ASTM D1418, ISO 1629 Designation: FKM
ASTM D2000, SAE J200 Type/Class: HK
Standard Colors: Black, Brown, Green
• DAI-EL® (Daikin Industries)
• Dyneon®, previously Fluorel® (Dyneon)
• Tecnoflon® (Solvay Solexis)
• Viton® (DuPont Dow Elastomers)
Relative Cost: High
General Temperature Range: -15° to +400° F
Also referred to as fluoroelastomers, fluorocarbon compounds are thermoset elastomers containing fluorine. Fluorocarbons make excellent general-purpose seals due to their exceptional resistance to chemicals, oil, and temperature extremes (-15° F to +400° F). Specialty compounds can further extend the low temperature limit down to about -22° F for dynamic seals and about -40° F in static applications. Fluorocarbons usually have good compression set resistance, low gas permeability, and resistance to ozone and sunlight. Over the last five decades, this remarkable combination of properties has prompted the use of fluorocarbon seals in a variety of demanding sectors. Though they were initially formulated for use in aerospace applications, FKM compounds are now widely used in the automotive, appliance, fluid power, and chemical processing industries.
Three main factors contribute to the remarkable heat and fluids resistance of fluorocarbon compounds. First, there are extremely strong bonds between the carbon atoms comprising the polymer backbone and the attached (pendant) fluorine atoms. Under most circumstances, these bonds cannot be broken, and thus the polymer is not prone to undergo chain scission (division of the macromolecular chains into smaller, weaker, more susceptible segments).
Second, fluorocarbons feature a high fluorine-to-hydrogen ratio. In other words, fluorine (rather than hydrogen) atoms fulfill the majority of the available bonds along the material’s carbon backbone. Polymers with a high level of fluorination have proven to be extremely stable. A stable compound is less inclined to react to, or be broken down by, its environment.
Third, the carbon backbone is fully saturated. That is, it contains only single bonds between the carbon atoms. It does not contain any of the covalent double bonds present in unsaturated compounds. Since double bonds are the focus for chemical attack, the saturated structure of fluorocarbons renders them impervious to harmful agents (such as oxygen, ozone, and UV light) that typically degrade unsaturated materials.
FKM performs well in:
- Aircraft engine applications
- Gasoline (and alcohol blends)
- Hard vacuum applications
- Petroleum products
- Silicone fluids and greases
FKM does not perform well in:
- Hot chlorosulfonic acid
- Hot hydrofluoric acid
- Hydrocarbons (nitro)
- Low molecular weight esters and ethers
- Fireproof hydraulic fluids (e.g. Skydrol)
Depending on the specific needs of your application, there are a number of different fluorocarbon formulations available for use. Though they may share some common characteristics, these different types are distinguished by their processing and end-use properties. Perhaps the most well-known fluorocarbon manufacturer is DuPont Dow Elastomers, to the point that the trade name for their compound, Viton®, is often used as if it was a generic term for FKM. In the interests of simplicity, the following descriptions of some of the most common FKM formulations will make use of the DuPont "type" names.
The original commercial fluorocarbon, Viton A, is the general-purpose type and is still the most widely used. It is a copolymer of vinylidene fluoride (VF2) and hexafluoropropylene (HFP). Generally composed of 66% fluorine, Viton A compounds offer excellent resistance against many automotive and aviation fuels, as well as both aliphatic and aromatic hydrocarbon process fluids and chemicals. Viton A compounds are also resistant to engine lubricating oils, aqueous fluids, steam, and mineral acids.
Viton B fluorocarbons are terpolymers combining tetrafluoroethylene (TFE) with VF2 and HFP. Depending on the exact formulation, the TFE partially replaces either the VF2 (which raises the fluorine level to approximately 68%) or the HFP (keeping the fluorine level steady at 66%). Viton B compounds offer better fluids resistance than the Viton A copolymers.
Viton GF fluorocarbons are tetrapolymers composed of TFE, VF2, HFP, and small amounts of a cure site monomer (Csm). Presence of the cure site monomer allows peroxide curing of the compound, which is normally 70% fluorine. As the most fluid resistant of the various FKM types, Viton GF compounds offer improved resistance to water, steam, and acids.
Viton GFLT fluorocarbons are similar to Viton GF, except that perfluoromethylvinyl ether (PMVE) is used in place of HFP. The "LT" in Viton GFLT stands for "low temperature." The combination of VF2, PMVE, TFE, and a cure site monomer is designed to retain both the superior chemical resistance and high heat resistance of the G-series fluorocarbons. In addition, Viton GFLT compounds (typically 67% fluorine) offer the lowest swell and the best low temperature properties of the types discussed here. Viton GFLT can seal in a static situation down to approximately -40° F. A brittle point of -50° F can be achieved through careful compounding.