MATERIAL WEAR AND EROSION AT THE SURFACE OF PLASTICS CAUSED BY WEATHERING

When exposed to the weather (moderate climate, Darmstadt), polymethylmethacrylate (PMMA) experiences weight changes, depending on the seasonal humidity, but no material loss. Polycarbonate (PC) on the other hand, and to a lesser degree PC with a “weather skin”, shows marked weight changes when exposed to artificial weathering (Xenotest 1200), which set in with some delay. Surface profile measurements across the borderline between exposed and covered material provide no evidence of material wear in the case of PMMA. With PC, and to a lesser degree with PC protected by a “weather skin”, wear does occur in the region of several μm. The extent of this material wear, which normally sets in with considerable delay after first exposure, is in good agreement with our weight loss measurements. The correlation between outdoor weathering and Xenotest weathering period (shortest wavelength 300 nm) is linear for all degrees of wear (time acceleration factor 13). Contrary to PMMA, scanning electron micrographs of PC show degradation morphologies dependent on the type of weathering which first develop after a certain weathering period. The degradation of PC occurs in two steps. Immediately after first weathering an embrittlement of the surface sets in, caused by molecular weight reduction and the formation of short‐chain degradation products. Moreover, as a result of weathering‐related dry/wet cycles and temperature fluctuations, the reduced elongation at break is exceeded locally and eventually cracking at the surface occurs, surface roughness and material loss increase and the gloss reduces. © 1990 Hüthig & Wepf Verlag, Basel