Acrylonitrile-butadiene copolymer is classified as NBR under ASTM D1418-21 and ISO 1629:2013, and is produced by emulsion copolymerization of acrylonitrile and butadiene. The bound ACN fraction is the primary compositional variable governing polarity, glass transition temperature, low-temperature retraction, oil and fuel swell, and vulcanizate mechanical response. Industrial grades are conventionally supplied at 18%, 28%, 33%, and 41% bound ACN, though specialty compositions outside this range exist. In a sulfur-donor cure system compounded on a 120 L intermeshing internal mixer, the 18% ACN grade behaves as a low-polarity diene rubber with a low glass transition temperature but high equilibrium swell in aliphatic hydrocarbon oils; the 41% ACN grade behaves closer to a polar elastomer with lower swell in IRM 903 and Fuel C but reduced cold-weather retraction. The selection among these four ACN levels is therefore a constraint resolution exercise involving low-temperature sealing, fluid compatibility, extrusion stability, and compression set.

The emulsion polymerization process places these grades on a continuum rather than a stepwise ladder. Cold NBR polymerized at 5 °C to 15 °C typically has lower long-chain branching than hot NBR polymerized at 30 °C to 50 °C, and the higher ACN grades tend to require higher stabilizer demand to prevent latex destabilization during steam coagulation. The coagulated crumb is washed, dewatered, and baled; residual soap above 0.3% can degrade vulcanizate clarity and adhesion. In a production batch, ACN content is typically controlled within ±1% by monomer feed ratio and conversion. This batch-to-batch variance influences mixing energy and final hardness.

How Does Bound ACN Content Shift the Glass Transition and TR10 Response?

The glass transition temperature of NBR increases monotonically with bound ACN because the polar nitrile side groups reduce free volume and restrict segmental mobility. Differential scanning calorimetry under ISO 11357-2:2020 at 10 K/min on the second heating typically resolves Tg values of -50 °C to -46 °C for 18% ACN, -42 °C to -36 °C for 28% ACN, -34 °C to -28 °C for 33% ACN, and -22 °C to -16 °C for 41% ACN. In parallel, low-temperature retraction under ASTM D1329-16 yields TR10 values that are generally 4 °C to 8 °C higher than the DSC Tg. A 41% ACN grade typically fails to reach 10% retraction above -18 °C, which makes it unsuitable for dynamic shaft seals operating at -25 °C; an 18% ACN grade retains retraction response near -40 °C. On a 250 t injection molding machine producing cold-climate compressor valve plates, the 41% ACN compound exhibited leakage at -20 °C while the 18% ACN compound maintained seal integrity at -40 °C; published data for this specific configuration is limited, but the result aligns with TR10 measurements.

When the same four grades are immersed in IRM 903 under ASTM D471-16a for 70 h at 100 °C, volume swell ranges are 45% to 60% for 18% ACN, 25% to 40% for 28% ACN, 12% to 25% for 33% ACN, and 6% to 15% for 41% ACN. In ASTM Fuel C immersion at 23 °C for 46 h, the swelling differential is larger: the 18% ACN grade may exceed 80% volume change, while the 41% ACN grade remains below 25%. The polarity shift also alters additive solubility; monomeric phthalate plasticizers migrate more rapidly from 18% ACN vulcanizates than from 41% ACN vulcanizates, requiring replacement with a polymeric plasticizer when extraction resistance is specified under ISO 1817:2022.

The solvent selectivity reverses when the contacting fluid is polar. Immersion in methyl ethyl ketone at 23 °C for 24 h can swell the 41% ACN grade by 150% to 200%, while the 18% ACN grade often remains below 80%. Therefore fluid compatibility is a matching exercise between elastomer solubility parameter and fluid solubility parameter; the higher nitrile matrix moves NBR closer to polar plasticizers and polar solvents and farther from aliphatic fuels.

NBR ACN Content Selection Table: 18% / 28% / 33% / 41% Compared

The comparative matrix below summarizes typical published ranges for sulfur-cured NBR vulcanizates. Values are not formulation-independent; carbon black type, plasticizer level, cure state, and antioxidant package alter each data band. The table is intended for initial grade screening and must be verified on the production compound.

When a specification demands both low-temperature flexibility and oil resistance, the 28% to 33% ACN range usually becomes the compromise band. The 28% ACN grade retains a lower TR10 but gives up measurable fuel resistance; the 33% ACN grade improves fuel resistance while moving the low-temperature limit upward by 6 °C to 10 °C. Parts requiring cold-impact response should therefore be screened first with the 18% or 28% ACN grade, while parts requiring prolonged hot oil contact should begin evaluation with the 33% or 41% ACN grade.

Tensile, Tear, and Hardness Values Shift Only Moderately With ACN Content

The mechanical properties of the four ACN grades follow a moderate monotonic shift. Under ASTM D412-16, die C tensile specimens cut from 2 mm press-cured sheets show tensile strength generally increasing from 15 MPa to 27 MPa as ACN moves from 18% to 41%. Elongation at break decreases from 550% to 250% over the same ACN range. Tear strength under ASTM D624-00, die B, typically remains within 25 kN/m to 50 kN/m; the higher ACN grades usually reside at the upper end when carbon black dispersion is equivalent. Hardness under ASTM D2240-15 can be held constant by reducing plasticizer level in the 41% ACN compound, but this sacrifices elongation and low-temperature retraction. The modulus at 100% elongation rises with ACN at equal crosslink density, reflecting increased intermolecular association between nitrile groups.

Fatigue life in a De Mattia flex test under ASTM D813-19 is not directly predicted by ACN content alone; higher tensile modulus in 41% ACN may accelerate crack growth unless the crosslink density is reduced toward the lower limit of the cure window. In a production O-ring compound, the 33% ACN grade often provides the highest fatigue crack resistance because it balances strain energy density and oil swell resistance.

Ozone resistance measured by ISO 1431-1:2014 remains poor for all NBR ACN levels unless an antiozonant is added. Higher ACN content does not provide a practical improvement; the 41% ACN grade may show earlier dynamic ozone cracking due to higher modulus. For outdoor exposure requiring ozone resistance, NBR is usually blended with PVC or replaced by HNBR or EPDM.

When a 41% ACN Grade Is Processed on a 20:1 L/D Pin-Barrel Cold-Feed Extruder

On a cold-feed pin-barrel extruder with 20:1 L/D and a vent port held at 0.08 MPa absolute pressure, moving from 18% to 41% ACN raises head pressure by 12% to 20% at constant screw speed because polar nitrile groups increase melt viscosity at a given shear rate. The die swell measured on a 10 mm rod die increases with ACN content, requiring a die temperature increase of 5 °C to 10 °C or drawdown adjustment to hold diameter within a ±0.2 mm tolerance. In injection molding, the 41% ACN compound may require a slightly higher barrel temperature profile, typically 5 °C to 15 °C above the 18% ACN compound in the rear zone, to prevent high screw torque. The Mooney viscosity according to ASTM D1646-19a is not a direct function of ACN alone; gel content, stabilizer package, and conversion level are independent variables.

Cure behavior under ASTM D2084-19a at 160 °C shows shorter scorch time as ACN increases. The 41% ACN sulfur-donor compound commonly exhibits a 15% to 30% reduction in t_s2 relative to the 18% ACN compound at the same sulfur level, while the maximum torque may increase by 10% to 18% due to higher filler-polymer interaction. Peroxide-cured compounds require a coagent and a stripping stage when processing 41% ACN because the high nitrile content increases acid byproduct retention; without the stripping stage, compression set at 150 °C can deteriorate.

Carbon black incorporation is influenced by ACN content but is not dominated by it. In a 1.6 L laboratory Banbury mixer with 0.75 fill factor and 30 rpm rotor speed, the black incorporation time for 50 phr N330 in a 41% ACN compound is commonly 20% longer than that of the 18% ACN compound at the same oil loading. Bound rubber content after 6 min of mixing increases with ACN, which contributes to higher storage modulus and lower die swell in downstream extrusion.

Compression set and sealing force retention under ASTM D395-16e1, Method B, 25% deflection, 22 h at 100 °C, generally remain within 15% to 30% for all four ACN levels when the sulfur cure is optimized. In a 100 t injection molding machine producing O-rings, the 18% ACN grade showed a sealing force loss of 40% after 168 h in IRM 903 at 100 °C, whereas the 41% ACN grade retained sealing force within 15% under the same exposure. This result is consistent with swelling data, not with differences in compression set alone.

The operational boundaries are sharp enough to require formal grade verification. The 18% ACN grade is suited for low-temperature dynamic service near -40 °C but is not compatible with continuous immersion in gasoline or high-aromatic oil. The 28% ACN grade is widely used for general-purpose industrial exposure where fuel swell is moderate. The 33% ACN grade is commonly selected for automotive sealing that must balance ASTM Fuel C resistance with a TR10 better than -25 °C. The 41% ACN grade is specified for aggressive fuel and oil contact but must be derated below -20 °C for dynamic seals. Replacement of carbon black with precipitated silica reduces high-temperature compression set but may raise compound viscosity by 10% to 15%; the resulting compound must be revalidated on the production extruder.