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

To obtain superior flame retardant poly(lactic acid) (PLA), a highly cross-linked poly (cyclotriphosphazene-co-4,4 '-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.