Physicochemical Analysis of Dyeing of Cotton Denim with Natural Indigo Dye from Strobilanthes cusia Using Green Reducing Agents and Alkalis

Abstract

Strobilanthes cusia is one of many indigo-producing plants with promising and attractive potentials as an alternative source of natural indigo dye due to its dyeability to cotton fibres with high colour strength, and due to its ease of cultivation. The aim of this research was to utilise natural indigo dye from Strobilanthes cusia to dye cotton denim fabric in an environmentally friendly way. The use of D-fructose and D-glucose as well as rice husk ash and lime solutions as reducing agents and alkali sources was explored in this study. The dyeing process was carried out at 30 °C for 30 minutes, followed by the oxidation process in open air for 45 minutes. Colour characteristics and fastness to washing and rubbing were evaluated. The results showed that D-fructose resulted in a higher colour strength than D-glucose, and that lime is better than rice husk ash and even sodium hydroxide in terms of colour strength. The highest colour strength was obtained from the combination of D-fructose and lime solution, with K/S and SUM(K/S) values of 16.02 (at 650 nm) and 8.5 respectively. The colour fastness to washing and rubbing was good for all dyed fabrics, with the staining scale ranging from 3/4 to 4. These results bring a new potential for natural indigo dye from Strobilanthes cusia beyond its current and limited use by traditional dyers to a much larger scale of denim dyeing for an eco-friendly fashion industry.