Green flame retardant strategy for thermoplastic polyurethanes: sustainable utilization of oyster shell waste through chitosan extraction combined with phytic acid and Prussian blue

In this work, we developed a green strategy to extract chitosan (CS) from oyster shell (OS) waste via a ball-milling exfoliation method, then modified with phytic acid (PA) and combined with Prussian blue analog (PBA) nanoparticles to serve as a flame retardant for thermoplastic polyurethane (TPU) composites. The resulting TPU/PBA-CS@PA composites exhibited remarkable improvements in fire safety performance, demonstrating 58.5% and 43.4% reductions in peak heat release rate (pHRR) and total smoke production (TSP), respectively, compared to the control TPU. Notably, the composite containing 6 wt% PBA-CS@PA achieved a UL-94 V-0 rating while exhibiting no melt dripping behavior. The flame-retardant mechanism involved PBA-catalyzed formation of a graphitized, thermally stable char layer, while PA and CS synergistically improved char compactness. Hydrogen bonding between additives and the TPU matrix enhanced interfacial adhesion, increasing tensile strength by 31.5% (30.16 MPa). This work presents a green strategy for converting biowaste into high-performance flame retardants, advancing eco-friendly polymer composites with balanced fire safety and mechanical properties.