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CATEGORIES (articles) > General > Finishings > Polytetrafluoroethylene (Teflon)

Polytetrafluoroethylene (Teflon)


Teflon is polytetrafluoroethylene (PTFE), a polymer of fluorinated ethylene.
3D model of a section of PTFE

Polytetrafluoroethylene (PTFE) is a fluoropolymer discovered by Roy J. Plunkett (1910–1994) of DuPont in 1938 and introduced as a commercial product in 1946. It is generally known to the public by DuPont's brand name Teflon.

PTFE has the lowest coefficient of friction (against polished steel) 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. Its melting point is 327 °C, 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; 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.

Teflon is also used as the trade name for a polymer with similar properties, perfluoroalkoxy polymer resin (PFA).

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 on account 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.

PTFE is the main ingredient behind the renowned Greased Lightning fuel additive series. ([1] page3.html)

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.E.-C. Koch "Metal-Fluorocarbon Pyrolants:III. Development and Application of Magnesium/Teflon/Viton" Propellants Explosives Pyrotechnics (2002),27(5),pp. 262-266.


Safety

Teflon is often used to coat non-stick frying pans as it has very low friction and high heat resistance.

While Teflon itself is chemically inert and non-toxic, Teflon begins to deteriorate after the temperature of cookware reaches about 500 °F (260 °C), and begins to significantly decompose above 660 °F (350 °C). These degradation products can be lethal to birds, and can cause flu-like symptoms in humans (see Teflon flu). By comparison, cooking fats, oils and butter will begin to scorch and smoke at about 392 °F (200 °C), and meat is usually fried between 400–450 °F (200–230 °C), but empty cookware can exceed this temperature if left unattended on a hot burner. A 1959 study, conducted before the FDA approved the material for use in food processing equipment, showed that the toxicity of fumes given off by the coated pan on dry heating was less than that of fumes given off by ordinary cooking oils..

A 1973 study confirmed the FDA findings and found that a 4-hour exposure to the pyrolysis products of butter in an uncoated pan were 100% toxic to parakeets at 260 °C, whereas no deaths were observed for exposure to Teflon pyrolysis products until the Teflon coated pan was heated to 280 °C.Griffith, F.D.; Stephens, S.S.; Tayfun, F.O. "Exposure of Japanese quail and parakeets to the pyrolysis products of fry pans coated with Teflon and common cooking oils." American Industrial Hygiene Association Journal (1973), 34(4), pp. 176-178. Over the 40 years non-stick cookware has been in widespread use, there is only one published case of a minor, short-lasting health effect linked to overheating non-stick cookware..

The EPA's scientific advisory board found in 2005 that perfluorooctanoic acid (PFOA), a chemical compound used to make Teflon, is a "likely carcinogen." This finding was part of a draft report.that has yet to be made final. DuPont settled for $300 million in a 2004 lawsuit filed by residents near its manufacturing plant in Ohio and West Virginia based on groundwater pollution from this chemical. Currently this chemical is not regulated by the EPA.

In January 2006, DuPont, the only company that manufactures PFOA in the US, agreed to eliminate releases of the chemical from its manufacturing plants by 2015, but did not commit to completely phasing out its use of the chemical. This agreement is said to apply to not only Teflon used in cookware but in other products such as food packaging, clothing and carpeting. DuPont also stated that it cannot produce Teflon without the use of the chemical PFOA, although it is looking for a substitute.

It is noteworthy that PFOA is not part of the finished product of nonstick cookware or bakeware. It is only used during the manufacture of the product and only a trace amount of PFOA remains after the curing process. There should be no measurable amount of PFOA on a finished pan, provided that it has been properly cured.

When PTFE coated stir bars are exposed to a solution of solvated electrons in liquid ammonia the surface is converted to carbon.






CATEGORIES (articles) > General > Finishings > Polytetrafluoroethylene (Teflon)

 
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