Dyes in Chemistry
Coloured substances used for colouring Textiles, Foodstuffs, Silk, Wool, etc., are called dyes.
But all coloured substances are not Dyes. For a substance to act as a dye, it must fulfill the following requirements.
It must have a suitable attractive colour, i.e., it should absorb light in the visible Region. It must be able to fix itself to the fabrics by either Physical or Chemical Bonding.
It must be fast to light; it must not be affected either by water, dilute acids or alkalies.
Different classes of dyes are discussed below.
Nitro Dyes : These are polynitro derivatives of phenol where nitro group acts as a chromophore and hydroxyl group as auxochrome.
These are less important industrially because the colours are not fast.
Azo Dyes : These are an important class of dyes and are characterized by the presence of azo group (-N = N-) as the chromophore.
The groups like NH2, NR2 or -OH etc, present in the molecule containing one or more azo groups act, as the auxochromes.
Triphenyimethane Dyes : These dyes contain the paraquinoid moiety as chromophore and -OH,-NH2 or -NR2 as auxochrome.
These dyes are not fast to light and washing and hence are mainly used for colouring paper or typewriter ribbons, e.g., malachite green which is used for dyeing wool and silk directly and cotton after mordanting with tannin.
Direct dyes: These include dyes which can be directly applied to the fabric when the latter is dipped in a hot aqueous solution of the dye.
Wool and silk are dyed by direct dyes.
The polar groups of the fabric (proteinic structure) unit with the polar groups of the dye and thus the dye is chemically fired upon the fibre.
Example of direct dyes are Mautius yellow, Naptho yellow S, Congo red, etc.
Mordant Dyes : Those dyes which are fixed on the fibre with the help of a modrant are known as mordant dyes.
Various mordants depending upon the nature of the dye are used.
For acidic dyes, basic mordants (such as hydroxides of iron, aluminium and chrornium) are used, while for basic dyes, acidic mordants (like tannic acid) are used.
Here the fabric is first dipped into a solution of mordant and then into the dye solution.
The colour produced depends on the nature of the mordants used.
Using alizarin as Mordant dye and aluminium, chromium and iron as mordants, red, brownish – red tones and black – violet colours, respectively are produced.
Vat Dyes : These are water insoluble dyes and are introduced into the fibre in its soluble) reduced form, also known as leuco form (colourless).
The parent dye is regenerated by oxidant with air or a suitable chemical.
These are called vat dyes because reducing operation (using sodium hydrosulphite) was formerly carried out in wooden vats.
Indigo is a vat dye and is used for Dyeing Cotton.
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