Determination of Bound Acrylonitrile Content in Nitrile Rubber (NBR): Methods, Standards, and Applic

Bound acrylonitrile content is the single most important compositional parameter of nitrile rubber (NBR), directly governing its oil resistance, low-temperature flexibility, and mechanical performance. This technical article provides a comprehensive overview of the determination of bound acrylonitrile content in NBR, covering applicable sample types, internationally recognized test methods such as the Dumas combustion and Kjeldahl procedures, relevant ISO and ASTM standards, and practical guidance for accurate, reproducible results.

Technical Overview

Nitrile rubber (NBR, acrylonitrile-butadiene rubber) is a random copolymer of acrylonitrile and butadiene produced by free-radical emulsion polymerization. The acrylonitrile (ACN) fraction that is chemically incorporated into the polymer backbone is defined as bound acrylonitrile, in contrast to residual (unreacted) acrylonitrile monomer, which is largely removed during devolatilization.

Bound acrylonitrile content is the primary structural variable that defines the performance envelope of NBR. Commercial grades typically contain between 18% and 50% bound ACN by mass:

• Low ACN grades (18–24%): superior low-temperature flexibility and dynamic properties, moderate oil resistance.
• Medium ACN grades (26–33%): balanced oil resistance, processing behavior, and mechanical strength; the most widely used family.
• High ACN grades (34–50%): excellent resistance to fuels, oils, and nonpolar solvents, very low gas permeability, but reduced cold flexibility.

Because every critical property of NBR — volume swell in oils, glass transition temperature, tensile strength, gas impermeability, and cure behavior — correlates strongly with bound ACN content, its accurate determination is essential for grade identification, quality control, specification compliance, and failure analysis. Since each acrylonitrile unit contains exactly one nitrogen atom, the determination is performed by quantifying total nitrogen in the purified polymer and converting it to bound ACN using the stoichiometric conversion factor of 53.06/14.01 ≈ 3.787.

Testing Samples

A wide variety of nitrile rubber materials can be evaluated, provided that the sample is properly conditioned and, where necessary, purified to remove non-polymer nitrogen-free components that would bias the result.

Raw NBR (Bale or Powder Form)

Untvulcanized raw rubber in bale, crumb, or powder form is the most common sample type. Prior to analysis, antioxidants, processing aids, and particularly oil extenders must be removed by solvent extraction (for example with acetone or an acetone/methanol azeotrope in accordance with ISO 1407), followed by drying to constant mass. This step is critical for oil-extended grades, because the extender oil dilutes the nitrogen signal and leads to an artificially low bound ACN value.

NBR Latex

Carboxylated and conventional NBR latices are first coagulated (typically with electrolyte or alcohol), washed free of serum solids and residual monomer, and dried before the nitrogen determination is performed on the isolated polymer film.

Vulcanized NBR Compounds and Finished Articles

Cured compounds, O-rings, seals, hoses, gaskets, and gloves can be analyzed, but carbon black and mineral fillers dilute the nitrogen concentration. In such cases, the filler content is determined separately (for example by thermogravimetric analysis or ash determination) and an appropriate correction is applied to express the result on a rubber-fraction basis.

NBR Blends

Blends of NBR with PVC, EPDM, SBR, or chloroprene rubber may be characterized, and the measured nitrogen content can be used to estimate the NBR fraction within the blend when combined with complementary techniques such as FTIR or TGA.

Testing Items

The core analytical deliverables of a bound acrylonitrile determination typically include:

• Bound acrylonitrile content (% by mass), reported on the extracted, dry polymer basis.
• Total nitrogen content (% by mass), the directly measured quantity from which bound ACN is calculated.
• Extractable content (% by mass), determined during sample purification, which validates the reliability of the bound ACN result.
• Optional supporting determinations: residual acrylonitrile monomer (by headspace GC), ash/filler content (for vulcanizates), and Mooney viscosity for complete grade characterization.

Testing Methods

Two primary reference methods are internationally standardized: the combustion (Dumas) method and the Kjeldahl method. Both are based on total nitrogen determination, with results converted to bound acrylonitrile by the factor 3.787.

Combustion (Dumas) Method — ISO 24698-1 / ASTM D3533

A pre-weighed sample (typically 100–250 mg of extracted, dried polymer) is combusted at high temperature (approximately 950–1,100 °C) in a stream of pure oxygen within a combustion reactor. The combustion gases are swept through a reduction furnace where nitrogen oxides are converted to molecular nitrogen, and after removal of water and carbon dioxide, the nitrogen is quantified by a thermal conductivity detector. Modern automated Dumas analyzers complete a determination in 5–8 minutes with excellent repeatability (typically better than ±0.3% absolute on bound ACN). Calibration is performed with certified nitrogen standards such as EDTA or acetanilide.

Kjeldahl Method — ISO 24698-2 / ASTM D3533

The classical wet-chemical procedure involves digesting the sample in concentrated sulfuric acid with a potassium sulfate/copper sulfate catalyst mixture until complete clarification, converting the bound nitrogen to ammonium sulfate. After alkalinization with sodium hydroxide, ammonia is steam-distilled into a boric acid receiving solution and determined by titration with standardized acid. The Kjeldahl method remains a robust, equipment-independent reference technique, although it is more labor-intensive and time-consuming than the Dumas method and requires careful control of digestion completeness.

FTIR Spectroscopy (Screening Method)

Infrared spectroscopy exploits the intense, isolated nitrile (C≡N) stretching absorption at approximately 2,237 cm⁻¹. Using a calibration curve built from NBR standards of known ACN content, FTIR provides a rapid, non-destructive estimate of bound acrylonitrile, well suited for incoming inspection, grade screening, and reverse engineering, though it is generally less precise than elemental analysis.

Calculation and Reporting

Bound acrylonitrile content is calculated as: bound ACN (%) = nitrogen (%) × (53.06 / 14.01) ≈ nitrogen (%) × 3.787. Results should state the basis of determination (extracted dry polymer), the analytical method used, and the applicable standard. Key factors affecting accuracy include sample homogeneity, completeness of extraction of non-rubber components, moisture control, combustion or digestion completeness, and, for filled compounds, correct application of the filler correction.

Applications

Determination of bound acrylonitrile content supports quality assurance, material selection, and product development across the rubber and elastomer industries:

• Grade identification and release testing at NBR manufacturing plants, ensuring conformance to product data sheets.
• Incoming material inspection by compounders and fabricators, detecting mislabeled or off-specification rubber before production.
• Specification compliance against ASTM D2000 / SAE J200 line call-outs, where oil-resistance classes (such as BF, BG, CH, and CK) are intrinsically linked to acrylonitrile level.
• Qualification of oil-resistant components: fuel hoses, hydraulic seals, O-rings, gaskets, diaphragms, and oil field packer elements.
• Medical and industrial glove manufacturing, where ACN content influences chemical permeation resistance and tactile properties.
• Failure analysis and competitive product reverse engineering of unknown elastomeric articles.

FAQ

Q: What is the difference between bound acrylonitrile and residual acrylonitrile?

A: Bound acrylonitrile is monomer that has been chemically polymerized into the NBR chain and defines the polymer's properties; residual acrylonitrile is unreacted free monomer remaining in the rubber, typically at ppm levels and determined separately by headspace gas chromatography.

Q: Which method should be chosen — Dumas or Kjeldahl?

A: The Dumas combustion method (ISO 24698-1) is preferred for routine and high-throughput testing due to its speed, automation, and small sample requirement. The Kjeldahl method (ISO 24698-2) remains valuable as a classical reference technique and in laboratories without combustion instrumentation. Both methods, correctly executed, deliver results that agree within normal laboratory repeatability limits.

Q: How do fillers and extender oil affect the result?

A: Neither carbon black, mineral fillers, nor paraffinic extender oils contain nitrogen, so they dilute the measured nitrogen concentration and depress the apparent bound ACN value. Extractable oils must be removed by solvent extraction before analysis, and filler content in vulcanizates must be corrected for by ash or TGA determination.

Q: How much sample is required and what is typical turnaround?

A: A representative sample of 10–20 g is generally sufficient to permit homogenization, extraction, and replicate analyses. Dumas determinations require only about 0.2 g per combustion and results are typically available within one to three working days, including sample preparation.

Q: What repeatability can be expected?

A: For properly extracted and homogenized raw NBR, duplicate determinations should agree within approximately ±0.3–0.5% absolute on bound acrylonitrile. Larger deviations usually indicate incomplete extraction, sample heterogeneity, or moisture contamination.