What is polytetrafluoroethylene (PTFE)?
Polytetrafluoroethylene is a thermoplastic polymer made from the monomer tetrafluoroethylene (CF2=CF2) with the general formula -[-CF2-]-n. A thermoplastic polymer is a material that can be repeatedly softened and hardened by alternately heating and cooling. Polytetrafluoroethylene is also known as PTFE and by the DuPont patented name Teflon® although they are also available under more than a hundred other trade names (Schlager, Weisblatt, Newton, 2006).
Discovery of Teflon®?
Polytetrafluoroethylene was invented in 1938 by Roy J. Plunkett (1910–1994) quite by accident. As a research chemist at DuPont’s Jackson Laboratory, Plunkett was studying compounds that might be used for refrigerants. He kept the compounds in steel tanks and was surprised on finding that the gas he wanted did not leave the storage tank when the valve was opened and found a waxy white material inside the tank. Upon analysis, the material turned out to be polytetrafluoroethylene. The gas stored in the tank, the tetrafluoroethylene, had undergone spontaneous polymerization within the tank, making it possible for Plunkett to discover one of the most remarkable synthetic products in the world (Schlager et al, 2006).
Manufacture and Properties of PTFE 1.5
The Manufacture of PTFE
PTFE is produced from the polymerization of tetrafluoroethylene (TFE). Molecules of TFE contain double bonds which give them the ability to form polymers (Graffte, 2005).
TFE itself is synthesized by combining fluorspar, hydrofluoric acid, and chloroform under high heat, a process known as pyrolosis. TFE is a colorless, odorless, nontoxic gas but is extremely flammable. It is stored as a liquid, at low temperature and pressure. PTFE manufacturers also make their own TFE on site because of the difficulty of transporting the flammable TFE (Ebnesajjad, 2000).
The PTFE polymerization process uses a very small amount of other chemicals as initiators. Various initiators can be used, including ammonium persulfate or disuccinic acid peroxide. The other essential ingredient of the polymerization process is water.
PTFE can be produced in a number of ways, depending on the particular traits desired for the end product. Many specifics of the process are proprietary secrets of the manufacturers. There are two main methods of producing PTFE. One is suspension polymerization. In this method, the TFE is polymerized in water, resulting in grains of PTFE. The grains can be further processed into pellets which can be molded. In the dispersion method, the resulting PTFE is a milky paste which can be processed into a fine powder. Both the paste and powder are used in coating applications.
Some Properties of PTFE
PTFE is a white solid at room temperature and according to DuPont’s technical specifications has a specific gravity of 2.15 and melting point of 327 °C (621 °F). It exhibits excellent chemical and solvent resistance and a weather resistance of 20 years. PTFE has a coefficient of friction of 0.1 or less, the second lowest of any known solid material after Diamond-like carbon (DeGaspari, 1999).
PTFE has excellent dielectric properties with a dielectric constant of 2.1 and dielectric strength of 18V/um according to DuPont’s technical specifications. This makes PTFE especially useful as an insulator in cables and as a material for printed circuit boards used at microwave frequencies. It is also important in the manufacture of semiconductors owing to its low electrical conductivity. Combined with its high melting temperature, this makes it the material of choice as a high-performance substitute for the weaker and lower melting point polyethylene that is commonly used in low-cost applications.
Uses of Fluoropolymers 3.5
Owing to polytetrafluoroethylene’s extremely low coefficient of friction, it is commonly used for applications where sliding action of parts is needed like bearings, gears, slide plates and on non-stick products like the widely-known Teflon®-coated cookware.
Application of Teflon® Non-Stick Coatings on Cookware
Because Teflon® does not stick to anything else, cookware manufacturers must use a special process to get it to stay on pots and pans. It requires the pan to be made of aluminum or an aluminum alloy with the pan surface specially prepared to receive the PTFE. First, the pan is washed with detergent and rinsed with water, to remove all grease. Then the pan is dipped in a warm bath of hydrochloric acid in a process called etching (Ebnesajjad, 2000). Etching roughens the surface of the metal. The pan is then rinsed with water and dipped in nitric acid. It is then washed with deionized water and thoroughly dried.
The liquid PTFE coating may be sprayed or rolled on. The coating is usually applied in several layers and begins with a primer. The exact makeup of the primer is a proprietary secret held by the manufacturers (Ebnesajjad, 2000). After the primer is applied, the pan is dried for a few minutes, usually in a convection oven. Then the next two layers are applied, without a drying period in between. According to the DuPont Teflon®’s web site, their typical pan has the following coating layers: a unique topcoat – for easy food release and easy cleanup; a tough midcoat – for excellent resistance to scratches and abrasions and a rugged primer – for long-lasting durability.
The pan is dried in an oven after all the coating is applied and then sintered. Sintering is the slow heating that is also used to finish the billet. So typically, the oven has two zones. In the first zone, the pan is heated slowly to a temperature that will evaporate the water in the coating. After the water has evaporated, the pan moves into a hotter zone, which sinters the pan at around 800°F (425°C) for about five minutes that gels the PTFE (Ebnesajjad, 2000) . Then the pan is allowed to cool. After cooling, it is ready for any final assembly steps.
Concerns Regarding Perfluorooctanoic Acid (PFOA)
Polytetrafluoroethylene has long been regarded as an essentially safe compound with no known health effects on humans or experimental animals. Recently, questions have been raised about possible health hazards of one of the compounds used in the manufacture of polytetrafluoroethylene, perfluorooctanoic acid (PFOA). PFOA is classified by the US Environmental Protection Agency (EPA) as a persistent chemical and a “potential” carcinogen (Williams, 2006).
Some studies suggest that PFOA may be responsible for birth defects and the development of cancer in people who have been exposed to the chemical (Schlager et al, 2006). Interestingly, the chemical appears to be present in the blood of 95% of all Americans even though the mechanism of exposure is little understood. One possible explanation is the deterioration of water- and grease-repellent coatings used on raincoats, takeout food containers and stain-resistant carpets (Williams, 2006).
Fluoropolymer Properties Useful in Medial Device Market
In the medical device market, the use of fluoropolymers centers on its two key properties: lubricity and biocompatibility. Fluoropolymers exhibit very good lubricity compared with other plastics. PTFE is the most lubricious polymer available, with its coefficient of friction (COF) of 0.1 or less, followed by fluorinated ethylene propylene (FEP), with 0.2. These two polymers represent the vast majority of all fluoropolymer tubing used in medical devices (Graffte, 2005).
The biocompatibility of any polymer used in a medical device is an obvious concern. Fluoropolymers, especially PTFE, excel in this area and have a long history of in vivo use. Medical-grade fluoropolymers should meet USP Class VI and ISO 10993 testing requirements before they can be used (Graffte, 2005).
Two Well Established Applications of Fluoropolymers as Medical Devices
Two of well-established medical devices applications of fluoropolymers are in guiding catheters and introducers.
Guiding Catheter. Used to deliver coronary stents and other devices, the guiding catheter has a well-established history in the medical device market. At the core of most guiding catheters is a PTFE inner liner. The superior lubricity of PTFE has made it the material of choice for this application, with the lowest dynamic COF of any polymer. Lubricity is so critical in this application that even FEP has not proven successful as a catheter liner. Guiding catheters are used in many percutaneous intravascular procedures to guide diagnostic and therapeutic devices or fluids, and the like, to a desired location within the patient. Percutaneous refers to any medical procedure where access to the inner organs or tissues is done via needle-puncture of the skin rather than using the “open” approach of exposing inner organs or tissue. During the construction of a guiding catheter, PTFE is chemically etched onto the tube’s outer diameter. This process allows for the bonding of materials to the outer diameter of the liner.
PTFE Introducer. Developed and patented in the 1970s by Cook Inc., the PTFE introducer utilizes a little-known property of PTFE. PTFE can be processed in a manner that allows for molecular orientation of the material. In such cases, the PTFE tube can be split and torn longitudinally. The tubing must first be scored precisely at the edges, enabling the tear to be easily facilitated by hand along the longitudinal grain of the molecules. The effect is similar to a tear notch on a food package. During use, a surgeon can remove a PTFE introducer from a patient while the primary device remains in place.
Four Other Applications of Fluoropolymers in Everyday Life.
Clothing. PTFE is also used to coat fibers to make them water-repellant and stain-resistant. Water will bead up and roll-off the surface of clothing and other materials coated with polytetrafluoroethylene instead of penetrating the fabric and possibly leaving a stain.
Furniture and carpets. Polytetrafluoroethylene is available as a spray treatment for carpets and furniture, forming a molecular shield to prevent water or oil-based stains from penetrating the material. Some carpets come pre-treated with a PTFE product to keep them clean and fresh. The compound can also be used on wood and plastic flooring to protect it from dirt, stains, and moisture.
Automobiles. Automobile manufacturers use polytetrafluoroethylene in a variety of ways. Windshield wiper blades coated with PTFE are smoother and stronger than uncoated blades. Automobile paint can be coated with PTFE to protect a car’s finish from tree sap, insects, and other residues. Automobile upholstery is often treated with PTFE to protect against stains caused by spilled drinks and dirty shoes.
Food Packaging. PTFE coatings are also used in food packaging to provide grease resistance in applications like microwave popcorn bags, pizza boxes and some other fast food containers. . PTFE’s excellent chemical resistance allows it to contain the release of grease from the packaged food and maintain the integrity of the packaging.
Cookware Safety 1.5
Concerns Regarding Safety of Cookware Use
According to DuPont, while PTFE itself is chemically inert and non-toxic, it begins to deteriorate after the temperature of cookware reaches about 500 °F (260 °C), and decompose above 660 °F (350 °C).
Polymer fume fever is the term used when individuals are sickened by toxic Teflon® emissions. Symptoms of polymer fume fever include chest tightness, malaise, shortness of breath, headache, chills, cough, sore throat and temperature between 100°F and 104 °F. All symptoms are characteristic of the common flu (Williams, 2006).
DuPont has not studied the incidence of sickness among the millions of people worldwide using coated pots and pans, nor has the US government assessed the safety of nonstick cookware (Williams, 2006).
In 1960, the US Food and Drug Administration (FDA) approved Teflon for contact with food based on a study of cooking hamburgers on a heat-worn pan. This did result in higher levels of Teflon in the meat, but was not considered to be a health risk at that time. The government has conducted no further safety tests of Teflon chemicals off-gassing in food or of the potential effects to humans from inhalation exposures, although several of the off-gas chemicals are considered highly toxic. The Environmental Protection Agency still indicates that using household products with non-stick coatings need not worry because scientific studies have not established an increased risk of cancer (Williams, 2006).
DuPont suggests consumers use nonstick cookware at low or medium temperatures. They further recommend that cookware should never be overheated or leave an empty cookware on a hot stove or in a hot oven.
Safe Temperature Limits of PTFE Coatings
DuPont acknowledges that heating empty Teflon-coated pans releases toxic fumes, but only at temperatures exceeding 660 °F (340 °C). There is currently a class-action suit filed by several states claiming Teflon releases PFOA under normal cooking use and that the company did not warn consumers about its dangers (Williams, 2005).
Independent tests show that preheating nonstick cookware can raise the temperature to 736 °F in three minutes and 20 seconds. DuPont’s own tests indicate Teflon off-gases toxic particulates at 446 °F. At 680 °F, Teflon pans emit six serious toxins. Coated drip pans easily reach 1000 °F, at which temperature, the coatings break down to a chemical warfare agent (PFIB) and phosgene, the chemical analog of a WWII nerve gas (Williams, 2006).
Benefits of Using Cookware Coated with Teflon® 0.5
The non-stick properties of Teflon®-coated cookware allows the user to cook food and gets the following benefits which also saves money and resources at the same time.
Less time and effort in clean-up. As compared to using cookware without the non-stick coating, less time and effort will be spent in cleaning up since there won’t be any charred left-over crumbs left on the cookware that has to be scoured out.
Healthier cooking. Teflon®-coated cookware facilitates healthier cooking since it only requires a minimum amount of oil or vegetable oil spray.
Safer cooking. A minimal amount of oil and fat in cooking reduces the risk of stovetop fires, the number one cause of house fires.
Since its accidental discovery in 1938, polytetrafluoroethylene has proven to be a versatile and useful polymer whose breadth of applications covers not only various industrial utilization but domestic usage as well. PTFE’s one-of-a-kind lubricious property has made it useful in applications where excellent sliding action among parts is required. Coupled with its superior resistance to chemicals and solvents and excellent dielectric properties, PTFE has provided a somewhat endless possibility of applications and has affected our lives in one way or another.
What comes with technology and invention is the responsibility to produce and use them safely. The safety issue raised with a chemical used in manufacturing fluoropolymers can be addressed by investing enough resources in developing a safer manufacturing process. The concern regarding the safety of fluoropolymer-coated cookware products still have to be substantiated by providing scientific evidence of the correlation. In the meantime, users of these products are still responsible in ensuring that they use them safely.
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DuPont Teflon®. Safety of Teflon® Non-Stick Coatings for Cookware. Retrieved October 24, 2008, from http://www.teflon.com/Teflon/teflonissafe/keyquestions.html#q3
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