Dyeing Principles
This domain tests the physical chemistry of dye uptake: why a dye goes onto a fibre, how it is fixed and how fastness is judged.
Core rules
- Affinity is the attraction between dye and fibre. Substantivity is the dye's ability to be absorbed. Exhaustion is the proportion of dye taken up from the bath.
- Fixation locks the dye in or on the fibre. Reactive dyes form covalent bonds; vat dyes are oxidised inside the fibre; direct dyes rely on hydrogen bonding and van der Waals forces.
- Control pH, temperature, electrolyte concentration, liquor ratio and time. Salt promotes exhaustion of direct and reactive dyes on cotton by reducing electrostatic repulsion. Alkali fixes reactive dyes.
- Levelness means even colour. Slow heating and good circulation prevent patchiness.
- Fastness includes wash, light, rub and perspiration fastness. Rinse and soaping remove unfixed dye.
Worked example
Cotton is dyed with a direct dye at 2 percent on weight of fibre. Reasoning: direct dye has affinity for cellulose but needs help to exhaust. Use a liquor ratio near 1:20, add salt at about 20 g per litre, raise the temperature slowly to 90 C, hold for 45 minutes, then rinse. Rapid heating would give an uneven result.
Traps that cost marks
- Dyeing without scouring, so wax and pectin block uptake.
- Heating too fast and producing patchy colour.
- Using the wrong pH for the dye class.
- Skipping soaping and rinsing, which leaves poor wash fastness.