Cross-linked polyethylene

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A cross-linked polyethylene (PEX) pipe

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 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.

In the 21st century, PEX 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.[1] Metric PEX is normally available in 16 mm, 20 mm, 25 mm, 32 mm, 40 mm, 50 mm and 63 mm sizes.

Properties[edit]

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.

Preparation methods[edit]

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, 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.

Raw material: XLPE powder used in Rotational molding in a factory.

Classification[edit]

North America[edit]

The current cell classification in North America for PEX tubing is either 0006, 0008, 1006, 1008, or 0306 (an ultraviolet blocker). 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[edit]

In European standards there are three classifications are referred to as PEX-A, -B, and -C. The classes are not related to any type of rating system.

PEX-A (PE-Xa, PEXa)
PEX-A is produced by the peroxide (Engel) method. This method performs "hot" cross-linking, above the crystal melting point. However, the process takes slightly longer than the other two methods as 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. After installation, PEX-B have the same properties as PEX-A.
PEX-C (PE-Xc, PEXc)
PEX-C is produced through electron beam processing, in a "cold" cross-linking process (below the crystal melting point). It provides less uniform, lower-degree cross-linking than the Engel method, especially at tube diameters over one inch (2.5 cm). When the process is not controlled properly, the outer layer of the tube 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.

Plumbing[edit]

Radiant heating system manifold using PEX tubing.
All pipes, including this copper exterior valve as well as PEX, can burst from freezing, although several reports suggest that PEX takes longer to burst under freezing conditions.
PEX's flexibility means fewer connections, better water flow, faster and easier installations.
A PEX compression fitting makes it possible to join copper and PEX pipes by simply pushing them together for a watertight fit.
Tools and fittings used in a plumbing installation with PEX piping. (1) crimping tool to squeeze a metal band to join a pipe and a fitting (2) compression coupling joining two 1/2 inch pipes (copper or PEX) (3) "T-joint" to connect 3/4", 3/4", and 1/2" pipes (4) Copper-to-PEX 1/2" connection (requires soldering) (5 and 6) tools to undo PEX connections (7) crimp rings to squeeze metal band to connect PEX to a fixture (8) PEX tube cutter.

PEX tubing is widely used to replace copper in plumbing applications. One estimate is that residential use of PEX for delivering drinking water to home faucets has increased by 40% annually,[2] and there is substantial evidence that PEX is or will soon become the dominant technology for carrying water in homes and businesses in the next decade or so. It is widely accepted among different groups, and has been used by volunteer organizations such as Habitat for Humanity in constructing homes.[citation needed] In 2006, The Philadelphia Inquirer recommended that plumbing installers switch from copper pipes to PEX.[3]

In the 20th century, mass-produced plumbing pipes were made from galvanized steel. As users experienced problems with the internal build-up of rust, which reduced water volume, these were replaced by copper pipes in the late 1960s.[4] Plastic pipes with fittings using glue were used as well in later decades. Initially PEX tubing was the most popular way to transport water in hydronic radiant heating systems, and it was used first in hydronic systems from the 1960s onwards.[2] Hydronic systems circulate water from a boiler or heater to places in the house needing heat, such as baseboard heaters or radiators.[5] PEX is suitable for recirculating hot water.[6]

Gradually PEX became more accepted for more indoor plumbing uses, such as carrying pressurized water to fixtures throughout the house. Increasingly, in the 2000s, copper pipes as well as plastic PVC pipes are being replaced with PEX.[4] PEX can be used for underground purposes, although one report suggested that appropriate "sleeves" be used for such applications.[6]

Benefits[edit]

Benefits of using PEX in plumbing include:

Drawbacks[edit]

Government approvals[edit]

PEX has been approved for use in all fifty states of the United States as well as Canada,[3] including the initially reluctant state of California, which approved its use in 2009.[6] California allowed the use of PEX for domestic water systems on a case-by-case basis only in 2007.[23] 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 2009, the Building Standards Commission approved PEX plastic pipe and tubing to the California Plumbing Code (CPC), allowing its use in hospitals, clinics, residential and commercial construction throughout the state.[6] Formal adoption of PEX into the CPC occurred on August 1, 2009, allowing local jurisdictions to approve its general use,[24] although there were additional issues, and new approvals were issued in 2010 with revised wordings of the 2007 act.[25]

Competitors to PEX[edit]

Alternative plumbing choices include:

PEX-AL-PEX[edit]

PEX-AL-PEX pipes, or AluPEX, 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.[26] The aluminium layer is thin, typically 1 or 2 mm, and provides some rigidity to the tube such that when bent it retains the shape formed (normal PEX tube will spring back to straight). The aluminium layer also provides additional structural rigidity such that the tube will be suitable for higher safe operating temperatures and pressures.

PEX tool kit[edit]

A PEX tool kit includes a number of basic tools required for making fittings and connections with PEX tubing. In most cases, such kits are either bought at a local hardware store, plumbing supply store or assembled by either a home owner or a contractor. PEX tools kits range from under $100 and can go up to $300+. A typical PEX tool kit includes crimp tools, an expander tool for joining, clamp tools, PEX cutters, rings, boards, and staplers.[further explanation needed]

Other uses for PEX[edit]

XLPE automotive duct

See also[edit]

References[edit]

  1. ^ Rafferty KD (2007). "Piping". Geo-Heat Center Quarterly Bulletin 19 (1). Archived from Scholar search the original on March 6, 2008. Retrieved 2008-06-12. 
  2. ^ a b c d e f g Jay Romano (September 3, 2006). "If Copper Pipes Are Too Costly ...". The New York Times. Retrieved 2011-07-09. "The price of copper has nearly quadrupled over the last four years, and plumbers and do-it-yourselfers are taking a fresh look at alternatives to copper tubing and fittings. And what some are turning to is a flexible synthetic material called PEX." 
  3. ^ a b c d e f g Alan J. Heavens (July 29, 2006). "Shortages Persist In Building Materials: Even as Demand for New Homes Falls, Cost of Cement and Copper Skyrockets`". The Philadelphia Inquirer. p. F25. Retrieved 2011-07-09. "Recommended alternatives to copper piping include: (1) Cross-linked polyethylene, which is known as PEX and has been adopted by installers of radiant-floor heating since it neither corrodes nor develops pinhole leaks. PEX also resists chlorine and scaling, and uses fewer fittings than rigid plastic and metallic pipe. The piping is approved for potable hot- and cold-water plumbing systems as well as for hydronic heating systems in all plumbing and mechanical codes in the United States and Canada. (2) Aluminum plastic composite, a multipurpose pressure piping that can distribute hot and cold water indoors and outdoors, and also is well-suited for under-the-floor heating and snowmelt systems. It is made of aluminum tube laminated to interior and exterior layers of plastic. (3) Corrugated stainless-steel tubing, which is used as an alternative to traditional threaded black-iron gas piping for residential, commercial and industrial applications. It consists of a continuous, flexible stainless-steel pipe with an exterior PVC covering. The piping is produced in coils that are air-tested for leaks." 
  4. ^ a b Barry Stone (July 22, 2006). "50-Year-Old House Warrants Special Scrutiny". The Washington Post. Retrieved 2011-07-09. "The use of galvanized steel water piping was abandoned in favor of copper in the late 1960s, and now the plumbing industry has moved from copper to PEX (cross-link polyethylene). The problem with old galvanized pipes is that they usually have internal rust build-up, which reduces water volume.... (Barry Stone => home inspector)" 
  5. ^ Al Heavens (January 20, 2011). "Trying to keep radiant floor project out of hot water". Chicago Tribune. McClatchy/Tribune News. Retrieved 2011-07-09. "Hydronic systems circulate water from a boiler or water heater through loops of polyethylene tubing, often called by the brand name Pex, but there are others. Tubing is typically installed on top of the subfloor in grooved panels or snap-in grids; clipped into aluminum strips on the underside of the floor; or embedded in poured concrete, or a lighter, concrete-like material in bathrooms or kitchens especially." 
  6. ^ a b c d e f Robert P. Mader (Sep 2, 2010). "California approves PEX for plumbing — again". Contractor Mag. Retrieved 2011-07-09. "PEX became part of the California Plumbing Code in August 2009, following the CBSC's January 2009 certification of an Environmental Impact Report (EIR) on PEX and the commission's ensuing unanimous adoption of regulations approving PEX water distribution systems.... The Commission's action allows the statewide use of PEX in hospitals, clinics, schools, residences and commercial structures.... The CBSC reinstated PEX with the caveats that underground PEX must be sleeved, the material had to stand up to recirculating hot water, the fittings won't de-zincify, and PEX systems had to be filled and flushed...." 
  7. ^ a b Alan J. Heavens (August 11, 2006). "No cool solution to removing heated tiles". The Philadelphia Inquirer. Retrieved 2011-07-09. "I assume that the radiant floor heating involves piping that is embedded in Gypcrete, a lightweight blend of concrete and gypsum that, in concert with a shift to flexible PEX piping, has made installation easy and operation problem-free." 
  8. ^ a b Television program Ed The Plumber, DIY Network, 2006
  9. ^ a b Jan Ellen Spiegel (April 20, 2008). "The House That Green Built". The New York Times. Retrieved 2011-07-09. "(Page 2 of 4) There is radiant floor heating, and the toilets use rainwater stored in a cistern. The floors, doors and wall paneling are reclaimed from vintage homes that were torn down elsewhere in the state. Instead of copper pipes, water will travel through Pex piping, less expensive flexible polyethylene tubes that are petroleum-based, but still may be greener than copper pipe. "It is a compromise," said Mr. Johnson, who said he worried a little about the health aspects of Pex. "I couldn't get a good read on that, to tell you the truth. I sort of got exhausted in asking a bunch of people."" 
  10. ^ a b John Kogel (2009-07-13). "Pex issues". Inspection News. Retrieved 2011-07-09. "Once you've worked with PEX, you'll never go back to that other stinky glue stuff. We see copper stubs at the water heater (sometimes), the rest is PEX. Also, when they use the red and blue colors, hot is hot and cold is cold" 
  11. ^ Michael Finnegan (February 21, 2011). "Officials probe structural issues in home where L.A. firefighter killed". Los Angeles Times. Retrieved 2011-07-09. "Running through the attic were plastic pipes for fire sprinklers. The fire melted the pipes, flooding the attic and filling the insulation with water, Peaks said. The weight of the insulation appears to have caused a large section of the ceiling to collapse, injuring Allen and five other firefighters, officials said." 
  12. ^ Jay Romano (January 28, 2009). "Before, and After, the Last Drop". The New York Times. Retrieved 2011-07-09. "Pipes, traditionally made of copper, can burst if the water inside freezes, because water expands when frozen, but copper does not. If the water expands too much, it has nowhere to go but out, forcing the pipe to burst at the frozen spot. Tom Kraeutler, a host of the syndicated radio show "The Money Pit", said most houses have one particular spot where the pipes tend to freeze. If there is fairly consistent freezing in an area, he said, it is wise to reroute the pipes and to replace them with PEX — a flexible plastic tubing that is much less likely to burst than copper. Like copper, though, PEX can freeze, as Mr. Carter, who moved in December, now knows. The house was built with modern materials, including PEX, but because the place was only six years old, he didn't think he had to worry about frozen pipes." 
  13. ^ Stacy Downs (February 24, 2006). "Frozen pipes can lead to flood of woe". Chicago Tribune. Knight Ridder/Tribune. Retrieved 2011-07-09. "Frozen pipes break differently depending on the material, Water said. Copper rips, PVC (polyvinyl chloride) shatters and PEX (polyethylene) swells and tears." 
  14. ^ a b Steve Maxwell (Jul 14, 2007). "Drywall may not work on waterfront". Toronto Star. Retrieved 2011-07-09. "Start by making sure that all runs of water supply pipe are sloped downwards slightly to central drain valves. Also, be sure to specify that all drain traps remain accessible, and be the kind that includes a removable plug on the bottom. As an added precaution, install PEX-al-PEX supply pipes instead of copper. If water accidentally remains in these pipes, they'll endure five or six freeze-thaw cycles without splitting. Copper pipe, on the other hand, splits apart promptly when it contains water that freezes." 
  15. ^ a b Steve Maxwell (Feb 28, 2009). "Put basement repair to wet weather test". Toronto Star. Retrieved 2011-07-09. "Q: Is it safe to use pipe wrap insulation on PEX water supply pipes? In a magazine put out by a home improvement retailer, it warns that a chemical reaction between insulation and PEX will eventually destroy the pipes. Is this true? A: To answer your question, I contacted one of the world's largest producers of PEX pipe. The only potential issue they know of has to do with certain types of adhesives touching the pipe surface. PEX includes antioxidants for stabilizing against chlorine, and these antioxidants can become destabilized in a reaction with adhesives, possibly aging the pipe prematurely. That said, they don't know of any issues relating to a chemical reaction between PEX and conventional foam pipe wrap insulation. I've installed foam insulation on PEX in my own house about a year ago, and there's no visible signs of trouble." 
  16. ^ a b Bill Kibbel (Historic & Commercial Building Inspections), Jim Katen, Nolan E. Kienitz (home inspectors) (2006-2007). "PEX and sunlight issues". The Inspector's Journal. Retrieved 2011-07-09. "Well, the manufacturers' instruction I've read and the Plastic Pipe Assoc. says it can't be installed where exposed to direct sunlight.... I've heard of some pretty serious problems with PEX that's exposed to sunlight. Your client's concerns are valid.... Another big factor is how the product has been "handled" from manufacture to site installation.... I had a client, with a new home, that was purchased back by the plumbing company due to mis-handling of the PEX that had caused over 10 leaks in less than 7 months." 
  17. ^ Bates, S.L. 2005. Damage to common plumbing materials caused by overwintering Leptoglossus occidentalis (Hemiptera: Coreidae). Canadian Entomologist 137: 492-496.[journals.cambridge.org/article_S0008347X00002807]
  18. ^ a b c Jeff Pope (Jan 22, 2009). "Pipe work begins in homes involved in Kitec lawsuit". Las Vegas Sun. Retrieved 2011-07-09. "The polyethylene pipes contained a thin layer of aluminum that held its shape as plumbers twisted and bent it. Plastic pipes without the aluminum require more anchoring because they spring back to a straight line. The pipes aren't failing though. It's the brass fittings that connect the pipes to copper fixtures on valves, water heaters and softeners. The problem is a chemical reaction known as dezincification, which accelerates corrosion in brass fittings when they are exposed to oxygen and moisture. Brass is an alloy primarily composed of copper and zinc. When dezincification occurs, zinc leaches out of the fittings, leaving a blockage of zinc oxide that leads to leaks, restricted water flow and breaks." 
  19. ^ Ted Menelly (2009-07-13). "Pex issues". Inspection News. Retrieved 2011-07-09. "Just a couple of leaks at poorly applied connections. Other than that I have not really seen any. Most, not all, but most new homes have PEX. There are some that still use only copper. have seen it used a lot in remodel with many homes I have inspected that have had repiping. It is easier to run through the attics and crawls. I guess it has been, what, 10 years or so since its major use. I guess only time will tell. There were many complaints in the very beginning but not much now." 
  20. ^ "California Building Standards Code". State of California. 2007. Retrieved 2011-08-15. "... PEX material is susceptible to chemical leaching, both from the outside environment and chemicals leaching out of the PEX material itself...." 
  21. ^ a b "Pipe Rollercoaster: After a recent exclusion, PEX pipe is back in the California Plumbing Code". Plumbing & Mechanical. October 1, 2010. Retrieved 2011-08-15. "... controversy in California ... resulting in a flurry of back-and-forth legal wrangling over health, safety and performance issues related to the flexible pipe.... That judicial rollercoaster finally came to a halt in mid-August when a coalition of consumer, environmental, public health and labor organizations reached an agreement with the state and the plastic pipe industry ... As a result, the California Building Standards Commission now allows the use of PEX in all occupancies..." 
  22. ^ Jack Sweet (October 1, 2010). "What was that flurry of acitivity this past summer?". Reeves Journal. Retrieved 2011-08-15. "It boiled up, came to a head and was then over almost as quickly as it takes to tell the tale. PEX, formally known as crosslinked polyethylene tubing-was given the administrative heave-ho from the California plumbing codes. Then, almost as quickly as the word could get passed out to the industry-at-large, PEX was back the state's good graces, albeit with a few stipulations on its use that weren't there before." 
  23. ^ 2007 CPC Table 6-4 Footnote 1; previously: 2001 CPC 604.1 #2
  24. ^ "(Press Release) PEX Plastic Pipe Unanimously Added to California Plumbing Code; State Officials Certify Favorable Environmental Impact Report". Reuters. January 27, 2009. Retrieved June 23, 2009. 
  25. ^ "Building Standards Commission". State of California. 2010. Retrieved 2011-07-09. "On August 16, 2010, the California Building Standards Commission certified the Final Environmental Impact Report and approved regulations allowing the use of PEX tubing. The Approved Final Express Terms document represents the final language that will be published into the 2007 California Plumbing Code and the 2010 California Plumbing Code (Effective Jan. 1, 2011) with the strikeout and underlining removed for clarity. All remaining agencies' rulemaking documents appearing on this page, were also approved by the Commission, but do not have the strikeout and underlined removed." 
  26. ^ "PEX choices". Home Heating Systems Newsletter. Archived from the original on June 11, 2008. Retrieved 2008-06-12. 

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