Acrylonitrile (CAS 107-13-1) is a flammable, water-soluble unsaturated nitrile produced industrially by vapour-phase ammoxidation of propylene, ammonia and air. The reaction occurs in a fluidized-bed reactor at 430–450 °C and approximately 0.5–2.0 bar over a bismuth molybdate-based catalyst. Crude acrylonitrile is quenched, absorbed and separated by extractive distillation to yield polymerization-grade monomer with a typical purity above 99.5 %. Storage and transport require nitrogen blanketing, closed-loop vapour recovery, and inhibition with 35–45 ppm hydroquinone monomethyl ether. The boiling point is 77.3 °C, the vapour pressure at 20 °C is 11.8 kPa, and the flammable limits in air are 3.05 % and 17.0 % by volume. Acrylonitrile must be isolated from strong bases, mineral acids, free-radical initiators and strong oxidizers to prevent exothermic polymerization or decomposition.

Within the packaging derivative chain, the most demanding application is Acrylonitrile Copolymers for High-Barrier Packaging (Barex-Type Resins). These materials are random copolymers of acrylonitrile and methyl acrylate, impact-modified with a butadiene-based elastomer, and retain typically 70–75 % acrylonitrile by mass. The high nitrile concentration increases cohesive energy density and reduces fractional free volume. This molecular architecture suppresses oxygen diffusion and provides chemical resistance toward hydrocarbons, oils, fuels and many oxygenated solvents. The trade-off is moisture sensitivity and a narrower thermal processing window than polyolefins.

What Makes Acrylonitrile Copolymers Effective Oxygen Barriers?

Gas transport in high-nitrile copolymers follows the solution-diffusion model, in which permeability is the product of diffusivity and solubility. The nitrile group creates strong dipole-dipole interactions between chain segments, limiting segmental mobility and reducing the free-volume elements available for oxygen movement. The random copolymer with methyl acrylate prevents crystallinity while preserving dense amorphous packing. Oxygen transmission through a 25 µm monolayer film tested per ASTM D3985 at 23 °C and 0 % relative humidity is reported in technical literature as approximately 5–15 cm³/(m²·day·atm). This places the material roughly one order of magnitude below uncoated PET and more than two orders of magnitude below high-density polyethylene film. Unlike ethylene-vinyl alcohol copolymers, high-nitrile barrier resins maintain measurable oxygen barrier at high relative humidity because the nitrile group is less extensively plasticized by water than the vinyl alcohol repeat unit.

Water vapour transmission remains the limiting property in high-humidity packaging. For the same 25 µm monolayer film, moisture vapour transmission measured per ASTM F1249 at 38 °C and 90 % relative humidity is commonly 8–15 g/(m²·day). This is higher than uncoated PET and significantly higher than polyolefin-based moisture barriers. Package designers therefore treat Barex-type resin as an oxygen and aroma barrier, not as a moisture barrier.

These values are indicative technical literature data for extrusion-grade high-nitrile copolymers and uncoated PET film. Actual results vary with molecular weight, impact-modifier content, sheet orientation and test temperature.

When High-Humidity Storage Changes the Barrier Stack

Because monolayer Barex-type film transmits moisture at a rate that is unacceptable for many pharmaceutical and cosmetic formats, the resin is commonly buried in a coextruded structure. A typical stack places the high-nitrile layer between polyolefin skins using maleic anhydride-grafted polyolefin tie layers of 8–20 µm thickness. The tie layer provides interfacial adhesion because acrylonitrile copolymers have poor inherent adhesion to polyethylene and polypropylene. The high-nitrile layer is positioned away from the high-humidity side of the package when possible. For pharmaceutical blister lidding requiring moisture ingress below 0.1 g/(m²·day), an aluminium foil or high-barrier overwrap is added.

At high relative humidity, oxygen transmission of high-nitrile copolymers increases less than that of ethylene-vinyl alcohol copolymers, but published data for this specific packaging configuration is limited to particular grades and thicknesses. Comparative testing should be conducted at end-use humidity because ASTM D3985 testing at 0 % relative humidity underestimates oxygen transmission at 80 % relative humidity for polar barrier resins. In rigid containers, the high-nitrile layer thickness is typically 50–100 µm, while dry pharmaceutical formats can function with 25–50 µm high-nitrile layers. The final barrier specification is normally verified by oxygen transmission rate per ASTM D3985, water vapour transmission rate per ASTM F1249 and seal strength per ASTM F88.

Extrusion-grade high-nitrile copolymers are processed on single-screw extruders with an L/D ratio of 24:1 to 30:1 and a compression ratio of 2.5:1 to 3.0:1. High-shear barrier screws with intensive mixing pins can raise melt temperature beyond the safe window and are generally avoided unless a vented barrel is used for devolatilization. Barrel profiles start at 160 °C near the feed throat, rise through 190–200 °C in the compression zone, and terminate at 205–215 °C at the die adapter. Melt temperature measured with a flush-mounted probe should remain below 225 °C. Total residence time from hopper to die should not exceed 10–15 min. Longer residence times produce progressive darkening, trace hydrogen cyanide evolution and a fall in melt viscosity.

Drying is mandatory before extrusion or injection molding. Barex-type copolymers hydrolyze at processing temperature when residual moisture is present, releasing ammonia and generating splay, bubbles and delamination in coextruded sheet. Desiccant drying for 4–6 h at 65–75 °C with a dew point below −30 °C reduces moisture to below 0.05 %. Wet resin alters melt flow and gauge control, and regrind must be re-dried under the same conditions. Uncontaminated, properly dried regrind can be incorporated at up to 20 % by mass in noncritical packaging layers; higher fractions reduce notched impact strength and darken the finished part.

Injection Molding, Thermoforming and Defect Control

Injection molding grades are processed at melt temperatures of 220–230 °C with mold temperatures of 40–70 °C. Clamp force, runner diameter and gate geometry are designed for short melt residence and low shear heating. Gate land lengths below 1.0 mm reduce local degradation and gate blush. Hot runner systems are used only when the manifold is held below 230 °C and residence time is minimized. Cold slug wells are recommended to trap degraded material after start-up. Purging with polypropylene or high-density polyethylene before shutdown prevents carbonized deposits in the barrel and hot runner.

Thermoforming of Barex-type sheet on production blister lines is performed at subsurface sheet temperatures of 120–150 °C. Forming pressure is generally below 0.6 MPa, and the forming die is held at 30–50 °C to prevent premature cooling. High-nitrile copolymers have high melt strength and low shrinkage, which allows deep-cavity forming with reduced corner thinning compared to polyvinyl chloride sheet. Heat sealing to lidding foil occurs at 150–180 °C with dwell times of 0.5–1.0 s. The high modulus of the nitrile layer requires controlled sealing pressure and flat anvil alignment to avoid microcracks at the seal edge. Seal strength is verified per ASTM F88 and leak integrity per ASTM F2096.

Regulatory Compliance Load for Food and Pharmaceutical Contact

In the United States, acrylonitrile copolymers for food-contact use are evaluated under FDA 21 CFR 177.1040. The regulation defines permitted copolymer compositions and imposes residual acrylonitrile monomer restrictions. Compliance testing uses headspace gas chromatography or solvent extraction followed by gas chromatography with nitrogen-specific detection. In the European Union, finished plastic food-contact articles must meet EU Regulation 10/2011, including an overall migration limit of 10 mg/dm² and a specific migration limit for acrylonitrile monomer of 0.01 mg/kg. Pharmaceutical packaging systems additionally require physicochemical screening and extractables assessment under USP 661.1.

For aggressive personal-care formulations, chemical compatibility is validated by storage trials at 40 °C and 75 % relative humidity for 12 weeks, with measurement of weight change, dimension change and extractable leachables by gas chromatography-mass spectrometry or liquid chromatography-mass spectrometry. Barex-type copolymers resist many hydrocarbons, oils, esters and alcohols but can soften or stress-crack in strong ketones, chlorinated solvents and high-polar aprotic solvents. These solvent classes require barrier replacement or an alternative material system.

Pharmaceutical blister lines running high-nitrile sheet require closed-loop moisture management because the sheet can regain surface moisture rapidly when relative humidity exceeds 60 %. The sheet is typically processed within a few hours after drying or stored in sealed polyethylene film. Operational boundaries are specific: moisture content above 0.08 %, melt temperature above 230 °C and regrind fractions above 20 % each reduce either barrier or impact performance. The material is not a drop-in replacement for PET in external packaging because of its higher water vapour transmission and lower continuous-use temperature under load.