Elastomer: PTFE and PEEK 4

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Elastomer: PTFE and PEEK

Hi All,

I would like to know which elastomer (in general) is better? PTFE or PEEK.

It appears to me that PEEK costs more than PTFE, so I am assuming it should be a better material?

May I also ask about the properties for PEEK in terms of:
(a) Sunlight
(b) Oxidation
(c) Gas Permeability &
(d) Compression Resistance

Many thanks for your help,
Jeffrey

RE: Elastomer: PTFE and PEEK

moltenmetal (Chemical)
Neither material you mentioned is an elastomer.

They are both rigid polymers.

Since you're talking about these as "elastomers," I'm going to assume you're interested in their use as "soft goods" - as sealing media in gaskets, valve seats or valve stem packings, etc. In these applications, sunlight resistance doesn't matter much, so perhaps you had another application in mind?

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, which 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 typically use it only where PTFE or reinforced TFE won't handle the heat.

RE: Elastomer: PTFE and PEEK

IRstuff (Aerospace)

Cost is not necessarily an indicator of quality; that's a myth propagated by highly compensated CEOs. Cost can also be dependent on scarcity or difficulty in achieving singular performance requirements unrelated to "quality." For example, you don't mention titanium, which is more expensive than either material, but it's not "better," is it?

RE: Elastomer: PTFE and PEEK

tbuelna (Aerospace)

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.

The OP seemed to imply that the material was intended for some type of sealing application, but did not provide any details. So it's hard to say which material - PTFE or PEEK - might be a better choice for this particular situation. Maybe Jeffrey can provide more details of his design.

RE: Elastomer: PTFE and PEEK

MintJulep (Mechanical)

You ask "which is better?"

But you won't tell us for what. So how do you expect a meaningful answer?

A Ferrari 488 certainly costs more than a Ford F150. Does that make it better? Certainly not if you need to transport a ton of manure.

RE: Elastomer: PTFE and PEEK

tbuelna (Aerospace)

Vespel is a very high-performance polyimide resin material that is far more costly than even PEEK. But that does not necessarily make it better for a specific application.

RE: Elastomer: PTFE and PEEK

IRstuff (Aerospace)

As an engineer, one "engineers" the proper and effective solution. If there was a material that was better "in general," there would be no need for engineers, because you would always use this "better" material.

There is no such thing as "in general." That's like asking whether one person is "better" than another "in general." "Better" can only be applied in the context of a specific engineering problem. One material is "better" than another in tensile strength. Another material is "better" than another in hardness.

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RE: Elastomer: PTFE and PEEK

1gibson (Mechanical)

Perhaps trying to reduce material selection expertise down to a "general table" is offensive.

I don't know what you will be using this for (maybe you don't either) but the best material isn't the most expensive; it's the least expensive that meets all the requirements.

RE: Elastomer: PTFE and PEEK

moltenmetal (Chemical)

If Jeffrey would tell us what applications or services he's interested in, we could be of more help. As far as the seats/seal applications I was talking about, there absolutely is a best selection- it's Teflon or reinforced Teflon, up to the temperature at which Teflon becomes too soft to be reliable long term. That temperature is in the range of 400-450 F, but might be stretched to 500 F in totally entrapped static seal designs. PEEK is only used in the narrow window between that range and its upper limit, which arguably is around 550-600 F depending on the application.

RE: Elastomer: PTFE and PEEK

tbuelna (Aerospace)

The table you linked includes materials that are not elastomers, including PTFE and PEEK polymers. However, there are elastomeric fluoropolymers such as ETFE (Tefzel), FKM (Viton), and FFKM (Kalrez) that provide high-performance properties for sealing applications. But these elastomers are also usually quite expensive.

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