Deterioration of a polyurethane coating studied by positron annihilation spectroscopy: Correlation with surface properties

Deterioration of a polyurethane coating by Florida natural environments as a function of time up to 16 weeks was studied by positron annihilation spec troscopy. Doppler broadening energy spectroscopy (DBES) of annihilation irr adiation and positron annihilation lifetime (PAL) were measured as a functi on of incident positron energy (0-30 keV). A significant decrease in the S- defect parameter from DBES and the intensity of orthopositronium from PAL w as observed as a function of weathering time. This is interpreted as a loss of free volume and holes as a result of the weathering process. The gloss and surface morphology in the same system were measured by glossimetry and atomic force microscopy (AFM), respectively. The gloss decreased and surfac e roughness increased as a function of weathering time. The AFM images show ed a new feature of a spherically coagulated microstructure on the surface after weathering. Direct correlations between the decrease in gloss and the increase in roughness as well as the decrease in the S-defect parameter fr om the DBES data and in the free volume from the PAL data were observed. Th ese results were used to discuss the weathering process in terms of chemica l and physical changes as a result of photodegradation in protective polyme ric systems. (C) 2001 John Wiley & Sons, Inc.