Vat and sulphur dyes are dealt with together here because their method of application
is similar. They differ entirely in their chemistry, however, and in their importance in
textile printing. Whereas in this field vat dyes are still widely used, the use of sulphur
dyes is restricted mainly to the production of black.
The dyeing mechanism in both cases depends upon the insoluble dye being
transformed into a water-soluble alkaline leuco compound under the influence of
alkalis and reducing agents. The alkaline leuco compounds diffuse from the thickener
into the swollen fibres, during the steaming process.
Oxidation then precipitates the
dye in a finely dispersed insoluble form inside the fibre, to give a print with typically
good fastness properties.
The formulae for indigo (CI Vat Blue 1) and indanthrone (CI Vat Blue 4) (5.2, 5.3)
illustrate two typical vat dye structures and their reversible reduction–oxidation
behaviour. The presence of at least two conjugated carbonyl groups is common to all
vat dyes [13]. Sulphur dyes are, in chemical terms, complex molecules produced by the
reaction of alkali sulphides with simple organic compounds, in which thiazine and
thiazole rings provide the chromophore groups.
It is not normally possible to print vat dyes in the leuco form, because exposure to air during and after printing results in premature oxidation. For the same reason, the reducing agent itself must either be stable in air or be introduced at a late stage in the process. Both approaches are important, the former in the ‘all-in’ method and the latter in the two-phase or two-stage process. All-in method: In 1905 it was discovered that sodium formaldehyde sulphoxylate, CH 2OHSO2Na, had the required stability at low temperatures and reducing action in steam to be used in vat print pastes.
A typical paste, as formulated in Chapter 8, can be stored for long periods, printed and dried with only a small loss of reducing power. When the print is steamed in air-free steam, reduction of most vat dyes is efficiently brought about. The control of conditions for optimum fixation is discussed in Chapter 8.
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| Vat and sulphur dyes |
It is not normally possible to print vat dyes in the leuco form, because exposure to air during and after printing results in premature oxidation. For the same reason, the reducing agent itself must either be stable in air or be introduced at a late stage in the process. Both approaches are important, the former in the ‘all-in’ method and the latter in the two-phase or two-stage process. All-in method: In 1905 it was discovered that sodium formaldehyde sulphoxylate, CH 2OHSO2Na, had the required stability at low temperatures and reducing action in steam to be used in vat print pastes.
A typical paste, as formulated in Chapter 8, can be stored for long periods, printed and dried with only a small loss of reducing power. When the print is steamed in air-free steam, reduction of most vat dyes is efficiently brought about. The control of conditions for optimum fixation is discussed in Chapter 8.
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