Understanding Fluoroelastomers (FKM/Viton): High-Performance Sealing

What are Fluoroelastomers (FKM)?

Fluoroelastomers (FKM) represent the premium tier of elastomeric sealing materials. Originally developed by DuPont in 1957 under the brand name Viton®, these synthetic rubber compounds contain fluorine atoms bonded to the polymer backbone, giving them exceptional resistance to heat, chemicals, and fuels that far exceeds conventional elastomers. Today, FKM compounds are manufactured by multiple companies including Chemours (Viton®), Daikin (Dai-El®), Solvay (Tecnoflon®), and 3M (Dyneon®).

Types of Fluoroelastomers

FKM materials are classified by their fluorine content and monomer composition, each offering different performance characteristics:

Type A (Dipolymer - 66% Fluorine)

The most widely used type, copolymers of vinylidene fluoride (VF2) and hexafluoropropylene (HFP). Offers excellent resistance to hydrocarbon fuels, mineral oils, and many industrial chemicals. Standard operating temperature range of -20°C to +200°C.

Type B (Terpolymer - 68% Fluorine)

Terpolymers incorporating tetrafluoroethylene (TFE) in addition to VF2 and HFP. Higher fluorine content provides improved chemical resistance, particularly to aromatic solvents, chlorinated hydrocarbons, and fuels containing high levels of aromatics.

Type F (Specialty - 70% Fluorine)

The highest fluorine content commercially available in standard FKM, offering the broadest chemical resistance. Excellent with aggressive fuels, low-molecular-weight esters, and ketone/aldehyde mixtures. More expensive than Types A and B.

Temperature Performance

FKM's heat resistance is one of its defining characteristics. Standard compounds operate continuously at 200°C (392°F) with intermittent exposure capability up to 250°C (482°F). This makes FKM the material of choice for high-temperature sealing in engines, exhaust systems, and industrial processing equipment.

Low-temperature performance has historically been FKM's weakness, with standard grades stiffening below -15°C. However, modern specialty grades have extended this to -40°C, making FKM viable for applications previously requiring other materials.

Chemical Resistance

The carbon-fluorine bond is one of the strongest in organic chemistry, and this directly translates to FKM's remarkable chemical resistance:

• Excellent: Mineral acids, petroleum oils, aromatic and aliphatic hydrocarbons, chlorinated solvents, diesel fuel, gasoline
• Good: Dilute bases, silicone fluids, synthetic lubricants
• Poor: Low-molecular-weight ketones (acetone, MEK), low-molecular-weight esters (ethyl acetate), amines, anhydrous ammonia, hot water/steam, phosphate ester hydraulic fluids

Applications

Aerospace

FKM is extensively used in aircraft fuel systems, engine seals, and hydraulic systems where exposure to Jet A fuel, Skydrol hydraulic fluid, and extreme temperatures is routine. AMS (Aerospace Material Specifications) documents define specific FKM compounds for various aerospace applications.

Automotive

Modern engines with turbocharging and emissions controls operate at higher temperatures than ever before, driving increased adoption of FKM for crankshaft seals, valve stem seals, fuel injector O-rings, and EGR system seals.

Chemical Processing

In chemical plants, FKM seals are used in pumps, valves, and reactors handling aggressive chemicals at elevated temperatures. The material's ability to resist a broad spectrum of chemicals makes it a go-to choice when NBR or EPDM fall short.

Semiconductor Manufacturing

Ultra-clean FKM compounds are specified for semiconductor fabrication equipment where the seals must withstand aggressive cleaning chemicals while maintaining extremely low outgassing and particle generation.

Cost Considerations

FKM typically costs 5-10 times more than NBR, which is why it should only be specified when its superior properties are genuinely required. For applications where NBR or EPDM can meet the performance requirements, those materials offer better value. However, when considering total cost of ownership — including reduced downtime, longer seal life, and fewer replacements — FKM often provides the best economic solution for demanding applications.

FFKM: The Ultimate Perfluoroelastomer

For the most extreme applications, perfluoroelastomers (FFKM) such as Kalrez® and Chemraz® offer near-universal chemical resistance approaching that of PTFE, combined with the resilience of a true elastomer. FFKM can operate continuously at 300°C+ and resists virtually all chemicals except molten alkali metals and elemental fluorine. The trade-off is cost — FFKM seals can be 50-100 times more expensive than equivalent FKM parts.