Page 276 - Basic Principles of Textile Coloration

PRE-METALLISED METAL-COMPLEX DYES 265

13.8.2 Pre-metallised 1:1 dye–metal complex dyes

The Neolan dyes, originally introduced by Geigy in 1919, are 1:1 metal-complex
dyes containing one atom of metal and one dye molecule with one or two
sulphonate groups per dye molecule. Although the chromium complexes of those
o,o¢-dihydroxyazo dyes with only one sulphonate group would have no overall net
electric charge, because the negative charges of two dissociated hydroxyl groups
and one sulphonate group neutralise the positive charge on the original chromic
ion, they behave as anionic dyes. This is because the residual positive charge on
the chromium atom in the complex is somewhat delocalised while the sulphonate
ion has a relatively localised negative charge. Alternatively, one of the non-dye
ligands that completes the coordination of the central chromic ion may be a
hydroxide ion rather than a water molecule or a neutral colourless ligand.

Dyeing wool with 1:1 metal-complex dyes involves a very acidic dyebath with
sulphuric acid (pH 2) and possibly sodium sulphate. Without other dyeing
assistants, the amount of concentrated sulphuric acid required is greater the
higher the liquor ratio and the greater the amount of the dyes. It can easily reach
8% owf H2SO4. When dyeing with such high concentrations of sulphuric acid,
there is a risk of hydrolytic damage to the wool fibres. Most manufacturers of these
dyes now recommend dyeing methods with reduced amounts of sulphuric acid or
the use of other acids such as formic acid. With lower concentrations of sulphuric
acid (4–6% owf), a polyethyleneoxide levelling agent in the dyebath retards dyeing
by complex formation with the dye. Alternatively, sulphamic acid can be used. At
the boil, this hydrolyses to ammonium bisulphate and increases the bath pH from
about 2.0 to 3.0–3.5, thus reducing fibre damage (Scheme 13.13). After dyeing,
the residual acid in the goods must be neutralised or buffered using sodium acetate
in the rinse.

NH2 SO3H + H2O NH4 + HSO4

Scheme 13.13

The Neolan P dyes (Ciba) are 1:1 dye–chromium complexes in which the
coordination of the chromic ion is completed with colourless hexafluorosilicate
ligands (SiF62–). They only require 3–4% owf of concentrated formic acid at a pH
of 3.5–4.0. This gives much less tendering of the wool compared to dyeing with
high concentrations of sulphuric acid and eliminates the need for neutralising or
buffering after dyeing.