The rates of dyeing were measured on 14 acid azo dyes on Amilan (nylon 6) in the infinite dyebath buffered in the range of pH 2-7, at the dye concentration between 1×10-5 and 1×10-8 eq/l, and at 59°C. The apparent diffustion coefficients D were obtained by using Hill's equation.
D, in each dye, varies with the equilibrium uptake of dye M∞. The decrease in D with the increased number of sulfonic acid groups and the enlargement of volume in the molecule (or affinity) of dye is observed, if θ=M∞/S <1 (S is saturation value), when compared with the same value of M∞ or θ. Since the variation in D with θ may be anomalous if θ>1, a comparison of the dyeing rates of different dyes, obtained at a constant concentration of dye and pH in the bath, does not give a regular relation between D and the molecular structure of dye.
In the range of pH investigated, the amino azo dyes with high pk-value produce ampho-ions in great quantities, leading to the significant irregularity in D-M∞ curves.