Page 395 - Basic Principles of Textile Coloration

384 VAT DYES

initially, or in portions during dyeing, to promote exhaustion. After dyeing the
goods are rinsed, the leuco dye oxidised and the dyeing is soaped as for a
conventional vat dye.

Sulphur dyes usually have acceptable substantivity, particularly in the presence
of salts, so that stripping in a fresh reducing bath is not easy. Dyeing is often
conducted at the boil but this decreases the degree of exhaustion. Sulphur dyes
require less salt than reactive dyes and usually have reasonable exhaustion. Low
sulphide leuco dyes require more salt and no polysulphide. They do not give good
exhaustion in heavy shades and the use of a low liquor ratio is recommended. For
popular shades such as black, it has long been common practice to use a standing
bath. This is a dye bath that is re-used for subsequent dyeings after addition of
more reduced dye. Any free sulphur that tends to accumulate is dissolved by
addition of sodium sulphite to give thiosulphate. This prevents it sticking to the
goods. The actual dyeing temperature can vary. At higher temperatures around
the boil, the bath exhaustion is less but penetration of the leuco dye into the fibres
is better than at lower temperatures.

17.12.3 Post-dyeing treatments

After dyeing, good rinsing before oxidation helps reduce bronziness and poor
rubbing fastness by removing loosely adhering surface leuco dye solution before
oxidation precipitates the insoluble pigment. Once the rinsing is completed, the
leuco dye in the fibres is oxidised to the insoluble pigment.

Some leuco dyes can be oxidised in air, others need chemical oxidation. Sodium
dichromate, hydrogen peroxide, or sodium percarbonate or perborate are used in
warm, weakly alkaline solution. The peroxy compounds used for vat dyes can be
used for sulphur dyes, but some leuco dyes (red-browns) are not oxidised by these
agents. Some blues are over-oxidised, probably by oxidation of the disulphide links
between the heteroaromatic units to form ionic sulphinate and sulphonate groups.
This increases the water solubility, decreases the wet fastness and results in staining
of other goods during washing. Even sulphur blacks oxidised with peroxides tend to
be bluer, lighter and somewhat less fast to washing. The best washing fastness is
obtained by oxidation of the leuco dye with sodium dichromate and acetic acid.
Sodium bromate (NaBrO3) is now more widely used as an oxidant, particularly in
North America. It requires a small amount of metavanadate ion (VO3–) as catalyst
for the oxidation of the more resistant red-brown dyes. This catalyst can be prepared
in situ during sodium bromate manufacture. Commercial products usually contain