Phosphorus‐Modified Castor Oil via Solvent‐Free Transesterification: A Green Strategy for Toughened, Flame‐Retardant Polylactic Acid Composites

ABSTRACT Developing polylactic acid (PLA) bioplastics that are both flame‐retardant and mechanically robust remains challenging, as most flame‐retardant approaches require high loadings or reduce toughness. Bio‐based modifiers rarely deliver flexibility, char formation, and effective flame inhibition at low levels, leaving a gap in achieving multifunctional PLA systems. Here, phosphorus‐modified castor oil (PMCO) was synthesized via solvent‐free transesterification of castor oil with trimethyl phosphate, yielding a bio‐based additive tailored for dual mechanical and flame‐retardant enhancement. PMCO, used alone or in combination with ammonium polyphosphate (APP), was incorporated into PLA via melt blending to evaluate synergistic effects. The phosphorus moieties in PMCO promoted char formation and gas‐phase radical quenching, while castor oil's long‐chain fatty acids imparted flexibility; APP reinforced condensed‐phase char development. Low PMCO loadings (1–3 wt%) enhanced tensile strength (up to 59.33 MPa) via plasticizing–reinforcing effects, whereas 5 wt% reduced stiffness; APP counteracted this in hybrid PLA/2.5%PMCO/2.5%APP, which restored tensile strength (58.15 MPa), doubled elongation at break (6.89%), and improved deformation resistance. Fire tests showed LOI rising from 19% (PLA) to 30% (hybrid), with UL‐94 reaching V‐0 and complete suppression of melt‐dripping. Cone calorimetry confirmed delayed ignition, lowered total heat release, and > 40% smoke reduction. This dual‐modification strategy achieves balanced toughness and strong flame retardancy in bio‐based PLA at low additive levels. The design of PMCO enables efficient dispersion, reduced brittleness, and multifunctional performance, offering a pathway toward high‐performance PLA composites for packaging, electronics, and automotive applications.