The Story of PTFE

Martin Pitt looks at the accidental discovery of polytetrafluoroethylene (PTFE), a versatile non-stick coating known as Teflon or Fluon, and one of the first per- or polyfluorinated alkyl substances (PFAS) in widespread use today

THE SLIPPERIEST solid and one of the most inert was discovered by accident – but thankfully by someone who had the knowledge and insight to understand what had happened.

A DuPont subsidiary called Kinetics Chemicals produced the first halogenated hydrocarbon refrigerant Freon commercially in 1935 but continued to explore other possible molecules. On 6 April 1938 an experiment went wrong. They were trying to react the gas tetrafluorethylene (TFE, F2C=CF2) with HCl to make F2CH-CClF2. However, the cylinder appeared to have run out, having lost pressure, but not weight. The head of the team, US chemist Roy Plunkett (1910–1994) ordered the valve to be removed. When turned upside down, a white powder came out. More was found inside when it was cut open. It was obvious to Plunkett that it was the polymer polytetrafluorethylene (PTFE), despite chemists having decided TFE could not polymerise. Its main characteristic was inertness. He finally determined the mechanism and patented it: catalysis by some metal salts under pressure over several days giving varying products of white or brown powder or a jelly. However, he had nothing further to do with the product, as DuPont put him in charge of tetraethyl lead and later Freon production.

PTFE was interesting, but so what? What was needed was a technical use and a method for full-scale production with a controlled degree of polymerisation.

The use came in 1942 during the Manhattan Project to build an atom bomb (see TCE 994). Uranium hexafluoride was an extremely toxic and reactive substance and DuPont employees on the project suggested the fluorinated lab curiosity PTFE might be a good material to prevent leaks. After an incredibly short development time, production began, and it was critical to the plutonium project, also lining pipes to handle fluorine.

The method came from DuPont chemist (and senior member of the AIChE) Malcolm Renfrew (1910–2013), in the form of suspension polymerisation [US Patent 2534058A Polymerization of tetrafluoroethylene with dibasic acid peroxide catalysts]. In this process, the monomer is dispersed as drops or bubbles in a non-miscible liquid containing a source of free radicals. Specifically, bubbles of TFE are introduced into water with peroxide, and the reaction occurs at 30 bar pressure and around 65°C in a semi-batch process. The aqueous solution is first loaded, followed by the addition of TFE with agitation to form bubbles. The mixture is then heated, causing PTFE grains to form within the bubbles, which agglomerate into granules as TFE is continuously added to maintain pressure. The concentration of peroxide affects the polymer chain length which in turn determines if the product is grease, wax, or solid. Renfrew scaled this up to a 200-gallon reactor, and an adjacent facility converted the granules into blocks to be sent for manufacturing at the nuclear site.

In 1946, Renfrew published the first paper and made the first public announcement of DuPont’s new wonder plastic, now named Teflon. However, at US$55 per pound (about US$1,400 today) it was not an immediate success, being limited to some extreme corrosive applications in industry.

In 1952, Plunkett was awarded the John Scott Medal “for improving the comfort, welfare, and happiness of mankind” and everyone attending was given a free muffin tin lined with Teflon. This may have given French engineer Marc Grégoire (1906–1996) an idea.

 

Grégoire worked at the French space agency, L’Office national d’études et de recherches aérospatiales, and used the work facility to coat his fishing rod with PTFE. His wife Colette said it would be nice if frying pans were also non-stick. They experimented covering a pan with PTFE film and frying an egg on it. To actually make this non-sticky plastic adhere to metal in a hot pan was more difficult. The aluminium was etched and the PTFE slightly melted into the surface. No company was interested, but in 1956 Marc and Colette started making and selling the pans. Then Marc left his employer to set up a business with the name Tefal (from Teflon and aluminium). In 1958, the French ministry of agriculture approved the material for cooking utensils, and the Grégoires sold a million pans. In 1960 Marc got a US patent, and the US Food and Drug Administration ruled that PTFE “did not present any problems under the Food Additives Amendment”. In 1961, the president’s wife, Jackie Kennedy (1929–1994) was photographed outside Macey’s department store with a Tefal pan in her hand, an early example of celebrity marketing. Sales rocketed in the US, and pans were imported by air to meet demand.

US inventor Marion A Trozzolo (1925–1992) had a company making plastic-coated laboratory ware, including what is now the ubiquitous PTFE-coated magnetic stirrer. He had heard of the growing interest in imported Tefal pans and produced his own coating method for cast iron, heavily advertising it in 1961 as the “Happy Pan”. It was at this time the urban myth began that PTFE had been invented by NASA as an offshoot of the Space Race (you might say the non-stick pan came indirectly from the French equivalent, and NASA certainly used PTFE extensively in space suits and fluid systems). Many US companies began making coated cookware of variable quality, so demand increased production and reduced the price of PTFE.

One outcome of the Manhattan Project was the production of fluorine on a large scale, and many companies began trying to develop products based on fluorinated hydrocarbons, for which the abbreviation PFAS for polyfluorinated alkyl substances came to be used. A major selling point was their lack of toxicity, like the Freons and PTFE.

In 1947, the 3M company produced two surfactants, perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) by electrochemical fluorination.

Meanwhile, DuPont had been working on dispersion (or emulsion) polymerisation (very small bubbles giving finer particles). Both suspension and dispersion methods were used, giving powders with different microstructures, suitable for different processing and applications, though now dispersion is usual. The dispersion really required a surfactant to ensure dispersion, and both were tested, with PFOA being adopted. Not only did it work well, but being highly fluorinated, any traces in the product should not affect its functional properties.

A 3M chemist, Patsy Sherman (1930–2008), was present when a fluoropolymer emulsion with PFOS was spilled in a lab worker’s tennis shoe. When wiped off, it was impervious to water, oil, and stains. She experimented further and ultimately had 13 patents. In 1956, 3M, a company made famous by its sticky tape, began a major business in anti-stick coatings. It still provides, among other products, milky dispersions of PTFE in water, which are applied to fabrics, dried, and baked to water- and dirt-proof them.

Teflon was made in the UK by ICI under the brand name Fluon. This soon encompassed a variety of materials including composites and copolymers, which generally had more suitable physical properties for different applications, but retaining most of the inertness and lubricating properties.

In 1957, one of DuPont’s chemical engineers, Wilbert “Bill” Gore (1912–1986), left the company to set up a business with his wife Genevieve making ribbon cables for computers insulated with PTFE. In 1969 his son Robert Gore (1937–2020), who was also a chemical engineer, was working to find the ideal conditions of heat and tension for stretching rods of PTFE in processing. In frustration he yanked it hard by hand, and it stretched about eight times. It also opened up, becoming porous. Instead of regarding this as a failure, the father and son’s inventive minds looked for applications as they developed the practical production process. The material was given the trademark Gore-Tex. The most famous application is its use as a waterproof membrane in clothing which still allows the passage of water vapour so does not become sweaty.

There are now a range of fluoropolymers, but about two-thirds of the market is PTFE. Their chief selling points, their inertness and durability are now a cause of concern with regard to microplastics. In addition, there has been pollution of the environment by some of the chemicals such as PFOA used in manufacture.

If heated, PTFE gives off harmful chemicals, leading DuPont to forbid smoking or even the possession of cigarettes or lighters in manufacturing areas. But in ordinary use, PFAS seemed extremely innocuous. Good enough to use in waterproof mascara or lipstick, and to help foundation powder stay on. PFOA or PFOS were essential for stain resistant carpets, clothes, and upholstery, and also used for hydraulic fluid in aircraft, in fire-fighting foams, inks, coatings on paper, and floor and car polishes. They also provide grease and water resistance to food packaging. It is only this century that concern has been raised and their production and use phased out, or sometimes replaced with other PFAS.

In the 1990s, American chemist Joseph DeSimone developed a “green” process for polymerisation of TFE in solution in supercritical CO2 which did not require the surfactant. In 2002, a US$40m pilot plant at DuPont’s factory in North Carolina began producing PTFE suitable for wire insulation, flexible tubing, and film. However, a planned full-scale one did not materialise.

In 1998, Robert Bilott (1968– ), an America corporate defence lawyer, was surprised to be asked by farmer Wilbur Tennant (1942–2009) to sue a chemical company for poisoning his cattle, not Bilott’s usual type or scale of activity. As his grandmother lived near the farm, he went to see her and found out the problem. DuPont had been dumping sludge from its nearby PTFE plant onto land adjacent to the farm. Some 190 cattle had died, many with blackened teeth, which can come from excess fluoride. A weakness of the US Environmental Protection Agency (EPA) was that it only dealt with “regulated substances” and PFOA, which had been found in the water supply to the farm and town, was not on the list. There then followed a 20-year legal battle to establish the harm of these supposedly innocuous chemicals and get government action to protect the general population.
In 2004, Tennant received US$4m compensation. Both he and his wife had terminal cancer. In 2005, the EPA ordered the company to phase out PFOA by 2015, which was done, with another PFAS replacing it. In 2009, a seven-year study established a positive correlation with a variety of severe medical conditions including cancer suffered by the locals and was strongly implicated in some deaths including Tennant’s. In 2017, DuPont paid US$620.7m in compensation to 3,550 claimants.

In 2024 the EPA set Maximum Enforceable Standards for 29 PFAS including PFOA and PFOS in drinking water. In January the Trump administration cancelled this before it became law.

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Martin Pitt CEng FIChemE is a regular contributor. Read other articles in his history series: https://www.thechemicalengineer.com/tags/chemicalengineering-history

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Image citation: Roy Plunkett with Teflon insulated cable (AVD_2004268_P00000217) and Roy Plunkett reenacts invention of Teflon, 1966 (AVD_2004268_P00000214) both DuPont Company External Affairs Department photograph files (Accession 2004.268), courtesy of the Audiovisual Collections and Digital Initiatives Department, Hagley Museum and Library, Wilmington, DE 19807