Classification of dyes

Dyes may be classified in two ways:

a. according to the chemical constitution of the dye molecule, or;

b. according to the method of application of the dye.

Acid dyes

Acid dyes are so called because they are usually applied under acidic conditions The fibres most readily coloured with acid dyes are man-made, synthetic, nylon fibres and the natural protein fibres (mohair, silk, wool, etc)

Dyeing with acid dyes: The following abbreviations are used when dyeing with acid dyes is described

DSO₃^-Na⁺ the generalized formula for the acid dye molecule

D the colour radical or component of the acid dye molecule

DSO₃^- the dye anion

Na⁺ the sodium ion

H⁺ the hydrogen Ion

nylon the nylon polymer

wool the wool polymer

The application of acid dyes to protein fibres results in an ionic or salt link between the dye molecule and the fibre polymer. The point of the fibre polymer at which the dye is attached is termed the dye site. In wool the dye sites are the many amino groups of the fibre. Under dyeing conditions, the amino group becomes positively charged and attracts the negatively charged dye anion. This can be represented as follows:

  wool-NH₂          +                H⁺             →          wool-NH₃⁺

wool polymer                hydrogen              wool polymer with

with amino group        or acid ion    positively charged amino group

then

wool-NH           +                   DSO₃^-        →      wool-NH₃^+-SO₃D

wool polymer                         dye                   ionic link formed

with positively                         anion                between positively

charged amino group                                       charged amino group

                                                                      on wool polymer and dye anion

Properties of acid dyes

Light-fastness: Dyed and printed acid colours have good light-fastness.

Wash-fastness: Dyed and printed acid colours have poor wash fastness.

Reactive dyes

Reactive dyes are so called because their molecules react chemically with the fibre polymers of some fibres to form a covalent bond between the dye molecule and fibre polymer.

The fibres most readily coloured with reactive dyes are the man-made and natural cellulose fibres, synthetic nylon, and natural protein fibres.

Dyeing with reactive dyes

The application of reactive dyes involves the formation of a covalent bond between the dye molecule and the polymer of the particular fibre. The process of applying reactive dyes is considered below for protein fibres.

Reactive dyes are applied to protein fibres under slightly acid conditions. 

Figure 1: Structural formula for a vinyl sulphone derivative reactive dye - Remazol Brilliant Blue.

Printing with reactive dyes

Reactive dyes can be used for printing textile materials such as cellulosics and wool.

Figure 2: The vinyl sulphone sodium sulphate radical of a Remazol-type reactive dye molecule and how it attaches itself, by way of covalent bonding, to fibre polymers.

Properties of reactive dyes

Light-fastness: Textile materials coloured with reactive dyes have very good light-fastness.

Wash-fastness: Textile materials coloured with reactive dyes have very good wash-fastness.

Washing-off: Textile materials which are coloured with reactive dyes have to be thoroughly rinsed and scoured.