Polysulfone or PPO for hot water service? 2

[btrueblood]

I need to design a small, lightly loaded part (essentially a bumper to prevent two other metal parts from making contact) for use in a hot water valve. May see glycol/water and some salts (corrosion inhibitors). Normally, I'd design this part as a teflon piece and attach it with fasteners, but the boss doesn't want a fastener in this location. This leaves me with a heat-staked or ultrasonically-staked version being the design, which has lead me to look at either polysulfone or PPO. Any experience with either of these in hot water with light grazing/contact/abrasion?

Thanks,

Ben T.

 

[tiffanyhoover]

http://www.us.piping.georgefischer.com/external/chemical_resistance_guide/

http://www.coleparmer.com/techinfo/chemcomp.asp

I've found these sites very useful in regards to chemical compatability. Unfortunately, my research/experience has been on fluoropolymer removal moreso than polymer strength, so I haven't had the experience you're looking for.

 

[btrueblood]

Tiffany,

Thanks, I hadn't seen the coleparmer site before. I was more interested in the abrasion resistance of either plastic, not so much the chemical compatibility, but all data is useful.

 

[Pud]

Hi,

Why polysulphone? Is it high pressure hot water above 100 deg C?

From your description i would (imho) suggest acetal (POM) would be a good choice - been used in water valves for many, many years. Easy processing, lot cheaper than PSU and I guess probably cheaper than PPO. A lot better wearing material than either too.

Rgds

Harry

 

[btrueblood]

Harry,

Yeah, sorry - high temp (over 100 C) water, pressurized to 10 or 20 bar. Acetal (POM) goes away under these conditions. I agree with you, the wear properties of the two candidate materials aren't too lovely (compared to Teflon), but how to "heat stake" Teflon? Sigh.

 

[patprimmer]

Acetal actually starts to go away at about 80 deg C in water due to hydrolysis as well as HDT.

It is also subject to attack in the presence of ZnCl, which is present in potable water in the presence of lower quality brass and chlorine, which is a situation that occurs often enough to cause unacceptable failure rates in some regions.

PPO works well in water meters, including hot water meters. Nylons 11 and 12 can also work under some conditions, and polysulphone should do it easy, but I would need to check some specs and I would need details of time temp and actual analysis of the extremes of chemicals likely to be in the water.

More than one hot water contact moulding has failed in use after passing extensive testing, because the "LAB" insisted on using distilled water to control their testing conditions for repeatability of results.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[btrueblood]

Chuckle. Yeah, I know about using "clean" systems when doing life testing. My preference for such tests is to throw any ungodly mixture I can think of at the parts, at least in a small sideline test, including from "every chemical on the shelf and under the sink" to adding floor sweepings. Usually I also end up using a mix of copper and iron pipe/fittings, just to add a nice corrosion byproduct mix into the whole mess too.

Hadn't heard about the ZnCl and POM. I tend to avoid nylons, not knowing the subtleties of the different formulations and what will or won't attack them. We've had poor success with run-of-the-mill nylons.

Pat, a somewhat related question: what is your opinion on the "best" way to keep the plastic material from sticking to the press/die during a heat-staking operation? Use Teflon coatings on the tool, use a stainless alloy for tooling, or just liberal applications of mold release? Or is it necessary to cool the tool & part down below the melt temperature before seperating them (ala injection molding)?

 

[patprimmer]

I have very little experience with heat staking.

Intuition tells me to use a Teflon coated tool, and keep the temperature as low as possible, and use more pressure if necessary.

It would be very difficult to cool, then reheat the tool quick enough to be economical I think.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[pierrig]

A PSU is the way to go. Try a polyphenyl-sulfone or a polyethyl-sulfone.