Cross-linked polyethylene, known by its abbreviations as PEX or XLPE, is a form of cross-linked polyethylene. It is used to make tubes and hoses, and is widely used in the construction of radiant heating systems that use water, domestic water pipes, and insulation for high-voltage electrical cables. It is also used for systems that handle natural gas and offshore oil facilities, chemical transportation, and wastewater and sewage transportation. Additionally, it has become an alternative to Polyvinyl Chloride (PVC), Chlorinated Polyvinyl Chloride (CPVC) or copper tubing as plumbing for home water systems. PEX pipes are manufactured in a series of measurements ranging from 1/4-inch to 32-inches in diameter, but 1/2-inch, 3/4-inch, and 1-inch pipes are the ones most commonly used in the real estate industry; in the mining industry, they are used from 4" to 32" in diameter for the transportation of water, transportation of abrasive fluids and fluids. corrosive.[1]​ In international units, PEX pipes are manufactured in the following diameters: 16 mm, 20 mm, 25 mm, 32 mm, 40 mm, 50 mm, 63 mm, 75 mm, 90 mm and 110 mm.

There are several grades of PEX depending on the cross-linking it has: PE-Xa (peroxide method) with a cross-linking greater than 70%, PE-Xb (silane method) with a cross-linking greater than 65% and PE-Xc (electron radiation method) with a cross-linking greater than 60%. Everything is included in the UNE EN ISO 15875 standard. The more cross-linking the pipe has, e.g. e.g. PE-Xa, the better it is and in more applications it can be used.

For the use of cross-linked polyethylene pipes in radiator heating installations or in invisible air conditioning (underfloor heating) systems, an extra bath of EVAL or EVOH (etivinyl alcohol) is usually given to provide the PEX pipe with protection against oxygen diffusion and thus avoid the generation of what is known as sludge in the closed circuit of this type of installation. The impermeability of the installation made with PEX pipe with anti-oxygen diffusion barrier (evalPEX) will also help us to ensure that the metal elements that make up said installation (boilers, radiators, etc.) suffer less wear and/or oxidation, lasting longer over time.