Flame-Retardant Properties of Nitrogen–Phosphorus Modified Lignin-Based Polyurethane Foams

Abstract As one of the most widely used polyurethane materials, the broader application and development of polyurethane foams are restricted due to their flammability. To enhance the flame-retardant performance of rigid polyurethane foam, in this work, nitrogen-modified lignosulfonate flame retardants (LGBF, LGMF), nitrogen–phosphorus-modified lignosulfonate flame retardants (LGBFP, LGMFP) were prepared and used as raw materials to synthesize the corresponding reactive-type rigid polyurethane foams (LGBF-PU, LGMF-PU, LGBFP-PU, and LGMFP-PU). Meanwhile, the nitrogen and phosphorus compounds as fillers were added to the foams to synthesize the additive-type foams. A series of characterizations of foams was conducted. The results showed that the reactive-type rigid polyurethane foams indicated much preferable mechanical properties and thermal stability, as well as a smaller pore size. Self-ignition behavior analysis and limiting oxygen index test show that LGMF-PU, LGBFP-PU, and LGMFP-PU produce self-extinguishing phenomena under the first single ignition source, and form a uniform and dense carbon layer with greater thickness and higher strength after combustion, demonstrating better flame-retardant effects. The results of cone calorimetry tests showed that LGBF-PU, LGBFP-PU, and LGMFP-PU exhibited lower heat release relative to the foam without flame retardant, where the pk-HRR decreased by 8.0%, 26.7%, and 17.1%, respectively, and the THR reduced by 35.3%, 41.4%, and 29.8%, respectively. The introduction of phosphorus also weakens the negative impact of LGBF and LGMF on the smoke release during the combustion of the foams. The ID/IG ratios of LGBFP-PU and LGMFP-PU are relatively low, being 1.91 and 1.90, respectively, indicating the formation of carbon layers with a more ordered structure and a higher degree of graphitization.