Reduction of Direct Dyes during the Dyeing of Viscose Rayon

The cause of reduction of certain direct dyes during the dyeing of viscose rayon at temperatures near the boil is the presence of alkali, and some direot dyes contain sufficient alkali to cause reduction of other direct dyes although they themselves may not be affected. With increase in temperature reduction increases, and at the boil considerably more reduction takes place than at 90dc. Dyeing in enclosed machines favours reduction owing to the restricted air space, but the effect of this can be overoome by blowing air. over the surface, or through the dye liquor. Lowering the pH of the dye liquor decreases the tendency of dyes to reduce during dyeing. Addition of such a substance as ammonium sulphate or chloride prevents reduction altogether even when dyeing at temperatures near the boil, in all but exceptional cases. The pH values of a number of aqueous dye solutions are listed, and a simple method is described whereby the susceptibility of direct dyes to reduction can be determined. A number of dyes have been tested, and are classified into four groups, according to whether they show considerable, moderate, slight, or no reduction.