Artificial aging of acrylurethane and alkyd paints: a micro-ATR spectroscopic study

The degradation of two paints during different standard tests was studied using micro-ATR spectroscopy. Standard test conditions do not give the same relative degree of aggressiveness depending on the binder. The IR results obtained with an acrylurethane top coat reveal that the degradation mostly proceeds through a direct or radical-induced scission of the urethane linkage. The chemical heterogeneity of the aged specimens was clearly underlined from high dispersal of their IR response. The advance in chemical degradation was found to be directly related to the UV light dose received by the samples, hence showing negligible influence of other exposure parameters (moisture, industrial atmosphere…). In contrast to the acrylurethane top coat, the behaviour of a glycerophthalic alkyd system appears to be dependent on exposure conditions other than UV radiation exposure. In that case, it was shown that with standard test cycles alternating UV, heat, moisture and SO₂ atmosphere exposures, the chemical change of the alkyd system was controlled by acidic hydrolytic backbone degradation of the phthalate moieties, hence resulting in chalking. This result was confirmed by an estimate of degree of chalking from IR results: chalking of the paint was related to the increase in the mineral IR absorption response relative to the organic one of the aged samples.