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

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.