Exterior durability: II

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 chemic al processes leading to degradation of coatings - photoinitiated oxidation and hydrolysis resulting from Exposure to sunlight, air, and water. These p rocesses are interrelated, including enhanced photoxidative degradation in high humidity ann enhanced hydrolytic degradation during photoexposure. Fur thermore, both processes are accelerated by higher temperatures. Hydrolytic degradation may be enhanced by exposure to acid, as from acid min. Other a tmospheric degradation include ozone and oxides of nitrogen and sulfur. Cha nges in temperature and humidity may result in cracking, which arises from the expansion and contraction of coatings or substrates. Rates at which the se processes occur vary, depending oil exposure site(s), time of year, coat ing composition, and substrate.