Site-specific weathering of coatings: II. Comparison of natural weathering and accelerated protocols for Central Europe and Florida

Abstract

Weathering protocols, which anticipate exposure behavior of coatings in the temperate climate in Central Europe (ZykStuttgart: ZS) and Florida (new protocol: NP), were applied for accelerated testing of melamine-, silicone resin/acrylate dispersion-, and polyurethane-based coating systems and compared to natural weathering at related sites in Stuttgart (Germany) and near Homestead (Florida/USA), respectively. Color parameters and infrared spectra were measured after defined exposure times and normalized to the irradiation doses. Color changes of an industrial standard with an orange pigment (ORWET) were found to depend for both artificial and natural weathering tests predominantly on irradiation dose. This confirms that the spectral power distribution of the lamp/filter configuration proposed in NP matches sufficiently well to both the sunlight spectra in Florida and Stuttgart. For a melamine-based standard crack coat, crack initiation was found only for NP and the late stage of ZS. For all coating systems studied, characteristic band intensities of infrared spectra and/or color parameters for the two protocols and from natural weathering at the related outdoor sites were shifted along the time axis to merge to master plots. To differentiate between thermally activated, photo-initiated, and hydrolytic degradation, the shift factors were plotted versus inverse annual- or cycle-averaged temperatures including irradiation ratios and averaged humidity ratios. Despite the different reaction mechanism for the acrylic component in the silicone resin/acrylate and the polyurethane coating, a dominance of thermally activated degradation was evident for both. The melamine-based binder showed coupled degradation mechanisms (photo-induced and thermally activated complimented by moisture effects). The use of irradiation doses, cycle-, and annual-averaged temperatures together with cycle- and annual-averaged humidities for data analysis provided simple possibilities for comparison of protocols and weathering sites and a rough differentiation between degradation mechanisms.