Fluorosilicone Rubber

Silicone is not only probably the best-known elastomer material among the general population, but is also widely used in various industries and medical applications due to its unique properties. A major strength of silicone rubber is its resistance to both very cold and very hot temperatures.

In applications requiring resistance to gasoline, fuel or oil, the elastomers NBR or FKM are often used. However, these two rubbers do not have the temperature properties of silicone and show weaknesses, especially at cold temperatures. Silicone rubber itself, however, is not resistant to fuels and oils, or only to a limited extent.

Fluorosilicone (FVMQ), which can be formed by introducing fluorine-containing groups to the polymer chain of silicone rubber, closes this gap and combines the excellent temperature properties of silicone rubber with resistance to fuel and oils.

In the following blog post, we go into more detail about the properties of fluorosilicone and describe the key differences between conventional silicone and fluorosilicone.

Mechanical and Thermal Properties: Silicone vs. Fluorosilicone

Articles made from fluorosilicone (the international abbreviation is FVMQ) are very similar in their mechanical and physical properties to finished parts made from silicone rubber. We have already created a detailed blog article about the properties of silicone and why the material is so popular. You can find the article here. It's worth it.

As with all elastomers, the mechanical properties can be influenced by the specific structure of a compound. In this respect, the following values should only be taken as guidelines.

In general, finished parts can be made from fluorosilicone with a hardness between 20 Shore A and 80 Shore A. The rebound elasticity is between 25 and 50 percent, depending on the compound structure. Compared to conventional silicone, fluorosilicone can have somewhat lower values and therefore has advantages, for example, in applications where good damping properties are required. Elongation at break is very high, especially in soft compounds, and can be as high as 700 percent, whereas tensile strength is weaker than in other elastomers.

Both silicone and fluorosilicone compounds are generally characterized by low compression set, which is maintained over a wide temperature range. Here, both types are superior to other elastomers.

In contrast, the abrasion properties of fluorosilicone (and silicone) are poor. In addition, both materials are limited in their applications by their high friction.

The gas permeability of fluorosilicone rubber is significantly lower than that of silicone rubber.

Even though the mechanical-physical properties of fluorosilicone (and silicone) appear at first glance to be weaker than those of other elastomers, fluorosilicone and silicone have one major advantage over other rubber materials: their properties remain virtually the same over a very wide temperature range, whereas the mechanical properties of finished parts made of other rubbers deteriorate significantly as soon as there is a change in temperature.

Both fluorosilicone and silicone exhibit excellent heat and cold resistance. The elastic properties of finished parts are almost completely retained in both hot and cold environments.

In continuous use, finished parts made of fluorosilicone, like silicone, can be subjected to temperatures of up to 200 degrees Celsius or 400 degrees Fahrenheit without any problems. Temperatures of 225 degrees Celsius or 440 degrees Fahrenheit to 250 degrees Celsius or 480 degrees Fahrenheit are also possible over certain periods of time. If the temperature is increased further, the service life of the vulcanizates decreases accordingly.

In terms of cold behavior, silicone rubber is slightly superior to fluorosilicone. While molded parts made of silicone can be used down to around -80 degrees Celsius or -110 degrees Fahrenheit, fluorosilicone can withstand temperatures as low as -65 degrees Celsius or -85 degrees Fahrenheit. This makes them significantly superior to other materials.

Chemical Resistance: Silicone vs. Fluorosilicone

In terms of chemical resistance, fluorosilicone clearly stands out from conventional silicone. In the development of fluorosilicone, the ultimate goal was to maintain the temperature resistance of silicone, but at the same time improve chemical resistance, especially with regard to fuels and oils. This was achieved by introducing fluorine-containing groups to the polymer chain of the silicone rubber, resulting in fluorosilicone.

Fluorosilicone achieves very good resistance to gasoline and other fuels, oil and solvents and water up to 100 degrees Celsius or 212 degrees Fahrenheit. In these areas, it is clearly superior to conventional silicone (see Figure 1).

Figure 1: Classification of elastomers according its heat and oil resistance following ASTM D 2000 (issue 2006). Source: Kraiburg Group - Compounder Info - Classification of rubbers and elastomers (brochure)

In detail, fluorosilicone has very good chemical resistance to engine and transmission oil, aromatic mineral oils, alcohol-free fuels, glycol-based brake fluids, flame-retardant hydraulic fluids (HFD-R and HFD-S), high-molecular-weight chlorinated hydrocarbons, animal and vegetable oils and greases, and water up to 100 degrees Celsius or 212 degrees Fahrenheit. 

On the other hand, resistance to low molecular weight aromatic hydrocarbons (for example benzene or toluene), low molecular weight esters and ethers, superheated water vapor above 120 degrees Celsius or 250 degrees Fahrenheit, acids and alkalis, and silicone oils is less good or insufficient.

The ozone and weathering resistance of finished parts made of fluorosilicone rubber is very good and comparable to products made of conventional silicone.

Areas of Application for Fluorosilicone Rubber

The price of fluorosilicone is usually about five times higher than the price of conventional silicone compounds. In this respect, fluorosilicone rubber is mainly used where the chemical resistance of silicone is no longer sufficient.

This is particularly the case when molded parts are to seal against gasoline, fuel or oil and the low-temperature flexibility of NBR or FKM is no longer given. Since the mechanical properties of both silicone and fluorosilicone are only average, finished products made of these materials are used primarily for static applications (for example, gaskets or seals) and are recommended only to a limited extent for dynamic applications. As a material for static seals, fluorosilicone is ideally suited due to its very good compression set.

Example areas for applications of molded parts made of fluorosilicone rubber are various petroleum and refinery applications, gasoline-carrying systems, systems in the jet fuel sector or applications with chlorinated solvents. Molded fluorosilicone parts are therefore widely used in aerospace applications and in the oil and gas industry. Typical products include O-rings, molded seals, hoses and bearings.

In the medical sector, on the other hand, fluorosilicone is used quite rarely. Here, silicone rubber (whether solid silicone or liquid silicone rubber LSR) is clearly the leading material due to its conformity to FDA regulations. In addition, silicone rubber is biocompatible and therefore well tolerated by humans.

Overview of Fluorosilicone properties

In conclusion, here is a recap of the properties of fluorosilicone rubber.

Remember, this is only a general guide and not to be used for your ultimate selection of materials. The individual properties of blends can be positively and negatively influenced by targeted formulation and as such may be different from the information presented here.

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For more details about properties or chemical resistance, or if you have a query about a particular application, please do not hesitate to contact us.  

If you have a question about this blog post or would like us to discuss a particular aspect of elastomers in an upcoming blog, please email us on info@hepako.de   

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