Non-Destructive Evaluation of Urethane-Ester Coating Systems Using the Scanning Kelvin Probe Technique

The research described herein examines the effect of hydrolytic decomposition on the work function of polyurethane rain erosion coatings (REC). Two groups of panels, one group coated with REC containing carbon black and one group coated in REC without carbon black, were exposed to high temperature and humidity conditions in a steam autoclave for various periods of time. The work function of each unique exposure time was measured with a scanning Kelvin probe (SKP) and compared to unexposed coated panels to determine the magnitude of change in the REC coating. For both sample groups, results from the SKP analysis showed that as the coated panels were exposed for longer periods of time, the work function increased compared to an identical panel that was not exposed. The extent of the polymer coating degradation was determined by Raman spectroscopy and thermomechanical analysis (TMA). Raman analysis indicated a decrease in ester bonds, while with TMA a decrease in polymer strength was observed with increasing exposure time. These results demonstrate that there is decomposition of the REC occurring from the exposure and it is due to hydrolysis of the ester bonds. The change in REC structure/composition was able to be detected by the Kelvin probe in a non-destructive manner, making it a valuable tool in assessing the decomposition of polyurethane rain erosion coatings.