Mordants, Assistants and Chemicals used in Dyeing
Mordants create a bridge between fibre and dye; assistants control how fast and how evenly the dye strikes. The exam checks named chemicals, their function and their correct stage of addition.
Mordants
Metallic salts that form a coordination complex with dye and fibre: potash alum (aluminium), copper sulphate, ferrous sulphate (saddening and darkening), stannous chloride (brightening, but harsh on wool), potassium dichromate (chrome, deep shades, toxic effluent). Tannic acid followed by tartar emetic gives cotton a cationic site for basic dyes. Application may be pre-mordanting, simultaneous (meta) or after-mordanting, and the order changes the shade.
Assistants and their jobs
- Electrolyte - sodium chloride or Glauber's salt; promotes exhaustion of direct and reactive dyes; added in portions.
- Alkali - soda ash or caustic soda; fixation of reactive dyes, dissolution of vat leuco.
- Acid - acetic, formic or sulphuric; exhaustion of acid dyes.
- Reducing agent - sodium hydrosulphite for vat and sulphur dyeing.
- Oxidising agent - hydrogen peroxide, air or dichromate to re-form vat pigment.
- Levelling agent - anionic or non-ionic surfactant that slows strike and aids migration.
- Wetting agent, sequestrant (EDTA), anti-creasing agent, soaping agent for removal of unfixed dye.
Worked example
A reactive red on cotton comes out streaky in hard water of 400 ppm. Calcium and magnesium ions complex with the dye and with soda ash, reducing available alkali and forming precipitates. Remedy: treat water or add a sequestrant, standardise the salt addition, and control pH at 10.5–11 with soda ash before raising temperature.
Traps
- Adding electrolyte too fast with reactive dyes - unlevel strike.
- Overdosing tin mordant and making wool brittle.
- Ignoring that dichromate is a toxic effluent problem, not just a chemical.