Textile Chemical Processing: Singeing, Desizing, Scouring, Bleaching, Mercerising, Dyeing and Printing
What the exam tests
The chemical processing domain follows grey fabric through preparation, colouration and finishing. Marks come from knowing why each stage exists, the chemicals and conditions used, and the fastness behaviour of dye classes.
Preparation
- Singeing burns off protruding fibres, giving a smooth, lustrous surface and less pilling. It is done before desizing so that size films are not heat-set.
- Desizing removes starch size, usually with amylase enzyme, or by oxidative or acid routes.
- Scouring boils the fabric in 2–4% sodium hydroxide with detergent, removing wax, pectin and oil; 4–8% weight loss and a good drop-absorption result show correct scouring.
- Bleaching uses hydrogen peroxide in alkaline liquor with a silicate stabiliser at 80–95 °C, destroying natural colour without tendering cellulose.
- Mercerising uses cold 18–25% sodium hydroxide under tension: fibres swell into cylinders, giving lustre, about 20% higher strength, better dye uptake and reduced shrinkage.
Dyeing and printing
Reactive dyes form covalent bonds with cellulose, so wash fastness is excellent provided unfixed dye is removed by soaping. Vat and azoic dyes also suit cotton; acid dyes need amino groups found in wool, silk and nylon; disperse dyes suit polyester; basic dyes suit acrylic. Printing applies dye with a thickener by block, flat or rotary screen, roller or digital inkjet, followed by steaming and washing.
Worked example
A customer reports poor wash fastness on a reactive-dyed cotton shirt. Diagnosis: hydrolysed unfixed dye was left in the fabric because soaping was skipped - not a wrong dye class. Remedy: hot soaping at the dyehouse and a fastness retest.
Traps
Reversing the order of singeing and desizing; saying bleaching strengthens fabric; forgetting tension during mercerising; ignoring the afterwash.