PAN Polymerisation and Wet Spinning of Acrylic Fibre
topic
PAN polymerisation for acrylic fibre employs aqueous suspension polymerisation or solution polymerisation of acrylonitrile with co-monomers to produce copolymer molecular weight Mn 70,000–120,000 g/mol (DP 1,300–2,300) — followed by wet spinning via concentrated polymer solution through submerged spinnerets into a coagulation bath. Aqueous suspension polymerisation (most common for staple fibre): AN monomer (85–92%) + methyl acrylate (5–10%, softens Tg from 100°C to 80°C for improved drawing) + sodium methallylsulfonate (1–2%, anionic dye site introduction for basic dye receptivity) + potassium persulfate / sodium bisulfite redox initiator → 40–60°C, pH 2.5–3.5, 4–6 hours → PAN copolymer precipitate filtered, washed, dissolved in solvent. Wet spinning with NaSCN solvent (Japanese process, Toray, Mitsubishi): PAN dissolved in 50% NaSCN aqueous solution at 80°C (12–15% polymer concentration, viscosity 50–150 Pa·s) → spinneret (50,000–100,000 holes, 0.05–0.10 mm diameter) submerged in coagulation bath (10–15% NaSCN, 10–15°C) → instantaneous coagulation by solvent dilution → gel fibre drawn 3–5× in steam or hot water bath (90–100°C) → washing (NaSCN recovery countercurrent washing cascade) → drying → heat relaxation. Wet spinning with DMF solvent (European process, Aksa, Dralon): 20–25% PAN/DMF solution, spinneret immersed in 50% DMF/water coagulation bath at 40°C — coagulation kinetics slower than NaSCN process → larger, more irregular fibre cross-section (kidney-bean or dog-bone shape versus round for NaSCN) → surface area higher → improved crimp uptake and wool-like hand. Bicomponent acrylic (self-crimping): eccentric core-sheath spinneret with two PAN copolymers of different molecular weight → differential shrinkage on relaxation → 3D helical crimp 8–12 crimps/cm → Woolmark-equivalent bulkiness without mechanical crimping → Dralon T (Bayer), Vonnel (Monsanto) self-crimping acrylic yarn used in hand-knitting premium segment. Crimp development of gel fibre: mechanically crimped (stuffer-box crimper, 3–6 crimps/cm, 38–51 mm cut) for standard staple; hot-air relaxation at 130°C develops latent crimp in bicomponent fibres.
Role
PAN wet spinning process knowledge explains the acrylic fibre structural features — bean-shaped cross-section from DMF process or round from NaSCN process — that determine surface area, dyeability, and hand feel, and understanding co-monomer composition design reveals how ionic dye sites (sulfonate groups from sodium methallylsulfonate) are engineered into the polymer chain to create the cationic (basic) dye receptivity that gives acrylic its distinctive brilliant colour performance unavailable in polyester or cotton.