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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. A fluoroelastomer has recently been adopted, as the material of choice, for the strap of the Apple Watch Sport.
Fluoroelastomers are categorized under the ASTM D1418 & ISO 1629 designation of FKM. This class of elastomers is a family comprising copolymers of hexafluoropropylene (HFP) and vinylidene fluoride (VDF or VF2), terpolymers of tetrafluoroethylene (TFE), vinylidene fluoride (VDF) and hexafluoropropylene (HFP) as well as perfluoromethylvinylether (PMVE) containing specialties. The fluorine content of the most common grades varies between 66 and 70%.
Main families of polymers
- Dipolymers of VF2/HFP, like Viton A and Tecnoflon: General purpose sealing. Automotive, Aerospace fuels & lubricants
- Terpolymers of VF2/HFP/TFE, like Viton B and Tecnoflon: Chemical Process plant, Power Utility Seals & Gaskets
- Terpolymers of VF2/HFP/TFE, like Viton F and Tecnoflon: Oxygenated Automotive fuels. Concentrated aqueous inorganic acids, water, steam.
- Peroxide curable polymers are similar to terpolymers. They offer better polymers are typically peroxide curable with an additional monomer of PMVE.
The performance of fluoroelastomers in aggressive chemicals depends on the nature of the base polymer and the compounding ingredients used for moulding the final products (e.g. O-rings, shaft seals). This performance can vary significantly when end-users purchase polymer containing rubber goods from different sources. Fluoroelastomers are generally compatible with hydrocarbons, but incompatible with ketones such as acetone and organic acids such as acetic acid.
Fluoroelastomers recently gained public attention when they were announced as the material chosen for the Apple Watch sport band, due to the considerable inertness of these polymers. However, as products such as watchbands are not often required to demonstrate such a broad spectrum of chemical resistance, this choice was most likely made for the purposes of texture and elasticity, rather than reactivity considerations.