# 1998 Gothenburg
An investigation has been carried out into butt welding and electrofusion welding of PEX pipes, which had been crosslinked using the peroxide, silane and electron beam processes (PEXa, PEXb and PEXc respectively). A new hypothesis was set-up to explain the good weld quality of PEXa pipes electrowelded with regular commercial couplers produced from MDPE and PE I00 materials. This "mixed crystal hypothesis" combined with the existing "adhesion" and "flow process" theories provides a theoretical basis to explain the welding behaviour of PEXa with MDPE and PE100 materials, both in the form of pipe and thin foils. It is based on modem views about the importance of tie molecules needed for good long-term properties. The mixed crystal hypothesis postulates. that the strength of a weld between MDPE or PElOO and PEXa depends on formation of "mixed crystals" which contain chain parts from both the uncrosslinked and the crosslinked material. In these mixed crystals, thought to be formed on the welding plane during crystallisation of the weld at around 1 1 5 "C, tie chains from both materials are incorporated. This provides the strong physical bond between the two 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 degrees of crosslinking of PEXa between 70 and 90%. this parameter does not affect the good weld qualities obtained. Moreover, the presence of residual uncrosslinked molecular chains in PEXa is not a necessary requirement to obtain a good weld strength. The good weld quality of PEXa pipes electrowelded using MDPE and PE100 couplers is illustrated by Scanning Elrclron Microscopy. However, the quality of electrofusion welds with PEXb and PEXc pipes was unsatisfactory. Butt welding of PEXa pipes to MDPE pipes is still very difficult, due to frozen-in extrusion stresses in the PEXa pipes, which lead to poor weld quality.