Exterior durability: II

Exterior durability of coatings refers to their resistance to change during outdoor exposure; such changes include changes of modulus, loss of strength, 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 chemical processes leading to degradation of coatings - photoinitiated oxidation and hydrolysis resulting from Exposure to sunlight, air, and water. These processes are interrelated, including enhanced photoxidative degradation in high humidity ann enhanced hydrolytic degradation during photoexposure. Furthermore, both processes are accelerated by higher temperatures. Hydrolytic degradation may be enhanced by exposure to acid, as from acid min. Other atmospheric degradation include ozone and oxides of nitrogen and sulfur. Changes in temperature and humidity may result in cracking, which arises from the expansion and contraction of coatings or substrates. Rates at which these processes occur vary, depending oil exposure site(s), time of year, coating composition, and substrate.