FlowGuard™ Flex

Types of PEX

FlowGuard™ Flex Pipe Means Performance...

FlowGuard Flex pipe is made from TempRite^® cross-linked PEX compounds, which are ideal for pipe applications. Cross-linked materials undergo a chemically-induced change in structure that ties the molecular chains together in a three-dimensional network. This network provides enhanced material performance over conventional polyethylene (PE), including increased mechanical strength at elevated temperatures, improved chemical resistance, enhanced stress-crack resistance and reduced creep.

There are three main categories of PEX piping, grouped by the method in which they are manufactured. PEX pipes can be produced by using processes that employ peroxide (Type A), radiation (Type C) or silane (Type B) to cross-link the polyethylene.

Peroxide (Type A)

Peroxide Method (Engel Process) is used by Wirsbo and Rehau to manufacture their PEX pipe. This is referred to as PEX-a in Europe.

Peroxides are chemicals with a “─O─O─” structure. When heated to their thermal homolysis temperature, free radicals are formed. These free radicals will start to abstract hydrogen from the polyethylene molecules. As a result, some of the carbons atoms will lose their hydrogen and become unstable. Two adjacent carbon atoms of such condition (unstable) will form a carbon─carbon bond to achieve stability, thus, forming a cross-link. This process propagates until the peroxide is consumed. This method requires these free radicals to form a cross-link product.

The main disadvantage of peroxide cross-linking is that certain additives (e.g. antioxidants, oils,) must be avoided because they consume free radicals. Therefore, in order to achieve the minimum required cross-link level, the use of antioxidant must be minimized. Hence, minimal stabilization for aggressive water condition (high chlorine) results.

As of January 1, 2007 no peroxide (Type A) PEX can or has passed and received NSF P171 Cl-R (chlorine-recirculation) test. For the most up-to-date NSF P171 listing, please go to www.nsf.org.

Note: Antioxidants are added and evenly distributed in the polymer matrix. They are required to protect the pipe from its natural aging, but more specifically to protect against the accelerated degradation caused by chlorine. 

Radiation (Type C)

This method uses high-energy electron beam or gamma ray for cross-linking. This method is used by Mercury and Roth to manufacture its PEX pipe. This is referred to as PEX-c in Europe.

Polyethylene pipe is first extruded, and then bombarded with radiation in a second step. Hydrogen atoms are ‘knocked off’ from the carbon atom by this high-energy radiation leaving behind a free radical polymeric chain. This free radical is unstable and will seek another free radical to form a stable bond or cross-link. As irradiation progresses, more and more bonds are formed to create a cross-linked structure. However, as mentioned above, the presence of antioxidants will consume free radicals. Therefore, the use of antioxidants must be minimized in order to obtain the desired cross-link level. Hence, minimal stabilization is possible.

As of January 1, 2007 no radiation (Type C) PEX can or has passed and received NSF P171 Cl-R (chlorine-recirculation) test. For the most up-to-date NSF P171 listing, please go to www.nsf.org.

Silane (Sioplast) (Type B)

Silane cross-linking uses a siloxane bridge to link polyethylene molecules. This method is used to manufacture the FlowGuard Flex PEX pipe. This is referred to as PEX-b in Europe.

After extrusion, the pipes are cross-linked by using moisture as the active reagent. The cross-linking involves a hydrolysis and condensation reaction. This process does not generate or require free radicals for cross-linking. Therefore, PEX pipe made by the Sioplast method can be properly stabilized with the appropriate antioxidants.  The antioxidants are not consumed in the cross-linking step and are available to protect the pipe in its end use application.   

This has proven itself out in chlorine testing. Only PEX pipes made by the Silane (Sioplast) method have been able to meet the continuous re-circulation requirement of NSF P171 Cl-R (chlorine-resistance) Protocol (the industry most stringent requirements).   

It's a fact: FlowGuard Flex 3 layer piping solution has passed NSF P171. Check it out at www.nsf.org.

All three methods can produce quality PEX pipe, and it is best to choose the one that best meets the requirements of a specific application.

Consideration in selecting the right PEX for your home or project:

• Is there chlorine present at your water source?
• How much chlorine is present on a daily average?
• Will the pipe be exposed to UV light prior to or during installation?
• What is the expected accumulated UV exposure time?
• What is the maximum allowed UV exposure? (Type A is typically limited to 15 days and 30 days.)

Verify your PEX pipe manufacturer

To verify what each PEX manufacturer has achieved in third party testing of its product for use in traditional domestic (NSF P171 Cl-TD) and re-circulating (NSF P171 Cl-R) potable water systems that contain chlorine, please refer to the NSF web site.  Site directions are:

1. Go to:  www.nsf.org
2. Select "Search Listings" at the bottom of the page.
3. Select "Plumbing Systems Components" at the bottom of the page.
4. Product Standard list, select "Protocol P171".
5. Select "Search".

Free Radical 101

Free radicals are nothing more than unstable molecules. Typically, stable molecules contain pairs of electrons. When a chemical reaction breaks the bonds that hold paired electrons together, free radicals are produced. This loss of an electron makes the molecule electrically unbalanced and the molecule is said to be "unstable". The unstable molecule is very active and demands to re-stabilize itself. The only way it can re-stabilize is to replace the electron it lost. The free radical regains its stability by robbing its neighbor. It physically rips an electron from another atom. It literally steals an electron from its neighbor. When the "attacked" molecule loses its electron, it becomes a free radical itself, beginning a chain reaction.

Free radicals are short-lived reactive chemical species having one or more electrons with unpaired spins.

Chemical antioxidants act by donating an electron to a free radical and converting it to a non-radical form. Antioxidants are molecules, which can safely interact with free radicals and terminate the chain reaction. These are in effect free radical scavengers.

Antioxidants are simply substances that prevent, neutralize, or kill free radicals.