Characterisation of peroxide crosslinked polyethylene

British Gas have been using polyethylene pipe and fittings for gas distribution since 1969. The medium density, and more recently high density polyethylene pipes are produced by a simple and well established extrusion process. Unfortunately the production of fittings, especially large diameter fittings is not so simple. Traditionally pipe fittings are made from either injection moulding or by other fabrication techniques which require subsequent cutting and welding of different parts to produce the finished article. Recently however the methodology has been developed to facilitate the production of fittings by rotational moulding. Incorporated into the new methodology is the ability to produce crosslinked fittings by peroxide inclusion. Crosslinking of the pipe fittings has a major advantage in that it produces a substantial improvement in the mechanical properties of the polymer. Of particular importance is the improvement in stress crack resistance. Stress cracking is presently the primary mode of failure in polyethylene pipelines. Cross linking should help to reduce the susceptibility of the pipeline to failure by this method. The purpose of this project has been to investigate the crosslinking process in two rotational moulding grades of polyethylene. In particular a study has been made of the relationships between various chemical and physical properties, and of how these change upon material modification. A series of compression moulded samples containing increasing levels of peroxide have been produced for both polymers. Characterisation of these samples was undertaken using such techniques as Differential Thermal Analysis, Fourier Transform Infrared Spectroscopy, Gel Permeation Chromatography, Gel Content and Microscopy. The results have shown that as peroxide concentration is increased gel content rises sharply before reaching a maximum value. In contrast to the gel content, the crystallinity of the samples was shown to decrease. Infrared analysis provided the facility to monitor the variation in molecule end group concentration with changing peroxide levels. It was found that increasing the level of peroxide resulted in a decrease in the concentration of terminal vinyl unsaturation in an inverse relationship to gel content results. Subsequent gel permeation chromatography analysis demonstrated that the terminal vinyl groups were being lost in a chain extension mechanism which resulted in an increase in the molecular weight of the samples