Different type of Fluoroelastomers (FKM) Rubber

Introduction to Fluoroelastomers (FKM)

Fluoroelastomers (FKM) are a type of synthetic polymer elastomer containing fluorine atoms in the carbon chain or side chain. These materials are known for their excellent high-temperature performance, chemical stability, weather resistance, oxidation resistance, oil resistance, and very low gas permeability. Common FKM brands include DuPont’s Viton®, Solvay’s Tecnoflon®, 3M’s Dyneon™, and Daikin’s DAI-EL™.

Structure and Properties of FKM

The molecular structure of FKM provides outstanding chemical stability and high-temperature resistance. Fluorine atoms surrounding the carbon chain and fluorocarbon bonds protect the carbon atoms from attack by other chemicals, molecules, ions, or atoms. The structure of tetrafluoromethane is shown below:

Molecular Structure of Tetrafluoromethane

The high bond energy of the fluorocarbon bond also means it is very stable and not easily broken.

Comparison of Bond Energies for Common Chemical Bonds (kJ/mol)

To maintain the characteristics of elastomers, rubber products must quickly recover after being deformed by external compression. Typically, elastomers are cross-linked to form a three-dimensional network structure, and the recovery force after deformation comes from the tendency of the molecular segments to return to a disordered state. Compared to hydrocarbon chains, fluorocarbon chains are more viscous, so FKM often exhibits slow relaxation.

FKM usually consists of two or more monomers. If the carbon chain polymerized from VF2 (or VDF), TFE, and ethylene monomers is long enough, it may crystallize. To form an amorphous polymer, some monomers with bulky side groups, such as hexafluoropropylene (HFP), perfluoromethylvinyl ether (PMVE), and propylene, are added. The molecular structures of common monomers are shown below:

Types of Fluoroelastomers

According to ASTM D1418-2022, fluoroelastomers can be classified into five types:

1. Binary Copolymers of Hexafluoropropylene (HFP) and Vinylidene Fluoride (VDF): These standard FKMs, also known as 26-type FKM or 2-type glue in China, account for over 80% of FKM usage. They offer balanced performance across various properties.
2. Ternary Copolymers of Tetrafluoroethylene (TFE), Hexafluoropropylene (HFP), and Vinylidene Fluoride (VDF): These FKMs have a higher fluorine content (typically 68%-69%) than binary copolymers, providing better chemical resistance, especially against hydrocarbons, and higher temperature resistance, suitable for long-term use above 250°C. They are known as 246-type FKM or 3-type glue in China.
3. Ternary Copolymers of Tetrafluoroethylene (TFE), Perfluoromethylvinyl Ether (PMVE), and Vinylidene Fluoride (VDF): The inclusion of PMVE improves elasticity at low temperatures. These FKMs have a fluorine content between 62% and 68%.
4. Ternary Copolymers of Tetrafluoroethylene (TFE), Propylene, and Vinylidene Fluoride (VDF): These FKMs offer improved resistance to polar solutions such as bases but have poorer swelling resistance. The fluorine content is about 67%.
5. Quinary Copolymers of Tetrafluoroethylene (TFE), Hexafluoropropylene (HFP), Ethylene, Perfluoromethylvinyl Ether (PMVE), and Vinylidene Fluoride (VDF): These FKMs are known for their resistance to polar solutions and high-temperature hydrogen sulfide.

Below is a chart showcasing the types of Viton fluoroelastomers from DuPont, which is representative of the various types of FKM:

It’s important to note that besides PMVE, MOVE is also used as a monomer in FKM.

Differences Between Binary and Ternary Fluoroelastomers

Binary Fluoroelastomers (FKM-2)

• Composition: Made from two monomers, typically hexafluoropropylene (HFP) and vinylidene fluoride (VDF).
• Properties: Standard performance across various properties, making them versatile for many applications.
• Applications: Common in automotive and industrial uses due to their balanced properties.

Ternary Fluoroelastomers (FKM-3)

• Composition: Made from three monomers, such as tetrafluoroethylene (TFE), hexafluoropropylene (HFP), and vinylidene fluoride (VDF).
• Properties: Higher fluorine content provides better chemical and temperature resistance.
• Applications: Preferred in more demanding environments, such as aerospace and chemical processing, where higher resistance is required.

• Binary Fluoroelastomers: Suitable for general high-temperature and chemical environments, with relatively lower cost.
• Ternary Fluoroelastomers: Suitable for more severe environments with higher chemical and heat resistance, but with higher cost.

Both types of fluoroelastomers have their advantages and disadvantages, and the choice of material depends on the specific requirements of the application.

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