Polytetrafluoroethylene

In chemistry, polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer which finds numerous applications. PTFE's most well known trademark in the industry is the DuPont brand name Teflon.

PTFE has an extremely low coefficient of friction and is used as a non-stick coating for pans and other cookware. It is very non-reactive, and so is often used in containers and pipework for reactive and corrosive chemicals. Where used as a lubricant, PTFE significantly reduces friction, wear and energy consumption of machinery.

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History

PTFE was invented accidentally by Roy Plunkett of Kinetic Chemicals^[1] in 1938.^[2] While Plunkett was attempting to make a new CFC refrigerant, the perfluorethylene polymerized in its pressurized storage container. (In this original chemical reaction, iron from the inside of the container acted as a catalyst.) Kinetic Chemicals patented it in 1941 and registered the Teflon trademark in 1944.^[3] The original patent number is US2,230,654.^[4]

Teflon was first sold commercially in 1946. By 1950, DuPont had acquired full interest in Kinetic Chemicals and was producing over a million pounds (450 t) per year in Parkersburg, West Virginia. In 1954, French engineer Marc Grégoire created the first pan coated with Teflon non-stick resin under the brandname of Tefal after his wife urging him to try the material which he was using on fishing tackle to be used on her cooking pans.^[5] In the United States Kansas City, Missouri resident Marion A. Trozzolo, who had been using the substance on scientific utensils, marketed the first frying pan "The Happy Pan" in 1961.^[6]

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.

The common statement that PTFE is a spin-off from the United States space program is an urban legend.

Properties

PTFE is a white solid at room temperature, with a density of about 2.2 g/cm³. According to DuPont its melting point is 327 °C (620.6 °F), but its properties degrade above 260 °C (500 °F).^[7]

The coefficient of friction of plastics is usually measured against polished steel.^[8] PTFE's coefficient of friction is 0.1 or less^[7], which is the lowest of any known solid material. PTFE's resistance to van der Waals forces means that it is the only known surface to which a gecko cannot stick.^[9]

PTFE has excellent dielectric properties. This is especially true at high radio frequencies, making it 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 lower melting point 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.

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 control unwanted creep, as well as to improve wear, friction, and other properties. Sometimes metal springs apply continuous force to PTFE seals to give good contact, while permitting some creep.

Applications

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 to 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.

Gore-Tex is a material incorporating fluoropolymer membrane with micropores. The roof of the Hubert H. Humphrey Metrodome in Minneapolis is one of the largest applications of Teflon PTFE coatings on Earth, using 20 acres (about 8 hectares) of the material in a double-layered, white dome, made with PTFE-coated fiberglass, that gives the stadium its distinctive appearance. The Millennium Dome in London is also substantially made of PTFE.

Powdered PTFE is used in pyrotechnic compositions as oxidizer together with powdered metals such as aluminum and magnesium. Upon ignition these mixtures form carbonaceous soot and the corresponding metal fluoride and release large amounts of heat. Hence they are used as infrared decoy flares and igniters for solid-fuel rocket propellants.^[10]

PTFE is also used in body piercings, such as a sub-clavicle piercing, due to its flexibility and bio-compatibility.

In optical radiometry, sheets made from PTFE are used as measuring heads in spectroradiometers and broadband radiometers (e.g. illuminance meter and UV radiometer) due to its capability to diffuse a transmitting light nearly perfectly. Moreover, optical properties of PTFE stay constant over a wide range of wavelengths, from UV up to near infrared. In this region, the relation of its regular transmittance to diffuse transmittance is negligibly small so light transmitted through a diffuser (PTFE sheet) radiates like Lambert's cosine law. Thus, PTFE enables cosinusoidal angular response for a detector measuring the power of optical radiation at a surface, e.g., in solar irradiance measurements.

PTFE is also used to coat certain types of hardened, armor-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 armor.

PTFE's low frictional properties have also been utilized as computer mice feet such as the Logitech G5 and Logitech G7 computer mice series from Logitech. The low-friction provided by PTFE allows the mice to be moved and glide across surfaces smoothly and with less effort.

Production

PTFE is either synthesized by the emulsion polymerization of tetrafluoroethylene monomer under pressure, using free-radical catalysts, or it may be produced by the direct substitution of hydrogen atoms on polyethylene with fluorine, using polyethylene and fluorine gas at 20 °C.^[11]

Safety

While PTFE itself is chemically inert and non-toxic, it begins to deteriorate after the temperature of cookware reaches about 500 °F (260 °C), and decompose above 660 °F (350 °C).^[12] These degradation products can be lethal to birds, and can cause flu-like symptoms in humans.^[13]

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 Food and Drug Administration 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.^[14]

Carcinogens in production

The United States Environmental Protection Agency'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.^[15] 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,^[16] but did not commit to completely phasing out its use of the chemical. This agreement is said to apply to not only PTFE used in cookware but also other products such as food packaging, clothing, and carpeting. DuPont also stated that it cannot produce PTFE without the use of the chemical PFOA, although it is looking for a substitute.

PFOA is used only during the manufacture of the product—only a trace amount of PFOA remains after the curing process. DuPont maintains that there should be no measurable amount of PFOA on a finished pan, provided that it has been properly cured.^[17]

Similar polymers

Other polymers with similar composition are also known by the Teflon name:

• PFA (perfluoroalkoxy polymer resin)
• FEP (fluorinated ethylene-propylene)

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.^[18]

References

• Ellis, D.A.; Mabury, S.A.; Martin, J.W.; Muir, D.C.G. (2001). "Thermolysis of fluoropolymers as a potential source of halogenated organic acids in the environment". Nature 412 (6844): 321-324.

See also

• Magnesium/Teflon/Viton
• Polymer fume fever
• Gore-Tex