Rubber Materials
NBR Nitrile Rubber Technical Guide: ACN Content, Oil Resistance Grades & Seal Applications
Complete NBR technical guide: how acrylonitrile content (ACN 18-50%) controls oil resistance vs low-temperature balance, ASTM D2000 BF/BG/BK classification, NBR vs HNBR vs FKM comparison, and applications in oil seals, O-rings, fuel hoses, and hydraulic seals.
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NBR Nitrile Rubber Technical Guide: ACN Content, Oil Resistance & Applications
Published: 2025-09-05 | Reading time: 8 minutes
Executive Summary
Nitrile Butadiene Rubber (NBR) is the industrial standard for oil-resistant rubber, produced by emulsion copolymerization of acrylonitrile (ACN) and butadiene. The acrylonitrile content (ACN%) -- ranging from 18% to 50% -- is the single most important parameter controlling the fundamental trade-off between oil resistance and low-temperature flexibility.
NBR accounts for approximately 10% of global synthetic rubber consumption, with billions of seals, O-rings, gaskets, and hoses produced annually. Its combination of excellent oil resistance, good mechanical properties, and moderate cost makes it the default choice for any application involving mineral oil, fuel, or hydraulic fluid contact at temperatures up to 100-120°C.
The ACN Content Trade-Off
The nitrile (-C=N) group is highly polar with a strong dipole moment. Increasing ACN content increases the overall polarity of the polymer, which:
- • Improves oil and fuel resistance -- greater polarity increases the solubility parameter (δ) mismatch with non-polar mineral oils (δ~15-16 MPa^½), reducing swelling
- • Increases glass transition temperature (Tg) -- Tg rises approximately 2°C for each 1% increase in ACN content because the polar nitrile groups increase intermolecular forces and reduce chain flexibility
- • Reduces low-temperature flexibility -- higher Tg means the rubber stiffens at higher temperatures
- • Improves gas barrier properties -- higher polarity decreases permeability to non-polar gases and vapors
- • Increases tensile strength and abrasion resistance -- stronger intermolecular forces improve mechanical properties
This trade-off is fundamental and unavoidable. The art of NBR selection is choosing the ACN content that provides adequate oil resistance while preserving sufficient low-temperature flexibility for the application.
ACN Content: The Critical Control Parameter
| ACN Content | 18-25% (Low) | 28-35% (Medium) | 36-50% (High) | 50%+ (Very High) |
|---|---|---|---|---|
| Oil/Fuel Resistance | ★★ Modest | ★★★★ Good | ★★★★★ Excellent | ★★★★★ Maximum |
| IRM 903 Swell (70°C/70h) | 30-50% | 15-30% | 5-15% | <10% |
| Low-Temp Flexibility | ★★★★★ (-45°C) | ★★★★ (-35°C) | ★★★ (-25°C) | ★★ (-15°C) |
| Brittleness Point | -50 to -55°C | -35 to -45°C | -20 to -30°C | -10 to -15°C |
| Resilience (Rebound) | ★★★★ High | ★★★ Good | ★★ Moderate | ★ Low |
| Heat Aging Resistance | ★★ Modest | ★★★ Good | ★★★★ Very Good | ★★★★ Very Good |
| Gas Permeability | ★★ High | ★★★ Moderate | ★★★★ Low | ★★★★★ Very Low |
| Tensile Strength | Moderate | Good | Very Good | Very Good |
| Abrasion Resistance | Moderate | Good | Very Good | Very Good |
Selection Guide:
- • Low-temperature fuel seals (<-40°C): Low ACN (18-22%) -- e.g., Arctic fuel handling equipment, aerospace fuel systems
- • General industrial oil seals: Medium ACN (28-34%) -- the most common grade; best balance of oil resistance, low-temp, and cost
- • Hot oil service (120°C+): High ACN (36-45%) or HNBR -- for transmissions, engine seals, oil coolers
- • Maximum oil/fuel barrier: Very high ACN (45-50%) or PVC/NBR blend -- for fuel hose inner liners, fuel tank diaphragms
ASTM D2000 / SAE J200 Classification
The ASTM D2000 / SAE J200 line call-out system is the standard specification framework for automotive and industrial rubber products. NBR grades are classified by their temperature resistance and oil swell characteristics:
| Grade | Temperature Rating | Oil Resistance (IRM 903, 70h) | Typical ACN% | Common Applications |
|---|---|---|---|---|
| BF | 100°C | Volume swell ≤120% | 28-34% | General purpose oil seals, gaskets |
| BG | 100°C | Volume swell ≤80% | 34-40% | Fuel hoses, higher oil resistance seals |
| BK | 100°C | Volume swell ≤40% | 40-50% | Maximum oil resistance; fuel system components |
| CH | 125°C | Volume swell ≤60% | 34-45% (heat-resistant NBR) | Higher temperature oil exposure |
The full line call-out format: ASTM D2000 M2 BG 610 A14 B14 Z1 (where M2 = metric units, BG = grade, 610 = hardness + tensile, A14/B14 = heat aging requirements, Z1 = custom suffix for additional requirements).
Typical Physical Properties
| Property | Typical Range | Test Standard | Notes |
|---|---|---|---|
| Tensile Strength | 10-20 MPa | ASTM D412 (Die C) | Higher ACN = higher tensile |
| Elongation at Break | 400-600% | ASTM D412 | Decreases with higher ACN and filler loading |
| Hardness Range | 40-100 Shore A | ASTM D2240 | Most common: 60-80 Shore A for seals |
| Tear Resistance | 30-60 N/mm | ISO 34-1 (Die B or C) | Higher with CV cure and fine carbon black |
| Compression Set | 15-30% (100°C/22h) | ASTM D395 Method B | EV cure reduces CS; peroxide even better |
| Service Temperature | -30 to +100°C (standard) | — | Continuous use rating |
| Low-Temp Grade Limit | -45°C | — | 18-22% ACN, special low-temp NBR grades |
| Heat-Resistant Grade Limit | +120°C | — | 40%+ ACN, EV or peroxide cure, heat-resistant antioxidants |
| Density | 1.0-1.2 g/cm³ | ASTM D297 | Depends on filler loading |
| Rebound Resilience | 30-50% | ISO 4662 | Lower ACN = higher resilience |
NBR Cure System Selection
| Cure System | Compression Set | Heat Resistance | Dynamic Fatigue | Best Application |
|---|---|---|---|---|
| CV Sulfur (2-3 phr S, low accelerator) | Poor (25-40%) | Moderate (100°C max) | ★★★★★ Best | Dynamic oil seals subject to movement |
| SEV Sulfur (1.5-2 phr S, medium accelerator) | Moderate (18-28%) | Good (110°C) | ★★★★ | General purpose seals and gaskets |
| EV Sulfur (0.5-1 phr S, high accelerator) | Good (12-20%) | Very good (120°C) | ★★★ | Static seals, O-rings, compression set priority |
| Peroxide | Best (8-15%) | Excellent (130°C) | ★★ Fair | High-temp seals, low-CS applications |
NBR vs. HNBR vs. FKM
| Property | NBR | HNBR | FKM |
|---|---|---|---|
| Temperature Limit | 100-120°C | 150°C | 200-250°C |
| Oil Resistance (IRM 903) | ★★★★ (15-30% swell for 28-34% ACN) | ★★★★★ (5-15%) | ★★★★★ (3-8%) |
| Low-Temp Flexibility | ★★★★★ (-45°C achievable) | ★★★★ (-40°C) | ★★ (-20°C std, -40°C GLT) |
| Mechanical Strength | ★★★★ (10-20 MPa) | ★★★★★ (20-30 MPa) | ★★★ (10-20 MPa) |
| Abrasion Resistance | ★★★★ Good | ★★★★★ Excellent | ★★ Poor-Fair |
| Ozone Resistance | ★ Poor (requires antiozonant) | ★★★★ Good | ★★★★★ Excellent |
| H₂S (Sour Gas) Resistance | ★★ Poor | ★★★★★ Excellent | ★★★ Good |
| Relative Cost | 1× | 3-5× | 10-20× |
Key Applications by ACN Content
| Application | ACN% | Key Requirements | Failure Mode to Avoid |
|---|---|---|---|
| Hydraulic cylinder seals | 28-34% | Oil resistance, wear resistance, low CS | Swelling beyond groove fill, compression set |
| Fuel hoses (inner layer) | 34-40% | Fuel permeation resistance, low extractables | Permeation (fuel vapors escape), extraction of plasticizers |
| O-rings (general purpose) | 28-34% | Balanced oil + cost | Compression set, hardening |
| O-rings (high temperature) | 36-45% | Hot oil, low CS, EV/peroxide cure | Thermal hardening, CS failure |
| Automotive shaft seals | 28-34% | Engine oil, long life, low friction | Wear under the lip, heat aging |
| Oilfield packers | 36-50% | H₂S resistance, HPHT conditions | Explosive decompression, chemical attack |
| Fuel pump diaphragms | 28-34% | Flex fatigue + fuel resistance | Fatigue cracking, swelling |
| Low-temp fuel seals (aviation/Arctic) | 18-22% | -45°C flexibility + moderate fuel resistance | Stiffening below Tg, loss of sealing force |
Why NBR Is the Workhorse
NBR's dominance in oil-resistant applications stems from its unmatched combination of:
- • Tunable performance: ACN content from 18-50% allows tailoring to specific oil/low-temp balances
- • Good mechanical properties: 10-20 MPa tensile, good abrasion, adequate tear
- • Processability: Mixes and molds well on standard rubber processing equipment
- • Cost-effectiveness: At 1.5-2× the cost of NR/SBR, NBR provides oil resistance at a fraction of HNBR/FKM cost
- • Wide availability: Produced globally by dozens of manufacturers, ensuring supply security
Its primary limitations -- poor ozone/weather resistance and temperature limited to 100-120°C -- are well understood and can be addressed by selecting HNBR or FKM when the application demands exceed NBR's capabilities.
NBR Compound Design Basics
A typical general-purpose NBR seal compound (60-70 Shore A) would contain:
| Ingredient | Loading (phr) | Function |
|---|---|---|
| NBR (28-34% ACN) | 100 | Base polymer |
| N550/N330 Carbon Black | 40-80 | Reinforcement; N330 for strength, N550 for processability |
| Plasticizer (DOP/DOA/TOTM) | 5-15 | Processing aid; adjust hardness and low-temp |
| ZnO | 3-5 | Activator for sulfur cure |
| Stearic Acid | 1-2 | Processing aid; ZnO solubilizer |
| Antioxidant (TMQ) | 1-2 | Long-term heat aging protection |
| Sulfur | 0.5-2.0 | Crosslinking agent (amount depends on EV/SEV/CV) |
| Accelerator (TBBS, CBS, TMTD) | 1-4 | Cure rate control (amount depends on cure system) |
| Antidegradant (6PPD or IPPD) | 1-2 | Ozone/antioxidant (if outdoor exposure possible) |
| Microcrystalline wax | 1-2 | Ozone barrier (if outdoor exposure possible) |
Inquiry & Technical Support
Nanjing Yuhang Rubber supplies NBR-based rubber sheets, hoses, sealing strips, O-rings, and custom seals across all ACN grades. For ACN selection guidance and compound recommendations matched to your specific fluid and temperature conditions: Products | Materials | Contact
FAQ
Can this article be used as the final selection basis?
It is intended for preliminary technical review. Final material or product selection should be confirmed with the actual medium, temperature, load, dimensions, drawings and sample testing when needed.
What information should be provided for an inquiry?
Please provide the application equipment, working medium, temperature range, dimensions, quantity, drawing or sample information so the technical discussion can be organized faster.