Dyeing of Wool with Indigo Dye: Effect of pH on Colorimetric Parameters, Tensile Properties and Felting Shrinkage

Wool fabric was dyed with indigo dye and the effect of dyeing conditions on color strength and mechanical properties was studied. The dyeing of wool with indigo dye in an alkaline condition causes hydrolysis of wool and thus a loss in strength and weight of the wool fabric. Therefore, dyeing process parameters were optimized, including pH, the concentration of sodium hydrosulphite and temperature. A broad range of pH from 6 to 11 provided a uniform shade with very good to excellent wash and rubbing fastness properties. The maximum color yield was obtained at pH 11. A temperature of 80 °C and sodium hydrosulphite concentration of 8 gpl were the optimum dyeing conditions for the maximum shade buildup. Both non-ionic and ionic forms of indigo dye have shown good substantivity for wool fibre. Dyeing at pH 6, at which the dye exists in leuco vat acid form, resulted in good color build-up and uniform shade with very good to excellent fastness properties. Additionally, due to acidic pH, there was no felting shrinkage and no loss in strength or weight after dyeing. Compared to pH 6, dyeing at pH 11 produced higher color depth. There was about 15% felting shrinkage, 17% strength loss (warp direction) and 7% weight loss. The dyeing temperature has significant effect on the mechanical properties of the wool fabric. At 80 °C and pH-9.5, the loss in strength was 13.94% which increased to 54.83% at 100 °C while weight loss increased from 3.2% at 80 °C to 18.4% at 100 °C with other dyeing parameters being constant.