Flame-retardant effect of hyperbranched phosphazene-based microspheres in poly(L-lactic acid)

A hyperbranched and amorphous microsphere (PZM) was synthesized with the hexachlorocyclotriphosphazene (HCCP) as the component unit. The PZM and ammonium polyphosphate (APP) as a synergistic flame retardant were incorporated into the poly(L-lactic acid) (PLA) to investigate the effects of the APP/PZM on the thermal stability, limiting oxygen index (LOI), UL-94 rating, combustion behavior, flame-retardant performance and crystallization behavior of the PLA. The PZM showed a much higher thermal stability and carbon residue than the HCCP, APP and 4,4'-diaminodiphenyl ether (ODA), but presented a lower initial degradation temperature. The PLA/10%APP/5%PZM exhibited the most excellent flame-retardant performance because it showed low total heat release, peak of heat release and total smoke release. By calculation, the PLA/10%APP/5%PZM presented the outstanding fire resistance. The PLA/10%APP/5%PZM formed a continuous, dense and relatively thicker char layer (which facilitated the isolation of the oxygen and heat flow, suppression of the smoke and prevention of the dripping) after the cone calorimeter (CONE) test, indicating that it showed a condensed-phase flame-retardant performance. In addition, the gas-phase flame retardance is a minor factor to enhance its flame-retardant effect. The crystallizability increased, and crystallization time shortened substantially for the PLA with loading of the 10%APP/5%PZM. Graphical abstract