Page 385 - Basic Principles of Textile Coloration

374 VAT DYES

The pigment form, however, has no substantivity for cotton at all. The pre-
pigmentation dyeing methods are simple processes in which a fine dispersion of
the vat pigment is uniformly circulated through the goods that are then treated
with alkaline hydros solution to cause reduction. Dyeing takes place around each
original pigment particle with almost no migration. The dyeing is subsequently
oxidised and soaped as before. Either of the two operations, the initial
pigmentation or the reduction, may be batch or continuous processes. This leads
to a variety of possible dyeing methods, some of which are described below.

17.7.1 Pre- and semi-pigmentation batch dyeing methods
In the pre-pigmentation method, a fine dispersion of the vat dye pigment is first
circulated through the goods. In the second step, reduction to the leuco dye is
carried out using NaOH and hydros. The latter may be added all at once or
gradually. This method is useful for light and medium shades where levelling and
penetration may be a problem in dyeing with a fully reduced vat dye. It is,
however, less suitable for compact forms of material. In package dyeing, the
pigment dispersion must not be filtered out in the packages but rather gradually
deposited into the package by salt addition that causes the particles to slowly
flocculate.

In the semi-pigmentation process, all the chemicals are circulated through the
goods at room temperature before gradual heating to start reduction. The rate of
vatting is very slow at the low starting temperature. As in the pre-pigmentation
method, a fine dispersion is required.

The vat acid process is another alternative for package dyeing. After reduction
of the vat pigment, in alkaline solution, the leuco vat acid is precipitated in the
presence of a dispersing agent by acidification. This is then circulated in the goods
and dyeing takes place by gradual addition of caustic soda to reform the soluble
and substantive leuco form.

17.7.2 Semi-continuous processes
In semi-continuous vat dyeing processes, the fabric is pigmented by padding with
the insoluble vat dye dispersion. Subsequent reduction to the leuco form is a batch
operation using a jig or beam dyeing machine. Some dye comes off the fabric
during development with the alkaline hydros solution and some vatted dye liquor
may be added to the bath in the machine beforehand. This prevents ending in jig