4.Chemical modification of cotton fibre prior to dyeing.
The chemical modification of cotton to improve dyeing with direct, reactive, sulphur or vat dyes, is another emerging area. This research has focused mainly on the introduction of cationic groups to the cotton fibre. Such modifications, usually referred to as cationisation, are achieved by treating cotton with low molecular weight cationic chemicals or with cationic polymers. Most of the cationic groups are quaternary, tertiary or secondary amino residues. In the reactive dyeing of cationised cotton, the anionic dyes are attracted by the cationic charges on the fibre. As a result, higher fixation efficiency and reduced use of salt can be achieved. Salt and alkali free reactive dyeing of modified cotton have also been reported.
In reactive dyeing, cationisation of cotton has been shown to be capable of eliminating the use of inorganic salt and alkali and leaving a reduced amount of unfixed dye. This offers significant environmental benefits. However, the technique has yet to be adopted by industry. That is mainly because cationisation is an additional process step and the treatment itself does not guarantee nontoxic effluent. Natural polymers such as cationic starch and chitosan have been explored as cationising agents. these may make the cationisation process environmentally safer.
Mercerisation of cotton fabric is a usual practice of continuous dye-houses. It is a pre-dyeing process that ensures the benefits of improved colour yield, tensile strength, lustre and dimensional stability. Mercerisation is an expensive process in terms of capital, maintenance, running and the caustic recovery costs. An experimental refinement into the cationisation of cotton fabric has the potential of replacing the mercerisation process, which can be carried out on a stenter without any capital and much of the running costs.