Classification of dyes by method of application.
The chemical classification of dyes is of interest to the chemist but the dyer is concerned mainly with the application of dyes to fabrics. The method used for application in a particular case depends on the nature of both the dye and the fiber to be dyed. The dyes are often classified on the basis of the technique employed for their application.
These can be connected to a piece of fabric by direct immersion in a water arrangement of the dye. A direct dye contains acidic or basic auxochrome which combines with the opposite polar group present in the chemical structure of the fiber. Wool and silk are readily dyes by this method.
Martius Yellow, a typical dye, has the acidic auxochrome-OH which interacts with the basic –NH2 group of wool or silk.
These dyes are insoluble in water but on reduction with sodium hydrosulphide in a vat form a colorless soluble compound which has a greater affinity for cotton and other cellulose fibers, The cloth is soaked in the solution of a reduced dye and then hung in the air, or treated with oxidants. As a result, the colorless compound is oxidized back to the insoluble dye which is now bound to the fabric. Indigo is a good example of a vat dye.
Indigo blue has auxochromes –OH which bind the dye fast to the cellulose fiber that contains ethereal oxygen and OH groups, by hydrogen-bonding.
This class of dyes have no natural affinity for the fabric and are applied to it with the help of salts e.g., oxides of aluminum or chromium. These salts are called Mordants. Fiber such as cotton is first treated with a mordant and then with the dye solution. The mordant forms a soluble coordination complex between the fiber and the dye and binds the two. The insoluble complex compounds appear in the form ‘lakes’ that are fast to light and washing. Alizarin is an example of a mordant dye.
The mordant dyeing is most suitable for wool and nylon.
Azoic Dyes (Ingrain Dyes).
In this method of dyeing, the water-insoluble azo dye is produced in the fabric itself. The cloth soaked in the solution of a coupling reagent-usually a phenol or naphthol. Then it is immersed in the solution of an appropriate diazonium salt. The dye that is developed in the fabric binds fast to via auxochromes. The azoic dyeing is particularly suitable for cotton and other cellulosic fibers yet may likewise be utilized for nylon.
These dyes are insoluble in the water yet can be scattered in a colloidal form in water. The fabric is immersed in the colloidal dispersion of the dye. The fine dye particles are retained into the ‘crystal structure’ of the fabric. Disperse dyes are used with modern synthetic fabrics such as nylon, orlon, polyesters, and cellulose acetate.