The design of flame-retardant biocomposites based on biobased flame retardants (FRs) represents a promising direction for creating a sustainable world. To date, it remains a major challenge to explore a green and scalable strategy for the design of highly effective, biobased FRs for bioplastics, such as polylactic acid (PLA). Herein, we have demonstrated a green, facile fabrication approach for a core–shell-structured biobased flame retardant (APP@CS@PA-Na) via layer-by-layer assembly using water as the assembly media. With electrostatic interactions, APP@CS@PA-Na was prepared by sequential assembly of ammonium polyphosphate (APP) with positively charged chitosan (CS) and then negatively charged phytic acid salt (PA-Na). The addition of APP@CS@PA-Na can enhance both the flame retardancy and the toughness of PLA. With the addition of 10 wt % APP@CS@PA-Na, the resultant PLA composite can pass an UL-94 V-0 rating and meanwhile shows an increased elongation at break by 28.4%, compared with that of neat PLA (8.1%). Through the analysis of the volatile gases and the residues, the flame retardant mechanism of APP@CS@PA-Na in PLA plays the key role in the condensed-phase. This work will broaden the practical application field of PLA, such as in electric and electronic and fibers fields.

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.