Technology Type

Of all the known polymerization processes, the main process used in the production of polyacrylonitrile (PAN) on an industrial scale for the manufacture of acrylic fibers is Suspension Polymerization.

The polymerization of acrylonitrile (AN) and its transformation into polyacrylonitrile occurs in the presence of a Redox system, most commonly based on persulfate, ferric iron and bisulfite ions at a pH between 2 and 3.5.

The process flowchart (illustration) shows the aqueous suspension copolymerization of acrylonitrile in the presence of a neutral comonomer (such as methyl acrylate or vinyl acetate).

• Using specifically computer-programmed pumps, the mixture of AN, comonomer and demineralized water is taken from the dosing tanks (1) into a pre-mixing tank (2) by a centrifugal pump (3) to the polymerization reactor (4), which consists of a vessel made of stainless steel or aluminum alloy, jacketed and closed at atmospheric pressure.
• The reactor (previously heated with steam from a boiler) is approximately ²/₃ of its volume filled with the mixture of demineralized water with the catalyst and activator (5). Thus, the solution in the premix tank meets the necessary conditions to begin copolymerization when it is released into the reactor.
• The AN copolymerization reaction is highly exothermic and, therefore, the reactor must be cooled by circulating water from a chilled water unit that generally maintains a constant reaction temperature between 55 and 60°C.
• The aqueous slurry that forms inside the reactor is continuously stirred by a propeller shaft at a rotation of 15 to 20 rpm (6), which maintains the viscosity of the suspension at around 50 cP.
• The polymerization continues until the reactor is completely full and it overflows through a tubular duct (7) coupled between the reactor lid and the stripping column (8). In this duct, the redox reaction that forms free radicals is interrupted by the addition of ethylenediaminetetraacetic acid (EDTA) (9), which acts as a chelating agent, forming stable complexes with iron ions. EDTA is constantly injected into the duct towards the stripping column.
• When it falls into the exhaust column, the aqueous sludge encounters an ascending current of superheated steam that acts to volatilize the monomers that did not react in the reactor, thus initiating a process of recovery of these non-polymerized monomers.
• The water vapor carries the monomers to a heat exchanger (10) cooled by the circulation of ice water, recovering the monomers by condensation (11), thus returning to the premix tank.
• The aqueous sludge passing through the exhaust column can be collected in a reservoir where it is pumped (12) for subsequent stages (13) that will culminate in the dry PAN powder polymer.
• A vacuum washing and filtration system removes the water, dissolved salts and residual EDTA.
• Then, the still-wet polymer mass is continuously pressed into pellets, which provides greater drying efficiency and better storage of the finished polymer in multi-ton silos.

After drying, PAN is a white powder, insoluble in water and only soluble in highly polar solvents such as dimethylformamide (DMF), dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), nitric acid, zinc chloride solutions and sodium and ammonium thiocyanates.

References

1. IGTPAN (Instituto Granado de Tecnologia da Poliacrilonitrila), Polimerização em Suspensão. (accessed 21^st Mar 2025)