Polytetrafluoroethylene
Polytetrafluoroethylene (PTFE) is a fluoropolymer discovered by Roy J. Plunkett (1910–1994) of DuPont in 1938.[1] It was introduced as a commercial product in 1946 and (in an example of a genericized trademark) is generally known to the public by DuPont's brand name Teflon®.
The coefficient of friction of PTFE is 0.1 or less, which is the lowest of any known solid material. It is used as a non-stick coating for pans and other cookware. PTFE is very non-reactive, and so is often used in containers and pipework for reactive chemicals. According to DuPont its melting point is 327 °C,[2] but its properties degrade above 260 °C.
Other polymers with similar composition are known with the Teflon® name: fluorinated ethylene-propylene (FEP) and perfluoroalkoxy polymer resin (PFA). They retain the useful properties of PTFE of low friction and non-reactivity, but are more easily formable. FEP is softer than PTFE and melts at 260 °C;[3] it is highly transparent and resistant to sunlight.
History
PTFE is sometimes said to be a spin-off from the U.S. space program with more down-to-earth applications; this is an urban legend, as Teflon® cooking pans were commonplace before Yuri Gagarin's flight in 1961. PTFE was discovered serendipitously by Roy Plunkett of DuPont in 1938, while attempting to make a new CFC refrigerant, when the perfluorethylene polymerized in its storage container. DuPont patented it in 1941, and registered the Teflon® trademark in 1944.
An early advanced use was in the Manhattan Project, as a material to coat valves and seals in the pipes holding highly reactive uranium hexafluoride in the vast uranium enrichment plant at Oak Ridge, Tennessee, when it was known as K416.
It was first sold commercially in 1946 and by 1950, DuPont was producing over a million pounds (450 t) per year in Parkersburg, West Virginia. In 1954, French engineer Marc Grégoire created the first Teflon®-coated cooking pan.
Teflon® has been supplemented with another DuPont product, Silverstone, a three-coat fluoropolymer system that produces a more durable finish than Teflon®. Silverstone was released in 1976
Properties and applications
Amongst many other industrial applications, PTFE is used to coat certain types of hardened, armour-piercing bullets, so as to reduce the amount of wear on the firearm's rifling. These are often mistakenly referred to as "cop-killer" bullets by virtue of PTFE's supposed ability to ease a bullet's passage through body armour. Any armour-piercing effect is, however, purely a function of the bullet's velocity and rigidity rather than a property of PTFE.
PTFE has excellent dielectric properties. This is especially true at high radio frequencies, making it eminently suitable for use as an insulator in cables and connector assemblies and as a material for printed circuit boards used at microwave frequencies. Combined with its high melting temperature, this makes it the material of choice as a high performance substitute for the weaker and more meltable polyethylene that is commonly used in low-cost applications. Its extremely high bulk resistivity makes it an ideal material for fabricating long life electrets, useful devices that are the electrostatic analogues of magnets.
Due to its low friction, it is used for applications where sliding action of parts is needed: bearings, bushings, gears, slide plates, etc. In these applications it performs significantly better than nylon and acetal; it is comparable with ultra high molecular weight polyethylene (UHMWPE), although UHMWPE is more resistant to wear than Teflon®. For these applications, versions of teflon with mineral oil or molybdenum disulfide embedded as additional lubricants in its matrix are being manufactured.
Because of its chemical inertness, PTFE cannot be cross-linked like an elastomer. Therefore it has no "memory", and is subject to creep (also known as cold flow and compression set). This can be both good and bad. A little bit of creep allows PTFE seals to conform to mating surfaces better than most other plastic seals. Too much creep, however, and the seal is compromised. Compounding fillers are used to control unwanted creep, as well as to improve wear, friction, and other properties.
Gore-Tex is a material incorporating Teflon® membrane with micropores. The roof of the Hubert H. Humphrey Metrodome in Minneapolis is the largest application of Teflon® on Earth, using 20 acres of the material in a double-layered white dome, made with PTFE-coated fiberglass, that gives the stadium its distinctive appearance.
Powdered PTFE is used in pyrotechnic compositions as oxidizer together with powdered metals such as aluminum and magnesium (see Magnesium/Teflon/Viton). Upon ignition these mixtures form carbonaceous soot and the corresponding metal fluoride and release large amounts of heat. Hence they are use as infrared decoy flares and igniters for solid fuel rocket propellants.[4]
Production
PTFE is either synthesized by the emulsion polymerization of tetrafluoroethylene monomer under pressure, using free-radical catalysts[5],
or it may be produced by the direct substitution of hydrogen atoms on polyethylene with fluorine, using polyethylene and fluorine gas at 20 °C [5]:
Safety
MEHMETS MUM
References
- ^ Roy J. Plunkett Chemical Heritage Foundation. Retrieved 10 September 2006.
- ^ Fluoropolymer Comparison - Typical Properties www2.dupont.com. Retrieved 10 September 2006.
- ^ FEP Detailed Properties Parker-TexLoc, 13 April 2006. Retrieved 10 September 2006.
- ^ E.-C. Koch "Metal-Fluorocarbon Pyrolants:III. Development and Application of Magnesium/Teflon/Viton" Propellants Explosives Pyrotechnics (2002),27(5),pp. 262-266.
- ^ a b Mike Orthner, Polytetrafluoroethylene/"Teflon" Synthesis, accessed on 02 Oct 2006.
- Ellis, D.A.; Mabury, S.A.; Martin, J.W.; Muir, D.C.G. "Thermolysis of fluoropolymers as a potential source of halogenated organic acids in the environment." Nature 2001, 412 (6844), pp. 321-324.
External links
Industry Information on Teflon®
- PFOA Facts (Society of the Plastics Industry, Inc. (SPI), 2005). PFOA Facts
- The Chemical Heritage Foundation (2000). Roy J. Plunkett. Retrieved Oct 7, 2005.
- DuPont Teflon PTFE Material Data Sheet Retrieved Sept 6, 2006
- DuPont (2005). Teflon News and Information. Retrieved Oct 7, 2005.
- DuPont (2005). Information on PFOA. Retrieved Oct 12, 2005.
Teflon® in the Kitchen
- DuPont (2005). Cooking Safety. Retrieved Oct 12, 2005.
- Environmental Working Group (2005). Canaries in the Kitchen - "Teflon Toxicosis" is deadly to pet birds. Are we at risk?. Retrieved Oct 7, 2005. More articles from EWG.
- Dr. Weil (2005-09-09). Teflon: The Sticky Business?
Environmental Debate on Teflon® / PFOA / C-8
- Nature Publishing Group (2001-07-19). Thermolysis of fluoropolymers as a potential source of halogenated organic acids in the environment. Retrieved Oct 7, 2005.
- Organic Consumers Association / New York Times (2004-08-08). Dupont, Dupont Denies Poisoning Consumers with Teflon Products - Now in the Frying Pan. Retrieved Oct 7, 2005.
- Washington Post (2005-06-29 page A04). Compound in Teflon A 'Likely Carcinogen'. Retrieved Oct 7, 2005
- CorpWatch / USA Today (2005-11-16). US: Engineer: DuPont hid facts about paper coating
- CorpWatch / Associated Press (2005-11-29). US: EPA, DuPont in Settlement Over Chemical
- CorpWatch / Associated Press (2005-12-14). US: DuPont fined more than $10M over Teflon
- Los Angeles Times (2005-12-15). DuPont Settles Charges That It Hid Toxic Risk Data. Retrieved 2005-12-15.
- Environment Writer / Los Angeles Times (2005-12-15). Du Pont Settles Charges That It Hid Toxic Risk Data
- ABC News (2006-01-25). Government Moves to Curb Use of Chemical in Teflon
- Washington Post (2006-01-26), Page A01. Harmful Teflon Chemical To Be Eliminated by 2015
Other Teflon® Issues
- Kopel, Dave (2004). Teflon Bullets. Retrieved Oct 7, 2005.
Chemical
