Acrylonitrile-Butadiene Copolymer (NBR) Chemical Compatibility & Swell Table: What It Cannot Handle

Acrylonitrile-butadiene copolymer compounds are qualified for fluid service through immersion testing under ASTM D471 and ISO 1817:2022. The resulting data set includes volume swell, Shore A hardness change, tensile retention, elongation retention, and mass change after exposure at defined temperatures and durations. Common screening conditions are 70 h at 23°C and 70 h at 100°C. In seal applications, the design limit is not a single swell threshold but the interaction of swell with groove fill, squeeze retention, and extrusion resistance. A volume swell below 10% is generally serviceable for static O-rings. Dynamic seals may tolerate 15–20% swell only when backup rings and wider groove widths compensate for dimensional growth. Swell values above 25% typically produce tensile loss above 40% and hardness loss sufficient to permit extrusion under pressure.

ASTM D471 does not provide a universal pass-fail criterion. Each product specification must define the acceptable volume swell, hardness change, and tensile retention. The repeatability of volume swell measurements can be affected by surface-to-volume ratio; standard test slabs of 2.0 mm thickness reach equilibrium faster than seal cross-sections of 5.0 mm or larger. A seal with thick cross-section may show lower apparent swell at the same time point despite identical polymer chemistry. This is a diffusion-controlled lag, not a different compatibility mechanism.

The term “incompatibility” in NBR service refers to more than volume increase. It includes plasticizer extraction, oxidative attack at the butadiene double bond, dehydrochlorination of halogenated solvents, and ozone stress cracking. The data below are representative ranges for a medium-ACN grade of 33% acrylonitrile, aggregated from published supplier technical bulletins; actual values depend on cure system, filler type, plasticizer loading, and immersion temperature.

Among the listed fluids, ketones, esters, chlorinated solvents, aromatic hydrocarbons, and phosphate esters fall into the D category. These are the first classes to screen out in NBR seal selection. Borderline B–C entries such as methanol and ASTM Oil No. 3 require additional tensile retention and dry-out data before approval.

What Drives Swell in the Butadiene Phase?

The swelling response of acrylonitrile-butadiene copolymer is governed principally by the solubility parameter distance between the fluid and the two phases of the vulcanizate. The butadiene repeat unit has a Hildebrand parameter near 17.0 MPa^1/2. Acrylonitrile units contribute a larger dipole density, raising the overall parameter to approximately 19.5 MPa^1/2 for a 33% ACN grade. Toluene has a solubility parameter near 18.2 MPa^1/2, benzene near 18.7 MPa^1/2, and methyl ethyl ketone near 19.0 MPa^1/2. The proximity of these values to the butadiene phase is sufficient for high solvent uptake.

Aliphatic paraffinic oils possess solubility parameters in the range 15.5–16.5 MPa^1/2, which is outside the favorable absorption window of the butadiene units. As a result, ASTM Oil No. 1 normally produces swell below 10% in medium-ACN NBR. ASTM Oil No. 3 contains aromatic and unsaturated structures that reduce the solubility parameter gap and raise swell into the 10–30% band. The same principle explains why ASTM Fuel C, with 50% toluene and 50% isooctane, creates much greater swell than straight-chain aliphatic gasoline components.

Temperature shifts the same mechanism. A fluid that yields 12% volume swell at 23°C may exceed 30% at 100°C because the polymer-solvent interaction parameter moves toward more favorable mixing and chain mobility increases. For this reason, a single room-temperature immersion result is not conservative for hot-flow service in fuel rails, transmission oil coolers, or engine sealing systems. Qualification programs under ASTM D471 therefore commonly include repeat testing at the maximum intended bulk fluid temperature.

In a rectangular O-ring groove, a seal with excessive swell loses contact stress because the rubber expands into the clearance gap. The resulting extrusion path widens under pressure, and spiral failure may initiate. Sealing standards such as ISO 3601-2 specify groove fill limits based on volume swell at the maximum service temperature. The incompatibility is therefore not limited to chemical degradation; it becomes a mechanical extrusion risk before the polymer network fully degrades.

Chlorinated solvents represent a separate incompatibility class that cannot be corrected by raising ACN content. Carbon tetrachloride and trichloroethylene absorb strongly into the butadiene phase; medium-ACN NBR typically swells 50–100% in carbon tetrachloride after 70 h at 23°C. At 70°C, the absorbed solvent can also extract sulfur-cured fragments and ester plasticizers, producing surface tack and reduced crosslink density. The failure is therefore additive loss and dehydrochlorination at elevated temperature rather than a reversible dimensional excursion. A dried component may be hard, shrunken, and cracked even though the immersed volume was large.

When Ketones and Esters Extract Plasticizer and Attack Nitrile Groups

When methyl ethyl ketone, acetone, or ethyl acetate contacts an NBR seal, swelling occurs rapidly because these polar solvents interact strongly with the nitrile group through dipole-dipole forces. A 33% ACN compound commonly exhibits 40–100% volume swell in methyl ethyl ketone and 40–80% in acetone. The swelling is accompanied by tensile loss that frequently exceeds 50% and Shore A hardness reduction of 10–25 points. These solvents are incompatible even with high-ACN grades because the polar nitrile functionality is insufficient to repel the solvent when the butadiene domains remain present.

Plasticizer extraction can mask the initial swell. In compounds containing dioctyl phthalate or dioctyl adipate, the solvent partitions the ester plasticizer out of the rubber matrix. After solvent evaporation, the part can show hardness increase of 8–12 Shore A and volume below the original value. This two-step response—initial swell followed by dry-out shrinkage—is a specific incompatibility marker in pump diaphragms, expansion joints, and painted-surface masking applications. Post-immersion dry-out mass and hardness checks are necessary because standard ASTM D471 reporting does not capture extraction unless the test plan includes them.

Strong oxidizing acids create irreversible chemical attack that volume swell data alone underestimates. Nitric acid at 10% concentration may show volume swell of 10–40%, but tensile retention below 30% after 70 h at 23°C appears in some supplier technical literature. Concentrated sulfuric acid above 50% chars and embrittles the butadiene phase. The unsaturated carbon-carbon backbone is the vulnerable site; increasing acrylonitrile content does not remove the butadiene double bonds, so NBR cannot be made sufficiently resistant to strong oxidizing acids by ACN adjustment alone. Replacement elastomers should be selected when the process stream contains nitric acid, concentrated sulfuric acid, or peroxides above trace levels.

Ozone Attack at the Unsaturated Backbone

Ozone at 50 pphm and 20% elongation produces visible surface cracking in unprotected NBR within 24–72 h under ISO 1431-1:2022 or ASTM D1149. The butadiene double bonds undergo ozonolysis, yielding chain scission and cracks perpendicular to strain. Antiozonant waxes, p-phenylenediamine antiozonants, or blending with polyvinyl chloride can lengthen service life in atmospheric exposure, but ozone remains a fundamental incompatibility for unprotected NBR. Static seals stored outdoors may fail before fluid exposure occurs. For continuous outdoor service, EPDM or chloroprene may replace NBR where oil resistance permits; otherwise a more ozone-resistant oil-resistant polymer such as FKM is required.

Phosphate ester hydraulic fluids and glycol-ether brake fluids are common fluid classes that NBR compounds cannot handle without excessive swell and additive extraction. DOT 3 and DOT 4 brake fluids contain glycol ethers that plasticize the NBR network and extract polar plasticizers, reducing lip seal contact stress and leading to leakage. Phosphate ester hydraulic fluids used in aircraft and power generation systems show representative volume swell of 30–80% and Shore A hardness loss of 10–30 points in medium-ACN NBR. These fluid systems normally require EPDM or butyl elastomers, not a higher-ACN nitrile variant. The substitution error is well documented in industrial maintenance literature and should be controlled through material-to-fluid verification under ISO 1817:2022 before seal installation.

Raising Acrylonitrile Content Does Not Convert Aromatic Solvents Into Service Fluids

Increasing ACN from 18% to 50% raises the polarity of the copolymer and improves resistance to straight-chain aliphatic oils, ASTM Oil No. 1, and aliphatic fuels. The change does not make aromatic hydrocarbons, ketones, esters, or chlorinated solvents serviceable fluids. A 45% ACN compound in ASTM Fuel C may show 25–50% swell instead of 30–70%, but that remains above the acceptable design limit for most dynamic seals and many static gaskets. Simultaneously, high ACN raises the glass transition temperature and reduces low-temperature flexibility; the TR10 value may rise from approximately −35°C to −25°C when ACN is increased from 28% to 45% in a conventional sulfur-cured compound.

The table shows a plateau in solvent resistance. ACN addition reduces Fuel C swell but does not bring the compound below the 20% threshold that many seal standards use for borderline service. In methyl ethyl ketone, even 50% ACN grades remain in the 30–85% swell band. The main benefit of higher ACN is improved containment of straight-chain aliphatic oils and fuels, at the cost of low-temperature sealing force retention. The correct selection logic is therefore not to force NBR into aromatic or polar solvent service, but to switch to FKM, FFKM, EPDM, or chloroprene depending on the complete fluid set and temperature profile.

Immersion Test Results Vary With Cure System and Filler Type

Crosslink density and cure chemistry modify the numerical swell response even when the chemical class remains incompatible. Peroxide-cured NBR with a relatively high network chain density can show 10–25% lower equilibrium swell in toluene compared with a low-sulfur conventional cure. The improvement does not convert toluene into a serviceable fluid, but it can move a marginal aliphatic-aromatic blend from one rating band to another. Sulfur donor systems that improve compression set may also produce more extractable low-molecular-weight fragments, which increases mass loss in hot oil or solvent exposure.

Filler choice also influences the measured dimensional change. Carbon black reinforcement reduces volume swell because the filler phase does not absorb solvent, so the apparent volume change is diluted by the filler volume fraction. A compound containing 60 phr carbon black may show lower swell than an unfilled gum compound, but the polymer phase may be equally swollen. Hardness retention and tensile retention are therefore more informative than volume change alone. These factors explain why supplier compatibility tables can disagree by 10–20 swell points for the same chemical class when ACN content, cure system, filler loading, and plasticizer are not specified.

Production-scale molding adds processing constraints when high-ACN compounds are selected for chemical resistance. A 45% ACN grade may have Mooney viscosity ML(1+4) at 100°C in the range 60–90 MU, requiring higher fill pressures and tighter barrel temperature control to prevent scorch in injection molding. Sulfur donor cure systems can reduce processing safety at 120°C below 4 min, constraining runner and gate design. No process adjustment overcomes the fundamental chemical incompatibility with aromatic hydrocarbons, ketones, esters, strong oxidizing acids, or ozone. The critical control point remains fluid chemistry and ACN grade selection before mold design is finalized.