Synergistic effect of flame retardants and graphitic carbon nitride on flame retardancy of polylactide composites

In order to improve the flame retardant of polylactide (PLA), the synergistic effect of graphitic carbon nitride (g-C₃N₄) with commercial-available flame retardants melamine pyrophosphate (MPP) and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) was investigated. The PLA composites containing 5 wt% g-C₃N₄ and 10 wt% DOPO had a highest limited oxygen index (LOI) value of 29.5% and reached the V-0 rating of UL-94 test. The cone calorimeter tests exhibited that DOPO had a better synergistic effect with g-C₃N₄ than MPP to improve flame retardancy of PLA. The peak heat release rate (pHRR) and total heat release (THR) of PLA composites containing 10 wt% DOPO could be reduced by 25.2% and 23.6%, respectively, as compared with those of pure PLA. The presence of rich phosphorus element and aromatic groups in DOPO contributed to obtain continuous compact char layer and increase the graphitization level of char residues, thereby, resulting in improving the flame retardancy of PLA together with g-C₃N₄. In addition, the incorporation of DOPO can serve as a plasticizer to reduce the complex viscosity, improving the processability of PLA composites.