Improvement of wet rub fastness in continuous dyeing with c.i. Sulphur Black 1 by ultrasonic treatment

C.I. Sulphur Black 1 is among the most important dyes for cellulose textiles, as this dye offers a maximum colour depth at reasonable costs. In its final form the dye is present as solid pigment, thus limited wet rub fastness can be an issue. Controlled re-oxidation of the leuco form into the oxidised dye pigment and intensive washing and soaping are measures to achieve acceptable wet rub fastness. In this study pad-steam dyeing with C.I. Sulphur Black 1 was used to investigate the effects of an intensified washing process with use of ultrasonification on rub fastness. Ultrasound was applied following to steam fixation of the dye. Introduction of ultrasound into the following process steps was investigated: water seal, warm rinse, oxidation, soaping, cold rinse. Best results in rub fastness were obtained by combination of soaping and ultrasound processing. The high shear rate near to the fibre surface led to a reduction of the thickness of the stagnant diffusion layer and more intense removal of loosely bound dispersed dye. As a result of the treatment an improvement in wet rub fastness of the dyeings was achieved. Duration of 60 – 120 s was found sufficient to remove the major part of surface bound pigments which allows an integration into continuous pad-steam dyeing processing. The results demonstrate that ultrasound washing may lead to improved rub fastness of other dyeings where a pigment is the final form of the dye. The method thus could be extended to vat dyes including indigo, or naphthol dyes.