FKM

This article has multiple issues. Please help improve it or discuss these issues on the talk page. (Learn how and when to remove these template messages) This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "FKM" – news · newspapers · books · scholar · JSTOR (January 2011) (Learn how and when to remove this template message) This article relies excessively on references to primary sources. Please improve this article by adding secondary or tertiary sources. Find sources: "FKM" – news · newspapers · books · scholar · JSTOR (March 2021) (Learn how and when to remove this template message) (Learn how and when to remove this template message)

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

• 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
  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.

• 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 (
  phosphonium salts
  are typically used to accelerate the curing process.
• 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.

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.

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.
• Semiconductor manufacturing;
• 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

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.

See also

• Magnesium/Teflon/Viton
• FFKM, perfluoro-elastomers
• FEPM, tetrafluoro ethylene/propylene elastomers
• PVDF
  , polyvinylidene fluoride

References