Crosslinked polyethylene (PEX) was first prepared in the 1930’s by irradiating the extruded tube with an electron beam. Since its initial introduction, PEX has become increasingly popular in the construction industry. In the last few decades, the manufacturing process has improved significantly.
There are currently three methods for producing crosslinked polyethylene (PEX) tubing.
- Engel or peroxide method (PEX-a)
- Silane method (PEX-b)
- E-beam (electron beam) or radiation method (PEX-c)
All three processes generate tubing that is crosslinked to varying degrees. Let’s take a closer at all three methods before diving into the benefits of the Engel method.
Engel Method (PEX-a)
PEX-a is produced by the peroxide (Engel) method. This method performs “hot” cross-linking, above the crystal melting point. The PEX tubing industry considers this tubing superior because the crosslinking is done during the manufacturing process when polyethylene is in its amorphic state (above the crystalline melting point).
The process takes slightly longer than the other two methods as the polymer has to be kept at high temperature and pressure for long periods during the extrusion process. Because of this, the degree of crosslinking reaches around 85%, resulting in a more uniform product with no weak links in the molecular chain.
Silane Method (PEX-b)
PEX-b tubing is crosslinked after the extrusion process by placing the tubing in a hot water bath or steam sauna. The degree of crosslinking for PEX-b is typically around 65 to 70%. This method is not as evenly crosslinked as the PEX-a method, nor does it have the same degree of thermal memory, which allows kinked tubing to be reshaped with the use of a heat gun.
E-beam Method (PEX-c)
PEX-c uses an electron beam to change the molecular structure of the tubing (i.e. crosslink) after the extrusion process. The PEX-c method requires multiple passes under the beam to reach a 70 to 75% degree of crosslinking. Side effects of this process are discoloration due to oxidation (from natural white to yellow, unless another pigment is added), and a slightly stiffer product.
The Benefit of Using PEX-a
PEX-a tubing is currently the leading crosslinked tubing on the market. It is the most flexible of all the PEX tubing types. This flexibility allows for the tightest bend radius possible, which can be as little as 31 ⁄2″ for 1 ⁄2″ tubing.
PEX-a also has little or no coil memory which greatly lowers the risk of kinks. Its high thermal memory means that if any kinks do occur it can easily be fixed with a simple shot of heat from a heat gun.
Shape memory ensures a much stronger connect due to the fact that it can expand and then shrink back to its original size, creating a durable and reliable fitting connection. Finally, it is much more resistant to crack propagation than PEX-b and PEX-c tubing. Effectively this means cracks are less likely to grow over time and cause leaks or damage.
In summary, here is a list of the features and benefits of PEX-a:
- Most flexible
- Smallest bend radius
- Least likely to kink
- Thermal memory
- Shape memory
- Highest degree of crosslinking
- Greater resistance to crack propagation