Page 265 - Basic Principles of Textile Coloration

254 ACID, PRE-METALLISED AND MORDANT DYES

wool dyeing, the conditions should always be as mild as possible, at as low a
temperature as possible, and in weakly acidic solution.

Chlorination is one process deliberately used to modify the scale structure of
wool fibres to prevent felting and shrinkage of woollen goods on washing (Section
7.4.2). The dyeing of chlorinated wool poses several problems. Acid dyes have
increased initial substantivity for chlorinated wool because of the severely
damaged scales. The rapidity of the reaction of chlorine with wool increases the
risk of uneven chlorination resulting in subsequent unlevel dyeings, even when
using levelling acid dyes. The dyeings also have lower washing fastness because of
the increased ease of desorption of dyes from chlorinated wool. The difference in
the dyeing behaviour of untreated and chlorinated wool appears to be entirely a
consequence of dyeing rates since the two fibres give the same equilibrium dye
absorption.

13.5.3 Use of wool dyeing assistants [3]

A wide variety of auxiliary products are used in wool dyeing to promote dye
levelling. Levelling agents function by retarding dye absorption. They are of two
main types: anionic and cationic/non-ionic. Wool adsorbs anionic levelling agents
and they retard dye absorption by initially blocking the cationic ammonium ion
sites. The more substantive dye anions eventually displace the anionic product.
Anionic surfactants with long alkyl chains have higher fibre substantivity and are
more effective, but they decrease the equilibrium uptake of the dye. Although the
ion exchange explanation may be an oversimplification of the action of anionic
levelling agents in wool dyeing, it adequately explains their retarding effect.
Typical products are the sulphate esters of fatty alcohols, R–OSO3Na.

The action of cationic/non-ionic levelling agents is quite different. Cationic
agents form a complex with the anionic dye in the dyebath and prevent its uptake
by the wool. The non-ionic portion of this type of product keeps the auxiliary–dye
complex (Dye– Aux+) dispersed in solution. This is important to avoid its
precipitation in the bath or on the material surface. Free dye molecules are
liberated as the dyeing temperature increases since the complex is less stable at
higher temperatures (Scheme 13.5). Strong levelling action can be achieved using
a cationic polyethoxylated amine (such as 1, Figure 13.4) with a short chain (n =
5–20), mixed with a purely non-ionic product to keep the complex in dispersion.
The greater cationic character of products with short polyethoxylated chains gives
strong complex formation, pronounced retardation of dyeing and a higher risk of