Fluorocarbon rubber is a polymer of vinylidine fluoride and hexafluoropropylene. Fluorocarbon rubber has a wide range of thermal application (-25 °C to +200 °C), is chemically stable, just slightly gas-permeable, and display minimum losses of weight in high-vacuum. Resistance to ozone, weathering and sun cracking is very good, as is its noninflammability. Fluorocarbon rubber shows good resistance to swelling in mineral oils and greases (including those with additives), motor fuels, aliphatic and aromatic hydrocarbons, some hardly combustible hydraulic fluids and synthetic aviation lubricants. Recently developed peroxide cross-linked materials are very resistant to media that display little or no compatibility with conventional fluorocarbon rubbers, e.g. alcohols, hot water, steam, alcohol based alternative motor fuels and SF mineral oils. Polar solvents and ketones, hardly combustible hydraulic fuels and brake fluids exert a strong swelling effect. Amines destroy the material. For material characteristics see materials for secondary seals.