Since its discovery in 1938, polytetrafluoroethylene (PTFE) has revolutionized the progress of modern man, as did the discovery of fossil fuels. PTFE has opened new doors for significant advancements in a variety of different technologies. From the aerospace industry to the food processing industry, PTFE has aided in many benefits to mankind. Most commonly known throughout the industry as Dupont’s brand name, Teflon®, PTFE has been commonly used in its virgin state. The material’s chemical inertness, low coefficient of friction and high temperature resistance are but a few advantages that it has to offer. As use of this material increased, so did the applications for it. Chemists soon began to add additional materials within PTFE to increase a variety of properties for their respective applications.
Processed in a variety of methods, normally through cold forming and a sintering process, this grade of PTFE resin offers several outstanding properties not found in other materials. The virgin resin has a large range of thermal applications since it can withstand temperatures from –450o to 500o F, making it excellent for most cryogenic applications. The fluorine atom and the carbon atom are the building blocks of PTFE. When combined, they form the PTFE molecule and it develops a tendency to repel other molecules, thus giving the material the “non-stick” property it is well known for. The virgin grade exhibits the lowest coefficient of friction of all solid materials making it extremely slippery. This tendency to repel also adds to the chemical inertness of the material. It has a chemical resistance to most chemicals except fluorochemicals and molten alkali metals. It has an extremely high electrical resistance and excellent dielectric strength. The virgin grade is also FDA approved for the food processing industry. Some virgin grades are also approved for insertion within the human body as possible prosthetics or even valves within an artificial heart.
This grade is manufactured from pre-sintered PTFE shavings, scrap etc. It exhibits most of the same properties that the virgin grade does but is subject to occasional contamination within the material. This is the grade of choice when cost is a major concern and cleanliness is not an issue.
The newest grade of fluoropolymers, modified PTFE offers the same thermal, chemical, low coefficient of friction and “non-stick” properties of a virgin PTFE but at the same time improves a variety of other properties. Modified PTFE offers a higher dielectric strength and improved performance in most electrical applications. Creep resistance and stiffness are improved making it an exceptional gasketing material. The modified grade offers an excellent weldability characteristic that makes the fabrication of complex parts economically feasible. This improved characteristic also lowers the porosity and improves permeation resistance. Also, environmental safety concerns are improved because of lower fugitive emissions.
Glass fibers are most commonly added to PTFE, normally in percentages that can range from 5% to 40%. The addition of the glass fibers improves the mechanical properties of PTFE as well as the compressive strength under load. The material’s wear resistance is also improved. The presence of glass fibers makes the material abrasive, so graphite is sometimes added to lower the coefficient of friction. The chemical and electrical properties virtually remain unchanged. The original chemical inertness remains the same for the PTFE base, but the glass fibers can be attacked by alkali and hydrofluoric acid. The temperature range for the glass filled PTFE is –4500° to +5500°F.
Carbon can also be added to PTFE, normally within a percentage range of 10 to 35%. In most applications, graphite is also added to increase lubricity. The electrical properties are altered but the consistent chemical resistance of PTFE remains the same. Since the mechanical properties improve so does the wear resistance of the material. The improved wear resistance makes this a good choice for most dry and wet applications. The temperature range for the carbon filled PTFE is normally –3200° to +5000°F.
When bronze powder is blended with PTFE it is normally in percentages ranging from 40 to 60%. Wear resistance is significantly improved along with a higher compressive strength and a lower creep value. The bronze within the PTFE is also a good thermal conductor making it a good choice in high abrasive applications. The drawback to this material is the decrease in chemical resistance and the electrical properties. The bronze filler is subject to corrosive environments.
Since graphite is a solid lubricant, it is sometimes added to PTFE, along with other fillers to lower the coefficient of friction. Frictional and wear properties are improved when the graphite is blended with other fillers within PTFE. When graphite is combined with another filler, it exhibits a good chemical resistance in most corrosive environments.
Molybdenum disulfide, commonly known as “moly” when discussed as a filler, improves surface hardness of PTFE. It decreases the coefficient of friction and is normally added to post-filled PTFE when the original filler makes the surface abrasive. It also has little to no effect on the chemical and electrical properties.
A variety of other fillers also exist, such as calcium fluoride, mica, stainless steel just to name a few. The blending capabilities are endless as long as the required application deems it necessary. The diversity found in PTFE, from the virgin grades thru the filled grades, offer unique characteristics not found in other materials, making it one of the most valued discoveries of our time.
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