Cross-linked polyethylene, commonly abbreviated PEX, XPE or XLPE, is a form of polyethylene with cross-links. It is used predominantly in building services pipework systems, hydronic radiant heating and cooling systems, domestic water piping, and insulation for high tension (high voltage) electrical cables. It is also used for natural gas and offshore oil applications, chemical transportation, and transportation of sewage and slurries. PEX is an alternative to polyvinyl chloride (PVC), chlorinated polyvinyl chloride (CPVC) or copper tubing for use as residential water pipes.
A low degree of crosslinking leads initially only to a multiplication of the molecular weight. The individual macromolecules are not linked and no covalent network is formed yet. Polyethylene that consists of those large molecules behaves similar to polyethylene of ultra high molecular weight (PE-UHMW), i.e. like a thermoplastic elastomer.
Upon further crosslinking (crosslinking degree about 80%), the individual macromolecules are eventually connected to a network. This crosslinked polyethylene (PE-X) is chemically seen a thermoset, it shows above the melting point rubber-elastic behavior and can not be processed in the melt anymore.
The degree of crosslinking (and hence the extent of the change) is different in intensity depending on the process. According to DIN 16892 (quality requirement for pipes made of PE-X) at least the following degree of crosslinking must be achieved:
· in peroxide crosslinking (PE-Xa): 75%
· with silane crosslinking (PE-Xb): 65%
· with electron beam crosslinking (PE-Xc): 60%
· in azo crosslinking (PE-Xd): 60%
PEX-AL-PEX pipes, or AluPEX, or PEX/Aluminum/PEX, or Multilayer pipes are made of a layer of aluminum sandwiched between two layers of PEX. The metal layer serves as an oxygen barrier, stopping the oxygen diffusion through the polymer matrix, so it cannot dissolve into the water in the tube and corrode the metal components of the system.[34] The aluminium layer is thin, typically 1 or 2 mm, and provides some rigidity to the tube such that when bent it retains the shape formed (normal PEX tube will spring back to straight). The aluminium layer also provides additional structural rigidity such that the tube will be suitable for higher safe operating temperatures and pressures.
The use of AluPex tubing has grown greatly since 2010 onwards. It is easy to work and position. Curves may be easily formed by hand. Tube exists for use with both hot and cold water and also for gas.
