Dyes and Dye Classes
This domain tests which dye class suits which fibre and why the fibre-dye bond differs between classes.
Core rules
- Natural dyes include indigo, madder, turmeric and cochineal. Indigo is a vat dye; madder and cochineal are mordant dyes.
- Direct dyes colour cellulose without a mordant but generally have moderate wash fastness.
- Reactive dyes form covalent bonds with cellulose and give good wash fastness.
- Vat dyes are insoluble, reduced to a soluble leuco form, applied to cellulose, then oxidised in the fibre. Indigo is the classic example.
- Sulphur dyes are used mainly on cellulose for deep blacks and browns.
- Azoic dyes are formed inside the fibre by coupling.
- Acid dyes apply to wool, silk and nylon. Basic dyes apply to acrylic and can also colour silk and wool. Disperse dyes apply to polyester and acetate.
Worked example
A polyester garment is offered for dyeing with a reactive dye. Reasoning: reactive dye reacts with hydroxyl groups in cellulose, and polyester has little affinity for it. The correct choice is a disperse dye, applied at about 130 C under pressure or with a carrier. Reactive dye would mostly remain in the bath.
Traps that cost marks
- Using direct dye on polyester.
- Using basic dye on cotton and expecting depth.
- Applying vat dye without reduction.
- Confusing acid dye with the use of acid on cotton.