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Dyeing process
Dyeing operations are used at various stages of production to add colour and intricacy to textiles and increase product value. Most dyeing is performed either by the finishing division of vertically integrated textile companies or by specialty dyehouses. The latter guarantee a cheaper product, a higher quality (mainly due to the fact that this is their only core-business and update their know-how more frequently and also solve more problems becoming more productive). Textiles are dyed using a wide rang of dyestuffs, techniques, and equipment. Dyes are sold as powders, granules, pastes, and liquid dispersions, with concentrations of active ingredients ranging typically from 20 to 80 percent (EPA Office of Compliance, 1997). Dyeing methodDyeing can be performed using continuous or batch processes. In batch dyeing, a certain amount of textile substrate, usually 100 to 5000 kilograms, is loaded into a dyeing machine and brought to equilibrium, or near equilibrium, with a solution containing the dye. Because the dyes have an affinity for the fibres, the dye molecules leave the dye solution and enter the fibres over a period of hours, depending on the type of dye and fabric used. Auxiliary chemicals and controlled dyebath conditions (mainly temperature) accelerate and optimize the action. The dye is fixed in the fibre using heat and/or chemicals, and the tinted textile substrate is washed to remove unfixed dyes and chemicals. Common methods of batch, or exhaust, dyeing include beam, beck, jet, and jig processing. Pad dyeing can be performed by either batch or continuous processes. Continuous dyeing processes typically consists of dye application, dye fixation with chemicals or heat, and washing. Dye fixation on the fibre occurs much more rapidly in continuous dyeing than in batch dyeing. 
Each dyeing process requires different amounts of dye per unit of fabric to be dyed. This is significant since colour and salts in wastewater from spent dyes are often a pollution concern for textile facilities. In addition, less dyes used results in energy conservation and chemical savings. The amount of dye used depends on the dyeclass, the affinity and quality of the yarn, the colourdepth and other factors (such as quality of the water, salt, auxiliaries…). The dyebath ratio is the ratio of amount of yarn vs. amount of water in the dyevessel. (Compliance, 1997). Principles of dyeing- Migration of dye molecule from liquor to fibre. This process is assisted by increasing temperatures and using auxiliaries – substances that help the dyeing process. - Diffusion of dye from the fibre surface into the fibre. This process is assisted by agitation of the fibre, dyebath or both together with heat. - Fixation ensures the dye molecule is attached to the fibre either by physical forces or chemical bonding. These forces may be weak or strong. - The unknown or less known variables. As until now no mathematical model exists by which with certainty the outcome of the process can be predicted and foreseen industrially. Most dyeing processes need heat to provide the energy for the dyeing to take place. This is commonly supplied by direct or indirect steam. Dyes used in the textile industry
Dyes for the textile dyeing can also be grouped according to the fibres to which they can be applied. The strength of the dye-fibre attraction is called “Affinity”. Each class of dye has a unique chemistry, structure and way of bonding to the fibre. Colour consents are now being enforced and therefore dye fixation levels are becoming more important. Selection of the right dyes helps to minimise effluent losses. The textile finishing industry has been put under immense pressure to reduce the usage of harmful substances. Especially mutagenic, carcinogenic, and allergenic effects of textile chemicals and textile dyes have been in the spotlight. 
The textile industry is comprised of a disperse, fragmented group of establishments that produce and/or process textile-related products (fibre, yarn, fabric) for further processing into apparel, home furnishings, and industrial goods. Textile establishments receive and prepare fibres; transform fibres into yarn, thread, or webbing; convert the yarn into fabric or related products; and dye and finish these materials at various stages of production. The textile chain is thus long and complex. Textile dyeing major bottlenecks
Spent dye baths (discontinuous dyeing), residual dye liquors and water from washing operations always contain a percentage of unfixed dye. The rates of fixation vary considerably among the different classes of dyes and may be especially low for reactive dyes (in the case of cotton) and for sulphur dyes. Moreover, large variations are round even within a given class of colorants. This is particularly significant in the case of reactive dyes. Fixing rates above 60% cannot be achieved, for example, in the case of copper (sometimes nickel) phthalocyanine reactive dyes especially used for turquoise-green and some marine shades. In contrast the so-called double anchor reactive dyes can achieve extremely high rates of fixation. The degree of fixation of an individual dye varies according to the type of fibre, shade and dyeing parameters. Therefore fixation rate values can be given only as approximations. However, they are useful to give an idea of the amount of unfixed dyes.
Dyes used in the leather industryLeather is an even difficult substrate to dye as textiles; it has structural differences, knurls, grooves and folds as well as other imperfections. To achieve the target of a level and uniform dyeing, the tanner therefore needs to be experienced and to have a thorough understanding of the dyeing properties of the dyes and auxiliaries used. The major reason for the different dyeing behaviour of dyes in leather is their varying affinity for the leather substrate and the variations between the dyes themselves (the same case happens in textiles when several subsequent different yarnlots have to get the same final colour). The behaviour of leather dyes is primarily determined by the charge of both the dye and the leather to be dyed. Differences in exhaustion rate or bath exhaustion and in the build up of the dye on specific leathers are the chief problems. In all these cases a balance has to be struck between the dyeing conditions and auxiliaries. The number of different types of dyestuff used by tanneries varies depending on their product range, and how much their products are influenced by the fashion world. Each tannery can use between 50 and in excess of 100 different types of dyestuffs. Even small amounts of dyestuff in the effluent stream can discolour the water exiting the effluent treatment plant (ETP). Many dyestuffs are hard to biodegrade (enhancing COD, BOD, SS) and in the case of some reactive dyes they could contribute to the aromatic organic compounds (AOX levels) in the wastewater effluents. Leather dyeing major bottlenecksThe main practical problems for the dyeing of leather are achieving full dye penetration. The second major problem would be achieving the desired shade. Frequently the batch of leather, which is being processed in the drum, has to be re-dyed to meet the desired shade, which is equired. The third biggest problem would be rub fastness (although this is not only a dye problem). Afterwards, comes the problem of light fastness and wet fastness of the dyed leather (this is related to the dye fixation). These last two problems are a lower priority because they are not obvious to the eye or to a sample rub test. Dyeing temperature is very important and is varied depending on the application that is being made, but more specifically if it is a penetration or surface dyeing method. A correctly applied dye will almost certainly exhaust from the bath and in many cases the float is virtually clear. The use of cationic fixation agents can improve the exhaustion and fixation. However, one tendency in practice with deeper and darker shades is to work well in the saturated region (i.e. use excessive dye) to avoid not reaching the correct depth of shade. Provided the typical leather dyes are applied correctly and in the correct amount then fixation percentages are similar, often in the 95-99% range. The exception is sulphur dyes and to some extent wood extract dyes which tend to have lower fixation percentages. Commercial dyes for leather are often mixtures of different molecules. Also when a product is namely homogeneous it is different from the homogeneous chemical concept. The dyes used for the leather industry are prepared with technical purity raw materials, with non quantitative reactions and with some by-product of reaction. Different commercial dyes with the same name and number from the colour index can give different results on the leather. The action of a colour on the leather is submitted, besides the pH, at the molecular sights of the component, their solubility in water, their aggregation capability, their affinity for the solid phase compared with that for the liquid phase, they diffusing speed, they reactivity with the functional group of the leather or the other molecules fixed on leather. Often to reach a particular colour on the leather it is necessary to mix more than two dyes and all the parameters above mentioned are to be added to their influence on the behaviour of each other. When a mix of colours is used the speed with the single component reach the reactive site and fix itself is decisive for the resulting colour of the leather. Often to overcome this problem large amount of dyes are used with large release in the waste waters. Another problem for the exhaustion of dyes is their slow affinity for leather treated with organic resins: the law affinity results in a request off large amount of dyes to use to dye leather with a large release. |
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