The Reduction of Vat Dyes in Printing Pastes

Abstract

The relation between the reduction properties of vat dyes measured under conditions akin to practice and their printing properties has been examined. Leuco potentials have been determined from electrometric titrations in aqueous solutions, and they have been found to govern the sensitivity of the dyes to the Formosul concentration. Complementary to these measurements are those in which the reducing strengths of the pastes have been determined from the potentials developed at a platinum electrode. These results have been related to the leuco potentials of the dyes, and it has been shown that the maximum potentials reached are adequate to reduce all the dyes. Their duration, however, is short for the low concentrations of Formosul and long for the high concentrations. This is due to decomposition of the reducing agent in the presence of gum, and some decomposition rates are given. The rates of reduction have been measured in print pastes using a model steamer. The times required for complete reduction cover a wide range, but for most dyes they are a substantial proportion of the time allowed for steaming, and must be considered when the printing efficiency of a dye is assessed. Finally, provided the dye is completely reduced, the maximum fixation is determined by its affinity; rates of diffusion in the fibre, solubility of the leuco dye in the paste, and acid leuco formation do not appear to be important.