Activation Energies of Light Fading of Non-ionic Disperse Dyes in Hydrophobic Polymers

Abstract

Eight non–ionic (disperse) dyes adsorbed in films of cellulose acetates, nylon and poly(ethylene terephthalate) (PET) have been faded by visible and near–ultraviolet radiation at temperatures in the range 15 to 100°C, and in atmospheres of relative humidities (r. h.) from zero to saturation. Fading is accelerated by rise in temperature, and, at r. h. above 5%, by rise in r. h. Activation energies of fading (Ef) (r. h. > 5%) range between about 3 and 18 kcal/mole, and depend on the nature of the substrate rather than on the structure of the dye. Values rise with increasing crystallinity of the polymer structure, except for PET which gives anomalously low values. The extent of dye–fibre interaction is at a minimum with PET and the rate–controlling step is a purely photochemical one. With the other substrates, the rate of fading is controlled by the dye–substrate interaction. The accelerating action of atmospheric humidity over normal ranges is partly due to its influence on the dye–substrate bonding and partly to its effect in swelling the substrate and thus increasing the diffusion coefficient of oxygen.