Full definition
A flexible structural adhesive based on polyurethane chemistry, providing high elongation (200-600%), excellent impact and vibration resistance, and strong bonds to a wide range of substrates: rubber, plastics, wood, metal, glass, and composites. Available as: one-component moisture-curing (cure by ambient humidity, 24-72h full cure), two-component (resin + isocyanate hardener, 1-24h cure, more controllable), and hot-melt reactive (applied molten, then moisture-cures for permanent cross-linked bond). Shear strength: 5-25 MPa depending on formulation. Temperature range: -40 to +80°C (standard), +120°C (high-performance). Key advantage: flexibility — PU adhesives absorb dynamic loads, thermal cycling stresses, and vibration without cracking, unlike rigid epoxies. Applications: automotive windshield bonding (the iconic PU adhesive application), conveyor belt cold splicing, rubber flooring installation, sandwich panel manufacturing, wood lamination, and flexible packaging. Limitations: moisture-sensitive before cure (foaming if excessive), isocyanate safety precautions (respiratory protection), and UV degradation without protection. Per ASTM D5868 (lap shear testing). Brands: Sika (SikaFlex, SikaBond), 3M, Henkel (Loctite), Bostik, H.B. Fuller.