatment of direct or reactive dyes – the aftertreatment of acid dyes on nylon – the chroming of mordant dyes on wool or nylon – giving protection against atmospheric influences, as in UV absorbers or inhibitors of gas-fume fading – back-scouring or reduction clearing (f) to enhance the properties of laundering formulations (fluorescent brightening agents).

Some auxiliaries fulfil more than one of the above functions. For example, an auxiliary to improve dye solubility may also accelerate (or retard) a coloration process, or an emulsifying agent may also act as a thickening agent; pH-control agents may both stabilise a system and also affect the rate of dye sorption. Thus the range of auxiliaries available is very large indeed, covering a multiplicity of uses for all stages of textile processing. However, a factor which has assumed great importance regarding the use of auxiliaries in recent years is that of their effects on the environment. In view of the extensive portfolio of products and processes, it is not surprising that good

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environmental management is complex. Undesirable effects from the use of auxiliaries may become evident during handling, through effluent discharge to surface waters, through discharge to the atmosphere (e.g. via stenter gases), through consumer contact with the finished product (e.g. skin sensitivity) or during the eventual disposal of solid wastes (e.g. incineration or landfill). All these factors need careful consideration in the selection of auxiliaries at all stages of processing. Compliance with good environmental practice may be voluntary (preferably) or enforced by legislation, some countries having introduced quite extensive and stringent requirements [2,3]. Many factors need to be considered: acute toxicity to mammals, toxicity to aquatic organisms (fish, daphniae, algae) and waste water bacteria, biodegradability (aerobic or anaerobic), abiotic degradability (hydrolysis, photolysis, oxidation), ground mobility, bioaccumulation, carcinogenicity, mutagenicity and teratogenicity. The textile wet processing industry produces particularly heavy discharges of effluent; hence the responsibility placed on it for environmentally good behaviour is indeed an onerous one, both technically and financially [4]. The preparation processes of desizing, scouring and bleaching, together with their associated wash-off processes, inevitably produce a heavy biological oxygen demand in the effluent [5]. Hence there has been, and continues to be, much research effort to improve the environmental performance of these areas. There are two general approaches to good environmental practice. The first, termed ‘endof-pipe’ solutions, requires all unacceptable matter to be removed from the effluent, or at least to be reduced to acceptable levels. This is relatively difficult and expensive, requiring the appropriate treatment facilities. The second attempts to minimise the need for end-ofpipe treatments by reducing the hazardous nature of the effluent in the first place. This can be achieved, for example, by recycling and reusing useful constituents, especially reducing water volumes through the use of low liquor ratios, and reducing the toxicity of the effluent by selecting ‘green’ chemicals and processing methods. In some cases, the volume of effluent can be reduced by combining some processes, e.g. desizing, scouring and bleaching. The possibilities inherent in the second approach have stimulated much research work amongst manufacturers and suppliers of auxiliaries, in terms of finding ‘greener’ products and more acceptable processes for their use. These efforts will continue for the foreseeable future. It is important to remember that auxiliaries nowadays are most frequently supplied as more or less complex mixtures. It is essential from an environmental point of view to consider the influence of the subsidiary components in the branded product as well as of the main substances. For example, even a small amount of solvent added to improve the stability of an auxiliary may pose problems of flammability or toxic vapours, necessitating careful storage and labelling. Constant monitoring of products and processes is necessary, since environmental requirements are continually changing as more information, matched by increasing awareness of hazards, becomes available. It has been suggested that the main concerns to date have been in response to controlling listed substances rather than in tackling fundamental environmental problems [5]. Wragg [6] has provided the following basic environmental check-list for textile wet processing: (1) Is the product free from species and/or by-products that are on the various control lists? Lists of controlled chemicals are being continually extended, together with controls on their use. Usage of heavy metals, solvents and AOX-containing products will gradually decrease.

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(2) Is the manufacturing process suitable for safety and environmental controls? Processing cycles are being placed under greater control in terms of machine operation. Auxiliary usage will be more specific and accurate to avoid over-consumption and to minimise waste. (3) Has the product or its components been assessed at any time for acute toxicity, carcinogenicity or irritant properties? (4) Are the materials easily dealt with in waste treatment systems? (5) Has the product or its components been assessed at any time for toxicity to aquatic species? The overall result of environmental awareness has been to increase the interplay of devolution and evolution. Devolution has seen increasing restrictions, sometimes amounting to a complete ban, on the use of certain substances (e.g. alkylphenolethoxylates, which were once widely used) and a corresponding evolution of new products which, at least for the present, are environmentally acceptable. This interplay between devolution and evolution is likely to continue indefinitely. Environmental factors as they affect specific types of auxiliary will be dealt with under the relevant sections of this volume. 8.2 THE GENERAL TYPES AND CHARACTERISTICS OF AUXILIARIES An auxiliary has been defined [7] as ‘a chemical or formulated chemical product which enables a processing operation in preparation, dyeing, printing or finishing to be carried out more effectively, or which is essential if a given effect is to be obtained’. It is much harder to devise a classification system for auxiliaries than it is for dyes. This is undoubtedly one of the main reasons why there has been no incentive to produce an auxiliaries index comparable with the Colour Index. It is difficult enough to put together a comprehensive yet manageable list of general application types; it becomes even more difficult to classify them chemically, especially as many of them are more or less complex mixtures, are of imprecisely known structure or are the subject of a good deal of trade confidentiality. Although the Society of Dyers and Colourists has shown reluctance to be involved in this area, a very useful biennial trade publication [8] has made considerable progress in the ordered listing of currently available commercial products. This first appeared in 1967. The seventeenth edition (2000) lists currently available products by trade name, application and suppliers. Unfortunately this publication does not include a listing by chemical type. Although the first section does give whatever chemical detail the manufacturers are prepared to divulge, these tend to be bland, broadly based descriptions such as ‘nonionic aqueous emulsion of modified wax’ or ‘quaternary ammonium compound, cationic’. In spite of these shortcomings it remains an indispensable guide to the vast range of products on the market today. The broadest classification of auxiliaries is achieved simply by dividing them into nonsurfactants and surfactants, as detailed below. Non-surfactants include simple electrolytes, acids and bases, both inorganic and organic. Examples include sodium chloride, sodium acetate, sulphuric acid, acetic acid and sodium carbonate, together with complex salts (such as sodium dichromate, copper(II) sulphate, sodium ethylenediaminetetra-acetate, sodium hexametaphosphate), oxidising agents (hydrogen peroxide, sodium chlorite) and reducing agents (sodium dithionite, sodium sulphide). Anionic polyelectrolytes such as sodium alginate or carboxymethylcellulose, used

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475

mainly as thickening agents and migration inhibitors, also fall within the class of nonsurfactants; so too do sorption accelerants such as o-phenylphenol, butanol and methylnaphthalene, although they normally require an emulsifier to stabilise them in aqueous media. Fluorescent brightening agents (FBAs) form another large class of nonsurfactant auxiliaries (see Chapter 11). Surfactants are, in general, substantially organic in nature and structurally more complex than most non-surfactants. It is difficult to define surfactants in a manner sufficiently precise to satisfy everyone. However, for the purposes of this book an adequate definition of a surfactant is given by the Society’s Terms and Definitions Committee [7]: ‘an agent, soluble or dispersible in a liquid, which reduces the surface tension of the liquid’. In coloration processes this reduction in surface tension usually takes place at a liquid/liquid or liquid/solid interface, although liquid/gas interfaces are also occasionally important. In general, a dramatic lowering of surface tension can be brought about by a relatively small amount of surfactant; as little as 0.2 g/l of a soap such as sodium oleate will more than halve the surface tension of water. This physical effect in solution is attributed to the molecular orientation potential of a relatively small hydrophilic moiety (a hydrophile) having strong polar forces, juxtaposed with a relatively large (usually linear) hydrophobic moiety (a hydrophobe) having relatively weak electrostatic forces (Figure 8.1). In aqueous solution or dispersion the polar hydrophile tends to be oriented into the body of the aqueous phase, whilst the hydrophobe, by nature subjected to forces of repulsion by the aqueous phase, is oriented towards (or at) the interfacial boundary, which may be that between the solution and air or between the solution and a fibrous (or other) substrate. The surfactants used as textile auxiliaries can be divided into four major groups, depending on the type and distribution of the polar forces, an arrangement broadly resembling the ionic classification of dyes. The general scheme is shown in Table 8.1.

Strongly polar hydrophile

Weakly polar hydrophobe

Figure 8.1 Schematic diagram of surfactant

Table 8.1 General classification of surfactants Degree of ionic charge on the

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Hydrophile (associated ion)

Class of surfactant

Hydrophobe

Anionic Cationic Nonionic Amphoteric

Weakly negative Strongly positive Weakly positive Strongly negative Uncharged Uncharged These possess balanced negative and positive charges, one or other of which dominates in solution depending on the pH

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FUNCTIONS AND PROPERTIES OF DYEING AND PRINTING AUXILIARIES

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

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H T Pratt, Text. Chem. Colorist, 19 (1987) 23. S Helman, Melliand Textilber., 72 (1991) 567. W Baumann, U Engler,. W Keller and W Schefer, Textilverediung, 27 (1992) 392. M Lomas, J.S.D.C., 109 (1993) 10. J Park and J Shore, J.S.D.C., 100 (1984) 383. P Wragg, J.S.D.C., 110 (1994) 137 Colour terms and definitions (Bradford: SDC, 1988). Index to textile auxiliaries, 17th Edn (Bradford: World Textile Publications, 2000).

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CHAPTER 9

The chemistry and properties of surfactants Terence M Baldwinson

9.1 INTRODUCTION A surfactant was defined in Chapter 8 as: ‘an agent, soluble or dispersible in a liquid, which reduces the surface tension of the liquid’ [1]. It is helpful to visualise surfactant molecules as being composed of opposing solubility tendencies. Thus, those effective in aqueous media typically contain an oil-soluble hydrocarbon-based chain (the hydrophobe) and a smaller water-solubilising moiety which may or may not confer ionic character (the hydrophile). The limitations of space do not permit a comprehensive detailed treatment of the chemistry of surfactants. The emphasis is therefore on a broad-brush discussion of the principal types of surfactant encountered in textile preparation and coloration processes. Comprehensive accounts of the chemistry and properties of surfactants are available [2–13]. A useful and lucid account of the chemistry and technology of surfactant manufacturing processes is given by Davidsohn and Milwidsky [14]. 9.2 HYDROPHILES The basic purpose of the hydrophile is to confer solubility (aqueous solubility is always to be understood unless otherwise stated). The simple moieties most often employed are as follows: (a) in anionic surfactants: sodium, potassium or ammonium cations, associated with negatively charged groups on the hydrophobe such as carboxylate, sulphonate, sulphate or phosphate (b) in cationic surfactants: chloride, bromide or methosulphate ions, juxtaposed with, for example, positively charged quaternary nitrogen atoms (c) in nonionic surfactants: ethylene oxide or propylene oxide moieties. More complex hydrophilic moieties are sometimes encountered, however, such as mono-, di- and tri-ethanolamine and the corresponding isopropanolamines in anionic surfactants. Morpholine, once employed, is now obsolete owing to its toxicity. 9.3 HYDROPHOBES There is a much wider choice of hydrophobes. Most are based on substantially linear longchain alkanes, either saturated or unsaturated. These were originally obtained from naturally occurring fats and oils such as castor, fish, olive, sperm, coconut and tallow oils, but these sources were later superseded by petroleum products which at that time were cheaper. More recently, not only has the price of crude oil escalated, but there has also been a growing 477

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awareness of the finite and diminishing nature of this resource. In 1995, some 75% fossilsourced raw materials were used in the production of synthetic anionic surfactants (90% if lignosulphonates are excluded) [8], but it is foreseen that more biological materials will be used in the future. It is evident that for some time there has been a systematic and large expansion of vegetable oil production, especially in South East Asia [8]. Typical vegetable oils include: tallow, coconut, palm kernel, palm, soybean, linseed, cotton, rape and sunflower. Most cationic surfactants are still obtained from petrochemical olefins, alcohols, paraffins and aromatics, although some are derived from fatty acids [9]. The most common hydrophobes used as the basis for surfactants are those containing eight to eighteen carbon atoms, such as those listed as carboxylates in Table 9.1. Some hydrophobes are aromatic (benzene or naphthalene) moieties, often containing lower alkyl substituents; dodecylbenzene (9.1) is a common example. Alkyl-substituted toluenes, xylenes and phenols, and mono- and di-alkylated naphthalenes (9.2 and 9.3), are also used. Table 9.1 Examples of hydrophobes No. of carbon atoms

Chemical name

Trivial name and formula

8

Octanoate

Caprylate CH3(CH2)6COO

10

Decanoate

Caprate CH3(CH2)8COO

12

Dodecanoate

Laurate CH3(CH2)10COO

12

9-Dodecenoate

Lauroleate CH3CH2CH=CH(CH2)7COO

14

Tetradecanoate

Myristate CH3(CH2)12COO

14

9-Tetradecenoate

Myristoleate CH3(CH2)3CH=CH(CH2)7COO

15

Pentadecanoate

Isocetate CH3(CH2)13COO

16

Hexadecanoate

Palmitate CH3(CH2)14COO

16

9-Hexadecenoate

Palmitoleate CH3(CH2)5CH=CH(CH2)7COO

17

Heptadecanoate

Margarate CH3(CH2)15COO

18

Octadecanoate

Stearate CH3(CH2)16COO

18

9-Octadecenoate

Oleate CH3(CH2)7CH=CH(CH2)7COO

18

9,12-Octadecadienoate

Linoleate CH3(CH2)4(CH=CHCH2)2(CH2)6COO

18

9,12,15-Octadecatrienoate

Linolenate CH3CH2(CH=CHCH2)3(CH2)6COO

18

12-Hydroxy-9-octadecenoate

Ricinoleate CH3(CH2)5CH(OH)CH2CH=CH(CH2)7COO

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ANIONIC SURFACTANTS

C12H25

C8H17

479

C4H9

H9C4 9.2

9.1

9.3

The hydrophobes are usually, though not always, used in the form of acids, alcohols, esters or amines. Commercial products rarely contain a single pure hydrophobe, however; most are mixtures containing a range of hydrophobes, since the raw materials from which they are made are generally themselves mixtures of homologues. For example, a batch of coconut oil, a rich source of the lauric hydrophobe, may have the approximate composition shown in Table 9.2, although the proportions of the individual components may vary by 1–3% between batches. As is the general rule in naturally occurring fats and waxes, only even-numbered carbon compounds are present; odd-numbered ones have to be made by synthesis. Clearly, a surfactant produced from such a mixture will contain a very large, and variable, number of homologues and isomers. Hence two products with the same nominal constitution, but from different manufacturers, often differ in details of composition and properties. This is one fundamental reason why a chemical classification of auxiliaries, analogous to that for dyes in the Colour Index, would be extremely difficult to devise. Table 9.2 Approximate hydrophobe composition of coconut oil Trivial name

No. of carbon atoms

Amount (%)

Caproate Caprylate Caprate Laurate Myristate Palmitate Stearate Oleate Linoleate

6 8 10 12 14 16 18 18 18

0.5 7.0 6.5 49.5 17.0 8.5 2.5 6.5 2.0

Any hydrophobe can yield each of the main (i.e. anionic, cationic, nonionic or amphoteric) types of surfactant in much the same way as the same chromogenic system can be used in anionic, basic or disperse dyes. This will be demonstrated in the following sections, dealing with each class of surfactant, using the cetyl-containing (C 16H33) hydrophobe. 9.4 ANIONIC SURFACTANTS Until recently this class accounted for by far the largest number of surfactants used in preparation and coloration processes. This dominance is now challenged by the much increased use of nonionic types. The essential feature of the class is a long-chain

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THE CHEMISTRY AND PROPERTIES OF SURFACTANTS

hydrophobe linked through an anionic grouping – usually carboxylate, sulphate (sulphuric ester) or sulphonate, but occasionally phosphate, carboxymethyl or other group – to a relatively small cation, generally sodium, although ammonium, potassium and other cations are also used. Carboxylates (9.4, where R is the long-chain hydrophobe and X the cation) represent the oldest type of surfactants, since they could be obtained from naturally occurring fats and oils long before the advent of the petrochemical industry; sodium heptadecanoate (9.5), for example, incorporates the cetyl group as hydrophobe. Sodium stearate, sodium palmitate and sodium oleate are the simplest carboxylates generally used as surfactants. Alkylaryl compounds (9.6) are also known. _ _ R

COO

X

+

_ C16H33COO

H25C12

+ Na

Na +

9.5

9.4

COO

9.6

Many carboxylates are used in the form of soaps, obtained by alkaline saponification of triglyceride fats and waxes of general formula 9.7. The three carboxylic ester groups (RCOO) may carry the same or different hydrophobes, generally containing eight to 22 carbon atoms, the most common being laurate, palmitate and stearate among the saturated types, and oleate and linoleate among the unsaturated ones. At ambient temperatures the unsaturated fats tend to be liquids and the saturated ones solids. R

CH2 R1

COO

OOC

R2

OOC

R3

CH CH2

HC

CH CH CH2

9.7

_ COO

_ COO

Na+ + Na

9.8

Particularly important as wetting agents are the disodium alkenylsuccinates (9.8), in which the saturated R group may contain from three to fourteen carbon atoms. The surfactant properties of these carboxylates, as with other types of surfactant, are dependent on the number of carbon atoms in the hydrophobe. Significant surfactant properties begin to appear in the C8 compounds, although the C8–C12 carboxylates are wetting agents rather than detergents. Better detergency and emulsifying properties become evident with C12–C18 alkyl groups. Solubility decreases with increasing length of the alkyl group; the solubility of soaps, for example, reaches its useful limit with the C22 compounds. The major disadvantage of the carboxylates is that they tend to be precipitated by acids and hard water, since the free acids and the calcium and magnesium salts of the carboxylates are insoluble. This disadvantage provided the main technical reason for finding alternative products that showed tolerance to a wider range of processing conditions. Modified carboxylates, in which the carboxylate moiety forms part of a carboxymethoxy group, are also available. These are made by reaction of selected nonionic surfactants with chloroacetic acid. The result is a useful hybrid range, lacking the sensitivity of simple

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carboxylates to calcium and magnesium whilst retaining excellent detergency; these compounds are more stable to electrolytes than are the conventional nonionics and more suitable for use at high temperatures as they are not susceptible to cloud point problems (section 9.8.2). Sulphates or sulphuric esters of the long-chain fatty acids were the first alternative to the carboxylates. They are essentially the half esters of sulphuric acid (9.9); the ester incorporating the cetyl hydrophobe (9.10) belongs to the important class of fatty alcohol sulphates. Such sulphates, using C8–C18 hydrophobes, are common. R

_ + OSO3 Na

_ C16H33OSO3 Na + 9.10

9.9

Just as there are mono-, di- and tri-carboxylate surfactants, the sulphates can also be prepared from products bearing mono-, di- and tri-hydrophobes. Indeed, the first sulphates to be used were analogous to soaps in that they were the sulphation products of triglycerides. Although their chemistry can be represented in simple terms, it is worth re-stating that most commercial products are highly complex mixtures. For example, a sulphated triglyceride may contain the following: – the sulphated glyceride proper – sulphated free fatty acid – unsulphated glyceride – unsulphated fatty acid – inorganic salts – traces of glycerol. The range of hydrophobes present may also be unexpectedly broad, since the raw materials often consist of mixtures of symmetrical and/or mixed glycerides. As little as 60% of an oil may be sulphatable; sulphation is never carried to theoretical completion and is often far below 100%. With these provisos in mind, the chemistry of the sulphated oils can be considered. Many oils are used as starting materials: olive, castor, tallow, neatsfoot, cotton seed, rape seed and corn oils are examples. Sulphated olive oil was the first sulphated oil to be produced and was used as a mordant in dyeing as long ago as 1834. Sulphation usually occurs at the double bonds of any unsaturated fatty acids in the glyceride (Scheme 9.1). On the other hand, in the preparation of the best-known of these products, Turkey Red Oil or sulphated (often wrongly termed ‘sulphonated’) castor oil, sulphation of the main component, the glyceride of ricinoleic acid (12-hydroxy-9-octadecenoic acid), takes place preferentially at the hydroxy group rather than at the double bond (Scheme 9.2). Such products possess useful wetting, emulsifying and dye-levelling properties.

CH

CH

+ H2SO4

CH2

CH

_ OSO3

Scheme 9.1

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THE CHEMISTRY AND PROPERTIES OF SURFACTANTS

CH

CH2

CH

CH

CH

CH2 CH OSO3 H+

+ H2SO4

CH

_

OH Scheme 9.2

At the present time, however, the long-chain alcohol sulphates already mentioned, such as structure 9.10, and particularly the sulphated ethers are of greater importance. The stability of the sulphates to mildly acidic conditions and to hard water is much better than that of the carboxylates and is sufficient for most purposes. Under more stringent acidic conditions, however, hydrolysis may take place. Another type of sulphated product, an ester sulphate, can be prepared by esterifying a fatty acid such as ricinoleic or oleic acid with a short-chain (C3–C5) alcohol and then sulphating. Such products are particularly useful foaming, wetting and emulsifying agents; an example is sulphated butyl ricinoleate (9.11). CH3(CH2)5CHCH2CH OSO H+

CH(CH 2)7COO(CH 2)3CH3

3

9.11

More recent developments amongst anionic surfactants are the sulphated polyethers or alcohol poly(oxyethylene) sulphates (9.12, 9.13), prepared by ethoxylating the fatty alcohol to give a polyether containing a terminal hydroxy group that is then sulphated. Aromatic hydrophobes may also be used to produce, for example, alkylphenol poly(oxyethylene) sulphates. In a general sense, a poly(oxyethylene) sulphate can be viewed as a partly anionic and partly nonionic surfactant, although the degree of ethloxylation of these products is generally much lower than that of the purely nonionic surfactants. Hence they are sometimes referred to as ‘lightly ethoxylated alcohol sulphates’; again, their actual composition may be a good deal more complex than indicated by their nominal structural formulae. There has been increasing use of these derivatives in domestic detergents. R

_ OSO3

(OCH2CH2)x

Na+

C16H33

(OCH2CH2)x

_ OSO3

Na+

9.13

9.12

Sulphonated anionic surfactants have the general structure 9.14, which should be compared with that of the sulphates (9.9). As well as the simple alkyl derivatives such as structure 9.15, aromatic and particularly alkylated aromatic (alkylaryl) types are technically and commercially important. Indeed, sodium dodecylbenzenesulphonate (9.16) has long been of great importance in domestic washing powders. Although it is no longer the only surfactant used in domestic washing powders, its economy, efficacy and environmental properties are such that it is likely to remain the dominant anionic surfactant in heavy-duty concentrated powder detergents for some time [15]. Environmental studies have shown that 90% of such linear alkylbenzenesulphonates are removed by conventional sewage treatment, the remainder being virtually completely biodegradable in topsoil with half-life values varying from 3 to 25 days [16,17].

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_ RSO3 Na+

_ + C16H33SO3 Na

9.14

9.15

483

_ SO3 Na +

H25C12

9.16

Nowadays these compounds are usually blended with other surfactants, including nonionic types (section 9.6). In 1990 a typical low- or non-phosphate domestic detergent contained 7% linear alkylbenzenesulphonate and 6% nonionic fatty alcohol ethoxylate [16]. There is increasing use of the long-chain fatty alcohol poly(oxyethylene) sulphates previously described (e.g. 9.12) as a partial or complete replacement for linear alkylbenzenesulphonates [15] since they are made from renewable feedstocks such as tallow and palm oil [16]. Naphthalene and other aromatic hydrophobes are also used to produce sulphonates, such as structure 9.17. Of greater importance, however, are the more complex condensation products that form the basis of many excellent dispersing, resist and aftertreating (syntan) agents. Typical examples are the condensation products of naphthalenesulphonates with formaldehyde, and the lignosulphonates derived from pulping processes; these are described in more detail in section 10.6.1. C3H7

_ + SO3 Na 9.17

Sulphosuccinates are of particular interest not only for their technical properties but also because structurally they combine the two hydrophile functions described earlier – the sulphonate and carboxylate moieties – in a single molecule (9.18). The sulphosuccinate diesters, however, are probably of greater commercial importance in textile processing than are the monoesters. The most important example is sodium dioctylsulphosuccinate (9.19), but the dinonyl, dimethylamyl and di-isobutyl analogues are also used commercially. As usual, a wide choice of hydrophobes is available and includes alcohols, lightly ethoxylated alcohols, alkanolamides and combinations of these. COOC8H17

COOR H2C CH

_

_ + SO3 Na

COO 9.18

Na+

H2C

_ SO3

CH

+ Na

COOC8H17 9.19

Phosphate esters (9.20) represent a different class of hydrophile-characterised anionic surfactants; mono- or di-esters can be formed depending on whether one or two alkyl groups are present. Most phosphate esters are based on alcohols and especially their ethoxylates,

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THE CHEMISTRY AND PROPERTIES OF SURFACTANTS

including aliphatic and alkylaryl types. Whereas the sulphates tend to be based on lightly ethoxylated alcohols, the phosphate esters are also made from more highly ethoxylated products. Commercial products are complex mixtures (9.21) of monoester, diester, free phosphoric acid and free nonionic surfactant [18]. O

O R

O

P

R

OH

O

P

R

O

OH

OH 9.20

O

O R(OCH2CH2)xO

P

R(OCH2CH2)xO

OH

P

O(CH2CH2O)xR

OH

OH Monoester surfactant

Diester surfactant 9.21

O HO

P

OH

R(OCH2CH2)xOH

OH Free nonionic surfactant

Free phosphoric acid

Phosphate esters are particularly useful for their alkali stability and wettability. It has been shown [18] that a certain amount of ethoxylation of the hydrophobe is required to obtain alkali resistance, and that as the relative molecular mass (Mr) of the hydrophobe increases, the proportion of ethylene oxide required also increases. The greater the degree of ethoxylation, the greater the degree of alkali resistance, few materials showing good alkali resistance with less than six moles of ethylene oxide per mole of hydrophobe. The same study [18] showed that wetting power decreased with increasing Mr of the hydrophobe. Incorporating 2–3 moles of ethylene oxide per mole of hydrophobe seems to give optimum wetting. It appears that as one attempts to increase the rate of wetting, alkali tolerance decreases. Other anionic surfactant types include the alkylisethionates (9.22), N-acylsarcosides (9.23), N-acyltaurides (9.24) and perfluorinated carboxylates, sulphonates (e.g. 9.25), sulphates and phosphates [13].

O

_ R

O

CH2CH2SO3 9.22

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+ Na

C

_ N

CH2COO

Na

+

CH3 9.23

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CATIONIC SURFACTANTS

O R1

C

N

_

CH2CH2SO3

F3C

+ Na

CF2

F3C

R2

C

CF3 C

C

CF2

F3C

485

O

_ SO3 Na+

CF3

9.24 9.25

9.5 CATIONIC SURFACTANTS By far the most important types of cationic surfactant used in textile processing are the quaternary ammonium salts (9.26), in which R is usually a long-chain hydrophobe and R1, R2, R 3 are lower alkyl groups. The most common anions in these and other cationic surfactants are chloride and bromide: thus cetyltrimethylammonium chloride (9.27) is typical of this class of cationic surfactants. In fact, however, all four alkyl groups on the nitrogen atom can be varied to alter the balance of properties of the products. In the alkyldimethylmethallylammonium chlorides (9.28), an unsaturated aliphatic group is used. Aromatic components are also used, as in the important alkyldimethylbenzylammonium chlorides (9.29), and both the aromatic nucleus and the alkyl groups in such products may contain substituents (9.30 and 9.31). As important as the quaternary ammonium surfactants are the pyridinium salts (9.32; R is a long-chain alkyl group), such as cetylpyridinium chloride (9.33). + R

N

CH3

CH3

_ R2

+

+

R1 X

H33C16

N

R3

_ CH3

R

Cl

N CH3

CH3

9.26

_ CH2

CH2

+ R

N

+ Cl

_

CH3

Cl

CH2

CH3

9.28

9.27

CH3

C

R

CH3

N

_ Cl

CH2

CH3

9.29 9.30

+ CH3 R

N

CH2CH2OH

CH3

N C16H33 + _ Cl

N R + _ X

_ Cl

9.32 9.33

9.31

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THE CHEMISTRY AND PROPERTIES OF SURFACTANTS

Imidazoles can be quaternised to yield cationic surfactants (such as structures 9.34 and 9.35). Long-chain alkyl primary, secondary and tertiary amines can also be used as cationic surfactants, but their use in textile processing is limited as a result of their insolubility in other than acidic aqueous media. The range of products available as cationic surfactants is truly enormous, including, for example, such complex products as alkylated mono- and diguanidines and polyamines (9.36) containing more than one basic nitrogen atom.

CH2CH3

CH3 H2C

N

H2C

N

_

+C

R1

+C

Cl

_

R

Cl

N

N CH2CH2

NHCO

R2

CH2

9.34

9.35 R

(NHCH2CH2)xNH2 9.36

Many of these cationic products, including the quaternary amines and imidazoles, can be ethoxylated (9.37, 9.38), forming cationic analogues of the ethoxysulphates and ethoxyphosphates in the anionic series. They are essentially cationic/nonionic hybrid surfactants, variously described in manufacturers’ promotional literature as ‘modified cationic’, ‘weakly cationic’ or even ‘modified nonionic’. Their value lies in the fact that the cationic nature can be controlled by varying not only the alkyl substituents but also the degree of ethoxylation. In addition the ethoxylate moiety confers useful emulsifying properties. Fluoro-containing cationic surfactants (9.39) can also be obtained [13]. +

CH3

CH3 R

N

_ (CH2CH2O)xH

H2C

Cl

(CH2CH2O)xH

H2C

R

_ Cl

N (CH2CH2O)xH

9.37

CH3 C7F15

N +C

CONH

(CH2)3

9.39

9.38

N CH3 + CH3 _ I

9.6 NONIONIC SURFACTANTS Nearly all nonionic surfactants contain the same type of hydrophobes as do anionic and cationic surfactants, with solubilisation and surfactant properties arising from the addition of ethylene oxide to give a product having the general formula 9.40. Usually, depending on the

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hydrophobe, aqueous solubility and detergent properties begin to be evident when x = 6, but in theory the degree of ethoxylation can be continued almost indefinitely. Optimal surfactant properties are generally found when x = 10–15, although higher homologues (for example, x = 50) are known. The ‘lightly ethoxylated’ sulphates mentioned earlier usually contain only 2–4 oxyethylene units per molecule. Thus a typical nonionic surfactant can be represented by structure 9.41. R

(OCH2CH2)xOH

H33C16

(OCH2CH2)12OH 9.41

9.40

Although there are other types of nonionic surfactant, the great majority are adducts of ethylene oxide with hydrophobes derived from three sources: – fatty alcohols and alkylphenols – fatty acids – fatty amines and amides. For many years the most common of these have been adducts with p-nonyl- and p-octylphenol, and to a lesser extent 2,4-dinonylphenol, p-dodecylphenol and 1-alkylnaphthols. Since the hydrophobes used may be variable products conforming to an average nominal structure, and since the quoted degree of ethoxylation can also only be regarded as an average value, products having the same name (such as, for example, p-nonylphenol dodecaoxyethylene) may in fact differ in detailed composition and properties when obtained from different manufacturers. These provisos should be borne in mind when considering the examples below, even though there is a trend in some cases towards the manufacture of narrower fractions. An example of an alcohol-based nonionic (9.41) has already been given. An alkylphenol adduct (9.42) is essentially similar; both alcohols and phenols give rise to the relatively strong and stable ether link, a valuable property of this type of product. Analogues based on alkylthiols (9.43) may also be used. (OCH2CH2)xOH

H19C9

R

(OCH2CH2)xOH H17C8

9.42

S

(CH2CH2O)xH

(OCH2CH2)xOH 9.44

9.43

Polyfunctional alcohols of varying complexity, such as polyethylene glycols (9.44) and polypropylene glycols of varying chain length, also provide useful nonionic agents. A polypropylene glycol molecule has a hydroxy group at each end to which ethylene oxide can be added, forming random segments of poly(oxyethylene) and poly(oxypropylene). This results in block copolymers, which can be engineered by control of starting materials and processing conditions to give products specifically suited to a wide variety of purposes by virtue of wide variations in segment length and degree of polymerisation. Whereas the alcohol and phenol derivatives are characterised by ether linkages, adducts of ethylene oxide with fatty acids give rise to both monoesters (9.45) and diesters. These are

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THE CHEMISTRY AND PROPERTIES OF SURFACTANTS

H33C16CO(OCH2CH2)xOH

H23C11NH(CH2CH2O)xH

9.45

9.46 (CH2CH2O)xH

(CH2CH2O)xH H23C11

N

H23C11 H23C11

C

(CH2CH2O)xH

NH(CH2CH2O)xH

C

N

O

(CH2CH2O)xH

O

9.47

9.49

9.48

less stable than the ethers in strongly acidic or alkaline media, however, hydrolysing to the original fatty acid and polyethylene glycol. Adducts of ethylene oxide with fatty amines can yield mono- (9.46) or di-substituted (9.47) products, as can the adducts with fatty amides (9.48, 9.49). In practice the products formed are far from being as simple or as symmetrical as represented by these formulae since, amongst other things, the ethylene oxide addition takes place randomly. A recent introduction in the area of nonionic surfactants is the alkylglycoside series. These are long-chain acetals of saccharides (9.50). Commercial products currently have an average alkyl chain length of 10–12 carbon atoms. These are manufactured from nonpetroleum sources, being synthesised from glucose and fatty alcohols. Such acetals are regarded as eco-friendly, being said to be completely biodegradable [19] and having low skin irritancy. These surfactants possess wetting, foaming and detergency properties similar to those of the corresponding alcohol ethoxylates but with higher solubility in water and in solutions of electrolytes. They are soluble and stable in sodium hydroxide solutions and show no inverse solubility characteristics. CH2OH CH HO

CH

O CH

CH

CH

OH

OH

O

CnH2n+1

9.50

The nonionic types so far discussed form the great majority used in textile processing. Of course, a great many more can be synthesised, as the possible range of permutations and combinations is truly enormous. Given appropriate conditions, ethylene oxide will react with almost any proton-donating compound, but the choice in practice is restricted by economic factors. Not all nonionic surfactants are ethoxylates, however. Analogous propylene oxide adducts are known; rather more different products include sucrose and sorbitan esters, alkanolamides and fatty amine oxides. The fatty acid esters of compounds such as sucrose and sorbitol exhibit surfactant properties. Some, such as the sorbitan fatty esters, are insoluble in water but being oil-soluble they can be used as emulsifiers in oil-based systems, or they can be ethoxylated to render them water-soluble. Mention has already been made of fatty amide poly(oxyethylene) adducts formed by condensation of a fatty acid with an alkanolamine which is then ethoxylated; some complex alkanolamides have in themselves (i.e. without ethoxylation) some surfactant properties, however. They are made by the

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reaction of a fatty acid (such as lauric acid or a coconut fatty acid) with a secondary alkanolamine (such as diethanolamine) to yield an amide, which then reacts further with diethanolamine to give the water-soluble alkanolamide surfactant. Typical fatty amine oxides (9.51 and 9.52) are derived, for example, from the peroxide oxidation of tertiary amines containing at least one fatty-chain group. O

CH3 H25C12

N

O

O

N C12H25

CH3 9.51

9.52

9.7 AMPHOTERIC SURFACTANTS As mentioned in Table 8.1, amphoteric surfactants contain both an anionic and a cationic group. In acidic media they tend to behave as cationic agents and in alkaline media as anionic agents. Somewhere between these extremes lies what is known as the isoelectric point (not necessarily, or even commonly, at pH 7), at which the anionic and cationic properties are counterbalanced. At this point the molecule is said to be zwitterionic and its surfactant properties and solubility tend to be at their lowest. These products have acquired a degree of importance as auxiliaries in certain ways [20–25], particularly as levelling agents in the application of reactive dyes to wool. The simplest type is represented by the higher alkylaminoacids, such as compound 9.53; disubstituted amines can also be synthesised (9.54). Ethoxylated products can also feature as amphoteric surfactants; an example is compound 9.55, an alkylamine poly(oxyethylene) sulphate. Of particular interest in textile processing are the trisubstituted alkylamino acids known as betaines; N-alkylbetaines (9.56; R = C8–C16 alkyl) and acylaminoalkylbetaines (9.57; R = C10–C16 alkyl) are typical [30]. Sulphate and sulphonate analogues of the carboxylates, such as the sulphobetaine 9.58, can also be used as amphoteric agents. _ + H33C16NH2CH2COO + H33C16HN

9.53

CH2COOH

(CH2CH2O)xH

_ CH2COO

H33C16

9.54

+ HN _ (CH2CH2O)xCH2CH2OSO3

R

CH3 _ + N CH2COO

9.55

CH3 9.56

R

CONHCH 2CH2CH2

CH3 _ + N CH2COO CH3

9.57

R

CH3 _ + N CH2CH2CH2SO3 CH3

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THE CHEMISTRY AND PROPERTIES OF SURFACTANTS

9.8 THE GENERAL PROPERTIES OF SURFACTANTS 9.8.1 Effects on the environment The widespread use of these products focused attention on their environmental properties long ago, owing to the persistent foam-creating tendency of many surfactants when discharged. However, the surfactants industry has a very good track record of responding to environmental problems, stretching back as long ago as the 1960s; that is, quite some time before the present environmental bandwagon began to roll. Karsa has provided a pointed reminder of this: ‘The most significant development in the West in the 1960s was the growing environmental awareness and concern for biodegradable components to overcome problems at sewage treatment plants and foam in watercourses. The result was an underpublicised and often forgotten fact that industry on both sides of the Atlantic voluntarily changed from branched-chain alkylbenzenesulphonates’. Since then, ‘detergents have been based on biodegradable components, contrary to the impression given with some of the information supplied with today’s ‘green’ detergents, which would have one believe biodegradability is something new and exclusive to these products. This was among the first major environmental moves by any industry and was ten years ahead of any UK or EEC legislation’ [16]. Major works dealing with environmental aspects of surfactants are available [26–30]. The excellent biodegradability of the linear alkylarylsulphonates has already been mentioned (section 9.4). The alcohol sulphates have low toxicity and alcohol poly(oxyethylene) sulphates are even less toxic. Alkane sulphonates have high COD, BOD and an MBAS degradation rate of 90%. Polyether carboxylates have excellent environmental properties and are non-toxic to the extent that they are used in cosmetics and household detergents. Sodium-α-olefin sulphonates show rapid biodegradation due to their linear structures. The α-sulphomonocarboxylic esters show good to excellent environmental properties and are also used in cosmetics and household detergents. Sulphosuccinates generally show 90% biodegradation after seven days and have a long history of safe use, being ranked as relatively non-toxic. Phosphorus-containing anionics are very mild to the skin and are used in cosmetics, shampoos and lotions. The toxicology of perfluorinated surfactants varies greatly; most are harmless, whilst some are amongst the most toxic non-proteins known, the structural differences between the two often being relatively slight. Hence caution is needed in their use, even though they are so strongly surface-active that they can be used in much smaller quantities than other surfactants. Cationic alkylammonium surfactants have shown 94% biodegradability [27]. Amongst the nonionics, the use of linear primary alcohol ethoxylates has grown rapidly since the 1970s, due in very large measure to their high degree of biodegradability under most test procedures, both rapid primary and ultimate degradation [28]. Biodegradation of such products is retarded by branching of the alkyl chain, this being cumulative. It is also retarded in secondary alcohol structures, by the addition of about 3 equivalents of propylene oxide to the ethoxylate moiety and by an ethoxylate chain of more than 20 units. However, as Talmage points out [28], products containing these features are not present in the simple alcohol ethoxylates most commonly used in detergent formulations. In contrast to the above trends, during the 1980s and 1990s there has been considerable environmental concern over the alleged effects of the nonionic nonylphenol ethoxylates.

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THE GENERAL PROPERTIES OF SURFACTANTS

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The concern seemed to be centred around possible bioaccumulation and the properties of nonylphenol itself, potentially one of the major metabolites (products of biodegradation) released during the environmental breakdown of nonylphenol ethoxylates. The incomplete biodegradation was attributed to branching in the nonyl group and the presence of the aromatic phenyl ring. In the 1980s, particularly in Europe, there were calls for restrictions and bans on the use of nonylphenol ethoxylates. This concern, not surprisingly, led to much careful and detailed research from which has evolved a clarified and much less alarmist picture. This research has been excellently reported by Naylor [31], who pointed out the limitations of laboratory methods (the extent of biodegradation of nonylphenol ethoxylates has been variously reported from 0% to 100%!) and the critical importance of determining biodegradability in conventional waste water plants using improved and streamlined analytical methods. This work, on American rivers and treatment plants, showed that nonylphenol ethoxylates exhibited high treatability under conditions of extremely high loadings on waste water treatment plants. It was confirmed that nonylphenol is indeed the metabolite of highest toxicity but it is not a significant metabolite except under anaerobic conditions. It was shown that nonylphenol ethoxylates are extensively biodegraded (92.5– 99.8% removal rates) in secondary treatment. Re-aeration studies have shown that nonylphenol and nonylphenol ethoxylates contained in sewage sludge degrade when the sludge is applied to soil. Thus there is a strong basis for the conclusion that nonylphenol ethoxylates are highly biodegradable, do not accumulate in water, sediment or aquatic organisms and do not pose a credible threat to the environment. Hence, in 1995, Naylor [31] was able to say that, in America, nonylphenol ethoxylates were by far the most important alkylphenol ethoxylates, accounting for 80% of the total volume and commonly found in formulations for fibre sizing, spinning, weaving, scouring and dyeing, as well as for water-based paints, inks, adhesives and many institutional and household cleaning products. Finally, in considering the environmental properties of surface-active auxiliaries generally, it should be borne in mind that they are more or less complex mixtures and hence the presence of other components, such as solvents, electrolytes or sequestrants, needs to be considered in addition to the surfactants present.

9.8.2 Application properties Anionic and cationic products generally tend to interact with each other, usually diminishing the surface-active properties of both and often resulting in precipitation of the complex formed. Amphoteric compounds can also be incompatible with anionics in acid solution but are generally compatible with cationics and nonionics. Interaction between anionic and cationic agents can sometimes be prevented by addition of a nonionic. In some cases, if an ethoxylated sulphate or phosphate is used as the anionic component a cationic compound produces no obvious precipitation, since the oxyethylene chain acts as dispersant for any complex that may be formed. The main disadvantages of the carboxylates are their tendency to react with calcium and magnesium ions in hard water to give insoluble precipitates and their insolubility in acidic media, although they generally have good wetting and detergent properties. The acylsarcosides are less affected by calcium and magnesium ions, however, whilst the

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THE CHEMISTRY AND PROPERTIES OF SURFACTANTS

carboxymethyl surfactants are unaffected. The sulphates were specifically developed to overcome the drawbacks of the carboxylates and, like the phosphates, are stable towards calcium and magnesium ions. As well as being outstanding detergents, the sulphonates are also unaffected by strongly acidic or alkaline conditions, and the higher-alkyl members have useful lubricating properties. On the other hand, the sulphates can be hydrolysed by acid and sulphated monoglycerides can also be hydrolysed by alkali. Their wetting properties tend to be inferior to those of the sulphonates but they are particularly valuable as emulsifying agents, especially in combination with nonionics. The sulphosuccinates have a high propensity to foaming and their solubility is not generally good, but the monoesters have good detergency properties and the diesters are particularly rapid wetting agents. As a group, the phosphates have good stability to acid and alkali for most purposes, have low foaming and good detergency properties and are biodegradable. They tend to be better wetting agents than the sulphates and their solubility in organic solvents makes them useful in, for example, dry cleaning. The perfluoroalkyl anionic surfactants are very expensive, but are powerful surfactants at very low concentrations and are stable in chemically hostile environments; they also exhibit surface activity in organic solvents. Cationic agents generally are less useful than anionics as detergents but they have useful properties as softeners, germicides and emulsifiers. Nonionic agents are generally compatible with both anionic and cationic types. They are also stable to calcium and magnesium ions. With the exception of the fatty acid esters, which are readily hydrolysed by acid and alkali, they are stable and effective over a wide range of pH values. A particular characteristic of nonionic surfactants is their inverse solubility: as the temperature rises the solubility decreases, until a point is reached at which the surfactant attains its limiting solubility and therefore begins to precipitate out, causing cloudiness of the solution. The temperature at which this occurs, known as the cloud point, depends on the number of oxyethylene units in the nonionic molecule in relation to the length of the hydrophobe. Thus, for any given hydrophobe, the cloud point increases with the increasing degree of ethoxylation; for example, dodecanol heptaoxyethylene C12H25(OCH2CH2)7OH has a cloud point of 59 °C, while that of the undecaoxyethylene homologue is 100 °C. Conversely, for a fixed number of oxyethylene units, the cloud point decreases with increasing size of the hydrophobe. The cloud points of nonionic agents are also generally lowered by the presence of electrolytes, the effect varying with the electrolyte and its concentration. It is important to bear this in mind when choosing nonionic agents for use in electrolyte-containing processes. This inverse solubility arises from the solubilisation of the nonionic molecules by hydrogen bonding of water with the ether oxygen atoms (9.59). As the temperature rises, the energy within these bonds becomes insufficient to maintain their cohesion and dehydration takes place, with a consequent decrease in solubility. A knowledge of the cloud point of a surfactant is useful, not only because of solubility effects but also because the surface activity tends to be optimal just below the cloud point. O

H

O

H O

H

H CH2 CH2

CH2

O

O

CH2

n

H 9.59

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H

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THE GENERAL PROPERTIES OF SURFACTANTS

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The tendency of nonionics to produce foam varies. Some, such as the block copolymers, are even used as defoamers. Their wetting, detergency and emulsifying properties also vary widely, depending to a large extent on the balance between the hydrophobic and hydrophilic (oxyethylene) portions. The amphoteric agents exhibit excellent compatibility with inorganic electrolytes and with acids and alkalis. Such is their stability in strongly acidic solution that they are even used in cleaning compositions based on hydrofluoric acid [14].

9.8.3 The theory of surface activity The physico-chemical theory of surface activity is a vast field and no more than broad principles can be touched on here; major reference sources exist for those who require more detail of the relationship between chemical structure and the various surfactant properties such as wetting, detergency and emulsification-solubilisation [32–36]. Surface activity is generally related to the balance between the hydrophobic and hydrophilic portions of the molecule. For example, among the anionic surfactants C8–C12 alkyl hydrophobes tend to be predominantly wetting agents, whilst the C12–C18 homologues exhibit better detergency and emulsifying properties. The alkylsuccinates and sulphosuccinates are particularly powerful wetting agents. Clearly, as the hydrophobic character of the surfactant is increased, aqueous solubility decreases and oil solubility increases. Thus the balance between the hydrophobic and hydrophilic moieties of a surfactant is a critical factor in determining its major characteristics. This is referred to as the hydrophile–lipophile balance, or HLB (the term ‘lipophile’, of course, being analogous to ‘hydrophobe’). Whilst the HLB value is of general use in expressing the characteristics of a surfactant, it is of particular value in describing the formation of emulsions. For some general purposes the HLB can be used qualitatively (referring, for instance, to low, medium or high HLB), but for more precise work it is preferable to use a quantifying scale. Such a scale, put forward in the 1940s [37], covers a range of values from zero (the lipophilic or hydrophobic extreme) to a hydrophilic extreme of 20 or higher, with a value of 10 approximately representing the point at which the hydrophilic and hydrophobic portions are in balance. This scale is especially useful in describing the properties of the nonionic ethoxylates. For example, a low HLB value (4–6) signifies a predominance of hydrophobic groups, indicating that the surfactant is lipophilic and should be suited for preparing waterin-oil emulsions. A value in the 7–9 range indicates good wetting properties. As the value shifts towards increased hydrophilicity other properties predominate, values of 8–18 being typical for surfactants that will give oil-in-water emulsions, and values of 13–15 for surfactants that show useful detergency. The HLB values required for solubilising properties are generally in the range 10–18. The HLB of a relatively pure poly(oxyethylene) adduct can be calculated from theoretical data [37]. For these agents the HLB is an indication of percentage by mass of the hydrophilic portion, divided by five to give a conveniently small number. For example, if the hydrophilic portion of a purely hypothetical nonionic agent accounted for 100% of the molecule (such a product cannot, of course, exist), its HLB is 20. Similarly, a more plausible product in which 85% of the molecule is accounted for by the hydrophilic portion has an HLB of 85/5 = 17. The ICI Americas Inc. method of calculating the theoretical HLB of a sorbitan monolaurate nonionic having 20 oxyethylene units per molecule is given in

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Equation 9.1 (total relative molecular mass = 1226, of which 1044 is contributed by the hydrophilic portion) [37].

HLB =

1044 1 ™ 100 ™ = 17.0 1226 5

(9.1)

As explained earlier, however, the actual constitution of a surfactant rarely conforms to its nominal structure. Consequently the theoretical method of calculation is of limited utility, practical methods being more reliable. The HLB value may be determined directly by analysis or by comparison with a range of surfactants of known HLB values. An analytical method for the sorbitan monolaurate described above uses Equation 9.2 [37]. SÛ Ë Ë 45.5 Û HLB = 20 Ì 1 - Ü = 20 Ì1 Ü = 16.7 Í Í AÝ 276 Ý

(9.2)

where S is the saponification number of the ester and A is the acid number of the recovered acid. The saponification value of a product is the mass in milligrams of potassium hydroxide required to saponify one gram of the product; it can be found by saponification of the product with an excess of potassium hydroxide, followed by back-titration of the remaining alkali with hydrochloric acid. The acid value of an acid is the number of milligrams of potassium hydroxide required to neutralise a standard quantity, and can again be found by titration. The comparative methods should always be used for the nonionic surfactants that are not based on ethylene oxide, and also for ionic surfactants since the hydrophilic influence of the ionic group exceeds that indicated by the mass percentage basis (this can lead to apparent HLB values higher than 20). Once the HLB values of a range of surfactants are known it is an easy matter to calculate the HLB value of a mixture as follows: Individual HLB 45% of surfactant A 16.7 35% of surfactant B 4.0 20% of surfactant C 9.6

Fractional HLB 0.45 × 16.7 = 7.52 0.35 × 4.0 = 1.40 0.20 × 9.6 = 1.92 Total HLB = 10.84

When preparing an emulsion, emulsification tends to be most efficient when the HLB of the agent matches that of the oil phase. Often a mixture of surfactants makes a more efficient emulsifying agent than a single product having the same HLB value as the mixture; similarly, if the oil phase to be emulsified is itself a mixture, its components will each contribute to the effective HLB value. It is this effective HLB that is the main criterion in designing a suitable emulsifying system. The effective HLB value can be found by carrying out preliminary emulsification tests with agents of known HLB values. A useful procedure [37] uses two such emulsifying agents of widely differing HLB values mixed in various proportions so as to give a range of intermediate HLB values. The HLB value of the mixture that gives the best emulsion of the oil phase under test then corresponds to the effective

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HLB value of the oil phase. Further tests can then be carried out with different chemical types of agents around this effective HLB value in order to find the optimum emulsifying system. 9.8.4 Micelle formation

Surface tension/N m–1

All surfactants in solution tend to form more or less ordered agglomerates of molecules, known as micelles. Pure water has a surface tension of about 72 × 10–3 N/m. As surfactant is added gradually to it, the surface tension falls quite rapidly (Figure 9.1) until, at a certain concentration of surfactant, it begins to level off more or less sharply. At the point at which this levelling out takes place, the critical micelle concentration (CMC in Figure 9.1), the surfactant molecules begin to orient themselves in clusters within the body of the solution, these clusters being more or less lamellar or spherical (Figure 9.2).

Concentration of surfactant/g l–1

CMC

Figure 9.1 Surface tension of water against surfactant concentration

Lamellar

Spherical

Figure 9.2 Micelle formation

In water the surfactant molecules orient themselves with their hydrophobes at the centre of the cluster. The CMC is typically quite low, perhaps 0.5–0.2 g/l. At concentrations lower than this the molecules orient themselves only at the interfaces of the solution, and it is this effect which brings about the lowering of surface tension. Once the CMC is reached the

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interfaces become saturated and as the concentration increases micellar clusters of molecules begin to form in the bulk of the solution; there is little further reduction in surface tension beyond the CMC, nor are there changes in the other surfactant properties such as wetting and foaming. In general, the CMC decreases with increasing size of the hydrophobe, and the CMCs of nonionic agents tend to be lower than those of ionic types, since with the nonionics micelles can form more easily in the absence of polar charges. This ability to form micelles is vital to the efficacy of surfactants as emulsifying, dispersing and solubilising agents. 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.

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Colour terms and definitions (Bradford: SDC, 1988). Kirk-Othmer encyclopedia of chemical technology, 3rd Edn., Vol. 22 (New York: Wiley, 1983). M R Porter and M Porter, Handbook of surfactants, 2nd Edn. (Glasgow: Blackie Academic and Professional, 1994). D R Karsa, J M Goody and P J Donnelly, Surfactants applications directory (Glasgow: Blackie Academic and Professional, 1991). M R Porter and M Porter, Recent developments in the technology of surfactants (Glasgow: Blackie Academic and Professional, 1991). K Y Lai, Liquid detergents (New York: Marcel Dekker, 1996). M J Rosen, Surfactants and interfacial phenomena, 2nd Edn. (New York: Wiley, 1989). H W Stache, Anionic surfactants: organic chemistry (New York: Marcel Dekker, 1995). J M Richmond, Cationic surfactants: organic chemistry (New York: Marcel Dekker, 1990). V M Nace, Nonionic surfactants: polyoxyalkylene block copolymers (New York: Marcel Dekker, 1996). E G Lomax, Amphoteric surfactants, 2nd Edn. (New York: Marcel Dekker, 1996). I Piirma, Polymeric surfactants (New York: Marcel Dekker, 1992). E Kissa, Fluorinated surfactants: synthesis, properties, applications (New York: Marcel Dekker, 1993). A S Davidsohn and B Milwidsky, Synthetic detergents, 7th Edn. (Harlow: Longman, 1987). G Bevan, Rev. Prog. Coloration, 27 (1997) 1. D R Karsa, Rev. Prog. Coloration, 20 (1990) 70. Summary of Aachen conference, Alkylbenzenesulphonates in the environment, J. Amer. Oil Chem. Soc., 66 (1989) 748; full proceedings in Tenside Surf. Det., (Apr/May 1989). A J O’Lenick and J K Parkinson, Text. Chem. Colorist, 27 (Nov 1995) 17. R H Mehta and A R Mehta, Colourage, 43/45 (1996) 49. A Riva and J Cegarra, J.S.D.C., 103 (1987) 32. H Egli, Textilveredlung, 8 (1973) 495. W Mosimann, Text. Chem. Colorist, 1 (1969) 182. J Cegarra, Proc. IFATCC, Barcelona (1975). J Cegarra, A Riva and L Aizpurua, J.S.D.C., 94 (1978) 394. J Cegarra and A Riva, Melliand Textilber., 64 (1983) 221. D R Karsa and M R Porter, Biodegradability of surfactants (Glasgow: Blackie Academic and Professional, 1995). M J Schwuger, Detergents in the environment (New York: Marcel Dekker, 1996). S S Talmage, Environmental and human safety of major surfactants (London: Lewis Publications, 1994). C Gloxhuber and K Kunstler, Anionic surfactants: biochemistry, toxicology, dermatology, 2nd Edn. (New York: Marcel Dekker, 1995). P Schöberl, K J Bock and L Huber, Tenside Surf. Det., 25 (1988) 86. C G Naylor, Text. Chem. Colorist, 27 (Apr l995) 29. K Tsujii, Surface activity: principles, phenomena and applications (San Diego: Academic Press, 1998). J C Berg, Wettability (New York: Marcel Dekker, 1993). A K Chattopadhyay and K L Mittal, Surfactants in solution (New York: Marcel Dekker, 1996). S D Christian and J F Scamehorn, Solubilisation in surfactant aggregates (New York: Marcel Dekker, 1995). A W Neumann and J K Spelt, Applied surface thermodynamics (New York: Marcel Dekker, 1996). The HLB system – a time-saving guide to emulsifier selection, Publication 103–3 10M (Wilmington: ICI Americas, 1984).

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CHAPTER 10

Classification of dyeing and printing auxiliaries by function Terence M Baldwinson

10.1 ELECTROLYTES AND pH CONTROL The simplest auxiliaries of all are the neutral electrolytes such as sodium chloride and sodium sulphate. These are used in large quantities for dyeing cellulosic materials with direct or reactive dyes and wool with anionic dyes. The major effect of electrolytes on dyes of this type is to increase the degree of aggregation of the dye anions in solution by the common-ion effect, the degree of aggregation varying markedly with dye structure (section 3.1.2). The electrolyte suppresses ionisation of the dye in solution, thereby effectively reducing its solubility in the dyebath and modifying the equilibrium in favour of movement of dye anions from the solution into the fibre. The objective, of course, is to use the optimum amount of salt to give the required rate and degree of exhaustion of the dyebath; too little electrolyte is ineffective whilst too much may aggregate the dye to an extent that may inhibit its diffusion into the fibre, thus giving a tendency to surface coloration only, or even bringing about precipitation. The aggregating effect of electrolytes varies, sodium chloride having a stronger effect than sodium sulphate, but it is generally decreased by raising the temperature. This effect, which we may term the ‘salting-on’ effect, is the result of interactions between electrolyte and dye. However, there may also be interactions between electrolyte and fibre, giving rise to a positive levelling action as electrolyte anions compete with dye anions for the cationic sites in the fibre. Ionic surfactants (Table 8.1) can of course be regarded as electrolytes, although by hydrophobic interactions they tend to form micelles in concentrated solution and hence may be referred to as colloidal electrolytes. In some respects their levelling action is analogous to that of simple inorganic electrolytes – that is, ionic hydrophobes compete with dye ions of similar charge for sites of opposite charge in the fibre. Electrolytes are used to promote the exhaustion of direct or reactive dyes on cellulosic fibres; they may also be similarly used with vat or sulphur dyes in their leuco forms. In the case of anionic dyes on wool or nylon, however, their role is different as they are used to facilitate levelling rather than exhaustion. In these cases, addition of electrolyte decreases dye uptake due to the competitive absorption of inorganic anions by the fibre and a decrease in ionic attraction between dye and fibre. In most discussions of the effect of electrolyte on dye sorption, attention is given only to the ionic aspects of interaction. In most cases, this does not create a problem and so most adsorption isotherms of water-soluble dyes are interpreted on the basis of Langmuir or Donnan ionic interactions only. There are, however, some observed cases of apparently anomalous behaviour of dyes with respect to electrolytes that cannot be explained by ionic interactions alone. 497

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The fact is, ionic interaction between dyes, fibres and electrolytes is only part of the story. As Yang [1] has pointed out, hydrophobic interactions also need to be taken into consideration. Whilst this has been accepted for many years in relation to dye–fibre interactions, the extension of the concept to interactions involving neutral electrolytes is novel. Yang quotes as one of several examples the fact that sodium chloride has a stronger effect than sodium sulphate on decreasing the uptake of CI Acid Red 1 by nylon, which cannot be explained on the basis of ionic interaction alone. It can, however, be explained in terms of the effect of the electrolytes on hydrophobic interaction, the same explanation also being applied to other examples. A lyotropic series is used to explain the effectiveness of hydrophobic interactions, which always coexist with ionic interactions. A semi-quantitative representation of the lyotropic series is shown in Figure 10.1. In such a series, neutral ions have little influence on hydrophobic interactions. Kosmotropes increase hydrophobic interaction and therefore tend to increase dye adsorption, whilst chaotropes decrease both hydrophobic interaction and dye adsorption. Thus, in the example quoted above of CI Acid Red 1 on nylon, sodium chloride has a stronger effect on decreasing dye adsorption than the more kosmotropic sodium sulphate. On this basis, Yang has introduced a modified Donnan model that quantitatively predicts the various effects of electrolytes on either decreasing or increasing dye adsorption.

anions

SO42– > CH3COO– > Cl– > Br– NO–3 SCN– H2PO–4

cations

Li+ > Na+ > K+ > Rb+ Cs+ kosmotropes (water structure-makers)

N E U T R A L

chaotropes (water structure-breakers)

Figure 10.1 Lyotropic series: effectiveness of hydrophobic interactions [1]

R

CH3 + N (CH2)n _ CH3 X

CH3 + N R CH3 X

_

n = 3–12 R = n-propyl, n-butyl or benzyl X = halide, e.g. bromide

10.1

Rather more complex compounds that are currently being researched are the bolaform electrolytes [2–4]. Bolaform electrolytes are organic compounds possessing two cationic or two anionic groups linked by a flexible hydrocarbon chain; the terminal groups may be aliphatic or aromatic (e.g. as in 10.1). Their interaction with sulphonated monoazo dyes in the presence of poly(vinylpyrrolidone) as substrate has been studied in detail. However, it remains to be seen what commercial developments take place with these interesting

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compounds. It seems likely that they would be used in complex formation rather than in the more traditional roles associated with electrolytes in textile processing. In whatever role electrolytes are used, their effects on the environment need to be considered, particularly when discharged to effluent. High salt loading is undesirable in waste water and sodium sulphate in particular causes corrosion of concrete pipes. It thus makes sense to choose electrolytes carefully and to use the minimum amounts consistent with obtaining the desired effects. Automatic dosing is helpful in this respect. The use of shorter liquor ratios has been promoted on the grounds of economy (less water to heat and less liquor to treat subsequently). However, it should not be overlooked that when dyeing in a short liquor more rinsing baths are required to give the same residual concentration as at a longer liquor ratio. Weible [5] has demonstrated this effect for the washing-off of reactive dyes from fabric having a retention capacity of 4 l/kg using 60 g/l of electrolyte. At a liquor ratio of 20:1, some 12 g/l and 2.4 g/l of electrolyte are found in the first and second rinses respectively. These rise to 30 g/l and 15 g/l when a liquor ratio of 8:1 is used. Equation 10.1 was used by Weible to calculate these concentrations.

Cs =

Cs = c = FV = R =

cR (g/l) FV

(10.1)

concentration in rinsing bath (g/l) concentration in treatment bath (g/l) liquor ratio (l/kg) retention capacity of goods (l/kg)

More detailed information on attempts to reduce the impact of electrolytes on the environment is given under the individual dye classes discussed in Chapter 12. The great majority of coloration processes demand some control over the treatment pH, which varies from strongly alkaline in the case of vat, sulphur or reactive dyes, to strongly acidic for levelling acid dyes. The concept of pH is a familiar one; its theoretical derivation can be found in all standard physical chemistry textbooks and has been particularly well explained in relation to coloration processes [6,7] both in theory and in practice. We are concerned here essentially with the chemistry of the products used to control pH and their mode of action. It has been stated [7] that: ‘Unfortunately, pH control appears simple and easy to carry out. Add acid and the pH decreases; add base (alkali) and the pH increases. However, pH is the most difficult control feature in any industry’. The control of pH in textile coloration processes is ensured by three fundamentally different techniques: (a) the maintenance of a relatively high degree of acidity or alkalinity (b) the control of pH within fairly narrow tolerances mainly in the near-neutral region (c) the gradual shifting of the pH as a dyeing proceeds. Approach (a) is normally the easiest to control, and is used in the application of levelling acid and 1:1 metal-complex dyes to wool or nylon, and of the reactive, sulphur or vat dyes to cellulosic fibres. The agents traditionally used are the stronger acids and alkalis such as sulphuric, hydrochloric and formic acids, sodium carbonate and sodium hydroxide. In

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certain operations, particularly fixation in steam (as in printing), steam-volatile acids are replaced with non-volatile products such as citric acid. Use of approach (a) can lead to the misconception already mentioned, that pH is easy to control, particularly as the dye– substrate systems involved are not normally sensitive to minor pH shifts. Nevertheless, strong acids and alkalis can react to produce quite drastic changes in pH; this can occur, for example, when alkali is carried over from wool scouring into initially acidic dyebaths. Wool and nylon absorb acid from dyebaths, thereby inducing a change in the dyebath pH, but wool will absorb significantly more acid than nylon [8] – a factor to be borne in mind when comparing results on these two fibres, especially in those systems using ‘half-milling’ acid dyes for which the controlling agent is generally the weaker acetic acid; such systems represent a compromise between approaches (a) and (b) and are moderately sensitive to change in pH. In this area, the organic formic and acetic acids are of interest. Formic, of course, is a stronger acid than acetic. Hence, acetic acid has been traditionally the preferred choice for the adjustment of slightly acidic media, down to about pH 4, whereas formic was the choice below this level. It has been demonstrated, however, that for general purposes formic acid is preferred to acetic acid, particularly on economical and environmental grounds [9]. Formic acid has an extremely low BOD, being biodegraded to carbon dioxide and water. In any case, being a much stronger acid, smaller amounts are needed, thus giving less load for disposal. For example, in order to obtain pH values of 4.5, 4.0 and 3.3, the amount of formic acid 85% needed is, respectively, about 62%, 50% and 12% of that of acetic acid 80%. Other advantages of formic acid are that it has a more powerful neutralising effect than acetic acid and it is less corrosive than mineral acids. Approach (b) needs greater awareness of the factors that not only determine pH but also help to stabilise it against interference. Most of the dye–fibre systems requiring approach (b) are operated in the near-neutral region (pH 4–9) and are much more sensitive to minor changes in pH. In addition, the pH of the water supply may vary, or drift during heating. Even the pH of pure water changes on heating, from 7.47 at 0 °C to 7.00 at 24 °C and 6.13 at 100 °C, but that of the process water used in dyehouses and printworks can change much more drastically, most commonly showing an increase. Changes in pH on heating may counteract the intended response of process liquors, especially in the central pH range associated with approach (b); even more critical can be the effect of any acids or alkalis carried over from previous processes. The dye–fibre systems of obvious interest for approach (b) are milling acid and 1:2 metalcomplex dyes on wool or nylon, basic dyes on acrylic fibres and disperse dyes on various fibres. With wool and nylon there is often some overlap with approach (c) (section 12.2). Where control is not too critical, simple electrolytes of weak bases with strong acids (such as ammonium sulphate) or strong bases with weak acids (such as sodium acetate) are often used to produce slightly acidic or slightly alkaline media respectively. Ammonium acetate is also commonly used, producing a less acidic effect than ammonium sulphate. Occasionally acetic acid and sodium carbonate are used, necessitating careful control and monitoring. These simple expedients are not suitable for systems requiring more sensitive control, however, and use of single electrolytes such as ammonium sulphate or sodium acetate more properly belong to control systems based on approach (c). More precise control is achieved by the use of buffering systems. By the use of electrolyte pairs, these systems set the initial pH and exert a protective action that tends to resist changes arising from contaminants entering by way of the substrate or the water supply.

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Buffering systems are generally based on combinations of: – a weak acid together with the salt of this acid formed from a strong base, or – a weak base together with the salt of this base formed from a strong acid. The most commonly used example of the first type is acetic acid/sodium acetate, which functions well over the pH range 3.8–5.8. Acetic acid with ammonium acetate is also used although it is less effective, especially in those boiling dyebaths from which ammonia can escape into the atmosphere, thus allowing the pH to fall. Such acetate buffers have the advantage of low cost. Somewhat more expensive are the phosphate buffers, of which the most commonly used is a mixture of sodium dihydrogen orthophosphate (NaH2PO4) with disodium hydrogen orthophosphate (Na2HPO4). Here, as with most polybasic acid systems, the distinction between the acid and its salt seems blurred at first sight. In fact, sodium dihydrogen phosphate is the ‘acting acid’ and disodium hydrogen phosphate is its salt. The tribasic orthophosphoric acid and its three salts can be used to produce a series of buffers, each active within a particular pH range: – orthophosphoric acid and the monosodium salt, main buffering region pH 2.5–3.5 – the mono- and di-sodium salts, main buffering region pH 6–8 – the di- and tri-sodium salts, main buffering region pH 10.5–11. This can be seen from the titration curve for phosphoric acid [6] shown in Figure 10.2. In practice the mono- and di-sodium salt system is used most extensively, since this covers the pH range over which precise control is most often needed. These phosphate buffers are more resistant than the acetate systems to temperature-induced changes. 12 Na3PO4

10

Na2HPO4

pH

8 Main buffer region

6 NaH2PO4

pH 6.2–8.2

4 2

H3PO4

Alkali added

Figure 10.2 Orthophosphate buffer system

The most common buffering system containing a weak base together with its salt formed with a strong acid is ammonia with ammonium sulphate. Some useful buffers are obtained from combinations of unrelated acids or bases with salts. The following combinations find occasional use in textile coloration processes, but the acetates and orthophosphates are most frequently used:

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– pyrophosphoric acid (H4P2O7) and its salts (pH 3–9) – orthoboric acid (H3BO3), sodium tetraborate (borax Na2B4O7) and sodium hydroxide (pH 8.1–10.1) – citric acid and sodium hydroxide (pH 2.1–6.4) – sodium carbonate and sodium bicarbonate (pH 9.3–11.3). The pyrophosphate buffer is of particular technical interest as it can be used over the relatively wide range of pH 3–9. Unlike the orthophosphate titration curve, that for the tetrabasic pyrophosphate system is almost straight [6]. This linearity (Figure 10.3) means that effective buffering action is available across the whole pH range simply by using various pairs of ionised components and varying their proportions; even so, however, it does not seem to be widely used.

9

Na4P2O7

pH

7

5

3

Strong acid added

Figure 10.3 Pyrophosphate buffer system

The mechanism of buffering can be described by reference to the acetic acid/sodium acetate system. In aqueous solution sodium acetate can be considered to be practically completely ionised (Scheme 10.1), the equilibrium being wholly to the right-hand side. Since acetic acid is a weak acid it is only slightly ionised, and the equilibrium represented by Scheme 10.2 lies mainly to the left-hand side. This low degree of ionisation is even further suppressed in the presence of sodium acetate as a result of the common ion (in this case acetate) effect operating through the law of mass action. The undissociated acetic acid is, in effect, a ‘bank’ of hydrogen and acetate ions that can be brought into play as a neutralising mechanism when either acidic or alkaline chemicals enter the system (either by deliberate addition or adventitiously). If a small amount of an acidic solute is added to the mixture, the added hydrogen ions combine with acetate ions to form undissociated acetic acid, which has only a minimal effect on the pH of the system. If an alkaline solute is added to the buffer, the added hydroxide ions react with the bank of hydrogen ions to form undissociated water and so again the ionic balance and hence the pH remain essentially the same. The mechanisms of other buffering systems are similar: buffering action is increased by adding more of the components, keeping their proportions constant.

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CH3COONa

Na + CH3COO

CH3COOH

H + CH 3COO

pH CONTROL

503

Scheme 10.1

Scheme 10.2

Approach (c) for pH control involves a deliberate shift of pH during the processing cycle, in a consistent direction rather than randomly. Systems of this type are particularly useful for non-migrating anionic dyes on wool or nylon and have long been known in this connection. More recently, similar systems have been adopted for reactive dyes on cellulosic fibres. The simplest and most widely used of these systems consist of the salts of strong acids with weak bases or of strong bases with weak acids, examples being ammonium sulphate and sodium acetate respectively. Ammonium sulphate, for instance, dissociates in aqueous media to yield the dominant strong-acid species of sulphuric acid, so lowering the pH (Scheme 10.3) at a rate that increases with temperature, especially when the ammonia formed can be released from an open dyebath. Ammonium acetate functions in the same way but does not yield as great a pH shift. Similarly, a solution of sodium acetate tends to produce the dominant strong-alkali species of sodium hydroxide (Scheme 10.4), thus increasing the pH.

(NH4)2SO4 + 2H2O

H2SO4 + 2NH4OH

NH4OH

NH3 + H2O

2H + SO42–

Scheme 10.3

CH3COONa + HOH

NaOH + CH3COOH

Na + OH Scheme 10.4

Acetic acid (b.p. 118 °C) is not boiled off from open dyebaths as readily as ammonia but is rapidly flashed off in steam or dry heat processes, thus developing the maximum degree of alkalinity under these conditions. The sodium salts of less volatile acids, such as sodium citrate, can be used to develop a lower degree of alkalinity. If the process demands a gradual shift from about pH 9 to a slightly acidic pH, ammonium sulphate together with ammonia can be used. This gives a safer, more uniform development of acidity than can be achieved by making additions of acid to an alkaline bath, although the degree of acidity developed will clearly depend on the ease with which ammonia can escape from the system. In enclosed or partially enclosed machines this system does not function so efficiently [10–12].

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Another method of obtaining a pH shift in the direction of acidity is to use an organic ester that hydrolyses to the alcohol and acid under the conditions of processing. Ethyl lactate (Scheme 10.5) and diethyl tartrate (10.2) have been recommended for applying milling and chrome dyes to wool [13]. 2-Hydroxyethyl chloroacetate (10.3) and γ-butyrolactone (10.4), which hydrolyses during processing to give 4-hydroxybutyric acid (Scheme 10.6), have also been recommended. Such hydrolysable esters may be used alone, beginning at a near-neutral pH, but more likely in conjunction with an alkali to give a higher starting pH. Thus γ-butyrolactone and sodium tetraborate (borax), giving a pH shift from about 8 to 5.6, have been recommended for the dyeing of wool [14], as has 2-hydroxyethyl chloroacetate with sodium hydroxide for the dyeing of nylon [15]. Such hydrolysable esters are sometimes sold under proprietary trade names. The disadvantages of hydrolysable esters have been their higher cost, a limited pH range and, where the dyebath is to be reused, the need for increasing quantities of ester to overcome the buffering effect caused by the accumulation of salts [7,16]. Interest in these systems has declined due to environmental pressures on the one hand and the increased availability and sophistication of automatic dosing and monitoring systems on the other. H3C

O CH

H2O

H3C

C

C

HO

OCH2CH3

HO

O CH

OH + HOCH2CH3

Scheme 10.5

O HO

C OCH2CH3

HC HC HO

O

OCH2CH3

C

C ClH2C

O 10.2

OCH2CH2OH 10.3

O O C O

CH2 CH2 CH2

H2O

C HO

CH2

HO CH2

CH2

10.4

Scheme 10.6

The advantages of automatic metering and monitoring devices were well described by Mosimann [17] and have recently been re-emphasised [9]. Such devices are clearly of great value in environmental terms since they are crucial in ensuring that the minimum quantity of agent is used, thus reducing the effluent load to a minimum. The use of strong acids and bases for control of pH-shift systems is obviously fraught with difficulties where the operation is carried out manually. If a sophisticated automatic monitoring and dosing system is used, however, the use of such compounds has certain very worthwhile advantages:

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(a) The adjusting chemicals are the cheapest available; (b) The entire range of pH values can be controlled by using just two chemicals; (c) Since a buffering system is not built up in the bath, the pH can be shifted in any direction to any degree, easily and with the minimum addition of chemicals; (d) As a result of (c), any variation in the intrinsic pH of the substrate or water supply can be easily neutralised; (e) Exhausted dyebaths can often be reused as there is no build-up of buffering agent; and (f) There are no environmental problems. It should be noted that automatic dosing and monitoring of dyes and chemicals can be used generally and is not restricted to pH control.

10.2 SEQUESTERING AGENTS The tendency of soaps and other carboxylates to form insoluble complexes with calcium and magnesium ions in hard water is mentioned in sections 9.4 and 9.8.2. Apart from decreasing the efficiency of the anionic surfactant, deposition of such insoluble complexes on the textile substrate can cause problems in subsequent processing and particularly in coloration. Even trace amounts of certain transition-metal or alkaline-earth elements may cause processing difficulties. The formation of ‘iron spots’, particularly in bleaching, is well known: multivalent transition-metal cations catalyse the decomposition of hydrogen peroxide (although divalent calcium and magnesium ions have a stabilising effect) and localised staining or tendering of the fibre may occur. In coloration trace-metal ions can react with certain dyes, giving rise to precipitation, discoloration, unlevel dyeing and reduced fastness. The processing water is the most obvious source of such extraneous metal ions, but other potential sources should not be overlooked. For example, trace metals may be dissolved from the surfaces of machinery and fittings. The substrate may already contain such metals, as may also any chemicals or dyes used. Hence these problems cannot always be avoided simply by ensuring the supply of suitable water – indeed, the overzealous treatment of water can actually lead to the presence of troublesome aluminium ions that were not originally present! Such problems can be solved using chemicals that react preferentially with the metal ions, effectively preventing them from interfering with the mainstream reaction or process. Such chemicals are aptly known as sequestering agents. Other terms frequently used in the literature include the derivative ‘sequestrants’ and ‘complexing agents’, although complexing does cover a wider field than just metal–ion chelation with which we are concerned here. Sequestering agents work by a mechanism of complex formation, often in the form of chelation. A chelating agent contains substituents suitably located to form one or more chelate rings by electron donation to the metal ion (section 5.2), the resulting complex remaining soluble and innocuous under the conditions of processing. The most useful donating atoms are nitrogen, as found in amines or substituted amines, and oxygen in the form of carboxyl, phosphate or ionised hydroxy groups. As in the formation of dye–metal chelates (such as chrome mordant and metal-complex dyes), at least two electron-donating atoms in the sequestering agent structure must be arranged so that a stable ring can be formed with the metal ion, the highest stability resulting from five- and six-membered rings.

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

A great many chemicals exhibit sequestering capability but not all are of commercial value in textile processing. Earlier literature [18,19] mentioned three main types: – aminopolycarboxylates – phosphates, mainly inorganic – hydroxycarboxylates. However, environmental awareness, in addition to commercial and technical exploitation, has resulted in considerable activity in this area, leading to a greatly expanded range of products in recent years, as well as some conflicting statements with regard to their environmental properties. The scheme of classification adopted here is as follows: – aminopolycarboxylates and their analogues, e.g. hydroxyaminocarboxylates – phosphates and phosphonates – hydroxycarboxylates – polyacrylic acids and derivatives. 10.2.1 Aminopolycarboxylates and their analogues These are powerful chelating agents, having good environmental properties [20]. Important members include: – ethylenediaminetetra-acetic acid EDTA (10.5) – diethylenetriaminepenta-acetic acid DTPA (10.6) – nitrilotriacetic acid NTA (10.7) These products are sold as free acids or sodium salts. Analogues of these aminopolycarboxylic acids include the hydroxyaminocarboxylic acids. These structures are derived

O Na +

O

_ C O CH2 N

_ Na+ O C

C H2C CH2CH2

N H2C

CH2

O

_ + O Na

_ O Na+ C O

10.5 EDTA

O O Na+

_

+ Na

C O

O

_ C CH2 O

CH2

C H2C

_ O Na+

N N Na +

CH2CH2

N _ + O Na

_ O C O

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506

H2C

CH2 10.6 DTPA

C O

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O Na+ Na+

_

C O

O

CH2 N

_ O C

CH2

C

CH2

_ + O Na

10.7

O

NTA

by replacing one or more carboxymethyl groups of the aminopolycarboxylate by a hydroxyethyl group. Examples include: N-(hydroxyethyl)ethylenediaminetriacetic acid HEDTA (10.8) in which one of the carboxymethyl groups of EDTA has been replaced by a hydroxyethyl group N,N-bis(hydroxyethyl)glycine DEG (10.9) in which two of the carboxymethyl groups of NTA have been replaced by hydroxyethyl groups. O

O

_ C + Na O CH2

C H2C

N

CH2CH2

CH2

HO

_ O Na+

HO

CH2 CH2

N H2C

CH2

C 10.8

N

_ O Na+

HO

CH2 CH2

O

HEDTA

O CH2

C

_ + O Na

10.9 DEG

These compounds are not persistent in the environment, NTA degrading slightly more quickly than EDTA, DTPA or HEDTA [20]. These aminopolycarboxylates act as sequestering agents by forming complexes in which each metal ion is chelated into one or more five-membered rings. It is often assumed that one molecule of sequestering agent interacts with one metal ion and for many practical purposes this is a valid assumption. The nature of the complexes actually formed, however, may depend on other factors such as the pH of the medium. It is difficult to represent such structures in detail, particularly as water of solvation is usually involved. It is convenient to adopt a simplified representation, omitting the water of solvation, as for the EDTA–calcium complex shown in structure 10.10, in which the arrows represent coordination bonds and the calcium ion is held by three five-membered rings. At pH values below 11 the structure tends to be more like that shown in 10.11, which also resembles the complex formed with NTA (10.12). O C O Na+

_ C O

H2C H2C

_ O

_ O Ca2+

N

N CH2CH2

O C CH2 CH2

O C

_ O Na+

10.10

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O

O

C _ O CH2 2+

Ca _

N

C CH2CH2

_

2+

Ca

N

C O

C

O

O

N

Ca

O

C

CH2 2+

_

H2C

CH2

O

O

_ O

H2C

_

O CH2

C

CH2

O

_ O

Na+

C O

10.11

10.12

A more elaborate representation of an EDTA-metal complex (10.13), which gives some indication of the three-dimensional aspects of the structure, shows a complex of five fivemembered rings [18]. A similar representation of a DTPA-metal complex shows a system of eight five-membered rings. O C CH2

O O C

CH2

O

N

CH2

M

CH2 O

N C O

CH2

CH2 O

C

10.13

O

10.2.2 Phosphates and phosphonates Various polyphosphates are effective sequestering agents under appropriate conditions. The best known of these is sodium hexametaphosphate (10.14), the cyclic hexamer of sodium orthophosphate. Further examples are the cyclic trimer sodium trimetaphosphate (10.15), as well as the dimeric pyrophosphate (10.16), the trimeric tripolyphosphate (10.17) and other linear polyphosphates (10.18). All of these polyanions function by withdrawing the troublesome metal cation into an innocuous and water-soluble complex anion by a process of ion exchange as shown in Scheme 10.7 for sodium hexametaphosphate. Hence these compounds are sometimes referred to as ion-exchange agents. The disadvantage of the polyphosphates is that at the temperatures (100 °C or higher) used in many textile processes they can be hydrolysed into simpler phosphates that cannot retain the metal atom in the sequestered form. For example, dicalcium disodium hexametaphosphate hydrolyses on prolonged boiling to yield the insoluble calcium orthophosphate. This is one of the main reasons why polyphosphate sequestrants are used much less extensively than the more versatile and stable aminopolycarboxylates.

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_ + O Na

O + Na Na+

P

O

_

P

O

O

O

O

_ P

O

O O

O

O P

O

_ O

O

P P

Ca2+ _ O

_ O Na +

P

_ + O Na

O

O _ P O Na

O

+ 2 Ca2+

P

O _ O

O O

O

O _ + O Na

O

O P

O

509

O

P

Ca2+ _ O

O P _ O Na+

+ + 4 Na

10.14 Sodium hexametaphosphate Scheme 10.7 _ O Na+

O + Na

_ O O P

P

O Na+

O O P O

O

_

O

Na+

_

P

O

O

Na+

O

O

_ P O

_

_ O Na+ Na+

10.16 Sodium pyrophosphate

10.15 Sodium trimetaphosphate

Na+

_

O

O

O _

O P _ Na+ O

O

P O P O _ _ O Na+ O Na+

+ Na

O _ O P _ Na+ O

+ Na

O O

P

O O

_ + O Na

P

_ O Na+

_ O

Na+

n

10.17

10.18

Sodium tripolyphosphate

Sodium polyphosphate

A structural compromise between these two types of compound can also give products with sequestering properties, although they are phosphonates rather than phosphates, since they contain C–P rather than C–O–P linkages. Examples of these aminopolyphosphonates are: – ethylenediaminetetramethylphosphonic acid EDTMP (10.19) – diethylenetriaminepentamethylphosphonic acid DETMP (10.20) – nitrilotrimethylphosphonic acid ATMP (10.21) – hydroxyethylethylenediaminetrimethylphosphonic acid HEDTMP (10.22) – hexamethylenediaminetetramethylphosphonic acid HMDTMP (10.23) Two sequestrants of the phosphonate class unrelated to aminopolycarboxylic acids are: 1-hydroxyethane-l,1-diphosphonic acid HEDP (10.24) 2-phosphonobutane-1,2,4-tricarboxylic acid PBTC (10.25)

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O HO

O

P

CH2

HO HO HO

CH2CH2

N

P

CH2

H2C

P

N

OH OH

H2C

P

OH

O

10.19

O

OH

EDTMP

HO

P

P

O

P CH2

CH2

HO

H2C

P

N

OH OH

H2C

P

N

HO HO HO

O

HO

O

CH2CH2

N

CH2CH2

CH2 10.20

O

OH

OH O

O

DETMP

HO

P

CH2

HO HO O HO

O

P

CH2

HO HO CH2

N

CH2CH2

CH2

H2C

P

N

OH OH

H2C

P

HO

N

P

OH

O CH2

OH

CH2

O

10.21 ATMP

OH

O

10.22 HEDTMP

O HO

O

P

CH2

HO HO HO

P

N

CH2CH2CH2CH2CH2CH2

H2C

P

N

OH OH

H2C

CH2 10.23

O

P

OH

OH

O

HMDTMP

OH

O

HO

O

CH3

O

P

C

P

HO

chpt10(2).pmd

OH

OH

O C

OH

CH2CH2

HO

O C

10.24

10.25

HEDP

PBTC

510

C

O

C

P

CH2 OH

OH

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OH

OH

SEQUESTERING AGENTS

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The consumption of these phosphonates in textile processing is small in relation to that of the aminopolycarboxylates; they are mainly used in detergent formulations [21,22] as sodium, potassium, ammonium or alkanolamine salts. Environmentally, phosphates generally have been a sensitive issue, not least because they can cause eutrophication of watercourses, and the situation is still not resolved completely. No aerobic or anaerobic bacterium has been found to date that will biodegrade aminopolyphosphonates under the treatment conditions used today, yet these products are not biologically persistent. They are partially eliminated photolytically, partially absorbed in sediment or eliminated by precipitation [23,24]. They show 50–80% elimination in the Zahn–Wellens test. They show low aquatic toxicity and are non-toxic to humans, animals and plants. Detailed ecological properties are listed by Schöberl and Huber [25]. Held [26] has investigated the ecological behaviour of sequestering agents based on phosphonic acids in detail, concluding that although they contain phosphorus they do show ecological advantages compared with other types and thus their use is justified. 10.2.3 Hydroxycarboxylates The hydroxycarboxylic acids provide a range of sequestering agents of which the best known are citric (10.26), tartaric (10.27) and gluconic (10.28) acids. The toxic oxalic acid (10.29) is now rarely used. However, these acids are much less important as sequestering agents for textile processes than either the aminopolycarboxylates or the polyphosphates. Hydroxycarboxylates are easily biodegraded but do have a high COD. It has been pointed out [20] that glucoheptanoic acid (2,3,4,5,6,7-hexahydroxyheptanoic acid; 10.30) is also used in the USA, on the grounds that this compound is less prone to browning at high temperatures although it possesses no other advantages, having less binding power as well as being more expensive than other hydroxycarboxylic acids.

O C HO

H

OH H

C

C

O

C

O

C OH

H C H O H

C

10.26 Citric acid

HO

H

OH

C

C

OH H

O C

10.27 Tartaric acid

OH

O

HO

H

H

OH H

OH

C

C

C

C

H

OH H

C

O C

C

HO

10.28 Gluconic acid

OH

OH H

O C

OH 10.29

Oxalic acid H HO

H

OH H

OH H

C

C

C

C

H

OH H

C

OH H

C OH

O C OH

10.30

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

10.2.4 Polyacrylic acids and their derivatives Recent research has led to some more complex sequestering agents, particularly the polymeric carboxylic acids referred to as polycarboxylates. These are, in effect, polyelectrolytes and as such have close similarities to the products described later as dispersing and solubilising agents (section 10.6), thickening agents or migration inhibitors (section 10.8). Common monomers used in the production of these compounds, either as homopolymers or as copolymers with each other, include acrylamide (10.31) and various unsaturated acids (10.32–10.34). The common and essential feature of these monomers is the carbon–carbon double bond. H H2C

H

C

H2C CONH2

H

CH3

C

H2C

C

COOH

H C

HOOC

COOH

10.31

10.32

10.33

Acrylamide

Acrylic acid

Methacrylic acid

C COOH

10.34 Maleic acid

Polymers which have been suggested for use as sequestering agents [23,27] include: – poly(butadiene-1,2-dicarboxylic acid) EMA (10.35) which is an ethylene–maleic acid copolymer – poly(α-hydroxyacrylic acid) PHAS (10.36) – poly(3-hydroxymethylhexatriene-1,3,5-tricarboxylic acid) (10.37) which is a copolymer of acrylic acid with hydroxymethacrylic acid – poly(3-formylhexatriene-1,3,5-tricarboxylic acid) (10.38) which is a copolymer of acrylic acid with 2-formylacrylic acid.

OH

CH2

CH2

CH

HOOC

CH2

CH

C COOH

COOH

n

n

10.35

10.36

EMA

PHAS

CH2OH CH2

CH

CH2

COOH

C

CH2

COOH

CH COOH

n

10.37

CHO CH2

CH

CH2

COOH

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C

CH2

COOH

CH COOH

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SEQUESTERING AGENTS

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These agents are generally described as ion-exchange reagents rather than complex-forming chemicals. They tend to operate by sequestering the metal by an ion-exchange mechanism and as a result of their polyelectrolytic character they keep the complex dispersed. Oligomers with a molecular mass of 1200–8000 (i.e. relatively low) are said to give optimum sequestering power. Polymers of high molecular mass (i.e. Mr = 106–10 7) are useful as flocculating agents or migration inhibitors. Although these acrylic oligomers and polymers show little decomposition in effluent treatment, they pose no significant threat to the environment since they can be removed easily by adsorption on activated sludge or by precipitation as an insoluble calcium complex [23]. Exhaustive tests have not revealed any adverse environmental influence. Their aquatic toxicity is negligible and toxicity to warm-blooded mammals is slight. Mutagenic, carcinogenic or teratogenic effects have not been found so far. A further development [27] is the formation of so-called sugar–acrylate copolymers in which acrylic acid is copolymerised with glucose or other saccharides. Unlike other sequestering agents these polymers are said to be readily biodegradable, this being the main reason for their development. An unusual type of sequestering agent is triethanolamine (10.39). This compound is cheap and exclusively useful for complexing iron(III) in strongly alkaline solutions, e.g. up to 18% sodium hydroxide. It does in fact remain active as a complexing agent even in more strongly alkaline solutions although solubility can be a problem. CH2CH2OH HOCH2CH2

N CH2CH2OH

10.39 Triethanolamine

10.2.5 The action of sequestering agents Sequestering agents are often used rather indiscriminately, in amounts far in excess of the stoichiometric quantities required by the particular set of conditions. Instructions often simply state ‘add 0.5–1.0 g/l of a suitable sequestering agent such as EDTA’. Whilst this is convenient for most purposes, it is worth bearing in mind that the action of sequestering agents is governed by physico-chemical factors that, among other things, determine a hierarchy of efficacy. When the type and concentration of trace-metal ions to be sequestered is known, a more discriminatory approach can be adopted regarding the choice of agent. In some cases, including the treatment of water, this more precise specification of type and quantity can be important. Little more need be said here about the simple ion-exchange reactions such as that between sodium hexametaphosphate and calcium ions (Scheme 10.7). It is useful, however, to consider in more detail those reactions involving chelation (Scheme 10.8). This is a reversible reaction, the equilibrium being dependent on the process pH and the concentrations of the reacting species (Equation 10.2). While chelated complexes are less stable at higher temperatures, this effect can be ignored in practice. The factors involved have been discussed in some considerable detail by Engbers and Dierkes [20,23].

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Sequestering agent (SA) + Metal ion (MI)

Chelated complex (CC)

Scheme 10.8

Ks =

[CC] [SA][MI]

(10.2)

The stability of the complex is generally expressed in terms of its stability constant, which is the logarithm of the equilibrium constant (Ks in Equation 10.2) of the reaction in Scheme 10.8. A high stability constant indicates a powerful sequestering effect. For example, amongst the aminopolycarboxylates the stability constant for a given metal ion generally increases in the order: NTA < HEDTA < EDTA < DTPA. Metals can also be listed in order of increasing stability constant: Mg2+ < Ca2+ < Mn2+ < Al3+ < Zn2+ < Co3+ < Pb2+ < Cu2+ < Ni3+ < Fe3+. The stability constants at 25 °C for six metals with four different aminopolycarboxylates are shown in Figure 10.4. 30 NTA

HEDTA

EDTA

DTPA

log Ks

20

10

Mg(II)

Ca(II)

Fe(II)

Zn(II)

Cu(II)

Fe(III)

Figure 10.4 Stability constants of aminopolycarboxylate chelates [20]

Thus for the series of sequestering agents and metal ions mentioned, the magnesium– NTA complex has the lowest stability and iron(III)–DTPA the highest. This scale of values effectively constitutes a displacement series. This means, in general, that in any system containing more than one metal it is the metal forming the most stable complex (that is, the complex having the highest stability constant) that chelates preferentially. When these ions have been completely chelated, any remaining sequestering agent then begins to sequester the metal that forms the complex having the next highest stability constant. Similarly if iron(III) enters a system in which, for example, calcium is already chelated, the iron will displace the calcium since the iron complex has the higher stability constant. The calcium will only remain chelated if sufficient sequestering agent is present to sequester both iron and calcium. Adding protons or hydroxide ions to the system will influence the position of the chelation equilibrium. The stability constant of a complex is thus influenced by the pH of

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the system and pH is an important consideration in the choice of sequestering agents. Figure 10.5 provides a very good illustration of the high sensitivity of the conditional stability constant (Kc) to the pH of the system, using the same four aminopolycarboxylates in the presence of iron(II) and iron(III). The pH-related stability constant is known as the conditional stability constant, log K c. The effects of pH on the conditional stability constants of the same four aminopolycarboxylates with other metal ions are shown in Figures 10.6 to 10.9. 20

log Kc

15

10

5

1

3

5

7

9

11

13

pH DTPA-Fe(III)

DTPA-Fe(II)

ETDA-Fe(III)

EDTA-Fe(II)

HEDTA-Fe(III)

HEDTA-Fe(II)

NTA-Fe(III)

NTA-Fe(II)

Figure 10.5 Effect of pH on the conditional stability constants at 25 °C of Fe(III) and Fe(II) chelates of aminopolycarboxylic acids [20]

20

DTPA-Fe(III) DTPA-Cu(II) DTPA-Fe(II)

log Kc

15

10

DTPA-Zn(II)

5

DTPA-Ca(II) DTPA-Mg(II)

1

3

5

7

9

11

13

pH

Figure 10.6 Effect of pH on the conditional stability constants at 25 °C of metal chelates of DTPA [20]

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

20

EDTA-Fe(III)

EDTA-Fe(II)

EDTA-Ca(II)

EDTA-Cu(II)

EDTA-Zn(II)

EDTA-Mg(II)

log Kc

15

10

5

1

3

5

7

9

11

13

pH

Figure 10.7 Effect of pH on the conditional stability constants at 25 °C of metal chelates of EDTA [20] 20

HEDTA-Fe(III)

HEDTA-Fe(II)

HEDTA-Ca(II)

HEDTA-Cu(II)

HEDTA-Zn(II)

HEDTA-Mg(II)

log Kc

15

10

5

1

3

5

7

9

11

13

pH

Figure 10.8 Effect of pH on the conditional stability constants at 25 °C of metal chelates of HEDTA [20] 20 NTA-Fe(III) NTA-Cu(II) NTA-Fe(II)

15

NTA-Zn(II)

log Kc

NTA-Ca(II) NTA-Mg(II)

10

5

1

3

5

7

9

11

13

pH

Figure 10.9 Effect of pH on the conditional stability constants at 25 °C of metal chelates of NTA [20]

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517

It should not be overlooked that stability constants can be affected significantly by the presence of electrolytes. For example, the stability constant of the Ca–EDTA complex changes from 12 in distilled water to 10.5 in 0.1N KCl and to 8.5 in 1.0N KCl. The inorganic polyphosphates tend to be most efficient under slightly acidic conditions whilst the aminopolycarboxylates generally work best under neutral or alkaline conditions, although they still show some usefulness, and are used, at pH values of around 4.5. Generalisations like these can be misleading, however, since efficiency varies from one metal ion to another at different pH values for each sequestering agent. The phosphates are good sequestrants for magnesium and calcium but are considerably less effective for trivalent cations, which can be successfully sequestered with NTA, EDTA and DTPA up to about pH 9. At higher pH values iron(III) tends to be precipitated from these complexes. It was mainly for this reason that the hydroxyaminocarboxylates were developed, this basically being their main use. For example, HEDTA will sequester iron(III) ions at pH 9 and DEG works well at pH 12. Although effective with most metal ions, DEG will not sequester calcium or magnesium, and HEDTA is also not as efficacious with these hard water ions as are the aminopolycarboxylates. At pH values above 12 iron(III) can be sequestered with triethanolamine (10.39), either alone or together with EDTA. Most divalent and trivalent ions, with the exception of the alkaline-earth metals, are effectively chelated by the hydroxycarboxylates citric and tartaric acid, and citric acid will also sequester iron in the presence of ammonia. Another hydroxycarboxylate, gluconic acid, is especially useful in caustic soda solution and as a general-purpose sequestering agent. Clearly, the efficiency of sequestering action must be optimised for a specific set of conditions. Thought needs to be given especially to the pH of the system and to whether broad-spectrum or specific sequestering is required. The extent of knowledge of the tracemetal ions present will determine whether a precise addition or an arbitrary excess of agent is appropriate. Finally, in some circumstances problems can arise from the use of certain sequestering agents that can remove the coordinated metal from a dye chromogen with subsequent changes in shade or fastness properties. Metal-complex acid dyes and mordanted dyes are obviously vulnerable, but also many direct and reactive dyes contain coordinated copper atoms.

10.2.6 The uses of sequestering agents Notwithstanding the comments made above in relation to the need to adopt a more or less sophisticated approach to the selection and use of sequestering agents to target known contamination by trace metals, an attempt will now be made to provide general recommendations [28]. This approach will also indicate the wide range of uses for these agents in textile wet processing. Sized warp yarns sometimes contain metal salts or complexes (e.g. copper or zinc compounds) as fungicides or bactericides and these can interfere with enzyme action in subsequent desizing. Phosphonates such as ATMP, DETMP, EDTMP and PBTC are suitable for use here, sequestering the heavy metals at pH 6.8–7.0, the effectiveness of each agent being dependent on the type of enzyme and the metal ions present. The use of the phosphonates HEDP or PBTC helps to reduce cellulose damage to a minimum in oxidative desizing with persulphate. In the kier boiling of cotton, the action of sodium hydroxide can be intensified by the use

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of sequestering agents. The phosphonates EDTMP, HEDP and PBTC have been recommended. In practice, however, the tendency is to use synergistic mixtures, such as: – phosphonate and gluconate – phosphonate and triethanolamine – aminopolycarboxylate and gluconate or triethanolamine. Polycarboxylates may also be added to increase dispersing power and so reduce the possibility of incrustations. Fine-tuning will again depend on the process details, the machine type and the degree of scouring necessary. Phosphonates are useful additions in acidification treatments after alkaline processing to assist in the removal of metal compounds that have limited solubility in alkali. Under the strongly alkaline conditions of mercerising, addition of either gluconate or triethanolamine with a little HEDP is useful. The presence of a polycarboxylate helps to prevent precipitation on machine components. Certain transition-metal salts catalyse redox reactions, leading to uneven treatment and perhaps damage to the substrate. Sequestering agents are therefore employed to complex these metal ions and so to inhibit their catalytic activity. In reductive bleaching with dithionite, PBTC acts as a stabiliser at pH 5.5–6.5 and EDTA also gives good results. At higher pH values, aminopolyphosphonates (e.g. EDTMP) and aminopolycarboxylates are useful. Only triethanolamine is effective at pH 13. For oxidative bleaching with hypochlorite or chlorite, the amine oxides of ATMP and PBTC may be used. Aminopolycarboxylates are less suitable, whilst the amine oxide of NTA is unsuitable. In the stabilisation of peroxide with silicate, the use of a polycarboxylate, perhaps in combination with a polyphosphonate such as DETMP (or PBTC in a lesser amount) helps amongst other things in preventing fibre damage and incrustations on fabric or machine. Conversely, aminopolyphosphonates such as EDTMP or DETMP may themselves be suitable as stabilisers in the absence of silicate. Combinations of sequestering agents may also be used to obtain a synergistic effect, the following mixtures having been suggested for use with silicates: – DETMP, HEDP and either gluconate or triethanolamine – DETMP, DTPA and gluconate. Polycarboxylates may also be added to help prevent incrustations. It should be borne in mind, however, that magnesium is an essential component in most cases of stabilisation in peroxide systems, so any mixture of sequestrants should have minimum binding effect on this metal ion. In the pretreatment and dyeing of synthetic fibres, the aminopolyphosphonates can assist in the removal of oligomers. Some dyes contain a coordinated transition metal as an essential part of their chromogenic structure and this must be left undisturbed by any sequestrant used to complex extraneous metal ions in the system. Hence a balance of properties is needed, phosphates and hydroxycarboxylates being useful. It is claimed that polycarboxylates can be molecularly engineered to give the required balance of properties. Wool is exceptionally prone to absorb metal ions, particularly in the weathered tips of the fibres, leading to shade differences on subsequent dyeing, especially if chelatable dyes are used. Hence sequestering agents can be essential additions to scouring, rinsing and dyeing

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baths in order to remove this absorbed metal. However, it is difficult to say which sequestrant or mixture of sequestrants gives the best results. Careful laboratory tests need to be carried out beforehand, taking into account the chemical structure of the dyes to be used, the type of metal ions involved, the pH value of the dyebath and the type and concentration of electrolyte present. In many dyeing and printing operations, livelier, more intense colours can be obtained with better reproducibility and perhaps better fastness to rubbing by careful choice of sequestering agent for use in the coloration process. The addition of triethanolamine with vat dyes can be beneficial in helping to prevent unnecessary loss of reducing agent or overoxidation. Triethanolamine together with EDTMP or HEDP can be used where the concentration of alkali is less than 10 g/l NaOH (or 22 ml/l caustic soda liquor 38°Bé). Bronzing of sulphur dyeings can often be prevented using triethanolamine with polyphosphate, polyphosphonate or polycarboxylate. Azoic combinations can be very sensitive to metal contamination. EDTA, or perhaps combinations of polyphosphate, polyphosphonate or polycarboxylate, can help in the solubilisation of naphtholates and in the stabilisation of their colloidal solutions, whilst EDTA, for example, can assist in protecting solutions of diazonium salts from metal-induced catalytic decomposition. Polyphosphates, or in lower amounts the very effective polyphosphonates, are helpful in applications of fluorescent brightening agents. Sequestering agents can be useful additions in the afterwashing of dyeings and prints, for example polyphosphates, polyphosphonates or polycarboxylates such as, amongst others, a polyacrylate of molecular mass 3000–4000 or an acrylic–maleic acid copolymer. In the soaping of vat or azoic dyeings, recrystallisation is accelerated and rubbing fastness improved by the use of sequestering agents, examples being mixtures of polyphosphonates and polycarboxylates, such as HEDP and an acrylic–maleic acid copolymer. The sugar–acrylate polymers [27] are recommended for applications similar to those mentioned above for polycarboxylate polymers and copolymers.

10.3 MACROMOLECULAR COMPLEXING AGENTS Macromolecular complexing agents have featured a good deal in recent research. Although they do not yet appear to have attained any significant commercial use, they possess interesting properties, not least their environmental advantages, that offer potential for future exploitation. The term macromolecule includes all large molecules, including textile fibres and polymers. The polyelectrolytes used as dispersing agents (section 10.6) or as thickening agents and migration inhibitors (section 10.8) are examples of linear macromolecules. Cyclic macromolecules are also known. An important feature of such macromolecules often responsible for their functioning as auxiliaries is their ability to form complexes, particularly with dyes or fibrous polymer segments. In the case of linear macromolecules, the complexes are generally formed by multipoint attachment with the smaller entity situated alongside the macromolecule. Cyclic macromolecules, on the other hand, may exhibit the interesting property of complexing another compound within its centre, the macromolecule completely surrounding the complexed entity. Thus such agents have some functional similarity with sequestering and chelating agents. However, whereas sequestering and chelating agents are generally used to complex simple metal ions, the macrocyclic complexing agents are usually

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engineered to complex with bigger molecules such as dyes, polymer segments or surfactants and do not act as simple metal-sequestering agents. It is these properties that have stimulated much research into the possible uses of macrocyclic complexing agents as auxiliaries in coloration processes or as agents for helping to clean up textile wastes. It is particularly interesting that certain macrocyclic agents can be obtained from natural replenishable sources. Four types of macrocyclic complexing agents are considered here: – cyclodextrins – cucurbituril – crown ethers – liposomes such as phosphatidylcholine derivatives. The discussion, however, is relatively brief in view of the fact that little commercial development seems to have taken place so far. 10.3.1 Cyclodextrins Cyclodextrins are obtained by enzymatic depolymerisation and extraction from starch. They comprise rings of D-glucose units and α-, β-, or γ-cyclodextrin can be obtained [29], depending on whether 6, 7 or 8 glucose units are present in the ring (10.40). The dimensions of the outer and inner surfaces increase as the number of glucose units increases (Figure 10.10). The important characteristic feature of these cylindrical structures that influences their properties lies in the different nature of their surfaces, the outer being essentially hydrophilic whilst the inner is essentially hydrophobic [29–31]. Thus the outer surface confers aqueous solubility whilst the inner surface is capable of attracting suitably configured hydrophobic moieties into the cavity, thus forming a complex, sometimes referred to as an inclusion complex. Obviously, the size of the hydrophobic moiety to be complexed must be such that it can fit into the cavity of the cyclodextrin used (i.e. α, β or γ). In addition, the versatility of cyclodextrin macromolecules as a group is enhanced by the fact that derivatives of cyclodextrins can be prepared [32]. This is achieved, for example, by OH O O HO

OH

O

O HO OH O

OH HO

OH OH

HO HO

OH HO

O

10.40a

O

α-Cyclodextrin (6 units)

HO OH O

HO

O

O

OH

O O HO

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OH O O O

HO

O HO OH

OH

OH HO

OH

O

HO

O

O

O HO

OH

HO

OH OH

HO HO

OH O 10.40b

HO OH

O

O

β-Cyclodextrin (7 units)

O O

O OH

HO HO

O HO

OH

O

O

O O HO

OH OH

O O HO

HO HO

OH

HO

OH

HO

OH OH

O 10.40c

O OH O

HO HO HO

γ-Cyclodextrin (8 units)

OH

O

OH

O O HO

O

O OH

O HO

-cyclodextrin

-cyclodextrin

-cyclodextrin

1370

1530

1690

500

650

850

780

780

Figure 10.10 Dimensions of cyclodextrins [29]

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substituting to various degrees the hydrogen atoms of the hydroxy groups (Table 10.1). Introduction of a hydroxypropyltrimethylammonium chloride grouping gives a cationic derivative, whilst the monochlorotriazinyl substituent gives rise to a derivative that is analogous to a reactive dye. This cyclodextrin derivative, therefore, is capable of reacting with fibrous macromolecules that contain nucleophilic groups.

Table 10.1 Typical derivatives of β-cyclodextrin [32] R Group β-cyclodextrin

H

CH2

CH

CH3

2-hydroxypropyl

CH2OH

2,3-dihydroxypropyl (glyceryl)

CH2CH2CH2CH3

2-hydroxyhexyl

OH CH2

CH OH

CH2

CH OH

CH2

CH

CH2

O CH2CH2CH2CH3

n-butylglyceryl

CH2

O CH2CHCH2CH2CH2CH3

2-ethylhexylglyceryl

OH

CH2

CH OH

CH2

CH

CH2CH3

CH2

O

phenylglyceryl

CH2

O

o-tolylglyceryl

OH

CH2

CH OH

H3C

CH2

COONa

CH2

CH

sodium carboxymethyl

CH3 OH

CH2

N CH3 CH3

2-hydroxypropyltrimethylammonium

Cl

Cl N monochlorotriazinyl

N N ONa

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OR O O RO

OR

O

O RO OR OR RO OR RO

O O RO

OR

RO

OR

RO

RO O

RO

O OR O

OR OR

O

O

RO O

O O RO

Once complex formation has taken place, the physico-chemical nature of both the cyclodextrin and the complexed substance is changed. For example, complexing diminishes the vapour pressure of volatile compounds and increases the stability of compounds that are sensitive to light or air [32]. The aqueous solubility of some sparingly soluble compounds can be increased by complexing, this being attributable to the influence of the hydrophilic exterior of the cyclodextrin. Complexing with cyclodextrin can also increase the hydrolytic stability of some compounds, including dyes [29]. The functionality of cyclodextrin complexes, like that of metal-sequestrant complexes, is governed to a large extent by the stability constant of the complex and this is markedly influenced by stereochemical factors. Cyclodextrins have ecologically advantageous properties. Not only are they produced from natural and replenishable sources, they are biodegradable, non-toxic and possess no allergenic potential [32,33]. They are commercially available in bulk quantities at an optional degree of purity and have been used for many years in pharmaceuticals. The general uses of cyclodextrins (i.e. non-textile as well as textile) have been reviewed [30]. Research into possible textile applications has been ongoing since the 1950s and has covered many aspects, from preparation of substrates through coloration processes to finishing, as well as effluent treatment. A brief review of more recent textile-related research is given here to demonstrate the wide range of potential applications of these interesting products. Substrate preparation It has been claimed that complexes of β-cyclodextrin with anionic surfactants, notably higher fatty alcohol ethoxylates, improve scouring efficiency on cotton and wool in laboratory-scale processing [34]. Residual surfactants carried over from preparation can have undesirable effects in subsequent processing. When cyclodextrins complex with surfactants, their surface activity is reduced. Hence cyclodextrins are potentially useful for the removal of residual amounts of surfactants from substrates [35]. The use of α- and β-cyclodextrins has been studied in this context with one cationic, one anionic and four

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nonionic agents. α-Cyclodextrin tended to form weaker complexes than β-cyclodextrin. Methylated β-cyclodextrin was especially useful. The hollow interior of a β-cyclodextrin molecule is capable of the optimum accommodation of a benzene ring and is thus particularly good for complexing ethoxylated phenols. Acid dyes The formation of complexes of the fluorescent tracer dye ammonium 1-phenylaminonaphthalene-8-sulphonate (10.41) with cyclodextrins has been investigated with favourable results, especially in environmental studies [33]. The ability of this dye to complex with cyclodextrins has been exploited mainly as an analytical tool in the study of cyclodextrin applications, since its fluorescence is easily measured. The interaction of α-, βand γ-cyclodextrins with azo acid dyes containing alkyl chains of different lengths has been reported [36,37]. The formation and isolation of solid complexes between β-cyclodextrin and CI Acid Red 42, CI Acid Blue 40 or Erionyl Bordeaux 5BLF (Ciba) have been reported [29].

NH

SO3NH4

10.41

Basic dyes The structure and formation constants of α-, β- and γ-cyclodextrin complexes with azoniabetaine dyes have been studied, the formation constants decreasing in the order: β-CD > γ-CD > α-CD [31]. Complexes of methylene blue (CI Basic Blue 9) with cyclodextrins have been examined, β-cyclodextrin giving the highest stability constant. This was indicative of almost ideal fitting of the dye molecule into the cyclodextrin cavity [38]. Complexing with cyclodextrins increased the fluorescence intensity of the dye, this effect also being highest with β-cyclodextrin. Direct dyes The application to cotton of CI Direct Orange 46, Red 81 and Blue 71 at 90 °C in the presence of a cyclodextrin, sodium chloride and borax (to give pH 9) gave results that varied with the dye and auxiliary combination present [39]. The formation and isolation of solid complexes between β-cyclodextrin and CI Direct Orange 40, Orange 46, Blue 86 or Green 26 and between γ-cyclodextrin and Green 26 have been reported [29]. The complexes of β-cyclodextrin with Orange 46 or Green 26 were evaluated for the dyeing of cotton with them in the presence of electrolyte at 90 °C. Greater amounts of the complexes were needed

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to obtain depths matching those obtained conventionally using the uncomplexed dyes. It is not clear whether complexing with the cyclodextrin interferes with the capability of the dye to interact with the fibre in the usual way. The stability of these complexes does not appear to be high. Disperse dyes Detailed dyeing studies have been carried out using complexes formed between disperse dyes (Table 10.2) and cyclodextrins. One study [40,41] evaluated the complexing and subsequent dyeing properties of five related disperse dyes (10.42) on polyester. The results indicated that dyes which have no ortho substituents in the diazo component formed 1:1-complexes with α-, β- and γ-cyclodextrins. Dyes having electron-withdrawing groups in both of these ortho positions formed 1:1-complexes with β-cyclodextrin and 2:2-complexes with γ-cyclodextrin. Dyes 3 to 5 have a substituted diazo grouping that is larger than the cavity of α-cyclodextrin and so they are unable to form complexes with this macrocyclic molecule. It was suggested that such complexing could be used as a retarding mechanism in dyeing with disperse dyes, although these studies were restricted to a maximum temperature of 90 °C. Table 10.2 Dyes used in complexing with cyclodextrins [40,41] Dye

X

Y

R

MSCS

1 2 3 4 5

H H Cl Cl NO2

H H Cl Cl Br

CH2CH3 CH2CH2OH CH3 CH2CH2OH CH2CH2OH

4.35 4.35 7.29 7.29 8.21

MSCS maximum size of cross-section of the substituted ring in the diazo component

X O2N

R

N N

N CH2CH2OH

Y 10.42

Buschmann et al. [29,42] have studied the formation and isolation of solid complexes between disperse dyes and cyclodextrins. Disperse dyes free from their usual diluents, such as dispersing agents, were used. Complexes were formed as listed in Table 10.3. CI Disperse Blue 79 did not form complexes with either β- or γ-cyclodextrin. The complexes with CI Disperse Orange 11, Orange 29, Red 82, Violet 1, Blue 165, Resolin Red FRL or Resolin Yellow 5GL were studied in the dyeing of polyester at 130 °C. An important aspect of these dyeings was that they were carried out using only the dye–cyclodextrin complex in water, without the addition of dispersing or levelling agents. Good level dyeings of high exhaustion were obtained, even though complexing with cyclodextrins increases the aqueous solubility

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of disperse dyes. Thus particularly important advantages are claimed for this process over the traditional method of dyeing with disperse dyes: the fact that no dispersing or levelling agents are needed coupled with higher exhaustion leads to much less pollution of the effluent. Table 10.3 Formation of complexes between cyclodextrins and disperse dyes [42] With β-cyclodextrin:

With γ-cyclodextrin:

CI Disperse Yellow 3 CI Disperse Yellow 42 CI Disperse Orange 11 CI Disperse Orange 29 CI Disperse Violet 1 CI Disperse Violet 31 CI Disperse Blue 56 CI Disperse Blue 165 Resolin Red FRL (DyStar) Resolin Yellow 5GL (DyStar) CI Disperse Orange 11 CI Disperse Orange 29 CI Disperse Red 82 CI Disperse Violet 1 CI Disperse Blue 56 CI Disperse Blue 165 Resolin Red FRL (DyStar) Resolin Yellow 5GL (DyStar)

Reactive dyes The formation and isolation of solid complexes between cyclodextrins and reactive dyes have been reported, but no dyeing results were presented [29]. Complexes were formed between β-cyclodextrin and CI Reactive Orange 16, Violet 5, Blue 38 or Blue 114 and between γ-cyclodextrin and CI Reactive Blue 38 or Blue 114. The use of β-cyclodextrin and its fibre-reactive heptasubstituted monochlorotriazine derivative (10.43) has been studied in an effort to minimise the amount of the environmentally undesirable hygroscopic agent urea that is necessary when printing with reactive dyes [43]. Although this detailed research covered many variables as regards print paste additives, it involved only one reactive dye. It was found that the 300 g/kg urea normally used could be reduced to 75 g/kg when 40 g/kg of the reactive cyclodextrin derivative was used or to zero when 80 g/kg of the cyclodextrin was used, comparable results being obtained in all cases. Such cyclodextrins may function in several possible ways. The exterior hydrophilic surface of the cyclodextrin may enhance dye solubility in the same way as traditional hygroscopic agents and the hydrophobic cavity may assist this by complexing with the dye. The reactive cyclodextrin derivative may first react with the fibre and then attract further dye through its ability to absorb dye into its hydrophobic cavity. Finishing It has been demonstrated [32,44] that the various β-cyclodextrin derivatives shown in Table 10.1 can be applied to the surface of appropriate fibres by dyeing methods traditionally used

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OR O O O

RO

O HO OH

OH HO

OH

OR O

HO

O

O

O

OH

RO

HO

OH HO

HO OH HO OH

O

OR O

Cl N

O

R=

N N

O O

O

ONa RO

RO

10.43

for those fibres. When the agent has become attached to the fibre surface, the cavity of the cyclodextrin derivative is still available for the accommodation of appropriate hydrophobic guest compounds. Such guest chemicals may be, for example, perfumed, bactericidal, waterproofing or stain-resist agents. Thus the concept offers a versatile potential for the development of special finishing effects. The mechanisms by which certain β-cyclodextrin derivatives can become successfully attached to respective fibres are analogous to those operating between dyes and fibres. The monochlorotriazine derivative can be applied to cellulosic fibres either: – by padding and fixing for 5 minutes in saturated steam at 100 °C or 3 minutes contact heat at 150 °C – or by printing followed by drying for 2 minutes at 100 °C and steaming in saturated steam for 8 minutes at 100 °C. The protective action of the alginate thickening agent in the print paste is believed to play a significant part in the success of this printing method. Application of the reactive β-cyclodextrin by an exhaust method, as used for hot-dyeing monochlorotriazine reactive dyes, was unsuccessful. This is probably because the cyclodextrin derivative, although a monochlorotriazine, does not possess the structural features, such as a planar molecule and a conjugated system of double bonds, that play an important role in the substantivity of reactive dyes for cellulosic fibres. Nor was padding followed by a cold (15 h at 25 °C) or hot (4 h at 80 °C) batching treatment successful. Several derivatives with hydrophilic (2,3dihydroxypropyl or 2-hydroxypropyl) or lipophilic (n-butylglyceryl, 2-ethylhexylglyceryl or o-tolylglyceryl) substituents showed evidence of fixation to polyester by exhaust application at 130 °C. The anionic sodium carboxymethyl derivative could be fixed to nylon and the cationic 2-hydroxypropyltrimethylammonium derivative became fixed to an acrylic fibre by exhaust application at atmospheric pressure and 95–98 °C.

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Effluent treatment Since cyclodextrins form complexes with various other substances, including many dyes and surfactants, it is clear that they could be useful in effluent treatment. They are potentially suitable for the reduction or removal of polluting substances either by immobilisation or by solubilisation and extraction and thus can accelerate detoxification [30]. Summary Cyclodextrins, discovered as long ago as 1930, have been the subject of much research since the 1950s, covering the whole field from textile preparation to effluent treatment. Their capability to form complexes with surfactants, dyes and segments of fibrous polymers is now well-established, yet little or no commercial exploitation appears to have grown out of their research and environmental potential. Most of the work has been done with β-cyclodextrin and its derivatives, followed by γ-cyclodextrin. The cavity of α–cyclodextrin appears to be too small for it to be widely used in textile applications. Thus there are limits to complexforming capability depending on the molecular size and nature of the cyclodextrin in relation to the molecular size, nature and configuration of potential guest compounds. Much of this research has been carried out with single guest compounds. Evidently, the behaviour of these systems is highly specific, leading to a need for considerable fine-tuning in extending the concept to more heterogeneous commercial conditions. For example, there have been few investigations of cyclodextrins with typical trichromatic mixtures of dyes, in which the compatible behaviour of all the components may be difficult to resolve. This is implicit in the work of Yun et al. [45], who studied the compatibility of β-cyclodextrin with 27 water-soluble dyes. They demonstrated marked specificity in cyclodextrin–dye interactions and found that this was influenced by electrolytes and surfactants, as well as by pH. Complexing with β-cyclodextrin protected some dyes from the effects of salts and acids; this could be desirable or problematic, depending on requirements. It was also found that β-cyclodextrin improved the levelling of certain acid dyes [45]. Further problems may arise from the possible effects of residual cyclodextrins on subsequent processes. For example, as described above, a cyclodextrin can be used to remove residual surfactants from a fabric after scouring, yet it is possible that any residual cyclodextrin could itself interfere with subsequent coloration or finishing processes to the same extent as the surfactants that have been eliminated.

10.3.2 Cucurbituril Cucurbituril, like cyclodextrin, is a macrocyclic complexing agent [46]. An advantage of this compound is its potentially low cost, being made from glyoxal, urea and formaldehyde. It is a cyclic hexamer (10.44) containing six acetyleneurein residues linked by methylene groups between the nitrogen atoms and is configured like a wristband (10.45), composed of six 8-membered rings alternating with pairs of 5-membered rings. Also like cyclodextrin, this cylindrical macromolecule has a hydrophobic cavity into which a dye molecule, or a part thereof, can be accommodated, the chemical structure of the dye having a distinct influence on the stability of the complex [46]. Cucurbituril can also complex with substances other than dyes, including alkaline-earth and alkali metal ions [47]. In the case of a typical dyebath composition containing dye, electrolyte and surfactants,

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O O

O N

N

C N H

H

H

C

N

cavity

N

C

O

N

C

H

N O

O

N

N

N

N

N

N

N

N

N

N

N

N

N C

N

N

O

C H

O 10.44

H 6

O

O O 10.45 Cucurbituril

and possibly alkaline-earth cations from hard water, there are many competing concurrent interactions involving cucurbituril. For example, the following possibilities coexist [46]: – the dye forms a complex with the cucurbituril and simultaneously is associated in micellar complexes with the surfactant – the surfactant forms a complex with cucurbituril – the electrolyte ions interact with cucurbituril. Each factor may affect the others, depending on relative concentrations and pH. Cucurbituril requires quite strongly acidic conditions for solubilisation, hence its use in textile processing is likely to be very limited. It has mostly been investigated in connection with the removal of colour from textile effluents [46,48–53]. 10.3.3 Crown ethers Crown ethers and related structures are macrocyclic organic compounds generally composed of repeating ethylene (CH2CH2) units separated by hetero atoms such as oxygen, nitrogen, sulphur or phosphorus [54]. Other alkylene sub-units such as methylene (CH2) or propylene (CH2CH2CH2) may also be included but are less common. In contrast to the cyclodextrins and cucurbituril, these macrocyclic complexing agents possess an electron-rich and highly polar cavity and a hydrophobic exterior. Usually they are readily soluble in organic solvents. They have been known since the 1930s. Two typical structures are 10.46 and 10.47. Oxygen is the hetero atom most commonly incorporated into the ring, but nitrogen (azacrowns), sulphur (thiacrowns) or phosphorus (phosphacrowns) are also known. Organic moieties sterically equivalent to the ethylene unit (such as 1,2-benzo) can be incorporated, as can most carbohydrates with vicinal dihydroxy groupings. Crown ethers are usually named as x-crown-y, from the number (x) of atoms composing the macrocyclic ring and the number (y) of hetero atoms contained within it. If one of the oxygen atoms in structure 10.46 is substituted by nitrogen, it becomes monoaza-12-crown-4. Not all crown ethers have been tested for ecological or toxicological properties. Some are irritants and some are known to be toxic, although those tested do not show high toxicity. Nevertheless, amongst those that have not been tested, some may be hazardous to health.

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H2C

H2C H2C

H2C

CH2

O

O

O

O

H2C

CH2

O

H2C

CH2 CH2

O

O

O

O

CH2 CH2

H2C

O

H2C

10.46 12-crown-4

CH2 CH2

10.47 Dibenzo-18-crown-6

The functional characteristic of these compounds that is of interest from the viewpoint of textile processing is their capability to accommodate alkaline-earth and alkali metal cations, as well as a variety of other species, within their cavities. Stability constants (Equation 10.3) are again used, both as a measure of ligand strength and as a hierarchical indicator of complexing capability. Ks =

kc kd

(10.3)

kc = rate constant for complex formation kd = rate constant for dissociation of the complex Ks = stability constant The stability constant is dependent, amongst other things, on the solvating medium. For example, for a simple crown ether kc is usually very large and kd also large, but in nonpolar solvents kd is much smaller than kc, so that KS increases with decreasing polarity of the solvating medium. It is generally accepted that for complexing to occur the cavity of the crown ether must contain convergent binding sites (as, for example, the inwardly directed oxygen atoms in 10.46 and 10.47), whilst the entity to be complexed must have divergent binding sites. An example is shown in 10.48, the formation of which is facilitated by the hydrogen atoms diverging from the central nitrogen of the methylammonium cation. It is the three N–H–O hydrogen bonds that stabilise the complex. The potential of crown ethers for use as auxiliaries in textile coloration processes does not appear to have been evaluated recently, although their potential to complex with alkaline-

H2C H2C O

H

H2C H2C

O

CH2

H CH2 + N CH3 O _ H Cl CH2 O

CH2

10.48

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531

CH2 O

CH2

H2C

O CH2

H2C D

N

H2C

CH2 O

H2C

CH2

O CH2 10.49

D = Styryl dye chromogen

earth and alkali metal ions has been demonstrated with styryl dyes containing an aza-15crown-5 macroheterocyclic moiety (10.49) [55]. 10.3.4 Liposomes Liposomes, also known as lipid vesicles, are aqueous compartments enclosed by lipid bilayer membranes [56,57]. Figure 10.11 shows how lipid bilayers are arranged in the liposome and the lipid structures in large unilamellar vesicles and multilamellar vesicles. Lipids consist of two components: – an elongated hydrophobic moiety – a hydrophilic end group. polar non polar

ca y

vit

polar

Liposome

MLV

LUV

Figure 10.11 Liposome structures, including multilamellar vesicles (MLV) and large unilamellar vesicles (LUV) [57]

When these lipids are dispersed in water, they spontaneously form bilayer membranes (also called lamellae) which are composed of two monolayer sheets of lipid molecules with their hydrophobic surfaces facing one another and their hydrophilic surfaces contacting the aqueous medium. In the case of phospholipids such as phosphatidylcholine (10.50), the structure consists of:

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– hydrophobic component: two hydrocarbon chains (R1 and R2) – hydrophilic component: glyceryl ester, phosphate and choline groups. O C O

R1

CH2 O

CH O

R2

C O

CH2

O

P

O _

CH2

O

CH2

CH3 + N CH3 CH3

10.50 Phosphatidylcholine

These structures are effective encapsulating systems for either hydrophilic or hydrophobic compounds. They can be obtained not only in uni- or multilamellar forms but also in different particle sizes with varying degrees of aggregation. They are particularly useful in biological and pharmacological applications. Recent research in various areas of textile wet processing has revealed further potential in these sectors. However, the methods of preparing liposomes so far reported are not readily adaptable to commercial processing conditions, as can be seen from the following typical procedures used by de la Maza et al. [57–60]. Clearly, substantial development work is needed before these techniques become compatible with bulk-scale textile wet processing. Large unilamellar vesicle liposomes Reverse-phase evaporation in a nitrogen atmosphere was used to prepare lipids. A lipid film previously formed was redissolved in diethyl ether and an aqueous phase containing the dyebath components added to the phospholipid solution. The resulting two-phase system was sonicated at 70 W and 5 °C for 3 minutes to obtain an emulsion. The solvent was removed at 20 °C by rotary evaporation under vacuum, the material forming a viscous gel and then an aqueous solution. The vesicle suspension was extruded through a polycarbonate membrane to obtain a uniform size distribution (400 nm). Multilamellar vesicle liposomes A lipid film was formed from a chloroform solution of egg phosphatidylcholine by rotary evaporation in a nitrogen atmosphere and under vacuum. An aqueous phase containing the dyebath components was then added to the lipid film. The solution was swirled to transfer the lipid from the flask and to disperse lipid aggregates; glass beads being added to facilitate dispersion. The resulting milky suspension was centrifuged for 5 minutes and then extruded through a polycarbonate membrane to obtain a uniform size distribution (400 or 800 nm). Liposomes made from pure phosphatidylcholine or containing lipids that are found in the cell membrane complex of wool (e.g. cholesterol) have been used to encapsulate aqueous chlorine solutions in chlorination processes [61,62]. The results showed improvements in

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the uniformity and homogeneity of oxidative treatment, minimising degradation of the wool and facilitating subsequent treatments. The application of acid dyes to wool using liposomes has also been researched. The dyes used were the milling acid dye CI Acid Blue 90 [57] and the neutral-dyeing 1:2 metalcomplex dye CI Acid Yellow 129 [60]. Dyeing conditions were 90 °C and pH 5.5, using various ratios of phosphatidylcholine:dye. In the work with CI Acid Blue 90, both uni- and multilamellar vesicles were used. Dye exhaustion decreased with increasing concentration of phospholipid (Figure 10.12) but the amount of bonded dye increased with increasing lipid concentration (Table 10.4). The percentage of dye bonded to wool (Cb) was expressed by Equation 10.4: Cb =

100(Ca - C e ) Ca

(10.4)

where Ca = mg/g dye absorbed by wool and Ce = mg/g dye extracted from wool by ethanol and ammonia. 100

100 A

B

80 Dye exhaustion/%

Dye exhaustion/%

80 60 40 20

60 40 20

20

60

100

20

60

Time/min Concentration (mmol/l)

100

Time/min 0.0

0.5

1.0

2.0

4.0

Figure 10.12 Exhaustion of CI Acid Blue 90 by untreated wool in dyeing with LUV (A) and MLV (B) liposomes [57] Table 10.4 Amounts of dye bonded to wool using LUV and MLV liposomes at different lipid concentrations with CI Acid Blue 90 [57] Bonded dye (%)

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LUV

MLV

4.0 2.0 1.0 0.5 0

84 78 77 68 62

77 74 73 68 62

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The work with CI Acid Yellow 129 used only unilamellar vesicles. The liposomes again suppressed exhaustion but increased dye–fibre bonding, leading to better fastness properties. It is claimed that liposomes can be used to control the rate of exhaustion. The application of CI Disperse Violet 1 to wool with phosphatidylcholine [58] and phosphatidylcholine/cholesterol [59] liposomes has been investigated. Figures 10.13 and 10.14 show that exhaustion decreases with increasing concentration of liposome, an effect which may be used to control exhaustion rate. It is claimed that liposomes enhance the dye dispersion efficiency, being superior to conventional dispersing agents. Dye–fibre bonding forces and levelling of the dye are also said to be improved. Exploration of the use of liposomes in wool processing stems from the similarity that exists between the bilayer structure of the cell membrane complex of wool and that of the liposomes. Merino wool contains about 1% by weight of lipids, these forming the hydrophobic barrier of the cell membrane complex. Cholesterol is one of the main lipid 100

Dye exhaustion/%

80

60

40 Phosphatidylcholine lipid concn (mmol/l)

20

20

60

40

0.5 1.0

1.5 2.5

80

100

120

Time/min

Figure 10.13 Exhaustion rates of CI Disperse Violet 1 on untreated wool in dyeing using liposomes at different lipid concentrations and constant dye concentration [58] 100

Dye exhaustion/%

80

60

40 Phosphatidylcholine/cholesterol lipid concn (mmol/l)

20

1.25 2.5

20

40

60

3.0

80

100

120

Time/min

Figure 10.14 Exhaustion rates of CI Disperse Violet 1 on untreated wool during dyeing in the presence of MLV liposomes at different lipid concentrations and constant dye concentration [59]

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components in wool; hence its use in combination with phosphatidylcholine (Figure 10.14). One of the ideas behind this research, which remains valid despite limited commercial prospects as yet, is to focus attention away from electrostatic forces of attraction towards hydrophobic interactions, which are now accorded greater importance. In dyeing, for example, the idea is that the hydrophobic liposome will encapsulate dye molecules by means of hydrophobic interaction, the liposome–dye complex then being absorbed into the hydrophobic centre of the cell membrane complex via further hydrophobic interaction. This is why claims are made for increased hydrophobic bonding of the dyes to the fibre. Along similar lines, synthetic cationic (10.51) and anionic (10.52) double-chain surfactant vesicles have been investigated for the dyeing of polyester with a monoazo disperse dye [63]. The results were moderately encouraging from technological, economical and environmental viewpoints, although problems and inconsistencies were observed. It is often difficult to explain results with disperse dyes on the basis of structural chemistry alone, since they can be influenced by variations in dispersion characteristics and instability during dyeing.

CH3(CH2)10CH2

CH3

CH3(CH2)10CH2

N+ CH3

10.51

_

CH3(CH2)14CH2

O

CH3(CH2)14CH2

O

X

_ O Na + P O

10.52

X = Br or Cl

10.3.5 Chitin, chitosan and their derivatives Chitin is the second most important natural polysaccharide produced by biosynthesis, exceeded only by cellulose, to which it is closely related in structure. It was first isolated by Braconnot in 1811 and thus its ‘original and spectacular’ properties have been recognised for a long time [64]. Chitin is found in crabs, lobsters and other crustaceans, spiders and other arthropodic insects, and the cell walls of fungi. Like cellulose (10.53), chitin (10.54) is a 1,4β-D-glucopyranose. Both have a linear sequence of pyranose rings linked by 1,4-glycosidic bonds, a non-reducing end group and a reducing end group in the cyclic hemiacetal form. The characteristic difference between them is that chitin has an acetylamino group in the 2-position, compared with the 2-hydroxy group in cellulose. Chitin exists in three polymorphic forms, depending on the directions of adjacent polymer chains, the alternating α-chitin structure being the most common [64]. CH2OH

CH2OH

CH2OH

O

OH

O

O

O

O

OH

OH

HO OH

OH

OH n–2

10.53 Cellulose

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OH

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

CH2OH

CH2OH

CH2OH

O

O

O O

OH

O

OH

OH

OH

HO NHCOCH3

NHCOCH3

NHCOCH3 n–2

10.54 Chitin

Chitosan (10.55) is a derivative of chitin made by alkaline hydrolysis resulting in deacetylation to give a primary amino group in the 2-position. Chitin is less hydrophilic than cellulose, whilst chitosan is more basic than either of the others. Structures 10.53–10.55 are representative only. Variations occur, depending on the source of the chitin and its treatment during and after harvesting. Chain length, average molecular mass and molecular mass distribution vary. There is also the question of impurities and the degree of deacetylation of chitosan, which is usually 75–95% [65]. Knittel and Schollmeyer [65] have outlined the nature, properties and uses of chitin and chitosan, including indications of their uses in textile processes [65]. Roberts has provided an excellent textbook [64]. There are also the proceedings of two symposia, although these do not deal with textile processing applications [66,67]. CH2OH

CH2OH

CH2OH

O

OH

O

O

O

O

OH

OH

OH

HO NH2

NH2

NH2 n–2

10.55 Chitosan

Numerous substituted derivatives of chitin and chitosan are known [67]; some important examples are shown in Scheme 10.9. The possibility of forming either anionic (5,7,8,11) or cationic (9,12) derivatives should be noted. The O-carboxymethyl (5) and N-carboxymethyl (11) polymers are of particular interest as they have stronger complex-forming capabilities with metal ions than either unsubstituted chitosan or EDTA [65]. In practice, derivatives formed by substitution via the 2-amino group of chitosan are more common than those substituted via the 6-hydroxy position of the glucopyranose grouping [65]. Chitosan features far more than chitin in research into applications. This is largely due to their difference in solubility characteristics, chitosan being more amenable to practical manipulation. Chitin is in fact rather more intractable than cellulose, since it is insoluble in those solvents, such as cuprammonium hydroxide, that are commonly used to dissolve cellulose. Chitin is soluble in hot concentrated solutions of certain inorganic salts capable of

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MACROMOLECULAR COMPLEXING AGENTS

CH2OCOR

CH2ONa

O O

OCOR

4

3

CH2OCH2COONa

CH2OCH2CH2OH

6

NHCOCH3

2

NHCOCH3

7

O

OH

NH2

11

O

O O

OPO3H2

NHCOCH3

8

NHCH2COONa

CH2OH

CH2OPO3H2

O

OSO3Na

O O

O O

CH2OH

OH

CH2OSO3Na

O

NHCOR

10

O O

1

O

OH

+NH3 _ OCOR

9 CH2OH

OH

NHCOCH3

OH

NHCOCH3

O O

5

O

OH

CH2OH

O

O

O O

ONa

NHCOCH3

OH

CH2OH

CH2OH O

NHCOCH3

O

OH

R

12

+ N CH3 CH3

_

X

R = alkyl Typical derivatives of chitin (1) and chitosan (2) [67]: alkali-chitin (3), O-acylchitin (4), O-carboxymethylchitin (5), O-hydroxyethylchitin (6), chitin O-sulphate (7), chitin O-phosphate (8), chitosan salt (9), N-acylchitosan (10), N-carboxymethylchitosan (11), trialkylammonium salt (12) Scheme 10.9

a high degree of hydration, the order of effectiveness being: lithium thiocyanate > calcium thiocyanate > calcium iodide > calcium bromide > calcium chloride. Chitin also dissolves, with some degradation, in concentrated hydrochloric acid, sulphuric acid (some O-sulphation taking place) or phosphoric acid, but not in nitric acid. Certain organic carboxylic acids, such as formic, dichloroacetic or trichloroacetic, will also dissolve chitin. Chitosan, on the other hand, interacts with inorganic acids to yield cationic polyelectrolytes, their solubility depending on the nature of the anion. Thus it is soluble in dilute hydrochloric, hydrobromic, hydroiodic, nitric or perchloric acid, but may be precipitated from hydrochloric or hydrobromic solutions as the acid strength is increased. Chitosan forms water-soluble salts with most carboxylic acids. Hence it is chitosan, rather than chitin, that has come to the fore in a remarkably wide range of end-uses, including such diverse fields as medicine, personal care, contact lenses, biotechnology, food, agriculture, effluent treatment, analysis, textile finishes and coatings. Although usage in textiles is relatively small as yet, its availability, environmental compatibility and remarkable versatility offer considerable potential. Both chitin and chitosan are manufactured commercially on a large scale. Chitosan is available in powder, gel, solution, film, membrane, fibre and bead forms. Interest in all forms and levels of purity is high and continuing to expand [67]. Chitosan is produced from amply replenishable biological sources and is readily biodegradable, non-toxic and non-allergenic. It has bactericidal and fungicidal properties and actively promotes wound-healing. The ability of chitosan to form complexes is of particular interest. Being slightly basic, it will readily form complexes with anionic compounds. Initially it forms into micelles with small amounts of anionic surfactants, leading to precipitation of a complex as the concentration of the anionic surfactant increases. Chitosan will complex with anionic

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polyelectrolytes leading to the formation of polycationic/polyanionic complexes of high molecular mass. This capability to complex with anionic substances is further enhanced if cationic derivatives of chitosan are used. The degree and strength of complexing depends on: – the nature and ionic strengths of the cationic and anionic species – the spacing of the charged ionic groups, as influenced by the relative molecular masses and spatial configurations of the components. This property is clearly of interest for the removal of anionic substances from effluent streams, for example. However, since such complexing often results in an increase in viscosity as complexing proceeds, such systems can be used to produce gels or viscous liquids. Hence there is the possibility of using these complexes as print–paste or pad–liquor additives to control migration. Weakly basic chitosan or its more strongly basic derivatives will complex with anionic fibres and can therefore be used as finishes or pretreatments to modify selected properties of the fibres. They are already used, for example, in hair sprays or for complexing with and isolating proteins. It is not surprising, therefore, that chitosan and its basic derivatives will complex with anionic dyes. Giles et al. [68,69] researched the use of chitosan for the removal of dyes from effluent as long ago as 1958. The binding capacity of chitosan for anionic dyes is pHdependent, but it has been reported [65] that in effluent treatment as much as 10 g dye per kg chitosan can be complexed at pH values above about 6.5. Similarly, chitosan has been used for the aftertreatment of direct dyeings on cotton to improve their fastness. The complexing of chitosan and its basic derivatives with anionic substances is paralleled by compatibility with cationic and nonionic compounds. Similarly, the anionic derivatives of chitosan show complex formation with cationic agents and are compatible with anionic and nonionic compounds. The capability of these chitosan derivatives to complex with certain metal ions, notably those of the transition series, is also important, having possibilities for the removal of metal salts from effluent. The hierarchy in terms of binding capacity is: Cr(III) < Cr(II) < Pb(II) < Mn(II) < Cd(II) < Ni(II) < Fe(II) < Co(II). Chitosan will readily react with formaldehyde via its primary amino groups [65]. The capability of chitosan and its basic derivatives to complex with anionic fibres has already been mentioned. In this context, the bactericidal and fungicidal properties of these chitosan compounds are useful. The fact that fibre-reactive chitosan derivatives can be prepared further increases these possibilities. Chitosan compounds containing long-chain alkyl groups exhibit fabric-softening properties and can be incorporated into finishing formulations for this purpose. The fact that charged chitosan derivatives can interact with appropriate fibre types gives scope for their use as levelling agents and to modify dye absorption in either a positive or negative sense, depending on circumstances and dyeing requirements. For example, they are claimed to reduce dye uptake variations between mature and dead cotton. The use of chitosan derivatives in print pastes, to reduce the content of the environmentally unfavourable hygroscopic agent urea necessary when applying reactive dyes, has been evaluated [43]. It was found that in recipes normally requiring 300 g/kg urea this could be reduced to 75 g/kg by adding either 20 g/kg chitosan or 4 g/kg N-hexylchitosan. Although this did not give a significant increase in dye yield, the replacement of most of the urea by a biodegradable chitosan polymer offered significant promise.

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539

10.3.6 Summary In conclusion, it is noteworthy that cyclodextrins, liposomes and chitin derivatives are all readily available from renewable biochemical sources and offer advantages of biodegradability and safety in use. However, it needs to be borne in mind that this fact alone does not necessarily mean that they are entirely environmentally innocuous in the long run. Demands on resources for the husbanding and processing of bioforms that may be necessary in order to sustain demand for commercially viable qualities and quantities can exert deleterious effects, not least because they may give by-products that present problems of utilisation or disposal [70]. 10.4 ENZYMES 10.4.1 Structure and properties of enzymes Enzymes are proteins, i.e. sequences of amino acids linked by peptide bonds. The sequence of amino acids within the polypeptide chain is characteristic of each enzyme. This leads to a specific three-dimensional conformation for each enzyme in which the molecular chains are folded in such a way that certain key amino acids are situated in specific strategic locations. This folded arrangement, together with the positioning of key amino acids, gives rise to the remarkable catalytic activity associated with enzymes. Their molecular masses range from about 10 000 to more than 1 000 000. Each enzyme can catalyse an indefinite amount of chemical change without itself being consumed or degraded by the reaction, although most enzymes lose their activity gradually under the conditions of use due to an inherent instability. Enzymes are produced by all living cells and are of two types: – exoenzymes: these are expelled by the manufacturing cell into the surrounding environment, where they can break down organic compounds such as proteins, starches and fats into more soluble components of lower molecular mass; and – endoenzymes: these remain within the living cell and are transformed or broken down by the action of coenzymes to produce relatively large amounts of energy and the cell components needed for cell processes. Clearly, it is the exoenzymes that are of interest in textile processing, an area which has seen considerable development in recent years. Originally used only in the preparatory processes of scouring and desizing, they are now also used to modify textile surfaces in finishing as well as in effluent treatment. Although enzymes are present in living systems they can exhibit powerful activity under certain conditions. Their behaviour within the cells of a correctly functioning (i.e. healthy) living entity is controlled by a sophisticated biochemical system designed to maintain optimum activity according to the needs of the living cells. In an external application such as textile processing, such biochemical control is not in place. Hence care is needed in the use of enzymes. Repeated inhalation of enzyme dust is associated with a comparatively high risk of respiratory allergy in susceptible persons. Undue exposure can cause irritation of moist skin, eyes and mucous membranes. The manufacture and use of enzymes is usually government-controlled.

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

More than 3000 different enzymes have been extracted from animals, plants and microorganisms. Traditionally, they have been used in impure form since purification is expensive and pure enzymes may be difficult to store and use. There is usually an optimum temperature and pH for maximum activity of an enzyme. Outside these optimum conditions, activity may simply be held in check or the enzyme may become ‘denatured’, i.e. altered in such a way that activity is lost permanently, although some forms of denaturing are reversible. Many enzymes are also sensitive to transition-metal ions, the effect being specific to particular metal ions and enzymes. In some cases, certain metal ions are essential for the stability and/or activity of an enzyme. In other cases, metal ions may inhibit the activity of an enzyme. Similarly, certain organic compounds can act as enzyme inhibitors or activators. An enzyme consists of a polypeptide chain with a particular spatial configuration specific to that sequence of amino acids. The molecule twists and turns, forming structural features that are catalytically active, these being known as active sites. There may be more than one active site per enzyme molecule. Sometimes an auxiliary catalyst, known as a coenzyme, is also needed. Apparently, only the relevant active site of the enzyme comes into contact with the substrate and is directly involved in the catalysed reaction. The active site consists of only a few amino acid residues. These are not necessarily adjacent to one another in the peptide chain but may be brought into proximity by the characteristic folding of the enzyme structure. The active site may also include the coenzyme. The remainder of the enzyme molecule fulfils the essential function of holding the components of the active site in their appropriate relative positions and orientation. Thus the alkaline protease obtained from Bacillus licheniformis with a molecular mass of about 27 000 consists of 274 amino acid residues and has serine and histidine as active sites. Pancreatic trypsin with a molecular mass of about 24 000 contains 230 amino acid residues and also has serine and histidine as active sites. Papain (molecular mass about 23 000 and 211 amino acid residues) has cysteine and histidine as active sites. The molecular folding of the backbone chain of the enzyme, as well as the distribution and content of amino acids [71] plays a decisive role in determining the characteristic specificity of an enzyme with regard to its reactions. This folding is markedly affected by temperature, for example. As the temperature rises, the chain gradually unfolds until a point is reached at which the enzyme becomes ‘denatured’ and the catalytic activity is lost. Most enzymes are highly specific, catalysing only one specific reaction. They may act upon only one isomer of a particular compound and are then described as stereospecific. Others are less specific, being able to catalyse several (usually related) reactions. Part of the active site is involved in binding to the substrate and another part is responsible for making or breaking chemical bonds. It appears that, for some enzymes, binding of the substrate produces a change in conformation which brings the key functional group of the enzyme into the required position for reaction to take place. Thus there must exist between the enzyme and the substrate a close stereochemical fit or complementarity, analogous to the situation between a lock and its key. Regarding stereospecific behaviour, certain enzymes exhibit a remarkable ability to discriminate between asymmetric right-hand and left-hand molecular configurations. Enzymes may be named trivially or more formally. Trivial naming tends to predominate in industry and two trivial systems exist: (1) a suffix (-in or -ain) is added to a root indicating the source of the enzyme, e.g. papain from papaya or pancreatin from pancreatic cells

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541

(2) the suffix -ase is added to a root indicative of the substrate or reaction involved, e.g. lactase acts on lactose, cellulase hydrolyses cellulose and glucose oxidase oxidises glucose. In formal nomenclature, a decimal numbering system is used. Only a brief description can be given here; more complete accounts can be found elsewhere [72,73]. The system requires four numbers. The first number gives the class of enzyme according to the following scheme: (1) Oxidoreductases (2) Transferases (3) Hydrolases (4) Lyases (5) Isomerases (6) Ligases. The second and third numbers give the subclass according to the type of reaction which is characteristic of the enzyme, e.g. 1. Oxidoreductases 1.1 acting on the CH–OH group of donors 1.1.3 with O2 as acceptor The fourth number is the serial number of the enzyme in its subclass. With regard to the specificity of enzymes, there are four main types: (1) Enzymes that catalyse the reaction of only one substrate are known as absolute enzymes. (2) Stereospecific enzymes catalyse reactions with one type of optical isomer but may also react with a series of related compounds of the same configuration. Many proteolytic enzymes hydrolyse only peptide bonds linking laevorotatory (L-) amino acids. (3) Enzymes that react with a specific type of ester linkage are known as general hydrolysing enzymes. Thus lipases hydrolyse a wide range of organic esters. Generally, phosphatases will break down phosphate esters into phosphoric acid and an alcohol. (4) This group is characterised by enzyme attack at a certain specific point in a molecule. Examples are: (a) Some proteolytic enzymes act at a location where the adjacent amino acid (e.g. phenylalanine) contains a benzene ring. (b) Some hydrolytic enzymes attack the interior bonds of a molecule. Thus α-amylase attacks the mid-chain region of the starch molecule and of glucosidic fragments formed from starch. (c) Other hydrolytic enzymes attack the end groups of saccharidic macromolecules. Thus β-amylase attacks the end groups in starch molecules, splitting off two glucose units in the form of a maltose residue. Amyloglucosidase attacks the nonreducing ends of starch or its hydrolysis products to split off single glucose units. As mentioned above, certain metal ions may be necessary for activity or stability. Thus calcium is needed for bacterial α-amylase. Magnesium or cobalt is needed with glucose isomerase. Calcium stabilises the starch-liquifying bacterial α-amylases but inactivates the glucose isomerase that may be used subsequently. Many enzymes contain an additional non-

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protein component, referred to as a coenzyme or prosthetic group. This may be an organic molecule, a vitamin derivative or a metal ion. In most cases the coenzyme participates directly in the catalytic reaction. The four main groups of enzyme activity mentioned above are covered by the six classes of enzymes already listed. Oxidoreductases Such enzymes catalyse reactions involving electron transfer. Oxidases use molecular oxygen as an electron acceptor (Scheme 10.10). Dehydrogenases remove hydrogen atoms from the substrate and transfer them to an acceptor other than oxygen. CH2OH

CH2OH CH O HO

CH

glucose CH OH

+ O2

oxidase

CH O CH

HO

C

O

CH CH

CH CH

OH OH

OH OH

10.56

10.57 D-Gluconolactone

D-Glucose Scheme 10.10

Transferases These catalysts bring about the transfer of a particular chemical group from one substance to another. Examples of groups that can be transferred include alkyl, formyl, carboxyl, aldehyde, keto, acyl, glucosyl, nitrogen-, phosphorus- or sulphur-containing groups. Thus a transaminase transfers an amino group and a transmethylase transfers a methyl group. Scheme 10.11 shows the transfer of an amino group using a transaminase. R H

C

R

R′ NH2

+

C

O

COOH

COOH α-Amino acid

C transaminase

R′ O

+

H

COOH

C

COOH

α-Keto acid

Scheme 10.11

Hydrolases Enzymes in this group are capable of hydrolysing a substrate. Examples include: Substrates starch proteins nucleic acids fats esters (e.g. phosphates)

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Types of enzyme amylases proteinases and peptidases nucleases lipases esterases (e.g. phosphatases)

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NH2

ENZYMES

543

Scheme 10.12 shows the action of cholinesterase in the hydrolysis of acetylcholine (10.58) to choline (10.59). CH3 H3C

N

CH3 CH3

H3C

N

CH2 H2O

CH2

cholinesterase

O C

CH3

CH2 CH2

+ CH3COOH

OH

O 10.59 Choline

CH3 10.58 Acetylcholine Scheme 10.12

Lyases These enzymes catalyse the non-hydrolytic cleavage of bonds in a substrate to remove specific functional groups. Examples include decarboxylases, which remove carboxylic acid groups as carbon dioxide, dehydrases, which remove water, and aldolases. The decarboxylation of pyruvic acid (10.60) to form acetaldehyde (10.61) takes place in the presence of pyruvic decarboxylase (Scheme 10.13), which requires the presence of thiamine pyrophosphate and magnesium ions for activity. CH3 C

pyruvic O

COOH 10.60 Pyruvic acid

decarboxylase and coenzyme

CH3 C

O

+ CO2

H 10.61 Acetaldehyde

Scheme 10.13

Isomerases These catalysts facilitate the interconversion of isomeric compounds and include racemases, optimerases, cis-trans isomerases, intramolecular oxidoreductases and intramolecular transferases. Scheme 10.14 shows the conversion of an aldehyde to a ketone by triose phosphate isomerase. Ligases or synthetases Such enzymes catalyse the condensation of specific compounds, accompanied by the breakdown of a pyrophosphate bond in adenosine triphosphate (10.64). Adenosine is the condensation product of a pentose (D-ribofuranose) and a purine (adenine). Scheme 10.15 shows the action of glutamine synthetase on a mixture of L-glutamic acid (10.65) and

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

CHO H

CH2OH triose

OH

C CH2

O

O

P

CH2

phosphate isomerase

OH

O

C

O

O

OH

P

OH

OH

10.62

10.63

D-Glyceraldehyde phosphate

Dihydroxyacetone phosphate

Scheme 10.14 NH2 N

O HO

P

N

O

N

CH2

HO

N

O HC

HO

CH

O

HC

P

O

CH

O

O

P

HO

OH

OH 10.64

Adenosine triphosphate

COOH

CONH2

CH2 + NH3 + ATP

CH2 H

C

CH2

glutamine

NH2

H

COOH

10.64

+ ADP + H3PO4

CH2

synthetase

C

NH2

COOH 10.66 L-Glutamine

10.65 L-Glutamic acid

Scheme 10.15

ammonia. Adenosine triphosphate (ATP) is converted into the diphosphate (ADP) and phosphoric acid is formed. More comprehensive accounts of enzyme chemistry, behaviour and technology are available [71,73–75]. In addition, general biochemistry textbooks contain more or less detailed accounts of these topics. 10.4.2 Enzyme applications in textile processing Enzymes have traditionally been closely associated with the desizing of cellulosic fabrics. In recent years, however, the sphere of possible, if not actual, uses has widened considerably.

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Further developments can reasonably be expected, particularly as much of this vigorous research is motivated by environmental concerns. An overview of this activity is given below. Mercerisation, scouring and alkali boiling of cellulosic fibres The traditional mercerising of cotton presents quite hostile conditions for enzymes. Hence it is not surprising that little use of enzymes has been reported, either in traditional mercerising or as an alternative means of obtaining similar effects. Cegarra [76] has concluded that, because of the strongly alkaline nature of mercerising solutions and the resultant transformation of the cellulose structure, it seems rather unlikely that enzymes will provide alternatives to alkali in the near future. Even so, it is pertinent to study the effects that mercerisation may have on any subsequent enzyme treatment. It has been shown [77] that enzymatic hydrolysis is accelerated on cellulose that has been mercerised without tension compared with stretch mercerisation. Possible uses of enzymes in scouring or alkali boiling offer somewhat more scope, although conditions can still be rather hostile as regards alkalinity and temperature. Nevertheless, there are some emergent signs. Various enzymes, such as cellulases, pectinases, lipases and proteases, have been compared [77], leading to the tentative conclusion that cellulases give the best results for the removal of impurities, together with slightly inferior whiteness, a similar loss in strength and less contaminated effluents, compared with the traditional alkaline scour. Use of either a pectinase or a pectinase/cellulase mixture for the removal of pectin from cotton has also been studied [78]. Effective removal using such enzymes was found at 40 °C and pH 4.5, giving a higher degree of whiteness than alkaline washing at the boil for the same degree of cellulose degradation. Cellulase can facilitate the alkaline scouring of viscose [79], enabling the concentration of alkali (36–60 g/l) traditionally used to be reduced by 5–10 g/l, and giving, moreover, a more uniform and consistent swelling process than when alkali is used alone. Another study [80] also demonstrated possibilities for using enzyme formulations in cotton scouring.

Desizing of cellulosic fabrics The enzymatic desizing of cellulosic fabrics is a long-established standard process. Amylolytic enzymes are used to convert any type of starch size into water-soluble products without affecting the cellulosic fibres. Using enzymes in their natural or modified state, products are available to allow desizing at 20–70 °C, 70–90 °C or 85–115 °C [81]. Cegarra [76] has intimated that, given the availability of such products, further studies are likely to be concentrated on formulations allowing simultaneous desizing and scouring in an alkaline medium, replacing the present two-stage process. Nevertheless, research continues to explore improved desizing processes. Advantages have been claimed for lipases [82] and traditional amylase desizing can be improved with the help of a thermostable lipase, giving both technical and environmental advantages [83]. Cotton sized with poly (vinyl alcohol) (PVA) is generally desized in water at about 80 °C. However, a mixture of two different PVA-degrading enzymes gives equivalent desizing at only 30–55 °C and pH 8.0. Extending the enzymatic treatment time to 4–6 hours (compared with one hour) resulted in minimum residual PVA [84]. Environmental benefits were also

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found, since the PVA content in the liquid waste after desizing for four hours was negligible. Advantages for oxidoreductases over amylolytic enzymes have been observed, since they break down lignin impurities and are effective over a wider range of temperature and of pH [85]. Bleaching of cellulosic fibres The possibility of catalysing the action of hydrogen peroxide by enzymes is an interesting one, but the need to avoid fibre damage is critical and so such catalysis by a peroxidase is not currently practical. However, the careful use of glucose oxidase in conjunction with an enzymatic desizing process is reported [86] to permit the novel and eco-friendly use of starch-containing effluent liquors for subsequent bleaching. This allows the use of hydrogen peroxide as the oxidising agent, together with gluconic acid (10.28) which has outstanding sequestering properties and good biodegradability. Ecological and economic advantages are claimed, including minimising the effluent pollution load, reducing chemical consumption and processing under mild conditions. After bleaching it is important to ensure that the fibre does not contain residual hydrogen peroxide since this can interfere with subsequent processes, particularly coloration. Certain enzymes, particularly catalases, used to eliminate peroxide are bio-friendly and time-saving [87], thus having significant advantages over traditional methods [88]. Such a technique can be used in either batchwise or continuous washing-off after bleaching to give rapid and complete decomposition of any residual peroxide [89]. Dyeing of cellulosic fibres Enzyme processing before, during or after dyeing is an active area of study. Enzyme pretreatment may have beneficial or adverse effects on subsequent coloration. The action of enzymes during coloration may improve the coloration process or provide a combined process, such as desizing/coloration or coloration/biofinishing. With the emergence of biofinishing techniques, it is important to know how such enzyme treatments are affected by any prior coloration process. Most of the published work deals with enzyme pretreatments or aftertreatments. In one study [90], enzyme pretreatment increased colour yield without affecting fastness properties. However, pretreatment of cellulosic fibres with cellulase lowered the subsequent fixation of homobifunctional triazine reactive dyes but did not impair the fixation of other types of reactive dyes [91]. Another study suggested that the enhanced brightness of reactive dyeings was greater with triazine dyes than with vinylsulphone types when cotton was pretreated or aftertreated with cellulase [92]. The enzyme biofinishing of cotton after dyeing was found to be inhibited by direct or reactive dyes but not by vat dyes [93]. In another investigation of reactive dyes in this context, biofinishing was variously influenced by the type of dye–fibre bond, the type of chromogen, the presence of metal ion, the number of reactive groups per molecule and even by the dye application method [91]. Yet another study [94] showed that cellulase pretreatment boosts dye exhaustion and cellulase aftertreatment increases the apparent depth of the dyeing. Interactions between cellulase enzyme pre- or post-treatments and reactive or direct dyes have been studied by Buschle-Diller et al. [95], with the objectives of

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elucidating the mechanism of enzymatic degradation and specifying optimum conditions for a combined dyeing/biofinishing process. Denim washing The practice of stonewashing of dyed cotton denim fabrics to give a ‘distressed’ or washeddown appearance was traditionally carried out with pumice stones. This was labourintensive, time-consuming, caused abrasion of the fabric surface and created debris. Several authors have described how the stonewashed effect can be produced more advantageously using cellulase enzymes to partially or wholly replace the stones [76,96–101]. The enzymes provide a controllable means of surface attack of the fibres, thus bringing about the desired uneven appearance. Advantages claimed include savings in time and labour, much less fabric abrasion and no debris. The enzymes used are neutral or acidic cellulase preparations, which may contain endo-, exo- or beta-gluconases. The finisher can exploit the differing characteristics of acidic and neutral cellulases by employing washing procedures that take advantage of each type of formulation [98]. Biofinishing of cellulosic fabrics Biofinishing, or ‘biopolishing’ as it is more popularly known, is similar to denim washing in its use of cellulase enzymes, although the effects intended are quite different. The process is designed to eliminate, by dissolution, the cellulosic fibrils projecting from the surface of the fabric. This treatment results in [76]: – a cleaner, smoother surface – a softer, cooler feel – improved resistance to pilling – brighter, sometimes deeper colours. The precise effects obtained are dependent on the fabric quality, the type of cellulase enzyme and the application conditions, but no mechanical forces are involved in removal of the fibrils. The process has attracted considerable attention and is now one of the main methods of defibrillating lyocell fabrics [94,101–114]. Simultaneous treatment with cellulase and protease enzymes has been applied to the biofinishing of wool/cotton blends [115]. Acidic cellulases at pH 4.5–5.5 and 45–55 °C or neutral cellulases at pH 6–8 and 50–60 °C are effective in biofinishing [106,107]. Heavier fabrics and lower enzyme concentrations need longer treatment times but 30–60 minutes is a typical duration. The treatment is terminated by inactivating the enzyme, either by raising the pH to 10 or by increasing the temperature to 75 °C for 10–15 minutes. The process is usually monitored by assessing the weight loss of the fabric; a weight loss of 3–5% usually represents an adequately finished effect without excessive loss of fabric strength [107]. Dissolution of the cellulose involves depolymerisation as illustrated in Scheme 10.16 [107].

Wool processing It is more difficult to control the enzymatic processing of wool. Hence there is a greater danger of fibre damage compared with cellulosic fibres. Since cellulose is a highly crystalline

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CH2OH

CH2OH

O

O

OH

O HO OH

OH

OH

OH

cellulase H2O enzyme

CH2OH

CH2OH O

O

OH

OH

+ HO

HO OH

OH

OH

OH

Scheme 10.16

material possessing only limited amorphous regions, it is relatively easy to restrict the action of enzymes to the surface of the fibre and to the amorphous material, thus leaving the strength of the fibre unchanged [116] In the case of wool, however, proteases and lipases catalyse the degradation of different components of the fibre. Proteases, having diffused into the interior of the fibre, hydrolyse parts of the endocuticle and proteins in the cell membrane complex. This is difficult to control and can lead to serious damage of the fibre. SEM micrographs have shown the complete damage of wool fibres and released cortical cells characteristic of uncontrolled attack by protease enzymes [116]. Three types of enzyme may be selected for the treatment of protein fibres [76,99]: (1) Proteases, which can be classified as either peptidases or proteinases. These cleave polypeptide chains eventually into their component amino acids. Peptidases can be further classified as endopeptidases (which act on the main-chain amido groups along the polypeptide molecule) or as exopeptidases (which act only at terminal amino acid residues). (2) Lipases, which mainly hydrolyse fatty esters, especially triglyceride esters of fatty acids. (3) Lipoprotein lipases, which act on the lipoproteic bonds of lipoproteins (combinations of proteins with fatty ester molecules), thus breaching the hydrophobic barriers formed by these compounds. The most widely used of these types are the proteases, but the others may be useful in some circumstances. A characteristic feature is that individual enzymes are highly specific in their action, so that although one protease may yield the required effect, another may fail to do so. Bleaching of wool A serine protease that is stable to hydrogen peroxide and is active in an alkaline medium has been found and marketed [76,117,118]. In fact this enzyme becomes more active with

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increasing concentration of peroxide. This enzyme increases the whiteness of wool directly by decolorising the natural yellowish hue of the fibres. Hence, depending on the degree of whiteness required, this enzyme can be used either alone or in combination with hydrogen peroxide to effect the bleaching of wool. Serine protease can also be applied with bleaching agents that operate by a reductive mechanism. Carbonising of wool The traditional method of carbonising with sulphuric acid is environmentally undesirable and can easily lead to fibre damage. Hence it is not surprising that research has been directed towards alternatives in which enzymes are used to remove the cellulosic impurities from wool. Cellulases and lignases are mainly used but others have been proposed [116]: (1) Removal of plant impurities by hydrolases, lyases or oxidoreductases. (2) Cellulolytic and pectinolytic enzymes used to reduce the amount of sulphuric acid required. (3) Incubation of wool with cellulases facilitated subsequent removal of burr with no chemical or physical damage to the wool. (4) Application of a mixture of cellulases, pectinases and lignases, again without damage to the wool. Dyeing of wool The effects of enzyme treatments on the subsequent dyeability of wool have been evaluated. One investigation included both chlorinated and unchlorinated wool [119]. Wool was treated with a protease at 50 °C and pH 7.5, followed by dyeing with CI Reactive Reds 28 and 116. The enzyme-treated wool showed more rapid dyeing and higher absorption with no effect on fastness. These effects were greater on the chlorinated wool than on the unchlorinated control. Alternatively, the enzyme-treated wool could be dyed at a lower temperature. The effect of pretreatment with a neutral protease on dyeing with acid dyes has also been examined [90,120], increased colour yields again being observed. It is essential, of course, to determine whether the economies of increased yield or lower dyeing temperature exceed the additional cost of enzyme treatment, and whether the durability of the wool is adversely affected. Shrink-resist finishing of wool This is an area of considerable research activity, comparable with the enzymatic stonewashing and biopolishing of cotton. However, there has been less success in translating this research into commercial processes. Evidently, the technical use of enzymes for wool fabrics will not become widespread for another five to ten years [121]. Since certain enzymes can remove cuticular scales from wool fibres, it is not surprising that they are of interest for shrink-resist finishing, either alone or in combination with traditional chlorination or resinapplication processes. Interest in this area is acute, because of the environmental disadvantages of chlorination procedures. These yield absorbable organohalogen (AOX) byproducts, which accumulate in the effluent and ultimately may give rise to toxicity problems in the food chain if taken up by aquatic organisms [116]. Hence there is considerable

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commercial potential for an enzymatic descaling process that could wholly or even partially replace chlorination. The critical factor is to achieve the optimal degree of descaling reproducibility, with minimal effect on fibre strength. Ideally, the anti-felting effect should be achieved using ‘soft chemistry’ without application of a synthetic resin and the entire process should be environmentally innocuous, producing no harmful substances [116,122]. This ideal has yet to be attained. It was originally thought that the large protease molecule would not be able to penetrate the fibre cuticle. If so, attack would be limited, as with chlorination, to the cuticular scales with only minor deterioration in mechanical properties attributable to damage in the interior [123] This proved to be too simplistic a viewpoint, however, as some proteases even attack the highly swellable cell membrane complex preferentially, possibly penetrating this region by channelling beneath the cuticular scales [123–125]. Moreover, microscopic examination has indicated that enzymatic action on wool is not uniform, some fibres remaining practically intact whilst others are damaged considerably [126]. Most anti-felting investigations have been carried out with proteases but other types have also been examined, e.g. a protein disulphide isomerase which rearranges the disulphide bonds of cystine residues [127] and transglutaminase which introduces new crosslinks into the keratin structure [128]. Protease activity towards wool can be increased by addition of sodium sulphite or bisulphite, either with the enzyme treatment or as a pretreatment [122]. Pretreatment with oxidising agents may also increase the effect of certain enzymes; hydrogen peroxide, dry chlorine, peracetic or performic acid, wet chlorination, potassium permanganate and peroxymonosulphuric acid (H2SO5) have been used in this way [122]. Sulphite reduction increases proteolytic activity by cleavage of cystine disulphide bonds in the cuticle to form thiosulphonic acid groups, a reaction known as sulphitolysis [122,129,130]. When preceded by oxidative treatment, the action of sulphite yields electron-withdrawing sulphonic acid groups in the sulphur-rich cuticular layers, selectively activating the nucleophilic degradative reaction catalysed by the protease and thus preferentially directing the enzyme action to the cuticle [122,129,131]. Not all proteases are activated by sulphite, however [126]. Although the present situation and the way ahead appear uncertain, it is clear that enzyme treatment alone does not fulfil the technical requirements for shrink-resist finishing. Even with enzyme treatment, some degree of chlorination (with the attendant AOX problems) and/or application of a resin will still be required. Two-stage or even three-stage processes have been proposed [116]: (1) (i) Treatment with permanganate; (ii) proteolytic enzyme treatment; This gave complete descaling. (2) (i) Treatment with papain (protease), monoethanolamine hydrosulphite and urea; (ii) treatment with dichloroisocyanuric acid; (iii) a second enzymatic treatment. (3) (i) A combined protease treatment; (ii) wet chlorination or oxidative treatment (using sodium hypochlorite and potassium permanganate); (iii) application of a polymer. (4) (i) Enzyme treatment; (ii) treatment in saturated steam. (5) (i) Enzyme treatment; (ii) high-frequency radiation. (6) The Schoeller Superwash 2000 process [132]: (i) so-called ‘black box’ pretreatment; (ii) enzyme treatment; (iii) application of a low-AOX polyamide resin. Most enzyme treatments of wool are carried out at about 50 °C for 30–60 minutes. The amount of enzyme required depends on the specific enzyme type and its commercial

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strength. Optimal pH also depends on the enzyme type. In a study of sixteen commercial proteases for which the optimal pH varied from 3 to 10.5 [122], it was found that only papain (optimal pH 6.5–7) and alkaline proteases conferred shrink-resistance on sulphitetreated wool and these tended to cause too much fibre damage. It is thus clearly apparent that in this area of enzyme activity there is still scope for further development to meet the desired targets.

Biofinishing of wool Enzymes can be used to modify the surface of wool fibres in order to improve lustre, softness, smoothness or ‘warmth’ of the fabric. Since such processes involve attack on the cuticular scales of the fibre, there is clearly a resemblance to shrink-resist treatments and similar methods are used [116]: (1) (i) Treatment with potassium permanganate, ammonium sulphate, acetic acid and bisulphite; (ii) treatment with a proteolytic enzyme. (2) Descaling by application of a heat-resistant neutral protease to confer a cashmere-like feel. (3) Combined use of dichloroisocyanurate and a proteolytic enzyme. (4) Complete removal of degraded or damaged portions of the wool (not merely the cuticle) using: (i) protease treatment; (ii) formic acid rinse and application of a softener. (5) (i) Treatment with dichloroisocyanurate; (ii) neutralising and incubating with papain; (iii) steaming at 100 °C. Only empirical tests have so far been carried out, however [76]. In a detailed but small-scale study, various options were examined for the sequence: (i) oxidative treatment; (ii) protease treatment; (iii) application of softener, including exhaust or pad application [133]. The following products were examined: Oxidising agents (a) (b) (c) (d)

dichloroisocyanuric acid, potassium peroxymonosulphate, magnesium monoperoxyphthalate hexahydrate, sodium hypochlorite.

Enzymes Papain and four protease formulations that varied from neutral to alkaline as regards optimal pH for activity. Softeners (a) a weakly cationic softener, (b) a cationic silicone micro-emulsion, (c) a cationic emulsion modified with a silicone elastomer.

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The results varied widely: – descaling: from none to full – fibre damage: from none to severe – strength loss: from -6% (i.e. a slight increase in strength) to +30% – Whiteness Index (original value -2): from -1.9 to +25.8. Irrespective of the descaling effect, development of a ‘soft lustre’ depends on application of a softener. These experiments were positive in demonstrating the possibility of descaling the fibres and in perceptibly improving lustre under mild conditions. In particular, it was shown that papain is effective at the remarkably low concentration of 50 mg/l, showing a high degree of specificity after chlorination. Degumming and desizing of silk The use of enzymes in silk degumming or desizing is well-established [76,99,134–136]. In a study of eight enzymes under optimised conditions [137], weight losses of 24 ± 3% were observed in most cases but trypsin and pepsin gave extremely poor results. Increasing the treatment time at the optimal concentration of enzyme gave no further significant weight loss. There was no significant strength loss in the case of degummed silk and lustre was improved. In earlier work, degumming with papain was as effective as alkaline degumming and superior to other methods [138]. Nevertheless, all the parameters must be carefully controlled to prevent attack on the silk itself [76]. Dyeing of silk Enzymes have been used to facilitate the dyeing of silk with reactive dyes [139]. Application of enzymes in laundering As surfactants are often used in textile processing, it is important to note that anionic or cationic surfactants can inhibit the action of enzymes, as has been reported in the case of cellulases used for the treatment of cotton [140]. Dyes can also inhibit enzyme activity: for example, CI Direct Red 28 has been shown to have a much greater inhibitory effect than CI Acid Orange 7 [141]. Many domestic and laundry washing formulations contain at least one enzyme. Alkaline proteases, with serine active sites and optimal activity at pH 9–10.5, are mostly used [73]. Much attention is given to the degree of temperature toleration and to compatibility with other components of the commercial product. In some countries (e.g. the USA) it is sufficient to have temperature tolerance up to 50–55 °C, whereas elsewhere (e.g. in Europe) toleration to 100 °C may be required for laundry detergent formulations. Papain, for example, has broad activity and is thermally stable but is unsuitable, as is trypsin, on account of incompatibility with perborate, many boosters and all bleaches. A protease derived from Bacillus licheniformis is much more suitable, this being compatible with surfactants, chelating agents such as phosphates, EDTA and NTA, as well as with fluorescent brightening agents and perfumes. Rather more offbeat investigations have centred on micro-organisms belonging to the group

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Archaea or Archaebacteria, which live in sulphurous waters around undersea volcanic vents. An extraordinarily stable enzyme which functions even at 135 °C and survives at pH 3.2–12.7 has been identified [142]. This enzyme has been termed STABLE (stalk-associated archaebacterial endoprotease). It is suggested that such exceptional stability may be attributable to unusually large Mr and tight folding of the protein chain. Suggested uses include washing powders and detergents, as well as industrial catalysts. It is even proposed that such remarkable properties may have contributed to the early evolution of life on earth [142].

10.5 PREPARATION OF SUBSTRATES By one of those perversities of terminology often encountered in textile wet processing, the term ‘auxiliaries’ generally includes all chemicals used in preparation and finishing processes, even though in these cases such chemicals often provide a primary rather than a secondary (auxiliary) function as in coloration processes. Thus the chemicals used in preparation are discussed in this section. It cannot be overstressed that the success of any coloration process relies on the state of the substrate presented for coloration; moreover, thorough preparation can often do much to reduce the need for auxiliaries in subsequent processes. Methods of preparation for cellulosic and wool substrates are also discussed elsewhere [11,143]. In this chapter the emphasis is on the chemistry of the products used rather than on the technology of processing. 10.5.1 Scouring The purpose of scouring is to reduce to an acceptable level the amounts of fats, waxes, oils and dirt present. Apart from the aesthetic benefits of a clean substrate, the major technical reason for scouring is to improve the extent and uniformity of absorbency for subsequent processes, especially coloration. Usually the objective is the complete removal of all extraneous matter but on occasion only partial removal is the aim, since a certain residue of oils, for example, will aid such processes as spinning, weaving or knitting. Scouring is particularly important with natural fibres, which obviously contain much more extraneous matter than do synthetic fibres. In scouring, surfactants function as primary, rather than auxiliary, agents. The basic requirements are for good wetting power and detergency, the latter property generally including the ability to remove, emulsify and suspend the extraneous matter in the liquor. Not all effective detergents possess good wetting properties; hence a combination of surfaceactive agents to provide both wetting and detergency may be preferable. Detergency can be significantly improved by the use of additional compounds usually referred to as ‘builders’, the chief of which is undoubtedly alkali in the form of sodium carbonate or hydroxide. Alkaline phosphates such as sodium orthophosphate, sodium pyrophosphate or sodium tripolyphosphate may also be used in those countries where they do not contravene the local environmental regulations. Alkalis function mainly through saponification of the waxes, fats and oils on the substrate, thus. rendering them water-soluble and more amenable to removal and suspension by detergents. If the processing water or the substrate contains cations such as those of calcium, magnesium or iron, a sequestering agent should be added to the scouring liquor. Apart from

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their ability to sequester metal ions, many of these agents also possess useful detergentenhancing powers. The aminopolycarboxylates are generally preferred, both for their sequestering ability and for their stability in warm to hot alkaline liquors. Polyphosphates are occasionally used at lower temperatures but are less efficient in alkaline media. When incorporated in the scouring medium certain organic solvents, such as pine oil, trichloroethylene, perchloroethylene, triethanolamine or glycols, can greatly aid the removal of greasy matter, particularly mineral oil which may be a component of any lubricating oils present on the substrate. However, the use of these additives must be in accordance with local environmental regulations. Soaps are occasionally still used for scouring, although anionic or nonionic synthetic detergents are almost always preferred. Among the anionics, fatty alkyl sulphates, sulphonates and phosphates are commonly used. Ethoxylated fatty alcohols are typical nonionic scouring agents. Ethoxylated nonylphenol was once the most common nonionic product used. This came under quite severe environmental scrutiny but, as explained in section 9.8.1, there is now a good deal of evidence to suggest that nonylphenol derivatives are not so environmentally damaging as at first thought. Proprietary scouring agents range from single-component surfactants to complex, specially formulated mixtures that contain some or all of the above mentioned types of component matched to give a balanced or compatible product. As mentioned in section 9.8.3, better emulsifying properties are generally obtained with a carefully selected blend of surfactants rather than with a single product. It has been demonstrated [144] that in the case of ethoxylated nonylphenol and ethoxylated fatty alcohol surfactants, the broader the molar mass distribution of the combined surfactants the better is the scouring efficiency, and that it is inappropriate to aim at homogeneity of the nonionic formulation in scouring. In selecting a suitable product, thought should be given to the ease with which it can be rinsed out of the substrate and to any effects that residual quantities may have on subsequent processing. Fabrics destined for printing, in particular, need the highest degree of uniform absorbency and cleanliness [145], free from residual surfactants that may cause bleeding or haloing of the printed design into the surrounding area. Scouring is of crucial importance in wool processing: important, because the raw fibre contains 20–60% of extraneous matter in the form of grease, suint, dirt, sand and vegetable matter; and critical, because the fibre is so easily damaged by hot alkaline treatments. There have been considerable changes in wool scouring practice, evolving mainly from corresponding changes in the types of lubricants used on the fibre, although environmental factors have also played a part. It has been pointed out that the pollution load from a wool scouring mill can be similar in magnitude to the average discharge from a small town [11]. Hence there are heavy environmental pressures on wool scourers. This impact has provided the impetus to develop systems which use as little water and energy as possible and reduce effluent contamination through the recovery of some of the components. Examples of such comprehensive systems are the WRONZ and Siroscour (CSIRO) techniques, which use minimal volumes of water whilst producing wool of optimal quality. The essential feature of the WRONZ treatment [146] is the passage of the greasy liquor from the first stage through a heavy solids separating tank and a centrifugal separator, from which the partially degreased liquor is returned to the first stage. The Siroscour system depends on concentration destabilisation to increase centrifugal recovery of grease and dirt and to reduce water usage even further [146]. In concentration destabilisation wool grease,

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dirt and suint salts are allowed to build up to very high levels in the scour bowls so that the resulting unstable emulsion can be cracked easily by heating to 95 °C, allowing the grease and dirt to be centrifugally separated. Separation occurs in three stages [147]: (1) superficial dirt (easy to remove) (2) grease (3) persistent dirt (difficult to remove). Advantages claimed for this process [148] include: improved whiteness, decrease in ash content, improved grease recovery and quality, optimisation of water consumption, efficient dirt removal from effluent and a reduction in treatment costs. An excellent review of wool scouring (and of wool processing generally) up to 1984 is available [146]. Soap and alkali were traditionally used and very greasy wools were scoured with alkali alone, forming a soap in situ by saponification of the wool grease. Such methods are now rarely used, having been supplanted by nonionic surfactants under neutral or alkaline conditions. Octa- and nona-ethoxylated nonylphenols are the preferred nonionics, providing environmental considerations permit their use, since they are unsurpassed for detergency. Alternatively, ethoxylated straight-chain fatty alcohols are preferred on the grounds of superior biodegradability. The advantages of nonionic detergents over soaps include greater efficiency under neutral conditions, stability in hard water, lower cost and more efficient removal of grease (although this is one area where over-degreasing can be a disadvantage). Syndets are desorbed more easily in difficult rinsing situations (in yarn cheeses, for example), although they are not as efficient as soap for the suspension of dirt. In place of the alkaline sodium carbonate, the tendency is to use neutral sodium sulphate as a detergent builder. Apart from the impurities present in raw wools, typical formulations [146] for lubricating wool fibres are: (1) water-miscible polyglycol lubricants; these are mainly used on carpet yarns and can be readily removed using a neutral nonionic surfactant; (2) mineral wool oil (mineral oil with a nonionic lubricant); normally extracted using a nonionic surfactant, although in some cases a little alkali may also be useful; (3) a combination of mineral oil, olein fatty acids and triglycerides; nonionic surfactant with sodium carbonate can be used, but for more complete removal (as where subsequent shrink-resist processes are carried out) soap and sodium carbonate must be used; (4) natural vegetable and/or animal oils; these are still used on woven worsteds scoured traditionally with soap and sodium carbonate, although even here there is a gradual trend towards more neutral systems. Nowadays, proprietary mixtures of lubricating agents are formulated with ease of removal in scouring very much in mind. Consequently, scouring processes are generally mild, using a nonionic surfactant at about 50–60 °C [11]. Aqueous scouring is expensive in terms of water use and effluent treatment and it can cause entanglement of delicate wool fibres. Solvent scouring offers an effective alternative but it is essential that the solvent does not enter the environment. Earlier solvent-based processes included the use of perchloroethylene in which 8–18% water had been emulsified with a surfactant. Current processes are based on hexane (de Smet process), 1,1,1-

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trichloroethane (Toa/Asohi process) [11,149] or a commercial product called Triwool (ICI) in the Wooltech process [150–152]. The Wooltech process is claimed to be environmentally friendly and to produce wool of superior quality. The Triwool solvent is non-flammable, does not deplete ozone from the upper atmosphere and is not a known carcinogen. The process specifically excludes water, being designed primarily to avoid fibre entanglement. Current use is for the scouring of wool tops, where it gives about 2–3% higher yields than aqueous scouring. The recovery of raw wool grease is claimed to be 99%. The process offers environmental advantages over conventional aqueous scouring and gives softer wool fibres of enhanced tensile strength and elasticity, which is beneficial for spinning and weaving. In the laundering of garments and household textiles, as opposed to the scouring of inprocess material, agents are often added to the liquor to prevent redeposition of soil extracted from the fabric during washing, typical products being the hydroxyethyl, hydroxybutylmethyl and carboxymethyl ethers of cellulose. It has been shown, using carbon black soiling on shrink-resist treated wool, that the lowest redeposition was obtained with hydroxybutylmethylcellulose and the highest with the carboxymethyl ether [153]. The other major natural fibre, cotton, contains a significant proportion of extraneous matter such as seeds, fats, waxes, colouring matter and dirt, as well as substances such as sizes and lubricants applied during processing. Unlike wool, however, it has outstanding stability in alkali and withstands strongly alkaline treatments ranging from severe caustic kier boiling to milder treatments with soap and soda [143,154]. It is difficult to detach the effect of scouring from the complete sequence of desizing, scouring, mercerising and bleaching, since they all contribute to improved absorbency and cleanliness. Traditional caustic treatment in kiers is carried out at the boil or in some cases at up to 120 °C, using 1–2% o.w.f. alkali. This treatment bursts the seed motes and saponifies fats and waxes, converting the fatty esters into sodium salts and glycerol. This in situ formation of soaps naturally aids cleaning. Nevertheless, synthetic detergents are often added to aid penetration through wetting and to increase detergency. The surfactants selected must be highly stable in the strongly alkaline conditions, as well as in hard water. Anionic surfactants of the fatty alkyl and alkylaryl sulphate types have been preferred, although the use of phosphate esters is increasing. A synergistic mixture is beneficial, one component (C10–C13) to aid wetting, the other (C14–C16 ) as a detergent. The sulphosuccinates, often a first choice for wetting ability, cannot be used here as they are hydrolysed under such strongly alkaline conditions. A sequestering agent is usually added in order to remove metal ions that would create problems in subsequent bleaching. Addition of a mild reducing agent guards against alkaline oxidative tendering of the fibre through oxycellulose formation and also promotes a degree of bleaching. The reducing agent will also reduce any iron(III) contamination to iron(II) ions, which are easier to remove by the sequestering agent. A commercial kier boiling additive may contain some or all of these components. Suppression of foam may also be a requirement. Semi-continuous and continuous scouring systems are more common nowadays [154]. The auxiliary needs in these pad–steam processes are generally the same as those for batch scouring, except that the selection and balancing of components is much more critical in order to secure optimal treatment during the short dwell times. Contrary to the usual practice of scouring cotton and its blends under alkaline conditions, McCaffrey and Santokhi [155] have cogently argued the case for scouring knitgoods under

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acidic conditions. This starts from the assertion that the degree of impurity removal resulting from conventional alkaline scouring is probably not really necessary for cotton knitgoods. In the case of knitgoods, as opposed to woven fabrics, a milder scour is desirable to retain some of the natural fats and waxes, resulting in a fabric with a softer handle and improved sewability [143]. It has been demonstrated [155] that acidic scouring (pH 4.5 with acetic acid) in the presence of a self-emulsifiable knitting lubricant can give results as good as traditional processes on knitgoods, except that impurity removal is inferior but acceptable. In addition there are significant advantages, particularly from economical and environmental aspects. Acidic scouring uses approximately one-third less water than an alkaline scour. Alkaline processes often require a separate acidic rinse to neutralise the alkali, whereas residual acid is easily rinsed out. Acidic scouring also offers an opportunity to reduce chemical consumption and hence costs. These savings minimise effluent treatment and reduce the effects of effluent on the environment. Acidic scouring gave a reduced processing time and lower weight losses, both factors that contribute to improved productivity. Compared with wool and cotton, the scouring procedures for synthetic fibres are relatively simple since these fibres contain fewer impurities. Most of these have at least some degree of water solubility; the most important are sizes and lubricants. The major sizes used are poly(vinyl alcohol), carboxymethylcellulose and poly(acrylic acid), all of which are completely or partially water-soluble. Sometimes aliphatic polyesters are used. Secondary acetate and triacetate fibres generally respond to a light scour with soap or synthetic detergent, usually at 60–75 °C, this being sufficient to remove soil, oil, sighting colour and any antistatic agent, although temperatures can range from 30 to 90 °C [156]. Anionic synthetic detergents, such as the poly(oxyethylene) sulphates, are preferred before dyeing with disperse dyes since low cloud-point nonionic scouring agents, if carried over into the dyeing process, can interfere with the stability of the dye dispersion at higher temperatures. Addition of a sequestering agent is helpful in hard water. Care should be taken if alkali is added, especially on secondary acetate, since these ester fibres can be hydrolysed to cellulose under hot alkaline conditions. Nevertheless, in the S-finishing process this alkali sensitivity is exploited to effect a carefully controlled surface saponification of the fibres to improve drape and antistatic properties [157]. Sodium hydroxide is applied together with an anionic surfactant to aid wetting and uniformity of treatment. S-finishing is more usually carried out on triacetate fibres, reducing the total acetyl content from about 62 to 59%. When scouring synthetic fibres that are to be dyed with disperse dyes, nonionic scouring agents are best avoided unless they are formulated to have a high cloud point and are known not to adversely affect the dispersion properties of the dyes. Conversely, when scouring acrylic fibres, anionic surfactants should be avoided [156] because they are liable to interfere with the subsequent application of basic dyes. These fibres are usually scoured with an ethoxylated alcohol, either alone or with a mild alkali such as sodium carbonate or a phosphate. Polyamide and polyester fibres are generally scoured using an alkyl poly(oxyethylene) sulphate and sodium carbonate. Some polyester qualities are subjected to a causticisation treatment with sodium hydroxide in the presence of a cationic surfactant to give a lighter fabric with a silkier handle [154,156]. This treatment involves etching (localised saponification) of the polyester surface and is broadly analogous to the S-finish used on triacetate fibres. The process has attracted considerable interest in recent years but its

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popularity is likely to decline as the availability of polyester microfibres increases. If the weight loss during caustic reduction of polyester is restricted to 15–17% there is little effect on the breaking strength or the viscosity of the fibre [158], since hydrolysis is limited to the surface of the fibre. If the weight loss is greater than 20%, however, deeper layers of the fibre are attacked and the fibre strength and viscosity are reduced. The crystalline regions of the fibre show considerable resistance to hydrolysis. These factors, however, may vary quantitatively depending on the structure and morphology of the particular polyester. The cationic surfactant normally used with the alkali acts as a catalytic accelerant [159]; quaternary ammonium compounds are most often used. The kinetics of the process have been studied [159], showing that careful control of all parameters is essential. The treatment results in an increase in surface polarity of the fibre. The substantivity for dyes and the rates of wetting-out and dyeing are increased. The increase in wettability arises from the formation of free carboxyl and hydroxy groups [160]. Although sodium hydroxide is the preferred alkali, other alkalis have been investigated [161], the order of activity being: KOH > NaOH >> Na2CO3. Saponification also increased at lower liquor ratios. Cationic accelerants vary in their efficacy [161]. Other types of accelerant have also been evaluated. In one study [162], comparisons were made between tetra-ethylammonium bromide, benzyltriethylammonium chloride, poly(diallyldimethylammonium chloride) and the diethyldimethylammonium derivative of a benzenesulphonate polyglycol ester. It was found that the cationic polymers had a greater effect than the simple quaternary ammonium compounds of lower molecular mass. This effect was attributed to the capability of the polymers to enter into hydrophobic interaction with the fibre surface. Ethylenediamine has also been found to accelerate the alkaline hydrolysis of polyester [163]. Alkaline hydrolysis in a solvent (dimethylformamide, dimethylsulphoxide or dimethylacetamide) containing sodium hydroxide has been investigated [164]. Fabric geometry [165] and the degree of heat setting of the polyester also influence the results. As the temperature of heat setting was increased, the accelerating effect of dodecylbenzyldimethylammonium chloride decreased [166]. Basic-dyeable polyester is particularly sensitive to alkaline hydrolysis [167]. In some cases, saponification has been used to produce special effects such as a leather-like finish [168]. 10.5.2 Desizing Desizing is an essential part of the purification process for woven fabrics. Sizes perform an adhesive and lubricating function. After drying, the size forms a protective film on the surface of the warp yarns, bonding the protruding fibrils to produce a smoother yarn with improved tensile strength and abrasion resistance. The objective of sizing is to improve weaving efficiency by reducing the number of yarn breakages, reducing frictional wear of loom parts and allowing increased running speeds. Individual size polymers may be used alone or in combination with one another and their performance may be further improved by the addition of other components such as waxes and lubricants. However, whilst sizing offers many benefits in the subsequent weaving of the yarns, it is anathema as far as wet processing is concerned. A typical sized yarn may contain as much as 34% of impurities, distributed as shown in Figure 10.15. These impurities can interfere with wetting-out and with bleaching. They may also affect coloration processes. Depending on the type of size and the dyes used, dye uptake may be increased or resisted;

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hence uneven distribution of size may lead to unlevel coloration. There may also be a deleterious effect on fastness properties, since coloured size is likely to be attached to the fibre only superficially. Consequently, it is invariably essential to remove sizes and lubricants thoroughly before further wet processing.

Approx 4% fats oils waxes abraded metal

Up to 20% size

Up to 10% hemicellulose pectins proteins seed husks fruit capsules colour salts

Figure 10.15 Cotton warp yarn and its impurities [169]

This removal of size residues inevitably raises environmental questions. Unfortunately, the various size polymers and their associated additives respond to different methods of removal. It is therefore highly desirable to know which sizes and other components have been used in a given case so that appropriate methods of removal can be formulated. This is not always easy, particularly in commission dyehouses or printworks where sizing has been carried out elsewhere. Analytical procedures are available but these require appropriate facilities and expertise. Once desizing has been carried out there arises the question of how to dispose of the effluent. The range of size polymers available has expanded to the point of being extremely complex [169], both in terms of the main types of size and the numerous combinational possibilities that they represent. The present overview of salient aspects covers the following: chemistry of size polymers, the use and properties of sizes, desizing methods, analysis of size polymers and environmental aspects. A summary of the equipment used for sizing is available [170]. Chemistry of size polymers Recent reviews dealing with the chemistry of size polymers are available [169,171]. Sometimes a distinction is made [172] between primary sizes, secondary sizes and binders. This distinction appears to be an arbitrary one depending mainly on the proportions present in a mixture, although other factors may also be pertinent. There is a great deal of similarity in essential chemistry between typical size polymers, thickening agents used in printing and migration inhibitors used in continuous dyeing. An overall summary [169] of the main types of size polymers available is given in Table 10.5. Similar but less comprehensive lists are given elsewhere [171,172].

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Table 10.5 Chemical types of main size polymers [169] Vegetable products Native Starches Resins Vegetable gums

Modified

Mosses and algae Pectin Starch derivatives

Cellulose derivatives

Potato, maize, wheat, rice, sago, tapioca Arabic, Locust bean, Senegal, Tragacanth Moss starch, sodium alginate Water-soluble starches British gum Starch ethers Hydroxyethyl starch Hydroxypropyl starch Starch esters Carboxymethyl starch Phosphate starch Starch carbamate Cellulose esters Carboxymethyl cellulose Methyl cellulose

Animal products Glue Gelatin Casein Synthetic products Polyvinyl compounds

Partially saponified poly(vinyl acetate) Fully saponified poly(vinyl acetate) Copolymers with crotonic acid Copolymers with vinyl acetate Acrylic acid copolymers with methacrylic acid with acrylic acid esters with acrylonitrile Maleic acid copolymers with styrene with ethyl vinyl ether with butadiene Poly(ethylene glycol) copolymers with isophthalic acid

It is important to recognise that the molecular characteristics (average molecular mass and distribution, possible degree of substitution) of such polymers can be varied quite widely, with attendant changes in properties. Thus polyacrylic acids and their salts may be described as either sizes or binders. The distinction is related to the proportions used: to be effective as a binder [171] such a polymer must constitute at least 10% of the dry weight of a size formulation. A product marketed specifically as a binder may have molecular characteristics that provide properties different from those of a similar product marketed as a size component. For example, an acrylic size may be engineered specifically for adhesion and film-forming properties whereas an acrylic binder may be designed to enhance film elasticity when added in smaller quantities (about 10%) to a size formulation. It is therefore important to bear in mind that different polymers based on similar chemistry can be engineered to provide suitability for specific uses. The chemistry of starches, galactomannans, modified starches, modified celluloses and alginates is discussed in section 10.8.1 on natural thickeners. The main starches used are

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potato starch in Europe, maize starch in America, and rice, maize, tapioca or sago starches in the Far East [169]. Animal glue is a complex colloidal mixture of proteins. The related gelatins are also complex heterogeneous mixtures of proteins. They are strongly hydrophilic and rich in the amino acids glycine, proline, lysine, hydroxyproline and hydroxylysine. Casein is a phosphoprotein obtained from the milk of mammals. Poly(vinyl alcohol) has the structure 10.67. Poly(vinyl acetate) is the fully esterified derivative of poly(vinyl alcohol), in which the –OH groups are replaced by –OCOCH3 groups. As indicated in Table l0.5, commercial polyvinyl sizes are effectively copolymers of poly(vinyl acetate) and poly(vinyl alcohol) that vary in the degree of saponification of the ester groups. These products may comprise 100% of either polymer, or combinations of the two monomers in any proportions. Crotonic acid (2-butenoic acid), widely used in the preparation of resins, may also be a component. This compound exhibits cis–trans isomerism (Scheme 10.17). The solid trans form is produced readily by catalysed rearrangement of the liquid cis isomer. CH2

CH

CH2

CH

OH

CH2

CH

OH

CH2

CH

OH

OH

10.67

H3C

H C

H3C C

C COOH

H

COOH

H

Crotonic acid mp 72°C bp 180°C

C H

Isocrotonic acid mp 14°C bp 169°C

Scheme 10.17

Polymers based on acrylic acid have gained considerable importance in recent years. Their essential chemistry is discussed in section 10.8.2 on synthetic thickeners. Copolymers of acrylic acid with acrylonitrile and methyl acrylate (10.68) contain a random distribution of cyano, ester and acidic sidechain groups [169]. CH2

CH

CH2

C O

CH

CH2

CH C

CN OCH3

CH

CH2

O

CH2

C OH

O

CH

CH2

C OCH3 O

CH CN

OH

10.68

The polyester sizes used have a much lower average molecular mass than polyester fibres. These structures (10.69) contain sulphonic acid groups and may be water-soluble or waterdispersible types. The degree of sulphonation is low [171]. If these resins are subjected to a high pH, the sulphonate groups can be hydrolysed, giving an insoluble resin that is very difficult to remove from the fibres.

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O

C

R1

C

O

X1

O

O

O

R2 X2

n

C O

R3

C O

10.69 (1) (2) (3) (4) (5)

R1 X1 R2 X2 R3

= = = = =

Aromatic ring —SO3–, —OCH2CH2CH2SO3– or —CO2CH2CH2SO3– substituent on the aromatic ring R1 CH2CH2 (n = 2–10), CH2CH2CH2 (n = 1–2) or CH2—cycloalkyl—CH2 —CH2OCH2CH2CH2SO3– substituent on aliphatic group R2, in which case X1 = H Cycloaliphatic or aliphatic hydrocarbon of 2–6 carbon atoms

Rather more specialised sizes are used in certain applications. For example, a reactive poly(dimethylsiloxane) (section 10.10.2) is recommended for the sizing of some industrial textile fabrics [173]. The application and properties of sizes As already mentioned, the essential aim of sizing is to increase productivity in weaving. This is achieved through a reduction in yarn breakages that permits increased running speeds. Indeed, the high speeds of modern weaving processes could not have been realised without corresponding improvements in sizing technology. The most important requirements of a size formulation can be summarised as follows [174]: – high adhesion and good film-forming properties on the yarn, together with good elasticity of the applied film – low tendency to foam in the application liquor – freedom from skin formation in the application liquor – good storage stability – good compatibility of wash-off liquors containing different size components – appropriate compatibility with alkalis and bleaching agents if desizing is not carried out separately from scouring and bleaching. To these may be added economy and ease of removal from the substrate, as well as a favourable response to effluent treatment. The adhesive strength of a size film is an important consideration as it has a bearing on stability during weaving [175]. Adhesive strength depends on such factors as type of size polymer, additives present (e.g. wetting agent), sizing liquor temperature, yarn characteristics, viscosity index of the size formulation and degree of saponification of poly(vinyl acetate) sizes. Size dissolution rate is also an important factor; this needs to be known if desizing is to be carried out effectively and efficiently [176]. Many factors will determine the choice of size type and formulation. A general scheme of size use relative to different fibre types is given in Table 10.6. It is useful to classify sizes according to those physico-chemical properties that largely determine the method of desizing, as shown in Table 10.7. Evidently, some degree of synergistic chemistry is involved in determining the specific suitability of size polymers for certain fibres. For example, there is an obvious similarity of molecular structure between starch and cellulosic fibres, offering

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substantial scope for hydrogen bonding between hydroxy groups. The similarity in molecular structure between polyester sizes and synthetic fibres facilitates hydrophobic bonding between hydrocarbon segments of the polymer chains. Table 10.6 Sizing agents for different substrates [169] Size:

Natural

Synthetic

Substrate

Starch

CMC

Gums

Staple fibre yarns Cellulosic Polyester/cellulosic Nylon/cellulosic Wool Polyester/wool Polyester, nylon

+ o o o o o

+ o o o o o

+ o o

Glue

AACo

AECo

PVA

+ + +

+ + + + + +

+ + + + + +

+

+ + + + +

+ + +

+

Filament yarns Viscose Acetate Triacetate Nylon Polyester

+

+

+

PE

PVCo

+ + + +

+ Alone o Only in combination with synthetic size CMC AACo AECo PVCo

Carboxymethylcellulose Acrylic acid copolymers Acrylic ester copolymers Polyvinyl copolymers

Gums PVA PE

Galactomannans Poly(vinyl acetate) Polyesters

Table 10.7 Size properties and methods of removal [169] (slightly modified) Physico-chemical characteristics

Type of size

Method of removal

Chemically degradable

starches modified starches

enzymatic or oxidative

Water-soluble

acrylic acid copolymers poly(vinyl acetate/alcohol) carboxymethylcellulose certain modified starches

rehydration and dissolution

Water-resistant

polyesters certain acrylic acid copolymers

neutralisation and dispersion

Economic factors play a major part in the selection of sizes. For this reason, starch sizes and their mixtures continue to be the most widely used, particularly on cellulosic substrates. Nevertheless, more costly size polymers may be economically justifiable if this can be offset by higher productivity in weaving. High productivity generally demands high elasticity and

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strong adhesion, provided mostly, if not exclusively, by synthetic sizes. Water-jet weaving machines require water-resistant sizes. Some sizes are adversely affected by high temperatures (as in heat setting) or by treatment at inappropriate pH values and these effects can make their removal more difficult. The choice and combinations of different size components must take account of many factors if optimum results are to be obtained. Much has been published regarding the optimisation of size formulations in relation to desizing processes [177–183]. Cotton warp yarns sized with starch are normally woven at high humidity (80% and above) to keep yarn breakages low, as the starch film is brittle at low humidity. It has been shown [183], however, that improved weavability at moderate relative humidity (e.g. 65%) can be obtained using: (a) starch/acrylamide or hydroxyethyl starch at not less than 15% add-on; or (b) poly(vinyl alcohol), which gave excellent results even at a low add-on of 5–6%. Addition of acrylamide to starch improved the performance of cotton yarn more than acrylamide alone, but addition of poly(vinyl alcohol) to starch lowered the performance of the yarn compared with poly(vinyl alcohol) alone. Overall, taking into account economic considerations, stringent pollution requirements and the needs of desizing, the singlecomponent hydroxyethyl starch showed optimum acceptability for weaving performance at moderate relative humidity. The incorporation of waxes or lubricants (section 10.10.1) is an important consideration that should not be overlooked. These components exert a significant influence on weavability and on the conditions and efficiency of desizing. The lubricant may be added as a component of the size formulation, or applied separately by kiss-roll after sizing. Lubricants may themselves be used as sizing agents, particularly on synthetic warp yarns. Indeed, it has been argued [184] that such treatments give results comparable with traditional sizes as regards fibre strength and weaving performance. They offer advantages of savings in heat energy during drying and in manpower and space, as no special sizing unit is required. Desizing methods As indicated earlier in Table 10.7, it is helpful to select desizing methods according to whether size polymers are chemically degradable, water-soluble or water-resistant. Desizing by chemical decomposition is applicable to starch-based sizes. Since starch and its hydrophilic derivatives are soluble in water, it might be assumed that a simple alkaline rinse with surfactant would be sufficient to effect removal from the fibre. As is also the case with some other size polymers, however, once the starch solution has dried to a film on the fibre surface it is much more difficult to effect rehydration and dissolution. Thus controlled chemical degradation is required to disintegrate and solubilise the size film without damaging the cellulosic fibre. Enzymatic, oxidative and hydrolytic degradation methods can be used. The traditional approach is enzymatic desizing, in which an α-amylase or diastase enzyme is used to attack the 1:4-glycosidic links in the starch, breaking down the macromolecules into small soluble saccharides such as maltose and glucose. Scheme 10.18 is a simplified representation based on hydrolysis of the amylose component of starch; similar reactions take place with the amylopectin component (section 10.8.1). In addition to the enzyme a surfactant is required to ensure rapid and thorough wetting, including penetration of the size film, and emulsification or solubilisation of lubricants. Nonionic surfactants are less likely to deactivate the enzyme than anionic agents. The enzyme liquor is generally applied by

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impregnation immediately after singeing. With modern equipment running at speeds up to 170 m/min, the role of the surfactant is a critical one [169]. A carefully formulated mixture of surfactants may give the best balance of properties for rapid and thorough wetting, together with the removal of lubricant waxes or oils. The concentration of enzyme needed depends on the type and amount of size present and particularly on the potency of the commercial brand used. CH2OH

CH2OH

CH2OH

OH

O

O

HO OH

O

O

O

OH

OH

OH

OH

OH

n

Amylose component of starch

enzymatic degradation CH2OH

CH2OH

CH2OH O

O

OH

O

HO OH

OH

OH

O

OH

HO

OH

OH

Maltose

OH

Glucose

Scheme 10.18

Temperature and pH are critical parameters that are also brand-related. Enzymes are available covering the range 20–120 °C [143,169] and from acidic to alkaline pH. Those requiring mildly alkaline conditions are usually preferred, since the degraded fragments from the branched amylopectin component of starch usually require such conditions for solubilisation, rice and tapioca starches having rather higher amylopectin contents [143]. The presence of salt or calcium ions can also influence the behaviour of the enzyme. Hickman [143] has summarised typical conditions (Table 10.8). The effect of electrolytes (NaCl, Na2SO4, CaCl2 and MgCl2) on the aqueous solubility of saccharidic size polymers, including potato starch, starch esters and galactomannans, has been studied in detail [185]. It is important to know whether a fungicide, such as a halogenated phenol, has been added to protect the size formulation against mildew, since such fungicides are toxic to enzymes. There are some circumstances in which enzymatic desizing is inefficient: – if insufficient space is available for the batching of enzyme-impregnated fabric – if branched-chain starches such as tapioca are present they can be difficult to degrade, depending on the degree of ageing – if fungicides have been used to protect the starch from mildew attack – if oxidative desizing can be adopted and combined with scouring or bleaching, thus minimising energy requirements.

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Table 10.8 Optimum conditions for enzymatic desizing [143] Effect of Enzyme

Optimum pH

Optimum temperature range (°C) NaCl

Malt diastase Pancreatic amylase Bacterial amylase Bacterial amylase

4.5–5.5 6.5–7.5 6.5–7.5 7.0–8.0

55–65 40–55 65–75 100–120

o + + +

Ca ions Time + + + +

12–24 h 12–24 h 1–4 h 1–2 min

+ Improved desizing o No effect

The primary alternative to enzymatic treatment is oxidative desizing. This is extremely popular worldwide, especially in the Far East [169], and its popularity is increasing [143], mainly for the economic reasons outlined above. The oxidants most often used are hydrogen peroxide and sodium or potassium persulphate. There may be a considerable risk of fibre damage by persulphate, however, as this oxidant tends to degrade both starch and cellulose. The mechanism of degradation includes the hydrolysis of amylose (see Scheme 10.18) and amylopectin, as well as the formation of aldehyde (10.70), carboxylic acid (10.71), keto (10.72) and diketo (10.73) groups. There may also be ring cleavage [169] to give diacids such as tartaric (10.74) and oxalic (10.75). H

O

HO

C

O

HO

C O

O

OH

HO OH

OH

10.70

O

OH

HO

OH

HO

O C

OH

HO

OH

O

10.71

O

HO

C

OH 10.72

O C

O

OH

HO

OH

HO O

O

HO

C

OH

O

10.73

10.74

OH C

C

O

O

10.75

This oxidative reaction is generally carried out simultaneously with a caustic scour at 100–130 °C, offering the economic advantages of a combined scouring and desizing process. If peroxide is selected as the oxidant, this also exerts a bleaching action. In addition to oxidant and sodium hydroxide, the auxiliaries required include a magnesium salt, sodium silicate or an organic stabiliser for the oxidant, a sequestering agent (e.g. DTPA;

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diethylenetriaminepenta-acetic acid) and a wetting agent. The role of the magnesium salt and oxidant stabiliser is discussed elsewhere (section 10.5.3). Recommended conditions for batchwise oxidative desizing at high temperature are given in Table 10.9. Table 10.9 Oxidative desizing at high temperature [143] Concentrations (% owf) Magnesium sulphate heptahydrate Sodium hydroxide (100%) Stabiliser DTPA (40% solution) Hydrogen peroxide (35%) or sodium persulphate Wetting agent Temperature (°C) Time (min)

0.005 2–6 0–1 0.2 5–15 0.2–0.5 0.2–0.5 100 <15

0.005 2–4 0–1 0.2 5–15 0.2–0.5 0.2–0.5 100 15–60

0.005 9–12 0–1 0.2 5–15 0.2–0.5 0.2–0.5 120–130 1–2

In this table, the lower amounts of sodium hydroxide provide desizing only, whereas the higher concentrations provide oxidative scouring. The concentrations of the various components are critical to ensure efficiency of action without fibre damage. Hickman [143] gives the following guidelines: (1) Rapid desizing treatments require more critical control of alkali and oxidant concentrations. (2) Increased alkalinity for a given oxidant concentration tends to increase chemical damage. (3) Increased oxidant concentration above the minimum required for desizing increases chemical damage. (4) Persulphates promote desizing, rather than bleaching, and require more critical control of concentration than does hydrogen peroxide. (5) Mixing of persulphates and hydrogen peroxide is not recommended in pad–steam desizing. (6) To desize oxidatively by a batchwise process, the oxidant must be added when the alkaline fabric reaches top temperature. It is possible to degrade and solubilise starch size residues using cold peroxide and alkali but batching times are long (typically 16–20 hours). This simultaneous desizing and bleaching process requires 30 g/l sodium hydroxide (100%) and 50 ml/l hydrogen peroxide (35%), together with wetting agent, detergent, stabiliser and sequestering agent. The desizing effect can be improved by addition of persulphate without risk of significant fibre damage at this temperature [169]. It has been demonstrated [186] that the inclusion of polyacrylamide in either enzymatic or oxidative desizing formulations results in increased pick-up of the liquor by the sized warp yarns. Desizing by hydrolytic degradation of starch during the traditional kier-boiling treatment using 3°Bé sodium hydroxide liquor at 110 °C is now rarely encountered as it is a slow and expensive process [169].

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In Table 10.7 are listed the following size polymer types in the water-soluble category: acrylic acid copolymers, poly(vinyl acetate/ alcohol), carboxymethylcellulose and certain modified starches. It is necessary to clarify what is meant by water-soluble in this context. All these polymers form colloidal solutions and the dissolving process is not instantaneous. When films formed by such polymers come into contact with water, there is an initial period during which they absorb water. The rate at which water diffuses into the polymer film varies widely, depending on the polymer structure and the heat treatments that the film has received. This phase is known as rehydration and is characterised by the formation of a gel. Only when the polymer has absorbed sufficient water does it actually begin to disintegrate and dissolve to give a viscous colloidal solution. Higher temperatures and the presence of surfactants generally increase the rates of rehydration and dissolution. Figure 10.16 illustrates relative dissolution rates in cold water of various water-soluble size polymers prepared as 60 × 15 mm films of 200 µm thickness [169]. The disintegration level represents the completion of rehydration. Thus in any desizing process relying solely on aqueous dissolution, it is always important to allow adequate time for rehydration. Film thickness 200 m

Dimensions 60 × 15 mm

Dissolution Disintegration

A

B

C

D E

4

8

12

16

20

Treatment time/min A B C D E

Size CB/CA Starch ether Partially saponified poly(vinyl acetate) Carboxymethylcellulose, salt free Fully saponified poly(vinyl acetate)

Figure 10.16 Dissolution rates of films of water-soluble sizing agents in water at 20 °C [169]

Not all modified starches are suitable for removal by aqueous dissolution alone. Such modifications of natural starches are carried out to reduce solution viscosity, to improve adhesion and ostensibly to enhance aqueous solubility. Commercial brands vary [169], however, from readily soluble types to those of limited solubility. Indeed, some may be as difficult to dissolve as potato starch if they have been overdried. It is thus very important to be sure of the properties of any modified starch present. If there are any doubts about aqueous dissolution, desizing should be carried out by enzymatic or oxidative treatment. Even if the size polymer is sufficiently soluble, it is important to ensure that the washing-off range is adequate. Whilst the above comments relate to modified starches, other size polymers such as poly(vinyl acetate/alcohol) and acrylic acid copolymers vary from brand to brand with regard to ease of dissolution. Carboxymethylcellulose, an excellent film-former, is a highly effective size on cellulosic substrates but has poor adhesion to synthetic fibres. It is easily desorbed, hot water generally being sufficient, although surfactant and alkali are usually added to increase the efficiency of

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removal. A further advantage of carboxymethylcellulose is that heat setting does not affect the ease of subsequent removal, nor is it sensitive to alkaline or acidic pH. Indeed, it has been stated [171] that if anyone has a problem removing a carboxymethylcellulose size, it is unlikely that they could remove much of anything from a piece of fabric! Nevertheless, Angstmann and Bassing [169] caution that this size does require much water and an initial swelling time. The poor adhesion of carboxymethylcellulose to synthetic fibres means that where such fibres are present, it can only be effective in combination with a synthetic size polymer (Table 10.6). This needs to be taken into account when considering suitable desizing procedures. If this cellulose derivative is to be used in conjunction with an electrolytesensitive acrylic acid copolymer, it is advisable to choose a salt-free carboxymethylcellulose. Poly(vinyl acetate/alcohol) sizes are also described as water-soluble and are widely used, either alone or in combination with most of the other types, across the whole range of fibres and blends [169,171]. However, this category covers a wide range of commercial products, differing greatly in quality and ease of removal. Indeed, some are quite difficult to remove, thus necessitating careful selection [187]. Detailed studies of factors affecting the removal of water-soluble sizes, particularly poly(vinyl alcohol) types, have been published [188–190]. Apart from molecular mass, the behaviour of polymers of this type depends strongly on alcohol group content, usually expressed as the percentage degree of saponification of the poly(vinyl acetate) from which it is derived. Poly(vinyl alcohol) forms a strong film that shows good adhesion to cellulosic fibres but poor adhesion to polyester. These films can usually be removed quite readily with hot water and detergent. The presence of a mild alkali may be acceptable [171] but is perhaps best avoided [169]. Poly(vinyl alcohol) is sensitive to alkali addition and is precipitated by strong alkali, making the residues very difficult to remove. The wash-off parameters should be optimised for efficiency of size removal, since high temperatures and long liquor ratios are environmentally and economically unfavourable. The amount of size polymer applied and the presence of any additives need to be carefully considered. Care should be taken in the heat setting of fabrics containing poly(vinyl alcohol) size. Too high a temperature for too long a time induces crystallisation, making the size residues much more difficult to remove. Poly(vinyl alcohol) can be modified with crotonic acid (Scheme 10.17) to give size copolymers that have higher solubility and lower sensitivity to alkali [169]. Although these sizes are generally regarded as water-soluble, they are more readily removed by alkaline oxidative desizing methods using either persulphate or peroxide [169], the polymer being degraded into smaller segments as indicated in Scheme 10.19. CH2

CH2

CH

CH

OH

OH

CH2

CH2

Na2S2O8 or H2O2

CH2

C

C

O

O

HO CH2 C O

Scheme 10.19

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C

OH +

O

CH2 C O

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CH2

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

Many acrylic acid copolymers are water-soluble but unlike poly(vinyl alcohol) they are not degraded by alkali. In fact they need alkali for effective desizing as they are more soluble at alkaline pH than in neutral solutions. They are sensitive to acidic media, which should not be used. Solubilisation occurs by the formation of sodium carboxylate groups from the anionic polyacid. The polyelectrolyte formed in this way is readily soluble and shows a rapid rate of dissolution. However, the presence of electrolytes such as magnesium or calcium salts from hard water can inhibit removal [191]. The desizing of water-soluble size polymers can be summarised as follows. Batchwise or continuous methods can be used; in both cases an adequate supply of hot water is needed during the washing-off. Hot water and detergent are needed to remove poly(vinyl alcohol) or carboxymethylcellulose. The addition of alkali may be beneficial with carboxymethylcellulose. Alkali is essential with modified starches and acrylic acid copolymers. Poly(vinyl alcohol) can be degraded effectively by alkaline oxidation. Water-resistant sizes include polyesters and certain acrylic acid copolymers. These are mainly used for the sizing of synthetic filament warp yarns, some formulations being particularly useful in water-jet weaving. The acrylic acid size copolymers described as watersoluble are generally partially neutralised during manufacture to give partial formation of the sodium salt; complete conversion to the sodium salt is carried out using alkali in desizing. Water-dispersible types, on the other hand, are normally partially neutralised with ammonia during manufacture and these are more suitable for water-jet weaving. When the sized yarn is dried the ammonia is evolved, leaving the less soluble acrylic acid form. This lower solubility is a significant advantage in water-jet weaving. A yarn sized with a sodium acrylate copolymer will have a typical pH of 6–7 when dry, compared with pH 4–5 for an ammonium acrylate type [171]. This difference in pH needs to be accounted for when desizing, so that more alkali is needed for the desizing of ammonium acrylate copolymer sizes. Typical desizing conditions include a detergent with up to 5 g/l sodium hydroxide or sodium carbonate at 70–90 °C [171]. The pH should be carefully monitored in order to maintain alkalinity throughout. A sequestering agent may be needed because transition-metal ions can insolubilise acrylic acid copolymers, one atom of trace metal reacting with two or three carboxylic acid groups in the size polymer. Polyester sizes differ from acrylic acid sizes in the nature of the solubilising groups present, namely sulphonic acid rather than carboxylic acid groups, solubility behaviour being dependent on the degree of sulphonation with respect to the average molecular mass of the polymer (10.76). In general, however, the degree of sulphonation is low (10.69) and the polymer is rather sensitive to alkali. Too strongly alkaline conditions result in hydrolytic cleavage of the ester groups (Scheme 10.20) to give some fragments without solubilising groups and these are difficult to remove in desizing [169]. Consequently, although alkali and detergent are needed for desizing, the amount of alkali is less than that needed for acrylic acid sizes and the alkalinity should not be allowed to rise above pH 9. Typical conditions are 0.5–1.0 g/l sodium carbonate with detergent at 90–95 °C [169]. A sequestering agent may also prove useful. Batchwise treatment, or more usually a pad-wash range, can be used. Polyester resins can be highly beneficial as additives to other size polymers, although a great deal of care and expertise is required in formulation [192]. Viscosity, for example, is an important factor in the warp sizing process. The viscosity of some sizes, such as poly(vinyl alcohol), is significantly affected by temperature fluctuations. The addition of a polyester resin tends to minimise such changes in viscosity. Surface tension is another important parameter

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PREPARATION OF SUBSTRATES

SO3Na (=X1) O

C

R1

O

C

O

X2

R2 X2

O

571

O

C

R3

O

C

O

O

C

SO3Na

O

R1

O

R2

C

O

O

(= X1)

10.76 hydrolytic cleavage

SO3Na HO

C O

R1

C O

X2 HO

OH HO

R2

OH

C O

R3

C

OH

HO

O

C O

HO

X2

R1

R2

C

OH

O

OH

SO3Na

In structure 10.76 (see also 10.69) either X1 or X2 contains a sulpho group, but not both. R3 is an unsulphonated aliphatic group Scheme 10.20

that is subject to variability and polyester resins can help to stabilise it within the optimum range. Such resins have beneficial effects on rheological behaviour, film formation and film characteristics, giving significant improvements in yarn abrasion resistance, good surface lubrication and a softening of the surface fibres. Certain acrylic acid or vinyl copolymer resins behave in a similar way to polyester resins in enhancing the performance of conventional size formulations. The degree of expertise required in formulating such complex mixtures of size polymers for optimal performance has led Mayfield [192] to emphasise the advantages of using proprietary products formulated by specialists. These products may contain up to thirty ingredients, including trace additives that can effectively influence the many components present. These formulations can be very accurately combined and evaluated for chemical and physico-chemical compatibility by specialist suppliers. Analysis and monitoring of size polymers and desizing processes The many technical factors involved in desizing and the need for economy and environmental accountability emphasise the importance of monitoring and analysis. However, only a brief outline can be given here. A review of analytical procedures and simple laboratory methods for size determination is available [193]. Methods are given for size determination directly on the fibre surface, for the extraction of components of low molecular mass and for their subsequent estimation in solution. Size polymers on polyester can be determined by staining tests with CI Basic Red 22, CI Reactive Red 12, iodine/potassium iodide solution, or a mixed indicator. The extraction of size components and their determination in solution using a variety of reagents to give a characteristic coloration or a coloured precipitate has been described. Methods using fluorescence spectroscopy with a fluorescent cationic dye (e.g. Pinacryptol Yellow or CI Basic Orange 14) were also described.

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

An extremely useful technique for measuring the amount of size applied is non-contact on-line determination of water absorption [194]. The moisture content of sized warps can be derived from microwave absorption by the water present. The identification of anionic poly(acrylic acid) sizes can be carried out by staining with a fluorescent cationic dye (CI Basic Orange 14) followed by spectroscopic measurement of excitation wavelength and fluorescence emission [195,196]. Such methods can also be used (with CI Basic Orange 14 or CI Basic Red 1) to detect and estimate carboxymethylcellulose, poly(vinyl alcohol) and starch derivatives [197]. A method is available, utilising on-line near-infrared reflectance spectroscopy, for controlling the uniform application of poly(vinyl alcohol) size [198]. The desizing of cotton can be monitored using a dyeing test supported by assessment of wettability [199,200]. The dyeing test requires a 10 g/l solution of CI Direct Red 83. This test can usefully be supplemented by the TEGEWA drop test to determine wettability. A simple method of estimating the efficiency of desizing with poly(vinyl alcohol) and starchbased sizes depends on the determination of TOC (total organic carbon) and COD (chemical oxygen demand) [201]. Environmental aspects Since size add-on is customarily in the region of 10–20%, desizing clearly produces quite a sizeable pollution load. Although many size polymers are biodegradable, they exhibit high biological oxygen demand (BOD) and chemical oxygen demand (COD). These high values are compounded by the major contribution of size residues. High liquor ratios or copious quantities of wash-off water facilitate the desizing process but these large volumes of waste liquor pose additional difficulties quite apart from the initial cost, both financially and environmentally. In Table 10.10 the water consumption, BOD and pollution load contributed by desizing are compared with those from other wet processes on cotton [202]. Typical BOD and COD values determined for the main classes of size polymers are given in Table 10.11 [203]. The COD of a typical cotton desizing effluent is composed of 42% from sizing agents and fibre fragments, 40% from impurities in the cotton and 8% from surfactants [204].

Table 10.10 Water and effluent data arising from the wet processing of cotton [202] Water consumption (% of total)

Process Desizing Scouring Bleaching Mercerising Dyeing Printing Washing-off Finishing

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5 1 46 2 8 7 30 1

BOD (% of total) 22 54 5 5 6 1 7

Pollution load (% of total) >50 10–25 3 <4 10–20 10–20 5 15

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Table 10.11 Specific COD and BOD values found for important sizing agents [203]

Sizing agent

Specific COD value (mg O2/g)

Specific BOD value (mg O2/g)

Starch Carboxymethylcellulose Poly(vinyl alcohol) Polyacrylate Galactomannan Polyester dispersion Protein size

900–1000a 800–1000a ca. 1700a 1350–1650a 1000–1150a 1600–1700a 1200a

500–600 50–90 30–80b <50 400 <50 700–800

a Taking account of moisture content of commercial product b With non-adapted inoculum

Clearly, one option to reduce the add-on is to use high-efficiency size formulations. However, there is a limit to what can be achieved by this approach. Even if the add-on is reduced to only 5%, the pollution load is still substantial. The two main options to facilitate disposal are: (a) recovery of size polymers; and (b) biological effluent treatment. Recovery of size polymers, particularly from water-soluble synthetic sizes, is based on extraction washing using the minimum quantity of water. Recovery rates in the region of 50% have been quoted for poly(vinyl alcohol) and carboxymethylcellulose size formulations. It is necessary to apply one of three concentration techniques: precipitation, condensation or ultrafiltration [205]. Precipitation is only really possible with poly(vinyl alcohol) and is seldom applied to textile effluents, normally only to eliminate size residues from effluent liquors. The condensation technique exploits heating to drive off water. This is energy-intensive and is therefore in decline. Ultrafiltration is long-established commercially and is the preferred concentrating technique. The principle of ultrafiltration, capable of separating particles in the range 0.05–0.15 µm, has been compared with other systems of filtration, such as reverse osmosis, nanofiltration and microfiltration [206]. A sequence of treatments can be used, such as ultrafiltration to recover most of the size followed by biological treatment of the residue. In some cases ultrafiltration renders an effluent acceptable for discharge to the municipal water treatment system, but this depends on local regulations. One major advantage of ultrafiltration is that it can facilitate the reuse of recovered materials [207]. In the case of sizing, however, this clearly depends crucially on the composition of the recovered material. If a blend of sizing agents is present, it does not necessarily follow that the recovered material will contain these components in the original (required) proportions for reuse. Nor is reuse a possibility where desizing has been carried out by a degradative process, such as enzymatic or oxidative desizing. Thus the scope for reuse is favoured where single-component formulations of non-degraded size polymer can be recovered. Simple formulations, however, may not meet all the weaving requirements. Hence arguments for and against the relative merits of simple and complex formulations are common. The basis of ultrafiltration is that a liquor is passed through a membrane many times until the required concentration of the permeate is attained. Fouling of the membrane can be a problem and regular cleaning and disinfection of the membrane is recommended. Ultrafiltration of poly(vinyl alcohol) and starch sizes offers economic advantages over

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

discharge to the sewer [208,209]. A starch carbamate size effluent had its entry COD of 19 159 mg O2/l reduced by ultrafiltration to 672 mg O2/l, a recovery yield of almost 97%. Starch ethers, such as hydroxypropyl and carboxymethyl derivatives, have given recovery values of 92–97% [209]. Size polymers suitable for recovery by ultrafiltration must exhibit the following characteristics: water solubility, thermal and mechanical stability, bioresistance, good washing-out properties and low to moderate viscosity [210]. Poly(vinyl alcohol), carboxymethylcellulose and certain acrylic sizes meet these demands. The ultimate benefits of ultrafiltration from economic and environmental aspects are that not only can size be recovered for reuse but also the water that passes through the membrane can be recycled into the washing-off range [211]. Biological treatment of a desizing effluent usually involves a two-stage anaerobic treatment using cultivated methane bacteria, possibly followed by a final aerobic treatment [209]. Typically, a dwell time of 12–17 days will give about 80% degradation of a starch size. Such treatment usually gives an effluent acceptable for discharge to the municipal sewage system. Several factors other than the nature of the size polymer can affect the efficiency of biological treatment, notably the degree of adaptation of the biological inoculum and the temperature at which digestion takes place. The method of evaluation can also influence the results obtained. Figures 10.17 and 10.18 illustrate the effects of adaptation of the inoculum on the bioelimination of a modified poly(vinyl alcohol) size [212], as assessed by the Sapromat [213] and Zahn–Wellens [214] test protocols respectively. The effect of digestion temperature on the same polymer using optimally adapted inoculum is shown in Figure 10.19. Thus seasonal temperature fluctuations need to be considered. It is possible to formulate mixtures of size polymers so that the effect of digestion temperature is minimised. Data showing the effects of temperature on the rates of biodegradation of hydroxypropyl starch and of a formulation containing poly(vinyl alcohol) and hydroxypropyl starch [215] reveal that the polymer mixture is much less sensitive to the temperature of digestion. The rate of influx and even of rainfall can also exert some influence on the rate of elimination.

100

Biodegradation/%

80 adapted

60

40

20 non–adapted

4

12

20

28

Time/days

Figure 10.17 Biodegradation rate curves for poly(vinyl alcohol) with adapted and non-adapted inoculum according to the Sapromat test [212]

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575

100 optimally adapted

DOC–elimination/%

80

60

40

20 non–adapted

2

4

6

Time/days

Figure 10.18 DOC-elimination rate curves for poly(vinyl alcohol) with optimally adapted and nonadapted inoculum according to the Zahn-Wellens test [212]

100

DOC–elimination/%

80

23 oC 15 oC 10 oC

8 oC

60

40

20

6

12

18

Time/days

Figure 10.19 DOC-elimination rate curves for poly(vinyl alcohol) with optimally adapted inoculum at various temperatures [212]

The following scale has been proposed to assist discussions of biodegradability [216,217]: Degree of biodegradation after 14 days 80–100% 60–80% 0–60%

Description easily biodegradable moderately biodegradable difficultly biodegradable

Many starches and modified starches are easily biodegradable, although some are less so. In an evaluation of the biodegradability of ten starch-based sizes over seven days [218], four

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

were degraded to 95–100%, two to 80–95%, three to 60–80% and only one to less than 60%. Whilst carboxymethyl starches are easily biodegradable [218], carboxymethylcelluloses are generally difficult to degrade [211,217]. The biodegradation of carboxymethylcellulose can be as low as 8–13% under aerobic conditions, although up to 54% can be eliminated under anaerobic conditions [211]. Galactomannan sizes are biodegradable to 80% within seven days [216,217]. It was mentioned earlier that poly(vinyl alcohol) sizes show wide variability in aqueous solubility. They are also extremely variable with regard to biodegradation characteristics, from 15% to 95%. It is not surprising, therefore, that they have been the subject of much research [187,203,211,212,219] and not a little controversy. Reference 203 lists twelve further references concerned with the microbial degradation of poly(vinyl alcohol). Misunderstandings have undoubtedly arisen through a failure to appreciate the need for an adequate digestion temperature and effective acclimatisation of the bacterium. There can be a drastic fall in efficiency of degradation at temperatures lower than 10–12 °C, since the desorption rate of the degrading micro-organisms at such temperatures exceeds the growth rate [203,219] and bioactivity is lower in any case. The fluctuations are influenced by variations in the composition of commercial size formulations, including the presence of organic chemicals such as methanol and acetic acid remaining from the polymerisation process, which may increase or decrease the biodegradability. In general, poly(vinyl alcohol) sizes can be degraded or eliminated by an optimally adapted inoculum in an appropriate sewage sludge phase, giving decomposition rates of 57–65% within seven days but with a potential for up to 100% elimination [211,212]. Trials have been carried out at a low temperature (8 °C) to demonstrate that even in winter poly(vinyl alcohol) can be eliminated by more than 90%, admittedly at a slower rate [211,212]. Acrylic size polymers generally show poor biodegradability (typically 3–10%), although recent research [211] has achieved up to 37% decomposition in special anaerobic cultures. However, these anionic polyelectrolytes can be readily eliminated from effluents by adsorption on sludge [220]. Acrylic size polymers may be precipitated in the third stage now included in many water treatment plants for the removal of phosphates, using a conventional inorganic precipitant such as iron(III) chloride. Some size formulations may require the addition of a specific precipitant of this kind, whilst others may be removed by bioelimination in the sludge. Polyester sizes may be disposed of by similar bioelimination methods [220]. Deliberate precipitation creates a need for subsequent disposal, but with acrylic size polymers this solid waste has high calorific value and is suitable for disposal by incineration. An advantage of acrylic size polymers arising from their ease of removal by precipitation is that they can be readily recycled, so that they need not enter the sewage system. Ultrafiltration is the preferred method of recycling [220,221]. 10.5.3 Bleaching The three primary oxidants associated with textile bleaching are hydrogen peroxide (H2O2), sodium hypochlorite (NaOC1) and sodium chlorite (NaClO2). There are other oxidising and reducing agents occasionally used (or proposed for use) in bleaching; these will be dealt with later. For the present, general factors affecting the use of the above three oxidising agents will be discussed. Many technical factors govern the selection of one bleaching agent over another:

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PREPARATION OF SUBSTRATES

– – – – – – –

577

generic type of fibre (cellulosic, wool, synthetic) physical form of fibre (yarn, woven or knitted fabric) product and process costs stability, and therefore reliability versatility of process (batchwise or continuous) degree of whiteness obtained extent of any fibre damage.

Environmental factors have now become important in bleaching and are having far-reaching effects on the choice of bleaching agent. A particularly important parameter is the absorbable organic halogen value, commonly referred to as AOX. This is generally expressed as the mass of organohalogen compounds absorbed per unit mass of activated charcoal, there being three main steps in the analytical procedure [222]: (1) Absorption of all organic halogen compounds by activated charcoal, taking care to avoid misleading results by uptake of inorganic salts such as sodium chloride (2) Combustion of the activated charcoal and collection of the halogens thus released (3) Quantitative determination of the halogens. This aspect came to the fore with the well-known and much publicised discovery of the potent toxin and carcinogen 2,3,7,8-tetrachlorodibenzo-p-dioxin (10.77), commonly but erroneously referred to simply as ‘dioxin’, in effluent from chlorine-based bleaching processes for wood pulp in papermaking. Although legislative requirements vary considerably worldwide the trend has been to ban or severely restrict the discharge of AOX-containing liquors. Typically, an AOX value of 0.5 mg/l must not be exceeded in waste water released from a textile wet processing works. This compares with a typical limit for drinking water of not more than 0.01 mg/l. In some legislative criteria there are limits on discharge from each processing line in a works, as well as on overall discharge. Typical values for three oxidising agents in cotton bleaching processes where no attempt has been made to minimise the AOX value are listed in Table 10.12.

Cl

O

Cl

Cl

O

Cl

10.77

Table 10.12 AOX values for cotton bleached with various oxidising agents [223]

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Oxidising agent

AOX (mg/l)

Sodium hypochlorite Sodium chlorite Hydrogen peroxide

27.00 2.40 0.92

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

The seriousness of the AOX problem is highlighted by the fact that there is currently no economically acceptable solution for the specific removal of halogenated organic compounds from effluents [224]. There are important technical reasons why, over the years, hydrogen peroxide has progressively gained ascendancy over sodium hypochlorite as a bleaching agent. In view of the AOX values in Table 10.12 it is clear that there will be additional pressure, this time on ecological grounds, to continue the phasing out of hypochlorite bleaching in favour of peroxide. This is in spite of the fact that there are some steps (to be mentioned later) that can be taken to reduce the AOX values of effluent from hypochlorite bleaching. The intention here is to concentrate discussion on the chemistry of the products used. Hydrogen peroxide, because of its pre-eminence in bleaching technology, will be dealt with first, followed by sodium hypochlorite, sodium chlorite and other bleaching agents. The initial discussion is concerned mainly with cellulosic fibres, with some reference to synthetic fibres; wool and silk are then dealt with separately. The processing details of bleaching and the machinery used are dealt with elsewhere [11,143,225,226]. Excellent historical accounts are also available [227,228]. Peroxide bleaching The advantages favouring the pre-eminence of hydrogen peroxide (over 90% of cotton goods are bleached with peroxide) include [143,225]: (1) Stability and consistency of supply Most of the hydrogen peroxide solution supplied for textile bleaching is acidic (pH 4.5– 5.0) because it shows maximum stability under these conditions. Additives are present to increase its stability further at this pH. (2) Environmentally friendly As indicated above, peroxide does not contribute to AOX values and potentially decomposes completely into water and oxygen during effluent treatment. (3) Versatility of application Peroxide can be used over a wide range of application conditions in batchwise and continuous methods, the latter being predominant. The success of continuous peroxide bleaching is attributable to the relatively rapid rate at which bleaching takes place, although long-dwell processes are also established. Peroxide baths do not cause significant corrosion of machinery. Peroxide gives a good white and can be used even without prior scouring of the material. It is particularly suitable for combining with other processes, e.g. scouring, desizing, application of fluorescent brighteners, as well as the bleaching of many blends. The technical disadvantages of hydrogen peroxide are relatively minor compared with the process costs: (1) Fibre damage This may occur by free-radical formation, especially in the presence of transition-metal ions such as those of iron or copper. Similar mechanisms can result in the decomposition of peroxide but there are means of controlling or avoiding this problem. (2) Quality of whiteness Higher levels of whiteness and more attractive tones (i.e. neutral or bluish rather than yellowish or reddish whites) are attainable with hypochlorite, although whites of the highest quality are produced using the two bleaching agents sequentially.

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(3) Cost-effectiveness If the costs for treating AOX-containing effluent are omitted, peroxide bleaching is more costly than hypochlorite bleaching. Data has been presented [222] showing that the chemical costs of a continuous bleaching process with alkaline hydrogen peroxide are on average about six times higher than those from sodium hypochlorite. In batchwise processing, peroxide can be twice as expensive as hypochlorite [222].

95

100

90

80

85

60

80

40

75

20

9.5

10.0

10.5

11.0

Residual H2O2/%

CIE whiteness

As mentioned above, hydrogen peroxide is available commercially as a stabilised liquid of pH 4.5–5.5. Solid peroxides have been proposed, with claims of bleached fabrics with greater strength and more stable whites [229], but it is difficult to foresee such solid peroxides gaining a commercial foothold at the expense of liquid hydrogen peroxide. Hydrogen peroxide exerts little bleaching action at pH 4.5–5.5 and requires activation to produce a bleaching effect. The principal means of activation is alkali addition. Given the required degree of alkalinity, temperature provides a further means of controlling the bleaching action, such as an overnight dwell at ambient temperature or from 3 to 20 minutes at 100 °C. Figure 10.20 shows the degree of whiteness attained at various pH [226], together with the corresponding curve for the amount of peroxide remaining in the treatment bath. This classic activation curve for hydrogen peroxide shows that bleaching is generally best carried out at pH 10.5–11.0. The amount of alkali required to give this pH will vary with the type of process, sodium hydroxide being the alkali most commonly added, although sodium carbonate or phosphate may be used.

11.5

Initial pH

Figure 10.20 Optimisation of pH in hydrogen peroxide bleaching [226]

Under alkaline conditions an additive is required for stabilisation of the peroxide, which is necessary to avoid undesirably rapid decomposition with loss of bleaching efficiency and/ or damage to the fibre. Traditionally, the most common stabilising agents have been the colloidal sodium silicates. The formulae of sodium silicates are best represented in terms of the ratio of sodium oxide to silica, which is 1 in sodium metasilicate (Na2O:SiO2) and 2 in the orthosilicate (2Na2O:SiO2). These silicates, however, are crystalline forms in which this ratio is 1 or greater. In the colloidal forms originally preferred for peroxide bleaching the

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

ratio is less than 1. For example, in the so-called ‘alkaline glass’ form the ratio is 1:2, whilst in the so-called ‘water glass’ form it varies from 1:1.6 to 1:1.38. The colloidal silicates are efficient and economical stabilisers but care is needed to ensure efficient washing-off in order to avoid silicaceous deposits on the fabric and equipment. A simple and rapid test procedure has been developed to determine the relative suitability of stabilisers, with a view to keeping the running surfaces of a continuous bleaching range free from deposits [230]. Although such colloidal forms have been preferred, the crystalline meta- and orthosilicates can also be used and may provide easier washing-off. The required degree of alkalinity is generally obtained by the addition of sodium hydroxide, sodium carbonate or a phosphate, the amount of alkali varying with the type and quantity of silicate used. Since the bleaching action yields acid, sufficient alkali is required for neutralisation as well as absorption by the cellulose. The mechanism by which these stabilisers act is complex, although the elements of buffering action and sequestering of transition-metal ions, such as those of iron(III) and copper(II), undoubtedly contribute. Magnesium ions also play an essential part in the mechanism and must be added (as the sulphate, for example) if sufficient are not already present in the system. It is important to recognise that whilst transition-metal ions catalyse the decomposition of peroxide, the alkaline earth elements stabilise it. In the absence of calcium and magnesium even silicates can act as bleach activators [19]. The problems associated with silicaceous deposits have led to the adoption of more costly organic stabilising agents that also aid in plant cleaning and reduce the incidence of reprocessing. These organic stabilisers are often commercially blended products which may or may not contain magnesium salts [143], the three main types being aminopolycarboxylate sequestering agents, protein degradation products and selected surfactants. The preferred sequestering agents, in terms of both sequestering ability and stability to oxidation, are DTPA (10.6), either as its sodium or its magnesium salt, and its hydroxy derivatives [18]. Relatively simple methods of evaluating the efficiency of stabilisers have been used, but more reliable results are obtained with statistical experimental methods involving a realistic simulation of the bleaching process [231,232].

The mechanism of peroxide bleaching The details of this mechanism have been the subject of much debate over the years. Dannacher and Schlenker [233] have reviewed the various hypotheses and applied experimental criteria to establish the most likely mechanism. The fact that the bleaching effect varies widely with reaction conditions suggests that the actual bleaching agent is not hydrogen peroxide itself but another species liberated from the peroxide under the influence of pH and temperature. This much is generally agreed; there is, however, much debate regarding the liberated species. Since molecular oxygen has no bleaching effect, nascent or atomic oxygen has been proposed as the active species on the basis that it is readily liberated from the perhydroxide anion (HOO–), according to Scheme 10.21. Dannacher and Schlenker reject this, since calculations show that the formation of oxygen atoms is energetically highly unfavourable and is not expected to occur under bleaching conditions. Singlet oxygen has also been proposed and this can indeed be formed, particularly in mixtures of hydrogen peroxide and sodium hypochlorite. However, carefully designed experiments showed that under these conditions sodium hypochlorite has neither a direct

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581

_ _

HO H2O2

_

HOO

HO

+ [O]

H+ Scheme 10.21

_ CH2CH2COO C CH

O

O CH CH 10.78

bleaching effect (because it is consumed in the reaction with peroxide) nor an indirect bleaching effect via singlet oxygen. Dannacher and Schlenker also examined the effect of singlet oxygen derived from the thermal decomposition of an endoperoxide (10.78). They concluded that singlet oxygen has no bleaching effect when homogeneously dispersed in a liquor and that alkaline hydrogen peroxide solution contains no singlet oxygen. The active species most often cited in recent times has been the perhydroxide anion mentioned previously (Scheme 10.21), even though it shows a decrease in redox potential with increasing pH. Hydrogen peroxide exists in aqueous solution in a dissociated equilibrium with the perhydroxide anion (HOO–) and the peroxo dianion (–O-O–), as in Scheme 10.22. In view of the predominance of this hypothesis, Dannacher and Schlenker subjected it to a thorough investigation, studying the bleaching effect of hydrogen peroxide at 60 °C over the range of pH 2 to 13 and obtaining the results shown in Figure 10.21. If the _

_ HO H2O2

_

HO

_ O

HOO

_ O

H+

H+

Scheme 10.22

100 Experimental values H2O2 HOO–

Stain removal/%

80

60

40

20

b

2

4

6

8

a

10

12

14

pH

Figure 10.21 Relative stain removal after 60 min bleaching at 60 °C [233] 6.5 × 10–3 mol/l hydrogen peroxide in buffer solution. Relative share (%) of the protolytic forms of hydrogen peroxide in the total amount at 60 °C (–O–O– negligible in this range)

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

perhydroxide anion were the effective agent one would expect the bleaching effect to increase with concentration and thus with increase in pH beyond the equilibrium point of pH 11. Figure 10.21, however, shows that at the point where the concentration of perhydroxide anions begins to exceed that of undissociated hydrogen peroxide, the bleaching effect decreases considerably, indicating that the perhydroxide anion is clearly not the effective bleaching agent [233]. As shown in Scheme 10.23, the perhydroxide anion can give rise to both perhydroxyl and hydroxyl free radicals. These free radicals have been proposed as the active bleaching agents. Dannacher and Schlenker carried out bleaching tests in the presence of scavengers for the hydroxyl free radical, 4-nitroso-N,N-dimethylaniline (10.79) and potassium hexacyanoferrate(II) (10.80) separately at a concentration of 10 –3 mol/l [233]. The bleaching effect was just as powerful in the presence of the scavengers as in their absence. It was concluded that the hydroxyl free radical has no bearing on the bleaching effect of hydrogen peroxide. _ H

O

O

+

Perhydroxide anion

H

H2O2

O

O

+

Perhydroxyl radical

Hydrogen peroxide

HO

+

Hydroxide anion

HO Hydroxyl radical

Scheme 10.23

K+ CH3 O

N

N

K

CH3

+

NC CN NC Fe CN NC

10.79

CN

K+

K+

10.80

Consideration of pKa values suggests that the perhydroxyl radical is dissociated almost quantitatively in the optimum pH range for peroxide bleaching, giving rise to the conjugate base .O–O– known as the superoxide radical ion (Scheme 10.24). When Dannacher and Schlenker carried out tests using hydroquinone (10.81) as a scavenger for the superoxide radical ion, it was found that the bleaching effect decreased with increasing concentration of scavenger (Figure 10.22). These results suggested that the effective species in hydrogen peroxide bleaching is the superoxide radical. The agreement between experimental and calculated results for superoxide concentration at different pH values was taken as further support for this mechanism [233]. _ HO H

O

O H

Perhydroxyl radical

+

_ O

O

Superoxide radical ion

Scheme 10.24

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PREPARATION OF SUBSTRATES

HO

583

OH

10.81 Hydroquinone

100 With 6.5 × 10–3 mol/l hydrogen peroxide Without hydrogen peroxide

Stain removal/%

80 60 40 20 0

0.5

1

2

3

4

5

Hydroquinone concentration/mmol l–1

Figure 10.22 Effect of increasing hydroquinone concentration on the relative stain removal after 60 min at pH 10.5 [233]

However, these conclusions have been vigorously challenged by Spiro and Griffith [234]. The bleaching tests of Dannacher and Schlenker were carried out on cotton stained with tea. Spiro and Griffith pointed out that tea stains are complex mixtures of coloured polyphenolic compounds with unknown protonation constants. They claimed that kinetic studies of peroxide bleaching are more easily interpreted using individual coloured compounds with known properties such as phenolphthalein (10.82), alizarin (10.83) or crocetin (10.84), a yellow polymethine colorant isolated from saffron. Spectrophotometric studies by Spiro and Griffith of the bleaching of these compounds with hydrogen peroxide demonstrated that none of these reaction rates was affected by addition of a scavenger for hydroxyl and perhydroxyl radicals, which were therefore ruled out as effective species. They do concur with Dannacher and Schlenker, however, that singlet oxygen can also be ruled out on the basis of scavenging experiments. Offering a radically different interpretation, Spiro and Griffith claimed that all the kinetic data can be explained quantitatively on the assumption that the perhydroxide anion (at higher pH values) and hydrogen peroxide itself (at lower pH values) are the only active oxidising species. Further studies of the bleaching of malvidine chloride (10.85) at pH 1.5–4.0 showed that the only significant oxidant was the hydrogen peroxide molecule, which was thus not as inert as Dannacher and Schlenker apparently believed. Spiro and Griffith concluded that in the peroxide bleaching of a wide range of colorants there is no need to invoke any oxidising species other than the perhydroxide anion and molecular hydrogen peroxide.

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

HO

OH

O

OH OH

O C O

O

10.83

Alizarin

10.82 Phenolphthalein

CH3

HO C

C

CH3 CH

CH

CH

C

CH3 CH

CH

CH

CH

C

CH3 CH

CH

CH

O

C

OH C O

10.84 Crocetin

OH OH OCH3

+ _ O

HO Cl

OH 10.85

OCH3

Thus the detailed mechanism of peroxide bleaching is not yet finally resolved. It should be borne in mind, however, that the work of Dannacher and Schlenker was carried out at 60 °C on tea-stained cotton, whereas Spiro and Griffith studied the decolorisation of individual colorants at 21–25 °C in the absence of a textile substrate. Fibre tendering and process control In an optimally controlled process free from transition-metal ions hydrogen peroxide bleaching is remarkably safe, there being no reported detrimental effects of bleaching at around 100 °C or for more than several hours [143]. Under such conditions, most of the peroxide appears to be consumed in the oxidation of chain end units of the cellulose macromolecule. The other major effect on the substrate is oxidation of secondary hydroxy to keto groups, accompanied by the formation of very few aldehyde or carboxyl groups [235]. Owing to the relative inactivity of keto groups in cellulose, the bleached effect is stable and is not susceptible to yellowing. This action of hydrogen peroxide is not entirely random amongst all cellulosic hydroxy groups but is directed mainly towards the C3 hydroxy group on randomly located glucose units in the amorphous regions of cellulose, as many as 8–17 keto groups being formed per macromolecule. The formation of keto groups is thought to occur as a result of transient generation of the free radical form of the C3 hydroxy group, as in Scheme 10.25. Free radicals can be formed from the C3 hydroxy groups following reaction

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585

between hydrogen peroxide and perhydroxide anions (Scheme 10.23) although other factors may be involved, such as a localised concentration of radicals in the amorphous regions of the fibre. A supply of free radicals can be generated by heat treatment, without requiring the presence of transition-metal ions as a catalyst. The number of oxygen atoms consumed per chain scission exceeds 100, compared with 26 for hypochlorite bleaching. The yield of oxidised hydroxy groups is less than 10%, whereas it is about 40% for hypochlorite treatment. Under these circumstances, the decomposition of hydrogen peroxide and its oxidative modification of cellulose can be adequately regulated by a well-formulated stabiliser, so that an unacceptable degree of fibre damage is avoided. R R

R

C

R H

+

HO

R

C

OH

OH

R

R

C OH

+

HO

R

C

+ H2O

+ H2O

O

Scheme 10.25

In the presence of certain transition-metal ions, however, notably Fe(II) or Fe(III), the decomposition of hydrogen peroxide and its oxidative attack on cellulose are accelerated. This results in uncontrolled damage, even to the extent in some cases of forming holes in a fabric. Although sequestering agents and other stabilisers, such as silicates, can decrease such severe degradation, in practice they cannot be relied upon to give consistent total protection in the presence of catalysing metals. In such cases, a pretreatment specifically to remove such metals before bleaching is recommended [143]. Various metals can be present in raw cotton, the amounts varying with source and treatment (Table 10.13). There has been a trend in recent years towards increased metal contamination of textiles, giving rise to a great deal of research in this area. Indeed, Reicher [236] cites 29 other references in the period 1987–91 alone. Rotor spinning has become more prevalent in recent years and this is known to give an increased incidence of localised critical concentrations of metal contamination. Despite the overall complexity of metallic contamination in cotton fibres, iron is the main culprit with regard to catalytic damage in peroxide bleaching [236,237] and hence is the only one considered here. Metal-ion catalysis of hydrogen peroxide decomposition can generate perhydroxyl and hydroxyl free radicals as in Scheme 10.26 [235]. The catalytic effects of Fe2+ and Fe3+ ions are found to be similar [235]. It is not necessary for the active catalyst to be dissolved [237], as rust particles can be a prime cause of local damage. The degradative free-radical reaction competes with the bleaching reaction, as illustrated in Scheme 10.27 [237]. Two adverse consequences arise from the presence of free radicals: – fibre damage – rapid decomposition of hydrogen peroxide, leading to a corresponding loss of bleaching action.

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

Table 10.13 Metal ion content of six different cotton samples obtained by atomic absorption studies [236] Sample: Metal (mg/kg)

1

Al Ba Ca Cu Fe K Mg Mn Na Ni P Pb S Sr Ti Zn

2

23 1.5 580 3 34 4500 490 3.2 220 nd 240 nd 340 5.4 0.3 4.5

3

21 32 1.5 1.6 600 840 4 2 100 110 4600 4500 510 520 3.5 4.0 230 230 1.2 nd 270 260 nd nd 360 360 5.6 5.6 0.25 0.7 7.3 5.3

4

5

47 1.2 830 5 790 5300 530 6.7 330 2.1 330 7.6 430 5.9 1.4 35

6

25 2.0 580 3 36 4700 520 3.3 330 nd 270 nd 380 5.7 0.8 4.9

22 1.5 630 2 94 4700 550 3.7 320 1.0 270 nd 380 6.0 0.7 13

nd Not detected in practice (Ni < 1 mg/kg; Pb < 3 mg/kg)

H

O

O

H

O

_ O

H

+

+

_

Fe2+

Fe3+

HO

H

O

+ HO

O

+

+

Fe3+

Fe2+

Scheme 10.26

Alkaline stabiliser

H

O

O

H

_ H O O Perhydroxide anion

Bleaching

Fe ion catalyst

Hydrogen peroxide Fe ion catalyst H O Hydroxyl radical

Scheme 10.27

Fibre damage

This is an important but very difficult area for research, particularly regarding the formulation and evaluation of stabiliser systems. Non-reproducibility of results is a serious problem and in commercial practice a single batch can unexpectedly give rise to damage despite using the same protective measures that had been successful with apparently similar

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batches. In research, it is difficult to obtain consistent metal-containing samples direct from nature, whilst artificially contaminated specimens do not always give results directly comparable with substrates contaminated naturally. In both research and practice, critical localised concentrations of metal contamination can be difficult to detect. Potassium hexacyanoferrate(II) (10.80) gives an intense deep blue coloration with iron(III), permitting extremely sensitive detection of tiny iron spots even by visual inspection. It is recommended as a quality control measure on batches of cotton destined for bleaching [237]. However, in view of the random distribution of metal traces, even the most sensitive test cannot guarantee freedom from contamination throughout a batch of goods to be bleached. Certain researchers have preferred soluble salts such as iron(III) nitrate [236] to represent deliberate contamination, whilst others have used insoluble forms. However, even iron(III) oxide in the form of rust is found to vary in catalytic activity depending on physical form. Although uniform distribution of the contamination, at least below a relatively low concentration, has been claimed to be less troublesome than localised concentrations, there is not even agreement on this. A further complication is that different studies have been carried out in either the absence or the presence of a cellulosic substrate. With these provisos in mind, the catalytic behaviour of trace metals and the effects of some preventive agents will be outlined. The parameter most commonly monitored in this research is the extent of hydrogen peroxide decomposition. Measurement of tensile strength and/or the degree of polymerisation can be useful indicators of fibre damage. The effect of iron(III) ion concentration in accelerating the rate of peroxide decomposition is shown in Figure 10.23, using a system comprising hydrogen peroxide, sodium silicate and magnesium sulphate at 95 °C and pH 12. The effects of pH and Fe(III) concentration on decomposition are indicated in Figure 10.24. Only slight changes in these variables can greatly influence the degree of decomposition.

Hydrogen peroxide decomposition/%

100

80

60 5 mg/l 1 mg/l 0

40

20

5

10

15

20

25

30

Treatment time/min

Figure 10.23 Effect of Fe(III) ion concentration on rate of hydrogen peroxide decomposition in absence of substrate [237]. Initial concentration 2.9 g/l H2O 2, Sodium silicate 5 g/l, Magnesium sulphate 0.2 g/l, 95 °C, pH 12

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

Hydrogen peroxide decomposition/%

100

80

20 mg/l 10 mg/l 5 mg/l 1 mg/l 0

60

40

20

9

10

11

12

13

pH

Figure 10.24 Effects of pH and Fe(III) ion concentration on hydrogen peroxide decomposition in absence of substrate [237]. Initial concentration 2.9 g/l H2O 2, Sodium silicate 5 g/l, Magnesium sulphate 0.2 g/l, 95 °C, 30 min

The effect on the decomposition rate of replacing sodium silicate and magnesium sulphate by a phosphonate stabiliser is shown in Figure 10.25. In this case, at a concentration of 2 g/l of the specific phosphonate used proved more effective in retarding Fe(III)-catalysed peroxide decomposition than the combination of 5 g/l sodium silicate and 0.2 g/l magnesium sulphate represented by Figure 10.23. However, doubling the concentration of silicate and magnesium sulphate also brought about a considerable improvement in stability (compare Figures 10.23 and 10.26).

Hydrogen peroxide decomposition/%

100 10 mg/l 5 mg/l 0

80

60

40

20

5

10

15

20

25

30

Treatment time/min

Figure 10.25 Effect of phosphate stabiliser on Fe(III)-catalysed decomposition of hydrogen peroxide in absence of substrate [237]. Initial concentration 2.9 g/l H2O2, Phosphonate stabiliser 2 g/l, 95 °C, pH 12

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589

Hydrogen peroxide decomposition/%

100 5 mg/l 1 mg/l 0

80

60

40

20

5

10

15

20

25

30

Treatment time/min

Figure 10.26 Effect of magnesium silicate stabiliser on Fe(III)-catalysed decomposition of hydrogen peroxide in absence of substrate [237]. Initial concentration 2.9 g/l H 2O 2, Sodium silicate 10 g/l, Magnesium sulphate 0.4 g/l, 95 °C, pH 12

The results shown in Figures 10.23 to 10.26 were obtained in the absence of a textile substrate. Those in Figure 10.27 were determined in the presence of cotton and involved measurement of the degree of polymerisation of the cellulose [236]. These curves show the effects of increasing the concentration (up to 40 g/l) of an unspecified commercial sequestering agent on both peroxide decomposition and the degree of polymerisation of the cellulose. The contaminant present on the cotton was iron(III) nitrate applied artificially. The initial concentration (3 g/l) of sequestering agent present was that recommended by the manufacturer. Under these conditions the loss of peroxide was 80%, accompanied by an unacceptable lowering of the degree of polymerisation to less than 1000. It was necessary to increase the concentration of complexing agent by several times the recommended amount in order to obtain acceptable protection. Such an increase may be impossible to justify, however. 2500 A Peroxide decomposition B Degree of polymerisation

80

A

2000

B

60

1500

40

1000 B

20

500

A

5

10

15

20

25

30

35

Average degree of polymerisation

Hydrogen peroxide decomposition/%

100

40

Sequestering agent/g/l

Figure 10.27 Effect of sequestering agent concentration on the decomposition of hydrogen peroxide and degree of polymerisation of cotton cellulose [236]

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

It is evident that careful optimisation of all the parameters involved is essential for efficient bleaching with minimal fibre damage. The need for close control of pH, minimisation of liquor ratio and coordination of the component concentrations with treatment time and temperature is critical [226]. Titration of samples of the liquor taken at regular intervals has been the traditional method of analysis. An obvious disadvantage is the unavoidable delay after sampling before corrective action can be taken. Hence the trend has been towards continuous monitoring. A method of continuous on-line monitoring and control of a continuous bleaching range, using measurement of pH, conductivity and temperature, has been described [238]. The use of biosensors and chemosensors to monitor peroxide in continuous ranges has been examined [239]. Biosensors incorporate electrochemical/enzymatic features with oxygen detectors. Chemosensors depend on anodic or electrolytic oxidation and are more robust under the conditions prevailing in the textile industry. This technique is limited to peroxide concentrations from 0.01 to 100 mg/l, so dilution of the test sample from an operating level of 10–50 g/l is necessary. Chemosensors offer a rapid means of determining the concentration of peroxide. Being small, they can be fitted quite close to the point of measurement with a direct link to the bleach liquor dispenser. In continuous flow measurement, where liquor is transported continuously through the measuring instrument, continual monitoring of the prevailing concentration is provided. By means of suitable electronic control equipment connected to the sensor system, automatic logging of measured values is possible and, if necessary, appropriate adjustment of the liquor can take place immediately [239]. An alternative electrochemical method is based on the detection of hydrogen peroxide at an electrode supplied with a reference voltage [240]. Many electrode materials, however, give proportional measurements only up to 0.02 mol/l but a special electrode material of the glassy carbon type provides linearity between current signal and peroxide concentration up to 50 g/l. The measuring cell has five components: the measuring, reference and back electrodes determine the current signal, whilst a pH and temperature probe provides compensation for the pH- and temperature-dependence of the current signal. In spite of the advantages offered by these systems most monitoring still depends on manual titration, with manual intervention at the dispensing stage where necessary [240].

Typical peroxide bleaching bath formulations The composition of bleaching formulations varies widely with the machinery available and the condition of the substrate. Typical formulations and conditions have been given [143] for batchwise processing (Table 10.14) and for continuous processing (Table 10.15). The need to conserve energy in recent years has led to growing interest in cold pad–batch peroxide bleaching, in which padded fabric is batched without uneven drainage or surface drying for 15–24 hours at ambient temperature. Table 10.16 gives typical pad liquor formulations [143,225,241]. A good wetting agent is required to ensure rapid and thorough wetting of grey fabrics at 25–35 °C, together with an efficient detergent to assist in removal of fats and waxes. A persulphate (up to 5 g/l) may also be added to assist desizing [143]. The addition of a peroxide activator based on 1,2,4-triazole has been suggested as a means of accelerating bleaching at about 30 °C [242].

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Table 10.14 Recommended conditions for batchwise peroxide bleaching [143] Kier (% owf)

Additives Magnesium sulphate heptahydrate Wetting agent Sodium silicate (79Tw) Organic stabiliser Caustic soda (100%) Hydrogen peroxide (35% solution) Liquor ratio Temperature (°C) Time (min)

Jig (% owf)

2–3

Winch or jet (g/l) 0.1 0.5–2 7 1–2 5–15 5–15

3–5 1–1.5 0.25–0.8 2–5

0.6–1.4 3–5 4:1 95 60–120

Package machine (g/l)

3:1 95 60–120

15–20:1 95 60–120

0.1 0.5–2 2–7 0.5–2 5–15 5–15 8–10:1 90 60–120

Table 10.15 Recommended conditions for continuous peroxide bleaching [143] J-box conveyor (woven goods in rope form)

Additives (g/l) Magnesium sulphate heptahydrate Wetting agent Sodium silicate (79Tw) or organic stabiliser Caustic soda (100%) Hydrogen peroxide (35% solution) Liquor ratio Temperature (oC) Time (min)

0.1 2–5

Roller-bed steamer

Pressure steamer

0.1 2–5

Jemco machine

0.1 2–5

5–10 2–5

10–20 5–15

5–10 2–5

15–30

45–60

1:1 95–98 60–120

1:1 95–98 10–30

0.1 0.5

J-box (knitgoods)

0.1 5

1.5 4

10 5

30–45

2

45

1:1 120–140 1–2

10:1 95–100 40–60

1:1 95–98 60–90

The above recommendations are for caustic scoured or peroxide desized fabrics. For fabrics not so treated, the amounts of caustic and peroxide should be increased by 50% [143]

Table 10.16 Recommended conditions for cold pad-batch peroxide bleaching References

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Additives (g/l)

225

Magnesium sulphate heptahydrate DTPA (10.6) (40%) Wetting agent Organic stabiliser Sodium silicate (79Tw) Sodium hydroxide (100%) Hydrogen peroxide (35%)

0.1 0.1 2–5 0–15 10–15 10–20 40–60

591

143,241

6–10 8–12 8–15 40–50

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

In some circumstances, hydrogen peroxide can be combined with an organic activator. One such process suggests the use of hydrogen peroxide and urea for the bleaching of linen [243]. Recommended additions are 7 g/l hydrogen peroxide, 8 g/l urea and 1 g/l nonionic wetting agent for 2.5 hours at pH 6, 95 °C and 50:1 liquor ratio. Under these conditions urea accelerates the decomposition of peroxide, which is normally slow at pH 6. In the presence of urea, peroxide decomposition is believed to proceed according to Scheme 10.28 [243]. In this scheme, hydrogen peroxide and urea interact to form an unstable complex which then decomposes to yield hydroxyl radicals. These attack more of the peroxide molecules to yield perhydroxyl radicals, hydroxyl radicals and molecular oxygen. It is also likely that urea interacts with perhydroxide anions from hydrogen peroxide according to Scheme 10.29. This anionic complex of urea can be a further source of hydroxyl radicals. The influence of temperature on the whiteness index of linen subjected to this process is shown in Figure 10.28. The process may be applied to scoured or unscoured linen [243]. The success of peroxy bleach activators such as tetra-acetylethylenediamine (10.86) in detergent formulations for low-temperature laundering has encouraged trials of this approach in peroxide bleaching [244,245]. TAED is colourless, odourless, non-toxic, nonsensitising, non-mutagenic and stable on storage. During biological treatment it is degraded to carbon dioxide, water, ammonia and nitrate; ethylenediamine is not detected on biodegradation [245]. TAED seems to be an ideal peroxide activator in the neutral to weakly alkaline range in textile bleaching, being entirely benign environmentally. Such an

H2N

H2N C

+

O

H

O

O

C H2N

H2N

C

C

H

O

O

O

O

H2N

OH

H2N H2N

OH

H

O

H

O

+ 2

OH

H2N

HO

+ H2O2

H2O

+

O

+ H2O2

HO

+ H2O

H

O

O + O2

Scheme 10.28

H

O

O

H

H

H2N

_ C

O

+

H

O

+

+

H

_ O

_ O

H2N C

O H2N

H2N

O

O

O

H

Scheme 10.29

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H

PREPARATION OF SUBSTRATES

593

60 Scoured fabric Unscoured fabric

Whiteness index

50 40 30 20 10

40

60

80

100

120

Temperature/oC

Figure 10.28 Effect of temperature on the whiteness index of linen bleached using urea-activated hydrogen peroxide [243]. Treated with 7 g/l hydrogen peroxide and 8 g/l urea for 150 min at pH 6 and 50:1 liquor ratio

approach permits bleaching at lower temperatures and near-neutral pH values. Advantages of this process over hot alkaline peroxide bleaching include: – at neutral or weakly alkaline pH fibre damage is only slight and catalytic fibre damage is almost unknown – low temperatures (60 °C or lower) permit energy savings – handle is improved, since cotton waxes remain on the fibre – the process can be used for coloured goods, since colour bleeding is minimal at low temperatures – the process is suitable for regenerated cellulosic fibres, giving less swelling of cellulose – the process is less sensitive to hard water. The disadvantages include: – TAED powder is difficult to dissolve in water – disintegration of cotton husks is not as good as in alkaline bleaching. The formulations listed in Table 10.17 have been suggested [244]. The mechanism of activation is believed to be as follows. In an alkaline medium, hydrogen peroxide yields the perhydroxide anion (Scheme 10.22), which reacts with TAED (10.86) to form diacetylethylenediamine (10.87) and the peracetate anion (10.88) as in Scheme 10.30 [244]. At pH 8–9, the peracetate anion is in equilibrium with free peracetic acid, as in Scheme 10.31 [244]. The peracetic acid reacts with the peracetate anion to form nascent oxygen which is the active bleaching agent, as in Scheme 10.32 [244]. Further possible activators suggested by Kleber [244] include: PAG penta-acetylglucose BOBS sodium p-benzoyloxybenzenesulphonate NOBS sodium n-nonanoyloxybenzenesulphonate TAGU tetra-acetylglycoluril

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

DADHT PAP

diacetyldioxohexahydrotriazine phthaloylaminoperoxycaproic acid.

Certain activated peroxide systems are specifically designed for bleaching wool and these will be mentioned later. The final step after peroxide bleaching is to ensure that the goods do not contain residual peroxide. Reducing agents have been used traditionally for this purpose. However, the possibility of using environmentally friendly catalase enzymes should not be overlooked [87–89].

Table 10.17 Recommended conditions for TAED-activated peroxide bleaching [244] Pad-steam Hydrogen peroxide (35%) Sodium carbonate TAED Stabiliser Wetting agent Defoamer

5.16 ml/l 6.36 g/l 3.42 g/l 3 g/l 2 g/l 1–2 drops

Pad to 100% liquor pick-up and steam for 30 min

Cold pad–batch Hydrogen peroxide (35%) Sodium bicarbonate (10% w/v) TAED Stabiliser Wetting agent Defoamer

8.6 ml/l 84 ml/l 5.7 g/l 10 g/l 4 g/l 1–2 drops

Pad to 100% liquor pick-up and batch for 2 hours at ambient temperature

O

O

CH3C

CCH3 N

CH2CH2

N

CH3C

+ CCH3

O

2H

O

_ O

Perhydroxide anion

O 10.86 TAED

O

O

2CH3C

_ O

Scheme 10.30

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O

10.88 Peracetate anion

594

+

O CCH3

CH3C NH

CH2CH2

NH

10.87 DAED

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PREPARATION OF SUBSTRATES

O

O CH3C

_ O

+ H2O

_

CH3C

+ HO O

O

595

O

H

Scheme 10.31

O

O

CH3C

+ O

O

H

CH3C O

O

O _ O

+

CH3C O

CH3C

H

_ O

+ 2 [O]

Scheme 10.32

Hypochlorite bleaching As indicated in Table 10.12, bleaching with sodium hypochlorite is the most environmentally damaging of all bleaching processes with regard to AOX values. Consequently, despite the economical and technical benefits of this bleaching process, the use of hypochlorite will continue to decline and may even be banned in some countries. The advantages of this bleaching agent include: (1) economical attractiveness (2) lower risk of catalytic fibre damage, although some chemical damage can occur depending on temperature and pH (3) powerful bleaching action. The disadvantages, in addition to the environmentally sensitive aspects already mentioned, include: (1) hypochlorite bleaching does not compete effectively with the rapid peroxide bleaching process (2) despite the intense whiteness that can be produced, hypochlorite bleached goods are prone to subsequent yellowing on storage (3) the substrate must be scoured before hypochlorite bleaching (4) many dyes and fluorescent brighteners are destroyed or degraded by hypochlorite bleaching (5) stock solutions of sodium hypochlorite are unstable and must always be chemically analysed before use. Sodium hypochlorite is commercially available as an alkaline solution, normally containing the equivalent of 12–14% available chlorine. However, this is so unstable that analytical testing of its strength is always necessary before use. Calcium hypochlorite (bleaching powder), stabilised by adding lime, has been used in the past but this product is no longer used in textile bleaching. The mechanism of hypochlorite bleaching appears to be considerably less controversial than peroxide bleaching. The pH-related active species in sodium hypochlorite are shown in Figure 10.29 and Scheme 10.33. The pH range 9–11 is the most suitable for hypochlorite bleaching. The active bleaching species is the hypochlorite anion ClO–. In fact bleaching

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

can be accelerated by lowering the pH but the liberation of hypochlorous acid HClO dramatically increases damage to the fibre. This reaches a maximum at pH 6–8 and such processes have never been exploited commercially. Higher temperatures also increase the rate of bleaching and especially the extent of fibre damage, so that hypochlorite bleaching is usually carried out at ambient temperature for several hours. The effect on cellulose is mainly one of depolymerisation as a result of chain scission. Degradation includes the nonspecific formation of a minor proportion of aldehyde, keto and carboxyl groups, the lastnamed predominating at the usual pH of bleaching (Figure 10.30). The formation of aldehyde groups, however, results in a tendency for the bleached fibre to yellow on storage. In the traditional process, the optimum pH of 10–11 is carefully controlled by addition of alkali, usually sodium carbonate. Typical bleach liquor formulations are given in Table 10.18. 100 Cl2

[ClO]–

HClO

Composition/%

80

60

40

20 HClO

HClO

2

4

6

8

9

10 11 12

pH

Functional groups/mequiv per 100 g per 10 equiv. O2

Figure 10.29 Effect of pH on the composition of sodium hypochlorite solutions [246]

1.0 COOH

0.8

0.6

0.4 CHO

0.2 CO

6

7

8

9

10

pH

Figure 10.30 Functional groups formed by oxidising cotton cellulose with sodium hypochlorite over the pH range 5–10 [235]

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pH > 10

_ H + + ClO

pH 5–6 HClO

HClO

+

H

+

_ +

_ + ClO

+ Na

NaClO

597

pH < 2 + H2O

Cl2

Cl

Scheme 10.33

Table 10.18 Recommended conditions for hypochlorite bleaching [143]

Cistern Available Cl2 (g/l) 2–4 Sodium carbonate (g/l) 2–4 Treatment time (h) 3–4

Jig

Winch

Package machine

J-box conveyor

2–4 2–4 1–2

1–2 1–2 1–2

1–2 1–2 1–2

2–5 2–4 1–2

Despite its technical usefulness, hypochlorite bleaching faces severe environmental pressures because it yields AOX values well in excess of permitted levels. The AOX value observed increases with the active chlorine content of the bleach liquor (Figure 10.31) and with the time of treatment (Figure 10.32). The state of the substrate, including the source of the cotton and its degree of purity, has a major influence on the AOX value. It has been shown [247] that under similar bleaching conditions pure cotton cellulose gives an AOX value of 8–9 ppm, scoured or desized cottons give intermediate values of 13–60 ppm and raw cotton 70 ppm or higher. Although pure cellulose already gives a measurable AOX value, much higher values arise from impurities in cotton. The concentration of hypochlorite applied and the treatment time can scarcely be

80

60

40

20

Whiteness AOX content/ppm

1

2

3

4

5

6

7

8

Active chlorine/g/l

Figure 10.31 Influence of active chlorine concentration on AOX content and whiteness [247]

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

Rate of increase/%

80

60

40 AOX content Whiteness Chlorine consumption

20

50

150

100

200

Bleaching time/min

Figure 10.32 Rates of increase of whiteness, AOX content and chlorine consumption with time of bleaching [247]

varied widely in practice, since they have been adjusted already to minimum values determined by the degree of whiteness required and the economy of the process [222]. This implies that attempts to reduce the AOX value must concentrate on removing impurities as much as possible during the pretreatments (scouring, desizing and washing-off) before hypochlorite bleaching [222,247,248]. The purification of cotton can be improved considerably by pretreatment with hydrogen peroxide and this gives even lower AOX values after hypochlorite bleaching [222]. This double bleaching, however, considerably increases process and chemical costs and demands additional processing capacity. Würster and Conzelmann [222] have estimated that such measures can lead to a reduction in pollution levels by about 70% at justifiable cost, offering the possibility of bleaching cotton with hypochlorite without exceeding permissible AOX loading of the effluent. Nevertheless, it seems inevitable that hypochlorite bleaching will continue to decline on environmental grounds. After bleaching with hypochlorite it is always necessary to remove or inactivate any residual chlorine present. The importance of this is highlighted by the fact that the concentration of undissociated hypochlorous acid reaches a maximum at pH 6, giving the greatest risk of damage to the fibre. Hence it is essential to avoid lowering the pH to neutral during washing off before an antichlor treatment has been given. The traditional antichlors have been sodium bisulphite, or less often the sulphite or dithionite, but the current trend is to use hydrogen peroxide (Scheme 10.34) on environmental grounds [143].

_

_

ClO

_ + HSO3

Cl

_ ClO

+ H2O2

Cl

_ + HSO4

_ +

H2O

+ O2

Scheme 10.34

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Chlorite bleaching Sodium chlorite is commercially available as an alkaline powder (80%) or as an alkaline liquid (26%) of specific gravity 1.25. As indicated in Table 10.12, chlorite gives considerably lower AOX values than hypochlorite, although not as low as peroxide. Bleaching with chlorite differs significantly from peroxide or hypochlorite bleaching, as it is generally carried out under acidic conditions. The general advantages of chlorite bleaching include: (1) no need for special cleaning treatments before bleaching, although such preparation does give rise to lower AOX values (2) the risk of chemical damage is low (3) minimal scouring action of the acidic bleach results in lower weight losses (4) softer handle and good sewability due to low degree of removal of fats and waxes (5) this oxidising bleach is the least sensitive to accelerated damage by metallic contamination (6) washing off is easier than with alternative bleaching agents (7) useful for synthetic fibres and particularly important for acrylic fibres. The disadvantages include: (1) toxic and unpleasant chlorine dioxide vapour can be liberated (2) acidic chlorite solutions are highly corrosive and thus demand highly specialised and expensive equipment (3) no rapid chlorite bleaching process is available (4) residual fats and waxes can be advantageous (see above), but they can also be disadvantageous by giving lower absorbency (5) many dyes and fluorescent brighteners are destroyed or degraded by chlorite bleaching. As with hypochlorite, the mechanism of chlorite bleaching does not appear to be controversial. The pH-related active species in sodium chlorite solution are shown in Figure 10.33. Initially, sodium chlorite hydrolyses in solution, as indicated in Scheme 10.35. Chlorous acid, however, dissociates in water to a limited extent only. Acidic or acidreleasing agents, sometimes referred to as activators, are needed to lower the pH and raise the concentration of chlorous acid to a level suitable for bleaching. However, as demonstrated in Figure 10.33, the reactions in solution are complex. Evidently the chlorite anions formed undergo the various reactions shown in Scheme 10.36, producing chlorine dioxide vapour (ClO2), chlorate anions (ClO3–), chloride anions (Cl–) and oxygen. Figure 10.34 illustrates the amounts of chlorite anions, chlorine dioxide, chlorate anions (which have no bleaching action) and oxygen produced at various pH values from a sodium chlorite solution (1 g/l) after 1 hour at 95 °C in a stream of nitrogen [235]. This demonstrates very clearly that decomposition according to Scheme 10.36 is very slow at pH 5 and almost ceases above pH 6. Below pH 5 the decomposition products are mainly chlorine dioxide and chlorate anions. Oxygen accounts for less than 5% of the products formed by decomposition. The rate of chlorite bleaching increases as pH decreases, but only between pH 2 and 9 is the rate proportional to the concentration of chlorous acid present in solution. At low pH values, evolution of the noxious and corrosive gas chlorine dioxide increases. In practice it is necessary to keep the pH above 3 in order to minimise the formation of chlorine dioxide. It is necessary to monitor the pH during chlorite bleaching because acid is liberated by the

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

100 [ClO2]–

80

Composition/%

HClO2

60

40 ClO2

20 HClO3

HClO2

2

4

6

8

10

pH

Figure 10.33 Effect of pH on the composition of sodium chlorite solutions [249]

12 [ClO2]–

Concentration/mmol l–1

10 8 6

ClO2

4 [ClO3]–

2

O2

2

3

4

6

5

pH

Figure 10.34 Decomposition of sodium chlorite solution as a function of pH [235]

NaClO2

+ H2O

NaOH

+ HClO2 Chlorous acid

Sodium chlorite Scheme 10.35

_ 5 ClO2 _ 3 ClO2 _ ClO2

_

+ 2 H+

4 ClO2 _ 2 ClO3

+ Cl

_ + 2 HO

_ + Cl

_ Cl

+ 2 [O]

Scheme 10.36

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PREPARATION OF SUBSTRATES

601

reactions, creating a need for careful adjustment or compensation. Bleaching is generally carried out at about pH 4. In some processes, particularly continuous or short liquor methods, this pH shift is allowed for by setting the initial pH somewhat higher (pH 4.5–6.5). Sodium nitrate is sometimes added to inhibit corrosion of stainless steel vessels to some extent. Sodium dihydrogen phosphate is a useful pH buffer able to act as an activator for sodium chlorite to improve the bleaching performance. Table 10.19 gives some typical processing conditions.

Table 10.19 Recommended conditions for chlorite bleaching [143]

Additives (g/l)

Cistern

Sodium chlorite (80%) Sodium dihydrogen phosphate Sodium nitrate pH Temperature (°C) Time (h)

Jig

2–4 0.5–2

2–6 0.5–2 1–2

3.8–4.2 80–85 3–4

3.8–4.2 80–85 1–2

Winch or jet

Package machine

1–3 1 1–3

2–4 0.5–1 1–2

3.8–4.2 80 1–2

3.8–4.2 80 1–2

J-box conveyor

Cold bleach

20

20–25

6–6.5 80–85 1–4

6–6.5 20 16–18

Aftertreatment with detergent (2–5 g/l) and sodium carbonate (2–5 g/l) often enhances whiteness and may improve fabric absorbency, particularly if the goods have not been scoured before bleaching. Antichlor treatment is unnecessary for white goods but may be required before coloration. A convenient antichlor treatment involves combining the detergent aftertreatment with sodium perborate, percarbonate or thiosulphate [143]. Traditional reductive antichlors such as sodium bisulphite are not recommended, since their residues can be just as troublesome as chlorite residues. Under optimum bleaching conditions, chlorite does not degrade cellulose but degradation can occur from excessive oxidant concentration, prolonged treatment time or non-optimum pH conditions, the main effect being depolymerisation. Formation of some aldehyde groups is suspected, since the bleached goods can be susceptible to yellowing on storage. The popularity of chlorite bleaching has always been restricted by the toxic and highly corrosive nature of chlorine dioxide, which even attacks stainless steel. Hence equipment costs for chlorite bleaching are high. Environmental aspects, in particular AOX values and the toxicity of chlorine dioxide, will increasingly mitigate against the process in future. As already noted (Table 10.12), chlorite bleaching reaches a significant AOX level, although this is only about one-tenth of that produced by hypochlorite bleaching of the same sample. It should also be borne in mind that chlorite bleaching is recommended for some synthetic fibres as well as cellulosics. Kleber [224] considers that consent levels for AOX can easily be met in practice and that chlorite bleaching will continue. It is important to examine the influence of impurities or additives, such as sizes and lubricants, since these are often prime sources of higher AOX values rather than the substrate itself. It is equally important to assess the contribution of auxiliaries in the bleaching bath to the total AOX value. Kleber [224] has reported several studies of systems requiring compliance with an AOX consent of 3 ppm. Table 10.20 shows that: (a) synthetic

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

Table 10.20 Effect of surfactant addition on AOX values after chlorite bleaching of synthetic fibres [224] AOX value (ppm) after chlorite bleaching Fibre

Without surfactant

With surfactant

Polyester Polyamide Acrylic

0.6 0.6 1.2

2.8 2.6 2.8

fibres are negligible contributors to AOX values; and (b) the inappropriate choice of an auxiliary (in this case a surfactant) can have an adverse effect. Similarly, the fibre manufacturer’s yarn finish may have a positive or negative influence on the AOX value. Thus careful attention should be given to all yarn additives and to the auxiliaries used in bleaching. The possible benefits of prescouring to remove such contaminants should also be considered. Alkaline pretreatments, including boiling off of cotton, have a profound effect on AOX values after chlorite bleaching (Tables 10.21 and 10.22). It can be beneficial, from the viewpoint of both AOX and whiteness, to follow a chlorite bleach with a peroxide treatment. Linen yarns after an alkaline scour and chlorite bleach gave a whiteness value of 63.9 with an unacceptably high AOX value of 8.0 ppm. These results were improved to 78.5 and 1.2 ppm respectively after peroxide treatment [224]. Peracetic acid bleaching In traditional peroxide bleaching, hydrogen peroxide is activated by alkali. Acids, both inorganic and organic, can also be used to activate peroxide by the formation of a peracid. Peracids can be effective oxidative bleaching agents and, at least potentially, offer an alternative to the environmentally sensitive chlorine bleaches. Although known for quite

Table 10.21 Effect of washing on AOX and whiteness values before and after chlorite bleaching [224] Whiteness (460 nm) Residual fat (%)

Substrate Cotton: untreated washed Viscose: untreated washed Nylon: untreated washed Acrylic: untreated washed

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2.2 0.1 0.9 0.7

AOX (ppm)

Before bleach

After bleach

3.8 2.4 6.2 2.8 2.6 1.2 2.8 1.5

54.6 61.5 68.7 72.0 82.3 81.6 70.9 72.5

81.3 87.7 81.1 84.1 80.6 84.9 75.5 78.5

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PREPARATION OF SUBSTRATES

603

Table 10.22 Effect of desize and boil-off of cotton on AOX and whiteness values before and after chlorite bleaching [224] Whiteness (460 nm) Pretreatment

AOX (ppm)

Before bleach

After bleach

Chlorite consumption

Bleach only Desize* and bleach Desize*, boil-off & bleach

4.4 7.2 2.8

58.0 61.2 62.7

87.9 89.1 89.9

51.4 57.3 55.0

* Starch sized and enzyme desized

some time, they have not achieved much use in practice, mainly because of their high cost and difficulties in preparing and handling them. Recent environmental concerns, however, have revived interest in their potential. Comparison with chlorine bleaches is one aspect often highlighted, but ultimately they will have to compete both technically and economically with peroxide bleaching. Peracetic acid is undoubtedly the peracid of greatest interest for textile bleaching, although persulphate and others have also been evaluated. Peracetic acid is produced in equilibrium with acetic acid by the reaction of acetic anhydride with hydrogen peroxide, as in Scheme 10.37. Alternatively, peracetic acid can be produced by acid-catalysed oxidation of acetic acid with hydrogen peroxide, as in Scheme 10.38. Characteristic of a peracid is the perhydroxide anion (H–O–O–), or in some instances the peroxo dianion (–O–O –). Originally, the bleacher had a choice of using either commercial peracetic acid (as a 38% solution) or preparing the peracetic acid in situ using acetic anhydride as in Scheme 10.37. The 38% peracetic acid solution has a pungent odour, attacks human skin and is highly inflammable; it requires special transport and storage facilities. Preparation of the peracid in situ is both hazardous and unpleasant, as it requires the handling and storage of acetic anhydride. One part of peroxide is reacted with six parts of anhydride for 4 hours at ambient temperature to give 80% yield [244,250,251], during which the explosive diacetyl peroxide may also be formed as a by-product, as in Scheme 10.39. Thus, although peracetic acid is environmentally friendly, hydrolysing to acetic acid and oxygen, its manufacture and use do pose quite severe hazards. Hence it is hardly surprising that it has been used only rarely. More recently, manufacturers have introduced safer versions containing only 12–15% active material [225]. Typical formulations of these so-called equilibrium mixtures are given in Table 10.23, together with their flash points [251–253]. Although these lower-strength O CH3C

O

O O

+ H2O2

CH3C

+ O

CH3C

O

CH3C

H

O

O Acetic anhydride

Peracetic acid

Acetic acid

Scheme 10.37

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H

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

O

H

CH3C

+

O CH3C

+ H2O2 O

+ H2O O

H

O

H

Scheme 10.38

O

O CH3C

CH3C

O O

CH3C

O O

CH3C

+

O

O

CH3C

+

O

H

O

H

CCH3

O O Acetic anhydride

Peracetic acid

Acetic acid

Diacetyl peroxide

Scheme 10.39

Table 10.23 Typical formulations of lower-strength peracetic acid equilibrium mixtures Peracetic acid Hydrogen peroxide Acetic acid

35–38% 7% 39%

15% 23% 16%

5% 27% 6%

Flash point (°C)

62

96

97

The remainder of these formulations consists of water, catalyst (e.g. sulphuric acid) and stabiliser

O

O CH3C

CH3C O

Scheme 10.40

O

Peracetic acid

+

O

O

H

H Acetyl radical

Perhydroxyl radical

mixtures are less hazardous than the 38–40% strength, they still pose problems in use. Oral ingestion, inhalation or contact with skin or mucous membranes leads to strong and sustained cauterising or burning and eczema that is difficult to heal [254]. They are quite stable when correctly stored but should not be stored in enclosed vessels or pipework, nor in contact with catalysing metals: ventilated containers of stainless steel or aluminium of at least 99.5% purity are suitable [254]. These hazard constraints are likely to continue to restrict the use of peracetic acid in comparison with relatively hazard-free peroxide. Bleaching is thought to take place through the perhydroxyl free radical (Scheme 10.40) [223]. Various attempts have been made over the last decade to elucidate the mechanism and demonstrate the potential of peracetic acid in bleaching. Temperature and pH are critical parameters with regard to the rate and degree of bleaching on the one hand and the

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605

extent of fibre damage on the other. It is difficult, however, to be dogmatic about these effects since the published results (and conclusions!) show variability because of the different conditions and substrates selected by the researchers. Examination of the influence of pH is complicated by the fact that the bleaching reaction lowers the pH due to the liberation of acetic acid [254]. Hence the results are affected by whether this effect is counteracted by the use of a buffer. It has been pointed out that this pH shift increases with the content of bleachable material present [254]. In theory, optimal bleaching should take place at pH 8.2, which corresponds to the pK value of peracetic acid [253]. The results in Table 10.24 have been obtained on knitted cotton bleached for 30 minutes at 60 °C with 2.5 g/l peracetic acid, the original degree of polymerisation of the cotton cellulose being about 2600. An investigation on linen [254] revealed no bleaching under acidic conditions. Whiteness reached a maximum at pH 7–8 but there was no increase beyond pH 8 as the peracetic acid decomposed to give acetic acid solution. Likewise, pH 8 has been recommended for the bleaching of cotton/acrylic blends with 2–10 g/l peracetic acid at 75 °C in the presence of a phosphate buffer [252]. Conversely, pH 3–6 has been indicated for the bleaching of nylon carpets with 1–5 g/l peracetic acid for 15–45 minutes at 60–70 °C [252]. Somewhat conflicting results [255] have been obtained on a starch-sized cotton fabric using 1% peracetic acid for 30 minutes at 75 °C and a long liquor ratio (Figure 10.35). Table 10.24 Effect of the pH of peracetic acid bleaching on the brightness and degree of polymerisation of cotton cellulose [251,253]

pH

Brightness value

Degree of polymerisation

3 5 7 9

37 43 62 65

2650 2720 2630 2430

80

Whiteness

75

70

6

pH

10

8

Figure 10.35 Effect of pH on the whiteness of starch-sized cotton fabric in a one-stage pretreatment with peracetic acid at a long liquor ratio [255]

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

As indicated in Table 10.25, markedly different effects can be obtained at alkaline pH values depending on whether sodium hydroxide or sodium carbonate is used to obtain the required pH [256]. These results were obtained on desized and scoured cotton bleached with 1% peracetic acid for 1 hour at 50 °C and 50:1 liquor ratio. It is evident that fibre damage was much higher in the region of the pK value of peracetic acid when sodium carbonate was used compared with sodium hydroxide. Sodium carbonate resulted in more rapid decomposition of peracetic acid above pH 6, although this was not accompanied by increased formation of the species active in bleaching, both alkalis giving virtually the same degree of whiteness for a given pH. The optimum conditions in most cases are found at pH 6–7. Table 10.25 Effect of alkali used with peracetic acid at various pH values on the degree of polymerisation of cotton cellulose [256] Degree of polymerisation pH

Alkali:

4 5 6 7 8 9

Na2CO3

NaOH

2750 2800 2850 2680 1850 1350

2780 2550 2550 2720 2760 2800

Table 10.26 Effect of temperature of peracetic acid bleaching on the brightness and degree of polymerisation of cotton cellulose [251,253] Temperature (°C)

Brightness value

Degree of polymerisation

20 40 60 80

38 44 60 66

2650 2600 2550 2500

The effects of temperature are shown in Table 10.26 for the bleaching of knitted cotton for 30 minutes at pH 6–7 with 2.5 g/l peracetic acid, the original degree of polymerisation being about 2600 [251,253]. Thus both whiteness and fibre damage increase progressively with bleaching temperature. In practice it is desirable to avoid excessive treatment temperatures, the preferred range being 50–80 °C [251,253]. At higher temperatures a pungent vapour is evolved. Under optimum conditions of pH and temperature, a treatment time of 20–60 minutes is generally adequate. The degree of whiteness increases with increasing concentration of peracetic acid. The optimum concentration is dependent on the type of process and

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substrate, but relatively high concentrations of peracetic acid have little effect on the degree of polymerisation. For example, increasing the oxidant concentration from 1.75 to 8.75 g/l improved whiteness by about 25%, whilst the degree of polymerisation was lowered by only about 9% [253]. This is in marked contrast to bleaching with sodium hypochlorite. Tests with 100 mg/l iron showed that this transition metal has no significant influence on whiteness or fibre damage [253] under otherwise optimum conditions. The addition of a surfactant can improve wettability and penetration. Indeed, this may exert more influence on whiteness than raising the treatment temperature [256]. In the context of peracid bleaching it is worthwhile recalling the reaction outlined in Scheme 10.30, in which peracetic acid is produced in situ by the action of the activator tetra-acetylethylenediamine (10.86) on hydrogen peroxide [244]. Peracetic acid bleaching is not widely practiced, so it is not possible to give typical formulations and conditions. However, various process recommendations have been given [251–256]. It has been demonstrated that peracetic acid bleaching combined with ultrasonic treatment gave higher whiteness values and less fibre damage than conventional bleaching with peroxide [257]. Low temperature bleaching with peracetic acid at 30 °C, catalysed by incorporation of 2,2′-bipyridyl (10.89) in an alkyl sulphate surfactant, has been proposed [258]. N N 10.89 2,2′-Bipyridyl

Combined bleaching processes Combinations of more than one bleaching process can be beneficial. The sequence of a hypochlorite bleach followed by a peroxide bleach is common: the second stage can considerably reduce the AOX value imparted by the preceding hypochlorite treatment. It has also been suggested that the sodium hypochlorite in this two-stage sequence can effectively be replaced by peracetic acid, lowering the AOX values even further. A review of combined bleaching processes for weftknit cotton fabrics is available [225]. Monitoring bleaching processes Accurate monitoring of bleaching processes is essential for efficiency, economy and protection of the environment. The importance of checking raw materials, including the substrate, should be obvious. There is also a need to monitor the actual process liquors and to evaluate the results obtained on the substrate. A useful account of such procedures is available [143]. Bleaching from the viewpoint of fibre type Most cotton is bleached with peroxide, by far the greater proportion by continuous methods. Synthetic fibres seldom require bleaching, but where it is necessary either peroxide or chlorite bleaches are recommended. Most regenerated cellulosic fabrics are only bleached

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

for fluorescent whites and pastel shades. Any of the three oxidative bleaches may be used [143]. Peroxide bleaching requires 5–15 g/l hydrogen peroxide (35%) at 80–100 °C, depending on the type of machinery selected, together with sequestrant, caustic soda and stabiliser for the peroxide. Chlorite bleaching on the winch requires 0.5–1.5 g/l sodium chlorite (80%) at pH 4, treatment conditions being 45–90 minutes at 90–95 °C. Peracetic acid (3 g/l) treatment for 60 minutes at 65 °C and pH 7 in the presence of sodium hexametaphosphate is also suitable. Wool bleaching Wool poses special problems, although only approximately 1.5% of the world production of wool is bleached directly. However, an unknown amount of so-called ‘top-up’ bleaching also takes place, usually by adding hydrogen peroxide to the last bowl of the scouring process. Reviews of wool bleaching, which includes oxidation, reduction and sequential oxidation– reduction processes, are available [11,259,260]. A particular problem with wool lies not so much in its initial natural colour as in its tendency to show yellowing under the influence of light, dry heat or wet alkaline treatments. It is particularly important to realise that bleaching often exacerbates this problem. The complex chemical mechanisms involved in the yellowing of wool have been reviewed in detail [259] and will not be discussed further here. Oxidative bleaching of wool is invariably carried out with hydrogen peroxide. The active species involved is likely to be the same as on cellulosic substrates but specific reactions with wool amino acid residues must be considered. The primary reaction is oxidation of cystine disulphide bonds leading to the formation of cysteic acid residues (Scheme 10.41). The rupture of disulphide crosslinks, with attendant increase in urea-bisulphite and alkali solubility values, adversely affects fibre properties. As the severity of bleaching conditions increases, the urea-bisulphite solubility remains little changed but the relationships between alkali solubility and cysteic acid (Figure 10.36) and between cystine and cysteic acid (Figure 10.37) are almost linear [259,261,262]. Tyrosine, tryptophan and methionine residues are oxidised by hydrogen peroxide [259]. In order to retain commercially acceptable fibre properties, the alkali solubility of bleached wool should not exceed 30% [259,263]. The degree of attack on the fibre depends on the nature of the wool itself. Chlorinated wool is more sensitive than untreated wool but Hercosett-treated wool is less sensitive (see Figure 10.38) [259,264]. As with cellulosic fibres, the bleaching of wool with peroxide requires careful optimisation of several parameters: peroxide concentration, pH, temperature, treatment time, choice and concentration of stabiliser and possibly choice and concentration of activator. A typical formulation for batchwise bleaching of wool is given in Table 10.27. Sodium pyrophosphate (10.16) or tripolyphosphate (10.17) are generally the stabilisers of choice but silicates can also be used. Bleaching for two hours at 50–60 °C and pH 8.5–9 is common, although in some cases careful optimisation can reduce the time to one hour [11]. Alkaline conditions are appropriate for cellulosic bleaching but may cause quite severe damage with wool and thus careful control is necessary. Alternatively, acidic conditions (pH 5 at 80 °C) can be chosen, using a suitable peroxide activator to produce a peracid. Only 15–25% of the hydrogen peroxide present is consumed in a typical bleaching process. In order to minimise wastage, some bleachers reuse liquors several times,

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replenishing the bath with fresh peroxide after determining the deficit by titration. The number of batches that can be bleached in the same replenished liquor is limited by discoloration of the bath by impurities desorbed from the wool [259]. There is a further economic benefit in that the peroxide replenishment decreases with each successive batch (Figure 10.39), possibly because of the build-up in the bath of soluble proteins, which have a stabilising effect. H2O2 CH2

S

S

CH2

CH2

_

SO3H

HO3S

+

CH2

HO Cysteic acid

Cystine

Scheme 10.41

Alkali solubility/%

80

60

40

20

1

2

4

3

Cysteic acid/%

Figure 10.36 Relationship between alkali solubility and cysteic acid content of peroxide-bleached wool [259,261] 12

Cystine/%

11

10

9

8

1

2

3

4

Cysteic acid/%

Figure 10.37 Relationship between cystine and cysteic acid content of peroxide-bleached wool [259,261]

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40 oC Alkali stability/%

Alkali stability/%

40

35 30 25 20

35 30 25 20

4

2

6

10

8

4

2

6

8

10

Time/h

Time/h 40 oC

Hercosett wool Chlorinated wool Untreated wool

50 oC

34 Whiteness index

34 Whiteness index

Chlorinated wool Untreated wool Hercosett wool

50 oC

40

33 32 31 30

33 32 31 30

4

2

6

10

8

4

2

Time/h

6

8

10

Time/h

Figure 10.38 Effect of treatment time on the alkali solubility and Jaquemart whiteness index of wool bleached with 2-vol. hydrogen peroxide at 40 °C and 50 °C [259,264]

Table 10.27 Recommended conditions for the peroxide bleaching of wool [259] Hydrogen peroxide (35%) Phosphate stabiliser Wetting agent

10-30 ml/l 2-4 g/l 1 g/l

Treatment for 1–16 hours at 40–60 °C

Hydrogen peroxide 27%/ml

1400

1000

600

200 1

2

3

4

5

Batch number

Figure 10.39 Bleach bath regeneration in the peroxide bleaching of wool [259]

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A typical liquor formulation for pad application in a continuous or semi-continuous system is given in Table 10.28. Batching for 24–48 hours is followed by backwashing, although the use of radio-frequency heating to give a batch temperature of 50–60 °C can reduce the treatment time to 2–4 hours. Table 10.28 Recommended conditions for the pad bleaching of wool [259] Additives

g/kg

Hydrogen peroxide (35%) Thickener Isopropanol Wetting agent Formic acid Antifoam Fluorescent brightener (optional)

40 8 16 10 2 3 5

Although various processes may have been subjected to optimisation in recent years as a result of economic pressures, a survey in the 1980s revealed disparities between different sectors of the industry as summarised in Table 10.29 for batchwise wool bleaching methods. Table 10.29 Worldwide practice of batchwise wool bleaching [259] Processing time:

<3 hours

>3 hours

Totals

Loose stock Slubbing Yarn Piecegoods

17.8 5.1 11.6 4.7

34.3 10.5 10.2 5.8

52.1 15.6 21.8 10.5 100.0

As with the peroxide bleaching of cellulosics, the presence of transition metals in the bleaching of wool with peroxide has a catalytic effect that can damage the fibre. This catalytic effect has been exploited in a process for the bleaching of heavily pigmented wools. The fibre is first mordanted with a ferrous salt in the presence of hypophosphorous acid (H3PO2), then rinsed to remove the iron from the wool keratin but not from the melanin pigment granules. Hydrogen peroxide is then added to destroy the pigment granules by a free-radical mechanism [11,259,265]. Reductive bleaching of wool is mostly carried out with stabilised sodium dithionite. Derivatives such as sodium formaldehyde-sulphoxylate (CI Reducing Agent 2) or zinc formaldehyde-sulphoxylate (CI Reducing Agent 6) as well as thiourea dioxide (CI Reducing Agent 11) are also used. The chemistry of these reducing agents is discussed in Chapter 12. Apparently little is understood about the reactions between these reducing agents and the pigments responsible for the natural colour of wool. Sulphitolysis of the wool is known to take place, the reducing agent reacting with the disulphide bonds of cystine to give a Bunte

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salt (Scheme 10.42) [266]. Rupture of the disulphide bonds results in an increase in alkali solubility of the fibre, to which the formation of sulphonic groups also contributes.

C CH

O

C CH2

S

S

CH2

NH

_ HSO3

O

C

CH

CH

NH

NH

O

Cystine

C

_ S

CH2

SO3

+

HS

CH2

O

CH NH

Bunte salt

Cysteine

Scheme 10.42

Typical conditions for bleaching with stabilised dithionite are given in Table 10.30 [11,259]. A warm rinse is then given, with an addition of 1 ml/l hydrogen peroxide (35%) in the final rinse to eliminate sulphurous odours. The sulphoxylates are used at pH 3.0 for up to 30 minutes at 90 °C, but these tend to produce a harsh handle as well as an unpleasant residual odour. Thiourea dioxide (1–3 g/l) can be used at 80 °C and pH 7 for one hour [11]. A sequestering agent is added to prevent metal-catalysed decomposition of the thiourea dioxide. The active bleaching species is sulphinic acid (Scheme 10.43) [11]. Thiourea dioxide has less effect on the physical properties of the fibre than other reductive bleaching agents [267]. Fluorescent brighteners can be applied together with a reductive bleach but they may increase the subsequent tendency to photo-yellowing. Table 10.30 Recommended conditions for the reductive bleaching of wool [11,259]

H2N

Stabilised sodium dithionite Wetting agent

2–5 g/l 1%

pH Temperature Time

5.5–6.0 45–65 °C up to 1 hour

O C

H2N

S

H2N

O H2O C

O

HN

Thiourea dioxide

H2N

S

C OH

Formamidinesulphinic acid

O

+

HO

S

OH

H2N Urea

Sulphinic acid

Scheme 10.43

Reductive bleaches are generally less costly than oxidative bleaches but tend to give a greenish white compared with the reddish white tones from peroxide. Combining the two in an oxidation/reduction sequence gives a more neutral white, this being known as ‘a full

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bleach’. The sequence is generally carried out in separate baths. However, an interesting attempt has been made to integrate the two processes [268–271]. This approach is of significant interest chemically. A combination of hydrogen peroxide and thiourea is used but the sequence of reactions is complex. Initially the wool is given a conventional oxidative bleach with 8 g/l hydrogen peroxide (30%) and 2 g/l sodium pyrophosphate (10.16), beginning at pH 9.0 and ambient temperature, then raising at 1 °C/minute to 60 °C and maintaining at this temperature for one hour. Titration shows that only about 10% of the peroxide is consumed and the redox potential is around +200 to +400 (i.e. oxidative). The objective in the second stage is to make use of the residual peroxide. The pH is lowered to 8.2–8.7 and then to 5.0–5.5 with acetic acid, whilst still at 60 °C. Thiourea (1.68 g/l) is then added and ten minutes is allowed for thiourea to react with peroxide to give the reducing agent thiourea dioxide, as in Scheme 10.44. Adequate time at this pH for the reaction to take place is essential. Ammonia is then added to give pH 6.8–7.2. The redox potential becomes -600 to -700 (i.e. reductive) and reductive bleaching proceeds at 60 °C for 25 minutes. The thiourea dioxide formed in Scheme 10.44 is hydrolysed to urea and the active reducing species, the sulphinate anion, during this phase, as in Scheme 10.45. The sulphinate anions in turn react with wool and with the coloured pigments present on the wool, being thereby oxidised to sulphate anions (Scheme 10.46). Any residual reducing activity can be quenched by adding a small amount of peroxide. H2N

H2N C

H2N

pH 5 C

S

SH

+ 2 H2O2

HN

H2N

O C

2 H2O +

S

HN

OH

Hydrogen peroxide

Thiourea

H2N

O C

H2N

O

Thiourea dioxide

Scheme 10.44

H2N

O C

H2N

H2O

S O

_ O

H2N

_ C

S

O

O

+

Urea

_ O

613

_ O

Sulphinate anion

_ O

O S

+ 2 [O] O

Sulphinate anion

Scheme 10.46

S

H2N

Thiourea dioxide

Scheme 10.45

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O_

Sulphate anion

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The control of conditions, particularly of pH and the molar quantities of reactants, is critical; otherwise, variant reaction routes are possible. Unfortunately, thiourea is a suspected carcinogen and this appears to be one reason why this two-stage redox process has not been readily adopted, even though thiourea is easy.to handle and the bath can be monitored colorimetrically to ensure complete conversion. In an attempt to overcome this problem, it has been found [272] that a cyclic analogue of thiourea, namely thiocyanuric acid or s-triazine-2,4,6-trithiol (10.90), can replace thiourea in this application. Thiocyanuric acid is commercially available, being used as a heavy metal precipitant in waste water treatment and as a vulcanising agent. It is best used in the bleaching process as a 15% solution of the trisodium salt. The process is claimed to produce exceptional whiteness with minimum fibre damage [272]. HS N N

SH N

HS 10.90 Thiocyanuric acid

Semi-continuous and continuous reductive processes are best carried out with activated hydroxymethanesulphinates, since dithionites (even when stabilised) are unsuitable due to being oxidised too quickly by air [273]. The environmental aspects of wool bleaching have recently been reviewed [273]. There is a tendency to replace phosphates by phosphate-free activators. Reductive bleaching agents are not environmentally friendly as they are oxygen-depleting, but pollution by dithionite is kept to acceptable levels if the residual concentration is less than 0.3 g/l. Thiourea dioxide offers the advantage of contributing only half the sulphur load from dithionite, but it also introduces detrimental nitrogen. The carcinogenic potential of thiourea has already been mentioned. Bleaching in particular is an area where many formulated commercial products are offered. These have the advantage of providing the bleacher with a single (optimised) product to simplify the preparation of bleach liquors, compared with having to store and prepare several separate components. Little information is available on the detailed constitution of such mixture products, although hints are sometimes given by the suppliers. Some bleachers, however, may prefer the greater freedom to adjust liquors according to requirements by the judicious use of separate components. Special considerations are needed where bleaching is carried out in association with other processes or as a means of ‘brightening’ goods that have been already dyed. 10.5.4 Mercerising Mercerising is an alkaline treatment often given to cotton yarns or fabrics, the objective being to increase fibre lustre, strength and dyeability. These effects are brought about by alkaline swelling of the fibres with or without tension. Accounts of practical aspects of mercerising treatments are available [143,235,274]. Processing options include: cold or hot,

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wet or dry, chain or chainless and batchwise or continuous. Consideration must be given to the position of mercerising in the overall sequence of preparation, since it may be carried out on grey, partially prepared or fully prepared goods. The maximum degree of swelling attainable decreases the later the stage at which mercerising is carried out. Accordingly, mercerising of grey-state goods gives the maximum potential for swelling but fibre penetration and uniformity of treatment are most difficult to achieve. Grey mercerising also has the severe disadvantage of giving maximum fouling of the caustic soda liquor. Mercerising is usually positioned either between desizing and boiling-off or between boiling-off and bleaching [274,275]. Care should be taken to avoid carry-over of iron impurities into the bleaching stage. It is also important to ensure thorough and uniform rinsing after mercerising, as any localised residual alkali will lead to uneven bleaching [275]. Whilst these are generally the two most favourable positions for mercerising, their disadvantage lies in interrupting the flow of the continuous preparation sequence, since mercerising is the slowest pretreatment stage. This is where hot mercerising (60 °C) can become an advantage, because the quicker rate of swelling allows this step to be more easily incorporated within the processing range as a whole [275]. Mercerising after bleaching gives the least fouling of the liquors but increases the possibility of fibre yellowing. Moreover, fibre swelling and absorptivity are less evident, sometimes leading to problems in subsequent processing, particularly in continuous processes where rapid uptake and maximum absorption are required [275]. In chain mercerising, the weft threads of the fabric are kept under tension on a clip stenter. In a chainless merceriser the fabric dimensions are controlled by a series of rollers. The fabric is usually woven slightly wider to allow for some weft shrinkage. Yarn is mercerised in hanks between two movable rollers which create the required tension whilst knitted constructions may be mercerised in either slit or tubular forms. Chain and chainless mercerising have been compared [276]. It is important to be aware of the machine configuration as this influences the target conditions, including the concentration of caustic soda applied and its subsequent washingoff. The three stages that are important from the viewpoint of auxiliaries are: mercerising zone, stabilising zone and washing zone. In the mercerising zone the goods are impregnated with caustic soda liquor and are subjected to the relevant means of controlling tension. In the stabilising zone the concentration of alkali is reduced to a level at which the fabric again becomes dimensionally stable, losing the plasticity imparted by the concentrated alkali in the mercerising stage. The counterflow washing principle is applied to reduce the alkali concentration. Only when dimensional stability has been restored are the goods ready for the washing-off section in which residual alkali is removed and the fabric neutralised. Traditionally, mercerising has been carried out cold (10–20 °C). This imparts the maximum degree of swelling but this is attained at the slowest rate. Hot mercerising has been introduced more recently and this is carried out at 60–70 °C. The characteristics of the two processes are compared in Table 10.31 [274,277]. Table 10.32 presents a comparison of these processes from the viewpoint of results obtained in relation to the stage at which mercerising is carried out, as well as analogous trends for modal fibres [275,277]. The characteristic feature of hot mercerisation is that the essential chemical and physical changes do not take place at the higher initial temperature but in the subsequently cooled fabric as it passes through a traditional tensioning process [143]. Hot mercerising, however, has not yet achieved significant commercial success [143]. In cold mercerising, the

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treatment time needs to be longer and is usually 40–45 seconds, compared with 25–35 seconds when mercerising at 60 °C [276]. The optimum effect is achieved when the fabric takes on a glassy, transparent appearance just before the stabilising section. A combination of hot and cold impregnation is possible [276]. Table 10.31 Comparison of hot mercerising with conventional cold mercerising of cotton [274,277] Conventional mercerisation (10–20 °C)

Hot mercerisation (70 °C)

strong fibre swelling slower swelling slower relaxation incomplete relaxation higher residual shrinkage surface swelling unevenness tighter fibre packing firmer handle slow NaOH diffusion less lustre less strongly swollen fibres in surface zone of yarn background less lustrous

less fibre swelling rapid swelling rapid relaxation good relaxation lower residual shrinkage complete swelling evenness looser fibre packing softer handle rapid NaOH diffusion optimised lustre moderately swollen fibres throughout yarn cross-section background equivalent lustre

Table 10.32 Comparison of cold and hot mercerising processes for bleached and unbleached cotton and for modal fibres [275,277]

Property

Substrate

Cold mercerised

Hot mercerised

Degree of polymerisation

grey cotton bleached cotton modal fibre grey cotton bleached cotton modal fibre grey cotton bleached cotton modal fibre grey cotton bleached cotton modal fibre grey cotton bleached cotton modal fibre grey cotton bleached cotton modal fibre grey cotton bleached cotton modal fibre

= = … = = < ≤ > > < < >/< < < < ≥ … > < … <

= = … = = > ≥ < < > > >/< > > > ≤ … < > … >

Breaking strength

Dyeability

Lustre

Dimensional stability

Handle (flexibility)

Crease recovery angle

= Similar to < Less than > Greater than the other method

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There is a so-called dry mercerisation process [275] in which the fabric is padded with caustic soda liquor at 20–25 °C and then dried in a stenter at about 130 °C. An immersion time in the pad trough of 7–10 seconds is sufficient but the goods need a total saturation time in the alkaline liquor of 30–40 seconds, i.e. from the nip to entry into the drying zone. The type of cotton and its condition determine to a large extent the degree and uniformity of mercerisation. Fibres with a relatively rounded cross-section, exceptional fineness and consistency tend to give the highest degree of mercerisation [274]. These properties also play a part in determining the mercerising conditions. Ideally, the maximum possible degree of mercerisation would be obtained if the goods were repeatedly mercerised (for example, twice at around 70 °C and then for a third time at 10–15 °C) but such a procedure is economically impractical [274]. In practice it is essential to aim for an optimum rather than the maximum degree of mercerisation, a compromise between what is desirable or ultimately possible and what is economically feasible on available machinery. In the context of the above basic requirements of the process, the chemicals used are sodium hydroxide as the primary agent and a surfactant-based auxiliary to aid rapid and even penetration as an important secondary requirement. The viscosity of solutions of sodium hydroxide increases with concentration and decreases with temperature as shown in Figure 10.40. It is the higher viscosity of cold concentrated solutions which makes fibre penetration so difficult in cold mercerising. Nevertheless, despite the slower rate of penetration at low temperatures, the ultimate degree of swelling is greater (Figure 10.41). Thus in this investigation, maximum swelling at 25 °C was obtained with 4–6 mol/l sodium hydroxide solution. These curves illustrate that the change in swelling behaviour with temperature becomes more critical at low concentrations of alkali. Traditionally, solutions of sodium hydroxide in the 6.25–6.5 mol/l range have generally been used in cold mercerising. In fact cellulose swells even in pure water but to a lesser extent than in alkali, the swelling in water being reversible whereas alkali-induced swelling is irreversible. Aqueous swelling affects only the readily accessible amorphous regions of cotton, whilst alkali also greatly affects the crystalline regions. 8

A

A B C

Dynamic viscosity/10–3 Pa s

B

6

531 g/l NaOH/42 oBé 297 g/l NaOH/30 oBé 167 g/l NaOH/20 oBé

4 C

2

10

20

30

40

50

60

Temperature/oC

Figure 10.40 Change in viscosity of NaOH solutions with temperature [274]

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

2.0

Degree of swelling

1.8

1.6

0 oC

1.4 25 oC

1.2 100 oC

2

4

6

8

10

Sodium hydroxide concentration/mol l–1

Figure 10.41 Effect of temperature on degree of swelling of cotton fibres by sodium hydroxide (molar sodium hydroxide solution contains 40 g/l NaOH) [278]

The mechanism is thought to be one of ionic hydration [143,279,280], arising from replacement of some of the water of hydration by cellulosic hydroxy groups as shown in Scheme 10.47. When a hydrated ion pair is absorbed by the cellulose, three molecules of water are released and are replaced by three hydroxy groups. These are unlikely to be located in the same anhydroglucose unit. The liberated water molecules now occupy a greater volume than when associated with the ion pair, thus causing swelling of the fibre [143]. It is important to realise that swelling involves only partial molecular disruption of cellulose; complete disruption would produce dissolution or at least dispersion. Hydrogen bonds in the crystal structure are ruptured but the van der Waals forces remain intact, thus enabling the cellulosic matrix to behave as mobile sheets held in close contact by the van der Waals forces. This parallel arrangement of the cellulose chains is enhanced by the tension applied during mercerisation and the surface of individual fibres becomes smoother, giving an increase in lustre [276]. _ + Na OH .nH2O

Scheme 10.47

+

[cellulose](OH)3

_ + [cellulose](OH)3.Na OH .(n–3)H2O + 3 H2O

The more highly crystalline the fibre, the more it will resist the mercerising treatment. Initial swelling always takes place in the amorphous regions, followed by penetration of the crystalline material by alkali. As indicated earlier in Figure 10.41, optimum conditions for swelling are obtained with about 4–4.5 mol/l NaOH at 25 °C, this being the most favourable concentration for penetration of the crystalline regions and thus for enhancing dye affinity. A further optimum is reached at a concentration of 6.5–7.5 mol/l NaOH, since it is at this concentration and 0 °C that the fibre cross-section changes from kidney-shaped to circular,

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giving enhanced lustre. This is why 6.25–6.5 mol/l NaOH has been the standard concentration in traditional cold mercerising. Greif considers that a concentration of at least 6.75 mol/l and preferably 8.75 mol/l NaOH is necessary for the highest degree of mercerisation [274]. In the so-called continuous addition mercerising process, a liquor concentration of about 13 mol/l sodium hydroxide is used at 70 °C, applied to squeeze-wet goods [274]. This facilitates quicker penetration and gives a higher degree of mercerisation. The active concentration, however, falls to the usual level of about 6.75–8.75 mol/l after a dwell time of about 15–25 seconds, since the fabric already contains about 60% moisture. Nevertheless, it is claimed that a better effect is obtained by this technique of ‘dilution from above’. Strictly speaking, regenerated cellulosic fibres cannot be ‘mercerised’ although they can be given a ‘causticisation’ treatment. There is a critical concentration of caustic soda that causes dissolution of regenerated cellulose: about 65 g/l. The much higher concentrations used in conventional mercerising do not cause dissolution of cotton cellulose, of course. Regenerated cellulosic fibres can be given a causticisation process using 15–50 g/l sodium hydroxide. There is some advantage in using higher temperatures as swelling is thereby reduced. Viscose staple fabrics for dresswear are often causticised to enhance dye receptivity, increase the brilliance of colour and obtain a softer handle [279]. Addition of a wetting agent to the mercerising liquor gives better penetration and more even treatment. However, the choice of wetting agent depends on the fabric to be mercerised and the position of mercerising in the preparation sequence as a whole. The need for this additive is greatest with grey yarn or piece goods. Goods that have been given an alkaline boil-off or bleach already have much better wettability, so the need for a wetting agent is not so great. Indeed, such goods may still be saturated from a previous process and if fed directly into the mercerising liquor, there is no need for a wetting agent [280]. The properties of a wetting agent for mercerising can be summarised as follows [235,280]: – good solubility and stability in 10M (400 g/l) sodium hydroxide solution; the stability should be maintained under the conditions of alkali recovery by centrifugal separation or vacuum evaporation – high wetting ability and high efficiency at low concentrations in the strongly alkaline solutions used; this is particularly critical in continuous processing and on grey goods – low affinity for the fibre; together with high efficiency at low concentrations this aids subsequent rinsing and recovery – low foaming. Suitable products include [235,280]: – alkylarylsulphonates – sulphated aliphatic alcohols, the most efficient being those of low molecular mass (i.e. 4–8 carbon atoms), such as sodium 2-ethylhexylsulphate (10.91); branched chains are more efficient than linear ones [235] – some short-chain alkylphosphonate esters, e.g. sodium methyloctylphosphonate.

CH3CH2CH2CH2 CHCH2OSO3Na CH3CH2 10.91

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A product such as 10.91 may require blending with about 10% each of butanol and unsulphated 2-ethylhexanol to give an effective formulation in terms of solubility, stability and wetting power. It is useful for a wetting agent to exert some degree of detergency across the whole range of caustic soda concentrations, including the lower concentrations for efficiency during washing-off [281]. Yarn shrinkage provides a good measure of the efficiency of a wetting agent under mercerising conditions [280]. Figure 10.42 illustrates results for an effective product, yarn shrinkage being complete in about 30 seconds. 20

B

15 Shrinkage/%

C D B

C

10 D

A B C D

5

Water only 200 g/l NaOH 280 g/l NaOH 300 g/l NaOH A

10

20

30

120 Time/s

Figure 10.42 Mercerising shrinkage rate using 5 g/l of a commercial wetting agent [280]

In the so-called stabilising zone, the concentration of the alkali is reduced, by a counterflow washing system for example, until the fabric regains dimensional stability. In the succeeding washing-off zone, most of the residual alkali is removed and the fabric is neutralised, typically with acetic acid [274,276]. The stabilising zone generally gives a spent liquor containing 40–80 g/l sodium hydroxide [274,282,283]. Desorption is more rapid the better the preparation of the cotton and the remarkable claim has been made that it is possible to remove more than 75% of the alkali in only 2 seconds [284]! Even after thorough rinsing, however, there is always a small amount of residual bound alkali in the fibre [285]. The spent liquors may contain lint and residual size that can be removed by filtration. Weakly alkaline liquors represent a cost problem, however. Although limited amounts of less dilute liquor may be recycled and used in boiling-off or scouring, the major proportion becomes a rather troublesome component of the effluent load. Neutralisation simply increases the salt content of the effluent. Recovery of the alkali by vacuum evaporation is the usual procedure [282,283]. Anhydrous liquid ammonia can also be used to enhance the absorption properties of cotton [143]. 10.5.5 Wool processing Milling, a process peculiar to wool, is carried out to develop its felting propensity. Traditional wool goods such as felt hats and blankets are milled under slightly acidic conditions, sulphuric acid being the main agent. Acid milling is particularly useful for dyed goods, which may not

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have adequate fastness to neutral or alkaline treatments. Alkaline milling conditions are still used for woven piece goods traditionally known as milling cloths, maximum milling taking place using soap at pH 10. The higher-melting soaps, such as those based on tallow and palm oils, have been preferred to give the required gelatinous solution and lubricating properties. Greasy woollens are often milled in sodium carbonate solution, which saponifies the natural grease to a soap. Even with such woven pieces, however, the trend is towards milling in almost neutral conditions, for which milling aids based on nonionic and anionic surfactants are useful. Some wool yarns are milled simply by tumble drying of the wet yarns, whilst knitted garments are milled in rotary-type machines using a nonionic surfactant with sodium bicarbonate or polyphosphate. Solvent-based systems in which a small amount of water is emulsified in the solvent by an appropriate surfactant have also been used [146], this often forming part of a sequence in which scouring, milling and shrink-resist finishing are all carried out in the same machine. Another process peculiar to wool is carbonising. This exploits differences between wool keratin and cellulose in their response to strong acid, wool being substantially more stable whereas cellulose is degraded. Hence strongly acidic conditions are required to remove cellulosic impurities from wool. Dilute sulphuric acid (4–8%) is most commonly used [286]. Other mineral acids (e.g. hydrochloric acid) or inorganic compounds that are strongly acidic in solution (e.g. aluminium chloride) may also be used. However, whilst aluminium chloride has been used to carbonise wool blends containing other fibre types such as polyester or cellulose acetate that are sensitive to sulphuric acid, it is not used commercially to any great extent [286]. Arylsulphonic acids and thionyl chloride (SOCl2) are also suitable. Thionyl chloride is hydrolysed to hydrochloric acid and sulphur dioxide, so it has been used in carbonising either as a vapour or as a solution in perchloroethylene [286]. All carbonising processes involve the following steps [286]: (1) Immersion (or spraying) to impregnate the wool with the appropriate acid solution (2) Drying to concentrate the acid (3) Baking to dehydrate and carbonise the cellulosic impurities (4) Crushing and dedusting to remove the charred cellulosic debris (5) Neutralisation of the residual acid. As already mentioned, sulphuric acid is by far the most common carbonising agent. In traditional processes, it is applied at 4–5% concentration with a dwell time of 3–5 minutes. So-called rapid processes apply 7–8% sulphuric acid with very short dwell times, typically 5 seconds. When used alone, there is a danger that localised droplets of highly concentrated sulphuric acid can be formed, with consequent damage to the wool. The critical conditions for this to occur are met when the acid concentration reaches 40–45% [286–288]. Auxiliaries are generally used to prevent this localised damage and to ensure efficient wetting and penetration. Many products have been suggested [286], surfactants being the most important. These must be stable to the hot acidic treatment, of course. Anionic surfactants such as alkylbenzenesulphonates have been used, as well as nonionic types such as nonylphenol polyoxyethylenes of 6 to 9 ethylene oxide units per molecule. Typically, the concentration of surfactant present is only 0.025–0.04%, depending on the composition of the surfactant [286]. Mixtures containing a polyethoxylated nonionic and an alkylarylsulphonate anionic, however, can significantly increase the risk of wool damage compared with the use of either component separately [286,289].

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

Since wool is attacked most rapidly by sulphuric acid of intermediate concentration, it is important that drying is carried out either at a relatively low temperature so that reaction of the acid with wool is slow, or very quickly so that the time of exposure of the wool to the critical acid concentrations is brief [146]. Ideally, all the sulphuric acid in the wool is absorbed chemically as bound acid that causes little hydrolytic damage. It is the free acid that can concentrate locally and cause serious degradation. The acid picked up by the vegetable impurities, on the other hand, is free acid that has the desirable effect of beginning the process of attacking the cellulose [286]. Large amounts of residual acid may cause damage to the wool, so that careful neutralisation after baking is an essential and important stage of the process. Carbonised fabrics allowed to accumulate without neutralisation at moderate humidity may suffer considerable damage, so it is essential that neutralisation should take place as soon as possible after carbonising. Neutralisation with ammonia or a mixture of ammonia and ammonium acetate is achieved more rapidly than with sodium carbonate or sodium acetate; the ammonia is best used cold [146,286,290]. The pH of carbonising effluent can be adjusted, of course, to meet discharge requirements. However, this can lead to undesirably high levels of sulphate. The slime in sewage pipes produced under anaerobic conditions in turn produces hydrogen sulphide from the sulphates present. This hydrogen sulphide gas is oxidised on the sewer walls, promoting the growth of sulphuric acid-producing anaerobic bacteria with consequent damage to concrete pipes [286]. It is possible to minimise sulphate levels by coagulation with a combination of aluminium salts and lime at pH 10 [286,291]. Mechanism of shrink-resist finishing Shrink-resist processes for wool came into prominence with the growth in importance of machine-washable wool. These processes have a decisive bearing on the selection of dyes as regards fastness demands. Numerous approaches have been suggested, normally involving oxidative modification of the epithelial scales, the application of a polymer to the fibre surface, or a combination of both. In the case of polymer deposition, two approaches are possible. The polymer may be applied either as a surface film masking the epithelial scales or to link together neighbouring fibres in a process sometimes known as ‘spot-welding’. These differences are clearly illustrated in Figure 10.43. Special equipment is needed for interfacial condensation polymerisation, however, and a further restriction is that it can only be applied

Scale masking by polymer deposition

Scale linking by interfacial condensation polymerisation

Figure 10.43 Mechanisms of shrink-proofing by polymer application [292]

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to fabric, whereas polymer deposition can be used at any stage of wool processing. For these reasons, polymer deposition remains by far the most popular method commercially. The degradative modification or removal of the wool scales is sometimes referred to as ‘subtractive’ shrink-resist finishing, whilst polymer deposition techniques are described as ‘additive’ [293]. Subtractive shrink-resist treatments The chemical basis of subtractive treatments is the oxidative breakdown of disulphide bonds in the cystine-rich epithelial scales of wool to form cysteic acid residues (Scheme 10.41). There is also some hydrolysis of amide groups in the main peptide chain. Acidified sodium hypochlorite was originally the preferred oxidising agent. However, difficulties in achieving consistent and uniform treatment without excessive fibre damage led to its replacement by sodium dichloroisocyanurate (10.92), also applied under acidic conditions. The use of this ‘chlorine generator’ provides a more gradual and controlled release of chlorine than can be achieved with hypochlorite. Additives include sodium dioctylsulphosuccinate as wetting agent, as well as the acidifying medium such as acetic acid with a buffer such as sodium formate/formic acid. O

Cl N

Cl

N

_ O Na+

N O

10.92

Liposomes (section 10.3.4) have been suggested as auxiliary agents in wool chlorination since they give improvements in the consistency and homogeneity of the oxidative treatment, minimising degradation of the wool and facilitating subsequent treatments [61,62]. Although chlorination with sodium dichloroisocyanurate is still by far the most commonly used method of shrink-resist finishing, there is considerable concern over the environmental influence of its AOX contribution. For this reason, its usefulness could decline in future and there has been considerable investigation of alternatives to this attractively cost-effective treatment. One possibility is to use an atmosphere of nitrogen containing 3% fluorine gas, for which a commercial-scale plant is available [294]. This dry fluorination process provides an effective subtractive treatment capable of replacing chlorination. Since it requires a halogen gas, it might be thought to be just as likely as chlorination to infringe AOX regulations. However, organofluorides cannot be detected by the current AOX test method. In any case, owing to the strength and stability of the carbon-fluorine bond, it is unlikely that carcinogenic species are formed. Furthermore, as this is essentially a surface modification, the formation of fluoride ions is limited. It is considered that current effluent legislation covering fluoride ions should not restrict commercial adoption of this process [294]. The design of the machine eliminates the risk of loss of gaseous fluorine. Corona discharge, bombardment of the wool fibre surface with electrons of sufficient energy to break covalent bonds, has also been applied to the improvement of shrink

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

resistance [294–296]. Collisions between electrons and molecules of oxygen and nitrogen in the atmosphere results in the formation of ozone and oxides of nitrogen. Subsequent reaction between free-valence species on the substrate surface and the corona atmosphere leads to the formation of a polar surface. This can facilitate adhesion of polymer if the subtractive stage is followed by an additive phase [294]. The plasma glow discharge treatment of wool in non-polarised gases like air, oxygen or nitrogen, usually combined with a polymer treatment, also shows promise as a zero-AOX treatment for the shrinkproofing of wool [297–299]. The plasma treatment does not damage the fibres, yet considerably reduces the felting potential of the wool, particularly by enhancing the effectiveness of polymer treatment. Enzymes have been proposed as a means of subtractive shrink-resist treatment. Their use has been discussed already in section 10.4.2. There are difficulties, however, in the commercially successful application of enzymes to wool at present. Alternatives to sodium dichloroisocyanurate as oxidising agent include potassium permanganate and peracids such as peracetic acid, peroximonosulphuric acid (Caro’s acid) and peroximonophthalic acid, of which by far the most important in terms of current interest is peroximonosulphuric acid (10.93), usually known as permonosulphuric acid The pure acid can be obtained in crystalline form, but its salts are unstable. In a detailed study of the effect of oxidising agents on wool, their relative reactivities generally corresponded with the degree of shrink resistance but this was not related to redox potential [300], the order of reactivity being: aqueous chlorine > dichloroisocyanurate = permonosulphate > permanganate and salt > peracetic acid > permanganate > persulphate = hydrogen peroxide. Permanganate does not produce adequate effects unless it is applied from saturated salt solution [301], a situation hardly likely to make it a preferred commercial choice. O

O

H

O

O

S O

H

10.93

Permonosulphuric acid looks quite promising and appears to have the greatest potential of all other oxidants as an alternative to chlorination. The high reactivity of permonosulphuric acid with wool makes it particularly suitable for continuous treatments, where only a short time is available for reaction. The reactivity can be controlled by pH adjustment (Figure 10.44), the most suitable range being pH 3–5. Lowering the pH increases the rate of oxidation but also increases the likelihood of uneven treatment. Compared with chlorination this product shows the following advantages, although it is less effective in minimising felting of wool fibres [301]: – No AOX problems – No yellowing of the wool – Superior uniformity of treatment – Almost odourless – Less likely to attack dyes by oxidative fading.

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625

5 pH 8 pH 6 pH 4 pH 2

Oxidant/%

4

3

2

1

5

10

20

30

40

50

60

Treatment time/min

Figure 10.44 Influence of pH value of treatment bath containing a commercial permonosulphate formulation on the rate of reaction with dyed wool at 25 °C and 20:1 liquor ratio [301]

Degree of yellowing (DIN 6167)

30 29

A

A Dichloroisocyanurate B Permonosulphate

28 27 26

B B

25

A

1

2

3

4

5

6

Oxidant concentration/%

Figure 10.45 Variation of degree of yellowing of wool with concentration of commercial dichloroisocyanurate and permonosulphate formulations [301]

The difference with regard to yellowing of the wool is quite marked (Figure 10.45). Permonosulphuric acid is suitable for use at any processing stage and on all the usual dyeing equipment. The goods are first treated in an acidic liquor at ambient temperature until virtually all the active oxygen has been consumed. A reductive treatment with sodium sulphite is then given in the same liquor at a slightly alkaline pH, followed by rinsing. There is some conflict regarding the parameters involved but careful investigation [301] has led to the following recommendations: (1) After wetting out, the wool is treated in the same bath with 4–6% of a commercial permonosulphuric acid formulation for 30–60 minutes at 25 °C and pH 4–5 (2) To the same bath are then added 0.5% sodium carbonate and 5% sodium sulphite (pH 8). The temperature is raised to 35–50 °C and maintained for 20 minutes.

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

Chlorination treatments, of course, are invariably followed by a reductive aftertreatment. In the case of permonosulphuric acid, this is even more important as the sulphite treatment significantly enhances the shrink resistance (Figure 10.46).

Shrinkage/%

40

30

20

10

0

1

2

3

4

5

6

7

8

9

10

Sodium sulphite/%

Figure 10.46 Influence of sulphite concentration on the felting shrinkage (IWS TM31) of dyed wool treated with 5% of a commercial permonosulphate formulation [301]; untreated wool shrinkage 60%

Permonosulphuric acid treatment confers only a modest shrink-resist effect which usually needs to be improved by a subsequent additive treatment. It has been suggested [300] that the most likely mechanism for inhibiting felting by permonosulphuric acid treatment is the removal of degraded protein from below the exocuticle, producing a modified surface with a reduced differential friction. The direct formation from cystine residues of low concentrations of Bunte salts has been confirmed, as indicated in Scheme 10.42. The use of peroximonophthalic acid (10.94) has been reported as a shrink-resist agent [302,303]. When comparing dichloroisocyanuric acid, permonosulphuric acid and permonophthalic acid, it was observed [303] that dichloroisocyanuric acid reacts so rapidly that it is difficult to control the evenness of chlorination. This treatment tends to cause stiffening and yellowing of the wool. Consequently, it is used at lower concentrations than would be needed for full shrink-resist properties, the balance being achieved by subsequent application of a polymer. Permonosulphuric acid does not cause these problems but can only be used successfully on woven cloth, unless a polymer is applied subsequently. Permonophthalic acid does not exhibit the disadvantages of dichloroisocyanuric acid. Furthermore, it can be applied to wool in all its forms to give an adequate degree of shrink resistance without the need for subsequent polymer treatment. O C

O

H

C

O

O

H

O 10.94

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627

Since the handle of the wool is not impaired, permonophthalic acid can be applied at a level that gives full protection from shrinkage, this being the reason why the cost of polymer treatment can be avoided. Currently however, there is a major disadvantage: although available for experimental work as its magnesium salt, this is expensive and not yet available in commercial quantities. However, a process has been proposed [303] whereby it can be readily and inexpensively prepared in about 80% yield at 25–30 °C by stirring together phthalic anhydride and hydrogen peroxide in water for about an hour, maintaining the pH at about 5. This process is analogous to that for the preparation of peracetic acid (Scheme 10.37), although phthalic anhydride is less hazardous and easier to handle than acetic anhydride. Nevertheless, the stability of the resulting permonophthalic acid is poor, its oxidising power decreasing through decomposition by about 5% per hour. As with permonosulphuric acid, a subsequent reduction stage using sodium sulphite is needed to eliminate residual oxidant. Additive shrink-resist treatments As mentioned previously, additive treatments involve the application of a polymer to the fibre. This is usually prepared before application and contains reactive groups. However, it is also possible to form the polymer in situ within the fibres. The traditional approach is to apply the polymer after a subtractive oxidation treatment but environmental concern over AOX problems is increasing demand for additive treatments that can stand alone. There is no denying that the oxidative step can facilitate subsequent treatment with a polymer, since the scission of cystine disulphide bonds to yield cysteic acid residues provides useful reactive sites for crosslinking or anchoring the polymer. The desirable properties of shrink-resist polymers are [301]: (1) The treatment must impart maximum shrink resistance. (2) High substantivity for wool. This is clearly linked with the foregoing requirement. These two factors are particularly important when application takes place after an AOX-free oxidative stage, since such treatments generally impart lower initial shrink resistance than chlorine-based subtractive treatments. Indeed, these two requirements may need to be fulfilled so effectively that the oxidative stage before polymer treatment can be omitted. (3) Uniform application properties on wool in all forms. (4) The treatment should not lead to harshness or stiffness of the fabric, thus obviating the need for a softener. Indeed, if the polymer itself provides a degree of softness, this is an added bonus. (5) Suitable for application by batchwise or continuous methods. (6) Should have no adverse effects on frictional characteristics of wool fibres or yarns. (7) Should have no deleterious effects on the colour or fastness of dyes. (8) Should not contribute to AOX values. No shrink-resist polymer developed so far meets all the above requirements [301]. There is clearly some similarity with easy-care finishing of cotton. Although effective crosslinking agents are readily available for application to cotton, the morphological complexity of the wool fibre is such that an equally effective polymer has yet to be identified for wool treatment [304].

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Still the most widely used shrink-resist polymer treatment is that associated with the Hercosett process. This treatment always follows on from an oxidative treatment with dichloroisocyanuric acid. Despite its popularity, it is doubly suspect environmentally because both the oxidative first stage and the polymer itself contribute to AOX values. The polymer has aqueous solubility and is a cationic polyamide-epichlorohydrin resin, the epichlorohydrin contributing to AOX values. Since the polymer is cationic it has substantivity for the anionic sites in wool, including those produced by the oxidative treatment. The polymer contains azetidinyl cationic groups, which are reactive with a variety of nucleophiles leading to insolubilisation of the polymer [11]. Covalent bonding is also possible through cysteine thiol groups. Despite the popularity of the chlorination-Hercosett route, it is clear that AOX problems will often enforce the adoption of alternatives, many of which have already been developed. Non-AOX polymers include polyethers, polyurethanes, polysiloxanes, polyquaternary compounds and multifunctional epoxides. Polyethers of various types are of particular importance and include the following types: – polyethers solubilised by reactive Bunte salts – polyethers solubilised by carbamoyl sulphonate groups – thiol-terminated polyethers – aziridine-terminated polyethers. The first two types are of long-standing commercial availability. They are applied using magnesium chloride as catalyst and are crosslinked by addition of ammonia. An important factor is that magnesium chloride induces a cloud point at about 50 °C, leading to the physical form essential for functioning of the polymer [11]. Polyurethanes have also been used for many years. They can be applied from a solvent such as perchloroethylene, but such solvents are increasingly under environmental scrutiny. An aqueous polyurethane formulation is normally applied by padding, followed by baking at 150 °C using sodium carbonate as catalyst. Polysiloxanes as shrink-resist finishes have been developed from their traditional uses as softeners and water repellents; as such their chemistry is discussed in section 10.10.3. This was a natural trend as many shrink-resist finishes tend to impart a harsh handle to wool. Polyquaternary compounds are useful in that as well as conferring shrink resistance they may also improve acid dye fastness to wet treatments. A useful and detailed comparison between specific examples of a polyether, a cationic polysiloxane and a polyquaternary compound is available [301]. This review includes details of practical application via various processing routes available for loose stock, tops, yarn, knitted garments and woven or knitted piece goods. As mentioned earlier no single polymer fulfils all requirements and combinations of different types are sometimes used. Some indication of this is given in Table 10.33. Whilst many methods have been proposed for preventing shrinkage of wool fibres, no satisfactory method operates without some damage to the hydrophobic nature of the fibre surface [305,306]. Nevertheless, it has been claimed that treatment of wool with a multifunctional epoxide, glycerol poly(glycidyl ether), in a saturated solution of sodium chloride gives excellent shrink resistance without impairing the wool surface [305,306]. Apparently this polymer is able to crosslink the cuticular cells and decrease the prominence of their edges. There must be doubts, however, about the commercial feasibility of using the

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Table 10.33 Comparison of typical shrink-resist polymers of the non-AOX types for application to wool [301] Polymer type

Advantages

Restrictions

Polyether with reactive groups

Outstanding shrink-resist effect on pre-oxidised wool Durable soft handle Suitable for piece goods, garments and loose stock. Cost advantage over polysiloxane type

Restricted range of applications Not applicable before dyeing Special dissolving requirements

Polysiloxane with amino groups Semi-microemulsion

Most effective shrink-resist silicone Durable handle after finishing. No crosslinking agent or catalyst required All processing stages suitable Long liquor application after pre-oxidation Padding possible without pre-oxidation

Costly but high-quality finish

Polyquaternary compound

Shrink-resist effect after pre-oxidation Exceptional improvement in fastness All processing stages suitable Improves shrink-resist effect of polyether and polysiloxane types

No improvement in handle Shrink-resist effect weaker than with other types Odour possible on chlorinated wool

saturated brine solution necessary to bring about effective treatment. Such a medium would be difficult to handle, costly and environmentally undesirable, even though it is AOX-free. The dyeing of wool with bifunctional reactive dyes can enhance its resistance to felting. This observation has been exploited in a shrink-resist process using a specifically designed ‘colourless reactive dye’ as the effective agent [307,308]. The product developed is the trifunctional reactive compound, dipotassium 2-chloro-4,6-bis(4′-sulphatoethylsulphonylanilino)-s-triazine, also known as XLC (10.95). This reactant has substantivity for wool and imparts shrink resistance through crosslinking, being an example of a non-polymeric additive treatment. The crosslinking activity is centred mainly in the low-sulphur microfibrillar proteins through their high content of lysine and histidine residues. A particularly interesting approach is to apply this compound together with reactive dyes, in which case it reacts and crosslinks to an even greater extent than in the usually shorter shrink-resist process [308]. Cl

N KO3SO

CH2CH2SO2

NH

N N

HN

SO2CH2CH2

10.95 XLC

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OSO3K

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

A variation of this technique utilises reactive surfactants based on Bunte salt acetate esters of dodecanol and dodecane-1,12-diol [309]. Results using mono-, bi- and trifunctional versions have been reported (10.96–10.98 respectively). These are watersoluble and highly surface-active compounds. They can be applied to wool by padding in combination with 20 g/l sodium sulphite and a locust bean gum thickener to give 100% pick-up, followed by batching at 20 °C for 24 hours. Rinsing completes the process. The results in terms of shrink-proofing are shown in Figure 10.47. It is evident that reactant efficiency increased with functionality: 15%, 10% and 3% of the mono-, bi- and trifunctional agents respectively were required to obtain zero shrinkage, from which it is predicted that only 1–2% of a tetrafunctional agent would be needed. The monofunctional

Area felting shrinkage/%

50

Monofunctional Bifunctional Trifunctional

40 30 20 10 0 1

3

5

8

10

12

15

Applied agent concentration/% owf

Figure 10.47 Effect on shrink resistance of functionality of agent applied by the pad–batch cold process [309]

C12H25

O C

CH2

NaO3SS

SSO3Na

CH2 C

O

CH2 O

O

10.96 Monofunctional

O

C O

O

O

C

C O

C12H24

CH

C12H24

C

CH2

SSO3Na

C

CH2

SSO3Na

O

OH

O

O

O CH2

C O

C12H24

O

10.98 Trifunctional

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C O

10.97

O

CH2

O

Bifunctional

CH2

NaO3SS

C12H24

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SSO3Na

PREPARATION OF SUBSTRATES

([wool]

S

S)n

[wool]

(NaO3SS)n

S

S

[wool]

[wool]

SNa

+

+ Na2SO3

[wool]

SSO3Na

SSO3Na

S

S

631

Wool protein Bunte salts Dodecanol derivative

[wool]

(SSO3Na)n

Mixed disulphides

Scheme 10.48

agent in fact gave better results than expected, showing clear evidence in scanning electron microscopy of agent–fibre spot-welding. This is believed to occur via the formation of mixed disulphides from the dodecanol derivative and wool protein Bunte salts from the reaction of disulphide-rich proteins in the wool with sulphite (Scheme 10.48) [309]. Mention has already been made of the effectiveness of corona or plasma treatment in increasing the influence of subsequent or concurrent polymer treatment. As examples of polymers used in this way, mention can be made of reactive cationic polysiloxane [294] and polymerisation on the fibre of tetrafluoroethylene or hexafluoropropylene [299]. Water repellency was also improved by the fluorinated polymers. Tetrafluoroethylene gave superior shrink resistance; this polymer covered the scale edges of the wool, whereas this did not occur with poly(hexafluoropropylene). Shrink resistance can be achieved by graft polymerisation of vinyl monomers onto wool [292]. Suitable monomers include methyl, ethyl and butyl methacrylate, particularly the methyl ester. Polymerisation is initiated by a redox system, potassium bromate and cobalt(II) acetate, with a co-solvent such as 2-butoxyethoxyethanol to improve both efficiency and selectivity. Grafting of 100% monomer is possible in three hours or less at 50 °C. Ultimately up to 950% can be achieved; such high levels, however, are impractical. There are some rather severe obstacles to the commercial development of this process: methyl methacrylate is flammable and is a respiratory irritant, whilst use of the co-solvent is also undesirable. It is suggested [292], based on a limiting volume model, that the mechanism is characterised by three stages: initial grafting and void-filling within the fibre, disruption of crystalline regions and polymer growth on or through the fibre surface with formation of interfacial bonds (i.e. spot-welding) at high levels of grafting. Whilst elimination (by oxidation) or masking (by polymer deposition on the cuticular scales) are the accepted mechanisms by which shrink resistance is achieved, there is evidence that other factors need to be considered, particularly as it is possible to obtain a shrink-resist effect without degradation or masking of the scales. A review is available [310] of the mechanism of chlorine-based shrink-resist processes. Whilst chlorine-based processes are well understood from a mechanistic viewpoint, there are differences between these and the permonosulphuric acid processes. Understanding of the mechanism of permonosulphuric acid treatment has improved in recent years but there are still aspects requiring elucidation [300]. An important difference between these two types of oxidative treatment is that chlorine-based processes lead to scale modification or

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

destruction with zero differential friction as the target. Permonosulphuric acid processes, even when followed by a sulphite treatment, leave the differential friction essentially the same as untreated fibres, apart from some longitudinal striations at the base of the scales and some detachment and rounding of scale edges [300]. Chlorine water acting on wool produces Allwörden bubbles or sacs by raising of the epicuticle [11]. This occurs through the formation of osmotically active oxidation products. Cleavage of peptide bonds and especially oxidation of the disulphide bonds to produce sulphonic acid residues result in the formation of soluble peptides responsible for the increase in osmotic pressure within the semi-permeable membrane. Permonosulphuric acid treatment, with or without a sulphite aftertreatment, also forms Allwörden sacs, as do hydrogen peroxide, potassium permanganate and peracetic acid [300]. On immersion in water, wool treated with permonosulphuric acid shows major decreases in magnitude of friction and differential friction compared with untreated fibres, especially following a sulphite aftertreatment. This may be attributed to degraded protein at the fibre surface acting as a lubricant or to changes in swelling properties of the wool surface. The sulphite aftertreatment is particularly important with permonosulphuric acid treatment. Evidence for the underlying mechanism is available from analysis of sulphur oxidation products formed in the various processes (Table 10.34). It is evident from these results that the concentration of [RSSO 3]– anionic groups necessary to change the hydration of the fibre surface is achieved by the reaction of bisulphite with cystine monoxide residues to give the required cysteine-S-sulphonate groups [311]. Table 10.34 Relative amounts of sulphur oxidation products formed during shrink-resist processing of wool [11,311] Treatment

Oxidation product

Frequency (cm–1)

Quantity

Chlorine-Hercosett Permonosulphate Permonosulphate + bisulphite

Cysteic acid (RSO–3)

1042

*** **

Chlorine-Hercosett Permonosulphate Permonosulphate + bisulphite

Cystine monoxide (RSOSR)

Chlorine-Hercosett Permonosulphate Permonosulphate + bisulphite

Cysteine-S-sulphonate (RSSO–3)

* 1076

* *** *

1024

** * ***

In the case of polymer deposition, it has been pointed out [293] that the masking effect at the scale edges may be less important than mutual adhesion of fibres in the yarn, since the thickness of the polymer film (0.1 µm) is much less than the average height of scale edges (1 µm). This effect is more analogous to spot-welding. The particular requirements of shrink-resist processes in relation to wool fabric printing have been described [312]. In addition to dimensional stability, there is a need for ease of

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diffusion and absorption of dyes in printing. Chlorination is particularly effective because the rapid reaction limits the oxidative attack to the fibre surface. Dye affinity and the hydrophilicity of the fibre are increased by the anionic sulphonic acid groups formed and the surface barrier to diffusion is lowered. However, this is an AOX-loading process. Permonosulphuric acid, although AOX-free, is an inadequate preparation for printing even with a polymer aftertreatment. Hydrogen peroxide, activated by the recyclable catalyst sodium tungstate, offers a suitable process [312] that is AOX-free. Oxidation remains concentrated at the fibre surface because of the high reactivity of peroxide, giving results analogous to those from chlorination. A subsequent polymer treatment may also be given. The process involves padding to 75% pick-up in a solution equivalent to 8% by weight of sodium tungstate and 3% by weight of hydrogen peroxide. The padded fabric is rinsed after a reaction time of two minutes and the tungstate can be recovered from the rinsing water. 10.5.6 Combined processes The economics, at least on paper, of combining two or more processes to gain major savings in time and energy have long been sufficiently attractive to motivate research in this direction. For example, one-stage desize–scour, scour–bleach, desize–scour–bleach, scour– dye and even (paradoxically enough) bleach–dye operations have been, and are, operated. The major disadvantage of such combined processes stems from difficulties of operation, especially compounded where there is a high probability of incompatibility, as in enzyme desizing–bleaching or in bleaching–dyeing. In addition, any process that is combined with scouring and/or desizing may be subject to interference from the products desorbed or decomposed by those processes. Under these circumstances the choice of chemical additions becomes critical. For example, in combined desizing–bleaching the enzyme must be stable to oxidation and both processes must be operable under the same conditions of time, temperature and pH. Similarly, other products added, such as surfactants for wetting and detergency or metal ion sequestering agents, must fulfil their primary task and not interfere with other functions, while themselves being unaffected by the conditions. Scour–dye operations in particular need careful planning backed by detailed knowledge, especially in regard to the effects of desorbed impurities on the stability and exhaustion of the dyes. In the combined scouring and dyeing of wool or nylon with acid dyes the detergent should ideally function as a levelling agent. The crucial factor in scour–dye processes involving disperse dyes is stability of the dye dispersion in the presence of the detergent and desorbed impurities. This becomes increasingly critical at higher temperatures (say 130 °C) and with difficult application conditions, such as tightly woven fabrics on beams. Hence such operations are more frequently used in jet dyeing, for example. Incompatible surfactants and greasy soil can have disastrous effects on dye dispersions, leading to agglomeration of dye particles and deposition of coloured oily stains on the substrate, with extensive breakdown of the dispersion in severe cases. Surfactants can have a highly selective effect on the rate and extent of exhaustion of disperse dyes and must therefore be selected accordingly. In spite of these difficulties successful combined processes are indeed operated, although they require vigilant monitoring. The promised economies must not be squandered by lost processing time and costs of damage and reprocessing. It would be surprising if the manufacturers of dyes and chemicals were more enthusiastic than textile processors about

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such combinations, in view of their understandable caution about guaranteeing the behaviour of individual products in circumstances for which they are not intended. Now that environmental and economic factors limit research into new products, the fusing of normally sequential processes into a single one will remain a worthwhile technical and economic goal. Possibilities for combining the three main preparatory processes for cotton (desizing, scouring and bleaching) remain economically attractive. One process for achieving this involves treatment of loomstate cotton with an aqueous solution containing 3 g/l sodium chlorite, 1 g/l hydrogen peroxide, 2 g/l nonionic wetting agent and 10 g/l disodium hydrogen phosphate for 90 minutes at 95 °C, pH 10 and 20:1 liquor ratio [313]. In another process the fabric is impregnated with 20 g/l sodium chlorite, 0.05 g/l potassium permanganate and 2 g/l nonionic wetting agent, followed by treatment for 30–60 minutes at 90 °C and pH 10 [314]. The concentrations of the oxidants, the pH and time of treatment were critical, however. In a modification of this process, impregnation with 20 or 30 g/l sodium chlorite, respectively 3 or 1 g/l potassium chromate and a nonionic wetting agent is followed by treatment for 90 minutes at 90 °C and pH 6 [315]. All these processes are subject to the usual criteria regarding machinery that is resistant to corrosion by sodium chlorite, whilst the discharge of effluent containing chromate has environmental implications in many countries. An AOX-free alternative [316] is impregnation with 4 g/l hydrogen peroxide, 8 g/l urea and 2 g/l nonionic wetting agent, then treatment for 60 minutes at 95 °C, pH 8 and 20:1 liquor ratio [316]. This results in a bleached fabric with excellent wettability and without serious fibre degradation. The urea interacts with hydrogen peroxide to form an unstable complex, which then decomposes to form hydroxyl and perhydroxyl radicals, according to Scheme 10.28 [316]. Urea exhibits undesirable environmental characteristics in some respects, however. In all the above processes, the optimised quantities of the chemicals indicated will be specific to the substrate quality evaluated. They would require further re-optimisation for each substrate to take account of the type and concentration of size, the presence of other impurities and the degree of natural yellowness. In particular, the amount of oxidant will need to be adjusted to give the optimum balance between oxidative desizing and the degree of bleaching required. A commercially established system for the combined desizing, scouring and bleaching of cellulosic fibres and blends is the Raco-Yet (Kieinewefers) system [317,318]. The fabric is impregnated successively with 23–26 ml/kg sodium silicate (38°Bé), 32–40 g/kg sodium hydroxide (100%), 20–26 ml/kg Cottoclarin AS and 40–65 ml/kg hydrogen peroxide (50%) before steaming. Cottoclarin AS is designed specifically for this process, having excellent wetting properties, freedom from foaming, high dispersing, emulsifying, complexing and detergency powers. The process is applicable to a variety of size polymers and their mixtures, including starch, hydroxypropyl starch, poly(acrylic acid), poly(vinyl alcohol), carboxymethylcellulose and polyesters. Desizing, boiling-off and bleaching can be achieved in 1–3 minutes. Savings in processing costs and exceptional flexibility of operation are claimed, since recipe changes can be made in less than one minute. It is essential to carry out a thorough analysis of each substrate and to adjust the recipe and processing speed accordingly. Table 10.35 gives an indication of the flexibility of the process together with typical results [318].

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Table 10.35 Examples of application of the Raco-Yet system to cellulosic fabrics and blends [318]

Polyester/ cotton 65:35 170 g/m2

Cotton/ polyester 88:12 290 g/m2

Polyester/ viscose/ linen 70:20:10 185 g/m 2

Starch

None

Fabric (130% pick-up)*

100% Cotton 320 g/m2

100% Cotton shirting 190 g/m2

Size type

Starch

None

PVA

26

26

23

26

26

40

40

32

40

40

26

26

20

26

26

65

65

40

65

65

Sodium silicate (38 Be) ml/kg Sodium hydroxide (100%) g/kg Cottoclarin AS ml/kg Hydrogen peroxide (50%) ml/kg Treatment time (min) Running speed (m/min) Evaluation

2

1.5

60 Grey

Absorptivity (mm) After 15 seconds 0 After 30 seconds 0 After 60 seconds 0 Whiteness: %REM 56.0 Berger 16.1 Degree of polymerisation 2760

1.5

80 Treated

Grey

3

80 Treated

Grey

1.5

40 Treated Grey

80 Treated Grey

Treated

15

0

18

0

15

0

32

0

18

20

0

24

0

20

0

41

0

25

25

0

31

0

25

0

52

0

33

90.2 104.5

112.5 152.6

50.1 43.3

80.3 64.0 2280

86.1 75.7

61.8 20.1

81.2 70.8

3080 2670

* concentrations are per kg of fabric

It is possible to use an enzyme with hydrogen peroxide in a combined desize–bleach but great care is needed in selection of the enzyme and optimisation of the concentrations [319]. Mercerising (high concentrations of alkali; cold) is particularly difficult to combine with desizing, scouring and peroxide bleaching (lower concentrations of alkali; hot). Nevertheless, a combined one-stage desize, scour, bleach and slack mercerise process has been attempted [320]. This involves impregnation of the fabric in a 10–30% sodium hydroxide liquor containing 20 g/l hydrogen peroxide and 50 g/l trichloroethylene for

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3 minutes, the results being dependent on impregnation temperature (20–100°c). The fabric is squeezed and (without predrying) is heated to 120 °C, the results being influenced by the duration of the heating step. It is claimed that, with a wet immersion treatment for 3 minutes at 40 °C and subsequent treatment for 30 seconds at 120 °C, the physical properties of the treated cotton are similar to those from a conventional two-stage approach. The use of trichloroethylene, of course, is highly sensitive both environmentally and as a health hazard (toxic to the liver). An electrochemical system combining scouring, mercerising and bleaching has been proposed. It is a non-polluting method based on an electrochemical cell, the cathode of which produces the base to mercerise and bleach, whilst the anode produces an acid to neutralise the base remaining after mercerisation [321]. A mercerising-type effect and bleaching of viscose fabrics can be achieved simultaneously using liquid ammonia, the bleaching agent being a peroxidated urea derivative that causes less damage than conventional oxidants, together with improved dye absorbency [322]. Mercerising has been combined with vat dyeing in a continuous process [323]. Cotton fabric is padded with an aqueous suspension of a vat dye in a sodium chloride solution containing caustic soda for mercerising. After drying, the dyeing is developed by padding in an alkaline solution of reducing agent, steaming and soaping. Further combined processes involving dyeing include: (1) Dyeing cotton yarn with selected direct dyes and simultaneous bleaching with peroxide. It is claimed that the peroxide also increases the colour yield [324] (2) Combined dyeing and easy-care finishing of cotton using bis-nicotinotriazine reactive dyes and DMDHEU in a pad–dry-HT steam process [325] (3) Combined dyeing and finishing of polyester/cotton using a liquid ammonia medium [326] (4) Dyeing polyester/cotton with reactive and disperse dyes and imparting a crease-resist finish [327].

10.6 DISPERSING AND SOLUBILISING AGENTS 10.6.1 Dispersing agents Dispersing agents are substances that promote the more or less uniform and stable suspension of relatively small particles in a given matrix. We are concerned here with the most common type of dispersion encountered in textile coloration, the solid-in-water systems typified especially by disperse dye technology, as well as the insoluble forms of vat and sulphur dyes. Pigments are also extremely important examples of solid-in-liquid dispersions but form a specialised case fully dealt with in Chapter 2. Also excluded from this section are other systems that depend mainly on a large increase in viscosity for their suspending action; these are more appropriately dealt with in section 10.8. An in-depth account is available [328], covering in particular the essentials of colloid science as applicable to dispersions, the preparation of dispersions (solid-in-liquid and liquid-liquid) as well as foams (section 10.11). An extensive account of the uses of dispersions is also available [329]; this includes pigments and the incorporation of colorants in polymer melts but is otherwise concerned with non-textile applications. Any formulation of solid particles in a liquid medium is more or less unstable as a result of

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(a) gravitational settling effects and (b) attractive forces between particles tending to lead to particles adhering, thus increasing the susceptibility of the system to gravitational effects. Two aspects need to be considered: the initial preparation of the dispersion, and its subsequent stabilisation during storage and use; only rarely will one agent satisfy the needs of both. Individual dyes vary widely in their requirements and any given dye may require different treatments depending, for example, on its microcrystalline form and the application processes for which it is intended. Therefore specific types of dispersing agents, or mixtures of them, are frequently needed to obtain the optimum dispersing action. There are two main groups of such agents: (1) Surfactants, mainly of the anionic and nonionic types (2) Water-soluble polyelectrolytes, most usually of the anionic type. The chemistry of surfactants has been described already. They usually play a subsidiary role in dispersions involved in textile coloration. The polyelectrolytes may be conveniently divided into two categories: (1) Acrylic acid copolymers, sulphonated polyvinyl compounds, alginates and carboxymethylcellulose. Some of these may require addition of other chemicals (e.g. alkali) in order to ensure aqueous dissolution. These polymers are less important as dispersing agents for disperse, vat and sulphur dyes than in areas such as pigment applications and as thickeners in textile printing or migration inhibitors in continuous dyeing (section 10.8). (2) The condensation products of formaldehyde with arylsulphonates or lignosulphonates, these being the major types of polyelectrolyte of interest in the manufacture and use of disperse dyes [330,331]. As with surface-active agents, the detailed chemistry of these products is a good deal more complicated than is indicated by the nominal structures frequently quoted. Most commercial products are mixtures of which the nominal structure represents a basic type only. Indeed, the detailed chemistry of the more complex products is still only partially understood. These provisos should be borne in mind when considering the structures given below. The sulphonated aromatic condensation products form a large and varied group, since formaldehyde will condense with many aromatic compounds [330], including sulphonated arylamines, phenols and aliphatic ketones; the range of commercially important products is relatively limited, however. One of the oldest is the condensation product of naphthalene-2sulphonic acid and formaldehyde (10.99), in which the degree of condensation is thought to

HOCH2

CH2

SO3Na

CH2

SO3Na

CH2

SO3Na

CH2OH

SO3Na

10.99

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OH

OH

OH CH2

HOCH2

OH CH2OH

CH2

CH2

CH2SO3Na

n

CH2SO3Na

10.100

OH

OH CH2OCH2

HOCH2

OH CH2OH

CH2OCH2

CH2SO3Na

CH2SO3Na

n

CH2SO3Na

10.101 OH

HO

CH2SO3Na

CH2

HO

CH2SO3Na

CH2 NaO3S 10.102

OH

HO

CH2SO3Na

CH2

HO

CH2 NaO3S 10.103

SO3Na

correspond to between two and ten naphthalene units, although the quantitative distribution of the condensates varies widely. Also of importance are the similarly structured condensation products of (a) phenols with formaldehyde and sodium sulphite (structures 10.100 and 10.101, depending on molar ratios) and (b) p-cresol and 2-naphthol-6-sulphonic acid with formaldehyde and sodium bisulphite (10.102 and 10.103). The types represented by structures 10.100–10.103 are also widely used as syntans (section 10.9.4). The lignosulphonates comprise a variable group of products derived from wood pulping.

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Their highly complex structures are only partially understood, although enough is known to enable representative structures to be proposed showing the major functional groups. There are two distinct processes whereby lignins are extracted. The first is an acidic digestion process in which the wood is pulped with sulphite or bisulphite. The second is an alkaline process, the so-called kraft process, in which the wood is treated [331] with sodium sulphide in autoclaves at pH 13 and 160–175 °C. The product is precipitated by careful addition of acid, filtered off and washed free from inorganic ions, then sulphonated to increase its aqueous solubility. Structure 10.104 has been proposed as being representative of kraft lignin prior to sulphonation [331]. An alternative partial representation of a lignosulphonate structure [330] is that of structure 10.105, which shows sulphonation as having taken place mainly at the CH=CH link. The molecular configuration is such as to give spherical particles. The final nature of the product varies enormously depending, amongst other things, on: (a) purity, especially the content of electrolytes such as sodium sulphate (b) the number of hydroxy groups present (c) the degree of sulphonation (d) the relative molecular mass (2000–1 000 000) and its distribution. This wide variability need not be a disadvantage provided it can be controlled to give reasonable consistency from batch to batch, since this enables products to be designed to give the optimum efficiency for the particular application concerned. The kraft lignins provide greater scope for modification, particularly of the degree of sulphonation and molecular size, and are also much more amenable to production in low-electrolyte forms [331]. Further information on the modification and behaviour of lignosulphonate dispersing agents is given in section 12.6.1. The basic similarity between these major types of dispersing agents and the surface-active agents discussed earlier lies in their amphiphilic nature (the possession of a combination of OH OCH3 HOCH2 CH2

CH

OH

HS

CH3O

CH2OH

CH

OCH3

CH H2C

OH HOCH2CH2

CH

O HC

CH

OCH3 OCH3

OCH3 HOOC

CH2

OH

HC

CH

HC

639

O O

CH OCH3

CH2 OCH3

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HO O

CH2 CH2

OH

CH2 HO

O H2C

CH HC

10.104

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CH3O SO3Na HOCH2

O

HOCH2

O

CH

OH

NaO3S

CH

OCH3

HOCH2

CH

O

OH CH SO3Na

CH

O

CH [lignin]

CH H2C

CH [lignin]

HO SO3Na CH3O

OCH3

CH CH HOCH2

OH 10.105

hydrophobic and hydrophilic moieties). The polyelectrolytes, however, are of much larger molecular size than conventional surface-active agents. When converting a conglomerate mass of relatively coarse particles into an aqueous dispersion (as in the manufacture of disperse dyes) there are two broad phases to be considered. In the first phase the dual aim is that of mechanically grinding the particles down to the required size and of obtaining as narrow a range of particle size as possible during the actual preparation of the dispersion. Maintaining these particles in a stabilised suspension constitutes the second phase. Particle size alone is not the main criterion; its distribution is equally important. This is because all dispersions are metastable. As well as tending to settle as a result of gravitational forces, there is also a thermodynamic tendency towards a reduction in the free energy of the system. This is manifest as a continuing increase in particle size leading ultimately to a severe deterioration in the dispersion quality (as already mentioned). Smaller particles tend to be attracted towards larger particles, with which they then form even larger particles. The opportunity for particle growth is therefore much less when all the particles are of similar size than when the range of sizes is large. The actual comminution of the coarse particles is usually carried out mechanically – for example, by grinding an aqueous slurry of the colorant in a rotatory mill containing relatively large, hard and inert grinding media such as pebbles. The process is facilitated by efficient wetting of the particles and lowering of interfacial tension. It is therefore preferable to add dispersing agents that have some surface activity and good wetting properties. Microfissures are created as the particles are broken down. The surface-active properties of the dispersing agent enable it to penetrate these microfissures, hindering agglomeration and facilitating further comminution. The amphiphilic agent becomes adsorbed and oriented on the surfaces of the particles, providing a protective sheath of like repellent charges, the forces of which eventually exceed the forces of attraction between the particles and thus stabilise the dispersion. This protective sheath becomes of critical importance during the second phase by stabilising the suspension of particles, both in storage and in subsequent

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use. Hence different dispersing agents may well be required to provide optimum dispersing and stabilising power in the two phases. The dual amphiphilic nature of the polyelectrolyte condensates and lignosulphonates described above, with their hydrophobic groups juxtaposed with many polarisable ionic groups, renders them highly efficient dispersing agents. It is necessary, however, to make a careful choice from the various grades of dispersing agents available with respect to their molecular size and charge distribution. Optimum dispersing ability depends on matching the steric, hydrophobic and ionic properties of the dispersing agents relative to the characteristics of the particles to be dispersed. The forces that may be operative in adsorption of surfactants onto disperse dye particles have been listed [330] as ion exchange, ion pairing, hydrogen bonding, van der Waals dispersion forces, polarisation of π–electrons on aromatic systems and hydrophobic interaction. It follows that anionic and nonionic surfactants, judiciously selected, may be used in conjunction with the polyelectrolytes to aid the dispersing mechanism [330].

10.6.2 Solubilisation It is necessary to differentiate between simple solutions and the process of solubilisation in colloidal solutions. A non-colloidal solution is a homogeneous single-phase system of a solute dissolved in a solvent, examples being an aqueous solution of sodium chloride or a solution of methylnaphthalene in acetone. The term solubilisation refers to the homogeneous mixing of an otherwise insoluble agent, the solubilisate, into a liquid medium by addition of a solubilising agent, invariably a surfactant. This agent acts as an amphiphilic bridge between solubilisate and medium. For example, methylnaphthalene will not dissolve to any significant extent in water, but its solubilisation in water to give an apparently clear colloidal solution can be brought about by the use of a surface-active agent such as nonylphenol poly(oxyethylene) sulphate. Hence the distinction between solution and solubilisation in non-colloidal and colloidal situations generally. Solubilisation can be viewed as one end of a reversible colloidal continuum that begins with wetting and proceeds through dispersion or emulsification to solubilisation, each of these stages being characterised by the size and nature of the particles. In this sense solubilisation is an extension of emulsification (or dispersion) in which the proportion of surfactant has been increased to the level where the discrete droplets (or particles) that characterised the emulsion (or dispersion) have become completely absorbed into the surfactant–medium phase. In some cases the surfactants used to produce an emulsion (or dispersion) may need modification (a change of hydrophile–lipophile balance) before complete solubilisation can be brought about. Similarly, if a solubilised system is diluted by addition of the liquid medium, a point will usually be reached at which the solubilised system changes to an emulsion (or dispersion). It is indeed possible to have all the stages of wetting, emulsification (or dispersion) and solubilisation present at the same time to different degrees. It is beyond the scope of this section to discuss the complex physico-chemical parameters of solubilisation in detail. Useful relevant works of reference are available [332–335]. It follows, however, that since solubilisation is essentially an extension of emulsification (or dispersion), the factors discussed in section 9.8.3 in regard to emulsification are also pertinent to solubilisation. Theory in this area is a useful guide but much still depends on

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empiricism. As in emulsification, each system to be solubilised will present its own specific requirements with regard to the type and amount of surfactant(s) required. In some cases solubilisation is aided by adding a small amount of a water-miscible solvent such as an alcohol or glycol, although the environmental and the health and safety aspects of such additions nowadays requires careful consideration. Conversely, the presence of electrolytes can have a deleterious effect. Temperature too can be important; a system that is an emulsion (or dispersion) at one temperature may become solubilised at a different (usually higher) temperature. There are two main approaches to solubilisation in textile wet processing. One is the deliberate preparation of a solubilised product for use as an auxiliary agent, as in the proprietary carriers formulated for dyeing polyester with disperse dyes. The other is as a concomitant of the process itself, as in the solubilisation of fats and oils during scouring processes and in the disperse dyeing process. In both situations more than one stage of the colloidal continuum of wetting-dispersion/emulsification-solubilisation may be present at any one time. 10.7 LEVELLING AND RETARDING AGENTS Level dyeing problems can be divided into two broad categories [336]: (1) Gross unlevelness throughout the material: this type of unlevelness is primarily related to the dyeing equipment or process; the substrate is often uniform in properties, both chemically and physically (2) Localised unlevelness: this is primarily related to physical and/or chemical nonuniformity of the substrate; typical examples are barriness in nylon or polyester dyeing and skitteriness in wool dyeing. There are also two fundamental mechanisms that can contribute to a level dyeing: (1) Control of rate of exhaustion of dye so that it is taken up evenly (2) Migration of dye after initially unlevel sorption on the fibre. Either or both of these mechanisms may operate to a greater or lesser extent in a given dye– fibre system, although a general trend towards better fastness properties has dictated the use of dyes that show low, if any, propensity to migration, thus placing the emphasis for level dyeing on the control of exhaustion rate. Physical factors such as temperature and frequency of liquor/substrate contact (governed by rate of liquor circulation in a jet, beam or package machine) can be used to exert some degree of control over these mechanisms. Slower rates of heating usually favour more even uptake of dye and higher temperatures tend to increase migration or diffusion. In some cases level dyeing can be influenced by dyebath pH and/or the presence of electrolytes. This section, however, is more concerned with the control of levelness by means of chemical auxiliaries, generally known as levelling or retarding agents. Since levelling agents are invariably surfactants, they may be anionic, cationic, nonionic or amphoteric in nature. Sometimes combinations of these are used. The chemical structure of commercial products is seldom revealed, however; hence only general principles can be covered here. The main mechanisms by which levelling agents operate [337–341] are as follows:

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(a) nonionic agents usually form water-soluble complexes with the dye, some degree of solubilisation being involved (b) ionic agents are primarily dye- or fibre-substantive; in the former case they tend to form complexes with the dye and there is competition between the levelling agent and the fibre for the dye, while in the latter case the competition is between levelling agent and the dye for the fibre. In complex formation the principle, as far as levelling action is concerned, is usually the same irrespective of whether nonionic or ionic agents are used, although the mode of complexing is different. The attractive forces between agent and dye create a counterbalancing mechanism against dye–fibre attractive forces, restraining the uptake of dye by the fibre. As the temperature of the dyebath increases the complex gradually breaks down, progressively releasing the dye for more gradual sorption by the fibre. Clearly, for an effective levelling agent that functions by this mechanism the stability of the agent–dye complex, governed by forces of attraction between agent and dye, is crucial. If these forces are so weak that a relatively unstable complex is formed, restraining or levelling action may be inadequate. On the other hand, strong forces of attraction may result in a complex that is too stable to break down as the temperature rises, so that the dye is effectively entrapped by the agent in the solution phase and is not available for sorption by the fibre. The objective therefore is to formulate the levelling agent so that it forms a dye complex of optimum, rather than maximum, stability relative to the conditions of application. This is done by adjusting the hydrophilic–lipophilic balance of the surfactant. The problem lies in the fact that the dye–agent interaction is so specific that different members of a range of dyes may each require a different balance. Hence commercial levelling agents may contain more than one surfactant. A difficulty that arises with ionic levelling agents is that they may form an insoluble precipitate with ionic dyes of opposite charge; this can be obviated in various ways. In the first instance attention should be paid to the concentration of the surfactant; where initial addition of surfactant to the dyebath causes precipitation of the agent–dye complex, further additions of surfactant often lead to its solubilisation. Alternatively, a further surfactant may be added to solubilise the complex; a nonionic agent will not itself react with either the dye or the original ionic surfactant to form a further insoluble complex, but its addition may further complicate the relationship between the hydrophobic–hydrophilic balance of the ionic agent and the dyes to be complexed. Due regard also needs to be paid to the cloud point of the nonionic agent under the conditions of use. This does not preclude the use of a relatively hydrophobic nonionic agent, since its cloud point may be effectively raised in the presence of the ionic agent (subject to possible interference from any electrolytes or solvents present in the dyeing system). Similarly, if there is any danger from the cloud point of a nonionic surfactant used as the primary levelling agent (as with disperse dyes, for example), a suitable anionic surfactant may be added to effectively raise the cloud point, again paying due attention to any effect the anionic agent may have on the complexing–liberating performance of the nonionic agent. The third method of obviating precipitation of an ionic agent–ionic dye complex is to choose what effectively amounts to a ‘modified’ ionic agent. Ethoxylated anionic and ethoxylated cationic agents are particularly useful in this respect. The ethoxylation tends to

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reduce the ionic character of the agent, thus giving rise to weaker but more controllable forces of attraction for dye ions, and the oxyethylene chain can further function as a dispersing–solubilising moiety for the agent–dye complex. In a sense this is basically similar to using a mixture of ionic and nonionic agents as described above except that a single agent is used, thus facilitating the aim of obtaining the optimum complexing–liberating balance. Dye–agent complexes of lower net charge are formed when the ionic agent is added to the ionic dye solution. As the concentration of agent is increased a point is reached at which all the dye is complexed and its ionic charge has been neutralised. Beyond this point, as more agent is added, the agent–dye complex takes on the charge of the complexing agent (i.e. the opposite to that of the dye itself). This brings about a change in the partition coefficient of the complex between water and organic solvents [336], modifying the electrical and solution properties of the dye and so altering its affinity for the fibre. Fibre-substantive levelling agents are usually of the same ionic type as the dye, that is anionic agents are used with anionic dyes and cationic agents with cationic dyes, the aim being to create a system in which levelling agent and dye both compete for the sorption sites in the fibre. Just as the complexing type of levelling agent has to be carefully chosen so as to obtain the optimum complexing–liberating properties, so must the competing type of levelling agent be chosen such that its ionic power gives the optimum level of competition relative to the dye– fibre system concerned. If the ionic power is too weak, it will not function as an effective levelling agent; if it is too strong, it may exert blocking effects, preventing sorption of the dye. Ideally the balance should be such that the smaller surfactant ions are adsorbed by the fibre more quickly than are the larger dye ions, but the agent–fibre interaction needs to be weak enough to permit subsequent displacement of the surfactant ions by the dye ions. As the forces of dye–fibre interaction vary from one dye to another, the ionic power of the levelling agent must be suitably adjusted through its hydrophilic–hydrophobic balance to give the optimum properties. This can be done either by careful choice of a single surfactant or by the use of mixtures, which has gained prominence in recent times. For example, the strongly anionic character of a long-chain alkyl sulphate or sulphonate can be modified (toned down) by mixing it with a more weakly anionic poly(oxyethylene) sulphate or with a nonionic agent. Some levelling agents operate both by complexing and by competition. For example, in the application of acid dyes a weakly cationic agent may be used to complex with the dye and an anionic agent may also be used as a competing agent. This combination is more versatile because unlevelness may arise from different mechanisms. Unlevelness arising from process or equipment variables can often be controlled by dye–agent competition, whereas localised dye uptake variations generally respond better to dye–agent complex formation. Evidently, in this combined system the balance of properties is highly critical. In particular the oppositely charged surfactants must not mutually precipitate; hence the more weakly ionic ethoxylates are of particular interest, since the oxyethylene assists solubilisation of any complex so formed. A purely nonionic agent may also be used to prevent coprecipitation of the ionic types. Amphoteric agents, in a sense, fall within this combined system. Theoretical considerations are clearly useful in formulating suitable levelling agents. Nevertheless, a good deal of empiricism is always involved in formulating well-balanced agents for specific dye–fibre systems. Table 10.36 shows the general types of levelling agents now being offered and their uses; more detail is given in Chapter 12 relative to each class of dye. Many, but not all, levelling agents promote migration of dye in addition to retarding dyeing, such agents will obviously be a further aid to level dyeing. In some cases, however,

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Table 10.36 Levelling agent types and their uses Recommended for use with Type of levelling agent

Substrate

Dye classes

Nonionic

Cotton Wool, nylon Polyester

Direct, vat, azoic Milling acid, metal-complex Disperse

Nonionic/anionic

Polyester Wool, nylon

Disperse Milling acid, metal-complex

Nonionic/cationic

Wool

Acid, metal-complex, reactive, chrome

Anionic

Wool, nylon Cotton Polyester

Acid Direct Disperse

Weakly anionic

Polyester

Disperse

Anionic/cationic

Wool, nylon

Acid, metal-complex

Cationic

Acrylic Wool, nylon

Basic Acid, metal-complex, reactive

Weakly cationic

Wool, nylon

Acid, metal-complex, chrome

Cationic/polymeric

Cotton

Vat, sulphur

Amphoteric

Wool

Acid, metal-complex, reactive, chrome

higher concentrations of levelling agent are needed to obtain significant migration and this may interfere unduly with dye sorption. Levelling agents are also widely used as stripping agents, either alone for non-destructive desorption or together with reducing agents such as sodium dithionite for destructive stripping. When used for this purpose, their hydrophilic– hydrophobic balance is not as critical as when they are used simply as levelling agents. Thus higher concentrations are often used in order to maximise rather than optimise desorption of the dye. It should not be overlooked that electrolytes can play an important part in levelling and retardation. In recent times the use of bolaform electrolytes (section 10.1), cyclodextrins (section 10.3.1) and liposomes (section 10.3.4) as complexing agents has been proposed.

10.8 THICKENING AGENTS, MIGRATION INHIBITORS AND HYDROTROPIC AGENTS USED IN PRINTING AND CONTINUOUS DYEING Most, if not all, textile printing and continuous dyeing processes entail the use of auxiliaries that considerably increase the viscosity of the application medium compared with conventional batchwise dyeing processes, the aim being to facilitate and stabilise the local application of colour prior to its actual fixation to the fibre. Such auxiliaries are generally known as thickening agents in printing and as migration inhibitors in padding operations.

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They are characterised by undergoing marked macromolecular swelling in solution due to solvation (hydration in aqueous systems). While the principal role of thickening agents is to increase the viscosity of print pastes or pad liquors, certain other properties are also of importance, such as stability and rheology of the print paste, adhesion and brittleness of the dried thickener film, the effect on colour yield and penetration, ease of preparation and removal, and not least cost. A succinct account of these factors and of their interrelationships is available elsewhere [342]. The following discussion is restricted to rheology of print pastes, since an understanding of the basic principles of fluid flow is essential in appreciating the fundamental mechanism taking place during printing. The number of recent publications dealing with rheology, particularly in relation to specific types of thickening agents, is evidence of its importance, both as a concept and in current research [343–352]. The essential fact concerning thickening agents is that they are viscoelastic, exhibiting properties associated with both fluids and solids and showing what is known as pseudoplastic (non-Newtonian) flow behaviour [352]. This is best understood by comparison with simple liquids such as water or alcohol, which show Newtonian flow behaviour. The apparent viscosity of Newtonian liquids does not change when a shear stress is applied (curve A in Figure 10.48). All thickening agents, however, are highly viscous in a static state but apparently show reduced viscosity when a shearing force is applied. This is indeed their modus operandi: they must flow under shear to allow transfer through the screen, then resume high viscosity when the shear is removed so that colorants remain where they have been deposited.

Rate of shear/s–1 × 103

10 A

8

B C D

6 4 A B, C D

2

Newtonian Shear thinning Thixotropic

0 5

10

15

Shear stress/kPa

Figure 10.48 Typical flow curves demonstrating behaviour of viscous liquids [342]

Most thickening agents are of the shear thinning type represented by curve B, the apparent viscosity progressively decreasing as the shear rate is increased. It is important that this change is reversible, viscosity returning to its original level as soon as the shear is removed. In some cases, shear thinning may not begin until a certain critical shear has been applied (curve C). Thixotropic fluids (curve D) show time-dependent effects in that apparent viscosity depends on both the rate and duration of shear, the return to original viscosity being delayed. The opposite of shear thinning is shear thickening, often referred to

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as dilatant behaviour; such behaviour is clearly not suitable for textile printing. An alternative method of representing flow behaviour is shown in Figure 10.49. 105

Apparent viscosity/mPas

Dilatant

104

Newtonian

103

102

Shear thinning

10 Shear stress/Pa

Figure 10.49 Relationships between apparent viscosity and shear stress [350]

Print pastes may be thickened by any of the following methods [342]: (a) a relatively low concentration of a long-chain thickening agent (b) a relatively high concentration of a shorter-chain thickener or one having a highly branched structure (c) an emulsion (d) a finely dispersed solid such as bentonite (derived from clay). The first two methods, particularly the first, are most frequently used today; combinations of these methods are also possible. Thickening agents can be of natural or synthetic origin. Various natural gums and starches have been used traditionally in many printing styles. The materials from which they are extracted are valuable sources of foodstuffs, so availability and cost can depend on fluctuating demand from the food industry. The properties required of an ideal thickener can be summarised as follows [352]: (1) Compatibility with colorants and other auxiliaries (2) Adequate solubility and good swelling properties in cold water (3) Good washing-off properties (4) High degree of purity and conformity to standard (5) Non-dusting (6) Biodegradable (7) Non-toxic (8) Manufactured from replenishable raw materials. 10.8.1 Natural thickeners Natural thickeners are derived from plants by extraction from part of the plant itself or from a plant secretion; their biosynthesis is now a possibility. These products are generally polysaccharides and are thus closely related to cellulose. They consist of homo- or

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heteropolymers of simple hexoses, most commonly glucose, mannose or galactose [353]. Linear and branched segments are normally present, the degree of branching being important in relation to the technical properties of the product. Polysaccharides bear some structural similarities to anionic polyelectrolyte dispersing agents (section 10.6.1) and sizing agents (10.5.2). In particular, the nature of the side groups (mainly, though not always hydroxy or carboxyl groups) has a decisive effect on viscosity and other technical properties. Some, such as native starch, are used as extracted from their sources; others, such as starch ethers, are derived by introducing substituents or undergoing controlled hydrolysis to lower their viscosity. As with other polymeric auxiliaries already discussed, their detailed structure is still not completely understood and the formulae given are only indicative of their structures. Although native starch is less important nowadays as a thickening agent for textile printing, some starch derivatives still make a significant contribution. Starch has two components, both of which are made up of linked α-glucoside units (10.106). In amylose, which accounts for some 20–30% of the polymer and has a relative molecular mass in the range 2–6 × 105 the α-glucoside units are linked in a linear 1,4 arrangement (10.107). Cellulose (10.108), by contrast, consists of β-glucoside chains. In amylopectin (Mr 4.5 × 104 to 4 × 105) the linear α-1,4-linked main chain is randomly branched at the 6-position every 15–30 glucose units to give an α-1,6-anchored side chain (10.109). CH2OH

6

O

OH

OH

1

3

OH

HO

O

5

4

2

10.106 α-Glucoside unit

CH2OH

CH2OH

CH2OH

OH

OH

OH

O

O

HO OH

O

O

O

OH

OH

OH

n

10.107 Amylose

CH2OH

CH2OH

CH2OH

O

O

O

O

O HO OH

OH

OH

OH

OH n

10.108 Cellulose

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OH

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THICKENING AGENTS, MIGRATION INHIBITORS AND HYDROTROPIC AGENTS

CH2OH O

O

O OH CH2OH

OH CH2OH

CH2

O

O

O

OH

O

O

O

O

OH

OH

OH

OH

OH

10.109 Amylopectin

The amylose component is substantially crystalline, forming helical structures that uncoil in an aqueous solution. It can also aggregate to give a gel or precipitate, an undesirable phenomenon known as retrogradation. Amylose is completely hydrolysed by the β-amylase enzyme. Amylopectin is substantially amorphous, having a globular structure that can expand considerably in aqueous solution. Its branched chains give rise to a much more stable solution, substantially free from retrogradation, and it is much more resistant to the action of β-amylase. Starches containing little or no amylose are known as ‘waxy starches’. The properties of starch can be improved from a printing viewpoint by conversion to British Gum (10.110). This is done by a dry roasting treatment at 135–190 °C, accelerated by trace quantities of acid, to give random hydrolysis of the 1,4-links to decrease the chain length but with the formation of 1,6-links (branching). The effect is to increase the solubility and stability although reducing characteristics, which can affect certain susceptible dyes, are enhanced by formation of more aldehyde end groups. Control of the hydrolysis and branching reactions yields a varied range of products.

CHO

CH2OH

O

O

CH2OH O

O

O OH

OH

OH CH2

OH

OH CHO O

O

O

O

O OH

O

O

HO

OH

OH

OH

10.110 British gum

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Besides roasting, other methods of modifying starch are available. Etherification and esterification, to give starch ethers and starch esters, are both practised although the ethers, being resistant to hydrolysis in acidic or alkaline media, are much the more important as thickening agents for textile printing. The starch may be first partially decomposed before etherification and the degree of etherification itself may be varied. The most important products are the carboxymethyl (10.111), hydroxyethyl (10.112) and methyl (10.113) starches (the structures illustrated use the glucose unit as a model, showing the primary hydroxy group substituted). The degree of alkylation is said to be low or high depending on whether it is less or greater than 0.3 substituents per glucose (or other) unit; the products are termed modified starches if the degree of substitution is low and starch derivatives if it is high. Crossbonded starches can be obtained by treating, for example, a starch ether of low degree of substitution with bifunctional agents such as ethylene oxide, propylene oxide, epichlorohydrin or phosphates. The corresponding derivatives of cellulose can also be made and used as thickening agents if the chain length is appropriate. The steric hindrance effect of the substituents gives thickening agents of improved all-round properties and certain derivatives have ousted their parent products in terms of commercial importance. CH2OCH2COOH

CH2OCH2CH2OH

O

O

OH

HO OH

CH2OCH3

OH

OH

HO OH

10.111

O

OH

OH

HO OH

10.112

OH

10.113

Galactomannans are another source of natural thickeners. Structurally related to starches they are polysaccharides composed of main-chain mannose and side-chain galactose units as in 10.114. Typical values are: locust bean gum (m = 3, n = 375) and guar gum (m = 1, n = 440). The distribution of galactose units varies with the source, as shown schematically in 10.115 [354]. Amongst other things, this distribution has an influence on ease of dispersibility. For example, warm water is required to effect complete dispersion of locust bean gum (the 1,4 form) but guar gum (the 1,2 form) disperses readily in cold water because of decreased molecular association arising from the greater frequency of side-chain substitution. As with the starches, modified gums can be obtained. In particular, etherification improves the cold water dispersibility of locust bean gum. In Table 10.37 various derivatives of galactomannans are listed together with their main applications [354]. Locust bean gum forms an interesting and unusual crosslinked complex by association of cis-dihydroxy groups in the mannose chains with borate ions, diagrammatically represented in structure 10.116. This complex forms a gel, which has been made use of in printing with vat dyes in a two-stage fixation process. The crosslinks are relatively weak, being in a state of dynamic equilibrium, and are ruptured in the presence of hydrotropes such as glycerol. The alginates derived from seaweed are of great importance as thickening agents. These are based on alginic acid (10.117; n = 60–600) of which the major commercial salt is sodium alginate, although calcium (particularly in mixture with sodium), magnesium and ammonium alginates, as well as amine salts, are also available. Their exceptionally low

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CH2OH O

HO

O OH

CH2OH

CH2

OH

O

O

O

O

O

OH

OH

OH

OH

m n

10.114 D-Galactomannoglycan

Polymannose M

M

M

M

M

M

M

M

M

M

M

Galactomannan-1,5 (m = 4) Cassia gum from Cassia tora/obtusifolia seeds M

M

M

M

M

G

M

M

M

M

M

G

M G

Galactomannan-1,4 (m = 3) Carob gum from Ceratonia siliqua seeds M

M

M

M

G

M

M

M

M

G

M

M

M

G

Galactomannan-1,3 (m = 2) Tara gum from Cesalpinia spinosa seeds M

M

M

G

M

M

M

G

M

M

M

G

M

M

G

Galactomannan-1,2 (m = 1) Guar gum from Cyamopsis tetragonoloba seeds M

M

G

M

M

G

M

M

G

M

M

G

M

M

G

M G

Galactomannan-1,1 (m = 0) M

M

M

M

M

M

M

M

M

M

M

G

G

G

G

G

G

G

G

G

G

G

10.115

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

Table 10.37 Composition and applications of galactomannan derivatives [354] Galactomannan

Chemical variant

Main applications

-1,2

Unmodified

Carpet printing/dyeing: acid, metal-complex dyes

Depolymerised

Carpet printing/dyeing: acid, metal-complex dyes Cotton, viscose: vat, direct, azoic dyes Polyester: disperse dyes Nylon: acid, metal-complex dyes Acrylic fibres: basic dyes

Hydroxyethylated

Carpet printing/dyeing: acid, metal-complex dyes Cotton: African prints with azoics Polyester: disperse dyes Nylon: acid, metal-complex dyes Acrylic fibres: basic dyes

Hydroxypropylated

Carpet printing/dyeing: acid, metal-complex dyes

-1.4

-1,5

Additionally depolymerised

Sizing

Carboxymethylated

Carpet printing/dyeing: acid, metal-complex dyes Cotton: vat, limited reactive dyes

Hydroxyethylated

Carpet printing/dyeing: acid, metal-complex dyes Cotton: African prints with azoics Polyester: disperse dyes Nylon: acid, metal-complex dyes Acrylic fibres: basic dyes Wool, silk: acid, metal-complex dyes

Carboxymethylated

Cotton, viscose: vat dyes Wool, silk: acid dyes

Hydroxypropylated Additionally depolymerised

Sizing

Carboxymethylated

Carpet printing/dyeing: acid, metal-complex dyes

Additionally depolymerised

Cotton, viscose: vat dyes Wool: acid dyes

O

H

O

O O

HOCH2

O

H B

_ O

O

O

CH2OH

O H

O

H

O

O 10.116

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reactivity with reactive dyes is a special advantage. This is a result of replacement of the primary hydroxy groups by carboxyl groups. As well as being non-reactive towards reactive dyes, the ionised carboxylate anions repel dye anions in alkaline or neutral media. Carboxylated polymers form gels with multivalent metal ions. This behaviour, like the locust bean gum borate complex mentioned earlier, has been exploited in the two-stage flash ageing process for vat dyes in printing. Alginate esters (such as the hydroxypropyl ester) have also been used. COOH

COOH

COOH

O

O

O

OH

O

O HO OH

OH

OH

OH

OH

OH

n

10.117 Alginic acid

Other natural polysaccharides used as thickening agents include gum arabic, gum tragacanth and xanthan gum, but these are of diminishing significance nowadays. Research and development in the area of natural thickeners continues to be active. Detailed studies of the rheology of starch derivatives, alginates, cellulose ethers and vegetable gums [343,344] have shown that thickeners with low solids content form loose networks with low convolution density whilst those with a high solids content show higher convolution densities. This results in differences in tackiness, shear sensitivity and viscoelastic properties, emphasising the major influence of flow properties and viscosity on the quality of prints obtained. Starch carbamate esters and carboxymethyl carob gums of varying degrees of substitution have been evaluated in the vat printing of cotton [355,356]. As the degree of substitution of the carboxymethyl carob gum was increased, the colour strength of the prints increased. Such thickening agents, either alone or in combination with alginates, showed shear thinning behaviour that became thixotropic after storage. Alkalimodified starches have also been assessed in vat printing [357,358]. The alkali-treated starches showed higher aqueous solubility and so were more easily removed during washing off, giving a fabric with a softer handle. Improved colour yields could also be obtained. It has been shown that carboxymethylcellulose thickeners can effectively replace emulsion systems in the application of pigments [359,360]. Research in Russia has been directed towards finding a carboxymethylcellulose thickener for reactive printing that would be efficient and economical, the latter resulting from the fact that low concentrations give excellent printing properties that cannot be achieved with other known thickeners [361]. The rheology of carboxymethylcellulose thickeners, including storage for up to seven days, has been studied in conjunction with their use in reactive printing [351], indicating suitable conditions for achieving optimum penetration, depth of colour, sharpness of outline and evenness of the prints. A review of uses of galactomannan thickening agents, of which some 23 000 tons were used worldwide in 1986, is available [354]. The use of borate ions to crosslink locust bean or guar mannose chains has been mentioned already. It has been shown that addition of borax

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to dry guar gum delays the development of viscosity, making it easier to prepare large stocks for continuous dispensing equipment [362]. The oxidative treatment of carob gum with sodium hypochlorite resulted in improved rheological properties and increased aqueous solubility [363]. No mention was made, however, of possible environmental problems arising from the use of hypochlorite. When applying reactive dyes, it is essential to use a thickener that does not react with the dyes, alginates being by far the most used. The rheological behaviour of alginate thickeners has been studied [346], showing that the gradient rule is not obeyed across the whole range of shear rates. Alginate thickeners may deteriorate on storage, notably as a result of biological attack. The significance of this for printing performance [364] and the addition and effects of bactericidal preservatives (sodium o-chlorophenate or chlorinated m-cresol) have been investigated [364]. Certain preservatives have a marked effect on the hues obtained, this being greater with dry heat than with saturated steam fixation, but formaldehyde does not give these effects [364]. The almost exclusive use of alginates with reactive dyes is threatened by uncertainty over sourcing of raw material and by drastic fluctuations in price and quality [365]. Hence there have been sustained efforts to find alternatives. In one study of various possibilities [366] a synthetic acrylic thickener was chosen, but another investigation [367] showed that although good results can be obtained with synthetic thickeners they cannot fully replace alginates because of poor fastness to rubbing. It has been claimed [368] that carboxymethylated derivatives of cellulose, guar gum or starch are quite suitable for reactive printing and actually give better performance. Presumably the ionised carboxymethyl groups, like the carboxyl groups in sodium alginate, inhibit reaction with the reactive dyes. Indeed, it has been confirmed that carboxymethylated guar gum derivatives do not react with reactive dyes, and that both carboxymethylated and unsubstituted guar thickeners give satisfactory results with vinylsulphone reactive dyes, especially for pale grounds and fine outlines [365]. 10.8.2 Synthetic thickeners Polymers based on acrylic acid have been known since the 1930s but it was not until the late 1970s that the use of thickening agents based on them came into prominence in textile printing [369]. Typical repeat units are shown in 10.118 and 10.119. Commercial linear products represented by 10.118 can have n = 50–750 but crosslinked grades of higher relative molecular mass are also available. The products represented by 10.119 cover a range of n values from 3200 to 30 000. Only the longer-chain grades are of significant interest for textile printing in the form of their sodium or ammonium salts. However, the scope for

O

OH

O

C CH2

CH

CH

CH2

CH2

CH

C OH

O

10.118

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OH C

CH2

CH

C n

O

OH n

10.119

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O

O

OH C

CH2

OH C

CH

CH2

CH

CH2

CH 100

100

CH

CH

CH2

CH2

CH

CH2

CH

C

CH2

CH

100

O

OH

100

CH2 10.120

C O

CH2

C OH

O

CH

655

OH

100

forming copolymers and crosslinked variants is virtually limitless. A schematic representation of a crosslinked copolymer, using acrylic acid and divinylbenzene at a molar ratio of 100:1, is shown in 10.120 [370]. It is important to make a clear distinction between acrylic binders used in pigment printing and acrylic thickeners. Binders are generally copolymers and usually contain an integral crosslinking agent [371]. The thickeners also find their greatest use in pigment printing. Their biggest drawback is their sensitivity to electrolytes, although this is less of a problem in pigment printing than in printing with dyes. The sensitivity of poly(acrylic acid) to electrolytes can be reduced by copolymerising with acrylamide [371], although only relatively small proportions can be incorporated before a deterioration in thickening efficiency occurs. Two important and interrelated parameters for acrylic thickeners are relative molecular mass and degree of crosslinking. Simply increasing the molecular mass of linear poly(acrylic acid) yields thickeners that give stringy pastes unsuitable for use in printing. Hence a degree of crosslinking is necessary to minimise stringiness by decreasing the water solubility and promoting dispersibility. Figure 10.50 illustrates the effect of crosslinking for three acrylic acid polymers of the same molecular mass [371]. The balance of molecular mass and degree of crosslinking influences other properties, such as degree of penetration, levelness of ground colours and sharpness of the print. These products are usually supplied to the printer as partially neutralised polyacids. Further neutralisation is carried out by the printer when making up the print pastes. This neutralisation is often a critical process. For certain applications, as with resin-bonded pigments, neutralisation is carried out with ammonia. This has the advantage that during subsequent baking the ammonia is driven off to liberate the free polyacid, which then catalyses activation of the resin binder. In other cases neutralisation is carried out with nonvolatile alkalis such as sodium hydroxide. It is particularly important to use the latter in reactive printing, since ammonia would be evaporated off during fixation leading to a lowering of pH and consequently poor fixation. Moreover, reactive dyes can be deactivated by reaction with ammonia to form their non-reactive amino derivatives. The commercial success of acrylic thickeners in pigment printing is attributable to the

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A

B

Viscosity

C

A B C

Highly crosslinked Moderately crosslinked Slightly crosslinked

Thickener concentration

Figure 10.50 Effect of crosslinking on thickener efficiency [371]

fact that they can be designed to give properties very similar to those of emulsion thickenings. These were previously the only systems used in pigment printing and they are dealt with in section 10.8.3. It is important to realise that an acrylic thickener intended for use with pigment systems may be unsuitable for use with dyes. This is because commercial thickeners, available as solutions, emulsions, liquid dispersions or powders, often contain additional chemicals to improve their stability and performance in particular systems. For pigment systems, for example, the thickener may also contain additives (surfactants or polyelectrolyte dispersing assistants), such as the ammonium or sodium salt of linear poly(acrylic acid) (10.121), which not only modify the behaviour of the acrylic thickener but also assist dispersion of the pigment [371]. Surfactant additions are undesirable with reactive dyes because they promote colour bleeding, whilst the ammonia is undesirable because of deactivation of reactive groups, the lowering of pH that occurs by its evolution during the fixation process and the subsequent difficulty in washing-off of the residual thickener, now bereft of its solubilising ammonium ions.

CH2

CH C O

_ O M+

n

M = Na or NH4

10.121

A major drawback of synthetic thickeners when used with dyes is their sensitivity to electrolytes. Most soluble dyes behave as highly ionised electrolytes and disperse dyes contain anionic polyelectrolyte dispersing agents unless they have been formulated with nonionic systems specifically for use with acrylic thickeners. Consequently there is a loss of viscosity; this can be quite pronounced although it depends on circumstances, particularly on the dye concentration. As already mentioned, this can be alleviated to some extent by copolymerisation with acrylamide during manufacture. Otherwise it is necessary to try to eliminate all electrolytes from the system or to increase the concentration of thickener. Such measures have their limitations in practice, however. Alternative synthetic thickening

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agents include poly(vinyl alcohol) and copolymers of maleic anhydride with alkenes (10.122). CH2

CH2

CH

CH

C O

C O

O

n

10.122

A detailed comparative study of the rheological properties of four acrylic thickeners varying in relative molecular mass from 1.25 × 10 6 to 4 × 10 6 and of two crosslinked ethylene-maleic anhydride copolymers has been published [345]. In respect of some properties, comparisons were also made with a starch ether and an alginate. Amongst other factors, the influence of molecular mass was demonstrated (Figure 10.51), showing that the higher the molecular mass of the acrylic polymer, the less the amount of thickener required to achieve a given viscosity. Nevertheless, earlier comments in relation to the stringiness of linear acrylic polymers should be borne in mind, i.e. factors other than viscosity need to be considered. Viscosity develops as water is absorbed, causing swelling and rearrangement of the polymer chains, a process that is assisted by, indeed is critically dependent on, neutralisation. Figure 10.52 gives a schematic representation of this swelling and also illustrates the similarity in behaviour with oil-in-water emulsions [371]. It is important that the degree of crosslinking is the optimum required to maintain the polymer in this swollen state and prevent it from dissolving, which would result in loss of desirable properties. 50

Acrylic thickeners Mr 4 × 106 Mr 3 × 106 Mr 1.75 × 106 Mr 1.25 × 106

Viscosity/Pa s

40

30

Crosslinked ethylene– maleic anhydride copolymers

20

No 1 No 2

10

0.2

0.6

1.0

1.4

Thickness concentration/%

Figure 10.51 Comparison of rheological behaviour of acrylic and copolymeric thickeners [345]

Base

Dispersed polyacid/pH 4

Neutralised, swollen polyacid/pH 9

Kerosene emulsion in water

Figure 10.52 Schematic diagram illustrating swelling of polyacid thickener and comparison with oil-inwater emulsion [371]

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A novel concept [371] was evaluated to seek a replacement for alginates in printing with reactive dyes. This approach utilised copolymers of poly(acrylic acid) onto which starch has been grafted by free radical polymerisation, the free radical initiator being a potassium persulphate redox system. Both native starch and starch pre-oxidised with sodium hypochlorite were included in the study. It was found that an acrylic–starch graft copolymer effectively replaced 25% sodium alginate, whilst a copolymer of poly(acrylic acid) and oxidised starch replaced 50% sodium alginate. A detailed rheological study was presented but the evaluation lacked economic analysis. 10.8.3 Emulsion thickeners When immiscible liquids are emulsified the viscosity increases and this can be exploited to prepare thickenings for textile printing. The emulsions used contain a hydrocarbon solvent (usually white spirit), surfactant(s) and water; the oil phase must account for at least 70% of the total volume [342]. The first pigment printing systems introduced in the late 1930s were water-in-oil emulsions (that is, at least 70% of the product was water) in which typical surfactants were ethoxylated alcohols, acids or amides with a low degree of ethoxylation, perhaps 5–8 oxyethylene units per molecule, morpholine/oleic acid or lauric, palmitic and stearic acid esters of sorbitol. Later, manufacturers developed oil-in-water emulsions for which appropriate surfactants are higher ethoxylated alcohols, acids or amides, or a wide variety of alkylaryl types. For any type of emulsion, the HLB of the emulsifying agent(s) is clearly of great importance. The size of the droplets in an emulsion is inversely related to its viscosity, typical diameters ranging from 100 to 7000 nm. Theoretically no more than 75% of oil can be incorporated in an aqueous emulsion, assuming uniformly spherical droplets, but distortion due to packing allows significantly higher proportions of oil phase to be added. Presumably the oil droplets are stabilised by a surrounding layer of like charges, the type and strength of the charge depending on the surfactant(s) used. Consequently the stability of the emulsion tends to be impaired by any additions that reduce the charge on the droplets. Emulsion thickeners can be mixed with low concentrations of either natural or synthetic thickeners, especially when applying fibre-substantive dyes rather than pigments; these additions act as film formers, taking the place of the binder used with pigments to increase retention of the dye by the substrate prior to fixation.

10.8.4 Continuous dyeing The foregoing discussion has concentrated on the use of thickening agents in textile printing. Similar types of product are used to thicken pad liquors in continuous dyeing processes, although then they are normally described as migration inhibitors rather than thickening agents. All polysaccharides mentioned previously, especially the alginates, locust bean, guar and xanthan gums, modified starches and celluloses, can be used in continuous dyeing, but by far the most widely used are the alginates. Concentrations tend to be significantly lower than in printing since the dyeing process requires a lower viscosity, permitting rapid and complete penetration into the fabric during padding. In addition to the alginates, polyacrylates (10.8.2), polyacrylamides and polyethoxylates are also used [373]. Polyethoxylates are mixed polyglycol ethers of fatty alcohols with ethylene oxide (or

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propylene oxide), the nonionic block copolymers described in section 9.6 that operate by virtue of a low cloud point (about 25 °C). The functions of these products are two-fold: (a) they should favour the uniform application of dye by padding; and (b) they should effectively inhibit any tendency of the dye to migrate during the subsequent intermediate drying process. In the absence of an inhibitor, dye liquor tends to migrate towards hotter regions of the fabric during drying, causing either patchiness or an undesirable two-sided effect. Whilst viscosity plays an important part in facilitating the uniform absorption of dye liquor, it is less effective than coagulation for inhibiting migration. The polyacrylates are particularly effective [373]. In continuous dyeing agents to assist rapid wetting and even penetration of the fabric are invariably added to pad liquors along with the migration inhibitor. Many types of surfactant can be used, including phosphate esters (which are particularly effective [373]), sulphonates, sulphates, sulphosuccinates and sulphated ethoxylates. Care should be taken to ensure that the type and concentration of product chosen do not depress the degree of fixation of the dyes. Wetting agents are rarely used in printing since they would tend to promote bleeding or haloing of printed areas. Methods available for assessing migration inhibitors have been reviewed [374,375]. Factors influencing dye migration during the drying phase include fibre type, liquor pick-up, drying temperature, running speed, type of dyes and the various pad liquor additives present. Those additives intended to increase substantivity and agglomeration of the dyes tend to inhibit migration. Interaction between dyes is demonstrated by the finding [376] that, using an alginate migration inhibitor on 50:50 polyester/cotton, the migration of an individual dye can be inhibited by the presence of other dyes in the mixture that have larger particle sizes or a greater tendency to flocculate. However, increasing the concentration of migration inhibitor progressively neutralises this tendency. When the concentration is high enough, specificity of particulate migration is eliminated as the migration of dyes in the mixture approaches zero. Polyacrylamides (10.123) of chain length (n) 7000 to 14 000, which is higher than normally suitable for migration inhibitors, are useful pad liquor additives [373] in that they increase liquor pick-up and sometimes colour yield, notably with pigments. When used with azoic dyes on cellulosic fabrics, they can eliminate the need for the intermediate drying process. In a study [377] using polyacrylonitrile saponified with alkali to form amide groups capable of being crosslinked with formaldehyde, it was found that migration during drying decreases as the degree of crosslinking is increased, this being attributed to the increased structural density of the polymer films. CH2

CH C O

NH2

n

10.123

The continuous dyeing of polyester/cotton blends inevitably results in staining of the cotton by disperse dyes, this effect being greatly influenced by the chemical nature and concentration of the migration inhibitor, the dyebath pH and the chemical nature and concentration of the dyes, whereas the presence of wetting agent or neutral electrolyte does not have much influence [378]. Normally in the dyeing of polyester/cotton blends a higher

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dyeing temperature minimises staining of cotton, since the higher temperature facilitates migration of disperse dye from the cotton to the polyester. Migration inhibitors can negate this effect, however, so it is important to establish the optimum agent concentration necessary to prevent migration but avoiding excessive concentrations that could lead to increased staining of the cotton. It has been shown that xanthan gum is an effective migration inhibitor for the application of water-soluble chemicals, leading to uniform distribution and more reproducible fixation [379]. Although this work was specifically concerned with the application of a soluble flame retardant to polyester, suitability for the application of reactive dyes or resin finishes is also claimed. 10.8.5 Hydrotropic agents In many printing and some continuous dyeing processes the colour yield of dyes can often be improved, sometimes markedly, by the use of an auxiliary that tends to increase the aqueous solubility of the dye, particularly when using highly concentrated pastes or liquors that tend to lose moisture under adverse conditions. There is clearly an analogy here with the mechanism of solubilisation discussed in section 10.6.2, since the hydrotrope acts as an amphiphilic bridge between the dye solubilisate and the aqueous medium. Surfactants can therefore function as hydrotropes and are sometimes used as such in continuous dyeing processes. Hydrotropes with much less powerfully surface-active properties are more suitable for use in printing, where surfactants are normally avoided because their concomitant powerful wetting properties would promote bleeding and haloing of the print. By far the most important of these compounds are urea (10.124) and thiourea (10.125). However, dye–fibre systems are so varied that many hydrotropes are of interest under specific conditions. Typical examples that have been mentioned [380] include: triethanolamine (10.126), N,N-diethylethanolamine (10.127), sodium N-benzy1sulphanilate (10.128), sodium N,N-dibenzy1sulphanilate (10.129), ethanol, phenol (10.130), benzyl alcohol (10.131), resorcinol (10.132), cyclohexanol (10.133), ethylene glycol, glycolic acid (10.134), 2-ethoxyethanol (10.135), diethylene glycol (10.136), 2-ethoxyethoxyethanol (10.137), 2-butoxyethoxyethanol (10.138), thiodiethylene glycol (10.139) and glycerol (10.140), the last-named in particular being useful with practically all classes of dyes. H2N

HOCH2CH2

H2N

C H2N

O

C

N

S

H2N

CH3CH2

CH2CH2OH

N

HOCH2CH2

CH2CH2OH

CH3CH2 10.124

10.125 Thiourea

Urea

10.126 Triethanolamine

10.127

CH2 N

H2C HN

SO3Na

10.128

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CH2 10.129

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HO

CH2OH

OH

OH OH

10.131

10.130

10.133

Phenol

Cyclohexanol

10.132 Resorcinol

HOCH 2COOH

HOCH2CH2OCH2CH2OH

CH3CH2OCH2CH2OH 10.135

10.134

CH3CH2OCH2CH2OCH2CH2OH

10.136

CH3CH2CH2CH2OCH2CH2OCH2CH2OH

10.137

10.138 CH2OH HOHC CH2OH

HOCH2CH2SCH2CH2OH

10.140

10.139

Glycerol

Most hydrotropic agents, though not surfactants in the usual sense of the word, do significantly lower the surface tension of water and this is an important prerequisite for their solubilising action. Hydrogen bonds, together with weaker dipolar and van der Waals forces, contribute to this interaction, the active centres in the hydrotropic molecules being protondonating groups (such as hydroxy, amino and amido groups) and proton-accepting atoms (such as the nitrogen atom of a tertiary amine). For this reason hydrotropes can also be added to the diluent system in the manufacture of certain dyes, particularly liquid brands, in which they not only increase the apparent solubility of the dye but also help to prevent the formation of surface skin. Another compound that has entered this area is 1-cyanoguanidine (10.141) popularly known as dicyandiamide. The main mechanisms whereby hydrotropes are believed to function have been mentioned above. However, there has been a good deal of discussion regarding possible mechanisms, mainly in connection with the interaction of urea with reactive dyes. Useful reviews of this topic are available [381,382]. H2N C

NH

CN

HN 10.141 Dicyandiamide

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The growth of environmental awareness has certainly impinged on this area, restricting the use of hydrotropes that are volatile during steaming or dry heat treatments, as well as those suspected of endangering health. These problems are particularly acute with urea, widely used for a long time and considered an essential hydrotropic assistant in printing and continuous dyeing with reactive dyes. This is considered in more detail in section 12.7.1. 10.8.6 Environmental aspects Reference to Table 10.10 in section 10.5.2 indicates printing to be responsible for some 10– 20% of the total pollution load from the textile wet processing of cotton. Other substrates, as well as continuous dyeing, also make significant contributions. Useful reviews of the environmental aspects of textile printing generally [383,384] and of pigment printing in particular [385,386] are available. There are two aspects to be considered: (1) airborne pollution from volatile components during steaming and dry heat treatments (2) aqueous effluent pollution from washing-off and washdown procedures. In both cases there are two basic methods of control: (1) elimination or minimisation of offending products, which may involve total or partial substitution by more benign products, or a redesigned process that does not require that type of auxiliary (2) remedial treatment of exhaust gases or effluent. In any case, it is always sound practice to optimise the concentration of an auxiliary so that no more than necessary is applied. This aim of using minimum quantities is assisted by the trend towards more efficient thickeners. For example, in the 1960s it was common for thickeners to be made up to a 20–30% concentration, whereas today 10% is more common and it is predicted that this will fall to an average of about 5% within the next decade [383]. Airborne pollution can arise from volatile fractions during drying, as well as dry heat curing or heat setting treatments. This aspect has been associated mostly, but not exclusively, with the use of emulsion thickeners in pigment printing, considerable volumes of organic compounds being released during drying and curing. Consequently, emulsion printing is now seldom carried out in the major developed countries [383]. Those acrylic thickeners that contain organic solvents or hydrocarbons are also sources of airborne pollution, but thickeners free from such additives are now available [386]. One method of dealing with the volatile emissions is incineration, but this is very costly. Scrubbing is highly efficient with water-soluble volatiles but not with hydrocarbons: hence it is best to avoid using polluting materials. A detailed account of the ecological factors in pigment printing is available [385], indicating the obligation to minimise or eliminate emissions of residual monomers, formaldehyde and unwanted solvents. Furthermore, alkylphenol ethoxylates formerly used as emulsifiers can be replaced by fatty alcohol ethoxylates or, to some extent, by anionic surfactants. Pigment printing also offers ecological benefits over other systems in that it saves time and energy, and washing-off is unnecessary [385]. In general (excluding pigment printing), however, the thickeners or migration inhibitors are present in waste waters, both from washing-off the dyeings or prints and from washingdown of equipment. The usual three methods of treatment (chemical degradation, bioelimination and recycling) have to be considered for dealing with them. Natural

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thickeners are not toxic in themselves but, in parallel with the analogous sizing agents (Table 10.11 in section 10.5.2), they show high chemical and biological oxygen demand [354,383,387]. Galactomannans [216,217,354] in particular are thought to have a good future in respect of environmental impact. Since the naturally occurring polymers present the least difficulties in environmental terms, it is thought that their derivatives will become less important [383]. The treatment and disposal of these wastes is not only expensive but entails the loss of potentially useful materials. Thus, as with sizing agents, recycling is an attractive consideration [388,389]. The basic principle is precipitation of the thickener in a suitable aqueous solvent system followed by isolation, often by ultrafiltration. Dyes can be separated from the thickener either continuously or discontinuously [388]. In a detailed study [389], the efficiency of precipitation of high-, moderate- and low-viscosity alginate thickeners in four solvent systems was compared with that of carboxymethylated galactomannans or celluloses and four synthetic thickeners, giving the results in Table 10.38. Thus all types of alginate thickener can be isolated quantitatively from the wash water and recycled without difficulty. These recycled thickeners are claimed [388,389] to give rheological properties and printing results equivalent to those of the original products. Pure acrylic thickeners can also be precipitated almost quantitatively [389]. Table 10.38 Precipitation (%) of printing thickeners using solvents [389]

Thickener

Ethanol

Low-viscosity alginate Moderate-viscosity alginate High-viscosity alginate

100 100 100

100 100 100

100 100 100

100 100 100

Carboxymethylgalactomannan Low-viscosity High-viscosity

>90 >90

>90 >90

>90 >90

70–90 70–90

Carboxymethylcellulose Low-viscosity High-viscosity

>90 >90

>90 >90

>90 >90

55–90 55–90

70–90 50 0

70–80 50 0

70–95 50–70 0

<20 <20 0

>90

>90

>90

>90

Synthetic thickener 1 Synthetic thickener 2 Synthetic thickener 3 Poly(acrylic acid)

Acetone

Aqueous methanol

Methanol

Natural thickeners are prone to biological attack and degradation, leading to a loss of thickening efficiency. It is common practice to add a small amount of a bactericide to protect against attack during storage [383,387,390]. About thirty classes of compounds have been listed as bactericides [387], but phenol derivatives (including chlorinated phenol, m-cresol or o-phenylphenol) and formaldehyde have proved particularly suitable, added either by the manufacturer of the thickening agent or by the printer during formulation of the stock thickener. As bactericides, such compounds are by definition toxic and hence

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environmentally undesirable if not actually prohibited in some countries. However, it has been pointed out [383] that they are used only at very low concentrations (usually below 0.1%) that are just about sufficient for bacterial efficiency. Together with the thickener they are then washed out with copious amounts of water, entering the effluent at such high dilution that they no longer have bactericidal action and can even be biologically eliminated during effluent treatment. Mention has already been made of the environmental undesirability of many hydrotropes (section 10.8.5) and this particularly applies to urea, the most widely used. This is discussed in more detail in section 12.7.1 in connection with reactive dyes. Suffice to note that the efficiency of urea as a hydrotrope is proving difficult to equal in every respect with substitutes that are environmentally acceptable [382]. 10.9 TREATMENTS TO ALTER DYEING PROPERTIES OR ENHANCE FASTNESS Treatments to improve the intrinsic fastness of dyeings have a long and prolific history. Most of these are aftertreatments, although some are concurrent with the dyeing process as in the application of UV absorbers to enhance light fastness. The treatments described in this section are essentially concerned with improving fastness properties or fibre stability. Treatments to modify the handle or other fibre properties are excluded, these being dealt with in section 10.10. Furthermore, such treatments are not an essential integral part of the dyeing process, thus excluding the afterchroming of chrome dyes and the oxidation of sulphur and vat dyeings. Many processes have been developed for improving fastness but relatively few are still in commercial use today. Their efficacy has to outweigh the extra processing cost and time. The emphasis will be on processes of current commercial or research interest. An excellent comprehensive review covering the period 1880–1980 is already available [391]. Of growing interest, at least in the research sector, are fibre pretreatments aimed primarily at modifying dyeing properties, although these may have secondary benefits in terms of improved fastness. There has always been a good deal of interest in pre- and aftertreatments amongst textile chemists and colourists, largely on account of the innate interest of the chemistry and the challenges involved. Commercial processors have been less enthusiastic, however, since these variations involve additional costs and scheduling difficulties. 10.9.1 Pretreatments One of the earliest fibre pretreatments for improving the dyeability of cotton is of course mercerisation (section 10.5.4). However, more recent research interest in this area has been generated by environmental concerns about reactive dyeing, aiming to enhance substantivity for the modified fibre so that higher absorption and fixation are obtained. This results in less dye (hydrolysed or still active) in the effluent. A further objective is to minimise the usage of electrolyte in the application process. This area has been thoroughly reviewed [392,393]. At the risk of over-simplification, most of the pretreatments covered by Lewis and McIlroy will be summarised in Tables 10.39 to 10.42. Their review [393] and the original works cited in it should be consulted for details, of course. The great majority of processes

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involve increasing the nitrogen content (i.e. the basicity) of the cellulose, there being an analogy here with the better dye-sorption properties of wool that naturally contains such built-in nitrogen. Table 10.39 presents structural modifications of cellulose not entailing the application of a polymer. Most of these reactions involve amination in one form or another. In fact interest in the amination of cellulose began in the 1920s, long predating current environmental concerns regarding reactive dyes. Hence it is not surprising that some of the earliest processes are environmentally suspect. Nevertheless, more acceptable processes have evolved, making cellulose more substantive to anionic dyes and enabling much less electrolyte and non-alkaline fixation conditions to be adopted for reactive dyeing. It has been pointed out that aminoethylcellulose produced by a two-stage dry process using 2-aminoethylsulphuric acid can even react covalently with hydrolysed vinylsulphone dyes, thus enhancing fixation and making washing-off easier [394]. Another cationic quaternary compound that has been used [395] is 3-chloro-2-hydroxypropyltrimethylammonium chloride (10.142). This compound and four analogous reactive cationic agents (10.143) have been proposed [396], prepared by reaction of the appropriate amine with epichlorohydrin. The cationisation of cotton cellulose using these agents takes place in two stages (Schemes 10.49 and 10.50), although from a practical viewpoint these occur concurrently in a single process by exhaustion for 20 minutes at 80 °C in the presence of sodium hydroxide as the base catalyst. In the first stage an epoxide is formed in the presence of alkali. In the second stage this epoxide reacts with a hydroxy group in the cellulose. The cationised cotton could be dyed with anionic dyes; only CI Acid Red 127 was used in this research. The degree of substitution of the cellulose and the amount of dye absorbed both decreased as the length of the hydrocarbon chains attached to the nitrogen atom was increased. Regardless of hydrocarbon chain length, the light fastness was slightly higher on cotton than on nylon or wool dyed with the same dye. Fastness to washing decreased with increasing length of the hydrocarbon chains. The cationised cotton showed enhanced antibacterial properties, the potency increasing with increasing hydrocarbon chain length [396].

CH3

CH3 + N CH2 CH3

CH2Cl _

Cl

CH OH

10.142

R2

R1 + N CH2 _ R3 Cl

CH2Cl CH

Trimethylamine:

R1 = R2 = R3 = CH3

Triethylamine:

R1 = R2 = R3 = CH3CH2

Tripropylamine:

R1 = R2 = R3 = CH3CH2CH2

Tripentylamine:

R1 = R2 = R3 = CH3(CH2)4

OH

10.143

Dimethyltetradecylamine: R1 = R2 = CH3 R3 = CH3(CH2)13

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R

R + N CH2 _ R Cl

CH2Cl

_ HO

CH

R

OH

R + N CH2 _ R Cl

CH2Cl CH

_ O

R

CH2

+ R

Scheme 10.49 R R

N

+

CH

N

CH2

R

Cl

_

_ + Cl

O

CH2 CH2

R

Cl

_

CH

+

HO

[cellulose]

O R

Scheme 10.50

R + N CH2 _ R Cl

CH2

O

[cellulose]

CH OH

Table 10.39 Pretreatments to modify cotton cellulose by substitution reactions [393] Pretreatment reaction Amination

Method

Comments

Tosylation, followed by treatment with amines (Scheme 10.51)

All amination treatments give improved dyeability

p-Nitrobenzoyl chloride, followed by reduction of nitro (Scheme 10.52) Acetylaminobenzenesulphonyl chloride in nitrobenzene or chloroform, followed by hydrolysis of the amide (Scheme 10.53) 2-Chloroethylamine and alkali-treated cellulose (Scheme 10.54) Sodium 2-aminoethylsulphate in aqueous alkali (Scheme 10.55)

More efficient in producing 2-aminoethylated cellulose. Relatively inexpensive and doesn’t require organic solvent

Sodium borohydride added to a sodium 2-aminoethylsulphate pad liquor

Gives white aminated cotton. Borohydride reduces yellow Schiff bases formed by reaction of aldehyde groups with amino groups

Polymerisation of ethyleneimine on fibre by various methods

Improved dyeability

Diethylaminoethylation 2-Chloroethyldiethylamine hydrochloride (Scheme 10.56)

Dyeable in absence of salt with direct, reactive or acid dyes Continued on next page

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Table 10.39 Continued Pretreatment reaction

Method

Comments

Esterification

Inorganic or organic acids

Improved dyeability

Esterification and amination

3-Chloropropionyl chloride, followed by amine (Scheme 10.57)

Primary, secondary, tertiary and quaternary derivatives can be produced. Dyeable with reactive dyes, neutral to slightly acidic without salt

Amination

Epoxides in alkali, including ethylene oxide, propylene oxide, glycidol (2,3-epoxypropan-l-ol). Scheme 10.58 shows glycidyltrimethylammonium chloride

Glycidyltrimethylammonium chloride marketed to enhance dyeability with direct and reactive dyes

Introduction of quaternary N groups

1,1-Dimethyl-3-hydroxyazetidinium chloride in presence of strong alkali by pad–bake (Scheme 10.59)

Dyeable with reactive dyes at pH 7 without salt, giving extremely high fixation

Acylation

Nicotinoyl-thioglycolate and alkali by pad–bake (Scheme 10.60)

Dyeable with monochlorotriazine reactive dyes at pH 3 without salt

Amination

N-Methylolacrylamide in presence of Lewis acid catalyst. Further modifications possible by addition to double bond (Scheme 10.61)

Improved dyeability with dichlorotriazine dyes at pH 5 without salt, giving 99% fixation

Amines with durable press resins

Some improvements in dyeability, especially with direct dyes, but light fastness can be a problem

[cellulose]

OH

[cellulose]

O

+ Cl

SO2

SO2

CH3

CH3

+ HCl

2 NH3

[cellulose]

NH2

+ + NH4

O3S

CH3

Scheme 10.51

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[cellulose]

OH

Cl

+

C

NO2

O

[cellulose]

O

C

NO2

+ HCl

O reduction

[cellulose]

O

C

NH2

O Scheme 10.52

[cellulose]

OH

[cellulose]

O

+

Cl

SO2

SO2

NHCOCH3

NHCOCH3

+ HCl

H2O

O [cellulose]

O

SO2

NH2

C

+ HO

Scheme 10.53

[cellulose]

CH3

OH

+ ClCH2CH2NH2

[cellulose]

O

CH2CH2NH2

Scheme 10.54

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TREATMENTS TO ALTER DYEING PROPERTIES OR ENHANCE FASTNESS

[cellulose]

669

OH + H2NCH2CH2OSO3Na

130°C NaOH 15 min

[cellulose]

O

CH2CH2NH2 + Na2SO4 + H2O

Scheme 10.55

CH3CH2 + NH

_

+

CH2CH2Cl

HO

[cellulose]

CH3CH2 Cl NaOH CH3CH2 N

CH2CH2

O

[cellulose] + NaCl + H2O

CH3CH2

Scheme 10.56

[cellulose]

OH

+

Cl

C

CH2CH2Cl

O NaOH

[cellulose]

O

C

CH2CH2Cl

+ NaCl + H2O

O

[cellulose]

O

C O

+ CH2CH2Cl + NHR3 _ Cl

100°C

[cellulose]

+ NR3 + HO _ Cl

C

CH2CH2Cl

O

Scheme 10.57

CH3

CH3 + N CH2 CH3

Cl

CH2 + HO

CH _

[cellulose]

O CH3

CH3 + N CH2 _ CH3 Cl

CH2 CH OH

Scheme 10.58

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O

[cellulose]

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

H3C

H3C + CH2 N CH2 H3C _ Cl

CH2 N

H

+ Cl

H3C

CH H2C

O

CH

OH

Scheme 10.59

N

+ C

S

HO

_

[cellulose]

N

HO

+ HS

CH2COONa

C

O

O

CH2COONa

[cellulose]

O

N

_ HO

+ H2O C

S

+ HS

N

CH2COONa

C

O

CH2COONa

OH

O

Scheme 10.60 O [cellulose]

OH

+

HO

CH2

NH

C

CH

CH2

ZnCl2, 150 °C O [cellulose]

O

Reaction with

CH2

NH

C

CH

CH2

+ H2O

Product O

Ammonia

[cellulose]

O

CH2

NH

C

CH2CH2

NH2

CH2CH2

NHCH3

CH2CH2

N(CH3)2

CH2CH2

_ + N(CH3)3X

CH2CH2

NHCH2CH2OH

O Methylamine

[cellulose]

O

CH2

NH

C O

Dimethylamine

[cellulose]

O

CH2

NH

C O

Trimethylamine.HX

[cellulose]

O

CH2

NH

C O

Ethanolamine

[cellulose]

O

CH2

NH

C

Scheme 10.61

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A somewhat different substitution reaction involved treating cotton with an aqueous solution containing 10 ml/l carbon disulphide, 1% sodium hydroxide and 0.5% wetting agent [397]. This thiocarbonate pretreatment was followed by dyeing with a direct dye in the presence of ammonium persulphate as free radical initiator, giving higher wet fastness than on alkali-treated or untreated cotton. This was explained in terms of involvement of the thiocarbonate groups in the formation of covalent bonds between cellulose and the direct dye by a free radical mechanism. Table 10.40 represents those pretreatments requiring application of a polymer, the fundamental objective again being to increase basicity by incorporation of amino groups. A point of interest is use of the cationic polymer Hercosett 125 (Hercules), formed by condensation of adipic acid and diethylenetriamine to give a polyamide subsequently partially crosslinked with epichlorohydrin, a resin more commonly associated with shrinkresist treatments of wool. Once again, improvements in dyeability are usually claimed but retaining good light fastness appears to be a common problem. Fibre-reactive quaternary ammonium compounds with chlorohydrin functionality have also been evaluated as epoxide pretreatments for wool/cotton blends in an attempt to facilitate union dyeing using anionic dyes for wool [398]. Since alkaline conditions were required, the integrity of the wool was preserved by treatment at ambient temperature for 3 hours at pH 11. Cotton grafted with 2-vinylpyridine followed by quaternisation using an excess of an alkyl bromide or epichlorohydrin showed markedly increased exhaustion with direct dyes [399]. Grafting alone gave a substantial effect, with further slight improvements being conferred by the quaternisation. Improved fastness to washing was also claimed.

Table 10.40 Pretreatments to modify cellulose using amino-containing polymers [393] Pretreatment polymer

Method

Comments

Chitosan

Cationic polymer applied by exhaustion from acidic solution

Improved coverage of immature fibres, increased exhaustion but decreased fastness unless further treated with fibre-reactive quaternary compound

Sandene (Clariant)

Cationic polymer applied by exhaustion under alkaline conditions

Enhanced dyeability with anionic and reactive dyes, the latter applied under neutral or slightly acidic conditions. Reduced light fastness and marked dulling with some dyes

Hercosett 125 (Hercules) reactive cationic polymer formed by condensation of adipic acid and diethylenetriamine, then partially crosslinked with epichlorohydrin

Pad-dry-cure for 3 min at 100 °C. Scheme 10.62 represents the reactive and nucleophilic sites that may exist on the surface of the treated fibre

Dyeable neutral without salt; good results with some high-reactivity dyes (dichlorotriazine and difluoropyrimidine) but not with some other types (monochlorotriazine and dichloroquinoxaline). Washing fastness very good but light fastness lower

Continued on next page

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

Table 10.40 Continued Pretreatment polymer

Method

Comments

Thiourea derivative of Hercosett (isothiouronium salt)

Scheme 10.63

Build-up good for dichlorotriazine dyes. Dyes of other types gave good results up to about 2% applied depths

Ethylenediamine derivative of Hercosett

Scheme 10.64

About 95% fixation of low- or high-reactivity dyes under slightly acidic conditions without salt, but light fastness still inferior

Polyepichlorohydrin and dimethylamine

Polymerisation of epichlorohydrin Good yields with direct dyes using in carbon tetrachloride with only 2 g/l salt. Excellent build-up boron trifluoride/ether catalyst, with most reactive dyes; only 10% then reaction with dimethylamine. of normal salt usage needed for Applied to cotton by exhaust low-reactivity dyes and none for method or pad–dry. highly reactive types. Washing Scheme 10.65 fastness very good but light fastness impaired. + CH2 N CH2 _ Cl

NH

N

CH2

CH

CH2

+ H _ HO

OH

OH

Azetidinium cation

+ NH2

+ 2H _ 2 HO

N

CH

+ NH

Secondary amino

CH2

CH

CH2

+ NH

OH Tertiary amino, secondary hydroxy

[cellulose]

OH

[cellulose]

O

Cellulose hydroxy

CH2

CH

CH2

N

Covalent link to cellulose

OH

Scheme 10.62

+ CH2 N _ CH2

NH2 CH

OH

+

S

C

N NH2

Cl Thiourea

Azetidinium cation

CH2

CH

CH2

OH

Isothiouronium salt

Scheme 10.63

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NH2 + S C _ NH2 Cl

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+ CH2 N _ CH2

CH

OH

+ NH _ Cl

CH2

CH

+

H2N

CH2CH2

NH2

NH

CH2CH2

NH2

673

Cl

+ CH2 N _ CH2

CH

CH2 OH

OH

+

H2NCH2CH2NH2

+

CH2 CH CH2

HO

Cl

N+ _ Cl

+ NH

CH2NHCH2CH2NH CH2 CH2

_

CH

CH HO

OH

Cl

Cl

Scheme 10.64

O H2C

CH CH2Cl

BF3 , EtOEt

CH2

+ NH _

CH2

CH

CCl4

O

CH2Cl

n

Epichlorohydrin CH3 HN CH3 CH2

CH CH2

O + NH

CH3 _ CH3 Cl

n

Scheme 10.65

Certain pretreatments depend on introducing sulphur rather than nitrogen, these being summarised in Table 10.41. So far these appear to have been much less successful than nitrogenous treatments. A more radical approach to pretreatment reverses the conventional reactive dyeing concept by preparing a reactive cotton cellulose capable of reacting covalently with suitable dyes containing, for example, aliphatic amino groups. In an initial attempt, cotton was

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

Table 10.41 Pretreatments to modify cellulose using sulphur-containing agents [393] Pretreatment reaction

Comments

Cotton treated with bis(2-isocyanatoethyl) disuphide in dimethylformamide at 80 °C, followed by reduction with tri-n-butylphosphine in methanol containing 10% water. This gives the 2-mercaptoethylcarbamyl ester, which is treated with methyl iodide to form sulphonium salts. (Scheme 10.66).

The greater the sulphonium content, the greater the uptake of direct dyes. Not an environmentally acceptable pretreatment, however.

Alkali-treated cellulose immersed in an arylsulphonium salt solution.

Dyeings with direct, reactive, sulphur and disperse dyes at pH 5 showed improved colour strength but detailed results were not reported.

Linen esterified with thioglycolic acid to incorporate thiol groups, these being more strongly nucleophilic than hydroxy groups.

Increased colour yields obtained with reactive dyes, but not enough to give adequate solidity on linen/wool blends.

[cellulose]

O

C

CH3I

NHCH2CH2SH

[cellulose]

O

C

NHCH2CH2

S

CH3

O

O

CH3I

[cellulose]

O

Scheme 10.66

C

NHCH2CH2

O

+ S CH3 _ CH3 I

O [cellulose]

O

CH2

NH

C

CH

CH2

+

H2NCH2CH2NH N R

NH

N N

D

HN

_ (SO3 )n

O [cellulose]

O

CH2

NH

C

CH2CH2NHCH2CH2NH N

Scheme 10.67

R

NH

N N

HN

D

_ (SO3 )n

treated with N-methylolacrylamide (Scheme 10.61). This is capable of Lewis acid catalysed etherification of hydroxy groups in cellulose and alkali-promoted Michael addition to nucleophiles [393]. The modified cotton was reacted with aminoalkylated triazine dyes (Scheme 10.67) at pH 10.5 in the presence of 80 g/l salt. In contrast to conventional halotriazine reactive dyes, these are stable to hydrolysis under these conditions. However,

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build-up proceeded only up to a limit, beyond which the absorbed dye appeared to act as a resist agent, preventing further uptake. In any case, this process still required the addition of a high concentration of salt. This approach was subsequently improved considerably by treating cotton with 2,4-dichloro-6-(2′-pyridinoethylamino)-s-triazine (Scheme 10.68). One of the chloro substituents reacts under mild conditions with cellulose and the reactivity of the other chloro is reduced [393], giving a fibre containing monochlorotriazine residues that can react covalently with aminoalkylated dyes. Pad–batch pretreatment gave better results than exhaustion, enabling application of the dye at pH 9 in absence of salt and giving similar fastness to conventional dyeing. A highly significant advantage is that full use is made of the dye applied because no dye hydrolysis can occur, resulting in shorter washing-off times and a reduction in coloured effluent. However, the process requires special dyes and at present no commercial range of these exists. Furthermore, there could be hydrolysis problems with the pretreating agent or with the pretreated fibre. Any unlevelness in the pretreatment process would clearly be disastrous in commercial terms.

[cellulose]

OH

Cl

+

N N

+ N _

NHCH2CH2 Cl

N Cl

[cellulose]

O N

+ N _

NHCH2CH2

N N

Cl

Cl

Scheme 10.68

A rather more perverse activity of researchers concerns their attempts to make cotton dyeable with disperse dyes so that, for example, polyester/cotton blends could be dyed or printed with a single dye class. This inevitably means invoking symbiotic chemistry; if cotton is to become dyeable with hydrophobic disperse dyes, it must itself be made more hydrophobic. This carries with it the risk that the very physical properties that make cotton so desirable, and for which it is incorporated into blends with hydrophobic fibres, are negated. Thus the moisture regain of a blend of polyester with hydrophobic cotton may be such that it has no advantage over a fabric made entirely from polyester microfibre. It is perhaps significant that moisture regain is hardly mentioned by researchers devising treatments for making cotton more hydrophobic. Little is also said regarding light fastness, although good fastness to washing is sometimes claimed [400]. Processes devised to make cotton hydrophobic are summarised in Table 10.42. These processes are undoubtedly successful in conferring substantivity for disperse dyes but attaining compatibility within a range of dyes across the entire colour gamut and on fibre blends of various blend ratios could be a problem. In addition, ester bonds can be saponified

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

during washing, typically losing 30% of the effect after one wash and 60–70% after five washes. Hence commercial exploitation is subject to severe limitations.

Table 10.42 Pretreatments to make cotton more dyeable with disperse dyes [393,400] Pretreatment reaction

Comments

Benzoylation with benzoyl chloride.

Proposed in the 1920s to confer easy-care properties. In 1976 process patents introduced for making cotton acceptable to disperse dyes in transfer printing.

Water-soluble acylating esters sodium benzoylthioglycolate and benzoylsalicylate (Scheme 10.69).

Applied by padding with alkali and baking at 200 °C. More acceptable for health and safety reasons than the lachrymatory agent benzoyl chloride.

Aliphatic acid chlorides.

Increasing the length of the aliphatic chain increased uptake of disperse dyes; maximum uptake needed at least eleven C atoms per molecule.

Aromatic substituents in cellulose confer greater disperse dye substantivity than aliphatic substituents.

Benzoylation required a degree of substitution of 15–25% for optimum substantivity; 30–40% was needed to give good fastness to washing.

Graft copolymerisation of styrene on partially carboxymethylated cotton using gamma radiation.

Higher graft yields of polystyrene gave higher colour yields with disperse dyes.

O C

SCH2COONa

+ HO

[cellulose] _ HO

Sodium benzoylthioglycolate O C

O

[cellulose] + HSCH2COONa

O

[cellulose] + HO

O C

O

+ HO

[cellulose] _ HO

NaOOC Sodium benzoylsalicylate

O C

NaOOC

Scheme 10.69

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Since wool dyeing is characterised by much higher levels of exhaustion than are typical for the dyeing of cellulosic fibres, as well as much lower concentrations of electrolytes, interest in pretreatments for wool is less concerned with the environmental aspects of dyeing processes. The motivation to modify wool is usually to increase or decrease dye absorption so that, for example, differential dyeing effects can be obtained [401]. As well as overdyeing fabrics containing some predyed wool, possibilities in this area include the use of prechromed and unchromed wool, pretreatment with a cationic agent to give increased dye uptake or pretreatment with a syntan (section 10.9.4) to give a dye-resist effect. Attempts around 1970 to commercialise the prechromed/unchromed system faltered as a result of poor reproducibility. In any case, nowadays there would be environmental questions to be addressed regarding chromium in the effluent. Pretreatment with a cationic agent or syntan has economic benefits over the spinning of predyed wools in the production of heathereffect yarns. Wool has been pretreated with glucose-derived crown ethers of the type shown in 10.144 and 10.145 [402]. Crosslinking occurs by interaction of amino groups in wool keratin with the glucosidic hydroxy groups in the crown ether. Increased dyeability with reactive dyes was achieved (Figure 10.53), a primary objective of the research being the low-temperature dyeing of wool. H2C

R O

O

H2C

CH O

CH2

O CH

HC

CH2

HO

O

HC HC

CH

H2C

CH

O

HO

OH

H2C

CH

CH2

O H2C

CH2

O

HC O

OH

HC R = H or CH3

O R

CH2

10.144 Bis-glucopyranoside-18-crown-6 ethers

O

O

O

MeO

OMe O

O

O

O O

O

O

O

O

O

O

O

O

O

OMe O

O

10.145 Bis-glucopyranosido-24-crown-8 ethers

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O

O O

O

O

O

O

O

O

MeO

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

100 Dye absorption/%

A

80 B

60 40 A Wool treated with glucosido crown ether B Untreated wool

20

30

60

90

110

130

Dyeing time/min

Figure 10.53 Rate of absorption of Lanasol Red G (Ciba; CI Reactive Red 83) by wool [402]

Wool dyeing at low temperatures has been enhanced by utilising the Maillard, Strecker and modified Strecker (P-Mannich) reactions as pretreatments [403,404]. In a sense, the interaction of wool with the glucose crown ethers already mentioned made use of the Maillard reaction. This is the reaction between the oxo group of an aldose molecule and the freely accessible amino groups of an amino acid or protein molecule, leading to the ultimate formation of the tautomeric forms of an N-substituted 1-amino-1-deoxy-2-ketose via the series of reactions shown in Scheme 10.70 [403]. This reaction is associated with the browning observed when cooking saccharide-containing foods. Hence it is necessary to control the conditions of application to wool so that yellowing or browning does not occur; 30 minutes at 90 °C gave satisfactory results [404]. The Strecker reaction is similar except that a glucose-cyanohydrin is used (Scheme 10.71). The modified Strecker or P-Mannich reaction typically uses dimethyl phosphite in place of the cyanohydrin [404], thus being more acceptable on health and safety grounds for use in a dyehouse (Schemes 10.72 and 10.73). R

NH2 +

H

NH O

R

N

NH

R

C

CHOH

CH

CHOH

CHOH

CHOH

CHOH

CHOH

CHOH

CHOH

CHOH

CHOH

CHOH

CHOH

CHOH

CH2OH

CH2OH

CH2OH

Aldose

Aminated aldose

Schiff base

R

NH

C

CHOH

CHOH

CH2

CHOH

CHOH

OH

CHOH

CHOH

CH2OH

CH2OH

Enol

Keto

O

CH

CH CH OH

Aldose amine

O

Amadori products

R = protein

Scheme 10.70

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R

COH

CH

HO

NH CH2

HO CH

R

CH

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TREATMENTS TO ALTER DYEING PROPERTIES OR ENHANCE FASTNESS

[wool]

[wool]

NaCN + H

C

CN O

+

CHOH

C

H2N

CN

CH

(CH2)4

CH

NH

CHOH

CHOH

O

CHOH

CHOH

CHOH

CHOH

CHOH

CH2OH

CH2OH

Glucose

Cyanohydrin

NH

(CH2)4

CHOH

[wool]

CHOH

C

O

CH NH [wool]

CHOH CHOH

lysine residue

CHOH

40°C

CH2OH

Scheme 10.71

COOH CH3O

H2N(CH2)4CH P

OH OH O CH3 P CH3O OCH3

CH3

CH3O

CH3 C

O

+

CH3 CH3O

NH2 +

COOH NH(CH2)4CH O NH2 CH3 P CH3O OCH3

CH3

L-Lysine

O

N-PhosphonomethylL-lysine

P CH3O

H

Dimethyl phosphite

Scheme 10.72

[wool] CH3O

CH3O P H

O

CH3O

C CHOH

OH

P

C O

+

H2N

(CH2)4

CH

CHOH CHOH

+

O

CHOH

CH3O

CHOH

CH3O

O P H

P CH

NH

C

O NH(CH2)4

CHOH

[wool]

CHOH

CHOH

lysine residue

CHOH

CHOH

80°C

CHOH

CH2OH

CH2OH

[wool]

CH3O CH3O

CHOH

CHOH

D-Glucose

CH3O

O

CH NH [wool]

CH2OH

Dimethyl phosphite

Scheme 10.73

When the Maillard reaction was evaluated using 10 g/l glucose for 30 minutes at 90 °C and 20:1 liquor ratio the fibre diameter increased by 3.5%; xylose gave an increase almost twice as much but showed some yellowing. In this process accessibility of the fibre for dye molecules is increased, since the glucose molecules penetrate between the peptide chains. The reaction also introduces primary alcoholic groups, making the wool more dyeable with

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

the reactive dyes normally used on cellulosic fibres, as well as with bromoacrylamide reactive dyes. Satisfactory dyeing was achieved at pH 5 and up to 80 °C on glucose-treated wool. Penetration was rapid, enabling the dyeing time or temperature to be decreased. The buildup and fixation of reactive dyes were increased. The Strecker reaction utilising glucose– cyanohydrin required only 20 minutes at 40 °C, whereas the modified Strecker reaction using glucose–dimethyl phosphite needed 30 minutes at 80 °C. All treatments gave improved dyeability, the order of efficacy being: Modified Strecker > Strecker > Maillard although differences were relatively slight. Dichlorotriazine dyes showed increased yield after the modified Strecker reaction. There was no decrease in light fastness. Treatment with dimethyl phosphite alone also improved dyeability but not to the same extent as glucose– dimethyl phosphite. Penetration of the peptide chains by glucose–dimethyl phosphite is shown in Scheme 10.74. The process is thought to occur in the following stages [404]: (1) Penetration of dimethyl phosphite into the fibre, accompanied by decomposition of salt linkages and elimination of structural water in a multi-step hydration process. New salt linkages are formed between cationic groups in wool keratin and anionic dimethyl phosphite. (2) Penetration of glucose molecules into the loosened structure, causing further increase in accessibility of the reactive sites. (3) Formation of covalent bonds between dimethyl phosphite, glucose and amino groups in wool keratin, stabilising the loosened fibre structure. Imaginative use has been made of triazine and sulphatoethylsulphone reactive dye chemistry in the application of pretreatments to nylon [405]. The concept resembles that used to make cotton cellulose reactive before dyeing with aminoalkylated dyes, as discussed earlier (Schemes 10.67 and 10.68). In this case, nylon becomes the reactive partner by pretreatment with a reactive multifunctional crosslinking agent: (a) 1,3,5-tris(acryloyl)hexahydro-s-triazine (10.146) or (b) the trifunctional reactive compound XLC (10.95), already discussed as a shrink-resist reactant for wool (section 10.5.5). The pretreated nylon then undergoes covalent fixation of dyes containing aminoalkyl groups. Interestingly, nylon treated with XLC showed markedly lower substantivity and reactivity with conventional dyes. If the pretreated nylon was reacted with ammonia, however, creating amino functionality at the reactive sites, normal reaction with a conventional reactive dye was restored [405]. Many of the syntan aftertreatments described in section 10.9.4 can be applied as pretreatments instead, mainly to confer dye-resist effects. This aspect is dealt with in section 10.9.4 rather than here.

10.9.2 Ultraviolet absorbers The use of polyester and nylon upholstery fabrics in automobiles results in their prolonged exposure to sunlight at high temperatures and humidities. This has created a demand for dyeings of very high light fastness and for these dyed fibres to have exceptional resistance to photodegradation. Although the selection of dyes of suitably high fastness under these

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Scheme 10.74

O

O

Lysine

Lysine

O

Serine

C

HN

C

HN

C

NH

CH

C

NH

CH

C

NH

CH

O

O

O (CH2)4

_

O

(CH2)4

C

O

+ H3N

CH2

O

O

CH2

O

+ NH3

CH2

+ NH3

(CH2)4

H H

NH

NH

HN

C

CH

C

HN

C

CH

C

HN

C

CH Serine

O

Aspartic acid

O

Lysine

O

O

O

O

C

HN

C

HN

C

NH

CH

C

NH

CH

C

NH

CH

O

O

(CH2)4

(CH2)4

CH2

O

O

O

CH2OH

CHOH

CHOH

CHOH

CHOH

CHOH

P

CH3O CH3O + NH3

P

CHOH

CHOH

CHOH

CHOH

CH3O

CH3O + NH3

H

CHOH

CH2OH

_ O

O

+ H3N

H

C

O

CH2

O

(CH2)4

O

CH2

NH

NH

C

HN

C

CH

C

HN

CH

C

HN

C

CH

O

O

O

TREATMENTS TO ALTER DYEING PROPERTIES OR ENHANCE FASTNESS

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

O CH2

CH

C N H2C

N N

CH2

CH

CH2

CH

CH2

C

CH2

O

C O 10.146

extreme conditions is of primary importance, additional protection can be achieved using so-called ultraviolet absorbers, these being colourless aromatic compounds with a high propensity to absorb the troublesome UV radiation. Such products are usually applied during dyeing and confer protection to both dyes and fibres. Although the main interest in UV absorbers centres on polyester and nylon, they are also recommended for use on other fibres, e.g. polypropylene, silk, wool (particularly bleached wools that have a tendency to photoyellowing) and cotton. More recently, due to the increased incidence of skin carcinomas induced by excessive exposure to sunlight, UV absorbers are being used together with modifications of fabric construction to give increased resistance to the passage of harmful rays through the fabric to the skin [406–408]. The principle involved is the same as that used in skin protection creams (that go under the commonly used misnomer of sun protection screens), as well as finding extensive use in plastics and surface coatings to enhance their durability to sunlight exposure. Two useful reviews are available [409,410]. Table 10.43 gives a comparison of the components of solar radiation. This illustrates very clearly the apparent paradox that those components present in least quantity nevertheless pack the greatest punch in terms of photon energy. Thus out of a total photon energy value of 1328 kJ/mol UV radiation of wavelengths 280–400 nm accounts for 1065 kJ/mol (80%), despite the fact that it represents only 6% of the total radiation intensity. This is the radiation that is primarily responsible for the degradation of fibres and colorants. This is also the region, or at least part of it, in which UV absorbers have to function. A useful definition [410] states that a UV absorber is a molecule that may be incorporated within a host polymer in order to absorb ultraviolet radiation efficiently and convert the energy into relatively harmless thermal energy, without itself undergoing any

Table 10.43 Intensity of global radiation (sum of direct and scattered radiation) at the earth’s surface and its classification (summer, vertical incidence) [409] Radiation intensity Regions of solar radiation UV-B UV-A Visible Infrared

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Wavelength (nm)

(W/m2)

(%)

Mean photon energy (kJ/mol)

280–320 320–360 360–400 400–800 800–3000

5 27 36 580 472

0.5 2.4 3.2 51.8 42.1

400 350 315 200 63

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683

irreversible chemical change or inducing any chemical change in the host macromolecules. Thus UV absorbers do not simply scavenge UV radiation preferentially and become themselves expended in the process, in the way that gas-fume fading inhibitors operate (section 10.9.3). UV absorbers convert electronic excitation energy into thermal energy by a rapid but reversible intramolecular proton transfer mechanism [410]. The fundamental nuclei of most of these compounds are represented by o-hydroxybenzophenone (10.147), o-hydroxybenzotriazole (10.148) or o-hydroxybenzotriazine (10.149). All of these structures exhibit keto–enol tautomerism, the keto form being favoured on irradiation. The characteristics of these skeletal structures can be modified by introducing substituents to make them more suitable for specific purposes. For example water-insoluble UV absorbers are preferred for application with disperse dyes for polyester, whereas water-soluble sulphonated derivatives are more suitable for use with anionic dyes on nylon. Thermal stability will be required if such agents are to be incorporated into molten polymers or intended for pad–thermosol application to polyester. Typical variants, together with other types of protective agents, will be mentioned later when dealing with specific fibres. Even certain dyes can act as UV protective agents, particularly in heavy depths, but other dyes may catalyse fading or fibre degradation [411]. Derivatives of certain dyes with built-in UV absorber moieties have been produced specifically with the aim of obtaining dyes of exceptionally high light fastness. For example, CI Disperse Yellow 33 (10.150; R = H) can be converted to a benzophenone derivative (10.151) and a benzotriazole analogue (10.152) of the important polyester dye CI Disperse Yellow 42 (10.150; R = Ph) is also readily available [410]. In some instances there is also a need for an antioxidant; these are discussed below in connection with nylon additives. H

HO O

hν C

O

O

OH

C

OH

10.147 Enol

Keto

HO

H hν

N

O

N

N

N

N

N 10.148

Enol

Keto

hν

N

O

N N

N R3

N R2

683

R3

N 10.149

Enol

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H

R1

HO

R1

R2 Keto

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

HO O2N

O2N

O C

HN

HN

SO2 HN

OCH3

SO2 HN

R

10.151

10.150

O2S HO

NH

O2N

N N

NH

N

Cl

CH3

10.152

Polyamides are characteristically prone to photochemical and thermal degradation, although much depends on the structure and purity of the polymer and the types and concentrations of additives present. Although the titanium dioxide delustrant is an effective UV absorber, it is also a sensitiser of the photodegradation of nylon. The decomposition is initiated mainly by free radicals from sensitising impurities. Although most polyamides are resistant to photochemical attack below 40–50 °C, they can degrade fairly rapidly at higher temperatures, particularly with higher concentrations of titanium dioxide present and when dyed in pale or medium depths. Certain metal-complex dyes, however, can act as photostabilisers, especially in full depths. UV absorbers afford protection to nylon only at black-panel temperatures below 40–50 °C. Since degradation is largely the result of oxidative free-radical attack, protective effects are shown by antioxidant addition, either alone or in conjunction with a UV absorber. Both agents may be incorporated during fibre manufacture or can be applied during dyeing or finishing. Some commercial formulations contain both types of agent. Water-soluble UV absorbers are preferred for nylon, particularly sulphonated benzotriazoles of the general type represented by 10.148, with halo, sulpho or sulphonated arylalkyl substituents in the benzotriazole nucleus and alkyl, alkoxy or sulpho groups in the phenolic nucleus. A specific example is provided by structure 10.153. Sulphonated derivatives of the dihydroxybenzophenone structure 10.147 are also suitable. One study of the influence of UV absorbers on light fastness involved a selection of nine acid dyes typical of those used in the dyeing of nylon carpets. The efficacy of four water-soluble agents applied by exhaust dyeing (10.154–10.156 and an undisclosed structure of the anionic o-hydroxyphenylbenzotriazole type) and three water-insoluble types applied from solution in tetrachloroethylene (10.157–10.159) was evaluated [412]. Some of the UV absorbers significantly improved light fastness, but others gave significantly lower ratings. Overall, the behaviour of the UV absorbers was dye- and hue-specific [412]. Conversely, in another investigation it was claimed that stabilisers applied to nylon during dyeing can markedly improve fibre stability and light fastness, independently of hue and depth of shade [413].

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HO

CH3 HO

685

O

CH2CH3

CH

HO

N

C

OCH3

N

SO3Na

N SO3Na

CH3O

10.154

SO3Na

10.153

HO O C

OCH3 SO3Na

HO O

HO

C

N

10.155

O(CH2)7CH3

N N CH3

10.156

10.157

HO O

O C

N

CH

C

N

CH3CH2O

OCH3

CH3 10.158

10.159

The additional protection given to nylon by antioxidants has already been mentioned. Since the need is to protect against oxidation by free radicals, antioxidants are essentially of two types: peroxide decomposers and radical scavengers. Reviews of these products are available [409,410,413]; these should be consulted for details of the mechanisms involved. Peroxide decomposer types include compounds of manganese(II) or copper(I) and copper(II) complexes, such as azomethine bridge derivatives of the type represented by 10.160, of which numerous water-soluble or water-insoluble variants are possible [409]. These products have a catalytic action and are therefore used in very small amounts. Y N

N

HC

CH Cu

R O X

chpt10(2).pmd

R O

10.160

685

X

R = H, halo, hydroxy, alkyl, alkoxy X = H, sulpho, carboxyl, sulphonamide Y = alkylene, cycloalkylene linkages

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

Conversely, radical scavengers have to be used in larger amounts because they lack the regeneration capability of catalytic types. The principal product types [409] are sterically hindered phenols (10.161) and sterically hindered amines (10.162). The latter are important because they act not only as scavengers but also as peroxide decomposers [410]. Further compounds include the semicarbazides 10.163 and 10.164, applicable by continuous and exhaust methods respectively. The product 10.165 is suitable only for continuous application. Most radical scavengers are suitable only for thermal stabilisation [410], photostabilising scavengers being restricted to transition-metal chelates, especially nickel(II) dithiocarbamates (10.166), and hindered amines such as 2,2,6,6-tetramethylpiperidine (10.167). Water-soluble triazine derivatives of hindered amines (10.168) have also been used [410]. H3C

CH3

C

HN

CH3

H3C

HO

R

A

CH3

R = H, oxy, hydroxy, acyl, alkyl, alkoxy A = O or NH X = H, sulpho, carboxyl Y = alkylene, arylene, substituted arylene (identical or different)

R = H, halo, alkyl, alkoxy X = H, sulpho, carboxyl, sulphonamide A = triazine or (CH2)nCONH linkages

H3C

CH3 NH

HN

X

10.162

10.161

H3C

HN

CH3

X

C

Y

N N

H3C

H3C

N

N

A

H3C

X

N

H3C R

Y

(CH2)6

NH

HN

N

N CH3

C

C

SO3Na

H3C NH

H3C

HN

C

NH

HN

O

CH3 N

C

CH3 O

O

O 10.163

10.164

OH CH3CH2CH2CH2

N CH2

S

N

H2C

Ni

C S

CH3CH2CH2CH2

10.165

CH2CH2CH2CH3

S C S

N CH2CH2CH2CH3

10.166

Cl

H3C H3C

CH3 N

CH3

H3C H

CH3

N NH

N

N N

HN

H H3C

10.167

chpt10(2).pmd

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CH3

10.168

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SO3Na

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TREATMENTS TO ALTER DYEING PROPERTIES OR ENHANCE FASTNESS

Water-soluble UV absorbers are preferred on bleached wool, mainly to retard the photoyellowing process. The sulphonated o-hydroxybenzotriazole type (10.169) is particularly effective [414]. When applying such products by the exhaust process it is best to avoid the use of sulphates as these lower the exhaustion, as does a pH higher than 5.5 [415,416]. These products are claimed to be fast to hand washing, shampooing and dry cleaning [416]. Nevertheless, polymeric UV absorbers have been developed with the intention of improving on the limited fastness of conventional types [417]. These have typical UV absorber moieties attached to an acrylate, methacrylate or methacrylamide polymer, examples of relevant monomers being 10.170–10.172. HO N

R N

X

HO

N X

O

O

10.169

C C

OCH2O

C

CH2

H3C R = alkyl, alkoxy, sulpho X = H, halo, sulpho, sulphonated arylalkyl

10.170

H3C H2C

HO

CH3

N

C

CH2

C

N

O

N N C H3C

CH3

(CH2)4CH3

10.171

HO

H3C

CH3 C

O

O

CH3

N

CH2CH2

C N

CH2CH2CH2

N

O

C CH

CH

O

CH

CH2

CH2CH2 10.172

Polyester fibres generally show high resistance to photodegradation providing exposure in the 280–320 nm range is avoided, so the main reason for using UV absorbers on this fibre is to enhance light fastness, particularly to the standards required for automobile furnishings. Indeed, marked improvements generally only become evident with prolonged exposure at high ambient temperatures [409]. Water-insoluble benzophenones and benzotriazoles, such as 10.173 and 10.174 respectively, are widely used. These products, however, have only moderate fastness to sublimation. Where a high setting temperature or pad–thermosol application is involved, o-hydroxyphenylbenzotriazoles of greater molecular mass offer higher fastness to sublimation [409]. Water-insoluble mono- (10.149) or bis(o-hydroxy-

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

phenyl)triazines are also of interest. Derivatives of o-hydroxyphenylpyrimidine and benzoxazin-4-one have also been patented [409]. In a recent series of papers [418,419], the effectiveness of the following UV absorbers on polyester was demonstrated: 10.147, 10.156, 10.175 and a benzotriazole of undisclosed structure.

HO

OH

O

HO

H3C

C CH3

N

C

CH3

N CH3O

N

Cl

OCH3

HO

H3C

CH3 C

HO

CH3

N

O N

Cl

CH3

10.174

10.173

C

N C H3C 10.175

CH2

CH3 CH3

O

10.176

Benzyl salicylate (10.176) at an applied concentration of 2% o.w.f. is reported [420] to have given good protection from fading to two anthraquinone acid dyes on silk. Water-soluble UV absorbers can be used on cotton to improve the photochemical stability of the fibre or to protect the skin from UV radiation but this does not generally improve the light fastness of dyeings. From a chemical viewpoint, bifunctional reactive UV absorbers (10.177) for cotton are of particular interest [421]. The vat dye 10.178 containing two imidazole rings is patented [422] for use by dyeing or printing on cellulosic fibres to improve solar protection both to the wearer and to the fibre. The use of 10.177 in combination with 10.178 is also patented [421]. Normal untreated cotton with a sun protection factor of 5 can be improved to factor 28 after applying 10.178 and to factor 187 when this dye is used in combination with the reactive UV absorber 10.177.

Cl OCH2CH3 O

N

NH C C HN NaO3S

NH

N N

HN

SO2CH2CH2OSO3Na

O 10.177

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TREATMENTS TO ALTER DYEING PROPERTIES OR ENHANCE FASTNESS

O

689

O Cl

O

NH

HN

N

O

N Cl 10.178

10.9.3 Gas-fume fading inhibitors Not all disperse dyes on cellulose acetate fibres are resistant to oxides of nitrogen that may be present in the atmosphere. Susceptible dyes, usually those containing primary amino or secondary amino groups, can undergo quite profound changes in hue depending on the reactivity or basicity of the susceptible group(s). The primary general mechanism of fading is believed to be the formation of N-nitrosamines [423]. The problem is best avoided by using dyes that do not fade, but this may not always be possible as these tend to be more costly and less easy to apply. Some protection can be obtained by treatment of susceptible dyeings, either during or after dyeing, with colourless agents that react preferentially with oxides of nitrogen. Such agents are known as gas-fume inhibitors. Since they act as scavengers of the acidic oxides of nitrogen, they need to be more basic in character than the dyes they protect. Many such basic compounds, generally applied from aqueous solution or dispersion, have been proposed [424]. The most popular and efficient are substantive to the fibre; typical examples are N,N′diphenylacetamidine (10.179), which tends to yellow on exposure to oxides of nitrogen, and particularly the diphenylated diamines such as N,N′-diphenylethylenediamine (10.180), which does not yellow. Non-substantive inhibitors applied by padding and drying, such as triethanolamine (10.126) and melamine (10.181), have also been used despite the fact that they are removed on washing. The demand for and commercial availability of gas-fume inhibitors have declined.

NH

NH2

C

CH3

N

NH

N

HN

H2N

CH2CH2 10.180

10.179

N N 10.181

NH2

Atmospheric ozone has also been reported as causing fading of certain dyes in some countries [425,426]; diallyl phthalate (10.182) used as a carrier in the dyeing of cellulose triacetate fibres, is said to be an effective ozone inhibitor [427]. Nylon, especially when dyed with certain amino-substituted anthraquinone blue acid dyes, can also be susceptible to ozone fading [428,429]. Selection of ozone-resistant dyes is obviously the best counteractive measure, although hindered phenols (10.161) and hindered amines (10.162) are said to provide some protection.

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

O C

OCH2CH

CH2

C

OCH2CH

CH2

O 10.182

10.9.4 Aftertreatments and resist treatments for acid dyes By far the most important aftertreatment for acid dyeings is the so-called syntan process widely used on nylon, which has superseded the classic full back-tan process. This involved treatment of the dyed nylon first with ‘tannic acid’ (a highly complex gallotannin or polygalloylated glucose, such as structure 10.183), which hydrolyses to give digallic acid (10.184) or gallic acid (10.185) [391,430], followed by further treatment with potassium antimonyl tartrate (tartar emetic; 10.186). It has been replaced on grounds of its high cost, instability to hot alkali, undesirable effects on fabric handle and light fastness, changes in colour (usually dulling) during treatment, and diffusion and degradation of the antimonyl tannate complex during subsequent steaming or dry heat setting, as well as for health and environmental reasons. Its presumed mechanism of action is complex [391,430]. OH

OH HO

O C

OH HO

OH

O

OH

OH C

O

O C

OH CH2

O

CH

O O

O

CH OH

C

HO

O

CH O

O

C CH

OH

O O

CH O

O

C

C

HO HO O HO

HO

C O

OH

OH OH

OH

HO

OH HO

10.183

HO

O C

OH

O OH HO

C O

chpt10(2).pmd

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10.184

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TREATMENTS TO ALTER DYEING PROPERTIES OR ENHANCE FASTNESS

O

OH

C

O C

OH

HO 10.185

OH

_ O [SbO] +

CH

OH

CH

OH

C O

691

10.186

_ O K+

The synthetic tanning agents (syntans) that have superseded the back-tan have the advantage of being applied in a single process but they tend to be polyphenolic compounds like the naturally derived gallotannins. In fact, compounds of this class have been described in section 10.6.1 as condensation products of formaldehyde with sulphonated phenols, naphthols or naphthylamines. Structures 10.100–10.103 are typical of these compounds although, as mentioned earlier, they form a large and varied group. The precise structural details of typical commercial syntans for nylon are hidden within a plethora of patent literature, much of which pertains to their use as tanning agents for leather [391]. In addition to the products mentioned in section 10.6.1 analogous sulphur-containing products, such as those derived from thiophenols, and heavy-metal phenolates were formerly used. A brief simplified mechanism whereby syntans are thought to operate can be described, at the risk of incompleteness. Their rapid sorption by the fibre under the (usually acidic) conditions of application is largely the result of electrostatic attraction between negatively charged sulphonate groups and protonated amino groups in the fibre. Hydrogen bonding between uncharged polar groups, and hydrophobic bonding between the nonpolar moieties in the syntan and the fibre, create conditions for the formation of complexes. Maximum improvement in wet fastness results when the complexes are formed at the fibre surface, since any treatment leading to diffusion of the syntan complex into the fibre tends to yield lower fastness. Hence a ‘barrier effect’ seems to be involved. Possibly multilayers can be formed via a Brunauer, Emmett and Teller (BET) mechanism. Some syntans will give a modest improvement in the wet fastness of disperse dyes on nylon but even with acid dyes the response to the syntan treatment in terms of improved wet fastness varies markedly from dye to dye. Empiricism has guided the screening of syntans for nylon. Both the affinity of the syntan for the fibre and its diffusion rate are important, and the required values are obtained by balancing the degree of sulphonation and range of molecular sizes present. Moreover, many of these products are used not only as aftertreating agents for the improvement of wet fastness, but also as blocking agents to inhibit absorption of dye either partially or completely – for example, in the dyeing of nylon/wool blends to restrain the usually preferential uptake of dye by the nylon, or to produce resist effects, particularly in printing. The balance of properties required in the syntan may therefore vary somewhat and is influenced by the characteristics of the dye–fibre system for which it is intended. Studies on nylon, using commercial syntans of undisclosed structure [431–433] showed that the absorption of syntan increased with decreasing pH, providing further evidence for electrostatic interaction. Evidence was also found for the BET mechanism, indicating that other forces are also operating, such as hydrophobic interaction. Uptake of syntan increased with applied concentration and increasing temperature, this being attributable to the higher

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

kinetic energy of the syntan molecules and the greater extent of fibre swelling. Syntan uptake was greater on nylon microfibres (greater surface area) than on conventional nylon staple fibres, but the fastness to washing was inferior on microfibres. Application of a cationic agent subsequent to the syntan treatment gave an additional significant improvement in fastness, particularly where multiple washes were involved [433]. Unsulphonated 1:2 metal-complex dyes showed less desorption of dye than disulphonated types after 1–3 washes but greater loss after 5–10 washes. The cost and inconvenience of these two aftertreatments, however, would not be justifiable under commercial conditions. Similar commercial syntans have been evaluated in dye–resist pretreatments on wool [434]. Evidence for both kinds of agent–fibre interaction was again found. The resist effect imparted to the wool was specific to the type of anionic dye applied, giving high resistance to four hydrophilic dyes and low resistance towards two hydrophobic dyes. Syntan desorption occurred during dyeing, indicating that these agents are relatively weakly attached to the fibre. The desorbed syntan had a restraining effect on the uptake of all six dyes examined. Section 10.9.1 includes an account of nylon pretreatments based on chemistry normally associated with reactive dyes for cellulosic fibres. Similar chemistry has been used to develop alternatives to syntans, both as aftertreatments to improve fastness and as pretreatments to give dye-resist effects. The trifunctional reactive compound XLC (10.95), already discussed as a shrink-resist reactant for wool (section 10.5.5), is hydrolysed under alkaline conditions to the active vinylsulphone form (10.187). This trifunctional crosslinking agent has been evaluated to improve the fastness of nylon dyed under mildly acidic conditions with specially prepared anionic aminoalkylated triazine dyes [435]. Such dyes do not bond covalently with nylon, but aftertreatment with this trifunctional reactive agent results in a significant degree of covalent bonding with a consequent improvement in wet fastness. A cationic aminoethyisulphonyl dye (10.188) was prepared and applied to nylon at the optimum pH 10, followed by treatment with the parent sulphatoethylsulphone crosslinker (10.95). In this way, the dye became fixed covalently via the agent to terminal amino groups in nylon, resulting in improved fastness to washing [435]. Traditional syntan treatments are rather ineffective for improving the wet fastness of anionic dyes on wool, mainly because of weakness of the interaction between syntan and fibre. It is therefore not surprising that covalent reactive systems have been explored to find more effective aftertreatments and dye-resist treatments for this fibre. In an initial study Cl N H2C

CHSO2

NH

N N

HN

SO2CH

10.187

CH2CH3 N H2NCH2CH2SO2

N

N

+ N _

CH2CH2 X 10.188

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CH2

TREATMENTS TO ALTER DYEING PROPERTIES OR ENHANCE FASTNESS

693

aimed at producing a system that would overcome the level dyeing problems associated with reactive dyes on wool [436], a specially prepared aminoalkylated dye (not fibre-reactive) was applied together with hexamethylenetetramine, from which formaldehyde was released (Scheme 10.75) to give covalent bonding between the amino groupings in the dye and in the fibre (Scheme 10.76). This process was successful up to a point but gave lower fixation when dyeing in full depths. Certain chromogens also showed marked changes of hue. N H2C

CH2

CH2 H2C

N

+ 6 H2O

H2C

N

6 HCHO + 4 NH3

N

CH2

Scheme 10.75

[dye]

CH2CH2NH2

[dye]

Scheme 10.76

+ HCHO + H2N(CH2)4[wool]

CH2CH2NHCH2NH(CH2)4[wool]

This approach was subsequently improved [437] by dispensing with the formaldehyde precursor and adding a trifunctional crosslinking agent to the dyebath after dye absorption was complete. The crosslinking agent was 1,3,5-tris(acryloyl)hexahydro-s-triazine (10.146). Scheme 10.77 represents an idealised reaction for the formation of dye–agent–wool linkages. Since reaction takes place in a random manner, however, the derivatives 10.189–10.192 could equally well be produced. Products 10.191 and 10.192 need to be minimised, because respectively they are linked only to the dye or to the fibre [437]. O CH2

CH

C N

2 [dye]CH2CH2NH2 +

H2C

N

CH

CH

CH2 + H2N(CH2)4[wool]

C

CH2

O

N

CH2

CH2CH2NH(CH2)4[wool]

N

CH2

N CH2

CH2

C O

O [dye]CH2CH2NHCH2CH2

C H2C

[dye]CH2CH2NHCH2CH2

Scheme 10.77

chpt10(2).pmd

N

C O

C O

693

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694

CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

O [dye]NHCH2CH2

O

C

[dye]NHCH2CH2

N N

N

C

N [dye]NHCH2CH2

C

CH2CH2X[wool] O

C

[wool]XCH2CH2

C

N

O

O

X = NH or S

C

X = NH or S

O

CH2CH2X[wool] N

10.189

10.190

O

O [dye]NHCH2CH2

[wool]XCH2CH2

C

N

N

CH2CH2NH[dye] N

N [dye]NHCH2CH2

C

C

N

O

[wool]XCH2CH2

C

C

O

O

CH2CH2X[wool] N

10.191

C O

X = NH or S

10.192

Subsequently, two more cost-effective bifunctional crosslinking agents, both commercially available, were used to give the same quality of dyeings [438], these being N,N′methylene-bis-acrylamide (10.193) and a quaternised precursor (10.194). Idealised reactions with functional groups in the dye and in wool are given in Schemes 10.78 and 10.79. Although these bifunctional reagents are cheaper than the trifunctional product, dyeing has to be carried out at pH 8 as their reactivities are lower than that of tris(acryloyl)hexahydros-triazine. However, this lower reactivity allows the bis-acrylamide to be added at the beginning of the dyeing process. The quaternised derivative is a more effective crosslinking agent, possibly because of its greater substantivity for wool under slightly alkaline dyeing conditions, but it cannot be added at the beginning of dyeing because of its cationic character. There has been active interest in exploring the potential of reactive triazines as dye-resist agents for wool. A model structure for reactive dye-resist agents (10.195) has been proposed [439], where Z is a suitable high-reactivity group, NH is a bridging group, Ar is an aryl group and [SO3] – is a solubilising and anionic dye-repelling group. Four dichlorotriazine compounds (10.196–10.199) based on this model structure were found to be effective as

+ HX

[wool]

[dye]CH2CH2NHCH2CH2CONHCH2NHCOCH2CH2X

[wool]

[dye]CH2CH2NH2 + CH2

CHCONHCH2NHCO CH

CH2

10.193

X = NH or S

Scheme 10.78

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TREATMENTS TO ALTER DYEING PROPERTIES OR ENHANCE FASTNESS

_ + A R3N CH2CH2CONHCH2NHCOCH2CH2

695

_ + NR3 A

10.194

CH2

CHCONHCH2NHCOCH

CH2

_ + + 2 NHR3 A

10.193 + HX

[dye]CH2CH2NH2

[wool]

[dye]CH2CH2NHCH2CH2CONHCH2NHCOCH2CH2X _ X = NH or S A = anion

[wool]

Scheme 10.79

dye-resist agents, although full white resist effects were difficult to achieve [439]. Pursuing this line of research further, several more complex naphthylamine-based dichlorotriazine dye-resist agents (10.200–10.206) were evaluated [440]. These agents each contained at least one dichlorotriazine reactive moiety as well as a sulphonated naphthylamine system that enabled the anionicity of the product to be varied. Using the trisulphonated dye CI Acid Red 27 (10.207), the resist efficiency improved significantly with increasing degree of sulphonation of the agent and with increasing relative molecular mass, enabling lower concentrations of the more effective agents to be used. The upper limit of resist achieved was 98.6%. Cl

_ Z

NH

Ar

N

[SO3 ]n

N

NH

SO3Na

N

10.195 Cl

10.196

Cl

Cl N

N

Cl

N NH

NH

N N

N

N

Cl

10.197 Cl

SO3Na

N

N Cl

O NH

C

N HO

HN

Cl

10.199

chpt10(2).pmd

10.198

N NH

SO3Na

695

COONa

Cl

N N

NH N

Cl

SO3Na

Cl

OH N

10.200

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SO3Na

696

CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

Cl N N

O NH

C

N

HN

Cl

SO3Na 10.201 NaO3S

Cl N

O

N

NH

SO3Na

C

N

HN

Cl

SO3Na 10.202

NaO3S

Cl N N

O NH

C

O

HN

N

C

Cl

HN

SO3Na

10.203

Cl N N

O NH

C

N

O

HN

C

Cl

HN SO3Na

10.204 NaO3S

Cl

Cl

SO3Na N

N

N NH

NH

N

N N

Cl

NaO3S

HN

SO3Na

10.205

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TREATMENTS TO ALTER DYEING PROPERTIES OR ENHANCE FASTNESS

Cl

Cl

SO3Na N

N

697

N NH

NH

N

N N

Cl

NaO3S

HN SO3Na 10.206 NaO3S O

NaO3S

SO3Na

HN N

10.207

SO3Na

CI Acid Red 27

A rather different approach [441] to obtaining resist effects on wool involved evaluation of the relatively simple compound sulphamic acid (10.208). From a comparison of curing temperatures over the range 100–150 °C, it was found that only by curing at 140 °C or higher was the sulphamic acid bound sufficiently to resist desorption during dyeing. There are several possible routes of decomposition of sulphamic acid [441], leading to the formation of various sulphonic acids and sulphonamides, as well as the possibility of scission to form sulphur trioxide and ammonia (Scheme 10.80). The reactions of sulphamic acid with wool are mainly through hydroxy groups and to a much lesser extent amino groups in the fibre (Scheme 10.81). Thus the dye-resist effect is provided by the presence on the modified fibre of anionic sulphate and sulphamate groups [442]. This approach was also examined on silk [443]. Compared with wool, silk has a much smaller proportion of nucleophilic amino groups but a substantial content of hydroxy groups with which sulphamic acid reacts preferentially. Excellent dye-resist effects were obtained with acid, 1:2 metal-complex and reactive dyes, better than when a commercial dichlorotriazine reactant (similar to 10.196) was used. 10.9.5 Aftertreatments for direct and reactive dyes on cellulosic fibres The fact that the aftertreatment of direct dyes has a long history is not surprising since wet fastness within this class is not particularly good. Their prime advantages are ease of application and economy compared with dyes of higher fastness (reactive, sulphur or vat) – hence the continued search for highly effective aftertreatments that improve wet fastness O H2N

S O

chpt10(2).pmd

697

O + H3N S

OH 10.208

_ O

O

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698

CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

O HO

S

O NH2

+ H2N

S

O

O

O

O

H2N

S

+

OH

H2N

S

O

O OH

HO

S

OH

H2N

S

OH

+ NH3

O O

S

NH

O

S

OH

+ H2O

O

SO2 OH

HN

NH

O2S

SO2

+ 3H2O

NH

O O + H3N S

NH

O

O H2N

S O

O

3

O

_ O

NH3 + SO 3

O

Scheme 10.80

[wool]

OH +

[wool]

NH2

HO3S +

HO3S

NH2

[wool] [wool]

NH2

_ SO3

O NH

+ NH4

_ + SO3 NH4

Scheme 10.81

without excessive additional cost. Some of the aftertreatments used for this purpose have also been applied to reactive dyes, as will be described later. One of the earliest aftertreatment processes employed diazotisation of free amino groups in the adsorbed dye, followed by coupling (development) with a suitable component such as a phenol, naphthol or amine; 2-naphthol, m-phenylenediamine, resorcinol and 1-phenyl-3methylpyrazol-5-one (10.209) were particularly popular. Obviously, this was only possible with dyes containing a diazotisable amino group, an example being CI Direct Yellow 59, the classic primuline (a mixture of structures 10.210a and b), which was converted on the fibre from a greenish yellow to a bluish red by diazotisation and development with 2-naphthol. The reverse approach, application of a dye containing a phenolic group and aftertreatment with a solution of a diazotised amine, such as p-nitrobenzenediazonium chloride, was also used.

N O

N

CH3 10.209

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SO3Na H3C

S

a

S

C

C

N

NH2

N SO3Na H3C

S S

C

b

S

C

N

C

N

NH2

N 10.210

Formaldehyde aftertreatment was employed to link together pairs of amino-substituted dye molecules by a methylene bridge (10.211). The required reactivity of the sites in the dye towards formaldehyde was ensured by pairs of o- and p-directing electron-donating groups provided by resorcinol, m-phenylenediamine or 3-aminophenol. Ar

Ar N

N

N

H2N

CH2 NH2

N NH2

H2N 10.211

A third approach utilised copper salts, especially copper(II) sulphate, in conjunction with dyes containing chelatable groupings such as salicylic acid or o,o′-dihydroxyazo moieties. Indeed, special ranges of ‘copperable’ direct dyes, for which the treatment with copper(II) sulphate was really part of the dyeing process rather than an optional aftertreatment, were introduced. In the past the main use of this chelation treatment was to enhance light fastness, but it is little used for this purpose nowadays. Since direct dyes have anionic structures, many cationic surfactants such as quaternary ammonium compounds were used as aftertreatments to form surfactant–dye complexes of reduced aqueous solubility and therefore higher wet fastness. The improved fastness related only to non-detergent agencies such as perspiration and water, however. In soap-based washing processes the stronger interaction between the anionic soap and the cationic agent tended to cleave the dye–cation complex, thus effectively negating the aftertreatment even after a single mild wash. The aftertreatment often brought about changes in hue and reduced light fastness, although the latter could sometimes be countered by a combined or subsequent treatment with a metal salt such as copper(II) sulphate, as described in the preceding paragraph. All the aftertreatment processes so far described have declined considerably in commercial significance and are now rarely carried out. Nevertheless, their common principle of creating on the fibre a dye–agent complex of larger size, reduced solubility and a

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correspondingly lower rate of desorption has survived in today’s use of resin-type fixing agents. The principle of cationic aftertreatment, in particular, has seen noteworthy development. The earliest polymeric cationic aftertreatments stemmed from the development of creaseresist finishes for cellulosic fibres. One such, promoted specifically for its colour fastness improvements when applied as an aftertreatment to direct dyeings, was a condensation product of formaldehyde with dicyandiamide (Scheme 10.82). Many similar compounds followed, such as condensation products of formaldehyde with melamine (10.212), poly(ethylene imine) with cyanuric chloride (10.213) and alkyl chlorides with poly(ethylene imine) (10.214; R = alkyl).

NH n H2N

C

N

C

NH + n HCHO + n HCl

CH2

NH

Cl

NH2

C

N

C

+ n H2O N n

Scheme 10.82

(CH3OCH2)2N

H(NHCH 2CH2)nNH N

N N(CH2OCH3)2

N

N N

N (CH3OCH2)2N

NH(CH2CH2NH)nH

H(NHCH 2CH2)nNH

10.212

10.213

_ + RNH(CH 2CH2NH)nCH2CH2NH3 Cl 10.214

These condensates were an improvement on the simpler cationic surfactants mentioned earlier. Their interaction still relies on electrostatic bonding between agent cation and dye anion; hence the major weakness remains fastness to washing with anionic detergents. This limitation of conventional cationic agents has been overcome by the development of bi-, triand tetra-functional agents which carry reactive groups capable of forming covalent bonds by reaction with other suitable groups in the dye and/or the hydroxy groups in the cellulosic fibre. Studies on the functionality and reactivity of various multifunctional crosslinking agents [442] led to the selection of 1,3,5-tris(acyl)hexahydro-s-triazines (10.215) as being particularly effective, their efficiency arising from the lability of the chloro substituents and the strongly polarising effect of the carbonyl groups. The tris(acryloyl) derivative (R is CH=CH2) was subsequently developed as a fixing agent for use with the Basazol(BASF) dyes in the printing of cellulosic fibres, in conjunction with urea as hydrotrope [444]. It has also been shown [445] that aftertreatment with 1,3,5tris(acryloyl)hexahydro-s-triazine or the tris (β-chloropropionyl) derivative improves the wet

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O C

R

N

R N

C

N

O

C

R

10.215 O

R=

CH

CH2

CH(Cl)

C(Cl) CH2Cl

or

CH2 CH2CH2Cl

fastness of direct dyes, providing the dyes contain suitable nucleophilic groups. The same compounds have useful properties with reactive dyes (other than the Basazols) in giving increased colour yield through fixation of hydrolysed dye, an aspect discussed in more detail below. The basic mechanism of action of multifunctional fixing agents has been well described [446] (Figures 10.54 and 10.55). However, the multifunctional products are no better than monofunctional types when used with conventional direct dyes that do not contain suitable nucleophilic groups through which to link the fixing agent. Direct dyes suitable for forming additional bonds have been termed reactant-fixable dyes and are mostly of the coppercomplex type. Thus brightness of shade is limited. A range of bifunctional, trifunctional and tetrafunctional fixing agents was developed [446,447] for use with a selected range of copper-complex (Indosol) dyes. The bifunctional type, which reacts only with the dye, was applied in a fresh bath at about 60 °C and gave fastness to washing at 50 °C through the formation of an extensive dye–agent complex within the fibre. The trifunctional type additionally forms covalent bonds with cellulose and is applied at 40 °C for about 15 minutes, followed by addition of alkali to bring about reaction; this confers a higher degree of fastness to washing at 60 °C even with deep shades. + Monofunctional type

Bifunctional type

Trifunctional type

Tetrafunctional type

+

+

+

Figure 10.54 Fixing agents showing various degress of functionality [446]

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cellulose

+

–

+

–

O3S–D

Monofunctional type

Bifunctional type

O3S–D

HO

O Trifunctional type

+

–

O3S–D

HO

O Tetrafunctional type

+

–

O3S–D

HO O

Figure 10.55 Modes of reaction of the various fixing agents [446]

Tetrafunctional reactant resins confer the highest fastness, even to washing at the boil. Typical polymers are made by the reaction of an amine such as diethylenetriamine with cyanamide, dicyandiamide (especially), guanidine or biguanide. A cationic polymer of this type is applied with an N-methylol reactant such as dimethyloldihydroxyethyleneurea and an acid-liberating catalyst such as magnesium chloride to give a commercial product sold as a cationic reactant resin. This formulation is applied to the dyeing by padding, at which stage a dye–agent complex is believed to be formed. The fabric is then treated at 175–180 °C, resulting in covalent reaction between the cationic agent and the N-methylol groups as well as crosslinking of cellulose chains by the N-methylol reactant, conferring not only excellent wet fastness but also improved crease resistance and good dimensional stability. The multifunctional agents offer an alternative aftertreatment for reactive dyeings [446], if problems arise from the presence on the fibre of unfixed hydrolysed dye, normally removed by ‘soaping’. This process is invariably lengthy, expensive and not always fully effective. Multifunctional cationic reactants will react with unfixed (still reactive) dye and with hydrolysed (hydroxy-containing) dye. Tri- and tetra-functional reactants will further fix them by covalent bonding to the fibre. Improved wet fastness can also ensue when tetrafunctional products are used, including considerably improved resistance to hydrolysis of the dye–fibre bond and better fastness to treatments involving chlorine or perborates. Deleterious effects on hue and light fastness still have to be carefully considered in the selection of dyes for aftertreatment, however. Most of the polymeric cationic products available [448] are based on the types described in Table 10.44. The ideal aftertreating agent must fulfil many requirements [448]. There is a high demand for: – improved fastness to washing and other wet treatments – low price

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– – – – – – –

703

applicability by exhaustion applicability with various dye classes and on various fibres and blends stability to electrolytes capability to minimise migration of hydrolysed reactive dyes stripping and overdyeing performance ease of cleaning of stained machinery biological elimination.

In addition, the ideal agent must not: – cause significant changes of shade of dyeings – increase chlorine retention or free formaldehyde content – impair the handle, hydrophilicity, light fastness, gas-fume fastness or sewability of the fabric – be toxic to humans or irritate the skin.

Table 10.44 Classes of polymeric cationic agents for aftertreatment of dyed cellulosic fibres [448] Amide–formaldehyde condensates

No defined structure

CH2 NH C NH C NH2 CH2 NH NH X

Polybiguanides

NH

CH2

Poly(ethylene imine)

CH2

n

NH2 CH2 CH2

n

X CH3

Quaternised poly(ethylene imine)

NH

CH2

CH2

N CH2 CH2 CH3

n

CH2

H2C HC Quaternised polyheterocycles

X

CH CH2

H2C N

CH3

H3C X

n

CH2

Quaternised polyacrylamides

O

CH C

CH3

NH CH2 CH2 N CH3 CH3

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R

R

NH2 + HClO

NHCl

+ H2O

T < 40°C

R

R NH

NCl + H2O

+ HClO

T < 40°C

R

R Scheme 10.83

heat R

NHCl

R

+ H2O

R

heat

+ HClO

R NH

NCl + H2O R

NH2

+ HClO

R

Scheme 10.84

Amines can give rise to chloramines during hypochlorite bleaching (Scheme 10.83). In addition to increasing AOX values, this can result in cellulose oxidative degradation by hypochlorite on subsequent hydrolysis of the chloramines (Scheme 10.84) [448]. Cationic fixing agents remain associated with the dyes on the fibre and are only partially removed by subsequent washing. Skin irritation is therefore an important consideration and in this respect they appear to have negligible influence. In common with cationic surfactants, however, they are readily absorbed by protein material and show moderately high toxicity to aquatic organisms, although interaction with excess anionic surfactant can considerably reduce this problem. In vitro they can show poor biodegradability, BOD values being typically not more than 10–20% of COD values, which are usually in the region of 40– 400 mg/g oxygen. However, they are highly absorbed by activated sludge, resulting in about 95–98% bioelimination accompanied by about 80% actual biodegradation. Studies of the effectiveness of cationic polymers for aftertreatment of dyeings continue to appear, although not all researchers disclose the detailed structures of the polymers evaluated. One comparison involved three types of commercial fixing agents for improving the fastness of six direct dyes on cotton, the agents being described as a dicyandiamide– formaldehyde condensate, a cationic nitrogenous polymer and a long-chain polyamine-type product. The long-chain polyamine imparted higher wet fastness than the dicyandiamide type, but fastness characteristics were found to be influenced by the number and position of solubilising and hydrogen bonding groups in the dye structures [449]. A polyacrylamide with a molecular mass of 1.87 × 105 was prepared by polymerising a 5% w/v aqueous solution of acrylamide monomer in the presence of 0.15% w/w benzyl alcohol and 0.025% w/w potassium persulphate for 45 minutes at 80 °C. This product was effective in preventing the bleeding of direct dyes and hydrolysed reactive dyes from dyed cotton, which was simply dipped in a 1% solution of the polyacrylamide and dried in air [450]. Ecological factors, as well as the improvement of wet fastness, were taken into consideration in a study of viscose dyed with direct dyes (CI Direct Yellow 126, Red 83:1 and Blue 90) and aftertreated with three cationic agents described as monofunctional, bifunctional and trifunctional [451]. For a system to be biodegradable, the BOD5/COD ratio should be at least 0.5. For the systems examined, this ratio ranged from 0.00 to 0.38. Thus, all of them proved difficult to treat, biodegradation taking at least 20 days. It was difficult to

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decide which of the three agents was least hazardous in environmental terms. The bifunctional agent gave the best overall performance, since it was not more environmentally hazardous than the others and gave the smallest colour change, together with good fastness to washing at 60 °C. There has been interest recently in comparing formaldehyde-free, low-formaldehyde and conventional N-methylol fixing agents with direct [452] and reactive [453] dyes. Advantages are claimed for formaldehyde-free products in giving less dye bleeding, less change in shade and excellent fastness to light and wet treatments [453]. The effect of conventional cationic aftertreating agents in improving the fastness of direct dyes can be further enhanced, albeit to only a small extent, by subsequent treatment with a syntan [454]. This may result from the formation of a larger electrostatically linked complex between the anionic syntan and the cationic fixing agent at the fibre surface, having low aqueous solubility and slow diffusion behaviour. Traditionally, cationic fixing agents have been applied to direct dyeings at neutral or slightly acidic pH. There is now some evidence [455] that they can be somewhat more effective if applied under alkaline conditions. The reasons for this remain speculative and more than one mechanism may be operating. 10.9.6 Aftertreatments for sulphur dyes on cellulosic fibres The fastness of sulphur dyes to the increasingly severe conditions of washing currently demanded, especially in the presence of peroxide-containing detergents, can sometimes be improved by an alkylation treatment. The best-known products for this purpose are adducts of epichlorohydrin with either ethylenediamine (especially) or ammonium salts. Typically the procedure involves the application of 2–3% o.w.f. of a proprietary alkylating agent with 1–2% o.w.f. sodium carbonate (pH 10 and 30–40 °C), raising the temperature to 90–95 °C and allowing the reaction to reach completion at this temperature, which it does after 10 minutes. This treatment usually replaces the traditional oxidation treatment, except for dyes having a distinctly yellow leuco form. There has been interest recently in extending the use of cationic fixing agents normally used with direct dyes to sulphur dyes. Five cationic polymers markedly improved the fastness to washing of solubilised sulphur, leuco sulphur and insoluble sulphur dyes when the cationic treatment was applied by a simple exhaust method typical of their application to direct dyeings [456]. The polymers were similarly effective when applied to leuco sulphur dyeings by pad–dry and pad–flash cure methods [457]. The mechanisms operating were thought to resemble those with direct dyes, i.e. the formation of electrostatically linked dye–agent complexes and/or the formation of a peripheral layer of polycations restricting dye mobility. The use of a reactive cationic agent has also been examined [458], this being of a type used successfully as a pretreatment to enhance dyeability. Washing fastness of reoxidised, solubilised sulphur dyes when aftertreated by an exhaust method was markedly improved. It was shown that the cationic aftertreatment could replace the usual reoxidation procedure, giving enhanced fastness. 10.10 AGENTS FOR FIBRE LUBRICATION, SOFTENING, ANTISTATIC EFFECTS, SOIL RELEASE, SOIL REPELLENCY AND BACTERICIDAL ACTIVITY Most of the title products are finishing agents rather than dyeing or printing auxiliaries. Although in principle they could be applied before, during or after coloration (with the

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exception of lubricants applied during textile manufacture), economic reasons firmly put the preference on conjunct application with colorants wherever possible. Whilst not dyeing auxiliaries as such, their inclusion often has a bearing on choice and behaviour of dyes as well as on possible problems encountered during coloration (compatibility or otherwise of the components) and afterwards (fastness properties). Many of these products have useful activity in more than one function. Thus an agent (and especially a composite commercial product) promoted chiefly as a softener may also have antistatic, yarn lubricating and soil release properties, and it is useful to bear this diversity of function in mind. 10.10.1 Fibre lubricants The main consideration here is lubricants applied during wet processing although some consideration will also be given to those included during textile manufacture. The basic requirement of a lubricant is that it should form a thin uniform protective coating around the fibres to lower their surface friction and flexural rigidity, thus assisting a process such as weaving or minimising the formation of durable creases during rope dyeing, particularly at high temperatures. In general practice a lubricant may, and usually does, contain more than one component, a typical composition comprising base lubricant(s), antistatic agent and surfactant(s). HPLC provides a useful means for the analytical separation and identification of the components of a lubricant formulation [459]. From the point of view of subsequent wet processing, it is usually preferable that fabrics be free from fatty substances such as oils and waxes. From the weaver’s viewpoint, however, it is usually contended that such fatty lubricants are essential to minimise wear of machine parts and to prevent yarn abrasion and breakages, more especially where high-speed weaving processes are concerned. Weaving lubricants of this kind are usually applied with the sizing agent (section 10.5.2) and they must operate in conjunction, one of the main functions of the lubricant being to increase the film-forming properties of the size polymer(s). However, such fatty lubricants are difficult to remove and may complicate subsequent desizing, bleaching and coloration processes. This practice of using fatty lubricants has been questioned in recent times, based on evidence that such hydrophobic agents can actually impair the performance of the size as well as creating difficulties in desizing [460–463]. This is the main reason why surfactants are often used with fatty lubricants, to improve compatibility with size polymers and to assist emulsification and removal. However, arguments have been advanced [463] for the use of carefully selected surfactants as lubricating agents. The fatty or hydrophobic moiety of a surfactant produces a low coefficient of friction between surfaces to which it is applied and so acts as a lubricant, whereas the hydrophilic moiety makes for built-in ease of subsequent removal. In addition, the use of inorganic salts such as sodium silicate and sodium carbonate, as routinely used in detergent formulations, enhances the performance of the surfactant as a lubricant, a typical formulation comprising 0.6% surfactant, 0.6–1.0% sodium silicate and 0.5% sodium carbonate. It is claimed that such a composition performs just as well as a fatty lubricant in sizing and yet it is much easier to remove during subsequent scouring or desizing. Anionic and nonionic surfactants were compared but unfortunately no details of structure were disclosed. Clearly a good deal of development work would be required to evaluate a range of surfactants varying in hydrophobicity, in order to obtain optimum performance with a specific size composition and substrate. This is essential in

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any case, irrespective of whether the surfactant is to replace a traditional lubricant or is in support of one. Compatibility with other components becomes a critical factor when lubricants are used in the dyebath. The properties of the ideal dyebath lubricant have been summarised as follows [464]: (1) excellent fibre-to-fibre and fibre-to-metal lubrication; (2) economical; (3) no effect on other physical properties such as handle, water-repellency and absorbency; (4) a foam suppressant or deaerating agent; (5) no effect on reproducibility of dyeing; (6) no effect on fastness properties; (7) non-yellowing; (8) easily washed out; and (9) biodegradable. When formulating a dyebath lubricant, particular attention must be paid to the type of substrate (hydrophilic or hydrophobic) and to substantivity of the lubricant for that substrate. Another important consideration is ionicity of the lubricant in relation to the components of the dyebath, since this has such an important bearing on compatibility (clearly, anionic lubricants should be avoided when basic dyes are used). Solubility and/or dispersibility in relation to dyebath composition and the conditions of dyeing (temperature, pH, liquor ratio) are also important, since the overall hydrophobic/hydrophilic balance has a major influence on compatibility. The behaviour of the lubricant during drying can be just as critical as during dyeing. Many lubricants promote undesirable thermomigration of disperse dyes on polyester during high-temperature drying and heat setting, leading to lower wet fastness. Natural products such as animal fats and vegetable oils still constitute an important share of the lubricants market although synthetic types are gaining acceptance [464]. Natural fats and oils include saponified fatty acids, fatty esters, fatty alcohols and fatty amides. Various anionic groups are suitable, including carboxylate, phosphate, phosphonate, sulphate or sulphonate, the last-named being the most widely preferred. Esterification of the fatty acids is particularly useful. For example, ethoxylation with ethylene oxide enables products of subtly graded character to be produced, depending on the degree of hydrophobicity of the fatty acid and the degree of ethoxylation. Sulphonates offer greater stability at higher pH and ionic strength but they can generate troublesome foaming. Synthetic-based lubricants include polyacrylates, acrylamide/acrylic acid copolymers, emulsified paraffin oils and waxes, modified silicones. The acrylates generally combine excellent solubility in the dyebath with fabric lubrication and such polymers can be designed to cover a wide range of solubility, rinsibility and lubricating performance. Ionicity can be varied, whilst acrylic esters offer scope for incorporating other functionalities along the polymer chain. Acrylic esters also allow the degree of anionicity to be varied. Acrylamide groups generally result in increased stability, especially in acidic media where the amido groups are partially protonated and thus mildly cationic, whilst in neutral media they behave substantially as nonionic moieties. Paraffin hydrocarbons of molecular mass 300 to 700 (i.e. 20–50 carbon atoms) require emulsification with surfactants. They show good resistance to oxidation and yellowing.

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However, the stability of these systems is critically dependent on effective emulsification. Viscosity is usually low, a concomitant of this being that the emulsifier constitutes a substantial proportion of the system. Ideally, the emulsifier should contribute to the lubricating power of the product. Related to the paraffins are the even more hydrophobic polyethylenes, primarily used as sewing lubricants or handle modifiers. These polymers may be difficult to wash out, although their ionic character can be varied according to whether anionic, cationic or nonionic emulsifying systems are chosen. Paraffin waxes and polyethylenes have an advantage over paraffin oils in that they are terminated with a mildly ionic functionality that assists their emulsification and, coupled with their high molecular mass, increases substantivity. This tendency, however, can make them difficult to wash out and leads to compatibility problems in dyeing. They are perhaps best regarded as softeners. Alkoxylated polysiloxanes are a relatively new class of dyebath lubricants. They have practically no substantivity for the substrate, yet combine adequate lubrication with water solubility and easy rinsability. If the silicones contain primary hydroxy groups, these can be modified by esterification, phosphation, phosphonation, sulphation, sulphonation or carboxylation. These anionic substituents confer substantivity for various substrates without losing rinsability. Anionic organic sulphates and sulphonates probably offer the best overall properties for dyebath lubricants, whilst other types can be more suitable for selected applications [464]. Certain problems associated with lubricants The manufacture of textile fabrics for automotive trims normally requires the application of a spin finish, a considerable percentage of which is still present in the finished fabric [465]. Residual lubricants can give rise to ‘fogging’ on the interior of car windows, caused by condensation on the glass of volatile components present: other similar products, such as antistats and softeners, may also be implicated. Fogging is measured by reflectance and is expressed as the ratio of the reflectance values of a glass plate before and after exposure to fogging (DIN 75021), a high value representing a low degree of fogging. Low-fogging compounds typically give values of at least 90%; some measured values [465] are shown in Table 10.45. Chemical type is not a direct indicator of propensity to fogging, since volatility varies with hydrophobic/hydrophilic balance as determined by chain length and structural type on the one hand and the degree of polar character, including ionicity, on the other. Yellowing can occur with certain lubricants during drying or ageing. This phenomenon has been investigated on wool and acrylic fibres and blends of them lubricated with a

Table 10.45 Fogging values of lubricants [465]

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Lubricant chemical type

Fogging value (%)

Alkylbenzene Poly(ethylene glycol) Fatty acid polyglycol ester No. 1 Fatty acid polyglycol ester No. 2 Poly(ethylene/propylene glycols) Phosphate monoester/diester

36 86 55 95 >95 >90

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cationic fatty acid condensation product, a cationic nitrogenous condensation product, a quaternary stearylalkylamino ester, a nonionic oxyethylated carbonate/phosphate and a polyglycol carbonate ester formulated with a nonionic surfactant [466]. Yellowing did not occur when the lubricant manufacturer’s instructions regarding concentration and drying temperature were followed. In some circumstances, however, such as radio-frequency drying, cationic compounds can induce the formation of sparks that may result in yellowing, indicating a need to reduce the power of the dryer and/or to use a lower concentration of lubricant. As already mentioned, some lubricants can be difficult to remove by washing and surfactants are often added to overcome this problem [463]. Lubricants can impair fastness properties, particularly those of disperse dyes. They may influence the uptake of dyes either positively or negatively, although seldom seriously except where it results in unlevelness. For example, knitting oils can increase the yield of relatively oleophilic reactive dyes on cotton and yet with highly hydrophilic types they may cause dye-resist effects [467]. When considering the environmental aspects of lubricants, it is necessary to bear in mind that commercial formulations are often more or less complex mixtures and that even small amounts of toxicologically questionable components can lead to restrictions on use, depending on local regulations. Thermally stable, readily biodegradable esterified oil emulsions can offer a compromise solution, being environmentally preferable to unmodified oils, fats and waxes, although problems can still arise in high-temperature processes such as texturising or heat setting, as a result of cracking reactions leading to the formation of harmful residues or condensates [468]. In order to overcome this problem of cracking, attention must be paid to the susceptible rupture points in the molecule. This has been done in the development [468,469] of a range of ecologically friendly lubricants by using carbonate ester bridging groups to combine fatty alcohols or their ethoxylates with poly(ethylene glycol) (10.216). Such products are readily water-soluble, self-emulsifiable, thermally stable compounds with excellent cracking behaviour. Their lubricity is such that they can be used in amounts 50–80% lower than normal. They show excellent biodegradability and skin compatibility properties. Emission values are 10–20 times less than with comparable esters or mineral oils. Environmentally friendly lubricants containing alkylglucoside with emulsifier (1:2) or amine oxide with emulsifier (1:2) have also been claimed [470]. O R

O

(CH2CH2O)x

C

O (OCH2CH2)y

O

C

(OCH2CH2)y

O

R

10.216

10.10.2 Antistatic agents Antistatic agents are useful to prevent garments accumulating charges of static electricity, particularly those made entirely from hydrophobic fibres. Whilst this is desirable for domestic wear, it takes on added importance in many other sectors, when working in explosion hazard areas, with micro-electronic components, in electronic data processing environments, near textile dust filtering systems, or in certain areas of military and space travel technology. Antistatic treatments may be impermanent (easily removed on washing)

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or have a degree of durability. They operate through their hydrotropic properties, creating a moist microclimate that discourages the build-up of static, or by having some degree of conductivity, thus dissipating any localised static charge into a wider area of lower potential. Impermanent antistats Surfactants may exhibit a degree of hydrotropy and thus function as antistatic agents. They are often used for this and for their emulsifying properties in conjunction with fibre lubricants, or may be used alone in a dual capacity as lubricant and antistat. Suitably active surfactants can be found amongst all four ionic types, some typical examples being [471]: – quaternary ammonium derivatives of fatty acids – polyethoxylated quaternary ammonium derivatives – quaternised alkylenediamines – alkyl sulphates, chlorides or phosphates – mixtures of mono- and di-esters of phosphoric acid – long chain amine oxides – polyethoxylated and polypropoxylated nonionics. Nonionic and cationic types are generally preferred, usually on grounds of fibre compatibility, higher hygroscopicity and higher oil solubility. The quaternary ammonium derivatives of fatty acids in particular impart static protection at a low level of application (i.e. 0.25%), whilst the polyoxyethylated versions give higher solubility in water. The nonionic and cationic types dominate particularly in spin finishes [471]. However, the cationic types may create subsequent problems if anionic surfactants are used at the washing stage. Impermanent antistatic agents that are not surface-active include triethanolamine, glycerol in combination with potassium acetate, as well as inorganic salts such as lithium chloride [471]. The major requirements for impermanent antistats are [47l]: (1) effective at low levels of humidity; (2) low volatility; (3) non-corrosive; (4) low toxicity; and (5) no yellowing. In addition, if used in spin finishes they must be thermally stable, oil-soluble and exhibit low migration on the fibre. Durable antistats There are two types of durable antistat. The first (so-called external antistatic agent) is applied at some stage after fibre manufacture whilst the second type is incorporated during manufacture. The former type exhibits varying degrees of durability and is not usually as durable as the latter. External antistatic treatments generally involve the formation of a crosslinked polymer containing ionic and/or hygroscopic groups, cationic polyelectrolytes containing poly(ethylene oxide) units being widely used. Many crosslinking agents are suitable to insolubilise hydrophilic

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components within a fibre, including poly(ethylene glycol) dihalides, epoxy compounds, cyanuric chloride, derivatives of piperazine, hexahydro-s-triazine, polyaziridines, polyacrylamide and polyamines [471]. Casein modified by graft copolymerisation with esters of acrylic acid and methacrylic acid has been traditionally used for the coating of leather. Casein modified in this way by grafting with ethyl acrylate [472] showed good film-forming properties on textile materials, particularly as an antistatic agent in the backing of carpets and as an additive in pigment printing. This polymer satisfied ecological requirements and its antistatic properties were substantially enhanced by incorporating electroconductive materials such as carbon black and powdered metals. In another evaluation [473], durable antistatic properties were obtained using watersoluble quaternary ammonium polyamines applied to polyester by padding and curing at 160 °C. These products were prepared according to Scheme 10.85. Epichlorohydrin was condensed with poly(ethylene glycol) 600 (known for its hygroscopicity and antistatic effect) using boron trifluoride as catalyst to give the poly(ethylene glycol) diglycidyl ether. This was then reacted with the highly electroconductive tetra-ethylenepentamine to give a watersoluble polyamine. Finally, in order to prevent gelation and to improve the stability in aqueous solution, this polyamine was cationised as the phosphate salt. An obvious means of increasing conductivity is to incorporate metals into the fabric. Thus fabric can be sprayed with a liquid resembling metallic paint, containing micron-sized metallic particles such as copper incorporated into a binder such as a polyester, epoxy or acrylic resin. During curing the metallic particles come into contact with one another, thus

H2C

CH

CH2Cl

+ HO(CH2CH2O)nCH2CH2OH

+ ClCH2

CH

O

CH2 O

BF3

H2C

CH

CH2

O(CH2CH2O)nCH2CH2O

CH2

O

CH

CH2 O

H2N(CH2CH2NH)3CH2CH2NH2

H2C

CHCH2O(CH2CH2O)n+1CH2CHCH2NH(CH2CH2NH)4CH2CHCH2(OCH2CH2)n+1OCH2CH CH2 O

OH

O

OH OH O

P

OH

OH HOCH2

+ + CHCH2O(CH2CH2O)n+1CH2CHCH2NH2(CH2CH2NH2)4CH2CHCH2(OCH2CH2)n+1OCH2CH CH2OH OH

OH

_ O O

OH

Scheme 10.85

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OH _ O

_ O

O

P

_

OH _ O

O

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providing excellent conductivity throughout the material [474]. The presence of copper, however, may present environmental problems when waste material of this kind requires disposal. An unusual method of imparting antistatic properties to acrylic fibres involves saponification of some of the nitrile groups to give amide and carboxyl groups, using sodium hydroxide under exhaust (80 °C), pad–batch (varying within 15–120 minutes at 160–60 °C) or pad–steam conditions. The pad–steam method in particular provides convenient surface modification of acrylic fibres, giving antistatic properties together with improved soil-release and improved dyeing properties with basic dyes [475]. This method may be substrate-specific since acrylic fibres vary widely in their sensitivity to alkali, some being quite easily discoloured. Methods of incorporating durable antistats during fibre manufacture include: – incorporation of a hydrophilic component, such as a surfactant – formation of a lattice of highly conductive components within the fibre. This approach is outside the scope of the present chapter but details may be found elsewhere [471,476,477]. 10.10.3 Softeners It is difficult to define unequivocally the quality of fabric handle or softness/firmness differences, since this involves many factors. It is often linked with lubrication, especially as similar products are often used for softening and lubrication. Whilst experienced assessors can be quite remarkable in the extent to which they can grade and assess softeners simply by means of a highly developed tactile sense, more objective methods are clearly desirable for scientific investigations. Since many factors combine in producing an overall sense of softness, it is not surprising that objective determination of softness involves more than one parameter of measurement. The details of assessment are outside the scope of this chapter, but descriptions and discussions are available elsewhere [478–481]. Suffice it to say here that the Kawabata system has acquired considerable importance in quantifying various aspects of fabric handle. The oils and waxes described as lubricants in section 10.10.1, as well as talc, can be used as softeners but have now been superseded by more effective products. These may be nonreactive or reactive and may be cationic, anionic, nonionic or amphoteric. Although many compounds have been patented, by far the most important are cationic quaternary ammonium compounds and various silicones. Until quite recently the field was led by the cationic types but there is now evidence that aminofunctional polysiloxanes have become the most important product group [482]. Before dealing with the detailed chemistry of softening agents, it is useful to consider some general factors. The desirable properties of an ideal softener can be summarised as follows [482]: – easy to handle (suitable for pumping, stable on dilution) – low foaming, stable to shearing, free from deposits on rollers – compatible with other chemicals – good exhaustion properties – applicable by spraying

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– – – – – –

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stable to high temperatures, non-volatile especially in steam no effect on shade or fastness no yellowing easily biodegradable non-toxic, dermatologically harmless, non-corrosive no restrictions for transport or storage.

No single product satisfies all the above requirements. Commercial softeners are usually more or less complex mixtures, available as aqueous emulsions with a solids content of 15– 25%. As well as the softener component(s) other additives are also present, including the emulsifying and/or dispersing agents necessary. It cannot be over-emphasised that emulsion stability is just as important as softener effectiveness, since it is crucial to the performance of the product. Nonionic emulsifying agents are especially useful, showing all-round compatibility with other substances even if they are different in ionicity. Consideration needs to be given to cloud point phenomena and stability to electrolytes when using nonionic surfactants. In such cases the emulsifying system itself may be a mixture. Softener emulsions are available in different commercial grades. Semimicro-emulsions have an average particle size of <0.1 µm, allowing for penetration to the fibre core and giving excellent distribution of the softener. Micro-emulsions have an average particle size of <0.01 µm, giving the substrate an excellent inner softness and a distinctive surface smoothness without looking greasy [482]. Micro-emulsions have good stability and a decreased tendency for deposition on rollers. Critically optimised products are particularly necessary when using micro-emulsions on high-speed equipment where high shearing forces are developed [482]. Irrespective of the basic chemical type, the fabric handle can be modified by varying the hydrophilic–hydrophobic balance of the softener. Thus increasing hydrophobicity by incorporating a longer alkyl chain imparts an increasingly greasy handle to the fabric. The ionicity of the softener influences its substantivity for different fibres. Whilst this can be significant in padding applications, it is of primary importance for application by exhaust methods. Cationic softeners are highly substantive to acrylic fibres, whilst anionic products are substantive to wool and nylon. Nonionic softeners, if water-soluble, have very low substantivity for any substrate and are easily lost by washing. Insoluble types may be substantive to polyester or cellulose acetate, as well as wool and nylon. The degree of softening with the nonionic products is only moderate but their nonionic nature makes them convenient for application simultaneously with any class of dyes or fluorescent brightening agents. The low-substantivity agents are preferable for repeat application in domestic washing. There has been a trend towards so-called multifunctional softeners in which account is taken of various other factors such as hydrophilicity, sewability, antistatic behaviour, lubrication and shearing stability [482]. For example, in jet dyeing a product will require, in addition to softening power: – extremely high emulsion stability to cope with high pump and nozzle shearing forces at high temperatures – a suitable defoaming system – good substantivity to ensure adequate exhaustion.

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Conversely, if good sewability is required, suitable products need to have good lubrication behaviour. Quaternary ammonium compounds and other cationic softeners Organic cationic softeners, as opposed to silicone derivatives, are usually quaternary alkylammonium compounds, the most important over many years being dimethyldistearylammonium methosulphate (10.217), on grounds of economy and availability. There are obviously many possible variants of such structures but typically the long-chain substituents are within the range C16 to C22 and may be fully or partially saturated [483]. These softeners possess aqueous solubility, substantivity for various fibres and, to some extent, antistatic and water-repellency properties. These properties can be modified by varying the substituents on the quaternary nitrogen atom. The softening effect is especially good. The balance of properties can be controlled more precisely using analogous ethoxylated or propoxylated amines (10.218), in which the degree of ethoxylation can also be varied. These are more expensive but provide high-quality industrial softeners. Further compounds available include quaternised imidazolines (10.219) and diamides or diurethanes containing a protonated amino group (10.220; R = alkyl or alkoxy).

CH3

CH3 + N (CH2)16CH3 (CH2)16CH3

CH3SO4

O (CH2CH2O)nH C + CH3 N CH2CH2NH _ CH2CH2NH CH3SO4 C R O

10.218

10.217

R

R1

_ CH3SO4

+ HN

O

N

CH2CH2NH C O

10.219

R2

H +H R C N HN CH2CH2 CH2CH2 _ X 10.220

O C

R

NH

The specific properties of such compounds obviously depend on the precise nature of the substituents but in general the degree of softening decreases as follows: dialkyldimethylammonium compounds > imidazolinium salts > aminoalkyl diamides or diurethanes [480]. In the case of the quaternised compounds the preferred anions are methosulphate or ethosulphate, since these have a less corrosive effect on steel vessels than the chloride salts. These compounds generally show maximum cation activity at around pH 3.5 but are usually applied at higher pH values. In some cases the cationic softening agent is mixed with a nonionic surfactant to serve as a lubricant/antistat, this being particularly common in fabric softeners for domestic use. It is also worth noting that most quaternary ammonium compounds have a degree of antibacterial activity.

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From a detailed study of three quaternary ammonium softeners based on hydrogenated tallow oil (10.221–10.223), adsorption of these softeners by cotton and acrylic fibres is shown in Figure 10.56. Adsorption of the three softeners by cotton did not vary greatly, apparently following a Donnan equilibrium mechanism. On the acrylic substrate a saturation point was reached at about 2 µmol of each softener per gram of fabric. A twenty-member panel using tactile sensation found no significant difference between the products for softness on either fibre. Measurement of bending rigidity indicated the following results for the same quantity of applied softener: on cotton fabric: imidazolinium quat > ester quat or alkyl quat on acrylic fabric: ester quat or imidazolinium quat > alkyl quat.

Softener adsorption on cotton fabric [NaCl] = 1.25 g/l, 30 oC, pH 4.5–5.0

Softener adsorption on acrylic fabric [NaCl] = 1.25 g/l, 30 oC, pH 4.5–5.0 0.20

0.6

A Q I Q E Q

0.4

0.2

0.2

0.4

0.6

0.8

Softener concentration/%owf

Adsorbed softener/%owf

Adsorbed softener/%owf

0.8

E Q I Q

0.15

A Q

0.10

0.05

0.2

0.4

0.6

Figure 10.56 Adsorption of cationic softeners by cotton and acrylic fabrics at 30 °C [483]

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0.8

Softener concentration/%owf

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50

Rewettability/%

40

30 A Q E Q

20

10 I Q

0.1

0.2

0.3

0.4

0.5

0.6

Softener concentration/%owf

Figure 10.57 Effect of cationic softeners on the rewettability of cotton fabric [483]; 20 °C, 60% relative humidity

50

Semi–discharge time/s

40

30

20

10

E–Q A–Q

0.1

I–Q

E–Q A–Q

I–Q

E–Q A–Q

0.2

I–Q

0.4

Softener concentration/%owf

Figure 10.58 Effect of cationic softeners on the antistatic behaviour of acrylic fabric [483]; 20 °C, 60% relative humidity

A problem associated with quaternary ammonium softeners is that they increase the hydrophobic nature of the fibre and thus interfere with rewetting, this effect being cumulative with each successive application of the softener. The rewettability of a cotton fabric treated with the above three softeners is shown in Figure 10.57, wettability being affected somewhat less by the alkyl or ester quat than by the imidazolinium quat. Antistatic effects on the acrylic fabric are illustrated in Figure 10.58, showing wide variations between the softeners at low concentrations [483]. Surprisingly, none of these softeners induced significant thermomigration of three disperse dyes on polyester [483]. Micro-emulsion formulations of cationic softeners are available and are claimed to have superior fibre penetration properties [484]. Figure 10.59 shows the sorption of a micro-

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emulsion formulation of dimethyldistearylammonium (10.217) chloride. Similar results were obtained with micro-emulsion formulations of N,N-dialkyl-N-2-hydroxyethyl-Nmethylammonium methosulphate (10.224) and N,N-bis(stearamidoethyl)-2-methylimidazolinium methosulphate (10.225), although the highest rate constant values were observed with the imidazoline derivative and the lowest with the dialkylhydroxyethylmethylammonium methosulphate (Table 10.46). Cotton washed 50 times in hard water absorbed the softeners rather more strongly than unwashed cotton. It is thought that this may be due to adsorption by the cotton of inorganic salts from the hard water and of acidic residues from the detergent used. Yellowing can occur with quaternary cationic softeners and this limits their use on white fabrics. This problem can be overcome to some extent, provided drying or fixation temperatures are not too high, using so-called pseudo-cationic softeners [482]. These products are analogous to the so-called weakly cationic surfactants described in section 9.5.

60 oC 40 oC 32.5 oC 25 oC

Sorption/g per kg cotton

3.0 2.5 2.0 1.5 1.0 0.5

0.25

1

2

3

4

Time/min

Figure 10.59 Sorption kinetics of micro-emulsion cationic softener formulation at different temperatures on cotton fabric [484]. Initial concentration 3 g/kg fibre, liquor ratio 15:1

CH3 _

CH2CH2OH + N (CH2)nCH3 (CH2)nCH3

CH3SO4

10.224

O CH3(CH2)16

C NHCH2CH2 CH3SO4–

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CH3 + N

O C

N

(CH2)16CH3

CH2CH2NH

10.225

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Table 10.46 Rate constants for sorption of cationic softeners at various temperatures on unwashed and washed cotton [484] Rate constants

Softener Dimethyldistearylammonium (10.217) chloride

Dialkylhydroxyethylmethylammonium methosulphate (10.224)

Bis(stearamidoethyl)methylimidazolinium methosulphate (10.225)

Temperature (°C)

Unwashed cotton

Washed 50 times

25 32.5 40 60

0.42 0.55 0.64 0.83

0.72 0.79 1.14 1.70

25 32.5 40 60

0.46 0.50 0.55 0.61

0.57 0.63 0.71 0.81

25 32.5 40 60

0.73 0.80 0.91 1.03

0.95 1.10 1.16 1.47

Silicone softeners Poly(dimethyl siloxane) (10.226) represents the simplest of the silicone softeners, which are usually applied from emulsion. This polymer, however, is unreactive and is not substantive to fibres. Therefore it is not fast to washing and is little used nowadays. More durable softeners can be derived by modification of this fundamental siloxane. The so-called ‘conventionally reactive’ silicones were the first to be introduced, typical examples being those containing activated silanic hydrogen (10.227) or silanol (10.228) groups. Such polymers react with crosslinking agents (10.229 being a typical example) during a curing process with an organometallic catalyst, typically a zinc or zirconium alkanoate. A crosslinked polymer network of high molecular mass is formed on the fibre, this being the mechanism necessary to achieve a soft finish of high durability. CH3 CH3

Si

CH3

CH3 Si

O

CH3

O

CH3

Si n

CH3

CH3

CH3

CH3

Si

H O

CH3

10.226

Si CH3

O

Si CH3 10.228

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O

CH3

Si n

10.227

CH3

CH3 HO

Si

CH3

CH3 Si

O n

CH3

OCH3 OH

CH3

Si

OCH3

OCH3 10.229

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The next development arose in the late 1970s with the introduction of aminofunctional silicone softeners. These contain aminoalkyl groups attached to the poly(dimethyl siloxane) backbone, leading to improved orientation and substantivity for the fibre [482]. This favourable orientation, illustrated in Figure 10.60, leads to an extremely soft handle, often described as ‘supersoft’. These polymers are highly cost-effective, very small amounts being required to obtain the desired properties, thus giving both economic and environmental benefits. Indeed, such are the advantages of these products that they have assumed a dominant role in the marketplace. Mainly based on a single type of amino group, more than 90% of all commercially available aminosilicone softeners are in fact aminoethyliminopropyl silicones (10.230) [485]. The softening effect of silicones results from their lubrication behaviour that affects both the surface and the interior of the fibre. The behaviour of polysiloxanes of the 10.230 type can be varied by adjusting the average values of x and y and the range of chain lengths present. Further variations are possible by varying the R groups. In view of the technical and

Negative zeta potential of cellulosic fibre

Figure 10.60 Attachment and orientation of an aminofunctional polysiloxane on a cellulosic fibre [482] CH3

CH3 R

Si

O

CH3

Si CH3

R Si

O x

CH3 O

(CH2)3

Si

R

CH3

NH 10.230

CH2 CH2

R = OH, CH3, OCH3

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

commercial importance of the aminofunctional polysiloxanes, it is worth exploring some structure/property relationships [485–489] even though there is as yet no conclusive correlation available for the most important aspect, that of softness as distinguished by tactile sensation. The presence of the aminofunctional group appears to be necessary to obtain a ‘supersoft’ handle. In a study [488] of nineteen variants of the aminoethyliminopropyl polysiloxane structure (10.230) differing in amino group content (0 to 1.5 milli-eq./g), viscosity (300 to 21 000 mPas) and emulsion particle size (50 to 350 nm), good agreement was found between test parameters and amino group content, for which there was a strong substrate dependence. The cotton fabric softness showed a marked dependence on amino group content, giving an optimum value at about 0.3–0.4 milli-eq./g polysiloxane. Polyester/cotton blends (65:35 to 35:65) gave an optimum at about 0.2 milli-eq./g with a strong dependence on amino content, whereas a 100% polyester fabric showed a much weaker dependence together with a relatively high optimum at 0.5–0.6 milli-eq./g. Emulsion particle size and viscosity played subordinate roles, although it is known that very low viscosities (indicating short polymer chains) and very small particle sizes can be detrimental to softness effects under certain conditions. Epoxyfunctional siloxanes are also useful as softeners. These may be derived from polysiloxane (10.231) or from aminopolysiloxanes (10.232). Further possibilities are represented by the polyalkoxylated epoxyfunctional silicones (10.233) and polyalkoxylated aminofunctional silicones (10.234). However, it has been pointed out [485] that the reaction of epichlorohydrin with aminopolysiloxanes is not very specific, since primary and secondary amine groups are usually randomly epoxidised resulting in viscous products that

CH3 CH3

Si

CH3 O

CH3

Si

CH3 Si

O

CH3

Si

O

CH2

x

CH3

CH3

y

Si

CH3

CH3

CH3 O

CH3

CH3

Si CH3

CH3 Si

O x

10.231

CH2

Si

CH3 O

CH3

CH3

CH

CH2

O

CH

CH3

R

Si

CH2

10.232 O

CH3

CH2 Si

O

CH3

x

CH3 Si

O

CH3

y

CH2

CH3 Si

O

(CH2)n

z

CH3

CH3

(OCH2CH2)a(OCH2CH2CH2)bOCH3

10.233

CH2CH2NHCH2CH2NH2 CH3 CH3

Si CH3

CH3 O

Si

CH2 Si

O

CH3

x

CH3

CH3

CH3 Si

O y

Si

O

(CH2)n

CH3

CH3 z

(OCH2CH2)a(OCH2CH2CH2)bOCH3 10.234

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Si

O

NH

CH O

CH3

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are difficult to emulsify. Thus they do not offer a significant improvement over the parent aminopolysiloxanes. Promising products, offering a variety of structural possibilities, are obtained by acylation of aminopolysiloxanes [485]. Suitable acylating agents include anhydrides, lactones and carbonates (Scheme 10.86), of which acetic anhydride is the most economical. The optimum combination of effects is obtained by 30–70% acylation; more than 70% substitution can reduce the softening effect to the level associated with conventional poly(dimethyl siloxane). No significant differences have been observed with respect to handle, whiteness or water absorbency, depending on whether acylation is achieved using acetic anhydride, butyrolactone or ethylene carbonate as the acylating agent. A slight decline in softness of handle is observed with the acylated products compared with that from normal aminopolysiloxanes but this is compensated for by better whiteness, water absorbency and soil release properties. A major drawback of the standard aminoethyliminopropyl polysiloxanes is their tendency to show yellowing, resulting from the formation of chromogenic species by oxidative thermal decomposition of the aminofunctional group. Acylation largely overcomes this problem and also gives improvements in water absorbency and soil-release performance.

Si

(CH2)3NHCH2CH2NH C O C O

Si

CH2

CH2CH2CH2OH

O

O

CH2

CH3C

CH2

O CH3C

(CH2)3NHCH2CH2NH2

O

Si

(CH2)3NHCH2CH2N H C

O O

C O

Si

CH3

O

CH2 CH2

(CH2)3NHCH2CH2NH C

OCH2CH2OH

O

Scheme 10.86

Further fine tuning of the properties of aminopolysiloxanes can be achieved [485] by substitution of the siloxane units with primary, secondary or tertiary amino functions (10.235). Whiteness, water absorbency and soil release properties are improved with increasing degree of amino substitution from primary through to tertiary amines (Figure 10.61). The improvement in whiteness compared with the aminoethylimino reference product is particularly noteworthy, this being attributed to retardation of the oxidative thermal degradation because of the protective effect of alkylamino substitution. However, only the secondary amines show softness values as good as those of the aminoethylimino reference. The cyclohexylamino function, in particular, gives rise to a most useful

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Handle Reference NHRNH2

Whiteness Primary NH2

Water Soil absorbency release Secondary Tertiary NHR NR2

Figure 10.61 Effect of aminofunctional polysiloxanes on the physical properties of treated cotton fabric [485]

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CH3

CH3 CH3

Si

Si

O

CH3

CH3 Si

O

CH3

x

723

CH3 Si

O

(CH2)3 O

CH3

CH3 y

CH H2C H3C

C

C N

H3C

10.236

CH2

CH3

H

Barrier to rotation (kcal/mol)

Bond dimensions

C

C

ca. 110o

O

C

ca. 110o

C

2.74

C

1.06

1.

Å

H3C—CH2—CH3

41

1.

54 Å

Typical compound

CH3

H3C—O—CH3

3Å

1.6

H3Si—O—SiH3

Si

O ca. 144o

Si

0.32

Figure 10.62 Molecular dimensions of alkane, ether and siloxane chain segments [489]

combination of effects: it gives optimum whiteness, as well as improved water absorbency and soil release (both properties superior to those of acylamino derivatives). In view of these results, it is not surprising to learn that hindered secondary amino substituents (10.236) also give rise to non-yellowing softeners [490]. An interesting attempt has been made to formulate a theoretical mechanism for the softening and fibre-substantivity characteristics of aminopolysiloxanes [489]. Their outstanding softening effects are the result of exceptionally high mobility of the polymer segments, this lubrication behaviour affecting both the surface and the interior of the fibre. Polyether chains are more flexible than polyalkyl chains, having a bond length of 1.41 Å and a bond angle of about 110° (Figure 10.62), thus providing a lower barrier to rotation.

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Polysiloxane chains have a longer bond length and a larger bond angle, giving rise to an even lower barrier to rotation. These softening agents have a binary nature; on the one hand there is the relatively hydrophobic polysiloxane backbone of the polymer, whilst on the other there is the aminoethyliminopropyl group capable of varying degrees of protonation. Increased protonation gives enhanced hydrophilicity and an increasing number of positive electrostatic charges. The hydrophobic backbone confers substantivity for hydrophobic fibres, whereas the protonated amino groups provide for electrostatic attraction to negatively charged fibres. These relationships are illustrated diagrammatically in Figures 10.63 to 10.68. In the case of poly(dimethyl siloxane) on cotton (Figure 10.63), the fibre–water hydrogen bonding forces are favoured and the weak polymer–fibre forces are inadequate to provide uniform deposition of a polymer film over the fibre surface. Thus the lubricating action to decrease fibre–fibre friction is inadequate and the handle of the finished fabric is firmer than untreated cotton.

Cotton fibre surface Dimethyl siloxane group

Figure 10.63 Poly(dimethyl siloxane) attached to cotton by weak polymer-fibre interactions [489]

Attachment to the cotton surface of an aminofunctional silicone containing relatively few partly protonated amino substituents is illustrated in Figure 10.64. The strong interaction between these groups and hydroxy groups in the cellulose does bring about some orientation of the silicone polymer segments close to the fibre surface but coverage of the latter is incomplete. The lubricating action of the polymer film, though more effective than in Figure 10.63, is somewhat limited and the fabric handle is not ‘supersoft’. Figure 10.65 shows the much more effective distribution over the fibre surface of an aminofunctional silicone containing the optimum proportion of partly protonated amino substituents. Coverage is uniform and complete and the length of the silicone polymer segments between the anchoring amino substituents is sufficient to permit their high flexibility to contribute to optimum lubrication and a ‘supersoft’ handle. In the case of an aminofunctional silicone containing an excessive proportion of partly protonated amino groups (Figure 10.66), coverage is complete but the thin silicone film composed of short and inflexible polymer segments between the anchoring points is inadequate to provide a ‘supersoft’ handle. Figure 10.67 indicates the probable distribution of a silicone containing the optimum content of aminoethyliminopropyl groups when applied to a polyester fibre surface. In this case the attachment is through hydrophobic polymer–fibre interaction and the mobility of the silicone chain segments is increased by electrostatic repulsion between neighbouring cationic groups. Dependence of softness of the treated polyester fabric on the proportion of

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Cotton fibre surface Dimethyl siloxane group Aminoethyliminopropyl group (partly protonated)

Figure 10.64 Aminoethyliminopropyl silicone attached to cotton by too few cationic amino groups [489]

Cotton fibre surface Dimethyl siloxane group Aminoethyliminopropyl group (partly protonated)

Figure 10.65 Aminoethyliminopropyl silicone attached to cotton by the optimum proportion of cationic amino groups [489]

Cotton fibre surface Dimethyl siloxane group Aminoethyliminopropyl group (partly protonated)

Figure 10.66 Aminoethyliminopropyl silicone attached to cotton by too many cationic amino groups [489]

Polyester fibre surface Dimethyl siloxane group Aminoethyliminopropyl group (partly protonated)

Figure 10.67 Probable orientation of an aminofunctional silicone on the surface of polyester [489]

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

Polyester fibre surface

Cotton fibre surface Dimethyl siloxane group Aminoethyliminopropyl group (partly protonated)

Figure 10.68 Probable orientation of an aminofunctional silicone between the surfaces of component fibres in a polyester/cotton blend [489]

partly protonated amino groups present is less sensitive than in the case of cotton and the optimum level of softness is reached at a higher proportion on polyester than on cotton. These concepts are extended in Figure 10.68 to the behaviour of an aminofunctional silicone applied to a polyester/cotton blend fabric. Hydrogen bonding and coulombic forces of interaction between the partly protonated aminoethyliminopropyl groups of the silicone and hydroxy groups at the cellulose surface are reinforced by hydrophobic interaction between the dimethyl siloxane units and the polyester fibre surface. The mobility of the silicone polymer segments is favoured by electrostatic repulsion between neighbouring cationic groups and the optimum degree of softness is achieved at a relatively low proportion of these groups, somewhat less than the corresponding optimum on cotton. Although fabrics made from microfibres generally have a softer handle and better drape than those from conventional fibres, these properties can be further improved to a significant extent by the application of silicone softeners, the best results being obtained with aminofunctional polysiloxanes [491]. Miscellaneous softening treatments Many other products can be used as softeners but are less important commercially because of greater cost and/or inferior properties. Examples are anionic surfactants such as longchain (C16–C22) alkyl sulphates, sulphonates, sulphosuccinates and soaps. These have rather low substantivity and are easily washed out. Nonionic types of limited substantivity and durability, usually applied by padding, include polyethoxylated derivatives of long-chain alcohols, acids, glycerides, oils and waxes. They are useful where ionic surfactants would pose compatibility problems and they exhibit useful antistatic properties, but they are more frequently used as lubricants in combination with other softeners, particularly the cationics.

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Some amphoteric softeners such as amino acids (10.237) and sulphobetaines (10.238) are more effective and durable than the nonionic types but less durable than the cationics; moreover, they tend to be expensive. Other amphoteric types include the zwitterionic forms of quaternised imidazolines (10.239); long-chain amine oxides (10.240) also exhibit softening properties.

R

_ O

+ NH2

CH2CH2

R

R

O

+ N (CH2)n

S

R

O

C O

10.237

10.238

R + HN

_ O N

_ O

R

C

CH2CH2OCH2CH2

R + N

_ O

R

O

10.240

10.239

There are reactive softeners, some of which are N-methylol derivatives of long-chain fatty amides (10.241) while others are triazinyl compounds (10.242). The N-methylol compounds require baking with a latent acid catalyst to effect reaction, whereas dichlorotriazines require mildly alkaline fixation conditions. The N-methylol compounds are sometimes useful for combination with crease-resist, durable-press, soil-release and waterrepellent finishes. In this context, the feasibility of using silane monomers such as methyltriethoxysilane (10.243), vinyltriethoxysilane (10.244), vinyltriacetylsilane (10.245) and epoxypropyltrimethoxysilane (10.246) in crosslinking reactions to give crease-resist properties and softness simultaneously has been investigated [492]. Cl N

O C HOCH2

NH

HN

OCH2CH3

H3C H2C

CH

OCH2CH3 O

10.244

OCH2CH3

OCH2CH3 10.243

10.242

OCH2CH3 Si

Si

Cl

O

CH

R

N

C

10.241

H2C

H3C

N

N

R

OCH2CH3

R

10.245

O C

CH3

Si

C

C

O CH3

CH2

OCH3 H3CO

Si

CH2CH2CH2 CH

OCH3

O 10.246

Softening treatments of a rather different nature include biofinishing enzyme treatments to modify the fabric surface. This has been dealt with already in section 10.4.2. Even more esoteric is the use of so-called telluric treatments using minerals (microliths) of precisely defined lithological and metamorphic properties. A detailed account of these complex materials is available [493]. In essence, an enzyme is micro-encapsulated within the mineral

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microlith. Under the action of strong mechanical forces this crystal structure is broken open, progressively releasing the enzyme. The process thus combines mechanical surface erosion of the textile with biochemical modification. Environmental aspects of softening treatments This account is concerned with environmental aspects of the application of softeners rather than their manufacture, although a discussion of environmental factors involved in the production of quaternary ammonium softeners is available [494]. Environmental aspects of cationic quaternary ammonium salts, nonionic surfactants and amphoteric compounds have been dealt with already in section 9.8.1. With regard to cationic quaternary softeners (Figure 10.56), it has been reported that the ester 10.221 can be considered to be biodegradable, in which respect it is superior to the tetra-alkylammonium salt 10.222 and the imidazolinium salt 10.223 [483]. A paradox has also been pointed out [494]: as the water solubility of quaternary compounds increases so does the rate of biodegradation and the fish toxicity, so that the requirement for the maximum rate of biodegradation can only be met by developing products that are more toxic to fish. Fish suffer through the interference of pollutants with gill breathing. In some mammals, however, there is a possibility of developing a subcutaneous toxicity that can cause neuromuscular problems and ultimately possible death. In this sense, quaternised silicone polymers can become highly toxic if certain neuronal distances are met, in the sense of a lock and key fit between them. The environmental compatibility of silicone softeners is generally favourable [495,496]. The discussion here concerns only the silicone component of the formulation and not the supporting emulsifying system. For the most part this is nonionic, preferably based on linear ethoxylated fatty alcohols, although alkylphenol ethoxylates are still used in some countries [496]. The salient points regarding the environmental influence of silicones can be summarised as follows: (1) Silicones are a minor part of discharges to waste waters. (2) Although highly resistant to biodegradation by micro-organisms, poly(dimethyl siloxane) derivatives are very effectively degraded via natural chemical processes [495] such as catalysed hydrolysis and oxidation during soil contact to produce siloxanols and silanols of lower molecular mass (Scheme 10.87). These are then susceptible to both biological and abiotic decomposition, ultimately oxidising to natural silica. CH3 CH3

Si CH3

CH3 O

Si CH3

Si

O x

CH3

CH3

CH3

CH3 CH3

soil water

HO

Si CH3

O

H

+

CH3

y

Si

OH

CH3

y = mainly 1

Scheme 10.87

(3) Silicones are ecologically inert, having no effect on aerobic or anaerobic bacteria. Thus they do not inhibit the biological processes taking place during waste water treatment. (4) Non-volatile silicones do not bioconcentrate in aquatic media. Their large molecular size prevents them from passing through the membranes of fish or other aquatic creatures. They readily become attached to particulate matter and are effectively

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removed by the natural cleansing process of sedimentation. Elimination rates from sewage sludge are very high. (5) Silicones give insignificant BOD values. Tests on aquatic plant and animal life revealed no measurable adverse effects even under highly exaggerated conditions. No significant change in growth rates of algae, plankton or other marine organisms has been found. (6) Silicones have not been found to pose a threat to insects or birds. (7) Volatile silicones are broken down by oxidative chemical processes on entering the atmosphere. The partially oxidised degradation products are less volatile and these are scrubbed out of the atmosphere by rain or deposited on the ground to be further diluted and degraded, the final products being natural silica, carbon dioxide and water. (8) Volatile methylsiloxanes degrade quickly, the atmospheric lifetime being 10 to 30 days, and have no potential to interfere with the ozone layer. (9) There is no risk of forming compounds that contribute to AOX values. (10) Formaldehyde can be produced only if the degradation temperature exceeds 200 °C and even then the amounts produced are significantly less than those from carbon–carbon polymers containing methyl groups.

10.10.4 Soil-release, soil-repellent and water-repellent agents Agents to protect against soiling were developed following the increasing use of hydrophobic fibres, particularly nylon and polyester, since experience demonstrated the tenacity of oily stains and oil-bound dirt for these fibres. Durable press cotton fabrics also tended to soil more easily than untreated fabrics. The subject of soiling and soil removal is more complex than might at first appear [476,497–500] and involves such aspects as soil resistance, soil adsorption, detergency, soil removal and soil re-deposition. We are concerned essentially with soils attracted to, and bound mainly at the fibre surface, as opposed to particulate dirt trapped within the interstices between fibres in the yarn. The primary objective is to modify the fibre surface (a) to increase the resistance of the fibre to soiling in the first place (soil repellency) and (b) to ensure that any soil that is deposited is more weakly bound and is hence more easily removed in washing (soil release). Most soils, as expected from a predominantly hydrophobic interaction, are held mainly by nonpolar bonding, although electrostatic forces may come into play with, for example, coloured anionic stains from food and beverages. The essence of any soil-resistant treatment is to render the surface of the fibres more hydrophilic. It also helps if the coating of the fibre is such as to reduce surface irregularity and surface energy. Whilst the two aspects of soil repellency and soil release are interrelated, the actual balance of these properties varies from finish to finish according to requirements. In carpet treatments for example, which are normally given a shampoo rather than washed, the emphasis must be on repellency, whereas soil release becomes of much greater importance in textiles that are frequently washed. Early soil-release agents, applied particularly to resin-finished cellulosic goods, were water-soluble polymers, many being related to thickeners (section 10.8) such as starch, hydroxypropyl starch, sodium carboxymethylcellulose, methylcellulose, hydroxyethylcellulose, alginates, poly(vinyl alcohol) and poly(vinylpyrrolidone). These functioned essentially as temporary barriers and ‘preferential reservoirs’ for soil, which was thus easily removed along with the finish in subsequent washing, when they then helped to minimise

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re-deposition. Obviously these finishes were only temporarily effective. More durable finishes have been developed and these are generally classified in three groups according to whether they feature (a) carboxyl groups, (b) oxyethylene and/or hydroxy groups and (c) fluorocarbon moieties. The fluorocarbon finishes in particular have also been developed as water-repellent treatments. The carboxylated polymers [476,499] include acrylic, methacrylic or maleic acid polymers (all obviously anionic in character) applied mainly from aqueous emulsion and particularly in combination with crease-resist or durable press resins. This type of chemistry has already been discussed in section 10.8.2. A particularly common example is the copolymer of acrylic acid with ethyl acrylate (10.247). In general the best balance of properties is obtained with 75–85% ethyl acrylate (y) and 25–15% acrylic acid (x), with an average chain length of about 1300 (x + y) units; 65–85% ethyl acrylate with 35–15% methacrylic acid is also suitable. When the content of the acidic comonomer increases above about 30% the durability to washing tends to decrease, whilst longer chains tend to give a stiffer handle [499]. CH2

CH

CH2

CH

C O

C OH x

O

y

OCH2CH3

10.247

Soil-release products containing oxyethylene or hydroxy groups may be anionic or nonionic. Many less durable water-soluble polymers have been mentioned already, such as the hydroxy-containing finishes poly(vinyl alcohol), starch, and derivatives of starch or cellulose. When applied together with N-methylol reactants, as in easy-care finishing, they give more durable soil-release properties. Typical of the oxyethylene-containing compounds are poly(ethylene glycol) and poly(ethylene oxide) adducts of carboxylic acids, amines, phenols and alcohols, which may be combined with hydroxy-reactive functional agents as used in easy-care finishes, such as N-methylol reactants or isocyanates. Essentially nonionic soil-release agents comprise polyesters, polyamides, polyurethanes, polyepoxides and polyacetals. These have been used mainly on polyester and polyester/ cellulosic fabrics, either crosslinked to effect insolubilisation (if necessary) or by surface adsorption at relatively low temperature. Polyester soil-release finishes have been most important, particularly for polyester fibres and their blends with cellulosic fibres. These finishes, however, have much lower relative molecular mass (1000 to 100 000) than polyester fibres and hence contain a greater proportion of hydrophilic hydroxy groups. They have been particularly useful for application in laundering processes. These essentially nonionic polymers may be given anionic character by copolymerising with, for example, the carboxylated polymers mentioned earlier; these hybrid types are generally applied with durable press finishes. Polyfluorinated chemicals now dominate in the fields of oil-repellent and water-repellent finishes. The earlier so-called conventional polyfluorinated products were of the type represented by poly(N-methylperfluoro-octanesulphonamidoethyl acrylate)(10.248) [499]. Such products presented a shield of closely packed fluoroalkyl groups at the fibre–air interface, thus giving low-energy surfaces with excellent oleophobicity. These showed excellent resistance to oil-based stains but were less satisfactory as soil-release agents during

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washing. The soil-release properties were subsequently considerably improved by copolymerising these conventional fluorochemicals with hydrophilic moieties [476,498,499] to give so-called hybrid block copolymers represented schematically by A–B–A–B–A–, where A represents the perfluoroalkyl-containing segment and B the hydrophilic segment. A typical example [498,499] is represented by structure 10.249, which is composed of alternating perfluorinated units of the type shown in structure 10.248 with hydrophilic oxyethylene moieties derived from the thiol-terminated copolymer of tetra-ethylene glycol dimethacrylate and hydrogen sulphide (10.250). CH2

CH

CH

CH2

C O

CH

CH2

C R

O

CH

CH2

C R

O

CH3 R=

C R

O

R

OCH2CH2

N SO2(CF2)7CF3

n

10.248

O H

CH

CH2

CH2

3

C O

S

CH

C

O O

(CH2CH2O)4

C

CH3

R

CH

CH2

S

CH2

CH

H

C

CH3

10

O

3

R

10.249

O HS

CH2

CH

C

O O

(CH2CH2O)4

C

CH

CH2

CH3

CH3

S

H 10

10.250

Once again these are only average schematic structures, in this case representing a block copolymer of alternating segments. The hydrophilic segments in themselves show no significant oil repellency and are not very effective as soil-release agents, yet when incorporated into such hybrid structures they considerably improve the soil-release properties without inhibiting the inherent soil-repellency of the perfluorinated segments. This is said [499] to result from the capability of these hybrid polymers to orient a specific moiety at the surface, depending on the polarity of the fibre–environment interface. Thus in air the fibre–air interface is dominated by the closely packed perfluoroalkyl chains, promoting good oil repellency, whilst in aqueous wash liquors it is the hydrophilic segments that orient at the fibre–liquid interface, thus enhancing soil release. In either case the nonactive moiety is said to be collapsed below the surface. In this way, the lowest interfacial energy, with respect to the particular environment, is attained in both cases. The example used here incorporated a perfluorinated polyacrylate and a poly(oxyethylene) hydrophilic moiety. Other fluorochemical and hydrophilic moieties can be used providing they display similar alternating surface orientation characteristics with respect to air and water. The essential character of the hydrophilic unit is that it should have polar groups capable of strong interaction with water, preferably by hydrogen bonding; examples are hydroxy, carboxyl and ether oxygen. Usually C5–C18 perfluoroalkyl groups are used, but

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

individual products may contain a mixture of homologues. Thus there is tremendous scope for designing a great variety of these complex copolymers. It is now opportune to consider the structure–property relationships of fluorochemical finishes in more detail [501,502]. Water repellency depends mainly on reducing the critical surface energy of the fabric surface. This parameter must be less than that of the wetting Table 10.47 Critical surface energies for low energy surfaces [502] Chemical groups on the surfaces

Surface energy (mN/m) at 20 °C

–CF3 –CF2– –CH3 –CH2–

6 18 21 31

Table 10.48 Surface tension of a range of liquids and surface energies of a range of textile fibres [502]

Liquid

Surface tension (mN/m) at 20 °C

Water Peanut oil Olive oil Petrol n-Octane n-Heptane Fluorocarbons

72 40 32 26 22 20 10–15

Textile fibre

Surface energy (mN/m) at 20 °C

Nylon Wool Cotton Polyester

46 45 44 43

liquid in order to create a physico-chemical barrier against penetration of the fabric by the liquid. Table 10.47 gives critical surface energies for various chemical groups at the surface of a fibre from which it can be seen that the CF3 group is by far the most effective for lowering this surface energy. Table 10.48 lists surface tension values of some liquids and surface energy values of various fibres. It is evident from these two tables that a high density of CF3 groups at the fibre surface will lower the critical surface energy sufficiently to create a barrier to penetration of all the liquids listed, particularly against water. It is also evident that perfluoroalkyl groups are essential to guarantee resistance to oily liquids. Thus the presence of CF3 terminal groups is crucial. Equally important, however, is the overall structure of the molecule [502]; the perfluorinated segments should be long enough to maximise the CF3 group density on the surface. It is this aspect that polyacrylic and polyurethane supporting structures have been able to satisfy, the longest chains producing the lowest surface energies (Figure 10.69). The results shown in Table 10.49 show how an increase in the perfluorinated segment length gradually enhances the resistance to oils and, to a lesser extent, to water. It is these considerations that have led to the preferred use of perfluoro-octyl or longer perfluorinated

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segments. Silicone emulsions, by comparison, typically give a surface tension of 25 mN/m and hence act only as water repellents.

Surface energy/mN m–1

25

20

15

10

5 1

2

4

6

8

10

12

(x+1) in acrylic polymer

Figure 10.69 Change in critical surface energy with length of the perfluoroalkyl groups in acrylic polymers [502]

Table 10.49 Oil and water repellency of cotton fabrics treated with perfluorinated acrylic polymers [502] Acrylic polymer CH2 O

1% Polymer applied to printed cotton

CH C

n

O CH2

R

Perfluorinated group R

Oil repellency (AATCC 118)

Spray test (ISO 4920)

–CF3 –CF2CF3 –(CF2)2CF3 –(CF2)4CF3 –(CF2)6CF3 –(CF2)8CF3

0 3–4 6–7 7–8 7–8 8

50 70 70 70 70 80

A rather more novel yet logical development in fluorochemicals has been the emergence of fluoro-silicone hybrid polymers [503,504]. A series of products (10.251–10.253) has been extensively evaluated [503] for various properties, including liquid and solid surface energies, micelle formation, wetting, contact angle, film-release and antifoam behaviour. A similar type of product is tridecafluoro-octyltriethoxysilane (10.254). This has been applied to polyester and to cotton fabrics, fixation being achieved by drying at ambient temperature [504].

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CH3 Si

O

R

O R

n

Si

O

O

CH2CH2CF3

R=

R

CH2CH2(CF2)3CF3

10.252

10.251

CH3 R1

Si

or

CH3 Si

O

CH3

2

CH2CH2(OCH2CH2)x

R2

OR3

10.253

R1

R2

R3

CH2CH2CF3

CH3

7

H

12

H H

x

CH2CH2CF3

CH3

CH2CH2CF3

CH2CH2CF3

7 12

CH2CH2CF3

CH2CH2CF3

CH2CH2(CF2)3CF3

CH3

7

CH3

CH2CH2(CF2)3CF3

CH3

12

CH3

H

OCH2CH3 F3C(CF2)5CH2CH2

Si

OCH2CH3

OCH2CH3 10.254

For application of these fluorochemical finishes to textile fabrics, an extremely important factor is their formulation into suitable aqueous emulsions or dispersions. The quality of the formulation has a critical influence on stability during storage and application, as well as the efficacy of treatment and durability [501,502]. In particular, the choice of surfactant(s) for emulsifying or dispersing must ensure good stability with freedom from deposition on rollers, yet must not impair the water and oil repellency of the finished fabric. No individual product fulfils all requirements; hence specifically formulated products are available for certain fibre types. Application is mainly by padding followed by curing at 150–180 °C, although minimum add-on techniques such as slop padding, spraying and foam application have been successful. They can also be applied by discontinuous methods, such as exhaust or dip-spin [501]. These fluoropolymers are also used as the basis of so-called stain-blocking treatments, applied especially to nylon floorcovering and upholstery [505–508]. In general, the fluorochemical is used in conjunction with an anionic syntan resist agent of the type described in section 10.9.4. The latter functions by blocking the cationic protonated amine sorption sites in nylon. Thus the fluoropolymer repels oil-based soils and facilitates their removal during cleaning, whilst the syntan inhibits electrostatic interaction between the cationic sites and many coloured anionic substances in food, drinks and human/animal excreta. The two product types may be applied later in the dyehouse, in which case the

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NaO3S

735

CH2OCH3

O

(CH2)zCH3

CH2 O CH3(CH2)x

O

S

O

(CH2)yCH3

O 10.255

x, y, z = 1–5

effect is less durable. For maximum efficacy the two component types must be carefully chosen after much empirical screening. Fluorochemicals having perfluoroalkyl groups containing 10–12 carbon atoms and yielding an overall concentration level of 200–800 ppm of fluorine on the weight of fibre appear to be optimal [508]. The syntans are typically formaldehyde–phenolic condensation products; one product has the structure 10.255, which is interesting in that all the hydroxy groups have been converted into alkylaryl ethers [508]. Polymers of methacrylic acid or maleic acid, either alone or as a blend or copolymer with the sulphonated aryl–formaldehyde condensation products, have also been evaluated as stain-blocking chemicals [508,509]. An interesting development is the use of a polystyrene– maleic acid copolymer, this being unusual because of the absence of sulphonic acid groups [508,510]. Although the maleic and methacrylic acid polymers do not have the durability of the conventional syntans, they have the advantage that they are non-yellowing. Although stain-blocking treatments were originally developed for nylon, there has been a good deal of emphasis over the last decade on extending their use to wool carpets [511– 515]. Whilst syntans similar to those used on nylon are also suitable for wool, larger amounts are required to block the greater number of dye sites in wool [512]. Environmental aspects of fluorochemical finishing agents In certain circumstances, organofluorine compounds can lead to the generation of AOX values, although a satisfactory method of measuring specific AOF values has yet to be developed [516]. Typical results of the environmental analysis of twelve fluorochemicals are shown in Table 10.50. 10.10.5 Bactericidal and insecticidal agents There are three areas to consider: (1) The use of insecticidal agents on wool to prevent attack by moth and beetle larvae. (2) The use of bactericides to prevent biodegradation of chemicals such as thickening agents. (3) The use of bactericides to inhibit bacterial activity on textiles. Insecticidal agents for wool Lists of the principal types of insect that lay their eggs in wool are available [11,517]. Damage to the fibre is caused by the larvae that emerge from these eggs. Hence any

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Table 10.50 Environmental analysis of fluorochemical agents at a product concentration of 1 g/l [516]

Product no.

Solids content (%)

1 2 3 4 5 6 7 8 9 10 11 12

21.2 40.9 20.3 18.7 5.3 17.0 18.5 18.0 32.3 10.1 23.4 31.2

AOX value (mg/l) 0.18 5.6 0.18 <0.05 1.3 <0.05 0.10 0.06 19.0 0.08 <0.05 0.06

COD value (mg/l)

BOD5 value (mg/l)

Bacterial toxicity (% resistance)

275 620 250 345 305 300 510 365 805 220 425 355

140 15 60 30 160 55 45 90 335 5 55 85

26 18 21 66 6 23 20 29 27 42 50 54

insecticide used must be effective against these larvae. Since these products have no insecticidal effect on insects that do not consume the wool, it seems likely that such products act only through the digestive tract of the insect larvae. In addition to health and safety considerations, fastness properties also need to be taken into account. Fastness requirements on carpets are not so stringent as on machine-washable wool. Environmental, as well as health and safety factors, have resulted in an almost total ban on the use of dieldrin (10.256), the first compound to be used for this purpose. It proved to be toxic to man, animals, fish and birds, and was highly persistent in the environment. Cl C

Cl

CH O

C

CH

CH

Cl

CH2

C

CH

CH

Cl

C HN

NH

Cl

Cl C

CH

Cl

O

Cl

O

Cl

C Cl

10.256 Dieldrin

NaO3S

10.257

Sulcofenuron

Environmental factors coupled with the relatively small size of the market are acting restrictively against several other products that were formerly used. In an excellent review of this subject [517], the compounds that have found use are divided into two categories. The first category comprises those compounds that were developed specifically as wool mothproofing agents, most of these being anionic multichlorinated aryl compounds; some of these, for the reasons cited above, are no longer available. One of the best known is sulcofenuron (10.257). The sulpho group confers water solubility and exhaustion behaviour similar to those of an acid dye. This product is relatively expensive but has very good

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fastness to washing and light. Sulcofenuron, up to a concentration of 4 g/kg wool, is one of only three types of insect-resist agent permitted for the GuT scheme of ecolabelling [518]. Chlorphenylid (10.258) has also provided insect-resist treatments but AOX generation during manufacture has led to the withdrawal of several such products since 1989. The chloromethylsulphonamido group requires at least pH 10 for aqueous solubility as the sodium salt. On acidification during application a dispersion of the free sulphonamide is formed and this is absorbed mainly through weak nonpolar interactions and hydrogen bonds. Combined chlorphenylid/sulcofenuron and chlorphenylid/flucofenuron (10.259) products were formerly used but have been withdrawn since 1989. Chloromethylsulphonamidotrichlorobenzene (10.260) has also been withdrawn since that date. Thus of the products developed specifically as insect-resist agents for wool only the sulcofenuron type remains in any significant use. Cl

O

Cl

C Cl

O NH

Cl

Cl

F3C

Cl

O2S

HN

NH

Cl CF3

10.259 Flucofenuron

10.258 CH2Cl

Cl

Chlorphenylid

Cl

NHSO2CH2Cl Cl

10.260

Compounds in the second group were originally developed as pesticides for agricultural use. These products have proved efficaceous and amenable to formulation as wool insect-resist agents. Most of them are pyrethroids, including permethrin (10.261), cyfluthrin (10.262) and cyhalothrin (10.263). A further effective compound is the hexahydropyrimidine derivative 10.264. Permethrin and cyfluthrin (together up to 210 mg/kg wool) are the other two types of insect-resist agent permitted for the GuT ecolabelling scheme [518]. H3C

CH3 C

O HC CH2

10.261

O

Cl CH

CH

C

C

Cl O

Permethrin

F H3C O

CH3 C

HC CH

O

Cl CH

CH

C

Cl O

NC

10.262 Cyfluthrin

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C

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

H3C

CH3 C

O HC CH

O

CF3 CH

CH

C

C

Cl O

NC

10.263

CH3

O

Cyhalothrin

Cl

NH

N

C Cl

O

O

N O

CH3

10.264

These synthetic pyrethroids mimic natural counterparts, of which the most important is pyrethrin 1 (10.265). Unfortunately, the natural products lack the photochemical and hydrolytic stability necessary for use as wool insect-resist agents. The synthetic products have the required stability, yet retain the low mammalian toxicity and low environmental retention of the natural products. Permethrin, however, is toxic to aquatic life and is therefore subject to increasingly severe discharge limits. There is some evidence that permethrin is less effective against larvae of a certain beetle. This can be compensated for by using a combination of permethrin with the hexahydropyrimidine derivative 10.264. Some possible alternative pyrethroids have been mentioned [517] as development products (10.266–10.269). O

H3C

CH3

HC

CH

Cl H2C

CH

CH

CH

O

CH2

CH

C

CH3

C Cl

O 10.265

Pyrethrin I

H3C O

CH3

HC CH NC CH3 CH

NC

C

O

O

C

Cl

O Fenvalerate

738

CH

Br

10.267

chpt10(2).pmd

CH

C

Deltamethrin

CH CH

O

10.266

H3C O

Br

C

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AGENTS FOR FIBRE LUBRICATION, SOFTENING, ANTISTATIC EFFECTS, SOIL RELEASE

H3C O

CH3 C

HC CH NC

O

C

CH3

C

CH3 O 10.268 Fenpropathrin

H3C O

CH3 C

HC CH

O

CH3 C

CH

C CH3

C

NC

O 10.269 Cyphenothrin

OCH2CH3

O C H2C

O

CH2CH2S

CHSO2CH2CH2

O

P

X

SCH2CH2CH3 10.270 X = O or S

Other chemicals evaluated but not yet adopted commercially include organophosphorus compounds, triphenyltin compounds, quaternary ammonium salts, imidazoles, benzimidazoles, carbamates and the precocene anti-juvenile hormones [517]. Although none of the above has found use as an insect-resist agent, several have been used as antimicrobial agents for textiles. Vinylsulphone fibre-reactive, insect-resist agents have been described, in which the insecticidal moiety is an organophosphorus grouping (10.270; X = O or S). The vinylsulphone group, by virtue of its nucleophilic addition reactions with wool keratin, confers excellent fastness. An interesting feature of these products is that they do not act as insecticides on wool until they become activated by hydrolysis of the ester bond during digestive processes within the insect. A development reported recently [519] involves reduction of the cystine disulphide bonds in wool with either thioglycolic acid or tetrakis(hydroxymethyl)phosphonium chloride to form thiol groups, followed by crosslinking with bifunctional reactive dyes. This gave improved insect resistance but had adverse effects on physical properties such as strength, shrinkage and stiffness, thus limiting the potential of the process for commercial use. Despite these interesting developments, it has been pointed out [11,517] that because of the relatively small market and the costs of registration and ecotoxicological testing, it is currently unlikely that novel agents designed specifically for wool could be marketed economically. Any further advances are likely to be spin-offs from agricultural pesticide developments. Application of these agents is best carried out from the dyebath to achieve the highest fastness ratings. This may not always be possible, however, and alternative stages (during scouring or application of spinning lubricants) are available [11,517]. Particular care is

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

necessary in the choice and application level of insect-resist agents when applying them to fibre blends, since their partition behaviour between the component fibres varies. Pyrethroids, for example, tend to partition in favour of nylon in wool/nylon blends [520]. It is not surprising, given their aquatic toxicity, that these agents are under continual environmental scrutiny [521]. In order to comply with minimum discharge requirements, it is obviously helpful to be able to apply the minimum levels needed for adequate functionality and this has led to the development of appropriate machinery and methods [522–524]. Bactericides for addition to fibres and other polymers Bactericides can be added to microbially nutritious polymers, such as certain size polymers, dispersing agents and thickening agents, in order to protect them against biodegradation. For the same reason, they may be incorporated into man-made fibres for geotextiles and awnings. They may be applied to medical fabrics, hosiery, underwear and sports clothing for reasons of hygiene, in order to prevent infection, promote healing or prevent the development of odours. A comprehensive index is available [525]. Although this index covers all uses of antimicrobials, there is a section devoted to agents for textiles, in which the following are listed: – Ammonium zirconium carbonate – 1-Capryl-2-hydroxyethylimidazoline (10.271) – Cis-N-[(trichloromethyl)thio]-4-cyclohexene-1,2-dicarboximide (10.272; Captan) – 2,2′-Dihydroxy-5,5′-1-dichlorodiphenylmethane (10.273; Dichlorophene) – Diiodomethyl-p-tolylsulphone (10.274) – Dimethylaminopropylricinoleamidobenzyl chloride (10.275) – Lauryl/stearyltrimethylammonium bromide/chloride (10.276) – Myristylamine (10.277) – Sodium 2-mercaptobenzothiazole (10.278) – 2,4,4′-Trichloro-2′-hydroxydiphenyl ether (10.279; Triclosan) – Zinc 2-pyridinethiol-1-oxide (10.280; Zinc pyrithone).

O

O

C

(CH2)8CH3

OH

C

N

N C

CH2CH2OH N

S

C Cl

Cl

10.271

I SO2

CH2

Cl

O

H3C

HO

10.272

10.273

Captan

Dichlorophene

CH I

10.274 Cl

CH2

NH C

(CH2)7CH

O

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CHCH2CH(CH2)5CH3

CH3

OCH2CH2CH2N 10.275

Cl

Cl

CH3

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R

CH3 + N CH3 _ CH3 X

CH3(CH2)13NH2

N C

10.277

R = lauryl / stearyl X = bromide / chloride

741

_ + S Na

S

Myristylamine 10.278

10.276 OH

N Cl

O

O

Cl Cl

Zn2+

S_ _ S

10.279 Triclosan

N

10.280

O

Zinc pyrithone

The environmental implications of adding biocides to polymers must always be borne in mind since, by definition, all effective biocides are more or less toxic. Thus addition of a biocide may render a normally biodegradable thickener less so. Hence biocides should be used at as low a concentration as possible. Natural size polymers, thickening agents and dispersing agents are particularly attractive targets for microbial attack, but degradation by this route is more rapid wet than dry. Thus printing pastes and liquid disperse dyes are sensitive targets. In the case of printing pastes, biological degradation on storage can lead to a significant loss of viscosity and rheological malfunction. Sensitive thickening agents, for example, can be protected by incorporating a biocide, usually at less than 0.1%, which is just about sufficient for effectiveness [383]. It has been suggested [383] that manufacturers of thickening agents will cease to incorporate preservatives in their products. The printer will then be responsible for selecting and using, in just the required amount, a preservative that is still just tolerable under the conditions of use. Formaldehyde has been widely used for this purpose but is now ecologically undesirable. Phenolic compounds such as sodium pentachlorophenate, ophenylphenate or chloro-m-cresol have also been used. Such nucleophilic compounds can adversely affect the yield and hue of certain reactive dyes [390]. If absolutely minimal quantities are used, just sufficient for the required bacterial efficiency, it is possible for these residues to be washed out with much water to give extremely dilute waste waters. These traces no longer exert significant bacterial action and may even be biodegradable under these conditions. Interest in the application of biocides to textiles has increased in recent years. They may in some cases be applied during manufacture of the fibre (in melt spinning) or as a finishing treatment. Although numerous papers on this subject have been published, many are unfortunately of little chemical interest as they disclose little other than commercial names. Poly(ethylene glycol) (HO[CH2CH2O]nH) crosslinked with dimethyloldihydroxyethyleneurea has been reported to give fabrics with antibacterial properties suitable for nonwoven protective surgical apparel [526]. Triclosan (10.279) kills a wide range of bacteria that cause food poisoning, dysentery, cholera, pneumonia, tetanus, meningitis, tuberculosis and sore throats. It also prevents the development of bacterially related odours and kills the yeasts responsible for candida ulcers

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and athlete’s foot. This compound can be incorporated during fibre manufacture to give durable antibacterial properties [527–529]. Despite the widespread use of Triclosan in toothpastes and acne creams, it is reported that it can cause allergic dermatitis in susceptible individuals, especially when used in products for the feet [525]. Poly(hexamethylenebiguanide hydrochloride) (10.281) has been used in the sanitisation of swimming pools. For textiles, it has been formulated into a finish capable of providing a range of antibacterial fabrics from medical products to odour-free socks [530,531]. This agent is mainly of interest for cotton; the polymeric cationic structure exhibits high substantivity for the negatively charged surface of the fibre. Application is by padding, optimally at pH 7–8, and the performance can be improved by subsequent wet-on-wet padding with an anionic fixing agent. No elevated curing temperature is required and approximately 1% of the antimicrobial agent on the weight of cotton is optimal for bactericidal performance. The product has a long history of use as a bactericide, exhibits low toxicity and is environmentally acceptable, being bioeliminated by adsorption.

CH2CH2CH2CH2CH2CH2

NH

C

NH

NH

C

NH

_

average n = 12

+NH2 Cl n

10.281

Novel bis-quaternary compounds have been reported for improving the microbial resistance of wool [532]. These products are described as a new class of bis-quaternary ammonium surfactants known as gemini quaternary ammonium compounds or bis-quats. They consist of two saturated hydrocarbon chains and a complex polar group consisting of two quaternary ammonium salts linked through an alkane spacer chain containing amide and optional disulphide bonds. Bacterial efficacy on wool was confirmed for two products: N,N′-bis(N-dodecyl-N,N-dimethylglycine)-1,4-diaminobutane dihydrochloride (10.282) N,N′-bis(N-dodecyl-N,N-dimethylglycine)cystamine dihydrochloride (10.283). These agents were applied at 0.0025–0.5% on the weight of wool by exhaustion from an aqueous solution at 40 °C.

H3C(CH2)11 H3C

N

+ NHCH2 _ H3C Cl

(CH2)11CH3 CH2CH2CH2CH2

N

C

C O

10.282

O

H3C(CH2)11 H3C

+ NHCH2 _ H3C Cl

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N

+ CH3 CH2NH _ CH 3 Cl

(CH2)11CH3 CH2CH2

SS

CH2CH2

N C

C O

10.283

O

+ CH3 CH2NH _ CH 3 Cl

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The bacterial resistance of silk fibroin can be improved by treatment at low pH values in aqueous solutions of metal ions and at high pH values in solutions of metal–amine complexes, using untreated silk or silk pretreated with an aqueous solution of tannic acid (10.183). Suitable metals include Cu, Zn, Ni, Fe and Ag [533]. It is to be expected that treatment with such metal compounds will be subject to restrictions in environmentally sensitive areas. Antimicrobial agents may adversely affect the light fastness of nylon dyeings or cause yellowing of the fibre. The six antimicrobial treatments listed in Table 10.51 have been Table 10.51 Comparison of antimicrobial finishes on nylon [534] Finish

Chemical class(es)

Structure(s)

1 2 3 4 5 6

Silicone quaternary ammonium salt Silicone quaternary ammonium salt Organo-tin Phenolics (mixture) Phenolic and organo-tin Organo-tin and quaternary ammonium salt

10.284 10.285 10.286 10.273/10.287 10.279/10.288 10.288/10.289

(CH2)17CH3 H3C

(CH2)9CH3

OCH3

+ N CH2CH2CH2 _ CH3 Cl

Si

H3C _ Cl (CH2)9CH3

OCH3

OCH3

10.284

Si

OCH3

OCH3

10.285

H3C (CH2)3 H3C(CH2)3

OCH3

+ N CH2CH2CH2

_ O

+ Sn H

CH C

C O

H3C (CH2)3

CH

(CH2)3CH3 _ O

+ H Sn (CH2)3CH3

O

(CH2)3CH3

10.286 OH

H3C (CH2)3 Cl

O Cl

H3C(CH2)3

Sn

O

H3C (CH2)3

Cl

(CH2)3CH3 Sn

(CH2)3CH3

10.288

10.287

CH2 R

+ N CH3 _ CH3

Cl

R = n-alkyl

10.289

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(CH2)3CH3

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

evaluated in relation to these two factors, each being applied at the manufacturer’s recommended concentration [534]. The effect on light fastness varied with each dye and only a few dyes were studied; some dyes were more sensitive to specific antimicrobial finishes. Overall, Finish 3 had the smallest effect on light fastness, followed by Finish 5. Finishes 6 and 4 gave the greatest reduction in light fastness, Finish 4 also causing appreciable discoloration of the undyed fibres. 10.11 FOAMING AND DEFOAMING AGENTS 10.11.1 Foaming agents The idea of using a matrix of foam to transfer chemicals and colorants to textiles had its origin in the growing need to conserve thermal energy and water in the aftermath of the socalled oil crisis and recession of the early 1970s. In a sense, it was an antidote to the feverish work carried out in the late 1960s on solvent dyeing. Foam dyeing began with an elegant short-liquor (about 1.5:1) dyeing process developed in 1972 mainly for garment dyeing in rotary machines. The application of foams to textiles has been widely investigated subsequently, although the degree of commercial acceptance has proved limited. Nevertheless, potential exists for the foam-based application of lubricants, stiffening agents, waxes, size polymers, mercerising liquors, durable press resins, water/oil repellents, softening agents, shrink-resist resins, dyes and print pastes. Although continuous dyeing and printing, mainly of carpeting, has attained significant commercial use, most foam processing is confined to the application of finishes where concentration tolerances and evenness of application are much less critical. A major account of the properties of foams and their general industrial applications includes a chapter devoted to textile applications [535]. Apart from reproducibility and uniformity of application, one of the main problems associated with foam application is dissolving or dispersing relatively large quantities of the principal and auxiliary agents in a very small volume of water, followed by the difficulty of maintaining compatibility of the components and the density and stability of the ‘loaded’ foam under such conditions. For example, resin finishing can typically require the dissolution of about 600 g of resin, softener and catalyst in only 400 g of water. Nevertheless, foam processing does offer advantages, notably in the conservation of water and energy and the reduction of effluent problems. The foam matrix used in textile wet processing is a stabilised air-in-water system. A foam cannot be made with pure water, however; a foaming agent, usually a surfactant, is needed to give a reasonably stable honeycomb matrix of air cells, each enclosed by a thin viscoelastic film of liquid. Clearly, a reduction in surface tension is one important factor in the creation of foam; others include the elasticity and viscosity of the film walls between the bubbles, and the size and uniformity of the bubbles themselves. Drainage, by gravity, of the liquid from the film walls leads to instability of the foam and tends to a maximum with larger spherical bubbles. In a system consisting of variously sized bubbles, the smaller ones tend to coalesce into the larger, which are thus further increased in size and become less stable because of the increased propensity for liquid drainage. Thus, for maximum stability, the bubbles should be as small and uniform as possible, leading to minimum diffusion of air from bubble to bubble, and maximum entropic and electrical double layer repulsion. The ideal state is not attainable in practice, however, and all foams are unstable to some degree. Nor is

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perfect foam stability particularly required in textile application, since at some stage during the process collapse of the foam is generally desirable to ensure maximum deposition of chemicals and/or colorants. The foaming propensity of surfactants generally reaches a maximum at the critical micelle concentration, beyond which there appears to be little further contribution to foam density. Foam stabilisers are also added in some cases. The two important steps in the foam treatment of textile materials are generating the foam and applying it to the substrate: (1) Generation is generally by high-speed rotors, with metered air and liquid flows and monitoring to control the density of the foam. (2) Application of controlled amounts of foam to the substrate is by knife-on-roller, knifeon-blanket, floating knife, horizontal pad or furnishing roller with doctor blade, or by squeegee across a printing screen. Subsequent collapsing of the foam is generally by collapse onto the fabric (controlled to some extent by the chemicals used), by vacuum suction of the foam into the fabric, or by means of a pad nip. The most important auxiliary used is, of course, the foaming agent. In theory any surfactant that will form a stable foam can be used. In practice the choice is usually between anionics (generally cheaper), nonionics, or a mixture of both. Consideration must be given to overall compatibility as well as to foaming characteristics: for example, anionic agents should generally be avoided when applying cationic products. Long-chain alcohol sulphonates and ethoxylates, as well as sulphates and sulphosuccinates, have been used; a typical selection is given below, the first three being anionic and the others nonionic: – sodium lauryl sulphate – ammonium lauryl sulphate – sodium dioctylsulphosuccinate – lauryl alcohol poly(oxyethylene) – decanol poly(oxyethylene) – tridecanol poly(oxyethylene). Particularly effective is a mixture of anionic and nonionic agents, such as a mildly anionic sulphated alcohol ethoxylate with a nonionic alcohol ethoxylate. Ideally, foaming agents should: – generate consistent foam easily – show optimum and uniform wetting – cover a wide range of wettability so as to be adaptable for different situations – show little or no effect on colour fastness – be compatible with the other components – be biodegradable. The main function of the foam stabilising agent is to reinforce the intercellular film wall by contributing rheological characteristics of viscoelasticity. The increased viscosity may also assist handling. The aim, as so often with auxiliaries, is to achieve an optimum balance. If the bubbles are too thin and wet too quickly they will collapse prematurely, whilst too stable a film could hinder uniform application. Examples of products used as foam stabilisers include thickening agents such as the polysaccharides, hydroxyethylcellulose, methylcellulose,

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

carboxymethylcellulose, poly(vinyl alcohol) and poly(acrylic acid), as well as other compounds such as sodium tripolyphosphate, sodium hexametaphosphate (detergent ‘builders’) and decanol. Ideally, foam stabilisers should increase the stability of the foam to the optimum controllable level whilst also allowing for subsequent controllable collapse. They should be compatible with the other components and effective at various concentrations, give pseudoplastic solutions, and should not affect the drape and handle of the fabric. Electrolytes can have positive or negative effects on foam stability [535]; for example, a low concentration of phosphate ions increases the stability of a sodium laurate foam, whereas sodium chloride decreases the stability. 10.11.2 Defoaming agents All chemical systems, to varying degree, tend to reduce free energy and foams are no exception. Hence foams are thermodynamically unstable, yet their stability varies from almost instantaneous collapse to prolonged persistence. Although foam can be useful, there are still many circumstances where its presence and persistence is enough of a nuisance to create a need for foam-destruction products, known as defoaming agents or antifoams. An authoritative reference work on the theory and applications of defoaming is available and this includes a chapter on applications in textile dyeing [536]. Reference 535 contains a chapter on the science and technology of silicone antifoams. Just as foams are stabilised by decreasing the rate of liquid drainage from the film walls, they can be destabilised by increasing this drainage, resulting in thinning and eventual rupturing of the film. Defoaming agents generally effect this by two mechanisms, the basic objective being to displace foam-stabilising substances from the liquid-air interface [537]: (1) Spreading alone is sufficient with light foams of high blow ratio (those having a high air-liquid ratio); in this case surfactants of relatively low surface tension (i.e. powerful surfactants) will spread over the large surface area of the intercellular film and displace the surfactants that are tending to stabilise the foam. (2) Denser foams of low blow ratio additionally require penetration of the thicker aqueous film by the defoaming agent. Such a defoamer consists of an emulsified hydrophobic substance, which when added to the foaming system disperses fine droplets of insoluble hydrophobic material within the liquid lamellar walls, thus entering the liquid-air interface, aided to some extent by the solubilising action of the foaming agent(s). This creates a weak link as a result of high interfacial tension, the foam then tending to rupture at the interface between defoamer and foamer. In practice, the system is a finely balanced one requiring careful formulation of the composite defoaming agent. The main requirement of an effective defoamer [537] is that the agent should be insoluble in the foaming system and should have a high rate of spreading. Spreading will be favoured if the defoamer has a lower surface tension than that of the foaming system. The interfacial tension between defoamer and foaming system must be high, but not so high as to inhibit spreading. A low degree of attraction between defoamer and foaming system (i.e. a high interfacial tension) is achieved by nonpolar defoamer systems that do not contribute positively to the surface viscosity of the lamellar walls. For maximum efficiency the defoamer should be added to the system as soon as foaming

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becomes troublesome, since its active life-span is inevitably limited by the system’s thermodynamic instability. Its principal action is on the liquid-air interface; therefore there is little point in adding it before sufficient liquid-air interfaces are formed. Secondly, the insoluble active ingredient of the defoamer, given the nature of the system in which it is working, must eventually become more or less solubilised, or at least emulsified, into the bulk liquid system, thus losing its activity and in some circumstances actually promoting foaming. Defoamers are generally anionic or nonionic systems and fall into two groups. The first group consists of water-soluble surfactants with polar and nonpolar moieties. These compounds are effective only over a narrow range of conditions, functioning simply as spreading agents, and are seldom used alone. Such surfactants are readily absorbed into the bulk of the foaming system where, not surprisingly, they contribute to the foaming problem. This system is more frequently used, however, as the vehicular or ‘carrier’ basis of the second group of defoamers, which are much more widely used. These more active defoamers are emulsions of water-insoluble silicones or organic-based compounds of low volatility and high spreading power. The general requirements for an ideal defoamer can be summarised [536]: – eliminates existing foam as well as preventing further foam from forming – easy to disperse in the dyebath – does not react or interact with dyes or auxiliaries present in the bath – chemically stable under dyeing conditions – no deposition on fabric or machinery causing spotting or staining of the fabric – no colour – no odour – stable to storage – safe both to humans and to the environment. The active organic-based defoamers include: (1) Fatty acids, their glycerides and other esters, including fats, waxes and oils such as mineral and vegetable oils; fatty alkylamines and acylamides. Alkaline earth metal or aluminium salts of fatty acids tend to leave deposits on machinery [536]. (2) Higher alkanols, including the isomeric octyl alcohols (2-octanol and 2-ethylhexanol), cyclohexanol, lauryl and cetyl alcohols. Aliphatic alcohols have relatively poor foam control and have an odour that can be nauseous [536]. (3) Polyglycols, especially poly(propylene-1,2- or -1,3-glycol). Poly(oxyethylene) and poly(oxypropylene) block copolymers have relatively poor foam control [536]. (4) Insoluble alkyl esters of phosphoric acid, especially tributyl phosphate. These phosphate esters have relatively poor foam control [536]. Certain limitations of organic defoamers can be minimised by judicious formulation of mixtures. The following system is said to overcome some of the drawbacks associated with aluminium salts of fatty acids [536]: – 87% Paraffin oil – 6% 2-Ethyl-n-hexanol – 4% Aluminium distearate – 3% Phosphoric acid esterified with polyethoxylated p-nonylphenol.

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

In the case of an aliphatic alcohol such as iso-octanol, a similar level of defoaming activity together with a much less offensive odour can be achieved by using a propoxylated ester of a branched aliphatic acid, such as propylene-1,2-glycol mononeodecanoate (10.290).

CH3 H3CCH2CH2CH2CH2CH2

C

O C

CH3

CH3 O

CH2

CH OH

10.290

The active ingredients in silicone-based defoamers have traditionally been a poly(alkyl siloxane), especially poly(dimethyl siloxane) (10.226), and silica (SiO2); the latter may be chemically bonded to the polysiloxane to render its surface hydrophobic. Some ‘spotting’ problems have been experienced with these defoamers owing to incompatibility of the antifoam emulsion with certain dye dispersions, especially at high rates of shear in the hightemperature dyeing of polyester with disperse dyes. This created a poor reputation for the early silicone antifoams in jet dyeing. The problems arose by destabilisation of the emulsion, resulting in cracking out of hydrophobic components which caused staining or spotting of the fabric, together with a drastic loss of defoaming action. The poly(dimethyl siloxane) products exert little foam suppression power alone. Their foam inhibition properties only become fully developed when combined with finely divided, hydrophobic silica particles [538]. Further improvements have been made to the emulsion system and to the silicone components themselves. Improved derivatives of poly(dimethyl siloxane) include [537] block copolymers with poly(oxyethylene) and poly(oxypropylene) segments represented schematically by structure 10.291. The solubility and other characteristics of these alkoxylated silicones (silicone polyglycols) can be adjusted by varying the proportions of dimethyl siloxane and oxyalkylene units. A specific advantage claimed for these compounds in high-temperature dyeing is their inverse solubility, analogous to the cloud point effect of nonionic surfactants. As the dyebath temperature approaches its maximum the solubility of the defoamer decreases, thus helping to maintain its effectiveness, whilst the increase in solubility on cooling after completion of the dyeing is claimed to overcome potential problems of subsequent spotting. The four most commonly used alkoxylated silicones are represented by structures 10.292–10.295 [536], where R is typically a methyl group, a is typically 1 and G is typically represented by formulae such as –(CH2)3(OCH2CH2)x(OCH2CH2CH2)yOR or –(OCH2CH2)x(OCH2CH2CH2)yOR, where R is an end-capping group such as methyl. A more recent development is the use of hybrid fluorochemical silicones, such as nonafluorohexyl-substituted siloxanes of the type represented by structure 10.296 [503]. CH3 CH3

Si CH3

CH3 O

Si CH3

CH3

CH3 Si

O

Si

O y

x

CH3

CH3

R1(CH2CH2O)a(CH2CH2CH2O)b

R2

10.291

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CH3 CH3

Si CH3

CH3

CH3

CH3 Si

O

Si

O

G

m

Si

O

CH3

n

CH3 G

CH3

CH3

Si

Si

CH3

Si

O

G

10.292

m

CH3 Si

O

CH3

n

G

CH3

10.293 CH3

Ra Si

CH3

CH3 O

749

O

CH3

CH3

Si

O

CH3

Si G

n

O m

Si

CH3

CH3 4–a

10.294

CH3 Ra

Si

O

CH3

Si

O

CH3

Si G

n

CH3 O m

Si

G

CH3

CH3

Si 4–a

O

CH2CH2CF2CF2CF2CF3 n

10.295

10.296

These copolymers have been mentioned already in section 10.10.4 as versatile and highly effective stain-resist, oil- and water-repellent finishing agents. Antifoams are generally supplied for textile use as carefully formulated, relatively dilute aqueous emulsions; this ensures that the hydrophobic phase is uniformly distributed within the foaming system and also helps to safeguard against overdosing, with the attendant danger of spotting. A typical emulsion generally contains emulsifying agent(s) and thickening agent(s) in addition to water and the insoluble hydrophobic defoaming agent. For emulsification, the most common system [537] is a combination of a low-HLB surfactant with one of high HLB value, such as glyceryl monostearate or sorbitan monostearate with poly(ethylene glycol) monostearate. Obviously, the surfactants selected need to be lowfoaming types and should provide an optimum level of emulsification, since overemulsification will tend to negate the activity of the defoamer by inhibiting its interaction with the foaming system. The size of the defoamer droplets, largely determined by the emulsification system and the degree of comminution during manufacture, is critical in relation to the efficacy of the product; too small a size gives inadequate activity and if it is too large the stability of the emulsion is adversely affected. The optimum droplet size generally appears to be in the range 2 to 50 µm [537]. The function of the thickening agent is to increase the viscosity and so contribute to the stability of the product, the aim again being to attain an optimum level of viscosity. Thickening agents that do not gel at the high temperatures used in textile processing are essential; hydroxyethylcellulose, alginates and synthetic poly(acrylic acid) derivatives may be used. A small amount of a bactericide such as methyl p-hydroxybenzoate is often added to safeguard against biological degradation during storage, particularly in the case of natural thickening agents.

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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION

Typical formulations of commercial composite antifoams have been detailed [536,537]. There are many products on the market but evaluation of their relative efficacy depends on the foaming problems to be overcome. Not only does the chemical type of the active defoamer have to be considered, but its state within the emulsion and the intrinsic properties of the emulsion are also of crucial importance. Methods of evaluating defoamers have been described [539,540].

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FLUORESCENT BRIGHTENING AGENTS

CHAPTER 11

Fluorescent brightening agents John Shore

11.1 INTRODUCTION Partly from an association of whiteness with cleanliness and partly for aesthetic reasons to achieve a more pleasing contrast with coloured goods, from the earliest times people have searched for methods of producing ever whiter textiles. The oxidative bleaching of vegetable fibres on exposure to air and sunlight, as well as the reductive bleaching of animal fibres by the action of fumes from burning sulphur, have been known for thousands of years. Natural blue minerals such as lapis lazuli (section 2.10.2), ground up and applied at very low concentrations, have been used for centuries to mask any residual yellowish colour remaining after bleaching, the overall greyish effect being perceived as ‘white’. Today whiteness is achieved by a combination of chemical bleaching and the use of so-called fluorescent brightening agents (FBAs). Fluorescent hydroxy-substituted derivatives of coumarin (benzo-α-pyrone) occur naturally, including aesculetin (6,7-dihydroxy) in horse chestnut, daphnetin (7,8-dihydroxy) in daphne and umbelliferone (7-hydroxy) in spurge laurel. Paul Krais in 1929, in the first recorded application of an FBA to textiles, carried out tests on half-bleached linen using an aqueous extract of aesculetin-6-glucoside (11.1) from horse chestnut bark [1]. The addition of blue-violet light to the total light reflected from the fabric produced a ‘whiter than white’ effect. Unfortunately, aesculetin showed poor fastness to light and washing. Nevertheless, Krais’s work stimulated research in this area and in 1934 Paine and Radley patented the use of 4,4′-bis(benzoylamino)stilbene-2,2′-disulphonic acid (11.2) in banknotes [2]. The first commercial bis(triazinylamino)stilbene derivative (11.3) was introduced by I G Farben in 1940 as a brightener for cotton.

O C

NH SO3H

CH2OH CH O HO

CH

HC

CH

CH CH OH OH

CH

O HO3S

HO

O

11.1 Aesculetin-6-glucoside

O HN

11.2

O

760

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761

NH N N

NH N SO3Na

HO HC

CH OH

NaO3S N HN

N N

11.3

HN

CI Fluorescent Brightener 113

After the Second World War, development of synthetic FBAs was extremely rapid. Several hundred commercial products, representing a wide variety of chemical types, have since been marketed, with FBAs probably corresponding to approximately 10% of world demand for dyestuffs. Several excellent books and reviews of the chemistry, application and properties of FBAs have appeared [3–13]. FBAs are used to brighten not only textile materials but also paper, leather and plastics. They are important constituents of household detergent formulations. More specialised areas of application include lasers, liquid crystals and biological stains. By far the most important uses for FBAs, however, are in applications to textiles and paper. Much of what follows will be concerned with these two categories. Descriptive terms such as ‘fluorescent whitening agents’, ‘optical brighteners’ and ‘optical bleaches’ have all been used for the products described in this chapter as FBAs. Many of these terms have validity and the term ‘fluorescent brightening agents’ is preferred here only because it has been adopted in the indexes of Chemical Abstracts. An FBA is a strongly fluorescent substance that absorbs ultraviolet radiation, emits light in the blue-violet region of the visible spectrum and is substantive to the substrate for which it is intended. The product should be applicable without undesirable side-effects, such as staining or subsequent photosensitisation of degradation, on any other substrate that may be present. The treated material should retain its properties during its working life and under the conditions in which it will be found. For commercial success the product will also need to be priced attractively and supplied to the customer in a form that is convenient and practical to use. The marketed product and the active brightener that it contains must not exhibit toxicity problems nor create an environmental hazard. 11.2 MODE OF ACTION OF A FLUORESCENT BRIGHTENER All dyes absorb light. Fluorescent dyes re-emit the absorbed energy as light of longer wavelengths. An FBA is a fluorescent chemical that absorbs in the ultraviolet region of the spectrum and emits blue-violet light. A typical FBA shows maximum absorption at a

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wavelength between 340 and 380 nm and emits visible light at a maximum emission between 425 and 450 nm. When describing a substrate treated with an FBA, the terms ‘remitted’, (reflected plus emitted) and ‘remission’ (reflectance plus emission), which take into account the fluorescence component of the total light emitted from the brightened substrate, are preferred to ‘reflected’ and ‘reflectance’. When present on a substrate an effective FBA increases the apparent reflectance of the article in the blue-violet region of the spectrum. The treated material remits more light in the visible region than does an untreated white sample and thus appears ‘whiter than white’. These effects are indicated in Figure 11.1, which illustrates the importance of thorough preparation of the substrate to be brightened. Curve C represents the remission of an unbleached ‘dirty’ fabric. On treatment with an FBA this material is brightened but the treated sample (curve D) may be less bright than a clean but unbrightened fabric (curve A) and much less bright than the same fabric after brightening (curve B). The details of fabric preparation and bleaching processes are discussed fully elsewhere [14]. In some cases brightening and bleaching can be carried out simultaneously and these possibilities will be discussed later in this chapter.

B

Clean brightened article Clean unbrightened article

Remission

A ‘Dirty’ unbrightened article ‘Dirty’ brightened article C D

Fluorescence

300

400

500

600

700

Wavelength/nm

Figure 11.1 Remission of brightened and unbrightened fabric

An efficient FBA must absorb strongly in the ultraviolet region and must also re-emit a major proportion of the absorbed energy as visible light, that is, it must have a high fluorescence efficiency. Although fluorescence can occur from the α-bonds of many organic compounds, strong fluorescence is associated with π-bonded electrons. All FBAs therefore contain a considerable number of conjugated double bonds. Processes occurring during absorption and fluorescence are shown in Figure 11.2, where

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S0, S1, S2, … represent so-called singlet states in which all the electrons have paired spins, and T1, T2, … represent triplet states in which two electrons have unpaired spins. The energy levels of both ground (S0) and activated states (S1, S2, …) are subdivided into vibrational and rotational energy levels. The vibrational energy levels are shown in Figure 11.2. Differences in rotational levels are very small and can be ignored for the present discussion.

Internal conversion

S2 T2 Thermal deactivation Intersystem

S1

crossing

S0

Intersystem crossing

Phosphorescence

Internal conversion

Absorption

Fluorescence

Energy

T1

S0

Figure 11.2 Absorption and fluorescence processes

When an FBA absorbs a photon of light an electron is raised from the ground state (S0) of the molecule to one of its activated singlet states (S1, S2, …). Transitions from a singlet to a triplet state are quantum-mechanically ‘forbidden’. Absorption occurs when the molecule is in its ground state, the vibrational level of the activated state reached by absorption being decided by the size of the quantum of energy (E) involved (E = hc/λ, where h is Planck’s constant, c the velocity of light in a vacuum and λ the wavelength). Vibrational energy levels are extremely close to each other and vibrational energy is lost very rapidly (within about 10–12 s) before fluorescence occurs, when the molecule returns from the lowest vibrational level of the activated state (S1) to one of the vibrational levels of the ground state (S0) whilst simultaneously emitting a photon of light. Fluorescence lifetime is typically about 10–9 s. Energy can also be lost from the activated singlet state by non-radiative processes (internal conversion) or by ‘forbidden’ intersystem crossing to give the triplet state. The triplet state in turn can lose energy, returning to the ground state by phosphorescence or by a further radiationless intersystem crossing. Phosphorescence always occurs at a longer wavelength than fluorescence because the energy difference between T1 and S 0 is less than that between S1 and S0. FBAs do not exhibit significant

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phosphorescence. For a more detailed discussion of the principles of fluorescence the reader is referred to books by Lumb [15] and Lakowicz [16]. Typical absorption and fluorescence spectra are shown in Figure 11.3. Since energy is lost in the activated state (S1) before fluorescence, the emission maximum always occurs at a lower wavenumber than the absorption maximum. The difference, which is termed the Stokes shift, can be calculated approximately from the absorption spectrum using the Pestemer rule [17,18]. This rule states that the Stokes shift is 2.5 times the half-bandwidth at the absorption maximum.

Relative absorption or emission

Stokes shift

Absorption

Fluorescence emission Halfbandwidth

Wavenumber

Figure 11.3 Typical absorption and emission curves for an FBA (polar solvent)

For the following reasons the Stokes shift of an FBA should not be too large: (1) The closer the maximum wavelength of light absorption of an FBA approaches the visible region, the greater is the energy content of sunlight at the earth’s surface that is available for fluorescence excitation and the greater the potential fluorescence. An FBA with a Stokes shift of 60 nm or less would have a maximum absorption at 370 nm or longer and it would still show maximum fluorescence in the blue region of the spectrum. (2) Although absorption and fluorescence spectra are not always (or even normally) symmetrical, a smaller Stokes shift reduces the chance of significant fluorescence in the green or yellowish green regions of the spectrum. Green or yellowish green fluorescence reduces whiteness. A possible method for predicting absorption bandwidths of chromogenic molecules or FBAs using PPP–MO theory (section 1.5) has been devised. It is based on the empirical linear relationship stated by the Pestemer rule. Thus theoretical Stokes shifts are computed by the PPP–MO method and related to bandwidths. The requisite MO parameters for various typical absorption bands have been developed for use in these calculations. Reasonable correlation between calculated and experimental half-bandwidth data was found, suggesting that this approach has practical potential in predicting colour tone and brightness intensity [19].

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The quantum efficiency of fluorescence of a molecule is decided by the relative rates of fluorescence, internal conversion and intersystem crossing to the triplet state. Up to the present time it has proved impossible to predict these relative rates. Thus, whilst it is now possible to calculate theoretically the wavelengths of maximum absorption and of maximum fluorescence of an organic molecule, it remains impossible to predict which molecular structures will be strong fluorescers. Design of new FBAs still relies on semi-empirical knowledge plus the instinct of the research chemist.

11.3 EVALUATION OF FBAs: MEASUREMENT OF WHITENESS Almost all man-made fibres destined for sale as white goods are producer-brightened by the manufacturer and white textiles are almost always laundered using detergents containing cellulose-substantive FBAs [20]. To evaluate an FBA it is necessary both to apply the product to the desired substrate and to measure the whiteness of the treated material. Measurement of the fluorescence intensity of the FBA-treated substrate provides useful additional, although different, information. Visual judgement of whiteness is highly subjective. Many factors, such as the observer’s age, sex and colour perception, and even the hue of ‘white’ materials normally encountered in the observer’s country, help to decide on individual preference for either a red-violet or a blue-green shade of white. Instrumental measurement of whiteness has been the subject of much research. The parameters needed for unambiguous characterisation in the assessment of whiteness and tint of fluorescent substrates have been reviewed [21]. The importance of seeking good correlation between different instruments is stressed [20]. Various trials have demonstrated that it is possible to adjust modern instruments used to measure the optical characteristics of FBA-treated samples of paper so that the results agree with a standard deviation of the order of one CIE whiteness unit [22]. Many whiteness (W) formulae have been proposed. All are based on CIE colour space and the X,Y,Z tristimulus values. Three of these equations are those of Berger [23] (Equation 11.1), Stensby [24] (Equation 11.2) and the CIE 1982 formula (Equation 11.3). W = 3B + G - 3R

(11.1)

W = L + 3a - 3b

(11.2)

W = Y + 800( x n - x ) + 1700(yn - y)

(11.3)

The R,G,B values of the Berger formula measured by tristimulus colorimeters are linearly related to the X,Y,Z tristimulus values of the CIE system. The Stensby formula incorporates the L,a,b tristimulus values of the Hunter system. In the CIE 1982 formula, xn and yn are the chromaticity coordinates of the D65 (2° or 10° observer) light source. Information on the hue of whiteness is provided by dividing CIE colour space, in the neighbourhood of the D65 achromatic point, into a series of parallel strips corresponding to variations in hue of whiteness. The principle is illustrated in Figure 11.4. According to this

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system, a neutral white has a nuance (NU) value of zero. Greener shades have values between 0 and +5, whereas violet shades have values between 0 and –5. The NU value can be calculated from chromaticity data. Typical values giving a result corresponding to the judgement of a standard observer are shown in Equation 11.4. NU = -1132 x + 725y + 115.45

(11.4)

The brightened specimen under test must be carefully prepared and meticulous attention to the instrumentation and state of equipment is necessary if reliable data are to be obtained. The subject is complex and has been well reviewed by Griesser and co-workers [25,26].

Greener NU values positive

y

D65 achromatic point

0.33

NU values negative More violet

0.28 0.28

0.31 x

Figure 11.4 Colour space and hue of whiteness

To an average observer a sample of white material brightened with a low concentration of an FBA will exhibit a bluish tone corresponding to a dominant wavelength in the vicinity of 467 nm. As the amount of FBA present in the substrate is increased this hue changes. At first the sample may become slightly more violet but as the maximum possible whiteness is approached there is a distinct change in hue towards green until the sample becomes overloaded with FBA, whiteness falls and the material is perceived as coloured rather than white. Typical effects are illustrated in Figure 11.5. In industry a direct comparison for strength between two FBAs is frequently required. Where both brighteners contain the same active component, or where they give a closely similar shade of white, such a comparison presents little difficulty. Where the two products yield quite different shades of white, however, comparisons between them are usually meaningless. A typical situation is illustrated in Figure 11.6. At concentrations below that yielding the maximum whiteness achievable there is an approximately linear relationship between whiteness and the logarithm of the concentration

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NU value

Brightened with green-hued FBA

+ve 0 Brightened with violet-hued FBA

–ve

Whiteness

Figure 11.5 Variation of hue with whiteness

Violet-hued FBA 1

Whiteness

Violet-hued FBA 2

Green-hued FBA

Log (concn of FBA on weight of substrate)

Figure 11.6 Dependence of whiteness on concentration

of FBA present on the substrate. If two FBAs are similar in hue the straight line portions of the graph are almost parallel and a single-strength relationship can readily be calculated. If two FBAs differ in hue, however, their relative effectiveness changes with the degree of whiteness, products that give greener hues generally being more effective at low concentrations. Methods for the instrumental measurement of whiteness are well established but visual comparison remains important, even in well-equipped laboratories. Some degree of quantification is achieved by the method of paired comparisons, in which a panel of observers is presented with pairs of FBA-treated samples and asked to decide, without undue delay, which is the brighter. The total of positive scores can be used as a measure of whiteness and the results presented graphically as shown in Figure 11.7. Although time-

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consuming to carry out, a paired comparison series produces results that can be regarded with considerable confidence and that usually correlate well with a comparison based on the CIE whiteness formula.

No. of positive scores

Brightened with FBA 1

Brightened with FBA 2

Log (concn of FBA on weight of substrate)

Figure 11.7 Presentation of results of a paired comparison

11.4 GENERAL FACTORS INFLUENCING FBA PERFORMANCE Apart from needing to be cost-effective, a good FBA must be capable of producing a high level of whiteness. As the amount of FBA on a substrate is increased, whiteness increases until a maximum value is reached (Figure 11.6). Further application of FBA results in lower whiteness. On polyester the fall in whiteness with increasing concentration of FBA is apparently not accompanied by a fall in total fluorescence [27]. On cotton both whiteness and total fluorescence fall, the decrease in whiteness occurring before that in fluorescence [28]. In general the main cause of the fall in whiteness with increasing concentration of FBA is an increase in aggregation of the FBA on the substrate, resulting in a shift in fluorescence hue. The effect is shown in Figure 11.8. Not surprisingly, FBAs that give a greenish hue at concentrations below the maximum whiteness tend to produce a lower maximum white than those exhibiting a violet hue. Other factors such as substantivity are of considerable importance, of course. A computer-based expert system has been devised as an aid to selection of the most suitable FBAs for specific applications to various fibres using the processing equipment available in any given finishing works [29]. The presence of salts and additives can have an important influence on the performance of an FBA. Traces of transition-metal ions such as iron and copper have an adverse effect on fluorescence [30], but this can be controlled using conventional polyphosphate or EDTAtype sequestering agents [31]. Other salts, even sodium sulphate or sodium chloride, have been claimed to enhance the fluorescence of FBAs in solution [32]. Apart from the normal

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769

Relative fluorescence

Low concentration FBA

Medium concentration FBA

High concentration FBA

Wavelength

Figure 11.8 Effect of FBA concentration on fluorescence hue

effect of such electrolytes in minimising electrostatic repulsion between anionic FBAs and the negatively charged surface of cellulose, their influence on the whiteness of a brightened substrate is doubtful. Surfactants, not surprisingly, exert a highly significant influence on the fluorescence of FBAs in solution. This effect is associated with the critical micelle concentration of the surfactant and may be regarded as a special type of solvent effect. Anionic surfactants have almost no influence on the performance of anionic FBAs on cotton, but nonionic surfactants may exert either positive or negative effects on the whiteness of the treated substrate [33]. Cationic surfactants would be expected to have a negative influence, but this is not always so [34]. No general rule can be formulated and each case has to be considered separately. The influence of additives needs to be kept under constant consideration when formulating commercial brands of FBAs. Liquid formulations are of considerable commercial importance and it is often necessary to use solvents such as diethylene glycol, poly(ethylene glycol) and alkoxylated alkylphenols in order to achieve stable solutions. Surfactants can help to stabilise FBA-resin bath combinations but in other circumstances they can adversely affect the performance of the formulation, not only in its capacity to brighten the substrate but also in the desired end-use of the brightened fabric. For example, nylon brightened by the pad–thermosol method using a liquid formulation containing a large amount of an alkoxylated polymer cannot be used as a substrate for colour printing unless previously washed. Without a wash-off, diffuse prints are obtained. Violet or bluish violet dyes can be used in combination with FBAs for shading purposes. These shading dyes are used sparingly at no more than 2% of the weight of FBA applied. They are of particular importance when the material to be brightened is slightly yellow. The shading dye converts the pale yellow hue of the substrate to a perceived grey, enhancing the effectiveness of the FBA. These effects can be considerable. Many types of violet or bluish violet dye may be used. Typical examples include crystal violet (CI Basic Violet 3) used in the brightening of paper stock or cotton, CI Disperse Violet 28 and CI Acid Violet 43 applied with cotton brighteners. Disperse dyes are also selected for shading with disperse FBAs on polyester and with basic FBAs on acrylic fibres.

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On slightly yellow cotton the addition of a shading dye is particularly convenient, since the substrate becomes bleached during the wash-wear cycle. Detergent formulations contain FBAs that are substantive to cotton and loss of shading dye during washing is unlikely to have a noticeable effect on the perceived whiteness of the article. Shading dyes are of limited use with thoroughly prepared and well-bleached cotton. The importance of the inherent degree of whiteness of cotton in determining its response to finishing processes has been emphasised. The effects of dry heat on peroxide-bleached cotton fabrics treated with brighteners that varied in substantivity were investigated. Remission spectroscopy was used to analyse the origins of thermal yellowing [35]. FBAs applied in combination may show a synergistic effect. Synergism is, at present, only of commercial interest with polyester brighteners. This phenomenon is discussed further in section 11.10. Conversely, the presence of a trace impurity in an FBA formulation may greatly reduce its effectiveness. In industrial laboratories much time and effort is expended in developing processes to minimise the content of such impurities or even to eliminate them completely. 11.5 CHEMISTRY AND APPLICATIONS OF FBAs FBAs are available for application to all types of substrate. Thus there are anionic FBAs for application to cellulosic materials in the presence of added salts, anionic types for application to nylon or wool in the presence of acid, cationic types for acrylic fibres, disperse types for polyester, and so on. Brighteners such as CI Fluorescent Brightener 104 (11.4), capable in principle of being fixed to wool or cellulosic fibres by reaction with nucleophilic groups in the substrate have been reported but have never achieved commercial importance. Such FBAs in their reactive forms show diminished fluorescence because the presence of labile halogen atoms leads to quenching. Hydrolysis or reaction with the fibre is accompanied by development of fluorescence but an aftertreatment step is essential to ensure that all the active chloro substituents present have been removed from the FBA absorbed by the substrate [36]. The chemical structures of FBAs are many and varied although, of course, they all contain some sort of extended π-electron system. In the following discussion all the main chemical types are mentioned, but mainly in terms of application rather than chemistry of preparation. 11.6 BRIGHTENERS FOR CELLULOSIC SUBSTRATES The earliest FBAs were developed for application to paper. Even today larger tonnages of brightener are marketed for application to cellulosics than to any other substrate. 11.6.1 FBAs for cotton Brighteners are applied to cotton by methods similar to direct dyes. By far the most common are triazinyl derivatives of diaminostilbenedisulphonic acid (DAS) of general formula 11.5, where M is an alkali metal, ammonium or alkylammonium cation. Examples of groups R1 and R2 are shown in Table 11.1. Most suppliers of FBAs market such compounds, often called DAST brighteners. Products in this class have sometimes been marketed because the supplier needed to offer something different for commercial reasons, or to avoid infringing a competitor’s patent, rather than for any real technological necessity.

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R2

O

N N

N

SO3M

R1

NH

N

NH N

N N

HC

SO3Na

Cl HC

CH

MO3S

CH

R1 N

NaO3S HN

N

HN

Cl

N

N

11.5

N

R2

N

11.4

N

CI Fluorescent Brightener 104 O

Table 11.1 Important FBAs of type 11.5 used to brighten cotton Substituents R1 and R2

SO3Na

Substantivity

Application

Lower

Padding

Higher

Exhaust

SO3Na

NH

NH

SO3Na NH

N

O

NaO3S SO3Na

N(CH2CH2OH)2

NH SO3Na

NHCH2CH2OH

NH

NH

SO3Na

N

O

NH

N(CH2CH2OH)2

NH

N

CH3 CH2CH2OH

NH

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OH ON

O2N

HN

2H

11.6

2H

SO3Na

SO3Na

SO3Na

CH3

CH3

CH3

NaOCl or air/catalyst self-condensation

H3C O2N

NaO3S O

SO3Na HC

N

N

11.8

CH

SO3Na

Reduction NaO3S

H2N

CH3

NO2

SO3Na HC

CH 11.7

NaO3S

DAS NH2

Scheme 11.1

DAS (11.7) is synthesised from 4-nitrotoluene-2-sulphonic acid (11.6) by the route outlined in Scheme 11.1. An important factor in the preparation of DAST brighteners in the purity necessary for good performance is the purity of the DAS used as starting material. At one time DAS made in this way contained significant amounts of yellow azoxy compounds similar to 11.8, which formed the main components of the obsolescent dye Sun Yellow (CI Direct Yellow 11) made by the partial reduction and self-condensation of intermediate 11.6. Today the major manufacturers supply DAS essentially free from these undesirable impurities [37]. It is almost impossible to give a comprehensive list of all the FBAs of type 11.5 that have appeared on the market since the first (11.3) of them was patented in 1940, but several important commercial products are shown in Table 11.1. The less water-soluble products have been widely used in the past as brighteners for detergent formulations and are generally used to brighten cotton by exhaustion. The more soluble, less substantive types are usually applied by padding in continuous bleaching, as a white ground for printing or in resin finishing of woven goods for white garments or household textiles. Application methods have been well described by Williamson [9].

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Where the groups R1 and R2 as defined in Table 11.1 are both derived from amines, variation of the amine has little influence on the hue of the white obtained. In general these products give slightly violet shades of white and the main technical justification for the existence of so many different structures in this class is the need for different levels of substantivity. The compound in Table 11.1 where R2 is methoxy gives a distinctly violet tone. All these products have light fastness values in the range 3–4 on cotton. In principle, all the more substantive types may be applied in conjunction with a hydrogen peroxide bleach. Their stability towards chlorine bleaches such as sodium hypochlorite varies, but they are all essentially unstable towards these reagents [4]. Nine DAST-type FBAs, including six of those listed in Table 11.1, were exhausted onto cotton fabric at two applied concentrations (0.05% and 0.5% on weight of fibre). Values of exhaustion, fluorescence intensity and whiteness index were determined, as presented for 0.05% o.w.f. in Table 11.2. Results at the lower concentration facilitated more precise comparisons between FBAs because of self-extinguishing of fluorescence and greater variability in exhaustion at the higher level [38]. Structures containing disulphonated anilino groups linked to either diethylamino or morpholino via the triazine rings were characterised by high solubility, poor exhaustion and relatively disappointing increases in whiteness and fluorescence intensity. Conversely, the combination of unsulphonated anilino and methoxy substituents yielded high fluorescence and whiteness values in spite of only moderate exhaustion, especially at the higher concentration. Average performance was characteristic of those products containing monosulphonated anilino and hydroxyalkylamino groups. The treated fabrics were exposed for various times in an Atlas Weatherometer and typical values of loss in fluorescence intensity and whiteness are given in Table 11.3. Relatively higher photostability was shown by the unsulphonated anilinotriazines, especially if associated with alkoxy substituents. The least stable combinations were disulphonated anilino groups with either diethylamino or morpholino i.e. those exhibiting poor exhaustion and low whiteness index in Table 11.2. As in the latter table, the presence of monosulphonated anilino and hydroxyalkylamino substituents on the triazine rings resulted in average levels of performance. A detailed study of the photostability of the DAST-type agent CI Fluorescent Brightener 85 on cellophane film was carried out recently. The initial fading reaction is a photosensitised trans–cis isomerisation of the stilbene grouping. The subsequent oxidative attack on the molecule is concentrated on the vulnerable ethene linkage at the centre of this moiety [39]. Substituted triazinyl derivatives of DAS are usually chosen for pad–dry–bake application to cotton in conjunction with an easy-care or durable-press finish. In these mildly acidic conditions (pH about 4) the FBA must show appreciable resistance towards the catalyst (usually magnesium chloride) necessary to cure the resin. The less substantive products in the upper half of Table 11.1 are important in this respect, as are compounds of type 11.9 where R = OCH3 or CH3NCH2CH2OH. It is likely that the hydroxyethylamino groups present in many of these compounds participate in condensation reactions with N-methylol groups in the cellulose-reactant resin. The performance of an FBA applied in conjunction with a resin finish can be modified and improved by careful formulation of the pad liquor but this lies beyond the scope of the present chapter. Alternatively, FBA and resin can be applied in two separate steps; most DAST-type brighteners would be suitable if applied in this way.

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Table 11.2 Exhaustion, fluorescence and whiteness shown by type 11.5 FBAs on cotton [38] Increase in fluorescence intensity at 0.05% o.w.f.

Increase in whiteness index (ASTM) at 0.05% o.w.f.

80

0.98

43

100

0.95

36

100

0.88

33

60

0.88

32

80

0.83

31

N(CH2CH2OH)2

80

0.78

28

N

O

40

0.82

26

N(CH2CH2OH)2

80

0.76

25

N(CH2CH3)2

40

0.69

19

Exhaustion (%) at 0.05% o.w.f.

Substituents R1 and R2

NH

OCH3

NH

N(CH2CH2OH)2

NH

N

CH3 CH2CH2OH SO3Na NH

NHCH2CH2OH

NH

OCH2HC

CH3 OH SO3Na NH

SO3Na NH NaO3S

NH

SO3Na SO3Na

NH NaO3S

11.6.2 FBAs for paper The paper industry is the second most important user of FBAs after the detergent industry, most of the products applied to paper being of the DAST type. Paper may be brightened during preparation, the FBA being added to pre-bleached pulp before the paper sheet is laid down, or during a subsequent sizing operation. Approximately one-third of the total FBAs used are applied to pulp and two-thirds at the sizing stage. An FBA selected for addition to the pulp must show high substantivity at low temperature, otherwise there would be excessive loss of brightener with the waste water from the process. Resistance towards acidic conditions as low as pH 3 can also be important. Fillers used in

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Table 11.3 Loss in fluorescence and whiteness of type 11.5 FBA-treated cotton on exposure to xenon light [38] Effect of 40 AATCC fading units % Loss in fluorescence at 0.05% o.w.f.

% Loss in whiteness (ASTM) at 0.05% o.w.f.

N(CH2CH3)2

70

92

N

O

58

84

N(CH2CH2OH)2

49

75

NHCH2CH2OH

49

73

NH

N(CH2CH2OH)2

44

73

NH

N

46

69

45

67

44

66

44

65

Substituents R1 and R2

SO3Na NH NaO3S SO3Na NH NaO3S

NH

SO3Na

SO3Na NH SO3Na

CH3 CH2CH2OH NH

N(CH2CH2OH)2

CH3 OCH2HC

NH

OH

NH

OCH3

papermaking, such as alum, chalk or china clay, may cause loss of fluorescence and the type and quantity of FBA added may have to be adjusted accordingly. For use from the size press it is necessary for the FBA to be compatible with the chosen size, such as starch, casein or urea-formaldehyde resin. Since sizes tend to be yellowish and to absorb ultraviolet radiation, brighteners are generally less effective in sized paper. The choice of FBAs and their methods of application to paper is highly complex, being almost as much an art as a science [40]. Examples of important FBAs for use with paper are listed in Table 11.4. This list is far from exhaustive, however, and there are other important products of type 11.5 used as FBAs for paper.

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FLUORESCENT BRIGHTENING AGENTS

N(CH2CH2OH)2 N N

NH N SO3Na

R HC

CH R

NaO3S N HN

N N

11.9

N(CH2CH2OH)2

Table 11.4 Important FBAs of type 11.5 used to brighten paper Substituents R1 and R2

Application

NH

N(CH2CH2OH)2

Pulp

N(CH2CH2OH)2

Pulp and size press

SO3Na NH

CH2CH2OH NH

N

SO3Na

CH2CH2CN

Pulp and size press

SO3Na NH N(CH2CH3)2

Size press

N(CH2CH2OH)2

Size press

NaO3S NH2

11.6.3 Preparation of DAST-type FBAs A major reason for the importance of DAST brighteners is their essentially straightforward manufacture from readily available and inexpensive intermediates. Products with widely different substituents and hence showing quite different application properties are easily prepared in a three-step, one-pot synthesis starting from diaminostilbenedisulphonic acid (11.7). The process is illustrated in Scheme 11.2. By suitable choice of reaction conditions the chloro substituents of cyanuric chloride (11.10) can be replaced in a stepwise fashion. In the first step DAS reacts with cyanuric chloride at a temperature in the 0–20 °C range, ideally at pH 5–6. In the second step an amine or alcohol (R1H) reacts within the range 20–50 °C under neutral or slightly alkaline

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BRIGHTENERS FOR CELLULOSIC SUBSTRATES

777

Cl N H2N

N

NH N

SO3Na HC CH

+ 2 Cl

SO3Na

Cl

Cl N

step 1

HC CH

N N

NaO3S

NaO3S

Cl

Cl N

11.10 11.7

NH2

HN 2 R1H

N N

step 2

Cl

Cl

R2 N N

N NH

N

N

NH N

SO3Na

R1

SO3Na

R1 2 R2H

HC CH

step 3

NaO3S

R1

HC CH NaO3S

R1

N

N N

HN

N

HN

N

N R2

Cl

Scheme 11.2

conditions. The third step with another amine or alcohol (R2H) is completed within the range 50–100 °C under alkaline conditions (pH 8–9). The exact conditions selected for the second and third steps depend on the nature of the attacking nucleophiles (R1H and R2H), as well as the substituents already present in the chlorotriazine intermediates. Scheme 11.2 illustrates the conventional sequence for the manufacture of DAST brighteners. However, it is not always necessary and may not be desirable for DAS to be the nucleophile selected for the first step. In principle the three nucleophiles can be reacted in any order, but it is preferable for the most nucleophilic amine to react last in order to avoid forcing conditions during removal of the last remaining chloro substituents. Alkylamines react more readily than alcohols, thus ensuring that alkanolamines yield their hydroxyalkylaminotriazine derivatives. The mechanism of these bimolecular nucleophilic substitution reactions is shown in Scheme 11.3 for the reaction between a primary amine and the intermediate dichlorotriazine. A corresponding scheme can be drawn for reaction of a secondary amine, an alcohol or any other nucleophile in any of the replacement steps. It follows from this mechanism that the rate of reaction depends on: (1) the nucleophilicity of the attacking species, the more nucleophilic reagents reacting more quickly or under milder conditions (2) the electronegativity of the substituent R1.

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FLUORESCENT BRIGHTENING AGENTS

H N

R1

_ HN+ R 2 N

R1

Cl slow

N

+

N

H2N

R2

N

N

Cl

Cl

Cl

N

R1 N

HN N

R2 + HCl

Cl

Scheme 11.3

The more electronegative the substituent, the more rapid the reaction or the milder the conditions required. Thus the reaction rate decreases in the order: PhO > MeO > EtO > PhNH > NH2 > MeNH > EtNH > Me2N > OH [4]. It also follows that protonation of the triazine ring makes it more susceptible to attack by nucleophilic reagents unless the reagent itself is also protonated. If the triazine ring remains unprotonated when a nucleophilic base, such as an alkylamine, is present as its acid salt the reaction is slower, of course. Cyanuric chloride itself is a very weak base that becomes protonated only under strongly acidic conditions. Thus step 1 in Scheme 11.2 can be carried out in aqueous solution even at pH 2 without risk of undesirable hydrolysis of cyanuric chloride, water being an extremely weak nucleophile. The manufacture of several important brighteners containing alkoxytriazine groups, such as the DAS derivative of highest substantivity in Table 11.1, does not follow the conventional sequence. The first step involves reaction of cyanuric chloride with excess methanol and excess acid acceptor, usually sodium bicarbonate. Under acidic conditions this reaction takes a quite different course and can become dangerously violent (Scheme 11.4).

H3CO N

NaHCO3 Cl

N N

N N Cl

Cl N

Cl

+ excess CH3OH HO

N

OH

Cl acid conditions

11.10

N

+ 3 CH3Cl(g) Methyl chloride gas

OH

Cyanuric chloride

Cyanuric acid

Scheme 11.4

chpt11(2).pmd

N

778

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BRIGHTENERS FOR CELLULOSIC SUBSTRATES

779

In the preparation of other DAST brighteners it may be advantageous to avoid reacting DAS with cyanuric chloride in the first step. It is difficult to suppress the reactivity of the second chloro substituent completely and undesirable by-products of the general type 11.11 can be eliminated if DAS is made to react with a dichlorotriazine intermediate in the second step. Very careful control of the reaction conditions, especially in steps 1 and 2, is also necessary in order to avoid formation of partially hydrolysed by-products such as structures 11.12 and 11.13. Unsymmetrical products of type 11.14 derived from DAS have been described but these are much more complicated and expensive to prepare than those with symmetrical structures. They have never become commercially important. R2

R2 N

N

N NH

N

HN

N

N SO3Na

R1 HC

R1

NaO3S

CH

HC

CH SO3Na

NaO3S N 11.11

HN

NH N

N

HO R2

N NH

N N

SO3Na

R HC

CH

R1 NaO3S

R2

R

HO

N

N N

NH

CH

NaO3S

R3 N N

HN

11.14

N R4

chpt11(2).pmd

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N R2

OH

SO3Na HC

N

N

N R1

N

HN

11.12

N

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11.13

780

FLUORESCENT BRIGHTENING AGENTS

11.6.4 Speciality FBAs for cotton Two commercially important brighteners for cotton are not of the DAST type. The distyryldiphenyl 11.15 is mainly of value for brightening cotton during laundering but it can also be used to brighten cotton by exhaust application. It has high aqueous solubility and has been recommended for use in combination with resin finishes, although its stability at the padding stage is suspect. It has found further uses in the brightening of polyester/ cotton and nylon/cotton blends. Both components of a nylon/cotton blend are brightened and on polyester/cotton the cellulosic component is brightened without any undesirable staining of the polyester. Its light fastness on cotton is 4, which is slightly superior to that of DAST brighteners. Compound 11.15 is also resistant to hypochlorite bleach but on cotton it has limited fastness to washing in soft water. The effect of humidity on the photostability of CI Fluorescent Brightener 359, a distyryidiphenyl structure similar to 11.15, has been studied on cellophane film recently. A kinetic analysis of fading rates under these conditions indicated participation in a bimolecular oxidative mechanism [41]. The manufacture of brightener 11.15 is shown in Scheme 11.5. The first chloromethylation stage can only be accomplished safely in a plant specially designed to CH2Cl

ZnCl2 HCHO + HCl

HOCH2Cl

O CH2Cl 11.16

2 HOCH2Cl ZnCl2

ClCH2

CH2Cl

Biphenyl

+ 2 H2O

2 P(OCH2CH3)3

Triethyl phosphite OCH2CH3 O

P

OCH2CH3

CH2

CH2

P

+ 2 CH3CH2Cl

O

OCH2CH3

OCH2CH3

SO3Na CHO

Base

SO3Na HC CH

HC CH

Scheme 11.5

chpt11(2).pmd

11.15

780

NaO3S

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781

BRIGHTENERS FOR CELLULOSE ACETATE AND TRIACETATE FIBRES

ensure that the highly carcinogenic by-product bis(chloromethyl) ether (11.16) formed from formaldehyde and hydrochloric acid does not escape. Otherwise the preparation proceeds without undue difficulty. A specific advantage of sulphonated distyrylarene brighteners such as 11.15 and similar structures is that the ethene double bonds are readily oxidised during effluent treatment, leading to the formation of soluble acids of low relative molecular mass (Scheme 11.6). Thus it can be inferred that these brighteners and their degradation products are relatively innocuous and are unlikely to accumulate in the environment. SO3Na O2 + H2O SO3Na

COOH

2

free-radical conditions

HC

+ CH

HC

OH

O CH

C HO

11.15

C O

NaO3S

Scheme 11.6

The introduction of heterocyclic rings as terminal groups in the 4,4′-positions of the stilbene nucleus intensifies the fluorescence of the conjugated system and shifts the fluorescence maximum to a longer wavelength. The vic-triazole 11.17 is a premium product and cotton brightened with it has a light fastness of 5. This structure is stable to bleaching with hypochlorite or chlorite. It has adequate fastness to washing and a liquid formulation has been marketed for use in combination with a resin finish. It is also of some importance as a component of household detergents. Unfortunately, it is also expensive and its main use is probably as an FBA for nylon, on which fibre it gives better value for money. Its preparation is shown in Scheme 11.7. A more soluble derivative of compound 11.17, the tetrasulphonated analogue 11.18, has been recommended for application to cotton in combination with a resin finish. Unlike DASTtype FBAs under these conditions, compound 11.18 is compatible with resin formulations containing zinc nitrate as latent acid catalyst. The brightness achieved is not high, however. Many other products of a variety of structures have been patented for the brightening of cellulosic substrates. The reader is referred to the reviews mentioned earlier for further information. 11.7 BRIGHTENERS FOR CELLULOSE ACETATE AND TRIACETATE FIBRES Cellulose acetate and triacetate fibres are brightened with disperse-type FBAs, including derivatives of 1,3-diphenylpyrazoline (11.19). These form a commercially important group of FBAs. If suitably substituted they can be applied to substrates other than acetate and triacetate. The commercially more important products of this type are used to brighten nylon and acrylic fibres. Their preparation and other aspects of pyrazoline chemistry are discussed in section 11.8. Examples of pyrazolines used to brighten acetate and triacetate

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FLUORESCENT BRIGHTENING AGENTS

H2N SO3H HC

CH

11.7

O C

HO3S H3C NH2

Alkyl nitrite (acid or alkoxide)

1. Diazotise 2. Na2SO3, then acid

H2N

NH O SO3H

HO

2

+

C N

HC

CH

CH CH3COONa

HO3S HN

NH2 HC

N OH

N

NH SO3Na HC

N N

N

CH

NaO3S SO3Na

HN

N

HO HC

CH

N

CH

NaO3S CH3OH / H2O / H2NCONH2 / (CH3CO)2O

11.17 N

N

N

Scheme 11.7

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BRIGHTENERS FOR CELLULOSE ACETATE AND TRIACETATE FIBRES

783

fibres include the sulphonamide 11.20 and the sulphone 11.21, the former giving greenish tones and the latter violet effects. A pyrene derivative (11.22), a naphthalimide (11.23) and benzoxazoles of smaller molecular size are also used and these are discussed in more detail in section 11.10. The naphthalimide ring system is highly stable, leading to products with good light fastness and stability to chlorite. Their main disadvantage, however, is relatively low fluorescence efficiency, which is primarily a result of low molar extinction coefficients. Stronger fluorescence arises when there is an electron-donating group such as methoxy (as in 11.23) or alkylamino in the 4-position, in which case the absorption and emission properties are associated with intramolecular charge transfer involving the donor group and the electronwithdrawing peri-carbonyl groups.

NaO3S N N

N SO3Na

N N

HC 11.18

CH 11.19

NaO3S

1,3-Diphenylpyrazoline

N

N

N SO3Na

N Cl

N

SO2NH2

N Cl

11.20

N

SO2CH3

11.21

OCH3 N

N N

CH3 OCH3

O

N

O

11.22

11.23

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OCH3

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FLUORESCENT BRIGHTENING AGENTS

Several interesting analogues of structure 11.23 were synthesised recently. These derivatives of 4-methoxynaphthalimide contained a triazine ring with an unsaturated polymerisable substituent capable of addition copolymerisation with other vinyl or acrylic monomers. Such brighteners can be incorporated into the synthesis of polymeric finishes and show exceptional durability to organic solvents and wet treatments [42–44]. 11.8 BRIGHTENERS FOR NYLON Disperse brighteners of the types used to brighten cellulose acetate, triacetate or polyester fibres can be used to brighten nylon. In practice, however, disperse types are little used and nylon is usually brightened with sulphonated compounds akin to acid dyes. Silk and polyurethane fibres can also be brightened with the FBAs normally used on nylon. Chief amongst these are products of the DAST type already discussed in section 11.6.3. Two series of DAST derivatives of the general structure 11.5 were evaluated recently, in which the R1 groups were morpholino and the R2 groups either arylurea or arylthiourea (11.24; Ar = aryl). These products showed high substantivity for both cotton and nylon, the ureido (X = O) or thioureido (X = S) residues providing scope for hydrogen bonding with polar groups in these substrates [45].

X C Ar

NH

NH

N N

NH N SO3Na

N HC

O

CH

O

NaO3S

N N HN

11.24

N N

HN HN

Ar

C X

Exhaust brightening of nylon is usually combined with a reduction bleach based on sodium dithionite. Important DAST-type FBAs suitable for this process are exemplified by the four most cotton-substantive compounds listed in Table 11.1. The methoxy-substituted derivative can be applied to nylon successfully at pH 6–6.5 but those with three NH groups attached to each triazine ring are best applied at pH 4–5. All these products show a light fastness rating of only about 3 on nylon. If a reductive bleach is unnecessary the DAST brightener 11.25 can be applied to nylon from an alkaline scouring bath. Good whites are achieved but the light fastness of this tetrakis(anilino) derivative is suspect on nylon.

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BRIGHTENERS FOR NYLON

785

NH N N

NH N SO3Na

NH HC

CH HN

NaO3S N HN

N

11.25

N HN

The premium FBAs 11.15 and 11.17 already mentioned are of importance on nylon because of their superior fastness properties. As on cotton, the distyryldiphenyl structure 11.15 has slightly higher light fastness than the DAST-type brighteners. In contrast to its performance on cotton, however, it has excellent fastness to washing on nylon. The light fastness of the vic-triazole 11.17 on nylon is 4–5; as on cotton this is significantly superior to that of the DAST derivatives. Unlike the DAST types, the victriazole is also stable towards a sodium chlorite bleach. Applied to nylon in combination with sodium chlorite, compound 11.17 can give exceptionally high whiteness and excellent fastness properties. Nylon can also be brightened using an anionic derivative of 1,3-diphenylpyrazoline, such as FBA 11.26, 11.27 or 11.28. Although these pyrazolines give excellent whites when applied to nylon by exhaustion, they are usually less cost-effective than the DAST brighteners. For continuous application by pad–thermosol or pad–acid shock methods the situation is reversed, however, and the pyrazoline FBAs are commercially important in this sector.

N Cl

11.26

N

SO2CH2CH2SO3Na

CH3

N Cl

SO2CH2

N

11.27

CH3

CH3 N

Cl

N

SO2CH2CH2SO3Na

11.28 Cl

chpt11(2).pmd

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C SO3Na

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FLUORESCENT BRIGHTENING AGENTS

On nylon these three pyrazolines (11.26–11.28) have light fastness values in the range 3– 4, slightly superior to the DAST types. Light fastness in the wet state is generally lower, however, and the pyrazolines suffer more in this respect than the DAST brighteners. Pyrazolines 11.26 and 11.27 in particular have very poor light fastness values of 1–2 in the wet state. The two electron-withdrawing chloro substituents in compound 11.28 have the effect of improving light fastness, especially in the wet state, but at the expense of a loss in solubility and slightly more difficult formulation and application. All three pyrazolines give violet-toned whites on nylon, compound 11.28 being slightly less violet than the other two. Electron withdrawal by the 3,4-dichloro substituents and the lactone ring in the 3′,4′positions of the diphenylpyrazoline derivative 11.29 enhances light fastness in both the dry and wet states. Thus the fastness rating of compound 11.29 is slightly higher than those of compounds 11.26–11.28 in the dry state and significantly higher on wet nylon. Pyrazoline 11.29 is also capable of giving brilliant whites of a pleasing bluish hue. It is complicated to manufacture, however. Increasing substitution also reduces solubility and thus adversely affects pad liquor stability, although this problem can be solved by suitable formulation. Pad liquor formulation can be especially important in the pad–thermosol process, where the brightened nylon fabric is often intended as a prepared white ground for colour printing with acid dyes. If too much surfactant has been applied together with the FBA in the pad liquor, ‘bleeding’ from the fabric at the printing stage can give an unacceptably blurred appearance to the printed design. CH3

O N

Cl 11.29

N Cl

O

CH2SO3Na

The general method for the preparation of diphenylpyrazolines is shown in Scheme 11.8, in which X is a suitable leaving group, usually chloro but sometimes dialkylamino. This reaction normally proceeds readily, although pH control may be important. Preparation of the substituted ketone and hydrazine intermediates needed for the synthesis may involve lengthy and complicated sequences. Further reactions are often required to modify the substitution in ring B after formation of the pyrazoline ring. The preparation of compound 11.26 shown in Scheme 11.9 illustrates one of the simpler instances. Derivatives of 1,3-diphenylpyrazoline have been used to brighten cellulose acetate (section 11.7) and acrylic fibres (section 11.11.1) as well as nylon. There has been much study of the effects of substituents on application properties and some general rules can be formulated: (1) The greater the electron-withdrawing character of substituents in ring A, the greener the hue of brightening. (2) The greater the electron-withdrawing character of substituents in ring B, the more violet the hue of brightening. (3) An electron-donating group in the 4-position of the pyrazoline ring has a slight hypsochromic effect, but an electron-withdrawing group has a bathochromic effect. (4) Light fastness is improved by the introduction of electron-withdrawing substituents into ring A, but is adversely affected by electron-donating substituents.

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BRIGHTENERS FOR NYLON

H2N

O C

A

HN

+ CH

CH

N

B

A

N

B

base

X

Scheme 11.8

O Cl

+

Cl

C CH2CH2Cl

Chlorobenzene

AlCl3

O Cl

H2N

C

+

SO2CH2CH2OH

HN

CH2CH2Cl base

N N

Cl

SO2CH2CH2OH

1. H2SO4 2. Na2SO3

N Cl

N

Scheme 11.9

SO2CH2CH2SO3Na

11.26

Theoretical explanations for the effects of substituents on the hue of diphenylpyrazoline brighteners have been published by Güsten and co-workers [46,47]. In practice almost all commercially important diphenylpyrazoline FBAs have the general structure 11.30, in which SO2R is a sulphone or sulphonamide grouping. CH3 N Ar

N

SO2R

Ar = Cl

or

Cl

Cl

11.30

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FLUORESCENT BRIGHTENING AGENTS

Since pyrazoline FBAs tend to stain cotton they cannot be used to brighten nylon/cotton blends, which require an FBA of the DAST type, the distyryldiphenyl 11.15 or the victriazole 11.17. The two most important of these are probably the methoxy-substituted DAST product in Table 11.1 and the distyry1diphenyl derivative. By careful adjustment of the dyebath pH these products will exhaust onto both nylon and cotton to give a good solid white. Nylon can also be brightened by incorporation of a thermally stable FBA in the melt, the FBA being added to the polymer before extrusion or shaping. Structure 11.31 is typical of the compounds selected for this purpose. The benzoxazole ring system is particularly important in intensifying the fluorescence of the stilbene system and conferring high fastness to light. H3C

C

CH3 N

H3C

HC O

CH 11.31

11.9 BRIGHTENERS FOR WOOL Wool is naturally yellower than other textile substrates and bleached wool gradually becomes yellow again when exposed to sunlight or other ultraviolet sources. The effect of light on wool is complex and depends on the conditions of exposure. The problem of yellowing is accentuated if the wool is left wet in sunlight. Since FBAs absorb ultraviolet light they accelerate this photo-initiated yellowing. The rate of yellowing of bleached wool increases with decrease in wavelength of the incident radiation, 300 nm being the approximate lower limit of ultraviolet reaching the earth from the sun. In contrast, the most important wavelengths for yellowing FBA-treated wool are within the range 340–420 nm. Maximum free-radical formation and acceleration of yellowing occurs in the region 350–410 nm, where brightener excitation takes place [48]. For satisfactory whiteness on wool, it is essential for the fibre to be well scoured and bleached, either oxidatively with hydrogen peroxide or by reduction using stabilised sodium dithionite. Brightener is usually applied together with the dithionite bleach. To achieve the highest possible whiteness, the wool should first be scoured to remove natural waxes and other contaminants, then bleached with peroxide and finally treated with FBA during a second bleach with dithionite. If wool containing a reducing agent is exposed to irradiation, the rate of yellowing is slower than on untreated wool. Thiourea dioxide is particularly effective in this regard, especially when used in conjunction with formaldehyde. Although thiourea dioxide retards the yellowing of wool treated with an FBA, it does not prevent destruction of the latter. It was shown that this reducing agent minimises yellowing by reducing the coloured products formed on photodegradation of the FBA and certain amino acid residues in the substrate [49]. The FBAs used to brighten wool are mainly DAST types and pyrazolines of the acid dyeing type already discussed in section 11.8. Examples include the three most cottonsubstantive DAST brighteners listed in Table 11.1, although on wool these give light

chpt11(2).pmd

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BRIGHTENERS FOR WOOL

789

fastness ratings of only approximately 2. The pyrazolines 11.26–11.28 have light fastness of 3–4 on dry wool, but very poor light fastness in the wet state. The coumarin derivative 11.32 is sometimes used on wool and can give exceptional brilliance, but unfortunately its light fastness is only 1. The important fluorescent coumarin derivatives almost invariably contain an electron-donating substituent in the 7-position, for example a dialkylamino group as in compound 11.32. Furthermore, an electron-withdrawing substituent in the 3- or 4-position, such as a cyano group, leads to shifts of the absorption and emission bands to longer wavelengths [50]. CH3

(CH3)2N

O

O

11.32

Although wool is most often brightened using FBAs containing stilbene or pyrazoline fluorescent systems, such compounds degrade rather quickly on exposure to sunlight and also sensitise photodegradation of the wool. Degradation on the surface of wool appears to involve interaction of the stilbene with wool keratin and this is not necessarily a photooxidative process. There is good evidence that adsorbed stilbene derivatives can sensitise the formation of singlet oxygen, which then reacts with indole rings (tryptophan residues) in wool. Like the pyrazolines, stilbenes have the facility to self-destruct in photochemical processes [51]. Novel fluorescent anionic surfactants of the types 11.33 and 11.34, where R represents alkyl groups of various lengths, have been applied to wool in order to study their distribution and effects on the physical and chemical properties of the fibre. Sections of the treated fibres were examined under a fluorescence microscope. The intercellular and cell remnant regions appeared to be the preferred locations of the adsorbed surfactants, but the distribution pattern was dependent on the length of the R chain of the surfactant and the conditions of application to wool [52]. CH3 KO3S

R N R

HO

O 11.33

N

SO3NH4

O 11.34

Fluorescence quenching studies of these alkylated FBAs in aqueous solution were carried out using spectroscopic techniques. For the higher members of each series, plots of fluorescence quantum yield against agent concentration showed a sharp decrease in fluorescence intensity at a specific concentration, in contrast to the smoothly decreasing curve characteristic of the lower alkyl or unsubstituted FBAs. The kinetics of fluorescence

chpt11(2).pmd

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790

FLUORESCENT BRIGHTENING AGENTS

quenching were consistent with the formation of micelles from the more surface-active higher alkyl derivatives [53]. Long-lived transient species have been detected during laser flash photolysis of solutions of the disulphonated distyryldiphenyl derivative 11.15. These transients are readily quenched by reducing agents but their yield is enhanced in the presence of oxidising ions. Such species are believed to be radical cations formed following monophotonic photoionisation. Transient quenching is observed in the presence of indole, tryptophan and tryptophyl peptides. These results are indicative of the sensitised photodegradation of wool keratin in the presence of FBAs of the distyrylarene class [54]. 11.10 BRIGHTENERS FOR POLYESTER FIBRES Much research has focused on the development of better brighteners for application to polyester. Huge numbers of patents have appeared and it is impossible to cover all the chemical variations in this chapter. Many of the more important commercial products and chemical types are discussed here but the reader is referred to published reviews [5,6,10,11] for more detail. Although polyester is always brightened with disperse-type products, the methods of application vary. FBAs are marketed for incorporation in the polymer mass, for exhaust application with or without carrier and for use in the pad–thermosol process at a temperature within the range 160–220 °C. Most products are applicable by more than one method, although none can be applied satisfactorily by all methods and cost-effective products introduced in the 1950s still remain important today. In general and as expected, brighteners of relatively small molecular size are most suitable for application by exhaustion. Less volatile compounds of larger molecular size tend to be preferred for pad–thermosol application or for incorporation in the polymer mass. Commercially important for exhaust application are the previously mentioned pyrene derivative 11.22, the naphthalimide 11.23, the bis(benzoxazolyl)ethene 11.35, the bis(benzoxazolyl)thiophene 11.36, the distyrylbenzene 11.37 and the stilbene bis(acrylic ester) derivative 11.38. Products of the 11.35 type show excellent light fastness but only moderate fastness to sublimation. In view of this volatility they can be used in the transfer printing of polyester. H3C

N HC O

O CH

11.35

N

CH3 N

CN

O

HC CH

HC

O S 11.36

CH 11.37

chpt11(2).pmd

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NC

15/11/02, 15:46

N

791

BRIGHTENERS FOR POLYESTER FIBRES

O CH3CH2O

CH C

HC

HC

CH CH

C OCH2CH3

HC

O 11.38

Polyester is brightened more effectively by exhaustion either in pressurised equipment at 125–130 °C or at the boil in the presence of a carrier. Small amounts of carrier may be added to assist levelling in the high-temperature process but the use of carriers is increasingly deprecated for environmental reasons. Commercially satisfactory results are obtained at the boil in the absence of carrier using rapidly diffusing FBAs of relatively small molecular size, such as the naphthalimide 11.23 or the benzoxazole derivatives 11.35 and 11.36. Polyester FBAs that are suitable for exhaust application are normally stable to sodium chlorite bleaching, although the pyrene derivative 11.22 and the bis-ester 11.38 are exceptions. Most of the FBAs used to brighten polyester by exhaustion may also be applied successfully by the pad–thermosol method at a baking temperature up to 190 °C. At temperatures higher than this the more volatile brighteners sublime and give poor yields. Compounds suitable for use with a baking temperature that exceeds 190 °C include the pyrene derivative 11.22, the benzoxazole 11.31, the distyrylbenzene 11.37 and the stilbene bis-ester 11.38. The temperature used during the baking stage in this process depends largely on the equipment available to the finisher. Thus FBAs showing optimum performance at temperatures ranging from 160 to 220 °C all have a niche in the market. Shorter baking times, leading to greater throughput of brightened fabric, are possible at the higher temperatures, although energy costs are greater. Products capable of giving good whiteness at relatively low thermosol temperatures appear to be gaining in importance [12]. Many other compounds have been marketed as polyester brighteners for application by exhaustion or in the pad–thermosol process. No account would be complete without mention of the important class of coumarin disperse FBAs, of which structure 11.39 is a typical example. Many commercial brighteners for polyester contain one or two benzoxazole groups, including compounds 11.31, 11.35, 11.36, 11.40 and 11.41.

N

N

O

O

N CH3

11.39

H3C

N

H3C

O

HC

OCH3 CH

11.40

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C O

792

FLUORESCENT BRIGHTENING AGENTS

H3C N

O

O

N

CH3 C CH3

H3C

OH CH3

C H3C 11.41

CH3

11.42

Polyester brighteners typically show excellent fastness properties. Light fastness is usually 5–6, the pyrene derivative 11.22 being an exception with light fastness of only 2–3. Although this compound gives a distinctly greenish brightening tone it is capable of producing remarkably brilliant whites and remains an important commercial product. Yellowing of white textiles in the presence of gas fumes (nitrogen oxides) has become important in recent years. The yellowing is often attributable to the formation of quinonoid compounds arising from reaction between the oxides of nitrogen and antioxidants such as di-t-butyl-p-cresol 11.42 present in packaging materials. Reinehr and Schmidt have shown that several polyester FBAs yellow in the presence of exceptionally high concentrations of nitrogen oxides, but they were unable to detect any significant yellowing at concentrations likely to be approached in practice [55]. Synergistic effects can often be observed with polyester brighteners and formulated mixtures of brighteners are increasing in importance. For example, mixture products containing the pyrene derivative 11.22 with either the naphthalimide 11.23 or the benzoxazole 11.35 have been marketed. This property may be exploited either to increase the maximum whiteness achievable or to attain a desired level of whiteness by applying a lower concentration of the synergistic mixture. The subject has been discussed by Martini and Probst [56], but the mechanism by which the synergy operates is not completely understood. In a recent evaluation of this phenomenon, the whiteness indices given by eleven individual brighteners on polyester were compared with those of their binary mixtures in various ratios. In many cases the whiteness performance of a mixture was markedly superior to that shown by the individual components [57]. A more specific investigation was confined to a series of benzoxazole FBAs. Their fluorescence spectra and fluorescence lifetimes were determined individually and in mixtures. The relationships between molecular structure and photophysical properties were discussed [58]. Much polyester fibre intended for curtain net or other white goods is sold in prebrightened form. The brightener is added to the polymer melt before or during extrusion, so it must exhibit high thermal stability. Two important FBAs used in this way are the triazolylcoumarin 11.43 and the bis(benzoxazolyl)stilbene 11.44. The latter structure provides brilliant whiteness with a violet tone, more pleasing to most observers than the slightly greenish hue given by compound 11.43. The coumarin, however, is stable under the conditions of condensation polymerisation and can be incorporated before polymer manufacture. The benzoxazole is not entirely stable and this has to be added to the polymer granules immediately before extrusion. Polyester brightened with the coumarin has a light fastness rating greater than 7, making it very important commercially. The presence of the

chpt11(2).pmd

792

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BRIGHTENERS FOR POLYESTER FIBRES

793

phenyl and naphthotriazolyl groupings, contributing π-electron mobility in positions 3 and 7 respectively, markedly amplifies the fluorescence intensity, as well as conferring exceptional photostability.

N

N

O

O

N 11.43

N HC O

O CH

11.44

N

The photochemical fading of disperse dyeings on polyester is retarded if an FBA has been applied by mass pigmentation before extrusion. The higher the concentration of FBA present, the greater is the protective effect on the light fastness of the disperse dyes [59]. Studies of the mechanism of photochemical decomposition of DAST-type brighteners have shown that stilbenes may act not only as sensitisers to produce singlet oxygen, but also as physical and chemical quenchers [60]. Owing to their capacity to self-destruct by such mechanisms, certain types of FBA can be preferentially destroyed in the presence of the photochemically more stable disperse dye molecules [59]. Since the structures of polyester FBAs are so varied, the reactions employed in their synthesis are also diverse. The organic chemistry can be complex and the intermediates required are often difficult to prepare. A full discussion is beyond the scope of this chapter. The reader is referred, in the first place, to the reviews mentioned in the introduction for further information [3–13]. A summary of the more important methods of manufacture follows. Those polyester FBAs containing a benzoxazole group are usually prepared from the appropriate o-aminophenol and carboxylic acid (11.45; Y = OH) or one of its derivatives, as shown in Scheme 11.10. The reaction proceeds via an intermediate amide and it can be advantageous to start from an acid derivative such as the acid chloride (11.45; Y = Cl) or ester (11.45; Y = OEt), which are both more effective acylating agents. The preparation of compound 11.36, shown in Scheme 11.11, illustrates this process, but the optimum conditions for ring closure vary considerably from one structure to another. The article by Gold contains a valuable and detailed summary [4]. Formation of the oxazole ring is not always the last step in synthesis of the brightener. Unsymmetrical compounds that contain both a benzoxazole group and an ethene linkage can be prepared by the anil synthesis [51], in which a compound possessing an activated methyl group reacts with a Schiff base. The preparation of brightener 11.31 is an illustration of this method (Scheme 11.12).

chpt11(2).pmd

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794

FLUORESCENT BRIGHTENING AGENTS

NH2

Y

NH

R

+ OH

R

C

C

O

O

N R O

OH

11.45 2-Aminophenol

Acylating agent

Benzoxazole

Amide

Scheme 11.10

O

HO C

C S

O

OH N

H3BO3

+

trichlorobenzene 150–220 °C

NH2

O S

O

N

11.36

2 OH

Scheme 11.11

(CH3)3C

N CH3

+

N CH

O Benzoxazole intermediate

Schiff base

KOH/dimethylformamide 40–60 °C

(CH3)3C

N HC CH

O

+

NH2

11.31 Aniline

Scheme 11.12

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794

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BRIGHTENERS FOR POLYESTER FIBRES

795

Most of the important class of coumarins used as polyester FBAs are made via 7-amino-3phenylcoumarin (11.46), which can be prepared by the Pechmann procedure from maminophenol. Conversion of intermediate 11.46 to FBAs may be achieved in various ways, two of which are shown in Scheme 11.13. The naphthalimide 11.23 is manufactured from acenaphthene by sulphonation, oxidation to the naphthalic anhydride derivative and conversion to 4-methoxy-N-methylnaphthalimide as outlined in Scheme 11.14. A Michaelis-Arbusov rearrangement followed by a Wittig-Horner reaction is involved in preparation of the distyrylbenzene derivative 11.37, as shown in Scheme 11.15. Precautions must be taken in the first stage to minimise formation of the carcinogenic by-product bis(chloromethyl) ether 11.16. The stilbene bis-ester 11.38 can be made by a similar procedure, or alternatively by the reaction of ethyl acrylate with 4,4′-dibromostilbene in the presence of a palladium-based catalyst (Scheme 11.16), a synthesis that yields the required trans form of the brightener. The important bluish mixing component 11.22 for whitening polyester is made by Friedel-Crafts acylation of pyrene (Scheme 11.17). This tetracyclic hydrocarbon is not unlike anthracene in its susceptibility to substitution reactions. The most stable bond arrangement in pyrene appears to be that shown as form 11.47a, which contains three benzenoid (b) rings. Canonical form 11.47b, containing only two such rings, contributes to a lesser extent (Scheme 11.18). In all monosubstitutions, pyrene is attacked initially at the 3-position, corresponding to the α-positions in anthracene or naphthalene. Brightener structures of only moderate molecular size are of interest for white grounds in the transfer printing of polyester fabrics. Derivatives of 6-acetamidoquinoxaline with an electron-donating substituent (X) in the 2-position (11.48) were prepared by converting quinoxalin-2-one to 2-chloro-6-nitroquinoxaline and condensation with amines (X = RNH), alcohols (X = RO) or phenols (X = PhO), followed by reduction and acetylation (Scheme 11.19). The nitro intermediates (11.49) are also of interest as low-energy disperse dyes for polyester [61]. FBAs for incorporation in the polymer melt are usually sold as the pure brightener without diluents. Most polyester FBAs, however, are supplied in the form of an aqueous dispersion. Considerable care is required in formulating these dispersions; not only must the dispersion be stable in transportation and storage, but the dispersing agents selected must not adversely affect the properties (such as light fastness) of the goods treated with the brightener. An FBA must be correctly formulated if it is to succeed commercially. White polyester/cotton fabrics represent a substantial segment of the market. Such blends can be brightened either by exhaustion or continuously by pad–thermosol or pad–steam processes. Suitable brighteners are selected from those intended for use on polyester or cellulosic substrates. Most polyester/cotton fabrics are woven constructions and it is essential to desize them before application of an FBA. Fabrics produced for sale as white goods must be chemically bleached before, during or after FBA treatment. In order to achieve the most solid white effects both fibre components of the blend require a brightener. The disperse and anionic brighteners selected must be compatible in hue. It is common practice, however, to brighten only one of the blend components for less critical end-uses. In a recent detailed evaluation of CI Fluorescent Brightener 393 on polyester, this product was incorporated into the polymer melt. The prebrightened fibre was blended with cotton and fabric knitted from these yarns was scoured and bleached. It was demonstrated

chpt11(2).pmd

795

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796

FLUORESCENT BRIGHTENING AGENTS

CH O

+ H2N

CH C

OH

CH3CH2O

O

Lewis acid catalyst, heat Inert solvent

1. Diazotisation 2. Tobias acid H2N

O

O N

N

O

O

11.46 NH2 1. Diazotisation 2. Na2SO3, then acid

Copper-containing catalyst Inert solvent, air

N H2N

NH

O

N

O

O

O

O

N

O

11.43

+ O C C H3C

N OH

Ring closure

N

N N

Scheme 11.13

chpt11(2).pmd

H3C

796

11.39

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BRIGHTENERS FOR POLYESTER FIBRES

797

CH3 O

O

O

Na2Cr2O7 / H2O

O

N

1. CH3NH2 2. CH3OH / NaOH

heat in an autoclave SO3H

SO3H

OCH3 11.23

Scheme 11.14

HCHO ClCH2

CH2Cl

HCl / ZnCl2 Benzene 2 P(OCH2CH3)3 Triethyl phosphite

OCH2CH3 O

P

OCH2CH3 CH2

CH2

OCH2CH3

P

O

+ 2 CH3CH2Cl

OCH2CH3

Ethyl chloride

CN CHO

2

base

CN HC CH

HC CH

11.37

NC

Scheme 11.15

chpt11(2).pmd

797

O

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798

FLUORESCENT BRIGHTENING AGENTS

O Br

HC

+ CH

CH

2

Br

C OCH2CH3

H2C

Ethyl acrylate Ph3P/Pd(OAc)2/NaOAc dimethylformamide

O CH

CH3CH2O C

HC

HC

CH CH

HC

C OCH2CH3

O 11.38 Scheme 11.16

OCH3 N

OCH3 N

AlCl3

N

N N

OCH3

+ Cl

N

OCH3

11.22

11.47 Pyrene Scheme 11.17

b

b

b

b

b

11.47a

11.47b

Scheme 11.18

that if an effective cotton bleaching process is applied in combination with producerbrightened polyester, the final degree of whiteness is sufficiently high to avoid the use of a cotton-substantive fluorescent brightener [62]. If a padding process is used to brighten a polyester/cotton blend, both the disperse and anionic brighteners may be applied from the same pad bath, even when a resin finish is applied simultaneously to the cellulosic component of the blend. Similarly, both types of

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BRIGHTENERS FOR ACRYLIC FIBRES

N

HNO3

N

O2N

N

O

N

H

POCl3

O2N

799

N

N

O

Cl

H HX

Quinoxalin-2-one

H2N

O2N

N

N

reduction N

N

X

X

11.49

acetylation

CH3CONH

N

N

X

11.48

Scheme 11.19

FBA may be applied by exhaustion from the same bath. If the polyester portion of the blend is to be bleached with sodium chlorite, the cotton is usually brightened in a second step since most FBAs for cotton are destroyed by sodium chlorite. Both types of FBA are normally compatible with a hydrogen peroxide bleaching process. 11.11 BRIGHTENERS FOR ACRYLIC FIBRES At one time disperse-type FBAs, such as pyrazoline, coumarin or naphthalimide derivatives, were commonly used to brighten acrylic fibres. Today all the important brighteners for these fibres are cationic in character and can be divided into two main categories: – type A: products that are oxidised by sodium chlorite – type B: products that are stable to sodium chlorite. Type B brighteners can be applied in the absence of bleach, of course, but show their best results when applied simultaneously with sodium chlorite, where they are capable of giving exceptionally high whiteness. Acrylic fibres are usually brightened from an exhaust bath in the presence of dilute organic acid. Application of brightener by a padding method, such as pad–roll or pad–steam, can be used but is uncommon. When these fibres are brightened from an exhaust bath, careful control of application conditions is necessary. Most acrylic fibres have a glasstransition temperature in the region of approximately 80 °C and the rate of absorption accelerates rapidly with increase in temperature over this critical region. Too rapid an increase in temperature can lead to unlevel absorption of the FBA. As an alternative to oxidative bleaching with sodium chlorite, acrylic fibres may be given a reductive bleach using sodium bisulphite in the presence of oxalic acid. This method is

chpt11(2).pmd

799

15/11/02, 15:46

800

FLUORESCENT BRIGHTENING AGENTS

necessary with Courtelle (Courtaulds) because this fibre would be damaged by chlorite bleaching. Both types of acrylic brightener can be applied with a bisulphite bleach. 11.11.1 Type A products This category mainly consists of derivatives of 1,3-diphenylpyrazoline, such as compounds 11.50–11.52. None of these substituted pyrazolines shows significant resistance to oxidative bleaching. The fluorescence stemming from the central ring disappears owing to dehydrogenation to the corresponding pyrazole (Scheme 11.20). The sulphones 11.50 and 11.51 are marketed as aqueous solutions of their formate salts. They produce violet brightening effects of light fastness 4 and are capable of producing excellent whiteness. The sulphonamide 11.52 gives greener effects and is incapable of producing the levels of whiteness attainable using either 11.50 or 11.51. In order to suppress the violet tone slightly and to achieve a higher visual level of whiteness from a given amount of FBA, the sulphone types are sometimes formulated with a small amount of a shading dye. The general method for the preparation of diphenylpyrazoline FBAs has already been discussed (Scheme 11.8). As a further illustration, the synthesis of the sulphone 11.51 is shown in Scheme 11.21.

N Cl

N H2C

X

CH2

oxidative bleach

N N

Cl HC

Scheme 11.20

X

+ H2O

CH

CH3 X= 11.50

SO2CH2CH2OCH

+ CH2NH(CH3)2

Cationic grouping X=

_ HCOO Anion + SO2CH2CH2CONHCH2CH2NH(CH3)2 Cationic grouping

11.51

X=

+ SO2NHCH2CH2CH2N(CH3)3

11.52

chpt11(2).pmd

Cationic grouping

800

_ Cl Anion

15/11/02, 15:46

_ HCOO Anion

BRIGHTENERS FOR ACRYLIC FIBRES

801

H2N

O +

Cl

SO3H

HN

CH2CH2Cl base N Cl

SO3Na

N

SOCl2

N Cl

SO2Cl

N

Na2SO3 N N

Cl

SO2Na

+

H2C

CHCONHCH2CH2N(CH3)2

HX N N

Cl

SO2CH2CH2CONHCH2CH2N(CH3)2 HCOOH

N Cl

N

+ SO2CH2CH2CONHCH2CH2NH(CH3)2

_ HCOO

11.51 Scheme 11.21

11.11.2 Type B products The benzimidazoles 11.53 and 11.54, both of which gave greenish brightening effects, were formerly used widely in the chlorite bleaching of acrylic fibres. The first of them to be introduced, the bis(benzimidazolyl)furan 11.53, gave excellent whites of light fastness 4 but a strongly acidic dyebath was recommended to give the best results. More recently, compounds 11.53 and 11.54 were supplanted by the improved benzofuranyl (11.55) and benzoxazolyl (11.56) benzimidazole derivatives, which give neutral shades of white with light fastness ratings slightly higher than the bis-benzimidazole 11.53. They are also easier to apply and are capable of producing a higher level of whiteness. For a time the parent compound 11.57, easier to prepare than its methylsulphonyl derivative 11.55, was also marketed. This was capable of producing brilliant whites on acrylic fibres that were exceptionally violet in tone. If violet brightening effects of good light fastness are required they can be achieved in combination with sodium chlorite using the

chpt11(2).pmd

801

15/11/02, 15:46

802

FLUORESCENT BRIGHTENING AGENTS

CH3

CH3

N + NH _ X

N

N

N

+ O

N

N

CH3

CH3

11.53

N O _ X

11.54

CH3

CH3

N

N

N

O

N

+ H3CO

O

+

N _ CH3

11.55

H3CO

SO2CH3

X

CH3

11.56 N N

CH3 N

N

+ H3CO

O

CH3CH2

N CH3

11.57

N

O

_ X

+ N CH3 _ X

O

N

_ X

CH3

O

11.58

CH3 CH3 _ N X CH3 N

N H3CO O

CH3

11.59

coumarin 11.58, which has been well-established for some years. Naphthalimide derivatives such as compound 11.59 can be used to obtain greenish shades of white on acrylic fibres in combination with a sodium chlorite bleach, but the effects are generally inferior to those produced by the preferred benzimidazoles 11.55 and 11.56. Acrylic fibres can also be brightened during manufacture by gel application during the wet spinning process. Special FBAs have not been developed for this purpose. Products such as the pyrazoline sulphone 11.50 and the benzofuranyl-benzimidazole 11.55 are suitable for this application. Some interesting organic chemistry is involved in the synthesis of chlorite-resistant brighteners for acrylic fibres. None of these compounds is easy to make and methods for preparation of the starting materials can be complex. Much manufacturing know-how is involved. One route for introduction of the benzimidazole nucleus into structure 11.55 is shown in Scheme 11.22. Preparation of the chemically rather simpler benzoxazole grouping in product 11.56 is shown in Scheme 11.23.

chpt11(2).pmd

802

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BRIGHTENERS IN DETERGENT FORMULATIONS

803

Synthesis of the coumarin derivative 11.58 containing two isomeric triazolyl rings is indicated in Scheme 11.24. The substituted pyrazolyl derivative of naphthalimide 11.59 is prepared by a reaction sequence somewhat similar in principle to that already shown in Scheme 11.14, using 4-amino-1,3,5-trimethylpyrazole in the penultimate step followed by quaternisation. 11.12 BRIGHTENERS IN DETERGENT FORMULATIONS The largest single commercial use of FBAs is in domestic detergents. Detergent technology is continually changing; modified, improved or even chemically new FBAs are still appearing on the market. The combination of large-volume sales by a limited number of detergent manufacturers with several suppliers of FBAs competing for the available business ensures that prices remain low. In the 1960s, FBAs for both cotton and nylon were incorporated into household detergents. Today FBAs for nylon are of negligible significance for the detergent industry. FBAs that are capable of effectively brightening polyester from a household wash at an acceptable laundering temperature (≤60 °C) remain undiscovered. Fibre types other than the cellulosics are essentially ignored from the viewpoint of FBA selection in the context of household detergents. There are fundamental differences in approach to the selection of FBAs for either household detergents or textile finishing. Brighteners in detergent formulations are intended to preserve the whiteness of fabrics that already contain FBAs during many successive wash and

H2N

Cl +

C O

H3CO

O

H2N

SO2CH3

base

NH

H3CO

O

N

SO2CH3

(CH3)2SO4 CH3 N + H3CO

O 11.55

N CH3

SO2CH3 _ CH3SO4

Scheme 11.22

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803

15/11/02, 15:46

804

FLUORESCENT BRIGHTENING AGENTS

NH2

NH +

H3CO

Cl3C N

OH

base

H3CO

N

NH

O

N

(CH3)2SO4

CH3 N

N

O

N

+ H3CO

_ CH3SO4

CH3

Scheme 11.23

N CH3CH2

N

OH +

C CH3

N

N

C O

H2N

NH

O

O

N CH3CH2

N

OH

N

NH

N

C C

CH3

O

1. Ring closure 2. (CH3)2SO4

N N N CH3CH2

N

O

+ N

CH3

_ CH3SO4

O

N 11.58 CH3

Scheme 11.24

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804

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O

N

BRIGHTENERS IN DETERGENT FORMULATIONS

805

wear cycles, whereas textile finishers apply FBAs to unbrightened material. If too much FBA is present during household washing then most of it is wasted. Using excessive amounts could even lead to deterioration in whiteness, as an excess of FBA builds up after several washes. If too little FBA is used, the goods will gradually suffer a loss in whiteness although it could be several months before this is noticed by the detergent user. Typically a household detergent powder contains 0.02–0.05% FBA, although the trend is towards the use of less FBA with activated oxidative bleaching agents that are effective at lower washing temperatures. The washing conditions and the type of surfactant present in household detergents vary from one part of the world to another. In some countries washing temperatures can be as low as 30 °C, whereas in others they can be as high as 90 °C. Chlorine-containing bleaches are routinely added to the wash in some countries but in others very rarely, if at all. The intensity of sunlight to which the washed goods are exposed during drying greatly influences the rate of fading of the FBA and this obviously varies considerably throughout the world. Since household detergents are marketed directly to the public, much attention has been given to packaging, physical appearance and handling of the various formulations available. Discoloration or development of odour on storage of the product, for example, would inhibit sales whatever the actual performance of the product in the wash. All these considerations influence the choice of type and quantity of FBA to be incorporated into a formulation. Furthermore, wash loads normally contain a variety of textile articles and an FBA designed to brighten cellulosic fibres must not adversely affect other fibre types present under the conditions of use of the detergent into which it has been incorporated. Safety in use and environmental impact are increasingly important factors for the selection of FBAs used in household detergents. The waste water from a household wash is discharged directly into the municipal drainage system. There is no opportunity for treatment of this effluent before disposal, in contrast to washing processes operated under factory conditions. The total quantity of waste waters from domestic washing is a major contribution to the effluent load on municipal effluent treatment facilities. Accordingly, the FBA and other components in a household detergent must be innocuous in the environment and free from toxic hazard. Evaluation of an FBA for use in household detergents is a lengthy and costly procedure. The information obtained from a standard washing test on unbrightened cotton is valuable but does not go far enough. Products must also be screened by measuring the build-up of whiteness during a series of successive washes using a detergent formulation containing the small proportion of FBA usually present in practice. Further tests continue on prebrightened textiles in machine washing cycles and in field trials. Extensive toxicity and environmental test protocols must also be followed. Although many different FBAs have been found acceptable for use in household detergent formulations, only a few products remain important today. The structures of typical examples have been mentioned already, such as the substantive DAST-type product 11.60, the distyryldiphenyl 11.15 and the vic-triazole 11.17. Certain FBAs, including these three compounds, can exist in relatively purer, near-colourless βcrystal modifications or in less pure yellowish crystalline forms. They are most easily prepared in the yellower form but incorporation of this material into a detergent formulation leads to unacceptable discoloration of the powder. Today, these products are supplied in a near-colourless form that may be prepared, for example, by heating an aqueous alkaline suspension of the yellowish material together with a co-solvent.

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805

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806

FLUORESCENT BRIGHTENING AGENTS

NH N N

NH N

HOCH2CH2

SO3Na

N CH3 HC

CH H3C N

NaO3S

CH2CH2OH

N HN

N N

11.60

HN

The DAST brightener 11.61 is the most important FBA used in detergent formulations and is probably the cheapest to manufacture. It shows excellent performance at temperatures of 60 °C and above but relatively poor solubility in cold water compared with the alternatives specified above. If it is to perform satisfactorily in low-temperature washing, it must be supplied in a finely divided form so that it will dissolve adequately during a typical household washing treatment. The necessary particle size can be achieved in various ways, one of which is wet milling in the presence of excess salt.

NH N N

NH N SO3Na

N O

HC

O

CH N

NaO3S N HN 11.61

N N HN

DAST-type FBAs may contain by-products (such as 11.13) derived from hydrolysis of one or more of the chloro substituents in cyanuric chloride (11.10). One such troublesome byproduct is 2,4-bis(anilino)-6-hydroxy-s-triazine (11.62). Not only is this compound environmentally undesirable, it may also interact with certain bleaching agents and its presence can lead to the development of unpleasant odours on storage of a detergent

chpt11(2).pmd

806

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BRIGHTENERS IN DETERGENT FORMULATIONS

807

powder. The proportion of this triazine present as an impurity in a brightener such as 11.61 can be kept to a minimum by careful control of the reaction conditions during manufacture. Alternatively, it can be extracted from the FBA using hot alkali.

HN

N N

NH N

OH

11.62

The instability of DAST-type brighteners towards chlorine-containing bleaches has been mentioned already. They also show limited stability towards per-acids. As recommended washing temperatures have tended to fall in recent years, a bleach consisting of sodium perborate activated by addition of tetra-acetylethylenediamine (11.63) has become an important component of household detergent formulations. This system is effective at temperatures as low as 40–50 °C. Since the FBA may be sensitive to the activated oxidant, however, in some formulations it is necessary to protect compounds such as 11.60 or 11.61 by encapsulating either the brightener or the activator, if adequate shelf-life is to be maintained. O

O CH3

C N

CH3

CH2CH2

C O

C

CH3

C

CH3

N O

11.63

The so-called ‘super brighteners’ 11.15 and 11.17 are generally much more stable towards activated per-acid bleaching systems. Both products, and especially the vic-triazole 11.17, offer higher light fastness than the DAST brighteners. The distyryldiphenyl 11.15 is an effective FBA when applied from a wash liquor at a temperature below 50 °C, but shows poor performance at higher temperatures and poor washing fastness in soft water. The victriazole 11.17 is effective at all temperatures but is expensive. Nevertheless, it may prove cost-effective in tropical climates where washed textiles can fade severely during drying. Compound 11.15 is particularly effective in enhancing the brightness of the detergent powder itself, although this in no way indicates its performance in the wash. Table 11.5 summarises the advantages and drawbacks of the four major FBAs discussed above. The poor performance of the distyryldiphenyl derivative 11.15 at higher washing temperatures is a serious drawback in some countries. In an attempt to overcome this disadvantage, product 11.15 has been marketed in admixture with an analogous FBA (11.64) derived from 4-chlorobenzaldehyde-3-sulphonic acid (see Scheme 11.5). This much less soluble variant is highly effective at high washing temperatures. Where resistance to chlorine bleaches such as sodium hypochlorite is required, the naphthotriazole 11.65 can be used. Formerly, this FBA was extremely important for use in

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Table 11.5 Advantages and disadvantages of FBAs in detergent formulations Product

Advantages

Disadvantages

11.61

Low price Effective at all temperatures

Unstable towards hypochlorite and activated perborate

11.60

Effective at all temperatures

Unstable towards hypochlorite and activated perborate

11.15

Stable in bleaching Good light fastness

Poor performance above 50 °C Poor wash fastness

11.17

Effective at all temperatures Stable in bleaching Excellent light fastness

High price

Cl

HC

SO3Na HC

CH NaO3S

Cl

CH 11.64

O

N

SO3Na

Cl

N

N N

OCH3

HC

N

C

CH

11.66

11.65

detergents but today it is much less so. It has the advantage of brightening both cotton and nylon from the wash bath. In the absence of hypochlorite bleach, pyrazolines such as the sulphonamide 11.20 and the ester 11.66 give brighter whites than the naphthotriazole 11.65 on nylon garments in the wash. Especially brilliant whites with a somewhat greenish tone are given by compound 11.66 but this FBA tends to stain polyester goods under wash bath conditions. The sulphonamide 11.20 gives less intense whites but stains polyester less. Neither pyrazoline derivative is effective on cotton, so they are not much used in detergent formulations nowadays. Domestic detergents in liquid form have become increasingly popular in recent years. This trend has created problems in the choice of suitable FBAs, as it is more difficult to devise liquid formulations of adequate storage stability. Liquid detergents containing FBAs can cause yellow ‘specking’ faults on the washed goods, which can be a serious problem. It has been claimed [63] that the use of an FBA such as 11.65 or 11.67, which contain only one sulphonic acid group in their stilbene residue, ameliorates the ‘specking’ problem. Classical approaches to the preparation of stilbenes or their symmetrical disulphonates are not applicable to the synthesis of their unsymmetrical monosulphonated analogues. Several

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condensation routes provide viable opportunities to introduce a single sulphonic acid group into the stilbene nucleus but these procedures do add substantially to the cost of manufacture [64].

NH N N

NH N

N

SO3Na

O

HC

O

CH N N

11.67

HN

N N HN

The disulphonated DAST derivative 11.25 containing four anilino groups per molecule is effective in liquid detergent formulations and much cheaper to manufacture than the monosulphonated DAST brightener 11.67, which was withdrawn from the market in the late 1980s. It has been necessary to purify compound 11.25 specially for use in detergents, in order to eliminate traces of residual unreacted aniline as far as possible, owing to the toxic properties of this impurity. 11.13 ANALYSIS OF FBAs Qualitative analysis of FBAs is best carried out by thin-layer chromatography (TLC). Silica gel is the usual stationary phase for the TLC of FBAs. Various eluants are available and these can be chosen according to the chemical nature of the FBA under test. Suitable standards are required. The plethora of possible brighteners of the DAST type, together with the various impurities present in such products, can cause difficulties in their identification by TLC. The technique can be used quantitatively, although costly instrumentation is required and considerable care must be taken in preparing and handling chromatograms. Chapters by Theidel and Anders in the book edited by Anliker and Müller [7] contain valuable information on the analysis of FBAs by TLC. More recently, Lepri and Desideri have described methods for the TLC identification of FBAs in detergent formulations [65]. If suitable standards are unavailable (for example, if the FBA has not been encountered previously) the active agent must first be isolated and purified. The pure compound can be characterised by the usual techniques, including elemental analysis and infrared, n.m.r. and mass spectroscopy. Final proof of structure demands synthesis of the FBA indicated by the analytical data. Once again, difficulties may be encountered with compounds of the DAST

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class. Optical densitometry using a Shimadzu CS 9000 flying spot scanner has been evaluated for the analysis of anionic DAST-type brighteners [66]. Once the FBA has been identified, ultraviolet absorption spectroscopy affords a rapid and accurate method of quantitative analysis. Care must be taken when interpreting the spectra of stilbene-type compounds, since trans to cis isomerisation is promoted by ultraviolet radiation. Usually, however, a control spectrum of the trans isomer can be obtained before the compound undergoes any analytically significant isomerisation. FBAs are often marketed on the basis of strength comparisons determined by ultraviolet spectroscopy. FBAs can also be estimated quantitatively by fluorescence spectroscopy, which is much more sensitive than the ultraviolet method but tends to be prone to error and is less convenient to use. Small quantities of impurities may lead to serious distortions of both emission and excitation spectra. Indeed, a comparison of ultraviolet absorption and fluorescence excitation spectra can yield useful information on the purity of an FBA. Different samples of an analytically pure FBA will show identical absorption and excitation spectra. Nevertheless, an on-line fluorescence spectroscopic method of analysis has been developed for the quantitative estimation of FBAs and other fluorescent additives present on a textile substrate. The procedure was demonstrated by measuring the fluorescence intensity at various excitation wavelengths of moving nylon woven fabrics treated with various concentrations of an FBA and an anionic sizing agent. It is possible to detect remarkably small differences in concentrations of the absorbed materials present [67]. High-performance liquid chromatography (HPLC) is being used increasingly to identify FBAs, to investigate product purity and for process control. HPLC has many attributes that TLC lacks, including greater sensitivity, better resolution and discriminatory power. Quantitative analysis can be carried out conveniently and rapidly using HPLC, providing the constitution of the FBA is known and a pure sample is available for calibration. Drawbacks of this approach, however, include the fact that samples have to be run sequentially rather than in parallel, substantially increasing the time for analysis. Care is needed to minimise the risk of cross-contamination caused by carry-over from one sample to the next. Although HPLC quickly became established for the analysis of organic compounds in many fields, the development of test procedures for textile dyes and FBAs took place more slowly. This is attributable to the number and variety of chemical classes represented and the fact that these structures may be anionic, cationic or nonionic. Attempts to devise general eluant systems to cope with this diversity in solubility characteristics met with considerable difficulties. Gradient elution systems using mixed solvents in various proportions could be used but the techniques were complicated and time-consuming. Multichannel detection of peak wavelengths using a variable wavelength UV-Vis detector is effective in enabling simultaneous monitoring of different components in mixtures [68]. Chromatographic problems associated with the separation of anionic dyes and FBAs were attributed to undesirable properties of silica-based packing materials in the column and better results were found using a chemically inert copolymer of styrene and divinylbenzene [69]. The addition of di-t-butyl-p-cresol (11.42) as an antioxidant was useful in ensuring chemical stability of oxidation-sensitive dyes and FBAs during extraction and analysis [70]. Established techniques of HPLC analysis are available for estimation of the relatively few FBAs that are widely used in detergent formulations [71,72].

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REFERENCES

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. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57.

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P Krais, Melliand Textilber., 10 (1929) 468. C Paine and J A Radley (ICI), BP 442 530 (1934). A K Sarkar, Fluorescent whitening agents (Watford: Merrow, 1971). H Gold in The chemistry of synthetic dyes, Vol. 5, Ed. K Venkataraman (New York: Academic Press, 1971). D Barton and H Davidson, Rev. Prog. Coloration, 5 (1974) 3. A Dorlars, C W Schellhammer and J Schroeder, Angew.Chem.Internat., 14(1975)665. Fluorescent whitening agents, Eds. R Anliker and G Müller (Stuttgart: Thieme, 1975). R Zweidler and H Hefti, Kirk-Othmer encyclopedia of chemical technology, 3rd Edn., Vol. 4 (New York: WileyInterscience, 1978) 213. R Williamson, Fluorescent brightening agents (Amsterdam: Elsevier, 1980). I H Leaver and B Milligan, Dyes and Pigments, 5 (1984) 109. A E Siegrist, H Hefti, H R Meyer and E Schmidt, Rev. Prog. Coloration, 17 (1987) 39. T Martini, Textilveredlung, 23 (1988) 2. B J Maier, Text. Asia, 21 (July 1990) 80. W S Hickman in Cellulosics dyeing, Ed. J Shore (Bradford: SDC, 1995) 81. Luminescence spectroscopy, Ed. M D Lumb (New York: Academic Press, 1978). J R Lakowicz, Principles of fluorescence spectroscopy (London: Plenum Press, 1983). M Pestemer, A Berger and A Wagner, Textilveredlung, 19 (1964) 420. X Qian, Y Zhang, K Chen, Z Tao and Y Shen, Dyes and Pigments, 32 (1996) 229. C Lubai, C Xing, H Yufen and J Griffiths, Dyes and Pigments, 10, No 2 (1989) 123. O Pitzurra, Melliand Textilber., 77 (1996) 331. R Griesser, Col. Res. Appl., 19 (1994) 446. J A Bristow, Col. Res. Appl., 19 (1994) 475. A Berger, Die Farbe, 8 (1959) 187. P S Stensby, Soap/Cosmetics/Chem. Specialities, 43 (July 1967) 80. R Griesser, Rev. Prog. Coloration, 11 (1981) 26. J Rieker, R Griesser and C Puebla, Textilveredlung, 31 (1996) 64. I Soljacic, A M Grancaric and K Weber, Textilveredlung, 10 (1975) 492. A M Grancaric and I Soljacic, Melliand Textilber., 62 (1981) 876. J R Aspland, J S Davis and T A Waldrop, Text. Chem. Colorist, 23 (Sep 1991) 74. I Soljacic and R Cenko, Melliand Textilber., 60 (1979) 1032. I Soljacic, D Kalovic and A M Grancaric, Textil Praxis, 46 (1991) 331. I Soljacic and K Weber, Textilveredlung, 9 (1974) 220. I Soljacic, A M Grancaric and B Luburic, Textil Praxis, 39 (1984) 775. K Seguchi, Y Ebara and S Hirota, Yukagaku, 34 (1985) 17. W Schürings, Textilveredlung, 29 (1994) 260, 291. R S Davidson, G M Ismail and D M Lewis, J.S.D.C., 104 (1988) 86. R Zweidler, Textilveredlung, 4 (1969) 78. R P Hurd and B M Reagan, J.S.D.C., 106 (1990) 49. H Ikuno, M Okuni, M Komaki and T Nakajima, Text. Res. J., 66 (1996) 464. Anon, Paper (June 1980) 33. H Ikuno, M Komaki and T Nakajima, J. Soc. Fibre Sci. Tech. Japan (1994) 87. I Grabtchev and T Philipova, Dyes and Pigments, 27 (1995) 321. I Grabtchev, P Meallier, T Konstantinova and M Popova, Dyes and Pigments, 28 (1995) 41. I Grabtchev and T Konstantinova, Dyes and Pigments, 33 (1997) 197. B R Modi, N M Naik, D N Naik and K R Desai, Dyes and Pigments, 23 (1993) 65. H Strahle, W Seitz and H Güsten, Z. Naturforsch., 316 (1976) 1248. H Güsten and G Heinrich, Ber. Bunsen-Ges., 81 (1977) 810. I H Leaver and G C Ramsay, Text. Res. J., 39 (1969) 730. R S Davidson, G M Ismail and D M Lewis, J.S.D.C., 103 (1987) 308. P Moechli, Dyes and Pigments, 1 (1980) 3. R S Davidson, G M Ismail and D M Lewis, J.S.D.C., 103 (1987) 261. L A Holt and I W Stapleton, J.S.D.C., 104 (1988) 387. L E Aicolina, I H Leaver and I W Stapleton, Dyes and Pigments, 11, No 3 (1989) 213. K J Smit and K P Ghiggino, Dyes and Pigments, 13, No 1 (1990) 45. D Reinehr and E Schmidt, J.S.D.C., 102 (1986) 258. T Martini and H Probst, Melliand Textilber., 65 (1984) 327. S Yongjia and R Shengwu, Dyes and Pigments, 15, No 3 (1991) 183.

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812 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72.

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Z Zhu, H Tian, X Zhang and S Ren, Dyes and Pigments, 21, No 4 (1993) 235. I N Bykova, Tekstil. prom., 8 (Aug 1987) 55. H Oda, N Kuramoto and T Kitao, J.S.D.C., 97 (1981) 462. D W Rangnekar and P V Tagdiwala, Dyes and Pigments, 7, No 6 (1986) 445. P L Moriarty, AATCC Internat. Conf. & Exhib. (Oct 1994) 297. J Wevers, L A Halas and P R Peltre, USP 4 559 169 (1985). R V Casciani et al., AATCC Internat. Conf. & Exhib. (Oct 1991) 93. L Lepri and P G Desideri, J. Chromatog., 322 (1985) 363. U Denter, K Tiroke, D Knittel and E Schollmeyer, Melliand Textilber., 73 (1992) 834. S Cleve and E Schollmeyer, Textilveredlung, 30 (1995) 18. J C West, J. Chromatog., 208 (1981) 47. P C White and A M Harbin, Analyst, 114 (1989) 877. B B Wheals, P C White and M D Paterson, J. Chromatog., 350 (1985) 205. B P McPherson and N Omelczenko, J. Amer. Oil Chem. Soc. (1980) 388. G Micali, P Curro and G Calabro, Analyst, 109 (1984) 155.

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CHAPTER 12

Auxiliaries associated with main dye classes Terence M Baldwinson

12.1 INTRODUCTION The aim in this chapter is to summarise the properties of auxiliaries normally used with each of the main dye classes. Where these agents have been dealt with earlier, the emphasis here is on application behaviour. Chemical details are included, however, for those auxiliaries that have not yet been mentioned; emphasis is given to the auxiliaries used rather than to processing details. 12.2 ACID DYES Only the products associated with acid and premetallised dyes are dealt with in this section. The auxiliaries used with mordant dyes are covered in section 5.8. Anionic acid dyes, applied principally to wool and nylon, vary widely in their fastness and level-dyeing properties (section 3.2.2); in general, the higher the wet fastness of a dye the more difficult it is to apply evenly. Hence it is not surprising that the use of auxiliaries with acid dyes is related mainly to level-dyeing properties. There are two basic aspects: (1) controlling the pH to give a satisfactory dyeing rate and ultimate exhaustion (2) using auxiliaries to give additional levelling, either through a competitive mechanism that exerts further control on absorption or through the promotion of migration and diffusion. Temperature provides another means of control although this is rarely the only technique employed. The control of pH is of particular importance, as the optimal pH varies with different types of acid dyes. This can be seen in Table 12.1, which shows the pH values generally required to give 80–85% exhaustion [1]. However, in some cases, either by modification of the dye type or by addition of certain auxiliaries, different pH values from those listed may be used. Table 12.1 Dyebath pH values to give 80–85% exhaustion [1] Dye class

pH

1:1 Metal-complex dyes Levelling acid dyes Chrome dyes Milling acid dyes Disulphonated 1:2 metal-complex dyes ‘Super-milling’ acid dyes Monosulphonated 1:2 metal-complex dyes Unsulphonated 1:2 metal-complex dyes

2.0–2.5 2.5–3.5 4.0–5.0 4.5–5.5 4.5–5.5 5.0–6.0 5.0–6.0 5.5–6.5

813

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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES

Levelling acid dyes and particularly 1:1 metal-complex types generally require an exceptionally low pH in order to promote exhaustion and levelling; up to 3% o.w.f. sulphuric acid is most commonly used for levelling acid dyes, although hydrochloric, formic and phosphoric acids are also effective. In the case of conventional 1:1 metal-complex dyes it is essential to use a sufficient excess of acid over and above the typical 4% o.w.f. sulphuric acid normally absorbed by the wool, otherwise there may be a tendency towards tippy dyeings and lower wet fastness. The actual excess required depends on applied depth and liquor ratio [2]; typical recommendations are given in Table 12.2. Table 12.2 Amounts of sulphuric acid used with conventional 1:1 metal-complex dyes [2] Sulphuric acid (96% solution) (% o.w.f.) Liquor ratio

<1% dye

>1% dye

10:1 20:1 30:1 40:1 50:1 60:1

4.7 5.4 6.1 6.8 7.5 8.2

5 6 7 8 9 10

Such high concentrations of strong acid may cause fibre damage at the boil. After dyeing it is essential to ensure that the acid in the fibre is adequately neutralised. Hence formic acid (8–10% o.w.f.) is sometimes used instead, a further advantage being that it leads to less chromium in the effluent. If the dyes are modified to have one or more of the three water ligands replaced by colourless inorganic complex anions such as hexafluorosilicate (SiF6)2– ligands, their dyeing behaviour is markedly altered. This facilitates dyeing at pH 3.5–4.0 with formic acid and an amphoteric auxiliary (a mixture, said to be synergistic, of quaternary and esterified fatty amine ethoxylates, polyaddition compounds of fatty amine ethoxylates and fluorosilicates) [3,4]. The use of sulphamic acid (12.1) has been recommended, resulting in a shift of pH from 1.8 to between 3.0 and 3.5 as the temperature approaches the boil, thus giving rise to less fibre damage. Typically, 6% o.w.f. sulphamic acid is added, together with an auxiliary and sodium sulphate. The change in pH arises as a result of hydrolysis of the sulphamic acid to give ammonium bisulphate (Scheme 12.1)[2,5].

NH2SO3H + H2O

NH4HSO4

12.1

Scheme 12.1

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With the so-called ‘half-milling’ or intermediate levelling dyes, values in the range 1.8– 3.5 would lead to too rapid a rate of exhaustion with consequent risk of unlevel dyeing. For these dyes, the optimal pH is 4.0–5.5, generally achieved using up to 2% o.w.f. acetic acid. Milling acid dyes and 1:2 metal-complex types are highly responsive to acid. Hence the tendency with these dyes is to use a pH-shift system (section 10.1), starting from neutral or slightly alkaline conditions and progressively decreasing the pH to the required level as dyeing proceeds. A hydrolysable organic ester or a latent-acid salt, such as ammonium sulphate or ammonium acetate, may be used, often with ammonia to give a higher initial pH. Whilst such techniques do not damage nylon, initially alkaline conditions can lead to some degradation of wool. Hence for wool it is preferable to choose dyes showing low substantivity at pH 7–8 but exhausting well at pH values of 6.2 or lower [2]. Figure 12.1 shows the ease with which wool is damaged in highly acidic, neutral or alkaline dyebaths [3,4]. The least damage occurs at pH 3.5–4.0, slightly lower than the isoelectric point of pH 4.5–5.0 (section 3.2.2). These considerations have led to the development of processes by which milling dyes and 1:2 metal complexes can be applied at pH values close to the isoelectric range. An effective surfactant-type levelling or retarding agent must then be used to counteract the high rate of exhaustion promoted by this degree of acidity [1–4,6–10].

Increasing hydrolysis of amide groups

Increasing hydrolysis of cystine links

Dyeing with 1:1 metal-complex dyes Region of least wool damage Isoelectric range

1

2

3

4

5

6

7

8

9

10

pH

Figure 12.1 Wool hydrolysis and region of least damage as a function of dyebath pH at the boil [3,4]

In general, rather less acidity is required on nylon than on wool for application of the same combination of dyes. However, there has been some discussion regarding the best means of controlling pH [11]. In some cases, starting at pH 7 or higher with ammonium sulphate or acetate can lead to variations in pH at the end of the process, with consequential variations in performance. Better end-point control is achieved by starting at pH 6.0–6.5 using a sodium dihydrogen phosphate/disodium hydrogen phosphate buffer and ensuring a slow rise of temperature. The improved consistency of dyeing may offset the higher cost of the phosphate buffer. In some regions, however, the use of phosphates is regarded as environmentally sensitive.

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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES

A neutral electrolyte, usually 10–20% o.w.f. sodium sulphate or sodium chloride, is often added with acid dyes to aid levelling. This action results from the competition for the dyeing sites in the fibre provided by this high concentration of inorganic anions. Ultimately these are replaced by the dye anions as a result of their higher affinity. Electrolytes are less effective as levelling agents in near-neutral dyebaths, however, since under these conditions the cationic charge on the fibre is too low to attract simple inorganic anions and dye sorption is generally through nonpolar rather than electrostatic forces. Nevertheless, it is still common to add an electrolyte when applying these dyes, although it functions primarily to boost exhaustion through a common-ion mechanism rather than as a levelling agent. The use of surfactant-type levelling agents is of importance with acid dyes on wool and nylon, especially with dyes of relatively high wet fastness. Anionic surfactants act by competing for the cationic sites and are mainly used to counteract fibre-oriented unlevelness due to physical and chemical irregularities in the fibre. Strongly cationic quaternary compounds readily form complexes with acid dyes, but may precipitate them if used alone. Weakly cationic ethoxylated tertiary amines do not suffer from this disadvantage and are of great importance in minimising unlevelness associated with rapid dye uptake. Combinations of anionic and weakly cationic types, carefully chosen according to the principles described in section 10.7, are of particular importance since they counteract both types of unlevelness. An incompatible combination of dyes is one that does not build up on tone because of the sequential sorption of individual components. A well-chosen levelling agent, or a combination of suitable agents, can effectively convert such a mixture into a compatible one. Amphoteric levelling agents, combining the properties of anionic and weakly cationic agents in the same molecule, have attained increasing importance [6–10]. Originally developed for the application of reactive dyes on wool, amphoteric agents have been exploited with acid dyes, particularly for dyeing at pH 4–5. They are especially suitable for the ranges of 1:2 metal-complex dyes containing ionic solubilising groups (carboxyl or sulpho) rather than the nonionised but polar groups (such as sulphonamide or sulphone) in traditional dyes (sections 3.2.2 and 5.1). These are often cheaper to manufacture and offer better wet fastness; their development and exploitation owed much to the use of amphoteric betaine levelling agents [12,13]. Although the behaviour of amphoteric agents has been studied with metal-complex and acid dyes on both wool and nylon, their main focus of interest has remained the application of reactive dyes to wool. For this reason, therefore, their application and mechanism of action are considered in more detail in section 12.7.2. Suffice to say here that the mechanisms observed are generally applicable to both reactive and acid dyes, including metal-complex types. By using more than the optimal amount of dye-complexing agent required for effective levelling, some of these products can be used as stripping aids either alone for partial nondestructive stripping or in combination with oxidising agents (such as sodium dichromate and sulphuric acid) or reducing agents (such as sodium formaldehyde-sulphoxylate or sodium dithionite) for more drastic destructive stripping. There has been long-standing interest in the so-called low-temperature dyeing of wool and, to some extent, of nylon. Generally this implies dyeing at 60–90 °C, most commonly at 80–85 °C, rather than at the traditional boil. This approach results in energy savings with both fibre types but is particularly attractive to wool dyers because it results in less damage to the fibre than when dyeing at the boil. Although some earlier methods involved the use

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of solvents such as benzyl alcohol or chlorinated hydrocarbons, now unacceptable on environmental, health and safety grounds, methods involving surfactants continue to generate interest, even though this does not result in widespread commercial use. Nonionic surfactants have been favoured [2,13], including ethoxylated alcohols, ethoxylated nonylphenol [2] and polyglycol esters and ethers [14]. Amphoteric auxiliaries have also given effective results. On nylon dyed at 75 °C with acid dyes, the best results were obtained [15] with the lauryl-substituted member of the fatty acylamidoethyl-N,N-dimethylglycine betaine series indicated in structure 12.2. Compounds of general formula 12.3 have been found effective with acid dyes on wool [16]. O O R

R1

C HN

CH2CH2

CH3 _ + N CH2COO CH3

12.2 R = lauric CH3(CH2)10 or palmitic CH3(CH2)14

C HN

R3 R2 N R4

12.3 R1 = long-chain alkyl R2 = short-chain alkylene R3, R4 = short-chain alkyl

The most effective auxiliaries for low-temperature dyeing processes are generally those that result in accumulation at the fibre surface of an auxiliary-rich phase of high dye concentration. This implies the formation of an auxiliary-dye complex at a concentration as close as possible to the critical micelle concentration and hence usually close to the limit of solubility for the system. This complex then migrates into the fibre; depending on the applied depth, dyeing time may need to be extended. In order to meet these criteria, the nonionic types selected should have a low degree of ethoxylation, typically 4–10 ethylene oxide units per molecule. This can lead to a high degree of auxiliary-dye specificity with consequent implications for the compatibility of dyes in mixtures, as well as possible problems of precipitation as a result of low cloud point phenomena. In spite of the potential attractions, such low-temperature dyeing methods have attained only limited commercial use. It is interesting to note that products effective in the low-temperature dyeing of wool and nylon tend to be effective in overcoming dyeability variations attributable to fibre irregularities, such as tippy or skittery dyeing in wool and barry dyeing in nylon. The amphoteric agents have proved to be particularly efficaceous in this respect, playing an important part in facilitating level dyeing with sulphonated 1:2 metal-complex dyes, which are otherwise rather prone to tippy or skittery dyeing. The phenomenon of barriness in nylon fabric dyeing has been reviewed [17,18] and the factors discussed in section 10.7 in regard to the action of levelling agents are pertinent here. Mixtures of auxiliaries are particularly effective, such as a sulphonated anionic with a fibre-substantive cationic type based on an aliphatic amine [18]. Care should be taken that such mixtures are compatible, either through the use of ethoxylated components and/or the addition of a solubilising ethoxylated nonionic agent. In a study of ethoxylated ethylenediamine derivatives (12.4) in the application of acid dyes to nylon, covering a range of ethoxylation from 40 to 180 units per molecule (average

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n = 10–45), the best initial restraining effect together with the highest uptake of dye at equilibrium was obtained with 180 ethylene oxide units (average n = 45) [19]. This highly ethoxylated ethylenediamine was found to increase dye uptake when incorporated into nylon 6 granules as an antistatic agent [20]. H

(OCH2CH2)n N

H

CH2CH2

(OCH2CH2)n

(CH2CH2O)n

H

(CH2CH2O)n

H

N

12.4

The use of liposomes as complexing agents in the application of premetallised acid dyes to wool has been investigated [21–24]. Liposomes are lipid structures containing aqueous compartments surrounded by bilayer membranes. However, the methods as yet available for the preparation of these agents are hardly practical in dyehouse terms (section 10.3.4). It is possible to increase colour yields on wool by the use of protease or hydrolase enzymes (section 10.4.2). Although some improvement in yield was observed at temperatures as low as 50 °C, the increase was insufficient to be of commercial interest, but yields with enzyme at 85 °C were close to those obtained without enzyme at 100 °C [25]. The presence in dyehouse effluents of typical dye-complexing metal ions is an environmentally sensitive issue, such metallic contamination arising mostly from the decomposition of metal-complex dyes [26]. The synthetic complexing agent cucurbituril (section 10.3.2) can be used to selectively extract such metal ions from the effluent. Continuous dyeing with acid dyes is most frequently carried out on loose fibre, tow or slubbing before yarn manufacture. Resilient fibres such as wool can cause problems at the immersion stage and during subsequent steaming, leading to unlevel results characterised chiefly by tippy or frosty effects. Similar effects can be observed with pile fabrics such as carpets owing to differing degrees of penetration of the pile. These defects are usually overcome using a hydrotropic agent such as urea with surfactant auxiliaries [9,12,27,28]. Certain anionic surfactants are claimed to be effective, particularly sodium dioctylsulphosuccinate and its 2-ethylhexyl and 1-methylheptyl isomers [27,28]. The mechanism involves formation of an agent-dye complex that wets the fibres evenly and forms a uniform film around them. The surfactant creates a foam during steam fixation, thus assisting the uniform transport of the dye throughout the fibre; the complex subsequently breaks down and the dye is then uniformly fixed. Detailed accounts of the printing of wool with acid dyes and metal-complex types are available [2,29]. Typical formulation details for print pastes are given in Table 12.3. A hydrotrope such as urea or thiourea is used to increase solvation of the dyes and to act as a humectant, thereby enhancing fixation. Additional solvents, such as thiodiethylene glycol (12.5) or sec-butylcarbinol (12.6), may also be added [2]. Locust bean or guar derivatives are used as thickening agents, either alone or in combination with water-soluble British gum; high solids content is preferred for fine line effects and low solids content for larger blotch prints, because of better levelling and freedom from crack marks. For generation of acidic conditions, a non-volatile acid such as citric acid (12.7), or an acid donor such as ammonium tartrate (12.8) or ammonium sulphate, is preferred. An acid or acid donor is not used with 1:2 metal-complex dyes of high neutral-dyeing affinity,

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819

Table 12.3 Typical formulation for the printing of wool with acid dyes [2] Concentration (g/l) Dye Urea Thiodiethylene glycol Wetting agent Antifoam Acid or acid donor Thickener (10–12%) Water to give

x 50–100 50 5–10 1–5 10–30 500 1000

CH2CH3 HOCH 2CH2SCH2CH2OH

HOCH2HC

12.5

12.6

CH3

O HO

O

C

C

CH2 HO

C CH2

HO

O

HO

CH

HO

CH

ONH4

12.8

C OH

C

C

O

ONH4

O 12.7

however, since this may lead to destabilisation of the print paste, aggregation, specking and unlevelness. Wool or the modified natural thickeners present may tend to promote reduction of certain sensitive azo dyes; to counteract this a small amount of sodium chlorate may be added to the print paste. Defoamers and surfactants to prevent frosting may also be required. In discharge printing a reducing agent is also required. The most widely used is zinc formaldehyde-sulphoxylate (CI Reducing Agent 6), since this functions in the weakly acidic pH range and thus gives less fibre damage. There can be problems in washing out the unsulphonated arylamines produced by reduction of certain azo dyes [2]. Sodium formaldehyde-sulphoxylate (CI Reducing Agent 2) may give excessive fibre damage since it requires an alkaline medium. The water-insoluble calcium formaldehyde-sulphoxylate (CI Reducing Agent 12) may cause screen blockage and inadequate penetration, although commercial formulation as a 30% dispersion is said to give better results. The calcium salt may be applied in admixture with the sodium salt. Thiourea dioxide is rarely chosen because of its low solubility (only 37 g/l at 20 °C) [2]. The washing-off of prints is best carried out with anionic polycondensation products of arylsulphonic acids [29] since these can improve the wet fastness of anionic dyes.

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820

AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES

12.3 AZOIC COMPONENTS There are three main demands for auxiliaries in the application of azoic components [30– 32]: (a) the composition of the naphtholate solution (b) the composition of the diazo solution (developing bath) (c) aftertreatments to develop hue and maximum fastness. These will be considered first in relation to batchwise application, followed by variations pertinent to continuous dyeing and printing. The discussion relates solely to cotton, by far the most important substrate for these dyes; application to other cellulosic substrates follows generally similar principles, the main difference being in product concentrations. 12.3.1 Composition of the naphtholate solution A primary requirement for naphtholate preparation is soft water; otherwise, insoluble calcium or magnesium naphtholates will be formed. If soft water is not available then a sequestering agent must be added, the sodium hexametaphosphate, EDTA or NTA types (section 10.2) being suitable. A little alcohol is generally added during the initial pasting and dissolving of the naphthol. Given water of suitable quality, the naphtholate bath in batchwise dyeing then usually contains the following additions: – alkali, most often sodium hydroxide, although in certain circumstances (particularly with regenerated cellulosic or bast fibres) sodium carbonate or trisodium orthophosphate may be used [31] – a protective colloid (dispersing agent) and perhaps a wetting agent – formaldehyde – electrolyte, either sodium chloride or sodium sulphate. The purpose of the alkali is to convert the insoluble free naphthol into its colloidally soluble sodium salt. An excess of sodium hydroxide is generally needed but too much will tend to promote hydrolysis of the amide groups present in most azoic coupling components. The actual amount required varies with the naphthol and processing conditions; the manufacturer’s detailed literature must be consulted. The protective colloid/wetting agent may be a single anionic agent; Turkey Red Oil, for example, combines both functions but is prone to form a precipitate in hard water. Only anionic types are suitable, since nonionic and cationic types generally cause precipitation [31]. Most protective colloids are of the following types: (a) lignosulphonates (b) protein-fatty acid condensates (c) sulphonated condensates of aromatic compounds, especially of phenols and naphthols with formaldehyde. The chemistry of these product types has been described previously (section 10.6.1). The anionic polyelectrolyte helps to stabilise the colloidal solution of the naphtholate, through a mechanism similar to that already described. Where the protective colloid itself does not give adequate wetting of the fabric a suitable wetting agent, which in batchwise dyeing must function well in the cold, should be added; the alkylnaphthalenesulphonate types are suitable.

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The formaldehyde plays an important role in counteracting the tendency of amidecontaining naphtholates to hydrolyse at high pH values to the o-carboxynaphthol, which couples to give coloured by-products of inferior fastness. Its protective action is in addition to that provided by the excess alkali and its use is recommended with most naphthols, exceptions being yellow acetoacetarylamides where coupling is inhibited. Formaldehyde operates through the reversible formation of a 1-methylol derivative at 40–50 °C, but at temperatures above 50 °C this derivative reacts with a second molecule of naphtholate to give a non-coupling dinaphthylmethane compound (Scheme 12.2) [30]. HO CH2 OH

OH HCHO

R

R CONH

CONH Naphtholate

Methylol derivative High temperature Naphtholate

R CONH

OH CH2 OH R CONH

Scheme 12.2

Insoluble methylene compound

The naphthols used in batchwise dyeing are moderately substantive and their exhaustion is improved by electrolyte addition, with consequent improvement in yield and fastness properties. The amount of electrolyte required varies with the substantivity of the naphthol, the depth applied, liquor ratio and substrate quality, but generally ranges from 10 to 40 g/l sodium chloride or sodium sulphate. Higher amounts are required for heavier depths of lowsubstantivity naphthols in long liquors. With high-substantivity naphthols on substrates that are difficult to penetrate and short liquor ratios, treatment is begun in a salt-free naphtholate solution and electrolyte is added later. After application of naphthols by batchwise techniques, excess surface naphthol is usually minimised or removed by hydroextraction, suction, squeezing or by rinsing in 10–50 g/l electrolyte and 0.3–0.6 g/l sodium hydroxide solution. Batchwise application of naphthols is generally carried out at 20–30 °C. Although a higher temperature may be chosen to improve the penetration of difficult substrates, it

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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES

should not be allowed to rise above 50 °C; substantivity decreases with increasing temperature. In continuous dyeing, however, in order to ensure uniformity of uptake from the pad bath, the requirement is for minimal substantivity. Hence application temperatures are generally high (80–95 °C) and naphthols of low to medium inherent substantivity are used. These factors indicate a need for the following modifications to the auxiliary formulations used: (a) formaldehyde should be omitted due to formation of the non-coupling dinaphthylmethane derivative at temperatures higher than 50 °C (b) electrolyte should be omitted (c) the amount of wetting agent can be reduced, or it may be omitted if the higher temperature together with the protective colloid provide sufficient wetting. Naphtholates in the form of ready-to-use solutions are now available [31,32]. These are formulated to contain the essential auxiliaries, are quite stable on storage and offer the following advantages: – no dissolving or boiling necessary – no dust during weighing and preparation and therefore cleaner working conditions – shorter times for setting up the process. 12.3.2 Composition of the diazo solution or developing bath This bath is essentially a dilute solution of a diazonium salt produced either by the diazotisation of an arylamine (Fast Colour Base) or by simply dissolving a stabilised diazonium compound (Fast Colour Salt). Soft water is desirable but not essential. General additions for batchwise dyeing with Fast Colour Bases include acid, sodium nitrite and possibly ice, together with a dispersing agent. Hydrochloric acid is the most widely used acid to effect dissolution of the base and activation of the sodium nitrite so as to bring about diazotisation. Temperatures must be kept low (5–15 °C) to avoid decomposition of the relatively unstable diazonium salt (section 4.3.1); hence ice is often added to the solution. A dispersing agent is used to ensure the fine and uniform dispersion of the azoic dye as it is formed during coupling. Only nonionic types such as fatty alcohol ethoxylates (section 9.6) are suitable, as anionic or cationic types may cause precipitation [31]. Once diazotisation is complete the excess hydrochloric acid must be neutralised before the diazonium salt is coupled with the naphthol, usually by addition of an alkali-binding agent. The agent most commonly used is sodium acetate, which by reaction with the hydrochloric acid produces acetic acid, so that the resultant mixture of acetic acid and sodium acetate acts as a buffer. The acetic acid/sodium acetate balance must be adjusted to suit specific needs related to the reactivity or coupling energy of the system (section 4.4), giving a pH varying from 4 to 5.5 for those having high coupling energy to 6–7 for those with low coupling energy. Examples of azoic diazo components and their relative coupling energies are given in Table 7.2 of reference [30]. Sometimes buffer systems utilising sodium dihydrogen phosphate and disodium hydrogen phosphate or sodium bicarbonate are preferred. When Fast Colour Salts are used hydrochloric acid and sodium nitrite are obviously not required, although some Fast Colour Salts do need an addition of acetic or formic acid. The nonionic dispersing agent is still necessary but as most Fast Colour Salts contain an alkali-

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AZOIC COMPONENTS

823

binding agent (aluminium sulphate, zinc sulphate, magnesium sulphate or, in a few cases, chromium acetate) to give the required pH, the only electrolyte additions to the developing bath are to correct any local variations in pH. In some cases, as in the batchwise application of diazo components, it may be advisable to add electrolyte to the developing bath to inhibit bleed-off of low-substantivity naphthols. Otherwise the auxiliaries for batchwise and continuous application of diazo components are essentially the same. Fast Colour Bases in the form of ready-to-use solutions or dispersions are now available [31,32]. As can be seen from their advantages listed below, their use has implications regarding the addition of auxiliaries: – the formulations are resistant to freezing temperatures and stable in use – all except one product can be diazotised without the addition of ice provided the bath temperature does not exceed 20 °C – diazotisation is complete in 2–5 min compared with up to 30 min for conventional Fast Bases – no nitrous gases are formed during diazotisation – these formulations already contain a dispersing agent and no further addition is needed – no dusting or formation of lumps as during dissolution of conventional Fast Bases. 12.3.3 Aftertreatments to develop hue and maximum fastness After the coupling (development) process is complete the goods are rinsed, acidified and given an alkaline soaping treatment. This not only substantially removes surface dye but also brings about a process of aggregation of dye molecules within the fibre, thus developing the full potential of hue and fastness. A combination of Marseilles (olive oil) soap (3–5 g/l) with sodium carbonate (batchwise 1–2 g/l, continuous 2–3 g/l) is traditionally used. A polyphosphate sequestering agent is needed if the water is hard. A second wash with a nonionic surfactant is also required. The conventional technique in printing is to apply the naphthol by padding as described for continuous dyeing, followed by printing with the diazo componenents using cellulose ether, locust bean or guar derivatives as thickening agents. In other respects the auxiliaries and general processing requirements are similar to those described above. A different system involves application of the naphthol coupling component and a stabilised diazonium salt in the same print paste followed by neutral steaming to effect development; a starch ether thickening agent is recommended for this process [29]. In certain resist styles aluminium sulphate is applied by printing onto naphthol-treated fabric; this brings about a localised reduction in pH that inhibits coupling during subsequent application of the diazo component, thus giving rise to a resist effect [29]. Overall, the application procedures for azoic dyeing are quite complex since many factors must be taken into account, such as: – the specific naphthol and diazo components selected, with regard to molecular characteristics, substantivity and applied depth – the application method, e.g. continuous or batchwise, paying attention particularly to liquor ratio and substantivity in the latter case – the substrate, including unmercerised or mercerised cotton, causticised regenerated cellulosics or bast fibres.

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824

AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES

All the above criteria influence the concentrations of the various components required. Some indication of how they are affected can be gleaned from reference [30]. It is important therefore to consult detailed information from the supplier of the naphthols and bases. Such information available on disc for use on a personal computer has been provided for the batchwise dyeing of cellulosic yarns [31] and the continuous dyeing of cellulosic fabrics [32]. The stripping of fully developed azoic dyeings can often be carried out using a hot solution of sodium hydroxide (1.5–3 g/l), sodium dithionite (3–5 g/l) and a surfactant; addition of anthraquinone (0.5–1 g/l) generally increases the effectiveness of the process. Yellow azoic dyeings are resistant, however, and can only be partially stripped [30]. On the other hand, stripping of naphtholated material before it has been coupled with the diazo component can be done quite effectively in boiling alkali. 12.4 BASIC DYES There are two major characteristics of basic dyes applied by exhaustion techniques to acrylic fibres: (1) below the glass-transition temperature (about 80 °C) exhaustion is very slight, becoming much more rapid at temperatures only a little above this (2) very little, if any, migration occurs at temperatures up to 100 °C. Consequently the rate of dyeing, and hence levelness, are very difficult to control; the degree of difficulty varies from fibre to fibre, generally tending to a maximum for readily dyeable fibres with a high glass-transition temperature. Owing to the sensitivity of some basic dyes to alkaline hydrolysis, these dyes vary in their response to dyebath pH, again depending on fibre type (section 3.2.4). The pH must be controlled to within 4.0 to 5.5 in order to obtain reliable, reproducible results across the range of dyes and fibres. Hence in the conventional batchwise application of basic dyes to acrylic fibres, auxiliaries must fulfil two functions: (1) to give the required pH (2) to control the rate of sorption in the critical temperature region and, as far as possible, to promote migration. A buffer system is preferred for the control of pH, the most common one being the relatively cheap acetic acid/sodium acetate system, although a simple addition of acetic acid may be adequate with water that does not show a significant pH shift on heating. The major variables are undoubtedly the rate of temperature rise and the use of retarding agents to control level dyeing. A review of acrylic fibres and their processing is available [33]; here we are concerned only with the essential chemistry of the auxiliaries used, particular emphasis being given to retarding agents. Cationic types of retarding agent are especially important. These function essentially as colourless cationic moieties competing for the anionic sorption sites in the fibre. Quaternary ammonium compounds (section 9.5) largely predominate; their fundamental structure (12.9) offers the possibility of varying up to four substituent groups around a quaternary nitrogen atom, and hence the variety of possible structures is enormous. A range of these compounds examined for their retarding effect in the application of basic dyes [34] gives some idea of the possibilities (Table 12.4). The selection of a retarder depends on several

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BASIC DYES

825

factors, however, of which the most important are the rate and extent of sorption of the retarder compared with those of the dyes. The dyeing kinetics of basic dyes in mixtures are now universally denoted by compatibility values covering the range from 1 to 5 [35–38]. Simply varying the substituents in otherwise structurally similar dyes can change their compatibility values [37]. The sorption properties of quaternary ammonium compounds can be varied and characterised similarly, as seen from the examples shown in Table 12.5 [39], which were obtained using a titration-spectrophotometric method [40]. The type of associated anion has only a minor effect on the properties of a cationic retarder. General practical experience [39,41] suggests that optimal control is achieved if the Table 12.4 Structures of some typical retarding agents [34] Substituents in quaternary ammonium compound 12.9 R

R1

R2

R3

Anion X

C12H25 (dodecyl) Coco* C16H33 (hexadecyl) C18H37 (octadecyl) Tallow** Coco* Coco* Coco*

CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3

CH3 CH3 CH3 CH3 CH3 CH3 CH3CH2 C6H5CH2

CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3

Cl Cl Cl Cl Cl CH3SO4 CH3SO4 Cl

* Consisted of approximately 47% C12 and 18% C14 with lesser amounts of C8, C10 , C16 and C18 hydrophobes. ** Consisted of approximately 48% oleyl, 27% cetyl and 13% stearyl, with minor quantities of others.

R

R1 _ + N R2 X R3 12.9

Table 12.5 Compatibility values of retarding agents [39] Substituents in quaternary ammonium compound 12.9

chpt12(2).pmd

R

R1

R2

R3

C14H29 C6H5CH2 CH3 C6H5CH2 C6H5CH2

CH3 CH3 CH3 CH3 CH3

C14H29 C14H29 CH3 C7–9H15–19 C6H5CH2

CH3 CH3 C14H29 CH3 CH3

825

Compatibility value assigned by experiment 1.0 2.5 3.0 5.0 >5.0

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826

AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES

retarder has a compatibility value equal to or slightly lower than that of the dyes, so that it will tend to be absorbed by the fibre either at the same rate as the dyes or somewhat more quickly. If the compatibility value of the retarder is significantly lower than that of the dyes, then there is a very real tendency for it to act as a blocking agent (with attendant problems), whilst if its compatibility value is much higher its efficacy is impaired. Once the substituents in the quaternary ammonium compound have been selected, consideration must be given to the applied concentration of retarder. Acrylic fibres vary significantly in the number of anionic sites available for sorption of cations but it is generally assumed that maximum likelihood of level dyeing occurs when the number of cations in the system (retarder as well as dyes) is just enough to saturate the anionic sites in the fibre. Thus the amount of retarder needed to achieve this theoretical saturation will vary from fibre to fibre, and also depends on the applied concentrations of the dyes. More retarder will be needed for fibres of high saturation value and for lower applied depths; the actual quantities required to satisfy the given conditions are generally specified by the dye manufacturers. However, the use of these theoretical quantities can lead to lower degrees of dye exhaustion within normal dyeing times. In any case level dyeing is not just simply a function of the ionic dye–fibre system but involves many other aspects, especially physical factors such as substrate form and machinery efficiency. It may well be that in a given practical situation there may be little or no level dyeing problem, so why use any more retarder than is necessary to ensure a level dyeing under practical conditions? Experience suggests that much less than the theoretical amount of retarder will often be adequate and this will help to alleviate any problems due to saturation if subsequent reprocessing for shade correction is needed. In addition to having an effect on the rate of dyeing, cationic retarders will assist migration to an extent that depends on the fibre and the substantivity of the dyes. Retarders tend to diffuse more quickly than dyes and to be absorbed at lower temperatures (typically 65–70 °C, compared with 80–85 °C), although the magnitude of these effects will depend on the structure and properties of both retarder and dye. In some cases, such as hank dyeing on machines with poor circulation or inadequate temperature control, it may be preferable to use a retarder that almost totally restrains the uptake of dye until the top temperature has been reached, after which dye sorption takes place gradually. A useful general classification of cationic retarders according to their properties has been given [42]: (a) strongly cationic with a strong blocking effect (b) moderately cationic with a weak blocking effect (c) weakly cationic with no blocking effect (d) products with little or no retarding effect but giving some levelling. Products in groups (b) and (c) allow for greater safety margins and give optimal exhaustion curves in bulk practice, although they may be more expensive than products in group (a). The main need for retarding activity arises during the critical exhaustion phase as the temperature increases from about 80 °C to the boil. Therefore some cationic retarders have been designed to hydrolyse progressively in this temperature region, so reducing the retarding activity in the later stages of dyeing and safeguarding against blocking effects. Subsequent shading and redyeing are then less problematical. Furthermore, the amounts of hydrolysing retarder used are perhaps less critical than with their non-hydrolysing

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BASIC DYES

CH2CH2OH RCOCO

N

RCOCO CH2CH2OH

12.10

CH3 _ + N CH2COO

CH3 RCOCO

CH3 12.11

827

N

O

CH3 12.12

counterparts, although more may be needed initially to obtain an equivalent retarding effect. A combination of hydrolysing and non-hydrolysing types may be used in some circumstances. On 100% acrylic materials the quaternary ammonium retarders are used almost exclusively. Other types have been evaluated, however. For example, saturated alkylamines (RNH2; R = C10, C12, C14 and C16 hydrophobes) were found to be just as effective as the quaternary types although other factors, such as aqueous solubility at the optimal dyebath pH and resistance to subsequent discoloration, favour the quaternary compounds [34]. On the other hand, bis(hydroxyethyl)cocoamine (12.10) had relatively little effect and the amphoteric carboxymethyldimethylcocoamine (12.11) none at all, although dimethylcocoamine oxide (12.12) was quite an effective retarder [34]. Other cationic compounds used [43,44] have included alkylpyridinium salts, imidazoles and imidazolinium salts, alkyldiamines, alkylpolyamines, as well as sulphonium and phosphonium derivatives. Polymeric cationic retarders that contain up to several hundred cationic groups per molecule have been proposed [45–47]. The early types [45,46] were described as quaternised polyamines (section 9.5) of relative molecular mass 1000–20 000, as compared with 300–500 for conventional quaternary ammonium compounds. Polyacrylamides of molecular mass 2500–780 000 have been evaluated more recently [47]. These agents, because of their large molecular size, do not diffuse into the fibre but are strongly adsorbed at the fibre surface, reducing its anionic potential. They retard the dyeing rate far more than does an equal concentration of a conventional quaternary agent, but do not assist migration. Some of these products can adversely affect the compatibility of dyes as a result of selective behaviour but are said to be free from blocking effects. They do not interfere with crimp development as conventional retarders sometimes do and are particularly effective in giving superior coverage of bicomponent fibres. Since they are not absorbed into the fibre but concentrate their activity at the surface, these polymers are effective at much lower concentrations than are conventional types and this favours their cost-effectiveness. Their retarding action is sustained throughout the dyeing cycle. Nevertheless, it does appear that the molecular mass of the polymer needs to be optimised in relation to the type of dyes used [46,47] With polyacrylamides [47] for example, a dye of small molecular size responds best to a retarder of large molecular size, and vice versa. These effects have been explained on the basis of the relative ease of diffusivity of the dyes through the polymeric retarder to reach the fibre surface. Thus a dye of small molecular size needs the extra resistance to diffusion of a retarder of great molecular mass, whilst such a retarder would present too effective a barrier to diffusion of a dye of larger molecular size. This relationship clearly has major implications for the selection of dyes to achieve close compatibility in mixture recipes. Despite the claimed advantages and the prediction as long ago as 1973 that polymeric retarders would rapidly become the preferred choice for dyeing acrylic fibres with a high content of dye sites [45], they have not attained a commercially significant role.

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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES

Retarders of opposite ionic charge to the dyes can be used [33,36,48–51]. Anionic retarders function by forming a thermally labile complex with the dye and thus lowering the substantivity of the dye for the fibre. Undesirable precipitation of this complex, which is one of the drawbacks of the system, can be inhibited: (a) by using excess anionic agent (b) by using an anionic agent that contains two or more sulphonate groups so that the resultant 1:1 complex retains solubility (c) by incorporating a nonionic agent as an antiprecipitant. Examples include sodium dinaphthylmethanesulphonates (section 10.6.1) and polyethoxylated alkylarylsulphates (section 9.4). Polymeric types, such as polystyrene sulphonate, have been tried but do not seem to offer any advantages. The advantages and disadvantages of anionic retarders can be summarised as follows. The advantages include: (a) the system is compatible with anionic dyes and anionic dispersing agents in the dyeing of fibre blends (b) there is no blocking of dyeing sites in the fibre (c) they have no adverse effects on the bulkiness of certain bicomponent fibres (d) they promote good migration of dyes (e) they can be used as stripping agents to reduce the depth of colour in reprocessing. The disadvantages include: (a) to prevent precipitation the quantity of anionic retarder should increase with increasing quantity of dye (the opposite of the situation with cationic retarders) and this conflicts with requirements for promoting exhaustion; hence exhaustion of dye when applying medium or heavy depths is poor (b) they show less levelling during the exhaustion stage (c) the use of cationic softeners in the dyebath is not possible. In practical terms the disadvantages outweigh the advantages, thus limiting the importance of anionic retarders. Electrolytes such as sodium chloride and sodium sulphate tend to retard dyeing [36,52,53] through preferential adsorption and subsequent displacement by the dye of the more mobile sodium ions, although the effect is relatively weak even compared with the weaker cationic retarders. Nevertheless, the use of up to 10% o.w.f. sodium sulphate in combination with a cationic retarder may enable the amount of the latter to be reduced by up to 20–30% [36]. The limitations of electrolytes, apart from this lower effectiveness, are that they reduce the final uptake of dye, their effectiveness decreases with increase in temperature and their effect is greatest with fibres containing weakly anionic groups such as carboxylate, rather than stronger ones such as sulphonate. Cationic softeners for acrylic fibres are sensitive to the presence of electrolytes, although sulphate-tolerant softeners may be used. The retarding effect of electrolytes in the application of basic dyes to acrylic fibres increases with increasing concentration of salt up to a certain level. Increasing the concentration beyond this point has no further effect on exhaustion with certain univalent anions, whilst with multivalent types there is an increase in dye sorption (Figure 12.2)

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BASIC DYES

829

[54,55]. These results have led to the conclusion that ionic mechanisms alone do not entirely explain the complex interactions that occur between basic dyes and acrylic fibres. Hydrophobic interaction also plays an important part and it has been demonstrated [55] that multivalent anions such as sulphate or phosphate can enhance the hydrophobic interaction, thereby also increasing dye sorption in some circumstances. Whilst such results are of interest in terms of dyeing theory, it is extremely doubtful whether there will ever be practical interest in exploiting the use of electrolytes at such high concentrations.

A Represents sulphates and phosphates B Represents chlorides, bromides and nitrates Dye sorption on fibre

A

B

Salt concentration in dyebath

Figure 12.2 The effect of salt on the equilibrium sorption of basic dyes on acrylic fibres [54,55]

The use of dye-solubilising agents such as urea or thiourea is more usually associated with continuous dyeing or printing. However, such compounds have also been investigated for their effects in exhaust dyeing [56,57]. These compounds increase the rate of exhaustion and exert their maximal accelerating effect at low temperatures (e.g. 80 °C) and low dye concentrations. Despite the higher acceleration factor at 80 °C as compared with 100 °C, the ultimate yields at 80 °C are lower than at 100 °C (Figure 12.3) [57]. When dyeing at 100 °C or above it is difficult to see any commercial reason for making these additions, particularly in view of environmental concerns regarding such compounds. Continuous dyeing of acrylic fibres with basic dyes [50,58,59] generally requires the use of saturated steam for fixation. As in batchwise dyeing there is a need to maintain an optimum pH of 4.5–5.0. If a sodium acetate/acetic acid buffer were to be used the acetic acid may volatilise in the steam, leading to development of alkalinity; hence it is usual to utilise a non-volatile acid such as citric acid (12.7) or tartaric acid (12.14). The thickening agent for use with basic dyes must not be anionic, a useful choice being galactomannanbased locust bean gum. Hydrotropes and fibre-swelling agents assist dye solubilisation and fixation; compounds used include thiodiethylene glycol (12.5), dicyanoethylformamide (12.15) and potassium thiocyanate (KSCN). A nonionic wetting and solubilising agent may also be useful. In general, for the pad–steam application of basic dyes to acrylic fibres, the auxiliaries are selected to increase the solubility of the dyes with minimal retardation and maximal improvement of fixation. In a detailed statistical study along these lines [60], it was

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830

AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES

Dyeings at 80 oC

100

Dyeings at 100 oC

100 Dye concn/ % owf

90

1.5 2.0 2.5 3.0

80

Dye exhaustion/%

Dye exhaustion/%

90

Dye concn/ % owf

70 60 50

1.5 2.0 2.5 3.0

80 70 60 50

0.4

0.8

1.2

1.6

0.4

Thiourea concentration/g l–1

0.8

1.2

1.6

Thiourea concentration/g l–1

Figure 12.3 Effect of thiourea on exhaustion of CI Basic Yellow 21 (12.13) by acrylic fibres [57]

H3C

CH3 CH N

CH

N

O

OH C

Cl

CH2CH2CN

H CH3

CH3

HO

CH

HO

CH

12.13

C

12.14

N CH2CH2CN

O

C

CI Basic Yellow 21

O

OH

12.15

concluded that the following represented the optimal composition of a mixed auxiliary formulation: 50% of a solubilising agent based on a branched-chain fatty alcohol with 8 units ethylene oxide per molecule 32% of a short-chain fatty alcohol with 2.5 units ethylene oxide per molecule 14% of a plasticising agent for acrylic fibres 12% of an ethoxylated long-chain alcohol (C10–C18) with 15 units ethylene oxide per molecule 12% of a short-chain fatty alcohol that increases the fixation of basic dyes. In addition to the conventional dyeing of acrylic fibres, there is considerable interest in socalled gel dyeing of acrylic filaments during the manufacturing process after extrusion. From the viewpoint of auxiliary usage this is outside the scope of the present work, but a useful account of the factors involved is available [61]. Basic dyes are used to dye acid-modified polyester fibres, in which case there is usually less need for a retarding agent. Glauber’s salt is often added, however, to guard against hydrolytic degradation of these fibres [62]. Cotton modified according to Scheme 12.3 can then be dyed with basic dyes, although commercial exploitation of such a process is unlikely [63].

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O C O

CH2OH CH

CH2 C

O

O

CH

O

Cl CH

CH CH

CH

OH

OH

CH

CH

acylation

CH

CH

OH

OH

oxidation NaIO4

O C

O C

HO

CH2

O

CH

O

CH2

O

CH NaHSO3

CH

CH

CH

CH

SO3Na

SO3Na

O

CH

OH

CH CH

CH

O

O

Scheme 12.3

O

OH

O

C

O

(CH2)7CH3

C

O

(CH2)7CH3

C HN

O 12.16

H2C

CH2 CH2 CH2

OH

CH2 12.17

12.18

In direct printing of acrylic fabrics a typical stock print paste [29] may contain the following components by mass: 0.5% citric acid (12.7) 1–2% dicyanoethylformamide (12.15) 3% thiodiethylene glycol (12.5) 7% acetic acid (30%) 50–60% locust bean thickener Dioctyl phthalate (12.16), caprolactam (12.17) and urea together with resorcinol (12.18) are also said to act as fixation assistants [64].A further addition of an anionic thickening agent such as carboxymethylcellulose [29] can act as a levelling agent when printing large blotches. A wash-off with anionic surfactant is usually given.

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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES

Discharge white styles are obtained using either formaldehyde-sulphoxylate or the weaker tin(II) chloride as reducing agent; crystal gum or British gum are recommended thickening agents, together with potassium thiocyanate as a fibre-swelling agent [29]. For coloured discharges tin(II) chloride is the recommended reducing agent, since formaldehydesulphoxylate would reduce the illuminant basic dyes; other additions are generally as for direct printing. Discharge styles, after steaming and rinsing, are given a clearing treatment at 40 °C in 1 ml/l ammonia (25%) and 1 g/l sodium dithionite [29], followed by rinsing and soaping with anionic detergent at 60–70 °C. Non-destructive partial stripping techniques for basic dyes on acrylic fibres are carried out at 100 °C (or higher if possible) using, for example, 1–10% o.w.f. anionic retarder and 1 g/l acetic acid (60%), or 1–5 g/l Marseilles (olive oil) soap. Destructive stripping requires acidified (pH 5.5–6.0) sodium hypochlorite, followed by an antichlor treatment in sodium dithionite or sodium bisulphite. In some cases a preliminary boiling treatment in 5 g/l monoethanolamine and 5 g/l sodium chloride is said to improve the effect of the stripping treatment. 12.5 DIRECT DYES Direct dyes represent one of the simplest dyeing systems, usually requiring only an electrolyte as an essential auxiliary for their application. Nevertheless a surfactant may sometimes be added to assist wetting and levelling, as well as a sequestering agent, since many direct dyes are sensitive to hard water. Control of pH may also be desirable. Certain traditional dyes require aftercoppering as part of their application procedure, whilst it is usual to aftertreat direct dyeings to improve their wet fastness properties. The dyeing of polyester/cellulosic blends with direct and disperse dyes requires application at temperatures higher than 100 °C. An up-to-date account of the application of direct dyes is available [30]. The main area to be considered in the batchwise application of these dyes is the use of either sodium chloride or sodium sulphate to promote exhaustion, although the sulphate can give rise to calcium sulphate deposits in hard water. Direct dyes vary enormously in their response to electrolyte; in general the more highly sulphonated dyes require greater amounts of salt. This is in line with the behaviour of dyes according to the universally used SDC classification [30,65,66], whereby dyes are allocated to three application classes. Class A dyes are generally the most soluble and least sensitive to salt, hence necessitating substantial additions of electrolyte to boost their low exhaustion values. It is advisable with class A dyes to add electrolyte to the rinsing water to inhibit the otherwise copious bleed-off of dye into the water. For this purpose magnesium sulphate may be more efficient than sodium salts since it can form the less soluble magnesium salt of the dye, but the acceptability of this will depend on whether magnesium can be tolerated in subsequent processing. Dyes in classes B and C are generally less soluble and are so responsive to electrolyte that salt must be added gradually over the dyeing cycle as otherwise the rate of strike will be so rapid as to give unlevel, poorly penetrated dyeings and there may even be salting out of the dye in the dyebath. More salt is needed in longer liquors, and for heavier depths. Dyes having the same CI generic name but made by different manufacturers may also require different amounts of electrolyte to be added to the dyebath, according to the amount of electrolyte present in the commercial formulation. A typical instruction is to use from 0 to

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20 g/l salt depending on the factors described above. Electrolyte may influence migration as well as exhaustion [67], an optimal concentration of electrolyte being found for maximal migration of class A and B dyes, whilst the migration of class C dyes decreases with increasing amounts of salt. As mentioned above, sodium chloride and sodium sulphate are the electrolytes most commonly used in practice and it is generally accepted that they exert their effect by means of the common ion effect. There is another aspect, however; electrolytes also modify the structure of water around the hydrophobic groups in dye molecules and around the surface of the fibre, creating a new order in solution as a result of solvation. This enables dye molecules to approach more closely to the fibre surface within the influence of short-range interactive forces [68]. Numerous electrolytes have been investigated in fact, although some of the research work is seriously limited by having been carried out with only a few dyes, sometimes just one. In an investigation of the relative effects of Zn, Mn, Cd, Sr, Al and Ce nitrates [68], it was found that the size of the cation, as well as its charge, played a part in the sorption process: saturation values and sorption rates increased with increasing size of the cation. A similar effect has been observed for the series: LiCl < NaCl < KCl [69], whilst in binary mixtures of these electrolytes [70] the larger cation has a strongly promotional effect on the activity of the smaller cation (Figure 12.4). It has to be admitted, however, that these experiments were carried out at temperatures lower than is typical in commercial dyeing with direct dyes. Mention has been made above of the use of magnesium sulphate to prevent bleeding of class A dyes during rinsing. With carefully selected dyes [71], magnesium salts can effectively replace the conventional sodium salts during dyeing. An optional mixture of

Equilibrium absorption at 34 oC

Rate of dyeing at 40 oC

0.30

8

0.25 0.20

6 Mt /M∞

Concentration of dye on fibre/(mol/kg) × 102

10

0.15

4 0.10 2

0.05

2

6

10

Concentration of dye in solution/(mol/l) × 104 KCl (0.1 mol/l) NaCl (0.1 mol/l) LiCl (0.1 mol/l)

14

2

6

10

1 t1/ (min /2) 2

KCl (0.05 mol/l) + NaCl (0.05 mol/l) LiCl (0.05 mol/l) + NaCl (0.05 mol/l)

Figure 12.4 Equilibrium absorption isotherms at 34 °C and rate of dyeing curves at 40 °C for CI Direct Blue 1 on viscose in the presence of electrolytes singly and in binary mixtures [70]

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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES

organic magnesium salts has been formulated. Since precipitation could be a problem, this proprietary mixture also contains a polymeric sequestrant. The rationale behind this process was to eliminate the conventional inorganic electrolytes from the dyeing process, since these have chronic toxic effects on freshwater organisms above certain concentrations. In addition, the usual electrolytes are quite difficult to remove from effluent. It is claimed that magnesium salts are used at lower concentrations and are easily removed from the effluent by precipitation. For environmental reasons, other attempts have been made to reduce the amount of conventional electrolyte added. Lowering the liquor ratio will in itself reduce the amount of electrolyte required. In one commercially feasible system [72], a range of direct dyes was successfully screened to select members that could be applied efficiently to give 95–100% exhaustion using significantly less electrolyte than usual. Thus at applied depths up to 2–3%, only 2–5 g/l salt is required; navy and black dyeings can be produced with only 7.5–10 g/l salt compared with the conventional 25 g/l addition. Ultrasonic irradiation has been shown in laboratory studies [73] to increase dye exhaustion, enabling salt levels to be reduced. However, it seems doubtful whether the higher effectiveness is sufficient to merit development to overcome the problems involved in scaling-up the ultrasonic equipment to bulk-scale processing. For example, in one experiment using 5% salt at 65 °C, ultrasound treatment increased the dye exhaustion from 77% to 82%. A sequestering agent is usually necessary in hard water to prevent the formation of sparingly soluble calcium and/or magnesium salts. These can lead to uneven deposits of lower fastness on the surface of the fabric as well as reduced yields due to precipitation in the dyebath. Polyphosphates are particularly useful in this respect. Organic sequestering agents such as EDTA must be avoided with metal-complex direct dyes as they tend to extract the metal from the dye molecules, resulting in a change in hue and a significant lowering of fastness, although they can be used safely with unmetallised dyes. In a rather unorthodox approach to the use of sequestering agents, ethylenediaminetetramethylphosphonic acid (EDTMP; 12.19) applied as a pretreatment for cotton at ambient temperature was shown to increase the exhaustion of CI Direct Red 79 (12.20) applied at 100 °C in the presence of salt [74]. The maximum effect was achieved with 3 mg of sequestering agent per g of cotton. On dyeings carried out for 30 minutes, this gave surprising improvements in exhaustion from 35% to 45% (for 10 mg dye per g cotton) and from 17% to 29% (for 40 mg dye per g cotton). Rate of dyeing was apparently increased, too. Only this one dye and one sequestering agent were examined, however. It was found that about 35% of the tetraphosphonate was absorbed by the cotton. It was postulated that the phosphonate groups are only partially ionised in the neutral dyebath and that the presence of the nitrogen atoms favours hydrogen bonding between nonionised phosphonate groups and anionic sulpho groups in the dye molecule (12.21). Increased fastness to washing was also claimed [74]. Some direct dyes are sensitive to reduction or hydrolysis under alkaline conditions, particularly if temperatures above 100 °C are used (section 3.1.3); pH 6 is frequently favoured for stability and this can usually be achieved using ammonium sulphate. A few dyes give optimal results under alkaline conditions, using sodium carbonate or soap; the tetraamino dye CI Direct Black 22 (12.22) is an example. Whether or not an addition is needed will depend on whether alkali is already present in the commercial brand.

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O

O HO

P

P

CH2

CH2

OH OH HO

835

N

CH2CH2

OH

OH OH

N CH2

CH2

P

P

OH

O

O 12.19 EDTMP

H3C

CH3

O

NaO3S

SO3Na

C HN

N

NH

N

N

N H3CO

OCH3 OH

HO 12.20

NaO3S

SO3Na

CI Direct Red 79

H P

S [dye]

O

O

O

NaO

O

O

N

H H2N

NH2

12.21 NH2 N

H2N

N

N

SO3Na O

H

H N

N

O

N

N

N NaO3S

SO3Na

NaO3S

12.22 CI Direct Black 22

Levelling and wetting agents for direct dyeing are mostly ethoxylated adducts, such as alkylaryl ethoxylates, although anionic types such as alkylarylsulphonates, phosphate esters and alkylbenzimidazoles are also marketed. Care should be exercised in the use of such agents; there is spectrophotometric evidence [75] that they interact with dyes, leading to lower exhaustion. Since this interaction is dye-specific, there may be problems with mixtures of dyes. Such auxiliaries also add quantitatively to the COD value of the effluent [76]. Various other auxiliaries have been investigated but their commercial use is either limited or

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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES

non-existent: urea, pyridine, amyl acetate, gelatin, carboxymethylcellulose [77]. Cyclodextrins [75,78] may have some potential for the future (section 10.3.1), although their action with direct dyes is dye-specific [75]. There is research interest in the potential to pretreat cotton with reactive compounds (section 10.9.1) followed by application of direct dyes or nucleophilic amino-containing dyes to give increased fixation and/or fastness. Attempts have also been made to apply such reactive compounds simultaneously with direct dyes [79]. Despite the inroads made by reactive dyes, there still remains considerable interest in the application of direct dyes, as evidenced by the introduction of the Optisal system by Clariant [80–82]. This is a carefully designed package of selected metal-free direct dyes, that require little salt to give high exhaustion and are stable up to 130 °C, together with a cationic formaldehyde-free fixation agent applied as an aftertreatment. The dyes may also be applied isothermally [81]. Environmental advantages are claimed, including high exhaustion, less pollution of effluent and low salt usage. Various techniques are available for the application of direct dyes by semi-continuous and continuous methods, such as pad–jig, pad–batch, pad–steam, pad–dry and pad–thermofix. The major problem arises from the high substantivity of direct dyes for cellulosic substrates, making it very difficult to avoid tailing problems. Hence concentrated brands of dyes having minimal electrolyte content are preferred; of these, the class B dyes offer better operating properties. The main methods of controlling uniform uptake remain careful selection of dyes for compatibility, speed of padding and the rate of supply of padding liquor. Low solubility of the dyes may also be a problem; use of a hydrotropic agent such as urea improves the solubility of certain dyes and may also improve fixation, particularly in dry fixation processes. The washing-off process after fixation may be combined with an aftertreatment to improve the wet fastness and avoid undesirable bleeding of dye. Treatment with durable-press resins and with cationic products, particularly of the multifunctional reactant type, is especially useful here. The aftertreatment of conventional direct dyeings to improve fastness to light and particularly to wet treatments, using copper(II) sulphate, formaldehyde, diazotisation and coupling techniques or cationic fixing agents, has been described in section 10.9.5 and will not be discussed further here. An interesting, if little-used, method of overcoming dye substantivity problems at the padding stage involved the use of certain amines, particularly those containing carboxyl or hydroxy groups such as structure 12.23 [83], in combination with 1:1 copper-complex direct dyes. A low-stability amine–copper–dye complex was formed. The complex diffused readily into the fibre and reverted to dye and amine during steam fixation, the amine being subsequently removed during washing-off. Careful selection of the amine, or mixture of amines, was necessary to achieve the desirable balance of properties. It is interesting to compare this use of an amine and 1:1 metal-complex direct dyes with the similar requirement for such dyes in the much more recent Indosol (Clariant) process (section 10.9.5). HOCH2CH2NHCH2CH2NHCH2CH2OH 12.23

Direct dyes are of limited interest for printing because of their restricted wet fastness, resulting in cross-staining of whites or pastel-dyed grounds when the prints are subsequently washed. Somewhat better results can be achieved by treating the prints after steam fixation

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with, for example, a cationic fixing agent, durable-press resin or, in the case of chelatable dyes, copper(II) sulphate as already described (section 10.9.5). Most azo direct dyes will discharge easily with reducing agents and can therefore be applied as dyed grounds for discharge print styles, although the limitations described above with regard to wet fastness are especially pertinent here. Partial non-destructive stripping of untreated direct dyeings can be accomplished with an alkaline solution of soap or synthetic detergent. Destructive stripping using a reducing agent such as sodium dithionite is also effective except with stilbene-type dyes. Aftertreated dyeings may additionally require a treatment to counteract the aftertreatment; for example, coppered dyeings can be treated with a sequestering agent such as EDTA and dyeings treated with a simple, non-reactive cationic agent may respond to treatment with an anionic detergent.

12.6 DISPERSE DYES Disperse dyes as a class are peculiarly sensitive to the influence of auxiliary agents, both as regards the quality and stability of the dispersion and the response of the dyes during the various coloration processes. Essential auxiliaries in batchwise dyeing include dispersing agents and chemicals to control the pH. Supplementary auxiliaries termed ‘carriers’ may be needed under certain circumstances to accelerate the otherwise inadequate rate of dyeing. Aftertreatment of the dyeings to remove surface dyes is important in many cases, as are the conditions of drying and finishing since these can influence fastness properties. 12.6.1 Dispersing agents The essential chemistry of dispersing agents has been discussed in section 10.6.1, where it was noted that different considerations may apply at the comminution stage of dye formulation compared with maintaining the stability of the dispersion during subsequent coloration processes. The dyeing of polyester at a temperature in the region of 120–135 °C in beam and package dyeing machines places severe demands on initial dispersion quality and subsequent stability under adverse conditions. Jig dyeing with a high concentration of dye in a very short liquor (as for navy blues and blacks) can also be the source of dispersion stability problems. The crux of the problem lies in the inherent thermodynamic instability of all dye dispersions, there being an overall tendency of fine particles to undergo Ostwald ripening with the consequent formation of larger particles. Although disperse dyes are generally considered to be virtually insoluble in water they are, in colloidal terms, sparingly soluble; indeed a low degree of solubility seems to be a necessary prerequisite for dyeing to take place from an aqueous medium. It is this limited solubility that favours Ostwald ripening. The detailed colloid chemistry of dispersions with particular reference to these phenomena has been thoroughly discussed [84]. The solubility of disperse dyes normally increases with temperature and dispersing agent concentration, although these effects vary enormously from agent to agent and from dye to dye. Most dispersing agents are of the anionic polyelectrolyte type, comprising various sulphonated condensation products of aromatic compounds and lignosulphonates (section 10.6.1). Increasing understanding of lignin chemistry with consequent improvements in manufacture, enabling lignins to be more economically and reliably ‘tailored’ for specific

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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES

end-uses, currently favours the use of these products, although by no means exclusively. Solid brands of disperse dyes contain a significant proportion of dispersing agent added during formulation; liquid brands contain rather less as they do not have to withstand the thermal and mechanical rigours of spray drying [85] and do not require redispersing at the dyebath preparation stage. Despite this, it is still advisable to add extra dispersing agent at the dyeing stage, more being required with liquid dyes to compensate for their lower intrinsic content. At the grinding stage of dye manufacture lignosulphonates with a high degree of sulphonation generally perform better. Less sulphonated types tend to give better stability at high dyeing temperatures [86], since they are more readily adsorbed onto and retained by the hydrophobic surfaces of the dye particles. The complexity of the relationships within a disperse dye system is well illustrated in Figure 12.5. The particle size distribution in a disperse dyebath and any transformations taking place during dyeing, including the three successive phases of heating up, maintaining top temperature and subsequent cooling, can exert critical effects on rate of dyeing, final degree of sorption and levelness. Whilst microscopy techniques are undoubtedly useful in studying dispersions, much more detailed information is obtained from photon correlation spectroscopy using a Coulter counter [88]. Figure 12.6 shows an example of particle size Molecular solution of dye Micellar solubilisation

Dispersing agent system Hydrotropy

Molecules Micelle Displacement Dissolving Crystallisation

Agglomeration Crystallites with dispersant Crystal formation from dispertion Aggregation Crystal formation from solution

Crystal

Figure 12.5 Model of the disperse dye system [87,88]

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50

839

A

Volume/%

40 30 B

20 10

C

0.4

0.8 1.2

2

3.2

5

8

12.6

20

Particle size/µm Treatment A 70 oC

5 min

100 oC

10 min

100 oC

10 min

70 oC

oC

20 min

130

oC

60 min

130

oC

15 min

70 oC

Treatment C 70 oC

20 min

130

oC

180 min

130

oC

15 min

70 oC

Treatment B 70

Figure 12.6 Effect of temperature changes on the particle size distribution in a dyebath containing 0.6% CI Disperse Orange 13 [88]

transformations taking place in a specific dispersion of CI Disperse Orange 13 on exposure to three different temperature profiles. The addition of auxiliaries, such as additional dispersing agents, levelling agents, carriers and electrolytes, brings about further changes that may be beneficial or otherwise, depending on circumstances. A method based on a ternary diagram (Figure 12.7) representing the relationship between dyeing properties and the concentrations of three dispersing agents used simultaneously in the dyebath has been described [89]. Of course, any ranges of concentration can be chosen that are appropriate for the dispersing agents present. The scheme is suitable not only for studying particle size distribution using a laser particle sizer as in this instance, but also for examining effects such as solubilisation, rate of dissolution, diffusion coefficient, dyeing rate and colour difference. Figure 12.8 shows how these various factors can be compared in a single diagram [89]. An alternative approach to evaluating the efficiency of dispersing agents depends on the partition effect [90]. In this simple and practical method a sample of the aqueous dispersion is extracted with a water-immiscible solvent (e.g. chloroform, methylene dichloride, monochlorobenzene, dimethyl phthalate, tetrachloroethylene) in which the dye but not the dispersing agent is soluble. Unprotected disperse dye particles dissolve immediately in the solvent layer. Dye particles protected by a sheath of dispersing agent are more hydrophilic and therefore favour the aqueous layer. The rate of extraction of a disperse dye from the aqueous layer into the solvent depends on the stability of the dispersion and the extraction conditions. However, there must be limits to the applicability of this method to the study of phase transitions during the dyeing process. The influence of dispersant structure on the thermal stability of dye dispersions has been illustrated as in Figure 12.9. Thermal stability appears to be related to the relative numbers and strengths of the adsorbing and stabilising groups in the dispersing agent. Partial blocking

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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES

Agent 1 1 0.5

0.1 8

0.4

0.2

0.3

0.3 2

6 7 0.4

0.2 10

9

0.1

0.5

4

3 0.5

Agent 3

0.4

5

0.3

0.2

0.1

Agent 2

Figure 12.7 Basic points of the ternary diagram in which individual parameters were determined experimentally; agent concentrations are quoted in g/l [89]

Agent 1 Rate of dissolution (mg/l min)

36

Rate of dyeing (mg/g min)

25

5 43

1 100

Solubilisation at 60 min (mg/1) BASF diffusion test (m2/s)

51

Colour difference (∆E)

5

0 25

42 0

7

2

100 28 63

15

25

Agent 3

0 54

7

9

17 100

6 45

0 80

43

83 78

10 61

15 59 96

3

62

68

0

3

56 44

47

36

25

61

8

59

100 57

4 57

58 100

68

5

57

100

Agent 2

Figure 12.8 Overall dependence of observed parameters in dyeing with CI Disperse Orange 21 in the presence of a mixture of three agents [89]. The value within each symbol represents a percentage of the maximal effect (= 100%) for that factor

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x

x

841

x

x

x x x

Dye crystal

Dye crystal

Very good thermal stability

Moderate thermal stability

x Dispersant stabilising group

Dispersant adsorbing group

Figure 12.9 Dependence of dispersion thermal stability on dispersing agent structure [91]

of phenolic groups in a lignosulphonate, for example, reduces the thermal stability by an amount that corresponds quite well with the lower concentrations of residual phenolic and carboxyl groups. The degree of adsorption can be determined by equilibriating a known mass of dye with a surfactant solution of appropriate concentration at constant temperature. The dye particles are then separated by filtration or centrifugation, followed by UV analysis of the supernatant liquid to determine the concentration of dispersant remaining. The amount of surfactant adsorbed per gram or per unit area of the dye particles is then calculated from the difference between the initial and final concentrations of the surfactant [91]. From the environmental viewpoint there are two important problems associated with dispersing agents: (1) dustiness of powder brands (2) inadequate and slow biodegradability. Since disperse dye powders supplied to the dyer have been treated already to render them essentially non-dusting, dustiness problems are mainly of concern during dye manufacture [92]. Nevertheless, some dyes remain inadequately treated, or after an initially adequate treatment they may deteriorate during storage, thus giving rise to hazards in handling associated with excessive dustiness. Methods of assessing dustiness have been reviewed [92]. Since dispersing agents are not significantly absorbed by the fibre, they remain in the exhaust dyebath and are discharged to effluent. As a result of their polymeric nature and the presence of stable benzenoid rings, most lignosulphonate and formaldehydenaphthalenesulphonate dispersing agents are only bioeliminated to about 30% [92–94]. They may also contain small amounts of residual starting materials that are toxic to fish [92]. More complete elimination can be achieved by precipitation with heavy metal salts or cationic surfactants, but this leads to problems of disposal of solid wastes. Dispersing agents based on mixtures of sodium salts of arylcarboxylates are claimed to offer superior bioeliminability (70%) and to show markedly improved application properties compared with traditional dispersing agents [93,94]. Dyebath pH exerts a marked influence on the efficacy of lignosulphonate dispersing agents, since this factor determines the degree of dissociation of phenolic and carboxylic acid groups, influencing the extent to which they are able to interact with the dye molecule. In general, the lowest pH that can be tolerated by the system (dye, fibre and auxiliaries) tends to give the

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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES

greatest dispersion stability during heating of sodium lignosulphonates [91]. The situation is somewhat different with amine salts of lignosulphonates, since these differ from the sodium salts in their degree of ionisation. For example, when the pH is lowered from pH 7 to 4, more amino groups are protonated, thus increasing the proportion of ionised sulphonate groups present. The solubility of the dispersant is increased and so the adsorption of agent by the disperse dye particles becomes more difficult. However, the overall situation is complex because lignin derivatives vary in their content of phenolic and carboxylic acid groups and these exhibit a range of pKa values according to their location within the macromolecule. Some azo dyes are susceptible to reduction under unfavourable conditions [95]. The least stable dyes tend to be those containing electron-withdrawing groups, such as nitro, chloro or cyano, ortho to the azo linkage. This instability to reduction is minimised by dyeing at the optimal pH, usually pH 4–5, in the presence of air, and by minimising the dyeing time at high temperature. Hence, under appropriate conditions, instability is not a serious problem. Decomposition is favoured, however, by various factors: (1) pH values greater than 6 (2) the absence of air (anaerobic dyeing conditions) (3) the presence of fibres containing reducing groups, such as wool or cellulose (4) the presence of reducing metal ions, such as copper(I) or iron(II), in the water supply (5) dispersing agents containing phenolic groups (6) conditions that tend to maintain the dye for longer periods in the liquor, such as slowerdyeing substrates (low-porosity sewing threads, for example) and auxiliaries that tend to solubilise the dye too much. Lignosulphonate dispersing agents tend to promote this reduction of sensitive dyes, much more so than the naphthalenesulphonic acid condensation types, probably owing to the presence in lignin of catechol residues and other easily oxidised functional groups [95] (structures 10.104 and 10.105). Commercial lignins vary considerably in their detailed constitution, however, and consequently in their reducing power. In certain cases the problem can be ameliorated by adding an oxidising agent (such as sodium dichromate) to the dyebath, but the effects can be variable and difficult to control. In printing applications, where steam fixation can have a pronounced reductive effect, stronger oxidising agents such as sodium chlorate are often added to the print paste. In theory the reductive tendency of lignosulphonates can be counteracted by chemical blocking of the active phenolic groups but this impairs the dispersing properties of the product [91,95]. Significant improvements can be achieved by replacing the conventional sodium ion in the lignosulphonate salts by other cations [91]; lithium is effective in this respect, but the most promising salt appears to be that of triethanolamine. This compound additionally acts as a chelating agent and so protects against the catalytic influence of iron(II) and copper(I) ions. Since high concentrations of electrolyte can adversely affect dispersion stability, low-salt formulations of dispersing agents have been developed [95]. These also help to minimise the lowering of viscosity by electrolytes with certain synthetic thickening agents in printing applications. In some cases it is necessary to choose dispersing agents that give minimal staining of the substrate. This applies particularly when dyeing nylon since anionic dispersing agents have significant substantivity for this fibre under acidic conditions. In general, lignosulphonates have a greater propensity to stain than have the naphthalenesulphonic acid condensation products.

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12.6.2 pH control and sequestering agents Although many disperse dyes give good results over an extensive pH range (pH 2–9 for example), some will only give satisfactory results over a narrower acidic range (pH 2–6) and a few require careful control to within pH 4 to 5.5. Since practically all dyes give good results at pH 5, this region tends to be regarded as the standard for exhaust dyeing conditions. A simple addition of acetic acid will be satisfactory where water quality permits; otherwise a buffered system is preferred. EDTA (section 10.2.1) is widely used to counteract the effects of metallic impurities, which not only affect the hue and fastness of a few susceptible dyes but may also catalyse dye reduction and promote deterioration of dispersion properties, as described above. In spite of the traditional preference for dyeing at about pH 5, the past decade has seen the promotion of polyester dyeing methods under alkaline conditions at pH 9.0–9.5 [96–98]. Various advantages are claimed for alkaline dyeing conditions. These include the benefits of economy and convenience that arise from dyeing at a pH closer to those used in preparation, including bleaching, mercerising of polyester/cotton and caustic weight reduction of polyester, as well as the clearing of surface dye after dyeing. This approach may eliminate the need for neutralisation or slight acidification after alkaline treatments. Further advantages claimed include improved handle of the substrate, more effective solubilisation and removal of oligomer, less frequent and easier cleaning of machinery and possible avoidance of reduction clearing. Alkaline conditions facilitate the simultaneous application of disperse and reactive dyes to polyester/cellulosic blends. Apart from the essential primary requirement of selecting disperse dyes that are stable to pH 9.5 at least, or preferably higher to ensure a safety margin, the choice of auxiliaries for this process is critical. The main requirement is a buffer system having sufficient reserve capacity to maintain pH 9 throughout most of the dyeing process. This is more difficult than might be assumed, since the polyester fibre and the oligomers present are partially hydrolysed by alkali at a high dyeing temperature to form carboxyl and other groups that cause a gradual lowering of the pH. Consequently, dye manufacturers have introduced alkaline dyeing ‘packages’ comprising a selected range of stable disperse dyes together with a purpose-designed auxiliary system to maintain the required pH. Little has been published about the detailed composition of such systems. However, they are claimed to contain more than just a suitable buffering system. For example, one such auxiliary is claimed [96] to be designed to (a) stabilise the dyes, (b) provide adequate buffering, (c) chelate metal ions and (d) assist dissolution of oligomers, whilst a second auxiliary is offered for use where there is an unusually high content of oligomer. Attention must be given to dispersion stability and to oligomer control. This is because certain types of dispersing agent have inferior efficiency under alkaline conditions and more oligomer is released into alkaline liquors. Unless the oligomers are adequately dispersed or solubilised they may contribute to dye dispersion problems and may be deposited onto the fibre or machinery. Hence, careful thought must be given to the selection of dispersing or solubilising agents. With regard to the buffer system, an extensive range of amino acid derivatives applied in combination with an alkali have been claimed [99]. From this extensive list, primary preference is given to N,N-bis(hydroxyethyl)glycine (12.24) in combination with sodium hydroxide. However, N,N-dimethylglycine, N-methylglycine and N-methylalanine are also listed as preferred compounds, whilst other possible alkalis include sodium carbonate,

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sodium bicarbonate or borax. Another system depends on the formulation of a mixture of a phosphonate and a polycarbonic acid in combination with sodium hydroxide, borax, sodium carbonate or bicarbonate [100]. Whichever buffer system is used, extensive empirical trials are required to determine the balance and concentration level needed to ensure stability of pH during the specific conditions of use. CH2CH2OH

O C

CH2

N CH2CH2OH

HO 12.24

12.6.3 Electrolytes Electrolytes are unnecessary for the application of disperse dyes alone. Nevertheless, electrolytes will be present when applying disperse dyes together with direct or reactive dyes in the dyeing of fibre blends. In particular, the high concentration of salt often used with reactive dyes can have an adverse effect on dispersion stability and may also interfere with the stability and/or efficacy of other auxiliaries, particularly those based on emulsion systems. These effects are often attributed to the destabilising influence of inorganic ions on the forces of attraction between disperse dye particles and dispersing agents. Manufacturers take these effects into consideration in marketing ‘electrolyte-stable’ formulations of disperse dyes or auxiliaries. 12.6.4 Levelling agents It is necessary to distinguish clearly between levelling agents and dispersing agents. The primary function of a dispersing agent is to maintain a stable dispersion. Since most of these agents enhance the low water solubility of disperse dyes they may improve level dyeing, although they vary significantly in this effect. Maximal dispersion stability is usually attained with agents that maintain dye particles at constant size and minimal solubility. Hence primary dispersing agents seldom enhance levelling; different auxiliaries are added where levelling action is needed. These are invariably anionic or nonionic surfactants and they tend to solubilise the dye much more effectively. Some anionic levelling agents are able to promote dispersion stability but nonionic types have a destabilising effect and great care is therefore required in selection. It is useful to consider how levelling agents can adversely affect dispersion stability. As dyebath temperature increases, thermal effects tend to cleave the film of dispersing agent protecting the dye particles. High shear rates in jet dyeing machines and additives such as electrolytes, fibre lubricants or sizes, as well as oligomers from the fibre, can contribute to this effect. Commercial batches of the same dye brand may behave differently according to the initial dispersion quality of the dye. A dispersion of a vulnerable dye then tends to deteriorate, resulting in crystallisation and agglomeration. The types of precipitation that can occur have been described [87,88,101] and are illustrated diagrammatically in Figure 12.5 (section 12.6.1). Suspended dye crystallites tend to agglomerate, eventually forming larger crystals. The dissolved dye molecules are able to diffuse into the fibre, but under adverse conditions

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Relative dye uptake after 120 min

crystallisation is favoured. Once seeded, the crystals may grow in size whilst retaining their original form, or they may undergo a transformation from the original thermodynamically metastable form to a more stable but less soluble form. On the other hand, such crystals may not form until the incompletely exhausted dyebath is cooled after the dyeing process; this problem may be avoided by blowing off the dye liquor at 125–130 °C. Precipitation by agglomeration tends to predominate with those dispersing agents that do not enhance dye solubility significantly, whereas crystallisation is more prevalent with levelling agents or dispersing agents having greater solubilising power. It is interesting that surfactant additions may be used during dye synthesis in order to obtain the dye in the optimal form for isolation and subsequent milling; for example, very fine crystals may clog filters, whilst thin needlelike crystals tend to mill more easily than platelets. Anionic levelling agents, especially the polyelectrolyte dispersing agents described previously, are generally preferred as the primary addition [102], particularly where it is desired to promote level dyeing by control of exhaustion during the heating phase of dyeing; higher concentrations have a greater retarding effect. Few of these anionic products promote dye migration, a characteristic that is useful if a more powerful levelling action is required. Nonionic surfactants, on the other hand, tend to solubilise the dye much more effectively and thus contribute to level dyeing both by a retarding effect and through the promotion of migration. Consequently they are generally more powerful levelling agents than anionic products although their effects are much more dye-specific. The dye-specific effects on retarding and restraining have been well-publicised [103–107], although the full extent of the variations is often overlooked in the industry. The restraining effects of a typical nonionic agent, a nonylphenol with an average of 20 ethylene oxide units per molecule, on five commercially important disperse dyes are illustrated in Figure 12.10. The spread of results between CI Disperse Blue 56 (only slightly affected) and CI Disperse Yellow 42 (much more affected) should be noted. Other nonionic auxiliaries would yield different effects and dyes may behave differently in mixtures compared with their response when tested in isolation. The retarding effects of the same agent on CI Disperse Blue 56 and Yellow 42 applied as a green 1:1 mixture are shown in Figure 12.11.

1.0 0.8 0.6 0.4 0.2

CI Disperse Blue 56 Yellow 5 Red 60 Red 82 Yellow 42

1

5

10

Agent concentration/g l–1

Figure 12.10 Relative dye uptake values for five disperse dyes on polyester at various concentrations of a nonylphenol 20 EO levelling agent [107]

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Dye uptake/mg dye per g polyester

CI Disperse Yellow 42

3

Agent/g l–1 1 5 10

2 1 CI Disperse Blue 56

3

Agent/g l–1 1 5 10

2 1

Temperature 90 Time

100

110

120

130 30

60

90

130 oC 120 min at 130 oC

Figure 12.11 Rate of dyeing curves for CI Disperse Blue 56 and Yellow 42 in admixture on polyester at various concentrations of a nonylphenol 20 EO levelling agent [107]

This complexity of response has critical implications for reproducibility of dyeing, particularly within the context of right-first-time dyeing. Careful prior evaluation and optimisation of each recipe, followed by consistent bulk use and monitoring, are essential for good reproducibility. Instrumental colour difference measurements are particularly useful for evaluating and monitoring responses [107,108]. On the other hand, a study of four different polyethoxylated sorbitan esters indicated that there were no significant differences between them in terms of desorption of disperse dyes from polyester [109]. Desorption, however, is only one aspect of level dyeing and migration. A major problem with nonionic agents arises from their inverse solubility. Thus an agent with a low cloud point may increase dye precipitation, although once again the effect is dyespecific. Published data suggest that a nonylphenol with a low degree of ethoxylation, having a cloud point of about 40 °C, should not be used as a disperse dye levelling agent [110]. A product of type 12.25, having a cloud point of about 105 °C, should be satisfactory for dyeing at any temperature up to 100 °C but should be avoided at higher temperatures. On the other hand, a carefully selected nonionic agent may be mixed with an anionic agent to raise its cloud point. For example, a mixture of the fatty acid ethoxylate 12.26 with 7–10% sodium dodecylbenzenesulphonate [110] has a cloud point of about 150 °C and is suitable for use in high-temperature dyeing. O CH3(CH2)15(OCH2CH2)17OH 12.25

CH3(CH2)7CH

CH(CH2)7

C (OCH2CH2)14OH

12.26

Surprisingly, other investigators were unable to confirm the adverse effect of nonionic surfactants of low cloud point in the high-temperature dyeing of polyester, even in the presence of electrolytes [111]. This was probably because of the rather low concentrations used. Adducts containing a C18–C20 hydrophobe and a decaoxyethylene hydrophile, as well

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as sorbitan ester ethoxylates were shown to be particularly effective levelling agents. Certain dyes were found to be sensitive to nonionic additives under the stringent conditions of the laboratory dispersion tests carried out in the absence of fibre, but it was pointed out that these systems still performed satisfactorily in actual dyeings [111]. An alternative type of levelling system contains a mixture of ethoxylates with aliphatic esters [112]. This combination exerts a retarding effect on many dyes during heating up to about 100–110 °C, especially if the dyes are present in low concentration. At higher temperatures this retarding effect is increasingly offset by the accelerating effect of the aliphatic esters. This temperature-dependent interaction is said to improve the compatibility of combinations of dyes applied with this system. The adverse effect of nonionic adducts of low cloud point can be avoided by the use of hybrid agents of the ethoxylated anionic type, variously and confusingly referred to as ‘modified nonionic’, ‘modified anionic’ or ‘weakly anionic’ types. Thus Mortimer [113] has proposed the use of products of the ethoxylated phosphate type (12.27). In this structure, R, as well as the degree of ethoxylation (n) may be varied to optimise the overall HLB value. The numerous ether groups are said to enhance the dye-solubilising and levelling capacity, whilst the polyphosphate grouping exerts several useful effects [113]. These compounds: (1) are sufficiently anionic to avoid most of the disadvantages of conventional nonionic agents with regard to high-temperature instability and the lack of an electrical double layer of value in dispersion stability (2) behave similarly to more orthodox polyphosphate sequestrants, thus offering some protection from hard water and other trace metal impurities (3) maintain effective stability in high concentrations of electrolyte (4) offer possibilities for pH control by varying the nature of M (5) are fully effective at pH 4–5, the most useful pH range for application of disperse dyes, whereas conventional nonionic types are said to become less effective as levelling agents at pH values less than 7. O R

(OCH2CH2)n

O

P

O

O

M

M x

12.27 R n x M

= = = =

hydrophobe typically 10–20 typically 1–3 H, alkali metal or organic base

Hence these agents are sophisticated multifunctional auxiliaries, which can take the place of separate additions of levelling agent, sequestering agent and a buffer to control the pH. Nonionic surfactants can be beneficial in minimising the redeposition of the sparingly soluble polyester oligomers that are released from polyester fibres during high-temperature dyeing. Ethoxylated multi-ester compounds (so-called ‘oligo-soaps’) have been promoted recently as dispersing/levelling agents [114]. These contain a multi-branched hydrophobe with

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pendant carboxyl groups that are esterified with poly(ethylene glycol). Thus they are similar in structural class to a mono-ester (so-called ‘mono-soap’) of structure 12.28, a conventional condensate of a fatty acid of high molecular mass with poly(ethylene glycol) but the multi-ester has a much higher relative molecular mass (Table 12.6). A micelle formed from an ethoxylated mono-ester is an aggregate of several molecules, whereas the individual molecule of an ethoxylated multi-ester is said to behave as a micelle, thus exhibiting a much lower critical micelle concentration. The thermal stability of a multi-ester is much greater because this macromolecular structure resists thermal agitation to a much greater degree. This maintains a more stable dye dispersion at high temperature under conditions of high shear. Further advantages include: (1) solubilisation of the dye takes place at a lower temperature (2) strike rates at lower temperatures in the dyeing cycle are much slower (3) solubilisation of oligomer and acrylic size (4) low foaming. O R

C (OCH2CH2)n

OH

12.28 R = fatty alkyl group

Table 12.6 Comparison of characteristic properties of mono-ester and multi-ester compounds [114]

Relative molecular mass Critical micelle concentration Foaming tendency Oligomer solubilisation Size solubilisation

Mono-ester

Multi-ester

Small 0.400 g/1 High No No

Large 0.004 g/1 Low Yes Yes

12.6.5 Carriers Although polyester or cellulose triacetate fibres are normally dyed at high temperatures, their blends with wool are still dyed at or near the boil. In such cases an auxiliary termed a carrier must be used to promote adequate exhaustion of disperse dyes by the ester fibre within a commercial dyeing time. Even in high-temperature dyeing, there are occasions when the usual maximum temperature (around 130 °C for polyester) cannot be used, as when dyeing qualities of texturised polyester that suffer loss of crimp at 130 °C. Carriers are then used to assist more rapid and complete exhaustion, using smaller amounts than at or near the boil. Carriers are sometimes employed to promote migration of unlevel dyeings. The active component of a carrier formulation is generally a nonionic compound of Mr 150–200 containing a benzenoid ring system. A comprehensive review listed the classes of compounds used together with their general properties, ideal requirements and the mechanisms that have been proposed for carrier action [115]. Carrier compounds fall into four main classes: phenols, primary arylamines, aryl hydrocarbons and aryl esters. Major

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representatives of these in commercial use include o-phenylphenol, biphenyl, methylnaphthalene, trichlorobenzene, methyl cresotinate, methyl salicylate (sometimes mixed with phenyl salicylate), butyl benzoate, ethers of 2,4-dichlorophenol, diethyl or diallyl phthalate and N-alkylphthalimide derivatives. Benzaldehyde has been used for the dyeing of aramid fibres [116,117]. Over the last decade the use of carriers has declined markedly and continues to do so, essentially for health, safety and environmental reasons [118–121]. In some countries these products are now virtually banned. Nearly all carrier compounds exhibit all or some of the following: toxicity, physiological irritancy or poor biodegradability (Table 12.7). Typical pollution loads for comparable high-temperature and carrier methods are given in Table 12.8. Table 12.7 Chemical and biochemical oxygen demand data for various types of carrier chemical [118] Carrier type

COD (mg/l)

BOD5 (mg/l)

o-Phenylphenol N-Alkylphthalimide Arylcarbonate ester Methyl cresotinate Dichlorobenzene Trichlorobenzene

1000–2000 1000–2100 900–1900 800–1700 500–1000 300–1000

200–800 100–200 700–800 200–800 0 0

Table 12.8 Chemical and biochemical oxygen demand data for hightemperature and carrier dyeing methods [120] Polyester dyeing method

Liquor ratio

BOD5 (mg/l)

High-temp. jet dyeing

40:1 40:1

165

Carrier dyeing on the winch

40:1 20:1

200 189

COD (mg/l)

BOD5: COD

Harmful factor*

584 722

1:4.4

140.2 72.2

2043 1888

1:10.2 1:10

408.6 188.8

* Harmful factor = g COD per kg of dyed goods

Harmful effects from carrier dyeing can arise in three ways: (1) residual carrier in the dyebath contributes to effluent pollution and may be environmentally harmful (2) carrier that is volatilised during dyeing or subsequent heat setting becomes an atmospheric contaminant (3) residual carrier in the fibre can be a health hazard, as well as causing an unpleasant odour on heating or during storage. The degree of carrier action is important in practice and varies considerably. For example, the phthalates have little action on polyester but are efficient on cellulose triacetate, for

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which they are the most widely used compounds. Ortho-phenylphenol and the chlorinated benzenes are generally powerful carriers for polyester, whilst methylnaphthalene and particularly butyl benzoate are less powerful, Although all carriers tend to promote the exhaustion of dyes, some degree of dye-specific behaviour results from the respective hydrophobic/hydrophilic balance of dye and carrier [102,109]. Some carriers, such as o-phenylphenol, tend to lower the light fastness of many dyes if carrier residues remain in the dyed fibre; others, such as the chlorobenzenes, have no effect on this property. Similarly, carrier residues differ considerably in odour. A dry heat treatment at 160–180 °C after dyeing, to volatilise the residual carrier, is the best method of minimising problems with light fastness and odour. The steam volatility of a carrier and its toxicity to human and plant life need careful consideration. For example, o-phenylphenol has relatively low volatility in steam and traditionally has been used in machines open to the atmosphere. The chlorinated benzenes, on the other hand, are readily steam-volatile and are toxic, so should not be used in machines where volatilised carrier is likely to condense (for example, on a cooler lid) since drops of condensate may cause ‘carrier spots’ if they fall onto the fabric. Biphenyl is relatively non-toxic to river life but is not readily biodegradable; methylnaphthalene, also of low toxicity, is moderately biodegradable, but halogenated benzenes are both toxic and difficult to biodegrade. Some carriers such as chlorinated benzenes and butyl benzoate are relatively efficient in promoting migration; others, such as o-phenylphenol, are less so. When dyeing a blend such as polyester/wool it is useful to consider the extent to which the carrier will promote migration of dye to polyester so as to minimise staining of the wool. All the carrier compounds mentioned above have little or no solubility in cold water. They are therefore used in the form of emulsions, many being marketed as ‘self-emulsifiable’ liquids that form stable emulsions on dilution in the dyebath. The choice of emulsifying system is very important, not only from the viewpoint of emulsifying the active carrier component, but also to ensure stability of the emulsion under dyebath conditions and compatibility with dyes and dispersing agents, as well as efficacy of carrier action. Thus two carriers of identical active components but with different emulsifying systems may well differ appreciably in behaviour. Two typical formulations [122] are given in Table 12.9, both being completely solubilised concentrates that on dilution in the dyebath give stable emulsions of good dyebath compatibility. The weakly anionic ethoxysulphates and ethoxyphosphates are especially useful emulsion bases for carriers. A small amount of a simple organic solvent such as ethanol may also be added to improve stability. Most commercial carriers are used in the dyebath at concentrations within the range 1–8 g/l depending on active strength of the carrier concentrate, applied depth, liquor ratio and

Table 12.9 Typical examples of carrier emulsions [122]

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Formulation 1

Formulation 2

90% Diethyl phthalate 10% Ethoxylated castor oil (40 mol ethylene oxide)

40% Phenyl salicylate 40% Methyl salicylate 20% Ethoxylated nonylphenol (20 mol ethylene oxide)

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other dyeing conditions. Although carriers exhibit dye-specific properties to some extent a particular carrier will generally have an optimal concentration in the dyebath to give maximum dye yield; higher concentrations will tend to solubilise the dye to such an extent that colour yield is depressed. 12.6.6 Aftertreatments and thermomigration Cellulose acetate and nylon dyed with disperse dyes are usually given a simple rinse with or without a synthetic detergent (anionic or nonionic) after dyeing. Most cellulose triacetate is similarly treated; however, some full depths are given a clearing treatment to remove surface dye so as to improve the fastness properties. Such a clearing treatment is more generally important with polyester dyeings. It most frequently takes the form of a reduction clear using 1–2 g/l sodium dithionite in alkaline solution. For triacetate the preferred alkali is ammonia (1–2 ml/l of s.g. 0.800) at temperatures up to 60 °C. Polyester will tolerate more severe conditions; hence the alkali is usually 1–2 g/l sodium hydroxide used at temperatures up to 70 °C, or even higher in continuous ‘short-dwell’ processes. This treatment works mostly by reductive fission of azo dyes and by converting anthraquinone dyes to their soluble leuco forms. It is also advantageous to use 1–2 g/l of a nonionic surfactant in the reduction clear to assist solubilisation of the reduction products and in some cases their thorough removal is ensured by a subsequent treatment with a nonionic detergent alone. Fatty acid ethoxylates of the type mentioned earlier (structure 12.26) are excellent nonionic agents for use in reduction clearing. This process, however, is not only expensive in itself, but creates additional expense through the need to deal with an environmentally unacceptable effluent. There is also the cost and inconvenience in carrying out two changes of pH: first from the acidic dyebath to the alkaline reduction clear, followed by neutralisation of the substrate after the reduction clear. It is not surprising, therefore, that reduction clearing is nowadays avoided as much as possible. One possibility is to use specialised dyes that can be cleared with alkali alone (section 4.9.2): this avoids the environmental nuisance of the reducing agent but still leaves alkali and the need for two pH changes. Alternative reducing agents are still sometimes proposed and evaluated. A detailed comparison of five reducing agents has been reported: sodium dithionite, thiourea dioxide, iron(II) chloride/gluconic acid, sodium hydroxymethanesulphinate and hydroxyacetone [123]. Results of fastness tests on black polyester dyeings variously aftertreated are given in Table 12.10. Hydroxyacetone must be used at temperatures above 80 °C on account of its sluggish action. Nevertheless it did not give adequate improvement of fastness to washing. It gives high COD values and has an unpleasant smell. The reducing power of sodium hydroxymethanesulphinate, even with anthraquinone as activator, is insufficient under these conditions. It did not give adequate improvement of washing fastness. Iron(II) chloride has the environmental advantage that it does not contain sulphur but the gluconic acid complexing agent results in relatively high COD values. Improvement of washing fastness was inadequate. Only thiourea dioxide gave results as good as sodium dithionite. It is three times more expensive but causes only half the sulphur pollution of dithionite. The relative usefulness of these two reducing agents really depends on the dyeing process. In the winch, the slow production of active species from thiourea dioxide is a disadvantage when working

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Table 12.10 Fastness of black polyester dyeings after various reduction clearing treatments [123] Fastness properties Perspiration Reduction clear

Washing at 60 °C

Acidic

Alkaline

Untreated control Hydroxyacetone Sodium hydroxymethanesulphinate Same, with anthraquinone activator Iron(II) chloride/gluconic acid Thiourea dioxide Sodium dithionite

1–2 2–3 2–3 3 3 4–5 4–5

2–3 4–5 4–5 4–5 4–5 4–5 4–5

2–3 4–5 4–5 4–5 4–5 4–5 4–5

with the counterflow. On the other hand it could be an advantage in jet dyeing machines, although this only arises if reduction of the dye occurs relatively quickly. In closed machines, sodium dithionite is more effective. Several reduction clearing auxiliaries have been introduced under commercial brand names, their composition in many cases not being revealed. At least one of these is designed to be used under acidic conditions [124]. Advantages claimed for this product include: no pH changes needed, low COD, high biodegradability, low toxicity, with further savings of time and water consumption. Moreover, since the agent is added directly to the exhaust dyebath any residual dye present is decolorised before discharge to effluent. Although highly effective with the majority of dyes, in a few cases (e.g. CI Disperse Yellow 29, Violet 35 or Blue 56) a higher concentration is needed. Polyester dyed with disperse dyes generally shows excellent fastness to wet treatments and rubbing after thorough reduction clearing and drying at low temperature (below 120 °C), irrespective of the dyes used. However, cost-effective production and the requirements for fabric dimensional stability demand the use of a combined drying and heat setting treatment at temperatures in the range 150–210 °C, most frequently at 180 °C. This causes some dyes to migrate from the core of the fibre to the surface, thus tending to negate the effect of reduction clearing. This surface dye is a potential source of lower fastness to rubbing and wet treatments [125–127], although the extent to which this occurs depends greatly on the dye and its applied depth. This phenomenon has been termed ‘thermomigration’, its effect on fastness varying considerably because of the generally adverse influence of surfactants, lubricants, softeners, antistats and so on. Similar problems occur on cellulose triacetate. A method for assessing the influence of auxiliaries on thermomigration has been published [128]. This is carried out with 1/1 standard depth CI Disperse Blue 56 (or other suitable dyeings) and the fastness to an ISO C02 washing test is determined after reduction clearing and stentering at specified temperatures. A detergent-based, rather than soap-based, washing test would be more critical [129]. The mechanisms operating during thermomigration in the presence of a surfactant have been evaluated experimentally [130]. The overall mechanism consists of four main processes:

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(1) extremely rapid attainment of equilibrium between dye in the surfactant layer and dye in the surface zone of the fibre (2) rapid diffusion of the dye molecules from the interior of the fibre towards the surface (3) slower diffusion of surfactant molecules into the substrate phase (4) eventual formation of a composite dye–fibre–surfactant phase in the surface region. In fact thermomigration readily takes place in the absence of surfactant, albeit usually to a lesser extent. In this case only the second of the above processes takes place. The term ‘surfactant’ in this model can be interpreted broadly to include any residual surfactant, reduction clearing assistant or applied finish, such as an antistat, lubricant or softener. No general correlations exist between the degree of thermomigration and the structure of a dye, its molecular mass, diffusion coefficient or fastness to sublimation. Nevertheless, to a limited extent, in a series of disperse dyes of closely related constitution there does appear to be some relation between the hydrophobic–hydrophilic balance of the dye molecule and its susceptibility to thermomigration. In such restricted series of related dyes, thermomigration decreases with increasing hydrophobicity [130]. This could be related to the strength of dye–fibre hydrophobic bonding, stronger bonding tending to limit migration of dye to the fibre surface. There seems to be a relationship between thermomigration and the degree of interaction between dye and surfactant, more specific interaction leading to greater thermomigration [130]. The degree of interaction in this work was deduced from specific conductivity measurements. It is important to recognise that the degree of thermomigration in itself is not necessarily indicative of any practical implications that may show up in fastness tests [129]. For example, CI Disperse Blue 60 thermomigrates to an appreciable extent, as measured by solvent extraction of the dyed fibre after stentering. Nevertheless, in most wet fastness tests it still gives excellent results simply because it has relatively low substantivity for adjacent fibres (especially nylon) in wet fastness tests. Conversely, a dye may show very little actual thermomigration yet give poor wet fastness after stentering on account of its high substantivity for nylon under the conditions of test. Direct measurements of dye diffusion behaviour generally have little practical significance in themselves, unless they can be related to the effects on fastness properties. Problems arising from thermomigration are best avoided by selecting dyes that show acceptable washing fastness after heat setting treatment and by ensuring that all surfactants from dyeing and afterclearing are completely rinsed out. Careful choice of finishing agents and finishing conditions is also important. In heat setting and curing, for example, temperature has a greater effect than time in promoting thermomigration [131]. Thus improved fastness to rubbing and wet treatments may be achieved using a selected durable press/softener finish (incorporating a rapidly reacting resin/catalyst system) giving the required finish effect at 140–160 °C. The longer curing time required at a lower temperature has a less deleterious effect than a higher temperature (such as 180 °C) for a shorter time. Another means of minimising the effects of thermomigration is to apply certain mildly reducing chemicals after dyeing. The application by padding of a polysiloxane and an organotin catalyst along with any other finishing agents [131,132] gives rise to a reducing effect during subsequent dry heat treatment that is capable of decomposing many dyes brought to the surface by thermomigration. Such products do not work successfully with all dyes and finishes, however, and can confer a degree of water repellency that is not always desirable or even acceptable.

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The presence in polyester fibres of polymerisation by-products (oligomers) can give rise to problems, particularly if their concentration is greater than normal. With polyesters based on ethylene glycol and terephthalic acid, the amount of oligomer is normally between 1.4 and 1.7%, consisting mainly of the cyclic trimer of ethylene terephthalate with smaller amounts of a pentamer, a dimer containing diethylene glycol residues and traces of other compounds. Significant migration of such oligomers from within the fibre can occur at dyeing temperatures of 110–135 °C, leading to deposits on the fibre and/or machine surfaces and sometimes also to interference with dispersion stability, since dispersed oligomer particles form potential nuclei for the crystallisation and agglomeration of disperse dyes. Discharge of the spent hot dye liquors without prior cooling is the best way of avoiding oligomer problems. Reduction clearing will normally remove any deposits from the fibre surface. Deposits on machine surfaces must be removed by regular cleaning at high temperature with strong solutions (5 g/l) of sodium hydroxide together with thermally stable surfactants and solvents. 12.6.7 Continuous dyeing The conventional method of continuous dyeing with disperse dyes is the pad–thermofix process [133,134], most frequently used for polyester/cellulosic blends although it can be used with 100% polyester (or cellulose triacetate) materials. The auxiliaries normally used at the padding stage include a thickening agent as migration inhibitor and a wetting agent. Alginates and other polyelectrolytes such as polyacrylamides are popular as migration inhibitors. Anionic sulphosuccinates are suitable wetting agents; since cloud point problems do not arise in continuous dyeing to the same extent as in batchwise processes, nonionic ethoxylates may also be used, often fulfilling a dual role as wetting and levelling agents. An addition of acetic acid to give pH 5–6 is usually adequate when applying disperse dyes alone, although certain processes may demand selection of dyes stable at higher pH, as in the combined alkaline application of disperse and reactive dyes to polyester/cellulosic blends. In some processes, too, the use of hydrotropes such as polyglycols and their esters, as well as of urea and related compounds, can be useful to enhance the degree of fixation. 12.6.8 Printing Printing with disperse dyes is generally carried out using a thickening agent and an acid donor to maintain a low pH during steam fixation. High-solids thickeners such as crystal gum or British gum give optimal sharpness of outline but suffer from the disadvantage of forming brittle films [29]. Hence low-solids thickeners such as alginates and locust bean ethers, which form more elastic films and are more easily removed in subsequent washingoff, are preferred. Further additions may include a fixation accelerator (hydrotropes such as urea, thiodiethylene glycol, cyclohexanol, dicyanoethylformamide) or a carrier and an oxidising agent, such as sodium chlorate or sodium m-nitrobenzenesulphonate, to inhibit the possible reduction of susceptible dyes during steaming. The dispersing agent present in the disperse dye formulation can have an influence on printing problems involving loss of viscosity of the print paste and reduction of some azo dyes. Loss of print paste viscosity is particularly associated with synthetic thickeners. Nonionic formulations of disperse dyes were developed to counteract this problem; in these brands the usual anionic dispersing agent was wholly or substantially replaced by a nonionic system.

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Freedom from electrolytes is also desirable for much the same reasons. The nature of the cations associated with the sulphonate groups in lignosulphonate dispersing agents is important, since it affects the hydrophilicity of the agent, this aspect having the primary influence on the absorptive behaviour of the dispersant. The concentration of ionised functions affected by the nature of these cations correlates directly with conductivity, printing paste viscosity and azo dye reduction [91]. A detailed investigation of three inorganic (Na, K, NH4) and six amine salts of lignosulphonates showed the triethanolamine salt in particular to offer the greatest benefits. The results of these conductivity and print paste viscosity measurements are shown in Table 12.11. Apart from the beneficial effects of the triethanolamine salt on print paste viscosity, this agent showed the least sensitivity to pH, did not promote reduction of azo dyes and was an effective sequestrant for dissolved iron and copper ions. Table 12.11 Conductivity and print paste viscosity data for various salts of lignosulphonate dispersant [91]

Lignosulphonate cationic salt

Conductance (m.mhos) at 5% dispersant concentration

Print paste* viscosity (cps) at 25 °C

Nonionic control Conventional low-sulphonated Na salt Low-electrolyte (Na) lignosulphonate Dimethylamine salt Trimethylamine salt Triethanolamine salt

– 9.80 5.26 4.75 4.34 3.31

71 000 1 800 29 000 23 000 27 500 41 000

* Viscosity sample: 8g dispersant in 970 ml water at pH 7 added to 30 g synthetic thickening

For polyester, the washing-off process to remove unfixed dye and thickening agent is generally a reduction clear as described in section 12.6.6. A simple wash-off with nonionic surfactant must be used on cellulose acetate or triacetate, although a mild reduction clear may be preferable on triacetate. Discharge effects on acetate are carried out by overprinting dyed grounds with a thickened paste containing the reducing agent thiourea dioxide and thiodiethylene glycol; a disperse dye stable to these reducing conditions may be added to a similar paste to give a contrast illuminated effect. Similar effects may be produced on polyester by printing a discharge (reducing) paste onto fabric that has been padded with dye; reduction of the discharge areas and simultaneous fixation of dye in the undischarged areas then takes place during subsequent steaming. The discharge paste may contain a reducing agent such as zinc formaldehydesulphoxylate or tin(II) chloride, although special ranges of alkali-dischargeable dyes are available that require only alkali (section 4.9.2). Reduction-stable dyes may be added to the discharge paste to create illuminated effects. More detailed recipes are available elsewhere. 12.6.9 Stripping Non-destructive stripping can be carried out at dyeing temperature with surfactants, a

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nonionic type with a high cloud point being particularly effective. In the case of polyester the stripping effect can be increased markedly by adding a carrier that has migrationpromoting properties, the chlorobenzenes and butyl benzoate being particularly effective, although this is now much restricted on health, safety and environmental grounds. The efficiency of destructive stripping depends on the fibre and dye types. The most usual method is to use a reduction process (alkaline sodium dithionite) together with a nonionic surfactant and, where possible, a carrier, the temperature being varied to suit the fibre. In some cases, particularly with anthraquinone dyes, an oxidation treatment (chlorite, hypochlorite or permanganate) may be more efficient. Occasionally a sequential combination of oxidation and reduction treatments may have to be used.

12.7 REACTIVE DYES 12.7.1 Cellulosic fibres As described in Chapter 7, the various ranges of reactive dyes for cellulosic fibres differ considerably in their reactivity and the number of application procedures is bewilderingly large, including numerous variants within each category of batchwise, continuous, semicontinuous and printing methods. Even a specific method may require modifications to suit a particular quality or form of substrate, dyeing machine or the specific dyes selected from a given range. Fortunately certain general principles are applicable to the great majority, if not all, of these methods. The characteristics of each range of reactive dyes and details of their application methods are fully described elsewhere [30], but in the working situation it is especially important with reactive dyes to consult the dye manufacturer’s literature. Recent years have seen considerable research into the modification of cellulose and reactive dyes, specifically to overcome some of the drawbacks of this dye–fibre system, including the limited degree of fixation in full depths, the need for alkali and relatively high concentrations of electrolyte. This research, which is driven by environmental considerations, was discussed in sections 7.10 and 10.9.1. Thus it need not be considered further here. Exhaust dyeing The critical importance of substantivity and its overriding influence on application properties has been described in sections 3.2.1, 3.3.2 and 7.5, as well as elsewhere [30]. The essential auxiliaries used to control reactive dyes in batchwise dyeing are electrolyte and alkali. Secondary auxiliaries may include sequestering agents, mild oxidising agents to prevent reduction of certain sensitive dyes and wetting or levelling agents. The classic procedure for dyeing with reactive dyes involves application under substantially non-reactive conditions by exhaustion with electrolyte at a temperature selected according to the reactivity of the particular dyes used, followed by addition of alkali to enhance absorption and, more particularly, to create the conditions through which the dyes can react covalently with the fibre. In the so-called ‘all-in’ process electrolyte and alkali are present together throughout to bring about simultaneous sorption and reaction, though this inevitably increases the opportunity for hydrolysis of the dye in the dyebath. Application temperatures vary from room temperature to the boil or even higher.

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There is as yet no official method of classifying reactive dyes according to their dyeing properties, unlike the situation with direct or vat dyes. Nevertheless, a promising preliminary scheme has been proposed [135] and it is worthwhile presenting the rationale here, since this scheme has a bearing on the use of auxiliaries insofar as they affect levelling properties. Group 1 – Alkali-controllable reactive dyes Dyes that have their optimal fixation temperature between 40 and 60 °C belong to this group. These dyes are characterised by relatively low exhaustion in neutral solution before the addition of alkali. This type of dye has high reactivity and careful addition of alkali must be made in order to obtain level dyeing. For these reasons, the name ‘alkali-controllable reactive dyes’ has been chosen. Typical examples of dyes belonging to this group are dichlorotriazine, dichloroquinoxaline, difluoropyrimidine and vinylsulphone dyes. Group 2 – Salt-controllable reactive dyes This group includes dyes that have their optimal fixation temperature between 80 and 95 °C. Such dyes show comparatively high exhaustion before fixation so it is important to ensure that dyeings are level. Salt should be added portionwise at specified stages during the exhaustion process, hence they are termed ‘salt-controllable reactive dyes’. Typical examples of dyes belonging to this group are aminochlorotriazine, bis(aminochlorotriazine) and trichloropyrimidine dyes. Group 3 – Temperature-controllable reactive dyes This group includes dyes that react with the fibre at 100 °C or above, without alkali present. Dyes in this group have self-levelling properties so there is no need to exercise control by means of dyeing auxiliaries. Good results can be obtained by controlling the rate of temperature rise. At present only the Kayacelon React (KYK) range of bis(aminonicotinotriazine) dyes represent this group. Sodium chloride is undoubtedly the most widely used electrolyte, a particular advantage being its ease of dissolution. Certain dyes, such as brilliant blue phthalocyanines and anthraquinones, are susceptible to aggregation and sometimes even precipitation in its presence, however, and in these cases sodium sulphate, which has a lesser aggregating effect, is preferred. The electrolyte should be free from calcium and magnesium salts and from alkali to avoid premature fixation in a two-stage process. The electrolyte functions with reactive dyes in a manner similar to that with direct dyes, but as reactive dyes are more highly sulphonated and hence less substantive than direct dyes, more salt is required to attain equivalent exhaustion. As with direct dyes, the higher the concentration of salt the greater the uptake of dye, provided over-aggregation and precipitation do not occur. The primary objective is to achieve maximal exhaustion over an optimal dyeing period, taking care to ensure level uptake since this is the only phase during which the rapidly diffusing reactive dyes can migrate. Hence electrolyte may be added in portions over the whole dyeing period. As might be expected with highly soluble dyes, liquor ratio has a pronounced effect on

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exhaustion and thus on the amount of electrolyte needed, especially with dyes of low substantivity. Decreasing the liquor ratio from 30:1 to 5:1 would justify lowering the electrolyte concentration to one-sixth but in fact even less than this can be used because of the marked effect on exhaustion of the reduction in liquor ratio. There is another important effect of electrolyte: initially the internal pH of the fibre is lower than that of the dyebath, a state that tends to favour hydrolysis of the absorbed dyes; adding electrolyte tends to equalise these pH values, thus protecting against hydrolysis. In general, the longer the liquor ratio, the lower the dye substantivity and the greater the applied depth, then the higher is the concentration of electrolyte required. Not long ago a rather cavalier attitude existed towards the consumption of electrolyte, based on the prevailing philosophy that ‘salt is cheaper than dye’ [136]. This outlook led many dyers to use as much as 20–30% more salt than recommended by dye manufacturers (100–150 g/l being not unknown) in an effort to secure maximum exhaustion. A graphic illustration of the total amount of salt used in reactive dyeing worldwide has been provided [137]: this is 1.8 million tons p.a., equating to 80 000 loaded rail wagons that would stretch for 1000 km from Paris to Berlin. This tendency towards excessive salt usage has been turned on its head, on both ecological and economic grounds. Considerable effort is now devoted to defining application conditions whereby the minimal amount of salt can be used, sometimes supported by computer programs supplied by dye manufacturers [136]. The trend towards lower liquor ratios evinced by machinery developments and the increasing use of bifunctional dyes with their higher average levels of fixation have both contributed to lowering of the amounts of salt used. Special ranges of ‘low-salt’ reactive dyes have been marketed, giving further emphasis to this important and worthwhile trend [137,138]. In spite of these advances, however, it has been suggested [139] that such developments have not proven to be totally satisfactory. Consequently, investigations were carried out to explore the potential of cationic surfactants, Groups IA, IIA and IIIA chlorides and carboxylate salts as alternatives to conventional electrolytes [139]. Cationic surfactants proved unsuitable as they promoted only surface deposition with low fixation, most of the dye being easily removed by washing. Groups IIA and IIIA chlorides were precipitated as hydroxides under alkaline conditions and were thus also unsuitable, although amongst Group IA salts potassium and caesium chlorides gave increased exhaustion and fixation of CI Reactive Red 180 (12.29) with increasing salt concentration (Figure 12.12). Fixation increased with the atomic size of the cation (Cs+ > K+ > Na+ > Li+) and equivalent fixation was achieved with about 20 g/l less of KC1 and about 40 g/l less of CsCl compared with a conventional concentration of NaCl [139]. Nevertheless, although potassium and caesium chlorides promoted higher exhaustion and fixation, it is difficult to agree that they are viable commercial replacements for sodium chloride. Significant amounts would still be left in the effluent; they will always be more costly and less readily available in commercial quantities, both short- and long-term. More promising results were observed with potassium salts of di-, tri- and tetra-carboxylic acids (Figures 12.13–12.15). Multicarboxylate salts facilitate much higher levels of dye exhaustion and fixation than sodium chloride, sodium citrate being particularly effective (Figure 12.16). Although sodium citrate is a chelating agent, it does not appear to affect metal-containing reactive dyes under these conditions [139]. Depending on the reactivity of the dyes used and the applied depth, the pH required for reaction with the fibre varies from 8 to 12 and in practice falls mainly between 9 and 11.

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Dye fixation/mg per g fibre

4 LiCl NaCl KCl CsCl

3

2

1

0 0.342

0.684

1.027

1.369

1.711

Salt concentration/N

Figure 12.12 Fixation of CI Reactive Red 180 with various concentrations of Group IA chlorides [139] O NaO3SO

CH2CH2

SO2

C SO3Na H

HN O

N N NaO3S

12.29 CI Reactive Red 180

4

O

Dye fixation/mg per g fibre

KCl K oxalate (12.30)

OK C

K tartrate (12.31) K phthalate (12.32)

C

3

O

OK

O

OK

C

2

H

C

OH

H

C

OH

12.30

C O

1

12.31

OK

O C

OK

C

OK

0 0.342

0.684

1.027

1.369

1.711

Salt concentration/N

O 12.32

Figure 12.13 Fixation of CI Reactive Red 180 with various concentrations of potassium salts of dicarboxylic acids [139]

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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES

O C

4

Dye fixation/mg per g fibre

KCl K citrate (12.33) K B3CA (12.34)

C

3

OK

H2C

O

C

OH

KO H2C C

OK

O

2

12.33

O

1

0 0.342

0.684

1.027

1.369

1.711

KO

C O

Salt concentration/N

C

OK

C

OK

O 12.34

Figure 12.14 Fixation of CI Reactive Red 180 with various concentrations of potassium salts of tricarboxylic acids [139]. K B3CA = Tripotassium benzene-1,2,4-tricarboxylate

O C

4

Dye fixation/mg per g fibre

KCl K BTCA (12.35) K B4CA (12.36)

O C

3

CH

KO

HC CH2

KO

2

O KO

C

KO

C

OK C O

C O

1

OK

H2C

12.35

O C

OK

C

OK

0 0.342

0.684

1.027

Salt concentration/N

1.369

1.711

O

O 12.36

Figure 12.15 Fixation of CI Reactive Red 180 with various concentrations of potassium salts of tetracarboxylic acids [139]. K BTCA = Tetrapotassium butane-1,2,3,4-tetracarboxylate; K B4CA = Tetrapotassium benzene-1,2,4,5-tetracarboxylate

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861

80 Na citrate NaCl

Exhaustion/%

60

40

20

0 20

40

60

80

Dyeing time/min

Figure 12.16 Exhaustion profiles for CI Reactive Red 180 in the presence of 1.711N sodium chloride or 1.711N sodium citrate [139]

The most widely used alkali is sodium carbonate, although sodium bicarbonate and sodium hydroxide are also used; these reagents may be used singly or in mixtures. Sodium bicarbonate, for example, can be used as a pH-shift agent, since the pH slowly increases on heating as sodium carbonate is formed (Scheme 12.4). The alkali induces ionisation of the cellulosic hydroxy groups, enabling the dye to react with these nucleophilic anionic sites to form covalent bonds with the fibre (section 7.3.1). It might be thought that increasing quantities of alkali would favour maximal reaction between dye and fibre, but in practice an optimum level of alkali, rather than a maximum, has to be sought. This is because the cellulosate anion tends to repel the reactive dye anion, thus decreasing the efficiency of dye uptake and so increasing the tendency towards hydrolysis of the dye. Consequently, the aim must be to keep the pH as low as possible consistent with maintaining complete reaction within a commercially acceptable dyeing time; however, fixation conditions vary widely according to the dye type and process used.

2 NaHCO3

Na2CO3 + CO2 + H2O

Scheme 12.4

In certain applications sodium silicate is preferred. Replacing sodium carbonate by sodium silicate can increase yields by 10–30%, as well as giving an improvement in fastness to washing. The increase in yield was higher with cold-dyeing than with hot-dyeing types. Cost savings were estimated at 15–35%, or 50–70% in some cases [140]. Occasionally, however, the lower alkalinity of silicate could result in greater hydrolysis with corresponding reduction in yield. It is opportune at this point to illustrate the combined effects of electrolyte and alkali (Figure 12.17). The initial stage with electrolyte alone at neutral pH approaches equilibrium primary exhaustion of dye; there is no fixation during this stage. Once the alkali is added (in this case after 20 minutes), fixation of the dye begins to take place; at the same time, the

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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES

alkali contributes to further exhaustion, which is termed secondary exhaustion. The difference between equilibrium exhaustion and fixation at the end of the process represents unfixed dye that requires washing off in order to maximise fastness properties. The profiles of these curves differ according to how the individual dyes respond to the conditions (type and concentration of auxiliaries, pH, temperature). Once the dye is fixed, it cannot migrate. Hence the difference between the exhaustion and fixation curves represents the potential for levelling during the alkaline treatment. Primary exhaustion

Secondary exhaustion

Exhaustion or fixation/%

100 Exhaustion curve

Fixation curve

50 S F P

0 20

40

60

80

Dyeing time/min P Degree of primary exhaustion S Degree of secondary exhaustion F Degree of dye fixation

Figure 12.17 Dye exhaustion and fixation profiles in exhaust dyeing with a typical reactive dye [141]

The traditional requirement for alkalinity can be a disadvantage on both environmental and cost grounds. Furthermore, this treatment is not fully compatible with dyeing requirements for other fibres used in blends with cellulosic fibres, particularly polyester, thus complicating the dyeing of such blends. This has favoured the development of so-called ‘neutral-dyeing’ reactive dyes. These do not require the addition of alkali for fixation but still need electrolyte for exhaustion [142–144]. With these dyes reaction does not occur at neutral pH below 100 °C but fixation becomes optimum at 120–130 °C and pH 6.5–7.5. A pH of 8 is recommended for the hank dyeing of yarn and jig dyeing of fabrics, where dyeing at high temperature is not feasible. This range of dyes is the only one to fall into Group 3 of the classification mentioned previously; hence their levelling is controlled by temperature rather than by additions of alkali or electrolyte. As mentioned at the beginning of this section, the so-called ‘all-in’ method may be adopted, treatment time being saved by adding the electrolyte and alkali together, albeit at the expense of lower fixation and (usually) inferior reproducibility. Studies of dichlorotriazine dyes applied with various alkalis or combinations of alkalis (NaHCO 3, Na 2CO 3/NaOH or Na2CO 3/NaHCO3) have shown recently that this process can be optimised to give enhanced dye fixation by buffering to give pH 8 or 9 [145]. Reactive dyes in general are not unusually sensitive to hard water. Nevertheless, the alkali used in most reactive dyeing processes may precipitate calcium or magnesium hydroxide on

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REACTIVE DYES

the substrate, to cause problems in later processes. Ideally, soft water with a pH not greater than 7 is preferred. Where the use of hard water is unavoidable, a sodium hexametaphosphate sequestering agent may be used in the minimal amount needed to overcome the hardness, since excessive quantities may bring about a significant reduction in dye yield. Organic sequestering agents of the EDTA type (section 10.2.1) are generally best avoided because they often result in colour changes and reduced light fastness, although they can occasionally be used successfully in minimal quantities [30]. It has been shown, at least with some reactive dyes, that hue changes due to traces of iron or copper in modal fibres (Table 12.12) can be prevented by the use of dimethylaminomethane-1,1diphosphonate (12.37), 1-hydroxyethane-1,1-diphosphonate (12.38), EDTA (12.39) or certain water-soluble polymers as sequestering agents [146]. However, as discussed later, the potential for trace metals to cause problems at the washing-off stage should not be overlooked [30]. As mentioned previously, once reactive dyes have reacted with the fibre no levelling is possible; hence all levelling must be achieved before the reaction has reached equilibrium. The preferred means is by controlled additions of alkali (Group 1) and electrolyte (Group 2) rather than by using a surfactant-type levelling agent. The latter causes restraining, although such products may be added in minimal amounts to aid wetting and to safeguard against rope marks through lubricating action [30]. A particularly detailed study of the effects of a range of nonionic aromatic and aliphatic ethoxylates confirmed the negative influence of these agents on the fixation of reactive dyes [147–149]. Figure 12.18 shows typical results

Table 12.12 Metal content of regenerated cellulosic fibres [146] Metal content (ppm)

Metal

Modal fibre

Schwarza modal fabric

Copper Iron Zinc Magnesium Lead Manganese

4.1 46.0 15.0 – 27.0 2.2

6.6 63.0 20.0 22.0 15.0 1.5

Ash content (%)

0.27

Lenzing modal fabric 2.0 13.0 38.0 9.0 <0.1 0.2

0.37

0.25

O

O

C HO

H3C

N

CH3

O HO

P

HO

O CH

OH

P

CH3 O

P

C

P

OH

OH

OH

N

OH HO

OH

C

CH2CH2

CH2

863

OH

N H2C

O

OH 12.37

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O

C H2C

CH2

OH C O

12.38

12.39

HEDP

EDTA

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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES

obtained with a nonylphenol ethoxylate (20 mols ethylene oxide) in dyeings of CI Reactive Red 2 (12.40). 80

No agent

Exhaustion/%

60

40

5 g/l agent

20

20

40

60

80

Time/min

Fixation/mg per g fibre

0.8

No agent

0.6

5 g/l agent

0.4

0.2

20

40

60

80

Time/min

Figure 12.18 Exhaustion and fixation curves for CI Reactive Red 2 in the presence and absence of a nonionic surfactant at 30 °C [147] Cl N N

Cl N

HN H

O SO3Na

N N NaO3S 12.40

CI Reactive Red 2

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However, the decrease in yield varied considerably with dye and surfactant. The magnitude of the effect increased with increasing degree of ethoxylation of the surfactant and varied considerably from dye to dye. Interestingly, this loss of yield occurred even though kinetic evidence indicated that the presence of the nonionic agent decelerated the rate of dye hydrolysis [149]. Spectrophotometric studies [148] showed positive evidence of interaction between the dye and surfactant. This interaction shifts the equilibrium between adsorbed dye and dissolved dye in favour of the dyebath [147]. Electrolyte and temperature also influenced the interaction [148]; higher temperatures tended to destabilise the dye–dye and dye–surfactant interactions, although these interactive effects were partially maintained at normal dyeing temperatures. Thus the possibility of using surfactants as levelling agents for reactive dyeing is best avoided. Similarly, any surfactants added during preparation should be completely rinsed out before dyeing. Provided goods have been prepared thoroughly, it should be unnecessary to add wetting or levelling agents to the dyebath [30]. Mention of the importance of thorough rinsing after preparation applies not only to the need to remove surfactants but also to prevent the considerable number of dyeing faults resulting from residual unevenly distributed alkali and residual peroxide [150]. Various attempts have been made to optimise the exhaust dyeing of reactive dyes. For example, some methods depend on pH control via precisely metered alkali dosing systems [151,152]. The curves in Figure 12.19 show how the fixation rate can be modified considerably by dye-specific alkali metering in order to increase the scope for levelling. Detailed attention to the precise effects of all dyeing parameters with a view to ultimate dyespecific control enables the minimal amounts of auxiliaries to be used that will give safe and effective control. Further advantages claimed include reduced water, steam and energy consumption together with substrate quality improvements [151–153].

Ideal fixation profile

Fixation

Conventional fixation curve

Metered alkali addition

Dyeing time

Figure 12.19 Control of reactive dye fixation rate by Remazol automet (DyStar) alkali addition [151]

Neps in cotton fabrics can pose a problem for reactive dyeing. Effective coverage of such dead or immature cotton can be achieved by pretreatment with poly(ethylene imine) on the jig at 10:1 liquor ratio before the dyeing process. The cationic polymer not only gave good coverage of the neps but also improved the colour yield generally. In addition, wet fastness could be improved by a low-temperature curing treatment with a cationic polymer emulsion [154].

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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES

Viscose fibres are particularly suitable for dyeing with reactive dyes, the differences in colour yield between dyed viscose and cotton varying between individual dyes and depending on the chromogen present [155]. Salt and alkali requirements are generally lower on viscose than on cotton, although most turquoise and green hues based on phthalocyanines are applied according to special recommendations [30]. A comparison of three types of reactive dye applied to cotton, viscose and lyocell showed little difference in colour between these fibres [156]. The colour strength was lowest on cotton, slightly higher on lyocell and marginally the highest on viscose. It should be borne in mind that the quality of today’s viscose is somewhat different from that produced in the 1970s: as regards response to preparation and dyeing parameters modern viscose is generally superior [157]. Hence some care is needed in relating current data to earlier results and many of the older recommendations should not be followed today. Certain dyes are susceptible to reduction, particularly under anaerobic dyeing conditions. An addition of 1–2 g/l sodium mnitrobenzenesulphonate is a useful palliative. Continuous and semi-continuous dyeing There are numerous variations of procedure for the continuous or semi-continuous dyeing of cellulosic fabrics with reactive dyes, viable reasons being evinced for the promotion of most of the major ranges of reactive dyes. We are concerned here only with rationale in the selection of auxiliaries; details of the many processing routes can be found elsewhere [30,158–163]. Low substantivity at the padding stage is often preferred in order to minimise tailing, but in fact dyes covering the whole range of reactivities can be used. Highly reactive dyes can be fixed in shorter times at lower pH and are often easier to wash off, whilst those of low reactivity offer greater stability in the pad liquor. The bifunctional types give highly efficient fixation and excellent fastness performance [30]. Many processes require only dye and alkali; for example, on cotton equal concentrations of dye and alkali over the range 5–30 g/l may be used with a pick-up of 60–80%, whilst on viscose the pick-up is usually higher (90–100%) and the alkali reduced to half the quantity of dye. With some aminochlorotriazine dyes both salt and alkali are used – perhaps up to 30 g/l sodium chloride and 10–15 g/l alkali. In some processes sodium silicate is preferred as the main alkali, in order to alleviate the problem of white selvedges. This fault can occur with sodium hydroxide, caused by neutralisation of the sodium hydroxide by carbon dioxide or other acidic atmospheric agents. A wetting agent is generally required in continuous processes to aid rapid wetting at the padding stage. A hydrotrope such as urea may be added, particularly for deep shades, to boost the solubility of less sulphonated dyes and to aid fixation through the mechanism of retaining moisture by hydrogen bonding, particularly in fully continuous thermofixation techniques. If a thickening agent is needed to minimise migration, sodium alginate is preferred since it does not interact with reactive dyes; electrolyte addition may serve the same purpose. An advantage of fully continuous procedures is that minimal quantities of alkali and water are consumed. Two-stage continuous processes, comprising two wet-on-wet paddings with the second pad applying the alkali, are applicable in certain circumstances. The paramount importance of efficient preparation in producing goods of thorough and uniform absorbency for continuous dyeing cannot be over-emphasised [157,158]; all continuous dyeing systems are heavily dependent on this prerequisite. Typical build-up curves on mercerised and unmercerised cotton fabrics in Figure 12.20 illustrate the

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significantly higher colour yields attained in a two-bath pad–steam process using sodium silicate (variant B) with vinylsulphone dyes. Recommended variant A and B chemical pad formulations are shown in Tables 12.13 and 12.14, for a steaming treatment of 30–60 seconds at 102–105 °C. In this process, important factors for the control of dye migration include the lowest possible pick-up of dye liquor in padding, minimum addition of wetting agent, sufficient electrolyte in the chemical pad liquor and selection of vinylsulphone dyes of high substantivity. If a one-bath, pad–steam process is used, the recommended variant A and B chemical formulations are as presented in Tables 12.15 and 12.16 respectively. Steaming treatment is again 30–60 seconds at 102–105 °C. Variant

B Bleached

100

and mercerised

A cotton Relative colour yield/%

80 B Bleached cotton

A fabric

60

40 20

20

40

60

80

100

Dye concentration/g l–1

Figure 12.20 Typical dye build-up curves for pad–steam process variants A and B on mercerised and unmercerised cotton [158]

Table 12.13 Chemical pad additions for two-bath salt variant A[158] Dye concentration (g/l)

<20

20–40

>40

Steaming time (s)

30

60

30

60

30

60

Sodium chloride or sulphate (g/l) Sodium carbonate (g/l) Caustic soda 32.5% (ml/l)

250 20 10

250 20 5

250 20 15

250 20 7.5

250 20 20

250 20 10

Table 12.14 Chemical pad additions for two-bath silicate variant B [158]

chpt12(2).pmd

Silicate strength (°Be)

Mass ratio Na2O:SiO2

Silicate addition (ml/l)

Caustic soda solution 32.5% (ml/l)

37–40 40–42

1:3.3 1:3.3

900 760

100 100

867

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868

AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES

Table 12.15 Chemical formulation for one-bath salt variant A[158] 30 g/l 20 g/l 5 ml/l 10 ml/l

Sodium chloride or sulphate Sodium carbonate Caustic soda 38°Be if <15 g/l Dye Caustic soda 38°Be if >15 g/l Dye

Table 12.16 Chemical formulation for one-bath silicate variant B[158] 95 ml/l Sodium silicate 37–40°Be 8 ml/l Caustic soda 38°Be (32.5%) if <15 g/l dye 15 ml/l Caustic soda 38°Be (32.5%) if >15 g/l dye

For the pad–batch fixation of heterobifunctional Cibacron C (Ciba) dyes four alkali variants have been suggested [159]: (1) Rapid fixation in 5–8 hours using a mixture of sodium hydroxide and sodium silicate, a metering pump being necessary. A high concentration of alkali gives rapid fixation but still allows excellent bath stability at temperatures up to 30 °C. (2) Fixation in 6–12 hours with a mixture of sodium hydroxide and trisodium orthophosphate, a metering device being necessary. This method is recommended for regenerated cellulosic fibres. This formulation contains the same total amount of alkali as method (1) with the same bath stability, but may be preferred where some buffering capacity is required and sodium silicate is undesirable. (3) A different combination of sodium hydroxide (reduced amount) and sodium silicate, a metering pump not being necessary. This method demands a longer fixation time (12– 24 hours). Bath stability is greater than two hours but the total amount of alkali is inadequate for application to grey goods, where the raw cotton consumes a great deal of alkali. (4) A mixture of sodium hydroxide and sodium carbonate, a metering pump being necessary. This method avoids the use of either silicate or phosphate and is popular for woven goods and in circumstances where silicate would pose problems. Ideally the carbonate should be free from bicarbonate. This system has less buffering capacity and gives slightly lower bath stability than methods (1) and (2). Methods (1) and (2) are the most widely used because they offer the most reliable results in bulk-scale processing. In pad–batch dyeing with the highly reactive chlorodifluoropyrimidine Drimarene R or K (Clariant) brands, it is equally possible to use either a weak alkali (sodium carbonate) for a long batching time or a strong alkali (sodium hydroxide) for rapid fixation. It is claimed that the versatility of monofunctional dyes of this established type makes the more expensive bifunctional types unnecessary [160]. Detailed studies of each dye in the range led to the generation of a series of three-dimensional graphs from which a computer-based optimised system has been developed and made available to dyers.

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868

15/11/02, 15:47

REACTIVE DYES

869

The Levafix E/EA/EN (DyStar) system based on computer-centred optimisation is also available [161]. Two main alkali formulations are involved: (1) Sodium carbonate with or without sodium hydroxide (2) Sodium silicate with or without sodium hydroxide. In general the prospect of working without silicate is regarded as a major advantage because the padding and washing-off of sodium silicate places considerable stresses on the machinery and on effluent treatment. However, the use of silicate in no way impairs the performance of Levafix dyes. Six optimised alkali recipes for this system are shown in Table 12.17, whilst Table 12.18 shows the relation between pad liquor pH, batching time and pad liquor stability. Continuous dyeing with Sumifix Supra (NSK) heterobifunctional dyes is claimed [162] to give reproducible dyeings exhibiting a high degree of fixation and good wet fastness. The dyed fabric has an attractive handle and good appearance. The high fixation contributes advantageously to low effluent loading. Typical pad liquor formulations and fixation conditions, in comparison with those for aminochlorotriazine and vinylsulphone dyes, are given in Table 12.19. There is a general tendency to abandon the ‘classical’ continuous reactive dyeing processes in favour of shorter pad–dry processes that is mainly driven by environmental and economic factors [163,164]. Particularly pertinent to this trend is the marked decrease in average length of run to a given shade. The latest developments in dyeing machinery allow greater control of the dyeing parameters and eliminate the former need for lengthy and

Table 12.17 Optimised alkali recipes for Levafix (DyStar) dyes at various applied depths [161]

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Recipe

Dye conc (g/l)

A

Sodium carbonate (g/l) pH 11.3

5

B

Sodium carbonate (g/l) Caustic soda 45% (ml/l) pH 11.5–12.1

C

Sodium carbonate (g/l) Caustic soda 45% (ml/l) pH 11.8–13.2

D

Sodium carbonate (g/l) Caustic soda 45% (ml/l) pH 12.6–13.5

E

Sodium silicate 37°Be (g/l) Caustic soda 45% (ml/l) pH 11.5–13.0

40 2.0

F

Sodium silicate 37°Be (g/l) pH 11.0

25

869

5

10

20

40

60

10

20

40

10 0.2

20 0.35

20 0.65

20 1.3

20 2.15

10 0.35

20 0.65

20 1.3

20 2.6

20 4.0

20 5.3

20 7.9

17 2.0

13.5 4.3

10.5 6.3

7 8.6

4 10.6

40 3.3

40 5.6

80 8.9

100 11.7

100 13.2

100 14.9

25

25

40

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80

120

870

AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES

Table 12.18 Relation between pad liquor pH, batching time and pad liquor stability for five optimised alkali recipes and Levafix (DyStar) dyes [161] Batching time (hours)

Recipe

pH

Pad liquor stability (min)

Bleached cotton

Mercerised cotton

Sodium carbonate without caustic soda

A

11.3

142

21

14

Sodium carbonate with a little caustic soda

B

11.9

41

14

8

Sodium carbonate with increasing caustic soda

C

12.2

18

10

6

Increasing caustic soda, decreasing sodium carbonate

D

12.7

8

5

3

Sodium silicate with increasing caustic soda

E

12.0

37

5

3

Table 12.19 Pad liquor formulations and fixation conditions for application of monofunctional and heterobifunctional reactive dyes by three continuous dyeing methods [162] One-bath pad–thermofix Reactive system Temp. / time ACT

200 °C 30 s

One-bath pad–steam

Two-bath pad–steam

Alkali

Temp. / time

Alkali

Temp. / time

Alkali

Na2CO3 10–20 g/l

103 °C 5–10 min

Na2CO3 10 g/l

103°C 60 s

NaOH 40°Be 20 ml/l NaCl 300 g/l

103 °C 3–8 min

Na2CO3 6–12 g/l

103°C 30 s

NaOH 40°Be 20 ml/l NaCl 250 g/l

103 °C 1–5 min

NaHCO3 10–20 g/l

103°C 30–60 s NaOH 40°Be 10 ml/l Na2CO3 20 g/l NaCl 100–150 g/l

Urea 100–200 g/l VS

150 °C 60 s

Na2CO3 10–30 g/l Urea 50–100 g/l

ACT-VS

180 °C 30–60 s Na2CO3 10–20 g/l Urea 50–100 g/l

ACT VS

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Aminochlorotriazine Vinylsulphone

870

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REACTIVE DYES

871

expensive conditioning of machinery at the beginning of each run. Liquor wastage is a major factor both economically and environmentally in continuous dyeing. Downtimes up to 60 minutes and waste liquor volumes up to 120 litres have been typical for traditional long runs to a shade [164]. Developments have reduced downtimes to as brief as 5 minutes and computerised systems inform management of the quantity and composition of pad liquors so that wastage is minimised to about 20 litres. By adopting pad–dry–bake or simple pad–dry methods chemical padding and steaming stages are eliminated, thus benefitting the environment by avoiding the demand for large amounts of salt. A typical development in this area is the Econtrol (Monforts and Zeneca) system [163,164], which consists of padding with dye, 1–2 g/l wetting agent and the alkali. With high–reactivity dichlorotriazine dyes this can be sodium bicarbonate (10 g/l). No urea, sodium silicate, electrolyte or other chemicals are required. After a short air passage the uniformly squeezed fabric is passed through a hot flue in which the carefully controlled relative humidity (25% moisture by volume) brings about fixation in two minutes. The humidity fulfils the function that otherwise would be provided by an environmentally problematical hydrotrope such as urea. This reduction of chemical consumption is indeed a major economic and environmental benefit. Acrylamide-based migration inhibitors are claimed to give more efficient fixation [165]. A comparison of the differences between cotton and viscose has revealed [157] that viscose requires significantly longer immersion times. For example, immersion time for thoroughly prepared cotton can be less than one second on today’s high-speed ranges, whereas 1–1.5 seconds is preferable for viscose. Contrary to some views, the addition of urea does not shorten immersion times and can in fact lengthen them [157]. Printing Similar general considerations apply to direct printing processes as to continuous dyeing. A single all-in or a two-stage, pad–steam process may be used. Alginates are the preferred thickeners because other carbohydrates react with the dyes. The non-reactivity of alginates, in spite of their hydroxy groups, is thought to be due to the presence on each mannuronic unit of a carboxyl group that tends to repel the dye anions. As discussed in section 10.8.1, difficulties in obtaining alginates have led to the evaluation of alternatives, in particular the synthetic anionic poly(acrylic acid) types that give higher colour yields but can be more difficult to wash off. These products have been slow to replace the alginates apparently because the synthetic acrylic thickeners show varying sensitivity to electrolytes. However, detailed comparative rheological studies [166] have indicated that even if electrolyte sensitivity were controlled the acrylic polymers might still prove unsuitable despite the greater fixation that they give. At the slow printing speeds necessary in these laboratory studies the amount of paste absorbed by the fabric was always greater with the synthetic thickener, but this effect did not give higher colour yields because penetration was greater. It was concluded, therefore, that synthetic thickeners should not be adopted because of the higher dye consumption that outweighs any price advantage offered by the synthetic thickener. This is expected to be even more relevant at the high printing speeds necessary in commercial production, because of the greater shear-thinning of the synthetic thickener under these conditions of application [166]. Emulsion thickenings, either oil-in-water or water-in-oil and including half-emulsions,

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872

AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES

have been important in the past but are now much less popular on environmental grounds. Hydrotropes and a mild oxidising agent such as sodium m-nitrobenzenesulphonate are commonly added, the latter to protect azo dyes from reductive decomposition. Choice of alkali depends on the reactivity of the dye. Sodium bicarbonate is usually preferred, being cheap and offering high print paste stability with all but the most reactive dyes, but for dyes of sufficiently high stability the stronger alkalis (sodium carbonate or sodium hydroxide) may be chosen to provide higher colour yields under more alkaline conditions. The amount of bicarbonate may be reduced with highly reactive dyes. A pH-shift agent such as sodium trichloroacetate can be selected; this hydrolyses during steaming to release sodium carbonate (Scheme 12.5). However, another product of this hydrolysis reaction is chloroform. As well as being a volatile AOX-generating compound, chloroform is hepatotoxic and its use may be severely restricted. Cl 2 Cl

C Cl

Cl

O + H2O

C ONa

2 Cl

CH

+ Na2CO3 + CO2

Cl

Scheme 12.5

In the two-stage process the dyes are first applied without alkali, using a thickening agent such as sodium alginate that gels on subsequent application of alkali, usually together with electrolyte. High concentrations of sodium silicate (Na2O/SiO2 = 1:2.1, 47°Bé) or mixed alkali solutions (for example, 185 g/kg sodium carbonate + 185 g/kg potassium carbonate + 30 g/kg sodium hydroxide 32.5% or 38°Bé) are often used. Undoubtedly of greatest concern in the printing of cellulosic fibres with reactive dyes is the essential demand for large quantities of the hydrotrope urea (typically 80–200 g per 1000 g of print paste). This auxiliary forms hydrogen bonds with water molecules, assisting in swelling of the fibre and the thickener, dissolution of the dyes and promotion of interaction between dye and fibre [167]. The most obvious benefits provided by urea are improved levelness, diffusion and colour yield but this compound is now environmentally suspect. Various attempts to replace urea over the last decade have led to contradictions, ranging from claims of complete success to the viewpoint that it has not yet been possible to find a satisfactory alternative and the prognosis for doing so is not particularly good. Claims have been made [168] that the two-phase flash age process is the only way to avoid urea problems. In this process, no urea or alkali is present when printing is carried out initially, followed by padding with alkali and perhaps electrolyte (to limit migration) prior to steaming. However, even this approach has its ecological drawbacks due to the alkali (which may include silicate) and electrolyte. Furthermore, not all dyes give a satisfactory response, in particular phthalocyanine turquoise blues [167]. It is possible to improve the versatility of the process by including small amounts (up to 50 g/l) of urea; whilst this does not attain the environmental objective of totally avoiding urea, it does give an improvement over more conventional procedures. Suitable recipes are given in Table 12.20. In conventional processes, the moisture content of the fabric at the fixation stage is about 20% or even less. However, in the flash age process it is significantly greater than this and the extent to which urea usage can be minimised is closely related to the amount of moisture present. Whilst traditional flash agers operated at around 50% humidity, more

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872

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REACTIVE DYES

873

Table 12.20 Print paste and fixation pad liquor formulations for two-phase flash age printing with reactive dyes [29] Print paste recipe Urea Sodium m-nitrobenzenesulphonate Sodium alginate thickener Water

Fixation pad recipe Potassium hydroxide Sodium carbonate Sodium hydroxide 38°Be Water

0–50 10 400–500 440–590 1000

g g g g g

185 185 30 600 1000

g g g g g

recent attempts to eliminate urea have seen increases in this level. For example, the inclusion of humidifying or moisturising equipment in front of an ager to increase moisture content by 30%, aided by adjustment of the viscosity of the thickener, gave satisfactory yields in the absence of urea, even with phthalocyanine turquoise blues [167]. One means of increasing moisture content in the single-phase system of printing is to pre-wet by means of foam [169]. This enables the complete elimination of urea, giving additional advantages of better flow properties of the print paste, shorter fixation times, reduced water consumption and savings of thickener. In another approach, the concentration of urea could be considerably reduced by minor additions to the print paste of cyclodextrin, chitosan derivatives or so-called ‘superabsorbers’ based on acrylates [170]. However, after careful consideration of the various roles played by urea, it was concluded that the prospects of finding an adequate substitute are not promising [171]. This echoes earlier views [172] resulting from a study of the fixation of reactive prints with various concentrations of urea by saturated (102 °C) or high-temperature (120 °C) steaming. It is important to remember that urea functions not just by attracting moisture but also by entering into various interactions with dyes, thickener and substrate. As regards environmental aspects, the use of print paste preservatives, as discussed in section 10.8.6, should be borne in mind. Conventional reductive discharge prints are applicable on grounds dyed with reactive dyes, especially if vinylsulphone dyes have been used, but there are difficulties with those non-azo blue and turquoise dyes that cede preference to resist processes [29]. Resists can be achieved by printing with a thickened paste containing a non-volatile acid (tartaric or citric acid, for example) or with an acidic salt such as sodium dihydrogen orthophosphate. The thickening agent used must be stable to acid; hydroxyethyl or methoxyethyl cellulose ethers, locust bean gum or tragacanth are suitable. The pre-printed and dried resist areas are then overprinted or padded with a solution of a highly reactive dye together with the minimum amount of sodium bicarbonate. The washing-off process An especially important and critical aspect of the application of reactive dyes, whether by dyeing or printing, is the washing-off process necessary to remove unfixed or hydrolysed dyes,

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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES

as well as other products such as alkali, electrolyte and thickening agents. After rinsing to remove the more readily soluble products such as alkali and electrolyte, the goods are soaped at a temperature close to the boil to remove the unfixed active or hydrolysed dyes; a high temperature is necessary to remove hydrolysed dye from the interior of the fibres. Surfactants, especially nonionic types, are often added to the soaping liquor and a sequestering agent is added if the water is hard. Where thickening agents have been used, sufficient time must be allowed during the initial rinsing for the thickener to become hydrated and easier to remove. This seemingly straightforward washing-off process has enormous environmental impact and has generated much research over the last few years. Washing-off accounts for as much as 50% of the processing cost of a reactive dyeing in terms of the washing-off process itself and subsequent treatment of the effluent. In addition to these environmental and economic consequences, the efficiency of washing-off has a critical influence on the fastness levels attained. Herein lies the primary rule: the extent of the washing-off sequence should be just sufficient to achieve the target fastness properties, since any further washing treatment only leads to unnecessary expenditure and greater effluent volumes. Seven parameters have been listed [173] as pertinent to the effects and efficiency of washing-off: (1) number of washing baths (2) duration of washing, varying in practice from a few seconds (continuous) to many minutes (batchwise) (3) temperature, varying from rinsing at about 20 °C up to soaping at the boil, or higher in special circumstances (4) liquor ratio, varying from 5:1 (continuous) to 100:1 (batchwise) (5) mechanical movement of liquor and/or goods is of major importance in influencing liquor interchange (6) composition of wash liquor: the electrolyte and chemical content of dyeings varies widely and these contaminants, together with any auxiliaries added to the washing baths, will considerably influence the result (7) the substantivity of the reactive dyes present markedly influences their washing-off behaviour. As a general illustration of the main effects, Table 12.21 shows the effect of a single washing treatment of varying time and temperature on an exhaust dyeing of the monoazo J acid scarlet CI Reactive Red 123 and a continuous dyeing of the copper formazan CI Reactive Blue 104. Both dyes contain a chlorodifluoropyrimidine reactive group, but consistently more dye was removed from the continuous dyeing, especially for shorter times at the lower temperatures. Even after six successive washes at 25 °C, the removal of unfixed dye from the exhaust dyeing only increased to 56, 62 and 74% respectively from the 24, 36 and 52% shown for one wash [173]. In general, these results emphasise the importance of longer times and higher temperatures. Further results demonstrated that electrolyte additions up to 10 g/l did not markedly influence the removal of dye but showed significantly increased effects at 20 g/l and especially 50 g/l. Mild alkaline washing was just as effective as mild acidic washing and the addition of a nonionic agent had a favourable influence on dye removal. These results are specific to the two dyes evaluated. However, it is abundantly clear from these trends and comments, bearing in mind the wide variety of reactive dyes, substrates and machinery, that optimising the washing-off procedure in each case for maximum economy and minimum environmental impact is no easy matter.

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874

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REACTIVE DYES

875

Table 12.21 Effect of a single washing treatment for various times at various temperatures on the removal of unfixed dye from exhaust or continuous dyeings with chlorodifluoropyrimidine reactive dyes [173] Removal (%) of unfixed dye from exhaust or continuous dyeing

Washing

Exhaust dyeing of CI Reactive Red 123 on bleached cotton

Continuous dyeing of CI Reactive Blue 104 on mercerised cotton

Temperature

25 °C

70 °C

98 °C

25 °C

70 °C

98 °C

Time 15 s 1 min 5 min

24 36 52

64 74 90

84 88 92

38 52 74

80 90 92

86 90 94

These difficulties have been thoroughly discussed in an exhaustive study [174]. Theoretical considerations were thoroughly explored and the economic and technical merits of four different washing systems were analysed: (1) individual washing baths (2) dyeing machines operating with continuous liquor exchange (3) continuous rope-washing machines (4) open-width washing machines. Three key stages applicable to most washing processes were identified: (1) In the first stage, unfixed dyes, salt and alkali present in the liquor phase must be removed and this is best done by replacing this liquor with fresh water. Sorption, desorption and diffusion processes play only subordinate roles in this stage, the key factors being liquor flow, mechanical action and liquor exchange. The dilution laws are generally applicable. (2) In the second stage, substantial amounts of alkali and unfixed dyes are desorbed and diffuse from the fibre pores into the liquor phase. This is the diffusion stage and the rate-determining step is diffusion of the labile dye molecules out of the fibre phase. This takes time and is accelerated by higher temperatures and perhaps by mechanical agitation of the substrate. (3) In the final stage, the electrolyte and unfixed dye concentrations are low but further changes of liquor must take place until almost all the unfixed dye molecules are desorbed. These three processes are not necessarily distinct consecutive stages but are major identifiable events that may be more or less separated or interlinked by transitional steps. It is argued that the washing-off requirements are different in each stage. In the first stage of liquor exchange, cold rinsing is preferred because higher temperatures offer only a slight advantage and alkalinity may hydrolyse dye–fibre bonds. Empirically, this stage may be regarded as completed when the electrolyte content reaches about 1 g/l; the number of liquor changes required is related to liquor ratio. A shorter liquor ratio demands more changes and involves only slightly lower water consumption, so is without advantage at this

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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES

stage. Continuous methods are more effective, since these give direct displacement of contaminated liquor by fresh water, rather than mixing of them. In the second stage of diffusion from the fibre, hot washing is necessary to ensure the major advantage of more rapid diffusion. Individual dye properties, such as substantivity and diffusion coefficient, are particularly pertinent here. In contrast to earlier conclusions [173], these studies involving several surfactants showed no particular advantage from the use of detergents [174]. The addition of a detergent is best avoided if possible, since it further contaminates the effluent. It is useful to compare the economics and fastness performance, particularly in heavy depths, of a thorough washing-off against a less than complete extraction of unfixed dyes followed by a cationic aftertreatment. To minimise effluent problems by treatment of liquors before discharge to effluent, it is preferable to concentrate the residual dyes in as small a volume as possible. This implies that removal of unfixed dyes should be restricted to as few baths as possible. In this context dye substantivity is an important consideration [175]. Cold initial rinsing removes much of the low-substantivity dye present but dyes of high intrinsic substantivity are difficult to remove in this way. If the initial rinse is hot, however, even these high-substantivity products are effectively removed, thus achieving concentration of these dyes in the initial stage and ensuring easier subsequent removal. Thus it was proposed that washing-off could be improved by using a hot rather than a cold initial wash [175]. This was substantiated in further optimisation studies which enabled the residual dyes to be confined to two or three baths, surfactant additions not being necessary for these water-soluble contaminants. These results were confirmed in bulk processing. It was found that lower liquor ratios were beneficial in washing-off [176] and similar control parameters were identified in continuous washing-off [177]. The benefits of hot initial washing (60–70 °C) have been demonstrated in yarn package dyeing, emptying of the vessel after dyeing being unnecessary [178]. From the viewpoint of energy conservation, the washing temperature should be chosen according to the dyes present [179]. Since dyes of low substantivity are desorbed more readily under mild conditions, a lower temperature should be selected for these dyes and higher temperatures for dyes of higher substantivity. In an evaluation of the washing-off behaviour of dichlorotriazine dyes from cotton at temperatures ranging from 20 to 98 °C with water alone, and at 98 °C with a surfactant, the surfactant-aided method was found to be the most effective [180] and the benefits of aftertreating with cationic agents were confirmed. A detailed comparison [181] of three vinylsulphone dyes included a low-substantivity monoazo N-acetyl H acid derivative (CI Reactive Red 35), a monoazo N-acetyl J acid type of higher substantivity (CI Reactive Orange 82) and a phthalocyanine turquoise somewhat prone to aggregation (CI Reactive Blue 21). Dyeings of these individual products were subjected to three wash-off procedures: (1) conventional sequence: cold, cold, warm, hot, cold (2) cold/hot sequence: 30 °C, followed by several baths at 95 °C (3) hot sequence: repeated treatments at 95 °C. Typical results are given in Table 12.22. The objective was to examine the relationship between the substantivity of the unfixed dyes and their response to changes in the conditions of the washing sequence.

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876

15/11/02, 15:47

REACTIVE DYES

877

Table 12.22 Effects of various washing–off sequences on the rate of desorption of dye in successive washing baths and the degree of staining of adjacent material in washing tests [181] *Colour value of wash liquor

Fastness to washing

(1)

(1)

(2)

(3)

CI Reactive Red 35 (low substantivity) W = 510 nm Washing bath 1 16 15 18 2 2 2 1 3 2 1 0 4 1 1 0

2 3 4 4–5

2 4 4–5 4–5

3 4 4–5 4–5

CI Reactive Orange 82 (moderate substantivity) W = 490 nm Washing bath 1 17 23 56 2 10 31 4 3 11 2 0 4 13 0 0 5 2 0 0

1 1–2 2 3–4 4–5

1 3 4 4–5 4–5

2 3–4 4–5 4–5 4–5

CI Reactive Blue 21 (phthalocyanine type) W = 664 nm Washing bath 1 14 15 33 2 3 15 6 3 2 2 1 4 6 0 0 5 3 0 0

2 2–3 2–3 3 4

2 3 4 4–5 4–5

2–3 4 4–5 4–5 4–5

Washing sequence

(2)

(3)

* Colour value given by 1000 E/d, where E is extinction at the peak wavelength (W nm) and d mm is the cell thickness

The low-substantivity red dye was desorbed relatively quickly, mainly in the initial cold rinse, and the acceptable fastness rating of 4–5 was reached after three or four washing baths. The greater difficulty of desorbing the more substantive orange dye was clearly evident. Only after the first hot bath of the sequence was most of the unfixed dye desorbed. A fastness rating of 4–5 was reached after the 5th, 4th and 3rd baths respectively of the washing sequences (1), (2) and (3). With CI Reactive Blue 21 seven baths were needed in the conventional sequence (1) to give a fastness rating of 4–5. Sequences (2) and (3) were more effective, giving acceptable fastness after four and three baths respectively. In all cases it is the hot washing sequence (without surfactant) that enables this target to be reached quickly and with the lowest water consumption. It was confirmed that the final bath need not be absolutely colourless after washing, nor is complete removal of unfixed dye necessary to attain the target fastness level. Similar results have been observed in the washing-off of reactive prints, in which it is necessary to monitor the removal of the alginate thickener as well as desorption of the unfixed dyes [182]. Batchwise washing showed that the thickener was rapidly washed out and that elevated temperatures increased the rate of removal. No significant dwell time for thickener swelling seemed to be necessary. Significantly more effective removal of thickener with increasing temperature was observed in continuous washing and most of the thickener was eliminated in the first wash.

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878

AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES

Confirmation of earlier work [180] on the temperatures of washing-off of dichlorotriazine reactive dyeings on cotton revealed that surfactant and sodium carbonate together were more effective than water alone, although there was little difference in performance between the six surfactants examined [183]. However, the addition of surfactant did not appear to be essential in order to achieve adequate dye desorption and fastness to washing, 2–5 g/l sodium carbonate being sufficient. These results were later substantiated with dichlorotriazine dyeings on lyocell [184] and for aminochlorotriazine dyeings on cotton [185]. Further studies involved different alkalis (ammonia, sodium bicarbonate, potassium hydroxide and sodium hydroxide) and unspecified buffers covering the range pH 7–13 with aminochlorotriazine dyeings on cotton. Although all these alkalis and buffer solutions were capable of removing unfixed dyes, the effectiveness of washing-off of the systems varied; potassium hydroxide and sodium bicarbonate were the most effective and the pH 8 buffer solution was the least. In general, effectiveness increased with increasing pH but the outstanding behaviour of potassium hydroxide and sodium bicarbonate could not be explained in terms of pH alone. Despite varying effectiveness in terms of washing-off, all of the washed dyeings were virtually identical in fastness to washing. When multiple alkaline washes were evaluated on aminochlorotriazine dyeings, potassium hydroxide and sodium carbonate were the most effective, although sodium bicarbonate was more attractive in terms of low cost, environmental impact and fastness to washing. However, some of these multiple alkaline treatments showed evidence that dye–fibre bond cleavage may have taken place in addition to removal of hydrolysed dye [185]. When washing-off in a jet machine, a combination of liquor exchange and continuous overflow rinsing is advantageous [186]. Optimal rinsing procedures depend on machine parameters and the cost structure of the plant, but in general the most economical system appears to be: (1) liquor exchange rinsing initially (salt removal) (2) continuous rinsing at a higher temperature (diffusion of unfixed dye) (3) liquor exchange rinsing finally (removal of dye liquor). The objective of stage (1) is to lower the salt concentration to 1–2 g/l and this requirement determines the number of initial baths given before subjecting the dyeing to a higher temperature. Salt removal is accelerated by liquor changes and dilution before commencing continuous rinsing. The duration of continuous rinsing is adjusted according to the depth of shade and the known diffusion properties of the dyes present. Calcium and magnesium ions present when washing in hard water make the unfixed dye anions more difficult to remove; poly(acrylic acid) derivatives are effective sequestering agents in mildly alkaline liquors [187]. Stripping The stripping of cellulosic materials dyed with reactive dyes is carried out by alkaline reduction followed by hypochlorite oxidation, preceded by a boiling treatment with EDTA if metal-containing dyes have been used. For example, a treatment with 5 g/l sodium carbonate or sodium hydroxide and 5g/l sodium dithionite at the boil is followed by a treatment in 0.5–1 °Tw hypochlorite, an antichlor and thorough rinsing.

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878

15/11/02, 15:47

REACTIVE DYES

879

12.7.2 Wool Although wool can be dyed with typical reactive dyes produced essentially for cellulosic substrates, the adoption of reactive dyes on wool has depended on the development of special ranges of dyes capable of covalent bonding under slightly acidic conditions. The use of surfactant auxiliaries is essential. The types of dyes used and the mechanisms of reaction have been discussed in section 7.7 and elsewhere [2]. These dyes are generally similar in response to dyebath pH to milling acid dyes, medium or full depths being applied at pH 5.0– 5.5 and pale depths at pH 5.5–6.0. At lower pH values adsorption and fixation may be irregular, whilst at higher values exhaustion is poor. Even at optimal pH fixation may still be incomplete within normal dyeing times, particularly when dyeing navy or black shades. Hence the bath is adjusted to a weakly alkaline state, which ensures complete fixation as well as helping to remove hydrolysed dye. A typical procedure is to dye at the boil and pH 5, followed by cooling to 80 °C and adjustment of the pH to 8.0–8.5 with ammonia, after which treatment is continued for some 20–30 minutes at 80 °C. A surfactant auxiliary is necessary to prevent tippy or skittery dyeing. Cationic or nonionic products have been used but the most useful have been the amphoteric N-alkylbetaines and alkylamidobetaines (described in section 9.7) and ethoxylated amphoteric types represented by structure 12.41 [188]. Compounds of this type can interact under slightly acidic conditions with both fibre and dye. The mechanisms have been investigated in detail [188–192]. The initial step appears to be the formation of an auxiliary–dye complex by interaction between the quaternary group in the surfactant and the dye anion, such a complex being less soluble than the dye alone. As the concentration of the auxiliary is increased to an excess over that required for complexing with the dye anions present, surfactant micelles are formed that tend to solubilise the auxiliary–dye complex. Unlike most levelling agents these amphoteric products often increase the rate of wool dyeing, to an extent dependent on the chemistry and concentration of the auxiliary and of the dye. Conversely, they may have a retarding effect if the concentration of auxiliary is so high as to solubilise the auxiliary–dye complex completely. These auxiliaries also tend to improve dye exhaustion at equilibrium. H3C(H2C)10H2C

(CH2CH2O)m

_ SO3 NH4+

(CH2CH2O)n

H

N + H3C _ CH3SO4 12.41 (m+n = 10–80)

The accepted explanation for this behaviour is that the zwitterionic auxiliary–dye complex is less electronegative than the dye anion; hence it exhibits more hydrophobic characteristics with increased molecular size and lower aqueous solubility. Consequently, the affinity of the dye for the undamaged roots of the wool fibres is enhanced relative to that for the more hydrophilic damaged tips. The auxiliary–dye complex is less sensitive to root-tip differences and thus gives more level dyeing. Furthermore, adsorption of the auxiliary by the fibre increases the electropositive charge on the fibre, thus increasing the attraction for anionic dyes. This will tend to further increase the rate of dyeing and is also the mechanism

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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES

whereby the total amount of dye absorbed is increased. On the whole, there is little evidence that these agents increase migration and indeed no migration can take place once the dye has reacted covalently with the fibre. The levelling action of betaines increases with concentration and with increasing length of the alkyl chain [188]; the effects on rate and total absorption disappear with products containing a very long alkyl chain (about C16) at a high concentration, although the levelling properties are maintained irrespective of the rate effects. In general, the betaines bring about a greater change in sorption characteristics than do the ethoxylated amphoteric compounds of similar alkyl chain length [188]. Thus higher concentrations of the latter are necessary to produce a similar effect; this can be an advantage, since the effects are not then so critically sensitive to the concentration of auxiliary. Furthermore, the manufacturer can vary the balance of properties of the auxiliary by varying the length of the oxyethylene chains as well as of the alkyl group. Using laser Raman spectroscopic techniques, it was confirmed that the primary mechanism involves ionic interaction between the anionic sulphonate groups of the dye and the cationic quaternary group of the auxiliary, although hydrophobic interactions were also significant [190]. However, investigations of the coverage of damaged wool indicated that interaction of the auxiliary with the fibre is mainly responsible for the improvement in levelness, both the surface and the interior of the fibre being involved [191]. Dye–auxiliary interaction does not seem to play a major role in levelling out root-to-tip variations. Studies with variously modified wools, including wool pretreated with an amphoteric agent, demonstrated that these auxiliaries mainly accelerate the rate of exhaustion on normal wool, whereas the effects on modified wools are minimal [192]. Although these agents promote dye uptake and fixation on normal wool, they do not enhance dye uptake to the same extent on Hercosett-treated wool [2]. Traditionally, the alkaline treatment given after reactive dyeing has been with ammonia at pH 8.0–8.5 and this is still the favoured method of removing unfixed dyes, although sodium bicarbonate is occasionally used. Indeed, it is claimed that the lower basicity of bicarbonate results in less fibre damage and no detrimental effect on fastness properties [193]. A specific disadvantage of ammonia is that it can cause uneven treatment in different parts of a yarn package because of irregular swelling of the fibre [2]. Several other agents have been suggested as potential aftertreating agents for reactivedyed wool: (1) Hexamethylenetetramine: this ammonia precursor does not cause fibre swelling and the unfixed dyes are removed efficiently at pH 6.5, compared with pH 8.5 with ammonia, thus causing less damage to the wool. However, the hydrolysis of this compound (Scheme 12.6) results in the formation of formaldehyde and this can modify the hue of certain dyes [2]. (2) Sodium bisulphite: this additive reacts by nucleophilic addition to the vinylsulphone group of dyes of this type, decreasing the substantivity and increasing the aqueous solubility of the unfixed dyes (Scheme 12.7). (3) A commercially branded product believed to be sodium trichloroacetate: this is added at a concentration of 5% to the dyebath 20 minutes before the end of dyeing. Sodium carbonate is formed by hydrolysis of the trichloroacetate (Scheme 12.5), accompanied by a change of pH from 5.0–6.0 to 6.7–6.9 [2]. This reaction also releases the volatile AOX-generating chloroform, however.

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(4) Various amine salts, such as hydrochlorides of 1,6-diaminohexane, 1,12-diaminododecane, 1-aminododecane and a bisulphate adduct of 1,6-di-isocyanatohexane. This was an attempt to introduce a final reactive step to form crosslinks between unfixed dye molecules on the fibre [193]. N H2C

CH2 H2C

4 NH3 + 6 HCHO

N

CH2

Scheme 12.6

[dye]

100 °C + 6 H2O

N H2C

N

CH2

SO2CH

CH2

+ NaHSO3

[dye]

SO2CH2CH2SO3Na

Scheme 12.7

Although all of these compounds were applicable in principle, treatment with sodium bicarbonate offers a much more economical and reliable option. The mechanism of reaction of α-bromoacrylamide dyes with wool has been investigated [194]. The application to wool of a range of heterobifunctional dyes normally used on cellulosic fibres has been promoted recently [195]. These dyes offer high exhaustion and fixation, as well as exceptional fastness. They are relatively insensitive to dyeing variables and suitable for dyeing wool–cellulosic blends by a one-bath method, with savings in energy costs and less wool damage. The choice of auxiliaries is essentially the same as when applying these dyes to cotton, although the pH is varied according to whether wool or wool– cotton is to be dyed. An ammonia aftertreatment is recommended for wool dyeings. The reactive dyeing of wool at 110 °C and pH 4.5 (ammonium sulphate and acetic acid) in the presence of a betaine auxiliary has been described [196]. Reactive dyes can be applied to wool by printing [2,29]; suitable ranges include chlorodifluoropyrimidine, α-bromoacrylamide, sulphatoethylsulphone and aminochlorotriazine. A typical print paste recipe is shown in Table 12.23. Derivatives of locust bean or guar gum, either alone or in combination with water-soluble British gum, are the preferred thickeners. Humectants, particularly urea, are essential to aid solubilisation and penetration as well as swelling of the wool. A wetting agent (5–10 g/kg) may also be needed if chlorination of the goods has been less than optimum and where complete penetration of the print design is desirable. Antifoam addition is generally necessary in machine printing. A non-volatile acid such as citric acid or an acid donor such as ammonium tartrate or ammonium sulphate is normally required to give optimum pH conditions; however, vinylsulphone dyes are applied under neutral to slightly alkaline conditions using sodium acetate (40 g/kg). The oxidising agent sodium chlorate is often added to counteract the reducing effect of the wool, although if vinylsulphone dyes are printed under alkaline conditions sodium m-nitrobenzenesulphonate is effective in protecting any reduction-sensitive dyes.

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Table 12.23 Typical print paste recipe for the printing of wool with reactive dyes [29] Dye Urea Thiodiethylene glycol Thickener (10–12%) Acid or acid donor Antifoam Water to

xg 50 g 50 g 500 g 10–30 g 1–5 g 1000 g

The most important reducing agents used for discharge printing on wool are the formaldehyde-sulphoxylates [2]. These agents are restricted to the minimum effective concentration within the range 30–180 g/kg. The print is washed in several baths containing 2 g/l disodium hydrogen orthophosphate, ammonia (to give pH 9) and an anionic surfactant in successive baths at temperatures increasing from 40 to 80 °C. The printing of silk with reactive dyes, using sodium silicate as alkali and sodium alginate as thickener, has been described [197]. Problems arising from the contamination of effluents with reactive dyeing and printing auxiliaries represent an important issue that has been fully discussed elsewhere [198–201]. 12.8 SULPHUR DYES The application of sulphur dyes to cellulosic substrates involves conversion to the substantive leuco form by alkaline reduction, followed by oxidation on the fibre to the insoluble disulphide form (section 1.6.2). Consequently, reducing and oxidising agents are essential auxiliaries. Secondary auxiliaries include wetting agents, sequestering agents, antioxidants, electrolytes and hydrotropes. Useful reviews of developments in sulphur dyes and their application are available [30,202,203]. The concern here is with the essentials of the auxiliary agents used in their application. It is convenient to deal separately with the aspects of reduction and oxidation. The discussion is confined to application by dyeing since the use of sulphur dyes in printing nowadays is restricted mainly to sulphur blacks applied by techniques similar to those used in vat printing [29]. 12.8.1 Reduction Sulphur dyes in the insoluble disulphide form and the CI Solubilised Sulphur brands are reduced by the dyer as part of the application procedure. In the case of the CI Leuco Sulphur brands reduction has already been carried out by the manufacturer, so that they are substantially in a form suitable for immediate application (section 1.6.2) The chemistry of the reduction of sulphur dyes is complex, as is the chemistry of the dyes themselves; it has been well described elsewhere [204]. It is possible to describe the state of a reduced sulphur dye in alkaline sulphide or polysulphide solution by the general formula 12.42, but there are certain complications. In many cases the chromogen is not itself reduced, but in others, notably reddish browns, blues and navy blues based on indophenols, the chromogenic

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quinonimine grouping can be reduced (Scheme 12.8). Additionally, the value of n in structure 12.42 varies, even for a given dye. It is not surprising, therefore, that the amount of reducing agent (and alkali) required varies from dye to dye, depending on the chemistry of the dye as well as on the concentration in the formulation. Hence the manufacturer’s literature must always be consulted for the amounts of auxiliaries to be used with particular dyes in the various batchwise, semi-continuous and continuous processes. D

Sn SX 12.42

m

D = chromogen n = 0–5 m ≥ 1 X = H or Na, depending on pH

reduction N

O oxidation

H N

OH

Scheme 12.8

The reduction process is invariably carried out in an alkaline medium, partly because of the instability of most reducing agents at low pH and partly because the acidic thiol groups react with alkali to give the much more soluble anionic thiolate form [204]. Traditionally the most widely used reducing agents have been sodium sulphide (Na2S) and sodium hydrogen sulphide (NaHS). Technically these are still the most widely preferred, not only for their efficacy but also because they are relatively inexpensive. Nowadays, however, they are increasingly subject to scrutiny on environmental grounds. At least 12 g/l sodium sulphide is required to dissolve the water-insoluble CI Sulphur dyes. The quantity added to the dyebath varies from dye to dye but is generally proportional to the amount of dye, with a minimum of 1.5–3.0 g/l [30]. When using sodium hydrogen sulphide the quantity is generally 0.6 times that of sodium sulphide, but it is also necessary to add alkali (10 g sodium hydroxide or 5 g sodium carbonate per 7 g sodium hydrogen sulphide). The pre-reduced CI Leuco Sulphur dyes usually contain a mixture of sodium sulphide and sodium hydrogen sulphide, together with hydrotropic and dispersing agents such as 2-ethoxyethanol, sodium 1,3-xylene-4sulphonate, sodium p-toluenesulphonate and sodium tetralinsulphonate [202]. The quantities of sodium sulphide and sodium hydrogen sulphide referred to above relate to the full-strength concentrated products and must obviously be proportionately adjusted if weaker commercial brands are available. In some applications, particularly in jet and winch dyeing, there is a danger that the reducing agent may be prematurely oxidised by air. Antioxidants, added along with the dyes and the primary reducing agent at the beginning of dyeing, can be used as palliatives. Polysulphides of general formula 12.43, such as disodium tetrathionite, have been widely used for this purpose and provide improved dyebath stability. These additives can be used with other reducing agents described below but are not compatible with dithionite. Another approach is to add a relatively more stable alkaline reducing agent such as sodium dithiodiglycolate 12.44 [205]. Environmental concerns are gradually curtailing the use of sulphides as reducing agents [203,206], although it has been indicated [207] that as late as 1995 some 90% of all sulphur

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XO3S

Sn

SO3X O

12.43

O C

X = Na, K, H or NH4 n = 1–4

CH2

S

S

NaO

CH2

C ONa

12.44

dyes applied worldwide were still reduced by sulphides. The environmental problems arising from sulphides include the toxicity of hydrogen sulphide, corrosion of the effluent drainage system, damage to the treatment works and the often associated high pH and unpleasant odours [208]. Sulphides cause no odour nuisance above pH 9 but at neutral or acidic pH values gaseous hydrogen sulphide is liberated. Neutralisation or acidification can occur in the dyehouse or during waste stream mixing. The MAK value of hydrogen sulphide is 15 mg/m3 (10 ppm), this being the maximum allowable concentration in the workplace at a continual contamination level to avoid impairment of health, the odour threshold being 0.035–0.14 mg/m3 of air [207]. Polysulphides yield free sulphur on acidification and this can lead to odours of sulphur dioxide on the dyed substrate. Nevertheless, sulphides can be quantitatively removed before discharge to the effluent, albeit at a substantial cost [208,209]. Several reducing agents have been suggested as environmentally more acceptable alternatives to the alkali sulphides. All are more expensive and exhibit other disadvantages. For example, the reduction may be more difficult to control, or a particular agent may be effective only with a limited range of dyes. Even then the alternatives may be less effective than the alkali sulphides in terms of colour yield. Nor is the fact that such compounds do not generate hydrogen sulphide a guarantee of freedom from environmental problems; for example, some give quite high COD levels. The most obvious alternative is sodium dithionite (12.45) with alkali, the reducing agent most widely used with vat dyes (section 12.9). When used with sulphur dyes, however, the process is difficult to control and some dyes may be partly destroyed through over-reduction. Nevertheless, dithionite is effective with CI Solubilised Sulphur and sulphurised vat dyes.

NaO

O

O

S

S

ONa

12.45 CI Reducing Agent 1

Sodium hydroxide is the alkali usually used in conjunction with dithionite. Sodium carbonate is a possible alternative when CI Solubilised Sulphur dyes are used but is insufficiently alkaline for the CI Sulphur brands, requiring careful control if over-reduction and the associated lower yields are to be avoided [30]. Typical concentrations are given in Table 12.24. The system of sodium carbonate and sodium dithionite used to reduce blue and black CI Solubilised Sulphur dyes is particularly suitable for flame-retardant viscose fibres that are sensitive to strong alkalis, since it preserves a satisfactory level of flame retardancy [30]. It is also possible to use a mixture of dithionite with sodium sulphide in alkaline media. Sodium formaldehyde-sulphoxylate (12.46; sodium hydroxymethanesulphinate) and alkali, although more stable than alkaline dithionite, tends to share the same disadvantages

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Table 12.24 Typical applied concentrations of sodium dithionite and alkali for exhaust dyeing [30] Liquor ratio 10:1

1% Dye

6% Dye

Caustic soda flakes Sodium dithionite

3.5 g/l 2.5 g/l

7.5 g/l 7.0 g/l

These amounts are decreased by 30–40% at liquor ratio 20:1 or increased correspondingly at liquor ratio 5:1 Sodium carbonate Sodium dithionite

0.5 –1.5 g per g of dye 0.25–0.75 g per g of dye

HOCH2

O

S

ONa

12.46 CI Reducing Agent 2

and is rarely, if ever, used with sulphur dyes owing to associated handling difficulties, inadequate cost-effectiveness and poor efficiency [207]. Sodium dithiodiglycolate (12.44) was mentioned above as an antioxidant. Such compounds may be used as the primary reducing agent in conjunction with alkali. Although they do not give rise to environmentally undesirable inorganic sulphides in the effluent, their chemical stability results in a high chemical oxygen demand, often causing more problems than those arising with sodium sulphide [202]. This system is rarely used for much the same reasons as sodium formaldehyde-sulphoxylate [207]. The most promising alternative to sulphides, from an environmental point of view, is the use of the reducing sugar glucose with sodium hydroxide or sodium carbonate. This system does not satisfactorily reduce all dyes, however. It is reasonably effective with CI Solubilised Sulphur brands [210], with which it may be used either as the sole reducing agent or in conjunction with sodium polysulphide, usually resulting in increased dye yields. It can be used as an additional reducing agent with CI Leuco Sulphur dyes, thus giving a lower sulphide content in the dyebath, or together with sulphide or polysulphide in the reduction of the traditional water-insoluble CI Sulphur brands [30]. The system is pH- and temperature-sensitive; hence performance may be good on jet machines but poor on the more temperature-sensitive jigs. Typical quantities recommended for a batchwise dyeing method [30] at liquor ratios of 10:1 to 20:1 are 3–8 g/l glucose, 4–10 g/l sodium carbonate and 2–6 g/l sodium hydroxide, depending on applied depth, a pH of 11–12 and a minimum dyeing temperature of 90–95 °C being necessary [206]. The glucose reducing system has a characteristic odour of burning sugar that many people consider preferable to the odour of an alkaline sulphide bath, although others dislike it, finding it excessively sweet and nauseous [30]. Nevertheless, the versatility of glucosebased binary systems has been emphasised [207]. The major problem with an alkaline glucose system is that it is gradually transformed into various decomposition products, thus losing its reducing action. The intermediate by-products possess some reducing action but are not sufficiently stable. More stable decomposition products are formed if dithionite is

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added to the system, this being the subject of patents more than sixty years ago [207]. Scheme 12.9 has been suggested to explain this effect. O

S

ONa

CHOH CHOH H

CH2OH

O C H

CHOH CHOH CHOH

sodium hydroxide

Sodium glyderylsulphinate

O C

2 CHOH

sodium

+ dithionite

CH2OH

CHOH

O

Glyceraldehyde

O

CH2OH

S

ONa

CHOH

Glucose

CHOH CH2OH Sodium glycerylsulphonate

Scheme 12.9

The reduction potentials of various reducing agents listed in Table 12.25 show that at 50 °C the dithionite/glucose system has a potential that is only slightly lower than that of dithionite alone, even though glucose has the lowest potential in this series. The addition of glucose reduces the potential of sodium dithionite to the point where full colour yield is obtained without the risk of over-reduction. Dyeing tests have confirmed that although sodium dithionite alone is exceptionally concentration-sensitive, the addition of glucose gives a more stable system. Optimal colour yield and good reproducibility are obtained even if dyeing temperature, time and concentrations of chemicals fluctuate within certain narrow limits. Similar results have been obtained with other glucose binary systems, including hydroxyacetone or sodium formaldehyde-sulphoxylate as stabiliser. Table 12.25 Reduction potentials of various reducing systems at 50 °C [207]

Reducing system

Reduction potential (mV) at 50 °C (platinum electrode)

Sodium formaldehyde-sulphoxylate Sodium dithionite Sodium dithionite/glucose Hydroxyacetone Sodium sulphide Sodium polysulphide Glucose

–900 –850 –700 –680 –650 –500 –200

5 g/l Reducing agent 10 ml/l Caustic soda 38°Be

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The environmentally innocuous properties of reducing sugars have been claimed for the dyeing of jute with sulphur dyes [211]. This claim is based on the influence of citric acid addition on the hydrolysis of cane sugar, giving a higher proportion of reducing sugars (Table 12.26). The optimum addition is 0.04% citric acid and treatment of the coarse brown sugar from the sap of palm trees (so-called ‘jaggery’) is for 24 hours at ambient temperature, giving a product termed ‘liquid jaggery’. This is added at 2.5 times the mass of sulphur dye in the dyeing of jute at 100 °C for 1 hour in a liquor containing 50 g/l sodium chloride. This product can also be utilised in printing, a typical print paste formulation being shown in Table 12.27. Table 12.26 Effect of citric acid addition on the composition of cane sugar after hydrolysis [211] Composition of product Citric acid concentration (%)

pH

Reducing sugars (%)

Non-reducing sugars (%)

0 0.01 0.02 0.03 0.04 0.05

5.9 5.8 5.5 5.3 5.3 5.0

21 30 33 36 39 43

40 38 36 32 28 24

Table 12.27 Typical print paste formulation for the application of liquid jaggery as a reducing system for sulphur dyes [211] Sulphur dye Sodium carbonate Liquid jaggery Thickening agent Water (if necessary) to

3–5 g 2–3 g 7.5–12.5 g 80 g 100 g

2-Mercaptoethanol (12.47) with alkali has been suggested as an alternative to sulphides [210], offering the advantages of no sulphides in the effluent and no odour from the dyebath, although the product itself can give off unpleasant and highly toxic fumes. This process is relatively expensive, with a tendency towards lower yields and a more restricted range of suitable dyes than when using traditional sulphides, so it has not achieved significant commercial use.

HSCH2CH2OH 12.47 2-Mercaptoethanol

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CH3COCH2OH 12.48 Hydroxyacetone

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Hydroxyacetone (12.48), originally introduced for vat dyeing, has proved moderately successful with sulphur dyes. This compound requires strongly alkaline conditions, the concentrations being critical. Colour yields are somewhat lower than with sulphide, the product is flammable and the dyebaths have an odour characteristic of acetone. Nevertheless, when used with CI Solubilised Sulphur dyes the effluent is free from sulphide. Mention has been made already of its possible use in a binary system with glucose [207]. Hydroxyacetone is the basis of the RD (Refine Dyeing) process of Ipose GmbH; this is a carefully optimised system incorporating automatic dosing of hydroxyacetone as the sole reducing agent [212]. Hydroxyacetone is suitable for application in indigo, vat and sulphur dyeing, including the continuous dyeing of cotton yarn with sulphur dyes or indigo, as well as the exhaust dyeing of knitgoods [212]. These processes are described in greater detail in section 12.9. Thiourea dioxide, also discussed further in section 12.9, has been regarded as a more environmentally friendly replacement for sodium sulphide in the application of sulphur dyes [213]. In an investigation of exhaust dyeing with eight sulphur dyes applied from alkaline dyebaths, thiourea dioxide gave colour yields and fastness ratings similar to those with sodium sulphide, although in some cases slightly different hues were observed. Typical analyses of exhaust dyebaths are detailed in Table 12.28. On balance, thiourea dioxide is much less hazardous to the environment than sodium sulphide; the major improvement is the decreased amount of oxidant required for chemical treatment of the effluent and a second advantage is the marked decrease in sulphate ion content. Although thiourea Table 12.28 Analysis of exhaust dyebaths after dyeing of cotton fabric with sulphur dyes using sodium sulphide or thiourea dioxide [213]

Colour pH Density at 20 °C (g/ml) Permanganate oxidation (mg/l O2) Dichromate oxidation (mg/l O2) Residue dried at 100 °C (mg/l) Residue baked at 600 °C (mg/l) Loss of mass at 600 °C (mg/l) Chloride (mg/l Cl–) Sulphate (mg/l [SO4]2–) Sulphide (mg/l [S]2–) Alkalinity (ml/l)

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3% CI Sulphur Yellow 1

10% CI Sulphur Blue 11

Sodium sulphide

Thiourea dioxide

Sodium sulphide

Yellow 10.2

Yellow 11.4

Blue 11.2

1.05

1.04

1.10

Thiourea dioxide Blue 9.1 1.06

937

433

1375

850

1853

965

2769

1692

7720

6416

13500

8255

6612

4924

12495

7130

1108

1492

1005

1125

Trace

Trace

Trace

Trace

3439

933

5813

1382

128

Nil

72

Nil

43

80

70

77

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dioxide is more expensive, calculations of processing costs must take into consideration the cost of treating waste liquors compared with those containing sulphide. Indirect electrolysis using regenerable redox systems, first proposed for use with vat dyes (section 12.9), can also be used with sulphur dyes [214]. Sulphur dyes develop a reduction potential of around –600 mV, compared with –900 mV for typical vat dyes (Ag/AgCl, 3M KCl). This is an important reason why a greater variety of reducing agents has been proposed for use with sulphur dyes compared with vat dyes. There are also wider possibilities for using various redox systems appropriate to the types of sulphur dye. Figure 12.21 shows the relationships between reduction potential and pH for vat, indigo and sulphur dyes, together with relevant redox systems. This indicates that anthraquinone derivatives and iron–gluconic acid complexes are suitable for the reduction of sulphur dyes by indirect electrolysis. Table 12.29 shows the important reaction paths involved in these two systems. Representative anthraquinone compounds include alizarin (1,2–dihydroxyanthraquinone), sodium anthraquinone-2-sulphonate or disodium anthraquinone-1,5-disulphonate. Table 12.30 shows typical results obtained with various sulphur dye types. Schemes 12.10 and 12.13 represent generation of the reducing species at the cathode. Schemes 12.11 and 12.14 indicate the mechanism of reduction of water-insoluble CI Sulphur dyes, whilst Schemes 12.12 and 12.15 show the release of the water-soluble leuco species from the CI Solubilised Sulphur precursor. Schemes 12.16 and 12.17 represent undesirable problems of precipitation that can occur in some cases; these may be preventable by addition of a sequestering agent. Excellent results, equivalent to those of conventional processes, were obtained [214]. This process offers advantages in terms of savings of chemicals, regeneration of the reducing species in situ, no undesirable by-products from the reducing agent, as well as possibilities for dyebath recovery and effluent recycling. Secondary auxiliaries used with the reducing system during preparation of the leuco dyebath may include wetting and sequestering agents; the water-insoluble CI Sulphur dyes may also require dispersing agents, The choice of wetting agent is not particularly critical as the behaviour of most sulphur dyes is unaffected apart from one or two that can show an

Fe–TEA = Iron–triethanolamine Fe–Gluc = Iron–gluconic acid

Reduction potential/mV

–1000

Fe–TEA Vat dyes

–800 Fe–Gluc Indigo

–600

AQ derivatives Sulphur dyes

10

11

12

13

14

pH value

Figure 12.21 Relationships between reduction potential and pH for various redox systems and dye classes [214]

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Table 12.29 Reaction paths involved in anthraquinonoid and iron-complex redox systems [214] ANTHRAQUINONE SYSTEMS Cathodic reduction of anthraquinone derivative Scheme 12.10 AQ + 2e– AQ2– Reduction of water-insoluble CI Sulphur dye Scheme 12.11 R–S–S–R + AQ2–

2R–S– + AQ

Reduction of CI Solubilised Sulphur dye Scheme 12.12 2R–S–SO3– + AQ2–

2R–S– + 2SO32– + AQ

IRON-COMPLEX SYSTEMS Cathodic reduction of iron-complex system Scheme 12.13 Fe3+L + e–

Fe2+L

Reduction of water-insoluble CI Sulphur dye Scheme 12.14 R–S–S–R + 2Fe2+L

2R–S– + 2Fe3+L

Reduction of CI Solubilised Sulphur dye Scheme 12.15 R–S–SO3– + 2Fe2+L

R–S– + SO32– + 2Fe3+L

Precipitation of iron sulphides Scheme 12.16 Fe2+L + 2Fe3+L + 4S2–

FeS↓ + Fe2S3↓ + 3L

Precipitation of water-soluble leuco dye Scheme 12.17 Fe2+L + Fe3+L + 5R–S–

(R–S)2Fe↓ + (R–S)3Fe↓ + 2L AQ L

R–S–S–R CI Sulphur dye R–S–SO3– CI Solubilised Sulphur Dye

Anthraquinone derivative Ligand (gluconic acid or triethanolamine)

adverse reaction (lower yield or even precipitation) with certain nonionic surfactants. Suitable sequestering agents include sodium hexametaphosphate and EDTA [30]. Electrolytes are added to assist exhaustion in batchwise dyeing. After application of the leuco dyes a thorough rinse is essential to remove any loose dye from the fibre surface. 12.8.2 Oxidation Chemically simple thiols and thiolates are readily oxidised to disulphides (Scheme 12.18). The situation is more complex with the leuco sulphur dyes [204]. In addition to having two or more thiol groups, they may contain other reactive functions that are potentially susceptible to oxidative attack, examples being primary, secondary or tertiary amino groups, hydroxy groups and aryl thioether (R–S–R) groups, as well as sulphonic acid groups in some cases. Furthermore, the high substantivity of the leuco dye for the substrate may inhibit free movement of the dye molecules and thus prevent complete conversion of thiol groups to

2 R

oxidant SH

R

S

S

R

Scheme 12.18

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Table 12.30 Suitability of various sulphur dye types for application with anthraquinonoid or iron-complex redox systems [214]

Sulphur dye types

AQ

Fe-GLU pH 12

Fe-GLU pH 13

Fe-TEA

Pre-reduced dyes CI Leuco Sulphur Black 1 CI Leuco Sulphur Black 2

+ +

o o

– –

+ +

Water-insoluble dyes CI Sulphur Blue 11 CI Sulphur Green 2

+ +

+ +

+ +

o o

Water-soluble dyes CI Solubilised Sulphur Blue 7 CI Solubilised Sulphur Blue 10 CI Solubilised Sulphur Green 3

+ + +

+ + +

+ + +

o o o

Sulphurised vat dyes Hydron Blue 3R Stabilosol (DyStar) Hydron Blue R Hydron Black CL Hydron Red GGF

+ + + +

+ + + +

o – – +

o o o +

+ o – AQ Fe-GLU Fe-TEA

suitable limited suitability precipitation at high concn. anthraquinonoid compounds iron-gluconic acid complex iron-triethanolamine complex

disulphide crosslinks; nevertheless, it is generally assumed that disulphide or polysulphide crosslinks are formed. From a purely technical viewpoint, ignoring environmental concerns, the most favoured and most widely effective oxidising system for sulphur dyes is sodium dichromate acidified with acetic acid. The concentration of dichromate is not critical [30], 1–2 g/l being generally recommended for batchwise processes carried out at 60–80 °C, but the pH is more important and should be controlled to within the range 4.5–5.5, giving good colour yield and fastness properties more reliably than all other oxidising systems. Up to 1 g/l copper(II) sulphate may be added to dichromate baths to give an improvement in light fastness, at the expense of some dulling of shade and a harsher handle. The copper salt should only be added to acidified and sulphide-free liquors, otherwise it may precipitate as the hydroxide and/or sulphide. The addition of copper should not be made when oxidising sulphur blacks, however, since it promotes acid tendering with these dyes. The addition of sequestering and dispersing agents to the dichromate bath may give an improvement in fastness to rubbing [30]. Dichromate oxidation does tend to give a harsher handle and a less hydrophilic fibre, tending to cause handling problems in subsequent processes such as weaving, and is therefore less suitable for yarn dyeings. The technical merits of dichromate are overshadowed by ecological considerations, however, since chromium compounds in surface waters pose a direct threat to health and are increasingly restricted by water treatment

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_ H3C

SO2 N

Cl

Na+ 12.49

authorities. Hence oxidation by dichromate is now only carried out in certain regions of the developing world. Despite considerable efforts to develop alternatives, no system so far devised matches the versatility, efficacy, economy and reliability of acidified dichromate [30,202,204,210]. Oxidising agents that are increasingly used include (a) hydrogen peroxide or other peroxy compounds, (b) iodates or bromates, (c) sodium chlorite, or (d) sodium Nchloro-p-toluenesulphonamide (12.49). It has been reported [207] that classes (b) to (d) account for 70% of the market. However, all of these produce organic halogen compounds, giving rise to high AOX values in the effluent. Air oxidation is obviously the most economical and environmentally friendly method but it has serious limitations. Peroxide systems are likely to become the most important method on both economic and ecological grounds, since peroxides do not generate AOX values. In fact peroxides are already established alternatives to dichromate, especially for yarn dyeing (giving a softer handle) and for sulphur blues and blacks. About 1–2 g/l hydrogen peroxide (130 vol.) is generally used at pH 10 and 40 °C [30,210]. Careful control is necessary, since under alkaline conditions oxidation is rapid and may go beyond the disulphide stage. In mildly acidic media oxidation is much slower but adequate under batchwise conditions for most dyes except CI Sulphur Red 10. It is too slow for rapid continuous methods and there is a danger of catalytic degradation of the fibre. Nevertheless, 1 g/l hydrogen peroxide (130 vol.) with 0.8 g/l acetic acid at 50–60 °C is widely used in yarn dyeing [30]. Oxidation under neutral conditions is occasionally carried out. Electronic metering and control is particularly useful with peroxide systems. The use of metal-ion catalysts with peroxide has been tried; vanadates offer some promise but iron and copper salts have not proved entirely suitable [210]. Peroxide oxidation has gained a reputation for giving somewhat lower fastness to washing than that obtained with dichromate. Although true in relation to traditional wash tests based on soap, tests based on the perborate-containing detergents in general use nowadays do not show this problem [210]. A concentration of 0.2–1.5 g/l potassium or sodium iodate [30,210] at 60 °C and pH 3.5– 5 (preferably 3.5–4.0) gives similar hues and fastness properties to those obtained with dichromate. Although expensive, the process shows good reproducibility and does not give the harsh handle associated with dichromate. The pH, controlled using acetic acid, is critical; oxidation is slower at values higher than pH 4.5 and stronger acids such as formic acid lead to precipitation of iodine which is corrosive to machinery and environmentally harmful. Higher temperatures lead to partial reduction of the iodate with consequent loss of efficacy. Under the same conditions bromates are less effective than iodates but perform equally well if a catalyst such as sodium metavanadate (NaVO3) is added [30]. Metavanadate, however, is increasingly under environmental scrutiny. Sodium chlorite is the basis of several proprietary oxidising agents for sulphur dyes. It is used at pH 10 (sodium carbonate) and 90–95 °C; careful control of conditions is important as the reaction is rather slow. The presence of additives is necessary for the successful use of chlorite. The proprietary formulations contain stabilisers, EDTA-type sequestering agents

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and detergents or dispersing agents. Other oxidising agents that have been proposed include: (1) Sodium m-nitrobenzenesulphonate in alkaline media (2) Sodium salt of N-chloro-p-toluenesulphonamide (12.49; chloramine T) (3) Sodium nitrite with sulphuric acid (this carries a danger of fibre degradation) (4) Potassium or ammonium salts of peroxydisulphonic acid with acetic acid. 12.8.3 Alkylation of thiol groups and other aftertreatments As an alternative or supplement to oxidising treatments, many leuco sulphur dyeings can be treated with alkylating agents such as those based on epichlorohydrin. This was discussed in section 10.9.6. The alkylating agent reacts with the dye thiol to yield an alkyl thioether, or with amino groups in the dye to yield substituted amines [204]. These are generally referred to as fixation treatments and give enhanced fastness to acid storage and to wet treatments although, strictly speaking, they are still oxidation mechanisms involving the removal of an electron from the dye thiolate. Several commercial cationic fixing agents for direct dyeings can be used to aftertreat sulphur dyeings, giving enhanced wet fastness; these treatments are claimed to be effective if applied in place of oxidation [215–217]. A soaping treatment after oxidation or other fixation treatment is recommended. 12.8.4 Continuous dyeing Continuous methods are far more important with sulphur dyes than is batchwise dyeing in terms of production volumes. There are many methods, including pad–steam, pad–sky, pad– dry–chemical–pad–steam and pad–dry–develop; summary descriptions of these processes are available [30]. In general the auxiliaries used, and particularly the redox chemicals, are the same as those used in batchwise dyeing although they are usually used in higher concentrations, especially reducing agents because they are more exposed to air in continuous processes. Wetting and sequestering agents are generally used. Some processes incorporate hydrotropes (such as urea) and migration inhibitors. Electrolytes may be used in a chemical (reducing) pad to assist fixation but are less frequently used when padding with leuco dyes, since they can promote tailing effects. 12.9 VAT DYES Vat dyes, like sulphur dyes, are applied to cellulosic fibres after initial conversion by alkaline reduction to the substantive leuco form, followed by reoxidation to the insoluble form on the fibre. Consequently the major auxiliaries are, once again, reducing and oxidising agents. Ancillary products include electrolyte, wetting, dispersing, levelling and sequestering agents; thickening agents and hydrotropes may feature in continuous dyeing and printing. An extremely important part of vat dye application is the final soaping treatment, which is essential for developing the ultimate colour and optimal fastness. Detailed accounts of vat dye application are available [30,218]. Here the reducing and oxidising stages of the process and the soaping treatment are considered separately; the main discussion deals with batchwise dyeing, after which the requirements for continuous dyeing and printing are considered.

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12.9.1 Reduction Vat dyes are available mainly in their water-insoluble pigment form, although a group of solubilised dyes, the vat leuco esters, are supplied in a stabilised pre-reduced form that does not require any reduction by the dyer. The chemistry of vat dyes, and of their reduction and oxidation, is much more clearly understood than is the case with sulphur dyes. In essence, the reduction process can be represented as the addition of an electron to each of the keto groups of the vat dye, with conversion to a conjugated dihydric quinol, the system being readily reversible by oxidative abstraction of electrons (Scheme 12.19). Vat dyes generally have larger negative reduction potentials than sulphur dyes and so require more strongly reducing conditions. In a detailed account of reduction and oxidation processes [219], various vat dyes are shown to have reduction potentials in aqueous media varying from –770 to –1000 mV, the majority being in the region –900 to –950. In contrast, sulphur dyes average only about –600 mV. Published reduction potentials for various reducing agents in aqueous alkaline media are shown in Table 12.31.

O

O HN

HN

+ 2e - 2e

NH

NH

O

O

Indigo derivative

Leuco indigo derivative

O

O + 2e - 2e

O

O Anthraquinone derivative

Leuco anthraquinone derivative

Scheme 12.19

Table 12.31 Reduction potentials of various reducing agents measured as 5 g/l solutions in 15 ml/l caustic soda 38°Be at 60 °C [220] Reduction potential (mV) Thiourea dioxide Sodium borohydride Sodium dithionite Hydroxyacetone Sodium formaldehyde-sulphoxylate

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–1100 –1100 –970 –810 –790

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895

Alkali is an essential component of the reducing system in order to ensure that the substantive ionised leuco species is formed, since the acid leuco form is usually insoluble in water and has limited substantivity for cellulose. To be successful a reducing agent should have a lower reduction potential than the compound it is intended to reduce but in practice there are a number of complicating factors. For example, the reduction rate of a vat dye (section 3.1.4) is critically dependent on the physical form of the dye as characterised by its crystal form and particle size distribution [221,222]. Table 12.32 indicates the magnitude of this effect for various samples of CI Vat Green 1. The stability of the leuco dye in solution is a function of the concentrations of dye and reducing agent, temperature, pH and liquor ratio. The influence of oxidation in air as it relates to different application systems has to be carefully considered. Hence the quantities of reagents suggested by the dye manufacturers can only be used as preliminary guidelines. By far the most important reducing system for the batchwise application of vat dyes is sodium dithionite (Na2S2O4) in a solution of sodium hydroxide. Obviously the theoretical concentrations required will depend on the number of keto groups in the dye molecule and on its relative molecular mass and concentration, but the reaction can be represented as in Scheme 12.20 for an anthraquinonoid dye with two keto groups. The effect of air oxidation on alkaline Table 12.32 Effect of particle size on rate of reduction of CI Vat Green 1 at 20 °C [221] Percentage of dye particles >1.05 µm

Time (min) to reduce 50% of the dye

66 42 22 6 2 1

206 120 70 42 28 7

CI Vat Green 1 (23%) Sodium dithionite Caustic soda 38°Be

100 mg/l 5 g/l 10 g/l O

+ Na2S2O4 + 4 NaOH

O

ONa

+ 2 Na2SO3 + 2 H2O

Scheme 12.20

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Na2S2O4 + 2 NaOH + O2

Na2SO3 + Na2SO4 + H2O

Scheme 12.21

dithionite must also be taken into account in practical situations and this can be represented by Scheme 12.21 [30]. In practice, more sulphite than sulphate tends to be formed. Clearly, this atmospheric oxidation results not only in loss of reducing agent but also of alkali, another reason why excess alkali as well as reducing agent is required. The influence of atmospheric oxidation varies enormously. Stability may be maintained for several hours in a large dyeing vessel providing the surface area of the liquor is relatively small, compared with only 30–60 seconds in a padding system. The ideal pH value for this system is between 12 and 13; below pH 12 there is an increasing danger that the dye will either revert to its keto form or yield the leuco acid, whereas above pH 13 there is a danger that the dithionite may decompose to form thiosulphate, sulphite or even sulphide if the temperature is high enough. Specifically in the case of indigo, however, better reproducibility is achieved when buffered to pH 10.6–11.4 [223], rather than the traditional application conditions of pH 12.1–12.9. Mention has been made above of the need to have both dithionite and alkali present to excess in most dyeing systems. Unfortunately this has often led to the indiscriminate use of these chemicals, particularly of dithionite, resulting in high chemical costs and excessive contamination of the effluent. Providing suitable analytical monitoring is available, cost savings up to 40% are achievable if the quantitative amount of dithionite is present in the system [224]. Automatic dosing systems can result in both cost savings and improved quality. This is exemplified in Figure 12.22, which indicates that the conventional process of topping up at each end in jig dyeing results in a fluctuating dithionite concentration, whereas automatic dosing enables the required minimum concentration of dithionite to be constantly maintained. Conventional method

Automatic dosing method

8

8

Sodium dithionite/g l–1

10

Sodium dithionite/g l–1

10

6

4 2

6

4 2

1 15

2 30

3 45

4 Ends 60 Time/min

1 15

2 30

3 45

4 Ends 60 Time/min

Figure 12.22 Variations in the concentration of sodium dithionite in jig dyeing using conventional and automatic dosing methods [225]

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Table 12.33 Classification of vat dyes and chemical requirements Group

Characteristics

IK

Relatively low substantivity for cellulose. Dyed at ambient temperature with a small amount of caustic soda and a high salt concentration. Higher substantivity for cellulose. Dye at 45–50 °C with more caustic soda and less salt (none for mercerised cotton or regenerated cellulosic fibres). Much higher substantivity for cellulose. Dyed at 60 °C with even more caustic soda but without salt. High substantivity and the maximum alkali concentration.

IW IN IN Special

Table 12.34 Guideline chemical concentrations for 2% o.w.f. vat dye at 10:1 liquor ratio [226]

Dye group

Temperature (°C)

Caustic soda (g/l)

Sodium dithionite (g/l)

Sodium sulphate (g/l)

IK IW IN

20–25 45–50 60

3.6 4.8 8.8

3.0 4.0 5.0

12.0 12.0 –

Electrolyte is sometimes used in the application of vat dyes from alkaline dithionite dyebaths. The quantities of chemicals required vary according to conditions and dye manufacturers’ literature should be carefully consulted, supplemented by local knowledge based on experience. Vat dyes are generally classified into several groups (Table 12.33). Guideline recommendations for IK, IW and IN dyes applied from a 10:1 liquor are given in Table 12.34. Another source [30] indicates typical amounts as varying from: 5 ml/l Caustic soda 27% (1.35 g/l NaOH) 2 g/l Sodium dithionite for pale depths of 1K dyes at 20:1 to: 25 ml/l Caustic soda 27% (6.75 g/l NaOH) 7 g/l Sodium dithionite for full depths of IN dyes at 10:1 Higher dyeing temperatures (up to 115 °C) are selected in some cases. These conditions require a more stable reducing agent of the hydroxyalkylsulphinate type, typical concentrations being 10–20 ml/l caustic soda 27% and 5–10 g/l reducing agent according to dye concentration, liquor ratio and temperature. A reduction inhibitor, such as glucose, may be added when applying dyes that are sensitive to over-reduction [30]. In the so-called pigmentation process, the finely dispersed vat pigments are first applied at 60–80 °C, usually with addition of electrolyte, followed by reduction to the vat leuco form using dithionite and alkali. The stability and efficacy of sodium dithionite can be enhanced by addition of polyacrylamide, a product more frequently used as a migration inhibitor in continuous dyeing processes [227]. This biodegradable polymer allows the amount of dithionite to be

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decreased, giving improved fixation with less contamination of effluent by dye and dithionite. In jig dyeing polyacrylamide helps to prevent oxidation spots and minimises endto-end and side-to-side variations. There are potential environmental problems associated with sodium dithionite, since in effluent it can produce sulphite and sulphate. Although the sulphite can be oxidised quite easily to sulphate, this does not entirely obviate the problems since high concentrations of sulphate can cause damage to unprotected concrete pipes. Thus there are environmental reasons for alternative reducing agents to dithionite and several are available, although mainly used for special purposes. In particular, their greater stability to atmospheric oxidation makes them of special interest for continuous dyeing and printing processes, rather than for batchwise dyeing. Thiourea dioxide (12.50) or formamidinesulphinic acid is a powerful reducing agent for vat dyes. This compound has a lower relative molecular mass than sodium dithionite and gives lower concentrations of sulphite and sulphate in the effluent, but shows certain disadvantages. The mode of action in hot aqueous alkali is represented in Scheme 12.22, showing first the rearrangement to sodium formamidinesulphinate (12.51), which then decomposes to yield urea and the active reducing species, sodium hydrogen sulphoxylate (12.52). Although thiourea dioxide is more stable than dithionic acid, the formamidinesulphinate formed in alkaline media is more readily oxidised than dithionite, thus negating any potential advantages. This agent can cause over-reduction of indanthrone vat dyes, against which inhibitors such as glucose or sodium nitrite have no palliative effect. Experimental work has indicated the possibility of using thiourea dioxide in combination with other compounds such as (a) sodium dithionite, formaldehyde and sodium hydroxide, or (b) saturated aliphatic ketones, but commercial exploitation has not been evident [218]. Greater commercial significance is attached to certain derivatives of sodium dithionite, including sodium formaldehyde-sulphoxylate (12.53; hydroxymethanesulphinate) (Scheme 12.23) and the less important sodium acetaldehyde-sulphoxylate (12.54; hydroxyethanesulphinate). These reducing agents have been of particular interest in printing, especially in the flash-ageing process [228–231]. The formation of sodium formaldehyde-sulphoxylate by reaction of sodium dithionite with formaldehyde is shown in Scheme 12.23; the bisulphite formed can be further reduced with zinc to produce another molecule of the sulphoxylate. H2N

O C

H2N

NaOH

S O

H2N

ONa C

HN

12.50

H2O

S O

H2N C

+

O

HO

S

H2N 12.52

12.51

Scheme 12.22 O O

O

HOCH2

H NaO

S

S

ONa

C

+ 2 H

Scheme 12.23

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O

+ H2O

O

S

+ O

HOCH2

12.53

898

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S

ONa

ONa

VAT DYES

899

CH3 HO

CH

O

S

ONa

HOCH2

O

S

HOCH2

OZn(OH)

S

OCa(OH)

12.56

12.55

12.54

O

These reducing agents are much more stable than sodium dithionite at lower temperatures; hence they can be used to prepare stable pad liquors and print pastes. At higher temperatures, as in steam fixation treatments, they are capable of bringing about rapid reduction of vat dyes. Sodium formaldehyde-sulphoxylate was used first in conventional steam fixation of vat prints, although the acetaldehyde analogue was initially preferred for the flash-ageing process. As vat dyes are invariably fixed under alkaline conditions, the sodium salts of the sulphoxylates are preferred to the basic salts of zinc (12.55) or calcium (12.56), which are unstable under alkaline conditions. Sodium formaldehyde-sulphoxylate has been used occasionally in combination with sodium dithionite [218) but other two-component or two-phase systems based on formaldehyde-sulphoxylates have generally depended on an accelerating or catalyst system. For example, a process that has been adopted to some extent in bulk practice [232,233] comprises a strongly alkaline solution of sodium borohydride (12.57; sodium tetrahydroborate), together with a second reducing system consisting of sodium formaldehydesulphoxylate and the catalyst sodium nickel cyanide. Various advantages have been claimed for this process, although there are misgivings regarding the environmental acceptability of sodium nickel cyanide [234]. Other accelerators used with sodium formaldehydesulphoxylate include sodium dimethylglyoxime complexes, anthraquinone and aminoanthraquinonesulphonic acids. Although sodium borohydride is itself a reducing agent, it generally reacts too slowly alone for use in vat systems; nor is there any evidence that it will act as a stabiliser for sodium dithionite [235], as has sometimes been suggested. Another reducing agent, suggested [236] for both flash-age printing and batchwise hightemperature package dyeing, is trisodium nitrilotriethanesulphinate (12.58). This does not appear to have attained commercial use, however.

12.57

NaBH4

N

CH2CH2

O

S

ONa

CH2CH2

O

S

ONa

CH2CH2

O

S

ONa

12.58

Hydroxyacetone (12.48), mentioned in section 12.8.1 in connection with sulphur dyes, is sulphur-free and biodegradable. This compound was originally proposed for use with vat dyes and continues to generate some interest. This agent can be used for the pad–steam application of vat dyes in the presence of high concentrations of sodium hydroxide (about 3.5–4.5 g/l). Hydroxyacetone does not cause over-reduction of indanthrone vat dyes but does give different shades with carbazole dyes, compared with sodium dithionite [218]. The optimised and metered use of this product is central to the RD (Refine Dyeing) process of Ipose GmbH [212]. This process, for which both environmental and economic advantages are claimed, is suitable for applying indigo on continuous yarn-dyeing ranges and for knitgoods

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dyeing with vat dyes that are difficult to reduce. Although colour yields are not quite as high as with dithionite, the advantages include biodegradability, decreased chemical usage, lower COD values and the effluent contains no sulphide, sulphite or sulphate. This process depends on optimal machinery technology coupled with optimised computer-controlled chemical dosing. Specific advantages in continuous yarn dyeing with indigo include [212]: (1) high quality with a good ring-dyeing effect, a prime requirement in indigo warp dyeing for denim jeans (2) greater elasticity of yarn with fewer yarn breaks and therefore higher productivity (3) higher dye exhaustion and thus less residual dye in the effluent (4) lower chemical usage and thus 20% less waste liquor (5) liquid form of indigo for ease of application (6) minimal effluent problems as only biocompatible chemicals are used, with possibilities for recycling of liquors and recovery of dye through ultrafiltration (7) no health hazard nor odour of hydrogen sulphide. Typical waste water analyses and relative costings for sodium dithionite and hydroxyacetone are given in Table 12.35.

Table 12.35 Continuous dyeing of cotton yarn with indigo using sodium dithionite or hydroxyacetone [212] Waste water analysis

Sodium dithionite

Dye (g/l) Sulphide (mg/l) Sulphite (mg/l) Sulphate (g/l) Phosphate (mg/l) *COD (mg/l) Surfactant (mg/l) Sequestering agent (mg/l)

1.5 12.5 64 19 60 7000–8000 18–19 6

0 1 1 0.2 6 700–1000 1.5 0

100% 100%

77% 140%

Dye and chemical cost Productivity

Hydroxyacetone

* COD values for hydroxyacetone are after treatment in a bioreactor; such treatment is not possible with dithionite.

Tests using indigo have shown that reduction with alkaline hydroxyacetone is most reproducible when the concentrations are as follows [237]: Hydroxyacetone 5.0–12.5 ml/l or 0.054–0.135 mol/l Sodium hydroxide 5.0–12.5 g/l or 0.125–0.312 mol/l It is interesting to note that the rate of vatting of indigo with hydroxyacetone under these conditions can be increased approximately fourfold using ultrasound, as illustrated in Figure 12.23. The ultrasound causes cavitation in the indigo pigment dispersion, accelerating the disintegration of the insoluble dye particles and thus increasing the probability of collisions between reducing agent and dye molecules.

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A With ultrasound B Without ultrasound

1.0 Absorbance at 710nm

901

0.75 0.5 A

B

0.25

500

1000

1500

2000

3000

Reaction time/s

Figure 12.23 Rate of reduction of indigo with and without ultrasound [237]. 0.1 g/l Indigo, 40 °C; 2.5 ml/l hydroxyacetone; 5.0 g/l sodium hydroxide, pH 12.7; 0.03 g/l anionic dispersing agent

Iron complexes have been investigated as alternatives to sodium dithionite in vat dyeing [238]. Iron(II) hydroxide Fe(OH)2 is a powerful reducing agent and this reducing power increases with increasing alkalinity of the medium. Alkalinity results in precipitation, however, and the hydroxide must be converted to a complex in order to maintain solution. The complex must be chosen to be reasonably stable, but not so inert that the iron(II) ion cannot exert its reducing action. Complexes with triethanolamine or gluconic acid are suitable, the latter being favoured on environmental grounds because it does not contain nitrogen. Investigation at a liquor ratio of 10:1 showed 50 ml/l caustic soda 38°Bé to be optimum, lower concentrations of alkali producing duller and weaker dyeings. The molar ratio of iron(II) ion to gluconic acid in the complex prepared by reacting iron(II) chloride tetrahydrate with gluconic acid was significant. Less reducing agent was required for vat dye reduction at a molar ratio of 1:2 than at 1:1 but too low a concentration of iron(II) ions resulted in paler and unlevel dyeings because of insufficient vatting of the dye pigment. It is environmentally advantageous to keep the gluconic acid concentration low to ensure a low COD value. A 1:1 molar ratio is probably the optimum. The formation of iron(III) ions lowers colour yield but levelness is not affected because the gluconic acid holds the iron(III) ions in solution. The use of iron(II)-gluconic acid did not cause overreduction of sensitive dyes even when dyeing was prolonged. With most dyes the colour yield was equal to that given by sodium dithionite. Precipitation of iron(III) hydroxide facilitates the elimination of dyes and auxiliaries from effluent after only a brief settlement time. The application of indirect electrolysis in conjunction with regenerable redox systems, already described in section 12.8.1 for sulphur dyeing, has attracted considerable research interest for the reduction of vat dyes [239–241]. In environmental terms, the advantages of this system for vat dyeing would be similar to those for sulphur dyeing. Figure 12.21 and Table 12.29 are essential to an understanding of the physico-chemical aspects of the process. The reduction potentials attainable with anthraquinone derivatives as mediators of the redox system are shown in Table 12.36. In general, these potentials are too low for vat dyes, as can be seen by reference to Figure 12.21.

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Table 12.36 Peak cathodic potentials of anthraquinone derivatives measured using Ag/AgCl/3M KCl in 4 g/l sodium hydroxide solution [239]

Anthraquinone derivative

Cathodic potential (mV)

1,2,5,8-Tetrahydroxy 1,2-Dihydroxy (alizarin) 1,8-Dihydroxy (chrysazin) 1,4-Dihydroxy (quinizarin) 1,2-Dihydroxy-3-sulphonic acid 2-Sulphonic acid (β acid) 1,5-Disulphonic acid 1-Amino-2-carboxylic acid 1-Amino-2-sulphonic acid 2,6-Disulphonic acid

–885 –880 –770 –760 –760 –750 –750 –750 –720 –560

Currently the most suitable system, that will generate potentials up to –1050 mV, is the iron-triethanolamine complex prepared from either iron(II) or iron(III) salts. Using iron(III) sulphate penta- or hexahydrate, for example, dyebaths are prepared by first dissolving sodium hydroxide in a small amount of water, to which is added the triethanolamine. Hydrated iron(III) sulphate is separately dissolved in a small amount of water and then added to the alkaline triethanolamine solution until the initially precipitated iron(III) hydroxide redissolves, after which the solution is diluted to full volume to give:

Sodium hydroxide Triethanolamine Hydrated iron(III) sulphate

g/l

mol/l

20 60 14

0.5 0.45 0.027

A schematic illustration of this process is shown in Figure 12.24.

Oxidised dye (insoluble)

Oxidised mediator

Reduced dye (soluble)

Cathode

Reduced mediator

Heterogeneous redox reaction

Figure 12.24 Schematic diagram of the cathodic reduction of the mediator that converts the insoluble vat dye into its soluble leuco form [239]

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903

The essence of the technique is as follows [239]. The cathode reduces the mediator which then reduces the dye. This mediator (the regenerable redox system) must continually produce a consistent reduction potential in the dye liquor, so that no reducing agent has to be added. The prevailing potential is defined by the Nernst Equation (12.2): Redox pair: Oxi + ne- ì Red n -

E = Eo +

E° E R T F [Oxi] [Red] n

= = = = = = = =

(12.1)

RT [Oxi] ™ ln nF [Red]

(12.2)

standard potential (mV) of the redox pair (under the experimental conditions) potential (mV) prevailing in the solution molar gas constant (8.314 J/°K.mol) absolute temperature (°K) Faraday constant (96 500 coulombs) concentration (mol/l) of the oxidised form of the redox pair concentration (mol/l) of the reduced form of the redox pair electrochemical valency.

If the concentrations [Oxi] and [Red] are identical, the potential prevailing in solution is identical to the standard potential E° of the redox pair under the conditions of use. If the ratio of these concentrations [Oxi]/[Red] is modified by electrolysis, the potential changes accordingly: if [Oxi] > [Red], E becomes positive and the action is one of oxidation if [Oxi] < [Red], E becomes negative and the action is one of reduction With conventional reducing agents the ratio [Oxi]/[Red] is determined by the decreasing concentration of reducing agent and the increasing concentration of oxidation products resulting from its decomposition. In an electrolytic process, however, this ratio can be adjusted so as to continually regenerate the redox pair and no secondary products are formed in the dye liquor, nor is there a build-up of unwanted oxidation products. After dyeing, the residual leuco dye can be reoxidised in the exhaust dyebath and removed by filtration; thus the dye liquor together with the mediator can be used several times, creating considerable ecological advantages. The requirements of a mediator or redox catalyst system are: (1) only a small loss (if any) in activity during useful life; therefore the maximum possible number of reaction cycles (2) rapid conversion at the electrode surface (3) no catalysis of side reactions or changes in colour (4) no affinity for the fibre (5) no reaction with the solvent (6) adequate solubility (7) non-toxic (8) no problems in effluent treatment (9) inexpensive.

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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES

A more detailed overview of the process is shown in Figure 12.25, highlighting the main steps of a vat dyeing process. The quotient of a pair of k values (rate constants) for a reaction in the forward and reverse directions is a measure of the equilibrium constant of the reaction step. The reduction of the suspended oxidised form of the dye is characterised by the pair of rate constants, k1 for reduction and k2 for reoxidation. For effective vatting of the dye to be achieved, k1 should be greater than k2. A measure of the affinity of the reduced dye is given by the quotient of k3 and k4; a high value of k3/k4 is characteristic of dyes with good affinity for cellulosic fibres in their reduced form, leading to a high degree of dye exhaustion. This can cause problems in the dyehouse resulting from rapid exhaustion of the dyebath. The quotient k8/k7 describes the stabilisation of the reduced form of the dye adsorbed on the fibre. A high k8/k7 quotient indicates that the adsorbed leuco dye remains stable in its reduced form, an important precondition in achieving equilibrium during exhaustion of the dyebath. When the quotients k1/k2 or k8/k7 are small, the dyeing process is disrupted by all oxidising agents, including air, leading to unlevelness in the dyeing. It is evident that the rates of dye reduction (k1 and k8) are of great importance in ensuring reproducibility of dyeing. The rate of reduction achieved by the iron(II)-triethanolamine complex is several orders of magnitude greater than that given by sodium dithionite; therefore the reduced leuco dye is effectively stabilised and the resulting dyeings are more reproducible in the case of electrochemical reduction.

Oxidation by air k5

Spontaneous decomposition Oxidised dye k6 in dyebath

Reducing agent

k1

k2 Reduced dye in solution

Exhaustion

k3

Reducing agent

k8

Reduced dye in the fibre Oxidised dye in the fibre

Oxidation by air Dyebath

k4

Desorption Cellulose fibre

k7

Oxidation by air

Favourable: k1 > k2 Disadvantageous: k1 < k2 High affinity: k3 > k4 k8 > k7

k8 < k7

Figure 12.25 Reaction scheme of the steps occurring in a vat dye reduction process [240]

The special requirements of the indigo dyeing of cotton warp yarns for denim are capable of being met by indirect electrolysis systems [241]. Examples of four suitable redox systems are shown in Table 12.37. Uniform build-up of depth was observed with each successive step, the results being at least equal to those from the conventional dithionite-based process. Apparently these processes are amenable to scaling up to bulk production levels [241]. In spite of the many reducing systems evaluated or proposed (often quite convincingly), sodium dithionite remains the agent almost universally preferred [242]; others such as thiourea dioxide, sulphinates and hydroxyacetone are only used for special purposes. The advantages and disadvantages of the most important reducing agents are summarised in Table 12.38.

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905

It is now opportune to consider additives for reduction systems. As mentioned above, the reduction process is invariably carried out in an alkaline medium, the most common alkali being sodium hydroxide particularly in batchwise dyeing. In printing and certain continuous dyeing processes sodium hydroxide enhances reduction potential but impairs the stability of the reducing agent, increasing the danger of premature loss of reducing action, particularly during the drying operation prior to steaming. In these processes carbonates give greater stability and hence are preferred [243]. There is little difference between sodium and potassium carbonates in terms of effect on the reducing agent, but there are advantages to be gained from using potassium carbonate to prepare relatively concentrated print pastes and pad liquors. Not only does potassium carbonate have a higher aqueous solubility at 20 °C (112% w/w) than sodium carbonate (21%), but the potassium leuco salts of vat dyes are also more soluble [29]. It is important to ensure the presence of excess alkali to counteract that consumed by the reducing agent and the substrate, as well as any reducing agent lost through atmospheric oxidation of the system [30]. Electrolyte may be used to enhance exhaustion of the leuco dye, particularly in batchwise dyeing. The use of electrolyte in the essentially ‘short-liquor’ printing and continuous dyeing processes is seldom necessary and could be inadvisable, as it may promote tailing during padding. However, electrolyte plays a positive part in those continuous processes in which a dispersion of the pigment form of the dye is applied first followed by separate application of reducing agent in the so-called chemical pad. The amounts of alkali and electrolyte

Table 12.37 Redox recipes used in indirect electrolysis application of indigo to cotton yarn [241]

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Recipe

Components

Concentrations

1

Iron(III) chloride Iron(II) chloride Gluconic acid Sodium heptagluconate Calcium carbonate Calcium chloride Sodium hydroxide

0.18 mol/l 0.06 mol/l 0.30 mol/l 0.12 mol/l 0.09 mol/l 0.09 mol/l to pH 11.5

2

Iron(III) sulphate hexahydrate Iron(II) sulphate heptahydrate Calcium heptagluconate Anionic dispersing agent Sodium hydroxide

3

Iron(III) sulphate hexahydrate Triethanolamine Sodium gluconate Sodium hydroxide

4

Iron(III) chloride hexahydrate Sodium gluconate Calcium chloride Calcium heptagluconate Sodium hydroxide

905

0.052 mol/l 0.060 mol/l 0.129 mol/l 0.5 g/l 17.1 g/l pH 11.5 61.3 g/l 227 g/l 2 g/l 69 g/l pH 13.5 54.0 g/l 54.5 g/l 11.1 g/l 24.5 g/l 20.0 g/l pH 11.4

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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES

Table 12.38 Summary of advantages and disadvantages of the three most important types of reducing agent for indigo, vat or sulphur dyeing [239] Sodium dithionite

Advantages (1) (2)

sufficient reduction potential for vat, sulphur or indigo dyeing good stability of leuco vat dyebaths

Disadvantages (1) (2) (3) Sulphinic acid derivatives

waste water loading; inhibits biological degradation and leads to a greater oxygen demand can cause over-reduction at higher temperatures special safe storage facility required

Advantages (1) (2) (3)

sufficient reduction potential for vat, sulphur or indigo dyeing especially suitable for high-temperature dyeing methods good resistance to oxidation by air

Disadvantages (1) (2) Hydroxyacetone

as for dithionite disadvantage (1) potentially temperature-dependent

Advantages (1) (2) (3)

biologically degradable ease of dosing in liquid form very good stability to storage

Disadvantages (1) (2) (3)

does not reach full reduction potential and thus mainly suitable for indigo and sulphur dyes persistent odour limited commercial production

obviously vary widely according to dyeing parameters (such as the liquor ratio) and the dyes used. A fully optimised computer-based program for recipe-specific calculation of quantities of sodium dithionite, sodium hydroxide and electrolyte has been published [244]. The advantage of such a program is that it enables exact quantities to be calculated, thus giving cost savings in chemicals and effluent treatment. Levelling agents are frequently used, since the initial strike by leuco vat dyes can be rapid. A nonionic surfactant such as cetyl poly(oxyethylene) alcohol of optimum chain length (such as structure 12.25) is particularly useful because it can also act as a wetting agent. Poly(ethylene imine) derivatives of fatty alcohols behave in a similar way. On the other hand, certain products that act through a complexing mechanism are not surface-active; for example, one such proprietary product is based on poly(vinylpyrrolidone). Wetting and dispersing agents are also useful, particularly with inadequately prepared substrates.

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906

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VAT DYES

907

Aliphatic sulphonates are widely used as wetting agents, particularly in combination with non-surfactant levelling agents. Any surfactants used must be stable under the alkaline reducing conditions. For continuous pad application they should also have a low propensity to foam; phosphoric esters are preferred to aliphatic sulphonates in this respect and in regard to the amount of pad liquor absorbed [221]. Dispersing agents, sometimes referred to as protective colloids, help to maintain the particulate distribution of the vat dye in pigment form and also inhibit aggregation of sparingly soluble leuco compounds; naphthalenesulphonic acid/formaldehyde condensates are especially useful. They are also essential additives in the application of vat dyes by the so-called pigmentation methods, using either the unreduced vat dyes themselves or, less frequently, acid leuco compounds [245]. A sequestering agent such as EDTA will prevent the formation of insoluble metal salts of vat leuco compounds. Trace metal ions would otherwise interfere with the application process and with level dyeing. It has been reported [246] that sequestering agents of the alkylphosphonate type can retard the uptake of vat dyes. An addition of pyrocatechol or tannic acid can help to counteract the well-known propensity of certain yellow and orange vat dyes to induce fibre tendering, whilst the addition of glucose or sodium nitrite can help to prevent the over-reduction of certain dyes. Rinsing after exhaustion of the leuco vat dye has a marked influence on the final result, especially as regards the final hue, depth of shade and fastness properties. Figure 12.26 illustrates the effect of rinsing temperature and pH on the colour yield of dyeings of pyranthrone (12.59; CI Vat Orange 9). In general, yield decreases significantly with increasing temperature and to some extent with increasing pH, particularly at 70 °C. Dyeings tend to be duller after rinsing at a high temperature and alkaline pH, although this effect varies from dye to dye. Overall, rinsing should be carried out thoroughly at a low temperature and at pH 7 [247]. Rinsing should be neither too brief (inefficient) nor too prolonged (wasteful of water and processing time). A study of aftertreatments in jet and overflow machines [248] has shown advantages from optimising the rinsing process in relation to the concentrations of residual dithionite at the end of the dyeing stage and after rinsing. The dithionite is measured by titration, either manually by iodometry or automatically in an atmosphere of nitrogen according to the hexacyanoferrate method. A nominal target value (between 2 and 4 g/l) is set for the dithionite concentration at the end of dyeing, depending on dyeing machine type, fabric quality and dyes present. Starting from this value, a defined amount of rinsing water is used to dilute the dyebath in a given time, aiming for a final concentration of 0.5 g/l after rinsing. This procedure implies that complete removal of dithionite is unnecessary. 12.9.2 Oxidation Various methods have been used for the reoxidation of vat leuco dyeings; atmospheric skying, hypochlorite, chlorite and acidified dichromate are now rarely employed. Atmospheric oxidation can be difficult to control and thus uneven; with some dyes it is also too slow, particularly for continuous methods. Sodium hypochlorite is used only for those few black dyes that tend to become dark green when oxidised with peroxide; obviously hypochlorite should be avoided with the many chlorine-sensitive dyes. Similarly sodium chlorite, acidified to below pH 5 with acetic acid, can only be used with certain dyes, although with these it certainly gives rapid oxidation. Dye selectivity is also a drawback with

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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES

13 30 oC

Colour yield (K/S)

12 11

50 oC

10 9

70 oC

8

4

5

6

7

8

9

pH of rinsing treatment

Figure 12.26 Effect of temperature and pH of rinsing on the colour yield of dyeings of CI Vat Orange 9 [247] O

O 12.59 Pyranthrone

acidified dichromate, since not surprisingly its chelating potential can give rise to hue variation, especially with sensitive blues and greens. The AOX-generating potential of the chlorine-containing oxidants and the toxicity of chromium also make these compounds environmentally undesirable. The most commonly used oxidising agents are hydrogen peroxide, sodium perborate and sodium m-nitrobenzenesulphonate [30]. Sodium nitrite acidified with sulphuric acid is used to hydrolyse and reoxidise the solubilised vat leuco ester dyes. Hydrogen peroxide and sodium perborate remain the mainstay of the vat dye reoxidation process; the amounts to be used vary widely according to the conditions of processing. Consequently, dye manufacturers’ recommendations can only be used as guidelines and may need modification according to the machinery and conditions used. Many important factors have to be considered, especially the liquor ratio, the contact frequency between liquor and goods, the particular dyes and the depth of shade, as well as the efficiency with which the reduction liquor has been rinsed out. For example, in a long-liquor (20:1 to 30:1) winch dyeing process as little as 0.5–1.0 g/l hydrogen peroxide (130 vol.) or sodium perborate at 50 °C is usually adequate; this may be increased to 3–5 g/l at 50–60 °C in short-liquor jig dyeing. The rate of oxidation can be controlled by addition of acetic acid to neutralise excess

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908

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VAT DYES

909

alkali carried over from reduction but care should be taken not to over-acidify as this would tend to convert the sodium leuco compound into the less soluble acid leuco form. Some blues are exceptionally sensitive to alkaline oxidation, however, and must therefore be rinsed thoroughly before oxidation in the presence of a small amount of acetic acid. Over-oxidation can often be corrected by again reducing the dyeing and then reoxidising under more carefully controlled conditions. Sodium m-nitrobenzenesulphonate has been proposed as an oxidising agent for vat dyes. It is available as a proprietary product and is claimed to react with leuco compounds more quickly than does peroxide. The solubilised vat leuco esters are most commonly hydrolysed and reoxidised to the insoluble parent dye using sodium nitrite and sulphuric acid. Alternative oxidising agents for vat leuco esters include hydrogen peroxide and ammonium metavanadate (NH4VO3), persulphates and nitric acid [218]. 12.9.3 Soaping The final soaping at the boil of reoxidised vat dyeings must be regarded as an integral part of the application sequence, rather than an optional extra, since it determines the reproducibility of colour and fastness. The traditional process was to use a boiling bath containing 3–5 g/l Marseilles (olive oil) soap and 2 g/l sodium carbonate, usually for 10–15 minutes in a batchwise process. Soaping times of less than a minute are the norm in continuous pad–steam ranges, however, and it is then preferable to select dyes that show only a slight change in colour during soaping [30,245]. Synthetic surfactants, particularly of the nonionic type, are now used instead of soap, although they contribute to an increased effluent load and should be used sparingly [247]. Boiling off in water alone is preferable as the dyeings obtained are similar and in no case do they exhibit fastness ratings inferior to those from washing with a detergent. This is clearly an important finding in view of the fact that vat dyeing effluent is already heavily contaminated [247]. Various mechanisms have been proposed to explain what happens during soaping. Investigations [249,250] suggest that dye molecules at first aligned along the fibre axis are converted by soaping to crystalline aggregates reoriented at right angles to the fibre axis. Evidence exists, however, to show that some dyes are present as amorphous aggregates both before and after soaping, whereas reoxidised indigo particles are crystalline even before soaping. Another possibility is that an initially metastable crystal modification of the dye is converted by soaping into a more stable crystalline form. 12.9.4 Continuous dyeing and printing In general, the reducing, oxidising and soaping agents for continuous dyeing are the same as those used in batchwise dyeing, although they have to be used at higher concentrations. In some cases there may be a preference for more stable reducing agents than dithionite, such as sulphoxylates; in continuous dyeing dithionite is still the most widely applicable product [221], although sulphoxylates are undoubtedly preferred in printing. In continuous pigment padding a migration inhibitor is essential to prevent tailing, twosidedness or general unlevelness, as well as to stabilise the pad liquor against pigment sedimentation [221]. It is important to select agents that promote the tendency of dye particles to agglomerate, as this is the mechanism whereby migration is inhibited. Poly(acrylic acid) derivatives, alginates,

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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES

branched polysaccharides and block copolymers of ethylene oxide and propylene oxide have proved useful. Conversely, some printing thickeners such as locust bean gum, starch ethers, guar derivatives and carboxymethylcellulose have been less successful in preventing particulate migration in continuous dyeing, thus indicating that a mere increase in viscosity is ineffective in controlling migration [221]. In direct printing by an all-in, single-stage process the most favoured reducing system is sodium formaldehyde-sulphoxylate with potassium carbonate as alkali. A thickening agent is required; British gum thickening may accelerate decomposition of the reducing agent but otherwise the choice of thickening agent is not critical. A hydrotrope helps to improve fixation in steam. A typical print paste recipe is given in Table 12.39. A reduction accelerator such as an aminoanthraquinonesulphonic acid may be added. The flash-ageing method is a two-stage process in which the initial print paste contains dye and thickener only [251]. The reducing agent and alkali are subsequently applied by padding. In this process exposure to air prior to steaming is minimal and the less stable sodium dithionite can be used as the reducing agent. The thickening agent should coagulate in the presence of alkali in order to inhibit bleeding of colour; alternatively a gelling effect can be obtained by adding a borate to a thickener such as locust bean gum, as described in section 10.8.1. Mixtures of thickeners are often formulated to attain the required degree of coagulation; if this is exceeded there can be problems at the washing-off stage. Table 12.39 Typical print paste recipe for a single-stage process with vat dyes Amount (% w/w) Vat dye Potassium carbonate Sodium formaldehyde-sulphoxylate Glycerol (hydrotrope) Thickening agent Water to

x 15 8 5 24 100

The solubilised vat leuco esters are particularly suitable for printing. There are two main methods of hydrolysis and reoxidation. In the first sodium nitrite is incorporated in the print paste and this is converted to nitrous acid by a short immersion of the printed goods in dilute sulphuric acid after drying and steaming. Sodium chlorate is used in the second method by incorporation into the print paste together with ammonia (to maintain alkalinity in the paste) and a steam-activated acid generator such as ammonium thiocyanate (NH4SCN); an oxidation catalyst, ammonium metavanadate (NH4VO3), is also used. Vat dyes can be used as illuminant colours in discharge styles where the discharged grounds may be azoic, direct or reactive dyeings. In such cases the quantity of reducing agent has to be increased since it is needed for discharging the other dyes as well as for reducing (but not discharging) the vat dyes. A further agent, known as a leucotrope, may also be needed if white discharges are required; leucotropes act as discharge (or stripping) promoters and are described below.

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12.9.5 Correction of faults Levelling and partial stripping may be carried out in a reducing bath using sodium dithionite and sodium hydroxide in combination with a suitable levelling agent such as those described earlier. Greater quantities of levelling agent can be employed to increase the stripping effect. The complexing agents are more effective than nonionic levelling agents. Certain quaternary ammonium compounds (such as structures 12.60 and 12.61) promote stripping by complexing with the leuco form of the dye. These products, often referred to as leucotropes, are used in conjunction with sodium dithionite and sodium hydroxide to give an effective stripping action. H2C

_ Cl CH3 + H3C N CH2

H2C

_ SO3

CH2

_

CH3 + H3C N (CH2)15CH3 Br

Ca

_

2+

SO3

CH3

12.60

12.61

CH

CH2

N H2C H2C

C CH2

CH2 C N

O

CH

O n

12.62

Poly(vinylpyrrolidone)(12.62) has an Mr range of 1–9 × 104 and a DP of 100–800. The macromolecular chains are considerably folded in aqueous solution. The pyrrolidone rings are oriented at right angles to the carbon chain, giving a hydrophilic series of keto groups on one side and hydrophobic propylene segments on the other [252]. This polymer readily forms complexes with vat leuco anions in alkaline dithionite solution to give a highly effective stripping action. 12.9.6 Ecological considerations Several ecological points have been mentioned above; an overall summary [220,253] will be useful here. Certain factors in dealing with ecological problems in dyehouses are common to all textile dyes, not just vat dyes. The primary approach, of course, is precise optimisation of all parameters so that the minimal quantity of all additives is used, thus ensuring minimum loading of the waste water. Dyeing at as low a liquor ratio as possible saves water and energy, as well as giving a lower volume of effluent. Shorter liquors give more concentrated waste water but this need not be a disadvantage, since it may facilitate subsequent treatments including precipitation, flocculation, filtration or centrifugation. The second factor is to choose products, both colorants and auxiliaries, that are the most environmentally favourable on the market; all reputable manufacturers and suppliers are working diligently in this direction. Indeed, the chemical industry and the textile wet processing sector have shown a responsible and increasingly effective approach to the varied and often difficult environmental problems that have arisen. As already noted, vat dyeings generally give more heavily loaded waste waters than direct or reactive dyeings but these loads are reasonably treatable. A specific advantage of vat

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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES

Table 12.40 An environmental assessment of commercial brands of vat dyes [253] Eliminability of organic halogen compounds

Ecotoxicity

Heavy metal impurities Dispersants and additives

Completely eliminated by adsorption onto sewage sludge; no organic halogen compounds in the outlet of the treatment plant Insoluble in water, therefore not toxic to bacteria, algae or fish; not bioavailable, so do not accumulate in living organisms Range-specific differences Range-specific differences

Table 12.41 Environmental properties of reducing agents for dyeing with vat dyes [253] Inorganic

Sodium dithionite

Advantages (1) (2)

Universal and highly effective Free from heavy metals

Disadvantage Sulphite/sulphate in waste water (oxygen-consuming, may damage piping) Organic

Hydroxyacetone (other products with improved properties being developed) Advantages (1) (2) (3)

Free from sulphite and heavy metals Nontoxic to aquatic life Readily biodegradable

Disadvantages (1) (2)

Increased TOC/COD Weaker reductive effect than dithionite

Table 12.42 Remedial action for operating with reducing agents for vat dyes [253] Reducing agents

Recommendations for dealing with the drawbacks

(1) Dithionite and derivatives

Minimising the concentration by optimising Oxidation of sulphite to sulphate (isothermal dyeing with oxidation in the dyebath for pale shades)

(2) Hydroxyacetone

In certain applications dithionite can be replaced by hydroxyacetone (e.g. indigo dyeing)

(3) Mixtures of dithionite and hydroxyacetone

Used in optimum proportions for the particular application, it is possible to reduce contamination of the effluent by sulphite to the minimum

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dyeing is that these insoluble dyes pose few hazards. The LD50 of vat dyes, determined by oral feeding to rats, exceeds 2000 mg/kg and they are non-toxic to fish. As yet, only one vat dye (CI Vat Green 9) has given a positive Ames test response and only one other (CI Vat Yellow 4) has given positive results for carcinogenicity [220]. Vat dyes exhibit another important advantage compared with direct and reactive dyes: their exceptionally high exhaustion leads to very little residual dye in the effluent. Furthermore, any dye that does enter the effluent is insoluble and can easily be removed by flocculation or precipitation; it is also readily adsorbed by sludge in biological treatment. Thus vat dyes that contain halogen pose no serious threat in the waste water [253]. Consequently the major contribution of vat dyes to effluent pollution arises from their content of dispersing agent, which may vary from one manufacturer to another. The difference between powder and liquid brands is pertinent here. Powder dyes, for example, can give a COD value of about 1500 mg/g, of which only 40% is from the dye and 60% from the dispersing agent. For liquid dyes the corresponding value is 600 mg/g, of which 50% is from the dye and only 25% from the dispersing agent and hydrotrope; the hydrotrope is readily biodegradable [220]. Thus, as far as the dyes themselves are concerned, the situation is summarised in Table 12.40. There are, of course, important contributions to effluent pollution from levelling agents, sequestrants, antifoams, migration inhibitors or thickening agents applied in conjunction with these dyes. Since the almost universal reducing agent is sodium dithionite, there is a serious problem of sulphite and sulphate ions in the effluent, the sulphite usually being mostly oxidised to sulphate. Directly added electrolyte may be present but this is usually less important than with direct or reactive dyes. Hydroxyacetone has the advantage of biodegradability, although it contributes to the organic carbon content. As already mentioned, there are environmental benefits to be gained from using mixtures of sodium dithionite and hydroxyacetone [253]. The overall situation with regard to reducing agents is summarised in Tables 12.41 and 12.42. The oxidation process using hydrogen peroxide poses no serious environmental problem. As indicated in section 12.9.3, there are obvious advantages to be gained from a purely aqueous boil rather than soaping with a surfactant. By employing an optimised isothermal dyeing process including metering of the dyes, reoxidation can be carried out in the exhaust dyebath giving savings in water, time and energy, as well as ensuring that all sulphite from the dithionite is oxidised to sulphate before discharge to effluent [253].

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

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J A Bone, J Park and J Shore, J.S.D.C., 104 (1988) 12. Wool dyeing, Ed. D M Lewis (Bradford: SDC, 1992). W Mosimann, Amer. Dyestuff Rep., 80 (Mar 1991) 26. C de Meulemeester, I Hammers and W Mosimann, Melliand Textilber., 71 (1990) 898, E408. BASF, German P, DE 3 440 968 A1 (1986). Sandoz, BP 2 147 319 (1985). J Frauenknecht, P C Hextall and A C Welham, Textilveredlung, 21 (1986) 289, 331. H Salathe, Amer. Dyestuff Rep., 74 (Jun 1985) 20. W Beal, Australian Text., 6 (Jun 1986) 28; Dyer, 172 (Feb 1987) 31.

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914 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. 63. 64. 65. 66. 67. 68. 69.

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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES

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