Information AboutPex |
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PEX is the common name for Cross-linked high-density Polyethylene . PEX is formed into tubing, and is used predominantly in hydronic Radiant Heating systems. It is also used in natural gas applications, offshore, oil and chemical transportation, and transportation of sewage and slurries. PEX contains Cross-link bonds introduced into the polymer structure, changing the thermoplast into an Elastomer . The high-temperature properties of the polymer are improved - adequate strength is maintained to 120-150 °C, its flow is reduced. Its chemical resistance is enhanced, it does not dissolve. Its low-temperature properties are enhanced as well. Its impact and tensile strength and scratch resistance are increased and its tendency to brittle fracture is reduced. The advantageous properties of this material make it a candidate for progressive replacement of metal and thermoplastic pipes, especially in long-life applications, as the expected lifetime of PEX pipes reaches 50-200 years. Almost all PEX is made from High Density Polyethylene which undergoes crosslinking during extruding of the tubing. The required degree of cross-linking, according to ASTM Standard F 876-93, is between 65-89%; too high degree would lead to brittleness and stress cracking of the material. The first PEX material was prepared in 1930's, using irradiation by an electron beam. This process was made feasible in 1970's, but was expensive; the cross-linking was introduced by irradiating the extruded tube. In 1960's, Engel cross linking was perfected; a peroxide is mixed with the HDPE before extruding, the crosslinking takes place during the passage of the melted polymer through a long heated die. In 1968, the Sioplas process using Silane was patented, followed by another silane-based process, Monosil, in 1974, and the process using Vinylsilane in 1986. {Link without Title} The peroxide (Engel) method performs "hot" cross-linking, above the crystal melting temperature. It provides more consistent and uniform cross-linking with better control over the process. The process takes longer and tends to be more expensive than the other two, as the polymer has to be kept under high temperature and pressure for long time during the extrusion process, but it produces the best material. The product is classified as PEX-A, '''PE-Xa''', or '''PEXa'''. The silane method, also called "moisture cure" method, performs the cross-linking in a secondary post-extrusion process, producing crosslinks between a cross-linking agent (eg. Vinylsilane ) and catalyst added into the polymer matrix during extrusion, in one of the several processes, eg. Sioplas, Spherisil or Monosil. The process is accelerated with heat and moisture. Unlike the two other methods, the bonds are not between the carbon atoms, but are realized as oxygen-silicon-oxygen bridges, principially similar to the sulfur bridges introduced during rubber vulcanization. These links are somewhat weaker than the carbon-carbon bond, slightly impairing the material's long-term chemical stability. It is classified as PEX-B, '''PE-Xb''', or '''PEXb'''. The electron irradiation method is the "cold" cross-linking, below the crystal melting temperature. It provides less uniform, lower-degree cross-linking than the Engel method, especially at tube diameters over 1", and when the process is not controlled properly, the outer layer of the tubes may become brittle. However it is the cleanest, most environmentally friendly method of the three, as it does not involve other chemicals and uses only high-energy electrons to split the carbon-hydrogen bonds and facilitate crosslinking. The irradiation takes place after the HDPE tube is produced, and may be performed in a separated facility. The resulting polymer is classified as PEX-C, '''PE-Xc''', or '''PEXc'''. {Link without Title} PEX tubing ranges in size from 1/4" to 4", but 3/4" and 1" are by far the most widely used. {Link without Title} PEX-AL-PEX pipes, or '''PEX/Aluminum/PEX''', are made of a layer of manufactures PEX-Al-PEX and compression fittings for the system, which they market as "IPEX AQUA". The compression fittings are made of brass which should theoretically never contact the aluminum inner layer of the pipe, however repeated expansion and contraction cycles may eventually wear away the protective polyethylene and result in contact with the brass, after which Galvanic Corrosion would occur. Furthermore, IPEX was forced to recall many PEX-Al-PEX fittings in late 2005. Many plumbers avoid PEX and PEX-Al-PEX in their own homes and contracting, citing unknown long-term reliability. {Link without Title} Highly crosslinked polyethylene under trade name Durasul is used in Artificial Joint s as a wear-resistant material. Manufacturers include Uponor . SEE ALSO |
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