Critical Solutions in the Dyeing of Cotton Textile Materials

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  • Abstract: Over the decades there have been several papers on the coloration ofcotton-based textiles. The number of articles dealing with the processing of cotton,including preparation, dyeing, and finishing, may be in the thousands. Aninvestigation of the possible causes of problems occurring in the coloration oftextiles revealed that a comprehensive review of case studies and scientificanalysis would be a welcome addition to the already rich pool of knowledge inthis area.

    Key words: Cotton, troubleshooting, pretreatment, dyeing, dyes, colorants.

    1. INTRODUCTIONCotton is the backbone of the worlds textile trade [1]. It has many qualities [2] andcountless end uses [3], which make it one of the most abundantly used textile fibresin the world [4]. It is a seed hair of plant of genus Gossypium [5], the purest form ofcellulose found in nature. However, cotton is one of the most problematic fibres as faras its general wet processing or dyeing is concerned. Quite frequently, the problemsin dyed cotton materials are not due to the actual dyeing process but due to somelatent defects introduced from previous production and processing stages. Often, theroot-cause(s) of a problem in the dyed material can be traced as far back as to thecotton field. This monograph will address problems in the dyeing of cotton textilematerials in various forms. An overview of various textile operations for cotton willbe given in the beginning. Then, various key stages and factors involved in theproduction of dyed cotton textile materials will be described in detail and problemsoriginating at each stage will be summarised.

    1.1 Overview of Textile Operations for CottonThe textile industry is comprised of a diverse, fragmented group of establishmentsthat receive and prepare fibres, transform fibres into yarn, convert the yarn into fabricor 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 includeyarn formation, fabric formation, wet processing and textile fabrication [6]. Textilefibres are converted into yarn by grouping and twisting operations used to bind themtogether [7]. Although most textile fibres are processed using spinning operations,the processes leading to spinning vary depending on whether the fibres are natural ormanmade. Figure 2 shows the different steps used in cotton yarn formation. Some of

    CRITICAL SOLUTIONS IN THE DYEINGOF COTTON TEXTILE MATERIALS

    R. Shamey and T. Husseindoi:10.1533/tepr.2005.0001

    The Textile Institute

  • The Textile Institute

    2 Textile Progress doi:10.1533/tepr.2005.0001

    Fig. 1 General steps in manufacturing cotton textile goods.

    YarnFormation

    FabricFormation

    WetProcessing

    Fabrication

    Warping

    Sizing

    Weaving

    Printing

    Finished Goods Sewing

    Cutting

    Finishing

    Dyeing

    Preparation

    Knitting

    Spinning

    Fibre Preparation

    Raw Cotton

    Fig. 2 General steps in yarn and fabric formation.

    Raw Cotton

    Cleaning

    Blending

    Carding

    Combing

    Drawing

    Drafting

    Spinning

    Yarn

    Knitting(Weft or Warp)

    Warping

    Sizing

    Weaving

    Fabric

  • doi:10.1533/tepr.2005.0001 Critical Solutions in the Dyeing of Cotton 3

    The Textile Institute

    these steps may be optional, depending on the type of yarn and spinning equipmentused.

    The major methods for fabric manufacture are weaving and knitting, althoughrecently nonwoven constructions have become more popular. Before weaving, warpyarns are first wound on large spools, or cones, which are placed on a rack called acreel. From the creel, warp yarns are wound on a beam wherefrom they are passedthrough a process known as sizing or slashing. The size solution forms a coating thatprotects the yarns against snagging or abrasion during weaving. Fabrics are formedfrom weaving by interlacing one set of yarns with another set oriented crosswise. Inthe weaving operation, the lengthwise yarns that form the basic structure of the fabricare called the warp and the crosswise yarns are called the filling, also referred to asthe weft [8, 9]. Knitted fabrics may be constructed by using hooked needles tointerlock one or more sets of yarns through a set of loops. The loops may be eitherloosely or closely constructed, depending on the purpose of the fabric. Knitting isperformed using either weft or warp knitting processes [10].

    Woven and knitted fabrics cannot usually be processed into apparel and otherfinished goods until the fabrics have passed through several water-intensive wetprocessing stages. Wet processing enhances the appearance, durability and serviceabilityof fabrics by converting undyed and unfinished goods, known as grey or greigegoods, into finished consumers goods. Various stages of wet processing, shown inFig. 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 onthe style of fabric being manufactured or whether the material being wet-processedis 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.

    Fig. 3 General steps in wet processing.

    FinishedFabric

    MechanicalFinishing

    ChemicalFinishing

    PrintingDyeing

    Mercerising

    Bleaching

    Scouring

    Desizing

    Singeing

  • 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 yarnsbefore weaving. Scouring is a cleaning process that removes impurities from fibres,yarns or cloth through washing, usually with alkaline solutions. Bleaching is a chemicalprocess that decolourizes coloured impurities that are not removed by scouring andprepares 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 textilesand increase product value. Dyeing can be performed using batch or continuousprocesses. Common methods of batch or exhaust dyeing include package, beam,beck, winch, jet and jig processing. Continuous dyeing processes typically consist ofdye application, dye fixation with chemicals or heat, and washing. Dyeing processesmay 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. Printingis a localized or patternised coloration of the fabrics. Fabrics are printed with colourand patterns using a variety of techniques and machine types. Finishing encompasseschemical or mechanical treatments performed on fibre, yarn or fabric to improveappearance, texture, or performance.

    2. PROBLEMS ORIGINATING FROM COTTON FIBRE

    2.1 Problems Caused by Immature and/or Dead CottonAlthough 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 cottonfibres. The normal mature cotton fibre is bean-shaped in cross-section and has a thickcell-wall. The other extreme, dead cotton, has virtually no cell-wall thickness. Theintermediate range between mature and dead is classified as immature. The immature(sometimes called thin-walled) fibre does have some secondary wall thickening. Thethinner wall of the immature fibre lacks the rigidity of mature cotton. This increasedflexibility of immature or dead fibres makes them prone to be mechanically knottedinto a clump during ginning, lint cleaning and carding. These neps or clusters offibres may resist dye and appear as white specks in the dyed material [1216].

    The distinction between dead and immature fibres is very important. Both dyelighter than fully mature fibres but only immature fibres respond to mercerization orany other swelling treatment. In contrast, dead fibres lack the ability to accept somedye even if pre-treated with a swelling agent.

    The white or light-coloured specks caused by immature/dead fibres may be of oneof the following three types. The first type of the defect occurs when a surface knotof 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 ofthe dyed material. Its greater reflectance makes the knot appear lighter at someviewing angles than the surrounding area although it has actually been dyed to thesame depth. The second type occurs when the fabric is poorly penetrated duringdyeing. 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

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    The Textile Institute

    the yarn behind it, is not properly penetrated during dyeing, a light spot will be seenwhen the clump changes its position. The third type is the classic case of the clumpof 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 anattempt can be made to decrease the amount of neps of the immature and/or deadfibres that are usually clumped together [