FKM 4408405 214286896 2008-05-22T21:53:29Z Delmlsfan 2835051 [[WP:UNDO|Undid]] revision 214129454 by [[Special:Contributions/82.37.94.29|82.37.94.29]] ([[User talk:82.37.94.29|talk]]) '''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 (company)|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. ==Crosslinking mechanisms== 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 [[adhesive|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]]. ==External links== * [http://www.zrunek.at/download/ZruElast%20FPM%20UK.pdf Designing with Fluoroelastomers] *[http://www.sealseastern.com/PDF/FluoroAcsChapter.pdf Fluorine-Containing Elastomers] [[Category:Elastomers]] [[Category:Materials science]] [[de:Fluorkautschuk]] [[it:Elastomeri fluorurati]]