Acrylonitrile-Butadiene Copolymer, described in the supply chain as NBR Polymer & Acrylonitrile-Butadiene Copolymer for Oil-Resistant Rubber, is an emulsion-polymerized random copolymer of acrylonitrile and butadiene. Bound acrylonitrile content, normally between 18 wt% and 50 wt%, is the primary compositional variable controlling resistance to aliphatic hydrocarbons, vegetable oils, and mineral-oil-based lubricants. The elastomer is designated NBR in ISO 1629:2013 and ASTM D1418-22. Low-acrylonitrile grades typically exhibit glass transition temperatures near −45 °C, while 40–50 wt% ACN grades may show Tg values around −20 °C to −15 °C by differential scanning calorimetry. The unsaturated butadiene units provide crosslinking sites for sulfur or peroxide vulcanization but also reduce resistance to ozone and thermal oxidative aging. NBR Polymer & Acrylonitrile-Butadiene Copolymer for Oil-Resistant Rubber is therefore selected when ASTM D471-16a volume swell in IRM 901 after 70 h at 100 °C must remain below approximately 15% for medium-ACN grades. Commercial forms include bales, crumb, and free-flowing powder; Mooney viscosity ML 1+4 at 100 °C per ISO 289-1:2018 normally ranges from 25 to 100. Higher-viscosity grades support filler extension and collapse resistance in extruded profiles, whereas lower-viscosity grades improve mold filling in injection molding.

How Does Acrylonitrile Content Govern Swell and Low-Temperature Flexibility?

The acrylonitrile unit increases cohesive energy density and reduces equilibrium uptake of nonpolar solvents in a manner that is approximately linear between 18 wt% and 50 wt% ACN. A representative 18 wt% ACN NBR compound containing 60 phr N550 carbon black and 20 phr diisononyl phthalate may show IRM 901 volume swell of 30–40%, while a 41 wt% ACN analogue may show 7–12% under ASTM D471-16a, 70 h at 100 °C. IRM 903 is a more aggressive oil with higher aromatic content; for the same NBR network, volume swell in IRM 903 after 70 h at 125 °C can be 1.5–2 times higher than in IRM 901. The trade-off is measured as brittleness temperature by ASTM D2137-11: low-ACN grades remain flexible at −45 °C, while high-ACN grades may fail at −20 °C. The data in Table 1 are representative of unfilled gum vulcanizates; compound-specific values depend on filler loading, plasticizer type, and crosslink density.

Table 1. Effect of bound acrylonitrile content on oil swell and low-temperature response.

Bound ACN (wt%)	Tg (°C)	Volume swell in IRM 901 after 70 h at 100 °C (%)	Brittleness temperature ASTM D2137-11 (°C)
18–22	−45 to −38	30–40	−45 to −40
28–33	−35 to −28	15–20	−38 to −30
40–50	−22 to −15	5–12	−22 to −15

Compounding of a 33 wt% ACN NBR masterbatch on a 75-litre tangential internal mixer with a two-wing rotor at a ram pressure of 0.4–0.6 MPa and rotor speed of 40–50 rpm follows a two-stage mixing sequence. Carbon black N330 or N550 is added after polymer breakdown and initial plasticizer addition; a dump temperature of 110–130 °C is maintained to avoid accelerator pre-activation in a single-pass masterbatch. The second stage is carried out on a two-roll mill with friction ratio 1.15:1 to 1.25:1; sulfur and accelerators are added below 90 °C to prevent scorch. Field data from production lines indicate that high-ACN grades exhibit greater viscous heat generation and rotor torque than low-ACN grades at equal Mooney viscosity, because the polar acrylonitrile units increase internal friction and reduce chain slip. A 41 wt% ACN NBR with 60 phr N550 can require 15–25% more mixing energy than a 28 wt% ACN grade with the same filler and plasticizer. If the stock temperature exceeds 130 °C during non-productive mixing, gel formation may occur, producing surface blemishes and lower tensile strength. Batch-to-batch Mooney viscosity after 6 min of mixing should be controlled within ±5 Mooney units; otherwise, die swell and extrusion gauge control in subsequent operations deteriorate. Pre-drying of crumb or powder NBR at 60–70 °C for 1–2 h is required when storage relative humidity exceeds 60%, because surface moisture causes porosity in vulcanized goods.

When Semi-Efficient Sulfur Vulcanization Is Applied to High-ACN NBR

High-ACN NBR grades respond to semi-efficient vulcanization formulations based on 1.2–1.8 phr sulfur, 1.0–1.5 phr sulfenamide accelerator, and 0.2–0.5 phr thiuram monosulfide. At 150 °C, a typical compound may show scorch time Ts2 of 3–6 min on an oscillating-disc rheometer per ASTM D2084-19a, while MDR cure curves per ISO 6502-3:2018 reach 90% cure in 8–14 min depending on accelerator ratio. Increasing sulfur to 2.5–3.0 phr in a conventional sulfur system shifts the crosslink distribution toward polysulfides, which improves fatigue and tear resistance but reduces heat-aging stability; compression set after 22 h at 100 °C per ASTM D395-18 Method B may exceed 35% with conventional sulfur, whereas peroxide-cured high-ACN NBR compounds may reach 15–20%. Peroxide curing with dicumyl peroxide 40KE at 2.0–3.0 phr is selected for steam and hot water service because sulfur-cured NBR can exhibit extractable cure residues and higher compression set. However, peroxide systems can be inhibited by acidic fillers or amine-based antioxidants; therefore, pH-neutral carbon blacks and non-amine stabilizers are used. Tensile strength per ASTM D412-16 of 33 wt% ACN NBR cured with semi-efficient sulfur is typically 12–18 MPa, elongation at break 300–450%, and tear strength 30–50 kN/m per ASTM D624-00(2020) Die B. The curing process is monitored by moving-die rheometer; optimizing process windows for high-ACN compounds often requires plateau cure between 160 °C and 180 °C for injection molding.

For rotary shaft seals produced from a 33 wt% ACN NBR compound, the material is qualified against the ASTM D2000-18 classification system using line call-out requirements for tensile, hardness, oil resistance, and compression set. Typical retained tensile after 70 h in IRM 903 at 125 °C should be at least 80% of original for high-ACN compounds, but high-ACN grades can harden due to plasticizer extraction; therefore, trioctyl trimellitate or diisodecyl phthalate are used instead of dioctyl phthalate where low-temperature flexibility and low volatility are required. Molded O-ring cross sections are also checked for flash, backrind, and trapped air using ASTM D2000-18 line call-outs; post cure at 120–150 °C for 2–4 h is used to improve compression set. The compliance matrix summarized in Table 2 is applied in release testing.

Table 2. Typical qualification matrix for NBR oil-resistant rubber compounds.

Property	Standard	Typical criterion
Mooney viscosity	ISO 289-1:2018	ML 1+4 at 100 °C: 30–80
Hardness	ASTM D2240-15e1	Shore A 50–90
Tensile strength	ASTM D412-16	≥ 10 MPa
Oil resistance	ASTM D471-16a	IRM 901/903 volume swell per grade
Compression set	ASTM D395-18 Method B	≤ 35% after 22 h at 100 °C
Low-temperature brittleness	ASTM D2137-11	No failure at −40 °C for 28 wt% ACN; −20 °C for 41 wt% ACN
Cure characteristics	ISO 6502-3:2018	MDR 180 °C, Ts2 ≥ 0.8 min, Tc90 ≤ 10 min

Ozone Resistance Through NBR/PVC Alloying

NBR has inherent ozone weakness because the butadiene segment contains 1,4-unsaturation. In dynamic applications such as cable jackets and automotive weather seals, unmodified NBR with 28 wt% ACN may exhibit surface cracking after 8–24 h of 50 pphm ozone at 40 °C under 20% elongation per ISO 1431-1:2017. Alloying with 30 phr of medium-molecular-weight PVC increases ozone resistance and reduces solvent swelling but raises low-temperature stiffness and reduces tear strength. A 70/30 NBR/PVC alloy is often processed at 160–170 °C on a two-roll mill or internal mixer with stabilizers to avoid HCl evolution from PVC degradation. The blend is not a solution alloy at all compositions; two-phase morphology can cause delamination if the compound is cooled too rapidly after sheeting. For compression-set applications, the PVC fraction should not exceed 30 phr because ASTM D395-18 Method B values increase above 40% after 22 h at 100 °C.

Fuel hose covers based on 41 wt% ACN NBR require a single-screw extruder with L/D of 20:1 to 24:1 and a compression ratio of 2.5:1 to 3.0:1. Barrel temperatures are normally set at 60–80 °C, head at 85–95 °C, and die at 100–110 °C to balance viscosity against scorch. At screw speeds above 80 rpm with a 60 mm screw, surface melt fracture and shark-skin defects occur unless the Mooney viscosity of the compound is reduced below 50 ML 1+4 at 100 °C or processing aids are added. High-ACN NBR compounds with 15–20 phr ester plasticizer show lower extrusion die swell than low-ACN grades because the polar matrix retains filler network under shear. However, plasticizer migration in fuel-contact layers is measured by extraction in ASTM D471-16a Fuel C; a high-ACN compound with 20 phr diisodecyl phthalate may show mass loss of 5–8% after 70 h at 23 °C, which affects permeability and low-temperature flexibility. For fuel hose inner liners, carboxylated nitrile rubber or NBR/PVC is used when lower gasoline permeability is required, but the base NBR Polymer & Acrylonitrile-Butadiene Copolymer for Oil-Resistant Rubber continues to be specified for oil cooler and transmission oil hose covers.

Processing Boundaries in High-ACN NBR Compounds

High-ACN NBR compounds have a narrow processing window between adequate mold flow and scorch. On a 180-ton injection molding machine, barrel temperature set points of 70–90 °C, screw back pressure of 0.5–1.5 MPa, and injection pressures of 60–120 MPa are typical. Mold temperatures of 150–170 °C reduce cycle times, but thick-section molded parts in 41 wt% ACN grades may develop internal porosity if the cure system is not adjusted. The thermal curing lag can be estimated from cure-rate data generated at 160, 170, and 180 °C on an MDR per ISO 6502-3:2018; high-ACN NBR compounds show higher molding shrinkage than 28 wt% ACN grades, with linear mold shrinkage of 1.6–2.2% depending on filler and processing orientation. Shrinkage anisotropy across flow and transverse directions can reach 0.3–0.5 percentage points; mold designers compensate by adjusting cavity dimensions in the flow direction. Moisture must be below 0.1 wt% before injection molding because water vapor causes porosity and increases screw slip. Continuous service limits for standard sulfur-cured high-ACN NBR in air are approximately 100–110 °C; short-term exposure to 125–130 °C is possible if antioxidants and antiozonants are selected. Contact with high-aromatic gasoline, ketones, chlorinated solvents, or glycol ether brake fluids leads to large volume swell and is outside the product's compatibility envelope.