Combustion and pyrolysis behaviors of flame retardant poly (lactic acid) composites containing phosphaphenanthrene and maleimide molecules

To improve the flame retardant performance of poly (lactic acid) (PLA), a phosphaphenanthrene (DOPO) -maleimide derivative (DNPMI) was synthesized. DNPMI based PLA composites were investigated for their flame retardant, thermal and crystalline properties. Moreover, the combustion and pyrolysis behaviors of PLA composites were investigated detailly. The results indicated that the introduction of DNPMI with DOPO and maleimide group in the molecular structure effectively improved the fire safety of PLA composites accompanied with good crystallization ability and transparency properties. With the addition of 7.5 wt% DNPMI, PLA composites reached to UL-94 V-0 grade with a high LOI value of 30.8 %. The excellent flame retardancy performance of PLA/DNPMI composite materials is mainly ascribed to the interplay of quenching and dilution effects in the gaseous phase. The maleimide, and phosphorus-containing radicals generated by the pyrolysis of DNPMI captured the chain radicals and the decomposed inert gases diluted the concentration of the combustion components of PLA. In addition, heat-resistant aromatic structures were generated by the reaction of maleimide radicals and reactive radicals from decomposition of PLA, thereby playing "cage effect" to inhibit the combustion process.