Acrylonitrile-Butadiene Copolymer (NBR) Technical Reference

Acrylonitrile-butadiene copolymer, designated NBR under ASTM D1418 and ISO 1629, is produced by emulsion polymerization of acrylonitrile and butadiene at acrylonitrile contents typically spanning 18 wt% to 50 wt%. The polar nitrile group raises cohesive energy density and resistance to aliphatic hydrocarbon swell, while butadiene segments provide elastomeric recovery, particularly at low acrylonitrile levels. Commercial grades are supplied with Mooney viscosity ML 1+4 at 100 °C ranging from 30 to 120, and specific gravity values between 0.98 and 1.02. Because monomer ratio determines glass transition temperature, Tg varies from approximately -10 °C for 50 wt% acrylonitrile to -45 °C for 18 wt% acrylonitrile. These compositional differences require distinct compounding and processing pathways, particularly where low-temperature sealing force and aliphatic oil resistance must be balanced.

Cold-polymerized grades, typically produced at 5–15 °C, yield lower gel content and narrower molecular weight distribution than hot-polymerized types. These characteristics reduce nerve on open mills and improve extrusion dimensional stability. Raw gum volatile matter is controlled to ≤0.5 wt% per ISO 248, while ash content is specified below 0.5 wt% per ISO 247. Bound acrylonitrile is measured by nitrogen analysis according to ISO 24698-1, and the result is reported on a dry polymer basis. The selection of raw gum Mooney viscosity influences shear heating response in internal mixers and the filler incorporation rate.

How Does Acrylonitrile Content Govern Oil Resistance and Low-Temperature Performance?

Acrylonitrile content remains the dominant compositional variable controlling equilibrium swell in hydrocarbon service. Under ASTM D471 or ISO 1817 immersion, a 28 wt% acrylonitrile grade typically exhibits higher volume change in reference oils than a 41 wt% acrylonitrile grade, while the 41 wt% material loses low-temperature flexibility earlier in ASTM D1329 temperature retraction testing. Compounds based on 33 wt% acrylonitrile NBR commonly show volume changes in IRM 901 below 30% after 70 h at 100 °C, while a 45 wt% acrylonitrile grade may reduce volume change to less than 10% under identical conditions. The inverse low-temperature effect appears in TR10 values: the lower ACN grade may reach -35 °C, whereas the higher ACN grade may measure -15 °C. This relationship is not linear because butadiene sequence distribution and cis/trans ratio affect crystallization and free-volume relaxation.

Representative published compound property ranges for carbon black–filled NBR compounds are listed below.

During mill mixing on a two-roll mill with a friction ratio of 1.15:1 to 1.25:1, sulfur-curative systems are added after zinc oxide and stearic acid have dispersed. The compound is cut and folded against the nip to maintain bank volume; the dump temperature is kept below 110 °C to avoid premature sulfur crosslinking. Carbon black types N550 and N774 are preferred for moderate heat build-up and extrusion smoothness. Mixing cycles on a laboratory internal mixer at 60 rpm with 70% fill factor typically reach a drop temperature of 105–115 °C. The resulting compound is sheeted to 6–8 mm and conditioned for 24 h at 23 °C and 50% relative humidity before rheometer testing per ASTM D5289.

Processing Windows for 33 wt% ACN NBR on Twin-Screw Extruders

For continuous extrusion of 33 wt% acrylonitrile compounds, a co-rotating twin-screw extruder with L/D 24:1 to 30:1 operates with barrel zone set points of 60 °C to 90 °C from feed to die. Screw speeds between 30 rpm and 80 rpm limit shear heating of high-filler formulations; die head pressure is maintained below 15 MPa by adjusting screw speed and feed rate rather than increasing zone temperatures. High-shear dispersion of N550 carbon black is monitored by dispersion rating per ISO 11345, with a target level of 8 or higher on the 10-point visual scale. Scorch safety is checked on a moving die rheometer at 180 °C; t5 values above 2 min are considered acceptable for stable extrusion. Published data for this specific configuration is limited to equipment-specific studies and should be confirmed with inline rheology measurements before transfer to production.

On injection molding lines, NBR compounds with high filler loading are processed in multicavity cold-runner molds with positive mold closures. Clamp force is matched to projected area and cavity count; flash formation is controlled by maintaining mold temperature uniformity within ±5 °C at 170–190 °C. Cure times are derived from MDR t90 values at the same mold temperature, typically with a safety margin of 10–20%. Pre-drying of the compound at 70 °C for 1–2 h is required when storage relative humidity exceeds 60%. Sulfur-donor cure systems with delayed-action sulfenamide accelerators reduce scorch in long runner systems, but mold residence time at elevated temperature must be limited to avoid reversion in thick sections.

Vulcanization kinetics are monitored from MDR isothermal data at 170 °C, 180 °C, and 190 °C per ASTM D5289. The activation energy for sulfur-cured NBR is calculated from the Arrhenius dependence of t90; this value is used to adjust cure time when mold temperature varies. Scorch time t5 is measured at the maximum mold temperature to ensure that the compound remains flowable during cavity filling. In multicavity tools, shear heating in hot runners can reduce effective scorch time by 15–20%; therefore gate size and runner diameter are balanced against pressure drop.

When Peroxide-Cured NBR Replaces Sulfur-Cured Compounds in Automotive Sealing

Peroxide cure is selected when compression set and heat aging dominate the service profile. Formulations use dicumyl peroxide or bis(t-butylperoxy isopropyl)benzene at 4–8 phr with a co-agent such as trimethylolpropane trimethacrylate at 2–3 phr. Under ASTM D395 Method B after 70 h at 150 °C, peroxide-cured NBR compounds typically exhibit compression set below 20%, whereas sulfur-cured analogues may exceed 35%. Tear strength per ASTM D624 Die C may be lower in peroxide systems unless particulate fillers are adjusted. Oxygen inhibition during open hot-air curing can produce tacky surfaces; therefore press curing is preferred. Avoid amine-based antioxidants and certain amine-containing processing aids because they scavenge free radicals and reduce crosslink density.

In hydraulic sealing applications, NBR compounds are qualified against fluid compatibility, compression stress relaxation, and abrasion under contaminated oil. Volume change in IRM 903 after 70 h at 125 °C is often specified below 35% for dynamic seals, while TR10 per ASTM D1329 remains above -30 °C for mobile equipment operating in cold climates. Migration kinetics of plasticizers in polymer matrices affect long-term seal swell and shrinkage; low-volatility ester plasticizers are preferred over phthalate plasticizers for retention. Compression stress relaxation per ISO 3384 after 168 h at 125 °C is used to evaluate sealing force decay. Tests are conducted on O-rings per ISO 3601-5 or buttons per ASTM D395 depending on the customer specification.

Fuel hose covers and liners based on 41 wt% or 45 wt% acrylonitrile NBR are specified for low permeation and high aliphatic fuel resistance. Permeation testing per SAE J30 or ASTM D814 uses reference fuel C with toluene and isooctane, and compounds with high ACN content show reduced fuel permeability relative to lower ACN grades. Extruded hose liners require smooth surface finish and low die swell; the compound is often plasticized with 10–20 phr of a low-temperature ester to balance TR10 and fuel resistance. Sulfur donor systems offer better dynamic flex fatigue, but peroxide cure is preferred when fuel exposure is combined with heat aging above 125 °C.

NBR Reorder & Annual Volume Program: Terms Structure for Repeat Buyers

The reorder structure for repeat buyers is based on a locked technical grade, an annual volume tier, and a rolling forecast that triggers call-off releases. Annual volume tiers may range from 1,000 kg to 50,000 kg, with tier-dependent lot sizes and lead-time windows. Every delivered batch is accompanied by a certificate of analysis reporting bound acrylonitrile per ISO 24698-1, Mooney viscosity ML 1+4 at 100 °C per ISO 289-1, ash per ISO 247, and volatile matter per ISO 248. REACH registration under Regulation (EC) No 1907/2006 and RoHS compliance under Directive 2011/65/EU are maintained at the shipment level. Batch lot traceability is retained for 24 months from dispatch. Grade substitution is not permitted without written technical approval, and Mooney viscosity deviations greater than ±5 MU from the locked specification trigger corrective action reporting.

Where FDA 21 CFR 177.2600 applies to repeat-use rubber articles such as dairy gaskets and conveyor belts, NBR formulations must be tested for extractives in water and n-heptane after the extraction cycles specified in the regulation. Plasticizer selection is restricted because low-molecular-weight esters can migrate into food simulants; polymeric plasticizers or high-molecular-weight acrylate plasticizers are used to reduce total extractives. The cure system must avoid thiourea accelerators and certain aromatic amine antioxidants that are not cleared under the regulation. Published migration data for NBR under 21 CFR 177.2600 are formulation-specific; each production lot requires extraction testing according to the conditions specified for the intended food type and temperature.

Batch-to-batch variance in high-ACN grades during high-shear mixing is controlled by pre-blending carbon black and oil prior to addition. Dispersion quality is assessed by visual comparison against the ISO 11345 reference photographs, and compounds failing the dispersion target are reprocessed or discarded. The oxygen permeability of NBR compounds is not negligible; therefore peroxide-cured seals stored in direct sunlight and high-humidity environments above 60% relative humidity should be protected from atmospheric moisture until vulcanization. The combination of NBR with amine-containing mold release agents is avoided in peroxide-cured systems because the amine group consumes free radicals and reduces crosslink density at the molded surface.