Cross-linked polyethylene

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. 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. Low-temperature impact strength, abrasion resistance and environmental stress cracking resistance can be increased significantly by crosslinking, whereas hardness and rigidity are somewhat reduced. PEX does not melt any more (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. Almost all cross-linkable polyethylene compounds (XLPE) for wire and cable applications are based on LDPE.XLPE-insulated cables have a rated maximum conductor temperature of 90 °C and an emergency rating up to 140 °C, depending on the standard used. They have a conductor short-circuit rating of 250 °C. XLPE has excellent dielectric properties, making it useful for medium voltage—1 to 69 kV AC, and high-voltage cables—up to 380 kV AC-voltage, and several hundred kV DC. Numerous modifications in the basic polymer structure can be made to maximize productivity during the manufacturing process. For medium voltage applications, reactivity can be boosted significantly. This results in higher line speeds in cases where limitations in either the curing or cooling processes within the continuous vulcanization (CV) tubes used to cross-link the insulation. XLPE insulation can be modified to limit the amount of by-product gases generated during the cross-linking process. This is particularly useful for high-voltage cable and extra-high voltage cable applications, where degassing requirements can significantly lengthen cable manufacturing time. Various methods can used to prepare PEX from thermoplastic polyethylene (PE-LD, PE-LLD or PE-HD). The first PEX material was prepared in the 1930s, by irradiating the extruded tube with an electron beam. The electron beam processing method was made feasible in the 1970s, but was still expensive. In the 1960s, Engel cross-linking was developed. In this method, a peroxide is mixed with the HDPE before extruding, the cross-linking taking place during the passage of the melted polymer through a long heated die. In 1968, the Sioplas process using silicon hydride (silane) was patented, followed by another silane-based process, Monosil, in 1974. A process using vinylsilane followed in 1986. A basic distinction is made between peroxide crosslinking (PE-Xa), silane crosslinking (PE-Xb), electron beam crosslinking (PE-Xc) and azo crosslinking (PE-Xd). Shown are the peroxide, the silane and irradiation crosslinking. In each method, a hydrogen atom is removed from the polyethylene chain (top center), either by radiation (hν) or by peroxides (R-O-O-R), forming a radical. Then, two radical chains can crosslink, either directly (bottom left) or indirectly via silane compounds (bottom right).