HISTORY

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.

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