Biobased Flame Retardants\nfrom Lactic Acid Ester:\nRoles of Alkyl Chain Length in Flame Retardancy and Mechanical Properties\nof Poly(lactic acid)

To enhance the fire resistance and mechanical properties\nof poly(lactic\nacid) (PLA), two biobased multifunctional flame retardants (HMLCP\nand HBLCP) were synthesized using methyl lactate, butyl lactate, and\nhexachlorocyclotriphosphazene. It was determined that HMLCP with short\nalkyl lactate shows stronger flame retardancy and reactivity but a\nweaker plasticizing effect on PLA and can better balance the flame\nretardancy and mechanical properties of PLA. The addition of 3 wt\n% HMLCP results in PLA achieving a UL-94 V0 rating and a limiting\noxygen index (LOI) of 26.6 vol %. On the other hand, PLA will require\n7 wt % HBLCP to obtain a V0 rating and 26.4 vol % LOI. The enhanced\nflame retardancy is primarily attributed to the self-extinguishing\ngas-phase effect of the phosphazene structure and the heightened melting-away\neffect induced by the lactic acid ester. HMLCP also displays a greater\ninclination to undergo transesterification with PLA, but both HMLCP\nand HBLCP are hard to leach after a long-time reaction with PLA. Both\nHMLCP and HBLCP slow the crystallization rate of PLA blends while\nenhancing the crystallinity. The results of this study have positive\nimplications for the future development of multifunctional biobased\nadditives for PLA, thereby expanding the range of applications for\nPLA.