Nylon 6 and Nylon 66 Polymerisation Chemistry
topic
Nylon 6 and nylon 66 polymerisation differ fundamentally in monomer type, polymerisation mechanism, and equilibrium chemistry — nylon 6 (PA6) via ring-opening polymerisation of ε-caprolactam, nylon 66 via step-growth condensation polycondensation of hexamethylene diamine (HMD) and adipic acid — creating polymers with identical –(CH₂)₅–CO–NH– and –(CH₂)₆–NH–CO–(CH₂)₄–CO–NH– repeating units respectively but different chain regularity, crystalline structure, and processability. PA6 polymerisation (hydrolytic ring-opening, VK-tube — Vereinfachter Kontinuierlicher tube reactor, BASF, Uhde Inventa-Fischer): caprolactam + 0.3–1.0% water (initiator/chain terminator) → 250–270°C, atmospheric then vacuum, 12–24 hours → PA6 melt IV 1.2–1.8 dL/g (Mn 18,000–25,000 g/mol) → extracted to remove 8–12% residual caprolactam monomer (hot water extraction 90°C, 12 hours, 4 stages) → dried chip IV 1.0–1.4 dL/g. PA6 equilibrium: ring-opening is reversible — at 220°C, equilibrium monomer content 8.3% in melt → residual caprolactam causes plasticisation (reduces Tg 5–10°C), fibre brittleness and yellowing if unextracted — extraction mandatory before spinning. PA66 polymerisation (aqueous salt polycondensation, DuPont/Invista autoclave batch or continuous): HMD + adipic acid → 1:1 'nylon 66 salt' (AH salt, 8.0% solution) → evaporation → autoclave at 280°C, 17 bar, 2 hours → atmospheric flash → vacuum polycondensation → MW Mn 12,000–18,000 g/mol, RV (relative viscosity) 40–60. PA66 stoichiometry critical: amine/acid imbalance of 0.1 mol% limits ultimate MW (Carothers equation: DP = 1/(1–p), at 99.9% conversion DP = 1,000 — any excess of one monomer caps chain ends at lower MW). Phosphorus heat stabiliser (0.05% phosphoric acid or sodium hypophosphite) prevents PA66 oxidative yellowing at 280°C processing temperature. Water content of PA66 melt: 0.2–0.5% moisture at spinning causes hydrolytic chain scission reducing RV 5–8 units → pre-dried chips (80°C vacuum oven, <100 ppm moisture) essential before melt spinning.
Role
Understanding PA6 versus PA66 polymerisation chemistry explains the structural differences that drive their respective application advantages — PA66's higher polymerisation temperature and stricter stoichiometric control requirements produce a polymer with higher Tg and melting point that justifies its $800–1,200/tonne price premium over PA6 in tyre cord and automotive applications requiring thermal stability above 200°C, while PA6's simpler ring-opening chemistry and easier caprolactam recycling make it more suitable for circular economy closed-loop systems like Aquafil's ECONYL regenerated PA6 from fishing net waste.