Fluorocarbon rubber – a long and successful history
Known as perfluoroelastomers (FFPM), these fully-fluorinated, high-tech elastomers are especially suitable for applications in contact with highly aggressive media. They are used in particular in areas with stringent safety or purity requirements or where possible high failure costs justify their use. Examples include the chemical industry, the oil production and refining industry, equipment engineering and power plant construction, the semiconductor industry, the food industry and aerospace.
Fluoro rubber categories: FPM types
There are currently five types of fluoro rubber, each with differing characteristics:
- Type 1: Di- or copolymers of hexafluoropropylene (HFP) and vinylidene fluoride (VDF). These are the standard FPM types. They generally contain around 66 percent by weight of fluorine.
- Type 2: Terpolymers of tetrafluoroethylene (TFE), HFP and VDF. These elastomers have a higher fluorine content than type 1 FPMs (between 68 and 70 weight percent), making them more resistant to chemicals and high temperatures.
- Type 3: Terpolymers of TFE, a fluorinated vinyl ether (FVE) and VDF. In comparison to type 1 and 2 fluorocarbon rubbers, these polymers have better flexibility at low temperatures. They contain between 62 and 68 weight percent fluorine.
- Type 4: Terpolymers of TFE, propylene and VDF. Base resistance is improved in this FPM type, although its swelling properties, especially in hydrocarbons, are less good. A fluorine content of around 67 weight percent is typical in these elastomers.
- Type 5: Pentapolymers of TFE, HFP, ethylene, an FVE and VDF. These polymers are also resistant to hydrogen sulfide, even at high temperatures.
General properties of fluoro rubber
Fluorinated polymers are characterised in particular by their high-temperature stability and good chemical resistance. Both of these properties can be attributed to the bulkiness of the fluorine atoms, which shield the main chain and protect the carbon-fluorine bonds of these polymers against attack. In addition, the high bond energy of the rubber molecules leads to good stability.
FPM has the highest density of all rubber types, at 1.80 to 1.87 g/cm³. Almost all FPM types are highly viscous, intolerant of plasticisers because of their physical and chemical inertness and can contain only small amounts of inactive fillers if good handling characteristics are to be maintained.
Key properties of fluoro rubber at a glance:
- High chemical resistance
- Very good thermal stability
- Excellent weathering and ageing resistance
- Resistance to mineral oils, fats and non-polar media
- Very good oxygen resistance and high ozone stability
- Low gas permeability
- Electrical insulating capability
- Operating temperature range: -40 to +250°C
Heat resistance and thermal stability of FPM
FPM can generally be used for extended periods at temperatures of up to 200°C without any difficulty. Higher temperatures are also possible, although the life of the rubber parts, e.g. seals or hoses, will be shorter as a consequence.
Low-temperature properties of fluorocarbon rubber
The low-temperature properties of fluoroelastomers are generally determined by two factors: the size of the fluorine atom and the substituent fluorocarbon molecules and the various intermolecular forces which come into play owing to fluorine's high electronegativity. Dynamic sealing applications with FPM-based products remain functional down to -40°C. Some FPM types have static sealing capability down to -60°C.
Crosslinking mechanisms of fluoro rubber
Since the polymer chains in fluoro rubber are saturated, it cannot be cured with sulfur. For that reason, other mechanisms are necessary for binding fluoro polymers to elastic networks.
Diamine crosslinking
Diamine crosslinking, in which blocked diamines are used to cure the polymers, is the oldest method of FPM vulcanisation. In a basic environment, vinylidene fluoride can cleave off hydrogen fluoride, freeing up a space for the amine. The hydrofluoric acid that forms in this process is usually trapped by magnesium oxide, giving rise to magnesium fluoride. This method is still in common use today, because diamine-cured elastomers lead to good rubber-metal adhesion. At the same time, however, diamine-cured FPM is susceptible to hydrogenation in aqueous media.
Bisphenol mechanism
The bisphenol or dihydroxy mechanism is a nucleophilic substitution mechanism. Bisphenol AF and a quaternary phosphonium salt are used as crosslinking components for the fluorocarbon polymer. This more modern method improves the resistance of the elastomers to hydrolysis and to elevated temperatures. It also results in superior compression set.
Peroxide crosslinking (triazine method)
Peroxide crosslinking (crosslinking by free radicals) is especially important for rubber containing perfluoromethyl vinyl ether (PMVE), since the other two methods could cause the polymer chains to break down by attacking the PMVE. In aqueous and non-aqueous electrolytes, peroxide-cured fluoroelastomers are superior to fluorocarbon rubbers cured by other mechanisms. The thermal stability of these polymers is slightly inferior to that of bisphenol-cured FPM, however.
Fluoro rubber in the automotive sector
One impact of tougher emission requirements is higher temperatures in the vicinity of the engine, while stricter fuel permeation standards are leading to increasingly aggressive fuels and fuel additives. Oxygen-rich fuels have higher volatility, and this can lead to increased swelling, a deterioration in properties and increased penetration of elastomeric materials. For that reason, seals and hoses have to be ever more resistant.
The following properties make fluoro rubber the material of choice for fuel seals, cylinder head seals and induction pipe gaskets, seals for fuel injection systems, quick-connect O-rings, sealants and components for fuel lines and turbocharger hoses:
- Resistance to hydrocarbons and acid fuels
- Good resistance to high and low temperatures (-40 to +230°C, with intermittent exposure up to 285°C)
- Resistance to solvents, acids and alkalis
- Low permeation rates
- Very good dynamic properties
- Low compression set
Fluoro rubber in the aerospace industry
In the air, and especially in space, absolute tightness is essential. Modern seals – as well as hoses and other parts – in commercial and military aircraft are often based on fluoro rubber. The usable temperature range of FPM O-rings is between -45 and +275°C. As such, these components can easily withstand the effects of the thermal cycles arising from a rapid ascent into and descent from the stratosphere. In addition, these rubber polymers offer good abrasion resistance and the ability to seal spacecraft against high vacuum.
Fluoro rubber seals are found in turbine engines, auxiliary power units and hydraulic actuators, for example. However, these elastomers are also used to manufacture clips for wiring harnesses in jet engines, suction hoses for hot engine oils, manifold gaskets and O-rings for pipe fittings, connections, pumps and valves.