Molecular Mechanisms of Disperse Dyeing of Polyester and Nylon Fibres

The kinetics of the dyeing of polyester fibres with disperse dyes have previously been discussed in terms of a model in which the only forces of interaction between dye and polymer are those that give rise to mutual solubility. The rate and extent of dyeing depend on the size and shape of the dye molecules and the detailed structure of the fibre. The concentration profiles of dye within single filaments of polyesters and nylons have now been determined by a new method termed ‘optical sectioning’, in which an image of a filament is scanned by a narrow slit. The dye distributions are found by comparing experimental transmission values with those calculated by a computer for a model system in which various parameters can be adjusted. For the polyester‐dye systems the rate constant of transfer of dye from dyebath to fibre ( k 1 ) was the same as the rate constant of diffusion away from the interface ( k 2 ). Some nylon‐dye systems, on the other hand, behaved as if k 2 = k 1 , whereas others behaved initially as if k 1 = k 2 but later as if k 2 > k 1 . Apparent diffusion coefficients were also derived from sorption measurements and were found to fall as the dye concentration in the fibre increased. Measurement of the orientation of the dye molecules by optical dichroism showed that the dye molecules entering the filaments late in the dyeing process were more highly orientated than the earlier ones. The decrease in diffusion coefficient has been interpreted in terms of the observed higher orientation, as resulting from a greater entropy of activation in the diffusion process.