Durability of polymeric coatings: effects of natural and artificial weathering

The durability of three commercially available polymeric coatings are investigated using slow positron beam techniques to monitor changes in sub nanometer defects during the degradation process. Doppler broadening energy spectra (DBES) measurements were performed on the slow positron beam at the University of Missouri-Kansas City. Results from UVB-313 artificial weathering are compared to the results for natural weathering for each of the coatings. It is shown that the slow positron beam technique is very successful in detecting the very early stages of coating degradation. In addition, the depth profile of the degradation process provides important information on the durability of the coatings and the kinetics of the degradation as a function of the depth from the surface. The artificial weathering technique is discussed as a predictor of the durability to natural weathering. It is shown that for each of the weathering systems differences are observed for each of the coatings. The loss of free volume during the degradation process is interpreted as being due to photo-oxidation of the polymer, with a net result of an increase in the crosslink density of the polymer.