Dyeing of cotton with indigo using alkaline protease and additives

Abstract

Indigo is invariably applied on cotton to produce an attractive blue shade, together with the desired wash-down effects. Because it is water insoluble, indigo is reduced and solubilised with sodium dithionite and NaOH to develop affinity for cotton. Sodium dithionite dissociates into hazardous by-products viz. sulphate, sulphite and thiosulphate with a harmful effect on the environment due to their toxicity, as well as a corrosive effect on waste lines. To overcome these problems, the eco-friendliness of alkaline protease, together with iron (II) sulphate (FeSO4) as a reducing agent, was studied as a replacement for sodium dithionite. Dyed samples were characterised by attenuated total reflection, using Fourier transformation infrared spectroscopy, scanning electron microscope and X-ray diff raction (XRD). It was observed that alkaline protease, together with iron (II) sulphate, is also capable of producing a comparable reduction potential in dye baths, reduction bath stability, and the surface colour strength and colour fastness properties of dyed cotton compared to those obtained using sodium dithionite.