Acrylonitrile, 2-propenenitrile, CAS 107-13-1, is a colorless to pale-yellow, low-viscosity unsaturated nitrile supplied as a stabilized vinyl monomer and chemical intermediate. The molecule has the formula CH₂=CH–C≡N and a molar mass of 53.06 g/mol. At atmospheric pressure the normal boiling point is 77.3 °C and the freezing point is -83.5 °C; the liquid density is 0.806 g/cm³ at 20 °C, the vapor pressure at 20 °C is 11.3 kPa, and the closed-cup flash point is -1 °C. The solubility in water is 73.5 g/L at 20 °C, substantially higher than that of styrene, which creates a different distribution profile in wastewater and firewater run-off. The substance is classified for transport as UN 1093, Class 3, Packing Group II, with a subsidiary inhalation toxicity hazard.

Commercially available product models are distinguished primarily as technical-grade, polymer-grade, and low-water precursor-grade material. A common polymer-grade specification is built around an acrylonitrile purity not less than 99.5 wt% and a monomethyl ether hydroquinone inhibitor loading of 35 mg/kg to 50 mg/kg. Technical-grade material may carry higher hydrogen cyanide and acetonitrile ceilings, while low-water precursor-grade material tightens water and color limits because residual moisture interferes with dimethylformamide and dimethyl sulfoxide spinning solvents in polyacrylonitrile fiber and carbon-fiber precursor operations.

Product Specification and Analytical Controls for Inhibited Polymer-Grade Acrylonitrile

Table 1 summarizes representative release criteria used for transport and site acceptance. The values are drawn from standard industrial specifications and should be read as typical limits rather than as a single manufacturer’s guaranteed values.

The water limit is not solely a product-quality parameter; in acid-catalyzed or anionic downstream chemistries water acts as a terminating agent, while in carbon-fiber solvent systems it reduces spinning-line stability and increases gel formation. Acetonitrile and acetone are chain-transfer and solvent impurities that can alter polymer molecular weight distribution, and hydrogen cyanide is controlled for both toxicological risk and oligomer color. The color and acidity limits detect oxidation and hydrolysis products that form when the inhibitor has been consumed or when the monomer has been exposed to copper alloys and acidic residues.

Bottom-unloading road tankers are connected to site storage using Type 316L stainless steel or lined carbon steel piping. Brass, copper, and copper-bearing alloys are avoided because copper ions can promote oxidative polymerization and color formation. Storage tanks are maintained at ≤25 °C and are fitted with refrigeration coils, pressure/vacuum vents, and flame arrestors. Dissolved oxygen is necessary for MEHQ inhibition; nitrogen blanketing or closed-loop inerting can collapse the induction period by removing oxygen. Tanks therefore remain under an air headspace rather than an inert pad. Agitated recirculation loops avoid dead legs because stagnant monomer can polymerize at the liquid line or behind valve seats. If a white insoluble popcorn polymer appears at the liquid-vapor interface, the seed can accelerate exothermic polymerization; the affected equipment is typically cleaned mechanically rather than by steam alone because steam can volatilize the monomer without deactivating the seed. Relief and emergency containment systems are sized for a two-phase runaway exotherm; the heat of polymerization for acrylonitrile is approximately 76.5 kJ/mol.

Why Does Acrylonitrile Need Different Storage Boundaries Than Styrene?

Acrylonitrile properties create a narrower storage envelope than those of styrene. The boiling point of 77.3 °C and closed-cup flash point of -1 °C place the liquid above its flash point at ordinary ambient temperatures, so the tank headspace can enter the flammable range unless the vapor space is managed. A comparison of monomer handling properties is shown in Table 2.

The high water solubility of acrylonitrile means that spills require treatment of dissolved monomer rather than floating-phase recovery. Styrene remains largely as a separate organic phase and can be recovered by skimming, although its lower solubility still exceeds environmental thresholds. Methacrylonitrile carries a methyl group at the alpha carbon, which lowers water solubility and alters the monomer’s copolymerization response; the nitrile group remains activating, but steric constraints reduce propagation rate relative to acrylonitrile in certain radical systems. In comparison with acrylic acid, acrylonitrile contains no carboxylic acid function and therefore does not require the same low-pH corrosion-resistant storage design, but its flammability and toxicological profile are more acute.

Acrylonitrile is not compatible with strong bases, amines, or concentrated acids without strict temperature control; these materials can initiate oligomerization or hydrolysis.

Feeding, Storage, and Inhibition Practice on Continuous ABS and NBR Lines

Feeding acrylonitrile into a continuous styrene-acrylonitrile or nitrile rubber polymerization line uses a sealed system with mass-flow metering and backpressure regulation. Because the vapor pressure at 20 °C is 11.3 kPa, pump suction lines are sized for low pressure drop to prevent cavitation. Positive-displacement diaphragm metering pumps or canned-motor centrifugal pumps with Type 316L wetted parts are used. The monomer is not distilled on-site unless necessary; off-spec recovered monomer is returned only after gas chromatographic determination of acetonitrile, acetone, hydrogen cyanide, and inhibitor. In bulk styrene-acrylonitrile mass polymerization, feed to the prepolymerizer is controlled so that the adiabatic temperature rise does not exceed the heat-removal capability of the boiling pool; prepolymerization conversion is typically maintained at 20–30% before transfer to a plug-flow finishing reactor. In aqueous emulsion lines, residual MEHQ above 1 mg/kg in the organic phase can extend the induction time and cause batch-to-batch variation in latex particle size.

Acrylonitrile enters the resin and synthetic rubber chain as a comonomer that contributes polar nitrile groups. In ABS and SAN production, the acrylonitrile feed is typically 20–35 wt% of the monomer mix, and the polar group raises the glass transition and melt strength while reducing unpigmented light transmission when compared with polystyrene. In nitrile rubber, bound acrylonitrile content is controlled from 18 wt% to 50 wt%; higher values improve volume swell resistance after immersion in ASTM D471 reference fuels and oils, but raise the low-temperature flexibility limit from approximately -50 °C to -5 °C. The conversion profile is influenced by the water-soluble nitrile; emulsion recipes therefore use staged addition or controlled latex viscosity to reduce reactor fouling.

Polyacrylonitrile for carbon-fiber precursor is produced from acrylonitrile with comonomer levels typically below 2 wt% methyl acrylate or itaconic acid. The precursor-grade monomer requires low non-carbonizable impurities because residual sodium, calcium, and nitrile by-products create defects after carbonization. In wet-spinning lines using dimethylacetamide or sodium thiocyanate, residual water in acrylonitrile above 0.10 wt% destabilizes polymer solution viscosity and can cause void formation in the coagulated filament.

When Hydrogen Cyanide and Nitrile By-Products Require Air Gap Monitoring

Acrylonitrile decomposes under fire conditions to release hydrogen cyanide, carbon monoxide, and nitrogen oxides. Combustion gas monitoring at storage flanges and pump alleys includes electrochemical sensors calibrated to 10 ppm hydrogen cyanide and photoionization detection for the monomer at 2 ppm. Area monitors are interlocked with deluge systems and remote block valves. Monomer vapor is denser than air and can accumulate in pits and drains. Under abnormal conditions, acetonitrile and hydrogen cyanide impurities partition into water and require treatment, whereas the parent monomer is stripped from wastewater under vacuum at temperatures below 40 °C to avoid thermal polymerization in the stripper reboiler. The operational boundary is therefore not solely flammability but the overlapping demands of toxicity, aqueous solubility, and low-temperature heat input.