Polyolefin are polymer materials made from small alkene monomers. 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. Physical Chemistry and Mechanical Properties of HDPE High-density polyethylene (HDPE)(0.941