Wet Processing (Dyeing, Printing and Finishing)
Preparation
Singeing, desizing, scouring, bleaching and mercerising prepare greige fabric. Each removes a specific impurity: singeing burns protruding fibres, desizing removes size, scouring removes waxes and oils, bleaching removes natural colour, mercerising improves lustre and dye uptake under tension with caustic soda.
Dyeing and printing
Dye classes are matched to fibres: reactive and vat for cotton, acid and reactive for wool and nylon, disperse for polyester, basic for acrylic. Understand exhaustion, fixation and fastness. Printing methods include direct, discharge and resist; screen and rotary printing dominate bulk production. Thickeners control paste rheology.
Finishing
Mechanical finishes (calendering, raising, sanforising) and chemical finishes (resin, softener, flame retardant, water repellent) change handle and performance. Sanforising controls shrinkage.
Traps
- Assuming reactive dyes are used on polyester; they are not.
- Confusing discharge (dye removed) with resist (dye prevented).
- Forgetting that mercerising requires tension.
Worked example
A 100 kg cotton batch requires 2% owf reactive dye. Dye mass = 0.02 × 100 = 2 kg. If exhaustion is 90% and fixation 70%, fixed dye = 2 × 0.9 × 0.7 = 1.26 kg, so about 37% is lost to wash-off - a useful check on shade reproducibility.