From: hclase@morgan.ucs.mun.ca (Howard Clase)
Subject: Re: Teflon: question about heating.
Organization: Memorial University of Newfoundland
Date: Thu, 23 Jul 1992 18:22:27 GMT

In article <1992Jul22.221548.17086@yrloc.ipsa.reuter.COM> loc@yrloc.ipsa.reuter.COM (Leigh Clayton) writes:
>
> I posted this article to sci.materials, and got one response suggesting
>that the gas Teflon *may* emit when heated too much is Phosgene. The respondant
>was a bit cautious, though, since Phosgene seems to contain Chlorine and it
>isn't obvious where that would come from. Since the interest in rec.food.veg
>seems to still be there, I'm trying again this time to sci.chemistry; maybe
>we'll get better luck. I don't read sci.chemistry though, so if anyone does
>decide to post an answer, please post to rec.food.veg as well.
>
>       -----------Begin previously posted article--------------
>
> In article <1992Jun11.152827.4156@bb1t.monsanto.com> tjfeue@bb1t.monsanto.com
>(I think. He may have been responding to an earlier post) suggests that Teflon
>should not be overheated because it will emit a dangerous gas that will kill
>pet birds (and presumably people).
>
> Teflon is (according to my American Heritage) a trade name for
>polytetrafluoroethylene. My organic chemistry is rather tentative, but I decode
>that name to look a bit like this:
>
>       F F F F F
>       | | | | |
>  ... -C-C-C-C-C ...
>       | | | | |
>       F F F F F
>
> This is sort of a chlorine-less, polymerised CFC. Containing Fluorine it
>clearly *could* be convinced to emit Fluorine gas, which is indeed very toxic
>(though why it would kill a pet bird and leave a human around to gripe about it
>is a bit of a mystery to me). But I would expect this compound to be quite
>heat-stable (and indeed, fluorine gas being well known to be so toxic I can't
>imagine regulatory agencies allowing anything likely to emit it in cooking
>utensils for home use). Of course, the indicted gas may not be fluorine, since
>there are lots of dangerous gases in the world (like cyanide, which is made out
>of just carbon and nitrogen, so there's likely some way to make it out of
>Teflon (or wood) and air too, but I'd be surprised to see a lot of it made just
>by heating the stuff too hot).
>
> This strikes me as a typical urban myth, but I'm cross-posting to the
>sci.materials group in the hope that someone there will know (or be willing to
>find out) exactly how thermally stable the stuff is.
>
>
>-----------------------------------------------------------
>Leigh Clayton                     loc@yrloc.ipsa.reuter.COM
>    -or-                        loc@ipsaint.ipsa.reuter.COM

	You are quite right, teflon contains no chlorine and cannot generate
phosgene, or carbonyl chloride, COCl2.

	The gas evolved when the polymer is heated at 500 - 600 Celcius is
simply tetrafluoroethylene, F2C=CF2.   This is toxic.   This reaction is given
as a convenient method of preparing tetrafluoroethylene in the lab.   500 C
corresponds to red heat, and is attainable on a kitchen range, so you do need
to be careful.   There have been deaths reported from people who smoked while
machining teflon and sucked the dust into their cigarettes, causing pyrolysis
of the polymer.

	There is an interesting story about the discovery of teflon in the
1940s.   A chemist at a lab working with the newly discovered
tetrafluoroethylene came in one morning to find that his cylinder refused to
emit any more gas even though he was sure it had been full the previous day.
Most people would have assumed that it had leaked or that the cleaners had been
fiddling with the valve, but he guessed what had really happened, had the
cylinder sawn open and discovered that it was lined with the new material now
known as teflon.   At room temperature the polymerisation reaction is
spontaneous once started, above 500 C the reverse reaction occurs.

	Howard Clase