Nitrile (NBR) is the default elastomer for petroleum fuel sealing, but its compatibility is sharply bounded by acrylonitrile (ACN) content and the alcohol fraction in the blend. Across 18-50% ACN grades, NBR resists gasoline, conventional diesel, biodiesel, and HVO 100, while failing in fuels containing more than 10-15% methanol or 20% ethanol [S1][S5].
The copolymer's ACN level is the single most useful spec handle for a buyer: higher ACN raises fuel and oil resistance, lowers low-temperature flexibility, and shifts the operating window toward the warm side. NBR is not a "one fuel, one grade" answer; the same NBR O-ring that survives diesel-B7 for years can swell and lose tensile strength in a few hundred hours in E85 [S2][S5].
Why ACN Content Sets the Fuel Window
ACN content in commercial NBR is commonly compounded between 18% and 50%, and the grade chosen for fuel service usually sits in the 33-50% range to maximise resistance to aromatic and aliphatic hydrocarbons [S4][S5]. Medium-to-high ACN NBR (about 33-45%) is the practical workhorse for gasoline, diesel, and biodiesel blends, while low ACN grades (18-25%) are kept where low-temperature flex matters more than chemical resistance.
Mechanically, this matters because the swelling and hardness shift on fuel immersion are directly tied to the ACN: as fuel penetrates the matrix, the polymer's tensile strength (TS), elongation at break (Eb), and compression set (CS) move with it. Reference immersion testing on ACM, HNBR, and FVMQ O-rings reports that mass, Shore A hardness, permanent deformation, tensile strength, and elongation at break are the five properties that track degradation across fuel families, including HVO 100, which did not produce significant seal degradation in those tests [S1].
Gasoline, Diesel, Biodiesel, and HVO: Where NBR Holds
Standard NBR handles conventional gasoline and neat diesel without issues and is widely used in fuel-injection O-rings, pump seals, and [mechanical seal](encyclopedia/mechanical-seal.html) secondary elements for those fluids [S2][S3]. HVO 100 (hydrotreated vegetable oil) ranks among the gentler modern fuels on NBR: in the same reference study, HVO 100 immersion produced the least degradation of any test fuel on HNBR and FVMQ O-rings, with no significant loss of mechanical performance reported [S1].
Biodiesel (FAME, e.g. B7 to B20) is more aggressive than fossil diesel because of its higher polarity, oxygen content, and tendency to carry entrained water, but high-ACN NBR is still the standard recommendation for in-line fuel hose and [oil seal](encyclopedia/oil-seal.html) service in automotive and stationary diesel equipment [S5][S7]. NBR's good compression set behaviour and abrasion resistance, paired with a 60 Shore A formulation, is the typical NBR spec pattern called out for petrol, diesel, and oil gasketing [S7].
Alcohol Blends: Where NBR Fails and What to Sub
Ethanol and methanol are the deal-breakers. Immersion testing of ACM, HNBR, and FVMQ O-rings in blended fuels found degradation was more pronounced for fuels containing ethanol, iso-butanol, n-butanol, methanol, and dodecanol, and the same logic carries forward to NBR as a more polar-vulnerable grade than HNBR or FVMQ in those fluids [S1]. For [industrial rubber](encyclopedia/industrial-rubber.html) sealing in flex-fuel and bio-blend service, NBR is commonly rated only for gasoline/alcohol blends up to about 15% methanol and MTBE/gasoline blends up to about 20% MTBE, and even then only with peroxide-cured, low-swell formulations [S4].
When alcohol content rises past those limits, the substitutes are FKM (Viton) for hot, alcohol-rich service, HNBR for improved oxidation and alcohol tolerance, and FVMQ for the broadest fuel compatibility [S1][S9]. FKM is also the typical first pick when temperature pushes past NBR's upper continuous limit near 100-110 °C, because FKM keeps fuel resistance at temperatures where NBR has already lost mechanical margin [S9].
Selection Criteria: NBR, HNBR, FKM, and EPDM at a Glance
For a fuel-system spec writer, four decision axes are enough to choose between the main options: fuel polarity, peak temperature, low-temperature flex, and price. Side-by-side, NBR leads on cost and oil/petroleum resistance but is weak on weather, ozone, and polar alcohol blends; HNBR is a tougher, more oxidative-stable NBR variant for higher-temperature and ozone-exposed fuel service; FKM (Viton) is the premium choice for hot, alcohol-rich, and aggressive fuel service; EPDM is the wrong family entirely for fuel service, because it has poor compatibility with petroleum oils and fuels, even though it is the dominant elastomer for [EPDM rubber](encyclopedia/epdm-rubber.html) cooling-system and outdoor sealing [S3][S9].
Specific service ceilings to anchor against: NBR is commonly used in the -30 °C to +110 °C continuous window for static fuel seals, with short excursions higher; HNBR extends that roughly to -40 °C / +150 °C; FKM runs from about -15 °C to +200 °C; and EPDM is fine on the same temperature axes but fails the fluid test, so it is excluded from any fuel-wetted gland, hose liner, or [bellows seal](encyclopedia/bellows-seal.html) face that sees hydrocarbons [S3][S4].
Spec Cautions, Failure Modes, and On-Site Pitfalls
The data-sheet trap is reading "compatible with fuel" and missing the alcohol content on the cert sheet. NBR's static compatibility values are typically generated against ASTM Reference Fuel C (an isooctane/toluene mix) and not against the E10, E85, M15, or B20 blends that actually arrive in modern fuel-system service, so the lab number is generous against the real duty cycle [S1][S5]. Spec writers should always ask for fuel-blend immersion data at the worst-case composition, not just neat-fuel data, and at the maximum operating temperature, not just at 23 °C.
Common failure modes on NBR fuel seals that pass a datasheet check but fail on site are: extrusion of the O-ring under pressure-pulse spikes in common-rail diesel; explosive decompression (ED) damage in high-pressure gas-rich or supercritical CO₂-admixed fuel systems, where NBR alone is not enough and an AED-grade HNBR or FKM is required; and rapid hardening followed by cracking on peroxidised biodiesel with high acid number, which ages NBR faster than the same service in fossil diesel [S4][S5]. On the mechanical side, [nitrile rubber](encyclopedia/nitrile-rubber.html) hardness is usually specified at 60-75 Shore A for static fuel seals, with 70 Shore A as a common default for pump and injector O-rings [S7][S4].
Sourcing, Standards, and Trackable Next Signals
Material certifications to request for any NBR fuel seal are: an ACN-content statement (often given as percent or as "low / medium / high ACN"), the cure system (peroxide-cured NBR is preferred for fuel service because of better heat ageing and lower compression set than sulfur-cured), hardness in Shore A, and an immersion test result in the actual fuel blend, not just Reference Fuel C [S4][S5]. For automotive and aerospace fuel-system service, AMS-R-6855, AMS 7273, and UL-listed material grades are commonly cited NBR specs, and FDA-compliant NSF 61 / WRAS grades exist for fuel-adjacent potable and food lines, though these are not the same fluid service [S4].
Trackable signals for the next review: biofuel and alcohol-blend fuel composition limits, since raising E10 to E15 or E20 in road fuels shifts the NBR-to-substitute crossover; and expanded reporting on HNBR and FVMQ immersion in HVO 100, where the current data set is small and the result (low degradation) is positive but limited in volume [S1][S5]. A practical spec rule worth carrying into procurement: NBR 33-45% ACN, peroxide-cured, 70 Shore A, immersion-tested in the actual fuel blend at maximum operating temperature; substitute FKM, HNBR, or FVMQ the moment alcohol content exceeds the NBR's published threshold, or the moment peak service temperature exceeds 110 °C continuous.
For component-level specifications, see nitrile rubber, bellows seal, and mechanical seal.
Background reading: Dock Leveler Embedment Angle: 3" Steel Coping Spec and Retrofit Rules.