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358 VAT DYES

CHAPTER 17

Vat dyes

17.1 INTRODUCTION
This chapter discusses vat, indigo and sulphur dyes together because their use in
dyeing involves the same principles. Vat dyes are one of the oldest types of dye.
Vat dyes in particular give dyeings on cellulosic fibres with the best overall fastness
properties. Because of the popularity of blue jeans, Indigo is still one of the most
important of all dyes in present use. Natural Indigo was obtained by extraction
from leaves, by fermentation in wooden vats, the origin of the term ‘vatting’.
Today, Indigo is synthesised from manufactured intermediates. Its application
involves reduction to the water-soluble leuco compound, dyeing the cotton and
re-oxidation of the leuco dye in the fibres to the insoluble pigment, the three
basics steps involved in vat dyeing (Figure 1.4).

Vat dyes are water-insoluble pigments. They are called dyes because chemical
reduction in alkaline solution converts the pigment into a water-soluble leuco
form with substantivity for cotton. The vat pigment and the leuco compound
often have quite different colours – blue and pale yellow in the case of Indigo – so
the progress of the reduction is often easy to observe. After dyeing with the leuco
compound, the pigment is regenerated in the dyed cotton by oxidation. The
overall process therefore involves three key steps:
(1) reduction of the pigment to the soluble leuco compound, a process called

vatting;
(2) absorption of the leuco compound by the cotton during dyeing;
(3) oxidation of the absorbed leuco compound in the cotton, reforming the

insoluble pigment inside the fibres.

The use of strongly alkaline solutions (pH 12–14) for vatting and dyeing limits the
use of most vat dyes to cellulosic fibres.

17.2 CHEMICAL CONSTITUTION OF QUINONE VAT DYES
In the case of Indigo, vatting involves reduction of a pair of conjugated carbonyl
groups (Figure 1.4). For most vat dyes, these carbonyl groups are present in an

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