August 11, 2026
Mixing Thermoplastic Piping Materials
Contributed by Chris Illsley, Senior Product Manager
For any application, there should, in theory, be a singular material that’s recognized as the best material for the given system. However, the best material for the job depends on perspective, and not every system component is available in every material option. Is the best material the one that has a 3x factor of safety and a 100-year service life? Or is it the one that meets the minimum factor of safety for the lowest cost? Perhaps it’s the one that would be easiest to install? An engineer’s job is to weigh the pros and cons of each material and select the best option based on the application’s needs and the owner’s constraints.
In most cases, this means specifying a single material that can withstand the operating conditions, with a modest safety factor, for the lowest cost of ownership. Yet, there are times when it may be prudent, or even necessary, to use more than one type of thermoplastic within a system. Different parts of a system may operate under significantly different conditions, so specifying one material may not be optimal, and significant savings may be realized by mixing materials. This approach isn’t without trade-offs, though, and it is essential that engineers understand what these challenges are and how to navigate them.
Example #1 – UPW
In our experience, there are a few applications where mixing materials is especially suited. Ultrapure water (UPW) distribution loops in semiconductor manufacturing facilities are a prime example. Due to their stringent water quality requirements, these applications use natural PVDF because it has the best leach-out properties of any commercially available polymer. The UPW supply lines, which transport UPW from the filtration system to the point of use, need to be made of PVDF to maintain water quality before use.
After reaching the point of use, unused UPW is transported back to the filtration system for polishing stages before being reused. Since water quality requirements are less stringent for the return loop, it is common to use high purity polypropylene (PP). This provides significant cost savings to the owner as PVDF pipe is approximately five to ten times the cost of PP. Using PVDF here would needlessly increase the cost of the system, since the filtration skid will remove any impurities that may be introduced by the PP.
While thermal expansion is something engineers need to account for in every thermoplastic system, different materials have different rates of expansion and must therefore be accounted for specifically. If the system is rigidly connected and experiences significant temperature differentials, it may put excessive stress on the areas where it transitions. The layout must be designed to account for this expansion, or a flexible connection must be used.
Another thing to keep in mind is that the wall thickness required to meet a UPW system’s pressure rating may differ between materials. In this case, an SDR 11 wall thickness is needed for PP, but only SDR 21 or SDR 33 is required for PVDF. This means that the inner diameters do not perfectly match. The existence of such a disparity may influence which type of joint is used for the transition. Also, the pressure drop across the materials will vary and must be accounted for in pump sizing.
Infrared (IR) or butt fusion welding are generally the best ways to join pipes, fittings, and valves in a thermoplastic piping system. When done correctly, it creates a homogeneous joint with no reduction in strength. However, in instances where the system transitions from one material to another, use of a mechanical joint is unavoidable. It is important to select a joint type that is suitable for the application. In the case of UPW, this should be a joint with minimal crevices or dead zones, though it is also common to utilize unions, threaded adapters, and tubing connections with flexible tubing bridging the gap.
Example #2 – Valves
Another common example of multiple materials being used is when a valve is integrated into a system with a body material that differs from that of the pipe and fittings. Not all materials are well-suited for valve applications, as they may lack the mechanical properties required to withstand the forces a valve experiences.
PVC valves are commonly used in HDPE or PE100-RC systems. While polyethylene (PE) ball valves are available, PVC valves are often preferred for their superior sealing performance and reliability.
There are limited valve options in certain materials when said materials are not as ubiquitous or when their material properties limit their application. One such case is with ECTFE. ECTFE is a specialty material generally used only in the most critical acid services and is not available in a wide range of valve options. Sometimes, a PVDF valve can be used in lieu of an ECTFE valve, with the understanding that its lifespan may not match that of the ECTFE piping system. As such, more frequent maintenance or early replacement may be necessary.
Again, since welding wouldn’t be an option for dissimilar materials, the components must be joined mechanically in that case. Ideally, for true-union valves, an end connector made of the piping material is provided and can be welded into the system. This is likely the most seamless and reliable installation method. Otherwise, flanged or threaded end connections may be suitable.
Some other quick examples of cases in which mixing materials could be considered include:
- UV Water Filtration: UV light may leak out of the metal housing of a UV filter and degrade a plastic fitting. Using one or two 90-degree stainless steel elbows helps trap the UV and limit the exposure to the plastic. A sanitary or tri-clamp fitting is a great way to transition from stainless steel to plastic.
- Natural PP to Pigmented PP: Natural, unpigmented polypropylene is a popular material for high purity water applications, but not all components are readily available. However, nearly all varieties of polypropylene are compatible for welding, so they can be fused directly as long as their OD and wall thickness match.
Whatever the scenario, it is important to perform due diligence and ensure that the materials are suited to the application. No single material is best in every sense, so consider if the application warrants a mixed installation. If this path is chosen, keep the following considerations in mind:
- Thermal expansion rates differ and must be accounted for in design.
- Varying wall thickness may affect your pump sizing.
- Never attempt to weld dissimilar materials; use the best mechanical connection for the application.
- Confirm chemical compatibility.
Asahi/America’s Technical Services and Sales teams are always available to review your application and help guide you to the best solution.