Types of EPDM O‑rings: peroxide and sulphur cured

EPDM (Ethylene-Propylene-Diene Monomer)

EPDM (Ethylene-Propylene-Diene Monomer) is a versatile elastomer produced by sulfur or peroxide vulcanization. Due to its properties (resistance to water, water vapor, ozone, UV radiation, hydraulic fluids), it is used worldwide.

Vulcanization

Vulcanization (curing) is a process in which long polymer chains are formed and linked together to create a highly flexible, homogeneous product. Rubber cures when a sufficiently high temperature is applied for a specified period. EPDM rubber is an elastomer crosslinked by sulfur or peroxide vulcanization. Both vulcanization processes have certain advantages and disadvantages.

EPR – What is it?

EPR is ethylene-propylene rubber, a copolymer of ethylene and propylene. EPDM synthetic rubber is a terpolymer of ethylene and propylene with a small amount of a third monomer (usually a diene) that enables sulfur vulcanization. EPDM offers unique properties, i.e., resistance to weathering, ozone, chemical resistance (e.g., to diluted acids and bases, polar solvents), excellent flexibility in low-temperature environments, and electrical insulation. The broad application of EPDM is useful in many industries.

Types of EPDM O-Rings

There are EPDM O-rings crosslinked via sulfur and peroxide vulcanization. The choice of curing method depends on the required properties for a given application. Peroxide-cured elastomers have better high-temperature resistance, while sulfur-cured elastomers provide better flexibility. Below we take a closer look at these two types of EPDM O-rings.

Peroxide-Cured EPDM O-Rings

EPDM synthetic rubber has exceptional resistance to ozone, weathering, and high temperatures. O-rings are exposed to aggressive chemicals, varying temperatures, weather conditions, and cyclic pressure fluctuations depending on the environment in which they are installed. EPDM O-rings are cured with sulfur or peroxide to enhance material properties in specific applications. Learn about the properties of peroxide-crosslinked EPDM and where it is most commonly used.

Peroxide Curing – Properties

Peroxide-cured EPDM withstands high temperatures up to 150 °C. Peroxide-vulcanized EPDM synthetic rubber also shows compression and aging resistance (longer service life), as well as resistance to alcohol, hot water, steam (up to 204.4 °C), liquid engine coolant, brake fluids, ketones, organic and inorganic acids. Peroxide-cured EPDM O-rings should not be used in applications involving contact with fuels, petroleum-based lubricants and oils, phosphate ester hydraulic fluids, aliphatic hydrocarbons, and mineral oils. Peroxide-cured seals are suitable for clean and hygienic applications such as drinking water, food, beverages (including dairy products).

Sulfur-Cured EPDM O-Rings

By mixing different types of rubber, it is possible to change the properties of finished rubber products. Sulfur vulcanization positively affects the thermal stability of O-rings and their chemical resistance. Sulfur vulcanization can be accelerated or non-accelerated. Accelerated sulfur vulcanization depends, among other things, on the rubber structure and the type of activators.

Sulfur Curing – Properties

Sulfur-vulcanized EPDM O-rings offer very good abrasion and tear resistance. Due to their flexible properties, they elongate under high loads, which protects the O-rings from breaking. Sulfur-vulcanized O-rings withstand a temperature range from -55 °C to +120 °C. Sulfur-vulcanized O-rings have good weather resistance and good dynamic properties. During the sulfur vulcanization process, a low-intensity odor is generated and production costs are much lower than in the case of peroxide curing. The use of sulfur during curing also allows the use of more fillers.

Sulfur- or Peroxide-Cured EPDM O-Rings – Differences

Sulfur-vulcanized EPDM material shows excellent flexibility compared to peroxide-cured EPDM. Unfortunately, it tends to harden more easily. EPDM crosslinked by peroxide vulcanization, on the other hand, offers better thermal stability and chemical resistance and has better compression properties. The advantage of peroxide curing is the formation of C–C (carbon-carbon) bonds that are thermally stable, providing higher and more effective heat resistance. O-rings made of peroxide-cured rubber withstand temperatures up to 150 °C, while those vulcanized with sulfur withstand up to 120 °C. Peroxide-cured O-rings are, however, more expensive to manufacture. In addition, peroxide-vulcanized rubber products have higher mechanical strength (compressive strength) than sulfur-vulcanized ones. Sulfur vulcanizates, in contrast, provide higher tensile, tear, and abrasion strength. Under high stress, the O-rings elongate much more, preventing tearing.

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Summary

Comparison with Other Materials

O-Rings Made from an EPDM and NBR Rubber Blend

Ethylene-propylene-diene monomer rubber offers the highest radiation resistance among elastomers. EPDM rubber exposed to the environment degrades. It also shows poor resistance in contact with aliphatic hydrocarbons. Nitrile rubber (NBR) is a polymer resistant to the effects of aliphatic hydrocarbons and resistant to radiation. O-rings made from an EPDM and NBR rubber blend increase the service life of a rubber product resistant to both UV radiation and aliphatic hydrocarbons. NBR O-rings also offer high resistance to lubricating oils and polar solvents (acids and bases). Due to these properties, NBR O-rings are often used in applications that come into contact with mineral oil-based oils and greases. For outdoor use, O-rings made of ethylene-propylene-diene rubber are definitely the better choice. This elastomer is definitely more cold-resistant than NBR rubber O-rings.

Viton O-Rings (FKM / FPM)

O-rings made of FKM fluoroelastomer (Viton) are, compared to EPDM rubber, resistant to hot water, steam, acids, and alkalis. Brake fluids also degrade Viton O-rings (FKM / FPM). On the other hand, such seals offer resistance to contact with mineral oils, fuels, and lubricants. Both EPDM O-rings and FKM (Viton) O-rings show comparable resistance to ozone and weathering.

Silicone O-Rings (VMQ)

Silicone O-rings provide very good insulating properties. Silicone O-rings are ozone-, weather-, and chemical-resistant (diluted salt solutions, vegetable and animal oils and fats). However, they are not mechanically strong. Due to their low strength, silicone O-rings wear quickly and are easily stretched and torn. Silicone reacts poorly to hot steam, acids and alkalis, or aromatic hydrocarbons; therefore, O-rings made of this material should not be used where they come into contact with these factors.

HNBR O-Rings

Hydrogenation of nitrile rubber (NBR) produces a synthetic HNBR polymer. O-rings made from this material have excellent mechanical strength and are wear-resistant. HNBR O-rings are resistant, e.g., to HFA, HFB, and HFC hydraulic fluids, hot water, ozone, atmospheric conditions, diluted acids and bases. HNBR O-rings are not compatible with chlorinated hydrocarbons, polar solvents (ketones, esters, ethers), and strong acids.

FFKM O-Rings

FFKM is a perfluoroelastomer that offers the highest resistance in an operating temperature range up to 320 °C and also high resistance in contact with chemical substances. O-rings made of FFKM are perfect for critical applications (e.g., in jet engines).

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