Fluorosilicone rubber and silicone rubber – differences and commonalities
Fluorosilicone rubber, which under international nomenclature is abbreviated to FVMQ or MFQ, was invented in the early 1950s by Dow Corning Inc., based in Midland, Michigan, USA. Fluorosilicone differs from pure silicone rubber at a submacroscopic level in that some of the methyl side groups are replaced by fluoroalkyl groups. Fluorosilicone usually contains 23 to 28 percent fluorine.
FVMQ has almost the same mechanical and physical properties as conventional silicone rubber and its low-temperature flexibility is just as good. However, owing to its fluorine content it has better resistance to many liquids such as oils and fuels, and as such it is frequently used in sealing technology. By contrast, the operating temperature range is slightly reduced: -50 to +250°C for fluorosilicone rubber as compared with -100 to +280°C, special types up to 300°C for silicone rubber.
The two materials also differ significantly in terms of electrical insulating capability, which is very good in the case of silicone rubber but can only be brought up to a similar level in fluorosilicone by means of appropriate formulation. The standard hardness of FVMQ is 20 to 80 Shore A, and the maximum tear strength is 10 N/mm².
Curing of fluorosilicone rubber
Like silicone rubber, fluorosilicone is also cured with peroxides or using platinum catalysts.
This typ of vulcanisation involves a radical mechanism with organic diacyl-, dialkyl-, oder diarylalkyl-peroxids. These peroxides splits at higher temperatures into high reactive peroxo-radicals with addition of the active peroxoradical on the double-bond of the polymer. As the the step follows a reaction of the free radicals by building up chemical crosslinks.
Addition curing with platinum catalyst systems
In this method the Si-H groups of the FVMQ are added to the double bonds of the vinyl groups of the polymer using a platinum catalyst, forming a three-dimensional network. The advantage of this method lies in the high curing speed, which can be controlled via the temperature. The addition curing of fluorosilicone, which produces no decomposition products, is one of the newest curing methods and is mostly used for liquid silicones.
Properties of fluorosilicone rubber
FVMQs offer good resistance to automotive and aviation oils, fuels and many other chemicals and solvents. As such, fluorosilicones are ideal for use in sealing technology.
Chemical resistance of FVMQ at a glance:
- Automotive fuels (unleaded/leaded petrol, diesel, various test fuels and rape methyl ester (RME))
- Mineral oil and synthetic oils
- Jet fuels and oils
- ATF (automatic transmission fluid)
The mechanical properties of the material, such as ultimate elongation for example, remain constant over a broad temperature range, so fluorosilicone is preferably used for diaphragms that are also subject to dynamic loading at low temperatures.
Other useful properties of fluorosilicones:
- Extreme high- and low-temperature resistance
- Good low-temperature flexibility
- Good weathering resistance
- High ozone resistance
- Good ageing resistance
- Good rebound resilience
- Good water resistance
- Very good dielectric properties
Use of fluorosilicone rubber
The potential uses of FVMQ in sealing technology and other sectors are limited only by its high gas permeability, low tensile strength and poor abrasion and tear properties. Typical applications include static seals in fuel systems for cars and in aerospace engineering as well as diaphragm valves for crankcase ventilation. Fluorosilicone rubber is also used in hoses, e.g. turbocharger hoses, and in O-rings.