PEX pipe is polyethylene pipe that is cross-linked (X-linked). Polyethylene (PE) consists of long molecules or polymer chains. When PE is heated it softens and can easily be deformed. Therefore PE pipe cannot be used for hot water service because exposure to hot water for long periods of time would cause the pipe to soften and deform (creep) under sustained water pressure inside the pipe. The deformation occurs because the polymer chains are pulled apart. This problem is solved by chemically tying the long polymer chains together so they cannot be pulled apart. This process is called cross-linking abbreviated using the letter X. Since the long polymer chains are chemically connected, they cannot separate so PEX does not deform or creep when used long term in hot water service.
There are three different processes used to manufacture PEX. The three processes are referred to as a, b, and c. The difference in the three processes is the chemistry used to create the X (cross-links) that ties the polymer chains together:
PEXa adds chemicals (peroxides = oxidants) to the PE which create highly reactive free radicals which attack the PE chains causing them to bond together. Unfortunately, free radicals are oxidizers and they actually make the PE less stable and more prone to oxidative degradation because the PE is partially oxidized (degraded) during its manufacture.
PEXb uses a process which is not based on free radicals and therefore the PE becomes cross-linked without exposure to oxidation processes.
PEXc process involves two steps. The first step involves manufacture of PE pipe coils. The PE pipe coils are then sent to another company which uncoils the PE pipe and exposes the pipe to high energy radiation. The radiation attacks the PE pipe making the PE polymer chains reactive with each other. Unfortunately the energized PE polymer chains are also reactive with oxygen in the air and also the antioxidant stabilizers in the pipe which stabilize the pipe against degradation during use. The PEXc X-linking process bombards the pipe with an uneven dose of high energy radiation so that the pipe is not uniform in composition around its circumference. The antioxidants in the plastic formulation are destroyed by the high energy radiation, and because the radiation dose is not uniform around the pipe, the final PEX pipe is not uniform in its resistance to oxidation during use, especially when the pipe is used in hot water service.
To summarize, PEXa and PEXc processes both involve oxidation chemistry which partially oxidizes (degrades) the pipe during the manufacturing process while PEXb process does not involve oxidation chemistry.
Which Type of PEX is Best?
PEXb is best because the antioxidants are not destroyed during the X-linking process and the pipe is not exposed to oxidative processes. Therefore, PEXb is inherently more oxidation resistant than PEX a or PEXc. Since, it has been our experience that the main cause of failure of PEX is oxidation, PEXb is best because it is inherently more oxidation resistant.