Design lignin doped with nitrogen and phosphorus for flame retardant phenolic foam materials

Developing lignin-based flame retardants for phenolic foam with exceptional flame retardancy and mechanical performances remains a huge challenge due to complicated chemical structure and unstable chemical activity of lignin. In this study, nitrogen‑phosphorus doped lignin flame retardant (NP-LMDL) was prepared by modifying reactive lignin with 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and triethylenetetramine (TETA), which was utilized to partially replace phenol to prepare flame retardant phenolic foams (NP-LMDLPFx). The results showed that nitrogen‑phosphorus doped lignin enhanced the flame retardant performance and compressive strength of the foam. The limiting oxygen index (LOI) and compressive strength of NP-LMDLPF15 increased by 33.3% and 43.7% comparing with pure phenolic foam (PF). The peak heat release rate (pHRR) and peak smoke release rate (pRSR) decreased by 48.0% and 53.2%. Moreover, the structure and elemental constitution of char residues were analyzed to discuss the flame retardant mechanism of nitrogen and phosphorus in foam. Nitrogen and phosphorus in NP-LMDLPFx worked together to promote polymers carbonization and generate rich nitrogen and phosphorus char residue layer, insulate heat and oxygen to play the role of flame retardant.