The wash-off of dyeings using interstitial water: Part 3. Disperse dyes on polyester

Poly(ethylene terephthalate), which had been dyed at 0.5, 1.0 and 2.0% omf depths of shade using three disperse dyes, was reduction cleared using a traditional, four-stage process that comprised two water rinses at 40 °C, treatment with aq Na2CO3/Na2S2O4 at 60 °C and one cold water rinse. A novel, two-stage wash-off method was also employed that consisted of treatment with damp nylon beads and surfactant at 70 °C and one cold water rinse. In terms of fastness to repeated washing at 60 °C and colorimetric characteristics, the traditional, four-stage reduction clearing treatment using aq., alkaline Na2S2O4 could be replaced by the two-stage, bead wash-off with detergent at 70 °C. As the detergent-based, bead wash-off process used lower amounts of water than reduction clearing and did not employ sodium dithionite, it avoided the environmentally unacceptable generation of aromatic amines in the case of the reduction clearing of azo dyes. Calculations indicated that considerably less heat energy was consumed in bead wash-off than reduction clearing not only because two, rather than four stages were involved but also since the bead process used only a 2:1 water:fibre ratio rather than the 20:1 liquor ratio employed in the reduction clear process; also, the much lower specific heat capacity of nylon than water meant that much less heat was required to heat the beads. The beads adsorbed vagrant disperse dye during wash-off, thereby offering the potential of a lower effluent load compared to a traditional reduction clearing treatment for disperse dyes on polyester.

► A novel two-stage wash-off method using treatment with damp nylon beads at 70 °C. ► The wash-off gives similar wash fastness to that using a four-stage reduction clearing process at 98 °C. ► The beads adsorb a large amount of vagrant dye removed during wash-off. ► The bead wash-off process consumes less energy than the reduction clearing process.