Polyacrylonitrile (PAN) is made by free‑radical polymerization of acrylonitrile, often with small amounts of co‑monomers, in a solvent or aqueous medium. The polymer is then dissolved or dispersed into a spinning dope, extruded, and coagulated to form continuous fibers that serve as carbon‑fiber precursors.
Monomer production and purification
Acrylonitrile is produced mainly by the Sohio ammoxidation of propylene, ammonia, and oxygen over a metal‑oxide catalyst. The raw product is distilled and scrubbed to remove impurities such as hydrogen cyanide and acetonitrile, yielding >99.5 % purity required for polymerization.
Choosing a polymerization route
Industries use either solution or aqueous suspension polymerization. In solution polymerization, monomers and initiator are dissolved in a solvent (DMF, DMSO, DMAc, or ZnCl₂). In suspension, monomers are dispersed in water with a water‑soluble initiator, forming fine PAN particles that must be washed and dissolved before spinning.
Controlling polymer structure with co‑monomers
Pure PAN tends to cyclize too quickly during stabilization, so 1–10 % of co‑monomers are added. Neutral co‑monomers (e.g., methyl acrylate) improve solubility and lower cyclization temperature, while ionic co‑monomers (e.g., acrylic acid) introduce built‑in thermal initiators that moderate the stabilization reaction.
Managing molecular weight and rheology
Target molecular weights of 100 000–300 000 g mol⁻¹ give the required tensile strength. Chain‑transfer agents (alkyl mercaptans) keep the polymer from over‑growing, ensuring the spinning dope has the right viscosity for extrusion.
Spinning the PAN dope into fibers
Wet or dry‑jet‑wet spinning is employed. The dope is extruded through a spinneret into a coagulation bath that solidifies the filament. The fiber is then washed, stretched (drawing), and stabilized in air or a controlled atmosphere before carbonization.
Post‑spinning stabilization and carbonization
Stabilization involves slow heating to 200–300 °C, inducing cyclization, dehydrogenation, and cross‑linking. This process turns the PAN filament into a thermally stable precursor. Subsequent carbonization at 1000–3000 °C removes non‑carbon elements, producing high‑strength carbon fibers.