Dyes and Dye Classes
Examiners test whether you can link a dye class to its fibre, its application conditions and its fastness. Learn each class by four facts: solubility, the reactive group or site, the auxiliary chemicals, and the fastness profile.
The main classes
- Direct - anionic, water-soluble, substantive to cellulose; salt promotes exhaustion; moderate wash fastness, improved by cationic fixing agents.
- Reactive - contains a reactive group (chlorotriazine, vinyl sulphone); forms a covalent bond with cellulose in alkali with heavy salt; excellent wash fastness; part of the dye hydrolyses and must be soaped off.
- Vat - insoluble pigment; reduced by sodium hydrosulphite and caustic soda to a soluble leuco form, absorbed by cotton, then oxidised back in the fibre. Very good fastness; high cost. Indigo is the classic example.
- Acid - anionic, applied to wool, silk and nylon at pH 3–5 with acetic, formic or sulphuric acid; acid acts as the exhaustion agent.
- Basic (cationic) - coloured cation, used on acrylic, also on silk and on cotton after tannic acid or other mordanting.
- Disperse - finely dispersed, almost water-insoluble; polyester, acetate, nylon; HTHP, carrier or thermosol methods.
- Azoic (naphthol) - two components applied separately: naphthol impregnation, then diazotised base coupling inside the fibre to build an insoluble azo pigment, typically deep reds.
Worked example
A cotton hand towel must survive repeated hot washing and bleaching at low cost. Direct dye is cheap but wash fastness is only moderate. Vat gives excellent fastness but is expensive and needs reduction–oxidation control. Reactive dye is chosen: covalent bonding gives the fastness, soaping removes unfixed dye, and despite salt and alkali costs it is the lowest-cost route that meets the fastness demand.
Traps
- Confusing azoic dyes with azo-containing direct dyes - different application route.
- Reporting reactive exhaustion as if it were fixation.
- Forgetting electrolyte in reactive dyeing, or adding it all at the start and causing unlevel strike.