Toughening, highly thermostable, and flame retardant polylactic acid enabled by polyphosphazene microsphere

Abstract To obtain superior flame retardant poly(lactic acid) (PLA), a highly cross‐linked poly (cyclotriphosphazeneco4,4′diaminodiphenyl sulfone) (CP‐DDS) microsphere was synthesized by the precipitation polymerization between hexachlorocyclotriphosphazene (HCCP) and 4,4′diaminodiphenyl sulfone (DDS). Thanks to the cooperative effect of flame retardant elements including P, N and S, this as‐synthesized CP‐DDS microsphere presented excellent fire resistance. The limiting oxygen index (LOI) value of the PLA containing 7 wt% CP‐DDS microsphere (PLA/FR7.0) was dramatically increased to 26.8% from 19.7%. Moreover, the dripping of PLA/FR7.0 sharply decrease and it passed V‐1 UL‐94 rating in the vertical combustion test. Cone calorimetry results presented that PLA/FR7.0 composites resulted in 20.85% and 23.02% reductions in the total heat release (THR) and the peak heat release rate (PHRR) respectively. Besides, the average effective heat combustion (av‐EHC) of PLA/FR7.0 composites is 14.24% lower than that of PLA. Surprisingly, the elongation at break of PLA/FR7.0 composites was enhanced by 40%, hardly without loss of the mechanical strength. Possible flame retardant mechanism from gas phase and condensed phase was proposed. The fire‐retardant materials prepared by this toughening microsphere are sufficient to cope with the challenges brought by complex environments, which are expected to be applied in more fields.