FKM and FFKM share many similarities. Understanding the differences is critical in helping determine the appropriate sealing solution for your application.
FKM is the ASTM designation for a class of fluorinated, carbon-based synthetic rubber, commonly known as fluoroelastomer. FKM has impressive heat resistance, allowing FKM seals to withstand temperatures greater than 200°C. FKM also exhibits extraordinary levels of resistance to high pressures, chemicals and other fluids (including several fuels).
FKM was originally developed in the late 1950s in response to demand for high performance seals in the aerospace industry. Development of FKMs continued through the 1980s, with advancements including greater thermal stability and improved heat, solvent, and compression resistance.
Today, FKM materials are commonly used to manufacture o-rings, seals, and gaskets for a variety of high-performance applications in the automotive, aerospace, energy, semiconductor, and industrial industries.
FFKM, or perfluoroelastomer, contains higher amounts of fluorine than standard FKM, and features higher temperature ratings, up to approximately 325°C. FFKM also has improved chemical resistance, with nearly universal chemical compatibility. This combination of high-performance capabilities makes FFKM seals the premium choice for the most challenging applications.
The first commercially available FFKM seal was produced in the late 1960s. However, widespread manufacturing of FFKM materials did not occur until the late 1980s due to patent restrictions.
FFKM is used in o-rings and seals in environments with high temperatures and/or harsh chemicals in the aerospace, semiconductor, energy, pharmaceutical and industrial industries.
FKM vs. FFKM Comparison
See a comparison chart for other material compounds here.
FKM and FFKM Solutions by Greene, Tweed
Greene, Tweed offers a broad range of FKM and FFKM sealing solutions to meet the specific demands of the industries and applications we serve.
Greene, Tweed’s proprietary FFKM material, Chemraz®, has the broadest chemical resistance of any elastomeric material, combining the resilience and sealing force of an elastomer with chemical resistance approaching that of PTFE.
| Material/Characteristic | FKM | FFKM |
|---|---|---|
| Estimated temperature range (°C/°F) | -20 - 250 (-4 to 482) | -40 - 325 (-40 to 617) |
| Heat resistance (ASTM D2000) (°C/°F) | 225/437 (Type G) | 250/462 (Type H) |
| Elongation @ break, % (ranges) | 70 – 300 | 100 – 300 |
| Compression set | 15-25% (on 214 o-ring after 70 hr @ 200°C) | 15-25% (on o-ring after 70 hr @ 200°C); better at higher temperatures: 15-30% (on o-ring after 70 hr @ 300°C) |
| Hardness range (Shore A) | 60 to 95 | 65 to 95 |
| Tensile strength range (psi/MPa) | 1,500/1.03 to 4,500/3.10 | 1,250/8.61 to 3,600/24.8 |
| Specific gravity | 1.8 to 2.0 | 1.9 to 2.2 |
| Fluorine content of polymer (%) | 66 - 70% | 71 - 73% |
| Chemical resistance | Broad range of chemicals: ga-soline, alcohol, fuel additives, acids, hydrocarbons, water, oils, oxidation, ozone, radiation | Broadest range of chemical resistance |
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Note: Characteristic metrics are provided as generally accepted industry ranges. Actual ranges may vary, depending on specific industry applications, such as additives, formulations, curatives used, etc.