Commercial acrylonitrile (CAS 107-13-1) is consumed in nitrile rubber synthesis as a polar vinyl monomer that is emulsion-polymerized with 1,3-butadiene. Polymer-grade acrylonitrile used for NBR production typically contains not less than 99.5 mass % acrylonitrile, with water below 0.5 mass %, acetonitrile below 100 mg/kg, and hydroquinone monomethyl ether inhibitor between 35 mg/kg and 50 mg/kg to prevent autopolymerization during storage. The resulting nitrile butadiene rubber contains bound acrylonitrile between 18 wt% and 50 wt%, and this variable controls glass transition temperature, equilibrium solvent uptake, and gas permeability. Hydrogenated nitrile butadiene rubber retains a similar bound acrylonitrile range but introduces a saturated backbone; carboxylated nitrile butadiene rubber incorporates a carboxylic acid termonomer at 2–10 wt% to modify tensile and abrasion response. Grade selection among NBR, HNBR, and XNBR begins with the bound acrylonitrile content and the associated Tg measured by differential scanning calorimetry in accordance with ISO 11357-2.

How Does Bound Acrylonitrile Content Shift Glass Transition Temperature and Solvent Uptake?

For amorphous nitrile elastomers, the bound acrylonitrile content raises the glass transition temperature through increased dipole-dipole interaction and reduced chain mobility. Commercial NBR data sheets typically show a Tg of approximately −50 °C at 18 wt% acrylonitrile, −30 °C at 33 wt%, −20 °C at 41 wt%, and −5 °C at 50 wt%. Dynamic mechanical analysis at 1 Hz and 0.1 % strain measured per ISO 6721-4 can report peak loss tangent values 5–10 °C higher than the DSC midpoint. The same acrylonitrile increase that raises Tg reduces equilibrium volume swell in ASTM IRM 903 at 100 °C from more than 60 % for 18 wt% acrylonitrile to below 15 % for 45–50 wt% acrylonitrile. Low-temperature brittleness also moves upward; grades with 50 wt% acrylonitrile can become brittle near 0 °C, while 18 wt% grades remain flexible below −40 °C. Consequently, seal selection requires balancing fuel resistance against the lowest service temperature required by the equipment.

In low-temperature hydraulic sealing units, 18–22 wt% acrylonitrile grades are specified only when the fluid is a straight aliphatic mineral oil and cold-start survival is controlling. The low polarity of these grades produces high swell in aromatic or oxygenated fuels, and the reduced polar interaction with carbon black lowers tensile strength unless high-structure N330 or N550 series is used at 40–60 phr. On a two-roll mill or internal mixer, low acrylonitrile grades exhibit lower heat build-up, allowing dump temperatures up to 120 °C without scorch; batch-to-batch Mooney viscosity ML(1+4) at 100 °C is generally held within ±4 MU for consistent extrudate dimensions on single-screw extruders with L/D 20:1–24:1. Published data for specific cold-seal configurations in silicone-free brake fluid is limited, so qualification must rely on ISO 1817 immersion and ASTM D2137 brittleness testing.

High-ACN Nitrile Grades in Aromatic and Oxygenated Fuel Service

For automotive fuel hoses, inlet seals, and injection system O-rings exposed to ASTM Reference Fuel C or oxygenated fuels containing methanol or ethanol, NBR grades with 36–50 wt% acrylonitrile are specified because the polar acrylonitrile units reduce aromatic and oxygenate penetration. The inner layer of a fuel hose extruded from a 45 wt% acrylonitrile NBR can reduce permeation by roughly 40–60 % relative to a 28 wt% compound when tested by SAE J30 or ASTM D814. In contrast, the higher Tg narrows the low-temperature sealing window; a 45 wt% acrylonitrile NBR cannot generally be guaranteed below −15 °C under dynamic lip seal conditions. Processing high-acrylonitrile grades requires tighter thermal control because the reduced butadiene content lowers sulfur cure reversion at 160 °C but also increases compound viscosity. On production injection molding machines, barrel temperatures above 100 °C and mold temperatures of 170–190 °C are used, while vented screw barrels with L/D 22:1 are specified to remove moisture that otherwise creates porosity in parts thicker than 6 mm.

In oxygenated fuel service, high-acrylonitrile NBR compounds also require careful plasticizer selection. Polymeric adipate or ether-ester plasticizers with low extraction in ASTM Reference Fuel C are preferred over monomeric dioctyl sebacate because the aromatics and methanol in the test fuel can extract monomeric plasticizer and increase hardness shift. A compound based on 41 wt% acrylonitrile NBR with 20 phr polymeric plasticizer typically shows volume swell below 30 % after 70 h at 23 °C in ASTM Reference Fuel C, while maintaining Shore A hardness between 65 and 75 points. The same compound tested by ISO 1817 in IRM 903 at 100 °C may still show volume swell of 12–18 %. Such data are normally generated on compression-molded slabs cured to 90 % of the maximum torque from an ISO 6502 moving die rheometer, then tested after 16 h rest at 23 °C.

When Hydrogenation Extends Service Life Beyond the Backbone Unsaturation

Selective hydrogenation of the butadiene units in NBR produces HNBR with residual unsaturation from less than 0.5 mol% to more than 20 mol% depending on grade. At equivalent 33–36 wt% acrylonitrile, an HNBR displays a Tg near −30 °C, similar to NBR, but the saturated backbone changes the dominant failure mechanism from oxidative chain scission to thermomechanical fatigue. HNBR compounds with 36 wt% acrylonitrile are specified for oilfield packers, automotive synchronous belts, and R134a air-conditioning O-rings where continuous service at 140–150 °C is required. In contrast, general-purpose NBR is normally limited to 100–125 °C in air and 125–135 °C in oil. The cure system changes from sulfur to peroxide because residual unsaturation is insufficient for effective sulfur crosslinking; typical peroxide levels are 5–8 phr of 40 % active dicumyl peroxide on a silica or carbon black carrier. Vulcanization kinetics measured by moving die rheometry per ISO 6502 show shorter scorch times for peroxide-cured HNBR than sulfur-cured NBR, requiring injection mold cold runner systems and mold temperatures controlled within ±2 °C.

Despite the improved oxidative resistance, HNBR does not overcome the swelling limits established by acrylonitrile content. A low-acrylonitrile HNBR with 17–21 wt% acrylonitrile still exhibits volume swell above 50 % in ASTM IRM 903 and is not specified for fuel contact. HNBR is also incompatible with some amine-based oil additives and certain sulfur-containing extreme-pressure additives, which can cause premature crosslinking or surface tack in long-term service. The premium cost of HNBR restricts its use to systems in which NBR fails by heat aging or reverse bending fatigue, not by room-temperature hydrocarbon swell alone.

Carboxylated Nitrile Grades for Abrasion Resistance Under Dynamic Sealing Conditions

XNBR is produced by polymerizing acrylonitrile and butadiene with a carboxylic acid termonomer, usually methacrylic acid, at 2–10 wt%. The carboxyl groups introduce ionic crosslink sites that independently raise modulus, tensile strength, and tear resistance without changing the acrylonitrile-to-butadiene ratio. A 33 wt% acrylonitrile XNBR can show tensile strengths above 25 MPa and tear strengths exceeding 70 N/mm when measured per ASTM D412 and ASTM D624, compared with 15–20 MPa and 30–50 N/mm for a similarly filled NBR compound. The penalty is higher compression set and water swell; XNBR grades typically absorb 2–5 times more water than NBR under ISO 1817 immersion in distilled water at 70 °C. Abrasion resistance per DIN 53516 improves by 30–50 % relative to an equivalent NBR formulation, making XNBR suitable for footwear soles, industrial roll covers, oilfield stators, and hydraulic seals exposed to high sliding friction. The cure system must be adjusted because the acid monomer can complex with zinc oxide and cause scorch; production mixing on tangential internal mixers usually requires dump temperatures below 110 °C and two-stage mixing with peroxide donors.

For progressive cavity pump stators handling crude oil with hydrogen sulfide and water at 120–140 °C, HNBR with 36–44 wt% acrylonitrile is specified rather than XNBR or high-acrylonitrile NBR. The HNBR grade provides adequate aromatic resistance, survives sour gas aging with a standard ISO 23936-2 test rating, and avoids the high water swell of XNBR. XNBR is retained for the same pump geometry only when the produced fluid contains abrasive sand and the operating temperature remains below 100 °C; in that case, the improved DIN 53516 abrasion resistance reduces stator surface erosion, but the operator must accept higher volume change in wet conditions. NBR is specified only for the low-pressure surface transfer of cool aliphatic hydrocarbons below 80 °C, where the acrylonitrile content is chosen at 28–33 wt% to balance cast elastomer processing and ISO 1817 swell limits.