| Polytetrafluoroethylene | |
|---|---|
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Poly(difluoromethylene) |
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Other names
Syncolon, Fluon, Poly(tetrafluoroethene), Poly(tetrafluoroethylene) |
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| Identifiers | |
| Abbreviations | PTFE |
| CAS number | 9002-84-0 |
| KEGG | C19507 |
| ChEMBL | CHEMBL1201480 |
| Properties | |
| Molecular formula | (C2F4)n |
| Density | 2200 kg/m3 |
| Melting point |
327 °C |
| Thermal conductivity | 0.25 W/(m·K) |
| Supplementary data page | |
| Structure and properties |
n, εr, etc. |
| Thermodynamic data |
Phase behaviour Solid, liquid, gas |
| Spectral data | UV, IR, NMR, MS |
| (verify) (what is: /?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) |
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| Infobox references | |
Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene that finds numerous applications. The most well 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 commonly believed that Teflon, like velcro, is a spin-off product from the NASA space projects. However, that is not so, even though both products have been used by NASA.[1]
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PTFE was accidentally invented by Roy Plunkett of Kinetic Chemicals in New Jersey in 1938. While Plunkett was attempting to make a new CFC 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 known polyethylene) in 1941[2] and registered the Teflon trademark in 1945.[3][4]
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 at the vast K-25 uranium enrichment plant at Oak Ridge, Tennessee.[5]
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.[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 urged him to try the material he had been using on fishing tackle on her cooking pans.[7] In the United States, Kansas City, Missouri resident 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]
PTFE is a thermoplastic polymer, which 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 (621 °F), but its mechanical properties degrade above 260 °C (500 °F).[9] PTFE gains its properties from the aggregate effect of carbon-fluorine bonds, as do all fluorocarbons.
| Property | Value |
|---|---|
| Density | 2200 kg/m3 |
| Melting point | 327 °C |
| Thermal expansion | 135 · 10−6 K−1 [10] |
| Young's modulus | 0.5 GPa |
| Yield strength | 23 MPa |
| 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.[11] PTFE's coefficient of friction is 0.05 to 0.10,[9] 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.[12]
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.
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.
Owing to its low friction, it 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 is one of the largest applications of PTFE coatings, 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 made with a substantial use of PTFE coatings.
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.[13]
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 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.
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.
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.
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.
PTFE is also sometimes used as feet for computer mice , to reduce the friction with a mousepad or other tracking surface.
The pyrolysis of PTFE is detectable at 200 °C (392 °F), and it evolves several fluorocarbon gases[14] 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),[15] although birds are proven to be much more sensitive to these decomposition products.[14][16]
While PTFE is stable and nontoxic, it begins to deteriorate after the temperature of cookware reaches about 260 °C (500 °F), and decomposes above 350 °C (662 °F).[17] These degradation by-products can be lethal to birds, and can cause flu-like symptoms in humans.[17]
Meat is usually fried between 200 and 230 °C (392 and 446 °F), and most oils will start to smoke before a temperature of 260 °C is reached, but there are at least two cooking oils (refined safflower oil and avocado oil) that have a higher smoke point than 260 °C. Empty cookware can also exceed this temperature upon heating.
Perfluorooctanoic acid (PFOA or C8), in the form of the ammonium salt,[18] is used as a surfactant in the emulsion polymerization of PTFE,[19][20] and has been detected in some PTFE products.[21][22] The levels that have been measured in nonstick cookware range from not detectable to 75 parts per billion.[22][23] These are lower than in PTFE products such as thread sealant tape (with 1800 parts per billion (1.8 parts per million) of PFOA detected) because nonstick cookware is heated to volatilize PFOA.[21]
A DuPont study on Teflon PTFE did not detect any PFOA above their detection limit of 9 parts per billion,[24] and DuPont says no PFOA is in Teflon brand cookware.[25] A 2009 USEPA study found levels of PFOA in nonstick cookware ranging from undetected (with a detection limit of 1.5 parts per billion) to 4.3 parts per billion.[22] DuPont says there should be no measurable amount on a finished pan provided it has been properly cured.[26] While PFOA has been detected in the low parts per billion range in the blood of people,[27] exposure from nonstick cookware is considered insignificant[28][29]—despite the marketing of other wares. However, at temperatures well above those encountered in cooking,[30] PTFE pyrolysis can form minor amounts of PFOA.[31][32]
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,[33] but did not commit to completely phasing out its use of the chemical. In the emulsion polymerization of PTFE, 3M subsidiary Dyneon has developed a replacement emulsifier[34] despite DuPont stating that PFOA is an "essential processing aid".[35] 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 nonstick cookware or all weather clothing, poses a concern."[36]
Other polymers with similar composition are also known by the Teflon trade name:
They retain the useful properties of PTFE of low friction and nonreactivity, but are more easily formable. For example, FEP is softer than PTFE and melts at 260 °C (500 °F); it is also highly transparent and resistant to sunlight.[37]
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