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Need help identifying a plastic Piston Ring material, Filled Teflon or PTFE I think

L

laminar-flow

Stainless
Joined
Jan 26, 2003
Location
Pacific Northwest
These rectangular section sealing rings are made of a plastic that looks like a Teflon or PTFE that is filled with something. I made some out of virgin PTFE and they are softer than the originals. The original rings are an off white light cream color, not the pure white of PTFE or Teflon. The original ones are also about twice the stiffness in bending than the PTFE ones that I made. This is for an air pump piston in which the piston is 1.250" Ø.

I would like some ideas on what the original material could have been. Thanks for looking.
 
It does not seem to be PEEK as it is still quite soft, but not as soft as virgin PTFE. Would filled PTFE be used for piston rings in an air pump, or would it wear the wall of the cylinder. The cylinder is aluminum but coated with some kind of teflon. There is a company, Performance Sealing, that uses a material called Duron which comes in many different grades, but I could not find any source. It might be a custom blend that they have rights to.
 
Mating part wear would depend on the fill material. PEEK filling would be low wear against most metals, but perhaps not bare Al. If the cylinder coating is truly just Teflon, that could be scrubbed by most fillings.

You could check out the IGUS line of polymer bearing materials, maybe one of them would have the right properties for a piston ring for your uses.
 
Most Rulon I recall as being a redish color. Delrin AF is also Teflon filled acetal and much less expensive than Rulon. It is usually a light brown color from my experience- darker than tan though.
Good luck!
 
Most non-metal air compressor seals are filled PTFE. Carbon can also be used, but it's very easy to break during installation. Depending upon duty and expected service life, you could use a combo of filled PTFE with a rubber o-ring as a backer. Or if you can find the right size, you could get a metal-reinforced filled PTFE coated seal. The idea is that, since PTFE doesn't have a great elastic modulus and can creep, the o-ring or metal spring adds some extra sealing force to the setup.

Glass and bronze are common filling materials.
 
Dan from Oakland said:
Most Rulon I recall as being a redish color. Delrin AF is also Teflon filled acetal and much less expensive than Rulon. It is usually a light brown color from my experience- darker than tan though.
Good luck!
Click to expand...

I think the material I recommend is indeed delrin AF, thanks for jogging my memory - very light brown, not redish at all. The Rulon was an error on my part.
 
One little trick that I have use. You can buy sample packs containing a myriad of different polymers i.e teflon rulon PTFE, etc. From Mcmaster. When I have to I.D. a part like your piston you have a set of known material to guide you and get you close. Also some times you can get a good amount of info using a soldering iron with digital temp read out. Use the iron to see what your piston material does at a given temp. Does it melt burn etc. and if so at what temp then compare to samples, not fool proof by any means but most of the time it will point in a + direction.
 
I have a few to make. I looked at the original under a microscope with a strong back light and I'd have to say it does look like it is filled. The glass filled PTFE is an off white so this might be what it is. Although I wonder about the glass if it would be a wear issue on the cylinder wall. I am having trouble finding 1.250 round Peek, (or something else that would double the hardness), filled PTFE, which I would feel better trying against the cylinder wall.
 
I forgot to mention that I tried some pure PTFE and pressure was lower after 1.5 years and when inspected, the PTFE ring was worn thin by the stepped overlap ring groove. This is the reason I'm looking for the original material.
 
laminar-flow said:
I have a few to make. I looked at the original under a microscope with a strong back light and I'd have to say it does look like it is filled. The glass filled PTFE is an off white so this might be what it is. Although I wonder about the glass if it would be a wear issue on the cylinder wall. I am having trouble finding 1.250 round Peek, (or something else that would double the hardness), filled PTFE, which I would feel better trying against the cylinder wall.
Click to expand...

Glass filled PTFE will indeed eat bare aluminum for breakfast, but you said it appeared to be coated. I would look at moly or bronze-moly (molybdenum disulfide, MoS2) as your filler. You say it's off-white, is there any "sparkle" to it? That would likely mean bronze. Moly can vary in color from a dark-ish cream color to something more like a grey-green.

Pictures of the seal and a measurement of the gap would help hone in on the answer as well.
 
doug8cat said:
One little trick that I have use. You can buy sample packs containing a myriad of different polymers i.e teflon rulon PTFE, etc. From Mcmaster. When I have to I.D. a part like your piston you have a set of known material to guide you and get you close. Also some times you can get a good amount of info using a soldering iron with digital temp read out. Use the iron to see what your piston material does at a given temp. Does it melt burn etc. and if so at what temp then compare to samples, not fool proof by any means but most of the time it will point in a + direction.
Click to expand...

So I worked at the DuPont plant that makes Teflon(R). I also have a PhD in Chemical Engineering, and my job right now is writing math models of human disease.

DO NOT use high temp testing to identify PTFE. PTFE doesn't melt, it degrades, and degradation products are in the highly toxic category. For example, perfluoroisobutylene. PFIB reacts with water to form fluorophosgene and then hydrofluoric acid. PFIB has an LC50 (lethal concentration, killing 50% of subjects) of 0.5ppm You might survive, but you'll feel miserable (the "PTFE flu") for a couple of days. Any pet birds you have will have gone the way of the "Norwegian Blue". Such decomposition is easily achieved with a soldering iron.

Just don't do it. Please.
 
Yes, I knew PTFE was bad to burn and breath. Did a really tiny piece with the windows open. But thanks for mentioning it for others.

Earlier I just ordered some teflon filled delrin and will give it a try. The softer pure PTFE smeared into the gap and wore quite bad.

Also, it could be that I was not the last person into this pump so what was in there might not be correct. So choosing the "off white bit stiffer than PTFE" might not have been a good idea.
 
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