Biobased Flame Retardants from Lactic Acid Ester: Roles of Alkyl Chain Length in Flame Retardancy and Mechanical Properties of Poly(lactic acid)

To enhance the fire resistance and mechanical properties of poly(lactic acid) (PLA), two biobased multifunctional flame retardants (HMLCP and HBLCP) were synthesized using methyl lactate, butyl lactate, and hexachlorocyclotriphosphazene. It was determined that HMLCP with short alkyl lactate shows stronger flame retardancy and reactivity but a weaker plasticizing effect on PLA and can better balance the flame retardancy and mechanical properties of PLA. The addition of 3 wt % HMLCP results in PLA achieving a UL-94 V0 rating and a limiting oxygen index (LOI) of 26.6 vol %. On the other hand, PLA will require 7 wt % HBLCP to obtain a V0 rating and 26.4 vol % LOI. The enhanced flame retardancy is primarily attributed to the self-extinguishing gas-phase effect of the phosphazene structure and the heightened melting-away effect induced by the lactic acid ester. HMLCP also displays a greater inclination to undergo transesterification with PLA, but both HMLCP and HBLCP are hard to leach after a long-time reaction with PLA. Both HMLCP and HBLCP slow the crystallization rate of PLA blends while enhancing the crystallinity. The results of this study have positive implications for the future development of multifunctional biobased additives for PLA, thereby expanding the range of applications for PLA.