Accelerated weathering of unsaturated polyester resins. Aspects of appearance change

This article describes aspects of appearance change during accelerated photoageing and weathering of a styrene crosslinked unsaturated polyester (UP) resin. The influence of oxygen, cure conditions, spectral irradiance and humidity on discolouration has been determined, and the role of moisture and oxygen on the gloss loss was studied. Discolouration kinetics of the cured resin is comparable both in the absence and in the presence of oxygen. Wavelength dependency of the discolouration process was determined using the sharp cut-on filter technique. Using cobalt compounds as peroxide decomposition catalysts induces a shift in UV-absorption to longer wavelengths in the cured resin and changes the mechanism of photo-yellowing. Gloss loss occurs after much longer exposure times compared with discolouration and is almost absent in inert atmosphere. Moisture appears to be irrelevant for the discolouration process but plays a crucial role in the mechanism of gloss loss. Extraction of products of photooxidation and/or hydrolysis and subsequent extraction of photoproducts are the main cause of gloss loss in weathering conditions.