Welding of peroxide, silane, and electron beam crosslinked polyethylene pipes

An investigation has been carried out into butt welding and electrofusion w elding of crosslinked polyethylene (PEX) pipes, which had been crosslinked using the peroxide, silane, and electron beam processes (PEXa, PEXb, and PE Xc, respectively). A new hypothesis was proposed to explain the good weld q uality of PEXa pipes electrowelded with regular commercial couplers produce d from medium density polyethylene (MDPE) and 'third generation' high densi ty polyethylene PE100 materials. This mixed crystal hypothesis combined wit h the existing adhesion and flow process theories provides a theoretical ex planation of the welding behaviour of PEXa with MDPE and PE100 materials, b oth in the form of pipe and thin foils. It is based on modern views about t he importance of tie molecules needed for good long term properties. The mi xed crystal hypothesis postulates that the strength of a weld between MDPE or PE100 and PEXa depends on the formation of 'mixed crystals' which contai n chain parts from both the uncrosslinked and the crosslinked material. In these mixed crystals, thought to be formed on the welding plane during crys tallisation of the weld at similar to 115 degrees C, tie chains from both m aterials are incorporated, providing the strong physical bond between the t wo types of material needed to explain the good weld quality. To test the model, measurements on welded thin foils of MDPE and PEXa were carried out. Peel tests showed that for 70-90% crosslinking of PEXa, this p arameter does not affect the good weld qualities obtained. In addition, the presence of residual uncrosslinked molecular chains in PEXa is not a neces sary requirement for good weld strength. The good weld quality of PEXa pipe s electrowelded using MDPE and PE100 couplers is illustrated by scanning el ectron microscopy. However, the quality of electrofusion welds with PEXb an d PEXc pipes was unsatisfactory. Butt welding of PEXa pipes to MDPE pipes is still very difficult owing to f rozen in extrusion stresses in the PEXa pipes, which lead to poor weld qual ity. PRCPA/1544 (C) 1998 The Institute of Materials.