Orings EPDM: Sulphur Cured (sulphur-networked)

EPDM O-rings crosslinked with sulfur are also referred to as sulfur-vulcanized EPDM O-rings. They are formed in the vulcanization process, during which the polymer chains are joined into a permanent three-dimensional structure. The method of crosslinking affects the mechanical properties, elasticity, temperature resistance and sealing behavior during operation.

EPDM is valued as an elastomer primarily for its resistance to water, water vapor, ozone, UV radiation and weather conditions. In the case of sulfur-crosslinked O-rings, elasticity and good mechanical properties also play an important role.

What is the vulcanization process?

The vulcanization process is so widespread that even people unrelated to the technical industry have heard of it. Few, however, know exactly what it involves and why it is so important — especially in the production of elastomers from which popular rubber products such as EPDM O-rings are made.

Although many people consider vulcanization a modern process, its roots go back to prehistoric times. The Aztecs already obtained latex from the Castilla elastica tree and mixed it with plant juices, creating a primitive form of rubber. Modern vulcanization was only discovered in the 18th century and perfected in the 19th.

Thanks to it, we now use rubber in tires, hoses, gaskets and O-rings. Natural rubber without curing is sticky and brittle at low temperatures. Vulcanization eliminates these problems, giving rubber elasticity, resilience and resistance to deformation.

Chemistry of sulfur vulcanization

In classic vulcanization, sulfur bonds to the double bonds between carbon atoms in the rubber structure, forming sulfur bridges. This process takes place at around 150 °C in calenders. The more sulfur used, the greater the hardness and abrasion resistance of the finished product.

Sulfur crosslinks are formed during sulfur vulcanization, joining EPDM polymer chains into a durable structure and affecting the elasticity and mechanical properties of the material.

Peroxide vulcanization

An alternative to sulfur is peroxide, used with silicones and special elastomers. It improves high-temperature resistance and is used, among other things, in tire retreading.

What are O-rings?

EPDM O-rings are closed rings with a circular cross-section used to seal liquids or gases. They are easy to install, take up little space and can operate both statically and dynamically — even up to 150 °C. They are produced from various elastomers, but EPDM stands out for its resistance to weathering, water vapor and many chemicals.

How are sulfur-crosslinked O-rings made?

Sulfur-vulcanized EPDM O-rings are made seamlessly, and their hardness depends on the amount of sulfur used. The vulcanization process allows properties to be tailored to specific applications.

Methods of producing EPDM O-rings

• Compression molding – manually placing the rubber in the mold. It is suitable for single-piece production and non-standard sizes.

• Injection molding – automatic and fast, ideal for large series in standard sizes.
   

Properties of sulfur-crosslinked EPDM O-rings

Properties of EPDM rubber

EPDM rubber (ethylene-propylene-diene rubber) is characterized by resistance to UV radiation, ozone, water vapor, moisture and many chemicals. For this reason, it is often chosen instead of SBR or NBR.

Operating temperature range depending on crosslinking method

• Sulfur crosslinking: from –45 °C to +120 °C

• Peroxide crosslinking: from –45 °C to +150 °C

Advantages of EPDM O-rings

• Wide operating temperature range

• High resistance to UV, ozone, steam and moisture

• Low permanent deformation after compression

• Resistance to ketones, alcohols, bases, polar solvents

• Compatibility with brake fluids DOT 3, 4 and 5 and HFD-R fluids

• Good chemical resistance to cleaning agents and low-concentration organic acids

What are EPDM O-rings not resistant to?

• Petroleum-based products (oils, fuels) – NBR will be better

• Organic and non-polar solvents

• Aliphatic and aromatic hydrocarbons – FPM (Viton) recommended

• Concentrated acids

Sulfur-crosslinked EPDM O-rings versus peroxide-crosslinked

EPDM can be crosslinked with either sulfur or peroxides. However, this does not mean that both types have identical properties.

Sulfur-crosslinked EPDM is often chosen for its good elasticity, mechanical properties, abrasion resistance and favorable production cost.

Peroxide-crosslinked EPDM can provide better thermal stability and performance properties in demanding temperature conditions.

In practice, the choice between the two systems should be based on the parameters of the specific application, not solely on the type of crosslinking agent used.

Applications and installation of EPDM O-rings

EPDM O-rings are widely used in the automotive industry – in braking systems, engines, fuel injection systems, turbochargers, thermostats, combustion chambers. They are also found in keyboards, fittings and industrial machinery.

Installation tips

• Avoid sharp edges – they can damage the O-ring

• Thoroughly clean the groove and the O-ring

• Do not use adhesives

• Do not use mineral oils with EPDM

• Allowable short-term stretch up to 20 %

• Store according to ISO 2230 – up to 10 years, in a dry and cool place (humidity below 70 %)
   

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We will help select the appropriate sealing solution for a specific application.