PTFE is the abbreviation for polytetrafluoroethene, a saturated fluorocarbon polymer, which was discovered serendipitously by Roy Plunkett, a 27-year-old research chemist working at the Du Pont Research Laboratories in Deepwater, New Jersey in 1938. He was actually doing some work for Kinetic Chemicals, a company founded jointly by Du Pont and General Motors to commercialise chlorinated fluorocarbon (CFC) refrigerants.
On the morning of 6 April of that year he went to use some tetrafluoroethene (TFE), a gas which was stored in a cylinder, which he needed to react with hydrogen chloride. The idea was that this would react with the C=C bond in TFE thus providing a route to hydrochlorofluorocarbon (HCFC) manufacture. To his astonishment, the cylinder which should have been holding 1000 grams of the gas only released 990 grams. Plunkett decided to dismantle the cylinder and, upon tipping it upside down, out came about 10 grams of a white waxy powder. Plunkett recorded in his laboratory notebook,
On cleaning up a cylinder which had contained approximately 1 kilo of tetrafluoroethylene1, a white solid material was obtained, which was supposed to be a polymerised sample of C2F2...Sample gave good Beilstein test for halogen.
Thus Plunkett had realised that the gas had polymerised to form a new polymer, polytetrafluoroethene.
Further investigation of this polymer revealed that it had some remarkable properties: it was not attacked by corrosive acids, even if they were hot; it did not dissolve in solvents; it could be cooled to -240°C without becoming stiff and brittle, and it could be heated to 260°C without impairing its performance. Furthermore, it could be heated to over 500°C without burning or charring2. In fact, PTFE is attacked only by molten sodium or fluorine gas under pressure; and so it rivals the noble metals, gold and platinum, in its unreactivity. Plunkett also noted that the substance had a slippery feel, and herein lay the secret of its later commercial success.
The difficulties of working with and characterising such an unreactive material were such that the development of processes for production of PTFE were prohibitively expensive, and Du Pont all but gave up on it. Indeed, unlike other plastics, it cannot be extruded, thermoformed, injection-moulded or rotomoulded. To 'work' it, techniques adapted from powder metallurgy must be used.
World War II and The Manhattan Project
This all changed in 1941 when the United States became embroiled in World War II and work on atomic bombs (the Manhattan Project) acquired a renewed urgency. Large quantities of fluorine were needed for the manufacture of uranium hexafluoride, from which the fissionable isotope 235U could be extracted in the gas-diffusion plant at Oak Ridge, Tennessee. However, both the fluorine and the uranium hexafluoride were so reactive that the provision of inert buffer gases, lubricants, coolants, gaskets, valve packings, reactor linings and pipes was critical. A saturated fluorocarbon, such as PTFE, would fit the bill perfectly. Teflon was also used for the diffusion membranes by which UF6 made from natural uranium could be isotopically enriched to 235UF6 for the atomic bomb.
During the war, a series of top-secret negotiations resulted in technical know-how and manufacturing rights being transferred to ICI (Plastics) in the United Kingdom. The ICI trade-name for PTFE is 'Fluon'.
Commercial Manufacture of PTFE
Du Pont gave the name 'Teflon' to its new polymer in 1945, and in 1950 they opened the world's first full commercial plant near Parkersburg, West Virginia.
Manufacture in the UK
ICI manufactured Fluon at their Hillhouse plant near Blackpool, UK; and this is now owned and operated by a Japanese company, Asahi Glass Fluoropolymers.
PTFE starts life as the blue mineral, fluorspar (known as 'Blue John') which used to be mined at Castleton, Derbyshire, but is now purchased from a number of global sources. Chemically, fluorspar is calcium fluoride3 and this is treated with concentrated sulphuric acid to yield hydrofluoric acid. Hydrofluoric acid is reacted with trichloromethane (chloroform) at 600°C to yield chlorodifluoromethane (CHClF2, also known as Arcton-22 or HCFC-22), an ozone-depleting refrigerant gas4. This is done by INEOS Fluor at its world scale production unit at Runcorn in Cheshire.
Arcton-22 is transported by road tanker to the Asahi facility in Hillhouse where it is subjected to steam pyrolysis using super-heated steam at 6 bar5 pressure and 900°C. A product of this is difluorocarbene (:CF2) which dimerises to form TFE. (Difluorocarbene is also able to insert into TFE and into subsequent higher polymers) to give by-products which include perfluoroisobutylene (PFiB)6).
Now TFE is highly unstable, and therefore it is made in situ when it is required and where required for polymerisation, to minimise the storage time between its production and polymerisation. Polymerisation is achieved by passing the TFE into water containing an initiator such as ammonium persulphate (ammonium peroxodisulphate, (HN3)2S2O8) at 310-350K and a pressure of 20 atmospheres. When the PTFE is required for granular resins or mouldings, 'suspension polymerisation' is used. Emulsion polymerisation is used to give fine-powder (dispersion) resins, used for extrusion processes.
The Non-Stick Frying Pan
The slippery feel of PTFE is due to the fact that it has extremely low intermolecular forces (van der Waals forces). The civil market for PTFE opened up in the 1950s once the American chemist, Louis Hartmann and the French engineer, Marc Grégoire independently discovered a way to bond PTFE to aluminium. This was achieved by treating the metal surface with acid, and applying the PTFE in emulsion form. The product is then baked at 400°C for a few minutes, allowing the polymer to melt and form a film over the surface. Following this, the Tefal7 company was set up in 1956 to market non-stick cookware.
In 1969, Dr Bob Gore found a way of expanding PTFE by heating and stretching it to form a membrane with microscopic pores in the structure. There are billions of these pores per square centimetre and they are small enough to keep water droplets out whilst allowing water molecules, present as vapour from sweat, out. Such a material is said to be 'microporous'. The PTFE membrane is sandwiched between the outer fabric and inner lining of the garment, whilst between the membrane and the inner lining is a layer of an oil-hating (lipophobic) polymer. This also allows the water vapour through, but prevents the natural oils of the skin from getting through and blocking the micropores in the PTFE. Gore-tex is now widely used in wet-weather gear and sportswear.
An interesting application of Gore-tex is as a biocompatible membrane to facilitate bone tissue restoration in patients with long-standing periodontal disease. In addition, it is possible to restore or regenerate bone prior to the placement of bridges or implants.
Membranes are created containing certain size pores and these are placed over damaged teeth that are being regrown. These membranes prevent 'non-desirable' cells from colonising the healing site, while allowing through osteoblasts8 which produce the new bone. The undesirable cells are fibroblasts9 essentially derived from gingival epithelium and gingival connective tissue; and immune cells which cause an inflammatory response. Such cells are somewhat larger than osteoblasts and can't pass across the membrane.
Other applications for Gore-tex in the bio-medical field include its use for artificial veins, arteries and trachea replacements. It is also used for artificial dentures and corneas (for example, for patients suffering from keratoconus) and as substitute bones for chin, nose, skull and hip.
The Space Race
It is an urban legend that Teflon was developed specifically for the space-race, and that the non-stick frying pan was a spin-off from this. Indeed, Roy Plunkett made his serendipitous discovery some 30 years before Neil Armstrong first set foot on the Moon; and the first non-stick frying pans were on sale a good five years before Yuri Gagarin became the first man in space.
However, the environments experienced in the upper atmosphere and in space are such that PTFE was the only material with the necessary range of properties to endure these. For example, in the upper atmosphere rocket components would have to endure the corroding effects of activated oxygen10, whereas in space they would have to withstand extreme cold and low pressures.
Other applications of PTFE
Domestically, PTFE is used as a stain-repellent on clothes, furniture covers and carpets, where it is marketed under such trade-names as Scotchgard and Zepel. It is also used on the underside of electric irons and as dental floss.
PTFE is well-known as the plumbers' tape for sealing joints in central heating systems. It is also an excellent electrical insulator and is thus used in electrical wires and cables.
PTFE has the lowest coefficient of friction11 of any solid material, due to having very low intermolecular (van der Waals) forces. Hence it finds use as lubricant-free bearings in motors. Scrap PTFE from industry is re-used by grinding it to a micro-fine powder and adding it to printers' ink where it facilitates ink flow.
A modification of the basic PTFE polymer has produced fluorocarbon rubber, which is used to keep aircraft wings free of ice at high altitudes. In tetrafluoroethene, the basic monomer for PTFE, all the hydrogen atoms in ethene have been replaced by fluorine. In fluorocarbon rubbers, however, the starting monomer is a hydrofluoroethene, in which only some of the hydrogen atoms have been replaced.
A related product is ETFE, a co-polymer of TFE and ethene that can be processed by conventional thermoplastic techniques, and which is used as roofing in sports arenas. A recent installation of this is at the Eden Centre in Cornwall, UK.