Page 27 - Basic Principles of Textile Coloration

16 AN INTRODUCTION TO TEXTILES, DYES AND DYEING

pores, or between fibre polymer molecules, depending on the internal structure of
the fibre. The overall process of adsorption and penetration of the dye into the
fibre is called absorption. Absorption is a reversible process. The dye can therefore
return to the aqueous medium from the dyed material during washing, a process
called desorption. Besides direct absorption, coloration of a fibre may also involve
precipitation of a dye inside the fibre, or its chemical reaction with the fibre. We
have already seen that these two types of process result in better fastness to
washing, because they are essentially irreversible processes.

For diffusion into a fibre, dyes must be present in the water in the form of
individual molecules. These are often coloured anions; for example, sodium salts
of sulphonic acids such as Congo Red (2, Figure 1.3). They may also be cations
such as Mauveine (1, Figure 1.1), or neutral molecules with slight solubility in
water, such as disperse dyes (3, Figure 1.6). The dye must have some attraction for
the fibre under the dyeing conditions so that the solution gradually becomes
depleted. In dyeing terminology, we say that the dye has substantivity for the fibre
and the dyebath becomes exhausted.

The four major characteristics of dyes are:
(1) intense colour;
(2) solubility in water at some point during the dyeing cycle;
(3) some substantivity for the fibre being dyed;
(4) reasonable fastness properties of the dyeing produced.

1.3.3 Dye classification and nomenclature

The Colour Index was first published in 1924 by the Society of Dyers and
Colourists (SDC) and is the major catalogue of dyes and pigments. The third
revised edition is published jointly by the SDC and the American Association of
Textile Chemists and Colorists (AATCC). In it, dyes are classified according to
chemical constitution (30 subgroups) and usage (19 subgroups) [2]. Table 1.4
gives partial classifications of dyes as presented in the Colour Index.

The first three volumes of the third edition of the Colour Index (CI) give
extensive information on the 19 subgroups of dyes classified according to usage. In
each subgroup, dyes have a Colour Index Generic Name based on the particular
application and hue. For example, CI Acid Red 1 is a red acid dye, with similar
dyeing properties to Orange II in Figure 1.2 (CI Acid Orange 7). CI Reactive Blue
4 is a blue reactive dye. Dyes in any one application subgroup will be used for
specific fibres using similar dyeing methods. For each dye listed, useful data on