The dyes which contain insoluble azo group(-N=N-) are known
as azoic dyes. These dyes are not found in ready-made form. They dyes are
produced through an in-situ process which created colored material directly on
the fabric by a reaction between two compounds namely
1. Diazo
compound or diazo base
2. Coupling
compound i.e. Naphthol
Azoic dye is one type of ingrain dyes .e. it is produced
during dyeing process inside the fibre. The general reaction of azoic dye
formation is given below:
R-N=N-Cl + R`-ONa R-N=N-R`
Diazo compound Naphthol Azoic color
The diazo compound is very often said fast base or diazonium
salt of fast base.
The colored substance formed from essential colorless
compound is insoluble in water and the washing fastness of this shade is
excellent. But it has poor rubbing fastness. Johann Peter Griess of German
discovered the diazotized reaction and subsequent reactions of a diazotized
reaction in 1858. This discovery was the foundation stone for the immerse
edifice of azo dyes. He found that Premuline can be diazotized on the fiber
which afterward takes part in couple reaction with an aromatic hydroxyl or
amino compound to give a new dye of greater wet fastness.the oldest application
of this method was in the dyeing of para red. This para red was produced by
coupling diazotized p-notro aniline with β-naphthol.
the formation of an insoluble azoic pigment was first patented by Thomas and
Holliday in 1880.