Nitrogen/Phosphorus Modified Polymer Toughener for Flame-Retardant, Complete Melt-Dripping Resistant, Tough Poly(lactic Acid)

Developing effective flame-retardant solutions for renewable poly(lactic acid) (PLA) remains a critical challenge in sustainable materials science. Conventional strategies often fall short in simultaneously overcoming PLA’s inherent drawbacks: high flammability, severe melt-dripping, and brittleness. Herein, we propose a molecular-level design that covalently integrates flexible poly(ε-caprolactone) (PCL), isocyanurate, and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO)-based flame retardants with PLA, creating a kind of flame-retardant star PCL-b-PLA block copolymer (IPCL-b-PLAD) tougheners for rigid PLA matrices. The optimized IPCL-b-PLAD system imparts a combination of exceptional flame retardancy, achieving Underwriter’s Laboratory Standard #94 (UL-94) V-0 rating and a limiting oxygen index (LOI) exceeding 33.1%, along with complete suppression of melt-dripping and a remarkable enhancement in mechanical toughness, exhibiting 293.2% elongation at break and 64.0 MJ/m3 tensile toughness. These improvements are achieved with only a marginal reduction in Young’s modulus (∼10.6%), preserving the material’s structural applicability. Through comprehensive fire safety assessments, including LOI, UL-94, and cone calorimetry tests coupled with mechanistic analysis, we demonstrate that the superior performance stems from the synergistic interactions among isocyanurate, DOPO, PCL, and PLA segments.