Scalable Manufacturing Green Core–Shell Structure Flame Retardant, with Enhanced Mechanical and Flame-Retardant Performances of Polylactic Acid

As a plastic that can be prepared from biomass resources, polylactic acid (PLA) has the characteristics of being degradable and renewable, and has shown great potential in replacing petroleum-based polymer materials. However, PLA material is flammable, the droplet phenomenon during combustion is still a disadvantage of PLA applications. In this research, a core–shell structure flame retardant was prepared based on polyacrylamide (PAM)/sodium alginate (SA) coated ammonium polyphosphate (APP) via layer-by-layer assembly using water as the assembly medium. The prepared core–shell flame retardant APP@PAM@SA-nDL (where “DL” is the PAM&SA double layer, and “n” represents 1-3DL) was molten compounded with PLA to improve its flame retardancy. APP@PAM@SA-nDL is added to PLA, which not only improves the flame retardancy of PLA composites, but also significantly improves the toughness, and the flame retardancy, flame retardant mechanism, mechanical performance and toughening mechanism of PLA composites were systematically studied. Among the core–shell flame retardants with different layers, the addition of APP@PAM@SA-3DL gives PLA composites excellent flame retardancy and tensile properties. Due to the formation of the cross-linked polymer network structure, 5% APP@PAM@SA-3DL can increase the elongation at break of PLA composites from 19.82 to 294.5%. In addition, 10% of APP@PAM@SA-3DL made the UL-94 of PLA composite materials reach V-0 level and increased the LOI from 19.7% (neat PLA) to 29.2%. The analysis of the carbon layer shows that the excellent flame-retardant performance of PLA composites can be attributed to the formation of a dense carbon layer during condensation and the dilution effect of P-containing free radicals, forming a flame-retardant mechanism with a condensed phase as the main component and a gas phase as a supplement. This high toughness flame-retardant PLA composite material has broad application prospects in the textile field. Graphical Abstract