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.

Almost all PEX is made from high density polyethylene (HDPE). 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. 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.

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 it was designed for.

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.[2] 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 2 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.

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.

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 25% of 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. 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.

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.