Microstructure Tuning and Performance Enhancement of Polystyrene Foams via Ball‐Milled Biochar–Induced Nucleation in Supercritical <scp> CO ₂ </scp> ‐Assisted Extrusion

ABSTRACT Developing sustainable thermal insulation materials is critical to reducing building energy consumption and greenhouse gas emissions. This study developed lightweight, high‐performance polystyrene (PS) composite foams reinforced with wood‐derived biochar using a continuous supercritical CO 2 (sc‐CO 2 ) extrusion process. Ball‐milled biochar (MBC) with a high surface area and porosity acted as an efficient heterogeneous nucleating agent, resulting in a 40% increase in cell density (4 × 10 8 cells cm −3 ) and a narrower cell size distribution (average cell size 75 μm) as compared to pristine PS foams. These refined microstructures reduced thermal conductivity by up to 4% (32 mW/m K) and enhanced specific compressive strength by 75%, reaching 3.8 MPa g cm −3 , suitable for load‐bearing insulation applications. Transmission electron microscopy (TEM) and micro‐computed tomography (CT) confirmed uniform biochar dispersion and preferential localization along cell walls, validating its role in nucleation and reinforcement. The PS‐MBC composite foam processed at 20.6 MPa exhibited superior mechanical and thermal performance compared with conventional foams. Overall, this scalable and solvent‐free sc‐CO 2 foaming approach provides a sustainable pathway to upcycle renewable wood residues into high‐performance polymer insulation materials, with improved mechanical reliability, thermal efficiency, and environmental responsibility.