Dyeing behaviour of hydrogenated indigo in electrochemically reduced dyebaths

The dyeing behaviour of hydrogenated indigo, in electrochemically reduced dyebaths, was studied on cotton yarn with particular focus on dark shades. Indigo concentration in the dyebath was studied from 0.9 g l⁻¹ indigo to 7 g l⁻¹ to achieve indigo fixation near the level of dyestuff saturation. The number of repetitive cycles of immersion into the dyebath and air oxidation was increased up to eight dips. The highest K/S value at 660 nm was measured with K/S 31 at samples dyed with eight dips in a dyebath containing 1.8 g l⁻¹ indigo and at a bath pH level of 11.7–12.1. When comparable amounts of indigo had been fixed on the yarn from baths containing 3.5 or 7 g l⁻¹ indigo, K/S values decreased; however, darkness in terms of L* value increased. Analysis of the reflectance curves between 400 and 700 nm proved that the visually perceived increase in darkness was caused by lowered reflectance in the wavelength range of 400–525 nm. This also explains the more reddish, duller shade of indigo dyeings obtained from baths that contain a relatively high indigo concentration. Comparison to reference experiments confirmed that cathodic dyebath reduction does not modify the dyeing behaviour of indigo.