Surface microcracking induced by weathering of polycarbonate sheet

Abstract

Polycarbonate sheet subjected to outdoor weathering for relatively short periods develops a network of surface microcracks on the side exposed to solar radiation. Artificial weathering and SEM microscopy were used to illustrate the process of microcrack formation. Microcracking occurs under the influence of light radiation in conjunction with cycling of either temperature and moisture or temperature alone. The use of radiation by itself, or even relatively severe cycling of temperature and humidity without radiation, does not induce microcracking. According to the proposed mechanism, the resin of the exposed surface undergoes, with weathering, a gradual reduction in strength owing to a lowering of its molecular weight as a result of photochemical degradation. Cyclic variation of temperature and humidity in natural and artificial weathering imposes on the surface material a type of stress fatigue. Such stress fatigue is caused by the non-uniform dimensional changes that result from thermal and moisture content gradients between the surface and the bulk of the resin and from inhomogeneities and defects. When the fatigue limit of the surface material at a microsite becomes lower than the physically induced stresses, the resin cracks.