Spatial Heterogeneity of Photo-Oxidation and Its Relation With Crack Propagation in Polyethylene Composites

The evolution of oxidation-depth profiles and section morphologies of high-density polyethylene (HDPE) composites during natural weathering have been investigated. Crack formation was found to be in good accordance with the development of the oxidation-depth profile. For stable composites such as HDPE/talc and HDPE/wollastonite, there was no obvious evidence of oxidation and no cracks appeared in the sections. For easily oxidizable composites such as HDPE filled with sericite, mica, kaolin, or diatomite, the oxidation depth profile increased with increasing exposure time, and cracks in the sections developed accordingly. Furthermore, determination of the molecular weight of the surface layer demonstrated that oxidation of the polyethylene chains caused significant chain scission. Recrystallization of the small molecules thus formed resulted in contraction of the surface layer, and the consequent tension led to the appearance of cracks. The transition region closely linked to the injection-molding process determined the thickness of the surface-oxidation layer, and hence the average length of the cracks. POLYM. ENG. SCI., 48:2270-2276, 2008. (C) 2008 Society of Plastics Engineers