Polyacrylonitrile fibers have high purity requirements for their raw material, acrylonitrile, with the total content of impurities to be below 0.005%. The monomers used for polymerization are primarily methyl acrylate, but also methyl methacrylate, with the aim of enhancing the spinnability and the hand, softness, and elasticity of the fiber; the third monomer mainly improves the dyeability of the fiber, typically using itaconic acid containing weakly acidic dye groups, sodium acrylate containing strongly acidic dye groups, sodium methyl acrylate, sodium p-methacrylamido benzenesulfonate, and methyl vinyl pyridine containing basic dye groups.

Aggregation

Aggregation processes are divided into two types: suspension polymerization using water as the medium and solution polymerization using solvents. The polymer obtained from suspension polymerization precipitates out as floc and needs to be dissolved in a solvent to form a spinning solution. The solvents used in solution polymerization can dissolve both monomers and polymers, and the obtained polymer solution is directly used for spinning. Solvents used in solution polymerization include dimethylformamide, dimethyl sulfoxide, sodium thiocyanate, and zinc chloride. The polymerization time using the first two organic solvents is generally over 10 hours, but the strong solvating ability of the spinning solution allows for higher concentrations, which can appropriately increase the spinning speed. The solvent recovery is also more convenient, resulting in better fiber properties and lower material requirements for equipment. On the other hand, using the latter two inorganic solvents, the polymerization time is only 2 hours, and the resulting fibers have better brightness.

Spinning

Spinning liquid is typically a polyacrylonitrile polymer with a number average molecular weight ranging from 53,000 to 106,000. The fibers have good brightness and a thermal decomposition temperature of 200 to 250°C, with a melting point reaching 320°C. Consequently, polyacrylonitrile fibers are produced through both wet and dry spinning methods using high polymer solution. The concentration of the spinning liquid for dry spinning is 25% to 30%, with a fast spinning speed, although the thin stream ejected from the spinneret solidifies slowly, leading to easy bonding before curing. This limits the use of spinnerets with a high number of holes. The spinning solvent is only dimethylformamide, resulting in a uniform and dense fiber structure suitable for weaving artificial silk fabrics. Wet spinning is ideal for producing short fibers, which are fluffy and soft, and are perfect for making artificial wool fabrics. Besides the solvent used for solution polymerization, dimethyl acetal, ethylene carbonate, nitric acid, and other solvents are used, most of which have high boiling points and are not easily evaporated during the spinning process. In the 1980s, research was conducted on melt spinning polyacrylonitrile fibers using water as the plasticizer for the polymer, eliminating the need to prepare the polymer stock solution and the solvent recovery and recycling system. The physical properties of the fibers are essentially the same as those produced by dry or wet spinning methods, and the technology is now in the intermediate test phase.