Functionalizing lignin by in situ solid-phase grafting ammonium polyphosphate for enhancing thermal, flame-retardant, mechanical, and UV-resistant properties of polylactic acid

Polylactic acid (PLA) exhibits intrinsic high flammability, inadequate toughness and UV resistance, which impedes its adoption into novel industry sectors. Although lignin-based fire retardants have shown high efficiency in PLA, their poor interfacial compatibility with PLA deteriorates mechanical properties and the preparation often involves use of plethora of organic solvents. Herein, a sustainable in situ solid-phase polycondensation strategy has been used to prepare a lignin-based fire retardant, Lig-APP, by grafting ammonium polyphosphate onto lignin without using any organic solvent. As-synthesized Lig-APP has shown an increased charring ability and excellent fire retardancy in PLA. With only 3 wt% loading of Lig-APP, PLA achieved a UL94 V-0 rating and a limiting oxygen index (LOI) of 27.6 %. Moreover, Lig-APP simultaneously strengthened and toughened PLA while increasing the UV resistance of PLA. This work opens an innovative eco-benign avenue to create biobased fire retardants and high-performance PLA biocomposites.