The influence of the exposure conditions on the chemical and physical changes of polyester-urethane coatings during photodegradation

The influence of the conditions of artificial degradation experiments on the photodegradation process of polyester-urethane clearcoats has been studied by comparing three types of exposure experiments with different conditions regarding the spectral power distribution (SPD), the exposure atmosphere (aerobic and anaerobic) and the presence of water. The presence of short wavelengths (λ < 295 nm) in the SPD largely influences the depth-inhomogeneity of degradation with respect to the optical properties and the chemical composition of the coating. The availability of oxygen in the exposure atmosphere determines the degradation pathway that is followed, such as to what extent the photo-oxidative breakage of urethane bonds or the formation of yellow chromophores due to aromatic crosslinking reactions occurs. Indentations at the surfaces of virgin and degraded coatings showed an increase in Young's modulus and hardness when degraded under aerobic conditions, while degradation under anaerobic conditions did not lead to significant changes. The presence of water is responsible for increasing the surface roughness of the coating during degradation, which directly influences the coating's gloss retention. Several time-independent correlations between the changes in chemical, optical and mechanical properties of coatings resulting from different exposure experiments have been established.