Textile Finishing and Dyeing
Preparation and finishing
The exam tests process sequence, purpose and fibre suitability. Preparatory wet processing for cotton usually follows desizing → scouring → bleaching → mercerising. Mercerising improves lustre, strength and dye uptake; it is not a shrink-proofing finish. Finishes are mechanical (calendering, raising, sanforising, embossing) or chemical (resin crease recovery, softeners, water repellents, flame retardants, antimicrobials).
Dyeing routes and dye classes
Dyeing can be done at fibre/stock, yarn, piece or garment stage. Match dye to fibre:
- Direct and reactive dyes for cotton; reactive forms covalent bonds and generally gives good wash fastness.
- Vat dyes for cotton, including indigo; require reduction and oxidation.
- Disperse dyes for polyester and other hydrophobic synthetics.
- Acid dyes for wool, silk and nylon; basic dyes for acrylic. Fastness is assessed for washing, rubbing, light and perspiration.
Worked example
A cotton bed sheet, a polyester curtain and a wool sweater need dyeing. Select reactive for cotton, disperse for polyester and acid for wool. Reasoning: the dye's chemistry must have affinity for the fibre; a reactive dye on polyester would not fix efficiently, and a disperse dye on wool would give poor colour yield.
Traps
Do not call mercerising a wash-and-wear finish. Do not confuse sanforising (controlled shrinkage) with calendering (surface lustre). Reactive dyes do not simply coat the fibre - they bond. Vat dyes are not soluble dyes applied directly; they need reduction.