General | History of PEX | FAQ | PEX Members | Publications
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
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.