G-C3N4-Orchestrated Multi-Component Synergy for Enhanced Flame Retardancy in PLA

Polylactic acid (PLA) is a biodegradable polymer with good processability, but its poor flame retardancy limits its wider practical use. In this work, an ethanol-assisted pre-assembled g-C3N4/fumed silica hybrid (gCNFS) was introduced into a PLA/ammonium polyphosphate (APP)/lignin system to improve its flame-retardant performance. The pre-assembly strategy was designed to improve the interfacial contact between g-C3N4 and fumed silica and regulate char formation and thermal decomposition during burning. The chemical structure and morphology of the composites were analyzed to study their thermal degradation behavior, volatile release, and post-burn char morphology. The results showed that the introduction of pre-assembled gCNFS improved the flame retardancy of the PLA/APP/lignin system. Among all formulations, PLA/Q-gCNFS-3 exhibited the best overall performance, achieving a limiting oxygen index (LOI) of 37% and a UL-94 V-0 rating. Compared with neat PLA, its peak heat release rate (pHRR) and total heat release (THR) decreased by approximately 50% and 24%, respectively, indicating effective suppression of burning intensity and total heat output. The flame-retardant mechanism of PLA/APP/Lignin with gCNFS enhanced both condensed-phase and gas-phase flame-retardant actions. In the condensed phase, a continuous and compact expanded char layer with better sealing ability and barrier performance was formed. In the gas phase, nonflammable gas release occurred, which helped reduce the concentration of combustible volatiles. Hence, the ethanol-assisted pre-assembled gCNFS is an effective strategy for improving the flame-retardant efficiency of PLA/APP/lignin composites.