A molecularly engineered bioderived polyphosphate for enhanced flame retardant, UV-blocking and mechanical properties of poly(lactic acid)

Polylactic acid (PLA) represents one of most promising bioplastics in response to the increasing global micro plastics pollution issue facing the entire ecosystem. Nevertheless, PLA is inherently flammable and also prone to UV-light attack, significantly restricting its wide applications in packaging, fibers and electronics industry. Despite high effectiveness in PLA, existing polyphosphate-based flame retardants are usually synthesized mainly from non-degradable petrochemical compounds and their functions are limited to improving flame retardancy of the polymeric matrix. We, herein, report the synthesis of a novel multifunctional bioderived polyphosphate (PPD) for PLA by using plant-derived diphenolic acid as a starting material. Our results reveal that the addition of 6 wt% of PPD enables PLA to achieve a desirable UL-94V-0 rating and a high LOI value of 27.1% due to its high gas-phase activity, which means that the final PLA fully meets the demanding flame retardancy requirement in industry. Moreover, the resultant flame-retardant PLA shows a high ultraviolet protection factor (UPF) value of 403, which indicates its excellent UV protection capability. Because of the fine and uniform dispersion of PPD within the resin matrix and their strong interfacial interactions, the PLA/6% PPD shows a 31% increase in impact toughness and a slight improvement in tensile strength compared to the bulk PLA. This work provides a strategy to synthesize a multifunctional bio-based polyphosphate flame retardant, which holds great promise for extending the practical applications of PLA with excellent flame retardancy, UV-blocking and mechanical performances.