Polyacrylonitrile fibers have a high purity requirement for the raw material acrylonitrile, with the total content of various impurities to be less than 0.005%. The monomer used for polymerization is mainly methyl acrylate, but methyl methacrylate can also be used, with the aim of improving spinnability and the hand, softness, and elasticity of the fiber; the third monomer primarily enhances the dyeability of the fiber, usually consisting of itaconic acid containing weakly acidic dye groups, sodium acrylate containing strongly acidic dye groups, sodium methacrylate, 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. Polymerization using the first two organic solvents typically takes more than 10 hours, but the strong solvating power of the spinning solution allows for higher concentrations, which can moderately increase spinning speed. Solvent recovery is also simpler, resulting in fibers with better properties and lower material requirements for equipment. On the other hand, using the latter two inorganic solvents, the polymerization time is only 2 hours, resulting in fibers with better brightness.

Spinning

Spinning liquid is typically a polyacrylonitrile polymer with a number average molecular weight of 53,000 to 106,000. The fibers have good whiteness, with a thermal decomposition temperature of 200 to 250°C and a melting point of up to 320°C. Consequently, polyacrylonitrile fibers are produced through wet spinning and dry spinning of high polymer solutions. The concentration of the spinning liquid in dry spinning ranges from 25% to 30%, with a fast spinning speed. However, due to the slow solidification of the fine stream emitted from the spinneret, the fibers tend to stick before solidification, and it is not suitable to use spinnerets with a high number of holes. The spinning solvent is only dimethylformamide, resulting in a uniformly dense fiber structure suitable for weaving artificial silk fabrics. Wet spinning is suitable for producing short fibers, which are fluffy and soft, ideal for weaving fabrics resembling wool. Besides the solvent used in solution polymerization, dimethyl acetamide, ethylene carbonate, nitric acid, and other solvents are used. Most of these solvents have high boiling points, making them difficult to evaporate during the spinning process. In the 1980s, research was conducted on melt spinning polyacrylonitrile fibers, using water as the plasticizer for the polymer. This method eliminates 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. The technology has now entered the pilot testing phase.