Fluoroelastomer (FKM)

Superb resistance to aggressive chemicals and excellent behavior at high temperatures has made fluoroelastomer highly important in a wide range of industrial sectors. Articles made of FKM are used in many applications and characterized by a long service life and excellent properties under difficult environmental conditions, and good performance under dynamic load stress. The first fluoroelastomer was manufactured in 1955 and has since been known primarily under the brand name Viton, marketed by the US chemical company DuPont (now DuPont de Nemours).  

In today’s blog post, we outline the different types of FKM and delve into the details of the mechanical, thermal and chemical properties of the material.

Fluoroelastomer (FKM) – the major types

FKM is the abbreviation for fluoroelastomer, defined by the standards ASTM D 1418 and DIN ISO 1629. Another of its well-known abbreviations is FPM, used particularly in Europe. 

Before we discuss the mechanical and thermal properties as well as the chemical resistance of fluoroelastomer (FKM), we would like to run through the different types of material as its properties are largely determined by choice of material.  The differences are most importantly in chemical resistance and flexibility at low temperatures.

The most important criteria for categorization of the FKM family of materials are the base polymer (and monomer composition), fluorine content and crosslinking mechanism.

Table 1: Overview of different FKM types

The first FKM type is the group composed of copolymers (VF2 and HFP monomers). This type is also known as A type. The fluorine content is usually about 66 percent. The cold temperature standard of this FKM type (TR10 value according to ASTM D-1329) is -17°C. The cross-linking mechanism of this FKM type is either diamine or bisphenol. Processing diamine crosslinking is relatively difficult due to the short scorch time, so most FKM compounds of the copolymer group are bisphenolically cross-linked.

The second category of FKM is the terpolymers. In addition to the monomers VF2 and HFP, the polymer chain of this type also contains TFE. The fluorine content is typically around 68 percent. It is slightly less resilient than the copolymer mixes with a cold temperature standard of -13°C. However, their performance at high temperatures and their media resilience is better. Crosslinking is generally bisphenol.

Elastomers of the third type of FKM type are the tetrapolymer group. These polymers are synthesized from VF2, HFP, TFE And CS monomers. A so-called cure-site monomer (CSM) that contains a reactive site allows crosslinking with peroxide and increases the resistance of the compounds to acids, amine-containing engine oil additives and fuels containing methanol. Similarly to the terpolymers, their compression set and flexibility at low temperatures is not as good as the copolymers. Formulations of this type tend to contain around 70 percent fluorine.

Fluoroelastomer ordinarily has poor flexibility in low temperatures but can be strengthened with PMVE (perfluoromethyl vinyl ether). Peroxide is used for crosslinking. FKM compounds can operate in dynamic applications at temperatures down to -35°C. Even lower temperatures are possible for static applications. As the requirement for flexibility in cold temperatures increases, the compounds become more expensive in terms of manufacture and purchase.

In practice, materials are tailored to the required application area and specific properties according to the formulation basis, that is, fluorine content, crosslinking system, fillers, etc. 

Mechanical properties of FKM

The mechanical properties of FKM can be described as average. If only its individual physical properties are considered, there are generally materials superior to FKM.

For example, in terms of tensile strength, natural rubber (NR), chloroprene rubber (CR) and nitrile rubber (NBR) are better. Regarding elongation at break, FKM is outclassed by natural rubber (NR), butyl (IIR) and NBR. With regards to tear resistance, natural rubber (NR) and chloroprene rubber (CR) take the lead. If abrasion resistance of the material is what matters for the application, then you are better off with NBR or HNBR.

Where Fluoroelastomer (FKM) wins is in its mechanical properties and the fact that the material essentially retains its properties at high temperatures.

In addition, FKM is characterized by an excellent compression set at both normal and high temperatures. In this instance, it is clearly superior to other materials and consequently frequently the first choice for seals. This does not, however, apply to cold temperatures under which its flexibility is weak.

FKM is often used in applications that require good damping. This is due to the very low rebound elasticity of fluoroelastomer.

The typical hardness of FKM is between 55 and 90 Shore A. Softer compounds such as 40 Shore A are possible for specialist compounds.

Thermal properties of FKM

Vulcanizates made of fluororelastomer (FKM) are characterized by their excellent resistance to high temperatures. Articles made of FKM to a large extent retain both their media resistance and physical properties and in this aspect are clearly superior to many other materials. FKM is popular for many applications specifically due to its excellent properties at high temperatures.

Rubber components made from FKM can be used without concern in conditions in which the temperature is continually up to 220°C. Even higher temperatures to 300°C are also possible sporadically (max. 70 hours) (ISO 4632, Part 2). However, depending on the type of FKM, the service life of the rubber component reduces as temperature increases.

FKM fares less well in low temperatures. As a rule of thumb, components made from FKM can be dynamically stressed down to a temperature of -20°C. A temperature of about -30°C is still entirely feasible for static stress. Specialist FKMs, which, however, are very expensive, allow a dynamic application down to -40°C.

FKM's weakness at low temperatures was the reason for the failed 1986 flight of the US space shuttle Challenger, which began disintegrating in the air shortly after take-off and cost the lives of all seven crew members. The cause of the crash was an FKM seal that lost the desired functionality (compression set) in the unexpectedly cold temperatures.  We have already covered the tragic accident in an earlier blog post. The details of which can be read here.

Media resistance of FKM

FKM is particularly valued for its excellent media resistance and performance at high temperatures.

FKM is resistant to a wide range of chemicals and superior to many other rubbers. Fluoroelastomers are resistant to mineral oils, aromatic hydrocarbons (e.g. benzene), aliphatic hydrocarbons (petrol, natural gas, etc.), synthetic hydraulic fluids and hydraulic oils, lubricating oils, fuels, kerosene and numerous organic solvents, making them an important material for the automotive industry and hydraulic applications. The suitability of FKM for fire resistant hydraulic fluids must always be tested, but usually the flame-retardant hydraulic fluids of the HFD group are adequate. If necessary, however, the expensive perfluorocarbon rubbers FFKM or EPDM must be used.

In the past, the automotive industry made several attempts to produce elastomer classification systems in order to make them more comparable. And although classification always involves some inaccuracy and a loss of flexibility, the classification system according to ASTM D 2000 reflects the importance of Fluoroelastomer FKM for the automotive industry. The image below shows the various elastomers classified according to their temperature and oil resilience.

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)

The excellent permeation resistance of FKM benefits the automotive industry.

FKM also exhibits good resistance to water and water-soluble solutions. 

Particularly for high-temperature applications, the media resistance of FKM is superior to potential alternatives such as silicone or fluorosilicone. For example, FKM easily outperforms conventional silicone with regard to aromatic and aliphatic hydrocarbons and is also slightly more suitable than fluorosilicone. This is even more true for concentrated acids, where the resistance of FKM depends very much on the type of FKM selected. However, in general, there is very good resistance to diluted acids. 

Parts made from FKM tend to be stable under environmental influences. Fluoroelastomer is highly resistant to oxidation, UV light, solar radiation and ozone. Vulcanizates made of FKM retain their original properties even after years of exposure to direct sunlight.

Noteworthy is also the very good flame retardance of FKM.

FKM is not resistant to polar solvents (e.g. acetone, methyl ethyl ketone), ketones, glycol-based brake fluids, ammonia gas, amines and alkalis. Likewise, there is no resistance to superheated steam.

FKM applications

In general, it can be said that FKM is used wherever very good media resistance or very good heat resistance is required. Although FKM rubber fetches a high price, the higher reliability of rubber parts made of FKM can increase maintenance intervals of machines or equipment, minimize downtime and reduce service costs.

The automotive industry has already been mentioned as an important consumer of FKM components. Rubber parts made of Fluoroelastomers are widely used in the aerospace and construction industries and in all areas related to extraction, production and transport of crude oil and natural gas.

In addition to the automotive sector, the high gas impermeability of fluororubber is particularly important in chemical process engineering in order to keep fugitive emissions as low as possible.

FKM products are also used in the food, medical and pharmaceutical industries. Formulations based on different FKM types can be created that comply with the requirements of the Food and Drug Administration (FDA).

The good compression set of FKM means it is often used in the above areas as O-rings or in molded seals of various designs.

In addition, molded parts and hoses that are exposed to high temperatures or aggressive chemicals tend to be made of FKM. These can be diaphragms, elastic connections, valves, valve stem seals, charge air hoses, dampers or bearings.

The application potential of molded parts made of FKM is limited primarily by their unreliable resistance to cold (see also: The informative value of technical data sheets). Although the physical properties of FKM are considerable, other rubbers are often preferable in the cold.

Overview of FKM properties

In conclusion, here is a recap of the properties of fluoroelastomer FKM.

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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