HNBR Rubber
Hydrogenated Nitrile Butadiene Rubber | HSN
HNBR is produced by selectively hydrogenating the butadiene segments of standard nitrile rubber, removing the double bonds that limit NBR’s temperature resistance, ozone resistance, and long-term mechanical property retention. The result is an elastomer that combines NBR’s oil and fuel resistance with a substantially higher temperature ceiling, excellent ozone resistance, and significantly better tensile strength and abrasion resistance, at a cost between standard NBR and FKM fluoroelastomer.
Wayne Rubber manufactures custom HNBR seals, o-rings, gaskets, and molded components in Wixom, Michigan. HNBR is the correct upgrade path from NBR when the application exceeds NBR’s temperature limit, requires outdoor exposure alongside oil resistance, or involves aggressive sour-gas or H2S-containing fluid environments where standard NBR degrades rapidly. This page covers the two-variable specification system (hydrogenation degree and ACN content) that determines every key HNBR property.
What Hydrogenation Does and What It Does Not Change
Standard NBR contains double bonds in the butadiene segments of the polymer backbone. These double bonds are reactive sites for ozone attack (causing surface cracking), thermal oxidation (causing hardening and embrittlement over time), and chemical attack by certain aggressive fluids, particularly hydrogen sulfide and amines encountered in oil and gas applications. The double bonds are also the primary reason NBR’s mechanical properties degrade progressively with heat aging.
Hydrogenation saturates these double bonds by adding hydrogen atoms across them, converting C=C double bonds to C-C single bonds. This saturation produces three critical changes: the backbone becomes resistant to ozone attack (like EPDM), heat aging causes significantly less property loss over time (because the oxidation sites are eliminated), and resistance to H2S and amine corrosion inhibitors is substantially improved. The nitrile groups (CN) in the ACN segments of the chain are not affected by the hydrogenation process; they remain intact and continue to provide the oil and fuel resistance that defines the nitrile elastomer family.
What hydrogenation does not change is the fundamental basis of NBR’s chemical resistance: the polar nitrile groups that repel petroleum hydrocarbons. HNBR’s oil and fuel resistance is governed by its ACN content exactly as standard NBR is, and the same ACN trade-off applies: higher ACN content means better oil resistance and poorer low-temperature flexibility. The hydrogenation process adds thermal and environmental durability on top of the existing oil resistance rather than replacing or changing it.
The mechanical consequence of saturation is significant beyond chemical resistance. Fully saturated HNBR compounds have tensile strengths of 20 to 31 MPa without heavy filler loading, substantially higher than standard NBR at equivalent durometer. This intrinsic mechanical strength makes HNBR the standard compound for high-load dynamic seal applications, timing belts, and oil field downhole tools where both mechanical and chemical demands exceed what NBR can sustain.
The Two Variables That Define Every HNBR Grade
HNBR properties are controlled by two independent variables: the hydrogenation degree (how completely the butadiene double bonds are saturated) and the ACN content (the acrylonitrile percentage in the base copolymer). These two variables control different properties and must both be specified correctly for the application. Specifying “HNBR” without stating both variables leaves critical compound decisions to the supplier.
Controls temperature resistance, ozone resistance, and cure system options.
Partially hydrogenated HNBR retains some residual double bonds. It can be crosslinked with either sulfur/accelerator systems or peroxide systems, giving more compound formulation flexibility. Its ozone resistance is improved over NBR but not as good as fully saturated grades. Temperature limit typically 250°F to 275°F (121°C to 135°C) continuous.
Fully hydrogenated HNBR has essentially no residual double bonds. It can only be crosslinked with peroxide systems (no sulfur cure possible on a fully saturated chain). Its ozone resistance equals or exceeds EPDM. Temperature limit 300°F (150°C) continuous, with short-term excursions to approximately 340°F (170°C). This is the grade required for the most demanding high-temperature and ozone-exposed applications.
Controls oil resistance and low-temperature flexibility: the same trade-off as in standard NBR.
Low ACN HNBR (17-25%) provides better low-temperature flexibility, useful to approximately -40°F (-40°C). Oil resistance is moderate. Used for cold-climate applications where HNBR’s high-temperature and ozone properties are needed alongside cold flexibility: outdoor equipment, arctic sealing, refrigeration compressor seals.
Medium ACN HNBR (28-36%) is the standard grade for most industrial and automotive applications, balancing oil resistance and low-temperature performance. The default specification for hydraulic seals, automotive engine seals, and oilfield downhole tools.
High ACN HNBR (39-44%) provides maximum oil and fuel resistance at the cost of reduced low-temperature flexibility. Used for aggressive fuel environments and high-aromatic fuel sealing where medium ACN HNBR shows unacceptable swell.
Partial vs. Full Hydrogenation: Practical Consequences
- Retains some C=C double bonds (typically 2-5% residual unsaturation) available as crosslink sites
- Can be cured with sulfur/accelerator systems or with peroxide; sulfur cure provides more compound formulation options
- Improved ozone resistance over NBR but not as complete as fully saturated grades
- Continuous service temperature approximately 250-275°F (121-135°C)
- Lower raw material cost than fully hydrogenated grades
- Used when the application needs HNBR’s improved heat aging over NBR but does not require the maximum temperature or ozone resistance of fully saturated grades
- Typical applications: automotive timing belts, power steering hoses, moderate-temperature oil seals
- Essentially complete saturation of butadiene segments; residual unsaturation below 1%
- Can only be crosslinked with peroxide cure systems; sulfur vulcanization is not possible on a fully saturated backbone
- Outstanding ozone resistance comparable to EPDM; no surface cracking in outdoor service
- Continuous service temperature 300°F (150°C); short-term excursions to approximately 340°F (170°C)
- Superior property retention after heat aging: tensile strength, elongation, and compression set degrade far less than partially hydrogenated grades at 300°F service
- Required for oil and gas downhole tools, geothermal seals, and any application with combined 300°F+ and aggressive fluid exposure
- Higher cost than partially hydrogenated grades; justified when the temperature and chemical exposure genuinely requires it
HNBR Chemical Resistance Summary
| Fluid / Environment | Resistance | Notes |
|---|---|---|
| Petroleum Oils and Fuels | Excellent | HNBR’s oil and fuel resistance equals or exceeds standard NBR at equivalent ACN content. The hydrogenation process does not reduce oil resistance; it adds heat and environmental resistance on top. Medium ACN HNBR is the standard for automotive engine oil seals, hydraulic seals, and fuel system seals requiring service above NBR’s 250°F temperature limit. |
| Hydrogen Sulfide (H2S) and Sour Gas | Excellent | This is HNBR’s most important advantage over standard NBR in oil and gas applications. H2S attacks the double bonds in NBR’s backbone, causing rapid chain scission and mechanical failure. HNBR’s saturated backbone is substantially more resistant to H2S attack. Fully hydrogenated HNBR is the standard elastomer for sour service downhole tools, wellhead seals, and gas processing equipment where H2S concentrations make standard NBR unsuitable. |
| Amines and Corrosion Inhibitors | Good to Excellent | Many oilfield corrosion inhibitors are amine-based. Standard NBR’s double bonds are reactive with amines, causing degradation in amine-inhibited fluid systems. HNBR’s saturated backbone is substantially more resistant to amine attack, making it the preferred compound for seals in amine-inhibited produced water and injection fluid systems. |
| Ozone and UV | Excellent (fully saturated) | Fully hydrogenated HNBR has no double bonds susceptible to ozone attack. Ozone resistance is comparable to EPDM for fully saturated grades. This is the key advantage that makes HNBR the correct specification for outdoor oil-wetted equipment: the application where both oil resistance (requiring a nitrile-type polymer) and ozone resistance are simultaneously required, and where EPDM’s petroleum incompatibility rules it out. |
| Heat and Heat Aging | Excellent | HNBR retains its mechanical properties after long-term heat exposure far better than standard NBR. A component that loses significant tensile strength and elongation after 1000 hours at 150°C in NBR will retain substantially better properties in fully hydrogenated HNBR under the same conditions. This property retention, not just the temperature rating, is what makes HNBR the preferred compound for belt, hose, and seal applications with long service intervals at elevated temperatures. |
| Refrigerants (HFC/HFO automotive) | Good | HNBR is widely specified for automotive air conditioning system o-rings in R-134a and R-1234yf (HFO) refrigerant service, where HNBR’s combination of refrigerant resistance, temperature performance, and low compression set provides better long-term seal integrity than standard NBR. |
| Steam and Hot Water | Moderate | Better steam resistance than standard NBR; adequate for intermittent steam contact and hot water service. Not suitable for continuous pressurized steam; EPDM is the correct specification for sustained steam sealing. Verify specific steam temperature and pressure conditions before specifying HNBR for steam-adjacent applications. |
| Ketones, Esters, Chlorinated Solvents | Poor | Same limitations as standard NBR. HNBR does not improve resistance to polar solvents. For ketone or ester environments, EPDM (if no oil contact) or FFKM (if combined with heat) must be specified. This is the most common over-specification of HNBR: engineers selecting HNBR as a “premium NBR” without checking compatibility with the specific chemical environment. |
| Glycol Brake Fluids | Poor | Like standard NBR, HNBR is attacked by glycol-based brake fluids. EPDM is required for brake fluid sealing. This incompatibility is inherited from the NBR base chemistry and is not eliminated by hydrogenation. |
NBR vs. HNBR vs. FKM: The Upgrade Decision
HNBR occupies the performance and cost space between standard NBR and FKM fluoroelastomer. The correct choice depends on which specific NBR limitation is being exceeded and how aggressively the chemical environment attacks the elastomer.
HNBR’s tensile strength advantage over FKM is a frequently overlooked specification factor. For dynamic seal applications in high-pressure environments (downhole packer seals, blow-out preventer seals, and high-pressure pump plunger seals), HNBR’s higher tensile strength and better fatigue resistance under cyclic loading often makes it the preferred compound over FKM even when FKM’s broader chemical resistance would be adequate for the fluid environment.
HNBR Applications
HNBR Products from Wayne Rubber
Wayne Rubber manufactures custom HNBR components in partially and fully hydrogenated grades across a range of ACN contents. Specify the hydrogenation degree, ACN content range, durometer, and service environment when requesting a quote.
Request Custom HNBR Parts from Wayne Rubber
Provide your part drawing, service temperature, fluid environment (especially H2S concentration, amine content, and fuel type), and hydrogenation degree requirement. Wayne Rubber will recommend the correct grade and quote.