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, insulation for high tension (high voltage) electrical cables, and baby play mats. 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. Properties Low-temperature impact strength, abrasion resistance and environmental stress cracking resistance can be increased significantly by crosslinking, whereas hardness and rigidity are somewhat reduced. Compared to thermoplastic polyethylene, PEX does not melt (analogous to elastomers) and is thermally resistant (over longer periods of up to 120 °C, for short periods without electrical or mechanical load up to 250 °C). With increasing crosslinking density also the maximum shear modulus increases (even at higher temperatures). PEX has significantly enhanced properties compared with ordinary PE. Crosslinking enhances the temperature properties of the base polymer. Adequate strength to 120–150 °C is maintained and chemical stability enhanced by resisting dissolution. Low temperature properties are improved. Impact and tensile strength, scratch resistance, and resistance to brittle fracture are enhanced. Almost all PEX used for pipe and tubing is made from high-density polyethylene (HDPE). PEX contains cross-linked bonds in the polymer structure, changing the thermoplastic to a thermoset. Cross-linking is accomplished during or after the extrusion of the tubing. The required degree of cross-linking, according to ASTM Standard F876, is between 65% and 89%. A higher degree of cross-linking could result in brittleness and stress cracking of the material, while a lower degree of cross-linking could result in product with poorer physical properties. A theoretically extreme degree of cross-linking is reached if only –CH2– bridges are placed between cross-linkings that are also –CH2– bridges and if the polymer structure's sum formula is (C3H4)n, then the structure is that of a diamond with all bonds replaced by –CH2– bridges. If 4 vertices of a cube c are placed where C-atoms of the diamond base are so that only one further diamond base atom lies inside c then 4 diamond base bonds lie in c and—because the bond angles are forced towards such maximal values that also here a diamond cubic crystal structure is already approached—it is expected that the C-atoms of these –CH2–