Page 373 - Basic Principles of Textile Coloration

362 VAT DYES

Because hydros reacts readily with oxygen (Scheme 17.1), the material being dyed
with the leuco dye should not be over-exposed to air. Oxidation of the excess
hydros can lead to subsequent oxidation of the leuco dye and precipitation of the
quinone, giving uneven surface deposits of pigment on the goods. A large excess of
hydros in the bath is usual. The amount used depends on the dyes and the type of
machine. The suppliers give suitable recommendations for their dyes. Other more
stable reducing agents, such as sodium sulphoxylate-formaldehyde
(HOCH2SO2Na), are used in printing with vat dyes to ensure stability of the
pastes. Steaming of the printed material activates this reducing agent and the vat
dye in the print paste is reduced (Scheme 23.3). Sodium sulphoxylate-
formaldehyde is a poor reducing agent at 25 °C and is more stable than hydros.
Other reducing agents such as thiourea dioxide (Scheme 23.3) and glucose are
now being suggested as more environmentally friendly alternatives to hydros.

17.3.3 Vatting

Traditional vatting involves adding hydros to a fine, relatively concentrated
aqueous dispersion of the vat dye pigment containing NaOH. The vatting
temperature is often higher than the subsequent dyeing temperature. This gives
rapid reduction. The concentrated solution of the leuco form is then diluted by
adding it to water in the dyebath. The bath water also contains NaOH and some
hydros to avoid any oxidation from dissolved atmospheric oxygen. Vatting may
also be carried out directly in the dyebath if the dye is readily reduced.

Dyeing takes place at temperatures lower than the vatting temperature to avoid
decomposition of the hydros. This also decreases the risk of over-reduction of
polyquinone dyes such as Indanthrone. Over-reduction is the reduction of more
than one pair of conjugated carbonyl groups in a polyquinone and often results in
poor colour yields and off-shade dyeings. Reduction inhibitors, such as sodium
nitrite or glucose, prevent over-reduction during vatting. Anthraquinone and
polycyclic vat dyes give leuco solutions with intense colours, often very different
from those of the original pigment dispersion. The vat dye flavanthrone is a yellow
pigment that produces a deep blue vat. It is often used in paper strips for the
detection of excess hydros.

Vatting conditions for quinone vat dyes vary widely. Three main types of
vatting process are used depending upon the dyes being used. These involve:
(1) a concentrated NaOH solution and a high vatting temperature (60 °C);