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Introduction
Discharge printing is a technique in which a printed pattern, either white or coloured, is obtained on a dyed fabric by applying a discharging agent. A coloured discharge can be achieved by using a discharge-resist colourant in the discharge paste (Panda, 2013). Different chemical agents have been used for the discharging of dyes on textile substrates. They can, broadly, be classified as reducing and oxidizing agents. Commonly used oxidizing agents are potassium permanganate (KMnO4 ), sodium hypochlorite, hydrogen peroxide (H2 O2 ) and sodium perborate/percarbonate. Similarly, reducing agents used for this purpose include formaldehyde sulphoxylate, thio-urea dioxide, sodium dithionite and tin chloride, which have now been abandoned due to environmental issues (Haggag et al. , 2013). Some enzymes have also been used successfully for discharging of different dyes (Ragheb et al. , 2013; Abd El-Thalouth et al. , 2008). The most important enzymes used for this purpose include laccase and glucose oxidase (Sharma, 2005; Soares et al. , 2006; Rafat et al. , 2013). The use of a discharging agent depends on the type of fibre, nature of dye, shade depth and construction properties of fabric (Haggag et al. , 2013).
Most of the denim fabrics comprise indigo-dyed warp yarns which are woven in a twill structure (Aspland, 1977). The discharge printing on denim fabrics can thus be performed using discharging agents that are active against indigo dyes. Oxidative discharge printing of indigo-dyed denim has been carried out using a combination of sodium ferro cyanide and sodium chlorate (Ying-Zhou, 2006). Similarly, reductive discharge printing of indigo denim using sodium formaldehyde sulfoxylate as a discharging agent has also been studied and used commercially. But, in the case of reductive discharging agents, the reduced colourless component might get oxidized again when exposed to air and light (Clark, 2011); some of these reducing agents are toxic in nature (Haggag et al. , 2013). Some enzymes like laccases derived from Trametes hirsute and Sclerotium rolfsii (fungi), can discharge indigo dyes on fabric through oxidation mechanism (Campos et al. , 2001). A hybrid process of oxidation of indigo has also been reported using KMnO4 , H2 O2 and ultraviolet (UV) light (Olya and Tarahomi, 2011). Therefore, the process of indigo discharge printing...