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 quaternary phosphonium 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) 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. 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. 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. 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; The automotive industry represents their main application sector, where constant reach for higher efficiencies push manufacturers towards