Batchwise Dyeing of Woven Cellulosic Fabrics

BATCHWISE DYEING OF WOVEN CELLULOSE FABRICS A PRACTICAL GUIDE G W MADARAS, G J PARISH and J SHORE Formerly of BTTG-Shir...

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BATCHWISE DYEING OF WOVEN CELLULOSE FABRICS A PRACTICAL GUIDE

G W MADARAS, G J PARISH and J SHORE Formerly of BTTG-Shirley, Manchester, UK

SOCIETY OF DYERS AND COLOURISTS PO BOX 244, PERKIN HOUSE, 82 GRATTAN ROAD, BRADFORD BDI 2JB 1993

Copyright © 1993 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 BDI 2JB, England.

Typeset by the Society of Dyers and Colourists and printed by Staples Printers Rochester Ltd.

Previous titles in this series published by the Society of Dyers and Colourists:

1.

A Practical Introduction to Yarn Dyeing, by James Park

2.

A Practical Introduction to Fibre and Tow Coloration, by Geoffrey Clarke

3.

An Introduction to Management in the Dyeing Industry, by James Park

4.

A Practical Introduction to the Dyeing and Finishing of Wool Fabrics, by Ian Bearpark, F William Marriott and James Park

5.

The Manufacture of Organic Colorants and Intermediates, by Gerald Booth

6.

Instrumental Colour Formulation - A Practical Guide, by James Park

ISBN 0 901956 55 4

Contents Chapter

Page

1

ECONOMIC BACKGROUND

1

2

TECHNOLOGY OF FABRIC PREPARATION

3

3

DYEING OF CELLULOSIC FABRICS

25

4

DYEING OF SYNTHETIC FABRICS

58

5

DYEING OF BLEND FABRICS

68

6

MACHINERY FOR PREPARATION AND DYEING

75

7

MACHINERY FOR DRYING AND MECHANICAL FINISHING

95

8

CHEMICAL FINISHING

109

9

SUPPORT SERVICES AND THEIR CONTRIBUTIONS TO OVERHEADS

134

GLOSSARY

140

Preface This further monograph in the Society of Dyers and Colourists’ series is intended to fill an outstanding gap in the literature covering the dyeing and finishing of cellulosic woven fabrics and blends. In an attempt to present as complete a picture as possible, there is some overlap of topic from other publications and it became necessary to provide some additional background information on dyeing which in isolation could be regarded as beyond the scope of the title. Furthermore it is relevant to refer to fabric preparation including continuous techniques as these are regularly used prior to batch dyeing. The authors are of the opinion that to have left these subjects out would have created an unnecessary void. It is also inevitable that in dealing with such a universally practised technology, certain process techniques, whilst internationally developed, are no longer universally applicable or acceptable in certain markets. The authors, mindful of some national or local restrictive legislation on chemical use and/or disposal, advise readers to be cautious in their interpretation of what were once, but may no longer be, acceptable practices. Chapter 6 incorporates information on machinery for both preparation and dyeing. Some illustrations are included, but a comprehensive illustrative account already exists in Engineering in textile coloration, edited by C Duckworth and published by the Society. Reference to this publication is strongly recommended. We acknowledge the help and advice received from several individuals and organisations, including the staff of the former Shirley Institute for their encouragement, to Ken Dickinson for helpful criticism and comment, and the several machine manufacturers who provided illustrations. A special ‘thank you’goes to Stuart Smith and his staff for editorial support and facilities in manuscript preparation.

CHAPTER 1

Economic background 1.1 INTRODUCTION During the conversion of fibres to yarns and fabrics, the coloration stage is determined by many factors, economics and fashion being the most important. This book is concerned with the batch dyeing of woven fabrics, an operation carried out after weaving, except for the special case of coloured woven designs. It is clearly desirable for the merchant converter or garment manufacturer to postpone the decision of coloration as long as possible in order to cater for the dictates of fashion, and fabric dyeing facilitates this. The stages to effect coloration of textiles are: ( a) Mass pigmentation (b) Gel dyeing (c) Tow dyeing (d) Loose-stock dyeing (e) Top dyeing (f) Yarn dyeing (g) Fabric dyeing: batch, pad-batch or continuous (h) Garment dyeing. Man-made fibres can be dyed at all the stages listed, whereas only stages (d) to (h) are suitable for natural fibres. Routes of coloration (a) to (e) have been discussed in an earlier book; likewise yarn dyeing and the dyeing of wool fabrics have been covered in previous titles. It is the intention here to extend the coverage of fabric dyeing specifically to cellulosics and blends in batchwise form. In volume terms, gel, tow and loose-stock dyeing are most important, followed closely by yarn and fabric dyeing. Fabric dyeing will always claim a high percentage share of the output of the dyeing industry. Dyeing textiles in fabric or garment form offers the processor maximum flexibility in responding quickly to changes in fashion and market demand. Fabric dyeing and finishing form an integral part of a sequence of operations in the conversion of cotton from fibre to saleable fabric or garment. Thus the dyer, printer or finisher occupies a position intermediate between weaving and making-up, and the dyeing and finishing industry therefore serves both the weaver and the garment manufacturer or maker-up. In a truly vertically arranged company, where raw cotton bales enter at one end, and dyed and finished fabrics or garments leave at another, fabric dyeing and finishing form a link in a chain of operational steps. In some sectors the textile industry is horizontally organised, that is companies engage only in one major processing operation, be that spinning, weaving, dyeing, finishing or making-up.

2

BATCHWISE DYEING OF WOVEN CELLULOSIC FABRICS

The merchandise in whatever form is often owned by merchants, sometimes by large vertical companies, and the goods are sent to spinner, weaver, dyer and finisher to be processed on commission. In general, the fastness requirements of a dyed fabric or article will vary according to the end use. A great deal of information is available concerning the properties and performance of dyed fabrics in different end uses and the large multiple retailers, vertical organisations and fibre manufacturers frequently have their own specifications which dyers and finishers are expected to meet.

CHAPTER 2

Technology of fabric preparation 2.1 INTRODUCTION All woven fabrics contain impurities that have to be removed prior to dyeing or printing. These impurities may be those present in natural cellulosic fibres, e.g. cotton waxes and natural colouring matter, or those added to facilitate spinning, weaving or knitting, i.e. warp sizes or lubricants. The removal of these impurities is called fabric preparation and includes the use of various processes: singeing, desizing, scouring, bleaching and mercerising. All or only some of these processes may be required and they can be applied either as separate stages or sometimes as combined stages, e.g. desizing and scouring, or scouring and bleaching. The objectives of fabric preparation may be summarised as follows: (a) To achieve high and uniform dye uptake (b) To reduce the natural impurities of cotton to very low levels (c) To impart good hydrophilic properties combined with high absorbency and uniform swelling (d) To cause little or no fibre tendering (e) To produce an acceptable degree of whiteness both for use as undyed fabric and also to give the required brightness of shade to the subsequently dyed fabric. Fabric preparation can be by batchwise or continuous processing, in open-width or rope form. The choice is dependent on fabric quality and amount to be processed. The extensive use of continuous open-width dyeing ranges and the major importance of polyester/cotton blend fabrics favour, where available, the adoption of continuous open-width preparation routes even for subsequently batch-dyed materials. Fabric preparation is crucially important, because the successful outcome of all subsequent operations, such as dyeing, printing and chemical finishing, are critically dependent on it. This vital role that fabric pretreatment plays in subsequent processing is frequently underestimated or overlooked. Inadequate or skimpy preparation can cause major problems for the dyer, printer or finisher. It is generally held that a high proportion of the faults in textile coloration can be traced to faulty preparation. 2.2 SINGEING Fabrics containing cotton or viscose staple yarns show protruding fibre ends at the fabric surface; these disturb the surface appearance of the woven fabric and in dyeing produce an effect known as frosting. It is therefore necessary to remove the surface fibres by passing the fabric through a gas flame, a process known as singeing. In the standard process, the fabric is passed rapidly over a row of gas 3

4


flames at high speed (50-300 m/min) and then immediately into a quench bath to extinguish the sparks and cool the fabric. The quench bath often contains a desizing solution and thus the final step in singeing becomes one of impregnation in a combined singeing and desizing operation. Particular care is necessary when singeing polyester/cotton blend fabrics. A balance must be struck between thorough singeing that gives good pilling resistance and excessive singeing that is likely to cause small surface hairs to shrink or melt, forming small globules of polyester; these are dyed more readily than intact polyester and thus give a speckled appearance especially after exhaust dyeing. 2.3 DESIZING Almost all woven fabrics contain size that has been applied to the warp threads to facilitate weaving. Efficient desizing is an absolutely essential requirement of good fabric preparation. Size normally contains an adhesive (film former) and a lubricant. For cotton fabrics the film former is usually starch or a starch derivative. All starches are, by their very nature, either water-insoluble or only sparingly soluble. For viscose the most important sizing agent is the cellulose derivative carboxymethylcellulose, which has good water solubility. In addition to natural products, such as starch and starch derivatives, synthetic sizes based on styrenemaleic acid copolymers, poly(vinyl alcohol), polyacrylates or polyacrylamides are used on polyester/cotton or polyester/viscose, as well as mixtures of starch and poly(vinyl alcohol). The synthetic polymer sizes and carboxymethylcellulose are also used on continuous-filament warps made from acetate, triacetate, nylon or polyester. The use of water-soluble size preparations opens up possibilities for size recovery and reuse. The lubricant in a size formulation is usually tallow, but spermaceti, paraffin wax and mineral oils are sometimes employed. These lubricants impart good smoothness and low frictional properties to the yarn and are therefore beneficial for weaving, but they are insoluble in water and difficult to remove from the fibre surface, which can lead to severe problems in desizing. Several chemical manufacturers offer waxlike products that are water-soluble for addition to size formulations to improve suppleness and smoothness of the yarn; being water-soluble these are relatively easily removed during desizing. 2.3.1 Removal of water-soluble sizes If fabrics containing warp yarns made from synthetic fibres or their blends are sized with a water-soluble size formulation, desizing is carried out in a mildly alkaline solution containing a detergent. Although the so-called water-soluble sizes are dissolved in water when preparing warp sizing liquors, during drying of the sized yarn and subsequent singeing the water solubility may become impaired, and removal by means of water or mildly alkaline desizing liquor may be incomplete. On impregnation with the desizing liquor, the size first swells and becomes gel-like; only when the fabrics with the size gel are brought into contact with more desizing liquor does dissolution of the size take place.


5

In bulk running the singed fabric passes through the desizing liquor at the same speed as through the singeing machine, hence the impregnation time is usually far too short. As it is desirable to let the fabric swell for 15-30 s before squeezing to allow maximum liquor uptake, experience has shown that a double-dip/double-nip process with an intermediate air passage (skying) allows maximum uptake. Some modification to the above process is required when size recovery is intended since it is desirable to recover the bulk of the size in as concentrated a form as possible. The desizing effect is helped by the addition of a good wetting agent to the liquor. For maximum removal of soluble size the following points should be observed: (a) The sized warp should not be overdried (b) Suitable auxiliaries should be incorporated in the desizing liquor, subject to limitations regarding size recovery (c) Adequate time must be allowed for immersion in the desizing liquor to ensure maximum uptake of the liquor and provide adequate time for the size to swell (d) A thorough hot water wash should be given to remove the solubilised size. 2.3.2 Acid desizing Acid treatments degrade starch-based sizes and offer the advantage of removing calcium and magnesium salts from cellulosic fabrics. The concentration of hydrochloric acid used may be as high as 2% for short times of steeping or as low as 0.2% for overnight steeping. Care must be taken to avoid any risk of localised drying out, otherwise hydrolytic damage of the cellulose may occur. 2.3.3 Enzymatic desizing Enzyme treatments are used widely on cotton fabrics sized with starch, bacterial amylases being particularly suitable. Formerly malt extracts (malt diastases) were widely used, but nowadays bacterial amylases are preferred. The stability and activity of enzymes depend on many factors, including pH, temperature, and the presence of activators (e.g. metal ions) and wetting agents. Under bulkconditions the fabric is padded in the enzyme preparation and batched, but in general the temperature of the enzyme liquor on the textile material is well below that required for optimum desizing. As the enzymatic degradation of starch size is a time- and temperature-dependent reaction, in practice a period of heating or an adequate dwell time in the cold will be necessary. Swelling, degradation and dissolution of the size degradation products are reactions that require up to 20 h to produce a homogeneous aqueous solution of starch degradation products on the fabric that can be readily desorbed. After enzymatic degradation some starch residues are water-soluble but some relatively long- or branched-chain residues require an alkaline treatment to solubilise these components. The removal of the soluble starch degradation products by washing has to be thorough, as residual degraded starch still causes problems in dyeing. Depending on fabric type and structure, fabrics are washed in rope form or open width on continuous washing ranges; alternatively washing on winch or jig is possible, although this is slow by comparison with continuous processing.


The iodine test, involving treatment with a mixture of potassium iodide and iodine, is used to determine whether or not the size has been completely removed. Spotting the desized fabric with this sensitive reagent shows up any traces of residual starch as a blue coloration. 2.3.4 Oxidative desizing Oxidative degradation of starch sizes, as an alternative to enzymes or acids, has been known for many years. Particularly prior to kier bleaching with hydrogen peroxide, dilute hypochlorite liquors were used for desizing by pad-steep processing. More recently hydrogen peroxide or persulphates have been generally associated with the term oxidative desizing. The concept of oxidative desizing is particularly useful as a response to the extensive use of poly(vinyl alcohol) sizes. As indicated previously, the water solubility of these can be impaired prior to desizing. Oxidative desizing ensures degradation of these sizes and their subsequent removal. While this is not possible when size recovery is required, subsequent oxidative treatment in scouring or bleaching ensures removal of otherwise insoluble residues. 2.4 SCOURING Scouring is a treatment with alkali that removes or destroys cotton waxes, coloured impurities and other non-cellulosic substances, such as pectic cell wall material and fragments of leaf and seed coat, that are present as impurities in cotton fibres. This leads to a more absorbent fibre with greatly enhanced wettability characteristics. Cotton and blend fabrics can be scoured in many different ways. The severity of treatment, i.e. time, temperature and concentration of alkali (caustic soda), is chosen to effect a predetermined degree of removal of impurities from the cotton fibre. The more complete the removal of impurities, i.e. the cleaner the cotton, the more severe the scouring treatment has to be. This in turn relates directly to increased costs, in terms of processing, chemicals or energy. Moreover, whilst in the scouring process impurities such as fats and waxes are saponified, some hydrolysis of the glycosidic linkages of the cellulose molecule is unavoidable, and therefore the cleansing operation also results in some chemical damage to the cotton. 2.4.1 Kier boiling Cotton fabrics are run together with the caustic soda scouring liquor into a vertical kier (section 6.2.1). This must be a carefully controlled operation in order to avoid channelling, i.e. the formation of channels through which the liquor may flow in preference to the load. Undue movement of the fabric during processing must be avoided, because contact with the kier walls is liable to cause abrasion marks (chafing) on the goods. If the goods are loaded into the kier in the dry state the hot scouring liquor is introduced from the bottom, so that the hot liquor wets the material rapidly and evenly and the air is free to escape through an open valve at the top of the kier. At this stage it is vital to expel all the air, as atmospheric oxygen and alkali


7

are liable to oxidise (damage) the cotton cellulose. The liquor is then heated to the scouring temperature and circulated through the load by an efficient pump working in conjunction with an external multi-tubular heater. Typical treatment conditions for an atmospheric scouring process are 10-20 g/l sodium hydroxide for 4-6 h at 9598°C. After completion of the boil it is still important to avoid contact with the air and it is therefore advisable to run in hot rinsing water as the alkaline liquor is drained off. In pressure boiling, an operation that has lost much of its former importance, a lower alkali concentration (5-10 g/l) can be used in view of the higher operating temperature. Pressures of up to 200 kPa (30 Ibf/in2) are employed, corresponding to a temperature slightly in excess of 130°C. Little or no degradation of the cotton fibre is observed, fats and waxes being more rapidly and efficiently removed from the cotton than in atmospheric scouring. For that reason pressure-boiled cotton, although showing excellent absorbency, can also exhibit a rough and harsh handle. The application of a softening finish overcomes this defect. Pressure boiling produces a degree of whiteness that is often adequate for dyeing medium to dark shades, even without further bleaching. After a pressure boil the liquor continues to circulate whilst cooling to 90°C, before dropping the liquor gradually and giving several rinses, hot and then cold. Use of caustic soda under atmospheric pressure can give rise to incomplete removal of pectins, waxes and coloured substances, and to tendering of the cotton. To overcome these shortcomings a combination of anionic surfactants is used to extract the impurities from cotton in the strongly alkaline medium. Sequestrants are also added to assist in removing inorganic impurities, such as calcium or iron salts. A reducing agent, such as a stabilised dithionite, is often included to destroy coloured organic impurities and to inhibit the formation of oxycellulose. 2.4.2 Semi-continuous processes Semi-continuous processes such as J-box, pad-roll or pad-batch, represent a significant advance over the classical long-liquor scouring processes described above. Reaction times have been reduced or treatment temperatures lowered considerably. In order to make this possible, the concentration of chemicals had to be significantly increased. Less than a decade or two ago it was generally considered that hot treatments of cotton with 50-100 g/l caustic soda would lead to unacceptable tendering of the cellulose. This view was based on experience gained with longliquor treatments in kiers, where relatively low concentrations of caustic soda in the presence of atmospheric oxygen could bring about severe tendering. J-box processing This popular system for the semi-continuous treatment of fabrics in rope form utilises one or more J-shaped steamers (section 6.2.2) arranged in sequence. It is an economical system for cotton goods and permits high production rates. Continuous scouring in rope form is not suitable for fabrics that have a tendency to crease, e.g. heavy, closely woven cotton fabrics or polyester/cotton blends. Fabrics to be steamed in the J-box are impregnated at 20-60°C in rope form. Their liquor pick-up

8


is therefore somewhat higher than that of fabrics impregnated in open width. The reaction time is fairly long (1-2 h at 95-100°C) and the capacity of the J-box is correspondingly large. In view of the reaction time, the amounts of chemicals required (e.g. 30-60 g/l sodium hydroxide) are somewhat less than those used in fully continuous pad-steam processes. Pad-batch and pad-roll methods The first unit of a semi-continuous open-width scouring range is effectively the impregnator (sections 6.3.2 and 6.3.3) in which the fabric is repeatedly immersed and squeezed (dipped and nipped) and finally passed through squeeze rollers resulting in a uniform pick-up of liquor across the width of the fabric. Nowadays it is considered that immersion should take no longer than 6-20 s and that after impregnation and squeezing the pick-up of a cotton fabric should be about 100%. Rapid impregnation is made possible by the use of alkali-stable wetting agents. Moreover, the complete penetration of the scouring liquor displaces the air entrapped in the fibre structure, so that as a result of subsequent thermal treatments, e.g. steaming, oxidative damage does not occur. In general, it is desirable to employ a caustic soda concentration and specific conditions of time and temperature that are known to give optimum effects in terms of extracting the non-cellulosic impurities and imparting a brightening effect. Practical experience has shown that in continuous and semi-continuous scouring all these parameters are directly related, so that different optimised scouring processes normally yield similar technical effects on a given fabric. In the warm batch process cotton fabric is padded with sodium hydroxide (50-80 g/l) at 50-90°C and then batched. The batch, wrapped in a plastic sheet, is rotated for 3-6 h and gradually cools during this time. In the less attractive cold batch process the fabric is impregnated with sodium hydroxide at room temperature and then batched, wrapped and rotated as before, but the reaction time must be extended up to 10-20 h. The pad-batch process is useful where no machinery is available for hot processing. Although optimum extraction of the impurities in cotton is not achieved, the process results in uniform swelling and fairly good absorbency of the fabric. In the less versatile and obsolescent pad-roll process, the impregnated cotton fabric (40-60 g/l sodium hydroxide at 40-90°C) is batched and the batch is slowly rotated in a chamber, often referred to as a caravan, in a saturated steam atmosphere at a fabric temperature of 90-100°C. The advantages of semi-continuous open-width scouring processes include the simple construction of the equipment, its reliability in constant operation and the scope for processing batches of varying sizes. The main disadvantage of the pad-roll process is insufficient uniformity of the pretreatment, which can cause listing and ending in dyeing. Reasons for lack of uniformity are: (a) The reaction time may vary from batch to batch (b) The reaction time varies between the beginning and the end of a batch (c) Variations in pressure and temperature in the batch are liable to cause liquor migration.


2.4.3 Continuous pad-steam processes Fully continuous processes involve the pad application of chemicals to the fabric, followed by steaming for sufficient time to ensure effective scouring. Reaction times previously measured in hours can be reduced to minutes. In a continuous steamer the initial heating of the fabric takes place under controlled tension as it passes through a roller system. Fabric may then fall on to a roller bed or plate conveyor system. Processing may be at atmospheric pressure (1-5 min at 100-103°C with 80-120 g/l sodium hydroxide) or in a pressure steamer at higher temperature (1-3 min at 125-135°C with 50-70 g/l sodium hydroxide). The pad-steam technique is widely adopted in scouring. It ensures that effective swelling of the cotton fabric is controlled by the tension on the fabric as it passes in open width over the rollers in the steamer in a serpentine fashion. This type of processing, which is commonly referred to as roller steaming, results in freedom from running creases and gives optimum results in terms of evenness of application and reproducibility. A potential cause of trouble in steaming is the dilution of the chemicals on the fabric because of steam condensing onto the material. This occurs if the steam is too wet or where the fabric enters the steaming chamber at too low a temperature. For this reason it is desirable to carry out the impregnation of the fabric at an elevated temperature (60-95°C), or to pass the fabric through a preheating zone prior to entry into the actual steaming unit. 2.5 WASHING Although an efficient washing process is a vital step in fabric preparation, its importance is so frequently overlooked that washing can become a neglected operation carried out in a perfunctory fashion. Yet without adequate washing, i.e. without a thorough removal of impurities, the effects of desizing and scouring can be diminished or even nullified. Considerable progress has been made in recent years in raising the efficiency of washing machines. This has been achieved by improving the interchange between water and fabric by vigorous agitation, and by the use of suction devices. To conserve water and energy, the countercurrent flow principle is now invariably used on open-width washing ranges. Care is necessary to guard against too strong a counterflow which can result in the lowering of temperatures if not properly controlled. Using this technique, the clean water is fed in at the exit end of the machine and gets progressively dirtier as it is passed in countercurrent fashion to the entry end of the machine, where the desized or scoured fabric enters the washing range (Figure 2.1). Good washing efficiencies have been obtained with a five-box washing range using softened water supplied at the rate of 400-600 I/h. The temperature of the wash liquor should be maintained at: 95°C First wash box Second wash box 95°C Third wash box 80°C Fourth wash box 60°C Fifth wash box add water spray.


Figure 2.1 Babcock Econ- Tex open- width washing machine On leaving the washing range, the fabric temperature should be at 30°C or below. 2.6 BLEACHING Bleaching of cellulosic fabrics fulfils a twofold purpose. Firstly it provides a high degree of whiteness for fabrics that need to remain white. For fabrics that are to be dyed to bright shades, prior bleaching enhances the whiteness of the cellulose and thereby imparts improved brilliance to the dyed effect. Secondly, oxidative treatment improves the uniformity of fabric appearance by helping to destroy residual impurities such as seed husks. Whatever chemical system is used to achieve the oxidative destruction of the natural colouring matter in cotton, the selection of bleaching agent depends on various factors: (a) Types of fibres and blends to be processed (b) The standard of white required (c) The need for fabric damage to be kept to a minimum (d) The processing machinery available (e) Chemical and processing costs as part of total preparation economics. At the present time bleaching systems based on the following agents have commercial significance for cellulosic and cellulosic blend fabrics: (a) Sodium hypochlorite (b) Sodium chlorite (c) Hydrogen peroxide (d) Peracetic acid. Of these, sodium hypochlorite was at one time the most widely used for cellulosic fibres, but the commercial availability of sodium chlorite and in particular hydrogen


11

peroxide made it possible to develop bleaching processes that offered both technical advantages and savings in costs over sodium hypochlorite. Current legislation in certain countries is now restricting the deposition of effluent containing chlorine and as a consequence hydrogen peroxide is becoming even more widely used. The last mentioned chemical, peracetic acid, can be used as a bleaching agent for polyamide and regenerated cellulosic fibres, but because of its specific field of application will not be considered further here. 2.6.1 Sodium hypochlorite bleaching Hypochlorite solutions provide a convenient form for handling chlorine, particularly for use in the concentrations required for textile preparation processes. Commercially hypochlorites have been supplied either as a solution of sodium hypochlorite (NaOCI) or as bleaching powder (mainly calcium hypochlorite). The strength of a solution of hypochlorite is normally expressed as the available chlorine content. This term, also referred to as active chlorine content, relates to the chlorine which is formed on reaction with acids as shown in Scheme 2.1. NaOCl + 2 H C I -

NaCl + H20 + Cl2

Scheme 2.1 The determination of available chlorine in commercial hypochlorite solutions is usually by standard iodometric analytical methods. Commercial products used in textile processing generally have an available or active chlorine content of either 150 g/l (for sodium hypochlorite liquor) or 35% by wt (for bleaching powder). Although Scheme 2.1 indicates the liberation of chlorine, hypochlorites are used for bleaching cotton under alkaline conditions that lead to the release of ‘active oxygen’. Sodium hypochlorite in solution is hydrolysed in a strongly alkaline reaction as shown in Scheme 2.2. The bleaching efficiency of the very slightly dissociated hypochlorous acid (HOCI) is based on its decomposition to hydrochloric acid and active (nascent) oxygen as in Scheme 2.3. In practice only a small amount of active oxygen is formed in the initial stages of the reaction. When the active oxygen is used up for bleaching, more hydrochloric acid and active oxygen are formed, allowing the reaction shown in Scheme 2.3 to continue. The release of increasing amounts of hydrochloric acid causes the pH to drop to a level where more hypochlorous acid is generated by hydrolysis and this in turn increases the formation of active oxygen and more hydrochloric acid. Thus bleaching with hypochlorite can soon get ‘out of hand’, unless careful control of pH is exercised. NaOCl + H2O -

NaOH + HOCI

Scheme 2.2 HOCI

Scheme 2.3

-

HCI + 0

12


In hypochlorite bleaching close attention must be paid to the following points in order to avoid fibre damage. (a) Bleaching must always take place under alkaline conditions, and the hypochlorite bleach liquors should be buffered with soda ash to avoid excessive formation of active (nascent) oxygen. The pH at the beginning of the reaction should be 11.5-12.0. (b) To avoid increasing the rate of reaction, i.e. formation and breakdown of the hypochlorous acid, the bleaching temperature should not exceed 20-25°C. (c) To prevent excessive formation of hypochlorous acid, the pH must never be allowed to drop below 10-11 until the very end of the bleaching process. A disadvantage of a hypochlorite bleach is that the bleached goods have atendency to yellow, whereas a peroxide bleach produces a stable white. Rope-form bleaching with hypochlorite Although the use of hypochlorite bleaching has diminished, in some areas of the world (e.g. China and India) it remains important. Cotton goods are never bleached with hypochlorite in the grey state, however; first they must be desized, followed by a thorough alkaline scour to remove fats, waxes, pectins and other impurities. Rope-form treatment is the most important technique for hypochlorite bleaching. After impregnation with the liquor (2-5 g/l active chlorine at pH 11.5) the cotton fabric is piled down in a cistern or J-box storage chamber for 30-90 min at ambient temperature. When bleaching is complete, the fabric is washed thoroughly and given an antichlor treatment with dilute acid or sodium metabisulphite. Alternatively smaller batches of fabric can be bleached satisfactorily on the winch at, say, 30:l liquor ratio for an hour at ambient temperature in a more dilute bleaching bath (l-2 g/l active chlorine at pH 11). Open- width bleaching with hypochlorite Jig bleaching with hypochlorite also requires treatment for about an hour at ambient temperature, depending on batch size. However, a more concentrated bath (2-4 g/l active chlorine at pH 11.5) is needed because of the short liquor ratio. Applying hypochlorite by a semi-continuous padding process is economical and produces good bleached effects; moreover, no additional equipment is required except for the impregnation system. The impregnated fabrics can be batched for 2 h at room temperature. The concentration of hypochlorite applied is similar to that used in the jig or J-box processes. 2.6.2 Sodium chlorite bleaching Sodium chlorite (NaCIO2) is available commercially in the form of a white free-flowing crystalline powder (50% by wt sodium chlorite) or a yellowish clear liquid (24-26% by wt sodium chlorite). The powder form contains sodium nitrate and has better storage stability. Sodium chlorite is a powerful oxidising agent and advice provided by the manufacturer regarding handling, safety and storage should be followed.


13

The powder form is hygroscopic and its decomposition is accelerated by heat, catalytic contamination, acids and reducing agents. Mixtures of sodium chlorite with combustible substances or reducing agents can create hazards, particularly if subjected to heat, friction or impact. Even the liquid product may cause combustion if allowed to dry out on flammable materials. An explosive and toxic gas, chlorine dioxide (CIO2), is liberated on contact with acids, and sodium chlorite decomposition is accompanied by the liberation of oxygen, which will support combustion. Moist sodium chlorite is very corrosive, the material acts as an irritant and can cause severe damage to the skin. Spillages should clearly always be washed away with large amounts of water. Bleaching with sodium chlorite is carried out under acidic conditions, but this gives rise to the major disadvantage of this system, namely the release of the above mentioned toxic, corrosive and even explosive gas chlorine dioxide. Moreover, as sodium chlorite bleach liquors are incompatible with many fluorescent brightening agents and as the system is unsuitable for rapid continuous processes, bleaching with sodium chlorite has given way to single- or multi-stage peroxide bleaching. Nowadays, chlorite bleaching is limited to specific areas for which it is particularly suitable, such as regenerated cellulose (viscose) because of this material’s low sensitivity to metallic impurities, cotton/acrylic blends and alkali-sensitive cellulose acetate. Chlorite must never be used on protein or polyurethane fibres. As already mentioned, sodium chlorite is only effective as a bleaching agent when its solutions are activated by the addition of an acid or acid donor. Depending on the pH, sodium chlorite reacts to form the active bleaching species chlorine dioxide and chlorous acid (HCIO2), as well as inactive sodium chlorate (NaCIO3) and sodium chloride. Chlorine dioxide formation is favoured at a very low pH of 10-2.5, whereas the less active chlorous acid is at its maximum concentration at pH 3. Bleaching at long liquor ratios is usually carried out at pH 3-4 with auxiliaries to minimise release of chlorine dioxide. This represents an optimum balance between effective bleaching and over-rapid or excessive chlorine dioxide formation. Alternatively bleaching can be commenced at a pH of 5-7, and with the aid of activators acid is slowly liberated to initiate the bleaching reaction. Activators commonly used include anhydrides of organic dicarboxylic acids, esters and aldehydes. It is well documented that an acidic chlorite solution produces chlorine dioxide as well as free hydrochloric acid, as illustrated in Scheme 2.4. As more acid is formed the rate of chlorine dioxide evolution increases. Moreover, a competitive reaction (Scheme 2.5) with a disproportionation of chlorous acid into chloric acid and hydrochloric acid takes place, and the free hydrochloric acid that is formed can cause fibre tendering. 5HClO 2 - 4ClO 2 +

HCI + 2H2O

Scheme 2.4 3HClO 2 -

Scheme 2.5

2HClO 3 + HCI

14


In practice therefore, in order to regulate the rate of evolution of chlorine dioxide and effect a controlled bleaching action from the chlorous acid, it is customary to maintain the bleaching temperature at 70-95°C and a constant pH with the addition of a buffer. The most commonly used buffers are salts of phosphoric acid, e.g. sodium dihydrogen orthophosphate (NaH2PO4). Formulation of chlorite bleach baths In addition to the chemicals already mentioned, chlorite bleaching baths contain two other components. Sodium nitrate is added as a corrosion inhibitor to protect stainless steel surfaces. An anionic surfactant is necessary to assist penetration and soil removal, but must be carefully checked for compatibility with sodium chlorite. When preparing a chlorite bleach bath, it is essential to avoid adding acid directly to the concentrated sodium chlorite solution. Chemicals should be added in the following order, allowing for thorough mixing at each stage: 1. Water 2. Sodium nitrate, previously dissolved 3. Buffer salts and/or other chlorite stabiliser 4. Anionic wetting agent 5. Sodium chlorite, predissolved if the solid product is used 6. Diluted acid and/or activator to give the required pH. A disadvantage of the chlorite bleach, apart from the unpleasant odour and toxicity of chlorine dioxide, is its strong corrosive action, even on stainless steel. Equipment in contact with chlorite solutions should preferably have a ceramic lining or be constructed of special corrosion-resistant material such as titanium alloy. Rope-form bleaching with chlorite Cotton and polyester/cotton goods should be desized and have undergone an alkaline pretreatment prior to chlorite bleaching in order to obtain satisfactory results in subsequent dyeing or printing. If desizing is omitted the hydrophilic properties will be poor and subsequent dyeings unlevel. If a chlorite bleach is carried out on goods that have been desized, but were not given an alkaline pretreatment, then the results may still be inferior for the following reasons: (a) Degree of whiteness: although a high degree of whiteness is achieved, the goods are liable to yellow subsequently (b) Removal of husks: although the husks are bleached, they are incompletely removed (c) Hydrophilic properties: as waxes, pectins and other impurities have not been properly removed, the hydrophilic properties may be inadequate for subsequent trouble-free dyeing. Desized and scoured cotton fabric for bleaching in the J-box is impregnated at 2040°C with 20-25 g/l sodium chlorite and an activator at pH 6.0-6.5. During storage for 1-2 h at 90-95°C the pH falls gradually to about 5 as hydrolysis of the activator proceeds. The bleaching stage is followed by a hot wash and antichlor treatment in a suitable washing range.


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Smaller batches of fabric can be bleached on the winch at about 30:l liquor ratio using 1-2 g/l sodium chlorite and 1-2g/l sodium nitrate at pH 3-4 for 1 h at 80-95°C. Hot washing and antichlor treatments are carried out in the same machine. Open- width bleaching with chlorite Jig bleaching with chlorite takes 1-3 h at 85-90°C and requires 5-7 g/l sodium chlorite and 2-3 g/l sodium nitrate at pH 3-4 and 5: 1 liquor ratio. Jig windows should be kept closed. After bleaching the fabric is given two ends in hot water, two ends antichlor treatment (0.2-0.4% sodium pyrophosphate or 0.5-1.0% sodium metabisulphite) and finally two ends in cold water. Chlorite bleaching on the pad-roll machine operates with essentially the same impregnation conditions as the J-box process, commencing at pH 6.0-6.5 and approaching pH 5 during storage of the batched roll for 2-4 h at 85-90°C. Hot washing and antichlor treatment follow on an open soaper. 2.6.3 Hydrogen peroxide bleaching Hydrogen peroxide is widely and increasingly used for bleaching natural and regenerated cellulosic fibres. Although it has no bleaching action on polyester fibres it is generally used for bleaching blends of cotton or viscose with polyester. No other bleaching agent has such a wide field of application. Commercial hydrogen peroxide solutions used by the textile industry range in strength from 27.5 to 50% by weight. It is a powerful oxidising agent that can cause combustion if allowed to dry out on readily oxidisable or flammable material. Uncontaminated hydrogen peroxide decomposes slowly at normal temperatures and it must not be stored in confined spaces. Decomposition may be accelerated by contamination with metals or dust. Such decomposition, when it occurs, is accompanied by the liberation of heat and oxygen. Hydrogen peroxide is an irritant to the skin and dangerous to the eyes. Advice provided by the manufacturer regarding safe handling and storage should be followed. Almost all bleaching of textiles with hydrogen peroxide is done under alkaline conditions, usually with caustic soda to provide the necessary alkalinity. In addition to the alkali, further stabilising chemicals are required but it should be noted that these are not the same as the chemicals used to provide stability during storage of commercial hydrogen peroxide solution. In spite of the considerable amount of research carried out into the actual mechanism by which bleaching occurs, the exact details are not yet fully understood. In particular the role of the stabiliser in promoting the controlled decomposition of hydrogen peroxide and preventing undesirable side reactions needs further clarification. It is generally agreed that in bleaching with peroxide the first step is an ionisation of the hydrogen peroxide molecules in the presence of hydroxide ions to perhydroxide ions (HOO-) and protons (H+) that immediately combine with hydroxide ions to form water (Scheme 2.6). From Scheme 2.6 it follows that the formation of active perhydroxide anions is favoured in the presence of hydroxide ions; in fact the maximum bleaching activity of hydrogen peroxide is at about pH 11.5.

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H2O2 + OH-

-

H + + OH -

HOO- + H+ H2O

Scheme 2.6 Colour destruction and breakdown of other non-cellulosic impurities result from oxidation reactions and these may be represented, albeit as an oversimplification, by the release of an ‘active’oxygen atom from the decomposition of the unstable perhydroxide ion (Scheme 2.7). HOO - -

OH - + 0*

o* + x - x-o where 0* is ‘active’(nascent) oxygen and X is an oxidisable substance

Scheme 2.7 Unfortunately the breakdown of hydrogen peroxide in alkaline media is not so straightforward and a number of side reactions can occur, particularly in the presence of metallic impurities. These can be summarised as a breakdown of hydrogen peroxide to water and molecular oxygen, the latter having no bleaching effect (Scheme 2.8). This breakdown is most rapid in highly alkaline solutions. The molecular oxygen escapes from the bleach solution and plays no further useful part in the bleaching process. However, intermediate free-radical species formed in these side reactions are very reactive and can cause fibre damage. To control bleaching a balance must be achieved and maintained between decomposition and stabilisation in the hydrogen peroxide bleach bath. 2H2O2

-

2H2O + O2

Scheme 2.8 Control of pH and temperature in peroxide bleaching The rate of decomposition of hydrogen peroxide is dependent on the following variables: (a) pH (b) Temperature (c) Types and concentrations of impurities (d) Type and concentration of stabiliser. As the pH increases so does the rate of decomposition of the peroxide, the effect of pH being most marked between values 9 and 10. The effect of increasing pH promotes not only the decomposition of hydrogen peroxide according to Schemes 2.6 and 2.7 but the undesirable side reactions summarised as Scheme 2.8 are also accelerated.


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Temperature also has a marked effect; increasing the temperature speeds up the decomposition of peroxide, resulting in enhanced bleaching but also accelerating the undesirable side reactions. Normally the time and temperature of processing are interrelated in this respect. Additives for peroxide bleach baths The decomposition of hydrogen peroxide can also be affected by the nature of the substrate, particularly the presence of certain impurities in the fibre that can have quite a dramatic effect. In this context it is appropriate to mention the catalytic effect of transition metals, their oxides and ions, especially iron, copper, manganese and cobalt. Even if present only in trace quantities, these can have a disastrous effect on the bleaching process. Such contaminants can initiate a free-radical decomposition of hydrogen peroxide by a chain reaction, whereby considerable oxidative damage is caused to the cellulosic substrate. To minimise the adverse effect of metallic contaminants it is advisable to use an appropriate sequestering agent during scouring and to select the least severe bleaching conditions that will provide adequately bleached fabric. An effective stabiliser for hydrogen peroxide must fulfil two requirements: (a) Give greater stability to the perhydroxide anions formed (Scheme 2.6), i.e. slow down their decomposition according to Scheme 2.7. (b) Prevent, as far as possible, the various side reactions leading to the wasteful formation of molecular oxygen (Scheme 2.8) and possibly to damage of the cellulosic substrate. From the earliest usage of hydrogen peroxide in bleaching it was known empirically that alkaline earth metal hydroxides and silicates, especially those of magnesium, were most effective in stabilising perhydroxide anions. However, the mode of action of the stabiliser in promoting stabilisation as the perhydroxide is still open to discussion. Most probably the effective stabilising entity is the magnesium ion that interacts with either one or two perhydroxide ions to form a complex and so retards the decomposition of the perhydroxide ion according to Scheme 2.7. Sodium silicate, commonly known as water glass, and magnesium salts provide the cheapest method of stabilising a hydrogen peroxide bleach bath. Stabilisation is attributed to the buffering action of the sodium silicate. This is capable of providing a reserve of alkali in the bleach bath without accelerating the decomposition of the peroxide. Furthermore, colloidal silicates in the presence of magnesium ions have a sequestering effect on transition metal contaminants that lessens the risk of catalytic decomposition of hydrogen peroxide, with the attendant damage to the cotton goods to be bleached. Colloidal magnesium silicates and hydrated silicas must be kept in colloidal form in the impregnation bath and during bleaching. As this condition is difficult to meet under plant conditions some deposition of sparingly soluble magnesium silicate and hydrated silica can occur. This can impair the handle of bleached goods and may cause subsequent problems in dyeing and printing. Problems associated with sodium silicate can be avoided by using auxiliary products referred to as organic

18


stabilisers. These are usually based on sequestering agents, protein degradation products or certain surfactants. For some bleaching methods, particularly on winch, jig and kier, organic stabilisers may be used alone; in others it is preferable to use them in conjunction with silicates. Organic stabilisers act primarily by forming complexes with transition metal ions, thereby removing the danger of catalytic decomposition of hydrogen peroxide. They also interact with the perhydroxide anions to form complexes that stablilise these ions. The use of an organic stabiliser, even in the presence of silicates, gives bleached fabrics with a much softer handle. Bleaching recipes based on hydrogen peroxide may also contain surfactants and inorganic sequestering agents. The surfactants provide wettability and detergency, care being taken to select products that are stable under alkaline conditions and that do not foam excessively. Inorganic sequestering agents minimise precipitation of insoluble calcium hydroxide and are useful in sequestering small quantities of transition metals present in the bleaching system, i.e. from the water supply, chemicals or fabric. The correct sequence for the addition of chemicals when preparing peroxide bleach liquors is: 1. Soft water; transition metal ions (iron, copper) should have been removed by ion exchange 2. All organic auxiliaries, e.g. organic stabiliser, surfactants, sequestrants, if required 3. Magnesium sulphate, to complex with silicate and/or organic stabiliser 4. Caustic soda 5. Sodium silicate, always added to an alkaline solution 6. Hydrogen peroxide. This procedure is necessary for batchwise bleaching or single-feed continuous systems. For continuous bleaching a minimum of two chemical stock solutions is recommended: one containing alkali, stabiliser and surfactant, the other being hydrogen peroxide alone. Most modern feed systems use three or four metered feed lines with chemicals diluted in a small intermediate feed tank just before use. Batchwise peroxide bleaching methods The combination of an alkaline scour followed by a peroxide bleach is the most effective of all preparation treatments for cotton and polyester/cotton blends. It produces excellent results with good reproducibility by whatever bleaching method is adopted. Because hydrogen peroxide bleaching is carried out under alkaline conditions, several options can be considered. Multi-stage systems involving separate desizing, scouring and peroxide bleaching can be used but for many fabric qualities, particularly polyester/cotton blends, two or more of these stages can be combined. When goods are prescoured the alkali and peroxide concentrations in the bleaching stage are much lower than in combined scour-bleach processes. It is essential that the bleaching equipment is compatible with hydrogen peroxide, e.g. 316 grade stainless steel. Some equipment, such as iron kiers, can be protected by suitable coating treatment.


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Whilst most progressive plants now use continuous machinery for woven preparation, kier bleaching with hydrogen peroxide has been traditionally and widely adopted. On many qualities a combined scour-bleach after preliminary desize can be used. A desize with acid or hypochlorite is preferred to enzyme desizing in order to ensure no carryover of enzyme to the peroxide bleach liquor. Care must be taken in loading fabric into the kier to ensure uniform application of the peroxide solution, and the temperature of the bleach liquor during loading should not exceed 50°C. When loading is complete the temperature of the liquor should be raised slowly to 90-95°C with an intermediate hold period of 10 min at 70-75°C to allow any trapped air to escape. After bleaching for about 3 h at 90-95°C the goods are washed with hot water on a rope washing range. If pressure kiers are available the process time can be reduced to 1-2 h at 120°C. The chemical requirements for kier bleaching are given in Table 2.1. For maximum whiteness some silicate stabiliser is desirable although for many qualities adequate bleaching is achieved using organic stabilisers. Table 2.1 Recipes for kier peroxide bleaching

Amount (% o.w.f.)

Agent

Desized only

Prescoured

Hydrogen peroxide (35%) Sodium silicate” Organic stabiliser Sodium hydroxide Wetting agent

3.0-5.0 2.0-3.0

1.0-2.0

a

29% SiO SiO22,,

0.6-1.4 0.1-0.2

0.4-0.8 0.4-0.8 0.1-0.2

b 8.8% Na2O

Jig bleaching of certain cotton fabrics is still widely practised. It is possible to use an organic stabiliser to the exclusion of sodium silicate, thereby giving the bleached goods a better handle. A further advantage is the easier removal of organic stabiliser during washing at about 5:1 liquor ratio on the jig. The chemicals (2-5% hydrogen peroxide, 0.5-1.5% organic stabiliser and 1.0.0to 1.5% sodium hydroxide) are added over two ends and the temperature is raised gradually to 80-95°C. After bleaching for 1-3 h at top temperature the fabric is washed thoroughly over two ends in hot water and several ends in warm water. In the winch bleaching of cotton at about 3O:1 liquor ratio the chemicals (3-12% hydrogen peroxide, 3-6% organic stabiliser and 3.0-4.5% sodium hydroxide) are added at 45-50°C and the temperature raised to 90-95°C over 30-45 min. After a further 1-2 h at top temperature the fabric is washed thoroughly in hot water.


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Bleaching in jet dyeing machines at about 8:1 liquor ratio may be desirable before jet dyeing. By using a suitable organic stabiliser, which prevents premature decomposition of the peroxide, the least possible tendering of the cotton goods is ensured. No sodium silicate is required and the dispersing action of the stabiliser prevents the filtering out of impurities from the cotton during the bleaching process, which is of considerable importance in jet processing. As in winch bleaching, the chemicals, typically 2-6% hydrogen peroxide, 1% organic stabiliser, 2% sodium hydroxide and 1% sequestering agent, are added at 45-50°C and the temperature raised gradually to 90-100°C. After a further 30-60 min at the boil the fabric is rinsed thoroughly in hot water (>70°C) to prevent redeposition of waxes. Semi-continuous peroxide bleaching In line with the search for lower energy costs the cold pad-batch bleach is becoming more and more important. Good effects can be achieved in terms of whiteness, wettability and seed removal. Nevertheless, the best results in terms of these criteria can only be obtained by recourse to a high-temperature treatment, and therefore the cold pad-batch bleach inevitably represents a compromise. However, cold bleaching is gaining wider acceptance as the benefits of energy saving are more widely understood and practised. It is most generally used without prior desize or scour treatments. Thus the recipe and processing conditions are devised to cater for processors with a minimum of sophisticated equipment and to keep chemical and processing costs as low as possible. The equipment required includes an open-width saturator, a storage facility such as A-frames and good open-width washing. Cotton fabric is padded at 20-30°C with a recipe such as that given in Table 2.2. The emulsifier is important to ensure maximum removal of fabric impurities, especially cotton waxes; persulphate may be added to improve the desizing action during bleaching. The batch is wrapped in a plastic film such as polythene and rotated slowly for 1 O-24 h at ambient temperature. After bleaching the fabric is washed off thoroughly on an efficient washing range using water that is as hot as possible. It is advisable to add 2 g/l sodium hydroxide with 2 g/l organic stabiliser to the first tank.

Table 2.2 Recipe for cold pad-batch peroxide bleaching

Agent

Amount (% o.w.f. at 100% pick-up)

Hydrogen peroxide (35%) Sodium silicate Organic stabiliser Sodium hydroxide Sodium persulphate Emulsifying agent Wetting agent

4.0-5.0 0.8-1.2 0.6-1.0 0.8-1.5 0-0.4 0.6-0.8 0-0.2


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Although the degree of whiteness obtained from a soundly based cold pad-batch process is acceptable for many end uses, it can be further improved if a hot treatment is incorporated in the processing sequence. A short passage through a steamer is more effective than an aftertreatment in an open-width washing range containing hot bleaching liquor in several tanks. The conventional pad-batch recipe detailed above can be used but the bleaching time on the batch can be reduced to approximately 5 h. Before running the fabric through a steamer, it can be reimpregnated in the bleach liquor if desired. Steaming can be carried out either at atmospheric pressure for 1-2 min at 100-103°C, or under pressure for 1-2 min at 105-110°C. The fabric is then washed off thoroughly as already described. Pad-roll bleaching was developed in Europe to process shorter lengths of fabric in open-width form where the capital expenditure associated with large conveyor steamers could not be justified. Owing to the disadvantages explained in section 2.4.2, this semi-continuous process has been largely replaced by roller-bed steamers. The prolonged bleaching stage made it necessary to stabilise the bleach liquor effectively. Cotton fabric is padded at 20-30°C with the chemicals (10-20 ml/l hydrogen peroxide, 3-5 ml/l silicate, 3-5 g/l organic stabiliser and 2-3 g/l sodium hydroxide) and batched for 3-4 h at 80-85°C before thorough hot washing. Continuous pad-steam peroxide bleaching Whether or not a caustic scour is needed prior to peroxide bleaching depends on fabric quality. For example, clean polyester/cotton blend fabrics with low cotton seed content can usually be processed with a one-pass scour-bleach after preliminary desize. In order to prevent chemical damage to the cotton cellulose it is recommended that the lowest amount of alkali is used in the bleach liquor consistent with obtaining the desired effect at a given bleaching time and temperature. Good absorbency and wettability can be positively influenced by the choice of suitable surfactants. Deposition of calcium and magnesium carbonates and silicates can be largely avoided or greatly minimised by the use of organic stabilisers and sequestering agents. Careful selection of stabiliser systems is necessary as some organic stabilisers are effective only in long-liquor processing. As a generalisation, processes involving retention times over 1 h require the use of silicate in combination with an organic stabiliser. In open-width J-box bleaching cotton fabric is padded at 20-30°C with the chemicals (20-30 ml/l hydrogen peroxide, 5-10 g/l silicate or organic stabiliser and 3-6 g/l sodium hydroxide) and after batching for about 20 min at 95-l 00°C the fabric is washed on an open soaper with two tanks at 95°C, one tank at 60°C and one tank of cold water. In conventional J-box or conveyor bleaching systems the reaction (bleaching time) is greatly extended, making it possible to reduce the concentrations of chemicals used. Thus after padding with the chemicals (10-20 ml/l hydrogen peroxide, 2-3 ml/l silicate, 3-5 g/l organic stabiliser and 3-5 g/l sodium hydroxide) the fabric is batched for 1-2 h at 85-90°C and then washed off as before. The availability of modern roller steamers operating at high fabric speeds led to the development of silicate-free bleaching techniques in which short steaming times of

22


only 1-5 min were common. To achieve a good degree of whiteness and high absorbency under these conditions, it was essential that the goods to be bleached had been well desized and scoured. Typically the fabric was padded with the chemicals (20-40 ml/l hydrogen peroxide, 4-8 g/l organic stabiliser and 5-10 ml/l sodium hydroxide), steamed for l-3 min at 100-103°C and thoroughly washed on an open soaper. A major drawback of the above method was that insufficient relaxation of the fabric caused problems of creasing, particularly in heavier fabrics. Longer steaming times of 15-20 min were necessary to overcome these problems and the fabric had to be plaited down. So-called Combi steamers were developed that incorporate a tightstrand section to give the fabric a chance to swell in open width before being plaited down and carried forward on either a conveyor or roller bed in a relaxed state. In this equipment the chemicals (30 ml/l hydrogen peroxide, 4 g/l silicate, 4 g/l organic stabiliser and 8 g/l sodium hydroxide) are applied by padding and the fabric is then steamed for 20 min at 100°C and washed off in open width. 2.7 COMBINED PREPARATION PROCESSES The basic considerations that govern all attempts to streamline preparation steps are aimed at simplified and shorter processing times with reduced processing cost and energy saving. The search for lower energy costs in preparation has been channelled into two main directions: (a) A reduction in the total number of preparation processes, e.g. three reduced to two, or two reduced to one (b) Low-temperature processing. An example of such streamlining is provided by the oxidative desize followed by peroxide bleach using two steamers. In principle this process consists of two padsteam operations with intermediate and final washing stages. It is particularly appropriate for cotton and polyester/cotton fabrics capable of being processed at high speeds in open width. The grey fabric enters at one end and is processed fully continuously, wet-on-wet without intermediate drying between the oxidative desizing and the peroxide bleaching stages. The method requires well maintained pad impregnation units, modern steamers and effective open-width washing-off ranges. Sophisticated control equipment is essential to synchronise the smooth operation of all units and to monitor the levels and concentrations of the chemicals used. Another simplified process consists of a pad-batch oxidative desize followed by a pad-steam bleach with intermediate and final washing. This appeals to processors who have only one steamer and have a lower throughput. A further advantage results from savings in steam utilisation and particularly in energy. 2.8 MERCERISING John Mercer discovered that the tensionless treatment of cotton fabrics with strong caustic soda improved the dye uptake and the tensile strength. It was later found that the application of tension during the process also improved lustre. These changes


23

in fabric properties result from swelling and internal reorientation of the cellulose structure, creating more sites for chemical and physical bonding in mature cotton fibres. Immature fibres are also restructured, thereby improving their dyeability and reactivity in general. The concentration of sodium hydroxide necessary to achieve the fully mercerised effect is a 25% by wt solution. This yields considerable swelling of the cellulosic fibre, accompanied by yarn shrinkage if the fabric is not held under tension. Mercerising may be carried out at various stages of the preparation sequence on one of two main types of mercerising machine, described as chain or chainless mercerisers. The chain merceriser applies tension directly to warp and weft whilst the chainless design only applies indirect tension across the weft. Consequently the fabric construction must allow for this width loss, or the fabric must be stentered to a greater width prior to mercerising, which in itself is difficult and sometimes impracticable. An important consideration in mercerising is the rate of extraction of the alkali during washing. The rate of removal of the caustic soda and the drying temperature both have an influence on the resultant accessibility of the treated fibres. The benefits of the process are improved lustre, tensile strength, dimensional stability, dye uptake and coverage of immature cotton. All these properties depend on the alkali concentration, dwell time in alkali and treatment temperature. The effects tend to be confined mainly to the surface since full penetration into the fabric by the viscous solution of caustic soda does not occur at low temperature, even with the aid of wetting agents. This is particularly true when mercerising cotton fabrics in the grey state. A further disadvantage of loomstate mercerisation is the fouling of the liquor by size residues, making caustic soda recovery and recycling difficult. In recent years the so-called hot mercerising process has been introduced. The basic principles are saturation with mercerising-strength caustic soda solution near to the boiling point, controlled hot stretching followed by controlled cooling, and finally traditional tension-controlled washing and rinsing. The advantages claimed for this process are: (a) Shortening of the process sequence (b) Increased efficiency and uniformity (c) Use of either chain or chainless mercerisers with less problems related to fabric width control (d) Improved lustre, tensile strength and dimensional stability since greater fabric stretching is possible (e) Good response from fabrics containing lower-grade cotton (f) Flash scouring effect obtained (g) Good desizing action (h) Better fabric penetration by the hot caustic liquor. The chemical and physical changes induced by hot mercerising occur mainly when the cooled fabric is being washed. The possibility of combined scouring and hot mercerising is of interest since the degree of scouring, as a preliminary to peroxide bleaching, is equivalent to a conventional caustic scour.

24


The use of anhydrous liquid ammonia to modify cotton in a similar way has also been developed. lnterfibrillar swelling is accompanied by a limited degree of intrafibrillar swelling, less than that produced as a result of caustic soda mercerisation. Treatment with dry heat followed by steam to remove the last traces of ammonia is generally preferred because the fabric, when subsequently crosslinked (particularly by a low wet pick-up method), has an improved balance of physical properties compared with fabrics from which the ammonia has been removed by water alone. The flat-setting properties imparted by the ammonia-dry-steam process are superior to mercerisation in respect of the smooth-drying appearance and crease-shedding properties of such fabrics. The changes in swelling occur more rapidly than in conventional mercerising and the tensions developed are several times greater, so that control of the fabric dimensions is important.

CHAPTER 3

Dyeing of cellulosic fabrics

3.1 FIBRE STRUCTURE AND DYEING PROPERTIES Cotton varies in its physical and chemical properties according to its origin. Varieties grown in different regions of the world vary in morphology and fine structure. These varieties absorb dyes at different rates and to different saturation levels. In order to produce a commercially acceptable yarn, the spinner resorts to blending different varieties of cotton and thus obtains a more homogeneous product. The literature on the physical structure of native cellulose (cotton) and regenerated cellulose (viscose) is extensive, sometimes conflicting and still open to changes in interpretation as new analytical techniques are developed. Nevertheless, for an understanding of dyeing and finishing it can be assumed that in all cellulosic fibres there are regions of differing degrees of molecular order and disorder resulting, at each extreme, in crystalline and amorphous regions respectively. The crystalline regions provide strength and rigidity, whereas the amorphous regions are associated with flexibility, sorption and reactivity. The relative proportions and distribution of the crystalline and amorphous regions are of considerable importance, as they determine the behaviour of the fibre towards chemical processing. It must be borne in mind that dyes and chemicals must diffuse into and be absorbed within the disordered (i.e. accessible) regions of the fibre. The crystalline regions of the fibres are inaccessible to the penetration of dyes and many other chemicals. Diffusion-controlled processes are restricted to the disordered regions. These regions range from areas of low degrees of order to those that are completely disordered (the so-called amorphous regions). The fine structure of the cellulosic fibre therefore determines the accessibility of the fibre to dyes and chemicals. Such important parameters as rate, extent and uniformity of dyeing depend on these factors. The properties of cotton and viscose depend on the fibre structure at three levels of complexity. These are: (a) Molecular level, i.e. the degree of order of the matrix of cellulose molecules (b) Fibrillar level, i.e. the orientation of the fibrils along the fibre axis (c) Morphological level, i.e. structural differences between the surface and interior of the fibre. Differences at all three levels can occur with cotton as a result of fluctuating conditions during growth, or with viscose fibres due to variations introduced in manufacture. The structure and hence the properties of the mature cotton fibre, or of the extruded and drawn viscose fibre, can be modified by wetting, swelling, 25

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steaming or drying processes. The dyeing properties of cotton fibres depend on two important characteristics: fibre diameter and internal structure. Different varieties, grown in different regions of the world, vary in linear density (fibre weight per cm) by as much as 3:1. Fibre diameter determines fineness of the staple and can vary by as much as 5:1. In all wet processes fibre fineness can be of critical significance: the finer the fibre, the faster the rate of dyeing! The heterogeneous nature of both cotton and viscose, particularly in a radial direction, affects the rates of diffusion of dyes and other chemicals into the fibre and can lead to ring dyeing in the case of cotton and skin-core differences in viscose. Cotton is composed of structural bundles known as fibrils. These are essentially crystalline and are built up from the close packing of a large number of identical units of cellulose I (D-anhydroglucopyranose). The unit cell is the smallest highly ordered unit that as a sequential repeat forms the underlying structure of the crystalline regions. A crystalline region forms the central core of each fibril, the less ordered material being confined to the fibril fringes. Accessibility and reactivity are therefore explained in terms of a system of interconnecting voids bounded by fibrillar surfaces. As chemical reactions are regarded as taking place at the surfaces of fibrils of different size and accessibility, an essential aspect of all coloration and finishing treatments of cotton is the access to fibrillar surfaces. The association and distribution of the fibrillar material within the surrounding voids are important factors that determine the chemical reactivity and morphology of cotton. Swelling treatments can exert a profound effect, as indeed can subsequent drying, by modifying both fibrillar association and void penetrability. 3.1.1 Immature and dead cotton fibres Under the microscope normal cotton fibres appear almost cylindrical after swelling in alkali, with no continuous lumen and no well defined convolutions. In ‘dead’ cotton fibres the wall thickness after swelling is one-fifth or less of that shown by normal fibres. ‘Immature’ fibres encompass a range of thin-walled fibres between the extremes represented by normal and dead cotton. A commercial sample of cotton is in fact a mixture of fibres of different degrees of maturity. Under favourable growth conditions mature fibres with fairly thick walls will predominate. During the preparatory stages of spinning the shorter immature and dead fibres, where little or no secondary wall thickening has taken place, roll up easily into knots and tangles known as neps. Certain direct dyes will dye neps to almost the same depth as normal cotton fibres, but most dyes colour the neps to a paler depth. Dead cotton fibres are only dyeable with highly substantive direct dyes. Mercerisation prior to dyeing generally enhances the dyeability of immature fibres but calendering often accentuates the unattractive appearance of a neppy dyeing. In many commercially dyed cotton fabrics the neps are readily apparent as lighter specks. In bulk practice equilibrium sorption is seldom reached and efficient washing and rinsing processes tend to result in lower retention of dye by the immature and dead fibres. Moreover, for optical reasons these thin-walled fibres will tend to appear paler, even when the dye concentration is the same as in the mature fibres.


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3.2 PRINCIPLES OF DYE SELECTION Textiles made from cellulosic fibres and blends can be found in numerous end uses and at all price levels. Such fabrics can meet the most exacting fastness requirements for military uses, workwear and furnishing fabrics. Using ranges of cheaper dyes, certain domestic textiles for which high wash fastness is not essential, e.g. bedspreads and curtains, as well as cheaper fashion clothing and leisurewear, can be satisfactorily processed to give adequate fastness properties. The differences between the major classes of cellulosic dyes will now be discussed, leading to an appreciation of the factors that determine which class is chosen for a given substrate and end use. The first question to consider is why several major classes of dyes are necessary for cellulosic dyeing. The reasons are partly historical, arising from the way the dyeing of cotton has evolved, and partly because the various dye classes tend to complement one another. No single dye class meets all requirements, each has its own strengths and weaknesses. Before the discovery of direct dyes, cotton had to be mordanted before applying either natural mordant dyes or the brighter synthetic basic dyes. There were several disadvantages to this process, e.g. inadequate fastness, limited colour range and problems of reproducibility with mixtures of dyes. The first direct dye, Congo Red, was discovered in 1884. Dyes of this class possess good inherent substantivity for unmordanted cotton, good levelling properties and are easy to apply, so they rapidly secured acceptance, even though early members of the range had limited fastness properties. These shortcomings were partly overcome by synthesising diazotisable direct dyes, which after development gave much improved wet fastness. The discovery of copper-complex direct dyes further improved the light and wet fastness. In the early decades of this century other dye classes, notably vat, sulphur and azoic dyes, with much superior all-round fastness properties, tended to displace direct dyes except at the cheaper end of the market. However, the widespread exploitation of viscose fibres in the 1930s created a strong demand for direct dyes, because direct dyeing represented a cheap and simple method that did not require strongly alkaline treatments. This led to the synthesis of dyes of improved brightness and fastness, making direct dyes once again competitive in terms of performance. Although sulphur dyes were discovered before direct dyes, the former became important only with the advent of sulphur blacks at the end of the last century. Sulphur Black T (Cl Sulphur Black 1) soon became the best selling individual dye for textile coloration and this is still true today. The range was soon extended to dull yellow, orange, brown, blue and green colours, satisfying the demand for full depths at moderate cost with better fastness than previously available from direct dyes. Another class, the azoic dyes, was being introduced about the same time as direct dyes. The first successful azoic combination was Para Red (Cl Pigment Red I), but other simple orange, brown and blue azoic dyes, synthesised from diazotised amines and coupled with beta-naphthol, were inferior to the reds in terms of brightness, fastness and cost. The discovery in 1912 of the more substantive Naphtol AS (Cl Azoic Coupling Component 2) laid the foundation for a range of azoic dyes with a broader colour gamut, excellent brightness, and improved build-up and fastness properties.


Although synthetic indigo, a natural forerunner of vat dyes, had become available by the turn of the century, the dearth of bright blue dyes with good all-round fastness properties was ultimately satisfied by the synthesis of lndanthrone Blue in 1901 and the era of vat dyes had begun. Further anthraquinonoid vat dyes followed in rapid succession and soon the range of vat dyes comprised orange, brown, red, violet, blue, olive, grey and black colours with standards of fastness to light and washing not hitherto attainable with other dye classes. After World War 1 the interest in vat dyes was rekindled by the discovery of Caledon Jade Green (Cl Vat Green I), a bright green dye with excellent all-round fastness, that filled a major gap in the dyer’s palette. Shortly afterwards Indigosol O, a water-soluble leuco sulphuric ester derived from indigo, was synthesised and this ultimately led to a new range of solubilised vat dyes. Until the 1930s these four main dye classes competed for the major share of the cellulosic dyeing market and in any given period technical and commercial considerations decided which class was predominant at the expense of the others. During World War 2, however, the demand for huge quantities of uniforms, tentage and camouflage fabrics, dyed and finished to the highest fastness standards, could only be met by vat dyes applied by continuous pad-steam techniques. As a result vat dyes regained much of their former dominance until the arrival of a new class of dyes in the mid 1950s. By this time the main deficiencies were the lack of bright greenish-yellows, scarlets or reds, and turquoises or greenish-blues with good all-round fastness properties. Reactive dyes, first discovered by ICI chemists in the mid 195Os, offered the potential for structural diversity capable of filling these gaps and their rapid growth was ensured by their bright shades combined with high wet fastness. Today the search for cost efficiency and conservation of resources greatly influences the choice of dyes and dyeing methods. The change has been towards shorter liquor ratios and lower consumption of chemicals, as exemplified by trends from winch to jet dyeing, and from jet or jig to the application of pad-batch dyeing techniques to woven fabrics, all of which favour the growth of reactive dyes at the expense of azoic and vat dyes. However, direct and sulphur dyes continue to dominate the dyeing of cotton and viscose in heavy and dull shades, mainly on economic grounds. 3.3 DIRECT DYES Direct dyes have been available for over a century. Their discovery marked a major advance in the dyeing of cotton, which until then had to be mordanted in order to make it dyeable. Despite their ease of application, the early direct dyes had low wet fastness and frequently poor light fastness. To overcome these shortcomings much work was done to synthesise direct dyes with new chemical structures, capable of aftertreatment in various ways to upgrade their fastness to light and washing. A further important stimulus for direct dyes in the 1920s was the introduction of viscose, a major new fibre readily and cheaply dyed with these dyes. It is probable that direct dyes had the major share of the market for dyeing cellulosic fibres until the arrival of reactives in the 1950s.


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Their relative loss of importance since then is in part due to higher standards of wet fastness demanded in the apparel and furnishing sectors of the textile trade. These needs have been met by the discovery of reactive dyes, which combine excellent wet fastness with good light fastness across a broad gamut of bright shades. Direct dyes are still used in many applications where high wet fastness is not essential. Examples include curtains, linings, cheap bedspreads, flannelettes and winceyettes, dressing gowns and tufted carpets incorporating a viscose component. 3.3.1 Chemical properties of direct dyes Most direct dyes are based on the azo chromophore. Of those listed in the Colour Index approximately 50% are disazo, 33% are polyazo, and the remainder are monoazo, copper-complex azo or based on other chromophores. Many disazo direct dyes of former importance based on benzidine have been withdrawn from manufacture because of the carcinogenicity of benzidine and certain of its derivatives. Dyes based on less hazardous diamine intermediates have replaced those prohibited. Certain important direct dyes, mainly yellows, oranges and browns, are based on derivatives of stilbene. Copper phthalocyanine derivatives form yet another group of direct dyes giving bright turquoise blue shades of excellent light fastness but only moderate wet fastness. In aqueous solution most direct dyes exist as aggregates of several dye molecules. An increase in the dye or electrolyte concentration increases aggregation, but an increase in temperature has a reverse effect. The addition of urea causes disaggregation by reducing hydrophobic bonding between dye molecules. Many direct dyes tend to decompose during dyeing at or above the boil. Decomposition is often attributable to reduction of an azo linkage in the dye molecule. It frequently occurs when viscose fibres are dyed under alkaline conditions, since these can exert a reducing action. The remedy lies in careful dye selection and control of the dyebath pH by using a buffer such as ammonium sulphate. Similar problems can arise in the high-temperature dyeing of the polyester component of a polyester/cellulosic blend, where the cellulosic component is being dyed with direct dyes. Dye decomposition is usually caused by the reducing conditions created by cellulose at alkaline pH. If reducing conditions can be avoided, e.g. by adding ammonium sulphate buffer to achieve a dyebath pH of 6, some direct dyes will withstand high-temperature dyeing. 3.3.2 Application behaviour of direct dyes Direct dyes are applied to cellulosic fibres in the presence of an electrolyte at or near the boil. There are four major factors governing their behaviour. (a) When cellulosic fibres are immersed in a solution of a direct dye, they absorb dye from the solution until equilibrium is approached; at this stage most of the dye is taken up by the fibres. (b) The rate of approach to equilibrium, i.e. the rate of dyeing, varies from dye to dye. (c) Dyebath exhaustion at equilibrium is directly related to the substantivity ratio, i.e. the proportion of the dye absorbed by the fibre to that remaining in the dyebath. (d) Exhaustion at equilibrium is therefore a measure of the substantivity of the individual dye for cellulose.

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The first stage in dyeing cellulosic fibres with direct dyes is initial uptake of the dye on the fibre surface, known as strike. The second stage is diffusion of the dye into the fibre to approach an equilibrium distribution of dye between fibre and dyebath. Complete penetration into the interior of the fibre is essential to fully attain the desired shade and fastness properties. Important factors affecting the absorption of direct dyes by cellulosic fibres include the time and temperature of dyeing, liquor ratio, salt concentration, and the solubility and aggregation behaviour of individual dyes. Strike and fibre penetration are temperature-dependent and are accelerated by an increase in temperature. Dyeing above the boil shortens the dyeing period, enhances levelling and produces fully penetrated dyeings. An increase in the temperature of dyeing raises the rate of dye absorption, but decreases the equilibrium exhaustion. For a fixed dyeing time there is an optimum temperature at which dye absorption is at a maximum. The production of level and well penetrated dyeings is favoured by an increase in the time of dyeing. Dyebath exhaustion is highly dependent on liquor ratio, but other factors such as solubility, levelling properties and strike have to be taken into account. In dyeing cellulosic fabrics considerable variations in liquor ratio may be found depending on the method of dyeing. The addition of electrolyte to the dyebath (Glauber’s salt or common salt) increases both substantivity and the rate of strike. The quantity of electrolyte required depends on the concentration of dye in the dyebath and the chemical structure of the dye, especially the number of sulphonic acid groups per molecule. Dyes of good solubility are required, particularly for padding processes at low liquor ratios and room temperature. It has long been known that the dyeing behaviour of individual direct dyes varies greatly and care has to be exercised in dye selection, particularly in mixtures, to avoid uneven dye uptake, poor penetration, listing and ending. Acharacteristic property of direct dyes, the time of half dyeing (i.e. the time taken to reach 50% of the equilibrium absorption under specified conditions), is a measure of the rate at which a direct dye is absorbed by the fibre. It was previously thought that dyes exhibiting similar times of half dyeing would have similar dyeing characteristics and would therefore be the preferred choice for mixtures. This view was subsequently shown to be incorrect when it was established that rate of dyeing by itself was insufficient to predict compatibility and that rate of migration and salt control are of equal importance. A detailed study by an SDC committee showed that the following four parameters were important in defining the dyeing properties and compatibility of direct dyes: (a) Migration or levelling power (b) Salt controllability (c) Influence of temperature on exhaustion (d) Influence of liquor ratio on exhaustion. The SDC committee recommended that direct dyes be classified as follows. Class A Dyes that are ‘self levelling’, i.e. dyes of good migration and good levelling properties.


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Class B Dyes that are not self levelling, but can be controlled by the addition of salt to give level results; these are described as ‘salt controllable’. Class C Dyes that are not self levelling and are highly sensitive to salt; exhaustion cannot be adequately controlled by additions of salt alone and they require additional control by temperature; they are known as ‘temperature controllable’. In exhaust dyeing the ABC classification is widely used to select dyes for compatible mixtures. Temperature range tests are also useful for determining the behaviour of individual direct dyes at various temperatures of dyeing. The percentage exhaustion of dye under standard conditions of electrolyte concentration, liquor ratio and time of dyeing is determined and the results are expressed graphically. The selection of compatible dyes for jig dyeing and padding is best carried out by simple dip or strike tests. Fabric samples are dyed briefly (2 min), removed from the dyebaths, replaced by fresh samples and this procedure repeated several times. The dyed patterns are mounted in series and are assessed visually for change of hue and depth. A marked change of colour indicates incompatibility. 3.4 REACTIVE DYES In general reactive dyes with the same functional reactive group or groups have similar dyeing characteristics and can therefore be applied from the same dyebath. Reactive dyes are usually classified according to the chemical reactivity of the characteristic reactive system. In addition substantivity and fixation temperatures are parameters that help to characterise reactive dyes. The major reactive systems are listed in Table 3.1 in descending order of relative reactivity in batchwise dyeing under moderately alkaline conditions, together with typical fixation temperatures. The trade names and manufacturers of the more important ranges of these reactive dyes are shown in Table 3.2. Table 3.1 Propertiies of major types of reactive dyes

Reactive group

Fixation temperature (°C)

Relative reactivitya

Dichlorotriazine Difluorochloropyrimidine Dichloroquinoxaline Monofluorotriazine Vinylsulphone Monochlorotriazine Dichloro- and trichloro-pyrimidine

30 40 50 50 60 80 95

1 2 3 3 4 5 6

a 1 - -most reactive,

6 - least reactive

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Table 3.2 Some commercially available ranges of reactive dyes

Reactive system

Trade name

Dichlorotriazine Difluorochloropyrimidine

Procion MX (Zeneca) Levafix P-A and E-A (BAY) Drimarene R and K (S) Levafix E (BAY) Cibacron F (CGY) Remazol (HOE) Procion H-E/H-EXL (Zeneca) Procion H (Zeneca) Basilene E (BASF) Cibacron E (CGY) Drimarene X (S) Cibacron T (CGY)

Dichloroquinoxalinecarbonamide Monofluorotriazine Vinylsulphone Monochlorotriazine

Dichloro- and trichloro-pyrimidine

Symmetrical dye structures with two identical reactive groups, e.g. two monochlorotriazine groups in Procion H-E/H-EXL dyes, have led to products with better compatibility, more reproducible exhaustion and higher fixation efficiency. More recently reactive dyes with two dissimilar reactive groups per molecule have been developed, including the Sumifix Supra (NSK) range, in which each dye molecule has a monochlorotriazine and a sulphatoethylsulphone group attached. A new range of Remazol reactive dyes is believed to be of a similar constitution. In the past reactive dyes were usually classified as cold dyeing (highly reactive) or hot dyeing (moderately reactive). In more recent times dyes have been developed for dyeing at intermediate temperatures or at long liquor ratios, as well as by continuous methods. 3.4.1 Chemicals used in reactive dyeing Common salt or Glauber’s salt (hydrated sodium sulphate) is used in large quantities in all batchwise dyeing. The choice depends on price and availability. In the UK common salt is available in a very fine and pure form and is widely used. Common salt, being appreciably more soluble than Glauber’s salt, is usually added to the dyebath in the dry state. Where salt consumption is high, the installation of a saturator to produce brine may have advantages. Metering solid salt or brine to the dyebath eliminates repeated weighings and reduces handling costs. As anhydrous sodium sulphate is difficult to dissolve, it is best slurried in very hot water followed by adding cold water with constant stirring. When using Glauber’s salt crystals account has to be taken of the water of crystallisation when calculating the quantity to be dispensed. Soda ash is the alkali most widely used in reactive dyeing; sodium bicarbonate and caustic soda also find application. These three alkalis, used either singly or in binary


mixtures, cover the pH range that is of interest in reactive dyeing. Sodium bicarbonate is widely used in dyeing viscose with reactive dyes. In cotton dyeing it can be mixed with soda ash to give intermediate pH values. Liquid buffer systems and alkali dosing agents in liquid form, such as Alkaflo (Tanatex), are now available for multiproduct injection systems. Provided fabrics are well prepared, it is usually unnecessary to add wetting or levelling agents to the dyebath. However, the addition of selected antifoam agents can eliminate unwanted foaming in jet dyeing machines. Selected nonionic dispersing agents have proved useful in overcoming problems of aggregation of some reactive dyes. Certain turquoise and green reactive dyes of the phthalocyanine type tend to show this defect. As reactive dyes are prone to hydrolysis in the presence of moisture, they will deteriorate unless carefully stored and handled. Cool, dry storage conditions are essential, and lids of drums and packages must be firmly replaced after use. Dry scoops, scales and containers must be used when weighing. Care in handling is advisable and the use of dust-excluding respirators is recommended. Reactive dyes are dissolved by one of the following two methods: (a) Pasting with cold water, then adding more water at the correct temperature with continuous stirring (b) Feeding a steady stream of dye powder into the vortex produced by a highspeed stirrer, using water at the correct temperature. 3.5 VAT DYES Vat dyes are predominantly used for dyeing cellulosic fibres and the cellulosic component in polyester/cellulosic blends. Dyeing with vats depends on converting the water-insoluble vat dye by reduction to the water-soluble leuco compound. This has substantivity for cellulose and is therefore absorbed by the fibre. After dyeing, the leuco compound is converted back to the original insoluble form by oxidation. Indigo and related vat dyes based on natural products have been known for a long time. However, vat dyeing as practised today is a development of this century. The first synthetic vat dye, Indanthrone, was synthesised in 1904 and many others have followed. Of the early vat dyes synthesised in the first decade of this century, several important products are still listed in the present Indanthren (BASF) range. 3.5.1 Chemical properties of vat dyes Vat dyes are usually derivatives of indigo, anthraquinone or highly condensed aromatic ring systems containing carbonyl groups. All vat dyes contain carbonyl groups, separated by a conjugated system of double bonds. Commercially available vat dyes are marketed as liquids, granules or dedusted powders. For batchwise pigmentation and semi-pigmentation processes and for continuous piece dyeing, particle size distribution has to be uniform, preferably an average particle size of less than 1 pm. Apart from uniform particle size, commercial vat dyes must have good shelf life and good stability in the preparation of dye liquors, both in the pigmentation process and during vatting.

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Popularly referred to as hydros or sodium hydrosulphite, sodium dithionite is the most widely used reducing agent in vat dyeing. The decomposition of dithionite in an alkaline solution in the presence of oxygen is of utmost importance for the practical application of vat dyes and can be represented by Scheme 3.1. Thus two moles of caustic soda are required for each mole of sodium dithionite oxidised. In practice, it is essential to ensure that an excess of caustic soda is present during vatting to prevent undesirable side reactions from taking place. Na2S2O4 + 2NaOH + O2 - Na2SO3 + Na2SO4 + H2O

Scheme 3.1 In the vatting stage the water-insoluble vat dye is reduced to the water-soluble leuco form by sodium dithionite and caustic soda. During dyeing additional caustic soda is required to keep the reduced vat dye in its leuco form, as the dye is absorbed only in its ionised form by cellulosic fibres. The dyebath must have a pH of 12-13 to prevent formation of the almost insoluble acid leuco compound. As caustic soda is consumed both in the vatting process and also by the action of atmospheric oxygen on the vat, a sufficient excess must always be present. Neutral salts, e.g. sodium sulphate or sodium chloride, improve the substantivity of the leuco dyes for the cellulosic fibre. Nonionic levelling agents form complexes with leuco dyes and slow down their rate of absorption by the fibre. When oxidising leuco-dyed fabrics and in the final soaping process, dispersing agents are used to prevent aggregation and flocculation of undissolved dye particles. 3.5.2 Application behaviour of vat dyes Before dyeing can take place the water-insoluble vat dye has to be converted into the water-soluble leuco form, which is substantive towards cellulosic fibres. This process is known as vatting and represents the reduction of the carbonyl or keto groups of the vat dye by sodium dithionite in the presence of caustic soda to the water-soluble ionised leuco dye, as shown in Scheme 3.2. ONa

+ Na2S2O4 + 4NaOH -

+ 2Na2SO3 + 2H2O ONa

Scheme 3.2 In practice the vatting rate is important and it is known that: (a) An increase in temperature of 10 degC almost doubles the rate of vatting (b) The vatting rate is independent of the hydroxide ion concentration above pH 12 (c) The higher the concentrations of dye and reducing agent, the more rapidly reduction takes place (d) The crystalline form and particle size of the dye determine the rate of vatting.


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To prepare a satisfactory vat an excess of both reducing agent and alkali must be used. The quantity of reducing agent is determined by the nature of the dye (e.g. number of carbonyl groups, molecular size, purity) and the amount of air present in the dyeing system. The quantity of caustic soda is determined according to Schemes 3.1 and 3.2 and the need to maintain high alkalinity (pH 12-13). In other words, the amount of caustic soda required depends on the number of carbonyl groups that have to be reduced and on the amount consumed by oxidation during dyeing. Vat leuco dyes are generally present in the dye liquor in monomolecular form or as small aggregates. The substantivity of a leuco dye is determined by its constitution and is unaffected by its degree of aggregation. The latter, however, can affect diffusion into the fibre and thus levelling properties. Data concerning solubility, sensitivity towards metal ions, and tendency to tender the substrate can be found in the dye manufacturers’literature. If a cellulosic substrate is immersed in an alkaline dyebath containing a leuco vat dye, the dye exhausts rapidly into the fibres until a state of equilibrium is approached. The higher the substantivity of a dye for the fibre, the higher the uptake. Vat dyes exhaust rapidly into cellulosic fibres even at relatively low temperatures. Kinetic studies have shown that the dye exhausts in two stages. The major portion (generally 80-90%) exhausts within a short time (about 10 min), whereas the remainder exhausts slowly over a period of up to 50 min. The rapid initial exhaustion of the dyebath is due to the high substantivity of the leuco dye, whereas the later stage represents slow diffusion of the leuco dye into the interior of the fibre. In vat dyeing therefore a real danger exists of the leuco dye being absorbed only into the outer, more accessible regions of the fibre, leading to uneven dye uptake, i.e. ring dyeing. When exhaustion is complete, the material is rinsed to remove unfixed dye, residual reducing agent and alkali. The leuco dye is then reconverted into the insoluble form by oxidation. Generally hydrogen peroxide or sodium perborate is used but sodium m-nitrobenzene sulphonate is occasionally preferred. After oxidation the dyeings are ‘soaped’ at the boil in an aqueous solution of surfactant. Soaping is an important process in vat dyeing, as it removes loose dye and thus improves the wash fastness of the dyed material as well as developing the final hue and optimum light fastness. Dyeings produced with vat dyes have higher overall standards of fastness than can be achieved with other classes of dyes for cellulose. Vat dyes are invariably specified for end uses where the highest fastness to light and wet treatments is demanded. 3.6 SULPHUR DYES It is not generally appreciated that in terms of quantity sulphur dyes still constitute the largest class of dyes used in cellulosic fibre dyeing. With the introduction of reactive dyes it was considered that usage of other classes would be greatly diminished, but this has not occurred. Sulphur dyes are also widely used on cellulosic fibre blends with polyester, such as cotton drill and corduroy fabrics. Sulphur dyes are dull in hue but relatively inexpensive. They find wide application in dyeing black, blue, navy, brown, khaki, olive and green colours in medium to heavy

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depths on a wide variety of apparel, workwear and military fabrics. Fastness properties vary; sulphur dyes have good to excellent light and wet fastness, although fastness to bleaching is poor. In the Third Edition of the Colour lndex sulphur dyes are classified into four groups: the sulphur dyes proper and the leuco, solubilised and condense sulphur types. Group 1 - Sulphur dyes These are water-insoluble; they contain sulphur both in the chromophore and in polysulphide chains. They are normally applied in the alkaline reduced (leuco) form from a sodium sulphide solution and are subsequently oxidised to the insoluble form on the fibre. This group comprises the traditional water-insoluble powder forms, the black grain types which contain some sodium sulphide and therefore possess limited substantivity, and the dispersed sulphur dyes (available in powder and paste form) that are non-substantive. Group 2 - Leuco sulphur dyes These dyes have the same constitution as the parent sulphur dyes. In the presence of a sufficient quantity of reducing agent, usually sodium sulphide or hydrosulphide, the leuco sulphur dye, having high substantivity, can be applied directly to the fabric. Leuco sulphur dyes are available in powder or liquid forms. Group 3 - Solubilised sulphur dyes Solubilised sulphur dyes are the thiosulphated derivatives of the parent dyes, also known as Bunte salts, and so have different Cl numbers. They are non-substantive but during dyeing are converted to the substantive, alkali-soluble thiol form. Group 4 - Condense sulphur dyes Dyes in this sub-class are sodium S-alkyl or S-aryl thiosulphates. Both constitution and dyeing methods are different from conventional sulphur dyes, but dyes of this class are of very limited significance. In the past powders have been the most important form in which sulphur dyes were sold. The dye powder is pasted with water and then dissolved by boiling with reducing agent and further additions of water. Pre-reduced powders are made from presscake paste to which a reducing agent has been added to solubilise the dye in water. Before drying into a powder, the dye paste is mixed with dispersing and stabilising agents. Grains are pre-reduced granules that have the advantage of being non-dusting. Dispersed powders are mainly used in pad-dry-chemical-pad-steam dyeing. They are milled to microparticles in the presence of dispersing agents. Dispersed pastes vary in strength, but can be readily poured from a container. Both partially reduced and fully reduced liquids are commercially available. The former are usually more concentrated, saving packaging and transport costs. However, they require further additions of reducing agent in order to convert the dye fully to the leuco compound. Fully reduced liquids, on the other hand, are ready to use and will give maximum colour yield. Neither type is adversely affected by low temperatures.


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Solubilised sulphur dyes can be prepared in both powder and liquid forms. These dyes show little or no substantivity for cellulosic fibres until a reducing agent has been added. They are used to obtain good penetration of tightly woven fabrics. The two most important reducing agents for sulphur dyes are sodium sulphide (Na2S) and sodium hydrosulphide (NaHS). These products are commercially available in different forms at various concentrations, but sodium sulphide is generally used at 60% strength and sodium hydrosulphide at 35% strength. The quantity of sodium sulphide required varies greatly and depends on the particular brand of dyes used. The amount is usually directly proportional to the dye weight, except for pale depths when a minimum concentration of 2.5-5.0 g/l sodium sulphide (60%) is used. The recommendations of the dye manufacturer should always be followed to obtain optimum results. Sodium hydrosulphide is widely used in place of sodium sulphide; the product is available in liquid and powder forms in several different concentrations. For many years caustic soda/sodium dithionite was the only sulphide-free dyeing system available for sulphur dyes. It has never been popular, as it is difficult to control and tends to give inconsistent results. Sodium carbonate/ sodium dithionite is only suitable for the water-soluble forms, particularly black and blue sulphur dyes. It is too weakly alkaline for the water-insoluble forms. The use of glucose as a reducing agent is on the increase where environmental considerations weigh against sulphide-based reduction systems. Glucose is suitable as an additional reducing agent for the pre-reduced liquid brands, or together with sodium sulphide for the water-insoluble types, to lower the total sulphide content of the dyeing system. With water-soluble sulphur dyes glucose can be the sole reducing agent, thereby creating a sulphide-free dye liquor. In all batch dyeing systems using glucose it is essential to maintain a temperature of 90-95oC for optimum results, irrespective of the alkali used. Many oxidising agents and methods are available for the reoxidation of leuco sulphur and sulphur vat dyes. The choice depends on: (a) Method of processing, i.e. batchwise or continuous (b) Fastness requirements (c) Effect on shade (some oxidising agents produce a dulling effect) (d) Available methods of control, particularly in continuous processing (e) Costs and environmental considerations. Depending on the oxidising system used, reoxidation requires a specific acid or alkaline pH that should be continuously monitored. Oxidation in an acid medium gives dyeings with better fastness to staining of adjacent whites. However, a greater change of colour takes place on washing in detergents containing peroxide. With dyeings oxidised in an alkaline medium the reverse applies. Traditionally potassium or sodium dichromate (pH 4.0-4.5) is the preferred system for oxidising sulphur dyes as it is rapid and reproducible. The addition of copper sulphate to oxidation baths improves the light fastness of fabrics dyed with sulphur dyes by as much as one point. Over-oxidation can have an adverse effect on handle and sewability. The presence of chromate in the effluent is environmentally undesirable. Oxidation with potassium iodate (pH 3.5-4.0) gives similar results. The use of

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sodium metavanadate (as a catalyst) shortens the reaction time. lnsufficient icient control can lead to the formation of undesirable iodine vapour. This substitute for sodium dichromate is itself being replaced on environmental and economic grounds. Oxidation with sodium bromate catalysed with sodium metavanadate (pH 4.0-4.5) gives a system that is similar in performance to those previously described. Fastness properties of the oxidised dyeings are also comparable. It is well known that oxidation with buffered (pH 4.0-4.5) hydrogen peroxide or sodium peroxide can be used to brighten the rather dull shades of sulphur dyes, especially of sulphur blues and blacks. The wet fastness of the resultant dyeings may be lowered by one point or more (staining of adjacent fabrics) by comparison with dichromate oxidation. Oxidation with mildly alkaline hydrogen peroxide (pH 10) proceeds rapidly, but requires strict control of peroxide content, temperature and pH to avoid variations in colour of the oxidised dyeings. The resulting shade is brighter than that obtainable from dichromate oxidation and there is little change of shade on washing with a peroxide-containing detergent. However, the wet fastness properties of the oxidised dyeing are inferior. Sodium chlorite is the active constituent of a number of proprietary oxidising agents. The method is suitable in both batch and continuous processing, provided oxidation is carried out at 90-95°C in an alkaline medium (soda ash) with sufficient time allowed for the oxidation reaction to proceed to completion. The oxidised dyeings have good wet fastness properties and a soft handle. Ludigol (BASF), the sodium salt of m-nitrobenzenesulphonate, in the presence of sodium carbonate gives complete oxidation on winches and jets. Chloramine T (Akzo), the sodium salt of N-chloro-p-toluenesulphonamide, can be applied in both batch and continuous processing. Complete oxidation is achieved and fastness properties of the oxidised dyeings are satisfactory. The treatment of leuco-dyed fabrics with alkylating agents based on epichlorohydrin results in much improved fastness to severe washing treatments using detergent/perborate formulations. The alkylation treatment also oxidises the leuco dye and may thus replace the normal oxidation treatment with most dyes. The dyeings may have lower light fastness. Crease-resist finishes improve the fastness to wet treatments of fabrics dyed with sulphur dyes by up to one point. The light fastness is not affected, but the colour of the dyeing usually becomes redder and duller. Sodium polysulphide is widely used as an antioxidant for reduced dyebaths, except for sodium dithionite with which it is incompatible. 3.6.1 Application behaviour of sulphur dyes The substantivity of sulphur dyes for cellulosic fibres varies; mercerised cotton and viscose are dyed more heavily than unmercerised cotton. Moreover, substantivity differs from dye to dye and from brand to brand. Dye liquors prepared from liquid brands, containing the leuco dye in liquid form, are virtually electrolyte-free and therefore exhaust much less readily than dye liquors prepared from the traditional powders containing electrolytes. The rate of exhaustion of sulphur dyes is very slow at low temperatures; in fact with liquid brands of leuco dyes electrolyte has to be added before exhaustion can begin. The rate of exhaustion of sulphur dyes is


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controlled by temperature rise and by the gradual addition of electrolyte, usually sodium chloride or anhydrous sodium sulphate. In batchwise processing the addition of electrolyte to the dyebath can be made either at the start of the dyeing process or on reaching top temperature. Addition at top dyeing temperature is favoured in jig dyeing. In machinery where additions are difficult the salt is added at the start of processing. The presence of electrolyte in continuous dyeing may cause tailing. The pre-reduced liquid brands, containing the leuco dye and a small amount of electrolyte, are frequently used. Padding is carried out at a low temperature consistent with good absorption of the dye liquor. As solubilised sulphur dyes are not substantive to cellulose, tailing is not a problem. Hence these are widely used in pad-dry-chemical-pad-steam processes. Dissolving sulphur dyes, particularly water-insoluble ones, is a most important step towards obtaining a satisfactory dyeing. The traditional sulphur dyes are dissolved by boiling for several minutes in a reducing solution (e.g. sodium sulphide). Alternatively they can be vatted with caustic soda and sodium dithionite in a similar manner to vat dyes. Dispersed powders or pastes are sprinkled into warm water with vigorous stirring to ensure uniform dispersion of the dye. The dispersion can be used directly as a pad or pigmentation bath, or after reduction as a solution of the leuco dye. When dyeing by the pad-dry-reduction method a migration inhibitor should be added to the pad liquor. Liquid dyes do not of course require dissolving, but care is needed in the preparation of the dyebath. Water-soluble dyes are dissolved by sprinkling into warm water containing wetting agent and possibly sequestering agent. After vigorous stirring the liquor is heated to the boil and allowed to simmer briefly to ensure complete dissolution, before diluting to full dyebath volume. The fastness properties of sulphur dyes are intermediate between those of direct dyes and vat dyes, but sulphur dyes are considerably cheaper than most vat dyes. In the area of medium to heavy depths, especially in black, blue and brown shades, sulphur dyes combine good to very good fastness with an economical price, guaranteeing their continued usage. As with most dye ranges, the fastness properties vary from dye to dye, but the following general comments apply: (a) Medium to heavy colours in most brown and khaki shades have a light fastness of 5, blues and navies reach 5-6 or 6, a full black dyed with Cl Sulphur Black will reach 7 (b) Good results are typically obtained in fastness to washing tests based on soap, but there is less resistance to laundering with detergents and perborates (c) Perspiration fastness is very good when tested at acid pH values; alkaline perspiration fastness is good to very good (d) Sulphur dyeings are destroyed in bleaching by sodium hypochlorite. The effluent from sulphur dyeing contains sulphides, the concentration of which depends on dyeing method, applied depth and dyes used. Discharge of sulphides to drain is not normally permitted because of danger of damage to the sewer, which can be caused by the bacterial oxidation of liberated hydrogen sulphide to sulphuric acid. Air oxidation treatment plants are effective in the treatment of such effluent.

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3.7 AZOIC DYES Azoic intermediates produce insoluble azo dyes in situ on textile materials. Azoic coupling components are generally referred to as naphthols. Azoic diazo components are primary amines (Fast Bases) or stabilised diazonium compounds (Fast Salts). Azoic compositions are mixtures of an azoic coupling component and a stabilised diazo compound. Azoic dyes are most frequently applied to cotton. Special precautions have to be taken when dyeing regenerated cellulose. There are four distinct stages in the application of azoic dyes to cotton: 1. Application of naphthol by treating the fabric in an alkaline solution 2. Intermediate drying to remove excess liquor 3. Development with a diazonium compound 4. Aftertreatment (soaping at high temperature, washing-off and final drying). Advantages of azoic dyes are: (a) A wide range of orange, red, bordeaux, dark blue and black shades (b) Bright full depths with good fastness properties (c) Good reproducibility and generally high fastness ratings (d) Good dischargeability (e) Economically viable processes. Disadvantages are: (f) Complicated and time-consuming application procedures (g) Limited versatility, with only a restricted number of azoic combinations of interest in respect of hue and fastness properties (h) Extremely restricted scope for formulating mixtures to produce intermediate shades (i) Wide variation in naphthol substantivity, and large differences in rate of coupling of diazo compounds. Naphthols are insoluble in water but their sodium salts (naphtholates) are watersoluble or can be dispersed in water. Dispersions require the addition of a protective agent to make them stable. Both hot and cold methods of dissolving naphthols are in use. In the cold-dissolving method the naphthol is pasted with industrial alcohol and warm water and then converted to the naphtholate by adding caustic soda. If required, formaldehyde can be added to the concentrated solution. The liquor is ready for immediate use following dilution and addition of a protective agent. The cold-dissolving process is used when preparing naphtholate solutions for batchwise application at room temperature. In the hot-dissolving method a protective agent and hot water are poured over the naphthol. After a brief boil the required quantity of caustic soda is added and the resulting clear solution is ready for application. Several naphthols of low to medium substantivity, normally difficult to dissolve, are marketed in a special form that dissolves easily and quickly. The hot-dissolving method is of importance for padding processes, as padding is normally carried out near the boil.


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3.8 FACTORS INFLUENCING CHOICE OF DYEING METHOD All classes of dyes find use in the dyeing of woven fabrics. For the cheaper end of the market direct and sulphur dyes are favoured, whereas quality fabrics with high fastness properties are generally dyed with reactive or vat dyes. Types of fabric made from cotton and polyester/cotton are listed in Table 3.3 in order of decreasingly stringent fastness requirements (polyester/cotton blended fabrics must also be dyed with disperse dyes). It is important to distinguish between batch and continuous (or semi-continuous) processes. In batchwise systems water-soluble dyes are simply exhausted onto the cellulosic substrate in the presence of salt. Typical dyeing machinery suitable includes winch, jig, beam and jet machines. Other classes, i.e. sulphur and vat dyes, can also be applied by batchwise methods but the dyeing systems are more complicated. Alkali and a reducing agent must be present to convert the dye into the leuco form that is substantive for cellulosic fibres. Table 3.3 Types of cotton and polyester/cotton fabrics and typical dyeing processes (in order of decreasingly stringent fastness requirements)

Application

Process

Military uniforms, sailcloth, awnings, tentage

Continuously dyed with vat dyes by the padsteam process to meet very high fastness requirements

Cotton and polyester/cotton shittings

Continuously dyed with vat leuco esters in pale shades and vat dyes in medium to dark shades for quality goods of high fastness

Furnishings and household textiles

Vat dyes used for the highest fastness, with reactive or copper-complex direct dyes for less exacting end uses

Cotton and polyester/cotton workwear

Vat and sulphurised vat dyes applied by padsteam or Thermosol/pad-steam provide resistance to repeated severe laundering.

Corduroys and velveteens

Sulphur dyes applied by a one-bath pad-steam process or reactive dyes by a pad-batch process are used for the top end of trade; cheaper qualities are jig-dyed with diazotisable direct dyes or azoic combinations

Outerwear, rainwear and coated fabrics

Generally vat, copper-complex direct or reactive dyes used to provide good light fastness

Fashionwear, leisurewear, sleepwear, candlewick bedspreads, cheaper curtains and tufted floor coverings

Direct dyes widely used either with a cationic after-treatment or a crease-resist finish to give improved wash fastness

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In continuous or semi-continuous processes, dyes of generally lower substantivity are applied by padding, usually at room temperature, followed by a fixation process, usually by steaming. Certain reactive dyes are eminently suitable for continuous processing but other classes, e.g. solubilised derivatives of vat and sulphur dyes, also perform well as they have low substantivity for cellulose. 3.8.1 Substantivity Substantivity for cellulose is crucial in determining whether a dye is likely to be applied by batch or continuous methods (Table 3.4). In batchwise dyeing substantivity determines the build-up properties and is thus related to the cost-effectiveness of the process. Table 3.4 Substantivity characteristics of dyes for cellulosic fibres

Dye class

Comments

Direct

High levels of substantivity make directs ideal for exhaust processes, but generally unsuitable for cold padding processes

Reactive

Wide range of substantivity; for exhaust dyeing substantivity factors of 6080% preferred, but in pad dyeing values of less than 40% are most suitable; in long-liquor dyeing processes high-efficiency dyes of the hot-dyeing type give best results in terms of yield, build-up and reproducibility

Vat

Solubilised vat leuco esters and vat pigments have little or no substantivity so are ideal for continuous processing; alkaline leuco form is highly substantive, yielding high exhaustion in batch dyeing; retarding agents required to control initial strike and promote levelling

Sulphur

Water-soluble leuco forms have high substantivity and are suitable for exhaust dyeing; solubilised derivatives have low substantivity and hence are primarily of interest in continuous dyeing

Azoic

Substantivity of coupling component determines which dyeing method is preferred; highly substantive naphthols favoured for batch dyeing; lowsubstantivity naphthols are better in continuous padding methods; in red sector available naphthols cover whole range from high to low substantivity

3.8.2 Solubility The solubility of a dye in water greatly influences its ease of application to cellulosic fibres. Hard water is generally undesirable and should be softened before use. Aftercopperable direct dyes are particularly sensitive to hard water. An increase in molecular size or a decrease in the degree of sulphonation tends to enhance the substantivity, but may adversely affect solubility, with increased risk of aggregation


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and possible precipitation. Heavy depths of low-substantivity direct dyes are difficult to apply by padding and can lead to aggregation and precipitation in the padding trough. A good balance between substantivity and solubility exists with most reactive dyes. Nevertheless, care is needed to avoid precipitation when applying lowsolubility cold-dyeing dyes in full depths by ‘all-in’or ‘salt-at-start’techniques. Insoluble reoxidised vats or sulphurs may cause problems of machine contamination on being deposited at the air-liquor interface of dyeing machines. A cause of precipitation in the acid leuco form and consequent contamination may arise from failure to keep the leuco solution of a vat or sulphur dye sufficiently alkaline. Maintenance of adequate solubility of azoic diazo and coupling components prior to the coupling stage is vital. Naphthols must be completely converted to the naphtholate with alkali, whilst the insoluble Fast Bases must be fully converted into the water-soluble diazonium compounds by the diazotisation reaction. 3.8.3 Compatibility In general dyes of different dye classes are rarely, if ever, combined for dyeing cellulosic fabrics. Even dyes from the same dye class, but belonging to different subclasses, can cause problems if used together in the same dyebath. Exceptions occur in the dyeing of blends of cellulosic and non-cellulosic fibres where two different classes of dyes are often applied together under carefully defined conditions. The classification of direct dyes into sub-classes, according to the need to control dyeing behaviour by temperature adjustment and salt additions, is well established (section 3.3.2). Specific combinations of dyes selected on grounds of colour and fastness properties from different sub-classes may cause major problems of incompatibility on attempting a compromise dyeing method. Reactive dyes with the same type of reactive group usually exhibit similar dyeing properties. It is usually inadvisable to combine dyes from different reactive systems in the same dyebath unless they can be fixed under the same conditions of pH and temperature. The classification of vat dyes into sub-classes according to dyeing behaviour is well established. Selection in mixtures on grounds of hue alone can lead to poor reproducibility if the leuco forms of the component dyes vary widely in substantivity. The leuco forms of sulphur dyes vary widely in substantivity. Incompatible combinations will give poor reproducibility, particularly in continuous dyeing. The scope for mixture recipes in azoic dyeing is extremely limited as only a restricted number of azoic combinations are of interest in respect of hue and fastness properties. Naphthols vary greatly in substantivity and diazo compounds differ widely in their rates of coupling. Hence competitive effects between like pairs of components may yield quite unexpected results in admixture. 3.8.4 Dyeing kinetics The aim of all conventional dyeing processes is to achieve the best possible penetration of the fibres. Thorough and uniform penetration can be equated with level dyeing and satisfactory fastness properties, within the limitations of the dye class employed. Good penetration results from adequate diffusion, provided that the

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dyes show satisfactory levelling characteristics. In terms meaningful to the the following parameters all influence the diffusion characteristics of a dye: (a) Molecular size and design (b) Degree of aggregation (c) Accessibility of fibrous substrate to dye molecules (d) Time of dyeing (e) Temperature of dyeing (f) Dyeing system (g) Presence of electrolytes and auxiliaries.

dyer,

The first three variables are not normally within the control of the dyer but the others are. Thus the correct choice of dyeing time, temperature control, chemical additions and dyebath parameters all contribute to achieving well penetrated and level dyeings. The comments in Table 3.5 may be helpful in comparing the behaviour of the various dye classes with regard to good penetration and level dyeing. Table 3.5 Diffusion, levelling and penetration of dyes into cellulosic fibres

Dye class

Comments

Direct

Aggregation can cause problems of slow diffusion and poor migration, especially with dyes of larger molecular size; levelling properties vary; class C dyes of lower solubility and higher wet fastness require careful control of temperature and salt addition; directs are unattractive for continuous application because of high substantivity and slow diffusion

Reactive

Designed to strike careful balance between substantivity, reactivity, levelling and penetration; high-reactivity dyes of relatively low substantivity recommended for low-energy processes, i.e. pad-batch and pad-jig; for conventional batchwise dyeing slow-diffusing dyes of low reactivity applied at higher temperatures preferred for optimum reproducibility and levelling

Vat

Highly substantive leuco forms show rapid strike and migrate incompletely; careful control of dyeing conditions necessary to achieve adequate penetration

Sulphur

Same comments as for vats

Azoic

No diffusion problems; diazo and coupling components penetrate readily and react together within the fibres; no levelling required

3.8.5 Build-up and cost-effectiveness The relationship between the various dye classes in terms of build-up and costeffectiveness is a complicated one. It depends on the colour sector concerned, the chemical constitution of the dyes, process costs and chemical costs, and last but not


45

least the cost of the dyes themselves. An approximate order of decreasing cost of the various dye classes and sub-classes used on cellulosic fibres is shown in Table 3.6. The old adage ‘you get what you pay for’ equally applies to the desirable characteristics of dyes and the dyeings obtained with them. The pros and cons of the various dye classes are presented in Table 3.7, again listed in decreasing order of cost. These brief comments are, at best, only a rough guide. Table 3.6 Relative dye costs of dyes for

cellulosic fibres Class

Relative Costa

Vat leuco ester 1 Vat 2 Reactive 3 Copper-complex direct 4 Diazotisable direct 5 After-copperable direct 5 Azoic 6 Conventional direct 6 Sulphur 7 a 1 - most costly, 7 - least costly

3.8.6 Ease of application Exhaust dyeing with direct dyes in the presence of salt is the most straightforward of all dyeing processes, but the wash fastness in moderate to full depths is inadequate without after-treatment. Reactive dyes, when applied by exhaust dyeing, need more salt than do direct dyes and must be fixed with alkali. A thorough washoff is required to remove unfixed dye, in order to attain the high wet fastness associated with reactive dyes. Vat and sulphur dyes require preliminary reducing treatment before exhaust dyeing and then oxidation and soaping aftertreatments. These processes make the application of these dyes a protracted and more complicated dyeing sequence. 3.9 BATCHWISE DYEING OF CELLULOSIC FABRICS

3.9.1 Direct dyes Batchwise dyeing is preferred for direct dyes. In jig dyeing careful selection of compatible recipes is important in order to avoid listing and ending. Such problems can frequently be minimised by resorting to the pad(dye)-jig(develop) alternative. Continuous dyeing processes create problems of solubility, substantivity and diff usion, and are of much less importance for direct dyes.


Table 3.7 Technical and commercial advantages and disadvantages of dye classes for cellulosic c fibres Dye class

Advantages

Disadvantages

Vat leuco ester

Water-soluble: excellent reproducibility; high fastness Less costly than vat leuco esters for moderate to full depths; good build-up; high fastness; less costly than reactive dyes in blue and green shades Azo reactive dyes in yellow to violet shades cheaper than vat dye recipes; technical advantages of brightness, wet fastness and versatility of application Economically competitive with azo reactive dyes, as well as vat blues and greens; wide range of hues available Cheaper than copper-complex direct dyes; some limitations on available structural types but reasonably wide gamut Cheap and easy to apply; wide range of hues available Full bright red hues cheaper than recipes based on direct or reactive dyes; good build-up and fastness; resistant to chlorine bleaches Economical in a restricted range of dull shades, notably navy and black; cheap to apply continuously

Poor build-up; high cost

Vat

Reactive

Copper-complex direct

Diazotisable and after-copperable direct Conventional direct Azoic

Sulphur

Careful control required to minimise rapid strike in exhaust dyeing; more costly than reactive dyes in all but blue and green shades Fastness to light and weathering significantly inferior to vat dyes; low chlorine resistance

Brightness and wet fastness inferior to reactive dyes

Inferior reproducibility and prolonged application sequence; only moderate wet fastness Poor wet fastness properties unless aftertreated In most hue sectors azoic dyes not fully competitive for brightness, ease of application and cost Batchwise methods often troublesome; problems of reproducibility; environmental drawbacks

Many direct dyes, but not alkali-sensitive ones, can be used to obtain pastel to medium shades on flannelette, winceyette and candlewick fabrics by a combined scour-dye process. Soda ash and a nonionic detergent are the usual scouring agents added to the dyebath. In combined scour-bleach-dye processes either hydrogen peroxide or sodium perborate can be used but copper-complex dyes and those sensitive to alkaline oxidation should be avoided.


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Direct dyes normally colour dead and immature cotton lighter than mature cotton fibres. With careful dye selection sufficient direct dyes can be found to adequately cover these fibres. Mercerising or causticising before dyeing swells the immature fibres and increases dye absorption, appreciably extending the range of suitable direct dyes giving satisfactory coverage. 3.9.2 Reactive dyes Reactive dyes can be applied to all types of cellulosic fabrics and blends by batch dyeing methods. Winches, jigs, beams and jets can all be used, depending on fabric construction and weight. Three distinct stages are recognised: (a) Exhaustion from an aqueous bath containing either common salt or Glauber’s salt, normally under neutral conditions (b) Addition of alkali to promote further dye uptake and chemical reaction between absorbed dye and the cellulose (c) Thorough rinsing of the dyed fabric, followed by soaping to remove electrolyte, alkali and unfixed dye. Dyeing temperatures can vary from ambient to the boil according to the dyeing properties of the reactive dyes used. In some dyeing processes stages (a) and (b) can be combined. Successful batchwise dyeing with reactives depends on correct preparation, i.e. a thoroughly desized and well scoured fabric. Fabrics for dyeing must be neutral, readily wettable and uniformly absorbent and have very low residual size levels. Suitable preparation processes have been described in Chapter 2. Reactive dyes do not invariably cover dead or immature cotton. It may sometimes be necessary to causticise or semi-mercerise cotton and viscose fabrics to enhance both appearance and colour value. Reactive dyes have good solubility and are not usually affected by neutral hard water, but as alkali is invariably used in dye fixation, it is essential to use soft water for dissolving dyes and in subsequent dyeing. Reactive dyes from different manufacturers’ranges should not be used in the same recipe, unless it is known that they contain the same reactive system. Even then consideration of individual dyeing profiles is essential to ensure dye compatibility and reproducibility of shade. The existence of many ranges of reactive dyes with different reactive groups has led to a multiplicity of recommendations in terms of temperature control, electrolyte and alkali usage. The guiding principle governing dye selection is to obtain the desired hue and colour fastness by an economical process that results in a good-quality dyed fabric. Shading additions and redyes significantly add to the total wet-processing costs. In spite of the popularity of pad-batch dyeing with reactive dyes, jigs are still widely used for dyeing cotton fabrics. Highly reactive dyes of medium to high substantivity have proved most effective. With dyes of low to medium substantivity, the dyes and salt may be added together over the first two ends. Dyes of low solubility and with a tendency to aggregate are best applied on enclosed jigs at 60°C with Glauber’s salt and a 1 :1 mixture of soda ash and sodium bicarbonate. Pad-jig development gives improved surface appearance and penetration of difficult fabrics.

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Advantages of highly reactive dyes in jig dyeing include: (a) Energy saving when dyeing under cold conditions (b) Good exhaustion and fixation at low liquor ratios (c) Good solubility and washing-off properties (d) Low salt and alkali usage (e) Alkalis employed are safe and simple to use (f) No listing or tailing (g) Good process reliability. If, for reasons of colour or fastness, hot-dyeing reactives are preferred, problems of listed selvedges and ending can be avoided by ensuring that variations in temperature between the dyebath and the fabric on the roll are kept to a minimum. This is achieved by: (a) Using enclosed jigs to provide as uniform a temperature as possible across the fabric width (b) Batching fabric evenly to avoid overlapping selvedges (c) Selecting dyes of similar reactivity/temperature profile. Winch dyeing with reactive dyes is of particular interest for lightweight woven cotton and viscose fabrics that can endure processing in rope form without creasing and without showing faults such as chafing, abrasion, rope marks, crow’s feet and running streaks. Variants to the standard salt/soda ash method include the ‘lowalkali’ and the ‘bicarbonate-ash’ processes, designed for difficult fabrics that do not respond satisfactorily to the standard process. The use of hot-dyeing reactive dyes has increased in recent years, because of their high exhaustion and fixation levels at long liquor ratios coupled with high standards of wet fastness. As dyeing is carried out at 80-85°C or the boil, penetration and levelling are good even with difficult substrates and constructions, Excellent liquor circulation and fabric movement are well known features of modern atmospheric jet and overflow machines. Other advantages include: (a) Shorter and simpler dyeing processes (b) Savings in water and energy consumption (c) Lower volumes of effluent (d) Lower salt and alkali consumption because of the short liquor ratios employed. Cotton fabrics have to withstand being processed at high speeds without creasing and yielding problems of abrasion and chafing. Excessive foaming, caused by vigorous turbulence at the jets, can be troublesome. Antifoam agents have to be carefully selected to minimise slippage, which interferes with circulation and can adversely affect dyeing quality. The jet machine is inferior to the winch in respect of washing-off, being approximately two and a half times less efficient than a winch for electrolyte dilution and removal. Reactive dyes giving high exhaustion and fixation are preferred for dyeing cotton fabrics in short-liquor atmospheric jets. The availability of automated machinery and the need for increased productivity has led to significant simplification of the


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previously established dyeing method, based on the portionwise addition of salt. Dyeing methods have been developed with shorter dyeing times and reduced labour and energy costs in mind. In the ‘salt-at-start’ method all the salt is added at the start of the dyeing cycle thereby reducing handling and saving time. This method is suitable for both mercerised and unmercerised cotton as well as viscose. In the ‘all-in’method all the alkali and the salt are added at the beginning of the dyeing cycle, which reduces handling even further. This method, developed for fully automated machinery, is suitable for unmercerised cotton only and may not necessarily achieve maximum exhaustion. High-substantivity dyes with poor migration properties should not be selected for the all-in process because of the risk of inadequate levelling and difficulties at the washing-off stage. 3.9.3 Vat dyes Classification of vat dyes into sub-classes according to the need to control dyeing behaviour by temperature control and additions of caustic soda and sodium dithionite is well established. Individual vat dyes have been classified as follows into five groups according to their dyeing properties. (a) IK dyes have relatively low substantivity for cellulosic fibres and are applied at ambient temperature with small amounts of caustic soda and large amounts of salt. (b) IW dyes have a higher substantivity and are applied at 45-50°C with more caustic soda and less salt. Regenerated cellulosic fibres and mercerised cotton are dyed without salt. (c) IN dyes have high substantivity, require even larger amounts of caustic soda and are therefore applied at 60°C without salt. (d) IN Special dyes are similar to IN dyes but require the largest amounts of caustic soda. (e) Other vat dyes require special application methods. Normally vatting takes place for 5-10 min in the reducing liquor at dyeing temperature. With many vat dyes, e.g. IN types, dyeing temperatures can be raised to 80°C or above to achieve better levelling, and also during shading. The fabric is entered into a freshly prepared dye liquor that contains the fully vatted dye, alkali (caustic soda), reducing agent (sodium dithionite), salt and auxiliaries (e.g. dispersing agent, complexing agent, levelling agent, as required). The quantities of caustic soda, sodium dithionite and salt and the processing conditions depend on the dyeing properties of the selected dyes, the depth of shade required and the liquor ratio. W ith specially selected dyes and reducing agents, and special precautions, the leuco process can be modified so that dyeing takes place at temperatures above the boil. The purpose of the HT process is: (a) To brighten the fabric while applying pale to medium depths at 115°C (b) To overcome levelling and penetration problems with certain fabrics and dye combinations (at 90-100°C) that may have been selected on grounds of hue and fastness properties, but where the leuco dyes differ widely in substantivity and dyeing properties.

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The pre-pigmentation method of application requires dyes with a fine particle size distribution, so that the vat dye becomes as evenly distributed as possible within the cellulosic fabric in the non-substantive insoluble form before vatting. Pigmentation is commenced at ambient temperature and the temperature is gradually raised to 60-80°C, with salt additions if necessary to promote exhaustion. Alkali and dithionite are added in portions to reduce the dye, which is gradually taken up by the fibre. Dyeing then proceeds in the normal manner. The semi-pigmentation process is based on the slower vatting rate of vat dyes at low temperature and, as in the pre-pigmentation process, the dyes must have a very fine particle size distribution. This process has proved valuable on winches and in overflow machines. The fabric is run at ambient temperature in a dispersion of the vat dye and caustic soda before addition of sodium dithionite over 10 min. The rate of dyeing is thus controlled by the slow rate of reduction of the dyes in the cold dyebath. The temperature is then gradually raised at 1 degC/min to the dyeing temperature. During this period any dye that is not yet reduced becomes absorbed in the insoluble form before being reduced to the leuco compound. From the outset this procedure results in a more uniform dyeing than is obtainable from the leuco process, where the leuco dye exhausts extremely rapidly and thus can result in uneven dyeing. The pigmentation process is the preferred method for jet machines. The fabric is run at 80-85°C in a bath set with a dispersion of vat dyes and caustic soda. Sodium dithionite is added to effect reduction of the well distributed vat dye to the leuco form. This method takes advantage of the superior levelling properties of vat dyes at elevated temperatures. Vat dyes can be applied on all types of dyeing machinery, i.e. jig, winch, beam, overflow and jet machines. Jigs should be the closed type, fitted with an automatic reverse mechanism. The dye liquor on the fabric must contain sufficient dithionite to prevent premature oxidation at the selvedges. Concentrations of sodium dithionite and caustic soda must be monitored frequently and additions made as necessary. Enclosure of winch and overflow machines is essential to keep sodium dithionite consumption to a minimum. The dyeing process most frequently used is the semipigmentation method. In beam dyeing the most important dyeing methods are the semi-pigmentation process for pale to medium depths and the leuco process for dark shades. In jet dyeing the pigmentation process finds wide application.

3.9.4 Sulphur dyes The leuco or solubilised sulphur dyes are preferred for dyeing on fully enclosed winches. The bath is set with alkali and sodium sulphide (or hydrosulphide) at 40-45°C and run for 10 min before adding the dye liquor. The fabric is run for a further 10 min before the temperature is raised gradually to 90-95°C. Dyeing is continued for a further 30 min before rinsing by overflow. This is followed by oxidation, rinsing and finishing in a bath containing lubricant and softener. Modern jet dyeing machines are generally suitable for sulphur dyes and give better results than winch dyeing. The enclosed conditions of the jet minimise the problems of premature oxidation. The sulphide-free reducing system based on glucose is ideal


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for jet dyeing as the required temperature of 95°C is easily attained. This reduction system leaves the jet machine in a particularly clean condition. The bath is set with the dissolved dye liquor at 40-45°C and run for 15 min. The pre-dissolved alkali and glucose are added and the process run for a further 15 min. The temperature is raised to 90-95°C and held for 10 min before adding salt over 15 min. Dyeing is continued for a further 20-30 min at top temperature, followed by rinsing by overflow and oxidation with 1-2 ml/l hydrogen peroxide at 40-45°C for 15 min. A final rinse and finish with softener completes the process. Sulphur dyes are widely used on enclosed jigs, batch sizes ranging from 200 to 1500 m. The essential requirement in jig dyeing is a uniform rate of dye uptake, since subsequent levelling is practically impossible. A typical dyeing process might consist of 18 ends as follows: (a) Dyes added over two ends and salt over two ends (b) Two ends run at top temperature with cold rinsing over four ends (c) Oxidising over two ends and hot rinsing over two ends (d) Soaping over two ends and rinsing over two ends. Bronzing of blacks and navies because of premature oxidation of the dye or precipitation by metals such as calcium, magnesium or iron is a fairly common problem. Remedies include the addition of an antioxidant, e.g. sodium polysulphide, or a sequestering agent. Other common jig dyeing problems include ending, listing and poor rubbing fastness. Correction of all these faults involves treatment in a blank bath of sodium sulphide for 30-60 min at 90-95°C with a sequestering agent. 3.9.5 Azoic dyes Naphthols of high substantivity are particularly suitable for batch dyeing. Naphthols of moderate substantivity can be applied both by continuous (where the undesirable substantivity can be suppressed by increasing the padding temperature and speed) and by batchwise (where the addition of electrolyte increases absorption) methods. Naphthols of low substantivity find their main application in continuous dyeing; with limitations they can be used in batch dyeing by the addition of common salt. The depth obtainable with azoic dyes depends on the amount of naphthol applied. The dyeings are defined by the quantity of naphthol fixed on the cotton fabrics as ‘g/kg naphthol’. In batch dyeing the depth of dyeing obtained with a specific combination of naphthol and diazotised base depends on: (a) Weight of the fabric to be dyed (b) Concentration of the naphthol solution (c) Liquor ratio (d) Amount of salt added. Dye manufacturers supply graphs and tables that give the concentration of naphthol at a specified liquor ratio to obtain the desired depth of shade. Water-insoluble naphthols have to be converted to the water-soluble naphtholates with caustic soda. Naphtholate solutions, irrespective of their method of preparation, contain an appreciable excess of caustic soda. The tendency for hydrolysis of the

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naphtholate, both in solution and on the fibre, to the free naphthol must be prevented as in this form no coupling will take place. Conversion to the insoluble free naphthol occurs on exposure to atmospheric carbon dioxide or in acid steam. Naphtholate solutions have limited stability, particularly on standing. It is best to prepare the solution immediately before use; standing baths are not recommended. Soft water should be used for dissolution and application of naphtholates, and if necessary a sequestrant should be added. Addition of a protective agent ensures that the naphtholate remains in solution. Wetting agents, based on alkylnaphthalene sulphonates, may be needed in batchwise processing to aid wetting-out of fabrics. Formaldehyde in the presence of an excess of caustic soda ensures good protection against the formation of free naphthol during hydroextraction and batching of naphtholated fabrics, as naphtholates are converted to stable methylol compounds. Using the cold-dissolving method for the preparation of the naphtholate, the addition of formaldehyde is recommended when naphthols are applied by batchwise exhaustion at room temperature. The protective action occurs at about 40-45°C; the reaction is accelerated by adding formaldehyde to a concentrated naphtholate solution. Additions of salt increase the exhaustion of naphthols, particularly those of low to moderate substantivity and at long liquor ratios. The amount of salt (10-40 g/l) required depends on the naphthol used. The exhaustion of naphthols decreases with increase in temperature; batchwise treatments are normally carried out at 20-30°C. Prior to the development stage it is necessary to remove excess processing liquor from the naphtholated fabric. Fabrics are hydroextracted if dyed on winch or jet, whilst jig-prepared fabrics are rinsed in common salt and caustic soda and then suction-extracted. All such fabrics are then dried in a hot-flue dryer at 90-11 0 o C avoiding overdrying, to a residual moisture content of 9-12%. Naphtholated fabrics are sensitive to light and should be protected during storage prior to development. At the development stage the naphthol-prepared fabric is entered into the developing bath containing a dilute solution of a diazonium salt. In the coupling reaction that ensues the insoluble azo dye is precipitated inside the fibre in a finely dispersed form. Diazonium salts are produced either by diazotising a primary aromatic amine, known as a Fast Colour Base, or by dissolving a Fast Colour Salt, which is a stabilised solid diazonium salt. Many different Fast Colour Bases and Salts are commercially available. They can be divided into four groups according to their coupling energies and optimum pH range of coupling. Fast Colour Bases, which are soluble in dilute hydrochloric acid, are diazotised by adding dissolved sodium nitrite (direct diazotisation); those which are insoluble have to be pasted with water and sodium nitrite prior to being added gradually to hydrochloric acid (indirect diazotisation). Diazotisation is normally carried out at 5-15°C and, as it is an exothermic reaction, care has to be taken not to exceed the recommended temperature by cooling with ice. The use of liquid Fast Colour Bases simplifies the diazotisation procedure. Dye manufacturers provide data for diazotisation, which is governed by the chemical constitution of the Fast Colour Base used. After diazotisation the diazonium solution contains excess hydrochloric acid and is stable in this form if protected from heat and light. The strongly acidic diazonium


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solution has to be neutralised before development, otherwise coupling will not occur. Sodium acetate is added either to the diazonium solution or to the developing bath prior to the addition of diazonium solution. Fast Colour Salts are stabilised solid diazonium salts and are readily soluble at 25-30°C in water containing a nonionic dispersing agent. The majority of Fast Colour Salts also contain an alkali-binding agent. This developing bath has the correct pH and is ready for use. Hydrochloric acid solutions of diazotised Fast Colour Bases are more stable than the neutralised acetic acid development baths. It is therefore common practice to store the strongly acid diazonium solution and neutralise it only shortly before use. Likewise it is advantageous to dissolve the Fast Colour Salts only when required. Addition of a dispersing agent to the developing bath is essential. Developing liquors, if kept cool and protected from light, remain stable for about one day. Hard water can be used in the preparation of developing liquors. Nonionic dispersing agents are suitable auxiliaries for dissolving Fast Colour Salts; they also help to disperse the insoluble azoic dye formed during development, preventing deposition of azoic dye particles on the fibre surface. The applied concentration (expressed in terms of g/l) of Fast Colour Salt or Base depends on the applied depth (in terms of g/kg) of naphthol and the liquor ratio. To ensure rapid and complete coupling with the absorbed naphthol, the development bath should be slightly acidic and contain excess diazo compound. Diazo components which are classified into groups according to their method of application and coupling energy. Group I products have the highest coupling energy and are best coupled at pH 4.0-5.5. Group Ill products have low coupling energy and require sodium acetate in the bath, the optimum coupling range being pH 6.0-7.0. Group II components occupy an intermediate position. A thorough after-treatment is an essential part of the azoic dyeing process and must be carried out carefully to obtain the desired hue and fastness properties. It consists of the following steps: rinsing, acidification, alkaline soaping, rinsing. Treatment in an alkaline boiling soap solution causes crystallisation of the insoluble azoic dye in the cotton fibre and results in some change in hue. Any loosely held surface dye is dispersed by the soap solution and is easily removed during rinsing. Alkaline soaping is carried out with Marseilles soap (olive oil soap) and soda ash using soft or softened water. With hard water it is necessary to use a polyphosphate sequestrant. 3.10 SEMI-CONTINUOUS DYEING METHODS 3.10.1 Direct dyes Direct dyes are not widely used in semi-continuous and fully continuous dyeing. With mixtures of dyes consistency of shade throughout the run of a pad-steam process is difficult to maintain because of differences in substantivity of the individual dyes. To minimise this problem selection of dyes with similar absorption characteristics is vitally important, as well as control of the concentration and rate of supply of dye to the pad bath, contained in a small trough of low capacity. Pad-steam dyeing of suitable cotton and viscose fabrics can be successfully carried out with class B dyes aftertreated with a cationic fixing agent.

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Fabrics dyed with direct dyes can be treated with a crease-resist finish to bring about an improvement in wet fastness but gradual removal of the resin in the washwear cycle lowers the wet fastness. Treatment with crosslinking reactants results in a notable improvement in wet fastness, but usually at the expense of a change in hue and a lowering of light fastness. A novel approach to achieving high wet fastness is represented by the lndosol (S) reactant-fixable dyes. These are copper-complex direct dyes. After dyeing, fixation with lndosol E-50 (S) powder takes place in afresh bath at 60°C. Dyeings are claimed to have good wash fastness up to 50°C. Better wash fastness (60°C) is obtainable using lndosol EF (S). The best wash fastness is obtainable from lndosol CR (a mixture of a cationic agent of the lndosol E type with a dimethyloldihydroxyethyleneurea reactant) applied by a pad-dry-bake treatment. Indosol-dyed fabrics have good light fastness and better wet fastness than is normally obtainable with aftertreated direct dyes. Aftertreatment with lndosol CR results in crosslinking of cellulose, improving crease recovery and enhancing dimensional stability. 3.10.2 Reactive dyes Reactive dyes with good solubility, low to moderate substantivity and controllable reactivity are particularly suitable for semi-continuous application. In batch dyeing liquor ratios range from 8:1 (jet) to 30:1 (winch) but in pad dyeing liquor ratios are as low as 1:1 or less. Dyeing at such short liquor ratios has real advantages in terms of rates of exhaustion and fixation. Depending on dye reactivity and pad liquor pH, reactive dye fixation can take place in a short (2 to 4 h) or a long batching time (16 to 24 h). The high-reactivity cold-dyeing dichlorotriazine dyes and the vinylsulphone systems are widely used because full fixation takes place quickly. The bifunctional Cibacron C (CGY) dyes containing both fluorotriazine and vinylsulphone groups have been introduced for pad-batch and continuous dyeing methods. They are applied with sodium silicate and caustic soda in the cold padbatch process and the optimum batching time is 6 h. The cold pad-batch process is a popular and economical dyeing method requiring relatively straightforward dyeing equipment and there is no need for intermediate drying. Pad-batch is a simple process with good reproducibility. In terms of labour cost, energy and water consumption, it represents the most economical approach to producing high-quality dyeings of high wet fastness on cotton and viscose woven fabrics. Pad-batch dyeing is particularly recommended for batch lengths of 100010 000 m per shade. Lengths in excess of 5000 m are frequently excessive for jig processing, and traditionally were considered too short to justify investment in fully continuous processing plant. For many woven fabrics, especially dressgoods and furnishings, the pad-batch route has substantially displaced jig dyeing. The processing sequence is as follows: 1. Impregnate the well prepared fabric in a cold solution of dye and alkali 2. Remove surplus liquor by passing the fabric through a mangle nip 3. Batch the padded fabric evenly and wrap in polythene sheeting 4. Slowly rotate the roll of wet fabric at ambient temperature for set period 5. Wash-off unfixed dye and dry.


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It is worth noting that in the cold pad-batch process the optimum temperature of both the pad liquor and the batched roll is around 25°C. If the pad liquor temperature falls below 20°C the colour yield will be reduced, but if it is allowed to rise above 30°C the bath stability will be impaired. For the standard short batching method (2-4 h) a dye/alkali mixer is an essential piece of equipment. In such a liquor feed device dye and alkali are brought together immediately before the mixed padding solution comes into contact with the material being dyed. Several reliable feed systems have been devised. On completion of the batch dwell period and depending on its length, the fabric can be washed on a jigger, an open-width washing range or in rope form in spiral winches. However, the most efficient washing-off process is based on elution with hot water. After padding the fabric is batched onto a specially designed perforated beam. When fixation is complete the batch is transferred to a suitable washing station. This method provides highly efficient washing-off with a minimum of labour, water and capital cost. Fixation times for dyes containing monofluorotriazine (Cibacron F) or vinylsulphone (Remazol) groups vary from 4 to 18 h, depending on the alkali used and the depth of colour to be dyed. The Remazol dyeing method uses sodium silicate as the major component of the alkali feed. It is claimed that the ‘light’ selvedge problem (caused by carbon dioxide during the batching period) is thereby virtually eliminated. However, great care in the washing stages is necessary to remove all residual silicate, otherwise an unacceptably harsh handle will result. Continuous dyeing methods are widely used where large quantities of fabrics have to be dyed to a limited range of shades. Historically it was usually accepted that an economic batch size should consist of about 10 000 m per shade but if downtime could be kept to a minimum, batches of 5000 m were economically viable. Today, as the result of high-tech match prediction operation and control, it is claimed that much shorter runs are now viable. Although the capital cost of installing a fully continuous processing line is high, the vast output that results from operating continuously at speeds above 100 m/min yields considerable savings in handling and labour costs compared with batch dyeing systems. The main technical advantages are: (a) Consistent shade stability over long runs and excellent shade reproducibility from run to run (b) Avoidance of listing and ending by careful attention to dyebath formulations and proper control of the processing conditions (c) W ide choice of fixation conditions offered by reactive dyes (d) Full range of shades from bright to deep ternary mixtures with good wet fastness properties. 3.10.3 Vat dyes In piece dyeing by the pad-jig method the material is impregnated with the vat dye dispersion containing a wetting agent on a two-bowl padding mangle. Unless the impregnated fabric can be immediately developed on the jig, the best procedure is to rotate the wet batch of padded material slowly and continuously, possibly wrapped in polythene, on a special frame. Intermediate drying is often adopted in pigment padding to enable a dyehouse to operate more efficiently, i.e. independently of the

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production of padded material. If intermediate drying is part of the processing sequence, an anti-migration inhibitor should be added to the pad liquor. In jig development the vat pigment is reduced in a blank vat and absorbed by the fibre. The liquor temperature in the jig (between 50 and 80°C) and the concentrations of caustic soda and sodium dithionite depend on the dyeing properties of the dye and the liquor ratio (from 2:1 to 5:1). In jig development, especially the wet-on-wet process (no intermediate drying), some of the insoluble vat dye is transferred from the fabric to the blank development bath during the first couple of ends. It is therefore necessary to add some pad liquor to the development bath and to allow this to vat before the first pass. An equilibrium is soon established between the insoluble dye that is being transferred into the blank bath and the leuco dye that is being absorbed. By this means ‘ending’ can be prevented, but it must be borne in mind that the larger the dye lot, the greater the risk of ending. The pad-jig process gives well penetrated dyeings with good levelness and is widely used. Other semi-continuous vat dyeing methods include the cold pad(dye)batch(develop) process. After padding with the vat dye dispersion (and intermediate drying if required), alkali and reducing agent are applied in a second padding process. The fabric is batched wet, wrapped in polythene and rotated for 2-4 h at ambient temperature. Oxidation and soaping is as before. The pad-steam process is the most reliable and popular continuous method for dyeing cotton with vat dyes. It was established during World War 2 for military fabrics in order to achieve consistent shade reproducibility coupled with outstanding fastness properties. Nowadays high-quality woven fabrics are still dyed by the padsteam route for exactly the same reasons. Conventional pad-steam ranges were designed for handling batches of no less than 10 000 m per shade. With the trend in high-income countries towards smaller dye lots, compact continuous pad-steam ranges have been designed that are capable of handling 2000 m or less economically, thereby minimising the need to resort to batchwise dyeing. 3.10.4 Sulphur dyes Solubilised and dispersed sulphur dyes are most suitable for the pad-jig process owing to their lack of substantivity for cellulosic fibres during impregnation. After padding the fabric is either stored on a rotating batch, or run directly into the jig already set with reducing agent and salt at 80-90°C and run for four to six ends before rinsing and oxidising. In all pad-jig processing some 10-20 ml/l pad liquor should be added to the jig in order to achieve equilibrium conditions more rapidly, therefore minimising ending. The pad-jig method is particularly useful in dyeing mercerised cotton or viscose fabrics, since the rapid strike of leuco dyes on these fibres would make subsequent uniform penetration and levelling difficult. The pad-batch process is used to a limited extent, the leuco liquid brands being preferred. Extra reducing agent and antioxidant are added to the pad liquor to counteract oxidation during batching. In pad-batch processing a minimum batching time of 3 h at ambient temperature is recommended. A pad liquor temperature of 40°C with a pick-up of 80% is normal practice, followed by rinsing and oxidation.


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Continuous dyeing represents the most widely used commercial method of applying sulphur dyes. The major outlet for sulphur dyes has been and still is cotton apparel, e.g. corduroys and velours. For continuous dyeing to be successful, fabric preparation must be consistently of a high quality, resulting in fabrics with a uniformly high level of absorbency. Cotton fabrics are frequently mercerised before continuous dyeing to minimise dye consumption.

CHAPTER 4

Dyeing of synthetic fabrics 4.1 PRINCIPLES OF DYE SELECTION All classes of dyes of interest for the dyeing of synthetic fabrics show some solubility in water, although disperse dyes are only sparingly soluble. They are absorbed by the synthetic fibres from aqueous solution by a reversible process, and the ease with which they desorb again varies enormously. This reversibility has two important consequences. Firstly an initially unlevel dyeing can be made more uniform by extending the time of dyeing. Secondly dye may be desorbed from the fabric during wet processing after dyeing, or in laundering treatments. These considerations are particularly important for disperse dyes on cellulose acetate and for disperse or acid dyes on nylon. Disperse dyes have substantivity for one or, usually, more types of hydrophobic fibre and they are normally applied from fine aqueous dispersion. Although almost all the dye is present as suspended particles, at least during the initial stage of dyeing, uptake of dye by the fibre takes place via the extremely dilute aqueous solution of dye that is continuously replenished by progressive dissolution of the suspended particles. The vapour of disperse dyes is readily absorbed by hydrophobic fibres. This is the basis of many continuous dyeing and printing processes in which the dye dispersion is deposited on the fibre surface by padding or printing and then drying. Dry heat or steam treatment is then given to vaporise the dye and promote diffusion into the fibre. Selection of disperse dyes for the dyeing of cellulose acetate, triacetate or polyester fabrics is strongly influenced by the choice of dyeing process conditions, the type of fabric to be processed and the equipment available. In the first instance, however, the dyes must be selected to provide fastness adequate for the required end use of the material, as typified by the very demanding requirements on polyester for the automotive trade. The washing fastness of disperse dyes on acetate fibres varies considerably from dye to dye, those dyes with inferior migration properties tending to show better fastness. It is also dependent on the fibre type, improving in the order: secondary acetate < unset triacetate < heat-set triacetate. Fastness to water and perspiration tends to parallel the fastness to washing, although some dyes of good fastness to washing exhibit unexpectedly low fastness to perspiration. Disperse dyes usually show good light fastness on acetate fibres, but anomalous fading in combination dyeings on acetate is quite well known. Certain disperse dyes on secondary acetate tend to fade when the dyed fibres are stored in areas where they can be affected by fumes from gas heaters, particularly red and blue dyes derived from aminoanthraquinones. It has been clearly demonstrated that the resistance to gas-fume fading is increased as the basicity of the dye is decreased. One of the advantages claimed 58


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for triacetate over secondary acetate is its suitability for heat setting by dry heat or steam. Consequently the ability of disperse dyes to withstand heat-setting treatments is of considerable importance in selection for use on triacetate. Traditionally the most important fastness requirements for disperse dyes on polyester fabrics have been fastness to light and heat treatment. However, increasing use of polyester and polyester blends for sports and leisurewear goods is putting more emphasis on wash fastness performance. Dyes selected for high heat fastness are usually found to have good fastness to washing, but the converse is not necessarily true. Dyeings subjected to relatively severe heat-setting conditions may show inferior fastness to washing, owing to the tendency for the dyes to diffuse to the fibre surface to some extent during the dry-heat treatment. Basic dyes are characterised by their substantivity for acrylic fibres containing a preponderance of acidic dyeing sites. Their advantages include their brilliance of hue, good light fastness on these fibres and the ability to build up to deep dyeings of very good wet fastness. Most basic dyes show very little migration on acrylic fibres, however. The dyeings should be as level as possible from the start and any unlevelness must be corrected during the exhaustion stage. If dyes in a combination are compatible then the dyeing time required is shorter, the reproducibility is better and the chance of obtaining a level dyeing is greatly improved. Acid dyes are water-soluble anionic colorants applied primarily to fibres such as wool, silk and nylon, all of which contain basic groups. Acid dyes have little or no substantivity for cellulosic materials, unlike direct or reactive dyes. Premetallised acid dyes of the 1 :1 and 1:2 metal-complex types are also classified as acid dyes in the Colour Index. Those acid dyes with high solubility in water tend to migrate readily but show relatively poor wet fastness, whilst the dyes of larger molecular size and a somewhat lower degree of sulphonation in general, including most of the metalcomplex dyes, often have inferior levelling properties but correspondingly higher fastness to wet treatments. Colour, fastness properties and batch-to-batch reproducibility are all important factors in selecting acid or metal-complex dyes for nylon. The dyes must be appropriate to the available equipment and dyeing process. The simplest commercially acceptable processing route and the cheapest available dyes are selected from those that meet the appropriate performance standards. If the nylon material exhibits variations in dyeability that are physical in origin, the dyes, auxiliaries and process parameters are carefully selected to minimise the visual impact of the warpway stripiness that often manifests itself when dyeability differences are present. Chemical variations leading to dyeability differences are less common, but it is difficult to improve significantly any stripiness arising in this way. The solution to this problem lies largely with the fibre manufacturer to ensure more careful control of the conditions of polymerisation and extrusion of the molten polymer. 4.2 DISPERSE DYES Most disperse dyes today are used to colour polyester and polyester-based blends, with relatively small amounts being used on acetate, triacetate, nylon and acrylics.


The dyes used for acetate and triacetate tend to be of intermediate molecular size owing to the limitations imposed by application conditions and the need for adequate levelling characteristics. The higher application temperatures necessary for polyester dyeing, as well as the often severe heat treatments given to dyed or printed polyester fabrics and blends, have necessitated the development of speciality dyes of high sublimation fastness. The recently developed polyester microfibres require more disperse dye to achieve a given depth and this can lead to inadequate fastness performance. On nylon the main advantage of disperse dyes is their ability to cover stripiness. They give only poor to moderate wet fastness, however, restricting their use mainly to lingerie and to pale depths on cheap apparel fabrics. They exhibit very good light fastness on acrylic fibres but are confined to pale depths because of limited buildup. The various manufacturers of disperse dyes have classified their products into three or four groups according to dyeing characteristics and heat fastness. Dyes of the smallest molecular sizes diffuse readily into the fibre and level well but their fastness to heat treatments is low. They are suitable for dyeing nylon, cellulose acetate and triacetate but are inadequate to meet the fastness requirements for dyed polyester. One application where their volatility is an asset, however, is in the transfer printing of polyester fabrics. Disperse dyes of intermediate molecular sizes show excellent dyeing properties on polyester, with good coverage of physical variations in textured polyester fabrics. Selected dyes from this group are also suitable for dyeing cellulose acetate and triacetate fabrics. Printers and dyers involved in the coloration of all three of the ester fibres find dyes of this class particularly useful. Dyes of the largest molecular sizes are primarily intended for dyeing polyester fabrics and blends where the highest fastness to heat treatments is required. Their main drawbacks are their poor levelling and migration properties. They also tend to be rather more sensitive to dyeing temperature and to diffuse much more slowly into the fibre than the more mobile dyes of smaller molecular size. The two main chemical classes represented amongst disperse dyes are monoazo (60% of consumption and rising) and anthraquinone (30% of consumption and falling). Other chromophore types are occasionally found (disazo, nitro, aminoketone, methine, quinophthalone). Traditionally monoazo structures have provided the yellow, brown, orange and red members of the range and the anthraquinone derivatives have dominated the sector from bluish-red to bluish-green. In recent years there has been a pronounced trend to introduce more economically attractive heterocyclic monoazo structures to replace some of the anthraquinone types in the blue sector. Anthraquinone dyes containing primary or secondary amino groups are particularly susceptible to gas-fume fading on secondary acetate or triacetate. Introduction of electronegative groups ortho to the amino position, or arylation of the amino group itself, increases the resistance to burnt gas fumes. The widespread use of heat treatments in the finishing of triacetate, polyester and their blends with cellulosic fibres makes the fastness to sublimation, dry-heat pleating and steam pleating on


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such fabrics of great importance. Fastness is improved by increasing the molecular size of the dye, especially by the introduction of polar substituents such as hydroxy or cyano groups. The aqueous solubility of typical disperse dyes ranges from 0.2 to 100 mg/l at 80°C. Since solubility increases logarithmically with temperature, in practice all the dye may be in solution in the dyebath at 130°C. Thus on cooling the dye may come out of solution as relatively large crystalline particles. Because disperse dyes are applied in the form of very fine aqueous dispersions, particle size and dispersion stability are extremely important. Ideally a disperse dye is required to disperse extremely rapidly when added to water and to give a stable dispersion of uniform particle size. This dispersion should remain stable throughout the dyeing process up to the top temperature reached. Under adverse circumstances aggregation of the dye particles may occur to such an extent that they become deposited on the fibre surface with consequent lowering of the fastness to rubbing, unless removed by reduction clearing. 4.3 BASIC DYES The classical basic dyes originally developed for application to tannin-mordanted cotton belonged to a variety of heterocyclic chromophoric classes, including azine, oxazine, thiazine, acridine and xanthene types. A few of these dyes are still of interest for dyeing acrylic fibres but the majority of the so-called ‘modified’ basic dyes of current interest are in the azo (45%), methine (15%), triarylmethane (10%) and anthraquinone (5%) chemical classes. In the classical heterocyclic chromophores the positive charge was delocalised within the chromophoric system, whereas in most of the modified types the positive centre is localised and isolated from the chromophoric system by an intervening saturated hydrocarbon chain. This has a direct bearing on the light fastness, which is generally considerably higher. The azo and anthraquinone basic dyes are often structurally related to disperse dyes of similar hue, tinctorial strength and light fastness. Although usually less brilliant than the basic dyes with a delocalised positive charge, their much superior fastness to light makes them extremely important, particularly for pale colours and whenever an unusually high standard of light fastness is demanded. 4.4 ANIONIC DYES FOR NYLON Ranges of acid dyes for dyeing nylon have been built up largely to include dyes that were originally introduced for dyeing wool; later these ranges were extended to include dyes specially developed for nylon. The major chemical classes represented are unmetallised azo (50%), metal-complex azo (30%), anthraquinone (10%) and triarylmethane (5%). Many of the unmetallised azo dyes are characterised by brilliance of hue and good to excellent fastness to light on nylon. Many dye makers have categorised their ranges of anionic dyes according to their dyeing behaviour on nylon, compatibility in mixtures and build-up properties. Monosulphonated acid dyes of relatively small molecular size tend to migrate readily and cover dyeabilityvariations well. They show fairly high substantivity under

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neutral-dyeing conditions but they often stain adjacent nylon badly in wet-fastness tests. Disulphonated acid dyes of markedly larger molecular size, on the other hand, migrate with difficulty and do not cover variations in dye substantivity so readily. Diffusion and levelling take place much more slowly but these give dyeings of much better fastness to wet treatments. The metal-complex azo acid dyes are coordination compounds in which one metal atom (chromium or cobalt) is complexed with one or two molecules of a monoazo dye containing hydroxy, carboxy, amino or methoxy groups in the 2,2 positions relative to the azo linkage. The 1:1 metal-complex dyes are of only limited interest for the dyeing of nylon because of the low application pH that is required. The 1:2 metal-complex types are much more important in nylon dyeing. Many of them contain no anionic solubilising groups (although some have non-ionised polar groups such as sulphonamide or methylsulphone) and thus are only sparingly soluble in water. They are applied to nylon from aqueous dispersion in a weakly alkaline dyebath (pH 8-9). The sulphonated unsymmetrical 1:2 metal-complex types are cheaper to manufacture and are relatively soluble in water. Compared with the unmetallised azo acid dyes, the premetallised types tend to be somewhat duller in hue, particularly the 1:2 metal-complex dyes. They exhibit high neutral-dyeing substantivity and build up well on nylon to give dyeings of excellent fastness to light and wet treatments. They diffuse slowly and migrate with difficulty only in high-temperature dyeing conditions. Thus they are particularly sensitive to dyeability differences in nylon. 4.5 DYEING METHODS FOR FABRICS DYEABLE WITH DISPERSE DYES Cellulose acetate is severely delustred if treated in near-boiling water and thus a temperature of 80-85°C is normally the maximum that can be tolerated when dyeing such fabrics. In contrast, triacetate is unaffected by boiling water and it is not unusual for a temperature of 115-120°C to be used, particularly when applying heavy depths. Strongly alkaline baths should be avoided, especially in the case of cellulose acetate. Surface saponification (S finishing) of triacetate is used to reduce the tendency of the fibre to acquire charges of static electricity. Polyester fibres show outstanding resistance to damage by most common chemicals under normal conditions of exposure. They will withstand the strongly alkaline conditions used in vat dyeing or in mercerising. This resistance may be attributed to the highly hydrophobic nature of the polymer. The effects of aqueous ionic reagents tend to be confined to the surface of the fibre. Advantage is taken of this property in applying severe chemical clearing processes for the removal of surface deposits of dyes remaining after dyeing, without affecting the dyes that have penetrated into the interior of the fibre. 4.5.1 Acetate and triacetate fabrics The use of individual disperse dyes in admixture sometimes yields unexpected results in terms of relative rates of dyeing and build-up properties. This arises from the fact that, for a given dyeing temperature, the rate of dyeing varies with the


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concentration of dye applied. With many disperse dyes the build-up of a dye in admixture is independent of the presence of the other dyes, and thus an additive effect is obtained that is most useful in obtaining full depths. It is sometimes the case that so-called homogeneous dyes in fact consist of mixtures of isomers and that these isomers give improved build-up by additive behaviour. The fastness to washing of disperse dyes on acetate and triacetate varies considerably from dye to dye. As might be anticipated, in order to meet more stringent standards of wet fastness, the dyer has to choose the more slowly diffusing dyes of larger molecular size, and these increase the problems of achieving level dyeing and satisfactory build-up. In general disperse dyes show good light fastness on cellulose acetate and triacetate. There is a tendency for the light fastness to be lower on dull (pigment delustred) acetate compared with that on bright fibre, whereas with triacetate there does not appear to be a difference in this respect. The main functions of dispersing agents are to assist in the process of formulation of the dye dispersion in warm water and to maintain the dispersion in a stable condition during the dyeing process. These are highly exacting requirements and it is often necessary to employ a mixture of agents in order to fulfil them. Dispersion stabilising agents of the lignosulphonate type are inexpensive but exhibit two disadvantages in practice. They are brown in colour and tend to cause discoloration because of partial uptake by the fibre during dyeing. They can also act as mild reducing agents when used in totally enclosed equipment above the boil, conditions that may apply to full depths on cellulose triacetate. Nonionic polyoxyethylene adducts are widely used as levelling agents in the dyeing of cellulose acetate with disperse dyes. These tend to increase the aqueous solubility of the dyes, resulting in enhanced migration, levelling and penetration. There is less tendency for listing and ending in the jig dyeing of acetate fabrics but the ultimate exhaustion value may be adversely affected. It is possible to increase the cloud point of a nonionic levelling agent by the addition of a relatively small proportion (10-20%) of an anionic surfactant. By such means speciality levelling agents that can be used at 110-120°C on triacetate fabrics have been formulated. Carriers have also been widely used in triacetate dyeing in the past, especially when dyeing at temperatures below the boil, but their use has declined considerably for environmental as well as purely health and safety reasons. The most popular chemical types of carriers for triacetate included diethyl phthalate and butyl benzoate, but many other aryl ester types have been recommended. Carriers produce a pronounced increase in both the rate of dyeing and the ultimate exhaustion of the slow-diffusing disperse dyes, while not adversely affecting the behaviour of the more rapidly diffusing components of a mixture recipe. Carriers also help to minimise listing and ending in the jig dyeing of triacetate fabrics. 4.5.2 Polyester fabrics Disperse dyes are absorbed by polyester fibres much less rapidly than by nylon or the acetate fibres. The rate of dyeing can be raised to the level of commercial acceptability either by the use of a carrier at the boil or by dyeing under pressure at

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about 13O°C. Apart from the special case of polyester/wool blends in which the wool fibre would normally be degraded at temperatures above about 105°C, the exhaust dyeing of polyester fabrics and blends is nowadays usually carried out under hightemperature conditions, mainly for environmental reasons. Under practical dyeing conditions the stability of the disperse dye/water/fibre system is often far from ideal. There is a tendency for the larger particles of dye present to grow at the expense of smaller particles, so that eventually less stable agglomerates are formed. As the dispersion deteriorates in this way there is an increased risk of deposition of agglomerated dye on the surface of the fabric. Thus it is important to choose processing conditions such that the dyeing process is completed before undue deterioration of the dispersion has occurred. The high substantivity of disperse dyes for polyester induces rapid initial adsorption of dye on the fibre surface, but the slow rate of diffusion into the interior imposes a limit on this surface adsorption when it reaches an equilibrium with the dyebath concentration. When this initial condition has been established, the rate of transfer from the dyebath is then governed by the internal rate of diffusion. The dye on the fibre is not yet evenly distributed throughout the fibre cross-section and dyeing must continue until a more uniform distribution has been achieved. The levelness achieved in the initial transfer phase is influenced by the relationship between the rate of dyeing and the rate of distribution of dye liquor throughout the batch, i.e. by the rate of flow in circulating-liquor machines or the speed of fabric transport in other types of dyeing machine. There are advantages to be gained by controlling the conditions of this transfer phase to minimise the time and energy required for the final levelling phase. For any polyester dyeing requirement it is possible to choose an optimum starting temperature, a rate of temperature rise (or a sequence of different rates) and a hold period at top temperature that will give a level and well penetrated dyeing, without prolonging the process or requiring excessive energy consumption at any stage. In principle the dyebath is heated as rapidly as possible to a chosen starting temperature at the lower limit of a critical temperature range. Within this range the exhaustion rate becomes rapid for the dye-fibre combination in question. The rate of temperature rise is then slowed sufficiently to ensure that dye uptake proceeds as uniformly and rapidly as is practicable. After more than 80% of the final exhaustion has been achieved, the temperature may then be raised to the top temperature chosen to complete the penetration of the fibre as quickly as possible. 4.6 DYEING METHODS FOR FABRICS DYEABLE WITH IONIC DYES 4.6.1 Acrylic fabrics Acrylic fabrics are processed mainly in the form of staple-fibre constructions. Dyeing with basic dyes is essentially an ion-exchange process. The dye cations are absorbed at the fibre surface from where they diffuse into the interior of the fibre and occupy anionic dyeing sites situated therein. The first two of these stages determine the rate of dyeing, and the number of available sites influences the attainment of dyeing equilibrium.


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The substantivity of basic dyes for acrylic fibres is very high and saturation of the fibre surface occurs rapidly at the dyeing temperature. With the exception of pale colours, dyeing proceeds from a layer of constant concentration of dye at the fibre surface throughout the dyeing process, and the rate of dyeing is virtually independent of liquor ratio. Above the glass transition temperature of about 80°C for acrylic fibres, the rate of dyeing with basic dyes is highly dependent on dyebath temperature. Careful temperature control, if necessary using retarding agents, is therefore essential if satisfactory levelness is to be achieved. Rates of dyeing under standard conditions differ considerably for different dye-fibre combinations, and these have been characterised individually in terms of measured constants published by the major manufacturers. A fibre saturation value has been established for each fibre type, representing the number of dye sites available per unit weight of fibre. Within the temperature range used in acrylic fibre dyeing, temperature does not affect equilibrium exhaustion appreciably, because the range of maximum temperatures used is narrow, affinities are high and the fibre saturation value is independent of temperature. For most acrylic fibres the saturation value increases slightly with increasing pH within the practical range (pH 4-6) butforfibres with weakly acidic (carboxy) sites there is much greater dependence on pH. The cationic retarders sometimes used are colourless quaternary compounds with moderate substantivity for acrylic fibres. They compete with dye cations for the dyeing sites within the fibre and at the fibre surface, thus reducing the dye concentration gradient effective for diffusion. They also increase the total number of cations effective for relative saturation. This double effect results in improved control of dye uptake and better levelness. Before dyeing, some acrylic fabrics are given a dry heat treatment either to impart stability against creasing (important in winch or jet dyeing) or to relax the material (important in beam dyeing). Unlike nylon or polyester, the setting effect is almost completely removed during the dyeing process owing to plasticisation of the acrylic fibre. The stenter frame is adjusted so that the fabric at take-off is free from tension. Winches, jet or overflow machines, and occasionally beam dyeing machines, are used for acrylic fabrics. In winch dyeing the tension must be as low as possible and the running speed controlled to avoid unlevelness and the development of permanent crease marks. The temperature throughout the material and liquor must be as uniform as possible and the machine should be enclosed. Attainment of levelness is often a problem, thus conditions must be chosen to promote maximum uniformity of uptake by selecting compatible dyes, a suitable retarder and close control of temperature over the critical range. Cooling to 70°C after dyeing must be slow to prevent creasing. Dyeing on jets or overflow machines entails considerably less severe problems of unlevelness and mechanical fluctuations. Fully flooded vessels are particularly suitable because of the low level of fabric tension, good liquor circulation and excellent uniformity of temperature throughout the goods. They can be used for most woven fabrics, including lightweight constructions.

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Acrylic fabrics must be relaxed completely prior to beam winding, since any shrinkage during beam dyeing will cause moire effects. Problems often arise with batching tension or sagging on the beam owing to plasticisation during dyeing but there is no risk of crease formation if the beam is carefully wound. After beam dyeing, acrylic fabrics are denser and harsher to handle than after dyeing in rope form. Jigs are unsuitable for dyeing acrylic fabrics because the material is so sensitive to undue tension at the top dyeing temperature. 4.6.2 Nylon fabrics Several interdependent factors must always be controlled in the dyeing of nylon with anionic dyes. Selection of dyes of low substantivity with good migration properties will give optimum levelness and coverage of variations in dyeability, but considerations of wet fastness may necessitate the use of more complex dye structures of higher substantivity. Dry heat setting under tension increases fibre crystallinity and reduces the rate of dyeing, but steam setting has the opposite effect. A pretreatment with an anionic blocking agent also reduces the rate of uptake of most acid dyes. The rate of dye uptake can be controlled by adjusting the rate of temperature rise, which should be relatively slow for dyes of higher substantivity. Most dyes do not exhaust uniformly over the whole temperature range and it is often economically advantageous to maintain a slow rate of rise (say 1 degC/min) over the most critical portion of the range (perhaps 50-70°C) and to use a mildly cationic levelling agent that forms a labile complex with the anionic dye. The choice of dyebath pH is governed by the substantivity level of the dye class being used. The higher the pH, the lower the dyeing rate and the final exhaustion level achieved. For dyes of moderate to high substantivity it is preferable to use acidliberating salts rather than free acids to ensure a slow rate of exhaustion during the early period of temperature rise over the critical range. As the temperature is raised the salt begins to dissociate and the pH progressively decreases, facilitating the attainment of optimum final exhaustion. Winch, jet, jig or beam equipment can be used satisfactorily to dye nylon fabrics, according to fabric type. Beam dyeing is preferred when the fabric construction is suitable, as it produces a finished fabric free from creases or abrasion marks. The use of high-temperature beams or jets also allows greater flexibility of choice of dyeing temperatures, and hence in the choice of suitable dyes. Winch dyeing is preferable when dyeing loop-raised and embossed constructions in which the surface effect would be flattened by dyeing on the beam. Textured nylon fabrics are normally scoured and relaxed before dyeing in order to achieve full bulk and handle. Winch equipment should be selected to ensure minimum fabric processing tension, since this can restrict the development of full stretch properties and fabric bulk. Fully flooded jet machines are ideal for processing textured nylon either at or above the boil, since support of the fabric by the liquor allows full relaxation to occur under conditions of minimum tension. Woven nylon is generally dyed by one of these four methods (jig, beam, jet or winch), depending on the type of fabric to be processed. Where suitable equipment


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is available it is customary to dye heavyweight constructions on the jig, lightweight or porous fabrics on the pressure beam, and more flexible or resilient constructions not prone to creasing on the winch or jet machine. Jig dyeing is normally carried out on unset fabric, since it is difficult to achieve a temperature sufficiently high to give an even and well penetrated dyeing on set fabric. Fully enclosed jigs are necessary to maintain a high temperature on the batch, maximum rate of dyeing and freedom from listing and ending. The main advantage of beam dyeing is the higher dyeing temperature attainable. Dyeing at 110-120°C gives better levelling and penetration, greater freedom from stripiness and unrestricted use of the high-substantivity dyes of optimum wet fastness. Before beam dyeing it is usually necessary to preset the fabric in order to avoid moire or water marking. Unset constructions such as taffetas should be interleaved with fine cotton to prevent these problems.

Dyeing of blend fabrics

5.1 PRINCIPLES OF DYEING METHODS FOR BLENDS The term blending is used by the yarn manufacturer to describe specifically the sequence of processes required to convert two or more kinds of staple fibre into a single yarn composed of an intimate mixture of the component fibres. To the dyer, however, the important type of staple-fibre blend is that in which the components are two different fibrous polymers, each with its own characteristic dyeing properties. The term blend has therefore been used widely by the dyer to refer generally to any combination of two or more fibre types, the essential difference between the components being that of dyeing characteristics. In this context the term includes folded yarns and union fabric constructions designed to achieve colour contrast effects. There are essentially three reasons that have been put forward to justify the replacement of a homogeneous textile material by a blend: (a) Economy: the dilution of an expensive fibre by blending with a cheaper substitute (b) Physical properties: a compromise to take advantage of desirable performance characteristics contributed by both fibre components (c) Aesthetics: the attainment of attractive appearance and qualities by development of fabric designs incorporating multicolour effects or combinations of yarns of different lustre, crimp or linear density. The kinds of coloured effect achieved by dyeing a blend of two fibres can be categorised as: (a) Solid: both fibres dyed as closely as possible to the same hue, depth and brightness (b) Reserve: only one fibre dyed and the other kept as white as possible Shadow: the two fibres dyed to the same hue and brightness, but the depth on one fibre is only a fraction of that on the other Contrast: the intention usually to achieve the maximum contrast in hue at approximately the same depth on both fibres, but sometimes more subtle contrasts preferred; in either case optimum brightness on both component fibres enhances the pleasing appearance of the contrast effect. A solid effect (also called union-dye effect) is most frequently the objective of dyeing a blend, since most of those developed for reasons of economy or physical properties, especially the blended staple type, are not intended for use in multicoloured designs.


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Reserve, shadow and contrast effects are mainly of interest for fabrics or carpets made from differential-dyeing yarns. Reserve effects are easiest to achieve on the acrylic or polyester component of their blends with cellulosic fibres. The shadow effect, which is also called two-tone or tone-in-tone, may be regarded as intermediate between solid and reserve effects. The most pleasing shadow effects are obtained when the paler depth is one-third to one-half the depth of the deeper component. Contrast effects, which are also known as cross-dye or two-colour effects, represent the primary justification for differential-dyeing variants. The best contrast is achieved with blends of variants of the same synthetic fibre that are dyeable with acid and basic dyes, where the physical and optical appearance of the undyed yarns is identical. It is useful to classify fibre blends in terms of their dyeing characteristics, based on the type of dye used to obtain fast dyeings in full depths on the component fibres. The dyeing characteristics in full depths are particularly important because many of the problems associated with dyeing blends are more serious under such conditions. The basic classification of the major fibre types by dyeing properties is: - D fibres (dyed with disperse dyes in full depths): secondary acetate, triacetate and polyester - A fibres (dyed with anionic dyes in full depths): nylon, wool and cellulosic fibres - B fibres (dyed with basic dyes in full depths): acrylic, ‘basic-dyeable’nylon and ‘basic-dyeable’polyester. Blends containing only ester fibres (D fibre blends) are especially suitable for shadow and reserve effects but solid effects are difficult to obtain and colour contrasts impracticable. Blends of A fibres, based on nylon and the natural fibres, are particularly important in the carpet industry and for leisure clothing. Solid dyeings are important and not difficult to obtain. These blends are ideally suitable for shadow effects but contrast and reserve effects are generally impracticable. Solidity is often the objective for blends of ester fibres with nylon or natural fibres (DA blends) but ester fibres with cellulosic types usually give good reserve effects if desired. Colour contrasts are limited by cross-staining of disperse dyes, especially on nylon or wool. The attainment of contrast or reserve effects by differential dyeing is the main justification for DB blends, which contain ester fibres with acrylics or other ‘basic-dyeable’ variants. Optimum reproducibility of bright complementary colour contrasts is achieved on AB blends, containing nylon or natural fibres with acrylics or ‘basic-dyeable’variants. Good reserve of ‘basic-dyeable’or cellulosic fibres, or solid effects, can be obtained if required. 5.2 BLENDS OF FIBRES DYEABLE W ITH DISPERSE DYES Secondary acetate, triacetate and polyester differ greatly in dyeability at a given temperature. Rates of dyeing on acetate are usually much more rapid, so that the difficulty of obtaining solid dyeings limits the usefulness of D fibre blends containing this component. Shadow effects represent the only possibility for acetate/triacetate blends; solidity, reserve and contrast effects are excluded. Dyeing at 60°C favours the acetate component. A blend of acetate with polyester can be dyed at this temperature with

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good reserve of the polyester component but shadow effects require a higher temperature for a longer time. Cellulose triacetate and polyester are sufficiently close in dyeing properties to give good solidity with selected dyes by various methods. Shadow effects and a limited degree of reserve of polyester are also possible using different dyes and dyeing conditions. Triacetate is dyed preferentially in the early stages of dyeing, but the dye migrates to the polyester later as it diffuses more slowly into the latter fibre. 5.3 BLENDS OF FIBRES DYEABLE WITH ANIONIC DYES Blends in which both fibres are dyeable with anionic dyes in full depths (A fibre blends) are usually developed because a desirable balance of physical properties and the attainment of solidity is required. Nylon/wool blends can usually be dyed by a simple method employing acid or metal-complex dyes and an anionic agent to control the distribution. Two classes of dye are usually preferred when dyeing blends of nylon and a cellulosic fibre. Ternary blends of nylon/wool/viscose are also sometimes encountered in carpets. The distribution of dyes between nylon and wool depends on dye structure, applied depth, pH and blend ratio. Acid dyes show a definite saturation limit on nylon, but saturation of wool is not approached at the applied depth necessary to saturate the amine end group content of the nylon. Pale depths on nylon/wool show a marked preferential dyeing of the nylon. As the applied depth increases, a critical depth is reached at which solidity is obtained. Anionic agents capable of controlling the uptake of anionic dyes by the nylon component below this critical depth are available and these make it much easier to adjust the application conditions so that a reasonably solid effect can be achieved. The nylon component of a nylon/cellulosic blend may be reserved by applying selected direct dyes at 80-90°C in the presence of a syntan to protect the nylon from cross-staining. Many suitable neutral-dyeing acid and 1:2 metal-complex dyes are available to give good reserve of the cellulosic fibre when applied conventionally to the nylon component using ammonium acetate. If these dyes are applied with direct dyes of the salt-controllable type, solid dyeings of good fastness can be obtained economically on these blends. Bright hues of excellent fastness can be achieved with reactive dyes on nylon/cellulosic blends but there are problems of incompatibility in control of distribution between the component fibres. However, many 1:2 metalcomplex dyes and some milling acid dyes are fast to soda boiling. This means that they can be applied with reactive dyes in a one-bath, two-stage process, provided no serious interaction occurs. Reactive dyes are applied in the presence of alkali and the metal-complex dyes for the nylon. Free acid is then added to give pH 7 and the temperature raised to the boil to fill in the nylon component. There is only slight staining of the cellulosic fibre under these conditions. 5.4 POLYESTER/CELLULOSIC BLENDS Blends of polyester and cotton fibres are widely used in fabrics for shirts, leisurewear, raincoats , workwear and household textiles. Polyester/cotton blends offer a com-


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bination of improved handle, comfort, durability and economy that is not available with the individual components, nor by any other fibres blended with cotton. Other polyester/cellulosic blends are also widely used, e.g. blends with viscose in lightweight suiting and fashionwear, and blends with linen in household tablecloths and sheeting. As the dyeing properties of polyester and cotton are so different, dyes from two different classes are used to obtain solid shades. In practice two separate dyeing operations, although giving excellent technical results, are rarely carried out because of the extended time of processing and the resultant high cost. Most current dyeing methods represent a telescoping of the separate methods and considerable progress has been made in developing cost-effective batchwise and continuous dyeing methods. The polyester component of a polyester/cellulosic blend is dyed with disperse dyes showing good resistance to heat so as to withstand subsequent durable-press or pleating operations. Staining of the cellulosic component by disperse dyes should be minimal, so that time-consuming and costly reduction clearing can be avoided. Disperse dyes with low substantivity for cotton show the least staining. It is usual to apply them under slightly acid conditions at about pH 5. The addition of small amounts of sequestering agent is recommended where disperse dyes are sensitive to metal ions. The use of speciality disperse dyes based on dicarboxylic esters and thiophene chemistry, which are alkali-clearable, have been developed by Zeneca. These are particularly beneficial for one-bath, two-stage exhaust application to polyester/ cellulosic blends, where the addition of alkali simultaneously fixes the reactive dye and clears the disperse dye from the cellulosic fibre. Similarly the speciality disperse dyes have been utilised in single-stage disperse/reactive dye continuous application, followed by an alkaline wash to clear the cross-staining of disperse dye from the cellulose to achieve the required colour fastness level. The cellulose component of these blends can be dyed with four main classes of dye: reactive, vat, sulphur or direct. The existence of several ranges of reactive dyes with different reactivity characteristics has given rise to a variety of dyeing methods. Disperse/reactive dye combinations provide the widest range of fast colours normally available on polyester/cellulosic blends. Liquor ratio is important in batchwise dyeing. Reactive dyes of high reactivity and substantivity giving high fixation at long liquor ratios are preferred. Interaction between components of disperse and reactive dyeing systems must be taken into account and avoided as far as possible. Reactive dyes can chemically react with disperse dyes or dispersing agents containing free amino or hydroxy groups. Vat dyes are widely used in continuous and batchwise dyeing and are suitable for one-bath, two-stage fixation processes. Vat dyes are applied to cotton after fixing the disperse dye, as in the oxidised form they are stable to high-temperature dyeing conditions and to thermofixation. In view of the excellent fastness of vat dyes on cellulosic fibres, vat/disperse systems represent the ultimate in fastness on polyester/cotton blend fabrics. Sulphur dyes are widely used, especially for dull heavy shades where cost is a major consideration. As their fastness properties are limited,

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the choice is dependent on performance standards that have to be met. Continuous methods of dyeing are most frequently employed. Direct dyes are often used in the dyeing of low-cost polyester/cellulosic fabrics, particularly those containing viscose. Normally batchwise dyeing methods are adopted. After the polyester has been dyed, direct dyes and salt are added to the exhausted dyebath to fill in the cellulosic component. Since the wet fastness of direct dyes is limited, freedom from staining of the disperse dyes on to the cellulosic component is less important. Vat leuco ester dyes are used to dye pastel shades on polyester/cellulosic blends by continuous processing. Resin-bonded pigments are frequently used to produce pale to medium shades on polyester/cellulosic blends by continuous processing. Light fastness is usually excellent, but rubbing fastness may not meet the more exacting requirements. 5.5 BLENDS OF FIBRES DYEABLE W ITH DISPERSE AND ANIONIC DYES The dyeing of DA blends is usually directed towards solidity rather than differential effects, but unfortunately the most troublesome problem of cross-staining is the staining of wool with disperse dyes. All the DA blends containing wool or nylon suffer from this to some extent. Cellulosic fibres are also prone to some cross-staining in this way, but they are less sensitive than wool to the relatively severe conditions applied to clear the stain. Solidity is not easy to achieve on acetate/nylon or triacetate/nylon because most disperse dyes show a marked bathochromic shift on nylon compared with their hue on the acetate fibres (yellows appear redder, reds bluer, blues greener on nylon). This effect is of little interest for contrast effects because the hue on the nylon is usually duller than on the acetate component. A good reserve of acetate or triacetate is obtained using 1:2 metal-complex dyes on the nylon; colour contrasts with the nylon heavier in depth are given by mixtures of disperse and anionic dyes. The staining of wool by disperse dyes is a serious problem in blends with any of the ester fibres. Secondary acetate suffers a loss in lustre if it is treated at the boil, as is necessary in dyeing the wool component. Migration from the acetate to wool increases with dyeing time and temperature of the wool dyeing stage. Migration from triacetate to wool is slower under these conditions, but after-treatment at 105°C with carrier to achieve full depths on the triacetate usually causes heavy staining of the wool. Polyester/wool presents the most difficult problem and investigation has revealed that many factors may affect the degree of staining observed. Dye structure has less influence on the initial level of staining than have dyebath conditions or the quality of the dye dispersion. The stain on the wool is dull in hue and exhibits poor fastness to light and wet treatment. Staining tends to decrease with concentration and anionic charge of the stabilising agents present in the dispersion, but the magnitude of the effect is specific to the dye and agent. At temperatures below the boil the wool is stained preferentially. Migration from wool to polyester occurs at or above the boil if dyeing is prolonged, although this may cause damage to the wool.


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5.6 BLENDS OF FIBRES DYEABLE WITH DISPERSE AND BASIC DYES Reserve and contrast effects are of relatively low interest on acetate/acrylic blend fabrics because they are usually made from staple yarns, and cross-staining is a problem. Solid effects in pale depths can be obtained with disperse dyes at 80°C, shaded if necessary with basic dyes for the acrylic component, although this fibre is only ring dyed under these conditions (required to preserve the lustre of the acetate). A good reserve of the triacetate component of a triacetate/acrylic blend is obtained by dyeing with basic dyes selected for minimum staining of triacetate using a nonionic dispersing agent. Solid and contrast effects are achieved by a one-bath method using selected disperse and basic dyes. At temperatures below the boil the basic dyes tend to stain the triacetate, but migration to the acrylic component proceeds at the boil. Pale to medium depths of selected disperse dyes can be applied to the polyester component of a polyester/acrylic blend with satisfactory reserve of the acrylic fibre using a methylnaphthalene carrier at the boil. In general, however, it is preferable for the acrylic fibre to be dyed more deeply if reserve, shadow or contrast effects entailing full depths are required. Solid or contrast effects in full depths should be applied by a two-bath method for improved fastness and control of colour matching. The polyester component is dyed first because a basic dyeing on the acrylic fibre may show slight bleeding in a disperse dyebath, especially at high temperature. For optimum solidity the polyester should be dyed slightly heavier than the target depth to allow for some transfer to the acrylic component in the second stage. 5.7 BLENDS OF FIBRES DYEABLE WITH ANIONIC AND BASIC DYES Blends of the AB type are of particular interest for reserve and contrast effects because both components are dyeable with ionic dyes, although good solidity can be obtained without difficulty when required. Usually either component can be reserved, but it is often more convenient to reserve the acrylic fibre using anionic dyes. The range of bright colour contrasts is much wider on AB blends than on all other types of binary blend because the fibres carry opposite charges and ionic dyes are much more selective than disperse dyes. The opposite charges carried by the dyes, however, lead to incompatibility in one-bath dyeing. Selected reactive or metal-complex dyes and basic dyes can be used on wool/ acrylic blends, either at pH 2.0-2.5 for 1 :1 metal-complex dyes or at higher pH for reactive dyes or neutral-dyeing milling acid and 1:2 metal-complex types. The zwitterionic character of the 1:1 complexes at low pH gives them better compatibility with basic dyes than the more anionic reactive and neutral-dyeing metal-complex types. Mildly cationic agents of the alkylamine polyoxyethylene type form watersoluble complexes with many anionic dyes under strongly acidic conditions, thus minimising any incompatibility. A two-bath method is preferred on nylon/acrylic blends for optimum fastness and freedom from any risk of coprecipitation. The basic dyes are applied first at the boil and the nylon dyed with neutral-dyeing milling acid or 1:2 metal-complex dyes at 80-85°C.

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There can be problems of compatibility between basic dyes and the various classes of anionic dyes required to meet appropriate fastness conditions on the cellulosic component of acrylic/cellulosic blends. Cross-staining is less of a problem, however, and a good reserve can be obtained on either component, although solid effects are most often required. The basic dyes are partly absorbed by the cellulose in the early stages of dyeing, but transfer to the acrylic fibre occurs as the dyeing temperature approaches the boil. It is not difficult to remove the residual stain from the cellulose by subsequent scouring. Staining of cellulosic fibres by basic dyes is increased if the cellulose suffers oxidation damage before dyeing.

Machinery for preparation and dyeing 6.1 SINGEING MACHINES Singeing is employed to remove from the fabric surface the projecting ends of fibres unavoidably produced by abrasion in the weaving process. The cleaner fabric that results is not only more acceptable in use, but is more satisfactory for all dyeing and finishing operations; singeing is therefore most commonly employed as the first preparatory process. The removal of projecting hairs requires the application of sufficient heat to burn off or char a single fibre without significant effect on the body of the material. This result can be achieved by passing the fabric over a heated metal plate, but much the commonest procedure employs a gas flame. A series of burners is mounted along the length of a gas pipe spanning the width of the machine, so that the fabric passes over a narrow strip of flame; this singes one face of the fabric while a second burner singes the other. To avoid fabric damage the process must be run at high speed, up to 200 m/min, and provision must be made to cut off the flame before the fabric stops. After singeing the fabric carries the charred residues of fibres, some of which will be smouldering and must be quenched. This may be done on the singeing machine itself, generally by a steam nozzle or chamber, which prevents any danger of fire without significant increase in moisture content, enabling the fabric to be batched to await further processing. Alternatively the fabric may be passed directly into the first wet-processing bath; this will normally be the impregnation stage of desizing. 6.2 ROPE-FORM PREPARATION MACHINERY 6.2.1 Kiers The machinery employed in fabric preparation must be matched to the requirements of the textile material, to its end use and to the needs of the chemical process involved. The kier (Figure 6.1) is the traditional pressure vessel used to provide the conditions needed for thorough preparation, in particular the thorough scouring, of those cotton fabrics that will withstand high-temperature processing in rope form. Although still to be found in use, kiers have been largely superseded by more productive continuous or semi-continuous machinery, which provides the less severe processing conditions now required. The essential feature of the kiering process is the treatment of a large quantity of fabric, typically several tonnes, over a period of some hours in a steam-heated pressure vessel. A typical kier has the form of an enclosed vertical cylinder with a relatively small opening at the top through which the fabric rope is loaded and unloaded. At one time fabric was loaded manually, with operatives actually standing in the machine; nowadays mechanical loading devices are employed. After the fabric 75

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Figure 6.1 Traditional kier used in fabric preparation is loaded, the process liquor is run into the kier, the vessel is closed and then heated to the treatment temperature. Kier boiling can produce very good results on fabrics for which this form of rope processing is suitable, but involves a substantial proportion of unproductive time in loading and unloading. 6.2.2 J-boxes These devices were developed to permit the long contact time required in fabric preparation at atmospheric pressure. The J-box (Figure 6.2) is essentially an openended tube of rectangular cross-section, formed into the shape of the letter J; the fabric is fed into the long arm of the J and out of the short one. The box is insulated or jacketed for steam heating. The relevance of the J-shape is simply that by suitably choosing the curvature and lengths of the arms, the fabric may be made to slide smoothly through the tube as material is withdrawn from the short end and fresh material fed in at the long one. The technique is particularly appropriate for rope-form processing, as the dimensions of the box do not need to be related to fabric width, but may be determined simply by processing speed, contact time and ‘packing density’of the fabric rope.


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Figure 6.2 J-box used in fabric preparation

In J-box operations the fabric is usually impregnated with processing liquors in a pad before it enters the box, but additional liquor may be added at the box itself. Since J-boxes are designed for continuous operation, a number may be combined to provide a continuous processing sequence. 6.2.3 Winches and jets Winches are essentially operated at atmospheric pressure although some pressurised versions have been introduced. Jets are widely used to process fabric in rope form both at atmospheric pressure and more commonly at temperatures above 100°C. Both types are primarily machines for dyeing and are described more fully in this context in sections 6.4.2 and 6.4.3. They may also be usefully employed in fabric preparation because, once the fabric is loaded, there are clear advantages in leaving it in place and performing a sequence of operations simply by changing the processing liquors. These machines are selected for dyeing where the relatively gentle conditions they provide may offer advantages in respect of fabric quality. It is likely that fabrics for which this is desirable will benefit also from relatively mild processing conditions in preparation. This can be a significant reason for carrying out both preparation and dyeing on the same machine.

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6.3 OPEN-WIDTH PREPARATION MACHINERY 6.3.1 Jigs The jig may be regarded in many respects as the counterpart for open-width processing of the winch, in that both machines permit inspection of and access to the fabric while processing is under way, with the facility for at least some degree of corrective action. Like the winch, the jig (Figure 6.3) is principally a dyeing machine and is described as such in section 6.51. Once the fabric is loaded into the machine it is often convenient to precede jig dyeing by preparation on the jig.

1 2 3 4 5

Dye vat for low liquor ratio both for small and large batches Nozzle for direct steam heating Coil for indirect steam heating Expander Main rollers

6 7 8 9

Rinsing tubes Dye liquor circulation pump Addition tank with rinsing rim, nozzle for direct steam heating and return pipe Dosing pump

Figure 6.3 Vald Henriksen J-19 jig Preparatory processes are performed in a manner similar to dyeing, with the fabric passed backwards and forwards through the liquor for a specified number of ends. After chemical processing the fabric is washed-off on the jig using two or three changes of water. Overflow rinsing was at one time popular, but is wasteful of water and should be avoided.


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6.3.2 Pad-roll units The various types of semi-continuous and continuous open-width processing systems described in this and the following sections all employ pad mangles to impregnate the fabric with process liquors and squeeze mangles in washing ranges to remove the unwanted products and surplus chemicals. It is a feature of all these systems that the impregnation step can be achieved very rapidly and the washingoff relatively rapidly, but the chemical reactions forming the heart of the process are inherently slower. They thus require sufficient time and a high enough temperature to run to completion. The function of the pad mangle is to impregnate the fabric with the correct quantity of process chemicals, with a reasonably thorough and uniform distribution. Uniformity, which is a critical requirement in dye padding, is not as critical in the application of preparation chemicals and some of the requirements for dye padders (section 6.5.3) may be relaxed in preparatory processes. Preparation is commonly performed on fabric that is not yet readily wettable. It is beneficial to employ a higher nip load than might be considered safe in dye padding; this will normally improve penetration but with some danger of loss of uniformity. A three-bowl padder with two immersion troughs will provide increased liquor uptake but precise control of uptake is not as critical as in dyeing, since preparatory processes are less sensitive to small changes in chemical content. In pad-roll systems the fabric is preheated as it is fed onto a batch in an enclosed chamber, in which it can be heated and rotated while the chemical reactions proceed. At the completion of the operation the fabric is washed-off in a continuous range, as discussed in section 6.3.6. 6.3.3 Saturators Saturators have a useful role in ensuring sufficient dwell time to bring about liquor interchange. They provide a longer immersion path for the fabric, which can be beneficial for those types that are not readily wettable. As with wash ranges, there have been developments towards minimising water usage by the introduction of spray techniques, as typified by the Babcock Super-Sat (Figures 6.4 and 6.5). 6.3.4 Pad-batch units The pad in a pad-batch sequence (Figure 6.6) is similar to that in the other openwidth processes discussed, and the conditions specified in the previous sections apply here also. After impregnation the fabric is simply batched onto the roller of an A-frame or similar support. The completed batch is wrapped to prevent drying out and moved to one side where it can be rotated slowly as the reaction proceeds. Further batches are prepared in the same way and effective utilisation of padder and washing range achieved, as in pad-roll operations. 6.3.5 Pad-steam ranges If a preparatory processing sequence is to be run continuously some means must be found to increase the rate of chemical reaction. Otherwise the central stage of the

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1 2 3 4 5

Fresh water Chemical solutions Mixing tank Level control Circulation pump

6 7 8 9 10

Volume adjustment valve Fabric Impregnation rollers Return pump Lint screen

Figure 6.4 Babcock Super-Sat impregnation unit

Figure 6.5 Babcock Super-Sat impregnation unit followed by a steamer process will not operate quickly enough to be compatible with the dye padding and washing-off operations. Essentially this involves raising the temperature at which the reactions occur, which must be done without the impregnated fabrics drying out. This is most readily done by using steam as the heating medium on a pad-steam range. An atmospheric pressure steamer, which operates at or close to 1OO°C, must have a chamber of large capacity to provide the required contact time at this temperature. The alternative is a pressure steamer operating at temperatures up to 130°C, which permits (and of course requires) a much more compact unit, but pressure seals are essential at fabric entry and exit points.


Alkali Dye storage storage

Padder (single or double dip)

Mixing pump (dye:alkali at 4:1)

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Counterbalanced driven batching roller

Perforated beam for batching and subsequent washing

Figure 6.6 Typical pad-batch sequence The atmospheric type of machine needs more space but is easier to maintain. The pressure vessel has the advantage of compactness but is more troublesome in the event of seal failure or fabric breakout. 6.3.6 Washing ranges A continuous washing range is the final element in the preparation sequence, its function being the removal of impurities and unused chemicals by rinsing with water. The standard range consists of a series of compartments, separated by mangle nips which draw the fabric through the machine and to limit the carry forward of liquor. This latter factor is important, because the effectiveness of washing is determined in a large part by the ratio of the water consumption to the carry forward of liquor with the fabric. The effectiveness also depends on the number of units in the range and on the efficiency of impurity interchange between fabric and wash water. Efficient use of water is achieved if all the wash water is run in the opposite direction to fabric flow through the whole length of the machine. In preparatory processes it should not be necessary to use more than 4-5 I of wash water per kilogram of fabric. The commonest design of washing compartment hasvertical fabric paths between upper and lower rows of guide rollers, with only the lower ones submerged. The low water level permits inspection windows in the side of the tank above the water line, for access in the event of a fabric breakout. A low water level also permits partitioning within the tank, offering some additional advantage in ‘contraflow’water usage. More recent designs have a horizontal fabric path with enhanced ‘contraflow’of water, whilst the use of spray techniques (as opposed to fabric being immersed in water) has the benefit of reducing the amount of wash liquor substantially. It is claimed that spray techniques give better mechanical push-through of the liquors into the fabric and enable the machine to be fully enclosed, without the need for intermediate squeezing between one wash section and the next.

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6.3.7 Mercerisers Mercerisation swells cotton fibres and so tends to cause shrinkage, the extent of which must be controlled if the process is to be operated reliably. Furthermore, the swelling does not occur instantaneously and the fabric must be allowed a few seconds in contact with the caustic liquor for the full effect to be achieved. The mercerising range must therefore have sections for impregnation, dwell time and caustic soda removal. The conventional range uses a series of cylinders to provide fabric control during the dwell period, followed by an open horizontal path with the fabric selvedges held by clips on travelling chains. In this latter section the fabric is washed by water flowing in a countercurrent direction to reduce the caustic soda concentration before the lateral tension is relaxed. Final washing-off may be performed on a conventional open-width washer. Several variations on this general plan have been employed, including a recent compact roll-to-roll unit claimed to be particularly suitable for small production requirements (Figure 6.7).

.;.

..:,.

Figure 6.7 Cibitex mercerising machine 6.4 ROPE-FORM DYEING MACHINERY 6.4.1 Basic principles Dyes may be applied relatively slowly in batchwise operations where the prolonged contact between fabric and dye liquor facilitates the level application of colour. Exhaust dyeing of this type requires machinery in which a given quantity of fabric may be brought into contact with the liquor containing the appropriate amounts of dyes required. Since the transfer of dye from liquor to fabric is relatively slow, treatment of fabric in rope form is no bar to a satisfactory result. In addition, since the operations of loading the fabric into the machine and removing it each take several minutes or longer, it is common practice to perform a sequence of processes (e.g. preparation, rinsing, dyeing, rinsing) in the same machine, simply by replacing one treatment liquor with the next. In this respect batchwise processing machinery differs radically from machinery for continuous processing, in which each machine element in the sequence performs a specific function in turn. Continuous dyeing requires the dye liquor to be applied initially in a uniform manner throughout the material, with fixation following in a


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subsequent process stage. The initial impregnation is performed almost exclusively by padding, and the requirement for uniform application of the pad liquor necessitates the fabric being in open width. It is obvious that batchwise processes lend themselves more readily to the treatment of relatively short lengths of fabric. Jigs and winches have for many years demonstrated their usefulness for this type of processing, among their most valuable features being simplicity and easy operator access. These machines were developed initially for dyeing and other processes at temperatures up to the boil. With the advent of higher-temperature processing for synthetics, pressurised versions of both types were introduced, but these have never become important commercially. Beam and jet machines, introduced to provide a high-temperature processing facility, may obviously be used at lower temperatures but the jig and winch have retained much of their traditional popularity. 6.4.2 Winches The winch is the rope-form equivalent of the jig in being a readily accessible, atmospheric pressure machine. In operation a length of fabric is loaded into the machine and the ends sewn together to form a continuous loop. This fabric rope is drawn from the liquor in the base of the winch by the front rollers and passed to the roller at the back, which plaits it down into the liquor. The back roller can be elliptical or circular in cross-section. Compared with open-width processing in the jig, there is much less warpway tension, but more mechanical action on the fabric, leading to changes in rope configuration. The winch is therefore extensively used for knitgoods since such processing leads to greater bulk and fuller handle. It is also suitable for fabrics loosely woven from cotton or man-made yarns. Such fabrics must obviously be able to withstand rope-form processing without creasing. Because of the construction of the machine the liquor ratio in the winch is inevitably much higher than in the jig, and is commonly in the region of 25:1, although it may be as low as 15:1. Hence water and heating costs are much higher and it is essential to avoid vigorous boiling or operation with doors left open. In some designs of winch the front access door opens first at the top, permitting steam to escape upwards at a safe height. 6.4.3 Jets In all true so-called jet dyeing machines the fabric rope is threaded through a ring or venturi, to which the processing liquor is supplied. The jet of liquor issuing from the ring serves both to transport the fabric and to bring the liquor into intimate contact with it. The original design by Gaston County used venturi nozzles mounted in tubes above the main vessel. The novelty of this design depended on a high-speed movement of liquor to transport the fabric rope from front to back of the dyeing vessel at remarkably rapid rates (100-300 m/min). The doffing jet nozzle plaited the fabric into the dyebath at the back of the vessel, and a set of driven metering rolls at the front lifted the rope from the top of the pile and controlled the rate at which it entered the jet nozzle tube.

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The force of the liquor movement was too great for delicate fabric constructions and creasing or tangling of the fabric ropes could occur in the voluminous bath at the base of the vessel. A few years later ‘fully flooded’or ‘hard stream’ machines of various configurations began to appear. Elimination of the large air space above the liquorand complete immersion of the fabric rope provided support for delicate fabrics and avoided the foaming problems characteristic of the Gaston County design. The Longclose Ventura (Figure 6.8) and Thies Jet Stream are typical fully flooded jet machines. The principle is to force the fabric rope through a high-speed venturi constriction into a flooded tube that opens i nto a much wider storage compartment. In this section both liquor and fabric move slowlyy until the fabric is withdrawn through a narrower tube and taken back to the jet nozzle. Dye liquor is drawn off from the system at both ends of the storage compa rtment and passed through a heat exchanger before being fed back to the jet nozzle. Lightweight fabrics usually requi re smaller nozzles and adjustable-diameter nozzles have become available.

1 2 3 4

Fabric transporting reel Variable speed drive Overflow reservior Dye liquor lubricated bearings

5 6 7 8

Fabric delivery tube Flow assistor Heat exchanger Main pump

Figure 6.8 Longclose Ventura jet 6.4.4 Overflow machines Early in the 1970s machinery makers in Japan and Western Europe began to introduce ‘soft flow’ and ‘partial immersion’ jets. These took advantage of the supporting influence of almost complete immersion of delicate fabrics but avoided the vigorous turbulence of the constricting venturi. The rope of fabric is lifted briefly from the dyebath by a driven circular reel and then carried along a transportation tube by means of a relatively gentle flow of liquor. This principle is available in both


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atmospheric and high-temperature pressurised machines. In most jets the transportation tube is filled with liquor and fabric/liquor movement along the tube depends on gravity. The transportation tube is usually outside and above the main vessel but in the ATYC Rapidsuau it is situated below the main dyebath. 6.4.5 Short-liquor machines The oil crisis of 1974 initiated a search for improved machinery designs that were more economical in energy usage. This led in the mid-1970s to the development of rope-dyeing machines that could be operated at liquor ratios as low as 5:1, requiring less than half the amounts of water, chemicals and energy that were necessary in the fully flooded and earliest partially immersed types. In many respects the essential layout of both the Thies Roto-Stream and Longclose Softflow (Figure 6.9) are similar to the original Gaston County jet but these later introductions have a different overflow/venturi configuration to provide better protection of the surface of delicate fabrics. The fabric is plaited down by a jet of dye liquor and slides down the perforated lining at the back of the vessel. At the front the fabric rope is lifted by a driven circular reel and guided into the venturi. Running speeds up to 400 m/min can be employed. The most recent developments in fabric dyeing machinery revolve around three particular objectives: (a) Minimising liquor usage (b) Avoiding fabric distortion, pilling and creasing (c) Improving rinse efficiency.

1

8 9

2 3 4

1 2 3 4 5 6 7 8 9

Figure 6.9 Longclose Softflow HT

Delivery tube Heat exchanger PTFE inner lining Pump Drive wheel Overflow chamber Accelerator chamber Loading door with inspection window Unloading reel


One example, primarily a development of short liquor ratio overflow equipment, is typified by the Longclose Supersoft Hitech machine. This utilises a double winch system to transport material in a low-lift, non-slip, tension-free manner, while allowing liquor treatment to be achieved with reduced pressure and improved surface quality. A novel system of squeezing fabric during the rinse cycle enables the time spent in rinsing to be minimised and water consumption to be reduced. These factors are becoming increasingly important to industry, and will continue to be so in the foreseeable future. Sclavos has approached the same three criteria in a different way with its Apollan Twin Soft Flow (Figure 6.10). This utilises a system that incorporates two distinctly separate jet nozzles in tandem. The consequence of this is that high fabric speeds can be achieved at very low liquor velocities, guaranteeing tangle-free operation. An ‘elbow-plaiting system’ensures that fabric folds are placed precisely on top of one another in such a way that all available storage space is occupied, whilst a liquor bypass system protects more delicate fabrics from crush-creasing.

Figure 6.10 Sclavos Apollan Twin A further innovation, typified by the Then Airflow (Figure 6.11), the Longclose AirTech and the Thies Luft-roto (Figure 6.12), is the circular-style machine; this incorporates fabric transport by winch and high-speed air flow. This is claimed to reduce liquor ratio, increase fabric running speeds and provide freedom from creasing. Originally applied to crease-prone woven fabrics, this technology is now also applicable to knitted fabrics of suitable construction.


Figure 6.11 Then Airflow

Figure 6.12 Thies Luft-roto

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The advent of microfibres has brought about the need for ultra-high-speed fabric dyeing machines that allow relaxation to occur within an extended storage zone, so avoiding rope marking and crowsfoot creasing. Typically these machines operate at liquor ratios of 4:1 upwards, with running speeds in excess of 600 m/min being achieved on ultra-light qualities. Double roping is utilised regularly to reduce fabric turnaround times, whilst adjustable nozzles, invetter pump flow control, synchronised winch/jet speed, seam detection and dosing systems could be considered essential. Such machines are typified by the Longclose Ventura Rapide and the Hisaka CUT FL. 6.5 OPEN-WIDTH DYEING MACHINERY 6.5.1 Jigs In machines of this type, long-established for open-width processing, the fabric is passed to and fro through the dye liquor from one batch roller to another (Figure 6.13). The jig is particularly suitable for woven fabrics that must not be creased in processing, e.g. poplins, satins, taffetas, suitings and ducks. The modern jig provides a variety of automatic controls. For example, the direction of fabric movement can be changed after a preset length, and the number of ends

Figure 6.13 Vald Henriksen jig


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and the fabric tension can be varied. Yet it still retains the essential features of the prototype machine, namely ease of operation and lack of complication. A jig of typical width of 1.8 m may hold a load of 150 kg of fabric and provide a liquor ratio of about 5:1. Although the parameters controlling the dyeing operation on this machine have been studied, practical experience is the most commonly used guide to satisfactory operation. The accessibility of the machine makes it relatively easy to take fabric samples for shade matching. The costs of dyes are normally greater than those of chemicals and processing (except labour), but the costs of heating and of water cannot be ignored. Jigs provide some degree of freedom to vary the depth of filling. This gives scope to accommodate variations in fabric load to maintain approximately the same liquor ratios. Energy consumption increases with increasing temperature and is pronounced if the liquor is allowed to boil vigorously. When machine hoods are left open it is nearly twice as great as with the hoods closed at any temperature. Thus in operation hoods must be closed whenever possible and vigorous boiling must be avoided. 6.5.2 Perforated beams In beam dyeing a length of fabric is wound on a perforated former and processed in a cylindrical autoclave. The system is fully enclosed and is normally pressurised to provide processing at temperatures up to 130°C (40 MPa, 4 atm). The process is best suited to lightweight woven synthetics and their blends. Dyeing and related processes take place while the liquor is being circulated through the layers of fabric. Both directions of liquor flow can be employed in the dyeing cycle to assist the attainment of uniform shade, but in-to-out is favoured, because out-to-in tends to compact the fabric, with the danger of water marking. This form of processing, in which liquor is circulated through a stationary batch of fabric, is inherently liable to non-uniformity, because of the higher rate of flow of liquor per unit area near the centre of the beam (where the diameter is smaller) and the danger of channelling through the more porous regions of the batch. Uniform winding of the batch of fabric is therefore crucial, and there is probably no alternative but to learn by experience the maximum loading of the beam practicable for a given combination of fabric quality and class of dyes. Although in general it can be said that an increased rate of liquor flow helps to promote uniformity, there is obviously a limit to the rate that can be employed, depending on pump capacity and fabric porosity. As the dyeing vessel is pressurised, access during the operation is strictly limited and dyeing must proceed by following a predetermined recipe. For this reason automatic process control has been taken further in these high-temperature processes than in some other operations (section 6.7). Beam machines must be fully filled with liquor and the liquor ratio will therefore increase if the fabric load is reduced for any reason; the cost per metre for water heating will rise under these circumstances. Most of the energy consumed is required for water heating, with a relatively modest supplement for losses, which comprise radiation/convection losses from the machine surface and the deliberate cooling necessary to permit circulation through the addition tank. There is none of the large evaporative heat losses that can be a major factor in jigs and winches.

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6.5.3 Pad mangles Much the commonest procedure in continuous dyeing for application of the dye solution or dispersion is padding. The pad mangle is the crucial element in the process, since it is essential to have the liquor uniformly distributed initially. A pad mangle is closely related to the squeeze mangle used for removing excess water before drying, and many years ago the same machine was often expected to perform both functions. Later it became recognised that the requirements are essentially different, with the emphasis in padding on the greatest possible uniformity in the nip. The function of the dye fixation machine elements in continuous dyeing processes depends directly on the type of dyeing involved. The simplest system mechanically is probably the semi-continuous cold pad-batch dyeing, in which the padded fabric is wound on a suitable former, rotated slowly while dyeing proceeds at room temperature, and washed off. Other processes may require much more sophisticated and specialised machine elements, such as infra-red pre-dryers and steamers. 6.5.4 Pad-batch units Reactive dyeings on cellulosic fabrics, wool or silk can be fixed at ambient temperature by rolling the fabric into a cylindrical batch and simply rotating the wet material for a prescribed time at an appropriate alkaline pH. Pad-batch dyeings are wrapped with polythene sheeting to prevent evaporation. When first introduced, this method was of prime interest for the application of colddyeing dyes, but modifications to suit the less reactive types followed quickly. The success of the sequence pad(cold)-batch(cold)-wash is attributable to several factors, including the scope for selective control of the dye-cellulose and dye-water reaction rates. It is generally necessary to employ a liquor feed device, whereby dye and alkali are brought together immediately before the mixed padding solution comes into contact with the material being dyed. The equipment is simple and application costs are low. The technique, in terms of energy and water consumption, and labour cost, is the most economical approach to the production of high-quality dyeings of high wet fastness on cellulosic fabrics. 6.5.5 Stenters Stenters are usually the most costly and versatile processing machines in a textile finishing plant. They have a major influence on the appearance, handle and properties of the finished fabric, since they provide exact control of the finished length and width of the material. Astenter is an open-width drying and finishing machine that heats the fabric by hot-air circulation. The moving fabric is held at the selvedges by a pair of endless chains fitted with pins (pin stenter) or clips (clip stenter) to maintain weft tension. The many uses of stenters include mercerising, drying after preparation, dyeing or washing processes, heat setting or fixation of dyes in synthetic fabrics, curing of pigment prints or durable finishes. The higher the running speed of the stenter, the more vital the need for an efficient entry system to ensure that the fabric is properly centred and the selvedges fed onto the pins at the correct speed. The rails of the stenter are divided into three sections


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that can be moved away from or towards the centre line for adjustment of fabric width. The tapering entry section (about 6 m long) exerts increasing weftway tension on the fabric before it enters the oven. The centre section (usually 10-25 m) is normally set at constant width and the delivery section (about 5 m) allows initial cooling to take place before the fabric is stripped from the pins by a roller. Various methods can be used to heat the air circulated through a stenter, each having certain advantages. High-pressure steam will provide a maximum temperature of about 165°C, which is adequate for drying and fixation of pigment prints or durable finishes on cellulosic fabrics, for example, but is insufficient for heat setting (dimensional stabilisation) or thermal fixation of disperse dyes on polyester. The rising price of oil and the increasing availability of natural gas in the UK has greatly favoured the installation of or conversion to gas-fired stenters in recent years. Stenters dependent on circulating hot oil or electricity are more expensive to install and maintain than gas-fired or combined steam and gas systems. 6.6 FABRIC HANDLING DEVICES For the general transport of open-width fabric within the dyehouse, its delivery to machines and removal from them, the traditional method is to employ trucks holding fabric in plaited form. This method has the advantage of simplicity, but the trucks are bulky and the fabric is inevitably laid down somewhat irregularly, with the lower layers in danger of creasing from the weight of the layers above. This procedure is being increasingly superseded by inter-process transport of large-diameter batches on A-frames. These provide much more compact carriage of fabric smoothly laid down, but the full batches can be extremely heavy and therefore more difficult to move in a site with any irregularity in the flooring. Tensioncontrolled take-up is required in batch building, as well as some form of controlled let-off at the entry to machines. One unavoidable feature of batch-to-batch processing is that the fabric changes direction in alternate processes, the leading end in one becoming the trailing end in the next. This is normally of little practical significance except in relation to the possible requirements of job identification, when it may be essential to label both ends of the order. Of more general significance is the fact that on a batch the inner end of the fabric is not accessible until the batch is unwound. For fully continuous processing it is therefore necessary to have some facility for delivering the last part of the batch into a stray to allow time for the trailing end to be sewn to the leading end of the next batch. Operation with fabric plaited into trucks avoids this complication since the end first into the truck may be left exposed and joined to the next truck load at any convenient time. Rope-form dyeing in winches or jets rarely calls for transport of large quantities of fabric about the dyehouse in this state. The fabric is commonly converted to rope form at the entry to the machine and the dyed fabric may be retained in this condition for mechanical drying by centrifuge. At some stage the conversion back to open width must be accomplished and here the presence of twist in the rope may be a problem. Little real twist will be introduced when the fabric is loaded into the dyeing machine, but the problem may arise if a substantial number of turns of false twist occur in one


Figure 6.14 Bianco automatic detwister direction before the reverse twist appears to unwind them. The twist may be held back satisfactorily by a simple beater, but it may be necessary to employ a detwister (Figure 6.14). The beater or detwister is used in conjunction with edge guiders which grip and position the selvedges of the opened fabric. Edge guiders are a common feature at the entry end of all open-width machines for the correct alignment of the fabric. Also found here and frequently throughout a continuous range, particularly in front of mangle nips, are curved bar or curved roller expanders used to minimise fabric creasing. Curved rollers used for this purpose are identical in construction to those employed to correct bow distortion in weft straighteners. It is therefore possible for an incorrectly set expander to introduce bow distortion. To achieve the expander action without any weft distortion the path length past the centre of the roller must be the same as that at the sides. This is provided if the axis of curvature of the roller bisects the angle formed by the lines of fabric. 6.7 AUTOMATION AND CONTROL The merest acquaintance with textile dyeing machinery will demonstrate that automation has reached a much higher level of sophistication and has been much more widely adopted in batchwise than in continuous processes. The reasons for these differences can be better appreciated by consideration of some general features of automatic control systems. Any control system must have three components: (a) A measurement of the value of some variable (b) Some means for comparing this measurement with a reference value (c) The means to effect some appropriate control action.


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Furthermore, and this point is particularly relevant in the present context, the control operation must act on the processing conditions, but the measurement may be either of the process or of the product. An example is a thermostat used to maintain the temperature of a process bath. Here the measurement is of the process temperature. Other examples involve measurement of pH and liquor level and control of the timing of process sequences. A feature of this type of automatic control is that the measurement on which the control depends is hardly to any extent ‘industry specific’; a temperature sensor suitable for use in a dyebath will be equally suitable in a wide variety of industrial situations. Product developments arise more readily when there is a wide market for the resulting product, and most of these types of control devices are products of the general instrument manufacturing industry. The same is definitely not the case when the measurement is of some property of the material being processed. The characteristics of the material determine the way in which the measurement must be made, and equipment suitable for one industry is very unlikely to be applicable in another. This is well illustrated in the field of moisture measurement, where various efforts to produce a universal instrument have failed; the only really satisfactory textile moisture meters have been those designed specifically for this application. In these circumstances the development and improvement of sensors is inevitably more difficult. Not only must the costs of development be borne by sales in one industry alone, but the measurements themselves depend on some specific property of the material, a property that may be related in some quite complex way to the characteristic which it is desired to represent. Thus in the example quoted in the previous paragraph, for on-line control purposes the moisture content of a textile is most commonly inferred from a resistance measurement, using knowledge of the various relations for different fibres between moisture content and resistance. 6.7.1 Batchwise processes The relevance of these considerations to the difference between batchwise and continuous processes will be apparent if it is appreciated that uniformity of dyeing within a given load in a batchwise process depends essentially on the design of the dyeing machine and on the dyeing recipe and procedure. There is nothing that can be done by on-line process control to influence the warpway uniformity, and the objective of control must be to ensure that the predetermined dyeing procedure is followed with sufficient accuracy to achieve the best possible uniformity within each batch and from one batch to the next. However complex and sophisticated they may be in practice, the requirements for the control of batchwise processes are conceptually simple, and rely commonly on simple and reliable sensors for the measurement of processing conditions. The correct performance of a batchwise dyeing process calls for the repetitive operation of a number of functions in predetermined order and at predetermined times. Automatic control has now progressed to the stage where all the operations between fabric loading and unloading may be effected under the control of an individual microcomputer, or a larger computer serving all the machines in a


dyehouse. The transition from a situation in which the available machinery was designed primarily for manual operation occurred conveniently in a series of steps, making the changes more readily intelligible than would have been the case with a rapid transition from one extreme to the other. The steps were briefly as follows: 1. Improving the ease and convenience of manual operation, by remote control of motorised valves and by grouping controls together, with automatic safety features and alarms 2. Providing automatic control of some operations previously performed manually, particularly of time/temperature characteristics by the use of ‘cut card’ controllers with a ‘follower’ whose position gave a visible indication of position in the dyeing cycle. 3. Devising more sophisticated operations involving punched cards or alternative programming techniques; at this stage it became necessary to provide some form of mimic diagram or read-out to show the state reached. The transition from this form of automation with machines individually controlled to large-scale computer control was essentially an electronic rather than a textile problem. 6.7.2 Continuous processes In continuous dyeing processes the fabric elements follow one another in sequence through the various process stages and the running fabric is repeatedly accessible for measurement. It is therefore possible to visualise a future situation in which online measurements including fabric colour will be employed to compensate for the inevitable small variations in processing conditions to achieve improved uniformity of coloration along the fabric. 6.7.3 Quality control On-line process controls rely on measurements of processing conditions. Quality control is a different matter with a wide variety of fabric properties measured under controlled conditions in the works laboratory. Such measurements are extensively employed for product selection or sorting, and commonly form an important item in the commercial arrangements between finisher and customer. The results of measurements such as these only become available after processing has taken place and can rarely be employed for immediate on-line control, but they are useful for the long-term control of machine function and operation. A series of quality control measurements, in particular, can be employed to give advance warning of a deterioration in some machine component, e.g. wear in pad mangle bowls.

CHAPTER 7

Machinery for drying and mechanical finishing 7.1 DRYING In the present context the function of drying operations is to remove the water from textile fabrics that have been thoroughly wetted in the dyeing process. When material in this condition is withdrawn from a dyebath it readily retains three or four times its own weight of water, a ratio which may fall to two or less if the textile is allowed to drain briefly. Drying processes must reduce this water content to the few percent of the dry fabric weight that is the normal ‘air-dry’condition. The later stages of drying must under all practicable circumstances be carried out by evaporation, but thermal drying is inevitably an energy-intensive process. Mechanical drying, achieving removal of liquid water, is much less costly, so the moisture content should be reduced as far as possible in this way. In practice considerations other than those relating only to drying costs will determine the balance between mechanical and thermal drying, but wherever possible maximum advantage should be taken of the cost-effectiveness of mechanical drying. Before dealing with the various techniques of drying it is useful to define the terminology to be adopted in this chapter. In the textile industry moisture content is most commonly quoted on a ‘dry weight’ rather than a ‘wet weight’ basis, i.e. the moisture is expressed as a percentage of the dry weight of the textile, rather than the total wet weight. This convention will be followed here, with the additional proviso that the dry weight is the ‘oven-dry’, not the less precise ‘air-dry’, value. ‘Oven-dry’ implies that a thermal process is employed to remove the last traces of water and leave the material completely water-free. In fact it is extremely difficult to be sure that all the water has been removed without any degradation of the fibre, and in any event the fibre will reabsorb some water from the air extremely rapidly. Thus the objective of drying processes is to reduce the water content from a value probably well in excess of 100% to a value close to the normal air-dry figure, which for cotton fibres is typically 6-7%. The moisture level attained after mechanical drying will be referred to as the ‘water retention’or just the ‘retention’. The same word could equally well be employed for the value after thermal drying and conditioning, but the longestablished name ‘regain’is so widely employed that it would only be confusing to introduce another. 7.1 .1 Mechanical drying The small amount of regain moisture associated with textile materials at normal levels of atmospheric humidity is bound very strongly to the fibre. Increasing the humidity increases the moisture content, but the water is held progressively less 95


strongly until eventually a stage is reached beyond which further uptake consists of effectively unbound water in capillaries and voids in the fabric structure. The distinction between bound and unbound water is generally considered to occur at the saturation regain, the regain at 100% RH. This distinction cannot be made precisely, nor can the critical regain be accurately determined, because condensation into the smallest capillaries can occur below 100% RH. Although the bonding cannot be precisely defined, the distinction between bound and unbound water is important because mechanical drying, under any practicable condition, can remove only unbound water, and in fact never quite the whole of that. Thus the minimum water retention value for cotton, in the region of 30-35%, represents an absolute limit for mechanical drying. Three methods of mechanical drying are commonly employed with textile fabrics: mangling, suction drying and centrifuging. Under the best conditions, which include favourable textile characteristics as well as ideal operating practice, all three methods will achieve a similar level of performance, and the lowest water retention attainable is in the region of 45% for cotton. Under what may be unavoidable circumstances the retention may be appreciably higher than this figure, but no one process has a clear technical edge over the others. The choice of method therefore depends on various factors, as indicated in the following sections. Mangling This is the most important method of removing water from open-width fabrics, and the general appearance of mangles is so well known that a detailed description is unnecessary. It is nevertheless useful to summarise the important features of the machine. Squeeze mangles are structurally similar to pad mangles, but in squeeze mangling nip uniformity is less critical and it is possible to relax this requirement in favour of a general increase in pressure, resulting in more water removal. This cannot be carried too far, and it is still necessary to have one relatively soft bowl in the nip, providing a cushioning effect to protect the fabric from damage in the event of creasing. Although it is possible to run a squeeze mangle with a rubber-covered bowl, it is now almost universal practice to fit harder, so-called ‘filled’ bowls running against metal for open-width fabrics. Somewhat more resilience may be desirable in a mangle for fabrics in rope form. Filled mangle bowls have been made from a wide variety of materials, their common feature being an appreciable radial thickness of the deformable material on a metal shaft, in contrast to the relativelythin layer of rubber on a metal shell in rubbercovered bowls. Modern bowl fillings commonly consist of discs cut from sheets of impregnated synthetic fibres, mounted on the bowl shaft and held with pressure between the end cheeks. These materials provide a good balance between the potentially conflicting requirements of hardness and resilience, and have good resistance to permanent deformation. One consequence of this form of bowl construction is a greater susceptibility to lengthwise bending under load, since only the metal shaft contributes to rigidity. In


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squeeze mangling the nip uniformity is less critical than in padding; so loss of rigidity need not be particularly serious, although it is desirable to have bowls of large diameter, permitting a larger diameter shaft. For particularly wide mangles some form of level pressure loading system may be required, but this is unlikely to be worthwhile below about 2.5 m face length. The combined effects of nip loading, bowl hardness and bowl diameter may be represented with reasonable accuracy by the average pressure developed in the nip. The nip pressure, in kg/cm2, say, is simply the nip loading in kg/cm divided by the nip width in cm. Bowl hardness and diameter influence this parameter by their effect on the nip width, a ‘tighter’nip being provided by a harder bowl or a smaller diameter. Diameter only has a significant effect when both bowls are 10 cm or less. Although this factor was utilised in some important machine designs in the 1950s, it is now largely ignored. Modern bowls, primarily by nature of their hardness, achieve nip pressures of the order of 100 kg/cm2 from applied loading of 60-80 kg/cm, and this represents a good level for efficient water removal. It is claimed that some modern bowl fillings achieve extra water removal by a ‘blotting’ or ‘sucking’ action at the exit from the nip (Figure 7.1). The evidence suggests that this may indeed be significant, but only with some fibres or fabric structures.

Figure 7.1 Roberto roll, designed to give better moisture removal than rubbercovered bowls Faster fabric throughput increases the amount of water retained by the fabric, but not to a marked extent. A useful practical consequence is that moderate speed changes can be made for other purposes without any significant effect on mangle performance. An increase in temperature at the nip leads to a reduced water retention. It is therefore advantageous where appropriate to mangle at an elevated temperature, if the fabric has come from a hot wash, for example. However, it is unlikely to be profitable to heat an immersion tank solely in order to achieve this mangling benefit, as the energy expended might well exceed that saved in the subsequent thermal drying.

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Differences in fibre and fabric structure can also influence water retention. These differences can be quite pronounced, e.g. fabrics of light weight and open structure retain, per unit weight of dry fibre, much more water than heavier, tightly woven structures. It is helpful to visualise the situation inside the nip, where the fabric structure effectively holds the bowls apart, with all the voids filled with water. More open structures have a greater proportion of void volume and hence leave the nip with a higher retention. This phenomenon may be related to the possibly beneficial ‘sucking action’ of some bowl fillings; it is certainly relevant when comparing mangling with suction drying and centrifuging. These other methods of mechanical water removal appear to be less influenced by fabric structure and may therefore be superior to mangling for lightweight, open fabrics. In one other respect mangling differs from the other processes, in being essentially a water-limiting operation. It does not matter how wet the fabric is as it reaches the nip; under given mangling conditions it will leave with virtually the same retention. Specifically, if a mangled fabric is passed a second time through the same nip, without rewetting, the water retention will decrease by no more than 1%. In mangles the same mechanism provides both water removal and fabric transport. Thus mangle nips are the almost inevitable choice for inter-stage drives in such machines as washing ranges. Suction drying In suction drying water removal is achieved by passing fabric over a narrow slot cut in a box connected to a vacuum pump. This technique is applicable only to openwidth fabric. In this process water is removed in the form of droplets driven from the fabric by the air flowing into the vacuum system. Some evaporation may take place subsequently, but within the fabric the removal is almost wholly mechanical. The rate of removal is remarkably rapid: perfectly satisfactory suction drying can be achieved with a slot 2-3 mm wide. At a speed of 60 m/min a fabric element passes this slot in 2-3 ms, in which time it may discharge more than half its own weight of water. The most important mechanical variables in suction drying are the vacuum achieved, the fabric speed and the slot width. In practical suction drying the vacuum pump will usually be run at a fixed rate, and the vacuum achieved will depend on the slot width and the density of the fabric, Only under controlled experimental conditions can the variables be studied separately. The effects of changes in vacuum roughly parallel those of nip pressure in mangling; the effects of fabric speed are generally similar in magnitude to those of mangling. It is natural to expect that suction time is a crucial parameter and that the effect of an increase in speed could be counterbalanced by a corresponding increase in slot width (assuming that the vacuum could be kept constant). In fact this turns out not to be the case, and at constant vacuum the slot width alone has very little effect on water retention. Investigation of this paradox has shown that virtually all the water removal occurs in regions close to the slot edges, the central region contributing very little, although it does influence the air flow into the vacuum chamber.


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In the interests of achieving the highest possible vacuum, therefore, it is desirable to employ a narrow slot. There is, however, no benefit in taking this policy to extremes, since the slot behaves, in respect of air flow, as if it were wider than its actual width. The enhanced air flow at the slot edges presumably accounts for the more effective water removal at these positions. In view of this phenomenon, the use of multiple slots might be expected to be advantageous and such arrangements have been devised. A second suction slot can remove more water from the fabric (a slot is not a water-limiting device, like a nip), but introduces more air flow and makes it more difficult to maintain the vacuum. In general, the achievement of an adequate level of vacuum is the main practical problem in suction drying. The lower vacuum resulting when more open structures are dried tends to offset the easier water removal that would otherwise be expected in this process. Suction drying requires a separate mechanism for fabric transport, and the power requirements of vacuum pumps are higher than those of mangles by a factor of two or more. Nevertheless, the process is less severe than mangling and can be employed on fabrics that would be damaged by a squeeze nip. If a fabric drive is already available, e.g. in a thermal dryer, a suction slot can be inserted in a very small space. In these circumstances suction drying, although much less widely used than mangling, has a useful role in mechanical drying.

Centrifuging Centrifuging is the natural choice for drying materials in batch form, such as loose fibre, yarn hanks or packages, and garments. It is also convenient and economical for piece goods that come from processing in rope form as relatively small batches, e.g. from a jet dyeing machine. The significant variables in this process are the centrifugal acceleration and the duration. The acceleration is determined by the rate of rotation and is proportional to the radius; the figure quoted, commonly in terms of gravitational units, is that for the radius of the centrifuge basket. The goods in the machine will, if free to move, be compressed into a fairly narrow band against the basket wall, but all the water removed must pass through the outer layers. It appears that, while the process is running, the layer adjacent to the basket must always be slightly wetter than the rest, but this difference rapidly disappears when the process stops. As a batch process, centrifuging readily lends itself to automatic control of duration, but if operated manually it is convenient to stop the operation on cessation of significant discharge from the machine drain. Being intermittent in nature, centrifuging is not readily comparable with continuous processes in terms of productivity, but energy costs can be roughly compared. In this respect the centrifuge is intermediate between mangle and suction dryer. 7.1.2 Thermal drying Thermal drying in some form is essential if a fabric is to be returned to an air-dry condition and the process is inevitably energy-intensive. Interest in its thermal efficiency is by no means a modern phenomenon.

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All the energy applied in thermal drying is immediately dissipated into the surroundings as heat (including the heat content of water vapour), but that part associated directly with water evaporation may reasonably be described as the essential heat requirement, in contrast to the losses ascribable, for example, to less than perfect insulation. Provided the energy consumption is related to the weight of water evaporated, rather than to the weight of textile material, there is little variation in the heat requirement over a wide range of conditions of drying temperature and moisture content. The average amount of energy used is about 2.7 MJ per kg of water evaporated. On top of this figure, different drying processes show different levels of heat loss, and different machines of a given type may also vary one from another. Cylinder drying This is the general ‘workhorse’drying method for open-width fabrics, heat being transferred to the fabric from steam-heated cylinders, typically at a surface temperature up to 160°C. This procedure offers no reliable facility for control of fabric width and tends to impart warpway stretch, or at least to limit any tendency to shrinkage on drying. Simplicity and thermal efficiency are the main advantages of the process, heat losses being restricted to radiation and convection from the exposed parts of the cylinders. Losses are typically in the range 0.6-0.8 MJ per kg of water evaporated under steady running conditions, giving a total energy consumption of approximately 3.4 MJ per kg. The rate of drying on cylinders is measured by the weight of water evaporated per hour per unit area of contact between cylinder and fabric. Defined in this way the parameter permits reliable comparison between machines comprising different numbers of cylinders of different sizes. Cylinder temperature is the major process variable influencing drying rate, but tension is also important and fabric structure has a marked effect. Stenter dying The stenter is the only drying machine that provides adjustment and control of fabric width in conjunction with the drying operation. The fabric selvedges are held by pins or clips carried by chains which travel the length of the machine, drawing the fabric through a horizontal oven in which it is heated by air jets from above and below. Stenters are widely employed for various operations other than drying, but in drying the air temperature is commonly restricted to no higher than 160°C (similar to that on cylinders), to minimise danger to the fabric should the machine stop. The rate of drying in stentering is measured as in cylinder drying, in terms of the evaporation of water per hour per unit area, the area now being the area of fabric within the oven. This measure is really more significant in stentering than in cylinder drying, because each square metre of heated fabric requires a square metre of floor space. A higher drying rate (achieved, for example, by a better design of air circulation fans or jets) enables a given drying performance to be obtained on a shorter, less expensive machine. The drying rate is also intimately related to thermal efficiency. In addition to radiation and convection losses from the oven casing, a heat loss results from the


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ventilation required to discharge the water vapour formed in drying. The problem is not one of loss of water vapour, but the heat loss represented by the air discharged with it. A lower rate of exhaust is clearly more thermally efficient, but increases the humidity inside the oven. This in turn reduces the rate of drying. Fortunately it is possible to define an optimum humidity that provides only a moderate heat loss together with a rate of drying only a little below the maximum. The optimum humidity is in the range 0.10-0.15 kg water vapour per kg dry air, but the losses rapidly increase if the humidity is allowed to drop much below this range. Control equipment is available to maintain the correct humidity automatically. Even under optimum exhaust conditions, heat losses in stentering are inevitably greater than in cylinder drying. In a sense this may be regarded as the penalty that must be paid for the specialised fabric control facilities offered by the stenter. The overall energy consumption in this process is likely to be in the region of 4.5 MJ per kg water evaporated. Radiation drying Infra-red radiation derived from electrical or gas-fired heating elements is another technique commonly employed in fabric drying, most usually to supplement cylinder or stenter drying or to provide rapid predrying. However the IR energy is generated, the source temperature is very much higher than the temperature of air in a stenter or at the cylinder surface. If the fabric stops for any reason there is an immediate danger of overheating, so care must be taken to protect the fabric from the heat source. If the source is a heated refractory or electrical element of high heat capacity, the only satisfactory procedure is some form of shutter. With a heater of low heat capacity it will normally be sufficient simply to switch it off; such a heat source will also heat-up rapidly when the process restarts. The efficiency of IR heating depends on the efficiency with which the fabric absorbs the incident radiation. Fortunately this does not depend too much on the visible colour of the fabric, since most surfaces have enhanced absorptivity in the IR region. Heat losses arise from reflected radiation and from air heated by the fabric in natural or forced convection. In general in this process efficiency is comparable to that of a well run stenter. Other forms of radiation drying by radio-frequency or microwave heating are only rarely employed on fabrics. The primary technical advantage of these techniques, their ability to penetrate the surface and deliver heat internally, is of little benefit with any but unusually thick fabrics. An economic limitation is the necessity to derive the heat energy by conversion from mains electricity in a process that can hardly be more than 60% efficient. 7.1.3 Heat recovery from thermal dryers It is not practicable here to attempt a detailed description of heat recovery schemes, but some general remarks are appropriate. Firstly, if waste heat is to be recovered, consideration must be given to where it is to be used. Preferably the heat should be returned to the process itself, since periods of supply and demand are then matched.


Heat delivery to other processes is less likely to be satisfactory, although some general-purpose heat recovery (e.g. in the form of hot water) may be more viable. Heat recovery from general heat dissipation (such as radiation or convection losses) is unlikely to be worthwhile. A stenter discharges hot air at 140-150°C, and an air/air heat exchanger will preheat cold air to 100-120°C. This can make a useful contribution to the heat demand of the process, provided the stenter can be made to accept this air in preference to obtaining ambient air. Much more heat could be recovered by condensing the water vapour generated by the drying process. Unfortunately, under the conditions normally existing in drying, a worthwhile degree of condensation can only be achieved at a relatively low temperature. A large amount of heat may be recovered, but at too low a temperature to be readily usable. Different conditions arise if drying can be carried out in what is effectively superheated vapour. Various designs and one or two experimental systems have been investigated, and these may form the basis of much more efficient thermal drying processes in the future.

7.1.4 Process control in drying It has long been recognised that overdrying reduces the productivity of thermal dryers and is wasteful of energy. Among the earliest of on-line process controls were moisture meters for attachment to the delivery end of dryers, particularly stenters, to enable fabrics to be dried to the correct air-dry regain. Numerous sensing techniques were evaluated to determine the moisture content of the fabric after drying, but the oldest established technique remains the most popular, namely a measurement of electrical resistance usually made through the thickness of the fabric between a series of small insulated rollers and an earthed guide roller. The output of such a device is an electrical signal representing the moisture content. It is nowadays the normal practice to link this signal to a device that controls the machine speed in order to maintain a predetermined final regain. The greatest benefits of automatic control of final regain are evident in stentering, the most expensive drying process. Additional types of control instrumentation have generally been developed with this process in mind. Automatic control of heat losses from the stenter exhaust may be derived from a sensor measuring the exhaust humidity. Certain other stenter functions, related to processes other than drying, may also be automatically controlled. The whole may be combined into an integrated control system that takes account of the interaction between the different individual process control loops.

7.2 MECHANICAL FINISHING Mechanical finishing is a general term to describe processes carried out on a prepared and dyed fabric in order to improve its suitability for the desired end use, the aftertreatment being provided purely by mechanical means. Such processes are designed either to change the dimensions of the fabric, or to change its surface appearance, handle or properties.


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Some of these mechanical processes do not give results that are permanent. In many instances this may not be a serious limitation, but if greater permanence is required it may usually be achieved by incorporating a suitable chemical reactant. This chapter, however, deals only with the mechanical operations themselves.

7.2.1 Dimensional changes: compressive shrinkage This operation results in a final product with a reduced tendency to shrinkage in use. Many of the processes in preparation and dyeing subject fabrics to warpway tension, so that a woven fabric reaches a state in which much of the crimp in the warp threads has been removed. In use such fabrics will tend to relax towards a condition with a more even balance of crimp between warp and weft, with consequent warpway shrinkage. This tendency can be reduced or even eliminated by subjecting the fabric to a shrinking process before it leaves the finishing works. There are several variants of compressive shrinking machinery, but all rely essentially on forcing a reduction in length of the fabric by holding it firmly in contact with a rubber blanket or similar surface that is initially stretched and then allowed to contract. The mechanism must incorporate a device for adjusting the stretch of the blanket at the fabric entry in order to control the degree of shrinkage imposed. The yarn movement necessary to accommodate the warpway shrinkage is facilitated by the application of heat and moisture during the process.

7.2.2 Raising This is a process in which fibres or fibre loops are pulled from the surface of the yarns to produce a nap on the surface of the fabric. The machine used almost universally to raise cotton and synthetic fabrics is the so-called double-action cylinder raising machine, in which the mechanical action is provided by rotating rollers covered with card wire. A number of effects can be produced by raising, this being as dependent on the characteristics of the fabric as on the differences that can be applied by the machine. Nevertheless, the operation of a raising machine requires quite subtle adjustments of processing conditions, and considerable experience and skill are needed. In particular it is important to recognise that this process is essentially destructive, since it depends on the pulling of fibres partially free from the yarn structure. The fabric must be initially strong enough to allow for the consequent loss in strength, and the process must be operated with fine control to avoid tearing the fabric to pieces. In atypical cylinder raising machine the raising rollers, which may number between 12 and 24, are mounted in bearings on two circular side plates so that they form in effect the outer surface of a cylinder. The fabric passes over this cylinder, making contact with the wire on each roller over a small arc. The card wire tips are angled, facing forward (in the direction of fabric travel) and backward on alternate rollers; the rollers are known respectively as ‘pile’and ‘counterpile’. This feature is the reason for the name ‘double action’. The fabric contacts the raising rollers first at the front of the machine, near the bottom of the cylinder, and leaves at the back, having been in contact for about three-quarters of the circumference.

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In operation the shaft carrying the side plates revolves and the individual raising rollers rotate in their bearings, driven by belts engaging with pulleys on their shafts. The pile roller drive is at one side, counterpile at the other. The fabric also moves so that the overall motion can be very difficult to follow. However, it may be deduced (or taken for granted if preferred!) that by suitably adjusting the roller speeds relative to that of the cylinder it is possible to achieve a balance such that the card wire tips at the outside circumference are moving forward at just the speed of the fabric. This is the ‘neutral point’, at which no raising action occurs and which forms the starting point for the process. Raising action is provided by slightly increasing the speed of the pile rollers or slightly reducing that of the counterpile, or more usually by both together. The result is that the card wire tips move at a speed slightly different from that of the fabric. Raising is not effected by a smooth combing action; it only occurs where a card wire tip becomes entangled in a yarn and, when suddenly released, draws a loosened fibre end with it. The noise associated with this action is the characteristic hiss of the raising process, which is not heard at the neutral point. Only a small proportion of the wire tips make effective contact with the fabric at any instant, and of these contacts only some will actually withdraw fibres, but the overall effect of the interaction between card wire and fabric is that the fabric is alternately under tension and relaxed between counterpile and pile rollers. This interaction demonstrates the benefit of the double-action system, namely that significant local tension changes may be developed within the process without any appreciable overall change of fabric tension. It is apparent from this description that the fabric is in contact with the card wire over a somewhat longer arc under raising conditions. Over this arc the ‘effective’ wires bend backwards and spring out suddenly at the point of release. It follows that the overall fabric tension has two potentially opposed effects: in the first place higher tension will tend to increase the number of contacts between fabric and wire, but on the roller it will tend to pull the fabric free after a shorter contact arc. It follows that the choice of optimum tension settings remains a matter that can really only be resolved by experience. Because the raised effect results only if a card wire tip becomes embedded in the fabric, the plucking-out of fibres occurs almost entirely from the weft yarns in a woven fabric. A wire tip that enters a warp yarn is likely to slip along it without becoming entangled. The weft yarns will inevitably suffer an appreciable loss in strength and some length reduction as a result of raising; they should initially be of sufficient strength to allow for this. Ease of raising is facilitated by incorporation in the fabric of short-staple weft yarns of relatively low twist and by the presence of a suitable lubricant. The raising of woven fabric proceeds by lifting individual fibre ends from the weft yarns. To achieve a uniform nap it is therefore essential to build it up gradually, with a gentle raising action. It is normal practice therefore to give the fabric several raising passes, either repeatedly through the same machine or by operating several machines in sequence. If both faces of the fabric are to be raised it is usual to give no more than two or three passes on one face before changing to the other, in order to keep the two faces more or less in balance. As raising proceeds, a given action


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(as measured by roller speed settings) will tend to have a reduced effect as the card wire is increasingly impeded by the nap already produced. At any single setting there is likely to be a limiting condition beyond which little further raising will take place. The action of the card wire on the fabric is balanced by reaction on the card wire rollers. Various techniques have been developed to quantify the raising action by measuring the force transmitted back to the roller drives. However, the effect of a given raising force depends intimately on the fabric characteristics. Hence measurements of this sort can only really be of value in assessing the relative rate of raising on a given fabric; they can make a useful contribution to control of the process provided this limitation is appreciated. The same proviso applies to measurements on raised fabric. The simplest measure of the amount of nap is the increase in thickness under low load or, a little more accurately, the increase of apparent specific volume (the reciprocal of overall density), which allows for width loss on raising. It will be apparent that the same thickness increase may be recorded from one fabric with a short, dense nap and another where the nap is longer but sparser. Here again the measurement is really only useful for following the progress of raising on a particular fabric or for assessing repeat runs under nominally identical conditions.

7.2.3 Calendering In this process dry fabric is passed in open width between rollers under pressure in order to alter its handle, surface texture and appearance (Figure 7.2). The machinery is similar in some respects to mangling, although the bowls are harder and the loads generally higher. The important similarity is that the nip through which the fabric passes is formed between two bowls, of which one at least is relatively soft, so that the area of contact is determined primarily by the indentation of this bowl. This protects thick regions in the fabric, and even creases, from severe damage. Modern calenders commonly have two or three bowls in a vertical arrangement. Some sixty or more years ago, when calendering was employed for a very wide Metal detector (optional) Cooling drums

Sear auto

Figure 7.2 Farmer Norton hydraulic calender

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variety of effects, stacks of six or seven bowls could be encountered, but a three-bowl arrangement is capable of providing the following processes, still regularly in use: (a) Ordinary calendering (swissing) (b) Schreinering (c) Moire effect calendering (d) Embossing (e) Friction calendering. The following is a summary of the important features of a typical machine. The top and bottom bowls are of chilled iron or steel, the middle bowl being the relatively soft one, comprising a central shaft with a substantial thickness of compressed cotton or compressed discs of woollen paper or impregnated synthetic fibre. The top bowl is fixed in its bearings and is directly driven, the other two bowls being raised into contact by pneumatic or, more often, hydraulic cylinders supplying loads up to about 800 kg per cm length of nip. The bottom bowl is normally driven through a slipping clutch to ensure that the bowls are rotating as the load is applied. The top bowl is heated, usually internally by gas flame or circulating oil, possibly externally by radiant elements. The surface temperature of this bowl may be set at 150°C or more. It is important to note that filled calender bowls suffer from the same defect as filled mangle bowls, namely their resistance to bending resides solely in the metal shaft, the filling making no contribution. These bowls are therefore much more prone to deflection than a metal bowl of the same overall diameter. If the filled bowl is in the central position in a three-bowl calender this problem is unimportant, but if it is the bottom bowl care must be taken to ensure that the effects of bowl deflection are not serious, either by limiting the applied load or by applying a suitable camber. This situation will obviously arise in a two-bowl calender, but also in a three-bowl machine if it is required to provide a filled-bowl/filled-bowl lower nip. Swissing In this, the simplest, calendering process, the fabric is passed through one or both of the calender nips in order to reduce its thickness somewhat and to provide what might in general be called a ‘crisper’ handle. The calendered effect is most pronounced if the fabric is slightly damp entering the machine and drier leaving it. A moisture content somewhat above the normal air-dry regain is useful, perhaps up to 12-15% for cotton. A top-bowl temperature of 100°C will readily supply the heat needed to evaporate this excess after the nip. Even in plain calendering the fabric face in contact with a metal bowl will always have a slightly glazed appearance. This may well be an additional advantage, but if it must be avoided the solution is to use filled bowls in bottom and middle positions and apply only the lower nip. Schreinering This process is essentially similar to plain calendering using a top bowl engraved with fine parallel lines. These lines should be at an angle that is as near as possible to the apparent lines caused by the twist of the warp yarns. To meet this requirement it is


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usual to have a variety of engraved bowls available. A fabric passed through such a nip will have this pattern impressed on its upper surface, resulting in a sheen possessed naturally only by fabrics woven from exceptionally regular yarns. Before the widespread availability of filament synthetic fabrics, the target appearance was that of silk, and schreinering was more widely used, with a variety of types of engraved line. At the present time the choice is likely to be much more restricted. Nevertheless, the process can still be most effective in appropriate circumstances. Moiré effect The characteristic moiré patterns may be produced by an engraved roller in a manner analogous to the schreinering process, but the result will show a warpway repeat at a pitch equal to the circumference of the engraved bowl. A truly random moiré effect is obtained by passing two lengths of the same fabric face-to-face through the calender. Each fabric generates a moiré pattern on the other as a result of the inevitable small variations in yarn spacing. As in plain calendering, if the fabrics are passed through a nip between metal and filled bowls, then one side will be glazed while the other is not. Normally this will not be a problem, but if identical sides are required then two similar filled bowls must be used in the calender. Embossing Embossing has some similarity to schreinering, but whereas schreinering is essentially a surface effect, the embossing process employs larger and deeper patterns, carried by both bowls. This requires a two-bowl calender in which the bowls are positively geared together. It is still necessary to have one relatively soft bowl, so that the fabric is not crushed in the contact between metal bowls. The pattern engraved in the metal bowl may be gradually run into the filled bowl, the positive gearing ensuring that the two bowls stay in synchronism. Friction calendering This process can be operated on a three-bowl calender in which both top and bottom bowls are positively driven in such a way that the top bowl has an appreciably higher surface speed, commonly by a factor of about two. In older calenders the speed differential is provided by a gear chain between the top and bottom bowl shafts. Certain modern designs incorporate separate drives to the two bowls, an arrangement that facilitates alteration of the friction ratio. When the top and bottom bowls are running at different speeds, the middle filled bowl takes up the speed of the bottom one. The reason for this is that the load on the lower nip is greater than that on the upper, because of the weight of the middle bowl. With similar coefficients of friction, the frictional force at the lower nip is the greater, and this determines the speed of the middle bowl. (That the speed differential is dependent on the coefficient of friction may readily be demonstrated on an empty machine by wiping a damp cloth across the face of the middle bowl, which will then be seen to accelerate briefly as the damp strip passes through the lower nip.)

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The friction calendering action takes place at the upper nip, where the fabric follows the middle bowl speed, and its upper surface is vigorously polished by the top bowl. The application of high nip pressure and high temperature to a fabric impregnated with starch or a synthetic alternative can, on repeated frictioning, produce a material that is quite transparent, having had virtually all the internal air spaces (and hence light-reflecting surfaces) eliminated. At one time friction calendering in this extreme form was important in the manufacture of tracing cloth and the process is still widely used for bookcloths and similar specialised fabrics. 7.2.4 Reducing stiffness Occasions may arise when it would be convenient to produce some modest improvement in fabric softness by mechanical means. Machines designed specifically for this purpose, such as the button breaker, may be encountered only very rarely, but simple makeshift arrangements will frequently prove quite effective. What is required, in essence, is to pass the fabric, under controllable tension, over a series of relatively sharp edges. An assembly of square-section bars or scrimp rails will normally suffice.

Chemical finishing 8.1 INTRODUCTION A dyed, washed-off and dried fabric is seldom an immediately saleable product. Additional processing is required to give the fabric and the garment made from it customer appeal. A well known company coined the phrase: ‘It is the finish that fits the fabric for its purpose’. Finishing of textiles falls into two categories: (a) Mechanical finishing (already discussed in Chapter 7) (b) Chemical finishing, the subject of this chapter. In order to obtain a desired finishing effect the fabric is frequently given several finishing steps, and these can be either mechanical or chemical in nature. 8.2 OBJECTIVES OF CHEMICAL FINISHING The purpose of finishing textiles is to impart the special properties that they must possess to meet appropriate in-service requirements and secure customer satisfaction. Dress goods, shirtings and leisurewear must have an acceptable handle, should not crease in wear and should display good easy-care properties. Workwear must be resistant to hazards encountered by the wearer, e.g. boiler suits worn by garage mechanics must have adequate oil and stain repellency, firemen’s uniforms must be flame-retardant and outdoor workwear must be water-repellent. 8.3 CHEMICAL FINISHING OF CELLULOSIC FABRICS 8.3.1 Dimensional stability Fabrics and garments made from cotton or viscose have to withstand washing, laundering or dry cleaning, part of the usual wash and wear cycle. If not chemically finished, cotton and viscose fabrics are dimensionally unstable when washed. Cellulosic fibres, when immersed in a wash liquor, readily absorb water and swell in a lateral direction. Cotton absorbs about 50-60% of its own weight in water, viscose up to 90-100%. The aim of a chemical finish is to reduce the water uptake or ‘imbibition’of the cellulosic fibres, thereby rendering the treated fibres resistant to swelling and therefore shrink resistant. Good dimensional stability is a prerequisite for cotton and viscose fabrics regularly washed or laundered, such as shirtings, apparel, leisurewear, bed linens, etc. 8.3.2 Easy-care finishing Since the 1950s there has been a marked trend towards automated domestic chores and more leisure time. One facet of this has been the need to develop apparel fabrics 109

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and household textiles that are easily washable and require no ironing, or only a minimum of ironing. The introduction of nylon and, particularly, polyester fabrics, with their inbuilt stability to deformation and creasing during wear, and their ‘smooth drying’ properties when washed or laundered, created new demands for higher standards of performance also from cellulosic fabrics. From the mid 1950s to the early 1970s a great deal of effort was devoted by the chemical manufacturers to developing new finishing agents that imparted the desired easy-care properties to cotton and, to a lesser extent, viscose fabrics. Chemical finishing of cellulosic fabrics was given fresh impetus by the availability of a host of new chemicals. The textile finishing industry responded by introducing new or improved application techniques, which imparted easy-care, wash-wear and minimum-iron properties demanded by the consumer. Easy-care finishing is still widely used on cotton, viscose and polyester/cotton fabrics. The extravagant claims made in the 1960s for superlative performance of easy-care-finished 100% cotton fabrics have long since been discounted, but easy-care finishing of certain types of cotton and polyester/cotton fabrics is here to stay. 8.3.3 Crease recovery of finished fabrics Easy-care finishing results in fabrics that resist creasing during wash and wear, yet a fabric’s resistance to and recovery from deformation due to creasing is a property that is frequently evaluated subjectively and assessed incorrectly. A chemically finished cotton or viscose fabric may have excellent recovery (snap-back) properties when creased by hand (clenched fist test) yet the same fabric may crease badly in wear (e.g. during prolonged sitting), or may have lost all its crease recovery after washing. It is clear therefore that two distinct processes exist: dry creasing and wet creasing. An in-between stage, creasing at high relative humidity and elevated temperature (as in sitting) is well known and difficult to overcome by chemical finishing Test methods exist to evaluate all forms of creasing, including creasing in wear and creasing during travel (box creasing). In evaluating creasing behaviour and crease recovery of a fabric, it is important to specify the type of creasing a fabric may be subjected to in use, so that the correct test method is applied. Thus to evaluate resistance to creasing in wear, samples cut from the fabric are creased under standard conditions of applied load and time. After removal of the load, the specimens are allowed to recover and the angular recovery is measured. The greater the recovery angle, the better will be the resistance of the fabric to creasing in wear. 8.3.4 Polymeric and crosslinking finishes In order to impart easy-care properties to cellulosic and blend fabrics a chemical finish has to be applied. The original crease-resist process, developed by Tootal Broadhurst Lee Co. in the late 1920s and early 1930s, entailed padding viscose fabric in a solution of a water-soluble precondensate of urea and formaldehyde with an acid catalyst, followed by drying and finally curing the resin-impregnated fabric at

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130°C. Treated fabrics had greatly improved dry crease recovery attributable to deposition of urea-formaldehyde (U/F) polymer in the interstices of the fibres. Chemical finishing agents that react mainly by self-crosslinking to form threedimensional polymer lattices in the fibres include those based on U/F and melamine/ formaldehyde (M/F). Commercially available U/F and M/F products include dimethylolurea (DMU) and trimethylolmelamine (TMM). These and similar resin products are still widely used by the finishing industry. Their main attractions are excellent dry crease recovery, simple application and low cost. The demand for cotton and viscose fabrics with good easy-care properties was met by the development of the so-called crosslinking reactants. These condense with the reactive hydroxy groups of the cellulosic fibres to form crosslinks durable to washing and laundering. In the past the chemical manufacturers devoted considerable effort to developing new N-methylol derivatives of ethyleneurea, propyleneurea, triazones and carbamates, to mention just some of the more important types. Of these the following have assumed special significance and are still used today: - Dimethylolethyleneurea (DMEU) - Dimethyloldihydroxyethyleneurea (DMDHEU), the most widely used reactant - Dimethylolpropyleneurea (DMPU) - Dimethylol-4-methoxy-5,5-dimethylpropyleneurea. In comparison with the traditional U/F and M/F self-crosslinking resin finishes, these and other cellulose reactants provide finishes that not only impart good easy-care properties, but may enhance or suppress other fabric properties according to the crosslinking reactant chosen. These include: (a) Enhance resistance to hydrolysis during laundering (b) Reduce chlorine retention during hypochlorite treatment (c) Suppress release of formaldehyde during storage, making-up and wear (d) Reduce change of shade of dyed or printed fabrics that can occur as a result of chemical finishing (e) Suppress formation of fishy odours during storage. Pad-dry-cure methods The conventional pad-dry-cure method is the process most widely used to impart easy-care properties to cellulosic fabrics. It is relatively simple and yields fabric with the properties outlined in Table 8.1. It is a characteristic feature of the classical pad-dry-cure method that an improvement in dry crease recovery is accompanied by a corresponding deterioration in physical properties. In fact a direct relationship exists between increase in dry crease recovery angle and loss in strength or abrasion resistance. Many modifications have been proposed to the basic method to obtain an optimum balance between improved easy-care performance and loss in strength. Some of these variants found commercial application, at least for a time. Variables at the disposal of the finisher include: (a) Composition and concentrations of the pad liquor: self-crosslinking resin precursors or crosslinking reactants, catalysts and auxiliaries

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Table 8.1 Advantages and disadvantages of the pad-dry-cure process for easy-care finishing of cellulosic fibres

(b) (c) (d) (e)

Advantages

Disadvantages

High dry crease recovery Moderately good wet crease recovery Good dimensional stability Good retention of shape Excellent easy-care properties

Reduction in tear strength Loss in abrasion resistance Change of shade of dyed fabrics Release of formaldehyde Danger of fishy odours

Application conditions: padding, lick roller, spraying or foam application Solids add-on: 3.5-5.0% for cotton, 7-10% for viscose Drying conditions: type of machinery, effect of time and temperature Curing conditions: as for drying.

All these variables have a significant effect on the final outcome. Amongst them, catalysts deserve special mention. The self-crosslinking U/F and M/F resins are best catalysed with ammonium salts, e.g. ammonium chloride or mono- or di-ammonium hydrogen phosphate, which at elevated temperature dissociate to liberate the free acid. Undesirable side effects associated with these catalysts include the development of fishy odours and degradation of the resin finish by absorption of chlorine during laundering and subsequent drying. The crosslinking reactants are best catalysed by metal salts of strong acids, e.g. magnesium chloride or zinc nitrate. Mixtures of metal salts with a-hydroxycarboxylic acids, e.g. tartaric or citric acids, are sometimes used. The chemical manufacturers provide detailed information about suitable resin and catalyst systems and application conditions. The classical resin finishing process consists of a minimum of three distinct processing steps, afterwashing sometimes being omitted on cost grounds. (a) Impregnation Fabrics are padded in an aqueous solution containing the crosslinking agent, catalyst and auxiliaries on a two- or three-bowl padding mangle. For cotton an expression or liquor pick-up of 60-70%, calculated on the dry weight of the fabric (o.w.f), is usual; for viscose the expression is generally 90-100% o.w.f. g Impregnated fabrics are dried at temperatures between 105 and 120°C, (b) Drying depending on availability of machinery. It is frequently desirable to dry fabrics to a known moisture content rather than to complete dryness. (c) Curing Dried fabrics are cured at elevated temperatures (130-180°C) to effect condensation/polymerisation or crosslinking with the fibre. Under these anhydrous conditions the cellulosic fibres are crosslinked in the unswollen state. (d) Afterwashing It is desirable to wash the cured fabric in hot detergent solution to remove surface resin deposits, uncured resin and catalyst residues. Washed fabrics are less prone to develop fishy odours or to release as much formaldehyde as the unwashed material.

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Moist-cure processes The disadvantages of conventional resin finishing, i.e. loss of tensile strength, tear strength and abrasion resistance, led to the development of new methods of curing. By curing fabrics with the fibres in a partially swollen state, satisfactory wet and dry crease recovery levels were obtained with less physical damage, thereby achieving a better balance of fabric properties. Moist crosslinking is carried out on partially swollen cellulosic materials having a residual moisture content of 5-8% in the case of cotton and 1.0-1.6% for viscose. After impregnation with a crosslinking reactant and catalyst, the fabrics are carefully dried to a predetermined moisture content, batched on an A-frame, wrapped in polythene sheeting and slowly rotated for 16-24 h. After completion of the reaction the fabrics are washed-off continuously, i.e. rinsed hot and cold, neutralised and dried. Crosslinking reactants and catalysts have to be carefully selected to guarantee reproducible results, e.g. DMDHEU or dimethylol-4-methoxy5,5-dimethylpropyleneurea give excellent finishes. The acid catalyst can be sulphuric acid or a mixture of inorganic salts and organic acids. The pH of the impregnating liquor must be between 1 and 2. In traditional dry curing processes the cellulosic fibres are in an unswollen state during crosslinking, which leads to relatively short crosslinks between cellulose chains, a relatively rigid crosslinked network and therefore relatively large losses in tear strength and abrasion resistance. In moist crosslinking processes the cellulosic fibres are in a partially swollen state and longer crosslinks are formed between cellulose chains, resulting in a more flexible or elastic network that can better accommodate to deformation during tearing or abrasion. This leads to an improved recovery/strength relationship. Using a traditional dry curing process a 10o rise of angular dry crease recovery produces a loss of tear strength of 5-9%. W ith a moist crosslinking process the loss of tear strength is reduced to 3.5-6.0% for the same rise in angular dry crease recovery. Wet crosslinking is a method designed to give wet crease recovery with little or no dry crease recovery, required for minimum-iron finishes. The method is a modification of moist crosslinking, insofar as the pH of the impregnation liquor is less than 1 and the padded fabric is not dried but is squeezed to a residual moisture content of 70-80%. In all other aspects processing is as for a moist-cure finish. Great care is required when working with a strongly acid impregnation liquor to prevent damage by spillage to operatives, machinery and fabric. Combined chemical and mechanical finishes Combined finishes generally use a calender to produce the desired mechanical effect. Three types of calendered finishes are generally recognised: (a) Schreiner, silk or moire finish using a metal roller with finely engraved lines (b) Chintz or glazed finish using a highly polished smooth metal roller (c) Embossed finish using an engraved metal roller. Schreiner finishing imparts a silk-like lustre an d handle; moire finishing gives a rippled appearance and im itates an irregular ‘watered’ effect. Chintz or glazed

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finishing confers a high gloss and is obtained by friction calendering. Embossed finishing imprints a design in relief consisting of three-dimensional raised (sculptured) and flat areas, Fabric construction is important in achieving a calendered effect. Moreover, fabrics to be calendered must have adequate strength to withstand the effect of the heated metal roller, often applied to fabrics with considerable pressure, as well as losses resulting from the chemical finish. To obtain washable mechanical effects, cellulosic fabrics have to be pretreated or sensitised with N-methylol derivatives, and curing must be delayed until after the calendering operation. For cotton fabrics a crosslinking reactant of the DMDHEU type is preferred; for viscose fabrics a combination of a crosslinking reactant with a self-crosslinking resin precondensate is usually selected. The choice of catalyst is equally important: for cotton mono- or di-ammonium phosphate or magnesium chloride is used, whereas for viscose ammonium chloride or ammonium sulphate is recommended. The pad liquor may contain up to five or six components, e.g. crosslinking reactant and/or self-crosslinking resin, catalyst, softening agent, additive to improve tear strength, hydrophobic agent to enhance resistance to water spotting, and dispersing agent. Cotton fabrics are padded to 70-80% expression; viscose fabrics to 90400% expression. Careful drying at temperatures of 100-l 20°C is essential to yield fabrics with a residual moisture content of 8-10% for cotton and 10-12% for viscose. As a general guide, an increase in residual moisture content yields a better mechanical effect with better wash resistance, but a harsher/stiffer handle and a higher loss in tear strength and abrasion resistance. Schreiner and chintz finishes require a pressure of 25-30 tonne across the face of the roller, the metal bowl being heated to 160-200°C. For embossed finishes the pressure is usually reduced to 15-20 tonne. After calendering the fabric is cured as previously described. The insertion of durable pleats represents another combined chemical and mechanical finish. To produce washable effects a highly reactive reactant or selfcrosslinking precondensate, e.g. DMEU or DMU, is selected. Fabrics are sensitised and dried as previously described, followed by insertion of pleats by a heated knife. Curing or condensation is limited to 1 min at 180°C by the design of the pleating machine. 8.3.5 Additives for crosslinking finishes Additives, when added to a chemical finishing liquor, bring about changes in physical or chemical properties of the treated fabrics. Additives are also used to enhance the stability and smooth running properties of the finishing liquors. Wetting, dispersing and antifoam agents These auxiliaries are usually added in small quantities (I-5 g/l) to finishing liquors. Wetting agents enhance the wetting-out properties and thus the absorptive capacity of fabrics to be treated. Both anionic and nonionic wetting agents can be used.

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Dispersing agents ensure stability and compatibility of the various components of a chemical finishing bath for prolonged periods (6-8 h). Nonionic dispersing agents are widely used, but cationic agents can find use in special cases when substantivity is required, i.e. when applying chemical finishes by exhaustion. Antifoam agents prevent foaming of the liquor in the pad trough or at the nip formed by the padding rollers. They act by modifying surface tension and many different products are available, several of them silicones. Softeners Softeners are used to improve the handle and smoothness of treated fabrics. Resilience (i.e. the ability to resist and recover from stretching, deformation and creasing) can also be improved. Softening agents are classified according to their ionic properties. Anionic agents are based on: (a) Sulphated oils or fatty acid esters (b) Alkyl sulphates (c) Fatty acid condensation products. Anionic softening agents impart a full handle to treated fabrics, but their softening effect is inferior to that obtainable from cationic or nonionic agents. Products based on (b) or (c) do not discolour under normal curing conditions, but their stability is limited below pH 7. Nonionic agents are based on: (a) Polyglycol ethers and esters (b) Ethoxylated phenols (c) Silicone products. Although the softening effect from products based on (a) or (b) is not as good as that given by cationic agents, the nonionic softening agents are universally applicable, as they are unaffected by pH and water hardness. Their resistance to discoloration is good, which is of importance in chemical finishing. Cationic agents are based on: (a) Quaternary ammonium or pyridinium derivatives (b) Aminoesters and amides. The softening effect is excellent on all fibres without affecting the fullness of handle. Cationic agents are widely used in chemical finishing as they are compatible with self-crosslinking U/F and M/F resins and crosslinking reactants. Reactive softeners are based on N-methylol derivatives of long-chain acylamides or of long-chain acyl-substituted ureas. These softeners are capable of reacting with cellulosic fibres and thus provide softening effects more durable to laundering. They can also impart a mild water-repellent effect to treated fabrics.

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Needle lubricants Needle lubricants improve the sewing properties, i.e. reduce needle cutting, of easycare cellulosic fabrics. They help to reduce losses in tear strength and abrasion resistance. These products function by lubricating the crosslinked fibre matrix. By making it more flexible, the fibre is better able to accommodate stresses imposed on the fabric structure during sewing, cutting and wear. In general three types of products are employed, outlined below. (a) Primary and secondary dispersions of polyethylene These products give excellent effects and are widely used. They do not materially affect handle or soiling and because of their excellent stability are almost universally applicable in chemical finishing liquors. (b) Silicic acid ester dispersions These agents impart similar effects to those obtained from polyethylene dispersions and give a smooth, silk-like handle to treated fabrics. (c) Silicone dispersions These are of considerable commercial importance and are discussed below. Handle modifiers Handle modifiers serve a multitude of functions. Firstly they enable the handle of fabrics to be varied from soft to firm according to demand. Secondly they restore bulk and firmness to fabrics that have become limp during preparation and dyeing processes. Thirdly they enable fabrics to be stiffened if necessary to facilitate making-up procedures. Finally a stiffening finish is required for end uses such as interlinings, workwear, table linen, bed linen, mattress covers, tapes and ribbons. The chemicals employed are similar to those used as handle modifiers and stiffening agents, but their effect depends on the amount of product applied to the fabrics. They can be natural or synthetic polymers, and finishes can have various degrees of durability, depending on their resistance to washing and laundering. Natural starches and their derivatives find wide application. Unless used in conjunction with polymeric or crosslinking reactants, the stiffening effects are not durable. Many synthetic polymers give washable effects. These products are usually based on polyacrylic acid and its derivatives, polystyrene, poly(vinyl alcohol), poly(vinyl acetate) and poly(vinyl propionate). Commercial products are frequently made from different monomers to give tailor-made copolymers with known chain lengths and degrees of polymerisation. Many washable stiff finishes are based on the self-crosslinking U/F and M/F derivatives. Polyacrylate and polysiloxane dispersions Commercial polyacrylate products are usually copolymers derived from alkylacrylates containing small amounts of reactive comonomers able to crosslink with both: (a) N-Methylol reactants or polymer-forming precondensates (b) Cellulosic fibres. The pH stability of these dispersions is good and reactive polyacrylate dispersions are widely used in easy-care finishing because of their beneficial effect on both 100%

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cellulosic and cellulosic blend fabrics. When added to the chemical finishing liquor they bring about a remarkable increase in crease recovery without adversely affecting tear strength and abrasion resistance. For any level of easy-care properties it is thus possible to substitute a given amount of crosslinking agent with a reactive polyacrylate dispersion, thereby obtaining a better balance of easy-care properties with a reduced loss of physical properties. Such products do not materially affect handle or soiling of easy-care finished fabrics. Reference has already been made to polyacrylate dispersions as handle modifiers. Commercial products known as silicones have been available since the 1950s. The early products were either solvent-based or aqueous emulsions and have found wide application as durable water-repellents for all classes of textiles. Silicones proved to be good lubricants and are extensively used in making-up to reduce needle cutting. However, their use as softeners has undergone many changes in the last two decades. From the inert polydimethylsiloxane softeners of the 1950s and 1960s, which did not penetrate the fibre structure and gave only surface lubrication, the 1970s saw the development of reactive polysiloxanes with hydroxy groups at the ends of the polymer chains, making crosslinking possible. In the 1980s aminosiloxane polymers with built-in crosslinking groups and catalysts were developed. These elastomeric reactive silicones are capable of crosslinking with themselves or with hydroxy groups in cellulosic chains. By penetrating into the fibre matrix to form a three-dimensional network, they provide internal lubrication and therefore superior softening to easycare finished fabrics. It is believed that 65% of all easy-care formulations nowadays contain elastomeric reactive silicone softeners. Amongst the latest developments in this field are the amine-functional silicone microemulsions. These are colourless silicone fluids emulsified by special techniques to reduce particle size to only one-hundredth of that of a conventional polysiloxane emulsion. Microemulsions, when applied to cellulosic fabrics as part of a crosslinking formulation, produce ‘inner softening’ by building a three-dimensional network or lattice inside the fibre. Such easy-care finished fabrics have a durable, soft and springy handle coupled with excellent physical properties. Silicone microemulsions do not affect the soiling behaviour of the treated fabrics 8.4 CHEMICAL FINISHING OF SYNTHETIC FABRICS In general the term chemical finishing is somewhat of a misnomer when applied to finishing of 100% synthetic fabrics. The lack of chemical reactivity of synthetic fabrics, particularly those based on polyester fibres, precludes chemical reaction between fibre and finishing agent. In fact chemical finishing processes devised for cellulosic fabrics (to impart dimensional stability, crease recovery, easy-care properties) are unnecessary for synthetic fabrics, as these desirable properties are already inherent in the fibre. Most finishing processes for synthetic fabrics are of a mechanical nature. When chemical finishes are applied to obtain certain specialised effects, the chemicals are generally deposited on the fibre surface without penetrating the structure. The durability of such finishes to washing and laundering is therefore only limited.

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8.4.1 Heat setting A heat setting treatment is often used to impart desirable properties to synthetic fabrics; this relaxes and stabilises the fibre structure and provides dimensional stability. Setting treatments require high temperatures and are generally carried out prior to dyeing, so as to present the dyer with a dimensionally stable and crease-free fabric during wet processing. Another advantage of presetting is that dyes cannot be adversely affected, or even decomposed, subsequently by the high temperatures used in postsetting. Nevertheless, heat fixation can be carried out as the final fixation step for disperse dyes or fluorescent brightening agents, and results in fabrics with excellent dimensional stability. Conditions for heat setting or dye fixation are: - Polyester fabrics: 20-30 s at 180-220°C - Nylon 6 fabrics: 15-20 s at 190-200°C - Nylon 6.6 fabrics: 15-20 s at 190-230°C. With nylon fabrics the lower temperature is used to obtain dimensional stability and the higher temperature is required to impart freedom from creasing.

8.4.2 Hydrosetting and steam setting Hydrosetting, usually confined to nylon fabrics, imparts a fuller and softer handle than dry heat setting in hot air. Fabrics are treated in an autoclave for 20-30 min with water at 125-l 35°C. Steam setting gives results intermediate between those obtainable from dry heat setting in hot air and hydrosetting. Using saturated steam, fabrics are steamed in autoclaves for 20-30 min at steam pressures of 180-200 kPa (1.8-2.0 atm) at 130132°C. The results are similar to those obtained from hydrosetting. With superheated steam, setting is carried out on a pin stenter, giving results similar to those obtained from thermofixation, except that steam, being a better heat-transfer medium than air, enables the treatment time to be cut down by 25% to I0-15 s. Moreover, in the absence of air superheated steam setting causes less yellowing of nylon fabrics.

8.4.3 Additives Many additives (section 8.3.5) are available for use on 100% synthetic fabrics, to impart the special effects described below. As previously stated, these finishes have only limited durability to washing and laundering on synthetic fabrics.

Filling and stiffening finishes Polyester and nylon fabrics used for interlinings, lace, nets, etc. are frequently stiffened with U/F and M/F resin precondensates; these adhere relatively well to the smooth fibre surface. Dispersions of acrylate copolymers can be used in combination with these resins to impart better elastic recovery from deformation.

Softeners Many anionic, nonionic and cationic softening agents are on the market and the choice depends mainly on method of application, type of handle required and cost

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(section 8.3.5). Some of these products besides conferring a soft and silk-like handle, also impart hydrophilic properties and improved sewing/cutting properties. ,

Hydrophilic finishes Synthetic fabrics have low regain values. Nylon takes up 5% moisture at equilibrium, polyester only 0.5%. To improve comfort in wear, particularly for underwear in contact with the skin, it is desirable to impart a hydrophilic finish. Fatty acid adducts and modified polyamide dispersions are widely used. Antistatic finishes Practically all chemical antistats, both temporary and permanent, function by providing a hydrophilic surface on the fibre, attracting a film of moisture from the atmosphere. The fibre surface thus becomes conducting and dissipates the charge. An elegant method is to apply a hydrophilic polymer, e.g. a polyether or polyglycol derivative, to the fibre and attach it permanently by a heat treatment. The polymer chain is thus fused into the fibre surface, whilst the hydrophilic groups make the surface conducting. Permalose TM (Zeneca) is an example, but other chemical manufacturers offer alternative products. An tipilling finishes Wear of synthetic fabrics may cause undesirable ‘pilling’by loose or broken fibres migrating to the fabric surface, where they form little knots or pills that are tenaciously held. Chemical finishing represents only one possible solution to the problem and it is not always fully satisfactory, Chemical finishing agents, e.g. polyacrylates and silicone elastomers, function by bonding loose fibres together at contact points, restricting their migration to the fibre surface. Antislip finishes Slippage of warp and weft yarns is a well known defect of loosely woven filament fabrics arising from the smoothness of synthetic yarns. The problem can be overcome by reducing fibre surface smoothness, thereby increasing interfibre friction, by applying silicic acid esters, sometimes in combination with polyacrylates.

The most popular blends are those of polyester with cellulosic fibres, either cotton or viscose, combining the excellent easy-care and hard-wearing properties of polyester with the comfort and freedom from static or soiling of cellulosic fibres.

8.5.1 Easy-care finishing The outstanding properties of polyester/cellulosic fabrics are widely recognised, with blend ratios varying from 50:50 to 70:30 . Cotton-rich blends have also been evaluated, but for such blends to be of technical importance a minimum of 35-40% polyester has to be present. As polyester has a price advantage over cotton, the 50:50 blend has been widely adopted.

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It has already been shown in section 8.3.4 that chemical finishing of 100% cellulosic fabrics represents a compromise between improvements in easy-care behaviour and losses in physical durability. Easy-care finishing of polyester/ cellulosic blends requires an even more difficult compromise, because the two constituent fibre types have diametrically opposite finishing requirements. Polyester fibres demand setting at high temperatures and no chemical finishing, whereas cellulosic fibres need to be chemically finished, but without exposure to such high temperatures. Although it might be thought necessary to resort to two distinct finishing steps, i.e. setting of the synthetic fibre followed by chemical finishing of the cellulosic component, simpler sequences are used in practice. Polyester/cellulosic fabrics are often heat set prior to dyeing to give dimensional stability during wet processing, or as part of the Thermosol process in the fixation of disperse dyes. In subsequent chemical finishing care is required to ensure that resins, catalysts and additives, as well as curing conditions, do not adversely affect the colour fastness of the dyed polyester fibres. Depending on the end use and the handle required, crosslinking reactants or U/F resin precondensates can be used. Reactant resins are often used on polyester/ cotton fabrics, where good easy-care properties coupled with good durability to laundering at 60°C are required. U/F resins may be preferred with polyester/viscose blends, where a resilient, springy handle is required with only moderate wash fastness. Resin precondensates, crosslinking reactants and additives have already been discussed in sections 8.3.4 and 8.3.5; those comments apply equally to polyester/ cellulosic blends. In calculating resin add-on to blend fabrics, it is important to base the calculation on the weight of the cellulosic portion in the blend and not on the total weight of fabric. In chemical finishing of polyester/cellulosic blends the weaker component, i.e. the cellulosic fibre, is further weakened. This is acceptable as long as the polyester portion of the blend compensates for the losses of tear strength and tensile strength. Likewise loss of abrasion resistance is tolerable in this way for white goods. With dyeings suitable additions must be made to the finishing liquors to minimise the change of shade that occurs when cellulosic fibre is lost by abrasion during wear.

8.5.2 Durable press Durable press is a generic term for a finishing process in which chemical or physical stabilisation of a fabric takes place after making-up in garment form. By delaying the final finishing step until making-up is completed, it is possible to stabilise the shape of a garment in its final saleable form. In durable press finishing a distinction is normally made between precure and postcure methods.

Precure This method is applicable only if the polyester content is at least 60% of the total fabric weight. Before making-up, the cellulosic component is chemically finished in

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fabric form, in the usual way. The polyester component is heat set after making-up, thereby imparting the desired shape to last the life of the garment. Figure 8.1 is a schematic representation of the precure method. Step 1 represents a conventional pad-dry-cure process using a crosslinking reactant with appropriate catalyst and additives. Step 2 represents making-up and hot-head pressing; as a result of the applied heat and pressure, the polyester component of the already resinfinished fabric is plasticised and moulded into the required shape. Hot-head pressing at 160°C is therefore the vital step which gives shape retention to the garment. Step 2

Step 1

Impregnation

Drying

Curing

I

Cutting Sewing

Pressing on hot-head press

Figure 8.1 Steps in precure process for durable press finishing

Postcure In this method blend fabrics are carefully impregnated with crosslinking reactant, catalyst and additives, but reaction between cellulosic fibres and crosslinking agent is prevented by appropriate selection of chemicals and close attention to working conditions. The chemically sensitised fabric is subsequently made up into garments, the required final shape of which is obtained by prolonged curing in an oven. Figure 8.2 is a schematic representation of the postcure process. In step 1 the blend fabric is treated with either a stabilised/buffered crosslinking reactant and a reactive catalyst or a reactive crosslinking agent with a stabilised/buffered catalyst. Fabric is dried after impregnation, at a sufficiently low temperature to prevent reaction between the cellulosicfibres and the crosslinking agent. In step 2 sensitised fabrics are made-up, the garments are pressed and then cured at elevated temperature in a specially constructed oven. In this final operation crosslinking of the cellulosic component and setting of the polyester take place simultaneously, giving the garment the shape that it retains for life.

Step 2

Step 1

53 Impregnation

Drying

Cutting Sewing

Pressing on hot-head press

Figure 8.2 Steps in postcure process for durable press finishing

Curing

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The postcure process removed the responsibility for curing or setting (albeit of garments and not fabrics) from the finisher to the garment maker, and this departure from normal practice caused considerable problems in the trade. It also took time to realise that all accessories used in garment making, i.e. linings, pocketings, waistbands and even sewing threads, had to have a similar response to the fabric panels so that the finished garments did not distort or pucker during hot-head pressing and oven curing. Garments finished to durable press specifications have to be dimensionally stable to washing and have good easy-care properties. The precure method has found acceptance as Fixaform and the postcure method is recognised under the Koratron trade mark.

8.5.3 Soil-release finishes The popularity of synthetics in blends with cellulosic fibres created a need for more effective washing methods. Blends, particularly those containing polyester or nylon, show considerable soiling that is difficult to remove even under the rigorous washing conditions used for cotton fabrics. This created a demand for finishes that facilitated the removal of soiling from blend fabrics. Two main types of finish are available to minimise soiling, as outlined below. (a) Stain- or soil-repellent finishes work by forming a barrier between fibre and soil. Such finishes are based on fluorocarbons that impart an oleophobic and hydrophobic character to the fabric surface (section 8.6.2). (b) Soil-release finishes decrease but do not prevent soiling from taking place. They facilitate soil removal during washing and laundering due to the presence of hydrophilic groups in the soil-release agent attached to the fibre surface. Certain soil-release agents function by preventing redeposition of the dispersed or emulsified soil from the wash liquor back onto the fabric. Hence the purpose of a durable soil-release finish is to facilitate the removal of soiling from polyester/cellulosic fabrics at moderate washing temperatures (40-60°C). This is achieved by combining the soil-release agent, usually an acrylate copolymer, with a crosslinking reactant, catalyst and other additives. In formulating recipes care is taken that all other chemicals are compatible with the soil-release agent and do not interfere with its effect on the substrate. Application is by the usual pad-dry-cure process (section 8.3.4).

8.6 SPECIALITY FINISHES 8.6.1 Water-repellent finishes Waterproofing is one of the oldest established finishing processes. In the early days waterproofing meant coating fabrics with linseed oil, glue and other materials, yielding stiff and tacky garments impervious to both water and air. Such fabrics were most uncomfortable to wear. Nowadays the rainwear trade produces water-repellent and shower-resistant garments that are no longer purely functional, but also satisfy consumer demands in respect of comfort and fashion. Rubberised waterproofs were discarded long ago and the consumer now requires water-repellent outerwear, i.e.

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rainwear, leisurewear and workwear, that not only offers protection from the elements, but also has an attractive appearance, a pleasing handle and is comfortable to wear. Several classes of chemical finishing agents are available to produce finishes that are water-repellent and yet permeable to water vapour and air, and the trade distinguishes between durable and non-durable water repellency. One of the early finishes (still used on canvas for tarpaulins and tentage and for reproofing after dry cleaning) is based on wax dispersions containing aluminium salts. On padding and drying the wax particles are deposited on the fabric surface and, together with the aluminium salts, form a water-repellent layer that penetrates to some extent into the fibre. The resultant finish shows good water repellency, is air permeable, but is removed on washing. The replacement of aluminium by zirconium salts, which complex more effectively with wax emulsions, results in an improved water-repellent effect with much better durability. Moreover, it is possible to combine paraffin wax/zirconium emulsions with easy-care finishing agents, based on either polymer-forming or crosslinking reactant resins. In fact simultaneous application of easy-care and water-repellent finishing agents provides enhanced water repellency and better durability to washing. Paraffin wax/zirconium salt emulsions are widely used because of their ease of application (pad and dry at 90-130°C), good technical effect, compatibility with other finishes and low cost. The availability of new compounds since 1940 heralded further developments in the field of water repellency. All these finishes had functional groups able to react with themselves, with other crosslinking agents and with cellulosic fibres. The waterrepellent effects were at least as durable to washing as those from paraffin wax/ zirconium finishes, but most of these new products were more costly and have since been replaced; therefore they will be mentioned only briefly. A water-repellent finish based on stearamidomethylpyridinium chloride was formerly used to proof cotton rainwear. Application was by the pad-dry-cure method, but processing had to be carefully controlled so that the hydrochloric acid liberated during curing did not degrade the cotton fabric. Another traditional type of water-repellent agent was a water-soluble organochromium complex. This gave good effects, but treated fabrics were discoloured as the chromium compound was itself green, so it was restricted to fabrics dyed to medium and dark shades. Other reactive water-repellent agents still used include products based on polymer-forming or crosslinking reactant resins. Typical commercial products are those based on fatty acyl-substituted methylolmelamines or monomethylolurea, an example of which is shown in Figure 8.3. This fatty acyl-substituted monomethylolNHR / O=C \ NHCH2OH

Figure 8.3 Typical reactive water-repellent agent, where R is a fatty acid group, e.g. C17H35CO

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urea compound is known to give excellent water-repellent effects on cotton and regenerated cellulosic fibres with good resistance to washing. Treated fabrics also have a soft handle. Such agents are particularly suitable for adding to an easy-care finish based on polymer-forming or crosslinking reactant resins; the combined finish reinforces the water-repellent effect and enhances resistance to laundering. A further development of this concept was Phobotex FT (CGY), a mixture of a hydrophobic agent and an aminoplast. It is applied by the usual pad-dry-cure sequence and is readily combined with polymer-forming or crosslinking reactant resins to impart both crease recovery and water repellency to treated fabrics. The development of siliconesin the 1950s represented the most important addition to the range of water repellents available to the textile finisher. Silicones have assumed considerable importance as the most effective and durable water repellent for natural, synthetic and blend fabrics. Moreover, they impart a smooth, silk-like feel to treated fabrics and garments, the so-called silicone handle. Commercial water-repellent silicones are based on polymerised siloxanes, primarily copolymers of hydrogenmethyl- and dimethyl-siloxanes, and are marketed as aqueous emulsions. Considerable expertise is required in formulating and manufacturing silicone emulsions. Early drawbacks included inadequate storage stability and emulsion breakdown caused by shear stress at the nip of a pad mangle. The choice of silicone catalyst affects: (a) Rate of crosslinking of polysiloxane chains (b) Water-repellent effect obtained (c) Resultant handle. Several catalysts are effective in crosslinking polysiloxane chains, but nowadays organometallic compounds, such as zinc alkylcarboxylates, zinc octoate, zirconium fatty acid derivatives and epoxy-amides or -amines, are mainly used. Zirconium catalysts can be incorporated in commercial silicone emulsions and additional zirconium or zinc salts are often added to the padding liquor. Organometallic compounds, such as fatty acid zinc salts, tend to impart a firm, full handle, with little risk of changes in shade of dyeings or yellowing in the case of white goods. The epoxy-amide or -amine catalysts are noted for their soft and supple handle and impart the highest water repellency with optimum wash fastness, but slight changes in shade may occur, depending on the dyes used. The mode of action of the catalyst is to orient the polysiloxane chains along the fibre surface. The hydrophobic methyl groups face away from the fibres and repel water molecules, whilst the silicone groups are anchored to the fibre surface by the organometallic compounds (Figure 8.4). A prerequisite for successful silicone finishing is for the fabric to be free from size and surfactants used in preparation and dyeing, such as wetting agents, detergents and softeners. All traces of hydrophilic agents must be removed as they mask the water-repellent effect. Cleanliness is vital in silicone finishing, and this applies to all equipment used. The general method of applying a silicone finish is to pad the silicone emulsion containing catalyst at room temperature, dry at 100-120°C and cure for 3-4 min at 150-155°C. This procedure is applicable to any fabric irrespective of fibre type.

125

0:@:

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O

Figure 8.4 Orientation of a siloxane polymer on a textile fibre

Cellulosic fabrics and blends show a marked improvement in water repellency and wash fastness if the silicone finish is combined with polymer-forming or crosslinking reactant resins to impart a combined easy-care and water-repellent finish, as previously described. 8.6.2 Oil- and soil-repellent finishes

Finishes imparting oil, stain and soil repellency are less important than the related water-repellent finishes. Their relatively high cost has mitigated against their adoption. Fields of application include workwear, military and automotive fabrics, carpets, primary upholstery and loose covers, table linen, etc. Typical products are based on fluorinated hydrocarbons, fluorinated acrylate esters, chromium complexes of perfluorocarboxylic acids and others. Considerable effort has been devoted to developing and extending the range of these chemicals. Their mode of action is similar to that of silicones, insofar as they form a hydrophobic and oleophobic film around fibres that is anchored to the fibre surface, the oil-repellent groups preventing ingress of oil, stains and soil into the fibre. A further water- and oil-repellent finish, using a fatty acylated methylolurea as a water repellent, has been developed for outerwear. The combined finish enhances the hydrophobic and oleophobic effects. 8.6.3 Flame-retardant finishes Early work on flame retardancy, mainly for military purposes, was primarily concerned with limiting flame propagation. More recently greater emphasis has been given to ana lysing the smoke and toxic gases evolved during burning. Processes for making cellulosic fabrics temporarily flame-repellent, based on ammonium phosphate and other inorganic salts, were soon established, but for apparel uses durable finishes were required that met well defined standards. Flame-retardant finishing of cotton and blends, to impart self-extinguishing behaviour in normal wear and after repeated launderings, creates problems far more complex than those encountered in any other finishing process. Firstly the amount of add-on, considerably more than required for any other functional finish, tends to impair handle and other desirable fabric characteristics, such as easy-care properties. Secondly process control has to be rigorous to ensure uniformity and reproducibility of finish, coupled with stringent laboratory testing to exacting standards. This is an expensive finishing process that depends on careful quality control.

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The most important group of flame-retardant compounds contain phosphorus; they function by decomposing into chemical species that alter the thermal degradation reactions of the substrate, decreasing the concentration of combustible products and increasing the amount of char produced. Phosphorus-based flame retardants only function effectively if the fibre structure is capable of undergoing transformation to char; melt dripping tends to prevent such transformation in synthetic fibres. It has also been shown that phosphorus-containing flame retardants function more effectively in conjunction with nitrogen-containing compounds. Nitrogen actually has an additive or a synergistic effect on the performance of phosphorus-containing compounds. Nitrogen-phosphorus systems are thought to act mainly in the solid phase by a mechanism of dehydration of the cellulosic fibre, modifying the pyrolysis so that only small amounts of combustible volatiles are formed. The second most important group of flame retardants are those containing halogens, notably chlorine and bromine. These are thought to operate mainly in the vapour phase by releasing hydrogen halide gas (HCI or HBr) and free radicals that suppress the flame reactions. The efficiency of these retardants is enhanced by the presence of either phosphorus or antimony (as antimony oxide). Both antimonyhalogen and phosphorus-halogen systems are reported to be synergistic. Table 8.2 summarises commercially available forms of durable flame retardants. Table 8.2 Durable flame-retardant finishes for cotton

Chemical type

Sample trade names

Nitrogen-phosphorus systems THPC or THPOH Phosphonopropionamide Vinylphosphonate

Proban (Albright & Wilson) Pyrovatex CP (CGY) Fyrol 76

Antimony-chlorine systems Antimony oxide + PVC, PVDC or polychloroprene

Timonox

Tetrakis (hydroxymethyl) phosphonium chloride (THPC) Phosphonium derivatives are one of the most important groups of durable flame retardants currently used, in particular on cotton. These agents are based on phosphonium salts having the general formula [(CH2OH)4P+Xn-, where X n- can be Cl - or OH -. Commercially the most important agent is tetrakis(hydroxymethyl)phosphonium chloride (THPC). The early technique for making cotton flame retardant was based on THPC applied by a pad-dry-cure process. The fabric was padded with an aqueous solution containing: (a) THPC and triethanolamine (to absorb any liberated hydrogen chloride) (b) Trimethylolmelamine and urea; after drying, curing took place at 140°C for 4-5 min.

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Disadvantages included release of formaldehyde, fabric stiffening and tendering. Fabric tendering was overcome by development of an ammonia-diffusion process, which suffered from low processing speeds and the need for a separate large ammonia curing unit. In the 1970s the Proban company developed an ammonia cure unit that was compact, of simple construction, cheap and capable of efficient curing at speeds of 60 m/min. The processing sequence of the present Proban process is: pad-dry-cure-oxidise-finish. The Proban padding composition is commercially available as a precondensate of THPC and urea, probably in a 2:1 molar ratio with a P:N ratio of 1 :1. The precondensate is padded, dried and then passed through an ammonia gas curing reactor, which exothermically forms a highly crosslinked phosphorus-nitrogen polymer within the fibre structure. To achieve satisfactory flame retardancy, the weight percentages of phosphorus and nitrogen in the polymer structure should be greater than 2%, depending on fabric density and construction. The ammonia-cured fabric is oxidised with hydrogen peroxide or sodium perborate to enhance the durability of the finish to commercial laundering, UV radiation and weathering. Further improvements in handle can be made using a softening agent in the final finish followed by compressive shrinkage. THPC or THPOH flame-retardant finishes are degraded by sodium hypochlorite, so Proban-treated fabrics or garments must not be bleached with chlorine-containing bleaching agents. N-methyloldiakylphosphonopropionamides Research carried out in the United States in the late 1960s showed that the hydrolytic stability of the phosphorus-carbon bond was greater than that of the phosphorusnitrogen bond. This led to the rapid development of the amido derivatives of methyland chloromethyl-phosphonic acids as flame retardants for cotton. Ciba-Geigy showed that effective flame retardants could be based on N-methyloldialkylphosphonopropionamides, and N-methyloldimethylphosphonopropionamide was chosen as the commercial flame retardant, marketed under the trade name Pyrovatex CP (CGY). The reaction with cotton can be represented by Scheme 8.1. O

cell-OH +

II

HO-CH2NHCO CHCH2P(OCH3)2 I CH3 -

O II

cell-O-CH2NHCO CHCH2P(OCH3)2 CH3

Scheme 8.1 Pyrovatex CP is applied with a methylolated melamine resin in the presence of phosphoric acid as a catalyst by a pad-dry-cure technique. To achieve the high levels of phosphorus (2-3%) required for durable flame retardancy, some fabric stiffening (depending on fabric weight and construction) may occur. Furthermore, to avoid an unacceptable loss in strength, efficient neutralisation using an alkaline aftertreatment is essential. This flame retardant functions as a condensed-phase retardant and promotes char formation in an analogous way to Proban.

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Antimony-chlorine systems Flame-retardant industrial fabrics must be able to prevent flame propagation and afterglow even if exposed for prolonged periods to weathering or to severe industrial environments. Handle is of lesser importance, but excessive fabric stiffness must be avoided. Processes developed in the 1950s and 1960s were based on antimony oxide and a halogen donor such as chlorinated paraffin, poly(vinyl chloride) or poly(vinylidene chloride). Afterglow was eliminated by incorporation of a phosphate or borate in the recipe. The finish was applied by a simple pad-dry procedure. It was never suitable for apparel fabrics because of the high add-on required. Flame-retardant finishes for polyester No success has yet been achieved in developing a flame-retardant finish for polyester fabrics that compares with those used for cellulosics. This is due to the difficulty of achieving high levels of finish add-on, and problems caused by softening and melting. Although polyester fibres have inherent flame retardancy, as shown by their ability to shrink away from an igniting source and to melt drip with consequent removal of energy from the flaming textile, this may cause an additional hazard by transferring flame and heat to another site, e.g. to adjacent or underlying fabric layers as well as to the skin of the wearer. Consequently, an effective flame-retardant finish for polyester fabrics should: (a) Promote char formation by catalysing pyrolysis (b) Enhance melt shrinkage but minimise the drip phenomenon. At present no satisfactory commercial finish exists, despite considerable research effort. In the 1970s considerable success was achieved with ‘tris’ or tris(2,3dibromopropyl) phosphate. Tris had to be withdrawn when toxicological testing showed that it was carcinogenic. Flame-retardant treatments for polyester/cotton blends The problem of finding a suitable flame retardant for polyester/cotton blends is even more difficult, because synthetic fibres (polyester) blended with char-promoting cellulosic fibres (cotton) create an increased hazard, the so-called ‘scaffolding effect’, where the molten component wicks onto the char of the cellulosic fibres and fuels combustion. To date satisfactory flame-retardant finishes can only be obtained on ‘cotton-rich’ blends, i.e. blends where the polyester component does not exceed 30% of the total fibre weight.

8.6.4 Bactericidal and fungicidal finishes Finishes that impart resistance to micro-organisms (bacteria and fungi) can be classified into three groups. Firstly the cellulosic fibre may be chemically modified to resist attack, e.g. by acetylation or cyanoethylation, but this approach is not within the competence of the finisher and can therefore be discounted. Secondly a resin that forms an impermeable barrier to micro-organisms may be applied to the fabric. This approach is used with industrial fabrics, e.g. tarpaulins and

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tentage, where appearance, permeability and porosity are secondary factors. More important are resin precondensates, e.g. halogen-substituted phenol-formaldehyde or N-methylolmelamine derivatives, followed by polymerisation inside the fibre structure. Cotton treated in this way has excellent resistance to micro-organisms. Thirdly an active antimicrobial finish can be deposited in the cellulosic fibres that is effective both as a bactericide and a fungicide. The finish must be effective, nondiscolouring, durable to washing and non-toxic to humans. Reagents such as copper naphthenate are effective but impart a green colour to treated fabrics. Tin compounds are colourless but exhibit varying degrees of toxicity. Mercury compounds have been ruled out essentially on environmental grounds, though phenyl mercuric acetate is most effective, even if applied at very low concentration (0.01%). Products established until recently included halogenated phenols, e.g. pentachlorophenol, copper-8-quinolinolate, N-(tributylplumbyl)imidazole, 4,4'-dihydroxyoctachlorodiphenyl diacetate and quaternary ammonium compounds.

8.7 METHODS OF APPLICATION 8.7.1 Conventional padding The general method of applying finishing agents to fabrics is by padding in open width. The padding operation consists of two steps: (a) Impregnation, i.e. guiding the fabric in open width in and out of the finishing liquor contained in a trough (b) Squeezing, i.e. passing the impregnated fabric through the nip formed by two bowls of the pad to leave a specific quantity of liquor on the fabric. The mechanical design of pad mangles, the pros and cons of two-bowl and threebowl pad mangles, the construction, composition and hardness of the bowls have already been covered in Chapter 6. Although padding is basically a simple operation, for it to be successful attention should be given to the following: (a) Fabrics must be uniformly wettable, free from creases, knots and foreign objects, e.g. pins, to prevent damage to the bowls (b) Fabrics must be batched with controlled tension and threaded correctly through the padder; slack or tight lengths of fabric lead to uneven running and cause problems at the nip (c) Pad mangles must be in good mechanical working order; pressure rams must be frequently checked for sticking and must deliver a uniform and reproducible pressure across the width of the bowls (d) Wear of bowls must be recognised and remedied; if the squeezing effect across the width of the bowls is not uniform, the pick-up by the fabric will vary, leading to lack of uniformity of the finished effect, requiring repair of the bowls. The pick-up of finishing liquor retained by the fabric in padding is referred to as the ‘expression’. It is the weight of liquid retained by a textile material after mangling, calculated as a percentage of either the air-dry or the oven-dry weight of the goods. Depending on construction, density and other fabric properties, as well as the speed

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of padding, hardness of the bowls, the pressure applied, etc., an average expression for a 100% cotton fabric is of the order of 60-75%, a 100% viscose fabric 80-95%, and a 70:30 polyester/cotton fabric 50-60%. Although in the following section alternative techniques are described, it should be borne in mind that an overwhelming number of finishers still use pad mangles because of their simple design, robustness and uncomplicated operation.

8.7.2 Low wet pick-up processing The initial objectives of low wet pick-up processing were to: (a) Seek improvements in the control of resin finishing (b) Promote distribution of resin precondensates or crosslinking agents in the fabric (c) Achieve better balance between easy-care and loss in wear properties. More recently other aspects of low wet pick-up processing, such as water and energy conservation, have assumed considerable importance. Investigations carried out by Triatex International showed that the relationship between easy-care performance and loss in wear properties could be beneficially influenced at the application stage of the easy-care treatment. By applying resin precondensates or crosslinking agents at higher concentration, migration of nonsubstantive finishing agents could be drastically curtailed. Triatex found that buildup of finishing agent on the fibres and embrittlement could be minimsed by reducing the liquor in the system to below 40% on the weight of the cotton. Further to this improved ratio of easy-care to wear properties, low wet pick-up processing allowed savings of chemicals and water, making it possible to streamline the drying and curing steps into one operation on the stenter. In the following sections various novel application methods are briefly described, but it should be noted that each system has advantages and disadvantages. The advantages claimed for low wet pick-up systems are summarised in Table 8.3. Kiss or lick roll application The first commercial low wet pick-up system was developed by Triatex International of Zürich and is known as the Triatex MA (minimum add-on) system. It is based on the well known technique of applying liquors by means of a lick or kiss roller. The novelty is that the speed of the lick roller and the fabric are controlled independently and this difference in speed enables the wet pick-up level to be carefully adjusted and controlled. Much background work had to be carried out by Triatex and auxiliary manufacturers in screening softeners, handle modifiers, etc. to develop non-foaming formulations capable of producing a liquor film of uniform thickness across the width of a lick roller. The major problem faced by Triatex was variation of the wet pick-up value. It was solved by incorporating P-particle gauges that continuously monitor the mass of textile material both before and after liquor application. The mechanism involved in transferring the liquor film from the lick roller to the fabric is fairly complex. In essence, when the fabric makes contact with the roller, the liquor film breaks up into small droplets. For uniform application these droplets of liquor must distribute themselves

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Table 8.3 Advantages and disadvantages of low wet pick-up systems

Advantages

Disadvantages

Economical use of chemicals Uniform distribution of agents in the fabric Better penetration of agents into the fibres Rapid processing, hence higher productivity Savings of water and energy (less water to evaporate in drying) Low volumes of effluent to treat or discharge

No universally applicable system (compared with padding) Considerable technical supervision required High capital cost of equipment

evenly throughout the thickness and plane of the fabric. The lowest wet pick-up value at which an apparent uniform distribution of finishing liquor is obtained is known as the critical application value (CAV), and for MA and other low wet pick-up systems a CAV of 35-40% is recommended. Extremely thorough preparation is required for cotton and blend fabrics destined for MA treatment. Mercerising leads to an increase in the CAV. Fabric construction affects the rate at which the droplets distribute themselves by capillary wicking. Closely woven fabrics made from fine regular yarns such as shittings have an excellent capillary network and therefore low CAVs, whereas more loosely woven fabrics made from less regular or coarse yarns have an inferior capillary network and tend to have higher CAVs. Curved-blade applicator Low wet pick-up application from a curved blade is represented by the West Point CBA system. The CBA (curved-blade applicator) system was developed by West Point Pepperell and the procedure is as follows. The finishing liquor is pumped to a distribution manifold and made to form a uniform film on an accurately machined curved blade. The liquor film flows down the curved blade and transfers to the fabric at the blade/fabric interface. The rate of application is regulated by a process control computer; this monitors fabric speed and regulates liquor pump speed so as to produce and maintain the desired wet pick-up level. Engraved or spiralled rollers Engraved-roller application systems have been adopted where long runs of identical or similar fabrics are available for finishing. The wet pick-up is controlled by the depth of engraving on the application roller and the type of doctor system in use. Machinery manufacturer Johannes Zimmer uses a spirally engraved metering or doctor roller to regulate the thickness of the liquor film on the applicator roll. The liquor film is subsequently transferred from the applicator roll to the fabric by contact. The transfer and distribution of the liquor within the fabric is controlled by magnetic rollers.

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Another machinery manufacturer, Max Goller, whose low wet pick-up system is marketed under the name Mini-Fluid, uses a magnetic device to control the metering roller, followed by a pneumatic nip to assist transfer and distribution of the liquor within the fabric. Vacuum applicators Vacuum systems maintain precise control over the volume of chemical liquor that remains in contact with the fabric. The vacuum slot pulls the finishing liquor into the fibres and removes the excess liquor to be recirculated. Commercially available vacuum systems include: (a) Vacuum Pad Applicator (Textile Vacuum Extractor, USA) (b) Evac Vacuum Applicator (Evac, USA) (c) Aqua-Vat (Babcock Engineering, UK). Spray applicators Application of finishing liquors by spraying is another approach to low wet pick-up processing. Commercially available spray units include: (a) SD applicator (Farmer Norton) (b) Weko minimal applicator (Weitmann & Konrad) (c) Spraymiser (Burlington Textile Machinery) (d) Rotajet (Bruckner). In the Farmer Norton and Weko units the spray is generated by pumping the finishing liquor to the centres of rapidly revolving discs or rotors, the direction of spray being controlled by baffles. W ith precise adjustment of all variables, e.g. rate of delivery of finishing liquor by metering pumps, angular velocity of revolving discs and fabric speed, a uniform distribution of fine droplets of finishing liquor over the full fabric width results. Application can be either single- or two-sided.

8.7.3 Foam application In the minimum add-on (MA) process, droplets of finishing liquor are distributed evenly throughout the fabric by capillary wicking. It was later realised that by using air to increase the volume of applied liquor, a more even application and therefore a better final distribution of finishing liquor would result. Although spraying was a step in the right direction, in the event foam finishing proved to be a moreversatile method. Foam is an emulsion of air in liquid. To produce a stable foam a surface-active agent must be present that reduces surface tension. Foam finishing is probably the easiest of the various low wet pick-up techniques to install and operate in conjunction with existing machinery. The basic equipment includes a foam generator and a foam applicator. In a foam generator flows of air and liquid are metered and the foam is produced by high-speed rotors. In applying foam to fabric it is necessary to control the rate of application, as for a given blow ratio (i.e. amount of air in the foam) the volume of foam applied determines the pick-up. Several techniques exist by which foam can be applied to a fabric.

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Direct method using pressurised foam The foam, contained under pressure in a distribution box or manifold, is applied directly to the substrate. With Gaston County’s FFT unit the fabric picks up the foam by passing over a slot of variable size. In the Stork RSF system the foam is applied through a rotary screen with the fabric pressed against a backing roller. A microprocessor monitors and controls blow ratio and metering of the foam. Direct method using non-pressurised foam The foam reservoir is not maintained under pressure, but pressure is used to apply and to destroy the foam. The Texicon Autofoam unit is a refined version of a knifeon-air unit, the foam generator being of a special design. Once the required details of fabric weight and foam pick-up are fed into the computer, foam delivery is automatically controlled so as to compensate for changes in stenter speed. Resin finishes have been successfully applied to furnishings and fashion fabrics using an Autofoam unit. The knife-over-roller applicator developed by United Merchants and Manufacturers (USA) is widely used. It requires fairly stable foams that have to be destroyed after the application stage. The same company has also developed a foam applicator using a horizontal pad technique. Indirect method The principle of indirect metering of foam onto a carrier roller or belt was adopted by several machinery manufacturers, notably Küsters, Monforts and Babcock. In the Küsters Janus machine the foam is doctored onto a drum and transferred to the fabric by contact. The Monforts Vacu-Foam machine uses a knife-over-roller to meter a foam layer onto a rubber blanket. Transfer of the foam onto and into the fabric is assisted by vacuum as the fabric passes round a perforated drum. Major advantages of foam finishing are: (a) Even application leading to uniform distribution of finishing agents: with easycare finishes an improved balance of fabric properties may be obtained (b) Maximum utilisation of chemicals (c) Minimum add-on: pick-up is reduced from 65 to 15% (d) Greater productivity: increases from 25 to 50% are possible (e) Energy savings: less water has to be evaporated and lower temperatures are possible in drying (f) Wet-on-wet application: possibility of multiple applications without intermediate drying.

CHAPTER 9

Support services and their contributions to overheads

9.1 SURVEY In this series of monographs it has been customary to consider the economics of the preparation and dyeing of the relevant substrates. Whilst this book is no exception, it should be stressed that what applies in a dyehouse in one part of the world may not be applicable in another, bearing in mind the ever-changing marketplace, legislation and differing local and national circumstances. Thus no attempt has been made to place specific values on particular process routes. Instead attention is focused on the general features of support services that contribute to the overhead costs at any site. There may or may not be a plentiful and inexpensive source of water. It may be possible to discharge effluent with no additional costs directly into the local sewer or alternatively, and ever more likely, it may be necessary to bear the cost of its treatment either on site or by a municipal authority plant. In health, safety and environmental matters certain locally prohibited products and procedures are not yet universally restricted. Some products banned in the UK, such as biocides based on chlorinated phenols, are frequently present in imported materials. Where local legislation controls its disposal, this results in serious financial constraints for the companies handling and processing such fabrics. On the other hand, education in the correct handling and storage of dyes and chemicals, and typically control of dye dusting characteristics, during the last decade has become generally accepted as a necessary basic requirement. The economics of batch and continuous processing and consideration of which is financially the more attractive have been discussed to elsewhere. To survive in a competitive market the modern dyehouse has to meet the needs of frequent fashion changes, and it is no longer uncommon to find a continuous dyeing technique being used for what in the past would have been a batch size lot. Except in the case of a green-field development, whichever route is followed today is determined by the equipment available and the experience and know-how of staff and operatives, and their determination to be more energy-efficient and economic. Most fundamental are the needs to be quality conscious and efficient in energy, machinery and manpower utilisation. A significant contribution in all this is the frequently undervalued role of a properly equipped and managed laboratory, including the now universally established computer match prediction facility. The laboratory is no longer just a support to the modern dyehouse; it should be the core and spearhead of process efficiency. 134


135

Laboratory equipment is inexpensive and there is no difficulty in evaluating its contribution. Pay-back time on a necessary investment can usually be calculated in months rather than years. For batch processing the investment is likely to be largely confined to dyeing and testing, as distinct from preparation machinery, because it is desirable to prepare the substrate for dye recipe formulation in the same way as for subsequent production dyeing. Exhaust dyeing and pad-batch systems are available from the various specialist manufacturers in this field, but the key to success is largely one of operative education and competence. The laboratory dyeing machine must be capable of imitating all process parameters available in the dyehouse. An efficiently equipped and managed laboratory will determine the optimum process parameters; management must ensure that the production department can match these parameters.

9.2 ENERGY CONSUMPTION IN THE DYEHOUSE The energy consumed in a dyeworks makes an appreciable contribution to processing costs, representing typically 15-20% of the total. Quite apart from the preservation of natural resources, this cost is itself sufficient justification for efforts towards energy saving. The ways in which energy may be saved are many, but it can be extremely difficult to evaluate the effectiveness of such measures, or indeed to assess the efficiency of energy usage at a given site. Energy-saving measures are best assessed by their benefit at that site, and in doing this due allowances have to be made for the effects of changes in operating practice, which may prove difficult. Although individual conservation measures may be numerous, there are three distinct areas where savings can be applied: before, during and after processing. Different conditions apply to energy saving in these and it is important to appreciate their significance.

9.2.1 Before processing This area is essentially concerned with steam raising and therefore boiler efficiency, heat recovery from flue gases, steam-line insulation, steam trapping, and heating and energy usage in offices. In implementation two important considerations apply. Firstly energy-saving measures correctly applied will have no adverse effect on the quantity and quality of steam required for production. Secondly the equipment employed in energy conversion is common to many industries, and both equipment and techniques can benefit from the scale of widespread application.

9.2.2 During processing Energy savings during processing, which include machine insulation, impinge directly on production processes. It is therefore important that production personnel help to decide possible operational changes to minimise energy consumption. Actions such as reducing water volumes, lowering temperatures or shortening process times are industry-specific. Any developments in this area should result from a pooling of ideas.

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9.2.3 After processing It is here where an attempt is made to claw back waste heat before it leaves the site completely. There is normally little scope for heat recovery from general energy dissipation, the only sources of recoverable heat being ducted hot air (including water vapour) and hot effluent. Although various techniques have been developed for heat recovery from exhaust gases, aqueous effluents carry the largest waste heat load and are more easily managed. While heat recovery from the total mixed effluent flow into the incoming cold water is practicable, the problem is more conveniently tackled for individual machines or groups of machines. Batch processes are less easily managed than continuous ones, since their effluent discharges occur intermittently and at high rates. Thus consideration must be given to the destination of the recovered heat and the form in which it is to be reused. An effective solution is to arrange for a group of machines to release their hottest effluents into a holding tank, which is subsequently discharged to waste through a heat exchanger. Energy cost savings are not of course confined to heat saving and recovery. Energy audits will highlight other areas that may not immediately come to mind, e.g. alternative fuel sources, their availability and the balancing of electricity charges so that the maximum demand load may be more economically managed.

9.3 WATER AND EFFLUENTS 9.3.1 Water usage Water consumption in batch processing is dependent on the type of machine employed, the complexity of the processing sequences and the machine loading, since many machines require a fixed water volume irrespective of the fabric load. The total water usage, including steam raising, washing down of machinery and so on, is likely to fall within the range 100-120 I per kg of fabric processed, but for the reasons stated a works may have a figure outside this range without deserving criticism for its extravagance or praise for efficiency. It is clearly sensible to eliminate obvious causes of waste such as running taps and overflow rinsing, and where practicable to employ machines and processes requiring less water, but it should be borne in mind that water is a renewable resource. There may be increasing demands on supplies in some geographical areas which may affect a works’ policy, but in most instances water represents only a small cost item, rarely more than 1-2% of total processing costs. It is therefore unwise to imperil the satisfactory outcome of a dyeing by over-eagerness to save water. A redyeing could not only involve costs much greater than that of the water saved, but actually require substantial additional water usage. When water is heated the associated energy consumption introduces a more significant problem, both financially and environmentally. A further proviso should be made in respect of boiler feed water in view of the treatment it requires. It is more valuable than untreated water, and efforts should be made to conserve it by returning to the boiler as high a proportion as possible of any condensate produced.


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9.3.2 Effluents Some water used in a dyeworks is lost by evaporation and leakage, but typically as much as 95% is discharged as aqueous effluent in a more or less contaminated form. Most of the dyes and finishing agents applied will remain on the fabric. However, virtually all the auxiliaries will be discharged in the effluent, mixing with the substances extracted from the greige goods during preparation. It is thus possible to obtain an indication of the average effluent load from a knowledge or estimate of these factors. Apart from instances of concern about specific individual chemicals, the information obtained in this way is most useful as a cross-check on the direct measurements of effluent composition, where the commonly used criteria are the concentration of: (a) Suspended solids (b) Total dissolved solids (TDS) (c) Dissolved organic matter. The dissolved organic matter in the effluent is more often determined as chemical oxygen demand (COD) rather than the time-consuming and more variable biological oxygen demand (BOD). Effluent treatment The aqueous environment into which treated effluents are discharged is tolerant to modest concentrations of inorganic salts, but calls for a pH close to neutral. It is much more demanding in respect of dissolved organic matter, the limit being largely related to the solubility of oxygen in water. The solubility of gases in water is extremely low, the saturation concentration of oxygen at 15-20°C being about 10 mg/l; the acceptable concentration of (biodegradable) organic matter should be of similar magnitude. The environment is highly sensitive to acute toxins, e.g. organic poisons or heavy metals, and has a less predictable sensitivity to certain persistent chemicals posing a long-term hazard. Against this background, typical untreated effluents may be acceptable in respect of their inorganic content, although likely to require pH correction. In most instances they carry no significant concentrations of acute toxins, but concern is felt over the long-term dangers associated with certain organic pollutants. It is in relation to degradable organic matter that textile effluents are often in excess of the acceptable level and require treatment. Concentrations of organic matter in textile effluents are comparable to those in domestic sewage and therefore the introduction of mandatory effluent treatment for textile wastes adopted techniques long-established for sewage treatment, either onsite or with domestic and other wastes at a sewage works. Promotion of the microbiological breakdown of organic matter by providing sufficient quantities of the oxygen required for metabolism is the common feature of techniques for sewage treatment. The processes are essentially biological and thus differ radically from the chemical processes familiar to dyeworks staff. A works that establishes its own treatment plant, whether from choice or because a sewage works cannot be used, must become involved in this science.


Where the choice is available, the arguments strongly favour delivering the effluent to a sewage treatment works. This will involve a charge and conditions imposed in respect of pH and temperature of the effluent, as well as colour standards and limitations on specific substances. Furthermore, it is inevitable that the list of prohibitions will continue to increase in the future. Nevertheless, the advantages of having the treatment done by professionals who accept responsibility for processing an accepted waste outweigh the disadvantages. Whether and for how long such a facility remains available is a matter of conjecture. Effluent recycling Irrespective of whether the effluent is treated on-site or at a sewage works, it represents an additional cost, incurred simply for legitimate disposal. It is natural then to consider reuse of the treated effluent, but water that is suitable for return to the environment is seldom acceptable for immediate reuse in preparation or dyeing. If process water is to be reused, the most appropriate treatments are physical or chemical, such as ultrafiltration, reverse osmosis, or adsorption on activated carbon or special resins. These processes are probably best applied to selected effluents only, such as the cleaner rinse water. At the present time the costs associated with treatment and recovery are generally likely to exceed the cost of fresh water. Recovery of cleaner effluents only will still oblige the works to discharge most of its waste. Ambitious schemes in which the whole of a works effluent may be recovered by multiple-effect evaporation, with the waste matter discharged as a relatively dry solid, have been considered and one or two examples constructed. Such schemes appear to be appropriate only under exceptional circumstances. 9.4 HEALTH AND SAFETY It is now a requirement that the workplace, be it a bleachcroft, dyehouse, printworks or finishing shed, should have eliminated the hazards caused by machinery, products and processes. In the foreword to the UK Health and Safety Executive publication Essentials of health and safety at work, it is stated that over 500 people die each year at work and several hundred thousand lose time through injury or illness. Quite apart from the suffering to workers and their families, the cost to a business can be huge. The Health and Safety at Work Act provides a duty to all those engaged in industry to improve the working conditions and protect the environment. Nevertheless, accidents that could be prevented still occur and are frequently caused by human error, ignorance and poor housekeeping. Running machinery always involves some risk and it is therefore management’s responsibility to train all operatives adequately and to ensure that they are fully conversant with all aspects of individual machines. Health and safety regulations make it compulsory for all moving parts of machinery to be guarded. Pad mangles, the cause of many avoidable accidents, must be fitted with effective nip guards, and emergency stop buttons must be clearly visible and handily sited.


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Electrical equipment must be installed in accordance with standards currently in force and used within the limits specified by the manufacturer, e.g. voltage, current, temperature, etc. as well as protection against impact, vibration, fumes, vapour or fluid. Electrical equipment should provide protection against electric shock, i.e. direct contact with ‘live’parts. This protection may be in the form of an enclosure or the insulation of the equipment. Further standards exist encompassing the use of electrical equipment in hazardous environments where vapours, gases or dust would be present. The three chemical hazards most commonly encountered are explosion, fire and exposure to toxic chemical substances. Of these explosions are relatively rare in dyeing, printing and finishing works, but fires occur only too frequently, sometimes with disastrous consequences. Escape routes and fire-fighting equipment must be readily accessible, and all personnel must be trained in and familiar with its use. It is well known that certain chemicals used in preparation, dyeing and finishing are dangerous if handled incorrectly or carelessly. Personnel engaged in such wetprocessing operations are usually aware of the risks from strong acids, caustic alkalis, solvents and noxious fumes and vapours. Yet hazards associated with less familiar other products and processes may be less obvious and therefore not recognised, particularly if harmful effects only become apparent over a prolonged period of time. Dye and chemical manufacturers have contributed significantly in preparation and presentation, ensuring that products are simple and safe to handle, and that packaging is easy to dispose of or recycle. The use of respirators, dust extraction booths and mobile trolley systems for chemical transportation have all contributed to ensuring a safer working environment.

Glossary Adduct The addition product or a reaction between molecules. Bathochromic effect A shift of the absorption spectrum of a dye towards the red end of the spectrum. Cheek The end flange of a filled mangle bowl. Ending A dyeing fault consisting of a change in colour from one end of a length of fabric to the other, commonly used with reference to batch-dyed material. Interfibrillar swelling Swelling which results from the penetration by water (or other polar solvents) of cotton fibres. The swelling agent may break hydrogen bonds between adjacent chain molecules. lntrafibrillar swelling A more intensive swelling, which can result in a molecule rearrangement of the cellulose chain molecules. lnterfibrillar swelling always precedes intrafibrillar swelling. Jet (1) Any machine in which some form of jet nozzle is used to provide vigorous liquor/fabric interaction and fabric transport. The term is sometimes applied to other machines in which circulating liquor is used to provide fabric movement (e.g. overflow machines). (2) A general term to describe a mechanism which employs the pressure of dye liquor to provide fabric movement in a dyeing machine. Mechanical details vary, but typically the nozzle comprises a ring through which the fabric passes, with the liquor directed inwards and in a forward direction (see Venturi). Labile complex An unstable (liable to change) coordination compound. Listing An uneven dyeing effect consisting of a variation in colour between selvedges and the centre of a dyed fabric.. Merchant converter A supplier of finished fabric who buys loom-state material and commissions its further processing. Neppy dyeings Clumps of tangled thin-walled immature cotton fibres (neps) scattered throughout the surface of cotton fabrics, which resist dye uptake and appear as white of pastel-coloured specks on the dyed fabric. Venturi A particular type of jet in the form of a tube which is narrower in the centre and wider at the ends. This shape produces a higher rate of fluid flow in the central region.

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