Sustainable dyeing technologies
A. Khatri1, M. White2 1 Mehran University of Engineering and Technology, Jamshoro, Sindh, Pakistan; 2School of Fashion and Textiles, RMIT University, Brunswick, VIC, Australia
5.1 Introduction The issues surrounding the sustainability of dyeing apparel textile materials are complex. In this chapter, dyeing is taken to include all of the preparation steps before dyeing. These two steps form the principal contributors to air and water pollution in the apparel manufacturing chain (Smith, 2003; Bide, 2007, Babu et al., 2007). Each step consumes water, energy, and chemicals and results in discharges to the environment. The steps prior to coloration are referred to as preparation. Depending on the fiber type these steps may involve heat setting followed by scouring to remove natural fiber contaminants and spinning and weaving lubricants. Some fibers require other specific preparation steps. For example, many cotton fabrics are processed under tension in a highly alkaline ammonia bath (mercerization). Mercerizing increases the brightness and depth of subsequent dyeings. It is also used to stabilize the fabric against shrinkage in industrial laundering. Thus, almost all cotton work wear is mercerized to meet customer expectations for performance. Preparation steps often include oxidative bleaching. Bleaching is important for white apparel fabrics. It is also frequently used where bright shades, especially pastel colors, are required. In some cases, the alternative approach of applying an optical brightener in the dye bath can be used. Apparel fabrics typically receive further chemical treatments past the dyeing operation to meet customer needs. Common examples include the application of acrylic copolymers to stiffen the fabric, to bring the fabric shade into specification through the process of pad-tinting, or to impart a specific performance characteristic such as water and oil repellency. Although finishing is the subject of first two chapters in this book, it is important to note that dyeing and finishing are invariably carried out in the same plant. There is usually only one liquid effluent stream to waste. Environmental discharge regulations will apply to the plant as a whole. The coloration or dyeing processes, which are the principal topics of this chapter, are complex and numerous. Each fiber type has its own specific group of dye classes. Typically, there are three to four different chemical classes of dye for two major apparel fibers: wool and cotton. Other apparel fibers such as nylon, acrylic, polyester, rayon, silk, and polypropylene have fewer dye classes available. Each dye class has its own “chemistry,” water usage, energy requirements, and effluent discharge volumes and characteristics. A further key consideration in looking at the sustainability of the dyeing process, including the preparation and postdyeing steps, is cost. The global
Sustainable Apparel. http://dx.doi.org/10.1016/B978-1-78242-339-3.00005-4 Copyright © 2015 Elsevier Ltd. All rights reserved.
apparel industry is intensely focused on cost. Economies of scale and least cost dyes and chemicals are often critical to the viability of the supply chain. Apparel items are frequently assembled from elements produced in different countries. For example, a cotton apparel item can have the fiber sourced from one country, spun and woven in a second, dyed and finished in a third, and made up in a fourth before supplying to a market in a fifth. The reasons for this are primarily influenced by process costs, process capabilities, environmental constraints, utility charges, distribution costs, and the distribution chain of the merchant buyer. Hence, attempting to gain a measure of the sustainability of a particular apparel item can be complex and difficult. Two approaches may be useful for determining the relative sustainability of an apparel item. The first is to “measure” the suitability or fitness for purpose of a particular apparel item. For example, is the garment over-engineered? Are all of the process steps used to bring the garment to market in line with the performance expectations in use? The second approach is to compare the relative sustainability of alternative processing methodologies and technology. For example, could an improvement in sustainability be achieved by switching to an alternative dye class or dyeing machine? In this chapter, the authors outline the key features of the steps involved in the preparation and dyeing of apparel fabrics. The dye classes and dyeing chemistry for the two major natural fibers for apparel are reviewed. A section then explores key technology issues to minimize waste, water consumption, energy, dyes, and chemicals. This is followed by an overview of effluent treatment and recycling issues. The chapter concludes with a review of future trends and emerging technologies together with sources for further information.
5.2 Apparel fibers and dyeing The term apparel covers a wide range of consumer end uses or applications. By far the greatest volume is in the fashion apparel market. Other important apparel fabric markets include corporate and uniform wear, leisure wear, and industrial, recreational, and specialized high-performance wear. Each of these areas will have different performance requirements. Each will have its own environmental footprint. Fibers used in the manufacture of apparel are predominantly cotton, wool, polyester, acrylic, and rayon. The fibers are used in both woven and knitted form with the exception of acrylic fibers, which are principally used for knitwear. Fiber blends are a feature of many items of apparel wear (e.g., polyester-rayon for corporate and evening wear, polyester-cotton for shirts, and polyester-wool for suits). Technologies for dyeing the two-fiber blends are well established. However, there can be some issues with effluent. Fibers from the bast or cellulosic family, namely hemp, jute, and linen, are occasionally used in fabrics in their own right but are more often used as feature blends in cotton and rayon apparel fabrics. These cellulosic fibers have the same basic fiber chemistry as cotton. Hence, the principles and practice for preparation, dyeing, and finishing processes are relatively similar. The specialty animal fibers such as alpaca and mohair are generally used in blends with wool as they have similar processing and
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coloration properties. Silk, although a protein fiber like wool, has its own relatively narrow range of coloration agents. There are other specialty fibers such as polypropylene (for fleece jacketings), carbon fibers, and Nomex® and Kevlar® for personal protection wear for police and military. Their coloration is not discussed in this chapter. Over 50% of the fiber consumed in apparel wear is cotton. When other cellulosic fibers such as rayon, linen, and the other bast fibers are included, then cellulose preparation, dyeing, and finishing dwarfs that of all other fibers. Hence, cotton becomes the dominant fiber discussed in this chapter. The importance of focusing principally on cotton (and by extension all the other cellulose fibers) is reinforced by the fact that for all apparel textiles the single largest dye–fiber volume is reactive dyes and cotton (King, 2007). Moreover, cotton dyed with reactive dyes consumes the highest volume of water per kilogram of any fiber. This combination also results in the highest discharge of salts, alkalis, and organic matter per unit fiber mass (Phillips, 1996).
5.3 Preparation processes Preparing fabrics for dyeing and printing is important because traces of spinning lubricants, processing aids, and dirt can have a marked impact on the quality of the subsequent coloration step. Batch-to-batch consistency in preparation is equally important for shade consistency. Redyeing or subsequent downgrading to second quality detracts from sustainability. Textile preparation and coloration require the use of a wide range of chemicals. Even in small to medium textile plants, say of the order of 20 M meters per year, the volumes and tonnages are significant. An often overlooked aspect of sustainability is the safe storage and use of dangerous chemicals such as caustic soda, ammonia, sodium hypochlorite, and reducing agents. Key issues in the management of all chemicals and dyes are well ventilated and bunded storage areas, emergency procedures for all spills, appropriate procedures for the disposal of aged stock, segregation of incompatible chemicals, and operator training and accreditation. Another often overlooked aspect of sustainability is the treatment of physical waste such as packaging materials, plastic wrapping, empty drums, and end cloths. These items should be segregated into appropriate streams for recycling. Only minimum quantities should be disposed to landfill or prescribed waste sites. Annual audits should be in place to ensure that best practice chemical storage and physical waste management standards are met.
5.3.1 Preparation of cotton fabrics Preparation of cotton woven fabrics normally consists of singeing, desizing, scouring, and bleaching (Hickman, 1995). Knitted fabrics are typically scoured only with an optional bleaching step for whites, pastels, and bright shades. Prior to weaving, cotton (and some other fabrics) requires the application of a natural or synthetic size to the warp. The size reduces the friction of the warp; in turn, this improves weaving productivity by decreasing yarn breakage and increasing weft insertion speeds. The size must be removed before dyeing by means of an enzymatic treatment or by boiling in
Typical composition of raw cotton
Cellulose Oils, waxes Pectins Carbohydrates Proteins Salts Water Other
Secondary wall Cuticle Primary wall Primary wall Lumen Lumen
86.8 0.7 1.0 0.5 1.2 1.0 6.8 2.0
Broadbent, (2005), Copyright of the Society of Dyers and Colorists.
a highly alkaline liquor with selected surfactants. Sizing agents contribute high levels of biological oxygen demand (BOD) to the effluent stream. In some large mills special plants to recover and reuse the size are in place. Bleaching, with oxygen donor chemicals, is carried out to destroy the natural yellowish-brown color present in some cotton fibers. Bleaching also produces a consistent base for whites, pastels, and bright shades. Table 5.1 shows a typical composition of the raw cotton fiber. Raw cotton is not sufficiently water absorbent because of its natural contaminants. To prepare cotton for dyeing, the natural contaminants are removed by scouring. At this stage, cotton is treated at the boil stage with alkaline solutions to emulsify waxes; hydrolyze oils, fats, and proteins; and solubilize any mineral salts present. The alkaline treatment also removes some low molecular mass noncellulose carbohydrates and results in around a 7% loss in mass of the fiber. For certain end uses and particularly for drills and work wear, the fabric is stabilized by treatment—under width and length tension—in baths of ammonia solution. Known as mercerizing, the process is also used to increase the brightness and dye use efficiency of subsequent dyeing. Discharge of ammonia to effluent is kept to very low levels by using distillation plants to recover and reuse the ammonia. High levels of process control, maintenance, and operator training are necessary to eliminate any reductions in sustainability through loss of ammonia to air or stream. Similar treatments with concentrated caustic soda are used in some plants.
5.3.2 Preparation of wool fabrics Like cotton, the key preparation processes for wool fabrics are scouring and bleaching. Scouring removes processing aids such as spinning lubricants and production contaminants such as oil, grease, and dirt. Uniformly and consistently removing all of these substances is a key prelude to the subsequent dyeing operation. Wool and the other protein fibers are different from cotton. They break down under alkaline conditions and are unable to withstand strong bleaching agents such as sodium hypochlorite. Oxygen donors, especially hydrogen peroxide, are the preferred bleaching agents for wool. After scouring, some wool apparel fabrics are milled to
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consolidate the fabric and to produce a surface nap. Skirtings, coatings, and jacketings are routinely milled by controlled felting. Ideally, fibers shed from the wool fabric in milling are trapped by a screen before the effluent is discharged to waste. Sustainability management issues for the preparation of wool fabrics are largely confined to good housekeeping.
5.3.3 Preparation of synthetic, rayon, and silk fabrics These fabrics generally only need mild scouring to remove yarn-spinning lubricants and soilage from the weaving or knitting room. The practice of heat setting woven polyester fabrics in the loom state has largely been discontinued. This practice resulted in high levels of thermally decomposed spin finishes in the air emissions. Some woven rayon fabrics can be warp sized and hence require additional processing. In most cases, warm detergent scouring prepares the fabric for dyeing or printing. Overall, sustainability issues are minimal.
5.4 Dye classes and dyeing process fundamentals 5.4.1 Dye classes for apparel fabrics Prior to the development of synthetic dyes, natural colorants were the only means of dyeing textile fabrics. Today, natural dyes are only used in handcrafted fabrics. Although some niche boutique fashion ranges also use natural dyes as part of their theme, the volume of such fabrics is extremely low. Natural colorants have four key weaknesses: they can provide only a limited range of colors, shades are difficult to reproduce, they have poor fastness to laundering, and they have very poor fastness to light. The synthetic dyes used today provide reproducible and consistent dyeing with broad shade ranges and light- and wash-fastness performance values between satisfactory to outstanding. The manufacture of dyes is almost entirely the domain of major and ethical global suppliers. In the mid-1900s, some azo-chromogen-based colorants were found to be carcinogenic. Although now banned, they have recently reappeared in apparel wear from illegal or unethical manufacturers. Sustainability audits should include an examination of supplier records. For cotton and the other cellulosic fibers, four key dye classes are available: direct, reactive, vat, and sulfur. Each dye class has advantages (e.g., shade range, brightness, cost, and ease of application) and disadvantages or limitations (e.g., limited shade range, lack of brightness, difficult to reproduce shades from batch to batch and wash and light fastness) (Shore, 1995a). Vat dyes have very specific applications especially for industrial apparel fabrics where very high levels of fastness and resistance to bleaching during laundering are required. Sulfur dyes present the opportunity for low-cost dyeing of a limited color range particularly for high-volume mass-apparel merchandise. The vat and sulfur dyes are insoluble in water. Their application requires conversion to a soluble form before application. After the dye has been applied, a chemical reaction is used to convert the dye back to its insoluble form. Direct and
reactive dyes are water-soluble anionic dyes. Direct dyes are low cost but suffer from a lack of brightness and poor wash fastness when used in dark shades. Their primary use in apparel wear is for pastel and some medium-depth shades. The dominant dye class for apparel is the reactive dyes. After the dye application, by exhaustion or padding, the dyes are fixed to the fiber by triggering a reaction between the dye and the fiber. There are a number of different classes of reactive dyes. The classes are based on the type of the reactive group as well as the number of functional groups present on the dye molecule. The early reactive dyes were based on mono-functional dyes. Since then, di- and tri-functional reactive dyes have been developed. For other major apparel fibers such as wool, silk, and nylon a dye class referred to as acid dyes is routinely used for coloration. Reactive dyes have also been developed for wool and are widely used for fashion apparel items because of their bright, broad color range. A range of mordant dyes is also available for wool and other animal fibers. The mordant dyes provide very high levels of fastness, but the shade range is limited, the shades are typically dull, and the application process is complicated. For acrylic fibers the dominant dye class is the basic dye. For polyester apparel, the insoluble disperse dye range is almost exclusively used.
5.4.2 The dyeing process and terminology Dyeing is fundamentally a process of transferring the required dye or dyes together with the appropriate dye auxiliary chemicals onto the fiber and then ensuring that the dye is level and fixed within the individual fibers and throughout the length and width of the fabric. The two principal application methods are appling the dye by exhaustion in a dyeing vessel or by padding the dye directly onto the fabric from a small volume bath. Uniformity of distribution of the dye is achieved by boiling (for exhaustion dyeing) or by time, or the application of heat or steam in the case of pad dyeing. For reactive dyes a fixation step is then used to generate a chemical bond between the dye and the fiber. Thorough rinsing is required to remove unfixed dye at the end of a dyeing process. This is important as unfixed dye will be removed on wear (poor rubbing fastness) or on laundering (bleeding and shade change). The extent and severity of the rinsing generally depends on the dye–fiber combination. The most severe rinsing or washing-off process is that for cellulose dyed with reactive dyes. Some important dyeing practice terms are discussed below. Each term contributes to sustainability through its impact on dyeing uniformity, dye usage, and batch-tobatch reproducibility. Redyeing to bring a batch back onto shade is an important contributor to sustainability and to the production cost. Depending on the product mix and batch sizes in a dyehouse, rework rates for off-shade dyeings can vary between 4% and 8% of all metreage. Dye substantivity The term substantivity refers to the ability of a dye to transfer from a solution onto a textile fiber and to “set” (Rouette, 2000). The quantitative measurement of the force with which the dye is captured by the fiber is described as “affinity.” In practice, substantivity is often used as a qualitative description of the affinity of a dye for a particular fiber. The substantivity of a dye generally depends on its solubility, its ionicity, and its molecular size and structure. Substantivity is favored
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by the formation of dye–fiber bonds (King, 2007). Thus, reactive dyes, because they increase the number of dye–fiber bonds, have higher levels of substantivity. Dye exhaustion In exhaust dyeing, the fabric starts absorbing dye as soon as dye is added to the bath. The term dyebath exhaustion describes the rate and extent of the dyeing process (Broadbent, 2005). Exhaustion is an important sustainability issue; the greater the absorption, the less dye that is discharged to waste. Dye diffusion The movement of a dye from the fiber surface into the fiber is diffusion (Broadbent, 2005). Optimal dyeing and hence maximum color yield requires uniform distribution of dye throughout the fiber (Khatri et al., 2014). In shades requiring two or more dyes, it is important for the dyes to have similar diffusion coefficients. Minor changes in the process conditions can lead to different distributions or take up each dye. This causes off-shade or batch-to-batch reproducibility problems. Dye migration The mobility of dye species within the fiber is referred to as migration. The extent of this mobility depends primarily on dye substantivity and dye– fiber bonding. It also can be influenced by the size of the dye molecule and the use of dyeing assistants. In the case of dyeing with reactive and acid dyes, dye species chemically fixed in the fiber cannot migrate during the dyeing process (Imada et al., 1992). Hence, good control of the dyeing to minimize premature fixation is important in achieving optimum sustainability. Dye fixation There are three main ways in which dye species can become attached to or fixed to the fiber: weak physical bonding, physical retention (by size), and chemical reaction (Hamlin, 1999). For cellulose, the vat, sulfur, and azoic dyes are fixed principally with mechanical retention, that is, the dye species is trapped as an insoluble pigment within the fiber. Direct dyes are held within the fiber by weak hydrogen bonds and by van der Waal’s forces. Reactive dyes are fixed through the formation of covalent bonds. Dye fixation is generally determined as the proportion of dye is applied to that actually fixed on the fiber (Rouette, 2000). The typical fixation percentages of cellulose dyes are given in Table 5.2 (Shukla, 2007). The lower fixation levels of reactive dyes are essentially due to dye hydrolysis during dyeing. That is to say that during the period before the fixation step, some of the reactive groups on the dye have hydrolyzed or otherwise been rendered inactive. There have been various analytical ways for estimating the extent of dye fixation and dye hydrolysis. Today, the percentage of dye fixation is usually determined by using absorbance measurements of dyebath solution and/or color strength measurements of the fabric during dyeing (Lewis and Vo, 2007; Montazer et al., 2007; Chattopadhyay et al., 2007). Table 5.2
Typical fixation percentages of cellulose dyes
Azoic Direct Reactive Sulfur Vat
83–93 70–95 50–80 60–70 80–95
The sustainability issue that arises from Table 5.2 is often in conflict with consumer needs. Clearly the more dye on the fiber, the less that is discharged to effluent. On this basis the preferred dye classes for cotton would be the azoic and vat dyes. The primary downside becomes a palette with low shade brightness and a very limited shade range.
5.5 Dyeing processes for apparel fabrics This section explores the practice and technology used to apply dye to the principal apparel fibers. This leads to Section 5.6, which provides an overview of sustainability issues arising from commercial practice in the dyeing of apparel fabrics.
5.5.1 Dyeing of cotton with anionic dyes When cotton is immersed in an aqueous dyebath, the fiber surface acquires an initial negative charge (Ribitsch and Stana-Kleinscheck, 1998). Anionic dyes, including direct and reactive dyes, are usually sulfonated to provide aqueous solubility and possess a negative charge in the dyebath. Therefore, an electrostatic barrier exists between the surface and the interior of the fiber. The practical method of overcoming this barrier is to use large quantities of inorganic electrolyte, such as salt or sodium sulfate. The electrolyte dissociates into its cations and anions, for example, sodium ions (Na+) and chlorine ions (Cl−) for salt. The negative charge on the surface of the fiber is suppressed by the cations enabling the dye anion to enter the fiber more readily (Gohl and Vilensky, 1983). Direct dyes have an inherent substantivity for cellulose fibers. The dyebath is gradually heated and electrolyte added to promote exhaustion and diffusion. The most attractive feature is the simplicity of the dyeing process. However, a cationic aftertreatment is necessary for most direct dyeings to enhance washing fastness (Cook, 1982; Shore, 1995b). The wash fastness of the apparel item, even with an aftertreatement, is only satisfactory for pale and some selected medium shades. Consequently direct dyes have been largely replaced by reactive dyes. Reactive dyes, developed in the 1950s, have become the major dye class for cotton, rayon, linen, and animal fibers (Holme, 2004). They offer high levels of washing fastness, a wide gamut of bright colors, and versatility for different application methods. The high fastness to washing is due to their reactive groups, which form covalent bonds with the hydroxyl groups of the cotton cellulose under alkaline pH conditions. The dyes also react with hydroxide ions present in the alkaline dyebath. This produces nonreactive hydrolyzed dye, which remains in the dyebath as well as in the fiber. To obtain the required level of washing fastness, it is necessary to remove all unreacted and hydrolyzed (unfixed) dye from the cotton fiber. This is achieved by “washing off,” which comprises a series of thorough rinsing and soaping steps. Around 50% of the dyeing cost is related to the washing off and effluent treatment (Mohsin et al., 2013). The dye remaining on the fiber after washing off is considered as dye fixed on the
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fiber. The dye fixation efficiency is typically in the range of 50–80% (Smith, 2003; Kalliala and Talvenmaa, 2000).
5.5.2 Dyeing of cotton with vat and sulfur dyes Sulfur and vat dyes are insoluble in water. They are chemically reduced using sodium sulfide or sodium hydrosulfite to produce their water soluble “leuco” form before application (Gohl and Vilensky, 1983). Once soluble, the dyes are applied to cotton in a way similar to dyeing with anionic dyes. Immediately after application, the fabric is oxidized to return the dye to its insoluble form. Thus, the dye molecules become physically trapped within the fiber. At the end of dyeing, washing off is carried out to remove unfixed dye. In such dyeing methods, reduction and oxidation steps pose sustainability issues due to their impact on dyeing effluent.
5.5.3 Dyeing wool, nylon, and silk with acid dyes Dyeing of protein and polyamide fibers with acid dyes is based on ionic attraction and then ionic bonding between the dye molecule and the fiber polymer. Acid dyes are anionic when dissolved in water, whereas amino groups of protein and polyamide fiber polymers are cationized in the acidic dyebath. Hence, the anionic dye is ionically attracted toward the fiber without the need for an electrolyte. Such dye–fiber attraction is often too high and can result in uneven dyeing. Hence salts or surfactant-based retardants are added to slow down the dye exhaustion rate, thereby controlling the dye levelness on the fabric. Once dye molecules are in the fiber they form an ionic bond with the wool fiber molecules. In addition, Van der Waal’s forces and hydrogen bonds also contribute toward dye fixation. Rinsing, to remove loosely bound dye, is the final step.
5.5.4 Disperse dyeing of polyester Today, almost all polyester is dyed with the small molecular-sized, nonpolar disperse dyes (Gohl and Vilensky, 1983). Before the advent of high-temperature pressurized jet-dyeing machines, disperse dyeing of polyester required the use of fiber-swelling agents such as phenolic-based chemicals. Now supplanted by pressure jet machines the use of such highly polluting carriers has been effectively discontinued. High dyeing temperatures, typically 120–130 °C, swell the fiber and allow the dye to penetrate. At the end of the dyeing, when fiber contracts to its original crystalline orientation, the dye becomes trapped within the fiber. Dye on the surface of the fiber is loosely held. This unfixed dye, especially in two-fiber dyeing (e.g., wool-polyester and cotton- polyester fabrics) is removed by reduction clearing followed by rinsing (Broadbent, 2005). Reduction clearing, akin to the reduction of vat dyes, poses sustainability issues.
5.5.5 Dyeing acrylic with cationic dyes Like dyeing protein with acid dyes, the dyeing of polyacrylic fibers with cationic dyes is based on the ionic attraction and bonding between the dye molecule and the
fiber polymer. In the aqueous dyebath, the cationic dye is attracted and then ionically bound to the anionically charged carboxylate and/or sulfonate groups within the acrylic fiber (Broadbent, 2005). There are no major sustainability issues with acrylic fiber dyeing.
5.6 Dyeing technology, machinery, and sustainability Dyeing can be carried out at any stage of the production pipeline. Hence, dye can be applied to fibers, yarns, fabric, or the final garment. Fiber dyeing is usually only practiced for cotton and wool apparel items where mélange and chambray effects are required. Yarn dyeing is used for the production of stripes and checks. Garment dyeing is practiced where short restocking times are a key part of the supply chain process. By far the greatest volume of dyeing is carried out in fabric form. Hence, this section of the chapter is focused on fabric. The major contributors to environmental sustainability in preparation and dyeing are water and energy, the nature and quantities of dyes and chemicals used, and the discharges to air and stream. The quantum of each of these contributors depends on the fiber type, the dye, and the associated chemical usage. The type and model of the machinery used for processing the fabric also have a material effect on dye utilization efficiency and chemical consumption. Discharge issues are principally confined to stream or effluent. Discharge to the air is an issue for some product manufacturing, but rarely apparel. In looking at sustainability there are two general contributing factors. (1) Water and energy consumption, which are influenced primarily by the type of dyeing machine and (2) chemical consumption and effluent discharge, which are more dependent on the fiber type and the dye class. These two contributing factors to sustainability are not totally mutually exclusive. For example, improved machinery design can be targeted at lower dye and chemical consumption. This is explored later in this section. The important technological issues and developments related to dyeing and machine technology for improved levels of sustainability are discussed in the following. Apparel fabric dyeing machinery can be divided into two broad classes: exhaust and pad. The winch, jigger, jet, and soft flow dyeing machines are the most common forms of exhaust dyeing machines. Exhaust dyeing is suitable for small runs (say 400–4,000 m). For this reason fashion apparel is usually dyed on exhaust machines. The jigger, where the fabric is passed, under tension, backward and forward, through a trough of dye liquor, between two fabric winding cylinders, is the only exhaust process where the fabric is treated in open width. Once common in dyehouses, jig dyeing is now reserved for special fabric–dye combinations and where the fabric is crease sensitive. All the other types of exhaust dyeing machine handle the fabric in rope form. Garment dyeing, that is, the dyeing after a garment has been made or substantially assembled, operates on the exhaust principle. It is predominantly used for fashion knitwear. For pad dyeing, the key machine component is the padding mangle. Therefore, open-width fabric is passed through a trough containing dye and auxiliaries and then squeezed between precision rollers under pressure. For some fiber–dye combinations,
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a padder with two dips and two sets of squeeze rollers is required. After padding, the fabric can be wound onto a batching frame (pad-batch/fix process) or passed into a continuous range for steaming, fixation, drying, and washing off. Continuous plants are generally designed for a specific dye–fiber combination. They are commonplace in almost all large apparel fabric dyeing plants. They are ideal for large runs (typically measured in tens of kilometers). The dominant apparel wear forms dyed in continuous form are mass merchandise wear, corporate style wear, and industrial work wear.
5.6.1 Exhaust dyeing In a typical exhaust dyeing of cotton with reactive dyes, the first phase of dyeing is carried out under neutral pH conditions to allow dye exhaustion and diffusion (Broadbent, 2005). Sodium chloride or sulfate is used to promote exhaustion. The temperature of the dyebath is gradually increased to aid penetration of dye into the fibers, and to assist uniform migration. Fixation of the dye is then achieved by adding a suitable alkali to the dyebath. The reaction phase of the dyeing occurs over 30–60 min, with typical dyeing temperatures in the range of 30–90 °C, depending on the type of reactive group and its reactivity. The fixation process results in additional dye transfer to the fiber, which is often referred to as secondary exhaustion (Srikulkit and Santifuengkul, 2000; Imada and Harada, 1992). The secondary dye exhaustion and dye–fiber reaction continue until no further dye is taken by the fiber. After completion of the dye–fiber reaction step, the fabric contains three forms of the dye: covalently bonded dye, absorbed but unreacted dye, and hydrolyzed dye. The unreacted and hydrolyzed dyes are unfixed. This causes poor wash-fastness and requires removal of all the unfixed dye in a wash-off step. Residual alkali and absorbed salts must also be removed. The wash-off process requires large quantities of water (Shukla, 2007). Approximately 75% of the water used in reactive dyeing occurs during washing off (Knudsen and Wenzel, 1996). The concentration of dyeing chemicals and auxiliaries is determined by the fabric mass per unit volume of dyeing liquor. Thus, the total consumption of chemicals can be reduced by reducing the volume of liquor (Kalliala and Talvenmaa, 2000). On this basis, a wide number of forms of low liquor-to-fiber ratio jet dyeing machines have been commercially introduced (Phillips, 1996; Wilbers and Seiler, 2002). Different machinery manufacturers offer specific equipment for particular fabric styles and weights, for example, jet airflow assist, special jets for sensitive fabrics, and creels to aid the rotation of fabric in the dye machine. As a result of these new technologies, the apparel dyeing industry moved from a 20:1 liquor ratio (liters of water per kilo of fabric) in the 1970s and 1980s down to ratios as low as 6–8:1 in the 1990s. Ultra-low liquor ratio jet dyeing machines now offer the opportunity to dye at ratios as low as 3:1. The reduction in liquor ratio also increases the driving force for the dye to move from solution into the fiber (Anderson, 1994). Thus, a lower concentration of salt is needed for dye exhaustion. The lower volume of water also results in less hydrolysis of the reactive dye thereby increasing the dye yield. Other advantages of low liquor dyeing machinery are reduced dye liquor wastage and lower steam consumption to heat the dyebath. The developments in low liquor dyeing machinery over the past
50 years have resulted in major improvements to sustainability, lower water and energy use, lower dye and salt use, and hence reduced disposal to the environment. Such dyeing machines are highly recommended as the best available technology for exhaust dyeing (European Commission, 2003).
5.6.2 Pad dyeing From a sustainability perspective, padding-based dyeing methods for cotton fabrics are the most preferred coloration pathway (Schramm and Jantschgi, 1999). Relative to exhaust dyeing, pad-based dyeing methods offer (Leube, 2003) the following: ●
Lower liquor-to-fiber ratio in the dyebath Lower amounts of leftover dyebath solution Faster dye application Easier control on dye levelness on the fabric No electrolyte required for exhaustion
The lowest possible liquor-to-fiber ratio in exhaust dyeing is 3:1 using ultra-low liquor-ratio dyeing machines. Pad dyeing reduces this to between 1:1 and 0.5:1. Such low liquor ratios further increase dye take-up. As a result further reductions in quantities of dye and water are discharged as effluent at the end of the process. However, these methods are normally avoided on knitted fabrics and small lots due to certain limitations (Smith, 2003) such as dimensional instability and higher pollution load (European Commission, 2003). In continuous pad-dyeing processes, the uniformity of dye applied across and along the fabric is critical to the final shade and acceptability of the dyed fabric. Key factors in achieving this are the dwell time in the padder, the type of fiber and the construction of the fabric, percent pickup, whether there is any preferential adsorption of dyebath components (this can produce tailing of the shade), reaction of reactive dyes in the dyebath, dye aggregation, and the transfer of impurities in the fabric from a pretreatment step into the padder. It is also important that the dye fixation is achieved within a short time frame (30–120 s) after padding. Fixation of the dyes in the impregnated fabric is achieved by baking or steaming in specialized continuous ranges. In pad-dyeing methods, the dyeing liquor left in the padding trough, pipes, and pumps are drained to the effluent in its concentrated form at the end of a dyeing process (European Commission, 2003). However, under certain circumstances, it may be drained to a sealable drum for later use. The volume of the liquor left depends on the padding trough capacity. A number of machinery manufacturers have developed trough volumes down as low as 10–15 L. This has provided meaningful improvements in dye utilization and disposal of waste (Phillips, 1996). Figure .5.1a shows a padder with a low-volume trough. Note that the fabric moves upward into the squeeze rollers, which allows surplus liquor to be returned to the trough. An alternative approach is to form a trough between the two squeeze rollers (Figure .5.1b). The European Commission (2003) recommends padding as the best available technology for reducing dyeing liquor wastes.
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Figure 5.1 U-trough (a) and nip (b) dye liquor pad application systems. Source: Benninger S-Roll Padders.
5.6.3 Pad-fixation processes A range of fixation technologies have been developed to take advantage of pad dyeing. Some have been evolutionary, whereas others have been developed specifically for a particular dye–fiber product. A summary of the key features of the various fixation technologies and their sustainability issues is given in the following sections. Pad-roll-batch Pad-batch dyeing processes are the most economical of all pad- dyeing processes for the reactive dyeing of cotton (Aspland, 1992a). In fact, for small lots of around 1000–10,000 m, this process is more economical than exhaust dyeing, mainly due to lower energy requirements. This process, also referred to as cold-pad-batch, involves padding the fabric with a dye solution containing a suitable alkali system and then winding the fabric onto a roller before batching for 6–24 h (Broadbent, 2005; Shore, 1995c). To avoid evaporation from the exposed edges of the roll, the fabric is wrapped with winder end cloth and then sealed with a plastic film. For dye fixation at ambient temperature, the dyes must have a high reactivity. During batching, the roller rotates slowly to avoid welling of the liquor within the batch. After batching, the fabric is washed to remove unfixed dye and residual chemicals. This is done either on a continuous washing range or on a batch dyeing machine. Relative to jet dyeing, cold-pad-batch dyed fabrics may have an improved handle and cleaner surface appearance. Prominent water and energy savings (Khatri et al., 2014), reduced consumption of dyes and chemicals, and less space and labor requirements of the pad-batch dyeing process (Stone, 1979) make it economical and ecologically sustainable (Van Wersch, 1992). Pad-dry-bake Pad-dry-bake dyeing of cotton with reactive dyes, as its name implies, involves padding, intermediate drying, and then dye fixation by baking at high temperatures of up to 160 °C for 60–120 s (Shore, 1995b). Dye selection based on the level of reactivity is important. The process is completed by washing off. The padding solution comprises the dye, an alkali (usually sodium carbonate), urea, and
an antimigrant (usually an alginate-based thickener). A high concentration of urea, in the range of 50–200 g/L is used to help the cotton retain water during drying and subsequent baking (Aspland, 1992a). The hot, dry environment provides a suitable medium for dye diffusion within the fiber. It also improves dye fixation and color yield (Krichevskii and Movshovich, 1970; Aspland, 1992b; Suss-Leonhardt et al., 2006; Shore, 1995c). Sometimes, urea is also used in pad dyeing to improve dye solubility. Urea is removed during the washing-off process but poses pollution issues as it is a source of nitrogen (Broadbent, 2005). Intermediate drying is a critical step in any continuous dyeing sequence. Excessive evaporation and dye migration are the key problems. An antimigrant is used in the padding liquor to avoid dye migration, which causes shade changes on both sides of the fabric. Predrying using hot cylinders or infrared heating are also used to minimize this problem (Broadbent, 2005). Pad-steam fix Pad-steam dyeing with selected reactives is the simplest and most economical process for producing pale to medium shade depths on cotton apparel fabrics (Aspland, 1992a; Broadbent, 2005; Phillips, 1998). Fabric is padded with a reactive dye solution containing inorganic electrolyte (for diffusion and subsequent levelness of the dyes into the fiber) and the appropriate alkali (to initiate the dye–fiber reaction) followed by 60–90-s steaming for dye fixation. Steamers used for fixation transport the fabric, as deep loops, on moving rollers. They provide an air-free environment by means of a water exit seal (as a “water-lock”). Each commercial range of dyes requires a particular set of steaming conditions. For reactive dyes on cotton, steam fixation is achieved using saturated steam at 101–102 °C and 100% humidity. Dye class selection is important; bifunctional reactive dyes tend to be more reproducible than monofunctional systems (Abeta and Imada, 1985). Finally, the fabric is washed off to remove unfixed dye and residual chemicals. Early steam fixation processes for reactive dyeing of cotton followed the sequence: pad (with neutral dye solution), dry, pad (alkali and salt), steam, and wash. This sequence is often referred to as the pad-dry-chemical pad-steam process. This process sequence is quite similar to the continuous vat dyeing of cotton fabrics. The fabric is first padded in a neutral reactive dye solution and dried in a hot chamber. After intermediate drying, the fabric is padded with a dilute solution of sodium hydroxide in saturated brine followed by steaming to complete the dye–fiber reaction. High concentrations of electrolyte are required to minimize color bleeding during the second alkali padding. Vacuum extraction of the padded fabric, in lieu of an intermediate drying, can be used to restrict dye bleeding into the chemical pad solution (Fortin et al., 1987). In recent decades, simpler and more environmentally sustainable dyeing sequences such as pad-dry-steam and pad-steam have been introduced by the major machinery manufacturers in collaboration with dye suppliers. The new sequences and machinery minimize bleeding and migration problems associated with pad-dye pad-chemical methods (Bright, 1990). Urea is used in the pad-dry-bake dyeing process to improve the extent of the dye– fiber reaction. Inorganic salt is used in pad-steam dyeing to improve dye levelness into the fiber. Salt is also used in the pad-dry-chemical pad-steam process to minimize dye bleeding during chemical padding.
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Although considerable progress has been made in improving the sustainability of continuous dyeing processes from the 1970s to the 1990s, there remained problems with the need to continue to use urea and inorganic salts. In the late 1990s, a joint machinery and process development by BASF and Monforts provided industry with a continuous cotton dyeing process that did not require urea and salt (Hyde, 1998). The machine, known as the Eco-Control range is based on the injection of moisture vapor into the hot fixation chamber. Typical fixation conditions for a dichloro-s-triazine dye have been reported as 120 °C temperature and 20–25% relative humidity. Only small concentrations of alkali (sodium bicarbonate) are required for these dyes. This development has been commended as the best technology for improving the ecological and economic impact of the pad-dyeing processes (European Commission, 2003). Padcxoxidation The production of black, navy, brown, olive, and green shades in medium to heavy depths on cotton apparel wear is most economically carried out with sulfur dyes on a continuous range. The attraction for the industry is the low cost of sulfur dyes. The sustainability issue centers on the extensive rinsing necessary to remove the unfixed organo-sulfur dyes. The effluent stream has a high sulfide content that results in high chemical oxygen demand (COD) and BOD demands in the effluent. More sustainable approaches to the current process system have been well studied (Božič and Kokol, 2008; Teli et al., 2001), but the effluent issue remains. Archroma (previously Clariant) recently developed a process modification that provides 100% fixation of the sulfur dye onto the cotton fiber (Khatri et al., 2012). The process development is based on the use of pad-oxidation steps in the rinse section of the sulfur dye range. The new technology is being increasingly adopted by industry.
5.6.4 Printing Textile printing is the one-sided application of a design or pattern onto a fabric. Two or more colors are usually applied using a viscous paste via a fixed flat or rotating cylindrical screen. Printing of apparel fabrics is generally confined to fashion-oriented women’s and children’s wear. Special applications include camouflage apparel wear for military use. Printing is divided into two broad classes: pigment printing (where the dye is bound to the outside of the fiber within a cured copolymer) and dye printing (where the dye is bound in the interior of the fiber in the same way as in fabric dyeing). The major sustainability issues in printing arise from the composition of the printing pastes (e.g., natural and synthetic polymers, hydrocarbon emulsions, and dye fixation chemicals). The following paragraphs provide a brief discussion on sustainability issues in the major printing technologies. Comprehensive reviews of printing are available (Gohl and Vilensky, 1983; Miles, 2003). In pigment printing, the permanency of the design and color depends on the degree of binding of the polymer to the fiber and yarn surfaces and on the binding of the pigment within the polymer film. Polymer selection and effective heat curing are important. Advantages of pigment printing are that the fabric does not require washing after curing and that it is a method highly applicable to the production of designs on finished garments.
Printing of cotton fabric with reactive dyes and polyester fabric with disperse dyes is also important but is confined principally to open-width fabrics. The usual process sequence for printing with dyes is printing, drying, steaming, and washing. The main chemicals are same as those when used during dyeing. Washing is important for the removal of the thickening agent, unfixed dyes, and other auxiliary chemicals. Urea, usually used as a humectant in the reactive printing of cotton, becomes an environmental concern when drained to effluent during washing. A variety of methodologies are available for printing fabric, block printing, roller printing, screen printing, transfer printing, and digital ink-jet printing. The most widely used are rotary-screen and flat-bed printing. Digital ink-jet printing of textiles is an emerging technology that offers a number of potential benefits over conventional screen printing methods (Momin, 2008). It eliminates the setup cost associated with screen preparation and can potentially enable cost-effective, one-off, and short-run production lots. It allows visual effects such as tonal gradients and infinite pattern repeat size, which cannot be practically achieved by screen printing.
5.7 Effluent treatment and recycling Effluent is a major issue for all dyehouses. Dyehouse effluents are multicolored discharge streams comprising nonbiodegradable inorganic salts and alkali along with organic matter such as surfactants, starches, and other process aids (Babu et al, 2007; Phillips, 1996; Smith, 2003; Ibrahim et al., 2008). Color in a discharge is undesirable because the reduced penetration of sunlight into the water decreases photosynthetic activity and lowers dissolved oxygen levels (Cooper, 1995). Salts, alkalies, detergents, and other processing chemicals cause high levels of BOD, COD, and TDS (for total dissolved solids) and hence present a threat to aquatic life. Heavy metals, especially chromium, can be present depending on the product profile of the dyehouse. They are unacceptable in their own right and can also interfere with effluent treatment processes. Effluents from cotton processing are generally the most polluted. A typical composition of the effluent from a cotton dyeing and finishing house is given in Table 5.3. The most common form of disposal is through the sewer and then to a central treatment plant. In some rare instances, some plants still discharge directly to streams Table 5.3
Typical composition of cotton dyehouse effluent
pH Total alkalinity BOD COD Total dissolved solids Total chromium
9.8–11.8 17–22 mg/l 760–900 mg/l 1400–1700 mg/l 6000–7000 mg/l 10–13 mg/l
Khatri et al. (2015).
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or the ocean. Dyehouses are typically charged a volume fee plus separate fees for alkalinity, TDS, BOD, and heavy metals. The increasing regulation of discharges from municipal plants, including demands for potable water output, has resulted in higher charges to textile companies. This has placed dyehouses under pressure to reduce both the volume output and the mass of dye, salts, and other processing chemicals discharged in their effluent. Dyehouses have a number of options for reducing water usage, for recycling, and for reducing discharge loads. In brief, the options include water usage, recycling, and reducing discharge loads. Water usage Appropriate machinery technology is the most commonly used approach to reducing total water consumption. Good housekeeping and minimizing the number of re-dyeings or shade corrections also assist in reducing water usage. Water consumption can also be reduced by recycling. Recycling Recycling rinse waters and other color-free liquors is becoming a frequently used practice. Suitable liquors are recovered, stored in tanks, and then used for scouring, first rinses, or new dyebath liquors. Reducing discharge loads Good housekeeping, maximizing dye uptake in the dyeing process, and minimizing residual liquors in padding are practices used by many dyehouses. Such practices reduce the cost of production and lead to lower volumes discharged. As discharge costs and fees have increased, many dyehouses have introduced stream separation methodologies. Some streams can be recycled either within the dyehouse or at the particular machine. Highly concentrated streams can be treated in-house. Intermediate streams can be disposed to the sewer. Some companies discharge to land if the effluent stream is deemed suitable for irrigation. In recent decades, a number of techniques and technologies have been developed for effluent treatments and recycling (Christie, 2007; Hauser, 2011; Babu et al., 2007; Vandevivere et al., 1998). A brief description of some of the more common technologies for the treatment of the dyehouse effluents is provided in the following sections.
5.7.1 Flocculation and sedimentation Flocculation and sedimentation treatments are based on the controlled addition of flocculating agents such as such as aluminum sulfate, ferric sulfate, ferric chloride, or special cationic polymeric compounds (Hauser, 2011). Settling tanks produce an organic sludge which requires subsequent disposal. The process is generally used within a large effluent treatment plant. The major sustainability issue is the disposal of the sludge to landfill.
5.7.2 Adsorption technologies Adsorption is a physicochemical effluent treatment in which effluent is mixed with, or passed through a bed of, activated carbon, a kaolin clay, or a silicon polymer. The different adsorbents have selective adsorption of different dyes. Activated carbon is regarded as the overall best adsorbent for dyeing effluent, but it is expensive (Christie, 2007). Absorption is only feasible in combination with a prior decantation or biological treatment because suspended solids rapidly clog the filter.
5.7.3 Biological treatment Biological treatment is a most effective means for removing biodegradable dyes and textile auxiliaries (Babu et al, 2007). Natural or regenerated bacteria are added to the effluent in a holding pond. Aerobic conditions are achieved by spraying the effluent into the air by means of large floating pumps. The output from the biological treatment pond is usually passed to either a flocculating plant or an activated carbon or clay filtration unit for further treatment.
5.7.4 Oxidation treatments Oxidation is a commonly used decolorization process. Oxidants such as chlorine, hydrogen peroxide, ozone, and chlorine dioxide are added to the effluent to break the dye molecules into colorless low molecular weight compounds. The process is efficient and the reaction time short (Christie, 2007). Oxidation of concentrated color streams on the dyehouse site is attractive as it allows delivery of a low-colored effluent to the main treatment facility. Advanced oxidation processes based on the treatment of effluent with activated hydroxyl radicals have been the subject of research by a number of scientists and technology developers. The activated hydroxyl radicals are generated by using various combinations of ultraviolet (UV) light, oxygen donors, and catalysts such as titanium dioxide and iron compounds. Such processes show good potential for decolorizing dyehouse effluents.
5.7.5 Electrochemical treatment of effluent Electrochemistry, through the use of special electrodes, offers good potential for the treatment of many effluent streams (Hauser, 2011). Although efficient in the removal of dissolved solids, color and BOD, electrode fouling, electrode consumption, and operating costs are high. Research into improved electrochemical technologies including electro-catalytic advanced oxidation process is ongoing.
5.7.6 Membrane technology Membrane separation processes, also known as reverse osmosis, nanofiltration, and ultrafiltration have become popular due to their simplicity and their high and selective separation efficiencies (Hauser, 2011). Membrane technology offers the only effective means for removal of dissolved inorganic salts (TDS) such as sodium chloride and sodium sulfate. Typical TDS outputs from a cotton dyehouse are on the order of 6000 mg/L. When diluted with streams from preparation and finishing, the TDS generally lies within the range 1500–2500 mg/L. Prescribed maxima for discharge to stream are generally 400–800 mg/L. For this reason membrane plants have become increasingly common. Capital costs, installation, maintenance, and membrane replacement or defouling are expensive.
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5.8 Future trends in dyeing There is little doubt that the refinement of dyes, process paths, machine technology recycling, and effluent treatment that have resulted in significant improvements to sustainability over past decades will continue. Some of these developments will be driven by the constant demand for cost reductions along the supply chain. Others will be driven by tighter national environmental regulations. Moves to more global uniformity of environmental regulations could have a profound effect on the sustainability of the industry as a whole. So, too, could consumer advocacy and pressure from brands and retail chains for more sustainable products. The key pathways for improving the sustainability of the dyeing is in three areas: 1. Development of synthetic dyes with improved chemistries. 2. The use of environmentally safer chemicals in dyebath formulations. 3. Chemical modification of fiber prior to dyeing.
Many of these pathways remain the province of research departments of major suppliers as well as universities and research institutes. The three areas are discussed below.
5.8.1 Developments in dyes and dyeing chemistry Most of the advancements have been in the area of reactive dyes as this area remains problematic in regard to fixation levels and the need for inorganic salts to drive the dye onto the fiber. The major area for R&D has been in optimizing dyeing performance through changes to the type and number of the functional groups on the dye molecule. The most widely used reactive groups, in the order of increasing level of reactivity, are trichloropyrimidine, aminochloro-s-triazine, sulfatoethylsulfone, dichloroquinoxaline, aminofluoro-s-triazine, difluorochloropyrimide, and dichlorotriazin. The extent of dye–fiber reaction and the ultimate discharge of unfixed dye varies widely with the type of reactive group, the number of functional groups, and the dyeing technology used. Improved fixation levels and hence lower dye costs and discharge quantities have been achieved through using two different functional groups on the dye molecule. These hetero-bifunctional dyes provide better fixation with more flexibility for the coloration method and the process parameters. Some bifunctional dyes are claimed to provide a fixation efficiency of up to 95% when applied on cotton by the pad-batch dyeing method (Shore, 2002; Luttringer, 1993). Several tri-, tetra-, and penta-functional dyes have been developed. Some have been introduced to the market (Taylor, 2000; Lewis, 2009). Ongoing improvements from further R&D in this area are expected. Some commercial polyfunctional reactive dyes are claimed to be “low salt” dyes. The requirement of a reduced amount of salt may be due to the high affinity and have high fixation efficiencies. The use of commercially available bi- or polyfunctional reactive dyes have been recommended as the best available technique for increasing the dye fixation efficiency (European Commission, 2003; Christie, 2007).
Reactive dyes based on a nicotinic acid residue allow dye–fiber reaction at a neutral pH of 7–7.5 (Morimura and Ojima, 1985). These dyes are particularly suitable for neutral, high-temperature exhaust dyeing, and for one-bath dyeing of polyester/ cotton-blended fabrics (Lewis, 1993). This continues to be an active area for R&D (Lewis and Vo, 2007; Lewis et al., 2008). Eliminating the need for salt by developing a reactive dye with cationic features has been successfully demonstrated in the laboratory (Hinks et al., 2001). Though cationic reactives are yet to be commercially developed, they remain an active area of research.
5.8.2 Developments in dyebath chemicals Biodegradable dyebath chemicals offer an interesting alternative to inorganic salts. The attraction is lower effluent loads. Betaine, an organic compound, allows reductions in the amount of inorganic salt (Liu and Yao, 2009). Organic surfactants, as inorganic salt substitutes, also reduce the effluent load (Rucker and Guthrie, 1997b). Mixtures of magnesium-based organic compounds in the dyeing of cotton with direct or reactive dyes has been patented (Moore, 1993). However, commercialization is problematic because magnesium increases water hardness and creates problems with color matching and dye process control (Patra and Gupta, 1995; Jain and Mehta, 1991). Sodium salts of organic acids have been explored as alternatives to sodium chloride and sodium sulfate. Trisodium citrate has been shown to be a viable alternative to traditional inorganic salts for exhaust dyeing of cotton with reactive, direct, and solubilized vat dyes (Prabu and Sundrarajan, 2002). Salts of polycarboxylic acids are also effective alternatives to salt (Rucker and Guthrie, 1997a; Guan et al., 2007). Tetrasodium ethylene diamine tetra-acetate and other alkaline polycarboxylic sodium salts have also been reported as a replacement for both salt and alkali in exhaust and continuous pad-steam dyeing of cotton with reactive dyes (Ahmed, 2005; Khatri, 2011). Research is expected to continue into salt substitutes as they produce meaningful reductions in effluent load. The cost of the salt alternatives remains the impediment to commercial use. Alternatives to the use of urea in printing and some continuous reactive dyeing methods are ongoing. Use of a dicyandiamide in the dyebath has been shown to reduce the amount of urea required (Phillips, 1996). Caprolactam products can result in partial or complete substitution for urea in reactive dyeing and in printing of cotton fabrics (Sheth and Musale, 2004). As discussed previously, the reduction and oxidation processes in vat and sulfur dyeing generate effluent pollution. Sodium dithionite, as the predominant reducing agent, produces large amounts of sodium sulfate (TDS) and environmentally undesirable sulfite and thiosulfate as by-products. Organic biodegradable reducing agents, such as some enediol compounds, have been shown to be more environmentally sustainable (Božič and Kokol, 2008) but are yet to be used in practice. Biotechnological treatments using enzymes for reduction and oxidation treatments do offer some potential for use in the industry (Cavaco-Paulo and Gübitz, 2003). Other emerging areas of focus are electrochemical reduction and oxidation along with technologies based on ultrasound, magnetic fields, and UV (Božič and Kokol, 2008).
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5.8.3 Chemical modification of cotton Chemical modification of cotton to improve dyeing with direct, reactive, sulfur, or vat dyes is an emerging area. Research has focused on the introduction of cationic groups to fiber (Lewis and McIlroy, 1997; Mughal et al., 2008; Fang et al., 2013; Hashem et al., 2005; Ameri Dehabadi et al., 2013). Such modifications are achieved by treating cotton with low molecular weight cationic chemicals or with cationic polymers. In the reactive dyeing of cationized cellulose, the anionic dyes are attracted by the cationic charges on the fiber. As a result, higher fixation efficiency and reduced (or no) use of salt can be achieved. In some cases, salt and alkali-free reactive dyeing of modified cotton have also been reported (Burkinshaw et al., 2000; Kannan and Nithyanandan, 2006; Blackburn and Burkinshaw, 2003). Natural polymers such as cationic starch and chitosan (Chattopadhyay, 2001; Zhang et al., 2005; Singha et al., 2012) are also effective. Research into eliminating the need for an extra process step may make this approach more potentially viable.
5.8.4 Emerging technologies Ultrasonic energy offers the opportunity to increase the intensity and speed of reaction of many wet textile processes. Past studies have demonstrated improvements to energy, water, and chemical consumption, improved color yields, and reduced effluent loads (Oner et al., 1995; Khatri et al., 2011; Ahmed and El-Shishtawy, 2010). Thakore constructed a specially mounted ultrasonic tube resonator and used it for exhaust and pad-batch dyeings of cellulose-based fabrics with reactive dyes on a production scale (Thakore, 2011). Electrochemical methods of reduction and oxidation of vat and sulfur dyes is an emerging area. Research to date has shown that electrochemical reduction and oxidation can be used to reduce effluent pollution (Božič and Kokol, 2008). Extension into the research in this area may provide interesting opportunities in the red-ox steps in vat and sulfur dyeings. Microwave heating has been shown to improve the uptake and fixation of reactive dyes on cotton (Lei et al., 2013; Badrossamay and Amirshahi, 2001). Research on potential applications is expected to continue. Plasma and supercritical carbon dioxide have been explored as ways to eliminate the use of water in dyeing (Ahmed and El-Shishtawy, 2010; Özdogan et al., 2002; Schmidt et al., 2003; Fernandez Cid et al., 2007; Patiño et al., 2011; Wang et al., 2012, 2013).
5.9 Conclusion This chapter has explored the principal sustainability issues in the preparation and dyeing of apparel fabrics. The subject and the issues involved are many, varied, and complex. Dyeing cotton apparel fabrics with reactive dyes represents the largest volume in all apparel dyeing. It is also the largest contributor to environmental pollution. The chapter has covered some of the many improvements in sustainability that have been made by dye manufacturers, machinery manufacturers, research institutes, and the industry.
Sources of further information Further information can be obtained from the short list of books, trade and professional bodies, and research groups provided in the lists below. Websites are recommended.
Books (a) Handbook of Sustainable Textile Production, edited by Marion I. Tobler-Rohr (Woodhead Publishing, 2011). (b) Environmental Aspects of Textile Dyeing, edited by R. M. Christie (Woodhead Publishing, 2007) (c) Wastes from textile processing by B. Smith, in Plastics and the Environment, edited by A. L. Andrady (John Wiley and Sons, Inc., 2003). (d) Sustainable Textiles, edited by R. S. Blackburn (Woodhead Publishing, 2009). (e) Basic Principles of Textile Coloration, edited by A. D. Broadbent (Society of Dyers and Colorists, 2005). (f) Cellulosics Dyeing, edited by J. Shore (Society of Dyers and Colorists, 1995). (g) Textile Science: An Explanation of Fiber Properties, edited by E. P. G. Gohl and L. D. Vilensky (Longman Cheshire, 1983).
Trade and professional bodies (a) American Association of Textile Chemists and Colorists. (b) Society of Dyers and Colorists, Bradford, UK. (c) The Textile Institute, Manchester, UK. (d) Environment Protection Agencies (USA, Australia and others). (e) Leading manufacturers of textile colorants and auxiliaries: Archroma, BASF, DyStar, Huntsman, and more. (f) Leading manufacturers of textile processing machines: Monforts, Benninger, Kuster, Goller, Brukner, Arioli, Cimi, Zimmer, Reggiani, Buser, CST, Lafer, Fongs, MCS, Scholl, Thies, Then, Mathis, Rapid, and more.
Research groups and activities (a) Color and Dye Chemistry and Sciences, and Energy, Environment and Sustainability, North Carolina State University, USA. (b) Color Research, Department of Materials Science and Engineering, Clemson University, USA. (c) Dyeing Research, Cotton Incorporated, USA. (d) Textile Chemistry and Coloration Group, School of Materials, University of Manchester, UK. (e) The Color and Polymer Science Group, School of Chemistry, University of Leeds, UK. (f) Color Science and Technology Research Group, School of Textiles and Design, Heriot Watt University, UK. (g) Textiles Research Group, School of the Arts, Loughborough University, UK. (h) Textile Chemistry Processing and Color Technology Research Group, Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Hong Kong. (i) Sustainable Textile Processing Research, Department of Textile Engineering, Mehran University of Engineering and Technology, Pakistan. (j) Coloration Technology Research, Department of Textiles, Ghent University, Germany.
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