Green reducing agents for indigo dyeing on cotton fabrics

A reducing agent is used in indigo dyeing to convert an indigo pigment to a water-soluble leuco form prior to its application on cellulosic fibers. Sodium dithionite (Na2S2O4), the most widely used reducing agent for industrial indigo dyeing, is considered environmentally unfavorable because sulfite and sulfate generated in the dyebath can cause various problems when discharged into wastewater. In this work, monosaccharides (glucose, fructose and galactose) and reducing disaccharides (lactose and maltose) were investigated as green reducing agents in order to be a substitute for Na2S2O4 in the technical indigo dyeing. The redox potential measurement coupled with photometric studies revealed that reducing sugars could be applied in the alkaline reduction of indigo at an elevated temperature. Within 10 min of adding reducing sugars, the redox potentials of the indigo dyebaths were lower than −700 mV and remained nearly constant for an hour. The dyebaths with use of sodium hydroxide led to greater color strength compared to experiments with use of calcium hydroxide. Replacing Na2S2O4 with reducing sugars in the technical scale denim production was found to be possible in the preparation of the stock vat. This can lead to substantial reduction in Na2S2O4 consumption.