More Predictive Spray/Wash Procedures for Xenon Arc Weathering

ABSTRACT The gentle, low-volume water sprays in commonly used accelerated weathering devices are unlike rain, and stable materials such as pigments (especially carbon black) can remain adhered to the surface as a polymer surface erodes during weathering. Data show that such residue does not accumulate under natural weathering conditions, presumably because of the washing effects of rain and wind, thus leading to poor correlation of accelerated weathering to outdoor exposures. We have investigated cycles with less frequent but longer duration sprays in the dark, standard and more aggressive nozzles, and periodic sponge washing on a series of samples that generally perform well outdoors but poorly under standard accelerated weathering protocols. Color shift and gloss loss were compared with Florida data assuming a correlation of approximately 3 MJ/m2/nm at 340 nm of daylight type I xenon arc exposure per year of Florida exposure. Color shifts of nonblack colors were well predicted under all conditions. Changing the spray cycle generally improved the predictability of gloss retention and color shifts of black samples. Adding the washing protocol improved the predictability much more, especially for black samples, but adding washing to the traditional 102-min light/18-min light and spray cycle resulted in much smaller improvements. The more aggressive spray nozzle was only slightly better than the standard misting spray nozzle. The results suggest that further development of the washing protocol is needed to improve predictability of gloss retention for samples containing large particle size pigments and that a modified protocol within the scope of current standards and in existing equipment can give much more predictive results for materials that undergo erosion during weathering.