Flame retardant and anti-dripping properties of polylactic acid/poly(bis(phenoxy)phosphazene)/expandable graphite composite and its flame retardant mechanism

Flame retardant polylactic acid (PLA) composites with poly(bis(phenoxy)-phosphazene) and expandable graphite are prepared by melt blending. Limiting oxygen index, UL-94 vertical burning test, cone calorimeter and thermogravimetric analysis are applied to characterize the flame retardant properties and thermal stability of PLA composites. This flame retardant system shows improved thermal stability, flame retardancy, synergy effect and anti-dripping performance. Raman spectroscopy, X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy are employed to investigate the chemical structure and composition of the residual char of flame retardant PLA composites after cone calorimeter tests. The residual char is composed of graphite and phosphorus-containing materials. The gaseous phase mechanism of the flame retardant system is investigated with thermogravimetry/Fourier transform infrared spectroscopy and mass spectrometry. The radical species, such as C6H5OP center dot, C6H5O center dot and PO2 center dot, are detected in the gaseous products of PLA composites. Thus, the flame retardant system exhibits both condensed and gas phase flame retardant action in the PLA composites.