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.
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.
Electron beam (irradiation) was made feasible in the mid 1970's. Of the three processes, the electron beam requires a substantially greater initial investment in tooling and machinery costing millions of dollars. No chemicals are added or used in the cross-linking process; cross-linking is accomplished after the tube has been extruded, coiled and placed in a special electron beam "oven".
Engel (peroxide) cross-linking, perfected in the 1960's, relies on peroxide being mixed with HDPE and fed into the extruder under high pressure. The product is then passed through a long heated die where the cross-linking takes place caused by a chemical reaction between the peroxide and the polyethylene under high temperature. While tooling cost are still substantial, initial cost are far less than electron beam processes.
Sioplas (silane) process was patented in 1968 followed by the Monosil (silane) process in 1974 and the Vinylsilane copolymers in 1986. The difference in these methods is primarily how the vinylsilane and catalyst are added to polyethylene before it goes through the extruder. While some of the cross-linking occurs in the extruder, the majority actually takes place in a water bath or in a sauna at elevated temperatures after the tube passes through the extruder.
Summary: There is a lot of discussion among manufacturers as to which process produces the best pipe, but a great deal rests on the ability of the one making the product. It appears to be far more important to look at the quality control, testing and support provided by the manufacturer than the method of cross-linking. Products that are manufactured to DIN (European), ASTM (United States) and/or CSA (Canadian) Standards must pass some very rigorous testing. This is a good way to judge the suitability of a product. The standards to look for are ASTM F876-93, ASTM F877-93 and CSA B137.5 (in Canada). The German DIN 16892 is also an excellent standard but may not be accepted locally due to dimensions which do not match North American standards. The requirements for the test methods include materials, workmanship, dimensions, tolerances, and hydrostatic sustained pressure strength. Also included are tests related to system malfunctions. They are designed for system components intended for 100 psi (0.69 MPa) water pressure up to a maximum working temperature of 180°F (82°C). PEX pipes meeting these standards are quite suitable for hydronic radiant heating.