A mathematical model, describing coating degradation mechanisms of thermoset coatings exposed to ultraviolet radiation and humidity at constant temperature, was extended to simulate the behavior of a coating with a low glass transition temperature. The effects of adding light stabilizers (a UV absorber and a radical scavenger) to the coating were also explored. The extended model includes photoinitiated oxidation reactions, intrafilm oxygen permeability, water absorption and diffusion, reduction of crosslink density, absorption of ultraviolet radiation, a radical scavenger reaction, and simulates the transient development of an oxidation zone. Simulations are in good agreement with experimental data for a fast degrading epoxy-amine coating with a glass transition temperature of −50°C. It was found that the degradation rate of the non-stabilized coating was influenced significantly by the diffusion rate of oxygen in the oxidation zone, whereas light absorption by the photoproducts formed was only a secondary effect. On the other hand, the degradation rate of the stabilized coating was mainly influenced by the light absorption capability of the coating and in this case there was no oxygen diffusion resistance. Finally, simulations showed that the rate constants of the photoinitiating and oxidation reactions, taking place within the epoxy-amine family of coatings, are strong functions of the specific crosslinker used and must be estimated, in each case, by calibration of the model against adequate experimental data series.
Progress in Organic Coatings, 2013, Vol 76, Issue 12, p. 1730-1737