In general, fluororubbers are characterized by high resistances in both the physical and chemical range. These properties vary depending on the copolymers used.

Physical characteristics

Fluororubbers are generally characterized by very low gas permeability. They also have a high volume resistivity, which means that they are electrically insulating. Finally, the material exhibits high abrasion resistance.

Mechanical characteristics

In general, fluoroelastomers are characterized by very good mechanical properties:.

• FKM have a very high tensile strength.
• FKM show a very low impact elasticity.
• The elongation at break of FKM is between 100% and can be up to 300%.
• The deformation due to external pressure, the so-called compression set, is low.
  
  

Thermal characteristics

FKM are resistant to high temperatures of up to 200 °C, but are only cold-resistant down to -20 °C, maximum -40 °C. Although the temperature resistance increases with higher fluorine content, brittleness also increases at lower temperatures. Fluororubbers are also non-flammable.

Chemical characteristics

Fluororubbers have a very high resistance to a whole range of chemicals, such as

• acids
• alkalis
• aliphatic, aromatic and halogenated hydrocarbons
• vegetable oils and animal oils
• fuels
• silicone and other mineral oils

Fluororubbers are not only resistant to fuels and mineral oils, but they also do not swell in these liquids. For this reason, they are suitable for all products that are exposed to these fluids for extended periods. Finally, fluororubbers also exhibit good weathering and aging resistance, as well as high resistance to other external influences, such as ozone and UV light. They are chemically unstable only to hot steam, alcohols and polar solvents.

FKM is a collective name for the following different types of fluororubber.

FKM copolymers

FKM copolymers consist of two different polymers. The best-known types consist of the monomers vinylidene fluoride and hexafluoropropylene.

Vinylidene fluoride (VDF)

Vinylidene fluoride, whose chemically correct designation is 1,1-difluoroethene, is a highly flammable, colorless gas. It has a double bond through which the crosslinking reaction takes place and which is responsible for the reactivity of the gas. At higher temperatures, vinylidene fluoride is chemically unstable and also easily polymerized.

Hexafluoropropene (HFP)

Hexafluoropropene, also known as hexafluoropropylene, is also a reactive, colorless and odorless gas. This also reacts via the double bond present in the molecule.

FKM Terpolymers

Analogous to copolymers, terpolymers consist of three different types of monomers. These include the already mentioned components vinylidene fluoride and hexafluoropropylene. Tetrafluoroethylene is very often used as a third monomer. This gives the resulting fluororubbers higher chemical and temperature resistance.

Tetrafluoroethylene (TFE)

Tetrafluoroethylene is a chemically unstable and very reactive gas due to its double bond. Since it decomposes very easily, it is stored with the addition of a stabilizer.

Fluororubber is formed in a polymerization reaction of vinylidene fluoride with the monomers already listed. Depending on which monomers are used for the reaction, different fluororubbers are formed. The polymerization reaction is carried out under high pressure to convert the monomers involved, which are gaseous at room temperature, into the liquid state. As a result, the actual polymerization can be carried out in solution as a so-called emulsion polymerization at temperatures between +80 °C and +125 °C.

Fluororubbers can be further processed and crosslinked in various ways. Three different processes are available.
So-called diamine crosslinking uses "blocked diamines" to link the individual polymer chains. "Blocked" here means that the reactive amine groups actually present for the reaction are linked to another molecule, a so-called protective group. Only under certain conditions, in this case under basic ones, is this linkage broken again; at the same time, vinylidene fluoride is also activated under basic conditions. In this way, the polymerization reaction can be controlled and take place under controlled conditions.

In the so-called bisphenolic mechanism, bisphenol AF and a so-called quaternary phosphonium salt are used. This is a phosphorus compound with four different residues and a single positive charge. With the help of these two components, which serve as crosslinkers in the polymerization reaction, the polymer chains are linked together.

A third process, known as the "triazine process," uses peroxides to start the crosslinking process. These decompose and form reactive components known as free radicals. Triazines belong to the aromatic compounds, the best-known representative of which is benzene. The triazine molecule contains three nitrogen atoms and serves as a crosslinking component. Polymerization by the triazine process is used, for example, when perfluoromethyl vinyl ether (PMVE) is one of the monomer components. This cannot be used in the other two processes, as it would itself be chemically attacked in these, so that no adequate crosslinking reaction would occur.

Fluororubber is used as a material whenever high temperature and chemical resistance are required. One of the first areas of application was aerospace, which requires special materials, especially for seals, due to its extreme conditions. A second major area of application is the automotive industry. Here, too, many components, such as hoses and seals, are made of fluororubber.

FKM in the aerospace industry

The world's first synthetic fluoro rubber under the name Viton was developed specifically to meet the demands of the aerospace industry. As a chemically and thermally resistant synthetic rubber that retains its elastic properties under extreme conditions, it particularly fulfilled the requirements of these industries. In the meantime, even higher-performance elastomers based on FKM have been developed.

O-rings made of fluororubber

O-rings made of fluororubber are used when high temperature and/or high chemical resistance are required. At the same time, this polymer has a low compression set, which means that it does not deform or only deforms very slightly under pressure. This property is also required for O-rings, as they are intended to seal two molded parts tightly against each other.

FKM seals

In addition to O-rings, there are also flat or profile seals. Seals made of fluororubber are used in many sectors, such as the automotive or petroleum industries.

FKM hoses

Hoses made of fluororubber are often used in the chemical industry in particular.
However, fuel hoses used at service stations or for transporting crude oil are also frequently made of fluororubber.

FKM gloves

Gloves made of 100% fluororubber are used for handling aggressive materials or when gas-tightness must be ensured. This means that it is mainly employees in laboratories or the chemical industry who use fluororubber gloves for handling chemicals.

Fluororubber is one of the most durable materials in plastics due to its built-in fluorine atoms.

EPDM and Viton in comparison

EPDM (ethylene-propylene-diene rubber), also belongs to the group of synthetic rubbers. Similar to FKM, EPDM is characterized by high temperature and chemical resistance. It is also very resistant to weathering, such as ozone or UV light.
An advantage over FKM is the additional resistance of EPDM to hot water and steam, which is why many hoses are made precisely from EPDM. On the other hand, EPDM is not resistant to fuel and mineral oils.
Both types of rubber are popular for seals, both flat gaskets and O-rings.

Viton and Buna in comparison

Buna is a synthetic rubber made from the two monomers butadiene and styrene. Another common abbreviation for Buna is SBR, which stands for styrene-butadiene-rubber. Buna exhibits very good mechanical properties, such as high strength and abrasion resistance. However, compared to FKM, Buna showed lower elasticity and reduced aging resistance. It is also much less resistant to mineral oils and greases. Buna is mainly used for the production of passenger car tires due to its good abrasion resistance.

Buna-N and Viton in comparison

Buna-N, which is also known as NBR or nitrile rubber, is made from the two components acrylonitrile and butadiene. This is also indicated by the abbreviation NBR, which stands for nitrile-butadiene-rubber. Like FKM, this synthetic rubber shows good resistance to mineral oils, greases and fuels. For this reason, NBR is often used as a low-cost alternative to FKM for seals, O-rings and various hoses that come into contact with the chemicals mentioned. However, Buna-N shows lower resistance to ozone, weathering and aging than materials made of fluororubber. Its resistance to high temperatures is also lower than that of FKM.

HNBR and Viton in comparison

The abbreviation HNBR stands for "Hydrogenated Nitrile Butadiene Rubber". Its chemical structure and properties are similar to NBR, but it is more resistant to weathering, ozone and UV light. Its thermal properties are also improved compared to NBR. HNBR is also used for seals, O-rings or hoses. Due to its very similar properties to FKM, it is a good alternative to fluororubber, especially from an economic point of view.

Aflas and Viton in comparison

Aflas, also known as FEPM, is a copolymer of the two components tetrafluoroethylene and polypropylene. Due to its high fluorine content, it resembles normal FKM in many properties. Chemical and thermal properties, for example, are almost identical, while mechanical properties, such as resistance to deformation under pressure, are somewhat poorer.