Static Friction of PEEK/PEEK; CFR PEEK/PEEK couples et. al.

[BiPolarMoment]

I'm working on a device where two PEEK surfaces are moved relative to each other in discrete fashion under load. In this situation overcoming the static frictional force is a concern as it is manually actuated. This motion is one time only for assembly so wear is not a significant concern.

I've been looking for frictional coefficients for these materials but I'm most interested in these specific pairing and that is not often given.

E.g. I've found one source for PEEK 450G dynamic coefficient as 0.4 and for 30% CFR PEEK dynamic coefficient as 0.19 (both dry vs. steel). What I don't know is the static coefficients which are presumably higher and perhaps not proportional. Additionally, I don't know the interaction between like/dislike polymers as in this case compared to vs. steel.

So if numbers can't be provided I'm still interested to discover what the heirechy of the friction couples might be.

CFR/CFR->CFR/PEEK->PEEK/PEEK?

or perhaps no notable difference?

 

[Demon3]

First, static COF is always more than dynamic.

Secondly, I would not worry at all about the COF as adding lubricant will have a far greater effect than changing material will.

If you can't use a lubricant applied topically then you can use solid additives in the PEEK to reduce friction. Hexagonal boron nitride is one, titanium carbide is another. There are others such as PTFE and silicones.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[dsheets05]

PTFE filled peek would be ideal for this application. "Soft: peek material used alot in ball valves exc, it is ~40-50%ptfe filled which greatly reduced the friction, I have some information I could possibly send you for this calculation. If you are interested send me a PM.

 

[BiPolarMoment]

Thanks for your suggestions guys but unfortunately this is for a long-term (i.e. permanent) medical implant and thus the lubricants involved are generally limited I apologize for not mentioning that up front.

Specifically, CFR PEEK and PEEK have a clinical history and master files with the FDA which is why I was so specific.

I tried to do a bench test with a CFR PEEK plate I had by inclining it but it was very unscientific (I don't have uniform shapes of each material to test). Suffice to say that the CFR PEEK chip I had seemed to require a larger angle to break static friction than the PEEK parts though I can't put too much stock in that result as it was not easily repeatable. Regardless, I'm having some identical prototype components made in each material so I'll be able to do a functional test anyhow.

Again, thanks for the help.

 

[dsheets05]

http://www.fast-houston.com/

They have testing/manufacturing culpabilities for peek products as well as any other polymer material that comes to mind. In addition to a friction/wear testing culpabilities they have a large engineering staff that can help you out. If you are interested send me a PM and I will dig up some contact information.

 

[Demon3]

You can measure the COF easily and accurately at home or in the office. This is how I do it. Go to Sears / Amazon or similar and buy a digital level for $30 (or use your iPhone).

Place the one material on the other and increase the angle until the top one slides. Repeat 5 times and write down the angle needed to make it slide.

Take the tangent of the angle. That is the static COF.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[prdave00]

Contact Invibio to see if they can refer you to any white papers or published journal articles. If this is long term implantable (>30 days) then sooner or later you'll have to get material from them since they are the only guys with a master file with the USFDA and have the largest presence OUS. Good luck. My guess is CFR on CFR will give you the lowest friction - especially with a pitch-based carbon fiber.

 

[BiPolarMoment]

Thanks Chris--that's what I was trying to do, unfortunately I did not have uniform samples to test (and no quick and easy way to get them).

Dave: I have contacted Invibio and they may be trying to establish this data for me. They admitted they don't have any data on PEEK-PEEK. I found a white paper associated with the NUBAC device (PEEK-PEEK) from Pioneer but unfortunately not any friction numbers. I'm still somewhat new to CFR PEEK but I have seen some data on Pitch vs. PAN CF--what exactly is the difference? Does Invibio only use pitch fibers?

Side note: Victrex now has a PEEK master file with the FDA but I do not believe any devices using their PEEK have been cleared yet--it's irrelevant to me due to contractual agreements with Invibio in any case.

 

[prdave00]

Pitch based fibers are more like "graphite" while PAN based fibers is your more standard filler for strength. The former has been shown to offer better wear resistance but not necessarily static friction. Invibio's Motis grade has milled pitch based fibers in a Victrex 150 PEEK resin matrix. It's a relatively new offering. You're right about other companies having master files but Invibio has established themselves with a long track record so companies like Evonik, Westlake, Solvay, etc. have a lot of catching up to do and it may be while before someone takes a gamble and tries to get a device cleared/approved with a non-Invibio grade (at least in the States). My apologies if I strayed too far from the OP.

 

[Demon3]

You can make either carbon or graphite fibers from PAN. I've seen the process running. You just need a higher temperature to make graphite.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry