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 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 1/4" to 4", but 1/2", 3/4", and 1" are by far the most widely used.

# Properties
Almost all PEX is made from high density polyethylene (HDPE). PEX contains cross-link bonds which into the polymer structure, changing the thermoplastic into an duromer. 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 120-150C 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 90C and an emergency rating of up to 140C, depending on the standard used to rate PEX-insulated cables. Cables insulated with PEX also have a conductor short-circuit rating of 250C. 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. This process was made feasible in 1970s but was still expensive. In the 1960s, Engel cross-linking was perfected. 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.

# Classification
In European standards as adapted also in North America, these three classifications are referred to as PEX-A, PEX-B and PEX-C, and are not related to any type of rating system. All the resulting PEX tubing products perform to standards and are rated for performance by the ASTM, NSF International, and Canadian Standards Association standards, 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.

## PEX-A (PE-Xa, PEXa)
PEX-A is produced by the peroxide (Engel) method. 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. The degree of cross-linking is above 70%.

## 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 between carbon and silicon atoms.

## PEX-C (PE-Xc, PEXc)
PEX-C is produced by the electron irradiation method, 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 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 20% less than alternatives, and installation is much less labor intensive.
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 joints as a wear-resistant material.
Some application of PEX has also been seen in dental restoration as a composite filling material.

# 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.