Acrylonitrile is classified under Harmonized System subheading 2926.10 in the WCO 2022 nomenclature. The WCO 2022 wording at heading level is “Nitrile-function compounds”; the subheading wording is the single term “Acrylonitrile.” The associated customs notes to heading 29.26 describe the subheading as covering prop-2-enenitrile, CAS 107-13-1, CH₂=CH—C≡N, molecular weight 53.06 g/mol, as a separate chemically defined compound. The notes exclude nitrile-containing polymers and copolymers, which are classified in Chapter 39. For import entry under 2926.10, stabiliser content does not change the six-digit code when the product remains a separate chemically defined unsaturated nitrile.

What Distinguishes Acrylonitrile Within Heading 29.26?

Heading 29.26 captures nitrile-function compounds in which the cyanide radical is present as a characteristic group. Acrylonitrile is the only named subheading within the heading at the international six-digit level; methacrylonitrile and acetonitrile fall to 2926.90 unless specifically enumerated elsewhere. The WCO 2022 explanatory notes emphasise that the subheading applies to the monomer only. A shipment containing polyacrylonitrile powder, acrylonitrile-butadiene-styrene resin, or an aqueous acrylonitrile copolymer dispersion is not entered under 2926.10 solely because acrylonitrile was the principal monomer; those products are classified in Chapter 39 because they are polymers rather than separate chemically defined compounds. The same logic applies to depolymerisation feedstock mixtures: if the acrylonitrile content is present in a blend with other nitriles at a level below separately defined commercial purity, the classification may shift to 2926.90 or a Chapter 38 preparation depending on the function of the mixture.

Commercial 2926.10-grade acrylonitrile is transported with a polymerisation inhibitor. Bulk specification typically calls for purity ≥99.5% by mass, water ≤0.5% by mass, acetonitrile ≤0.1% by mass, and monomethyl ether of hydroquinone at 0.003–0.1% by mass. Density at 20 °C, measured by ASTM D4052, is 0.806 g/cm³. Boiling point at 101.3 kPa is 77.3 °C; freezing point is −83.5 °C; vapour pressure at 20 °C is 11.7 kPa. The closed-cup flash point is −1 °C, and flammability limits in air are 3.0 vol% to 17.0 vol%. Autoignition temperature is 481 °C. These physical boundaries are relevant to customs verification because a certificate of analysis showing density or boiling range inconsistent with acrylonitrile should trigger laboratory confirmation before entry acceptance.

The WCO 2022 customs notes are not a substitute for transport classification. Import documentation should identify the product as UN 1093 Acrylonitrile, stabilised, with the appropriate flammable liquid and acute toxicity classification. In the United States, OSHA 29 CFR 1910.1045 establishes a permissible exposure limit of 2 ppm as an 8-hour time-weighted average and a 10 ppm ceiling for short-term excursions. Under EU CLP, acrylonitrile is carried as a category 1B carcinogen. Closed-loop transfer, vapour return lines, and continuous photoionisation detection are specified for tank sampling and drum transfer operations to maintain personnel exposure below the 2 ppm threshold.

Vapour Pressure and Explosion Limits in Bulk Storage

Bulk storage of 2926.10-grade acrylonitrile requires simultaneous control of flammability and polymerisation stability. The vapour pressure of 11.7 kPa at 20 °C places the product within the scope of fixed-roof tanks with internal floating decks or nitrogen blanketing. Relief and venting devices sized according to API 2000 are installed to manage normal breathing loads and emergency relief; tank design pressure is typically maintained below 17–20 kPa to prevent bulkhead deformation. Vapour space oxygen concentration is held below 5 vol% to remain outside the flammable range, while dissolved oxygen in the liquid is monitored because acrylonitrile polymerisation inhibitors require a minimum residual oxygen level to remain effective. The storage temperature is maintained below 20 °C to limit dimer formation and vapour evolution; prolonged residence beyond 30 days requires inhibitor re-certification by gas chromatographic analysis before the tank is offered for customs release or downstream transfer.

Flammability testing of the vapour space may be performed by closed-cell methods according to ASTM E681. The reported limits of 3.0 vol% to 17.0 vol% in air are not static under process conditions; elevated tank temperatures and nitrogen dilution shift the fuel-air ratio. Therefore, a single oxygen reading is insufficient for safe release of the tank into service. Continuous analysers are positioned at the tank vapour outlet, and interlock logic closes the inlet valve when oxygen exceeds 5 vol%. The same interlock logic is used during tank cleaning: water washdown must be followed by nitrogen drying to −20 °C dew point before reintroduction of acrylonitrile, because residual water at inhibitor concentrations below 0.003% can reduce the stabiliser package and permit premature polymer formation.

When Propylene Ammoxidation Falls Outside the 420–480 °C Window

Industrial acrylonitrile is produced predominantly by fluidised-bed ammoxidation of propylene on bismuth molybdate and multi-metal oxide catalysts. The nominal reaction C₃H₆ + NH₃ + 1.5 O₂ → C₃H₃N + 3 H₂O is exothermic. At reactor temperatures of 420–480 °C and gauge pressures of 0.5–2.0 bar, per-pass propylene conversion is typically 85–95 mol%, with acrylonitrile selectivity in the range of 80–85 mol%. Process excursions below 400 °C increase acrolein formation and unreacted propylene recycle; excursions above 500 °C increase ammonia oxidation to nitrogen oxides and afterburning of propylene to carbon oxides. In a production-scale fluidised bed, the temperature window is therefore treated as a process constraint rather than an optimisation variable.

The reactor effluent is quenched with water, neutralised with sulfuric acid to recover excess ammonia, and distilled to separate acrylonitrile from acetonitrile and hydrogen cyanide. Because hydrogen cyanide is a by-product, the recovery section is designed with closed-loop stripping and vapour destruction capacity. Extractant distillation uses water to separate acrylonitrile from acetonitrile by altering relative volatility; the acrylonitrile overhead is then dried and inhibited before storage. The final product is transferred under a nitrogen cap into tanks that meet the storage requirements described above. The WCO 2926.10 subheading does not distinguish between ammoxidation-derived acrylonitrile and material produced by obsolete acetylene-hydrogen cyanide chemistry; customs treatment follows the chemical identity, not the route of manufacture.

In the manufacture of acrylonitrile-butadiene-styrene resin, acrylonitrile is copolymerised with butadiene and styrene in a grafted rubber morphology. Acrylonitrile content in the final resin is typically 15–35% by mass. The polar nitrile group raises chemical resistance to aromatic and chlorinated solvents, but the processing window narrows when the acrylonitrile content exceeds 35% because melt viscosity increases and thermal degradation is observed as yellowing. Melt flow index is measured according to ISO 1133-1:2022; tensile properties are verified by ASTM D638, and notched Izod impact is tested by ASTM D256. In emulsion polymerisation, the acrylonitrile feed rate is controlled to avoid composition drift; a rate mismatch between butadiene and acrylonitrile leads to high-rubber grafting defects and reduces Izod impact by more than 20% relative to specification in some published production data.

Styrene-acrylonitrile copolymers, used as transparent moulding compounds, contain acrylonitrile in the range of 20–35% by mass. The copolymerisation is carried out in continuous bulk or suspension reactors, and residual acrylonitrile monomer is stripped to below 1 ppm in the pelletised resin to comply with food-contact migration limits where applicable. The acrylonitrile monomer contributes to glass transition temperature and barrier performance, but it also increases moisture uptake relative to pure styrenics. Drying before injection moulding is specified at 80 °C for 2–4 h when relative humidity exceeds 60%, because moisture in the melt reduces surface gloss and can hydrolyse residual nitrile groups at high barrel temperatures.

Why NBR Producers Specify Bound Acrylonitrile Content

Nitrile-butadiene rubber is produced by emulsion polymerisation with bound acrylonitrile contents from 18% to 50% by mass. The selection of bound acrylonitrile determines the balance between oil resistance and low-temperature flexibility. A grade with 18% acrylonitrile has a brittle point near −45 °C and swells substantially in mineral oil; a grade with 50% acrylonitrile resists aromatic fuel but loses elasticity below −10 °C. Producers specify bound acrylonitrile by Fourier-transform infrared spectroscopy or nitrogen analysis, and the polymer is sold under Chapter 40 for vulcanised rubber compounds rather than under 2926.10. The customs boundary is therefore clear: the liquid monomer is classified in 2926.10; the rubber is not.

Acrylonitrile is also converted to adiponitrile by electrolytic hydrodimerisation, which is then hydrogenated to hexamethylenediamine for nylon 6,6. In this route, the nitrile functionality is retained through the dimerisation step. The adiponitrile produced by hydrodimerisation is classified separately, and its shipment is not covered by 2926.10. Acrylonitrile for this use often requires strict limits on acetonitrile and propionitrile impurities, because these saturated nitriles can interfere with the electrolytic reaction and reduce current efficiency. Published data for impurity limits specific to this configuration are limited, but suppliers routinely provide a dedicated purity specification because standard polymerisation-grade acrylonitrile may contain inhibitor levels that are incompatible with electrochemical dimerisation.

Carbon fibre precursor production accepts only narrow-specification acrylonitrile. Polyacrylonitrile tows are solution-polymerised with 1–6 mol% of a comonomer such as methyl acrylate or itaconic acid to modify the cyclisation exotherm during oxidative stabilisation at 200–300 °C. The precursor is then carbonised at 1000–1500 °C under inert atmosphere. Defect-free carbon fibre requires transition-metal impurities in the precursor below 10 ppm and insoluble gel particles below the detection limit of a 5 µm filter test. The oxidative stabilisation step is sensitive to inhibitor residues; therefore, acrylonitrile supplied to carbon fibre precursor plants may be inhibitor-free or may use a volatile inhibitor that is removed before polymerisation. The customs declaration under 2926.10 should record inhibitor identity and concentration because national import authorities may request that information for safety and verification of the tariff line, particularly when the product is shipped under inert gas and marked as polymerisable.