DIRECT DYES AND THEIR RELATED PROPERTIES

Direct dyes are aromatic azo compounds which are solubilized by the in troduction of sulphonic acid groups into the aromatic nuclei and are u sed in the form of sodium salts. Azo dyes have always been the major c hemical class of dyes with which relatively long and linear molecular structures can be built up by mean of successive diazotization and cou pling sequences. Good wet-fastness properties of direct dyes on cotton are generally believed to be due to the unique linear structure which allows them to aggregate within the cellulose structure once they hav e penetrated the fibre. The dyeing mechanism of cellulose fibre with d irect dyes depends primarily on the pore structure of die fibres and d ye properties such as diffusion and substantivity. Different auxiliari es are used to assist in dyeing, by wetting and levelling and their in teraction with dyes plays a very important role in achieving level dye ing. The mechanism of interaction depends on the chemical composition of the auxiliary. Levelling agents control the rates of dye sorption, thereby producing uniformity in the supply of dye to the fibre surface . The role played by various electrolytes in promoting dye bath exhaus tion has been reported in relation to dye structural features. The int eraction between the surfactant and direct dyes and the effect of alka li-metal ions and many other cations on the adsorption of direct dyes has also been reported.