Cotton dyeing by indigo with the borohydride process: Effect of some experimental conditions on indigo reduction and dyeing quality

Classical process for dyeing by indigo uses generally the sodium dithionite as reducing agent. In this paper, a new technique of reduction of indigo by sodium borohydride in the presence of a catalyst and without the addition of alkali was developed. This reduction was carried out at a temperature range of 40–70 °C. In order to determine the leuco-indigo concentration in the bath, a potentiometric titration procedure has been established. The effect of some main reaction parameters was studied: the temperature and the amount of catalyser. This study indicated that maximum yield of indigo reduction took place at 55 °C with an amount of 1% of catalyser.The dyeing study of a cotton fabric was carried out at the same temperature of the reduction reaction. A “6-dip–6-nip” technique was used to study the performance of this dyeing. The dyeing results were evaluated by measuring the colour yield (K/S) at 660 nm. The best results were obtained at a temperature of 40 °C and with an amount of 1% of catalyser.