PEX tubing is manufactured using polyethylene (PE). PE is a thermoplastic. Thermoplastics soften when heated allowing them to melt and flow under pressure. Softening under high temperature causes dimensional changes (creep) to occur which is not acceptable for pipes being used for hot water as the diameter of the pipes needs to remain constant over time. For the PE pipes to remain dimensionally constant over time, the long polymer chain molecules in the PE are chemically coupled (crosslinked or X-linked) or tied together. The “X” in PEX stands for x-linked.
A CASE STUDY
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PROBLEM: PEX Failures due to Manufacturing Defects
PEX tubing is manufactured using polyethylene (PE). PE is a thermoplastic. Thermoplastics soften when heated allowing them to melt and flow under pressure. Softening under high temperature causes dimensional changes (creep) to occur which is not acceptable for pipes being used for hot water as the diameter of the pipes needs to remain constant over time. For the PE pipes to remain dimensionally constant over time, the long polymer chain molecules in the PE are chemically coupled (crosslinked or X-linked) or tied together. The “X” in PEX stands for x-linked. Once the long-chain polymer molecules are tied together, the material is no longer thermoplastic but instead becomes a thermoset. Thermosets are dimensionally stable. Crosslinking of the PE to make PEX is achieved using three different chemistries.
The PE polymer chains in PEXa are coupled together using peroxides to form free radicals along the PE polymer chains. Free radicals couple together to tie the PE chains together. This process has the disadvantage that the peroxide depletes some of the antioxidants added to the PE resin to protect it against oxidative degradation thereby jumpstarting the oxidation process in the material.
The PE polymer chains in PEXb are coupled together using silane chemistry. Since silane chemistry does not involve the formation of free radicals, the antioxidant stabilizers in the PE are not consumed during the crosslinking. Therefore, PEXb tends to be inherently the most oxidation resistant of the three types of PEX.
The PE polymer chains in PEXc are coupled together using high-energy electron beam radiation. The PE tubing is passed through a high-energy beam of electrons. Like the PEXa process, free radicals are formed along the PE chains causing the chains to couple together and a significant amount of the antioxidant stabilizers in the PE to be consumed. Also, PEXc tubing tends to lack uniformity in the extent of x-linking around its circumference because the electrons are penetrating a curved surface. The sides of the tubing are exposed to fewer electrons than the top surface that is directly exposed. This not only causes non-uniformity in both extents of x-linking around the tubing circumference but also the local concentration of antioxidant stabilizers.
As mentioned in the previous section, non-uniformity is a common cause of failure for PEXc. However, non-uniformity can also be a cause of failure for any type of PEX tubing. As mentioned previously, PEX will degrade by oxidation and become brittle if it is not stabilized by mixing antioxidants into the wall of the tubing. It is very challenging to uniformly mix antioxidants into plastic. Intense mixing is required. Over time, the mixing elements in the tubing extruder wear out, and the extent of mixing is reduced. When Mixing elements wear out, they must be replaced.
The Solution to Maximizing the Service Life of PEX Tubing
The most common cause of PEX failure is the oxidative degradation of the inside surface of the tubing caused by a chemical attack by the chlorine disinfectant added to municipal water supplies. The chemical attack is accelerated by heat and stress. Therefore, running the water temperature and pressure as low as possible helps to increase the longevity of the tubing. Water temperature should not be allowed to exceed 140F. Water pressure should be maintained in the 60 – 80 psi range.
There are many different grades of PEX. We recommend only using PEX tubing having the highest chlorine resistance rating. There should be a series of four digits printed somewhere along the print line on the tubing, The series of four numbers normally follow the letters PEX. The first digit is the chlorine resistance rating of the PEX. The highest chlorine resistance rating is “5”. To achieve a chlorine resistance rating of “5”, the tubing is supposed to be designed to last 50 years when exposed continuously to hot chlorinated water. We recommend only using PEX with the highest chlorine resistance rating even though the tubing is being used for cold water service. This will help to minimize PEX tubing failures caused by oxidation.
The use of PEX tubing is expected to continue and even grow. When failures do occasionally occur, we can determine the cause of the failure and can offer advice on how best to remediate the tubing to restore the system to a reliable condition.