Reduction Clearing and Aftertreatment
topic
After HT/HP dyeing, surface-adsorbed (unfixed) disperse dye is removed by reduction clearing: sodium hydrosulphite (2 g/L) + NaOH (2 g/L) at 70–80°C for 20 min, reduces and solubilises surface dye. Critical for deep shades (high surface dye load) and for improving wash fastness (ISO 105-C06). Without RC, dye migrates onto adjacent fibres in washing (staining). RC bath requires neutralisation and treatment before discharge.
Role
Reduction clearing is an essential step in polyester dyeing that distinguishes dyeing quality and wash fastness. Its chemistry — reductive decomposition of azo and anthraquinone disperse dyes — is important in textile chemistry and effluent treatment curricula.