Scouring, is a chemical washing process carried out on cotton fabric to remove natural wax and non-fibrous impurities (eg the remains of seed fragments) from the fibres and any added soiling or dirt. 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 colouration and remaining trace impurities from the cotton; the degree of bleaching necessary is determined by the required whiteness and absorbency. 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 mercerising 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.
Many other chemical treatments may be applied to cotton fabrics to produce low flammability, crease resist and other special effects but four important non-chemical finishing treatments are:
First of all, singeing which is designed to burn off the surface fibres from the fabric to produce smoothness.
The second additional 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 rollers to generate smooth, polished or embossed effects depending on roller surface properties and relative speeds.
Finally, mechanical shrinking (sometimes referred to as sanforizing), whereby the fabric is forced to shrink width and/or lengthwise, creates a fabric in which any residual tendency to shrink after subsequent laundering is minimal.
Finally, cotton is an absorbent fibre which responds readily to colouration processes. Dyeing, for instance, 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, on the other hand, is the application of colour in the form of a paste or ink to the surface of a fabric, in a predetermined pattern. It may be considered as localised dyeing. Printing designs on to already dyed fabric is also possible.
It is evident, therefore, that there is no such material as a simple cotton textile. The choices available in raw cotton sources, the many and variable processes available at the spinning and fabric-forming stages coupled with the complexities of the finishing and colouration processes ensure that cotton textiles may be produced having wide ranges of characteristics. In turn, these enable cotton textiles to respond to continuing changes in consumer, fashion and market demands. It is the versatility that helps cotton command, still, the important position that it does within the world's textile markets.
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