A process for dyeing cotton with direct dyes possessing primary aromatic amino groups furnishing wash fastness exhibited by reactive dyes

Direct dyes constitute one of the major classes of azo dyes. These dyes due to their structural characteristics possess affinity for cellulose fibres when dyed in the presence of electrolytes. However, they bind with cellulose fibres through weak physical forces due to which cotton dyed with direct dyes bleeds when washed. In the present study, cotton fabric dyed by using direct dyes possessing primary aromatic amino groups followed by treatment with nitrous acid and soda ash was shown to exhibit a remarkable improvement in wash and rubbing fastness. With this process, any azo dye with primary aromatic amino groups possessing moderate substantivity for cellulose can be used to obtain dyed cotton exhibiting fastness properties comparable to that of reactive dyes. The dyeing process can, in principle, be made continuous thereby offering dyers the possibility of achieving good to excellent wash fastness by using inexpensive direct dyes.