Fyndium
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Stirrer bearing and vacuum, adapter?
Because PTFE has much higher thermal expansion than glass, I'm worried that stirrer bearing could cause the glass joint to crack when subjected to
vacuum and high heat. I don't wanna take any risks cracking expensive multi-neck flasks, hence, I though I can get a sacrificial adapter between. By
my tests, the radial support does not suffer too much with few cm increase in bearing height to insert depth in flask, and stirrers in homogenous
fluids seem to self-stabilize rather well when the rotational speed reaches equilibrium with the reaction mass.
Would it be of any sense to use smaller, like 14/20 adapter with 14/20 > 29/32 (or any size) if starting from scratch instead of simple 29>29? I
would think that smaller the size, smaller the stress? I know this is just schematics, but a thought for an idea. 
What we are talking here, in pics.
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Sulaiman
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I've used this type of adapter at up to 100C for extended periods
a 24/29 adapter in a 3-neck 500ml rbf for overhead stirring
and a 19/23 adapter in the side neck of a 5l rbf as a thermometer adapter
So far no problems.
P.S. I would not trust these adapters to hold a thermometer with vacuum applied.
[Edited on 4-8-2021 by Sulaiman]
CAUTION : Hobby Chemist, not Professional or even Amateur
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Fyndium
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They do not hold thermometer or a bleeder tube. I always wrapped them with thick layer of tape to the neck to prevent them shooting into the flask.
For thermometers, use a thermowell.
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macckone
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my experience is they slip in the joint and with a vacuum applied the glass expands faster on heatup.
and on cooldown the teflon slips outward as the glass cools. It visibly moves if the glass gets really hot even with a metal keck clamp.
With no vacuum, the teflon will literally pop out if not held with a keck clamp.
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Fyndium
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I have had to use pliers to get the teflon cap out of flasks many times, so it gets really tight.
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monolithic
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If you're not using it for vacuum work, wrap it in a single layer of PTFE plumber's tape and then secure with Keck clips. The seal is "good enough"
and it will slide out without issue.
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Fyndium
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Sounds like a reasonable solution. It fits in when cold, but when heated, it will get stuck very tight. I have never dared to cool it down without
loosening it first.
Should lubrication actually be beneficial or detrimental in case of teflon-glass-joint?
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macckone
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Depends on the lubricant.
Fluorinated silicone grease is the usual vacuum grease.
It won't react with the teflon and it won't allow gases in either.
Like I said my teflon stirring adapter is quite slippery naturally.
I wonder if there are different grades of teflon being used where mine acts like it is lubed all the time but yours doesn't.
Of course mine is relatively new so that may change over time but the previous one was several years old and didn't.
One note however, when teflon gets hot it does get sticky and starts outputting HF gas.
If it gets too hot bad things happen and you have to buy a new one and a new flask too because once it sticks to glass it isn't coming off. And then
you have a new adapter and a new flask. You can guess why I have a fairly new adapter.
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Dr.Bob
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If you want to hold a themometer, there are adapters for that which work well and can hold a therm under vacuum pretty well. If you want to use a
glass stir shaft, it is very difficult to do that under vacuum and keep any sort of seal. It can be done, in large reactors, but most use special
seals and metal shafts, not a glass rod and bearing. Most are designed for use under inert positive pressure, which they can hold with a small
leak, which is manageable. There are few vacuum greases thick enough to hold a vacuum but thin enough to allow a shaft to spin, and even the Teflon
bearings don't hold much of a vacuum, unless you have really worked at it. You might be able to modify a rotovap type seal to hold a vacuum (they
make one for Buchi kogelrohs that is 10 mm diameter I think), but even those are limited to the vacuum you can pull on a rotovap.
I always try to decide if I want a good vacuum or good stirring, both at once it tough to achieve.
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monolithic
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The industrial solution is a magnetic coupling stirrer. Relatively high torque transmission with a fantastic seal for high pressure or vacuum work.
Most typically seen on larger reactors but some are designed to work with ground glass necks. Unfortunately they are very expensive.
Some examples of various designs:
https://www.deheerbv.com/products/premex-magnetic-stirring-c...
http://www.haage.com/EN/products/high-pressure-autoclaves/ma...
http://www.kenkimble.com/products/magnetic-couplings/
https://www.parrinst.com/products/stirred-reactors/options-a...
It seems they're mostly Inconel or Hastelloy on the process side with graphite/PTFE bushings. Some manufacturers add a cooling jacket to reduce
process heat transmission to the magnets. If one had access to a sturdy lathe (nickel/chromium alloys are a bitch to machine) it would definitely be a
fun project to DIY.
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Sulaiman
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A minor correction to my post above,
the ptfe adapter for the 5l rbf was 19/26,
not 19/23.
Inconsequential, just correcting a minor error.
CAUTION : Hobby Chemist, not Professional or even Amateur
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