From:         loc@yrloc.ipsa.reuter.COM (Leigh Clayton)
Subject:      Teflon - summary at long last - *Interesting* (I think)
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Date:         4 Sep 92 00:07:30 UT


 Several weeks ago I undertook to find out about Teflon (r): it's safety,
especially. I've been away (and my news system broke down too), but there was a
lot of information in sci.chem that I think will interest at least the people
who were interested in the subject here.

 I'm going to summarise first, and then include parts of messages I think are
interesting. To begin with, Shankar suggested that I simply get in touch with
DuPont. As expected :-), this was a very sensible suggestion. I still haven't
done it because the last posting in sci.chem looked quite convincing to me and
(more importantly) DuPont don't appear to have an office in Toronto so the call
wouldn't be local. If anyone cares to call them and verify/expand this
information I'd be very interested in the results.

 The bottom line appears to be that Teflon is capable of being very nasty stuff
indeed, heating it in air can give rise to fluorophosgene and
tetrafluoroethylene (as well as even nastier stuff in smaller amounts and at
higher temperatures). These are toxic gases (and fluorophosgene is *seriously*
dangerous) so the risk is not something to take lightly. The good news is that
it appears to be perfectly safe if used *sensibly*. The first posting I'm
including is from a poster who actually did a heat-in-air analysis of Teflon,
and clearly shows that at temperatures used in normal cooking there is no
appreciable danger. One poster noted that self-cleaning ovens get *very* hot,
and it looks to me like leaving a Teflon pan in one during the clean cycle
would be a seriously foolish thing to do. Leaving one on a red-hot electric
element also looks a bit dangerous, but that seems a bit more survivable. **I
stress** that these evaluations are my own and I am neither a chemist nor a
physicist, I may have miscinstrued the results presented below (or they may be
wrong). I plan to keep using my Teflon frypan (the fat intake it saves me is
likely a more serious threat to my health than the risk of poisoning myself)
but I will treat it with greater respect than I had previously known it
deserved.

 The last posting I received, I now include. It is first because it seems the
most informative, for anyone who doesn't want to read this whole posting.

-From: dluznie@bingsuns.cc.binghamton.edu (dluznie)
-Subject: Heating TEFLON
-
-
-Approximately 2 weeks ago there was a discussion about possible toxic
-products formed from teflon when it is heated to high temperatures.
-The question was related to the use of teflon as coating of cookware.
-
-I said I might be able to run a set of DTA/TGA runs on a sample of
-teflon. I did it recently and I will summarize the results here.
-If somebody is interested in more detailed results I can e-mail results
-of the runs in form of numerical data, that can be imported to some
-spreadsheet (eg LOTUS) or plotting program. This data will be in ASCII
-form, so there should be no problem in processing them. Beware though:
-the files are big - each contains over 1500 data points, 4 numbers per
-point.
-If someone prefers the copies of plotted data send SASE to:
-       Tomasz Dluzniewski
-       Chem. Department SUNY
-       Binghamton, NY 13902-6000
-       USA
-
-
-Now short summary of the results:
-I used two samples of teflon: one cut of a solid block of material and
-the other being a piece of the thin tape like the one used for sealing
-threaded pipe connections. Both samples gave practically identical
-results.
-
-The samples were heated at the rate of 20 deg. C / min. from room
-temperature to 600 C in a flow of air or argon (50 ml/min).
-The weight loss starts at well over 400 C. The DTA curve shows
-endothermic effect at ca. 350 C which is not related to any weight
-change (melting?? - I thought that teflon does not melt, maybe it is
-recrystallization?). Second large heat effect is connected with the
-final weight loss and starts at approximately 450 C. The sign of this
-effect depends on the atmosphere. In inert gas it is endothermic ie.
-decomposition and evaporation. In a flow of air it becomes strongly
-exothermic, which means that the products of decomposition
-(depolymerization) undergoes oxidation.
-
-To test longer term stability of teflon I have heated the sample of teflon
-in a flow of air at the slowest available heating rate (1 deg/min).
-The results were similar to the ones obtained in air at 20 deg/min.
-Most important was that the weight loss was not observed up to 440 C.
-That means that at temperatures much lower eg. 280 C no decomposition
-shoud be expected. During the discussion someone said that at this
-temperature teflon ferrules dissapeared completely from his gas
-chromatograph, and that they probably decomposed. It seems to me highly
-unlikely in view of TGA results.
-
-On the other hand, what happens to teflon around 330 C surprised me a
-little. If this really is melting or kind of softening then the material
-may disappear above this temperature (leak out or something).
-
-Tomek
-

 This posting did not address the identity of the gas(es) emitted above
440C(820F); the following postings do so.

-
-The gas given off could as small as the CF2 monomer, or perhaps small chains
-of polymers as a soot or the monomer.
-
     ......
-
-
-Teflon should *not* be overheated.  Beyond 400C it decomposes back into its
-monomer, tetrafluoroethylene and beyond that, probably down to CF2.
-
-In the Merck Index, it is only cited to cause mucous membrane irritation,
-but hey, it could kill your birds...
-
-
-  ....       I have heated teflon in the lab and have encountered
-the fumes. Have you ever gotten a whif of NO ( =>NO2 ) ? It's pretty rough
-and, to my knowledge, can cause pulmonary edema the same way NO2 can.
-
-As I remember, when we've ordered teflon samples, the book sized warning labe
-l
-that comes with every order, included warnings about toxic gases  if overheat
-ed.
-Personally, I'll take their word for it.  Merck does also say "..There
-have been reports of fume fever in humans exposed to unfinished products.
-Pyrolysis products are irritating to mucous membranes."
-
-As far as birds go, yes, they are more sensitive to airborn toxins than human
-s.
-This according to my brother who is an 'avian endocrinologist and reproductiv
-e
-physigist', which certainly *sounds* good.  It has something to do with the
-fact that birds respirate through the insides of their *wing* bones!
-(hey, this is just what I'm told)  You've certainly heard of a 'canary in
-a coal mine'. They didn't bring canaries into coal mines just to hear them
-tweet, they brought them there because they made darn good toxic gas detector
-s.
-Dead bird -> leave soon.
-

-From: hclase@morgan.ucs.mun.ca (Howard Clase)
-Subject: Re: Teflon: question about heating.

-
-       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 give
-n
-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 be
-en
-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
-


-From: mvp@hsv3.lsil.com (Mike Van Pelt)
-Subject: Re: Teflon: question about heating.

->Ligon@macgw1.ge.com (Woody Ligon) writes:
->: When heated above about 280 C in air (not vacuum), Teflon(R) decomposes to
- [ nasty stuff ]
->This is a little hard to believe, since a regular cooking oven can get that
->hot without any problem. There are many products designed for oven use that
->are Teflon coated.
-
-280C is 536F, which is a bit higher than normal oven cooking
-temperatures.  The teflon would be in contact with food, too,
-which would tend to keep it at a lower temperature.
-
-Of course, you shouldn't leave your teflon in a self-cleaning
-oven when you run the clean cycle; that's about 800F.


-From: eddie@csa2.lbl.gov (EDWARD J MOLER)
-Subject: Re: Teflon: question about heating.

-       I have teflon in my Ultra-High Vacuum chamber and have heated it to at
-least 150 degrees C with no sign of outgassing (as read in a mass spectromete
-r
-in the UHV chamber).  Even with "new" teflon.  I regularly achieve 1x 10**(-1
-0)
-torr like this and would be VERY sensitive to small outgassing rates.  I look
-at masses as high as 50 amu's most of the time.
-


-From: Ligon@macgw1.ge.com (Woody Ligon)
-Subject: Re: Teflon: question about heating.

-When heated above about 280 C in air (not vacuum), Teflon(R) decomposes to
-give (in part) fluorophosgene (carbonyl difluoride). Unlike dioxin, asbestos,
-PCBs and Alar, etc., etc., this really is a genuinely and very seriously toxi
-c
- material. The term "toxic" has been so completely devalued that I think this
-kind of comparisons are necessary to provide perspective.
-
-Regarding the suggested production of NO, this is not particularly
-relevant. Practically any process which heats a combination of N2 and O2 to
-a sufficiently high temperature gives small amounts of NO, but Teflon itself
-would have long since disappeared before any temperature was reached which
-gave more than traces of NO. Even automobile engine combustion temperatures
-produce only small amounts of NO.
-


-From: Ligon@macgw1.ge.com (Woody Ligon)
-Subject: Re: Teflon: question about heating.

->
-> You are right, it will release fluorophosgene, but the temperature
-> needed as I recall must be much higher. Teflon(R) decomposition
-> temperature as I remember is 416 deg. C. Fluorinated hydrocarbons
-> released during decomposition of polytetrafluoroethylene
-> at high temperatures will oxidize to fluorophosgene (similarly to
-> chlorinated hydrocarbons which will form phosgene eg. chloroform vapors
-> on contact with red hot wire or hot end of a cigarette oxidizes in air
-> with formation of phosgene).
->
-> Tomek Dluzniewski
->
-
-What we have here is a question of rates.  Of course, Teflon (R) decomposes
- more quickly at 400 C (or 600 as someone else suggested) but
-the reaction also goes as low as 280 C albeit much more slowly.  Anyone who
- has ever tried using teflon ferrules in a gas chromatograph above 280 for an
-y
-extended period of time will know that the Teflon disappears entirely at
-these temperatures in a matter of about a month. Ferrules are far thicker
-than the coatings we are talking about on cookware so extended use of a
-coating above 280 C (about 500 F) is definitely going to destroy such coating
-s
-in short order.
-
-I don't have a copy of the TGA (thermogravimetric analysis) of Teflon (R)
-handy but I'd strongly suspect that even with a 20 C/min temperature ramp mos
-t
-of the weight would be lost long before 400 C provided the experiment was
-done in air.
-
-I was reporting the temperature at which the reaction would proceed at a
-noticable rate, not the temperature at which the reaction would be complete
-in a few seconds!
-

-From: dluznie@bingsuns.cc.binghamton.edu (dluznie)
-Subject: Re: Teflon: question about heating.

-Yes, we are talking about the rates. You may be right that the decomposition
-would be fast enough at lower temperatures. I am not sure if the temperature
-of
-280 C will be sufficient for oxidation ie. formation of fluorophosgene. Anywa
-y
-whatever decomposition products would be formed they could accumulate in food
-,
-especially in fat. Whether concentration would be harmful? Apparently not ver
-y.
-After all many people are using Teflon(R) coated pans. But does anybody know
-what are long term effects of doing so.
-
-[...]
->
->I don't have a copy of the TGA (thermogravimetric analysis) of Teflon (R)
->handy but I'd strongly suspect that even with a 20 C/min temperature ramp mo
-st
->of the weight would be lost long before 400 C provided the experiment was
->done in air.
->
->Woody Ligon
-[...]
-
-I may be able to run a TGA analysis of a piece of teflon, maybe I'll even
-scratch it from old frying pan to be closer to real life. But not sooner than
-after Aug. 5. I will post the results if I find time to really do this.
-
-Tomek Dluzniewski
-



 Well, that's it. I did get a few other messages, but they're not of as general
an interest so I haven't included them in the summary. I hope some of you find
this interesting, and as I said above, if anyone calls DuPont for their opinion
I'd be very interested to hear what they say (if no-one does, perhaps I'll send
them a hard-copy of this posting by real-mail and see what they say).

 The one question that someone had that I am only able to infer an answer for
is "is it safe to use a teflon (r) pan that I know has been overheated badly at
some point". My take on this discussion is that if there has been no obvious
appreciable loss of the Teflon, it would be perfectly safe to use it but I
stress that this is a very lay interpretation of this information (which is
itself provided under disclaimer) so if you do so, and it kills you, don't come
whining to me in ectoplasmic form about it :-(.

../Leigh

-----------------------------------------------------------
Leigh Clayton                     loc@yrloc.ipsa.reuter.COM
    -or-                        loc@ipsaint.ipsa.reuter.COM