Polytetrafluoroethylene ( PTFE) is a synthetic fluoropolymer of tetrafluoroethylene, and has numerous applications because it is chemically inert.[
]
Polytetrafluoroethylene is a fluorocarbon solid, as it is a high-molecular-weight polymer consisting wholly of carbon and fluorine. PTFE is hydrophobic: neither water nor water-containing substances Wetting PTFE, as fluorocarbons exhibit only small London dispersion forces due to the low polarizability of fluorine. PTFE has one of the lowest coefficients of friction of any solid.
Polytetrafluoroethylene is used as a non-stick coating for pans and other cookware. It is Chemically inert, partly because of the strength of carbon–fluorine bonds, so it is often used in containers and Piping for reactive and Corrosion chemicals. When used as a lubricant, PTFE reduces friction, wear, and energy consumption of machinery. It is used as a graft material in surgery and as a coating on .
PTFE and chemicals used in its production are some of the best-known and widely applied per- and polyfluoroalkyl substances (PFAS),
which are persistent organic pollutants. PTFE occupies more than half of all fluoropolymer production, followed by polyvinylidene fluoride (PVDF).
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]
For decades, DuPont used perfluorooctanoic acid (PFOA, or C8) during production of PTFE, later discontinuing its use due to legal actions over Ecotoxicology and health effects of exposure to PFOA.
History
Polytetrafluoroethylene (PTFE) was accidentally discovered in 1938 by Roy J. Plunkett while he was working in Chemours Chambers Works plant in New Jersey for DuPont. A team of DuPont chemists attempted to make a new chlorofluorocarbon refrigerant, called tetrafluoroethylene. 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.
By 1948, DuPont, which founded Kinetic Chemicals in partnership with General Motors, was producing over of Teflon-brand polytetrafluoroethylene per year in Parkersburg, West Virginia.[ American Heritage of Invention & Technology, Fall 2010, vol. 25, no. 3, p. 42] 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 enriched uranium plant in Oak Ridge, Tennessee.
In 1954, Colette Grégoire urged her husband, the French engineer Marc Grégoire, to try the material he had been using on fishing tackle on her cooking pans. He subsequently created the first PTFE-coated non-stick pans under the brand name Tefal (combining "Tef" from "Teflon" and "al" from aluminium).
The brand name Zepel was used to promote its stain resistance and water resistance when applied to fabrics.
In the 1990s, it was found that PTFE could be radiation cross-linked above its melting point in an oxygen-free environment.[ Electron Beam Processing of PTFE E-BEAM Services website. Accessed 21 May 2013] This radiation-induced chain scission allows it to be more easily reground and reused.
Corona treatment of the surface to increase the energy and improve adhesion has been reported.
Production
PTFE is produced by
free-radical polymerization of tetrafluoroethylene.
The net equation is as follows:
- n F2C=CF2 → −(F2C−CF2) n−
Because tetrafluoroethylene can explosively decompose to tetrafluoromethane () 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 with
Peroxydisulfate, which homolyzes to generate sulfate radicals:
- O3SO−OSO32− ⇌ 2
The resulting polymer is terminated with
sulfate ester groups, which can be
hydrolyzed to give OH
.
[Carlson, D. Peter and Schmiegel, Walter (2000) "Fluoropolymers, Organic" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim. ]
Granular PTFE is produced via suspension polymerization, where PTFE is suspended in an Aqueous solution medium primarily via agitation and sometimes with the use of a surfactant. PTFE is also synthesized via emulsion polymerization, where a surfactant is the primary means of keeping PTFE in an aqueous medium.
Surfactants in the past have included perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS). More recently, Perfluoro 3,6 dioxaoctanoic acid (PFO2OA) and FRD-903 (GenX) are being used as alternatives.
Properties
PTFE is a
thermoplastic polymer, which is a white solid at room temperature, with a density of about 2200 kg/m
3 and a melting point of .
It maintains high strength, toughness and self-lubrication at low temperatures down to , and good flexibility at temperatures above .
[ Teflon PTFE Properties Handbook. Retrieved 11 October 2012.] 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 highly reactive metals like the
, at higher temperatures, such metals as aluminium and magnesium, and fluorinating agents such as
xenon difluoride and cobalt(III) fluoride.
At temperatures above PTFE undergoes depolymerization.
However, it begins to decompose at about through , and
pyrolysis occurs at temperatures above .
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| Glass temperature | |
| Thermal expansion | 112–125×10−6 K−1 |
| Thermal diffusivity | 0.124 mm/s |
| Young's modulus | 0.5 GPa |
| Yield strength | 23 MPa |
| Coefficient of friction | 0.05–0.10 |
| Dielectric constant | , |
| Dielectric constant (60 Hz) | , |
| Dielectric strength (1 MHz) | 60 MV/m |
| Magnetic susceptibility (SI, 22 °C) | −10.28×10−6 |
The coefficient of friction of plastics is usually measured against polished steel.[ Coefficient of Friction (COF) Testing of Plastics. MatWeb Material Property Data. Retrieved 1 January 2007.] PTFE's coefficient of friction is 0.05 to 0.10.[ PTFE's resistance to van der Waals forces means that it is the only known surface to which a gecko cannot stick.][" Research into Gecko Adhesion", Berkeley, 2007-10-14. Retrieved 8 April 2010.] 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 of formicarium. There are surface 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.[ PTFE was not known to crosslink like an elastomer, due to its chemical inertness. Therefore, it has no "memory" and is subject to creep. 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.]
PTFE is an ultraviolet (UV) transparent polymer. However, when exposed to an excimer laser beam, it severely degrades due to heterogeneous photothermal effect.
Processing
Processing PTFE can be difficult and expensive because its high melting temperature, , is above its decomposition temperature. Even when molten, PTFE does not flow due to its exceedingly high melt viscosity.
Some PTFE parts are made by cold-moulding, a form of compression molding.[ After a settling period, lasting from minutes to days, the mould is heated at ,][ allowing the fine particles to fuse (sintering) into a single mass.]
Applications and uses
Wire insulation, electronics
The most common use of PTFE, consuming about 50% of production,[ This application exploits the fact that PTFE has excellent dielectric properties, specifically low group velocity dispersion,][ making it suitable for use as an excellent insulator in connector assemblies and Electrical cable, and in printed circuit boards used at microwave 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-point polyethylene commonly used in low-cost applications.
]
Bearings seals
In industrial applications, owing to its low friction, PTFE is used for , , , seals, gaskets, bushings,
Electrets
Its extremely high bulk resistivity makes it an ideal material for fabricating long-life , the electrostatic analogues of magnet.
Composites
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 with bismaleimide 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 the Hubert H. Humphrey Metrodome in Minneapolis, US, was one of the largest applications of PTFE coatings. of the material was used in the creation of the white double-layered PTFE-coated fiberglass dome.
Chemically inert liners
Because of its extreme non-reactivity and high temperature rating, PTFE is often used as the liner in hose 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.
Tensioned membrane structures
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 in tensile structures.
Musical instruments
PTFE is often found in musical instrument lubrication products, most commonly valve oil.
Lubricants
PTFE is used in some aerosol lubricant sprays, including in Micronization 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.
Kitchenware
PTFE is best known for its use in coating non-stick and other cookware, as it is hydrophobic and possesses fairly high heat resistance.
The sole plates of some Clothes iron are coated with PTFE.[ Fers à repasser semelle teflon - Fiche pratique - Le Parisien. Pratique.leparisien.fr. Retrieved on 2016-11-17.]
Others
niche applications include:
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It is often found in as a non-mechanical AFD (anti-friction device)
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It can be stretched to contain small pores of varying sizes and is then placed between fabric layers to make a waterproof, breathable fabric in outdoor apparel.
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It is used widely as a fabric protector to repel stains on formal school wear, like uniform blazers.
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It is frequently used as a lubricant to prevent captive insects and other from escaping.
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It is used as a coating for medical and healthcare applications, formulated to provide strength and heat resistance to surgical devices and other medical equipment.
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It is used as 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.
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Expanded PTFE membranes have been used in trials to assist trabeculectomy surgery to treat glaucoma.
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Powdered PTFE is used in pyrotechnic compositions as an oxidizer 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. They are used in infrared decoy flares and as for solid-fuel rocket .
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Powdered PTFE is used in a suspension with a low-viscosity, azeotropic mixture of siloxane ethers to create a lubricant for use in .
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In optical radiometry, sheets of PTFE are used as measuring heads in spectroradiometers and broadband radiometers (e.g., illuminance meters and Ultraviolet ) due to PTFE's capability to diffuse a transmitting light nearly perfectly. Moreover, the optical properties of PTFE stay constant over a wide range of wavelengths, from UV down to near infrared. In this region, the ratio 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.
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Teflon-coated bullets are coated with PTFE to reduce wear on the rifling of firearms that uncoated projectiles would cause. PTFE itself does not give a projectile an armor-piercing property.
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Its high corrosion resistance makes PTFE useful in laboratory environments, where it is used for lining containers, as a coating for magnetic stirrers, 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.
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PTFE tubes are used in gas-gas heat exchangers in gas cleaning of waste incinerators. Unit power capacity is typically several megawatts.
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PTFE is widely used as a thread seal tape in plumbing applications, largely replacing paste thread dope.
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PTFE membrane filters are among the most efficient industrial air filters. PTFE-coated filters are often used in dust collection systems to collect aerosol from air streams in applications involving high temperatures and high particulate loads such as coal-fired power plants, cement production, and steel foundries.
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PTFE grafts can be used to bypass stenosis artery in peripheral vascular disease if a suitable autologous vein graft is not available.
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Many bicycle lubricants and greases contain PTFE and are used on bicycle chain and other moving parts subjected to frictional forces (such as hub bearings).
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PTFE is used for some types of dental floss.
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PTFE can also be used when placing dental fillings, to isolate the contacts of the adjacent tooth so the restorative materials will not stick to the adjacent tooth.
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PTFE sheets are used in the production of butane hash oil due to its non-stick properties and resistance to non-polar solvents.
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PTFE, associated with a slightly textured laminate, makes the plain bearing system of a Dobsonian telescope.
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PTFE is widely used as a non-stick coating for food processing equipment;
[ dough hoppers, mixing bowls, conveyor systems, rollers, and chutes. PTFE can also be reinforced where abrasion is present – for equipment processing seeded or grainy dough, for example.]
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PTFE has been experimented with for electroless nickel plating.
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PTFE tubing is used for Bowden tubing in 3D printers because its low friction allows the extruder stepper motor to push filament through it more easily.
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PTFE is commonly used in aftermarket add-on mouse feet for gaming mice to reduce friction of the mouse against the mouse pad, resulting in a smoother glide.
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PTFE foils are commonly used with laser printers everywhere, in their fuser unit, wrapped around the heater element(s), and on the opposite pressure roller to prevent any kind of sticking to it (neither the printed paper nor toner waste)
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PTFE is also used to make body jewellery as it's much safer to wear compared to materials like acrylic, that release toxics into the body at 26.6 °C, unlike PTFE at 650–700 °C.
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PTFE is used to make bookbinding tools for folding, scoring, and separating sheets of paper. These are typically referred to as Teflon bone folders.
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PTFE is commonly used for the tip of desoldering pumps due to its high melting temperature.
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Liquid PTFE is used by ant hobbyists to contain ants within their habitat and prevent escape.
Safety
While PTFE is stable at lower temperatures, it begins to deteriorate at temperatures of about , it decomposes above , and pyrolysis occurs at temperatures above .[
]
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 .[
]
Most cases of polymer fume fever in humans occur due to smoking PTFE-contaminated tobacco,[ although cases have occurred in people who have welding near PTFE components.][ PTFE-coated cookware is unlikely to reach dangerous temperatures with normal use, as meat is usually fried between , and most (except refined safflower oil and avocado oil oils) start to smoke point before a temperature of 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 was lethal for parakeets, although that was a higher temperature than the required for fumes from pyrolyzed butter to be lethal to the birds.]
Perfluorooctanoic acid (PFOA), a chemical formerly used in the manufacture of PTFE products such as non-stick coated cookware, can be for people who are exposed to it (see Ecotoxicity).[ Non-stick coated cookware has not been manufactured using PFOA since 2013,][
]
Ecotoxicity
Living Building Challenge
PTFE was added to the Living 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).
Trifluoroacetate
Sodium trifluoroacetate and the similar compound sodium chlorodifluoroacetate can both be generated when PTFE undergoes thermolysis, as well as producing longer chain polyfluoro- and/or polychlorofluoro- (C3-C14) carboxylic acids, which may be equally persistent. These products can accumulate in evaporative 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.
PFOA
Perfluorooctanoic acid (PFOA, or C8) has been used in the manufacture of PTFE as a surfactant during its emulsion polymerization, although several manufacturers have entirely discontinued its use.
PFOA persists indefinitely in the environment.
As a result of a class-action lawsuit and community settlement with DuPont, three Epidemiology conducted studies on the population of Parkersburg, WV 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), and gestational hypertension (pregnancy-induced high blood pressure).
Overall, PTFE cookware is considered a minor exposure pathway to PFOA.
GenX
As a result of the lawsuits concerning the PFOA class-action lawsuit, DuPont began to use GenX, a similarly fluorinated compound, as a replacement for perfluorooctanoic acid in the manufacture of , such as Teflon-brand PTFE.[ and it has proven to be a "regrettable substitute";][
]
The chemicals are manufactured by Chemours, a corporate spin-off of DuPont, in Fayetteville, North Carolina.
The North Carolina Department of Environmental Quality (NC DEQ) records
Similar polymers
The Teflon trade name is also used for other polymers with similar compositions:
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Perfluoroalkoxy alkane (PFA)
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Fluorinated ethylene propylene (FEP)
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 ; it is also highly transparent and resistant to sunlight.[ FEP Detailed Properties, Parker-TexLoc, 13 April 2006. Retrieved 10 September 2006.]
See also
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BS 4994, PTFE as a thermoplastic lining for dual laminate chemical process plant equipment
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Dark Waters, a film about litigation related to PFOA
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The Devil We Know, documentary on PFOA's health and environmental effects
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ETFE
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Gore-Tex waterproof fabric
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Magnesium/Teflon/Viton, pyrolant thermite composition
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Polymer adsorption
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Superhydrophobic coating
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Surface treatment of PTFE
Notes
Further reading
External links
