Cross-linked Polyethylene (PEX) is a plastic material made from medium- or high-density polyethylene that is modified to have improved properties such as enhanced heat resistance and long term strength and stability that make it an excellent choice for use in service lines, hot- and cold-potable water distribution systems, residential fire-sprinkler systems and hydronic heating applications. Pipes made from these materials are produced to meet the requirements of ASTM F876, ASTM F877, AWWA C904 and/or CSA B137.5.
PEX piping has been used in hot- and cold-water distribution systems and for hydronic radiant heating in Europe for decades. Introduced into the United States in the 1980's, PEX is the most widely-used flexible plumbing piping for plumbing and radiatn floor heating applications.
PEX tubing is manufactured by extrusion in sizes from ¼"-3" nominal diameter. It is controlled to OD controlled to (Outside Diameter) dimensions in what is commonly called CTS (copper tube size). The wall thickness is based on a Standard Dimension Ratio 9 (SDR 9), meaning that the ratio of the OD to the wall thickness of the tubing is 9:1. It is available in coils of various lengths and straight lengths.
A significant benefit of PEX and other plastic pipe is its small environmental impact compared to other materials used for similar applications. To learn more click here.
PEX tubing can be used in service lines or potable water distribution systems provided it has been tested in accordance with the governing standard, meets the requirements of ANSI/NSF Standard 61, and bears proper certification from a recognized testing agency. PEX tubing is also widely used for heat transfer applications - both low-temperature (radiant floor heating and/or cooling, snow melting, turf conditioning and permafrost protection), as well as distribution piping for temperatures up to 200°F (hot-water baseboard, convectors, radiators, etc.) and some is approved for use in residential fire-sprinkler systems.
PEX tubing is recognized as acceptable for water distribution piping in all major model plumbing codes.
PEX tubing is available from plumbing wholesalers, building centers, and manufacturers throughout the U.S.A. and Canada.
PEX tubing must be labeled as follows:
- The manufacturer's name or trademark
- The standard to which it conforms (ASTM F876, F877, AWWA C904 and/or CSA B137.5)
- Tube size and CTS
- Material designation code (PEX0006 or similar)
- Pressure/temperature rating(s)
- If the tubing is for potable water, a laboratory seal or mark attesting to suitability for potable water
- ASTM fittings designations approved for use by the tubing manufacturer
The marking interval on the tube is required to be no more than five feet.
The Online Piping & Usage Specification by the Mechanical Contracting Education & Research Foundation (MCERF) informs users on pipe types, materials, applications and joining methods.
PEX results from chemically joining individual polyethylene molecules in order to improve the performance of the original base resin in higher temperatures. The primary reason for cross-linking polyethylene (PE) is to raise the thermal stability of the material under load. This substantially improves environmental stress crack resistance and resistance to slow crack growth.
In the late 1950s scientists worked on the structure of polyethylene to strengthen the connections between the polymer chains. They developed ways to create additional ties between the PE molecules through covalent or chemical bonding. The result was a PE structure that did not "flow" or move to a softened state as quickly when the temperature is increased.
PEX Manufacturing Methods
There are three primary methods for producing PEX tubing.
1. The "Engel" or peroxide method employs a special extruder with a plunger action where peroxide is added to the base resin and through a combination of pressure and high temperature the cross-linking takes place as the tubing is produced.
2. The "Silane" method of PEX production involves grafting a reactive silane molecule to the backbone of the polyethylene. The tubing is produced by blending this grafted compound with a catalyst which can be done using either the Sioplas method or by using a special extruder it can be done using the Monosil method. After extrusion the tubing is exposed to either steam or hot water to induce the final cross-linking reaction in the tubing.
3. Electron Beam crosslinking takes place when very high energy radiation is used to initiate molecular cross-linking in high density polyethylene. This product is extruded like normal HDPE then taken to an E-beam facility and routed under a beam or ray in the accelerator where it is dosed with a specific amount of radiation to release the hydrogen atoms and cause polymer cahins to bond or link to the open carbon sites.
In European standards these three methods are referred to as PEX-A, PEX-B and PEX-C, respectively and are not related to any type of rating system. All the resulting PEX tubing products perform similarly and are rated for performance by the ASTM, NSF and CSA standards for which they are tested and certified. The listings and certifications met by each product are printed on the printline of the tubing itself to ensure the product is used in the proper applications it was designed for.