A novel phosphorus-containing lignin-based flame retardant and its application in polyurethane

A novel lignin-based flame retardant (LMD) is synthesized in this work via the in-situ reactions of lignin, diphenylmethane diisocyanate (MDI), and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO). Flame-retardant polyurethane (FLPU) is prepared by the co-curing of LMD and prepolymer synthesized by lignin, MDI, and polyether polyol 204. Polyethylene glycol (PEG) 2000 is also added to adjust the flexibility of the lignin-based polyurethane (LPU). The limiting oxygen index (LOI) value of the FLPU gradually increases with the LMD content and can reach 29.9% for F30-L20-L15-80PU (L20MD content is 30% in L15-80PU), exhibiting excellent flame-retardant properties. The hardness of the FLPU for the optimized formulation is 2H, the adhesion is 0, and the flexibility is 2 mm. And the impact resistance is greater than 100 cm/1000 g. These outstanding properties illustrate the LMD is excellent flame retardants and reinforcing materials for LPU. The interface can be strengthened by the co-curing between the LMD and the LPU prepolymer.