Degradation of polymer coating systems studied by positron annihilation spectroscopy. 3. Wavelength dependence of UV irradiation effect

Photodegradation of a polyurethane coating induced by accelerated UV irradiation is studied using three different light sources: 340 nm UVA, 313 nm UVB, and Xe lamps. Doppler-broadened energy spectra (DBES) and positron annihilation lifetime (PAL) are measured as a function of incident positron energy (0-30 keV) and of irradiation time (0-800 h). The photodegradation of the coating is characterized in terms of subnanometer defect changes. Significant variations of S parameter, orthopositronium lifetime, intensity, and lifetime distribution are observed as a function of depth and of exposure time. These results indicate a decrease of free volume and holes at the atomic level during the degradation process. The UV irradiation at 313 nm shows a significant effect compared with that at 340 nm on the photodegradation: (1) the magnitude of S reduction increases a factor of 2 near the surface; (2) the photodegradation rate increases similar to 25 times near the surface; (3) the irradiation effect attenuates with depth, and a larger extinction coefficient is observed with shorter wavelength UV irradiation. The observed effect by Xe light irradiation is the strongest among these three light sources. Enhanced degradation at shorter wavelengths is also indicated by increased free radical production, as observed by electron spin resonance spectroscopy.