Engineering phosphorus-containing lignin for epoxy biocomposites with enhanced thermal stability, fire retardancy and mechanical properties

Fabricating a high-performing thermoset using bio-based flame retardant is critical for the sustainable development of engineering materials with superior fire safety and robust mechanical properties . Herein, the epoxy (EP) composites with the industrial requirements are manufactured with a novel high-efficient, lignin-based flame retardant named DAL- x , which is fabricated by grafting 9, 10-dihydro-9-oxa-10-phosphaze-10-oxide (DOPO) onto lignin. The resulting DAL- x /EP composite exhibits excellent flame retardancy with a desirable UL-94 V-0 rating and a satisfactory limiting oxygen index (LOI) of 29.8% due to the appropriate phosphorus content of DAL- x with adjustable molecular chain structure. Moreover, the DAL- x /EP composite shows an unexpected improvement in the elastic modulus (∼36%) and well-preserved strength and ductility compared with those of pure EP. This work offers a feasible strategy for creating efficient bio-based flame retardants utilizing industrial waste lignin and preparing high-performance EP composites that meet the demanding requirement of fire retardancy in industries, contributing to the circular economy and sustainability.