PHYSICAL ASPECTS OF THE THERMAL-OXIDATION OF CROSS-LINKED POLYETHYLENE

The thermal oxidation of radiation crosslinked polyethylene (XLPE) was studied at 130, 150, 160, 170 and 180-degrees-C. By FTIR analysis of microtome sections, it was established that carbonyl formation after the end of the induction period occurred only in a superficial region whose thickness was a decreasing function of the temperature as predicted by a simple kinetic model of the diffusion controlled process. The density increases in the oxidized layer to reach values of about 1.2 in the surface layer. This is consistent with the hypothesis of incorporation of about one oxygen atom per three carbons whereas IR analysis gives about one carbonyl per 30 carbon atoms (shortly after the end of the induction period). This density increase is accompanied by a weight loss. A simple calculation shows that the quantity of oxygen evolved in volatile oxidation products is of the same order of magnitude as the quantity of oxygen atoms found in the surface layer. Density increase and weight loss result in shrinkage, whose consequences for subsequent mechanical behaviour must be considered.