IndependentBoffin
Hazard to Others
Posts: 150
Registered: 15-4-2011
Member Is Offline
Mood: No Mood
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PTFE-glassware thermal expansion incompatibility mishap
I was boiling a mixture containing acetic anhydride and hexamine (some of you clever guys might know what my end product is!) in an oil bath, using a
custom made PTFE stopper with a 6mm hole, made in my workshop on a lathe, leading to 6mm PTFE tubing. The objective was to distill out the acetic
anhydride out at 140 C.
First accident was the PTFE stopper broke the borosilicate test tube's neck because of thermal expansion. Yeah I know I should have been using a
conical flask but it was only about 5-10ml, and it was a large test tube.
Second time I didn't put the PTFE stopper as deep so it wouldn't crack the test tube's neck. This time the stopper popped out mid-boiling, splashing
acetic anhydride and the smelly test tube contents everywhere  Luckily none got
into my eyes and whatever got onto my bare skin was quickly washed off. The oil bath foamed a lot as the stuff got into it as well. If I was making a
larger quantity, hot oil + whatever I was heating splashing everywhere is a possibility. My garage now smells fishy/vinegary despite my best efforts
to locate all splattered ingredients.
BTW I was using PTFE stoppers and tubing because my next experiment will involve making white fuming nitric acid under vacuum aspiration. PTFE is the
only material that would withstand attack by acetic anhydride and WFNA.
Lessons learnt:
1) If using PTFE with glassware involving heating, pre-heat the PTFE in boiling water.
2) Wear full face masks when doing any boiling work.
3) I am an aero engineer by training, and it has been some time since I did chemistry lab work (Year 12 actually). I might need to get more lab
experience with more benign chemicals and use proper rather than custom equipment (e.g. all glassware -> less issues with CTE mismatches) before
moving on to WFNA.
4) This stuff is miles more fun than report writing, finite element/smoothed particle hydrodynamics simulations or CAD  . I am going to a rifle range tonight to test another invention of mine . All being well I should have a new product to sell within a few months.
P.S. If I go quiet suddenly something terrible has happened  . I just joined but
hope to become a regular here 
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watson.fawkes
International Hazard
Posts: 2793
Registered: 16-8-2008
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Standard taper for
glassware is 1:10. A stopper of that taper held in by a spring will deal with thermal expansion over a wide temperature range.
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Lambda-Eyde
National Hazard
Posts: 859
Registered: 20-11-2008
Location: Norway
Member Is Offline
Mood: Cleaved
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Save up a few hundred quid and invest in some ground joint glassware if you're going to do distillations and organic synthesis. B24 and B19 are nice
sizes, I recommend a mix of them.
Messing around with custom made PTFE/plastic apparatus and test tubes just isn't worth it. Don't trade safety for cost.
This just in: 95,5 % of the world population lives outside the USA
You should really listen to ABBAPlease drop by our IRC channel: #sciencemadness @ irc.efnet.org
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