Cotton, like all natural fibres, has some natural colouring matter, which confers a yellowish brown colour to the fibre. The purpose of bleaching is to remove this colouring material and to confer a white appearance to the fibre. In addition to an increase in whiteness, bleaching results in an increase in absorbency, levelness of pre treatment, and complete removal of seed husks and trash. In the case of the production of full white finished materials, the degree of whiteness is the main requirement of bleaching. The amount of residual soil is also taken into consideration because of the possibility of later yellowing of the material. In the case of pretreatment for dyeing, the degree of whiteness is not as important as, for example, the cleanliness of the material, especially the metal content. Similar demands refer to the production of medical articles. In this case, too, the metal content as well as the ash content are important factors. If whiteness is of primary importance, it requires a relatively large amount of bleaching agent as well as a high operating temperature and a long dwell time.
Accurate regulation of the bleaching bath is a further obligatory requirement. Where the destruction of trash, removal of seed husks and an increase in absorbency is a prime necessity (e.g. for dyed goods), a high degree of alkalinity is all important. It is, however, not the alkali alone that is responsible for these effects. The levelness of pretreatment can only be guaranteed if cotton of the same or equal origin is processed in each bath. If this is not the case, suitable pre-treatment will have to be undertaken to obtain, as closely as possible, the required uniformity. A pre-treatment with acid and/or a chelating agent will even out (better still eliminate) varying quantities of catalytic metallic compounds.
Although there are different bleaching agents that can be used for bleaching cotton, hydrogen peroxide is, by far, the most commonly used bleaching agent today. It is used to bleach at least 90% of all cotton and cotton blends, because of its advantages over other bleaching agents. The nature of the cotton colour, its mechanism of removal with hydrogen peroxide and the basic rules for formulation of bleaching liquors have been presented in detail elsewhere. The mere formulation of the correct initial bath concentration is not sufficient to ensure a controlled bleaching process. Of equal importance are regular checks of the bath composition during the operation. Such checks do not only contribute to an economic bleaching operation but also allow an early tracing of the defects and failures of the system. The important parameters for bleaching with hydrogen peroxide are as follows:
- Concentration of hydrogen peroxide.
- Concentration of alkali.
- Nature and quality of the goods.
- Water hardness and other impurities.
- Types and concentration of auxiliaries.
- Desired bleaching effect.
- Available equipment and stabilizer system employed.