Page 275 - Basic Principles of Textile Coloration

264 ACID, PRE-METALLISED AND MORDANT DYES

dyes may therefore give differences in colour depth, or even two different hues, on
the roots and tips of ‘tippy’ wool.

In the metachrome method, dyeing and mordanting the wool occur
simultaneously, dyeing in the presence of sodium dichromate, ammonium sulphate
and sodium sulphate at a pH around 6–7. Only a limited number of dyes are stable
in hot solutions of dichromate and have reasonable substantivity at pH 6–7. The
procedure has the advantage of facilitating shading and requires less time than the
after-chrome procedure. It gives higher levels of residual chromium in the effluent.
Only a few dyes of this type are now used (CI Mordant Brown 48) and the method
is almost obsolete.

13.8 PRE-METALLISED METAL-COMPLEX DYES [4]

13.8.1 Chemical structures

Pre-metallised dyes are anionic 1:1 or 2:1 dye–metal complexes (Figure 13.8) used
for dyeing wool and nylon under conditions typical of acid dyes. The properties of
1:1 and 2:1 metal-complex dyes resemble those of levelling and milling acid dyes,
respectively. The fastness properties of both types on wool approach those of
chrome dyes. Complexing a dye with chromic ion produces a bathochromic shift
and dulls the shade. The dyeings are, however, slightly brighter than those of after-
chromed mordant dyes. The dye–metal complex is pre-formed and stable. Hence,
these dyes do not need any aftertreatment and are therefore much easier to use in
colour matching.

In dyeing with all types of metal-complex dyes, soft water is preferred. If it is
necessary to counteract any hardness, a polyphosphate sequestering agent is used
rather than an EDTA type. The latter are capable of removing the metal from the
dye–metal complex, thus liberating a non-metallised acid dye of different hue.

The 2:1 metal-complex dyes for wool, along with fibre-reactive dyes, are
gradually replacing the other types of acid dyes, because of their superior fastness
properties. Pre-formed metal-complex dyes are gradually replacing after-chrome
dyes whose lengthy dyeing procedure tenders the wool, and which pose a serious
environmental problem. With increasingly stringent surface water quality
standards imposed by governments, there is now concern that dyeing with metal-
complex dyes will have to be optimised to reduce chromium and cobalt levels in
dyehouse waste water even further. If future water quality standards cannot be
met, the use of metal-complex dyes will decline in favour of the metal-free fibre-
reactive dyes for wool (Section 16.6).