A holistic strategy based on a phosphorus–nitrogen benzimidazole zwitterion for flame-retardant and self-catalytically closed-loop recyclable polylactic acid

The inherent flammability of poly (lactic acid) (PLA) severely restricts its range of applications, whereas circular economy principles require full component recovery of waste PLA components. Conventional flame-retardant modifications often induce excessive PLA degradation during melt processing and contaminate monomer streams in chemical recycling, creating a long-standing conflict between fire safety and recyclability. In this work, a phosphorus/nitrogen-containing flame retardant, designated CEDCE, was synthesized. With only 3.0 wt% CEDCE loading, the PLA/CEDCE composite attained a limiting oxygen index of 28.2% and achieved a UL-94 V-0 rating, accompanied by suppressed melt dripping and markedly reduced heat release rate and total heat release, all while preserving favorable mechanical properties and processability. During chemical recycling, the zwitterionic nature of CEDCE not only enables its own efficient recovery through pH adjustment but also becomes activated under methanolysis conditions to catalyze PLA depolymerization, thereby allowing simultaneous recovery of high-purity methyl lactate and reusable flame retardant without any external catalyst. This study provides an innovative strategy to simultaneously improve the functionality, comprehensive performance, and chemical recyclability of PLA-based materials for sustainable circular development.