Polytetrafluoroethylene (PTFE) is a potent polymer that is used in an incredible variety of applications. Its popularity, particularly as a seal or sealant, is largely due to its resilience. Whether exposed to extreme heat, cold, or pressure, PTFE retains its mechanical properties and remains relatively inert.
The critical difference between PTFE and other sealing materials, such as elastomers or polyurethane, is that it is a fluoropolymer, using fluorine instead of hydrogen in the polymer chain. It is the strong bond between fluorine and carbon that drives the advantageous thermal and chemical properties of PTFE, allowing the manufacture of durable seals.
Properties and Sealing Applications of PTFE
PTFE can tolerate extreme temperatures, from -200o C to 260o C, with a melting point as high as 327o C. Additionally, it has a very low coefficient of friction, reducing potential frictional heating in dynamic applications. While these properties of “virgin” PTFE deliver reliable performance for many types of seals, the addition of filler creates different grades of PTFE for more specific uses.
- Carbon Fiber Filled: Carbon grants greater mechanical strength and thermal conductivity to PTFE.
- Graphite Filled: Graphite conveys self-lubricating properties and keeps a seal’s coefficient of friction low. This grade of PTFE is commonly used in non-lubricated applications, such as compressor piston rings.
- Carbon-Graphite Filled: The best of both worlds, combining the mechanical and thermal advantages of carbon fiber with the self-lubricating properties of graphite. It is used for piston rings, rider rings, and rod packing in industrial compressors.
- Bronze Filled: Bronze provides excellent electrical and thermal conductivity, well-suited to high-pressure environments that start and stop frequently, such as hydraulic piston rings. The bronze filler can be substituted with other metals for slightly different material properties.
- Stainless Steel Filled: Steel makes an abrasive grade of PTFE, but excels in static applications where high loads are a greater concern. Its high wear resistance makes it useful in products like flow control valves.
Choosing PTFE for an Application
A long list of other fillers can be used in combination with PTFE if the filler can maintain its properties through the high-temperature curing process. Manufacturers provide numerous grades of PTFE with properties tailored to common applications.