Acrylonitrile-butadiene copolymer, designated NBR under ASTM D1418 and ISO 1629, is a random emulsion copolymer of acrylonitrile and butadiene. Commercial grades typically range from 18% to 50% acrylonitrile content, with the remaining diene contribution providing crosslinkable unsaturation for sulfur or peroxide cure. The polarity of the nitrile group imparts resistance to aliphatic hydrocarbon swelling and reduces permeation, but it simultaneously raises glass transition temperature and reduces low-temperature flexibility. NBR is produced by emulsion polymerization and supplied as bale, crumb, or oil-extended slab. Mooney viscosity ML(1+4) at 100 °C commonly falls between 30 and 90 MU depending on molecular weight and oil extension. In applications where a single polymer must satisfy either hot-oil containment or water/glycol exposure, the selection logic frequently reduces to NBR vs EPDM: The Oil-vs-Water Decision in One Table.

When the Seal Must Survive Hot Hydrocarbons

Seal compounds for transmission and engine-oil service are specified against ASTM D471 immersion in IRM 903 or equivalent service fluids. A sulfur-donor-cured NBR with 34% ACN and 70 Shore A hardness typically exhibits volume swell of 8–15% after 70 h at 100 °C, while an EPDM control swells beyond 80% under identical exposure. The NBR compound is filled with reinforcing carbon black at 40–80 phr and protected with zinc oxide at 3–5 phr, stearic acid at 1–2 phr, and an antioxidant package appropriate to the service temperature. Plasticizer selection is governed by volatility and extraction limits; dioctyl sebacate is used for low-temperature sealing, while polymeric ester plasticizers are specified where hot-oil extraction would harden the compound. Plasticizer migration kinetics in the polymer matrix control hardness change during hot-oil ageing: low-molecular-weight esters migrate more rapidly into oil and produce shrinkage, while polymeric esters remain in the network. The vulcanization kinetics of a semi-efficient sulfur cure with CBS accelerator at 1.0–1.5 phr and sulfur at 0.8–1.2 phr typically produce a T90 of 3–8 min at 170 °C on a moving die rheometer under ASTM D5289. Peroxide-cured NBR with dicumyl peroxide at 3–5 phr generally exhibits longer T90, lower compression set, and poorer hot tear. Peroxide-cured compounds should not use amine-based antioxidants because these scavenge peroxy radicals and reduce crosslink density.

Does Acrylonitrile Content Shift the Low-Temperature Limit?

The inverse relation between ACN content and low-temperature flexibility is the central design conflict. Published glass transition data place the onset of Tg near −50 °C for 18% ACN grades and near −5 °C to −10 °C for 50% ACN grades. Low-temperature sealing behaviour is more reliably tracked by TR-10 retraction temperature under ASTM D1329 or by Gehman stiffness under ISO 2921. A typical 34% ACN sulfur-cured compound with 70 Shore A hardness can show a TR-10 around −25 °C; raising ACN to 45% with the same plasticizer system may move TR-10 toward −10 °C. Simultaneously, oil volume swell in IRM 903 drops. This is a cliff-edge trade: a grade chosen for aggressive Fuel C or higher aromatic oil may fail a low-temperature specification unless the compound contains high monomeric plasticizer loading, which then increases compression set. Published data for all possible fuel-oxygenate combinations is limited; each compound must be validated in the actual service fluid under ASTM D471 because oxygenates can deviate from hydrocarbon swell models.

On a 60:1 L/D twin-screw extruder running a 45% ACN NBR compound, die-head temperature is controlled below 140 °C. Higher stock temperature generates localized gel from premature crosslinking, visible as surface lumps in the extrudate and downstream blemishes. Batch-to-batch Mooney variation is typically held within ±5 MU; wider variation alters die swell, screw backpressure, and final hardness. Pre-drying is required at relative humidity above 60% if crumb has been stored in unheated warehouses. Moisture above 0.2% causes porosity in extruded profiles; a 70 °C desiccant hopper or hot-air tray is then used for 2–4 h before continuous extrusion. Injection molding of 70 Shore A NBR seals on a 180-tonne clamp-force press typically uses a barrel temperature profile of 60–80 °C rear and 80–100 °C front, mold temperature 170–190 °C, and injection pressure 60–100 MPa. Raising mold temperature above 200 °C can produce gas accumulation near the parting line and void formation.

NBR vs EPDM: The Oil-vs-Water Decision in One Table

The comparative matrix below uses standardized cured-sheet and finished-part tests. Values are compounded ranges for medium-hardness industrial articles, typically 70 Shore A, and are not procurement specifications. Compound formulation, plasticizer type, filler loading, and cure state shift the measured values.

Mobile hydraulic cylinder rod seals and coolant-system seals sit on opposite sides of the same table. A rod seal bathed in mineral oil requires the NBR cell; a coolant flange seal in a 50:50 water-glycol mixture at 110 °C selects EPDM. Brake-fluid service is the reversal case: NBR swells excessively in glycol ether brake fluids, and EPDM becomes the preferred polymer. Diesel fuel injector return lines reject EPDM due to severe hydrocarbon swell. A seal exposed to both hot oil and glycol ether therefore cannot be resolved by either polymer alone; the system must be separated or a composite with an HNBR body and EPDM cover must be specified.

Why NBR Linings Fail in Ozone-Rich Air at 50 pphm

Unsaturation in the butadiene segment is the ozone-sensitive site. Under ISO 1431-1 exposure at 50 pphm ozone, 40 °C, and 20% elongation, a medium-ACN NBR vulcanizate without antiozonant develops visible surface cracks within 24–48 h. The addition of 1.5–3.0 phr 6PPD and a microcrystalline wax bloom can delay cracking to 168–336 h, but protection is finite because the antidegradant is consumed at the surface. For continuous outdoor exposure where ozone concentration exceeds 0.1 ppm, an NBR/PVC blend containing 30–40 phr PVC or an EPDM sealing surface should be specified. This is an operational boundary: accumulated surface cracks on a compressed O-ring can propagate under dynamic service and create leakage channels.

Mixer Rotor Speed, Fill Factor, and the Risk of Scorch

In a 1.6 L laboratory Banbury mixer, NBR batches are mixed at fill factor 0.75–0.85 and rotor speed 35–50 rpm. High-ACN grades generate additional shear heat because polar acrylonitrile raises compound viscosity and reduces filler wet-out rate. Dump temperature is held below 140 °C for sulfur-donor systems; peroxide systems require dump below 110 °C to prevent premature decomposition of dicumyl peroxide. A second-pass mill at 70–80 °C is used to form sheet. Mooney scorch at 121 °C is held above 10 min for injection molding and above 5 min for compression molding. If scorch time drops below 5 min, injection-molding barrel temperature is lowered and screw speed reduced to avoid filled runners and burnt gate remnants.

Regulatory specifications for nitrile rubber are fluid-dependent. 21 CFR 177.2600 permits rubber articles for repeated food contact under extraction limits, but high-ACN NBR is generally restricted to aqueous or low-fat service because oil-soluble acrylonitrile oligomers and plasticizers migrate more readily into fatty food simulants. REACH and RoHS obligations are compound-specific: free acrylonitrile monomer must remain below applicable workplace and food-contact limits, and carbon black and process oil require PAH compliance under EU regulations. The table below anchors the main test and classification references used in NBR compound datasheets.

Rotary oil-well blowout preventer ram seals are typically specified with 34–45% ACN NBR when the service fluid is oil-based mud. Water-based mud at elevated temperature shifts the requirement toward higher ACN or hydrogenated nitrile because NBR hardens and loses elastomeric recovery in hot brine. In diesel fuel service, 40–50% ACN is required to limit aromatic swell, but low-temperature operability at −30 °C becomes the limiting test. The process boundary is set by mixing and extrusion of high-ACN grades. When the final component must survive both oil and water extremes, the choice cannot be resolved by a single polymer; fabric-reinforced NBR with an EPDM cover layer is used, combining the oil-resistant body with a water- and ozone-resistant exterior.