during their conventional application to cellulosic fibres, are converted by reduction under alkaline conditions to the corresponding, water soluble, ‘alkali leuco’ formwhich is applied to the substrate. At the end of dyeing, the alkali leuco form is oxidised, so as to regenerate the insoluble, parent vat dye in situ within the fibre (Fig. 1).
However, the alkali leuco form is readily converted to the sparingly water-soluble ‘acid leuco’ variant (Fig. 1), which has been used to dye nylon fibres [10e12]. In effect, the acid leuco variant resembles a disperse dye in terms of its adsorption characteristics and, when oxidised at the end of dyeing, is converted into the insoluble parent
vat dye in situ within the nylon substrate.