The purpose of the article is to present basic data on O-ring properties, not to provide an in-depth review of the topic. The additional resources at the foot of the page cover the topic in significantly greater detail. The table below shows the recommended operating temperature, durometer hardness and compression set of 12 commonly used O-ring materials. The graph below provides a visual representation of the recommended operating temperatures presented in the table.
O-ring temperature compatibility
The temperature ranges provided are only estimates because O-ring properties vary slightly between manufacturers. Furthermore, the minimum and maximum operating temperatures are not well defined. Operating outside of these temperature ranges will not cause immediate and catastrophic failure of the O-ring. In fact, it is often permissible to operate at above the maximum operating temperature for short periods of time. However, doing so is likely to reduce the sealing force and increase the compression set of the O-ring. Furthermore, the fatigue life will be negatively impacted. Kalrez O-rings have the highest operating temperature, at over 300 °C. Beyond this temperature requires use of high temperature gaskets made of materials such as graphite and mica.
A materials hardness is its resistance to localized plastic deformation. Each O-ring material is available in a range of hardness values. Manufactures vary the hardness of a material by controlling the degree of polymer cross-linking. The hardness of O-rings is most commonly assessed using a shore A durometer, which measures the depth of indentation caused by a cone shaped indenting tool, to which a fixed static force is applied. An O-ring of low hardness easily conforms to the geometry of the surface being sealed. This is an ideal characteristic for achieving a good seal with minimal sealing force and so low hardness O-rings are popular in low pressure applications. However, low hardness O-rings do not provide sufficient sealing force for high pressure applications. Furthermore, low hardness O-rings used in high pressure applications are more likely to be extruded through gaps, resulting in seal failure. Finding the correct hardness requires trial and error. Shore A hardness of 70 is considered an intermediate hardness which provides good performance across many applications.
O-ring Compression set
Compression set is the permanent deformation that occurs when an O-ring is compressed for a period of time and subsequently relaxed. It is defined as the percentage change in O-ring thickness due to a compressive force over a period of time (e.g. 1 kN applied for 60 minutes). It is an important property because permanent deformation causes stress relaxation which results in a decrease in sealing force and therefore an increased probability of leakage. Materials with high compression set are less suitable for use in high pressure and high force applications. Although it is possible to counteract the effect of compression set by placing the O-ring under a larger initial load, this increases the probability of mechanical failure of the O-ring. The maximum recommended initial compression when installing an O-ring is 40% compression.
O-ring Chemical compatibility
Chemical compatibility data for O-ring materials is not provided here because there are 100’s of substances that would need reviewing. However, chemical compatibility is an extremely important parameter to consider when selecting an O-ring. Symptoms of poor chemical compatibility include swelling, discoloration, softening, blistering and in extreme cases dissolving. Depending on the severity of the chemical attack, the lifetime of the O-ring could be reduced to 6 months or to 15 minutes. Common fluids used in industry include acids, alkalies, organic solvents and fuels. A material may be compatible with strong acids and yet incompatible with other substances that are seemingly harmless. For example, Kalrez has far superior chemical resistance than NBR and EPDM. However, Kalrez has poor chemical resistance to fluorocarbons whereas NBR and EPDM generally posses good chemical compatibility with fluorocarbons. Teflon (PTFE) is the closest there is to a universally chemically resistant O-ring material. However, Teflon is a rigid material (not an elastomer) and therefore surfaces to be sealed with Teflon O-rings must be manufactured to high tolerances. As a result, Teflon is used predominantly in applications where other common O-ring materials are unsuitable.
|Name||Temperature||Hardness (Shore A)||Compression set|
|Fluorosilicone (FVMQ)||-60 to 205 °C||40-80||B|
|Kalrez (FFKM)||-25 to 315 °C||65-90||B|
|Neoprene (CR)||-40 to 120 °C||40-90||B|
|Teflon (PTFE)||-150 to 260 °C||98||A|
|Viton (FKM)||-25 to 205 °C||55-90||A|
|Silicone (VMQ)||-60 to 215 °C||10-85||B|
|Butyl rubber (BU)||-60 to 120 °C||30-90||C|
|(EPDM)||-40 to 150 °C||30-90||A|
|Nitrile rubber (NBR)||-30 to 105 °C||20-100||A|
|Polyurethane rubber (AU)||-40 to 80 °C||10-100||C|
|styrene-butadiene rubber (SBR)||-45 to 100 °C||40-90||B|
|Natural rubber (NR)||-50 to 105 °C||30-90||A|