Scouring, is a chemical washing process carried out on cotton fabric to remove natural wax and non-fibrous impurities e. Scouring is usually carried in iron vessels called kiers. The fabric is boiled in an alkali , which forms a soap with free fatty acids saponification. A kier is usually enclosed, so the solution of sodium hydroxide can be boiled under pressure, excluding oxygen which would degrade the cellulose in the fibre.
If the appropriate reagents are used, scouring will also remove size from the fabric although desizing often precedes scouring and is considered to be a separate process known as fabric preparation. Preparation and scouring are prerequisites to most of the other finishing processes. At this stage even the most naturally white cotton fibres are yellowish, and bleaching, the next process, is required. Bleaching improves whiteness by removing natural coloration and remaining trace impurities from the cotton; the degree of bleaching necessary is determined by the required whiteness and absorbency.
Cotton being a vegetable fibre will be bleached using an oxidizing agent , such as dilute sodium hypochlorite or dilute hydrogen peroxide. If the fabric is to be dyed a deep shade, then lower levels of bleaching are acceptable, for example. However, for white bed sheetings and medical applications, the highest levels of whiteness and absorbency are essential.
A further possibility is mercerizing, during which the fabric is treated with caustic soda solution to cause swelling of the fibres.
This results in improved lustre, strength and dye affinity. Cotton is mercerized under tension, and all alkali must be washed out before the tension is released or shrinkage will take place. Mercerizing can take place directly on grey cloth, or after bleaching. Color is a sensation caused when white light from a source such as the sun is reflected off a pigment on the surface.
The pigment selectively reflects certain wavelengths of light while absorbing others. A dye can be considered as a substance that can be fixed to a material that has these properties. The colour it reflects is defined by the structure of the molecule, and particular the parts of the chromogen molecule called the chromophore group. Cotton is an absorbent fibre which responds readily to colouration processes. Dyeing is commonly carried out with an anionic direct dye by completely immersing the fabric or yarn in an aqueous dyebath according to a prescribed procedure.
For improved fastness to washing, rubbing and light, other dyes such as vats and reactives are commonly used. These require more complex chemistry during processing and are thus more expensive to apply. Printing is the application of colour in the form of a paste or ink to the surface of a fabric, in a predetermined pattern.
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It may be considered as localised dyeing. Printing designs on to already dyed fabric is also possible. The common processes are block printing , roller printing and screen printing. Another finishing process is raising. During raising, the fabric surface is treated with sharp teeth to lift the surface fibres, thereby imparting hairiness, softness and warmth, as in flannelette. Calendering is the third important mechanical process, in which the fabric is passed between heated rollers to generate smooth, polished or embossed effects depending on roller surface properties and relative speeds.
Many other chemical treatments may be applied to cotton fabrics to produce low flammability, crease resist and other special effects. From Wikipedia, the free encyclopedia. Mittal, K. Open Share Save.
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