Flame retardant effect and mechanism of nanosized NiO as synergist in PLA/APP/CSi-MCA composites

Intumescent flame retardants (IFRs) are widely used in preparing functional polymer composites, but it is still a challenge to improve their flame retardant efficiency. Herein nanosized nickel oxide (NiO) as synergist was incorporated into poly(L-lactic acid)/ammonium polyphosphate/silicone-containing macromolecular charring agent (PLA/APP/CSi-MCA) composites with the aim of improving their flame retardancy. According to limited oxygen index (LOI), vertical burning (UL-94) and cone calorimetry test results, the obtained PLA composites showed a LOI value as high as 33.0%, V0 ratting in UL-94 and 63.1% reduction for PHRR with the incorporation of 1 wt% NiO. The flame retardancy mechanism was also discussed by analyzing residual char structure and the composition of the liquid degradation products. It was found that the carbon layer contained two types of chars: APP and CSi-MCA could form char 1 via esterification and cross-linking reaction, while the PLA matrix could produce char 2 via the catalyzing carbonization of CSi-MCA and NiO to generate MWCNTs. The combination of char 1 and char 2 with better barrier effect resulted in the improvement on flame retardancy of PLA composites.