Practical Dyeing Volume 1 - Dye Selection and Dyehouse Support By James Park and John Shore
2004 Society of Dyers and Colourists
Copyright © 2004 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.
ISBN 0 901956 84 8
Contents Volume 1 – Dye Selection and Dyehouse Support Chapter 1 Globalisation of Textile Coloration and Related Industries
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Chapter 2 Impact of Dyeing And Finishing on the Environment
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Chapter 3 Services and Resources
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Chapter 4 Control, Automation and Robotics
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Chapter 5 Product Evaluation
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Chapter 6 Colour Communication, Colorimetry and Match Prediction
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Authors’ Preface The original idea of practical monographs was conceived in the 1970s as a result of an on-going debate as to what constituted a practical paper and the lack of such papers within the pages of the Journal of the Society of Dyers and Colourists. There is of course no absolute definition of a practical paper since this depends on the interests of the individual reader, location within the industry, topicality and, not least, the burning issues of the day. The Society of Dyers and Colourists attempted to rectify this lack of practical information by encouraging such papers for publication in the Journal, as well as initiating a series of practical monographs, authored by experts in various areas of textile coloration. Between the years 1981 and 1993, nine such monographs were published. Only two of these are still available: 1. Batchwise Dyeing of Woven Cellulosic Fabrics, by G W Madaras, G J Parish and J Shore (1993) 2. Instrumental Colour Formulation, by J Park (1993). For several reasons, not least the diminishing educational resources available for textile coloration, sources of practical, current information are increasingly required. This was the incentive behind the production of this practical e-book intended to assist practitioners occupying ‘hands-on’ positions at all levels within the industry. Copious recent references are included in each chapter. Two further e-books by the current authors will augment the information in this publication: 1. Dyeing Laboratory Practice, by J Park and J Shore (in preparation) 2. Dyehouse Management Practice, by J Park and J Shore (in preparation).
Chapter 1 Globalisation of Textile Coloration and Related Industries 1.1 Impact of the Oil Crisis on Global Fibres Production Although religious tradition [1] describes that, having eaten of the forbidden fruit of the tree of knowledge, man required some form of cover for reasons of modesty, it is more likely that primitive Homo sapiens required protection from the elements. In earliest times, skins and fur from animals gave some protection but skins in particular were inflexible and did not fit the body contours snugly. At some point in time, it was found that the long thin fibres from plants or animals could be twisted together and that the thread produced in this way could be interlaced to form more flexible clothing. The domestic origins of textile manufacture are lost in prehistory, but natural fibres are known to have served man’s needs for thousands of years. Recent archaeological evidence shows the imprint of woven materials on clay pots estimated to be 27000 years old, made long before settled farming and domestication of animals first began. More specific records suggest that woollen garment making began in Central Asia around 9000 BC, linen in Europe about 8000 BC and silk cultivation in China about 5000 BC [2]. Natural fibre processing remained a cottage industry until the industrial revolution in the second half of the eighteenth century. Man-made fibres were first produced at the end of the following century by regeneration of cellulose filaments from solution. Synthetic fibres began with the discovery of nylon in the 1930s, followed by polyester and acrylic fibres during the Second World War, although it was not until the 1950s that these completely new fibrous polymers achieved significant commercial use for civilian purposes. Today worldwide synthetic fibre production exceeds 28 megatons annually [3]. Establishment of the OPEC cartel by the major oil-exporting nations following the 1973 Yom Kippur war between Israel and neighbouring Arab states had a devastating immediate effect on the world’s chemical industries. The price of crude oil was increased at a stroke from US$ 3 to US$ 12 per barrel. Chemical companies were tightly squeezed by the discrepancy between the soaring costs of energy and raw materials and the declining selling prices for their finished products. New plants erected at great expense could not be filled as demand fell dramatically. These effects were aggravated further by the second oil crisis of 1979 following the fundamentalist revolution that deposed the Shah of Iran, when the oil price doubled again [4]. Operating costs had to be lowered by closing unprofitable units, laying off personnel and saving energy. World overcapacity in petrochemicals and polymers was even more serious and its effects extremely far-reaching. The 1980s saw strong growth in exports of cheap synthetic fibres, especially polyester, from Turkey, Mexico, Eastern Europe and the Asia Pacific region. Rationalisation measures were easier to organise in Japan than elsewhere because of the discipline that the trade ministry (MITI) was able to exert through a cartel that had been formed between firms producing petrochemicals. Between 1978 and 1982 a 17% cut in production of synthetic fibres was achieved, although this did not restrict the range of fibres manufactured by each producer. In Western Europe there was more regard for the principles of free competition. Bilateral arrangements between individual firms in the 1980-84 period led to the
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closure of unprofitable units and increased specialisation by each producer. In the field of synthetic fibres, a European Community agreement allowed each company to specialise in certain polymer types while giving up production of unprofitable ones. Such efforts reduced the total capacity for synthetic fibres in Europe by almost a million tons. American companies were obliged to act alone during this period of global reorganisation because of anti-trust legislation. The multinational corporations tended to close down their chemical fibres and plastics subsidiaries in Europe and divert this production to their USA factories. Some petrochemicals capacity was taken over by smaller companies with lower overheads that could still operate profitably. Diversification into speciality products and cost-cutting measures allowed the reorganised major producers to survive and return to profitability in the mid-1980s after several difficult years. Strong growth in synthetic fibre capacity during the mid-1990s, particularly in the Far East, led to a sharp decline in prices, aggravating the restructuring amongst fibre producers. Chemical conglomerates in Western Europe and the USA began to develop into life science companies. Hoechst gave up fibre manufacture by selling its Trevira division. DuPont now regards life science as the apex of its business profile, although still remaining involved in synthetic fibres production [5]. World cotton prices rocketed by 60% in the early 1990s, following poor harvests in PR China and Pakistan, encouraging farmers around the world to grow more cotton. Cotton consumption boomed in the 1990s as environmental concerns grew and consumer fashion in developed countries shifted back towards natural fibres. Chinese cotton output peaked in the mid-1980s and had halved by the mid-1990s, mainly because of pesticide-resistant bollworm infestation. Cotton demand will continue to grow, as the expansion of textile and clothing exports to developed markets in Western Europe and North America will come from the largely cotton-based industries of the Indian subcontinent and the Pacific Rim [6]. Cotton and wool textiles can be regarded as biodegradable speciality products increasingly retained for finishing in growth-oriented regions and designed for export as luxury goods to the markets of industrialised nations. Synthetic textiles, on the other hand, are essentially standard products manufactured in high-tech oil-based factories by industrial economies for sale at low prices for mass consumption in developing countries [7]. The success of textile goods in the future will be dependent on life-cycle acceptability and utility from the initial raw materials through to ultimate disposal. It is widely perceived that natural fibres are environmentally benign. Whilst it is true that natural fibrous polymers are biodegradable, the negative effects of pesticides, fungicides and fertilisers used in their cultivation have often been overlooked. Synthetic fibres can take many years to be transformed into breakdown products that can be assimilated by the natural environment. Research is under way to dramatically accelerate that process by developing biodegradable fibre variants. It seems likely that synthetic fibres will only retain market share if they can be converted back to their raw materials at an acceptable cost, as in the interconversion of caprolactam and nylon 6, or can be rendered biodegradable. In the case of polyester, uncontaminated waste can either be remelted and reextruded, or alternatively converted back to the starting intermediates by methanolysis. Recycling technologies for nylon 6.6 are also well advanced and waste nylon carpets can be recycled. The conversion of acrylic and modacrylic
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copolymers back to the monomer units for recycling is not yet possible, however. Accordingly, these fibres are expected to continue to lose market share [2]. Table 1.1 indicates fibre production data worldwide in 1996 and 2001. These trends emphasise the growing importance of cotton and polyester fibres whether alone or in blends, these two fibres accounting for over 70% of world fibre production. Manufacture of synthetic fibres is now more than 50% of the total. It is estimated that by the year 2050, polyester consumption will have overtaken that of cotton. The earth’s population is currently growing at about 1.3% per annum but improved living standards for many people ensure a growth rate of 2% in the average consumption of textile goods per head. Many of the textile products that characterised the rapid growth of the synthetic fibres industry in the 1950s and 1960s have become mature commodities, providing only slow growth and low profit margins. Intense global cost pressure, higher consumer expectations, a highly diverse customer base and reduced spending on research have contributed to sluggish growth in the fibres business. Environmental concern favours products that exhibit properties such as biodegradability and recyclability. Future fibres must be manufactured by safe, energy-efficient and zero-pollution processes. Figure 1.1 ranks the level of structural sophistication in various natural or manmade materials, illustrating how future fibres will acquire higher value through precise solutions to complex requirements. Ultimately the capability to build up highly specialised materials is determined at the molecular level. At the base of the pyramid in Figure 1.1 are essentially simple materials of industrial value with properties defined largely by the composition and macromolecular sequence. At the next level are high-performance fibres such as cellulose, aramids and carbon fibre. In these, both molecular conformation and spatial features are important property determinants. Still higher in structural organisation are natural materials such as silk or leather, with properties that are enhanced by the participation of individual macromolecules in specialised lamellar or helical structural arrangements. At the apex of the pyramid are biocomposites of natural origin, probably the most sophisticated materials known. In addition to the features already mentioned, these cellular components of living organisms are arranged in interconnected patterns that allow for highly specialised properties and functionality. The refinement of recombinant DNA methodology has made it possible to manipulate and direct the specificity of the biosynthetic process in certain fibreforming polymers, notably proteins. By utilising structure and sequence information from naturally occurring macromolecules, it is possible to design similar polymers from modular building blocks, such as amino acids, each providing a specific end-use performance. Using this approach it has become possible to construct genetic templates that encode bioengineered analogues to spider and moth silks [8,9]. These templates are expressed from a host organism to provide polymers that can be processed into highly lustrous fibres suitable for textile applications. In 1997-98 there was a loss of financial confidence in some of the boom markets of the Asia Pacific region. Thailand, Malaysia, Indonesia and the Philippines suffered rapid devaluation of their currencies and high national debt, fuelled by excessive borrowings [10]. Intense global competition ensured that this monetary disorder worsened the already difficult trading conditions for European man-made fibre producers. The build-up of excess capacity for intermediates and fibres
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production depressed prices and led to attempts to export fibres overseas at dumped prices. Local selling prices in Asia fell to levels that did not cover production costs. European producers risked paying twice over during the crisis, as substantial contributors to IMF (International Monetary Fund) support programmes and later in lost jobs as a result of dumping of low-price fibres into European markets [11]. This economic crisis in the Asia Pacific region seriously affected wool prospects in Western Europe. Overproduction in Asia deepened the plight of wool producers at every stage. Combing plants had been constructed or expanded during more prosperous times in Asia, Australia and South Africa. Tops and other products from these plants were now offered in Europe at prices low enough to destabilise the market. The situation was particularly worrying in Japan, a leading market for high-quality woollens and worsteds, as well as the most important buyer of pure cashmeres. Slowdown of consumer demand in these sectors was also evident in the USA and Western Europe [12].
1.2 Diversification of Dye Manufacture Worldwide The oil crises of the 1970s presented dyemakers and dye users with acute difficulties because of the dependence of dyes and auxiliary products on petroleum-based intermediates. Energy costs accelerated at 20 to 30% per year, more than doubling throughout the period 1975 to 1981. Many of the American chemical companies withdrew from the dyes sector in the late 1970s because of lack of profitability [13]. Chemical plants in the USA were centred mainly on large-scale continuous processes and were reliant on imported intermediates that had risen dramatically in price. Sales were mainly restricted to the domestic market with no tradition of the export marketing of dyes. Dyemakers in Europe had traditionally dominated the market and were better placed to weather the storm. Inevitably, however, they had to slim down operations to counter the effects of world overcapacity. This resulted from the decline in the traditional export markets and the emergence of new producers of commodity dyes in Eastern Europe and the Far East. Established manufacturers in Europe and Japan had to rely much more on discovering new speciality products, but the costs of research and development were much increased by the need to meet increasingly strict requirements for hazard testing. Japan’s vigorous economic growth brought about problems of pollution in the 1970s. As a result the country took an initiative, ahead of the rest of the world, in applying strict environmental regulations. Dyemakers and dye users consumed much of their corporate resources in taking the required measures to meet the challenges of environmental protection. The oil crises of the 1970s caused dye consumption to decline substantially. An industry-wide rationalisation became essential and the top five Japanese dyemakers cooperated in establishing a system of concerted production of disperse dyes and basic dyes through a consignment arrangement between them [14]. Long and costly test programmes now have to be carried out on newly discovered dyes or chemicals before they can be marketed. Many familiar and longestablished fine chemicals would not necessarily be available if they had had to be subjected to the test procedures now enforced by legislation. Safety testing has become so time-consuming and costly that research and development costs increased fourfold during the 1980s, greatly limiting the number of new speciality products reaching the market-place. It now takes much longer to attain this point
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from the date of discovery, so that the patent protection remaining when the new product is launched gives the discovering firm a much shorter lead over its rivals. The impact of the oil crises on the structure of the dyemaking industry was less immediate and far-reaching than for fibres manufacture, because the pattern of production of dyes and their intermediates is such that rationalisation within or between companies is a more complex process that takes longer to resolve. Optimistic forecasts of growth in synthetic fibres had led to overcapacity in dyemaking plants, notably for disperse dyes, and reactive dyes failed to live up to early expectations that they would soon achieve a predominant position on cellulosic fibres. Non-traditional suppliers of dyes based in low-wage nations, notably in the Asia Pacific region, began to take a significant share of local demand for commodity dyes and pigments. Profits were mostly reinvested to expand production capacity, enabling these companies to target world exports as a new area for growth [15]. The recession of the 1990s resulted in decreased demand for dyes just as this new capacity came on stream, leading to overcapacity problems. These fluctuations in supply and demand created a chaotic market-place with an increasing number of aggressive suppliers competing for a variable but always limited demand. This situation initiated a new round of industry restructuring with alliances, joint ventures, acquisitions and closures being made to combat competition. Non-traditional suppliers now represent more than one-third of the textiles dyes business worldwide [16], as indicated in Table 1.2. The main centres of dye manufacture are Western Europe, the USA, Russia, Japan, India and P R China. Asian capacity is growing at the expense of Europe and America. Table 1.3 shows the distribution of dyemaking activity throughout the world in 1996 [17]. Total production capacity was estimated to be about 900 kilotons but total demand was running at only 650 to 680 kilotons, representing an overcapacity of about 30%. It is estimated that by 2006 the Asian textile industry will account for more than half of world dye consumption [10]. Demand in Europe and the Americas is static or slowly declining. P R China and India are expected to show the most active growth in textile dye usage. At the beginning of the 1990s there were six major European dyemakers, four Japanese and one in the USA. Ten years later, after considerable rationalisation, there remained only four European and two Japanese dye manufacturers [15]. Table 1.4 lists several reasons why in developed economies there are increasing demands for quality dyes and processes but a diminishing consumption of commodity products. Greater amounts of disposable income mean that there is more demand for all fibre types in quality textiles but low economic growth rates operate in the opposite direction. Environmental legislation is driving the development of products with improved ecological profiles. The growth in activity in the Asia Pacific markets in the early 1990s encouraged the German dyemaker BASF to transfer the headquarters of its dyes and chemicals business for textiles and leather to Singapore in 1996, not long before the economic crisis of 1997-98 in the region. The other major producer DyStar continued to consolidate its activities in Germany. Despite high local costs in terms of wages, taxes and environmental protection, as well as stringent operating conditions, major advantages were claimed to justify remaining in this traditional location. These included innovative customers in a high-quality market, well-trained and well-motivated personnel, together with opportunities for close collaboration with research institutes and progressive textile machinery makers
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[18]. Since then BASF and DyStar have merged to form the world’s largest dyemaking group. Table 1.5 indicates that the drive for industrialisation in the developing world creates the potential for greater competition between importers and local manufacture. The low labour costs make it easier for local dyemakers to compete successfully against the traditional suppliers. The lowering of tariff barriers allows easier access to other trading regions and increases the attractiveness of imported quality products. Industrialisation leads to improved educational opportunities, higher skill levels and the potential for higher quality production. Production capacity for dyes in India is estimated at 75-80 kilotons p.a., with most producers concentrated in Gujarat and Maharashtra. The 48 larger dyemaking plants account for 70% of this total and about half of their total production is destined for export. There are about 900 small-scale independent units also producing dyes for local usage. In the 1990s the Indian dye industry went through a difficult phase, owing to low growth in demand, low profitability and environmental restrictions [15]. India has adopted a relatively hardline policy regarding ecological impact, with closures of chemical plants and dyehouses that are unable to treat effluent to satisfactory standards. In 1997 P R China produced 255 kilotons p.a. of dyes, the vast majority of which were commodity products. Exports of dyes from China have risen from 95 kilotons in 1996 to 156 kilotons in 1999, almost all the increase being in disperse dye presscake. It is not surprising, therefore, that this remarkable surge in activity is seen as the major threat to the established dyemakers elsewhere [15]. During the 1990s about 5000 small-scale chemical units in China were closed down, either on pollution grounds or to facilitate consolidation of the industry into larger units [10]. Taiwan’s major capacity is in reactive and disperse dyes, estimated at over 20 kilotons in 1998. The leading Taiwanese producer, Everlight Chemical Industrial, has a growing international reputation for reactive dyes and has steadily increased capacity above its 1996 level of 11 kilotons. Everlight has been collaborating with BASF since 1998 on joint production of selected Procion dyes [15]. The main characteristics of the Brazilian dye industry are: 1. almost total dependence on imported intermediates 2. intense competition, resulting from the presence in Brazil of all the major producers worldwide 3. substantial overcapacity in the industry 4. approximately 33% of Brazilian dye consumption is imported 5. about 25% of local production is exported, mainly to other Latin American markets 6. lack of specialisation in specific dye classes or sophisticated products. The low prices offered by Asian producers of intermediates, mainly from India and PR China, make local Brazilian investment in capacity for these chemicals highly unlikely. Foreign trade plays a major role due to low import duties [19].
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1.3 Globalisation of Textile Manufacture and Processing Since 1970 the capacity of the UK textile industry has halved: bleaching has shrunk to 44%, dyeing to 56% and printing to 35% of their former size [20]. This trend is not untypical of the situation in the European Union generally. The UK industry is still mainly run on a commission basis. Typically the woven fabric is imported in the grey state by a textile merchant and sold to a textile garment manufacturer, who sends it to a dyer and finisher for wet processing. Developing countries are increasingly equipped to dye and finish their own woven fabric, putting a great deal of pressure on the remaining UK commission finishers, who are competing against low-wage operations carried out in state-of-the-art, highly automated finishing plants. The knitted fabric industry has suffered less from fabric imports but almost all yarn supplies are imported. Companies in this sector are increasingly plagued by imported made-up garments, however, such as Tshirts [20]. According to a 1990 world survey of the major importers and exporters of textiles and clothing [21], the combined output of the five leading Far East producers exceeded that of the five major European countries (Table 1.6). As globalisation evolves, the importance of the Asia Pacific region in the world textile market becomes more evident. This growing dominance stems from the fact that tropical and coastal Asia is the most densely populated region in the world. It has substantially lower costs of production, including labour, energy and raw materials. Initially, developing countries have low labour costs and by the late 1980s, there was a factor of 80 between the rates paid in developed countries and those of developing countries. This differential has been eroded with time as third world countries have developed. Once modern machinery with automation and robotics have been installed, processing costs decrease as a result of lower labour requirements. During the early 1980s there was a pronounced movement of the centre of the world’s textile industry from the highly industrialised nations towards the lowercost economies of the developing world. Textile production in countries such as Taiwan and South Korea expanded at an incredible rate. The shift of the textile centre to the Asia Pacific zone has profound implications for suppliers outside this region. For suppliers to be effective in penetrating the rapidly developing and exceptionally price-sensitive Asian market, they must be capable of selling products that offer real value for money. The globalisation of textile manufacture implies that suppliers must be able to operate internationally in a highly competitive economic environment [22]. International trade in textiles and clothing is governed by the multi-fibre arrangement (MFA), established in the early 1970s as an interim measure to control imports from low-cost developing markets during the restructuring of the mature textile producers. The treaty has been amended at intervals but the established industries in Europe and the USA claim that the developing nations have maintained their trade barriers and supported their industries to gain significant commercial advantages. After 2005 the world textile trade will operate without the MFA. Following a phased reduction in tariffs, duties and quotas, the global industry will operate as a free trade zone, inevitably resulting in further import penetration of the developed markets [23]. An interesting comparison of the cost structures for textile finishing in Germany and South East Asia has been made in the context of the global response of dye suppliers to these differences [24]. Striking variations are evident between South East Asia, Japan, Western Europe and the USA in terms of their approaches to
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effluent legislation and degree of concern about different types of contaminant or ecological hazard. Unduly sensational reports in the German media have been blamed for allegedly damaging the image of the local textile finishing industry, subsequently provoking the rash of ecolabels introduced to reassure customers. Nevertheless, the challenge posed by environmental issues is probably the most important problem facing the wet processing industry; it will shape the development of acceptable chemical technology and be a major driving force for change in processing methods [25]. The 1990s became the age of the healthconscious and discerning consumer who demands quality products that offer value for money. European consumers, in addition to desiring quality products at attractive prices, also require them to be innocuous. Safety legislation and its enforcement have compelled dyers to be cautious in introducing new processes and equipment. Environmental pressures and the growing popular interest in the materials used to manufacture clothing have persuaded suppliers of chemicals and equipment to design and develop processes and machinery that are less polluting and more energy-efficient. In the 1990s the textile finishing industries of such developing countries as Pakistan and Turkey have revolutionised their operations. Pakistan opened nine new factories in five years, with European technologists training the local workforce. Phenomenal growth trends are evident in P R China but the Chinese are facing intensified competition from other Asian producers, including Pakistan, Turkey, India and Indonesia. With a workforce estimated at 15 million and relatively low labour costs, the Chinese textile and clothing industry has become the biggest in the world. P R China, now incorporating the Hong Kong SAR, is already the dominating supplier of these products to the lucrative markets of Western Europe and the USA, accounting for more than 25% of world exports (Table 1.6). This huge country has tremendous potential for future growth. China forms a quarter of the world’s population but accounts for only 17% of world fibre consumption. The retailing sector is set for rapid expansion in the coastal cities and economic zones as incomes improve over forthcoming years. China may have to divert some output away from exports in an attempt to satisfy fast-rising domestic demand. Textile production in P R China exceeds 11 megatons p.a., an increase of about 50% over 1993 levels. The average growth rate has been above 5% p.a. throughout the two decades since 1980. During the 1980s textile material exports rose by 11% p.a. in volume terms, whilst made-up clothing exports shot up by an astonishing 70% from a low base [26]. However, China’s rapid expansion, coupled with a succession of poor cotton harvests, created serious raw material shortages in the mid-1990s. Many textile mills across China had to be closed, forcing the world’s largest fibre producer to import major quantities of raw cotton and synthetic fibres. There is increasing resistance in developed markets to growing incursions from Chinese imports of manufactured goods, including made-up garments. Overmanning and inefficiency will continue to be a drain on the industry, weakening the cost-competitiveness of Chinese textile and apparel exports in the face of rising competition from elsewhere. The capital needed to fully re-equip the Chinese industry is immense and depends on substantial foreign investment. Plagued by obsolete equipment, P R China has had to restrain growth in textile output in order to update selected mills and consolidate the organisational structure of the industry. Old plant has been scrapped and only the best and most vigorous enterprises supported. Transport, power supply, communications and
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distribution facilities have been overwhelmed by the mushroom growth of the economy. The southern provinces possess a more developed infrastructure with easier access to the important export market and distribution centre of the Hong Kong SAR. However, costs there are far higher than in the more remote inland provinces [26].
1.4 Definition of the Global Consumer The global consumer shares with other individuals elsewhere in the world certain characteristic traits, such as lifestyle preferences and reactions to purchasing stimuli, that are independent of traditional geographical markets. All consumers are subject to a myriad of influences on shopping behaviour and product choice. Some factors, such as energy, the environment and the increasingly rapid flow of information, cut across national frontiers. These pressures exert a considerable impact on the business environment but may not be recognised or articulated by the individual consumer. Factors that may be considered in defining different segments of a retail market include: age profiles, household patterns, earning power, lifestyles, work and leisure time requirements. Table 1.7 displays current changes in the demographic composition of the UK population profile, which is broadly similar to other developed countries in Western Europe. It clearly indicates an ageing pattern, with the over-35s (postwar baby-boomers) growing in numbers at the expense of teenagers to thirty-somethings (post-pill babies). Table 1.8 categorises the shopping population into the traditional socio-economic groups based essentially on a classification of professional and manual occupations. Table 1.9 indicates general trends of consumer expenditure on clothing in various categories of the UK population. Taking the broad-scale figures in these three tables it is clearly evident that an important target segment for this market would be garments for women (purchasing 55% of all clothing) aged 35 to 54 (most rapidly growing sector of the population) with occupations in groups C1 and C2 (more than 50% of working adults). This example is a simple analysis of broad-scale trends but by utilising more refined statistics in a similar way it is possible to derive much more precise target customer profiles [27]. The results of marketing surveys into the major factors that are influential in clothing purchase decisions made by the most active sector of the shopping population are indicated in Table 1.10. These factors apply, to a greater or lesser degree, in all of the world’s developed markets. Socio-cultural trends are active agents of change present throughout the developed world. Five of these are listed in Table 1.11 as specifically relevant to clothing, fabrics and fashion, reflecting certain facets of the individual consumer’s interaction with the environment [28].
1.5 Organisation of Textile Production in a Global Economy A global market is one in which the individual consumer has access to information about goods and services available from around the world. Markets are defined by the appeal of products to global consumers rather than restricted within traditional cultural or geographical boundaries. The relatively affluent consumers in the interlinked economies of Europe, North America and Japan share many characteristics: comparable income levels, well-educated, similar leisure and travel interests, access to the same sources of media-generated information.
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Worldwide product identity, high visibility of brandnames, a universal logo and shared promotional messages are features that contribute to the growth of a global business. Corporate value structures, management style and marketing strategies ignore political borders but respect cultural differences specific to each geographical region in the ‘borderless world’. Businesses operating in this trading situation must offer products that have global market appeal and be demonstrably committed to serving a global network of demanding customers [28]. Tailoring product ranges to meet the requirements of different sectors of the global market-place is expensive. Many exporting companies face a dilemma because they are not sufficiently capitalised to acquire the necessary manufacturing facilities and global marketing organisations required to remain competitive. The more they penetrate the global network, the more they must respond to the numerous local differences between market sectors. Therefore companies have to find new ways to exploit company strengths while keeping costs under control, tackling strategic opportunities without seriously depleting resources. In other words, to do more with less. Possible approaches to facilitate this include licensing agreements, franchises, company alliances, joint ventures, mergers and acquisitions [22]. The globalisation of the textile industry has resulted from the application of telecommunication systems to enable computerised analysis of sales data from retailing in any region of the world to be readily and instantly transferred to a textile manufacturing facility and its associated dyeing, printing and finishing services, so that supply and demand can be much more closely integrated. Similar communication links enable textile designers in different countries to collaborate simultaneously on design editing and modification using advanced computer-aided design (CAD) systems. To be successful in today’s economic environment, the retailer must have a clear picture of the consumer that is to be served. Much time, effort and resources are devoted to the analysis of market statistics in order to understand the purchasing profiles of today’s consumers. Larger retailers aim to cater for a range of different market sectors, whereas smaller and more specialised suppliers focus on a clearly defined market niche. Lively display layouts, making shopping precincts and retail outlets more exciting places to visit, together with the introduction of niche marketing areas within larger retail stores, have been important in stimulating market developments. Technology has revolutionised the consumer/retailer transaction. Electronic barcoding of individual garments and the development of electronic point of sale (EPOS) data capture through computerised checkout tills now allows the scheduling of deliveries of individual products on a daily basis. The more advanced retail organisations communicate directly with their suppliers using electronic data interchange (EDI) systems. This minimises the stock levels carried by the retail outlets and has a significant impact on the working capital needed to finance the business, minimising lost opportunities and price mark-downs, as well as allowing rapid and accurate monitoring of the sales pattern for newly introduced items. Delaying colouring and garment sizing instructions to a date as near as possible to the EPOS transaction increases the accuracy of demand forecasts, reduces lead times and further minimises mark-downs and lost opportunities. The early 1990s saw the onset of economic recession. Demand in developed markets declined and retailers challenged traditional sales patterns to retain
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market position. The historical approach towards seasonal buying, based on spring/summer and autumn/winter fashion collections, maintained by the leading fashion houses of Western Europe, has been significantly eroded. These have been replaced by four or sometimes six shade palettes in the year. The impact of this trend means that many more new shades have to be programmed into the production schedule [23]. The market-place is now highly reactive to new ideas and trends, irrespective of the calendar. Speed of response to sudden volatility in demand and the ability to influence fashion styles are critical to the retailer’s success [29]. These developments favour those suppliers who can offer flexibility of response and short lead times to supply fabric or garments. Recession conditions have seen the disposable income of the average consumer decrease, so that garment price levels play a more important role in the decision to purchase. Many retailers have been unable to raise the selling prices for core products over several years. In the ultra-competitive market-place, retailers must offer value for money and differentiate through quality, availability and customer service. The increased influence of retail organisations has put pressure on garment manufacturers and fabric producers, creating a highly competitive environment in which the overwhelming need to satisfy the retailer’s demands is paramount. The discerning consumer is swayed by marketing, reputation, style, availability and price. To survive, retailers must be able to offer the right product at the right time and at the right price. The textile chain consists of a sequence of various interlinked operations that follow a definite route; Figure 1.2 is a simplified representation of the relationships involved. The process begins traditionally about two years in advance with forecasts of the target season’s designs, fabrics, colours and finishes. Much later, merchandising decisions are taken to promote selected products and to define which garment styles, sizes and colourways will be manufactured. This stage is delayed as late as possible in order to minimise the risks of making wrong choices. This requirement means that the manufacturing and processing links in the chain are under constant pressure to reduce their lead times to the retailer. The provision of detailed EPOS information helps the retailer and suppliers to plan ahead and respond quickly to the market-place. For the retailer sales patterns act as the basis for re-order and price mark-down decisions. For the manufacturer and processor such data give vital information for production planning (PP) and materials requirements planning (MRP) decisions. The purpose of sharing this commercial information is to persuade suppliers to cooperate with the retailer towards shared objectives and to focus on the task of getting the correct assortment of goods on the sales floor when the customer is ready to buy. Typical American experience indicates that initial order lead times can be reduced by 30% or more. In-season re-orders can be programmed so that retail outlets can receive frequent consignments of best-selling lines. This ensures that the suppliers are making what the retailer knows his customer wants [31]. Each supplier link in the textile chain must evaluate its own and its competitors’ strengths and weaknesses, as well as devising a strategy to consolidate partnerships with other links in the chain. It is important for such partnerships to examine critical aspects of the service provided and to define specific measures that can be taken to generate a competitive advantage in the market-place. The driving force is the retailer’s commitment to the needs of the customer. Having established these operating relationships it is essential that these principles are
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carried through so that suppliers respond to the retailer and the customer in the most dynamic and cost-effective manner [29]. The textile marketing chain is long and complex. The traditional business environment was so full of variables and imponderables that failure to produce to specification frequently occurred and allowance for non-conformance was routinely accepted. However, the increasingly stiff competition in the global market-place has changed the climate in which industry has to operate. Enhanced customer choice between wider ranges of products and retail outlets has emphasised the need for the retailer, garment maker, fabric supplier and finisher to improve their financial performance. The industry can no longer tolerate the possibility of one link in the marketing process adversely affecting the efficiency and viability of the total supply chain as it attempts to fulfil the customer’s requirements. Total quality management (TQM) may be regarded as an ideal that is ultimately unattainable, but quality control measures should be established and continually refined so that effectiveness is seen to approach closer and closer to the optimum. The focus on TQM has provided an alternative means of defining the value of controlled production performance. Using what is now becoming a standard analytical technique, the cost of quality (the cost of conforming to the required standard of performance) is defined and expressed as a percentage of sales revenue. The key factors contributing to this measure are: 1. Preventive maintenance – calibration and procedures
costs
of
laboratory,
quality
management,
2. Shading additions and re-dyes – internal costs of non-conformance 3. Appraisal – goods inspection and quality control systems 4. Reprocessing – internal and external costs 5. Claims and debits – external costs of non-conformance 6. Consequential – lost opportunities for profit. It is quite common for quality costs to be as high as 25% of the sales revenue [32]. Quick response (QR) is a mode of operation by which manufacturing or service industry attempts to supply products or services to its customers in the precise quantities, variants and time frames that they require [31]. The objective is to do this on a continuing basis to achieve minimum lead times and avoid commercial risks, whilst retaining maximum competitiveness and flexibility. QR entails stricter disciplines and immediate communications but it will yield improved performance in terms of turnover and sales, combined with an increased level of service to the customer. Typically in America the initiative for QR projects started at the retail end of the textile supply chain. Major retail organisations have shown a willingness to adopt creative approaches to long-range planning, supplier selection, stock control and distribution, transportation and handling. The adoption of QR by the manufacturing sector thus becomes an instrument of survival and maintenance of competitive position. The full benefits of QR operation can only be realised if all links in the supply chain are committed and actively involved in the system. The most dynamic response performance has been achieved in the knitwear sector; a greater proportion of dyeing and finishing in garment form has
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dramatically shortened lead times. Piece dyers have reorganised production schedules to ensure quicker response to market demands and progress has even been possible in yarn and loose stock dyeing. Continuous open-width dyeing was traditionally reserved for long runs per colour and semi-continuous methods (such as pad-batch) for batch sizes intermediate between exhaust and fully-continuous methods. The economic climate together with the need for quick response has eroded demand for continuous and semi-continuous dyeing and such plant may often run uneconomically because of short runs. It seems likely that small-batch QR production with focused marketing and dynamic response to fashion demands will enhance added value, leading to higher levels of profitability. The QR system targets the production of saleable merchandise that is actively demanded in the market-place. Stock inventory is not created until orders and reorders are placed, so that the inventory of finished goods is set up at the correct time. The concept of just-in-time (JIT) means having only the right products in the right place at the right time, with consistency of quality [31]. It is vital in these circumstances to be absolutely sure that the inventory set up is free from all defects. Thus TQM is an essential prerequisite for successful JIT. In the current economic climate the risks and consequential costs of carrying or creating excess or substandard stock in any sector of the supply chain immediately creates serious problems. In a complex market such as textiles and apparel, the problem of forecasting fluctuating demand brings dangers to the adoption of JIT that make close liaison between the various links of the supply chain not merely desirable but absolutely vital. Incorrect chemical pretreatment has been shown to be implicated in about 70% of all faults in finished fabrics [33]. The last two decades have seen major improvements in control systems to improve the reproducibility of dyeing by minimising deviations from standard conditions. Nevertheless, many factors must be taken into account if troublefree processing is to become routine. Changes in consumer lifestyle and the widespread use of central heating systems have favoured the growth in sportswear and leisure clothing, which now account for more of the apparel market than formal wear [34]. Greater problems in respect of soiling of sportswear and leisure garments dictate the use of dyes of higher fastness to washing, particularly where perborate-activated detergents may be used at lower laundering temperatures [21]. The concept of right-first-time (RFT) processing gained rapid acceptance in the 1980s, when it was originally termed blind dyeing. The RFT philosophy is intended to provide higher quality with quick response, as well as maximising productivity, process efficiency and profitability. To minimise fibre damage, high-quality textiles must be subjected to the shortest possible time of wet treatment compatible with achieving the target properties. The cost of non-conformance is a simple but effective measure of the losses incurred by the production of unacceptable goods that do not meet the required specification [33]. On average, a single shading addition can add 10 to 30% to the cost of an exhaust dyeing. If an unlevel or off-shade dyeing has to be stripped and re-dyed, the cost of processing increases by more than 100%, guaranteeing either financial loss or an uncompetitive quote if this degree of inefficiency is allowed for in the original price. Not only are shading additions and re-dyes bad for profit, they also interfere with productivity and cause delivery dates to be missed, resulting in loss of goodwill and future business.
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In the dyehouse the total cost of production can be calculated by adding the cost of additional processes necessary to achieve the required performance to the standard cost for routine processing. Table 1.12 shows the cost of nonconformance calculated by means of computer modelling techniques for a typical knitgoods dyehouse operating on a commission basis. The blind dyeing process is the ideal that represents the processing cost, productivity and profitability for troublefree RFT production. If, after checking the shade, a small shading addition to the cooled dyebath is required, the total cost of production is increased by 10% but productivity is reduced by 20%. More importantly, the profit for this production batch is reduced by more than 50%. If a large shading addition is needed, calling for the exhaust dyebath to be drained, rinsed and re-set, then the impact on processing cost, productivity and profitability is even more dramatic. In this case a substantial loss of nearly 50% is incurred. The most costly scenario is to have to strip and re-dye a faulty dyeing. To recoup this loss it is necessary to dye another four RFT production batches, all of which must be problem-free. The figures in Table 1.12 are only part of the story since they do not allow for the reduction in revenue which occurs through loss of production when machines which should be processing the next batches are tied up with corrective treatments. These factors also have a bearing on the ‘designed capacity’ of the plant and impact on capital expenditure. By improving the capacity for RFT production, wasteful additional processing to achieve the required performance can be eliminated. For the knitgoods dyehouse model from which Table 1.12 was calculated, the average number of batches per machine per week represents the full capacity of the dyehouse when operating at 60% RFT production. Improving this figure will reduce the variable costs (excluding labour) and improve return on sales (ROS) as shown in Figure 1.3. For example, improving RFT production from 60 to 90% will increase the ROS from the original 100% to 130% and generate additional capacity accordingly. In order to obtain RFT production consistently, there are numerous factors that the dyer must take into account, as indicated in Table 1.13. Factors associated with the inherent constitution of the starting materials are best monitored by laboratory testing on a regular basis. Those factors arising from the requirements of the dyeing process and the associated colour control measures should be dealt with in a series of standard operating procedures (SOP) that need to be designed in accordance with the substrates, dye classes and dyeing methods currently operated within the dyehouse. The factors which can be controlled by SOP, plus dye purchasing, are matters of company policy and cannot be changed without senior management approval. Similar lists can be established for preparation, aftertreatment and finishing processes. The financial rewards from improved quality and efficiency include [32]: 1. Increased sales from RFT production 2. Increased productivity from the same equipment and time frame 3. Reduced working capital required because of lower stocks of material awaiting processing, shorter process routes and less strategic stocks of finished goods awaiting despatch because of quicker response to market trends 4. Reduced occurrence of shading and re-dyeing operations. This not only frees additional production capacity but ensures that production planning is much less disrupted by unforeseen delays. It is therefore easier to plan for on-time
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deliveries with only short lead times, maximising the opportunities available to respond to fast-moving trends in the market-place 5. Cost savings generated by lower processing costs and reproducibility of guaranteed quality. In order to control a process effectively it is necessary to identify and measure all relevant process variables. Microprocessor control has been successfully established for: 1. Time/temperature profile of the dyeing cycle 2. Speed of fabric rope movement in jet machines 3. Dyeing machine speed/output 4. Product addition according to controlled profiles 5. Redox potential, pH and specific gravity can be controlled by on-line systems. The target set values together with predetermined tolerances are stored in the control system; providing that the process controller detects values within these acceptable limits, the processing cycle moves on to the next step. If the value detected lies outside the pre-set tolerance range, an alarm device calls for on-line attention so that corrective action can be taken. Systems are available to synchronise liquor circulation, fabric speed and process cycle in jet dyeing.
References [1]
Genesis, chapter 3, verse 7.
[2]
J P O’Brien and A P Aneja, Rev. Prog. Coloration, 29 (1999) 1.
[3]
J Rupp, Internat. Text. Bull., 48 No. 4 (Aug 2002) 28.
[4]
J Park and J Shore, JSDC, 115 (1999) 298.
[5]
H J Koslowski, Chem. Fibers Internat., 48 (1998) 174.
[6]
D Morris and A Stogdon, JSDC, 111 (1995) 341.
[7]
H K Rouette, Textilveredlung, 32 (1997) 108.
[8]
S R Fahnestock and S L Irwin, Appl. Microbiol. Biotechnol., 47 (1997) 23.
[9]
S R Fahnestock and L A Bedzyk, Appl. Microbiol. Biotechnol., 47 (1997) 33.
[10]
P Kelshaw, JSDC, 114 (1998) 35.
[11]
C Purvis, Chem. Fibers Internat., 48 (1998) 7.
[12]
Anon, Wool Record, 157 (April 1998) 1.
[13]
G N Mock, Rev. Prog. Coloration, 32 (2002) 80.
[14]
S Abeta and K Imada, Rev. Prog. Coloration, 20 (1990) 19.
[15]
P Bamfield, Rev. Prog. Coloration, 31 (2001) 1.
[16]
A X Rad, Dyer, 185 (Nov 2000) 12.
[17]
K V Srinivasan, Colourage Annual, 46 (1998) 79.
[18]
A X Rad, Melliand Textilber., 79 (1998) 530.
[19]
J Falzoni, Rev. Prog. Coloration, 25 (1995) 64.
[20]
C Smith, Rev. Prog. Coloration, 29 (1999) 37.
[21]
I Holme, Rev. Prog. Coloration, 22 (1992) 1.
[22]
K Cheuk, JSDC, 111 (1995) 135.
[23]
M J Bradbury, P S Collishaw and S Moorhouse, SDC Biennial Conf., Blackpool (Oct 2000).
[24]
W Reddig, Melliand Textilber., 78 (1997) 834.
[25]
I Holme, Textile Month, (Sept 1990) 55.
[26]
J Glasse, JSDC, 111 (1995) 98.
[27]
F Moore, JSDC, 111 (1995) 212.
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[28]
D Siegel, JSDC, 113 (1997) 231.
[29]
M J Bradbury and J Kent, JSDC, 110 (1994) 173 and 222.
[30]
K Parton, JSDC, 110 (1994) 4.
[31]
J Hobson, JSDC, 107 (1991) 305.
[32]
P S Collishaw, D A S Phillips and M J Bradbury, JSDC, 109 (1993) 284.
[33]
W Prager and M J Blom, Text. Chem. Colorist, 11 (1979) 11.
[34]
P W Leadbetter and A T Leaver, 15th IFATCC Congress, Lucerne, (Jun 1990).
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Table 1.1 Global production of various fibre types [3]
Fibre type Cotton Polyester Nylon Acrylic Wool Silk Other synthetics* Other cellulosics Total Global population (billions) Average consumption (kg/head)
Production (megatons) 1996 2001 20.03 18.73 19.24 13.30 3.73 3.93 2.58 2.62 1.36 1.49 0.086 0.071 3.16 1.96 2.63 2.87 44.97 52.81 5.77 6.16 7.79 8.57
Change +1.30 +5.94 -0.20 -0.04 -0.13 +0.015 +1.20 -0.24 +7.84 +0.39 +0.78
* Mainly polypropylene fibres
Table 1.2 Market share (%) of the global textile dyes industry held by various suppliers [16] Dye manufacturers DyStar Large non-traditional suppliers Ciba Small non-traditional suppliers Japanese dyemakers Clariant Small traditional suppliers Medium traditional suppliers
Market share (%) 23 22 15 14 9 8 5 4
Table 1.3 Production of dyes and pigments worldwide [17] Country Germany USA Russia and CIS Japan Switzerland UK India Other countries
Production (%) 22.4 18.0 11.3 9.3 8.3 8.2 6.6 11.1
Worldwide production in 1996 was about 700 kilotons
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Table 1.4 Factors governing the pattern of dye consumption in developed countries [10] Factor Increased disposable income Fashion demands Low economic growth rate Focus on high-quality, high-value products Environmental legislation
Market impact Enhanced requirements for all fibre types Requirement for high-quality dyes and processes Depressed growth of standardquality goods Reduced demand for commodity dyes Boost in demand for environmentally safe products
Table 1.5 Factors governing growth of dyemaking in developing countries [10] Factor Increasing disposable income Political will to create an industrialised economy Low labour costs Lowering of tariff barriers Improving education and technical skills Greater access to developed export markets
Market impact Demand for textile goods increasing Rapid rise in local manufacture Local manufacture able to compete with importers Importing of quality products becomes more attractive Improving quality Growth in export-oriented producers
Table 1.6 Top ten world importers and exporters of textiles and clothing [21]
Country USA Germany Hong Kong UK France Japan Netherlands Italy Belgium Switzerland Total
Imports (US$ billion) 29 565 23 175 12 169 11 942 11 768 10 631 6 833 6 681 5 796 4 364 122 924
Country Hong Kong Italy Germany South Korea PR China Taiwan France Belgium Japan UK Total Asia Total Europe
Exports (US$ billion) 18 182 16 564 15 903 13 540 11 327 9 324 7 904 6 311 6 060 5 797 58 423 52 479
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Table 1.7 UK population age profile 1992 to 2001 (millions) [27]
Age (years) 0-4 5-14 15-24 25-34 35-44 45-54 55-64 65+
1992 3.9 7.3 8.0 9.2 7.8 6.9 5.8 9.1
1996 4.0 7.6 7.3 9.3 8.0 7.6 5.8 9.5
2001 3.85 7.9 7.3 8.3 8.9 7.8 6.2 9.3
Change (%) 1992-2001 -1.3 +7.6 -9.6 -10.8 +12.4 +11.5 +6.5 +2.2
Table 1.8 Shopping population by socio-economic group [27] Group A B C1 C2 D E
Proportion of adult population (%) 3.1 15.7 25.6 26.0 17.0 12.6
Classification by occupation Upper professional Middle professional Lower professional Skilled manual Unskilled manual Subsistence level
Table 1.9 UK consumer expenditure on clothing (£ million) [27]
Category Womenswear Menswear Girlswear Boyswear Infantswear Total
Expenditure 1989 9 500 4 800 1 200 950 700 17 150
1994 9 700 4 500 1 400 1 000 650 17 250
Change (%) 1989-1994 +2.1 -6.7 +14.3 +5.0 -7.7 +0.6
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Table 1.10 Results of surveys into influential factors in clothing purchase decisions by female customers aged 18 to 45 [28] Factor Fashion
Comment A constant factor, whatever other criteria are involved Great importance attached to all three of these factors, which greatly influence the decision to buy Many customers are prepared to pay more for clothes that they perceive to be of higher quality Three of every four customers want information on clothing that they are interested in purchasing
Fit, comfort and shape retention Price
Product information
Table 1.11 Socio-cultural trends relevant to clothing, fabrics and fashion [28] Experiencing with all the senses – a fundamental shift away from the dominance of the visual to savouring with all five senses. In fabric terms, texture and handle are becoming as important as colour and pattern Well-being – concern for the ‘total self’, both mental and physical. In clothing terms, this translates into expectations of comfort and visual appeal Personal appearance – dress is used as a means of self-expression and personal statement, but fashion and style have become more cross-cultural Networking – an interactive social dynamic in which individuals belong to several mobile and diverse groups, adapting clothing to the activities they are engaged in at various times Intelligent shopping – individuals are better informed about the value, quality and features of products in the market-place. The constitution of components of a product are as significant as its external appearance or function
Table 1.12 Cost of non-conformance to blind dyeing method (RFT=100) [33]. Process Blind dyeing Small addition Large addition Strip and re-dye
Cost 100 110 135 206
Productivity 100 80 64 48
Profit 100 48 -45 -375
Practical Dyeing, Volume 1 Table 1.13 Important factors influencing RFT production Starting materials Purity of water Dyeability of textile substrate Preparation of textile substrate Standardisation of dye supply Moisture content of dyes Dyeing process control Weighing of substrate batch to be dyed Weighing and dispensing of dyes Weighing and dispensing of chemicals Control of liquor ratio Control of pH Time / temperature profile Control of liquor flow Control of substrate movement (if relevant) Colour control Selection of dyes Behaviour of dyes in combination Accuracy of laboratory dyeing recipe Accuracy of transfer to bulk-scale recipe Batch to batch reproducibility in bulk-scale dyeing Method of colour assessment Determination of metamerism index
21
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Figure 1.1 Architectural hierarchy of fibres and biocomposite materials [2] Increasing structural sophistication.
Bones, teeth, wood, shells, wool fibres
Quaternary – folding patterns multiple components in a composite structure.
Collagen, silk
Cellulose, aramids, carbon
Glass, polyester, nylon, ceramics
Tertiary – supermolecular helices, lamellae. Secondary – conformation.
Primary – composition and sequence.
Figure 1.2 Primary steps in the textile chain [30] Fashion forecasting
Merchandise ordering
Yarn manufacture
Re-ordering
Fabric manufacture
Garment manufacture
Retailing
Customer
Dyeing and finishing
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Figure 1.3 Relationship between RFT production and return on sales [32]
150
Return on sales, %
140
130
120
110
60
70
80
RTF production, %
90
100
Chapter 2 Impact of Dyeing and Finishing on the Environment 2.1 Developments in Health and Safety Legislation Health and safety were almost never regarded as significant by the industrial entrepreneurs of the nineteenth century and earlier. Risks were taken with chemicals and equipment that would now be regarded as horrific. Accidents must have occurred with alarming frequency and there was no system in place to compensate the inevitable victims or even to record the events adequately so as to avoid future disasters. No doubt workers had to learn the hard way by practical experience how to treat moving machinery and corrosive chemicals with due respect, but this knowledge was of limited value to individuals forced to work hastily in dangerous surroundings. Early UK legislation under the Alkali Act (1863) created the world’s first national pollution control agency, the Alkali Inspectorate, to control atmospheric emissions of acidic gases primarily from factories producing caustic alkalis. More insidious, however, were the potential health risks resulting from longer-term exposure to toxic or carcinogenic chemicals. Operatives were expected to provide and wash their own working clothes. These must have become heavily contaminated with chemical stains, putting the worker’s family also at risk of exposure to hazardous vapours [1]. It was not until the 1940s that a causal connection was established between the high incidence of bladder cancer in employees of firms making or using dyes and their prolonged exposure to certain arylamine intermediates, following epidemiological studies of individuals who had spent their working lives in these industries. Following a period of unprecedented growth and optimism in the chemical industries worldwide during the 1950s and 1960s, a sociopolitical reaction set in during and after the oil crises of the 1970s. Public demand, often fuelled by media misinformation and speculation [2,3], became insistent that the manufacture and use of industrial chemicals should be more closely regulated and monitored. A series of tragic accidents occurred over this period, involving cyclohexane at Flixborough (UK), dioxins at Seveso (Italy), mercury compounds at Minamata (Japan), and methyl isocyanate at Bhopal (India). These were followed in 1986 by the Chernobyl (USSR) nuclear explosion that polluted much of Northern Europe and the Schweizerhalle (Switzerland) fire that polluted the Rhine basin in the heart of Europe, reinforcing this trend towards a political climate of reform and control. The UK Control of Pollution Act (1974) applied the first controls to the disposal of industrial wastes. The Environmental Protection Act (1990) was the first governmental commitment to protection of the environment and the Water Resources Act (1991) covered the disposal of effluent to controlled waters. This sweeping legislation was designed to impose financial constraints on the release of contaminated air or waters, according to the principle that the polluter must pay to clean up the polluted environment [4]. A major step forward in legislative protection against health risks in the working environment was the UK Health and Safety at Work Act (HSWA) in 1974. This placed a duty on employers to safeguard the health and safety of all personnel entering the workplace. The Control of Substances Hazardous to Health (COSHH) regulations provide a framework aimed at protecting workers against health risks
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from hazardous substances. The main requirement of the Personal Protective Equipment (PPE) regulations is that gloves, nasal masks, eye protection and safety footwear must be used wherever there are relevant hazards to health and safety [5]. Much helpful advice and data have been published by the UK Health and Safety Executive (HSE). Possible hazards to the general public from traces of chemicals in the environment have been increasingly the subject of active debate in the media and elsewhere in recent years. Since 1992 a Textiles Group of the Institute for Health Protection of Consumers and Veterinary Medicine in Germany has focused on the issue of health risks from textiles. Topics that have been studied include allergic reactions caused by textile materials, carcinogenic and mutagenic substances present in dyeings and finishes, azo dyes capable of being reduced to form hazardous arylamines, dye carriers, flame retardants, traces of dioxins in textile fibres, assessment of exposure to textile contaminants and the toxicological investigation of colorants and auxiliaries in textiles [6]. 2.1.1 Risk evaluation and prevention Indispensable to the management of risk by reduction or avoidance is a knowledge of the risk and the controlling factors determining its magnitude. Risk in this context is a function of the potentially harmful effects arising from inherent toxicological properties of the chemical and the extent of its bioavailability to the organism exposed. Risk is also a function of degree of exposure and the probability of its occurrence. Obviously, the risk of experiencing harmful effects can be lowered by limiting the degree of exposure and this approach affords a means of improving safety [7]. Industry has a substantial interest in helping the risk assessment approach to operate. Failure to do so will encourage regulation based on hazard considerations alone. There is already a trend in favour of the precautionary principle and the introduction of various ‘black lists’. Such discriminatory actions undermine the agreed basis for chemicals control and warrant an unreserved rejection by the chemical industry [8]. Thus two components, exposure and hazard, must be evaluated together in determining the level of risk posed by a given colorant or other chemical. Risk management may therefore be regarded as a series of interdependent steps (Figure 2.1). The process of risk evaluation for personnel working with dyes and textile chemicals has been discussed in detail [9]. The more extensive the database covering the toxicological, physical, chemical and application properties of the product, the easier it is to evaluate the risks involved. Although exposure levels are just as important as hazard potential for the risk evaluation, the quality of the exposure data is often the weak point in the data available. Consequently, in many instances it is not possible to be fully confident of the reliability of the risk evaluation, which may tend to be in error on the side of over-estimation. Particular attention should be given to improving aspects of exposure assessment (occupational exposure, consumer exposure, environmental release) [10]. The widespread use of colorants creates a great diversity of exposure situations. The most serious exposure potential exists for operatives in colorant manufacture and those employees of dye user firms engaged in weighing and dispensing. Dust particles less than 7 µm in size can gain access into the lungs and pose the greatest problem. It is not feasible to market all colorants in liquid form and considerable efforts have been devoted to the development of low-dusting solid forms. The monitoring of amine excretion in the urine of individuals exposed to
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arylamines, or dyes expected to be metabolised in the body to such amines, offers a possibility of checking the adequacy of safety precautions [7]. The conventional classification of organic colorants into broad chemical classes or application ranges is of limited help in hazard assessment. It is not possible to generalise about the toxicological properties of entire groups like these. Biological activity can vary dramatically in spite of close structural similarities. Nevertheless, an observed toxic effect can often be attributed to a specific structural feature within a narrow subclass of colorants or to a specific metabolite produced from them, as in the formation of benzidine from its parent disazo dyes. Organic colorants generally exhibit relatively low acute toxicity. This is especially true of organic pigments because of their extremely low bioavailability. Of major concern, however, are the potential carcinogenic and allergic effects of specific dyes and intermediates. As a basis for the determination of risk it must be assumed that the colorants are properly handled and applied. It is not appropriate to estimate risk primarily on the basis of exposure values obtained under improper working conditions, or where appropriate plant and equipment are not available. Ensuring satisfactory operating conditions and training of operatives to handle products correctly is essential nowadays for technological success as well as for health and safety requirements. In this way, exposure levels can be kept below the threshold of unacceptable risk. It is reasonable to accept that for practical purposes levels of exposure exist below which the risk becomes trivial [7]. The various measures to reduce risk are an integral part of risk management. A state of ‘zero risk’ cannot be reached, but efforts to maintain exposure levels below the threshold of unacceptability must be unremitting, in order to increase the margin of safety. An essential prerequisite for effective risk control is the provision of readily accessible hazard information on the computer disc, in safety data sheets and on warning labels. It is prudent to minimise exposure to all chemicals through good working practice. Respiratory protection by approved equipment must be worn wherever dusts or aerosols are being generated or disturbed. A constructive approach to reducing risk is the replacement of hazardous products by safer ones. This cannot be achieved quickly in most instances, because of the complex profile of technical and economic requirements that governs selection of a colorant for a specific purpose [11]. 2.1.2 Safety data sheets From 1974 onwards, the member companies of ETAD (Ecological and Toxicological Association of the Dyestuffs Manufacturing Industry) issued safety data sheets for all organic colorants in their selling ranges. Subsequently, various EU directives have enforced the provision of material safety data sheets (MSDS) for all hazardous substances and preparations. The standard 16-heading format covers the composition of the material in question, possible hazards, first aid and firefighting measures, steps to be taken in the event of unintentional release, handling and storage facilities, limitation of exposure using personal safety equipment, physical and chemical properties, stability and reactivity, toxicological and ecological data, disposal instructions, transportation information and regulations [12]. Ecological test data include biodegradability, BOD (biochemical oxygen demand), COD (chemical oxygen demand), DOC (dissolved organic carbon) and TOC (total organic carbon) [13].
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When a potentially hazardous substance or preparation is first placed on the market the producer must make available the MSDS for that product to all organisations purchasing or receiving that material for their potential use or testing purposes. All supplying companies in the EU are responsible for issuing appropriate MSDS to their customers and these must be in the appropriate languages of the member states in which the recipient firms are situated. The EU format is recommended as an appropriate model for all countries outside the EU that do not mandate other specific MSDS formats. 2.1.3 Ecolabelling The current multitude and diversity of ecolabelling schemes (national or regional, non-governmental, privately sponsored and company labels) has led to the confusion of consumers and raised questions about potential trade distortion. Criteria have been applied by ETAD to the evaluation of ecolabelling schemes [10], which should preferably: 1. be risk-based rather than discriminatory 2. provide objective criteria of evaluation 3. be regional or international in scope 4. apply to consumer goods rather than specific dyes 5. require dye selection only if there is real environmental benefit. The main deficiency of existing schemes is that they focus on the exclusion of certain hazardous dyes, but do not take into account that it is the conditions of manufacture and use that are the most important determinants of environmental impact. For an ecolabelling system to be worthwhile, it should really take account of the full history of the product from material sourcing through manufacture, distribution, application and disposal. The EU Ecolabelling Scheme is a voluntary system for consumer products, designed to assist product selection and to encourage manufacture of products that are less damaging to the environment. Criteria for a specific product group are developed by applying a lifecycle assessment (LCA) to gauge their impact at every stage of a product’s life. The scheme applies across all member states of the EU and is intended eventually to replace existing national labelling initiatives. The Nordic Swan label is another important multinational ecolabelling scheme, designed to guarantee an objective standard for products that must satisfy strict requirements. It is administered in Norway, Sweden, Finland and Iceland; the green swan logo is based on the emblem of the Nordic Council. Important national ecolabelling programmes include Blue Angel in Germany (the world’s first in 1977), Eco-Mark in Japan and Green Seal in the USA. The two leading private textile ecolabels operating in Western Europe are GuT and Oeko-Tex. Prominent companies in the European carpet industry founded the Gemeinschaft umweltfreundlicher Teppichboden (Association of Eco-friendly Carpets!) in 1990, with the aim of optimising the production cycle of textile floor coverings. Approximately 75% of the carpets manufactured in Western Europe are produced under the control of GuT. In 1991 the Austrian Textile Research Institute and the Hohenstein Research Institute jointly established Oeko-Tex, the International Association for Research and Testing in Textile Ecology. The OekoTex Standard 100 contains detailed analytical procedures for specific substances
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that are ecologically hazardous, stipulating individual limit values based on research data. The detailed features of these various ecolabelling schemes have been reviewed [14] and an analytical comparison of the EU Ecolabel, Nordic Swan, Eco-Mark and Green Seal systems gave rise to a set of eight criteria to be taken into account in any effective ecolabelling programme [15].
2.2 Colorants in Waste Waters The high visibility of water-soluble dyes released to the environment ensures that only extremely low concentrations in watercourses would not be noticed. A typical limit of visibility in a river would be as low as 0.1 to 1 mg/l. but this varies with the illumination, colour and clarity of the water. The human eye can detect a reactive dye at a concentration down to 0.005 mg/l in pure water, particularly in the red to violet sector [16]. There is considerable debate, however, about what level of environmental hazard is represented by visible colour in a watercourse. The view has been expressed that dyestuffs should not be regarded as water pollutants because at concentrations of the same order of magnitude as these visibility limits their harmful effects are negligible [17]. Nevertheless, even though this colour problem is mainly if not entirely an aesthetic one, the fact is that the general public will not tolerate coloured amenity water and the problem therefore has to be addressed and rectified [18-20]. It has been estimated that although about 450 kilotons of organic dyes are manufactured annually worldwide, some 9 kilotons (2%) are wasted in manufacture and another 41 kilotons (9%) wasted during application [21]. The extent to which dyes from various classes are lost in exhaust dyebaths and wash liquors has been assessed (Table 2.1). Such losses vary considerably, however, according to depth of shade, liquor ratio and application technique. On average for all classes of dyes, losses are typically 10% for a deep shade, 2% in a medium depth and negligible for a pastel dyeing [22]. Residual colour in treated waste liquors from disperse dyeing may be attributable to the presence of dispersing agents of the formaldehyde-naphthalenesulphonate type. These agents contribute colour in two ways. Not only do these chemicals absorb visible light to a limited extent they also undergo photochemical reactions on exposure of the contaminated waste liquors to sunlight, resulting in the formation of yellowish oxidation products [23]. The main consideration regarding the environmental impact of residual dyes is concerned with toxicity to aquatic organisms. This is normally expressed in terms of the LC50 value, which represents the concentration of the substance under test that is required to kill 50% of the organisms exposed. With the exception of a small minority (about 2%, mainly basic dyes), organic dyes generally show only low toxicity to fish (Table 2.2) and other organisms such as Daphnia magna. There is little or no published data for the effects of dyes on this freshwater invertebrate [22]. Bioaccumulation is also important, defined as the factor F = Ca/Ce, where Ca is the concentration of the pollutant in the fish species and Ce that for the general environment [24]. The partition coefficient (P) of the colorant in an noctanol/water mixture can be used as an indicator of bioaccumulation. If P is less than 1000 it can be predicted that F in fish will be less than 100, a level at which no problems are foreseen. More than 75 dyes investigated by ETAD gave F values
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of less than 100. Water-soluble dyes do not bioaccumulate and even those disperse dyes and pigments that give P values above 1000 still show no evidence of bioaccumulation in fish [22]. Algae form an important part of the aquatic ecosystem, with algal photosynthesis a critical source of oxygen. The adverse effects of dyes in inhibiting growth of green algae (Selenastrum capricornutum) do not parallel the effects on fish, so that no conclusions about the one can be drawn from the other. Nevertheless, those basic dyes that yield low LC50 values in fish toxicity tests also tend to inhibit algal growth at concentrations as low as 1 mg/l. In an investigation by the American Dye Manufacturers Institute (ADMI), among a series of 56 dyes tested, 15 inhibited algal growth and 13 of these were basic dyes. Investigations have been carried out by ETAD and ADMI to determine whether dyes have an adverse effect on waste water bacteria and hence whether dyes could have any deleterious effect on the operation of effluent treatment plants. ETAD developed a screening test, later adopted by the OECD, to detect the effect of dyes on the respiration rate of aerated sludge. Using this test, it was found that only 18 of the 202 dyes examined had an LC50 value of less than 100 mg/l and these were all basic dyes. Of the 30 basic dyes tested only 12 did not show an inhibitory effect at the test level of 100 mg/l. At this limit dyes are unlikely to adversely affect the bacteria in polluted water or soil. In view of the good to excellent fastness of most colorants, it is not surprising that they are not readily biodegradable. Biodegradability may be defined as the degree of decomposition of an organic contaminant after biological treatment under specified conditions [25]. With the brief retention times normally prevailing in effluent plants, there is practically no evidence of biodegradation of colorants under aerobic conditions. Bioelimination includes removal of the colorant by adsorption on the biomass as well as that undergoing biochemical decomposition. A large majority of dyes can be absorbed by the biomass to the extent of 4080%. High adsorption occurs with basic dyes, direct dyes, disperse dyes and most of the premetallised and milling acid dyes [26,27]. The only dye types that are not substantially absorbed during biological treatment are the highly soluble multisulphonated levelling acid dyes and virtually all reactive dyes, which share similar characteristics.
2.3 Reuse of Dyes Reuse of dyebaths was once common practice, especially when applying natural dyes by traditional methods. When synthetic dyes were introduced and standard methods of manufacture were developed, there was a greater justification and demand for reproducibility at the dyeing stage. The pressure of dwindling resources and the economic and environmental benefits of recycling effect chemicals where practicable have revived interest in the possibility of dyebath reuse. Instrumental colour matching can be adopted to measure the content of dyes in a partly exhausted dyebath and to provide an appropriate shading addition to adjust the concentrations back to their original levels. Recycling is worthy of consideration for vat dyebaths because the redox process is reversible and only the dye absorbed by the cellulosic fibres is oxidised at a later stage of processing [28]. One of the dyehouses in North Carolina has been successfully recycling indigo dyebaths for many years [29]. Sulphur dye effluent from traditional dyeing systems contains sulphides and thiosulphates as well as
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the residual unfixed dye. The discharge of sulphide liquors to drain is not normally permissible because of the toxicity of hydrogen sulphide vapour that would be released under acidic conditions. Stringent standards are required for consent and sulphide waste is normally separated from other effluent streams and treated by oxidation or precipitation. Reactive groups in dye molecules that fail to react with the substrate are hydrolysed during dyeing or discharge of the residual dyebath. Recycling is therefore not a viable option in the case of reactive dyeings [28]. Bioelimination (the sorptive removal of dyes during biological treatment of effluent) is also ineffective for reactive dyes, which show little adsorption in this way. This behaviour is independent of the degree of sulphonation or the ease of hydrolysis of the reactive dye molecules. There is no chemical change to direct dyes during orthodox application, so their exhaust dyebaths are eminently suitable for recycling. Membrane processes have been used successfully to remove direct dyes from dyehouse effluents. There are possible cost savings associated with reuse of the electrolyte, depending on the rejection properties of the membrane [28]. Considerable adsorption of direct dyes occurs during biological treatment of dyehouse waste liquors containing them. This effect is not dependent on the degree of sulphonation of the direct dye molecules [22]. Disperse dyes remain unchanged during orthodox application and their exhaust dyebaths are suitable for recycling. Such dyes were successfully removed from an effluent stream by a microfiltration membrane module on an industrial scale [30]. In this trial the permeate was reused but not the dyes, although other work has demonstrated that recycling of disperse dyes is possible [31]. Moderate to high adsorption of disperse dyes takes place during biological treatment of dyehouse effluent. Basic dyes do not undergo chemical change during dyeing, but the proportion remaining in the exhausted dyebath is low (typically 2-3%) and scarcely justifies isolation for reuse. Recycling of the process water, however, may allow recovery of the inorganic salts and other auxiliary chemicals present [28]. There is normally a high degree of sorptive removal of residual basic dyes during biological treatment of effluent. This is important, because basic dyes tend to exhibit toxicity to aquatic organisms. Acid dyes remain unchanged during dyeing and are highly suitable for reuse. Removal of acid dyes from dyehouse effluent has been achieved by membrane processing on an industrial scale. The process water and auxiliary chemicals are also suitable for recycling to the dyebath [28]. The sorptive removal of acid dyes during biological treatment of effluent varies with their degree of sulphonation. Levelling acid dyes of high solubility exhibit low sorption, whereas the more hydrophobic neutral-dyeing dyes are bioeliminated to a much greater extent [22]. Reuse of the permeate from a reverse-osmosis membrane process on dyebath effluent containing premetallised acid dyes has been achieved [30].
2.4 Arylamines from Azo Dyes In the debate about the toxic effects of dyes and chemicals, there is no doubt that carcinogenic effects are perceived by the general public as the most threatening. Chemicals remain a focus for this concern in spite of the weight of evidence that they make only a minor contribution to the incidence of cancer
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[3,6,32]. The generally accepted estimate of cancer causation, based on mortality statistics, indicates that only 4% of all cancer deaths are attributable to occupational exposure. Another 2% are considered to arise from environmental causes and 1% from other forms of exposure to industrial products. As by far the largest chemical class, it is perhaps not surprising that azo dyes have attracted most attention with regard to carcinogenicity. Some structurecarcinogenicity trends for azo dyes and their metabolites have been discussed with a view to attempting prediction of dye carcinogenicity [33,34]. If an azo dye is carcinogenic and is relatively stable in the hydroxyazo tautomeric form, the dye itself is likely to be the active carcinogen. In contrast, those dyes that exist predominantly in the ketohydrazone form are more readily reduced to metabolites. In this case, the pro-carcinogen is likely to be an arylamine and the ultimate carcinogenic potential can thus be deduced from the availability of a suitable active site on the metabolite. Azo pigments, because of their extreme insolubility and low bioavailability, are unlikely to be metabolised even if they exist preferentially in the hydrazone form [33]. Most water-soluble azo dyes do not form carcinogenic arylamines when reductively cleaved. In many cases, the reduction products are arylaminesulphonic acids, which have little or no carcinogenic potential. Anaerobic conditions, such as apply in digesting sewage sludge or when residual colorants are present in river sediments, favour biodegradative reactions. Under these circumstances the biodegradation of dye chromogens is a primary cause of colour removal. Dyes are quite readily absorbed by sludge, suspended solids or sedimental matter. With azo colorants these conditions render the azo group susceptible to reductive cleavage, giving rise to arylamines as breakdown products [35,36]. There is concern that arylamine metabolites formed under such anaerobic conditions could be desorbed later into the aquatic aerobic environment and thus represent a hazard. The arylamines, however, are generally susceptible to aerobic degradation. Aniline and its monosubstituted derivatives, such as anisidines, phenetidines and toluidines, are readily degraded. Diaminobiphenyls, including benzidine, dianisidines, tolidines and dichlorobenzidines, are more resistant but still inherently biodegradable [37]. A wider selection of arylamine metabolites from azo dyes, including arylaminesulphonic acids, gave broadly similar results [38]. Epidemiological studies first alerted the colorants industries to causal links between certain manufacturing operations and an increased risk of bladder cancer among workers [3,39,40]. Regulations were passed in the 1960s that placed a virtual ban on the importation and use of certain dye intermediates, such as benzidine and 2-naphthylamine, and certain processes, including auramine manufacture [41]. Most responsible colorant manufacturers in Europe, Japan and the USA ceased production of benzidine-based dyes in the early 1970s due to inability to ensure their safe handling in the dyehouse. However, owing to the attractive economic and technical merits of such dyes on leather and cellulosic fibres, manufacture continued in other parts of the world (for example: Latin America, India and the Asia Pacific region). The voluntary cessation of manufacture of benzidine and the dyes derived from it by major manufacturers in the developed economies created a series of research targets to find replacements with the corresponding technical properties. Alternative non-mutagenic diamines were sought [42] and found to yield dyes exhibiting satisfactory performance [43]. Unfortunately, these were almost always substantially less cost-effective than the analogous benzidine-based
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products they were intended to replace and which were still available commercially from non-traditional suppliers. Following the emergence in the early 1990s of conclusive evidence of animal carcinogenicity from CI Acid Red 114 derived from o-tolidine and CI Direct Blue 15 derived from o-dianisidine, several dyemakers ceased production of these and other dyes made from these two diamines. A thin-layer chromatographic method was developed to identify benzidine, o-tolidine, o-dianisidine and related non-sulphonated benzidine derivatives and optimised to separate benzidine from these analogous diamines [44]. In 1994 the German government issued an amended regulation concerning consumer goods involved in direct body contact, including clothing, shoes and bedlinen. This banned the use of azo colorants that could be reduced to give any of twenty specified arylamines. These amines have been classified by the German MAK organisation as substances that have been unequivocally proven to be carcinogenic. Following the German ban, the EU ecolabelling scheme adopted the same list of twenty arylamines and added two more that have been classified by the EU as category 2 carcinogens [45]. A full list of these specified amines is given in Table 2.3, including the actual names on the official list and some alternative systematic names. The German regulation embraced the concept that an azo dye capable of being cleaved to yield a carcinogenic arylamine is itself a carcinogen. A group of 278 azo dyes has been recognised as being carcinogenic according to this definition [9]. Most of these are derived from o-toluidine, benzidine and its symmetrically 3,3'-disubstituted derivatives o-dianisidine and otolidine (Table 2.4). When the implications of this unilateral ban by the German authorities began to be realised, it seems to have become a prime example of how not to enact legislation in such a commercially and technically complex area. Although the major dyemakers in Western Europe, Japan and the USA had already ceased manufacture of benzidine-derived azo dyes in the early 1970s, the 1994 ban had an immediate and substantial impact on certain sectors of the dye-using industries, notably cotton textiles and leather. The globalisation of trade in the 1990s ensured that there were many repercussions both inside and outside the EU. No risk analysis had been carried out and there was no consultation with interested parties outside Germany before the ban, is spite of notification procedures required by EU regulations [46]. Although the 1994 regulation specified consumer goods, suppliers were asked by German retailers to guarantee that all the materials supplied were free from the banned dyes. Thus companies higher up the supply chain became involved, if their goods containing such dyes were to be imported into Germany. A major stumbling-block initially was the lack of an official list of dyes to be banned. This left dye users uncertain about the availability of acceptable dyes from suppliers, especially colorant merchants who do not manufacture the products that they sell. Until 1996, there was no officially recognised test method to isolate and identify all the specified arylamines after extraction from textiles or leather. Analysis was sometimes undertaken by testing organisations lacking the necessary skills or expertise. It was not surprising, therefore, that spurious results (false positives) were sometimes obtained [46]. For example, 2-naphthylamine has been detected occasionally in dyes derived from 1-naphthylamine, owing to contamination of the intermediate [47].
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Benzidine may be formed by reduction of 4,4´-dinitrobiphenyl, which can arise by homolytic dediazoniation of the 4-nitrobenzenediazonium ion and dimerisation of the resulting 4-nitrophenyl radicals [47,48]. In a similar way, 4-aminobiphenyl can result from dediazoniation of the benzenediazonium ion and subsequent coupling of the resulting phenyl radical with aniline [47,49]. Hydrolysis of the amide linkage in CI Pigment Red 8 can release p-chloroaniline and decomposition of the non-azo dye CI Acid Blue 150, made from bromamine acid and benzidine, can regenerate benzidine [50]. It is also possible for 2-naphthylamine to be formed by desulphonation from water-soluble dyes derived from Tobias acid (2-naphthylamine-1-sulphonic acid) [50,51]. In 1996 an official German analytical method was published for the detection of banned amines in relation to cotton, viscose, wool and silk; a second method was introduced for leather [52]. The official procedure recommends reduction at 70°C with a sodium dithionite solution buffered to pH 6 with citrate [53]. More aggressive conditions are known to produce false positive results where the amine detected is an artefact of the test procedure, resulting from chemical reactions other than azo cleavage [47,50]. The thin-layer chromatographic separation depends on differences in rates of diffusion of the arylamines with various eluants on sorption layers of varying polarity [54,55].
2.5 Sensitisation and Acute Toxicity of Dyes Allergic contact dermatitis or skin sensitisation by dyes or other chemicals appears as a persistent irritating rash. The presence of certain dye stains on the skin has been known to accelerate the reddening effect of sunlight exposure (erythema). The causation of skin sensitisation by dyes, both in animal tests and in exposed workers or the general public, has been reviewed [56,57]. Cases of occupational skin sensitisation [58] attributable to dyes are uncommon [59], even in those workplaces where inadequate handling precautions have been taken. Occasionally, reports of organic pigments causing skin sensitisation have arisen but such cases appear to arise from the presence of soluble impurities. Typically, disperse dyes of the nitrophenylazo (e.g. CI Disperse Red 17) or aminoanthraquinone (e.g. CI Disperse Blue 3) types have been implicated in cases of contact dermatitis [60,61]. Circumstances common to such cases appear to be heavy depths of low fastness on nylon (rather than polyester) and occurring in articles of clothing that are in direct contact with the skin, often in areas that are likely to become moistened by perspiration. Hosiery, socks, blouses and close-fitting athletic or fashion wear, such as velvet leggings, are representative of the types of garment where this problem has arisen [40]. A list of nine such dyes that may be sensitising has been drawn up: CI Disperse Yellow 3, Orange 3, 37 and 76, Red 1 and Blue 1, 35, 106 and 124. Garments containing any of these dyes should carry a hazard warning label and for contact clothing such as hosiery they should not be used. The possible mechanism of sensitisation in the case of nitro-substituted azo dyes is thought to be the production of the quinonimine derivative by reduction and oxidation [59]. Sensitisation of the respiratory tract by inhaling dust particles from various chemicals has frequently been reported in industry. It is likely to result in symptoms of respiratory disease or distress when the sensitised individual is exposed to a specific allergen. Respiratory allergy is the clinical manifestation of this state, with bronchial asthma or allergic rhinitis (resembling hay fever) constituting typical disease symptoms. It is believed that relatively high exposure
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levels are important in the induction phase. There is also evidence that a predisposition to respiratory allergy may be caused by genetic or other factors. As there is no suitable animal test system for respiratory sensitisation, ETAD has provided guidance on the hazard labelling of reactive dyes in this context [62]. Instances of severe sensitisation to the dust from reactive dyes have been reported [63]. These prompted the UK Health and Safety Executive to initiate a study involving about 440 workers in 51 dyehouses who were in contact with reactive dye powders. About 15% of them showed work-related respiratory or nasal symptoms. In 21 individuals their allergic reactions could be attributed to contact with one or more specific reactive dyes [64]. Reactive dyes are capable of reaction with amino, hydroxy and thiol groups in proteins. Such a reaction seems to be the initial step of the sensitisation process. The reactive dye may react with human serum albumin (HSA) to form a dye-HSA conjugate, which behaves as an antigen. This in turn gives rise to specific antibodies and these, through the release of mediators such as histamine, produce the allergic symptoms [6,64]. Acute toxicity refers to effects that occur within a brief time after a short-term exposure, such as a simple oral administration. The generally low acute oral toxicity of colorants is well-established [65-67]. This is normally expressed in terms of the LD50 value, a statistically derived dose that is expected to cause death in 50% of treated animals (typically rats) when administered over a prescribed period in the test. In 1974 ETAD began a programme to generate a systematic toxicological database. More than 80% of commercial dyes have an LD50 value (rat, oral) greater than 5000 mg/kg. In response to an EEC Council Directive of 1979 regarding the labelling of dangerous substances, ETAD in 1986 decided to publish a list of twelve colorants that have been classified as toxic on the basis of their acute peroral LD50 values. These varied within the range 25 mg/kg (CI Basic Red 12) to 205 mg/kg (CI Basic Blue 81) and the list included six basic dyes, three azoic diazo components, two acid dyes and one direct dye. Although such data provide an essential basis for advice on safe handling procedures, long-established experience indicates that dyes, and even more so organic pigments, present few acute toxicological risks providing good practices are followed.
2.6 Heavy-Metal Contaminants in Dyes Much effort has been devoted to minimising the trace metal content of colorants and in effluents from dyemaking plants. Heavy metals are widely used as catalysts in the manufacture of dyes. Mercury is used when sulphonating anthraquinones, copper when reacting arylamines with bromoanthraquinone and dichromate as oxidant when making triphenylmethane dyes. Certain basic dyes and stabilised azoic diazo components (Fast Salts) are marketed in the form of tetrachlorozincate complex salts. Metal salts are used as reactants in dye synthesis, particularly for the ranges of premetallised acid, direct or reactive dyes, which usually contain copper, chromium, nickel or cobalt. Difficulties arising particularly from the presence of chromium residues in effluents from factories involved in the manufacture or use of premetallised dyes have stimulated research on dye complexes of iron (III), which is unlikely to give rise to significant pollution problems because of a much higher permitted level in effluent compared with that for chromium [68-70].
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The toxic effects of trace metals towards animals or aquatic life are highly dependent on the physical and chemical form of the contaminant [71]. For example, dissolved copper (II) or chromium (III) ions are highly toxic, whereas the same atoms coordinated within stable organic ligands such as dye molecules are not harmful. Unfortunately this is not widely acknowledged in setting limits for consent conditions, where the total metal content is often specified rather than the forms in which it is present. The permitted levels for trace metals in dyehouse effluents vary from one country to another and even between different areas in the same country [1,25]. When restrictions on the contamination of effluent by chromium residues were imposed in the 1970s, the initial reaction in the wool dyeing industry was to predict the rapid demise of chrome dyes. This expected decline did not materialise because of the outstanding fastness of chrome dyeings and the efforts made to minimise effluent pollution [1,21]. In 1996 member firms of the GuT carpet ecolabelling scheme introduced a voluntary ban against the use of metalcomplex dyes on certain nylon floorcoverings. Extension of this ban to all carpets made from nylon, wool or their blends has been predicted [72]. Ecolabelling schemes covering apparel and household textiles must also take account of the presence of premetallised dyes because of the obvious risk that extraction into perspiration or saliva can take place from dyeings of inadequate wet fastness [73]. The presence of residual unbound transition-metal ions on a dyed substrate is a potential health hazard. Various eco standards quote maximum permissible residual metal levels. These values are a measure of the amount of free ions extracted by a perspiration solution [74]. Histidine is an essential amino acid that is naturally present as a component of perspiration. It is recognised to play a part in the desorption of metal-complex dyes in perspiration fastness problems and in the fading of such dyes by the combined effects of perspiration and sunlight. Virtually all of the chrome dyes that remain of major commercial importance are simple monoazo structures. These products are easy to manufacture from lowcost intermediates, readily water-soluble and build up well to heavy depths. Owing to their relatively small molecular size they show good level-dyeing properties when applied to wool at pH 4 and the boil. The unique combination of level-dyeing behaviour and outstanding wet fastness offered by chrome dyes made them increasingly important in the 1970s, when shrink-resist wool knitwear suitable for laundering in household washing machines was introduced. Since then there has been a growing awareness of the environmental hazards associated with chromium compounds, especially the hexavalent chromium form [75]. Although 10 mg per day of chromium (III) in food is normal for good health, it is important to concede that chromium (VI) is highly toxic to mankind and aquatic life [76]. Chromium is more toxic in soft than in hard water and game fish are more susceptible than coarse fish, factors that are relevant in determining consents [22]. Typical limiting values in the UK regarding permissible amounts of chromium for discharge to effluent are 0.2-0.5 mg/l as the more potent chromium (VI) dichromate ion and 2-4 mg/l as the chromium (III) cation. Proposed ecological criteria for the EU ecolabel with respect to chromium baths are 0.5 mg/l for chromium (VI) and 5 mg/l for chromium (III) ions [77]. Legislation covering the release of chromium-containing effluents is becoming increasingly strict, especially in Germany, the UK and the USA [78]. Draft regulations indicate that no more than 0.1 mg/l total chromium will be tolerated in future [76].
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Originally the amounts of dichromate used in the traditional afterchrome process varied between about 25 and 50% of the total amount of chrome dye present, with the lower and upper limits set at 0.25% and 2.5% of the mass of wool. These quantities were well in excess of the stoichiometric amount required even for formation of the 1:1 complex. This approach resulted in excess dichromate remaining in the aftertreatment bath for discharge to effluent, as well as on the wool fibre where it contributed to further oxidative degradation. Users of chrome dyes are increasingly concerned with dyeing under mild conditions, typically at pH 4.5 and 85°C followed by chroming at pH 3.5-3.8 and 90°C to minimise wool damage [79,80]. Chroming in a fresh bath tends to give lower residual chromium content but does increase the processing costs. Every effort should be made to exhaust the dyebath as much as possible, because any residual mordant dye will complex with chromium (III) ions in the dye liquor. Apart from complicating effluent treatment, this raises the possibility of lower fastness resulting from deposition on the surface of the wool. Concern regarding exposure to chromium is not just related to effluent discharge. It is obvious that residual unbound chromium present on the fibre is also a potential hazard. The Oeko-Tex ecolabel specifies 1 ppm total chromium or cobalt on babywear and 2 ppm chromium or 4 ppm cobalt on other garments. These figures represent the amount of free metal extracted by a standard perspiration solution. In general, typical 1:2 metal-complex dyeings will satisfy these requirements in full depths, chrome dyeings only to medium depths and premetallised 1:1 complexes only in pale-depth dyeings.
2.7 Halogenated Colorants During the 1990s, several environmental agencies and activist groups argued that the banning of chlorine and all chlorinated organic chemicals will be necessary to protect the environment. The impact of such a comprehensive ban would be immense, particularly for those organic dyes and pigments that are predominantly dependent on chlorine-containing intermediates used in their manufacture. Approximately 40% of all organic pigments contain chloro substituents in the pigment structure itself, although this corresponds to only 0.02% of total chlorine usage. In the EU and Japan, controls on the discharge of absorbable organohalogen (AOX) compounds are becoming increasingly severe. Certain shrink-resist treatments and insect-proofing agents for wool, trichlorobenzene carriers for polyester dyeing and reactive dyes of the chloroheterocyclic types certainly fall into this category [81,82]. Interestingly, organofluorine compounds do not fall into the AOX classification since the fluoride ion liberated as soluble silver fluoride according to the test protocol is not detected. It seems likely, therefore, that reactive dyes containing vinylsulphone or fluoroheterocyclic groups will become more important [21]. Many direct, disperse and vat dye structures contain chloro-substituted aryl nuclei and some have trifluoromethyl substituents to enhance light fastness. It is to be hoped that rational evaluation of the available evidence will convince regulatory authorities that the mere presence of an inert chloro substituent in a molecule does not mean that it will pose an environmental risk [40].
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2.8 Natural Dyes In Germany during the 1970s there was a growing demand by supporters of the Green movement for greater use of natural dyes of vegetable origin to dye natural fibres such as wool, silk and cotton. This trend has been taken up enthusiastically in tropical countries with climates suitable for growing such dyeyielding plants [83,84], including woad [85], lac dye [86], lichens [87] and even tea plants [88]. Research has been carried out with a view to minimising the amounts of mordanting chemicals necessary to apply natural dyes [89-91], in order to offset criticism that such processes would be as polluting as the use of premetallised synthetic dyes. Contamination of effluent streams with residual heavy metals from mordanting [92,93] is by no means the only drawback of a return to the multi-stage coloration methods that prevailed before the discovery of synthetic dyes [94,95]. Natural dyes are tinctorially weaker and duller; variability of harvest, climate and location makes definition of a standardised product extremely difficult. They often give level-dyeing problems and show inferior fastness to light and washing compared with their synthetic counterparts. Only madder red and indigo blue are able to meet typical commercial standards of fastness. From the viewpoint of application, most natural dyes fall into the mordant class and require a variety of metal salts to assist in their fixation to natural fibres. There are also a few that can be regarded as vat, direct or acid dyes [96] but they serve only to supplement the limited colour gamut attainable by a two-stage mordant dyeing process. Reactive, disperse, basic and premetallised dyes are entirely absent, so that natural dyes are of zero interest for the dyeing of polyester, polyester/cotton, nylon or acrylic fibres. Calculations show that about 400 kg of cultivated dye plants are required to yield enough dye for dyeing the same depth as given by 1 kg of synthetic dye on cotton or wool, at a cost ratio of about 100:1. Vegetable dyes have to be isolated from leaf extracts and the large volume of residual biomass requires disposal [97]. Furthermore, if the present worldwide consumption of dyed cotton were coloured with natural vegetable dyes rather than synthetic ones, approximately 30% of the world’s agricultural land would be needed for their cultivation [92,94]. This is more than 13 times the area currently in use to grow the cotton and does not take into account what would be required if the other textile fibres, paper and leather were also to be coloured in the same way. The extraction of natural dyes from animal sources is just as wasteful of resources, time-consuming and by no means environmentally friendly. To obtain 1 kg of cochineal scarlet requires the harvesting of 150,000 insects reared on cactus plants. The living insects are swept off the leaves into bowls or cloths and executed by immersion in steam or hot water, then dried by prolonged exposure to sunlight. If production of the classical vat dye Tyrian purple were to be restored on a large scale, isolation of 1 kg of this colorant would demand the slaughter of about 10 million specimens of a Mediterranean mollusc (Murex brandaris). Vast quantities of these discarded shells that develop an obnoxious odour when exposed to the sun would become unsightly spoilheaps, extremely offensive to the coastal environment [72,94].
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2.9 Reducing Agents The most important reducing agent that is necessary for several key dyeing processes is sodium dithionite (Na2S2O4, sodium hydrosulphite, CI Reducing Agent 1). In conjunction with caustic soda, it is almost always the preferred reducing system for vat dyes and the most effective reduction clear for disperse dyeings on polyester. With sulphur dyes, however, the alkaline dithionite system is difficult to control and some of these dyes may be partly destroyed by overreduction. Nevertheless, it is effective with sulphurised vat dyes and the CI Solubilised Sulphur brands. There are serious potential environmental problems associated with sodium dithionite, since in effluent it produces sulphite and sulphate ions. Although the sulphite can be readily oxidised to sulphate, this does not alleviate the major problem, since high concentrations of sulphate can cause damage to unprotected concrete pipes and drainage chambers. Thus there are environmental reasons for seeking effective alternatives to dithionite, although in most instances their higher cost and greater stability to atmospheric oxidation makes them of particular interest for continuous dyeing and printing rather than for exhaust dyeing or reduction clearing. Certain derivatives of sodium dithionite are commercially significant, particularly sodium formaldehyde-sulphoxylate (HOCH2OSONa, sodium hydroxylmethanesulphinate, CI Reducing Agent 2) made by the reaction of sodium dithionite with formaldehyde. The corresponding product of reaction with acetaldehyde, sodium acetaldehyde-sulphoxylate (HOCH(CH3)OSONa, sodium hydroxylethanesulphinate), is less important but both of these agents have been used in vat printing, especially the flash-ageing process. They are much more stable than sodium dithionite at lower temperatures and can be used to prepare stable pad liquors and print pastes. At higher temperatures in steaming or flash ageing they are capable of bringing about rapid reduction of vat dyes. As vat dyes are invariably fixed under alkaline conditions, the sodium salts of sulphoxylic acid are preferred to the basic salts of zinc or calcium. These are HOCH2OSOZn(OH), zinc formaldehyde-sulphoxylate (CI Reducing Agent 6), and HOCH2OSOCa(OH), calcium formaldehyde-sulphoxylate (CI Reducing Agent 12), which are unstable under alkaline conditions. Sodium formaldehyde-sulphoxylate has been used occasionally in combination with sodium dithionite but other two-component systems based on formaldehydesulphoxylates have usually depended on an accelerator or catalyst system. For example, a process that has been adopted to some extent in bulk practice [98,99] comprises a strongly alkaline solution of sodium borohydride (NaBH4, sodium tetrahydroborate) together with a second reducing agent consisting of sodium formaldehyde-sulphoxylate and the catalyst sodium nickel cyanide, NaNi(CN)2. Various advantages have been claimed for this process, but there are misgivings regarding the environmental acceptability of sodium nickel cyanide [100]. Alternative accelerators used with sodium formaldehyde-sulphoxylate include sodium dimethylglyoxime, anthraquinone and various aminoanthraquinonesulphonic acids. Although sodium borohydride is itself a reducing agent, it generally reacts too slowly for use alone in vat dyeing systems; nor is there any evidence that it will act as a stabiliser for sodium dithionite [101], as has sometimes been suggested. Trisodium nitriloethanesulphinate, N(CH2CH2OSONa)3, has been proposed as a reducing agent for flash-age printing and batchwise package dyeing with vat dyes
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on cotton at high temperature [102]. This agent does not appear to have attained commercial application, however. Sodium formaldehyde-sulphoxylate is rarely, if ever, used for the application of sulphur dyes owing to handling difficulties, inadequate cost-effectiveness and poor efficiency [103]. The performance of this rather stable reducing agent in the reduction clearing of disperse dyeings on polyester is also unsatisfactory. In a detailed comparison with four other reducing systems, formaldehyde-sulphoxylate showed inadequate improvement of fastness ratings even when applied in the presence of anthraquinone as activator [104]. Thiourea dioxide (NH2.CSO2.NH2, formamidinesulphinic acid, CI Reducing Agent 11) is a powerful reducing agent for vat dyes. It gives lower concentrations of sulphite and sulphate anions in effluent than sodium dithionite, but shows certain practical disadvantages. In hot aqueous alkali sodium formamidinesulphinate decomposes to yield urea and the active reducing agent species sodium hydrogen sulphoxylate (HOSONa). Although thiourea dioxide is more stable than dithionic acid under acidic conditions, sodium formamidinesulphinate formed in alkaline media is more readily oxidised than sodium dithionite, thus negating the potential advantage of the higher stability in acid solution. Thiourea dioxide can cause over-reduction of indanthrone dyes. Experimental work has indicated the possibility of using thiourea dioxide in combination with other agents such as (a) sodium dithionite, formaldehyde and caustic soda, or (b) saturated aliphatic ketones, but commercial exploitation has not been evident. In a comparison of five reduction clearing systems for disperse dyeings on polyester, only thiourea dioxide gave results as good as sodium dithionite. It is three times as expensive for this purpose but causes only half the sulphur pollution of dithionite in effluent [104]. In a detailed evaluation of exhaust dyeing with sulphur dyes, thiourea dioxide gave colour yields and fastness ratings similar to those with sodium sulphide, although is some cases slightly different hues were observed. On balance, thiourea dioxide is much less hazardous to the environment than sodium sulphide; the major improvement is the decreased amount of oxidant required for chemical treatment of the effluent and a second advantage is the markedly lower sulphate ion content. Although thiourea dioxide is more expensive, calculations of processing costs must take into consideration the cost of treating waste liquors compared with those containing sulphide [105]. Hydroxyacetone (CH3COCH2OH, acetol) is a sulphur-free reducing agent originally introduced for vat dyeing that has also proved moderately successful with sulphur dyes. It requires strongly alkaline conditions at elevated temperatures because of its relatively sluggish reducing rate; the vapour is flammable and dyebaths have a penetrating odour characteristic of acetone. Hydroxyacetone is suitable for application in dyeing with indigo, vat or sulphur dyes, including the continuous dyeing of cotton yarn with sulphur dyes or indigo, as well as the exhaust dyeing of knitgoods with vat dyes that are difficult to reduce. Hydroxyacetone does not cause over-reduction of indanthrone vat dyes but does give different tones with carbazole vat dyes compared with conventional dithionite methods. Although colour yields are not quite as high as with dithionite, the advantages include biodegradability, lower COD values and decreased chemical usage. The effluent contains no sulphide, sulphite or sulphate, but hydroxyacetone does contribute to the dissolved organic carbon content. There are environmental benefits claimed for the use of mixtures of sodium dithionite and hydroxyacetone [106]. Organic complexes of iron have been investigated as alternatives to sodium dithionite in vat dyeing [107] and the reduction clearing of disperse-dyed
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polyester [104]. Iron (II) hydroxide Fe (OH)2 is a powerful reducing agent and this reducing power increases with higher alkalinity of the medium. Alkalinity results in precipitation, however, and an iron (II) complex must be formed in order to maintain a homogeneous solution. The selected complex must be reasonably stable but not so inert that the iron (II) ion cannot exert its reducing action. Complexes of iron (II) chloride with triethanolamine or gluconic acid (an oxidation product of glucose) are suitable, the acid complex being favoured on environmental grounds because it does not contain nitrogen or sulphur. It is advantageous to keep the gluconic acid concentration low to ensure a lower COD value in the effluent, but too low a concentration of iron (II) ions results in paler unlevel dyeings because of insufficient vatting of the pigment. The use of iron (II)-gluconic acid did not cause over-reduction of sensitive dyes even when dyeing was prolonged. With most dyes the colour yield was equal to that given by sodium dithionite [107]. Traditionally the most widely used reducing agents for sulphur dyeing have been sodium sulphide (Na2S) and sodium hydrosulphide (NaHS). Technically these are still the most widely preferred, not only for their effectiveness but also because they are relatively inexpensive. Environmental concerns are gradually curtailing the use of sulphides as reducing agents [108,109], although as late as 1995 some 90% of all sulphur dyes applied worldwide were still reduced by sulphides [103]. The environmental problems arising from sulphide usage include the toxicity of hydrogen sulphide, corrosion of the effluent drainage system, damage to the treatment plant and the often associated high pH and unpleasant odours. Sulphides cause no odour nuisance above pH 9 but at neutral or acidic pH values gaseous hydrogen sulphide is liberated. Neutralisation or acidification may occur in the dyehouse or during waste stream mixing. In some applications, particularly in jet and winch dyeing, there is a danger that dissolved sulphides may be prematurely oxidised by air. Antioxidants, added along with sulphur dyes and primary reducing agent, may be applied in order to minimise this problem. Polysulphides (such as Na2S2 or Na2S4) or sodium tetrathionite (Na2S4O6) have been widely used for this purpose and provide improved dyebath stability. However, polysulphides yield free sulphur on acidification and this can lead to acrid odours of sulphur dioxide on the dyed substrate. An alternative approach to sulphide dyebath stabilisation is to add a relatively more stable alkaline reducing agent such as sodium dithiodiglycolate (NaOOCCH2SSCH2COONa) [110]. Indeed, such compounds may be selected as primary reducing agents in conjunction with alkali. Although they do not give rise to environmentally undesirable inorganic sulphides in the effluent, their chemical stability results in high COD values, often causing more problems than those arising from sodium sulphide [111]. 2-Mercaptoethanol (HSCH2CH2OH) with alkali has been suggested as an alternative to sulphide methods [112], offering the advantages of no sulphides in the effluent and no odour from the dyebath, although the product itself can give off unpleasant and highly toxic fumes. This sulphur dyeing process is relatively costly with a tendency towards lower yields and a more restricted range of suitable dyes than when using traditional systems, so that it has not achieved significant commercial exploitation. The most promising alternative to sulphides, from an environmental point of view, is the reducing sugar glucose with sodium hydroxide or carbonate. This system does not satisfactorily reduce all sulphur dyes, however. It is reasonably effective with the CI Solubilised Sulphur brands [112], with which it may be used
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either alone or in conjunction with sodium polysulphide, usually resulting in increased dye yields. It can be used as a supplementary reducing agent with the CI Leuco Sulphur brands, thus giving a lower sulphide content in the dyebath, or together with sulphide or polysulphide in the reduction of the traditional waterinsoluble CI Sulphur brands. The glucose reducing system has a characteristic odour of burning sugar that many people consider preferable to the odour of an alkaline sulphide bath, although others find it excessively sweet and nauseous. Nevertheless, the versatility of glucose-based binary systems has been emphasised [103]. The glucose system is pH- and temperature-sensitive, becoming transformed into various decomposition products and gradually losing its reducing power. The intermediate by-products possess some reducing action but are not sufficiently stable. The binary dithionite/glucose system has a reduction potential only slightly lower than that of dithionite alone, even though the potential of glucose alone is much lower. The careful addition of glucose lowers the potential of sodium dithionite to the point where full colour yield is achieved without the risk of over-reduction. Dyeing tests have confirmed that although sodium dithionite alone is exceptionally concentration-sensitive, the addition of glucose gives a more stable system. Optimal colour yield and reproducibility are obtained even if dyeing temperature, time and chemical concentrations fluctuate slightly, within narrow limits. Similar results have been observed for other binary systems of glucose with hydroxyacetone or sodium formaldehyde-sulphoxylate as stabiliser.
2.10 Carriers Although polyester or cellulose triacetate fibres are usually dyed under pressure, their blends with wool are still dyed at or near the boil. Under these conditions a carrier must be used to ensure adequate exhaustion and build-up of disperse dyes within a commercial dyeing cycle. Even in high-temperature dyeing the usual maximum temperature (about 130°C for polyester and 120°C for triacetate) may be excessive, as when dyeing textured polyester fabrics that may suffer loss of crimp at 130°C. Smaller amounts of a suitable carrier may be added in these circumstances to promote migration and assist more rapid and complete exhaustion. Carrier compounds fall into four main classes: phenols, chloroalkyl compounds, aryl hydrocarbons and aryl esters. Typical examples in commercial use include biphenyl, trichlorobenzene, dichlorophenyl ethers, o-phenylphenol, Nalkylphthalimide, methylnaphthalene, methyl cresotinate, methyl salicylate, butyl benzoate, diethyl or diallyl phthalate. Mechanisms proposed for carrier action and ideal requirements of carrier chemicals have been comprehensively reviewed [113]. Over the last decade the use of carriers has declined markedly and continues to do so, essentially for health, safety and environmental reasons [114-117]. In some countries these products are now virtually banned. Nearly all carrier compounds exhibit some or all of the following hazards: toxicity, physiological irritancy or poor biodegradability (Table 2.5). Harmful effects from carrier dyeing can arise in three ways: 1. carrier that is volatilised during dyeing or subsequent heat setting becomes an atmospheric contaminant
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2. residual carrier in the fibre can be a health hazard, as well as causing an unpleasant odour on heating or during storage 3. residual carrier in the dyebath contributes to effluent pollution and may be environmentally harmful. Typical pollution loads for comparable high-temperature and carrier methods are given in Table 2.6. Carrier residues differ considerably in odour. A dry heat treatment of dyed polyester at 160-180°C after dyeing, to volatilise the residual carrier, is the most effective method of minimising problems of odour and anomalously low light fastness. The steam volatility of carrier chemicals and their toxicity to human and plant life need careful consideration. For example, o-phenylphenol has relatively low volatility in steam and traditionally has been preferred for use in machines open to the atmosphere. The chlorinated benzenes, on the other hand, are readily steam-volatile and are toxic, so should not be used in machines where volatilised carrier is likely to condense inside the roof of the machine, forming ‘carrier spots’ where they fall onto the fabric. Biphenyl is relatively non-toxic to river life but is not readily biodegradable; methylnaphthalene, also of low toxicity, is moderately biodegradable, but halogenated benzenes are both toxic and difficult to biodegrade.
2.11 Volatile Organic Compounds Carrier chemicals represent an important subclass of the large group of volatile organic compounds (VOCs). The toxicity of many aryl hydrocarbons and their chloro derivatives is well-established but they are often useful as specific low-cost solvents and extractants. They may be involved in a range of industrial syntheses and are often found in effluents from chemical plants. Appropriate waste water treatment is essential and it is advisable to separate such effluents into three streams: (a) aryl hydrocarbons, (b) chlorinated compounds, and (c) low concentrations of mixed contaminants. Each stream can be processed individually to avoid problems arising in subsequent biological treatment [118]. A 1997 EU Directive defined a VOC as any organic compound having a vapour pressure of at least 10 Pa at 20°C, or corresponding volatility at other operating temperatures in industry. A rapid technique for determining whether any given organic liquid should be characterised as a VOC has been described [119]. The 1990 Environmental Protection Act defined a VOC as any carbon compound that participates in atmospheric photochemical reactions. Carbon monoxide, carbon dioxide, carbonic acid, ammonium carbonate, metal carbonates and carbides are all excluded. A broader description of atmospheric contaminants is that of hazardous air pollutants (HAPs) and a programme of regulations (MACT standards) is being developed, based on maximum admissible control technology. The best source of information that signals the potential hazards of a solvent or other VOC is the SDS available from the product supplier (section 2.1.2). Precautions in handling, storage, transportation and disposal are provided. Recommended occupational exposure conditions are defined in terms of threshold limit values (TLVs). The prospect of fire hazard becomes important whenever the temperature of a solvent exceeds its flash point [120]. An EU commission proposal covering the use of VOCs does not significantly affect typical textile dyeing and finishing plants but textile coating and printing
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processes that exceed a defined threshold of VOC consumption are included within the legislation [4]. Only a few specific solvents are used in the formulation of aqueous-based coatings. They can be quantified individually and the total VOC content determined directly. A solid-phase micro-extraction technique with gaschromatographic analysis of the extract has been recommended [121]. The Environmental Protection Act obliges printers of paper and packaging materials to prevent the atmospheric release of solvents by: 1. reduction at source and eventual elimination or replacement of solvents 2. abatement of emissions using biological digestion, thermal oxidation or solvent recovery. Carbon adsorption systems for solvent recovery offer the most attractive solution for factories using only one or two solvents [122]. Dry-cleaning establishments and even spot removal using solvents, as frequently necessary in garment processing and finishing plants, are included in EU legislation covering VOCs [4]. Traditional dry-cleaning with perchloroethylene has been subjected to increasingly stringent limitations and research has been carried out to seek eco-friendly alternatives. More than 130 organic solvents were screened for this purpose and twelve of them were selected for further study. Excellent cleaning performance was achieved using ultrasound together with a relatively dilute aqueous solution of a propyl, n-butyl or t-butyl monoether of propylene glycol (ROCH2CH2CH2OH) [123].
2.12 Formaldehyde For decades the presence of formaldehyde in textiles and other consumer products has given rise to much controversial discussion. Consumer protection agencies have been extremely critical about the alleged health risks, whereas producers have claimed that it is relatively innocuous and readily detectable by its strong odour and eye irritation at tolerably low concentrations (0.01-0.1 ppm). However, workers exposed frequently to the vapour soon become acclimatised and can tolerate substantially higher levels (1-10 ppm). The present UK maximum atmospheric exposure limit in the working environment is 2.0 ppm. Formaldehyde vapour causes coughing and temporary shortness of breath. However, its aqueous solubility results in rapid absorption mainly in the upper airways, so that the irritant effect does not reach the lungs. It is a severe eye irritant, dissolving in the ocular fluids and causing inflammation. After more than a century of commercial exploitation and extensive use by the medical profession, there is still no evidence that exposure to formaldehyde has caused cancer in humans [124]. The main cause of allergic contact dermatitis from cotton and its blends with synthetic fibres is the presence of residual free formaldehyde and N-methylol reactant finishes capable of releasing formaldehyde [125]. Similar derivatives of formaldehyde are sometimes used as stabilising agents to minimise wool fibre damage in the dyeing of wool blends above the boil. Repeated contact of the skin with formaldehyde-treated textiles can result in dermatitis at concentrations as low as 2% by mass of the formaldehyde derivative on the textile material. The release of formaldehyde from resin-finished textiles into the atmosphere is governed by several factors, including: 1. choice of resin/catalyst system
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2. inadequate curing of the applied resin 3. omission of the washing stage after curing 4. extensive use of steam pressing and forming in garment manufacture 5. prolonged storage of finished fabrics or garments in confined spaces 6. efficiency of local ventilation system 7. atmospheric conditions of storage, high temperature and humidity favouring formaldehyde release. Regulations covering formaldehyde in textiles and clothing vary significantly between member nations of the EU but efforts are being made to seek harmonisation by carrying out a complete risk analysis. This is likely to include an obligation to warn the consumer to wash a garment before first use if there is a high level of free formaldehyde present [4]. Several test methods are available for determination of formaldehyde release from textiles. These vary in terms of ease of operation, consistency of test results and practical significance in relation to problems of (a) exposure to formaldehyde vapour in the working environment, and (b) the propensity for skin irritation from resin-treated textiles [124].
2.13 Biocidal Agents Formaldehyde is just one of a large group of biologically active compounds with a variety of applications in textile dyeing and finishing. It is not surprising that chemicals selected for their biocidal activity are the focus for particular concern with regard to their potential impact on the environment. The most far-reaching EU legislation for the aquatic environment is the Dangerous Substances Directive, which established the List I (Black list) and List II (Grey list) substances. List I covers products which should be eliminated on account of their toxicity, persistence and bioaccumulation. List II is for products with a less deleterious effect. Typical examples of substances that feature strongly in these lists are given in Table 2.7. All the Black-listed organic chemicals exemplified here are chlorinated structures, unlike those of the Grey list examples. In the UK the 1989 Water Act made specific reference to the discharge of prescribed substances (Red list in Table 2.8), consents for which are the responsibility of Her Majesty’s Inspectorate of Pollution (HMIP). Twelve of these substances also appear in List I of the EU Directive (Table 2.7). There are three major areas in the textile processing industry where biocidal agents are of practical interest: 1. insecticides used on wool and other animal fibres to prevent attack by moth and beetle larvae 2. bactericides used as preservatives to prevent biodegradation of natural polymers, such as thickening agents or starch sizes 3. bactericides applied to textile materials in order to inhibit bacterial activity during storage or end-use applications. Damage to wool garments during storage can occur when certain moths or beetles lay their eggs within the wool yarns and fibrous material is consumed by the larvae that emerge from these eggs. Any insecticide applied must be directly effective against these specific larvae. Since effective agents are not harmful to other insects that do not consume wool, it seems likely that such products operate only through the digestive tract of the larva. In addition to health and
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safety requirements, fastness properties also need to be taken into account. Fastness demands on wool carpets are not so stringent as for machine-washable wool clothing. Environmental, as well as health and safety factors, have resulted in an almost total ban on the use of dieldrin, the first product to be used for this purpose. This hexachlorinated multicycloalkyl compound proved to be toxic to humans, animals, fish and birds, and was highly persistent in the environment. Similar factors coupled with the relatively small size of the market for insectproofing of wool are acting restrictively against several other well-established agents used for this purpose. In an excellent review of the subject [126], the important insect-proofing agents in commercial use are placed in two categories. The first category includes those compounds that were developed specifically as wool mothproofing agents, most of these being anionic multichlorinated aryl structures suitable for application to wool together with typical milling acid dyes. One of the best known is the tetrachloromonosulphonated compound sulcofenuron. This product is relatively costly but has very good fastness to washing and light. Sulcofenuron is one of the only three types of insect-resist agent permitted in the GuT ecolabelling scheme (section 2.1.3). Compounds in the second category were originally developed as pesticides for agricultural use. Most of these have pyrethroid structures, including permethrin and cyfluthrin which are the other two insect-resist agents permitted for the GuT label. These synthetic pyrethroids share certain structural features with their natural counterparts, the most important of which is pyrethrin I. Owing to the increasing costs of registration and ecotoxicological testing of new products in relation to only modest market demands, it is unlikely that novel agents designed specifically for wool could now be developed [126]. Any further advances in this field are likely to be spin-offs from agricultural pesticide development, although this research sector is also unfavourably affected by trends in favour of organic farming methods. Natural pyrethroids lack the photochemical and hydrolytic stability necessary for use as insect-resist agents for wool. The synthetic analogues possess satisfactory stability and exhibit the low mammalian toxicity and lack of bioaccumulation of the naturally occurring agents. Permethrin, however, is toxic to aquatic organisms and is therefore subject to increasingly severe discharge limits. There is some evidence that this agent is not fully effective against certain beetle larvae and mixtures of permethrin with other compounds are sometimes preferred [127]. Vinylsulphone fibre-reactive insect-resist agents have been described, in which the insecticidal feature is an organophosphorus group. The vinylsulphone group reacts with nucleophilic sites in wool keratin, conferring excellent wet fastness. An interesting aspect of these agents is that they do not act as insecticides on wool until they become activated by hydrolysis of an ester bond that links the reactive group to the biologically active portion of the molecule, when enzymatic wool digestion processes take place within the insect larva. A development reported more recently involves reduction of the cystine disulphide bonds in wool with thioglycolic acid to form nucleophilic thiol groups, followed by crosslinking of the wool keratin using bifunctional reactive dyes [128]. Although this approach conferred improved insect resistance there were adverse effects on physical characteristics of the wool such as tensile strength, shrinkage and stiffness, thus limiting the potential of the process for commercial use. To achieve the highest fastness ratings, insect-resist agents should be applied to wool from the dyebath. This may not always be possible, however, and
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alternatives include addition to the scouring bath or together with spinning lubricants [126]. Particular care is necessary in the choice and application level of these agents for fibre blends, since their partition behaviour between the component fibre types varies. Pyrethroids, for example, tend to partition in favour of nylon in wool/nylon blends [129]. It is not surprising, given their aquatic toxicity, that these agents are subjected to continual environmental scrutiny [130]. In order to comply with minimum discharge requirements, it is obviously helpful to be able to apply the minimum levels needed for adequate functionality. Bactericides are often added to microbially nutritious polymers, such as natural thickeners, dispersants and size polymers, in order to protect them against biodegradation during storage [131-133]. In the case of printing pastes, biological degradation on storage can lead to a significant loss of viscosity and rheological malfunction. Various biologically active compounds have been utilised as bactericides, but formaldehyde and certain phenol derivatives, such as cresols, chlorophenols or phenylphenols, have proved particularly suitable. These are added either by the supplier of the thickener concentrate, or by the printer during formulation of the stock thickener paste. The environmental implications of adding biocides to polymer formulations must always be borne in mind since, by definition, all effective biocides are more or less toxic. Thus addition of a biocide may render a normally biodegradable polymer less so. Hence biocides should be added at as low a concentration as possible, although the rate of microbial attack of the polymer increases with the humidity during storage. Sensitive thickening agents, however, can be protected by incorporating a suitable biocide, usually at less than 0.1% on the mass of thickener, which is just about sufficient for effectiveness [132]. Together with the thickener, this small amount of biocidal contaminant is washed out with copious volumes of water after print fixation, entering the effluent stream at such high dilution that it no longer has any bacteriological effectiveness. It has been suggested that suppliers of dispersants and polymer concentrates will cease to incorporate preservatives in their products [131]. The printer or warp sizer will then be responsible for using, in exactly the correct amount, a preservative that is just about tolerable under the conditions of application. Formaldehyde has been widely used for this purpose but is now ecologically undesirable. Phenolic compounds have also been popular and effective, but the nucleophilic OH group present can adversely affect the yield and hue of dyes containing highly reactive groups [133]. Commission textile processors need to be aware of the possible use of biocides by fabric suppliers, yarn spinners or cotton growers higher up the supply chain. Instances have arisen where cotton finishers in the UK have unknowingly discharged effluent containing pentachlorophenol (PCP) even though they have not been applying it in their fabric processing routines. This problem came to prominence in 1990 when it was revealed that overseas weavers of these imported fabrics had been adding PCP to their warp sizing baths as a preservative to avoid starch size degradation in hot climates. When the starch is removed during fabric desizing and scouring, the soluble sodium pentachlorophenate is also carried into the effluent stream. PCP can be difficult to treat in a conventional biological treatment plant because of its adverse effect on waste water bacteria. This stable compound is not degraded by bacterial action and is bioaccumulative, causing progressive damage to the aquatic environment [4]. Some imported cottons were found by analysis to contain PCP levels as high as 500 ppm. A typical jig load of fabric containing only
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5 ppm of PCP can give rise to a level of 1.7 ppm in the effluent volume from that dyeing. This compares with a consent level of only 0.02 ppm of PCP in effluent after treatment and discharge [134]. Eventually elimination of PCP usage by the Asian weavers was achieved in the early 1990s by international negotiation but at the cost of greatly increased claims for fabric damaged by mildew during transit to Europe, an occurrence that was relatively rare before the ban on using PCP in sizing baths [4]. Biocidal chemicals have been applied to cotton for many years as rotproofing finishes for awnings, tents and geotextile materials. There is increasing incorporation of bactericidal agents in medical textiles, hosiery, underwear and sportswear, in order to prevent infection, promote healing or prevent the development of odours. A comprehensive index of antimicrobial chemicals is available [135]. Although this volume covers a variety of applications, there is a section devoted to ten agents suitable for textile treatment (Table 2.9). Only three of these are chlorinated structures. Poly(ethylene glycol) crosslinked with dimethyloldihydroxyethyleneurea (DMDHEU) has been reported to give fabrics with antibacterial properties suitable for nonwoven protective surgical apparel [136]. Triclosan kills a wide variety of bacteria that cause food poisoning, dysentery, cholera, pneumonia, tetanus, meningitis, tuberculosis and sore throats. It is also capable of inhibiting the development of bacterially generated odours and deactivates the yeasts responsible for mouth ulcers and athlete’s foot. This compound can be incorporated during fibre production to give durable antibacterial properties [137]. Despite the widespread use of Triclosan in toothpaste and acne creams, apparently it may cause allergic dermatitis in susceptible individuals, especially when used in products for foot treatment [135]. Poly(hexamethylenebiguanide) has been used for the sanitisation of swimming pools. On textiles, it has been incorporated into antibacterial fabrics ranging from medical products to odour-free socks [138,139]. This cationic polymer is mainly of interest on cotton and is applied as a solution of the hydrochloride by padding at neutral pH. The positively charged polymer exhibits high substantivity for the negatively charged fibre surface. Approximately 1% of the agent on the mass of the cotton is optimal for bacterial performance. No thermal curing treatment is necessary but a wet-on-wet padding with an anionic fixing agent can be given to enhance fastness of the antibacterial effect. This polymer has a long history of use as a bactericide, exhibits low toxicity, does not contribute to AOX values and is environmentally acceptable, being bioeliminated by adsorption on the biomass.
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V A Shenai, Colourage, 44 (Dec 1997) 41.
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D Brown and P Laboureur, Chemosphere, 12 (1983) 405.
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D Brown and B Hamburger, Chemosphere, 16 (1987) 1539.
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C E Searle, Chem. Brit. 22 (1986) 211.
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E A Clarke and D Steinle, Rev. Prog. Coloration, 25 (1995) 1.
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Carcinogenic substances regulations (London: HMSO, 1967).
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J S Bae and H S Freeman, AATCC Review, 1 (Sep 2001) 67.
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V G Yadav, Colourage, 45 (Jan 1998) 53.
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A Puntener, D Mausezahl and C Page, J. Soc. Leather Technol. Chem., 77 (1993) 1.
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P A Turner, JSDC, 111 (1995) 53.
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K T W Alexander, JSDC, 112 (1996) 341.
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S W Oh, M N Kang, C W Cho and M W Lee, Dyes and Pigments, 33 (1997) 119.
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C T Page and J Fennen, J. Soc. Leather Technol. Chem., 82 (1998) 75.
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A Puntener, J Fennen and C T Page, J. Soc. Leather Technol. Chem., 80 (1996) 1.
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U Sewekow and A Westerkamp, Melliand Textilber., 78 (1997) 56.
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W B Achwal, Colourage, 44 (May 1997) 29.
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A Hudson and P A Britten, Rev. Prog. Coloration, 30 (2000) 67.
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A Geisberger, JSDC, 113 (1997) 197.
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B Küster and U Wahl, Textilveredlung, 32 (May/Jun 1997) 121.
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K Hübner, E Schmele and V Rossbach, Melliand Textilber., 78 (1997) 720.
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F Klaschka, Melliand Textilber., 75 (1994) 193.
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K L Hatch, Text. Chem. Colorist, 30 (Mar 1998) 22.
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F Gallagher, JSDC, 113 (1997) 307.
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V A Shenai, Colourage, 45 (Oct/Nov 1998) 62.
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K D Wozniak, A Keil and D Müller, Textil Praxis, 45 (1990) 965.
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P Elsner, Textilveredlung, 29 (1994) 98.
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H Motschi, JSDC, 116 (2000) 251.
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J M Wattie, JSDC, 103 (1987) 304.
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A Docker, J M Wattie, M D Topping, C M Luczynska, A J N Taylor, C A C Pickering, P Thomas and D Gompertz, Brit. J. Industrial Medicine, 44 (1987) 534.
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E A Clarke and R Anliker, The handbook of environmental chemistry, Vol. 3A, Ed. O Hutzinger (Berlin: Springer-Verlag, 1980) 181.
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E A Clarke and R Anliker, Rev. Prog. Coloration, 14 (1984) 84.
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R Anliker in Toxic hazard assessment of chemicals, Ed. M L Richardson (London: Royal Society
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H S Freeman, L D Claxton and V S Houk, Text. Chem. Colorist, 27 (Feb 1995) 13.
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J Sokolowska-Gajda, H S Freeman and A Reife, Dyes and Pigments, 30 (1996) 1.
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J Sokolowska-Gajda, JSDC, 112 (1996) 364.
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G M P Morrison, G E Batley and T M Florence, Chem. Ind. (1989) 791.
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B C Burdett, Dyer, 180 (Sep 1995) 16.
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U Sewekow, Text. Chem. Colorist, 28 (1996) 21.
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K Parton, JSDC, 114 (1998) 8.
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K Schaffner and W Mosimann, Textilveredlung, 14 (1979) 12.
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D M Lewis, JSDC, 113 (1997) 193.
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L Benisek, Wool Record, 158 (Apr 1999) 42.
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P A Duffield, R R D Holt and J R Smith, Melliand Textilber., 72 (1991) 938.
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G Meier, JSDC, 95 (1979) 252.
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A Hoyes, Wool Record, 151 (Apr 1992) 49.
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B M Müller, Rev. Prog. Coloration, 22 (1992) 14.
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W S Hickman, JSDC, 109 (1993) 32.
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P M Chan, C W M Yuan and K W Yeung, Text. Asia, 28 (Oct 1997) 58: 29 (May 1998) 59.
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M Ryder, Wool Record, 157 (May 1998) 41.
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K D Casselman, Wool Record, 157 (Oct 1998) 57.
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H T Deo and B K Desai, JSDC, 115 (1999) 224.
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G Dalby, JSDC, 109 (1993) 9.
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U Sewekow, Melliand Textilber, 76 (1995) 330.
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H Bürger, Melliand Textilber., 76 (1995) 910.
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B Glover, JSDC, 114 (1998) 4.
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B Glover and J H Pierce, JSDC, 109 (1993) 5.
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B Glover, Text. Chem. Colorist, 27 (Apr 1995) 17.
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D J Hill, Rev. Prog. Coloration, 27 (1997) 18.
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G Horstmann, JSDC, 111 (1995) 182.
[98]
M M Cook, Amer. Dyestuff Rep., 68 (Mar 1979) 41.
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G L Medding, Amer.Dyestuff Rep., 69 (Sep 1980) 30.
[100] L C Ellis, AATCC Nat. Tech. Conf. (Oct 1981) 266. [101] U Baumgarte and U Keuser, Melliand Textilber., 47 (1966) 286. [102] P Senner and J Schirm, Textil Praxis, 20 (1965) 1006. [103] M Hähnke and C Schuster, Melliand Textilber., 76 (1995) 414. [104] S Anders and W Schindler, Melliand Textilber., 78 (1997) 85. [105] W Czajkowski and J Misztal, Dyes and Pigments, 26 (1994) 77. [106] H Schlüter, Melliand Textilber., 76 (1995) 143. [107] B Semet and G E Grüninger, Melliand Textilber., 76 (1995) 161. [108] R A Guest and W E Wood, Rev. Prog. Coloration, 19 (1989) 63. [109] W Marx, Textilveredlung, 26 (1991) 74. [110] C Heid, Z. ges. Textilindustrie, 70 (1968) 626. [111] W E Wood, Rev. Prog. Coloration, 7 (1976) 80. [112] R Klein, JSDC, 98 (1982) 106. [113] A Murray and K Mortimer, Rev. Prog. Coloration, 2 (1971) 67.
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[114] D Fiebig and K König, Textil Praxis, 32 (1977) 694. [115] G Dürig, Textilveredlung, 11 (1976) 62. [116] P Richter, Textilveredlung, 13 (1978) 134. [117] R C D Kaushik, J K Sharma and J N Chakraborty, Colourage, 40 (Apr 1993) 33. [118] F Rey and V Oles, Textilveredlung, 32 (1997) 252. [119] C Nielsen, B Hogh and E Wallstrom, J. Oil Col. Chem. Assoc., 80 (1997) 467. [120] R L Stout, J. Coatings Technol., 70 (Oct 1998) 161. [121] A C Censullo, D R Jones and M T Wills, J. Coatings Technol., 69 (Jun 1997) 33. [122] B Mills, Professional Printer, 41 (Nov/Dec 1997) 10. [123] R E McCall, F M A Patel, G N Mock and P L Grady, AATCC Internat.Conf. and Exhib. (1997) 150. [124] M Hewson, JSDC, 110 (1994) 140. [125] M H Beck, Text. Horizons Internat., (Oct 1992) 96. [126] D M Lewis and T Shaw, Rev. Prog. Coloration, 17 (1987) 86. [127] J Barton, Dyer, 185 (Sep 2000) 14. [128] J L Burtness and B M Gatewood, AATCC Internat. Conf. and Exhib. (1995) 324. [129] R J Mayfield, JSDC, 101 (1985) 17. [130] J Haas, SDCNZ 13th Internat. Symp. (Oct 1992) 37. [131] F Bayerlein, Melliand Textilber., 70 (1989) 948. [132] W Tiedemann, P Hülsberg, P Horlacher and D Kinast, Textil Praxis, 47 (1992) 337. [133] F Gähr, G Schulz, C Leibold and J M Engel, Melliand Textilber., 77 (1996) 398. [134] A P Lockett, JSDC, 108 (1992) 474. [135] M Ash and I Ash, The index of antimicrobials (Aldershot: Gower, 1996). [136] R S Jinkins and K K Leonas, Text. Chem. Colorist, 26 (Dec 1994) 25. [137] J W McCurry, Text. World, 147 (Jan 1997) 52. [138] J D Payne and D W Kudner, AATCC Internat. Conf. and Exhib. (1995) 341; Amer. Dyestuff Rep., 85 (Jun 1996) 26. [139] J D Payne, Text. Chem Colorist, 28 (May 1996) 28; JSDC, 113 (1997) 48.
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Table 2.1 Extent to which dyes are lost in exhaust dyeing and washing-off [22]
Dye class Reactive Sulphur Acid Direct Vat Disperse Metal-complex Basic
Proportion of applied dye lost in effluent (%) 20-50 30-40 7-20 5-20 5-20 1-20 2-5 2-3
Table 2.2 Fish toxicity of dyes in common use [22]
LC50 value (mg/l) <1 1-10 10-100 100-500 >500
Proportion of tested dyes (%) 2 1 27 31 28
Table 2.3 List of specified arylamines classified as carcinogens [9] Name on official list o-Aminoazotoluene 4-Aminobiphenyl Benzidine p-Chloroaniline 4-Chloro-o-toluidine 3,3´-Dichlorobenzidine 3,3´-Dimethoxybenzidine 3,3´-Dimethylbenzidine 4-Methoxy-m-phenylenediamine 6-Methoxy-m-toluidine 4,4´-Methylene-bis(2-chloroaniline) 4,4´-Methylenedianiline 4,4´-Methylene-o-toluidine 4-Methyl-m-phenylenediamine 2-Naphthylamine 5-Nitro-o-toluidine 4,4´-Oxydianiline 4,4´-Thiodianiline o-Toluidine 2,4,5-Trimethylaniline
Alternative systematic name 2-Amino-4,4´-dimethylazobenzene 4,4´-Diaminobiphenyl 4-Chloroaniline 4-Chloro-2-methylaniline o-Dianisidine o-Tolidine 2,4-Diaminoanisole 2-Methoxy-5-methylaniline 4,4´-Diamino-3,3´-dichlorodiphenylmethane 4,4´-Diaminodiphenylmethane 4,4´-Diamino-3,3´-dimethyldiphenylmethane 2,4-Diaminotoluene 2-Aminonaphthalene 2-Methyl-5-nitroaniline 4,4´-Diaminodiphenyl ether 4,4´-Diaminodiphenyl thioether 2-Methylaniline
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Table 2.4 Azo dyes that yield the specified arylamines on reduction [9]
Specified arylamines 4,4´-Diaminobiphenyl (benzidine) 3,3´-Dimethoxybenzidine (o-dianisidine) 3,3´-Dimethylbenzidine (o-tolidine) 2-Methylaniline (o-toluidine) Other arylamines Total
Number of azo dyes 59 80 51 51 37 278
Proportion (%) 22 29 18 18 13 100
Table 2.5 Chemical and biochemical oxygen demand data for various types of carrier chemical [116] Carrier type o-Phenylphenol N-Alkylphthalimide Arylcarbonate ester Methyl cresotinate Dichlorobenzene Trichlorobenzene
COD (mg/l) 1000-2000 1000-2100 900-1900 800-1700 500-1000 300-1000
BOD5 (mg/l) 200-800 100-200 700-800 200-800 0 0
Table 2.6 Chemical and biochemical oxygen demand data for high-temperature and carrier dyeing methods [117] Polyester dyeing method Carrier dyeing on the winch High-temperature jet dyeing
COD (mg/l) 1888-2043 584-722
BOD5 (mg/l) 189-200 165
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Table 2.7 Important examples of dangerous substances in the Black and Grey lists [22] List 1 Cadmium Mercury Carbon tetrachloride Chloroform Dichloroethanes Trichloroethylene Perchloroethylene Hexachlorobutadiene Hexachlorobenzene Pentachlorophenol Monochlorobenzene Dichlorobenzenes Trichlorobenzenes DDT Aldrin Dieldrin Endrin Lindane
List II Arsenic Boron Chromium Copper Inorganic tin Iron Lead Nickel Vanadium Zinc Ammonia Sulphide Organotin compounds Benzene Toluene Xylenes
Table 2.8 Initial Red list of prescribed substances in the Water Act [22] Cadmium Mercury Dichloroethanes Hexachlorobutadiene Hexachlorobenzene Pentachlorophenol Trichlorobenzenes Polychlorobiphenyls Tributyltin compounds Triphenyltin compounds DDT
Aldrin Dieldrin Endrin Lindane Atrazine Azinphos-methyl Dichlorvos Endosulfan Fenitrothion Malathion Simazine Trifluralin
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Table 2.9 Typical biocidal finishing agents applied to textile materials [135] Caprylhydroxyethylimidazoline Captan Dichlorophene Diiodomethyl-p-tolylsulphone Dimethylaminopropylricinoleamidobenzyl chloride Lauryl/stearyltrimethylammonium halide Myristylamine Sodium 2-mercaptobenzothiazole Triclosan Zinc pyrithone
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Figure 2.1 Key steps for chemical risk management [10] Hazard assessment Risk evaluation
Exposure assessment
Risk prevention
Chapter 3 Services and Resources 3.1 Organisation and Stages for Coloration The application of colour to textile substrates not only increases significantly the added value of the product but is a major factor by which textile manufacturers and retailers can differentiate their products from those of their competitors. Thus the wet processing sector is an important component of the textile manufacturing chain that provides the design, colour, finish and performance characteristics of the finished product. Wet processing plants are established to apply colour at various stages of the manufacturing chain. The stages at which coloration can take place are illustrated in the flow diagram in Figure 3.1. These facilities may be part of a vertical organisation or they may be an independent company providing a commission service to a particular sector of the textile industry. The structure of the industry and the involvement of commission dyers vary in different sectors of the textile industry, as shown in Table 3.1. Traditionally, commission processors were located in close proximity to their major customer base but, as indicated in section 1.5, this is no longer necessarily the case as the textile manufacturing chain is increasingly organised on a global basis. Large textile manufacturing groups have conventionally carried out their wet processing within the group, with coloration being carried out at various possible stages throughout the processing sequence. In view of the infrastructure required, smaller manufacturing companies tend to rely on commission processors. Various factors, including problems with quality or quick response, sometimes encouraged the smaller manufacturers to establish in-house wet processing plants, although this can incur a significant cost penalty. The need for RFT and quick response production has already been emphasised and justified in section 1.5. To achieve such objectives, wet processing requires a significant infrastructure supported by adequate services and resources, the subjects of this chapter. The emphasis placed on these aspects often differs between vertical groups and commission dyers. In the current harsh economic climate, services and resources can be fertile areas for cost savings. Whilst some cost reductions are acceptable, others may significantly interfere with the attainment of RFT production and quick response. Despite the importance of services and resources, provision of them is often less than ideal. Reasons for this include the installation of wet processing in unsuitable buildings, the impact of technological change and the emergence of new legislation.
3.2 Water Supplies Although sporadic attempts have been made to develop solvent or solventassisted methods of dyeing, water remains the principal solvent used in textile processing. For this reason, dyeing and finishing plants were traditionally sited close to abundant supplies of water of suitable quality for textile processing. Water is no longer a cheap resource, however, and now this geographical factor seldom applies. Water quality may have changed for various reasons, such as construction of roads or housing estates where watercourses have been diverted, the incidence of acid rain or drainage of fertilisers from farmland.
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Most of the world’s water is saline, only 2.7% being directly available for use. No less than 88% of this is needed for agriculture, 7% for industry and only 5% for human use. For more than 90% of the earth’s population, water is a limited resource with many regions having less than 10 l/day available. In contrast, the textile processing industry requires 50-150 l/kg of textile material processed [1]. Water for textile processing is usually obtained from one of three sources: 1. surface water from reservoirs or rivers (the quality of river water may vary greatly according to season and flow of water) 2. ground water from wells or boreholes 3. water from a public or municipal source in urban areas (this must be legally potable but it may not be suitable for textile wet processing). 3.2.1 Water Specification Several attempts have been made to establish a specification for water suitable for textile processing. Hardness salts give faults in scouring and dyeing, whilst high solids cause filtration problems in package dyeing of loose fibre, yarn or fabric. A typical specification for process water is given in Table 3.2 [2]. 3.2.2 Water Treatment The treatment of water supplies is usually carried out for the following reasons: 1. removal of solid matter by sedimentation and filtration 2. correction of the pH value 3. elimination of residual chlorine, if this has been added by the water authority 4. removal of hardness (calcium and magnesium ions) by softening processes 5. removal of heavy-metal ions (iron, manganese and copper). These treatments are based on simple neutralisation and precipitation reactions. Water containing suspended humic acids from peat requires treatment with an aluminium salt followed by removal of the coagulated impurities by settling. Chlorinated water can be treated with a reducing agent (sulphite or thiosulphate), before softening using zeolites or synthetic ion-exchange resins. Water for steam raising is softened using lime-soda precipitation containing polyphosphate conditioners. 3.2.3 Water Consumption Detailed surveys of water usage in the 1970s indicated that there were marked differences in water consumption (10-550 l/kg) between companies processing different substrates, as shown in Table 3.3, although use had been reduced over the preceding decade [3]. It has been estimated [3] that 50-60% of dyehouse water is used for processing, 25-35% for cooling and 5-10% for steam raising, although this varies with the process, equipment used and efficiency of processing. Water consumption according to fibre type and process is shown in Table 3.4. In an efficient package dyehouse processing synthetic-fibre yarns, process water requirements may be as low as 8 l/kg with cooling water consumption three times this amount. A similar relationship applies to jet dyeing of fabric [7].
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Welbeck Fabric Dyers Ltd (UK) commission dyers and finishers process about 17.5 M metres p.a. of fabric entailing considerable costs for water and effluent services. Commencing with a thorough audit in 1995 the water usage was assessed in four areas: preparation, dyeing, finishing and non-metered uses. Preparation accounted for about 20% of the total water consumption. By fitting optimised valve settings, the annual usage in the first year was reduced by about 30% (34,000 m3), with incidental savings on water heating [8]. 3.2.4 Water Savings and Conservation Recurring water shortages and the increasing cost of water have encouraged development of measures to reduce water consumption, as listed in Table 3.5. Fluctuations in water requirements, especially the peaks and troughs associated with demands of production, can be alleviated by increasing the storage capacity for water. Since it is recognised that water quality is an important factor in achieving RFT production (Table 1.13), a treatment plant may be required if the water supply is not of adequate quality for textile processing. The costs of this treatment (including considerable capital expenditure) are incentives to minimise water consumption and to reuse water whenever possible, with or without treatment after use. Pressures have increased to improve the effluent quality, providing a further incentive to reuse effluents (section 3.3.2) especially if operating to a standard acceptable for disposal into a river. 3.2.5 Water Recycling Reuse of water from selected continuous preparation, washing and rinsing operations is not new. Counter-current washing has long been used in continuous preparation where rinse water is fed to the previous bath in the processing range. The use of standing baths has long been practised for dyes of relatively low substantivity, such as indigo and sulphur blacks dyed on cotton. The systematic reuse of dyebaths was adopted in the solvent-assisted dyeing of wool [9] to minimise the consumption of the costly solvent (benzyl alcohol). In aqueous dyeing systems successful dyebath reuse demands high exhaustion, dyebath auxiliaries that do not interfere with subsequent dyeings, and estimation of the concentrations of residual dyes, chemicals and auxiliaries. The reuse of dyebaths without intermediate treatment can give water savings up to 90%, depending on the number of times the bath is reused. Reproducibility, fastness and levelness were not impaired by the reuse of dyebaths without treatment when dyeing wool and wool/nylon carpet and handknitting yarns [10]. Substantial savings were obtained in chemicals (57-80%), water (27-62%), energy (20-43%) and effluent charges (11-58%). Similar results have been achieved with dyebath reuse in the dyeing of nylon pantihose with disperse dyes [11], jet dyeing of Nomex with disperse dyes [12], jet dyeing of polyester knitted fabrics with disperse dyes [13] and nonwoven microfibre nylon fabrics with 1:2 metal- complex dyes [14]. A major concern is the possible build-up of additives or impurities, which might interfere with dyeing behaviour. Control of dyebath pH becomes increasingly important and buffer systems will give stable and reproducible conditions [15]. Where water recycling is being implemented, one option is to segregate colourless wastes containing low levels of contamination from coloured wastes requiring treatment before reuse, providing the infrastructure costs of
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implementation on existing sites do not mitigate against this option [16]. A procedure for the reuse of residual finish liquors has been described, based on the application of simple test methods for repeated assessment of the quality of the liquor after storage for various times up to six weeks. Test parameters include pH, electrical conductivity, specific COD, fluidity and visual examination. By comparing these results in relation to storage time, a profile of storage instability can be derived. Hence the residual degree of usefulness of a given batch of liquor can be predicted. Correlation between the residual usefulness defined in this way and the finish effects attainable can be demonstrated. To some extent it is possible to enhance this residual quality level in order to broaden the versatility of the reusable liquors [17]. 3.2.6 Hot Water Supply The availability of a hot water supply at about 50°C, suitable for preparing dyebaths and carrying out rinses, can assist in shortening processing cycles. A high grade of hot water can be obtained by discharging the liquors from polyester dyeings at top temperature through high-temperature drains and then through a heat exchanger. Dyebaths from other dyeing processes can be treated similarly, although too much low-grade hot water may be obtained if the flow rate through the cooling coils is not sufficiently low. Such low-grade hot water is often useful in continuous preparation plants. A level probe can be inserted in the hot water storage tank and a temperature probe in the cooling water return and these are coupled to a microprocessor. High-grade hot water, above a specified temperature, can then be returned directly to the hot water storage tank and heated further, if required, to the target temperature by a heating coil. Water significantly below the target temperature is returned to the cold water supply tank. Water between the specified and target temperatures can be run to waste, unless the hot water supply tank requires topping-up, when this water is added and heated.
3.3 Effluent Effluents from wet-processing plants are complex and their composition varies according to the range of processes carried out. Traditionally most textile effluents (about 90%) were discharged to water authority sewage works but larger companies often operated their own effluent treatment plant. This latter option is likely to increase, not only as effluent treatment charges increase but also as consent limits for discharge to sewer or watercourses become more stringent and cover a wider range of contaminants. Additionally, water recycling becomes a serious option if the costs of the raw water, pretreatment for wet processing and effluent treatment to enable discharge to a river or watercourse are taken into consideration. 3.3.1 Effluent Composition In establishing consent limits and, in the case of sewage plant operation, setting charges for effluent treatment, the important factors are volume, pH, suspended solids and the biological and chemical properties of the effluent that express the degree of pollution, as defined in Table 3.6.
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Determination of COD is more reproducible than BOD. TOC determination may replace BOD and turbidity may replace suspended solids in discharge limits. Typical consent limits are shown in Table 3.7. Apart from the obligation to meet these limits, there may also be a restriction on the volume that can be accepted, especially peak loadings likely to be reached from time to time. Legislation has been introduced in which five processes and twenty-three chemicals (the so-called Red list) are prescribed processes and substances which require separate consideration as regards effluent. The prescribed chemicals include agents such as DDT, pentachlorophenol and dieldrin, the latter two having been widely adopted as mothproofing agents for wool (see also Table 2.8). Companies treating effluent use the so-called Mogden formula to calculate the total cost, although the component charges may vary between different treatment companies. The Mogden formula is explained in Figure 3.2. On average, effluent costs account for about 10% of total production costs in the textile industry [18]. A detailed comparison of water consumption and effluent pollution from various wet processes for cotton (Table 3.8) indicated that bleaching and washing-off require 75% of the water but contribute only 6% of the BOD and 8% of the pollution load. Conversely, desizing and scouring account for about 75% of the BOD and pollution load but use only 6% of the water [19]. However, the pollution load can be significantly decreased by oxidative desizing, which converts carbohydrate residues to carbon dioxide and water [20]. Coloration processes take a 10-20% share in all these distributions and this is also broadly true of the wool dyeing sector. The dyeing and finishing of synthetic-polymer fibres require just as much water, but the BOD is usually substantially lower, owing to the relatively low level of impurities present compared with natural fibres. The major classes of contaminants in textile effluents can be classified into five groups and even the most problematical can be isolated and treated individually [21]. There are advantages in separating effluents into individual streams for treatment according to their characteristics before combining them for discharge. Production-integrated methods of minimising contamination and quantity of waste liquors are now favoured, rather than the traditional reliance on end-of-pipe treatments to deal with the entire pollution load arising from the textile processing sequence. For example, useful by-products such as wool grease can be recovered and recycled. Treatment of concentrated dyebaths is more economical than end-of-pipe treatment of composite waste water [22]. Although the latter approach does not interfere with the choice and flexibility of the textile production pattern, it is the least attractive option economically [18]. 3.3.2 Effluent Treatment Methods Methods of effluent treatment may be classified into three main categories: physical, chemical and biological, as listed in Table 3.9. Primary stages of treatment are mainly physical and include screening, sedimentation, flotation and flocculation to remove fibrous debris, undissolved chemicals and particulate matter. Primary treatment does not significantly remove colour. Secondary stages are designed to eliminate the organic load and consist of a combination of physico-chemical separation and biological oxidation. Biological treatment does not remove sufficient colour, COD and electrolytes to be satisfactory on its own but is often the least costly method of treatment. Physico-chemical separation depends on the forces of chemisorption to extract the colloidal organic
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compounds from the liquid phase. Tertiary stages of treatment have become more important but they make a major contribution to treatment costs. This stage is important for the removal of colour and no one treatment will deal with the removal of all types of colour. Biological processes must be protected from shock loads of industrial pollutants, especially those containing toxic constituents. Preliminary stages include equalisation, neutralisation and disinfection. Major reasons for poor results from settling tanks include significant fluctuations in flow, temperature and composition of waste liquors which cause convection currents and stratification in the tanks and interfere with normal sedimentation. Balancing of flow and composition together with cooling or heat recovery is necessary. Disinfection with chlorine may protect the micro-organisms in the biological stage from toxic contamination. Industrial effluent treatment alternatives are indicated in Figure 3.3. As indicated above, there are many physical and chemical processes available for the treatment of textile effluents. The removal of colour has become a major issue over the last ten years and if treated water meets all acceptable quality standards, recycling becomes a serious consideration. Economic viability is the most important factor in selecting treatment for a dyehouse effluent to remove colour and achieve successful reuse in wet processing. The basic processes used by dyehouses to remove colour, although not necessarily to make it suitable for reuse, include those listed in Table 3.10 [23]. The possibility of water recycling has been extensively reviewed [16]. Aerobic activated sludge processes have been used either on the dyehouse site or at a local sewage treatment plant, to remove biodegradable chemicals measured as either BOD or COD. Effluent containing soluble anionic dyes, such as acid, reactive and direct dyes, is not decolorised by this process since dyes are only partially removed by adsorption on to activated sludge. The chromophoric systems in many dyes are stable to the mildly oxidising conditions in activated sludge and further specific colour removal processes are required before recycling. Oxidation using chlorine dioxide or ozone, coagulation/flocculation, membrane and adsorption processes have all been used in conjunction with activated sludge to meet regulatory consent conditions. Combinations of such processes do not necessarily give a treated water of sufficient consistency for recycling. Anaerobic biological treatments will remove the colour of azo dyes by reduction of the azo groups. A strong oxidant such as chlorine, sodium hypochlorite, chlorine dioxide or ozone may be effective in decolorising dyehouse effluents by destroying chromophoric systems. Catalytic oxidation, using hydrogen peroxide catalysed with iron (II) sulphate in acidic solution (Fenton’s reagent) produces a water quality suitable for recycling for specified applications. The removal of colloidal iron after settling is critical for recycling. This process destroys water-soluble dyes and reduces COD by oxidation of organic matter. Electrochemical variations of the process have been developed and alternative catalytic oxidation processes, such as photochemical systems using ultraviolet light, have been proposed to decolorise effluent. In a recent series of trials, effluents from six dyehouses containing acid, disperse or metal-complex dyes were treated on a pilot plant using various advanced oxidation processes based on hydrogen peroxide or ozone, with or without UV radiation. Decolorisation occurs more quickly than COD reduction under these conditions. The most effective lowering of COD, TOC and AOX requires
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UV/peroxide treatment in the presence of a trace level of iron (II) sulphate as free-radical initiator. The oxidation conditions may have to be varied according to the dye classes or chromogenic groups present. A combination of abiotic oxidation followed by a biological activated sludge process was shown to yield an effluent quality that would permit reuse of the waste water in commercial rinsing or washing processes [24]. Inorganic coagulants based on multivalent calcium, magnesium, aluminium or iron (II or III) salts that form voluminous precipitates with alkalis have long been used to partly remove soluble anionic dyes from coloured effluents. Anionic chemical auxiliaries may also be removed. Water-soluble polyelectrolyte flocculants facilitate separation of suspended flocs containing complexed dyes. A plant for this operation, together with the associated separation and sludge disposal facilities, occupies a large area. As in many physico-chemical dye removal processes, conditions established for optimum removal from one effluent may not be effective with another. To ensure flexibility and consistency, refining treatments may be necessary, particularly for recycling. Activated-carbon adsorbents and ion-exchange resins have been used for water purification, including colour removal. These media are expensive for highly coloured effluents but can be employed either as a principal colour removal method or as a refining treatment. Other adsorbents evaluated include lignite coke, chitin-containing polymers and acid-treated clays. Membrane processes, such as nanofiltration and reverse osmosis, are interesting when recycling is the objective. The effluent is passed through pores small enough to remove dissolved dyes. Pre-filtration is carried out to prevent blocking of the membranes and reverse osmosis will remove acid and reactive dyes of high solubility, whilst also removing salt. However, each effluent requires the selection of an optimum pore size, configuration, membrane type and operating pressure to produce the required water quality, especially if recycling is involved. The practical and economical feasibility of using membrane technology to recycle dye liquors has been assessed. Factors affecting the practicality of recycling dyes include: the dyeing process, classes of dyes used, effluent volume, frequency of use of specific dyes and the auxiliaries added to the dyebath. Similar considerations apply to the suitability for recycling of the process water associated with the dyeing process. Other relevant factors include the water quality attainable before recycling and the process stages for which this water is acceptable. To assess the economics of dyebath effluent reuse, water company treatment and supply charges as well as dye and chemical costs must be considered [25]. In a recent evaluation, dyebath effluents from several dyehouses representing various dyeing processes were treated in a nanofiltration test unit to compare membranes from different manufacturers. Membrane retention values in excess of 99% were attained with regard to residual colour from reactive, disperse, acid and metal-complex dyebaths. The COD and TOC loadings were thereby reduced by about 90%. Laboratory dyeing tests demonstrated that permeates and concentrates yielded by nanofiltration are reusable in the dyehouse. Nonrecyclable concentrates can be subjected to biological aftertreatment to meet the limit values for discharge to the municipal purification plant. The economics of nanofiltration recycling systems depend essentially on the point at which nanofiltration can be rationally integrated into the production process [26]. It is neither feasible nor desirable to attempt to deal with the total effluent flow in this way because of the large-area membranes needed to cope with such
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quantities. Emulsified polymeric materials with low stability to shear stress put severe limitations on this technique by rendering the membrane impermeable, especially washings containing a high content (>10%) of residual acrylic binder. With complex mixtures of contaminants ultrafiltration is applied first to separate out dispersed pigments and polymeric constituents, followed by reverse osmosis to remove dissolved substances such as dyes, salts and surfactants [27]. Available methods of removing colour from effluent were reviewed and descriptions were given of two full-sized treatment plants [23]. One of these is sited at a dyehouse processing about 180 tonnes of fabric per week with a weekly water consumption of 50,000 m3. The basic treatment consists of screening to remove coarse suspended solids, followed by a fine rotary screen to remove fibres and lint. The effluent is then pumped into a balancing tank, holding about 1200 m3, representing 3-4 hours of production. This tank, maintained at about 40°C, evens out the composition and protects the living organisms in the subsequent biological tanks from any shock treatment. Variable-speed pumps transfer the mixed effluent into the activated-sludge tanks. During a three-hour treatment at 40°C in the presence of air, injected through diffusers, the waste material is consumed to produce an increasing amount of biomass. The mixed liquor is then passed to a clarifier. The biomass is allowed to settle by gravity and the clear purified water is drawn off from the top. The settled sludge is pumped into a re-aeration tank, where the biomass completes the digestion of absorbed nutrients before it is contacted with fresh effluent. A cationic polymer is dosed into the coloured water, interacting with the anionic dyes to form an insoluble product. The coloured particles are treated with a flocculant in the presence of any suspended solids to produce large flocs in a reaction tank. These are separated from the clean liquid by dissolved air flotation, in which fine bubbles of air are released into a tank containing the suspension, lifting them to the surface to form a stable layer. The solids from this process are combined with the surplus sludge from the biological tanks for disposal. It is estimated that this plant to treat 50,000 m3 of effluent per week would now cost in excess of £ 3 million. The various methods of removing colour from effluent have been reviewed and an indication given of the capital and running costs involved [28]. Water to be recycled, especially for coloration processes, must at least meet the specification given in Table 3.2. A suggested general quality requirement for treated and recycled effluent to be used in scouring and washing-off processes is given in Table 3.11. Possibilities for recycling and descriptions of successful installations have been discussed [16]. A schematic diagram of a water treatment plant including reverse osmosis for removal of colour is shown in Figure 3.4 [29]. This plant is claimed to recycle 95% of the effluent input. The last twenty years have seen tremendous activity in the areas of water usage and effluent quality. The supply, consumption, recovery and disposal of water were reviewed [21] and many predictions have since become realities. The impact of legislation regarding effluent disposal was reviewed [30]. Many governments in the EU and elsewhere have tightened legislation over the years but have not always imposed this because of a lack of affordable treatment. Apparently this honeymoon period is now over [31] and legislation is likely to be vigorously enforced. There has obviously been a significant improvement in the quality of river water. By 2001, wildlife protection agencies in the UK were rejoicing that many attractive species, including otters, kingfishers and salmon, had returned to areas where they had not been seen for up to 30 years [32]. Some of this improvement
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is a direct result of the demise of the dyeing and finishing industry. However, those dyehouses still in business in Leicestershire now treat their discharge onsite and Severn Trent Water Authority (STWA) has been able to switch off all colour removal plants at its sewage treatment works [32]. The signs are ominous for other dyehouses in the SWTA region since the successes in Leicestershire will prompt action in other areas. Although many dyehouses now treat their effluent prior to discharge, very few actually recycle the treated effluent to achieve a major payback. Simple flocculation will meet the consent limits but the cost of chemicals and the increase in suspended solids contribute significant costs which cannot be passed on to the customer. Crossflow membranes provide the highest quality, allowing up to 95% of the treated water to be recycled. Costs of treating incoming water are decreased and energy savings achieved through heat recovery. Membrane filtration will reduce the COD, suspended solids and colour. Based on a model dyehouse working on three shifts for five days per week and producing 15m3 of effluent per hour, a saving in excess of £100,000 per year has been calculated. The Cognis Securyl EFW system allows water to be recycled with a demonstrated cost saving [33].
3.4 Steam Textile wet processes consume much energy, mostly supplied as steam. Steam heating can be supplied as low-, medium- or high-pressure steam, the latter being essential for the processing of synthetic-fibre materials at high temperature. A useful review of the properties of steam and its use, particularly in steamers used for various textile processes, has been given by Hickman [34]. 3.4.1 Boiler Plant Until the 1940s, coal-fired Lancashire boilers were used for steam raising. These were relatively cheap and easy to construct, simple to operate and did not require high-quality feed water or sophisticated water treatment. Fluctuating steam demands were not a problem since the boiler had good storage capacity for water and steam. A thermal efficiency of about 75% was obtained. This was acceptable while coal was cheap but these boilers required full-time stokers. The Lancashire boiler was replaced by the economic boiler fitted with a combustion chamber and tubes to allow two or three passes of the furnace gases and thereby give a larger heat-conducting surface. However, the steam and water capacity is reduced, so that the standard economic boiler is highly sensitive to fluctuations in steam demand. Lancashire boilers were designed to produce from 2000 to 5500 kg of steam per hour at a working pressure of 150 to 200 psi, whilst economic boilers have a capacity from 450 to 11000 kg per hour at a working pressure of 250 psi. As the temperature of the exit flue gases from either boiler type can be as high as 500°C, an economiser is usually fitted to recover some of this heat. An economiser is a multi-tubular heat exchanger and this is fitted in the gas stream between the boiler and the chimney. The hot gases pass around the outside of the tubes and feed water passes through the tubes. It is estimated that a saving of 1% in fuel costs is obtained for every 4.5°C increase in feed water temperature.
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Both types have been superseded in modern plants by so-called packaged boilers. These are basically modified economic boilers but, as the name implies, the boiler together with all the ancillary equipment (such as feed-water pump, forceddraught fan, oil heater and pump) are mounted on a common base so that the minimum of civil engineering work is required for their installation. Several such units can be installed to meet short- and long-term demands, such as seasonal requirements for space heating. A major advantage of packaged boilers is their ease of installation. Already mounted on a solid base, connections are readily made to the fuel, power and water supplies and also the steam system. Provision is required for disposal of ‘blow-down’ and the chimney, usually an integral part of the boiler, need be only as high as required by the local authority. Packaged systems are usually treble-pass ‘wetback’ boilers, in that the combustion chamber is completely enveloped in the boiler shell and surrounded by the boiler water, whereas in a conventional economic boiler the combustion chamber is outside the boiler shell. With so much heating surface in the packaged boiler, an economiser is not required but the boiler feed water should be preheated in the feed-water tank or hot well. Pre-heating in the hot water tank at atmospheric pressure is limited by the elevation of this tank above the feed-pump suction. The higher this tank, the higher the possible temperature of the feed water. The extensive heating surface in the boiler is achieved at the expense of water and steam storage capacity, so fluctuations in steam demand can still cause problems even with the relatively fast response of the boiler. Treatment of the feed water is necessary, since scale-forming salts in the feed water are rapidly precipitated on to the internal boiler surfaces. Closed-coil heating systems in dyeing machines are essential to minimise the consumption of boiler feed water and to prevent contamination of process liquors with water treatment chemicals. Since only the raw water or feed water requires treatment, it is advantageous to return the maximum condensate to the boiler from the processing plant. Start-up procedures for packaged boilers are relatively simple and modern units are highly controlled or automated so that the boiler plant will run unattended. To overcome the problem of fluctuating steam demand, the thermal storage boiler was developed. This is basically a large-sized economic boiler with a very high water capacity. In a modern plant, the alternative of using a steam accumulator is more popular. This is a large cylindrical vessel filled to about 75% capacity with cold water. Steam is fed from the boiler plant and injected into the water via a series of nozzles. When the water boils, steam is produced and as the temperature of the water continues to rise, the steam pressure rises until it equals that of the steam being supplied from the boilers. When the accumulator pressure equals the supply pressure, the accumulator is fully charged. Much sensible heat is now stored in the compressed water and steam can be drawn off to supply processes. As accumulator pressure starts to decrease, the sensible heat stored in the water is gradually converted into steam and this continues until the accumulator pressure reaches the process steam pressure. The accumulator has then been discharged. However, the accumulator would again produce steam if the process steam pressure is lowered. In practice, accumulators are not usually operated over the full pressure range, but over a narrower range to ensure that at the minimum there is a reasonable head of pressure on the process steam system and at the maximum the pressure is not too near the point of blowing-off. Control and reducing valves are fitted to regulate charge and discharge of steam. The accumulator is usually operated in parallel with the boiler plant supplying the process steam, so that when boiler steam production is
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greater than demand, the excess steam charges the accumulator. When demand exceeds boiler plant output, the accumulator increases supply by discharging. Packaged boilers are widely used in textile wet processing plants, whereas the highly efficient water-tube boilers are seldom installed. In these boilers the steam pressure and temperature are as high as possible and they are often used to supply power generation plant with high-pressure superheated steam. In textile wet processing plants, the working pressure of the boilers is usually four to six times the process steam pressure, the boiler producing steam in the range of 150 to 200 psi. This is controlled by reducing valves between the boilers and the process equipment to give an operating pressure of 25 to 40 psi. A revolutionary system of water heating was developed at the University of Florida during the energy crisis of 1974. This technology is now available in Europe under the name of Direct Contact Water Heating (DCWH). All indirect energy-transfer interfaces, such as gas/steel and steel/water, are eliminated by using the gas flame to impinge directly on the fresh water. Water is injected downwards into a cylinder and the gas flame injected upwards from the other end. DCWH can be guaranteed to yield a remarkable 99.7% thermal efficiency [35]. A high efficiency gas-fired steam generator has been developed which can be particularly useful as a satellite in-line heater for preparation and washing ranges [36]. Steam generation can be decentralised and the equipment can be incorporated into new machinery, making such units independent. Operating efficiencies are in excess of 80% and energy costs are reduced by at least 50%. 3.4.2 Properties of Steam To discuss the qualities of steam, it is necessary to understand some basic definitions and these are given in Table 3.12 [37]. 3.4.3 Fuels With the obsolescence of traditional coal-fired boilers, influenced by the increasing cost of imported coal, the demise of the UK coal industry and environmental considerations, boiler plants are now usually oil- or gas-fired. Dual firing is also possible, the method adopted depending on the economics of fuel supply at a given time. If natural gas is chosen, the gas supply industry usually insists on a contract that is liable to possible three-month periods of interruption since domestic users have first priority. This usually means that there must be storage capacity for a three-month supply of oil and these storage tanks need to be contained in an enclosed bund in case leakage or spillage occurs. Premium prices may be demanded for this oil since it is not on a regular contract supply basis. 3.4.4 Steam Distribution The essential link between steam-raising and processing plants is the steam distribution system. The benefits of efficient steam production can be lost through inefficient distribution. The steam main must be of the correct size for the volume and flow rate of steam to be distributed. Under-sizing of steam pipes will result in starvation at the processing equipment, whilst over-sizing will increase the initial capital cost and aggravate radiation losses. Pipe sizes are usually determined on the basis of the flow of steam required or on the pressure drop. Having assumed
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a given flow rate, the volume of steam is calculated on the basis of the crosssectional area of the pipes. For dry saturated steam, the flow rate required is usually between 24 and 36 m/s. Sizing of steam pipes on the basis of flow takes no account of the distance the steam has to travel. Due to frictional resistance in the pipes, uneven pressure variations can arise throughout the network of piping. This is unacceptable since it is important to have a constant steam pressure at the processing end of the pipework. With the requirement of this constant pressure, pipe sizes are best based on pressure drop. In this approach, the effective length of the pipe run should be known or estimated and account taken of restrictions such as curves, elbows, junctions, valves and expansion loops. The plant layout should be arranged to favour short lengths and straight pipe runs. Future requirements should also be considered. There are specialist companies to advise on and install the necessary pipework. Steam pipes are usually manufactured from hot- or cold-rolled seamless steel tube and lengths can be joined by screwed or welded couplings but preferably flanges with joints between are used. The thickness of the tube depends on working pressure and to a lesser extent on steam temperature. Standards are available in different countries regarding the various components required in a steam line. Valves are another important component of the distribution network and should be chosen with care. The parallel-slide valve is possibly the most useful since it offers little resistance to steam when fully open and does not leak when fully closed. This type of valve is not suitable as a control valve since opening to and closing from a partially open position will cause ‘hunting’ of the valve and considerable vibration in the steam line which can cause damage and leaks. Globe valves are suitable for controlling equipment and the size should be selected according to the amount of steam required. For equipment requiring a lower steam pressure than that supplied from the main distribution system, a reducing valve is fitted in a branch line from the main line and this enables a steady but lower pressure to be obtained. Valves may stick in the open position, so that it is important particularly with reducing valves to have an adequate safety valve. Steam mains and branch lines should be fitted with pressure gauges; steam meters and thermometers may also be useful. Allowances must be made for expansion when fitting a steam main, since a pipe carrying steam at 150 psi will expand by about 0.3% (1 cm in every 30 metres). The pipes must be supported and anchored. The supports should allow for free movement of the pipe in all directions and should be placed near junctions. Spacing of supports depends on other features of the building to enable rigid support to be obtained. Expansion is usually arranged via expansion bends or bellows. Provision must also be made for condensate drainage and return. Steam pipes usually slope slightly to allow this to occur. Water remaining in steam pipes causes ‘hammer’ due to the water being carried along the pipe with the steam. This is particularly dangerous at start-up since the flow of water can be rapid enough to fracture a valve. Steam is advantageously circulated within the processing plant by means of a ring main. Steam from the boiler plant is supplied in a single pipe into the processing buildings. A T-junction fitted at this point feeds the steam in both directions into a ring of pipework. The advantages of using a ring main include:
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1. the ability to use piping of smaller diameter with some reduction in capital cost 2. there is no machine at the end of the main which could be starved of steam 3. steam demands on a branch can be served in both directions, eliminating a pressure drop in that branch. Valves are fitted at various points in the ring main so that sections of the plant can be shut down for maintenance without affecting the rest of the plant. Machinery suppliers are able to quote steam requirements fairly accurately so that branch lines can be accurately sized. Connections to machinery should be made from the top of the main to provide steam as dry as possible and these connections are preferably fitted with a valve. Expansion and draining of branch lines are necessary and steam traps to discharge condensate should be fitted with strainers on the inlet side to remove dirt. Steam lines and condensate returns should be efficiently insulated to minimise heat loss, save fuel and lower running costs. Rigid sections of fibreglass or mineral wool, held in position by light-gauge steel and fixed with metal bands, are widely used.
3.5 Pipework As seen in previous sections, considerable pipework is required for each section of processing plant, particularly dyeing machines. Services requiring pipework include hot and cold water supply, steam line, condensate return, cooling-water supply and return, compressed air and pre-dissolved dyes and chemicals from the dispensary (sections 3.12 and 4.2). Effluent may flow into a common drain or through heat-exchangers to recover sensible heat. Pipework for all these services can be sited on a pipe bridge located at a high level above the machines. This allows ease of access and is much more convenient than sinking most of it in the floor. Pipework to convey supply water or recovered water to be reused should be made from corrosion-free material, preferably stainless steel, to prevent water contamination. Short, straight pipe runs are preferred wherever possible and this can be assisted by the appropriate positioning of the boiler, softening and recovery plants within the building complex. Dye-cycle times can be significantly decreased and dyeing machine utilisation increased if the inlet/outlet pipework allows rapid filling and draining of machines.
3.6 Electricity Supply Electricity is typically supplied from public generating sources at 11000 volts and an alternating frequency of 50 Hz. For industrial use, this is stepped down by transformers to 415 volts at the same frequency. The supply is normally threephase and neutral, giving 415 volts phase-to-phase and 240 volts phase-toneutral. There are significant differences throughout the world in voltage and frequency so that machinery suppliers need to fit motors and ancillary equipment to meet the local requirements. Although alternating-current and direct-current motors of different types are available, the selection of appropriate motors is normally made by the machine supplier. Where continuous dyeing and finishing processes are operated, it is crucial that driving motors have matched characteristics, such as starting response and acceleration ramp.
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Adequate switching arrangements are necessary so that lights and machinery can be switched off when not in use. The correct motor should be used for the equipment to be driven. Capacitors are usually installed in the supply so that the balance of capacitive-inductive-resistive loads (power factor) is close to unity, thereby saving cost. Few, if any, textile plants now generate their own electricity supply with the option of selling surplus power to the local electricity authority. However, where so-called ‘brown-outs’ (frequent interruptions to the electricity supply) are anticipated, it is worthwhile installing an oil-powered generator to start up and supply critical needs (such as dyeing machines) within seconds of a power failure occurring. Some large modern plants in developing countries have installed a dedicated power plant on-site in case national supplies become intermittent or unreliable.
3.7 Compressed Air Valves on processing equipment, whether automated or not, are actuated pneumatically. Compressed air is also required for lid seals on dyeing machines and for the pressure required on nip rollers. An adequate supply of clean, dry compressed air is generated by rotary oil-sealed compressors delivering about 10 cubic metres/minute and reduced to the required pressure at the valve according to requirements. Generators are usually automatically controlled and incorporate after-coolers, refrigerated air dryers and submicron filters to ensure that the supply is free from water, oil and dirt particles.
3.8 Heating, Ventilation and Air-Conditioning Traditional wet processing factories must have been the darkest (and wettest) of all of the satanic mills. Modern dyehouse buildings are likely to be windowless structures to conserve energy, with a high level of artificial illumination. Machines are totally enclosed as a further energy-saving measure to prevent the escape of steam and effluent is discharged into closed drains or pipework. Pleasant working conditions are therefore possible, with facilities for control of heating, cooling, ventilation and full air-conditioning. The atmosphere in wet processing areas is totally changed ten times per hour to provide fresh air of a suitable quality. Humidity control in the grey storage area may assist in achieving consistent results on dyed wool [38]. In both wet and dry areas fresh-air intakes, filter sections, automatic modulating damper boxes, heating batteries and centrifugal fan sets can be installed in the roof to save space. Air is usually distributed through galvanised sheet-metal ducting which terminates in adjustable aluminium air grilles. The automatic modulating damper box controls the amount of fresh air mixing with recirculated air. Switches on the damper motors operate extractor fans to balance the amounts of input and extract air. Heater battery outputs are automatically controlled by room thermostats which operate control valves in the heater pipework.
3.9 Buildings Modern textile wet processing machines and their ancillary equipment are unlikely to be housed satisfactorily in traditionally designed textile buildings nor, indeed,
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in modern conventional factory sheds erected on an industrial estate. The needs for copious water supply and effluent disposal are additional factors that complicate the selection of a suitable location. It is unfortunate that many modernised wet processing facilities have been located in existing unsuitable buildings, thereby detracting from their effectiveness. Custom-designed and -built facilities are the best option for a modern wet-processing plant, although probably difficult to justify in many cases (section 3.15). Good external roadways are needed to allow for tanker deliveries of commodity chemicals such as acids, alkalis, salts, detergents and bleaching agents. The delivery doorways should be large enough to provide easy access for complete machine items or modular units during installation. Many of the services discussed in this chapter will be supplied by specialist contractors, particularly with a greenfield site installation since the wet processing company is unlikely to have the manpower and expertise available. An installation of this kind together with the use of specialist services has been described [39]. Steel and reinforced concrete are the materials of choice for construction of a wet processing plant. The design is likely to be based on Portal steel-frame construction to give a wide clear-span area with the roof pitch, span and height being flexible to the requirements of each processing facility. Ancillary equipment can be slung and pipe bridges supported from girders beneath the gently sloping roof. Portal frame units can be separated by conventional brickwork whilst process control offices, laboratories and dispensaries can be built at a higher level within the building. Steelwork will usually be coated to minimise corrosion, although this problem is less likely with enclosed machines. Such buildings will have roof insulation and air-conditioning (section 3.8) to eliminate condensation and give a pleasant working environment. Although modern dyehouse floors can be kept virtually dry, they usually slope slightly to a drain (either in the centre or edge of the unit) to allow for cleaning up spillages. Floors, usually concreted up to 20 cm thick, must be of sufficient load-bearing capacity for equipment placed on them plus the movement of handling devices such as fork-lift trucks. Floors are usually sealed to repel water and chemicals. Easy-clean surfaces are advantageous for ceilings, walls and floors, whilst electrical supply equipment and lights must be protected from water and heat. Modern stenters and similar machines are usually fitted with independent exhaust equipment, including scrubbing, extraction and heat recovery. Dyehouses usually include areas for grey storage, preparation, dyeing, drying, finishing, final inspection and finished goods storage. Dimensional requirements of each Portal frame unit will be influenced by the processing sequences used, the textile materials to be handled and the sector of the industry. Plant layout is considered in section 3.10. Batchwise dyeing operations can benefit from a two- or three-tier design of building. In a Portal frame building, a three-tier design is useful for a packagedyeing plant for fibre or yarn where vertical kier/vertical spindle machines are used (section 13.5). The machines are positioned on the floor of the building (tier 1) and an operating platform is erected, usually in checker-plate, at the machine operating level (tier 2). The dispensary is built in a mezzanine (tier 3) from which dyes and chemicals can be fed by gravity to the dyeing machines. This gives easy access to pumps and services in tier 1. For package-dyeing machines such as horizontal kier types which can be placed directly on the floor and for fabricdyeing machines such as winches, jets and jigs, a two-tier installation is appropriate. In the three-tier configuration, the building height may be up to nine
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metres to allow for the headroom required by cranes for unloading the frames. However, this height leaves ample space for the ancillary areas mentioned above.
3.10 Plant Layout The plant layout should be such that work flow is unrestricted and bottlenecks are avoided. The various stages within the processing sequence must also be in balance. A traditional U-shaped arrangement can be useful. Siting of boiler and pipework is a key factor to give short, straight pipe runs. It is perhaps easier to design a dyehouse for batchwise operations [40] and possible layouts for fibre and yarn dyeing operations are given in later chapters (Figure 12.9 and Figure 13.15 respectively). Batchwise operations for fabric and garment dyeing could perhaps be based on similar concepts. In continuous dyeing and finishing plants, process stages can be close-coupled to give long machine runs without the need to break-out fabric between process stages. The length of the building is thus a key factor. The direction of fabric movement can only be altered at points where the fabric is wound onto A-frames for transit or storage. Potential layouts for preparation, dyeing and finishing of woven fabrics using 1960s technology have been given [41,42].
3.11 Handling Devices When package dyeing loose fibre, yarn or fabric, cranes are used to load and unload the carriers in which the substrate is contained. These need to be of a sufficient safe working load (SWL) to accommodate the total weight of carrier, substrate and water. The dyed material can then be transported by crane to the drying machine, usually a hydro-extractor, thereby minimising manual handling. Fabric rope-dyed by batchwise methods is run from the dyeing machine into trucks over an exit reel. Such fabric is then untwisted and dewatered, either continuously or by rotary hydro-extraction. Sufficient suitably designed trucks are required to accommodate the likely volume of work in progress. Fabrics from continuous or semi-continuous dyeing (section 16.9) are usually rolled as batches of large diameter, using A-frames with independent batching devices. After continuous processing in rope form the fabric is opened into full width using a scutcher (section 15.3) before drying in open width. Stenters are usually equipped with edge-uncurling devices and weft straighteners.
3.12 Dye And Chemical Dispensing Major factors in achieving reproducibility in dyeing are the weighing, measuring and dispensing of dyes and chemicals. Sumner [43] identified inaccurate weighing of dyes and measuring of chemicals as the two most important causes of offshade batches. The location, design, layout and equipping of the ‘drugroom’ or dispensary, therefore, are vital factors in the quality management of a dyeing operation. This facility also impacts on health and safety requirements, as discussed in section 2.1.1. Many advantages and savings can be gained by the selection of rationalised ranges of dyes, auxiliary products and chemicals, as discussed in section 5.8.1. In view of the demonstrated importance of these factors, it is surprising that they have been neglected in many dyehouses.
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It is standard practice to isolate the dispensary from the main production area of the dyehouse to avoid contamination of work in progress. The removal of all dye handling from dyeing machine operatives, moving the dispensary into a dedicated area and ensuring that only suitably trained personnel work there, probably achieves the greatest single step in improving accuracy. A saving in labour costs follows but these actions are also essential on health and safety grounds. Early dyestores were primitive ‘kitchens’. Even when dyes and chemicals are dissolved or dispersed manually, it is advantageous to locate the dispensary on an upper mezzanine of a two- or three-tier dyehouse, so that dye/chemical solutions/ dispersions can be gravity-fed to the process equipment. Where it is not possible to site the dyestore on an upper floor because of building restrictions, mobile dispensing tanks can provide a first step towards centralising dye and chemical handling. Dispensing to processing equipment sited on the same level is possible but requires the use of pressure pumps to transfer solutions or dispersions and this complicates cleaning procedures [44]. In an ideal design, a bulk storage area on the ground floor holds unopened containers of dyes and auxiliaries. A lift is provided for transferring these to the higher level dispensary when needed. On this upper mezzanine, a dry room with suitable storage devices is available for products in immediate use. Another dry room, equipped with a laminar-flow or down-draught extraction booth, is used for weighing. It is preferable to convey product containers to this second room for weighing in turn directly from a delivery container into a clean, dry, stainlesssteel bucket using a clean, dry scoop. A wet room is provided for dissolving or dispersing and dispensing from mechanically stirred mixing vessels. The floor in the dispensary should be chemically resistant and of sufficient weightbearing capacity to allow for the traffic of heavy containers. Walls and floors should allow for easy and regular cleaning. Suitable storage for dyes ranges from simple shelving to paternoster devices. The wide variations that exist in container design and size, often influenced by the physical properties of solid or liquid brands of dyes and auxiliaries, complicate the design of storage systems. The variability which can occur in the moisture content of disperse dyes caused by changes in ambient conditions has been studied [45] and the influence that such changes have on the colour of wool dyed with acid dyes has been measured instrumentally [38]. The simplest expedient is to keep containers tightly closed between weighings. Air-conditioning and ventilation should be considered in the dispensary, since recent work [38] indicates that this investment gives a short payback period through improved consistency in wool dyeing. Complete air changes should occur six times per hour, with simple ‘air-lock’ door systems (often photo-electrically controlled) under slightly negative pressure being provided at entrances to maintain the atmospheric conditions while preventing the escape of contamination. Extraction equipment with air-conditioning and ventilation in the dispensing area removes steam from mixing and dispensing tanks. Weighing of dyes, chemicals and auxiliaries is crucial. Repeatable and accurate weighing to within 1% of target is required. At least two balances are required, one to handle amounts in grams up to 1 kg and the other for 1 kg or more. To weigh small quantities for pastel dyeings and shading additions, dilute solutions of the dye can be used. The weighing operation raises serious concerns for health and safety since the disturbance of powders produces aerosols which can be inhaled by the weighing operative (section 2.5). Reactive dyes have been the main cause for concern
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[46]. Engineering solutions are the preferred approach to this problem with the installation of weighing booths and extraction cabinets. The selection of suitable physical forms, such as liquids, grains or non-dusting powders, also alleviates the problem. ‘Space-suiting’ including nasal masks, goggles, gauntlets and overalls that must be regularly laundered are provided individually for each operative. Standard operating procedures should be established for dissolving or dispersing and dispensing dyes. The modern dispensary is equipped with stainless-steel dispensing tanks, these being either conical or cylindrical in shape, the number required for each dyeing machine depending on the process details. Such tanks are usually equipped with the facilities listed in Table 3.13. Dyes, chemicals and auxiliaries are gravity-fed to the dyeing machine through stainless-steel or glass tubes. When dispensing from the mezzanine level, sufficient time must be given at the appropriate phases of the dyeing routine to allow homogenisation of the liquors and travel to the dyeing machine. The increasing use of ultra-low liquor ratio (ULLR) dyeing machines and centralised dispensing emphasises the need for dyes and chemicals of adequate solubility or dispersibility and stability properties. Large weights of salt and alkali associated with the dyeing of cellulosics are added at the dyeing machine level. The packaging, storage and handling of dyes and chemicals have been reviewed [47].
3.13 Laboratory Support The works or support laboratory must be the nerve centre of the modern coloration operation. The laboratory plays a major role in establishing the limits of accuracy required to obtain a high degree of reproducibility and in maintaining the SOP required in bulk processing to ensure that a high success rate of RFT production is achieved. The laboratory can therefore make a major financial contribution to the well-being and profitability of the company. Unenlightened management has often regarded the laboratory as a financial drain on the company and this should certainly not be the case. Exact simulation of production processes is neither possible nor necessary, but it is essential to establish reliable transfer of lab recipes into bulk-scale production. The penalties of achieving wrong-first-time (WFT) production have been summarised in Table 1.12. By taking these figures, it has been shown that the cost of jet dyeing a 300 kg batch of fabric WFT and having to give a single shading addition equates with the cost of between 3 and 25 laboratory dyeings or a 3 to 10 kg sample dyeing, depending on the substrate/dye combination. The cost of a single redyeing equates with the cost of between 25 and 85 laboratory dyeings [48]. The nature of work carried out in such laboratories, together with their design, organisation and equipping have been extensively discussed and working methods for the various activities have been given [49]. The laboratory is situated in close proximity to the dispensing and production facilities, one option being on a mezzanine floor above the production area.
3.14 Management Strategy for Success An important, and perhaps an increasingly scarce, resource is an efficient and effective management team to control the plant supported by the necessary labour force. The various aspects of dyehouse management have been discussed [40].
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Market research will be carried out continuously by the successful company. There are a number of management tools and techniques which can assist in this essential market research. Competitor profiling systematically analyses the competition to learn from their strengths and weaknesses. Customer retention (the duration of customer relationships) is a measure of customer satisfaction and reasons for loyalty or defection can be established. Customer satisfaction can be measured by asking customers to define the company’s performance. Strategic alliances can often be built between companies prepared to commit resources to achieve common objectives. Such alliances can be with customers, suppliers or even competitors and benefits can include entry into new markets, supplementing skills or sharing financial risks and commitments. Value chain analysis is used to identify potential sources of a company’s economic advantage in its industry. Process re-engineering, the redesign of a company’s core business, may be required to achieve dramatic improvements in productivity, turn-round time or quality. There are five major forces that impact on the long-term success of a business, as listed in Table 3.14. The successful company develops core competencies, based on the knowledge and skill of its workforce, to create a differentiable customer value. Total quality management (TQM) defines the performance requirements of products and services based on customer needs in order to meet these specifications with zero defects. Having established the market potential the production facility must conform to the TQM criteria and must be balanced between processes. Operation of the plant must be based on a well-defined business philosophy including the selection of appropriate customers and product lines. Management is about change, not preserving the status quo. Failure to change soon enough or radically enough can result in eventual extinction. It is perhaps worth discussing briefly some of the changes that impact on dyehouse management. The costs of dyes, chemicals, resources and labour are levelling out on a worldwide scale and modern technology is available to all. As automation and robotisation are adopted more widely, labour costs decrease considerably. A major criticism of the UK textile and coloration industries has been lack of investment. Developing countries are investing, often obtaining development bank funding to do so, but their workers may lack the training, education or attitude to achieve high efficiencies from modern plant. Investment in these countries is often confined to machinery and hardware. The need for adequate and appropriate procedures, people skills and software may be neglected. Undue deference to the status quo often exists. Any significant change (and consequent improvement) in current practice implies a criticism of established procedure and a ‘loss of face’ for senior individuals opposing the innovation. There is therefore little opportunity in such an environment to learn from previous mistakes. A company cannot readily or quickly launch major changes in products, processes, markets or customer base and thus requires well-defined long-term objectives. This corporate strategy includes the actions listed in Table 3.15. Capital expenditure may be required for the reasons shown in Table 3.16. The gradual replacement of equipment is the important sector for investment, since it can permit the production range to be expanded and enable the company to evaluate the latest machines and processes. This approach allows facilities and machines to be updated within the financial capabilities of the business. The
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company can thereby remain viable without the need for an abrupt and painful injection of large amounts of capital which have to be financed. When identifying outstanding companies, there are usually two important factors which lead to success: 1. qualifiers: those basics which a company must get right in order to survive 2. differentiators: those factors which give the company an edge over its competitors. In most cases the successful company has the attributes shown in Table 3.17. As regards strategy, most successful companies have formulated a clear statement of their goals and aspirations, have communicated these to everyone in the organisation so that they can ‘buy-in’ and have monitored and communicated the outcomes frequently. Such companies ensure that all employees act on the belief that customers want a product that is right for them and want it now. This approach secures business from the most exacting customers by adopting exacting standards, exploits special niches, gets products right-first-time and responds to external changes such as environmental concerns. Key strategies are listed in Table 3.18. A participative management style will build a dedicated and responsive workforce with high morale so that everyone identifies with the company’s objectives. In the production plant, especially where there is a significant technical input, the chief executive is unlikely to be a desk-bound manager and ‘management by walking about’ (MBWA), making regular contact with staff at all levels, is highly effective. Continuous management development is essential and the company must clarify its needs for management talent in the future. A SWOT (strengths, weaknesses, opportunities and threats) analysis defines those areas of competence and those in which efforts are required to achieve the desired level. Success depends on the company developing the attributes listed in Table 3.19. Empowerment is an important concept and requires the motivation listed in Table 3.20. All personnel should foster a climate of honesty and fairness in dealing with problem situations. Isolated incidents involving faulty production or breakdowns may be attributable to chance, but multiple events of a similar kind usually signal incompetence or inadequate training. It is vital to adopt a positive approach to teamwork in problem solving that has no need for witch-hunts or the blaming of individual operatives.
3.15 Economic Considerations As will be seen from this chapter and the volume 3 chapters, machinery and processing developments have been much concerned with cost savings and particularly the resources discussed in this chapter, including labour. The latter topic will be discussed further in section 4.4. Process optimisation is, in general, a fruitful means of cost reduction [50]. Feasibility studies are required before re-equipping an existing operation or a totally new facility is considered. If major changes have occurred in processing needs, building a new plant may be more economical than updating an existing one. However, it is more difficult to justify minor improvements on the basis of modest savings requiring prior capital expenditure.
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The possibility of building a new plant on a greenfield site is attractive since it allows the latest equipment to be installed throughout. A greenfield location may have other attractions, since many existing plants are on ‘island’ sites surrounded by other premises, including domestic accommodation. Ease of access and parking are often difficult, space for future development is seldom available and residents may not be sympathetic to new construction. Noise and odour restrictions, water availability and effluent disposal may make the current site unattractive. The disruption caused by building the new plant whilst maintaining production should not be underestimated. It has been previously noted that existing buildings may be unsuitable and the cost of modifying these may be prohibitive. The costs of repairs and maintenance for old buildings may also be high, so that a new building of the correct design and size in a new location may be a cost-effective option. Few feasibility studies have been published, not only because of their uniqueness to a given investment and company strategy but also because they are usually shrouded in secrecy (if not mystery). Two UK papers covered the economics of bleaching, dyeing and finishing of woven cotton in the late 1960s. In one [41], it was concluded that installation of the latest equipment in modern buildings could be justified if market conditions were favourable and specialised production was undertaken. In the other [42], it was argued that investment in modern equipment could be at least as successful in existing buildings. One difficulty can be the escalating cost of machinery, however, the equipment evaluated in these two papers having risen in cost by a factor of ten in the inflationary period 19551970. Two feasibility studies in the 1980s dealt with the establishment of yarn package dyehouses on greenfield sites, one being in the UK [51] and the other in the USA [52]. There was a surprising degree of agreement between these studies, both concluding that investment on a greenfield site was feasible for a new business but unlikely to be so for an existing operation due to poor market prices. This underlines the need to carry out feasibility studies for each option at a time as close as possible to construction. Many of these views expressed in the 1980s regarding automation would now be quite different.
3.16 Capital Investment – Quo Vadis? The economic considerations of capital investment are discussed in several of these chapters. It was shown that on a greenfield installation, 65% of the capital investment is required for the resources and services discussed in this chapter [49]. Although the actual capital required has probably increased by at least a factor of ten over the intervening years, the distribution has probably remained fairly consistent. It is appropriate to discuss the future of capital investment in this chapter. Major changes have occurred in the global textile industry over recent decades with developing countries, often already major agricultural producers, grasping textile production as a major industry. Technology and machinery are readily available for use by relatively low-cost labour, with there being a ready local market for the product. The governments of developed countries have often been unsympathetic to their textile industries. Nowhere is this more apparent than in the UK where few indigenous spinning and weaving firms have survived and knitters, dyers and finishers are dependent on imported yarns and fabrics. Much of the demise of what are considered in some quarters as ‘mature’, ‘smokestack’
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or ‘sundown’ industries has been blamed on the low cost of production in developing countries. These volumes should dispel this pejorative view, since the wet processing industry is in many respects a high-technology sector. It has been shown that there is a levelling-out of global wage rates and that it is feasible for a developed country to invest in spinning and weaving, based on current technology (open-end spinning and projectile weaving machines) [53]. From the year 2005, the global textile industry will operate without the restrictions of the multi-fibre arrangement (MFA) and the industry will thus become more competitive with free trade being established without tariffs, duty or quotas being imposed on textiles or garments. The successful company will need to exploit management technology, with the development of appropriate investment plans and production strategies. Success depends on the company possessing distinctive competencies in the areas listed in Table 3.21, that will lead to significant added value. The absence of these means that the company is never likely to be viable and should seek a better method of using shareholder’s capital elsewhere immediately. The results of piecemeal installations are often disappointing. New dyeing plants that have been installed without adequate support facilities and the necessary management philosophy are known to be achieving less than 50% RFT production. Many new investments are based on misleading calculations from preceding production, when high levels of RFT were not achieved and much reprocessing existed. As discussed in section 1.5, RFT production gives a significant increase in output, as a result of new work being produced, in addition to the elimination of corrections. Labour costs are levelling out and these will assume even less significance as modern, labour-saving machines are installed equipped with control equipment and robotics. It is believed that feasibility studies carried out similar to those described above would conclude that a modern building housing modern equipment together with control technology and robotics, where appropriate, would be viable provided the criteria listed in Table 3.22 are met. Needless to say, no such feasibility studies have been published and it takes a courageous and strong management to build, commission and run such a facility. Evidence confirms that the approach given in Table 3.22 is being followed for new facilities for niche products, for example, a package dyehouse to dye yarn for the automotive industry [54], although the financial performance of such developments remains an unknown quantity.
References [1]
W B Achwal, Colourage, 45 (Oct 1998) 36.
[2]
I Gailey, JSDC, 96 (1980) 600.
[3]
W Dürig, Amer. Dyestuff Rep., 70 (Feb 1981) 26.
[4]
A H Little, JSDC, 87 (1971) 137.
[5]
S M Jaeckel, J Knight and P Pyle, JSDC, 92 (1976) 157.
[6]
G Horstmann, Australasian Textiles, 6 (Jan/Feb 1993) 34.
[7]
J Park, JSDC, 95 (1979) 400.
[8]
Anon, Colourage, 45 (Aug 1998) 45.
[9]
W Beal, K Dickinson and E Bellhouse, JSDC, 76 (1960) 333.
[10]
Water quality requirements and waste water recycling in the UK textile industry, Textile
[11]
F L Cook, W C Tincher, W W Carr, L H Olson and M Averette, Text. Chem. Colorist, 12 (1980)
Research Council (1978). 1.
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[12]
W C Tincher, F L Cook and L A Barch, AATCC Nat. Tech. Conf., (1981) 271.
[13]
F L Cook, R M Moore and G S Green, AATCC Internat. Conf. and Exhib., (1988), 184.
[14]
J Koh, J Cho and J Kim, AATCC Review, 1 (2001) 27.
[15]
J Koh, G Shim and J Kim, Color. Technol., 117 (2001) 337.
[16]
J K Skelly, Rev. Prog. Coloration, 30 (2000) 21.
[17]
R Teichmann. Textilveredlung, 32 (1997) 131.
[18]
W Köhn, Melliand Textilber., 79 (1998) 647; Textilveredlung, 33 (Sep/Oct 1998) 24.
[19]
W Dürig, IFATCC Symposium, Barcelona, (1975).
[20]
W S Hickman, JSDC, 109 (1993) 34.
[21]
J Park and J Shore, JSDC, 100 (1984) 383.
[22]
W S Perkins, AATCC Internat. Conf. and Exhib., (1996) 354.
[23]
Colour in dyehouse effluent, Ed. P Cooper (Bradford: SDC, 1995).
[24]
K H Gregor, Melliand Textilber., 73 (1992) 526; 79 (1998) 643.
[25]
C Diaper, V M Correia and S J Judd, JSDC, 112 (1996) 273.
[26]
T Schäfer, J Trauter and J Janitza, Textilveredlung, 32 (Mar/Apr 1997) 79.
[27]
U Wehlmann, Melliand Textilber., 78 (1997) 249.
[28]
Anon, Dyer, 183 (Jun 1998) 28.
[29]
K Jeavons, Proc. textile ind. dyehouse water recovery and reuse conf., Huddersfield (May 1999)
[30]
I G Laing, Rev. Prog. Coloration, 21 (1991) 56.
137. [31]
S Cronshaw, Dyer, 187 (Jan 2002) 25.
[32]
A Wakeling, Dyer, 187 (Jan 2002) 18.
[33]
Anon, Dyer, 187 (Jan 2002) 21.
[34]
W S Hickman, Rev. Prog. Coloration, 29 (1999) 94.
[35]
W Dunlop, Dyer, 187 (May 2002) 25.
[36]
Nordsea Gas Technology Ltd., JSDC, 101 (1985) 381.
[37]
W Ronald in Engineering in textile coloration, Ed. C Duckworth (Bradford: SDC, 1983).
[38]
V M Adamiak, J-H Dittrich, S Struckmeier and R D Reumann, Color. Technol., 117 (2001) 313.
[39]
Anon, Dyer, 162 (Aug 1979) 170.
[40]
J Park and J Shore, Dyehouse management manual
[41]
C Duckworth and J J Thwaites, JSDC, 85 (1969) 225.
[42]
J M Bainbridge, A Burgess and G Milns, JSDC, 86 (1970) 345.
[43]
H H Sumner, JSDC, 92 (1976) 84.
[44]
W S Hickman, Rev. Prog. Coloration, 31 (2001) 65.
[45]
P L Adamczyk, Text. Chem Colorist, 6 (1974) 183.
(Bombay: Multi-Tech Publishing Co.,
2000)
[46]
J M Wattie, JSDC, 103 (1987) 304.
[47]
R M Brown, Rev. Prog. Coloration, 21 (1991) 1.
[48]
D Hildebrand and F Hoffmann, Text. Chem. Colorist, 25 (Apr 1993) 24.
[49]
J Park and J Shore, Dyeing laboratory manual (Upperhulme: Roaches International Ltd. 1999).
[50]
W Boyd, J Park, T M Thompson and T Warbis, JSDC, 96 (1980) 497.
[51]
H Chaplin, J Park and T M Thompson, JSDC, 96 (1980) 580.
[52]
Institute of Textile Technology, Charlottesville Va., Private communication (1985).
[53]
A Ormerod, Textile Month (Oct 2000) 18.
[54]
P Lennox-Kerr, Textile World, 147 (Aug 1997) 85.
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Table 3.1 Vertical and horizontal organisation Stage for dyeing Fibre, tow and top Yarn
Fabric
Garments Totally vertical operation
Wet processing options Commission In-house by spinner Commission Merchant dyer (buys yarn and dyes for sale) In-house by spinner, knitter, weaver or carpet manufacturer Commission Knitter as part of knit, dye, cut, make-up, trim Weaver Commission By knitter or garment producer Fibre to finished fabrics or garments within one company
Table 3.2 Process water specification [2] Parameter Colour pH value Total hardness (ppm, CaCO3) Alkalinity to Methyl Orange (ppm, CaCO3) Iron (ppm) Manganese (ppm) Total dissolved solids (ppm) Suspended solids (ppm) Chloride (ppm) Sulphate (ppm)
Limits Hazen No. Clear 2-5 7.0–7.5 10–25 35–65 0.02-0.1 0.03 65–150 Nil 0–30 0–30
Table 3.3 Average water consumption by fibre type [3]
Fibre type Wool Cotton Synthetic-polymer
Consumption 1966 150–550 100–300 25–200
(l/kg) 1975 75–300 50–200 10–100
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Table 3.4 Water consumption by process [4-6]
Fibre type Cotton
Wool Synthetic-polymer Acrylic Wool Nylon Nylon Wool Cotton Cotton Cotton Cotton Cotton
Process Scouring Bleaching Jig dyeing Winch dyeing Hank dyeing Fabric dyeing Garment dyeing Sock dyeing Sock dyeing Hose dyeing Loose stock dyeing Continuous dyeing Yarn dyeing Printing Fabric dyeing Garment dyeing
Consumption (l/kg) 10-80 10–130 10-60 100–450 40-65 65–190 100-230 265–465 125–150 100-240 40-60 60-80 100-140 140–200 100–180 80–140
Reference [4]
[5]
[6]
Table 3.5 Measures to reduce water consumption Development of efficient standard operating procedures (SOP) Following SOP exactly, assisted by automation Installation of machines operating at low liquor ratios Operating at minimum liquor ratio by processing full machine loads Monitoring liquor levels, assisted by automation, to avoid overfilling Improved production scheduling to avoid unnecessary machine cleaning and downtime Replace direct heating by heat exchangers to avoid liquor volume increases Control rate of flow of cooling water through heat exchangers Return cooling water from closed-coil systems Use of water-soluble lubricants to eliminate separate scouring Combine processes or use one-bath methods whenever possible Avoid aftertreatment or excessive rinsing, especially overflow rinsing Allow drainage time between rinses Adopt counter-current rinsing methods in continuous processing Reuse relatively clean rinsing water for processes not requiring high-quality water Partial purification and bath reuse Total water purification and recycling
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Table 3.6 Definition of degree of pollution Factor Biochemical oxygen demand (BOD)
Chemical oxygen demand (COD) Total organic carbon (TOC) Dissolved organic carbon (DOC) Biodegradability
Bioelimination
Definition Atmospheric oxygen consumed by microorganisms during the biochemical decomposition of the contaminants, often during a five-day test Oxygen consumed when the contaminants are oxidised in a boiling aqueous acidic dichromate solution Total organic contaminants, both suspended and dissolved, expressed in terms of the carbon content of the effluent Organic carbon present in the dissolved phase after centrifuging or membrane filtration Degree of decomposition of an organic contaminant after biological treatment under specified conditions This includes the material removed by adsorption on the biomass as well as that which undergoes biochemical decomposition
Table 3.7 Effluent consent limits Parameter BOD (ppm) COD (ppm) Suspended solids (ppm) pH Temperature Toxic metals total (ppm) Toxic metals soluble (ppm) Ammonia as N (ppm) Colour – absorbance at 500 nm
Sewer 800 400 6 to 10 Not above 43°C 30(a) 10
(a) Sum of concentrations of Cr, Zn, Cu, Cd, Sn, Pb.
River 15 25 5 to 9 Not above 30°C 0.5(a) 10 0.05
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Table 3.8 Water usage and pollution loads in cotton finishing processes [19]
Process Desizing Scouring Bleaching Mercerising Dyeing Printing Wash-off Finishing
Water usage (l/kg) 20 4 180 7 30 25 110 5
Water usage (%) 5 1 46 2 8 7 30 1
Approx. BOD (mg/l) 4500 11000 1000 30 1000 1200 200 1500
BOD (%) 22 54 5 5 6 1 7
Pollution load (%) >50 10-25 3 <4 10-20 10-20 5 16
Table 3.9 Methods of effluent treatment [21] Physical Sedimentation Filtration Flotation Foam fractionation Coagulation Reverse osmosis Solvent extraction Ionising radiation Adsorption Incineration Freezing Distillation
Chemical Neutralisation Reduction Oxidation Catalysis Ion exchange Electrolysis
Table 3.10 Treatments to remove colour [23] Biochemical and chemical oxidation or reduction Coagulation and flocculation using inorganic or organic agents Adsorption/absorption using natural or synthetic inorganic or organic agents Membrane techniques
Biological Stabilised ponds Aerated lagoons Trickling filters Activated sludge Anaerobic digestion Fungal treatment
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Table 3.11 Suggested quality requirements for recycled effluent [18] Parameter Colour Absorbance at 450 nm Absorbance at 500 nm Absorbance at 550 nm Absorbance at 600 nm pH COD Total hardness (as CaCO3) Iron Chromium Copper Aluminium Inorganic salts
General application
Neutral <20–50 mg/l <90 ppm <0.1 ppm <0.1 ppm <0.005 ppm <0.20 ppm <500 mg/l
Scouring and washing-off 0.02-0.04 (in 1 cm cell) 0.02-0.05 0.01-0.03 0.01-0.02 7.0-8.0 <200 mg/l <0.01 ppm <0.01 ppm <0.05 ppm <500 mg/l
Table 3.12 Steam technology [37] Term Absolute pressure Gauge pressure
Sensible heat
Latent heat
Dry saturated steam Superheated steam
Wet steam
Definition The total pressure of steam above a complete vacuum, measured in pascals (Pa) or pounds/inch2 (psi) The traditional unit used in steam generation and the pressure of steam above atmospheric pressure quoted on pressure gauges in psi The heat transmitted from burning fuel to raise the water temperature or transferred in a heat exchanger. The amount of sensible heat that can be absorbed by the water is directly proportional to the pressure and temperature of the steam. The temperature of the steam at any given pressure is given in steam tables The heat in joules to convert 1 kg of water to ‘saturated’ steam at a constant temperature. The latent heat is steam can only be fully utilised by condensing the steam back to water Steam at the temperature of boiling water (at a particular pressure) which does not contain free water droplets. It is the ideal quality of steam (rarely found in practice) Saturated steam from the boiler to which extra heat has been added by passing through a superheater. This is dry steam and its temperature is higher than the temperature of saturated steam at the same pressure Condensation occurs in steam as it is conveyed through the distribution pipework and wet steam is a mixture of steam vapour and water droplets – not to be confused with saturated steam
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Table 3.13 Dispense tank requirements Automatic level control for filling Steam heating and temperature control Stirring device of the correct design Gravity feed to the dyeing machine Drain to waste for cleaning Rinse ring, usually a perforated pipe in the top of the vessel Hot and cold water, often supplied through the rinse ring
Table 3.14 Major market forces Intensity of the competition Threat of new entrants in the market-place Threat of low-cost substitution products Bargaining power of buyers Bargaining power of suppliers
Table 3.15 Strategic actions Defining the scope of the company’s activities Matching these activities to the business environment Matching these activities to its resource capability The allocation and re-allocation of major resources The values, expectations and goals of those influencing strategy The direction the company will move in the long term Implications for change throughout the organisation – they are therefore likely to be complex
Table 3.16 Capital expenditure requirements Diversify by entering a new business or making new products Improve productivity Improve consistency of processing Improve quality Improve customer service Obtain cost reductions
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Table 3.17 Attributes of a successful company A visionary, enthusiastic leader who is a champion of change and has generated the necessary commitment at all levels Flexible and motivated employees Learns from its customers, competitors, suppliers and academia Seeks continuous innovation and improvement Seeks processes and practices to achieve low-cost product customisation while retaining competitive prices, quality and service.
Table 3.18 Key strategies Supply chain management – just-in-time delivery People management – lean production, employee effectiveness Introduction of new products Information management Financial management
Table 3.19 Attributes for successful management The ability to identify niche (high-added-value) products and markets for them Realistic investment and capital expenditure based on adequate feasibility studies The ability to provide a quality service – RFT, quick response, JIT processing At the right price Exploiting the latest technology and following best practices Giving adequate training and experience to staff at all levels Empowering employees Developing strategic partnerships within the organisation Developing strategic partnerships with suppliers (dyes, chemicals, water, energy, equipment) Developing strategic partnerships with customers Strategic alliances (off-shore making-up with low cost) The use of appropriate control strategies. The ability to learn from mistakes A positive mental attitude High levels of personal and technical integrity
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Table 3.20 Empowerment of personnel Operatives to
be multi-skilled have input into planning, quality, safety, productivity, cost saving participate in problem-solving work closely with colleagues and other departments
Supervisors to
help operatives to develop initiative assist operatives in developing problemsolving skills share responsibility for quality, safety and cost saving and contribute to production planning and decision making
Managers to
be proactive (not fire-fighting) make time for strategic planning trust colleagues and delegate responsibility listen (not talk) and invite opinions and contributions from others
Table 3.21 Distinctive competencies Quality of relationships with suppliers and customers Speed of response Quality of product Speciality product features Innovation Service Price
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Table 3.22 Operational criteria. Parameter Resources
Criteria Water of appropriate quality available Consent conditions available for effluent discharge on greenfield site selected New buildings of correct size and architectural design
Products
A niche product market selected Small product range based on standardisation of processes
Technology
Changes in fibre consumption likely to lead to changes in machinery for preparation and dyeing Small production runs likely with the need for specialist finishing machines Modern machinery (such as ULLR machines) to give reduced labour requirements and savings in water, effluent, electricity and steam Full-capacity operation based on RFT processing Use of cost-effective dyes and equipment which guarantee RFT production; machine supplier to give maintenance, service and spares support
Productivity
Correct machine sizes required – miniaturisation of plant RFT plus control/robotics to reduce costs: productivity could increase by up to eight times in terms of kg/operative hour
Quality
High-quality support laboratory
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Figure 3.1 Stages for coloration Natural fibres
Synthetic fibres Mass pigmentation Extrude Gel dye
Staple fibre
Continuous filament
Loose stock dye or Tow/top dye Spin Yarn dye Fabric formation Fabric dye or print Garment making Garment dye or print
Figure 3.2 The Mogden formula for effluent charges
C=R+V+
St.S Ot.B + Bv + Ss Os
where: C R V Ot B Os Bv St S Ss
= = = = = = = = = =
total cost per m3 reception and conveyance primary treatment COD of effluent after one hour quiescent at pH 7 BOD of settled waste COD of crude sewage after one hour of quiescent settlement additional volume charge if there is biological treatment total suspended solids (mg/litre) in effluent at pH 7 treatment and disposal cost per m3 of primary sludge total suspended solids (mg/litre) in crude sewage
Charges for B/Os and S/Ss are expressed as costs relative to standard strength (concentration, usually expressed in mg/litre); standard strengths vary between treatment companies
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Figure 3.3 Treatment alternatives Waste process stream
Equalisation
Neutralisation
Chemical oxidation
Ultrafiltration Biological treatment
Adsorption Physico-chemical treatment
Discharge
Figure 3.4 Schematic diagram of reverse osmosis treatment plant [29] Dyehouse waste stream
Waste stream balance/storage
Pre-filtration unit
Recovered waste storage
Reverse osmosis water recovery unit
Recovered water to dyehouse
Colour inactivation unit
Effluent disposal to municipal sewer
Heat-exchange unit
Reverse osmosis CIP
Chapter 4 Control, Automation and Robotics 4.1 Control and Limits of Accuracy Interpretation of measured data from a process carried out numerous times has to consider precision (all measured values close to the actual value) and accuracy (all measured values similar but not necessarily close to the actual value). This leads to consideration of repeatability (accuracy of measured values obtained with repeated use of the same equipment) and reproducibility (agreement of measured values obtained using different items of similar equipment). To achieve satisfactory reproducibility in dyeing, numerous factors must be either controlled or monitored. These factors have to be within certain acceptable tolerances, often referred to as the ‘limits of accuracy’, in order to obtain a colour match within prescribed colour matching limits to either the standard or a previous production batch. Several authors have discussed the factors where control must be exercised [1–5]. An early paper [2] showed that some factors could be directly controlled within the dyehouse whereas others were outside its influence. Gailey later graded these and other factors into levels of importance in the dyeing of cotton with vat dyes or polyester sewing thread with disperse dyes and this grading is given in Table 4.1 [3]. The limits of accuracy for these factors directly involved in these dyeing processes were given [3] and in many cases a deviation of 1% or less from the mean value is required to achieve a commercial match. Water quality requirements and the need to control the moisture content, weighing and dispensing of dyes have been discussed in section 3.12. Work carried out on polyester fabrics [6], indicated that the factors listed in Table 4.2 were the most important. The experimental work described above and carried out carried out by Coats at Paisley [2,3] and Shirley Institute (now BTTG) in Manchester [5,6] specified the factors to be controlled and the magnitude of control that must be imposed. Each dyehouse, or at least each specific type of operation, must determine its own important factors and the limits of accuracy that must be applied. The degree of control that is exercised is dependent on the matching tolerance acceptable for the particular product and end-use involved. A general procedure for determining the limits of accuracy for any dyeing process was published by the Shirley Institute. This method of determining the sensitivity of the dyed shade to controlled variation of each factor is outlined in Table 4.3 [5,6]. Few dyers seem to have carried out this work and even fewer have published the results. Those who set their own limits of accuracy tend to be over-restrictive, leading to unnecessary costs. The factors that must be controlled in the dyeing operation to achieve RFT production have been given in Table 1.13. Similar factors must be defined for preparation and finishing processes and the necessary limits of accuracy determined and applied. These factors must be incorporated into standard operating procedures (SOP) for both laboratory and production techniques and applied rigorously. Most of the factors listed in Table 1.13, often referred to as ‘assignable’ variables, are controlled directly by the SOP and relatively few, known as ‘random’ variables, require routine and regular testing. Random variables include water quality, substrate dyeability, moisture content and standardisation (strength and hue) of dyes. Certificates of conformity can be obtained from the dye supplier for strength and hue as an alternative to in-house testing. Failure to test these parameters regularly is a major reason for failure in achieving RFT dyeing. The process water specification given in Table 3.2 is 90
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sufficiently strict to ensure satisfactory reproducibility of dyeing. The limits of accuracy which were applied to other important factors to achieve a ∆E(CMC) of 1.0 or less have been determined for acrylic dyeing and are shown in Table 4.4, expressed in terms of % deviation from the mean value [7]. Dye selection, as discussed in section 5.8.1, makes an important contribution to reproducible dyeing and dyes should be robust (i.e. have low sensitivity to dyebath variables), compatible, stable and consistent. As discussed in section 3.12, it was shown [8] that two major causes of poor reproducibility of shade in reactive dyeing were inaccurate weighing of dyes or chemicals. It was later shown that colour mapping of dyes would allow combinations of dyes to be selected within limited regions of colour space to minimise the effect of dye weighing errors [9]. A similar technique was used to select dye combinations based on ‘internal primaries’ [10]. From the foregoing, it is apparent that colour measurement has played an important role, and this technology is discussed in section 6.1. A survey of the variability found in important factors over a number of dye/fibre systems is summarised in Table 4.5 [11]. It is thus apparent from Table 4.4 and Table 4.5 that the installation of an advanced control system is essential to ensure a high degree of reproducibility. To obtain the maximum benefits, it is necessary to evaluate and select the most appropriate control technology and to have a well-defined technical and management philosophy [1]. Many factors have to be considered, particularly if a re-equipping programme is involved [12]. In a typical dyehouse using only a rudimentary control system, wide variations occur between bulk batches by applying a so-called ‘proven’ formula. For example, 29 batches which were blinddyed by a dyeing operation of this type gave a mean ∆E(CMC) of 1.8 between batch and standard with a range of colour differences of 0.4 to 2.8 units.
4.2 Machine Control The earliest systems introduced to control batchwise dyeing machines were electronic devices operated by punched cards. One of the first of these was developed by Kilsunds AB in Sweden in the 1950s, providing specific timetemperature profiles for the dyeing of different fibre types [13]. As early as 1968, Bunting pointed out that automation of the dyeing process was essential. Initial areas that could already be improved by automation at that time were loading and unloading of the goods, automatic dye-cycle corrections and recording of the dyeing conditions [14]. If optimised dyeing procedures are carefully developed and thoroughly documented in SOP, it is possible to achieve high reproducibility of dyeing in a manually operated dyehouse. Many who have introduced control systems or automated equipment have been disappointed with the lack of improvement in reproducibility, because the essential development and documentation of optimised procedures has not been carried out in advance of the installation. Nowhere does GIGO (garbage in, garbage out) apply more effectively than when automated equipment is installed in a dyehouse that has not yet established reproducible, optimised and fully documented dyeing procedures. If a mess becomes automated, one obtains an automated mess, since the control equipment faithfully reproduces unreliable procedures, ‘warts and all’, and an automatic dyeing faults generator is created.
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By installing controllers on process equipment, two major advantages are obtained: 1. the process variables as documented in the SOP are strictly followed and controlled 2. interference time, arising from an operative manually controlling several machines, is eliminated or reduced. It was established that optimisation and rationalisation of dyeing processes led to considerable cost savings [15] and many authors have realised the importance of the limits of accuracy, the savings obtained by RFT production and the penalties incurred by getting it wrong-first-time [3,15-18]. The advantages of installing controllers dedicated to individual dyeing machines, often referred to as semiautomatic control (SAC), are given in Table 4.6. For complete control of the dyeing machine operation, the functions required in Table 4.7 are required. The background to instrumentation and process control has been discussed [19] and control engineering in the dyehouse has been reviewed, concentrating mainly on semi-automatic control applications in the 1980s [1,4,7]. The selection of equipment and typical installations were described [4]. Contact-free devices are available to monitor fabric speed and dimensions, yarn count, weight per unit area, weft distortion, dwell time and moisture content. Sensors can be fitted on wet processing machines to measure pressure differentials, liquor flow, pH, conductivity, peroxide concentration, refractive index and liquor pick-up [20]. The results of recent sensor development and process studies have been reported, including computer control of the peroxide bleaching of cotton, the relation between colour intensity and colorant concentration in screen printing with pigments, on-line measurement of the oxygen content of steam, control of residual alkali in continuous washing-off using conductivity sensors and monitoring of fabric geometry using optical sensors [21]. A data-acquisition system has been developed to study dyeing processes and rapidly determine the compatibility of dyes. The system senses variables in real time and incorporates a VAX lab minicomputer and Compaq Deskpro microcomputer to analyse and graphically present the real-time data via a video display. Dyeings are carried out on an Ahiba Texomat machine fitted with a range of sensing probes, including a guided-wave spectrophotometer, pH and conductivity probes with transmitter and signal conditioner, and a thermocouple with signal conditioner [22]. A serious problem with control systems at any level of sophistication is the major addition of chemicals (salt and alkali) required in dyeing cellulosic fibres with reactive dyes. These are usually added manually at the dyeing machine but various alternatives exist. Saturated brine and liquid caustic soda (to replace solid salt and solid alkalis such as soda ash or caustic soda) can be added by controlled dosing. Solid chemicals can be fed from a bulk storage silo or a local bulk storage bin using an Archimedean screw. The local storage bin can be filled using a bagemptying device. Dissolving of the chemicals can be carried out by a machineside automated system mounted on a load-cell equipped with recirculation and dispensing pump. Multi-product injection (MPI) systems were developed so that metering of dyes and/or chemicals can be carried out during dyeing to aid levelling. The addition of the liquid is carried out by means of a machine-side mobile dispense tank with microprocessor control. Metering can be either a linear function or an exponential (progressive or regressive) function and chemicals are added over a period of
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time from 10 to 60 minutes. The Automet system was jointly developed by Adcon AB of Sweden and Hoechst [23]. This is a constant-temperature dyeing system operated at 40 or 60°C or at 80°C for phthalocyanine dyes. The process sequence is the same for all three dyeing temperatures. The Adcon ADC 201 metering unit has been developed for control of the dosing cycle [24]. The alternative Levametering system was a joint development by Bayer and Thies [25]. A dyeing control strategy based on the sequential addition of dye and alkali during the dyeing process, often referred to as integration dyeing, may minimise the degree of hydrolysis of reactive dyes during exhaustion. The minute-byminute control of the exhaustion and fixation rates provides opportunities to limit the amounts of dyes that otherwise, because of hydrolysis, would have to be added to achieve a satisfactory depth of shade. These parameters are controlled according to pre-defined profiles and closed-loop feedback systems [26]. Metering systems designed for optimising continuous preparation, dyeing and printing processes have been reviewed. In continuous systems the liquid dye brands can be dosed either from containers as supplied or from stock tanks fitted with dedicated pumps and piping. This arrangement should include a closed circuit for part of the flow to ensure effective circulation of the stock liquors and to prevent settling. The stock liquors are piped to valves, usually with three flow rates available plus a single-drop delivery into a container positioned on an electronic balance. The valves may be arranged in a cluster with a single point issue and the balance in a fixed position. Alternatively, if the valves are arranged in line, the balance moves on tracks into position under the appropriate valve. Clustered valves with a fixed balance give maximum weighing accuracy but inline valves with a mobile balance favour accessibility [27].
4.3 Automation In addition to the control of dyeing machines, a fully-automatic control system performs various other functions, as summarised in Table 4.8. An alternative approach to totally automatic weighing is to use simple or paternoster storage systems with manual weighing in a down-draught booth, controlled by a check-weighing system. This is followed by dissolving or dispersing and distribution from dispense tanks that can be linked into the control system (section 3.12). Developments in control systems have centred around increases in the power of microprocessors with a reduction in their size and cost [28]. The dyehouse manager can interrogate the control system from home using the Internet. Information from the system can be used to optimise the areas of: planning and scheduling, process control, selection of recipe and process, quality and production information. Systems can be operated from keypads or touch-screen colour displays and can include machine diagrams, on-line process diagrams, displays of future scheduling and various analytical information functions (such as efficiency and consumption analysis). Virtually any function can be automated at a cost, needless to say, but some of the control parameters that are available to assist in obtaining RFT processing are listed in Table 4.9. As indicated in section 3.12, the dispensary is a major key to success in obtaining reproducibility of dyeing and has received a great deal of attention [29]. Dispensing, dosing and metering of dyes and chemicals have been thoroughly reviewed recently [30], with discussion of all the elements involved in
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commercially available systems. Dispensing and automated handling systems for dyes and chemicals have been reviewed and a tabular guide given of the functions available on commercially available systems for which contact details are given [31]. The ColorService TRS automatic weighing and dissolving system for solid dye brands has been described. The main advantages include sealed, dry and safe storage of dyes, preventing moisture fluctuations commonly encountered with open containers, elimination of health hazards to operatives because of dust-free storage, weighing and dissolving conditions so that < 1 ppm dye dust is detectable in production areas [32]. Control systems can be stand-alone or include colour physics capabilities [33] and several options for the use of automation in both the laboratory and production were suggested [34]. A typical control system is illustrated schematically in Figure 4.1 and details of the Process ITM (Datacolor International) system [35] are given in Figure 4.2.
4.4 Robotisation Loading and unloading of dyeing machines was traditionally carried out manually but the circular-kier machine was an early candidate for robotised handling since the machine operating system included functions for opening and closing the lid. By the 1980s, there were relatively few missing links for the ‘human-free’ (or ‘lights-out’) dyehouse to become a reality. The availability of automated hydroextractors situated immediately in front of radio-frequency (RF) dryers has already reduced the handling and labour requirements for package drying after dyeing. The development of a human-free facility for dyeing wool and syntheticfibre tops and yarns was reported [36] with claims of a productivity improvement of 200% and energy consumption reduced to one-third. Although total robotisation is feasible for most package-dyeing operations, irrespective of substrate form, the main concentration of development has been in the area of yarn dyeing, where both hanks and packages are amenable to this approach. Package dyeing in either vertical or horizontal machine configurations can be robotised for loading, dyeing, unloading, water removal and drying. The advantages of horizontal-kier machines include elimination of the needs for high headroom and the siting of vertical machines in pits (as discussed in section 13.5). Since the machines can be floor-mounted, there is no need for overhead cranes and handling can be carried out by mobile shuttles on tracks or by robots. With both hank and package processing, it is possible to fully automate from ‘dry in’ to ‘dry out’ material. Most of the elements of automation are applicable to fabric dyeing and it is possible to robotise garment dyeing (section 17.2.7). Robotisation is not possible with winches and jets since an automated technique has not yet been devised for loading, sewing-in and unloading the fabric lengths, although tangle detectors and automatic untangling systems are available (section 14.6). Much of the development in robotised operations, including weighing and dispensing of dyes and chemicals and in dyeing machine operation, has been carried out in Italy. The reasons for this include a very active textile machinery producers association (ACIMIT) which publishes brochures and videos, as well as generous grants from the Italian government for companies carrying out robotics research and to those companies prepared to install such equipment. By 1990, fifteen robotised plants had been established worldwide for dyeing either tops or yarn. Details were given of robotised yarn dyehouses using either vertical or
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horizontal spindle machines [37]. Robotisation introduces some rigidity into technical aspects of the process and work scheduling but can greatly increase productivity, quality and service. The elements in the robotisation of a package-dyeing operation are outlined in Table 4.10. Robotised handling has also been developed for hank-dyeing processes and the outline is given in Table 4.11. Obem have recently introduced a system (the TMB/CNM) based on a rotating-arm hank-dyeing machine. Several papers have described robotised dyehouses. The installation of a robotised operation based on Obem API/O horizontal kier package-dyeing machines has been described [38] and includes package loading and pressing, with centrifuging and RF drying. In systems based on Obem machines, the robot shuttles run on tracks positioned in the dyehouse floor. An installation based on Bellini RBNO (vertical spindle, horizontal kier) machines has been discussed [39] together with a ColorService TRS system for the weighing, dissolving and dispensing of dyes [40]. The entirely new fully-automated yarn and top dyeing plant established in 1996 by Zwickauer Kammgarnspinnerei has been described. The Thies Eco-bloc HX dyeing machines provide a total capacity of 1.5 kilotons p.a. Dye and chemical dispensing, laboratory and dyehouse control, pressure drying, transport and handling equipment, energy supply and wastewater management are all designed to take full advantage of automatic systems [41].
4.5 Continuous Processing Little has been published regarding the limits of accuracy that must be applied in continuous dyeing to achieve acceptable reproducibility. Apparently the Asia Pacific countries have been more interested in this subject than their counterparts in Europe and America, even to the exclusion of work on the limits of accuracy for exhaust dyeing. An insight into the methods used in Japan for continuous dyeing control was given [42]. In order to control a continuous process, it is necessary to consider the machine operation as a series of functional zones, to define the parameters to be controlled in each zone and to determine the number of controllers for each zone. The functions that can be controlled include those listed in Table 4.12. The Küsters system of dye liquor consumption analysis for a pad dyeing unit has been described [43]. Process control and production monitoring as applicable to continuous dyeing and stenter operation have been discussed [44]. Process visualisation is an important aspect of control with machines of this type. The need to carry out shorter runs, involving a greater number of processes and variables, together with the importance of quick colour changes in continuous dyeing, makes automatic control essential. It is claimed that with such control, runs as short as 300 metres per shade can be processed economically with colour changes in 3 to 6 minutes. Quality improvements, greater machine efficiency and major cost reductions are also claimed from automation of such equipment.
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4.6 Ancillary Applications Many of the services discussed in Chapter 3 are amenable to control technology and these include water and effluent processing, boiler plant, compressed air supply, heating, ventilation and air-conditioning. The application of automation in the dyeing laboratory was discussed [45] and it was shown that improved accuracy could be obtained within the laboratory and in the transfer of laboratory results into bulk [46,47]. The Aladys (Automated Laboratory Dyeing System) developed by Ciba provides capacity for 250 exhaust dyeings per day on woven or knitted fabric. The components of the system include the preparation line, dye liquor formulation, dye and chemical dispensing, dye-cycle and sample distribution (drying, unwinding, labelling and reporting). The Aladys was developed in close cooperation with the robot manufacturer Demaurex SA [48]. Robotic laboratory testing has not been widely adopted but automated methods have become part of standard laboratory practice [49]. Software for the Autolab (Datacolor) dispensing systems is operated in the Windows system. Multitasking provides dynamic data exchange for all products in use, with the capability to group products together according to compatibility and suitability for specific substrates. A recipe dispense queue is maintained to ensure optimum working priorities. Autolab systems incorporate solution-preparing devices to make up stock solutions accurately and reproducibly. Computercontrolled dispensers formulate the required dye recipes quickly and automatically. Automatic agitation and tube cleaning ensure consistent concentrations [50].
4.7 Economic Considerations Virtually any function can be controlled, automated or even robotised but at a cost. As with any proposed capital investment, the evaluation of sophisticated control equipment requires a feasibility study, with a cost-benefit analysis and calculation of the payback period. The benefits to be expected from investment in control technology include those listed in Table 4.13. The need for an individual feasibility study is obvious for the reasons listed in Table 4.14. Irrespective of how thoroughly this is carried out, hindsight will eventually show that only some decisions were correct and stood the test of time, as a result of the dynamic interaction between technology, equipment, company products and policy. However, anticipated changes should never be reasons for failing to implement a financially feasible study with short payback periods. The opportunity to build a plant on a greenfield site is attractive since it allows state-of-the-art technology to be adopted. This includes building design, plant layout, the latest processing machinery and current processing techniques. Although in this instance the capital required is high, if there have been major advances in any or all of the above factors, it may be a much more financially feasible option than to introduce modern equipment and technology piecemeal into an old building. The maintenance, repair and spares costs of old buildings and equipment can soon surpass the cost of installing new versions. Because of the staying power of top-quality stainless steel [51], many existing wetprocessing machines are almost immortal. Of course, control equipment can be fitted onto existing machines in an established plant. This is obviously a less capital-intensive option. However, management may be loath to invest in
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sophisticated control equipment for what were relatively cheap machines at the time of purchase, compared with current prices. In general, computers and most control equipment yield short payback periods. A machine control system plus a semi-automatic dispensary on a mezzanine will have a payback period within two years. Increased reproducibility and the capability to achieve consistent production by blind dyeing reduces the cost of each dyelot by 30% for each correction eliminated (as discussed more fully in section 1.5). Typical benefits obtained from installing control equipment include a 50% reduction in labour, a 30 % increase in productivity, savings of 15 to 20% in dyes, chemicals and resources, together with an improvement in quality. It may be difficult to justify robotisation following the introduction of a semi- or fully-automatic control system since this already offers major savings. However, robotisation can appear more attractive when advancing from a manuallyoperated to a fully-automatic one, especially if this is on a greenfield site. Key decisions may be significantly influenced by the average dyelot size and the number of production machines. A fully-automated dispensary is very desirable on quality and reproducibility grounds but a typical system can prepare about twenty dyebaths per hour. Installing such a system can only be justified for operating many machines with short processing times. Thus sixty machines with a cycle time of 4 hours would enable the dispensary to reach an occupancy of 75%. The investment would be impossible to justify for a ten-machine operation, particularly dyeing reactive dyes on cotton. With relatively long cycles for the complete process, occupancy of the dispensary could not exceed 10%. Labour saving is unlikely to occur in these circumstances. It has been argued that greater savings and shorter payback periods are obtained by automating many small machines, such as found in a typical sewing thread dyehouse where the average dyelot size is 35 kg [52]. Conversely, short payback periods were achieved in a package dyehouse with a few large machines and an average dyelot size of 340 kg [7]. Productivity is governed by the actual machine capacity as determined by such factors as package size and density. The count of the yarn will influence the productivity of hank dyeing, whilst the average dyecycle time, defined by the fibre being dyed and the dye class being used, will be an important factor in both hank and package dyeing. A comparison of productivity in two dyehouses processing hanks and packages, both with average dyelots sizes of 320 to 340 kg, is shown in Table 4.15.
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I Gailey, Text. Chem. Colorist, 9 (1977) 25.
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Table 4.1 Factors requiring control in the dyeing of sewing thread [3] Factors of prime importance
Dyeability of substrate Heat-setting conditions (polyester): time, temperature and tension Dry weight of substrate Colour value of dyes Weight of dye Moisture content of dye Accuracy of dissolving or dispersing dye Dyebath temperature
Factors of major importance
Liquor-to-goods ratio Weight of electrolyte (cotton dyeing) Weight of auxiliary product Time of dyeing pH of dyeing
Factors of minor importance
Yarn construction (e.g. twist) Degree of bleaching (cotton) Degree of mercerisation (cotton) Size content Temperature of drying before dyeing Water quality
Table 4.2 Most important factors for polyester fabric dyeing [6] Substrate dyeability Heat treatments above 150°C before dyeing Fabric preparation Dyebath pH Metal-ion content of the dyebath Dye concentration
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Table 4.3 Determination of the shade sensitivity of dyeings to variation in selected factors of the dyeing process [5,6] Stage 1 2
3
4 5
Procedure Carry out dyeings at four or five variations of each factor, including the standard value for the conventional process Using regression analysis, determine the slopes of the straight lines relating lightness (L), chroma (C) and hue (H) to the series of values of each factor selected From each series of dyeings, determine the amount of variation of each factor that will give a ∆E of one unit, using the CMC (or appropriate) colour difference formula Calculate the ∆L, ∆C and ∆H values that contribute to this ∆E of one unit From the slopes linking these colour differences to variations in each process factor, determine the limiting values for control of the dyeing process
Table 4.4 Limits of accuracy for batchwise dyeing of acrylic yarn with basic dyes [7] Factor Moisture content of dye powder/grains Moisture content of substrate Weighing of substrate Weighing of dyes, chemicals and auxiliaries Dye standardisation for strength Dyebath pH
Limit applied ±3.5% ±0.5% ±0.5% ±0.5% ±2.5% ±0.35 pH units
Table 4.5 Summary of important factors in dyeing processes requiring control to minimise variability in colour difference [11] Factor Visual limit of trained colourist Substrate variability Water supply variation Instability of dye solutions and dispersions Variations in dye weighing of 2.5% Quality control assessment of dyeings Batch levelness Reproducibility in bulk Laboratory dyeing reproducibility Laboratory to bulk reproducibility
Variability ∆E(CMC) 0.6 up to 4.0 up to 3.0 3.0 to 5.0 2.5 0.15 0.2 0.2 0.12 0.3
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Table 4.6 Advantages of semi-automatic control (SAC) systems Increased productivity
Improved quality
Increased flexibility Improved safety Cost reduction
Same production with less labour More production from same labour Shortened dyeing cycles RFT production by blind-dyeing techniques Elimination or reduction of colour corrections Elimination or reduction of reprocessing Improved reproducibility Reduction in off-shade Improved levelness Preservation of substrate quality Less material loss Improved plant management and supervision Improved production planning Less risk of human error Improved working conditions Overall reduction in processing costs Savings of dyes and chemicals Improved utilisation of labour force
Table 4.7 Control functions for dyeing machine Function Machine fill Raise to initial temperature at rapid rate of rise Circulate material or liquor Raise to top temperature at controlled ramp Flow reversal sequence Holding time at top temperature Controlled cooling ramp Drain
Requirement Actuated valve and level sensor Actuated steam valve and temperature sensor Controlled electric motor or pump
Actuated valve on cooling water Actuated valve on drain
Table 4.8 Outline of fully-automatic control (FAC) system 1. Transfer of recipe details to colour and chemical dispensary 2. Totally automatic weighing (liquid or solid), dissolving or dispersing and distribution of dyes and chemicals 3. Control of additions of dyes and chemicals 4. Updating of planning, scheduling and stock inventories 5. Storage of diagnostic and management information, including consumption and utilisation of resources 6. Graphical presentation of stored data to generate reports
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Table 4.9 Control parameters Liquor ratio by analogue control or electromagnetic flow meter Time/temperature profile pH measurement, monitoring and adjustment using retractable electrode Rate of change of pH Conductivity Specific gravity Redox potential Liquor flow rate Rope circulation time Addition time and profile for dyes, auxiliaries and chemicals
Table 4.10 Outline of robotised package dyeing Precision wind to measured length to give standard weight of package – automatic doffing Robotised loading on to yarn frame(s) Robotised weighing and dispensing of dyes and chemicals Robotised transfer of yarn frame(s) into dyeing machine Automated dyeing process Unload yarn frame(s) from machine by robot Unload packages from yarn carriers Robotised hydro-extraction and RF drying
Table 4.11 Robotised hank dyeing Manual weighing of dyelots Robotised loading of hanks onto sticks of dyeing machine Loading sticks onto dyeing frame Loading hank frame into dyeing machine Robotised weighing and dispensing of dyes and chemicals Automated dyeing cycle Hank carrier removed from dyeing machine by robot Sticks removed from hank frame and squeezed by robot to remove excess water (eliminates rotary hydro-extraction) Robotised drying Robotised wrapping and packing of hanks for despatch
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Table 4.12 Control functions in continuous processing Bath temperature control Fabric temperature control Steamer pressure Water and chemical flow Liquor pH control Conductivity of liquor or fabric Specific gravity of process liquor Pad pressure and nip expression Compensator pressure Liquor level Fabric speed Chemical dosing Automatic filling of baths Data for steam, water, chemical and electricity consumption Process visualisation/schematics Automated mangle rinsing Automated I R predryer heat control to speed up batch changeover Control of oxidation baths Control of final fabric pH Rapid substitution of operating process details to cope with changes in substrate quality being processed
Table 4.13 Anticipated benefits from investment in control technology Factor Shade reproducibility Levelness Minimal damage of the substrate RFT dyeing Low liquor ratios Water recovery and reuse Low-temperature dyeing Controlled equipment Consistent dye and chemical weighing and dispensing Reliable recipe calculation Shorter cycle times Less downtime and less cleaning through scheduling Reduced manning levels Rationalised/standard products Better stock control Instrumental colour measurement
Benefit Improved quality
Savings in resources (steam, water, effluent, power)
Elimination of errors
Productivity increases
Savings in dyes and chemicals Improvements in productivity and reproducibility
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Table 4.14 Factors influencing feasibility studies Each company tends to use technology specific to its own products and processes The costs incurred depend on geographic location Technology and the economic climate are constantly changing, so that capital requirements will change over the development period Competitive quotations will be obtained from a number of suppliers for similar equipment and these will change with market requirements It must be established that a market exists for current products and production levels attainable, with some consideration being given to likely future trends The number of production machines and their designed capacity may influence costs and planning decisions
Table 4.15 Productivity comparison between two dyehouses Dyeing method Hank
Package
Degree of control Manual Machine control system and dispensary Robotised hank handling, automated weighing and dyeing Manual Machine control system and dispensary Totally robotised
Productivity (kg/operative/hour) 40 50 100 70 125 270
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Figure 4.1 Schematic of control system options Recipe prediction and correction
Quality control
Computer colour matching system
Recipes and process details
Order entry
Stock control
Dyeing cycle
Dispensary (can be dedicated system)
Dyeing machines (can be dedicated system)
Central controller
Figure 4.2 Process ITM Orders received New fabric batches
New shades
ITM Process Recipes for new shades formulated Production work sheets issued Recipes sent to dispensary Programs sent to controllers Inventory updated Standard shades stored Dyed/finished/despatched dyelots checked against standard Continuity files kept for all shades Every dyelot costed Quality control of fabric Recipes adjusted for fabric variation Recipes always updated
Dispensary Correct weighing checked Correct mixing made
Machine control Details of every dyelot reported Production schedule
Chapter 5 Product Evaluation 5.1 Interrelated Factors Two major product areas that impact on the successful operation of the dyehouse are discussed in this chapter. First are the substrates which start as raw materials and are converted into dyed and finished goods. This is achieved by using the second range of products, namely dyes, chemicals, auxiliary products and finishing agents. Control and monitoring of these major product areas are important in achieving right-first-time results, so that the finished goods meet the required quality and fitness for purpose. As shown in Figure 3.1, dyeing can be carried out at various stages in the manufacturing sequence. Several interrelated factors determine the stage at which dyeing is carried out, as summarised in Table 5.1 and discussed further in Volume 3.
5.2 Substrates When designing a fabric or garment a specification must be developed so that the final product meets the end-use requirements and is fit for purpose. Goods supplied are tested by the purchasing organisation for conformity against this specification. The product specification must define various parameters to be met during manufacturing; the most common are listed in Table 5.2. The design of textile materials is still largely based on traditional subjective techniques of trial and error, experience and intuition. This implies that time and cost are major elements, depending on the availability of experienced designers and the range of their expertise. In reality, routine textile production is based on a relatively few woven or knitted fabric designs that are almost commodity items. Product differentiation is achieved mainly by colour selection (either solid shade or patterned) and appropriate finishing routines. Computer-aided design (CAD) technology has been successfully applied to patterned fabrics. Various design approaches using this technology have been reviewed [1]. Computer graphics has proved useful in colour or design patterning for apparel fabrics. Computer designs can be transmitted directly from the computer screen to knitting machines or weaving looms. CAD techniques are applicable to garment design and patterning for cut, make-up and trim (CMT) manufacture of garments. Only limited information is available concerning the relationship between structural and functional properties relevant to product design. CAD is able to simulate fabric appearance or texture and this can be applied to ‘colour on screen’ systems as discussed in section 6.8. Databases have yet to be built for translating the mechanical properties of textiles into design parameters. Typical parameters that define a textile product design in terms of both effect and function are summarised in Table 5.3. Satisfactory preparation of the substrate before dyeing and finishing makes major contributions to consistent attainment of the desired end-product quality. It is perhaps worth repeating the adage that ‘well prepared is half-dyed’. The impurities present in various fibre types and the preparatory processes to remove these are extensively discussed in Chapters 7 and 8 and the Volume 3 chapters. The product specification at each stage of manufacture and the methods used for
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preparation, dyeing and finishing must be such that the end-product specification is not impaired. Three major objectives that must be achieved are: 1. each process used must leave the product in a suitable condition for the next operation 2. processes must not so degrade the material that the final specification cannot be achieved 3. fastness properties of the dyes must be such that the product will withstand subsequent processes in addition to meeting end-use requirements. Fastness requirements are discussed in section 5.15. The vertical processor has maximum control of the composition, construction, manufacture, dyeing and finishing of the textile product. This is not the case for commission processors; testing and analysis may be necessary at appropriate stages to confirm the authenticity of the product received and its conformity to specification at each stage of processing. The importance of sourcing and adequate preparation of cotton fabric by the commission dyer for dyeing with reactive dyes has been discussed [2], although many of the principles apply equally to other dye/fibre systems. The testing of cotton fibres was extensively reviewed [3] including a discussion on fibre colour and impurities together with the use of small-scale yarn spinning as a method of quality assessment. As indicated in section 13.1, textured yarns remain an important sector of the industry and the production of these by falsetwisting and other methods [4] has been reviewed. Air-textured yarns have been increasingly used in the production of automotive fabrics and silk-like effects have been obtained by texturing methods. Yarn hairiness became an increasing problem when machine speeds were increased and as new spinning techniques were developed. It has been estimated that 30% of weaving stoppages are associated with yarn hairiness which is also responsible for lint formation in knitting [5,6]. The quality of the fibre, whether natural or synthetic, the spinning technique used and winding operations can affect the degree of yarn hairiness. Careful control of sizing operations can reduce the hairiness produced but where the problem is severe, fabrics must be singed. The production and properties of sewing threads is highly specialised; this has been extensively reviewed recently [7]. Singeing of cotton yarns and heat-setting of synthetic yarns are usually integral parts of the process. Continuously increasing demands on sewing and seam performance require on-going development. Adequate thread lubrication is necessary to permit high-speed sewing with dissipation of the heat generated at the sewing needle. Incorrect thread storage can have an adverse effect on sewing performance. This may be strength loss due to photodegradation or increased friction and sewing temperature because of lubricant migration. Fastness properties must meet enduse requirements. Yellowing of textiles in storage can be a problem [7,8]. Factors which either alone or in combination may give rise to yellowing complaints are listed in Table 5.4. The application of finishing agents after dyeing is carried out at each stage where dyeing takes place, whether this is in fibre, yarn or fabric form. The main objective is to give the necessary lubricity for the following processes. Such agents should not lower productivity or performance in subsequent treatments or impair properties such as handle, sewability or fastness. Selection of an appropriate finishing agent is thus complex. Product changes are difficult to
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justify and should only be considered after extensive evaluation, including wearer trials.
5.3 Fibre Identification and Analysis Qualitative and quantitative analysis of the fibre composition is important for the dyer so that appropriate dyes and application techniques can be selected. The combinations and amounts required can be adjusted to produce the desired colour when dyeing fibre blends. Fluctuation in blend ratio results in off-shade dyeings, the component fibres being dyed to different colours when a previously established standard formula is applied. This also occurs when any component in a blend is changed to an alternative fibre source or merge, which usually exhibits different dyeability. Methods for fibre identification have been described using staining, burning, solubility and microscopic methods. Quantitative chemical analysis of fibre blends is usually based on the selective solubility of different fibre types in individual solvents [9]. These methods are most likely to be employed by the works support laboratory. More sophisticated analytical techniques can be applied using equipment not usually available in the works laboratory. The key to success with these methods often depends more on the skilled laboratory technician rather than on the equipment [10]. Scanning electron microscopy (SEM) produces images of the surface topography of the fibre with a far greater depth of field and resolving power than optical microscopy, and therefore usefully improved magnification. Transmission electron microscopy (TEM) is largely confined to studying fine internal structures of single fibres or parts of fibres and depends on the careful preparation of ultra-thin axial or cross-sectional samples, or a replica of the fibre. Conventional physical techniques can be augmented by melting point determinations and gravimetric methods. Instrumental techniques including infrared (IR) spectroscopy, pyrolysis gas chromatography, differential thermal analysis, thermogravimetric analysis and X-ray diffraction techniques are employed. As discussed in section 8.1, DNA analytical methods can be employed to identify hair fibres. The monitoring and testing of so-called ‘manufactured’ fibres at various stages of the textile production chain have been reviewed [11].
5.4 Physical and Chemical Testing of Textiles Physical and chemical tests on fibres, yarns or fabrics may be required for the following purposes: 1. to assess the raw material and the effects of subsequent processes 2. as part of a product development project 3. to establish a product specification 4. as a customer service 5. to ensure that the product conforms to a purchasing or sales specification 6. the investigation of faults, complaints and processing problems. A comprehensive list of chemical and physical tests for fibres, yarns, fabrics and floor coverings, including flammability tests, has been given including references to the standard test methods employed [9]. Not all of the tests cited are likely to
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be used on a regular basis by the works laboratory, the most common being listed in Table 5.5. Tests may be required before and after a given treatment. Developments in chemical testing and analysis and in physical testing and quality control have been discussed [12]. Pilling has been reviewed [13]. Changes in fibre structure may result in significant effects on textile properties. Many important physical properties of textiles are dependent on the fine structure of the fibres. The relationship between structure and textile properties has been examined [14]. Over the last two decades, much research has been directed to the objective measurement of fabric properties whereby parameters ranging from handle to total product specification can be determined without the use of traditional subjective assessments. In the Kawabata Evaluation System for Fabrics (KES-F) measurements are made on four test instruments: tensile/shear, bending, compression and surface-friction/geometrical-roughness. Fabric assurance by simple testing (FAST) can be used in place of the KES-F system. The FAST system, developed by CSIRO in Australia, uses three instruments which measure compression, bending and extensibility. This latter system was developed specifically to measure the properties of wool and wool-blend fabrics that affect their tailoring performance and the appearance of such garments during wear. This technology, often referred to as ‘Fabric Objective Measurement’ (FOM) has been extensively reviewed [15].
5.5 Substrate Degradation Degradation of textile substrates is a major problem and a complex phenomenon. The fibre begins to degrade during manufacture or harvesting and this continues throughout its lifetime until the textile is rejected as useless, even though no deliberate ill-treatment of the material has occurred. Textile degradation has been extensively reviewed and the principal causes include those listed in Table 5.6. The mechanisms by which degradation occurs and the effects obtained include those listed in Table 5.7 [16]. It is important that processes are designed to minimise degradation during dyeing and finishing. For minimum degradation, treatment time and temperature should be kept to an effective minimum that gives acceptable performance. This is assisted by the development of reproducible processes which are documented in SOP. Control of pH and the use of optimum chemical concentrations are also essential. Possible causes of degradation and methods of reducing damage are discussed where relevant in Volume 2 chapters. Several reasons for damage are well-known, including the photosensitisation of cellulosic fibres by certain vat dyes, the embrittlement and losses in tear strength and abrasion resistance that can occur in the application of resin finishes to cellulosic fabrics and the oxidative degradation of nylon by certain 1:2 metal-complex dyes, although some of these dyes give a protective effect [17]. Typical tests carried out by the works laboratory to examine textile degradation have been listed [9] but a comprehensive range of sophisticated tests is available, including those given in Table 5.8.
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5.6 Dyeability Tests Dyeing tests are frequently used as an aid to fibre identification, to assess fibre damage or structural modification, or to reveal dye-affinity variations; these methods have been well documented [9,18]. Bayer (now part of DyStar) was the first dye manufacturer to introduce the concept of systematic dyeing procedures [19]. Dyeability parameters are determined to enable the necessary calculations to be made to optimise the process. Such tests have been discussed [9] and further details of these processes are given in Volume 2 chapters. A major reason for failure to achieve RFT dyeing is the false assumption that once a colour has been matched and a ‘standard formula’ obtained in bulk, all subsequent batches of substrate will dye in the same way as this first production. To ensure RFT production, it is necessary to compare the dyeability of each delivery of substrate with a standard kept for this purpose, usually taken from the first bulk delivery [9].
5.7 Identification of Size Polymers and Finishing Agents Sizing agents are applied to yarns to improve weaving efficiency and these must be effectively removed as part of the preparation process. Removal depends on the identification [20] of the size so that the correct desizing process can be carried out. Finishing agents are often applied at various stages of wet processing and methods for identifying the type of finish and its distribution on the substrate have been discussed [18]. Simple staining tests have been described to identify size polymers or resin finishes on textile fabrics. Although resin finishes are normally applied after dyeing, knowledge of the distribution of the resin may be valuable to indicate the uniformity and durability properties of the resin-treated fabric. An alternative to dye staining is to use a fluorescent brightener as staining agent and to examine the treated sample under a fluorescence microscope. Another technique useful in this context is a combination of swelling and staining treatments. Highly crosslinked cellulosic material is relatively insensitive to the swelling agent and a pretreatment of this kind tends to increase the degree of contrast revealed by the subsequent staining test.
5.8 Dye Standardisation The standardisation of dyes and their conformity to specification are important for the purchasing and use of these products. Reputable dye manufacturers have established procedures to ensure consistent standardisation [21,22] and the dye user must also follow certain principles if the dyes selected are to contribute to RFT processing. Dye evaluation and selection are important factors; there is much in common in the techniques used by the dye manufacturer in evaluating speculative or competitive products and those used by the dye user in selecting products suitable for processing and to meet end-use requirements [23]. The important parameters are shown in Table 5.9. The factors that influence the behaviour of dyes in the dye-fibre system are of particular relevance to the dye user. The dye user will obtain optimum reproducibility from a combination of dyes, if they are: 1. robust- unaffected by slight changes in processing conditions (such as pH, liquor ratio, temperature and time)
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2. compatible- function in combination as if they were single dyes 3. stable- not attacked by contaminants in the water supply or substrate 4. consistent- insensitive to slight changes in substrate quality Whilst all factors listed in Table 5.9 are relevant in research and the early stages of development, fewer factors need to be considered in the routine evaluation of dyes, either in monitoring of deliveries or in the evaluation of competitive products. This restriction is essential on time and cost grounds alone and a suitable protocol of tests is defined, depending on dye class, substrate, application method, finish, end-use and fastness. Having selected a range of dyes according to the parameters outlined above, quality assurance of deliveries must be achieved. The quality testing of each individual manufacturing batch against a master batch as control is an integral part of the dye standardisation process. When a partnership is developed between a reputable supplier and a regular customer, the dyemaker will provide the user with a certificate of conformity to standard for individual batches or deliveries. This certificate is an essential part of the quality accreditation procedures for the dye user (for example to ISO 9000). Alternatively, routine delivery testing can be carried out, either by the works laboratory or by an independent accredited laboratory [9]. This is possible on a routine basis for only a small number of parameters, the most important being selected from those listed in Table 5.10. To obtain results that are accurate and consistent between laboratories, it is essential to have standard test procedures; these have been discussed and methods are available [9]. Standard depth is a term used to describe a subjectively defined property of colour, yet it is of great importance both in fastness testing and standardisation of dyes. Analysis of databases from the four major European dyemakers resulted in a new method of defining 1/1 Standard Depth for each angle around the hue circle [24]. Dye suppliers normally quote the fastness properties of their products to various agencies and usually provide solubility data in pattern cards. However, there are numerous parameters listed in Table 5.9 and Table 5.10 for which information is not readily available. There is even less guidance on acceptable tolerances between batch and standard, the noteworthy exceptions being strength and moisture content. Dye manufacture is normally a multi-stage synthesis from several intermediates [25] and it is seldom feasible to guarantee optimum yields at every stage. Isolation of the product in the desired physical form may also be difficult to reproduce precisely. Hue and strength are the most important control parameters and methods have been described [21,22,25] by which the dyemaker evaluates this at various stages of manufacture. By the time the press cake stage has been reached, the ideal situation is to identify two manufacturing batches of the product that, when mixed, will give an on-standard product for hue and strength. This is seldom possible in practice and shading colour has to be added to the batch to obtain the correct hue and strength while maintaining an acceptable balance of other relevant properties. This standardisation procedure is subject to the following restrictions: 1. only specified dyes can be used as shading components for each individual dye 2. only a limited amount of each shading component can be used 3. only a specified diluent can be used to reduce strength 4. only limited amounts of other specified agents (such as dispersing or dedusting agents) can be added.
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Within each dyemaker’s range of dyes, the same or compatible dyes, diluents and auxiliary agents are employed. As already mentioned, compatible dyes must be used in the dye application process if consistency is to be achieved. In addition, any auxiliary products added to the dyebath, such as levelling or dispersing agents, should be compatible with the dyes and with all agents and diluents added during dye formulation and standardisation. Two papers [27,28] have focused on the influence of such additives in the application of disperse dyes. If such compatibility is taken into consideration, further factors in dye evaluation can be added to Table 5.9, as indicated in Table 5.11, especially when dyeing fibre blends. Unless much work is to be undertaken initially to assess all factors impacting on dye compatibility, there is a strong argument in favour of selecting a package of dyes and associated auxiliary products from one manufacturer. This approach is unlikely to be cost-effective, however, compared with a selection from all potential suppliers. The dye user also becomes vulnerable if supply problems arise. Changes in dyes or auxiliaries can only be undertaken following extensive assessment of the effects of such changes on compatibility. Whichever policy is adopted towards suppliers, such product changes cannot be undertaken lightly. It has been questioned whether so-called ‘identical’ (disperse) dyes are really identical [29]. Dyes listed in the Colour Index (C.I.) as equivalent are only similar with respect to the major coloured component present, this correspondence being disclosed by the manufacturers. Whilst the C I designation may be used as a guide to the equivalence of water-soluble dyes of relatively simple structure, such as unmetallised monoazo acid dyes, this information is of less value for other ranges of dyes and pigments. With disperse or vat dyes formulated as liquid brands, the colour content may be as low as 10 to 15% of the product marketed. Equivalence of C.I. generic name or number does not imply any similarity of colour strength or in the nature or amounts of other components present [23]. Unless reactive dyes are synthesised, isolated and stabilised carefully, the content of active (non-hydrolysed) dye present may vary, leading to deficiencies in fixation and fastness properties. Ecotoxicological data are strictly applicable only to the specific dye formulation under test. Such criteria are not transferable between products sharing the same C.I. number. Thus the dye user needs to carefully consider the implications whenever any one product is substituted by another. The dye manufacturer takes great care in the standardisation process, not only to correct dyes for hue, strength and other parameters, but to select additives for dye formulations that do not adversely affect compatibility within a given dye range. If the dye user does not take the same care in the evaluation and selection of dyebath additives, this can nullify the efforts of the manufacturer. Much laboratory work and testing is necessary to ensure that compatible products are chosen. Compatibility can be guaranteed (at a price) by selecting an exclusive package of dyes and auxiliaries from one source. An ultimately more costconscious and versatile approach is to assemble a compatible selection from several suppliers, although this entails substantial compatibility evaluation. In any case, frequent product substitution on price grounds is no longer a viable option. Thorough evaluation in the laboratory, followed by pilot-scale confirmation and bulk-scale trials, is the only viable route.
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5.8.1 Dye selection Dye selection is based on screening many available dyes to pinpoint which products give outstanding performance and cost-effectiveness. A positive purchasing policy for dyes and chemicals must be adopted to rationalise the number of dyes purchased, so that a complete colour gamut can be matched on any one substrate using the minimum number of dyes. Technical and financial advantages are thus obtained. Supplier selection should also be carried out [30]. Colour matching can normally be achieved using only three dyes, but four- or five-membered combinations are occasionally necessary to meet a particularly difficult degree of metamerism. Depending on the class of dyes and the colour gamut attainable with them, matching methods can be based on the following approaches: 1. A trichromatic combination of bright dyes plus a small range of support dyes compatible with the main trio 2. Internal primaries; for example, a brown colour matched with yellowish, reddish and mid-brown components with colour coordinates close to target 3. A dye close in hue to the target colour plus small amounts of compatible shading dyes. Computer colour-matching software usually operates inefficiently with data based on approach (2). Colour mapping, by which the colour coordinates of individual dyes are plotted in colour space, can be a useful aid to dye selection. Colours with coordinates located inside the triangle joining those of the component dyes can be matched by those three [31-33]. There are a number of interrelated factors that affect dye selection, as shown in Table 5.12.
5.9 Solubility and Physical Form Dyes are available in liquid or solid form. Solid brands include conventional and modified powders, microgranules, granules or treated granules and are mainly used in exhaust dyeing. Liquid brands are used in continuous dyeing and printing. Vat dyes are usually pastes or powders, disperse dyes may be granules, powders or liquid dispersions, whereas reactive dyes are marketed as powders, granules or liquids. The proportion of liquid-brand reactive dyes is expected to increase in future for the following reasons: optimum operational safety, unrestricted application by various methods with special advantages in padding or printing, designed metering concepts and variations to comply with customer demands and environmental restrictions [34,35]. The aqueous solubility of ionic dyes has become increasingly important as dispensary operations have been automated and dyeing is carried out at low liquor ratio or by semi-continuous methods. Aqueous solubility is determined at a series of temperatures related to the processes used [9]. Related properties such as wetting characteristics of the dye powder, and the specific gravity, viscosity and surface tension of the dye solution or dispersion are of practical significance in automated dye-dispensing systems. The selection of disperse dyes depends on application properties determined by: 1. the particle size distribution in the initial dispersion and the stability of the dispersion to subsequent changes in temperature, pH or ionic strength 2. concentrations of dispersing agents and electrolytes present
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3. liquor flow and 4. the form of substrate to be dyed. Four types of instability of a dye dispersion can be distinguished: 1. agglomeration of particles 2. crystal growth 3. change of crystal modification and 4. recrystallisation. Improvements in the physical forms of disperse dyes became necessary for jet and package yarn dyeing at lower liquor ratios, particularly for textured yarns. Ultrafine particles of certain aminoanthraquinone disperse dyes have been microencapsulated using polyvinylpyrrolidone as shell material, leading to improved dyeability and dispersion stability [36]. Environmental concerns (section 2.5), initially associated with the inhalation of reactive dye powders [37], have led to developments in physical form of dyes to minimise dusting. Low-dusting granular and powder forms containing dedusting agents, such as dodecylbenzene, paraffin oils or mineral oils, are available. Dusting problems can be eliminated entirely if liquid brands are selected [38]. The supply of dyes from non-traditional suppliers (NTS) with the associated distances involved in transport means that the effects of dedusting agents are lost due to settling or adsorption onto the dye particles.
5.10 Storage and Handling Storage of dye powders in a moist atmosphere can lead to caking but variation in the moisture content of the dye has more serious implications for the accuracy of weighing and the ability to achieve consistency [39]. Modern dyestores are airconditioned with double-door entry systems to maintain a suitable atmosphere. The storage and handling of dyes has been reviewed [40] and discussed in sections 3.12 and 4.1.
5.11 Analysis and Identification of Dyes The analysis of dyes by a wide variety of techniques has been thoroughly discussed [41] and methods for identification of the class of dye on the fibre have been described [42]. Methods of sampling from bulk deliveries have been detailed together with the dye tests likely to be used in the works laboratory, as listed in Table 5.9 and Table 5.10, including schemes for identification of dyes in substance and on the fibre [9]. Dye strength is determined by spectrophotometric analysis [43] of the batch against standard, followed by confirmatory dyeings as discussed in section 6.3 [9,23]. Chromatographic techniques [9,41] are widely used to separate the colour components of dye samples and to confirm that approved shading components are used in deliveries. Advances in chromatography as applied to dyes have been reviewed, concentrating on the techniques of thin-layer and high-performance liquid chromatography and capillary electrophoresis [44].
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5.12 Stripping of Dyed Fibres For the analysis of dyes or dyed fibres, dyes may need to be stripped from the substrate by either destructive or non-destructive methods. Non-destructive stripping is essential when the amount of dye on the fibre has to be estimated, or the dye has to be identified or analysed by techniques such as chromatography. Non-destructive methods may be based on blank dyebath treatments with high concentrations of levelling agents and such methods for nylon and polyester have been given [45]. Solvent extraction techniques [9] are more likely to be used, however. Laboratory methods for stripping dyes seldom mirror those used in bulk processing, since these are usually destructive methods based on strongly reductive or oxidative methods [46].
5.13 Cost-Effectiveness Evaluation Increasing economic pressures have forced dye users to select dyes with greater care so that a complete gamut of colours can be obtained from the minimum number of dyes, thereby reducing stocks and gaining bulk purchasing price concessions. More stringent technical requirements have meant that for certain end-uses only a few dyes will meet the required specification, for example, in the dyeing of automotive fabrics [17]. Automated and cost-effective processes favour the use of rationalised small ranges of dyes. Dye application must be controlled by imposing the limits of accuracy as discussed in section 4.1. Relative price at equivalent strength (cost-effectiveness) is an important factor in dye cost comparisons but there is little point in buying a product at lower unit cost for a given strength if this jeopardises reproducibility. The limited scope for substitution with so-called equivalent dyes has already been discussed. Differences in exhaustion, reactivity, fixation and build-up of reactive dyes greatly influence their cost-effectiveness. The effect on other factors such as the cost of effluent disposal cannot be ignored either. It has been claimed that the development of a truly universal standard method for determining the relative strengths of disperse dyes may be impractical because of the diversity of physical form, formulation and dispersion stability of these brands [47]. A computer-based technique for the value analysis of dyes was developed [48] whereby the application method to give the minimum cost of dyeing can be selected. A curved surface in ANLAB colour space representing equal colour strength (Control Depth) irrespective of hue or brightness is defined in terms of a constant Integ value. This is determined by an integration with respect to wavelength of a weighted function of the measured reflectance values of a dyeing throughout the range of visible wavelengths. The amount of dye that must be applied to a substrate by a specific dyeing method to give the target Control Depth may be determined by experiment and is called the Control Strength of the dye. The relative colour values of dyes of a given colour on this substrate can be estimated and compared, irrespective of minor differences in hue, by multiplying together the Control Strength and the selling price in each case. The use of optimised dyeing techniques can achieve major cost savings [49]. Dyes should be selected that are applicable by so-called rapid-dyeing techniques to give the specified fastness without lengthy processing or aftertreatments. Chemicals used in a dyeing process are normally purchased against a specification and methods are available for analysing these [50]. Purchasing a lower quality chemical at a reduced price may give rise to dyeing problems, with
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a reduction in RFT production. Inferior qualities of common salt, for example, may contain significant amounts of hardness salts. Dyebaths for reactive dyes should be checked for pH, hardness and salt concentration (by measurement of specific gravity) to ensure that the correct conditions are reproducibly achieved.
5.14 Dye Application Tests Many dye properties can only be assessed by actual dyeing tests and reproducible dyeing methods that correlate with bulk processing have been developed [9]. Level dyeing is an essential requirement for commercial acceptability but it may be difficult to reproduce bulk-scale unlevelness in laboratory dyeings. Many tests have been developed, under the auspices of the Society of Dyers and Colourists, to classify dyes according to their dyeing properties and these have been summarised and documented [9,23]. In view of the importance of reactive dyes, it is unfortunate that no equivalent standard tests have been developed. The compatibility of dyes is important for cost-effective dyeing since it affects dyeing time, reproducibility and levelness. Auxiliaries that are claimed to improve compatibility, strike or migration behaviour and thereby level-dyeing properties can be evaluated by relevant dyeing tests with and without the agent present [9]. Poor reproducibility in the dyeing process may be due to dye instability under certain pH conditions. This can be assessed by measuring dye exhaustion at various pH values, preferably from absorbance data for the dyebath as well as reflectance of the dyed substrate. Variations in substrate and its ease of coverage are assessed by dyeing tests using dyes known to differ in their sensitivity to dyeability variations. Cross-staining onto other fibres is an important factor in blend dyeing since this can be a major reason for poor reproducibility and a reduction in fastness properties.
5.15 Fastness Fastness requirements vary according to the stage in the manufacturing sequence at which dyes are applied, the subsequent finishing processes and the proposed end-use. Fastness demands relevant to end-use are detailed in specifications issued by purchasing organisations. Dyes must be selected to give satisfactory fastness to the agencies of interest. These can be categorised as indicated in Table 5.13. Standard methods for most of these tests have been published [51], identical formats being available in versions issued by the British Standards Institution (BSI), the Society of Dyers and Colourists (SDC) and the International Standards Organisation (ISO). These tests are used worldwide and useful background information on their development and the equipment required has been given [52]. An independent series of tests has been developed in the USA through the efforts of the American Association of Textile Chemists and Colorists (AATCC) [53] and most countries have their own national versions. Fibre producers and major textile retailers often develop and publish their own test methods. These standard tests are intended only to define the method by which the test should be performed and the results assessed. They do not give a ‘fitness for purpose’ specification or a pass/fail rating. This must be specified by the purchasing organisation and agreed with the supplier.
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A method has been proposed for the rapid assessment of light fastness by expressing the results of standardised exposures in terms of grades on the ISO Grey Scale. The technique is applicable to textile samples treated with fluorescent brighteners. The relationship between Grey Scale assessments of fading and traditional Blue Scale ratings has been discussed [54]. Certain combination dyeings with reactive dyes on cellulosic fibres show catalytic wet fading, typically the accelerated fading of certain azo yellows when used in admixture with phthalocyanine blues. This phenomenon has been investigated recently by comparing Blue Scale fastness ratings with detailed colour difference measurements on dyeings prepared with sensitive dyes applied alone and in mixtures that show anomalous fading [55]. Outstanding fastness to light and weathering is a crucial requirement for automotive textiles but each of the major car manufacturers operates with individual test specifications. Even within the same company it is possible that different test procedures have been developed originally for specific production models [56]. Different national test regimes prevail in various parts of the world. European test standards generally operate at the highest temperatures but with only a moderate spectral distribution. In the USA there is more emphasis on accelerated test methods by including more shortwave UV radiation [56]. In contrast, the Japanese carmakers do not use these low UV wavelengths and tend to increase intensity of irradiation to achieve accelerated effects [58]. During dye screening and selection, a protocol of appropriate tests must be selected from the lists in Table 5.13 to which selected dyes must exhibit specified fastness ratings This ensures that the rationalised small ranges of dyes held in inventory for each fibre, dyeing class and end-use specification will not give rise to fastness problems during finishing and will subsequently meet the specification issued by the purchasing organisation. Many tests listed in Table 5.13 are long-established and these methods are seldom changed, although the pass/fail tolerance or specification tends to tighten over time as the consumer’s expectations increase and as more varied finishing routines are introduced. Fastness to washing has seen the greatest changes, since these tests must reflect changes in washing practice. An early range of tests, the so-called ISO 1 to 5 series [59], was developed to simulate washing procedures of different degrees of severity based on soap with or without the addition of alkali. These conditions ranged from low-temperature hand washing to prolonged high-temperature (95°C) alkaline washing of cotton. These ISO tests have been used in dye screening for many years. More recently tests were developed to simulate the effects of domestic or commercial laundering in EU countries. This set of C06 tests [60] was based on the use of a specifically formulated EU detergent without FBA and contained sixteen test variations, covering temperature, liquor volume, addition of chlorine or sodium perborate, time, pH and the addition of steel balls [61]. Retailers sometimes specify either ISO 1 to 5 or C06 series tests but replacing the standard EU detergent by a commercial product. This can give conflicting results between laboratories, since commercial detergents are often formulated to meet local washing conditions in different geographical areas. Independent tests have been developed in the USA [62] to reflect the washing conditions used there, which are different from European practice. Environmental pressures to reduce water and energy consumption in the domestic washing cycle have resulted in corresponding changes to washing
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machine design. Detergent formulations have been modified so that the same degree of cleansing is achieved at the lower liquor ratios and temperatures employed. Developments in washing detergent formulation have depended on the introduction of four types of additive, as shown in Table 5.14 [63]. Detergent formulations containing such additives can modify the shade of coloured garments after only a few (4 or 5) repeated domestic washes and established test methods [60,62] do not predict the degree of colour fading that is likely to be observed in practice. Single-stage tests have been developed [64-68] to predict the fading which will occur after many (25) domestic washes. Suitable dyes can then be selected based on the results of these tests [63]. The so-called ‘colour-safe’ detergents are not just standard heavy-duty detergents with the activated bleach omitted. The exclusion of this component has a significantly negative effect on washing performance. The modification to render a formulation colour-safe confers some efficacy in preventing dye transfer but is much less effective in avoiding dye loss. Active components other than the bleach system play significant roles in combining colour consistency with effective detergency. Fluorescent brighteners tend to modify the hue of pastel shades but the surfactant system exerts solubilising effects. Enhancing colour-safe behaviour involves adding polyvinylpyrrolidone as a complexing agent to inhibit dye transfer, omitting the fluorescent brightener and increasing electrolyte, sequestrant and enzyme content [69]. It is recognised that some fading complaints are due to the combined effects of exposure to light and washing liquor which occurs during outdoor drying. Although wet fading tests are not currently part of retail specifications, it is likely that these could be included in the future based on the use of either water or alkaline perborate solution. A testing protocol frequently specified in the Asia Pacific region for functional sportswear is the perspiration-light test [70]. This is a very severe test that assesses the combined effects of water, perspiration and light [71]. It has been suggested that screening tests should be carried out at the laboratory matching stage to select dye combinations that will not give rise to fading [63]. The suggested tests and the proposed tolerances are summarised in Table 5.15. On-tone fading should occur in all tests, except the oxidative hydrolysis test. The conventional tests for contact staining, wash fastness and rubbing will be used to monitor the effectiveness of production procedures. Wet contact tests such as fastness to water and perspiration are important in assessing fastness performance in active wear. The effects of changing the test variables, particularly the buffer formulation chosen to maintain the test pH, have been examined. Perspirometer and M&S Plate techniques were included in the evaluation, together with a comparison of visual and instrumental assessments. There was a tendency for more severe ratings to be recorded when using the M&S Plate technique combined with instrumental assessment of samples showing staining in the 1-2 region of the Grey Scale [72]. The colour and bursting strength of swimwear fabrics made from nylon, elastane or blends of elastane with nylon, polyester or polyester/cotton were measured before and after exposure to a traditional chlorinated pool water formulation and an alternative chlorine-free formulation. The test conditions included exposure in artificial light to simulate the swimming pool environment. Both treatments resulted in colour changes and strength losses, but the combination of light and chlorine-free pool water caused less deterioration than the traditional exposure test [73].
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5.16 Reproducibility and Accreditation Many of the parameters discussed in this chapter have a bearing on reproducibility and RFT processing as indicated in section 1.5 and Table 1.13. This is particularly true of dyes, chemicals, auxiliary products and those substrate parameters that impact on dyeability. As already mentioned in section 5.8, those companies that enlist in quality accreditation procedures, such as ISO 9000, require certificates of conformity from their suppliers. Alternatively, quality testing of deliveries in-house or by an independent accredited laboratory is essential [9,18].
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Table 5.1 Factors in the selection of dyeing stage Dyeing stage Fibre
Positive factors Highest fastness dyes can be used Resulting unlevelness can be corrected by blending in spinning Special effects can be produced such as ‘heather’ mixtures
Yarn
Colour woven effects can be produced Solid colour or blend-dyed effects can be produced
Negative effects Long lead times necessary between shade selection before dyeing and retail in garment form High fastness necessary to ensure satisfactory processing in yarn and fabric form
Higher fastness necessary to ensure satisfactory processing in fabric form
Quick response is possible through package-dyeing route Fabric and garment dyeing
Quick response is possible to market and fashion trends
Level dyeing is essential and this implies compromise with fastness to meet market requirements
Table 5.2 Product specification Stage of operation Fibre
Yarn
Fabric
Dyeing stage
Parameter Fibre content Fibre diameter and length Lustre levels Spinning system or texturing method Count and twist Relaxation shrinkage Cloth construction Fabric weight per unit area Strength Shrinkage potential Abrasion resistance Pilling propensity Chemical finishes to be applied Physical finishes Flammability Tactile properties Fastness requirements To produce target shade, appearance and texture when combined with finishing routine
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Table 5.3 Effect and function parameters [1] Appearance
Handle
Easy care
Dimensional factors
Drape
Bending modulus
Washability
Shrinkage
Lustre
Compression
Dry-cleanability
Crease recovery
Texture
Surface friction
Drying
Deformation
Optical effects
Surface contour
Ironing
Pliability
Shear properties
Pilling Snagging Oil retention Physiological
Biological
Safety
Mechanical
Stretchability
Mildew resistance
Flame retardancy
Tensile strength
Thermal insulation
Insect resistance
Injury protection
Tearing strength Wear resistance
Lightness
Abrasion resistance
Air permeability Perspiration transport Microclimatic Electrostatic Skin irritation Clogging Dyeing
Ease of finishing
Tailoring
Dyeability
Heat treatment
Sewability
Colour fastness
Pressing
Formability
Surface treatments Chemical finishes Lamination Pressing
Cost
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Table 5.4 Factors causing yellowing of textiles [7, 8] The presence of butylated hydroxytoluene (BHT) as an antioxidant in film wrapping material Lubricants that are miscible with BHT Exposure of garments to atmospheric oxides of nitrogen, possibly after migration of BHT from film material Yellowing of textile substrate induced by oxides of nitrogen Oxides of nitrogen reacting with textile finish The presence of certain fluorescent brighteners with or without interaction with oxides of nitrogen Hydrogen sulphide in polluted atmosphere Migration of oligomers from film material Oxidation of cationic softeners Scorching of the textile substrate or finish Fungal or bacterial action on storage Loss of shading dyes on washing Light fading to give yellow breakdown products Thermal or photochemical oxidation of nylon
Table 5.5 Common test procedures [9] Test type Chemical
Physical: fibres
Physical: yarn
Physical: textured yarn Physical: fabric
Commonly used test Moisture content Fibre composition pH Content of oils, fats, waxes Size content Strength Length Diameter Count Twist Strength Shrinkage Number of filaments Crimp rigidity Effective steam-setting temperature Weight per unit area Pilling Abrasion Strength Shrinkage Washing stability Seam slippage Flammability
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Table 5.6 Principal causes of textile degradation [16] Inherent molecular structural factors Processing conditions – fabric production, dyeing and finishing Storage Maintenance procedures – washing and laundering Normal use – mechanical action, UV radiation, weathering, microbiological attack, thermal treatment Severe use (protective clothing) – acid or alkaline hydrolysis, salts
Table 5.7 Mechanisms and effects of degradation [16] Mechanisms Mechanical
Effects Mechanical and structural change
Photodegradation
Surface changes
Thermal Chemical
Changes in morphology, visco-elasticity and degree of polymerisation (DP)
Microbiological
Gas evolution, oxidation and weight loss Yellowing and changes in photochemical stability Water absorption Hydrolysis
Table 5.8 Methods for assessing degradation [16] Mechanical property changes
Tensile modulus, tearing strength and abrasion resistance
Chemical methods
Solubility tests Viscosity or fluidity Degree of polymerisation Molecular mass Chemical analysis and reactivity Diagnostic dyeing
Thermal methods
Pyrolysis/gas chromatography Pyrolysis/mass spectroscopy
Instrumental methods
Ultraviolet spectroscopy Infrared spectroscopy Gas chromatography Gel-permeation chromatography Optical and electron microscopy X-ray and electron diffraction
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Table 5.9 Factors affecting dye evaluation and selection [23] Response of dyes to environment
Behaviour in dye-fibre systems
Standardisation
Cost-Effectiveness
Homogeneity
Shade area
Absorbance in solution
Colour value
Analysis and identification
Build-up Reproducibility
Storage Stability
Dye Application Properties
Variation in moisture content
Levelling and migration
Storage conditions
Substantivity and diffusion Reactivity and fixation Sensitivity to temperature pH and redox behaviour Compatibility Cross-staining Transfer and vapour pressure Efficiency of wash-off
Solubility and Physical Form
In-Service Requirements
Aqueous solubility
Coverage
Crystal modification
Penetration
Particle size
Fastness
Commercial form
Tendering of substrate
Health and Safety
Influence of finishes
Dustiness Trace metals Eye and skin irritation Acute toxicity Long-term hazards Biodegradation Sludge adsorption Fish toxicity
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Table 5.10 Routine dye evaluation tests [9] Moisture content Strength by reflectance of dyeings Strength by transmission of dye solutions Paper or thin-layer chromatography Build-up test pH-sensitivity test Reactive dye fixation Thermomigration test Strike-migration test SDC migration test Temperature strike test Strike test for disperse dyes Dispersibility Dispersion stability Dusting Solubility and solution stability Electrolyte sensitivity of reactive dyes Cold water solubility Coverage properties Fastness (selected from over 30 agencies)
Table 5.11 Dye selection for each fibre type Dye combinations for each fibre type Compatibility between dye classes Cross-staining of each fibre type Compatibility of chemicals and auxiliaries
Table 5.12 Interrelated factors in dye selection Factor Substrate
Effect Substantivity Ease of penetration Coverage properties
Machine characteristics
Flow rate Rate of substrate movement
Application technique
To give level dyeing properties
Fastness
To subsequent processing Effect of chemical finishes End-use requirements
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Table 5.13 Fastness tests Processing agencies
End-use agencies
Dry-cleaning
Light
Organic solvents
Washing
Potting
Dry-cleaning
Decatising
Organic solvents
Steaming
Water
Alkaline milling
Sea-water
Acid felting
Chlorinated water
Stoving
Perspiration
Bleaching agencies
Spotting with water, acid and bleach
Dry heat
Hot water
Pleating
Gas fumes
Vulcanising
Ozone
Carbonising
Bleaching agencies
Chlorination
Rubbing (wet and dry)
Mercerising
Carpet shampooing
Soda-boiling Cross-dyeing Degumming Formaldehyde Hot pressing Dyebath metals Sublimation
Table 5.14 Washing detergent additives [63] Enzyme-based systems that digest organic stains or remove surface hairiness to prevent dulling of colours so that garments remain in pristine condition The use, in some regions, of silicate-based alkalis as partial or full replacement of soda ash so that washing is carried out at a higher total alkalinity Peroxy-based bleaching systems (perborate or percarbonate) Peroxy-based bleaching systems that are activated with tetraacetylethylenediamine (TAED) or sodium nonanoyloxybenzenesulphonate (SNOBS)
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Table 5.15 Summary of screening tests [63] Test method Washing BS 1006 UK-TO Light ISO 105 B02 Alkaline perborate wet fading: 24 hours exposure Perspiration-light JIS L-0888 Oxidative hydrolysis
Outerwear
Leisurewear
Sportswear
3-4
3-4
3-4
4
3-4
3-4
4
4
-
4
4
3 4
Chapter 6 Colour Communication, Colorimetry and Match Prediction 6.1 Development of Colour Measurement Technology Colour measurement is well-established in the dye manufacturing and textile coloration industries and is an essential tool for efficient operation of modern dyeing and finishing plant. This technology is used in production, quality control and in the laboratory. As indicated in section 4.3, the colour measurement system in the dyehouse may be either stand-alone or integrated into a total control system. In common with other control equipment, there has been a reduction in size and price of hardware with an increase in the range of functions available. The advent of the Internet enables colorimetric data to be transferred by e-mail, thereby speeding up colour communication. The areas in which colour measurement is used include those listed in Table 6.1. The fundamentals of instrumental colour measurement were developed in the 1930s. It was not until the 1960s, with the development of robust and accurate spectrophotometers together with computers capable of carrying out the many calculations necessary at high speed, that computer colour matching (CCM) became a reality. The important developments are summarised in Table 6.2.
6.2 Background to Computer Colour Matching The practical colourist has several criteria to fulfil when applying a coloured substance to a substrate, including those listed in Table 6.3. This is a daunting task, bearing in mind that conventional matching is carried out by trial and error, usually in the laboratory, using accumulated experience, colour libraries, illustrations of single colorants and records of previous batches. The need for RFT processing, every time, just-in-time (JIT), together with the concept of quick response to the needs of the marketplace, has not eased the pressures imposed by this conventional approach. The task is made simpler, however, by using a limited range of colorants, selected after careful and exhaustive screening, which can give a wide colour gamut, have satisfactory application properties and possess fastness characteristics adequate for the proposed end-use of the product, as discussed in section 5.8. To fulfil the criteria listed in Table 6.3 could take numerous trials and even when all the technical factors have been met, it is still a race against the clock. Most laboratories using conventional matching techniques, employing only manual methods for establishing a suitable recipe and visual methods of assessment, have a perpetual backlog of colours to be matched. This situation changes rapidly on the successful introduction of computer-based matching techniques. The computer system is able to obtain matching formulations quickly and comment on their colour constancy or metamerism. The latest software will also provide information on compatibility, level dyeing and fastness properties of the predicted dyeing [20]. Available recipes are also evaluated for cost-effectiveness. Substantial financial savings can be made by introducing CCM technology as discussed in section 6.9 and improved customer service can be provided using rapid matching techniques.
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Many other potential cost-saving capabilities are enhanced by the availability of a CCM system, including those listed in Table 6.4. As already mentioned, full stock control, automated machine control and management information are features that can be linked with a CCM system with the availability of the necessary software and interfacing with the relevant equipment. Computer prediction of colorant recipes is an analytical problem and is based on a definable relationship between a measurable physical parameter of the substrate, namely its reflectance, and the colorant concentration. Although this relationship is extremely complex, a practical approximation can be found in the theory developed by Kubelka and Munk [5]. The theory of computer match prediction has been discussed [21-25]. The Kubelka-Munk theory has shortcomings, notably that calibration curves on textile substrates are non-linear, but most software in commercial CCM is based on this theory. The simplifying assumption is made that dyes behave in the same way singly or in mixtures. Various attempts have been made to improve such systems by collating the results from previous predictions and dyeings, such as the unpublished J&P Coats ‘feedback’ approach. Another system of this type is the SmartMatch (Datacolor) technique. A comparison between conventional and SmartMatch systems in recipe calculations for combinations of Terasil (Ciba) disperse dyes on polyester fabric has been provided. It has been shown that application of the SmartMatch system ensures first-match success rates at well over 90%, even under difficult conditions [15]. Artificial intelligence systems or artificial neural networks (ANN) are based on the biological model of the nerve cells. Signal transfer within the network resembles similar mechanisms in the natural model. The artificial network has a learning capability based on algorithms, the so-called ‘learning rules’. Learning takes place by ‘self-organisation’, classifying input patterns into categories [26-28]. A neural network was applied to prediction of the concentrations of three dyes from their absorption spectra alone and in mixtures. The ANN system performed significantly more effectively than two other models based on the traditional Beer’s law approach [29]. Neutral networks were trained using yellow, red, blue and black members of the Astrazone (DyStar) basic and Resolin (DyStar) disperse dye ranges. The results of colour matching trials were satisfactory in most cases, but the Astrazone black network gave less reliable predictions because only 57 data sets were available. Training with 500 data sets takes about 45 minutes of computer time on Windows 95. This approach to colour match prediction is only appropriate for some routine tasks because of the considerable volumes of training data required [30]. A similar exercise with fluorescent acid dyes also confirmed that, although timeconsuming, this approach is viable and accurate [31]. Although graphical methods of recipe formulation were an early development, the iterative technique outlined by Park and Stearns [8] is still the basis for most computer programs. Computer colorant formulation has been a practical and commercial reality since the 1960s (Table 6.2).
6.3 Colorimetric Instruments An important component of the CCM equipment is the colour measuring device, which in modern CCM systems is usually a spectrophotometer that measures the UV and visible radiation reflected or transmitted by the material. This information is transformed by tristimulus integration into numerical values of colour at a
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series of wavelengths. The detailed features of spectrophotometers, including measurement geometry, have been discussed in detail [24,25,32-34]. Modern instruments have microprocessors as an inherent part of their design so that the spectrophotometric data are retained in the memory and can be read or printed out digitally or graphically in a number of formats, for example, reflectance or tristimulus data. This information is also retained for future calculations such as those required for colour-difference measurements or in formulation calculations. Reflectance is usually measured between the wavelengths of 400 and 700 nanometers, at either 10 or 20 nm intervals, the measurement being completed in a few seconds. The selection of an instrument depends on the requirements of the individual user and on a range of operating factors, including those listed in Table 6.5. It is claimed that recent developments have increased the correlation between instruments of the same type and between instruments from different manufacturers [35], as shown in Table 6.6. Nevertheless, the apparent lack of standardisation between modern colorimetric instruments may be an obstacle to the spread of colour quality control as a means of product quantification. Colour difference measurements taken using eleven different models of spectrophotometer were compared with the aim of analysing their correlation and dispersion. A high degree of correlation was found for the instruments but for the red, yellow, green and blue regions of colour space examined, significant variability was found [36]. There are about 20 manufacturers offering up to 100 colorimetric instruments for the European market. These models differ in measurement principle and geometry, size of the test area, type of illumination, selection of standard observer and colour-difference formula. Factors mainly influencing differences in the results obtained include the type of illumination, selection of standard observer, measurement geometry and colour coordinates. When evaluating different spectrophotometers for their degree of concordance, the important criteria are measurement of the same samples with the various instruments, calibration of colour distances against their specified values and measurements of colour distances between pairs of samples [37]. Practical methods for monitoring the performance of tristimulus colorimeters, solid-state array spectrophotometers and scanning spectrophotometers have been detailed. These procedures are recommended for monitoring these instruments in terms of short-term repeatability and long-term reproducibility [38]. Regular maintenance of instruments is required together with calibrations of accuracy and reproducibility, for which a series of ceramic tiles is available [39]. Hand-held spectrophotometers [40-42] are available from several major suppliers, proving useful in the laboratory and in portable CCM systems. They are widely used for quality control, where it is inconvenient to take a sample to the laboratory, such as in a car body paint shop. Measurements are made by the portable instrument and downloaded later to a personal computer. A colour television recording involves the automatic colorimetry of very many spatial elements every second. Video cameras offer significant practical advantages over conventional colorimeters: 1. Measurement of colour without a need to contact the surface to be measured 2. Measurement of many coloured elements simultaneously. A technique has been developed to obtain signals of sufficient resolution at low levels, so that small colour differences can be detected even amongst strongly or
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brightly coloured samples. This raises the possibility of measuring such colour differences using a video camera, which offers important advantages over standard colorimeters or the human eye [43]. The strength of a colorant is an important factor in the evaluation and purchasing of dyes. Dye strength against either a standard or a competitive product can be evaluated by optical transmission measurement of dilute solutions [1]. The spectrophotometers can be supplied with CCM systems that will undertake such measurements and carry out the necessary calculations [25]. Guidance on the preparation of the solutions has been given [44]. In view of the large volume of information that is available on dye strength testing by transmission methods [1], it is perhaps surprising and disappointing that inter-laboratory agreement in such tests is poor [45,46], apparently associated with systematic laboratory errors [46]. Reflectance measurements on dyeings are invariably used as the final arbiter of dye strength, perhaps as an indirect result of the inconsistency of transmission data. Practical guidance on carrying out transmission measurements has been given [25].
6.4 System Selection The functions carried out in a colour matching laboratory using a modern CCM system include those listed in Table 6.7, together with an indication of the necessary software required. The system may be interfaced with two or more spectrophotometers and may either be a stand-alone system or linked into a total control system, as discussed in section 4.3. For the functions listed in Table 6.7, the system will include the components listed in Table 6.8. The calibration of spectrophotometers, monitors, printers and viewing cabinets is an important aspect for colour communication which is discussed in section 6.8. The system is usually operated in an air-conditioned atmosphere, with a working environment of 20°C and 65% relative humidity and a change of air six times per hour. Air is usually filtered and recycled with air-lock entrances and antistatic mats being beneficial. The location should be vibration-free since this can affect both the optics of the spectrophotometer and the computer drives.
6.5 Laboratory Dyeing Procedures and Database Preparation The consistency of results obtained from the match prediction programs of the CCM system depends on the quality and accuracy of preparation of database (also referred to as primaries or calibration data) fed to the computer by the spectrophotometer, together with the reproducibility of laboratory procedures. The methods used in the laboratory must be correlated with or simulate those used in bulk and the importance of SOP has already been stressed. Guidance on the preparation of database and the assessment of its quality has been given [25,47] and laboratory equipment and methods have been detailed [48]. Database must be prepared using the conditions and materials (water, dyes, chemicals, substrate) that are used in practice. Poor performance from database may arise from incompatible dyes, dye-auxiliary interaction or changes in factors which are thought to be already ‘known’. Routine laboratory dyeing must be carried out to the same accuracy as that used for database preparation. A standard procedure must be available for the drying, conditioning, preparation, presentation and measurement of all sample types. In a well-operated CCM
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system, the majority of target colours should be matched within two laboratory dyeings. Failure to achieve this is usually due to an inappropriate sample for measurement (usually as regards size), inadequate measurement procedure, selection of an incompatible dye combination or inaccuracies in laboratory dyeing. If this stage is reached, the matching procedure should be restarted.
6.6 High-Quality Recipe Prediction and Total Colour Control By the mid 1970s, the equipment and technology were in place and the discipline could be instilled by appropriate management to achieve a high success rate in coloration processes using blind-dyeing techniques. Instrumental colour measurement was successfully applied to define the limits of accuracy that had to be achieved to obtain a given colour matching tolerance and these limits have been incorporated into SOP, as discussed in section 4.1. The ideal approach to colour matching depends on following the SOP and allowing for variations in water quality, substrate dyeability and dye quality, so that high-quality recipe predictions can be achieved. These can be transferred directly into bulk processing without the need for laboratory matching. A less satisfactory quality of prediction is likely to be obtained if correlations for the above factors are not made. Although the preliminary laboratory work may be less, laboratory matchings and corrections are necessary and it is unlikely that predictions from this second route will be satisfactory for use directly into bulk. The quality of the prediction will determine which of four options is chosen: 1. the first laboratory dyeing can be transferred to bulk production 2. a visual correction is made on this first laboratory dyeing before transfer to bulk 3. a computer correction is made and this is transferred to bulk without a further laboratory dyeing 4. a computer correction and second laboratory dyeing is necessary before transfer to bulk. By the early 1980s, the concept of total colour control had been established and a typical system which was operating by 1981 [49] is shown in Figure 6.1. Key elements of this approach were the adoption of instrumental colour difference assessment and the reliance on non-physical standards, both of which are discussed in section 6.7. The scheme illustrated in Figure 6.1 could be operated in either a manual or automated plant but more usually in the latter, as discussed in section 4.3. The concept of total colour management was established, including the elements illustrated in Figure 4.1 and Figure 4.2. A flexibly programmed system of optimised recipe management and calculation has been described, in which dye recipes and procedural details are tailored to account for fluctuations in production parameters. The system was developed inhouse for the package dyeing of cotton yarn with vat dyes at the H Eing GmbH dyehouse, but it is suitable in principle for any dyeing or finishing process. As far as possible, all processing factors that may influence the dyed result are expressed in empirical mathematical functions incorporated in the control program. Data from previous batches of the target shade are included and reappraisal of the empirical functions takes place regularly. The system has been in operation since 1990 and a blind dyeing level of 95% has been achieved. The problem of liquor ratio variability has been solved satisfactorily but automatic correction factors for substrate dyeability are not yet practicable [50].
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Bureaux matching services can be a cost-effective alternative for the small dyehouse with a few matchings or as an entry method into colour physics technology. This approach has been given a new lease of life with the advent of the worldwide web, as discussed in section 6.8. Dye selection is an important factor in obtaining high-quality predictions and the CCM system can assist in this selection process. Reducing the number of illuminants in which a match is required and increasing the matching tolerances yields increasing numbers of acceptable predicted recipes. Enhanced software has been provided to improve the quality of predictions using accumulated previous experience [15]. Changes in substrates and dyes can occur on a short-term basis and these necessitate frequent correlations to upgrade the database. Longer-term changes in application techniques (such as time, temperature, chemical changes, liquor ratio) usually demand preparation of new database. When a comprehensive colour library has been created in the computer, together with established, reproducible formulations, recipes can be quickly formulated for new colours by finding the nearest and correcting the filed recipe – a search and correct technique. This archival approach can be quicker and more reliable than using a raw prediction approach, especially where the recipes held by the computer have been successfully blind-dyed in bulk more than once. The main method of controlling exhaust dyeing by adjusting the time/temperature profile is a major function of dyeing machine control systems, as discussed in section 4.2. Dye or chemical dosing, pH control, flow rate or reversal could also be used [51]. These alternatives imply that a method of continuously monitoring dye exhaustion is available, since on-line control can only be achieved once the effect of changes made can be measured [52]. Considerable experimental effort is required to determine the individual colour change sensitivity of trichromatic combinations of dyes to a specific dyeing parameter [53,54]. Lack of accurate, robust and reliable monitoring equipment has retarded the adoption of these alternative parameters. Control of these parameters would undoubtedly improve process efficiency and reproducibility but control of the exhaustion rate alone may not necessarily yield a dyeing of the required degree of levelness. Personal computers are being used increasingly in the dyehouse to calculate and optimise the quantities of chemicals required in dyeing recipes, conversion of laboratory to bulk recipes, adaptation of recipes for application to different fabric qualities, optimisation of dyeing processes and selection of cost-saving recipes and processes [55]. DyStar’s Compudye system of computer-aided dyeing is intended to replace existing non-optimised procedures by a computer program to automatically optimise the recipe and procedure for each individual batch. Thus bulk production becomes the focus for development and optimisation [56,57]. Equipment has been developed for the control of dyebath exhaustion using online measurement of colour in the dyebath, particularly for acrylic fibres dyed with basic dyes [58,59]. As already noted, measurement of the transmission of dye solutions is not without difficulty and this equipment has been mainly applied to the development of application methods on small-scale machines. Poor machine circulation resulting in temperature gradients within the machine would result in inadequate control of changes in other physical parameters. Although much work has been reported in this area [51,60,61], there has been little, if any, commercial exploitation of the results [62]. On-line colour measurement has not been tackled with enthusiasm, although some equipment suppliers have developed on-line pH measurement.
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6.7 Instrumental Quality Control Procedures An important factor in the quality control of coloured materials is maintaining the colour of subsequent batches close to standard within an agreed tolerance. This has traditionally been carried out visually, but raises problems associated with the subjective nature of the human observer, as listed in Table 6.9. The tolerance that can be allowed between batch and standard is to some extent determined by the observer. However, it also depends on the reproducibility of the colouring process, the cost of applying strict tolerances in relation to the value of the product and the magnitude of the colour difference regarded as acceptable before customer complaints are received. The variability of natural daylight due to geographical location, weather conditions, season of year and time of day means that reproducible colour assessment must be carried out in cabinets fitted with the necessary illuminants, usually daylight (D65), tungsten filament (Illuminant A) and store light (TL84). These cabinets should be operated in ‘darkroom’ conditions to exclude extraneous light and samples should be conditioned in a standard atmosphere or conditioning cabinet before assessment. A measuring principle has been developed by which dyed samples can be rapidly conditioned after rinsing and aftertreatment, so that in the intermediate sampling stage they are not over-dried but still in a moist state almost corresponding to standard humidity conditions [63]. The physical, mental and psychological state of the observer affects the colour decisions made. Even a trained colourist will change approximately 20% of previous assessments if given a second chance to do so, leading to ‘wrong decisions’ which can have costly consequences. It often happens that considerable differences are found between visual and colorimetric assessments of dyeings on goods with unusual texture or lustre. The possibility of excluding the substrate surface as an influential factor by measuring the colour alone and then incorporating a fibre surface parameter into the calculation has been examined. It was found that although colorimetric measurement is possible with exclusion of surface effects, the introduction of a specific surface parameter into the calculation is only possible to a limited extent [64]. If a professional colourist is presented with an array of colours, it is impossible to arrange these logically using a two-dimensional area such as a table top. The colours can only be arranged logically in three dimensions, in the form of a colour solid or colour space. The first colour space was devised by the CIE in 1931 and was based on the XYZ tristimulus values and the associated x, y, Y colour space. The concept of the XYZ system is based on the three-component theory of colour vision in which the eye possesses receptors for three primary colours (red, green and blue) and that all other colours are seen as mixtures of these three primaries. The CIE also defined a colour space for the graphical arrangement of colours in two dimensions using the chromaticity co-ordinates x and y (calculated from XYZ) and the lightness value, Y, effectively a third dimension perpendicular to the x,y plane. The tristimulus values are useful for defining colour, but the results are not easily visualised. The CIE XYZ system was designed only to indicate if colours match. If two samples have the same x, y and Y values, they will be an exact match for a given illuminant and observer. However, the two samples may have different reflectance curves and they are defined as being metameric, since they will no longer match if the illuminant, observer or viewing conditions are changed. General indices of metamerism based on reflectance data show poor correlation
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with visual assessments for both illuminant-dependent and independent equations [65,66]. For a pair of dyed samples to be metameric their reflectance curves must differ and intersect at multiple wavelengths throughout the visible spectrum [65]. Metamerism exhibited by a pair of samples should not be confused with the colour constancy of a single sample whose perceived colour does not change when viewing conditions or illuminants are changed. A recent project on this topic was initiated to derive reliable special indices for predicting metamerism and colour constancy. These were based on the three advanced colour-difference formulae (CMC, BFD and CIE 94) and gave satisfactory correlation with visual results. The degree of precision from these special indices was equal to or higher than typical observer precision [67]. The major disadvantage of the CIE colour space is that it is non-uniform; visually equal colour differences will be represented by different distances in colour space depending on the shape of the colour solid involved. Alternatively, ellipsoids drawn in various regions of the colour solid containing acceptable matches will differ in size [68]. Early attempts to overcome these difficulties resulted in the CIELAB and LCH systems, the latter being typified by the Munsell colour order system [69,70]. In the LCH system, colour position is defined by chroma (C) or brightness and hue (H) or hue angle. Lightness (L) is a third dimension perpendicular to the CH planes running from white (at the top = 100) to black (at the bottom = 0). Chroma or brightness increases as the distance increases from the centre of the colour solid. An advantage of the LCH system is that the terms used (lightness, chroma or brightness and hue) are those familiar to practical dyers in defining colour differences. Having defined the position of a pair of samples in colour space (such as LCH) the colour difference is easily quantified by the three-dimensional Pythagorean Equation 6.1. ∆E = [(∆L)2 + (∆C)2 + (∆H)2 ]0.5
Equation 6.1
where ∆ signifies ‘difference in’ and E is the initial letter of the German word ‘Empfindung’ meaning sensation. A major advance was the development of the so-called optimised colour difference equations which effectively compensated for the visual non-uniformity of the colour solid. The most important of these was the JPC 79 equation, introduced by J & P Coats [12]. The development of these optimised equations has been discussed [21] and many of the colour difference equations available have been reviewed [71,72]. A ranking of the reliability of available equations, using seven colour-difference data sets, indicated that the JPC 79 equation was the most reliable, as determined by the percentage of wrong decisions between visual and instrumental assessments [21]. With minor modifications the JPC 79 equation became the CMC (2:1) equation [73] which gained the status of a British Standard [14]. Similar equations were developed by Datacolor and Marks & Spencer but unfortunately the details of these were not published. Since the introduction and widespread adoption of the CMC equation, three further colour-difference equations have been derived, which are claimed to give improvements, namely BFD (l:c) colour difference formula [74], the CIE 94 formula [75] and CIE DE 2000 formula [76]. Overall the BFD formula gave the best fit to experimental results in a detailed comparison with particular reference to lightness differences. However, the lightness scales used to derive the CMC and CIE 94 formulae are based on extensive application without serious complaint
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[77]. In a further evaluation, the level of disagreement between the BFD and CIE 94 formulae was generally lower than that between the CMC and CIE 94 systems. In the latter case, the discrepancies covered a large region of colour space, especially in the orange sector [78]. Although such innovations are admirable, a huge volume of data, including limits of accuracy and pass/fail information, has been amassed using the CMC equation, which works effectively in practice. It is often costly and difficult for organisations to change formula when so much data has been collected in this way, unless the perceived benefits are substantial. With the availability of optimised colour-difference equations, the difficulties associated with visual assessment can be eliminated. It is also possible to undertake single-number shade passing (SNSP), which allows a single numerical tolerance to be applied to any component of colour difference (such as ∆E) and this colour tolerance is then satisfactory for all regions of colour space. The numerical tolerance is defined only by the end-use criteria as regards the tightness of matching and ∆E (CMC) = 0.8 is widely used. Many pass/fail decisions are based on the use of ∆E only but ∆H may also be specified. The adoption of SNSP as the sole means of assessing colour in a textile dyehouse has been advocated [79]. An SOP must be available for carrying out colour measurement, particularly as part of the quality control procedures. The parameters that should be considered include those listed in Table 6.10. The value to the practical dyer of a means of readily converting measured colour differences between individual dyeings and the target shade into concentration differences of the component dyes in the recipe has been emphasised. A mathematical model has been developed that enables the coordinates of CIELAB colour space to be linked with changes in the dye component concentrations by means of non-linear mathematical functions. Further refinement of this system is in progress to provide an improved approach to recipe formulation, shade correction and analysis of off-shade dyeings [80]. Correct setting of the pad liquor is an essential element in the quality assurance of continuous dyeing. Transmission measurement has been proposed as an alternative to control dyeings prepared in the laboratory. Practical trials have confirmed that consistent liquor monitoring is possible for dye dispersions as well as solutions. With extensive automation of the measurement technique, high operational reliability and short times of measurement can be attained. It is claimed that the system virtually rules out faulty batches arising from incorrectly set pad liquors and appreciably reduces dependence on laboratory control dyeings [81]. In addition to batchwise quality control, on-line measurement can also be undertaken on continuous dyeing ranges. Many dyeing faults can be recognised by installing a robust colorimetric monitor following the padding stage. The application of such devices combined with automatic control of nip pressure during padding has been successful in bulk. Careful analysis of the dyeing procedure and equipment operation is necessary before adopting this approach [82]. Most weftway shading and warpway tailing is attributable to non-uniformity of the residual moisture content, absorbency, whiteness or fabric temperature. On-line colorimetric devices can make a major contribution to recognition of the typical fault patterns arising from these variations [83–85]. The Mahlo Colorscan has a sensor head mounted on a weftway traversing mechanism located 18 mm above the moving fabric. The compact design of this
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unit allows for easy integration within existing continuous dyeing equipment. Colorimetric scanning can be fitted at any of seven possible locations in a typical continuous dyeing range (Figure 6.2). The final dyeing can be measured at the inspection point (1) but there is no further possibility of intervening in the dyeing process. Measurements taken immediately after final drying (2) can be misleading because of variations in fabric temperature and residual moisture content. Similar objections arise if attempts are made to monitor warm wet fabric after washing (3), warm dry fabric after thermosol treatment (4) or damp fabric after IR predrying (5). The most favourable position for the scanning head is after the liquor has become uniformly distributed within the fabric during the skying section (6). Although measurements immediately downstream of the padding nip (7) provide no significant information in terms of the final tone, depth and uniformity of the dyeing, this measuring location provides highly effective indications of tailing and weftway shading problems. Scanning at these positions just downstream from the colour application stage rapidly signals whether the fabric will be uniformly dyed, with obvious advantages over later scanning points in terms of speed of response when faults seem likely to occur [84]. Initial samples for matching should be of a sufficient size to give meaningful results in spectrophotometric measurement. The inexperienced manipulation of small, lustrous, lace or pile fabric samples can give rise to inaccurate measurements. Satisfactory colorimetric reproducibility can be expected for plainwoven fabric samples using spectrophotometers with either spherical or bidirectional geometry. Such performance is often inferior, however, in the case of velvet or corduroy samples with non-uniformity of surface texture [86]. Measurements of fibre blends also present difficulties, but a technique involving pressing out of felt-like discs that retain the colour characteristics of both fibre components has been described. These discs are readily prepared and are preferred to yarn windings for calibration purposes. Details are given of the application of this technique to colour matching for the pile and backing of a cotton/acrylic velour fabric [87]. Physical standards must be handled and stored with care since frequent, careless handling leads to soiling and abrasion. Such standards should be stored in a conditioned atmosphere and any material used in mounting or covering should not give rise to colour changes. Non-physical standards should be used whenever possible. This is an important factor in the scheme illustrated in Figure 6.1. Improvements in the absolute accuracy, repeatability between instruments and in calibration techniques allows reliable data to be routinely transferred in this way, as discussed again in section 6.8. A logical extension to instrumental quality control of colour is the sorting of batches that can be grouped together for either fabric or garment manufacture. Instrumental methods allied to the necessary software can supersede traditional visual methods. Regression analysis was applied to investigate the correlation between visual colour assessment and instrumental colour acceptance. Three colour-difference equations (CIELAB, CMC and CIE 94) were compared to determine the most appropriate for generating a uniform colour space, in order to allocate the colour population in shade sorting. Both CMC and CIE 94 showed better correlation than the CIELAB equation [88]. Shade sorting techniques have been reviewed [89]. The earliest system widely applied was probably the HunterLab 555 technique, in which the region of colour space occupied by the batches to be sorted was divided into equally sized
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rectilinear boxes. The faces of these blocks were aligned parallel to the L, a and b axes of colour space and the size of the blocks depended on the tolerances for ∆L, ∆a and ∆b. Blocks had to be of different sizes in different regions of colour space. Optimised colour-difference equations allow the same block size to be used for any colour and the lightness, chroma and hue tolerances are the same. The rectilinear boxes of the 555 system now become cubes but these can result in relatively inefficient sorting. The application of a uniform colour micro-space based on one of the optimised equations has resulted in more efficient sorting. A micro-space based on ∆L, ∆C and ∆H can also be used. The maximum dimension of the shade-sorting volume represents the maximum permissible colour difference tolerance allowable in making-up, for example between panels in a garment. The most efficient volume will contain every batch that is not more than half of this distance from the batch at the centre. The ideal volume is therefore a sphere of this maximum diameter but these cannot be close-packed without leaving interstitial spaces. Up to 26% of batches may fall into the spaces. In a uniform colour space, the brick-shaped block becomes a cube, as already mentioned, but this occupies only 37% of the volume of the sphere that intersects the corners of the cube. Several regular solids are capable of closer packing and the twelve-faced rhombic dodecahedron and fourteen-faced truncated octahedron have been used [21]. These occupy 47 and 68% respectively of the volume of the sphere that intersects the corners of the enclosed polyhedron. If the shade distribution in the octahedron is regular, 46% fewer cutting lots are required, in comparison with the cube. The use of shade sorting has not been widely accepted but it may well yield the greatest single saving through the use of instrumental methods. Savings which equate with 10% of the annual purchase cost of finished substrate are possible.
6.8 Colour Communication Traditionally, shade ranges were created by a designer collecting coloured specimens of materials from various sources and these physical samples were then sent to the matching laboratory for conversion of these colour ideas into dyeings on a textile substrate. The problems associated with visual assessment, the interpretative nature of this process and possible conflict with the designer’s personal preferences often made this a lengthy and involved process. The use of colour atlases or specification products illustrating many coloured specimens, preferably on a textile substrate, can greatly assist in the initial colour selection decisions and in the matching process [90]. Atlases based on colour order systems [70] are preferred and commercial colour specification products have been developed. Some of these are in-house products with a relatively small circulation, for communication between an in-house designer and colourists, whilst others have been commercially sold in large numbers [91]. The development of a uniformly-spaced colour atlas based on the CMC colour space has been described [16]. This was dyed on cotton fabric with commercially available reactive dyes to ensure adequate fastness of the atlas in use and so that colours of satisfactory fastness could be accurately reproduced commercially. Two further advantages of this atlas are that colour recipes can be readily formulated in the spaces between existing colours and the colours illustrated on cotton are readily obtainable on other substrates using the appropriate class of dye. A major high-street retailer found that by using this colour specifier [16], the time taken
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to generate master standards, to which suppliers had to match products, was reduced from eight to two weeks. Developments in spectrophotometry, together with improved accuracy in computer colour matching and laboratory dyeing, has allowed non-physical standards in the form of reflectance data [92,93] to be communicated by fax or e-mail as part of the matching process [17]. Even when spectrophotometers from two manufacturers were involved, the average variability between instruments was found to be only ∆E(CMC) = 0.3, when assessed by means of ceramic tiles [39]. High levels of colour acceptance were achieved, provided that a match was obtained in the primary illuminant of ∆E (CMC) below 0.5 and that colour constancy was obtained in secondary illuminants. In a population of 2500 colours, matching difficulties were experienced with only three colours; one being a paper standard, the second containing a fluorescent brightening agent and the third being a multifibre colour mixture. This first approach to ‘colouring by numbers’ allowed a significant shortening of lead times (Figures 17.1 and 17.2) and a quicker turnaround in matching. A major development in the use of non-physical standards and ‘colouring by numbers’ has been the ability to communicate, visualise, evaluate and manipulate colour on screen [94,95]. This was pioneered by UMIST with their ShadeMaster system [96,97] subsequently commercialised as Colorite, this eventually becoming part of Datacolor International. The system development has been described [98] and its use by a major high-street retailer discussed [99]. The equipment required is listed in Table 6.8. The features and capabilities of the system include those listed in Table 6.11. Major time-savings in product development have been demonstrated. By loading an equally-spaced colour atlas [16] into the system, the Colorite system can generate eight equally-spaced colours between any pair of original colours, increasing the size of the atlas to approximately 64,000 equally-spaced colours. This atlas can be communicated via the website [19]. Impressive time and cost benefits can be made in the communication of colour for matching, standard generation, communication with suppliers and quality control in the end-product from either the laboratory or bulk production, as outlined in Figure 6.3. The standards can be loaded onto a website and password-protected, granting availability to authorised users only. So-called digital colour communication is becoming established as another means of achieving quick response in colour selection and product development, enabling the assessment of development and production samples without physically despatching them. A further advantage of the procedure illustrated in Figure 6.3 is that the virtual match will define the limits of shades that can be matched using commercially available dyes contained in the prediction programmes of the CCM system. This avoids attempts to match brighter colours unobtainable using the actual dyes on textile substrates. Digital colour communication and the equipment required have been reviewed [100]. It is concluded that all necessary hardware and software are available but real success depends on adequate training of operators and application of correct procedures by the systems manager. The textile industry has changed dramatically over recent decades from mainly vertical organisation to retail-specified manufacturing [101]. In traditional vertical manufacturing, garments were designed by manufacturers who had the responsibility of selecting yarns, fabric constructions, colour, finish and the final garment design. In theory at least, a garment was never designed that could not
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be readily made and costs were kept within a pre-determined budget to arrive at a profitable selling price. Many of these factors are now specified by the retailer but nevertheless an informed input is required from other members of the manufacturing chain, including dyers and finishers. Following this change, however, there is increasing demand for techniques such as digital communication of both colour and design. Web-based colour management software involves relatively low installation and running costs [102] and a complete service can be provided in this way, including computer matching, quality control, ‘digital swatches’ and dye purchase. This is not to say that the dyed standard or sample will disappear, since it is impossible to touch non-physical samples. Whereas specifiers, buyers and technologists have welcomed digital communication, designers tend to see it as a denigration of their artistic flair and talents. Important factors in the use of nonphysical standards (NPS) are given in Table 6.12.
6.9 Economic Benefits from Investment in Computer Colour Matching Some of the benefits obtainable by introducing colour measurement have been mentioned throughout this chapter. It is more difficult to quantify in financial terms the benefits provided by digital colour communication and the quicker generation of shade ranges and novel textile designs. Quick response from wet processing production improves customer service and just-in-time delivery, so that the customer can reduce inventory levels with an ultimate cost saving. Typical savings attainable in the dyeing operation are shown in Table 6.13. In a vertical textile group involved in cut, make-up and trim (CMT) manufacture, a significant saving of fabric wastage can also be obtained. As with all capital expenditure projects, a feasibility study is necessary but with CCM investment, a short payback period is likely. The savings made in a typical project during the late 1980s are illustrated in Table 6.14, which gives details for a fabric dyehouse processing 50 tonnes per week [25] The improvements and financial savings presented are those achieved within a year of the introduction of a CCM system. This gave a payback period of under six months for the system and the laboratory improvements in equipment and buildings. By the end of the second year of operation, colour additions per dyelot had been reduced to below 0.1 (above 90% RFT) with an increase in production of 25% and a reduction of 10% in labour requirements. In today’s economic climate, with a reduction in system cost and significant increases in the various contributions to processing costs, greater savings and even shorter payback periods are likely. Some purchasers of CCM systems have been disappointed with the results obtained. This is usually due to a lack of standard procedures and discipline, which must be first instilled within the organisation. The selection of a rationalised range of dyes with well-documented and appropriate procedures is essential. The absence of an adequate database, accurately prepared by the methods laid down in the SOP, is often a major reason for the poor performance of matching systems. Poor database quality and inadequate selection, measurement and care of standards soon result in GIGO generation. In countries such as the UK, where equipment manufacturers, dyemakers, colour users, retail and research organisations have been actively involved in research and application of colour measurement, it is estimated that 70 to 80% of textile
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colouring operations have invested in CCM technology. On a worldwide basis, market penetration of CCM systems into the textile coloration industry is no more than 40%. In view of the financial and other benefits to be gained from this technology, it is surprising that market penetration is no higher. The reasons for this antipathy appear to be: 1. some potential users do not believe the claims made for such systems, although they are well-proven and documented, as shown earlier in this chapter 2. many potential users do not have enough management information to ascertain the real performance of their company and, 3. consequently, they are unable to quantify the advantages and benefits or calculate payback periods. A spreadsheet approach is available to readily calculate this latter factor [103], but this is still impossible if basic management information is unavailable. It is inconceivable that a modern coloration unit can function without the benefits of CCM technology. The rapidly increasing importance of digital communication will mean that such organisations do not survive in the market-place.
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W J Jasper, E T Kovacs and G A Berkstresser, Text. Research J., 63 (1993) 545.
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G Faes, Melliand Textilber., 79 (1998) 462.
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C M Bezerra and C J Hawkyard, JSDC, 116 (2000) 163.
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P T F Chong, Rev. Prog. Coloration, 18 (1988) 47.
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R Herold, Die Farbe, 35/36, No. 1 (Aug 1990) 77.
[34]
D L Randall, Color Technology in the Textile Industry, 2nd Edition, (AATCC,1997) 10.
[35]
Macbeth Inc., private communication.
[36]
A Raggi and G Barbirolli, Color Res. Applic., 18 (1993) 11.
[37]
K Schläpfer, Die Farbe, 35/36, No. 1 (Aug 1990) 113.
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SDC Colour Measurement Committee, Col. Technol., 118 (2002) 149.
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Ceramic Colour Standards, Series II, (BCRA).
143
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K Hilscher and J Rieker, Melliand Textilber., 74 (1993) 53.
[41]
D S Reininger, AATCC Internat. Conf. and Exhib., (1994) 273.
[42]
R L Connelly and R W Harold, Color Technology in the Textile Industry, 2nd Edition, (AATCC,
[43]
C Connolly, W Leung and J H Nobbs, JSDC, 111 (1995) 373.
[44]
T R Commerford, Color Technology in the Textile Industry, 2nd Edition, (AATCC, 1997) 34.
[45]
B L McConnell, R Besnoy and R S D Wagner, Text. Chem. Col., 24 (Feb 1992) 23.
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C D Sweeny, Color Technology in the Textile Industry, 2nd Edition, (AATCC, 1997) 60.
[47]
D L Randall, Color Technology in the Textile Industry, 2nd Edition, (AATCC, 1997) 72.
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J Park and J Shore, Dyeing laboratory manual (Upperhulme: Roaches Internat.Ltd., 1999).
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J Park and T M Thompson, JSDC, 98 (1982) 74.
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K H Orriens, Melliand Textilber., 78 (1997) 725.
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A Gilchrist, Rev. Prog. Coloration., 25 (1995) 35.
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A Gilchrist and J H Nobbs, JSDC, 116 (2000) 154.
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D P Oulton and P B Chen, JSDC, 111 (1995) 237.
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P B Chen, C W M Yuen, K W Yeung and C K Yeung, JSDC, 115 (1999) 378.
[55]
F Hoffmann, H Schubert and J Fiegel, Textil Praxis, 47 (1992) 233.
[56]
F Hoffmann, AATCC Internat. Conf. and Exhib., (1997) 288; Text. Chem. Col., 30 (Oct 1998) 19.
[57]
B Drechsel, F Hoffmann and S Ottner, Melliand Textilber, 79 (1998) 328.
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K H Bauer, J Boxhammer, D Kockett and P Toldrian, Textilveredlung, 14 (1979) 183.
[59]
A Gilchrist and J H Nobbs, JSDC, 113 (1997) 327; 114 (1998) 247.
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S Li, B White and W C Tincher, AATCC Internat. Conf. and Exhib., (Oct 1996) 58.
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M R Shamey and J H Nobbs, Adv. Col. Sci. Technol., 2 (1998) 46.
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M F Comelo, Rev. Prog. Coloration, 32 (2002) 1.
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J Rieker and T Guschlbauer, Textilveredlung, 30 (1995) 251.
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J Rieker and P Ehrler, Textilveredlung, 31 (1996) 161.
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A K R Choudhury, Colourage, 45 (Mar 1998) 25.
[66]
F S Chow, K Chan, C L Chong and J H Xin, JSDC, 115 (1999) 173.
[67]
W G Kuo and M R Luo, JSDC, 112 (1996) 312, 354.
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M Mahy, L van Eycken and A Oosterlinck, Col. Res. Applic., 19 (1994) 105.
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A Broadbent, Canadian Text. J., 111 (Oct/Nov 1994) 19.
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A K R Choudhury, Rev. Prog. Coloration, 26 (1996) 54.
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R McDonald, J Oil Col. Chem. Assoc., 65 (1982) 43, 93.
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[74]
M R Luo and B Rigg, JSDC, 103 (1987) 86.
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R McDonald and K J Smith, JSDC, 111 (1995) 376; Color Technology in the Textile Industry, 2nd Edition, (AATCC 1997) 113.
[76]
M R Luo, G Cui and B Rigg, Col. Res. Applic., 26 (2001) 340.
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B Rigg, JSDC, 111 (1995) 267.
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D Heggie, R H Wardman and M R Luo, JSDC, 112 (1996) 264.
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J Park and T M Thompson, JSDC, 97 (1981) 523.
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J Rieker, K Wobser and W Ruttiger, Textilveredlung, 32 (1997) 261.
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H P Locher and H Firmann, Textilveredlung, 26 (1991) 393.
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J Rieker and T Guschlbauer, Textilveredlung, 25 (1990) 387.
[84]
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R F Willis, Color Technology in the Textile Industry, 2nd Edition, (AATCC, 1997) 135.
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P T F Chong, AATCC Internat. Conf. and Exhib., (1993) 323.
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P Medilek, Melliand Textilber., 75 (1994) 822.
Textilber., 74 (1993) 49; JSDC, 111 (1995) 139.
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Y S W Li, C W M Yuen, K W Yeung and K M Sin, JSDC, 115 (1999) 22, 95.
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J R Aspland and R W Harold, Color Technology in the Textile Industry, 2nd Edition, (AATCC,
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K C Lau, JSDC, 111 (1995) 142.
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M R Luo, P Rhodes, J H Xin and S Scrivener, JSDC, 108 (1992) 516.
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C J Hawkyard and D P Oulton, JSDC, 107 (1991) 309.
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K J Smith, SDC Biennial Conf., Blackpool (2000).
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C J Sargeant, SDC Biennial Conf., Blackpool (2000).
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Table 6.1 Applications of colour measurement Colorant manufacture Quality control
Formulation procedures
Product evaluation
Colorant use Formulation procedures
Transmission measurements on solutions of colorant against the standard sample [1] – may be checked by reflectance measurement after application of batch and standard to substrate [2] Production of dye mixtures, correction of ‘offshade’ batches, technical service to customers in formulating new shades Testing of development products or competitive samples for technical performance, including value analysis
Matching of new or existing colours to obtain the least-cost, best-match formulation, including elimination of metamerism or to give a colourconstant match Laboratory or production colour correction
Quality control
To check possible alternative dyes To check colorant deliveries Checking of coloured batches against the standard
Colour communication
Transmission of new standards, predicted ‘virtual’ matches, actual matches and quality control of production batches
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Table 6.2 Important developments in colour measurement Year 1931 1935 1940 1944 1948 1952-55 1958 1961 1964 1965 1967-68 1970 1973
1978 1979 1980 1987 1988 1989
1995
1998
Development Concept of CIE standard observer [3,4] Publication of Kubelka-Munk theory [5] Availability of Hardy recording spectrophotometer [6] Colorimetry first applied to dyeing theory studies [7] Iterative technique published by Park and Stearns [8] First tristimulus colorimeter for measuring colour difference (Hunter) Prediction of recipe by curve fitting First analogue colour-matching computer (Comic) [9] Match prediction system based on digital computer developed by ICI using Elliott 803B [10] RFT by blind dyeing established in dyeing industry Automatic data transfer device in analogue computer Direct interfacing of spectrophotometer and digital computer (ICS and Varian) Companies established to provide commercial colour matching systems – ACS (USA), Datacolor (Switzerland), ICS (UK) Tristimulus colorimeter interfaced to minicomputer for measurement of colour differences (ICI colour difference meter) Hatra colour-difference data published [11] Hatra colour-difference system Pulsed xenon spectrophotometer developed Early dyehouse automation systems installed Replacement of minicomputer by in-built microprocessor in colorimeter J&P Coats published colour-difference equation based on uniform colour space [12] ICI developed Integ system for dye strength evaluation [13] Laboratory automation developed CMC colour-difference equation published as BS [14] Merger of three main computer companies to create Datacolor International Hand-held spectophotometers became available Distance focus instruments for colour measurement on production machines Smart software to improve colour matching using previous data [15] Equally spaced colour atlas published [16] Wider introduction of non-physical standards (colouring by numbers) [17] ‘Colour on screen’ communication [18] Colour atlas available on website [19]
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Table 6.3 Matching requirements The end-product must be the desired colour, within a prescribed tolerance, often when evaluated under more than one illuminant (non-metameric match) The process must give uniform coloration and the colorants must adequately cover any irregularities in the substrate Relevant fastness requirements must be satisfied The physical properties of the substrate must not be impaired The finished product, including coloration, must meet the required target cost The product must be delivered on time
Table 6.4 Cost-saving features of a CCM system Screening and selection of colorants from existing ranges Evaluation of new or alternative colorants Checking of colorant deliveries for shade and strength (using transmission and reflectance methods) Checking the transmission characteristics of the application liquor to ensure that it has been prepared correctly Pass and fail decisions on attempted matching (in a variety of illuminants) Quality control of production batches Shade sorting of those production batches that can be blended or used together Assessment of batch levelness Communication of colorimetric data without physical samples Evaluation of fastness test results Assessing relative dyeability of substrates
Table 6.5 Spectrophotometer requirements Forms of output – reflectance, tristimulus values, transmittance Wavelength range – visible or including UV Number of illuminants Specular reflectance Viewing geometry Speed of operation Recording or non-recording Precision Repeatability Sample sizes Sample forms Cost
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Table 6.6 Spectrophotometer comparison [35] Primary instrument Macbeth Coloreye 3000 Macbeth Coloreye 7000
Comparison instrument DCI SF 600 DCI SF 600 CM 3700 D CE 2180
∆E(CMC) 0.3 0.18 0.13 0.17
Table 6.7 Laboratory functions Function Recipe prediction and correction
Quality control of dyes Quality control of laboratory and production dyeings Colour communication Recipe storage and retrieval
Software Prediction/correction program with facilities to input changes in dye strength and substrate dyeability Colour difference based on transmission or reflectance measurements Colour difference program which can be extended to colour sorting functions Ability to accurately display colour on screen and transfer this to distant locations Ability to find nearest colour from shade library and correct to target colour required
Table 6.8 Components of computer matching system Spectrophotometer – dual-beam, diffuse/8° geometry, capable of transmission and reflectance measurements Personal computer (minimum specification) – 10 GB hard drive capacity, floppy disc and CD drives for data communication, minimum 256 MB RAM, 65 MB graphics card Screen/monitor - 45 cm (such as Sony Trinitron FST) Screen calibrator for monitor screen (for example, Minolta) Printer – capable of reproducing colour from screen display (such as Epson Stylus 1200) ISDN link via a modem to give rapid communication of data via Internet Calibrated viewing cabinet with adjustable illuminant levels situated next to calibrated screen
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Table 6.9 Problems with visual methods Absence of standard illuminants and viewing conditions Subjectivity, psychological nature and fatigue of human observer Difficulty in agreeing standards Difficulty in establishing standard tolerances Wrong decisions by individual observers Disagreement between observers Soiling of physical standards
Table 6.10 SOP for colour-difference measurement Parameter Drying of samples Conditioning of samples Colour difference equation Illuminants
Sample preparation and presentation Aperture size Number of measurements Specular component UV component Standard observer data
Comment Slightly over-dried to below the natural regain of the fibre Conditioning cabinet at 65%RH and 20°C for 30 to 60 minutes Usually an optimised equation such as CMC D65 (daylight) TL84 (store light) A (tungsten filament) Carding of fibre Card wrapping or knitting of yarn Number of layers of fabric Large whenever possible Minimum of two with sample positions specified Included Included 10°
Table 6.11 Features of ImageMaster Screen calibration to allow worldwide communication Input of images by scanning or from digital image data Availability of fully-modelled images or articles, having shape, form, areas of light and shade, gloss and texture Colour specification by tristimulus systems, colour libraries, reflectance data (from a keyboard or a CCM system), computer-generated reflectance data that are colour constant and realisable using real dyes Colours may be viewed as they will appear under any illuminant of interest Colour manipulation on screen to obtain desired modified colour Rapid and precise communication and visualisation of colours and differences
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Table 6.12 Factors in the use of NPS No more accurate than physical data Not a replacement for trained colorists Limited acceptance by some designers and others Physical samples still required for handle and colour visualisation Need for standardisation, maintenance, calibration of equipment and SOP since differences can occur due to: spectrophotometer, operator, method of preparing and presenting samples, sample conditioning, substrate history and type
Table 6.13 Typical cost savings [25] Saving in total dyeing cost of 30% (ranges from 24 to 36%) for each bulk correction eliminated through RFT dyeing Reduction of dye inventory to 25% of original Reduction in dye purchases by 30% (ranges from 15 to 45%) by selecting leastcost recipes Elimination of dyeing full to standard (10% of dye cost) Reduction in laboratory dyeings to achieve target colour (typically 8 to 2) Fabric saving due to colour sorting (10% of annual fabric purchases)
Table 6.14 Actual savings made [25]
Factor Laboratory dyeings to achieve target Laboratory submits accepted per customer (%) Additions per dyelot Reprocessing (%) Savings in dye cost Total
Before 6 30 Up to 3 4
After 2 86 0.3 1.5
Cost savings per year (£) 13 500 6 750 113 28 60 222
400 350 000 000
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Figure 6.1 Total colour control [25] Colorant and material data
Colorant screening and selection
Substrate and colorant delivery testing
Recipe storage and retrieval
Prediction of shades
Application process
Colour correction program
Colour difference measurement
Fail
Pass
Use of numerical colour standards
Colorant and chemical inventory
Batch sorting into those that can be mixed
Figure 6.2 Possible locations for on-line colorimetry in continuous dyeing [84]
Padding
Inspection 1
7
2
Skying
Drying
6
3
IR Predrying
Washing
5
4
Drying thermosol
Cooling
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Figure 6.3 Colorite system in action Precise colour on screen
Request for virtual matching by e-mail and predict appropriate recipe
Send virtual match by e-mail
Assess virtual match
Accept or modify
Laboratory dyeing of approved virtual match
Production dyeings assessed (QC) using lab dyeing as standard
Practical Dyeing Volume 2 - Fibre Types and Dyeing Processes By James Park and John Shore
2004 Society of Dyers and Colourists
Copyright © 2004 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.
ISBN 0 901956 84 8
Contents Volume 2 – Fibre Types and Dyeing Processes Chapter 7 Dyeing of Cellulosic Fibres Chapter 8 Dyeing of Wool, Silk and Other Animal Fibres
1 82
Chapter 9 Polyester Dyeing
134
Chapter 10 Nylon Dyeing
176
Chapter 11 Acrylic Dyeing
198
Authors’ Preface The original idea of practical monographs was conceived in the 1970s as a result of an on-going debate as to what constituted a practical paper and the lack of such papers within the pages of the Journal of the Society of Dyers and Colourists. There is of course no absolute definition of a practical paper since this depends on the interests of the individual reader, location within the industry, topicality and, not least, the burning issues of the day. The Society of Dyers and Colourists attempted to rectify this lack of practical information by encouraging such papers for publication in the Journal, as well as initiating a series of practical monographs, authored by experts in various areas of textile coloration. Between the years 1981 and 1993, nine such monographs were published. Only two of these are still available: 1. Batchwise Dyeing of Woven Cellulosic Fabrics, by G W Madaras, G J Parish and J Shore (1993) 2. Instrumental Colour Formulation, by J Park (1993). For several reasons, not least the diminishing educational resources available for textile coloration, sources of practical, current information are increasingly required. This was the incentive behind the production of this practical e-book intended to assist practitioners occupying ‘hands-on’ positions at all levels within the industry. Copious recent references are included in each chapter. Two further e-books by the current authors will augment the information in this publication: 1. Dyeing Laboratory Practice, by J Park and J Shore (in preparation) 2. Dyehouse Management Practice, by J Park and J Shore (in preparation).
Chapter 7 Dyeing of Cellulosic Fibres 7.1 Properties of Cellulosic Fibres Cellulose is the most important structural material in nature and the most abundant of the natural polymers. It is formed by photosynthesis in growing vegetation and has been exploited for many purposes over many thousands of years in the forms of wood, paper and cellulosic textiles. Nevertheless, it was not until the 1930s that it was proved beyond doubt to be a linear polymer of anhydroglucose units [1]. It is remarkable that the manufacture of regenerated cellulosic fibres and the complexities of bleaching, mercerising, dyeing and printing of natural and regenerated cellulosic textiles were already developed and understood, within the limitations imposed by this uncertain knowledge of cellulose structure. 7.1.1 Cotton Cotton is the seed hair of plants of the genus Gossypium. The many species grown commercially may be divided into three general types: 1. fine cottons with staple lengths (average fibre lengths) varying from 25 to 60 mm, including high-quality varieties such as Egyptian, Sudanese and Sea Island cotton, 2. coarser species with shorter staple lengths (typically 13-33 mm), such as American Upland cottons, 3. coarse fibres of even shorter length (about 9-25 mm), commonly grown in Central Asia and the Indian subcontinent. Major cotton producers include the USA, China, India, Pakistan and the Central Asian nations. Smaller but not insignificant amounts are grown in Egypt, Sudan, Turkey, Mexico and Brazil. During the major growth period for synthetic fibres (1950-75) the demand for cotton declined substantially, so that by 1975 cotton had retained only 34% of the US market for apparel and domestic fabrics at the retail level. The oil crises of the 1970s, disappointment with some features of all-synthetics and the influence of promotional campaigns for natural fibres brought about a marked resurgence in demand for cotton, which had recovered to claim a 50% share of the US market by 1990 [2]. The mature cotton fibre forms a flat ribbon varying in width between 12 and 20 µm. It is highly convoluted, probably as a result of the twisting that takes place when the tubular shape formed during growth collapses on drying. The number of convolutions varies between four and six per mm, reversing in direction every mm or so along the fibre length. The bean-shaped cross-section of the cotton fibre may be described as a bilateral structure, because the density of packing of the cellulose chains is not uniform across the sectional area. Hence the accessibility of the chain segments to swelling reagents varies across the fibre (Figure 7.1). The three main zones (A, B and C) have been identified by studying enzymic degradation [3]. The rate of degradation increases in the order A
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Cotton fibres normally develop in two stages (Figure 7.2). In the primary stage the fibre grows steadily in length; only after the lengthwise growth is almost complete does the secondary wall begin to form from the outside inwards, causing the cotton fibre to thicken widthwise. When this normal wall thickening process is interrupted or slowed by adverse influences from insects, plant disease, drought stress, inclement weather (especially early frost) or premature harvesting, the maturing process is retarded or halted. A fully mature cotton fibre may be defined as one that swells to a tubular shape in aqueous caustic soda, with virtually no lumen and free from well-defined convolutions. The lumen is what remains of the central channel from which the layers of cellulose were laid down in the wall during the secondary growth stage. In dead fibres the wall thickness after swelling is one-fifth or less of the maximum ribbon width (usually found about midway between two convolutions). Immature fibres are thin-walled (Figure 7.3) because the secondary wall has not fully developed between cessation of lengthwise growth and bursting open of the cotton boll. The boll is the pod remaining on the plant after the flower has fallen; it may contain twenty or more individual seeds. It is instructive to compare the composition of outer primary wall material with that of the entire cotton fibre, since it is the primary wall that becomes disintegrated during the preparation of cotton for dyeing or printing. Although the primary wall accounts for only 5% of the mass of the fibre, it contains most of the non-cellulosic material present (Table 7.1). The residual substances are mostly water-soluble sugars and organic acids. Cotton fibres have a fibrillar structure. The primary wall consists of a network of cellulose fibrils covered with an outer layer or cuticle composed of pectin, protein, wax and mineral salts. The wax renders the fibre impermeable to water unless a wetting agent is present. The secondary wall forms the core of a mature fibre and consists almost entirely of cellulose fibrils arranged spirally around the fibre axis, the direction of the spiral reversing many times along a single fibril. Thus the cotton fibre consists of an assembly of fibrils in which the cellulose is accessible to chemical reagents only at fibrillar surfaces via a system of voids and channels. Some surfaces of the elementary fibrils are so close to one another that they are inaccessible to reagents unless swelling has been induced. Others are relatively disordered and more accessible; there are also readily accessible surfaces in the amorphous tilt-twist zones that connect the relatively ordered regions [5]. 7.1.2 Bast Fibres The bast fibres are obtained from the stems of certain dicotyledonous flowering plants, the most important of which is Linum usitatissimum, generally known as flax. They form a strengthening protective layer around the woody central core of the stem, and are themselves protected by an outermost cuticle containing waxy substances. The fibres consist of bundles of thick-walled cells held together by an adhesive material containing hemicellulose, lignin and pectin. The term flax is generally used to denote the plant and the fibre extracted from it, whereas yarns and fabrics manufactured from flax fibres are described as linen. Like cotton, linen textiles suffered from increasing restrictions in research and development effort during the expansion of the synthetic fibres industry but in the early 1980s a fashion-led movement towards an unstructured casual look brought natural fibres back into the spotlight. With the emphasis on relaxed clothing, linen’s tendency to form creases was no longer a serious problem.
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Nevertheless, the limited demand for linen in more formal apparel is undoubtedly attributable to the failure to find an effective finishing treatment that would improve crease resistance without adversely affecting the capability of linen to withstand abrasion in normal wear [6]. Linen is important in high-quality household textiles and is used in automotive trims [7]. The growing of flax and hemp is encouraged by the EU as an alternative to surplus food crops. Cutting of these exceptionally long fibres allows them to be processed on cotton spinning machinery. Irregularities in these bast fibres have prompted the introduction of flax/hemp blended staple yarns. Modern cultivation of both plants has been transformed: the less the sowing density, the higher the quality [8]. Research on the cultivation, harvesting and extraction processing of hemp [9] and ramie [10] has been reported. Jute is grown mainly in the Indian subcontinent and is widely used in the manufacture of hessian sacking, carpet backing, ropes and tentage. More recently, because of its economy, ready availability and biodegradability, jute is being used increasingly in other end-uses, such as upholstery, paper and viscose manufacture [11]. An economical denim fabric has been developed from a combination of 40% mixed cut jute fibres and 60% cotton. The blend produced in the blowroom was spun conventionally at 40,000 rpm, warp dyed and then woven on an air-jet loom. The enzyme-desized fabric met the required standards for mass/area, breaking load, bending length and crease recovery [12]. Kenaf is a short-fibre alternative to jute in typical end-uses [13] and interest is being shown in the peroxide bleaching and direct dyeing of the bast fibre extracted as a byproduct from sugar cane cultivation [14]. The morphology of a bast fibre may be regarded as an assembly of the ultimate fibres held together as a bundle by means of the non-cellulosic components forming the intercellular material. The ultimates vary in dimensions from one species to another. For example, the individual cells of hemp, flax and especially ramie are much longer than those of the other bast fibres, such as coir, jute, kenaf or sisal (Table 7.2). On the other hand, the diameters of ultimates from all of these species (except ramie) are not significantly different, varying between 15 µm and 25 µm in all cases. In the bast bundles the ends of the individual fibres overlap, forming continuous fibrils extending along the stem. Most bast fibres are used as full-length bundles but flax is separated into its ultimate fibres for the production of fine linen yarns and ramie is also separated similarly before spinning. Raw bast fibres contain far less cellulose than grey cotton fibres, as indicated in Table 7.3. The intercellular material consists of a complex mixture of polysaccharides such as hemicellulose and pectin, together with the crosslinked network of lignin. This complex polymer is based on substituted hydroxyphenylpropane units and is a major component of wood. The wax content of jute and ramie is lower than that of cotton, whereas raw flax contains a significantly higher amount. Flax fibres have a high elastic modulus but, paradoxically, a low extensibility. This is because the flexibility is derived from the nodes or bending points. It is this irregular structure in bending mode that makes linen such an unusual fibre, with the fissures in the cell wall responsible for its high rate of moisture absorption and the resulting coolness to the touch. Although the nodes bring flexibility to the fibre structure, they are also points of weakness. Repeated flexing at these points results in fibrillation and eventually fracture, accounting for the relatively poor resistance to abrasion of linen [6].
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Separation of the fibre bundles from the harvested stems is complicated and time-consuming. The first stage is retting or soaking until the effect of bacterial action on the intercellular material loosens the fibres for mechanical separation. The finest flax came originally from the Courtrai region of Flanders, where the flax bundles were retted by soaking in the river Lye and then spread on fields to dry. In Holland, Germany and Ireland the retting was carried out in trenches, whereas the dew retting technique in France and Russia involved allowing moisture and soil bacteria to act directly on the flax straw, producing a darker coloured flax. Water retting in tanks at 27-32°C is much quicker than dew retting and enzyme stimulators are now used to accelerate the process [8]. More efficient machinery has been developed to replace traditional scutching, hackling and decorticating processes for removing residual sprit (woody debris) from the retted straw. Jute is frequently used without further purification, but flax and ramie are usually scoured and often bleached. Jute for dyeing is prescoured but considerable amounts of lignin remain, leading to poor light fastness. Reductive scouring of linen with alkaline dithionite has been advocated before oxidative bleaching [16]. The traditional lime boil and hypochlorite bleach has been abandoned in favour of chlorite or peroxide bleaching [6]. Detailed studies of optimised bleaching conditions for chlorite, hypochlorite or peroxide treatment have been carried out to ensure their utilisation in the most effective manner with respect to the functional characteristics of the linen [16,17]. The scope for bleaching linen will be restricted to peroxy chemicals if chlorine-containing bleaching agents are eventually withdrawn from use on the grounds of AOX formation. It has been shown that peracetic acid is an effective replacement for both bleaching and delignification [15]. Mercerising of linen enhances the quality of the fabric. This treatment can be given on a jumbo jig following scouring and bleaching. Mercerising does not improve the lustre but it strengthens the fibres at their nodal points, increasing the abrasion resistance by about 30% [6]. 7.1.3 Regenerated Cellulosic Fibres Wood is the most important source of cellulose, which forms 40 to 50% of its mass. Wood fibres are too short to be spun into textile yarns, so they must first be dissolved in a suitable solvent from which they can be regenerated by extrusion through a fine orifice or spinneret to form a continuous filament. Pioneering research on electric light bulbs a century ago was aimed at producing more regular carbon filaments for lamps than those obtained by the carbonisation of cotton [18]. Cellulose was nitrated and dissolved in an ether/ethanol solvent. This solution was extruded into either water (wet spinning) or hot air (dry spinning) to form cellulose nitrate filaments. This flammable product was denitrated with ammonium hydrosulphide for textile use. Nitro fibres were the first regenerated cellulose fibres to achieve commercial success in 1890, but they were soon rendered obsolete. Cuprammonium hydroxide, made from copper sulphate and ammonia, is capable of dissolving natural cellulosic materials. Cupro filament yarn was first produced commercially in 1901, although it had been made experimentally in the 1890s. Wood pulp or cotton linters (short-staple fibres) are dissolved in cuprammonium hydroxide and this solution is extruded into water. The filaments are stretched and passed into dilute sulphuric acid to complete the regeneration of the cellulose. The viscose process was also discovered in the 1890s but it was not until 1910 that appreciable quantities of viscose filament yarn were produced commercially
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by Courtaulds. Alkali cellulose was reacted with carbon disulphide to yield the xanthate ester. A solution of sodium cellulose xanthate dissolved in aqueous caustic soda was extruded through a spinneret into a coagulating bath containing sodium sulphate and sulphuric acid. The filaments are stretched mechanically during regeneration and their properties are determined by the concentration of the viscose solution, the rate of coagulation and the rate of stretching. The coagulation process is controlled by temperature and additives such as zinc sulphate or glucose. Regular viscose differs from cotton in being non-fibrillar, having no central lumen and a much lower degree of polymerisation (DP). Although consisting wholly of cellulose, the skin and core of viscose filaments differ in morphological structure. The relative proportions of skin and core vary according to the conditions of coagulation. In 1910 the niche market for ‘artificial silk’ was led by the nitro and cupro variants. However, viscose was cheaper and safer to produce on a commercial scale and by the 1920s it had achieved dominance and widespread popularity [18]. Manufacture via the nitrate route eventually ceased but there remained a limited demand for cupro yarn. This solution process did not require esterification and hydrolysis stages, so there was less degradation of the cellulose. Cupro yarn always offered the highest quality but was the most expensive route. Compared with cotton, regular viscose suffers from the disadvantage of much lower breaking strength, particularly when wet. This is seldom a problem in apparel, but renders viscose unsuitable for more critical end-uses. High-tenacity viscose fibres have been produced for at least a half-century. They are highly oriented all-skin filaments with a near-circular cross-section. Originally developed for reinforcing cord in rubber tyres, they have been supplanted for this purpose by synthetic fibres. The early high-tenacity fibres still had the disadvantage of a low wet modulus. It was to overcome this problem that modified high-wetmodulus (HWM, so-called because of ‘modifiers’ in the spinning dope) and polynosic fibres were introduced. The polynosic fibres were first developed in Japan. They have been defined as having low wet extension even under alkaline conditions, high knot strength and a higher DP than regular viscose. The distinctive features of their method of production include: 1. a xanthate solution of viscosity higher than that used in the manufacture of regular viscose, achieved using an aged pulp of intrinsically higher DP rather than a more concentrated solution, 2. a coagulating bath of low salt concentration with no modifiers or other additives, 3. a lower temperature of extrusion than regular viscose. Under these conditions, high stretch (up to 300%) can be achieved. Polynosic fibres are highly oriented and have stress-strain curves closely similar to those of cotton, rather than to other regenerated cellulosics. They appear to be fibrillar in structure and are largely unaffected by dilute solutions (up to 8%) of sodium hydroxide, which will dissolve as much as 25% of regular viscose. Modal fibres are made with an above-normal concentration of zinc sulphate in the coagulating bath and ‘modifiers’ (such as dimethylamine or cyclohexylamine) in the spinning dope. The initial wet modulus of these fibres is higher than regular viscose but lower than that of the polynosic fibres. The term ‘modal’ was introduced to describe all regenerated cellulosic fibres having a linear density of D
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dtex, a breaking strength B cN in the conditioned state and a force F cN for 5% extension in the wet state that satisfy Equations 7.1 and 7.2. B ≥ 1.3D1/2 + 2D
Equation 7.1
F ≥ 0.5D1/2
Equation 7.2
Numerous variants of regular viscose have been developed for special purposes. Chemically crimped viscose is produced by modifying the regeneration conditions so that the skin of the filament ruptures while in the spinning bath. The liquid viscose exposed is thus regenerated under different conditions from the skin, resulting in a bicomponent structure. A permanent crimp develops as a result of the differential shrinkage in subsequent washing and drying. In the wet processing of such fibres, excessive tension results in loss of crimp and should be avoided. Chemically crimped viscose is usually used in staple form and has an attractive handle. Hollow viscose fibres, such as Viloft (Courtaulds), have been produced in an attempt to simulate the natural lumen of cotton. Sodium carbonate is incorporated in the spinning dope. When this is extruded into the acidic coagulating bath the carbon dioxide formed inside the filament creates a continuous hollow central channel. Careful control of the conditions of the carbonate decomposition reaction is necessary to obtain a reproducible product [19]. The hollow structure of the fibre imparts high torsional rigidity, leading to an attractive handle with higher bulk and fabric cover than regular viscose of the same fabric density. Hollow viscose fibres have a lower density (1.15 g/cm3) and higher water imbibition (130%) than regular viscose (1.52 g/cm3 and 90%), giving good insulation, extra absorbency and comfort. By a careful choice of spinning conditions it is possible to produce hollow fibres with water imbibition values as high as 200% - the so-called super-absorbent fibres designed for surgical and sanitary purposes. An alternative approach is to add water-retentive polymers such as sodium polyacrylate or sodium carboxymethylcellulose to the viscose dope before spinning. Flame-retardant fibres have been produced by dispersing a suitable agent into the viscose solution before spinning [20]. One of the most successful agents was tris(2,3-dibromopropyl) phosphate used in such fibres as Darelle (Courtaulds), but in the late 1970s this agent was suspected of being carcinogenic and had to be banned. Flame-retardant viscose is no longer manufactured for apparel, although some containing less hazardous flame-retardant additives is still produced for other end-uses. Viscose fibres of modified dyeability were marketed in the 1970s for the one-bath dyeing of wool/viscose blends [21]. Additives with free aminoalkyl groups were incorporated into the viscose dope, resulting in fibres that were dyeable with acid dyes. Although effective they were too costly to achieve commercial success. Mass-coloured viscose is often called ‘dope-dyed’ yarn because insoluble pigments are dispersed in the viscose mass from which it is extruded. These fibres exhibit excellent fastness to light and laundering but represent a tiny fraction of viscose fibre production because of the limited shade range available. The objectionable effects of viscose manufacturing plants on the environment have been universally recognised for a long time, but it was not until the 1970s that the use of an alternative solvent for wood pulp emerged as a serious possibility. The solvent power of certain cyclic tertiary amine oxides was reported at this time. Structural criteria for the capability of dissolving cellulose were
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established and the crucial importance of having the right amount of water present was recognised. The solvent found to be most suitable for development of regenerated fibres was N-methylmorpholine-N-oxide (NMMO) and eventually in 1988 Courtaulds established full-scale manufacture of Tencel, the first lyocell fibre [22]. Tencel is obtained by continuous dissolution of wood pulp in mesomorphic NMMO and extrusion into a dilute aqueous solution of the amine oxide to precipitate the regenerated fibre. As the solvent is washed out, the fine filaments formed are collected as a tow from which the staple fibre is produced. More than 99% of the solvent is purified and reused at the continuous dissolving stage, so that the process is environmentally innocuous. The fibre has a smooth surface and a circular cross-section, providing high lustre in the raw state. The tenacity (wet and dry) of lyocell fibres is markedly higher than that of cotton or any other type of regenerated cellulosic fibre. The wet tenacity is only about 15% lower than the dry value and is markedly higher than that of cotton. The exceptionally high wet modulus results in very low shrinkage, about 2% in warp and weft yarns. Lyocell is fibrillar in structure and resembles cotton even more closely than modal fibres in behaviour under stress and capacity for absorbing liquid water. The mechanical properties and water imbibition of cotton and various regenerated cellulosic fibres are compared in Table 7.4. Because of the close similarity between stress-strain curves of lyocell and cotton, the regenerated fibre can contribute to the strength of the blended yarn even at low blend levels. An interesting feature of lyocell is that the conversion of fibre strength to yarn strength is considerably higher than for other cellulosic fibre types, because of the high cohesion between the closely packed fibres of circular cross-section in the yarn. Lyocell improves the performance of blends with cotton by enhancing strength, lustre, yarn regularity, spinning and wear properties. Since its commercial introduction in 1988 Tencel has been characterised by its fibrillation properties, which have been exploited in providing ‘peachskin’ surface effects. Although this fibrillation can be avoided using open-width processing it is the surface finish that has seen most development, especially in woven goods. Tencel has often been chemically crosslinked by the dyer as a pretreatment for yarn or fabric. This effect has now been achieved in Tencel A100 (Acordis) by crosslinking the fibre in tow form after extrusion and curing before being crimped and cut [24]. Tencel A100 is a non-fibrillating option of particular interest in the jersey and knitwear sectors. It is characterised by giving remarkable stitch clarity and a positive bulkiness and warmth to fabrics and garments. In a somewhat similar development, NewCell (Akzo Nobel) is a lyocell fibre that does not show macrofibrillation, so that the enzymic defibrillation process can be eliminated. Only a microfibrillation finish by tumbling is recommended. The priority market segment for NewCell is outerwear for women, including business clothing and elegant evening wear. Further target applications are being evaluated in underwear, nightwear and even the fine hosiery sector [25]. 7.1.4 Cellulose Structure Cellulose (7.1) is a condensation polymer of β-D-glucopyranose with 1,4glycosidic bonds between successive pyranose rings. All the substituents, including the glycosidic links, project from the pyranose ring in the same plane and only the four C-H bonds are perpendicular to this plane. The essential features of the polymer chain are the main sequence of main-chain units (7.1A),
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the non-reducing end-group (7.1B) and the reducing end-group (7.1C). The reducing group is a cyclic hemiacetal that exhibits the characteristics of both a secondary alcohol (7.1C) and an aldehyde (7.1D) under appropriate conditions. The degree of polymerisation (DP) of cellulose varies with the source and is usually expressed as an average, since a wide distribution is found. In raw cellulosic material it can be as high as 14,000 but purification treatments involving alkali usually lower this to about 1000 to 2000. Furthermore, the hemiacetal end-groups are oxidised to carboxylic acid groups under these conditions, so that purified cotton normally has no reducing power if degradation has not taken place. The DP of the cellulose in viscose regenerated from the xanthate ester is only about 250 to 300, but that in modal fibres is higher (500 to 700). All cellulose regenerated in this way contains a very small proportion of aldehyde, ketone and carboxylic acid groups introduced during manufacture. Thus regenerated cellulose does differ from cotton cellulose chemically, but only slightly. 7.1.5 Degradation of Cellulose An industrially important feature of cellulosic fibres is chemical stability, enabling them to withstand degradation with its consequential loss of tensile strength and abrasion resistance under normal conditions of use. Six different degradative agencies have been identified: acids, alkalis, oxidants, enzymes, heat and radiation. Ultimately the complete degradation of cellulose yields carbon dioxide and water, but it is the early stages of partial degradation that are important in textile processing. The fibrous products of the action of mineral acids and oxidising agents may be still loosely described by the traditional trivial names of ‘hydrocellulose’ and ‘oxycellulose’ respectively. Acidic degradation The primary reaction of cellulose in aqueous acidic media is the acid-catalysed hydrolysis of the 1,4-glycosidic links between neighbouring pyranose rings. The properties of the hydrocellulose formed depend solely on the number and distribution of these broken linkages, which vary with acid concentration, time and temperature of treatment. Once the readily accessible bonds have been randomly hydrolysed the decrease in DP reaches a plateau but a much slower hydrolytic reaction continues at chain ends in the fringes of the more crystalline regions of polymer structure. Alkaline degradation Remarkably, the glycosidic linkages in cellulose are not attacked by alkali at temperatures below about 170°C. However, the hot alkaline preparation of cotton for dyeing usually causes a greater loss in mass than can be accounted for by the extraction of non-cellulosic impurities. These excessive losses are attributable to the stepwise removal of anhydroglucose units from the reducing ends of the cellulose chains. The reducing end-group is first converted to a fructose residue that is more readily removed, leaving a new exposed end-group that is then transformed in the same way. This progressive erosion of the end-groups, often known as the ‘peeling’ reaction, would continue indefinitely were it not halted by the simultaneous conversion of some reducing end-groups into non-reducing acidic residues still attached to the cellulose chain. This competing ‘stopping’
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reaction eventually limits the overall loss in mass of the alkali-treated cellulose to a tolerable level. Enzymic degradation The enzymic degradation of cellulose is an essential feature of the carbon cycle by which life on Earth is sustained. Biodeterioration of textiles is a serious problem. If damp cotton is exposed to air, mildew may gradually develop and cause staining that is difficult to remove. Odour formation and gradual loss in strength renders mildewed cotton unusable. The micro-organisms act on localised sites that are rendered water-soluble, leaving most of the cellulose unchanged. Many cellulase enzymes extracted from fungi hydrolyse natural cellulose to only a limited extent unless it has become more accessible by mercerisation or other swelling treatment. Oxidative degradation Each main-chain unit in cellulose contains three hydroxy groups, so the number and variety of oxidation products is considerable. Only a small proportion of a cellulosic fibre is readily accessible to most oxidising agents, which therefore react rapidly at first and then very much more slowly. The action of most oxidising agents is non-specific and complex, so it is frequently impossible to either elucidate its mechanism or determine precisely the constitution of the oxycellulose produced. The formation of aldehyde groups in oxidised cellulose renders it inherently unstable. Cotton that has been overbleached with sodium hypochlorite goes yellow on storage, even in the dark at ambient temperature. Viscose fibres always contain a small proportion of reducing groups arising from the ageing of the alkali cellulose during manufacture. These groups can cause yellowing problems in the dyeing of pastel shades by pad-thermofix methods. Thermal degradation Cellulose can be heated for many hours at 120°C without serious deterioration. In dry air at higher temperatures, however, considerable depolymerisation and strength loss occurs, accompanied by the formation of carbonyl and carboxylic acid groups, together with the evolution of water, carbon dioxide and carbon monoxide. At about 250°C pyrolysis to numerous products becomes significant and these have been extensively studied because of the fire hazard associated with cellulosic textiles. Dehydration probably leads to the formation of intra- and intermolecular ether linkages, the latter constituting a form of crosslinking. Photodegradation In practice, the most important type of photochemical degradation to affect cellulose is that caused by visible and near-UV radiation. This is always a photosensitised reaction because only UV radiation of wavelength less than about 340 nm has sufficient energy to cleave the C-C and C-O bonds in cellulose. Thus pure cellulose is scarcely affected by daylight (400-700 nm). However, in the presence of oxygen and a photosensitiser considerable degradation occurs and this is enhanced by the presence of moisture. The oxycellulose formed is of the alkali-sensitive type and can be particularly problematic in curtains or other domestic textiles. Although there is little change in tensile strength during exposure to light, serious losses may occur during subsequent laundering. The
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most important group of photosensitisers includes yellow, orange and red vat dyes, although sulphur dyes and basic dyes in this colour sector, as well as the oxides of titanium and zinc, are also active. The direct photolysis of cellulose is a simpler reaction brought about by UV radiation of wavelength shorter than 340 nm. In contrast to photosensitised degradation, direct photolysis is unaffected by the presence of oxygen and is inhibited by water or vat dyes. An interesting feature is that this degradation may continue for several weeks after irradiation has ceased, at a rate that increases with the storage temperature. 7.1.6 Effects of Water and Alkalis on Cellulose Water is essential for the processing of cellulosic materials, including bleaching, dyeing, finishing and papermaking. The comfort of clothing made from cellulosic fibres is closely associated with moisture absorbency. These fibres normally contain regain moisture (expressed in g water per 100 g bone-dry cellulose and typically within the 7 to 13% range) at a level determined by the ambient temperature and relative humidity. The tenacity and elongation of the fibres vary with their moisture content (Table 7.4). The numerous hydroxy groups on the surface and in the disordered regions of the fibre structure are responsible for the adsorption of water vapour from the atmosphere but it is generally accepted that water molecules do not penetrate the crystalline regions. Under typical ambient conditions in the UK (65% relative humidity at 20°C) scoured cotton has a regain value of about 7.5%. Fully mercerised cotton and regular viscose have regains of about 11% and 13% respectively under these conditions. Cellulosic fibres imbibe considerably more water when immersed in the liquid than they absorb from the atmosphere at 100% relative humidity. Outside the cell walls almost all the water is held by condensation in capillary voids within the fibrillar structure of the fibres. The most convenient method of measuring water imbibition is to weigh the amount retained after the sample has been centrifuged under standard conditions (such as 30 minutes at 900 g). Cotton yarn gives a water imbibition of about 50% and regular viscose about 90% by this method (Table 7.4). Greater retention is likely in the wet processing of fabrics since excess liquor is usually removed by less vigorous means, such as a mangle nip. When bone-dry fibres are immersed in water their average diameter may increase by as much as 20%. In practice, however, cellulose already containing regain moisture is of much more interest than bone-dry material and the term ‘fibre swelling’ usually relates to an increase in dimensions greater than that produced by water adsorption. The degree of swelling is expressed as a percentage increase in either diameter or cross-sectional area. Mercer in 1844 discovered that the dyeability of cotton could be enhanced by a swelling treatment in 25-30% sodium hydroxide solution at ambient temperature. Cotton yarn treated in this way swells in diameter and shrinks in length, so that fabrics become denser. Similar effects were observed using 62% sulphuric acid at room temperature or 59% zinc chloride at 70°C. Lowe in the 1880s found that if alkali-swollen cotton fabric was stretched to its original dimensions during subsequent washing-out of the alkali, or if it was held under tension to prevent shrinkage during treatment, it acquired a much improved lustre and smoothness. This process became known as mercerisation and remains a valuable pretreatment before dyeing or printing of cotton.
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The degree of swelling of cotton in aqueous alkali depends on the amount of water imbibed, which in turn depends on temperature, alkali concentration and the nature of the cation. The essential feature distinguishing the alkali-metal cations from one another is the degree of hydration under comparable conditions. Thus maximum swelling at a given temperature decreases in the order LiOH >NaOH >KOH >RbOH >CsOH, which is also the order of decreasing hydrating power of the respective cations. Hydration of the cation and swelling both decrease as the temperature increases (Figure 7.4). These curves pass through a maximum at an alkali concentration in the range 3M to 5M, this point increasing with temperature. The exceptionally sharp peak at 0°C and 3M NaOH has been attributed to bursting of the primary wall in the fibres. 7.1.7 Biopolishing New garments or household textiles made from cotton or polyester/cotton often have a rather unattractive handle and appearance. Plain-woven fabric surfaces may lack smoothness because of the fuzzy effect of small microfibrils protruding from them, whereas terry towelling may feel too stiff and prickly to the skin. After a relatively short period of use, pilling may develop on the garment surface giving it an unappealing and shabby look. Singeing of grey cotton fabrics is intended to minimise surface fuzziness, fabrics that form pills during wet processing can be cropped and treatment of terry towelling with softeners may enhance the handle but impair the water absorbency. Such measures may improve the initial impression given by the newly purchased goods but are essentially temporary expedients and do not counteract similar adverse effects that develop on repeated laundering. Biopolishing is a durable finishing treatment introduced by Novo Nordisk in the early 1990s to overcome these problems. Bacterial α-amylase has been established for the enzymic desizing of starch-sized woven fabrics for around half a century. Cellulase enzymes exert a specific action on the 1,4-glycosidic linkages that connect successive pyranose rings in the cellulose molecule. Cellulases can be isolated from the fermentation of various organisms, notably the Trichoderma, Aspergillus and Fusarium moulds. Enzyme molecules are too large to penetrate into the interior of a cotton fibre, so the hydrolysis reaction only takes place at or close to the fibre surfaces [26]. In this reaction, the microfibrils protruding from these surfaces and any pills formed by entanglement of fibrous debris are weakened and thus become detached much more easily, leaving the fabric appearance and handle much smoother than before the enzyme treatment. Fabric softness, drape and moisture absorbency are also improved. These changes are durable to subsequent repeated laundering because of the fundamental nature of the hydrolytic effect of the enzyme. The biopolishing process is suitable for bast fibres, regenerated cellulosics and their blends, as well as cotton and polyester/cotton goods [27]. Cellusoft L (Novo Nordisk) treatment is best applied immediately after bleaching, although if preferred it can be given after dyeing or printing. Application during jet or overflow dyeing may be possible if the processing conditions are compatible, but the possibility of interaction between high-reactivity dyes and functional groups in the enzyme molecule must be borne in mind. Two types of cellulase enzyme may be used: 1. acid-type cellulases (such as Cellusoft L) which give their best effect at pH 4.5 to 5.5 and 45 to 55°C,
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2. neutral-type cellulases applicable at pH 6 to 8 and 50 to 60°C [28]. Treatment time is typically 30 to 60 minutes at an enzyme dosage level of 5 to 30 g enzyme preparation per kg fabric. Heavyweight fabrics processed at low enzyme dosage require longer treatment times, whereas lightweight goods can be finished more quickly. Termination of the catalytic reaction can be achieved in two ways: 1. raising the temperature to 70 to 75°C for 10 to 15 minutes, or 2. adding sodium carbonate to adjust to pH 9 to 10. The biopolishing of cotton, linen or regenerated cellulosic fibres can be monitored directly or indirectly by determining the rate of loss in mass of the fabric. On cotton, a loss of 3 to 5% usually indicates an effective biopolishing treatment without undue lowering of fabric strength (less then 5%). The loss of mass can be kept within safe limits by terminating the hydrolysis reaction sufficiently early. In the case of polyester/cotton and polyester blends with other cellulosic fibres, the loss in tensile strength is negligible and control of the loss in mass is thus less critical [28].
7.2 Preparation and Dyeing of Cotton 7.2.1 Cotton Impurities and Preparation Chemicals Unless cotton fibres are uniformly high in whiteness, absorbency and cellulose content, dyes, chemicals and finishing agents will not be absorbed readily and evenly. Effective removal of the impurities associated with the primary wall material (Table 7.1), particularly the cotton wax that interferes considerably with uniformity of absorbency and dyeability, is the major objective of the preparation stage before coloration. Successful preparation depends on four factors [29]: 1. the amounts of the various impurities present, 2. the purity of the water supply, 3. the chemicals used in the various preparation processes, 4. the machinery available for processing of the goods. At least 5 to 6% by mass of a batch of cotton consists of non-cellulosic impurities (Table 7.1). Pectins are polygalacturonic acids and their calcium, magnesium and iron salts. The inorganic ash contains calcium, magnesium and potassium phosphates and carbonates. To these natural impurities must be added 10 to 15% of size polymer, spin finish or knitting lubricant. The spin finish and knitting oil contain mineral oil and surfactants applied to decrease friction on machinery parts. Sizing agents are film-forming polymers applied to warp yarns before fabric weaving in order to minimise yarn breakage. Metallic ion contamination, particularly iron and copper, is of serious concern during oxidative bleaching processes as this can lead to chemical damage, often manifesting itself as highly localised resist spots or strength losses. Many chemicals have been used in cotton preparation, including enzymes, alkalis, bleaching oxidants, stabilising agents, sequestrants and reducing agents. Surfactants play an important role in wetting, scouring and detergency. From the viewpoint of environmental impact, the impurities in cotton and the chemicals
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necessary to remove them share two general characteristics in common. Both are high in their content of oxidisable organic material and of inorganic electrolytes. Approximately 75% of the biochemical oxygen demand from typical preparation, dyeing, printing and finishing effluents is contributed by the desizing and scouring processes (Table 3.8). Scouring, bleaching and mercerising liquors are all strongly alkaline and these waste streams must be effectively neutralised before disposal, simultaneously producing further amounts of undesirable inorganic salts. Good wetting is necessary for rapid and complete saturation and penetration of the substrate. The rate of wetting is highly critical in continuous pad-fixation processes. Commercial wetting agents are usually anionic or nonionic surfactants, or a blend of the two. For most applications it is important that the surfactants discharged to effluent are biodegradable, linear alkyl compounds being more readily degraded than their branched isomers. Important factors regarding the selection and use of surfactants in preparation include [29]: 1. anionic wetting agents used in the desizing stage must be compatible with the enzyme preparation, 2. detergents selected for scouring must be stable at the temperature and concentrations of alkali and electrolyte required, 3. surfactants added to bleach liquors must be stable to these strongly oxidising conditions, 4. residual surfactants retained in the treated fabric must not cause problems in subsequent printing or water-repellent finishing, 5. the cloud point of any nonionic surfactant used must be high enough to avoid impairing the wetting or detergency performance, 6. surfactants present must be low-foaming to avoid risks of pump cavitation in circulating-liquor systems and loss of traction in conveyor or roller-bed steamers, 7. the viscosity of the surfactant solution should allow satisfactory performance in automatic dosing systems. Amylase enzymes are highly effective catalysts for the hydrolysis of the amylose and amylopectin components of starch size. Traditionally they are applied for several hours at 65 to 70°C but thermostable hydrolytic enzymes have been introduced, allowing brief dwell times at temperatures up to 120°C. Common salt and calcium ions increase the rate of hydrolysis but amylase is deactivated by copper or zinc ions, as well as most anionic surfactants. There is some interest in the use of pectinases as scouring agents and lignases to degrade the lignin in bast fibres, but as yet no commercial processes have been developed. Sodium hypochlorite, sodium chlorite and hydrogen peroxide are the three oxidants used for bleaching cellulosic fibres. Hypochlorite bleaches rapidly at ambient temperatures. It is less sensitive to traces of transition-metal ions than peroxide but it does easily chlorinate organics present. Thus it yellows grey cotton by chlorinating the pectins and proteins present. It is essential, therefore, that the cotton is cleared of natural impurities by thorough scouring before bleaching. Hypochlorite bleaching must be followed by washing and an antichlor treatment with bisulphite. It is a prolonged, labour-intensive batchwise process with relatively low chemical costs but high water consumption. Chlorinated impurities are detected in effluent streams as absorbable organohalogens (AOX) and
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chloroform, which are environmentally unacceptable. It is likely that hypochlorite bleaching will be abandoned for this reason [29]. Sodium chlorite is used under acidic conditions and bleaching can be carried out on grey or desized cotton. Bleaching under acidic conditions removes only a small proportion of the natural cotton wax, which can be advantageous in softening knitgoods and knitting yarns. Chlorite bleaching is relatively unaffected by iron or copper contamination. Adequate fume extraction is essential, however, to ensure protection against evolution of toxic chlorine dioxide gas from the acidic chlorite solution. The corrosive nature of this chemical demands exotic constructional materials such as titanium. The AOX generated by chlorite is much less than the amount generated by hypochlorite, but even this is more than twice that present after bleaching with hydrogen peroxide (Table 7.5). Hydrogen peroxide is an extremely versatile bleaching agent, applicable over a wide range of temperatures (ambient to 130°C) and times (minutes to days) by batchwise or continuous methods. Bleaching is carried out under alkaline conditions and this allows combined scour-bleach processes to be used. Peracetic acid is of increasing interest as a prebleach for knitted fabrics [31], in denim washing and the bleaching of cotton/acrylic blends. In situ production has been recommended, but storage of acetic anhydride is more inconvenient than the commercially available peracid. Bleaching with hydrogen peroxide is controlled by adding a stabiliser. Various formulations are commercially available, including magnesium salts and anionic polyelectrolytes such as silicates, polyacrylates or protein degradation products to give stabilisation, often including sequestering agents to counter the adverse effects of transition-metal ions and surfactants to provide detergency during bleaching. Most of these products give adequate stabilisation in batchwise conditions but stabilisation in continuous pad-steam processes is more difficult. 7.2.2 Desizing, Scouring and Mercerising The application of film-forming polymers to warp yarns minimises frictional problems during weaving. Natural starch, starch ethers, cellulose ethers and polyacrylates are usually used for this purpose. Economical starch-based formulations are effective for cotton yarns. Poly(vinyl alcohol) is often preferred for sizing polyester/cotton blends. Waxy plasticisers may be added to the size mix. Desizing is more predictable in a vertical organisation, since there is complete control of product selection and size recovery processes may be feasible. In most cases, however, the commission finisher seldom knows the fabric source or the size formulation used. Size removal depends on several factors: 1. concentration and viscosity of the size formulation, 2. nature and amount of plasticiser present, 3. fabric construction, 4. ease of dissolution of the size film, 5. washing-off procedure and temperature. Enzyme desizing is highly effective for degrading starch-based sizes. The αamylase is often applied in the quench box after singeing. The halogenated phenol preservative added to the size mix to avoid mildew formation on damp fabric is toxic to enzymes, as are most anionic surfactants. The rate of enzymic
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hydrolysis of starches increases with temperature. The linear amylose component is degraded more readily than the branched amylopectin fraction. Rice and tapioca starches contain more amylopectin and are thus more difficult to remove than corn starch. Scouring is the hot alkaline process necessary to remove the non-cellulosic impurities. The main effects of this treatment are a 5 to 10% loss in mass and a dramatic improvement in wettability and absorbency. The change in mass results from degradation of protein to amino acids, conversion of pectin to soluble sodium salts, hydrolytic dissolution of hemicellulose and a limited amount of oxidative degradation of cellulose. Saponification of the cotton wax is incomplete but it has to become no longer capable of forming a continuous film over the fibre surface. Scouring above 100°C is essential for maximum absorbency of loose stock, yarn or woven fabrics. Knitted cotton fabrics are usually cleaner and can be scoured using a milder and more rapid process [32]. Considerable quantities of dissolved or suspended impurities build up in the liquor during scouring. This is particularly important in batchwise processes because rapid cooling or changes in pH can result in deposition of these contaminants onto the goods. In a similar way, saturators can show a build-up of impurities and filters in the circulation loop must be checked regularly. Antifoams are best avoided because they tend to aggravate instability problems. Persulphates or perphosphates have been recommended as additives in caustic soda scouring to accelerate the degradation of starch sizes. Oxidative desizing offers the possibility of reducing the number of fabric preparation stages, an important means of minimising the overall energy consumption. The oxidant can be added to the hot caustic scour liquor and little or no magnesium silicate or organic stabiliser is needed. Rapid desizing treatments require more critical control of alkali and oxidant concentrations. Increased oxidant above the minimum necessary for effective desizing and increasing the alkalinity for a given oxidant concentration both tend to increase the degree of chemical damage. Persulphates promote desizing rather than bleaching and require more critical control of concentration than does hydrogen peroxide. Bioscouring is a process in which an alkali-stable pectinase enzyme is applied to selectively remove pectin and waxes from cotton fibres. By hydrolysing the pectin material between the waxes and the fibre surface, the enzyme exposes the waxes to emulsification when the scouring bath temperature exceeds their melting range. Bioscouring does not eliminate motes (cottonseed fragments) or the natural colour of the cotton, which can be beneficial when scouring for a ‘natural look’ [33,34]. Mercerisation of cotton yarn or fabric is normally achieved by saturation with cold caustic soda solution at about 25% by mass and containing a good wetting agent. Considerable swelling of the fibres takes place, accompanied by shrinkage unless the fabric or yarn is held under tension. Mercerisation may be carried out on the grey, scoured or bleached substrate. When mercerising loomstate fabrics, penetration by the alkali is slow at ambient temperature and tends to give surface effects. Grey mercerising also fouls the liquor with size residues, causing instability and making caustic recovery for recycling difficult. Mercerisation enhances fabric lustre, smoothness, tensile strength, dyeability, dimensional stability and coverage of dead cotton. All these properties are influenced by alkali concentration, temperature and dwell time in alkali prior to washing-off. An
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important consideration for reproducibility is the rate of desorption of alkali at the washing-off stage. 7.2.3 Bleaching of Cotton All natural fibres are coloured and the colouring matter in cotton and the bast fibres confers a yellowish or brownish discoloration. The objective of bleaching is to destroy these coloured impurities by oxidation and solubilisation, giving a pure white appearance to the fibres. Cotton bleaching today usually means hydrogen peroxide treatment, since hypochlorite and chlorite methods have lost ground because of their environmental impact. The advantages and disadvantages of these chlorine-containing bleach chemicals are listed in Table 7.6 and Table 7.7. Three important factors are responsible for the dominance of hydrogen peroxide as a bleaching agent for cotton: 1. exceptional versatility of application by batchwise or continuous methods, 2. a wide range of possible means of activation and stabilisation, 3. exceptionally innocuous to the environment, decomposing into oxygen and water. Hydrogen peroxide, normally marketed as a 35% or 50% aqueous solution, is extremely stable, losing only about 3% of its activity after storage for a year at 40°C. Bleaching of cellulose with hydrogen peroxide alone would be very slow. The most important activator for textile bleaching is sodium hydroxide but other alkalis, mineral acids, and O-acyl or N-acyl compounds exert broadly similar effects [35]. Transition-metal ions and UV irradiation bring about homolytic fission of the peroxide molecule into two hydroxyl radicals; the problems created by these radicals have been discussed [36]. Correct process stabilisation conditions control the rate of bleaching and ensure a residual peroxide content prior to washing-off, making the most economical use of the chemicals. In this way the required standards of whiteness with minimum chemical damage are achieved. Four parameters have to be carefully balanced: temperature, time, pH and stabiliser content. The effects of alkali concentration on the reflectance of peroxide-bleached cotton are shown for batchwise and continuous processes (Figure 7.5 and Figure 7.6 respectively). High pH and temperature lead to decomposition of hydrogen peroxide and degradation of the cellulose, both reactions being catalysed by transition-metal ions [36]. The role of the peroxide stabiliser is to control these effects by acting as a buffer, metal ion sequestrant and dispersant. The sequestering action inhibits the catalytic effect of trace metal ions from the fabric or water supply. The stabilising action of sodium silicate or other anionic polyelectrolytes is enhanced by addition of magnesium sulphate. The degree of preparation prior to peroxide bleaching influences alkali and stabiliser concentrations required in the bleach bath. When pretreatment is limited to the oxidative removal of size the presence in the cotton of natural impurities helps the stabilisation and so the bleach liquor may need less stabiliser or more activator. Acid treatment to neutralise scouring or remove transitionmetal contamination may also remove natural calcium and magnesium ions from cotton and so, in this case, an increase in magnesium sulphate addition may be necessary. Alkaline scouring may result in carry-over of alkali into the bleach bath and so the alkali addition may need to be reduced or omitted altogether.
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The rate of peroxide bleaching increases with temperature. Providing the reaction time is sufficiently prolonged, such as an overnight dwell period, satisfactory results can be achieved under ambient conditions. When bleaching is carried out at 100°C, however, the dwell time can be reduced (typically to between 3 and 20 minutes). The capability of achieving effective bleaching in such a brief time is unique to hydrogen peroxide and makes rapid continuous bleaching processes possible. Pretreatment before bleaching is desirable but not always essential. This process depends on the end-use and type of textile material, as well as the machinery available. Woven fabrics are normally desized and scoured, whereas yarn and knitted fabrics are simply washed-off to remove spinning or knitting lubricant before bleaching. Loose stock destined for high-absorbency end-uses requires a full caustic scour before peroxide treatment. The advantages and disadvantages of bleaching with hydrogen peroxide are summarised in Table 7.8. The ECObleach (Monforts) pretreatment of cellulosic fibres combines desizing, scouring and bleaching in a single process of 2 to 3 minutes in a dryer using a controlled steam/air mixture as the treatment medium. A detailed comparison of this process with the cold pad-batch peroxide bleach on cotton corduroy, terry and woven qualities, as well as lyocell and polyester/cotton fabrics, gave fully satisfactory results. The possibility of immediately following the ECObleach with the Econtrol process (section 16.2) for dyeing cellulosic fibres with reactive dyes is being explored, using the alkali from the pretreatment stage as the fixation medium for the dye-fibre reaction [39]. To achieve high-quality dyeings, it is essential to eliminate residual peroxide after bleaching. Repeated hot rinses are energy- and water-intensive but cannot guarantee complete removal. An enzymic process using catalase for 15 minutes at 30°C after an initial hot rinse at 80 to 95°C offers substantial economic and environmental advantages. Costs of energy, water and labour are reduced by up to 20%, giving overall annual savings of 6-8% compared with conventional multiple rinsing [40]. 7.2.4 Cotton Structure and Dyeing Properties Cotton varieties grown in different parts of the world show highly significant variations in quality, fibre dimensions and physical properties. Therefore it is not surprising that there are also corresponding differences in dyeability. Table 7.9 shows the colour differences observed when nine different varieties of cotton were dyed separately under identical dyeing conditions with C.I. Direct Green 27. Closest together in quality and colour were the Egyptian and Sudanese cottons. When investigating the sources of dyeability variations in cotton it is essential to bear this aspect in mind. The importance of uniform blending of fibres before yarn manufacture is evident. The most obvious structural feature to influence the dyeability of a fibre is its specific surface area, which is inversely related to its fineness (usually expressed in linear density). It is well known that the rate of dyeing increases with fibre fineness because of the higher ratio of surface area to fibre mass. Fineness is another physical characteristic that varies considerably with the source of the cotton. The linear density of St Vincent Sea Island is about 1.05 µg/cm, more than three times finer than the much coarser Indian Bengal type at about 3.3 µg/cm. Most American, Egyptian and Sudanese cottons lie between these two extremes.
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The accessible internal surfaces of a cotton fibre include the voids between microfibrils and the space between elementary fibrils, as represented by regions B and C in Figure 7.1. One measure of the extent of these voids is the water imbibition value of 50% for cotton (Table 7.4), but this does not correlate closely with the absorption of larger entities such as dye anions. It might be expected that cotton fibres, because of their bilateral structure (Figure 7.1), would absorb dye more rapidly in regions B and C than in region A, but this does not appear to have been investigated. A detailed study by Courtaulds of the factors that influence the dyeability variations in cotton demonstrated the effectiveness of the spinner’s blending process (Table 7.10). A sampling programme was initiated at successive stages of processing and the dyeability was assessed using C.I. Direct Green 27 in a standard test. The dyeings were subjected to colorimetric measurement and the degree of variation expressed as a percentage: shade variations of less than 5% are not perceptible even to a trained colourist. Cotton fibres from an identical source were used to spin yarns of differing count. The colorimetric data from this series of sampling trials demonstrated that yarns of increasingly finer count gave a progressively increasing depth of colour in the standard dyeing test (Table 7.11). Although marginal differences in the degree of colour variation could be detected in sampling physically identical yarns produced by different spinners (Table 7.12), the level of consistency of output achieved is remarkable in view of the variability that exists in the raw material (Table 7.9). The achievement and maintenance of this standard of quality is neatly illustrated in the Figure 7.7 histogram. The conditions of wetting and especially subsequent drying processes during the preparation of cellulosic fibres have an important bearing on attainable dye uptake with direct or reactive dyes. For example, after ten wet/dry cycles (each 1.5 minutes at 150°C in a convection dryer), the water retention value of cotton fabric had diminished by 9% and the relative colour strength of a 3% owf dyeing of C.I. Reactive Red 120 by 25%, both properties decreasing progressively with drying temperature. The relative loss in colour strength was much more evident on cotton than on viscose. At drying temperatures above 60°C, the drying of cotton at 70% relative humidity lowered this value significantly further than in the absence of moisture [43]. Any sample of cotton is likely to contain at least some exceptionally thin-walled fibres, either dead or immature (Figure 7.3). These fibres lack a fully developed secondary wall. The substantivity of many dyes for the primary wall material is less than for the secondary, so the dyed thin-walled fibres appear paler than the normal material. Even in those cases where there is no significant difference in dye uptake, thin-walled fibres still appear paler because the average path length of the illuminating light is less. If the paler fibres are uniformly distributed in a yarn or fabric, the variations in dye uptake are obscured. During ginning and carding, however, they tend to form small clumps or knots of tangled fibres known as neps. Flattening of such neps may form small, highly reflective flecks. Poor dye penetration may leave undyed areas if the loosely attached neps change position after dyeing. The dyeability of immature cotton can be improved by mercerisation but the lack of secondary wall development in dead cotton makes such treatment ineffective [44]. Direct dyes vary considerably in their ability to cover dead or immature cotton, but by careful selection a sufficiently wide range can be found suitable for this purpose. Cotton fabrics containing both immature fibre neps and process
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neps composed of damaged mature fibres were given various mercerising treatments. After dyeing with ten different direct dyes, fully mercerised fabrics showed a 95% increase in visible coverage of neps, but causticised fabrics only an 85% increase in coverage. After liquid ammonia treatment, significant improvement in nep coverage occurred with only five of the ten dyes [45]. The differences between direct dyes in their behaviour towards nep coverage are most evident with those neps composed of immature fibres with some secondary wall development [46]. Direct dyes that contained more than one amino or amido group in their structure were found most likely to achieve relatively good coverage of neps [47]. Although good coverage bears no obvious relationship to molecular size or shape, aggregation and migration properties of direct dyes do seem to be related to nep coverage [46]. The marked increase in dyeability that accompanies the swelling of cotton with sodium hydroxide is generally attributed to an increase in the volume of accessible regions within the fibres. Nevertheless, it has also been demonstrated that even when the amounts of dye absorbed by mercerised and unmercerised cotton are identical, the mercerised sample often appears more deeply coloured [48]. This effect of mercerising on the visually apparent depth has been interpreted in terms of changes in the internal scattering of the incident light [49]. The conditions of mercerising have a significant effect on dyeability. In general, the higher the alkali concentration the greater is the dye uptake (Table 7.13). Usually (but not always) mercerised fibres absorb dyes more readily if they are not dried before dyeing (Table 7.14). Pretreatment of the cotton before mercerising also plays an important part. Scoured-only or scoured and bleached cotton yarns were slack mercerised, restretched and then dyed with a disazo direct blue at 70°C. The exhaustion and rate of dyeing of the mercerised yarns were appreciably higher than those of the scoured yarns but these differences decreased as the degree of restretching was increased. A hypochlorite bleach after scouring also decreased the difference in dye uptake between scoured and mercerised yarns [50]. Liquid ammonia treatment is a highly effective and well-controlled alternative to caustic soda mercerisation, but the high capital cost of the necessary equipment for recovery and reuse of the ammonia as well as the application step limits the adoption of this sophisticated approach more widely. The effect of liquid ammonia treatment on the dyeing of cotton depends on the way that the ammonia is removed. Aqueous washing gives a product almost as dyeable as mercerised cotton. As with mercerising, treated yarns and fabrics appear more deeply dyed than untreated material having the same amount of dye present [51]. Bleached cotton fabrics given either conventional mercerising or liquid ammonia treatment were examined for changes in morphology by X-ray analysis, fibre cross-section and measurements of the sorption of steam, iodine and C.I. Direct Blue 1. Marked differences between the two treatments were found and liquid ammonia swelling does not always result in a stable fibre structure [52]. Highquality cotton shirting subjected to liquid ammonia treatment followed by durable-press finishing showed good easy-care performance with only modest strength loss. Changes in dyeing behaviour and morphology during this sequence were investigated [53], using the pair of direct dyes of markedly different molecular size that are recommended for the red-green test to indicate immaturity in raw cotton (C.I. Direct Red 81 and Green 26).
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7.3 Preparation and Dyeing of Bast Fibres Ramie has a low content of non-cellulosic impurities compared with other bast fibres (Table 7.3) and can be processed much like cotton. The effects of caustic soda mercerisation, either slack or with tension, on the direct dye uptake and colour yield of ramie, linen and cotton yarns were investigated. Treated and untreated ramie exhibited lower dyebath exhaustion than cotton or linen, but for a given dye content the visual depth of dyeing on ramie was greater [54]. The scouring of ramie with sodium metasilicate, alone or in combination with sodium carbonate and trisodium phosphate, has been evaluated and optimum conditions determined. Relative effectiveness was assessed in terms of loss in mass, whiteness index and Methylene Blue absorption [55]. The other bast fibres contain more impurities and are usually brownish in colour because of the presence of lignin. In the processing of flax it is the fibre bundles that exhibit the characteristic properties of linen fabrics. If the fibre ultimates are large, as in the case of flax (Table 7.2), the intercellular material can be removed without undue strength loss, although the loss in mass is substantial. In hemp, jute and sisal the ultimates are much smaller and removal of the impurities causes serious weakening of the fibres. It is usual to remove the lignin from flax but not from jute, which makes the latter sensitive to photoyellowing [56]. In all cases the greater the whiteness of the substrate, the greater the loss in mass. Unbleached flax varies widely in colour because of climatic differences during growth and variations in the conditions of biological retting of the fibre from the plant stem. An under-retted flax often has a reddish brown hue whereas a fullyretted flax is almost grey in colour. Over-retting is accompanied by biodeterioration of the cellulose. The retting process creates substantial BOD/COD loading of effluent and it is preferable to adopt a more reproducible warm retting process in tanks. The preparation of linen is a compromise between the whiteness level desired (often defined in quarter steps up to full white) and the loss in mass tolerated. The presence of residual woody debris (sprit) may be desirable in certain enduses or tolerated for low loss in mass. In such cases, careful dye selection is necessary to give uniform coverage. High whiteness demands multiple-step processing and there is a danger of separating the fibre bundles, called cottonisation. The risk of this fault is minimised by repeated mild treatments rather than one severe stage. After appropriate preparation, linen can be dyed with reactive dyes by conventional methods developed for cotton. Residual lignin takes up more dye than the cellulose, resulting in a speckled appearance and sometimes an off-tone shade. These deeply dyed specks decrease in depth by desorption of the poorly fixed dye during subsequent alkaline fixation and washing treatments. Every effort should be made to remove these impurities as much as possible before reactive dyeing. In package dyeing linen yarns are more readily penetrated using reactive dyes than by vat dyeing. Dye-fibre reactivity and ultimate fixation efficiency are closer to those of unmercerised rather than mercerised cotton and thus appreciably less than on viscose. Careful attention is required during the critical levelling and diffusion stage just before and during the addition of alkali. Causticised woven linen fabrics generally give satisfactory results under normal conditions for cotton. Pad-jig development is regarded as the best approach for
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the coverage of slubs or other surface irregularities. The pad-batch process is especially attractive for many linen qualities because of the excellent dye diffusion in reasonably short batching times. Careful dye selection is necessary for woven linen that is to be given a crease-resist finish. Limited amounts of ramie are dyed with reactive dyes by methods similar to those for cotton. Bleached jute is batchwise dyed with reactive dyes using salt and careful addition of soda ash. Jute is somewhat sensitive to alkali and control of alkaline pH and temperature is important. Sisal is sometimes dyed with reactive dyes because of their attractive hues, good light fastness and excellent fastness to water, although poor penetration of certain constructions can present problems. Unsatisfactory results are obtained on coir and other bast fibres, however, owing to their deep natural colours and high content of non-cellulosic impurities. Linen to be dyed with vat dyes is pretreated and bleached only before pale depths, since all alkaline processes remove the non-cellulosic material and result in an undesirable loss in mass. This amounts to 5 to 10% after dyeing at 60 to 80°C but can reach 20% if the linen is given an alkaline treatment under hightemperature conditions followed by a reductive bleach. In package dyeing it is difficult to achieve good penetration of the yarn at crossover points, particularly in the case of conical packages on rigid tubes. Hence it is best to use flexible springs, together with dyes giving good levelling performance. All dyeing processes for cotton yarn may be used on linen. The best result from the viewpoint of penetration on linen fabrics is achieved by beam dyeing at high temperature, but pad-jig and pad-batch processes are also satisfactory with care. Blends of linen or ramie with cotton will withstand an alkaline scour at the boil, followed if necessary by a combined peroxide/chlorite bleach. Dyeing in rope form with direct dyes is followed by conventional resin finishing. Treatment with liquid ammonia can be carried out before or after dyeing to enhance the performance of the final finish. It is essential to carefully neutralise any retained ammonia by treatment in rope form with acetic acid solution. Effective finishing is also important to achieve optimum wet fastness of the direct dyes [57]. The traditional growing of flax has been resumed in Saxony since 1993. Fabrics woven from open-end yarns spun from 40% short-staple flax and 60% modal fibres are being produced. Various preparation sequences have been evaluated, including enzyme desizing in a jet machine and cold pad-batch bleaching, or continuous pad-steam scour-bleach treatment with alkaline peroxide. The preferred dyeing process is exhaust dyeing with reactive dyes, which offer excellent reproducibility, levelness, penetration and fastness performance [58]. Jute fabrics can be bleached effectively with peracetic acid in the presence of tetrasodium pyrophosphate. A buffered medium is essential during bleaching in order to achieve fibre brightness [59]. The effectiveness of aftertreatments for enhancing fastness performance of direct dyes on jute has been examined. Photostability was improved by treatment with copper(II) sulphate and potassium permanganate, whilst cationic fixing agents gave higher wash fastness ratings. Yellow and red dyes with peak wavelengths of 400 to 550 nm yielded good light fastness but difficulties were encountered with blue and green direct dyes [60]. Vat dyes are rarely used on jute, mainly because of their cost and poor light fastness associated with the high absorbency of UV radiation in the 300 to 400 nm region by the lignin present in jute fibres. A selection of 29 vat dyes was evaluated on jute, 30:70 jute/cotton and cotton yarns. The jute/cotton blend was
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markedly superior to the all-jute yarn, giving satisfactory fastness to light and washing with no tonal variation [61]. When jute is dyed with sulphur dyes according to the conventional sodium sulphide process the effluent has high BOD and releases hydrogen sulphide and thiosulphates. A more eco-friendly alternative system based on glucose and caustic soda offers high exhaustion and satisfactory fastness [62]. The incorporation of kenaf bast fibres in blends with cotton confers mildew resistance and contributes to higher strength. Conventional peroxide bleaching and mercerising treatment, either slack or with tension, can be given to 1:1 kenaf/cotton fabrics and there are no significant problems in achieving satisfactory dyeability and colour fastness [63]. In order to achieve effective water repellency and flame retardancy, it is necessary to apply these two finishing formulations in a one-step process [64]
7.4 Preparation and Dyeing of Regenerated Cellulosic Fibres Fabrics made from typical regenerated cellulosic fibres are desized by the same processes as for cotton. Care should be exercised with oxidative desizing in terms of both alkali and oxidant concentrations, which should be no more than half of those used for cotton. Most warps contain starch-based sizes and so enzyme desizing is customary. However, carboxymethylcellulose is sometimes used and in this case a cold swelling process followed by hot washing with a detergent is adequate [29]. Viscose and most other regenerated cellulosic fibres dissolve in caustic soda liquor at about 6.5% by mass (70 g/l), so causticisation rather than mercerisation is carried out using 3.3 to 5.5% NaOH by mass (35 to 60 g/l) to enhance wetting or dyeability. The addition of salt minimises the risk of damage. However, causticisation can cause uneven swelling of the fibres and lead to unlevel dyeing. Gel permeation chromatography has demonstrated that treatment with a cellulase enzyme (Penicillium funiculosum) brings about widening of the voids in viscose fibres, making more internal surface available to alkali during swelling. A saving of 5 to 10 g/l caustic soda is attainable, as well as a more uniform and consistent swelling effect [65]. The swelling action of alkali on the strength and handle of viscose is less pronounced with more tightly spun yarns and woven fabrics. Better stabilisation is possible by fibre blending; for example, viscose can be dyed with vat dyes without difficulty when blended with cotton or polyester. Polynosic fibres or highwet-modulus (HWM) fibres are more resistant to alkali than regular viscose. Cotton/polynosic blends form an alkali-stable substrate that can be mercerised. Although the HWM fibres are more sensitive to alkali, they are sufficiently stable to be dyed with vat dyes. The strong swelling of viscose results in an increase in the diameter of the fibre. Particular attention must be paid to this change in the dyeing of cross-wound packages, since it increases the resistance to liquor flow. For this reason, packages of viscose staple yarn must be wound at a lower tension than cotton yarn. Since leuco vat dyes have higher substantivity for viscose than cotton, there is a greater risk of obtaining unlevel dyeings; this can be counteracted by increasing the quantity of levelling agent used. Viscose yarns are dyed in package or hank form by the leuco or prepigmentation process. Dyeing at as high a temperature as possible minimises swelling of the fibre and promotes levelling.
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On completion of dyeing the material must be thoroughly acidified because alkali is only removed slowly from these fibres by rinsing. Since woven viscose staple fabrics are readily deformed in the wet state, care must be taken not to stretch the material during dyeing. Most regenerated cellulosic fibres are used as staple and bleached in fabric form. Bleaching is only necessary for fluorescent whites and pastel shades. Chlorite or hypochlorite can be used but peroxide bleaching is preferred. Bleaching processes for regular viscose and HWM fibres have been compared [66]. Polynosic and HWM fibres in 50:50 blends with cotton have been evaluated for their response to alkaline bleaching, causticisation and mercerisation processes, in terms of the effects on physical properties and direct dye uptake [67]. Since the differences in fine structure between different regenerated cellulosic variants are much greater than between varieties of cotton, precise relationships between fibre structure and dyeability are difficult to formulate. When considering dyeability it is important to distinguish between rate of dyeing and the final equilibrium uptake. As with cotton, the finer the fibre the more rapid the dyeing rate. Moreover, at equal dye content there may be visual colour differences between fibres differing in linear density because of internal optical effects. Hollow viscose fibres dye more quickly than regular viscose because of the much greater surface area of the internal channel, but the equilibrium uptake is essentially the same. An important characteristic of regular viscose and some other regenerated cellulosics is the difference between skin and core. The skin contains more crystalline material than the more readily swollen core. The most important special feature of modal and polynosic fibres is their fibrillar structure, which means that they behave more like cotton than regular viscose. They usually have a high degree of lateral order; that is, their fibrils are uniformly distributed across a section of the fibre. Direct dyes usually show higher substantivity on viscose or mercerised cotton because a greater surface area is available on these substrates than on unmercerised cotton. The order of increasing substantivity for various cellulosic fibres does not differ significantly from one dye to another, except in the case of phthalocyanine blues. These show higher substantivity for cotton than for viscose or modal fibres. This is because their inherent affinity for regenerated cellulose is lower in spite of its greater accessibility [68]. The higher dyeability of mercerised cotton is attributable to the lower surface charge on this substrate. This difference in behaviour is less marked at higher salt concentrations but under these conditions direct dyes show slower migration and inferior levelling. Blends of mercerised cotton and viscose will often give good solidity with direct dyes even in full depths by dyeing at the boil with little or no salt because of the inherently higher dyeability of mercerised cotton. The development of viscose microfibres has enabled colour yields and reflectance values to be obtained with direct or reactive dyes that are close to those on mercerised cotton [69,70]. As a result, the attainment of solidity on fabrics containing these two fibre types is now easier than on conventional cotton/viscose blends. The proportions of crystalline material in regenerated cellulosic fibres are about 40% in regular viscose, 50% in modal fibres and 65% in polynosics, compared with 70% in cotton. As crystallinity increases the water imbibition and dyeability decrease accordingly. Thus direct dye uptake under a given set of conditions generally increases in the order: cotton<polynosics<modal fibres
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viscose. The uptake of selected direct dyes by the hollow viscose fibre Viloft (Courtaulds) and two crimped modal fibres Avril (Avtex Fibers) and Prima (ITT Rayonier) has been compared with cotton and regular viscose (Table 7.15). C.I. Direct Red 80 has unusually high substantivity, whereas Blue 218 and Yellow 106 have only moderate substantivity for cellulose. Exhaustion at equilibrium was consistently lower on cotton than on any of the regenerated cellulosic fibres, as expected. The dyeability of Viloft was closely similar to that of regular viscose. Prima was consistently more dyeable than Avril, which was in turn more dyeable than viscose [71]. Exhaustion and fixation of reactive dyes on viscose or modal fibres are normally higher than on cotton or linen, but this is not generally true for polynosic or hightenacity fibres. The substantivity of individual dyes for these less amorphous regenerated fibres is somewhat more variable. Many reactive dyes show significantly higher fastness to light on viscose than on cotton at equivalent applied depths. Salt and alkali requirements are generally lower for reactive dyes on viscose than for the corresponding dyeings on cotton but most green or turquoise hues based on phthalocyanine dyes are applied according to special recommendations. Lightweight viscose materials can be dyed at relatively low temperatures with high-reactivity dyes using processes that are not much different from corresponding methods for cotton. Yarns and fabrics made from coarser denier viscose or heavyweight fabric constructions may show poor penetration and surface frostiness under these conditions, however. Dyes of low to moderate reactivity requiring fixation temperatures of 60°C or higher give better results on materials that show inferior levelling or inadequate penetration at lower temperatures. Under alkaline conditions in enclosed machines, viscose tends to give problems of reduction with certain sensitive azo reactive dyes. Addition of a reduction inhibitor from the start of dyeing is essential. The effects of traces of transition-metal ions in modal fibres on the hues of dyeings produced with three monochlorotriazine reactive dyes (C.I. Reactive Yellow 35, Orange 5 and Red 45) were examined using additions of Cu(II) and Fe(III) sulphates. It was demonstrated that EDTA and various phosphonate-type sequestering agents were effective in overcoming the shading problems attributed to these fibre impurities [72]. Any of the dye classes used for cotton dyeing can be applied to Tencel (Courtaulds) lyocell fibre. Consistency of Tencel in terms of dyeability is routinely monitored using dyes known to be sensitive to potential variations in this important property [22]. Various studies of reactive dye yields on the main cellulosic fibre variants have demonstrated that lyocell gives significantly higher yields than polynosic, modal, viscose or mercerised cotton [73]. Thus care is necessary when dyeing cotton/lyocell blends because of preferential uptake by the lyocell component; viscose/lyocell blends, on the other hand, readily give solid effects. The yield of reactive dyes on lyocell is exceptional by all dyeing methods and especially by printing. Thus alkaline treatment analogous to causticisation of viscose or mercerisation of cotton is not necessary for lyocell. Blends of cotton and lyocell can be mercerised, however. Open-width dyeing represents the most economic and ecologically sound technique for coloration [74]. Although lyocell fibres absorb reactive dyes extremely well, the higher substantivity can be controlled by appropriate dyeing conditions to ensure level dyeing performance similar to other cellulosic fibres. Reproducibility in mode shades is excellent, particularly with respect to moderate
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changes in liquor ratio. In pad dyeing systems lyocell displays negative tailing behaviour similar to that of regular viscose. Negative tailing is the tendency of the substrate to absorb water preferentially from the pad liquor, so that the dye concentration increases gradually (by about 7% at equilibrium in both instances). The fastness to light, washing and perspiration of reactive dyeings on lyocell is excellent but rub fastness can sometimes appear lower than on other cellulosic fibres, depending on the finish. Where a fibrillated finish has been given, many fine fibrils protrude from the surface of the fabric. Fragments from these can be transferred to the adjacent cotton in the rub fastness test, giving the impression of inferior colour fastness. However, dyed lyocell given a non-fibrillated finish has identical ratings for fastness to rubbing as other cellulosic fibres dyed and finished in the same way [73]. Tencel A100 (Acordis) and Newcell (Akzo Nobel) are non-fibrillating modified lyocell fibres. The dyeing and finishing characteristics combine deep, vibrant shades with remarkable stitch clarity and a positive bulkiness and warmth in garment form. Tencel A100 has a more open structure than unmodified Tencel, giving a higher water imbibition but about 10% lower tenacity and modulus. In the key areas of fabric strength, dimensional stability and pilling tendency, Tencel A100 performs at least as well as other cellulosic fabrics. The handle is fuller than modal or viscose fabrics, with greater softness and less stiffness than cotton. Scouring with sodium carbonate at concentrations up to 20 g/l causes no loss of non-fibrillation performance. Pad-batch bleaching with 25 g/l hydrogen peroxide and 4 g/l sodium hydroxide is recommended for pastel shades and fluorescent whites. Full mercerisation is not recommended but causticisation treatment with 11% by mass of caustic soda is acceptable at ambient temperature. Stripping of dyeings on Tencel A100 is satisfactory using 3 g/l sodium dithionite and 3 g/l sodium carbonate at 80°C [24]. Colour yields with typical chlorotriazine or vinylsulphone dyes on Tencel A100 are consistently higher than Tencel, cotton or viscose (Figure 7.8). As with direct dyes the results are dependent on dye type and applied concentration. Phthalocyanine dyes give particularly high exhaustion and fixation on Tencel A100 compared with unmodified Tencel. Novel effects can be obtained in striped or jacquard colour wovens by exploiting the differences in colour yield and fibrillation tendency between these two variants. Particularly impressive results have been obtained with the novel Procion XL Plus (DyStar) polyfunctional reactive dyes on Tencel A100 (Figure 7.9). These dyes exhibit high tinctorial strength and fixation efficiency resulting from the incorporation of two chromogenic groups and at least two reactive groups per molecule. Less dye is required to achieve full depths, reducing the salt requirement to 35% less than for cotton. This shortens the wash-off cycle and minimises the water and energy consumption [73]. Wet fibrillation is the abrading of fine fibrillar hairs from the fibre surface by subjecting water-swollen cellulose to mechanical stress. Unmodified lyocell fibres are particularly susceptible. Interfibrillar crosslinking induced by a cellulose reactant resin early in the wet processing sequence prevents further fibrillation of lyocell fibres. Certain multifunctional reactive dyes exert a similar effect, but specific molecular characteristics must be present. These include steric orientation and separation of the reactive groups, degree of reactivity, type and size of chromogen, molecular flexibility and diffusion properties. Cibacron LS (Ciba) applied by exhaust dyeing and Cibacron C (Ciba) dyes in the pad-batch process
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largely fulfil these requirements [75]. The crosslinking effect provided by black exhaust dyeings based on C.I Reactive Black 5 is adequate to eliminate postfibrillation and greying on laundering [76]. Characterisation of the fibrillar behaviour of lyocell materials is possible using a specially developed wet rubbing test method [77]. Lenzing Lyocell knitgoods have an attractive soft handle, excellent drape, high dimensional stability and outstanding comfort in wear. Overflow and softflow jet dyeing machines are particularly suitable and a crease lubricant must be used. Temperatures lower than 50°C lead to chafe marks and creasing. In conventional finishing, primary fibrillation in alkaline solution at 90°C is followed by reactive dyeing, defibrillation with a cellulase enzyme and finally crosslinking with a reactant resin to avoid post-fibrillation during household laundering [78]. NewCell (Akzo Nobel) is a lyocell filament yarn that in woven fabrics shows markedly lower fibrillation than unmodified Tencel or Lenzing Lyocell. Two finishing routes are recommended: 1. desizing and causticising in open width, rope dyeing in an airflow-type machine, fibrillation by tumbling and finally finishing, or 2. similar preparation, tumbler fibrillation, followed by dyeing in either rope or open-width form [25]. The inherent tendency of unmodified lyocell woven fabrics to fibrillate is decisive from the viewpoint of machinery selection, depending on the target degree of fibrillation desired. In the Thies Roto-stream, maximum fibrillation is attained at high fabric speed with intensive mechanical stress applied to the fabric rope. Finishing conditions to achieve modified surface effects have been outlined, such as a smooth ‘classic touch’, the ‘peachskin’ effect or a chemically modified ‘sand wash’ finish. Secondary fibrillation may be conferred by mechanical aftertreatment in a Roto-tumbler at 140°C and fabric speeds of up to 1000 metres/minute. Operation at 3.5 bar pressure achieves an intensive tumbling effect at low power consumption [79]. The sequence to produce a ‘peachskin’ finish on woven Tencel involves three stages: primary fibrillation, enzyme biopolishing and secondary fibrillation. Owing to the stiffness of lyocell fibres, Tencel yarns are hairy. The wet fabric is subjected to mechanical action in the primary fibrillation stage. The hairs are the most accessible fibres and fibrillation occurs predominantly there. They can become entangled, giving a characteristic pilled appearance. A cellulase enzyme removes the fibrillated hairs and pills from the fabric surface [80]. The preceding primary fibrillation created weak points in the fibres, allowing the enzyme to depill the fabric effectively. Further wet processing brings about secondary fibrillation on the fabric highpoints. There are no longer any protruding hairs, so no pilling occurs. Final softening and tumbling allow the surface fibrillation to lift, giving a frosted appearance from the light scattering behaviour of the fibrils and the characteristic handle of the goods [23]. A troublesome problem with unmodified lyocell woven fabrics is the greying that is observed after only a few repeated washes. Testing of wet abrasion resistance (WAR) in a modified Martindale test is useful for assessing the degree of fibrillation. Crosslinking treatments improve WAR, reduce fibrillation and minimise greying. Thus WAR increases from ca. 400 (untreated) to ca. 1000 after dyeing with multifunctional reactive dyes, or to ca. 2500 for typical low-formaldehyde crosslinking agents in reactant finishes. These are more effective than selfcondensing resins or formaldehyde-free polycarboxylic acid reactants for
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increasing WAR values. In all cases, however, the inverse relationship between wet and dry abrasion values as the degree of crosslinking is increased must be taken into account [81]. A yarn-on-yarn abrasion test method has been developed to effectively monitor the WAR value of lyocell. The test system is sensitive to the effects of reactive dyes and crosslinking agents. Under the test conditions employed, the dry yarn breakdown of the lyocell appeared to occur by a different mechanism to the wet abrasion effect [82].
7.5 Reactive Dyes The highest demand for textile apparel exists in the leisurewear, casualwear and active sportswear market segments. This demand has been stimulated by the move away from traditional functional garments to much more fashion-oriented designs with a high colour content or contrast trims. Reactive dyes with their brilliant shades and high fastness to washing are ideally suited for these garments, which are subjected to frequent wash-wear cycles. The consumption of reactive dyes for various types of cellulosic or polyester/cellulosic substrate is indicated in Table 7.16. Owing to the physical nature of most of these substrates and, to a lesser extent, the limited versatility of dyeing machinery with respect to dyelot sizes, exhaust dyeing accounts for more than half the demand for reactive dyes (Table 7.17). The requirements of reactive dyes for application by exhaust methods are distinctly different from those for pad-batch and continuous methods, although these differences have declined with the development of more sophisticated microprocessor controls. 7.5.1 Reactive Systems The characteristic features of a typical reactive dye molecule include: 1. one or two chromogenic groups, contributing the colour and much of the substantivity for the fibre, 2. one or more reactive systems, enabling the dye to form covalent bonds with the fibre and often contributing some substantivity, 3. bridging groups forming links between reactive systems and chromogenic groups, often exerting important influences on reactivity, stability and substantivity, 4. typically between two and eight solubilising groups, usually sulphonic acid substituents on the aryl rings of the chromogenic groups, 5. in some instances, as with the important monohalotriazine reactive systems, a colourless arylamino or other grouping attached to the triazine ring that further modifies solubility and substantivity. All the important chemical classes of chromogen have been included in reactive dye structures. The sulphated ester precursor of the vinylsulphone reactive group contributes significantly to aqueous solubility. The nature of the bridging links, especially in dyes of the haloheterocyclic type, greatly influences the reactivity and other dyeing characteristics of such dyes [84]. The structure of the reactive grouping and substituents attached to it is decisive with regard to the chemical stability of the dye-fibre bond that is formed.
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Numerous factors have to be taken into account in designing reactive dyes of commercial interest [85]. Some of the more important are: 1. Economy – any reactive system selected for a range of dyes must enable them to be produced at acceptable cost, 2. Availability – the system selected must be free from patent restrictions, health hazards or other limitations to exploitation, 3. Versatility – it must be possible to attach the reactive system to a variety of dye chromogenic groupings in manufacture, 4. Storage stability – dyes containing the reactive system must be stable to storage under ambient conditions worldwide, 5. Efficiency – the manufacturing yield must be economically viable and the dye fixation must be high under conventional conditions of application, 6. Bond stability – the dye-fibre bonds must be reasonably stable to a range of relatively severe fastness tests. Only a relatively few reactive systems (Table 7.18) have met these requirements sufficiently well to become commercially established in a significant segment of the market for reactive dyes. Evidence soon emerged in the 1960s that monofunctional reactive systems containing two reactive centres, such as the dichlorotriazines, were capable of forming crosslinks between adjacent cellulose chains. It was not until around 1970, however, that ICI introduced two ranges of high-fixation dyes containing two monochlorotriazine groups per molecule. If other factors are equal, the application of a reactive dye containing two reactive groups rather than its analogue with only one reactive group per molecule increases the fixation from a typical 60% to approximately 80% on average in exhaust dyeing. In the pad-batch process the corresponding fixation efficiency levels are about 75% and 95% respectively [86]. Bifunctional systems containing two different kinds of reactive group are popular in exhaust dyeing and gaining ground, especially on account of their relative insensitivity of fixation to fluctuations in dyeing temperature [87]. Dichlorotriazine Dyes This reactive system, the first to be introduced commercially by ICI in 1956, is the most reactive of those listed in Table 7.18. The two active C1 substituents in a dichlorotriazine dye 7.2 are equivalent and capable of reacting with OH groups in water or cellulose under alkaline conditions. Hydrolysis to the hydroxychlorotriazine 7.3 is rapid but this monochloro product is converted to the dihydroxytriazine 7.4 much more slowly. Reaction of dye 7.2 with cellulose initially gives the rather unstable dye-fibre linkage 7.5, which still contains an active Cl substituent and is sensitive on exposure to mildly acidic conditions. However, the product 7.5 generally hydrolyses to the stable dye-fibre linkage 7.6. Some crosslinking to give 7.7 may also occur under severe conditions of fixation. Dichlorotriazine dyes can be readily fixed by pad-batch dyeing at ambient temperature or by batchwise methods at 40°C. This means that relatively small chromogens are preferred to ensure adequate mobility on the fibre during the exhaustion stage. This requirement makes these dyes eminently suitable for bright shades but less satisfactory for deep tertiary hues, since the larger-size chromogens often fail to give acceptable performance by low-temperature application.
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Monochlorotriazine Dyes Reaction of a dichlorotriazine dye with an arylamine at about 25 to 40°C yields a much less reactive monochlorotriazine dye. More energetic reaction conditions, typically 80°C and pH 11 for batchwise application, are necessary for efficient fixation. Early studies of the relationships between structure and substantivity of monochlorotriazine dyes revealed that the NH bridging groups linking the chromogen and the uncoloured arylamino substituent to the triazine ring had marked effects on the solubility and dyeing properties of the dyes [88]. Methylation of the NH group tended to lower the substantivity for cellulose. The presence of a sulphonated arylamino substituent in this position was helpful to enhance solubility and modify the dyeing behaviour. Monofluorotriazine Dyes A fluoro substituent is used as the leaving group in the Cibacron F (Ciba) reactive system. The greater electronegativity of fluorine compared with chlorine results in a markedly higher level of reactivity for these dyes than for the monochlorotriazine analogues. The substantivity and solubility of the dye structures can be modified considerably by introducing appropriate substituents on the chromogen, the uncoloured arylamine and the NH bridging groups. Trichloropyrimidine Dyes The chloro substituents in the pyrimidine ring are much less activated than those in the triazine system. Fixation to the fibres by batchwise application requires treatment at the boil rather than the 80°C found to be optimum for the monochlorotriazine dyes, but the dye-fibre bond containing a pyrimidine ring is more stable than that containing a triazine ring. Difluoropyrimidine Dyes As with the monofluorotriazine dyes, the introduction of F substituents into the pyrimidine ring results in a markedly higher level of reactivity compared with the corresponding Cl-substituted pyrimidine dyes. Batchwise dyeing temperatures for optimal fixation of difluoropyrimidine dyes are about 40 to 50°C. The dye-fibre bond formed by reaction of this system with cellulose is more stable to acidic conditions than that of the competing dichlorotriazine dyes. However, the dyefibre bond from difluoropyrimidine dyes does tend to undergo oxidative cleavage more readily under the influence of light exposure in the presence of peroxy compounds. Dichloroquinoxaline Dyes The reactivity of this system is much higher than that of the corresponding dichloropyrimidine dyes and comparable with that of the dichlorotriazine and difluoropyrimidine systems, optimal fixation being achieved by batchwise dyeing at about 50°C. Unlike all other important haloheterocyclic reactive systems, the bridging link between chromogen and reactive grouping is amidic and thus expected to be readily hydrolysed under acidic conditions. The diazine ring in the dye-fibre bond tends to undergo oxidative cleavage when exposed to light or heat under peroxidic conditions. In spite of these potentially severe defects, the continued commercial success of the dichloroquinoxaline dyes suggests that they do not give rise to serious practical problems under normal circumstances [88].
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Vinylsulphone Dyes In contrast to all the haloheterocyclic systems mentioned above, the vinylsulphone dyes function by an addition mechanism rather than substitution. Before this can occur, however, alkaline elimination of the sulphate ester precursor grouping 7.8 is necessary to release the reactive vinylsulphone system 7.9. Polarisation of the carbon-carbon double bond by the sulphone group confers a positive character on the terminal carbon atom, favouring addition to OH groups in water or cellulose under alkaline conditions. Hydrolysis to the hydroxyethylsulphone 7.10 competes with the fixation reaction to form the dyefibre bond 7.11. A much smaller proportion of the vinylsulphone 7.9 reacts with the hydrolysis product 7.10 to yield the dimeric dye 7.12. Vinylsulphone dyes are intermediate in reactivity between the high-reactivity systems, such as dichlorotriazine or difluoropyrimidine, and the low-reactivity ranges, such as monochlorotriazine or trichloropyrimidine. Exhaust dyeing temperatures between 40 and 60°C may be chosen, depending on the pH, since caustic soda is often selected to bring about alkaline hydrolysis of the precursor sulphate ester. Vinylsulphone dyes are applicable by a wide range of dyeing or printing processes. The substantivity of many of these dyes is markedly lower than that of typical haloheterocyclic dyes. Not only has the vinylsulphone group, unlike the heterocyclic ring systems, little if any inherent affinity for cellulose, but the terminal sulphato group enhances the aqueous solubility of the precursor form before elimination to the vinylsulphone. In contrast to the haloheterocyclic systems, the dye-fibre bonds formed by the vinylsulphone dyes are at their weakest under alkaline conditions. A novel trichromatic combination of Remazol Yellow RR, Red RR and Blue RR has been introduced by DyStar, supplied as low-dusting granules to improve safety in handling and suitability for automatic dosing or manual application. The reproducibility of applying mode shades by exhaust dyeing at 60°C under fluctuating conditions is exceptionally good. This trichromatic combination is suitable for producing more than 50% of the dyed shades popular for leisurewear [89]. The Remazol RR dyes give high fixation and consistent washing-off behaviour. A high standard of light fastness is achieved and the dyeings fade on tone. The dye-fibre bonds formed are extremely stable to acid treatment, so that these dyes are readily applicable to polyester/cellulosic blends by dyeing the cellulosic component first, followed by disperse dyeing of the polyester at a mildly acidic pH [90]. Bifunctional Dyes Bis-monochlorotriazine dye molecules are approximately twice the size of analogous monochlorotriazine structures because they contain two chromogenic groups per molecule. Their high substantivity ensures excellent exhaustion at the preferred dyeing temperature of 80°C, leading to fixation values of about 70 to 80%. High-temperature application guarantees good levelling and the high fixation leads to better utilisation of the dyes applied, with less hydrolysis and less coloration of the effluent. Unfortunately, the rate of removal of unfixed dyes at the washing-off stage is slow owing to the high intrinsic substantivity of these dyes. A range of bis-monofluorotriazine Cibacron LS (Ciba) dyes has been marketed for long-liquor dyeing, requiring only a low salt (LS) concentration in the dyebath. The patent literature indicates that the most likely structures to meet this target
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contain two high-substantivity chromogens linked through a central unit carrying both fluorotriazine reactive groups [91]. This system offers an environmental advantage over the chlorotriazine analogues; organofluorine compounds do not fall into the AOX classification because the fluoride ion liberated as soluble silver fluoride according to the test protocol is not detected [92]. The most commercially successful reactive dye of all, C.I. Reactive Black 5, contains two sulphate ester precursor groups 7.8 that contribute notably to its initial solubility. When these are hydrolysed in alkali to release the vinylsulphone groups 7.9, the considerable increase in substantivity (Table 7.19) leads to highly efficient fixation. Further hydrolysis of the vinylsulphone groups to give the inactive hydroxyethylsulphone groups 7.10, however, lowers the substantivity and hence contributes to favourable washing-off performance. Reaction of a dichlorotriazine dye 7.13 with an anilino intermediate 7.14 containing a sulphate ester precursor group is the preferred route to a heterobifunctional reactive dye 7.15 of the Sumifix Supra class, introduced by Sumitomo in 1980. These dyes are capable of reacting with cellulose via either the monochlorotriazine system or the vinylsulphone group released by the precursor sulphate ester. The presence of two reactive groups that differ in reactivity gives dyes that are less sensitive to exhaust dyeing temperature than any of the typical monofunctional reactive systems. They can be applied over a wider range of temperatures (50 to 80°C) and reproducibility of hue in mixture recipes is improved. Moreover, they show minimal sensitivity to electrolyte concentration and are less affected by changes in liquor ratio [94]. Low dyeing temperatures favour reaction via the vinylsulphone group, whereas at higher temperatures the contribution of the chlorotriazine system to fixation becomes more important [95]. This was demonstrated by controlled enzymic degradation of a mechanically milled cotton fabric that had been exhaust dyed at 60°C, generating the following conclusions after analysis of the products isolated [96]: 1. about 80% of the vinylsulphone groups had reacted with OH groups in cellulose, 2. about 50% of the chlorotriazine groups had not reacted and only about half of these had been hydrolysed to the hydroxytriazine, 3. a considerable proportion of the dye molecules had formed crosslinks by reacting via both reactive systems. The incorporation of two different reactive systems and the high substantivity of heterobifunctional dyes favour the attainment of unusually high fixation. Although this leads to better utilisation of the dyes applied, with less of the hydrolysed byproducts to colour the effluent, removal of these unfixed dyes at the washing-off stage may present difficulties because of their high intrinsic substantivity. The formation of two different types of dye-fibre bond has beneficial consequences for fastness performance. Heterobifunctional dyes show superior fastness to acid storage compared with dichlorotriazine or dichloroquinoxaline systems and better fastness to peroxide washing than difluoropyrimidine or dichloroquinoxaline dyes [87]. In 1988 Ciba launched the Cibacron C range of bifunctional reactive dyes. They contain a novel aliphatic vinylsulphone system and either a monofluorotriazine bridging group or an arylvinylsulphone function [97]. Owing to the small size of the fluorine atom, the difluorotriazine precursor reacts more smoothly with an
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alkylamine carrying the sulphate ester precursor grouping. The Cibacron C dyes are designed mainly for pad applications and are characterised by low to moderate affinity, good build-up, ease of washing-off and high fixation (often more than 90%). Their good stability under padding conditions, high solubility, efficiency of reaction and outstanding fixation make them especially suitable for the pad-batch process [98]. The stability of the dye-fibre bonds in these dyeings is high to both acid and alkali, compared with monohalotriazine dyes, because of the major contribution of the vinylsulphone function to the fixation mechanism. The fluorotriazine group confers much higher stability to alkali than is shown by vinylsulphone dyes [99]. A characteristic feature of the Sumifix Supra MCT-VS system is the major difference in reactivity between the MCT group and the much more reactive vinylsulphone. There are some practical conditions, notably in pad-batch application, that do not allow full advantage to be taken of both types of reactive group present. The combination of MFT and VS groups in the synchronised Cibacron C system, both groups offering effective fixation under virtually the same conditions, exploits the concept of bifunctionality more effectively.
7.5.2 Physical Form and Dyeing Properties All reactive dyes tend to hydrolyse in the presence of moisture, especially the high-reactivity ranges, and they may deteriorate unless carefully handled and stored. Cool, dry conditions are essential and the lids of containers must be replaced firmly after use. Since reactive dyes in powder form may release dust when disturbed, it is always possible for respiratory allergies to arise with some workers who handle them. Suitable dust-excluding respirators should be used and weighing or dissolving procedures should be carried out in ventilated enclosures. Conventional dye powders are usually dissolved by one of the following techniques: 1. pasting with cold water followed by the steady addition, with stirring, of the required amount of water at the correct temperature, 2. sprinkling of a steady stream of dye powder into the vortex formed by rotary agitation of the required volume of water at the correct temperature Few ranges of reactive dyes require boiling water, although Remazol (DyStar) vinylsulphone dye powders are dissolved in boiling water followed by passing immediately through a fine sieve into the required amount of cold water. Highly reactive dyes, including DCQ, DCT or DFP types, should be dissolved at a temperature no higher than 50°C. Most dyes of lower reactivity, such as the bisMCT, MCT or TCP ranges, are usually dissolved at 80°C. The dusting problem with some reactive dye powder brands can be avoided by selecting granulated or liquid formulations. Troublefree weighing and handling of small amounts for batchwise dyeing is ensured by using cold-dissolving granules such as the Drimarene CDG (Clariant) brands [100]. These are non-dusting, freeflowing grains that dissolve readily in cold water and offer ease of handling in automated dissolving and metering devices. The marked tendency of reactive dyes to undergo hydrolysis in solution has delayed the development of liquid formulations of the highly reactive ranges. For continuous dyeing or printing, however, especially where automated metering equipment is installed, liquids are particularly convenient. Liquid brands of the relatively stable types are well
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established in commercial use. They are essentially isotropic aqueous solutions of the dyes, often with auxiliaries such as a buffer, a hydrotropic agent such as urea or caprolactam, and often a polymeric stabiliser to inhibit settling out on storage. The fixation reaction takes place under alkaline conditions between the cellulosate anion and the reactive system in the dye anion. This reaction can only occur when the dye anions have been absorbed into the water-swollen cellulose phase. Thus the rate of fixation is strongly influenced by the rate of dye exhaustion under alkaline conditions. Hydrolysis of the reactive system by reaction with hydroxide ions will always compete with the desired fixation reaction. The efficiency of fixation [101] is a function of: 1. the reactivity ratio (Rr), the ratio of rate constants for fixation (Kf) and hydrolysis (Kw), is a constant for a given dye over a wide range of alkaline pH values (Equation 7.3), Rr =
Kf Kw
Equation 7.3
2. the substantivity ratio (Sr), the relative concentrations of dye absorbed into the fibre and remaining in the dyebath at equilibrium, is a constant for a given salt concentration over a wide range of alkaline pH values (Equation 7.4), Sr =
Df Dw
Equation 7.4
3. the rate of diffusion of the dye anions in the cellulosic fibre, 4. the liquor:goods ratio, 5. the surface area of the substrate available for dye absorption. The substantivity ratio is the most influential of these factors. Dyes of higher substantivity diffuse more slowly than less substantive dyes. Changes in dyebath conditions that increase substantivity tend to retard the rate of diffusion. Lowering the liquor ratio tends to increase the fixation efficiency but the full effects of this cannot be realised because the higher dye concentrations at low liquor ratios tend to decrease the substantivity ratio, offsetting the potential gain. Finer fibres with a relatively low linear density have a greater surface area per unit mass, which favours an improved efficiency of fixation. Substantivity ratio remains approximately constant within the pH range 7 to 11 at a given salt concentration, but above pH 11 there is a marked fall in substantivity, especially with highly sulphonated dye anions. As the applied concentration of dye is increased at constant salt concentration, the substantivity ratio and hence the fixation efficiency is lowered. Thus full-depth dyeings require longer for completion of the reaction. More salt can be added to counteract this decline in efficiency, but this increases the risk of aggregation and possible precipitation with dyes of limited solubility. An increase in dyeing temperature lowers the substantivity ratio and accelerates the rate of dye hydrolysis, both effects reducing the fixation efficiency. However, the rates of dye diffusion and reaction with cellulose are accelerated by temperature increase and both favour efficient fixation. An increase in salt concentration enhances substantivity without impairing reactivity, providing the dye remains completely dissolved. The beneficial effects of salt are most evident with the more highly sulphonated dyes at relatively high pH and applied depth.
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The Cibacron LS (Ciba) range of bis-monofluorotriazine dyes was developed to minimise the large amounts of salt applied in the conventional reactive dyeing of cellulosic fibres and polyester/cellulosic blends. These dyes have been designed to have inherently high substantivity, so that only about 25% of the salt dosage usually used is required for the Cibacron LS dyes [102]. There are also savings in labour and handling costs. Further advantages include high tinctorial strength, problem-free exhaustion and fixation behaviour, as well as low sensitivity to changes in liquor ratio [103]. A helpful classification of reactive dye systems has been devised, based on three of the important controlling parameters in the reactive dyeing process [104]: Group 1: Alkali-Controllable Reactive Dyes These dyes have optimal temperatures of fixation between 40 and 60°C. They are characterised by relatively low exhaustion in neutral salt solution before alkali is added to bring about fixation. They have high reactivity and care in addition of alkali is necessary to achieve level dyeing, preferably at a controlled rate of dosage. Typical examples of dyes belonging to this group include DCQ, DCT, DFP and VS reactive systems. Group 2: Salt-Controllable Reactive Dyes Dyes in this group show optimal fixation at a temperature between 80°C and the boil. Such dyes exhibit relatively high exhaustion at a neutral pH, so it is important to add salt carefully to ensure that dyeings are level. Salt addition is often made portionwise or preferably at a controlled rate of dosage. Dyes with these properties typically have low-reactivity systems such as MCT, bis-MCT or TCP. The MFT and bis-MFT ranges of Ciba have high substantivity and should thus be regarded as salt-controllable but they are sufficiently reactive for fixation at 60°C by batchwise application. Group 3: Temperature-Controllable Reactive Dyes This group is represented only by the Kayacelon React (KYK) range of bisnicotinotriazine dyes. These react with cellulose at temperatures above the boil in the absence of alkali, although if desired they can be applied conventionally with alkali at 80°C. Dyes in this group have self-levelling characteristics and good results can be achieved by simply controlling the rate of temperature rise. The performance of a reactive dye can be defined in terms of the Reactive dye Compatibility Matrix (RCM), which encompasses five key criteria [105]. The relationships between them are indicated in Figure 7.10. Substantivity (S) is the amount of dye absorbed by the fibre during the migration phase (D30), expressed as a percentage of the total dye in the system (Dt). Equilibrium exhaustion (E) is given by the total absorption in the migration and fixation phases (D90), calculated in the same way. Equilibrium fixation (F) is the proportion of this amount that becomes fixed (Df), again as a percentage of the total dye applied. The Migration Index (MI) is determined by a dye transfer test under the conditions of the migration phase; the ratio between the amount transferred to the originally undyed specimen (Du) and that remaining on the dyed one (Dd) is expressed as a percentage. An index of 50% is reached when one-third of the
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total dye has transferred and complete migration (100%) when the dye concentrations on the two specimens are equal (Du=Dd). The Level Dyeing Factor (LDF), a measure of the impact of exhaustion on level dyeing performance, is the product of the migration index (MI) and the ratio of substantivity to equilibrium (S/E). Finally the index of reactivity of the dye (T50) is the time taken after addition of the alkali to attain 50% of the equilibrium fixation (F/2), a measure of the rate of fixation. 100 D30 Dt
Substantivity
S=
Exhaustion
E=
Fixation
F =
Migration Index
MI =
Level Dyeing Factor
LDF =
100 D90 Dt 100 Df
Equation 7.5 Equation 7.6
Equation 7.7
Dt
100 Du Dd
100 S.MI 100 D30 . Du = E D 90 .Dd
Equation 7.8
Equation 7.9
The target exhaust dyeing profile for reactive dyes, to ensure right-first-time (RFT) performance in terms of productivity, compatibility and level dyeing behaviour, is defined in Table 7.20 in terms of the five key criteria discussed above. Combinations of dyes with this optimised profile will exhaust, migrate and fix at closely similar rates, producing level and reproducible dyeings. Using this profile as a model enables ranges of highly compatible reactive dyes to be developed. Mixture shades produced with such a range will be insensitive to small practical variations in dyeing parameters because these variations will have the same effect on all components of the combination. Slight fluctuations in conditions will thus tend to cause slight differences in depth rather than hue or brightness. Table 7.20 also lists the actual ranges of values for these criteria that have been determined for the ten members of the Procion H-EXL (DyStar) range, the first to be developed using this approach in the early 1980s [83]. A useful measure of the degree of compatibility of a trichromatic combination of reactive dyes is provided by the so-called Robustness Index (RI). This is a statistical indicator of the differences in chroma (∆C) and hue angle (∆H) produced by deliberate variations in liquor ratio, time and temperature of fixation from a control dyeing using standard conditions. Four pairs of variations in conditions are defined: 1. Fixation time 15 minutes less or more than standard. 2. Fixation temperature 5°C lower or higher than standard. 3. Liquor volume 10% underfill or overfill relative to standard. 4. Liquor volume 20% underfill or overfill relative to standard. The calculation used to determine the Robustness Index is: ⎡ RI = ⎢ ⎣⎢
n
⎤
1
⎦⎥
∑ (∆Cn )2 + (∆Hn )2 ⎥
1 /2
Equation 7.10
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A Robustness Index of unity or lower may be regarded as indicative of a trichromatic combination that can achieve target levels of RFT production greater than 95%. 7.5.3 Batchwise Dyeing
Reactive dyes can be applied by any conventional batchwise dyeing method for cellulosic fibres and blends, including circulating-liquor machines for loose stock, yarn or woven fabrics, as well as jets, jigs or winches for piece dyeing. The conventional dyeing process consists of three stages: 1. Exhaustion from an aqueous bath containing electrolyte, normally under neutral conditions, 2. Addition of alkali to promote further uptake and chemical reaction of absorbed dyes with the fibre at the optimal pH and temperature, 3. Washing-off of the dyed material to remove salt, alkali and unfixed dyes. Numerous variants of this basic procedure in terms of chemical conditions and dyeing conditions have been devised to take account of the characteristic properties of the various ranges of reactive dyes. Except in special circumstances, batchwise preparation before reactive dyeing is carried out in the dyeing machine itself or in equipment of similar design reserved for the preparation stage. The essential requirements are that the material must be made available for dyeing in a neutral, uniform and readily absorbent state. In contrast to the application of vat or sulphur dyes, typical reactive dyeing processes will not eliminate fats and waxes. Acceptable results are often possible on knitgoods without lengthy pretreatment, as in pad-batch or hot batchwise dyeing in the presence of a powerful wetting agent. Residual size must always be removed from woven goods because of the risk of dye wastage by reaction with OH groups in size components. Owing to the brilliant hues of many reactive dyes, sufficient brightness may be attainable on thoroughly desized and scoured woven cotton without prebleaching. Where bleaching is necessary it is imperative to check that all traces of residual oxidant are removed prior to dyeing, otherwise loss of reactivity and even partial destruction of some dyes can occur. With few exceptions, reactive dyes have good solubility in water. Although seldom sensitive to neutral hard water, precipitation by reaction with alkaline-earth cations can occur at the alkaline fixation stage and thus soft water should be used for all dissolving and dyebath additions. Common salt (sodium chloride) or Glauber's salt (sodium sulphate decahydrate), in large amounts, is essential to all batchwise dyeing processes for reactive dyes. Common salt is widely used, but Glauber’s salt is preferred with certain blues and greens based on anthraquinones or phthalocyanines. Common salt is more soluble and easier to dissolve than Glauber’s salt. Provided goods have been prepared efficiently for exhaust dyeing, it is unnecessary to add wetting or levelling agents to the dyebath. In jet or overflow dyeing of tubular-knitted cotton, however, minimal addition of a wetting agent can provide a lubricant action for the avoidance of rope marks. Unwanted foam in jet or overflow dyeing can be minimised by careful addition of selected antifoam agents. When dyeing in enclosed machines at temperatures higher than 70°C, certain azo reactive dyes may undergo reduction owing to the combined effects of
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heat, alkali and reducing end-groups in cellulose, particularly when dyeing regenerated cellulosic materials. This problem can be counteracted by addition of a mild oxidant (2 g/l sodium m-nitrobenzenesulphonate). Reactive dyes are highly suitable for dyeing all cellulosic fibre types in circulatingliquor machines as loose stock, yarn packages or beams of woven fabric. The more thorough the access of dye liquor to the material, the better the levelness and penetration. Rapid dyeing cycles require uniform liquor circulation, which in turn depends on the efficiency of machine loading and the quality of winding of the package or beam. These conditions depend on winding tension and density of the goods, which must ensure uniform and consistent flow of liquor. If circulation is not entirely uniform there is a risk of unlevelness arising, especially at the critical stages of migration in salt solution and subsequent dosage of alkali. Where penetration problems are anticipated, as when dyeing certain high-twist or multiply mercerised cotton yarns, it is advisable to use low-reactivity dyes at temperatures above 70°C, withholding the alkali addition until at least 30 minutes after the final addition of salt. The flow rate in circulating-liquor equipment, the amount of liquor pumped through the material in unit time, depends on the resistance to flow of the substrate and the efficiency of the pump. The contact number, or rate of exchange of the liquor content of the goods, is one of the most important parameters in circulating-liquor dyeing. The higher the contact number, the better the conditions for level dyeing [106]. In short-liquor package dyeing (liquor ratio 4:1 to 6:1) the total liquor volume passes through the substrate mass much more frequently than in conventional equipment (10:1 to 15:1). Short-liquor processing offers the possibility of tolerating faster rates of heating and cooling. A short liquor ratio can be achieved by either lowering the liquor level in the vessel (sump dyeing) or filling the chamber as full as possible with yarn packages. In the sump method, the packages are not immersed completely in the dyebath and dye liquor can only be pumped in one direction (out-to-in). This has no adverse effect on the quality of the dyeings but packages must be correctly prepared and leakages avoided. This is achieved using large cylindrical packages wound on suitable tubes, fitted together and compressed into a continuous column in a hydraulic press [107]. In spite of the importance of pad-batch dyeing with reactive dyes (Table 7.17), jig machines are still used for smaller dyelots of woven cotton. They pose a special problem for reactive dyeing in that marked variations in temperature can exist between the dyebath and the exposed selvedges of the fabric batched onto the draw rollers. Such differences result in listed selvedges that are either paler or off-tone relative to the remainder of the dyed fabric. Ending faults can also be apparent because the draw roller (initially at ambient temperature) tends to abstract heat from the adjacent layers of fabric. The equilibrium uptake of dye is not greatly affected but the dye-cellulose reaction rate is appreciably retarded at lower temperature. The use of high-reactivity dyes with medium to high substantivity is the most effective solution to this problem. Dyebath temperatures of 30 to 40°C are sufficiently close to ambient conditions, resulting in negligible temperature differentials throughout the goods even under the most adverse conditions. Such dyes exhibit good build-up on the jig at operating liquor ratios in the range 3:1 to 6:1. Pad-jig development gives improved surface appearance and penetration with difficult fabrics. A high pick-up should be avoided to minimise the likelihood of seepage defects. A padded cloth may be passed directly to the jig, set with the
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full amount of salt, for immediate development. Alternatively, quality can be improved by holding on the turning roller for about 30 minutes before passage to the prepared jig. When reactive dyes were first introduced in the 1950s for dyeing knitted and lightweight woven fabrics suitable for processing in rope form, these goods were dyed at long liquor ratios in the range 20:1 to 30:1 on unsophisticated winch machines. These lacked positive liquor circulation or separate addition tanks and poor reproducibility of temperature and liquor ratio was unavoidable. Particular care was necessary to avoid abrasion damage, rope marks, crowsfoot creasing and running streaks. The long liquor ratio consumed large amounts of chemicals and energy for dyeing and washing-off, as well as producing considerable volumes of waste dye liquors. The discovery and rapid exploitation of the pressure jet dyeing machine for polyester fabrics and blends in the late 1960s led to the development of atmospheric jet machines for the reactive dyeing of cotton knitgoods in the 1970s. The powerful pump and venturi jet provided rapid and efficient liquor circulation and these machines were equipped with time-temperature controllers, a separate addition tank and mechanical features to minimise creasing problems. It was possible to operate at lower liquor ratios in the range 8:1 to 12:1 and to achieve improved fabric movement and level dyeing. These developments promoted liquor exchange and increased the contact number so effectively that differences in temperature and concentration within the system became negligible, making it possible to reduce the liquor ratio further and satisfy demands for a more cost-effective process. This reduction of liquor ratio brought problems, however, with manual addition of salt to control levelling and of alkali to bring about fixation [69]. There were restrictions on the addition of salt during running, so the salt-at-start technique became established. The development of metering technology then facilitated accurate control of the reaction at constant temperature. A direct result of short-liquor application is to enhance substantivity at a given salt concentration, or to attain the same substantivity with less salt. A liquor ratio of 5:1 is regarded as the effective minimum for conventional shortliquor dyeing. Dyeing at a liquor ratio as low as this can reduce the total cost of processing by 25% compared with a machine operating at a 10:1 liquor ratio [105]. Lowering of the liquor ratio is limited by the solubility of the dyes, the levelness attainable and the availability of liquor for dissolving and addition of chemicals. In fabric rope-dyeing equipment the decrease in liquor ratio has not stopped at the conventional limit of 5:1. As the development of the Then Airflow and Thies Rototherm machines demonstrates (section 14.6.2), liquor ratios as low as 3:1 are attainable by rotating the goods in a chamber that is no longer flooded with dye liquor, using continuous impregnation and recirculation of excess dye liquor from the sump below this chamber. This method is known as the ultra-low liquor ratio (ULLR) technique. The advantages and disadvantages of operating at low liquor ratio in ULLR dyeing machines are summarised in Table 7.21. A detailed study of the performance of five navy reactive dyes with different reactive systems under ULLR dyeing conditions established the controlling relationships between dye substantivity, salt concentration and liquor ratio [110]. Substantivity S (%) and salt concentration [NaCl] (g/l) are related by a linear equation of the form:
S = a[NaCl] + b
Equation 7.11
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For example, Procion Navy H-EXL (DyStar) obeys this equation when the two constants have the values a = 0.5 and b = 48, making it possible to predict the exact salt concentration required at 5:1 liquor ratio to attain the same substantivity value in the migration phase as at 10:1 liquor ratio with the normal salt recommendation. Liquor volume V (in litres for a 300 kg batch) and salt concentration [NaCl] (g/l) are also related linearly: [NaCl] = xV + y
Equation 7.12
For example, a dyeing of 2% Procion Navy H-EXL requires a salt concentration of only 39.3 g/l at 5:1 liquor ratio to maintain the same substantivity value as given with 60 g/l salt at 8:1 liquor ratio, the relevant values of the two constants being x = 0.023 and y = 4.8 in this case. Table 7.22 indicates the substantial reductions in salt concentration that become possible by lowering the liquor ratio from 10:1 in the case of 4% applied depths of the five dyes included in the evaluation. Delays in production of reactive-dyed cotton are often caused by inadequate services (water, steam, cooling water and drainage). The services to and from the dyehouse need to be designed so that, for example, a jet machine operating at 8:1 liquor ratio can be filled within one minute or less, can be heated or cooled at 8 to 10°C per minute and can be discharged to drain at high temperature [105]. An option now available is to fit the machine with a full-size charge tank capable of pumping hot water into the dyeing vessel at a rate to fulfil these requirements. Simply by improving the rate of filling the total process time can be cut by up to 45 minutes, representing a 10% improvement in productivity. Modern dyeing machinery almost always offers the option of the power drain. This design feature allows the vessel to be drained within one minute or less, but it demands that the surface drains are capable of discharging large volumes of water. In some cases it is advantageous to drain at high temperature (80°C or above), but the drains must be enclosed for safety reasons. Power drain technology contributes a 5% improvement in productivity. Designing the dyehouse infrastructure so that services can meet the demands of the dyeing machines makes a significant contribution to process predictability and productivity [105]. Considerable progress has been made in reducing the need for manual intervention in batchwise dyeing processes for reactive dyes by automating the marginal operations. Isothermal process cycles based on automated control, compared with traditional techniques involving portionwise additions of dyes and chemicals, have been devised. The dyes, salt and alkali may be added according to predetermined profiles over a given time period and additions are controlled from dyeing programmes stored in the microprocessor memory. This type of control contributes to the high degree of reproducibility essential for RFT production [32,111]. Multi-product injection (MPI) dosing systems enable an exact amount of product to be added to a process both over a specific period of time and in a selected manner, including intermittent, linear, progressive or exponential rates, by means of an electronically controlled system. Thus the dyer can control the dosage of dyes and chemicals as well as temperature and process time, thereby directly influencing the course of the chemical and physical reactions taking place in the dyeing system. Reproducible dyeings can thus be achieved, since dosing always takes place under identical conditions. Liquid and solid dosing equipment can be
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connected permanently to the dyeing machine, or may be utilised as a portable unit, servicing several machines [112]. In summary, the MPI dosing system offers the following advantages: 1. Comprehensive automation of the dyeing process, saving time and avoiding human errors 2. Ensuring that the necessary chemicals are always available at their prescribed concentrations 3. Increased control and reproducibility of the dyeing process 4. Improved levelness of the resulting dyeing 5. Reduced risk of dye aggregation or precipitation during short-liquor dyeing of full-depth colours 6. More efficient rinsing and washing-off, since there is less hydrolysis of the dyes during the process. Methods of control of parameters relating to the physical characteristics of the material to be dyed, taking into account the influence of mechanical and hydraulic forces exerted by the dyeing machine during processing, have been introduced [113]. Process control devices now allow the action of these forces to be adjusted for each stage of the dyeing process. Thus the total energy transmitted to the substrate can be reduced, with a consequent improvement in surface appearance because there is less fibre disturbance and abrasion. In the processing of yarn packages, mechanical and hydraulic influences may impair the physical properties of the yarn. In order to preserve the quality of the yarn, the pump delivery should be reduced to avoid undue turbulence [114]. The consequent risk of unlevel dyeing makes adoption of MPI dosing essential. Only by the controlled addition of dyes and chemicals at a lower liquor throughput can satisfactory dyeing be achieved. In some instances the turbulence and frictional forces in a jet dyeing machine can adversely affect the surface appearance of the fabric. This reduction in quality can be avoided by reducing the speed of movement of the fabric rope and the rate of liquor flow through the jet nozzle. In doing this there is a risk of unlevel uptake. In order to apply a synchronised dyeing system under jet dyeing conditions [115], it is necessary to determine the kinetics of the dyeing process and to understand the interactions with fundamental parameters of machine operation: 1. circulation pump (variable flow) 2. transporting reel (variable speed control) 3. multi-product injection system 4. fully automatic microprocessor control. Machine controllers have moved away from simple time-temperature devices to sophisticated multifunctional and multitasking units. All assignable variables can be measured and controlled on-line, as well as the machine operating parameters such as lifter reel speed, jet pressure, jet aperture setting and rope circulation time. The multitasking capability provides interesting options for application profile design that increase productivity without threatening RFT repeatability. Simultaneous adjustments can be made to the matrix of operating parameters so that the application profile can be closely linked to the contact number, the frequency of passage of the rope through the jet venturi [105].
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7.5.4 Continuous Dyeing
The lower limit of liquor ratio attainable in batchwise dyeing is about 5:1 or, in the case of ULLR equipment, possibly 3:1. Padding methods extend this limit further to the region of 1:1 to 1:2. Thus the advantages of enhanced exhaustion and fixation characteristic of short-liquor exhaust dyeing can be significantly improved by adopting semi- or fully-continuous processes. Slow fixation in padbatch dyeing is of particular interest to achieve controlled diffusion, penetration and levelness on certain types of substrate, whereas rapid fixation by pad-steam is of greater relevance for optimal economy and productivity. The pad-batch process offers a means of producing runs of moderate length per colourway at low capital cost. In effect, this process is an exhaust method at extremely low liquor ratio and ambient temperature in which all the dye liquor is entrained within the substrate. The impregnated batch is stored separately from the padding equipment and this allows wide variation of dwell time according to the reactivity of the dyes selected and the pH of the impregnated fabric. All pad-batch processes follow the general sequence: 1. impregnation of the uniformly absorbent dry fabric in a solution of the dyes and alkali at ambient temperature, 2. uniform squeezing of surplus liquor from the fabric as it passes through the padding mangle, 3. wrapping of the batched roll of impregnated fabric in polythene film and continuous rotation at ambient temperature for a specified dwell time between 2 and 24 hours, depending on dye reactivity and pH, 4. washing-off of unfixed dye, 5. drying of the washed dyeing. The success of this technique is attributable to the low capital cost of the equipment, low consumption of water and energy, excellent reproducibility and scope for selective control of the rates of dye fixation and hydrolysis. Dyelots in the range 1000 to 10,000 metres per colourway, excessive for exhaust dyeing but inadequate to justify investment in fully-continuous installation, can be processed economically. With high-reactivity dyes it is essential to employ a liquor feeding device, to bring dye and alkali streams together immediately before the mixed pad liquor contacts the fabric to be dyed. The initial approach was a pneumatically operated stainless-steel tipping bucket into which solutions of dye and alkali were metered separately and mixed en route to the padding trough. Various designs of electrically operated dual metering pumps and valves are now available, whereby the solutions are mixed and delivered to the trough at a known rate of flow [116]. When dyeing viscose or modal fibres, their different swelling behaviour compared with cotton must be taken into account. On such fabrics and certain qualities of mercerised cotton, dye penetration and surface appearance can be improved significantly by using a slower fixation treatment. Fabrics dyed by this method have a smoother surface than jet-dyed goods and this gives them enhanced lustre. Corduroy fabrics are ideally suitable for pad-batch dyeing [117]. Tubularknitted cellulosic fabrics can be dyed using specialised impregnation units, although the capital cost of open-width washing ranges has retarded adoption of these techniques in the knitgoods field [118]. The problem of folded edge marking and measures to overcome this have been outlined [119].
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With skilled organisation and management, fully-continuous reactive dyeing methods offer economic advantages when long runs are required in a limited range of colours. Listing and ending are avoidable and excellent reproducibility is possible, with notable savings in handling and labour costs. The minimum length to each colour on a fully-continuous range is traditionally estimated at about 10,000 metres, depending on local circumstances and avoidance of excessive downtime. The dye-fibre reaction takes place extremely rapidly in the presence of minimal amounts of concentrated alkali at elevated temperature. With highly reactive dyes the dwell times are sufficiently brief to permit fixation simply by passing the impregnated fabric through a conventional dryer. The first pad-dry process originally developed for applying dichlorotriazine dyes together with sodium bicarbonate is still important for the high-reactivity ranges. Urea is added to enhance dye solubility in full depths, as well as salt or sodium alginate to minimise migration problems during drying. Urea is widely used in continuous dyeing and printing with reactive dyes as a dissolving assistant, a disaggregating agent, a swelling reagent for cellulose and a hygroscopic auxiliary. It is difficult to specify which of these effects is the most important. Nevertheless, it certainly enhances colour yield, accelerates diffusion and improves levelling, especially on viscose fabrics. Although cheap and readily available, urea is not an ideal chemical from the environmental viewpoint and more costly alternatives have been evaluated. Many viscose fabrics show excessive migration and poor diffusion of the dyes, evident as inadequate penetration of colour. These materials can be handled far more satisfactorily by the pad-batch route. Pad-steam application of low-reactivity dyes normally follows the sequence: pad (neutral dye solution)-dry-pad (caustic soda in brine)-steam-wash-dry. After intermediate drying in a hot flue, highly efficient absorption of the alkali/salt pad liquor is achieved. Where small-capacity troughs are fitted, few colour-bleed problems are encountered. Vacuum extraction of the padded fabric is a viable alternative to intermediate drying. It restricts bleeding of the dyes into the chemical pad liquor and the wet vacuumed fabric retains sufficient alkali-salt liquor to give satisfactory fixation at the steaming stage [120]. Several designs of roller-bed steamer with a cold water exit seal are suitable. Each commercial range of reactive dyes requires an appropriate set of steaming conditions. Selected vinylsulphone dyes are suitable for the pad-wet fixation sequence: pad (neutral dye solution)-dry-impregnate (alkali/salt)-wash-dry. This is of particular interest where a steamer is unavailable or fully occupied for other requirements. Caustic soda in sodium silicate solution is usually preferred as the chemical development bath, typically for 5 to 15 seconds at 95 to 100°C. A specially designed wet-fixation trough, with indirect heating to avoid dilution via condensation, is necessary for the boiling alkali/salt treatment. A one-bath paddry-steam route was developed for these dyes, requiring air-free steaming at 100 to 102°C for only one minute to minimise migration and avoid problems associated with chemical pad application [121]. 7.5.5 Washing-off and Aftertreatment
Effective washing after reactive dyeing is crucially important. At this stage the substrate contains residual hydrolysed dyes and possibly some active dyes. The dyeing cannot show satisfactory wet fastness until this loose colour is removed or rendered insignificant in amount. It is remarkable that only about 0.003% by mass of reactive dye on cellulose can produce a visible stain equivalent to a grey scale rating of 4.
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All washing processes are essentially techniques for achieving progressive dilution, but the effect of substantivity forces on the rate of removal must always be borne in mind. For example, if an exhaust dyeing of a low-reactivity dye is carried out at a temperature between 80°C and the boil, an initial rinse at ambient temperature will be inefficient because the dye is more substantive at 20°C than at the dyeing temperature. Hot rinsing at 60 to 80°C is recommended for optimal desorption of bis-monochlorotriazine dyes from cotton yarn dyed at 80°C [122]. An important function of the initial rinsing steps in a batchwise washing sequence is to lower the total electrolyte concentration by progressive dilution and thus to lower the substantivity of the residual unfixed dyes. This dilution, as well as an increase in temperature above 60°C for the first washing step, greatly enhances the rates of desorption of the dyes into the washing bath. Reducing the carryover of entrained liquor between successive steps of the sequence is another highly effective means of increasing washing efficiency. Washing efficiency is often checked using a small sample taken from the dyed material before unloading the machine. This is either hydro-extracted or passed through a mangle nip to remove the excess of entrained wash liquor, placed between two layers of white absorbent cotton fabric and pressed with a hot iron. The stain on the interior surfaces of the white adjacents is an approximate indication of the degree of staining likely to arise in wet fastness tests on the finished dyeing. The decision whether further washing is needed to meet target quality depends on this result. Reasons for poor wet rubbing fastness of reactive dyeings on cotton have been investigated. For fabrics that have been correctly washed-off, the major cause of unsatisfactory fastness is removal by abrasion of small particles of dyed fibre. Wet rubbing fastness is almost always inferior to dry rubbing because the outermost layers of exposed fibres are swollen by wash liquor and become even more sensitive to the shearing forces [123]. The Thermoflush technique was developed in the 1980s by Bayer AG and Thies GmbH for the intensive washing-off of reactive dyeings. Once the dyebath has been drained, water is injected without circulation to overflow rinse. A steam treatment then causes unfixed dyes to migrate to the hot surface of the material. After repeating these water/steam injection cycles several times, virtually all of the hydrolysed dyes have diffused out of the fibres and have been rinsed off immediately [124,125]. Washing-off process time can be saved using the Thies CCR (combined cooling and rinsing) system, in which the cooled water passing through the heat exchanger is fed by the main liquor pump into the process liquor circulation via the overflow nozzle, simultaneously rinsing and cooling with an overflow to the drain. This combined rinse/cool principle can be used when cooling from a hot alkaline fixation step or from a soaping treatment at the boil [32]. A further development from Thies GmbH is programmable COR (controlled overflow rinsing), in which the rinse liquor is preheated in a tank at 85°C and then circulated via the main liquor pump through the heat exchanger (further boosting the temperature) to the overflow nozzle at a rate of approximately 200 litres/minute per nozzle. Savings of up to 60 minutes per batch are achievable [105]. The combination of Thies CCR and COR has been termed Smart Rinsing [126]. The washing-off profile based on Smart Rinsing principles for a Then AFS jet machine is illustrated in Figure 7.11.
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A new system for washing-off reactive dyeings and prints has been described recently. An enzyme is used to selectively decolorise the hydrolysed dye desorbed from the dyed or printed fabric and the residual dye present in the wash liquor. After this enzyme treatment the rinsing baths are virtually colourless and there is no intact dye present to be re-adsorbed by the coloured fibres. However, this process is not suitable for all reactive dyes [127]. 7.5.6 Laundering with Activated Peroxide Detergents
It is estimated that more than 70% of domestic washing of textiles is carried out at 50°C or lower temperatures. In the USA, washing at 35 to 40°C and much longer liquor ratios than in Europe is quite common. This trend to lower washing temperatures has developed to save energy and minimise colour loss from leisurewear and sportswear garments that need frequent laundering. The hydrogen peroxide generated from the sodium perborate present in most detergent formulations is most effective for stain removal only at 70°C or higher temperatures, so a bleach activator is necessary to ensure efficient performance at lower temperatures. The activator most commonly used in Europe is tetra-acetylethylenediamine (TAED). This reacts with hydrogen peroxide to form peracetic acid (CH3CO.O.OH), the effective bleaching agent at low temperatures [128,129]. The milder washing conditions in the USA (lower temperature, briefer dwell time, longer liquor ratio) necessitate the selection of a more substantive bleaching activator, sodium pnonanoyloxybenzenesulphonate (SNOBS). In the presence of hydrogen peroxide this generates pernonanoic acid (C8H17CO.O.OH). In view of this difference in degree of peroxide activation, many reactive dyes regarded as sensitive to oxidative bleaching under European conditions may be assessed as satisfactory in American laundering tests [130]. The mechanism of oxidative attack on reactive dyeings by these activated peroxy species is two-fold. Chromogens containing unprotected azo groups are highly vulnerable but structures in which each azo group is protected from attack by the presence of an ortho-sulpho substituent generally show satisfactory fastness [131]. However, certain reactive systems (TCP, DFP, DCQ) are sensitive to peroxidic attack of the heterocyclic ring in the dye-fibre bond grouping (section 7.5.1), so this can also be a cause of inadequate fastness [132]. It has been recognised for many years that dyed or printed cellulosic garments or household textiles, representing approximately 80% of the total domestic washing load, undergo significant shade changes after only about five domestic washing cycles. The established wash fastness tests such as the ISO 105 C06 series do not correlate with the degree of colour change observed in practice. These test procedures were designed to examine the effects of washing with standard detergent and perborate at various temperatures but do not reflect the more severe combination of perborate and a bleach activator. Accordingly, the BS 1006 105 UK-TO washing test was introduced in the late 1990s. This involves treatment for 30 minutes at 60°C and 100:1 liquor ratio with standard detergent (10 g/l), perborate (12 g/l) and TAED (1.8 g/l). The results of this test have been correlated with the effects of 25 domestic washing cycles using a commercial domestic washing powder. The UK-TO procedure has been fully adopted by major retailers such as BhS and Marks & Spencer. It has been ratified by ISO in 2001 as the ISO 105 C09 standard in Parts A (TAED) and B
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(SNOBS). The AATCC 190-2001 (SNOBS) test reflects the domestic washing conditions prevailing in the US market [83]. 7.5.7 Reactive Dyeing Residues in Waste Liquors
Approximately half the cost of a typical reactive dyeing may be attributed to the washing stage and treatment of the resulting effluent. The contribution of hydrolysed dyes to the overall reactive dyeing process is most evident during washing-off and in contamination of dyehouse effluent. With dyeings of highly substantive dyes, boiling water is not fully effective in removing all of the hydrolysed dyes present. If dye concentration and COD value are determined for each bath after washing, it may be advantageous to separate the waste waters into high- and low-load reservoirs for treatment [133]. Reactive dye hydrolysates are not easily adsorbed by sewage sludge in a biological clarification plant. It is desirable to decolorise the liquor at source if practicable. A post-scouring step designed specifically to remove hydrolysed dyes as effectively as possible offers scope for concentrating the waste dye liquor into as small a volume as practicable, simplifying the treatment of low-load effluent from rinsing [134,135]. Apart from contamination by hydrolysed reactive dyes, probably the most serious ecological problem arising from reactive dyeing is the high salt load in the effluent. Dispensing salt in solid form without significant change in liquor ratio offers wider scope for optimising quality and cost-effectiveness in the package dyeing of yarns and the jet dyeing of fabrics. Improved dye yield and levelness, shorter process time, lower salt usage and environmental benefits are claimed for this technique. An exceptionally fine grade of Glauber’s salt crystals is selected for this application. The salt is transmitted via a screw device from a storage tank outside the dyehouse to tanks positioned above the dispensing vessels, from where it is fed to the dyebath at a controlled rate. In this way the transfer problems associated with large volumes of electrolyte solutions are also solved. The size of the storage tank, fitted with a special heating unit, depends on dyehouse production. It is periodically refilled from a salt delivery vehicle. The heating unit prevents caking of the salt crystals by condensation of entrapped moisture, since this complicates the transfer and controlled dosing procedures [106]. A more fundamental approach to the objective of minimising contamination of reactive dyeing effluents by excessive salt content is to adjust the dyeing parameters so that only the lowest possible concentration needs to be added at the dyeing stage [136]. There are four main ways in which the salt consumption can be decreased [69]: 1. lowering the liquor ratio, 2. avoiding high dyeing temperatures, 3. optimising the dye recipe, 4. developing novel dyes of high substantivity. The design of modern batchwise dyeing equipment takes account of this need to operate at as low a liquor ratio as possible, since this brings many other important technical and economic benefits. The choice of low dyeing temperatures implies a preference for dyes of high reactivity, although these may not be ideal when dyeing substrates that are difficult to penetrate or to dye level. Optimising recipes offers some scope to lower the salt content and the development of homogeneous dull-hued reactive dyes with high intrinsic
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substantivity to replace trichromatic combinations of bright components is another interesting approach [69].
7.6 Direct Dyes Direct dyes are defined as anionic dyes with substantivity for cellulosic fibres, normally applied from an aqueous dyebath in the presence of salt. The simplicity of the direct dyeing process is attractive, but most direct dyeings require a separate aftertreatment to achieve commercially acceptable wet fastness. The two most significant non-textile outlets for direct dyes are the batchwise dyeing of leather and the continuous coloration of paper. The standard of fastness to washing of direct-dyed cotton, even when aftertreated, does not meet the requirements of more demanding end-uses. Only about 0.0015% owf of C.I. Direct Red 80 on cotton is visually equal to a rating of 4 on the SDC Grey Scale for staining of adjacent fabric [137]. Consequently, direct dyes have been replaced to a great extent by reactive dyes, which have better wet fastness and exceptional brightness. Nevertheless, there are still applications in the textile industry for goods dyed with cheap direct dyes where high wet fastness is not essential. Resin finishing after direct dyeing produces a marked improvement in wet fastness, especially on regenerated cellulosic fabrics. During the 1980s, specialised aftertreating agents and crosslinking reactants were developed for use with selected direct dyes of high light fastness, enabling these dyes to compete more effectively with reactive dyes in meeting severe wet fastness requirements [138]. Many direct dyes of former importance made from benzidine were withdrawn from manufacture on account of the occurrence of carcinoma of the bladder in operatives after prolonged exposure to this chemical. Epidemiological studies carried out in the early 1950s revealed this increased incidence of bladder cancer, but it was not until the 1970s that the major dyemakers agreed to phase out the manufacture of benzidine-derived dyes. An intensive search was undertaken into alternatives produced from less hazardous intermediates [139]. A more general objective has been to identify and utilise precursor intermediates that have been shown to be non-genotoxic. Increasing use is being made of the published results of the mutagenicity and carcinogenicity testing of azo dyes and their intermediates. It is clearly helpful in the design of non-carcinogenic azo dyes to ensure that the intermediates are non-mutagenic and to take into account the potential genotoxicity of the metabolites resulting from the reductive cleavage of azo linkages. Certain direct dyes, primarily sulphonated copper phthalocyanines, exhibit a photochromic change on cellulosic fibres, the hue changing from bright turquoise to violet or reddish blue on prolonged exposure to sunlight or UV radiation. The hue reverts gradually to the original turquoise when the illuminant is withdrawn. This effect is accentuated by the presence of dye-fixing agents or crease-resist finishes, particularly if the dyed material is stored in an acidic condition. The degree of photochromic change is related to the intensity of the UV radiation and the ambient humidity. The drier the atmosphere, the slower the reversion to the original turquoise. Direct dyes exist in aqueous solution as aggregates of several anions or molecules rather than as individual molecules. It is not necessary to assume that all component molecules in a dye aggregate are in the same state of ionisation. Dye
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aggregates should be envisaged as amorphous in composition with zones of more or less polar character distributed within them, although there will still be a tendency for individual anions to become oriented with their ionised groupings towards the aqueous phase. Dimers are formed before larger aggregates, growing further by accretion of more dye anions to form lamellar micelles in which the dye units are stacked like cards in a pack [140]. Planar chromogens, especially the sulphonated phthalocyanines, are particularly prone to such stacking. Most measurements of aggregation number (the average number of molecules or anions per aggregate) have been carried out on solutions of direct dyes, since these dyes aggregate more readily than most of the other classes of watersoluble dyes. Regrettably, aggregation numbers determined for the same dye under the same conditions using different methods of measurement are seldom consistent. Most investigators agree, however, that such solutions contain a mixture of aggregates of various sizes in dynamic equilibrium. Individual anions, dimers or trimers are removed by adsorption onto the fibre surface during dyeing and larger aggregates break down to maintain a similar overall distribution of aggregate sizes. The extent of aggregation of a direct dye decreases with increasing temperature and increases with increasing concentration of salt or of the dye itself. Although many direct dyes are highly aggregated at ambient temperature, the degree of aggregation is often negligible under normal dyeing conditions at the boil even in the presence of salt. 7.6.1 Dyeing Properties
Direct dyes are usually applied at or near the boil with the addition of salt. Much less often, dyeing may be carried out at temperatures above the boil, as in the package dyeing of polyester/cellulosic yarns. When cellulose is immersed in a direct dye solution, dye is absorbed until an equilibrium is reached with most of the dye in the fibre phase. Measurements of the absorption spectra of direct dyes on cellulose film have shown that dyes enter the cellulose as single anions which then form aggregates within the voids of the substrate. The variations in behaviour between individual direct dyes necessitate care in selection for mixture recipes, in order to achieve optimum compatibility and prevent the occurrence of faults such as unlevel or insufficiently penetrated dyeings on all types of material and listing or ending on jig-dyed fabrics. The determination of four dyeing parameters is necessary to assess compatibility: migration or levelling power, salt controllability and the influence of temperature and liquor ratio on dyebath exhaustion. Selection of direct dyes for the exhaust dyeing of cellulosic fibres has been aided by their ABC classification: Class A Self-levelling: dyes with inherently good migration or levelling behaviour Class B Salt-controllable: dyes that are not self-levelling but can be controlled using appropriate additions of salt Class C Temperature-controllable: dyes that are highly sensitive to salt and require additional control by appropriate temperature changes. Control of initial strike and fibre penetration can be improved by raising the dyeing temperature. Pressure dyeing above the boil offers the advantage of shortening the time required at top temperature and yielding improved levelling and penetration, which are important when dyeing yarn packages. Dyebath exhaustion gradually decreases with increasing liquor ratio but other factors such as dye solubility, levelling properties and initial strike have to be taken into
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account. The extent to which direct dyes are affected by the electrolyte is known as salt sensitivity. Different dyes vary considerably with respect to this property. The presence of salt markedly increases the initial strike rate. The dyeing behaviour of a broad selection of direct dyes was examined in the presence of various relatively low concentrations of Glauber’s salt at different dyeing temperatures and liquor ratios. Commercially satisfactory levels of dyebath exhaustion could be achieved with almost all of the dyes examined using minimal salt concentrations (2 to 5 g/l, even at applied depths up to 2 or 3% dye). Dyeing at the lowest practicable liquor ratio should be an integral factor in any programme to minimise salt contamination of dyehouse effluent. When salt discharge into the effluent is a critical issue direct dyes offer an advantage over reactive dyes, which generally require 10 to 15 times as much salt to achieve high exhaustion levels [141]. 7.6.2 Batchwise and Continuous Dyeing Methods
Direct dyes are dissolved by pasting with cold water, then stirring the paste into boiling water. To ensure that dissolution is complete, the solution should be sieved before addition to the dyebath. Pastel hues are preferably dyed without salt. There is increasing interest in the controlled dosage of salt crystals in solid form for the direct dyeing of cotton yarns and fabrics. The effects on levelness in package dyeing of variations in salt dosage, process and machine design factors have been examined. Electrical resistance measurements were used to monitor salt concentrations in the dyebath [142]. Many direct dyes are suitable for application by combined scouring and dyeing of either knitted fabrics on jets or woven fabrics on jigs. The usual practice is to add soda ash and a nonionic detergent to provide the scouring action. However, dyes containing amide groups should be avoided because of the risk of hue change as a result of alkaline hydrolysis. Combined peroxide bleaching and dyeing with selected direct dyes is another long-established process. There are potential savings of process time and energy but more care is necessary to ensure satisfactory levelness and reproducibility. The essential factors are thorough and effective preparation, as well as selection of suitable dyes. The process is of special interest for dyeing pastel colours and Class A or B dyes are preferred to attain satisfactory levelling. Sodium carbonate is preferred to caustic soda because the risk of oxidative degradation of the dyes is greater at higher pH. An organic stabiliser for the peroxide is preferred to sodium silicate, which tends to cause harshness of handle on terry towelling, for example. Direct dyes are much less suitable for continuous processing than for batchwise dyeing. Tailing is a serious problem in pad application and prolonged diffusion is necessary in pad-batch or pad-steam methods. As far as possible, close attention should be given to selecting dyes with similar absorption behaviour and to controlling the rate of supply of feed liquors. Recommendations have been given with regard to control of fabric moisture content and pad liquor temperature, dye selection for optimum compatibility and the use of appropriate auxiliaries [143]. Padding assistants that facilitate the padding of cellulosic fabrics with direct dyes without tailing problems have been developed. A method for determining in advance the exchange factor in the pad dyeing of cotton and viscose fabrics with direct dyes has been described [144].
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7.6.3 Aftertreatment of Direct Dyeings
The wet fastness properties (particularly washing, water and perspiration) of virtually all direct dyeings are inadequate for many end-uses but notable improvements can be brought about by aftertreatments. All such treatments, however, incur increased processing costs because of the extra time, energy, labour and chemicals involved. Many long-established direct dyes containing primary amino groups could be diazotised and coupled on the fibre with a variety of developers to give larger molecules with improved wet fastness. A change in hue usually occurred, depending on the developer selected, and frequently the light fastness was impaired. The complexity of this approach and the associated problems of hue change and light fading have rendered this technique virtually obsolete. The aftertreatment of dyeings with metal salts to confer improved fastness was exploited by dyers long before direct dyes were discovered. Subsequent washing or alkaline treatment tended to decompose the metal complex, however, so that the light fastness reverted to the original rating. Many dyes of this type are pHsensitive before metallisation and this necessitates care in handling. Control of application is essential, since once complex formation has taken place unlevel results can only be rectified by stripping. Cationic fixing agents interact with the sulphonate groups present in direct dyes, conferring enhanced wet fastness in all tests at temperatures below 60°C. Hue changes may occur and, in some cases, light fastness may be reduced. The dyeauxiliary complex usually dissociates in hot detergent solution above 60°C. Efficiency indices have been devised to assist users of fixing agents to evaluate cost-effectiveness in improving wet fastness, but also taking account of their adverse effects on hue and light fastness [145]. Fastness to wet rubbing can be a serious problem in full-depth dyeings aftertreated with a cationic agent. Mechanical destruction of fibres on the surface of the dyed material produces microscopically small fragments that stain the adjacent white fabric. 7.6.4 Effect of Finishing on Direct-Dyed Fabrics
Improvements in wet fastness can be ensured by treatment of direct dyeings with cellulose reactants or amide-formaldehyde resins. In the case of wash fastness, resin-treated direct dyeings will withstand washing at temperatures below 60°C, even in full-depth dyeings. This notable improvement in wet fastness on resin finishing means that direct dyes can be used for end-uses in which they would be quite unsuitable without this treatment. Subsequent removal of the resin by acid hydrolysis, however, leaves the unfixed direct dyes on the fibre with their originally low ratings of wet fastness. Treatment with N-methylol crosslinking agents in resin finishing improves the wet fastness, but hue and light fastness may be adversely affected, the effect in both cases varying with the individual direct dye and the crosslinking system. Studies of the relationship between dye structure and light fastness after resin treatment have shown that OH and NH2 groups are normally the most sensitive, although their positions in the dye molecule play an important role. Unbound formaldehyde together with the inorganic catalyst reduce the light fastness of azo dyes with an NH2 group located ortho to the azo group. The higher the residual amount of unbound formaldehyde on the finished fabric, the greater the adverse effect on light fastness. Magnesium chloride and zinc nitrate are used as catalysts in resin finishing, with curing conditions showing variations in respect of humidity
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and temperature. Zinc nitrate has a greater effect on both hue and light fastness than magnesium chloride, but metal salt catalysts show less influence on light fastness than ammonium salts. The effects of other durable finishes on the hue and fastness properties of direct dyeings depend on the chemicals and treatment conditions applied, as well as on the individual dyes. No significant impact on hue or light fastness of direct dyes has been reported with either fluorocarbon stain-repellent finishes or the acrylate resins often used in soil-release finishing. Amongst compounds that have been used for water-repellent finishing, only those containing melamine derivatives adversely affect either the hue or the fastness of direct dyeings. The variations in the UV transmittance spectra of samples of a lightweight cotton fabric dyed with a representative selection of direct dyes have been examined. The objective was to determine whether the UV protection factors of such fabrics can be enhanced by the judicious selection and application of dyes in full depths. The results indicated that black dyes do not necessarily provide the best protective effect. Other colours may also significantly improve UV protection, depending on their absorption characteristics in the UV region [145]. 7.6.5 Direct Dyeing Residues in Waste Liquors
Although direct dyes are particularly easy to apply, the presence of chelated copper in many structures of above average fastness to light and the frequent need to apply cationic aftertreating agents means that care must be taken to minimise difficulties in effluent treatment. Trace metals can affect the performance of many dyes, not least the copper-complex directs. Although sequestering agents are used to minimise the influence of alkaline-earth or transition-metal ions on unmetallised dyes, they obviously exert undesirable effects on the colour and fastness of metal-complex dyes. The rising costs of water procurement and effluent disposal are forcing dyers and finishers to re-examine the potential for reuse of dyebaths and other process liquors. Equations have been derived to show how the salt concentration builds up during repeated use of direct dyebaths. They can be used to calculate the amounts to be added after each dyeing or the amount of the bath to be discharged. Studies of the sorption of anionic dyes from dyehouse wastes by activated carbon demonstrated that the saturation adsorption of direct dyes increased with: 1. increasing relative molecular mass of the dye, 2. salt concentration in the waste water, 3. integral pore volume of the activated carbon. Methods of measuring the colour of waste dye liquors and minimising the contribution of dyeing and printing processes to this problem have been reviewed. A survey found that reactive red dyes and sulphur blacks caused most difficulty in this regard, but direct dyes can be readily removed by adsorption or precipitation [146].
7.7 Vat Dyes Dyeing with vat dyes is based on the principle of converting a water-insoluble keto-substituted colorant 7.16 by reduction to the water-soluble sodium enolate
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leuco form 7.17 that is substantive to cellulose. This penetrates into the fibre, where it is reoxidised back to the original insoluble form. All vat dyes contain two or more keto (C=O) groups, separated by a conjugated system of double bonds. The leuco potential of all vat dyes lies between –650 and –1000 mV; thus satisfactory vatting (reduction to the enolate leuco form) can only be achieved with a reducing agent that has a more electronegative reduction potential than this. After it has been synthesised, a vat dye is not yet in a suitable state for commercial use, because reduction to the leuco form is normally an extremely slow process at this stage. It must first be converted into a suitable commercial form, usually by a wet milling process with appropriate dispersing agents. The demands placed on vat colorant formulations have increased considerably, particularly for batchwise pigmentation and semi-pigmentation processes in package dyeing and for continuous piece dyeing, where a high standard of control of the particle size distribution is necessary. The top-quality dyes available generally have an average particle size of well below 1 µm. Apart from particle size, other important properties of all commercial formulations include their storage stability, behaviour during preparation of the dye liquors and the pigmentation process, and their rates of vatting. The vatting rate is determined by the particle size distribution and the crystalline form of the dye. The most important reducing agent in vat dyeing is sodium dithionite, generally referred to as hydrosulphite or hydros. It has a reduction potential that is sufficiently negative for all practical purposes. More stable derivatives, the hydroxyalkane sulphinates, are widely used in vat printing and for dyeing at temperatures above 100°C. Various other reducing systems (section 2.9) have been evaluated in this way for vat dyeing but the cost-effectiveness of sodium dithionite ensures that it remains of primary importance. Since sodium dithionite is sensitive to atmospheric oxygen, an excess of dithionite must always be present. The amount of this excess depends on the application conditions, particularly on the influence of certain factors governing the rate of oxidation: 1. Temperature: under otherwise constant conditions, the rate of oxidation increases with liquor temperature. 2. Dithionite concentration: under otherwise constant conditions, a specific amount of dithionite is oxidised in a given time, so that a more concentrated solution takes longer to become deactivated. 3. Movement of liquor and air: the greater the agitation of the liquor in the presence of atmospheric oxygen, the more rapid the rate of decomposition. 4. Surface area of the liquor: the greater the specific surface area (the ratio of surface area to volume), the more rapidly is the dithionite oxidised. 5. Leuco dyes: under otherwise constant conditions, the reducing agent is oxidised more rapidly in the presence of leuco dyes. Before dyeing the water-insoluble vat colorant must be converted into the watersoluble substantive form. This is achieved by vatting for 5 to 10 minutes with a reducing agent in the presence of caustic soda to form the sodium enolate leuco compound. This reaction takes place in a heterogeneous system, the dissolved reducing agent attacking the finely dispersed colorant particles. The rate of vatting depends on various factors: 1. The rate of vatting approximately doubles for each 10°C rise in temperature.
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2. Above pH 12 the rate of vatting is independent of the hydroxide ion concentration. 3. The higher the concentrations of vat colorant and reducing agent, the more rapidly the reduction proceeds. 4. The rate of vatting depends on the crystalline form and particle size of the vat colorant. The leuco dyes are usually present as the monomolecular anion or as aggregates of a few planar dye anions in the dye liquor. Absorption spectrophotometric studies have indicated that most of these leuco compounds are present mainly as single anions or dimers, depending on concentration and temperature [147,148]. This low degree of aggregation does not significantly influence substantivity but can affect the rate of diffusion within the fibre and the levelling behaviour. With some leuco dyes, chemical changes may take place in the alkaline medium, such as hydrolysis of carbonamide groups, dehalogenation or over-reduction of the indanthrone blues. The tendency towards over-reduction increases with the concentrations of caustic soda and sodium dithionite, as well as the temperature and duration of the dyeing process. 7.7.1 Dyeing Properties
It is characteristic of vat dyes that they exhaust rapidly, even at relatively low temperatures. The time of half-dyeing (time to reach 50% of the equilibrium exhaustion) is typically only 2 to 3 minutes and most of the dye (generally 80 to 90%) exhausts within about 10 minutes. The final approach to equilibrium is much slower, however, as it is governed mainly by the rate of diffusion into the interior of the fibre. The initial rate of exhaustion is virtually independent of the rate of circulation but is influenced considerably by the type of textile material. The greater the surface area for adsorption, the more rapid the initial strike. Rapid strike is favoured by a low dye concentration, a short liquor ratio and a high dyeing temperature [148]. In practice, the quicker the initial rate of uptake the greater the risk of obtaining an unlevel dyeing. The rate of penetration of the individual fibres is determined by the rate of diffusion of the leuco dye from the fibre surface into the interior. Diffusion behaviour can be characterised by an empirical diffusion factor. Dyes with good levelling properties have a diffusion factor between six and eleven, whereas those with only limited levelling capacity give values of four or less [148]. The rate of diffusion is markedly dependent on dyeing temperature, rising exponentially with increasing temperature. In the absence of a levelling agent, the behaviour of leuco vat dyes is mainly determined by the initial rapid strike and the attendant risk of unlevel dyeings. This problem is particularly critical with pale dyeings at low liquor ratios on mercerised cotton or regenerated cellulosic fibres. The addition of a nonionic levelling agent shifts the initial distribution of the dyes in favour of the dyebath, retarding the rate of absorption and promoting the migration of dye that has already been absorbed. These levelling agents are typically fatty alcohol polyoxyethylene adducts capable of forming associated complexes with the leuco dye anions [147,148]. When exhaustion is complete, the dyeing is rinsed to remove loose dye and most of the residual alkali and reducing agent. The leuco dye is then reoxidised to the insoluble vat colorant using hydrogen peroxide or sodium perborate. There can be
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problems with the indanthrone blues due to over-oxidation, caused especially by atmospheric oxygen in the presence of a high alkali concentration. The higher the temperature and the lower the alkaline pH, the quicker the rate of reoxidation. After oxidation the dyeings are treated at the boil in an aqueous solution of an anionic detergent, generally referred to as ‘soaping’. This process is a crucial one; loose dye is removed and the essential characteristics of the vat dyeing are developed. Fastness to light and washing is usually enhanced and there may be a tonal change, particularly with dyeings on mercerised cotton or regenerated cellulosic fibres. Two factors are important in practice: 1. The efficiency of the soaping process is determined entirely by the temperature and time of treatment; it is not influenced by pH. 2. Dyes showing only a slight tonal change are particularly suitable in continuous dyeing, where only a brief dwell time is practicable. Vat dyeings on cellulosic fibres exhibit an overall standard of fastness superior to that attainable using other classes of dye. This applies to factors important in textile preparation, such as soda boiling, oxidative bleaching and mercerisation, as well as end-use requirements such as fastness to light, washing and weathering. In view of their exceptional fastness to cotton preparation processes, vat dyes are ideal for coloured woven goods such as shirting, handkerchiefs, tablecloths and towelling. The light fastness is excellent with dyes having a multinuclear ring system with integral NH groups. However, certain combinations of a yellow with a blue or green dye may show anomalous fading on exposure to light or weathering, resulting in the blue or green component fading more rapidly than anticipated. This phenomenon is commonly referred to as catalytic fading and is influenced by: 1. dye selection, 2. applied depth and proportions of the component dyes present, 3. the UV content of the incident radiation, 4. the nature of the textile material, 5. the moisture content of the substrate. Dyeings on mercerised cotton or bright viscose show these effects to a lesser extent than on unmercerised cotton or delustred viscose. Vat dyeings based on these sensitive combinations exhibit virtually no catalytic fading on fabrics that have been given a crease-resist or water-repellent finish, because such goods absorb much less moisture. As already noted (section 7.1.5), certain yellow, orange and red vat dyes tend to sensitise the photochemical degradation of cellulosic textiles. 7.7.2 Batchwise and Continuous Dyeing Methods
Since the vat dyeing of cellulosic fibres is carried out in relatively strongly alkaline conditions, it is possible in certain circumstances to scour-dye grey goods, although a preliminary oxidative bleach is often necessary to achieve the target level of brightness of hue. However, leuco vat dyes must be reoxidised with alkaline peroxide or perborate after dyeing, so that an oxidative prebleach may be unnecessary for dyeings in medium or full depths.
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In the leuco dyeing process, the goods are entered into the dye liquor containing the leuco dye, caustic soda and dithionite, together with salt, dispersant, sequestrant and levelling agent, if required. The chemical additions are determined by the liquor ratio, together with the type and amounts of dyes applied. There are three main groups of vat dyes (IK, IW and IN) applied at different temperatures, as well as a few individual dyes requiring special procedures. The IK dyes have relatively low substantivity and are dyed at ambient temperature with minimal alkali and a high salt concentration. The IW dyes have much higher substantivity and are applied at 45 to 50°C with more alkali and less salt. The IN dyes are dyed at 60°C with even more alkali but no salt. No salt is required with IW dyes on mercerised cotton or regenerated cellulosic fibres. With most dyes the temperature can be raised to 80°C if necessary to achieve better levelling. It is usual to raise the temperature to 80°C for shading. With vat dyes that are sensitive to over-reduction it is necessary to add an inhibitor such as glucose when dyeing above 60°C. In the pigment padding process the vat colorant is distributed throughout the substrate as evenly as possible in the non-substantive form prior to vatting. Only brands with a fine particle size distribution are suitable for this method of application. The process is begun at ambient temperature before raising slowly to 60-80°C, salt being added if necessary to promote exhaustion. Alkali and dithionite are added to generate the soluble leuco dye. Dyeing continues to completion at the usual appropriate temperature. A prerequisite for all continuous dyeing processes with vat dyes is that the fabric should be highly absorbent so that it wets out rapidly and uniformly. Since the differences in substantivity of the leuco dyes have less influence, dye selection is less restricted than in batchwise dyeing. Highly substantive dyes in the IW and IN groups have proved the most suitable. The pad-steam process is the most reliable and popular continuous dyeing method for vat dyes. The fabric is padded with the colorant dispersion and dried immediately. Alkali and dithionite are applied by chemical padding, after which the fabric is treated in saturated steam to form the leuco dye and complete the fixation process. This is followed by reoxidation, rinsing and soaping in an open-width washing range. In the wet-steam process, pad application of the vat colorant dispersion is followed by wet-on-wet impregnation with alkali and dithionite and then immediate development by steaming, thus eliminating the intermediate drying step. The padding trough containing the chemicals can be situated (a) directly in front of the steamer, (b) at the entry slot or (c) inside the chamber close to the entrance. The usefulness of the wet-steam option is restricted to certain qualities, including voluminous or heavyweight unmercerised woven goods such as terry towelling or corduroy. The merits and limitations of this approach have been discussed [149]; equipment is available for dyeing short runs by this method, which is particularly economical compared with the traditional jig dyeing alternative [150]. 7.7.3 Indigo Dyeing
Blue denim jeans have been popular over a longer period than any other item of apparel. Originally developed as a heavy-duty workwear fabric, denim is inexpensive, durable, versatile and socially egalitarian. The blue-dyed warps wash down to an attractive blue without undue staining of the white weft yarns. Synthetic indigo was first marketed in 1897 after twenty years of research, but within a few years it had replaced the natural product previously grown mainly in
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the Bengal region of the subcontinent. The Heumann-Pfleger synthesis by alkaline fusion of phenylglycine and sodamide was devised in 1901 and has been used at BASF Ludwigshafen since 1926, giving an overall yield of 85% from aniline. By recycling most of the by-products, less than 7% of the chemicals used are discharged to waste-water treatment. BASF in 1982 initiated the world’s first manufacture of Indigo Microgranules and in 1994 marketed Indigo Solution 20%, the disodium salt of leuco-indigo produced by hydrogenation and supplied in containers inertised with nitrogen [151]. Leuco indigo has only low substantivity and only pale depths are attainable by exhaust dyeing. Indigo is therefore applied in a series of ‘dips’ with intermediate squeezing and atmospheric oxidation. Repeated application in this way eventually gives deep blue dyeings with relatively low fastness to rubbing. On cotton, indigo dyeings have a light fastness rating of 3 at standard depth. Although yellow decomposition products are formed during exposure to light, these are watersoluble and the original blue is restored on washing. Indigo is usually vatted using dithionite and caustic soda. Cotton yarn in the form of ball warps or warp beams is dyed continuously by passing through several vats, each followed by ‘skying’, an air passage for reoxidation. The processing speed is typically 20 to 30 metres per minute, with an immersion time of 20 to 30 seconds in each vat. After squeezing to a liquor pick-up of approximately 100%, the reoxidation step requires about 2 minutes. To ensure that the dyebath composition is consistent in the multiple dip tanks they are coupled together via pumps and piping, including a feed tank to continuously replenish the leuco dye, caustic soda and dithionite consumed. Good flow volumes with minimum turbulence are essential to ensure uniformity [152]. Indigo was in short supply during the 1970s and development work at Dan River Inc (USA) demonstrated that the consumption of indigo in the dyeing of cotton yarn for denim could be markedly lowered using a sodium carbonate/hydroxide buffer rather than caustic soda alone to generate lower dyebath pH values. Dyebaths produced in this way were less stable, however, resulting in a variable pH, more deposition of inadequately dissolved dye and a greater risk of unlevel dyeing. More recently the effect of dyebath pH on the yield and penetration of indigo dyeings has been studied in greater detail [153]. Measurements of apparent colour yield, expressed in terms of the reflectance absorptivity coefficient of the indigo-dyed yarn, showed this to reach a maximum close to pH 11 and to decrease progressively with pH, until at pH 13.5 the colour yield was only about one-sixth of that at pH 11 (Figure 7.12). The degree of penetration of the dyeings was also highly dependent on dyebath pH, poorly penetrated ring dyeings being obtained at pH 11 and almost fully penetrated ones at pH 13. This is attributable to a rapid rate of strike onto the fibre surface under conditions of much higher substantivity at the lower pH. It is well-known that the reduction of indigo 7.18 gives firstly the monoionised sodium hydrogen enolate 7.19 and then the disodium enolate 7.20. The relative amounts of these species present is governed by the dyebath pH. Sorption studies have revealed that both equilibrium sorption and the ratio of the monoionised to diionised forms of indigo are markedly higher at pH 11, where the highest colour yield is achieved in indigo dyeing. Above about pH 12.7, a greater proportion of the leuco dye is present as the disodium enolate 7.20. Improved formulation of buffered alkalis has enabled the degree of ring dyeing obtained in the commercial indigo dyeing of cotton yarn for denim to be closely controlled [153].
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7.8 Sulphur Dyes Sulphur dyes are important for black, navy, brown, olive and green colours in medium and heavy depths, being relatively inexpensive. The liquid brands are ideally suitable for continuous dyeing in long runs. Their fastness properties vary markedly throughout the range; light fastness, for example, increases from yellow at about 3 to black at 7. Fastness to wet treatments is good in general, although fastness to bleaching is poor with certain notable exceptions. Sulphur dyes are still widely used on cellulosic fibres and their blends with synthetic fibres. Cotton and polyester/cotton drill and corduroy are dyed continuously or on the jig, whilst knitted and pile fabrics are jet-dyed. Yarn dyeing is carried out in warp, hank or package form, mainly as navy or black. There are four main classes of sulphur dyes: 1. C.I. Sulphur dyes: water-insoluble brands that require application with a reducing agent. 2. C.I. Leuco Sulphur dyes: brands containing the soluble leuco form of the parent dye and a reducing agent, usually sodium sulphide or hydrosulphide. Compared with the parent sulphur dye, a typical pre-reduced formulation has a dye content of 25 to 30% [154]. 3. C.I. Solubilised Sulphur dyes: water-soluble brands containing the thiosulphuric acid derivative of the parent dye, non-substantive but converted to the substantive leuco form during dyeing. 4. Selected members of C.I. Sulphur and C.I. Vat classes that exhibit superior wet fastness and higher resistance to bleaching than traditional sulphur dyes. These products are normally applied with caustic soda and sodium dithionite as reducing system. C.I. Sulphur powders were traditionally the principal form in which sulphur dyes were marketed, particularly where high-strength dyes were transported over long distances. These were prepared for dyeing by pasting with water and dissolved by boiling with a solution of sodium sulphide. C.I. Leuco Sulphur powders or grains contain reducing agent, salt and dispersing or stabilising agents to assist application. Dispersed powder or paste brands are mainly intended for continuous dyeing. The dye pressscake is milled to microparticle size in the presence of a dispersing system. Drying is strictly controlled to prevent agglomeration of the finely ground particles. Partially reduced liquid brands are usually more concentrated than fully reduced liquids, thus saving packaging and transportation costs. A further addition of reducing agent to the dyebath is necessary to achieve the full colour value, but this need not be a sulphide. Glucose or dithionite may be selected in order to minimise the sulphide content of the effluent. Fully reduced liquids are ready for use, since the amount of sulphide present has been carefully adjusted to give maximum stability on storage and optimum colour yield in use. These brands clearly predominate for continuous dyeing owing to their ease of handling [154]. C.I. Solubilised Sulphur dyes can be prepared as powders or liquids, the latter being cheaper to produce since no drying costs are involved [155]. They are made by warming the polysulphide-free paste with sodium sulphite or bisulphite until the soluble thiosulphuric acid derivative is formed. This is salted out from solution or isolated by drum or spray drying of the liquor. Aqueous solutions of these derivatives exhibit little or no substantivity for cellulosic fibres until a reducing agent is added. This also assists the penetration of tightly woven goods.
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These brands give more level dyeings with less tendency to bronziness. They are widely used by commission dyers and on speciality goods, or where effluent pollution problems have been indicated. The two most important reducing agents for sulphur dyeing are sodium sulphide (Na2S) and sodium hydrosulphide (NaHS). Particularly when dissolving C.I. Sulphur brands, it is essential that sufficient reducing agent is present to dissolve the dye fully. When dyeing to heavy depths at low liquor ratio it may be necessary to use the full dyebath volume to dissolve the dye. The amount of reducing agent required varies markedly for different dyes and the details provided by the manufacturer should be followed. For many years alkaline dithionite was the only sulphide-free reducing system for sulphur dyeing, although it was never universally suitable. The system is difficult to control and tends to give inconsistent results. With the class 4 sulphurised vat dyes, however, satisfactory conditions analogous to vat dyeing methods can be achieved. Black and blue C.I. Solubilised Sulphur dyes are well suited to a less alkaline system based on dithionite and soda ash. Alkaline glucose is in increasing use where environmental considerations prevent the use of sulphide-based systems (section 2.9), although it does present some problems of its own. Jig dyeing is particularly difficult, because it may not be possible to maintain a temperature high enough for satisfactory reduction of all sulphur dyes. Sodium polysulphide acts as an antioxidant, greatly assisting in overcoming bronzing of the dyeing and premature oxidation at the surface of the dyebath. Sodium borohydride (NaBH4) additions are also effective in preventing selvedge bronzing when dyeing sulphur blacks on the jig [155]. Thioglycol (HSCH2CH2OH) has been evaluated for use with C.I. Solubilised Sulphur dyes, both by exhaust dyeing and by one-bath pad-steam processes, although the colour yield is lower than from sulphide-reduced systems. The advantages of thioglycol are the absence of sulphide from the effluent and the lack of odour from the dyebath. The main drawback is that the dyeing method is more costly overall. Many of the C.I. Sulphur brands do not dissolve completely and the sulphurised vat dyes are not applicable [154]. The environmental impact of conventional unstable reducing agents can be avoided by replacing them with electrochemically regenerable reducing systems. For example, a study of the redox behaviour of C.I. Leuco Sulphur Black 1 demonstrated that the leuco form produced by cathodic reduction had a reduction capacity of sufficient magnitude to serve as a combined reducing agent and antioxidant. Application by the one-bath pad-steam process gave depths of shade corresponding with standard dyeings produced using conventional reducing systems [156]. In exhaust dyeing with either C.I. Sulphur or C.I. Leuco Sulphur brands, colour yields equal to standard were achieved using a redox system based on sodium anthraquinone-2-sulphonate. Inferior yields and precipitation of iron(II) sulphide occurred in an iron-triethanolamine complex system. Excellent results with C.I. Solubilised Sulphur dyes were obtained in anthraquinone-2-sulphonate or -1,5disulphonate systems. Sulphurised vat dyes normally require dithionite reduction but satisfactory performance was achieved in this case using systems based on either alizarin or an iron(II)-glucose complex [157].
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Electrochemical reduction systems offer significant advantages: 1. process control by measurement of electrical potential, 2. control of dyebath potential by regulation of cell current, 3. precisely defined and reproducible dyeing conditions, 4. greater precision in the control of mass balances, 5. consistent concentrations of all chemicals present, 6. predictable behaviour of the dyes during exhaustion, 7. no accumulation of decomposition products from reducing agents in the dyebath, 8. no chemicals involving safety problems, 9. savings in chemicals and water consumption, 10. easier and less costly waste water treatment, 11. improved ecological profile of vat and sulphur dyes [158]. Traditionally, acidified dichromate solution is the preferred system for reoxidising sulphur dyeings. Chromium(IV) compounds oxidise all leuco sulphur dyes rapidly and completely. Colour yields and fastness ratings are good and taken as standard. Nevertheless, chromium residues are being banned or restricted by many water authorities (section 2.6). Dichromate oxidation is usually avoided when dyeing in yarn form because of its adverse effects on handle and sewability. Addition of copper(II) sulphate to dichromate oxidation baths improves the light fastness of sulphur dyeings by up to one grey-scale unit. However, this treatment has a dulling effect and gives a significantly harsher handle. The legislation governing the discharge of effluents containing chromium compounds has now become so strict that in future it may be no longer possible to reoxidise with dichromate. Iodate-based systems were introduced in the USA to replace dichromate but other alternatives have been preferred for environmental, economic and procedural reasons. Sodium bromate in the presence of sodium metavanadate as catalyst gives a reoxidising system with properties approaching those of acidified dichromate. Unfortunately, pollution by metavanadate is also being subjected to restriction by some water companies in the UK. Alkaline hydrogen peroxide or sodium perborate is now widely used in the dyeing of sulphur blacks, although as a result of over-oxidation the wet fastness may be lowered by one grey-scale unit in respect of staining of adjacents when compared with the dichromate-treated standard [159]. Under mildly acidic conditions hydrogen peroxide gives a slower rate of oxidation but the activity is not high enough for reoxidation of some red sulphur dyes. Most blue dyes give slightly duller and weaker dyeings than when oxidised using dichromate [154]. 7.8.1 Batchwise and Continuous Dyeing Methods
The substantivity of leuco sulphur dyes for cellulosic fibres varies with the substrate and between different brands of the same dye. Dye liquors prepared from C.I. Leuco Sulphur liquid brands, being virtually salt-free, are exhausted to a much lower extent than those prepared from traditional C.I. Sulphur powders, which invariably contain salt added during dye standardisation. This diluent is insufficient to give optimum exhaustion in batchwise dyeing, however.
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The rate of exhaustion of sulphur dyes is slow at temperatures below 50°C and can be controlled by temperature or by gradual addition of salt. In jig dyeing it is preferable to add the salt at dyeing temperature because some absorption takes place during the heating stage. In enclosed dyeing systems the salt is usually added at the start, using temperature rise to control the exhaustion rate. A sequestering agent should be present because traces of iron salts or water hardness tend to cause precipitation and lower the rub fastness of the resultant dyeing. In continuous dyeing the presence of electrolyte may cause tailing, so the prereduced liquid brands are preferred at a padding temperature that is as low as possible consistent with satisfactory absorption. Fabric preparation should always ensure that the substrate has high and uniform absorbency. C.I. Solubilised Sulphur brands are suitable for the pad-dry-chemical pad-steam process on mercerised cotton. They have very low substantivity in the thiosulphuric acid form, so tailing is not a problem even at elevated padding temperatures [154]. Traditional C.I. Sulphur brands are dissolved by boiling for several minutes in a sodium sulphide solution. The dye liquor should be thoroughly stirred, heated to the boil and allowed to simmer for 5 minutes with occasional agitation to ensure complete dissolution. Sulphurised vat dyes are pasted with water and a suitable wetting agent before diluting with aqueous caustic soda. This liquor is heated to vatting temperature and the sodium dithionite added. After stirring for 15 minutes to ensure complete dissolution, the leuco dye is added to the dyebath preset with alkaline dithionite. Liquid C.I. Leuco Sulphur brands do not require heating as they are already in solution. They are added to a dyebath already preset with alkali, reducing agent and sequestrant. C.I. Solubilised Sulphur dyes are dissolved by sprinkling into a warm solution of wetting agent and sequestrant. After stirring vigorously, the liquor is heated to the boil and allowed to simmer for 2 minutes to ensure complete dissolution. The most suitable brands for package dyeing of yarn or beam dyeing of fabric are the non-substantive C.I. Solubilised Sulphur dyes for optimum penetration or the C.I. Leuco Sulphur liquids which require no pre-boiling. Sulphur dyes may be applied under pressure at 120°C with consequent savings in time and improved levelness. Dyebaths should be cooled to 80°C before rinsing by overflow and reoxidation as for conventional dyeings. The early jet dyeing machines were largely unsuitable for sulphur dyeing because the excessive turbulence in the jet effectively destroyed the reducing agent so rapidly that antioxidants were ineffective. Modern softflow machines, however, are quite suitable for sulphur dyes, especially when glucose-based reduction systems are utilised. High-temperature equipment is preferred, even when dyeing at 90 to 95°C, as foaming tends to be a greater problem in atmospheric machines. As with all foaming problems, it is usually easier to prevent foam forming than to destroy it once it has formed. Sulphur dyes are widely applied to woven fabrics on the jig. The essential requirement in jig dyeing is a constant rate of uptake of the dyes, since there is little scope for subsequent levelling. The dye liquor is normally divided into portions, two-thirds at the start and the remainder added after the first end. All the chemicals, apart from any reducing agent necessary for the second portion, are added at the start. This sounds simple but may be difficult in practice, especially when applying medium or heavy depths of insoluble C.I. Sulphur
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Practical Dyeing, Volume 2
brands, since the solubility limit may necessitate dissolving the dyes in the jig trough. In these circumstances the jig roll should be ‘swung’ whilst the second portion is prepared. In contrast, the liquid brands may be simply added in equal portions over two ends without stopping the jig action. Although the alkaline glucose reduction system gives satisfactory results in jet or package machines, it has proved unsatisfactory on the jig, giving inconsistent results and poor colour yields. Dispersed C.I. Sulphur dyes and the C.I. Solubilised Sulphur brands are the most suitable for application by the pad-jig method owing to their lack of substantivity during impregnation [160]. After padding, the goods are stored on a rotating batch or run directly into the jig already set with reducing agent and salt at 80 to 90°C, then run for four to six ends before rinsing and reoxidising. Liquid C.I. Leuco Sulphur brands are sometimes applied by this method to achieve better penetration. The pad-jig process is particularly useful for dyeing mercerised cotton or viscose, where the rapid initial strike makes subsequent uniform penetration of the fabric more difficult. Sulphur dyes, especially blacks, are much more important for continuous dyeing than for exhaust application. Traditionally, cotton pile fabrics have been a major sector for sulphur dyeing. Corduroy is generally dyed in muted hues, such as olive, navy, dark brown and black, for which sulphur dyes are the most economical option. The one-bath pad-steam method without intermediate drying is especially suitable for this purpose since it preserves a particularly attractive appearance by avoiding migration [161]. For mercerised cotton, however, the pad-dry-chemical pad-steam process is claimed to give a better surface appearance. It is applicable to polyester/cotton blends by inclusion of a thermofix stage for the disperse dyeing after the drying stage. C.I. Solubilised Sulphur brands or the dispersed C.I. Sulphur types may be applied with the appropriate reducing system. A third option is to use liquid C.I. Leuco Sulphur dyes and to pre-reduce with sulphide in the chemical pad. However, the drying stage gives rise to more unpleasant fumes than from the other dye types. The addition of salt and some of the dye liquor to the chemical pad helps to establish a state of equilibrium by minimising initial bleed-off of dye from the fabric into the chemical pad. 7.8.2 Fastness and Acid Tendering of Sulphur Dyeings
The fastness characteristics of sulphur dyes in general are superior to direct dyes but inferior to vat dyes. The fastness to light increases throughout the range from about 3 in the yellow to orange sector up to 6 in navy blue and 7 in full blacks. Sulphur dyeings show good fastness to the traditional soaping tests but are less resistant to laundering with detergent and perborate at 60°C or higher. Fastness to bleaching with hypochlorite or peroxide is generally poor. Ratings of fastness to rubbing are variable and depend on the substrate and the dyeing process, especially the efficiency of rinsing before reoxidation. The fastness to dry rubbing is normally quite good (4-5 in full depths) but ratings as low as 2-3 for wet rubbing are often recorded for navy and black dyeings. To achieve optimum rub fastness it is essential to maintain the leuco dyes completely in solution throughout the dyeing stage and to rinse thoroughly to remove as much loose dye as possible before reoxidising according to the standard procedure.
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The phenomenon of acid tendering of sulphur black dyeings is well-known. Under adverse storage conditions of elevated temperature and humidity sulphuric acid can be formed by oxidation of disulphide groups in the dye molecule or residual sulphide in the fabric. This problem may be alleviated or even eliminated by thorough rinsing before reoxidation and an alkaline rinse or softening treatment as the final stage of processing. Resin finishes are claimed to exert a buffering action to inhibit acid tendering by sulphur blacks. No evidence has been reported of sulphuric acid formation and damage from other sulphur dyes. In a detailed evaluation of the problem, the effects were examined of three different sulphur black dyes, aftertreatment with various reoxidising systems, pH variations at the neutralisation stage, traces of transition-metal ions, finishing with various softeners and differences in moisture content during storage of the treated dyeings [162]. 7.8.3 Sulphur Dyeing Residues in Waste Liquors
For many years sulphur dyes suffered from an image problem, regarded as cheap but foul-smelling and ecologically unsafe. Much has changed more recently, owing to marked improvements in product synthesis and design of the dyeing process. Modern ranges of sulphur dyes offer notable advantages, including freedom from transition metals, AOX content or the hazardous arylamines specified in the German ban [163]. Developments in application techniques include exhaust dyeing under a nitrogen atmosphere, indirect electrochemical reduction and padsteam dyeing using a flash ager [164]. Reducing agents such as sodium sulphide and hydrosulphide are in decline for ecological reasons. Glucose is now widely favoured in conjunction with a polysulphide as antioxidant. The dyeing temperature should be at least 90°C and the pH in the 11 to 12 region to obtain optimal colour yields and level dyeing [159]. The glucose system has a characteristic odour of burning sugar that some consider preferable to the odour of a sulphide-reduced dyebath, although others dislike it, finding it too sweet and nauseous. Sulphur dye effluent from traditional reducing systems contains unfixed dye, sulphides, thiosulphates and residual organics. All of these contaminants contribute to the chemical oxygen demand. The discharge of sulphides to drain is not normally permissible because of the danger to life or damage to the sewer structure from the liberation of hydrogen sulphide and its bacterial oxidation to sulphuric acid. The treatment of sulphur dyeing effluent is essentially the oxidation and removal of sulphides, the concentration of which depends on the dyeing method, applied depth and individual dyes used. Sulphide-bearing waste streams should not be allowed to come into contact with acidic effluents, such as those arising from wool dyeing, because hydrogen sulphide will be evolved. Conversely, mixing sulphide effluent with bleaching liquors is useful because both hydrogen peroxide and sodium hypochlorite are highly efficient oxidants and hence have a beneficial effect. In general, however, sulphide wastes should be kept separate from other effluent streams [165]. Air has been the traditional oxidising agent to treat sulphide liquors and it is still the most cost-effective and is widely used. The oxygen in air converts sulphide to thiosulphate. Typically 1 kg of sulphide requires about 1.1 kg of oxygen for complete removal, indicating that the sulphate arises from further oxidation of the thiosulphate formed initially (Figure 7.13).
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Practical Dyeing, Volume 2
The rate of oxidation is almost always linear and the rate-controlling step is the dissolution of oxygen in water. During oxidation the pH gradually falls from 12 to about 10, often marked by a slight increase just before the reaction is complete. It is absolutely essential that the pH is not allowed to fall below 9 during aeration, otherwise toxic hydrogen sulphide will be evolved. Whilst air oxidation is ideal for treating large volumes of sulphide liquors, for smaller amounts chemical oxidation using either hydrogen peroxide or sodium hypochlorite is highly effective and economically acceptable. Below pH 8.5 the reaction of hydrogen peroxide with hydrogen sulphide predominates to yield elemental sulphur as the major product, whereas under more strongly alkaline conditions sulphide anions are oxidised to sulphate anions (Figure 7.14). Sodium hypochlorite solution can sometimes be obtained cheaply as a by-product from the oxidation of coloured effluents with gaseous chlorine. The unreacted chlorine gas is absorbed into caustic soda liquor to give hypochlorite solution (Figure 7.15) [165].
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E Miosga, Melliand Textilber., 79 (1998) 532; Internat. Text. Bull., 43 No. 4 (1997) 32.
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Practical Dyeing, Volume 2
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[131] D A S Phillips, M Duncan, E Jenkins, G Bevan, J Lloyd and J Hoffmeister, JSDC, 112 (1996) 287. [132] I D Rattee and J I N Rocha Gomes, JSDC, 101 (1985) 319. [133] D Fiebig and G Schulz. Lenzenger Ber., 58 (1985) 109. [134] D Fiebig and D Soltau, Textil Praxis, 43 (1988) 644; 44 (1989) 1124. [135] D Fiebig, Chemiefasern, 39/91 (1989) 686. [136] E Polasek and V Cip, Melliand Textilber., 68 (1987) 1254. [137] C Boardman and A N Jarvis, Rev. Prog. Coloration, 30 (2000) 63. [138] W B Egger and T Robinson, Chemiefasern, 31/83 (1981) 852; Textilveredlung, 18 (1983) 41: Text. Chem. Colorist, 15 (Oct 1983) 189. [139] C V Stead, Rev. Prog. Coloration, 6 (1975) 1. [140] E Coates, JSDC, 85 (1969) 355. [141] M A Herlant, AATCC Internat. Conf. and Exhib., (1991) 287; Amer. Dyestuff Rep., 82 (Apr 1993) 19. [142] A Lubbers, I Souren and H K Rouette, Melliand Textilber., 71 (1990) 371. [143] H Lehmann, Melliand Textilber., 67 (1986) 189. [144] S Dugal, K F Elgert and G Heidemann, Melliand Textilber., 68 (1987) 61. [145] M Srinivasan and B M Gatewood, AATCC Internat. Conf. and Exhib. (1998) 361. [146] B W I Textil, Textilveredlung, 20 (1985) 123. [147] J Wegmann, Amer. Dyestuff Rep., 51 (1962) 276. [148] U Baumgarte, Melliand Textilber., 55 (1974) 953; 59 (1978) 311.; Textilveredlung, 15 (1980) 413. [149] W Braun and J Rieker, Textil Praxis, 40 (1985) 1325. [150] F Somm and R Buser, Textilveredlung, 17 (1982) 15. [151] H Schmidt, Melliand Textilber., 78 (1997) 418. [152] J A Greer and G R Turner, Text. Chem. Colorist 15 (Jun 1983) 101. [153] J N Etters, Text. Chem. Colorist, 21 (Dec 1989) 25; 27 (Feb 1995) 17; Amer. Dyestuff Rep., 79 (Sep 1990) 19; 81 (Mar 1992) 17; 83 (Jun 1994) 26; JSDC, 109 (1993) 251. [154] R Klein, JSDC, 98 (1982) 106. [155] R A Guest and W E Wood, Rev. Prog. Coloration, 19 (1989) 63. [156] T Bechtold, E Burtscher and A Turcanu, Text. Chem. Colorist, 30 (Aug 1998) 72. [157] T Bechtold, A Turcanu, E Burtscher and O Bobleter, Textilveredlung, 32 (1997) 204 [158] T Bechtold, E Burtscher, A Turcanu and F Berktold, Melliand Textilber., 81 (2000) 195. [159] W Marx, Textilveredlung, 26 (1991) 74. [160] G Krauzpaul, Textil Praxis, 42 (1987) 140. [161] H U von der Eltz, JSDC, 101 (1985) 168. [162] V Kuhn, Textilveredlung, 26 (1991) 108. [163] M Hähnke and C Schuster, Melliand Textilber., 76 (1995) 414. [164] O Annen, Melliand Textilber., 79 (1998) 752. [165] James Robinson Ltd., JSDC, 111, (1995) 172.
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Practical Dyeing, Volume 2
Table 7.1 Composition by mass of a typical cotton fibre [4]
Component Cellulose Protein Pectin Wax Inorganic ash Water-soluble residue
Proportion (%) of dry mass Total fibre Primary wall 94.0 54 1.3 14 1.2 9 0.6 8 1.2 3 1.7 12
Table 7.2 Dimensions of bast fibre ultimates [15] Fibre type Ramie Flax Hemp Manila Urena Mesta Jute (Cor. olitorius) Sisal Kenaf Jute (Cor. capsularis) Coir
Diameter (µm) 28.1-35.0 17.8-21.6 17.0-22.8 17.0-21.4 15.6-16.0 18.5-20.0 15.9-18.8 18.3-23.7 17.7-21.9 16.6-20.7 16.2-19.5
Length (mm) 125-126 27.4-36.1 8.3-14.1 4.6-5.2 2.1-3.6 2.6-3.3 2.3-3.2 1.8-3.1 2.0-2.7 1.9-2.4 0.9-1.2
Table 7.3 Composition by mass of some important bast fibres
Component Cellulose Intercellular material Wax Inorganic ash Water-soluble residue
Proportion (%) of dry mass Decorticated ramie Raw flax Jute 71.3 80.1 83.3 27.1 10.5 7.5 0.4 2.6 0.2 0.8 1.5 2.1 0.4 5.3 6.9
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67
Table 7.4 Typical physical properties of cellulosic fibre types [22,23]
Fibre type Cupro Viscose Modal Lyocell Polynosic Cotton
Tenacity (cN/tex) Wet Dry 17 10 24 13 35 20 42 36 37 28 22 28
Elongation (%) Wet Dry 7-23 16-43 22 27 14 14 15 17 12 12 8 13
Wet modulus(a) (cN/tex)
Moisture regain (%)
50 110 270
13 12.5 11.5
100
8
Water imbibition (%) 100 90 75 65 55 50
(a) At 5% extension
Table 7.5 Typical AOX values for cotton bleached with various oxidising agents [30] Oxidising bleach Sodium hypochlorite Sodium chlorite Hydrogen peroxide
AOX (mg/l) 27.0 2.4 0.92
Table 7.6 Advantages and disadvantages of bleaching with sodium hypochlorite [29] Advantages
Disadvantages
Low risk of catalytic damage
Necessary to prescour before bleaching
Low chemical cost
No pad-steam bleach process available
Low energy input
Risk of chemical damage (depending on temperature and pH) High levels of AOX and chloroform Degradation of most dyes and FBAs Risk of yellowing on storage after bleaching
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Practical Dyeing, Volume 2
Table 7.7 Advantages and disadvantages of bleaching with sodium chlorite [29] Advantages
Disadvantages
Prescour before bleaching is unnecessary
Costly chemical and equipment
Low chemical damage
No pad-steam bleach process available
Least sensitive to metal-ion contamination
Liberation of the toxic gas chlorine dioxide
Lower loss in mass due to non-scouring action of acid bleach
Exotic construction of equipment to avoid corrosion
Acid process makes washing-off easier
Poor absorbency because of residual cotton wax
Soft handle and good sewability due to non-removal of wax
Control needed in multichemical baths
Incompatible with most dyes and FBAs
Suitable for bleaching blends of cotton with synthetic fibres
Table 7.8 Advantages and disadvantages of bleaching with hydrogen peroxide [29] Advantages
Disadvantages
Excellent storage stability
More costly than hypochlorite
Exotic materials of construction unnecessary
Sensitivity to transition-metal ions
Severe pretreatments unnecessary Exceptionally versatile processing Preparation stages can be combined: desize-scour, scour-bleach or even desize-scour-bleach Exceptionally stable whites and good absorbency Compatible with many dyes and FBAs Decomposition products are oxygen and water No AOX formation even in the presence of salt
Multichemical baths require careful control
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Table 7.9 Colour differences between various types of cotton fibre after dyeing with C.I. Direct Green 27 [41]
Cotton fibre type Pakistan Dessai Tanguis India Bengal Middling American Sudan Sakel Egypt Ashmouni Egypt Karnak Mexico El Paso Malaki
Colour coordinates
Staple length (mm) 15.9 31.8 12.7 27.0 31.8 34.9 33.3 30.2 38.1
x 0.237 0.235 0.236 0.240 0.238 0.238 0.238 0.238 0.236
y 0.298 0.297 0.297 0.297 0.298 0.298 0.297 0.297 0.297
Y 9.85 10.40 10.70 9.60 11.00 11.15 11.20 12.05 12.95
Colour difference (∆E) 0 1.05 1.46 1.47 1.87 2.08 2.24 3.50 4.93
Table 7.10 Degree of variation in dyeability with C.I. Direct Green 27 at successive stages of fibre processing [42] Process sampled Bale to bale Ribbon lap Sliver Ring tube
Variation (%) 16-20 13-15 7-9 <4
Table 7.11 Increase in dyeability with C.I. Direct Green 27 for cotton yarns of increasing fineness [42] Yarn count 24 30 34 38
Dyeability (%) 99-100 102-105 104-110 110-112
Table 7.12 Statistical variation in dyeability with C.I. Direct Green 27 of physically identical yarns produced by two different spinners [42] Spinner 1 2
Mean value 101.1 100.7
Range 3.4 5.2
Standard deviation 0.6 1.4
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Table 7.13 Effect of alkali concentration on absorption of C.I. Direct Red 2 by mercerised cotton [41] NaOH concentration (%) 0 0.85 13.5 17.5 22.5 27.0 31.5
Dye absorption (mg/g) 17.7 23.9 29.5 31.5 33.8 35.6 36.0
Table 7.14 Effect of drying after mercerising on absorption of direct dyes [41]
Fibre type Unmercerised Mercerised and dyed: without drying after drying in air after drying at 110°C for 1 hour
Dye absorption (mg/g) C.I. Direct C.I. Direct Red 2 Yellow 12 3 8 25 16 13
10 8 5
Table 7.15 Differences in dyeability with direct dyes between various cellulosic fibres [71]
Fibre type Prima Avril Viscose Viloft Cotton
Equilibrium exhaustion (%) of C.I. Direct Yellow 106 Red 80 Blue 218 74 99 80 66 92 74 66 90 66 66 88 66 50 84 59
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Table 7.16 Consumption of reactive dyes in the dyeing of various substrate types [83]
Substrate types Knitted cotton Woven cotton Terry towelling Woven polyester/cotton Cellulosic yarns Cellulosic blends Woven polyester/viscose Others
Reactive dye consumption (%) 33 15 10 8 8 7 5 14
Annual worldwide production of reactive dyes is about 115 kilotons
Table 7.17 Application of reactive dyes by various dyeing and printing methods [83]
Application method Exhaust dyeing (warm) Textile printing Exhaust dyeing (hot) Pad-batch Continuous dyeing
Reactive dye consumption (%) 42 21 13 13 11
Table 7.18 Important reactive systems Monofunctional Dichlorotriazine (DCT) Monochlorotriazine (MCT) Monofluorotriazine (MFT) Trichloropyrimidine (TCP) Difluoropyrimidine (DFP) Dichloroquinoxaline (DCQ) Vinylsulphone (VS)
Homobifunctional Bis-MCT Bis-MFT Bis-VS Heterobifunctional MCT-VS MFT-VS DFP-VS
Table 7.19 Relationship between substantivity and nature of the sulphonyl substituent in C.I. Reactive Red 22 [93] Sulphonyl substituent -SO2CH2CH2OSO3Na -SO2CH=CH2 -SO2CH2CH2OH
Primary exhaustion (%) 20 80 38
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Table 7.20 Key criteria of performance for Procion H-EXL (DyStar) reactive dyes in exhaust dyeing [83] Reactive dye Compatibility Matrix (RCM) Substantivity (S) Exhaustion (E) Migration Index (MI) Level Dyeing Factor (LDF) Time of half-fixation (T50)
Key criteria 70-80% 85-95% ≥90% ≥70% ≥10 min
Procion H-EXL dyes 72-80% 87-94% 90-100% 73-86% 10-14 min
Table 7.21 Advantages and disadvantages of reactive dyeing under ULLR conditions [108,109] Advantages
Disadvantages
Higher equilibrium exhaustion
Dye solubility problems
Higher degree of fixation
Risk of unlevelness because of rapid exhaustion
Improved reproducibility of shade Lower energy requirements Lower salt concentration Less effluent pollution Facilitates RFT production Easier transfer of recipe
Inferior migration at high exhaustion levels Addition of dyes and chemicals according to a careful sequence Need for low-foaming auxiliaries Need for running aids and creasing inhibitors Accumulation of fibrous debris Inefficient wash-off conditions
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Table 7.22 Possible reductions in salt concentration by dyeing with typical reactive dyes under ULLR conditions [110]
Reactive dye(a) A B C D E (a)
A B C D E
Reactive system VS MCT-VS Bis-MCT DFP MFT
Constants in equation 7.12
Salt concentration (g/l) at liquor ratios of:
x 0.024 0.01133 0.0133 0.01 0.00466
5:1 44 33 50 65 53
Remazol Navy Blue GG Sumifix Supra Navy B-F Procion Navy H-EXL Levafix Navy Blue E-BNA Cibacron Navy F-G
y 8 16 30 50 46
6:1 51 36 54 68 54
7:1 58 40 58 71 56
10:1 80 50 70 80 60
Reduction (%) from 10:1 to 5:1 LR 45 34 29 19 12
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Figure 7.1 Bilateral structure of a mature cotton fibre in cross-section showing zones that differ in fibrillar packing density [3]
B C
D
D
A
A
Figure 7.2 Typical development of fibre length and secondary wall thickening in cotton fibres [2] 100
Development, %
80
60
40
Length Secondary wall
20
0
10
20
30
40
50
Days from bloom
Figure 7.3 Cross-sectional features of cotton fibres with different degrees of maturity
Mature
Immature
Dead
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Figure 7.4 Effects of temperature and alkali concentration on swelling of cotton by sodium hydroxide solutions 2.0
Degree of swelling
0 oC 25 oC 100 oC
1.5
1.0 0
2
4
6
8
10
Sodium hydroxide conc., mol/l
Figure 7.5 Effect of sodium hydroxide concentration on fabric reflectance after winch bleaching with hydrogen peroxide [37] 81
Reflectance, %
79
77
75
73 0
1
2
3 NaOH conc., g/l
4
5
6
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Figure 7.6 Effect of sodium hydroxide concentration on fabric reflectance after J-box pad-steam bleaching with hydrogen peroxide [38]
Reflectance, %
89.5
88.5
87.5
86.5 0
2
4
6
8
10
NaOH conc., g/l
Figure 7.7 Statistical variation in dyeability with C.I. Direct Green 27 at successive stages of processing from raw material through to dyeing and finishing [42] 25 Range Standard deviation Variation in % dyeability
20
15
10
5
0 Bale
Ribbon
Sliver
Ring
Knitted
Dyed
Finished
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Figure 7.8 Relative colour yields using 2% owf Procion H-EXL (DyStar) bismonochlorotriazine dyes on various cellulosic fibres [24]
Relative colour yield, %
100
Cotton Viscose Tencel Tencel A100
80
60
40
20
0 Yellow
Red
Navy
Figure 7.9 Relative colour yields using 3% owf Procion XL Plus (DyStar) polyfunctional reactive dyes on various cellulosic fibres [73]
Relative colour yield, %
100
80
60 Cotton Modal Tencel Tencel A100
40
20
0 Yellow
Rubine
Blue
Cyan
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Figure 7.10 Key criteria of performance for reactive dyes in exhaust dyeing [105] 100 Alkali addition
E
80 Dye on fibre, %
S
F
60
40
MI F/2
20 Migration phase
Fixation phase T50
0 0
10
20
30
40
50
60
70
80
90
Dyeing time, min
Figure 7.11 Smart Rinsing profile for a Then AFS jet machine [126] 100 oC
90 oC Smart rinse 110 l/tube/min 5 contacts
Grad rinse 1 contact
Contact time 2 minutes Contacts (total) 23 Process time 46 minutes Water consumption 15 litres
Smart rinse 110 l/tube/min 5 contacts
Grad rinse 10 contacts
60 oC Smart rinse 110 l/tube/min 2 contacts
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Figure 7.12 Effect of dyebath pH on apparent colour yield in a five-dip indigo dyeing process [153]
Reflectance absorpticity coefficient
200
150
100
50
0 10.5
11
11.5
12
12.5
13
Dyebath pH
Figure 7.13 2Na2S + 2O2 + H2O
Na2S2O3 + 2NaOH
Na2S2O3 + 2NaOH + 2O2
2Na2SO4 + H2O
Figure 7.14 H2O2 + H2S
S + 2H2O
4H2O2 + S2–
SO42– + 4H2O
Figure 7.15 Cl2 + 2NaOH 4NaOCl + S2–
NaOCl + NaCl + H 2O SO42– + 4NaCl
13.5
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OH HO HO
OH
HO
O
HO
O
O
HO
O OH
7.1B
O HO n–2
7.1A
7.1C
OH O
OH O
HO
OH
HO
HO
O
OH
OH HO
HO
CHO
7.1D
7.1C
OH N
H2O
N
D NH Cl
7.3
OH N
H2O
D NH
N
7.4
Cl
N N OH
N N
D NH 7.2
N Cl
Cl N
cellulose OH
D NH 7.5
N
OH H2O
N D NH
N
7.6
O cellulose
N N O cellulose
O cellulose cellulose OH
N D NH 7.7
N N O cellulose
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D SO2CH2CH2OSO3Na
81
NaOH
D SO2CH CH2 + Na2SO4 + H2O
7.8
7.9
D SO2CH2CH2OH
H2O
7.10
D SO2CH CH2 7.9
D SO2CH2CH2OCH2CH2SO2
D
7.12
D SO2CH2CH2O cellulose 7.11
cellulose OH
Cl N D NH
N
7.13
N
+ H2N
SO2CH2CH2OSO3Na 7.14
Cl
Cl N D NH
N
7.15
N NH
SO2CH2CH2OSO3Na
O–Na+
O + 2NaOH + 2H+
+ 2H2O O–Na+
O 7.16
7.17
O N H
H N O
7.18
Na+O– H N N H
OH
7.19
Na+O– H N N H
O–Na+
7.20
Chapter 8 Dyeing of Wool, Silk and Other Animal Fibres 8.1 Properties of Wool and Hair Fibres Wool accounts for about 90% of the production of animal fibres and the majority of this is obtained by shearing the sheep twice a year. Wool is also removed from the pelts of slaughtered sheep, by a depilatory process, usually based on lime and sodium sulphide; this process loosens the wool fibres which can then be pulled from the pelt. Bacterial action can also be used to loosen the wool fibres. The wool produced by these fellmongering processes is usually inferior in quality to that of sheared wool with which it is often blended. Adequate rinsing to remove the depilatory chemicals is a necessary part of preparatory processes. Until as late as the 1980s, the world production of new wool was insufficient to meet demands and use was made of recovered wool. Shoddy was wool recovered from fabrics which had not been excessively milled during manufacture whilst mungo was wool recovered from heavily milled or felted materials such as velours and meltons. Extract was wool recovered from cotton/wool union cloths. In recent years the use and production of animal fibres has declined so that these fibres now account in volume for about 2.5% of fibre usage. As a result, the worldwide sheep population has declined as has the sales price per kilo of virgin wool. Nevertheless, the relative value of animal fibres is higher than the volume percentage as a result of these fibres being used for luxurious and high addedvalue products. This is particularly the case for hair fibres, such as cashmere, mohair, alpaca, llama, rabbit fur and vicuna. The quality of wool varies greatly according to the breed of sheep, the climatic and geographical conditions under which wool growth has occurred, the health and age of the individual sheep and the region of the sheep’s body in which the wool has grown. Important parameters in wool quality include fibre diameter and length, handle, colour and lustre. Grading and sorting of wool is an essential preliminary to spinning processes. The mean fibre diameter for wool can range from 17 microns for fine fibres to 40 microns for coarse fibres with corresponding staple lengths ranging from 60 to 280 mm [1]. Hair fibres are generally very fine, cashmere having an average fibre diameter of 15 microns. The hair fibres have particular cross-sectional features when viewed under the microscope, often with a distinctive medulla, which aids identification. A breakthrough in the objective analysis of speciality fibres was made [2] when it was demonstrated that deoxyribonucleic acid (DNA) is not only present in hair roots but can easily be extracted from animal hair shafts. This was an important development since some fibres, such as wool, are sheared rather than combed. DNA of sufficient quality for analysis can be isolated from scoured, bleached and dyed materials as well as from raw fibre samples. The fineness of the hair fibres together with the presence of a medulla means that they require significantly more dye than wool to obtain a given colour. Traditionally, wool fibres were spun on either the woollen or worsted system. The woollen system consists essentially of carding and spinning operations and the fibres are distributed in a random manner within the yarn. Woollen yarns are thick and full and the coarse fibres are held loosely within the yarn since only a limited twist level is inserted during spinning. Such yarns are used for the manufacture of woven tweeds, blankets and for knitted fabrics. Worsted yarns are smoother and finer. The combing process, carried out before spinning, produces a
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top in which the wool fibres lie parallel and from which short fibres have been removed, as noils. Ring spinning is the most commonly used spinning operation. Open-end and other modified spinning processes (such as the Sirospun method) are also used. The physical characteristics of fabrics manufactured from woollen or worsted spun yarns are significantly different. Worsted fabrics are usually ‘clean-cut’ in appearance whilst woollen fabrics have a much more ‘random’ surface appearance. Worsted fabrics usually require less finishing than woollen fabrics which are given extensive finishing routines. It is often stated that whereas worsted fabrics are manufactured in the loom, woollen fabrics are manufactured in finishing.
8.2 The Composition, Chemistry, Physics and Morphological Properties Of Wool Proteins are natural polymers of high relative molecular mass (Mr) formed by condensation of α-amino acids through their carboxyl and amino groups. Proteins are widespread in nature, being essential components of animal and plant tissue and they have thus been the subject of much research. It is perhaps not surprising that an early text [3] on the chemistry and physics of wool was authored by two medical scientists (P Alexander and R F Hudson). The 1952 Nobel laureates in chemistry were two WIRA scientists (A J P Martin and R L M Synge) [4] for the development of partition chromatography, an important method for amino acid analysis. Wool belongs to a class of proteins known as keratins. The α-amino acids have the general formula (8.1) and are distinguished from one another by the Rgroup, which can be basic, acidic or nonpolar [3]. Examples include the amino acids proline (8.2) and cystine (8.3). Cystine plays a key role in the properties of wool but is absent from the structure of silk. Each protein chain contains a primary amine (NH2) group at one end of the protein chain and a carboxylic acid (COOH) group at the other end. Complete hydrolysis of wool yields a mixture containing eighteen amino acids [5,6], although significant variations in the amino acid composition can exist, even along the length of a fibre or between fibres from the same animal. The diet of the sheep and weathering effects can cause variations in the amino acid content, notably of cystine. The method of cleansing the sample for analysis may also have an effect and various methods are available for amino acid analysis including acid hydrolysis and enzyme digestion. Much research has centred on the arrangement of the amino acids along the chain length. The general structure of a wool polypeptide is shown schematically (8.4) where R1, R2, R3 represent amino acid side-chains. A significant proportion of the polypeptide chains in wool are believed to be in the form of an α-helix and this ordered arrangement is responsible for the characteristic X-ray diffraction pattern of unstretched wool (αkeratin). β-Keratin (found in feathers) gives a different x-ray pattern, similar to that obtained from stretched wool. The side-chains of the various amino acids vary in size and chemical nature. The nonpolar hydrocarbon side-chains of glycine, alanine, phenylalanine, valine, leucine and isoleucine are of varying hydrophobic character and low chemical reactivity. Serine, threonine and tyrosine contain hydroxyl groups which make their side-chains polar in nature and thus more chemically reactive than the
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hydrocarbon residues, especially under alkaline conditions. The side-chains that probably have the most marked influence on the properties of wool, including its dyeing properties, are those containing acidic or basic groups. Acidic carboxyl groups are contained in residues of aspartic and glutamic acids, whereas histidine, arginine and lysine residues contain basic side-chains (the imidazole, guanidino and amino groups respectively). The individual peptide chains in wool are held together by covalent crosslinks and various noncovalent interactions (8.5) [7]. The majority of the sulphur content of wool occurs in the form of cystine. This is formed within the hair follicle in the skin of the sheep during keratinisation (hardening) of the fibre. The disulphide bonds of cystine form crosslinks, either between different protein chains or between different parts of the same protein chain. These crosslinks are responsible for the greater stability and lower solubility of keratin, compared with other proteins. Cleavage and rearrangement of the disulphide bonds in wool is involved in shrinkproofing and setting processes. Keratin fibres are not chemically homogeneous, consisting of a complex mixture of widely different polypeptides and it has been estimated that wool contains about 170 different types of protein molecule. Despite the overall classification of wool as a keratin, wool proteins have been defined as keratinous and nonkeratinous on the basis of their cystine content. Non-keratinous proteins have a lower cystine content and as a consequence fewer disulphide linkages making them less resistant to chemical attack. The amino acid sequences of many types of proteins have been determined [8-11]. Wool and other keratin fibres are also physically heterogeneous and are considered to be biocomposite materials (section 1.1) consisting of regions that differ from each other both physically and chemically. The complex morphological structure of wool is shown schematically in Figure 8.1 [12]. Fine wools contain two types of cell: the cells of the external cuticle and those of the internal cortex. Together these constitute the major parts of the mass of clean wool. Coarse keratin fibres may contain a third type of cell, those of the medulla. This is a central core of cells, arranged either continuously or intermittently along the fibre axis and wedged between the cortical cells, often in a ladder-like manner; airfilled spaces lie between the medullary cells. The presence of a medulla increases the light-scattering properties of fibres. The cuticle cells or scales constitute the outermost surface of the wool fibre and are responsible for important properties such as wettability, tactile properties and felting behaviour. Approximately 10% of a fine wool fibre consists of cuticle cells which overlap like tiles on a roof, with the edge of every scale pointing from the root to the tip of the fibre. A consequence of the ratchet-like arrangement of cuticle cells is that the coefficient of friction along the fibre surface is much less in the root to tip direction than it is from the tip to the root. This differential friction effect, (DFE), under the correct conditions of pH, moisture and temperature, is responsible for wool’s unique attribute of felting. The cortex comprises almost 90% of keratin fibres and is largely responsible for mechanical behaviour. The cortex of fine wool consists of closely packed, overlapping cortical cells arranged parallel to the fibre axis. Each cell is surrounded by the cell membrane complex which is a continuous phase that extends throughout the entire fibre. The cells of the cortex are composed of rodlike elements of crystalline proteins (microfibrils) surrounded by a relatively amorphous matrix. The cortex consists of orthocortical and paracortical cells, each having a different distribution of non-keratinous material within the cell.
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Orthocortical and paracortical cells also differ in the composition and arrangement of the microfibril/matrix system within each macrofibril. The relative proportions and arrangement of the different types of cortical cell in wool vary with fibre diameter and often within a fibre. The bilateral segmentation of fine wools is associated with the highly desirable natural crimp of the fibres. In these wools, the orthocortex is always oriented towards the convex surface of the crimp, the two segments of the cortex twisting around the fibre in phase with the crimp. Basic dyes, cationic agents and surfactants and many high-Mr. ions containing heavy metals preferentially stain the more accessible orthocortex. Acid dyes do not favour either cortex. The presence of readily swollen non-keratinous material in the orthocortex makes it more wettable and more susceptible to acid hydrolysis. The lower crosslink density of the orthocortex leads to higher rates of stress relaxation and setting in the orthocortex. Wool will react with many chemicals and contains three main types of reactive group, namely: peptide bonds, the polar side-chains of amino acid residues and disulphide crosslinks. The highly reactive nature of wool has enabled many industrial treatments to be developed, including bleaching, dyeing, shrinkproofing, flame-retarding and other finishing processes.
8.3 Fibre Structure as it Impacts on Wool Dyeing Early studies of the mechanism of wool dyeing were mainly concerned with the thermodynamics of the dyeing process in which the wool fibre was treated as a cylinder of uniform composition. This led to theoretical models such as those developed by Gilbert and Rideal or Donnan [3,13], which were mainly concerned with the situation when dyebath equilibrium had been reached and provided little information on the kinetics of the actual dyeing process. The importance of the diverse morphological structure of wool in determining its dyeing behaviour is now recognised. The exhaust dyeing process involves three stages [13]: 1. the diffusion of dye through the aqueous dyebath to the fibre surface 2. transfer of dye across the fibre surface 3. diffusion of the dye from the surface throughout the whole fibre. The rate at which dye is supplied to the fibre surface is determined by the circulation rate of the dye liquor. With good dyebath circulation, diffusion of the dye to the fibre surface is unlikely to be a critical factor in determining the overall dyeing rate [13]. Important factors which affect the sorption of dye at the fibre surface include the characteristics of the individual dye, the pH of the dyebath and the presence of inorganic salts or surfactants. To achieve satisfactory colour yield and fastness, complete penetration of the dye into the fibre interior is essential. The rate at which this occurs depends on the rate of transfer across the fibre surface and then diffusion through the fibre. If the wool is treated as a uniform cylinder Fick’s laws of diffusion [14] apply, so that a plot of dye uptake against the square root of time would be linear over most of the dyeing process [15]. The actual wool dyeing curve, however, is initially concave and only becomes linear after some time, from which it was deduced that a slow-permeability barrier exists at the fibre surface and that this was responsible for the non-Fickian curve. There has been much discussion regarding the nature of this barrier, including various components of the fibre
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epicuticle and the presence of lipids at the cuticular junctions. It would appear that the cuticle, probably the highly crosslinked A-layer of the exocuticle, is a barrier to dye penetration, in that the dyes must penetrate the gaps between the scales in order to reach the cortex [16]. It appears that lipids present at the intercellular junctions are also a barrier to diffusion of dyes into the nonkeratinous regions of the cell membrane complex. This intercellular mode of dye penetration only applies to intact wool. Fibres which have been chemically or physically modified, for example, by damage to the Alayer of the exocuticle, severe surface abrasion or complete removal of the cuticle, show dyeing behaviour that differs from undamaged wool. After initial penetration into the wool fibre, the dye must diffuse throughout the entire cortex and it has been suggested that the continuous network of the cell membrane complex provides a pathway for the diffusion of reagents into the wool. The cell membrane complex swells in formic acid to a much greater extent than other components of the fibre and this disproportionate swelling explains the rapid uptake of dye from concentrated formic acid [17]. Dyeing rates of specifically modified wool fibres also confirm the importance of the cell membrane complex in the mechanism of wool dyeing. Transmission electron microscopy [18,19] demonstrated the importance of the non-keratinous components of the fibre in wool dyeing. After dye has entered the fibre between the cuticle cells, diffusion occurs throughout all the non-keratinous regions of the cell membrane complex, the endocuticle and the intermacrofibrillar material. As dyeing proceeds, dye is progressively transferred from the non-keratinous regions into the sulphurrich proteins of the matrix surrounding the microfibrils within each cortical cell. Dye is also transferred from the endocuticle into the exocuticle. It appears that the hydrophobic proteins located in these regions have a higher affinity for wool dyes than the non-keratinous regions. At the end of the dyeing process the nonkeratinous regions, although important in the early stages of dyeing, become virtually devoid of dye. For non-reactive dyes, thermodynamic equilibrium with wool is not established until the process of dye transfer into the keratinous regions is complete. This stage is not reached until some time after the dyebath is exhausted. This explains why prolonged treatment at an elevated temperature is necessary to produce satisfactory wool dyeings. If dye remains in the non-keratinous regions, rapid diffusion out of the fibre can occur, resulting in poor wet fastness. Reactive dyes probably show a different equilibrium distribution between the keratinous and non-keratinous regions. Since these dyes are capable of covalent bonding with proteins in the latter region, at equilibrium reactive dyes may be present in the cell membrane complex and endocuticle to a greater extent than non-reactive types.
8.4 Fibre Damage in Wool Dyeing In conventional methods of wool dyeing, relatively prolonged treatment at elevated temperatures (at or near 100°C) is required to achieve satisfactory penetration into the fibre, levelling and fastness properties. Depending mainly on the class of dye used, the application pH is usually in the range 2 to 7. Under these conditions, wool damage can occur, resulting in unacceptable levels of yellowing, lower yields in spinning, decreased production due to yarn breakages and inferior product performance, such as lower abrasion resistance.
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Damage to wool in hot, aqueous, acidic solutions occurs mainly by hydrolysis of peptide bonds, particularly at aspartic acid residues. Tryptophan and amide sidechains are also susceptible to acid hydrolysis. Chemical attack on wool in an alkaline medium is less selective and more rapid than under acidic conditions. Peptide linkages are cleaved but other linkages, notably cystine, are also readily hydrolysed. Under alkaline conditions, reactions with cystine can produce lanthionine and lysinoalanine crosslinks and it is believed that the formation of the former leads to fibre embrittlement and lower abrasion resistance. The non-keratinous components of wool, particularly the cell membrane complex, are regions of relative weakness in the overall composite structure of the wool fibre. It is believed that preferential attack on these readily swollen regions is a major factor in the damaging of wool during dyeing. When wool is dyed at the boil, soluble proteins (termed ‘wool gelatins’) are extracted from the fibre. These have a low cystine content and are believed to originate from the cell membrane complex and other non-keratinous regions. The yield of wool gelatins is regarded as a measure of the extent of wool damage. At a given liquor pH, the mass of non-keratinous material extracted is proportional to the treatment time [20]. Ionic bonds (salt linkages) between side-chains of opposite charge are important to stabilise the structure of wool and their concentration depends on the pH of the fibre. The influence of dyebath pH on damage during dyeing has long been recognised [21-25]. It has been demonstrated that fibre damage is at a minimum when wool is dyed at a pH value within the isoelectric region of the fibre (pH 4 to 5), when the concentration of salt linkages is at a maximum and their stabilising effect greatest. The effect of liquor pH on the yield and composition of the soluble proteins extracted when wool is boiled for a fixed time has been determined [23]. The effect of liquor pH is changed markedly by the addition of electrolytes. In the absence of salt, the yield of wool gelatins appears to be independent of pH in the range 3 to 8, with a dramatic increase below pH 3. In the presence of electrolyte, the amount of soluble protein extracted shows a distinct minimum around pH 3.5 to 5.0, coinciding approximately with the isoelectric region of the fibre and the maximum concentration of salt linkages. In the absence of electrolyte, the internal pH is markedly different from the pH of the external solution, whereas neutral salts bring the internal and external pH values closer. In an acid medium the effect of electrolyte is thus to decrease the internal pH, whereas in an alkaline solution the internal pH is raised. In both cases, the shift in pH away from the isoelectric region will result in an increase in damage and an increase in the yield of wool gelatins. In a comprehensive study, Peryman [24] measured the effects on various physical and chemical properties of treating wool for three hours in boiling aqueous liquors over the pH range 1.5 to 9.0. The presence of sodium sulphate in the liquor had little effect on damage in the pH range 1.7 to 6.8 but above pH 6.8 the electrolyte caused a marked increase in damage. Peryman’s results [24] in alkaline liquors agreed with those of Baumann and Mochel [23] but the difference between the two sets of results in acidic liquors is difficult to explain. These differences may be due to the less selective nature of alkaline damage and the sensitivity of different tests used by the two groups of workers. Peryman [24] concluded that minimum fibre damage occurs when wool was dyed at pH 3 to 3.5, whereas Elöd and Reutter [22] suggested that pH 4.5 to 5.0 gave optimum results. Baumann and Mochel [23] have suggested that the optimum pH is 3.5 to 4.0.
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Although the effect of dyebath pH on fibre damage has long been known [21-25], it is only fairly recently that dye ranges have been specifically marketed for application at pH values within the isoelectric region, together with the use of levelling agents with these ranges or with conventional dyes (section 8.10). It has long been recognised that fibre damage caused by dyeing can be significantly reduced by lowering the dyeing temperature below 100°C [26-30]. Various dyeing procedures have been developed for dyeing at lower temperatures, usually in the range 85 to 90°C. Some methods require the addition of surfactants to the dyebath, whilst others depend on fibre modification before or during dyeing. Some methods only allow a lower temperature by extending the dyeing time. A typical modification involves pretreatment under mildly alkaline conditions with a special amphoteric surfactant, allowing dyeing to proceed at pH 4.5 and a temperature of 80 to 90°C [31]. The practical aspects of low-temperature dyeing have been discussed [32], culminating in what is claimed to be a realistic dyeing process, giving cost savings with improved product quality and productivity. Careful dye selection is necessary together with the use of selected auxiliary products and dyeing can be carried out in the range 85 to 95°C. This approach could perhaps serve as a useful template for the development of such processes. Damage in wool dyeing has been comprehensively reviewed [33]. Fibre protective agents were traditionally added to dyebaths for loose wool to minimise fibre losses in carding. Dyers had noted that the protein hydrolysates remaining in exhausted wool dyebaths gave this beneficial effect. Such agents, however, may lower the wet rubbing fastness of wool dyeings. DuPont proposed the use of formaldehyde to protect the wool component when dyeing polyester/wool blends. Formaldehyde-treated wool may have a harsher handle and certain azo dyes are sensitive to reduction by formaldehyde. There are also health and safety objections to the use of this agent (section 2.12), but N-methylol reactants developed for cellulose crosslinking can be used safely as wool protective agents without fastness problems. Certain reactive dyes are also capable of exerting a protective effect by reaction with the thiol groups in reduced wool.
8.5 Theory of Wool Dyeing Comments have already been made in section 8.3 regarding the impact of fibre structure on dyeing and a detailed treatment of wool dyeing theory is outside the scope of a practical manual of this nature. As discussed previously, wool has a complex molecular and morphological structure. Theories of wool dyeing that regard the fibre as a homogeneous cylinder should be treated with caution. During dyeing, wool dyes apparently diffuse through the intercuticular regions first and then into the non-keratinous regions of the endocuticle, the intercellular matrix and the intermacrofibrillar material. The surface of the wool scales acts as a barrier to dyeing. Neutral-dyeing dyes for wool can be highly aggregated in the aqueous dyebath system, even at the boil. Certain dyeing auxiliaries are able to disaggregate the dyes and thereby increase both the dyeing rate and levelling. Traditional theories of wool dyeing, based on ionic attraction between the dye and ‘dyeing sites’ in the fibre, are only partially valid. Apparently the ionic forces of attraction mainly control the rate of dyeing, whereas hydrophobic interactions largely determine the affinity of the dyes for wool and their wet fastness
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properties. Comprehensive reviews of the theory of dyeing, including wool, have been given [13,34,35].
8.6 Impurities Present in Wool Raw wool may contain between 25 and 70% by weight of impurities. These consist of wool grease, perspiration residues (suint), dirt and vegetable matter such as burrs and seeds picked up by the fleece during wool growth. Wool grease is a complex mixture of various fatty acids and their esters, whereas suint is composed mainly of potassium salts of fatty acids, sulphate, phosphate and nitrogenous material. The composition of wool wax has been extensively reviewed [36]. The composition of various types of greasy wool [1] is shown in Table 8.1. Vegetable matter is removed during carding in the woollen spinning process or during combing in the worsted spinning process. Alternatively, a carbonising treatment destroys the cellulosic impurities. Suint, grease and dirt are removed by scouring. These processes (see section 12.4.2) were traditionally carried out in the wool-using countries. However, there has been a rationalisation of wool scouring internationally, with more scouring being operated in the grower countries (principally in New Zealand, Australia and South Africa). Approximately 65% of the domestic clip is scoured in New Zealand. This development has increased the added value of wool exports and reduced transport costs, but has also created effluent disposal problems in the grower countries. Whilst larger, more economical plants have been built in the grower countries, raw wool scouring has declined in user countries.
8.7 Processes to Remove Impurities from Wool The first stage in wool fibre processing is to blend and open the fibre so that it is in a suitable condition for subsequent cleansing processes. The vegetable matter can be removed at this stage by passing the wool through fluted or Peralta rollers and the crushed matter falls out on shaking. Although carbonising can also be carried out in fabric form (section 8.11.2), heavily contaminated wools are best treated at the fibre stage. The extensive literature on wool carbonising was reviewed [37], revealing that conflicting results had been obtained by various workers on the effect of adding surfactants to the acid bath as a means of reducing wool damage. Preferential absorption of the acid by the vegetable impurities occurs at temperatures between 10 and 30°C, whilst the moisture content of the wool as it reaches the baking stage has an effect on the degree of damage. A 5.5% uptake of sulphuric acid by the wool is optimum. A later review [38] indicated that the ideal carbonising conditions included the use of wetting agent in the acid bath and drying at 70°C to a moisture content of less than 10%. Baking is carried out at 125 to 150°C for about one minute to char the vegetable matter and the charred material is removed by crushing through a pair of rollers. Neutralising should be carried out immediately. The equipment and processes for raw wool preparation have not changed much over the past hundred years. The carbonising and scouring of wool fibres are described in section 12.4.2.
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8.8 Fibre Modifications 8.8.1 Shrink-Resist Treatments When wool garments are washed in aqueous media dimensional changes can occur because of relaxation shrinkage and felting of the fibres. Relaxation effects occur when strains introduced into the material during manufacture are released when the garment is agitated in water. Relaxation shrinkage is reversible and can occur in garments made from any fibre. Felting shrinkage is not reversible and is peculiar to the wool fibre, largely associated with the differential frictional effect (DFE) of the scale structure of the fibres (section 8.2). Most commercial shrink-resist processes control felting shrinkage by either modifying the scale edges to eliminate the DFE or by bonding the fibres together so that relative movement is not possible. Machine-washable wool garments are well-established, particularly from knitted fabrics. Many commercial processes were developed, originally based on chlorination, although chlorinated residues are now most unwelcome from the environmental viewpoint. Fibre bonding is achieved by resin treatment, one of the most successful originally being Hercosett 125 (Hercules), a reactive polyamide-epichlorohydrin resin [39]. About 75% of shrink-resist wool is treated in top form and the remainder is processed in fabric or garment form. Shrink-resist treatment of wool fibres is discussed in section 12.4.2 while the processing of garments is described in section 8.14.4. Important parameters for the successful finishing of machine-washable wool have been specified: 1. The pH of the fabric before finishing must be within the range 7 to 8 after hydro-extraction. 2. Decatising is preferably carried out before shrink-resist finishing. 3. Curing of a silicone-modified polyurethane resin should be at a temperature between 150 and 160°C. 4. The warp yarn construction is more important than the weft; single and crepe yarns are often selected but a two-ply warp yarn avoids seam shrinkage and puckering. 5. Fluorochemical application is normally under acidic conditions but optimum crosslinking of polyurethane occurs at alkaline pH. However, combined application is possible with only slight sacrifice of water- and oil-repellency performance [40].
8.8.2 Insect-Resist Treatments Unlike other textile fibres, wool and animal fibres are subject to attack by the larvae of certain moths and beetles. Various chemicals have been applied to wool to resist such attack. Considerable environmental restrictions now apply to the type of agent employed (section 2.13). Such treatments have been reviewed [41]. Suitable products can be applied during processes such as scouring or dyeing. The main concern with such chemicals is their effect on mammalian and fish toxicity and thus they must be selected with care.
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8.8.3 Flame-Retardant Treatments The wool fibre is relatively resistant to burning but with the introduction of stringent test methods for aircraft carpets, upholstery and contract furnishings, chemical methods of imparting flame-retardancy have been improved. The most successful treatments are based on the use of fluoro complexes of titanium and zirconium, especially hexafluorotitanate and hexafluorozirconate. As the potassium salts, these compounds are readily absorbed by the wool fibre. The treatment is usually carried out as an aftertreatment after dyeing using 8% (owf) of potassium hexafluorozirconate at pH 3 for 30 minutes at 60°C. A range of dyes which will withstand the treatment is available. 8.8.4 Fibre Photostability With the increasing use of wool for automotive fabrics, the tendency of wool to degrade in ultraviolet light is a disadvantage, since fading tests are based on the use of a hot xenon lamp which has a high UV content. Carmakers apply a stringent high-temperature fastness test, where the rates of photodegradation of both the dye and the wool fibres are assessed. It was found [42] that 1:2 cobaltcomplex dyes decreased phototendering of wool. Ultraviolet absorbers have been developed, notably Cibafast W (Ciba) which is based on sulphonated benzotriazole. This product can be applied from acid dyebaths and as well as minimising the phototendering of wool, is claimed to increase the light fastness by one to two points [43,44].
8.9 Stages for Wool Dyeing Wool materials can be dyed at various stages in the manufacturing sequence, as shown in Figure 8.2. Several factors impact on the selection of the stage for dyeing. Dyeing of wool early in the manufacturing sequence allows dyes of high intrinsic fastness properties to be used, whilst any unlevelness can be blended out at the combing or carding processes in yarn manufacture. However, these processes are associated with long lead times and quick response is hardly possible with these dyeing methods. Dyeing later in the manufacturing process, as garments, fabric or even yarn, will facilitate quick response but level dyeing is essential. Market requirements usually dictate that there has to be a compromise between levelness and fastness. Heather mixture effects can only be obtained through fibre dyeing, whilst colour woven fabrics are based on yarn or fibre dyeing processes. With the exception of garment dyeing, all the processes listed in Figure 8.2 can be carried out by exhaust or continuous dyeing methods. Traditionally, for many of the reasons cited above (including the need for high fastness and levelness) most wool used to be dyed at the early stages of the manufacturing sequence (as fibre, top or tow). It was foreseen in 1969, in what subsequently became known as the Atkins report [45], that yarn dyeing (particularly in package form) would begin to replace the earlier-stage dyeing processes. This has in fact occurred, as shown in Table 8.2 for 1994 [46]. 8.9.1 Wool Dyeing Processes Machinery for the dyeing of wool fibre and top is described in sections 12.5 and 12.6. After dyeing, loose stock is dried and blended before proceeding to the
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spinning process. Following dyeing, tow-dyed material is usually back-washed, dried and re-combed before proceeding to the worsted spinning process. Woollen and worsted spun yarns will normally contain a significant amount of spinning lubricant, unless dry-spun. Conventionally spun yarns were lubricated with saponifiable or mineral oils, the former being removed by the action of soda ash and the latter by using a detergent/alkaline scour. Water-soluble lubricants have now become more common. Yarn in hank form is scoured on a continuous brattice or tape scouring machine. Yarns for package dyeing should be dry-spun or lubricated with a water-soluble oil, since adequate scouring is difficult to achieve in package-dyeing. Machines for dyeing wool yarns in hank and package form are discussed in sections 13.4 and 13.5. The dyeing of wool fabrics is carried out in machines described in sections 14.6 and 15.5. The preparation, dyeing and finishing of wool fabrics is discussed in sections 8.11, 8.12 and 8.13.
8.10 Application of Dyes to Wool Acid, chrome, metal-complex and reactive dyes are the main types used for dyeing wool (Table 8.3) [47]. Virtually all acid dyes are the sodium salts of aromatic sulphonic acids whilst a few contain carboxyl or phenolic groups. Although dyes may be synthesised from many different intermediates, they are usually classified on the basis of their application properties such as dyeing method, levelling or wash fastness properties. Acid dyes are thus classified as levelling, milling or super-milling types [48]. There is some degree of correlation between molecular mass and dyeing properties, such as wet fastness or level dyeing. The inverse relationship between migration and wet fastness is represented in the ‘wool dye grid’ illustrated in Figure 8.3. As the intrinsic wet fastness of the dyes increases, the migration and level dyeing properties are adversely affected. Dyes of higher fastness must be applied uniformly to the substrate from the start of the dyeing process since their inability to migrate does not allow subsequent levelling to take place. As the intrinsic wet fastness of the dyes increases, greater control is required over the dye application procedure. The technical and economic factors that govern the selection of dyes for wool were discussed [48], with reference both to shade area and fastness requirements. The interconnected relationships between dye and process costs, level dyeing, limits of metamerism and fastness requirements are such that difficult compromises in dye and process selection become inevitable. Process costs in relation to effective utilisation of the available machinery should not be viewed in isolation without considering the maintenance of product quality, such as fabric strength and handle after fabric dyeing, or spinning yield and yarn strength after loose stock or top dyeing. It is unlikely that a single dye class will meet the various requirements in any particular stage of dyeing. Typical application methods for the various types of acid dyes, including the various ranges of 1:2 metal-complex dyes, are based on commencing dyeing at 40°C and raising the temperature to 100°C at a rate of temperature rise of between 1.0 and 2.0°C per minute. The optimum rate is dependent on the efficiency of circulation of the dyeing machine used, as influenced by the rate of liquor flow or substrate movement. Dyeing at 100°C is continued for 30 to 60 minutes, often depending on depth of shade, to allow for exhaustion, penetration
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and levelling by migration when this is possible. Traditional methods of control depended on the adjustment of the dyebath pH, with level dyeing acid dyes being applied in the pH range 2.5 to 3.5 or 1:2 metal-complex and milling acid dyes in the range 5.0 to 8.0. Acid donor systems are now often employed to adjust pH gradually as dyeing proceeds. As discussed in section 8.4, these traditional dyebath pH values resulted in wool damage. Levelling agents (section 8.10.1) were developed, not only to improve the levelling properties of the dyes at conventional pH values but also when the dyes were applied in the isoelectric region. Chrome dyes are exceptional (Figure 8.3), since they exhibit level dyeing properties but produce dyeings of high fastness. These dyes are effectively level dyeing until they react with chromium salts to give a chromium complex of high wet fastness. Chrome dyes can be applied by three methods: 1. on a chrome mordant, when the wool is first treated with the chromium salt and then the dye is applied in a fresh bath 2. by the metachrome mordant process where chroming and dyeing are carried out simultaneously in a single bath 3. by the afterchrome method when the dye is applied and then the chroming process is carried out as a second stage, but often in the same bath. The chrome mordant method is lengthy and expensive in time and resources, whilst the prechroming process can lead to significant fibre damage. Although the metachrome process is quicker (similar in time to acid dyeing), few dyes are suitable, limited exhaustion restricts the method to pale or medium depths and residual chromium levels are high. Accordingly, the afterchrome method is now the only significant technique for applying chrome dyes to wool. The main disadvantage of the afterchrome method is the difficulty of matching colours, because major shade changes occur during chroming. In addition, the richness, bloom and ‘through shade’ obtained with chrome dyes on wool can seldom be reproduced with other dye classes on wool or other fibres. Chrome dyes were traditionally applied by a method similar to that used for levelling acid dyes and then sodium or potassium dichromate was added, normally equal to one-third of the weight of dye employed. This chrome dyeing method has become unacceptable (section 2.6), partly as a result of residual chrome in the substrate and the amount of chromium discharged in the effluent. Modified methods have been developed, whereby the optimum amount of chromium required can be calculated [48-52]. An early attempt to avoid the prolonged chrome dyeing sequence whilst maintaining high levels of wet fastness was the development of the 1:1 metalcomplex dyes, such as the Neolan dyes (Ciba) in which one atom of chromium was complexed within the dye molecule. To ensure level dyeing, however, these dyes had to be applied at pH 2 with sulphuric acid and fibre damage could result. A later development of this type of dye was the Neolan P(lus) range, which are modified 1:1 metal-complexes containing fluorosilicate anions. This rationalised range of eight dyes is applied to wool at pH 3.5 to 4.0 in the presence of Albegal Plus (Ciba), a synergistic mixture of quaternary and esterified alkylamine ethoxylates and fluorosilicates [53,54]. The 1:2 metal-complex dyes, developed from 1953 onwards, contain two dye molecules complexed with one chromium or cobalt atom. Although originally
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applied at a neutral pH, the use of levelling agents allows these dyes to be applied to wool in the isoelectric region. The increasing cost of dye research and ecotoxicological testing means that few new chemical entities are likely to be marketed, so that novel dye ranges are likely to depend on the screening, selection and rationalisation of existing dyes. Several developments for wool dyeing have been obtained by this route: 1. Sandolan MF dyes [55] (Sandoz) – a range of selected level dyes to give improved fastness when applied in the presence of Lyogen MF as levelling agent at pH 4.5 to 5.0 2. Lanaset dyes [56,57] (Ciba) – a compatible range of fifteen existing milling acid and 1:2 metal-complex dyes selected to give good reproducibility, levelness and fastness when applied with Albegal SET as levelling agent at pH 4.5 to 5.0. The time of dyeing loose wool can be reduced significantly when using Lanaset dyes by the use of Miralan T (Ciba) as a dye accelerator. 3. Lanasan CF (Sandoz) dyes – a range of six dyes within the Optilan (Sandoz) concept [58,59] applied with Lyogen UL as a dyeing auxiliary at pH 4.5 to 5.0. Although reactive dyes were originally developed for cellulosic fibres, a minority of commercial dye ranges have been specifically developed for application to wool [60]. Several of them are listed in Table 8.4. However, the annual consumption of about 2 kilotons for wool dyeing is dwarfed by the corresponding figure of 120 kilotons for the dyeing of cellulosic fibres [61]. Reactive dyes tend to produce skittery dyeings on wool and specific auxiliary products have been developed to assist level dyeing or to improve dye uptake. These dyes are mainly used for dyeing machine-washable garments. The Lanasol (Ciba) dyes are undoubtedly the market leaders. Careful application is necessary in the presence of de-aerating and levelling agents, whilst an alkaline aftertreatment is required to remove unreacted dye. Sales of the metal-free Lanasol CE (Ciba) reactive dyes have developed well since their launch in 1997, a reflection of the increasing demand in the industry for chrome-free dyeing [62]. With their high fastness, good levelling and low price, chrome dyes still retain a market share of about 30% of wool dyes (Table 8.3), mainly for navy and black dyeings. Reactive black dyes have to be mixture products, since it is very difficult to synthesise a homogeneous metal-free black chromogen with adequate light fastness. Lanasol Black CE is the basis of costeffective blacks, with dyeing being carried out at pH 4.0 to 4.5 [63]. Bayer (now part of DyStar) was the first dye manufacturer to develop the concept of systematic dyeing procedures, the most important probably being that developed for acrylic fibre dyeing (section 11.4). This company has developed the Supra S [64] process for wool dyeing with acid dyes, in which the rate of dyeing is controlled by selecting an optimum pH and an adjusted temperature profile. This includes starting temperature, rate of temperature rise and final temperature, based on the selection of dyes according to their K factor or (if in the presence of the levelling agent, Avolan SCN 150) their K’ factor. A fibre affinity factor (AF) is used instead of the more usual saturation factor (SF) since with wool there is no saturation effect but different wool qualities can have different affinities. A corresponding system has been developed for 1:2 metalcomplex (Isolan) dyes. In the Optilan (Sandoz) concept [58], three ranges of dyes are used, namely: the Sandolan MF range, 1:2 metal-complex together with milling acid dyes and Drimalan F (reactive) dyes. Dyes are selected from within a compact range to
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achieve a high level of reproducibility, fastness, minimum fibre damage and level dyeing. Within the Lanasan CF range, 1:2 metal- complex and milling acid dyes are selected according to a rate of dyeing index (CV) and an SK value, which is a measure of the ability of the dye to cover root to tip dyeing differences. Dyeing is carried out in the presence of a levelling agent, Lyogen UL, at pH 5.0. 8.10.1 Levelling Agents in Wool Dyeing Levelling agents are employed in wool dyeing to assist in level dyeing, especially to enable dyes of high intrinsic wet fastness to be applied at a pH within the isoelectric region and thereby minimise wool damage. Anionic surfactants have a strongly negative polar group and as a result have affinity for wool at an acidic pH. Levelling agents of this type function by acting as blocking agents due to their preferential absorption by the wool. As dyeing proceeds and the bath temperature is raised, these colourless anions are displaced by the dye anions of higher affinity. Weakly cationic surfactants have a positive polar group and readily interact with the anionic dyes. The complex formed between dye and levelling agent slowly liberates the dye as the dyeing temperature is raised, allowing the dye anions to enter the fibre. Nonionic surfactants are uncharged but levelling agents of this type are able to complex with the dye by hydrogen bonding with polar groups in the dye molecule. The properties of amphoteric surfactants are pH-dependent; they tend to act as cationic agents in an acid medium and as anionic agents in an alkaline medium. 8.10.2 Dye Selection for Wool Traditionally, it was usual to select dyes of the highest fastness for dyeing processes near the beginning of the manufacturing sequence so that any unlevelness could be corrected in subsequent processing. Dyes with good levelling properties were preferred for dyeing processes later in the manufacturing sequence. This approach has been eroded for various reasons including: 1. the market-place demands high fastness and uniformity from dyeing processes, irrespective of the stage at which dyeing takes place 2. the need for ‘quick response’ dictates that more goods are being dyed by latestage methods 3. the high level of reproducibility necessary for right-first-time (RFT) dyeing is usually attainable with high-substantivity dyes that exhibit high wet fastness, providing dye migration or decomposition is negligible 4. metamerism between batches dyed to the same shade by various routes must be avoided, so that the same dyes have to be used throughout. However, it is possible to achieve level, fast and reproducible dyeings on wool at all stages of the manufacturing sequence using any of the dye classes discussed, without incurring significant fibre damage. This can be done by the selection of the correct dyeing machinery and control equipment, using a compatible combination of dyes, a rate of temperature rise determined by the limitations of dyebath circulation, together with the use of an appropriate levelling agent and pH control [48].
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8.11 Preparation Routines and Processes for Wool Fabrics Thorough preparation of fabric is essential for successful dyeing to be achieved and this is especially true of raw wool fabrics, from which lubricating oils and associated dirt must be removed by scouring. It is worth repeating here the often quoted maxim that ‘well-prepared is half-dyed’. The greige fabric should be carefully inspected as soon as possible after manufacture so that obvious faults can be marked and an assessment made of the amount of mending required. The fabric is accurately weighed and the weight and dimensions of the fabric clearly marked on the face of the fabric together with the piece number. The quality identification number identifies the preparatory processing required, as defined by standard operating procedures (SOP). Similar qualities are usually assembled and sewn together for subsequent processing. 8.11.1 Processing Sequences For woven fabrics, the important preparation processes are scouring and milling; colour wovens and fabrics to be piece-dyed require these processes. For certain constructions, such as velours, partial raising or other mechanical treatment may be given before dyeing. Setting processes may be undertaken to improve fabric stability during dyeing. Improvements in farm practices have reduced the need for carbonising which is mainly carried out in the fibre form, particularly for Australian wools. If carbonising is required in fabric form, this may be carried out at various stages in the preparation process, as shown in Figure 8.4. The stage at which carbonising is carried out will influence fabric quality [65]. Other significant aspects can include chemical savings that result from the carry-over of residual carbonising acid into the subsequent dyebath, the ease of impurity removal and the attainment of satisfactory dyeing quality. Greasy carbonising (sequence A) allows for the maximum removal of cellulosic contaminants but the lubricating oil must withstand acidic treatment. This, in practice, restricts the choice of oil to saponifiable types but by carbonising before scouring and alkaline milling, the wool is then close to the pH for dyeing by all the major dye classes. Scouring before carbonising (sequence B) means that a larger range of lubricating oils can be used by the spinner, but burr removal may be more difficult with highly contaminated materials, because consolidation of the fabric may occur in scouring. Acid and alkaline milling (sequences B and C) give fabrics of different quality, acid milling being chosen when dense felting is preferred. When carbonising is carried out just before dyeing (as in sequences D or F) neutralising is necessary if dyes other than 1:1 metal-complex or levelling acid dyes are being used. Sequence G gives significant economies in acid consumption. It has often been claimed that greasy carbonising gives least damage to the wool, due to the protective action of the grease, but long-term storage of fabric in highly acidic or alkaline conditions before dyeing can cause wool damage and yellowing. Acid treatment followed by an alkaline treatment, without due care, can cause serious damage. It has been suggested that greasy carbonising is best used in conjunction with solvent scouring. A practical evaluation was carried out [65] of the various processing routes given in Figure 8.4. This indicated that the best results, based on carbonising efficiency, dyed fabric quality, colour fastness, physical properties and handle of the finished fabric, were produced by sequence D, although all sequences gave
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fabrics of commercially acceptable quality. The sequence selected may depend on: 1. amount of burrs present and their ease of removal 2. storage time envisaged before neutralising or dyeing 3. types of dye or dyeing process to be used 4. relative cost and saving in chemical consumption of the individual method 5. the ability to have fabric ready for dyeing as a contribution to ‘quick response’ 6. achieving RFT processes in both preparation and dyeing operations
8.11.2 Carbonising The comments made in section 8.7 regarding carbonising in fibrous form apply equally to fabric processing. Conventional modern practice is to employ a continuous process for acid impregnation, drying and baking with crushing being carried out batchwise, for example on a milling machine. Neutralising is usually incorporated into the subsequent scouring process. A range for continuous carbonising in this way is illustrated in Figure 8.5. Acid treatment is carried out for 2 to 5 minutes at 35°C with a 7 to 8% solution of sulphuric acid containing 0.01 to 0.02% nonionic wetting agent. Squeezing to a 75 to 80% expression gives an acid content on the fabric of 5%. Drying to a moisture content less than 10% is carried out at 90°C, followed by baking for up to 3 minutes at 130°C. Running speeds are typically 5m/min for heavyweight fabrics and 25 m/min for lightweight fabrics. An integrated solvent scouring and carbonising process, the Carbosol process, has been developed by Sperotto Rimar [66]. The fabric is first scoured in perchloroethylene, then impregnated with sulphuric acid followed by drying and baking. It has been found that wool fabric containing this solvent retains much lower concentrations of sulphuric acid after carbonising, whilst the acid is preferentially absorbed by the cellulosic impurities. This gives savings in acid consumption and easier washing off whilst the entire process causes less fibre damage. The principle of the Carbosol process is shown in Figure 8.6. 8.11.3 Scouring A major contribution by Moxon and his co-workers at WIRA (now part of BTTG) was to recognise by a series of surveys that wide variations existed in practical scouring methods [67]. Following from this, recommended methods for wool fabric scouring were published [68]. Scouring is often the first process in the preparation sequence (Figure 8.4) with the objective of producing a thoroughly clean fabric, free from processing oils and dirt. The chemistry of wool fabric scouring depends largely on the type of lubricant used in yarn spinning. Traditionally, widespread use was made of saponifiable oils, which are rich in free fatty acids. A saponification scour removes these impurities, the fatty acids being converted to soap in situ by the addition of sodium carbonate (soda ash). The chemical reactions between the fatty acids and the alkali form soaps, with the liberation of carbon dioxide. Both reaction products are claimed to enhance the fullness of the fabric handle. With non-saponifiable oils, scouring depends on the combined action of a surfactant and alkali to extract the oil from the fabric and to keep the oil and associated dirt in emulsion. Watersoluble lubricants are sometimes present.
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The historically important methods [68] are still largely viable although widespread use is now made of synthetic detergents. Carbonised fabric is usually run in cold water for 15 to 30 minutes followed by draining to remove the excess acid. This will normally decrease the acid content to about 3%. Saponification scouring is generally carried out cold for 20 minutes with the addition of soda ash, whereas emulsion scours are carried out at 40°C. Hot and cold rinses complete the process. Colour woven fabrics are scoured cold to minimise colour bleeding. After the scouring stage, emulsified oil and dirt must not become redeposited on the fabric since their removal becomes difficult. Rinsing is carried out by slow dilution of the scouring liquor. Stains can be removed by solvent spotting. For heavily stained fabric, a solvent-based detergent may be used in the scouring liquor. Scouring is generally carried out at a liquor ratio of 2:1. The amount of alkali required for saponifiable oils is calculated so that the following four functions are accomplished: (a)
the free residual acid from carbonising is neutralised
(b)
the acidic side-chains in wool keratin are neutralised
(c)
the alkali acts as a detergent builder
(d)
the oil is fully saponified to free fatty acids.
The total required = (a + b + c + d) g of soda ash per kg of wool, with b + c being approximately 5g/kg of wool and d is calculated from Equation 8.1. d=
FC g/kg 50
Equation 8.1
where F is the free fatty acid content of the oil (%) and C is the oil content of the wool (%). For non-saponifiable oils, soda ash must be added to satisfy factors a, b and c and scouring is carried out with 1 to 2% detergent. Excess alkali can cause harshness of handle and yellowing during storage. Sufficient must be present to give a stable emulsion, so that oil and dirt are not redeposited, since dirt is difficult to remove if no longer in close contact with the oil. For satisfactory further processing, the following limits are usually applied to scoured fabrics: Oil content:
below 0.5%
Soap content:
0.75 to 1.0%
pH of fabric for dyeing:
6.5 to 8.0
pH of fabric for raising before dyeing:
4.0 to 5.0.
Aqueous scouring techniques are carried out on machines such as the dolly which is illustrated in Figure 8.7. This is a rope-scouring machine in which A and B are squeeze rollers, C is the main trough and D is the internal or suds trough, in which it is possible to reduce the effective liquor ratio for scouring by preventing the return of scouring liquor to the main trough. E and F are guide rollers and G is the draft board with an automatic knock-off mechanism if fabric becomes entangled; H shows the fabric path. Rollers A and B can be made of the same or different material. Fabric speeds are in the region of 100 m/minute. Fabrics which are prone to develop
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creases or running marks should be scoured in open-width machines. Modern machines are fitted with controls, addition tanks for chemicals, pneumatic adjustment of roller pressure and are run at high speed(up to 200 m/minute). A baffle plate can be lowered into the fabric path to produce a semi-milled effect. Solvent treatments and continuous methods have been discussed [66,69]. 8.11.4 Milling Milling or felting is a unique property associated with the scale structure of the wool fibre (section 8.2). The differential frictional effect (DFE) allows the fibre to preferentially move in the direction of its root during milling. Milling is carried out under alkaline (pH 10) or acidic (pH 2) conditions in the presence or absence of detergent. Each process yields its own characteristic handle, acid milling giving a dense felt. The soaps formed in saponification scouring can improve the effectiveness of the subsequent milling process. The objectives of milling are to: 1. improve fabric strength 2. enhance surface appearance 3. increase the fabric density 4. improve fabric handle 5. prepare fabric for subsequent raising processes. There are three important milling procedures in practice: grease, soap and acid milling. Grease milling is a cheap but dirty method of carrying out the milling operation in which the fabric is run through a liquor containing 25 to 30 g/l soda ash and squeezed to a liquor ratio of 1:1 or 2:1. The fabric is then milled to the required cover and dimensions. Soap milling is considered to give the softest and most attractive fabric handle. Sodium or potassium soap is dissolved to give a 5% solution which is used for milling. Specially blended milling soaps give more uniform results, with ease of removal giving savings in cost, energy and effluent. A typical processing sequence is summarised in Table 8.5. Acid milling is often preferred as a milling medium, since the resultant fabric has a greater tensile strength, extensibility and stretch. The process is more rapid than the alternatives and gives denser felting. Less fibre damage occurs, the low pH minimises dye bleeding and grey mixtures are less likely to yellow. Washingoff can be quicker, giving a water saving. Power costs are lower because less weight is required on machine lids and rollers. Wet fabric is less likely to bleed or result in bacterial damage. Whenever the fabric pH is changed from alkaline to acid, a potential processing problem exists. This is more likely to happen in soap milling (Table 8.5) than in acid milling (Table 8.6). Factors requiring control during milling include temperature, pH, milling agents, liquor ratio and fabric dimensions. Significant parameters outside the finisher’s control include wool fibre length and fineness, yarn tenacity, extensibility and elastic recovery, density and structure of yarn and fabric. Studies at WIRA of the milling process have been summarised [70]. Milling is carried out in batchwise aqueous processes and the earliest machines have been described [71,72]. Milling is usually carried out in rotary machines,
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such as that illustrated in Figure 8.8 in which the milling action depends on the generation of heat during the process, the control of pH and liquor ratio and the careful application of pressure. An increase in roller pressure causes weft shrinkage, whereas greater pressure in the spout gives warpway shrinkage. Combined scouring and milling machines, such as that illustrated in Figure 8.9, are rapidly replacing conventional milling machines. These machines are highly productive and economical due to savings in water, whilst control equipment assists in improving the reproducibility of the process. When investing in new machinery for the milling of wool fabrics, two options would now be given serious consideration in the selection of equipment and routines. 1. Solvent scour and carbonise using Carbosol technology, followed by neutralising, milling and washing-off on a combined scouring and milling machine and finally tenter or stenter 2. Greasy carbonise and bake, then neutralise, scour, mill, wash-off in one operation in the combined scouring and milling machine and finally tenter or stenter.
8.11.5 Setting Processes Fabrics likely to develop cockling and ‘crowsfoot’ creasing (width distortion of the design and weave) are crabbed before scouring. During the crabbing operation, the fabric is wet set so that the pre-scoured appearance is retained, this applying particularly to the successful finishing of fine-weave fabrics such as mohairs, tropicals and clear-finished gabardines. In this process, the fabric is permanently set by treatment in roll form with boiling water and steam, shown in Figure 8.10 and Figure 8.11. The machine often consists of two crabbing units, two steaming units and a cold water quench. The fabric, to which cotton end pieces have been attached, passes under even tension in open width by way of a guide roller through the boiling water bath and is wound on to the underside of a roller about 60 cm in diameter, so that the rotating fabric roll is half immersed in the water. The fabric then passes to a second crabbing unit to ensure uniformity of treatment. It is then wound on a perforated steaming cylinder covered with cotton fabric, and steaming is carried out for 10 to 15 minutes with steam at 0.1 to 0.5 MPa (1 to 5 atm) pressure, with the fabric rotating throughout the process. To ensure uniformity, the fabric is run to a second steaming cylinder and the process repeated. Finally it is quenched in cold water. Crabbing and steam blowing must be carried out evenly to ensure level dyeing. Potting is a process somewhat similar to crabbing and is used for high-quality woollen fabrics. The fabric is wound carefully onto a cotton-wrapped perforated iron roller and the roll is placed in a tank of cold water. The temperature of the water is then raised slowly, either to 80°C or the boil, and treatment continued for 3 to 5 hours. After cooling and rewinding to reverse the fabric, the operation is repeated. Rust from the rollers can give staining problems. 8.11.6 Drying Before Storage Fabrics may be stored for prolonged periods before dyeing and finishing, particularly if manufacturing instructions are received early, with colouring instructions to be given later, often as part of a quick response service. Before
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storage, water is removed from the fabric by squeezing or rotary hydro-extraction followed by drying on a tenter (section 8.13.1). It has been found that ‘hot mangling’ can give increased drying efficiency and then the tenter can be run at higher speed. The temperature of the hot water should be 70°C for undyed fabrics. If the technique is used for colour wovens or piece-dyed qualities the temperature should not exceed 45°C. Adverse storage conditions can increase the risk of subsequent dyeing faults [73] and possible microbial contamination. Fabric, usually in roll form or plaited at this stage, should be covered to avoid damp and draughts, with the fabric having a moisture content close to the natural regain and the isoelectric pH of 4 to 5. Water spotting on wool containing residual acid can cause hydrolytic degradation. 8.11.7 Knitted Fabrics Knitted fabrics are normally produced from cleaner yarns and contain small amounts of emulsifiable oils. Scouring is usually carried out in the dolly or in a combined scouring and milling machine at 40°C with synthetic detergent and light milling is given to obtain the necessary degree of cover. Fabrics which do not require milling can be scoured in the dyeing machine prior to dyeing. 8.11.8 Bleaching Wool fabrics are bleached by batchwise methods in a dyeing machine for four hours at 50°C in 4.5 volume hydrogen peroxide stabilised to pH 8 to 9 with sodium pyrophosphate. Semi-continuous processes may be used in which the fabric is padded through an acidic hydrogen peroxide bath, followed by drying and storage in the dark for 48 hours.
8.12 Dyeing Processes for Wool Fabrics The selection of the dyeing class and method for wool fabrics depends on the following factors: 1. the preparatory processes treatments have been given
and
whether
carbonising
or
shrink-resist
2. fabric density, related to the degree of milling 3. required fastness to further processing 4. design and efficiency of the available equipment 5. end-use, with particular reference to health and safety. The comments made earlier in section 8.10 apply equally to fabric dyeing and require only some expansion here. The 1:1 metal-complex dyes can be used with advantage on carbonised material without pre-neutralisation, will give good fastness and cover neps. Acid levelling dyes will also give good penetration of carbonised goods in bright colours of high light fastness but lower wet fastness. Milling acid, 1:2 metal-complex and reactive dyes will give high fastness and are suitable for shrink-resist end-uses but carbonised fabric must be neutralised before dyeing. Control of level dyeing by means of pH, temperature and leveldyeing assistants is required. Chrome dyes are economically attractive for the production of high fastness in dark colours.
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Adequate wetting-out is essential at the start of the dyeing process. The selection of dyes and dyeing method may often be an exercise in compromise. Minimum damage occurs in the isoelectric region and at temperatures below 100°C. Much lower pH values apply when dyeing carbonised fabrics without neutralisation. To obtain levelling and penetration of difficult fabrics, processes have been developed for dyeing at 110°C in the presence of a wool protecting agent (section 8.4), such as Irgasol HTW (Ciba). Shrink-resist materials can often be dyed with 1:2 metal-complex dyes together with suitable complexing-type levelling agents (section 8.10.1) in the isoelectric region. Dyes of this type with anionic solubilising groups give high wet fastness which can be enhanced by aftertreatment with 10% Sandopur SW (Sandoz). Developments in 1:2 metal-complex dye chemistry have been discussed [74]. Reactive and chrome dyes are also widely used for shrink-resist materials. The production of RFT dyeings on wool fabrics has been less successful than with other fibres, perhaps not least because wool dyers have been slow to install new machinery and the associated process control equipment, including dye weighing and dispensing. Fabric preparation, dye selection and the use of an appropriate dyeing technique are essential. The latter may be a compromise between levelness, fastness and reproducibility. Reproducibility is easier to achieve when applying high-substantivity dyes such as the 1:2 metal-complex and super-milling types. Woven wool fabric is dyed using the equipment discussed in section 14.6. The winch dyeing machine is still widely used, including shallow-draught types. The low purchase price of the original machine, probably purchased many years ago, has discouraged investment in control equipment, adding to the problems mentioned above. Jet machines have been used (section 8.12.1) and these are also preferred for weft-knitted wool fabrics. Such machines with associated control equipment and lower liquor ratio lead to improved levelness with highsubstantivity dyes, reduce costs and facilitate production of RFT dyeings. Two particular developments in wool fabric dyeing are worthy of note. 8.12.1 Jet Dyeing of Wool Fabrics With only a small share of the total fabric dyeing market, there have been no specific jet dyeing machine developments for wool. Jets were seen as eventual replacements for traditional winches and also as a means of overcoming problems associated with the dyeing of wool fabrics, especially to high fastness requirements. A survey was carried out, followed by practical dyeing trials, on the five basic types of jet dyeing machines (section 14.6.2) [75]. This indicated that machines of the combined overflow/jet nozzle type with a driven winch reel, which are normally partially flooded, offered considerable advantages for dyeing wool fabric. There was a limit to the weight of woollen fabrics that could be successfully dyed but worsted-type fabrics could be dyed without any problem. Adjusting fabric transport by controlling the overflow cascade, nozzle pressure and the winch reel eliminates running marks and over-working of the fabric surface. The Thenflow jet machine illustrated in Figure 14.10 has been used successfully. 8.12.2 Cold Pad-Batch Dyeing The runs to a colour in wool fabric dyeing are seldom sufficiently long to regard fully-continuous dyeing as a viable technique. Cold pad-batch techniques have
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been developed [76] for the application of reactive dyes to wool fabrics and this can be an economical process for relatively small batches. The fabric is padded in open width by passing through a liquor containing the dye, migration inhibitor, wetting agent and urea. Excess liquor is squeezed out at the nip of the padder to give an expression of 50 to 70%. The fabric is then rolled on to an A-frame or similar storage device, wrapped with polythene sheeting to prevent drying out and stored with rotation to allow dye fixation to occur. The batching time depends on the type of reactive dyes used and the depth of colour. After storage the material is washed off in a dilute solution of ammonia, followed by thorough rinsing. The addition of sodium bisulphite to the padding liquor gives higher yields with some dyes, improved levelness and a reduction in batching times. Cold pad-batch dyeing is discussed in section 16.5.
8.13 Finishing Processes and Routines 8.13.1 Drying After wet treatments, including preparation, dyeing or wet finishing processes, the fabric must be dried before storage or transport to the next dry stage. A fabric may thus be dried several times during the wet processing sequence. This is necessary not only to extract the moisture but also to remove creases and stabilise the fabric dimensions for the next operation. The drying requirements of the plant, especially for thermal drying machines such as tenters, must be calculated with some care. The bulk of the water from the interstices of the fabric is removed by a relatively cheap mechanical method. This is either by rotary hydro-extraction or by mangling in open width. High-speed rotary extractors will normally reduce the moisture content of wool fabrics to about 25% (on dry weight of fabric) whereas mangling, even when using hot mangling methods, will leave about 45% moisture on the fabric. The latter method is less labour-intensive, avoids the risk of creasing and allows the fabric to be plaited mechanically for transfer to the next process. Thermal drying of woven fabrics is traditionally carried out on a multi-layer tenter as illustrated in Figure 8.12, although single-layer stenters, with infra-red predrying, are now widely used in worsted finishing. These facilitate the setting of wool/polyester fabrics. Thermal drying consumes much energy and every effort should be made to reduce the moisture content to a minimum before the fabric enters the tenter. There is a tendency to over-dry and the tenter speed should be adjusted so that the dry cloth exits the machine with a moisture content approximately equal to the natural regain of the wool (about 30%). Tentering also removes creases and stretches the fabric, but at the pre-tentered width it is in the fully relaxed state and if the fabric is over-stretched, subsequent shrinkage problems are likely to occur. If the pre-tentered width is significantly less than the desired final width, tentering to width may have to be followed by a setting process such as crabbing. It is usual to tenter woollen fabrics 5 to 9 cm wider and worsted fabrics about 5 cm wider than the desired finished width. Approximately 10% overfeed is normally applied.
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Modern tenters are fitted with control equipment for fabric speed, temperature, moisture content and relative humidity, weft-straightening devices, a heat exchanger to recover sensible heat and a filtration system to remove fumes. Heat setting capacity is necessary for processing polyester and its blends. Circular-knit fabrics are hydro-extracted as for wovens and thermally dried over a perforated heated tube (Figure 14.11). Alternative methods for drying circularknit fabrics include suction-drum drying, the use of a brattice dryer or slitting of the fabric before stenter drying. Fabric from flat-bed knitting is dried on a stenter, which in construction is similar to a single-pass tenter. Uncurling devices ensure that the selvedges are opened out and flat before the fabric is pinned. 8.13.2 Mechanical Finishing Wet finishing includes processes such as carbonising, scouring, milling and crabbing, as described earlier. After dyeing and drying, woollen fabrics normally receive several dry finishing treatments, these being fairly extensive compared with those for worsted fabrics. After drying, the first operation may be damping the fabric by either a water spray or steam to make it more receptive to mechanical finishing processes. As mentioned above, intermediate drying may be carried out to minimise creases or skewing of the weft. Raising Raising treatment can dramatically alter the appearance of the fabric by either raising or depressing the pile surface. The fabric is passed over the surface of a cylinder covered with either natural teasels or card wire. Care is required in this process since it impairs the fabric strength. The card-wire raising machine (Figure 8.13) is more productive than the teasel gig. Raising is usually started on the former and completed on the latter, although certain effects can only be obtained on the teasel gig. The speed and effectiveness of raising depends on factors such as fabric construction and tensions, fibre length, yarn count and twist, previous processes such as milling, presence of moisture, pH and softeners, machine settings and the age of the card wire or teasels. Raising should begin gradually to obtain a uniform finish. Moisture assists raising but the process can be carried out either wet or dry. In wet raising the effect is to lay the raised pile in the teasel direction on the fabric. Dry raising produces a more spongy effect on fabrics that have not been milled. It is customary to raise after milling so that raising takes place on a consolidated fabric. The warpway stress causes an increase in fabric length and some weft shrinkage, whilst a weight loss occurs by the complete removal of some fibres. The weight loss is less in wet raising than in dry raising. Wet raising gives a more even effect and has a gentler action. Teasel raising is accepted as the best method to raise woollen fabrics since the alternative metallic wire methods cause more fabric damage. Raising is often started with used teasels to give a gentle action before changing to new ones. Excellent dense, laid-pile doeskins and billiard-table baize fabrics are produced in this way. With card-wire machines, single or double-action raising techniques are possible as shown in Figure 8.14. The single-action machine consists of 18 to 24 rollers covered with card wire and revolving in the opposite direction to the drum upon which they revolve. The
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fabric passes over the surface of the drum or barrel against the card rollers, so that one side of the weft yarn is raised. Several passes of the fabric over the barrel with the same end leading results in the formation of a laid pile, but it has a much fuller handle than that produced by wet teasel raising. By alternating the leading end, velour mossy and velvety finishes are produced. Typical running speeds are about 15 m/minute. In double-action raising, the machine can have twice the raising capacity of the single-action machine. The raising barrel and the fabric run in the opposite direction to the pile and counterpile card-wire rollers. The intensity of raising action of the pile and counterpile rollers can be varied, many finishes being attainable in this highly productive and versatile machine, which can also incorporate a wire laying brush. Fabric speed can be varied but is typically 20 m/minute. A slightly raised effect is obtained on wool fabrics using a sueding machine consisting of a series of rollers covered with sandpaper or emery cloth. The degree of contact between the abrasive surface and the fabric can be varied using guide rollers. Fabrics which do not require dry raising are simultaneously steamed and brushed to lift the surface fibres before shearing. It is usually desirable to brush again after shearing to remove fibrous debris and restore the uniformity of the pile. Cropping Shearing, cutting or cropping is carried out to trim and level the surface pile produced by raising and to remove the surface fibres in clear-finish fabrics. The machine consists of revolving knives and an adjustable bedplate, as shown in Figure 8.15. The back of the fabric is firstly sheared to remove knots and tails which have been pushed through to the back during greasy mending and burling. If these were not removed, the fabric would pass unevenly over the bedplate leading to damage by the cutting blade. Shearing of the face then follows, either once or twice. A close crop will result in a crisp, stubbly handle, whereas a longer trim gives a softer handle. Modern cutting machines are fitted with a metal detector to prevent damage to the blade, automatic adjustment of height of cut, sharpening without removing the blade, automatic seam detection, automatic lubrication of the cylinder felts and direct drives so that the tension between cutting heads can be varied. Running speeds are between 5 and 20 m/minute and a cutting machine usually has one backing blade, a turnover and two face cuts, although other combinations are possible. Laid-pile finishes should be wet set after the raised pile has been cropped to the height required. The pile should be relaid to its original pre-cut state before setting. This process reduces the sensitivity of the pile to gentle friction and enhances the durability of the finish. The best results are obtained by potting in water at 75 to 80°C, as described earlier; the longer the immersion, the more durable the finish. The Hemmer Konticrab can also be used and this process is not so time-consuming as potting. Subjective visual assessment of the effects of raising, cropping or emerising finishes is still widespread but it is prone to error and is not a suitable basis for process control or optimisation. In a newly developed objective method, CCD matrix cameras record grey-shade images of the fabric nap and these are processed in digital-image systems. This is followed by image interpretation,
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where various parameters are computed from the images. Detailed evaluation programmes were necessary to correlate objective data obtained in this way with the visual assessments reached by a panel of experienced observers. The objective system has been adapted and refined for on-line operation on the raising machine [77]. Napping Napping may be carried out after raising or cutting. The fabric is drawn forward at about 4 m/minute on to a stationary plush-covered table above which is suspended a mobile pad covered with rubber or glass paper. Naps or beads are formed on the fabric surface by the action of the pad, this being controlled by eccentric cams to give a variety of effects. The fabric is treated stepwise in lengths of 0.5 to 1 metre at a time. Pressing Pressing is carried out on all clear-finish woollens and worsteds to make them firm, solid and smooth. This finish is often lost at the first stage of making-up. In a hydraulic or vertical press, the entire piece or several pieces are pressed simultaneously after plaiting mechanically, with electrically heated papers inserted manually between the layers of fabric. A pressure of about 3 to 3.5 MPa (430 to 500 lbf/in2) is applied and heating times vary from 10 minutes to 3 hours. The fabric is then refolded so that the edges are in the middle and the process is repeated. An automatic flat press consists of a fixed top plate, an intermediate plate and a rising plate, all of which are heated. A ram below the rising plate applies the pressure. The fabric is passed to and fro about five times between the plates via guide rollers, so that five thicknesses are pressed at one time. Pressure is applied when the rollers and the fabric are at rest. After a given time, the pressure is released and the plates separated. The cycle may be repeated four times per minute and the temperature and pressure are adjusted according to the fabric and the finish required. Creasing of fabric folds is avoided and, unlike rotary pressing, the fabric is not stretched. Flat presses are now seldom used, however, whereas rotary pressing is much more common. A rotary press consists of a steam-heated cylinder, 40 to 60 cm in diameter, which fits into the hollow of a steam-heated bedplate. The fabric is carried through the press by rotation of the cylinder and a pressure of 3.5 to 7.0 MPa (500 to 1000 lbf/in2) is exerted by pressing the bedplate against the cylinder. The machine operates continuously at about 5 to 10 m/minute. Double-cylinder machines are available to give the fabric two pressings. The press may be provided with a belt or apron to allow for light pressing, thus giving fabrics some degree of stretch. Gentle steaming will eliminate any excess lustre or the papery handle associated with pressing. Different effects can be obtained: 1. a high lustre with the face to the bedplate 2. a lower lustre with the face to the cylinder 3. a more papery and firmer handle using a high pressure.
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Decatising Continuous decatising has been the most important worsted finishing process over the last two decades. The objectives of decatising fine wool fabrics after shearing are to: 1. decrease the undesirable lustre caused by pressing, 2. allow any necessary shrinkage and 3. set the fabric dimensions. In wet decatising, hot water is adequate to achieve these desired effects. The decatising characteristics of the wet finish are more durable and less easily reversed than dry decatising, which only gives acceptable results with steam [78]. Dry setting can be carried out by blowing with steam, autoclaving or kier decatising, or by the addition of chemicals. Steam blowing improves the appearance, handle and drape of the fabric while removing creases and crimps. The fabric is run between a cotton wrapper onto a perforated cylinder; steam is passed through the wrapper for 2 to 3 minutes and the steam removed by pumping. Wrapping tension, steaming conditions and the pumping cycle must be controlled. Adequate moisture must be present and this is often obtained by dewing followed by a twelve-hour dwell wrapped in polythene. For a more durable set, the fabric is wound as for steam blowing but the autoclave is evacuated to a pressure of about 60 KPa (0.6 atm). Intermittent bursts of steam are introduced to pre-heat the package and create a passage for the steam through the fabric. At the end of the evacuation cycle, steam is injected steadily through the batch until a predetermined pressure of about 70 KPa (0.7 atm) is reached. The steam is extracted from the batch until the autoclave is again evacuated to 60 KPa (0.6 atm), when the cycle is complete. Woollen fabrics are generally steam-decatised at atmospheric pressure to consolidate the structure. Appearance and handle can be varied by modifying the wrapper pressure, times of evacuation and steaming, and the direction of steam flow - inwards or outwards through the batch. Prolonged steaming with inwards flow at high wrapper pressure gives a durable lustrous finish. Conditions at the opposite extreme confer softer and less lustrous effects, but at low wrapper pressure the batch may collapse. Handle is greatly improved by conditioning in a cold damp atmosphere for 24 hours. Chemical Setting Chemical setting can be achieved by treating the wool with a reducing agent which is capable of breaking the pattern of cystine disulphide crosslinks and these can form again in a new structural arrangement after setting in steam. The usual processing sequence is: scour and mill, shrink-resist, dry and set. A typical setting process is to pad the fabric to give an uptake of 50% of a 1% solution of the reducing agent at pH 4.5 to 6, followed by immediate steaming for 5 minutes on a blowing machine at 0.5 to 0.6 MPa (5 to 6 atm) pressure. Sodium bisulphite, ammonium thioglycolate, tetrakishydroxyphosphonium chloride (THPC) and monoethanolamine sulphite (MEASAC) have been used. MEASAC has been used to set durable creases in trousers and skirts using a spraying technique followed by steaming. Careful dye selection is required.
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Many of the processes discussed above are applicable to knitgoods but these seldom require mechanical finishing. Milling to give a surface cover is important, whilst shrink-resist treatments for knitgoods are discussed in section 8.14. Inspection Inspection of fabrics as part of an overall quality control system is carried out at several stages in the processing sequence. Greige inspection is almost always carried out, with intermediate inspections before and after dyeing. Final inspection is always undertaken and faulty fabric rejected for reprocessing or downgrading. Inspection is carried out on a perch (a well-lit framed sloping window) that allows the fabric to be measured for length and examined by both transmitted and reflected light. Knotting, Burling and Mending Following inspection, these operations are necessary to overcome blemishes in the fabric: 1. knots are eliminated by interlacing the yarn into the woven fabric 2. burling removes debris such as any cellulosic residues that survive carbonising 3. mending involves repair of any holes that have emerged during processing.
8.13.3 Finishing Routines The individual stages in the preparation and finishing of wool fabrics have been discussed in earlier sections of this chapter. An optimum sequence has to be established to process a given fabric quality and this is often almost unique to the individual manufacturer and finishing plant. The possible variations in sequences have already been highlighted. Figure 8.16 indicates typical procedures for specific woollens and worsteds. Finishing routines for woollens are usually longer than those for worsteds, with knitgoods requiring the least finishing. There is thus a great deal of handling and the assembly of batches differs according to the processes being used. Single pieces are run into the machine in rope form and sewn head to tail for batchwise scouring, milling and winch or jet dyeing. Many single pieces are sewn head to tail in a long run for open-width processing, as in carbonising, scouring, drying and most dry finishing processes. 8.13.4 Continuous Processing It is perhaps not surprising that attempts have been made to develop continuous finishing methods for wool fabrics, since these would reduce the handling required and open-width processing eliminates running marks and creases associated with rope processing. Much longer runs are required for continuous operation and greater control of fabric quality and width is required. Continuous processing on modern equipment would favour reproducibility by the elimination of variations in dyeability arising from competing fabric lengths prepared at different times by batchwise techniques but dyed together. Continuous processing for certain wool fabric qualities has been available for many years. Continuous solvent scouring and carbonising can be carried out by the Carbosol technique and crabbing with drying if required can be followed by cold pad-batch dyeing, washing-off, suction extraction and tenter drying.The
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sequence of finishing processes, typically brushing, shearing, humidification and conditioning, pressing, decatising and shrinking, could all be carried out in-line in open-width. Hemmer has developed a continuous decatiser (the Kontiset) and a continuous dry finishing machine (the Kontipress). The milling process in rope form may remain the main obstacle to fully-continuous processing. For continuous open-width operation a single-storey modern building is desirable and this would probably necessitate investment on a greenfield site with a major capital outlay for buildings, services, machinery and technical support. Long runs at speeds in excess of 20 m/minute would be possible. Such an investment would be difficult to justify nowadays. An economic study in the 1960s [79] indicated significant savings in capital cost and the cost of labour and resources, although improved laboratory support facilities would be required. Developments in this area have been reviewed [80].
8.14 Garment Processing Woollen knitgoods are produced in two forms: half-hose (such as socks of various kinds) and garment blanks (such as pullovers, cardigans, jumpers and their accessories). The hosiery industry produces fabrics in seamless form on circular knitting machines or fully-fashioned goods on flat-bed machines. A major problem for the garment dyer is the penetration of seams and linkages. There is also a tendency for knitted fabrics to roll at the selvedges and this causes the seams to roll on one another. 8.14.1 Dyeing and Finishing of Half-Hose After knitting, linking and mending, socks are received for dyeing with the ‘right’ side outwards. Coarse qualities and those containing cellulosic or synthetic fibre blends are usually processed in this way. Finer qualities are turned inside-out to protect the ‘right’ surface. Scouring is carried out in the dyeing machine (normally side-paddle or rotary-drum machines) with alkaline detergent. The treatment time varies with the degree of consolidation required. Almost all wool socks are given a shrink-resist treatment (section 8.14.4) applied before or after dyeing. Treatment before dyeing avoids colour changes attributable to the process or ancillary treatments with chemicals such as sodium bisulphite. Improved handle and wearing properties are obtained if the socks are treated after dyeing but the dyes used must be fast to the treatment. Whichever method is selected, dyes of high wet fastness must be used since most shrink-resist treatments cause modification of the wool fibre so that dye desorption occurs more readily. Treatments can be carried out in side- or overhead-paddle machines at room temperature. The socks are run in water before dyeing is commenced, the pH checked and then adjusted according to the class of dyes to be applied. The dyes and methods for applying them have been discussed in section 8.12. After dyeing at top temperature for 20 minutes, the penetration of the heels and toes is checked and, if satisfactory, dyebath exhaustion is completed by the addition of acid. The dyebath is cooled, the socks are given a cold rinse and a cationic softener is applied in a fresh bath for 15 minutes at 50°C. The socks are then rotary hydroextracted and tumble dried.
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During the final ‘trimming’ process, the socks are sorted in sizes, placed on wooden formers of the required size and pressed between two metal surfaces at about 115°C for 1 minute. 8.14.2 Dyeing and Finishing of Garments Cardigans, pullovers and sweaters are frequently made up as garment blanks in the ecru (undyed) state or containing dyed yarn if the blanks are for finishing only, with necks and stoles knitted separately for sewing on after finishing. The various sizes must be identified by tagging and the accessories kept with the appropriate blanks. Generally necks are linked in dozens and the garments turned ‘right’ side inwards for processing. The accessories are usually placed in bags for dyeing. Occasionally the garments may be placed in bags in dozens, if a minimum of face marking is required or if distortion is a problem. Garments are usually dyed to one of three in-service specifications, namely, dryclean only, hand-wash and machine-washable. The processing route depends on this specification and the stage at which the wool is shrink-resist treated. Three types of wool are typically used for garments: 1. botany (worsted-spun, merino, usually for machine-washable) 2. lambswool (either woollen- or worsted-spun) 3. Shetland (normally loose stock or yarn dyed). Processing sequences are shown in Figure 8.17. An anti-cockle treatment is necessary for certain knitted qualities that distort during wet processing. Treatment is given on a side-paddle for 10 minutes at the boil at a liquor ratio of 30:1 with 1 to 3% sodium metabisulphite and 3% nonionic detergent. The steam is then turned off and the garments allowed to stand for 10 minutes. The liquor is circulated again, the bath cooled to 60°C for 10 minutes and the garments rinsed and drained. Scouring may be carried out in the dyeing machine with alkaline detergent or with 2 g/l detergent, 2 g/l tetrasodium pyrophosphate and 1 g/l sodium sulphate for 6 minutes at 40°C followed by rinsing. Milling may be carried out with 4 g/l soap and 1 g/l tetrasodium pyrophosphate at 40°C to attain the desired cover, followed by two rinses. Process times vary from 20 to 40 minutes in a side-paddle to 2 to 5 minutes in a rotary-drum machine. One-bath scouring and milling is also practised where the oil content of the fabric is low or where self-emulsifiable lubricants have been used. Proprietary products for scouring and milling containing detergent, water-softening agent, solubiliser and pH buffers are often used. Excess water is removed by rotary hydro-extraction and the garments tumbledried for 5 to 15 minutes at 70 to 75°C. Garments are then ‘trimmed’ and ‘framed’ to size and shape. These frames are pre-heated to avoid condensation. The garments are placed on a flat-bed press operating at a steam pressure of 0.15 to 0.2 MPa (1.5 to 2.0 atm), steamed on the open table for 3 to 5 seconds and then cooled. The garment is then removed from the frame, laid out on the steam table and pressed at 102 to 105°C with the sequence: steam for 5 seconds, cool, steam for a further 2 seconds if necessary, cool and blow for 3 seconds and cool.
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8.14.3 Solvent Techniques for Garments Scouring, milling and shrink-resist treatments can be carried out from a solvent medium using machinery similar to that used for industrial dry-cleaning. Machine loading for wool is usually lower (60% or less) than conventional loading. As in dry-cleaning, the recovery and reuse of solvents is an important factor. Scouring is carried out by simply immersing the goods in dry-cleaning solvent at room temperature in the absence of water or processing aids. Treatment is for 2 to 3 minutes, after which the soiled liquid is removed by extraction. So-called charged systems, with water present in the solvent, are necessary for solvent milling. Three techniques have been described [81]. In the steam milling procedure, steam is injected while the cage is rotated and the goods are milled for a pre-determined period before pure solvent is pumped into the machine and the goods are given a conventional solvent scour. The goods are centrifuged and dried. In the conventional solvent milling process, the goods are scoured in clean solvent and extracted at high speed for 3 to 4 minutes. Milling is then carried out to give the desired cover with an emulsion of milling aid, water and perchloroethylene followed by high-speed centrifuging, the milling liquor being returned to the addition tank. Low-surfactant milling is similar to the above process but a lower quantity of milling aid is present. Much antiquated equipment has been used for processing both garments and hose and some of this has been described previously [71,81]. Side-paddles are still widely used and modern versions are available complete with control equipment and automatic unloading. Rotary-drum machines have gained in importance, since high-quality products are obtained by this method. These machines can be highly automated and will accommodate the complete process sequence. Garment dyeing and relevant machinery are discussed in sections 17.1 and 17.2. 8.14.4 Shrink-Resist Treatment for Garments Most shrink-resist treatments are applied to wool tops, as discussed in section 8.8.1, but wool knitwear is shrink-resist treated with aqueous liquors in sidepaddle and rotary-drum machines. Solvent techniques are also available. Traditional aqueous procedures were based on chlorination, using acidified sodium hypochlorite, but the rapid reactions of chlorine and hypochlorous acid with wool made the process difficult to control. Excessive oxidation led to dye decomposition, accompanied by wool yellowing and loss of strength. The chlorine generators commonly used more recently are alkali-metal salts of dichloroisocyanuric acid (DCCA), typified by Basolan DC (BASF). In acidic solutions these precursors are capable of liberating approximately 35% of their own mass as available chlorine. In the presence of wool, the salt may react directly with the fibre as well as being decomposed into chlorine and hypochlorous acid, which subsequently react with the fibre. Typically, wool is chlorinated with DCCA at pH 3.5 to 4.0 and a temperature of 20 to 25°C. The process is outlined in Figure 8.18. Where a high level of machine washability is required, it is usual to apply a basic polymer subsequent to the chlorination process. Dyeing may be carried out after chlorination followed by the application of the cationic resin. This then performs the dual purpose of improving shrink-resistance and dye fastness. A process recommended by BASF is based on chlorination with Basolan DC, antichor and rinse, dye and finally apply Basolan F.
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Although the chlorination-polymer sequence has been widely applied, it can cause yellowing of the wool and dye decomposition, resulting in colour changes and a reduction in wet fastness. Chlorination treatments are now in decline for ecological reasons and chlorinated residues are unwelcome (section 2.7). Chlorine-free processes have been developed to overcome these problems, based on the application of reactive precondensates to the wool which are subsequently polymerised on the fibre surface. Commercial processes are based on the use of Nopcolan SHR3 (Henkel) [81] and Synthapret BAP (BAY) [82]. Organic solvent processes are available for applying reactive precondensates, such as the DC 109 (Dow Corning) process. This is a two-pack system consisting of a poly(dimethylsiloxane diol) and a low Mr alkoxysilane compound which, in the presence of atmospheric moisture, hydrolyses and reacts with the diol to form a crosslinked polymer. Since solvent processing can be carried out under essentially anhydrous conditions, polymerisation does not begin until the goods are removed from the solvent and exposed to the atmosphere, thereby overcoming the problem of polymer depositing inside the machine. The treatment level is 3.5 to 4.0% polymer on the weight of the goods with a polymer/catalyst ratio of 9:1. The garments are saturated in the resin solution and extracted to 100% liquor retention. The catalyst solution is sprayed on to the garments which are then tumbled for 10 minutes followed by drying. The silicone elastomer is formed on exposure to moist air. This process does not impair the fabric handle, pilling propensity is reduced and wet fastness is increased.
8.15 Speciality Animal Fibres Wool is by far the most important of the animal fleece fibres but several others are of interest. These include mohair and cashmere from species of goat, alpaca and vicuna from camel species, angora fur from rabbits, wool from the llama that is native to the Andean plateau and yak hair from the humped Tibetan ox. Most of these speciality fibres, notably cashmere, angora and mohair, are relatively scarce and costly but they may be blended with high-quality wool to increase lustre and give a distinctive appearance. Demand for such blends is largely subject to the dictates of fashion. Hand-knitting yarns are luxury items and processing costs tend to be low relative to retail prices, so that more attention can be paid to high quality rather than productivity and material cost. Some cashmere is diluted with fine wools for economic reasons but may still carry a cashmere label. Following an upsurge in the late 1980s and steady growth for the next decade, the cashmere industry in P R China suffered a temporary collapse during the severe recession in Asian markets at the end of the 1990s. China is home to 120 million goats and accounts for more than 60% of the global raw cashmere output [83]. Exports decreased 16% by weight and 30% by value. Demand fell markedly in Japan and Korea, normally major consumers of cashmere, silk and angora. Cashmere prices in Inner Mongolia, source of over 20% of the world’s supply, dropped to 30% of the early 1980s levels. Even this golden opportunity to obtain these attractive fibres at historically low prices failed to kick-start market demand until the 2000s [84]. Yak hair is a warm fibre with elasticity, good resistance to moisture and a softness comparable to silk. The Chinese province of Qinghai holds vast herds of yaks and considerable development is in progress to exploit this resource. Yak wool is being used increasingly for knitwear garments in China and demand is
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buoyant because this fibre is substantially cheaper than cashmere. However, with some blends so far containing as little as 5 – 10% of yak hair, strenuous efforts are being made to improve overall quality standards [85]. The natural colour of animal fur is closely related to the environment in which the species normally lives. Although most animals for speciality fibre production are domesticated and bred by large-scale ranching, on smaller farms in China or the Andes there may be little control of the natural colour [85]. Major problems faced by processors of speciality fibres arise from this natural pigmentation, such as: 1. non-uniform distribution of the pigment in the material 2. the natural colour is often too dark for pastel colours or bleaching to white 3. small proportions of heavily pigmented fibres may be present in light-coloured or near-white fleeces [86]. In all instances, however, bleaching processes can help to solve these problems. On the other hand, bleaching is the most delicate and hazardous wet treatment during the entire processing of speciality animal fibres. Any risk of inadequate control can give rise to serious fibre damage. Careful control of process parameters in bleaching is essential. Peroxide bleaching conditions for a range of speciality keratin fibre types have been recommended [87], as indicated in Table 8.7. In the case of Persian lamb (karakul) fibres, bleaching at 70°C for a shorter time may be advantageous. It is important to bear in mind that the inherent variability in these materials dictates that quality and behaviour may differ greatly from lot to lot. Preliminary laboratory trials are essential before embarking on the processing of new batches. The outstanding properties of angora and cashmere in knitwear apparel are well known. These fibres will not withstand prolonged boiling, so reproducible colour matching and first-class levelness are essential [88]. Angora is only processed in blends with wool, sometimes with the addition of a small proportion of nylon to improve the durability. For economic and technical reasons 1:2 metal-complex and milling acid dyes are preferred. Chrome dyes and 1:1 metal-complex types are seldom used because strongly acidic dyebaths may damage the angora. A simple test using CI Acid Red 18 and CI Basic Blue 9 can be used to assess the degree of oxidation damage to the angora. The bluer the staining with this mixture, the greater the degree of damage [89]. Solidity can be achieved using a retarding agent, the addition needed being dependent on the applied depth and blend proportion, particularly the amount and quality of any nylon present. The recently introduced organo-chromium mordant SCA-Cr has been applied in the bulk-scale chrome dyeing of cashmere for more than two years. About 80 tonnes of loose stock have been dyed in this way in P R China, where more than 60% of the world’s cashmere is produced and processed. The new method has replaced the conventional dichromate aftertreatment. The concentration of Cr (VI) in the residual dyebath is greatly decreased (to less than 0.01 mg/l). There is much less oxidative damage of the cashmere during treatment, so that the quality of the final product is greatly enhanced [90]. Major outlets for wool/mohair blends are worsted outerwear and suitings. Such fabrics may be made from intimate blends for both warp and weft, but they often consist of a mohair warp with a botany wool weft. The enhanced lustre and good wear properties make these blends suitable for lightweight suitings and dresswear. Wool/mohair fabrics may be piece-dyed with 1:2 metal-complex dyes or more economically with levelling acid dyes. If the blend is to be used in
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suitings, it is customary to dye the wool and mohair separately in sliver or top form for subsequent blending. The rate of dyeing and equilibrium exhaustion on mohair fibres are higher than for wool fibres of similar diameter. Visual and instrumental assessment of colour yield, however, shows little difference between the two fibre types dyed separately with the same dyebath concentration of an acid dye. These measurements support the view that the pronounced surface lustre associated with mohair is responsible for its apparently lower content of absorbed dye when compared with other less-lustrous wools dyed from the same bath [91]. Mohair and wool show a similar tendency to yellow as a result of aqueous oxidation or thermal treatments. Urea-bisulphite solubility data indicate that mohair suffers less modification under mild conditions but this position is gradually reversed with increasing severity of treatment [92]. Thus the loss in mass during aqueous treatment is ultimately greater for mohair than for wool.
8.16 The Cultivation and Properties of Silk Real silk filaments are produced by silkworms, the larvae or caterpillars of the moth Bombyx mori, which feed on the leaves of the mulberry bush. This insect larva generates a double filament from two spinnerets, the individual filaments being surrounded and held together by sericin, the silk gum. These filaments are used to form a cocoon or protective covering for the chrysalis or pupa, the next stage of development towards the moth imago. The pupae within the harvested cocoons are killed by steaming or baking and the cocoons softened by treatment in a hot soaping bath. The silk is gathered by reeling off, the filaments from 4 to 12 cocoons being reeled together, and the thread thus produced is called greige silk. At this stage the filaments are held closely together by sericin, which still accounts for approximately 20 to 30% by mass of the silk threads [93]. Silk is produced in several countries, mainly in the Asia Pacific zone, as indicated in Table 8.8. The fibres from different regions contain varying amounts of sericin proteins that differ somewhat in chemical and physical properties [94]. Until the late 1970s Japan was the world’s major producer of raw silk but this place is now held by P R China. However, in the late 1990s production in China declined again, as a result of disastrous floods in Hubei province and the need to boost food production [95]. The demand for silk textiles from Japan and Korea also declined markedly for economic reasons, leaving a surplus that Europe and India failed to absorb. Raw silk prices tumbled by 20 to 25% in 1998. Different qualities of silk yarn are produced for various end-uses by doubling, trimming and twisting greige silk threads. The top-quality yarn is tightly twisted for warps under the name organzine and voluminous high-quality threads are combined to make trame silk for wefts. Only the middle third of a typical cocoon (approximately 300 to 1200 m of filament) is suitable for producing organzine and trame yarns. The remainder, as well as damaged cocoons that cannot be reeled off, is more variable in quality and classified as floss silk suitable for staple yarn production. Schappe is worsted-spun silk with staple lengths in the range 50 to 250 mm, whereas bourette is woollen-spun from combed floss and has staple lengths of 10 to 50 mm. The residual material from these operations is worked up and used under the name of silk noils. Apart from these various forms of cultivated silk, lower-priced dresses, saris and scarves are made from Tussah or wild silk, filaments exuded in
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a similar way by the larvae of the oak moth Antheraea, which feed on oak tree leaves [96]. Tussah threads and the associated sericin have a distinctly brown colour and thus require bleaching because of their tannic acid content. In contrast to mulberry silk, Tussah threads are more variable in physical properties and contain only 12 to 20% sericin. All forms of silk are prone to mechanical damage (chafing and pilling), requiring careful handling through all wet processing treatments. Silk fibroin, like wool keratin, is a protein fibre but there are major differences in composition, most notably the virtual absence of cystine crosslinks (section 8.2). A detailed comparison between silk fibroin and wool keratin in terms of physical and chemical properties has been provided [97]. A summary of the broad chemical differences is shown in Table 8.9. Basic and acidic side-chains occur much less frequently along the silk fibroin peptide chains and therefore electrostatic links between them make a much lower contribution to the internal structure of the fibre.
8.17 Silk Degumming Sericin forms the firm outer covering of greige silk and is classified as a kind of scleroprotein or protective sheath. The sericin content of the filaments varies according to their position within the cocoon, those in the outermost layer having the highest content. Sericin is not a homogeneous protein but can be separated into four fractions on the basis of their differential solubilities in hot water [98]. Sericin and fibroin differ considerably in their chemical composition and accessibility, sericin being comparatively easily accessible to degumming chemicals [94]. Raw silk does not exhibit the lustre or soft handle normally associated with silk garments. These characteristics emerge when the sericin is removed by the degumming or boiling-off process. Depending on the degree of degumming, three categories of silk have been defined [99]: 1. Ecru or bast silk is lustreless raw silk with an intrinsic yellowish brown colour. It is only slightly degummed, resulting in a loss in mass of 4% or less, mainly grease, wax and resin. 2. Half-boiled or souple silk is partially degummed, containing 11 to 14% sericin with a degumming loss of approximately 6 to 12%. 3. Fully degummed or cuite silk has a soft handle and high lustre. The degumming loss is 18 to 30% [100]. Optimum conditions for the partial degumming of mulberry silk yarn with soap and sulphuric acid at the boil have been evaluated. The objective was to achieve an economical process capable of yielding souple silk suitable for weaving fabric constructions with the characteristic stiffness and scroop (or rustling sound) given by acid treatment of real silk. This souple silk showed enhanced colour strength compared with cuite silk, without significant change in fastness to washing [99]. Various methods are available for degumming: aqueous extraction, boiling-off in soap, degumming with alkalis, enzymes or in acidic solutions. The use of soap or synthetic detergent in alkaline solution has been standard practice for many years. The raw silk may be given a preliminary soak for one hour at 50°C, then cooled and treated overnight in 2 g/l soda ash solution. The actual degumming stage that follows may be a one- or two-bath procedure. In the one-bath process,
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treatment is for 2 to 3 hours at 90 to 95°C with 5 g/l olive oil and 0.3 to 0.5 g/l soda ash. Both methods are usually followed by a mild alkaline rinse in 1 g/l soda ash at 50 to 60°C and a final warm rinse in soft water. In view of the rarity of silk, it is perhaps surprising that continuous degumming ranges have been developed [101]. The relative effectiveness of various degumming processes has been examined. Mulberry silk fabric was treated by five different methods (acid, alkali, enzyme, triethylamine and soap) and the results compared in terms of handle and mechanical properties. Soap was taken as the standard method and significant differences in low-stress behaviour of the silk were observed. Alkali, amine and soap methods scored over the acid and enzyme methods in terms of softness and handle. Inferior shear and surface properties characterised the acid- and enzymetreated silks, indicating non-uniform removal of sericin from these samples. Whiteness index was marginally higher for the enzyme- and amine-treated silks than for the other three samples [102]. In a similar investigation, silk degumming processes using Marseilles soap, a syndet Miltopon SE (Henkel) and an enzyme Alcalase (Novo Nordisk) were compared for physical properties, whiteness and dyeing behaviour. The air permeability and transparency of the woven fabrics were increased to different extents according to the effectiveness of the three processes. Their influence on dyeing behaviour was much less critical. Colour differences between the silks after degumming and dyeing were mainly attributable to the differences in whiteness index after degumming [103,104]. The loss in mass and volume that occurs during degumming has been compensated traditionally by ‘weighting’ treatments using metal salt solutions. In recent years the environmentally unattractive phosphate-silicate weighting process has been largely supplanted by graft polymerisation using methacrylamide as the monomer and ammonium peroxydisulphate as initiator. Other acrylic monomers have been evaluated but they do not impart the desired physical characteristics to the weighted silk. The influence of acidic pH, temperature and time of treatment on the weighting effect conferred by methacrylamide grafting was examined [105]. Bleaching of cultivated silk is normally carried out only for pale to medium dyeings or fabric sold as white. Reductive bleaching with hydrosulphite can be combined with alkaline degumming [106] but it is more usual to give an oxidative bleach separately after degumming. A typical process would be for 2 to 4 hours at 70°C or 1 to 2 hours at 80 to 85°C, using 10 to 15 ml/l hydrogen peroxide (35% solution), 1 g/l EDTA (30% solution) and 3 g/l sodium pyrophosphate to pH 9, followed by thorough rinsing.
8.18 Dyeing of Silk The dyeing behaviour of silk is similar to that of wool but silk typically requires between two and four times as much dye as wool to achieve a similar visual depth because of the fineness of the silk filaments. Milling acid, 1:2 metalcomplex, direct and reactive dyes are the most important classes for silk dyeing [106,107]. Hank and package methods are suitable for yarn dyeing. Fabrics may be dyed on winches, jets, jigs, beams, or in traditional star frames to ensure a gentle action with delicate constructions. Pad-batch, pad-roll and pad-steam methods are available if sufficiently long lengths per colourway are required.
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Traditionally, the concentrated soap solution remaining after degumming was used to provide an anionic levelling system for the application of direct and acid dyes. This approach has been superseded by dyeing with 1:2 metal-complex and milling acid dyes in the presence of levelling and penetrating agents [107]. Typically, dyeing is commenced at 30°C in the presence of 1 to 2% of a weakly cationic levelling agent at pH 4.5 to 5.0. The temperature is raised at 1°C per minute to a top temperature in the range 70 to 85°C, depending on dye exhaustion and the type of equipment available. Dyeing is continued for 45 to 60 minutes at top temperature before cooling and rinsing. Lanaset (Ciba) dyes have been widely used with Albegal SET as levelling agent for the dyeing of silk under these conditions. Virtually all reactive dyes are applicable on silk. Published research on the mechanisms of fixation and optimum application conditions of the major classes of reactive dyes has been reviewed in detail [108]. The formation of crosslinks in silk by bifunctional reactive dyes and their effects on fibre solubility and physical properties have been investigated [109]. The market leaders on wool and silk are the Lanasol (Ciba) bromoacrylamide dyes. Dyeing is commenced at 30°C and the temperature raised at 1°C/minute to 80°C. Sodium sulphate (20 to 80 g/l depending on applied depth) is added portionwise during the temperature-rise stage. After 20 minutes at 80°C, soda ash (1 to 2 g/l) is added and fixation completed in a further 20 minutes. The dyeing is cooled, rinsed, soaped for 15 minutes at 80°C, rinsed and dried.
8.19 Finishing of Silk The finishing processes applied to silk fabrics are usually restricted to those intended to restore the desirable features of degummed silk. Treatment with acetic, formic or lactic acid in warm water will enhance the unique scroop of silk, which is also conferred by acid degumming and application of acid dyes under mildly acidic conditions. A soft handle is obtained by applying up to 2% of a cationic softener at 35°C and pH 6.0 to 6.5. Crease-resist and easy-care properties can be achieved by conventional pad-dry-bake application of Nmethylol reactants of the types used for crosslinking cellulose. Formaldehyde-free reactants such as butane-1,2,3,4-tetracarboxylic acid and polyurethane coatings have also been applied successfully to silk [110]. The finishing of silk by sandwashing resembles the stone-washing of denim jeans. The abrasive action of pumice stones modifies the fabric surface to give enhanced softness and enables the silk garment to be washed rather than dry-cleaned. The process is carried out in a rotary-drum washer. Alternative techniques include emerising with sandpaper or sand powder, or the use of an enzyme such as Bactosol SI (S). Dyeing with Lanaset, metal-complex or reactive dyes may be carried out before or after sandwashing. However, when dyed silk is sandwashed there may be slight changes in colour and dye uptake may be unlevel if prior sandwashing is applied without adequate control [111].
8.20 Spider Silk Industrial and protective fabrics require materials that are both strong and resilient, properties that are often mutually exclusive. The search for improved fibre structures has led to the study of spider silks in recent years [112]. Not only
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is a spider’s web remarkably strong, it functions as a highly sensitive information system enabling the spider to detect an intruder weighing only 0.1 milligram. These sinewy protein threads, only a few thousandths of 1 mm in diameter, will withstand higher loads than comparable steel wire filaments. These fibroins appear to have a well-ordered helical structure reinforced by fibrils [113]. Although spider threads and Kevlar (DuP) aramid filaments are similar in wet and dry tensile strength, their elongation at break values are 30% and 4% respectively. Moreover, spider filaments remain elastic even at -40°C. DuPont and US Army researchers are working on the potential laboratory production of man-made spider silk filaments. A feasibility study has analysed the cloning and recombinant expression of fibrous proteins from segments of webs produced by the orb-web spider Nephilia Madagascariensis. The native spider silk fibroin could be reproduced with 96% identity using the synthetic gene Spidroin 1. Using advanced computer simulation techniques and recombinant DNA, a biosilk fibre has been prepared [114]. Preliminary solubility and coagulation behaviour studies using native silk protein from the mulberry silkworm Bombyx mori as a model material have revealed problems concerned with optimal rates of the coagulation process and simultaneous improvement of the degree of orientation of the fibroin macromolecules during the extrusion process [115]. Spider silk fibroins are of great interest as potential high-performance materials that are biocompatible, biodegradable and recyclable. Bulletproof vests, parachute cords and lightweight helmets are potential uses, as well as civil aviation, bridge cables and composite materials. In biomedical applications, the human body is unlikely to react less favourably to spider protein than to conventional synthetic polymers [116].
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W Mosimann, Amer. Dyestuff Rep., 80 (Mar 1991) 26.
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A C Welham, Dyer, 169 (May 1984) 38.
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A C Welham, Wool Record, 147 (Apr 1988) 73..
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D M Lewis, JSDC, 98 (1982) 165.
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J A Taylor, Rev. Prog. Coloration, 30 (2000) 93.
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K Hannemann, Wool Record, 159 (Jun 2000) 48.
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K Hannemann and P Runser, Dyer, 184 (Apr 1999) 22.
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F Hoffmann, W Langmann, H Elbertzhager, W Gruttke and M Schnee, Textilveredlung, 15
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J Park, JSDC, 87 (1971) 114.
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P Zanaroli, Dyer, 183 (Apr 1998) 28; Melliand Textilber., 79 (1998) 446.
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A survey of scouring and milling in the wool industry. B J F Moxon and J A Barritt, Wira
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The scouring of woollen and worsted pieces. B J F Moxon, Wira Publication 223 (July 1964).
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Anon, Wool Science Rev., 23 (1963) 13.
(1980) 130.
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An account of Wira’s work on milling. B J F Moxon, Wira Publication 216, (March 1961).
[71]
The dyeing and finishing of wool fabrics. I Bearpark, F W Marriott and J Park (Bradford: SDC,
[72]
Wool fabric finishing. H K Rouette and G Kittan, (Ilkley: WDI, 1991).
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D R Lemin, JSDC, 86 (1970) 169.
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D M Lewis, Wool Sci. Rev., 54 (1977) 30.
[75]
R R D Holt and F J Harrigan, IWS/CSIRO Publication (1979).
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Anon, Wool Sci. Rev., Special edition, ITMA, Paris (1971).
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H Mehlhorn and H Beier, Melliand Textilber., 81 (2000) 404.
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K Wasser and K H Lehmann, Melliand Textilber., 81 (2000) 293.
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M A White, Text. Progress, 13 (2) (1983) 1.
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K R F Cockett, Wool Sci. Rev., 56 (1980) 2
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K R F Cockett, JSDC, 96 (1980) 214.
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Anon, Wool Record, 157 (Nov 1998) 31.
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P Meyer. Wool Record, 157 (Oct 1998) 23.
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Anon, Textile Asia, 28 (Nov 1997) 88.
[86]
A Bereck, Rev. Prog. Coloration, 24 (1994) 17.
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A Bereck, 2nd Internat. symposium on speciality animal fibres, Aachen (1989); Reports of the German Wool Research Institute at RWTH Aachen, 106 (1990) 20.
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J A Galek, Dyer, 163 (Feb 1980) 133.
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F Sakli, M van Parys, R Dubois and J Knott, Melliand Textilber., 69 (1988) 191.
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J Xing and M T Pailthorpe, JSDC, 116 (2000) 91.
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M B Roberts and E Gee, SAWTRI Bull., 11 (Sep 1977) 32.
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M B Roberts, SAWTRI Tech. Report, 351 (1977).
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Chemistry of natural protein fibres, R S Asquith (New York: Plenum Press, 1977) 53.
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J Prabhu, S Sanne and T H Someshekar, JSDC, 111 (1995) 245.
[100] J Hilden, Internat. Text. Bull., Dyeing/Printing/Finishing 31(1985) 41. [101] D H Wyles in Engineering in textile coloration, Ed. C Duckworth (Bradford: SDC, 1983). [102] S Chopra, R Chattopadhyay and M L Gulrajani, J. Text. Inst., 87 (1996) 542. [103] W B Achwal, Colourage, 44 (Mar 1997) 33. [104] E Schmode, S S Truckmeier, J H Dietrich and R D Reumann, Textilveredlung, 31 (1996) 90. [105] J M Marzinkowsky, J Trappe and B Hambsch., Melliand Textilber., 72 (1991) 538. [106] R Hofstetter, Melliand Textilber., 72 (1991) 366. [107] K Y Chu and J R Provost, Rev. Prog. Coloration, 17 (1987) 23. [108] M L Gulrajani, Rev. Prog. Coloration, 23 (1993) 51. [109] D Agarwal, K Seri and M L Gulrajani, JSDC, 112 (1996) 321. [110] B Tsang and R C Dhingra, Textile Asia, 27 (May 1996) 56.
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[111] K P S Cheng and K C Yeung, Textile Asia, 24 (Aug 1993) 32. [112] P F Hamlyn and B J McCarthy, Rev. Prog. Coloration, 31 (2001) 15. [113] F K Ko, Textile Asia, 28 (Apr 1997) 38. [114] A P Aneja, Textile Asia, 29 (Oct 1998) 36. [115] K Heinemann, K H Gührs and K Weisshart, Chem. Fibers Internat., 50 (2000) 44. [116] K Kälberer, Chem. Fibers Internat., 48 (1998) 489.
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Table 8.1 Composition of greasy wool (%) Grease and suint 15–30 15–30 5–15
Wool type Merino Medium crossbred Long–staple wool
Sand and dirt 5–40 5–20 5–10
Vegetable matter 0.5–10 1–5 0–2
Wool fibre 30–60 40-65 60–75
Table 8.2 Wool dyeing by method (1994) [46] Dyeing process Loose stock Slubbing Hank Package Fabric and garments Printing Other
% of total 15 40 11 15 16 1 2
World production 1.75 megatons p.a.
Table 8.3 Estimated proportion of wool dyed worldwide in 1995 with various ranges of dyes [47] Dye range
Proportion (%) 29 29 20 9 7 6
Chrome mordant 1:1 Metal-complex Milling acid Levelling acid 1:2 Metal-complex Reactive
Table 8.4 Reactive dyes for wool
Dye range Lanasol (Ciba) Drimalan F (S) Verofix (Bayer) Hostalan (Hoe)
Year launched 1966 1969 1970 1971
Reactive group α-Bromoacrylamide 2,4-Difluoro-5-chloropyrimidine N-Methyltaurine-ethylsulphone β-Sulphatoethylsulphone
Auxiliary Albegal B Lyogen FN Avolan REN Eganol GES
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Table 8.5 Processing sequence for soap milling of fabrics Stage Scouring Carbonising Neutralising Soap milling Washing-off Levelling acid dyeing
pH 10 2 7 10 7 3
Table 8.6 Processing sequence for acid milling of fabrics Stage Scouring Carbonising Washing-off and acid milling Levelling acid dyeing
pH 10 2 3 3
Table 8.7 Preferred bleaching conditions for the peroxide bleaching of pigmented speciality keratin fibre types [87]
Fibre type Angora Alpaca Cashmere Karakul Vicuna Yak Mohair
Hydrogen peroxide (ml/l 35%) 20-40 25-45
Time (minutes) 45-120 45-180
Temp (°C) 50-60
pH 8–8.5
30-50
60-180
60-70
8.5
Liquor ratio 15:1; sodium pyrophosphate (10 g/l) and ammonia to required pH
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Table 8.8 Worldwide production of raw mulberry silk [94] Country China India Japan Korea Turkestan Brazil Others
Proportion (%) 56 14 11 7 6 3 3
World annual production 62.95 kilotons in 1988
Table 8.9 Comparison between wool and silk in terms of content (%) of the main amino acid types [97] Amino acid types Nonpolar Hydroxy-containing Basic side-chains Acidic side-chains Sulphur-containing
Wool keratin 34 22 10 24 10
Silk fibroin 76.6 17.0 3.4 2.7 0.3
Practical Dyeing, Volume 2 Figure 8.1 Schematic diagram of the morphological components of wool.
Figure 8.2 Stages for wool dyeing Fibre Loose stock dye Combing process Top/sliver dye Spin Yarn dye Fabric formation (weave, knit) Fabric or garment dye
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Figure 8.3 Wool dye grid
Leveldyeing Acid
1:1 Metalcomplex
Chrome
Migration
Half-milling Acid
Milling Acid Super-milling Acid 1:2 Metal-complex Reactive
Wet fastness
Figure 8.4 Typical preparation sequences Sequence A
Sequence B
Sequence C
Sequence D
Carbonise Scour Alkaline mill Dye
Scour Carbonise Acid mill Neutralise Dye
Scour Carbonise Alkaline mill Dye
Scour Alkaline mill Carbonise Neutralise Dye
Sequence E Scour Alkaline mill Dye Carbonise
Sequence F Solvent scour Carbonise Neutralise Dye
Sequence G Scour Carbonise Semi-neutralise Acid mill Dye with levelling acid dyes
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Figure 8.5 Continuous carbonising range 7
2
9
3 8 4
5
1
1 2 3 4 5
6
Guider Set of cleaning brushes Impregnation tank High squeezing padder J-section for intermediate storage
6 7 8 9
Equipment to neutralise selvedges Drying carbonising chamber Cleaning chamber Exit pleater
Figure 8.6 The Carbosol process
Figure 8.7 Scouring dolly A B C D E F G H
F A E B G D Floor H C
Squeeze roller Squeeze roller Main trough Internal (or suds) trough Guide roller Guide roller Draft board Fabric path
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Figure 8.8 Rotary milling machine 3.55 m
1
4 2
5
3
6
6
11 10
1 2 3 4 5 6 7 8 9 10 11
9 7 8
Delivery winch Soap and soda inlets Light Expanding mouthpiece Milling trough Inspection door and window Main drive Main tank Adjustable draft gate Slippage sensing detector Safety knock-off
Figure 8.9 Combined scouring and milling machine 5 2
4
1 3
6 7
10
9
8 3.38 m
1 2 3 4 5 6 7 8 9 10
Milling boards Main drive Water spray Safety knock off Delivery winch Slippage detector Draft gate Expanding mouthpiece Main tank Trough
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Figure 8.10 The Yorkshire crab
Figure 8.11 The Konticrab (Hemmer)
1 2 3 4 5 6 7 8 9 10
Straining beam Cloth guider Hot water trough Water trap Expanding roller Steam case Guide roller Endless rubber blanket Fabric expander device Cooling trough
Figure 8.12 Multi-layer tenter
11 12 13 14 15 15a 16 17 18
Squeezing rollers Guide roller Transport hasp Plaiter Heating drum Condensate drain Cleaning device for endless blanket Cleaning device for heating drum Regulating device for endless blanket
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Figure 8.13 Card-wire raising machine
Figure 8.14 Principles of single- and double-action raising Fabric
Cylinder Fabric
Cylinder
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Figure 8.15 Principle of a shearing machine
A
D
A B C D
Revolving knives Fabric path Bedplate Doctor blade
B
C
Figure 8.16 Typical processing sequences Woollen velour
Woollen twill
Worsted/woollen
Scour Carbonise/bake Dry mill Neutralise Soap mill Dye Tenter Knot Decatise Raise Cut Felt Cut Steam
Scour Carbonise/bake Dry mill Semi-neutralise Acid mill Dye Tenter Knot Steam Cut Decatise
Crab Scour Dye Tenter Crop Blow
Worsted serge
Worsted gabardine
Cavalry twill
Scour Mill Dye Tenter Damp Crop (2x face,1 x back) Rotary press Kier decatise
Crab Scour Dye Tenter Crop (twice) Steam Kier decatise
Crab Open-width scour Dye Tenter Brush and damp Crop (3x face, 1x back) Blow
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Figure 8.17 Processing routes for garments Botany
Lambswool
Shetland
Anti-cockle Scour Chlorinate (if not done) Dye Resin treatment Soften Finish out
Scour/mill Chlorinate (if required) Dye Resin treatment Soften Finish out
Scour/mill Shrink-resist Soften Finish out
Figure 8.18 Typical sequence for SR treatment of wool garments Stage 1 Scouring
Details
2
Milling
3
Chlorination
DCCA, 2 to 5% owf, pH 3.5 to 4.0, 40 to 60 minutes at 20 to 25°C
4
Antichlor/neutralisation
Sodium sulphite, 5% owf Soda ash, 3% owf 20 minutes at 30°C
5
Rinsing
Heavy depths – pH 8 with ammonia 5 minutes at 50°C
6
Polymer application
Hercosett 125 – 16% owf or Basolan SW – 3% owf pH 4 to 6 for 20 minutes at 25 to 30°C
7
Softener application
8
Hydro-extraction
9
Drying
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R H2N
COOH
8.2
O
H
H
R2
8.3
R3
N
N O
CH2 S S CH2
HOOC
H
8.1 R1
H2N
COOH
N
N O
H
8.4
HN R O
O
O
R
HN
O
phenylalanine
valine
(CH2)4NH3+ O
O
NH
HN
ionic bond
–
lysine
OOC CH2
aspartic acid
NH
CH2 CH2C O
HN
O
glutamine
hydrogen bond
HO CH2
CH2
O
S
S cystine
O
HN CH2
HN
CH2 CH2 CO NH CH2 CH2 CH2 CH2 O
N-(γ-glutamyl)lysine
O
isopeptide crosslink
CH2 CO NH CH2 CH2 CH2 CH2 N-(β-aspartyl)lysine
8.5
R O
R O
NH
O isopeptide crosslink
O
NH
serine
disulphide crosslink
R
NH
O
NH2
NH
HN R
CH2
HN
NH O C
O
hydrophobic bond
CH3
NH
HN
R
CH3
NH
HN R
CH
R O
NH
HN
R
NH
O
NH2 COOH
Chapter 9 Polyester Dyeing 9.1 Production and Properties of Polyester Fibres The discovery of poly(ethylene terephthalate) fibres by Whinfield and Dickson of the Calico Printers Association in 1941, by the condensation of ethylene glycol and terephthalic acid, was a major step forward in the emergence of the synthetic fibres industry [1]. Carothers and his team at DuPont had investigated a series of aliphatic polyesters from simple glycols (HO-R-OH) and dibasic acids (HOOC-RCOOH) in the late 1920s but had turned to aliphatic polyamides with a view to achieving higher melting points. This approach had proved more successful, eventually leading to the discovery and commercialisation of nylon fibres. The important concept established by Whinfield and his co-workers was that the incorporation of benzene rings within the polymer backbone of terephthalic esters (9.1) raised the melting point by strengthening the forces of cohesion between the chain segments. 9.1.1 Production and Recycling of Poly(Ethylene Terephthalate) Polyester fibres are manufactured throughout the world and marketed under many trade names. Modified forms of poly(ethylene terephthalate) 9.1: R = (CH2CH2) and other polyester variants derived from different dicarboxylic acids or diols are produced. Moderate- or high-tenacity filament yarns of various types are available, as well as staple fibres varying in dimensions according to the requirements of the spinning machinery for which they are intended. Staple fibres are normally drawn sufficiently to yield moderate tenacity but may be extruded from molten polymer of relatively low molecular mass to give improved pilling performance accompanied by a tolerable loss in abrasion resistance. Continuousfilament tow of unusually high total denier is produced for processing on tow-totop convertors or on direct-spinning machines. Filaments and staple fibres are usually smooth in appearance and circular in cross-section, although trilobal, pentalobal or other types may be encountered. Staple fibres are usually crimped and many filament yarns are bulked or textured by false-twist or other treatments during manufacture. World polyester production continues to increase steadily, most of the new manufacturing capacity being located in the Asia Pacific region. Average annual growth of 7% is forecast for the period up to the year 2008. The industry is highly cyclical and margins swing wildly. Economic difficulties in some Asia Pacific nations resulted in oversupply and excess capacity in the late 1990s but production and demand were expected to return into balance early in the 2000s. Polyester materials drive the demand for purified terephthalic acid (PTA), which in turn controls demand for the key intermediate p-xylene. The four major polyester market segments are fibres, plastic bottles, films and speciality resins, representing 72, 18, 6 and 4% of PTA demand, respectively [2]. Growth rates (13%) in the plastic container segment are twice those for polyester fibres (6%). The proportion of integrated production has doubled to 40% over the last decade, either by extending ownership within the production chain or by intercompany alliances and partnerships. DuPont is still deeply involved globally in polyester fibres, films, intermediates and recycling/regeneration, regarding such integration measures as the best chance for success in this challenging market [3].
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Worldwide usage of polyester plastic bottles has quadrupled since 1990 and poses a serious waste disposal problem. Montefibre and the European Replastic Consortium (formed to recycle plastics) are involved in a project to manufacture polyester fibres from post-consumer recycled bottles. After removal of nonplastic contaminants, PVC bottles are separated by an X-ray detector to identify chlorine. An optical detector distinguishes opaque polythene bottles from transparent polyester, as well as clear from coloured ones. Polymer flakes are made from the bottles by washing, grinding and flotation. Commercial production of Terital Eco staple fibres from these flakes commenced in 1996. Physical properties and dyeing performance closely resemble virgin polyester, although whiteness of the Terital Eco staple is somewhat below standard [4]. 9.1.2 Chemistry and Properties of Polyester Oligomer In common with other condensation polymers, the reaction between ethylene glycol and terephthalic acid gives rise to a small proportion of oligomers of low molecular mass. The most important of these is the cyclic oligomer tris(ethylene terephthalate) (9.2), together with much smaller amounts of linear oligoesters such as the dimer and the pentamer. Commercial samples of polyester fibres typically contain between 1.5 and 3.5% of the cyclic trimer, depending on the source of the sample and the method of solvent extraction, using dioxan or a chlorinated hydrocarbon. The proportions and types of cyclic and linear oligomers present is determined by an equilibrium established in the molten state. Thus a polymer sample from which the oligomers have been extracted will tend to regenerate the original equilibrium if remelted. Cyclic tris(ethylene terephthalate) is a crystalline solid that melts at 314 to 319°C. Although it can be hydrolysed by strongly alkaline solutions, the reaction is very slow because the cyclic trimer is almost insoluble in aqueous media (Table 9.1). It is readily soluble in certain organic solvents and in molten polyester but on heat treatment of polyester fibres the oligomer present tends to diffuse slowly towards the polymer surface. Conditions that favour the formation of surface deposits of oligomer include high-temperature dyeing or the steam setting of polyester yarns. Little migration is observed in steaming or aqueous treatment under pressure at a temperature below 110°C, but the degree of oligomer release increases rapidly between this temperature and typical heat setting or pressure dyeing temperatures in the range 125 to 135°C. After high-temperature dyeing for about two hours at 130°C, a deposit of about 0.2% by mass of cyclic trimer is present on the surfaces of typical polyester fibres. An approximately equal amount can be detected as a dispersion in the dyebath and deposited on the internal metal surfaces of the dyeing vessel. These amounts of oligomer released by the polymer tend to increase progressively with dyeing time and temperature. The dispersed oligomer present in the dyebath will deposit crystals in regions of high hydrodynamic shear when the temperature is lowered at the end of the dyeing period. If the dye liquor is discharged without cooling, via drains that are specially modified for the purpose, the degree of deposition on the fibre surface and the interior of the dyeing vessel can be minimised. This method yields a much cleaner dyeing because discharging with pressure release avoids the recrystallisation and filtration of mixed crystalline deposits of disperse dyes and oligomer on the polyester fibre material in a circulating-liquor machine. Difficulties arise when oligomer deposits interfere with the frictional characteristics of polyester yarns in
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spinning and fabric manufacture, or when localised high concentrations arise by filtration or drainage effects. Crystalline oligomer deposits can build up on the interior surfaces of dyeing vessels, pumps and pipework, from which they are difficult to remove without severe cleaning treatments. If the incidence of such deposition is to be minimised, cleaning of the dyeing machines at regular intervals is necessary, preferably after no more than about 100 hours of polyester wet processing. At 130°C the cyclic trimer deposits are attacked by 5 g/l sodium hydroxide solution and this concentration can be adopted as the basis of an effective machine cleaning treatment. The intensity of the cleaning effect is enhanced more by the temperature increase from 100 to 130°C than by increasing the concentration of alkali to much higher levels. 9.1.3 Development of More Readily Dyeable Polyester Variants Unlike poly(ethylene terephthalate) 9.1: R = (CH2CH2), or to a much lesser extent poly(butylene terephthalate) 9.1: R = (CH2CH2CH2CH2), poly(trimethylene terephthalate) 9.1: R = (CH2CH2CH2) has not achieved significant importance as a raw material for textiles. This is probably because the necessary glycol intermediate has not been available in sufficient quality and purity until recently [6]. Propane-1,3-diol is now available from Degussa in Germany using a novel and more cost-effective process [7]. Shell Chemical has established a new facility in West Virginia to produce Corterra, a fibre-forming polymer based on poly(trimethylene terephthalate) or PTT. This is claimed to combine many of the advantages of both conventional polyester (PET) and nylon fibres [8]. PTT fibre exhibits the chemical resistance of polyester and the outstanding elastic recovery and resilience of nylon. It has excellent abrasion resistance and generates only low levels of static electricity. Corterra possesses an inherent resistance to staining and a low soiling tendency. The rate of water absorption is slow but the glass-transition temperature is lower than that of poly(ethylene terephthalate) fibres. Thus PTT fibres are dyeable with conventional disperse dyes at the atmospheric boil without addition of a carrier and pressure-dyeing equipment is not essential. Dyebath exhaustion is high and continuous dyeing, space dyeing and printing methods of application have been developed [9]. The dyeings and prints show good fastness to UV radiation, ozone and oxides of nitrogen [10]. Conventional PET fibres must be dyed under pressure at about 115°C in order to achieve the same colour yield at a given applied depth as a dyeing on PTT fibres at 99°C. Furthermore, the extent of dye penetration at a given dyeing temperature is markedly greater for PTT dyeings than for conventional PET dyeings. These effects were demonstrated in dyeing trials using the high-energy monoazo diester CI Disperse Blue 139 and the low-energy anthraquinone dye CI Disperse Red 60 at 85, 95, 105 and 115°C. Cross-sections of dyeings of CI Disperse Blue 139 at 95 and 120°C on both fibre types clearly showed the difference in penetration [11]. Polymer chips made by blending PET and PTT variants were melted and extruded to give a compromise balance of dyeing and mechanical properties [12,13]. Fibres containing only 10% of PTT blended with 90% of PET exhibited markedly improved elastic recovery without significant loss of tenacity and modulus. With 30% PTT present, the dyeability of the blend was much superior to pure PET and even significantly better than the pure PTT control. Blends of poly(butylene
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terephthalate) with PTT polymer were less interesting because both of these variants are more costly to manufacture than conventional PET and the differences between these PBT/PTT blends and the pure components were less significant [12]. Blended PET yarns containing varying amounts of PBT fibres were analysed for oligomer and their mechanical properties investigated. Drawn yarns with 10% or 25% PBT present were found to contain the least oligomer by extractive methods and were selected for disperse dyeing tests. The presence of PBT increased dye uptake to such an extent that similar depths of shade were obtained on 75:25 PET/PBT at 100°C and on pure PET at 130°C [14]. Elasticity is essential in sportswear, underwear and leisure clothing. It is usually achieved with elastic-core yarns or, more economically, textured nylon or polyester yarns. Textured poly(butylene terephthalate) has elastic recovery values intermediate between those of polyurethane and conventional textured polyester (PET). Polyesters from butylene glycol were patented in 1941 along with those from ethylene glycol, but commercial production of PBT fibres did not commence until the early 1980s in Japan and the USA. Despite initial optimism, usage was modest in lingerie, beachwear, sportswear, stretch jeans, carpets and rugs. Renewed interest arose in the 1990s, when price levels were lowered much closer to standard polyester. In addition to traditional uses, apparel stretch yarns produced by folding a textured PBT yarn with a natural staple yarn are being promoted. The elastic recovery and the relative ease of dyeing of PBT at the boil are seen as important assets for the future success of PBT textiles [15]. During the 1980s Hoechst introduced a flame-retardant variant of poly(ethylene terephthalate) under the tradename Trevira CS (comfort and safety). A small proportion of the terephthalic acid units in the conventional polymer were replaced by P-methylcarboxyethylphosphinic acid units (9.3). The improvement in flame retardancy given by this reactive additive rose quite steeply until a level of about 0.4% phosphorus was reached, but further amounts produced little additional benefit. With such a minor modification the existing melt spinning and drawing techniques required little adjustment to accommodate Trevira CS production. By 1990 an annual usage of about 22 kilotons of this variant had become established. The presence of the phosphinate ester units lowers the melting point and glass-transition temperature by about 40°C. Colour yields on Trevira CS at 120°C are about 20% higher than conventional PET dyeings and pale depths can be dyed at the boil without carrier. Fibres pre-set at 180°C are more readily dyeable than unset Trevira CS but the dyeing temperature must not exceed 120°C or a loss in fibre strength may result [16].
9.2 Development and Characteristics of Polyester Microfibres 9.2.1 Physical and Chemical Properties of Polyester The absorption of water by polyester fibres is very low. Thus polyester garments dry quickly at ambient temperature and there are no significant variations in the tensile properties of polyester yarns and fabrics with humidity. The moisture regain of poly(ethylene terephthalate) is approximately 0.4% at 65% relative humidity and 20°C [17]. Polyester fibres have a high initial modulus of elasticity, high resistance to bending deformation and good recovery behaviour, negligible creep under low extension and high resistance to abrasion. Certain staple-fibre
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variants have below-average resistance to abrasion, if this property has been sacrificed to a controlled extent in order to confer a lower propensity for the formation of surface pills from loose fibres during wear. Polyester fibres show thermoplastic behaviour and thus their tensile properties vary with temperature. The tenacity of poly(ethylene terephthalate) fibres at 180°C is approximately half that at ambient temperature and the extensibility is higher. Unset filament yarns shrink when heated and the degree of shrinkage increases with yarn tenacity. The shrinkage of an average-tenacity unset yarn is approximately 10% in saturated steam or in water at 140°C but only 8% in dry air at the same temperature. Commercial polyester yarns, however, are normally given a thermal stabilisation treatment during manufacture or subsequent heat setting, so that the observed shrinkage of these set yarns is usually much lower than 8 to 10%. Nevertheless, if the setting temperature is approached or exceeded in a later process, shrinkage is again observed. Polyester fibres show outstanding resistance to a wide variety of chemicals. Dilute solutions of mineral acids and concentrated organic acids cause little or no loss in fibre strength even on prolonged treatment at or near the boil [17]. These fibres exhibit excellent stability in oxidising or reducing media. They are unaffected by the strongly alkaline conditions necessary for bleaching, mercerising or vat dyeing of cotton. Hydrolysis of ester groups in the polymer is not significant in dilute alkaline solutions at the boil. More concentrated alkaline or organic basecatalysed treatments are sometimes applied, however, to modify the fibre surface layer and so produce a softer, more silk-like handle. Hot solutions of sodium sulphide normally exert a similar action to aqueous sodium hydroxide of equivalent concentration, but unexpectedly rapid attack of the polyester is sometimes observed when applying sulphur black to the cotton component of a polyester/cotton blend. Chemical attack causing progressive chain scission of the ester groups in the polymer can be distinguished from surface erosion produced by hydrolytic agents at lower temperatures by measuring changes in the intrinsic viscosity of a solution of the polymer in a suitable nonhydrolysing organic solvent. Intrinsic viscosity (IV) provides an indicator of the relative molecular mass of the polymer and is usually calculated from viscosity data for a 1% solution of the fibres in redistilled o-chlorophenol (BP 175-176°C) measured at 25°C, although other solvents have been used in similar methods of test. Poly(ethylene terephthalate) fibres of average tenacity usually have an IV in the range 0.60 to 0.67, although low-pill staple variants may have an IV as low as 0.4. If the IV is lower than about 0.55, the abrasion resistance is likely to be significantly inferior to that of standard fibres with average tenacity. The rate of hydrolysis of the ester groups in a polyester fibre is extremely slow but becomes significant during exceptionally prolonged treatments in steam or boiling water. There is no detectable difference between the degradative effects of liquid water or saturated steam at the same temperature. The degree of hydrolysis is virtually independent of filament denier or staple fibre type. The rate of loss of tenacity increases by a factor of 1.082 per degree C rise in temperature. Thus this rate approximately doubles for a temperature rise of 9°C. The rate of loss is 0.12% per hour at 100°C, increasing to 1.28% per hour at 130°C. The effect of pH is particularly important: at pH values less than 4 or greater than 8, high-temperature dyeing conditions can cause serious degradation of polyester fibres if the treatment is prolonged. Figure 9.1 indicates the effects on the tenacity of a polyester staple yarn of dyeing for 4 hours at 130°C in blank
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dyebaths buffered to various pH values. The optimum range for stability is between pH 4.5 and 6.0, which is also the optimum region for ensuring the stability of those disperse dyes that are sensitive to variations in dyebath pH. Enzymic surface modification of polyester fibres under mild conditions improves several unattractive features of the polymer. The enzyme polyesterase is a serine esterase that cleaves ester bonds in the main chain to release water-soluble fragments. This treatment decreases lustre, removes residual polyester size, enhances hydrophilicity, increases the uptake of basic dyes, minimises pilling and improves release of oily stains [18]. 9.2.2 Dyeing and Finishing Behaviour of Microfibre Variants The trend towards the increasing use of finer deniers in polyester filament yarns and staple fibres for blended yarns has been evident since the 1970s but developments in spinning technology have facilitated this progressive trend in the 1990s [19]. With the arrival of polyester microfibres, synthetic fabrics that resemble natural silk in handle and appearance have become possible. Silk has long been regarded as the most attractive fabric for dresswear garments owing to its characteristic drape and lustrous appearance but silk lacks the wash-wear durability of synthetic fabrics [20]. Trevira Finesse and Micronesse (Hoechst) polyester microfibres are three times finer than flax or wool, twice as fine as cotton and finer than the finest silk [21]. A microfibre has been defined as a variant with a cross-sectional diameter that is less than half the diameter of a conventional fibre [22]. As Table 9.2 indicates, the fineness range of polyester textile filaments in commercial use has decreased from 3 to 5 dtex in the 1970s to only 1 to 3 dtex at the present time. The introduction of microfibres with a fineness of 1.0 dtex or less in the 1990s has accelerated this trend and greatly extended the potential applications of the new generation of synthetics (or shin gosen in Japanese). Estimates of worldwide polyester microfibre production amounted to about 80 kilotons in 1992, less than 1% of total polyester textiles [23]. Growth towards an interim target of around one million tons p.a. has continued since then but it is rather difficult to separate out data for the microfibre market because of differences of interpretation as to what qualifies as a microfibre. Polyester microfibres can be used to design fashionable outerwear fabrics with a silk-like handle and smooth drape. They arouse a pleasant feeling when in contact with the skin, providing the convenience of a synthetic with respect to washing, drying and ironing, combined with the comfortable softness and attractive appearance of a natural fibre. Microfibres are particularly adaptable to sportswear with improved transmission of moisture and one of their best-known applications is in the polar fleece, providing excellent thermal insulation. The fineness of microfibre yarns is especially suitable for the tightly woven constructions necessary in waterproof garments [24]. Further varied end-uses include velour and suede fabrics, imitation leather, heat insulation, filter fabrics, liquid absorbents, paper, ion-exchange materials and certain biological treatments [25]. Desizing, scouring, bleaching and heat setting treatments suitable for optimum handling of polyester microfibre fabrics before dyeing have been reviewed [26,27]. The differences from conventional qualities become clearly evident at the dyeing stage. The colour yield achieved is closely related to the fineness of the individual filaments or staple fibres present in the yarns. The amount of dye required to give the target colour yield is inversely proportional to the square root
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of the filament denier [28,29]. This relationship was first demonstrated in the 1940s and the relative percentages of dye required for filaments of different average fineness shown in Table 9.2 were calculated accordingly. The implications of the effect of increasing fineness on the applied concentrations of dye required are self-evident. In practice there are many factors that can modify the accuracy of this relationship, including the content of delustring agent present, variations in cross-sectional shape, differences of crystallinity and orientation of the polymer structure and the effect of texturing on dye uptake [30]. Nevertheless, the relationship remains a useful, if approximate, means of estimating the amounts of dye required for different yarn types. The rate of dyeing of microfibres is more rapid than that of conventional polyester at a given temperature because of the higher specific surface area per unit mass of filaments. Compatible dyes with good build-up and level dyeing properties are required. Precautions during the rise to top temperature are advisable, limiting the rate of dye uptake between 90 and 100°C over a period of 20 to 25 minutes to promote migration before the exhaustion stage at 130°C [29]. Selection of auxiliaries for jet dyeing is important: a deaerating agent Albatex FFC (Ciba), a lubricant such as Cibafluid W or Irgalube BOA (Ciba) to minimise creasing and Tinegal NT (Ciba) as a dispersing and levelling agent have been recommended [26,27]. Typical end-uses for polyester microfibres can be accommodated by optimising dye selection and designing suitable application methods to achieve level dyeing and adequate fastness to light and wet treatments. The relatively greater amounts of disperse dyes required on microfibres have a highly significant influence on build-up performance. Minor differences in build-up between different component dyes in a combination dyeing on conventional polyester are exaggerated when applying the same combination of dyes on polyester microfibres. The build-up and fastness properties of six disperse dyes were compared on conventional and microfibre fabrics under the same dyeing conditions. The microfibre substrate required three- or four-fold increases in applied concentrations of dye to match the corresponding dyeings on conventional polyester [31]. The relative amounts of dye required to produce dyeings of identical visual depth on conventional and ultrafine polyester were determined for CI Disperse Blue 60 and Reds 82, 86 and 302 at 130°C. The temperature sensitivity of these effects at 110 to 130°C was also studied [32]. In a more detailed investigation, three series of polyester microfibre yarns were yarn-dyed at 130°C with CI Disperse Red 60: 1. ca. 90 dtex yarns containing from 150 to 32 filaments of dtex 0.6 to 3, 2. 64 fil yarns of 38 to 192 dtex from the same series of filament diameters, 3. 19 to 5.7 dtex yarns containing 32 to 95 filaments of 0.6 dtex. The results provided ample evidence to confirm that the finer the microfilaments, the paler is the visual colour yield on the substrate for the same dyebath concentration in equilibrium with a given concentration of absorbed dye in the polyester filaments. The number of filaments in the yarn has no significant influence on the visual colour yield [33]. It is important to select disperse dyes of high fastness to light for microfibre polyester as the rate of fading tends to be significantly more rapid on microfine filaments. Fastness to light is influenced by the amount of delustrant present, as well as the fineness and cross-sectional shape of the microfilaments [32,34]. The
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optimum degree of stability to photofading on a microfibre polyester fabric was found at applied depths of approximately 2% or higher for typical disperse dyes [31]. The levels of fastness to sublimation, rubbing and washing on microfibres are significantly inferior to those on conventional polyester because of the greater amounts of dye necessary to achieve target colour yields [32,34]. Standards for fastness to light, washing, perspiration, rubbing, hot pressing and dry-cleaning of outerwear garments made from microfibres have been tabulated [21]. The degree of thermomigration during heat setting on a stenter after dyeing was found to be the most important factor governing the attainable wet fastness on polyester microfibres [35]. Table 9.3 indicates the marked difference in performance between disperse navy dyeings on conventional and microfibre fabrics after giving a typical heat setting treatment at 170°C after the dyeing process. Polyester microfibre fabrics are best dyed in jets or overflow machines because this type of equipment allows the fabrics to develop maximum bulk and softness of handle. Various jet machine types, including ultra-low liquor ratio units, are suitable for these qualities. Neither package dyeing of yarn nor beam dyeing of fabrics is recommended because of the dense packing and closely woven constructions that tend to impair the uniform liquor flow necessary for level dyeing under these conditions [29,36]. Levelling problems in jet dyeing of microfibre sportswear fabrics may be attributable to starting the process at too high a temperature, dye exhaustion at too rapid a rate in the temperature-rise period, the use of incompatible dyes, too slow a rate of fabric movement through the jet, or overloading of the vessel with too much fabric. Improvement can be ensured by application of optimised dye selections, starting at a lower temperature, increasing the temperature at a slower rate, more prolonged dyeing at top temperature, the use of rapid-flow equipment at the maximum rate of circulation and loading of the vessel with shorter lengths of fabric [37]. The production of staple blends of polyester microfibres with cotton for knitted sportswear fabrics has been inhibited by problems at the opening, carding and spinning stages, as well as a tendency of the finished garments to show pilling defects [23]. In the field of woven blends, filament yarns containing polyester and viscose have achieved importance as the basis for lightweight blouses and dressgoods. Several dyeing procedures have been found satisfactory for these blends, including the customary prolonged two-bath sequence of jet dyeing with disperse dyes at 130°C, reduction clearing if necessary, dyeing of the cellulosic component with reactive dyes and finally washing-off, all stages occupying the jet machine. Substantial savings of time, water and chemicals consumption are achieved by the reverse sequence involving reactive dyeing of the cellulosic component using either a pad-batch or exhaust method, followed by jet dyeing of the polyester at 130°C that also removes the unfixed reactive dyes from the cellulose [21]. The anticipated growth in consumption of polyester microfibres for blending with viscose, lyocell, cotton or nylon [34] puts further emphasis not only on the fastness performance of the disperse-dyed polyester but also the cross-staining characteristics of the disperse dyes selected. Higher proportions of dye applied inevitably mean more pronounced staining of the cellulosic component when dyeing polyester/cellulosic blends. Disperse dyes of the azothiophene or dicarboxylate ester types are capable of being rendered alkali-soluble by the action of a mildly alkaline aftertreatment. These offer considerable benefits when dyeing polyester/cellulosic blends containing microfibres, including higher
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productivity, improved wet fastness and minimal cross-staining of the cellulosic component by the disperse dyes [23].
9.3 Disperse Dyes and Auxiliaries for Polyester Dyeing 9.3.1 Dyes and Dye Selection At the end of the disperse dyeing process, the dyes that have been absorbed by the polyester fibres are present essentially in the monomolecular state. Transfer of dye to the fibre takes place by surface adsorption of individual dye molecules from solution, the low concentration of which is maintained virtually constant during the initial phase of the dyeing process by the progressive dissolution of solid dye particles dispersed in the dyebath. The successive stages of the disperse dyeing process are therefore as follows: 1. The dye dispersion forms a dynamic equilibrium by dissolution of a small proportion of the smaller particles present, 2. Individual molecules of dye are absorbed by the fibre surface, 3. The equilibrium in the dyebath is restored by gradual dissolution of more small particles, 4. The adsorbed dye molecules slowly diffuse into the interior of the fibre, allowing more surface adsorption to take place from solution. The aqueous solubilities of disperse dyes are quite low, ranging between about 0.5 and 200 mg/l at 100°C and between 1 and 1000 mg/l at 130°C [38]. The hydrophobic crystals of pure disperse dyes dissolve with difficulty but in a commercial dyeing process dissolution is facilitated by the very fine state of subdivision maintained in the dyebath using highly effective dispersing agents. The crystal form of the dye is also an important factor, the proportion present in the monomolecular state being dependent on the crystal structure of the dye particles [39]. Under practical dyebath conditions the stability of the dispersion system is often less than ideal. There is a tendency for the larger particles in the dispersion to increase in size at the expense of the smaller particles and crystal growth can occur in the cooling dyebath during the concluding phase of the dyeing process. These adverse effects are promoted by the presence of electrolytes or certain nonionic levelling agents, eventually leading to deterioration of the quality of the dispersion and the deposition of some of the smaller particles on the surfaces of the dyed fibres. For these reasons it is important to adjust the processing conditions in such a way that the dyeing process is completed before the quality of the dispersion has deteriorated to an appreciable extent. Polyester fibres have high affinity for disperse dyes and thus the initial adsorption of dye on to the fibre surface proceeds as rapidly as the aqueous solubility of the dye will permit. The slow rate of diffusion into the interior of the fibre tends to inhibit the rate of transfer as the concentration of adsorbed dye on the fibre surface approaches a limiting value that corresponds to equilibrium with the dilute solution in the dyebath. When this condition becomes established, the rate of transfer of monomolecular dye from solution into the fibre is governed by the rate of diffusion within the polyester structure. The concentration at the fibre surface then remains virtually constant until the end of this transfer phase. The dye remaining in the depleted bath consists mainly of dissolved molecules in dynamic
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equilibrium with the saturated fibre surface and with residual dye particles from the original dispersion. At this stage the adsorbed and partly diffused dye is not yet uniformly distributed within the fibre and dyeing must continue until complete penetration into the interior has been attained. As with all exhaust dyeing systems, dye selection is strongly influenced by the choice of dyeing process conditions and this is determined by the requirements of the material to be processed and the types of dyeing equipment available. In the first instance the dyes are selected to provide dyeings of adequate fastness for the end-use of the material, whilst achieving the target colour and metamerism response at an acceptable cost. The processing conditions and the selection of shading dyes and dyeing auxiliaries must then be adjusted to suit the predominant dye or dyes in the recipe, so as to obtain a level and well-penetrated dyeing in the most economical way. The most important fastness requirements for polyester materials are fastness to heat, light and wet treatments. Dyes selected for satisfactory performance in these tests will generally be found to show acceptable resistance to most other agencies encountered in typical end-uses for apparel and furnishing fabrics. In selecting dyes for fastness to sublimation, account must be taken of heat treatments involved in the making-up of garments or other articles, as well as the more obvious drying and heat setting processes necessary in the finishing of woven or knitted fabrics. Domestic washing conditions are relatively mild but commercial laundries handling contract-supplied work-clothing or uniforms use much more severe conditions. For critical end-uses the dyes must be chosen on the basis of individual testing for satisfactory performance under the required conditions. Fastness to washing is markedly dependent on thermomigration effects that take place when the dyed polyester is subjected to heat treatment during finishing. Many of the long-established red, violet or bright blue disperse dyes are based on substituted anthraquinone structures, whereas those in the yellow, orange, red, brown or navy blue sectors of the gamut are often substituted benzenoid monoazo compounds. With few exceptions, it is possible to classify the application characteristics and fastness properties of typical anthraquinone disperse dyes under general headings: 1. Good to excellent fastness to light, 2. Good coverage of dyeability variations, 3. Good level dyeing characteristics. These properties were highly regarded in the early days of polyester and polyester/cellulosic dyeing [40]. Carriers were necessary to accelerate the exhaustion and diffusion of disperse dyes applied under atmospheric conditions but they tended to lower the light fastness of the dyed goods. By selecting dyes of high light fastness it was possible to achieve satisfactory results on carrierdyed fabrics. The properties of good coverage and level dyeing behaviour shown by anthraquinone-based dyes were appreciated in both atmospheric and hightemperature dyeing equipment. In the dyeing machines available at that time, the efficiency of liquor interchange between substrate and dyebath was usually poor and the yarns and fabrics variable in physical and chemical properties, so that irregular dyeability problems were all too common. Anthraquinone disperse dyes made a significant contribution to overcoming these faults [40].
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With the introduction of detergent-based washing tests, many anthraquinone disperse dyes were shown to be less than satisfactory. If fabric or yarn dyed to a full depth with a dye of this class as major component is subjected to a detergent wash test at 60°C or above after heat setting, pronounced staining of the nylon portion of a multifibre adjacent specimen is likely to occur. This problem has led to a decline in popularity of anthraquinone disperse dyes on polyester. The decline has been accelerated by the development of more cost-effective monoazo disperse dyes derived from heterocyclic amines, more efficient high-temperature dyeing equipment with improved liquor interchange and substrates with more uniform dyeing properties. In terms of meeting the demands for higher levels of fastness to washing on post-set polyester and polyester/cellulosic blends, many monoazo dyes show significant advantages over anthraquinone dyes in the red-violet-blue sectors of the gamut. Nevertheless, there are several specific end-uses for which anthraquinone disperse dyes are particularly suitable. Polyester is finding increased use for automotive upholstery and trim because the outstanding light fastness of these dyes makes them ideal for this outlet. They are still used extensively for the dyeing of cellulose acetate, triacetate, nylon and blends of polyester with other synthetic fibres. In order to meet the most critical international washing fastness specifications, the Dispersol XF (extra fast) (DyStar) range of disperse dyes was developed for the dyeing of polyester and polyester/cellulosic blends. Produced in granular form, they are especially suitable for dull ternary dyeings in full depths [41]. As Table 9.4 indicates for Dispersol Navy XF Grains and four other azo navy disperse dyes on woven polyester, members of this range of high-fastness dyes are outstanding in terms of freedom from staining of adjacent nylon, wool and ester fibres in detergent-based washing tests at 60°C. Cross-staining of the cotton component by Dispersol Rubine XF Grains and three other azo rubine dyes in the dyeing of a 50:50 polyester/cotton blend is compared in Table 9.5. These ratings demonstrate that a simple aqueous rinse eliminates much of the surface deposit in the case of Dispersol Rubine XF, whereas a full reduction clear treatment is essential to ensure removal of the staining shown by the other three rubine dyes. The range of twelve Terasil W (Ciba) dyes offers outstanding fastness to washing and sublimation, meeting the highest requirements of the industry on polyester and polyester/cotton blends. In order to achieve the best reproducibility, these dyes must be applied with certain precautions. The pH has to be set at 4 to 5 using acetic acid or formic acid with ammonium sulphate. Cibatex ALK (Ciba) is necessary under conditions that may present a risk of azo dye reduction. For the reasons already discussed, the traditional high-temperature exhaust dyeing process for polyester fabrics was unduly prolonged because of the limitations of the equipment available and variability in the properties of the substrate. Dyeing was usually commenced at 50-60°C and pH 5.0 to 5.5 (obtained using acetic acid) in the presence of dispersing agent. The dyebath temperature was raised slowly to 125 or 130°C and then held at this temperature for up to 60 minutes to ensure thorough penetration and levelling, the time being determined by the dyes selected and the applied depth. The dyebath was then cooled slowly to minimise creasing of the fabric, until the vessel could be opened up at atmospheric pressure. To remove surface dye from the polyester fibres, a reduction clearing treatment was carried out for 30 minutes at 70°C using sodium
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dithionite and caustic soda. Thorough rinsing to neutralise the dyeing was necessary to complete the process. The focus of development since then has been to improve the quality and performance of the substrate and the selected dyes, to design more suitable and reliable dyeing equipment and to accelerate or combine process stages in order to minimise the total processing time. To be applicable in rapid dyeing processes, disperse dyes must be compatible by sharing similar rates of exhaustion at all dyeing temperatures. With such dye combinations, it is possible to economise on dyeing time by rapidly raising the temperature in the heating-up phase. Dyes that exhibit rapid diffusion and high exhaustion permit a shorter dwell period at top temperature to achieve complete penetration of the fibres. An essential requirement is that all dyes in the recipe are stable to the dyebath conditions. Accelerated dyeing processes put extra pressure on machine operation and control. If the key parameters of machine design, process control and substrate quality are less than optimum, the benefits from rapid dyeing techniques are not attainable. Indeed, the level of reprocessing could become worse than with the traditional prolonged dyeing process [42]. 9.3.2 Processing Techniques and Dispersing Systems Polyester fibre materials may be dyed at almost any stage in the manufacture of yarns and fabrics. The coloration stage selected for any specific end-use is dictated by a balance of economic and technical considerations. The choice is influenced by the processes in use for other stages of manufacture, as well as by the equipment available and end-use performance required in the finished goods. The mass coloration of polyester by incorporation of pigments in the polymer melt is restricted to the production of extremely large batches in a limited range of standard colours, because of the high cost of changing over from one colour to another. Apart from the mass-coloured product, polyester staple fibres are available for dyeing as cut-fibre, continuous filament tow of exceptionally high denier, or as sliver such as tops produced by worsted-type spinning processes. Fine-denier short-staple fibres are sometimes spun and woven as 100% polyester materials, but are more commonly blended with cotton, linen, viscose, modal or lyocell fibres. Most often these blends are dyed in fabric form but they may be yarn-dyed for coloured woven effects. Careful handling is necessary when dyeing staple fibres in the form of loose stock, in order to obtain dyed fibre of good spinning quality. Polyester tow is even more sensitive to handling conditions than staple fibre and the cost of downtime when changing colours limits tow dyeing to relatively large batches, although these are much smaller than batches of mass-coloured fibre. Polyester slubbing in the form of tops is less sensitive to handling than tow, although care must be taken to avoid setting-in faults arising by folding or twisting. Filament yarn materials may be dyed as bulked yarns or woven fabrics in circulating-liquor vessels, or as woven or weft-knitted goods in rope form using jet or overflow machines. Dyed textured yarns are mostly used in the knitting of multicoloured double jersey jacquard constructions. It is not always necessary to scour polyester materials before dyeing. Loose stock, tow and slubbing are supplied in a clean state with only a light coating of processing additives, which are normally compatible with disperse dyebaths. Such substrates may be simply rinsed with water or immersed directly into the
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dyebath. Yarns and knitted fabrics, on the other hand, may be heavily oiled or lubricated with wax and woven fabrics contain sized warps. Some yarns may have been treated with cationic softeners that could interact with the anionic dispersion in the dyebath. These contaminants must be removed before dyeing and the dyer must verify what types of impurity are present on the raw material as received so that appropriate action can be taken. An aqueous scouring bath containing 1-2 g/l anionic detergent and 2 g/l sodium carbonate is satisfactory for most purposes. Suitable detergents include fatty alcohol sulphates, alkylarylsulphonates and their mixtures with ethoxylated fatty alcohols. At the low liquor ratios applicable in scouring on the jig these concentrations should be increased threefold. The processing time and temperature may be adjusted according to substrate type, the degree of contamination and the available equipment, although 30 minutes at 60°C is adequate in many cases. Woven or lightweight knitted fabrics prone to creasing should be treated in open width at 50 to 60°C but many textured weft-knit qualities can be scoured in rope form for a shorter time at the boil. Similar conditions are suitable for the scouring of polyester blends with viscose, modal or lyocell staple fibres. In the case of blends of polyester with cotton or linen, however, the preparation sequence is determined by the quality and condition of the natural cellulosic fibre rather than the polyester component. If the grey goods contain starch size or other polymers not readily soluble or dispersible in water, it is necessary to extract or solubilise these contaminants before scouring. Even with the water-soluble size polymers it is often advisable to carry out an enzyme desizing process, because they may become difficult to solubilise after drying at temperatures above 100°C. Heavyweight long-staple woven polyester and polyester/wool qualities are best scoured in rope form to develop the desirable softness of handle, using cool (20 to 40°C) scouring liquors at high concentration and relatively low liquor ratio. The scouring process for polyester/wool blends should be somewhat more vigorous than conventional treatment of all-wool fabrics, because oils and waxy lubricants are more difficult to extract from the polyester component. The most important factors in the scouring of oil-spun goods are process time and detergent concentration; sodium carbonate concentration is less important and no advantage is gained by increasing the alkalinity. Oil, grease or wax contaminants are often difficult to remove from polyester fibres. It is therefore important to ensure that all the lubricants used in fibre processing and cloth manufacture contain emulsifiers that facilitate their easy and complete removal during normal scouring processes. Fibre-processing oils are usually made self-emulsifying by adding nonionic surfactants. The conflicting requirements of antistatic properties, oil compatibility and detergent activity are met using a balanced mixture of surfactants differing in hydrophile-lipophile balance formulated to remain stable as a single homogeneous phase. Disperse dyes are particularly sensitive to the influence of surfactants and polyelectrolytes, as regards both the quality and stability of the dispersion and the response of the dyes in exhaust dyeing systems. The dyeing of polyester at a temperature in the range 120 to 135°C in circulating-liquor machines places severe demands on initial dispersion quality and subsequent stability under adverse conditions. The crux of the problem lies in the inherent instability of all dye dispersions, there being an overall tendency of fine particles to adhere together with the consequent formation of larger particles. Although disperse dyes are generally considered to be virtually insoluble in water they are, in
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colloidal terms, sparingly soluble; a low degree of solubility is a necessary prerequisite for dyeing to proceed from an aqueous dyebath. It is this limited solubility that favours growth of particle size [43]. The solubility of disperse dyes normally increases with temperature and dispersing agent concentration, although these effects vary greatly from agent to agent and from dye to dye. Most dispersing agents for disperse dyes are anionic polyelectrolytes, mainly various sulphonated condensation products of aromatic compounds or lignosulphonates from wood pulp. Increased understanding of lignin chemistry with consequent improvements in manufacture, enabling lignins to be more economically and reliably modified for specific end-uses, currently favours greater use of these products. Solid brands of disperse dyes contain a significant proportion of dispersing agent added during formulation; liquid brands contain rather less as they do not have to withstand the thermal and mechanical rigours of spray drying [44] and do not require redispersing at the dyebath preparation stage. Nevertheless, it is advisable to add extra dispersing agent at the dyeing stage, more being required with liquid brands to compensate for their lower content. Lignosulphonates with a high degree of sulphonation generally perform well during milling of disperse dye particles. Less sulphonated types tend to give better stability in high-temperature dyeing, since they are more readily adsorbed and retained by the hydrophobic dye particles [45]. The particle size distribution in a disperse dyebath and any transformations taking place during heating-up of the dye liquor, maintaining top temperature and subsequent cooling, can exert critical effects on rate of dyeing, final degree of exhaustion and levelness. The addition of auxiliaries, such as additional dispersing agents, nonionic levelling agents, carriers and electrolytes, brings about further changes that may be beneficial or otherwise, depending on circumstances. Thermal stability of a dispersion appears to be related to the concentrations of adsorbing and stabilising groups in the dispersing agent. Partial blocking of phenolic groups in a lignosulphonate, for example, reduces the thermal stability by an amount that corresponds quite well with the lower concentration of residual phenolic groups. Some azo disperse dyes are susceptible to reduction under unfavourable conditions [46]. This instability is minimised by dyeing at the optimum pH (usually pH 4 to 5) in the presence of air and by minimising the dyeing time at top temperature. Lignosulphonate dispersing agents tend to promote this reductive decomposition of sensitive dyes, much more so than the naphthalenesulphonate condensation products. Commercial lignosulphonates vary considerably in their detailed constitution, however, and thus in their reducing power. In certain cases the problem can be ameliorated by adding an oxidising agent (such as sodium dichromate) to the dyebath, but the effects can be variable and difficult to control. In theory, the reductive tendency of lignosulphonates can be counteracted by chemical blocking of the active phenolic groups but this impairs the dispersing properties of the product [46]. 9.3.3 Alkaline Pretreatment and Dyeing Techniques Controlled alkaline hydrolysis of the surface layer of the fibres confers a limited degree of silk-like softness to conventional poly(ethylene terephthalate) materials. The resultant loss in mass is accompanied by an increase in surface polarity arising from the additional hydroxy and carboxyl groups formed by ester hydrolysis. The rates of wetting-out and uptake of disperse dyes are increased by the treatment, which also inhibits the inherent tendency of the fibres to adopt a
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static charge. The Methylene Blue test is a useful indicator of the increase in carboxyl group content. This test has been used in examining the influence of variations in caustic soda concentration during treatment for one hour at 90°C on the changes in physical and chemical characteristics [47]. More recently, the influence of additions of ethylenediamine to accelerate the hydrolysis by caustic alkali was examined in a rapid process for only 1 to 5 minutes at the boil. The treated polyester fibres were assessed in terms of moisture regain, loss in mass and decreased tenacity. The deterioration in strength under these conditions made this process difficult to control in practice [48]. The effects of prior heat setting at temperatures in the range 100 to 220°C on the degree of hydrolysis during treatment of PET fibres with 10% aqueous sodium hydroxide for 1 or 2 hours at 90°C were examined. Loss in mass gradually decreased with increasing temperature of heat setting up to 140°C, but then progressively increased again with the greatest loss in mass being observed for material pretreated at 220°C [49]. These differences were attributed to changes in the fine structure of the amorphous regions of the polyester. The degree of crystallinity increases with heat setting temperature but above 140°C this stabilising effect is offset by slow thermal decomposition that makes the pretreated fibres increasingly vulnerable to alkaline hydrolysis. The influence of the cationic accelerant benzyldimethyldodecylammonium chloride (9.4) on the alkaline hydrolysis of heat-set polyester has been evaluated. The rate enhancement shown by this surfactant gradually decreased with increasing temperature above the boil, owing to the increase in degree of crystallinity of the polymer [50]. A detailed investigation of the effects of key parameters of this process on the dyeability characteristics of the alkali-treated polyester emphasised the importance of the choice of alkali and cationic accelerant. Disperse dyeing was evaluated using CI Disperse Red 82 applied at 95°C with carrier and carboxyl group content using CI Basic Violet 33 applied at 90°C. The effectiveness of the alkali selected increased in the order: Na2CO3 << NaOH < KOH. Increasing effectiveness of the cationic accelerant was as follows: Irgasol AR (Ciba) < Merse RTD (Tanatex) < Rematard AC (Hoechst) < Tinegal B (Ciba). As expected, the degree of hydrolysis as expressed in terms of either decreased mass or enhanced dyeability was found to increase with alkali concentration, accelerant concentration, time or temperature of treatment, and to decrease with increased liquor ratio, with all other variables held constant [51]. In a more recent study of this process, the accelerating effect of ultrasonic treatment was examined at various alkali concentrations and reaction times from 10 to 60 minutes. The enhanced dyeability of the mass-reduced substrate was assessed by application of CI Disperse Red 50, which showed satisfactory fastness performance [52]. The growing interest in pretreatment of polyester with caustic alkali to improve the softness of handle and other properties mentioned above has increased the risk of possible carry-over of alkali into the dyebath, giving rise to problems of varying pH and inadequate reproducibility. The traditional sequence of processes in the exhaust dyeing of polyester involves several changes of pH: (a) alkaline scouring and/or mass-reduction, (b) neutralisation, (c) acid dyeing, (d) alkaline reduction clearing, (e) neutralisation before drying. The possibility of dyeing polyester under alkaline conditions offers scope for simplifying and contracting this sequence. Improved reproducibility and quality of the dyed material may be anticipated, as well as freedom from the release of dispersed oligomer particles into the dyebath (section 9.1.2).
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The flow chart shown in Figure 9.2 outlines the numerous process steps accompanied by deliberate changes in pH. The adoption of an alkaline dyeing stage would allow all wet processes between fabric manufacture and finishing to be carried out in alkaline media, significantly limiting the amount and variety of chemical additions to the system and simplifying the treatment strategy required to deal with the waste liquors from the successive stages. The problems associated with alkali carry-over into acidic dyebaths are eliminated and the adoption of alkaline dyeing conditions enhances the scouring action of the liquor, contributing to an improvement in quality [53]. An essential prerequisite of an alkaline dyeing system is the availability of a full range of disperse dyes that show adequate dispersion stability and freedom from chemical decomposition at the dyebath pH selected [53-56]. Certain alkalisensitive dyes, particularly some of the monoazo types derived from heterocyclic diazo components, are unsuitable for use in an alkaline dyeing system. More stable structures, notably the anthraquinone disperse dyes, are already available, however, and these are selected for compatibility and rapid diffusion into the fibre under the preferred application conditions. A preferred selection of twelve alkalistable dyes from the Terasil and Teratop (Ciba) ranges is available, all of them being satisfactory for use on polyester automotive fibres. Dispersing agents of the lignosulphonate type should be avoided if possible because of their tendency to accelerate the reductive decomposition of disperse dyes (section 9.3.2). Auxiliary products such as Cibatex ALK (Ciba), Levegal DLP (Bayer) or JPH 95 (Hoechst Mitsubishi Kasei) have been developed to fulfil the specific requirements of the alkaline dyeing process. These are composite products that perform several functions. including the protection of disperse dyes from the impact of alkali. They are capable of buffering the dyebath pH in the region of pH 9, chelating trace metal ions and dissolving any oligomer present [54]. The initial dyebath pH of 9.5 gradually decreases to about 9 at the end of the dyeing stage. This careful control of mildly alkaline conditions throughout the dyeing cycle promotes excellent reproducibility. Alkali-stable dispersing agents and ultraviolet absorbers are also available. In conventional polyester dyeing under acidic conditions, the release of oligomer into the high-temperature dyebath is responsible for various practical problems, including interference with stability of the dye dispersion, deposition on the surfaces of equipment and the dyed fibres, as well as adverse effects on the frictional behaviour of dyed yarns and build-up of white deposits on spinning machinery (section 9.1.2). When dyeing at about pH 9 in the presence of a solubilising agent such as Cibatex ALK, Levegal DLP or JPH 95, a considerable proportion of the oligomer present can be solubilised by alkaline hydrolysis [53]. Table 9.6 indicates the effect of addition of JPH 95 on the oligomer composition of the dyebath after dyeing polyester yarn for 30 minutes at 135°C and 15:1 liquor ratio in the presence of 1 g/l of a nonionic levelling agent. More than 90% of the total oligomer released becomes solubilised by 2 g/l of the JPH 95 product. Traditionally, disperse dyes have been formulated for optimum performance under mildly acidic dyeing conditions and many of them show inferior dispersion stability when applied at an alkaline pH. When dyeing today’s high-density fabrics woven from fine-denier polyester yarns it is increasingly difficult to remove residual sizing agents and other contaminants completely at the scouring stage and if a caustic mass-reduction treatment is given residual alkali may be carried over into the dyebath. Table 9.7 shows that alkaline dyeing of a polyester satin fabric in the presence of JPC 95 for 20 minutes at 130°C removes approximately
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40 to 50% of the 1.6% size polymer content. A scouring treatment with 2 g/l trisodium orthophosphate, 2 g/l sodium carbonate and 2 g/l anionic detergent is necessary for removal of 90% of the size residue. More than half of all dyeing faults can be attributed to inadequate pretreatment before dyeing and the degree of shade reproducibility attainable from the alkaline dyeing process on polyester is claimed to be excellent [54,57]. Process rationalisation becomes possible if the dyebath pH is increased from about 5 to about 9. Neutralisation between the preparation and dyeing stages (Figure 9.2) can be eliminated. The favourable effect of alkaline dyebath conditions on the control of released oligomer extends the possibility of dispensing with the reduction clearing step except for dyeings at full depths. As Figure 9.3 displays, the traditional sequence for scouring, high-temperature acidic dyeing and reduction clearing of polyester could take as long as six hours from loading to unloading of the vessel. This can be decreased to about four hours or even less by incorporating an alkaline dyeing stage, giving only one rinsing step between scouring and dyeing, eliminating the reduction clear and limiting the washing-off step to a single warm rinse (Figure 9.4). It has been claimed that further savings of time, water and chemicals are possible when dyeing in pale depths or for less demanding end-uses, by combining the scouring and dyeing stages and then scouring in the exhaust dyebath (Figure 9.5). This approach would not be suitable for dyeing woven goods or heavily oilcontaminated knitgoods. Despite the attractive benefits attainable from the alkaline dyeing process, acceptance has been rather slow so far because of limitations in the selection of dyes, inadequate cost-effectiveness of some recipes and reports of disappointing reproducibility if careful control of pH is not ensured. Preliminary trials are essential before this technique can be applied with confidence [58]. The alkaline dyeing method is of particular interest for dyeing navy and black shades, because the beneficial influence on oligomer solubilisation facilitates the attainment of minimal deposition and optimum fastness to rubbing in circulating-liquor equipment. The one-bath dyeing of polyester/cellulosic blends with reactive or direct dyes is also compatible with disperse dyeing of the polyester component at pH 9. There are substantial differences between various alkaline buffer systems to give effective control and reproducibility in the pH 9 region. Development of the Domapal PH (Dohmen) buffer system to ensure optimum pH constancy and dye stability has been described [59]. When applied together with the preferred sequestering agent Doragen A (Dohmen), satisfactory commercial dyeing of polyester yarn has been established using this alkaline system. Even production for the automotive industry with minimum batch-to-batch shade variations can be guaranteed. As mentioned in Chapter 8, Bayer researchers were leaders in the development of so-called systematic procedures for dyeing. For polyester, such a procedure allows the total dyeing time to be calculated in advance, thereby leading to shorter and more reproducible dyeing cycles with reductions in cost and fibre damage. In the Resolin S process [60,61], the Bayer disperse dyes were classified into four groups, principally based on diffusion coefficient and affinity. The reference temperature (TR) at which the rate of exhaustion of the dyes is equivalent to 1% per minute is determined for each group of dyes as a function of the dye concentration. The rate of dyeing, V, for each polyester type and the efficiency of the dyeing equipment being used are also determined. From these
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factors, it is then possible to calculate the starting and final temperatures, the rate of temperature rise and the holding time at the final temperature. The addition of a carrier/levelling agent can also be incorporated into the calculation. It was shown [62] that the rate of dyeing of an individual, essentially homogeneous, disperse dye is dependent on the depth of dyeing but the rate of dyeing at a given depth can be characterised by a single factor Vx, based on the time of half dyeing. If the component dyes in a given formulation have similar Vx values, the mixture will be rate-compatible and build up on tone. However, no combination of individual dyes will be compatible over a range of depths and proportions. This led to the formulation of dye mixtures containing dyes over a range of Vx values for Serilene V/VX (Yorkshire) dyes. In the Palegal process (BASF), the use of an auxiliary Palegal SF was claimed to improve the compatibility of disperse dyes in a mixture, allowing faster rates of rise to be used. This agent has a restraining action within the range 100 to 110°C and a migrating action at higher temperatures. In the Suproma technique (Sandoz) [63], the concept of contact number was used, based on the liquor circulation in package-type machines or through the venturi in a jet dyeing machine. Dye selection, substrate dyeability and temperature control were other important factors. 9.3.4 Fastness Properties of Disperse Dyeings Generally speaking, correctly dyed polyester materials exhibit outstandingly good all-round fastness properties to all common agencies. The unusually severe conditions of application required to induce disperse dyes to diffuse to the interior of polyester fibres are matched by the corresponding slowness with which they diffuse outwards again when subjected to even relatively extreme conditions of wet fastness testing. The fastness to light and weathering of polyester dyeings is also exceptionally good. This is one of the main reasons why polyester is preferred for especially demanding outlets, including automotive textiles and outdoor fabrics such as sailcloth. Fastness to sublimation is probably the most important requirement of dyed polyester, apart from fastness to light. The migration behaviour and wet fastness of disperse dyes on polyester are closely involved with their response to heat treatments. Adequate fastness to heat is essential so that the dyed material will withstand the conditions encountered in heat setting, in durable pleating and in ironing or pressing of the goods during the making-up of garments. Most of the polyester dyed in fabric form is pre-set before dyeing but some qualities are postset after the dyeing process. Most garment lengths or panels and almost all multicoloured woven or knitted constructions made from dyed yarns must be heat-set in the dyed state; in some cases under relatively severe conditions. Even a slight stain on white or pale-dyed yarns in a patterned design may greatly impair the attractiveness of the goods. Consequently, it is essential to select dyes most carefully for such outlets so that the sublimation fastness of all component yarns is satisfactory. When interpreting the results of laboratory tests for fastness to sublimation, it must be borne in mind that the effect of a contact heat test on a dyeing is much more severe than that of hot air on the same dyeing running through a stenter for the same exposure time at the same temperature. During heat treatment on a stenter much of the exposure time is occupied in heating-up the fabric to the treatment temperature. Thus the loss of dye by sublimation from the dyed
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polyester under these conditions is much less than in the contact heat test, where the initial fabric heating step is extremely rapid. Generally speaking, experience suggests that a contact heat test at x°C gives a result approximately equivalent to a stenter treatment for the same time at (x + 20)°C. It is a sensible precaution, however, to carry out contact heat tests at a temperature known to be significant with regard to subsequent processing of the goods, in order to provide a margin of safety that will ensure detection of borderline cases. This broad correlation in terms of fastness to sublimation should not be extended to the interpretation of thermomigration behaviour. The behaviour of individual dyes with respect to the staining of adjacent materials in wet fastness tests is much more significant in this case. Customer complaints of inadequate fastness to perspiration, water or mild washing of polyester materials are always associated with the presence of virtually insoluble dye particles on the fibre surface. At the temperature range (20 to 60°C) within which these fastness tests are conducted, the rates of diffusion of disperse dyes in polyester are extremely slow and therefore internal migration towards the fibre surface is negligible. The presence on the fibre surface of dye particles at a concentration of only 20 mg/kg fibre, however, can result in significant staining of adjacent white polyester threads in contact with the dyed yarn in the wet state at ambient temperature. The availability of a relatively severe aqueous reduction clearing process that is capable of cleansing the fibre surface without significantly affecting the dye molecules that have already diffused into the interior of the polymer ought to make it easy to avoid producing dyed polyester materials that still contain surface deposits of particulate dye. In practice it is indeed quite simple to ensure this for goods that have been dyed with relatively vigorous agitation in jet or overflow machines. In contrast, yarn dyed in package form or fabric dyed on the beam often forms a rather efficient filter, which collects on its inner layers any particulate dye or oligomer crystals coarse enough to lodge there [64]. These crystalline particles may be surprisingly resistant to chemical attack. Although they will eventually yield to the correct treatment it is necessary for the dyer to be aware of the staining capability of trace residues of dye particles on such goods. Even when the fibre surfaces are thoroughly clean after dyeing and clearing, there remains a tendency for dyes to migrate outwards from the fibre interior whenever they are given sufficient energy to initiate a significant rate of diffusion. The extent to which disperse dyes migrate to the fibre surface during heat treatment of the dyed material depends on the dye, the depth of the dyeing and the duration and temperature of the process [65-67]. Fastness problems arising from this thermomigration effect are most commonly associated with deep colours, so that it is very difficult to preserve surface cleanliness with black, navy blue, olive green and dark brown shades if these goods have to be heat-set after dyeing. Surface deposits formed by thermomigration in this way are usually desorbed by the first laundering treatment given to the made-up garment, so that complaints of poor fastness arising from this fault are more commonly associated with quality control testing rather than with poor performance in practical use of the goods. If the problem is detected before the suspect dyeings leave the dyehouse, the surface deposit can usually be removed by scouring or reduction clearing and then drying at a low temperature (90 to 100°C). The finisher should be alerted to such effects, especially when handling new or unfamiliar qualities, so that heat
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setting conditions are adjusted to be as mild as possible and any adversely affected batches can be intercepted and reprocessed. The effect of thermomigration on fastness performance varies considerably because of the generally adverse influence of surfactants, lubricants and other finishing agents. A method for assessing the influence of auxiliaries on thermomigration has been published [68]. This is carried out with a standard depth dyeing of CI Disperse Blue 56 and the fastness to an ISO C02 washing test is determined after reduction clearing and stentering at specified temperatures. A detergent-based, rather than soap-based, washing test would be more critical [69]. The processes involved during thermomigration in the presence of a surfactant have been evaluated experimentally [70]: 1. Extremely rapid attainment of equilibrium between dye in the surfactant layer and dye in the surface zone of the fibre, 2. Rapid diffusion of the dye molecules from the interior of the fibre towards the surface, 3. Slower diffusion of surfactant molecules into the substrate phase, 4. Eventual formation of a composite dye-fibre-surfactant phase in the surface region. Thermomigration readily takes place to a lesser extent in the absence of surfactant, when only process (2) takes place. There seems to be a relationship between thermomigration and the degree of interaction between dye and surfactant, more specific interaction leading to greater thermomigration [70]. The term ‘surfactant’ can be interpreted broadly to include any residual surfactant, reduction clearing assistant or applied finish, such as antistat, lubricant or softener. Problems arising from thermomigration are best avoided by selecting dyes that show acceptable fastness to washing after heat setting and by ensuring that all surfactants from dyeing and afterclearing are completely rinsed out. Careful choice of finishing agents and finishing conditions is also important. 9.3.5 Reduction Clearing and Alkaline Clearing Methods The reduction clearing process takes advantage of the hydrophobic character of polyester fibres, which prevents penetration of the polymer by almost all ionised water-soluble chemicals at temperatures below the boil, as well as the very slow diffusion of disperse dyes outwards from the fibre interior at temperatures below 80°C. The dyed goods can thus be treated at 70°C in a strongly reducing bath containing 2 g/l of sodium dithionite in a solution of 2 g/l sodium hydroxide, without affecting the dyes that have penetrated the fibre. This process solubilises anthraquinone-type disperse dyes in the form of their alkali-soluble leuco compounds and destroys azo disperse dyes and other reduction-sensitive structures. The larger crystals of some disperse dyes are relatively hydrophobic and are attacked only slowly, but become detached from the fibre surface by the action of caustic alkali and nonionic detergent. The effectiveness of the clearing treatment is much improved by addition of 1-2 g/l of an ethoxylated fatty alcohol. With heavy deposits it is advisable to follow the reduction clear treatment by a separate soaping with the nonionic surfactant alone. At the end of the reduction clearing process the dyebath should still give an alkaline reaction to
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phenolphthalein indicator and should give a positive reaction for reducing power when spotted onto a test paper impregnated with CI Vat Yellow 1. Low-pill polyester variant staple fibres are generally more susceptible to alkaline treatments than are conventional fibres of normal intrinsic viscosity (section 9.2.1). Thus when reduction clearing low-pill polyester or polyester/wool blended materials it is necessary to replace the caustic soda by 1-2 ml/l of 0.88 ammonia solution in order to avoid damage to the wool or the low-pill variant polyester. It is difficult to thoroughly clear all the disperse dye stain from the wool component of a polyester/wool blend but the process is useful especially in the two-bath dyeing of full depths. For medium or pale depths on this blend it is usually sufficient to scour with a nonionic detergent alone. The reduction clearing process for polyester dyeings is not only an expensive one, but it incurs further costs because of the need to deal with the environmentally unacceptable waste liquors (section 2.9). There is also the cost and inconvenience of carrying out two changes of pH, first from the conventional acidic dyebath to the alkaline reduction clear, followed by neutralisation of the substrate after this process. It is not surprising, therefore, that dyers prefer to avoid a reduction clear whenever possible nowadays. One possibility is to select specialised dyes that can be cleared using alkali alone; this avoids the environmental nuisance of the reducing agent but still requires alkali and the need for two changes of pH. Alternatives to sodium dithionite as reducing agent are sometimes proposed and evaluated, although most of these are less effective and more selective in their action on disperse dyes. A detailed comparison of five reducing systems has been published: hydroxyacetone, Formosul (sodium hydroxymethane-sulphinate), the redox system iron(II) chloride/gluconic acid, thiourea dioxide and sodium dithionite as control. Their relative reactivities with atmospheric oxygen and with hydrogen peroxide solution at pH 12 were determined [71]. When used at equimolar concentrations for the reduction clearing of black-dyed polyester yarn, dithionite and thiourea dioxide were more effective than the other systems (Table 9.8). Hydroxyacetone must be used at temperatures above 80°C because of its sluggish action. Nevertheless, it still did not give an adequate improvement in fastness to washing at 60°C. This compound gives high COD values and has an unpleasant odour. The reducing power of Formosul, even together with anthraquinone as activator, is insufficient under these conditions to adequately improve wash fastness. Iron(II) chloride offers the environmental advantage that it does not contain sulphur but gluconic acid used with it as a complexing agent results in relatively high COD values. Improvement of washing fastness was inadequate. Only thiourea dioxide gave results as good as sodium dithionite. It is three times more expensive but causes only half the sulphur pollution of dithionite. The relative usefulness of these two reducing agents is dependent on the dyeing process. In the winch, slow production of the active species from thiourea dioxide is a disadvantage when working with counterflow. On the other hand it could be advantageous in jet machines, although this only arises if reduction of the dye proceeds relatively quickly. In closed machines, however, sodium dithionite is more effective. Several reduction clearing auxiliaries have been introduced under commercial brandnames, their composition not being revealed in most cases. Sapolib DCR (Allied Colloids) is a one-pack product that replaces separate additions of caustic
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soda, sodium dithionite and a nonionic surfactant. It is readily water-soluble at ambient temperature and contains a highly efficient dispersing agent that prevents redeposition of undesirable residues on the goods or equipment and enhances the fastness properties of the dyed goods [72]. Many practical dyers prefer a liquid product suitable for automatic metering. Cyclanon R (BASF) is a liquid organic reducing agent that must be used in strongly alkaline solution. Cyclanon TX 5179 (BASF) has been introduced as a liquid anionic clearing agent that can be used at an acidic pH. Advantages of this product include a low COD value, high biodegradability and very low toxicity. Savings of 30% in time and 40% in water consumption are claimed, by avoiding the pH changes and separate baths necessary in a conventional reduction clear. This product can be metered directly into the acidic exhaust dyebath after conventional dyeing at pH 5 to 6 and the goods are treated for 10 to 20 minutes at 70 to 80°C. By application in this way, a high degree of waste-water decolorisation is achieved before discharge to drain [73]. Although highly effective with the majority of dyes, in a few instances (e.g. CI Disperse Yellow 29, Violet 35 or Blue 56) a higher concentration is needed. With the introduction of disperse dyes (9.5) containing two carboxylate ester groups in the coupling component that could be readily solubilised by forming anionic sodium carboxylate groups (9.6), the concept of alkaline clearing to replace the environmentally unattractive reduction process became feasible [42]. These dyes are applied under conventional exhaust dyeing conditions at a mildly acidic pH and a dyeing temperature of 125 to 130°C. The dyed polyester or polyester/cellulosic material is subjected to a hot alkaline treatment at 70 to 80°C with sodium carbonate and/or caustic soda to clear any superficial disperse dye on the surface of the polyester fibres or any disperse dye cross-staining on the cellulosic component of a blend [23]. The range of alkali-clearable Dispersol C (DyStar) dyes of the diester type has been supplemented by other structures containing an alkali-sensitive heterocyclic ring forming part of the chromogenic grouping, such as azothiophene, azothiazole, azopyridone or benzodifuranone derivatives [42]. These dyes are ideally suited to the dyeing of polyester/cellulosic blends for which the alkaline clearing stage can be used as the fixation stage for reactive dyes applied to the cellulosic component. In addition to improved productivity from the dyehouse of approximately 30% compared with that of the conventional two-bath dyeing method for these blends [74], these dyeings show very good all-round fastness properties Disperse dyes of the anthraquinone class do not always respond completely to reduction clearing and on polyester such dyeings have traditionally been given an oxidative clear using neutral or mildly alkaline hypochlorite. This is not possible on polyester/cellulosic blends dyed with reactive dyes because of the bleaching effect of the hypochlorite. However, interest in oxidative clearing techniques has been revived recently because of the high values of COD and conductivity characteristic of waste waters produced from the conventional reduction clear with alkaline dithionite. A clearing treatment with 1.5 g/l sodium perborate and 2 g/l detergent for 20 minutes at the boil has given highly satisfactory results on polyester/cotton knitgoods dyed with disperse and reactive dyes [75].
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9.4 Thermal Characteristics and Finishing of Polyester Textiles 9.4.1 Physical Structure and Effect of Heat Treatment The application of heat and mechanical stress to polyester yarns or fabrics before dyeing has a major influence on subsequent dyeing behaviour. The effects of variations of temperature or tension during heat treatment can produce highly significant differences in colour yield if the materials treated differently are dyed in the same dyebath. The characteristic curve that relates the dye uptake of a heat-set polyester filament fabric to the temperature of prior heat setting is shown in Figure 9.6. These dyeings were carried out with the low-energy monoazo dye CI Disperse Red 1 for 90 minutes at the boil in the absence of carrier in order to emphasise the differences in dyeability that result from thermal changes in the fine structure of the polymer. During setting, the fabric was held to fixed dimensions and therefore the tensions that developed in the yarns varied considerably over the range of setting temperatures, with maximum tension being observed in yarns heat-treated in the region 150 to 170°C. The curve in Figure 9.6 shows a marked decrease in dye uptake as the setting temperature was increased from 120 to 150°C because of the increase in degree of crystallinity produced in the polymer structure. The observed progressive increase in dyeability with setting temperature for fabrics pre-set at 190°C or above is attributable to thermal degradation and softening of the polyester structure as it approaches the melting region (above 250°C). In the central portion of the curve, between 150 and 190°C, dye uptake is virtually independent of heat setting temperature. Under these conditions the crystalline structure of the polymer remains unchanged and the treatment temperature is low enough to avoid thermal degradation. This region therefore represents the most favourable conditions to ensure consistency of dye uptake in the subsequent coloration process. Unset polyester textiles shrink when heated in water or in air. This tendency can be reduced or eliminated by controlled heat treatment applied via any suitable contact medium, although hot air and steam are the agencies most commonly used. Figure 9.7 and Figure 9.8 illustrate the free shrinkage of a standard unset polyester filament yarn in hot dry air and in saturated steam respectively. If unset polyester is allowed to shrink freely in this way up to a given temperature, it will not shrink again subsequently until the temperature reached during this setting treatment is exceeded. As a result of shrinkage during setting, the extensibility of the polyester increases and the breaking load diminishes. In general, the setting conditions should be chosen to eliminate the possibility of uncontrolled shrinkage in succeeding processes, whilst retaining acceptable mechanical properties in the material. It is common practice, however, for several setting or relaxation processes to be applied to the same polyester material during the course of yarn and fabric manufacture and finishing processes. Polyester staple fibres usually receive a relatively mild relaxation treatment designed to improve the durability of the crimp inserted during manufacture. This treatment confers sufficient stability for the dyeing of loose stock or slubbing. The slivers from crush-cutting tow-to-top converters can be processed unset but those from stretch-breaking converters should be recrimped and steam-set before dyeing. Polyester yarns are normally set by relaxation in saturated steam. Tension-stable yarns with no stretch/bulk characteristics should be wound onto collapsible centres and steamed at a temperature about 5°C higher than the
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maximum temperature to be reached in dyeing. The shrinkage of yarns that are steamed on collapsible centres is usually about 1% less than their free-relaxation shrinkage at the same steaming temperature and the procedure yields yarns that are fully stable at the dyeing temperature. In practice it is found that on packages wound at a relatively steep pitch, especially on conical supports, the fullystabilised yarns may slacken off slightly during high-temperature dyeing under the action of the circulating liquor. This effect can lead to collapse of the outer layers of filament yarn packages and consequently these are often made from yarns twist-set at a temperature a few degrees below that to be used in dyeing, so that the slackening tendency may be offset by a small residual shrinkage. Short-staple polyester yarns are commonly twist-set by steaming whilst still on the spinner’s ring tubes and the temperature must then be restricted to 110°C to avoid distortion of the tubes, which would prevent their reuse. The yarns are restrained by the rigid package centres and because of this restraint and the low steaming temperature employed they may shrink during dyeing. Some 3 to 4% yarn shrinkage can be tolerated in cylindrical packages wound from relatively porous staple yarns, many of which can be dyed without pretreatment. However, yarns with high potential shrinkage, such as standard polyester filament yarns, core-spun filament-staple mixture yarns and blended yarns containing a component that can swell in the dyebath, such as polyester/viscose, must be set as fully as possible before winding onto the package support for dyeing. Yarn steaming is carried out in autoclaves fitted with a vacuum pump, so that the chamber can be evacuated before and after steaming. Pre-evacuation assists penetration of the yarn package by the steam and some finishers prefer to operate a double process cycle, each of the steam treatments preceded by evacuation and with a final evacuation to assist cooling of the yarns. Steam setting must be carried out carefully and precautions taken to ensure that the treatment is uniform. Variations in steaming temperature of as little as 2°C can be detected as minor variations in depth when using high-energy disperse dyes and any greater variation in temperature could produce package-to-package colour differences that would be difficult to correct. Steamers are jacketed to reduce loss of heat from the walls of the pressure vessel and fitted with automatic control of temperature and devices that ensure a slow constant circulation of steam throughout the vessel to avoid the formation of stagnant regions. Polyester garments, garment lengths and hosiery are stabilised by steaming, preferably using a double cycle, in much the same way as for yarns. In the steaming of fabrics, it is difficult to avoid variations in the degree of shrinkage between the inside and outside of a large batch. Consequently, steaming treatments are confined mainly to the mild conditions necessary to preset the wool in polyester/wool blends and polyester fabrics are normally pre-set in hot air on a pin stenter. The dry blowing and crabbing processes used for polyester/wool fabrics differ mainly in the magnitude of their effects, crabbing being the more severe. They are used to reduce processing shrinkage and rope marking in scouring or piece dyeing and to eliminate the distortion or cockling that can arise from small differences in shrinkage, either between yarns of different type or origin in woven designs or within the wool component of blended yarns, especially those containing crossbred wools. Dry blowing is carried out by passing steam through the rolled batch of fabric for a total time of ten minutes, whilst rotating the batch on a perforated drum. Preferably the process should be interrupted after five minutes and the batch rewound in the reverse direction before steaming for a further five minutes.
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Several layers of cotton cloth should be wrapped onto the blowing drum before batching on the polyester/wool fabric to avoid producing spots at the dyeing stage that correspond with the drum perforations. Crabbing may cause dyed yarns to bleed in coloured woven designs but may be used on fabrics intended for piece dyeing. Two-bath crabbing is satisfactory for most purposes, using water at 80 to 95°C in the first bath, boiling water in the second and giving 3 to 5 minutes treatment in each bath. When even more severe treatment is necessary, the goods may be steamed wet (wet blown) whilst still hot after crabbing. The position in the processing sequence to be selected for the setting of polyester fabrics or garment lengths depends on the state of cleanliness in which the goods are received by the dyehouse and the types of fibres or yarns present. Residual oil or wax contamination is markedly more difficult to remove from the fabric after heat setting. The scour-set-dye sequence ensures that the goods are clean before heat setting, prevents shrinkage during dyeing and avoids the need for high-temperature treatment after dyeing. This is the most satisfactory sequence for polyester fabrics in general and is essential for those in which stretch or bulk are developed in a carefully controlled scour-relaxation process. The disadvantage is the central position of the heat setting step, incurring the extra cost of having to dry the fabric twice. For goods that require little or no scouring, it is possible to adopt the sequence set-scour-dye. This route is useful for warp-knit qualities that carry only minor amounts of yarn lubricant and for delicate constructions prone to distortion or thread slippage if handled in rope form before setting. This is the preferred sequence for setting woven fabrics before scouring and dyeing on the beam. If heat setting is postponed, as in the sequence scour-dye-set, the conditions of setting may modify the handle of the fabric. Qualities made from tension-stable yarns tend to stiffen and may develop a crisp, papery handle if the tension becomes excessive. Textured fabrics tend to acquire a thin, impoverished handle if the setting temperature is too high as the yarns begin to lose their bulk. Clip stenters and cylinder dryers have been used for heat setting but hot air pin stenters are generally preferred because of their versatile dimensional control. The stenter conditions selected vary markedly for different fabric qualities and must be carefully chosen according to the end-use of the goods and the thermal history of the yarns present. A balanced approach is necessary to achieve satisfactory stability, handle and dimensions of the finished fabric. Attainment of the most suitable setting conditions may be restricted by the thermal sensitivity of bulked yarns or non-polyester fibres present. For the best possible combination of handle and thermal stability the setting temperature should be as high as possible consistent with these limitations and the setting tension should be as low as possible whilst still providing good dimensional control. The time of exposure in the setting zone of the stenter varies between ten and thirty seconds depending on the fabric construction. Most of this time is occupied in heating the goods to the setting temperature and once this is reached the actual setting effect is attained within a fraction of a second. More rapid treatment can result in variations of dyeability in a subsequent dyeing process. Fabrics that have been fully stabilised by a high-temperature dyeing process are usually able to withstand subsequent pressing in garment making, laundering and ironing without shrinkage. Nevertheless, the handle and crease recovery of such materials are improved by stentering with low tension at 150 to 170°C and this process is rarely omitted from the finishing routine. Fabrics containing both dyed yarns and unset white yarns are prone to differential shrinkage and must be set
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at the highest temperature permitted by the sublimation fastness of the dyed yarns. Undyed polyester qualities may be heat set at 220°C if the yarns are stable to moderate tension. It is difficult to control the running of fabric without imposing a small degree of tension and the high shrinkage involved in full relaxation leads to considerable loss of yield. Thus it is necessary to impose some restraint on shrinkage and the best and most uniform finished effect is achieved by restraining some 4 to 5% of the potential yarn shrinkage, allowing an increasing degree of relaxation shrinkage to take place in the stenter as the setting temperature rises. Table 9.9 shows how the degree of restraint can be maintained at an approximately constant level over a wide range of setting temperatures. It may be used as a guide in the selection of setting conditions for filament fabrics and other qualities woven or knitted from yarns stable to tension. Staple fabrics shrink less than filament fabrics and stability adequate for apparel is conferred by setting at 170 to 180°C, although higher temperatures are possible unless restrictions are imposed by the properties of other fibres present in a blend. Polyester/wool fabrics should be set for 30 seconds at 170°C, allowing 3 to 5% relaxation shrinkage in worsted-spun goods and 1 to 2% for woollenspun materials. Some finishers prefer higher temperatures but little further increase in stability to conventional garment-making presses is gained. After setting, the goods may be moistened or steamed to restore the equilibrium moisture regain of the wool fibres as rapidly as possible. Under normal conditions short-staple polyester/cellulosic blends are typically heat-set for 30 seconds at 180°C because of the tendency of cellulosic fibres to become discoloured at higher temperatures. If increased stability is required, as in the case of fabrics to be dyed by a pad-thermofix method, setting for 20 to 30 seconds at 200°C is possible without serious risk of thermal degradation of the cellulose. The free weft shrinkage of a typical polyester/cotton shirting fabric is 4% at its normal setting temperature but this is restrained by 2 to 3% in order to ensure the removal of creases and to gain control over weft straightness. The choice of setting conditions for textured polyester materials depends on the type of bulking and yarn stabilisation processes that have been employed. High tensions must be avoided, of course, in order to preserve maximum bulk. Textured polyester yarns have a critical temperature above which the force required to produce an irreversible extension of the yarn becomes comparable with fabric weight. Even when the fabric is supported by balancing airflows within the stenter chambers, some loss of bulk can occur above this temperature and it is usually necessary to accept some compromise between handle and stability when dimensional stability is required. 9.4.2 Finishing of Polyester Fabrics An important aspect of polyester finishing is the re-application of fibre-processing assistants to tow, loose stock, sliver or yarn after scouring and dyeing. These formulations must have both lubricant and antistatic properties and must be applied uniformly at the correct level within narrow limits of tolerance. Not least important, they should be readily and completely removed during fabric finishing. To achieve reliable and reproducible spinning quality it is necessary to establish standardised conditions and carry out careful testing of the resultant application levels.
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A convenient and economical method for accurate and uniform re-application of fibre-processing assistants to polyester loose stock is in a centrifuge fitted with auxiliary tanks arranged for the application and recovery of an aqueous dispersion of the agents. Polyester tops for worsted spinning can be re-dressed by spraying and this application method is the easiest to control with accuracy. The metering spray nozzle is mounted above the entry to the first gilling machine after dyeing. Both tops and tows can be re-dressed by immersion, followed by uniform squeezing, at the last bath of a back-washing machine. The re-lubrication of dyed yarns is carried out most effectively by oiling or waxing during a re-winding process after dyeing. Staple yarns require only light lubrication for most end-uses and this can be provided by applying a wax emulsion from a warm final rinse bath. Self-scouring oils are used for textured filament yarns. Warp-knitters and weavers generally prefer relatively low oil levels (1 to 2%) but weft- knitters normally require higher levels (3 to 5%). Probably the most important aspect of the finishing of polyester staple materials is to minimise pilling. All staple fabrics form small balls or pills of entangled fibres on the cloth surface as a result of abrasion during wear. The high resistance of polyester fibres to flexural abrasion promotes retention of these pills in sufficient concentration to produce an unpleasant handle and appearance. Heat treatment reduces the tendency of loose fibres to migrate and removal of protruding hairs from the fabric surface minimises the pilling propensity. The singeing process is suitable for the treatment of lightweight polyester/cotton shirting, sheeting and similar goods. Polyester/wool worsteds, textured fabrics and other heavier constructions can be cropped successfully and may be cropped before singeing. The effect of both processes is the removal of hairs protruding from the cloth surface but these treatments must not be severe enough to damage the yarns themselves or the pilling propensity may be further increased. Visual inspection of the treated goods may be misleading and the results must be assessed by carrying out a suitable pilling test. Emerising (also known as sanding, peaching or sueding) is a physical finish for synthetic fabrics in which tightly woven or knitted constructions are passed over high-speed driven rollers covered with emerycloth or sandpaper (Figure 9.9). These rotate against the fabric surface, abrading the filaments and changing the appearance and feel of the cloth. The final effect resembles that of suede or a peachskin in which the damaged filaments form a soft nap or pile. Careful consideration of fabric design is critical to ensure a successful finish. Microfibres have more filaments available for surface cover and thus develop a softer handle more quickly. Prior to emerising, it is essential that the polyester is treated with a suitable lubricant to assist the cutting action. Colour variations on emerised fabrics can be problematic. Light reflections create a substantially paler appearance and it is necessary to apply a higher concentration of dye to compensate for this effect, thus increasing the limitations of fastness characteristic of polyester microfibres (section 9.2.2). Modification of the surface appearance caused by flattening of the nap or variation in nap length may also give rise to colour continuity problems [78]. Emerised velour fabrics are commonly used in outdoor clothing, sportswear, fashion shirts and blouses as well as high-performance garments such as breathable waterproofs. Emerising softens fabric handle and confers a greater degree of warmth, substantially increasing fabric bulk, but this is balanced by a consequential loss of fabric width. Careful control of the process is vital; if a
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major percentage of the fabric’s filaments are damaged or broken severe losses of tensile strength and extensibility will result. Unlike emerising, the creation of a more pronounced surface nap by loop raising does not damage the filaments. The effect is achieved by teasing out individual filaments or fibres from the yarns so that they stand proud as numerous small loops on the fabric surface. The raising machine is fitted with hooked card wires set in a strong support fabric that forms the wrapping of rotating drums. The raised effect is produced by the differential speeds of movement of the fabric and the drum. As the wire hooks dip into the fabric structure this difference causes lateral movement so that the raised loops are formed and released as the hooks move away again. Control of the raising process is dependent on card wire shape and angle, as well as the density of raising points per unit area. Friction calendering of synthetic fabrics involves passing the material between heating or cooling hydraulic cylinders that rotate in the direction of cloth movement but at different speeds, so that the faster calender roller skids over the fabric surface to produce a more lustrous or glazed appearance. The handle of the material also changes, becoming smoother, thinner and more compact, as the fibre crimp is modified and the fabric interstices become much smaller. Calendering is essential for feather-resistant duvet covers to confer satisfactory resistance to penetration. Controlled reduction of interstice size is a crucial step in the production of outdoor fabrics. This increases resistance to severe weather conditions whilst retaining the high degree of breathability required [78]. Emboss calendering is a similar process that exploits the thermoplastic properties of polyester materials. The high glass-transition temperature of drawn polyester fibres makes it possible to achieve durable embossed or pleated effects on fabrics that have previously received a setting treatment at a higher temperature (200 to 210°C) and are therefore free from shrinkage during the secondary finishing process at 190 to 200°C. Alternatively, garment panels may be pleated or embossed at a lower temperature, sufficient to make them durable, and then rolled between paper formers and set by steaming in an autoclave. Thermosetting resins have only limited durability on polyester fibres because of poor resin-fibre bonding but poly(vinyl acetate) or melamine resins are sometimes used to confer stiffness, crease recovery and shrink resistance. Careful drying is critical and the recommended curing conditions, typically 3 to 5 minutes at 150°C, should not be exceeded because of the risk of yellowing or loss of tear strength and abrasion resistance. Vinyl or acrylate polymeric binders have been used to apply pigments for economical coloration of polyester or blended fabrics in pale colours. Lubricants and antistatic agents are often applied to polyester fabrics as a final finish to improve their performance during garment making. Certain qualities, such as tightly-knitted textured fabrics used in suitings, are particularly susceptible to needle damage during sewing. This tendency can be controlled by the application of a polyethylene emulsion as a lubricant. Cationic or nonionic antistatic agents are usually applied by padding immediately before the final stenter drying. Such treatment is useful to protect polyester goods temporarily from fog marking during storage and against static electrical effects during garment making. These agents depend for their action on the absorption of moisture from a humid atmosphere to form a conductive film on the hydrophobic surface.
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Durable antistatic finishing of polyester is attainable using block copolymers of linear oligomeric poly(ethylene terephthalate) condensed with poly(ethylene glycol). When applied to the fibre surface and heat treated, the oligoester segments of the copolymer become closely attached to the chemically similar polyester fibre. The polyglycol segments are more hydrophilic and they confer these characteristics to the newly created surface of the coated fibre. The antistatic and soil-release properties acquired in this way are durable to repeated washing under the mildly alkaline conditions of domestic laundering but the finish is stripped fairly readily by the action of strong alkali. Softeners act on synthetic fibres by coating the individual fibres or filaments with a low-friction smooth film, ensuring that the fabric feels considerably softer and drapes well. Most softeners are anionic or cationic, often based on silicone polymers, but repeated washing gradually diminishes the effect by depleting the coating of the fibres. Silicone resins also provide the most effective waterrepellent finishes for polyester materials, normally applied as aqueous solutions after dyeing. These are padded, dried and usually require curing at 150°C under tension on a hot air stenter to bond the finish to the fibre surface. Fluorocarbon finishes are applied as dispersions in a similar way to confer combined water repellency and resistance to oil-borne stains or soiling. A problem associated with durable hydrophobic finishes is that the goods cannot be rewetted to facilitate further wet processing should the need arise. Fluorocarbon finishes are very difficult to remove once they have been applied and cured. They may also cause slight shading effects, making dyed or printed colours a trace yellower or duller after finishing. Waterproof coatings for outdoor synthetic materials include breathable types based on polyurethane and stain-resistant laminates derived from polytetrafluoroethylene [78]. 9.4.3 Dyeing and Finishing of Polyester Automotive Textiles Polyester and nylon are the dominant fibre types used in textiles for car interiors, although a variety of other fibre types occupy niche positions where natural fibre appeal is important. It is difficult to ascertain exact amounts of each fibre type used in automotive upholstery, although the worldwide market of about 75 kilotons p.a. divides up approximately into: polyester 66%, nylon 26%, others 8% [79]. These figures indicate a reversal of the situation in the early 1980s when nylon occupied a dominant position. Standards for colour fastness and strength retention under the extreme conditions encountered inside vehicles were sharply increased by car makers and disperse-dyed polyester protected by the application of UV absorbers proved to be the most stable system in this respect. Although the selection of dyes of suitably high fastness during prolonged exposure to sunlight at high temperature and humidity is of primary importance, additional protection is achieved using UV absorbers, which are colourless aromatic compounds with a high propensity to absorb damaging UV radiation. A useful definition states that a UV absorber is a molecule that may be incorporated within a host polymer in order to absorb UV radiation efficiently and convert the energy into relatively harmless thermal energy without itself undergoing any reversible chemical change or inducing such change in the host polymer [80]. Such products are usually applied to polyester from the disperse dyebath and are water-insoluble compounds mostly derived from o-hydroxy-substituted benzophenones, benzotriazoles or benzotriazines [81,82].
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The full range of woven and knitted fabric manufacture is utilised for the automotives industry. Flat woven cloths in air-jet textured and staple polyester have been important for many years. These are usually made from dyed yarns woven into either plain or patterned constructions but without any surface finish. Such fabrics are continuously scoured and stenter dried, since residual contaminants are liable to contribute to fogging problems due to temperature fluctuations in the car interior. On patterned designs, particular care must be taken with uniformity of appearance because of the straight-edge sewlines or components of car furnishings that make deviations especially obvious [83]. Woven or circular-knit pile constructions are produced from dyed yarns and then cropped to various degrees, with or without steam, before being subjected to stenter treatment to stabilise the fabric. These high-pile goods are sensitive to crushing and must be carefully handled until the pile is stabilised. Weft knitting has been one of the important growth areas. Computer-controlled jacquard machines for pile fabrics have met demands for good design versatility. Short runs offer greater flexibility and a capability to produce specialised fabrics quickly and more economically than by other routes. Warp-knit fabrics for headliners are processed as wide as possible and cut to multiples of the correct width after final processing. The knitted material is preset and then either beam- or jet-dyed before removing excess moisture and applying a lubricant to assist raising. After drying to predetermined dimensions on a stenter the wide fabric is passed down a brushing line. The raise of the fabric has to be uniform to ensure good cover and a soft appearance of the fitted headlining. The fabric is finally post-set and split to the desired dimensions. Warp-knit backs and bolsters designed to complement the seat facings may be brushed, sueded or dappled. Often these fabrics are pre-set before brushing or sueding, then jet-dyed and stenter prepared for brushing prior to post-setting. It is possible for such products to be given four stenter passes and two brushing runs for the correct control to be exerted over the resultant finish. The goods must be carefully inspected for all faults, including continuous monitoring of colour tolerance. After the fabric is laid and cut, any piece from any dye lot could be sewn with any other, forcing a tight tolerance on the dyer and finisher [83]. Warp-knit facing fabrics have more design and colour features than those for backs and bolsters, requiring a finish to impart a more luxurious but hard-wearing look. They may be pre-set, depending on the shrinkage potential of the yarns, jet-dyed and dried whilst applying a lubricant. After passing several times over brushing machines in line to create a compact pile, this is then sheared to confer a velvet appearance. The pile height and angle are important in ensuring continuity of shade and finish. Double needle-bar raschel constructions for seat facings are produced using knitting machines with two needle bars. Two fabrics are generated simultaneously, joined together by some of the yarn crossing from one side to the other. They are separated into the two fabrics off the machine and then both must be finished, often by differing routes, to give the same product in terms of design, colour and appearance. The finishing route may include a continuous scour and then several cropping and brushing operations to obtain the desired pile height, angle and lustre. The fabric is set at some stage to stabilise the yarns. The necessity to separate a double-backed fabric inhibits design and the difficulty of achieving two split halves of equal pile quality can impose limitations.
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If standards of fastness to light and sublimation continue to increase, further research on dyes and auxiliaries for use with automotive textiles will be an area of essential activity. For polyester the handful of disperse dyes that meet test criteria is already under pressure from existing tests in certain shades. Clearly, improved dyes and protective agents are essential. Less volatile UV absorbers of higher activity that are less sensitive to application conditions must remain an important research target. Abrasion resistance tests should indicate likely loss of fabric mass, degeneration of appearance due to felting or pilling and anticipated effective fabric life. There is a conflict between cost and speed of test in relation to performance of the textile material in actual use. Fogging tests are likely to become more common and more demanding, putting pressure on chemical manufacturers to supply less volatile UV absorbers and cleaner finish formulations.
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Table 9.1 Variation of aqueous solubility of cyclic tris(ethylene terephthalate) crystals with temperature [5]. Temperature (°C) 100 120 130 150
Aqueous solubility (mg/l) 0.4 2 5 16
Table 9.2 Relative dye consumption for dyeing polyester filaments of decreasing fineness (taking the 1970 average as unity) [23]
Fibre/year Conventional fibre in 1970 Conventional fibre in 1990 Conventional fibre in 2000 Microfibre Super microfibre
Filament fineness (dtex/fil) 3-5 2-3 1.5-2.2 0.3-1.0 0.1-0.3
Average (dtex/fil) 4 2.5 1.85 0.65 0.2
Applied depth (%) of dye for equal colour yield 1.00 1.27 1.47 1.82 4.48
Table 9.3 Comparison of wash fastness ratings on conventional and microfibre polyester fabrics dyed with Dispersol Navy D-3GR (DyStar) [23] Dye applied (%) 3.0 6.0
Woven polyester Conventional Microfibre
C06/B2 at 50°C with multi-fibre strip A B C D 5 4-5 4 4-5 5 3-4 3 4
M&S C4A at 60°C with multi-fibre strip A B C D 5 3-4 3 4-5 4-5 2-3 2 4
Dyed for 60 minutes at 130°C, reduction cleared, post-stentered for 30 seconds at 170°C Conventional 1/76 f 24; 3.2 dtex per filament Microfibre 1/50 f 88; 0.57 dtex per filament A – Effect, B – Polyester, C – Nylon, D - Cotton
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Table 9.4 Fastness to washing of various azo navy disperse dyes on woven polyester [23]
Dye Dispersol Azo navy Azo navy Azo navy Azo navy
M&S C4A at 60°C with multi-fibre strip Acetate Cotton Nylon Polyester 4-5 5 5 4-5 3 4-5 2-3 3 3 4-5 2 3 3-4 4-5 2 3 2-3 4 2 3
Navy XF dye A dye B dye C dye D
Acrylic 5 4-5 4-5 4-5 4-5
Wool 5 3-4 3-4 3-4 3-4
Table 9.5 Cross-staining of cotton by various azo rubine disperse dyes on polyester/cotton [23] Dye Dispersol Rubine XF Azo rubine dye A Azo rubine dye B Azo rubine dye C
Rinse only 3-4 1-2 2-3 2
Alkali clear 4 3 3 3
Reduction clear 4-5 4-5 4-5 4-5
Table 9.6 Composition of the oligomer released under various dyebath conditions in the high-temperature dyeing of polyester yarn [54]
Dyebath pH Acidic Alkaline Alkaline
JPH 95 concn (g/l) 1.5 2.0
Total oligomer (A) 45 84 92
Dissolved oligomer (B) 11 67 84
Crystalline oligomer (A-B) 34 17 8
Table 9.7 Degree of removal of residual sizing agent from polyester satin fabric during scouring or high-temperature dyeing [54]
Stage Grey goods Acid dyeing Alkaline dyeing Alkaline dyeing Scouring
JPH 95 concn (g/l)
4 6
Residual size (%) 100 94 64 49 10
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Table 9.8 Fastness of black dyeings on polyester yarn after various reduction clearing treatments [71]
Reduction clear Untreated control Hydroxyacetone Formosul Formosul with AQ activator Iron(II) chloride/gluconic acid Thiourea dioxide Sodium dithionite
Washing at 60°C 1-2 2-3 2-3 3 3 4-5 4-5
Perspiration Acidic Alkaline 2-3 2-3 4-5 4-5 4-5 4-5 4-5 4-5 4-5 4-5 4-5 4-5 4-5 4-5
Table 9.9 Shrinkage allowance for constant restraint of potential yarn contraction [77] Setting temperature (°C) 150 160 170 180 190 200 210 220
Residual shrinkage (%)(a) 4.5 5.5 5.8 6.0 7.4 8.7 9.8 10.8
(a) Assuming 6% scouring shrinkage.
Shrinkage allowance (%) 0 1.0 1.5 1.5 3.0 4.0 5.5 6.5
Restraint (%) 4.5 4.5 4.3 4.5 4.4 4.7 4.3 4.3
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Figure 9.1 Effect of dyebath pH on the tenacity of polyester staple yarn [17] 100
Retained tenacity, %
90
80
70
60
50 0
2
4
6
8
10
12
Dyebath, pH
Figure 9.2 Traditional process sequence for polyester fabrics [54] Spinning Oils, size polymers Lubricants
Weaving or knitting
Alkali, oxidising agent
Scouring and bleaching
Oils, size by-products
Caustic alkali
Caustic mass-reduction
Alkali, oligomer
Acid Acid, disperse dyes Alkali, reducing agent Acid Finishes
Neutralisation Disperse dyeing Reduction clearing Neutralisation Finishing Drying and making-up
Salts Oligomer, dye residues Oligomer, dye by-products Salts Finish residues
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Figure 9.3 Traditional acidic dyeing process for polyester 140
Dyeing
Dyeing temperature, oC
120
100
80
Scour
Reduction clear Warm rinse
60
Warm rinse
Rinse
40
Cold rinse and unload
20 0
100
200
300
400
Dyeing time, min
Figure 9.4 Alkaline dyeing process for polyester [54] 140
Dyeing
Dyeing temperature, o C
120 100 80
Scour
60
Warm rinse
Warm rinse
40
Cold rinse and unload
20 0
100
200 Dyeing time, min
300
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Figure 9.5 Rapid scour-dye process for polyester [42] 140
Dyeing
Dyeing temperature, oC
120
100
80
Scour
Scour Warm rinse
60
40
Cold rinse and unload
20 0
100
200
300
Dyeing time, min
Figure 9.6 Influence of heat setting temperature on subsequent uptake of CI Disperse Red 1 by a polyester filament fabric [76] 80
Dye uptake, %
70 60 50 40 30 20 120
140
160
180
Heat setting temperature,
200 oC
220
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Figure 9.7 Free shrinkage of standard unset polyester yarn in hot dry air [77] 20
Shrinkage, %
15
10
5
0 60
80
100
120
140
160
Air temperature,
180
200
220
oC
Figure 9.8 Free shrinkage of standard unset polyester yarn in saturated steam [77] 12 10
Shrinkage, %
8 6 4 2 0 100
105
110
115
120
125
Steam temperature,
oC
130
135
140
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Figure 9.9 Principle of operation of emerising machine [78] Sueding rollers Fabric entry
Fabric exit
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...
O
O
O
O R O C
C O R O C
C O R O C
C
O
O
O
9.1
O
O CH2CH2 O O C C
O
O C O CH2
C O CH2
CH2
CH2 O C
C O
O
O 9.2
CH3 HO P CH2CH2COOH O 9.3
CH3 CH2
N+ CH3
9.4
(CH2)11CH3 Cl–
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[aryl] N N 9.5
N
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CH2CH2COOCH3 CH2CH2COOCH3 2NaOH
[aryl] N N
N
CH2CH2COONa CH2CH2COONa
+ 2CH3OH
9.6
Chapter 10 Nylon Dyeing 10.1 Production and Properties of Nylon Fibres Commercial development of the first-ever synthetic fibre arose from the fundamental research on condensation polymerisation carried out by Wallace Carothers and his team at DuPont from 1929 to 1937. This polyamide (10.3) made by condensing hexamethylenediamine (10.1) with adipic acid (10.2) was the first fibre made by extrusion of the molten polymer rather than dissolution in a suitable solvent and extrusion into a precipitating medium. Structure 10.3 was given the name nylon 6.6 (from the number of carbon atoms in the starting materials) to distinguish it from other synthetic polyamides. In 1938 Paul Schlack of I G Farben succeeded in synthesising nylon 6 polymer (10.5) from caprolactam (10.4), a reaction that Carothers had considered but abandoned, and development of Perlon fibres rapidly followed this discovery. DuPont and I G Farben concluded an agreement in 1939 that facilitated the development of nylon as a superior replacement for silk in parachutes and other wartime requirements. An interesting account of the development of the nylon 6 industry during the turbulent 1940s in Germany appeared recently [1]. The demand for nylon intermediates is influenced mainly by fibres (textile, industrial, carpet) representing 75% of nylon polymer use, but nylon resins for plastics (engineering, food packaging) will increase their share from 25% to 35% over the next few years [2,3]. The nylon fibres market is stagnant and the present production ratio of about 60% nylon 6 to 40% nylon 6.6 will continue. North America at 34% has the largest share of global consumption of nylon, mainly because of the popularity of thick plush carpets in the USA. Western Europe is a static nylon market at about 20%; many Europeans move house only once or twice in a lifetime, whereas US citizens generally move more often (7-8 times). Asia, with a large population and low labour costs, is showing strong growth at 30%, although carpet usage is mainly found in air-conditioned buildings (hotels, offices) and nylon 6.6 accounts for only 4% in this region [4]. Annual global production of nylon amounts to about 4000 kt. Total caprolactam capacity is 3400 kt p.a., of which about 60% is non-captive, since there are 29 producers around the world [5,6]. This gives caprolactam a logistic advantage, especially for Asia Pacific nations. The technology for continuous polymerisation of nylon 6 can be bought from various engineering companies, which can sell knowhow to the nylon 6 producers without restriction. Nylon 6 waste can be recycled back to caprolactam, without deterioration of the quality of the monomer. DSM/Honeywell have started up the first nylon 6 recycling plant in Augusta, Ga [3]. A caprolactam recycling plant to utilise waste nylon 6 floorcoverings was developed by Lurgi Zimmer and constructed at Premnitz in 2000. The Sulzer falling-film crystallisation process is especially suitable for the production of high-purity intermediates and enables the recovered caprolactam to be isolated in a quality similar to virgin material synthesised from oil. The main advantages of this crystallisation process are minimal maintenance, small space occupation and low production cost [7]. Nylon 6 polymer recycled via caprolactam is of particular interest in components used in the automotive industry. Global capacity for hexamethylenediamine (HMD) is 2400 kt p.a.; this is the essential raw material for nylon 6.6 because adipic acid is readily available. The eight major producers in the nylon 6.6 industry are involved in the entire
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business chain from raw materials to fibre products. Thus only a limited share (only about 20%) of HMD supplies are available for non-captive use. DuPont is the world’s leading producer of nylon 6.6 from HMD and BASF the second largest producer of nylon 6 from caprolactam. These two companies have announced a new route to caprolactam via adiponitrile, which is also converted to HMD for nylon 6.6. These reactions are carried out in two reactors; in the first adiponitrile (10.6) is hydrogenated to yield an equimolar mixture of HMD and aminocapronitrile (10.7) and in the second the latter product is hydrolysed to give caprolactam (10.4) and ammonia. Fractionators are required to separate and recycle the products and intermediates [8]. The two important routes to adiponitrile (10.6) are (a) the nickel-catalysed addition of HCN to butadiene (10.8) and (b) the electrohydrodimerisation of acrylonitrile (10.9). Adiponitrile plants are the most capitalintensive, especially route (a) where the greatest economy of scale can be realised. In the innovative Asahi synthesis of adipic acid (10.2), benzene is converted into cyclohexene by partial hydrogenation and this is hydrolysed to cyclohexanol. The product is then oxidised to adipic acid using nitric acid with a copper/vanadium catalyst. A more unorthodox route devised at Purdue University, Ind. [9] utilises D-glucose from cellulose waste. This raw material is converted by microbial enzymes to muconic acid (10.10), which can be hydrogenated to adipic acid. Reproducibility of the polymerisation reaction and the melt extrusion process has a major influence on the characteristics of the nylon fibre and hence on the dyeing and finishing behaviour of nylon textiles. In the formation of AH salt (hexamethylenediammonium adipate) prior to the polymerisation to give nylon 6.6 polymer, the components 10.1 and 10.2 have to be mixed with an accuracy of better than 0.01% [4]. Nylon 6 melts at 215°C and nylon 6.6 at 250°C. The condensation polymerisation must be controlled to maintain the average molecular mass (M) of the polymer within fairly well-defined limits (15,000 to 18,000). The degree of polymerisation n in structure 10.3 should be within the range 67 to 80 and for the much smaller repeat unit in structure 10.5 the value n should be 130 to 160. Polymers with M lower than 15,000 yield brittle fibres of low tensile strength, whereas the higher melting and softening points of those with M higher than 18,000 present problems in the extrusion and drawing processes. To control the degree of polymerisation within the desired limits, a carboxylic acid is added as a chain stopper prior to the condensation reaction. The addition of a chain stopper implies that there will be an excess of carboxyl over amino endgroups in the polyamide. In industrial practice, the polymerisation conditions and the physical and chemical properties of the polymer are carefully controlled and this should ensure a uniform product. However, relatively minor fluctuations in the purity of the raw materials and the temperature and time of polymerisation can lead to significant variations in the nylon fibres and these may influence dyeing characteristics. Dyeability is also dependent on the degree of stretch imparted during the drawing process. Dye uptake decreases as the draw ratio increases, as a result of the structural changes that the fibre undergoes during stretching. Effects similar to those caused by variations in the degree of stretching are liable to occur during heat setting if unevenly stretched fibres are heat treated under otherwise uniform setting conditions. Variations in degree of crystallinity and orientation within the polymer structure result in differences in the shrinkage behaviour of the fibres.
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end-groups, whereas 1:2 metal-complex dyes tend to be more sensitive to physical differences arising from variation in the conditions of stretching and heat setting processes. Figure 10.1 shows the effect of variations in false-twist heater temperature on the uptake of a typical 1:2 metal-complex dye by nylon [10]. Disperse dyes are relatively insensitive to physical and chemical variations in fibre structure. In order to suppress the natural brightness of nylon, it is necessary to add a suitable delustrant to the polymer before extrusion. The appropriate amount of titanium dioxide is normally added as an aqueous dispersion. The quantity and quality of the added delustrant both influence the physical properties of the treated nylon. Increasing the amount of delustrant present within the fibrous polymer results in increased scattering of light, the opacity of the fibre increases and a higher concentration of dye is required to produce a specific depth of colour. The presence of delustrant also results in a small but significant lowering of the light fastness of dyeings and resistance of the nylon fibres to photodegradation. Nylon fibres are normally stabilised against oxidation and photodegradation by the addition of very small amounts of transition-metal salts and organometallic compounds. The presence of these antioxidants exerts a beneficial influence on the resistance of the polyamide to oxidative degradation and yellowing during dry heat setting treatments. The amino end-groups are protected from oxidation and free-radical photo-oxidative attack on the polymer chain is strongly retarded. The protective action of these additives can be adversely affected by strongly oxidative bleaching agents and these products should therefore be avoided in nylon processing. Nylon is inert to all common organic solvents at ambient temperature and shows excellent resistance to dilute acids, alkalis and dilute solutions of oxidising agents. Degradation may occur during prolonged treatments at low pH or under oxidising conditions at elevated temperatures. Nylon is dissolved by concentrated mineral acids, formic acid or phenol. To produce nylon fibres of consistent quality and dyeability performance, it is normal practice to blend polymer batches carefully prior to melt extrusion. However, polymer characteristics are often deliberately modified to give variant products that must be clearly identifiable in order to avoid accidental mixing of different types of yarn in the finished fabric. The changes in polymer quality are routinely distinguished by giving each distinct polymer variant a different merge number. Yarns with different merge numbers should never be mixed during yarn processing and fabric manufacture. In contrast to standard nylon yarns, microfilament nylon materials cannot be dyed in circulating-liquor machines because the high density of the yarn package or fabric beam prevents adequate liquor flow. Nylon fabrics of this type are preferably dyed on the jig or in a jet or overflow machine. Microfibre nylon fabrics tend to float and hence do not absorb the dye liquor uniformly, but this can be avoided by dyeing at 105 to 110°C [11]. Problems associated with processing such fabrics include inadequate fabric preparation, unlevelness, limited build-up performance, inferior fastness to light and wet treatments, less effective aftertreatment and inadequate softening. Four parameters can be manipulated carefully to promote the conditions required for level dyeing: levelling agents, high-temperature dyeing, pH control and dyeing process controllers [12].
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Akzo Nobel has recently developed a high-tenacity Stanylenka filament yarn from nylon 4.6, made by polycondensation of adipic acid and diaminobutane. This polymer has been used by car manufacturers worldwide for the injection moulding of car parts since the early 1990s. The higher concentration of amide groups compared with nylon 6.6 results in a higher melting point (285°C), crystallinity and rate of crystallisation. Various applications envisaged for Stanylenka include woven airbags, sewing threads, conveyor belts, cord fabrics and protective clothing [13]. High-performance aramid fibres made by polycondensation of aryldiamines and aryldicarboxylic acids have exceptionally high melting points, crystallinity and durability. Meta-aramid fibres based on poly(m-phenylene isophthalamide) exhibit excellent thermal and chemical resistance. They possess a non-rigid structure attributable to the non-linearity of the polymer chains. Para-aramids based on the linear poly(p-phenylene terephthalamide) show much higher crystallinity. Technora (Teijin) aramid based on poly(p-phenoxyphenyl terephthalamide) adopts a non-linear conformation. Optical birefringence reveals that para-aramid fibres are more oriented and contain larger-size crystallites than Technora and especially the meta-aramid polymers. Poor compressional behaviour, fibrillation, poor fatigue and wear resistance have been major drawbacks of the highly crystalline para-aramid structure [14].
10.2 Preparation of Nylon Differences in the degree of swelling of nylon fibres arising from the uneven absorption of moisture or localised drying out of moist goods can result in unlevel shading faults, especially when applying metal-complex or milling acid dyes of high wet fastness. A heat setting treatment prior to dyeing normally prevents such problems, but if the heat treatment is not sufficiently uniform this will again show up as unlevel shading particularly when fast dyes are applied. To obtain reproducible results in nylon dyeing and finishing, it is normally essential to remove all spin finishes, lubricating agents, oil and random soiling from the goods. The scouring and rinsing liquors should be free from traces of iron, copper, calcium or magnesium salts that might be liable to precipitate on the goods and form stains. Scouring for 30 minutes at 60°C with 2 g/l detergent and 1 ml/l ammonia 25% will give satisfactory results in normal circumstances. Treatment with a higher concentration of detergent at a higher temperature may be necessary to remove heavier soiling or oil contamination. Addition of an appropriate sequestering agent will be necessary if the water hardness is significant or when traces of transition-metal ions are present. A reducing agent such as sodium dithionite will be helpful if soiling with graphite or molybdenum disulphide is suspected. Nylon fabrics are almost always subjected to a setting treatment at some stage in preparation, dyeing or finishing. The objective of this treatment is to release strains imposed during knitting or weaving, minimise creasing during wet processing and impart the high degree of dimensional and configurational stability required in finishing and subsequent use. In order to achieve a satisfactory degree of set in nylon, it is necessary to apply sufficient energy to the fibre to weaken or rupture interchain bonds and allow new bonds to be formed whilst the material is held at the desired dimensions. Thus the goods must be heated to a temperature close to the softening point of the nylon polymer and simultaneously appropriate widthway or lengthway restraint is imposed on the fabric. Under
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these conditions, the degree of fibre orientation and crystallinity is increased and this will significantly modify the behaviour of the nylon during dyeing and finishing. Three methods of setting are available for the stabilisation of nylon: 1. with hot air in a stenter 2. with superheated steam in an enclosed chamber 3. with hot water in a high-temperature dyeing machine. After heating, the material is given a shock cooling treatment. Setting is a relative term and if a fabric is treated at a temperature higher than that at which it was pre-set, the effects of the earlier treatment will be essentially eliminated. It is possible, however, to pre-set nylon material and enhance the degree of set during the dyeing process, as the effects of hydrosetting and heat setting treatments are normally additive. The various methods of heat setting have characteristic effects on the material. Goods that have been set with hot air exhibit a relatively thin handle, whilst those set in hot water have a full and soft handle. The characteristics of steam-set goods lie between those two extremes. The advantages and disadvantages of steam and dry heat setting processes are compared in Table 10.1. Heat setting of nylon fabrics is normally carried out on a pin (or clip) stenter with steam injection, the treatment temperature being maintained by indirect gas, oil or electrical heating. Injection of high-pressure steam into the stenter chambers improves processing efficiency by: 1. increasing the rate of transfer of heat to the fabric 2. allowing higher setting temperatures and shorter dwell times to be maintained 3. scavenging air from the setting chambers and degradation, stiffening and yellowing of the nylon.
minimising
oxidative
The degree of setting depends mainly on the temperature reached and, to a lesser extent, on the time of treatment at this temperature. Therefore, the duration of the process depends mainly on the time required to achieve the desired setting temperature throughout the goods being treated. Nylon 6.6 fabrics are typically set at 210 to 225°C and nylon 6 fabrics at 190 to 205°C with dwell times of about 15 to 20 seconds. Optimal efficiency is achieved when a six-chamber stenter is used with pre-heating occurring in the first two units, the setting temperature being maintained throughout the next two and cooling taking place in the last two chambers. The heat setting of nylon fabrics can be carried out either before or after the dyeing process. For economic reasons certain fabrics, such as warp-knitted qualities, are often set in the grey state without prior scouring or relaxation treatment. The advantages and disadvantages of these various approaches are compared in Table 10.2. Alternative methods are available for the dry heat setting of nylon fabrics, such as processing of the flat filament fabrics on heated metal cylinders, but it is generally difficult to control the width dimension accurately or to avoid surface glazing and stiffening on this type of equipment. In view of the rapid heating-up of the goods, this method is highly economical. Occasionally, fabrics that have been set in a stenter are aftertreated on a bank of setting cylinders to stabilise fabric selvedges to curling.
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Steam and hot water are excellent media for the setting of nylon, since the swelling action of moisture assists in the weakening and rupture of interchain bonds. Under pressure dyeing conditions the degree of set can be equivalent or superior to that achieved by dry heat setting. Several factors influence setting but, as an approximate guide, hydrosetting at the boil is roughly equivalent to dry heat setting at 185°C. Processing tensions are released, fabric bulk and softness are developed; fabric discoloration and stiffening, on the other hand, are virtually negligible after hydrosetting. In the case of flat filament fabrics, excessive shrinkage and cockling will occur if grey fabric is batched and subjected to hydrosetting at an elevated temperature. Consequently, it is advisable to pre-set such qualities in dry heat or steam to control fabric dimensions and minimise shrinkage. Using this procedure the degree of stability achieved during presetting can be significantly enhanced by the hydrosetting effect of the subsequent dyeing process. The treatment of nylon in saturated steam at temperatures above 100°C is carried out as a batchwise process in an autoclave equipped with automatic control of temperature, steam pressure and vacuum extraction of air. Some degree of dimensional restraint must be imposed if shrinkage is to be controlled to achieve the target dimensions. In practice, weft-knitted fabrics are calendered to approximately 5 cm wider than the target dimension and batched under slight tension onto perforated tubes. The setting cycle is commenced by a vacuum extraction down to a pressure of 62 cm of mercury. Saturated steam is then injected at the desired temperature and treatment continued for approximately 10 minutes. This is followed by a further vacuum extraction and a prolonged steaming (20 minutes) at the selected temperature. A final vacuum extraction completes the process. Since the steam-set goods usually have to withstand subsequent wet processing, it is necessary to set at a temperature about 20°C above that likely to be encountered later. Typical steam-setting conditions are 115 to 130°C for nylon 6.6 and 110 to 120°C for nylon 6. Woven or warp-knit fabrics are batched in open width on perforated centres before carrying out the steaming operation. In the case of nylon hose or tights, the goods are framed onto suitably designed aluminium formers that allow the material to shrink into shape during the steaming treatment in an autoclave. The steaming cycle is usually 1 to 3 minutes duration and a temperature restricted to 110°C for nylon 6.6 or to 105°C for nylon 6. The chemical intermediates and processing conditions for producing nylon are carefully controlled, so that the fibre is almost white and relatively free from impurities. Consequently, strongly active oxidising bleaches are not normally necessary. In the case of weft-knitted fabrics stabilised by steam setting under pressure, fibre discoloration is minimised and a mild bleaching treatment is entirely adequate. In the case of woven or warp-knit qualities stabilised by dry heat setting in the grey state, yellowing of the nylon under these conditions may cause problems. The techniques available for improving the whiteness of heat set or unset nylon are as follows: 1. Reductive bleaching with sodium dithionite under acidic or neutral conditions 2. Oxidative bleaching with alkaline hydrogen peroxide, or with peracetic acid or sodium chlorite under acidic conditions
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3. Fluorescent brightening agents either alone or in the presence of a reducing agent. Oxidative bleaching treatments are the most effective in overcoming the yellowness of grey-set nylon but analysis reveals that some amino end-groups are oxidised and dye substantivity is lowered. This effect significantly impairs the resistance of the nylon to photodegradation and lowers the light fastness of dyeings and prints produced on the bleached substrate. The more drastic the oxidative bleaching treatment, the greater the sensitivity of the treated nylon to photodegradation. In most instances nylon is bleached under reducing conditions using sodium dithionite and a suitable fluorescent brightener under acidic conditions at pH 4 to 5.
10.3 Dyeing of Nylon Disperse dyes exhibit good build-up, levelling and migration properties on nylon. Fastness to washing is moderate at best, but these dyes show very good coverage of physical and chemical variations of dyeability. Owing to their ease of application to nylon, disperse dyes are the first choice whenever their limited wet fastness is adequate for end-use requirements, such as the dyeing of nylon hose and tights. Disperse dyes on nylon give more bathochromic hues than the same dyes applied to cellulose acetate (yellows redder, reds bluer and blues greener). The resistance to gas-fume fading of disperse dyes on nylon is far superior to that on cellulose acetate. Dyes of low molecular mass may be prone to sublimation during dry heat setting and care must be exercised in selecting dyes for goods to be given a post-setting treatment. Nylon 6.6 fibres have a more crystalline structure than nylon 6, resulting in a slower rate of dye uptake at a given temperature. For similar reasons, the wet fastness of an acid dye on nylon 6.6 is slightly superior to that of the same dye on nylon 6. The more compact structure of nylon 6.6 fibres restricts the migration and levelling properties of acid dyes. More care is required in dye selection and in control of the dyeing process. It is generally true that acid dyes of rather larger molecular size are preferred on nylon 6 in order to achieve satisfactory fastness. Because of their mode of attachment by electrostatic bonding to positively charged amino groups in nylon, acid dyes are sensitive to fluctuations in the amino end-group content and this can give rise to stripiness in the finished fabric. The coverage of physical variations in nylon is dependent on molecular size, so that metal-complex and milling acid dyes of relatively higher molecular mass are particularly sensitive to dyeability differences of this kind. Even when nylon fibres exhibit pronounced variations in dyeability, it is possible to improve the coverage of potentially stripy fabric by: 1. careful dye selection 2. modification of the rate of dye uptake 3. selection of levelling agents 4. control of pH and temperature 5. high-temperature dyeing. It is difficult to significantly overcome stripiness arising from chemical variations within the fibres and the solution to this problem is prevention, which is largely in
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the hands of the fibre manufacturer. Fortunately, careful control of polymerisation and melt extrusion conditions can virtually eliminate this problem. Acid dyes vary widely in their affinity for nylon, but in general they are all applicable at some region within the pH range 4 to 7. The amino end-groups are protonated under acidic conditions and these positive charges are initially neutralised by association with the highly mobile inorganic ions. These are gradually displaced by the slowly diffusing dye anions of much higher affinity. The dye anions are initially adsorbed on the fibre surface and gradually diffuse into the interior, forming electrostatic linkages with the protonated dyeing sites. Fixation of relatively hydrophobic dye anions is enhanced by the formation of nonpolar dye-fibre bonds. In the case of levelling acid dyes applied at about pH 4, the sorption of dye anions corresponds quite closely with the content of protonated amino endgroups. With the larger and more hydrophobic metal-complex and milling acid dye ions showing higher neutral-dyeing affinity at pH 5-7, dye sorption is substantially in excess of that necessary to saturate the available amino endgroups present. Owing to competition for these dyeing sites between dyes that differ in affinity, practical difficulties are encountered when attempting to produce full-depth dyeings. In mixture recipes the individual dye components compete for the limited number of dyeing sites and a high-affinity dye that is preferentially adsorbed can partially inhibit uptake of the other components present. Owing to these effects, selection of a trichromatic combination of compatible dyes is quite difficult. In view of structural differences between nylon 6 and 6.6, a given acid dye will exhibit a slower rate of exhaustion and higher wet fastness on nylon 6.6 than on nylon 6. Conversely, on nylon 6 the same dye will build up more readily and show superior levelling properties. With nylon 6 it is normally necessary to aftertreat medium-depth dyeings with a syntan in order to achieve comparable fastness ratings. It is important to thoroughly rinse the dyeings to avoid transfer of loose dye into the aftertreatment bath, as this can lead to poor fastness to rubbing and perspiration. The infinity process for the exhaust dyeing of nylon with acid dyes is so named because exhaustion takes place under conditions approximating to infinite dilution, with the supply of dye to the dyebath being controlled to correspond with the rate of dye uptake by the fibre. Demonstration dyeings of C.I. Acid Blue 45 have been discussed in detail [16]. Fibre cross-section tests confirmed that the infinity dyeings were fully penetrated and showed significantly higher colour yield than conventional dyeings, without sacrifice of fastness to light or wet treatments. Closed-loop control of the dosing of dyes and chemical auxiliaries has been used to achieve on-tone build-up in the dyeing of nylon with a binary combination of monosulphonated acid dyes. Computer-controlled dosing pumps were adjusted to regulate the gradual change in pH, the individual concentrations of the component dyes and their rates of sorption throughout the dyeing process. It is practicable to analyse a partially exhausted dyebath containing residual dyes and chemicals and then to reuse this in a subsequent dyeing process after appropriate adjustment of the relevant concentrations [17]. In another evaluation of the reuse of acid dyebaths for nylon, it was found that pH control using the latent acid donor γ-butyrolactone (10.11) offered marked advantages compared with the use of ammonia and ammonium sulphate, in terms of freedom from dye aggregation and less risk of unlevelness as the reuse cycle proceeded [18].
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There has been some discussion regarding the best means of controlling pH in the dyeing of nylon with acid dyes [19]. In some cases, starting at pH 7 or higher with ammonium sulphate or acetate can lead to variations in pH at the end of the process, with consequential variations in performance. Better end-point control is achieved by starting at pH 6 to 6.5 using a mixed phosphate buffer and ensuring a slow rate of rise in temperature. The improved consistency of dyeing may offset the higher cost of the phosphate buffer. In some regions, however, the use of phosphates is regarded as environmentally sensitive. In most instances, metal-complex and acid dyes are applied by temperature control at constant pH. The temperature profile is usually based on experience depending on fabric quality, depth of shade and equipment available. To increase dyebath exhaustion and to improve levelling and reproducibility, an approach that coordinates a reduction in pH with a controlled temperature rise is preferable. The critical temperature range is slightly higher for this pH-sliding method and the dwell time at top temperature may be longer. A statistical evaluation of 300 jet dyeings to optimise this technique revealed that average process times were reduced by 27%, mainly resulting from a 65% reduction in corrections to faulty dyeings [20]. The use of surfactants as levelling agents is particularly important with acid dyes on nylon, especially with dyes of high wet fastness. Anionic agents act by competing for the cationic dyeing sites and are mainly used to counteract fibreoriented unlevelness due to physical and chemical irregularities in the fibre. Strongly cationic quaternary compounds readily form complexes with acid dyes but may precipitate them if used alone. Weakly cationic ethoxylated tertiary amines do not suffer from this disadvantage and are of great importance in minimising unlevelness associated with rapid dye uptake. Carefully chosen combinations of anionic and weakly cationic types are particularly useful because they counteract both types of unlevelness. An incompatible combination of dyes fails to show on-tone build-up because of the sequential sorption of individual components. A well-chosen levelling agent, or a combination of suitable agents can effectively convert such a mixture into a compatible one. In an evaluation of ethoxylated ethylenediamine derivatives (10.12) in the application of acid dyes to nylon, covering a range of ethoxylation from 40 to 180 units per molecule (average n = 10 to 45), the best initial retarding effect together with the highest uptake of dye at equilibrium was achieved with 180 ethylene oxide (EO) units (average n = 45) [21]. This highly ethoxylated ethylenediamine was found to increase dye uptake when incorporated into nylon granules as an antistatic agent [22]. Ethoxylated diethylenetriamine derivatives (10.13) containing 4, 20, 60 and 100 EO units per molecule (average n = 1, 5, 15 and 25) were evaluated with milling acid dyes on nylon 6 fibres. The product with only 4 EO units/mol was the most powerful retarder but it invariably caused marked restraining of the final yield. The relative performance of these agents was dependent on constitution and applied concentration, dye concentration and dyeing temperature. The best compromise between initial retarding effect and final colour yield was observed with 0.3 g/l of the agent containing 100 EO units/mol [23]. Systematic dyeing procedures have been developed by Bayer for the exhaust dyeing of acid dyes on nylon. In the Telon S process [24,25], the fibre saturation value (SF) and the rate of dyeing (V) are determined for the nylon fibre to be dyed. For these values, the starting temperature at which dyes begin to be absorbed by the fibre, the pH value to give maximum dyebath exhaustion and the
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dyeing time at top temperature to give full penetration are calculated. The amount of a levelling agent (Levegal FTS) required is related to the amount of dye applied. The dyes are classified for compatibility according to K value (or K' value in the presence of levelling agent). By far the most important aftertreatment for acid dyeings on nylon is the socalled syntan process. The dyed material should be rinsed well and then treated in a fresh bath containing the syntan and sufficient formic or acetic acid to bring the pH within the range 3.5 to 4.5. The temperature of the liquor is raised to 80°C and the treatment continued for 20 minutes at this temperature. The material is finally cooled and rinsed. Dyeings aftertreated with syntans tend to exhibit slightly lower fastness to light but show significantly improved fastness to washing. Dry heat or steam setting treatments reduce the overall washing fastness of aftertreated dyeings but this drop in fastness is minimised if dyes of high affinity for nylon have been used. Structural features of these anionic aftertreating agents have been described elsewhere [26]. Their rapid sorption by nylon under acidic conditions is largely the result of electrostatic attraction between their negatively charged groups and protonated amino groups in the surface regions of the fibre. Maximum improvement in wet fastness results when the syntan is adsorbed at the fibre surface, since diffusion of the agent into the fibre results in lower fastness ratings. The response to the syntan treatment in terms of improved wet fastness tends to vary markedly from dye to dye. Chrome mordant dyes can be used to produce economical full-depth dyeings on nylon with excellent wet fastness and very good fastness to light. They give good coverage of physical differences in nylon but are sensitive to variations in amino end-group content. Chrome dyes are applied at the boil in the presence of an acid-liberating salt such as ammonium acetate or sulphate. After one hour the pH is gradually lowered by the addition of formic acid to ensure a high degree of exhaustion. Any residual mordant dye present in the bath during the metallisation stage will form a metal complex that may be deposited on the fibre surface, resulting in inferior fastness to rubbing and perspiration. When chrome dyes are applied to nylon it is essential to include a mild reducing agent such as sodium thiosulphate in the metallisation bath. This ensures that the dichromate anions are reduced to chromium(IV) cations capable of chelating with the mordant dye on the fibre. The dyed material is rinsed thoroughly and metallisation carried out in a fresh bath containing formic acid and potassium dichromate. The temperature of the liquor is raised to the boil and treatment continued for 45 minutes. Sodium thiosulphate is then added to ensure reduction of the dichromate anions and metallisation is continued for a further 30 minutes. When applying chrome dyes the true hue is not attained until metallisation has been completed. Therefore it is necessary to develop sample cuttings of the dyed fabric in the laboratory before colour matching can be carried out. When the target colour has been achieved, the goods are rinsed and then afterchromed in a fresh bath. The 1:2 metal-complex dyes have proved particularly attractive for dyeing nylon because of their excellent build-up, freedom from significant blocking effects, good compatibility in combination shades and very good fastness to light and wet treatments. However, their aqueous solubility, dyeing behaviour and capability to cover dyeability variations in nylon vary significantly with molecular size, shape and degree of sulphonation. Some traditional 1:2 metal-complex dyes contain no sulphonic acid groups and are applied from aqueous dispersion. All metal-complex
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dyes tend to accentuate physical variations of affinity in nylon. Nevertheless, by the use of specific auxiliaries, combined with control of dyebath temperature and pH, the rate of dye uptake can be controlled. High-temperature dyeing conditions are particularly advantageous for improving coverage of affinity variations of physical origin. The 1:2 metal-complex dyes form low-solubility complexes with strongly cationic auxiliaries, which should therefore be avoided. However, selected weakly cationic agents will form unstable complexes with these dyes and can significantly reduce the rate of dye uptake. Furthermore, this dye/auxiliary system enables metalcomplex dyes to be applied by rapid dyeing techniques on the beam at 110°C (for nylon 6) or 120°C (for nylon 6.6). Using rapid dyeing procedures, the dye and complexing agent can be added to the initial dyebath at the boil without inducing unlevelness. Amphoteric levelling agents, combining the characteristics of anionic and weakly cationic surfactants in the same molecule, are especially suitable for controlling the 1:2 metal-complex dyes containing ionic solubilising groups (carboxyl or sulpho) rather than the nonionised but polar groups (such as sulphonamide or sulphone) in traditional metal-complex dyes. The sulphonated types are often cheaper to manufacture and offer better wet fastness; their development and exploitation owed much to the use of amphoteric betaine-type levelling agents [27]. In 1983 ICI Fibres launched Tactel in the apparel market offering enhanced aesthetic and comfort properties combined with the intrinsic benefits of nylon 6.6 fibres. Tactel Micro fine-denier filament yarns were introduced in 1989 for apparel fabrics that are ideal for modification during dyeing and finishing by such processes as sueding, coating and the application of surface-specific finishes [28]. A further recent development is DuPont’s Tactel Colorsafe, which has outstanding wash fastness (to ISO 4 to 5) and requires fewer chemical additions than conventional dyeing processes. This technology offers the distinctive features of fine-filament Tactel in combination with reactive dye systems to provide washfast fabrics with high colour values [29].
10.4 Stain Blockers for Nylon Carpet Fibres Application of nylon fibres in carpets exceeds 25% of total nylon usage [30] because of their resilience, durability, dyeability and aesthetics. An inherent drawback is their lack of resistance to soiling and staining. Treatment with perfluorinated finishes imparts effective soil repellency and improved stain resistance, provided the stains are rapidly removed. However, stain blocking agents are necessary to facilitate the ease of removal of more stubborn stains, such as the anionic colorants found in food, beverages and cosmetics. The most important class of stain blockers are the syntans, reaction products of sulphonated phenolic compounds with formaldehyde [26], that are also used in aftertreatments to enhance the wet fastness of acid dyes on nylon. It is widely accepted that this type of anionic polymer becomes attached to protonated amino groups in the surface layer of the nylon fibre by electrostatic bonding. The adsorbed syntan forms a surface barrier that retards the diffusion of anionic dyes out of the treated fibres during the washing process. By a similar mechanism this negatively charged barrier is capable of stain blocking by inhibiting the uptake of either acid dyes or anionic stains by treated nylon [31].
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Syntan stain blockers are mainly condensates of formaldehyde with phenolsulphonates, naphtholsulphonates or sulphonates of dihydroxydiphenylsulphone (10.14), or their mixtures. The preferred average relative molecular mass is within the range 250 to 700, representing only about three aryl rings per molecule. Condensates of substantially larger molecular size have inadequate aqueous solubility [31]. Some syntans contain other polar groups, such as carbonamide, sulphonamide, sulphone or ureide [32,33]. The traditional phenolic syntans are rather prone to give a reddish discoloration on prolonged exposure of the treated nylon to heat or light; this effect is attributed to oxidative degradation with the formation of quinonoid rings in the syntan molecule. Nevertheless, these condensates remain of interest because of their favourable cost-effectiveness. Trichromatic Carpets have developed a onepot product based on phenol-formaldehyde condensation, sulphonation and phosphation to yield a resol product. Enhanced stain resistance and satisfactory light fastness are claimed [34]. Syntans can be derived from dihydroxydiphenylsulphone (10.14) by acetylation, monosulphonation of each aryl ring, hydrolysis to regenerate the phenolic groups and finally condensation with formaldehyde. Although requiring this multistage sequence, such syntans do not impair light fastness significantly, permit the dyeing of nylon with acid dyes at elevated temperatures and yet retain the capability to prevent absorption of fruit stains at ambient temperature. Not all stain blockers are derived from arylsulphonic acids. Thus branched alkylsulphonic or cyclo-alkylsulphonic acids have been patented [31]. Carboxylated polymers derived from maleic or methacrylic acid have been developed for this purpose more recently. These are less durable to washing than conventional syntan stain blockers but do not discolour on prolonged exposure to light or oxides of nitrogen [35]. Stain blockers may be applied as a spin finish to nylon polymer as it is being melt extruded. Application is usually carried out in conjunction with a fluorocarbon finish in order to combine optimal stain resistance with dry soiling protection. The twist in nylon carpet yarn is typically steam set at 130 to 140°C or dry heat set at 195 to 205°C, which increases the adhesion of the stain blocker to the fibre surface. Alternatively, the stain blocker may be applied to nylon carpeting in a winch after dyeing. In this case the dye liquor is drained from the vessel and replaced with an aqueous solution of the stain blocker. The pH is adjusted to 4.5 by addition of acetic acid and treatment continued for about 30 minutes at the boil. In another commercially significant method the stain blocker is applied continuously to nylon carpeting using a specially designed applicator such as the Küster Flex-Nip or Otting Thermal Chem, followed by a dwell period with steaming. The application pH is normally maintained below 2.5, which improves exhaustion of the agent onto the fibre surface [36]. Addition of a divalent metal salt such as magnesium sulphate apparently enhances stain resistance by a mechanism that is not fully understood.
10.5 Fastness and Photodegradation of Nylon In spite of their inherently high tensile strength and remarkably good chemical resistance, unstabilised nylon fibres can suffer substantial losses in strength when exposed to sunlight for relatively short periods. The mechanism of
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photodegradation is still not fully understood owing to the extreme complexity of the reaction and the conditions of exposure. When nylon curtain materials are exposed to direct sunlight they may be in simultaneous contact with light, heat, air, water vapour, ozone and gas fumes. All these factors can catalyse or modify the fibre degradation reaction. The fibre itself is directly influenced by the presence of delustrants, antioxidants and, not least, by preparation, dyeing and finishing treatments to which it has been subjected. The influence of sunlight is dependent on the amount of incident light energy reaching the textile material, varying according to the weather and season of the year. In practice, nylon curtains and furnishings are normally exposed behind windows and the transmission characteristics of the glass have a marked influence on the incident light reaching the fibre surface. Relative humidity has a major effect on the rate of photodegradation, the loss in tensile strength of nylon increasing with the average humidity during exposure. Oxygen and ozone have a pronounced effect on the rate of photodegradation; strength loss is much slower if nylon is exposed in the absence of oxygen. Factors that enhance the resistance of nylon to photodegradation are listed in Table 10.3. The use of nylon upholstery fabrics in automobiles results in their prolonged exposure to sunlight behind glass at high temperatures and humidity. This has created a demand for dyeings of very high light fastness and for these dyed fabrics to show exceptional resistance to photodegradation. Although the selection of dyes of suitably high fastness under these extreme conditions is of primary importance, additional protection can be provided using ultraviolet absorbers, these being colourless aryl compounds with a high propensity to absorb the troublesome UV radiation. Such products are usually applied during dyeing and confer protection to both dyes and fibres. A useful definition states that a UV absorber is a molecule that may be incorporated within a host polymer to absorb ultraviolet radiation efficiently and convert the energy into relatively harmless thermal energy, without itself undergoing any reversible chemical change or inducing any chemical change in the host macromolecules [37]. Thus UV absorbers do not simply scavenge UV radiation preferentially and become themselves expended in the process, in the way that gas-fume fading inhibitors operate. The fundamental nuclei of most of these compounds are represented by ohydroxy-substituted derivatives of benzophenone (10.15), benzotriazole or benzotriazine. All of these structures exhibit keto-enol tautomerism, the keto form being favoured on irradiation. The characteristics of these compounds can be modified according to application; thus water-soluble sulphonated derivatives (10.16) are most suitable for use with anionic dyes on nylon. UV absorbers afford protection to nylon only at temperatures below 40 to 50°C. Since degradation is largely the result of oxidative free-radical attack, protective effects are shown by antioxidant addition, either alone or in conjunction with a UV absorber. One study of the influence of UV absorbers on light fastness involved a selection of nine acid dyes typical of those used in the dyeing of nylon carpets. The effectiveness of four water-soluble agents applied by exhaust dyeing, a product similar to compound 10.16 and three water-insoluble types applied from solution in tetrachloroethylene, was evaluated. Some of the UV absorbers significantly improved light fastness, but others gave significantly lower ratings. Overall, the behaviour of the UV absorbers was dye- and hue-specific [38]. Conversely, in
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another investigation it was claimed that stabilisers applied to nylon during dyeing can markedly improve fibre stability and light fastness, independently of hue and depth of shade [39]. Since the need is to provide additional protection for nylon against oxidation by free radicals, antioxidants of two types are available, namely, peroxide decomposers and radical scavengers. Peroxide decomposer types include salts or organo-metal complexes of manganese(II), copper(I) or copper(II). These products have a catalytic action and are therefore used in very small amounts. Conversely, radical scavengers have to be used in larger amounts because they lack the regeneration capability of catalytic types. The principal structural types are sterically hindered phenols or amines. The latter are more important because they act not only as scavengers but also as peroxide decomposers. Most radical scavengers are suitable only for thermal stabilisation, photostabilising scavengers being restricted to transition-metal chelates, especially nickel(II) dithiocarbamates and hindered amine derivatives [37]. Not all disperse dyes on nylon are resistant to oxides of nitrogen that may be present in the atmosphere. Sensitive dyes, usually those containing primary amino or secondary amino groups, may undergo marked changes in hue depending on the reactivity or basicity of the susceptible group. The primary general mechanism of fading is believed to be the formation of N-nitrosamines. The problem is best avoided by using dyes that do not fade, but this may not always be possible for economic reasons. Some protection can be obtained by treatment of susceptible dyeings with colourless agents that react preferentially with oxides of nitrogen. Since they act as scavengers of the acidic oxides of nitrogen, they need to be more basic in character than the dyes they protect. Stabilisers of the antioxidant and UV absorber types were incorporated into delustrant-free nylon 6.6 filament yarns at the melt extrusion stage. The objectives were to improve the thermal stability and fastness to gas-fume fading of the nylon. Both types of stabilising agent were effective in minimising yellowing of fibres, both in the presence and absence of oxides of nitrogen [40]. Atmospheric ozone has been reported as a significant cause of fading in certain dye-fibre systems [41,42]. Nylon, especially when dyed with various aminosubstituted anthraquinone blue acid dyes, is particularly susceptible to ozone fading [43,44]. Selection of ozone-resistant dyes is obviously the most satisfactory means of overcoming the problem, although hindered phenols and amines provide some protection. Selected reactive dyes are also claimed to significantly improve resistance to fading by ozone [44]. The effects of chlorinated swimming pool water and simulated sunlight on dyed nylon knitgoods was examined. Chlorination of the pool resulted in more serious changes in hue and losses in fabric strength than exposure in unchlorinated water. The extent of photodegradation of the nylon was greater than the influence of chlorine water on fabric strength [45]. The influence of oxidant concentration in swimming pool water disinfected by chlorination was investigated for knitted nylon and nylon blends, in order to define the relationship between hue changes and degree of chlorination [46].
References [1]
H Bode, Chem. Fibers Internat., 50 (2000) 129.
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[2]
P Driscoll, Chem. Fibers Internat., 48 (1998) 469.
[3]
W Bongard, Chem. Fibers Internat., 50 (2000) 254.
[4]
B de Soyres, Chem, Fibers Internat., 47 (1997) 195.
[5]
J Prins, Chem. Fibers Internat., 48 (1998) 291.
[6]
P Booms, Chem. Fibers Internat., 47 (1997) 121.
[7]
A Nikzad, J. Asia Text. Apparel, 11 (Jun/Jul 2000) 66.
[8]
Anon, Chem. Fibers Internat., 48 (1998) 117.
[9]
Anon, Wool Record, 152 (May 1994) 4.
[10]
Anon, Hosiery Research Bull., 5 No.1 (Jan 1961) 11.
[11]
J C Dupeuble, Chemiefasern, 40/92 (1990) 986.
[12]
K Parton, Dyer, 181 (Jun 1996) 14.
[13]
A Konopik and O Meister, Chem. Fibers Internat., 48 (1998) 207.
[14]
I Karacan, J. Asia Text. Apparel, 9 (Aug-Sep 1998) 77.
[15]
P Ginns and K Silkstone, Dyeing of synthetic-polymer and acetate fibres, Ed. D M Nunn
[16]
A D Broadbent and F Motamedian, Canadian Text. J., 112 (Sep-Oct 1995) 20.
(Bradford: SDC, 1979) 241. [17]
R McGregor, M S Arora and W J Jasper, Text. Research J., 67 (1997) 609.
[18]
M Bide, X Wang and C Yuan, AATCC Internat. Conf. & Exhib., (1997) 337.
[19]
F Little, Amer. Dyestuff Rep., 81 (Jun 1992) 18.
[20]
F Hoffmann, M Woydt, J H Heetjans and J Hennemann, Melliand Textilber, 81 (2000) 284;
[21]
R Garvanska, V Lekova and R Lasarova, Textil Praxis, 44 (1989) 1212.
Dyer, 185 (Jun 2000) 34. [22]
R Lasarova and R Garvanska, Textilveredlung, 22 (1987) 423.
[23]
V Lekova, R Garvanska and R Lasarova, Textilveredlung, 32 (1997) 210.
[24]
The systematics of dyeing synthetic fibres in exhaust dyeing. W Beckmann, Bayer Publication Sp 627 (1994)
[25]
Telon S Procedure. Bayer Publication Sp 502 (1977)
[26]
T M Baldwinson in Colorants and Auxiliaries, Vol. 2, Ed. J Shore, 2nd Edition (Bradford: SDC
[27]
K R Schneider, Dyer, 172 (Sep 1987) 13.
2002). [28]
L Jacques, JSDC, 109 (1993) 315.
[29]
J P O’Brien and A P Aneja, Rev. Prog. Coloration, 29 (1999) 1.
[30]
J Carr, Text. Horizons, 8 (Jun 1988) 43.
[31]
T F Cooke and H D Weigmann, Rev. Prog. Coloration, 20 (1990) 10.
[32]
J Shore, JSDC, 87 (1971) 37.
[33]
M Tomita and M Tokitaka, JSDC, 96 (1980) 297.
[34]
Y Elgarhy, Canadian Text. J., 114 (1997) 25.
[35]
R C Buck, Textilveredlung, 33 (Mar-Apr 1998) 57.
[36]
X X Huang, H D Weigmann and L Rebenfeld, Text. Asia, 24 (Dec 1993) 54.
[37]
J C V P Moura, A M F Oliviera-Campos and J Griffiths, Dyes and Pigments, 33 No. 3 (1997) 173.
[38]
W M Rich and P C Crews, Text. Research J., 63 (1993) 231.
[39]
G Reinert and F Thommen, Textilveredlung, 24 (1989) 182; Text. Chem. Colorist, 23 (Jan
[40]
B Küster, Textil Praxis, 46 (1991) 558.
1991) 31. [41]
V S Salvin and R A Walker, Text. Research J., 30 (1960) 381.
[42]
V S Salvin, Amer. Dyestuff Rep., 53 (Jan 1964) 12.
[43]
J C Haylock and J L Rush, Text. Research J., 46 (1976) 1; 48 (1978) 143.
[44]
J Bowles, A Puntener and J R Aspland, Text. Chem. Colorist, 26 (Mar 1994) 17.
[45]
H H Eppo and K S Grise, Amer. Dyestuff Rep., 81 (Jul 1992) 34.
[46]
K Kitamura and M Ichimura, J. Japan Res. Assoc. Text. End-uses, (1994) 363.
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Table 10.1 Advantages and disadvantages of steam and dry heat setting of nylon fabrics [15]
Characteristic
Setting in saturated steam
Dry heat setting
Degree of set
Equivalent set achieved at lower temperatures than in dry heat
Good set achieved with steam injection
Nylon stiffening and yellowing
Minimal
Slight stiffening and yellowing; minimised by steam injection
Economics of process
Relatively expensive batchwise process
High-speed continuous process
Effect on chemical and structural characteristics
Minimal effect on endgroups. Fibre physical structure is modified.
Decreased amino content but increase in acidic end-groups; minimised by steam injection
Influence on dyeing properties
Increased rate of dyeing. May slightly accentuate stripiness. Pre-setting slightly lowers wet fastness. Post-setting can markedly lower wet fastness by impairing the effectiveness of syntan aftertreatments.
Slightly decreased rate of dyeing. May increase physical and, to a lesser extent, chemical variations. Pre-setting slightly increases wet fastness. Post-setting can lower wet fastness and increase yellowing by degrading the syntan fixing agent.
Influence on fibre lubricants
Only slight fixation. May oxidise unsaturated lubricants and hence lower the dyeability with acid dyes
Tends to fix lubricants to some extent. Unsaturated lubricants and certain size polymers become difficult to remove
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Table 10.2 Advantages and disadvantages of pre-setting and post-setting of nylon fabrics [15] Setting process
Advantages
Disadvantages
Pre-setting of fabric in the grey state
Enables maximal yield to be attained at any given stability level. Reduced risk of creasing and shrinkage in subsequent processes. Fabric yellowing or stiffening is minimised during subsequent processing. Minimal effect on fastness properties. Significantly simplifies processing route. Minimal cost of process
Much increased risk of fabric stripiness. Slightly reduced rate of dye uptake. Set must withstand dyeing process and high temperatures are essential. Grey setting tends to fix spinning oils and knitting lubricants. Certain size polymers may become fixed. Volatilisation of lubricant residues may contaminate the stenter and subsequent batches.
Pre-setting after relaxation and scouring
Reduced processing problems. Enables yarn bulking to take place. Minimal yellowing of nylon and this is further reduced during wet processing. Ensures removal of soil and lubricant residues and thus avoids stenter contamination. Suitable for sized fabrics. Minimal effect on fastness properties.
Increased risk of fabric stripiness, although less than after grey setting. Slightly reduced rate of dye uptake. High setting temperatures can be applied. Increased processing costs as fabric must be dried prior to setting.
Post-setting after dyeing process
No problems of residual lubricants. Textured, figured and stretch fabrics can be treated satisfactorily. Weft-knit fabrics can be partially set to attain good stability and crease recovery without sacrificing stretch properties.
Higher temperatures must be applied to overcome hydrosetting effects. Setting may have an adverse effect on syntan fixing agent and thus lower fastness. Dyes of inherently higher fastness must be selected. The yellowing effect must be allowed for in colour matching. Slight fabric stiffening occurs.
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Table 10.3 Factors that enhance the resistance of nylon to photodegradation [15] Optimal purity of starting materials for polymerisation Minimal temperature of polymerisation, with exclusion of oxygen Inclusion of protective inorganic salts, particularly those of transition metals such as manganese (II) Inclusion of protective organic compounds as antioxidants Exclusion of fluorescent brighteners from the polymer Increased average molecular mass of the polymer Increased content of amino end-groups, as in deep-dye nylon variants Minimal temperature of the polymer melt, with exclusion of oxygen Minimal concentration of delustrant, using the optimal grade of titanium dioxide Selection of suitable spinning lubricants Draw ratio as high as possible to ensure maximum crystallinity, consistent with other commercial requirements Storage of grey fabric away from direct sunlight and away from heating systems Minimal temperature of heat setting in the grey state, consistent with the attainment of adequate set Treatment in a stenter designed for steam injection Alkaline rather than neutral or acidic conditions of scouring Oxidative bleaching should be avoided, but fluorescent brighteners can be applied under reducing conditions Dyes of high fastness to light (6-7) are preferred, with particular care in selecting suitable trichromatic combinations Certain metal-complex and milling acid dyes exert a protective effect on nylon, whereas others may catalyse photodegradation [15] Preferable to dye at or near pH 5, since more strongly acidic conditions can adversely affect antioxidants Inclusion of thiourea or hydroxylamine in the dyebath eliminates dissolved oxygen and minimises the risk of degradation in high-temperature dyeing.
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Figure 10.1 Influence of false-twist heater temperature on subsequent uptake of Cibalan Blue BRL (Ciba) by nylon 6.6 hosiery yarn [10] 13
Dye uptake
12
11
10
9
8 Untreated
130
170 Heater temperature
210
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H2N (CH2)6 NH2 + HOOC (CH2)4 10.1
COOH
10.2
– H2O
H [NH(CH2)6NHCO(CH2)4CO]n OH 10.3
N C
H H [NH(CH2)5CO]n OH O
10.5
10.4
2 NC (CH2)4 CN
6H2
H2N (CH2)6 NH2 10.1
+
10.6
H2N (CH2)5 CN 10.7
H2N (CH2)5 CN
H2O
10.7
(a) 2 HCN +
N C
H + NH3 O
10.4
H2C CH CH
CH2
Ni
10.8
(b) 2 H2C CH CN 10.9
H2
NC (CH2)4 CN 10.6
NC (CH2)4 CN 10.6
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HOOC CH CH CH CH COOH
2H2
HOOC (CH2)4 COOH
10.10
C O
O
H2O
10.2
HO CH2CH2CH2 COOH
10.11
H (OCH2CH2)n H (OCH2CH2)n
N CH2CH2 N
(CH2CH2O)n H (CH2CH2O)n H
10.12
H (OCH2CH2)n H (OCH2CH2)n
N CH2CH2 NH CH2CH2 N 10.13
O HO
S O 10.14
OH
(CH2CH2O)n H (CH2CH2O)n H
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HO
HO N C O 10.15
OH
N
H3C
CH2CH3
N SO3Na
10.16
Chapter 11 Acrylic Dyeing 11.1 Fibre Production and Properties Acrylonitrile (11.1) was first made in Germany in 1893 but remained virtually a laboratory curiosity until the late 1920s when IG Farben developed Buna N synthetic rubber. This copolymer of acrylonitrile and butadiene, a petroleum byproduct, was made by addition polymerisation. Acrylonitrile is manufactured by any one of four routes: 1. Ethylene chlorohydrin (11.2) and sodium cyanide are reacted together to produce ethylene cyanohydrin (11.3) which is dehydrated at 250 to 350°C in the presence of an alkaline catalyst, or at 350°C in the presence of alumina, to produce acrylonitrile. 2. Acrylonitrile is made directly from acetylene (11.4) by the addition of hydrogen cyanide. This synthesis was devised in 1948 by Otto Bayer and Peter Kurtz at Bayer Leverkusen. 3. Hydrogen cyanide is added to acetaldehyde (11.5) to form acetaldehyde cyanohydrin, which is dehydrated to acrylonitrile 4. Propylene (11.6) is oxidised to acrolein (11.7) which is then reacted with ammonia to form a propenolamine. This is dehydrated and dehydrogenated to acrylonitrile. Sohio developed this propylene ammoxidation route in the 1950s and it has been a major manufacturing method. In 1996 BP Chemicals announced the revolutionary propane ammoxidation process. This offers 20% lower production costs, valuable co-products and higher yields [1]. Acrylonitrile (11.1) readily undergoes free-radical addition in a suitable solvent medium to form polyacrylonitrile (11.8). Alternatively, an aqueous suspension of the monomer can be treated in the presence of catalyst and surfactants, when the water-insoluble polymer is formed and precipitated in the form of a slurry. This is filtered and the polymer is washed and dried. Suspension methods are widely used by Asahi, Bayer, DuPont, Monsanto and Montefibre, whereas solution polymerisation is preferred by Acordis, Toray and most Eastern European producers [1]. Polyacrylonitrile and its copolymers do not yield a stable molten phase, so filaments are extruded from solvent by dry or wet spinning processes. Dry spinning is carried out by dissolving the polymer in a suitable organic solvent, such as dimethylformamide, to form a 25 to 40 % solution. This is de-aerated, filtered, heated almost to boiling point and then extruded through spinnerets. The fine jets of solution emerge into a vertical tube or spinning cell, through which air or another gas at high temperature (about 400°C) is circulating. As the jets fall through the tube, the solvent evaporates to give filaments of polymer. The dry spinning technique was favoured by those companies in the USA and Germany who pioneered the production of acrylic fibres in the early 1950s, including DuPont, Bayer and Hoechst. In the wet spinning process, the polymer is dissolved in dimethylformamide or an alternative organic solvent and the solution is de-aerated and filtered. It is then pumped through spinnerets into a coagulating bath containing a liquid in which the solvent is soluble but the polymer is insoluble. The jets of polymer solution coagulate into fine filaments, forming a tow which is washed as it emerges from the bath. Monsanto introduced the wet spinning process with dimethylacetamide
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as the solvent. American Cyanamid and Dow Chemicals also adopted wet spinning for their fibre types, selecting aqueous sodium thiocyanate solution as solvent. Courtaulds and the Japanese producers of acrylic fibres have also given preference to wet spinning technology. The extruded polymer filaments are collected at the base of the spinning cell and hot stretched from 3 to 10 times their original length. The drawn filaments are collected and uncut tow is used for conversion. If staple fibre is required, the tow is crimped, heated to relax the filaments and then cut to the required staple length. Worldwide production data for acrylic fibres in 1998 are given in Table 11.1. The annual growth rate is forecast to be between 2.6 and 3.1% until 2005. World annual production is expected to reach approximately 3000 kilotons in 2005. No increase in production is anticipated for Western Europe. For the USA and Japan, production rate declines of 3.5% and 1.5% respectively are forecast. A production rate increase in this period will be sustained only by the Asian countries, excluding Japan [2]. Dry-spun fibres virtually always have a dog-bone or kidney-shaped cross-section and as a result give better cover, higher lustre, softer handle and better soilhiding properties than wet-spun fibres, which have a circular cross-section. Acrylic fibres are generally more expensive to produce than other synthetic fibres as a result of the high energy input required, although the cost of acrylonitrile itself is relatively low. Stringent operating standards for levels of residual acrylonitrile monomer are enforced in fibre manufacture to minimise hazards from occupational exposure. Acrylic fibres are unusual in their ability to attain a metastable state on hot stretching. When hot-stretched fibres are cooled, they will remain in their stretched state until subsequently heated, when they will revert to their unstretched state. High-shrinkage fibres may be made in this way, with shrinkages of 30% or higher. By blending these high-shrinkage fibres with normal staple, followed by steaming, high-bulk effects are obtained. The processing of high-bulk yarns is discussed in Chapter 13. Staple acrylic fibres produced from copolymers share some of the characteristics of wool. The handle of acrylic fibres becomes softer if finer counts are used and firmer with coarser counts. The high degree of bulk gives low density and makes it possible to produce lightweight clothing with good recovery capacity, heat retention and good covering power. As acrylic fibres do not swell at high humidity, garments made from them remain air-permeable even when damp. The macromolecules in an acrylic fibre are associated into well-ordered crystalline regions and less well-ordered amorphous regions. Even fibres spun from pure polyacrylonitrile do not show the same degree of order as that present in nylon or polyester fibres. Acrylic fibres do not develop much additional crystallinity or orientation on heat setting, and they are much more difficult to heat set. Thermal yellowing is a characteristic defect of polyacrylonitrile and its copolymers, caused by the formation of yellowish quinonoid chromogens by cyclisation reactions involving adjacent pairs of nitrile groups under oxidative conditions. The effects of six different types of lubricant on the thermoyellowing of acrylic fibres and their blends with wool were examined. No unacceptable yellowing was detected provided the lubricant concentration and drying temperatures recommended by the product manufacturers were strictly followed. However,
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problems occurred in radio-frequency drying when applying cationic lubricants; these may cause sparking during drying and this results in localised yellowing [3]. The term ‘microfibre’ is understood to apply to a fine filament with an individual count below 1 dtex. Using these fine-count fibres, lightweight synthetic textiles that are exceptionally soft and breathable can be provided to compete with classic knitwear. Acrylic fibre producers have quickly followed the trend established for polyester and nylon, extending their ranges to microfibres of 0.6 to 0.9 dtex. Details have been given of laboratory research on wet stretch and spinning conditions leading to successful industrial production by Montefibre of Myoliss (worsted-spun) and Leacril Micro (cotton-spun) yarns with counts in this range [4]. In bicomponent fibres, two filaments of differing chemical composition are fused during spinning, forming a composite filament in which each component behaves differently with respect to its swelling effect. This results in a three-dimensional crimping effect. This behaviour is similar to that shown by wool, in which the fibre crimp results from the bicomponent morphological structure of the cortex of the fibre. Bicomponent acrylic filaments are typically formed from homopolymeric and copolymeric variants. If the hydrophilic and mainly ionic group concentrations differ markedly in the two components, the crimping effect is reversible. With closely similar concentrations of hydrophilic groups, however, the crimp becomes irreversible or permanent. Fabrics knitted from these yarns are distinguished by high bulk, an attractive wool-like handle and a clear stitch structure [2]. In attempts to enhance the wearing comfort of textiles, fibremakers have attached particular importance to moisture-absorbent fibres. Such garments have to be able to absorb water vapour from humid air, at best by means of a system of pores in the fibre interior. It is important that this pore system is protected by a sheath of correctly balanced strength in order to ensure problem-free processing. The sheath must have numerous fine channels that convey the moisture into the porous fibre interior. Bayer researchers succeeded in developing a moisture-absorbent Dralon variant of low density and high absorbency, as well as rapid transport and evaporation of the absorbed moisture, but commercial interest in this variant has been unspectacular. More recently, copolymers of acrylonitrile and vinyl acetate or methyl acrylate were synthesised by solution polymerisation in the presence of sodium p-toluenesulphonate. Porous terpolymers formed by blending these copolymers with cellulose acetate were extruded to form wet-spun filaments that were hot stretched and dried to yield modacrylic fibres with high moisture retention and improved porosity [5]. When it became established that asbestos dust can cause cancer in humans, efforts were made to replace this material as a reinforcing fibre in cement constructions. Such fibres must have high tensile strength and elastic modulus, resistance to alkaline media, dispersibility in cement slurry and, not least, a low price. By increasing the average molecular mass of the polymer, combined with a high degree of hot drawing up to 20:1, such criteria could be met by acrylic homopolymer and copolymers [2]. High-tenacity terpolymeric fibres have been produced by the dry-jet-wet spinning technique after synthesis from acrylonitrile, methyl acrylate and either methacrylic or itaconic acid. The effects of extrusion conditions, draw ratio and heat treatment on the microstructure of the terpolymer were evaluated. Fibres obtained after plasticised stretching in glycerol and subsequent collapsing on heater plates showed enhanced strength and elongation at break [6].
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The precursor material designed for carbon fibre production must not melt during thermal decomposition, carbon loss should be small and the carbon skeleton formed by pyrolysis should recombine easily into graphitic structural units. Precursor yarns from homopolymeric and copolymeric acrylics have proved best for producing high-grade carbon fibres. Both wet- and dry-spun yarns, synthesised by free-radical or anionic polymerisation of acrylonitrile, usually with a small amount of vinyl bromide or acrylic, methacrylic or itaconic acid as comonomer, are used as starting materials [2]. Clean skin typically contains bacteria and fungi at the 100-1000 microbes/cm2 level. Under hot, moist conditions for 8 hours, one bacterium can generate 1.6 million offspring, resulting in odour and possible infection. Fungi multiply more slowly but are more difficult to remove by washing. Antimicrobial acrylic fibres have been under development since the mid-1970s. At that time, however, the active agents showed poor fastness to washing and dry cleaning. Improved formulations with much more durable antimicrobial efficiency are now available and these fibre variants are achieving greater commercial success in the production of socks, bath mats and blankets. Amicor (Courtaulds) antimicrobial acrylic fibres have been manufactured by the Courtelle wet-spun thiocyanate process, which is particularly suitable for producing functionalised variants. Amicor AB (antibacterial) fibre contains Irgasan DP (Ciba), a bacteriostat widely used in cosmetics and toiletries. Amicor AF (antifungal) fibre contains a mild antifungal agent developed to combat trichophyton (athlete’s foot). Amicor Plus is a fibre blend (AB/AF) giving dual activity [7]. Most antibacterial fibres contain organic active agents but these may have drawbacks in production or end-use. As alternatives, special zeolites containing Ag or Zn ions have been proposed but these have limited use in low dtex fibres with good abrasion resistance, especially if wet-spun. Montefibre has evaluated a titanium silicate as an alternative to the classic aluminium silicate zeolites. By increasing the percentage of Ag ions exchanged, the amount of additive required is reduced but effective antibacterial action is maintained [8].
11.2 Modification to Improve Dyeability Copolymeric acrylic fibres can be subdivided into two groups. Those containing at least 85% of acrylonitrile monomer units are described as acrylic fibres, whereas those fibres containing less than 85% but more than 35% of acrylonitrile units are classified as modacrylic fibres. This is a useful subdivision since it separates two groups of acrylic copolymers having little in common from a practical standpoint but allows within each group for great variations in the individual characteristics of fibre types. Thus the term ‘acrylic’ does not distinguish between fibres that are copolymers of acrylonitrile with a small proportion of a second monomer and fibres spun from a graft copolymer of acrylonitrile. Numerous acrylic and modacrylic fibres were developed during the 1950s; some of the most important early variants are listed in Table 11.2. Union Carbide introduced filament Vinyon N, a dry-spun acetone-soluble copolymer (60:40) of vinyl chloride and acrylonitrile in 1948. Three years later this was followed by a wet-spinning process using the same polymer to make Dynel staple, the most successful of the modacrylic variants. Dynel is delustered during dyeing at temperatures above 80°C but the lustre can be restored by subsequent treatment at a higher temperature, i.e. by drying at 120 to 130°C or by hydrosetting in salt solution. The controlled shrinkage of Dynel can be turned to practical use in the
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manufacture of bulked yarns and of backing yarns in knitted pile fabrics. Verel, a copolymer of acrylonitrile and vinylidene chloride, is a modacrylic fibre of high flame resistance and a soft handle. It is preferably dyed at 80 to 90°C (or at 70 to 80°C in the presence of an organophosphate carrier). If dyed at or near the boil, however, loss of lustre and deformation of the fibre can take place, resulting in fabric creasing. DuPont’s original Orlon 41 (staple) and 81 (filament) yarns consisted of homopolymeric material produced from 100% acrylonitrile monomer. This was exceptionally difficult to dye because of the high polarity of the macromolecules and the rigid helical conformation of the ordered regions. It was soon recognised, however, that improved solubility and more favourable spinning performance could be achieved if acrylonitrile is copolymerised with small amounts (5 to 10%) of other monomers to act as internal plasticisers in order to open up the physical structure. Acrylamide, methyl acrylate and vinyl acetate were typical comonomers selected for this purpose. Dyeability could be improved by incorporating anionic comonomers such as styrenesulphonic or allylsulphonic acid to provide dyeing sites for basic dyes or vinylpyridine to give sites for dyeing with anionic dyes. The introduction of these comonomers also enhances the dyeing rate at conventional dyeing temperatures by decreasing the glass-transition temperature. Acrylic fibres have outstanding resistance to sunlight, micro-organisms, insects and ageing as well as excellent outdoor weathering resistance. They have good resistance to all the chemicals likely to be encountered in textile wet processing, including bleaching and dry-cleaning, although strong alkali and concentrated mineral acids will attack the fibre. Acrylic copolymers have a low glass-transition temperature (Tg), or second order transition temperature) in the range 60 to 80°C. Dyes are not absorbed below this temperature, so that the starting temperature of the dyeing process can be selected to fit in with the Tg of the individual acrylic fibre. Above this temperature the fibre is in a thermoplastic form. Process liquors have to be cooled slowly to below this temperature, otherwise fibres and yarns develop deformations, so that fabrics and garments form creases which are not easily removed. The cuprous-ion dyeing technique was devised in the USA for Dynel in 1951. It depended on the ability of cuprous ions to coordinate with nitrile groups in the fibre. Dynel was treated at the boil with cupric sulphate and hydroxylamine sulphate to generate the cuprous ions in situ at pH 2 to 3. Monosulphonated levelling acid dyes showed high substantivity for cupronitrile groups in the treated fibre but unlevel dyeing was a serious problem. Certain reduction-sensitive azo dyes suffered partial decomposition in the presence of hydroxylamine. Accelerated yellowing of the fibre when heated in the presence of copper ions caused dulling of bright shades. The cuprous-ion dyeing method was used on the early acrylic fibres but was abandoned when the more dyeable variants became available. From a dyeing standpoint, the most important properties of acrylic fibres are their inertness to chemical attack, hydrophobic nature, tendency to soften above the glass-transition temperature and their excellent dyeability and fastness properties when dyed with cationic dyes. These outstanding fastness properties are believed to arise from the chemical inertness and hydrophobic nature of the fibre, since with minimal water absorption there are no active species inside the fibre to promote dye decomposition. This stability is assisted by high substantivity and the ionic nature of the dye-fibre bonds, which participate in the dissipation of
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energy. Moreover, no diffusion or desorption of dye is possible below the Tg (60 to 80°C). Many of the variables in acrylic fibre production influence the dyeability, colour yield and fastness. These include factors such as the comonomers used, the method of spinning (wet or dry) and the optical properties (dependent on the distribution of voids and particles of delustring pigment present). These factors also influence the lustre of bright, semi-dull and dull fibre variants.
11.3 End Uses and Stages for Dyeing Acrylic fibres have been used in numerous fabric constructions and end-uses. Yarns are spun using a variety of systems, worsted spinning often being used for hand- and machine-knitting yarns. Carpet yarns for both weaving and tufting are spun on the woollen, semi-worsted and worsted systems, usually with watersoluble lubricants. Open-end and truncated spinning methods, such as the Sirospun method, have been widely used with these fibres. Woollen carding and spinning machines must be free from conventional lubricants, since these can cause severe streaking problems due to soiling. Sliver knitted, flocked and nonwoven products have been based on these fibres. Acrylic fibres are mainly used in apparel, for garments with a woollen character such as sweaters, knitted jackets, jersey dresses, outerwear, scarves and thick socks. Acrylic fibres are much less suitable for woven fabrics owing to their tendency to form creases that are not readily removed by conventional heat treatments. Sales of hand-knitting yarns have declined in recent years. In household textiles, acrylic fibres are well represented in upholstered furniture, covers, blankets, quilts, synthetic furs and soft toy plush. Modacrylic fibres are important in blends for furnishings and floor coverings because they impart adequate flame resistance when present as 30% or more of the blend composition. However, the usage of acrylic fibres in the carpet sector has dropped sharply in recent years. Acrylic materials can be dyed at all stages of the manufacturing sequence, as discussed for wool in section 8.9. Acrylic fibres are dyed mainly in yarn form. Approximately 2 to 3% of total production is mass-coloured by dispersing organic or inorganic pigments in the spinning solution before extrusion. Titanium dioxide is added in this way as a delustrant. Another 10% of acrylic filament production is continuously dyed in gel form before the hot-stretching process [9]. Liquid formulations of basic dyes are applied from aqueous solution at 40 to 50°C, as described in more detail in section 12.3. Gel dyeing is particularly beneficial with wet-spun variants, since the highly swollen gel filaments produced in wet spinning make rapid dye uptake possible.
11.4 Dyes and Application Methods The improved copolymeric variants of polyacrylonitrile fibres that were introduced in the mid-1950s contained enough accessible dyeing sites to give full depths at the boil. Acrilan was designed to be readily dyeable with anionic dyes and the cuprous-ion method was used for fibres of the Orlon type. The situation changed, however, when it was found that the Astrazone basic dyes, originally developed by Bayer in 1938 for use on cellulose acetate, gave much higher light fastness ratings on acrylic fibres than had been anticipated. This led other dyemakers to
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extend their ranges of traditional basic dyes. Many of the novel basic dye structures designed for acrylic fibres contained a monoazo or anthraquinone chromogen with a quaternary ammonium or pyridinium group insulated from it, in contrast to the traditional types in which the cationic nitrogen atom formed part of the chromogen itself. Commercially, the basic-dyeable copolymeric variants have become the most important. These variants can be dyed with disperse dyes but build-up is poor and fastness properties not outstanding, so that their application is limited to the production of pale colours and the use of disperse fluorescent brighteners for whites. Basic dyes, on the other hand, build up to full depths, are tinctorially strong and give dyeings of high fastness to light and wet treatments. Dyers experienced great difficulty in the early days of dyeing acrylic fibres with basic dyes due to the high incidence of unlevel dyeing. Many were led to believe that an acrylic fibre, because of its wool-like appearance, was a form of synthetic wool and could be dyed accordingly. The rate of uptake of basic dyes by acrylic fibres, however, was found to be highly sensitive to dyeing temperature in the 90 to 100°C region. A slow rate of temperature rise together with a retarding agent was essential to control the dyeing process. Much pioneering work by fibremakers and dyemakers, notably Bayer, resulted in suitable dye application techniques being made available. Without such developments, the success of acrylics might have been short-lived as a result of dyeing difficulties. A pioneering discovery [10] was that the substantivity and diffusion behaviour of the modified basic dyes commercially available varied considerably. The dyes can be conveniently classified on a 1 to 5 scale, this value being designated as the compatibility value (CV) or compatibility constant (K) of the dye. Basic dyes are regarded as compatible when they have the same CV or K value. A test for the assessment of this factor [11] depends on the use of selected dyes with known CV or K values. Yellow and blue basic dyes with compatibility values ranging from 1 to 5 have been listed [12]. Dyes with the lowest CV or K values exhaust most rapidly, exhibit higher wet fastness properties and usually have a greater tinctorial strength. If the components of a combination dyeing exhaust at the same rate, the dyeing is seen to build up on tone, i.e. the depth increases with dyeing time without any perceptible change in hue. Retarding agents are required to control the rates of exhaustion of the dyes on to the fibre. Such agents have been discussed [13] and can be classified into two main types. Anionic retarders form labile complexes with the dye cations, whilst cationic retarders are preferentially absorbed by the fibre and liberated as dyeing proceeds with increasing dyebath temperature, becoming displaced by the larger dye cations with higher affinity. Cationic retarders are favoured in practice since the system is readily adjustable to allow for different fibre types. The CV or K value of the retarder should be only slightly lower than that of the dyes being applied. Hydrolysable cationic retarders are available that gradually lose their cationic character as dyeing proceeds. Cationic retarders can be categorised into four groups: 1. strongly cationic with a strong blocking action 2. moderately cationic with a weak blocking tendency 3. weakly cationic with no blocking effect 4. products which give little or no retarding effect but give some levelling.
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Products in groups 2 and 3 are preferred since they provide a wider safety margin but are likely to be more expensive than those in group 1. The rate of exhaustion of basic dyes can also be reduced by adding an electrolyte, such as sodium sulphate. Control of pH will also assist in level dyeing, since the degree of association between the cationic dyes and anionic groups in the fibre is reduced by lowering the dyebath pH. This decreased substantivity results in a slower rate of dyeing, thereby improving level dyeing. However, basic dyes are prone to decomposition outside the pH range 4 to 6 and this approach has to be applied with care. Control of the rate of temperature rise is another important aid to level dyeing, with rates of rise as low as 0.25 to 0.5°C per minute having been maintained. Two important parameters define the dyeing characteristics of each individual acrylic fibre type. These are the rate of dyeing (V) and the saturation value of the fibre (Sf). Methods have been given for determining these factors [14,15], and typical values for various acrylic and modacrylic fibres are listed in Table 11.3. Dyeing rates and saturation values on modacrylic fibres are generally lower than on many commercial acrylic fibres. These parameters can vary considerably between different variants of nominally the same fibre type [16]. It is therefore highly desirable to determine the correct values experimentally for the specific substrate under evaluation rather than relying on published data measured elsewhere. The dye saturation factor (f) is a parameter used to calculate the saturation concentration (CS) in % of a given dye or combination of dyes attainable on a given acrylic fibre variant of fibre saturation value (Sf). The calculation is as shown in Equation 11.1.
Saturation concentration =
S Fibre saturation value = Cs = f Dye saturation factor f
Equation 11.1
Having values for these parameters, together with those defining the properties of the dyes used, will allow the amount of retarder required to be calculated. As mentioned in Chapters 8 to 10, Bayer pioneered the development of systematic dyeing techniques for calculating the dyebath variables. The first in this series of techniques, and probably the most useful and widely used, was that for basic dyes on acrylic fibres [17,18]. In the calculation of the cationic retarder amount required the basic premise is that, for a given fibre, there is a total number of dyeing sites which must all be occupied by either dye or cationic retarder molecules to give level dyeing. The steps required in this calculation are as follows: 1. using the Sf and V values for the fibre to be dyed, the total concentration value is obtained from the table supplied by the dye manufacturer 2. the dye concentration values are calculated by multiplying the percentage of each of the component dyes (selected to have similar compatibility values) by its saturation factor f, as supplied by the dye manufacturer 3. the sum of the dye concentration values (see 2) is subtracted from the total concentration value (see 1) to give the percentage of cationic retarder required. Such calculation systems are provided by suppliers of basic dyes and are often available as standard programs that can be run from a lap-top computer. Rapid
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dyeing methods are available in which time savings are obtained, for example, by allowing for the Tg when selecting the starting temperature. Dyes with compatibility values close to 3 are widely used for dyeing acrylic fibres at all stages in the manufacturing sequence. A small range of six to ten dyes will yield a wide gamut of colours. These dyes can be used for gel dyeing and continuous dyeing, as well as exhaust dyeing. Dyes with compatibility values close to 1 are often used in loose stock dyeing, whilst those with compatibility values close to 5 are occasionally employed for the production of pale colours on fabric or garments. It may be difficult to obtain isomeric matches, however, when trying to formulate the same colours using various tertiary combinations of dyes with different compatibility values. Dyeing is usually commenced at about 80°C in the presence of a cationic retarder with acetic acid to give pH 4.5 to 5.0. The temperature is raised to 95 or 100°C at a rate of 0.5 to 1.0°C per minute and dyeing continued for up to 45 minutes. The dyebath is then cooled slowly (0.5 to 1.0°C per minute) to 75°C and the dyeing rinsed. A cationic softener may be added to the dyebath or applied as an aftertreatment in a fresh bath. Dyeing is carried out on appropriate machinery as described in Volume 3. For exhaust dyeing methods, these machines must have adequate control systems to ensure slow and even rates of temperature rise and fall to be applied, the cooling being by means of heat exchangers rather than overflow rinsing. The dyeing of fabrics on conventional winches is a recipe for disaster in terms of unlevel dyeing and creasing. Shallow-draught winches, as a minimum machine design, or preferably jet machines are used for fabric dyeing. Auxiliaries are usually necessary in the continuous dyeing of acrylic tow by the pad-steam process to enhance colour yield and the solubility of basic dyes in the pad liquor. It is important to ensure, however, that the retarding effect of solubilising agents is minimised as much as possible, whilst maintaining the optimum improvement in dye fixation. The selection of suitable basic dyes and auxiliaries for this process has been discussed in detail [9,19]. Following dyeing, loose stock or yarn is processed in a similar manner to other synthetic fibres. Fabrics are usually dried on the stenter, where a further alternative is to apply a soft finish by padding and drying. Any creases may be removed at this stage and mechanical finishing is usually restricted to raising, brushing and cropping. Some of the nitrile groups in the surface regions of an acrylic fibre polymer (11.9) can be hydrolysed to acrylamide (11.10) or ammonium acrylate (11.11) units. These reactions can be facilitated by treatment in an aqueous or methanolwater solution of caustic soda. Exhaust application is carried out at 80°C; padbatch or pad-steam processes are also available. Surface modification of acrylic fabrics in this way not only reduces the accumulation of static charges but also improves soil-release properties and dyeing behaviour [20].
References [1]
Anon, Chem. Fibers Internat., 47 (1997) 280.
[2]
B von Falkai, Chem. Fibers Internat., 50 (2000) 144.
[3]
I Müllejans, K Schäfer and H Höcker, Textilveredlung, 31 (1996) 100.
[4]
R Tedesco, L Console and M Cavallini, Man-made Fiber Year Book, (Sep 1997) 26.
Practical Dyeing, Volume 2
[5]
B Soni and V Gupta, AATCC Internat. Conf. and Exhib., (1994) 24.
[6]
P Bajaj, T V Sreekumar and K Sen, Chem. Fibers Internat., 48 (1998) 308.
[7]
D Service, Chem. Fibers Internat., 48 (1998) 486.
[8]
R Stevenato and R Tedesco, Chem. Fibers Internat., 48 (1998) 480.
[9]
G Puhlmann and A Keil, Textil Praxis, 45 (1990) 817.
[10]
W Beckmann, F Hoffmann and H G Otten, JSDC, 88 (1972) 354.
[11]
SDC Basic Dyes on Acrylic Fibres Committee, JSDC, 88 (1972) 220.
[12]
S Sostar and S Jeler, Textilveredlung, 26 (1991) 10.
[13]
H Kellett, JSDC, 84 (1968) 257.
[14]
SDC Basic Dyes on Acrylic Fibres Committee, JSDC, 89 (1973) 292.
[15]
Bayer Publication Sp 447 (1974).
[16]
W Beckmann, 11th Internat. Text. Seminar, Kingston Ontario (1968); Z.ges Textilind., 71
207
(1969) 603. [17]
The systematics of dyeing synthetic fibres in exhaust dyeing. W Beckmann, Bayer Publication
[18]
Bayer Publication Sp 433 (1974).
Sp 627 (1994). [19]
L Kostova, R Iltsheva and R Detsheva, Textilveredlung, 28 (1993) 398.
[20]
K Sen, P Bajaj and J S Rameshbapin, Melliand Textilber., 72 (1991) 1030.
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Table 11.1 Worldwide production of acrylic fibres in 1998 [2] Region Asia (including Turkey) Western Europe South America North America Eastern Europe Africa
Production (%) 54 26 9 6 4 1
World annual production 2501 kilotons in 1998
Table 11.2 Marketing of some important acrylic and modacrylic fibres Year marketed 1948 1950 1951 1953 1954 1956 1957 1958 1958 1958 1961 1962
Trade name Vinyon N Orlon Dynel Dralon Acrilan Verel Courtelle Creslan Darvan Zefran Beslon Teklan
Manufacturer Union Carbide DuPont Union Carbide Bayer Monsanto Eastman Courtaulds American Cyanamid Goodrich Dow Chemical Toray Courtaulds
Fibre type Modacrylic Acrylic Modacrylic Acrylic Acrylic Modacrylic Acrylic Acrylic Modacrylic Modacrylic Acrylic Modacrylic
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Table 11.3 Fibre saturation (Sf) and dyeing rate constants (V) of commercial acrylic and modacrylic fibres [16] Trade name Exlan Courtelle Cashmilon Toraylon Leacril Dolan Creslan Acrilan Beslon Acribel Modacrylic M Vonnel Orlon Dralon Teklan Crylor Kanekalon Dynel
Manufacturer Exlan Courtaulds Asahi Kasei Toray Montefibre Hoechst American Cyanamid Monsanto Toray Enka-Glanzstoff Monsanto Mitsubishi DuPont Bayer Courtaulds Crylor SA Kanegafuchi Union Carbide
Sf 2.3 2.5-3.5 2.2 2.5 1.7 3.0 2.0 1.2-1.5 3.0 3.4 2.6 1.4 2.5 2.3 0.9-4.6 2.3 1.2-2.0 0.9
V 4.6 1.8-4.2 3.6 3.5 3.0 1.6-3.0 2.5-2.8 1.4-2.7 2.6 2.5 2.5 2.3 2.0 1.7 1.0-1.5 1.3 0.6-1.0 0.7
Fibre type Acrylic Acrylic Acrylic Acrylic Acrylic Acrylic Acrylic Acrylic Acrylic Acrylic Modacrylic Acrylic Acrylic Acrylic Modacrylic Acrylic Modacrylic Modacrylic
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HOCH2CH2CN
HOCH2CH2Cl + NaCN 11.2
HC
350°C
H2C CH
11.3
CH + HCN
Catalyst
H2C CH
11.1
CN
11.1
11.4
O H3C C 11.5
– H2O
OH + HCN
H3C CH
H
H2C CH CH3
– H2O
H2C CH
CN
O2
11.6
O H2C CH C H
CN
11.1
NH3
OH H2C CH CH NH2
11.7
– H2O H2C CH CH NH – H2 H2C CH CN
2n H2C CH CN 11.1
CH2 CH CH2 CH CN 11.8
CN n
CN
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CN CH2 CH CH2 CH CN 11.9
H2O OH–
211
CONH2 CH2 CH CH2 CH CONH2 H2O
11.10
OH–
COO–NH4+ CH2 CH CH2 CH COO–NH4+ 11.11
Practical Dyeing Volume 3 - Dyeing Equipment and Textile Form By James Park and John Shore
2004 Society of Dyers and Colourists
Copyright © 2004 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.
ISBN 0 901956 84 8
Contents Volume 3 – Dyeing Equipment and Textile Form Chapter 12 Producer Coloration, Loose Fibre and Tow Dyeing
1
Chapter 13 Yarn and Narrow Fabric Dyeing
32
Chapter 14 Dyeing of Knitted Fabrics
68
Chapter 15 Preparation and Dyeing of Woven Fabrics
92
Chapter 16 Continuous Dyeing of Woven Fabrics
130
Chapter 17 Garment Dyeing
165
Chapter 18 Carpet Dyeing
182
Chapter 19 Closing Comments
199
Authors’ Preface The original idea of practical monographs was conceived in the 1970s as a result of an on-going debate as to what constituted a practical paper and the lack of such papers within the pages of the Journal of the Society of Dyers and Colourists. There is of course no absolute definition of a practical paper since this depends on the interests of the individual reader, location within the industry, topicality and, not least, the burning issues of the day. The Society of Dyers and Colourists attempted to rectify this lack of practical information by encouraging such papers for publication in the Journal, as well as initiating a series of practical monographs, authored by experts in various areas of textile coloration. Between the years 1981 and 1993, nine such monographs were published. Only two of these are still available: 1. Batchwise Dyeing of Woven Cellulosic Fabrics, by G W Madaras, G J Parish and J Shore (1993) 2. Instrumental Colour Formulation, by J Park (1993). For several reasons, not least the diminishing educational resources available for textile coloration, sources of practical, current information are increasingly required. This was the incentive behind the production of this practical e-book intended to assist practitioners occupying ‘hands-on’ positions at all levels within the industry. Copious recent references are included in each chapter. Two further e-books by the current authors will augment the information in this publication: 1. Dyeing Laboratory Practice, by J Park and J Shore (in preparation) 2. Dyehouse Management Practice, by J Park and J Shore (in preparation).
Chapter 12 Producer Coloration, Loose Fibre and Tow Dyeing 12.1 Dyeing Stages and Options for Fibre Dyeing For aesthetic reasons the coloration process forms an essential step in virtually all manufacturing routes for textile materials destined for use as apparel or household furnishings. The point at which coloration is applied depends on various economic and technical factors, including considerations of fashion trends and customer demand. Figure 12.1 specifies the significant manufacturing steps in which coloration can be achieved. The choice is wider for synthetic fibres than for natural fibres. The selection of a suitable coloration method (excluding yarn, fabric or garment dyeing) for the main fibre types is summarised in Table 12.1. Some of the important factors to be taken into consideration when deciding which coloration route to adopt for a given textile product are outlined in Table 12.2. Mass coloration fits neatly into the production of melt-spun synthetic fibres, involving the relatively simple incorporation of insoluble coloured pigment particles into the molten polymer before extrusion. This technique results in a product of exceptionally high all-round fastness suitable for numerous end-uses. Mass coloration does not allow for rapid reaction to changes in fashion, however, the time-scale from coloration to garment manufacture being several months. If the consumption data for individual colours in a typical shade range for a conventional end-use are analysed, it is usually found that 80 to 90% of the total consumption can be represented by only 8 to12% of the colours in the range, often including brown, deep red, green, navy and black. The remaining 10 to 20% of total volume comprises fashion-oriented shades, including pastel tones, that consume only small amounts of colour [2]. Production levels for mass-coloured polyester and nylon variants have been in slow decline for decades because of the low flexibility of colour selection but the process remains most important for polypropylene (Table 12.1), which cannot be dyed readily by conventional exhaust or continuous dyeing methods. Gel dyeing is the favoured route in the manufacture of producer-coloured acrylic fibres and this method represents the outstanding performer in terms of lowest dyeing cost per kg of fibre. It satisfies the traditional requirements of those major retailers operating with a limited range of standard large-volume shades supplemented by a fluctuating series of fashion colours in smaller volumes. The cost of gel dyeing is essentially a dye-only cost plus a small standard cost for in-plant charges. Conventional dyeing costs, incorporating contributions for labour, chemicals, water and energy, are virtually absent from the gel dyeing approach. Tow dyeing mainly provides worsted-spinning systems with coloured starting materials, whereas loose fibre dyeing processes yield those required for cotton, semi-worsted and woollen spinning systems. Dyers of tow and loose fibres therefore provide a service to yarn spinners and must offer dyed material designed to undergo yarn and fabric manufacture without causing stoppages of production. To achieve this, the dyer must be clearly aware of the fibre or tow finish to be applied to ensure that the dyed material performs satisfactorily. When drying, the dyer must conform to strict limits of moisture content. Considerations of colour fastness must include the effects of known subsequent textile process conditions that the dyed goods have to withstand. This places a responsibility on
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the dyer to select dyes of adequate fastness to all subsequent processing and ultimate end-use. The continuous dyeing of synthetic fibre tow provides a consistent quality for conversion into a combed top. Costings are highly competitive and costs of labour and energy are low. Equipment costs are high, however, so that effective utilisation is essential to achieve adequate return on investment. Where smaller amounts of dyed tow in each colour are required, batchwise methods of tow dyeing are available. The economics can be made competitive even allowing for a re-packing system after the dyeing process. Top dyeing remains the most expensive of the processing routes available but provides a high-quality product for sophisticated end-uses (Table 12.2). The needs of fancy yarn manufacture are best met by this dyeing method. Processing costs are high because of the care necessary to preserve the configuration of the top throughout dyeing, to ensure ease of subsequent processing of the dyed yarn. Batchwise and continuous methods are available for the dyeing of acrylic or polyester tow. Both fibres, and also wool, may be dyed in the form of tops or as loose stock by batchwise or continuous methods. Loose cotton, linen, viscose, silk or nylon fibres (Table 12.1) are normally batch dyed. Circulating-liquor machines are used for all of these, but the dyeing temperature varies according to the fibre and the class of dyes selected. High-temperature dyeing equipment is required for polyester or acrylic fibres but the other fibres can all be dyed at an atmospheric boil or at lower temperatures in most instances. Continuous dyeing systems for wool, acrylic or polyester materials in these forms entail pad application of the dye liquor, steam fixation, washing-off and reapplication of a finish to the dyed substrate. The variable factors include the choice of appropriate dyes and pad-liquor additives, fixation conditions, washingoff sequence and type of surface finish. An important advantage of continuous dyeing is the scope for dry-to-dry operation, with dry undyed fibre entering the range and dry dyed fibre emerging from the delivery end. Important criteria for successful continuous dyeing of these materials include: 1. Dyes should be compatible, with similar substantivity and diffusion properties, to avoid variations in colour during the run 2. The pad liquor must have optimum viscosity to ensure uniform application and to avoid ‘frostiness’ arising from dye migration 3. Pad-liquor additives should be compatible and must not give rise to unlevel dyeing by interaction with specific dyes 4. The fixation unit must be of adequate length to ensure satisfactory fixation of all dyes applied 5. The washing-off sequence and finish re-application must give satisfactory results with all dyes applied 6. The drying equipment should be capable of drying the dyed material adequately and preserving the physical quality of the loose fibre, tow or top.
12.2 Mass Coloration of Synthetic Fibres The addition of organic or inorganic pigments to the polymer mass before extrusion is a relatively simple and consistent method of colouring syntheticpolymer filaments. The product is characterised by exceptionally high colour fastness because the pigment particles become entrapped within the polymer
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matrix. Injection of a concentrated dispersion of the pigment into an ecru spinning line is an effective alternative to addition of particles directly into the polymer mass. Black filaments obtained using carbon black as the pigment offer exceptional fastness at low cost and provide a useful means of disposal of offstandard material from pale colours. The inert nature of carbon black ensures attractive contrast effects of high fastness when black-pigmented yarns are incorporated into woven or knitted designs with other brightly coloured yarns. There are limitations to the effects that can be achieved by this method of coloration. The pigmented filaments have a matt appearance, since the addition of an insoluble colorant suppresses lustre in a similar way to titanium dioxide particles incorporated into ecru filaments as delustrant. Outdoor textiles such as awnings, banners, tents, sunblinds and sailcloth, as well as artificial grass, are particularly appropriate for mass-coloured fibres because of the excellent fastness to weathering attainable. The main economic drawback of mass coloration is the high proportion of offshade material that inevitably results from a bulk-scale production run. The total melt-spinning system must be fed initially with the pigmented polymer, gradually flushing out all the residual ecru polymer still present. When the run of masscoloured polymer has been completed this procedure must be repeated, this time using ecru polymer to clean out residual coloured material prior to resuming ecru spinning. Both of these cleansing steps result in significant quantities of filament that is much weaker in colour than the target shade. This is only saleable at much reduced prices or may be convertible to black by recycling with carbon pigment. The reduction in revenue must be offset against the production of standard material, so that only production runs of substantial volume are economically viable. Criteria for the suitability of products developed for use in mass coloration include: 1. pigment particle size and distribution in the polymer mass and in the filament 2. solubility or dispersibility and compatibility of the pigment particles in the polymer mass 3. solubility or compatibility of dispersants and other additives in the polymer mass 4. stability of the colorant and additives under the conditions of preparation and extrusion 5. fastness properties of the colorant in the extruded filaments. The degree of dispersion of the pigment particles in the polymer mass and the extruded filaments must be sufficiently fine to avoid adverse influences on filterability, extrusion, stretching and further textile processing. The average pigment particle size must be no greater than 1 µm, although a few individual particles of 2 to 3 µm size can be tolerated. Polymer melts are pressed through a combined filter system made from fine-mesh steel sieves, the smallest mesh being about 25 µm diameter. The polymer solutions required in viscose or acrylic fibre manufacture are normally passed prior to extrusion through capillary filters made from modified cellulosic material. Mass coloration results in homogeneous colour yields with the pigment particles distributed evenly throughout the filament in terms of both penetration and uniformity along the length, provided that the melt-spinning conditions are kept constant. In contrast to conventional dyeing, structural variations in crystallinity
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and orientation within the extruded polymer do not have any significant influence on the uniformity of mass coloration. Pigment preparations available for this purpose are solid or liquid formulations containing the finely divided pigment, dispersing agents and stabilisers. These agents are adsorbed on the surface of the pigment particles, inhibiting them from forming larger agglomerated particles during storage and use. The fastness properties of mass-coloured fibres must comply with standard requirements relevant to their respective end-uses. In virtually all instances, the fastness ratings shown by mass-coloured materials are generally superior to those obtained on conventional dyeings. This trend is evident for fastness to light and weathering, to processing conditions (bleaching, cross-dyeing, heat setting…) and to end-use requirements (washing, perspiration, rubbing, ironing, dry cleaning…). The important advantage of superior fastness performance is achieved at no additional cost. For certain applications the outstanding ratings on the mass-coloured substrate cannot be achieved by dyeing techniques. Various methods are available for incorporation of the pigment preparation into the polymer mass, including: 1. Batch pigmentation of the polymer melt 2. Injection of a colour concentrate into the polymer mass 3. Homogeneous mixing of polymer chips with the colour concentrate in chip form prior to melting. Polypropylene is a special case, because uniform coloration in full depths can only be achieved by incorporation of coloured pigments into the polymer mass. Many attempts have been made to develop dyeing methods for homogeneous or modified ‘dyeable’ polypropylene variants but these techniques have suffered from serious economic and technical limitations: 1. Considerably higher costs for polymer modification and dyeing 2. Uneconomic build-up and unsatisfactory levelness in full depths 3. Inadequate fastness in relatively demanding end-uses. A schematic diagram outlining the major steps in the mass coloration of polypropylene is given in Figure 12.2. An important consideration for the application of organic pigments in the mass coloration of polyester fibres is the relatively high temperature of the polymer melt. This limits the selection of colorants to those with satisfactory stability under these severe conditions of melting and extrusion. Figure 12.3 illustrates the main process steps from polycondensation of the esterified intermediates, formation of polymer chips, injection of colour concentrate, quenching of the extruded filaments, to stretching and take-up of the coloured filaments. The design and operation of the compact spinning unit for the mass coloration of polyester have been described [3]. Chip dyeing is a special process applied to the mass coloration of nylon 6 fibres. In this technique the nylon 6 chips are pre-coloured using polymer-soluble dyes and then melt spun in the conventional manner. Clearly, the selected colorants must be capable of withstanding the conditions of melting and extrusion of the molten polymer. In its simplest form, nylon 6 chip dyeing is carried out in a heated circulating-liquor vessel and the coloured chips after drying are blended with ecru chips by melting just prior to the extrusion stage. Figure 12.4 shows these operations in a schematic diagram.
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The mass coloration of viscose fibres by addition of a pigment dispersion to the cellulose xanthate solution prior to extrusion and then coagulation by means of sulphuric acid treatment is no longer of much practical significance. The exceptionally high fastness levels attainable in this way are not matched by corresponding demands for viscose materials in long runs of standard colours in critical outdoor end-uses. Lyocell regenerated cellulosic fibres have substantially replaced viscose in fashionable apparel outlets for environmental reasons (section 7.1.3). The pigments selected for incorporation into cellulose xanthate solution had to be stable under the conditions of strongly alkaline solution and strongly acidic coagulation processes. The process sequence for mass coloration in viscose manufacture is indicated in Figure 12.5. The mass coloration of acrylic fibres is now essentially of historical interest only. Gel dyeing with basic dyes (Table 12.2) is equally suitable for achieving long runs of high fastness to each desired colour at low cost, with the exception of carbon pigmentation for black filaments. If solvent dyes or organic pigments are used in the wet-spinning process for acrylic fibre manufacture they must not bleed into the aqueous sodium thiocyanate or other solvent medium and must not interfere with regeneration of the acrylic filaments. Figure 12.6 shows a typical layout of a mass coloration unit for acrylic fibres.
12.3 Gel Dyeing of Acrylic Fibres The gel dyeing process involves passing an acrylic tow whilst in the gel state through a dyebath containing basic dyes that are substantive for acrylic fibres. These dyes are absorbed rapidly and efficiently because of the readily accessible structure of the polymer in the unstretched aqua-gel state prior to developing the higher crystallinity and orientation necessary to confer adequate strength for normal end-uses. Solutions of individual dyes can be fed into the dye applicator and the entire coloration process can be accurately controlled by instrumental means. Responses fed back from measuring heads located on the moving tow will activate metering pumps to increase or decrease the amount of each individual component dye as it responds to minor fluctuations of shade. In this way, relatively short runs dyed by the gel system become economically viable. A typical acrylic wet-spinning procedure is shown schematically in Figure 12.7. Polymer solution is pumped through spinnerets and the extruded filaments emerge into a bath containing water as a non-solvent for the acrylic polymer. During transit through this medium the filaments precipitate at a low temperature. Stretching in hot water and/or steam follows to orientate the structure and improve mechanical properties. Residual solvent is washed out of the filaments so that on-line dyeing can take place. The application conditions can be varied but the temperature is well below the second-order transition temperature, the dwell time minimal and the dye liquor is in constant circulation. Additives to the dye liquor are kept to a minimum, the pH of the solution being the main criterion for reproducible results. Application of a soft finish follows and finally the fibres are dried, crimped and packaged. When the spinning solution enters the precipitation bath the composition of the emerging filaments changes through diffusion processes, until the polymer is no longer soluble in the surrounding liquid medium. The porosity of the filaments formed on precipitation is determined by this limiting composition. The density of the filament material is greatest near to the exterior surface where the diffusion process is initiated. Precipitation is very rapid, being essentially complete within
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one second. Water enters the filament at first but this process is counteracted by sodium thiocyanate diffusing outwards. These factors together result in a contraction of volume in the later stages of precipitation [4]. The engineering features of colour application at the aqua-gel stage are complex but the relatively simple diagram in Figure 12.8 illustrates the essential principles. A water-coagulable solution of the acrylic copolymer, after filtration and deaeration, is passed under pressure along a supply line (1) by means of a pump (2). The solution is extruded through a series of spinnerets (3) into the coagulation bath (4). The angle of the spinnerets varies from horizontal to vertical, dependent on the take-up arrangement for the filaments (5). The temperature of the coagulation bath is maintained at a steady 10°C by means of a cooling jacket (6) around the vessel (7). The liquid in this bath must be in constant circulation so that fresh water is always available to achieve efficient and regular formation of the filaments. A guide roller (8) takes up the filaments from the spinneret face and they pass through the bath to a travelling wheel or godet (9), which conveys them into a hot washing bath (10), around another godet (11) and out again to a third one (12). Controlled tensions can be applied between godets (9), (11) and (12) to stretch the filaments before they pass through a delivery mechanism (13) into the dye application unit (14) maintained at ambient temperature. This unit contains a series of baths to carry out dyeing, washing and finishing operations as a continuous sequence. DyStar operates a laboratory-scale pilot plant at Leverkusen to simulate the spinning and gel dyeing of acrylic fibres. Apart from in-house use for the evaluation of solvents, Dralon polymer qualities, dyeing behaviour and other process parameters, this experimental equipment is also available for customer trials and process optimisation studies [6]. The operation and application of the CIR dispensing system in the Neochrome process for gel dyeing of Courtelle has been outlined. Facilities for rapid and efficient colour changes are described and it is claimed that unequalled control and consistency of shade reproduction are assured [7]. The basic dyes selected for gel dyeing must have good solubility at low liquor ratio as well as sufficient stability to avoid any possible slight changes in tone of the coloured tow when subjected to subsequent processing. Ratings for fastness to light and wet treatments on gel-dyed material are equal to control values on conventional dyeings. An important criterion of selection of dyes for gel dyeing is that satisfactory fixation must take place in the gel state during the few seconds of immersion at the application temperature. The availability of liquid brands is important in view of the large quantities of dyes to be handled and dispersed during continuous operation of this method of coloration. The porosity of the outer skin of the filaments and the degree of orientation of the structure in the aqua-gel state have important influences on dyeing behaviour. The dye cations must travel through the water-swollen pores in the relatively denser surface region. Water imbibition values give a direct measure of porosity and this factor decreases during steam stretching. The exposure to steam collapses the pore structure and this has a profound effect on the subsequent gel dyeing rate. Variations in the rate of heating-up of sections of a thick commercial tow during transit through the steam stretch tunnel can lead to differences in dye uptake. Such random fluctuations result in the formation of streakiness in the acrylic tow after gel dyeing [4].
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12.4 Batchwise Dyeing of Loose Fibres The dyeing of loose fibres, often called stock dyeing, is one of the oldest methods of coloration that was introduced many centuries ago. It involved manual stirring of textile material in a heated cauldron. Highly sophisticated machinery is available nowadays but in many ways the dyeing of loose stock is less critical than methods designed for yarn, fabric or garment dyeing. The shade can be corrected when the loose fibres are blended prior to yarn spinning, permitting an exceptionally high proportion of right-first-time dyeings and ensuring maximum utilisation of the dyeing machinery. A simulation model has been developed for the analysis of unlevelness in dyed loose stock. In pale depths acceptable colour tolerances can be achieved by blending but in dark shades the elimination of unlevelness is more difficult [8]. Preparation of loose man-made fibres before dyeing is seldom necessary. Suitable combined scour-dye processes are available to remove the minimal impurities present. Dyes used in stock dyeing are selected mainly from the viewpoint of economic advantage but the normal fastness levels to withstand subsequent wet processing and to meet the requirements of the relevant end-use must be satisfied. When densely packed bales of loose fibre are opened up the material expands in volume. It is desirable to open up residual denser regions whilst loading into the dyeing container in more uniform layers. The pack should be wetted down as each layer is added and then pressed down to give the required density. Some types of fine-staple fibres are liable to consolidate during dyeing. In these circumstances it may be necessary to open up the machine after processing for 10 to 15 minutes at 70°C (if pre-scouring) or at 80 to 90°C (if circulating in a blank bath prior to scour-dyeing), so that further material may be added on top of the consolidated pack to prevent channelling. Short-staple spinning processes are particularly sensitive to the state of surface cleanliness of the dyed fibres. High-temperature dyeing processes for polyester fibres should be kept as brief as possible to avoid the build-up of surface deposits of oligomer or disperse dyes. It is advisable to discharge the dyebath without cooling and to reduction clear all dyeings, irrespective of applied depth. The ‘dressing’ or spin finish applied after dyeing is important and must provide the spinner with a starting material that will process satisfactorily. Short-staple fibres should be re-dressed with fibre processing aids whilst still in the wet state after dyeing. The accuracy with which this process is carried out is of critical importance for the spinning system. The spin finish confers the desired handle and builds antistatic properties into the dyed fibres. Application is usually from the exhaust dyebath on the cooling cycle to minimise the process time. Concentrations are largely determined by the demands of the spinner. The dyed and finished fibres must have the correct amount of moisture and a suitable handle, with sufficient lubricant and antistat to ensure efficient and consistent processing. Dyeing cycles are reduced to a minimum duration and the dyed fibres are often merely hydro-extracted prior to delivery to the spinning mill. After hydro-extraction, staple fibres are dried sufficiently by the blowing action of the fibre-blending process, which should be applied prior to re-baling. The batchwise dyeing of loose stock is essentially a system of package dyeing under circulating-liquor conditions. A high degree of reproducibility is attainable by automation and the general approach is to dye most of an order on a rightfirst-time basis and then to bring the total consignment on shade by means of
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one or more ‘correction’ dyelots. Colour physics has been instrumental in providing a sophisticated tool to achieve this routinely. Much automated handling is possible in fibre dyeing and little manual handling is necessary between presspacking and the emergence of dry, dyed fibre. The layout of a typical stock dyeing plant is shown in Figure 12.9. A dyepack of uniform density throughout is essential for a level dyeing to be obtained; this is mainly attributed to the elimination of liquor channelling. The fibre is thus press-packed into the dyeing cages, usually by wet-stamping techniques. The wet-stamping device can be fed by a brattice arrangement to assist handling of the fibre. Pack density for dyeing depends on fibre type and the flow characteristics of the machine. Values as low as 250 g/l may be necessary for coarser qualities of natural fibres but pack density can be as high as 500 g/l for acrylic fibres, which tend to deform and consolidate during dyeing. Pack dyeing is carried out in a cylindrical vessel capable of being pressurised if required. The design permits the circulation of hot dye liquor through the central zone at the base of the vessel, through the mass of the fibres and back to the pump. The vessel is fitted with an annular stainless-steel basket or cage with perforated sides and the dye liquor is forced through the packed substrate horizontally. A typical cage may be about 150 cm in overall diameter with a race (annular compartment) about 40 cm wide and about 80 cm deep. The bottom of the cage is secured firmly to the bottom of the vessel and a top plate fitted to prevent dye liquor flowing over the top of the substrate. The cage is lifted into and out of the vessel by overhead crane. Sampling and colour additions do not present problems, since a high-pressure injection pump is used for additions and a pressure-lock sampling device enables samples to be taken. A disadvantage of the pack system of loose wool dyeing is that excessive fibre damage during treatment may result in lower productivity, less effective operation of the equipment and impaired product performance. Fibre damage is attributable to the chemical conditions and mechanical forces operating during dyeing. Excessive packing density may contribute significantly, especially for fibres in a bent or folded configuration. Investigations by Wronz and IWS confirmed that dye liquor flow pressures in loose stock dyeing were often too high, particularly in conical-pan or radial-flow machines. The Wronz Soft-Flo system [9] was developed to regulate the flow of the liquor in the loose stock dyeing of wool. This microprocessor control system monitors the flow rate so that high flow rates are used only in the critical stages of the dyeing process. When the dyeing reaches a predetermined set point (often 60°C), the dyeing process continues at a constant minimum flow pressure throughout the rest of the cycle. The flow is sufficient to give level dyeing but with reduced mechanical damage to the wool. Bale dyeing offers an alternative to pack dyeing in appropriate circumstances. Bales of loose fibre obtained from the supplier are placed directly into a specially constructed cage that is designed to hold four bales. The loaded cage is lowered into the conventional dyeing vessel and dyeing carried out according to the usual procedure. After dyeing the bales are either hydro-extracted or vacuum extracted to remove excess moisture. This system is ideal for adopting in a newly established plant that processes a limited range of fibre qualities obtained from a small number of regular suppliers. The advantages are almost all economic, since the stages of bale opening, loading of the cage with loose fibre and press-packing to achieve uniform density are eliminated, with obvious cost savings. It is claimed that there is minimal
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disturbance of the fibre, leading to less risk of channelling causing unlevel dyeing. Disadvantages for an existing dyehouse that may consider switching to bale dyeing are the need to invest in specially designed cages, the existence of markedly different bale sizes produced by various suppliers and the problems that some dyeing vessels will not accommodate unusually large bales. Moreover, techniques of drying or semi-drying in bale form are inadequate for certain qualities of man-made fibres, where careful control of drying is essential. 12.4.1 Preparation of Loose Cotton Traditionally, loose cotton stock is bleached at atmospheric pressure in batch form on pack machines or in autoclaves. Bales of grey cotton fibre are mechanically opened up, an essential cleaning operation, and packed into cakes. This packing is consolidated using a wet-stamping machine and the fibres are formed into cakes on stainless-steel rings or packed into a perforated steel basket. Packing densities of 250 to 300 g/l are typical and limited by the strength of the container. Short-staple cotton waste from ginning, carding or combing can be processed in this way. The traditional scouring and peroxide bleaching of loose cotton has been reviewed recently. Although whitening of the cotton cellulose is easily accomplished without commercially significant strength losses, the removal of ligneous impurities from residual cotton plant debris is more difficult to achieve. Test methods for evaluation of key parameters of the bleached fibres, including whiteness, chemical modification and absorbency, were assessed [10]. A continuous processing range for loose cotton bleaching was introduced by Cotton Incorporated in the USA during the 1970s. The fibres are formed into a web or sheet (described as a ‘batt’) and this material is then processed with minimum tension, as if it were a light and delicate fabric [11]. 12.4.2 Preparation of Loose Wool Wool fibres can contain between 25 and 70% of non-fibrous impurities (see Table 8.1), depending on the source and quality of the raw material, and these must be removed before dyeing. Vegetable matter is removed by carbonising, whilst grease and suint are removed by scouring. The practical process of carbonising consists essentially of the following stages: 1. impregnation with 5% sulphuric acid (96%) at 10 to 30°C 2. drying at 100 to105°C, followed by baking at 125 to 150°C for one minute to char the cellulosic impurities 3. crushing through rollers to remove the charred debris 4. immediate neutralisation of the residual acid and rinsing. Scouring is usually carried out in a continuous scouring range using alkali (builder) and a detergent. The range consists of up to five or six tanks (called ‘bowls’), each with a capacity of 104 litres, and with a pair of squeeze rollers (or ‘nips’) between each of the bowls. The wool is fed into the first bowl by a brattice feed and is moved through each bowl by means of harrows or, for coarser qualities, by swinging rakes. A perforated tray forming the base of the bowl allows dirt and sand to fall through. One bowl of a typical scouring range is illustrated in Figure 12.10 [1]. The conditions of application in a six-bowl machine operating at 40 to 55°C are detailed in Table 12.3.
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The important factors in the efficient removal of contaminants from the wool fibres are: 1. opening before scouring 2. number of bowls 3. amounts and type of detergents and builders used 4. water quality 5. time of immersion in the liquor 6. temperature 7. amount of mechanical action 8. efficiency of the squeeze rollers. If the suint content is high, this is removed in the first bowl at 30°C. Soda ash and detergents based on either nonyl- or octyl-phenol ethoxylates or fatty alcohol ethoxylates are normally used. Soft water gives optimum scouring conditions but sequestering agents must be added if the total hardness exceeds 70 mg/l calcium carbonate. The temperature of the scouring liquors should be above the melting point of wool grease (37 to 38°C) and is usually in the range 50 to 65°C. The cloud point of the detergent is also typically 50 to 60°C. Final drying is carried out in the temperature range of 80 to 100°C. Modern installations will have control equipment, including the following: 1. weighbelt feeding to give a constant flow of wool through scouring 2. microprocessor control of bowl liquor levels and temperature, up to full computer control 3. controlled detergent and builder additions 4. addition of lubricants and antistatic aids after scouring. Machinery developments have included the increased use of stainless steel, widewidth bowl designs up to three metres and the well-known Fleissner suction-drum principle for both scouring bowls and dryers. Mini-bowl technology, first developed by Wronz [12] but now available from a number of manufacturers, was based on the use of bowls of two-metre diameter and two-metre width, resulting in savings of water, energy and space. The incorporation of a steep hopperbottomed design for each bowl is claimed to give easier and more uniform removal of dirt from the bottom of the bowl. Scouring, carbonising, neutralising, rinsing and drying can be a continuous in-line series of processes. Bleaching of wool with hydrogen peroxide can be carried out by either a batchwise process or at the end of a continuous range followed by rinsing and drying. In a recent investigation, merino wool top was peroxide bleached in alkaline or acidic media. For the same concentration of peroxide, bleaching in an alkaline medium leads to a much whiter wool than acidic bleaching but causes more intense chemical attack. For the same degree of chemical attack, however, wool bleached in alkaline peroxide is markedly whiter than acid-bleached wool [13]. The pigment present as the natural colour of animal hair and fur can be removed by means of a ‘mordant’ bleach in which a metal salt, usually a compound of iron or copper, is employed as a catalyst to enhance the bleaching with hydrogen peroxide in a subsequent bath [14]. Following treatment with the metallic salt, bleaching with between 25 and 50 g/l hydrogen peroxide (35%) is necessary, at temperatures of 50 to 70°C for times of up to three hours, depending on the fibre
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type. In view of the value and rarity of such fibres, treatments are carried out by batchwise methods. The Wooltech solvent cleansing process for wool tops has been introduced recently. Conventional processes are hazardous for the environment (section 2.11) but this novel system yields wool of high quality and is cost-effective and environmentally innocuous. The solvent Triwool is non-flammable, does not deplete the ozone layer nor exacerbate the greenhouse effect and is not a known carcinogen. The strength, elasticity, softness and machine washability of the scoured wool are enhanced [15]. Shrink-resist treatments to produce machine-washable wool are applied to loose stock or tops, using a continuous sequence involving chlorination and resin treatment. A modified suction-drum back-washing range is often used for this continuous process. In each bowl, the web of slivers passes around a perforated drum submerged in the treatment liquor. The liquor is circulated through the wool and the drum and then returned to the bowl by an impeller positioned inside the drum. This continual circulation ensures that the liquor penetrates the centre of each sliver so that all fibres are evenly treated. A typical shrink-resist process is shown schematically in Figure 12.11 and details of the application conditions in this five-bowl sequence are given in Table 12.4. Softlaine, developed jointly by IWS and Stephenson Thompson Textile Chemicals, is a new system of shrinkresist and soft lustre treatment of wool tops. It does not affect natural regain or the rubbing fastness of dyed tops [16] The Woolmark Company and Fleissner have jointly developed the System-2 shrink-resist processing line for wool tops. This process yields an easy-care wool top with savings in water consumption, higher productivity, increased running speed, uniform reproducibility and stability of process compared with conventional processing lines [17]. This plant will produce 500 kg/hour and a two-bowl chlorination unit improves uniformity of application. A six-bowl backwasher and a six-drum dryer are used and there is an automatic chemical metering system. Improved suction-drum machinery is employed. An innovative machine developed recently by IWS and CSIRO is designed to stretch wool fibres, coupled with chemical and physical treatments, to yield a combed sliver of Optim fibres. In the Optim-fine process a typical 19-micron wool is converted to 16 microns diameter. These fibres are used in trans-seasonal lightweight fabrics with soft drape and handle, a subtle lustre and silk-like touch. Opti-max is a retractable fibre that can be made to contract in length by 20 to 25%. Blending with untreated wool yields yarns with 20% more bulk for lightweight bulky knitwear [18]. 12.4.3 Processing Aids for Loose Fibre and Tow The processing aids applied to man-made fibres by the producer are mainly removed during bleaching or dyeing. In the case of natural fibres, agents must be applied to the scoured and bleached or dyed stock to ensure satisfactory performance in subsequent processing. The dyed fibre or tow must run efficiently with minimum stoppages in conversion or spinning. The process sequence of conversion, carding, drawing, spinning, yarn twisting and winding must result in a yarn with the required physical characteristics. The processing aids should optimise the frictional properties and dissipate the electrostatic charges encountered during this sequence from fibre or tow through to yarn.
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The control of friction between fibres, and between fibres and metal surfaces, is one of the most important aspects of the performance of a fibre lubricant. From the initial extrusion via the spinneret through to the fibre staple or filament as used in yarn or fabric form, the control of friction is of the utmost importance. There are two fundamentally important fibre frictional properties: friction between fibres and other surfaces (particularly stainless steel) and fibre-to-fibre friction. Friction between Fibres and Other Surfaces (Particularly Stainless Steel) In virtually all stages of staple or filament processing it is of prime importance to achieve a low coefficient of friction between the fibre and machine parts. This results in: 1. minimal breakages, abrasion and damage to fibres, filaments or tapes 2. maintenance of high productivity with as few machine stoppages as possible 3. preservation of fibre length 4. reduction in fly and short fibres during processing 5. reduction in static build-up 6. reduction in wear on machine parts and guides.
Fibre-to-Fibre Friction Not all fibre lubricants lower the inter-fibre frictional forces adequately. Generally it is important that a level of fibre-to-fibre friction and cohesion is achieved by a processing lubricant commensurate with the requirements of the particular fibre quality and processing machinery in question. Optimisation of the correct fibrefibre, yarn-yarn or tape-tape friction results in: 1. adequate lubricity and fibre mobility to facilitate opening prior to blending or carding 2. adequate fibre-to-fibre friction and cohesion to provide good fibre assemblies, including webs, slivers, tops and rovings 3. control of the correct friction level after stock dyeing commensurate with subsequent processing 4. assurance of package-build stability and appearance in yarn and tape processing. The adverse effects produced by static charges in textile processing can interfere with production efficiency, the quality of slivers and tops during processing and the end-use performance of the finished goods. The troublesome retention of static charge is caused by the inadequate conductivity of the textile material. This retained charge can be reduced or eliminated by increasing the conductivity of the fibres. Antistatic agents are designed to promote moisture absorption, thus reducing the initial charge formation and accelerating the dissipation of any charge generated. Both ionic and nonionic agents are available. In many instances the nonionic agents containing polyoxyethylene groupings ensure efficient moisture retention on the fibre surface to ensure dissipation of static charge. Under more extreme conditions, at low humidity and with materials of very high resistivity such as polyester, ionic systems are preferred. The chemical types and application properties of such agents are outlined in Table 12.5.
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Various commercial derivatives of proteins (hydrolysates or condensates with fatty acids or fatty amines) were evaluated as wool protective agents by application during bleaching or during dyeing with 1:1 metal-complex or reactive dyes. By determination of the sliver cohesive forces, a lower value was found for sliver treated with protein-based agents than for wool treated with conventional surfactants. Protein derivatives labelled with fluorescein-5-isothionate were found by fluorescence microscopy to be distributed evenly over the wool fibre crosssection after application by either bleaching or dyeing [19]. 12.4.4 Dye Selection and Dyeing of Loose Stock or Sliver Cotton is seldom dyed in the form of loose stock. However, if this procedure is used, dyeing is always carried out by the pack system. The fibrous material is opened up, sprayed with hot water and loaded into the dyeing vessel, in which the fibres must be packed very tightly and uniformly to prevent channelling dyeing dyeing. The loose stock is not usually pretreated prior to vat or sulphur dyeing, which is mostly carried out by the leuco process at an appropriate temperature. Cotton can be vat-dyed in the form of sliver when a large quantity of yarn is required with optimum levelness of colour and a high bulk, or when a fully penetrated dyeing is required on a high-quality yarn. Sliver can be dyed: 1. wound on beams 2. like loose stock, by the pack system 3. like wool tops, in the form of wound packages. Sliver is usually vat-dyed by the leuco process or by the pigmentation method with simultaneous bleaching. In view of the natural origin of wool, controlled pre-scouring is essential to ensure level dyeing and a clean product in the designated end-use. Stock dyeing, in circulating-liquor machines or by continuous methods, is one of the most important methods of wool dyeing (section 8.9). For relatively clean material, a pre-scour can be given in the dyeing machine with soda ash and a detergent for 15 minutes at 70°C. Selection of the class of dyes for wool is determined mainly by end-use requirements and fastness to the conditions of wool finishing (section 8.10). Chrome, 1:2 metal-complex, milling acid and reactive dyes are generally used. The application of chrome dyes is restricted mainly to low-cost, dull hues of high fastness in full depths. They are applied by low-chrome methods to ensure acceptable levels of residual chrome in the effluent. Reactive dyes are especially suitable for bright colours on machine-washable wool to be given a shrink-resist finish. Many reactive, milling acid and 1:2 metal-complex dyes of high fastness are sensitive to tippiness and other dyeability variations in wool but adequate coverage can be achieved to ensure satisfactory levelness after blending. In the stock dyeing of nylon, milling acid dyes are usually selected for bright shades and 1:2 metal-complex dyes for full depths and mode shades. Dyes of high intrinsic wet fastness are preferred to minimise the need for syntan aftertreatments. If both wool and nylon fibres are dyed in the same dyehouse, many of the same dyes can be applied to both fibre types and this limits the number of dyes that have to be stocked to achieve a complete gamut.
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The pack dyeing of loose polyester fibres can be carried out successfully at 135 to 140°C to achieve full depths and satisfactory penetration and levelness, providing the time at top temperature is not excessive. It is important to avoid the build-up of surface deposits of residual oligomer or disperse dyes. The loading of stock dyeing machines requires similar precautions as for other fibres and a compact pack must be layered down to avoid channelling and consequent unlevel dyeing. Cooling must be carefully controlled but is less critical than for acrylic fibres. Reduction clearing of all dyeings is advisable to ensure satisfactory yarn manufacture. Acrylic fibres can be dyed with basic dyes and, in pale depths, with disperse dyes of adequate fastness for economic reasons. Dye selection depends on end-use requirements and suitability for either batchwise or continuous application. Compatibility in terms of dyeing rate is an important criterion because the levelling of basic dyes demands careful control in bulk-scale operation. Methods of calculation are available to determine the optimum amount of cationic retarder required to yield satisfactory levelling properties according to the applied depths of the three component dyes in any combination (section 11.4). The higher the proportion of dyeing sites saturated by the dyes and retarder, the greater the probability of level dyeing. For shading it is advisable to cool the dyebath from the dyeing temperature to the initial temperature before making an addition.
12.5 Continuous Dyeing of Loose Fibres The essential stages of a continuous stock dyeing process are padding, fixation, removal of unfixed dye and application of spin finish. The principle of the system is illustrated in Figure 12.12 which shows the Smith Petrie pad-steam range. The loose fibres are well opened out and fed onto an endless rubber belt conveyor that runs around the lower bowl of the two-bowl vertical pad mangle. The sheet of fibres is sprayed with dye liquor before passing through the nip, the upper bowl being made from soft rubber. After padding, the saturated fibres are conveyed to the entry aperture of the piston steamer. As the material falls into the chamber, a horizontal piston fitted alongside presses the loose stock into a plug and pushes it forward, step by step, through the pre-drying and steam fixation zones of the double-walled horizontal cylinder. Steaming time is within the range 30 to 45 minutes, depending on applied depth. After leaving the steamer, the dyed fibres are washed to remove unfixed dyes, residual chemicals, migration inhibitor, antifrosting agent and levelling agent. Spinning assistant, antistat and antisoil agent can be applied by padding immediately before final drying. In processing ranges supplied by Fleissner, widespread use is made of the perforated suction-drum principle for fixation, washing and drying, as illustrated in Figure 12.13. The Fastran radio-frequency fixation system (Figure 12.14), developed by Dawson International and Smith Textile Engineering Machinery, exemplifies developments in this area. This range offers optimum fibre condition after dyeing, substantial energy savings, quicker process times, reduced water consumption, lower manpower requirements and minimum space occupancy. The principle of the Smith piston steamer is combined with a Smith-Fastran EDF (electronic dye fixation) unit to give a continuous dyeing system suitable for wool, nylon or acrylic fibres. The system ensures uniform dye penetration throughout the mass of fibres with good fixation in minimum time. A further benefit is the good
Practical Dyeing, Volume 3 condition of dyed manufacture.
fibre,
15 which
improves
efficiency
in
subsequent
yarn
The undyed fibres are fed onto an endless rubber belt conveyor running beneath a metal detector and then sprayed with dye liquor, before passing through a nip to give uniform impregnation. The saturated fibres are fed via a hopper into a horizontal cylindrical chamber in which a piston pushes the plug of fibres into a tubular pressure section surrounded by the radio-frequency (dielectric) field. Heating of the saturated fibres under the influence of radio-frequency energy takes place quickly and uniformly throughout the material. When fixation is complete the dyed fibre is automatically conveyed to a conventional washing-off sequence. RF can be used for final drying. The Lanapad (IWS) system combines the advantages of the IWS pad-store method of dyeing wool sliver with the energy-efficient radio-frequency technique of fixation. Full depths can be achieved economically using sulphonated 1:2 metal-complex dyes. The same equipment can be used for economical wool bleaching and simultaneous pastel dyeing and bleaching processes. Advantages of the Lanapad system include minimal fibre damage, improved spinning performance, reduced labour requirement and lower consumption of energy and water. Wool sliver is fed from a conventional creel through a horizontal pad mangle 50 cm wide and cuttled into specially constructed insulated containers. The pad liquor contains dye, urea, thickener and wetting agent. The containers are passed into radio-frequency heating units, in which charged upper and lower electrode plates raise the temperature of the impregnated wool uniformly to 60°C. After heating, the insulated containers are sealed to maintain this temperature during storage overnight. The process is completed by washing-off in a conventional back-washer.
12.6 Tow, Top and Sliver Dyeing The dyeing of tops is a necessary service to the manufacturers of multicoloured fancy yarns. The top, whether made from natural fibres or converted man-made fibre tow, must be kept in good condition throughout wet processing prior to its conversion first into sliver and finally into singles yarn. Coloration at the tow or top stage allows the spinner to obtain the same quality in various colours suitable for blending into a multicoloured yarn. Although top dyeing is expensive it still retains a place in the industry. The equipment used for dyeing is almost always batchwise with the top being loaded into a top can. This perforated container is fitted into the dyeing vessel that is provided with a circulation pump capable of reversing cycles. Layering of the tops into the top can ready for dyeing is critical, since disturbances of the material at this or any subsequent stage will result in the top becoming unsuitable for further processing. At worst, this results in the top being cut for staple or being re-combed. Both routes are expensive and so the top dyer is able to charge a premium for his services. As top dyeing was originally developed to serve the worsted spinning industry, wool was the first fibre type to be processed via this route. Polyester staple soon became a top dye candidate, mainly because of the importance of blended polyester/wool suitings. Acrylic tops are also dyed by this route for incorporation in fancy yarns for knitwear.
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Practical Dyeing, Volume 3
Tow dyeing has been of great interest in the worsted industry for many years. High-quality coloured yarns can be produced via this route that compare favourably with yarns produced by conventional routes. Initially tow was mainly dyed batchwise but the development of sophisticated continuous dyeing equipment revolutionised this approach to coloration. Once it had been proved that the tow-dyed product would satisfactorily convert on the various stretch/break machines in use, the worsted industry invested heavily in this area. The batchwise dyeing of tow still remains viable for the production of small lots of commission-dyed tow, however. The batchwise dyeing of tow or tops is carried out in circulating-liquor machines operating with either radial flow (Figure 12.15) or vertical flow (Figure 12.16). In radial flow machines, the tops (2 to 7 kg in size) are mounted on perforated spindles and protected by a perforated cage. Liquor circulation is rapid and efficient. Tops, tows and slivers are all sensitive to handling conditions and require considerable care in processing in order to ensure that the dyed material will pass satisfactorily through the subsequent stages of yarn manufacture. A typical synthetic-filament tow for worsted processing may contain up to 125,000 continuous filaments laid together with great care, forming a cohesive ribbon capable of being converted at a high level of running efficiency directly into sliver of good quality. Crush-cutting convertors fitted with spiral cutters may produce over-length fibres if the lay of the tow is disturbed by the presence of twists or of rolled edges. Some forms of disturbance produce random clumps of tangled fibres that are difficult to separate in the drafting processes during yarn spinning. Tow faults lead to the production of uneven, neppy yarns unless the faulty material is removed by extra gilling processes or, in serious cases, by combing. Special creels are required to lift tows vertically from the producer’s supply boxes and to feed them correctly into the dyeing zone. At any stage where the tow must be laid down, scanning devices are required to ensure that it is laid in precise and even layers that will not drag or trap when the tow is next fed forwards. When continuous tow is loaded into cages, special care must be taken to avoid inserting twist. The cage is mounted on a turntable and rotated very slowly whilst the tow is run through a light nip, sprayed with hot water and mechanically stamped to consolidate the pack. After dyeing the tow is carefully withdrawn whilst the cage is rotated in the opposite direction to avoid inserting twist. A 160 cm diameter machine with a liquor capacity of 3200 litres has a nominal loading of about 365 kg of loose stock (about 9:1 liquor ratio) or 550 kg of continuous tow (about 6:1 liquor ratio) packed into cages. Two-way flow is normally employed during dyeing and suspended impurities or undissolved dye must be rigorously excluded as the pack behaves as a highly efficient filter. The processing quality of dyed synthetic-fibre slubbing is much less sensitive to the conditions of handling than is that of continuous tows. Nevertheless, it is necessary to exercise care to avoid setting-in durable kinks and twists, which can influence the regularity of the yarn and may introduce neps if the incidence of the fault is severe. If it becomes necessary to unwind a top for any process, this should be done in a manner that avoids the insertion of twist. With coiled bump tops the problem does not arise, but ball tops should always be creeled and unwound from the outside and never drawn out from the centre of the ball. Bump tops or tops that have been hanked from the ball may be dyed in the annular container of a machine of the radial flow type. Ball tops and suitably prepared bump tops may be dyed in perforated canisters with a central
Practical Dyeing, Volume 3
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perforated spindle. Machines with special canisters are available for the handling of exceptionally large bump tops, weighing up to 40 kg per package. Tops in cylinder/spindle containers should be compressed mechanically to about 80% of their initial depth. Packing densities in the range of 280 to 320 g/l have given good results under practical conditions in bulk processing. A radial flow vessel of 1.6 m diameter with a liquor capacity of 3200 litres will take about 320 kg of coiled tops (10:1 liquor ratio). The economics of tow or top dyeing are dependent on the size of batches to be dyed to each target shade. In all continuous and semi-continuous dyeing, the cost of downtime whilst changing colour has a rapidly increasing influence on the dyeing cost per kg, once the size of the batch falls below a critical level. In semicontinuous equipment, such as that at one time manufactured by Vanysol, padding and plaiting down into cans, as well as washing-off and drying, were continuous processes, whereas steam fixation was carried out batchwise in a separate autoclave. This approach offered a transitional alternative between either batchwise or fully continuous sequences. Advantages of this approach were that manipulation was simple and brief steaming times were possible even when the amount of tow was allowed to accumulate 100 kg in the vacuum steamer. An output of 350 kg/hour was possible. Loading and unloading of individual tops add significantly to the cost of batchwise dyeing, so that continuous methods are attractive on this account when suitable batch sizes are available. The typical processing range shown in Figure 12.17 illustrates the essential principle of these systems. Conventional vigoureux printing of wool sliver has been used for many years to produce multicoloured fancy yarns. Disadvantages of this traditional approach include low productivity, colour bleeding and poor reproducibility of colours. A novel system of multicolour printing of wool has been developed to enable application of colours in stripes along the sliver. Advantages of the system include reproducible multicolour prints, printing without gilling, clarity of the printed effect, stability of print paste supply and systematic colour changes [20]. The conventional continuous steaming of wool slivers is carried out in equipment of the J-box or the conveyor type. Drawbacks of J-box steaming include uneven treatment of the slivers and potential entanglement of the material. The conveyor type of steamer requires extensive operating space and is also apt to cause entanglement. A novel continuous steaming unit has been developed that is capable of transporting slivers smoothly without entanglement and steaming them efficiently and uniformly. High productivity, quicker delivery and reduced costs are claimed [20]. Equipment for the continuous dyeing of tow consists essentially of the following process units: 1. A creel arrangement ensures that the tow enters the padding stage in a uniform condition. If the tow has not been allowed sufficient time and travelling distance in the creel to eliminate kinks and other irregularities, the pick-up of liquor will be variable and unlevel dyed tow will result. 2. Padding is usually carried out at ambient temperature with a pick-up of 80 to 100%. The capacity of the pad trough is kept to a minimum, resulting in regular replenishment of the liquor and minimum build-up of residual unabsorbed dye that would contribute to tailing problems. 3. A pre-heating zone is incorporated is some continuous ranges and this has the effect of minimising dye migration that may result in frosting faults.
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4. Various designs of steaming chamber are available, utilising saturated steam at atmospheric pressure (100 to 105°C) or under pressure (110 to 130°C). Steamer capacity must be adequate for dwell times of at least 30 minutes to achieve satisfactory fixation of full-depth dyeings. 5. Typically, four to six perforated-drum washing units are required. Excess colour, thickener and auxiliaries must be removed without disturbing the configuration of the tow. It is important to avoid shock cooling of synthetic tows immediately on exit from the steamer. This is achieved by maintaining the first washbowl at an elevated temperature. 6. The final washbowl is usually a finish applicator. Finish is applied at this stage to replace that removed during the washing-off of excess colour and auxiliaries. 7. The most popular method of drying utilises the perforated-drum dryer with hot air being passed through the wet tow. Over-drying or baking of the tow must be avoided as this would result in handling problems during subsequent tow conversion. 8. The minor re-packing function is an important one because the dried tow must retain the same parallel configuration that gives economic running speeds on the stretch/break equipment. Elaborate plaiting arrangements are necessary to lay down the tow into cartons for transporting. The actual physical operation of dyeing tow is not especially difficult; it is the mechanical handling of the tow that can present serious problems. Tow in 3, 5 or 8 denier form is suitable for direct tow to top conversion. The actions of dyeing, washing-off, drying and re-packing can impair the physical properties of the dyed tow, rendering it uneconomical to convert because of a high wastage figure and slow conversion rates. The Fleissner WAF atmospheric steaming arrangement is shown in Figure 12.18. After suitable tensioning devices and opening creels the sheet of tows is fed vertically into a horizontal padding nip. High-speed fixation is carried out in a small J-box prior to full fixation being completed on a belt steamer. The conveyor belt is continuously cleaned to avoid deposition of dyes. The belt steamer is designed to be capable of processing tow laid down lengthwise or plaited crosswise or both. The steaming time at 100 to 105°C can be varied from a few minutes to three hours. The tow is then washed and finish re-applied in Fleissner back-washing units, before drying on a four-drum Fleissner unit. The Serracant continuous tow dyeing range was a sophisticated unit. After the usual tensioning and creeling devices, two tows of 400,000 total denier were fed into a horizontal nip of a two-bowl pad compartment formed by steel and rubber bowls, the liquor level being regulated pneumatically. The padded tows passed into the twin tunnels of a horizontal steamer. The speeds of the nip rollers and the steamer feed rollers were widely variable by means of a synchronised photoelectric cell control device. The tows were transported through the steamer on two endless pre-heated stainless-steel conveyor chains. Since the speed of the chains was much less than that of the feed rollers, the tow became compressed into each tunnel to form a plug. Almost immediately after the tows entered the tunnels they were pre-heated with live steam and indirectly heated to prevent the undesirable formation of condensed water when the tows entered the main steaming zone. In this section the diameter of the tunnels widened so that steam could quickly enter into the tows, opening them up slightly to ensure an even and rapid rise of temperature. When the compressed tows entered the tapering ends of the tunnels, they formed efficient steam locks enabling pressure steaming to
Practical Dyeing, Volume 3
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take place. The tows emerged from the steamer into an indirect cooling zone, followed by washing-off and a finish re-application bath. Finally, the dyed, washed and re-finished tow dropped off the endless chain, down a slide into a suitable container for transportation to a suction-drum dryer.
12.7 Drying of Loose Fibre, Tow and Top Mechanical methods of removing water from fibrous materials are much less costly than thermal methods, which consume far more energy. The two important mechanical drying methods are rotary hydro-extraction and the application of squeeze rollers. Rotating the wet material at high speed in a centrifuge will reduce the moisture content of synthetic materials to between 4 and 8%, a level sufficiently low to ensure good spinnability without further treatment in a thermal dryer. This method is extremely convenient if the dyed load can be lifted as an undisturbed pack from the dyeing vessel and lowered directly into a nearby rotary hydro-extractor. In the squeezing technique, the wet material is tipped from the dyeing cage into a brattice/hopper arrangement that feeds a squeeze nip. These rollers are usually situated at the entry end of a thermal dryer so that final drying can be completed in-line without further handling. Squeezing methods lower the moisture content only to about 50 to 70%, so that a thermal treatment is always required, even for synthetic fibres. The traditional thermal dryers, in which the material on a slowly moving brattice was passed through a heated chamber, have been superseded by much more efficient perforated-drum dryers with circulating fans creating a suction action to retain the fibrous mass on the drum surface. Such machines can be used to dry loose fibres, tow or top and this type of dryer is often incorporated into a continuous dyeing range. Radio-frequency heating (Figure 12.14) is an alternative that offers considerable energy savings and preserves the fibre quality. The operation is simple and instrumental control of moisture content ensures a consistent quality for delivery to the spinner or convertor. Whereas conventional drying equipment requires warm-up time, radio-frequency treatment is instantaneous both in start-up and shutdown, resulting in lower labour costs. The economics of fibre dyeing in various forms has been discussed in section 12.1. Probably the most important development in fibre dyeing has been the introduction of RF drying since this not only gives an improvement in quality but a means whereby considerable cost savings can be obtained. With the development of modern hydro-extraction equipment, the conventional route for dyed worsted tops of dye-wash-dry can be superseded by the use of RF drying. Although this latter route introduces a hydro-extraction stage, back-washing can be eliminated. The advantages claimed for using RF in top drying are given in Table 12.6 [21].
References [1]
Fibre and tow coloration, G Clarke (Bradford: SDC, 1982).
[2]
J Park and J Shore, JSDC, 100 (1984) 150.
[3]
A Schweitzer, Chemiefasern und Textilindustrie, 40/92 (1990) 1066.
[4]
S J Law, Chemical Fibers Internat., 50 (2000) 65.
[5]
A Cresswell, American Cyanamid, USP 2 558 735.
[6]
Anon, Textile Asia, 29 (Oct 1998) 95.
[7]
I Holme, Dyer, 176 (Jan 1991) 10.
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[8]
S H Amirshahi and M T Pailthorpe, Dyes and Pigments, 26 (1994) 237.
[9]
IWS Tech. Information Bull., DPB 24 (Apr 1988).
[10]
W S Hickman, AATCC Internat. Conf. & Exhib. (Oct 1997) 229; Text. Chem. Colorist, 31 (Jan
[11]
A Winch, Text. Research J., 50 (1980) 64.
[12]
R G Stewart, Wool scouring and allied technology, Wronz, p65.
[13]
J Gacen and D Cayuela, JSDC, 116 ((2000) 13.
1999) 17.
[14]
A Bereck, Rev. Prog. Coloration, 24 (1994) 17.
[15]
Anon, Text. Horizons, 16 (Dec 1996/Jan 1997) 34.
[16]
Anon, Dyer, 179 (Jun 1994) 12.
[17]
R Kettlewell and J Jackson, Dyer, 186 (Jun 2001) 23.
[18]
Anon, Wool Record, 157 (Jun 1998) 21.
[19]
K Schäfer and H Höcker, Melliand Textilber., 77 (1998) 402.
[20]
M Kaimori, N Nobukuni and K Kitamura, J Text. Machinery Soc. Japan, 41 (1995) 75, 109; 42
[21]
Anon, Progress in wool coloration, IWS (1986).
(1996) 36.
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Table 12.1 Selection of coloration methods for various fibre types [1]
Fibre type Cotton Linen Wool Silk Viscose Acetate Nylon Polyester Acrylic Polypropylene
Mass coloration
Gel dyeing
● ● ● ●
Tow dyeing
Loose fibre dyeing ● ● ● ●
Top dyeing
Sliver dyeing
●
●
● ● ●
● ● ●
●
●
● ●
● ● ●
●
Table 12.2 Factors influencing the selection of coloration methods [1] Gel dyeing
Tow dyeing
Stock dyeing
Top dyeing
Low-cost continuous coloration by producer Long runs (>6000 kg) to a colour necessary for viability High-quality basic dye liquids required Increased downgrading compared with ecru production Slow response to fluctuations in market demand Shorter runs (1000-6000 kg continuously) to a colour Dye cost 10% lower than gel dyeing Moderate flexibility to market demand Short runs (<1000 kg) to a colour are possible Lowest dye cost, wide choice of dyes Moderate flexibility to market demand Short runs (<1000 kg) to a colour are possible More costly than gel, tow or stock dyeing Particularly suitable for blend yarns Moderate flexibility to market demand
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Table 12.3 Wool scouring conditions [1]
Bowl 1 2 3 4 5 6
Temp. (°C) 55 50 45 45 40 40
pH 9.5-10 9.5 9
Initial soda ash (g/l) Water 1-3 0.5-2 Water Water
Initial detergent (g/l) Water
Soda ash feed (g/l per hr)
Detergent feed (g/l per hr)
0.5–1 0.2-5 0.3 Water Water
0-0.5 0-0.5 0–0.2
Table 12.4 Application conditions for shrink-resist treatment [1]
Process stage Chlorination
Temp. (°C) 20
pH 1.5-2.0
Neutralisation
20-30
8.5-9.0
Rinsing Resin treatment Softening Drying
20-30 35-50
7.4-7.8
35-45 80
Initial charge 5 g/l sulphuric acid 0.3 g/l available chlorine 1-2 ml/l wetting agent 10 g/l sodium carbonate 5 g/l sodium bisulphite Water 5 g/l shrink-resist resin 2.5-4g/l softener Moisture content to be below 10%
Feed 1.5-2% available chlorine on mass of wool plus acid
1.5-2% on mass of wool
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Table 12.5 Chemical and application characteristics of fibre processing aids [1] Auxiliary Anionic antistats
Cationic antistats
Nonionic antistats
Anionic emulsifiers Nonionic emulsifiers
Nonionic lubricants
Characteristcs Phosphate esters derived from the reaction of either phosphorus pentoxide or tetraphosphoric acid with fatty alcohols, their ethoxylates or alkylphenol ethoxylates, which may be in the free acid or the neutral form. They are multifunctional and highly effective with emphasis on their emulsification, antistatic and corrosion inhibition properties. The potassium salts of such phosphate esters impart good antistatic protection and yarn wettability, with little tendency to migrate into nylon. Those derived from fatty alcohol ethoxylates are used as spin finish lubricants for both filament and staple fibres. Quaternised derivatives of ethoxylated fatty amines, and fatty alkylimidazolinium salts, are highly effective antistatic agents. The amine-derived products have excellent initial colour, are thermally stable, and give acceptable results in skin irritation tests. Mainly ethoxylated fatty amines or fatty alkanolamides derived from, for example, coconut oil. The amine-derived products are nonyellowing, non-corrosive, thermally stable and confer good antistatic properties on filament or staple fibres. Potassium salts of fatty alcohol phosphate esters. Conventional alkylbenzene sulphonates and sulphated or sulphonated alcohols and esters are added to provide wetting and emulsifying properties. Mainly ethoxylates of fatty alcohols, which vary from highly oil-soluble to highly water-soluble depending on the oxyethylene content. They combine low foaming properties with good wetting and emulsification. Like the high-temperature lubricants, they are thermally stable. Alkylphenol ethoxylates, as well as fatty ethers or esters of higher molecular mass, such as polyoxyethylene esters of lauric, oleic or stearic acids, or oxyethylene/oxypropylene adducts condensed as random or block copolymers, are widely recommended as base lubricant components for spin finish formulations, including hightemperature applications.
Table 12.6 Advantages of RF drying of tops [21] Labour costs reduced by 80% Improved working conditions Different colours can be dried at the same time Drying time reduced by 75% Floor space required reduced by 33% Energy costs reduced by 65% Reduced noise and radiated heat Improved working conditions Bottlenecks at dryer eliminated Steam raising requirements reduced Same equipment can be used for yarn drying
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Figure 12.1 Process stages at which coloration may be applied [1] Natural fibres
Synthetic fibres Mass coloration Gel dyeing Tow dyeing Loose fibre dyeing Top dyeing Yarn dyeing Fabric dyeing Garment dyeing
Figure 12.2 Mass coloration of polypropylene filaments [1] Binding agent
Polypropylene chips
First pre-mix
Second pre-mix
Pigment preparation powder
Final mixing
Pigment preparation or masterbatch granules
Extruder
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Figure 12.3 Mass coloration of polyester filaments [1] 1 2
3
6
4 7
5
1 2 3 4 5 6 7 8 9 10 11
Terephthalic acid 8 Ethylene glycol Esterification reactor Polycondensation reactor Chips production Dryer Silo Extruder Colour concentrate Quenching and spinning tube Take-up machine
9
10
11
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Figure 12.4 Mass coloration of nylon 6 filaments [1]
1
2
3
4
5
6
7
1 2 3 4 5 6 7 8 9 10
Autoclave Cooling bath Chip cutter Hopper Chip dyeing unit Dryer Silo Extruder Manifold Take-up machine
8
9
10
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Figure 12.5 Mass coloration of viscose filaments [1] 4
5
1 7 2 1 2 3 4 5 6 7 8 9 10 11 12
3
6
Cellulose/NaOH Steeping press Shredder Alkali cellulose Carbon disulphide/NaOH Solubilisation Ripening tank Filter Storage tanks Extrusion Coagulation Take-up unit
8
9
9
10 12
11
Figure 12.6 Mass coloration of acrylic filaments [1]
1 4
2
5
7 6
3
9 8
10 11
12
1 2 3 4 5 6 7 8 9 10 11 12
Acrylic polymer solution Filter Storage tank Filter Pump Mixer Storage tank Extrusion and coagulation Pump Pump Pigment dispersion in solvent medium Temperature-controlled jacketed storage tank at 60°C
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Practical Dyeing, Volume 3
Figure 12.7 Acrylic wet-spinning sequence [4]
Crimper
Water Water
Dye mix
Steam stretch
Spinning
Soft finish
Dryer Packaging
Figure 12.8 Formation and coloration of acrylic filaments in the gel state [5] 1
2 12
9
13
10
4
5 14
3
8
11
7 6
Figure 12.9 Typical layout for loose stock dyeing [1]
Dispensary on mezzanine Wet stampers
Dyeing machines
Control room on ground floor
Hydro-extraction
Bale breaker and dryer
Fibre in
Fibre exit
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Figure 12.10 Layout in a bowl of a raw wool scouring range [1]
Immersion drum
Swinging rakes for propelling the wool
Nip
Pump or steam inlet
Sediment channel
Figure 12.11 Process sequence for shrink-resist treatment of wool [1]
1st bath
2nd bath
3rd bath
4th bath
5th bath
Suction drum
Suction drum or satellite bowl
Suction drum or satellite bowl
Suction drum
Suction drum or satellite bowl
Chlorination
Neutralisation
Rinsing
Resin application
Softening
Figure 12.12 Smith Petrie pad-steam piston system [1]
A
B
C E D A Rubber conveyor belt B Nip rollers C Tubular steamer
D Front of steamer E Piston
Dryer
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Figure 12.13 Fleissner range for washing-off and drying [1]
Feed-in
Washing range
Perforated-drum dryer
Figure 12.14 Fastran EDF radio-frequency fixation system [1]
Metal detector Dye spray Electrodes
Pressure chamber
Spray pipes
Hopper feed
Dye padder
Smith Fastran EDF unit
Wash-off unit
Figure 12.15 Liquor circulation with radial flow [1]
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Figure 12.16 Liquor circulation with vertical flow [1]
Reversible pump
Figure 12.17 Typical continuous dyeing range for tops [1]
Dye padder
Drum/belt steamer
Perforated-drum wash bowls
Perforated-drum dryer
Figure 12.18 Fleissner continuous tow dyeing system [1]
Padding Feed-in
Steaming
Washing
Perforated-drum dryer
Chapter 13 Yarn and Narrow Fabric Dyeing 13.1 Yarn-Dyed Products Textured yarns have revolutionised the world of textiles and clothing. By giving synthetics a look and feel that is more akin to natural fibres, they have facilitated the penetration of markets that were the domain of cotton and blended yarns. The global market for textured yarns continues to expand in the USA and Japan, but the largest and fastest-growing markets are in the developing East and South-east Asian countries. The world’s biggest market for these yarns is PR China; from a mere 8.2 kilotons in 1980, the last two decades have seen phenomenal growth to a current level of about 850 kilotons per annum [1]. Dyed yarn is needed for sewing threads, hosiery, carpets, towelling and a wide variety of colour woven or knitted designs in outerwear, sportswear, workwear and home furnishings. Although a much smaller market segment than fabric dyeing, the range of shades required is just as large. Virtually all fibre types can be dyed in yarn form. This includes staple-spun natural fibre yarns, continuous filament, both flat yarn and textured, and staple-spun yarns manufactured from synthetic fibres. As shown in Figure 8.2, yarn dyeing is situated almost midway in a typical manufacturing sequence. By the suitable selection of dyes and processing routines, level dyeings of high fastness together with a high degree of reproducibility can be produced, leading to right-first-time (RFT) production. It is economically advantageous to spin undyed fibre, showing improvements in spinning efficiency and the elimination of coloured waste. The availability of undyed yarn in a form suitable for dyeing gives shortened delivery times, leading to a ‘quick response’ processing route. The principal methods of dyeing yarn are either as hank or in package form. Hank dyeing tends to produce a yarn with a fuller handle and bulk, but tangling may occur and the technique is not readily suitable for singles yarn. Hank reeling and subsequent rewinding (back-winding) after dyeing are costly and may generate waste. Levelness may be inferior to that obtained from package dyeing due to channelling of the liquor in the dyeing machine, whilst the payload for hank is much less than that for package in a machine of a given size. Package dyeing gives better fabric definition but has often been criticised for yielding a leaner yarn. However, by suitable yarn engineering this leanness can be overcome. Methods have been developed for reducing the liquor ratio during dyeing. These include the use of larger package dimensions with higher package densities produced by press-packing techniques. Faster back-winding with the generation of less waste is possible. By the incorporation of suitable lubricants in the dyebath, back-winding can even be eliminated and the dye package can be utilised directly as the supply package for warping, weaving, knitting and tufting processes. High degrees of levelness and reproducibility can be achieved, using dyes of intrinsically high fastness properties. Many of the developments in package dyeing lead to savings in energy, water, effluent, labour and space. Whilst the traditional demarcation between hank and package dyeing routes has been eroded, hand-knitting yarn, high-bulk acrylic yarns and carpet yarns were usually dyed in hank form. These can now be dyed successfully in package form. On the other hand, singles yarn, particularly cotton yarns and singles yarn for the production of marls have been traditionally package-dyed. Viscose cake, sewing
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threads and continuous filament yarn are most successfully dyed in package form.
13.2 Hank Dyeing of Yarn 13.2.1 Introduction Hank dyeing as a process dates back to antiquity when yarn, simply suspended from wooden poles, was immersed in the dyebath, probably heated by an open fire, and the poles turned by hand. The history of machine developments in the dyeing of textile yarns has been reviewed in detail [2]. The development of hank dyeing machines is also discussed in section 13.4. It was predicted [3] that there would be a decrease in the importance of hank dyeing but this forecast has not been fulfilled for a number of reasons, including those listed in Table 13.1. As already mentioned, hank dyeing is usually judged to give a yarn with a more fully developed bulk and a fuller handle than that obtained by package dyeing routes. The same comments are often made about fibre-dyed products. This is probably because the hanks, as suspended on the poles or sticks of the dyeing machine, are free to relax completely and are not constrained in any way. This not only allows for full bulk development but allows a certain freedom for twist in the yarn to run and find its equilibrium, thereby removing spinning tensions. Hank dyeing is still preferred in many carpet yarn dyehouses because it favours relaxation and bulking, to yield excellent cover and tuft burst in the carpet [4]. Certain yarns, such as high-twist carpet yarn (often referred to as ‘kinky’ yarn), are dyed in baskets of the dyeing machine to allow full development of the yarn properties. A degree of skill is required for loading the yarn on to the dyeing frames, since this can have a significant effect on the quality of yarn produced as regards bulk and levelness. The number of hanks per pole must be determined and in rectangular-type machines, the same number of hanks must be placed on each pole to give an even loading. The hanks must be correctly placed or ‘dressed’ on the poles so that the yarn is opened out and any twists within the hank removed. Tangling of hanks may still occur during dyeing and to minimise this the liquor in single-stick machines may be circulated in one direction only. The care with which frames are unloaded may determine the level of waste generated in subsequent back-winding. Hank-dyeing machines are relatively simple and this can give economies in maintenance. Payloads are generally lower (by about 50%) for a given machine size compared with package-dyeing payloads. Much more space is required for hank handling, both in terms of machine space and ancillary operations. Hankdyeing machines may have a relatively slow liquor flow rate so that the levelness and fastness properties obtained may be restricted. The process is labourintensive, since much handling is required, not only in the dyeing operation itself but in associated processes, as shown in Table 13.2. With a yarn that has to be scoured before dyeing (section 13.2.4), it may be handled up to eleven times between the end of the spinning line and becoming available as dyed yarn on cone. During the dyeing process, hanks may be turned manually on the sticks of the dyeing machine one or more times to improve levelness and avoid the risk of ‘stick-marking’. Hank-reeling for dyeing and back-winding on to cone are costly processes which can generate significant waste.
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13.2.2 Hank Preparation Following spinning and doubling, the yarns are reeled into hanks on hank-reeling machines which are fitted with swifts (revolving frames) having a circumference in the range 120 to 230 cm. The length of the hank produced is influenced by the section of the industry to be supplied. It is important that this length can be accommodated in the dyeing machines and on the swifts of subsequent backwinding machines. In the case of high-bulk acrylic yarns, which will shrink by 18 to 25% during dyeing, due allowance must be made for this change in hank length. The average length of hank produced is 140 to 180 cm. Conventionally, hank weights are in the range 340 to 450 g and these are fastened at various places around the hank with interlaced tie-bands to preserve the integrity of the hank and prevent tangling. Tie-bands which are too tight will give rise to undyed places in the hank. Tie-bands may be eliminated by wrapping the hanks in heat-set polyester stockinette which protects the yarn during processing and prevents tangling. Jumbo hanks have been developed, up to 2 kg in weight for hosiery yarn and up to 10 kg in weight for carpet yarn. The advantages of using jumbo hanks are listed in Table 13.3. The processing of jumbo hanks, particularly in the carpet industry, has achieved many of the advantages claimed for package dyeing and has reduced the level of interest in entry into package dyeing, with its associated high capital cost. Jumbo hank processing involves a relatively modest capital requirement in the winding plant. Nevertheless, investment trends favour the package dyeing of carpet yarns and this may be the preferred alternative when hank dyeing plants reach the end of their useful life. Considerable tension and strains can be set up in the individual yarns in jumbo hanks during reeling. It is recommended that tumbling be carried out before dyeing to remove such tensions and insure against unlevel dyeing. This will manifest itself as stripes in the fabric produced. Tumble drying of jumbo hanks has been adopted also to achieve maximum bulk development. Reeling of hanks ready for dyeing is carried out by the spinner and not by the dyer, which is contrary to the usual practice for package dyeing. Hank dyeing appears to be preferred in certain countries, such as Italy, often on the grounds of quality but also influenced by the availability of low local labour costs. However, as seen in Chapter 4 and in later sections, modern, sophisticated hankdyeing machines have been developed with a high level of control and robotisation. 13.2.3 Hank Dyeing of Narrow Fabrics Narrow fabrics, including ribbons and tapes for a variety of end-uses, are dyed in hank form [5] and these are manufactured from all types of textile fibres. Narrow fabrics are also made in numerous widths. Ribbons for trimming fashion garments may be required in small quantities of below 100 metres so that hank dyeing is an obvious choice of dyeing method. The ribbons are reeled into hanks, often containing about 100 metres, but this depends on the width and weight of the fabric. Although hanks of this kind have been dyed in baskets and cages, the more usual method is to carry out the process on conventional single-stick machines. The high density and the width of the fabric usually demands frequent turning of the hanks during dyeing to achieve level dyeing and avoid stick marks, making this method labour-intensive with relatively low productivity.
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13.2.4 Hank Processing before Dyeing Most synthetic-fibre yarns and also yarns spun from natural fibres are dry-spun with only a small amount (about 0.2%) of spinning lubricant or are spun with a water-soluble lubricant. These yarns may be given a rinse in water in the dyeing machine or require no scouring before dyeing. An alkaline detergent scour at 65 to 70°C for 15 minutes will remove soiling and lubricant. One advantage of hank processing is that heavily contaminated or lubricated yarns can be scoured effectively in a continuous scouring machine. This may be required for woollen and worsted-spun wool yarns. Wool yarns produced on other spinning systems, such as open-end or the Sirospun technique, are usually dry-spun. In older machines, the woollen yarn hanks were carried through the scouring baths by means of a pair of brattices whereas in modern machines the yarn transport is by means of a pair of tapes. A scouring range consists of three or four scouring troughs with a pair of squeeze rollers between each trough. The upper roller, usually covered with wool or cotton sliver, is in contact with the lower steel roller. High pressure is applied to these rollers, giving an expression as low as 60%. The principle of a tape scouring range is shown in Figure 13.1. The two machine types have approximately the same dimensions but the immersion time in the brattice machine is longer because of its slower speed. A tape machine runs at about 9 metres per minute with an immersion time of 6 to 12 seconds in each trough. With a loading of 0.5 kg of woollen yarn per metre of tape, an output of 270 kg per hour can be obtained. Although requiring a higher manning level, these machines can be run with 1.5 kg of yarn per metre of tape, giving a production of 810 kg per hour. The troughs are steam-heated and modern machines have temperature and level controls. 13.2.5 Hank Setting Wool yarns can be wet set before scouring and hank dyeing to prevent cockling occurring in the fabric. This process can be carried out by stretching the hanks tightly in a frame, for example in a two-stick dyeing machine, which is then immersed in boiling water for 30 minutes, turning the hanks at least once. The yarn is cooled under tension before removing from the frame. Wet setting of this kind has been shown to prevent loss of strength in subsequent processing. The results shown in Table 13.4 are indicative of the effects obtained with 2/30s worsted yarn. 13.2.6 Cotton Yarns Cotton yarns, especially fine counts for knitting and woven shirting fabrics, are singed by passing the yarn through a pair of gas-fired burners, as the yarn is wound from cone to cone. The yarn is then reeled into hanks and mercerised in a hank mercerising machine in which the hanks are placed over arms that apply tension. The arms rotate as the yarn is immersed in a 350 g/l solution of caustic soda (60° Tw) for two minutes at a temperature not exceeding 18°C, usually with a controlled refrigeration system. The alkali is then washed out and recovered. A hank mercerising machine is illustrated in Figure 13.2. Rinsing, bleaching and dyeing of the hanks are often carried out in two-stick cabinet-type machines. Traditionally, cotton yarn has been kier-boiled after singeing and before mercerising. This has been superseded mainly by scouring in the dyeing machine.
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Scouring and bleaching can be carried out in cabinet-type or conventional hankdyeing machines, or alternatively in the annular cages (or baskets) of fibredyeing machines. Scouring with alkaline detergent is often adequate for dark shades, or when the seed content of the yarn is low. For light colours and whites, a scouring process followed by bleaching with hydrogen peroxide is given. A combined scour/bleach can also be employed, thereby reducing the processing time. Cotton contains a high proportion of so-called ‘hardness’ salts, mainly calcium, magnesium and manganese, from the fertilisers, insecticides and harvesting-aids applied during growth. These salts can have deleterious effects in dyeing and must be reduced to a low level, often using an alkaline demineralising process incorporating speciality chemicals. Alternatively, a sequestering agent such as ethylenediamine tetra-acetic acid (EDTA) can be used, in virtually all wet processing baths.
13.3 Package Dyeing of Yarn 13.3.1 Introduction The advantages of package dyeing compared with hank processing include the factors listed in Table 13.5. Criticisms of package dyeing have generally centred around the increased cost of plant and the risk that a less bulky yarn will result. This has been overcome in many areas, by either modifying the process or engineering the yarn to be suitable for package dyeing, whilst obtaining the target bulk, handle and desired properties in the final fabric. A major disadvantage with a traditional package-dyeing process was that it could introduce two additional winding operations into the total process route, with a consequent increase in cost. The preparation of dye packages from spinner’s or doubler’s bobbins or from a two-for-one twisting machine eliminates one of these processes, as does the preparation of dyepack by the bulked continuous filament (BCF) yarn producer. The application of suitable soft finishes and lubricants in the dyebath eliminates the second winding process since the dyeing package can be used as the supply package for the next process. The high cost of purchasing and installing package-dyeing equipment has been an obstacle, as has the development of less costly production methods such as producer coloration or continuous tow dyeing. A major advantage of package dyeing is its contribution to ‘quick response’ processing, in which yarn can be prepared on package, ready to dye as orders are received, thereby shortening the processing cycle. Many important products are package-dyed, including those listed in Table 13.6. Successful package dyeing of wool yarns for various end-uses, including hand-knitting yarns, has been described [6]. 13.3.2 Package Preparation The success of package dyeing, in terms of levelness, reproducibility and yarn quality, is greatly influenced by the degree of care and control exercised in the preparation of uniform packages of a suitable density. Package density is controlled during the winding process by adjusting the yarn tension and winding speed, allied to a suitable yarn path through the winding machine. Numerous designs and materials, including metal springs and plastic formers, have been used as support media for yarn packages for both natural and
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synthetic fibre yarns. Different package types, including rockets, cones, muffs and parallel-sided dyepacks (PSDP), have all found favour at various times. Stainlesssteel formers can involve a major capital cost initially but they are virtually indestructible and thus represent in the long term a low cost per kg of yarn processed. Plastic centres range from one-trip devices to recyclable units, the ultimate cost per kg of yarn depending on the number of cycles that the centre can withstand. Textured yarns are frequently wrapped in stockinette for dyeing since this prevents filamentation (the separation and breaking of filaments) and helps to keep the yarn clean. Selection of unsuitable stockinette material or prolonged recycling of these wrappers can cause problems due to staining. Textured yarn has been traditionally package-dyed, with methods varying from packing yarn in annular cages to loading packages on spindles, with or without support centres. It was shown [7] that inside/outside crimp rigidity variation was least when yarn was dyed on a former loaded on to the dyeing machine spindle. A major advantage of textured yarn packages is that they are prepared on the texturising machine and therefore eliminate one winding process for the dyer. A package specification must be agreed between the throwster and the dyer. Various package centres have been used, but stainless-steel springs and multi-trip or single-trip plastic centres (such as the Aflex) are widely preferred. Packages of textured nylon carpet yarn on stainless-steel springs, weighing up to 4 kg, have been prepared by the BCF yarn producer. A typical specification for textured yarn packages is given in Table 13.7. Staple yarns were originally delivered to the package dyer on perforated cones ready for dyeing. The conicity of the cone was usually 4°20’ or 5°57’. The main claim for cone dyeing was that a winding process was eliminated and that the dyeing cone could be the supply package for the next process. In reality, the problems associated with cone dyeing are listed in Table 13.8. A major achievement in staple yarn dyeing was the concept of the parallel-sided dyepack. It had already been recognised [8,9] that these gave better flow characteristics, although soft-wound packages must be avoided since dye liquor channelling can occur due to package distortion, preventing direct use of the packages for the next process. Stable, high-density packages are suitable for use without rewinding. Increased package sizes allied to press-packing eliminate variations between packages, giving increased dyelot sizes and at the same time level dyeing. Parallel-sided packages can be prepared on flexible stainless-steel springs. An alternative is to use a winding machine with a mandrel of the same diameter as the spindle on the dyeing machine so that the packages can be prepared using a polypropylene non-woven centre of either a reusable or throw-away variety. The detachable plastic mandrel functions as a push-out centre on which the package is transported to the loading area. The packages are dyed on the spindle on nonwoven supports and these can be bleached and recycled. The technique can be used satisfactorily for most fibre types and end-uses. Using either springs or nonwoven centres allows press-packing techniques to be used with a compression of up to 20%. With the development of spinning techniques, such as open-end methods, it is possible to prepare dyeing packages directly from the spinning frame. PSDP can be prepared directly from the two-for-one twister or alternatively from spinner’s bobbins. All of these options effectively eliminate a winding operation from the conventional process.
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Following from the use of PSDP, bi-conical formers made from polypropylene were developed, originally by Rost. Glass fibre reinforcement of these centres allows their use at dyeing temperatures up to 140°C. The conicity of the bi-conical former is 4°20’ which means that many conventional cone-winding machines can be used to produce the parallel-sided packages. Grooves in the base of the formers correspond with vanes in the top so that press-packing techniques, with a compression of up to 15%, can be used. This development gives the advantages of press-packing associated with PSDP without the disadvantages of cones, while allowing the spinner to prepare the dyeing packages. This eliminates a winding process if the packages can be supplied to an appropriate specification. Many suitable package specifications are possible, depending on the winding machine used and the dimensions of dyeing frames and spindles. Typical specifications for cone, PSDP and BI-KO packages are shown in Table 13.9 together with some of the machine loading advantages. In dyeing trials, the cones gave unacceptable levelness, whereas the other two package types were satisfactory. Various methods of presenting packages of staple yarn were compared [7]. Package winding of staple yarns can be carried out by two techniques: 1. cross-wound packages which are then press-packed on the dyeing machine spindle to even out the variations in density between individual packs 2. precision-wound packages which are much more uniform Precision and random dyepack-winding methods have been compared [10] and precision winding is now preferred. This assists in achieving highly reproducible dyeings with good levelness and allows a package of approximately 20% more weight and density to be prepared. The uniformity between packages virtually eliminates the need for press-packing. This allows perforated, plastic, parallelsided formers to be used for dyeing. These have a flanged design, whereby the top of one former sits inside the base of another. This gives a uniform and parallel column of yarn on the dyeing machine spindle. The principle of BI-KO centres is shown in Figure 13.3. Yarn packages with spindle densities in the region of 400 to 500 g/l can be successfully dyed on machines with adequate flow rate. 13.3.3 High-Bulk Acrylic Yarns Preparation of high-bulk acrylic-fibre yarn packages by the dyer is justified since continuous relaxation of the yarn is carried out at the same time. High-bulk acrylic yarns were conventionally dyed in hank form to allow for the approximately 20% relaxation shrinkage which occurs in water at about 85°C and above. Various machines have been developed for yarn relaxation using steam or dry heat. The principle is to wind the unrelaxed yarn from cones through the relaxation chamber, the yarn then being wound on to suitable packages for dyeing. The main methods used commercially are: 1. the stuffer box/J-tube technique developed by Hacoba 2. the conveyor belt method of Superba 3. the conveyor band, originally developed by Horauf Süssen and expanded by Savio. Selection factors are listed in Table 13.10 whilst a continuous relaxation machine is shown in Figure 13.4.
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A comparison was made between the three types of relaxation equipment listed above, using a 2/30s worsted count yarn spun from 3-denier Acrilan by conventional ring, Repco and Sirospun spinning methods. All relaxation methods reduced the yarn strength by between 25 and 30% with a further small reduction in strength occurring in subsequent package dyeing. However, all the methods evaluated gave a low residual shrinkage level suitable for package dyeing with the Superba method giving the lowest values. All techniques gave yarns which knitted satisfactorily and produced commercially satisfactory fabrics as judged by handle, appearance and physical properties. There was no evidence to suggest that any particular type of relaxation machine is more suitable for processing yarn manufactured by any particular spinning process. The low residual shrinkage obtained by the Superba technique is extremely beneficial for hand-knitting yarns and for a high-quality product. The yarn may be relaxed on this equipment before or after package dyeing. Modern machines consume much less steam than their older counterparts. Package size and density are important and these are influenced by the take-up tension. In preparing packages of high-bulk acrylic yarn for dyeing, the feed-on tension of the yarn and the force existing between the driving barrel and the winding package must be adjusted to achieve two primary targets: 1. to produce a yarn with an elliptical cross-section, as opposed to a flat-ribbon type yarn, as laid on the package; 2. to obtain a package of even density throughout. A perfectly vertical, flush edge is required on the package so that adjacent packages bed down together on the spindles. The comments above on selection of former and winding process apply. 13.3.4 Carpet Yarns Staple-spun carpet yarns can be package-dyed, since the yarn can be wound on to dye packages from spinner’s or doubler’s tubes, thereby eliminating a winding operation. Packages are usually up to 1.5 kg in weight and can be used, after dyeing and drying, as the supply packages for carpet weaving without further winding. Continuous filament carpet yarn can be processed on springs which are prepared by the fibre producer. However, both yarn types can be wound by the dyer to give a package with a traverse of 200 to 260 mm weighing up to 5 kg. These packages are ideally suited for direct tufting. The package dyeing of carpet yarns has been discussed [11] from a practical standpoint. 13.3.5 Knit-Deknit Yarns Continuous filament yarn for the production of ‘crinkle-type’ fabrics is processed by the knit-deknit (KDK) method. The yarn is knitted into tubular fabric approximately 8 cm in diameter. Rolls of this fabric can be handled as dye packages on spindle machines and are deknitted after wet processing and drying. 13.3.6 Press-Packing References have been made to the philosophy of press-packing, in which dye packages are prepared to a given specification. These are loaded manually on to the dyeing spindles, then additional packages are loaded by means of a mechanical press giving a compression of 20 to 40%, depending on the yarn, the
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package density and the target spindle density. This technique is essential for cross-wound packages; it gives increased payloads and reduced liquor ratios, minimises package density differences, prevents liquor channelling and contributes greatly to level dyeing. When the packages have been press-packed, a smooth, vertical column of yarn is produced in which any variation in package preparation can be readily seen. With BI-KO centres, the package density at winding is adjusted to give a maximum compression of 15%. A typical package press is shown in Figure 13.5. Precision winding is now being widely adopted and this gives an increase in package weight and density of about 20%. Since the packages are uniform, press-packing is not required. 13.3.7 Preparation for Dyeing Scouring is difficult in package-dyeing machines, due to filtration by the yarns. Excessively contaminated yarns cannot be successfully scoured without the redeposition of dirt and oil. Yarns should therefore be dry-spun or spun with watersoluble lubricants. The lubricants and spin finishes from synthetic-fibre yarns are readily removed by an alkaline detergent scour. Solvent-soluble contamination of yarn for package dyeing should be below 0.5% and evenly distributed for level dyeing to be obtained. Heavy deposits on the inside of packages can cause serious problems and scouring is usually carried out with two-way flow. The comments made in section 13.2.6 regarding the preparation of cotton yarns in hank form apply equally to packages. 13.3.8 Shrink-Resist Wool Wool is normally given a shrink-resist treatment, when required, either in top or garment form. However, this can also be carried out as part of the packagedyeing process. The outline of a typical process is shown in Table 13.11. Yarn packages with a spindle density of 400 g/l have been successfully processed in this way. The hues of selected reactive and 1:2 metal-complex dyes are unaffected by this treatment and the dyeings retain a high level of fastness. The method is expensive, not least since a processing time of between 7 and 10 hours is required, depending on the process details. 13.3.9 Package Dyeing of Narrow Fabrics Narrow fabrics can be dyed in package form [5]. This method has been widely used for polyester since high-temperature methods can be used. Setting of fabrics can be eliminated. Fabrics less than 3 mm wide are often dyed in this way. The method has been widely used for Velcro fastenings and for zip tapes when the support fabric (the narrow fabric component) and the ‘chain’ (the zip) are dyed at the same time. Packages are usually prepared by winding the fabric directly on to the spindle of the dyeing machine, using a specially adapted winding machine. Methods have also been developed using a spindle similar to that in the beam dyeing of wide-width fabric. The narrow fabric is wound directly on to the spindle, the large diameter favouring level dyeing. Dyeing is carried out in conventional package-dyeing machines for both types of spindle.
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13.4 Hank-Dyeing Machines For many centuries, until the early decades of the twentieth century, hank dyeing was carried out in primitive equipment. The yarn hanks were suspended from wooden poles in a rectangular wooden vat. The sticks were moved from end to end of the vat to give some degree of agitation and at the same time the hanks were turned manually to prevent the formation of undyed areas if the yarn remained in contact with the sticks. The gradual mechanisation of this system and the replacement of wood by metal, ultimately by stainless steel, resulted in the development of the ‘Hussong’ machine. This type of machine has been made by most manufacturers and is still widely used, especially in the carpet industry. Machines of various sizes are made, ranging from single hank to four one-tonne machines. A machine of this type is illustrated in Figure 13.6. Heating is by open or closed-coil steam pipes positioned below a perforated false bottom. The dye-liquor is circulated over a weir and through the yarn by means of a reversible impeller. In modern machines, the yarn is suspended from Vshaped sticks with perforations to prevent stick marking. Even so, the yarn may be turned manually on the sticks one or more times during the temperature rise of the dyeing cycle and sticks placed in different positions to overcome levelling problems. Most variations in the design of hank-dyeing machines have probably been intended to overcome these problems of stick-marking and the generally poor flow characteristics of the machine. Rectangular, single-stick machines have a relatively slow flow rate, leading to ‘dead spots’ in the corners of the vessels. In rectangular machines, an equal number of hanks must be placed on each stick and both over- and under-loading will give rise to unlevel dyeing. Many designs are now obsolete and some manufacturers have ceased operation. These machines, therefore, will only be given a brief mention. Two obsolete machines, formerly representing advances in hank-dyeing methods, were based on the principle of rotating the sticks. In the Gerber type of rotatingarm machine, the hanks were suspended into the dyebath from smooth horizontal rods that were rotated while at the same time being given a reciprocating vertical motion. The rods were reversed automatically. The Klauder-Weldon was a twostick machine, in which the hanks were held on wooden rods in a carrier consisting of two larger and two smaller concentric wheels. The hanks were moved through the dye liquor by rotating the whole wheel assembly. By means of projections on the outer sticks and fixed strikers fitted to the side of the bath, the sticks were revolved with every turn of the wheel assembly. This machine was useful for wet-setting processes. Improvements were made to the rectangular-type machine, but none is still in production. Early improvements included the use of closed steam coils which allowed a more consistent liquor ratio to be maintained. Double-scroll propellers running at slow speed to reduce turbulence, a central impeller compartment to reduce the distance of flow and perforated sticks were all aimed at improving level dyeing and eliminating stick marks. In the Pegg Pulsator, the liquor flow was given a pulsating action which raised the hanks off the sticks. This was an early impetus to introduce controls so that the frequency and length of the pulsating mechanism could be varied and the direction of flow reversed. Due to the compact machine load, liquor ratios of 10:1 were achieved, hank dyeing normally taking place at a liquor ratio of 20:1. Bottom sticks could also be fitted, converting the machine into a two-stick type.
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In single-stick machines, the direction of flow is mainly in one direction. The use of bottom sticks allows two-way flow and thus minimises severe tangling. Hightemperature versions of two-stick machines were introduced. The Pegg GSH machine was a two-stick type designed especially for dyeing high-bulk acrylic and hand-knitting yarns. A unique feature was that a regulated proportion of the dyeliquor was circulated through the top sticks. In some versions, the bottom sticks were removed without detriment to the results. With a suitable frame and a slightly higher capacity pump, the Pegg GSH could be converted to a packagedyeing machine. Packages were loaded on to horizontal spindles which meant that press-packing techniques could not be used. The cabinet type of hank-dyeing machine, as illustrated in Figure 13.7, is still widely used. This is a two-stick, two-tier machine claimed to give a gentle action and to be suitable for a wide range of yarn types. Several manufacturers have developed frames for dyeing hanks, to fit into circular machines originally intended for dyeing in other forms. This type of multipurpose approach allows the dyehouse to be versatile in that fibre and yarn in various forms can be dyed in one machine provided the necessary types of carrier are available. The two-stick principle is used so that flow reversal is possible and high-temperature methods can be used. These machines, however, have a higher capital cost than conventional hank-dyeing machines. The principle of the frame is shown in Figure 13.8. The hank sticks are situated in concentric circles round the frame and each consecutive circle accommodates a different number of hanks. The distance between the top and bottom sticks is adjustable to allow for different hank lengths and shrinkage. The circular cages are not easy to load and the use of any two-stick machine makes the loading process more labour-intensive. In all twostick machines, it is necessary to leave a gap of about 4 cm between the bottom stick and the yarn to allow for the hank to lift when the flow direction is reversed. Hanks can also be dyed in cages or in annular-type loose-stock baskets. The comparative payloads of circular machines are given in Table 13.12. Modern hank-dyeing machines are heated and cooled by closed coils so that reuse of cooling water is possible. Machines are amenable to a high level of control as discussed in Chapters 3 and 4. For hank processing, working space is likely to be 200 square metres per tonne of weekly production, if scouring, dyeing and back-winding are involved.
13.5 Package-Dyeing Machines Yarn has been hank-dyed for many centuries, but package dyeing dates back to around 1882, when Otto Obermaier was granted the first patent for a machine of this type. In an analogous situation to hank dyeing, many designs of packagedyeing machine have become obsolete and many machinery manufacturers have ceased operation. Package-dyeing machines are of five major types as listed in Table 13.13. The characteristics of the vertical kier/vertical spindle type are listed in Table 13.14. A machine of this type is illustrated schematically in Figure 13.9 and one in operation is shown in Figure 13.10. Machines based on horizontal kiers into which frames are inserted with vertical spindles have most of the characteristics shown in Table 13.14, with the added advantage that much less headroom is required. A machine of this type is
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illustrated in Figure 13.11. Rectangular machines used for package dyeing have often been developed from hank-dyeing equipment, as discussed in section 13.4. They have a long liquor ratio and relatively slow flow rate so that there is often a restriction on package sizes and densities; with horizontal spindles, press-packing cannot readily be carried out. This type of machine can be economical to install but requires much headroom, since the frames are lifted from the top of the machine. Beam-dyeing machines fitted with package carriers require less headroom since they are side-loading, but only limited press-packing is possible and packages tend to sag on the horizontal spindles. Machines in which spindles are inserted in individual kiers have many of the characteristics listed in Table 13.14, with the advantage of lower installation costs since they can be installed in standard buildings and the horizontal version requires little headroom. Kiers can be linked in a variety of configurations to give either single-spindle or multi-spindle systems. The most widely used machine of this type is illustrated in Figure 13.12. When selecting package-dyeing machines, the design of the machine must be assessed in terms of the total concept of package dyeing. Larger spindle diameters favour level dyeing and this dimension must coincide with that of the mandrel diameter on the winding machinery used. Spindle diameters are now in the range 55 to 72 mm. The distance between spindle centres must be such as to accommodate the diameter of package to be processed. Large packages, often up to 1.5 kg or more, are now the norm with even larger packages for carpet yarns. Spindle centres are often 210 to 230 mm. The traverse of the package on the winding machine must be such that sufficient complete packages will fit on to the effective spindle length in the dyeing machine to allow for press-packing if necessary. A decision must be made, therefore, as to whether cross-wound packages are to be press-packed or precision-wound packages (without presspacking) are to be used instead and the winding equipment selected accordingly. The number of packages in the dyeing batch is determined by the creeling requirements of the next process, such as warping or tufting. The spindles themselves vary in design from simple spears with a Y cross-section through to perforated or fluted spindles. Machines are manufactured in various sizes from single-package machines to those accommodating at least one tonne of yarn. Machines can be coupled but the flow rate between them must equal the flow rate within individual machines if level dyeing within and between them is to be achieved. Package-dyeing machines will normally be manufactured to give high-temperature dyeing capability up to 140°C. There has been continuing improvements in spindle and pump design with circular kier machines being readily pressurised. When selecting a dyeing machine, methods of pressurising, pump type, floor space and headroom are important factors, together with price. The philosophy of package dyeing is to operate machines at full loading if possible to obtain consistent liquor ratio and flow rate. Techniques are available, however, with vertical spindle machines, particularly vertical kier types (Figure 13.9 and Figure 13.10), to improve the versatility of the machine regarding both batch size and number of packages in the batch. In the blanking-off technique, for example, unwanted spindles are replaced by solid blank-off devices screwed into the base of the frame. This reduces the payload but is likely to increase the liquor ratio and the flow rate. Such methods may also give leakage problems through inadequate sealing of the blanking-off devices. Alternatively, yarn frames can be designed with a shorter spindle length and a stainless-steel dummy placed in the bottom of the kier. This method reduces the payload but allows consistency of
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liquor ratio and flow rate to be maintained. So-called ‘short-liquor’ dyeing systems have been developed in which a pressurised cushion of compressed air or nitrogen is built up above the dye liquor. It is claimed that a liquor ratio as low as 7:2 can be achieved by this technique when dyeing full loads or part loads dyed at the normal liquor ratio used in the fully-flooded machine. Machines with individual kiers for each spindle (Figure 13.12) are much more versatile in achieving different batch sizes since complete kiers can be isolated. The flow rate of early package-dyeing machines was slow and usually inadequate. The flow rate attainable depends on the resistance of the substrate and the efficiency of the pump being used. Centrifugal pumps are widely used, with efficiency values in the range 80 to 85% [12]. Apart from the flow rate, the contact number, representing the frequency of exchange of liquor through the load, is one of the important parameters in the package dyeing process (Equation 13.1).
Contact number (exchanges/minute) =
Flow rate (l/kg/min) Liquor ratio (l/kg)
Equation 13.1
It is now accepted that a flow rate of 30 litres/kg/minute at a typical liquor ratio of 15:1 is a minimum requirement. This gives a contact number of two exchanges of dye liquor per minute, or a complete circulation of liquor through the goods every 30 seconds. Conventional package-dyeing machines now have flow rates in the range 30 to 90 litres/kg/minute with contact numbers of 2 to 6 at 15:1 liquor ratio and complete exchange every 30 to 10 seconds respectively. Rapid-dyeing machines with flow rates up to 150 litres/kg/minute and liquor ratios in the range 15:1 to 5:1 have been introduced [13]. These give contact numbers of 10 to 30 and complete circulation times as brief as 6 to 2 seconds, allowing extreme rates of temperature rise as steep as 16°C per minute, compared with only 1 to 3°C per minute in a conventional machine. Many dye combinations in virtually all dye/fibre systems will not tolerate extreme conditions, however. Unlevel dyeing becomes highly likely if the dye exhaustion rate exceeds 3% per liquor circulation. When applying basic dyes to acrylic yarn, for example, the rate of exhaustion is highly sensitive to temperature and rapid rates of temperature rise must be avoided. In package dyeing, there is obviously a relationship between the flow rate of the liquor in the dyeing machine, the ramp (constant rate of temperature rise) and package density. These factors together with other important parameters which influence level dyeing of acrylic yarn were investigated [14]. Linear relationships were found between these three factors and the rate of exhaustion per circulation of the dyebath, expressed as either the overall rate or as separate rates for the two successive stages of the exhaustion process, namely surface exhaustion and diffusion. The equations obtained from this work relating rate of exhaustion to package density, ramp and the reciprocal of the rate of liquor flow were combined into a single equation. This enabled the relative effects of the three variables to be calculated. The equation can be used to decide which combination of ramp and flow rate will give a pre-determined rate of exhaustion by a package of a given density. Alternatively, the corresponding equation relating the instrumental colorimetric criterion of degree of unlevelness to package density, ramp and rate of flow can predict the effects that these three variables will have on the risk that unlevel dyeing will occur.
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This work [14], which had been carried out on one acrylic fibre type, was extended [15] to cover the behaviour of various basic dyes on a number of acrylic fibre types, under dyeing conditions that were expected to give borderline levelness. The shape of the rate-of-dyeing curve depended on the type of acrylic fibre and, to a lesser extent, on the cationic retarder used but not on dye type within the group of CV 2.5 to 3.5 basic dyes examined. Evidence of a different type of random unlevelness, not related directly to dyeing rate, was obtained from dyeings on the various fibre types at a slow ramp. The results from these laboratory studies [14,15] were confirmed by bulk dyeings carried out on three different types of acrylic fibre [16]. This showed that significant improvements in productivity can be obtained by adopting faster ramps when dyeing acrylic yarns in package form, providing other factors are satisfactorily controlled. The appropriate ramp can be predicted from its known relationship with package density and rate of liquor flow. For a given package preparation technique and a specific dyeing machine, the package density of the yarn and the flow rate will usually remain constant. It is also essential to make a selection of compatible dyes capable of covering short-term dye-affinity variation in the yarn and to calculate the correct quantity of a suitable retarder in order to ensure a controlled rate of dyeing and a satisfactory degree of exhaustion. Integration dyeing is a novel technique in which the dyes and auxiliaries are continuously dosed into the dyebath during the exhaustion process, the objective being to achieve a dynamic balance between the rate of input of dye into the system and the rate at which it is being absorbed by the substrate. In a study of this technique using the milling acid dye CI Acid Red 111 on cross-wound packages of wool yarn, levelness was found to depend on the instantaneous exhaustion of dye at any time during the dyeing process. Empirical relationships were derived to link levelness with dosing time, temperature, liquor flow rate and dye concentration. Increasing dosing time and process temperature adversely affected levelness, whereas increasing rate of flow improved levelness [17]. Acrylic packages are usually dyed with one-way flow, inside to outside, during the entire dyeing process. Most yarn dyeings are carried out using two-way flow, varying in profile from equal times in both directions to a predominance of flow in one direction. The flow direction profile is often based on personal preference and experience with times ranging from 3 to 10 minutes in each direction being common. Pumps may be either co-axial or centrifugal and since the latter may take a long time to complete the reversing sequence, this should be taken into account. With plastic tubes of the Aflex type, liquor flow can be restricted by the reduced volume through which the liquor can flow as a result of pressing the centre, so that flow should be predominantly in the out-to-in direction for level dyeing. The detailed pattern of liquor flow within cross-wound packages of cotton yarn has been investigated recently using nuclear magnetic resonance tomography. The technique was demonstrated by examining axial and radial tomographic sections through cross-wound packages prepared with hard-wound inner and soft-wound outer regions. Gradual drainage of water from the flooded system is informative, since the water remaining within the package distributes itself according to the porosity of different regions. Initial trials with cotton packages under laboratory-scale flow conditions allowed laminar, turbulent and stagnant liquor regions to be clearly distinguished [18]. Modern machines are heated by closed steam coils and cooled using a heat exchanger. The latter technique allows for the recycling and recovery of hot
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water. In machines for dyeing polyester, high-temperature drains are usually installed so that the dye liquor can be discharged at top temperature, thereby eliminating the deposition of oligomer on the yarn and within the machine. Package-dyeing machines must be purchased on technical merit and performance. The costs of similar machines from different suppliers may vary but this may equate to a very small difference in cost per kg of production. This cost increases as the payload of the machine decreases [19]. Machines may be fitted with full-sized stock tanks to enable preparation of the next dye liquor.
13.6 Drying 13.6.1 Hank Drying Following dyeing, the hanks are removed from the frames of the dyeing machine and the bulk of water is removed mechanically by either: 1. squeezing the hanks through a pair of rollers similar to those in a continuous scouring machine. Even with several tons of applied pressure, the residual moisture content is unlikely to be less than 60%. However, with a suitable trough, this method can be used to apply chemicals, such as soft finishes, to the yarn. 2. rotary hydro-extraction. This is undoubtedly the cheapest method and depending on the fibre type and whether high- (1450 rpm) or low-speed (375 rpm) extractors are used, the moisture content can be reduced to between 4% (for synthetic-fibre yarns) and 35% (for natural-fibre yarns). This is usually sufficiently dry to allow synthetic-fibre yarns to be wound without further drying. Thermal drying is necessary for natural-fibre yarns; the hanks are loaded on sticks and passed through a heated chamber. In addition to drying, this process also allows the crumpled yarns to straighten out. Conventional machines were straight-through dryers with an operative at each end, allowing a drying capacity of about 350 kg per hour. More modern machines allow the yarn to be loaded and unloaded at the same point so that only one operative is required. In machines for handling jumbo hanks of carpet yarn, squeezing is included and thus two operatives are required for the claimed production of 2800 kg per hour. Following hydro-extraction, radio-frequency (RF) drying is viable on a conveyortype machine with an operative at each end. RF drying requires less energy than thermal methods and yarn quality is preserved since over-drying and yellowing do not occur. In the Cool Dry system [20], originally developed by Fastran, combined RF/vacuum dryers are used in which a combination of dielectric heating and air flow ensures that the drying temperature does not exceed 60°C. For optimum performance in an RF dryer, all yarn packages in the load must have the same dimensions and package density. They must be identical in fibre composition and moisture content before treatment. The same pretreatment and dyeing process conditions must have been given and the residual concentrations of salt and auxiliaries present must be similar [21].
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13.6.2 Package Drying Rotary hydro-extraction is used to remove the bulk of the water mechanically. Thermal drying is carried out in chambers with the packages loaded on sticks or perforated spindles. Alternatively, RF drying can be used [22] and an automated hydro-extractor at the front end of the dryer allows it to be run by two operatives. Automated and robotised drying assemblies are available for both hanks and packages based on the above methods, as discussed in section 4.4 and illustrated in Figure 13.13. An alternative method for package drying is to use the so-called rapid-drying technique. The cage of wet yarn is transferred directly from the dyeing machine into the dryer where a sequence of cold ‘squeezing’ followed by thermal drying takes place. This method is costly in energy but low in labour content. However, good column seals are required for effective drying and initially this process was criticised for non-uniform drying. The latest machines are equipped with a conditioning device and it has been confirmed [23] that this produces a more uniform moisture content throughout the packages. Drying is complete in 60 minutes followed by a 15-minute conditioning period, which eliminates the normal four-hour conditioning time. This compares with 8 to 12 hours in a cabinet dryer. Despite the high labour content of hydro-extraction, this method is still the cheapest way of removing water. A comparison of costs of the various drying methods and their productivity is given in Table 13.15. The costs include labour, electricity, steam, depreciation and other overheads.
13.7 Rewinding Following dyeing and drying, hank-dyed yarn is invariably rewound on to a suitable supply package for the next process or for sale. With machine knitting and weaving yarns, this is usually on cone, the conicity being 9°15’ or 5°57’. Waxes are generally applied during cone winding. Hand-knitting yarns are often wound on to cone as a preliminary to the balling operation. Carpet yarns for weaving are often wound by the carpet manufacturer on to bobbins or spools to suit the type of loom used. Yarns for tufting are wound on to large cones. In cone winding, the hank is carefully loaded on to the swift of the winding machine to minimise waste. Manual machines are frequently used with a running speed of up to 350 metres per minute. After dyeing and drying, it is conventional practice to rewind package-dyed yarn on to cone for delivery. It is believed that the back-winding operation can act as a secondary quality inspection point, but yarn can be cleared at this stage and large knots replaced by splices. Lubricants can be applied to give the correct frictional properties for subsequent processing; waxes are usually applied to staple yarns, whereas a lubricating oil is applied to textured filament yarns. Automatic winding machines, which can operate at speeds up to 1200 metres per minute, are often employed with automatic doffing, measured length facilities, electronic clearing and splicing. Automatic controls ensure uniform operation of all spindles by regulating tension, waxing pressure, package counterweighting and speed changes. The selection and application of lubricants for yarn back-winding processes has been discussed [24]. As previously mentioned, a major objective is the elimination of winding after dyeing by making a stable, parallel-sided package to which a softener or dyebath
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lubricant is applied. After drying, this becomes the supply package for the next process.
13.8 Costs and Process Comparison As discussed earlier, a conventional package-dyeing process may introduce an additional two winding operations compared with competitive dyeing systems and, therefore, on economic grounds alone, the elimination of winding processes is an attractive option. Comparative costs are given in Table 13.16. Package dyeing, however, offers a highly reproducible dyeing process able to produce a high degree of levelness and fastness whilst giving a quick response.
13.9 Continuous Dyeing Methods Most of the package-dyeing methods already discussed have been concerned with relatively small packages loaded on spindles. Entire beams of yarn after warping and before weaving can be package-dyed on large perforated spindles. Beams of cotton yarn were often dyed in this way using vat dyes. Warp yarns, either as a full-width slasher warp or in bundles (ball warps) of 350 to 400 individual yarns, can also be dyed continuously [25,26]. Several attempts have been made to dye yarn continuously, and this approach was often seen as a method for producing small batches. Such developments included the French OPI equipment and the WIROC (Roaches) machine for wool and nylon yarns. CSIRO developed a dye applicator for dyeing yarn continuously, claiming a speed of 250 to 500 metres per minute. Successful application of basic dyes to acrylic yarn was achieved by applying the dyes in foam with a moisture uptake of only 17.5%. The dye was fixed by steaming on a Hacoba continuous relaxation machine. Thus high-bulk acrylic yarn could be dyed and relaxed in one continuous operation at a speed of 400 metres per minute. Whilst this system was successful for acrylic yarns, much higher moisture contents were necessary for wool and nylon yarn, making the process unworkable. In the Accu-Strand process [27], yarn is knitted in a manner similar to that used in the knit-deknit process, although flat-knitted fabric can also be processed. Dye is applied to give about 325% pick-up at a speed of 4.5 to 5 m/minute, then processed for a minimum of eleven minutes in a unique design of vertical steamer. Washing is carried out on a counterflow washer, followed by squeezing, a second washing process and the application of any aftertreatment required. The fabric is re-steamed and dried in a tensionless, flow-through dryer at 170°C before de-knitting onto cones. High fastness is obtained on nylon yarn for floorcoverings using this process with major cost savings in labour (70%), energy (70%) and water (85%) compared with hank dyeing. Production is 115 kg per hour and whilst there is a minimum batch size of 70 kg, there is virtually no upper limit. A continuous yarn dyeing process with a long and successful history is the dyeing of cotton warps with indigo in the manufacture of denim fabrics for blue jeans. World production of blue denim fabric is estimated at 2200 million metres per annum. A classical indigo blue represents about 90% of the worldwide market, with ecru, black and other colours accounting for the remainder [28]. In 1982 BASF initiated the world’s first manufacture of indigo microgranules and in 1994 marketed Indigo Solution 20%, the disodium salt of leuco-indigo produced by
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hydrogenation of the pigment form and supplied in containers inertised with nitrogen [29]. The well-established Sucker and Müller indigo warp dyeing range has been described in detail. Advantages include low cost production at high quality levels, reproducibility to AATCC Blue Bell standards, low consumption of indigo and chemicals, with high weaving efficiency. Metering of indigo, dithionite and caustic soda solutions is carried out under controlled conditions using a Siemens OP20 automatic device. Optimum dyeing performance is achieved by allowing sufficient time in the immersion bath (about 6 minutes) and oxidation section (at least 60 seconds), as well as a rapid circulation rate and a relatively low dye concentration. The installation of measuring systems for the control of liquor concentration and pH, redox potential and liquor temperature is an option. To obtain consistent operational air temperatures, the Quick Oxidation TCFA (temperature-controlled fresh air) system is available. A caustic soda mercerising device can be incorporated between the wetting trough and the indigo dyeing line [30]. The Ben-indigo (Benninger) indigo warp dyeing range is controlled by AMS (automated monitoring system) software, modelled on detailed studies of the kinetics of vatting, liquor retention, diffusion and reoxidation from leuco-indigo to indigo. The AMS program calculates target values for each stage in the process. Just-in-time provision of pad liquor is ensured by vatting to 95% conversion in an enclosed 160-litre reactor without excess dithionite or access of atmospheric oxygen. Savings of 20-40% in chemicals usage are claimed. Exclusion of oxygen from the absorption and diffusion stages improves the colour yield. Freedom from excess dithionite facilitates the reoxidation stage with further savings of oxidant, water and energy. The final washing stage is simplified by the virtual absence of residual unfixed dye and electrolytes on the dyed yarn [31]. Continuous dyeing methods are used successfully for narrow fabrics [5]. This method is almost exclusively used for the dyeing of harness and safety belt materials. Narrow fabrics woven from nylon or polyester yarns are successfully dyed by continuous methods. A much simplified processing route with the elimination of heat setting is possible by the continuous dyeing method compared with the batchwise process. A continuous dyeing machine for narrow fabrics is illustrated in Figure 13.14. Such equipment has much in common with the laboratory equipment used by continuous dyers of wide-width fabric. Information on this equipment and its operation has been given [32].
13.10 Multicoloured Effects The whims of fashion designers occasionally dictate that multicoloured yarn should be used in the production of fashion items. Various processes have been developed to achieve these effects: 1. random dyeing of yarn hanks, in which a part of the length of the hank is dipped in the dyebath and this is repeated on various portions of the hank with contrasting colours 2. knitted fabric is printed with stripes along its length using a small continuous dye applicator ; after fixation and washing, the yarn is deknitted in the usual way 3. space dyeing effects by injecting dye into yarn packages or applying dye to webs of yarn. Space dyeing methods for yarn have been reviewed [33,34].
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13.11 Yarn Quality A modern spinning plant will carry out on-line monitoring of yarn quality parameters such as evenness, thick and thin analysis and weight variation. This monitoring will be supported by off-line (laboratory) testing of raw materials and finished product [35]. The results from the on-line monitoring can be benchmarked against international data. Precision-winding of this high quality yarn allows for parallel-sided packages to be produced containing a measured length of yarn which has been electronically cleared and in which splicing has been used. The package density of a random selection of packages will normally be checked before dyeing. Even so, a small percentage of packages may exhibit unlevel dyeing as a result of density variations. Total inspection of packages for density before dyeing can be carried out using rapid weighing allied to a non-destructive, contact-free optical imaging technique [36]. This method requires a film camera, a weighing scale and a computer of the necessary speed and accuracy which will enable ten thousand packages per hour to be assessed on a conveyor scale. Changes in the weight and the silhouette against a standard indicate a potential unlevel-dyeing package. The production of precision-wound packages facilitates the elimination of backwinding after package dyeing, particularly if the dyer is not equipped with sophisticated winding machinery. After lubrication, whether this is carried out in the dyebath or at back-winding, the yarn must exhibit adequate frictional properties for subsequent warping, weaving, knitting or tufting operations. Package-dyed yarn often has to meet stringent fastness specifications, such as the exceptional light fastness requirements of the automotive industry or high wet fastness for yarns used in striped fabrics. Levelness of dyeing is a prime parameter. This is usually assessed by knitting on a single-feeder machine. With hanks, a selection may be taken from various positions in the dyeing machine, including the corners of rectangular vessels. The yarn is wound onto a package for knitting. After package dyeing, packages are sampled from various positions on selected spindles and split by winding into outside, middle and centre of the dyepack. These split packages are then knitted into five-centimetre panels to assess the levelness throughout and between packages. A panel of the standard colour is knitted on the end of the stockinette to assist colour assessment, this usually being carried out instrumentally for both colour acceptability and levelness.
13.12 Dye Application It has already been noted that package dyeing is an efficient filtration system. Water for package dyeing must meet the specification given in Chapter 3, usually being softened and filtered to remove hardness and particulate impurities. Metal ions must be removed since these can complex with anionic dyes or auxiliary products and may result in agglomeration and filtering out on the yarn. Dyes for package dyeing should be of high solubility or dispersibility with a small particle size. Dye manufacturers will usually designate dye batches that are suitable for package dyeing. As indicated in Chapter 9, oligomer formation is a serious problem in polyester dyeing and particularly in package dyeing since the oligomer contaminates the
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yarn, the interior of the dyeing machine and the back-winding plant. Several solutions have been proposed, including the addition of nonionic agents to the dyebath and dropping the exhausted dye liquor at high temperature. The latter procedure requires high-temperature drains with heat exchangers which provide a source of hot water in addition to removing oligomer. The alkaline application of disperse dyes has been advocated [37] as a method of reducing the oligomer produced; it is also claimed that a cost saving of up to 30% can be obtained (section 9.3.3). Non-aqueous systems for dye application have been examined, particularly on polyester fibres [38]. Dyes have to be specifically formulated for this purpose. These systems require the design of special dyeing equipment to eliminate loss of solvent and enable its total recovery. Such processes have seldom reached commercial maturity but the latest in this lineage is the use of supercritical carbon dioxide [39-41]. Special equipment has been designed and a range of disperse dyes developed. The process has been used successfully for dyeing sewing threads in quantities of 1 to 5 kg.
13.13 Plant Layout Yarn dyeing, particularly in package form, is amenable to a high level of control, automation or robotisation and the comments made in Chapters 3 and 4 are important in this context. The concept of a three-tier dyehouse was specifically developed for package dyeing based on circular, vertical kier, vertical spindle machines, as discussed in section 13.5 and illustrated in Figure 13.9. These machines required a great deal of headroom so that it was convenient to place the pumps and services in a low-level basement, with the dyeing machine itself at ground level and the dispensary and control room on a second floor or mezzanine. The support laboratory can also be situated on this higher level. For hank-dyeing machines and the more recently developed horizontal kier/vertical spindle machines, as illustrated in Figure 13.11, a two-tier configuration is adequate. The machines and services can be located directly on ground level and the dispensary, lab and control room on the upper level. The number of dyeing machines and the distribution of dyelot sizes over these are influenced by the business sector in which the dyehouse operates and whether it is a commission or an in-house facility. These factors impact on the overall plant layout, which should be such that work flow is unrestricted and bottlenecks avoided. For a plant carrying out preparation, dyeing and winding, a working space of 100 to 150 square metres per tonne of weekly production is required for a package operation and even more for hank processing. A traditional U-shaped layout can be used to advantage since this allows an adequate work flow, with straight pipe runs from a suitably sited boiler plant and with control room and laboratory on an upper floor, as illustrated in Figure 13.15.
13.14 Economic Considerations Hank dyeing is more costly than package dyeing since it always has two winding operations associated with it. The first winding process is to reel the yarn from spinner’s bobbins onto hank and the second is required to wind the hank onto a delivery package such as cone or parallel-sided package. These winding operations can generate as much as 5% of waste, whereas the total waste in
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package dyeing can be below 1.5%. When a dual-purpose dyeing machine, capable of dyeing yarn in either hank or package form, is installed in a modern dyehouse, the hank capacity can be as low as half that for packages, as indicated in Table 13.12. Since hanks are dyed at much higher liquor ratios than packages, the cost of all resources (including depreciation) is much higher per kg of yarn. As indicated earlier in this chapter, the total cost of package dyeing can be reduced significantly by eliminating the two winding processes associated with yarn dyeing. Several package-dyeing machine designs (section 13.5) have been developed; the vertical spindle machine with a circular vertical kier configuration is still widely used. Developments with this machine have been designed to reduce liquor ratio, giving major savings in resources, and shorten the dye-cycle to increase productivity. Shorter cycle times have often depended on the use of more efficient pumps. A development of this type has resulted in the COS series of package-dyeing machines from Fong; the important features of these machines are given in Table 13.17 [42]. A comparison of the relative costs of processing cotton yarn between the COS machine and a conventional package-dyeing machine operating at a liquor ratio of 8:1 to 10:1 is shown in Table 13.18. The drying of cotton yarn is relatively costly and conventional package dyeing machines should be fitted with vacuum extraction to minimise cotton drying times and costs. Conditioning of cotton, particularly knitting yarns, after drying is essential to ensure efficient fabric production, adding costs to the process. Another attractive objective is to reduce the lengthy cycle times associated with cotton dyed with reactive dyes, either in package or on beams, particularly the prolonged washing-off required after dyeing. The joint efforts of Thies and Bayer (now DyStar) introduced the Thermoflush process [43]. After dyeing and overflow rinsing the yarn is steamed, causing the hydrolysed dyes to migrate from the inside of the fibres to the surface, from which they are readily removed by a short shock rinse. Repeated steaming and rinsing give efficient washing-off. It is claimed that this process reduces the complete dye-cycle time by 20% and washing-off time by 50%, whilst 20% less water and 30% less steam are required. The final steaming treatment decreases the residual moisture content of the yarn. This lowers drying costs and may also allow immediate sizing without drying. The availability of such a steaming process on a package-dyeing machine allows steaming before dyeing, which improves yarn wettability.
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W Schelenker, D Werthemann, P Liechti and A Case, U S Patent No. 5 199 956 (April 1993).
[41]
D. Knittel, W Saus and E Schollmeyer, J. Text. Inst. 84 (1993) 534; Text. Res. J., 63 (1993)
No. DE 3906724 A1 (Sep 1990).
135. [42]
S Shang, Dyer, 187 (Apr 2002) 23.
[43]
W Lohnert, Bayer Farben Review, 36 (1984) 40.
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Table 13.1 Reasons for retention of hank dyeing Many spinners, especially outside the UK, are equipped with hank-reeling machines as the last operation in spinning As spinning capacity has increased in developing countries, the cost of shipping press-packed hanks to other countries is much less than that for yarns on packages The development of large (so-called ‘jumbo’) hanks has given some of the advantages associated with package dyeing There are still many hank-dyeing machines, albeit of dubious performance and life expectancy, in operation The high cost of purchasing and installing more sophisticated, replacement machines (and ancillary equipment) in times of economic recession has slowed the decline of hank processing The introduction of multi-purpose machines to dye both hank and package has prolonged the life of the process It is often claimed that, for certain products, hank dyeing gives a better quality
Table 13.2 Handling of yarn in hank form Remove yarn from spinning or doubling frame Hank reel Load and unload from continuous scouring machine Load and unload from hank-dyeing machine Load and unload from centrifuge Load and unload from thermal dryer Back-wind
Table 13.3 Advantages of jumbo hanks Up to 30% increase in capacity of the dyeing machine Better yarn quality and less entanglement Less waste Increased efficiency in back-winding with machine efficiencies up to 95% Fewer knots Back-winding at faster speeds with less labour Higher machine loadings give significant cost savings in dyes, chemicals, labour, water, effluent and energy.
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Table 13.4 Effects of processing on yarn strength and extensibility
Process details Ecru yarn in oil Scoured and dyed without setting Set with 5% extension, scoured and dyed
Tensile strength (g) 360 191 346
Extension at break (%) 15.0 6.9 13.3
Table 13.5 Advantages of package dyeing Elimination of hank reeling Reduction in waste Faster back-winding speeds More controllable dyeing process with better levelness and higher fastness Savings in water, effluent, energy, dyes and chemicals at lower liquor ratio Less space required Improved utilisation of labour More production from a machine of a given size High-temperature dyeing possible Quicker and more economical drying Readily controlled, automated and robotised procedures
Table 13.6 Package-dyed products Textured yarns for both weaving and knitting Sewing thread Singles yarn for use as such or for the production of marl effects Regular-spun acrylic yarns for machine knitting or weaving Weaving yarns in wool and acrylic fibres for apparel and furnishing fabrics High-bulk acrylic yarns, after continuous relaxation, for machine knitting Cellulosic yarns for weaving and machine knitting Automotive yarns, principally nylon and polyester Dyeing and shrink-resist treatment of wool yarn
Table 13.7 Package specification for textured yarn Parameter Package weight Package diameter Package centre diameter Package traverse Angle of traverse wind Package density Spindle density of packages after press-packing
Value 1.0 kg 220 mm 56 mm 140 mm 15°40’ 220 g/l 280g/l
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Table 13.8 Problems with cone dyeing Dyer has no control over package specification Expensive spacing devices are necessary Spacing devices often slip during dyeing Channelling and unlevel dyeing will result Payload is less with cones than other package types (see Table 13.9) Rounding of cone shoulders is necessary Limitation to drying methods used Yarn usually rewound and not used as supply package Singles yarn often dyed for two-fold twisting to mask unlevelness
Table 13.9 Specifications for staple yarn packages Parameter Pack diameter (cm) Pack traverse (cm) Pack weight (kg) Pack density (g/l) Spindle density (g/l) Packs per spindle Weight (kg) of yarn/spindle Effective liquor ratio Dyeing cost (% of PSDP)
Cone 20.3 (base) 15.2 1.3 350 5 6.5 21:1 148
BI-KO 20.3 15.2 1.3 335 375 7 9.1 15:1 118
Table 13.10 Factors in selecting relaxation machines Space required Output required Package size and traverse Cost Reliability Availability of spares and service Sophistication of take-up winder Residual shrinkage Labour content (number of ends per operative) Steam consumption
PSDP 20.3 15.2 1.3 340 405 9 11.7 11:1 100
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Table 13.11 Shrink-resist process in package form Detergent scour and rinse Dye with reactive or suitable 1:2 metal-complex dyes Chlorinate using dichloroisocyanuric acid Anti-chlor with sulphite Alkali rinse Water rinse Application of Basolan SW (BASF) resin Rinse with hydrogen peroxide Rapid dry or hydro-extract followed by hot air or RF drying
Table 13.12 Comparative payloads of circular machines
Carrier form Hank
Basket
Package
Material type Wool High-bulk acrylic Cotton Fibre Tops Muffs Wool or cotton Continuous filament Acrylic
Relative machine capacity 100 75 170 190 225 125 220 200 200
Table 13.13 Major categories of package-dyeing machines Circular, vertical kier with vertical spindles Circular, horizontal kier with vertical spindles Rectangular, horizontal kier with horizontal spindles Circular, horizontal kier (beam type) with horizontal spindles Spindles, either vertical or horizontal in individual kier
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Table 13.14 Characteristics of circular, vertical kier with vertical spindles Advantages
Disadvantages
Maximum payload
Requires a high headroom to accommodate cranes to remove the cages; best operated in a three-tier dyehouse
Minimum liquor ratio Savings in resources, dyes and chemicals High degree of levelness with fast dyes High degree of reproducibility High-temperature dyeing possible
High capital and installation cost as a result
Adjustment of flow rates and differential pressures possible High flow rates Rapid-dyeing versions available Can be run fully or partially flooded (air pad) Kiers can be coupled to increase dyelot sizes Rapid drying methods possible Package integrity maintained so that dye package can be supply pack for next process Cages for other substrate forms (fibre, tow/top, hank) – versatile machine Readily automated and robotised Either precision-wound packages or cross-wound packages with press-packing are used
Table 13.15 Comparison of drying methods Method Hydro-extract only Hydro + cabinet dry Rapid dryer Hydro + RF
Relative cost 1.0 7.0 10.0 6.0
Production (kg/hour) 250 260 400 250
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Table 13.16 Comparative costs of processing routes Processing route Loose stock dyeing Continuous tow dyeing Package dyeing with two winds Dyed package delivered on dyepack
Relative cost 40 30-40 100 77
Table 13.17 Features of Fong COS machines [42] Low liquor ratio – down to 5:1 Rapid filling and draining – 1 and 2 minutes respectively for a two-tonne machine High-temperature draining Multifunctional intelligent rinsing without complete draining of machine Preparation tank Pressurised hydro-extraction in dyeing machine giving particularly significant savings in rinsing time Unloading basket to reduce time and labour in unloading
Table 13.18 Percentage saving on COS machine [42] Process Bleaching Dyeing Rinsing
Time 50 44 25
Water 35 37 14
Steam 28 20 -
Electricity 13 28 -
Chemicals 38 38 -
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Figure 13.1 Principle of tape scouring range Top tape return
Bo t
to m
ta pe
re tu rn
Hanks
Intermediate bowl Top tape returning to start
p To
Bo tt om
pe ta
t re
n ur
Bottom tape return ta p
e
re tu r
Feed
n
Delivery
Figure 13.2 Hank mercerising machine
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Figure 13.3 BI-KO centres
Figure 13.4 Continuous relaxation machine
1
2
17
3 4
18
5 6 7
19
8
20
9
21
10
22
11, 12, 13
23
14
24
15
25
16
26
1 2 3 4 5 6 7 8 9 10 11
Ventilator Illumination Automatic tube loading group Package roller Grooved roller Feeding package Feeding needle Yarn tensioner Smoke suction pipe Over-feeding roller Push-button starter for bands and take-up head
12 Stop button 13 Control button for yarn introduction and starting of yarn transport group 14 Tube unloading 15 Unwinding ring 16 Photoelectric cell for correct positioning of the small hank outside the shrinking chambers 17 Electric and electronic cables 18 Package unloading, with transport belt
19 20 21 22 23 24 25 26
Hot air and dust suction duct Rotating distributor Yarn over-feeding group Yarn deposit and transport group Control motor of yarn deposit and transport group Fan control motor of forced air circulation Yarn shrinking chamber Fan for forced air circulation
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Figure 13.5 Package press
Figure 13.6 Hussong hank-dyeing machine Hoist for raising and lowering
Sticks
Suspended hanks Reversible impeller
Open or closed steam pipe
Practical Dyeing, Volume 3 Figure 13.7 Cabinet-type hank-dyeing machine
Figure 13.8 Circular hank frame
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Figure 13.9 Schematic of vertical kier/vertical spindle machine
Figure 13.10 Vertical kier/vertical spindle machine
Practical Dyeing, Volume 3 Figure 13.11 Bellini horizontal kier/vertical spindle machine
Figure 13.12 Obem horizontal individual kier machine
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Figure 13.13 In-line automated hydro-extractor and RF dryer
Figure 13.14 Continuous dyeing machine for narrow fabrics
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Figure 13.15 Yarn dyehouse layout Boilers, engineering, etc.
Despatch warehouse
Exam Final winding and packing
Dyehouse
Service corridor
Ecru warehouse
Relaxing
Holding area
Laboratory and dyehouse offices (2 floors)
Dye and chemical store (2 floors)
Chapter 14 Dyeing of Knitted Fabrics 14.1 Knitgoods Knitting is a highly productive method of fabric manufacture, particularly in comparison with weaving, and knitgoods offer in-built properties of comfort, crease-resistance, stretch and recovery. The fabrics are porous and give good cover, thickness and high insulation because of their loop structure. Weft-knitted fabrics are readily deformed in both length and width directions and have relatively low dimensional stability, since the yarn is inserted under significant tension. Wet processing results in considerable fabric shrinkage and weft-knitted fabrics have a strong tendency to curl at the edges. Yarn breakage can cause ‘ladders’ by the elimination of a long series of knitting loops from the structure of the fabric. Careful handling is thus necessary. Weft-knitting is carried out on flatbed or circular-bed knitting machines, ranging from single-feeder to multi-feeder yarn insertion. The handle and appearance of weft-knitted fabrics depends on several factors, including the fibre type, fineness and spinning method, with combed yarns giving less hairiness than carded yarns. With synthetic-fibre fabrics, the stretch, handle and appearance are governed by the texturing or bulking process used. Compressibility, which affects comfort in wear, increases with fineness of filament yarns and with porosity of the fabric structure. Warp-knitted fabrics are produced by feeding an array of warp yarns from a beam, with each yarn passing through an individual knitting needle. Warp-knitted fabrics are less prone to snagging and deformation. Table 14.1 summarises machines available for producing weft-knitted fabrics and garments, and also lists the knitgoods obtained from each machine type together with the usual dyeing stage employed. Products from flat-bed machines are seldom dyed in fabric form, not least because of their tendency to curl. Much of the production from both flat- and circular-bed knitting machines has already been dyed in either fibre or yarn form as discussed in sections 12.4 and 13.3 respectively. Garment dyeing is important for both types of knitgoods as discussed in section 8.14, with particular reference to wool, and in section 17.1. This chapter is thus mainly concerned with weft-knitted fabrics that are produced in tubular form from circular-bed knitting machines. Warp-knitted fabrics are usually dyed in fabric form. In the past, circular-knitted nylon was important, often produced from textured yarns, using false-twisting, air-texturing or edgecrimping methods. Nylon fabric dyeing has greatly declined, as implied by the statistics quoted in Table 1.1. Fabric dyeing is largely concerned with cotton, polyester and their blends. The importance of fabrics knitted from these fibres has increased with the growing demand for casual-, sports- and leisure-wear fabrics which demand a high comfort factor. The concept of ‘smart casual’ wear in the office has further increased this demand. The need for frequent washing demands high fastness properties.
14.2 Processing Routes for Tubular-Knitted Fabrics An outline of the optional processing sequences for tubular-knitted fabrics is shown in Figure 14.1. Most knitted fabrics are scoured and dyed in tubular form as received from the knitting machine. This allows ballooning of the fabric to occur during rope dyeing processes and assists in level dyeing, in an analogous
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way to that of ‘bagged’ woven fabrics. After dyeing the fabric may be finished in tubular form, often dictated by the end-use. For example, T-shirts are made directly from tubular fabric, this having been knitted to the necessary diameter to give the correct size, allowing for shrinkage and finishing. For sensitive fabrics in tubular form, the fabric may be turned inside out for dyeing. Alternatively, fabric can be slit after dyeing, stentered and finished in open-width. Open-width fabric is essential if destined for mechanical finishing such as brushing and cropping. Open-width fabric or tubular fabric can be used for multi-layer cutting operations. Tubular fabrics may be slit before dyeing, so that heat-setting can be carried out in open width on the stenter at 190 to 210°C for polyester fabrics and up to 200°C for elastomeric blends. Fabrics containing more than 8% of elastomeric yarns, such as Lycra (DuP), are usually slit and heat-set but fabrics containing less than this amount of elastomeric yarn can be processed in tubular form. Knitted fabrics are usually prepared and dyed in rope form, since processing in open width by batchwise, semi- or fully-continuous methods is seldom successful, owing to the dimensional instability of knitted fabrics, which tend to curl at the edges. Lightweight warp-knitted fabrics are beam-dyed but heavier qualities are processed in rope form.
14.3 Processing of Elastomeric Fabrics The experimental production of elastomeric fibres based on segmented polyurethane was first reported in the early 1950s by Bayer. This was followed by DuPont’s introduction in 1962 of the world’s first elastane fibre under the trade name Lycra. Until that time, natural rubber thread was the main elastomeric component used in stretch apparel. Rubber thread had numerous deficiencies. It had to be wrapped with an outer textile covering for protection during processing and was unsatisfactory for many textile applications because of the relatively low retractive force, limited denier range and poor durability to abrasion, heat and oxidative degradation. By overcoming many of these deficiencies, elastanes rapidly displaced rubber thread and greatly broadened the scope for stretch in apparel fabrics, offering improved comfort and better fit than less flexible constructions could provide. Early growth was driven by the replacement of rubber in waistbands and foundation garments. This was followed by expansion into intimate apparel and swimwear, later continuing into hosiery and sportswear. Increased design flexibility and imparting better fit to garments using elastane-containing fabrics have transcended fashion in staying power because they provide functional rather than purely aesthetic benefits [1]. Worldwide production of elastanes in 1997 was 120 kilotons [2], approaching 0.2% of worldwide fibre consumption. The elastane content of stretch fabrics may vary according to end-use, as indicated in Table 14.2. Elastane may be blended with nylon, cotton or polyester. Polyester/elastane blends are particularly critical in process requirements because of the adverse effects of high temperatures and carrier additions on the recovery properties of the elastane component. Elastanes have an elongation at breaking of more than 200% (normally 400 to 800%) and stretchability values in elastane-blended fabrics range from 18 to 25% in outerwear, 35 to 60% in sportswear and 80 to 120% in foundation garments [3].
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By definition, an elastane (or spandex) fibre contains at least 85% of a segmented polyurethane, which is an oriented block copolymer consisting of long flexible chains (soft segments) of low glass-transition temperature linked by tie points (hard segments) to form a crosslinked network. In natural rubber the flexible polyisoprene chains are held together by rigid disulphide bonds. The tie points in elastane copolymers are hydrogen bonds that can be separated and rearranged by heat, permitting elastane-blended fabrics to be heat-set. A breakthrough in polymer chemistry by DuPont in the early 1990s yielded a new class of high-value elastanes known as Soft Stretch. Lycra fibres spun from these polymers exhibit more efficient stretch and recovery properties. In the early 1960s, raschel warp knitting was the dominant fabric-forming route utilising elastane fibres. Bare Lycra was strong enough to withstand the forces of warp knitting, so it did not need covering like rubber and therefore fabrics of much lighter weight could be produced. Since positively driven beams on warpknitting machines became available, accurate delivery of bare spandex to the knitting mechanism became feasible. Processing Lycra was easier because the knitter did not have to cope with the solid powder finish used on the rubber thread to reduce tackiness. Elastane fabrics were whiter and did not have the objectionable odour associated with rubber-containing fabrics. The emergence of finer dtex elastanes in the late 1960s permitted the development of stretch and were suitable for swimwear tricot (Charmeuse) fabrics. These warp-knit constructions provided true two-way stretch and access to swimwear and intimate apparel garments of higher value and better fit. Finer machine gauges and higher knitting speeds have greatly increased the productivity and sheerness of stretch warp-knits, placing heavier demands on elastane toughness. Machine speeds have risen from about 600 courses/minute in the 1960s to over 2500 courses/minute in the 1990s. Traditionally lagging behind warp-knits in the 1970s and 1980s, conventional circular-knits containing elastanes have experienced rapid growth in the 1990s. Circular-knit constructions with continuous filament companion yarns have featured in these developments. Bare elastane knitting techniques with positive feeders, low-friction yarn paths and elastane plating technology have greatly stimulated growth in single- and double-jersey fabrics. The development of horizontal softflow jets for dyeing knitted fabrics has been a key advance in preserving the elastic properties of stretch fabrics during dyeing. All elastanecontaining fabrics undergo a degree of set or ‘lean-out’ when tensioned during dyeing. Softflow low-profile jet machines help to minimise the force of gravity on the fabric and thus preserve more of its inherent power and stretch. Hosiery knitting became a second major fabric-forming route for elastanes in the late 1960s, mainly for the production of pantihose waistbands. Initially these were produced using covered elastane (as for rubber waistbands) but the introduction of direct knitting with bare elastane eliminated the cost of covering and permitted new design flexibility. Pantihose legs are mostly knitted with elastane yarns (single-covered with nylon for protection) in alternate courses. However, there has been a trend in the 1990s towards pantihose in which the elastane is knitted into every course. This produces more attractive hosiery with better fit and counter appeal. For this application DuPont has developed a modified elastane with the high settability in steam that is necessary for boarding [1].
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14.4 Processing of Lace Fabrics Stretch lace is being used increasingly in lingerie and hosiery manufacture. The stretch properties are built into the lace and strict control is necessary to ensure that the desired performance is obtained. Stretch lace such as Helanca, widely used in underwear as well as tights and stockings, has to be processed in such a way that the finished fabric reaches its final target dimensions after having been allowed to relax progressively during treatment [4]. Lace usually has a pattern or motif incorporated into its construction as part of the design. This means that the dimensions of the pattern repeat in both length and width have to be strictly controlled. The reason for this is not merely to produce an aesthetically well-proportioned design but also to guarantee a uniform and controlled set of dimensions that enable the garment maker to fit in the panels accurately. The colour fastness to washing of dyed lace is clearly important since lingerie has to be laundered frequently. Adequate fastness to perspiration and water contact staining must also be considered. Lace can be structurally quite delicate and so its bursting strength must be monitored closely. Shrinkage during laundering can be critical in closely fitting structured underwear. This may be controlled by heat setting of synthetic-fibre lace but different approaches are necessary for cotton lace. When a garment is designed, it is assumed that all the various components will be sewn together to yield a composite entity that will retain its shape and performance throughout its end-use life. The lace designer and the garment maker play important roles to ensure reliable efficiency in processing and subsequent use. Due attention must be given to possible changes in appearance of the lace, such as shrinkage or distortion during dyeing and finishing. Fitness for purpose can only be guaranteed if the required parameters are inherently achievable. The degree of stretch and the tensile strength are directly related to yarn selection and fabric construction. Stretch laces that contain elastomeric fibres tend to be constructed in such a way that the bare elastane is not masked by the other fibres present. For this reason the elastane has to be coloured or otherwise it will ‘grin’ when the other fibres have been dyed. A substantial proportion of lingerie items are dyed black, which tends to aggravate this problem. The designer can help by selecting a clear elastane rather than a fully delustred one that may exhibit poor light fastness when coloured to disguise its presence. The control of colour matching in lace dyeing has been greatly aided by the introduction of instrumental methods. After overcoming the initial problems of accurately measuring substrates that contain a complicated pattern of holes varying in size, shape and depth, highly successful colorimetric systems have been developed [4].
14.5 Fabric Preparation As with all fabric types, adequate preparation is a prerequisite for successful dyeing and finishing of knitted fabrics. Yarns for knitting are seldom sized, in contrast to warps for weaving, and therefore desizing is not part of the process sequence. Mercerising of tubular weft-knitted cotton, with or without a previous singeing operation, is carried out to improve the lustre, handle, elasticity, shrinkage potential, wearing properties and dyeability of the goods. Singeing reduces the hairiness of the fabric and thereby also increases the lustre. The
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appearance, lustre and handle of the mercerised fabric depend on the yarn type (carded or combed) used to knit the fabric, whether singeing is an integral part of the process and on mercerising parameters such as caustic soda concentration, temperature and degree of fabric stretch during the alkaline treatment and washing-off. Machinery has been developed by Dornier [5] for the continuous singeing and mercerising of tubular-knitted fabrics. The SMA singeing machine and the ASM mercerising machine involve passing the fabric over floating circular expanders, usually referred to as ‘cigars’ because of their shape. These units give uniform treatment of the circular-knit fabric in both the length and width directions and allow fabric diameters of 250 to 1200 mm to be processed. The CMB machine enables combined mercerising and bleaching to be carried out. In the singeing machine, eight swivel burners, which can operate on natural gas, propane, butane or liquefied petroleum gas (LPG), are situated around the cigar so that the fabric is totally exposed to the flame. On leaving the cigar, the cloth passes between a rotating drum and belt which extinguishes any residual flames. In the ASM mercerising range, which is shown in Figure 14.2, the fabric is first impregnated with 20% w/w caustic soda solution containing a suitable wetting agent, with a dwell time of 60 seconds to give a liquor uptake of 140 to 180%. Cold mercerising is the usual process and impregnation is carried out at 14 to 18°C. The fabric then passes over a driven circular expander to uniformly stretch the fabric, the degree of stretch determining the lustre level and tensile strength obtained. The fabric is washed with a spray of water at 80°C as it passes over several expanders positioned in series. Processing speed is 25 metres per minute. Hot mercerising may be carried out at 60°C to give greater fibre swelling and better absorbency but lower lustre levels. In a further recent application of tubular expanders, Dornier has produced the Ecofix equipment for heat-setting tubular fabrics [6]. The diameter of the tubular expander is adjustable from 185 to 1050 mm and the obvious advantage is the elimination of slitting for heat-setting on a stenter, followed by possible re-sewing into tubular form for dyeing. In addition to this cost saving, other advantages include elimination of selvedge marks, the ability to process fabrics containing elastomerics, improved fabric handle and dimensional stability with the elimination of crease marks in subsequent wet processes. Fleissner [2] has produced a heat-setting machine for tubular fabrics based on the perforateddrum principle and claims similar advantages. The lubrication of yarn before knitting is important, not only to ensure knitting efficiency but also to eliminate hairiness and to reduce fly generation during knitting. Yarn lubrication has been discussed in depth for staple [7] and filament yarns [8]. Cotton contains significant amounts of heavy metal salts and ‘hardness salts’. These originate from the fertilisers, insecticides and harvesting aids applied to the cotton during growth, often by spraying, to enhance crop yields. The quality of the water used in such spraying operations also influences the metal content of the fibre. The amounts of trace metals present in cotton vary with the geographical location of cultivation, from season to season and with cotton variety [9-12]. Sequestering agents may be added in almost all wet processing baths, both preparation and dyeing, to render these metallic impurities harmless, but choice of the appropriate sequestering agent is critical [9]. A separate demineralising process has been advocated [10], involving alkaline pretreatment in the presence of a wetting agent and a phosphonate sequestering agent, or a strongly acidic
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sequestering formulation containing phosphonic acids. The incorporation of a separate demineralising process in what is already a time-consuming sequence has not been met with enthusiasm. The impurities present in cotton, including hardness salts and waxes, may interact with knitting lubricants and lower the yield of reactive dyeings. This appears to be associated with lubricant type but the effect is not related simply to the ease of removal of the needle oil [13]. Cotton fabrics must be scoured before dyeing to remove natural fats and waxes, together with the spinning lubricants applied to the yarn before knitting. As indicated above, metallic impurities should also be removed by scouring or in a separate demineralising process. Provided the seed content of the cotton is low, scouring may be adequate to prepare knitted fabrics for dyeing to medium depths. Scouring is usually carried out with alkaline detergent at 95°C for 60 minutes, followed by rinsing. Fabric lubricants are also employed to minimise fabric creasing and abrasion. The chemistry of suitable agents and their availability from various suppliers has been reviewed [14]. Although prolonged alkaline scouring at the boil is the long-established traditional method of preparation for cotton knitgoods, the waste liquors produced impose a high chemical oxygen demand on the effluent treatment system. A milder and more economical process involving treatment with 2 g/l Depsolube ACA (ICI Surfactants) for only 10 minutes at the boil and pH 4.5 has proved successful in commercial operation since the 1980s. A feature of this approach is that selected disperse dyes can be applied to the polyester component of polyester/cotton knitgoods in a combined scour-dye system. The success of this process is favoured if only self-emulsifiable knitting lubricants are present [15]. Hickman has advocated the use of acidic bleaching for cotton knitgoods, claiming that the reduced scouring action and consequent retention of natural fats and waxes results in fabrics of softer handle and improved sewability [11]. For pale colours, whites and where there is much cotton seed present, bleaching is also necessary. Bleaching of cotton weft-knitted fabrics has been thoroughly reviewed by Hickman [11]. Various bleaching agents are suitable including sodium chlorite, sodium hypochlorite, hydrogen peroxide, peracetic acid and sodium hydrosulphite, both singly and in combination. Sodium chlorite attacks the stainless steel of processing equipment and chlorine bleaches are now considered to be ecologically unacceptable due to the generation of absorbable organohalogen compounds (AOX). Hydrogen peroxide is greatly preferred for bleaching cotton as it is relatively economical, especially since scouring and bleaching can be combined into a single process. As well as the alkali, detergent, fabric lubricant, sequestering agent and hydrogen peroxide, the bleaching bath contains a stabiliser for the peroxide. For white goods, a suitable fluorescent brightening agent (FBA) can be incorporated into the bleaching bath. Rinsing after peroxide bleaching usually includes a bath containing a peroxide quencher to remove residual oxidant from the fabric so that this does not impair subsequent dyeing performance. Polyester/cotton fabrics are prepared in a similar way to 100% cotton. Syntheticfibre fabrics require only a scouring treatment, usually with alkaline detergent at 60°C, with a suitable FBA applied to whites for sale as such. Spin finishes used on synthetic fibres, particularly elastomerics, are often based on silicones. These must be thoroughly removed using a specially selected detergent. A residue of as little as 0.2% may produce unlevel dyeing with elastomeric blends. Residual
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traces of silicone also result in poor bonding performance and low peel bond strengths in laminating processes. The pretreatment methods used worldwide for knitted fabrics are distributed as shown in Table 14.3. Recent developments in processing conditions for all methods have been discussed [16,17]. A typical knitgoods dyehouse has only a relatively small capacity (in the range two to ten tonnes of fabric per 24 hours) and often forms part of a vertical operation where the main activity is knitting, supported by dyeing and finishing together with a cut, make-up and trim (CMT) facility. Dyeing is often seen by such a company as principally a service operation to give quick response to CMT, where cost is not the main factor. In a relatively small dyehouse, preparation is by batchwise processing at a long liquor ratio, followed by dyeing and washing-off carried out entirely on the batchwise dyeing machinery. This traditional approach results in processing times of approximately nine hours for RFT production, requiring three hours each for preparation, dyeing (e.g. with reactive dyes on cotton) and washing-off, including soaping and soft finish application. The precise time often depends on the filling and draining times of the machines, dictated by the sizes of pipework, drains and valves together with the circulation capabilities of the machine to distribute the baths uniformly. Such limitations were applicable to traditional winch machines with relatively long liquor ratios. Modern knitted-fabric dyehouses are equipped with jet-dyeing machines of one or more types and these are discussed in section 14.6.2. Developments in the dyeing of knitgoods with reactive dyes, using the various systems discussed in Chapter 7, depend on the use of jet machines. Such machines operate at a low or ultra-low (ULLR) liquor ratio, of 5:1 or less, thereby giving significant savings in water, effluent, electricity, steam and chemicals. In the Colorstar (Scholl) machine, cotton interlock or single-jersey fabrics can be dyed at 5:1 LR, woven fabric blends at 4:1 LR and fully-synthetic wovens at 3:1 LR. At these liquor ratios less liquor needs to be heated and cooled, giving savings of water and energy [18]. The exhaustion of many reactive dyes is markedly sensitive to liquor ratio, so that the dyeing of a given shade under such ultra-low conditions provides substantial economies in dyes, chemicals and effluent treatment [19]. The fully-flooded design and gentle mechanical action of the Colorstar makes it applicable to a wide range of sensitive fabric constructions. Controlled relaxation and troublefree running of knitgoods ensure level dyeings in short dyeing times. The considerable capacity but low space requirements of the machine make possible high utilisation of available dyehouse floor space [20]. Pretreatment conditions have been specified [16] in which ecologically acceptable chemicals are employed. These methods produce good absorbency, a good white (Berger value 70 to 75) using an alkaline peroxide process and carry-over of chemicals does not affect the reproducibility of dyeing. A batchwise preparation process for dyeing can be completed in two hours. Controlled application conditions based on the latest optimum dye selection results in a dyeing time of two hours. Combined cooling and rinsing (CCR), achieved by introducing clean, pre-heated water into the main machine circulation and through the jet/overflow nozzle, is a feature of the Smart Rinsing (Thies) procedure in ULLR machines [21]. Rinsing times can be reduced to one hour which contributes to a load-tounload time of five hours for RFT production [16,21].
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The principle of carrying out preparation on specialised equipment away from expensive jets is an attractive proposition [11]. As shown in Table 14.3, some preparation of knitted cotton and polyester/cotton is carried out by cold padbatch techniques [22] but this is limited because of fabric edge-curling and the tendency of the padding process to flatten knitted fabrics with loss of fabric softness. The Delphin I (Brückner) rope processing range developed in 1990 was based on a novel cloth run with short fabric paths ensuring low-tension treatment. Some acrylic or polyester microfibre fabrics, however, did not exhibit the required running performance. The more versatile Delphin II design offers several new features: a higher transportation system, overflow funnel, deflector plate, elimination of creasing or looping, increased fabric capacity and liquor ratio, extended dwell times for bleaching or aftertreatment [23]. The Spray-Flow (Babcock) unit is used for the intensive washing of tensionsensitive and permeable knitted fabrics. The Knit-Sat (Babcock) unit was developed for impregnating knitgoods with bleach liquor in conjunction with the Polykomat (Babcock) multi-component metering unit. The Store-Tex (Babcock) compartment is suitable for wet treatments requiring lengthy dwell times. The Mini-Store (Babcock) short batching unit, which also permits immersion, was developed to optimise aftertreatment processes. Ranges incorporating these modular units can be equipped with a microprocessor-based process control and monitoring system [24]. Hickman [25] has discussed the theory and practice of continuous washing and reviewed relevant methods for both woven and knitted fabrics. In a previous paper [11] he reviewed the continuous preparation of knitted fabrics, including the use of conveyor steamers, J-boxes, spiral winches and spiral jets. Spiral winches as typified by the Brückner Colorado and Jemco machines have probably had the widest application. Processing conditions for fabric preparation on such machines have been defined [11,26]. Serious disadvantages include the major capital expenditure and prodigious output, which is beyond the needs of all but the largest dyehouses. The Jemco III is claimed to produce 900 kg/hour [11] and with conventional cycle times of nine hours (load-to-unload for preparation, dyeing and washing), this machine could supply fabric to 27 jet dyeing machines of 300 kg capacity. If the latest technology is adopted [16], with dyeing times of two hours and preparation [11] and washing-off [25] being carried out continuously, processing in these machines becomes more attractive and costeffective. The cost and productivity advantages of continuous preparation were defined [19]. A spiral winch machine is illustrated in Figure 14.3. A cost comparison [11] of various preparation methods was based on data provided by Jemco, presented in monetary terms for a year’s operation. It must be assumed that the throughput of the individual methods was identical. There was nothing in the original data to indicate purchase cost or depreciation of the capital equipment. In Table 14.4 the component costs are shown as percentages of the total cost for each technique. Although the distribution of costs is similar for most processes, there is a large cost saving offered by the totally continuous Jemco III spiral winch method.
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14.6 Dyeing Machines for Knitted Fabrics 14.6.1 Winch Dyeing Machines The traditional preparation and dyeing of knitted fabrics, particularly cellulosic fabrics, was carried out on winch dyeing machines which were then in widespread use. Many dyehouses still utilise winches but it is unlikely that a new dyeing operation would install them, even if they could be obtained from the diminishing number of machinery makers. A deep-draught winch dyeing machine is illustrated in Figure 14.4a. The winch is one of the earliest fabric dyeing machines and it is still widely used. Several lengths of fabric are run over the winch reel into the liquor and sewn end to end. During dyeing the ropes of fabric run over the winch reel and collect in a relaxed state in the bottom of the vessel. They are then drawn through the peg rail to prevent entanglement and over the jockey roller back to the main reel. The dyebath is heated by the steam pipe situated behind the perforated partition, in the so-called ‘stuffer box’. Modern machines are constructed in stainless steel and hooded. Time/temperature controllers are a basic requirement. In winch design, many of the dimensions are critical to obtain level and crease-free dyed material. These include the distance and the slope of the fabric between the main reel and the jockey roller and the distance between the jockey roller and the dyebath. Developments to improve levelness include elliptical or variable-geometry reels, shallow-draught winches (Figure 14.4b), driven jockey rollers, lowering of the main reel and pumps to aid liquor circulation. Fabric speed is usually about one revolution of the rope around the reel per minute (about one metre per second). A device known as the Autoloda to improve levelling by loading one continuous length of fabric into the machine is illustrated in use on a shallow-draught winch in Figure 14.5. Winches operate at relatively long liquor ratios (15:1 to 25:1) and atmospheric pressure, the dyebath temperature seldom exceeding 95°C. The Dynawash (Fong’s/Henriksen) inflating dyeing machine offers the typical characteristics of conventional winch dyeing. However, it is capable of operating with the most delicate knitted fabrics to give outstanding quality and levelness, avoiding pilling and crease marks. Jet nozzles are avoided and the circulated dye liquor is sprayed onto the goods at four different levels. A ballooning device creates an air pocket of consistent size within the fabric rope. This gentle treatment eliminates abrasion of the fabric surface, minimises foaming and ensures uniform distribution of the dye liquor. A rubber-coated squeeze roller removes contaminated water at the rinsing stage to prevent mixing with clean water, giving more efficient rinsing and a substantial reduction in water consumption [27]. 14.6.2 Jet Dyeing Machines From 1971 onwards there was rapid development of jet dyeing machines, although the original patents date back to 1958 and the first production Gaston County machine appeared at ITMA in 1967. Jet dyeing machines were initially developed for processing fabric in rope form at high temperatures, particularly to overcome the problems of dyeing textured polyester in conventional or hightemperature winches. The history of jet machine development has been thoroughly documented [28-34]. Despite many developments and the construction of machines designed for certain types of fabric, the basic principle
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of the jet dyeing machine remains unchanged and is shown diagrammatically in Figure 14.6. The design of this type of machine depends on a high-speed jet of liquor to transport the fabric rope through the machine. Rope speeds of up to 300 metres per minute are possible. A doffing jet plaits the fabric into the dyebath at the back of the machine and a set of power-driven metering rollers is situated at the front of the machine to lift the rope from the top of the fabric pile, controlling the rate at which it enters the jet tube. Tangling and creasing of the fabric may occur in the dyebath and the action of the machine is too vigorous for delicate fabrics. These problems were ameliorated by the development of fully-flooded machines, which also eliminated foaming problems. A typical machine of this type was the Thies Jet Stream which is illustrated diagrammatically in Figure 14.7. The fabric rope is moved through the liquor by means of a high-speed jet venturi into a flooded tube that opens out into a much wider storage compartment, in which both the liquor and fabric move slowly until the fabric is drawn through a narrower tube and taken back into the jet. Dye liquor is drawn off from the system at both ends of the storage section and passed through a heat exchanger before being fed back to the jet. Lightweight fabrics require smaller jet orifices and replaceable or adjustable jet sizes are a standard feature of modern jet machines. The so-called softflow machines were developed particularly for processing delicate lightweight fabrics without distortion or undesirable changes in surface appearance [35]. The fabric is supported by almost complete immersion in the liquor and the vigorous action of the jet is replaced by a power-driven cylindrical reel which lifts the fabric briefly from the dyebath. The fabric is then carried forward by a relatively gentle liquor flow. The transport tube is filled with liquor using an overflow system and the fabric and liquor movement along the tube depends on gravity. Atmospheric and high-temperature versions of this machine type are available and the transportation tube is usually outside the main dyebath, either above or below the machine. A typical machine of this type is illustrated in Figure 14.8. Spurred by the energy crises of the 1970s, jet machines were developed to enable processing at low liquor ratios (as low as 5:1), thereby achieving savings in water, effluent, energy and chemicals. Typical of this type of machine is the Thies Roto Stream, shown diagrammatically in Figure 14.9, in which the fabric is plaited down by a jet of dye liquor and then slides down the perforated lining at the back of the vessel. The fabric is lifted at the front of the machine by a cylindrical power-driven reel and guided into the jet. Fabric speeds of up to 400 metres per minute can be achieved. By 1979 it was possible to identify jet dyeing machines of five basic types [36]: 1. partially or fully-flooded machines with hydraulic transport by means of a venturi nozzle with a high-velocity flow 2. machines with a driven winch reel and jet nozzle 3. overflow machines with a combination of hydraulic transport and a driven winch reel, usually partially flooded with a gentle action 4. combined overflow and jet nozzle machines with a driven reel, usually partially flooded (Thenflow: see Figure 14.10) 5. machines using some form of mechanical conveyance to assist fabric transport in addition to a winch reel or jet/overflow system. Carousel machines with conveyor belts were a common approach.
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There are three main types of drive system in piece dyeing machines: winch drive only (now rare), air-jet drive only (gaining an important market share) and winch drive with jet assistance. Calculation of the necessary forces and capacities shows that at high fabric speeds the lifting height between storage compartment and drive is of secondary relevance. The acceleration force is considerably higher and is the determining factor for fabric transport. The limitations on winch drive mechanisms at high fabric speeds have been quantified [37]. Air-only drive, as exemplified by the Aero-Dye (Krantz) machine, is capable of transporting heavyweight fabric ropes reliably and quickly, with only minimal risk of abrasion and consistent fold deposition. Jet dyeing is now a mature technology and is the leading method of batchwise dyeing in rope form, certainly in terms of number of dyelots if not metreage. By ITMA 1995, there were at least 43 manufacturers providing over 100 variations of jet dyeing machines, ignoring size variations [38]. The selection of an appropriate machine for a given task has now reached nightmare proportions, in spite of the gradual disappearance of many machinery manufacturers. Jet machines are amenable to a high level of control and automation. The factors discussed in Chapter 4 are applicable. Each dyehouse with its existing equipment has to exert considerable ingenuity to develop methods of processing new fabric qualities presented to it, since the option to install new machinery for a specific quality seldom exists. Machine selection has perhaps eased in that two main types of jet dyeing systems may be categorised, depending on the design approach of the vessel: 1. elongated machines with a long shallow bath, giving minimum fabric creasing and a gentle action, with most machines operating at liquor ratios of 8:1 to 14:1 and rope speeds of 600 metres per minute, 2. compact (J-box related) machines with relatively dense fabric packing, running at relatively short liquor ratios (about 3:1 to 10:1) and fabric speeds of 350 metres per minute for liquor-driven machines or up to 1000 metres per minute for air-flow machines. These machines have been designated [39] as ‘banana’ and ‘apple’ configurations. Banana machines are generally suitable for woven and knitted synthetic-fibre fabrics, whereas apple machines are preferred for woven and knitted cellulosics. The apple machines, with short-liquor capabilities, special lifter reel, jet nozzle design and employing hydraulic (water) and/or pneumatic (air) fabric drives are appropriate for high-added-value goods such as viscose, polynosic, lyocell, polyester or nylon microfibres and elastane-containing stretch fabrics. These two machine configurations are recognisable in the early machines illustrated in this chapter. Design developments have been evolutionary rather than revolutionary [40]. Many improvements are concerned with water, energy and time savings, increasing machine loadings, reducing dyeing cycles while improving the quality, levelness or appearance attainable on specific fabrics [41,42]. Such developments have included: 1. ease of changing jet nozzles or variable jet adjustment, allied to machine control 2. self-cleaning filters for the removal of lint 3. automatic untangling devices 4. spray systems to give more efficient machine cleaning
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5. semi-automated salt dissolving and dispensing 6. a major development has been the use of air, either in place of or in conjunction with liquor to drive the fabric rope, giving effective liquor ratios as low as 3:1 7. synchronised dyeing control for flow rate, jet nozzle pressure and winch speed in conjunction with automatic untangling, automatic jet nozzle variability, self-cleaning filters (Thies Ecosoft Plus) 8. twin-jet systems (Sclavos Apollon and Venus machines) [43] 9. full-size tanks to prepare the liquor for the next stage of the process 10. single-rope multi-bath machine (MCS) and processes based on the number of fabric circulations in unit time rather than process duration alone. It is claimed to give better reproducibility between batches, especially for fabric appearance. Controlled overflow rinsing (COR) is a further aid to reducing process times and improving fastness [44,45] and forms part of the Smart Rinsing (Thies) procedure in ULLR machines as mentioned earlier [16,21]. Conventional wisdom was originally to run fabric ropes at a speed of one complete revolution through the jet venturi every minute, limiting the rope lengths to about 100 metres. In a modern jet machine, longer rope lengths, higher speeds and longer turn-round times are possible while achieving level results [38]. For example, a lightweight narrow fabric of 100 g/m2 and a rope length of 1750 metres can be run at 500 m/min on a modern jet, giving a rope turn-round time of 3.5 minutes. The performance of the machine can be altered by varying the jet pressure and the degree of assistance obtained from the reel. With liquor flow as the driving force, there is an upper limit to the speed at which the fabric can circulate. With air flow driving the fabric, the rope speed can be controlled to achieve a greater range of effects with a given fabric. On modern machines, a complete preparation, reactive dyeing and washing-off sequence can be completed in 4.5 hours, assisted by a heating rate of 4°C per minute and a cooling rate of 5.5°C per minute. From a mechanical viewpoint the factors in jet dyeing that influence fabric quality, as assessed by level dyeing and crease-free fabric, include: 1. length of fabric rope and its passage time through the machine, as influenced by reel speed 2. liquor ratio 3. liquor movement as influenced by the pump speed 4. jet nozzle size. Machines of less sophisticated design (and usually lower cost) often have a fixed pump speed. Thies has given guidance [46] on the optimum settings of the above factors. The mathematical relationships in bath-change and continuous methods of rinsing in jet machines have been evaluated [47]. The critical parameter in bath-change rinsing is the dilution factor, i.e. the carry-over as a fraction of the total bath volume. In continuous rinsing the dyebath is drained to the rinsing volume and then rinsing proceeds with a defined quantity of fresh water flowing into the vessel. Knowledge of the liquor exchange factor is essential for the evaluation of continuous rinsing. The optimum rinsing principle depends on the machine parameters and the cost structure of the plant. In general, a combined process is
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the most economical: bath-change rinsing at the start (salt exchange), continuous rinsing at higher temperature (diffusion of dye hydrolysate), then bath-change rinsing at the end (hydrolysate exchange) [47]. 14.6.3 Beam Dyeing Machines Warp-knitted fabrics, especially lightweight fabrics, can be dyed on a beam dyeing machine, of which both atmospheric and high-temperature versions are available. This is in effect a form of package dyeing and the fabric must be wound carefully on to a perforated beam which is loaded into the dyeing vessel. This method avoids the creasing associated with rope-dyeing methods. Uniform fabric density must be achieved during winding so that the loaded beam has adequate porosity and satisfactory dimensional stability to withstand the flow pressure. Beam dyeing is discussed in section 15.5.2.
14.7 Dye Application Application methods for the dye/fibre combinations encountered in knitted-fabric dyeing have been discussed in the Volume 2 chapters. However, further comments are necessary on the dyeing of cellulosic fabrics. These are dyed principally with direct or reactive dyes. Knitted-fabric dyers seldom utilise vat or sulphur dyes, although jet dyeing machines have been developed for the application of these dye classes [39]. Contamination of the internal surfaces of the machine by the reoxidised pigment forms of these dyes can be a troublesome problem. The Then Airflow AFS aerodynamic jet machine offers improved dyeing quality, significant savings in dye consumption, up to 40% increase in production, up to 50% reduction in water, chemicals and steam consumption, improved repeatability, reduction in effluent and lower installation costs due to its modular construction [48]. This machine is especially suitable for the processing of lightweight fashion knitgoods with a soft silk-like handle made from regenerated cellulosics, polyester or their blends, as well as circular-knitted cotton or cotton/polyester fabrics. Emphasis is given to lyocell, polynosic and cupro types of regenerated cellulosics that show enhanced potential for controlled surface fibrillation to achieve opalescent peachskin effects. The Then Airflow design offers the facility of fabric tumbling at 140°C in the dry or moist condition, as well as a special Spectra-dyeing function for multicoloured rainbow-dyed effects [49]. Extensive dyeing trials with vat dyes on knitted and woven cotton fabrics using a Then Airflow AFS 225 machine have been reported [50]. The suitability of the aerodynamic system for vat dyeing was confirmed, in spite of the extremely low liquor ratio during dyeing and aftertreatment. It was demonstrated that the air in the processing chamber is saturated with water vapour, which behaves essentially as an inert gas. There is no significant difference in temperature between the vapour stream and the surface of the fabric. The existence of this state of equilibrium and the continual exchange of liquor through the injection system ensure uniform absorption of the vat leuco dye by the substrate [50]. The critical phase of the application of vat dyes to knitted fabrics in rope form is the aftertreatment process, during the transition from reducing to oxidising conditions. Excessively brief or excessively prolonged treatment can both cause problems. There is a pronounced lack of suitable control parameters for this process, because conductivity and redox measurements have shown serious
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limitations under these conditions. However, it has been found that dithionite anions have satisfactory stability when air is excluded, as in the closed system of a flooded jet machine. By means of a titration procedure the dithionite concentration of the dyebath at the end of the vatting stage can be predicted and set at a desired level. This target concentration has proved to be a useful control parameter by which the aftertreatment process can be optimised [51]. Several commercial ranges of reactive dyes based on various reactive systems are used in practice to dye cellulosic fabrics [52]. The cost of dyeing a particular colour is not related merely to the costs of the individual dyes and their proportions in the recipes. The application conditions for the various dye ranges are significantly different and the real cost of dyeing is influenced by the chemical concentrations required, the number of baths and the dye-cycle profile. The costeffectiveness of the process is seriously impaired if shading and reprocessing is required. The cost-effectiveness of different dyeing machines, for example winches and jets, is dependent on factors including depth of shade, machine occupancy and shift pattern. It may not be economically sensible to replace an unsophisticated but still viable winch by a costly jet machine unless the available workload and technological control systems can justify this decision by ensuring that the new machine is kept fully and effectively occupied. It has already been noted that changes in fabric qualities do not justify investment in new machinery. All capital expenditure in this area has to be justified by feasibility studies. The relevant options in preparation and washing-off, for example jet processing compared with continuous machines, have already been discussed.
14.8 Drying and Finishing Following dyeing, the fabric is removed from the dyeing machine and the bulk of the water can be removed by rotary hydro-extraction. Circular-knitted fabrics are usually dried over a perforated heated tube as in Figure 14.11. However, it is more convenient and cost-effective to carry out this part of the finishing operation by a semi-continuous method. The methods employed are as follows: 1. fabric which has been slit and dyed in open width is hydro-extracted by passing over a vacuum slot followed by stentering continuously 2. tubular fabric is untwisted, suctioned, followed by slitting and stentering 3. tubular fabric is untwisted, suctioned, spread into open-width and then dried and compacted as a tubular fabric. This complete operation can be carried out in-line continuously. Finishes and FBAs can be applied by padding either on the stenter or on the continuous in-line range. Figure 14.12 illustrates a tubular fabric finishing range. The uniform application of a fabric finish of the correct type is an essential part of this process, by either padding at the entry end of the stenter or at the suction stage of the in-line process. Such finishes are necessary to achieve the desired handle and to confer good sewability in the fabric during cut, make-up and trim operations. It is important to dissipate the heat transferred to the fabric from the high temperature attained by the needle in high-speed sewing processes. Sewing damage can be minimised by using softeners formulated as micro-emulsions. Their efficiency is controlled by substantivity, cationic charge strength, chemical structure and pH of the formulation applied [53].
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As already noted, shrinkage is a serious problem with knitted fabrics. Even a target shrinkage of 5% maximum in both length and width directions is difficult to achieve. Slit (open-width) fabrics are stentered with overfeed and without undue stretching, then often Sanforised to reduce potential shrinkage. Both open-width and tubular fabrics are dried on what is effectively a continuous open-width ‘tumbler’ dryer in which the fabric passes through the machine in a relaxed state on a continuous belt and air is blown from jets through the fabric. Both types of fabric can then be subjected to compacting in which the fabric is overfed and steamed. It is claimed [5] that tubular, mercerised and compacted knitted fabric can have a shrinkage as low as 1%. Shrinkage control is based on the following factors [54,55]: 1. correct knitted construction is essential 2. excessive tensions should be eliminated during processing (from grey inspection to finishing) 3. untwisting and extraction to below 65% moisture content 4. padding the correct softener on to the fabric 5. spreading with overfeed and pre-drying to approximately 30% moisture content 6. step by step shrinkage reduction. Computer programs have been developed [56] in an attempt to reliably predict the shrinkage and dimensional properties of finished, knitted cotton fabrics using a database of processed fabrics of known construction, processing sequences and performance. This was given the name Starfish – start as you mean to finish! Dry finishing processes are usually restricted to brushing (particularly for fleece fabrics) and cropping. These processes and their associated machinery have been discussed in section 8.13.2.
References [1]
J E Bollek, Chem. Fibers Internat., 50 (2000) 154.
[2]
J Heller, AATCC Rev., 1 (Nov 2001) 32.
[3]
H Hüber, Melliand Textilber, 79 (1998) 243.
[4]
B Condon, JSDC, 108 (1992) 52.
[5]
G Euscher, Dyer, 182 (Jul 1997) 25.
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G Euscher, Dyer,186 (Nov 2001) 25.
[7]
Anon, Hatra Research Reports, 22 (Feb 1972) and 23 (Aug 1972).
[8]
Anon, Hatra Research Report, 24, (Apr 1973).
[9]
S Charlton, Dyer, 184 (Oct 1999) 27.
[10]
H Behnke, Colourage, 43 (Sep 1996) 27.
[11]
W S Hickman, Rev. Prog. Coloration, 26 (1996) 29.
[12]
P S Collishaw, B Glover and M J Bradbury, JSDC, 108 (1992) 13.
[13]
M S Elliott and D Whittlestone, JSDC, 110 (1994) 266.
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J Barton, Dyer, 186 (Jul 2001) 7.
[15]
S McCaffrey and G K Santokhi, JSDC, 115 (1999) 167.
[16]
D Angstmann and M J Bradbury, Dyer, 183 (Mar 1998) 11; Melliand Textilber., 79 (1998) 755.
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F Schleger, Dyer, 186 (Jan 2001) 17.
[18]
Anon, Textile Month, (Jan 1991) 42.
[19]
P S Collishaw and D T Parkes, JSDC, 105 (1989) 201.
[20]
E Brenner, Melliand Textilber., 72 (1991) 757.
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[21]
M J Bradbury, P S Collishaw and S Moorhouse, JSDC, 116 (2000) 144.
[22]
W S Hickman and R J Chisholm, Australian Text., 13 (Mar/Apr 1993) 24.
[23]
G Kolmer, Melliand Textilber., 79 (1998) 848.
[24]
K Meyer, Melliand Textilber., 79 (1998) 538.
[25]
W S Hickman, Rev. Prog. Coloration, 28 (1998) 39..
[26]
W S Hickman in Cellulosics Dyeing, Ed. J Shore (Bradford: SDC,1995), 81.
[27]
Anon, Melliand Textilber., 77 (1996) 328.
[28]
M D Paterson, Rev. Prog. Coloration, 4 (1973) 80.
[29]
J D Radcliffe, Rev. Prog. Coloration, 9 (1978) 58.
[30]
W J Marshall, Shirley Institute Publication S33 (1979).
[31]
V Simborowski, JSDC, 96 (1980) 111.
[32]
J D Radcliffe, JSDC, 96 (1980) 94.
[33]
D H Wyles in Engineering in Textile Coloration, Ed. C Duckworth (Bradford: SDC, 1983) 1.
[34]
H Quas, Internat. Text. Bull., Dyeing/Printing/Finishing, 36 No.3 (1990) 55.
[35]
I Holme, Wool Record, 157 (Oct 1998) 55.
[36]
R R D Holt and F Harrigan, IWS/CSIRO Publication (1979).
[37]
B Böhnke, Melliand Textilber., 79 (1998) 346.
[38]
M White, Rev. Prog. Coloration, 28 (1998) 80.
[39]
M J Bradbury and J A Bone, BASF ITMA 1999 Review.
[40]
Anon, Dyer, 179 (Nov 1994) 10.
[41]
Anon, Dyer, 182 (Nov 1997) 17.
[42]
A Dayal, Colourage, 44 (Dec 1997) 55.
[43]
A C Welham, Canadian Text. J., 111 (Oct/Nov 1994) 32; Colourage Annual, 42 (1995) 47.
[44]
Anon, Dyer, 182 (May 1997) 12.
[45]
J H Heetjans, Dyer, 181 (Nov 1996) 14.
[46]
A handbook for the piece dyer, P Schomakers, S Sutcliffe and R Tindall (Thies, 1996).
[47]
F Hoffmann, K Siedow, M Woydt and J H Heetjans, Melliand Textilber., 77 (1996) 852.
[48]
Anon, Chemiefasern, 43/95 (1993) 416.
[49]
R Adrion, Textilveredlung, 32 (Jan/Feb 1997) 16.
[50]
K Beck, W Christ, H Dörfer and B Stetter, Melliand Textilber., 80 (1999) 60.
[51]
N Klose and W Schindler, Melliand Textilber, 76 (1995) 430.
83
[52]
J Park and J Shore, JSDC, 102 (1986) 90.
[53]
K Poppenwimmer, Textilveredlung, 26 (1991) 119.
[54]
B W Gordon, D L Bailey, B W Jones, R L Stone and R D Noell, Text. Chem. Colorist., 16 (1984)
[55]
R M Tyndall, D L Bailey and L K Tuck, AATCC Internat. Conf,. and Exhib., (1989).
232. [56]
S A Heap, P F Greenwood, R D Leah, J T Eaton, J S Stevens and P Keher, Text. Res. J., 53 (1983) 109.
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Table 14.1 Weft-knitting machines and knitgoods Machine type Flat-bed knitwear blank machines
Fully-fashioned flatbed machines Circular-bed hose machines Half-hose circular-bed machines Circular-bed knitwear blank machines (capable of making a welt) Circular-bed fabric machines
Typical knitgoods Jumpers, pullovers Cardigans, dresses, suits, trouser suits, fabric rolls Trimmings (cuffs, collars) Fully-fashioned hose, jumpers, sports shirts, underwear Seamfree hose, tights, fabric for knit-deknit processes Socks, stockings and tights
Dyeing stage Garment Pre-dyed
Jumpers, pullovers, cardigans, dresses, suits, trouser suits, vests, panties, briefs Rolls of fabric, jackets, tunic tops, sports and T-shirts, suits and dresses, swimwear, underwear. Furnishing and upholstery fabrics
Pre-dyed or garment
Table 14.2 Typical elastane contents [2] Fabric type Woven fabrics Underwear Swimwear and sportswear Stockings Foundation garments Medical hose
Elastane (%) 2-8 2-5 12-20 2-12 10-45 35-50
Table 14.3 Pretreatment methods for knitted fabrics [16] Method Batchwise (long liquor) Continuous (pad-steam) Semi-continuous (pad-batch)
Production (%) 60 35 5
Pre-dyed or with fabric Pre-dyed or garment
Pre-dyed or garment Pre-dyed or garment
Fabric dyed, trimmings knitted on flat- bed machines and fabric-dyed Pre-dyed for colourknitted designs
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Table 14.4 Cost comparison of preparation methods [11]
Machine Winch LR 20:1 Jet LR 10:1 Argathen spiral jet Jumbo jet LR 5:1 Pad-batch, rope wash Jemco III
Chemicals (%) 60 56 60 62 45 60
Labour (%) 19.5 23 22 25 30 22
Resources (%) 20.5 21 18 13 25 18
Relative total cost 100 86 85 59 72 31
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Figure 14.1 Optional processing sequences for tubular-knitted fabric Tubular fabric from knitting machine
Turn (optional) Scour
Slit
Dye in tubular form
Heat-set (optional)
Slit
Tubular dry
Dye in open width
Stenter dry
Finish
Stenter dry
Finish
Finish
Figure 14.2 Dornier ASM mercerising system for circular-knitted fabric
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Figure 14.3 Jemco spiral winch continuous preparation machine 4
1
2
1 2 3 4 5 6 7
5
3
Cradle Sewing scray Control panel Dry J-box Bleach kier Jet washer Folder
6 7
Figure 14.4 Winch dyeing machines
Window Winch reel Window
Winch reel
Jockey
Jockey
Dyebath
Steam coil Steam pipe
Gate
Dyebath
(a) Deep-draught winch
(b) Shallow-draught winch
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Figure 14.5 Shallow-draught winch with Autoloda
Figure 14.6 Principle of jet dyeing in the original Gaston County design
Jet
Metering rolls
Heat exchanger
Pump
Loading port
Dyeing vessel
Drain
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Figure 14.7 Diagram of Thies Jet Stream
Loading port Jet
Heat exchanger
Dyeing vessel
Pump unit Drain
Figure 14.8 ATYC Rapidsuau softflow machine
90 Figure 14.9 Thies Roto Stream
Figure 14.10 Thenflow jet machine
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Practical Dyeing, Volume 3 Figure 14.11 Tubular dryer
Calendar bowls
Stretching device
Vertical tube
Pile of tubular fabric
Hot air
Figure 14.12 Santex range for tubular fabric finishing
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Chapter 15 Preparation and Dyeing of Woven Fabrics 15.1 The Weaving Process The handicraft of weaving has been practised for thousands of years and the mechanised production of woven fabrics in textile mills for more than two centuries. Weaving is the transformation of yarns into a fabric on a loom that interlaces two sets of threads; the array of size-reinforced warp threads passes from a flanged roller or warp beam. Lease rods separate the yarns to ensure smooth operation and prevent entanglements. A harness lifts some yarns and depresses others to form a gap known as the shed. The shuttle containing the weft yarn is propelled through the gap, traditionally mechanically or by water-jet (hydraulic) or compressed-air (pneumatic) action, crossing the fabric width from selvedge to selvedge. The process of weaving consists of five movements: 1. shedding, vertical separation of the sets of warp threads to allow the shuttle to pass through the shed between them 2. picking, in which the shuttle carrying the weft is projected between the separated sets of warp yarns 3. beating-up, a consolidating motion that pushes the weft thread into the fell of the fabric against the previously inserted series of wefts 4. letting-off, release by the warp beam of a sufficient length of the array of warp threads to enable the next picking to take place 5. taking-up, in which the consolidated length of newly formed woven fabric is gradually wound onto the cloth beam. The rate of take-up controls the number of picks or weft threads per inch. Steps (1) to (3) are the primary movements and these proceed consecutively, whereas the secondary motions (4) and (5) take place simultaneously. Automatic controls ensure that the entire sequence is extremely rapid. The motion of the loom is stopped instantly if a thread breaks and the shuttle is automatically replaced when empty. The buildings and services required in a modern weaving plant have been discussed together with the processes of winding, warping, sizing and weaving [1]. The weaving section is mainly concerned with automatic and shuttleless machines. Shuttleless looms are usually considered in four categories: rapier weaving, projectile looms, water-jet and air-jet machines. Air-jet weft insertion has been reviewed [2].
15.2 Typical Woven Fabric Constructions Most woven fabric constructions are based on three basic weaves and their variations: plain, twill and satin. They may be combined or grouped to form highly ornate designs. There is a huge number of different types of woven fabric; Table 15.1 lists some of the descriptive terms for characteristic qualities, others are mentioned in the following paragraphs. Some of these terms, marked with an asterisk, derive from the town or region from which the fabric originated, often in France (Cambrai, Creton, de Nimes, Laon, Tulle), North Africa (Fostat, Maroc) or Asia (Bokhara, Calicut, Damascus, 92
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Mosul, Shantung). It is necessary to give the edges of a woven fabric a stronger and firmer construction than the body of the cloth to avoid fraying; these are called selvedges. It is often sufficient to incorporate 15 to 30 warp threads at each side to form the selvedge. Two-fold warps or yarns of coarse count may be employed but it is also possible to gather normal single warp ends together two or three at a time. Selvedge design is important because it contributes to fabric dimensional stability and a faulty or inadequate selvedge can increase the incidence of running marks. Plain woven cloth has the maximum number of intersections and is woven on the principle of alternate threads being shed up and down. A square plain is a weave in which identical yarns of the same count are found in both warp and weft, as well as the same number of warp ends and weft picks per inch. About 70% of woven fabrics are based on the plain weave and numerous qualities are possible by varying the types of warp and weft yarns and the number of threads per inch. Cord stripes can be incorporated by introducing folded yarns or single threads gathered together to lift as one during shedding. The poplin weave, particularly important for cotton and polyester/cotton shirting, has more warp ends per inch and the weft yarn is relatively coarser; this produces a fine rib across the fabric. The term poplin (popeline in French) relates to the former papal city of Avignon, where the cloth was originally woven. In repp fabric, often selected for curtains and upholstery, alternate coarse and fine picks interlace with alternate coarse and fine ends so that the coarse/coarse crossover points produce a corded surface effect. Ribbed effects are produced in the hopsack weave by lifting two or more warp ends simultaneously and inserting two or more picks into the same shed. Weaving becomes very difficult when warp or weft yarns are separated by spaces slightly less than their own diameters. The degree of cover provided by woven cotton fabrics can be compared in terms of the cover factor (Equation 15.1): Cover factor
K = t/N1/2
Equation 15.1
where t is the number of threads per inch and N the yarn count. A continuous sheet of closely packed yarn ends of circular cross-section would have a theoretical warp cover factor of 28 but in such a fabric the weaving action would jam when the weft cover factor reached 14. Closely woven constructions, such as poplin, canvas (a strong coarse cloth made from flax or hemp for sails, tents and paintings) or duck (a strong linen or cotton cloth for sails and sailing garments), have cover factors that exceed 14. Exceptionally lightweight fabrics such as cheesecloth (a thin loosely woven cotton originally used to wrap cheeses) or voile (a diaphanous material intended for veils and other semi-transparent dresswear) have cover factors of 8 or lower. Interlacing of warp and weft yarns in the twill weave is designed to form diagonal lines across the fabric. These are produced by each successive warp end advancing upwards or downwards one pick for each pick in the repeat of the pattern; this results in short floats and a decreased number of intersections. The 2/2 twill weave (two ends up followed by two down) is most often encountered, being used in serge (a durable worsted construction for suitings or uniforms), sheetings and Harvard shirtings, the diagonal line running from left to right at 45 degrees. The angle of the twill line may be varied, either by increasing the number of ends per inch or by stepping more than one thread at a time in the repeat. Gabardine (a smooth and durable worsted or cotton cloth) is an example
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of a twill weave with a twill line running steeper than 45 degrees. Pointed twills are produced by reversing the direction of the diagonal line at a specific distance; the herringbone twill changes from a warp line to a weft line at the reversal. The satin structure is characterised by the exceptionally smooth surface of the fabric; this is brought about by binding the floating threads into the cloth in such a way that twill or rib lines are excluded. Strictly, the term satin relates to warpfaced qualities and weft-faced structures are called sateens. Construction of these weaves depends on the rule that the number of threads in the repeat design have no common factors. Typically these are small primes, such as 3 and 5, or 3 and 7. Traditional satin-stripe curtaining is woven with alternating bands of plain and satin weaves. Attractive and ornate effects can be achieved by alternating areas of satin and sateen weaves. The two main classes of pile fabrics are velvet, a closely woven structure with a thick short warp pile on one side, and velveteen, which is similar but has a weft pile. In the velveteen weave, there is a ground weave or foundation, generally plain or twill, from which numerous extra weft floats project. This weft pile interlacing usually follows a twill or sateen pattern. After the velveteen has been woven, the floats on the surface are cut and brushed upright to form the tufts on the fabric face. Corduroy is a further development of velveteen in which the weft pile yarns interlace with typically three ends of warp and then float; hence when the pile floats are cut a corded effect is produced. The fact that a warp pile is needed for the velvet weave necessitates the use of two beams in a velvet loom, one for the ground warp and the other for the pile warp. As the pile warp is raised on shedding, a wire of appropriate dimensions is inserted. The pile warp is interlaced with the ground weft to form a series of floats or loops that are cut by a knife attached to the wire as this is withdrawn from the shed. Velvets may also be woven in the form of a double fabric, face to face, with the warp pile between; cutting of the pile thus produces two cloths simultaneously. Uncut velveteens or corduroys have been widely used as durable cloths with a soft nap on one side, represented by fustians, swansdowns and others. There is also an important demand for warp pile fabrics with uncut loops, which are not merely uncut velvets. On the contrary, these form the versatile terry weave, which has been consistently popular for towelling fabrics. Two warps are required, the ground warp being woven under considerable tension whereas the pile warp is held under only minimal tension.
15.3 Handling and Processing of Woven Fabrics The choice of handling techniques for woven fabrics is strongly influenced by their structural and flexural properties, which determine the practicality of processing the cloth in rope form. Rope processing normally generates a softer, fuller handle than can be attained in open-width processes. It is preferred for those classes of woven materials that are less prone to thread slippage or the formation of durable running creases. More delicate or crease-prone constructions must be dealt with in open-width form under controlled tension on jigs, beams, batching units and other suitable equipment. Most staple-fibre goods and some resilient constructions loosely woven from filament yarns can be successfully processed in rope form. Densely woven goods with high cover factors, most filament fabrics
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and many polyester/cellulosic blends should be maintained in open width throughout the process sequence. Although fabrics are invariably produced and used in open width, they are most easily processed and piled down for temporary storage in rope form. Threading of the fabric rope through machinery units is simple and changes of direction are readily made via annular ceramic apertures known as pot-eyes. The traditional cotton preparation sequence was based on rope processing, allowing convenient storage in kier or cistern for prolonged reaction times. A typical tight- or slackrope washing unit is capable of giving the fabric several dip-squeeze sequences within a compact space. These conditions are particularly suitable for washing out residual chemicals and other contaminants. A high running speed ensured good productivity at low capital cost. Semi-continuous and continuous stages were eventually introduced into the rope preparation sector. Thus the J-box was developed from the Gantt piler and Tensitrol machine (Figure 15.1) provided tension-controlled washing and impregnation conditions. The driven roller in this unit rotates more quickly than the exit nip to provide continual overdrive. When contraction of the rope of fabric begins the traction on the driven roller increases and the overdrive margin decreases, so that the upward speed of the moving strands increases. Conversely, if the fabric rope begins to extend there is less traction on the driven roller and overdrive increases, with the result that the upward speed slows down. Therefore the running fabric in the Tensitrol section never suffers excessive tension and the exit nip discharges the fabric rope at a uniform speed. For drying and further open-width processing, a rope of fabric must be opened out to full width. The procedure, known as scutching, requires the application of an automatic scutcher. The method depends on the fact that however many twists the fabric has acquired in one direction since it was first gathered together as a rope, there must be somewhere along its length an equal number of twists in the opposite direction. Strong, resilient cloths can be scutched using a rapidly rotating beater device. The rope is drawn out along a free trajectory, usually upwards to the roof of the mill, and on returning is vigorously agitated by the beater. The twists resolve themselves by meeting in the free-running length and the cloth is gradually stretched out to full width by a series of driven scroll rollers. In more sophisticated automatic scutchers suitable for handling more delicate fabrics the direction of twist in the rope is detected in advance by a feeler device and the rope is rotated accordingly as it passes through a guiding mechanism. Alternatively, a wagon containing the load of cloth may be rotated on a turntable to untwist the rope according to the direction of twist. The characteristics of many woven fabric constructions demand open-width processing. Table 15.2 indicates whether certain fabric types are best processed in open width (O) or rope (R) form. Tightly woven cloths made from two-fold yarns, such as the traditional shirting poplins, are prone to form persistent ‘ghost’ running creases, as a result of minor physical changes in the fabric. Polyester/cotton blends become exceptionally stiff when wet because of swelling of the cotton fibres that exacerbates the tight packing of the threads. Drying, heat-setting and fixation of dyes and finishing agents are examples of processes that have to be carried out in open width to ensure uniform treatment. With few exceptions, such as where the fabric quality demands little or no residual warp crimp, the normal objective is to process woven fabrics with only enough warpway tension to ensure smooth running. Inevitably, but unfortunately, there is a somewhat variable relationship between the dimensions in the warp
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and weft directions that is difficult to determine precisely. Hence warpway tension extends the fabric length and simultaneously causes weftway contraction. Compensator rollers with pneumatic loading perform the dual function of synchronising machinery units coupled in series and applying a desired warpway load to the cloth. Probably the best method of loading is by a pneumatic diaphragm. In spite of the inertia effect of the added weight, compensator rollers should be sufficiently robust to prevent deflection and consequent creasing. The arms must be stiff in the lateral direction to avoid vibration developing during running. Cylinder drying machines present a difficult challenge to the maintenance of minimal tension, because the cloth tends to shrink gradually as it dries. If the cylinders are driven at an identical speed of rotation, the tension in the fabric increases as it dries and distortion of the weave may occur. It is desirable to provide separate spiral-spring belt drives for the individual cylinders in the drying machine so that shrinkage strains on the fabric can be compensated. Open-width washing machines are often fitted with tension-control drives on the upper guide rollers, sometimes with light auxiliary pressing nips known as jockey rollers, to minimise slippage. This can be a successful system, particularly at modest running speeds. Maintenance must be meticulous, however, because the small circumference of jockey rollers results in vibration and rapid wear when run at excessive speeds. Unless the selvedges of a woven fabric are held apart at constant weftway tension, it is impossible to achieve positive control of the fabric width. Much can be done to minimise width loss during running, however, by employing rubbercovered bowed roller expanders, usually immediately before nips or drying cylinders. Low-friction bearings and wear-resistant coverings are important in extending the life of these devices. Expanders are fitted to free the cloth from creases and to stretch it slightly in the weft direction. A simple curved expander is formed by a rubber sleeve covering springs mounted on a curved bar. Revolving and spreading expanders consist of floats that move laterally from the centre of the expander to its extremities; they are mounted to form a hollow cylinder with cam wheels at the ends to bring about lateral movement as the expander rotates. The floats may comprise grooved brass rails or felt-covered metal supports for delicate fabrics. Scroll opening rollers are highly efficient in eliminating warpway folds and are driven in pairs in the counter-direction to the cloth passing between them. A scrimp rail can be simply a grooved metal bar, the ridges of which diverge from the centre, or consist of two sets of oval metal discs with washers between them threaded onto a square-section steel bar. The divergence of the grooves or discs straightens out the cloth as it runs over the scrimp rail under slight tension. The possibility of correction of weft distortions during finishing is limited by the effectiveness of mechanical straightening devices and the strain that may be placed on the fabric structure. Probably the most important cause of woven fabric distortion is differential shrinkage of selvedges relative to the body of the cloth during drying. Most mechanical straighteners are designed to correct for skew (one selvedge lagging behind the other) and bow (both selvedges lagging behind the centre) by applying two bow rollers followed by three skew rollers [6]. Controlled fabric tension (Figure 15.2) is an essential aspect of weft straightening. The cloth must be in constant contact with the straightening rollers to minimise slippage or further distortion. The objective is to make those portions of the weft that are too advanced follow a longer path than the remainder.
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15.4 Preparation of Woven Cotton Fabrics All cotton fabrics contain impurities that have to be removed before dyeing or printing. These may be cotton wax and natural coloured impurities, as well as spinning lubricants or warp size polymers added to facilitate weaving. Fabric preparation involves a sequence of processes, including singeing, desizing, scouring, bleaching and mercerising. The objectives of fabric preparation may be summarised as follows: 1. to achieve optimum and uniform dyeability 2. to remove virtually all the natural and added impurities from the fibres 3. to confer high absorbency, hydrophilic properties
effective
swelling
behaviour
and
uniform
4. to achieve satisfactory whiteness in goods intended for sale as white and to allow brilliant colours to be produced by dyeing or printing 5. to attain these desirable properties without incurring strength loss or fibre degradation. Fabric preparation can be carried out in open-width or rope form by batchwise or continuous methods, the choice depending on fabric quality and amount to be processed. The availability of continuous preparation and dyeing ranges and the importance of blended polyester/cotton fabrics favour the adoption of continuous open-width preparation where practicable, even for goods destined for batchwise dyeing. Consistent preparation is crucially important because the successful outcome of all subsequent dyeing, printing and chemical finishing processes critically depends on uniformity. It is generally agreed that a high proportion of the faults encountered in fabric coloration can be traced back to faulty preparation. 15.4.1 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 after dyeing may produce an unattractive speckliness known as frosting. It is therefore necessary to remove these projecting hairs by passing the fabric rapidly over a series of burners mounted along the length of a gas pipe spanning the width of the machine. The fabric thus briefly contacts a narrow strip of flame; this singes one face of the fabric while a second burner singes the other. The gas/air mixture is regulated to give a gentle colourless flame to remove the projecting fibre ends without overheating the body of the fabric. A high running speed (100 to 300 m/min) is essential. The angle of contact between flame and fabric, the height and intensity of the flame and the speed of passage are all controllable factors that determine the efficiency of the process [7]. After singeing the fabric carries the charred residues of fibre ends, some of which are smouldering, and provision must be made to cut off the flame before any stoppage of the fabric. Singeing machines have been responsible for many fires and safety precautions must be strictly observed. The singed fabric runs immediately into a quench bath to extinguish the sparks and cool the cloth. This 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 blends. A balance must be struck between thorough singeing that confers effective pilling resistance and excessive singeing that may cause surface polyester fibres to shrink or melt,
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forming small globules that are dyed more readily than intact polyester and giving a speckled appearance especially after exhaust dyeing. 15.4.2 Desizing Almost all woven fabrics contain a sizing agent that has been applied to the warp threads to facilitate weaving. Sizing reduces the frictional properties of warp yarns by coating them with a film-forming polymer. The process improves weaving productivity by increasing weft insertion speed and decreasing yarn breakages. Efficient desizing is an absolutely essential requirement of good fabric preparation. Sized cotton warps usually contain starch or a starch derivative and a lubricant. The lubricant imparts smoothness and low surface friction but is insoluble in water and difficult to remove from the fibre surface, sometimes leading to serious problems in desizing. In bulk running the singed fabric passes through the desizing bath at the same speed as the singeing treatment, so that the impregnation time is often far too short. As it is desirable to allow the fibres to swell for 15 to 30 seconds before squeezing to attain maximum uptake of liquor, experience has shown that a double-dip/double-nip sequence with an intermediate air passage (skying) achieves optimum absorption. The swelling effect is accelerated by adding an efficient wetting agent. Size removal depends essentially on the following factors [4,8]: 1. viscosity of the size polymer in solution 2. amount of sizing liquor applied 3. nature and amount of lubricant in the liquor formulation 4. the sized warp should not be overdried 5. fabric construction and absorbency 6. adequate time must be allowed to permit fibre swelling and maximum uptake of liquor 7. a thorough hot wash should be given after desizing to remove the solubilised size polymer. Enzyme treatments are widely used on cotton sized with starch, bacterial amylases being particularly suitable. Although frequently applied in the quench box after singeing, they may be used on almost all types of batchwise or continuous equipment. Under bulk conditions the fabric can be padded in the enzyme preparation and batched, but in general the temperature of the absorbed liquor is well below that required for the optimum rate of desizing. Depending on fabric quality and construction, the goods are usually washed-off in rope form or open-width on continuous washing ranges. Oxidative desizing usually refers to the application of hydrogen peroxide in alkaline scouring liquors. This process was developed for fabrics sized with poly(vinyl alcohol) where desizing at about pH 9 is recommended [9]. Starchbased sizes can be removed by treatment in hot caustic soda solution and it is possible to combine size removal with grey mercerising, although this approach precludes recycling of the mercerising liquor [10]. Simple aqueous washing removes certain synthetic size polymers. Carboxymethylcellulose, poly(vinyl alcohol) and the polyacrylates swell when immersed in water. If fabric containing these swollen size residues is agitated in a high-efficiency washing unit, complete size removal can be achieved.
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15.4.3 Scouring On initial wetting-out the yarns in a woven fabric tend to relax, contracting in length and causing crumpling. Qualities woven from cellulosic filament yarns in particular may form durable creases arising from uneven contraction, so that it is highly desirable to wet out such fabrics whilst they are being held in open width under slight tension. Lightweight crepe dress fabrics and similar delicate qualities containing high-twist or fine-denier cellulosic filament yarns require careful handling to ensure uniform relaxation. A typical festoon relaxing machine is illustrated in Figure 15.3. The fabric is suspended in loops about 80 cm long from a series of parallel rods mounted in tracks that move slowly along just below the liquor surface. The rods rise up successive saw-tooth ramps and drop 5 cm each time, allowing the fabric in contact with the rod to be displaced. The fabric is immersed for up to 20 minutes before being drawn off by a roller and sprayed with warm and cold water, then plaited down. The liquor in the tank is heated by steam coils. After carrying the fabric through the machine the rods move to the bottom of the bath and return to the entry end. For delicate filament fabrics the festoon machine provides excellent conditions for relaxing weaving strains. Scouring is a treatment with strong alkali that extracts cotton waxes and destroys other non-cellulosic impurities, resulting in a more absorbent fibre with greatly enhanced wettability behaviour. The severity of treatment, in terms of temperature, time and concentration of caustic soda, is intended to achieve a predetermined degree of removal of impurities from the cotton fibres. More severe conditions can be applied at higher cost to fully saponify the fats and waxes, but this does increase the risk of possible chemical damage to the cotton. Kiers are the traditional pressure vessels formerly used to provide the conditions necessary for thorough scouring of those resilient cotton fabrics that will withstand high-temperature processing in rope form. Although occasionally still found in use, kiers have been superseded by more productive continuous or semicontinuous scouring equipment. The essential feature of the kier boiling process is the treatment of several tons of fabric for 4 to 6 hours at the boil or under pressure at 130°C for a shorter time. Caustic soda concentration is typically 10 g/l or more at the boil or somewhat lower than this in the pressure boil. A typical kier is an enclosed vertical cylinder with a relatively small entry port at the top through which the fabric rope is loaded and unloaded. Probably the best known pressure steamer designed for scouring is the Mather & Platt Vaporloc, which is essentially a roller conveyor located within a pressure vessel. The chamber is sealed along the top on both sides of a central beam. Each flexible seal is inflated pneumatically and presses on a low-friction covering. The pressure inside the seal need be only slightly greater than that inside the vessel. Positively driven draw rollers at the entry end feed the fabric via a plaiting mechanism to the roller-bed conveyor of capacity about 200 metres. At the delivery end the fabric is lifted from the bed and drawn through the lip seal by delivery rollers outside the vessel. The entry seal has a nip effect that sets an upper limit on the amount of liquor taken into the steamer by the fabric. Interlocking devices ensure that the door is closed and locked before compressed air can be fed to the seals and only then can steam be admitted. A modulating valve is fitted to ensure that a steady pressure is maintained. The continuous pressure steamer is used mainly for scouring woven cotton and polyester/cotton fabrics in dilute caustic soda solution for 2 to 3 minutes at 130°C. Care must be taken when sewing fabric lengths
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together to avoid multiple thicknesses that can damage the seals. In the event of a seal being damaged severely the loss in air pressure cuts off the steam and opens an exhaust valve. The demand for open-width equipment that allows effective scouring of woven fabrics in a crease-free condition has encouraged the development of various ingenious devices. The essential function of a continuous washing range is the removal of impurities and unused chemicals by aqueous rinsing. Without adequate washing, the effects of desizing and scouring can be diminished or even nullified. The standard range consists of a series of compartments, separated by mangle nips to draw the fabric through the machine and to limit carry-over of absorbed liquor. This is important, because the effectiveness of the washing sequence is largely determined by the ratio of the water consumption to the carry-over of liquor by the fabric. The effectiveness also depends on the number of compartments, the efficiency of contaminant interchange between fabric and wash liquor, as well as the rapid movement of the fabric (about 100 m/min) through the slow-moving wash liquor. Considerable ingenuity has been displayed by machinery makers in devising means to promote fabric-liquor interchange and apply flexing and squeezing forces to the fabric [11]: 1. fluted, grooved or planetary rollers to agitate the wash liquor and direct it through the fabric; these are simple, low-cost modifications of existing units 2. passing the fabric between corrugated baffles to distort liquor flow and fabric configuration, as in the Gaston County alternator (Figure 15.4) 3. passing the fabric around a perforated cylinder (diameter 30 cm) immersed in the wash liquor; the axle of the cylinder vibrates to cause vigorous radial oscillations in the washbox. The Küsters Vibrotex (Figure 15.5) is intended primarily for open-weave fabrics and knitgoods that are relaxed effectively by this treatment 4. passing the fabric around a perforated drum containing an inner fluted or baffled roller, independently controlled. Rotation of the inner roller creates surges in the liquor that have a pulsating action on the fabric 5. ultrasonic agitation has been demonstrated to enhance the effectiveness of the continuous open-width washing of a mercerised cotton twill fabric. The same washing efficiency was achieved at 50°C with ultrasound compared with 90°C under control conditions [12]. The traditional design of an open-width washing machine (Figure 15.6) consists of a series of top and bottom parallel guide rollers with only the lower set submerged. The low liquor level permits inspection windows in the sides of the tank above the liquor surface, for ready access in the event of a fabric break-out. The fabric passes alternately between the top and bottom rollers, thus being subjected to repeated immersions in the wash liquor. The top rollers are driven as a bank of five or six that makes up each compartment. The bottom rollers freewheel on sealed self-lubricating bearings. Both top and bottom sets should be of the largest practicable diameter (typically 12 to 15 cm) and the distance between the sets kept to a reasonable minimum to reduce the tendency for fabric creasing or selvedge curling. Where compact washing compartments are desirable to utilise the working space more effectively, double threading may be employed by installing additional freerunning top and bottom rollers, as in the Benninger Becoflex design shown in
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Figure 15.7. This arrangement essentially halves the liquor ratio and doubles the fabric capacity per compartment, the frequency of immersion in the liquor and the effective duration of washing for a given running speed. Where double threading is practised, the depth of the scouring liquor has to be increased to cover both bottom rollers. To conserve water and energy, as well as improving working efficiency, the counter-current principle of liquor flow is now invariably used on open-width washing ranges. Vertical metal plates separate the liquor between the bottom rollers and clean water is sprayed in at the fabric delivery end of the machine. Liquor flows in the opposite direction to fabric travel, by either a cascade or a serpentine arrangement of plates. Each successive immersion of the fabric is in cleaner liquor, whereas the liquor becomes increasingly contaminated as it approaches the discharge point close to the fabric entry end of the range. Care is necessary to guard against too strong a counterflow as this may result in lowering of the wash temperature if not properly controlled. Continuous open-width washing demands approximately half of all the energy consumption of a typical fabric dyeing and finishing plant [13]. The rate of removal of soluble impurities from the fabric increases with the temperature of the wash liquor, mainly because of more rapid diffusion and the lowering of viscosity at higher temperatures. For most woven fabrics, a temperature of about 95°C appears to give the optimum balance between rate of removal of contaminants and cost of the heat input. Most machines are fitted with a lightweight cover to retain heat and water vapour. Typical washing conditions for a five-box washing range are as follows [8]: First washbox
95°C
Second washbox
95°C
Third washbox
80°C
Fourth washbox
60°C
Fifth washbox
water spray input
At the exit from the washing range, the fabric temperature should be 30°C or lower. With counter-current operation, heating is primarily at the liquor entry to the machine, with heating coils in the remaining compartments to maintain the temperature. In many cases hot liquor discharged from the washing range passes through one side of a heat exchanger to raise the temperature of the incoming water. For rapid heating direct steam addition through a perforated pipe is effective; steam coils are preferred for more controlled heating and economy. An important factor in open-width washing is the amount of liquor carried over by the fabric from one immersion to the next. As the fabric is lifted upwards, liquor streams downwards and a small proportion is expressed on making contact with the upper roller. Carry-over can be minimised by fitting a pressing roller above each driven roller. Good washing efficiency has been achieved with a five-box washing range using softened water supplied at a rate of 400 to 600 litres/hour [8]. The rate of water inflow should be controlled (outflow is by overflow) to avoid waste of heat and water. Coupling inflow to machine speed ensures that when the machine slows or stops the inflow of water is automatically restricted. More sophisticated devices link the rate of water input to the concentration of contaminants in the discharged
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liquor, as indicated by opacity measurements. In preparatory processes it should not be necessary to use more than 5 litres of wash water per kilogram of fabric. 15.4.4 Bleaching The bleaching of cellulosic fabrics provides a high degree of whiteness for goods that are to be sold in the white state, as well as conferring optimum brilliance for fabrics to be dyed or printed in bright colours. Another important function of this oxidative treatment is to enhance the uniformity of fabric appearance by destroying residual impurities such as seed husks or leaf debris. Hydrogen peroxide is widely used for bleaching natural and regenerated cellulosic fibres. Although it has no bleaching action on polyester fibres it is generally preferred for polyester/cellulosic blends. Almost all peroxide bleaching of textiles requires alkaline conditions, usually using caustic soda to provide the alkaline pH. In addition to alkali, sodium silicate and a magnesium salt are required to stabilise the hydrogen peroxide solution. There is growing interest in the use of acidic peroxy compounds for the bleaching of cellulosic fibres. The most important of these is peracetic acid (PAA), which can be made by reacting hydrogen peroxide with acetic acid or its anhydride [14]. Applications of PAA include the prebleaching of knitted fabrics, the bleaching of blends of cotton with viscose, spandex or acrylic fibres, denim washing and lowtemperature laundering. There are transportation and storage problems associated with PAA, so that launderers usually prepare this bleaching agent in situ by reacting the precursor tetra-acetylethylenediamine (TAED) with hydrogen peroxide. The cold pad-batch bleaching process offers good whiteness, wettability and seed removal. Energy costs are low but higher concentrations of chemicals are necessary to compensate for the relatively slow oxidation reaction rate at ambient temperature. Thus the pad-batch process represents a compromise, combining satisfactory quality for most end-uses with only modest capital expenditure. It is generally used without prior desize or scour treatments, so it is less effective for lower-quality cottons that are heavily contaminated. The long dwell times allow desizing and scouring to be combined with bleaching, however, offering substantial savings when preparing cottons of higher quality [15]. The essential equipment required includes an open-width saturator, substantial storage capacity in the form of A-frames and an efficient open-width washing range. After impregnation with the alkaline peroxide liquor the fabric is simply batched onto the beam of an A-frame or similar support (Figure 15.8). The roll of fabric is wrapped in a plastic sheet to avoid drying out. As the bleaching reaction proceeds the roll is kept slowly rotating to prevent drainage of liquor through the roll, which would result in irregular treatment. Further batches are similarly prepared in turn to keep the saturator occupied efficiently and to provide a steady supply of bleached fabric to the washing and drying stages. As A-frames are released after the bleaching stage they become available to receive further batches of impregnated cloth. Pad-roll bleaching was developed in Europe to process relatively short lengths of woven fabric in open width when the capital expenditure associated with the large roller-bed or conveyor steamers used in the USA could not be justified. The fabric is padded with alkaline peroxide solution and warmed by passing through a preheater before batching onto an A-frame enclosed in a movable container, often called a caravan. As the bleaching reaction takes place the roll of fabric is rotated
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and the chamber heated to maintain a constant temperature. After treatment the fabric is unrolled, washed off in an open-width washing range and dried. Bleaching proceeds more quickly than in the pad-batch process, so fewer pad-roll units are needed to keep the washing facility fully occupied. The main disadvantage of pad-roll bleaching is the inadequate uniformity of the treated goods, which may result in listing or ending in subsequent dyeing. Ending is particularly difficult to avoid because the time difference between the leading and trailing ends of the batched roll can be as much as two hours, so that the leading end receives a much longer reaction time. Listing problems are associated with swelling of the fibres in the roll when subjected to prolonged hot alkaline treatment. This generates tensions in the tightly wound batch that exerts a squeezing action, particularly in the inner layers near the centre of the fabric mass. Thus the effective liquor ratio becomes higher in regions where tension is lower and subsequent dyeing shows up the irregularity of the bleaching process. A characteristic limitation of semi-continuous processes such as pad-batch and pad-roll is that the bleaching reaction does not proceed quickly enough to be compatible with the impregnation and washing-off operations. Overcoming this limitation involves raising the reaction temperature without drying out of the impregnated fabric or the development of variations in temperature or pressure within the treated goods. This is most readily achieved using steam as the heating medium in a pad-steam range. An atmospheric steamer operating at 100 to 105°C must have a chamber of large capacity to permit the required contact time at this temperature. The alternative is a pressure steamer operating at 125 to 130°C. This can be a much more compact unit but pressure seals are essential at the fabric entry and exit ports. Atmospheric steamers require more space but are easier to maintain. Pressure vessels offer the advantages of compactness but are more troublesome in the event of seal failure or fabric break-out. Consistent, uniform application of chemicals makes a major contribution to successful preparation. Saturators have a useful role in ensuring sufficient dwell time to bring about liquor interchange. They provide a longer immersion path for the fabric than padding troughs and this can be beneficial for cotton qualities that are not readily wettable. As with washing ranges there have been developments towards minimising water usage by the introduction of spray techniques, as typified by the Babcock Super-Sat impregnation unit. Steam and chemicals are applied together directly to the fabric in the Kleinewefers Raco-Yet. This is a system for the simultaneous desizing and bleaching of woven cotton and polyester/cotton, based on alkaline peroxide and Cottoclarin (BASF), a scouring agent free from foaming and with excellent wetting, dispersing, emulsifying, complexing and soil-extracting properties [16]. The open-width saturator (Figure 15.9) can be used to process closely woven fabrics that would suffer unacceptable creasing in rope form. The rollers and tension bars of such saturators are often essential components in promoting interchange and penetration of the liquor applied. The Dip-Sat Vario (Max Goller GmbH) impregnator system for the open-width application of bleaching chemicals in the continuous pad-steam process has been described recently. The advantages claimed for high add-on saturation at high running speeds using this system include greater absorbency and improved whiteness of the bleached goods, rapid liquor interchange, less risk of edge marking and improved reliability, with economic and ecological benefits from the small volume of the impregnation bath [17].
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The rope saturator (Figure 15.10) is used for both woven and knitted goods in rope form. Invariably there needs to be some type of liquor pick-up control on exit from the saturator. Traditionally this was a conventional mangle nip but the Babcock Super-Sat [19], Küsters Flexnip and Optimax [20] are modified developments in saturator design that not only give increased control of liquor pick-up but add versatility to the saturator. Preparation invariably involves fibre swelling and dissolution of impurities, so the higher the liquor retention the better. The J-box was developed in the USA to reconcile fully-continuous operation with the prolonged contact time required in fabric preparation at atmospheric pressure. The basic design evolved from the Gantt piler, an open-ended wooden structure of rectangular cross-section formed into the shape of the letter J. Idling rollers were fitted at the foot of the curved portion to reduce friction and facilitate movement of the piled fabric. The significance of the J-shape is simply that by choosing a suitable curvature and the lengths of the two arms, the fabric rope piled into the long arm may be made to slide smoothly through the tube as the treated material is gradually withdrawn from the shorter end. Two distinct types of J-box were developed for bleaching woven fabrics in rope form: they differ mainly in the method of fabric heating. In the Becco or openwidth J-box (Figure 15.11) heating is by the direct application of steam through manifolds at various levels into the pack of plaited ropes. With the DuPont or enclosed J-box (Figure 15.12) the fabric is heated by passing through a steam atmosphere in a heater tube forming the entrance passage of the J-box. The box is insulated by jacketing to minimise heat losses. Most machines are made from stainless steel with a smooth internal finish so that the fabric slides down easily. The technique is particularly appropriate for processing in rope form as the dimensions of the box need not be related to fabric width, but are determined simply by operating speed, contact time and packing density of the fabric rope. The use of sodium silicate as a stabiliser for hydrogen peroxide in J-box bleaching led to the formation of sparingly soluble salts of calcium or magnesium from the water supply. These salts not only became deposited on the fabric and thus formed resist marks in subsequent dyeing but also built up as a colourless layer on inner surfaces of the J-box, contributing to abrasion of some fabric qualities. These deposits were difficult to remove and the problem was eventually overcome by using an organic stabiliser in place of the silicate. Certain fabric types have a tendency to bunch up and snag in the curved portion of the J-box; this is usually corrected by forming a wet bottom, in which water is introduced via a standpipe flowing at about 40 l/min. This can be swivelled to control the amount of liquor immersing the fabric rope in the curved section. The added water not only acts as a lubricant but also commences the rinsing stage. In J-box bleaching the fabric is usually impregnated with process liquor in a saturator before entering the J-box unit, but additional liquor may be added by spraying onto the fabric rope in the longer arm of the tube. The FMC continuous kier evolved from the wet bottom J-box. In this the water flow is replaced by bleach liquor circulated to the J-box from a heel tank, at a concentration between 30 and 50% of the saturator concentration [9]. Rope J-boxes have a capacity of 5 to 10 km of fabric for a dwell time of 1 to 2 hours at an operating speed of 100 to 150 m/min [4], so that productivity is approximately 1500 kg/hour of bleached fabric. The weight of fabric saturated with bleach liquor pressing down on the rope lengths in the curved section is considerable. Sharp creases are formed under this pressure and withdrawal of
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fabric from the shorter exit arm can sometimes be difficult. The U-box was designed to handle fabrics in open width and to minimise the pressure on the lowermost portions of the load (Figure 15.13). The capacity of this unit was much lower than that of the J-box, providing a dwell time of only 10 to 20 minutes. Unfortunately U-boxes were more difficult to operate because of distortion of the open-width configuration and the development of weftway creasing at the folds, particularly when bleaching polyester/cotton blends [11]. The first open-width steamers to be evaluated in preparation were the atmospheric tight-strand steamers already in use for fixation in continuous dyeing. The goods are threaded between two horizontal sets of rollers, typically about two metres apart. As the fabric is in close contact with all the rollers, the dwell time can only be extended by slowing down the operating speed. At the slowest practicable speed the dwell is only 1 or 2 minutes, too brief to extract seed debris from woven cotton fabrics. Thus this large-volume steaming chamber is suitable for dye fixation but not generally for preparation processes. Single-layer conveyor steamers (Figure 15.14) were developed in the USA for bleaching woven goods. Whilst clearly unsuitable for dye fixation, these units offered the delay periods necessary for preparation treatments. The Brugman conveyor was used for bleaching linen and cotton/linen blends, where the long dwell time of 10 to 20 minutes made this an attractive design [21]. The fabric is plaited down onto a slowly moving smooth or woven-mesh continuous belt, forming a bed of folded fabric about 10 to 20 cm deep. This system has the disadvantage that there is no relative movement between the fabric and the supporting belt, so that the areas of contact between them may receive a different treatment from the remainder of the goods. The roller-bed steamer (Figure 15.15), consisting of a curved bed of rollers that rotate much more slowly than movement of the fabric through the chamber, was introduced to provide longer dwell times (3 to 5 minutes) than the tight-strand steamer without loss of productivity. The fabric enters the steamer via a restricted opening designed to minimise access of air and is plaited directly onto the roller bed. Problems have been encountered with slippage on the entry pull rollers but most fabrics behave satisfactorily when allowed to fall freely for one metre or more so that they ripple onto the rollers. Low-pressure steam is injected into the chamber through a water sump to provide saturation. The moving bed transports the goods in a relaxed state to the take-off point. Regaining control of the fabric as it is lifted from the bed is not difficult if it was laid down correctly on entry. Pull rollers may be used or the fabric pulled out by an exit nip. Any tendency to crease may be countered by a coarse-pitch scroll roller driven against the fabric and a pair of guiders used as expanders. The plaiter feed and take-off rollers can be operated independently. The fabric contracts slightly in length during relaxation, so the take-off speed is marginally slower than the plaiter feed. In a typical machine about 600 metres of fabric can be accommodated on the bed. The dwell time available is not a function of the machine operating speed but of the thickness and length of the cloth layer above the roller bed. This is dependent on the rate at which fabric is plaited onto the bed and the rotary speed of the rollers. Less flexible fabrics bunch up to form compact folds and thus produce a thicker layer of cloth, whereas smoother qualities do not often do so. The thicker the cloth layer, the greater the risk of trapping fabric at the take-off point. Küsters overcame this difficulty by plaiting the fabric onto a separate entry roller
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rather than the bed. As this rotates to deposit the fabric down to the bed, the pile inverts [20]. Even under otherwise favourable conditions in the roller-bed steamer, creasing may occur when processing polyester/cotton fabrics. This problem was eliminated by utilising the space above the roller bed to accommodate a short tight-strand section, based on the view that pre-heating the cloth under low tension allows swelling and dimensional changes to occur before plaiting down onto the bed in the relaxed state and thus minimising the risk of creasing. This arrangement is characteristic of the so-called Combi-steamer (Figure 15.16) and it makes dwell times of 10 to 15 minutes feasible, aiding seed removal in the combined scourbleach process. The first conveyor steamer was the Mathieson Alkali multi-layer conveyor type (Figure 15.17) introduced in 1938 for the chlorite bleaching of cotton. The impregnated fabric enters the steamer chamber and is plaited onto the first of three conveyor belts moving towards the exit end of the chamber. At the end of the first traverse the cloth drops onto a second conveyor moving in the opposite direction and, after another traverse, down to a third conveyor before reaching the take-off point and the fabric exit. A treatment temperature of 95°C and dwell times of 15 to 60 minutes are typical. The Kleinewefers pressure steamer has entry and exit openings with lip seals to retain vessel pressure. It is a tight-strand steamer in which expander bars are used to avoid crease formation. Normal dwell times vary in the range 1 to 3 minutes, which is adequate for a scour-bleach at 125 to 130°C, and production speed is in the range 60 to 100 m/min. In spite of their high productivity, such steamers are expensive and have not proved popular [4]. 15.4.5 Mercerising Mercer discovered that the tensionless treatment of cotton fabrics with a cold solution of strong caustic soda improved dye uptake and tensile strength, but it was Lowe who found later that mercerising under tension markedly improves the lustre of the fibres. These changes result from swelling of the fibres and this causes fabric shrinkage if the tension is too low to prevent this occurring. The extent of these dimensional effects must be controlled if the process is to be operated reliably. Fibre swelling by alkali brings about internal reorientation of the cellulose structure, creating more accessible sites for dyeing or chemical reaction. Immature fibres are also modified so that they become less different in properties from mature fibres. A mercerising range must have facilities for impregnation in a saturator with caustic soda solution of adequate strength (about 25% w/w NaOH), sufficient dwell time for the full effects to be achieved, followed by removal of the caustic soda and efficient washing-off on an open-width washing unit. In conventional chain mercerising the impregnated fabric is taken around a series of metal cylinders to keep it crease-free by maintaining warpway tension during the dwell period of 40 to 50 seconds. The selvedges are then gripped by clips in a strongly built stenter chain about 25 metres in length and weftway tension applied to pull the fabric back to its original width, whilst washing out the caustic soda using overhead washwater sprays augmented by suction boxes immediately beneath the fabric. Washing is arranged according to the counter-current principle, the liquor collected below the fabric being pumped up to the preceding spray. The
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stenter chains, operating under strongly alkaline conditions, require frequent maintenance. A chainless mercerising machine consists of a series of tanks each fitted with horizontal metal rollers partly immersed in the liquor, with rubber-covered rollers situated above and between the metal rollers. This arrangement assists liquor penetration, restricts weft shrinkage and provides fabric traction. Each tank containing about twelve rollers has its own drive operating on intermediate pad nips. The first two or three tanks are used to saturate the fabric with caustic soda liquor, giving a dwell time of 40 to 50 seconds, and the remainder of the range provides stabilisation as the alkali is removed using hot wash water. Further washing then takes place on a conventional open-width washing range. The chainless merceriser only applies indirect weftway tension across the fabric width. Consequently the fabric construction must be designed to allow for the loss in width caused by weft shrinkage and the fabric must be stentered sufficiently wide prior to mercerising. The Perfecta chain merceriser and the Optima chainless machine have been described in detail [22]. The important variables that determine the effectiveness of the mercerising treatment are alkali concentration, dwell time, treatment temperature, rate of removal of alkali and the drying temperature. Pressure on costs is forcing finishers to adopt wet-on-wet mercerising techniques, which demand more careful control of alkali concentration and process conditions. The advantages of the Benninger Ben-Dimensa mercerising range with a combined chainless-chain-chainless arrangement for this technique have been advocated. The importance of the ratio of g NaOH to kg fabric in relation to fabric density and running speed was emphasised, with particular relevance to the mercerising of heavyweight goods at high speeds of operation [23,24]. The benefits of mercerisation include improvements in lustre, fabric smoothness, tensile strength, dimensional stability, dyeability, colour yield and coverage of dead cotton. Depending on fabric density and yarn twist, most of the convolutions in the cotton fibre are eliminated to give a near-circular cross-section. There is evidence that subsequent reactive dyeing and crosslinking reactions in finishing take place more uniformly on mercerised cotton. Mercerisation may be carried out on the grey, partially or fully-prepared substrate. When mercerising cotton fabrics in the grey state, however, the effects tend to be confined mainly to the surface since full penetration by the viscous solution of caustic soda does not occur at low temperatures, even in the presence of wetting agents. A further disadvantage of loomstate mercerisation is fouling of the liquor by size polymer residues, making caustic soda recovery and recycling difficult. However, it is preferable to impregnate certain fabric qualities with caustic soda while they have maximum strength. Nevertheless, in general it has been convincingly demonstrated that the best position for mercerising is after scouring and bleaching [25]. Since the mid-1970s there has been sporadic interest in the potential of the socalled hot mercerising process. The essential steps are [26,27]: 1. saturation with mercerising-strength caustic soda solution at a temperature close to its boiling point 2. controlled hot stretching for a brief dwell period 3. controlled cooling and dilution of the caustic liquor 4. traditional tension-controlled washing and final rinsing.
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The main chemical and physical changes to achieve the desired improvement in fabric properties do not take place at the high temperature of the saturator but when the cooling fabric passes through the traditional caustic dilution and washing-off stages. This approach has been advocated as a route to combined scour-mercerising of cotton, particularly as the degree of scouring is said to be equal to a conventional caustic scour before peroxide bleaching. The advantages claimed for hot mercerising include [8]: 1. shortening of the process sequence to provide cost savings 2. increased efficiency and reproducibility 3. use of chain or chainless mercerisers with fewer problems related to fabric width control 4. improved lustre, tensile strength and dimensional stability because greater stretching of the fabric is practicable 5. increased dye uptake at moderate tension, but excessive stretching can result in lower dyeability 6. improved results from fabric qualities containing lower-grade cotton 7. flash scouring effect obtained 8. good desizing action 9. enhanced penetration of the fibre by the hot caustic liquor. Despite these apparently impressive claims, hot mercerising has not succeeded in achieving significant commercial success [4]. The impact of ecological considerations on the development of textile processes has focused attention on optional techniques such as mercerisation, particularly in view of the highly polluting nature of the effluent from grey-state mercerising. The earlier the stage at which caustic soda treatment is applied, the greater the degree of swelling that can be attained [28]. The Küsters Ecomerse addition mercerisation system is a wet-on-wet process whereby strong caustic liquor (530 g/l NaOH solution) is added via a Flexnip unit to squeezed wet goods at a temperature dependent on the cotton quality, fabric density and dwell time. The novel stenter design and longitudinal stretching section produce noteworthy improvements in lustre, dimensional stability and uniformity [29]. 15.4.6 Treatment with Liquid Ammonia The inherent properties of cotton can be enhanced by treatment in anhydrous liquid ammonia at -33°C. In the original Prograde (J & P Coats) process developed in the 1970s for cotton sewing threads, the ammonia was removed subsequently in a hot water bath. The process conferred higher tensile strength and improved resistance to photodegradation, but the changes in lustre and dyeability were inferior to those given by caustic soda mercerisation. The Beau-Fixe NH3–wet (Verametex) and Sanfor-Set NH3–dry (Martini) processes are of particular interest for high-quality woven cotton shirting or blouse fabrics, as well as cotton and linen workwear [30]. Liquid ammonia treatment should not be regarded as an alternative to mercerising. Thorough preparation is essential and the perceived benefits are mainly associated with chemical finishing. Heavyweight fabric qualities develop a softer handle, enhanced crease recovery and smooth drying appearance, as well as improved dimensional stability in washing and tumble drying [31]. Less resin is needed for subsequent durable-
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press finishing but dye uptake may be increased or decreased by the liquid ammonia treatment. If necessary, this treatment can be applied after dyeing. For maximum crease recovery it is customary to remove the ammonia by dry heat but a sequence of dry heat and then steam may be preferable to achieve a more favourable balance between crease recovery and loss in tear strength after crosslinking. The effects of swelling in liquid ammonia on the morphological characteristics of cotton have been studied [18]. Interfibrillar swelling is accompanied by a limited degree of intrafibrillar swelling, less than that resulting from caustic soda mercerisation. These swelling effects occur more rapidly than in the conventional mercerising process and the tensions developed are much higher, so greater control of fabric dimensions is necessary.
15.5 Batchwise Dyeing of Woven Fabrics All classes of cotton dyes find use in the dyeing of woven cellulosic fabrics. Direct and sulphur dyes are favoured for the cheaper end of the market, indigo and sulphur black are used in warp dyeing for denim and quality fabrics are dyed to high fastness standards using reactive or vat dyes. Typical cotton and polyester/cotton fabric qualities are listed in Table 15.3 in order of decreasingly stringent fastness requirements (appropriate disperse dyes are applied to the polyester component of blends). Particular care is needed when processing flat filament fabrics woven from synthetic-polymer yarns because of their high potential shrinkage. Most of these qualities readily form creases and should be maintained in open width, especially before heat setting. The tensions imposed during setting should be as low as possible, but sufficient to ensure firm pinning on the stenter and to avoid any tendency towards bowing or skewing of the weft. If setting tension becomes too high, a stiff papery handle results and the yarns adopt an elliptical cross-section. This gives rise to an effect resembling chalk marking, if the uniform reflectance of the fabric surface is disturbed by folding or touching. Fabrics woven from bulked synthetic-polymer yarns are more stable to moderate tensions but exceptionally high tension tends to cause loss of crimp and must be avoided. The handle, cover and draping quality of bulked woven fabrics, as well as performance characteristics such as inherent crease recovery and snag resistance, depend on control of the degree of bulk during finishing. Preferably, optimum bulking should be developed during scouring and the bulk level preserved during stentering. Maximum bulking action and shrinkage are developed by overfeeding the fabric into the scouring bath whilst subjecting it to controlled agitation. Before dyeing, the cloth should be heat set with slight width tension to remove creases but without true overfeed, so as to avoid the formation of rippled selvedges. Setting stabilises the relaxed fabric and reduces the risk of rope creasing during jet dyeing. When dyeing in a jet machine (section 14.6.2), the maximum length of cloth that can be handled satisfactorily is determined by the running speed of the jet nozzle. This length should be adjusted so that the cloth in each jet tube completes each running cycle in not more than three minutes and in a multi-jet machine the cloth lengths in different tubes should match within 2%. The load of fabric within the pressure vessel should be loosely packed so that the jet action draws the rope smoothly forward without snatching. It is important to select a jet nozzle of the
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correct size for the cloth to be dyed, in order to attain the most efficient use of the jet transport effect. If the jet tube is too large, lightweight qualities tend to float forwards in a doubled condition and the nozzle may become jammed. Jet dyeing at high temperature brings about further yarn bulking, to an extent that depends on the pre-setting temperature, resulting in simultaneous warp and weft shrinkage. As these changes take place whilst the cloth is being thrust forwards in rope form, various creasing problems may arise. Machine overloading must be avoided and fabric qualities prone to crease formation because of their structure should be stentered at a higher temperature than usual before dyeing, with some loss of yarn crimp being accepted. In particularly difficult cases it may be necessary to reduce the rate of temperature rise at the dyeing stage. The bulk and handle required in certain polyester/viscose suiting fabrics can only be attained by jet dyeing and cannot be achieved by open-width processing. Many different fabric types have been woven from flat or bulked filament warps with staple or blended weft yarns. In general, their handling routines in dyeing and finishing must reach a compromise between the often conflicting needs of their component yarns. These filament/staple goods have been produced in a variety of qualities, ranging from lightweight filament warp/cotton-spun weft batistes and lawns to heavy poplins and repps with worsted-spun wefts. Face fabrics, such as satins, have been made with polyester/modal staple wefts for use in rainwear. Heavy stretch fabrics have been woven with bulked filament warps and blended staple wefts. Fabrics woven on filament warps must be handled in much the same way as allfilament goods. This involves scouring in open width and heat setting at the highest practicable temperature before dyeing on the beam. Few filament-warp qualities are suitable for processing in rope form and some are prone to thread slippage, especially before heat setting. These must be handled gently with great care at the scouring stage. So-called fancy fabrics are those in which surface-textured or three-dimensional effects are developed during cloth finishing. Synthetic fibres can be used to supplement the incorporation of traditional differential-twist techniques by weaving alternate bands of set and unset filament yarns. Fancy fabrics generate their characteristic crepe, seersucker or blister effects during a relaxation treatment, which releases the torque in sized high-twist yarns and causes differential shrinkage to appear between heat-set and unset yarns or between tighter and looser woven areas of the cloth. The extent of the weave distortions produced depends on the number of threads involved in each of the alternating active sections of warp or weft yarns. After development of these textured effects, fancy fabrics must be handled under the least possible tension, especially when being heat-treated. The application of high temperatures when stentering or hot pressing at the making-up stage can cause some loss of texture if the effective setting conditions of the stabilised yarn sections are exceeded. In effect, the stentering and hot pressing temperatures for twist crepes and seersuckers are restricted to the region of 150 to 160°C. Such fabrics are therefore unsuitable for end-uses that require high dimensional stability. In other sectors, however, their characteristic elasticity makes it possible to tolerate a moderate degree of shrinkage on washing.
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15.5.1 Jig Dyeing of Woven Fabrics The jigger or jig-dyeing machine (Figure 15.18) has been established for many years in the open-width treatment of woven fabric qualities that must not be creased during dyeing. These constructions include quality apparel cloths, typically dress satins and taffetas, suiting fabrics such as serge or gabardine, and tightly woven goods of high cover factor, including poplin, duck and canvas. The principle of jig dyeing involves a series of immersion and batching steps, called ends. The fabric is batched onto one so-called draw roller, passed into a relatively small-volume dyebath and wound onto the other draw roller. When this batching step is complete, the cycle is repeated for the predetermined number of ends. The rollers are normally 1.8 to 2.0 metres in length. The draw rollers were traditionally 10 to 20 cm in diameter and typical loads of about 500 metres were dyed in dyebaths of about 150 to 200 litres, corresponding to a liquor ratio in the 3:1 region. Modern larger jiggers are fitted with draw rollers about one metre in diameter, accommodating heavier loads up to about 5000 metres of lightweight taffeta requiring a dyebath volume of around 1000 litres. The trough is designed to be as narrow as practicable to ensure a relatively small liquor surface area, which is advantageous in minimising premature oxidation by air when dyeing with vat or sulphur dyes. Several metres of cheap calico end-cloth are butt-end sewn to each end of the batch to be dyed, to avoid possible overlap and mark-off of the image of an untidy seam onto adjacent layers of the batched fabric. When loading the jigger, the leading end is threaded beneath the free-running rollers in the trough, upwards again and round the empty draw roller. It is essential for all the fabric in the batch to have the same width and to be run-up precisely straight so that the sides of the rolled batch are uniformly perpendicular to its axis. Running with neatly rolled batches and keeping the hood of the jigger closed as much as practicable are important to ensure that the temperature of the saturated fabric remains as consistent as possible throughout the dyeing process. Cooling at the selvedges of irregularly wound or inadequately enclosed batches is the main cause of listing faults, where the edges of the dyed fabric are usually paler than the centre. The duration of each process stage, in terms of the number of ends, is programmed after the first passage through the machine and thereafter the direction of fabric movement is reversed automatically. Both the unwinding and take-up draw rollers are driven to minimise warpway tension, which adversely affects some fabric qualities under hot, wet conditions. The drive control system normally has facilities to control the important variables, including length of the first run, cloth speed (40 to 100 m/min), warpway tension and the number of ends to be run. The concentrated dye liquor is usually introduced directly into the dyebath in two equal portions, which are added just before commencing the first and second ends. The liquor is agitated by the movement of the fabric through the dyebath. Several horizontal spray pipes are fitted across the full width of the trough in order to expedite fabric rinsing. Heating is usually by low-pressure steam from a perforated pipe situated in the lowest portion of the trough. Some jiggers have a dual heating system with direct steam injection for rapid heating up to the dyeing temperature and then closed-coil heating to maintain that temperature. When running wet fabric from roll to roll there is always a tendency to form warpway creases that must be eliminated before winding onto the roll. Expander bars are fitted so that their convex surfaces are in direct contact with the fabric
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shortly before it reaches the take-up roll. In order to allow for the increase in batch diameter as winding proceeds, expander bars are usually mounted in pairs on a frame that varies in position depending on fabric tension and batch diameter. When dyeing lightweight filament fabrics the batched roll may develop warpway ripples, possibly leading to unlevel streaking. If a free-running scroll roller is fitted to rest on top of each winding batch, this flattens out the fabric ripples whilst the spiral thread permits an even flow of liquor between the roller and the fabric surface. When jig processing is completed, the fabric is run onto an A-frame via a nip or suction device to remove extraneous water during unloading. Jiggers require modification to handle velvets and other woven pile qualities. These fabrics are positioned so that the expander bars act only on the back rather than the pile face. Pile fabrics take up a large volume of dye liquor and this is forced preferentially to the base of the pile by pressure and centrifugal forces, leading to marked shade differences between root and tip of the pile. To compensate for this effect, the threading-up arrangement is modified for velvets so that the fabric is taken up with the pile inwards on one draw roller and outwards on the other (Figure 15.19), the drive being modified to provide contra-rotation of these rollers. As the fabric passes through the dyebath the pile is laid back slightly in the direction of travel and runs smoothly into the fabric nip formed on the roll. The high-temperature Jigger-Tiro developed jointly by Funke GmbH and Thies GmbH operates with a hermetically sealed airtight system. The fabric is transferred from one draw roller to the other by essentially the same means (Figure 15.20) as in a conventional jigger but dyeing temperatures up to 140°C can be attained. An integrated fabric transfer system with expanders ensures crease-free loading under controlled tension. A floor-level track mechanism assists loading and unloading of the kier. A fabric speed (10 to 150 m/min) and tension (10 to 100 kg) control system guarantees precise monitoring of the fabric passage through the liquor trough and allows automatic selection of the sampling point. The economical liquor ratio trough can permit a capacity variation between 55 and 100%, whilst maintaining a constant liquor ratio. Uniform dosing of dyes and chemicals is controlled by the batch length passing through the liquor trough. An addition pump is fitted for shading corrections and dosing under pressure. An optional salt-dissolving device with electric stirrer is available for automatic brine dosing into the pressurised kier. The external pump and heat exchanger provide uniform liquor and heat circulation for regulation of pH and temperature. This machine is particularly recommended for processing crease-sensitive or impermeable fabric constructions in open width. Fabric widths up to 3.6 metres and batch diameters up to 1.2 metres can be processed successfully. The Henriksen high-temperature Vacu-Jigger has been introduced for the dyeing of woven polyester and polyester/cotton fabrics. It has been proven excellent for high-tenacity woven nylon, giving improved depth and penetration. The vacuum system consists of a suction slot mounted above the liquor level in the centre of the jigger. As the fabric passes over the slot, dye liquor is extracted and pumped back to the dyebath. During the washing-off stage, contaminated liquor is extracted in the same way and pumped to drain, whilst clean water is sprayed in micro-mist form onto the fabric as it is wound around the main roller. Both of these procedures save up to 25% of total processing time and during rinsing up to 65% savings in water consumption can be achieved. An analogue dosing system provides gradual progressive addition of dyes and chemicals to maintain the predetermined concentrations in the dyebath. The HighSpeed control system ensures constant fabric tension and maintains the centrifugal force on the
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batching roller by gradually accelerating the fabric speed up to a maximum of 200 m/min. The Vacu-Jigger is suitable for fabric widths in the range 1.6 to 3.2 metres and batch diameters up to 1.2 metres. The capacity (C) in metres is given by a simple formula (Equation 15.2), where t mm is the fabric thickness. C =
1000 t
Equation 15.2
Dyeing on the jigger may be regarded as a series of immersions in the dyebath, each followed by a dwell period in the rolled batch during which dye absorption and diffusion take place. The factors controlling the rate of dye absorption are: 1. the amount of interstitial dye liquor retained in the interstices of the fabric weave 2. the exhaustion of the interstitial liquor in the dwell period between successive immersions 3. the degree of interchange of liquor during one immersion (interchange factor). The interchange factor measures the proportion of exhausted liquor replaced by fresh dye liquor during one immersion of the fabric through the dyebath. Under normal dyeing conditions it is remarkably independent of fabric speed, warpway tension, immersion time, dyeing temperature and distribution of the guide rollers in the dye trough [11]. The amount of interstitial liquor in the goods may be quoted as a linear density value (mass of liquor per unit area of fabric) or expressed as a percentage of the total mass of fabric (interstitial ratio). Experimental data for three typical filament fabrics are given in Table 15.4. The interchange factor decreases with increasing density (mass of fabric or interstitial liquor per unit area). These figures show that the effective liquor ratio in the batched fabric on the roll is around 1:1. Under these conditions at typical dyeing temperatures, exhaustion of most of the dye from the interstitial liquor takes place within about 5 seconds after the immersion. Diffusion of dye from the fibre surface to the interior proceeds much more slowly. Ending faults arise from the relationship between dye diffusion rate and the dwell period on the batched roll. Fabric in the middle of the batch has successive dwell periods of roughly similar duration but both ends tend to give colour yields slightly weaker than the target shade because two immersions in rapid succession are followed by a dwell period extending over most of two successive ends. Loss of surface dye in the second immersion often nullifies the benefit of this extended dwell. Within the rotating batch of fabric the combination of gravity and centrifugal forces generates a vertical pulsing motion of the interstitial liquor, the magnitude of which is highly dependent on the porosity of the fabric. This liquor movement contributes significantly to levelling action through the load, providing the liquor temperature is consistent, but it can accentuate the marking-off of several repeats of imperfect joins in the fabric length if these take up excessive amounts of dye liquor. Microprocessor-controlled metering devices are now available to regulate the addition of sodium dithionite and caustic soda in the vat dyeing of cotton on the jig. The metering rates can be adjusted according to the fabric structure, mass, width and running speed, vat dye type and applied depth, dyeing temperature and liquor ratio. Metering procedures are designed to ensure constant concentrations of alkali and reducing agent throughout the dyeing process. This
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approach results in up to 50% savings of dithionite consumption, less effluent pollution and improved productivity. Metering is claimed to minimise dyeing faults, improve level dyeing and optimise fastness to rubbing and wet treatments [32]. The requirements of shade uniformity in the making-up of clothing are especially stringent for the combination of a plain-dyed lining with a one-colour dress, suiting or outerwear garment. Even the slightest listing fault (selvedge/centre variation) will lead to claims. About twenty sources of variation that can give rise to such problems have been specified, with particular reference to the pad-jig dyeing of viscose filament lining fabrics with direct dyes. Individual dyes can vary considerably in their response to substrate or process variables. The four most important variables in this context are dye selection, substrate preparation, moisture content and temperature fluctuations [33]. The presence of neps of immature or dead cotton fibres on the surface of woven fabrics for low-price garments can be particularly troublesome when dyeing with commodity direct or reactive dyes. Desized, scoured and bleached cotton fabrics containing neps were treated on a jig with Polymin P (BASF), a cationic poly(ethylene imine) emulsion, for 30 minutes at 60°C before conventional dyeing with typical reactive dyes. Coverage of the immature cotton neps was good and colour yields were enhanced. Resin aftertreatment together with Polymin P gave improved fastness to wet rubbing but light fastness of some of the dyes was impaired [34]. 15.5.2 Beam Dyeing of Woven Fabrics Pressure beam-dyeing machines became established during the 1950s in response to the growing demand at that time for a crease-free method of dyeing delicate lightweight fabrics woven or warp-knitted from nylon, acetate or triacetate filament yarns. Relatively porous polyester, polyester/wool and polyester/cellulosic cloths are also eminently suitable for beam dyeing and these have become the most important fabrics dyed in this equipment. Woven cotton, linen, woollen and worsted goods do not require high-temperature processing and are seldom dyed on the pressure beam because of their rather low porosity, the flattening effect of the rolled batch and the high capital cost of these machines. The first beam-dyeing machines were developed for atmospheric dyeing processes and consisted simply of an open tank containing a perforated metal beam around which the fabric was batched. The dye liquor was pumped through this assembly from inside the beam, the dyeing temperature being limited to about 95°C. When the limitations of carrier dyeing of polyester and polyester/cotton became evident in the 1960s, high-temperature versions were developed, in which the open tank was replaced by a cylindrical pressure vessel and a more complex pumping circuit provided two-way flow (Figure 15.21). Pressure vessels can vary in internal diameter from only 30 cm for sample dyeings up to 2 metres for full-size batches of woven or warp-knitted fabrics. The length of the perforated beam can range from 2 metres up to 4.5 metres for wide warp-knits. At the centre of the back of the vessel the dye-liquor inlet from the pump feeds directly into the interior of the perforated beam and below the centre is the dye-liquor outlet back to the pump. The rolled batch mounted on a cradle is transported into and out of the vessel on metal rails. The access door at the front of the vessel is fitted with a rubber seal and is securely locked during operation of the machine.
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A standard vessel about 2 m long and 1 m diameter for wovens has a nominal liquor capacity of about 3000 litres and will accommodate about 250 kg of lightweight fabric at a liquor ratio of 12:1. A jumbo machine 4 m long and 1.5 m diameter will hold about 750 kg lightweight fabric (15:1 LR) or up to 1500 kg of a denser quality (only 15:2 LR). Pumps and motors of larger capacity are necessary on jumbo machines. The axial or centrifugal main pump for circulating the liquor must deliver a large volume rather than develop a high pressure. Separate gauges indicate the pressure both inside and outside the rolled batch. Adequate circulation of liquor is essential for successful dyeing on the beam and the system is designed to minimise restrictions to rapid flow. The pump is fitted as close as possible to the entry point at the back of the vessel to provide effective delivery to the interior of the beam and to collect the outflow. Although pressure systems permit two-way flow, in-to-out flow through the batch is greatly preferred. Out-to-in flow has the disadvantage of compression, causing flattening and glazing of the surface particularly on the inside layers. Even modest pressure on the outermost layers imposes high pressures on the innermost ones compressed against the rigid steel perforated beam. During operation of the machine a small amount of liquor leaves the vessel and passes through a cold-water condenser into an expansion tank alongside. This open tank is useful for adding dissolved dyes and chemicals during the process. Liquor is fed from the base of this tank via a small high-pressure pump back into the main circulation system. As the temperature of the liquor in the system is raised the level of liquor in the expansion tank also rises. Heating is usually applied by piped high-pressure steam. In order to provide a large surface area for heating and minimum resistance to liquor flow, the pipes are fitted longitudinally along the bottom of the vessel below the rails. They are joined at both ends and arranged in an arc so that condensate drains into the lowest pipe and out of the vessel to the condensate return pipe. Cooling is usually conducted via the same system. After the valves for steam and condensate are closed, cold water is passed through the pipes in the opposite direction, preferably to a hot-water storage tank for subsequent reuse. The preparation of a uniformly wound batch of fabric is an essential prerequisite for level dyeing on the beam. Perforated beams are made in a range of diameters to fit the particular size of the vessel; for example, about 50 cm to fit a vessel of 1.3 m in diameter or about 60 cm for a 1.6 m vessel. Apart from a 20 cm smooth section at each end, the beam is perforated with several thousand holes about 5 mm in diameter through which the liquor flows. To accommodate fabrics narrower than the maximum width, excess holes near the ends of the beam have to be blanked-off by bolting on flexible stainless-steel sheets positioned so that the inner edges are 2 to 5 cm inside the rolled batch of fabric. If inadequate overlap (< 2 cm) is allowed there will be leakage of dye liquor from the sides of the fabric batch, but if there is excessive overlap (> 5 cm) the fabric close to the selvedges will be starved of dye and therefore listed relative to the centre of the batch. Before loading with the fabric to be dyed, the perforated beam is wrapped with a few layers of cheap calico fabric. These innermost layers provide a resilient bed for the fabric batch and a favourable medium for the dye liquor to diffuse through, whilst preventing the beam perforations from showing up as over-dyed spots on the inner layers of the dyed batch. The end-cloth also serves as a fine-
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mesh filter to remove any suspended particles of dye inadequately dispersed in the liquor. For fabrics of average to low permeability, the optimum size for the rolled batch is governed by Equation 15.3: N = KP
Equation 15.3
where N is the number of layers of cloth that can be wound onto the beam as the largest batch that will give a dyeing free from ending under the optimum dyeing conditions. P is the measured porosity of the fabric expressed in terms of the air permeability of unit area of a single layer in unit time at unit pressure difference. K is an empirical constant, characteristic of the pumping capacity of the machine and the porosity value. Typical beam-dyeing machines designed for processing woven goods give K values in the region of 50. The value of P is highly sensitive to small changes in fabric structure. Thus a change of 2 to 3% in the cover factor (section 15.2), too small to be detectable in terms of other fabric properties, can bring the performance of a difficult fabric well into the range where successful beam dyeing is practicable. Polyester rainwear fabrics with porosity values as low as 12 have been beam-dyed successfully in 1000 m batches and shorter lengths of qualities with P as low as 7 have given satisfactory dyeings. Fabrics to be beam-dyed must have reasonably good dimensional stability throughout the dyeing process. If they suffer warpway extension under the influence of hot dye liquor being forced through the layers of fabric the liquor may begin to channel outwards through the sides of the batch, resulting in gross unlevelness. Fabric qualities with a tendency to contract, such as unset nylon filament goods, become flattened by the build-up of high pressure close to the perforated beam. This gives rise to moire (water-marking) faults visible as fluctuating light reflections. Such problems are particularly associated with nylon taffeta fabrics woven from lustrous filament yarns and dyed in dark colours. Accordingly, it is usual to give nylon a dry heat setting under tension or a hot scour to relax any inherent shrinkage before winding onto the beam for dyeing. The essential equipment for beam winding consists of a mechanism to rotate the beam and a take-up system capable of delivering the fabric free from creases. The perforated beam is usually surface-driven by being mounted on a pair of parallel rubber-covered driving rollers, which control the rotation at constant speed. The feeding mechanism consists of scroll rollers and uncurlers as on the entry end of a stenter. The fabric tension during winding must be kept to a minimum consistent with achieving a stable batch. When winding of the batch is complete a short length of slightly wider calico or polypropylene net is neatly butt-end sewn on, wrapped around the batch and re-sewn. This provides additional stability and protects against soiling both before and during dyeing. The wound batch of fabric on its cradle is loaded into the dyeing vessel and the inside end of the perforated beam fixed securely to the liquor inlet pipe. The door is securely locked, safety devices checked, water admitted through the inlet and the pump started on in-to-out flow. It is crucial initially to remove all air from the wound batch as trapped bubbles would result in undyed patches. Reversal of the flow direction every few minutes is desirable so that out-to-in flow helps to compact the layers of fabric and gives improved stability during the conventional scouring and dyeing procedures that follow. Dyeing faults in woven fabrics typical of the beam-dyeing process may be classified as [11]:
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1. gross unlevelness caused by channelling in a slack batch 2. weakly dyed patchiness caused by residual air bubbles 3. variation near the selvedges on the innermost layers caused by misjudged overlap of blanked-off portions 4. ending with the innermost layers slightly deeper caused by excessive in-to-out flow during dyeing. As dye liquor is pumped through the batch in the in-to-out direction the pressure and liquor flow per unit area are reduced. The pressure drop is greatest across the innermost layers and the pressure is halved after the passage through about 36% of the batched layers. To avoid ending faults no more than 2% exhaustion of dye should take place at each circulation of the liquor through the batch. It is therefore highly desirable to increase the rate of liquor flow as much as practicable. The flow rate is determined mainly by four factors: 1. pump characteristics 2. fabric porosity (P) 3. number (N) of fabric layers 4. liquor viscosity, which decreases with increasing temperature. Powerful pumps and motors are necessary for rapid flow. Where woven fabrics of only moderate porosity are being dyed, the number of layers should be limited to that given by Equation 15.3. Thus a typical 50-cm perforated beam will tolerate about 1200 m (or 600 layers) of 100 g/m2 nylon taffeta, giving a total batch diameter of about 80 cm [11]. The beam dyer learns by experience what is the maximum tolerable batch size of a specific fabric quality that can be dyed uniformly with an appropriate selection of dyes and dyeing procedure. Useful advice on troubleshooting in the beam dyeing of wool fabrics has been given. This included care in preparation of the wound batch, precautions in the dispensing of dyes and chemicals, problems arising with dyes that are borderline for dispersion stability and the control of foaming using proprietary antifoams [36]. Warp stripiness is a troublesome fault in filament fabrics that may only become visible after the goods are dyed. This is particularly irksome when all normal precautions have been taken in the beam-dyeing stage. A system has been described for analysing the presence of stripiness in the undyed material, involving the use of a high-resolution black/white video camera and an image evaluation unit [37].
References [1]
A Ormerod, Modern preparation and weaving machinery (London:Butterworths, 1983).
[2]
L Vanghelwe, Text. Prog., 29 No. 4 (1999).
[3]
J Heaton in Engineering in textile coloration, Ed. C Duckworth (Bradford: SDC, 1983).
[4]
W S Hickman in Cellulosic dyeing, Ed. J Shore (Bradford: SDC, 1995).
[5]
B A Evans, Text. Chem. Colorist, 13 (Nov 1981) 254.
[6]
W Carruthers, Rev. Prog. Coloration, 19 (1989) 57.
[7]
J Pashley, JSDC, 109 (1993) 379.
[8]
G W Madaras, G J Parish and J Shore, Batchwise dyeing of woven cellulosic fabrics (Bradford:
[9]
M H Rowe, AATCC Nat. Tech. Conf., (Oct 1977) 64; Text. Chem. Colorist, 10 (Oct 1978) 215.
SDC, 1993).
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[10]
L A Sitver, AATCC Nat. Tech. Conf., (Oct 1977) 71.
[11]
D H Wyles in Engineering in textile coloration, Ed. C Duckworth (Bradford: SDC, 1983).
[12]
N H Rathi, G N Mock, R E McCall, P L Grady and L T Farias, AATCC Internat. Conf. & Exhib., (Oct 1997) 254.
[13]
M E Atkinson, AATCC Internat. Conf. & Exhib., (Oct 1991) 63.
[14]
W S Hickman, Rev. Prog. Coloration, 32 (2002) 13.
[15]
K Dickinson and W S Hickman, JSDC, 101 (1985) 283.
[16]
B D Bähr, Textil Praxis, 46 (1991) 780, 973; 47 (1992) 1041.
[17]
R Ott, Melliand Textilber., 78 (1997) 243.
[18]
Solvay Interox Brochure AO. 2.5 Continuous preparation of cellulosic and blended fibres
[19]
H C Paulsen, Internat. Text. Bull., 36 No.3 (1990) 67.
[20]
Anon, Internat. Text. Bull., 36 No.3 (1990) 83; Melliand Textilber., 71 (1990) 398; 72 (1991)
(1986).
775. [21]
C A Theusink, Deutscher Färbenkalender, 81 (1977) 102.
[22]
G Gebhardt, Melliand Textilber., 74 (1993) 44; Amer. Dyestuff Rep., 84 (Sep 1995) 76.
[23]
J Ströhle, Melliand Textilber., 79 (1998) 40.
[24]
J Ströhle and H P Weber, Dyer, 185 (Feb 2000) 22.
[25]
G Rösch, Textil Praxis, 43 (1988) 61, 264, 384, 515, 615, 847; 44 (1989) 38.
[26]
C Duckworth and L M Wrennall, JSDC, 93 (1977) 407.
[27]
D Bechter and G Kunz. Textil Praxis, 34 (1979) 965, 1369.
[28]
W Schumacher, Colourage Annual, 45 (1998) 135.
[29]
S Greif, Melliand Textilber., 72 (1991) 753; 77 (1996) 594.
[30]
K Bredereck and A Commarmot, Melliand Textilber., 79 (1998) 64.
[31]
K Bredereck and A Bluher, Melliand Textilber., 72 (1991) 446.
[32]
G Schnitzer, Textilveredlung, 26 (1991) 78.
[33]
C W Meyer, Melliand Textilber., 79 (1998) 342.
[34]
R D Mehta and P Salame, Textile Asia, 24 (Jun 1993) 41.
[35]
I Bearpark, F W Marriott and J Park, Dyeing and finishing of wool fabrics (Bradford: SDC, 1986).
[36]
R Weisberg, Amer. Dyestuff Rep., 81 (Jan 1992) 34.
[37]
J Wolfram, U Bolz, M Salama and H Weinsdorfer, Textil Praxis, 47 (1992) 619.
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Table 15.1 Descriptive terms for typical woven fabrics Fabric Barathea Bombazine Brocade Buckram* Calico* Cambric* Chambray* Charmeuse Chiffon Crepe Cretonne* Damask* Denim* Drill Fustian* Georgette Gingham Grenadine Grosgrain Lawn* Marocain* Moquette Muslin* Plaid Plush Pongee Poult Seersucker Shantung* Taffeta Tulle* Tweed*
Description Fine woollen cloth, sometimes blended, typically for suitings Twilled worsted dress fabric, sometimes blended, formerly for mourning dress High-quality fabric woven with a raised design, often for furnishings Coarse linen or cotton stiffened with adhesive, used as interfacing and in bookbinding Plain-weave bleached or grey cotton cloth, usually for printing Fine linen or bleached cotton fabric for lightweight garments Linen-finish gingham with a white weft and coloured warp Soft and smooth silk dress fabric Diaphanous fabric woven from silk or man-made filament yarn, often for scarves Fine, often gauze-like, fabric with wrinkled surface produced using high-twist yarns Heavy cotton fabric with printed pattern, usually floral, on one or both sides Twilled linen or silk dress fabric woven with a floral satin pattern Hard-wearing cotton twill with a white weft and indigo blue warp, fashion fabric but traditionally for workwear Coarse twilled cotton fabric, often for workwear or uniforms Thick twilled cotton cloth with a short uncut nap, traditionally for slippers Thin silk or man-made filament crepe dress or scarf fabric Plain-weave cotton cloth, especially in colour/white striped or checked pattern Loosely woven silk or silk/wool dress fabric Corded dress fabric, traditionally woven from silk Fine bleached cotton or linen fabric, often for handkerchiefs Dress fabric of ribbed crepe construction from high-twist yarn Thick uncut loop-pile fabric, usually for furnishings Fine translucent delicately woven cotton dresswear fabric Woollen twilled cloth, usually in a chequered or tartan pattern Cotton or traditionally silk cloth with a long soft nap, often for furnishings Soft and usually unbleached silk or man-made filament dress fabric Fine corded silk or man-made filament taffeta dress fabric Linen or cotton crepe fabric with a puckered surface from high-twist yarns Soft unbleached and usually undyed silk dress fabric Fine lustrous silk or man-made filament dress fabric Soft fine silk or man-made filament net, for veils and dresswear trimmings Rough-surfaced twilled woollen cloth, usually in blended fleck colours
* See text for description
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Table 15.2 Processing of various fabric types in open-width (O) or rope (R) form [4,5] Fabric type Sheetings Table linen Twill fabrics Interlinings Furnishings Ticking Shirtings Canvas Corduroy Pile fabrics
Cotton O/R O/R O/R O/R O/R O/R O O O O
Polyester/ cotton O/R
Viscose
Linen O
O O
O O
O O O O
O
O
Table 15.3 Typical dyeing processes for various cotton and polyester/cotton fabric qualities [8] Fabric qualities Military uniforms, sailcloth, awnings Cotton and polyester/cotton shirtings Furnishing and household textiles Cotton and polyester/cotton workwear Corduroys and velveteens Outerwear, rainwear and coated fabrics Fashionwear, leisurewear, sleepwear, cheap curtains and bedspreads
Processes Continuous dyeing with vat dyes by pad-steam for very high fastness Continuous dyeing with vat dyes for quality goods of high fastness Vat dyes for high fastness; reactive or coppercomplex direct dyes for less critical end-uses Vat and sulphurised vat dyes applied by pad-steam for high fastness to repeated laundering Sulphur dyes by pad-steam or reactive dyes by pad-batch; cheaper qualities dyed with aftertreated direct dyes Generally vat, copper-complex direct or reactive dyes selected for good light fastness Direct dyes widely used with crease-resist finish or cationic aftertreatment for good wet fastness
Table 15.4 Experimental values of interstitial density, interstitial ratio and interchange factor [11] Fabric quality Triacetate taffeta Acetate poult Nylon taffeta
Fabric density (g/m2) 126 110 62
Interstitial density (g/m2) 185 102 59
Interstitial ratio (%) 147 93 96
Interchange factor 0.59 0.63 0.77
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Figure 15.1 Principle of the Tensitrol unit [3]
Fabric rope entry
Idler spools Exit nip
Driven roller
Figure 15.2 Principles of weft straightening for bow and skew Fabric movement
Fabric movement
Straightening forces
Tension
Tension
Bow
Skew
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Figure 15.3 Festoon-type open-width relaxing/scouring unit Draw rollers Draw rollers
Prewetting bath
Exit roller Entry trough Rods on sawtooth ramps
Feed control
Steam-heated immersion bath
Centrifugal pump
Figure 15.4 Gaston County alternator washer [11] Fabric entry Compensator roll Fabric exit
Corrugated baffles
Mangle nip Expander roll
Screens
Cascade washer
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Figure 15.5 Küsters Vibrotex washing unit [11] Fabric exit
Fabric entry
Perforated cylinder
Agitated liquor
Wash compartment
Figure 15.6 Mather & Platt Aquatex units with exit nip [11] Top-driven pressing rollers
Fabric entry
Expander roll
Carryover roller
Free-wheel rollers
Fabric exit
Mangle nip
Counter-current liquor flow
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Figure 15.7 Benninger Becoflex washing compartment [11] Mangle nip
Driven pressing rollers
Fabric entry
Fabric exit
Expander roll
Free-wheel rollers
Figure 15.8 Typical pad-batch application unit [8] Fabric input
Driven batching roller
Padding mangle
Peroxide solution
Alkali solution
Mixing pump
A-frame
Figure 15.9 Principle of the open-width saturator [18] Fabric entry
Exit nip
Entry nip Driven upper rollers
Impregnation bath
Rotating batch
Fabric exit
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Figure 15.10 Principle of the rope saturator [18] Fabric entry
Fabric exit Exit nip
J-piler
Impregnation bath
Figure 15.11 Becco-type open-top J-box unit [18] Fabric entry Fabric exit
J-tube
Steam heating
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Figure 15.12 DuPont-type enclosed J-box unit [18]
Fabric exit
Heater tube
Fabric entry
J-tube
Figure 15.13 Principle of the U-box bleaching unit [4]
U-box Fabric entry
Fabric exit
Idling rollers
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Figure 15.14 Principle of the single-layer conveyor steamer [3] Fabric entry
Fabric exit
Plaiting roller
Conveyor belt
Figure 15.15 Principle of the roller-bed steamer [3] Fabric entry
Fabric exit Plaiting roller
Roller-bed support
Figure 15.16 Principle of the Combi-steamer [18] Tight-strand section
Fabric entry
Fabric exit Plaiter
Roller-bed support
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Figure 15.17 Principle of the multi-layer conveyor steamer [18] Tight strand
Plaiting roller Fabric exit
Fabric entry
Conveyor supports
Figure 15.18 Principle of the jig-dyeing machine [11] Fabric batches on draw rollers with scroll rollers rotating on fabric batches
Idling guide rollers
Dyebath in trough
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Figure 15.19 Threading arrangement for the jig dyeing of velvets [11] Fabric batches on contra-rotating rollers
Pile outwards
Pile inwards
Dyebath in trough
Figure 15.20 The Thies/Funke high-temperature Jigger-Tiro dyeing machine
Figure 15.21 Principle of the beam-dyeing machine [35] Rolled batch of fabric on perforated beam
Expansion tank
Hinged door
Pressure vessel
Heat exchanger
Chapter 16 Continuous Dyeing of Woven Fabrics 16.1 Semi- and Fully-Continuous Dyeing Most of the dyeing equipment and methods discussed elsewhere in this volume have been designed for batchwise processing, in which both application and fixation are highly dependent on preferential exhaustion of the dyebath and thus on the substantivity of the dyes for the substrate. The two previous chapters have focused on the batchwise dyeing of knitted and woven fabrics on the beam (sections 14.6.3 and 15.5.2), jet (sections 14.6.2 and 15.5), jig (section 15.5.1) and winch (section 14.6.1). Fully-continuous piece dyeing methods depend much less on the forces of substantivity, but rely on immediate impregnation of the fabric with dye liquor followed by rapid fixation within only a few minutes after the impregnation stage. The pad-dry-thermofix-wash-dry sequence illustrated schematically in Figure 6.2 (section 6.7) is a typical example. Semi-continuous methods of piece dyeing invariably involve continuous impregnation of the fabric, followed without undue delay by a much slower fixation treatment. The dye liquor, already in close contact with the substrate after padding, gradually becomes exhausted and the dyes diffuse into the fibre interior where the fixation process is gradually completed. Several fully- and semi-continuous coloration methods for textile materials at various stages before fabric manufacture have been described elsewhere in this book. These include the continuous producer coloration of synthetic fibres (sections 12.2 and 12.3), the continuous dyeing of loose fibres (section 12.5), semi- and fully-continuous dyeing of tow or top (section 12.6) and the dyeing of cotton warps for denim manufacture (section 13.9). Mention has also been made of the pad-batch dyeing of machine-washable wool (section 8.12.2), the pad-thermofix process for narrow fabrics made from nylon or polyester (section 13.9) and the pad-steam dyeing of nylon carpeting (section 18.8.4). Typical examples of continuous processing sequences for cellulosic fabrics and polyester/cellulosic blends are listed in Table 16.1. Some of these (azoics, leuco esters) are now essentially of historical interest. Continuous piece dyeing is seldom an option for goods made from other fibres and blends, either because of practical difficulties or of insufficient demand for long runs (more than 5000 metres) to a single shade. Wide-width fabrics woven or knitted from synthetic fibres suffer from insuperable migration problems during drying after padding. The continuous dyeing of wool fabrics has never been commercially viable because of the preponderance of colour-woven or –knitted designs and heatherblended yarns. Silk, linen and regenerated cellulosics could be dyed continuously but there is very seldom a demand for adequate amounts per dyelot of these goods to justify installing or modifying the necessary equipment to adopt this approach. It has often been claimed that the visual appearance, freedom from creasing, uniformity of shade and reproducibility of fabrics dyed continuously are normally significantly superior to goods dyed by alternative batchwise methods. This is particularly often found when applying vat or sulphur dyes to cellulosic fabrics. The classical pad-dry-steam route was originally introduced by DuPont in the 1940s to produce long runs of vat-dyed cotton for military uniforms [1] and it was later adapted to apply other classes of dyes. Vat dyes still represent an important part of the continuous dyeing sector where high fastness to light, weathering, bleaching and severe repeated laundering is required.
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Dyehouses equipped for continuous dyeing have faced difficult problems in recent years as the nature of the global market-place has changed. The dramatic growth in demand for woven leisurewear, sportswear and even fashion-related brightly coloured workwear has brought major production problems. The need for rapid response to fashion changes, to meet short lead times and to maintain minimal stocks of dyed goods has led to a greater variety of individual shades and to shorter runs per dyelot [2]. Before 1980 the average length of run to a single shade was as much as 50,000 to 100,000 metres in the USA and typically 10,000 to 20,000 elsewhere in the world [3]. A decade later the growing need for frequent changes of fashion had brought about a marked decrease in these average values (see Table 16.2). The problems that short runs bring have been summarised [2]: 1. increased downtime 2. more changes of shade 3. reproducibility problems 4. more wastage of dye liquor 5. increased environmental problems 6. less efficient utilisation of machinery 7. decreased cost-effectiveness Figure 16.1 shows how machine utilisation falls dramatically with shorter production runs and more prolonged downtimes. Taking 75% (horizontal dashed line) as a realistic and commercially viable target for effective machine utilisation, it is seen that downtime is a crucial factor in defining the minimum length of run that can still achieve this level of efficiency. Thus a downtime of one hour requires a run of 10,000 metres or longer, whereas an average run per shade as short as 1000 metres can be tolerated if downtime can be brought down to only 5 minutes. These important relationships have been outlined for a specific case study of a dyehouse producing 60 million metres of dyed goods annually, about 20% comprising polyester/cotton blend fabrics dyed by the pad-dry-thermofix-padsteam route. Although an average length of 5000 metres per shade produces an annual profit exceeding £1 million, calculations show that when the run length falls to only 1000 metres this conventional process sequence ceases to be profitable. Continuous dyeing methods for blends that demand an intermediate reduction clear are particularly severe loss makers [4]. Another significant factor is attributable to the loss of equilibrium in processing conditions when a continuous dyeing range has to be stopped because of fabric damage or equipment failure, for example. It can take several hundred metres for stable processing conditions to be restored when the range is restarted, resulting in a long length of unacceptably variable or off-shade material. Approximately 800 metres of cloth are required to completely fill all units of a classical continuous dyeing range [5]. When runs per shade as short as 1000 metres are to be dyed, it is obviously totally unacceptable to incur off-shade or unlevelness faults of a similar magnitude. Machinery developments have to be focused on the essential requirement of maintaining the plant in operating status whilst changing shade or dealing with in-process modifications, thus keeping downtime to an absolute minimum. In these circumstances dyehouse organisation becomes extremely important. A vital consideration is the condition and availability of the fabric to be dyed. This
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must have been thoroughly prepared, consistent in quality and available at the time it is required. Careful scheduling of production to ensure a consistent supply of top-quality fabric must be guaranteed before any machinery modifications to minimise downtime can be effective [2].
16.2 Continuous Dyeing of Cellulosic Fabrics The use of thermal fixation in the continuous application of reactive dyes has been established since the first dichlorotriazine and monochlorotriazine dyes were introduced in the 1950s. Both of these dye classes were found suitable for application by the pad-dry-bake process sequence illustrated in Figure 16.2. In recent years homobifunctional bis-monochlorotriazine dyes such as the Procion CX (BASF) range have been introduced into this process to give deeper shades at higher fixation levels [2]. Similar results have also been reported when applying heterobifunctional monochlorotriazine-vinylsulphone dyes by the pad-dry-bake route [6]. This method of dry heat fixation has become well-established over the years but there are serious environmental drawbacks related to the necessary use of urea. This auxiliary acts as a humectant to maintain a humid environment at the fibre surface and to promote diffusion and fixation of the reactive dyes. At the traditional temperature range required for optimum fixation (150 to 180°C) the excessive emission of urea fumes into the dyehouse working area can be a problem. Elimination or replacement of urea by alternative humectants has proved difficult because of its low cost and outstanding effectiveness. It has long been known that dichlorotriazine dyes could be fixed efficiently in the absence of urea providing a level of 20 to 25% (vol) relative humidity could be maintained in the hot flue dryer. However, the problem of monitoring and controlling this level of humidity had always proved difficult. Recent improvements in the design of control systems have facilitated the joint development of the Econtrol process by Monforts and BASF [7]. The fabric is padded with the dichlorotriazine dyes, 1-2 g/l wetting agent and 10 g/l sodium bicarbonate. No urea, sodium silicate, common salt or other chemicals are necessary. After a short air passage the uniformly wetted and squeezed fabric is transported into a hot flue dryer. It is treated continuously for 2 to 3 minutes at 110 to 130°C with 25% (vol) steam content in the dryer (Figure 16.3). A fabric temperature of 68°C is attained during drying and this is adequate to fully fix such high-reactivity dyes. The Econtrol Thermex system was introduced in the ITMA exhibition at Milan in 1995 [8]. This novel machinery was designed to inject steam into the hot flue at the impulse of the computer which continually monitors and controls the humidity level. Further development of the equipment is continuing [9]. The presence of 25% (vol) steam in the Thermex chamber, together with the rapid rate of fixation of dichlorotriazine dyes, ensures that migration problems are virtually eliminated. This not only makes the Econtrol method suitable for plain woven constructions but has given good-quality dyeings on pile fabrics [7]. Although dyes of lower reactivity can be applied on the Econtrol equipment they require stronger alkali and the use of a dye/alkali metering pump. Furthermore, several of the key advantages of the Econtrol process are sacrificed when these less reactive types are applied [10].
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16.3 Continuous Dyeing of Polyester/Cellulosic Blends When dyeing a polyester/cellulosic blend fabric continuously there is a more difficult requirement to dye each of the fibre types present with a different class of dyes and to ensure that the process conditions for both are mutually compatible. The pad-dry-thermofix process for dyeing polyester with disperse dyes was introduced commercially during the 1950s and it has changed little since then. A fixation temperature in the range 200 to 220°C is necessary and attempts have been made to use these conditions to colour the cellulosic component of the blend with an appropriate class of dyes. In 1959 DuPont was recommending that pastel shades on polyester/cotton should be dyed with vats only by the pad-dry-steam process, pale to medium depths with only disperse dyes applied by pad-dry-thermofix and deep shades should be matched with a prolonged three-bath sequence of pad-dry-thermofix disperse (Figure 16.4), vat pigment pad-dry and finally reducing pad-steam-reoxidisewash-dry (Figure 16.5) [11]. A more economical approach was to apply the disperse and vat dyes simultaneously but this brought with it the problem of some of the vat dye becoming fixed on the polyester during the thermofix treatment, making shade matching more difficult [12]. In 1961 considerable work was carried out in the laboratories of Cassella in Germany on the selection of vat dyes from the extensive Indanthren range that would dye both polyester and cotton to similar depths of shade [13]. These were marketed under the trade name of Polyestren, but disperse/vat combinations still had to be applied for deep shades. Initially the need to reduction clear the standard disperse dye brands available at the time precluded combining disperse and reactive dyes in the same process stage, as the reduction clearing bath of sodium dithionite and caustic soda would destroy the reactive dyes present. The disperse/vat pad-dry-thermofix-reducing pad-steam method had the advantage that there was no need for a separate reduction clear stage, as the disperse dyes on the polyester surface were cleared during the reducing pad-steam stage, which utilised dithionite and alkali to reduce the vat dyes to their soluble leuco forms. When applying reactive dyes to the cellulosic component, however, it was often necessary to clear the disperse dyes from the polyester before colouring the cellulosic fibres in a separate pad-batch, pad-dry-bake or pad-dry-alkali pad-steam process. With the gradual trend towards shorter runs in a wider range of shades, such elaborate multi-stage processes made polyester/cellulosic blends increasingly difficult to operate efficiently. Around 1980 the first effective one-bath continuous dyeing processes were developed for dyeing polyester/cellulosic blends. Essentially two distinct approaches were adopted. In the late 1970s the Procion T (ICI) acid-fixing phosphonate reactive dyes were introduced. These required the same conditions of fixation (pH 5 to 6 and 200 to 220°C) as the disperse dyes on polyester. Although these dyes achieved limited success in the 1980s and were an elegant approach to polyester/cellulosic dyeing, they were eventually withdrawn from the market. In the 1980s Nippon Kayaku marketed the Kayacelon React bisnicotinotriazine dyes that also gave excellent fixation on cotton under mildly acidic conditions at thermofix temperature. The second approach was to utilise conventional reactive dyes in a one-stage process. In the late 1980s several methods of this kind were introduced, known as the AT (Bayer) [14], 2DA (Sandoz) [15] and RTN (ICI) [16] processes. The
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general principle was to select suitable reactive dyes that could be fixed efficiently using mild alkali in the thermofix treatment at 200 to 220°C. These conditions would avoid the deleterious effects of stronger alkali on disperse dye yields and minimise the dulling of shade attributable to yellowing of the cellulose at high pH and temperature. Careful dye selection is the key to success with this approach. The presence of reactive dyes precludes the reduction clearing step, but inclusion of the alkali-clearable Dispersol PC (ICI) dyes enables surface disperse dye and hydrolysed reactive dye to be removed simultaneously at the wash-off stage [17]. In the early 1990s a detailed cost comparison was carried out between the RTN (ICI) process and five other continuous dyeing methods for polyester/cotton blends. Calculations were completed for average run lengths of 5000, 2500 and 1000 metres in order to assess the influence of run size on cost per metre. An important trend was that the cost advantage of a one-stage treatment, such as the RTN process, over more prolonged conventional sequences increased markedly as run size decreased. When these data were extrapolated to profitability comparisons in a specific case study, it was evident that if run lengths averaging only 1000 metres were dyed using the longer conventional sequences incorporating a reduction clearing step, production ceased to be profitable [2].
16.4 Dye Impregnation 16.4.1 Dye Padding It is preferable to feed woven fabric from a movable A-frame of large diameter rather than in plaited form from wagons. The cloth must be crease-free with flat selvedges and should be run under only moderate tension; excessive tension adversely affects wetting-out, absorbency and liquor retention. Fabric is drawn from the batch by a pneumatic draw nip with compensator control feed to the padding unit. The goods must be readily and uniformly absorbent because the immersion time in the pad liquor is normally less than 10 seconds. Bone-dry cotton or pre-set polyester/cotton blends are notoriously slow to wet out, so it is important for sufficient wetting agent to be present during padding. Batch changing within a dyelot is accomplished without significant variation in range speed by means of an accumulator scray. The operator manually increases the speed of the draw nip to accumulate sufficient fabric in the scray to give time to change batches and stitch the fabric ends together. When replacing batches between dyelots, the range normally has to be stopped to change dye liquors and for appropriate cleaning of the equipment. Productivity of continuous dyeing processes is greatly influenced by the duration of this downtime to change colour. The uniform application of dye liquor to the fabric is the most critical stage of a continuous dyeing process and satisfactory performance of the padding unit is crucial to success. Pad mangles for dyeing are often termed padding units or padders; the description mangle is more appropriate when referring to the squeezing of water from the cloth, as between successive rinses of a washing range or for hydro-extraction prior to drying [18]. Alternatives to the dye padder have been developed, such as spray systems, foam application, multicolour feed systems (Polychromatic technique, Küsters TAK method) and the Standfast Molten Metal unit, but the various forms of dip-and-nip application remain overwhelmingly the most important.
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The padding operation itself consists of two essential steps: thorough impregnation by immersion of the absorbent fabric in a dye solution containing a wetting agent, followed by squeezing of the wet fabric between rollers to expel air and replace it with dye liquor, as well as expressing surplus liquor back down the sloping fabric surface to the pad trough. The fabric must be centred accurately for delivery to the nip and prevented from wandering by strategically positioned driven scroll rollers. For dye or chemical padding a bowed sleeve expander is obligatory to remove or prevent creasing. The usual line of entry to the nip is just a few degrees below the horizontal. The fabric path on leaving the nip is equally important. This should be a horizontal path for a vertical nip, i.e. the fabric exits along the tangent to the pad rollers, because the dyed shade will show two-sidedness (back to face differences) if the wet fabric touches the surface of either roller. A roller guide therefore follows the nip and the path is then to a compensator for speed/tension control. For some dyeing systems a diffusion time between padding and predrying is beneficial and this requires an airing frame or accumulator scray. All the various units of a continuous dyeing range have critical aspects of construction and operation but the padding unit is probably the most important as it controls the initial application of the dye liquor, not only in terms of uniformity of absorption but also the amount of dye liquor retained by the padded fabric. The liquor retention (also sometimes called the nip expression, pick-up or add-on) is determined by the fabric construction, thoroughness of preparation and the physical conditions of the padding operation, such as fabric speed, nip loading and temperature. The liquor retention (R) is the mass of liquor absorbed during padding expressed as a percentage of the mass of dry fabric entering the immersion trough. It is usually calculated from weighings of fabric samples before (Fd) and after (Fw) impregnation. It is also equal to the ratio between the concentration of dye absorbed by the fabric (Df g/kg) and the dye liquor concentration (Ds g/l) in the padding trough (Equation 16.1): F − Fd D R = w = f 100 Fd Ds
Equation 16.1
Apart from its importance in recipe formulation, the liquor retention must be accurately known and recorded for each dyelot in order to calculate how much liquor must be available in the stock tank to ensure that the pad trough remains full until the end of the run. Not only would insufficient be disastrous, but to make up too much is a direct waste that for short runs would add substantially to the cost of dye usage. Typically, liquor retention values for standard woven fabrics would be 45 to 55% for polyester/cotton, 60 to 70% for cotton and 80 to 90% for regenerated cellulosics [18]. Variations outside these limits are observed with non-standard constructions, however, such as 70-75% for a lightweight cotton voile but as low as 50% for a heavier cotton satin [19]. The nip rollers (often called bowls) are the key to successful pad dyeing. In general, two-bowl nips are preferred for lightweight or standard fabrics running at moderate speeds, whereas three-bowl arrangements are intended for heavier or more densely woven qualities that may be more difficult to wet out and thus require a double-dip and double-nip treatment. High-speed running of standard fabrics also demands a longer immersion time. Two-bowl nips can be arranged vertically, inclined in ascending or descending order, or horizontally opposed. The ascending bowl arrangement gives better observation of the nip than the descending-bowl system, but the latter has a slight advantage in that the fabric
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path does not change as much. Various three-bowl arrangements are possible (Figure 16.6) and these are usually designed to provide maximum impregnation on the first dip-and-nip, followed by maximum uniformity at the second squeeze. It is normally necessary to take the fabric over a free-running, smooth roller before entering a vertical or inclined nip. This should be positioned so that the fabric path leaving the trough is nearly, but not quite, vertical and so that the fabric enters the nip at a suitable, near-tangential angle. The desirability of ensuring minimum contact with the impregnated goods led to the introduction of the horizontally opposed pad, in which the fabric rises vertically from the pad trough directly into the nip and straight through into an infrared pre-dryer mounted above the nip. An alternative application that exploits a unique feature of the horizontally opposed arrangement is the so-called Peter pad. In this system the space immediately above the nip can be used to form the liquor trough, with plates fitted to retain the dye liquor at the ends of the rollers. In this instance the incoming fabric passes vertically downwards into the liquor and through the nip. The advantages of this Peter pad system are that the dyebath has a very small volume and there is no drainage of depleted liquor back into the trough caused by preferential absorption. However, the Peter pad or ‘wedge nip’ is mainly restricted to lightweight goods, such as polyester or polyester/cellulosics dyed by the paddry-thermofix process. Thicker qualities tend to suffer from liquor seepage at the selvedges and the setting of the end plates has to be adjusted exactly to maintain a good seal with the pad rollers rotating in opposite directions. Threading-up and cleaning of the horizontal nip system are more difficult and the continual expulsion of air from the incoming fabric tends to cause frothing at the surface of the pad liquor. The pad trough is usually a deep U-shaped vessel with a single roller attached to the base of a displacement block that leaves a narrow passage to accommodate the moving fabric (Figure 16.7). The liquor volume should be as small as possible to allow complete replenishment of dye liquor within about 3 minutes and thus minimise tailing problems. Infeed of the liquor is via a perforated pipe running across the back of the vessel, the perforations pointing downwards away from the fabric. Depending on the fabric speed, the immersion time usually varies from about 2 seconds at 120 m/min to 5 seconds at 50 m/min. Less than 2 seconds poses problems in achieving uniform saturation. The liquor volume is influenced by the slot width and by the separation between the inner surface of the trough and the outer surface of the displacement block. In theory this separation need not be much wider than the fabric thickness but in practice it should allow the occasional rough seam to pass without the fabric touching the metal surfaces. The interior of the pad trough must be smooth and polished so that it is easy to clean with a pressure hose; dye carry-over from one shade run to another would be disastrous. The bottom roller rotates in sealed bearings and is typically about 10 cm in diameter. An alternative trough system contains an assembly of rollers disposed vertically to displace a substantial volume of liquor (Figure 16.7). For a given vessel size, this assembly gives a slightly longer immersion length than the displacement block with a single roller. The flexing of fabric around these rollers is claimed to assist liquor penetration but multi-roller systems are more difficult to clean and there are more bearings to maintain. A conventional trough of the displacement or multi-roller type is about 1.8 metres wide and contains about 45 litres of pad liquor [20]. Even wider padders and larger troughs are required when processing furnishing fabrics. In comparison,
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the 26-litre economy trough (Figure 16.7) has only one roller and an immersion time of only about 2 seconds at 50 m/min. This means that thorough penetration and consistent absorbency of the fabric is even more important when operating with this smaller vessel. The wedge nip of the horizontal-bowl Peter pad is even smaller with a liquor volume of approximately 6 litres. As already noted, this limits its applicability mainly to polyester and polyester/cellulosic blends. The installation of a 12-metre skying unit immediately following the Peter pad has been recommended in appropriate circumstances to facilitate adequate penetration of the fabric before commencing pre-drying. To obtain consistency of shade it is most important that the fabric running speed and the length of immersion of fabric in the dye liquor remain constant throughout the padding run. Control of the immersion length is achieved by means of automatic liquor level devices. Although these work quite well in a conventional trough with an immersion length of several metres, they are less accurate when applied to an economy trough. The three main types of automatic level control are: 1. Float switches: these are reliable and are unaffected by foam but they are relatively bulky 2. Conductivity probes: these are small and neat but foaming of the pad liquor adversely affects their performance 3. Differential pressure detectors: those with a hollow tube projecting downwards from the liquor surface are difficult to clean and the preferred type is that with a closed diaphragm set in the base of the vessel. The control of pad liquor temperature is highly desirable to achieve consistent results. Temperature variations can arise if the roll of fabric being delivered from the A-frame has been dried a short time earlier following the last stage of preparation, so that heat has been retained within the batch of bone-dry cloth. Pad trough temperature fluctuations ranging between 20°C and 30°C have been observed under bulk-scale operating conditions [21]. Differences in temperature of this magnitude can cause variations in depth when applying mixtures of disperse and vat dyes to polyester/cotton [20]. Greater instability of the pad liquor may also arise when operating with mixtures of disperse and reactive dyes on these blends. At the end of the padding run the dye liquor in the trough and feed pipes, as well as that remaining unused in the stock tank, is discharged to drain. The cleaningdown procedure should be carried out as follows [22,23]. The pad trough is emptied of liquor and, together with the bowl system and associated rollers, is sprayed with water two or three times. These sprays wet out the following absorbent end-cloth which is made to traverse the rollers and so clean those parts of the rollers that were outside the dyed fabric width. The rollers can be repeatedly braked automatically and released to facilitate cleaning. Residual dregs of water are removed from the pad trough by means of compressed air jets. About 200 metres of a highly absorbent end-cloth are required for this procedure. The entire cycle takes less than 5 minutes when changing from a pale shade but when deep shades have been run more vigorous manual cleaning may be necessary. The liquor wastage factor, which is the unused dye discharged as effluent expressed as a percentage of the total amount of dye initially prepared for that dyelot, can be quite high and the trend towards shorter dyelots has aggravated the wastage problem. Table 16.3 gives an example of 1000 metres each of two
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different (1.6 m wide) fabrics (120 g/m2 dresswear and 250 g/m2 workwear) with a liquor retention value of 50% using conventional and economy troughs. An estimate of 10 litres of dye liquor in the feed pipes has been added to the nominal volume of each trough but there is no allowance for any residual liquor remaining in the stock tanks. With the exception of creasing, shading across the width of the fabric is the largest single cause of customer complaints in continuously dyed goods. To test that the width of the nip (the contact zone where the rollers are directly touching) is consistent across the full width of the roller assembly, a 15 cm-wide strip of soft carbon paper sandwiched between sheets of white paper is laid along the top of the stationary lower roller. The upper roller is slowly lowered and pressed on the sandwich at the full working pressure for a few seconds before release. The width of the carbon impression on the white paper is then measured at different points along the length of the nip area. Although rollers made from very soft rubber may cause the sandwiched paper to split, this method is usually highly successful and has the advantage that the marked paper can be readily numbered and stored for future reference. An alternative technique involves spraying a suitably fine white powder into both sides of the nip already set at normal working pressure. The upper roller is then lifted, the width of the dark rectangular nip area measured at various points and the results carefully recorded. Although it is necessary to confirm that a parallel nip width has been achieved, the setting of the nip pressure under static conditions does not guarantee consistency of behaviour under actual running conditions. Monitoring moisture content of the moving fabric just before immersion and immediately after the nip using Mahlo or Pleva detectors at the fabric centre and near the selvedges will provide continuous trace readings on a line chart. Automatic adjustment of the nip pressure to compensate for gradual variations in incoming moisture level can be arranged in order to improve the consistency of liquor retention after padding [24]. Since it is the final shade that matters, on-line colorimetry has been evaluated as an effective means of control but this brings problems of where to locate the Mahlo Colorscan or Macbeth Eagle Eye colorimeter and how to correct for the effects of moisture and temperature on the shade [23]. Positioning the colorimeter at the final drying stage would give a direct measure of the dyed shade achieved but by the time this was found to be off-shade the continuous dyeing range would be full of unacceptable cloth. The question of colorimeter location has been carefully considered (section 6.8 and Figure 6.2), with the conclusion that the measuring device should be sited close to the wet fabric downstream from the padding unit and thus provide an early warning of off-shade trends or other shade faults [24]. Nip rollers used for continuous dyeing of woven fabrics are typically 170 to 200 cm in length and 30 to 40 cm in diameter, although for regular high-speed running (120 m/min) larger diameters are favoured. Increasing the diameter reduces the rev/min for a given fabric speed but heavier rollers necessitate larger bearings. When padding with dye solutions, a minimum contact width of 1 cm is desirable. The greater this width, the more rapid the wear of the roller surface. Increasing the area of contact for a given loading reduces the pressure applied to the fabric. To compensate for this effect, increased force has to be applied to the ends of the rollers when trying to achieve a relatively low liquor retention.
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Nearly all nip rollers are made from a steel mandrel covered for most of its length by hard rubber with a surface layer of relatively soft rubber about 15 mm thick. The hardness of this surface is measured using a Shore Durometer; pad rollers for dyeing are typically 55 to 70° Shore. Both rollers must be of similar hardness or two-sidedness will occur. The softer the rubber layer the more readily it deforms under pressure to give a greater area of contact. Soft rollers will deform around slubs or other thick places in a fabric and with relatively open constructions will lower the liquor retention in the fabric interstices. Unfortunately, a roller surface that is easily deformed is also somewhat prone to rapid wear and a suitable compromise must be reached. The chemical composition of the rubber surface must be resistant to dye liquors and associated auxiliaries. Immediately after a run, the nip rollers must be hosed down thoroughly, allowing time for all chemicals to diffuse out of the surface layer. Soft rollers tend to harden during use and frequent cleaning is essential to avoid dye particles becoming engrained in tiny blemishes on the surface. Organic cleaning solvents are not recommended; aqueous detergent solution and bristle brushes are preferred. To preserve the rubber coating in prime condition, rollers should be covered during storage to minimise oxidation of the surface. Excessive heat, cold, dampness and direct sunlight should be avoided. After prolonged storage, grinding to remove the outer layer of oxidised rubber is recommended. The application of pressure to the roller assembly is usually by a pivoted lever system linked with pneumatic cylinders and operates on the mandrel at both ends of the upper roller which is forced, through a fulcrum, to press against the lower roller. In three-bowl mangles the central roll is fixed and the outer two press against it. The maximum pressure is about 400 N/cm (50 kg/cm) of bowl width, or a total of about 10 tons across the entire area of contact between the rollers. For a given force, the softer the rubber surface layer of the roller the larger is the area of contact at the nip and the less the actual pressure applied (Figure 16.8). The contact area also increases with bowl diameter. For very soft bowls of large diameter a point is reached at which an increase in force will not result in increased nip pressure. Increasing the force applied to the steel mandrel at both ends of the upper roller naturally causes some deflection of the roller, providing lower pressure in the middle of the nip and higher pressure near the selvedges. This results in greater liquor retention in the middle of the fabric, producing paler selvedges. The amount of deflection can be calculated and depends on the diameter of the rollers, the length of the contact area, the inherent stiffness of the steel mandrel and, of course, the force applied. The deflection can be measured using a Shirley Bowl Deflection Indicator. The Suchy K02 two-bowl and K03 three-bowl highperformance units are designed to guarantee a high squeeze effect with satisfactory uniformity across the full width of the fabric [19]. In order to try to counteract the effect of bowl deflection the surface camber of one or both bowls is modified so that the surface profile nullifies the deflection. This is usually done by grinding down the surface to achieve slight tapering of 1-2 mm towards each end of the roller. This is a poor expedient as the amount of camber is only correct for one specific pressure at the nip. An alternative approach to correcting for bowl deflection is to skew the axis of the pressure roller relative to that of the fixed roller. This arrangement slightly reduces the nip pressure near the selvedges relative to the fabric centre, the amount of skew being varied with the bowl deflection. An advantage of this system is that the bowl surfaces are ground parallel but it is mechanically unsound because it
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substantially increases the load on the bearings and accelerates the wear of the rubber coverings [3]. The introduction of the Küsters ‘swimming-roll’ design in the 1960s was a major step forward in the attainment of a consistent nip width. The principle of this padding unit is to have a stationary steel mandrel around which the steel shell rotates in bearings. The space between these components is divided into two compartments by pressure seals. The smaller one, pressurised by oil, is always directed towards the nip. Oil is pumped continually into this compartment, some passing the seals and circulating back to the oil reservoir in the larger compartment. Increasing the oil pressure in the smaller compartment raises the surface pressure at the middle of the roller to compensate for the deflection of the mandrel. By controlling the internal oil pressure and the pneumatic force exerted at the ends of the mandrel, the configuration of the system can be adjusted to give a uniform linear pressure at the surface. The maximum internal pressure is about 500 kPa. There is a danger that the deflecting internal mandrel will come into contact with the internal surface of the swimming shell. A graph is therefore supplied with each machine, showing the permitted settings of internal oil pressure and compressed air pressure applied to the mandrel. Swimming rollers are normally used in pairs but can be fitted in configuration with a conventional roller. Most of the alternative padding units developed to counter nip roller deflection operate on similar principles, deflecting the outer surface of the upper roller either mechanically [23] or pneumatically [25] in such a way as to ensure a consistent nip width. A recent exception is the Monforts Matex Color padder, which has a continuously variable crown roller system. It is an important component of the Econtrol continuous dyeing range (Figure 16.3). Adjustments to the nip profile are made by moving the rollers in a transverse direction. Advantages of the Matex Color system include simple mechanical design, excellent squeezing effect, ease of operation, optimum reproducibility in setting of the desired nip profile and automatic nip profile monitoring during running [26,27]. 16.4.2 Vacuum Impregnation A major problem when impregnating heavyweight fabrics is the presence of air bubbles occluded within the interstices of the construction, which seriously impedes the rapid absorption and saturation of the fibres with dye liquor. Following the observation that a textile fabric under vacuum can be very rapidly saturated with a dye solution [28], development work was commenced on equipment designed to apply vacuum impregnation continuously [29]. This led to the commercialisation by Farmer Norton of a vacuum impregnation machine in which the fabric was carried on a rubber belt beneath a perforated rotary screen, in the first half of which suction was applied to the fabric and in the second half dye liquor was fed at atmospheric pressure. The saturated fabric then passed through a nip to squeeze off excess liquor, which could be either returned to the dye feed system or discharged to drain. Trials showed that effective impregnation was achieved even on loomstate cotton containing sized warp yarns. The early vacuum extraction devices tended to gain a reputation for unpredictability and difficulties of control. There was gradual accumulation of lint at the vacuum slot or screen and the vacuum performance varied with fabric quality, width and running speed. Although much improved penetration of thick, heavy fabrics could be achieved, increased quantities of dye were required to
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attain the target depth of shade. Commercial acceptance was limited by these drawbacks, because virtually all fabric qualities gave satisfactory results in the two-dip, two-nip impregnation sequence on a conventional three-bowl padding unit (Figure 16.6). A recent detailed study of the influence of machine- and fabric-dependent parameters on the degree of hydro-extraction achieved by a vacuum extraction assembly was carried out on cotton, viscose, nylon, polyester and polyester/cotton fabrics. It was demonstrated that the most significant parameters included the operating vacuum, slot width, fibre type, fabric quality, width and running speed. Closer tuning between the vacuum setting and fabric parameters was advocated to achieve consistent results [30]. Modern vacuum extraction devices give highly reproducible performance. The Optivac (Optitexma) vacuum bars minimise frictional forces because of the special nozzle construction. These systems are equipped with high-performance cyclone separators and frequency-controlled pump motors to maintain a constant vacuum level [31]. The EVAC LVLM (low volume, low moisture) vacuum impregnation dyeing unit is claimed to be suitable for the pad-batch or pad-dry application of reactive dyes, the pigment pad-jig develop process for vat dyes and pad-dry-bake coloration with pigment formulations [32]. In practice, the viability of vacuum impregnation techniques is limited by the reuse capability of the colorant dispersion or solution extracted by the vacuum slot. It is vital to recycle as much as possible because the discharge of this liquor to drain results in high wastage factors. With reactive dye solutions this is seldom practicable because of premature hydrolysis, resulting in inferior fixation of the recycled dyes. Vacuum impregnation has been most often used for pad-jig dyeing with vat dyes and the application of pigments by the pad-dry-bake process. When operating with pigment dispersions, vacuum extraction assists greatly in minimising migration at the dyeing stage [2].
16.5 Pad-Batch Dyeing The equipment and process sequence for pad-batch dyeing of cotton with reactive dyes (Figure 16.9) is essentially the same as the pad-batch bleaching process using alkaline peroxide (section 15.4.4). The processing stages are impregnation with the alkaline dye solution on a conventional dye padding unit, batching on an A-frame for a suitable dwell period of 4 to 24 hours depending on dye reactivity, followed by washing-off on an efficient open-width washing range. The saturated roll of dyed fabric is wrapped in a plastic sheet and the roll kept slowly rotating during the fixation stage to prevent drainage of dye liquor through the roll. It is vital to have sufficient A-frames available to cope with the production rate of the padding unit and to provide a steady supply of dyed fabric to the washing and drying range. The pad-batch dyeing process represents a particularly useful alternative for woven fabrics between exhaust dyeing on the one hand and continuous methods such as pad-dry-steam or pad-dry-bake on the other [33]. Most woven constructions are suitable for pad-batch dyeing but top-quality results are less often achieved on knitted cotton. Nevertheless, the Monforts Matex Color padding unit (section 16.4.1) is claimed to be fully satisfactory for both knitted and woven goods [34]. Table 16.4 lists the main advantages of the pad-batch process.
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Important factors that must be taken into account in order to achieve satisfactory results by this method are specified in Table 16.5. High-reactivity dyes (dichlorotriazine, dichloroquinoxaline, difluoropyrimidine types) have been widely used in the pad-batch process because of the short batching times that can be achieved using only moderate alkaline pH values in the pad liquor. Dyes of intermediate reactivity (monofluorotriazine, vinylsulphone and the bifunctional Cibacron C fluorotriazine-vinylsulphone dyes) are also important, since they can be fixed at long dwell times using mild alkali or more quickly at higher alkaline pH [33]. Dye substantivity has an important bearing on compatibility and the reproducibility of dyed shades, as well as the relative ease of washing-off after the batching stage. Substantivity data can be expressed in terms of the exchange factor (E) when the dyeing system has reached equilibrium (Equation 16.2): E = Di / D e
Equation 16.2
where Di is the initial dye concentration and De is the value observed when equilibrium fixation has been achieved. Values of E have been recorded for typical high-reactivity dyes on cotton alone and in binary or ternary combinations [35]. Radio-frequency (RF) heating of the saturated fabric during batching after padding with dye liquor greatly accelerates the rate of fixation. Thus a dwell time of 24 hours for low-reactivity dyes can be drastically reduced to 1 to 2 hours in this way. The rate of RF power absorption increases with moisture content and the electrolyte concentration in the dye liquor applied. The migration behaviour of dyes during RF treatment is more problematical than in conventional hot-air drying. RF heat treatment combined with short-dwell pad-batch processing offers little or no savings of dyes or chemicals [36].
16.6 Infrared Pre-Drying Conventional drying of a wet textile fabric entails the application of heat to both surfaces, resulting in a rise of temperature and the evaporation of water as steam into the surrounding atmosphere. The water lost from the surface in this way is replaced by diffusion from the interior of the textile material and this process continues until evaporation ceases when the fabric becomes dry. The higher the temperature of heating and the greater the amount of water initially present in the goods, the more vigorous is this evaporating process. When dissolved or dispersed dyes are present in the wet fabric, movement of the liquor in which they have been applied results in migration and irregular deposition close to the fabric surface. General blotchiness or irregular streakiness, haloes around slubs or other features of the woven design, as well as two-sidedness of shade, are typical unlevelness problems attributable to migration. Two-sidedness occurs when one face of the fabric is subjected to a higher temperature than the other; dye movement takes place preferentially towards the hotter surface to give a slightly deeper shade than on the cooler one. Anionic polyelectrolytes, typically sodium alginate, are often added to the pad liquor to act as migration inhibitors. These increase the viscosity of the liquor and facilitate a more gradual process of evaporation at the fabric surface. If an impregnated fabric is allowed to dry slowly and uniformly at ambient temperature no dye migration takes place but this is obviously not a commercially practical approach to the problem.
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As drying proceeds a critical moisture content is reached below which migration ceases. This critical value depends mainly on the fibre types present and, to a lesser extent, on the nature of the fabric construction. Thus a typical cotton fabric with a liquor retention of 75% after padding will exhibit migration phenomena until a critical moisture regain of about 25% is reached, when about two-thirds of the absorbed water has been evaporated. Corresponding approximate values for other substrates include: viscose or lyocell 40%, polyester/cotton 20%, nylon 10%, polyester 5%. If infrared (IR) radiation is applied evenly to both sides of the fabric it is possible to carefully evaporate the water content from the liquor retention level down to the critical regain level without inducing unacceptable dye migration. The difference in temperature between the interior and the surface of the fabric during drying by thermal radiation is much less than in drying by conduction or convection. IR heaters are mounted as vertical arrays and may be gas- or electrically-heated. The length of each array of heating elements ranges from 1.5 to 3 metres and, in the case of gas-firing, the elements are estimated to reach a temperature of 800°C. An IR radiation of wavelength about 3 µm is required, as this is preferentially absorbed by the textile fibres. The thermal output is high and difficult to control. The simplest method is to measure the temperature of the air being removed at the top of the machine, which gives a quicker response than a fabric moisture controller at the exit of the machine. It is impractical to use an IR unit to remove the critical regain moisture as well as the pre-drying stage because of the process cost and difficulties of control. It is particularly suitable when applying disperse dyes to lightweight polyester or polyester/cellulosic qualities from a horizontal-nip padding unit because the IR pre-dryer immediately above the nip commences the drying process very quickly after the impregnation step. The guide rollers fitted to convey the fabric upwards between the IR heaters should be shielded from the radiation to avoid overheating. The intensity of the thermal input from the infrared heaters causes serious problems in the event of a fabric stoppage; the fabric can quickly suffer scorching damage and fabric ignition would be a real possibility. Safety systems have been designed so that if a stoppage occurs not only can the gas or power to the heaters be switched off but also metal shutters drop down between the fabric and the heating elements. In modern IR pre-dryers these heat shields are so effective that the heating elements can remain switched on, so that when making changes of shade between dyelots the equipment does not have to be allowed to cool down and then be heated up again before production can recommence. The latest IR units apply a combination of radiant and convective heat to the incoming fabric, as a result of recirculating hot moist exhaust air from the top of the unit by means of ducts and fans which feed this into a nozzle box [37,38]. It is now possible to govern the thermal output of an IR unit in order to maintain the target moisture regain on the fabric [37]. Moisture detectors from Mahlo or Pleva are positioned following the IR unit and just in front of the dryer. These record the residual moisture remaining on the pre-dried fabric and the thermal output of the heaters can be regulated (by altering the gas supply or the voltage) in order to keep the moisture regain at a steady value.
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16.7 Drying 16.7.1 Drying Cylinders and Hot Flues Contact heating is the simplest and cheapest method of fabric drying and it is widely used for woven goods. The fabric is passed around a series of ‘cans’ or metal cylinders about 60 cm in diameter filled with steam under pressure in the range 200 to 600 kPa. The direct contact between the hot surface of the cylinder and the wet fabric under tension resembles a hot ironing treatment and this produces an attractive finishing effect on certain fabric qualities, such as cellulose acetate taffeta. The cylinders are arranged in vertical or horizontal banks and positioned in echelon so that they provide maximum contact with the fabric threaded around them. The first few cylinders in the series should be PTFE-coated to minimise friction and their temperatures scaled between 75°C and 100°C to gradually heat up the incoming wet fabric. The last few cylinders should be scaled between 100°C and 140°C to complete the drying process. Drying cylinders have good thermal efficiency and occupy less space than alternative fabric drying machines. Their main limitation is the lack of control of fabric width and the degree of warpway tension that has to be imposed. A stenter treatment is essential after cylinder drying in order to achieve the target finished width consistently. In pad-dry-bake processes the dryer and the thermofix unit should be closely coupled in order to minimise heat losses but a compensator is necessary to control tension in the fabric passing between them. Rotary perforated suction-drum dryers are suitable to complete the drying process instead of drying cylinders, but the infrared pre-drying step must be controlled carefully in order to minimise the risk that the drum perforations will mark the fabric. This Fleissner system is less versatile in a dyehouse processing a wide range of fabric qualities. The air flow is necessarily unidirectional (out-to-in) and when thick fabrics with low air permeability are being dried there is a serious risk of developing two-sidedness faults. Hot flue dryers, which contain a series of driven top rollers and a lower set of free-running rollers, are more generally useful. In this case the drying rate is controlled by temperature, the velocity of the air circulation fans and the direction of air flow as it impinges on the fabric surface [23]. The air velocity should be only moderate rather than too rapid because a consistent rate of drying across the fabric width is essential. Hot flue dryers have been regarded as less reliable than cylinder dryers in this respect. In recent years, however, preference has tilted strongly towards hot flue units for both drying and fixation. Units have been developed that operate with fan speeds and air flows permitting finely balanced drying to reduce the risks of migration, listing or two-sidedness [27]. Control systems are now available to monitor and maintain the temperature and humidity consistently in the drying chamber. The latest infrared probes are capable of measuring the actual cloth temperature during treatment, giving greater accuracy and control of the uniformity of drying. The humidity levels in hot flue dryers are critical from the viewpoint of maximising energy efficiency. Devices are available for monitoring and adjusting the humidity so that the desired rate of drying is achieved with each fabric quality processed. The cleaning of hot flue rollers was traditionally a lengthy procedure, especially after the application of dark shades. To permit rapid changes of shade with minimal downtime, the latest versions include self-cleaning roller systems. An intermediate wet end-cloth is passed through to wipe them clean at the
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operating temperature. The rollers are fitted with an intermittent braking mechanism to assist the cleaning operation as the end-cloth wipes over the temporarily halted rollers [2]. 16.7.2 The Remaflam Process During the 1980s an interesting and potentially spectacular method of fabric drying was introduced by Brückner and Hoechst as the Remaflam process, which became commercially established for a time at selected dyehouses in Germany. Approximately one-third of the pad liquor was methanol; after passing through the nip the fabric was heated and this ignited the methanol vapour as it evaporated from the fabric surface. This heat energy generated so close to the goods rapidly evaporated water from the fabric as steam. Providing the wet fabric did not dry out completely it suffered no significant damage. However, changes in the relative costs of methanol and other energy sources, as well as the potential problems of flammability and toxicity during the storage and dispensing of substantial amounts of methanol, eventually curtailed the further development of this remarkable technique. 16.7.3 Radio-Frequency Drying Radiant energy in other forms, such as microwave or radio-frequency radiation, has been evaluated for the heat treatment stage of various dyeing processes. Radio-frequency drying became established in the package dyeing sector during the 1980s. Development work with reactive dyes on cellulosic fabrics also showed promise, results comparable with conventional pad-batch or exhaust application being obtained [39]. However, the cost of generating microwave or radiofrequency waves is a serious drawback and it is unlikely that these techniques will become commercially significant methods of reactive dye fixation.
16.8 Thermofixation The Thermosol process for the simultaneous disperse dyeing and heat setting of polyester fabrics was introduced by DuPont in 1949. It became widely established for dyeing the polyester component of polyester/cellulosic blends during the 1960s. The fixation step of the pad-dry-thermofix process involved heat treatment for 30 to 60 seconds in hot air at 200 to 220°C. Temperature variation across the width of the fabric should not exceed + 3°C. Pin stenters have been successfully used for this sequence because they eliminate the problems of width losses due to warpway tension on the fabric in other types of thermofixation equipment. Stenters are still used in this way by commission dyers who may not be able to afford to invest in more specialised units. By operating at an air-circulating temperature of 230°C on a four-bay stenter it has been possible to thermofix disperse dyes on polyester/cotton workwear by running at the relatively slow speed of 30 m/min [20]. However, when running at higher speeds it may well be necessary to use an exceptionally long stenter with eight or more heating bays. Such a machine occupies a great deal of working space and often incurs high costs of repairs and maintenance because of the extremely long pin chain. Contact of the fabric with the hot pin plates can give non-uniform treatment close to the selvedges. However, specially designed pin plates can be fitted that do not allow the fabric to come into contact with the base plate.
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Most thermofixation units are of the hot flue type. The fabric is looped around top and bottom parallel sets of rollers mounted in an oven. The top rollers are driven and the lower ones are free-running. The distance between the two sets should not exceed 120 cm. Rollers of large diameter (18 cm) are used to minimise the risk of fabric creasing. The capacity of a typical hot flue oven is 80 to 100 metres of fabric, yet it occupies relatively little space compared with a full-length stenter. There is no control of fabric width, which is effectively heat set at the thermofixation temperature. Compensators are fitted to allow for the warpway dimensional changes that take place during this high-temperature treatment. There is a significant degree of tension in the upward half of each loop but this helps to maintain traction. The roller surfaces are smooth, so there is significant slippage between the fabric and the rollers. The driven rollers close to the exit slot are set to run slightly faster in order to minimise the warpway tension. It is important that the fabric is fully dry and hot when it enters the fixation oven. The entry and exit slots are narrow to minimise the ingress of cold air and the egress of hot air with the fabric. A moderate flow of air is circulated through heat exchangers filled with hot oil at the bottom of the machine and upwards between the loops of fabric. The rate of heat transfer is inferior to that in a stenter. The fabric selvedges tend to heat up more quickly than the body of the fabric because of greater exposure to hot air and radiant heat from the inner surfaces of the oven. Production stoppages arising from mechanical breakdown or a fabric break-out means serious damage of the fabric trapped in the oven and several hours of lost production while the oven is cooled, re-threaded with fabric and heated up again. Contact heating provides much more rapid and uniform heat transfer, allowing overall treatment time at 200 to 220°C to be reduced from 30 to 60 seconds down to 15 to 20 seconds. This minimises the machine capacity required and offers energy savings for a given rate of production. Cylinder diameters are less than in conventional cylinder dryers, to give more frequent reversals of fabriccylinder contact. Direct gas-firing inside the cylinders is preferred to hot-oil circulation for economy, simplicity and ease of maintenance. Fabric emerging from the unit must be cooled immediately by passing over water-cooled cylinders. Thermofixation on heated cylinders tends to give a glazed finish to the fabric, which is only acceptable for certain robust and resilient qualities. Fleissner perforated-drum machines offer a highly effective means of hightemperature fixation treatment. A typical range contains four, six or eight drums of 100 to 140 cm diameter arranged in horizontal sequence. Dye fixation time is typically 15 to 20 seconds, with a six-drum range operating at about 60 m/min. An exhaust fan positioned at the drum axis withdraws air through the fabric as it contacts each rotating perforated drum. This air is heated by a hot-oil or gas-fired heat exchanger and recirculated, the air flow through the unused arc of the drum being restricted by internal baffles. The rate of heat transfer is exceptionally good, combining contact heat with hot-air flow through the fabric in alternate directions as it passes around successive drums. On leaving the heating chamber, the fabric comes into contact with a cooling cylinder filled with cold water. The perforated drums are completely covered by a wire mesh screen to help diffuse the air flow around the perforations. Owing to the high temperature of thermofixation, charred debris becomes embedded in the wire mesh and lowers the fixation efficiency. High-pressure hot water jets are an effective means of removing the contamination.
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16.9 Stenter Treatment Stenters are the most expensive, important and versatile machines in a dyehouse and their cost is increased by much ancillary equipment that has become standard on modern stenters. These items include a padder to uniformly wet-out fabrics and apply finishing agents, weft-straightening devices and control equipment. Irrespective of the number of times a fabric is dried by other means, stenter treatment is essential as the only way to fully control the attainment of the target dimensions of width and length. Stentering has a strong influence on the final appearance, handle and physical properties of the finished goods. With the high cost of stenters and their ancillary equipment, careful studies of the stenter capacity required must be based on the logistics of the individual dyehouse. Fabric may pass through stentering several times not only for drying but also to remove creases, to batch fabric onto A-frames and to edge-gum, particularly with knitgoods after slitting before processes such as printing. All modern stenters have operator keyboards and manual switches have virtually disappeared. The selvedges are held by two endless chains typically 40 to 60 metres in length and the fabric is conveyed through a series of heated compartments or bays. Hot air is directed onto the fabric equally from above and below. The selvedges are held either on pins about 5 mm long mounted in base plates or by clips that grip the selvedges between smooth surfaces. Lubrication of the chain is important and this takes place automatically during running. The lubricant must be completely stable during continuous high-temperature operation. Clip stenters are now usually confined to the drying of woven fabrics with a robust selvedge, particularly furnishings. The use of clip stenters has declined because of the difficulty of applying overfeed [18]. Overfeed is frequently necessary to compensate for warpway stretch in previous processing and to minimise shrinkage in washing and dry-cleaning. Stenter speeds range from only 10 m/min for heavyweight furnishings up to 100 m/min for lightweight dressgoods. The higher the operating speed, the more vital the need for careful feeding of the fabric selvedges onto the pins. In the stenter entry section the fabric passes around a set of free-running rollers to allow creases to drop out, descends behind and beneath the operator platform, ascends again via pairs of scroll rollers to uncurl the selvedges and then through smooth idling rollers to approach the pin chains. Finally the selvedges pass through detectors mounted on pivoted arms that guide the fabric onto the pin chains with the aid of free-running circular brushes (pinning brushes) that press the fabric onto the pins. The speed of the fabric through the stenter is controlled by the motion of the pin chain, but the fabric entry can be adjusted independently by the entry rollers. For many apparel fabrics, which must have good dimensional stability in laundering, the warpway tension acquired in fabric processing requires compensation by overfeeding, that is by feeding the fabric onto the pins slightly more rapidly than the stenter chain is moving. The degree of overfeed selected demands skilled judgement, a level of 5% being typical but greater amounts up to 20% in rare instances may be imparted. However, excessive overfeed reduces the yield of finished fabric obtained and, in extreme cases, may result in cockling at the fabric selvedges. On modern stenters, the classical compensator roller for fabric entry has been replaced by a precision tension control synchroniser that permits extremely short fabric paths. Separate drives in the overfeed shrinkage device for the two selvedges allow elimination of the need for a mechanical
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equalising system. Temperature uniformity over the entire area of fabric is ensured by means of a large-volume high-turbulence nozzle arrangement on which the fabric floats as if on a cushion of air [40]. The first 3 metres of the pin rails are pivoted so that they can move rapidly inwards or outwards to meet the incoming fabric. The tapering entry section up to the first heating bay is usually about 5 metres long for woven fabrics or 7 metres for warp-knits. The main parallel section extends the full length of the oven, which may have from three to eight heating bays each about 3 metres long. The delivery section about 5 metres long provides sufficient cooling before the fabric is unpinned by making contact with the take-off roller. At the delivery end a pair of draw rollers transfer the fabric to an overhead plaiting mechanism or to an Aframe driven by a pneumatic or hydraulic motor. On some pin stenters the pin plates can be tilted outwards about 10 degrees after the fabric has been fed onto the pins. This prevents the fabric from becoming detached by the air flow in the heating bays. At the delivery end the pin plates resume the vertical position before the fabric is removed from the pins by the take-off roller. Broken pins should be quickly replaced before problems of snagging or selvedge damage arise. The Econ-Air system of internal air flow with central fresh-air feed and central exhaust-air extraction was developed in the 1970s. This system maximises the evaporation capacity by creating an extremely dry atmosphere in the drying chamber [40,41]. The mechanism of relaxation shrinkage under the influence of heat, moisture and tension has been discussed in detail for woven and knitted fabrics. The shrinkage force is doubled as a result of doubling fabric speed, hotair circulation or length of the heat treatment zone. The Johann Müller shrinkage stenter incorporates an innovative double-belt nozzle dryer technique. This enables warpway shrinkage values in laundering and tumble-dry tests to be improved by about 5% in absolute terms [42]. Various methods are used to heat the circulating air in the stenter bays. Highpressure steam from the main boilers can be fed to a heat exchanger and condensate returned to the boilerhouse. This method is clean and economical but the temperature attainable is only adequate for drying or resin finishing. A steam pressure of 1000 kPa. at the stenter will provide a maximum air temperature of 165°C. This is completely inadequate for heat setting or dye thermofixation. For oil-fired stenters a thermostable oil is heated to 250°C in a separate boiler and circulated through the heat exchanger. Provision of this boiler and well-lagged pipework, pumps and storage facilities increases substantially the capital and maintenance costs. Gas is the most important fuel used for thermofixation stenters. The burnt gas fumes are fed directly into the stenter bays. Radial gas burners give more complete combustion and improved thermal efficiency. To avoid build-up of unburnt gas in the stenter, the exhaust fans must operate for 30 to 60 seconds before turning on the gas supply. If the pilot flame goes out the gas supply is shut off automatically. Few stenters have been installed with electrical heating systems because the running costs are too high [18]. In the 1990s there has been intensive debate and legislation concerning exhaust air purification and these developments are still in progress. Operational studies, including performance analyses during normal production runs, have been carried out on stenters and other dryers in seven typical finishing plants. The exhaust air from conventional heat treatments can yield measured values of volatile emissions and odours in excess of the relevant regulatory limits. Purification trials on a pilot-plant scale have demonstrated that by means of condensation in
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tandem with an air scrubber or bio-washer, or by adsorption on active carbon, the odour emission and TOC content of the exhaust can be effectively reduced. However, combustible methane gas residues in the exhaust air are not eliminated by these purification techniques [43].
16.10 Chemical Padding The continuous chemical pad-steam range was first developed by DuPont in 1944 when there was an exceptional demand in the USA and elsewhere to produce military uniforms to a consistently high standard using vat dyes on cotton. Many of the practical problems likely to occur in this process were encountered at that time [44]: 1. the need to keep the pad liquor cool in order to retard decomposition of the alkaline dithionite reducing system 2. location of the pad trough and nip as close as possible to the steamer entry in order to minimise air oxidation of the alkaline dithionite absorbed by the fabric 3. a dwell time of 60 seconds at 102°C in the steamer to give maximum fixation 4. the need to exclude air from the steamer during the fixation step 5. a heated roof inside the steamer to prevent condensation droplets marking the dyed fabric 6. a multi-stage reoxidation and soaping sequence to complete the dyeing process. These recommendations have since been incorporated into modern chemical padsteam units widely used for the dyeing of polyester/cotton blends. In the absence of air, sodium dithionite is extremely stable in alkaline solution. In practice, the important factor is its behaviour in solution in the presence of atmospheric oxygen. It is particularly important to note that sodium hydroxide is consumed in the oxidation reaction and thus an excess of dithionite and alkali must always be present. The amount of this excess depends on the application conditions [45]: 1. temperature – given that the other conditions are constant, the rate of oxidation increases with liquor temperature 2. dithionite concentration – given that the other conditions are constant, a specific amount of reducing agent is oxidised in a given time; thus the more concentrated the initial solution of dithionite, the longer it takes to become deactivated 3. relative movement of liquor and air – the greater the agitation of the liquor in the presence of atmospheric oxygen, the more rapidly the decomposition proceeds 4. specific surface area – the greater the specific surface area of the liquor (the ratio of surface area to volume) the more rapidly is the sodium dithionite oxidised. Since the time required for half of the reducing agent to be oxidised is inversely proportional to the specific surface area, complete oxidation may take many hours in a large vessel with a relatively small surface area. On the other hand, because of the high specific surface area of the absorbed liquor, oxidation on the impregnated fabric takes place within 30 to 60 seconds, depending on dithionite concentration and the prevailing conditions of application
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5. presence of vat leuco dyes – given that the other conditions are constant, the reducing agent is oxidised more rapidly in the presence of vat leuco compounds than in their absence. The oxidation reaction is exothermic and as the temperature rises decomposition of the dithionite accelerates. It is therefore essential to keep the stock solution as cool as possible and covered to exclude air. Great progress has been made with the introduction of the Pesch mixing unit. A stock solution of sodium dithionite and caustic soda is prepared automatically using a 29:1 ratio by mass of dithionite to caustic soda. The container is water-cooled, covered and kept cool during storage. This solution is fed into a small covered supply tank located very close to the pad trough of the chemical pad-steam range. There it is diluted with caustic soda solution to give a 1:1 ratio by mass of dithionite to caustic soda and fed into the pad trough fitted with an automatic level control. The pad trough is normally a 26-litre economy unit for a typical fabric 1.8 metres in width, thus ensuring that there is minimum decomposition of the alkaline dithionite solution before padding onto the fabric [20].
16.11 Steam Fixation Tight-strand roller steamers are mainly used to provide suitable conditions for the continuous fixation of vat, sulphur or reactive dyes on cotton and polyester/cotton blends. The operating principle is that the uniformly distributed dyes located mainly at the fibre surface diffuse quickly into the interior of the cotton fibres during treatment for 20 to 120 seconds in saturated steam. Reactive dyes of the low-reactivity classes require treatment for 60 to 90 seconds at 102°C for optimum fixation [46]. Vat or sulphur dyes require treatment in dry saturated steam at 102 to 105°C with the minimum of superheat. The steam must be completely free from air when fixing the leuco forms of vat or sulphur dyes. The steamer must be closely coupled with the chemical pad unit. If the passage time from the reducing bath nip to the steamer entry slot is substantially in excess of 2 seconds, the vat dye yield is progressively lowered because of the rapid oxidation of the dithionite in the absorbed liquor retained in the fabric. This rate of oxidation is not mitigated by operating at a higher dithionite concentration or a higher liquor retention value. Before commencing a run with vat or sulphur dyes it is essential to confirm that there is no air remaining in the steamer or the colour yield will be adversely affected [47]. The problem with consistently achieving air-free steam depends on the pattern of steam flow within the steamer chamber. This flow must flush out the initial air content and prevent further ingress of air during running. Steam is less dense than air and tends to rise upwards inside the chamber, thus displacing the air downwards. Air venting is effected mainly through steam traps, preferably of the thermostatic type. Reliable instruments are available to detect extremely small proportions of air in steam atmospheres [47]. The steam just inside the fabric entry slot is sampled and tested, where the residual air is most likely to collect. The automatic testing device contains a known volume of cold water that is displaced by any air present in the sample. Steam condenses immediately into the cold water, so that air-free steam does not lower the water level. The sample flows into the test device for 60 seconds. If the test for air is positive, another sample is taken and tested after 15 minutes. Fabric processing must not begin until the test for air yields a negative result [20].
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The main steam supply is typically dry saturated steam at around 90 kPa. and can be fed directly into a water sump with an automatic level control at the bottom of the chamber. This is rapidly raised to the boil and helps to maintain an equilibrium in the steamer chamber. Reducing to the supply pressure from the mains pressure tends to produce superheat and it may be necessary to leave part of the supply pipe unlagged to dissipate the superheat content. Only slight superheat (2 to 5°C) can be tolerated as it adversely affects colour yield. Steam with too much superheat also causes excessive soiling of rollers and other internal surfaces within the steamer [2]. The input of steam is usually controlled by a thermostat located well away from the heating plates in the chamber roof which tend to promote superheating. Condensate removal from the steamer is usually via a U-type drain trap. It is preferable to pass the main steam supply through an external sump to minimise superheating. This device is activated when the steamer temperature rises above the target setting [20]. Textile fabrics heat up very quickly (within about one second) in saturated steam. This immediately begins to condense onto the fabric, raises the moisture content and thus facilitates better contact with the steam vapour. The steam temperature should be in the range 102 to 105°C but the attainment of this does not necessarily indicate the absence of air. Although superheating raises the temperature the ingress of air has the opposite effect. When the steam supply has been correctly adjusted, however, there is a slight excess pressure of 1 or 2 mm of water and air is absent from the steam. Steam requirements vary for different fabric qualities and widths, so the steamer should be fitted with a steam volume regulator to control a bypass of the main steam supply. This device ensures the safe and economical use of steam. The shell of the steamer should be well lagged and the roof heated by steam coils or preferably heating plates to avoid condensation. A steam supply at 200 kPa. is adequate for these steam plates that feed into a condensate line [18]. Automatic rapid-change spray washing systems enable rapid cleaning of the inside surfaces of the steamer and the water seal at the exit slot. The capacity of the steamer chamber is typically 30 to 60 metres of fabric running at 40 to 80 m/min. In an orthodox tight-strand steamer the fabric is threaded alternately between a top row of driven rollers 15 to 20 cm in diameter and one metre apart and a similar lower row of free-running ones. A compensator speed control is necessary to maintain consistent tension in the fabric. The compensator must be located inside the steamer chamber because of the close coupling of the chemical pad and the entry slot. Important aspects of fabric entry into and exit from the steamer chamber are illustrated in Figure 16.10. The narrow entry slot has a heated lip to prevent any possibility of steam condensing and dripping onto the fabric as it approaches the slot. The excess steam is mainly ejected from the extraction pipe with only slight evolution via the entry slot to prevent the ingress of air through this aperture. When leaving the steamer the fabric passes through a wet leg, feeding directly into a cold water seal of capacity 50 to 80 litres to eliminate any possibility of air entering at this point. The wet leg is fed uniformly across the width with a constant supply of cold water at a rate of 3 to 5 litres/kg fabric or about 5 m3/hour [20]. The water temperature must not exceed 35°C or the wet leg will become a dilute leuco dyebath and shade uniformity will be jeopardised. The cold water flows into the steamer over a weir with a slight external overflow to remove any accumulation of deposits.
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Immediately following the wet leg exit from the steamer, a low-pressure nip may be required to act as a draw-nip to deliver fabric to the next process stage. In the case of vat and sulphur dyes diffusion of the leuco compounds in the steaming treatment is followed by washing to remove alkali and reducing agent, reoxidation of the leuco dye to the insoluble pigment form and soaping at the boil. It is thus advisable to pass the fabric, still full of steam, directly into the first hot washbox in which the steam condenses to assist penetration of the liquor into the fabric.
16.12 Washing-Off A suitable sequence for washing-off reactive dyeings in an eight-box range might be [3]: First washbox
cold water
Second washbox
water at 95°C
Third and fourth washboxes
soaping at 95°C
Fifth and sixth washboxes
water at 95°C
Seventh and eighth washboxes
cold water.
Agitation and excessive turbulence during rinsing or soaping at a vigorous boil (100°C) is wasteful of steam and would lead to creasing problems. The aqueous rinsing stage before soaping is mainly intended to extract the salt and alkali from the fabric so that the removal of the hydrolysed reactive dyes during subsequent soaping and hot rinsing is facilitated. However, no single washing sequence can be generally recommended. Attention must be given to the substantivity, reactivity and fixation profile of the reactive dyes used, the fabric running speed and capacity of the washboxes in the range, which determine the dwell times achievable for the successive process steps. A suitable sequence for aftertreatment of vat dyeings in an eight-box range might be [20]: First and second washboxes
washing at 40°C
Third and fourth washboxes
reoxidation at pH 9 and 60°C
Fifth and sixth washboxes
soaping at 95°C
Seventh and eighth washboxes
washing at 95°C.
The initial washing stage reduces the high pH before commencement of the reoxidation with peroxide in the next two washboxes. Above pH 9 indanthrone blues may become greener and duller as a result of over-oxidation. Caustic soda removal can be a problem when aftertreating heavyweight fabrics. The final four washboxes are operated on a counterflow system at 95°C with the detergent input being into the sixth washbox. The washing efficiency of ‘open soapers’ or washing-off ranges has been greatly enhanced by introducing counterflow of washwater within each washbox or compartment. Within each washbox, each bottom roller is separated by dividing plates so that counterflow washing is possible within each division. Liquor flow between divisions is facilitated through cut-outs in each dividing plate. It is desirable to couple washboxes so that the overflow from one becomes the feed to
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the next, with an obvious saving in water consumption. Successive washboxes in a range should be installed at a slightly higher level to facilitate liquor flow against the direction of fabric travel (Figure 16.11). Methods of mechanical agitation to create controlled turbulence of the liquor are important, as well as intermediate nips between washboxes to minimise carryover of liquor with the fabric. The space requirements for individual process steps may be reduced by specifying double threading in which a second set of rollers of slightly smaller diameter is interposed between the usual top and bottom rows, as in the Benninger Becoflex design (Figure 15.7). Access for threading-up is more restricted, however. Pressing rollers are rubber-covered and form a series of nips with the conventional top row of rollers in each washbox. The efficiency of the washing process is improved by these light nips that promote liquor interchange and reduce carry-over of liquor. At high running speeds, excessive liquor carryover can completely cancel the benefits of a counterflow washing system. At low speeds it is adequate to use self-weighted rollers but at higher speeds it is essential to exert a positive pressure provided by pneumatic or hydraulic means. Rollers are seldom perfectly circular and at high speeds without a positive nip the pressure rollers vibrate and may cause creasing. Modern washing machines have automatic spray cleaning. It is important for these ranges to have large-diameter drains to allow rapid change of wash liquors or the benefits of rapid changes between dyelots will not be fully realised [2]. Heat supply to each washbox is usually by direct steam injection for rapid initial heating-up, with maintenance of temperature by a closed coil. Heat exchangers are installed to recover heat from discharged liquors. The final nip before drying must remove as much water as possible and a sturdy mangle frame is used with loading up to 45 kg/linear cm. The top bowl is usually of porous but hard composition, working against a stainless-steel driven bowl. Finally the fabric is carefully dried on a graded-temperature bank of drying cylinders.
16.13 Continuous Dyeing Faults 16.13.1 Creasing This is probably the most common fault to occur in continuous dyeing. Sometimes these are retained visible folds formed during preparation or the dyeing process itself, but more often they are temporary folds that have appeared and vanished again in a preparatory process, their presence only being revealed later by differential dye uptake. The source of such problems may be difficult to identify with certainty. Excessive, insufficient or variable tension in running fabrics can be important causes of crease formation. Tight selvedges, differential shrinkage, the development of bowed or skewed weft (Figure 15.2), incorrectly bowed expanders and worn or badly rotating rollers can all produce rippled or irregular patterns of creasing. The build-up of lint, loose threads or other insoluble debris as hard deposits on roller surfaces is another potential source of variability in fabrics running over them. 16.13.2 Shade Matching Faults Shade matching in continuous dyeing is a challenge quite different in scale from that of automated batchwise dyeing, where a mis-matched dyelot can often be given a minor correction without removing the dyed batch from the dyeing vessel.
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If a continuous dyeing arrives at the delivery end of the range slightly off-shade, some form of reprocessing is usually inevitable. Even if the dyeing range is equipped with correctly positioned on-line colorimetric control (section 6.8 and Figure 6.2), substantial amounts of off-shade material are produced whilst a colour correction step is being implemented. Slightly off-shade but otherwise commercially acceptable fabric can often be disposed of by negotiation with the original customer or sale elsewhere as seconds quality. More serious divergencies from the target shade are usually corrected in batchwise open-width equipment rather than attempting to apply a correction in a second run on the continuous range. However, it may be technically and commercially preferable to cover the faulty goods by re-dyeing to navy or black, because these deep shades are normally in demand and usually readily disposable as seconds. 16.13.3 Lab-to-Bulk Reproducibility Level bulk-scale dyeings that unexpectedly fail to match the relevant laboratory dyeing for shade are often attributable to human error in formulating the dyebath, either in bulk or in the lab. Such errors can be cross-checked by repeating the lab dyeing and testing the bulk-scale liquor in a lab-scale dyeing. If the same shade has shown satisfactory lab-to-bulk reproducibility in a previous dyelot, the off-shade result may have a different cause, such as a change in substrate dyeability, liquor ratio, water supply, non-standard preparation, or a failure in the bulk-scale control of dyebath pH, temperature or chemical additions. Modern dyeing control systems are so reliable that substrate variability is far more likely to be involved. Consistency of substrate dyeability can be readily monitored in a vertical organisation with a limited number of fabric suppliers to the dyehouse, but the wide variety of fabric qualities dealt with in a typical commission dyehouse demands constant vigilance to keep the frequency of offshade faults down to an acceptable level. 16.13.4 Two Sidedness This occurs when one face of the fabric is subjected to a higher temperature than the other during the pre-drying stage. Dye migration takes place preferentially towards the hotter surface to give a slightly deeper shade than on the cooler one. If the component water-soluble dyes in a trichromatic combination differ in substantivity, however, the least substantive will tend to migrate more readily than the most substantive component and the two sides of the dyed fabric will show differences in hue as well as depth. 16.13.5 Listing Listing is the term used to describe weftway differences in hue or depth across the fabric width, normally a gradual shading from one selvedge to the other, or a difference between the centre of the fabric and both selvedges. Possible causes of listing include weftway variation in: 1. nip loading in a preparation treatment 2. residual size content after preparation 3. heat setting before dyeing 4. moisture regain after padding 5. nip loading at the padding stage
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6. temperature or moisture content during drying 7. temperature during thermofixation. 16.13.6 Ending Ending is the term used to describe warpway variations in hue or depth along the fabric length of a dyelot. Possible causes of ending include random variations of: 1. desizing, scouring or bleaching conditions 2. heat setting before dyeing 3. moisture regain before padding 4. nip loading at the padding stage 5. migration during pre-drying after padding 6. temperature in drying, thermofixation or steaming treatment 7. dwell time during fixation or aftertreatment. 16.13.7 Tailing This term refers to the depletion of dye concentration in the pad liquor that takes place gradually during continuous running. The higher the dye substantivity and the lower the applied depth, the more pronounced is the depletion or tailing effect. If the component dyes in a trichromatic combination differ significantly in substantivity, tailing may be more obvious because it manifests itself as a gradual change in hue. This fault can be minimised by rapid recirculation of the pad liquor from the trough back into the stock feed tank. However, it is also essential to consider carefully the relationship between the laboratory pad, stock tank and pad liquor formulations. A lab-scale padding gives a similar shade to that of the first few metres dyed in bulk, whereas the equilibrium shade reached after several minutes of bulk-scale running may be significantly paler or off-shade relative to the lab result. Quantification of these differences can be used to calculate allowance factors, so that the stock feed and lab-scale formulations can be adjusted to ensure that the pad liquor at equilibrium yields the target shade on the finished goods. 16.13.8 Chemical Pad Bleeding Another substantivity-dependent fault of a similar nature can arise in the paddry-thermofix-reducing pad-steam process for vat or sulphur dyes and the paddry-thermofix-alkaline pad-steam application of reactive dyes to polyester/cotton blends. Although the electrolyte concentration of these chemical pad liquors is invariably high, there may be significant desorption of unfixed dyes from the dried goods during immersion. Lab-scale dip tests may give qualitative confirmation, but as with tailing problems they cannot reproduce the equilibrium state reached on prolonged running. Nevertheless, quantitative measurements of colour differences do enable allowance factors to be determined for defining the relationship between dye padding formulation, chemical pad composition and target shade on the finished goods. 16.13.9 Staining Faults These are of almost infinite variety and tend to occur randomly, but careful analysis of the processing history of the fabric batch in question often pinpoints
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the source of the problem [48]. The fault may appear in a repeat pattern along the fabric length and it is important to record the exact circumference of all rollers and other cylindrical components that come into contact with the running fabric. Staining faults may be roughly categorised as random staining, resist marks, spotting and foam marks. Random Staining This may be variously described as mealiness, swealing, patchiness or blotchiness. When these effects are encountered on continuously dyed fabric the source of the fault is frequently migration during pre-drying after padding. The migration of water-soluble dyes at this stage is inversely related to substantivity and can be minimised by careful incorporation of electrolytes and migration inhibitors [49]. The degree of reactivity of reactive dyes is also significant, because highly reactive dyes become partly fixed during pre-drying and this effect competes with migration. Thus high-reactivity dyes with high substantivity are the least prone to migration problems [50]. The migration of an individual disperse dye can be restricted by other dyes present in combination that may have a larger particle size or a tendency to flocculate. However, the higher the concentration of migration inhibitor present, the less apparent are these differences in the migration behaviour of individual dyes [51]. The reduced liquor retention attainable by vacuum impregnation with disperse dyes greatly suppresses the extent of dye migration at the subsequent pre-drying stage [52]. In the absence of a migration inhibitor, vacuum impregnation is more effective than infrared treatment as a means of inhibiting migration but it is important to recycle the extracted dye liquor back to the pad trough [53]. Resist Marks Most of these faults arise from inadequate preparation but it can be quite difficult to deduce precisely where or how the fault originated. Occasionally, chemical analysis may confirm that the poor dye uptake was caused by the presence of residual cotton wax, size polymer, oil or other contaminant before dyeing but far more often the dyeing and washing-off processes extract the offending impurities and such tests are then negative. Localised acidic or alkaline resists are normally caused by soluble contaminants that are soon extracted or neutralised at the dyeing stage. Light Spots Repeated white or pale-coloured spots may be attributable to localised deposits on roller surfaces that interfere with the application of uniform pressure to the fabric containing absorbed dye liquor. Random light spots can arise as a result of water droplets falling onto the moving fabric after condensation has occurred on roof surfaces within or above hot and wet processing equipment. These wet spots dilute the unfixed dyes and chemicals locally, thus inhibiting full fixation in the spotted region.
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Dark Spots These spots or specks are usually local concentrations of deposited dye attributable to unsatisfactory dissolution or dispersion of the dyes and inadequate sieving before feeding to the pad trough. Incompatibility between dyes and auxiliaries, or between different classes of dyes when dyeing blends, pH fluctuations, variations in the water supply and desorption of impurities from the fabric have all occasionally been found to contribute to dye spotting problems. Airborne dye particles released when weighing, dissolving or dispersing must not be allowed to contaminate fabric or machinery in the vicinity; this is taken care of in modern dispensary facilities. Low-energy disperse dyes may volatilise and contaminate the interior surfaces of thermofixation equipment, so thorough cleaning between dyelots can be critical. Foam Marks These faults often arise when a scum or foam on the surface of a dye liquor contains undissolved dye particles that can become airborne or collapse as a random deposit on the fabric surface. The presence of excess migration inhibitor, wetting agent or other surfactant, accompanied by turbulence of the dye liquor when operating at high speeds, may contribute to such problems. Similar faults can arise during the reoxidation of vat or sulphur dyes when leuco compounds desorbed from the fabric surface become reoxidised in particulate form if foam is building up at the liquor surface.
16.14 Pad-Dry-Bake Application of Pigments Pigments are extensively used in textile printing using the relatively simple printdry-bake sequence. The corresponding technique for the continuous coloration of woven fabrics is by pad-dry-bake application, usually on a stenter. This approach is being chosen increasingly for those fabric qualities and end-uses not adversely affected by the presence of the cured pigment binder [54]. This process is ideal on polyester/cotton blend fabrics destined for household goods, such as curtains and bedlinen. The pigment dispersions contain particle sizes in the 1 µm region and these colorants have no substantivity, so migration during pre-drying can be troublesome [2]. However, pigment dispersions have excellent stability and can be readily recycled back to the pad trough if vacuum extraction is the preferred method of pre-drying. No special curing equipment is required and satisfactory wet fastness can be achieved without washing-off after thermofixation. This factor is becoming increasingly attractive for environmental reasons. Light fastness is excellent even in pale shades but fastness to rubbing and severe laundering is limited in full depths. The selection of an appropriate binder system is the key to achieving an acceptable handle and all-round fastness properties [2].
References [1]
W von Dietrich, Textil Praxis, 4 (1949) 333.
[2]
R F Hyde, JSDC, 109 (1993) 142; Rev. Prog. Coloration, 28 (1998) 26.
[3]
J Park and S S Smith, A practical introduction to the continuous dyeing of woven fabrics
[4]
R F Hyde and G Thompson, JSDC, 108 (1992) 122.
[5]
C B Palmer, Amer. Dyestuff Rep., 75 (Aug 1986) 18.
(Upperhulme:Roaches Engineering Ltd, 1990)
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[6]
G Lippert and M Schulze-Braucks, Textilveredlung, 24 (1989) 355.
[7]
K van Wersch, Dyer, 181 (Jan 1996) 28; Melliand Textilber, 78 (1997) 76; 79 (1998) 166.
[8]
L S Moser, Text. Chem Colorist, 28 (1996) 31.
[9]
K van Wersch, Internat. Text. Bull., 43 (1997) 21.
[10]
R F Hyde, G Ashton, G Thompson and K A Stanley, Colourage, 44 (1997) 67.
[11]
M Santymire, Amer. Dyestuff Rep., 48 (1959) 12.
[12]
J J Iannarone and W J Wygand, Amer. Dyestuff Rep., 49 (1960) 81.
[13]
H Musshoff, JSDC, 77 (1961) 89.
[14]
W Marschner, Textil Praxis, 43 (1988) 637.
[15]
F Somm, Textilveredlung, 17 (1982) 463.
[16]
H Burchardi and R F Hyde, Internat. Text. Bull., 34 (1988) 48.
[17]
P W Leadbetter and A T Leaver, Rev. Prog. Coloration, 19 (1989) 33.
[18]
D H Wyles in Engineering in textile coloration, Ed. C Duckworth (Bradford; SDC, 1983).
[19]
R Petschner, Melliand Textilber., 78 (1997) 340.
[20]
H D Moorhouse, Rev. Prog. Coloration, 26 (1996) 20.
[21]
G R Wangen and P Senner, Melliand Textilber., 70 (1989) 194.
[22]
K van Wersch, Melliand Textilber., 69 (1988) 431.
[23]
H Lehmann and G Meyer, Melliand Textilber., 70 (1989) 927.
[24]
K van Wersch, Melliand Textilber., 70 (1989) 46; Internat. Text. Bull., 36 (1990) 21; JSDC,
[25]
Anon, Dyer, 179 (Jun 1994) 25.
[26]
R Fischer, Melliand Textilber., 72 (1991) 459.
111 (1995) 139.
[27]
K van Wersch, Melliand Textilber., 70 (1989) 677.
[28]
H R Hadfield and D R Lemin, JSDC, 77 (1961) 198.
[29]
M R Fox, W J Marshall and N D Stewart, JSDC, 83 (1967) 493.
[30]
B Hellwich, Textilveredlung, 32 (Jan/Feb 1997) 21.
[31]
I Raether-Lordieck and B Grauers, Melliand Textilber., 79 (1998) 363.
[32]
P W Mickler, Dyer, 180 (Jun 1995) 21.
[33]
J M Sire and P Browne, Melliand Textilber., 72 (1991) 465.
[34]
K van Wersch, Melliand Textilber., 73 (1992) 431.
[35]
U Denter, S Dugal and E Schollmeyer, Melliand Textilber., 74 (1993) 166.
[36]
M S Carlough and W S Perkins, JSDC, 109 (1993) 65.
[37]
P Meyrahn and M H Lauger, Textil Praxis, 43 (1988) 989.
[38]
H C Paulsen, Textil Praxis, 44 (1989) 409.
[39]
R M Perkin and N Catlow, JSDC, 100 (1984) 274.
[40]
W Hartmann, Melliand Textillber., 79 (1998) 338.
[41]
K H Gottschalk, Melliand Textilber., 78 (1997) 434.
[42]
K Müller, Textilveredlung, 32 (1997) 25.
[43]
J Janitza and S Koscielski, Textilveredlung, 32 (1997) 84.
[44]
P L Meunier, Amer. Dyestuff Rep., 53 (1964) 49.
[45]
F R Latham in Cellulosics dyeing, Ed. J Shore (Bradford; SDC, 1995).
[46]
W S Hickman, Rev. Prog. Coloration, 29 (1999) 94.
[47]
J C Isarin and R D M Holweg, JSDC, 109 (1993) 28.
[48]
L Boyd, AATCC Internat. Conf. & Exhib. (Oct 1991) 49.
[49]
H Herlinger, Melliand Textilber., 72 (1991) 784.
[50]
D Fiebig, H Herlinger and P Schafer, Textil Praxis, 46 (1991) 550.
[51]
J N Etters, Amer. Dyestuff Rep., 79 (Oct 1990) 15.
[52]
P Sarabi, AATCC Internat. Conf. & Exhib. (Oct 1990) 140.
[53]
L Cleveland, AATCC Internat. Conf. & Exhib. (Oct 1990) 25.
[54]
E Haug, Chemiefasern/Textilindustrie, 42/94 (1992) 900.
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Table 16.1 Typical semi- and fully-continuous dyeing of woven fabrics Process Pad-batch Pigment pad-jig develop Pad-batch-beam
Type Semi-continuous Semi-continuous Semi-continuous
Pad (coupler) – jig develop (diazo) Pad (leuco ester and oxidant) – jig develop (acid) Pad-dry-wash
Semi-continuous Semi-continuous Continuous
Pad-dry-bake Pad-dry-thermofix Pad-dry-steam
Continuous Continuous Continuous
Dye pad-dry-alkaline pad-steam Pigment pad-dry-reducing padsteam Pad-dry-thermofix-alkaline padsteam Pad-dry-thermofix-reducing padsteam
Continuous Continuous Continuous Continuous
Example Reactive on cellulosics Vat, sulphur on cellulosics Disperse/reactive on polyester/cellulosics Azoics on cellulosics Vat leuco ester on cellulosics High-reactivity dyes on cellulosics Reactive on cellulosics Disperse on polyester Reactive, vat, sulphur on cellulosics Reactive on cellulosics Vat, sulphur on cellulosics Disperse/reactive on polyester/cellulosics Disperse/vat or /sulphur on polyester/cellulosics
Table 16.2 Average length of run per shade in continuous dyeing [2,3]
Market region United States Asia Pacific and Western Europe Japan
Average run per shade (metres) 5000-7000 1000-3000 500-2000
Table 16.3 Comparison of liquor wastage factors for polyester/cotton fabrics [20]
Dye liquor retention Economy trough (26 litres) Dye liquor prepared Wastage factor Conventional trough (45 litres) Dye liquor prepared Wastage factor
Dye liquor volume (litres) Dresswear Workwear 96 200 36 36 132 236 27% 15% 55 55 151 255 36% 22%
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Table 16.4 Advantages of the pad-batch dyeing process [33,34] More economical than pad-steam or pad-bake for small dyelots Higher productivity than exhaust dyeing Low capital investment for relatively simple equipment Smaller space occupancy than fully-continuous dyeing range Minimal labour costs for relatively simple dyeing procedure Lower energy costs than fully-continuous dyeing processes Good fabric appearance and dyeing quality Excellent reproducibility of dyed shades Rapid colour change between successive dyelots Most suitable for short dwell times with high-reactivity dyes No intermediate drying step between impregnation and fixation Chemical savings (especially salt) compared with exhaust dyeing Less environmental pollution (no salt or heat treatment)
Table 16.5 Important factors that influence performance in pad-batch dyeing [33,34] Fabric construction and quality Fabric running speed Dye impregnation technique Liquor retention in padding Liquor renewal rate from stock tank Pad liquor formulation (dyes, alkali, wetting agent) Physical form of dyes (non-dusting, cold water-soluble, granules or liquid) Applied depth and dye solubility Dye reactivity (high-reactivity types give short dwell times) Dye hydrolysis (low-reactivity types give higher pad-liquor stability) Liquor stability profile (effect of alkaline pH on hydrolysis rate) Dye/alkali mixing unit and metering pumps Temperature control in padding and batching on the roll Dwell absorption and fixation times Effect of alkaline pH on fixation rate Recipe reproducibility and dye compatibility Dye substantivity and tailing problems Washing-off to remove hydrolysed dye
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Figure 16.1 Machine utilisation in the continuous dyeing of short runs at a fabric running speed of 60 metres/minute [2] 100
Machine utilisation, %
80
60 Downtime 5 min 20 min 60 min
40
20
0
0
0.5
1.0
2.5
5.0
7.5
10
15
20
Length of run, thousand metres
Figure 16.2 Typical pad-dry-bake process sequence [2]
Infrared pre-dry A-frame
Pad
Wash
Drying and baking
Dry
Figure 16.3 Layout of the Econtrol continuous dyeing range [2] Wetting unit Washing option
A-frame
Scray
Matex Color padder
Thermex hot flue
Steam injection
Scray
A-frame
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Figure 16.4 Typical pad-dry-thermofix process sequence Batching option
Infrared pre-dry Pad A-frame
Scray
Hot flue treatment
Wash
Dry
Figure 16.5 Typical chemical pad-steam-wash-dry process sequence
A-frame
Scray
Chemical pad
Steam
Wash
Figure 16.6 Two-bowl and three-bowl padding arrangements Ascending
Vertical
Descending
Descending-horizontal Horizontal
Ascending-horizontal
Dry
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Figure 16.7 Design of displacement, multi-roller and economy pad troughs Displacement trough
Multi-roller trough
Economy trough
Figure 16.8 Relationship between applied force, area of contact and nip pressure [3] Force x
Force x
Very soft rubber
Standard rubber hardness
Area of contact y1
Pressure P1 =
x y1
Pressure P2 =
x y2
y1 < y 2 P1 > P2
Area of contact y2
Figure 16.9 Typical pad-batch dyeing unit [6] Driven roller
Pad mangle
A-frame
Scray
Dye solution
Alkali solution
Mixing pump
Rotating batch
A-frame
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Figure 16.10 Entry and exit features of a vat dyeing steamer [20,46] Extraction pipe
Fabric exit
Fabric entry
Overflow pipe
Sampling point
Weir and water seal
Entry slot with heated lip
Figure 16.11 Counterflow washing in an eight-box range [20] Fabric exit
Fabric entry Cold water feed
Hot water overflow
Drain Cold water supply
Heat exchanger
Chapter 17 Garment Dyeing 17.1 Advantages and Technology of Garment Dyeing Garment dyeing has become an increasingly important sector of the dyeing and finishing industry in recent years [1]. However, dyeing in garment form as an alternative process route has been available for many years and has been an established tradition for certain articles particularly suited to this approach [2]. Ever since the formation of major retail organisations offering ranges of textile garments there have been commission dyers specialised in processing fullyfashioned garments of the highest quality. The global market-place now has a tremendous demand for quality leisurewear and casualwear goods varying greatly in colour and fashion details. As an alternative to dyeing fabric lengths (‘pieces’ traditionally about 100 yards long) that are subsequently cut to shape and made into garments, a significant proportion of the knitgoods wet processing industry is concerned with the dyeing and finishing of garments. Originally, garment dyeing was mainly concerned with underwear, sweaters, pullovers, socks, tights, pantihose and stockings knitted from ecru yarn. Table 14.1 gives an indication of the various knitgoods processed in this way. Originally, the fibre types processed were silk, mainly in the form of stockings, and wool. The need for wool products to survive garment dyeing without shrinking and felting gave the impetus required to develop shrink-resist treatments [3], leading ultimately to the standards of machine-washability which are currently expected of wool garments in terms of dimensional stability and fastness to washing. The processing of wool garments, including shrink-resist treatments, has been discussed in section 8.14. Many of the forces serving in the Second World War were thankful for socks, sweaters and headgear that had been shrink-resist treated and garment dyed, so this technology is not exactly new. The dyeing of knitted garments was extended to other fibres, particularly nylon and acrylic fibres, as these became commercially significant from about 1950 onwards. Elastomeric yarns are now a major feature of garments (section 14.3). The processing of these synthetic fibres had much in common with the technology already developed for the dyeing and finishing of wool garments. The successful processing of polyester garments had to await the development of hightemperature machines, especially the rotating-cylinder machines discussed in section 17.2.4. At that time the dyeing of cotton garments was largely limited to the work carried out by laundries and dry-cleaners to give such articles a new lease of life by over-dyeing. The advantages of garment dyeing include: 1. the capability to produce small batches in a range of colours, which would be uneconomical by other processing routes 2. reduction in coloured waste textile materials 3. many desirable effects required in knitgoods can only be obtained by garment dyeing and finishing 4. colour decisions can be left until late in the process, thereby responding quickly to changes in fashion and providing opportunities to re-order the most popular colours 5. long before it became a ‘buzz’ term, quick response was possible by what is a ‘fast track’ method of coloration late in the processing cycle (Figure 8.2).
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The problems associated with the dyeing of traditional garment types include [4]: 1. the knitted garment may contain panels from different knitted batches, giving rise to bulk variations and difficulties of penetration and levelness 2. inadequate yield, patchy dyeing and poor penetration of seams 3. the possibility that the garment dyeing method can be eroded by other techniques on the basis of cost, ease of dyeing blends, better colour continuity and higher fastness. The third problem has been overcome as the importance of ‘quick response’ was realised, facilitating changes in colour and fashion to meet current demands. Indeed, the success of garment dyeing has been assured by developing products not traditionally dyed in this form. These are complete garment assemblies made from woven fabrics and processes to simulate the ‘distressed look’ associated with worn denim. The dyeing of such woven garments is a much more difficult task than that of dyeing knitgoods, since the woven garments are dyed with ancillary components, such as buttons, zips, linings and trims of different fibres. The garment dyer has become a ‘fashion dyer’ and garment dyeing presents a greater challenge than conventional fibre blends. Extensibility and dimensional instability of fabrics are found to play a major role in poor tailoring performance and unattractive garment appearance [5]. Garments subjected to wet processing have to be designed specifically to allow for changes in dimensions and physical properties. Suitable selections of the various components are necessary if body fabric, trims, linings, buttons and sewing threads (all differing in composition) are to be dyed and matched satisfactorily. The selection of sewing threads and interlinings has been discussed in this context [6,7]. Cotton goods destined for garment dyeing are normally made up using unmercerised cotton threads to give inconspicuous solid-dyed seams. Conversely, subtly contrasting effects can be deliberately achieved using either mercerised cotton thread to show up darker against the unmercerised garment panels or polyester thread to remain undyed when the cotton panels are garment-dyed. Sewing tensions should be kept to a minimum in lock-stitch and chain-stitch operations, deliberately producing slacker stitches, to help avoid seam pucker resulting from shrinkage of threads during subsequent garment dyeing. Many cotton casualwear garment designs incorporate metal components such as zips, buttons or press-studs, which may pose specific problems in garment dyeing. Where metal components are present in the undyed garment, the selection of prepared fabric for making-up is recommended to avoid possible interaction with peroxy-based bleaching systems [8]. Catalytic decomposition of the bleaching agent by metal ions can lead to severe fabric degradation that shows up as dye-resist marks or strength losses, particularly in areas close to the metal components. Ferrous-based materials should be avoided, since corrosion from attack by electrolytes may be evident, as well as the risk of rust stains. Discoloration of garment dyeings may arise in the immediate vicinity of metal press-studs. These are made of brass, coppered, blackened with sodium sulphide and then polished. Many direct and reactive dyeings are sensitive to copper and will form complexes of markedly different hue by localised reaction with traces of copper from the studs. Sequestering agents such as EDTA may help to minimise these effects but do not provide a guarantee. The best option in these circumstances is to replace the offending press-studs with nickel-plated brass. The dyer of the goods is seldom made aware that press-studs will be incorporated
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in the finished garment. A mild detergent should be selected when laundering studded garments and perborate or other oxidants avoided. Where garments contain metallic components the manufacturer should give appropriate washing instructions on the label [9]. The dyeing of cotton garments has been a major growth area in the last twenty years, associated with the increasing popularity of casual-, sports- and leisurewear for which the relaxed appearance achieved by garment dyeing is fashionable. The comfort and aesthetic appeal of such garments favours the selection of cotton. A major advantage of dyeing in garment form is the much shortened lead time from fabric production to electronic point of sale (EPOS) compared with traditional routes. Thus garment dyeing gives quick response for the replenishment of orders based on EPOS information. Lead times can be eight weeks for traditional routes, compared with two weeks when garment dyeing is chosen for multi-component cut-and-sew garments. Lead times for fullyfashioned cotton garments can be reduced from sixteen weeks to two or three weeks by garment dyeing. Processing routes for these two fabric types are illustrated in Figure 17.1 and Figure 17.2 respectively [10]. Garment dyeing costs can be two or three times the cost of fabric dyeing, depending on batch sizes. The payback for garment dyeing comes from a reduction of up to 50% in inventory and mark-down costs. If these costs represent 10 to 15% of the value of the goods sold, garment dyeing has been calculated to have a payback period of two to three years [11]. Garment dyeing may be carried out by commission dyers either specialising in this activity or carrying out a range of dyeing services, by in-house dyeing operations or even by laundries. Garments handled belong to four categories [12]: 1. fully-fashioned garment dyeing by major commission dyers 2. cut-and-sew garments, both woven and knitted, dyed to high fastness standards 3. boutique-trade cotton goods of low fastness suitable for hand washing only 4. washing, desizing, bleaching or stone-washing of denim garments.
17.2 Garment Dyeing Machinery 17.2.1 Paddle Machines Traditionally, side-paddle or overhead-paddle machines have been used for dyeing garments in all their various forms, including shrink-resist wool and garments requiring milling in addition to dyeing. The side-paddle machine, as illustrated in Figure 17.3, consists of an oval stainless-steel vessel with a central island and a large paddle along one side of the machine to agitate the liquor. The liquor/garment interchange is relatively slow, since this is the only method of agitation. This machine gives satisfactory results with loosely-knitted fullyfashioned garments but thick constructions can give rise to problems of seam penetration. Dyes and chemicals are added to the machine through a hopper situated on the central island. Paddle speed is usually variable and heating is by either open or closed steam coils. The overhead-paddle machine, shown in Figure 17.4, is less versatile with a lower degree of agitation, often resulting in uneven distribution of dyes and chemicals.
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Socks, stockings and tights are usually loaded into mesh bags for dyeing in paddle machines. The paddle rotates at 5 to 10 rpm and paddle machines operate at a liquor ratio from 25:1 to 40:1. Traditional paddle machines seldom have sophisticated control systems and are labour-intensive in operation, particularly in unloading. The Italian company Flainox manufactures a modern side-paddle machine featuring control equipment and a gate in the side of the machine which can be opened to facilitate unloading of the garments into a truck, as shown in Figure 17.5. Machine sizes range from 4 to 150 kg of garments. 17.2.2 Rotary-Drum Machines The garments are contained in a perforated horizontal cylinder which rotates slowly in a vat of a slightly larger size. The drum is usually divided into four segments and has been widely used for dyeing stockings, pantihose and socks. Heating is by direct steam injection and the drum rotates at a speed of up to 5 rpm, with drum reversal about every 20 to 30 seconds. These machines, widely known as Smith drum machines, operate at a liquor ratio of 20:1. The lack of sufficient agitation is the main limiting factor. A typical machine is illustrated in Figure 17.6. 17.2.3 Toroid Machines In these machines, typified by the Pegg Toroid and its later version the Pegg Ktype, the garments circulate in the liquor in a toroidal path with the aid of an impeller situated below the perforated false bottom of the vessel, as illustrated in Figure 17.7. Movement of the goods depends completely on the pumped action of the liquor. High-temperature versions of this machine operating at 120 to 130°C were developed in the 1970s for dyeing fully-fashioned polyester or triacetate garments. The liquor ratio of such machines is about 30:1. 17.2.4 Rotating-Cylinder Combined Dyeing and Hydro-Extraction Machines The development of garment dyeing machines with markedly lower liquor ratios has given major savings in resources. This machine resembles an industrial version of a front-loading domestic washing machine. The principle of the machine is shown in Figure 17.8. Liquor ratios of 10:1 or lower are obtained and dyeing is carried out with the cage rotating at 3 to 35 rpm, with drum reversal every 20 to 30 seconds. Hydro-extraction is carried out at 500 rpm. The advantages of this machine type include the following [10,13]: 1. low liquor ratio with savings in resources 2. dry loading to hydro-extracted unloading 3. optional basket geometry of open, D- or Y-pocket configurations allowing one, two or three compartments 4. external liquor circulation 5. high-speed hydro-extraction 6. high-temperature capability 7. control systems for time/temperature, rotation profile and dispensing dyes and chemicals
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8. sophisticated design for heating and cooling with heat exchanger and heat recovery 9. variable rotation speed for the dyeing operation with drum reversal 10. in-line lint filter 11. tilting mechanism for ease of unloading. Despite the fact that garment dyeing accounts for only 6% of textiles production [13], there has been intensive development of rotating-cylinder dyeing and hydro-extraction machines, particularly in Italy. A machine of this type, the Flainox NRP, is illustrated in Figure 17.9. Such machines have control and dispensing systems and a capacity of 50 to 150 kg, operating at a liquor ratio of 8:1. In the Bellini Robotel, a liquor ratio of 3:1 is possible for the dyeing stage and 15:1 for washing-off. Dry-cleaning machines used for batchwise solvent processing were developed from industrial dry-cleaning equipment, operating on a similar principle and including solvent recovery. Solvent dyeing of polyester was of some practical interest in the 1970s but is no longer viable. However, full pieces knitted from yarn-dyed polyester can be solvent-scoured in such machines, followed by hydroextraction before unloading for stentering. The Sancowad (S) process [14] utilised aqueous foam application at liquor ratios as low as 2:1 to minimise the cost of resources. Densely foaming surfactants provided a foamed dyebath containing water-soluble dyes. This process was superseded by development of the modern rotating-cylinder dyeing machine. 17.2.5 Hosiery Dyeing Machines More recently, circulating-liquor machines such as the Bellini TCC have been used for dyeing stockings and tights [15]. This avoids creases, ladders and fabric abrasion, which are common problems with rotary machines, and this improvement in quality is further enhanced by boarding prior to dyeing. In the TCC machine, the hosiery is loaded into circular, perforated carriers, removed after dyeing and transferred directly to a rotary hydro-extraction unit, thereby saving handling. Cabinet machines, in which the hosiery is loaded on trays, have also been used to preserve the quality and appearance of the goods. The traditional process for dyeing and finishing nylon stockings was to heat set (pre-board), drum dye and finally set (post-board) the stockings in an autoclave, each stocking being mounted on metal formers (boards). This final operation was expensive and labour-intensive, so dye-boarders were developed [16] to combine the operations of dyeing and boarding. Presentation of the stockings on boards provided an ideal opportunity for dye liquor application by spraying from the roof of a pressure chamber. The complete process consisted of steam relaxation at atmospheric pressure, spraying with a two-minute dwell to allow the dye liquor to run down the former, discharge of the dye liquor, treatment in saturated steam and finally hot-air drying. The process was limited to the use of disperse dyes (see section 17.3). 17.2.6 Drying Hydro-extraction facilities are not an integral part of the dye-boarder, so the goods are hydro-extracted in a rotary centrifuge and then dried in a tumble dryer.
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Conveyor-type radio-frequency dryers can also be used for hosiery and garments after rotary hydro-extraction. 17.2.7 Automation As already mentioned, the handling of garments in conventional paddle machines is labour-intensive and rotary machines have as a minimum a tilting device to aid unloading. An automated system for garment handling has been described [17] in which the goods are dyed in tilting, open-pocket rotating machines which are sling-loaded from overhead rails. When dyeing and hydro-extraction is complete, the machine tilts forward and unloads the goods onto a transfer shuttle which takes the goods to a tumble dryer. When drying is complete, the goods are automatically unloaded from the opposite side of the drying machine onto a flatbed conveyor for transfer to the finishing operations.
17.3 Dye Application and Processing Garments and hosiery can be dyed successfully in all types of machinery described in the previous section but the automated, rotating-cylinder combined dyeing and hydro-extraction machine is finding most favour. Loading of the goods into mesh bags is often an integral part of the procedure to preserve the quality of the goods by preventing abrasion and creasing. The processing of wool materials in garment form has been discussed in section 8.14 and the methods described for handling and trimming are widely used for other fibre types. Differences in wrinkle-shedding behaviour between wool, cotton and synthetics have been discussed in terms of the thermomechanical characteristics of these various fibres [18]. Nylon garments are usually heat-set by a boarding-type process, before or after dyeing. This is basically for the prevention or removal of creases and to impart dimensional stability. Nylon hosiery, including tights, pantihose and stockings, was traditionally dyed with disperse dyes using a tertiary combination of lowenergy dyes. This dye selection effectively covered barre attributable to differences in the prior thermal history of the nylon and allowed the dye-boarding process to be introduced. Disperse dyes were satisfactory when only relatively pale mode shades were required. More stringent fastness demands and fashion requirements for dark or bright colours have meant that milling acid and 1:2 metal-complex dyes have virtually entirely replaced disperse dyes. Optimum results with the current selections are achieved using the widely popular rotatingcylinder machines with control systems. Acrylic garments are dyed with basic dyes taking all the precautions discussed in terms of dye selection, retarder concentration and time/temperature control (section 11.4). Dyeing machines with adequate control systems are necessary. Polyester can only be dyed satisfactorily in high-temperature versions of the Pegg Toroid or rotating-cylinder machines. Fibre blends are widely encountered in garment dyeing, especially in socks, where wool/nylon, wool/acrylic and nylon/cotton blends are popular. The dyeing of blends requires careful dye selection and appropriate dyeing techniques have been well documented [19].
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17.4 Cotton Garments The pressures of quick response have encouraged garment dyeing of knitted and woven goods, including multi-component designs incorporating woven or knitted panels (as compared in Figure 17.1 and Figure 17.2). The difference in lead time is significant by using garment dyeing routes for fully-fashioned garments. Knitted fabrics require less preparation than woven fabrics and are usually scoured or bleached/scoured, relaxed and dried using a conveyor dryer before delivery to the garment maker. Heavy-gauge knitwear is normally processed as a complete garment whereas fine-gauge knitwear is dyed as garment blanks and separate trims for subsequent finishing and trimming. Woven cotton fabrics are usually prepared continuously before delivery to garment manufacture by a sequence that will include singeing, desizing, scouring and/or bleaching and often mercerising. This is an added benefit towards quick response since it shortens processing time for the garment dyer. Alternatively, grey (unprepared) fabric can be delivered to the garment manufacturer, so that the garments have to be desized, scoured and/or bleached, depending on the brightness of shade required [20]. In this case an allowance should be made for shrinkage by cutting the components oversize. This approach precludes singeing and mercerising from being undertaken. Desizing in garment form is carried out by a fairly rapid (15 to 60 minutes), high-temperature (80 to 90°C) enzyme process and bleaching is carried out with hydrogen peroxide. During preparation, water-soluble or self-emulsifiable lubricants may be applied to assist in cut-andsew operations [10]. Alternatively, durable elastomers may be applied. The use of specialised chemicals in garment processing has been discussed [21]. For pale colours and when fastness requirements permit, knitted or woven garments can be dyed with direct dyes, using SDC Class A and B types to aid level dyeing and seam penetration. Widespread use is made of reactive dyes since these provide a wide gamut of colours, including bright shades, of high fastness properties. The monochlorotriazine reactive dyes are ideally suited for garment dyeing since their relatively low reactivity favours level dyeing, with high-temperature application assisting diffusion and migration. This is particularly the case if a migration technique is used where dyeing is carried out at a top temperature of 90 to 95°C, which aids seam penetration and levelness, before cooling to 80°C for alkaline fixation. Limited use is made of vat dyes if fastness to industrial laundering or chlorine is required. The hot pigmentation technique is generally used. The vat dyes are applied as dispersions under alkaline conditions at 60 to 80°C. After a levelling period, sodium dithionite is added to reduce the dyes to their leuco forms and penetration into the cotton fibres takes place. After overflow rinsing to pH 10, the leuco dyes are reoxidised to the pigment form and soaped to give optimum fastness [10]. As already mentioned, certain woven garments contain many ancillary components that must be selected with care. The dyeing of such assemblies is an extreme case of blend dyeing, often involving unusual combinations of components [19]. Since many of these components are present as small percentages of the total garment, they are dyed at what are effectively very long liquor ratios, giving rise to exhaustion and reproducibility problems. Cotton garments are preferably dyed in rotating-cylinder machines, in which the level of control equipment is satisfactory for the application of reactive dyes. After hydro-extraction, the garments are tumble-dried to give complete relaxation. Conveyor and RF dryers may also be used. Garments may be steam-trimmed to improve appearance.
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The term ‘biopolishing’ has been coined by the Danish firm Novo Nordisk, a leading company in the enzyme production sector, for enzyme finishing treatments of natural fibres in fabric or garment form. The objective of such processes is the elimination of surface pills formed during preparation and dyeing by the accumulation of fibrous debris and superficial microfibrils. The effectiveness of these treatments depends on the enzyme type and application conditions, including pH, temperature and treatment time; no mechanical action is necessary. The main characteristics imparted to the goods are: 1. improved appearance and freedom from pilling faults 2. cleaner, smoother surface, conferring a cooler feel 3. softer and more attractive handle 4. enhanced luminosity of coloured goods [22]. Cellulase enzyme preparations for this purpose contain three active ingredients: exo-cellulase, endo-cellulase and emulsin (β-D-glucosidase). These catalyse a controlled hydrolysis of the more accessible cellulose material to consolidate the superficial microfibrils, leaving the fibre surfaces smoother and conferring a more uniform appearance [23-25]. Enzyme preparations for the biopolishing of wool garments are based on alkaline proteases. These can be applied before or after dyeing depending on the finishing characteristics required [26].
17.5 Wet Processing of Denim Fabrics Denim jeans were originally introduced for gold miners during the California Gold Rush and were subsequently marketed as workwear with emphasis on their durability and practicality. Jeans eventually were discovered and appreciated as casual wear and thus became a fashion garment. Techniques were developed to enhance denim garments as a fashion item, including washing to eliminate the traditional stiffness of the fabric together with stone-washing, with or without chlorine, ice-washing with permanganate or cellulase enzyme washing. These techniques are carried out in rotary-drum machines of the Smith drum type because of its robustness. The warp yarns of denim garments are dyed continuously by repeated dipping into the leuco indigo dye liquor followed by air oxidation (section 13.9). The more times the dipping occurs, the darker the shade that is produced. Black sulphur dyes are also used either alone or in combination with indigo. The warp is sized to assist weaving and an undyed weft is inserted during the weaving process. After weaving and garment making, the garments are desized with an α-amylase enzyme and scoured with alkaline detergent. Additives may be included in the scouring bath to accelerate the washdown process. The garments are tumble washed for 20 to 60 minutes and a softener applied. Tumble drying and pressing complete the process. Garments may be processed inside out to minimise abrasion marks. This processing provides comfort and softness, compared with the firm finish of traditional unbroken workwear. Current fashion favours the ‘broken-in’ or ‘distressed’ worn and faded look. Stone-washing accelerates production of the washdown effect and gives the garments a scuffed appearance and softer handle by introducing abrasive stones into the wash liquor. Starch residues and some dyed surface fibrous material are removed by the abrading action of the stones and hot water. Natural or manmade stones are available but pumice and volcanic rock are the most widely
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used. As washing proceeds, the stones slowly disintegrate, reducing the severity of the process with time [27]. The stones abrade the fabric and eventually corrode the rotary-drum machine, which cannot be used for other processing. The process sequence is desize, stone-wash at 50 to 70°C for 30 to 90 minutes, apply softener, de-stone and tumble dry. By incorporating hypochlorite into the stone-washing procedure, the colour intensity of the indigo, or other chlorine-sensitive dye present, is reduced. Residual chlorine must be removed before drying to avoid fibre degradation and an anti-chlor is given using sodium bisulphite or hydrogen peroxide. The so-called ‘ice-wash’ process is carried out with stones impregnated with potassium permanganate. This oxidises the dye present in the surface of the yarns in the garment. Stones are soaked for one to two hours in potassium permanganate solutions (1.5 to 5.0%). The stones should be drained before use. The fabric is desized and scoured, then introduced slightly damp into a machine containing the stones. The ratio of stones to garments is usually 1:2 to 3:1. Treatment is for 10 to 30 minutes. Residual manganese dioxide must be removed from the garments. Many of the drawbacks of stone-wash finishing, with or without oxidant additions, can be avoided by controlled enzyme treatment under neutral or acidic conditions. Advantages of this approach [28,29] include: 1. process times can be reduced by 30 to 50% 2. garment loading of the machine can be increased by up to 50% 3. much less damage of the garments or the machine parts 4. labour-intensive removal of stone dust from the treated garments is eliminated 5. no problems associated with the disposal of oxidant-contaminated stones 6. avoidance of oxidants and stone dust ensures much less contamination of waste-waters. Acidic or neutral cellulase preparations are available. The different activities of such products have been evaluated. Acidic preparations have been recommended for rapid but relatively superficial treatments and neutral types to achieve more severe modifications of the denim [30]. Three types of cellulose-hydrolysing enzyme may be included in such preparations [31]: endo-cellulase, exo-cellulase and emulsin (β-D-glucosidase). The factors that determine the degree of effect obtained include the type and concentration of enzyme applied, temperature and time of treatment. The pH of the wash liquor must be carefully controlled and the dyebath pH in the warp dyeing process is also important because of its influence on the degree of penetration of indigo into the warp yarns [31]. The Retro Dye (T S Chemicals/Jeans Care/Denykem) system is claimed to replace completely the stone-wash finish using pumice stones. Treatment for 20 minutes at 5:1 liquor ratio with Bioprep TBS (cellulase enzyme preparation) and Lyoprep Extra (combined scour and anti-crease agent) is followed by a 3-minute rinse to ensure maximum removal of size polymer [32]. For a highly ‘distressed’ look, partial substitution of pumice stones with enzyme treatment can achieve the appearance required [28]. In this case, the ratio of stones to garments is typically 1:2 to 2:1. The ‘weathered’ or ‘distressed’ look for casual clothing can also be obtained by surface coloration of the garments with pigments. As pigments are nonsubstantive, the garments have to be pretreated with a polymer to provide sites for adsorption of the pigment particles [33,34]. Washing processes to achieve the
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desired effects are carried out as described above, before thermal treatment to cure the pigment binder system. A varied selection of analytical tests to determine the chemical parameters that indicate the response of cotton denim garments to stone-washing treatments, with or without the addition of enzymes or oxidising chemicals, has been described [35]. Indigo fading may be aggravated by incorrect application of silicone softeners in finishing, but selected agents from other classes may exert a slight protective effect [36]. The phenomenon of indigo fading on exposure to photochemical smog is often encountered. Owing to the pronounced ring dyeing that results from continuous dyeing of warp yarns with indigo, most of the dye present in blue denim is highly accessible to free-radical destruction by noxious gases. The yellowing of denim jeans that may occur in processing plants or during storage before retail has been attributed to exposure of the goods to oxidative gases (ozone or oxides of nitrogen), but it is suspected that by-products of the oxidation of indigo are implicated. Measures to minimise this yellowing in storage include [37]: 1. avoid silicone softeners that may enhance fading and photoyellowing 2. pack the jeans in BHT-free plastic film before storage 3. ensure that harmful gases are excluded from storage areas 4. ban the use in warehouses of vehicles powered by internal combustion engines. Subsequent yellowing of denim jeans has often been associated with the technique of ‘ice-washing’ in the presence of stones pretreated with potassium permanganate. Analytical studies have shown that although thorough rinsing markedly lowers the residual metal content of ice-washed denim, substantial amounts often remain [38]. Photoyellowing of the rinsed goods may persist and it is difficult to correlate this with the presence of specific trace metals [39]. Solutions of nitrous acid and oxalic acid were found to be the most effective media for rinsing out trace metal contaminants [38]. The presence of anthranilic acid and isatin as by-products of the oxidation of indigo has been confirmed in denim jeans after the ice-wash process. Both of these can contribute to yellowing faults if not adequately removed by hot water rinsing [40]. Metal-ion contaminants are capable of forming salts with anthranilic acid that are more difficult to wash out of the goods. Manganese dioxide is a brown product of limited aqueous solubility readily formed by the reduction of permanganate anions. Brownish oxides of iron (rust) may arise from abraded metal surfaces inside the machine or metallic accessories attached to the garments [41]. Tests with various agents of the aminopolycarboxylate type indicated that EDTA was the most efficient means of sequestering dissolved transition-metal ions during rinsing of ice-washed denim, at a cost of only 1% of the total cost of washing-off [42]. Photoyellowing has also been attributed to the retention of arylamine by-products of indigo oxidation by complexing with aluminium ions present in the cotton [38]. However, a rise in calcium content of denims as a result of washing also appeared to be linked with a yellowish discoloration [39]. Furthermore, the mineral vesuvianite from igneous rocks (the source of pumice) is a complex silicate of Ca, Mg, Fe and Al. In spite of the long history of indigo dyeing, it must be assumed that the jury is still out on the question of precisely which constituents are responsible for the yellowing phenomena observed in this context.
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17.6 Dry-Cleaning of Garments Chlorinated solvents, especially perchloroethylene and trichloroethylene, have been widely used in the traditional dry-cleaning of garments because of their low cost and useful technical performance, especially their non-flammable property. However, alternatives have been sought because of health and safety concerns, including possible carcinogenic hazards and the effects of AOX compounds on the environment. Several commercially available (but flammable) solvents (isooctane, limonene, isopropyl lactate and certain dibasic esters) were found to perform better than perchloroethylene in many but not all respects when applied as dry-cleaning solvents [43]. Aqueous-based alternatives to dry-cleaning have been evaluated in order to avoid the technical problems and costs of equipment and processing associated with dry-cleaning solvents. The Aquatex (John Linthwaite Associates, Ripponden) cleansing system is designed for microprocessor-controlled aqueous washing and drying in conjunction with non-hazardous detergents and finishing agents specifically formulated by Oils and Soaps (Bradford). This system is applicable to a wide range of appropriate goods, including suitings, evening wear, wedding dresses, delicate garment constructions, waxed textiles, suede, leather, curtains and duvets. Tests have confirmed that for all water-based stains aqueous cleansing shows higher degrees of stain removal from suede, leather and textile garments than conventional dry-cleaning with solvents [44]. The Drywash (Global Technologies LLC) process is an alternative dry-cleaning technique based on the application of supercritical carbon dioxide. It is claimed to be quicker, cleaner and superior in performance to existing commercial dry-cleaning, posing no threat to human health or the environment. Flammable solvents are avoided and the process does not generate hazardous liquid or solid wastes, or contribute to depletion of the ozone layer [45].
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G Cawood and J Scotney, Rev. Prog. Coloration, 30 (2000) 35.
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H W Partridge, Rev. Prog. Coloration, 6 (1975) 56.
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J.Park, Dyer, 142 (Jul 1969) 115.
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D C Gore and J H Settle, Amer. Dyestuff Rep., 83 (May 1994) 24.
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D H Tester and A G de Boos, Melliand Textilber., 73 (1992) 358.
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A D Wilcox, Text. Prog. 19 No. 2 (1988) 44.
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P J Judd, Text. Prog., 19 No. 2 (1988) 52.
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B Hill and W S Hickman, Dyer, 171 (Mar 1986) 21.
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H J Flath, Melliand Textilber., 73 (1992) 344.
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J A Bone, P S Collishaw and T D Kelly, Rev. Prog. Coloration, 18 (1988) 37.
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W Reed, Amer. Dyestuff Rep., 76 (Nov 1987) 35.
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K Scott, Dyer, 171 (Nov 1986) 31.
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F Krämer, Dyer, 176 (Jun 1991) 8; 182 (Jul 1997) 21.
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G H Lister, JSDC, 88 (1972) 9.
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Anon, Dyer, 185 (Jun 2000) 10.
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T D Kelly, Dyer, 174 (Sep 1989) 27.
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J Rayment, Text. Manuf., (May 1964), 200.
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A G de Boos, K W Fincher and A M Wemyss, Text. Chem. Colorist, 29 (Oct 1997) 28.
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J Shore, Blends dyeing (Bradford: SDC, 1998).
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J M Murphy, Amer. Dyestuff Rep., 76 (Nov 1987) 41.
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M Higgins, Text. Prog., 19 No. 2 (1988) 33.
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[22]
J Cegarra, JSDC, 112 (1996) 326.
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U Hotz, Tenside, 30 (1993) 388.
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G L Pedersen, G A Screws and D M Cedroni, Melliand Textilber, 72 (1993) 1277.
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C L Chong and P C Yip, Amer. Dyestuff Rep., 83 (Mar 1994) 54.
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M N Sun and K P S Cheng, Textile Asia, 24 (Jun 1993) 38.
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D Kochavi, T Videbaek and D M Cedroni, Amer. Dyestuff Rep., 79 (Sep 1990) 24.
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Anon, Dyer, 181 (Jun 1996) 8.
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S Klarhorst, A Kumar and M Mullins, Text. Chem. Colorist, 26 (1994) 13.
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B Schmitt and A K Prasad, Colourage, 45 (Oct 1998) 20.
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J McKenna and V Gallagher, Dyer, 181 (May 1996) 27.
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C L Chong, S Q Li and K W Yeung, Amer. Dyestuff Rep., 81 (May 1992) 17.
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T Lever, JSDC, 108 (1992) 477.
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W Y K Kwok, K P S Cheng and L S H Cheng, J.Asia Text. Apparel, 8 (Oct/Nov 1997) 52.
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S Thumm, Internat. Text. Bull., Dyeing/Printing/Finishing, 43 (1997) 33.
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P Maier, R Kruger and G Grüniger, Melliand Textilber., 77 (1996) 786.
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J W Rucker, H S Freeman and W N Hsu, Text. Chem. Colorist, 24 (Oct 1992) 21.
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M N Larson, AATCC Internat. Conf. and Exhib., (Oct 1991) 36.
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A H Reidies, D Jensen and M Guisti, Text. Chem. Colorist, 24 (May 1992) 26.
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T Rydberg, J. Text. Inst., 85 (1994) 402.
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Anon, Dyer, 181 (Jun 1996) 33; Text. Horizons, 16 (Jun/Jul 1996) 36.
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Anon, Wool Record, 155 (May 1996) 65.
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Figure 17.1 Processing routes for multi-component cut-and-sew cotton garments Traditional route
Garment dyeing route
Woven
Knitted
Woven
Knitted
Continuous prepare
Batchwise prepare and dye
Continuous prepare
Batchwise prepare
Dye
Finish
Dry
Relax dry
Deliver to garment manufacturers
Finish
Deliver to garment manufacturers
Cut-and-sew
Cut-and-sew
Stock
Stock
Garment dye and finish
Retail sale
Retail sale
Lead time about eight weeks
Lead time about two weeks
Figure 17.2 Processing routes for fully-fashioned cotton garments Traditional route
Garment dyeing alternatives
Prepare yarn (hank/package)
Knit garment blanks
Yarn dye and dry
Make-up
Stock
Wind
Stock
Dye garment blanks and tumble dry
Knit garment blanks
Garment dye and tumble dry
Make-up and finish
Finish
Make-up
Finish
Stock
Lead time 12 to 16 weeks
Retail sale
Retail sale
Lead time 2-3 weeks
Lead time 5-6 weeks
178 Figure 17.3 Side-paddle garment dyeing machine
Figure 17.4 Cross-section of overhead-paddle machine
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Practical Dyeing, Volume 3 Figure 17.5 Flainox side-paddle machine
Figure 17.6 Smith drum machine
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Figure 17.7 Pegg Toroid machine A B
C
E
A B C D E F
D
F
Water spray Liquor level Distributor Steam pipe Impeller Drive
Figure 17.8 Rotary-drum dyeing and hydro-extraction machine
C B D G A E F
A B C D E F G
Perforated drum (with Y-pocket) Machine controller Sampling port Liquor-level indicator In-line lint filter Heat exchanger Addition tanks
Practical Dyeing, Volume 3 Figure 17.9 Flainox NRP rotary-dyeing machine
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Chapter 18 Carpet Dyeing 18.1 Carpet Production The technology of carpet coloration generally resembles other textile sectors but it is often regarded as a separate industry, possibly since product specifications and properties have more in common with the furnishing fabrics sector. The carpet industry is a major consumer of dyes, although the sector accounts for only 5% of world fibre usage. Conversely, the carpet industry utilises a high proportion of certain fibre types, such as nylon and polypropylene. An indication of world production of carpets is given in Table 18.1. Over the period shown there has been a growth rate of about 4% per annum. Production is concentrated in North America (45%) and Western Europe (32%), where much of the consumption takes place. Contract end-uses account for 33 and 22% of carpet production in North America and Western Europe respectively.
18.2 Fibre Distribution The wear characteristics of a carpet are important and these properties are influenced by the fibre type, the composition of the blend, the yarn specification and the mass of carpet per unit area. The distribution of fibre usage is shown in Table 18.2. Nylon continues to dominate as the principal carpet fibre. Polypropylene has increased in importance and all other fibres have declined. Acrylic fibre consumption in carpets is negligible. Consumption of polyester, including carrierfree dyeable types, has declined steeply in the carpet sector. Wool’s share of the market is decreasing overall but it is still significant in woven constructions where blends such as wool/nylon are used, although these carpets are almost exclusively manufactured in the UK.
18.3 Carpet Production Methods Traditionally most carpets were woven but from the 1960s onwards tufting and other non-woven constructions became the major methods of manufacture [1], almost to the exclusion of weaving in the USA, as shown in Table 18.3. Nylon carpets are mainly tufted (90%), wool carpets mainly tufted (49%) or woven (47%), and polypropylene may be needlefelt (43%), tufted (38%) or woven (19%) [2]. Processing routes for both woven and tufted carpets are shown in Figure 18.1, together with the stages at which coloration can take place. Traditional dyeing of loose stock or yarn for woven carpets has given way mainly to piece dyeing and printing of tufted constructions, providing much improved economy and flexibility of production. Approximately 70% of tufted nylon carpeting is coloured in piece form and approximately half of this is dyed or printed continuously [2].
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18.4 Coloration Processes The distribution of production between the various coloration processes specified in Figure 18.1 is given in Table 18.4. Over the period shown, producer coloration and continuous piece dyeing have grown in importance whilst batchwise methods of carpet dyeing have declined, particularly in Western Europe. Carpet printing has declined significantly in North America. This topic will not be discussed further here, especially since it has been admirably discussed elsewhere [3]. The traditional methods for coloration of woven carpets include loose stock dyeing, especially for achieving exceptional levelness and fastness in solid shades for Wiltons, and hank dyeing, which is widely preferred for Axminsters since many small lots are required for the numerous colourways. Staple yarn for tufted carpets is traditionally dyed as loose stock, whereas filament yarn is producercoloured or space-dyed. Continuous dyeing and printing are important for tufted nylon carpets and package dyeing has been developed successfully for most types of yarn. Producer coloration of carpets has increased in importance from the late 1980s onwards. This can be associated with the emergence of polypropylene as the second most important fibre, but producer-coloured nylon, both masspigmented and chip-dyed, is also increasing in importance. Phosphorescent polypropylene or nylon carpet yarns have been produced by Afterglow Accent Yarns Inc of Chatsworth, Ga. Chips are moistened and coated with a finely divided zinc sulphide in powder form followed by melt spinning, resulting in the production of phosphorescent filament yarns [4]. Variations in the metering of pigment master-batches, in the texturing, heat setting and winding of bulked continuous filament yarns and in the tufting process itself, become visible as distinct longitudinal streaks in plain-coloured cut-pile carpets. By contrast multicoloured cut-pile, loop-pile and staple cut-pile carpets are less sensitive [5,6]. Nylon yarns are usually dyed with acid dyes under carefully controlled conditions. Polypropylene, on the other hand, is normally mass-coloured with pigments and streak problems can originate from inadequate control of pigment master-batches. The tufter as carpet producer often takes the largest share of responsibility for these faults but variations can arise in production of the pile yarn. Streaks in the dyed nylon yarn or carpeting may be associated with variability in polymer dyesite content and delustrant level. Further causes of shade streaking include crosssectional variations of individual filaments, bulk-related differences, crimp variability, fluctuations in yarn setting temperature, tension and moisture content of the yarn, tufting tension and cutting variations [7]. Texturing conditions for nylon 6.6 filament yarns were systematically varied and the resulting bulked yarns characterised for crimp, relative dyeability and propensity to form streaks in two-ply saxony carpets. The streak formation was highly correlated with crimp variations rather than dye uptake differences [8].
18.5 Fastness Requirements In common with automotive and furnishing fabrics, a high level of light fastness is required in finished carpets. Other properties of importance for carpets are fastness to wet and dry rubbing, shampooing and water (or sea-water). The application of stain-blocking agents demands the use of fast dyes. For contract end-uses, such as aircraft carpets, the dyes must be fast to the flame-retardant
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finishes applied. Carpets are usually given an elastomeric coating on the back of the carpet in the form of a latex (to lock the tufts into the backing fabric) or a foam (to give the carpet improved resilience). Dyeings must withstand these processes, whilst the presence of traces of copper or manganese in the dyed fibre may adversely effect the latex-curing process. The fading of dyed nylon carpet fibres is an oxidative degradation catalysed by UV radiation and sensitised by titanium dioxide if present as a delustrant. Ultramid UV (BASF) is a modified nylon 6 derivative containing a photostabiliser integrated into the polymer chain. This functions as a radical trap that suppresses essential steps in the degradative process. Ultramid UV heat-set carpet yarns are available as regular, deep-dye and basic-dyeable variants. Multicoloured Ultramid UV carpets exhibit markedly higher fastness to light than conventional differentialdyed nylon 6 carpets [9].
18.6 Dye Selection for Batchwise Application Dye selection and methods of application to wool or nylon have been discussed in Chapters 8 and 10. Increasing use has been made of acid donors that allow the pH to decrease gradually as dyeing proceeds. Monosulphonated levelling acid dyes can be applied with an anionic levelling agent at 80 to 85°C and a pH of 6.0 to 7.0, depending on the depth of shade. This low-temperature method and controlled pH gives good reproducibility and colour corrections are seldom necessary. Superior tuft definition is obtained and side-to-centre colour variations are eliminated. To save energy, various low-temperature dyeing methods are used, based on pH control and an appropriate dyeing assistant. Control of pH is essential in nylon dyeing and various buffer systems are available for this purpose (Table 18.5) [10,11]. Alternatively, acid donors may be used; these are usually esters of organic acids that hydrolyse to give a gradually decreasing pH as the process proceeds through the time/temperature profile. Initially, tufted carpeting was plain-dyed on winches, variety being obtained using textured or sculptured effects by varying the height of the pile during tufting. Tufting machine developments, including scroll pattern attachments, sliding needle bars and streak-breaking have improved the design and patterning potential. Colour contrast or shadow effects can be obtained using differentialdyeing nylon variants. This differential uptake is achieved by varying the number of amino end-groups in the nylon polymer, since this determines the dyeability with anionic dyes. The introduction of sulphonic acid groups into the polymer structure confers dyeability with basic dyes and resistance to anionic dyes [12]. A wide range of contrast effects can be obtained from the various yarns available (Table 18.6), with selected disperse dyes being used to provide a ground shade on all yarn types when required. To achieve a high degree of contrast, acid dyes should be selected that do not stain the basic-dyeable yarn under the preferred dyeing conditions at pH 6 and basic dyes that do not stain the deep-dyeing yarn. Slight staining of the normal yarn will always occur and thus acid dyes should be used for deep colours, so that only slight dulling arises from the basic dye stain on the normal yarn. Carpeting containing differential-dyeing nylon is given a combined scour-bleach to remove the sighting colours (applied to identify the various yarn components) and lignin impurities. After rinsing, the carpet is placed in a cold bath containing a nonionic dispersant and a weakly cationic levelling agent acidified to pH 6. The
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dispersant acts as an anti-precipitant to prevent interaction between the oppositely charged dyes and the levelling agent increases the contrast between the acid- and basic-dyeable yarns. The basic dye is added, followed by the acid dyes after a suitable running time. The temperature is raised to 95°C at 1.5°C per minute and dyeing continued for 45 to 60 minutes. Acid should not be added to increase exhaustion since this can seriously impair the migration properties of the dyes. Monosulphonated levelling acid dyes are preferred for plain colours in the Brückner Carp-O-Roll process and differential effects can also be obtained. Guidance on dye selection for winch dyeing of various forms of nylon carpet has been given [13]. Interest in polyester carpets has waned but disperse dyes are applied either by carrier-dyeing methods on the winch or in high-temperature jets as described earlier. Carrier selection is important since some give inadequate dye yields and light fastness, as well as posing environmental problems. The introduction of polyester fibres, such as Trevira 210 (Hoechst), which can be dyed at the boil without carrier, was a significant development that may yet result in a revival of polyester carpets [14]. Piece dyeing of wool carpets is generally carried out with levelling and 1:1 metalcomplex dyes [15] and has been discussed earlier. Acrylic carpets in piece form would suffer severe deformation that would be set into the carpet during winch dyeing. Blend dyeing is relatively unimportant in winch dyeing, although 50/50 polyester/nylon (using selected disperse dyes) and 80/20 wool/nylon (using levelling acid dyes) are encountered.
18.7 Dye Selection for Continuous Application Nylon carpets can be dyed continuously using metal-complex, milling or levelling acid dyes together with auxiliaries for levelling, wetting and prevention of frosting, antifoams and anti-precipitants. Monosulphonated levelling acid dyes give high colour yields and moderately good wet fastness. Intrinsically fast dyes, such as milling acid or metal-complex types, diffuse more slowly during steam fixation and may give lower wet fastness if penetration into the nylon is inadequate. Differential-dyeing carpets can also be dyed continuously.
18.8 Dye Application Processes 18.8.1 Loose Stock Dyeing Fibre dyeing methods have been widely used to produce yarns for both weaving and tufting of plain carpets, where a high standard of both levelness and fastness is required. Loose stock dyeing is also appropriate before spinning multicoloured yarns for tweed or heather-mixture effects. Fashion-oriented carpets based on traditional Berber styles and the Wooltweed concept [16] have been developed. Fibre dyeing, using either batchwise or continuous methods (sections 12.4 and 12.5), is appropriate when large weights of fibre per colour are required. In the batchwise method, large dyelots can be blind-dyed and the final colour obtained by dyeing correction lots. Continuous dyeing methods are available to give savings in labour, energy, water and effluent compared with conventional exhaust methods. A continuous range consists of padding, dye fixation, continuous back-
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washing and drying. In the Smith-Fastran EDF, a hydraulic ram was used to feed the padded fibre into the fixation unit under pressure and fixation was achieved by radio-frequency energy in 15 minutes. A production of 750 kg per hour could be achieved, whilst a radio-frequency dryer after back-washing gave a controlled moisture content. Fibre lubricants and softeners are applied after dyeing and these must be chosen with care, since they are unlikely to be removed by scouring in a later process if dry-spun yarns are used for tufting. Lubricants that give adverse soiling or static properties must not be used, whilst uneven application may result in stripiness on plain-dyed carpets. 18.8.2 Yarn Dyeing Hank dyeing has been widely used for the production of yarns for Axminster weaving. The designs in such carpets usually require many colours in relatively small dyelots. Other carpet styles (such as tufting) require large dyelots and, to achieve these, several hank-dyeing machines may be coupled together. A carpet hank dyehouse usually has a wide spread of machine sizes. Hank dyeing and the necessary machinery have been discussed in sections 13.2 and 13.4. The introduction of jumbo hanks and the use of RF drying have been of particular relevance to the carpet industry. Automatic handling devices for loading and unloading hanks from the dyeing machine have been developed and used in the carpet hank dyehouse to reduce labour costs; these are discussed in section 4.4. With yarns spun in oil, adequate scouring before dyeing is essential to minimise the risk of stripiness in the carpet as a result of differential soiling during wear. Uniform hydro-extraction is desirable before thermal drying on poles, since differential moisture content at this stage, leading to variations in yarn tension, may be another cause of stripy carpets. Hank dryers for carpet yarns have been designed so that they can be operated with a minimum of labour by having entry and exit points situated close together. Robotic handling of jumbo hanks, together with a squeezing unit to reduce moisture content to 50%, then unloading and packing after drying can be incorporated into a system such as that shown in Figure 18.2. The carpet industry has been slow to adopt yarn dyeing in package form, even allowing for the advantages of ‘quick response’ production which can be important in the contract sector. The high capital cost involved and the investment in jumbo hank technology have been two reasons for this but a few package-dyeing units have been installed, usually when traditional hank-dyeing facilities have reached the end of their useful life. The technology and advantages of package dyeing of carpet yarns have been well documented [17-19] and discussed in section 13.3. Savings in handling, including a reduction in winding processes, make a major contribution but improvements in carpet manufacture and planning are also possible. Jumbo packages can be processed and the preparation of packages to a suitable specification is important for the success of the process. Most, if not all, yarn types can be package-dyed and the method is extremely useful for dyeing singles yarn used in the production of marl or ‘stipple’ yarns, which are difficult to hank dye. Intermingled filament yarns containing differential-dyeing components can be dyed in package form ready for tufting. Package-dyeing machines are amenable to a high level of control and this can be allied to automation and robotics as discussed in Chapter 4.
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Continuous dyeing methods for yarn have been particularly successful in the carpet sector. Superba, well-known for its continuous relaxation equipment for yarn, developed a continuous yarn dyeing range, whilst Mageba has demonstrated that yarn can be processed on its narrow fabric ranges. The setting of twist in carpet yarns is necessary to give improved tuft definition in cut-pile carpets, to develop special pile textures, to improve carpet stability in piece dyeing or printing and to improve the appearance retention during wear. Setting can be carried out in hank form, either in hank-dyeing machines, in basket cages (using boiling water with or without the addition of chemicals), in autoclaves or in tape-scouring machines. Yarn setting can be carried out as an integral part of package dyeing. A continuous process for wool yarn was developed based on a four- or five-bath unit for package-to-package processing, designated the ANDAR/WRONZ Chemset process [20]. Handling is simplified, since the yarn from a package is scoured, set, insect-resist treated and back-wound on to a package ready for tufting. Chemical setting involves treatment in sodium bisulphite at pH 7.0, which permits cut-pile wool carpets to resist the mechanical action that can cause texture loss in winch dyeing [21]. 18.8.3 Batchwise Piece-Dyeing Methods Batchwise and continuous methods of piece dyeing are essentially restricted to the coloration of tufted carpets, particularly those manufactured from nylon, and are important application methods as indicated in Table 18.4. Traditionally, long runs were considered necessary for continuous dyeing. It was estimated that a carpet producer required per colourway the equivalent of the production capacity from six winches before continuous dyeing became viable. However, shorter runs are now the trend. Continuous methods can still be cost-effective in resources and labour because rapid-change applicators have reduced downtime, making the process more competitive. Carpet-tufting machines are highly productive and piece dyeing avoids many inventory problems that occur with fibre- or yarn-dyed materials. Filament yarns in loop-pile constructions are robust and versatile for piece dyeing, since fibre shedding does not occur and sculptured designs, typically based on multi-level tufting techniques, can be produced. However, the formation of ‘ladders’ can be a problem with loop-pile carpets. Staple yarns are used in cut-pile constructions and fibre shedding usually occurs in batchwise dyeing, although ladders are not a problem. Certain finishes for cut-pile carpets can only be obtained by piece dyeing. As indicated above, nylon is the principal fibre for tufted or woven piece dyeing. Carpets containing differential-dyeing nylon yarns are processed by batchwise or continuous methods. Polyester carpets are seldom piece-dyed, despite the development of high-temperature versions of the carpet winch. Not much wool carpeting has been dyed in piece form because of loss of tuft definition and felting. However, the chemically set yarn described above avoids these problems and details of wool carpet processing on the winch have been given [22], although reel speeds are reduced to 20 m/min. The batchwise dyeing of carpets is carried out on large stainless-steel winches similar in design to those used in other sectors of the industry, described in section 14.6.1. Machines for carpet dyeing need to be more robust in construction and more sophisticated in design. Pumps or impellers are fitted to aid liquor
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circulation, whilst scroll openers and centring devices are provided for open-width processing. A conventional carpet winch is illustrated in Figure 18.3. Discussion has arisen as to whether rope or open-width form gives the best results in batchwise carpet dyeing. Rope dyeing gives better levelling but causes more creasing, although it may be necessary for re-dyeing if unlevel dyeings are obtained. Carpet lengths of 100 to 200 metres in widths up to five metres (weighing up to 2500 kg) are normally processed and reel speeds are in the range of 60 to 100 m/min. Modern machines are fitted with automatic controllers and rates of temperature rise are adjusted to 1 or 2°C per rope revolution. External heat exchangers and automatic pH controllers with dosing systems [23] have been installed on some versions. An automated dispensary is necessary because of the large quantities of dye to be dispensed. The internal surfaces of the winch are regularly smoothed by buffing to prevent snagging and ladder formation. Because of the heavy weights involved, power-driven reels are necessary to unload the wet carpet into heavy-duty trucks, usually mechanically hauled to transport the carpet in the dyehouse. A recent development [24] is the NTM Obermaier Novacarp equipment for dyeing carpet in open-width at liquor ratios in the range 10:1 to 20:1, using troughs that follow the contour of the carpet package. Variable load sizes can be dyed at a constant liquor ratio and savings in water (30%), electricity (95%) and time (70%) compared with winch dyeing, have been claimed. The maximum fabric speed is about 120 m/min and the dyelot size 2000 metres2. The machine is illustrated in Figure 18.4. Jet dyeing machines, such as the Then Carpetflow, have been developed [25] for the high-temperature dyeing of polyester carpets but these machines can also be used for other tufted carpets in rope form. Cost reductions, as high as 70% compared with winch dyeing, have been claimed as a result of low liquor ratios (down to 12:1) and the consequent savings in water, effluent and energy. High quality in terms of level dyeing, reproducibility, development of bulk and freedom from running marks is claimed; by coupling jet tubes, batches of up to 2400 m2 can be dyed successfully. Nylon tufted carpet has been dyed by a cold pad-batch method using the Brückner Carp-O-Roll process. The method is claimed to favour sensitive fabrics, such as velours, where pile deformation could occur in winch or continuous dyeing. Differential-dyeing materials can be successfully processed but, as expected, variable pile-height material cannot be dyed successfully. The processing costs compare favourably with batchwise and continuous methods, especially due to the energy savings obtained. The carpet is pre-steamed before it passes to the dye applicator, where a metered dye supply is applied by a nozzle system to give a dye liquor uptake of 150 to 300%. 18.8.4 Continuous Piece-Dyeing Methods Continuous dyeing ranges involve considerable capital investment and are almost exclusively used to colour tufted nylon carpets. Since many sections of the range are common to both continuous dyeing and printing, it is not unusual for printing and dyeing for plain colours to be applicable in the same line [26]. A continuous dyeing range is illustrated schematically in Figure 18.5. The general trend towards shorter production runs requires flexible equipment capable of quick colour changes, short downtimes and minimum dye-liquor waste.
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Continuous dyeing of carpets is carried out before the latex coating or foam backing is applied and a typical unit will include the following sections: 1. dye-liquor preparation area (colour kitchen, usually housed away from the application unit) 2. carpet sewing-in 3. compensator and lint removal unit 4. pre-steamer or jet bulker 5. colour application unit 6. steamer 7. washing unit with vacuum extraction 8. chemical finish application 9. dryer. Colour applicators are based on a number of techniques, including the following [24]: 1. padding 2. dip-drain methods 3. curtain coating 4. Küsters Fluidyer 5. spray methods 6. foam application 7. hot dye-liquor techniques. Continuous dyeing ranges usually operate at speeds in the range 5 to 30 m/min but this depends on the weight per unit area of the carpet, which in turn will influence the efficiency and, therefore, the speed of the drying unit [26]. Carpet widths are typically 4 or 5 metres and considerable quantities of dye-liquor must be prepared. For example in continuous dyeing a 4-metre wide carpet weighing 1 kg/m2 and running at 10 m/min with a 300% pick-up will require 120 litres of dye-liquor per minute [2]. Colour kitchens and their ancillary equipment are discussed in section 3.12 and several companies can supply this equipment for continuous dyeing of carpeting. Liquid brands of dyes are an attractive option in view of the large volumes to be handled. For continuous dyeing, tanks for mixing and storing prepared thickener are required together with dissolving and mixing tanks for dyes and chemicals, with at least two dye-liquor storage tanks per colour station [2]. Automated dosing, recipe calculation, recipe storage and dye usage statistics are standard data. Carpet lengths must be sewn together efficiently and without skewing before loading into a J-box compensator of adequate size to give enough sewing time, particularly in high-speed machines. Hours of production time can be lost by inadequate stitching. Combined beater/vacuum devices are used to remove lint, which would give resist marks on finished carpet if allowed to remain on the surface. A short steaming treatment for 5 to 20 seconds is given to the carpet on the pre-steamer, using low-pressure steam at temperatures of 100 to 105°C. This gives a uniform moisture content, slight pre-bulking and removes creases which can give rise to uneven dye uptake. Alternatively a jet bulker is used to give a short vigorous continuous washing treatment to the carpet, using either cold water or hot water up to 70°C. Detergent may be added, but this is preferably avoided to prevent foaming. Jet bulking removes lubricants, sighting colours and
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creases to deliver a clean, uniform carpet. The carpet is passed over a vacuum slot to reduce the moisture content to 40 to 50%. The main problems with pad application for coloration are tailing and side-tocentre colour variations over the extensive width of the carpet. Swimming-roll padders, as developed by Küsters, assist in overcoming these problems [26]. Most conventional padders are used to pre-wet the carpet for subsequent dye application on a swimming-roll pad. Pick-up is in the range 80 to 120%, with padding at ambient temperature to minimise tailing. Dye retarding agents applied at a high pH may assist in minimising tailing effects. In dip-drain methods as developed by the BDA the carpet is impregnated with dye-liquor by passing under a roller in the colour trough. There is a slight squeezing action resulting in a liquor uptake of 400 to 800%, controlled by liquor viscosity and carpet speed. After impregnation, the carpet rises vertically about 4 m to the steamer entry and excess dye-liquor drains back down the carpet into the trough. High drying energy requirements arising from the high pick-up and large volume of dye-liquor necessary to fill the trough, together with dye wastage (about 20%) make this an expensive process. Curtain coating methods, as developed by Fleissner and Küsters, are aimed at giving a uniform application of dye-liquor by ensuring that a continuous film of dye-liquor falls evenly from a ‘weir’ reservoir applicator onto the carpet surface [26]. Liquor pick-up is 300% on to pre-wetted carpet, giving a total uptake of 400% and thus less liquor loss in the steamer. Since there is no exchange of liquor, there are no tailing problems associated with the actual applicators. The equipment allows for quick colour changes (about 20 minutes). By applying multiple dye streams, space-dyed or random-colour effects, such as the TAK [27] and Polychromatic systems [28] are possible. In the Küsters Fluidyer, liquor or aerated liquor is applied via a special distributor to a slot held in contact with the carpet pile by an inflated air bag [29]. Hot dyeliquors can be used and, with suitable ancillary equipment, quick colour changes are possible [30]. Dye uptake is adjustable over the range 50 to 400% for the production of either plain dyeings or ground shades for printing. Lower pick-up gives no liquor loss in the steamer and lower drying energy requirements. Dyeliquor is applied to dry carpet and since there is no liquor interchange there are no tailing problems. Spray application techniques are available at various levels of sophistication [31-33]. In the Otting version, oscillating sprays are mounted about 1 cm apart and deposit a liquor spray, the density of which is adjustable pneumatically, onto the carpet surface. The Millitron (Deering Milliken) [31], Chromotronic (P Zimmer) [32] and Titan [33] units were developed for jet printing. Depending on the effects required, pick-up can be varied from 50 to 400%. Foam application machines have been developed to reduce liquor uptake and thereby minimise energy, chemical and thickener costs [34] but frosting can be a problem at low pick-up levels. Certain applicators, including the Küsters Fluidyer and Otting spray, can be operated with hot dye-liquor to give superior solidity of shade with steaming times of only 1 to 2 minutes. Although there are no significant energy cost savings, a higher production rate can be obtained using a shorter steamer [35]. Festoon steamers are preferred for uniformity in solid colours and to achieve good contrast on differential-dyeing nylon [36]. Steaming times of 2 to 5 minutes for nylon and 4 to 10 minutes for wool are required. Cut-pile and wool-rich carpets generally need longer steaming times to achieve full penetration of dye [2]. At
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high liquor pick-up, it is preferable for the carpet to enter the steamer at the top to reduce liquor flowing back onto cold carpet, thereby avoiding problems of air pockets and steam losses. With lower pick-up, the carpet can enter at the bottom of the steamer; roof entry of the steam is a more efficient system, since air pockets are eliminated as the steam forces the air downwards [29]. Unfixed dye, auxiliaries and thickeners must be removed by washing before drying and finishing. This is essential to improve the wet fastness, handle and freedom from soiling. Modern washing systems require efficient spray/vacuum systems with variable slot widths [29]. Perforated-drum washing systems have been developed by Fleissner, as illustrated in Figure 18.6. Wool and wool-rich cut-pile carpets need a gentle action during passage through the washing stage to avoid excessive pile burst [2].
18.9 Finishing of Carpets Antistatic or antisoil finishes can be applied by spraying or padding between the washer and the dryer. Spraying is mostly preferred but gives limited penetration down the pile. Dip-and-nip padding methods give good penetration and a higher uptake, thus requiring a higher energy requirement in drying. Stain-blocking agents (section 10.4) can be applied in the dye-liquor prior to the steamer or before drying. Application conditions can be critical for optimum stain-blocking and to minimise subsequent yellowing [26]. The effluents from mothproofing treatments of wool, especially those based on permethrin, have adverse effects on the environment if discharged to drain (section 2.13). In a finishing process developed by WRONZ, the permethrin-based agent Eulan SPA (BAY) was sprayed onto carpeting as an improved alternative to application on loose wool or yarn, in order to minimise effluent pollution. Spray/vacuum systems for finish application are claimed to overcome some of the limitations of conventional spray or foam applicators and offer the advantage of requiring no further steaming to confer adequate fastness. A disadvantage, however, is the small volume of liquid effluent remaining to be treated and discharged. A further possibility is the dry carrier technique, in which the active agent is encapsulated in an inert dry powder (typically talc) as carrier medium. This approach has the advantage that no liquid effluent is formed but poststeaming is necessary to give adequate fastness [37]. In the foam application sector, the Autofoam Advanced Carpet Applicator (Datacolor International) system is claimed to solve the environmental problems associated with insect-resist treatments for wool carpets. Hitherto, mothproofing agents have been applied mainly in the scouring or dyeing stages. The closedloop system ensures that effluent pollution is eliminated and facilitates insectproofing. The automatic control system maintains the chemical composition, density and pressure of the foam applied to the carpet surface, so that the pile is thoroughly penetrated and the agent absorbed is evenly distributed at the required concentration. Carpets treated with permethrin-based mothproofers by this process are resistant to photodegradation and problems with carpet cleaning or effluent treatment are avoided [38]. Apart from insect-resist finishing, the Autofoam ACA system is suitable for the application of stain-blocking agents, pile appearance enhancers, softeners, antistats, flame-retardants and fluorochemical soil-repellents [39].
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After application and fixation of finishing chemicals, vacuum slot extraction removes most of the water and final drying is carried out on horizontal or drum dryers, which are at their most efficient when used in conjunction with heat exchangers and moisture-level indicators [40]. Over-drying is costly and can also cause thermochromic changes which complicate colour-matching procedures. Temperature and air velocity in the dryer should be uniform with upper limits on both, so that satisfactory handle and tuft definition are obtained. The fabric is then inspected over a suitable frame and any ladders in loop-pile carpet are repaired. A latex coating or foam backing is then applied continuously and finishing agents applied by spraying at the entry end of the range. Steaming and back-beating may be carried out to raise the pile and enhance the bulk. Shearing to remove surface fibres is carried out after (or in line with) the backing process, followed by final inspection. Modern shearing machines have two or four cutting cylinders with automatic machine control in which processing details are recorded, with seam and metal detectors being standard features. Infrared treatment is a viable alternative to gas-fired ovens and hot-air dryers for the curing of latex coatings. Bulk trials using the Hereaus Noblelight unit for this purpose resulted in a 40% increase in output and improvement in carpet quality because of more thorough curing of the latex [41].
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G N Mock, Text. Chem. Colorist, 30 (Aug 1998) 66.
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H Peak, Rev. Prog. Coloration, 18 (1988) 12.
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P Lennox-Kerr, Internat. Carpet Bull., 255 (Jan 1995) 2.
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K Berhalter, Chem. Fibers Internat., 48 (1998) 65.
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K Berhalter and W Hoof, Melliand Textilber., 79 (1998) 764.
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M L Honeycutt, AATCC Internat. Conf. and Exhib., (Oct 1995) 107.
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R W Miller and J H Southern, Text. Research J., 61 (1991) 61.
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P M Bever, U Breiner, G Conzelmann and B S von Bernstorff, Chem. Fibers Internat., 50 (2000)
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P Charles and J Park, Dyer, 140 (1968) 371.
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T L Dawson, Dyer, 162 (Aug 1979) 19.
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J Shore, Blends dyeing (Bradford: SDC, 1998).
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J Park, JSDC, 109 (1993) 133.
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H Beiertz, Chemiefasern, 42/94 (1992) 64.
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M Stursa, Melliand Textilber., 70 (1991) 954.
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J Park, Wool Record, 146 (Aug 1987) 23.
[17]
K Limbert, JSDC, 91 (1975) 299.
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C Philpott, Dyer, 153 (1975) 298.
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J Park, Dyer, 158 (1977) 481.
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A J McKinnon, Wool Sci. Rev., 66 (Dec 1989) 31.
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G H Crawshaw, Text. Chem. Colorist, 23 (Jun 1991) 13.
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C T Page et al., Australian Text., 11 (Feb 1990) 52.
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Ciba-Geigy Brochure, 9126 (1977).
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P Owen, AATCC Review, 1 (Oct 2001) 20.
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W Christ, Internat. Text. Bull., Dyeing/Printing/Finishing, 27 No.3 (1981).
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A Keller, Internat. Carpet Yearbook, (1995) 27.
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K Zimmerli, Amer. Dyestuff Rep., 68 (1979) 19.
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T L Dawson, JSDC, 90 (1974) 235.
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K Zimmerli, Amer. Dyestuff Rep., 75 (Jun 1986) 28.
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[30]
D Ward, Dyer, 185 (Mar 2000) 15.
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Deering Milliken, USP 3 696 779 (1974).
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P Zimmer, Dyer, 156 (1976) 554.
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T L Dawson, JSDC, 97 (1981) 262.
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P E Ingham and C K Rowan, Textile Asia, 21 (Sep 1990) 112.
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Table 18.1 World carpet production (m2 x 106)
Geographical area North America Western Europe Rest of the World Total
Year 1989 1130 940 630 2700
2000 1670 1220 890 3780
Table 18.2 Consumption (% by mass) of fibres in the carpet industry
Fibre Nylon Polypropylene Wool Polyester Other
Western Europe 1989 2000 41 53 32 40 15 5 6 2 6 -
North America 1989 2000 74 62 16 30 1 1 9 7 -
Table 18.3 Production of carpets by construction (%)
Construction Tufted Needlefelt Woven Others
Western Europe 1989 2000 66 57 22 25 10 12 2 6
North America 1989 2000 95 87 3 5 1 3 1 5
Table 18.4 Production (%) by various coloration methods
Coloration method Producer Batchwise Printing Continuous
Western Europe 1989 2000 31 45 32 8 23 25 14 22
North America 1989 2000 24 37 34 22 17 4 25 37
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Table 18.5 Systems for pH control in nylon carpet dyeing
pH value 4.5 5.0 6.0 7.0 8.0
Agents and concentrations (g/l) required Disodium Sodium Sodium dihydrogen hydrogen phosphate acetate phosphate 0.5 1.0 1.5 0.25 0.6 1.2 0.04 1.8
Table 18.6 Nylon fibre variants for carpets Nylon type Extra deep Deep Normal (standard) Low Light basic Basic
Dye class used Acid Acid Acid Acid Basic Basic
Acetic acid (80%) 1.2 1.0
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Figure 18.1 Processing routes and stages for coloration Tufted
Woven
Staple fibre
Fibre
Producer coloration
Loose stock dye
Spin
Continuous filament
Space or package yarn dye
Spin Tuft
Yarn dye Piece dye or print Weave
Figure 18.2 Minetti hank dryer for carpet yarns
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Figure 18.3 Gaston County Super Beck
Figure 18.4 Obermaier Novacarp carpet dyeing equipment
Figure 18.5 Continuous carpet-dyeing range
Colour application
Steaming
Washing
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Figure 18.6 Application of perforated-drum technology
Dye applicator
Steamer combination perforated drum/loop steamer
Perforated drum washing machine
Perforated drum dryer
Chapter 19 Closing Comments 19.1 An Expanding Market As a result of a continuing harsh economic climate in the textile industry of the developed world, it is readily overlooked that both household textiles and clothing manufacture are in fact growth areas as a result of both the increasing global population and the increasing value of disposable income available, particularly in developed countries. Between 1960 and 1998, textile manufacturing grew by an average of 2.1% per annum, the growth rate having peaked at 3.7% per annum in the 1960s. Total global fibre consumption grew from 15,153 kilotons in 1960 to 46,612 kilotons in 2000, an annual increase of 2.8% in fibre consumption over this period. These figures represent a somewhat lower average than that for worldwide manufacturing as a whole, which grew at an average of 4.2% per annum over the same period [1]. Although this continuing trend is good news for textile production, the main areas to benefit from this growth in dyes and textiles manufacture have been in developing countries, as discussed in Chapter 1. The reasons behind this have been listed in Tables 1.4 and 1.5 and discussed elsewhere [2]. They include the recognition by developing countries that textiles can be a sunrise industry if supported by an adequate labour supply and initially a captive local market demand that is eventually expanded into export.
19.2 A High-Tech Industry The preceding chapters have demonstrated that textile production in general and the dye manufacturing, textile dyeing and finishing sectors in particular are hightech operations. This technology is accessible to all countries, often made more readily available to developing countries as a means of entry for the marketing of relevant products. Developing countries, often with appropriate financial assistance, have been quick to invest in modern dyeing and finishing plants, including control equipment, automation and robotics. This has resulted in considerable reductions in the labour required, so that the low labour cost in developing countries is no longer the dominant issue that it was in the past. This sophisticated technology, including appropriate dye selection, the latest equipment and ancillary services and the refinement of application techniques leads to the capability to achieve virtually total RFT processing by blind dyeing techniques. This facilitates the establishment of ‘lights out’ operations by the use of robotics. By these means, major cost reductions, substantial savings and increases in profitability and productivity are attainable. RFT production is necessary to fulfil the ‘quick response’ demanded by the market.
19.3 Need for Investment To establish a successful dyeing and finishing operation by exploiting the latest equipment and techniques, it is necessary to initiate capital investment in the most suitable facilities. Developed countries have failed to encourage such investment whereas some developing countries have made major investments. The existing differences between these various countries will gradually be eroded,
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as indicated above, and a ‘level playing field’ will eventually be reached. Developed countries will once more be able to compete on price as well as quality, but appropriate investment is required. Success may often depend on identifying ‘niche’ products for specialised markets. Such venture capital investment must be substantiated by feasibility studies which will include factors such as whether to upgrade existing facilities or to establish a greenfield operation. Alternative processing routes and equipment available for different routes or processes need to be evaluated. Health, safety and environmental issues must also be addressed. Investment in people, not least the education of young entrants into the industry, is of primary importance.
19.4 An Era of Change Fundamental changes have occurred in the textile industry, not least in the wet processing sector, over recent decades. These have effectively made many established practices and facilities obsolete, with the obvious need to replace these with modern buildings, plant and attitudes. The textile industry has evolved into retail-specified manufacturing demanding RFT production and quick response. This in turn has resulted in fashion changes throughout the year compared with the traditional two-season approach. This has enforced shorter runs per colour – a further factor to be considered in any feasibility study. Advances in colour communication, such as colour on screen and the use of the internet, as described in section 6.8, are likely to continue apace. Fibre usage is likely to evolve further; the forecast that polyester will take over from cotton as the prime fibre is already becoming firmer. The consumption of natural fibres, wool and cotton in particular, is likely to decline as land is turned to the production of alternative arable crops. Much of the cotton produced in future is likely to be genetically modified varieties. However, it is unlikely that there will be major developments in dyes, chemicals or fibres, not least because of escalating costs of R&D together with health, safety and environmental testing. These latter issues are likely to remain major concerns in the modern wetprocessing industry.
References [1]
Industrial Statistics Yearbook. New York, United Nations, published annually.
[2]
J. Park and J. Shore, Dyehouse management manual (Bombay: Multi-Tech Publishing Co., 2000).
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