Elastane Dry Spinning Process and Polyurethane Chemistry
topic
Elastane dry spinning is the dominant commercial production process (95% of global production), involving synthesis of polyurethane-urea polymer in DMAc (dimethylacetamide) solvent, extrusion through spinneret into hot air column to evaporate solvent, and collection of solid elastane filaments. Polymer synthesis (two-stage): Stage 1 — prepolymer synthesis: PTMEG (Mw 2,000) + MDI (2.05:1 molar ratio NCO:OH excess) → 50–60°C, 2 hours → NCO-terminated prepolymer (% NCO 3.5–4.5%); Stage 2 — chain extension: prepolymer dissolved in DMAc (25–30% solids) + EDA (ethylene diamine, chain extender, 0.95:1 NH₂:NCO ratio) → immediate chain extension forming polyurethane-urea block copolymer solution (Mw 80,000–120,000 g/mol, solution viscosity 1,000–5,000 Pa·s). DMAc solution spinning: polymer dope pump → spinneret (10–100 holes, 0.1–0.5 mm diameter) → vertical dry spin column (220–235°C, nitrogen atmosphere, 4–5 m long) → DMAc evaporation (boiling point 165°C — supercritical evaporation in column at >165°C) → solid filament formed → DMAc recovery (>99.5% recovery by condensation required for environmental and economic reasons) → finish application (mineral oil 0.5–1.5% OWF + PTFE powder anti-stick agent) → filament coalescence (multiple filaments from multi-hole spinneret coalesce to single multifilament yarn) → winding at 300–800 m/min. Filament coalescing: 10–32 individual monofilaments (1–3 dtex each) coalesce by mutual adhesion at spinneret face into a single 20 dtex, 40 dtex, 78 dtex, or 156 dtex yarn — coalesced yarn behaves as monofilament in knitting but individual filaments prevent total adhesion (allows separation for recycling processing). Decitex range: 11 dtex (ultrafine, lingerie, sheer hosiery), 20–44 dtex (standard swimwear, intimate apparel), 78–156 dtex (compression sportswear, surgical support), 310–620 dtex (industrial elastic tape, orthopaedic). Chlorine resistance: PTMEG-MDI-EDA elastane degrades in chlorinated pool water (HOCl attacks –NH–CO–NH– urea linkages) → grade 2–3 ISO 105-E03 → modified polyester diol soft segment or chlorine-stabilising antioxidant (hindered amine) improves chlorine resistance to grade 3–4 for swimwear elastane.
Role
Elastane dry spinning chemistry knowledge explains the structure-property relationships that make commercial elastane engineering possible — the PTMEG soft segment molecular weight (1,000 versus 2,000 g/mol) controls elastic modulus and elongation range, the hard segment concentration (15–35% by weight) controls recovery force and heat setting temperature, and the DMAc solvent system's 99.5% recovery requirement drives the capital-intensive solvent recovery infrastructure that creates a $50–80 million minimum viable scale for new elastane production facilities.