Bio-based chitosan-cerium synergist: Achieving high-efficiency flame retardancy and smoke suppression in thermoplastic polyurethane

Thermoplastic polyurethane (TPU) faces significant fire safety challenges due to its inherent flammability and toxic smoke emissions. To address this issue, a novel bio-based flame retardant, APP@CS-Ce, was developed via electrostatic self-assembly of chitosan (CS) and cerium ions (Ce 3+ ) onto ammonium polyphosphate (APP). This core-shell structure used chitosan derived from biomass as the carbon source and synergistically utilized the catalytic carbonization and free radical quenching functions of the rare earth element cerium (Ce 3+ ). Incorporating only 15 wt% APP@CS-Ce enabled TPU composites to achieve a UL-94 V-0 rating and a high limiting oxygen index (LOI) of 27.6 %. The char residues surged from 0.5 wt% (pure TPU) to 24 wt%, while peak heat release rate (PHRR) and total smoke production (TSP) decreased by 76 % and 61 %, correspondingly. Critically, CO emissions were reduced by 45 %, confirming effective toxicity suppression. The cerium ions further promoted graphitization of the char layer and catalyzed CO-to-CO 2 conversion, enhancing barrier properties and combustion efficiency. Unlike conventional additives, APP@CS-Ce preserved mechanical integrity, with elongation at break decreasing merely 0.5 % due to improved interfacial compatibility. This work demonstrates a sustainable, high-performance flame-retardant strategy combining biomass resources with rare-earth catalysis for next-generation TPU applications.