Polytetrafluoroethylene

Polytetrafluoroethylene
IUPAC name
Poly(tetrafluoroethene)
Systematic name
Poly(tetrafluoroethylene)
Other names
Teflon, Syncolon, Fluon, Polytetrafluoroethene, Poly(ethylene tetrafluoride)
Identifiers
Abbreviations PTFE
CAS number 9002-84-0 YesY
Properties
Molecular formula CnF2n+2
Density 2200 kg/m3
Melting point

327 °C
Thermal conductivity 0.25 W/(m·K)
 YesY (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 that 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 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.

Contents

History

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 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 brand name 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 US-made Teflon coated 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 at the vast K-25 uranium enrichment plant at Oak Ridge, Tennessee.

Properties

PTFE is often used to coat non-stick frying 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 2.2 g/cm3. According to DuPont, its melting point is 327 °C (621 °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. 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.[9]

Due 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 Teflon. For these applications, versions of Teflon with mineral oil or molybdenum disulfide embedded as additional lubricants in its matrix are being manufactured.

Property Value
Density 2200 kg/m3
Melting point 327°C
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

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 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 made substantially of PTFE.

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.[10]

PTFE is also used in body piercings, such as a subclavicle piercing, due to its flexibility and biocompatibility.

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.

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, etc.). 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, 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 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[11][12] 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),[13] although birds are proven to be much more sensitive to these decomposition products.[12][14]

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).[15] These degradation byproducts can be lethal to birds, and can cause flu-like symptoms in humans.[15]

Meat is usually fried between 200–230 °C (392–446 °F), and most oils will start to smoke before a temperature of 260ºC is reached, but there are at least two cooking oils (safflower oil and avocado oil) that have a higher smoke point than 260°C. 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.[16]

PFOA

Perfluorooctanoic acid (PFOA or C8), in the form of the ammonium salt,[17] is used as surfactant in the emulsion polymerization of PTFE,[18][19] and has been detected in some PTFE products.[20][21] The levels that have been detected in nonstick cookware range from not detectable to 75 parts per billion.[21][22] They are lower than PTFE products such as thread sealant tape (with 1800 parts per billion of PFOA detected) because nonstick cookware is heated to volatilize PFOA.[20]

A DuPont study on Teflon PTFE did not detect any PFOA above their detection limit of 9 parts per billion,[23] and DuPont says no PFOA is in Teflon cookware.[24] 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.[21] DuPont says there should be no measurable amount on a finished pan provided it has been properly cured.[25] While PFOA has been detected in the low parts per billion range in the blood of people,[26] exposure from nonstick cookware is considered insignificant[27][28]—despite the marketing of other wares. However, at temperatures well above those encountered in cooking,[29] PFTE pyrolysis can form minor amounts of PFOA.[30][31]

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,[32] but did not commit to completely phasing out its use of the chemical. In the emulsion polymerization of PTFE, 3M subsidiary Dyneon has a replacement emulsifer[33] despite DuPont stating PFOA is an "essential processing aid".[34] 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."[35]

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:

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.[36]

See also

Footnotes

  1. ↑ US 2230654, Plunkett, Roy J, "Tetrafluoroethylene polymers", issued 1941-02-04 
  2. ↑ "History Timeline 1930: The Fluorocarbon Boom". DuPont. http://www2.dupont.com/Refrigerants/en_US/sales_support/history_1930.html. Retrieved 2009-06-10. 
  3. ↑ "Roy Plunkett: 1938". http://www2.dupont.com/Heritage/en_US/1938_dupont/1938_indepth.html. Retrieved 2009-06-10. 
  4. ↑ "Teflon History ", home.nycap.rr.com, Retrieved 2009-01-25.
  5. ↑ Robbins, William (December 21, 1986) "Teflon Maker: Out Of Frying Pan Into Fame ", New York Times, Retrieved 1986-12-21 (Subscription)
  6. ↑ 6.0 6.1 Fluoropolymer Comparison - Typical Properties Retrieved 10 September 2006.
  7. ↑ Coefficient of Friction (COF) Testing of Plastics MatWeb Material Property Data Retrieved 1 January 2007.
  8. ↑ "Research into Gecko Adhesion ", Berkeley, 2007-10-14, Retrieved 2010-04-08.
  9. ↑ O-Ring Design and Materials Guide: Tetrafluoroethylene R.L. Hudson & Company Data Retrieved 1 August 2010.
  10. ↑ E.-C. Koch (2002). "Metal-Fluorocarbon Pyrolants:III. Development and Application of Magnesium/Teflon/Viton". Propellants Explosives Pyrotechnics 27: 262–266. doi:10.1002/1521-4087(200211)27:5<262::AID-PREP262>3.0.CO;2-8. 
  11. ↑ Teflon (PTFE) Thermal Decomposition Products. Fluoride Action Network Pesticide Project.
  12. ↑ 12.0 12.1 Teflon offgas studies|Environmental Working Group
  13. ↑ Zapp JA, Limperos G, Brinker KC (1955-04-26). "Toxicity of pyrolysis products of 'Teflon' tetrafluoroethylene resin". Proceedings of the American Industrial Hygiene Association Annual Meeting. 
  14. ↑ Can Nonstick Make You Sick? — ABC News
  15. ↑ 15.0 15.1 DuPont, Key Questions About Teflon, accessed on 3 December 2007.
  16. ↑ Dale Blumenthal. "Is That Newfangled Cookware Safe?". Food and Drug Administration (US). http://www.enotalone.com/article/7805.html. Retrieved 2010-04-08. 
  17. ↑ "Substance flow analysis for Switzerland ", (UW-0922-E), pp.40–41, 2009-12-03, 1787 kB, Swiss Federal Office for the Environment, Retrieved 2008-04-08
  18. ↑ Sandy, Martha. "Petition for Expedited CIC Consideration of Perfluorooctanic Acid (PFOA)". The State of California, Office of Environmental Health Hazard Assessment, Cancer Toxicology and Epidemiology Section, Reproductive and Cancer Hazard Assessment Branch. http://www.oehha.ca.gov/Prop65/public_meetings/pdf/PFOACIC%20Slides121206.pdf. Retrieved 2008-09-27. 
  19. ↑ Lau C, Anitole K, Hodes C, Lai D, Pfahles-Hutchens A, Seed J (October 2007). "Perfluoroalkyl acids: a review of monitoring and toxicological findings". Toxicol. Sci. 99 (2): 366–94. doi:10.1093/toxsci/kfm128. PMID 17519394. http://toxsci.oxfordjournals.org/cgi/reprint/99/2/366.pdf. 
  20. ↑ 20.0 20.1 "PFOA in Norway TA-2354/2007". Norwegian Pollution Control Authority. 2007. p. 18. http://www.sft.no/publikasjoner/2354/ta2354.pdf. Retrieved 29 August 2009. 
  21. ↑ 21.0 21.1 21.2 Guo Z, Liu X, Krebs KA (March 2009). "Perfluorocarboxylic Acid Content in 116 Articles of Commerce" (PDF). USEPA. p. 40. http://www.epa.gov/nrmrl/pubs/600r09033/600r09033.pdf. 
  22. ↑ Begley TH, White K, Honigfort P, Twaroski ML, Neches R, Walker RA (October 2005). "Perfluorochemicals: potential sources of and migration from food packaging". Food Addit. Contam. 22 (10): 1023–31. doi:10.1080/02652030500183474. PMID 16227186. 
  23. ↑ Powley CR, Michalczyk MJ, Kaiser MA, Buxton LW (September 2005). "Determination of perfluorooctanoic acid (PFOA) extractable from the surface of commercial cookware under simulated cooking conditions by LC/MS/MS". Analyst 130 (9): 1299–302. doi:10.1039/b505377c. PMID 16096677. 
  24. ↑ "Teflon firm faces fresh lawsuit". BBC News. 19 July 2005. http://news.bbc.co.uk/2/hi/business/4697939.stm. Retrieved 24 January 2009. 
  25. ↑ "About Teflon". DuPont. http://web.archive.org/web/20080229143203/http://www2.dupont.com/PFOA/en_US/about_teflon/. Retrieved 2010-02-09. 
  26. ↑ Houde M, Martin JW, Letcher RJ, Solomon KR, Muir DC (June 2006). "Biological monitoring of polyfluoroalkyl substances: A review". Environ. Sci. Technol. 40 (11): 3463–73. doi:10.1021/es052580b. PMID 16786681.  Supporting Information (PDF).
  27. ↑ Trudel D, Horowitz L, Wormuth M, Scheringer M, Cousins IT, Hungerbühler K (April 2008). "Estimating consumer exposure to PFOS and PFOA". Risk Anal. 28 (2): 251–69. doi:10.1111/j.1539-6924.2008.01017.x. PMID 18419647. 
  28. ↑ "Nonstick pans: Nonstick coating risks". Consumer Reports. http://www.consumerreports.org/cro/home-garden/kitchen/cookware-bakeware-cutlery/nonstick-pans-6-07/overview/0607_pans_ov_1.htm. Retrieved 4 July 2009. 
  29. ↑ “Cooking up a storm in a frying pan ”, Royal Society of Chemistry, Chemistry World, September 2005, Retrieved 2010-04-08
  30. ↑ Ellis DA, Mabury SA, Martin JW, Muir DC (July 2001). "Thermolysis of fluoropolymers as a potential source of halogenated organic acids in the environment". Nature 412 (6844): 321–4. doi:10.1038/35085548. PMID 11460160. 
  31. ↑ Ellis DA, Martin JW, Muir DC, Mabury SA (June 2003). "The use of 19F NMR and mass spectrometry for the elucidation of novel fluorinated acids and atmospheric fluoroacid precursors evolved in the thermolysis of fluoropolymers". Analyst 128 (6): 756–64. doi:10.1039/b212658c. PMID 12866900. 
  32. ↑ Juliet Eilperin (2006-01-26). "Harmful PTFE chemical to be eliminated by 2015". Washington Post. http://www.washingtonpost.com/wp-dyn/content/article/2006/01/25/AR2006012502041.html. Retrieved 2006-09-10. 
  33. ↑ Michael McCoy (November 2008). "Dyneon Phasing Out Perfluorooctanoate". Chemical & Engineering News 86 (46): 26. http://pubs.acs.org/isubscribe/journals/cen/86/i46/html/8646busc7.html. 
  34. ↑ "Learn More About DuPont Teflon". DuPont. http://www2.dupont.com/Teflon/en_US/keyword/pfoa.html?src=search_us_pfoa. Retrieved 16 May 2009. 
  35. ↑ "Failure to Report Chemical Risks Can Result in Major Fines, EPA Office of Civil Enforcement". Environmental Protection Agency. 2008-08. http://www.epa.gov/compliance/resources/newsletters/civil/enfalert/8e-tsca-0807.pdf. Retrieved 2009-01-19. 
  36. ↑ FEP Detailed Properties Parker-TexLoc, 13 April 2006. Retrieved 10 September 2006.

References

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