Influence of climatic factors on the weathering of coated wood

To evaluate a new test method for accelerated natural weathering of exterior wood coatings and European standardization a round robin test was carried out on nine different locations in Europe. Part of the evaluation consisted of investigating the relationship between the weathering results and the meteorological conditions on each site. The aim is to develop a climatic index, reflecting differences in severity of climate in different areas within Europe with respect to the weathering of coatings on wood. Preferably the relationship should be uniformly applicable to all coatings systems, making it possible to transfer test results from one location to another using this climatic index, in order to reduce the need for natural weathering tests on different sites. From the preceding literature study, in which the climate elements relevant for ageing effects of exterior wood coatings have been characterised, it was concluded that a climate index relevant to this purpose should be composed of global irradiation, total precipitation and number of days with more than 0.1 mm precipitation. For all tested coating systems together the correlation between the calculated climatic index and weathering results appeared to be very poor. No unambiguous relation could be found, which was valid for all systems. Thus, transferring weathering results from one location to another, independent of the coating system, does not seem possible on the basis of these results. When seperating the results obtained from the standardized coating 'Internal Comparison Product (ICP)', the relationship was considerably better. Transferring results by means of the ICP could therefore constitute a more suitable approach. Of the climate factors involved, only global irradiation showed reasonable correlation to the weathering results. Including the data on precipitation hardly improved the model.