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| Names | |
|---|---|
| IUPAC name Poly(1,1,2,2-tetrafluoroethylene)[1] | |
| Other names Fluon, Poly(tetrafluoroethene), Poly(tetrafluoroethylene), Teflon | |
| Identifiers | |
| Abbreviations | PTFE |
| ChEBI | |
| ChemSpider |
|
| ECHA InfoCard | 100.120.367 |
| KEGG |
|
| UNII | |
| Properties | |
| (C2F4)n | |
| Density | 2200 kg/m3 |
| Melting point | 327 °C |
| Electrical resistivity | 1018 Ω·cm[a] |
| Thermal conductivity | 0.25 W/(m·K) |
| Hazards | |
| NFPA 704 (fire diamond) | |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Polytetrafluoroethylene (PTFE) is a syntheticfluoropolymer oftetrafluoroethylene, and has numerous applications because it ischemically inert.[3] The commonly known brand name of PTFE-based composition isTeflon byChemours,[4] aspin-off fromDuPont, which originally invented the compound in 1938.[4]
Polytetrafluoroethylene is afluorocarbonsolid, as it is a high-molecular-weightpolymer consisting wholly ofcarbon andfluorine. PTFE ishydrophobic: neither water nor water-containing substanceswet PTFE, as fluorocarbons exhibit only smallLondon dispersion forces due to the lowelectric polarizability of fluorine. PTFE has one of the lowestcoefficients of friction of any solid.
Polytetrafluoroethylene is used as anon-stick coating forpans and othercookware. It isnon-reactive, partly because of the strength ofcarbon–fluorine bonds, so it is often used in containers andpipework for reactive andcorrosive chemicals. When used as alubricant, PTFE reduces friction, wear, and energy consumption of machinery. It is used as a graft material in surgery and as a coating oncatheters.
PTFE and chemicals used in its production are some of the best-known and widely applied per- and polyfluoroalkyl substances (PFAS),[5] which arepersistent organic pollutants. PTFE occupies more than half of all fluoropolymer production, followed bypolyvinylidene fluoride (PVDF).[5]
For decades, DuPont usedperfluorooctanoic acid (PFOA, or C8) during production of PTFE, later discontinuing its use due tolegal actions overecotoxicological andhealth effects of exposure to PFOA.[6][7] DuPont's spin-off Chemours currently manufactures PTFE using an alternative chemical it callsGenX, another PFAS. Although GenX was designed to be less persistent in the environment compared to PFOA, its effects may be equally harmful or even more detrimental than those of the chemical it has replaced.[8][9]
Polytetrafluoroethylene (PTFE) was accidentally discovered in 1938 byRoy J. Plunkett while he was working in Chemours Chambers Works plant in New Jersey forDuPont. A team of DuPont chemists attempted to make a newchlorofluorocarbon refrigerant, calledtetrafluoroethylene. The gas in its pressure bottle stopped flowing before the bottle's weight had dropped to the point signaling "empty". John J. Beall (chemist), noticing a weight differential in his test cylinder, brought it to the attention of Roy Plunkett. The chemists in the lab sawed the bottle apart and found the bottle's interior coated with a waxy white material that was oddly slippery. Analysis showed that it was polymerized perfluoroethylene, with the iron from the inside of the container having acted as a catalyst at high pressure.[10] Kinetic Chemicals patented the new fluorinated plastic (analogous to the already knownpolyethylene) in 1941,[11] and registered the Teflon trademark in 1945.[12][13]
By 1948, DuPont, which founded Kinetic Chemicals in partnership withGeneral Motors, was producing over 910,000 kilograms (2,000,000 lb) of Teflon-brandpolytetrafluoroethylene per year inParkersburg, West Virginia.[14] An early use was in theManhattan Project as a material to coat valves and seals in the pipes holding highly reactiveuranium hexafluoride at the vastK-25uranium enrichment plant inOak Ridge, Tennessee.[15]
In 1954, Colette Grégoire urged her husband, the French engineerMarc Grégoire, to try the material he had been using on fishing tackle on her cooking pans. He subsequently created the first PTFE-coatednon-stick pans under the brand nameTefal (combining "Tef" from "Teflon" and "al" from aluminium).[16] In the United States,Marion A. Trozzolo, who had been using the substance on scientific utensils, marketed the first US-made PTFE-coated pan, "The Happy Pan", in 1961.[17] Non-stick cookware has since become a common household product, now offered by hundreds of manufacturers across the world.
The brand name Zepel was used to promote its stain resistance and water resistance when applied to fabrics.[18]
In the 1990s, it was found that PTFE could be radiationcross-linked above its melting point in an oxygen-free environment.[19]Electron beam processing is one example of radiation processing. Cross-linked PTFE exhibits enhanced high-temperature mechanical properties and improved radiation stability. That was significant because, for many years, irradiation at ambient conditions has been used to break down PTFE for recycling.[20] This radiation-inducedchain scission allows it to be more easily reground and reused.
Corona discharge treatment of the surface to increase the energy and improve adhesion has been reported.[21]
PTFE is produced byfree-radicalpolymerization oftetrafluoroethylene.[22] The net equation is as follows:
Because tetrafluoroethylene can explosively decompose totetrafluoromethane (CF4) and carbon, a special apparatus is required for the polymerization to prevent hot spots that might initiate this dangerous side reaction. The process is typically initiated withpersulfate, whichhomolyzes to generate sulfate radicals:
The resulting polymer is terminated withsulfate ester groups, which can behydrolyzed to give OHend-groups.[23]
Granular PTFE is produced viasuspension polymerization, where PTFE is suspended in anaqueous medium primarily via agitation and sometimes with the use of asurfactant. PTFE is also synthesized viaemulsion polymerization, where a surfactant is the primary means of keeping PTFE in an aqueous medium.[24] Surfactants in the past have includedperfluorooctanoic acid (PFOA) andperfluorooctanesulfonic acid (PFOS). More recently, Perfluoro 3,6 dioxaoctanoic acid (PFO2OA) andFRD-903 (GenX) are being used as alternatives.[25]
PTFE is athermoplasticpolymer, which is a white solid at room temperature, with a density of about 2200 kg/m3 and a melting point of 600 K (327 °C; 620 °F).[26] It maintains high strength, toughness and self-lubrication at low temperatures down to 5 K (−268.2 °C; −450.7 °F), and good flexibility at temperatures above 194 K (−79.15 °C; −110.5 °F).[27] PTFE gains its properties from the aggregate effect ofcarbon-fluorine bonds, as do all fluorocarbons. The only chemicals known to affect these carbon-fluorine bonds are highly reactive metals like thealkali metals, at higher temperatures, such metals as aluminium and magnesium, and fluorinating agents such asxenon difluoride andcobalt(III) fluoride.[28] At temperatures above 650–700 °C (1,200–1,290 °F) PTFE undergoes depolymerization.[29] However, it begins to decompose at about 260 °C (500 °F) through 350 °C (662 °F), andpyrolysis occurs at temperatures above 400 °C (752 °F).[30]
| Property | Value |
|---|---|
| Glass temperature | 114.85 °C (238.73 °F; 388.00 K)[31] |
| Thermal expansion | 112–125×10−6 K−1[32] |
| Thermal diffusivity | 0.124 mm2/s[33] |
| Young's modulus | 0.5 GPa |
| Yield strength | 23 MPa |
| Coefficient of friction | 0.05–0.10 |
| Dielectric constant | ε = 2.1,tan(δ) < 5×10−2 |
| Dielectric constant (60 Hz) | ε = 2.1,tan(δ) < 2×10−2 |
| Dielectric strength (1 MHz) | 60 MV/m |
| Magnetic susceptibility (SI, 22 °C) | −10.28×10−6[34] |
Thecoefficient of friction of plastics is usually measured against polished steel.[35] PTFE's coefficient of friction is 0.05 to 0.10.[26] PTFE's resistance tovan der Waals forces means that it is the only known surface to which agecko cannot stick.[36] In addition, PTFE can be used to prevent insects from climbing up surfaces painted with the material. For example, PTFE is used to prevent ants from climbing out offormicaria. There aresurface treatments for PTFE that alter the surface to allow adhesion to other materials.
Because of its chemical and thermal properties, PTFE is often used as a gasket material within industries that require resistance to aggressive chemicals such as pharmaceuticals or chemical processing.[37] However, until the 1990s,[19] PTFE was not known to crosslink like anelastomer, due to its chemical inertness. Therefore, it has no "memory" and is subject tocreep. Because of the propensity to creep, the long-term performance of such seals is worse than for elastomers that exhibit zero, or near-zero, levels of creep. In critical applications,Belleville washers are often used to apply continuous force to PTFE gaskets, thereby ensuring a minimal loss of performance over the lifetime of the gasket.[38]
PTFE is anultraviolet (UV) transparent polymer. However, when exposed to anexcimer laser beam, it severely degrades due to heterogeneousphotothermal effect.[39]
Processing PTFE can be difficult and expensive because its high melting temperature, 327 °C (621 °F), is above its decomposition temperature. Even when molten, PTFE does not flow due to its exceedingly high melt viscosity.[40][41] The viscosity and melting point can be decreased by inclusion of small amount ofcomonomers such as perfluoro (propylvinyl ether) andhexafluoropropylene (HFP). These cause the otherwise perfectly linear PTFE chain to become branched, reducing its crystallinity.[42]
Some PTFE parts are made by cold-moulding, a form ofcompression molding.[43] Here, fine powdered PTFE is forced into a mould under high pressure (10–100 MPa).[43] After a settling period, lasting from minutes to days, the mould is heated at 360 to 380 °C (680 to 716 °F),[43] allowing the fine particles to fuse (sinter) into a single mass.[44]
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The most common use of PTFE, consuming about 50% of production,[45] is for the insulation of wiring in aerospace and computer applications (e.g. hookup wire, coaxial cables).[46][45] This application exploits the fact that PTFE has excellentdielectric properties, specifically lowgroup velocity dispersion,[47] especially at highradio frequencies,[47] making it suitable for use as an excellentinsulator inconnector assemblies andcables, and inprinted circuit boards used atmicrowave frequencies. Combined with its high melting temperature, this makes PTFE the material of choice as a high-performance substitute for the weaker, higher dispersion, and lower-melting-pointpolyethylene commonly used in low-cost applications.
In industrial applications, owing to its low friction, PTFE is used forplain bearings,gears,slide plates, seals, gaskets, bushings,[48] and more applications with sliding action of parts, where it outperformsacetal andnylon.[49]
Its extremely high bulkresistivity makes it an ideal material for fabricating long-lifeelectrets, theelectrostatic analogues ofpermanent magnets.
PTFE film is also widely used in the production of carbon fiber composites as well as fiberglass composites, notably in the aerospace industry. PTFE film is used as a barrier between the carbon or fiberglass part being built and, in breather and bagging materials, is used to incapsulate the bondment when debulking (vacuum removal of air from between layers of laid-up plies of material) and when curing the composite, usually in an autoclave. The PTFE, used here as a film, prevents the non-production materials from sticking to the part being built, which is sticky due to the carbon-graphite or fiberglass plies being pre-pregnated withbismaleimide resin. Non-production materials such as Teflon, Airweave Breather, and the bag itself would be considered F.O.D. (foreign object debris/damage) if left in layup.
Gore-Tex is a brand of expanded PTFE (ePTFE), a material incorporating a fluoropolymer membrane with micropores. The roof of theHubert H. Humphrey Metrodome inMinneapolis, US, was one of the largest applications of PTFE coatings. 20 acres (81,000 m2) of the material was used in the creation of the white double-layered PTFE-coated fiberglass dome.
Because of its extreme non-reactivity and high temperature rating, PTFE is often used as the liner inhose assemblies,expansion joints, and in industrial pipe lines, particularly in applications using acids, alkalis, or other chemicals. Its frictionless qualities allow improved flow of highly viscous liquids and for uses in applications such as brake hoses.
PTFE architectural membranes are created by coating a woven glass-fibre base cloth with PTFE, forming one of the strongest and most durable materials used intensile structures.[50] Some notable structures featuring PTFE-tensioned membranes includeThe O2 Arena in London,Moses Mabhida Stadium in South Africa,Metropolitano Stadium in Spain and theSydney Football Stadium Roof in Australia.[51]
PTFE is often found in musical instrument lubrication products, most commonly valve oil.
PTFE is used in some aerosol lubricant sprays, including inmicronized and polarized form. It is notable for its extremely low coefficient of friction, its hydrophobicity (which serves to inhibit rust), and for the dry film it forms after application, which allows it to resist collecting particles that might otherwise form an abrasive paste.[52] Brands include GT85, Tri-Flow and WD-40 Specialist.[53][54][55]
PTFE is best known for its use incoating non-stickfrying pans and other cookware, as it ishydrophobic and possesses fairly high heat resistance.
The sole plates of someclothes irons are coated with PTFE.[56]
Other niche applications include:
While PTFE is stable at lower temperatures, it begins to deteriorate at temperatures of about 260 °C (500 °F), it decomposes above 350 °C (662 °F), andpyrolysis occurs at temperatures above 400 °C (752 °F).[30] The main decomposition products arefluorocarbon gases and asublimate, includingtetrafluoroethylene (TFE) anddifluorocarbeneradicals (RCF2).[30]
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 250 °C (482 °F).[72] Above those temperatures the degradation by-products can be lethal tobirds,[73] and can causeflu-like symptoms in humans (polymer fume fever),[74] although in humans those symptoms disappear within a day or two of being moved to fresh air.[75]
Most cases of polymer fume fever in humans occur due to smoking PTFE-contaminated tobacco,[75] although cases have occurred in people who havewelded near PTFE components.[75] PTFE-coated cookware is unlikely to reach dangerous temperatures with normal use, as meat is usually fried between 204 and 232 °C (399 and 450 °F), and mostcooking oils (except refinedsafflower andavocado oils) start tosmoke before a temperature of 260 °C (500 °F) is reached. A 1973 study by DuPont's Haskell Laboratory found that a 4-hour exposure to the fumes emitted by PTFE cookware heated to 280 °C (536 °F) was lethal forparakeets, although that was a higher temperature than the 260 °C (500 °F) required for fumes from pyrolyzed butter to be lethal to the birds.[76]
Perfluorooctanoic acid (PFOA), a chemical formerly used in the manufacture of PTFE products, such as non-stick coated cookware, can becarcinogenic for people who are exposed to it (seeEcotoxicity).[77] Concerning levels of PFOA have been found in the blood of people who work in or live near factories where the chemical is used, and in people regularly exposed to PFOA-containing products, such as someski waxes and stain-resistant fabric coatings. However, non-stick cookware was not found to be a major source of exposure, as the PFOA is burned off during the manufacturing process and not present in the finished product.[75] Non-stick coated cookware has not been manufactured using PFOA since 2013,[78] and PFOA is no longer being made in the United States.[77]
PTFE was added to theLiving Building Challenge (LBC) Red List in 2016. The Red List bans substances prevalent in the building industry that pose serious risks to human health and the environment from construction that seeks to meet the criteria of the Living Building Challenge (LBC).[79]
Sodium trifluoroacetate and the similar compoundsodium chlorodifluoroacetate can both be generated when PTFE undergoesthermolysis, as well as producing longer chain polyfluoro- and/or polychlorofluoro- (C3-C14) carboxylic acids, which may be equally persistent. These products can accumulate in evaporativewetlands and have been found in the roots and seeds of wetland plant species, but have not been observed to harm plant health or germination success.[75]
Perfluorooctanoic acid (PFOA, or C8) has been used in the manufacture of PTFE as asurfactant during itsemulsion polymerization, although several manufacturers have entirely discontinued its use.
PFOA persists indefinitely in the environment.[80] PFOA has been detected in the blood of many individuals of the general US population in the low and sub-parts per billion range, and levels are higher in chemical plant employees and surrounding subpopulations. PFOA andperfluorooctanesulfonic acid (PFOS) have been estimated to be in every American person's blood stream in the parts per billion range, though those concentrations have decreased by 70% for PFOA and 84% for PFOS between 1999 and 2014, which coincides with the end of the production and phase out of PFOA and PFOS in the US.[81] The general population has been exposed to PFOA through massive dumping of C8 waste into the ocean and near theOhio River Valley.[6][82][83] PFOA has been detected in industrial waste, stain-resistant carpets, carpet cleaning liquids,house dust,microwave popcorn bags, water, food and PTFE cookware.
As a result ofa class-action lawsuit and community settlement withDuPont, threeepidemiologists conducted studies on the population of Parkersburg, West Virginia, surrounding the (former DuPont) Chemours Washington Works chemical plant that was exposed to PFOA at levels greater than in the general population. The studies concluded that there was an association between PFOA exposure and six health outcomes:kidney cancer,testicular cancer,ulcerative colitis,thyroid disease,hypercholesterolemia (high cholesterol), andgestational hypertension (pregnancy-induced high blood pressure).[84]
Overall, PTFE cookware is considered a minor exposure pathway to PFOA.[85]
As a result of the lawsuits concerningthe PFOA class-action lawsuit, DuPont began to use GenX, a similarly fluorinated compound, as a replacement for perfluorooctanoic acid in the manufacture offluoropolymers, such as Teflon-brand PTFE.[86][87] However, the EPA has classified GenX as more toxic than PFOA[8] and it has proven to be a "regrettable substitute";[88]its effects may be equally harmful or even more detrimental than those of the chemical it was meant to replace.[88][9]
The chemicals are manufactured byChemours, a corporate spin-off of DuPont, inFayetteville, North Carolina.[89] Fayetteville Works was the site where DuPont began manufacture of PFOA after the lawsuit in Parkersburg WV halted their production there. When the EPA asked companies to voluntarily phase out PFOA production, it was replaced by GenX in Fayetteville Works. In June of 2017, The Wilmington Star-News broke the story[90] that GenX was found in the Cape Fear River – the drinking water supply for 500,000 people. The source of the pollution was determined to be the Fayetteville Works site, which had been run by DuPont since its founding in 1971 and then managed by DuPont spinoff, The Chemours Company, since 2015. The water utility indicated they could not filter these chemicals from the drinking water.
The North Carolina Department of Environmental Quality (NC DEQ) records[91] indicate that DuPont started release PFAS into the area beginning in 1976 with the production of Nafion, and that PFAS including GenX had been released as a byproduct of the production of Vinyl Ethers since 1980, exposing the Cape Fear Basin for decades. A small nonprofit called Cape Fear River Watch sued NC DEQ for not taking swifter and stronger action, and sued the polluter, Chemours, for violations of the Clean Water Act and the Toxic Substances Control Act. The result was a Consent Order,[92] signed February 25, 2019, by Cape Fear River Watch, NC DEQ, and Chemours.[93] The order has required Chemours to stop wastewater discharge, air emissions, groundwater discharge, provide sampling and filtration options to well users, and required sampling that proved there were upwards of 300 distinct PFAS compounds being released from Fayetteville Works.[94]
The Teflon trade name is also used for other polymers with similar compositions:
These retain the useful PTFE properties of low friction and nonreactivity, but are also more easily formable. For example, FEP is softer than PTFE and melts at 533 K (260 °C; 500 °F); it is also highly transparent and resistant to sunlight.[95]
