Page 387 - Basic Principles of Textile Coloration

376 VAT DYES

17.7.4 Wet-on-wet processing
The usual pad–steam process cannot be used for corduroy or terry cloth because of
flattening of the pile and excessive migration during drying. A wet-on-wet dyeing
process is used. In this, the cloth is padded with the pigment dispersion and then,
while still wet, it is padded again with the alkaline hydros solution. The solution
pick-up at the chemical pad may go from values of around 70 up to 150–200%.
The goods then go through the steamer and the usual aftertreatments.

17.8 FASTNESS PROPERTIES OF VAT DYES
Many quinone vat dyes provide outstanding colour fastness. The colour gamut is
somewhat restricted as there are no bright colours, particularly in the red and
turquoise areas. Only azoic and sulphur dyes have smaller colour gamuts. The
colour gamut is the size of the ‘colour space’ for the colours obtainable with a
particular type of dye either alone or in combinations (Section 21.8.2).

Vat dyes are very expensive, like reactive dyes. A high degree of overall colour
fastness is characteristic of vat dyes. They usually have excellent fastness to
washing, chlorine bleaching, alkali boiling and light. Vat dyes also have the
advantage of being the only dyes for cellulosic materials that do not give shade
changes on application of topical reactive finishes for cotton, such as flame resist
finishes (Section 25.3.1). One minor problem is that of photochemical catalytic
fading or cotton tendering by a small group of vat dyes. These are mainly yellow
and orange vat dyes that photosensitise the tendering of cotton, as in drapery
fabrics. The dye functions as a catalyst for the photo-oxidation of the cellulose.
Others photosensitise fading of other vat dyes.

17.9 DYEING WITH INDIGO AND INDIGOID VAT DYES
Natural Indigo was rapidly displaced from the market once the synthetic product
became available. The quality of natural Indigo was quite variable because of the
presence of other coloured impurities. The fastness properties of Indigo dyeings are
not up to the standards expected from the vat dye class as a whole. It is, however,
the appearance of faded Indigo in denim that is so fashionable today. After dyeing,
various wet processes, such as stone washing, deliberately enhance this faded
effect. Indigo builds up primarily on the cotton fibre surface. This is one reason for
the somewhat inferior fastness properties of Indigo compared to quinone vat dyes.