Polytetrafluoroethylene
Polytetrafluoroethylene is a synthetic fluoropolymer of tetrafluoroethylene, and has numerous applications because it is chemically inert. The commonly known brand name of PTFE-based composition is Teflon by Chemours, a spin-off from DuPont, which originally invented the compound in 1938.
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 wet PTFE, as fluorocarbons exhibit only small London dispersion forces due to the low electric 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 non-reactive, partly because of the strength of carbon–fluorine bonds, so it is often used in containers and pipework for reactive and corrosive 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 catheters.
PTFE and chemicals used in its production are some of the best-known and widely applied per- and polyfluoroalkyl substances, which are persistent organic pollutants. PTFE occupies more than half of all fluoropolymer production, followed by polyvinylidene fluoride.
For decades, DuPont used perfluorooctanoic acid during production of PTFE, later discontinuing its use due to legal actions over ecotoxicological and health effects of exposure to PFOA. DuPont's spin-off Chemours currently manufactures PTFE using an alternative chemical it calls GenX, 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.
History
Polytetrafluoroethylene 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, 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. Kinetic Chemicals patented the new fluorinated plastic in 1941, and registered the Teflon trademark in 1945.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. 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.
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. 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. 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.
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 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. This radiation-induced chain 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.
Production
PTFE is produced by free-radical polymerization of tetrafluoroethylene. The net equation is as follows: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 persulfate, such as potassium persulfate and ammonium persulfate, which homolyzes to generate sulfate radicals:
The resulting polymer is terminated with sulfate ester groups, which can be hydrolyzed to give OH end-groups.
Granular PTFE is produced via suspension polymerization, where PTFE is suspended in an aqueous medium primarily via agitation and somewhat commonly 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 and perfluorooctanesulfonic acid. More recently, Perfluoro 3,6 dioxaoctanoic acid and FRD-903 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/m3 and a melting point of. It maintains high strength, toughness and self-lubrication at low temperatures down to, and good flexibility at temperatures above. 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 alkali metals, at higher temperatures, such metals as aluminium and magnesium, and fluorinating agents such as xenon difluoride and cobalt fluoride. At temperatures above PTFE undergoes depolymerization. However, it begins to decompose at about through, and pyrolysis occurs at temperatures above.| Property | Value |
| 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 | , |
| Dielectric strength | 60 MV/m |
| Magnetic susceptibility | −10.28×10−6 |
The coefficient of friction of plastics is usually measured against polished steel. 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. 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 formicaria. 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. However, until the 1990s, 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 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. The viscosity and melting point can be decreased by inclusion of small amount of comonomers such as perfluoro and hexafluoropropylene. These cause the otherwise perfectly linear PTFE chain to become branched, reducing its crystallinity.Some PTFE parts are made by cold-moulding, a form of compression molding. Here, fine powdered PTFE is forced into a mould under high pressure. After a settling period, lasting from minutes to days, the mould is heated at, allowing the fine particles to fuse into a single mass.