Which Elastomer In General Is Better? PTFE Or PEEK

June 03 16:32 2021

As the vast array of industries had become more demanding with higher temperatures, caustic environments, higher pressures, etc… the standard elastomeric seal could no longer offer the life expectancy needed for companies to remain competitive. Polytetrafluoroethylene (PTFE) offers the opportunity to run at higher RPMs, higher and lower temperatures, and much higher pressures than an elastomeric seal.

This is due to the fact that PTFE is an inert material, developed to withstand severe temperature, has a lower coefficient of friction, and when combined with select fillers, PTFE offers exceptional wear characteristics and other mechanical properties that elastomer seals cannot.

PTFE is more chemically resistant than PEEK. It is MUCH softer and hence takes less clamping force to achieve a seal, but it also creeps at a much lower temperature than PEEK does. It is also slippery- having a low coefficient of friction means that parts can slide past one another easier, resulting in lower torque in rotating seal applications etc.

PEEK is a very hard, dense, rigid polymer. It’s expensive. It is also resistant to quite high temperatures. You use it typically only where PTFE or reinforced TFE won’t handle the heat.

Moltenmetal is correct, neither PEEK or (solid) PTFE are elastomers, they are thermoplastic polymers. 

PEEK has excellent mechanical strength, high temperature capability, chemical compatibility, and wear resistance. PTFE has excellent low/high temperature capability, low friction properties, and is chemically inert. Both materials can be fabricated by machining or molding. PEEK is usually more expensive than PTFE on a $/lb raw material cost basis, but most seal applications would require modest amounts of material if molding is used to fabricate the seal element. For high-performance seal applications the raw material cost difference should not be a major concern. But for a high-volume, low-cost product application I could see where it might be an issue.

For a dynamic seal application, where pressures are not too high, PTFE might be a good choice since it has excellent friction and conformability characteristics. One especially nice thing about a PTFE seal running against a metal surface is that over time a layer of PTFE is transferred to the metal surface, which results in a PTFE-on-PTFE contact with very low friction and leakage. The one major limitation of using unfilled PTFE for pressure sealing is that it has a tendency to cold-flow when exposed to sustained pressure over long periods of time.

ETFE is a rigid thermoplastic. FEP fluorinated ethylene propylene) is also a rigid thermoplastic, though it is softer than PFA or PTFE – it is not an elastomer. Polytetrafluoroethylene-co-propylene (Aflas) is an elastomer, as the typical fluoroelastomer (tradename Viton – (FKM)) and perfluoroelastomers like tradename Kalrez (FFKM). All can be used in sealing applications (packing a, gaskets, valve seats) but only the elastomers can be used in self energizing seals such as o rings. O rings can be encapsulated with the nonelastomeric materials but it can and does affect their ability to seal gas.

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