Core–Shell Bioderived Flame Retardants Based on Chitosan/Alginate Coated Ammonia Polyphosphate for Enhancing Flame Retardancy of Polylactic Acid

Bioderived flame retardants represent one class of the most promising sustainable additives for creating flame retardant polylactic acid (PLA) because of their marginal impact on the biodegradability of PLA. Ammonium polyphosphate (APP) has demonstrated high flame-retardant effectiveness in PLA but its flame-retardant efficiency remains unsatisfactory even if after modifications. Herein, we report the facile fabrication of core–shell bioderived flame retardants by using APP as the core, and the chitosan (CS)/alginate (AA) bilayer as the shell through self-assembly in aqueous solution. The resultant core–shell flame retardant, APP@CS@AA-nBL (where "BL" is a CS&AA bilayer and "n" denotes 1–3 BL), can endow PLA with improved flame retardancy without negatively affecting the thermal properties. The PLA containing 10 wt % APP@CS@AA-3BL shows the highest LOI value (30.6%) and achieves a UL94 V-0 rating in the vertical burning test. Meanwhile, the cone calorimetry results demonstrate that the peak of the heat release rate and total heat release are respectively decreased by 23% and 11% relative to the PLA bulk. Such enhanced flame retardancy is mainly due to the excellent char-forming capability of APP@CS@AA. Moreover, the inclusion of 10 wt % APP@CS@AA-3BL gives rise to ∼23% increase in the impact strength of PLA possibly because of their interfacial hydrogen-bonding interactions. This work provides a facile and green strategy for preparing highly effective bioderived flame retardants for PLA and thus is expected to expand the practical applications in industry.