3.6 Aftertreatment to fix hydrolysed dye on the fibre.
A hydrolysed dye is not able to react with cotton and is removed through washing-off. A dye fixation efficiency of up to 99% is sometimes claimed for some reactive dyes with pad-batch dyeing processes. However, there is still a need for washing-off to remove any residual hydrolysed and unreacted dyes on the fibre. The use of certain aftertreatment agents to trap small amounts of hydrolysed dye in unfixed dye in the discharge and to improve colour yields. Cationic aftertreatment agents have been studied for immobilising the unfixed dye present on the fibre. Improvements in colourfastness to washing were found. However, such treatments may lead to slight reduction of colourfastness to light. Dye manufacturers market such cationic aftertreatment agents.