Abstract Enhancing the fire safety of polystyrene (PS) insulation foams remains challenging due to their inherent flammability and environmental concerns associated with halogenated flame retardants. In this study, halogen‐free flame‐retardant PS foams are produced using a pilot‐scale supercritical CO 2 extrusion process incorporating expandable graphite, melamine polyphosphate, biochar, and 10 wt% recycled PS. The optimized composite exhibits a refined cellular structure with the highest cell density (5.8 × 10 8 cells cm −3 ) and the smallest average cell size ( ca 50 μm). Transmission electron microscopy analysis confirms uniform dispersion of carbon‐based fillers, providing effective heterogeneous nucleation sites, while rheological measurements indicate increased melt viscosity and elasticity, supporting stable cell growth. This engineered morphology results in a low thermal conductivity of 39 mW m −1 K −1 and enhanced load‐bearing capability, with a specific compressive modulus of 74 MPa g −1 cm 3 and a specific compressive strength of 4.8 MPa g −1 cm 3 . Flame‐retardancy testing showed no melt dripping, rapid self‐extinguishment within 12.5 s, and a limiting oxygen index of 25%. Char analysis indicated that biochar reinforces and densifies the expandable graphite–polyphosphate network, forming a compact protective layer. Overall, this work demonstrates a scalable, sustainable strategy for producing mechanically robust, halogen‐free PS insulation foams under industrially relevant conditions. © 2026 The Author(s). Polymer International published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.
Gaidhani et al. (Sun,) studied this question.