Cross-Linked Polyethylene Tubing (PEX)
Definition: PEX is hydronic tubing manufactured from polyethylene plastic which has, as part of the manufacturing process, a three dimensional molecular bond created within the structure of the plastic which dramatically improves a large number of properties such as heat deformation, abrasion, chemical and stress crack resistance. Impact and tensile strength are increased, shrinkage decreased and low temperature properties improved. Cross-linked tubes also have a shape memory which only requires the addition of heat to return it to it's original shape when kinked, PEX is a thermoset material made from medium or high density, cross-linkable polyethylene meeting the requirements of ASTM F876 and F877.
Relevance: Tubing used in hydronic heating systems must withstand the rigors of long term use in a difficult environment. While non-cross-linked polyethylene has many good qualities, it is limited primarily by temperature and pressure. Cross-linking adds a great deal of safety margin and produces a pipe which can easily meet the specifications for hot water heating set by code bodies, and provide lasting confidence in its ability to perform.
Discussion: Almost all PEX tube begins as High Density Polyethylene (HDPE) resin which is put through the cross-linking process during the manufacturing of the tubing. PEX tubes are never fully cross-linked because this would make them too brittle and highly prone to stress cracking. Too little cross-linking can have equally serious effects or result in a product which is no better than the original material. The ideal is to find a degree of cross-linking which will produce the best strength while retaining the flexibility needed for working with the tube and the elimination of stress cracking. Depending on the method and care in manufacturing, the ideal degree of cross-linking will vary. ASTM Standard F 876-93 Cross-linked Polyethylene (PEX) Tubing requires a degree of cross-linking in the range from 65% to 89%.
The first cross-linking was done in the 1930's using irradiation (electron beam). Since that time several chemical processes have been developed, the most common of which use peroxide or silane to instigate the cross-linking.
Calcium Carbonate Formation on Cross-Linked Polyethylene (PEX) and Polypropylene Random Copolymer (PP-r)
The accumulation of calcium carbonate (referred to as scale) on the surface of cross-linked polyethylene and polypropylene random copolymer tubes is compared to that on copper. Water with total calcium and carbonate concentration of 3×10–3 M and a pH of approximately 9.1, yielding a supersaturation of 7.8, was pumped through the tubes at a velocity of 0.07 m/s for 2.5, 5, and 7.5 h. Flow was laminar with Reynolds numbers of <1000. Sections of the tubes were analyzed at the designated time points to determine the extent of scaling. Results include scanning electron microscope images of the tube surfaces before and after exposure to the supersaturated water and chemical analysis to determine the mass of calcium carbonate per unit surface area. Measured scaling rates of the two polymer tubes are similar to that of copper.
Cross Linked Polyethylene (PEX) is an increasingly popular material for a variety of piping applications. PEX is hydronic tubing that features a stable three-dimensional molecular bond within the structure of the plastic.
Advantages of PEX Pipes
Due to this three-dimensional molecular bond, PEX pipes offer a number of advantages over traditional metal pipes.
The bond improves the pipes' resistance to:
• Heat deformation
• Stress cracks.
In addition, impact and tensile strength are increased and both high and low temperature properties improved. No special additives or water treatment is necessary for either soft or hard water; PEX pipes ensure years of constant water flow and pressure.
Also, PEX pipes are:
• Quieter than copper and other metal pipes due to their flexibility and ability to absorb pressure surges
• Convenient to install, requiring no solders, flames, or chemicals.