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Deterioration of most plastic materials starts at the outer surface and may take the form of discoloration, pitting, exudation of ingredients, fiber prominence, and microcracking. The deleterious effects of weathering consist of a complex set of processes in which the combined action of ultraviolet (UV) light and oxygen are predominant. When the energy of an excited group or segment of a molecule cannot be released through a photophysical process, it causes the dissociation of a chemical bond to produce free radicals. This event, with or without the participation of oxygen, can lead subsequently to one or more chemical changes. Light-initiated degradation makes the plastic more susceptible to fracture by stress fatigue induced by changes in humidity and temperature; the resulting surface microcracks cause a loss in mechanical properties. Fiber prominence can be produced by stress fatigue alone, but its formation is accelerated by UV irradiation.
deterioration mechanisms, photo-oxidative degradation, photodegradation, weathering, plastic materials, plastic, polymers, chemical processes, physical, processes, environmental stress fatigue, surface microcracks, cracks, building materials, durability
Research officer, National Research Council of Canada, Ottawa, Ont.