# Polytetrafluoroethylene

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Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene that has numerous applications. The best known brand name of PTFE is Teflon by DuPont Co.

PTFE is a fluorocarbon solid, as it is a high-molecular-weight compound consisting wholly of carbon and fluorine. PTFE is hydrophobic: neither water nor water-containing substances wet PTFE, as fluorocarbons demonstrate mitigated London dispersion forces due to the high electronegativity of fluorine. PTFE has one of the lowest coefficients of friction against any solid.

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. It is also commonly used as a graft material in surgical interventions.

It is commonly believed that Teflon is a spin-off product from NASA space projects. Although it has been used by NASA, the assumption is incorrect.[1]

## History

Teflon thermal cover showing impact craters, from NASA Ultra Heavy Cosmic Ray Experiment (UHCRE)

PTFE was accidentally discovered in 1938 by Roy Plunkett, in New Jersey while he was working for Kinetic Chemicals. As Plunkett was attempting to make a new chlorofluorocarbon refrigerant, the tetrafluoroethylene gas in its pressure bottle stopped flowing before the bottle's weight had dropped to the point signaling "empty." Since Plunkett was measuring the amount of gas used by weighing the bottle, he became curious as to the source of the weight, and finally resorted to sawing the bottle apart. Inside, he found it coated with a waxy white material which was oddly slippery. Analysis of the material showed that it was polymerized perfluoroethylene, with the iron from the inside of the container having acted as a catalyst at high pressure. Kinetic Chemicals patented the new fluorinated plastic (analogous to the already known polyethylene) in 1941,[2] and registered the Teflon trademark in 1945.[3][4]

DuPont, which founded Kinetic Chemicals in partnership with General Motors, was producing over two million pounds (900 tons) of Teflon brand PTFE per year in Parkersburg, West Virginia, by 1948.[5] An early use was in the Manhattan Project as a material to coat valves and seals in the pipes holding highly reactive uranium hexafluoride at the vast K-25 uranium enrichment plant in Oak Ridge, Tennessee.[6]

In 1954, French engineer Marc Grégoire created the first pan coated with Teflon non-stick resin under the brand name of Tefal after his wife Collete urged him to try the material he had been using on fishing tackle on her cooking pans.[7] In the United States, Marion A. Trozzolo, who had been using the substance on scientific utensils, marketed the first US-made Teflon coated frying pan, "The Happy Pan", in 1961.[8]

In the 1990s, it was found that PTFE can be radiation cross-linked above its melting point in an oxygen free environment.[9] Electron beam processing is one example of radiation processing. Cross-linked PTFE has improved high temperature mechanical properties and radiation stability. This is significant because for many years irradiation at ambient conditions has been used to break down PTFE for recycling. [10] The radiation induced chain scissioning allows it to be more easily reground and reused.

## Production

PTFE is produced by free-radical polymerization of tetrafluoroethylene. The net equation is:

n F2C=CF2 → 1/n —{ F2C—CF2}n

Because tetrafluoroethylene can explosively decompose to tetrafluoromethane and carbon, special apparatus is required for the polymerization to prevent hot spots that might initiate this dangerous side reaction. The process is typically initiated with persulfate, which homolyzes to generate sulfate radials:

[O3SO-OSO3]2- ${\displaystyle {\overrightarrow {\leftarrow }}}$ 2 SO4-

The resulting polymer is terminated with sulfate ester groups, which can be hydrolyzed to give OH-end-groups.[11]

Because PTFE is poorly soluble in almost all solvents, the polymerization is conducted as an emulsion in water. This process gives a suspension of polymer particles. Alternatively, the polymerization is conducted using a surfactant such as PFOS.

## Properties

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

PTFE is a thermoplastic polymer, which is a white solid at room temperature, with a density of about 2200 kg/m3. According to DuPont, its melting point is Template:Convert.[12] It maintains high strength, toughness and self-lubrication at low temperatures down to Template:Convert, and good flexibility at temperatures above Template:Convert.[13] PTFE gains its properties from the aggregate effect of carbon-fluorine bonds, as do all fluorocarbons. The only chemicals known to affect these carbon-fluorine bonds are certain alkali metals and most highly reactive fluorinating agents.[14]

Property Value
Density 2200 kg/m3
Melting point 600 K
Thermal expansion 135 · 10−6 K−1 [15]
Thermal diffusivity 0.124 mm²/s [16]
Young's modulus 0.5 GPa
Yield strength 23 MPa
Bulk resistivity 1016 Ω·m [17]
Coefficient of friction 0.05–0.10
Dielectric constant ε=2.1,tan(δ)<5(-4)
Dielectric constant (60 Hz) ε=2.1,tan(δ)<2(-4)
Dielectric strength (1 MHz) 60 MV/m

The coefficient of friction of plastics is usually measured against polished steel.[18] PTFE's coefficient of friction is 0.05 to 0.10,[12] which is the third-lowest of any known solid material (BAM being the first, with a coefficient of friction of 0.02; diamond-like carbon being second-lowest at 0.05). PTFE's resistance to van der Waals forces means that it is the only known surface to which a gecko cannot stick.[19] In fact, 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.

Because of its chemical inertness, PTFE cannot be cross-linked like an elastomer. Therefore, it has no "memory" and is subject to creep. This is advantageous when used as a seal, because the material creeps a small amount to conform to the mating surface. However, to keep the seal from creeping too much, fillers are used, which can also improve wear resistance and reduce friction. Sometimes, metal springs apply continuous force to PTFE seals to give good contact, while permitting a beneficially low percentage of creep.{{ safesubst:#invoke:Unsubst||date=__DATE__ |\$B= {{#invoke:Category handler|main}}{{#invoke:Category handler|main}}[citation needed] }}

## Applications and uses

The major application of PTFE, consuming about 50% of production, is for wiring in aerospace and computer applications, e.g. hookup wire, coaxial cables. This application exploits the fact that 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.

Another major application is for fuel and hydraulic lines, due to its low resistance against flowing liquids. The colder temperatures at high altitudes cause a reduction in flow of these fluids, and coating the interior surfaces of the fuel and hydraulic lines improves flow.[11]

In industrial applications, owing to its low friction, PTFE is used for applications where sliding action of parts is needed: plain bearings, 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 PTFE, for these applications, versions of PTFE with mineral oil or molybdenum disulfide embedded as additional lubricants in its matrix are being manufactured. Its extremely high bulk resistivity makes it an ideal material for fabricating long-life electrets, useful devices that are the electrostatic analogues of magnets.

Gore-Tex is a material incorporating a fluoropolymer membrane with micropores. The roof of the Hubert H. Humphrey Metrodome in Minneapolis, USA, is one of the largest applications of PTFE coatings, using Template:Convert of the material in a double-layered white dome, made using fiberglass with a PTFE coating.

### Other

PTFE (Teflon) is best known for its use in coating non-stick frying pans and other cookware, as it is hydrophobic and possesses fairly high heat resistance.

PTFE tapes with pressure-sensitive adhesive backing

### Niche

PTFE is a versatile material that is found in many niche applications.

• a film interface patch for sports and medical applications, featuring a pressure-sensitive adhesive backing, which is installed in strategic high friction areas of footwear, insoles, ankle-foot orthosis, and other medical devices to prevent and relieve friction-induced blisters, calluses, and foot ulceration.
• Powdered PTFE is used in pyrotechnic compositions as oxidizers 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.[20]
• 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 down 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.
• coating for certain types of hardened, armor-piercing bullets, so as to prevent the increased wear on the firearm's rifling that would result from the harder projectile; however, it is not the PTFE itself that gives the bullet its armor-piercing property.[21]
• High corrosion resistance favors the use of PTFE in laboratory environments as containers, as magnetic stirrer coatings, and as tubing for highly corrosive chemicals such as hydrofluoric acid, which will dissolve glass containers. It is used in containers for storing fluoroantimonic acid, a superacid.{{ safesubst:#invoke:Unsubst||date=__DATE__ |\$B=

{{#invoke:Category handler|main}}{{#invoke:Category handler|main}}[citation needed] }}

• PTFE tubes are used in gas-gas heat exchangers in gas cleaning of waste incinerators. Unit power capacity is typically several megawatts.
• PTFE is also widely used as a thread seal tape in plumbing applications, largely replacing paste thread dope.
• PTFE membrane filters are among the most efficient used in industrial air filtration applications. Filter coated with a PTFE membrane are often used within a dust collection system to collect particulate matter from air streams in applications involving high temperatures and high particulate loads such as coal-fired power plants, cement production, and steel foundries.
• PTFE grafts can be used to bypass stenotic arteries in peripheral vascular disease, if a suitable autologous vein graft is not available.

## Safety

The pyrolysis of PTFE is detectable at Template:Convert, and it evolves several fluorocarbon gases[22] 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 Template:Convert.[23] More recently, however, a study documented birds having been killed by these decomposition products at Template:Convert, with unconfirmed reports of bird deaths as a result of non-stick cookware heated to as little as Template:Convert.[22][24]

While PTFE is stable and nontoxic at lower temperatures, it begins to deteriorate after the temperature of cookware reaches about Template:Convert, and decomposes above Template:Convert.[25] These degradation by-products can be lethal to birds, and can cause flu-like symptoms in humans.[25] In May, 2003, the environmental research and advocacy organization Environmental Working Group filed a 14-page brief with the U.S. Consumer Product Safety Commission petitioning for a rule requiring that cookware and heated appliances bearing non-stick coatings carry a label warning of hazards to people and to birds.[26]

Meat is usually fried between Template:Convert, and most oils will start to smoke before a temperature of Template:Convert is reached, but there are at least two cooking oils (refined safflower oil and avocado oil) that have a higher smoke point than Template:Convert. Empty cookware can also exceed this temperature upon heating.

### PFOA

{{#invoke:main|main}} Perfluorooctanoic acid (PFOA, or C8) is used as a surfactant in the emulsion polymerization of PTFE. Overall, PTFE cookware is considered an insignificant exposure pathway to PFOA.[27][28]

## 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 trade name:

These retain the useful PTFE properties of low friction and nonreactivity, but are more easily formable. For example, FEP is softer than PTFE and melts at Template:Convert; it is also highly transparent and resistant to sunlight.[29]

## References

1. NASA Spinoff under Are Tang, Teflon, and Velcro NASA spinoffs?
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7. "Teflon History ", home.nycap.rr.com, Retrieved 25 January 2009.
8. Robbins, William (21 December 1986) "Teflon Maker: Out Of Frying Pan Into Fame ", New York Times, Retrieved 21 December 1986 (Subscription)
9. Modification of PTFE by radiation J.Z. Sun, Y.F. Zhang, X.G. Zhong, X.L. Zhu, Modification of polytetrafluoroethylene by radiation. 1. Improvement in high-temperature properties and radiation stability, Radiat. Phys. Chem. 44 (1994) 655–679.
10. Electron Beam Processing of PTFE E-BEAM Services website. Accessed May 21, 2013
11. D. Peter Carlson and Walter Schmiegel "Fluoropolymers, Organic" in Ullmann's Encyclopedia of Industrial Chemistry 2000, Wiley-VCH, Weinheim. Template:Hide in printTemplate:Only in print
12. Fluoropolymer Comparison – Typical Properties Retrieved 10 September 2006.
13. Teflon PTFE Properties Handbook Retrieved 11 October 2012.
14. DuPont Teflon® Coatings. plastechcoatings.com
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