Page 277 - Basic Principles of Textile Coloration

266 ACID, PRE-METALLISED AND MORDANT DYES

A typical dyeing procedure for dyeing wool with 1:1 dye–metal complex dyes
follows the scheme in Figure 13.2 but using 6% owf of sulphuric acid in the
dyebath.

The fastness to washing of these dyes on wool is lower than for after-chrome
dyes. It is comparable to that of fast acid dyes. Light fastness is usually good. These
dyes are still important for dyeing loose wool, yarn for carpets, and for dyeing wool
fabric after carbonising or acid milling, when the acid remaining in the goods
serves directly for dyeing. It is unlikely that ligand groups in the wool bond to the
1:1 dye–metal complex since they have only moderate to good washing fastness
and migrate readily under the acidic dyeing conditions. Because of this, the
exhaustion is not optimal at pH 2. When applied at a higher pH of around 4, they
give higher exhaustion, but produce skittery dyeings without a levelling agent.
Their shades are often brighter than those of after-chrome dyes, but duller than
those of non-metallised acid dyes. Shading and colour matching are relatively
straightforward as there are no aftertreatments.

13.8.3 Pre-metallised 2:1 dye–metal complex dyes

These dyes have two dye molecules per metal atom and the molecular structures
may be symmetrical, with two identical tridentate dye ligands, or unsymmetrical,
with two different dyes. There are three types of 2:1 metal-complex dyes:
(1) weakly polar types that do not have sulphonate groups in the dye molecules.

Their limited water solubility comes from polar methyl sulphone or
alkylsulphonamide groups in the dye molecule. The complexes are anionic
since the trivalent chromic ion reacts with four anionic ligand groups, such as
the four hydroxyl groups from two o,o¢-dihydroxyazo dyes;
(2) monosulphonated 2:1 dye–metal complexes that have an unsymmetrical
structure since the two dye ligands are not identical and only one is
monosulphonated;
(3) disulphonated 2:1 dye–metal complexes with symmetrical molecular
structures since both identical dye ligands are monosulphonated.

The weakly polar 2:1 dye–metal complex dyes are applied to wool from a nearly
neutral dyebath in the presence of ammonium acetate or sulphate. They require
careful temperature and pH control since they have only limited levelling ability.
In some cases, acetic acid may be added to the dyebath towards the end of dyeing.
This decreases the pH to around 5.5 and promotes exhaustion. These dyes have