A case study exploring the ongoing problem of PP pipe failure
Prior to 1970, the most commonly used piping for potable water was copper. However, the high cost of copper and the ease of installation of plastic piping caused an exponential increase in the use of plastic over the past 50 years.
In the late 1970s, polybutylene piping emerged. Due to the low cost of the material, polybutylene piping systems were viewed as “the pipe of the future” and it rapidly became the preferred potable water piping.
However, in the 1990s, polybutylene piping got a bad reputation because of the high rate of failure. The high failure rate prompted a class action lawsuit and Shell (the company that developed the piping) stopped selling the product. The failure of the pipe was determined to be oxidative degradation by chlorinated water.
The demise of polybutylene pipe caused a void in the plastic pipe business, which was replaced by the development of Kitec piping. Kitec was a tri-layer pipe consisting of two layers of polyethylene (PE) with aluminum pipe sandwiched in between. However, Kitec piping also had a high failure rate and, like polybutylene piping, ended up being put out of business because of a class action lawsuit.
The newest piping product on the market is polypropylene (PP) pipe. PP pipe is being touted for its durability. In 2014, an article showcased the benefits of PP piping by touting installation of a new PP piping system in a high rise condominium tower in Seattle.
Yet, after less than four years in service, the entire piping system is now being replaced because of extensive degradation of the piping. Why?
Because like polybutylene pipe, PP pipe is highly oxidatively unstable.
“Polypropylene plastic is about 10 times more prone to oxidative degradation than PE and PEX. Therefore, in order for PP pipe to stand up against embrittlement when exposed to chlorinated water, higher levels of antioxidants must be added to the PP than to PEX.”Dr. Duane Priddy, Plastic Expert Group, Founder & CEO
Causes of polypropylene (PP) pipe failure
When the inside surface of a PP pipe is exposed to high levels of chlorine, the antioxidants are “burned off” the inside surface in direct contact with the chlorinated water. Once the antioxidants are gone from the surface, because of the poor inherent oxidation resistance of PP, the surface layer becomes degraded and brittle. As soon as the surface turns brittle, it is just a matter of time before the surface cracks propagate deeper into the wall of the pipe.
Once cracks are initiated, the cracks as “stress concentrators” and they continue to penetrate deeper and deeper into the wall of the pipe. The surface embrittlement phenomenon of PP pipe is similar to surface embrittlement of other PP products that are utilized in an oxidizing environment.
For example, several medical device companies manufacture medical mesh implants using PP fibers. A short time after implantation in the body, the outside surface of the fibers looks very similar to the inside surface of the PP pipe. Once the antioxidants have been consumed from the PP surface, rapid oxidative degradation and embrittlement takes place.
Professional plastic pipe failure analysis and testing
At Plastic Expert Group, we are highly experienced in performing forensic investigation into the root cause of failures of all kinds of plastic piping including ABS, CPVC, PEX, PP, and PVC. If you have PP piping and are experiencing failures of the piping, our A2LA certified laboratory is set up to run ASTM tests and evaluate plastic piping against applicable ASTM and AWWA standards including:
- ASTM D543 (Chemical Resistance of Plastics)
- ASTM D3895 (Oxidative Induction Time (OIT)
- ASTM F442 (Specification for CPVC Fire Sprinkler Piping)
- ASTM D1784 (Specifications for rigid PVC Piping)
- ASTM D1599 (Quick Burst Testing for Plastic Piping)
- ASTM F876/877 (specifications for PEX Piping)
- ASTM F2389 (Specifications for PP piping)
- ASTM 12454 Cell Classification
- AWWA C905 (Specifications for large diameter PVC Piping)