O-ring material selection —
NBR, EPDM, FKM, SBR,
silicone and neoprene compared
Fit the wrong material and the O-ring may appear to seal initially, then swell, harden or crack under service conditions. This article covers six common O-ring elastomers you may encounter — what each one is, what each one resists, and where each one typically belongs.
Six O-ring elastomers you may encounter
The most widely used O-ring elastomer in general industrial and hydraulic applications. Good resistance to petroleum oils, mineral oils, greases and many mineral-oil hydraulic fluids. Often used in fuel-system duties, but fuel compatibility depends on fuel blend, additives, temperature and compound data. The default choice for many pneumatic and mineral-oil hydraulic systems where oil contact is present.
Poor resistance to hot water, steam and ozone. Not appropriate for outdoor weathering applications or heating system water contact without confirming compound compatibility.
The standard starting choice for water, hot water and heating system applications. Good resistance to water, ozone, weathering and many water treatment chemicals including corrosion inhibitors. Selected EPDM compounds may also be suitable for steam, but only where the compound data supports the actual steam temperature, pressure and exposure time.
Poor resistance to petroleum oils, fuels and most hydrocarbon solvents. Do not use where oil or fuel contact is present. Potable-water, food-contact or gas applications still require the relevant compound documentation or approval.
High-performance fluoroelastomer with excellent resistance to oils, fuels, solvents, aromatic hydrocarbons and high temperatures. A common next material to assess where temperature, hydrocarbon or selected chemical resistance requirements exceed the NBR or EPDM range, provided the service medium is also compatible with the chosen FKM grade.
Higher cost than NBR or EPDM. Poor resistance to low temperatures, steam above certain grades, and some ketones. Should be specified where its specific properties are genuinely required, not as a blanket premium choice.
An older general-purpose synthetic rubber, now largely superseded by NBR and EPDM for most O-ring applications. May be encountered in some legacy specifications. Brake-system seals should be replaced only with the material and compound specified by the equipment or vehicle manufacturer.
Moderate water resistance, poor oil resistance. Lower performance than NBR in most hydraulic applications and lower water and steam resistance than EPDM. Where SBR is specified in an existing system, confirm compatibility with the actual service medium before substituting another material.
Wide temperature range and good electrical insulation properties. Used in food contact, medical, pharmaceutical and some high-temperature static seal applications. Excellent for low-temperature flexibility down to −60°C in some grades.
Poor mechanical strength and low tear resistance — not suitable for dynamic applications or high-pressure duties. Not typically preferred for fuel, steam or high-stress chemical duties, and usually limited to static applications with compatible media.
Good weathering, ozone and UV resistance with moderate oil resistance — better than SBR or EPDM for oil contact, but not as good as NBR. Used in refrigeration and air conditioning systems, outdoor sealing applications and some marine environments.
Used in some refrigeration applications, but refrigerant and lubricant compatibility should always be checked for the specific system. Not suitable for petroleum fuels or aromatic hydrocarbons at higher concentrations.
Compatibility matrix — common media
This matrix is a screening tool, not a compatibility approval.
| Medium | NBR | EPDM | FKM | SBR | Silicone | Neoprene |
|---|---|---|---|---|---|---|
| Mineral oil / hydraulic fluid | Good | Poor | Excellent | Poor | Poor | Moderate |
| Petroleum fuels / petrol blends | Often suitable / check blend | Poor | Excellent | Poor | Poor | Limited |
| Water (cold) | Moderate | Excellent | Moderate / check grade | Moderate | Good | Good |
| Hot water / heating circuit | Poor | Good (compound dependent) | Check grade | Poor | Poor | Moderate |
| Steam | Poor | Grade dependent | Limited (grade) | Poor | Poor | Poor |
| Ozone / weathering / UV | Poor | Excellent | Good | Poor | Good | Good |
| Corrosion inhibitor (heating) | Poor | Generally good | Check grade | Poor | — | Poor |
| Brake fluid systems | Poor | Glycol brake fluid grades | Poor | OEM specification only | Good | Poor |
| Aromatic solvents | Poor | Poor | Good | Poor | Poor | Poor |
| Refrigerants (HFC) | Limited | Poor | Check grade / system | — | — | Moderate (check) |
| Food contact (general) | Grade specific | Grade specific | Grade specific | Not typical | Grade specific / common in documented grades | Not typical |
Compatibility ratings are general indications for standard grades of each elastomer. Actual performance depends on compound formulation, temperature, concentration and exposure duration. Always confirm compatibility against the specific compound data sheet for critical applications.
General compatibility tables are a starting point, not a specification. The same elastomer family can have very different performance depending on compound formulation, cure system and fillers. An EPDM compound optimised for steam may perform differently from one optimised for outdoor weathering. Always verify against the actual compound data sheet for the specific product and application.
The key mismatches — where wrong material selection fails
NBR in hot water or steam. This is a common material error in heating system maintenance. NBR looks like a general-purpose rubber and is cost-effective and widely available. In unsuitable hot-water service, general-purpose NBR can harden, lose recovery and weep. Steam service is normally outside general-purpose NBR selection and requires a separate material check. The joint that sealed fine in cold water starts to weep within weeks of the heating season starting. EPDM is the standard starting material for closed heating circuits and hot water systems, subject to compound suitability.
EPDM in oil or fuel contact. EPDM has good water resistance and selected compounds can handle steam, which leads some to treat it as a general-purpose upgrade. In petroleum-based oils and fuels, EPDM swells significantly. EPDM in oil or fuel contact is generally unsuitable and can lead to severe swelling and loss of sealing performance. NBR is typically the starting material there.
Silicone in dynamic applications. Silicone's wide temperature range attracts specification in demanding environments. Its poor mechanical strength means it extrudes under pressure and wears rapidly in dynamic sealing — rotating shafts, reciprocating pistons. For static seals in temperature-demanding environments it can be appropriate. For dynamic applications, it is typically not.
SBR where EPDM or NBR is available. SBR is an older material that persists in legacy systems. Where replacement is possible, both NBR and EPDM offer better performance in their respective application areas. Before repeating a legacy SBR specification, confirm whether a better-suited modern elastomer exists for the actual medium.
EPDM in heating systems — compound matters
Within the EPDM family, the cure system significantly affects performance at sustained elevated temperature. Sulphur-cured EPDM compounds develop higher compression set over time at elevated temperature compared to peroxide-cured grades. Compression set — the permanent deformation the O-ring accumulates under load — directly determines how long the seal remains effective.
Peroxide curing creates carbon-carbon cross-links rather than sulphur bridges. These are more thermally stable, giving lower compression set at sustained elevated temperature. For heating system O-rings that remain under load at 60–110°C for extended periods, the cure system is a meaningful performance variable — not just a processing choice.
Peroxide-cured EPDM with published compression set data is a strong starting specification for many heating system O-ring applications. Nominal temperature range alone does not indicate how the compound performs under sustained load at elevated temperature — compression set data does. See the Kinetics Line compound section below for confirmed M534 values.
Field check: Before choosing an O-ring material, identify what actually touches the seal, the minimum and maximum temperature, whether the seal is static or moving, the pressure, the groove condition and any documentation requirement. Do not select from colour, hardness or "rubber type" alone. The same black O-ring can be NBR, EPDM, FKM or another compound — and those materials fail in opposite media.
Selection sequence
When selecting an O-ring material, work through these questions in order:
- What is the medium? Oil, fuel, water, steam, refrigerant, chemical. This eliminates most wrong choices immediately.
- What is the temperature range? Minimum and maximum, not average. Cold start and peak operating temperature both matter.
- Is it static or dynamic? Dynamic applications eliminate silicone and some softer compounds.
- What is the pressure? Higher pressure requires harder compounds or backup rings to prevent extrusion.
- Are there documentation or compliance requirements? Potable water, food contact and gas require compound-specific documentation, approvals or compliance declarations depending on the application and jurisdiction — not just elastomer family compatibility.
- What is the service interval? Long service life under sustained load demands lower compression set — which points to compound formulation, not just elastomer family.
The general elastomer guide above is enough to narrow the material family. For heating applications, long-term performance depends on the specific compound — which is where published compression set data becomes more useful than nominal family temperature range alone.
Kinetics Line O-rings — compound data and documentation
Most O-ring suppliers list elastomer family and a nominal temperature range. Compound-level test data — compression set values at defined temperatures over defined time periods — is less commonly published. For heating system applications where the O-ring remains under load at elevated temperature for extended periods, compression set data is the relevant specification parameter, not the nominal temperature range alone.
EPDM peroxide — compound M534
Kinetics Line EPDM O-rings use peroxide-cured compound M534. Peroxide curing creates carbon-carbon cross-links rather than sulphur bridges. These are more thermally stable at sustained elevated temperature, giving lower compression set over time compared to sulphur-cured EPDM grades.
Confirmed compound data (source: Kinetics Line EPDM M534 technical data sheet, rev. 37, 2024-05-02):
- Hardness: 72 ShA (nominal 70)
- Density: 1.15 g/cm³
- Tensile strength: 17 N/mm²
- Elongation at break: 210%
- Modulus 100%: 4.5 N/mm²
- Temperature range: −65°C static / −45°C dynamic to +150°C continuous / +160°C peak
Compression set values (ISO 815-1 Method A unless noted):
- 10% @ 22h / 100°C
- 20% @ 70h / 100°C
- 10% @ 22h / 125°C
- 20% @ 70h / 150°C
- 10% @ 1000h / 110°C in water — DVGW W534
- 15% @ 2000h / 110°C in water — DVGW W534
- 19.5% @ 3000h / 110°C in water — DVGW W534
The DVGW W534 long-term test data — 3000 hours at 110°C in water — is the relevant benchmark for heating system O-ring specification. A compression set of 19.5% after 3000 hours at 110°C in water is a strong published data point for comparing compounds in hot-water O-ring applications. Actual service life still depends on groove design, squeeze, pressure, medium, installation quality and duty cycle.
What the DVGW W534 data means in practice: most heating system O-rings sit at load continuously through the heating season — typically 6 to 9 months per year. The 3000-hour / 110°C test is a severe long-duration hot-water benchmark. It gives a stronger basis for comparison than short-duration compression set data, but it is not a direct service-life guarantee. A compound with published data at this level has stronger evidence for this application than a compound with only nominal temperature range data.
Frequently asked questions
What is the most common O-ring material?
NBR (nitrile butadiene rubber) is the most widely used O-ring elastomer in general industrial, hydraulic and pneumatic applications. It offers good resistance to petroleum oils, greases and many mineral-oil hydraulic fluids. It is also used in fuel-system duties where the fuel blend and compound are compatible. Typical temperature range is approximately −30°C to +100/+120°C depending on compound. EPDM is the more common choice for water and heating applications, where NBR's poor resistance to hot water makes it unsuitable.
What O-ring material for hot water and heating systems?
EPDM is the common starting material for water, hot water and many heating applications. It offers good resistance to water and many water treatment chemicals, including the corrosion inhibitors used in closed heating circuits. Steam service is compound-specific and must be checked against the actual grade data. General-purpose NBR is normally avoided for hot water and steam service, where it can harden, lose recovery and degrade. For higher-temperature or more demanding applications, alternative compounds may be required, but selection should be based on the actual medium, temperature and compound data rather than cost tier alone.
What is the difference between FKM and Viton?
Viton™ is a registered trade name for FKM fluoroelastomer materials. FKM is the ISO/ASTM designation for the same elastomer family. They refer to the same class of material. FKM/Viton O-rings typically offer higher temperature resistance than NBR or EPDM in standard grades, together with strong resistance to many fuels, oils, hydrocarbons and selected solvents, depending on grade.
Medium first. Temperature second. Groove, pressure and movement immediately after that.
NBR is typically the starting family for oils and hydraulics. EPDM is the standard starting family for water, hot water and heating duties — with steam compound-specific. FKM is commonly assessed where temperature or chemical resistance requirements exceed those ranges, provided the service medium is compatible with the chosen grade. SBR only where legacy system compatibility requires it. Silicone for static, food-contact-grade or low-temperature duties. Neoprene for weathering and some refrigeration. Within each family, the compound formulation — particularly the cure system for EPDM — determines long-term sealing performance at elevated temperature.