PEI surface condition changes- isopropyl alcohol 2

[subsearobot]

Hello all!
We have an interesting situation. We have a light- interference fit between an ultem shaft and a polypropylene socket. The interference creates an airtight seal, and retains the polypropylene part.

We carefully measure the force required to insert the ultem into the pp socket. We have noticed that the force roughly doubles or triples when the ultem part is cleaned with ISO alcohol (and allowed to air dry) prior to insertion. We have noticed this effect on multiple high-temp, inert polymers substituted for PEI (Peek, PSU, others). Some polymers (especially fluoropolymers) react less severely in regards to insertion forces & the ISO cleaning. The effect is greater with the ultem surface is roughed (sandblasted) prior to testing.

I'm a mechanism guy, so please forgive my conjecture if it is wildly speculative...
I've done some preliminary testing. Rotational instead of insertion testing- because I could look at a longer distance of interface interactions. (the insertion length is ~.030", and I wanted to see a longer wear case- just curious more than anything)

I think that I notice that when distilled water is wiped over the surface (and dried), it mitigates the increased contact friction due to ISO. this is preliminary, I have not had a chance to measure the forces because of both machine time and this seems crazy. Is this observation crazy, or is it worth machine time to observe?

What is the deal with ISO on supposedly "inert" surfaces? any idea? Can you suggest any references that may help me understand what is happening? and ultimately, how to mitigate?

thanks!!

 

[Compositepro]

The standard abbreviation for isopropyl alcohol is IPA. All polymers will absorb some small amounts of solvent. The result is some swelling. There are a number of reasons why the water may affect how PEI reacts to IPA. Humidity in the air may have a similar, but slower effect. An interference fit would be very sensitive to surface swelling. The duration of exposure and the degree of drying would be important factors to control in any study. You might try reading-up on "swelling in polymers".

 

[IRstuff]

Have you considered the possibility that the IPA is removing something that is lubricating the process, rather than interacting or affecting the surface itself?

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[Pud]

I can't think anything with a truly 'Inert' surface. Polymers especially. They tend to resist a range of chemicals and will operate at a range of temperatures. The criteria tends to be mechanical properties after immersion test rather that frictional characteristics.
Irstuff may have the answer though. Many plastics have a lubricant added to enable mould release. Some, especially PC, have a particular affinity to steel. Most of these have a grade described as 'easy release'. Years ago it used to be a metallic soap (e.g. Zinc stearate) Not sure about PEI, but it may contain a lubricant which is soluble in alcohol. The lubricants are configured to appear at the surface for obvious reasons.
I don't suppose the material manufacturers will tell you what's in it but might be worth asking?

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[subsearobot]

yes, IPA not ISO. thanks.

we mold the ultem parts inhouse without any release agent. the lower friction returns after a period of time (several hours). For both those reasons, I don't think it is a surface coating.

either or both surface swelling and water molecules at the interface still seem plausible.

Googling "swelling in polymers". good tip. thanks!!

 

[Pud]

I wasn't referring to externally applied agents. It might be in the material itself. You clean it off and more migrates to the surface over time. Like some colourants migrate in some polymers.

www.tynevalleyplastics.co.uk

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[IRstuff]

IPA is often used to displace adsorbed water, so it's possible that it's displacing the water for a short time, but over time, the water gets adsorbed again.

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[mighoser]

What is the IPA purity. Perhaps a residue to forming. If you water rinse and vacuum bake at 100C do you get lower friction. I would suggest careful measurement of your fits would help to resolve some of these possibilities.

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