Irradiation crosslinking of oriented plasticised PVC compounds
Plasticised polyvinyl chloride (PPVC) compounds were biaxially stretched, annealed in the drawn state and subsequently exposed to electron beam irradiation. During sample irradiation the crosslinking reaction was promoted by a radiation sensitising monomer, trimethylolpropanetrimethacrylate (TMPTMA), included in the PVC compound formulations. The influence of stabiliser type and level, TMPTMA level and irradiation dose on the network structure produced was investigated. A tin stabiliser, Stanclere TL, was selected for this study as it promoted the crosslinking reaction and it showed no interfering peaks in the wide angle x-ray diffraction (WAXD) traces which were used to follow the development of structural order. The appearance of gel, the material insoluble in THF, in irradiated oriented samples proved that a crosslinked structure was created. The gel formation increased with TMPTMA level and irradiation dose. The gel content was found to affect mechanical properties at elevated temperatures and produced an increase in the area shrinkage temperature. Sample thickness and the plasticiser content were found to have a major effect upon gel formation. The thicker the sample was, the smaller the quantity of gel that was formed. Samples containing 46.5phr plasticiser showed greater gel content than samples containing 25phr plasticiser as a result of the higher molecular mobility in the more plasticised samples. It was also found that the orientation of the film decreased the efficiency of network formation in PVC formulations with a 46.5phr plasticiser content but no significant change in gel content could be detected in those containing 25phr plasticiser. Tensile properties and impact strength at room temperature were slightly improved in irradiated samples. The modified structure resulted in an improvement in tensile strength and penetration resistance at higher temperatures and an increased area shrinkage temperature. Crosslinking did not affect room temperature recovery or crystalline orientation; however irradiation crosslinking reduced crystallinity.