Exterior durability: III

Exterior durability of coatings refers to their resistance to change during outdoor exposure; such changes include changes of modulus, loss of strengt h, embrittlement, discoloration, loss of adhesion, chalking, loss of gloss, and environmental etching. Thus, both aesthetic and functional properties are involved. The terms outdoor durability and weatherability are also used . This series examines exterior durability, focusing on the most common chemi cal processes leading to degradation of coatings-photoinitiated oxidation a nd hydrolysis resulting from exposure to sunlight, air, and water. These pr ocesses are interrelated, including enhanced photoxidative degradation in h igh humidity and enhanced hydrolytic degradation during photoexposure. Furt hermore, both processes are accelerated by higher temperatures. Hydrolytic degradation may be enhanced by exposure to acid, as from acid rain. Other a tmospheric degradants include ozone and oxides of nitrogen and sulfur. Chan ges in temperature and humidity may result in cracking, which arises from t he expansion and contraction of coatings or substrates. Rates at which thes e processes occur vary, depending on exposure site(s), time of year, coatin g composition, and substrate.