An investigation into the uv breakdown of thermoset polyester coatings using impedance spectroscopy

The breakdown of powder polyester coatings when exposed to UV_b radiation (270-390 nm, peak ~313 nm) has been explored by monitoring changes in their ion transport properties using impedance spectroscopy. The properties of the coatings were measured immediately after formation and also after artificial weathering (with UV radiation, temperature cycling and high humidity). Impedance spectroscopy demonstrated that one manifestation of weathering was the development of an increased level of porosity in the films which could be measured quantitatively. The mechanism of breakdown of the coatings has been shown to be different for clear and pigmented systems, in that the clear coats showed very little change in porosity after initial detection whereas the pigmented systems failed by catastrophic cracking, probably right through the coating. After weathering, the pigmented films consist of a thin brittle surface layer over a thicker more elastic layer, whereas for the clear coatings there is a gradual decrease in brittleness with coating depth. The results from impedance spectroscopy were supported by SEM and gloss loss measurements.