Elastomer: PTFE and PEEK 4

[JHWC]

Hi All,

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

It appears to me that PEEK cost 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

 

[moltenmetal]

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 those 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 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.

 

[IRstuff]

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 difficultly 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?

TTFN
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[tbuelna]

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 JeffreyHo can provide more details of his design.

 

[JHWC]

Hi all,

Thanks for the input and many apologies for the late reply.

For this current work, I am comparing the thermoplastic polymer matrix; PTFE vs PEEK.

I thought PTFE is a more superior material compare to PEEK, and therefore should cost more. However, when I see the price between these 2 materials, PEEK is more expensive.

Thanks,
Jeffrey

 

[MintJulep]

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.

 

[tbuelna]

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.

 

[JHWC]

Hi MintJulep,

Like I mentioned, I am comparing the thermoplastic polymer matrix. It is a table of comparison, not for any design purpose. If you have read my earlier statement, I am asking in "general". Please do not sound so offensive. In here, I am seeking the expertize of all professionals, asking for their views and opinions.

Hi tbuelna,

Thanks for the input once again. You have made me realize that cost of the material isn't everything. Cheers. =)

 

[IRstuff]

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.

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.

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[JHWC]

Hi IRstuff,

Thanks. Well noted on that.

Cheers,
Jeffrey

 

[1gibson]

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.

 

[moltenmetal]

If JeffreyHo 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 application.

 

[JHWC]

Hi 1gibson,

Thanks, noted on that.

Hi Moltenmetal,

As mentioned, just drafting out a table which is similar to the handbook of valves and actuators found online.

The link is found here -->

https://books.google.com.sg/books?isbn=0080549284

Thanks,
Jeffrey

 

[tbuelna]

JeffreyHo-

The table you linked includes materials that are not elastomers, including the polymers previously noted such as PTFE and PEEK. 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.

Here is a good technical reference for PTFE o-rings It describes the issues (cold flow, creep, limited compliance) involved with using un-filled PTFE as a pressure seal.

 

[JHWC]

Hi tbuelna,

Thanks for the information provided. You are right. Tefzel, Viton and Kalrez do provide high performance.

However, I am curious regarding the cost of these materials mentioned. I am wondering what are the cost for these elastomers/polymers mentioned, including PTFE and PEEK.

Thanks,
Jeffrey

 

[tbuelna]

JeffreyHo-

If you read the previous posts, you'd have noted that PTFE, PEEK, Vespel and Torlon are thermoplastics that can be molded or machined from stock. Since these materials are quite costly on a $/lb basis, for large production quantities it would most cost efficient to mold the part. But for small quantities it would be better to machine them from stock.

Elastomers like Tefzel, Kalrez or Viton must be molded. And this requires some added cost for the tooling.

If you want a more helpful answer to your situation, it would help to provide more explicit details of your design.

 

[moltenmetal]

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.