Effective reduction clearing parameters involving alternative reducing agent

Abstract

Dyeing of polyester is done by using disperse dyes. Some of the disperse dyes remain unfixed during dyeing and create problems in shade and colorfastness properties. Reduction clearing (RC) is a process to remove these unfixed dyes and to enhance colorfastness properties. In the Pakistani textile industry, reduction clearing is being done by using sodium dithionite (Na₂S₂O₄), which has the best results in reducing dyestuff.  The problem with sodium dithionite is that it is sensitive to air and also creates a lot of environmental issues. Its sensitivity causes inconsistent results of reduction clearing. Different dyers used different processes and recipes of reduction clearing. Thiourea Dioxide (TUD) is a green reducing agent and can be used as a replacement for sodium dithionite. TUD (CH₄N₂S) is a stable product with a comparable result with sodium dithionite. Interlock knitted fabric is used in this project whose composition has recycled polyester. The fabric was first dyed using black dyestuff and then reduction clearing was done on a lab-scale with different factors (Shade depth, the concentration of TUD, RC temperature). Three colorfastness tests were conducted to evaluate the different levels of each factor i.e. colorfastness to washing, fastness to perspiration, and fastness to water according to standard numbers ISO 105-C06, ISO 105-E04, and ISO 105-E01, respectively. It was observed that shade depth does not affect the results while concentration and RC temperature have a significant effect on the results. Moreover, the effective concentration of TUD was also determined.