Microstructural Characterisation of Polyethylenes for Pipe Application [PE]

A detailed investigation has been carried out on the microstructure of various polyethylenes recently developed for applications in the pipe industry. Of particular interest have been the PElOO formulations, and samples of these have been studied both from commercial piping and from laboratory produced plaques. Attempts have been made to relate the crystallinity and morphology to the cooling rates. In order to achieve some control over the cooling rates, a special laboratory-based rig was constructed to allow the compression moulding of polyethylene plaques containing embedded thermocouples. This equipment was designed to enable plaques to be produced at the rapid cooling rates that occur industrially, as well as slower rates, and it was therefore possible to provide a broad range of samples for analysis and comparison with the through pipe microstructural variations. Numerous etching techniques were used to characterise the morphologies of the various polyethylenes, and some striking differences between the materials were observed. Some of the samples showed the typical spherulitic microstructure normally associated with polyethylenes crystallised from the melt. However, it was found that, despite having relatively high crystallinities, some materials, even at the slowest cooling rates, did not exhibit such a regular arrangement of the lamellae, but only displayed a wheatsheaf-like arrangement reminiscent of embryonic spherulites. Attempts have been made to understand this behaviour by collecting data from various other physical techniques such as DSC, molecular weight measurements etc.