One-Pot, Solvent- and Catalyst-Free Synthesis of Polyphosphoramide as an Eco-Benign and Effective Flame Retardant for Poly(lactic acid)

Despite many merits, biobased and biodegradable poly(lactic acid) (PLA) suffers from inherently high flammability, significantly impeding its practical applications in the areas of packaging, fibers, electric and electronics, etc. Phosphoramides-based flame retardants have demonstrated exceptional flame retardancy in PLA only at very low loading levels. Unfortunately, exiting synthetic processes of phosphoramides often involve the use of a huge amount of organic solvents, presenting a potential threat to the ecosystem and human beings, in addition to high production costs. Herein, we report a one-pot, solvent- and catalyst-free, and green approach for the synthesis of a polyphosphoramide (DM-H) via two-step polycondensation between dimethyl methylphosphonate (DMMP) and 1,6-hexanediamine (HDA). As-synthesized DM-H exhibits exceptional flame-retardant efficiency in the PLA while preserving the mechanical properties of the PLA. The addition of only 2 wt % DM-H enables the PLA to achieve a desirable V-0 rating during vertical burning tests and a high limited oxygen index (LOI) value of 29.7%, because the DM-H can act both in the gas and condensed phases. Moreover, the tensile strength is of 59.7 MPa comparable to 62.1 MPa of the bulk PLA. This work offers an innovative and green approach for the synthesis of polyphosphoramides to create high-performance sustainable PLA materials with balanced flame retardancy and mechanical performances.