Enhanced flame retardancy and accelerated degradation of polylactic acid using a chitosan-based additive with phosphorus and zinc

Polylactic acid (PLA) is a bio-recyclable plastic, but its high flammability limits broader applications. Here, a novel flame retardant (Zn-CHP) is synthesized from chitosan (CH), diethylenetriaminepenta (methylenephosphonic) acid (DTPMP), and ZnCl₂ using a simple, solvent-free process. The Zn-CHP additive is melt-blended with PLA, achieving excellent flame retardancy at just 2 wt% loading. The modified PLA exhibits a limiting oxygen index (LOI) of 25.5 % and meets , though minor non-combustible dripping is observed, which cools significantly around 200 °C compared to the untreated PLA. Cone calorimeter tests show that the addition of 2 wt% Zn-CHP reduces the peak heat release rate (pHRR) by 16.8 %, total heat release (THR) by 9.0 %, and fire growth rate by 23.1 %. Importantly, the tensile performance of the PLA is little affected, while the additive accelerates its degradation. We believe this work offers a promising approach to enhancing flame retardancy and promoting faster recycling of PLA.