Optimization of Indigo Dyeing on Modal Fabrics Using Thiourea Dioxide as an Environmentally Friendly Reducing Agent

This investigation examines the potential of thiourea dioxide as an environmentally reducing agent for dyeing of natural indigo on modal fabrics. Although traditional sodium dithionite is effective, its sulfur by-products generate environmental concerns. Thiourea dioxide, which provides a more environmentally friendly alternative, was studied under various dyeing conditions. In this study, six dyeing parameters, including thiourea dioxide and sodium hydroxide concentrations, reduction temperature, reduction duration, dyeing time, and reduction-oxidation cycles, were optimized to enhance color strength. Results indicated that the most favorable reduction conditions were achieved when 80 g/L of thiourea dioxide and 2 g/L of sodium hydroxide were heated to 50°C for 30 min and at the dyeing duration of 30 min. Moreover, the repeated reduction and oxidation cycles continuously improve color strength of the dyed fabric. The indigo dyed fabrics had excellent wash fastness, good light fastness, and slightly lower rub fastness.