Improving Flame-Retardant Properties of Poly (Lactic Acid) Composites Using Maleic Anhydride-Modified DOPO and SiO2

Polylactic acid (PLA) is well regarded for its excellent environmental friendliness; however, its flammability presents a significant barrier to wider applications. Developing highly flame-retardant PLA (with a limiting oxygen index, LOI, greater than 30%) using simple, general methods remains a challenge. In this study, maleic anhydride (MA) was used to modify 9,10-dihydro-9-oxa-10-oxide (DOPO), enhancing its thermal stability and interaction with the PLA matrix. Additionally, by leveraging the carbonization effect of nano-SiO2, DOPO-MA was compounded with nanosilica to further improve PLA's flame-retardant properties. As a result, a highly flame-retardant PLA with an LOI of 31.3% and a UL-94 V-0 rating was successfully prepared using 5 wt.% DOPO-MA and 3 wt.% SiO2. Furthermore, total heat release (THR) and peak heat release rates (pHRR) of PLA decreased by 7% and 25%, respectively. Detailed analysis of the flame-retardant mechanism revealed that nano-SiO2 significantly increased the residual char content and promoted the formation of a more stable graphite carbon structure. Moreover, silica effectively suppressed the generation of combustible gases in the pyrolysis products of PLA, collectively enhancing its flame-retardant properties. This study presents a straightforward and scalable approach for designing highly efficient phosphorus/silica-containing synergistic flame retardants, offering valuable insights into PLA flame retardancy.