Acrylonitrile-butadiene copolymer (NBR) is an emulsion-polymerized elastomer composed of acrylonitrile and 1,3-butadiene repeat units. The bound acrylonitrile content in commercial grades ranges from 18% to 50% by mass and controls glass transition temperature, low-temperature flexibility, and equilibrium swelling in aliphatic hydrocarbon fluids. Cold-polymerized NBR produced at 5–15 °C provides narrower molecular weight distribution and lower branching than hot-polymerized grades produced at 30–50 °C. Mooney viscosity ML(1+4) at 100 °C across commercial grades commonly spans 20–140 under ASTM D1646-19. The polymer is classified as NBR in ASTM D1418-21 and ISO 1629:2013. Residual unsaturation from the butadiene segments permits both sulfur and peroxide cure systems, while the nitrile group confers resistance to petroleum-based oils, fuels, and selected solvents.

What Happens When Acrylonitrile Content Crosses 34% by Mass?

The 34% bound acrylonitrile threshold is significant because fluid resistance in ASTM Reference Fuel C improves while low-temperature compression set and flex fatigue degrade. At 28–32% bound acrylonitrile, equilibrium volume swell in ASTM Reference Fuel C after 70 h at 23 °C is commonly reported in the range of 25–40%; at 34–40% bound acrylonitrile, the same exposure yields volume change of 10–25%. The glass transition temperature rises from approximately −40 °C to −20 °C as bound acrylonitrile increases from 28% to 40%, measured by differential scanning calorimetry per ISO 11357-2:2020. This creates a processing conflict in production lines: high-acrylonitrile compounds require higher mixing energy input, generate more frictional heat in internal mixers, and show increased die swell during extrusion. In injection molding, high-acrylonitrile NBR exhibits higher compound viscosity at equivalent Mooney value and may require barrel temperatures of 70–90 °C in the feed zone and 85–100 °C in the metering zone to prevent scorch. Table 1 summarizes representative property ranges for emulsion-polymerized NBR across four acrylonitrile bands.

Compounding of NBR on production-scale equipment is generally carried out as a two-stage operation. Stage-one masterbatches are mixed in an intermeshing internal mixer with a fill factor of 0.70–0.78 and ram pressure of 0.35–0.55 MPa; dump temperatures are held at 110–130 °C for sulfur-containing formulations and below 100 °C when peroxide curatives are added in a second pass. Carbon black dispersion is assessed by visual microscopy per ISO 11345:2016 or by surface roughness after extrusion; poorly dispersed N550 or N774 aggregates reduce tensile strength and increase compression set. The addition of 20–40 phr of a medium-acrylonitrile NBR-compatible ester plasticizer such as dioctyl adipate lowers compound Mooney viscosity by 5–15 ML(1+4) units depending on base polymer and black loading, but plasticizer migration in contact with polycarbonate or acrylonitrile-butadiene-styrene must be evaluated using ISO 177:2016 or an OEM-specific extraction method. Sulfur-donor systems based on 2.2 phr tetramethylthiuram disulfide and 0.4 phr dibenzothiazyl disulfide are used where low compression set and heat resistance are secondary, while peroxide cure with 2.5–4.0 phr dicumyl peroxide and 1.0–1.5 phr triallyl cyanurate coagent yields lower compression set at 125 °C. NBR compounds should be stored below 40 °C and protected from ultraviolet exposure because ozone attack occurs rapidly in unsaturated NBR unless 2–4 phr of N-alkyl-N′-aryl-p-phenylenediamine antiozonant is added.

NBR Long-Term Supply Agreement: Price Mechanism & Volume Tier Table

Supply contracts for NBR are typically structured with annual volume commitments and quarterly or monthly price adjustments because butadiene and acrylonitrile feedstock costs exhibit substantial volatility. In a typical long-term supply agreement, the contract price per metric ton is established by combining a negotiated base price with feedstock index movements. A representative adjustment formula may allocate 55–65% of the price movement to butadiene, 20–30% to acrylonitrile, and 10–20% to a fixed conversion margin. The volume tier table defines the commercial mechanism, not a fixed list price; actual base prices are confidential and should be specified in the contractual annex. Quality documentation per lot typically includes certificate of analysis against ISO 4658:2019 and ASTM D3187-16, with the buyer retaining the right to reject material outside agreed Mooney viscosity and bound acrylonitrile tolerances. Published data for a single uniform NBR price formula across all producers is limited; the mechanism shown is a composite of common contract structures and must be validated against the seller’s cost model and the buyer’s audit rights.

When Peroxide Cure Replaces Sulfur in Closure Sealing Profiles

Peroxide cure changes vulcanization kinetics and demands stricter temperature control than sulfur-donor systems. Dicumyl peroxide exhibits a half-life of approximately 10 h at 117 °C and 1 min at 171 °C, so mixing and extrusion must remain below the activation threshold. Peroxide-cured NBR closure profiles are selected when compression set below 25% after 22 h at 120 °C is required under ASTM D395-18 Method B, compared with 35–45% for standard sulfur systems. The replacement of amine antioxidants such as 6PPD with peroxide-compatible oligomerized 2,2,4-trimethyl-1,2-dihydroquinoline at 0.5–1.0 phr reduces radical scavenging and prevents undercure at the part center. Internal mixers require two-pass mixing so that the peroxide is introduced at a dump temperature below 90 °C. Extrusion of peroxide-NBR closure profiles is performed on cold-feed pin barrel extruders with L/D ratios of 14:1 to 18:1 and screw speeds of 20–40 rpm; at screw speeds above 45 rpm, frictional heat can initiate premature crosslinking and surface roughness. The hot-air vulcanization line is operated at 190–220 °C for 5–12 min depending on profile wall thickness. Scorch safety is monitored by Mooney scorch MS-t5 at 121 °C under ISO 289-1:2018; production lots with t5 below 10 min are rejected for successive extruder campaigns.

Fuel System Seals Require a Different Cure Chemistry than Industrial Gasketing

Fuel system sealing elements manufactured from 33–36% bound acrylonitrile NBR are specified for resistance to oxygenated fuels containing up to 15% methanol or ethanol by volume when the compound is made with low-extractable plasticizers and a peroxide or sulfur-donor cure. Volume swell in ASTM Reference Fuel B after 70 h at 50 °C is typically limited to ≤15% under ISO 1817:2015; higher acrylonitrile grades reduce swell but lose low-temperature sealing. Low-temperature retraction TR10 values above −25 °C, measured per ASTM D1329-16, often disqualify formulations with over 40% bound acrylonitrile for cold-climate fuel filler neck seals. Long-term heat exposure at 135 °C causes embrittlement within 7–14 days unless the formulation contains zinc oxide and magnesium oxide with trimethylquinoline antioxidant. NBR is not recommended for continuous immersion in aromatic hydrocarbons, ketones, esters, chlorinated solvents, or glycol-based brake fluids; the polar acrylonitrile group is insufficient to resist these fluid classes. In folded seal lip regions, ozone cracking is a known field failure mode. Static ozone exposure at 50 pphm, 40 °C, and 20% strain per ISO 1431-1:2017 produces visible cracking within 24 h for unprotected NBR. Consequently, 2–4 phr of mixed diaryl-p-phenylenediamines plus microcrystalline wax are essential for underhood sealing parts. Continuous service limits commonly assigned by compounders are −30 to 100 °C for sulfur-cured NBR and −20 to 120 °C for peroxide-cured NBR, beyond which compression set or thermo-oxidative hardening becomes unacceptable.