ASTM D1418 Designation: FKM

ASTM D 2000, SAE J200 Type / Class: HK

RELATIVE COST: High

GENERAL TEMPERATURE RANGE: -15° to +300° F

Fluoroelastomers (FKM) are thermoset elastomers containing fluorine (see Figure 37). Fluoroelastomers make excellent general-purpose seals due to their exceptional resistance to chemicals, oil, and temperature extremes (-15° to +300° F). Specialty compounds can further extend the low temperature limit down to about -25° F for dynamic seals.

Fluoroelastomers usually have good compression set resistance, good oil resistance, and resistance to ozone and sunlight. FKM compounds are widely used in the automotive, appliance, fluid power, and chemical processing industries.

Three main factors contribute to the remarkable heat (see Table 6) and fluid resistance of fluoroelastomers. 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, fluoroelastomers 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 either 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 fluoroelastomers renders them impervious to harmful agents (such as oxygen, ozone, and UV light) that typically degrade unsaturated materials.

FKM PERFORMS WELL IN:
• Petroleum oils & fuels
• Acids
• Aircraft engine applications
• Hard vacuum applications
• Silicone oils & greases
• Solvents

FKM DOES NOT PERFORM WELL IN:
• Amines
• Methanol
• Hydrocarbons (nitro)
• Ketones
• Low molecular weight esters & ethers
• Fireproof hydraulic fluids (e.g. Skydrol®)

Depending on the needs of your application, there are a number of different fluoroelastomer formulations available. Though they may share some common characteristics, these different types are distinguished by their processing and end-use properties. The best-known fluoroelastomer manufacturer is DuPont Dow Elastomers; the trade name for their compound, Viton®, is often used as if it were 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 fluoroelastomer, 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 fluoroelastomers are terpolymers combining tetrafluoroethylene (TFE) with VF2 and HFP. Depending on the formulation, the TFE partially replaces either the VF2 (raising the fluorine level to about 68%) or the HFP (keeping the fluorine level steady at 66%). Viton B compounds offer better fluids resistance than Viton A copolymers.

Viton F fluoroelastomers are terpolymers combining TFE, VF2, and HFP (as in Viton B), except the fluorine level is approximately 69%. Viton F compounds have the best fluid resistance of the various Viton types, but Viton F compounds (and Viton GF compounds, see below) also have the poorest low temperature properties of the various Viton types.

Viton GF fluoroelastomers are tetrapolymers composed of TFE, VF2, HFP, and small amounts of a cure site monomer. 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. As mentioned above, Viton F and GF compounds have the poorest low temperature properties of the Viton types.

Viton GLT fluoroelastomers are engineered to combine the heat and chemical resistance of general Viton types (like A, B, and F) with improved low temperature properties. The “LT” in Viton GLT stands for “low temperature.” The glass transition temperature (Tg) is generally accepted as an indicator of low temperature capabilities; the Tg of Viton GLT is typically 8° to 12° C lower than that for the general-use Viton types.

Viton GFLT fluoroelastomers combine VF2, perfluoromethylvinyl ether (PMVE), TFE, and a cure site monomer. The result retains both the superior chemical resistance and high heat resistance of the GF-series fluoroelastomers. Viton GFLT compounds (typically 67% fluorine) also offer the lowest swell and the best low temperature properties of the types discussed here (see Table 7). As with Viton GLT, the “LT” in Viton GFLT stands for “low temperature.” Table 8 compares both the low temperature flexibility and fuel swell for several Viton formulations.

“FKM makes excellent general purpose seals due to its exceptional resistance to chemicals, oil, and temperature extremes.”

Figure 37

Table 6

Table 7

Table 8