Bio-based flame-retardant plasticizers derived from l-lactic acid: Effects of valence states of phosphorus structure on fire safety, flexibility and transparency of poly(lactic acid)

Bio-based multi-functional additives can not only reduce the types and addition of additives, but also prevent damage caused by leaching during the use of plastic products. In this study, three bio-based flame-retardant plasticizers (PBL-1, PBL-3, and PBL-5) derived from l-lactic acid and phosphorus-containing monomers with different phosphorus valence states were synthesized by two-steps. The effect of three additives on the flame retardancy, mechanical properties, crystallization, and optical transparency of poly(lactic acid) (PLA) resin was investigated. The findings indicated that the fire resistance of PLA exhibited differences with the variation of phosphorus valence states of additives. The lower phosphorus valence states of additives generate more phosphorus-containing free radicals to capture H• and OH• free radicals in the gas phase; By contrast, the higher phosphorus valence states of additives produce more phosphorus-containing substances, which facilitate to the formation of char layer in the condensed phase. Meanwhile, the incorporation of three additives significantly mitigated the peak heat release rate, total heat release, average rate of heat emission, fire propagation index, and flame retardancy index of composites. As the addition of three additives was 20 phr, all composites reached UL-94 V-0 rating, and their LOI values exceeded 30.0%. Additionally, the long alkyl ether chains and significant lactate groups present in the three additives chains confer the excellent flexibility of PLA, and their elongation at break reached about 200%-350%. Interestingly, three additives effectively enhance the crystallization of PLA and maintain excellent optical transparency of their composites. It can be predicted that three additives have a potential as an eco-friendly option for future green electronic products.