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I have read that teflon coatings undergo decomposition at temperatures above 300 degree Celsius. What do they decompose to? And what other conditions apply for the decomposition? Is this decomposition harmful for humans as cooking goes beyond that temperature?

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Like any other polymer decomposition process, the products of PTFE decomposition depends on the chemical species present while PTFE is undergoing the process and temperatures.

General process is this : Decomposition is initiated by random-chain scission, followed by depolymerization. Termination is by dis-proportionation. And all of this happens rapidly above 600 K (~326 C). There are no other conditions applied for the decomposition to take place hence can happen in dry or aqueous environments, which would give different by products. Cooking below 200 C, it would be completely safe since the mass loss below 300 C is undetectable. Only above the glass transition temperature (~326 C) is the mass loss significant.

Decomposition products will depend on the environment and usually Oxygen usually does not enter the cycle directly but through water, to give species like,

  • Carbonyl fluoride
  • Carbonyl difluoride

As expected other species like fluorinated alkanes and alkenes are obtained like,

  • the monomer, tetrafluoroethylene
  • Hexafluoroethane
  • Octafluorocyclobutane
  • Octafluoroisobutylene
  • Perfluoroisobutylene
  • Tetrafluoroethylene and more.

However, the chemicals (species) listed above are for controlled lab experiments. What happens in real life cooking scenario is not (or cannot) be anticipated. But, it can be readily said that the above species can react with other chemicals in food to give fluorinated compounds that will be harmful for humans. As a precaution, one should not cook above 200 C on PTFE coated utensils to be completely safe.

Check this extremely detailed page for abstracts and toxicity remarks from research papers on PTFE decomposition on Fluoridealert.org where the conditions and level of toxicity is reported in an organized manner.

Polymer decompositions- check PTFE section for general comments.

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    $\begingroup$ Tetrafluoroethylene is the monomer $\endgroup$ – Mithoron Feb 27 '17 at 21:53
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    $\begingroup$ Although the question is about harm in humans, it's worth noting that pet birds are very sensitive to PTFE toxicosis. (The proverbial "canary in the coalmine.") The dangers of overheated teflon to birds has been public since the 1980s. See sources for Teflon Toxicity (PTFE Toxicosis) in Birds. $\endgroup$ – mwoodman Feb 27 '17 at 22:31
  • $\begingroup$ Carbonyl fluoride is dangerous stuff. It's pretty similar to phosgene, a chemical weapon used in World War 2. $\endgroup$ – Pritt says Reinstate Monica May 14 '17 at 9:46
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Teflon is a mono polymer.

The monomer is Tetrafluoroethylene — $\ce{C2F4}$

Pyrolysis of PTFE is detectable at 200 °C (392 °F), and it evolves several fluorocarbon gases and a sublimate. An animal study conducted in 1955 concluded that it is unlikely that these products would be generated in amounts significant to health at temperatures below 250 °C (482 °F)

Source: ( Zapp JA, Limperos G, Brinker KC (26 April 1955). "Toxicity of pyrolysis products of 'Teflon' tetrafluoroethylene resin". Proceedings of the American Industrial Hygiene Association Annual Meeting.)

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I had, until recently, machined pure Teflon (PTFE) in industry. I know the MSDS sheets I was provided with mentioned, at least under heat of cutting, one thing it breaks down into (among others) was hydrofluoric acid. Stuff you do NOT want to breathe! I know that as a highly poisonous acid that attacks glass.

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