Fyndium
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Registered: 12-7-2020
Location: Not in USA
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Risk of cracking joints with stirrer adapters?
PTFE has thermal expansion rate 100 times larger than boro, and I have noticed that at room temp practically loose stirrer adapters get stuck to the
point when heated it is impossible to remove them, even with carefully using pliers for torque. I'm afraid this could cause the glass joint to crack.
Is this a real issue, and how it could be avoided?
Chemglass(among others likely) sells dual piece adapters that have glass joint and thread attachment for the ptfe bearing part. I see that they might
be onto something here.
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Sulaiman
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Registered: 8-2-2015
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At home (UK) I used a ptfe stirrer and here (Malaysia) I use a ptfe stirrer adapter as a thermometer holder,
I've only used them up to around 100oC and not tried to remove them when hot,
but once cooled down I've not noticed this problem.
I guess that the expansion of the ptfe will not cause excessive stress on the glass as it is plastic and deformable, more so when hot.
CAUTION : Hobby Chemist, not Professional or even Amateur
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Fyndium
International Hazard
Posts: 1192
Registered: 12-7-2020
Location: Not in USA
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I wouldn't want to test it in practice, as a swell will cost me 400€ per crack. Pre-testing this with cheap/already broken glass by exposing the
joint to extremes of hot and cold by applying boiling water, heat gun and ice to it would show if it cracks, but all glass is unique, then, so it
would be little for reference.
One way would be to pre-heat it before applying, but I noticed that the stirrer vibration will push it deeper anyways, so it's somewhat useless.
The most arduous way would be to get glass threaded joints for thermometers or similar and turn DIY PTFE adapters from stock bar, but I'm sort of
tired of making everything myself. I could soon as well start a multi-workshop that makes anything from metal, plastic or glass. 
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