NBR Grade Selector: Match ACN Content and Mooney to Your Application

Acrylonitrile-butadiene copolymer grade selection for a production compound begins with two primary specification values: bound acrylonitrile content and Mooney viscosity ML(1+4) 100°C. Bound ACN, reported as mass percent, determines the polarity balance that controls volume swell in ASTM D471 reference fluids and low-temperature retraction behaviour. Mooney viscosity determines the shear stress transmitted through a 45-L intermeshing internal mixer, the die swell at a single-screw extruder head, and the pressure drop in a multicavity injection mold. The selector is therefore not a single-point tool but a two-axis map in which a high-ACN grade may fail processing before it fails service.

Commercial nitrile grades are polymerized with bound ACN between 18% and 50% and supplied at ML(1+4) 100°C values from 20 to 130, determined according to ISO 289-1:2015 or ASTM D1646-19a. The polymer nomenclature is defined by ISO 1629:2013 and ASTM D1418-22. The selection boundary for a given component is constrained by the way these two values interact: raising ACN from 28% to 38% reduces equilibrium swell in IRM 901 by approximately 40% to 50%, while the same change shifts the glass transition temperature upward and narrows the low-temperature sealing window.

What Happens to Swell Resistance and Low-Temperature Recovery as Bound ACN Increases?

Bound ACN increases the solubility parameter of the polymer chain and raises the barrier to nonpolar oil and fuel absorption. In ASTM D471 immersion testing in IRM 901 at 100°C for 70 h, a sulfur-cured 18% ACN compound may exhibit equilibrium volume swell above 60%, while a 50% ACN compound under identical cure conditions typically drops below 15%. The same trend appears in fuel resistance: lower ACN grades swell rapidly in Reference Fuel C, whereas grades above 38% ACN are preferred for fuel-contacting liners and gaskets. The trade-off appears in low-temperature retraction measured by ISO 2921:2019. TR-10 values move from below -45°C for 18% to 22% ACN gum vulcanizates to approximately -10°C to -15°C at 45% to 50% ACN. DSC glass transition temperatures measured by ISO 11357-2:2020 follow the same ordinal trend; the exact value shifts with cure type and plasticizer loading. This boundary is not symmetrical: oil swell improves nonlinearly with ACN, but low-temperature flexibility degrades more steeply above 35% ACN.

Mooney viscosity is the second axis of the selector. At ML(1+4) 100°C below 30, the polymer flows rapidly into shallow injection-molded seal grooves but may lack sufficient chain entanglement to hold green strength on a two-roll mill or during transfer molding. At values above 80, the polymer tolerates high carbon black and plasticizer oil loadings and retains dimensional stability in calendered roll covers, but the same molecular weight raises mixing dump temperatures and narrows the scorch time at the curing temperature. A compound made from a 50 Mooney grade can be run on a 90 mm single-screw extruder with stable head pressure; the same formulation with an 80 Mooney grade may require a 20% reduction in screw speed to control a temperature rise of more than 15°C at the die. The selector therefore treats Mooney not as a polymer property but as a processing window constraint.

Establishing Mooney Limits for Mixing, Extrusion, and Injection Sequencing

Production experience on intermeshing internal mixers shows that high-Mooney, high-ACN grades generate greater viscous heating than low-ACN grades of identical ML(1+4) 100°C. The mixing cycle is typically split into two stages: a sulfur-free masterbatch dropped at 120°C to 140°C, followed by a final-stage addition of sulfur and accelerators on a cooled mill or second mixer pass at 90°C to 100°C. For a 45% ACN grade with ML(1+4) 100°C of 80, the dump temperature may exceed the first-stage ceiling 10°C to 15°C earlier than a 28% ACN grade of equivalent Mooney. This forces a reduction in rotor speed from 50 min⁻¹ to 35 min⁻¹ or a reduction in fill factor from 0.75 to 0.70. In extrusion of fuel hose covers through a 42:1 L/D single-screw extruder, grades below 50 Mooney may show a head-pressure oscillation of less than ±0.3 MPa at 60 min⁻¹; grades above 80 can exceed ±0.7 MPa and require a barrier screw with deeper feed channels.

For an injection-molded O-ring classified as ASTM D2000 M2BG 70, the grade boundary is usually a 28% to 34% ACN polymer with ML(1+4) 100°C of 40 to 60. At this ACN range, volume swell in IRM 901 is moderate, low-temperature retraction remains acceptable for automotive sealing, and the Mooney range permits filling of a 32-cavity cold-runner mold at injection pressures below 120 MPa. Raising Mooney to 80 improves tear strength and green strength but increases gate freeze time and may require a larger clamp force for the same projected area. Published data for this specific configuration is limited, but production measurements on similar multicavity tools indicate that cavity-to-cavity tensile variation can double when Mooney increases from 50 to 80 and flow length decreases.

Raw NBR bales with ML(1+4) 100°C below 30 may cold-flow in storage at temperatures above 35°C, causing bale deformation and feeding inconsistencies in automatic hoppers. Grades above 80 may require mill warming before cutting and can stall a bale cutter that is not rated for high-viscosity elastomers. The grade selector places a lower Mooney boundary at 25 for compounds that must retain preform shape in transfer pots and an upper Mooney boundary at 95 for automated cut-to-weight feeding systems.

When High-ACN Grades Are Paired with Peroxide Cure Systems

The cure system introduces a third constraint that is not visible in the raw polymer specification. High-ACN grades are frequently selected for sealing components that require low compression set after hot air aging. A peroxide-cured 38% ACN compound can hold compression set below 20% after 22 h at 120°C when tested according to ISO 815-1:2014, while a sulfur-cured equivalent may exceed 30%. Peroxide cure, however, is sensitive to acidic residues, certain processing oils, and residual accelerators that can reduce crosslinking efficiency. Scorch safety at the processing temperature is evaluated by ISO 6502-1:2018; the use of a 40% ACN grade with ML(1+4) 100°C of 70 reduces t_s2 at 125°C relative to a 28% ACN grade because the higher mixing temperature required for filler incorporation consumes part of the scorch reserve. A safer peroxide selection for high-ACN seal compounds pairs a 50 to 60 Mooney grade with 1.5 to 2.5 phr trimethylolpropane trimethacrylate coagent and limits final-stage mixing to below 95°C.

In a fuel hose inner liner that must pass fuel permeation requirements under SAE J30 and related OEM specifications, the inner liner grade is typically 38% to 42% ACN and ML(1+4) 100°C of 60 to 80. Higher ACN reduces Reference Fuel C volume swell and lowers permeation. Below 35% ACN, fuel swell in ASTM D471 may exceed the limit after 70 h at 40°C. Above 42% ACN, the same liner becomes too stiff for the mandrel bend radius and may crack at -20°C. A 70 Mooney grade gives sufficient green strength for continuous vulcanization lines; a 50 Mooney grade may be acceptable for short-batch compression molding but may sag in hot-air continuous lines with unsupported spans beyond 1.2 m.

Roll Cover Compounds, Fuel Line Formulations, and the Compression Set Trade-Off

Roll cover compounds for printing and industrial rollers use high-ACN NBR because the polar nitrile group reduces swell in the ester-based or ketone-containing cleaning agents encountered on the press deck. The Mooney selection in this application is inverted relative to injection molding: a 50% ACN polymer with ML(1+4) 100°C of 80 to 100 is often calendered at 70°C to 80°C to obtain the required green strength. The same grade cannot be injection molded into a thin-wall seal without excessive shear heating. Compression set, measured by ASTM D395-18 Method B, increases with decreasing Mooney in high-ACN compounds; a 50 Mooney grade may show compression set 5 to 8 percentage points higher than an 80 Mooney grade after 22 h at 100°C. This creates a direct conflict for designers who require both low compression set and long injection flow paths. In such cases, the selector boundary falls between 60 and 70 Mooney, with process adjustment required to keep the compound temperature below the cure activation threshold.

Implicit in the selector is the exclusion of certain service environments. NBR compounds, including high-ACN grades, remain incompatible with continuous exposure to ketones, chlorinated solvents, and strong oxidizing acids. Immersion in methyl ethyl ketone according to ASTM D471 can produce volume swell above 200% even at 50% ACN, leading to dimensional failure before chain degradation. Uncompounded bales and mixed compounds should not be stored for extended periods above 40°C because the scorch reserve depletes and may shorten the safe processing interval. When storage humidity exceeds 60% RH, compounds containing mineral fillers require pre-drying at 60°C to 70°C for 2 to 4 h before injection molding to prevent surface voids and porosity.