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 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.^[1] In metric PEX is normally available in 16 mm, 20 mm, 25 mm, 32 mm, 40 mm, 50 mm and 63 mm sizes.

Contents 1 Properties 2 History 3 Classification 3.1 North America 3.2 Europe 4 Uses 4.1 Residential plumbing 4.2 Benefits And Drawbacks 4.3 Government approvals 4.4 Other uses for PEX 4.5 Competitors to PEX 5 PEX-AL-PEX 6 Pex tool kit 6.1 Properties 7 See also 8 References 9 External links

Properties

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.

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, 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.

Classification

North America

The current cell classification in North America for PEX tubing is either 0006, 0008, 1006, 1008, or 0306 a 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

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.

Uses

Residential plumbing

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.^[3] In 2006, the Philadelphia Inquirer recommended that plumbing installers switch from copper pipes to PEX.^[4]
In the twentieth century, the first plumbing pipes were made from galvanized steel water pipes, but users experienced problems with the internal build-up of rust which reduced water volume, but these were replaced by copper pipes in the late 1960s.^[5] 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.^[6] PEX is suitable for recirculating hot water.^[7] 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.^[5] PEX can be used for underground purposes, although one report suggested that appropriate "sleeves" be used for such applications.^[7]

Benefits And Drawbacks

Benefits of using PEX in plumbing include:

• Flexibility. PEX has become a contender for use in residential water plumbing because of its flexibility.^[8] 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 can not 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.^[2]

• 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.

• Less materials cost. Cost of materials is approximately 25% of alternatives.^[9][10] One account suggested that the price of copper had quadrupled from 2002 to 2006.^[2]

• Easier installation. Installing PEX is much less labor intensive than copper pipes, since there is no need to use blow torches to solder pipes together, or to use glue to attach pipes to fittings.^[9] One home inspector wrote that "Once you've worked with PEX, you'll never go back to that other stinky glue stuff."^[11] Builders putting in radiant heating systems found that PEX pipes "made installation easy and operation problem-free."^[8] 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.^[2] Generally, fewer connections and fittings are needed in a PEX installation.^[4]

• Reliable. It neither corrodes nor develops so-called "pinhole" leaks.^[4]

• No fire risk during installation. Copper piping required soldering using blowtorches, 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.^[12] Overall PEX piping is much safer to install; according to the U.S. National Fire Prevention Agency, blow torches used for soldering metallic plumbing ranked as one of the "top-ten leading causes of house fires each year."^[13]

• Acceptance by plumbers. There are routinely advertisements for plumbers specifically seeking ones with PEX experience.^[3]

• 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,^[2] 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.^[11] Typically, red PEX tubing is used for hot water while blue PEX tubing is used for cold water.^[4]

• 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.^[14] One account suggested that PEX water-filled pipes, frozen over time, will swell and tear; in contrast, copper pipe "rips" and PVC "shatters".^[15] 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.^[16] 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 constructions, it is recommended that all water pipes be sloped slightly to permit drainage, if necessary.^[16]

• No corrosion. Copper and iron pipes can experience corrosion leaks but PEX does not have these problems.^[13]

• 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.^[10]

• 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.^[17]

Drawbacks of PEX piping include:

• Degradation from sunlight. PEX tubing cannot be used in applications exposed to sunlight, as it degrades fairly rapidly.^[18] 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.^[18]

• 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 which caused a negative interaction between its mineral-rich hard water^[19] and so-called "yellow brass" fittings.^[7] What had happened was that zinc in the fittings leached into the pipe material in a chemical reaction known as dezincification, causing some leaks or blockages.^[19] A solution was to replace the yellow brass fittings which had 30% zinc with red brass fittings which had 5% to 10% zinc.^[19] 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.^[7]

• 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.^[20]

• 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.^[17]

• 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.^[2]

• Potential problems for PEX radiant heating with iron-based components. If PEX tubing is used in a radiant heating system which 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.^[21] 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.^[22][23] An environmental impact report and subsequent studies determined there were no causes for concerns about public health from use of PEX piping.^[22]

Government approvals

PEX has been approved for use in all fifty states of the United States as well as Canada,^[4] including the initially reluctant state of California, which approved its use in 2009.^[7] California allowed the use of PEX for domestic water systems on a case-by-case basis only in 2007.^[24] 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.^[7] Formal adoption of PEX into the CPC occurred on August 1, 2009, allowing local jurisdictions to approve its general use,^[25] although there were additional issues, and new approvals were issued in 2010 with revised wordings of the 2007 act.^[26]

Other uses for PEX

• Artificial joints. Highly cross-linked polyethylene under the trade name Durasul is used in artificial joints as a wear-resistant material. Cross-linked polyethelene is preferred in hip replacement because of its resistance to abrasive wear. Knee replacement, however, requires PE made with different parameters because cross-linking reduces mechanical stregnth and there is greater weight-bearing in knee joints.

• 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 as 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 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.

• Power cable insulation. PEX is widely used as electrical insulation in power cables, especially for medium voltage. It is the most common polymeric insulation material.

Competitors to PEX

Alternative plumbing choices include:

• Aluminum plastic composite are aluminum tubes laminated on the interior and exterior with plastic layers for protection.^[4]

• Corrugated stainless steel tubing which is continuous flexible pipes made out of stainless steel which have a PVC exterior and are air-tested for leaks.^[4]

PEX-AL-PEX

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.^[27] 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 is a small briefcase which includes a number of basic tools which are 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+.

Properties

A typical PEX tools kit includes Crimp tools, clamp tools, PEX cutters, rings, boards and staplers.

See also

• 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

Plastics Polyacrylic acid (PAA) · Cross-linked polyethylene (PEX, XLPE) · Polyethylene (PE) · Polyethylene terephthalate (PET, PETE) · Polyphenyl ether (PPE) · Polyvinyl chloride (PVC) · Polyvinylidene chloride (PVDC) · Polylactic acid (PLA) · Polypropylene (PP) · Polybutylene (PB) · Polybutylene terephthalate (PBT) · Polyamide (PA) · Polyimide (PI) · Polycarbonate (PC) · Polytetrafluoroethylene (PTFE) · Polystyrene (PS) · Polyurethane (PU) · Polyester (PEs) · Acrylonitrile butadiene styrene (ABS) · Poly(methyl methacrylate) (PMMA) · Polyoxymethylene (POM) · Polysulfone (PES) · Styrene-acrylonitrile (SAN) · Ethylene vinyl acetate (EVA) · Styrene maleic anhydride (SMA)

References

External links

• 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, [1]