Functionalized Soybean Oil as a Bio-based Flame Retardant for Poly(lactic acid): Role of Phosphorus Content

Bio-based phosphorus flame retardants offer significant promise in enhancing the flame retardancy of bioplastics without compromising their environmental friendliness. In this study, a bio-based phosphorus-based flame retardant (PFR) synthesized from epoxidized soybean oil polyol (PESO) was used to enhance the flame-retardant properties of poly(lactic acid) (PLA), aiming to establish the correlation between phosphorus content and the fire behavior of PLA composites. Notably, the study determined that the critical phosphorus content value for improving the PLA rating from NR to V-0 was as low as 0.1 wt % in the composites, with the limited oxygen index (LOI) of PLA increasing from 21.0% to 25.2%. Meanwhile, the introduction of PESO with high phosphorus contents decreased the peak heat release rate (PHRR) and total heat release (THR) of PLA by 5.33% and 9.66%, respectively. Nevertheless, with the complete replacement of the epoxy group by phosphorus ring-opening, PESO loses its plasticizing effect, making it challenging to enhance the elongation at the break of PLA. Finally, the flame-retardant mechanism of PLA/PESO composites was comprehensively analyzed. It was found that the mechanism was also related to the phosphorus contents; high phosphorus was not favorable for the char forming in the condensed phase but promoting the melt dripping and formation of PO^•, PO₂^•, and HPO^• which could trap radicals in the gas phase. This study provides valuable insights into designing PFRs from renewable resources for PLA.