The role of auxiliaries in the immersion dyeing of textile fibres: Part 9 practical aspects of the role of inorganic electrolytes in dyeing cellulosic fibres with pure reactive dyes

The colour strength, as well as the extents of both exhaustion and fixation, achieved for three pure reactive dyes on cotton were promoted by the addition of 50 gl⁻¹ NaCl at each of seven liquor ratios employed (1:50, 1:20, 1:10, 1:6, 1:3, 1:2 and 1:1.5). The depth of shade of dyeings obtained in the absence of added electrolyte using a1:1.5 liquor ratio were of similar magnitude to those secured using 1:10–1:20 liquor ratios in the presence of 50 gl⁻¹ NaCl. The promotional effect on dye uptake imparted by both added electrolyte and reduced liquor ratio was interpreted in terms of their effects on the substantivity of the reactive dyes towards the cotton substrate. The two, seemingly different actions of adding electrolyte such as NaCl or Na₂SO₄ to the reactive dye dyebath and reducing the liquor ratio used for immersion dyeing have the same result, namely that of encouraging dye aggregation in the dyebath, which reduces the solubility of the direct dye in the dyebath, which, in turn, shifts the inherent preference of the dye to favour the aqueous phase towards the fibre phase. It is thus possible to dye cotton using pure reactive dyes in the complete (ie 100%) absence of added inorganic electrolyte. The ensuing dyeings were of realistic depths of shade and displayed excellent levels of wash fastness.