Cross-linked polyethylene
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 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 pipe for use as residential water pipes. PEX tubing ranges in size from where 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.[1] In metric PEX is normally available in 16 mm, 20 mm, 25 mm, 32 mm, 40 mm, 50 mm and 63 mm sizes.
Properties
Almost all PEX is made from high density polyethylene (HDPE).[2] PEX contains cross-linked bonds in 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°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 irradiating the extruded tube with an 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. 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.[3]
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 for which it was designed.
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.[4] 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 two 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
PEX has become a contender for use in residential water plumbing because of its flexibility. It can be bent into a wide-radius turn if space permits, or by using elbow; PVC, CPVC and copper all require elbow joints. It also has the capacity to be routed from a distribution point to an outlet fixture without cutting or splicing; this reduces the need for potentially weak and costly joints and reduces pressure drop due to turbulence induced at transitions. The cost of material can also be approximately 25% of alternatives, and installation is much less labor intensive.[5]
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 years. However, the longest warranty offered by any PEX producer is 25 years. There have been some claimed PEX systems failures in the U.S., Canada and Europe resulting in several pending class action law suits. The failures are claimed to be a result of the brass fittings used in the PEX system.
PEX tubing cannot be used in applications exposed to sunlight, as it degrades fairly rapidly. Prior to installation it must be stored away from light, and needs to be shielded from daylight after installation. Leaving it exposed to direct sunlight for as little as 30 days may result in premature failure of the tubing due to embrittlement.
Highly cross-linked polyethylene under the trade name Durasul is used in artificial joints as a wear-resistant material.
Some application of PEX has also been seen in dental restoration as a composite filling material.
The 2007 California Plumbing Code (CPC), effective January 1, 2008, allowed the use of PEX for domestic water systems in the State of California on a case-by-case basis only.[6] This was mostly due to issues with corrosion of the manifolds, not the tubing itself, and was allowed in California when used in hydronic radiant heating systems. In January 2009, the California Building Standards Commission (CBSC) approved the addition of PEX plastic pipe and tubing to the California Plumbing Code (CPC), allowing its use in hospitals, clinics, residential and commercial construction throughout the state of California. Formal adoption of PEX into the CPC occurred on August 1, 2009, allowing local jurisdictions to approve its general use.[7]
PEX is also used in many canoes and kayaks. The PEX is listed as the name Ram-X and other brand specific names. Because of the properties of Cross-Linked Polyethylene, repair of any damage to the haul is rather difficult. Some adhesives such as 3M's DP-8005 is able to bond to PEX, while larger repairs require melting and mixing more Polyethylene into the canoe/kayak to form a solid bond and fill the damaged area.
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 aluminum.[8][dead link]
See also
References
- ^ Rafferty KD (2007). "Piping" ([dead link] – Scholar search). Geo-Heat Center Quarterly Bulletin. 19 (1). Retrieved 2008-06-12.
{{cite journal}}: External link in|format= - ^ [1]
- ^ Cross-Linked Polyethylene Tubing Master Hydronics
- ^ [2]
- ^ Television program Ed The Plumber, DIY Network, 2006
- ^ 2007 CPC Table 6-4 Footnote 1; previously: 2001 CPC 604.1 #2
- ^ "(Press Release) PEX Plastic Pipe Unanimously Added to California Plumbing Code; State Officials Certify Favorable Environmental Impact Report". January 27, 2009. Retrieved June 23, 2009.
- ^ "PEX choices". Home Heating Systems Newsletter. Retrieved 2008-06-12.
External links
- History of PEX on Plastic Pipe and Fittings Association (PPFA) homepage
- PEX FAQs on Plastic Pipe and Fittings Association (PPFA) homepage