The following whitepaper is the property of Plastic Expert Group and cannot be copied or distributed without prior permission. Our analysis does not imply that specific plastic failures will happen systematically. Every case is unique and should be treated accordingly
Polybutylene (PB) Pipes: Unveiling Three Critical Features Affecting Reliability
Polybutylene (PB), once heralded as a groundbreaking material for plumbing pipes, has faced a barrage of scrutiny in recent decades due to a series of high-profile failures. These failures have raised significant doubts about the wisdom of continuing to manufacture pipes from Polybutylene, particularly for use in potable municipal chlorinated water applications. The cause of the poor reliability of Polybutylene piping has been extensively studied in depth over the last 30 years. The results of dozens of studies aimed at understanding the causes of failure Polybutylene pipe have been published in peer reviewed journals. In this article, we discuss three structural features that challenge the suitability of Polybutylene for plumbing pipes including its inherent poor oxidative stability making it vulnerable to chlorine degradation, its polymorphic nature, and its variable crystallization behavior and kinetics. By examining these shortcomings, our goal is to shed light on the ongoing debate surrounding the use of Polybutylene in potable water piping.
NORTH AMERICAN EXPERIENCE
In the 1970s, Shell developed Polybutylene for use in the manufacture of piping. Due to the low cost of the material and ease of installation, Polybutylene piping systems were viewed as “the pipe of the future” and were used as a substitute for traditional copper piping. PB pipe was used extensively in North America for water supply piping from 1978 until 1995.
Within a few years of the introduction of Polybutylene pipe in the US, many Polybutylene piping systems experienced premature oxidative (brittle) failures resulting in significant property damage and lawsuits. Most of the Polybutylene pipe failures were attributed to the Polybutylene material itself having insufficient resistance to hot chlorinated water.
Due to litigations and class action settlements in North America totalling about one billion dollars, Shell chose to stop supplying pipe grade Polybutylene resin. The Polybutylene product standard ASTM D3309 was withdrawn in 2010, and the material has since been removed from plumbing and mechanical codes in US and Canada. However, even though PB piping has ceased to be used in North America, Polybutylene continues to be used as a plastic pipe material in other parts of the world where reports of failures continue .
 See article “The Rise and Fall of Polybutylene Piping”
Susceptibility to Chlorine Degradation: Molecular Structure:
Why Some Types of Polyolefin Pipes are more Prone to Oxidative Degradation
One of the primary issues plaguing Polybutylene pipes is their susceptibility to chlorine degradation, driven by the unique molecular chain structure of Polybutylene. This vulnerability becomes particularly pronounced in hot chlorinated water applications.
CH is much more prone to oxidation than CH2
All polyolefins contain secondary CH2 bonds. Some polyolefins (e.g., Polybutylene) also contain tertiary C-H bonds along the entire polymer chain opposite to the pendant ethyl branches. This is depicted in Figure 1. The tertiary C-H atoms are highlighted in red. Oxygen reacts with tertiary C-H bonds much more rapidly than with secondary CH2 resulting in scission of the polyolefin polymer chain leading to embrittlement and failure. Polyethylene (PE) contains primarily CH2 and only a few random tertiary C-H while Polybutylene contains C-H on alternating carbon atoms along the entire polymer backbone. The presence of tertiary C-H in Polybutylene pipe causes the pipe to be much more oxidatively unstable than Polyethylene or PEX pipe. Consequently, the hydrogen atom shown in red is more easily abstracted (i.e., removed) than the other hydrogen atoms along the chain. Therefore, manufacturers of Polybutylene pipes add much higher levels of antioxidant stabilizers (AO) to compensate for the inherent poorer oxidation resistance of the resin. Using high levels of AO to compensate for the inherent poor oxidative stability of water pipe has its challenges as it relies on the permanence of the AO in the inside surface of the pipe. Unfortunately, AO are extracted by warm water as it flows through the pipe. Once the AO is depleted, the antioxidant protection diminishes, and the inside surface of the pipe becomes brittle and cracks develop. The loss of AO has led to many pipe failures and litigations against both Polypropylene and Polybutylene pipes.
Figure 1. Drawings showing the chemical structure of Polybutylene and Polyethylene. The highly reactive tertiary C-H located on alternating backbone carbons are highlighted in red.
Manufacturers of Polybutylene piping advertise the benefits of Polybutylene pipe over competitive piping products. Since stress accelerates pipe failure processes, pipe manufacturers provide detailed installation and operation instructions to minimize stress on the pipe. When pipe failures occur, pipe manufacturers investigate the installation and operation in order to find alternate causes other than the inherent nature of the PB pipe itself. For example, one European pipe manufacturer teaches that ALL polyolefin pipe will degrade when exposed to “harsh oxidizing agents” (e.g., chlorine in water). However, a paper published in 2017 in a Society of Plastic Engineers Jouurnal presented data comparing the chlorine resistance of Polybutylene and PEX pipe. They reported extensive cracking of the Polybutylene pipe but no cracking of the PEX pipe under identical conditions .
Polymorphic Nature of Polybutylene
Multiple Crystalline Forms
Polybutylene can exist in different crystalline forms, each with distinct physical properties. This polymorphism can result in inconsistent material behavior, complicating efforts to predict how Polybutylene pipes will perform under different conditions.
The presence of various crystalline states can make Polybutylene pipes less homogeneous, which can impact their overall strength and durability. Variability in crystalline structure can introduce weak points and reduce the material’s resistance to stress .
Stress Accelerates Degradation and Failure
When pipe is first manufactured, Polybutylene first forms the kinetically preferred Type II crystal structure. Immediately after manufacture, the pipe is generally coiled and stored for distribution. As the pipe coils set in the warehouse, the crystal structure changes to the more thermodynamically stable Type I crystal structure. During the transition of the pipe crystal structure from Type II to Type I, the material shrinks/increases in density and the material takes on the permanent coil shape. When the pipe is uncoiled for installation, it becomes stressed as it is forced away from its coiled shape . Because of this, one pipe manufacturer recommends a 3 times higher bend radius when reverse bending coiled pipe compared to straight pipe .
 Rui Zin et al., “Polymorphic Behavior and Phase Transition of Poly(1-Butene) and Its Copolymers,” Polymers, 2018, 10, 556.
 See article “Defect in Polymer Materials as Design Process Consequence“
The once-promising Polybutylene plumbing pipes, is now marred by critical concerns regarding its vulnerability to chlorine degradation, polymorphic nature, and variable crystallization behavior and kinetics leading to non-uniformity of the pipe. These issues have cast a shadow of doubt over the continued use of Polybutylene in plumbing applications.
As plumbing technology advances and safety standards become more stringent, it is essential to critically evaluate the suitability of Polybutylene pipes for modern plumbing systems. Alternative materials like copper and PEX (cross-linked polyethylene), offer more reliable and consistent performance, particularly in the face of challenging conditions. In making decisions about plumbing materials, it is paramount to prioritize the long-term structural integrity and safety of plumbing systems, weighing the risks and benefits of each material option carefully.