PEX Tubing for Potable Water Plumbing

PEX or Cross-linked polyethylene is a form of polyethylene with crosslinked bonds. It is formed into plumbing tubes used for potable plumbing applications as a replacement to copper and PVC, and CPVC piping. PEX tubing ranges in size from imperial sizes of ¼” to 4″ in diameter however ½” ¾” and 1″ are most commonly used. PEX tubing works directly with PEX fittings. Metric PEX is 16/2025/32/40 mm in diameter. PEX tubing for plumbing has been used for much longer in Europe but is not becoming the plumbing material of choice for the North American market as well.

Almost all PEX is made from high density polyethylene (HDPE). PEX contains cross-linked bonds in the chemical polymer structure, changing the thermoplastic characteristics to a thermoset state. Crosslinks are accomplished at the point of or directly after the extrusion of PEX tubing. The degree of cross-linking, (to comply with ASTM Standard F 876-93, is from 65 to 89%. A higher degree of cross links could result in brittleness/stress-cracking of the PEX.

The high temperature properties of the PEX are improved by cross links. Adequate strength to 120-150°C maintains by reducing the flow tendencies. Chemical resistance is enhanced by dissolution resistance. Tensile strength, scratch resistance, and brittleness fracture resistance are also enhanced. PEX cables have a maximum conductor temperature of 90°C and a high rating to 140°C. This rating is the same on PEX fittings as well.

PEX material for plumbing projects was first prepared in the 1930s. The process was to irradiate the extruded tubes with electron beams. The beam-processing method was made developed in the 1970s but was still quite expensive. In the 60s, Engel PEX-linking was deployed. With this method, a peroxide chemical is mixed with the HDPE before extrusion, the cross-linking happens during the passage of a melted polymer through a long heated die. In 1968, the Sioplas process incorporating silane was patented, followed by another silane process called Monosil in 1974. Also a manufacturing process using vinyl-silane was developed and used in plumbing supplies in 1986.

The current classification in the US and Canada for PEX tubing is 0006, 0008, 1006, or 1008 numerals. In North America PEX tubing products perform to certain standards and should be rated for performance by ASTM (F876, F877), NSF body (NSF-pw, NSF 61, NSF 14, CL-TD, CL-R), and Canadian Standards Association CSA standards B137.5, for which they are tested. The certifications/listings met by the products are printed onto the tubing to be sure that the product is used for the application to which it was intended.

In the 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.

After installation, PEX-B material will have the same properties as PEX-A. PEX-C is produced with electron beams process, in “cold” linking (under the crystal melting point). It provides lower-degree crosslinks that are less uniform than through the Engel method, especially at diameters over 1″ (2.5 cm). When the process is not controlled well, PEX outer layers of the tubing becomes brittle over time affecting pex plumbing for potable plumbing water applications. Yet, it is still the cleanest, most environmentally efficient method of the three processes, because it does not involve any other chemicals and uses only high density energy electrons to split carbon/hydrogen bonds to facilitate cross linking.