Use of ESR spectroscopy for detection of photodegradation of coating and their components
The selection and development of coating systems for outdoor application requires testing of their weathering durability. Since ""natural"" outdoor weathering is quite time-consuming, use of accelerating artificial (laboratory) weathering tests is common in industrial practice. Within the last century, several significant achievements were made regarding improvement of the correlation of such tests with outdoor weathering. However, it became apparent that accelerating such tests by intensifying the impact of the loading parameters is generally critical regarding preservation of the correlation to outdoor weathering. In contrast, use of highly sensitive detection techniques monitoring coating degradation already in its early stages does not affect the degradation mechanism and may contribute to shorter weathering duration, if the results of these detections allow reliable conclusions on the degradation progress even before its consequences become apparent macroscopically. This suggests searching for such highly sensitive detection techniques, which deliver reliable data about the progress of degradation even already in its early stages. Such methods should be already available, applicable in industrial practice, and their data should be interpretable reliably. In case, such methods could be found, it would be highly relevant determining their potential regarding early recognition of initiating weathering defects. We think that Electron Spin Resonance (ESR) spectroscopy could be such a method. Since ESR spectroscopy detects species with unpaired electron spin exclusively, and since radicals are such species and are the earliest possible messengers of an initiating photodegradation, ESR spectroscopy could principally be used for an early recognition of coating degradation induced by weathering. Within the work to be presented, two preparation methods are introduced, by which samples for determining the photodegradation tendency of coating systems as well as of their components can be manufactured in a way, which allows a reproducible ESR measurements. ESR monitoring of coating systems during a weathering period (e.g. in outdoor exposure) followed by a quantitative and exposure time resolved interpretation of this data resulted in the finding, that the progression of the radical intensities detected indicates the photodegradation processes occurring in the respective degradation phase. Thus, it became possible to gain information about the progress of degradation of each coating system at any time of exposure, even before first damages of the coatings became apparent. Moreover, dispersing pigments in a test binder matrix and adding some spin trapping agents, UV-induced formation of radicals initiated by the adsorption of the pigments can be detected by ESR spectroscopy even at room temperature. Thus, a reliable estimate about the photocatalytic effect or (alternatively) the extent of "UV innocence" of pigments, fillers etc. becomes accessible.