CHANGE OF MECHANICAL-PROPERTIES OF CROSS-LINKED POLYETHYLENE DURING ITS THERMOOXIDATIVE AGING

The thermal aging of extruded samples of electron beam-crosslinked lin ear polyethylene has been studied in the 110-170-degrees-C temperature range. Short and long term changes of mechanical (tensile) properties were observed, and the corresponding mechanisms are discussed on the basis of complementary sol-gel and gravimetric measurements on whole s amples, and density, FTIR and tensile testing on microtome sections. T wo mechanisms are involved in short term changes: a post-crosslinking and a change of the crystalline morphology (for samples exposed above the melting point). The latter effect is especially important in the c ase under study where the polymer was crosslinked in the solid state a nd leads to a noticeable decrease (approximately 5 MPa) of the yield s tress. The long term changes take place after the end of the induction period of oxidation. They can be interpreted in terms of ductile-brit tle transition induced by structural changes occurring on the macromol ecular level (chain scission) as well as macroscopically (core-shell s tructure resulting of the diffusion control of oxidation kinetics). It appears that noticeable changes of ultimate elongation occur just bef ore the end of the induction period as detected by IR or DSC method.