Electrochemical Impedance Spectroscopy on UV-Aged Polyester Coatings: Possibilities and Limits of Modeling Water Diffusion

The water uptake behavior of three different, white pigmented polyester topcoats after UV‐A testing is studied with electrochemical impedance spectroscopy. One topcoat is investigated more in detail to explore the influence of short testing times (<500 h). Diffusion coefficients derived from fitting the capacitance curves with a common equation containing a swelling factor show a dependence on the initial film thickness for UV‐aged and pristine coatings. It is suggested that diffusion coefficients should be calculated with the initial slope method to better compare the influence of testing on water uptake. Coatings with a thickness of 20 μm show sigmoidal capacitance transients, and in general, the diffusion coefficients decrease after UV‐A testing. This is attributed to a superficial layer, on top of the coating which is affected by photooxidation and which has a retarding effect for water uptake. The first 500 h of UV‐A testing seems to be most important for the amount of water taken up by the coating. After the entire experiment scanning electron microscopy (SEM) micrographs are made on cross‐sectional polished samples. SEM cannot reveal any changes in porosity after UV‐A testing. Thermomechanical analysis (TMA) does not show a significant change in glass transition temperature after UV‐A testing.