Acrylonitrile-Butadiene Copolymer (NBR): Technical Grade Selection and Pre-Order Parameter Matrix

Acrylonitrile-butadiene copolymer is designated NBR under ISO 1629 and is produced by free-radical emulsion polymerization of acrylonitrile and butadiene. Commercial grades span bound acrylonitrile contents from 18% to 50% by mass, with Mooney viscosity ML(1+4) at 100°C typically controlled between 20 and 120. The polymer is supplied as bales or crumb, with volatile matter held below 0.5% by mass and ash below 0.5% by mass on as-delivered values. Bound acrylonitrile content is the primary selection parameter for seals, gaskets, O-rings, and fuel-system components because it governs the trade-off between swelling resistance in hydrocarbon fluids and low-temperature flexibility.

Acrylonitrile-butadiene copolymerization in emulsion is controlled by monomer charge ratio and conversion. The reactivity ratios of acrylonitrile and butadiene generally produce a random copolymer, but at conversion above 70% drift in monomer composition can create localized ACN heterogeneity. To maintain reproducible oil resistance, producers limit conversion or use programmed monomer addition. Residual free acrylonitrile in finished polymer is controlled below 1–10 ppm depending on grade, with lower limits for food-contact and potable-water applications. Cold-polymerized NBR produced at 5–15°C generally has lower gel content, narrower molecular weight distribution, and lower branching than hot-polymerized NBR produced at 30–50°C. Gel content is determined by dissolution in methyl ethyl ketone after 24 h at room temperature and is specified below 5% for most extrusion grades. Branching affects processability: linear cold-polymerized grades with high acrylonitrile content exhibit sharper shear-thinning behavior in capillary rheometry at 100°C, with apparent shear viscosity at 100 s⁻¹ ranging from 500 Pa·s to 1,500 Pa·s depending on Mooney viscosity and filler loading. Raw NBR specifications are typically governed by supplier certificate of analysis. Mooney viscosity tolerance is ±5 ML units, bound acrylonitrile tolerance is ±1.5% by mass, volatile matter 0.5% max, ash 0.5% max, and soap residue 0.3% max. Emulsifier residues are controlled because they affect water resistance and cure rate.

How Does Bound Acrylonitrile Content Shift Swelling and Low-Temperature Performance?

Swelling resistance is characterized by immersion in ASTM D471 reference fluids. In IRM 901 oil after 70 h at 100°C, representative unplasticized sulfur-cured NBR formulations with 50 phr carbon black show volume swell of approximately 40–60% at 18% bound acrylonitrile, 10–20% at 34% bound acrylonitrile, and below 5% at 50% bound acrylonitrile. The corresponding impact brittleness temperatures under ASTM D2137 shift from near -55°C for low-ACN grades to -10°C or higher for high-ACN grades. Fuel resistance follows a similar pattern: volume swell in ASTM D471 Fuel C after 70 h at 23°C can range from 25–40% at 18% ACN to 10–20% at 45% ACN, depending on plasticizer type and filler loading. Low-temperature flexibility measured as TR10 under ASTM D1329 shows a corresponding shift from approximately -45°C at 18% ACN to -25°C at 34% ACN and -5°C at 50% ACN. These ranges are formulation-dependent and are not intrinsic polymer constants.

Compounding of a 33% bound acrylonitrile grade with Mooney viscosity ML(1+4) 100°C of 50 is typically performed in an intermeshing rotor internal mixer with 45 L chamber volume, fill factor 0.70–0.75, rotor speed 30–40 rpm, and dump temperature 110–130°C. Carbon black N330 or N550 is introduced at 40–70 phr, plasticizer such as dioctyl phthalate or alkyl sulfonic ester at 10–20 phr, and zinc oxide with stearic acid at 3–5 phr and 1–2 phr respectively. Sulfur donor systems based on tetramethylthiuram disulfide are limited by scorch; Mooney scorch MS t5 at 121°C should remain above 15 min for safe extrusion and injection molding. High-ACN grades above 41% bound acrylonitrile generate greater frictional heat during mixing. For those grades, rotor speed is reduced and dump temperature is held below 120°C to avoid premature crosslinking and gel formation. Production-scale behavior shows that batch-to-batch Mooney viscosity variation exceeding ±5 ML units can alter mixing energy by more than 15% and should be controlled by blending or grade selection. Below 20 phr carbon black, NBR compounds exhibit low tensile strength and high die swell; above 80 phr, compound Mooney viscosity can exceed 100 ML(1+4) 100°C and requires high processing temperatures. Plasticizer migration becomes problematic above 20 phr in static sealing applications: extraction in ASTM D471 IRM 901 after 70 h at 100°C can exceed 10%, leading to shrinkage and loss of sealing force.

When High-ACN NBR Is Processed on Twin-Screw Extruders

High-ACN compounds with Mooney viscosity ML(1+4) 100°C above 70 are often compounded on a twin-screw extruder with L/D ratio 42:1, segmented screws, and barrel temperature profile from 60°C at the feed zone to 90°C at the die. Vacuum devolatilization at -0.08 MPa removes residual moisture and volatile by-products. Screw speed is maintained between 80 rpm and 120 rpm to control residence time and die swell. For injection molding of NBR seals, clamping force is typically specified from 500 kN to 2,500 kN, with injection temperature 70–90°C, mold temperature 160–190°C, and injection pressure 80–120 MPa. Thick-section seals require cure time of approximately 2–3 min per millimeter of wall thickness; cold-runner residence times above 10 min at 100°C are avoided because of scorch initiation. On production-scale calenders, NBR friction ratio is set between 1.0 and 1.2; higher friction ratios generate heat and feed instability. In extrusion, screw compression ratio is typically 2.5:1 to 3.5:1 for NBR compounds. Die swell at 100°C and shear rate 100 s⁻¹ is 15–30% for carbon-black-filled NBR; higher ACN content and lower plasticizer reduce die swell.

Vulcanization kinetics are measured with a moving die rheometer under ISO 6502. A sulfur cure at 160°C using 2.0 phr sulfur and 1.5 phr N-cyclohexyl-2-benzothiazolesulfenamide commonly reaches t90 in 6–12 min. Peroxide cure using dicumyl peroxide at 2.0–2.5 phr with trimethylolpropane trimethacrylate coagent at 1.0–1.5 phr requires 170–180°C and yields compression set after 70 h at 125°C of 15–25% under ASTM D395 Method B. Tensile strength per ASTM D412 for black-filled sulfur-cured formulations is commonly 12–18 MPa, with elongation at break 250–450%. These values apply to compounds containing 50 phr carbon black and no plasticizer; addition of 15 phr plasticizer typically reduces tensile strength by 15–25% and hardness by 5–10 Shore A. Unfilled NBR gum stocks have tensile strength below 3 MPa and are not used structurally. Reinforcement with carbon black at 40–70 phr increases tensile strength to 12–18 MPa and tear strength to 25–45 N/mm under ISO 34-1 for trouser tear specimens. Hardness is adjusted from 40 Shore A to 90 Shore A by filler and plasticizer levels; standard sealing grades are commonly 60–75 Shore A.

Thermal Aging Resistance in Sulfur- and Peroxide-Cured NBR Formulations

Thermal aging in air at 125°C for 70 h under ASTM D573 produces different failure modes depending on cure chemistry. Sulfur-cured NBR compounds generally show elongation retention of 50–70% and hardness increase of 5–12 Shore A, while peroxide-cured grades retain 70–90% elongation and increase hardness by 3–8 Shore A. The limiting service temperature for continuous exposure is typically 100°C for sulfur-cured and 120–130°C for peroxide-cured NBR, with peak service in oil not exceeding 150°C for short periods. Peroxide-cured NBR compounds should not be formulated with high levels of amine-based antioxidants because amine radicals consume peroxide and reduce crosslink density. Acidic fillers with pH below 6.5 retard thiazole-accelerated sulfur vulcanization and are neutralized with zinc oxide or stearic acid before accelerator addition.

NBR is not recommended for continuous contact with strong oxidizing acids, ketones, chlorinated solvents, or fuels with high aromatic content. Immersion in acetone at 23°C for 70 h under ASTM D471 can produce severe swelling and loss of tensile strength. O-rings for hydraulic and pneumatic systems are tested for compression set under ISO 815-1 or ASTM D395. A typical sulfur-cured 70 Shore A compound shows compression set after 70 h at 100°C of 20–30%; peroxide-cured 70 Shore A compounds achieve 10–15%. Fuel-system components require low extractables; total extractables in ASTM D471 Fuel C after 70 h at 23°C are specified below 10% by mass for many fuel-contact seals. Gas permeability coefficients measured by ISO 2782 for NBR decrease as ACN content increases. At 23°C, oxygen permeability coefficient for 18% ACN is approximately 5–8 ×10⁻¹⁷ m²/s·Pa, while for 45% ACN it falls to 0.5–1.5 ×10⁻¹⁷ m²/s·Pa. High-ACN grades are therefore selected for barrier layers, although low-temperature flexibility is reduced.

NBR bales are stored at 5–30°C with relative humidity below 60%. Condensation on cold bales is controlled by pre-drying at 60°C for 2–4 h when ambient relative humidity exceeds 60%. Unvulcanized mixed compounds are consumed within 14 days when stored at 15–25°C, because sulfur donor and thiuram systems lose activity. Contact with copper, manganese, or strong acids accelerates oxidative degradation. Specific food-contact NBR grades may comply with FDA 21 CFR 177.2600 for repeated-use rubber articles, subject to acrylonitrile monomer migration limits. Regulatory documentation for industrial grades commonly includes REACH pre-registration and RoHS recast 2011/65/EU Annex II declarations when requested.

NBR Pre-Order FAQ: MOQ, Lead Time, Payment and Sampling in One Table

The pre-order parameter matrix below consolidates the commercial and sampling conditions most frequently requested for NBR grade selection. Values are representative supplier boundaries and are confirmed in the proforma invoice; they do not constitute an offer until batch-specific specifications are agreed.

Pre-shipment samples are tested for Mooney viscosity, bound acrylonitrile content, ash, volatile matter, and cure rheometer torque. Batch release documentation includes certificate of analysis, safety data sheet, and, if required, a EN 10204 3.1 inspection certificate. Customer-specific compound development requires a separate technical agreement covering formulation ownership and scale-up on customer equipment.