FKM is the designation for about 80% of fluorinated elastomers 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 (Fluorel) and Solvay-Solexis (Tecnoflon).
Despite the higher prices compared to other kinds of elastomers, fluoroelastomers are used when superior heat and chemical resistance is needed.
FKMs can be divided into different classes on the basis of either their chemical composition, their fluorine content or their crosslinking mechanism.
Types of FKM
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, HFP, TFE, 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.
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.
- Bisphenol AF crosslinking (dihydroxy crosslinking) was the next step in curing FKMs. It provides superior heat resistance, improved hydrolytic stability and better compression set than diamine curing. In contrast to diamine curing the bisphenolic mechanism is not an addition mechanism but an aromatic nucleophilic substitution. Typically quarternary phosphonium salts are used to accelerate the curing process.
- Peroxide crosslinking (triazine 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.