Abstract: Phosphating guar gum （DTGG） was obtained through the modification of guar gum （GG） with 9，10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide （DOPO） and 1，2，3，6-tetrahydrophthalic anhydride （THPA）， and its chemical structure was determined using Fourier-transform infrared spectroscopy and nuclear magnetic resonance spectroscopy. Flame-retardant poly（lactic acid） （PLA） blends were prepared through melt-blending PLA with DTGG as a carbon source and ammonium polyphosphate （APP） as acid and gas sources. The flame retardant properties of the resulting blends were investigated through vertical burning experiments （UL 94）， limiting oxygen index （LOI）， and the cone calorimeter tests， and their thermal properties and morphology were characterized through thermogravimetric analysis and field-emission scanning electron microscopy （FESEM）. The results indicated that the blends obtained an LOI value of 33.4 vol% at a DTGG/APP mass ratio of 9/11 and achieved a UL 94 V-0 classification in the vertical burning experiment. Meanwhile， the blends showed the lowest total heat release （THR） value， which decreased by 33.8% compared to that of pure PLA. The uniform distribution of DTGG and APP in the PLA matrix was found from the FESEM observation. The synergistic effect of DTGG and APP effectively enhanced the flame retardancy of PLA. The synergistic mechanism of DTGG and APP for the flame retardancy of PLA was also proposed.

Key words: poly（lactic acid）, guar gum, ammonium polyphosphate, flame retardancy