Influence of Different Curing Systems on the Properties of FKM O-Rings

FKM is mainly divided into two grades: binary VDF-HFP and terpolymer VDF-HFP-TFE. Both grades can be cured by bisphenol or peroxide curing systems. Different combinations of polymer grades and curing systems lead to drastically different resistance to steam, weak acids and high temperatures, which directly defines their industrial application scope.

Ⅰ. Differences in Molecular Structure and Intrinsic Properties

Binary FKM only contains VDF and HFP monomers. It can be crosslinked with bisphenol; after halogen modification, it is also suitable for peroxide curing. Terpolymer FKM incorporates additional TFE monomers with higher fluorine content. Its long-term service temperature is 20–30°C higher than binary FKM, and it exhibits far less swelling when exposed to fuel, aromatic hydrocarbons and organic acids. Disadvantages: With the same curing system, terpolymer FKM has higher molecular rigidity, resulting in generally larger compression set and poorer low-temperature flexibility. Binary FKM features flexible molecular chains with superior resilience at ambient and moderate temperatures at a lower cost. The two polymers follow the same crosslinking mechanism and only differ in baseline chemical resistance.

Ⅱ. Bisphenol-AF Curing System

For both binary and terpolymer FKM, bisphenol curing delivers smooth processing and low scorch tendency, offering the best compression set at room temperature. Nevertheless, the ether linkages formed during crosslinking are susceptible to hydrolysis under high-temperature steam and prolonged immersion in weak acids, resulting in swelling, hardening and gradual loss of elasticity. Terpolymer FKM outperforms binary FKM in fuel resistance and heat resistance but suffers worse compression set. Neither grade is recommended for steam pipelines or pickling equipment.

Industrial Applications

Auxiliary seals for external cooling circuits of semiconductor equipment, general pneumatic and hydraulic valves, and non-wetted cabinet gaskets. Suitable for moderate-temperature oil environments without continuous steam or weak acid exposure. Binary FKM is preferred for its cost performance and better resilience.

Ⅲ. Peroxide Curing System

Carbon-carbon crosslinks have no hydrolysis vulnerability. Peroxide-cured binary and terpolymer FKM both withstand high-temperature steam, dilute weak acids and alcohol-based fuels, with minimal metal ion leaching. Terpolymer FKM achieves overall superior heat and solvent resistance for severe corrosive service, while binary FKM provides an economical solution for mild acid and alkaline conditions. Limitations: Peroxide curing requires vacuum compression molding with precise temperature control and has a narrow scorch window; its high-temperature compression resilience is inferior to the bisphenol system.

Ⅳ.Industrial Applications

• Semiconductor and food industries: low-extraction high-purity sealing components.
• New energy industry: gaskets for high-temperature moisture pipelines. Peroxide-cured terpolymer FKM is the first choice for heavily corrosive environments.

Secondary post-curing is mandatory for both grades to extend service life. Bisphenol-cured FKM removes residual hydrogen fluoride to improve compression resilience. Peroxide-cured FKM eliminates residual peroxides to prevent progressive thermal ageing under long-term high-temperature service.

Ⅴ. Material Selection Summary

• Ambient-temperature oil circuits, low compression set and cost control: binary FKM, bisphenol cured.
• High-temperature oil circuits, fuel resistance, no steam or weak acid: terpolymer FKM, bisphenol cured.
• Continuous steam exposure, weak acid service and low leaching requirements: Mild corrosion → binary FKM with peroxide curing; High temperature & severe corrosion → terpolymer FKM with peroxide curing.

The fundamental property gap between binary and terpolymer FKM is determined by fluorine content, while resistance to water and acids as well as elasticity depend on the chemical bonds formed during crosslinking.