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.