Azoic dyes are insoluble azo dyes produced in situ on the
fiber. In this dyeing process two components are necessary. The first component
is Naphthol and the second is Fast base.
The first component which is nearly colorless or faintly
yellow or brown in color has to be dissolved in water, containing NaOH and
applied to cotton as if it were a direct dye but at room temperature and not at
the boil.
The second component has to be subjected to a section called
diazotization and then applied to cotton which has already been treated with a
first component. These two compounds react (couple) with each other almost
immediately, producing usually a bright colored pigment, which is insoluble in
water. Since an azo group (—N=N—) is formed during the coupling of the two
components, so these dyes are called ‘Azoic dyes’. They have excellent washing
fastness.
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