polytetrafluoroethylene

Polytetrafluoroethylene
IUPAC name	Poly(tetrafluoroethene)
Systematic name	Poly(tetrafluoroethylene)
Other names	Teflon, Syncolon, Fluon, Polytetrafluoroethene
Identifiers
Abbreviations	PTFE
CAS number	9002-84-0 Y
Properties
Molecular formula	CnF2n+2
Density	2200 kg/m3
Melting point	327 °C
Supplementary data page
Structure and properties	n, εr, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

In chemistry, polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene which finds numerous applications. PTFE is most well known by the DuPont brand name Teflon.

PTFE is a fluorocarbon solid, as it is a high molecular weight compound consisting wholly of carbon and fluorine. Neither water and water-containing substances nor oil and oil-containing substances are wet by PTFE, as fluorocarbons demonstrate mitigated London dispersion forces due to the high electronegativity of fluorine.

PTFE is used as a non-stick coating for pans and other cookware. It is very non-reactive, partly because of the strength of carbon–fluorine bonds, and so it is often used in containers and pipework for reactive and corrosive chemicals. Where used as a lubricant, PTFE reduces friction, wear and energy consumption of machinery.

History

PTFE was accidentally invented by Roy Plunkett of Kinetic Chemicals in 1938. While Plunkett was attempting to make a new CFC refrigerant, the perfluorethylene polymerized in its pressurized storage container, with the iron from the inside of the container acting as a catalyst. Kinetic Chemicals patented it in 1941^[1] and registered the Teflon trademark in 1945.^[2][3]

By 1950, DuPont had acquired interest in Kinetic Chemicals and was producing over a million pounds (450 tons) of Teflon 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 urged him to try the material he had been using on fishing tackle on her cooking pans.^[4] 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.^[5]

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 K-25.

Properties

PTFE is often used to coat non-stick frying pans as it is hydrophobic and possesses fairly high heat resistance.

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).^[6] PTFE gains its properties from the aggregate effect of carbon-fluorine bonds, as do all fluorocarbons.

The coefficient of friction of plastics is usually measured against polished steel.^[7] PTFE's coefficient of friction is 0.1 or less^[6], which is the second lowest of any known solid material (diamond-like carbon being the first). PTFE's resistance to van der Waals forces means that it is the only known surface to which a gecko cannot stick.^[8]

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". 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 can be compromised. Compounding fillers control unwanted creep and improve wear, friction, and other properties. Sometimes metal springs apply continuous force to PTFE seals to give good contact, while permitting some creep.

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 (81,000 m²) 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 aluminium 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.^[9]

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 meters and UV radiometers) 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 bullet, so as to reduce the amount of wear on the firearm's rifling. These are often referred to as "cop-killer" bullets by virtue of PTFE's supposed ability to ease a bullet's passage through body armor. However, this is simply an urban myth as PTFE has no effect on a bullet's ability to penetrate soft body armor; its virtue is in the reduced damage to the rifling of the weapon from firing very hard ammunition (which provides the armor piercing capability).

PTFE's low frictional properties have also been used as 'feet' for computer mice such as the Logitech G5, Logitech G7 and Logitech G9 series and most Razer gaming mice (e.g. the Deathadder, Lachesis, ...). The low friction provided by PTFE allows the mice to glide across surfaces more smoothly and with less effort.

PTFE's high corrosion resistance makes it ideal for laboratory environments as containers, magnetic stirrer coatings, and as tubing for highly corrosive chemicals such as hydrofluoric acid, which will dissolve glass containers.

PTFE is also widely used as a thread seal tape in plumbing applications, largely replacing paste thread dope.

PTFE grafts can be used to bypass stenotic arteries in peripheral vascular disease, if a suitable autologous vein graft is not available.

PTFE can be used to prevent insects climbing up surfaces painted with the material. PTFE is so slippery that insects cannot get a grip and tend to fall off. For example PTFE is used to prevent ants climbing out of formicaria.

Safety

The pyrolysis of PTFE is detectable at 200 °C (392 °F), and it evolves several fluorocarbon gases ^[10][11] and a sublimate. Animal studies indicate that it is unlikely that these products would be generated in amounts significant to health at temperatures below 250 °C (482 °F).^[12], although birds are proven to be much more sensitive to these decomposition products.^[11][13]

While PTFE is stable and non-toxic, it begins to deteriorate after the temperature of cookware reaches about 260 °C (500 °F), and decompose above 350 °C (660 °F).^[14] These degradation by-products can be lethal to birds, and can cause flu-like symptoms in humans.^[14]

Meat is usually fried between 200–230 °C (400–450 °F), and most oils will start to smoke before a temperature of 260 degrees is reached, but there are at least two cooking oils (Safflower oil and Avocado oil) which have a higher Smoke point than 260 degrees. Empty cookware can also exceed this temperature upon heating.

A 1959 study (conducted before the U.S. 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.^[15]

Perfluorooctanoic acid (PFOA or C8)

The United States Environmental Protection Agency's scientific advisory board decided in 2005 that 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.^[16] 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,^[17] 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.^[18] A 2005 U.S. Food and Drug Administration (FDA) study detected PFOA in finished PTFE products including PTFE/Teflon cookware.^[19] A February 2007 New York State Department of Health study detected PFOA in the gas phase coming from new nonstick cookware and microwave popcorn bags;^[20] this research was funded by a 2005–2006 $17,700 grant from the Consumers Union.^[21]

As of August 2008, the EPA's position was that it "has no information that routine use of household or other products using fluoropolymers, such as non-stick cookware or all weather clothing, poses a concern."^[22]

Similar polymers

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

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.^[23]

See also

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

Footnotes

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. doi:10.1038/35085548. 

External links

• EPA: Compound in Teflon may cause cancer [1], Tom Costello, NBC News, June 29, 2005
• DuPont (2005). Teflon News and Information. Retrieved 7 October 2005.
• Plasma Processes and Adhesive Bonding of Polytetrafluoroethylene