Aftertreatments to improve the wash fastness of sulphur dyeings on cotton

The effectiveness of five cationic agents, when applied to the oxidised dyeings of four CI Solubilised Sulphur dyes, four CI Leuco Sulphur dyes and four CI Sulphur dyes, in improving the fastness of the dyeings to the ISOC06/C2 wash test was assessed. Each of the five cationic agents markedly improved the wash fastness properties of the twelve dyes used. Whilst aftertreatment caused a slight flattening of shade if had little effect on the colour strength (K/S) of the dyeings. As the enhanced wash fastness clearly indicates that the polycationic agents were adsorbed on to the dyed substrate, it is postulated that this adsorption may have arisen by virtue of electrostatic forces operating between the cationic agents and anionic carboxyl groups in the fibre and/or anionic groups in the dye molecules, alternatively intermolecular forces other than ion-ion may have contributed towards uptake of the polycations. The mechanism by which the improvement in wash fastness was effected may be the formation of a large molecular size, dye-cationic agent complex of reduced aqueous solubility or the presence of a peripheral 'layer' of polycation molecules having reduced the diffusion of dye out of the dyed, treated fibre during washing. Further work is required to clearly establish the mechanism by which the observed enhanced fastness is achieved.