FKM is a family offluorocarbon-basedfluoroelastomer materials defined byASTM International standard D1418,[1] andISO standard 1629.[2] It is commonly calledfluorine rubber orfluoro-rubber. FKM is an abbreviation of Fluorine Kautschuk Material.[3] All FKMs containvinylidene fluoride as the commonmonomer, to which different other monomers are added for specific types and functionalities, fitting the desired application.
Originally developed byDuPont (under the brand name Viton, now owned byChemours), FKMs are today also produced by many other companies, including:Daikin (Dai-El),[4]3M (Dyneon),[5]Solvay S.A. (Tecnoflon),[6]HaloPolymer (Elaftor),[7]Gujarat Fluorochemicals (Fluonox),[8] and several Chinese manufacturers. Fluoroelastomers are more expensive thanneoprene ornitrile rubberelastomers. They provide additional heat and chemical resistance.[9] FKMs can be divided into different classes on the basis of either their chemical composition, theirfluorine content, or theircross-linking mechanism.
On the basis of their chemical composition FKMs can be divided into the following types:
Type-1 FKMs are composed ofvinylidene fluoride (VDF) andhexafluoropropylene (HFP).Copolymers are the standard type of FKMs showing a good overall performance. Their fluorine content is approximately 66 weight percent.
Type-2 FKMs are composed of VDF, HFP, andtetrafluoroethylene (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, andperfluoromethylvinylether (PMVE). The addition of PMVE provides better low temperature flexibility compared to copolymers andterpolymers. Typically, the fluorine content of type-3 FKMs ranges from 62 to 68 weight percent.
Type-4 FKMs are composed ofpropylene, TFE, and VDF. While base resistance is increased in type-4 FKMs, their swelling properties, especially inhydrocarbons, are worsened. Typically, they have a fluorine content of about 67 weight percent.
Type-5 FKMs are composed of VDF, HFP, TFE, PMVE, andethylene. Known forbase resistance and high-temperature resistance tohydrogen sulfide.[10]
There are three establishedcross-linking mechanisms used in the curing process of FKMs.
Diamine cross-linking using a blocked diamine. In the presence of basic (alkaline) media, VDF is vulnerable todehydrofluorination, which enables the addition of the diamine to the polymer chain. Typically,magnesium oxide is used to neutralize the resultinghydrofluoric acid and rearrange intomagnesium fluoride and water. Although rarely used today, diamine curing provides superior rubber-to-metalbonding properties as compared with other cross-linking mechanisms. The diamine's capability to be hydrated makes the diamine cross-link vulnerable in aqueous media.
Ionic cross-linking (dihydroxy cross-linking) was the next step in curing FKMs. This is today the most common cross-linking 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 aromaticnucleophilic substitution. Dihydroxyaromatic compounds are used as the cross-linking agent, andquaternaryphosphonium salts are typically used to accelerate the curing process.
Peroxide cross-linking was originally developed for type 3 FKMs containing PMVE as diamine andbisphenolic cross-linking systems can lead tocleavage in a polymerbackbone chain containing PMVE. While diamine and bisphenolic cross-linking areionic reactions, peroxide cross-linking is afree-radical mechanism. Though peroxide cross-links are not as thermally stable as bisphenolic cross-links, they normally are the system of choice in aqueous media and nonaqueouselectrolyte media.
Fluoroelastomers provide excellent high temperature (up to 500°F or 260°C[11]) and aggressive fluids resistance when compared with otherelastomers, while combining the most effective stability to many sorts of chemicals and fluids such as oil,diesel,ethanol mix or body fluid.[4]
They can be easily distinguished from many other elastomers because of their highdensity of over 1800 kg/m3, significantly higher than most types of rubber.[12][13]
Food and pharmaceutical, because of their low degradation, also in contact with fluids;
Aviation andaerospace: high operating temperatures and high altitudes require superior heat and low-temperature resistance.[4]
They are suitable for the production of wearables, due to low wear and discoloration even during prolonged lifetimes in contact with skin oils and frequent exposure to light, while guaranteeing high comfort and stain resistance;[14]
Theautomotive industry represents their main application sector, where constant reach for higher efficiencies push manufacturers towards high-performing materials.[15] An example are FKM o-rings used as an upgrade to the original neoprene seals on Corvair pushrod tubes that deteriorated under the high heat produced by the engine, allowing oil leakage. FKM tubing or lined hoses are commonly recommended in automotive and other transportation fuel applications when high concentrations of biodiesel are required. Studies indicate that types B and F (FKM- GBL-S and FKM-GF-S) are more resistant to acidic biodiesel. (This is because biodiesel fuel is unstable and oxidizing.)[citation needed]
FKM O-rings have been used safely for some time inscuba diving by divers using gas blends referred to asnitrox. FKMs are used because they have a lower probability of catching fire, even with the increased percentages of oxygen found in nitrox. They are also less susceptible to decay under increased oxygen conditions.
While these materials have a wide range of applications, their cost is prohibitive when compared to other types of elastomers, meaning that their adoption must be justified by the need for outstanding performance (as in the aerospace sector) and is inadvisable for low-cost products.
FKM rubber is widely used in the watch strap industry due to its superior resistance to chemicals, extreme temperatures, and wear. It is commonly found in premium aftermarket watch straps, offering durability and a comfortable fit.[16]
FKM/butyl gloves are highly impermeable to many strong organic solvents that would destroy or permeate commonly used gloves (such as those made withnitriles).
At high temperatures or in a fire, fluoroelastomers decompose and may releasehydrogen fluoride. Any residue must be handled using protective equipment.
