Fading characteristics of some monoazo dyes on cellulose diacetate and polyamide films: Part II

The fading rate curves and the characteristic fading curves of seven azo dyes on cellulose diacetate (CDA) and polyamide (PA) films were examined. Fading was enhanced by prolonging the time of irradiation and decreased by increasing dye concentration on the substrate, irrespective of the nature of the latter or the dye used. However, the magnitude of fading was governed by structural characteristics of both dye and substrate. The results obtained suggested that dyes of higher ability to aggregate forming large and uniform particles were more resistant to fading than those forming smaller and/or a non-uniform size of dye aggregates. Similarly, substrates of higher polarity and porosity acted in favour of association of dye molecules, thereby impeding fading. These suggestions were, indeed, substantiated by measurements of the extinction $\text{y}\text{x}$ ratio after different irradiation periods.