Multiphase Structure and Interface Engineering of Polyurethane‐Based Composites With Enhanced Acoustic Properties

In this work, polyurethane (PU)‐based acoustic composites were systematically designed through multiscale structural and interfacial engineering to achieve broadband sound absorption and enhanced sound insulation. Dibutyltin dilaurate (DBTDL) and antioxidant AO‐80 were employed to regulate foaming kinetics and pore morphology, while multiwalled carbon nanotubes (MWCNTs) and hydroxyl‐functionalized MWCNTs were introduced as interfacial damping modifiers. Furthermore, 316 L stainless steel hollow spheres were incorporated to activate local mass–spring resonance effects. Scanning electron microscopy revealed that DBTDL promoted highly open‐cell structures with high porosity, enabling efficient viscous damping and improved mid‐frequency sound absorption. In contrast, AO‐80 suppressed cell formation and induced dense, closed‐cell morphologies, resulting in enhanced sound insulation but limited absorption capability. The incorporation of MWCNTs introduced interfacial viscoelastic damping, broadening the effective absorption bandwidth above 1500 Hz. Notably, hollow spheres significantly enhanced low‐frequency acoustic performance through local resonance, while surface modification with hydroxylated MWCNTs further strengthened interfacial coupling and energy dissipation. As a result, the optimized composites exhibited simultaneously improved sound absorption and sound insulation over a wide frequency range. The coupled structure‐dependent acoustic response—combining porous viscous loss, local mass–spring resonance, interfacial viscoelastic damping, and multiscale scattering—provides a scalable design strategy for lightweight, high‐efficiency polymer‐based acoustic materials.