Phosphorus-Modified Biobased Epoxy Soybean Oil Enables Flame-Retardant and Toughened Polylactic Acid Composites

Developing flame-retardant polylactic acid (PLA) that maintains mechanical integrity, suppresses melt dripping, and complies with green processing remains a key challenge, as conventional flame-retardants often require high loadings and compromise material performance. Herein, a solvent-free, biobased phosphorus–silicon–nitrogen (P–Si–N) hybrid flame-retardant system is developed via surface modification of ammonium polyphosphate (APP) with amino-silane coupling agent to obtain silane-modified ammonium polyphosphate (MAPP), followed by its integration with 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO)-functionalized epoxidized soybean oil (ESO–DOPO). The resulting ESO–DOPO–MAPP hybrid additive is synthesized without organic solvents and incorporated into PLA through melt blending. Owing to improved dispersion and interfacial compatibility, the composites exhibit enhanced flame retardancy while largely preserving their mechanical properties. At 5 wt % loading, the composite retains a tensile strength of 53.4 MPa, a Young’s modulus of 1.48 GPa, and an elongation at break of ∼4.0%, comparable to neat PLA. Simultaneously, the limiting oxygen index increases from 19% to 30%, and a UL-94 V-0 rating is achieved with suppressed melt dripping. Cone calorimetry reveals a reduction in peak heat release rate from 401.6 to 324.2 kW m –2 and a decrease in total heat release from 75.7 to 71.7 MJ m –2, accompanied by enhanced char formation. The improved fire performance is attributed to synergistic gas-phase radical quenching and condensed-phase char reinforcement. This work provides a scalable, environmentally friendly strategy for producing flame-retardant PLA with balanced fire safety and mechanical performance.