The relation between the diffusion coefficient of acid dyes in nylon 6 fiber, and the amino end group content in the fiber (AGC) and the affinity parameter A was studied. The affinity parameter A, defined as the [slope of tangent of absorption isotherm at origin]/[AGC], corresponds to the first binding constant of the dye on the substrate. The diffusion coefficient was determined by the use of Hill's equation from the rate-of-dyeing curve. AGC was found to be 25, 54 and 95meq./kg for Nylon (1), (2) and (3), respectively. Results are as follows.
1) The diffusion coefficient is nearly constant in the range θ=0.35-0.50, where θ is the fraction of AGC occupied by the acid dye. This result is in contrast with the well known results that the diffusion coefficient of acid dye in nylon depends on θ.
2) The diffusion coefficient at pH=4-6 decreases with increasing AGC.
3) The diffusion coefficient depends on the shape, size and the dissociation state of the dye.
4) Affinity parameter A increases as AGC is increased and the diffusion coefficient of the dye used at various pH decreased with increasing the parameter A.
From these results, we conclude that the main factor which influences the diffusion coefficient is the affinity parameter A.