Textile Finishing and Dyeing
Textile Finishing and Dyeing
This domain tests the order of wet processing, the purpose of each finish, and the match between dye class and fibre.
Preparation and routine finishes
Singeing burns projecting hairs; desizing removes size; scouring removes oils and waxes; bleaching removes natural colour; mercerizing uses cold concentrated sodium hydroxide under tension to increase lustre, strength, and dye uptake in cotton. Mechanical finishes include calendering, raising, and sanforizing, which controls shrinkage. Functional finishes include flame retardant, water repellent, and anti-microbial treatments.
Dyeing stages and dye classes
Dyeing may be done at fibre, yarn, piece, or garment stage. Reactive and vat dyes are used for cotton; acid dyes for wool, silk, and nylon; disperse dyes for polyester; basic dyes for acrylic.
Worked example
A cotton canvas bag must withstand repeated washing and rubbing. Choose a dye class and justify it. Reactive dyes form covalent bonds with cellulose, so they give good wash fastness on cotton; direct dyes are simpler but bleed more. Vat dyes are also fast, but reactive dye is the standard choice for the stated requirement. For polyester, the same reasoning would point to disperse dyes because the fibre is hydrophobic and needs high-temperature dyeing.
Traps that cost marks
Mercerizing is not a dye; it improves dye uptake and lustre. Do not dye polyester with reactive dyes. Do not place bleaching before scouring in the routine sequence. Sanforizing controls shrinkage, while calendering mainly presses and lustres the fabric.