Polymer Bellows and Diaphragms
PTFE (polytetrafluoroethylene), aka Teflon, is typically the first choice polymer for bellows and diaphragms, but did you know that PTFE isn’t the only polymer you can choose from? In this article we are going to compare two other polymers – PFA and TFM – to PTFE as a material choice for bellows and diaphragms.
PFA, or Perfluoroalkoxy, is sometimes referred to as Teflon PFA. It has properties that are similar to PTFE, including outstanding chemical resistivity and has extremely low gas permeability. It is not hydrophobic like PTFE, however, and absorbs slightly more water. One reason it might be chosen over PTFE is its ability to maintain its mechanical strength at high temperatures, even when combined with caustic chemicals. It also possesses both excellent creep, fatigue properties and thermal stability. Its maximum continuous service in temperature is 260°C.
PFA is also more versatile when it comes to how parts can be manufactured; for example, it lends itself well to extrusion, injection molding, transfer molding, blow molding, and compression molding. You will often see PFA used for plastic lab equipment because of its outstanding chemical inertness, and its flexibility had made it a popular choice for tubing in many chemical applications. It’s also popular for semiconductor and pharmaceutical applications.
You may have heard of TFM, or PTFE-TFM. TFM is a second generation PTFE that includes an additional modifier called Perfluoro(propyl vinyl ether). This modifier makes its polymer structure denser than PTFE, lowering its gas permeability below that of PTFE but not quite as low as PFA. The same wall thickness of TFM has twice the barrier effect as PTFE. Its water absorption is comparable to that of PTFE, which is very good. Like PFA, it performs well at high temperatures and is very chemically inert. TFM is also well adapted to applications that combine high temperatures with vacuums. Like PFA, it works well for pharmaceutical and semiconductor applications.
Compared to PTFE, it has an even better surface characteristics, is stiffer, and is less susceptible to creep and has improved fatigue properties. Compared to PTFE, it also exhibits better stress recovery. Its maximum continuous service temperature is 250°C, which is slightly below PFA.
Both of these polymer options are excellent choices for high purity applications, including medical, pharma, semiconductor bellows. So, the next time you are selecting polymer bellows or diaphragm, don’t forget that PTFE isn’t your only material option.