Why is weather resistance critical for unsaturated polyester resin in outdoor applications?

Why is weather resistance critical for unsaturated polyester resin in outdoor applications?
UPR's unsaturated double bonds and residual reactive groups make it prone to photo-oxidation under UV exposure, leading to yellowing, chalking, and mechanical degradation. This limits outdoor use in landscape sculptures, photovoltaic brackets, and automotive parts. Traditional stabilization relies on hindered amine light stabilizers (HALS) and antioxidants, but these offer limited long-term protection. Benzotriazole-type UV absorbers, such as UV-1351, absorb strongly in the 300-400 nm range while remaining transparent to visible light. They convert UV energy into heat and also scavenge free radicals, providing more comprehensive protection. This is increasingly important as UPR applications extend to infrastructure and exterior building components with 10-20 year service life requirements.