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Mechanically robust biochar-modified polystyrene foam with enhanced flame retardancy using halogen-free additives
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DOI：10.1002/pi.70155.png)
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Abstract
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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 CO2 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 × 108 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 cm3 and a specific compressive strength of 4.8 MPa g−1 cm3. 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.
Keywords:
polystyrene foam
biochar
halogen-free flame retardants
supercritical CO2 extrusion
thermal insulation materials
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