Acrylonitrile (CAS 107-13-1) is the polar vinyl monomer that enables nitrile butadiene rubber (NBR) to resist hydrocarbon swelling. Commercial acrylonitrile is produced by fluidised-bed ammoxidation of propylene, ammonia, and oxygen over bismuth molybdate catalysts at 400–500 °C and 0.5–2.0 barg; per-pass propylene conversion is typically 60–80 % with acrylonitrile selectivity above 80 %. The monomer is inhibited with 35–50 ppm hydroquinone monomethyl ether before storage and transport. Acrylonitrile exhibits a boiling point of 77.3 °C, a closed-cup flash point of -1 °C, an autoignition temperature of 481 °C, and flammability limits of 3.0–17.0 vol % in air. In NBR emulsion polymerisation, acrylonitrile and 1,3-butadiene are copolymerised at 5–15 °C for cold-polymerised grades or 30–50 °C for hot-polymerised grades. The bound acrylonitrile content, not residual monomer level, is the compositional parameter that determines oil resistance after vulcanisation.

Oil-resistance differences track the solubility parameter of the cured NBR. Bound nitrile groups contribute a strong dipole and raise the total solubility parameter from approximately 18.5 MPa^0.5 at 18 % ACN to 22.0 MPa^0.5 at 50 % ACN. Nonpolar aliphatic oils such as ASTM IRM 903 have solubility parameters near 16.0–17.0 MPa^0.5. A larger polymer solubility parameter increases the Flory-Huggins interaction parameter and reduces equilibrium solvent uptake. ASTM D471-16 immersion testing quantifies the effect as volume change, extracted soluble matter, and hardness change after exposure.

NBR is classified by bound acrylonitrile content rather than by molecular weight alone. The standard elastomer designation is ASTM D1418-21; the nitrile content is measured by Fourier-transform infrared spectroscopy using the nitrile absorbance at 2237 cm⁻¹ according to ISO 24698-1:2018. The three loose families are low-nitrile (18–24 % ACN), medium-nitrile (25–35 % ACN), and high-nitrile (36–50 % ACN). Low-nitrile grades are selected for low-temperature flexibility; high-nitrile grades are selected for service in ASTM IRM 901, IRM 902, and IRM 903 oils, as well as aliphatic hydrocarbon process streams.

NBR (Nitrile Butadiene Rubber): How the Acrylonitrile % Sets Oil Resistance

Bound acrylonitrile content in NBR is ordinarily controlled between 18 % and 50 % by adjusting the monomer charge ratio in the emulsion polymerisation reactor. The oil-resistance effect is most clearly demonstrated by ASTM D471-16 immersion in ASTM IRM 903, a high-naphthenic oil used to simulate industrial mineral oil. At the low end of the ACN range, the cured rubber absorbs a larger fraction of the oil because the polymer matrix is less polar and more compatible with aliphatic hydrocarbon fluids. At the high end, the dense nitrile phase reduces the free-volume fraction available for oil uptake and slows the diffusion front. This is not a linear dilution effect; the empirical relation between bound ACN and equilibrium volume change is stronger above 33 % ACN because the polar interaction approaches the solubility parameter of the oil, and further nitrile addition moves the cured network into a thermodynamically less favourable state for aliphatic absorption.

Flory-Rehner swelling theory provides the quantitative basis for this shift. The equilibrium solvent volume fraction φ₁ is a function of crosslink density ν_e and the Flory-Huggins interaction parameter χ. For NBR-oil systems, χ scales with the square of the difference between polymer and solvent solubility parameters. Increasing bound ACN from 18 % to 50 % widens the solubility-parameter gap against ASTM IRM 903, reducing φ₁ and therefore the volumetric swell. This thermodynamic effect is supplemented by kinetic resistance because the polar nitrile groups lower the diffusion coefficient of aliphatic hydrocarbons through the free-volume network.

The values in Table 1 are representative supplier data from carbon-black-filled NBR and do not constitute a specification. The inverse relation between bound acrylonitrile content and volume change in ASTM IRM 903 is reproducible across production lots. At 50 % ACN, equilibrium solvent uptake is controlled primarily by polar interactions rather than chain mobility; however, the increase in glass transition temperature imposes a minimum service-temperature penalty. The low-ACN compound shows a volume change of 35–45 %, which is unacceptable for most static seal applications but tolerable in vacuum hose and low-temperature gasket compounds where oil exposure is intermittent.

What equilibrium swelling and dynamic mechanical data distinguish 18 % ACN from 50 % ACN?

Dynamic mechanical analysis per ISO 6721-4:2019 records the loss tangent peak, a more sensitive low-temperature flexibility indicator than DSC glass transition temperature. At 18 % ACN, the tan δ peak lies near -50 °C; at 33 % ACN, near -30 °C; at 50 % ACN, near -5 °C. This shift of roughly 1.5 °C per percentage point of bound acrylonitrile between 20 % and 45 % ACN is the central trade-off in NBR product selection. The same polar nitrile groups that block aliphatic oil penetration stiffen the polymer backbone and reduce chain flexibility at sub-zero temperatures.

For oilfield blowout preventer packer elements, equilibrium volume change in ASTM IRM 903 at 135 °C for 168 h is the primary screening test. Compounds with 40–45 % ACN maintain volume change below 10 % and hardness change below 8 IRHD; compounds with 18 % ACN exceed 40 % volume change and lose compression-set resistance after prolonged exposure. Crosslink density modifies the swelling response. The addition of 5–10 phr trimethylolpropane trimethacrylate to a peroxide-cured high-ACN compound raises crosslink density and lowers equilibrium swell by a further 5–10 %, but low-temperature brittleness becomes more severe.

Conversely, high-ACN NBR is more sensitive to polar fluids and high-aromatic fuels. The nitrile group increases the polar component of the Hansen solubility parameters. ASTM Reference Fuel C, which contains 50 vol % toluene, produces more swelling than IRM 903 at the same ACN; the table shows volume changes of 25–35 % at 50 % ACN, whereas the IRM 903 value is below 5 %. This incompatibility matters in gasoline systems; methanol and MTBE blends require ACN levels above 40 % but still show excessive swell at aromatic contents above 50 vol % toluene.

In automotive fuel injector seal compounds with 40 % ACN NBR, the production route is typically a Banbury internal mixer with a 0.75 fill factor, two-wing rotor, and ram pressure of 0.5 MPa. The first-pass masterbatch is dropped at 130–135 °C and final curatives are added on a two-roll mill at 40–50 °C. Injection moulding is performed on a 200 t clamp-force cold-runner machine with a 20:1 L/D general-purpose screw, melt temperature of 95–110 °C, and mould temperature of 175–180 °C; tc₉₀ is typically 120–180 s. Compounds with 40 % ACN require pre-drying at 70 °C for 2–3 h when exposed to relative humidity above 60 %. Residual moisture above 0.1 wt % produces steam porosity at the gate and reduces tensile strength by more than 10 %.

O-rings produced from 34 % ACN NBR for mineral-oil hydraulic systems are compression-set tested per ASTM D395-18 Method B at 125 °C for 70 h. In a 90 Shore A compound with 80 phr N550 carbon black, compression set at 125 °C is typically 18–22 % for 34 % ACN and 12–16 % for 45 % ACN; however, the same high-ACN part exhibits poor low-temperature recovery at -25 °C. This operational boundary is why many hydraulic O-ring specifications accept 34–36 % ACN as a compromise rather than the highest available nitrile level.

Thermal-oxidative ageing complicates the oil-resistance picture. In air-oven ageing per ISO 188:2023 at 125 °C for 72 h, a 45 % ACN sulfur-cured compound retains 75–85 % of tensile strength, whereas a 33 % ACN compound retains 85–90 %. The polar nitrile groups reduce oxidative stability because residual double bonds adjacent to nitrile units are more susceptible to radical attack. This is controlled with polymerised 2,2,4-trimethyl-1,2-dihydroquinoline at 1–2 phr; loadings above 2 phr bloom to the surface and interfere with adhesion to metal inserts.

When high-ACN NBR replaces lower-nitrile material in torque converter seals

Replacing 33 % ACN NBR with 45 % ACN NBR in a torque converter seal compound raises Mooney viscosity ML(1+4) at 100 °C from 70–80 MU to 90–110 MU at equivalent carbon black and plasticiser loading. On a 40:1 L/D counter-rotating twin-screw extruder with barrel temperatures of 70/80/90/95/95 °C, specific energy input rises from 0.15 kWh/kg to 0.19 kWh/kg, and die head pressure increases by 20–30 %. Vulcanisation kinetics are modified because bound nitrile content increases accelerator adsorption on zinc oxide. The scorch time t_s2 at 121 °C per ISO 6502:2020 decreases from 18 min to 12 min. To preserve processing safety, tetramethylthiuram disulfide is reduced from 1.0 phr to 0.6 phr while N-cyclohexyl-2-benzothiazole sulfonamide is increased from 1.0 phr to 1.4 phr.

Higher ACN also changes filler networking. In a moving die rheometer at 177 °C, minimum torque increases from 2.5 dN·m to 3.2 dN·m for the same N330 loading because the polar matrix improves filler-matrix wetting. High-shear dispersion of N330 carbon black in high-ACN NBR is achieved during the Banbury first pass; the polar matrix improves filler wetting but increases mixing torque by 10–15 % relative to low-ACN. The practical effect is faster viscosity build during mixing and higher heat generation; for compounds with 45 % ACN, a two-stage mixing cycle with a cooling water temperature of 15–20 °C is required to keep final stock temperatures below 120 °C.

Mixer, injection moulder, and triple-roll calender constraints for 45 % ACN stock

Calendering of 45 % ACN NBR sheet for blowout preventer packers is run on a triple-roll calender with roll surface temperatures of 70/80/70 °C and a bank width controlled to 25–35 mm. The higher nitrile compound has greater nerve and requires a higher friction ratio of 1.15:1 between the top and middle rolls to prevent surface roughness. Sheet stock is then preformed at 25–30 °C and compression moulded under 15 MPa at 150–160 °C for 20–30 min. Production-scale batch-to-batch variance in bound ACN is typically controlled to ±1.0 percentage point by redox initiator feed and monomer ratio control; a shift of 2 percentage points is sufficient to change ASTM D471-16 volume change by 2–3 % and low-temperature brittleness by several degrees.

In brake-fluid contact, NBR is generally not used; glycol ether brake fluids cause plasticiser migration and surface cracking under ASTM D471-16 immersion at 121 °C. Ethylene-propylene diene monomer is preferred for that duty. High-ACN NBR also has limitations in wet sour service at temperatures above 100 °C; hydrogenated nitrile or fluoroelastomer grades are required when hydrogen sulfide and amine-based corrosion inhibitors exceed the stabilisation window of standard NBR. The polar nitrile groups alter plasticiser migration kinetics in polymer matrices; ester plasticisers such as dioctyl sebacate are retained more strongly in high-ACN NBR, but extraction after 168 h in ASTM Reference Fuel C at 40 °C reaches 3–5 wt %.