Preparation of a nitrogen and phosphorus containing lignin-based flame retardant using oxidized kraft lignin and its application in phenolic foam

In order to improve the flame retardancy of phenolic foam (PF), it is a feasible method to add flame retardants. Due to the similar structure of lignin to phenol, this provides the possibility of developing lignin-based flame retardants to partially replace phenol. In this work, we used kraft lignin as the raw material and prepared oxidized kraft lignin (OL) through the oxidation reaction to increase the carbonyl content in the side chain. Then, a nitrogen and phosphorus lignin-based flame retardant (NP-OL) was prepared by chemical grafting of the side chain with 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and pentaethylenehexamine (PEHM), and then nitrogen and phosphorus modified phenolic foam (NPOLPFX) was prepared by partially replacing phenol with NP-OL. The lignin-based flame retardant was then utilized to partially replace phenol for preparing modified phenolic foam. The addition of NP-OL could effectively improve the limiting oxygen index (LOI) and compressive strength of phenolic foam. The LOI and compressive strength of modified phenolic foam (NPOLPF5) with a 5% replacement rate were much higher than those of pure phenolic foam. The cone calorimeter data also showed that NPOLPF5 had good flame retardancy, with significantly lower peak and total heat release rates than PF. This work provides a novel strategy for improving the flame retardancy of phenolic foam and promoting the utilization of lignin.