Chemical Principles of Synthetic Fibre Dyeing by S. M. Burkinshaw


Chemical Principles of Synthetic Fibre Dyeing
by S. M. Burkinshaw

Chemical Principles of Synthetic Fibre Dyeing

1. Polyester 1
1.1. Introduction
1.2. Disperse dyes 2
1.2.1. Aqueous phase transfer 9
1.2.2. Thermodynamics of dyeing 10
1.2.3. Kinetics of dyeing 19
1.2.4. Effect of crystal form of the dye on dye adsorption 23
1.2.5. Effect of particle size and distribution on dye adsorption 25
1.2.6. Effect of dispersing agents on dye adsorption 26
1.2.7. Effect of levelling agents on dye adsorption 28
1.2.8. Effect of temperature on dye adsorption 29
1.2.9. Isomorphism 32
1.2.10. Oligomers 34
1.2.11. Carrier dyeing 35
1.2.12. Solvent-assisted dyeing 57
1.2.13. Solvent dyeing 58
1.2.14. High-temperature dyeing 61
1.2.15. Thermofixation 66
1.2.16. Afterclearing 68
1.3. Azoic colorants 69
1.4. Vat dyes 70
References 70
2. Nylon 77
2.1. Introduction 77
2.2. Anionic dyes 80
2.2.1. Barre effects 81
2.2.2. Acid dyes 83
2.2.3. Mordant dyes 126
2.2.4. Direct dyes 130
2.2.5. Reactive dyes 133
2.3. Cationic dyes 134
2.4. Non-ionic dyes 135
2.4.1. Disperse dyes 135
2.4.2. Disperse reactive dyes 140
2.4.3. Azoic colorants 148
2.4.4. Vat dyes 149
References 150
3. Acrylic 157
3.1. Introduction 157
3.2. Cationic dyes 159
3.2.1. Thermodynamics of dye·adsorption 160
3.2.2. Kinetics of dye adsorption 165
3.2.3. Effect of pH on dye adsorption 167
3.2.4. Effect of electrolyte on dye adsorption 168
3.2.5. Effect of temperature on dye adsorption 169
3.2.6. Effect of water on PAN fibres 173
3.2.7. Carrier dyeing 175
3.2.8. Retarding agents 181
3.2.9. Dye-fibre characteristics 183
3.2.10. Migrating cationic dyes 185
3.2.11. Gel dyeing 185
3.3. Disperse dyes 186
3.3.1. Thermodynamics of dyeing 186
3.3.2. Kinetics of dyeing 188
3.3.3. General considerations 189
References 190
4. Microfibres 194
4.1. Introduction 194
4.1.1. General considerations 196
4.1.2. Microfibre production 199
4.2. Polyester microfibrcs 201
4.2.1. Mass-reduced polyester fibres 205
4.3. Polyamide micro fibres 211
References 216
Dye Index 219
Subject Index 221

This book is based on a series of lectures given to final year undergraduates and MSc students of the Department of Colour Chemistry and Dyeing at The University of Leeds. As such, the main intention, in writing the first three chapters of the book, was to provide these students, and others who have a fundamental understanding of dyeing, with an overview of the theories of dyeing the three major synthetic fibres, namely polyester, nylon and acrylic. In the case of a reader who has less knowledge of dyeing or one who may be newly entering the field, each of the first three chapters includes an introduction to the various dye classes that have been used to dye the three fibre types.

The subject of the theory of dyeing polyester, nylon and acrylic fibres has attracted enormous interest over several decades and, whilst none of the first three chapters represents an exhaustive account of the vast amount of published literature, it was intended that the references presented should provide the reader with a broad, general background of knowledge. Accounts are provided of the dyeing of the three types of fibre with several dye classes; whilst, nowadays, some of the classes of dye included either enjoy little usage or are not commercially employed owing to various reasons, it was decided to include such dye classes so as to provide a balanced view of the dyeing of these fibres. In a similar manner, although carrier dyeing is currently little used owing, primarily, to environmental reasons, this subject, which has attracted a great deal of attention in published literature, has been included. Furthermore, as the subject of carrier dyeing spans each of the Chapters 1-3 and as carrier dyeing was of greatest importance with regard to the dyeing of polyester, an account of the carrier dyeing of this fibre in Chapter I comprises a detailed description of this subject that included references to work carried out using other fibres, this having been done in an attempt to provide an account of the evolution of the theory of carrier dyeing.

The final chapter of the book concerns the dyeing of microfibre, which, although a relatively recent textile substrate, continues to grow in importance. The intention in writing Chapter 4 was to demonstrate why the dyeing of microfibre differs to that of conventional decitex fibre by the use of explanatory accounts of the effects that decreasing linear filament density have upon the colour yield and fastness properties of dyeings.

1. Polyester
1.1. Introduction

The undeniable outstanding success of polyester fibres, which were commercially introduced by ICI in 1948 under the trade name ‘Terylene’, can be attributed to their excellent textile properties and generally very high chemical resistance under typical dyeing and finishing conditions. The poly(ethylene terephthalate) (PET) fibres, which, typically, are prepared from terephthalic acid and ethylene glycol [1-3], now enjoy world-wide production and are marketed under a variety of trade names. Although other polyester fibres, such as the ‘Kodel II’ (Eastman) range prepared from 1,4-dimethylolcyclohexane and terephthalic acid [2, 4] are also available, this account concerns only the dyeing of PET fibres.

Owing to the compact and highly crystalline structure of the hydrophobic fibres the rate of dye diffusion within the fibres is very low. Copolymerisation of ethylene glycol and terephthalic acid with a third comonomer such as ethylene oxide, isophthalic acid or 4-hydroxybenzoic acid, reduces the structural regularity of the homopolymer thereby improving the dyeability of the fibre with disperse dyes. The use of nonionic comonomers results in ‘non-carrier dyeing’ or ‘deep-dyeing’ PET fibres [5] and the use of anionic comonomers, such as 5-sulphoisophthalic acid imparts basic-dyeability to the fibres while the use of certain nitrogen- containing comonomers confers substantivity towards anionic dyes [3]; high-shrink PET fibres are also obtained by copolymerisation [6]. Additionally, the pilling performance of the fibre is enhanced by means of copolymerisation. Physical modifications employed in fibre production, such as modifications of the spinning conditions or the use of shorter molecular chains [6] also alters the fibre structure thereby modifying the dyeability and pilling performance of the fibre [2]. The ‘Kodel II’ (Eastman) range of poly(l,4-dimethylolcyclohexane) fibres [2, 4] also are modified in a similar manner as PET fibres [4].

PET fibres possess high resistance to oxidising and reducing agents and also to many organic solvents at room temperature although some solvents, at or near their boiling points, and in some cases at room temperature, induce shrinkage of unset fibres [7]. The fibres are prone to hydrolysis in the presence of dilute acids or alkalis as well as water [1]; however, this is of relatively little significance even under high-temperature dyeing conditions (130°C) provided that the pH is maintained within the range 4.5 to 6 [5]. Although possessing reasonable resistance to dilute aqueous acids, they are slowly degraded by concentrated acids; whilst the fibre will withstand typical alkaline conditions encountered in vat dyeing of PET/cotton blends, hot, concentrated alkali solutions hydrolyse the polymer although this is limited to surface saponification of the fibre at temperatures up to the boil [5]. Controlled saponification by means of treatment with hot aqueous sodium hydroxide is used to enhance the aesthetics of PET fabrics. Although it was observed in 1950 [8] that treatment of PET filaments with potassium hydroxide increased the fineness of the filaments, the use of sodium hydroxide to reduce fabric mass and thus enhance fabric aesthetics was first disclosed in 1952 [9] and later developed to impart high lustre [10]. Caustic treatment is now mainly used to improve the suppleness [11] of the fibre, the change in aesthetics, which is a direct function of fabric mass reduction [11], being dependent upon the temperature, duration and concentration of sodium hydroxide employed [11-13]. It has been demonstrated [14] that such alkali treatment enhances the dyeability of the fibre and it is considered [5] that alkali-treated PET is dyed more rapidly and to deeper shades than untreated fibre. A more detailed discussion of the mass reduction of PET fabric and its effects on dyeability is given in Chapter 4.

Barriness in PET fibres arises mainly from variations in crystallinity introduced during primary spinning, drawing, texturising and heat setting prior to dyeing; generally, barre effects introduced during texturising and heat setting are more prevalent than those imparted during drawing. Although an increase in crystallinity reduces the accessibility of the dye to those regions in which dye adsorption occurs, and thus rate of dyeing, the availability of such regions is aiso reduced, thereby reducing the saturation value of the fibre.

Although acid- and basic-dyeable variants have been developed for use in blends with conventional PET fibres, these variants enjoy considerably less usage than their disperse-dyeable counterpart. Nowadays, virtually all PET fibres are dyed using disperse dyes, although the fibres are dyeable with vat dyes, azoic colorants and, in the case of anionic-modified PET, cationic dyes.

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