Page 409 - Basic Principles of Textile Coloration

398 DYES SYNTHESISED IN THE FIBRE

CHAPTER 19

Dyes synthesised in the fibre

19.1 INTRODUCTION
An ingrain dye is an insoluble pigment formed in situ by means of a chemical
reaction between colourless, soluble precursors already inside the particular fibre.
There are a number of different types of ingrain dye. The Colour Index includes
ingrain or developed dyes under different headings:
(1) azoic dyes prepared by impregnating the material with an alkaline solution of

an azo coupling component and subsequently reacting this with an
appropriate diazonium ion to produce an azo pigment within the fibres. The
Colour Index lists Azoic Coupling Components and Azoic Diazo
Components. The latter are given as the primary aromatic amines from which
the diazonium ions are prepared;
(2) condensed sulphur dyes (Colour Index Condensed Sulphur Dyes). These are
polythiols that behave like the leuco compounds of sulphur dyes. On
oxidation inside the fibre, they form an insoluble polymeric dye (reverse of
Scheme 17.4);
(3) black pigments based on the oxidation of aniline or other primary aromatic
amines (Colour Index Oxidation Bases);
(4) copper phthalocyanine pigments synthesised in the fibre by reaction of copper
salts and appropriate isoindole derivatives under reducing conditions, or
alternatively by precipitation of water-soluble copper phthalocyanine
precursors (Colour Index Ingrain Dyes).

The developed azoic dyes used in cotton dyeing are the most important ingrain
dyes but have gradually lost their market share. Reactive dyes have superseded
them, apart from a few deep red and bordeaux combinations that give shades
unobtainable with any other class of dye. The oxidation bases and ingrain dyes are
used mainly for printing but are not particularly significant.

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