Vanillin-derived bio-based Schiff base for simultaneously enhancing flame retardancy, crystallizability and UV-shielding of poly(L-lactic acid)

Poly(L-lactic acid) (PLA) is a biodegradable, biocompatible, and mechanically strong biomacromolecule with broad applicability across multiple areas. Nevertheless, its high flammability, tendency to melt-drip during burning, and inherently low crystallization rate limit its practical use. To address these issues, a halogen-free bio-based Schiff base flame retardant (VDPC) derived from vanillin and containing nitrogen and phosphorus was synthesized. With the incorporation of 10 % VDPC into the PLA, the composite achieved a limiting oxygen index (LOI) of 31.6 %, representing an increase of 56.4 %, and attained a V-0 rating in the UL-94 vertical burning test without dripping. Additionally, the peak heat release rate (pHRR), total heat release (THR), and carbon dioxide production (CO 2 P) reduced by 15.9 %, 36.8 %, and 29.7 %, respectively, confirming significantly enhanced fire safety. These improvements are attributed to synergistic flame-retardant mechanisms operating in both the condensed phase–through the formation of a relatively continuous and dense char layer–and the gas phase–via radical quenching by phosphorus–oxygen species. Furthermore, the composite showed accelerated crystallization rate, exceptional UV-blocking performance, and improved hydrophobicity. This study presents a sustainable and multifunctional flame-retardant system and demonstrates a viable approach to extending the utility of PLA-based materials in demanding applications.