Enhancement of Flame Retardant Performance of Bio-Based Polylactic Acid Composites with the Incorporation of Aluminum Hypophosphite and Expanded Graphite

A series of flame retarding biodegradable polylactic acid composites (FR-PLA) based on aluminum hypophosphite (AHP) and expanded graphite (EG) were prepared by the melt-compounding method. The effect of AHP and EG on the flame retardancy and thermal stability of PLA were investigated by thermogravimetric analysis (TGA), UL-94 vertical burning test, limiting oxygen index, microscale combustion calorimetry and cone calorimeter testing. The gas products of PLA and FR-PLA composites were investigated by TG-IR technology. TGA results showed that the FR-PLA composites presented higher char residue and reduced mass loss rate than neat PLA. Significant improvement of flame retardancy of PLA/AHP/EG composites indicated an excellent synergistic effect between AHP and EG. Cone calorimeter tests gave clear evidence that the combination of AHP and EG resulted in a significant reduction of the heat release rate (HRR) and total heat release (THR). TG-IR revealed that the combination of AHP and EG resulted in significant decrease of gas products. The char residue of FR-PLA composites were also investigated by scanning electron microscope, Fourier transform infrared spectra spectroscopy and X-ray photoelectron spectroscopy. Based on the analysis, possible flame retardant mechanism was discussed.