Material of the month – Teflon
This month, Anna Ploszajski investigates a material most commonly found in the kitchen.
Polytetrafluoroethylene (PTFE) is in the family of polymers called fluoropolymers and comprises a carbon backbone surrounded by coordinating fluorine atoms. It was stumbled upon accidentally in 1938 by 27-year-old Roy Plunkett, who was working to develop a new chlorofluorocarbon refrigerant at Kinetic Chemicals, a subsidiary of DuPont. A blockage in his pressure bottle full of tetrafluoroethylene gas prompted him to investigate the cause, and when, he opened it up, he found that its interior surface was coated with a white, waxy, slippery solid. The polymerisation of the gas had been catalysed by the iron surface of the container at high pressure. The new material was patented in 1941 and given the registered trademark Teflon in 1945.
This early Teflon was destined for just one place. During World War II, DuPont reserved its entire yield of the material for the US Government, and around two thirds went towards the Manhattan Project, the programme that brought about the atomic bomb. Teflon was essential to the project’s success, thanks to its extraordinary corrosion resistance. It was used to coat valves and seals in the pipework, which contained a highly caustic and toxic substance, uranium hexafluoride. Today, Teflon is still exploited in laboratories, and can be used for lining containers and tubing to hold corrosive chemicals, such as hydrofluoric acid, which dissolves glassware receptacles.
Since the Manhattan Project, the use of Teflon in an industrial setting has diversified from containers and pipework. With superior chemical and lubricative properties over wide temperature ranges, Teflon is also used in high-friction applications, such as bearings, gears and slide plates in machinery, reducing wear and energy consumption. New materials innovations include embedding mineral oil or molybdenum disulphide to provide even better lubricated surfaces. PTFE air filters are efficienct in removing particulate matter from high-temperature industrial air streams, such as in coal-fired power plants, cement production and steel foundries, making our air cleaner to breathe. Today, half of the PTFE produced is destined for wiring in aerospace and computing applications. Thanks to its dielectric properties, PTFE is used for insulation around cables and printed circuit boards at radio and microwave frequencies and in harsh chemical environments.
PTFE has retained its military roots in a couple of discrete applications. Powdered PTFE is used in pyrotechnics as an oxidiser with powdered magnesium. When ignited, the mixture forms magnesium fluoride and releases large amounts of carbon and heat. Such mixtures are used in infrared decoy flares, which make use of the high burn temperature to attract infrared-guided missiles away from the host aircraft’s engines. Another defence application of PTFE is in projectiles. Teflon-coated bullets reduce wear on the interior surface of the barrel of firearms, extending their functional lifetime.
But, of course, the most favoured application for Teflon since the war has been in non-stick cooking applications. The idea for a Teflon-coated pan came from Colette Gregoire in 1954. She challenged husband and engineer Marc Gregoire to find a way of bonding PTFE to aluminium, and they began to market their unique product in France. In 1956, they founded the Tefal Corporation and, by 1960, had begun to sell in the US market. Other manufacturers soon followed suit, tweaking the PTFE formula to include diamond and titanium reinforcements for additional integrity.
But how do you make something that is inherently non-sticky stick to the surface of a pan? The aluminium pan is first dipped in a warm hydrochloric acid bath, which etches the surface, roughening it, followed by a dip in nitric acid before being washed and thoroughly dried. A primer is then applied, followed by several layers of liquid PTFE, which can be sprayed or rolled onto the surface. The product is then dried by heating slowly and sintered at 425°C for five minutes to set the PTFE layers and form a smooth, continuous coating.
We experience first-hand the non-stick characteristics of Teflon every day, but what makes this remarkable material so effective? These properties originate from the presence of highly electronegative fluorine atoms, which also give Teflon its high temperature capabilities. These atoms stick out from the carbon backbone and line up so as to be as far apart as possible. It is energetically unfavourable for the polymer chain to bend or rotate, which locks the molecule into a long, rod-like formation with the fluorine atoms gently spiralling around its length. These rods can efficiently pack together to form a crystalline microstructure and this is what gives PTFE a particularly high melting temperature for a polymer.
The fluorine atoms are so electronegative that they hold the electrons in the carbon-fluorine bonds closely. Having a layer of immobile and slightly negative fluorine atoms along the polymer exterior means that chains repel one another, so there are unusually weak inter-molecular dispersion forces. These carbon-fluorine bonds are so strong that it is very difficult for other molecules to react with them, therefore minimal chemical bonding can occur and PTFE is chemically inert.
Health and safety
This strong repulsion and resistance to dispersion interactions is what makes Teflon the only known surface to which a gecko is unable to stick. For non-stick cookware, as food slides over the PTFE, tiny lumps are transferred to the food. These are spread out as the food continues to slide along the PTFE surface, to form a thin film and the PTFE surface is flattened into an organised layer. The two contact surfaces are now both coated with well-organised PTFE molecules to slide over each other with ease, thanks to fluorine’s electronegativity.
Despite multiple investigations into the safety of Teflon-coated kitchenware, no legislation has emerged to control the sale of such products. Nevertheless, DuPont has advised its consumers that Teflon products should not be heated beyond 260°C to avoid the release of harmful gases, and to replace any pans that are visibly degrading.
Production and processing
PTFE’s popularity calls for industrial-scale synthesis. The first stage of production is to make the monomer, tetrafluoroethylene (TFE). It is synthesised from fluorspar, hydrofluoric acid and chloroform combined under high heat. PTFE has poor solubility in almost all solvents, so the polymerisation is conducted as an emulsion in water. Liquid TFE and an initiator are added to distilled water, and free-radical polymerisation forms grains of solid PTFE that float to the surface, or a milky suspension of much smaller PTFE particles, depending on how violently the reaction chamber is mechanically shaken.
PTFE can be applied directly in this suspension form to fabrics as a stain-resistant coating, or it can be dried and milled into a powder. In the latter case, the powder is agglomerated into pellets by heating in a rotating drum, before being moulded into the desired shape and sintered to coalesce the particles and form a solid product.
In 1966, John Cropper, from New Zealand, constructed a machine capable of producing stretched PTFE tape. Three years later, father and son team Wilbert and Robert Gore invented microporous thermo-mechanically expanded PTFE (ePTFE) which they named Gore-Tex. It is used in breathable rainwear and high-performance fabrics. In the medical field, Gore-Tex has found use in vascular grafts since 1975 on account of its biocompatibility and inertness in the body. Kink resistant and twist tolerant, ePTFE-based grafts offer superior handling for surgeons and protection against patient aneurisms. Outside the body, PTFE patches in footwear provide additional lubrication and protection from blisters, calluses and foot ulcers in high-friction areas.
Since the happy accident that led to its discovery, Teflon’s unique properties have seen it applied in a diverse range of uses, from cutting-edge military technologies to everyday domestic cookware. With its record-breaking low-friction properties, we are undoubtedly still only scratching the surface of Teflon’s capabilities.