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. 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 used for pipe and tubing 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 F876, is between 65 and 89%. A higher degree of cross-linking could result in brittleness and stress cracking of the material while a lower degree of cross-linking could result in product with poorer physical properties.
Crosslinking improves the elevated-temperature properties of the base polymer. Adequate strength to 120–150 °C is maintained by reducing creep, 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.
Almost all cross-linkable polyethylene compounds (XLPE) for wire and cable applications are based on [low-density polyethylene] LDPE. 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. XLPE 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.
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 the 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.
Materials used in PEX pipes in North America are defined by cell classifications that are described in ASTM standards, the most common being ASTM F876. Cell classifications for PEX include 0006, 0008, 1006, 1008, 3006, 3008, 5006 and 5008, the most common being 5006. Classifications 0306, 3306, 5206 and 5306 are also common, these materials containing ultraviolet blockers and/or inhibitors for limited UV resistance. In North America all PEX tubing products are manufactured to ASTM, NSF and CSA product standards, among them the aforementioned ASTM standard F876 as well as F877, NSF International standards NSF 14 and NSF 61 ("NSF-pw"), and Canadian Standards Association standard B137.5, to which the pipes are tested, certified and listed. The listings and certifications met by each product appear on the printline of the pipe or tubing to ensure the product is used in the proper applications for which it was designed.
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.
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, 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. In 2006, The Philadelphia Inquirer recommended that plumbing installers switch from copper pipes to PEX.
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. 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. Hydronic systems circulate water from a boiler or heater to places in the house needing heat, such as baseboard heaters or radiators. PEX is suitable for recirculating hot water.
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. PEX can be used for underground purposes, although one report suggested that appropriate "sleeves" be used for such applications.
Benefits of using PEX in plumbing include:
- Flexibility. PEX has become a contender for use in residential water plumbing because of its flexibility. It can bend into a wide-radius turn if space permits, or accommodate turns by using elbow joints. In addition, it can handle short-radius turns, sometimes supported with a metal brace; in contrast, PVC, CPVC and copper all require elbow joints. A single length of PEX pipe cannot handle a sharp 90-degree turn, however, so in those situations, it is necessary to connect two PEX pipes with a 90-degree PEX elbow joint.
- Direct routing of pipes. PEX can run straight from a distribution point to an outlet fixture without cutting or splicing the pipe. This reduces the need for potentially weak and costly joints and reduces the drop in pressure due to turbulence induced at transitions. Since PEX is flexible, it is often possible to install a supply line directly from the water source to an appliance using just one connection at each end.
- Greater water pressure at fixtures. Since PEX pipes typically have fewer sharp turns, there is greater water pressure at the sinks and showers and toilets where it is needed.
- Lower materials cost. Cost of materials is approximately 25% of alternatives. One account suggested that the price of copper had quadrupled from 2002 to 2006.
- Easier installation. Installing PEX is much less labor-intensive than copper pipes, since there is no need to use torches to solder pipes together, or to use glue to attach pipes to fittings. One home inspector wrote that "Once you've worked with PEX, you'll never go back to that other stinky glue stuff." Builders putting in radiant heating systems found that PEX pipes "made installation easy and operation problem-free". PEX connections can be made by pushing together two matching parts using a compression fitting, or by using an adjustable wrench or a special crimping tool. Generally, fewer connections and fittings are needed in a PEX installation.
- No fire risk during installation. Copper piping required soldering using torches, and there was a risk of flame and heat causing a fire; but with PEX there is virtually no danger from fire. However, there was an unfortunate counter-incident in 2011 in which authorities suspect that six firefighters were injured when a fire melted the plastic PEX pipes, causing water to soak into ceiling insulation, adding greater weight, which caused the ceiling to collapse; but the PEX tubing was not blamed as the cause of the fire. Overall PEX piping is much safer to install.
- Acceptance by plumbers. There are routinely advertisements for plumbers specifically seeking ones with PEX experience.
- Ability to merge new PEX with existing copper and PVC systems. Manufacturers make fittings allowing installers to join a copper pipe on one end with a PEX line at the other, as well as have options to reduce or expand the diameter of the pipes.
- Longevity. 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.
- Suitable for hot and cold pipes. A convenient arrangement is to use color-coding to lessen the possibility of confusion. Typically, red PEX tubing is used for hot water while blue PEX tubing is used for cold water.
- Less likely to burst from freezing. The general position is that PEX plastic materials are slower to burst than copper or PVC pipes, but that they will burst eventually since freezing causes water to expand. One account suggested that PEX water-filled pipes, frozen over time, will swell and tear; in contrast, copper pipe "rips" and PVC "shatters". Home expert Steve Maxwell suggested in 2007 that PEX water-filled pipes could endure "five or six freeze-thaw cycles without splitting" while copper would split apart promptly on the first freeze. In new unheated seasonal homes, it is still recommended to drain pipes during an unheated cold season or take other measures to prevent pipes from bursting because of the cold. In new construction, it is recommended that all water pipes be sloped slightly to permit drainage, if necessary.
- No corrosion. Copper and iron pipes can experience corrosion leaks but PEX does not have these problems.
- Environmental benefits. One account suggested that PEX used in radiant heating was better for the environment than a copper choice, although it noted that the pipes were based on petroleum products.
- Pipe insulation possible. Conventional foam wrap insulation materials can be added to PEX piping to keep hot water hot, and cold water cold, and prevent freezing, if necessary.
- Degradation from sunlight. PEX tubing cannot be used in applications exposed to sunlight, as it degrades fairly rapidly. Prior to installation it must be stored away from sunlight, 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.
- Perforation by insects. PEX tubing is vulnerable to being perforated by the mouthparts of plant-feeding insects; in particular, the Western conifer seed bug (Leptoglossus occidentalis) is known to sometimes pierce through PEX tubing, resulting in leakage.
- Problems with yellow brass fittings. There have been some claimed PEX systems failures in the U.S., Canada and Europe resulting in several pending class action lawsuits. The failures are claimed to be a result of the brass fittings used in the PEX system. Generally, builders and manufacturers have learned from these experiences and have found the best materials for use in fittings used to connect pipe with connectors, valves and other fittings. But there were problems reported with a specific type of brass fitting used in connection with installations in Nevada that caused a negative interaction between its mineral-rich hard water and so-called "yellow brass" fittings. Zinc in the fittings leached into the pipe material in a chemical reaction known as dezincification, causing some leaks or blockages. A solution was to replace the yellow brass fittings, which had 30% zinc, with red brass fittings, which had 5% to 10% zinc. It led California building authorities to insist on fittings made from "red brass" which typically has a lower zinc content, and is unlikely to cause problems in the future since problems with these specific fittings have become known.
- Initial adjustment to a new plumbing system. There were a few reported problems in the early stages as plumbers and homeowners learned to adjust to the new fittings, and when connections were poorly or improperly made, but home inspectors have generally not noticed any problems with PEX since 2000.
- Can't use adhesives for pipe insulation. One source suggested that pipe insulation, applied to PEX using certain adhesives, could have a detrimental effect causing the pipe to age prematurely; however, other insulating materials can be used, such as conventional foam wrap insulation, without negative effects.
- Fittings somewhat more expensive. Generally, PEX fittings, particularly the do-it-yourself compression ones, are more expensive than copper ones, although there is no soldering required. Due to the flexibility of PEX, it generally requires fewer fittings, which tends to offset the higher cost per fitting.
- Potential problems for PEX radiant heating with iron-based components. If PEX tubing is used in a radiant heating system that has ferrous radiators or other parts, meaning they are made out of iron or its alloys, then there is the possibility of rust developing over time; if this is the case, then one solution is to have an "oxygen barrier" in these systems to prevent rust from developing. However, in new installations PEX pipes and iron-based components are not intermixed.
- Possible health effects. There was controversy in California during the 2000s about health concerns. Several groups blocked adoption of PEX for concerns about chemicals getting into the water, either from chemicals outside the pipes, or from chemicals inside the pipes such as methyl tertiary butyl ether and tertiary butyl alcohol. These concerns delayed statewide adoption of PEX for almost a decade. After substantial "back-and-forth legal wrangling", which was described as a "judicial rollercoaster", the disputing groups came to a consensus, and California permitted use of PEX in all occupancies. An environmental impact report and subsequent studies determined there were no causes for concerns about public health from use of PEX piping.
PEX has been approved for use in all fifty states of the United States as well as Canada, including the initially reluctant state of California, which approved its use in 2009. California allowed the use of PEX for domestic water systems on a case-by-case basis only in 2007. 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. Formal adoption of PEX into the CPC occurred on August 1, 2009, allowing local jurisdictions to approve its general use, although there were additional issues, and new approvals were issued in 2010 with revised wordings of the 2007 act.
Competitors to PEX
Alternative plumbing choices include:
- Aluminum plastic composite are aluminum tubes laminated on the interior and exterior with plastic layers for protection.
- Corrugated stainless steel tubing, continuous flexible pipes made out of stainless steel with a PVC exterior and are air-tested for leaks.
- Polypropylene Pipe, similar in application to CPVC but a chemically inert material containing no harmful substances and reduced dangerous emissions when consumed by fire. It is primarily utilized in radiant floor systems but is gaining popularity as a leach-free domestic potable water pipe, primarily in commercial applications.
PEX-AL-PEX pipes, or AluPEX, or PEX/Aluminum/PEX, or Multilayer pipes 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. 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
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
- Artificial joints. Highly cross-linked polyethylene is used in artificial joints as a wear-resistant material. Cross-linked polyethylene is preferred in hip replacement because of its resistance to abrasive wear. Knee replacement, however, requires PE made with different parameters because cross-linking may affect mechanical strength and there is greater weight-bearing in knee joints. Manufacturers start with ultra high molecular weight polyethylene, and crosslink with either electron beam or gamma irradiation.
- Dental applications. Some application of PEX has also been seen in dental restoration as a composite filling material.
- Watercraft. PEX is also used in many canoes and kayaks. The PEX is listed by the name Ram-X, and other brand specific names. Because of the properties of Cross-Linked Polyethylene, repair of any damage to the hull is rather difficult. Some adhesives, such as 3M's DP-8005, are 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.
- Power cable insulation. Cross-linked polyethylene is widely used as electrical insulation in power cables of all voltage ranges but it is especially well suited to medium voltage applications. It is the most common polymeric insulation material. The acronym XLPE is commonly used to denote cross-linked polyethylene insulation.
- Automotive Ducts & Housings. PEX also referred to as XLPE is widely used in the aftermarket automotive industry for cold air intake systems and filter housings. Its properties include high heat deflection temperature, good impact resistance, chemical resistance, low flexural modulus and good environmental stress crack resistance. This form of XLPE is most commonly used in rotational molding; the XLPE resin comes in the form of a 35 mesh (500 µm) resin powder.
- Low-density polyethylene (LDPE)
- Linear low-density polyethylene (LLDPE)
- High-density polyethylene (HDPE)
- Medium-density polyethylene (MDPE)
- Ultra-high-molecular-weight polyethylene (UHMWPE)
- Stretch wrap
- 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.
- 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."
- 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."
- 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)"
- 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."
- 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...."
- 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."
- Television program Ed The Plumber, DIY Network, 2006
- 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.""
- 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"
- 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."
- 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."
- 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."
- 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."
- 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."
- 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."
- 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]
- 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."
- 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."
- "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...."
- "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..."
- Jack Sweet (October 1, 2010). "What was that flurry of activity 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."
- 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". Reuters. January 27, 2009. Retrieved June 23, 2009.
- "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."
- "PEX choices". Home Heating Systems Newsletter. Archived from the original on June 11, 2008. Retrieved 2008-06-12.
- History of PEX on Plastic Pipe and Fittings Association (PPFA) homepage
- PEX FAQs on Plastic Pipe and Fittings Association (PPFA) homepage
- PEX Association, The History and Influence of PEX Pipe on Indoor Environmental Quality, 
- As a viable Polybutulene repipe option.
- Analytical techniques to characterize crosslinked polyethylene