Vat dyes are used in the dyeing
and printing of all types of cellulosic fibre, and also of blends of cotton
with polyester. In their coloured form they are insoluble pigments, so their
application depends on reversible reduction–oxidation (redox) reactions (page
42). In the dyebath the pigment is converted into a water-soluble form using a
strongly alkaline solution of a powerful reducing agent. This forms the sodium
‘leuco’ compound of the dye, which is soluble in water but often different in colour
from the original pigment. It is then allowed to dye the cellulose in this
water-soluble form; once exhaustion is completed the leuco compound is oxidised
to the coloured form.
The large majority of vat dyes are based on the anthraquinonoid or the indigo (or thioindigo)
chromophores; indigo, one of the oldest dyes still in use, remains popular through the wide use of
indigo-dyed denim. Several of the anthraquinone dyes are complex polycyclic quinones (Appendix
1), and they all possess two carbonyl groups ( C=O) linked by alternate single and double bonds in
a conjugated chain. This molecular arrangement is responsible for the easily reversible redox
reactions on which the application of vat dyes depends.
chromophores; indigo, one of the oldest dyes still in use, remains popular through the wide use of
indigo-dyed denim. Several of the anthraquinone dyes are complex polycyclic quinones (Appendix
1), and they all possess two carbonyl groups ( C=O) linked by alternate single and double bonds in
a conjugated chain. This molecular arrangement is responsible for the easily reversible redox
reactions on which the application of vat dyes depends.
No comments:
Post a Comment