Bleaching is an important and essential step in pretreatment of Textiles. It helps 'whiten' the textile material by removing undesired inherent coloring components. Generally, the natural fibres and their blends comprise about 50 % of total textile substrates and are invariably bleached, while the regenerated and synthetic goods are given this treatment considering the specific requirement. Depending on the substrate, a number of chemicals and application methods are employed to achieve optimum bleaching effect, however, it is also imperative to remove and get rid of the excess and leftover bleaching agent from the textile goods before subsequent wet processing. In this article an attempt is made to review various bleaching chemicals used in textile industry and the methods of their removal and cleaning.
Inherent colorants in textile
Natural vegetable fibers like cotton, linen, jute, etc. contain varying extent of pigments like Chlorophyll, Xanthophyll and Carotene. This imparts off-white, yellowish brown hue and depends on the area of cultivation, climatic conditions and varies from crop to crop. Similarly, the animal fibers like wool and silk too contain non protein impurities which impart characteristic color.
Need of Bleaching
Bleaching is the process of decolorization of raw textile material by removing inherent and or acquired coloring components from the fiber. It provides base whiteness to the textile material which could be further whitened with the help of optical brighteners or dyed | printed depending on the desired end use. Even in case of regenerated and synthetic fibers, bleaching step is incorporated for achieving full white or extra bright shades. Though, in case of material to be dyed with dark and dull shades bleaching step could be omitted.
Bleaching can be carried out at various stages of conversion of fiber to garment i.e. from fiber, yarn, hank, woven, knit, towel, sewn up garments, etc. on various types of machines by simple hand processing to sophisticated bleaching ranges and by different application processes from exhaust to continuous.
Though bleaching is an age old process, the major commercial chemicals were developed in the last few centuries. The modern bleaches resulted from the work of 18th century scientists Claude Berthollet, who developed bleaching with sodium hypochlorite, later Louis Thnard produced hydrogen peroxide which gained wide commercial acceptance.
Basically, two type of chemical processes are employed - oxidative and reductive, and are employed depending on the substrate to be bleached and the nature of inherent coloring impurities.