Three Birds With One Stone: Facile Synthesis of a Bio‐Based Phosphorus–Nitrogen Flame Retardant for Polylactic Acid Toward Enhanced Fire Safety, Toughness, and Biodegradability

ABSTRACT Polylactic acid (PLA) suffers from high flammability and brittleness, limiting its practical applications. In this study, a bio‐based multifunctional flame retardant (PPM) was synthesized via a one‐pot method using phytic acid, 1,4‐phthalaldehyde, and melamine. Incorporation of PPM into PLA markedly enhanced the material's flame retardancy, mechanical performance, and biodegradability. At a loading of 4.0 wt%, the composite achieved a UL‐94 V‐0 rating, with the limiting oxygen index increasing to 28.7%. The peak heat release rate (PHRR) and total heat release of PLA/PPM‐4.0 decreased from 641.8 kW/m 2 and 87.8 MJ/m 2 to 565.3 kW/m 2 and 79.7 MJ/m 2 , respectively, while the time to PHRR was extended by 14 s, indicating a substantial improvement in fire safety. The good compatibility of PPM with PLA and the formation of hydrogen bonds enabled the composite to maintain a tensile strength comparable to that of neat PLA, while significantly increasing the elongation at break. Specifically, the tensile strength of PLA/PPM‐4.0 remained at 94.1% of neat PLA, whereas elongation at break increased from 3.76% to 5.84%. Moreover, the polar hydrophilic groups in PPM enhanced the water uptake of PLA, thereby facilitating enzymatic degradation. This work provides a straightforward and effective strategy for developing high‐performance PLA‐based materials.