FKM Fluoroelastomer
Fluorocarbon Rubber | FPM (ISO/DIN)
FKM is specified when no standard elastomer can survive the operating environment: when temperature, chemical aggressiveness, or both exceed what nitrile, EPDM, neoprene, or silicone can sustain. Its fluorine-rich polymer backbone provides resistance to petroleum fuels, aggressive solvents, concentrated acids, ozone, and continuous temperatures to 400°F that hydrocarbon-based elastomers cannot approach.
Wayne Rubber manufactures custom FKM seals, gaskets, o-rings, and molded components from Wixom, Michigan. FKM is the generic ASTM D1418 designation; “Viton” is a registered trademark of Chemours. Wayne Rubber does not use proprietary trade names in specifications; FKM grade type (A, B, F, GF, GLT) and fluorine content are the correct specification parameters.
The Chemistry Behind FKM’s Performance
The carbon-fluorine bond is one of the strongest single bonds in organic chemistry, stronger than the carbon-hydrogen bonds in hydrocarbon rubbers, and substantially stronger than the carbon-chlorine bonds in neoprene. FKM’s polymer backbone is composed predominantly of carbon-fluorine bonds, which gives it two fundamental properties that no hydrocarbon elastomer can replicate: thermal stability and chemical inertness.
Thermal stability means the C-F bond does not break down at temperatures that degrade C-H and C-Cl bonds in other elastomers. Standard FKM grades maintain their mechanical properties and sealing performance at continuous temperatures up to 400°F (204°C). Specialty grades extend this to 500°F (260°C) for short-term exposure. This is approximately 150°F higher than the continuous limit of nitrile and 100°F above neoprene.
Chemical inertness means fluorine’s high electronegativity repels polar and non-polar solvents, acids, and oxidizing agents that attack other elastomers. Petroleum oils and fuels that swell EPDM, aromatic solvents that swell nitrile and neoprene, and concentrated acids that attack most organic polymers all have limited effect on the FKM backbone. The fluorine atoms essentially shield the polymer chain from chemical attack.
The fluorine content of an FKM compound (typically 66% to 71% by weight depending on grade type) is the primary driver of its chemical resistance. Higher fluorine content generally provides better resistance to aggressive solvents and fuels, at the cost of reduced low-temperature flexibility. This is the central trade-off in FKM grade selection, analogous to the acrylonitrile content trade-off in nitrile rubber.
FKM’s high density (approximately 1.8 to 1.9 g/cm³, compared to 1.0 to 1.2 g/cm³ for most hydrocarbon rubbers) is a direct consequence of its fluorine content and is a useful field identification characteristic. An FKM o-ring feels noticeably heavier than an NBR or EPDM o-ring of the same dimensions.
The cost of FKM reflects both the manufacturing complexity of fluoromonomer production and the high fluorine content. FKM compounds are typically 5 to 15 times the cost of equivalent nitrile or EPDM compounds. This cost premium is justified where the application genuinely requires FKM’s capabilities; it is not justified as a “safe default” for applications that standard compounds could handle. Correct compound selection saves cost without compromising performance.
FKM Grade Types: A, B, F, GF, and GLT
FKM is not a single compound; it is a family of fluoroelastomers with different monomer compositions, fluorine contents, and performance profiles. The grade type system (originally defined by Chemours for the Viton brand and widely adopted generically) describes the polymer composition and determines which specific chemical environments and temperature ranges the compound handles. Specifying “FKM” without a grade type leaves the compound selection to the manufacturer and may result in a grade inadequate for the application’s specific chemical exposure.
The most widely used FKM type. Composed of vinylidene fluoride (VDF) and hexafluoropropylene (HFP). Good overall chemical and heat resistance for most standard FKM applications. The default grade for fuel system seals, general petroleum chemical service, and hot oil sealing to 400°F. Limited resistance to methanol-blended fuels and polar solvents.
Adds tetrafluoroethylene (TFE) to the Type A copolymer, raising fluorine content and improving resistance to aggressive fuels, aromatic solvents, and acids relative to Type A. Used in more demanding chemical processing applications and fuels with higher aromatic content. Slightly improved low-temperature performance over Type A in some formulations.
Higher fluorine content than Type B, providing significantly improved resistance to aggressive organic acids (formic, acetic), amines, and some ketones that degrade lower-fluorine grades. The correct specification when chemical resistance at the FKM limit is the primary driver. Used in pharmaceutical, specialty chemical, and demanding solvent service where Type A/B swell is unacceptable.
Incorporates perfluoromethylvinylether monomer, which dramatically improves low-temperature flexibility relative to standard FKM grades without sacrificing chemical resistance. Service to approximately -40°F (-40°C), a significant improvement over the -15°F limit of standard Type A. Used in cold-climate automotive applications, outdoor fuel system seals, and aerospace applications requiring both low-temperature flexibility and broad chemical resistance.
Specialty formulation providing FKM-level chemical resistance with service to approximately -40°F (-40°C) or below. The correct FKM grade for arctic-climate applications, cold-start automotive sealing, and cryogenic-adjacent environments where standard FKM would harden and lose sealing contact at the minimum operating temperature. Higher cost than standard FKM grades; specify when the low-temperature requirement is genuine.
Not technically FKM, but the natural upgrade path when FKM’s chemical or temperature limits are exceeded. Near-universal chemical resistance (ketones, amines, steam) and continuous service to 600°F. Cost is substantially higher than FKM. See the FFKM material page for full details. Specify only when FKM genuinely cannot meet the requirements.
FKM Cure Systems and Post-Cure
FKM requires a two-stage cure process: an initial press cure (vulcanization) during molding, followed by a post-cure in an oven at elevated temperature. The post-cure is not optional; it is required to achieve the published compression set, tensile strength, and chemical resistance values. FKM parts that have not been properly post-cured will have significantly higher compression set and lower chemical resistance than specification, which is a common source of premature seal failure when purchasing from suppliers who skip or shorten the post-cure step.
The standard cure system for most FKM applications. Provides the best high-temperature compression set resistance and heat aging properties of the three systems. Used for the majority of Type A, B, and F grade compounds. Requires the presence of magnesium oxide and calcium hydroxide as acid acceptors in the compound. Not recommended for steam service or aqueous environments where the ionic crosslinks can hydrolyze; peroxide cure is preferred for those conditions.
Peroxide crosslinks are more hydrolytically stable than bisphenol ionic crosslinks, making peroxide-cured FKM the correct specification for steam, hot water, and aqueous acid environments where bisphenol-cured FKM would degrade. Required for Type GF and GLT compounds. Slightly lower heat resistance than bisphenol cure at very high temperatures but superior in wet chemical service. FKM cured with peroxide provides significantly better resistance to aqueous media than bisphenol-cured grades.
The original FKM cure chemistry, now rarely used in new compound development. Provides superior rubber-to-metal bonding properties compared to bisphenol and peroxide cures, historically used for FKM bonded assemblies. Diamine crosslinks are vulnerable to hydrolysis and have lower heat resistance than bisphenol cure. Primarily encountered in legacy specifications; most new bonded FKM applications use peroxide or bisphenol cure with appropriate adhesive systems.
FKM Chemical Resistance: What It Handles and What It Does Not
FKM’s chemical resistance is broader than any other commodity elastomer, but it has specific, well-defined failure modes. The most common and most costly specification errors involve exposing FKM to the fluids it cannot resist: particularly steam, ketones, and amines, under the incorrect assumption that “FKM is resistant to everything.”
| Fluid / Environment | Resistance | Notes |
|---|---|---|
| Petroleum Oils and Fuels | Excellent | All FKM grade types provide excellent resistance to mineral oils, hydraulic fluids, gasoline, diesel, jet fuel, and high-aromatic-content fuels that attack standard nitrile. Type GF and GLT grades maintain performance in modern biofuel blends including high-ethanol content fuels. |
| Concentrated Acids (mineral) | Excellent | Excellent resistance to concentrated sulfuric, nitric, and hydrochloric acids, far superior to nitrile and EPDM. Type F grades provide the best acid resistance of the FKM family, and the correct specification for chemical plant seals in concentrated inorganic acid service. |
| Aromatic Solvents (toluene, xylene) | Excellent | FKM is resistant to aromatic hydrocarbons that severely swell nitrile and neoprene. Correct specification for fuel systems with high aromatic content and for chemical processing with aromatic solvent contact. |
| Ozone, UV, Weather | Excellent | FKM has outstanding resistance to ozone, UV, and outdoor weathering, better than EPDM in most conditions. Long outdoor service life without surface degradation. |
| Steam and Hot Water | Poor (standard grades) | This is FKM’s most important and most frequently overlooked incompatibility. Standard bisphenol-cured FKM undergoes dehydrofluorination in continuous steam service; the fluorine atoms are stripped from the backbone, releasing hydrofluoric acid and destroying the compound. EPDM is the correct specification for steam service. Peroxide-cured FKM grades have somewhat improved steam resistance but are still not appropriate for continuous steam. This failure mode is potentially hazardous due to HF release. |
| Ketones (acetone, MEK, MIBK) | Poor | Ketones attack standard FKM grades significantly. This is the most common chemical specification error with FKM: engineers specify FKM as a “premium” material and then expose it to ketone-based cleaning solvents or process fluids. EPDM has good ketone resistance. FFKM (perfluoroelastomer) is required for true ketone compatibility alongside high-temperature service. |
| Low-Molecular-Weight Organic Acids | Poor (standard grades) | Formic acid and acetic acid attack standard Type A FKM despite FKM’s excellent resistance to concentrated inorganic acids. Type F and GF grades have improved resistance to these short-chain organic acids. Verify the specific organic acid against the Chemical Resistance Guide before specifying any FKM grade for organic acid service. |
| Amines (primary and secondary) | Poor | Amines are highly reactive with standard FKM, causing rapid degradation. Applications involving amine-based corrosion inhibitors, amine scrubbers, or amine-catalyzed reactions require FFKM or EPDM (for non-petroleum amine service). This is a common failure mode in chemical plant applications where amine-based process additives are used. |
| Skydrol and Phosphate Ester Hydraulic Fluids | Poor | Phosphate ester hydraulic fluids (Skydrol, Pydraul, HyJet) attack standard FKM. EPDM is the correct specification for phosphate ester hydraulic fluid service. This is a critical aviation maintenance point; FKM is commonly used in aircraft hydraulic systems with mineral-oil hydraulic fluid and must not be used when the system is converted to phosphate ester fluid. |
| Low-Temperature Flexibility | Limited (standard grades) | Standard Type A FKM becomes rigid and loses sealing contact below approximately 15°F (-9°C). For cold-climate applications, specify Type GF or GLT grades. This is a frequently overlooked limitation in outdoor equipment that uses FKM for its chemical resistance but must also seal at low ambient temperatures during cold-start conditions. |
When to Specify FKM and When Not To
- Continuous service temperature exceeds 250°F where nitrile reaches its limit
- Petroleum fuel system with aromatic content that causes unacceptable swell in high-ACN nitrile
- Concentrated inorganic acid contact (sulfuric, nitric, hydrochloric) at any temperature
- Simultaneous high temperature and aggressive chemical exposure that neither nitrile nor EPDM can survive
- Aromatic solvent service (toluene, xylene, benzene) where neoprene and nitrile fail
- High-pressure gas sealing where low gas permeability is required alongside chemical resistance
- UV and ozone resistance are required alongside chemical resistance that EPDM cannot provide
- The service fluid is steam or hot water: use EPDM
- The process involves ketones (acetone, MEK): use EPDM or FFKM
- The service involves primary or secondary amines: use EPDM or FFKM
- The fluid is phosphate ester hydraulic fluid (Skydrol): use EPDM
- The application is within nitrile’s chemical and temperature capability; FKM cost is not justified
- The operating temperature is below 15°F without specifying a GF or GLT grade
- The service involves formic or acetic acid without verifying Type F or GF grade compatibility
FKM Fluoroelastomer Applications
FKM Fluoroelastomer Products from Wayne Rubber
Wayne Rubber manufactures custom FKM components in Type A, B, F, and GF compound grades via compression, transfer, and injection molding. Each page below covers FKM-specific specifications, grade options, and design guidance for that product form.
Request Custom FKM Parts from Wayne Rubber
Provide your part drawing, operating temperature, fluid exposure (including any cleaning agents and secondary fluids), and any low-temperature requirements. Wayne Rubber will recommend the correct FKM grade type and cure system and quote.