Abstract
Understanding the degradation phenomena of a coating is paramount in determining its service lifetime. A coating may experience a multitude of extreme environments; humidity, temperature and UV radiation, with UV radiation being the acute cause of coating degradation. With aircraft flying at altitudes between 9 and 13 km, their exposure to UV and ozone rise significantly. However, the effects of UV and ozone combined are seldom explored within literature. A polyurethane (PU) military aircraft topcoat was exposed to UV radiation, within a QUV chamber, and to UV/ozone, using a novel method of exposure that was developed herein. To compare the traditional exposure test with the novel approach, the coatings were analysed with x-ray photoelectron spectroscopy (XPS), time of flight secondary ion mass spectrometry (ToF-SIMS) and atomic force microscopy (AFM). Significant degradation is observed within 2 minutes of UV/ozone exposure which equates to 56 days of UV degradation. The main changes observed are the rise of inorganic components with exposure time, as the polymer resin decomposes. The topography also shows an increase in roughness at the microscale level. By combining ToF-SIMS with principal component analysis (PCA) a more detailed analysis of the data can be obtained. PCA analysis enabled the positive identification of cluster ions representing the degradation product as well as the reactant ions. The degradation product was identified to be an azo compound, which enabled a degradation mechanism to be proposed. Using the detail provided by the surface analysis, two successful techniques have been developed for the examination of degradation of in-service coatings through an economic, straight forward device. One, is through the use of a portable ATR-FTIR spectrometer and applying PCA to the collected data. Two, using a more simplistic economic approach, through UV irradiance, where increasing degradation displays greater fluorescence upon UV irradiance.