Cross-linked polyethylene, commonly abbreviated PEX or XLPE, is a form of polyethylene with cross-links. PEX is made from high density polyethylene (HDPE). PEX contains cross-linked bonds in the polymer structure, changing the thermoplastic to a thermoset. Crosslinking is accomplished during or after the extrusion of the tubing. The required degree of cross-linking, according to ASTM Standard F 876-93, is between 65 and 89%. A higher degree of cross-linking could result in brittleness and stress cracking of the material.