Printing and Dyeing
This domain tests dye-to-fibre matching, print methods and print styles, plus the arithmetic of a dye recipe. Examiners look for the correct dye class before anything else.
Dye-fibre pairing
- Cellulosic (cotton, viscose): reactive, vat (indigo), direct, sulphur.
- Protein (wool, silk) and nylon: acid, metal-complex.
- Polyester: disperse, applied at high temperature (HTHP) or with a carrier.
- Acrylic: basic/cationic.
- Pigment: no affinity for any fibre; it is fixed by a binder film and cured by dry heat.
Print methods and styles
- Methods: block, flat screen, rotary screen, roller, transfer (sublimation for polyester) and digital inkjet.
- Styles: direct, discharge (a dyed ground is stripped to white or a coloured reserve), resist (batik wax, tie-dye, bandhani) and burnt-out.
- Fixation: saturated steam at 100-102 C for reactive; dry-heat cure 150-180 C for pigment. Washing-off removes unfixed dye and is what makes fastness real.
Worked example
A 3% shade reactive black is required on 50 kg cotton at a liquor ratio of 1:10. Find dye and salt quantities, with salt at 60 g/L. Dye = 3% of 50 kg = 1.5 kg. Bath volume = 50 kg x 10 = 500 L. Salt = 60 g/L x 500 L = 30,000 g = 30 kg. Reasoning: the shade percentage is on the weight of goods, but auxiliaries quoted in g/L are on the bath volume, which comes from the liquor ratio.
Traps that cost marks
- Specifying reactive dye for polyester, or disperse dye for cotton.
- Confusing discharge (destroys existing colour) with resist (prevents dye uptake).
- Ignoring the liquor ratio when scaling auxiliaries.
- Judging fastness before washing-off.
- Assuming digital printing needs no fixation or after-treatment.