Page 410 - Basic Principles of Textile Coloration

AZOIC DYES 399

19.2 AZOIC DYES

19.2.1 Introduction

These usually are water-insoluble mono- or bisazo compounds precipitated in the
fibre by reaction of a diazonium ion with a suitable coupling component. The
name ‘azoic dye’ is somewhat confusing, since many soluble dyes are also azo dyes.
The name ‘azoic combination’ is preferable.

The production of dyeings with Para Red, one of the first azoic combinations
(1880), illustrates the basic principle of dyeing with this type of dye. Cotton fabric
was first impregnated with an alkaline solution of 2-naphthol (1, in Figure 19.1). It
was dried and then padded with an acidic solution of diazotised p-nitroaniline (2).
A deep red pigment formed in the cotton fibres (3). A final soaping at the boil
removed pigment particles adhering to the fibre surfaces. This gave the optimum
fastness to washing, light and rubbing, although these were not outstanding.
Intermediate drying was necessary because 2-naphthol has poor substantivity for
cotton and tended to bleed out of the wet fabric into the diazonium ion bath
causing pigment formation, particularly on the fibre surfaces. The introduction of
coupling components with much higher substantivity for cotton overcame this
problem, and also decreased the migration of the coupling component to the yarn
surfaces during the intermediate drying.

NO2

N N
N+ N

OH OH

NO2
1 23

Figure 19.1

Azoic combinations are still the only class of dye that can produce very deep
orange, red, scarlet and bordeaux shades of excellent light and washing fastness.
The pigments produced have bright colours, and include navies and blacks, but