Studies in the Mechanism of Dyeing of Polyamide Fibers with Acid Dyes

Abstract

The dyeing of nylon 66 with five acid dyes of differing chemical constitution and affinities has been studied under dyebath conditions of pH 1 and 2 and at 60°. 68°, and 75° C. With all the dyes studied and under different conditions of application. the rate-of- dyeing curves can be resolved into three distinct portions: rapid dye uptake during the mitial stages, constant rate of uptake during the mtermediate stages, and very little further increase during the final stages of dyeing. The higher the temperature of dyeing. the lower the pH of the dye liquor. and the higher the affinity of the dye for nylon, the greater is the uptake of dye at each stage of dyeing. Simultaneous with dye adsorption. depolymerization of nylon takes place, as indicated by the fact that end amino and end carboxyl groups increase and the intrimsic viscosity of formic acid solutions containing dyed samples decreases. In the absence of acid dye, depolymerization of nylon is limited, but the presence of acid dyes catalyzes the depolymerization to an extent depending on the affinity of the dye for the fiber stibstance. In the case of mixture dyeing of nylon with Solway Green G and Lissamine Fast Yellow 2G, at each stage of dyeing the uptake of each dye from a mixed dyebath is always less than is obtamed in the absence of the other dye, and the number of end groups formed as a result of dyeing from a mixed dyebath is always less than the total number of end groups formed when the t\\o dye components are used singly. A probable mechanism mvolving dye combination at positively charged amino groups in nylon, dye uptake at charged amide groups, and adsorption through secondary valency forces of non-ionic type has been suggested to explain the dyeing behavior of acid dyes when apphed from dyebaths at low pH.