Critical solutions in the dyeing of cotton textile materials

CRITICAL SOLUTIONS IN THE DYEING OF COTTON TEXTILE MATERIALS R. Shamey and T. Hussein doi:10.1533/tepr.2005.0001 Abstract: Over the decades there have been several papers on the coloration of cotton-based textiles. The number of articles dealing with the processing of cotton, including preparation, dyeing, and finishing, may be in the thousands. An investigation of the possible causes of problems occurring in the coloration of textiles revealed that a comprehensive review of case studies and scientific analysis would be a welcome addition to the already rich pool of knowledge in this area. Key words: Cotton, troubleshooting, pretreatment, dyeing, dyes, colorants. 1. INTRODUCTION Cotton is the backbone of the world’s textile trade [1]. It has many qualities [2] and countless end uses [3], which make it one of the most abundantly used textile fibres in the world [4]. It is a seed hair of plant of genus Gossypium [5], the purest form of cellulose found in nature. However, cotton is one of the most problematic fibres as far as its general wet processing or dyeing is concerned. Quite frequently, the problems in dyed cotton materials are not due to the actual dyeing process but due to some latent defects introduced from previous production and processing stages. Often, the root-cause(s) of a problem in the dyed material can be traced as far back as to the cotton field. This monograph will address problems in the dyeing of cotton textile materials in various forms. An overview of various textile operations for cotton will be given in the beginning. Then, various key stages and factors involved in the production of dyed cotton textile materials will be described in detail and problems originating at each stage will be summarised. 1.1 Overview of Textile Operations for Cotton The textile industry is comprised of a diverse, fragmented group of establishments that receive and prepare fibres, transform fibres into yarn, convert the yarn into fabric or related products, and dye and finish these materials at various stages of production. Figure 1 shows some of the general steps involved in manufacturing cotton textiles. Textiles generally go through three to four stages of production that may include yarn formation, fabric formation, wet processing and textile fabrication [6]. Textile fibres are converted into yarn by grouping and twisting operations used to bind them together [7]. Although most textile fibres are processed using spinning operations, the processes leading to spinning vary depending on whether the fibres are natural or manmade. Figure 2 shows the different steps used in cotton yarn formation. Some of © The Textile Institute 2 Textile Progress doi:10.1533/tepr.2005.0001 Raw Cotton Fibre Preparation Yarn Formation Spinning Warping Sizing Knitting Fabric Formation Weaving Preparation Printing Dyeing Wet Processing Finishing Cutting Fabrication Finished Goods Fig. 1 Raw Cotton Sewing General steps in manufacturing cotton textile goods. Yarn Cleaning Blending Carding Combing Drawing Warping Sizing Weaving Knitting (Weft or Warp) Drafting Spinning Fabric Fig. 2 General steps in yarn and fabric formation. © The Textile Institute doi:10.1533/tepr.2005.0001 Critical Solutions in the Dyeing of Cotton 3 these steps may be optional, depending on the type of yarn and spinning equipment used. The major methods for fabric manufacture are weaving and knitting, although recently nonwoven constructions have become more popular. Before weaving, warp yarns are first wound on large spools, or cones, which are placed on a rack called a creel. From the creel, warp yarns are wound on a beam wherefrom they are passed through a process known as sizing or slashing. The size solution forms a coating that protects the yarns against snagging or abrasion during weaving. Fabrics are formed from weaving by interlacing one set of yarns with another set oriented crosswise. In the weaving operation, the lengthwise yarns that form the basic structure of the fabric are called the warp and the crosswise yarns are called the filling, also referred to as the weft [8, 9]. Knitted fabrics may be constructed by using hooked needles to interlock one or more sets of yarns through a set of loops. The loops may be either loosely or closely constructed, depending on the purpose of the fabric. Knitting is performed using either weft or warp knitting processes [10]. Woven and knitted fabrics cannot usually be processed into apparel and other finished goods until the fabrics have passed through several water-intensive wet processing stages. Wet processing enhances the appearance, durability and serviceability of fabrics by converting undyed and unfinished goods, known as grey or greige goods, into finished consumers’ goods. Various stages of wet processing, shown in Fig. 3, involve treating greige goods with chemical baths and often additional washing, rinsing and drying steps [11]. Some of these stages may be optional, depending on the style of fabric being manufactured or whether the material being wet-processed is a yarn, or a knitted or woven fabric. Some of the key steps in the treatment of cotton material include singeing, desizing, scouring, bleaching, mercerizing, as well as dyeing and finishing. Singeing Desizing Scouring Bleaching Mercerising Dyeing Printing Mechanical Finishing Chemical Finishing Finished Fabric Fig. 3 General steps in wet processing. © The Textile Institute 4 Textile Progress doi:10.1533/tepr.2005.0001 Singeing is a dry process that removes fibres protruding from yarns or fabrics. Desizing is a wet process that removes the sizing material applied to the warp yarns before weaving. Scouring is a cleaning process that removes impurities from fibres, yarns or cloth through washing, usually with alkaline solutions. Bleaching is a chemical process that decolourizes coloured impurities that are not removed by scouring and prepares the cloth for further finishing processes such as dyeing or printing. Mercerization is a chemical process to increase dyeability, lustre and appearance. Dyeing operations are used at various stages of production to add colour to textiles and increase product value. Dyeing can be performed using batch or continuous processes. Common methods of batch or exhaust dyeing include package, beam, beck, winch, jet and jig processing. Continuous dyeing processes typically consist of dye application, dye fixation with chemicals or heat, and washing. Dyeing processes may take place at any of several stages of the manufacturing process (fibres, yarn, piece-dyeing). Stock dyeing is used to dye fibres; yarn dyeing is used to dye yarn; and piece/fabric dyeing is done after the yarn has been constructed into fabric. Printing is a localized or patternised coloration of the fabrics. Fabrics are printed with colour and patterns using a variety of techniques and machine types. Finishing encompasses chemical or mechanical treatments performed on fibre, yarn or fabric to improve appearance, texture, or performance. 2. PROBLEMS ORIGINATING FROM COTTON FIBRE 2.1 Problems Caused by Immature and/or Dead Cotton Although it a common practice to use the terms ‘dead’ and ‘immature’ interchangeably, it is useful to use these terms to indicate two different levels of maturity in cotton fibres. The normal mature cotton fibre is bean-shaped in cross-section and has a thick cell-wall. The other extreme, dead cotton, has virtually no cell-wall thickness. The intermediate range between mature and dead is classified as immature. The immature (sometimes called thin-walled) fibre does have some secondary wall thickening. The thinner wall of the immature fibre lacks the rigidity of mature cotton. This increased flexibility of immature or dead fibres makes them prone to be mechanically knotted into a clump during ginning, lint cleaning and carding. These neps or clusters of fibres may resist dye and appear as white specks in the dyed material [12–16]. The distinction between dead and immature fibres is very important. Both dye lighter than fully mature fibres but only immature fibres respond to mercerization or any other swelling treatment. In contrast, dead fibres lack the ability to accept some dye even if pre-treated with a swelling agent. The white or light-coloured specks caused by immature/dead fibres may be of one of the following three types. The first type of the defect occurs when a surface knot of entangled immature fibres is flattened during processing and takes on a glazed, shiny appearance. The knot then becomes a small, reflective mirror on the surface of the dyed material. Its greater reflectance makes the knot appear lighter at some viewing angles than the surrounding area although it has actually been dyed to the same depth. The second type occurs when the fabric is poorly penetrated during dyeing. Since the clumps of immature fibres are often loosely attached to the material, they can be moved or knocked loose during subsequent processes. If the clump, or © The Textile Institute doi:10.1533/tepr.2005.0001 Critical Solutions in the Dyeing of Cotton 5 the yarn behind it, is not properly penetrated during dyeing, a light spot will be seen when the clump changes its position. The third type is the classic case of the clump of immature or dead fibres not dyeing to the same depth as the surrounding material. The coverage of immature cotton depends upon the following factors: Fibre preparation: There are several stages in the fibre preparation where an attempt can be made to decrease the amount of neps of the immature and/or dead fibres that are usually clumped together [17]. It is important to try to remove these clumps prior to the carding process. Once past the main cylinder of the card, the clumped fibres go into the subsequently formed yarn and the fabric. Preparation sequence: The preparation sequence has little, if any, impact on the coverage of immature cotton. Only pre-treatments that swell the cell wall, giving it greater thickness, are effective in improving the dyeability of immature cotton. Swelling pre-treatment: Treatment with swelling agents at optimum concentration (e.g. caustic soda with a 14% or greater concentration) is effective in swelling the secondary wall of immature cotton, and improving its dyeing affinity. On the other hand, dead cotton lacks the necessary cell-wall thickness to be effectively treated by any type of swelling pre-treatment system. Dye selection: Dyes vary widely in their ability to effectively eliminate the white or off-shade specks. It is recommended that dye suppliers be consulted for data on the immature cotton coverage capabilities of specific dyes. Since caustic pre-treatment is ineffective in eliminating white or off-shade specks caused by dead cotton, dye selection is the best alternative in this case. Although the exact mechanisms are unknown, one theory is that dyes that cover dead cotton are those which do not penetrate into the cellulose of the fibre (the core) but are deposited mainly in the outside layer. This gives the dead fibre a ‘coloured’ skin. After-treatments: Swelling treatments such as mercerization or ammonia treatment may be effective after dyeing, as well as before, if the problem is the presence of reflective surfaces and not a genuine difference in dye uptake by the immature cotton. However, such a procedure is justified only in extreme cases, as there is an inevitable change of shade even when the fabric is dyed with dyes that are resistant to strong alkalis. 2.2 Problems Caused by Dyeability Variation in Cotton The results of research [18] confirm the dyeability variations in cotton obtained from different sources. It has been suggested that the substrate should be obtained from a single source, wherever possible, in order to keep the dyeability variations to a minimum. Since some dyestuffs are more sensitive to dyeability variations than others; those dyes should be selected for dyeing which are less sensitive to dyeability variation. 2.3 Problems Caused by Contaminants in Cotton While cotton fibre may be as much as 96 % cellulose, there are other components present which must be removed in preparation for a successful dyeing. Table 1 gives a summary of naturally occurring impurities in cotton [19]. The level of contamination in cotton is affected by: geology of cultivation area; soil constitution; weather conditions during the maturing period; cultivation techniques; © The Textile Institute ... - tailieumienphi.vn