FKM is a family of
fluorocarbon-based
fluoroelastomer materials defined by
ASTM International standard D1418,[1] and
ISO standard 1629.[2] It is commonly called fluorine rubber or fluoro-rubber. FKM is an abbreviation of Fluorine Kautschuk Material.[3] All FKMs contain
vinylidene fluoride as the common
monomer, to which different other monomers are added for specific types and functionalities, fitting the desired application.
Originally developed by
DuPont (under the brand name Viton, now owned by
Chemours), 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 than
neoprene or
nitrile rubberelastomers. They provide additional heat and chemical resistance.[9] FKMs can be divided into different classes on the basis of either their chemical composition, their
fluorine content, or their
cross-linking mechanism.
Types
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 is approximately 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. Known for
base resistance and high-temperature resistance to
hydrogen sulfide.[10]
Cross-linking mechanisms
There are three established
cross-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 to
dehydrofluorination, which enables the addition of the diamine to the polymer chain. Typically,
magnesium oxide is used to neutralize 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 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 aromatic
nucleophilic substitution. Dihydroxy
aromatic compounds are used as the cross-linking agent, and
quaternaryphosphonium salts are typically used to accelerate the curing process.
Peroxide cross-linking was originally developed for type 3 FKMs containing PMVE as diamine and
bisphenolic cross-linking systems can lead to
cleavage in a polymer
backbone chain containing PMVE. While diamine and bisphenolic cross-linking are
ionic reactions, peroxide cross-linking is a
free-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 nonaqueous
electrolyte media.
Properties
Fluoroelastomers provide excellent high temperature (up to 500°F or 260°C[11]) and aggressive fluids resistance when compared with other
elastomers, while combining the most effective stability to many sorts of chemicals and fluids such as oil,
diesel,
ethanol mix or body fluid.[4]
The performance of fluoroelastomers in aggressive chemicals depends on the nature of the base polymer and the compounding ingredients used for molding the final products (e.g.
o-rings). Some formulations are generally compatible with
hydrocarbons, but incompatible with
ketones such as
acetone and
methyl ethyl ketone,
ester solvents such as
ethyl acetate,
amines, and
organic acids such as
acetic acid.
They can be easily distinguished from many other elastomers because of their high
density of over 1800 kg/m3, significantly higher than most types of rubber.[12][13][14]
Applications
Because of their outstanding performance they find use in a number of sectors, including the following:
Chemical process and
petroleum refining, where they are used for seals, pumps, gaskets and so on, due to their resistance to chemicals;
Analysis and process instruments: separators, diaphragms, cylindrical fittings, hoops, gaskets, etc.
Food and pharmaceutical, because of their low degradation, also in contact with fluids;
Aviation and
aerospace: 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;[15]
The
automotive industry represents their main application sector, where constant reach for higher efficiencies push manufacturers towards high-performing materials.[16] 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 in
SCUBA diving by divers using gas blends referred to as
nitrox. 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/butyl gloves are highly impermeable to many strong organic solvents that would destroy or permeate commonly used gloves (such as those made with
nitriles).
Precautions
At high temperatures or in a fire, fluoroelastomers decompose and may release
hydrogen fluoride. Any residue must be handled using protective equipment.