Page 379 - Basic Principles of Textile Coloration

368 VAT DYES

There are various other classifications of vat dyeing methods. The SDC
recommend tests to determine the best dyeing method [1]. In this, the colour
strengths of dyeings produced under different dyeing conditions are compared
with those of standard dyeings using a grey scale. This test applies only to
anthraquinone dyes. There are also SDC tests for determining the strike,
migration and levelling characteristics of vat dyes. Different companies have
different classification systems for their vat dyes. Because vatting and dyeing
conditions vary from one dye to another, the suppliers’ recommendations should
be consulted.

17.4.3 The high substantivity of leuco vat dyes

The origin of the substantivity of leuco vat dyes for cotton is far from clear.
Hydrogen bonds between cellulose hydroxyl groups and phenolate ion groups, or
amino or amide substituents, in the leuco dye may be involved in dye–fibre
binding. The substantivity of the leuco dye is very dependent on the position of
the phenolate ion groups and on the actual molecular structure. Substantivity
tends to be greater the larger the surface area of the leuco dye molecule and van
der Waals interactions between the dye and fibre are important. Molecules of
leuco vat dyes tend to be large and, although coplanarity of the structure is
essential, the structures are often not long and linear as for direct dyes. Although
the planar molecular structures of leuco vat dyes should promote aggregation,
there is not much evidence of this in dyeing solutions.

In the dyeing process, the influence of salt addition and increasing dyeing
temperature are similar to the effects found in dyeing cotton with direct dyes.
Unlike direct dyes, however, dyeing with many leuco vat dyes is very rapid at
relatively low temperatures and usually gives high exhaustion at equilibrium. The
ability to control the dyebath exhaustion by salt addition is very limited since the
bath initially contains a high concentration of sodium ions from the added hydros
and caustic soda. Because of this, many dyes have a rapid strike. The dyebath may
be 80–90% exhausted within 10 minutes. Because there is little diffusion into the
fibres within such a short period, there is a strong tendency for cotton yarns and
fibres to be ring dyed in the early stages of dyeing. Diffusion only becomes
important well after the time of half dyeing. The high strike must be controlled to
avoid unlevel dyeing since the low dyeing temperatures do not favour migration.
The behaviour of the leuco dyes is assessed by the usual migration and strike tests
in comparison with the behaviour of standard dyes [1]. The dyeing methods