Hydrogenated nitrile butadiene rubber (HNBR) occupies a specific and well-defined position in the elastomer selection hierarchy: it is the material of choice when standard NBR is technically insufficient but the cost and lower-temperature limitations of FKM make it impractical. HNBR is not a general-purpose upgrade from NBR — it is a targeted solution for three specific engineering challenges: elevated temperature above +120°C in petroleum environments, ozone and outdoor exposure, and sour gas service where hydrogen sulfide is present.
Understanding where HNBR wins, where NBR is still adequate, and where FKM is required prevents both under-specification (failures in the field) and over-specification (unnecessary procurement cost).
Quick answer: HNBR continuous temperature range: −30°C to +150°C (standard grade); −40°C to +150°C (low-temperature grade); −25°C to +165°C (high-heat RDB ≤ 0.9% grade). Compression set at +120°C/22h: 25–38% (vs NBR 40–55%). Primary advantages over NBR: ozone resistance, H₂S (sour gas) compatibility per NACE MR0175, automotive refrigerant (R-134a, R-1234yf) service, and petroleum hydraulic service above +100°C. HNBR is a direct dimensional drop-in for NBR at the same Shore A hardness — no groove modification required. Cost: 1.5–3× standard NBR. Upgrade to FKM when continuous temperature exceeds +150°C or chemistry includes ketones, strong acids, or high-aromatic fuels.
What Is HNBR? The Hydrogenation Process
HNBR (Hydrogenated Nitrile Butadiene Rubber) starts as NBR — a copolymer of acrylonitrile (ACN) and butadiene. The butadiene units in the NBR backbone contain carbon–carbon double bonds (C=C) that are the reactive sites for ozone attack, thermal oxidation, and sour gas degradation.
The hydrogenation process saturates most of these C=C bonds with hydrogen using a palladium or rhodium catalyst under elevated temperature and pressure (typically +80°C to +150°C, 2–10 MPa hydrogen partial pressure). The result is a polymer with:
- The same ACN content as the parent NBR (same oil and fuel resistance)
- A predominantly saturated backbone (dramatically improved ozone, thermal, and chemical stability)
- Slightly modified mechanical properties (higher tensile strength, improved tear resistance)
Degree of hydrogenation: Commercial HNBR is typically described by its residual double bond (RDB) content. Standard-grade HNBR has RDB ≤ 4% (≥96% hydrogenated). Premium low-RDB grades (RDB ≤ 0.9%, ≥99% hydrogenated) provide the best thermal stability and sour gas resistance for the most demanding oilfield applications.
Commercial polymer brands: Therban® (LANXESS), Zetpol® (Zeon Chemicals), and Tornac® are the major HNBR polymer trade names. These are raw polymer brands — finished O-ring compounds from different suppliers may use the same polymer with different compound formulations (carbon black loading, plasticizers, cure agents) that affect the final properties.
Temperature Performance: Grade-by-Grade Data
| Grade | Continuous Service | Short-Term Peak | Low-Temperature Limit | Notes |
|---|---|---|---|---|
| Standard (RDB ≤ 4%) | +150°C | +165°C | −30°C | General hydraulics, automotive |
| Low-temperature grade | +150°C | +165°C | −40°C | Cold-climate equipment, arctic service |
| High-heat (RDB ≤ 0.9%) | +165°C | +180°C | −25°C | Automotive AC, oilfield downhole |
| Peroxide-cured grade | +150°C | +170°C | −30°C | Better compression set at elevated temp |
Compression Set Performance
Compression set is the most critical high-temperature property for sealing. An O-ring that has set to more than 40% (ASTM D395 Method B) has typically lost enough elastic recovery to cause leakage.
| Temperature / Duration | NBR (Standard 70 ShA) | HNBR (Standard 70 ShA) | HNBR (High-Heat Grade) |
|---|---|---|---|
| +100°C / 22 hours | 25–35% | 18–28% | 14–22% |
| +120°C / 22 hours | 40–55% | 25–38% | 20–30% |
| +150°C / 22 hours | >70% (out of range) | 35–52% | 28–40% |
| +165°C / 22 hours | Not applicable | >60% (marginal) | 38–50% |
These data show why +120°C is the practical boundary between NBR and HNBR: at that temperature, NBR compression set reaches levels where sealing reliability becomes uncertain, while HNBR remains within serviceable limits.
Acrylonitrile (ACN) Content and Its Effects
HNBR is available in different ACN content grades, which control the oil resistance vs. low-temperature flexibility tradeoff — the same way it works in NBR.
| ACN Content | Oil/Fuel Resistance | Low-Temp Flexibility | Typical TR10 | Applications |
|---|---|---|---|---|
| 28–30% | Good | Excellent | −48°C | Cold-climate hydraulics, refrigeration service |
| 34–36% (standard) | Good to excellent | Good | −36°C | General hydraulic and fuel service |
| 39–40% | Excellent | Limited | −25°C | High-aromatic fuel, hot oil service |
For most industrial hydraulic and automotive applications, 34–36% ACN HNBR is the standard specification. When specifying HNBR for cold-climate service (outdoor equipment in northern regions, cold soak conditions), confirm the ACN content — a high-ACN HNBR compound will perform worse at cold start than a standard NBR compound.
Chemical Compatibility
Excellent Resistance
| Fluid / Chemical | Notes |
|---|---|
| Mineral hydraulic oil (ISO VG 32–100) | Primary application |
| Engine oil (SAE 0W–40, mineral and semi-synthetic) | Excellent |
| Diesel fuel (including low-sulfur ULSD) | Good |
| Gasoline (up to ~25% aromatics) | Good |
| Biodiesel FAME (B5–B20 at temperature) | Good; better than NBR at elevated temp |
| Sour crude oil (H₂S present) | Excellent; major advantage over NBR |
| Water and water-glycol hydraulic fluids | Good |
| Dilute acids and bases (pH 4–10) | Good |
| Amines (primary and secondary, at moderate temperature) | Better than NBR; not as good as FKM or FFKM |
| Ozone (unlimited concentration) | Excellent |
| Refrigerants R-134a, R-1234yf with lubricant | Excellent; industry standard for automotive AC |
| R-410A, R-404A, R-32 refrigerants | Good to excellent (verify per compound) |
Limited Resistance
| Fluid / Chemical | Notes |
|---|---|
| Ethanol blends (E85+) | Significant swell; FKM preferred |
| Aromatic hydrocarbons >25% | Moderate swell; check specific compound |
| Biodiesel B100 at high temperature | Marginal; FKM preferred |
Poor Resistance (Avoid)
| Fluid / Chemical | Notes |
|---|---|
| Ketones (acetone, MEK, MIBK) | High swell; FKM or FFKM required |
| Esters (ethyl acetate, FAME at high concentration) | Swell and degradation |
| Chlorinated solvents (TCE, DCM) | High swell |
| Strongly oxidizing acids (concentrated HNO₃, H₂SO₄) | Degradation |
| Phosphate ester hydraulic fluids (Skydrol) | Not suitable; EPDM required |
| Silicone fluids | Swelling |
Sour Gas (H₂S) Service and NACE Compliance
This is HNBR's most significant advantage over NBR in oilfield applications. Hydrogen sulfide attacks NBR through its residual double bonds — in sour environments, NBR undergoes rapid degradation through a combination of sulfur crosslinking (initially stiffens the compound) followed by chain scission (eventually brittle failure). The timeline varies with H₂S concentration and temperature, but degradation in high-concentration H₂S at +80°C can occur within days for NBR.
HNBR's fully saturated backbone has no reactive C=C sites for H₂S attack. The primary failure mode shifts from chemical degradation to mechanical degradation under pressure and temperature — which HNBR's improved mechanical properties also handle better than NBR.
NACE MR0175 / ISO 15156 compliance: This standard specifies material requirements for equipment used in H₂S-containing petroleum environments. HNBR compounds that meet NACE requirements must demonstrate:
- Resistance to H₂S at specific concentrations and temperatures
- No cracking or embrittlement in H₂S partial pressure environments
- Tensile and elongation retention within specified limits after sour gas exposure
HNBR compounds qualified to NACE MR0175 are available with full material test documentation (hardness, tensile, elongation, compression set before and after H₂S exposure). Specifying NACE-compliant HNBR without the documentation does not constitute compliance — the test data must accompany the lot.
Oilfield applications using NACE-grade HNBR:
- Downhole packer elements and packing stacks
- Wellhead seals and connectors
- Blowout preventer (BOP) seals and rams
- Sour gas pipeline valve seals
- Subsea connector and tree seals (shallow H₂S service)
- Completion equipment: bridge plug seals, frac plug elements
Automotive Air Conditioning: A Key Specific Application
HNBR has replaced NBR as the industry standard for automotive air conditioning refrigerant seals in most OEM programs. The reason is the combination of refrigerant and compressor oil compatibility with thermal performance.
Why HNBR for automotive AC:
- R-134a (HFC-134a): Standard automotive refrigerant. HNBR shows low volume swell (typically 8–15% in R-134a at +100°C for 70 hours) and good compression set. NBR is adequate at lower temperatures but degrades faster at AC compressor discharge temperatures (+100–130°C).
- R-1234yf (HFO-1234yf): Low-GWP replacement for R-134a in newer vehicles. More aggressive than R-134a toward elastomers. HNBR with appropriate compound formulation (specific ACN content and curing system) is the standard specification for R-1234yf systems. NBR is not recommended.
- PAG oil compatibility: Modern automotive AC systems use polyalkylene glycol (PAG) compressor lubricant. HNBR is compatible with PAG oil at AC operating temperatures; NBR shows marginal performance with PAG at elevated temperatures.
AC sealing positions using HNBR:
- Compressor shaft seal
- Compressor manifold O-rings
- Service port O-rings (Schrader valve seals)
- Quick-connect fitting O-rings (A/C hose connections)
- Expansion valve O-rings
SAE J2064 and MAC specifications: Many automotive OEM AC component specifications reference SAE J2064 (refrigerant hose and coupling) which specifies HNBR or equivalent for refrigerant-side seals. Aftermarket AC O-ring kits for R-1234yf systems require HNBR — using NBR O-rings from a generic kit in an R-1234yf system is a documented failure cause.
Comparison: HNBR vs NBR vs FKM
| Property | NBR | HNBR | FKM |
|---|---|---|---|
| Continuous temperature maximum | +120°C | +150°C | +200°C |
| Ozone resistance | Poor | Excellent | Excellent |
| Compression set at +120°C | 40–55% | 25–38% | 15–28% |
| Sour gas (H₂S) resistance | Fair | Excellent | Good |
| Petroleum oil resistance | Excellent | Excellent | Excellent |
| Aromatic fuel resistance | Good to ~20% aromatics | Good to ~25% aromatics | Excellent |
| Ethanol resistance (E85) | Poor | Poor | Good |
| Abrasion resistance | Good | Very good | Good |
| Low-temperature limit (dynamic) | −25°C | −30°C (standard), −40°C (LT grade) | −15°C (standard), −25°C (GF grade) |
| Ketone resistance | Poor | Poor | Limited |
| Cost index | 1× | 1.5–3× | 5–12× |
| Standard AS568 availability | Excellent | Good | Good |
The decision framework:
- Below +120°C, indoor, no ozone, no H₂S, petroleum oil service → NBR
- +120–150°C, or ozone exposure, or H₂S service, petroleum oil → HNBR
- Above +150°C, or aromatic/ethanol fuels, or aggressive chemical environments → FKM
HNBR vs Viton (FKM): When to Choose Which
HNBR and FKM (Viton) overlap in many oil and fuel applications. The choice depends on temperature, chemistry, and cost.
| Selection Factor | HNBR | FKM (Viton) |
|---|---|---|
| Continuous temperature | +150°C (+165°C premium grade) | +200°C |
| Petroleum oil / mineral hydraulic fluid | Excellent | Excellent |
| Diesel fuel | Excellent | Excellent |
| Gasoline with aromatics | Good to ~25% aromatics | Excellent |
| Ethanol blends (E85) | Poor | Good |
| Ketones / esters | Poor | Limited |
| Steam / hot water | Not suitable | Not suitable |
| Ozone / weathering | Excellent | Excellent |
| Sour gas (H₂S) | Excellent | Good |
| Low-temperature flexibility | −30°C (standard), −40°C (LT grade) | −15°C (standard), −25°C (GF grade) |
| Cost | 1.5–3× NBR | 5–12× NBR |
Choose HNBR when: temperature is below +150°C, sour gas is present, low-temperature flexibility matters, or cost is a constraint. Choose FKM when: temperature exceeds +150°C, fuel contains high aromatics or ethanol, or aggressive chemicals are present.
HNBR for Diesel, Gasoline, and Fuel Systems
HNBR is widely used in automotive and industrial fuel systems where NBR's temperature limit is too low but FKM's cost is unnecessary.
| Fuel Type | HNBR Compatibility | Notes |
|---|---|---|
| Diesel (ULSD, biodiesel B5–B20) | Excellent | Standard for fuel injection seals |
| Gasoline (E0–E10) | Good | Acceptable for most automotive fuel seals |
| Gasoline (E85) | Poor | FVM required |
| Biodiesel B100 | Marginal | Check compound; FKM may be needed at high temp |
| Fuel oil / heating oil | Excellent | Common for burner seals |
Green O-rings and fuel resistance: Green-colored O-rings are often HNBR or EPDM. HNBR green O-rings are common in automotive AC and fuel systems because HNBR resists fuel and refrigerant. EPDM green O-rings are common in water and brake systems. Color alone is not definitive — verify material by compound specification before using a green O-ring in fuel service.
High-Temperature Hydraulic Service
Industrial hydraulic systems operating above +100°C present specific challenges for seal materials. Heat sources include:
- Hydraulic pump heat generation (typically 30–50°C above ambient after steady state)
- Proximity to process equipment (heated dies, ovens, furnaces)
- High-duty-cycle systems with limited oil cooling
For continuous service at +100–150°C in mineral hydraulic oil (ISO VG 46–68), HNBR's performance advantage over NBR is clear:
| Performance Parameter | NBR at +130°C | HNBR at +130°C |
|---|---|---|
| Compression set (22 hours) | 55–70% | 30–45% |
| Hardness change | +5 to +15 Shore A (hardens) | +3 to +8 Shore A |
| Tensile retention | 60–75% | 75–90% |
| Practical service interval | 3–6 months | 9–18 months |
For hydraulic presses, injection molding machines, and die casting equipment where hydraulic circuits operate at +130–150°C continuous, HNBR is the standard specification. The longer service interval reduces planned maintenance frequency and eliminates unplanned failures.
Mechanical Properties and Dynamic Service
HNBR's improved mechanical properties relative to NBR make it the preferred material for demanding dynamic sealing:
| Property | NBR (Standard 70 ShA) | HNBR (Standard 70 ShA) |
|---|---|---|
| Tensile strength | 15–22 MPa | 20–30 MPa |
| Elongation at break | 250–350% | 200–300% |
| Tear resistance (Die C) | 25–40 kN/m | 35–55 kN/m |
| Abrasion resistance (DIN 53516) | Moderate | High |
| Shore A hardness range | 40–90 | 50–90 |
In high-cycle dynamic applications — reciprocating hydraulic cylinders, piston pump seals, compressor seals — the higher mechanical properties of HNBR contribute to longer service life between seal replacements. The improvement is most pronounced at elevated temperature where NBR's mechanical properties degrade faster.
Design Guidelines
Groove Dimensions
HNBR O-rings use the same groove dimensions as NBR for the same cross-section (CS) diameter. AS568 and ISO 3601 standard groove dimensions apply. Compression targets:
- Static seals: 15–25% compression
- Dynamic reciprocating seals: 12–18% compression (keep at lower end for high-cycle applications to reduce heat generation)
- Dynamic rotary seals: 10–15% compression
Surface Finish
| Surface Type | Recommended Ra | Notes |
|---|---|---|
| Rod (reciprocating) | 0.10–0.25 μm | Circumferential grind preferred |
| Bore (piston seal) | 0.20–0.40 μm | — |
| Gland wall (groove) | 0.80–1.60 μm | Non-critical; static contact surface |
| Rotary shaft | 0.10–0.20 μm | Must be concentric; no axial marks |
Gland Fill
Keep gland fill below 85% at maximum operating temperature. HNBR has a slightly higher thermal expansion coefficient than FKM; verify fill percentage at maximum temperature for systems that see large thermal swings.
Procurement and Availability
Standard sizes: HNBR O-rings are available in AS568 and ISO 3601 standard sizes in 70 and 80 Shore A from distributor stock with 3–7 day lead time. 90 Shore A and specialty low-temperature or NACE-grade compounds may require 7–15 days depending on stock levels.
Custom sizes: Available with MOQ starting at 50 pieces for compression-molded standard compounds; 100–500 pieces for NACE-certified or specialty compound custom sizes.
Color coding: Most HNBR O-rings are black. Green is used by some suppliers as an HNBR identification color in mixed-material service kits. Custom colors are available for large production volumes.
Certification: NACE MR0175 / ISO 15156 compound qualification documentation, material test reports (hardness, tensile, elongation, compression set, H₂S immersion data), and certificates of conformance are available per lot for oilfield applications.
Test Methods and Standards Referenced
The property values in this guide are reported according to the following test methods:
| Property | Test Method |
|---|---|
| Hardness (Shore A) | ASTM D2240 |
| Compression set | ASTM D395 (Method B, 22 h) |
| Tensile strength & elongation | ASTM D412 |
| Tear strength | ASTM D624 (Die C) |
| Fluid immersion / swell | ASTM D471 |
| Low-temperature retraction (TR10) | ASTM D1329 |
| Sour gas (H₂S) qualification | NACE MR0175 / ISO 15156 |
| Automotive AC refrigerant (R-134a, R-1234yf) | SAE J2064 / SAE J2842 |
| Metric / inch size designations | ISO 3601 / AS568 |
Verify that a supplier's reported values quote the same test method, temperature, and duration — compression-set and immersion figures are only comparable when test conditions match.
FAQ
Q1: What does HNBR stand for?
HNBR stands for Hydrogenated Nitrile Butadiene Rubber. The hydrogenation process saturates the carbon–carbon double bonds in the NBR polymer backbone, producing a compound with significantly better heat, ozone, and sour gas resistance while retaining NBR's petroleum oil and fuel compatibility.
Q2: What is the temperature range of HNBR O-rings?
Standard HNBR operates from approximately −30°C to +150°C continuously, with short-term peak capability to +165°C. Low-temperature grade HNBR extends the lower limit to −40°C. High-heat premium grades (RDB ≤ 0.9%) provide continuous service to +165°C and short-term capability to +180°C.
Q3: Is HNBR compatible with R-1234yf automotive refrigerant?
Yes — HNBR is the OEM-recommended elastomer for R-1234yf systems. R-1234yf (HFO-1234yf) is more aggressive toward elastomers than the R-134a it replaces, and NBR-based AC kits from the NBR era are not suitable. Confirm that the HNBR compound specifies R-1234yf compatibility — different compound formulations vary in their refrigerant resistance, and the manufacturer's specific immersion data in R-1234yf with PAG oil should be reviewed.
Q4: Can HNBR replace NBR in hydraulic systems without groove modification?
Yes. HNBR in the same hardness (Shore A) and same AS568 or ISO 3601 size is a direct dimensional substitute for NBR. No groove modification is required. Match the hardness exactly — a 70 Shore A HNBR replaces a 70 Shore A NBR. If the application originally used 80 Shore A NBR (for higher-pressure extrusion resistance), specify 80 Shore A HNBR.
Q5: Is HNBR good for sour gas (H₂S) service?
Yes — HNBR is one of the best oil-resistant elastomers for sour gas environments. The hydrogenated backbone resists H₂S attack through the mechanism that destroys NBR (sulfur crosslinking via double bonds). HNBR compounds qualified to NACE MR0175 / ISO 15156 are the standard specification for O&G equipment in H₂S service. Request compound-level NACE qualification documentation, not just a material type declaration.
Q6: When should I specify FKM instead of HNBR for hydraulic service?
Specify FKM when: (1) continuous service temperature exceeds +150°C; (2) the hydraulic fluid or process fluid includes aromatic solvents at high concentration, strong acids, or E85+ ethanol blends that HNBR resists poorly; (3) very low compression set is required at extended high temperature (FKM compression set at +150°C is typically 15–25%, better than HNBR's 35–52%); (4) the application specification (aerospace OEM, automotive OEM) requires FKM by name. Within HNBR's service window (+100–150°C, petroleum oil service, sour gas), HNBR is preferred over FKM for its lower cost, better low-temperature flexibility, and equivalent or better mechanical properties.
Q7: What is the difference between Therban and Zetpol HNBR?
Therban (LANXESS) and Zetpol (Zeon Chemicals) are the two main HNBR polymer suppliers. Both produce HNBR across multiple ACN content grades and hydrogenation levels. The differences lie in specific molecular weight distribution, cure chemistry, and compound formulation details that affect processing and certain performance characteristics. From an end-user perspective, O-rings compounded from either polymer to the same specification (ACN content, hardness, compression set requirement) will perform comparably. Specify performance requirements, not polymer brand names, unless a specific OEM qualification requires a named polymer.
Q8: How do I interpret HNBR immersion test data to evaluate compatibility with a specific hydraulic fluid or fuel?
Immersion data for HNBR is typically reported following ASTM D471 (Effect of Liquids on Rubber Properties). The test exposes five O-ring specimens to the test fluid at a specified temperature and duration (commonly +100°C for 70 hours for hydraulic oil; +60°C for 168 hours for fuels). Four parameters are reported. Volume change (%): the percentage increase in specimen volume due to fluid absorption. Acceptable range for HNBR in hydraulic oil: +5 to +15% swell; in fuel: +5 to +20%. Values above +25% indicate significant fluid penetration and long-term softening. Hardness change (Shore A): acceptable ±5 Shore A; values beyond ±10 Shore A indicate material incompatibility. Tensile strength retention (%): must remain above 75% of original tensile strength — values below 70% indicate polymer chain degradation. Elongation retention (%): must remain above 70% of original elongation — low retention with high volume swell indicates plasticization rather than crosslink degradation. When interpreting compound-supplier data sheets, verify that the test temperature and duration match your service conditions — data at +70°C is not predictive for service at +130°C. For new fluid formulations (e.g., new-generation E-fluids or sustainable hydraulic fluids), request or conduct 70-hour immersion testing at the actual service temperature before specifying HNBR.
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Need HNBR O-rings for hydraulic, refrigerant, or sour gas service? Request a quote with your size, hardness, service temperature, and fluid — we supply NACE MR0175-qualified HNBR with lot-specific test data, automotive AC grades for R-134a and R-1234yf, and standard AS568/ISO 3601 sizes from stock. MOQ as low as 1 piece; lead time 7–15 business days.
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Related engineering resources: HNBR Products | Oil Seals | Hydraulic Seals