| Abstract: | Published evidence relating the rate of fading of a colorant to its physical state in the substrate, and to the photochemical layer effect, is discussed, and a new theoretical Model of light fading is proposed. In this the rate of oxygen diffusion from the atmosphere is considered, first in traversing the polymeric substrate, and then in entering an embedded colorant particle, under illumination. Expressions are derived which show that the rate of fading should vary with I/a2 (a = radius of particle), for large particles. As the particles become smaller, the rate tends towards a 1/a dependence, and eventually, with very small particles, it is independent of their size. Recently reported rates of fading of pigments of measured particle size gave results in agreement with the present treatment; the smallest particles thus considered are of the same order of magnitude as those of high light fastness direct dyes in cellulose, as observed by electron microscopy. Thus the model may be applicable to at least some water-soluble dyes as well as pigments. |
