Nitrile Rubber (NBR)

What Nitrile Rubber Is

Nitrile rubber is a synthetic copolymer of acrylonitrile and butadiene. The acrylonitrile units carry a polar nitrile group, and that polarity is the reason NBR resists nonpolar petroleum oils and fuels: the polymer and the oil are chemically dissimilar enough that the oil does not dissolve into the rubber and swell it. The butadiene units supply the elasticity and the rubbery resilience that let a seal recover its shape after compression. Adjusting the ratio between these two monomers, the ACN content, is the single most important lever a compounder has, and it is why one rubber family can serve duties from low-temperature cold-flex seals to aggressive fuel-system parts.

Under the ASTM D1418 nomenclature, NBR sits in the R class, the rubbers whose backbone retains unsaturated carbon-carbon double bonds from a diene. Those residual double bonds are both an asset and a liability: they provide the cure sites for sulfur vulcanization, but they are also the points where ozone and heat attack the chain. Removing them by hydrogenation produces HNBR, a higher-temperature, ozone-resistant relative covered later in this guide. Adding carboxyl groups produces XNBR, a higher-strength, abrasion-resistant relative. NBR, HNBR, and XNBR together form a small material family that covers most oil-resistant sealing duties in industry.

Commercially, NBR is one of the oldest synthetic rubbers. It was developed in Germany in the 1930s under the Buna-N trade name, where Buna referred to the butadiene-plus-sodium (Na) polymerization route and N to the nitrile comonomer. That historical name survives today as a generic synonym for NBR, alongside the producer brand names Perbunan, Krynac, and Nipol. The polymer is supplied as raw bales or crumb to compounders, who mix in carbon black, plasticizers, curatives, and antioxidants before molding or extruding the finished part.

The industrial footprint of NBR is large precisely because automotive and hydraulic machinery run on petroleum fluids. Engine crankshaft seals, fuel-pump diaphragms, transmission seals, hydraulic rod and piston seals, O-rings in oil circuits, and the inner liner of fuel hoses are overwhelmingly NBR or one of its relatives. Where the duty stays inside NBR's temperature and chemical envelope, it is almost always the lowest-cost qualified choice, which is why a procurement engineer encounters it more often than any other oil-resistant elastomer.

Four parameters dominate any NBR selection decision: ACN content (sets oil resistance versus cold flexibility), hardness in Shore A (sets sealing force and extrusion resistance), the continuous service temperature window, and the specific fluids the part will contact. The remaining chapters decode each of these in turn, because getting any one of them wrong, ordering a high-ACN grade for a cold climate, or a plain NBR part for brake fluid, produces a seal that fails in service even though it met its written hardness and dimensional spec.

Grade Families and ACN Classes

NBR is not a single material but a family of grades defined primarily by acrylonitrile content. Commercial ACN runs from roughly 18 to 50 percent, and the industry buckets it into low, medium, and high nitrile. The classification matters because ACN content trades oil resistance against low-temperature flexibility along a continuous curve: every percent of acrylonitrile that improves fuel resistance also raises the glass transition temperature and stiffens the rubber in the cold. The table below summarizes the three ACN classes and how each behaves.

Low-nitrile grades keep more butadiene in the chain, so they stay flexible to roughly -40 degrees Celsius and suit cold-climate dynamic seals where the part must keep sealing in winter or in refrigeration circuits. The cost is reduced oil and fuel resistance, so these grades are not the first pick for hot, aggressive hydrocarbon service. Medium-nitrile grades are the default for general industry: they balance oil resistance and cold flex well enough that most catalog O-rings, oil seals, and hydraulic gaskets use ACN somewhere in the 33 to 40 percent band.

High-nitrile grades maximize resistance to oils, fuels, and nonpolar solvents and also minimize gas permeability, which makes them the choice for aggressive fuel-system parts and barrier applications. Their penalty is poor cold flexibility, so they are avoided in low-temperature dynamic duty. Beyond ACN content, NBR grades also differ in Mooney viscosity (which governs processing and flow during molding) and in whether the polymer is hot or cold polymerized, but for a procurement engineer the ACN class and the finished-part hardness carry most of the selection weight.

Two important relatives extend the NBR family beyond the temperature and wear limits of the base polymer. HNBR (hydrogenated nitrile) selectively saturates the butadiene double bonds, raising continuous service to about +150 degrees Celsius with peaks near +165 degrees and adding strong ozone and heat-aging resistance, at several times the cost. XNBR (carboxylated nitrile) grafts acrylic-acid groups onto the chain for extra cross-link sites, raising tensile strength, modulus, and especially abrasion resistance at the expense of some cold flexibility. The table below positions the three side by side.

Mechanical and Physical Properties

NBR is supplied as a compound, not a pure polymer, so the published numbers reflect the carbon-black loading, plasticizer, and cure system as much as the base rubber. The values below are representative ranges for general-purpose vulcanized NBR across the common hardness band; any single ordered compound will sit at a specific point inside these ranges, and the controlling document is the supplier datasheet. The Key Specifications table compares the headline mechanical and physical parameters.

Hardness in Shore A governs sealing force and extrusion resistance. A softer compound (60 Shore A) conforms better to imperfect mating surfaces and seals at lower force, while a harder compound (80 to 90 Shore A) resists being squeezed into the extrusion gap under high pressure. O-ring and oil-seal catalogs most commonly stock 70 Shore A as the default, with 90 Shore A reserved for high-pressure backup duty. Hardness, tensile, and elongation are interdependent: raising carbon-black loading hardens the compound, raises tensile strength, and lowers elongation, so the three are specified together rather than optimized in isolation.

Tensile strength for general NBR compounds runs roughly 10 to 25 MPa depending on filler and cure, while elongation at break spans about 200 to 600 percent, dropping as hardness climbs. These are useful as compound-quality indicators but are rarely the limiting parameter for a static seal, where compression set matters more. For dynamic seals and abrasion duty, tear strength and the XNBR modulus advantage become more relevant than peak tensile.

Compression set is arguably the most important spec for a static seal: it measures how much of its deformation a compressed sample fails to recover, expressed as a percentage. A seal with high compression set takes a permanent flat and eventually leaks even though it still passes a hardness check. Quality static-seal NBR compounds target compression set well below 25 percent at 22 hours and +100 degrees Celsius. Set climbs steeply once the part runs above its rated temperature, which is the physical mechanism behind hardened, leaking seals in overheated oil circuits.

Specific gravity sits near 1.0 to 1.25 g/cm3 for typical compounds, rising with carbon-black and ACN content, and is mainly used to verify a delivered batch matches the qualified compound. It is a quick incoming-inspection check: a density that drifts from the datasheet figure flags a substituted or off-spec compound before the part ever reaches assembly.

Chemical Resistance and Media Compatibility

NBR is specified almost entirely for its chemical behavior, so getting the media match right is the core of selection. The polarity of the nitrile group makes the rubber resistant to nonpolar petroleum fluids but vulnerable to polar solvents and to ozone. The practical rule is simple: NBR loves oil and hates everything that dissolves nail polish. The table below is a first-pass compatibility lookup, intended for initial screening only.

What NBR resists. The polymer is excellent with petroleum-based fluids: motor and lubricating oils, mineral hydraulic fluids, diesel, gasoline, fuel oil, and most greases. This is exactly the chemistry of automotive and machinery duty, which is why NBR dominates oil seals, fuel-system parts, and hydraulic gaskets. It also handles water and dilute aqueous acids and alkalis at ambient temperature acceptably, though not as well as EPDM handles hot water.

What attacks NBR. Polar solvents swell and soften it: ketones such as acetone and methyl ethyl ketone, esters, and chlorinated and aromatic hydrocarbons like benzene and toluene. Strong oxidizing acids degrade it. Glycol-ether automotive brake fluids (DOT 3 and DOT 4) are a classic trap, because the seal looks correct but swells in service, so brake hydraulics use EPDM, not NBR. The selection error in all these cases is matching a fluid name loosely instead of checking the immersion data at the real temperature and concentration.

Ozone and weathering. The residual double bonds in the butadiene backbone make untreated NBR crack under combined ozone exposure and mechanical tension, and it weathers poorly in sunlight. The three engineering fixes are: compound in antiozonant and protective wax, switch to HNBR with its saturated backbone, or specify EPDM for any part that lives outdoors or near ozone-generating equipment such as electric motors and high-voltage gear. Static seals hidden inside housings, shielded from light and moving air, tolerate plain NBR well; exposed dynamic parts in weather do not.

Because compatibility depends on concentration, temperature, and exposure time, this table is a screening tool, not a final answer. Before committing a compound, obtain the producer or seal-maker fluid-immersion chart and verify the specific medium at its operating temperature and concentration, and for fuel service confirm whether the fuel contains aromatics or ethanol blends that shift the requirement toward a higher-ACN grade or toward FKM.

Standards and Spec-Sheet Designations

Three standards systems govern how NBR is named and specified, and a procurement engineer needs to read all three. ISO 1629 and ASTM D1418 supply the polymer nomenclature (the letters NBR, HNBR, XNBR themselves), while ASTM D2000 and its SAE counterpart J200 supply the line-call code that pins down a compound's properties on a drawing. Reading these correctly is what turns a vague request for nitrile into an orderable, verifiable part.

ASTM D1418 and ISO 1629 are the nomenclature standards. ASTM D1418 classifies rubbers by the chemical makeup of the polymer chain into classes such as M, N, O, R, Q, T, U, and Z. NBR falls in the R class, the diene rubbers with an unsaturated backbone, where the same class also contains NR, SBR, BR, IIR, CR, and the nitrile relatives HNBR and XNBR. The X prefix on XNBR marks the carboxylic-acid modification, and the H in HNBR marks hydrogenation. ISO 1629 defines an equivalent international symbol system, so the two are read together and the letter codes match.

ASTM D2000 and SAE J200 are the classification standards that actually appear on engineering drawings, and the two are identical in intent. They encode a compound as a line call built from a Type letter and a Class letter followed by hardness, tensile, and suffix requirements. The Type letter sets heat-aging resistance and the Class letter sets oil-swell resistance. NBR is the polymer behind the BF, BG, BK, and CH groupings, where, for example, B Type means the material survives 100 degrees Celsius heat aging and G Class limits oil swell to a maximum of 40 percent volume. The table below decodes the first two fields most relevant to NBR.

A full line call such as M2BG710 reads field by field: the leading M signals metric (SI) units, the digit 2 is the grade number, BG sets B Type and G Class (100 degrees Celsius aging, 40 percent maximum oil swell), 70 is the durometer in Shore A, and the final 10 is the minimum tensile strength, which in a metric (M) call is read directly as 10 MPa (about 1,450 psi). In an imperial call the same field would instead carry the first two digits of the psi value. Suffix codes appended after the basic call then add specific requirements for heat-aged properties, compression set, low-temperature brittleness, and fluid immersion, each suffix letter paired with a test condition. The single most common procurement mistake is to specify a bare hardness and Buna-N without a D2000 line call, which leaves oil swell, heat aging, and compression set unconstrained and lets two suppliers ship physically different rubber against the same drawing.

For sealing parts there are also dimensional and tolerance standards layered on top of the material spec. O-ring sizes follow AS568 (the standard inch series) and ISO 3601, and these govern the groove and the ring geometry independently of which NBR compound fills them. The complete specification of an NBR part therefore stacks a nomenclature code (NBR per ISO 1629), a material line call (ASTM D2000 / SAE J200), and a dimensional standard (AS568 or ISO 3601 for O-rings), and a drawing that omits any one of the three leaves room for a non-interchangeable substitution.

Selection Decision Factors

To turn the preceding five chapters into an ordered part, work through the decision sequence below. The most common failures come not from a single wrong number but from skipping a level: picking a compound on hardness alone, or matching a fluid by name without checking the immersion data. These steps double as an RFQ template for an NBR seal, gasket, or molded part.

1. Confirm the fluid and rule out NBR killers: identify every medium the part contacts. If the list includes brake fluid (DOT 3/4), ketones, esters, aromatics, or strong oxidizers, stop and move to EPDM or FKM. Only proceed with NBR once the chemistry is inside its envelope.
2. Set the temperature window: establish both the maximum continuous temperature and the minimum cold-start temperature. Above +100 to +120 degrees Celsius continuous, step up to HNBR or FKM. Below about -30 degrees Celsius, you will need a low-ACN grade and should verify the brittle point.
3. Choose the ACN class: balance step 1 against step 2. Aggressive fuels push toward high ACN; cold climates push toward low ACN; most general oil duty lands on a medium-ACN grade in the 33 to 40 percent band.
4. Select hardness: 70 Shore A is the catalog default. Go softer for better conformability on rough surfaces, harder (80 to 90 Shore A) to resist extrusion under high pressure, where a backup ring may also be needed.
5. Decide whether a relative is required: specify HNBR if heat, ozone, or weathering exceed plain NBR, or XNBR if abrasion and wear life are the limiting factor. Both cost more, so use them only where base NBR genuinely cannot serve.
6. Write a full ASTM D2000 line call: never order bare hardness plus Buna-N. Pin Type, Class, durometer, tensile, and the suffix requirements for compression set, heat aging, and fluid immersion so two suppliers ship interchangeable rubber.
7. Specify the dimensional standard: for O-rings cite AS568 or ISO 3601 and the groove geometry; for gaskets and molded parts give the drawing and tolerance class. Material spec and dimensional spec are independent and both are mandatory.
8. Confirm compliance and certification needs: food or potable-water contact (FDA 21 CFR 177.2600, NSF/ANSI 61, EC 1935/2004), drinking-water approval, or industry-specific qualifications must be called out up front, because a compliant compound is a different formulation, not a paperwork add-on.

One dimension that is easy to overlook is supplier traceability and serviceability: whether the maker can supply a certificate of conformance against the cited D2000 line call, whether the compound is consistent batch to batch, and whether replacement parts will still match years later. For the raw polymer, ARLANXEO (Perbunan, Krynac, Therban), Zeon (Nipol, Zetpol), Synthos, LG Chem, and Kumho Petrochemical are the principal global producers, and finished NBR seals are stocked to published specs by Parker Hannifin, Trelleborg, Freudenberg, SKF, and NOK. For high-volume or non-critical parts, qualified regional seal makers supply equivalent D2000 grades at a fraction of imported prices, provided the line call and dimensional standard are specified tightly enough to make the parts interchangeable.