Chemical Processing of Textiles (Dyeing, Printing and Finishing)
Questions here cover the preparatory sequence, correct dye class for each fibre, printing styles, and the standard finishes with their chemical effect. Numericals usually involve on-weight-of-fibre (owf) dye or chemical recipes and liquor ratio.
Preparation before colour
- Singeing burns off protruding fibre for a smooth surface; desizing removes starch size, usually with enzymes; scouring with alkali removes wax, pectin and oil, giving absorbency; bleaching with hydrogen peroxide in alkaline medium (with silicate stabiliser) whitens without severe damage.
- Mercerisation uses strong caustic soda under tension: lustre, strength, dye uptake and dimensional stability improve. Without tension the same treatment only causes caustic shrinkage.
Dye selection and printing
- Cotton: reactive (needs salt for exhaustion and alkali for fixation), vat, sulphur, azoic. Wool, silk, nylon: acid and reactive. Polyester: disperse, applied at high temperature under pressure or with a carrier. Acrylic: cationic (basic).
- Printing styles: direct, discharge (white or coloured on a dyed ground) and resist. Methods include flat screen, rotary screen, transfer and inkjet; thickener choice matters (alginate for reactive dyes); steaming fixes the print before washing.
Finishing
Mechanical: calendering, sanforising, raising, stentering. Chemical: resin/cross-linking for crease recovery, softeners, water repellents, flame retardants.
Worked example
Dye 50 kg cotton at 2% owf, liquor ratio 1:10, salt at 60 g/L. Dye = 0.02 × 50 = 1.0 kg. Water = 10 × 50 = 500 L. Salt = 60 g/L × 500 L = 30 kg. Reasoning: owf is calculated on fibre mass, g/L on liquor volume - they use different bases.
Traps
- Forgetting that mercerisation needs tension.
- Using disperse dyes on cotton or reactive dyes on polyester.
- Mixing owf and g/L additions.
- Omitting salt or alkali in a reactive dyeing recipe.