PROS&CONS OF PEX-A, PEX-B, PEX-C TUBING

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The cross-linked polyethylene tubing(PEX) that provides an excellent option for plumbing, radiant and hydronic systems in both residential and commercial plumbing applications is widely used in America, Europe as well as Australia. Each area has its standard in PEX installation which is searchable on www.astm.org. Hope you can help yourself choosing the best PEX type for your upcoming domestic or industrial projects after reading this essay.

Manufacture Process of PEX Tubing

The PEX-A & PEX-B & PEX-C are all processed by Crosslinking, which is a chemical reaction that occurs between polyethylene polymer chains.

Crosslinking causes the HDPE to become stronger and resistant to cold temperature cracking or brittleness on impact while retaining its flexibility.

The three methods of crosslinking HDPE are:

1. PEX-A : Peroxide (Engels) method. During the manufacturing process, free radicals are created when HDPE polymer is melted and cross-links between molecules occur at temperatures that exceed the decomposition temperature of the polymer
2. PEX-B : Silane (Moisture Cure) Method. After the extrusion process, cross-linking where links between the molecules of the HDPE using a catalyst and exposing PEX tubing to hot water. This type of PEX  is  the  most
3. PEX-C : Radiation (Electronic Irradiation) method. Cross-linking of the molecules is done after the process of extrusion by exposing the pipe to an electron radiation beam. The radiation emitted allows to break the existing links between molecules of the polymer and initiate cross-linking process.

Pros&Cons of PEX-A, PEX-B, PEX-C Tubing

PEX-A

• The most flexible and highest quality tubing on the market, which makes it the most expensive of all those 3 types.
• The kinks in the pipe could be repaired by merely heating the pipe with a heat gun for it is more elastic than PEX-B.
• The wall thickness of PEX-A systems is normally greater than PEX B because it has a lower burst strength and hence requires the greater thickness to cope with the same operating pressures.

As a result of the thicker wall of PEX-A pipe, it will generally have a lower SDR and hence a lower flow rate.

• The pipe is expanded around the fitting rather than the fitting being fit inside the pipe. This may make fittings harder to install if you’re working in cold conditions.

 

Tips: This PEX is slightly more flexible than the other types at temperature above 60oF. However, below this temperature, PEX-B tends to be more flexible and offer an advantage during installation in colder climates.

Several industry participants claim that the PEX-a method yield more flexible tubing than the other methods

PEX-B

• The cheapest one of those 3 types.
• The expansion fitting s couldn’t work with PEX-B.
• The flow rate is superior in comparison to PEX-A, as it is often rated for higher pressure than PEX-A.
• The most commonly use PEX in North America, including compression, pushfit and copper crimp rings. It is more affordable because is is more rigid.
• The higher crystallinity molecular structure results in PEX B product having a higher surface hardness, better  scratch  resistance, higher  thermo  stability, higher rigidity, harder body, better resistance to chlorine solutions and a higher burst strength.
• Though all PEX types are vulnerable to high chlorine counts, it performs better in chlorine testing than other types.
• The pipe can degrade if exposed to high levels of the chemical.
• The kinks in the PEX-B must be cut and repaired with a fitting.

 

 

PEX-C

• Similar price to PEX-B, similar quality to PEX-A, but not enough to be used with full-flow fittings.
• The pipe is more flexible than PEX-B, although it may feel slightly more “plastic-like” than other PEX types.
• The more environmentally-friendly manufacturing process it may in fact become a more popular choice in the future.
• The cross-linking process for this type creates the least amount of bonds, which makes it closer to a regular PolyEthylene plastic pipe as opposed to true PEX tubing PEX A and B.

 

 

 

Comparison of Main Attributes

Property	PEX-A	PEX-B	PEX -C
Flexibility	Better flexibility than PEX-B	Stiffer to work and bend than the other 2	Varies
Kinking resistance	Good	Poor (Most vulnerable to be kinked than PEX-A or PEX-C)	Fair (More vulnerable to be Kinked)
Kink Reparability	Excellent	Not Possible	Decreases after heating to transparency
Strength	Meets ASTM	Meets ASTM	Meets ASTM
Homogeneity	Not very good	Good	Good
Thermal Memory	Good	Poor	Poor
Barrier property	Meets DIN 4726	Meets DIN 4726	Meets DIN 4726
Thermostability	Better than ASTM requirements	Better than ASTM requirements	Better than ASTM requirements
Memory effect of being coiled	Fair – Better than PEX-B	Poor	Fair – Better than PEX-B
Minimum bending radius	Good – Better than PEX-B	Poor – The worst of all PEX types	Varies
Density	0.938 – higher	0.941 – highest	0.941 – highest

 

 

CROSSLINKED POLYETHYLENE

In the early 1980s, crosslinked polyethylene tubing (PEX) made its way from Europe to North America. At first, it was generally viewed as a product limited to use in radiant floor heating systems. Today, PEX tubing is largely recognized as a universal piping material, with a wide variety of applications in both hydronics and potable water distribution.

Most of the PEX tubing used in North American hydronic systems conforms to the ASTM F876/877 standard. It is commonly available in nominal sizes of ” to two-inch, and with pressure temperature ratings of 80 psi at 200F, and 100 psi at 180F. The type of PEX used in most closed loop hydronic systems is known as “barrier PEX.”  It has a thin layer of a compound called EVOH (ethylene vinyl alcohol), which greatly reduces the rate at which oxygen molecules can diffuse through the pipe wall. This reduces the potential for oxygen based corrosion of ferrous metal components, such as cast-iron circulators or steel expansion tanks.

PE-RT

A relative newcomer in North America, PE-RT stands for polyethylene raised temperature. Used for over 20 years in Europe, PE-RT is certified to ASTM F2623, and provides pressure/temperature ratings of 160 psi at 73F and 80 psi at 180F. PE-RT tubing is available with an oxygen diffusion barrier for closed hydronic systems. It is available in sizes ranging from ” to one-inch nominal inside diameter.

One key difference between PE-RT and PEX, is that PE-RT is a thermoplastic, whereas PEX is a thermoset polymer. A thermoplastic can be heated and melted, whereas a thermoset polymer cannot. This allows PE-RT to be joined to compatible polyethylene fittings using socket fusion. The outside surface of the tube, and inside surface of the fitting are simultaneously heated using a special tool, such as that shown in Figure 5.

At temperatures in the range of 450F to 465F, PE-RT assumes a semi-molten state. After a specified heating time, the tubing and fitting are simultaneously pulled off the heating tool and pushed together to form a very strong and permanent bond. PE-RT is also used in combination with an aluminum core to create a composite pipe referred to as PERT-

 

 

Todd Forner  |Technical Sales & Design  | Johnstone Supply Muskegon Group

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