Enhanced flame retardancy of poly(lactic acid) with ultra-low loading of ammonium polyphosphate

We have developed a flame retardant poly(lactic acid) (PLA) composite with the addition of only 2 wt% of ammonium polyphosphate (APP) and 0.12 wt% of resorcinol bis(diphenyl phosphate) (RDP), which achieved the V-0 rating of UL-94 test and could be drawn into filaments for Fused Deposition Modeling (FDM) 3D printing. The composite was designed by analyzing the work of adhesion (W_a) between RDP/PLA, RDP/APP and PLA/APP, where RDP was found to act as the compatibilizer to improve the dispersion of APP particles within PLA matrix, resulting in enhanced flame retardant efficiency. These results were confirmed by thermal imaging of the UL-94 burning process and cone calorimetry. Scanning electron microscopy imaging and EDS mapping also confirmed the better dispersion of APP particles and the reduced particle size with the addition of RDP. Furthermore, with RDP localized on the PLA/APP interface, the relatively high W_a between RDP/PLA and RDP/APP minimized the negative effect of APP and RDP on mechanical properties. Impact and tensile testing showed that the PLA/APP/RDP composite has mechanical properties comparable to those of neat PLA, which enabled the composite to be drawn into thin filaments for 3D printing application.