FKM
Encyclopedia
FKM is the designation for about 80% of fluoroelastomer
Fluoroelastomer
A fluoroelastomer is a special purpose fluorocarbon-based synthetic rubber. It has wide chemical resistance and superior performance, especially in high temperature application in different media....

s as defined in ASTM D1418. Other fluorinated elastomers are perfluoro
Fluorocarbon
Fluorocarbons, sometimes referred to as perfluorocarbons or PFCs, are organofluorine compounds that contain only carbon and fluorine bonded together in strong carbon–fluorine bonds. Fluoroalkanes that contain only single bonds are more chemically and thermally stable than alkanes...

-elastomers
(FFKM) and tetrafluoro ethylene/propylene rubbers (FEPM). All FKMs contain vinylidene fluoride as a monomer. Originally developed by DuPont
DuPont
E. I. du Pont de Nemours and Company , commonly referred to as DuPont, is an American chemical company that was founded in July 1802 as a gunpowder mill by Eleuthère Irénée du Pont. DuPont was the world's third largest chemical company based on market capitalization and ninth based on revenue in 2009...

 (Viton
Viton
Viton is a brand of synthetic rubber and fluoropolymer elastomer commonly used in O-rings and other molded or extruded goods. The name is a registered trademark of DuPont Performance Elastomers L.L.C.....

), FKMs are today also produced by Daikin Chemical (Dai-El), 3M
3M
3M Company , formerly known as the Minnesota Mining and Manufacturing Company, is an American multinational conglomerate corporation based in Maplewood, Minnesota, United States....

's Dyneon (Dyneon Fluoroelastomers) and Solvay
Solvay (company)
Solvay S.A. is a Belgian chemical company with its head office in Ixelles, Brussels, Belgium. It was founded in 1863 by Ernest Solvay to produce sodium carbonate by the solvay process. Since then the company has diversified to two major sectors of activity: chemicals and plastics...

-Solexis (Tecnoflon). Fluoroelastomers are more expensive than neoprene
Neoprene
Neoprene or polychloroprene is a family of synthetic rubbers that are produced by polymerization of chloroprene. Neoprene in general has good chemical stability, and maintains flexibility over a wide temperature range...

 or nitrile rubber
Nitrile rubber
Nitrile rubber, also known as Buna-N, Perbunan, or NBR, is a synthetic rubber copolymer of acrylonitrile and butadiene. Trade names include Nipol, Krynac and Europrene....

 elastomers partly because 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.

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
    Hexafluoropropylene
    Hexafluoropropylene is a compound with the formula C3F6. It is a fluorocarbon alkene in which all of the hydrogen atoms in propylene are replaced by fluorine atoms. It is used as a chemical intermediate....

     (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
    Tetrafluoroethylene
    Tetrafluoroethylene is a chemical compound with the formula C2F4. It is the simplest alkene fluorocarbon. This gaseous species is used primarily in the industrial preparation of polymers.-Properties:...

     (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
    Propylene
    Propene, also known as propylene or methylethylene, is an unsaturated organic compound having the chemical formula C3H6. It has one double bond, and is the second simplest member of the alkene class of hydrocarbons, and it is also second in natural abundance.-Properties:At room temperature and...

    , 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
    Ethylene
    Ethylene is a gaseous organic compound with the formula . It is the simplest alkene . Because it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon. Ethylene is widely used in industry and is also a plant hormone...

    . Type 5 FKM is known for base resistance and high temperature hydrogen sulfide
    Hydrogen sulfide
    Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...

     resistance.

FFKM

FFKMs are perfluoroelastomeric materials. They have excellent resistance to high temperatures and chemicals. Certain grades have a maximum continuous service temperature of 327 °C (620.6 °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 are encountered.

Crosslinking mechanisms

There are three established crosslinking mechanisms used in the curing process of FKMs.
  • Diamine
    Diamine
    A diamine is a type of polyamine with exactly two amino groups. Diamines are mainly used as monomers to prepare polyamides, polyimides and polyureas. In terms of quantities produced, 1,6-diaminohexane, a precursor to Nylon 6-6, is most important, followed by ethylenediamine...

     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
    Magnesium oxide
    Magnesium oxide , or magnesia, is a white hygroscopic solid mineral that occurs naturally as periclase and is a source of magnesium . It has an empirical formula of and consists of a lattice of Mg2+ ions and O2– ions held together by ionic bonds...

     is used to catch the resulting hydrofluoric acid
    Hydrofluoric acid
    Hydrofluoric acid is a solution of hydrogen fluoride in water. It is a valued source of fluorine and is the precursor to numerous pharmaceuticals such as fluoxetine and diverse materials such as PTFE ....

     and rearrange into magnesium fluoride
    Magnesium fluoride
    Magnesium fluoride is an inorganic compound with the formula MgF2. The compound is a white crystalline salt and is transparent over a wide range of wavelengths, with commercial uses in optics.-Production and structure:...

     and water. Although rarely used today, diamine curing provides superior rubber-to-metal bonding
    Adhesive
    An adhesive, or glue, is a mixture in a liquid or semi-liquid state that adheres or bonds items together. Adhesives may come from either natural or synthetic sources. The types of materials that can be bonded are vast but they are especially useful for bonding thin materials...

     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
    Nucleophilic substitution
    In organic and inorganic chemistry, nucleophilic substitution is a fundamental class of reactions in which an electron nucleophile selectively bonds with or attacks the positive or partially positive charge of an atom or a group of atoms called the leaving group; the positive or partially positive...

    . Dihydroxy aromatic compounds are used as the crosslinking agent and quaternary phosphonium
    Phosphonium
    The phosphonium cation describes positively charged polyatomic cations with the chemical formula . Salts of the parent PH4+ are rarely encountered, but this ion is an intermediate in the preparation of the industrially useful tetrakisphosphonium chloride:Organic phosphonium salts are common...

     salts are typically used to accelerate the curing process.
  • Peroxide
    Peroxide
    A peroxide is a compound containing an oxygen–oxygen single bond or the peroxide anion .The O−O group is called the peroxide group or peroxo group. In contrast to oxide ions, the oxygen atoms in the peroxide ion have an oxidation state of −1.The simplest stable peroxide is hydrogen 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.

External links

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