Degradation studies of crosslinked polyethylene. 2, Aged in water

Silane crosslinking of polyethylene was carried out by grafting an organofunctional silane (vinyltrimethoxysilane) onto polyethylene and by subsequent moisture crosslinking in hot water using a tin catalyst. This study focuses on the degradation processes, which occurred in the material after water ageing in an autoclave; ageing temperatures ranged from 90 to 190°C, while ageing times ranged from 2h to 500h. Significant changes in the chemical structure of the material were observed by FTIR, as carbonyl group concentration increased and different structures formed in the region of absorbance of groups containing silicone; the structural changes affected significantly the mechanical properties as shown by the tensile data. A chemical analysis of the extracts in chloroform of water aged samples carried out by using FTIR, LIMA and GPC techniques and some optical microscopy evidence, suggested that the mechanism of degradation in water is different from the one in air, as during water ageing the antioxidants are washed away by water and hydrolytic oxidation also occurs. ECHIP experimental design software was used in order to optimise the number of experimental trials and to model the results obtained. Keywords: PE-crosslink, water ageing, carbonyl ratio, silane, antioxidants.