Page 278 - Basic Principles of Textile Coloration

PRE-METALLISED METAL-COMPLEX DYES 267

very good light fastness and good washing fastness, but, for deep shades, the latter
is lower than for dyeings with after-chrome dyes. They are used mainly on loose
wool, slubbing, yarn and knit goods because they tend to highlight irregularities in
woven materials. The 2:1 dye–metal complex dyes have high substantivity for
wool, even in neutral solution. Dye levelling and penetration are usually good,
provided that proprietary levelling agents are present in the dyebath. Uniformity of
the initial fabric pH is critical and the goods are carefully neutralised after
carbonising or other acidic processes.

The polar types usually have improved wet fastness compared to the weakly
polar dyes, and have good light fastness. They can replace mordant dyes where
there is a risk of damage to the wool from the extended boiling required in the
after-chrome process. All the 2:1 dye–metal complex dyes have large molecules
that tend to aggregate in solution and have slow diffusion in the wool fibre. The
presence of sulphonate groups in the dye molecules leads to decreased
substantivity in neutral solution, an indication of the importance of hydrophobic
interactions in dyeing with the non-polar types. With increase in the degree of
sulphonation, the dyeing pH must be lower to increase ionic interaction with
ammonium ion sites in the wool fibre. The weakly polar dyes dye wool at pH 6–7
using ammonium acetate or sulphate in the dyebath, the monosulphonated dyes at
pH 5–6 with ammonium acetate plus acetic acid, and the more polar
disulphonated dyes at pH 4–5 using acetic acid. Dyeing is usually carried out at
temperatures close to the boil, with careful control of the temperature during the
heating phase. Alternatively, both low (80 °C) and high temperature (110 °C)
dyeing methods are possible.

The disulphonated dyes possess the highest all-round fastness. The levelling
capability of the disulphonated 2:1 complexes is poor, somewhat better for the
monosulphonated type, and better still for the weakly polar dyes. The ability to
migrate during dyeing increases with increasing dyeing pH and thus decreasing
interaction with the diminishing number of ammonium ion sites in the wool. The
dyes of lower migration therefore tend to give skittery dyeings and are not suitable
for piece dyeing because of this.

All types of 2:1 metal-complex dye are suitable for dyeing shrink-resist wool
from the Hercosett process (Section 7.4.2). Because of the cationic nature of the
polymer deposited on the fibre surface and residual acid from the required pre-
chlorination, strict control of the initial fabric pH and temperature are necessary
to obtain level dyeings because of the greater substantivity of the dyes for the
treated wool. An ester that slowly liberates an acid on hydrolysis, such as ethyl