Construction of environmentally friendly multifunctional core-shell flame retardant and its application in flame retardant polylactic acid.

The flammability and poor ultraviolet (UV) aging resistance of polylactic acid (PLA) limit its applications outdoors and in fields requiring flame retardancy. To address these limitations, this study designed ammonium polyphosphate (APP) as the core, the biopolymer chitosan (CS) as the inner shell, and lignin (LK) as the outer shell. CS and LK are deposited on the surface of APP via electrostatic interaction in the aqueous phase to prepare a core-shell structure flame retardant APP@CS@LK with anti-UV aging properties. The resulting PLA/APP@CS@LK composite achieved a UL-94 V-0 rating in vertical burning tests. The limiting oxygen index (LOI) of neat PLA was 19.5 %, which increased to 30.7 % upon incorporation of 10 wt% APP@CS@LK. Furthermore, cone calorimeter data revealed that compared to pure PLA, the peak heat release rate (PHRR) of PLA/APP@CS@LK was reduced by 23.5 %, while CO₂ and CO emissions were reduced by 31 % and 74 %, respectively. Simultaneously, the phenolic hydroxyl groups in LK act as free radical scavengers, imparting exceptional UV resistance. The composite blocked nearly 99 % of UV light (UPF 64.8). This study provides a safe and green preparation method of multifunctional flame retardant, which can be used to prepare flame retardant PLA composites with anti-ultraviolet aging property, thereby expanding the potential applications of PLA.