Fundamentals of Dyeing and Colour Theory
Dyeing is the transfer of colour from a bath into a fibre and its retention there; colour theory explains how that colour is described, matched and judged. The exam tests the vocabulary of substantivity, affinity, exhaustion and fixation, the variables that decide levelness, and the basics of colour measurement.
Movement of dye into fibre
- Substantivity - practical attraction of a dye for a given fibre in the dyebath.
- Affinity - the thermodynamic driving force; substantivity is its working measure.
- Exhaustion % = dye absorbed ÷ dye originally in bath × 100.
- Fixation - dye actually locked in (covalent bond for reactive, trapped pigment for vat, ionic site for acid).
- Migration - dye moving from heavily dyed to lightly dyed areas; slow strike plus good migration gives level shades.
Colour language
- Hue = colour name; value = lightness; chroma = saturation.
- Subtractive primaries in dyeing are cyan, magenta and yellow. Two primaries make a secondary; adding the complementary colour dulls and darkens.
- Metamerism - a pair matching under one light and mismatching under another. Judge shades under a standard daylight cabinet and confirm with a spectrophotometer, reporting difference as ΔE.
Worked example
A 5 kg cotton lot is dyed at 2% shade owf with material-to-liquor ratio 1:10. Dye required = 0.02 × 5000 g = 100 g. Liquor = 50 L, so bath strength = 2 g/L. If the same lot runs at 1:20, dye stays 100 g but liquor becomes 100 L, i.e. 1 g/L. Strike is slower and levelness improves, but exhaustion falls unless electrolyte (salt) is raised. Reasoning: shade percentage is on weight of fibre, never on bath volume.
Traps that lose marks
- Writing g/L when the question states % owf.
- Claiming complementary colours brighten a shade - they grey it.
- Treating exhaustion and fixation as the same number; reactive dye exhaustion is often 70% while fixation may be 55%, the rest being hydrolysed dye washed off.