O-ring designations and material codes

Overview: 12 O-ring designations

With twelve common O-ring designations, choosing the right material can be difficult. To make the decision easier, this page lists all twelve designations with their most important properties in a clear overview.

For each material you will find the relevant details on material designation, resistance and mechanical load capacity. This lets you quickly identify which material suits your application.

Differences in raw material price

Compared with the other O-ring materials, NBR is the cheapest material. The factor in the following comparison indicates how much more expensive a material is than NBR.

These factors explain why most suppliers do not keep expensive materials such as FFKM in stock. At NH O-RING, things are different.

FFKM (perfluoroelastomer)

FFKM is by far the highest-performing O-ring material. FFKM combines almost universal media resistance with the highest temperature resistance, from −40 °C to +340 °C depending on the compound.

Unlike FEP or PTFE, FFKM is elastic and therefore suitable for every type of application, that is, for flange, piston and rod seals. In addition, FFKM is available in a wide range of compounds, for example for chemical process technology, the food sector (FDA), the oil and gas industry (NORSOK), the semiconductor industry as well as for low and high temperatures.

The disadvantage: the raw material price is extremely high, as a reference value around 100x higher than FKM. For this reason, only a few suppliers keep FFKM O-rings in stock, which makes short-term availability difficult. Important to note: under no circumstances may FFKM come into contact with molten alkali metals, as this can produce toxic gases.

FEP / PFA (fluorinated ethylene propylene)

FEP is a plastic used to encapsulate O-rings. The finished O-ring is a combination product consisting of the seamless FEP encapsulation and an elastic core. FKM (black) or silicone (red) are typically used as the core.

This combines the almost universal media resistance of FEP with the elasticity of rubber. The FEP encapsulation complies with FDA requirements and Directive EG 1935/2004, so that FEP O-rings can be used in the food and pharmaceutical industries. FEP O-rings can be manufactured in almost any size and in any small quantity. Our largest FEP O-ring to date had an inner diameter of almost 5 meters.

As a combination product, however, FEP also has disadvantages. No other material shows such large quality differences. You can recognize good quality by the fact that there is little air between the core and the jacket and that the ring is cleanly and seamlessly welded. The encapsulation must not be damaged, so FEP O-rings are not suitable for heavy abrasion loads or dynamic applications. Due to their partial elasticity, they should only be used as a flange or rod seal and stretched by no more than 3 % during installation.

FKM (fluoroelastomer)

FKM is the all-rounder among the O-ring materials. FKM has good media resistance and at the same time good temperature resistance of −25/+200 °C.

There is a wide range of FKM compounds in various Shore hardnesses, colors and approvals, for example FDA or BAM. In addition, compounds for low-temperature applications down to −40 °C or high-temperature applications up to +225 °C are available. With these properties, FKM offers an excellent price-performance ratio.

FKM has hardly any serious disadvantages. FFKM is superior in every respect, but FKM is significantly cheaper. If no extreme requirements call for an FFKM O-ring, FKM is an excellent alternative. One small drawback is its comparatively low low-temperature flexibility. However, FKM is available in LT compounds (LT for low temperature) that are resistant down to −40 °C.

A classic mistake is confusing the designation. So that you do not fall into this trap, here is the explanation:

• FKM is the current, official designation for fluoroelastomer.
• FPM is an outdated designation for fluoroelastomer.
• Viton is FKM and merely a trademark of the DuPont company.

PTFE (polytetrafluoroethylene)

PTFE has almost universal media resistance and generally complies with the FDA guidelines. In addition, PTFE has an extremely broad temperature range of −250/+250 °C and is known for its excellent coefficient of friction. This makes PTFE outstandingly suitable for dynamic applications. PTFE O-rings can also be manufactured in any size.

Unlike all other O-ring materials, PTFE is not an elastomer but a thermoplastic. This results in the high hardness of 60 Shore D, which corresponds to about 100 Shore A. Because of this hardness, PTFE O-rings cannot be stretched and are used exclusively as flange seals. During compression they deform plastically and can therefore be compressed only once. In addition, PTFE O-rings must be stored more carefully, as otherwise they can lose their round shape over time.

NBR (nitrile butadiene rubber)

NBR is probably the best-known O-ring material and is used most frequently. A major advantage is its very low price, which is why NBR is produced in large quantities.

NBR has good resistance to oils and fuels and is mechanically robust. This is why NBR is popular in the automotive industry. NBR is also interesting for vacuum applications, because like FFKM and FKM it has comparatively low gas permeability.

The biggest disadvantage is its low resistance to temperature, ozone, weathering and a large number of chemicals. When designing, you should therefore check whether NBR meets the requirements. NBR O-rings must be stored in the dark and airtight, because their service life is only around 6 years and decreases further with improper storage.

HNBR (hydrogenated nitrile butadiene rubber)

HNBR is hydrogenated NBR and very comparable in its properties. However, HNBR has higher temperature resistance of −30/+150 °C and higher chemical resistance.

HNBR is peroxide cross-linked, whereas NBR is sulfur cross-linked. This is not a quality difference as such, but there are applications in which no sulfur may be present in the compound. In addition, HNBR is more resistant to ozone and weathering and has a longer service life of around 8 years.

The biggest disadvantage is its poor availability. Very few sizes are available from stock and must first be produced when required. Because the raw material first has to be mixed, this has a negative effect on the price. In most cases it therefore makes sense to use an FKM instead of HNBR, as there are only a few chemicals to which HNBR is resistant but FKM is not.

EPDM (ethylene propylene diene rubber)

EPDM is a versatile O-ring material. Its biggest advantage is its resistance to a wide range of chemicals as well as to the aggressive cleaning processes CIP (Clean in Place) and SIP (Sterilization in Place). This is why EPDM is popular in the chemical, food and pharmaceutical industries.

EPDM is excellently resistant to hot water and steam up to +170 °C. For comparison: FKM is only resistant to steam up to +120 °C. EPDM also has good low-temperature flexibility and can be used down to −50 °C. Compounds with FDA, USP Class and 3-A Sanitary approvals are available.

The biggest disadvantage is its lack of resistance to oils and greases. If an EPDM O-ring comes into contact with them, it soaks up and grows significantly in volume. As a result, the O-ring loses its feel and surface structure, and sooner or later the sealing effect fails. In addition, EPDM is resistant only up to +120 °C, or up to +150 °C when peroxide cross-linked. For comparison: FKM and silicone are permanently resistant up to +200 °C.

VMQ (vinyl methyl silicone)

VMQ, i.e. silicone, is very frequently used in industry. This is due to its comparatively low price and its attractive properties for many industry sectors.

Silicone has high elasticity, excellent low-temperature flexibility down to −60 °C, excellent weathering resistance and generally complies with the FDA guidelines. This is why silicone is used in particular in the food and pharmaceutical industries as well as in plant engineering.

The biggest disadvantage is its comparatively low media resistance. Silicone O-rings fail after a short time when exposed to many chemicals. Its mechanical load capacity is also low: the elongation at break is high, but the tensile strength is very low. Silicone O-rings can therefore be stretched wide, but only with little force. In addition, silicone has a high coefficient of friction and is therefore not suitable for dynamic applications.

FVMQ (fluoro vinyl methyl silicone)

FVMQ is a fluorinated VMQ. In fluorosilicone, silicone (VMQ) is combined with fluorine (F). This creates a material that combines all the advantages of silicone with the excellent media resistance of fluorine.

Thanks to the fluorine, FVMQ is even resistant to oils and fuels and is therefore popular in the automotive industry and the oil and gas industry.

There are only a few application areas in which the use of FVMQ makes sense, mainly where high resistance to oils and fuels coincides with very low temperatures down to −60 °C. Availability is correspondingly low, and FVMQ O-rings are rarely a stock item. In most cases they are new production, which drives the price up.

FEPM (tetrafluoroethylene rubber)

FEPM is comparable to FKM in its basic properties. Its biggest advantage is its excellent resistance to a wide range of chemicals. In addition, FEPM has outstanding mechanical properties, excellent weathering resistance and very good paint resistance.

The biggest disadvantage is its low availability and the comparatively high price associated with it. In most applications FKM is the better choice, because most FKM sizes are available from stock and considerably cheaper. There is also the low low-temperature flexibility of −10 °C.

CR (chloroprene rubber)

CR, also known as neoprene, is chosen comparatively rarely as an O-ring material. However, there are applications in which CR makes sense, because CR has excellent resistance to a wide range of refrigerants.

In addition, CR has outstanding mechanical resistance and can therefore be used as a dynamic seal. CR is mainly used in air-conditioning and refrigeration technology as well as in the marine sector.

The biggest disadvantage is its low availability and the comparatively high price associated with it. Overall there are only a few sensible use cases, as CR has only low temperature resistance of −40/+100 °C and average media resistance. At around 6 years, its service life is also rather short.

ACM (acrylate rubber)

ACM is a material developed for the automotive industry. ACM has excellent resistance to heating oil and lubricants as well as outstanding resistance to ozone and hot air.

A further advantage is its low gas permeability, which is why ACM is also used in vacuum applications.

ACM is the least in demand in the world of O-rings. This is mainly because fluorine-based materials such as FKM, FEPM or FFKM have gradually displaced the ACM material. Nowadays only a few automotive manufacturers still use ACM. Accordingly, ACM is almost never a stock item, which results in comparatively high costs for new production.

Frequently asked questions