Crosslinked Polyethylene (XPE) Materials
Crosslinked Polyethylene (XPE)
“Cross-linked polyethylene, commonly abbreviated PEX or XLPE, is a form of polyethylene with cross-links. It is formed into tubing, and is used predominantly in building services pipework systems, hydronic radiant heating 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. Recently, it has become a viable alternative to polyvinyl chloride (PVC), chlorinated polyvinyl chloride (CPVC) or copper tubing for use as residential water pipes. PEX tubing ranges in size from imperial sizes of 1/4-inch to 4-inch, but 1/2-inch, 3/4-inch, and 1-inch are by far the most widely used. In metric PEX is normally available in 16 mm, 20 mm, 25 mm, 32 mm, 40 mm, 50 mm and 63 mm sizes.”
“Almost all PEX 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 F 876-93, is between 65 and 89%. A higher degree of cross-linking could result in brittleness and stress cracking of the material.
“The high-temperature properties of the polymer are improved. Adequate strength to 120-150°C is maintained by reducing the tendency to flow. Chemical resistance is enhanced by resisting dissolution. Low temperature properties are improved. Impact and tensile strength, scratch resistance, and resistance to brittle fracture are enhanced.
PEX- or 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 - 10 to 50 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. PEX insulations 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.”
(Wikipedia, Crosslinked Polyethylene, 6/25/2012)
“Crosslinked polyethylene, sometimes referred to as PEX or XL-PE, is a well known plastic material having many common uses. PEX is commonly used in the production of tubing, conduits, and piping for use in a variety of applications such as fire protection, plumbing, heating, gas distribution, and the like. Due to the flexibility and strength of PEX at temperatures ranging from below freezing up to 93.degree. C. (200.degree. F.), PEX is an ideal piping material for hot and cold water plumbing systems, hydronic radiant heating systems, snow melting applications, ice rinks and refrigeration warehouses.
A common use of PEX is in the production of barrier pipes. Barrier pipes are plastic water pipes that are used in domestic heating systems. The pipe is manufactured with a barrier that prevents oxygen from penetrating the material and entering the water system, reducing the risk of corrosion. The oxygen barrier layer is usually a resin material bonded between the outer and inner layer of the pipe itself.
Another common use of crosslinked polyethylene is in wire and cable applications including coatings such as, for example, insulation or jacketing.
In the production of PEX, crosslinks between polyethylene macromolecules are formed to make the resulting molecule more durable under temperature extremes and chemical attack, and more resistant to creep deformation.”
[Runyan et al, US Patent 8,192,813 (6/5/2012)]
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Roger D. Corneliussen
Maro Polymer Links
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Copyright 2012 by Roger D. Corneliussen.
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* Date of latest addition; date of first entry is 6/25/2012.