FKM (by ASTM D1418 standard) (equivalent to FPM by ISO/DIN 1629 standard) is the designation for about 80% of fluoroelastomers as defined in ASTM D1418. Other fluorinated elastomers are perfluoro-elastomers (FFKM) and tetrafluoro ethylene/propylene rubbers (FEPM). All FKMs contain vinylidene fluoride as a monomer. Originally developed by DuPont (Viton), FKMs are today also produced by Daikin Chemical (Dai-El), 3M's Dyneon (Dyneon Fluoroelastomers), Solvay Specialty Polymers (Tecnoflon) and HaloPolymer (Elaftor). Fluoroelastomers are more expensive than neoprene or nitrile rubber elastomers. They provide additional heat and chemical resistance. FKMs can be divided into different classes on the basis of either their chemical composition, their fluorine content or their crosslinking mechanism.
On the basis of their chemical composition FKMs can be divided into the following types:
- Type 1 FKMs are composed of vinylidene fluoride (VDF) and hexafluoropropylene (HFP). Copolymers are the standard type of FKMs showing a good overall performance. Their fluorine content typically ranges around 66 weight percent.
- Type 2 FKMs are composed of VDF, HFP, and tetrafluoroethylene (TFE). Terpolymers have a higher fluorine content compared to copolymers (typically between 68 and 69 weight percent fluorine), which results in better chemical and heat resistance. Compression set and low temperature flexibility may be affected negatively.
- Type 3 FKMs are composed of VDF, TFE, and perfluoromethylvinylether (PMVE). The addition of PMVE provides better low temperature flexibility compared to copolymers and terpolymers. Typically the fluorine content of type 3 FKMs ranges from 62 to 68 weight percent.
- Type 4 FKMs are composed of propylene, TFE, and VDF. While base resistance is increased in type 4 FKMs, their swelling properties especially in hydrocarbons are worsened. Typically they have a fluorine content of about 67 weight percent.
- Type 5 FKMs are composed of VDF, HFP, TFE, PMVE, and Ethylene. Type 5 FKM is known for base resistance and high temperature hydrogen sulfide resistance.
A useful overview of different types of FKM-compounds is listed here.
FFKMs (by ASTM 1418 standard) (equivalent to FFPMs by ISO/DIN 1629 standard) are perfluoroelastomeric compounds containing an even higher amount of fluoride than FKM. They have further improved resistance to high temperatures and chemicals and even withstand environments where Oxygen-Plasma are present for many hours. Certain grades have a maximum continuous service temperature of 327 °C (621 °F). They are commonly used to make O-rings and gaskets that are used in applications that involve contact with hydrocarbons or highly corrosive fluids, or when a wide range of temperatures is encountered.
For vacuum applications, demanding very low contamination (out-gassing and particle emission) as well as high temperature operation (200 - 300 °C) for prolonged out-baking or processing times and where a copper or metal sealing is not possible or very inconvenient/expensive, a custom-made, clean-room manufactured, sealing such as Kalrez 9100 or SCVBR can be used. After manufacturing, they are O-plasma vacuum cleaned (and/or vacuum baked) to reach out-gassing performance similar to Teflon while reaching vacuum leak tightness (permeability rates) similar to FKM (Viton) compounds. This combination of properties allows FFKM seals to reach well into UHV pressures without the use of metal sealing. However, they are significantly more expensive than standard FKM O-rings.
There are three established crosslinking mechanisms used in the curing process of FKMs.
- Diamine crosslinking using a blocked diamine. In the presence of basic media VDF is vulnerable to dehydrofluorination which enables the addition of the diamine to the polymer chain. Typically magnesium oxide is used to catch the resulting hydrofluoric acid and rearrange into magnesium fluoride and water. Although rarely used today, diamine curing provides superior rubber-to-metal bonding properties as compared with other crosslinking mechanisms. The diamine's capability to be hydrated makes the diamine crosslink vulnerable in aqueous media.
- Ionic crosslinking (dihydroxy crosslinking) was the next step in curing FKMs. This is the most common crosslinking chemistry used for FKMs. It provides superior heat resistance, improved hydrolytic stability and better compression set than diamine curing. In contrast to diamine curing the ionic mechanism is not an addition mechanism but an aromatic nucleophilic substitution. Dihydroxy aromatic compounds are used as the crosslinking agent and quaternary phosphonium salts are typically used to accelerate the curing process.
- Peroxide crosslinking was originally developed for type 3 FKMs containing PMVE as diamine and bisphenolic crosslinking systems can lead to cleavage in a polymer backbone containing PMVE. While diamine and bisphenolic crosslinking are ionic reactions, peroxide crosslinking is a free radical mechanism. Though peroxide crosslinks are not as thermally stable as bisphenolic crosslinks, they normally are the system of choice in aqueous and nonaqueous electrolytes.