Textile Finishing and Dyeing
Textile Finishing and Dyeing
This domain covers preparatory finishing, dye selection by fibre type, and fastness. Exam questions often test the purpose of each wet process and the difference between a percentage shade and a g/L recipe.
Preparatory finishing
- Singeing burns off surface fibres; desizing removes sizing; scouring removes oils and waxes; bleaching removes natural colour.
- Mercerizing treats cotton with 18–25% sodium hydroxide under tension. It improves lustre, strength, dye uptake and dimensional stability. It is not a wool shrink-resist process.
Dyeing and fastness
Dye choice follows fibre: reactive, direct and vat dyes for cotton; acid dyes for wool and silk; disperse dyes for polyester. Reactive dyes form covalent bonds with cellulose and generally give good wash fastness. A 2% shade means 2 g dye per 100 g fabric. Fastness is tested for washing, rubbing and light.
Worked example
A dyer needs a 2% shade on 500 g cotton fabric. Dye required = 2% × 500 g = 10 g. If the recipe uses 10 g/L electrolyte at a liquor ratio of 1:20, water = 10 L, so electrolyte = 100 g. Reasoning: percentage shade is based on fabric mass, while g/L is based on dye-liquor volume.
Traps
Do not use acid dyes as the main class for cotton. Do not say mercerizing shrinks cotton-tension prevents shrinkage. Do not confuse desizing with scouring. Do not omit the soaping after-treatment for reactive dyes; it affects colour and fastness.
Key points
- Singeing, desizing, scouring and bleaching are preparatory processes.
- Mercerizing uses strong alkali under tension on cotton.
- Reactive dyes bond with cellulose; acid dyes suit wool and silk; disperse dyes suit polyester.
- Percentage shade is on fabric mass; g/L is on liquor volume.
- Wash, rub and light fastness are separate properties.