Dyes

Dyes are coloured compounds use for important colour to the textile ,silk, woo,l foodstuffs etc however all substances not act as dye for a substance to act as dye certain condition must be satisfied

1) It must have a suitable colour

2) It must be able to fix itself from the solution to the fabric

3) When fixed it must be fast to light resistance to the action of water during washing or to the organic solvents during dry cleaning .

How a dye exhibit colour

A dye is an organic compound which can absorb a certain portion of the visible light falling on it and the rest of the light is reflected  the colour of the dye give the colour of the dye which is complementary to the colour of the light absorb in fact no dye reflect only one band of wavelength Hence no  dye gives a pure shade.

Classification of dyes

Natural dyes  -   indigo and alizarin are the two well known example of natural dyes

Synthetic dyes are of the following types

Direct dyes. These are those dyes which can be directly applied to the fabric from an aqueous solution . For example Congo red and martius yellow.

Disperse dye. Water insoluble dyes which are dispersed in suitablereagents before application to the synthetic fibre. These dyes are used for nylon, polyester

Fibre reactive dyes.  These are those dyes which contain a reactive group which combine directly with the the hydroxyl or amino group of the fibre because of the chemical reaction the colour is fast and has a long life

VAT dyes. These are are insoluble dyes which are first reduced to a colourless soluble form in large vats with a reducing agent such as alkaline sodium hypophosphite and then applied to the fabric a well known example of this dye is Indigo

Mordant dyes. These dyes do not dye the fabric directly but require a mordant the mordant act as binding agent between the fibre and the dye. For acid dyes metal ions act as mordant and for basic dyes tannic acid is used as mordant these are used to dye wool .for example alizarin gives Rose red colour with aluminium ions but gives blue colour with barium ions

By Rajesh Prashar