Fibres and Their Properties
Dyeing works only where fibre chemistry and fibre structure allow it. This domain is tested through questions on polymer type, amorphous versus crystalline regions, moisture regain, dye sites and the practical consequence - which dye class suits which fibre under which conditions.
Cotton (cellulose)
β-1,4-linked glucan, high degree of polymerisation, roughly two-thirds crystalline. Reactive sites are the hydroxyl groups; there are no ionic dye sites, so reactive, vat, sulphur, direct and azoic dyes are used with electrolyte and alkali. Moisture regain about 8.5%; fibre swells in water, opening the structure.
Wool and silk (proteins)
- Wool - keratin with disulphide cross-links, overlapping scales that cause felting, amphoteric character; below pH 4 the amino groups are protonated and take up acid dyes. Regain about 16%. Alkali and strong agitation damage it.
- Silk - fibroin, smoother and with fewer dye sites; acid and reactive dyes at mild pH, short times, no harsh alkali, which degrades the fibre and dulls lustre.
Synthetics
- Polyester - hydrophobic and highly crystalline; disperse dye only, applied at 130 °C under pressure, with a carrier, or by thermosol at 180–210 °C.
- Nylon - amide linkages, dyes with acid and disperse dyes.
- Acrylic - anionic sites that take cationic (basic) dyes; heat above the glass transition causes shrinkage and harsh handle.
Worked example
Why does direct dye give only a pale stain on polyester? The fibre is hydrophobic and tightly packed, so water and the large direct dye molecule cannot penetrate; there is no ionic site to hold the dye. Polyester must be dyed with a small disperse molecule above its glass transition temperature (about 80 °C), so the structure opens and dye dissolves into the fibre.
Traps
- Naming vat dyes for wool as a first choice.
- Assuming silk behaves exactly like wool - it is far more sensitive to alkali.
- Forgetting that crystallinity limits dye uptake even when the chemistry is right.