An effective multi-hydroxy-containing ammonium phosphate towards flame-retarding poly(lactic acid): Flame retardance, thermal and pyrolysis behaviors

To retard the flammability of poly(lactic acid) (PLA), a multi-hydroxy-containing ammonium phosphate (DPPT) derived from 2-chloro-5,5-dimethyl-1,3,2-dioxaphosphinane-2-oxide (DOP) and trihydroxymethyl aminomethane (TRIS) through a one-pot reaction was introduced. By comparison with PLA, the limiting oxygen index (LOI) of PLA modified with 2.5% DPPT increased from 19.5% to 32.7%, and UL-94 vertical rating was improved from no rating to UL-94 V-0 rating. Besides, the results of cone calorimetric tests (CC) showed that DPPT could suppress the heat release of PLA during combustion to some extent. Although the thermal behaviors of PLA were affected by DPPT, DPPT could promote the degree of crystallinity of PLA. According to the relevant results as well as the analyses of thermogravimetric analysis coupled with fourier transform infrared spectroscopy (TG-FTIR) and pyrolysis-gas chromatograph/mass spectrometer (py-GC/MS), the flame retardant mechanism of DPPT was also disclosed that it could decompose to generate the incombustible compounds such as water and ammonia, and simultaneously release phosphorus-containing free radicals; Besides, the removal of nonflammable dripping could also bring away heat and cut off the heat-transfer to the matrix.