Temperature dependence of structural alteration by ultraviolet irradiation in acrylic-urethane coatings studied by positron annihilation spectroscopy and solvent swelling behavior

Abstract The structural alteration of acrylic-urethane coatings, induced by photo-aging, and its temperature dependence, was investigated using positron annihilation lifetime spectroscopy and solvent swelling experiments. The results revealed a slight structural alteration due to photo-aging at 20 °C. On the other hand, a significant structural alteration occurred due to photo-aging at 60 °C, which resulted in a decrease in the pore size and total pore volume. To study the mechanism of such morphological changes, infrared spectra were measured before and after aging the samples. The results revealed the breakage of the urethane crosslink points and the generation of various photo-products including peroxide species that can form hydrogen bonds. This indicates the foundation of new physical and chemical crosslinks (different from the initial urethane linkage). Based on the chemical modifications, probable mechanisms of the internal morphological changes are proposed. Moreover, the changes are mainly due to an enhancement in the chain-mobility, due to the destruction of the polymer network and an increase in the inter-chain interactions. The results of this study reveal that significant internal structural alterations can occur after sufficient damage to the initial network structure. The proposed explanation is reasonable in relation to the temperature dependence of photo-induced structural alterations, and with respect to the molecular mobility and reaction ratio of photo-oxidation.