Some Experiments on the Overdyeing of Nylon

Rate‐of‐dyeing curves have been measured for nylon dyed under very acid conditions (pH ca. 1.5). The curve falls into two parts; in the initial stages the uptake is rapid, and this is followed by a slow increase in dye concentration which does not reach a maximum. By stripping the dye from the fibre and redyeing under conditions of high pH, the number of free amine ends has been estimated. From the results it has been shown that during the second part of the rate‐of‐dyeing curves hydrolysis occurs, giving an increased number of amine groups. The amount of dye taken up at the low pH values is, how‐ ever, far in excess of that required to saturate the amine group and must be accounted for by “amide group” dyeing. Viscosities have also been used to confirm the hydrolysis. The picture of the dyeing process at low pH is, therefore, initial absorption of dye on to sites provided by the peptide groups either as the free dye acid or by absorption of the hydrogen ions in the dyebath followed by the dye anion. Simultaneous with the absorption is a slow but definite hydrolysis of the nylon molecules with consequent degradation of the fibre.