Fusion Welding of Crosslinked PE [PE]

This paper summarises a comprehensive study examining the factors which influence the weldability of crosslinked polyethylene W E ) whilst gaining an understanding of the nature ofjoints at a molecular level. Butt welding of XLPE to XLPE was investigated using both chemical @eroxide)and radiation crosslinked PE. The compatibility of both types of XLPE with all of the conventional PE pipeline grade materials was assessed using both bun and electrohion welding. The findings were used to weld a selection of large diameter, thick walled fittings, manufactured using the low production cost rotational moulding technique. Materials used were high flow, low grade polyethylene's which were subsequently crosslinked during production by peroxides or after production by irradiation. The crosslinked fittings produced bad improved stress crack resistance and were welded to conventional MDPE. This work led to the development of the interface layer technique of jointing, which involves the bun welding of two XLPE pipes interposed with a layer of MDPE. Methods of introducing the MDPE interface layer were developed into a single stage butt weld operation using pre-formed MDPE caps. It was concluded that XLPE could best be described as a modified thermoplastic with chemical linkages introduced between the long polymer chains to create a three dimensional network. The resulting material retains many of the characteristics of the parent polymer but some physical properties can be drastically changed. In particular, flow of the material above its melting point can be prevented, effectively changing a thermoplastic material into an elastomer. This limited mobility of the bulk material is the cause of faulted butt welds between XLPE and XLPE. Although having slits in the bead region, centrally, strong bonding was observed, indicating the possibility of material flow on a micro level. This can be aided by more precise weld preparation, reduced dwell times and jointing to a higher flow thermoplastic. Experimentation has proved that although crosslinked materials do not flow in their melt state, they can still be welded. Microshuctural examination has shown that this is only made possible due to an ability for molecular scale movement, albeit limited. The scale of this effect varies with the crosslinking process and more specifically with the actual crosslink sites. It is believed that chemical XLPE will in general, be more difficult to weld than radiation XLPE. This is due to the crosslinks occurring in the amorphous regions only, which become more prevalent at the weld interface during welding. This will funher hinder the already sensitive wetting and diffusion reactions taking place at the weld interface on a molecular level, the ability and existence of which results in bonding.