pH control for dyeing polyamide in a reused acid dyebath

The pH control that can be achieved in a reused dyebath was investigated. The study involved a phosphate buffer system and four pH sliding systems, including ammonium sulphate and three hydrolysable organic esters. Instead of discharging the dyebath after each dyeing cycle, the residual dyebath was analysed using a UV‐Vis spectrophotometer and reconstituted to the required concentration of dyes, auxiliaries and acid donors. The dyebath was reused for 10 cycles and colour reproducibility, levelness and fastness of the dyed samples were measured after each recycling. In comparison with ammonium sulphate and sodium dihydrogen phosphate, hydrolysable organic esters gave a stable and effective pH shifting in the dyebath reuse system. In addition, hydrolysable organic esters resulted in a very low amount of salt in residual dyebath. No deterioration in colour fastness of the dyed fabrics over 10 cycles of dyebath reuse was evident.