Cross-linked polyethylene
2416422
223192327
2008-07-02T23:55:21Z
121.72.241.142
[[WPMOS]]
{{redirect|PEX||pex}}
'''Cross-linked polyethylene''', commonly abbreviated '''PEX''' or '''XLPE''', is a form of [[polyethylene]] with [[cross-link]]s. It is formed into tubing, and is used predominantly in hydronic [[radiant heating]] systems, domestic water piping and [[electrical insulation|insulation]] for high tension electrical cables. It is also used for [[natural gas]] and offshore [[petroleum|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]] pipe for use as residential [[water pipe]]s. PEX tubing ranges in size from 1/4-[[inch]] to 4-inch, but 1/2-inch, 3/4-inch, and 1-inch are by far the most widely used.<ref>{{cite journal | journal=Geo-Heat Center Quarterly Bulletin | url=http://geoheat.oit.edu/bulletin/bull19-1/art59.htm | volume=19 | issue=1 | year=2007 | author= Rafferty KD | title=Piping |accessdate=2008-06-12}}</ref>
== Properties ==
Almost all PEX is made from [[high density polyethylene]] (HDPE).<ref name="Holohan">{{cite web |url=http://www.heatinghelp.com/newsletter.cfm?Id=18 |title=PEX choices | work=Home Heating Systems Newsletter |accessdate=2008-06-12}}</ref> PEX contains cross-link bonds which into the [[polymer]] structure, changing the [[thermoplastic]] into an [[elastomer]]. 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 65-89%; a higher degree would lead to brittleness and stress cracking of the material.
The high-temperature properties of the polymer are improved. Adequate strength is maintained to 120-150[[Celsius|°C]] by reducing the tendency to flow. Its chemical resistance is enhanced by resisting dissolution. Its low temperature properties are also improved; its impact and tensile strength, scratch resistance, and resistance to brittle fracture are enhanced.
PEX-insulated cables have a rated maximum conductor temperature of 90°C and an emergency rating of up to 140°C, depending on the standard used to rate PEX-insulated cables. Cables insulated with PEX also have a conductor short-circuit rating of 250°C. PEX has excellent dielectric properties making it useful for a large range of voltage applications from 600 V to 500 kV.
Numerous modifications in the basic polymer structure can be made to maximize productivity during the cable manufacturing processes. In medium voltage applications, the reactivity can be boosted significantly. This results in higher line speeds in the cases where there are limitations in either the curing or cooling processes within the continuous [[vulcanization]] (CV) tubes used to cross-link the insulation. PEX insulations can also be modified to limit the amount of by-product gases that are generated during the cross-linking process. This is particularly useful for high voltage and extra-high voltage cable applications, where degassing requirements can significantly lengthen the time required to manufacture the cable.
== History ==
The first PEX material was prepared in the 1930s, by [[irradiation|irradiating]] the extruded tube with an [[cathode ray|electron beam]]. The [[electron beam processing]] method was made feasible in 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, with the cross-linking taking place during the passage of the melted polymer through a long heated [[die (manufacturing)|die]]. In 1968, the Sioplas process using [[silane]] was patented, followed by another silane-based process, Monosil, in 1974. A process using [[vinylsilane]] followed in 1986.[http://www.masterhydronics.com/info/pex.html]
== Classification ==
=== North America ===
The current cell classification in North America for PEX tubing is either 0006, 0008, 1006, or 1008. In North America all PEX tubing products perform to standards and are rated for performance by the ASTM (F876, F877), [[NSF International]](NSF-pw, NSF 14, NSF 61, CL-R, CL-TD), and [[Canadian Standards Association]] standards (B137.5), for which they are tested and certified. The listings and certifications met by each product are printed on the printline of the tubing to ensure the product is used in the proper applications it was designed for.
=== Europe ===
In European standards there are three classifications are referred to as PEX-A, -B, and -C: these classes are not related to any type of rating system.
==== PEX-A (PE-Xa, PEXa) ====
PEX-A is produced by the peroxide (Engel) method.<ref name="Holohan" /> This method performs "hot" cross-linking, above the crystal melting temperature. It provides more consistent and uniform cross-linking with better control over the production process. However the process takes longer and tends to be more expensive than the other two methods, because the polymer has to be kept at high temperature and pressure for long periods during the extrusion process. The cross-linked bonds are between [[carbon]] atoms.
==== PEX-B (PE-Xb, PEXb) ====
The [[silane]] method, also called the "moisture cure" method, results in PEX-B. In this method, cross-linking is performed in a secondary post-extrusion process, producing cross-links between a cross-linking agent. The process is accelerated with heat and moisture. The cross-linked bonds are formed through silanol condensation between 2 grafted vinyltrimethoxysilane (VTMS) units, connecting the polyethylene chains with C-C-Si-O-Si-C-C bridges.
==== PEX-C (PE-Xc, PEXc) ====
PEX-C is produced through [[electron beam processing]], in a "cold" cross-linking process (below the crystal melting temperature). It provides less uniform, lower-degree cross-linking than the Engel method, especially at tube diameters over one inch (2.5 cm), 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, since it does not involve other chemicals and uses only high-energy electrons to split the carbon-[[hydrogen]] bonds and facilitate cross-linking.
== Uses ==
[[Image:Col.lector terra radiant.JPG|thumb|right|Radiant heating system manifold using PEX tubing.]] PEX has become a contender for use in residential water plumbing because of its flexibility. It can be turned 90 degrees either by a wide turn or using an adapter; PVC, CPVC and [[copper]] all require elbow joints. It also has the capacity to run tubing directly from a distribution point continuously to the desired outlet fixture without cutting or splicing; this reduces the need for potentially weak and costly joints. The cost of material can also be approximately 25% of alternatives, and installation is much less labor intensive.<ref>Television program ''Ed The Plumber'', [[DIY Network]], 2006</ref>
PEX tubing has also become the most popular way to transport water in hydronic radiant heating systems. In these systems PEX is used to carry hot water from the heat source to the baseboard or radiators. PEX tubing with an oxygen barrier must be used when [[ferrous]] components are present in a radiant heating system. If an oxygen barrier is not used in these systems, the ferrous components will rust. However, if a radiant heat system does not contain ferrous components standard PEX tubing may be used.
The advantageous properties of PEX also make it a candidate for progressive replacement of metal and thermoplastic pipes, especially in long-life applications, because the expected lifetime of PEX pipes reaches 50-200 years.
Highly cross-linked polyethylene under the trade name Durasul is used in [[artificial joint]]s as a wear-resistant material.
Some application of PEX has also been seen in dental restoration as a composite filling material.
2007 California Plumbing Code (CPC) (effective 1/1/08) does NOT allow the use of PEX for domestic water systems in the State of California. (ref: 2007 CPC Table 6-4 Footnote 1; previously: 2001 CPC 604.1 #2). Note that this is mostly due to issues with corrosion of the manifolds, not the tubing itself, and PEX ''is'' allowed in California when used in hydronic radiant heating systems.
== PEX-Al-PEX ==
PEX-Al-PEX pipes, or PEX/Aluminum/PEX, are made of a layer of [[aluminum]] sandwiched between two layers of PEX. The metal layer serves as an [[oxygen]] barrier, stopping the oxygen diffusion through the polymer matrix, so it cannot dissolve into the water in the tube and corrode the metal components of the system. It also bends better, because the [[shape memory]] of the PEX is offset by the aluminium.<ref name="Holohan" />
== References ==
<div class="references-small"><references /></div>
== See also ==
* [[UHMWPE]]
== External links ==
* [http://www.ppfahome.org/pex/historypex.html History of PEX at the Plastic Pipes and Fittings Association (PPFA) page]
<!-- Above is almost linkspam, but may provide some content. -->
[[Category:Dental materials]]
[[Category:Plastics]]
[[Category:Plumbing]]
[[Category:Polyolefins]]
[[de:Polyethylen#Vernetzung]]
[[fi:PEX]]
[[sv:PEX (plastmaterial)]]