The role of auxiliaries in the immersion dyeing of textile fibres: Part 5 practical aspects of the role of inorganic electrolytes in dyeing cellulosic fibres with direct dyes

Both the colour strength of 2% omf dyeings and the extent of dye exhaustion achieved for three commercial grade direct dyes on cotton were promoted by the addition of 20 gl−1 NaCl at each of eight liquor ratios (1:50, 1:20, 1:10, 1:6, 1:3, 1:2, 1:1.5 and 1:1). The extent of electrolyte-enhanced dye uptake decreased with decreasing liquor ratio. For each of the three dyes, unlevel dyeings were obtained at 1:1 liquor ratio only in the presence of added electrolyte, whereas level dyeings were secured even at 1:1 liquor ratio in the complete absence of added electrolyte. The depth of the dyeings obtained in the absence of added electrolyte using both 1:1 and 1:1.5 liquor ratios were of similar magnitude to those secured using 1:10, 1:20 and 1:50 liquor ratios in the presence of 20 gl−1 NaCl. The promotion of dye uptake imparted by both added electrolyte and reduced liquor ratio were interpreted in terms of their effects on the substantivity of the direct dyes towards the cotton substrate. It is proposed that adding electrolyte to the dyebath and reducing the liquor ratio employed for dyeing have the same consequence in terms of enhancing dye uptake, namely that of encouraging dye aggregation in the dyebath which reduces the aqueous solubility of the anionic dye, which, in turn, results in the inherent preference of the dye to favour the aqueous phase shifting towards the fibre phase.