Dye-fibre association

The first three usually act in conjunction to varying degrees in most dye classes, although depending on the colorant and the fibre and the dyeing conditions used, one of these physical forces usually predominates. Covalent forces are only present between fibres and reactive dyes.

Some of the important dye classes include:

Direct dyes

The first direct dye was called Congo red and was introduced in 1884. It was called a direct dye because it was the first dye to become available for colouring cellulose 'directly', without the use of a mordant. Direct dyes are applied in a boiling dyebath in the presence of salt.
Advantages:
· Cheap
· Simple to apply
· Wide range of colours
· Very good fastness to light
· Disadvantages:
· Poor wet fastness
· Moderate brightness

Reactive dyes

Developed in the 1950s, these were the first dyes produced capable of reacting chemically with the fibre (usually cellulose) under alkaline conditions. The dye thereby becomes part of the fibres, rather than merely remaining as an independent chemical entity within the fibre. Reactive dyes are applied at relatively low temperatures, with controlled amounts of salt and alkali.
Advantages:
· Simple to apply
· Wide range of bright colours
· Good fastness to light and washing

Disadvantage:
· Relatively expensive