The Power of PEX Tubing

By Lawrence Drake
Executive Director, Radiant Panel Association

The world of hydronic radiant heating and domestic water piping was destined to change when, in 1933, an explosion in an English chemical company laboratory produced polyethylene (PE) plastic. By 1939, the process was under control. Early experiments with cross-linking were conducted in the 1930s, using irradiation or an electron beam. Thirty-five years after the explosion, in 1968, a German inventor, Thomas Engel, developed a revolutionary method of chemically cross linking the molecules of polyethylene, resulting in a plastic that was much stronger and could tolerate much higher temperatures than PE. Thus the X was added to PE, to become what we know today as PEX.

PEX is hydronic tubing manufactured from polyethylene plastic that has, as part of the manufacturing process, a three-dimensional molecular bond created within the structure of the plastic that dramatically improves a large number of properties, such as heat deformation, abrasion, chemical and stress crack resistance. Impact and tensile strength are increased, shrinkage is decreased and low temperature properties are improved. Cross-linked tubes also have a shape memory that requires only the addition of heat to return kinked tube to it's original shape.

By the mid-1980s, PEX was dominating the European hydronic heating and domestic water market. It made its way to North America and found a niche in the U.S. radiant heating market. It was first introduced to this market because there were far fewer codes and standards concerning hydronic radiant heating than those that applied to domestic water systems.

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 that can easily meet the specifications for hot water heating set by code bodies, and it can provide lasting confidence in its ability to perform.

Almost all PEX tube begins as high-density polyethylene (HDPE) resin, which is put through the cross-linking process during the manufacture 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 that is no better than the original material. The ideal is to find a degree of cross linking that will produce the most strength, while retaining the flexibility needed for working with the tube and eliminating 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%.

Several processes have been developed over the years for cross linking polyethylene pipe. In recent years they have become known as PEX-A, PEX-B and PEX-C, primarily for easy reference.

The PEX-A Engel (peroxide) cross-linking method, perfected in the 1960s, 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 at a high temperature. While tooling costs are still substantial, initial costs are far less than early electron beam processes.

The PEX-B Sioplas (saline) cross-linking process was patented in 1968, followed by the Monosil (saline) process in 1974 and the Vinylsaline copolymers in 1986. The difference in these methods is primarily in the way that the vinylsaline 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.

The PEX-C Electron beam (irradiation) method was made feasible in the mid-1970s. 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."

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 at 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. The German DIN 16892 is also an excellent standard but may not be accepted locally, due to dimensions that 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.

PEX has the lion's share of the current radiant heating market and has recently made great inroads into the domestic water market as well. While much of the radiant heating tubing is PEX, there are also variations on the pure PEX pipe.

When plastic pipe was introduced to the radiant industry, it took some time to realize that the permeability of plastic could present problems. All hydronic heating and cooling systems are susceptible to oxygen entering the system through numerous sources, such as threaded fittings, air vents and gas permeable materials. Excess amounts of oxygen in a system can lead to premature failure of ferrous metal components due to corrosion. While copper tube is, for all practical purposes, impervious to oxygen migration through its walls, all synthetic tubes display a degree of permeation unless specifically designed to prevent the intrusion of oxygen. Whether this characteristic can lead to problems for ferrous metals in radiant floor heating systems is dependent on a variety of factors. The amount or length of tubing in a system can play a significant role, since it is the available wall surface that makes the tubing suspect. Water temperature must be given strong consideration because, as the temperature of the tube wall increases, it becomes more permeable. The internal system pressure and the speed of flow are less important factors. Water quality can also greatly impact the corrosion process and cause acceleration beyond acceptable limits.

Although oxygen permeation is measurable in the laboratory, there has been much debate as to the degree of long term effects in actual installations. With radiant floor systems numbering in the hundreds of thousands over the last twenty years in the United States, there has not been strong evidence to indicate that oxygen permeation in synthetic tubing has contributed significantly to widespread system failures. It should be noted, however, that there have been a number of incidences of premature expansion tank failure and excessive corrosion of cast iron pumps and cast iron boilers that can be linked directly to excess oxygen in the system.

Today, most manufacturers offer synthetic tubing, including PEX, which incorporates an oxygen diffusion barrier that dramatically reduces the measurable amount of oxygen permeation to the point where it is no longer in question. The RPA Standard Guidelines and the European DIN Standard calls for an oxygen permeation rate of less than 0.1 gram per cubic meter per day and is generally accepted as the measure of an oxygen tight radiant heating tube. Keep in mind that this is only a concern when ferrous metals are present in the system.

The growth of radiant heating in North America has been significant over the last fifteen years. Even with a slowing of the economy and the falling off of the housing market, radiant heating continues to be strong. PEX will continue to play a large role in this growth, worldwide. What started out as an almost exclusively European product is now produced all around the globe. Competition has driven prices down to where the once-expensive PEX has become an affordable commodity.

Quality control and customer service have become the leading sales points for suppliers of PEX products. PEX has also been incorporated into composite piping, utilizing an aluminum core as an oxygen diffusion barrier. There is no doubt that we will continue to see advancement in the development and use of PEX in both the hydronic heating market and the domestic plumbing market.