Constructing Novel Biobased Phosphorus-Containing Oligomeric Lactates toward Synchronously Enhancing the Toughness and Fire Safety of Poly(lactic acid)

Simultaneously improving the flame retardancy and toughness of polylactic acid (PLA) remains a huge challenge. In this work, we report a simple method for synthesizing PLA blended with biobased phosphorus-containing oligomeric lactates. Four phosphorus-containing oligomeric lactates (PEL, PBL, PHL, and POL) were synthesized with l-lactic acid, aliphatic dihydric alcohols (ethylene glycol, butane-1,4-diol, 1,6-hexanediol, or 1,8-octanediol), and phenyl phosphorodichloridate via esterification and acylation reactions. Accordingly, flame-retardant and ultratough PLA composites can be achieved by direct blending with phosphorus-containing oligomeric lactates and commercially available plasticizer acetyl tributyl citrate (ATBC). The results exhibited that the toughness of the PLA composites was obviously enhanced on adding 10 phr oligomeric lactates and 10 phr ATBC, and its elongation at break reached about 550–670%, indicating their ultratoughness. This is ascribed to the fact that after the ATBC is diluted, the oligomeric lactates can be fully dispersed in the PLA matrix so as to reduce the intermolecular force of PLA and enhance its toughness. Simultaneously, the fire resistance of the composites was enhanced distinctly. The phenyl phosphoro group of the oligomeric lactate structure can enhance the fire resistance of PLA, which is conducive to the capture of radicals in the gas phase and the formation of residual char in the condensed phase. All composites reached the UL-94 V-0 rating. Besides, the limited oxygen index of PLA increased from 20.0 to 34.8–41.0% with the addition of 20 phr oligomeric lactates. Additionally, both the heat release rate and the total smoke release of composites were effectively reduced. Intriguingly, four phosphorus-containing oligomeric lactates could improve the crystallization of composites and retain their outstanding transparency. This work offers a new method for manufacturing green flame-retardant, ultratough, and transparent PLA composites.