Enhancing Rigid Polyurethane Foam Properties with Lignin-Based Core–Shell Intumescent Flame Retardants

Lignin, an abundant polyphenolic biomass, has excellent carbonization potential, making it highly promising for developing ecofriendly flame retardants. This study combined ammonium polyphosphate (APP) and alkaline lignin (AL) to create a novel core–shell flame retardant, APP@AL. APP@AL was then applied to rigid polyurethane (RPU) foam to improve its flame-retardant performance and interfacial compatibility with the matrix. Results showed that the mean heat release rate and total heat release of flame-retardant RPU foam decreased by 76.8% and 65.9%, reaching 58.61 kW/m2 and 19.09 kW/m2, respectively, along with a significant improvement in smoke suppression, and TSP was decreased to 1.67 m2. The flame retardant content for each RPU was 25 wt % of the polymethylene polyphenyl isocyanate. The flame-retardant mechanism was further explained by analyzing the char layer structure and the pyrolysis gas-phase products. Moreover, APP@AL enhanced the interfacial compatibility of RPU, as verified by digital image correlation, which demonstrated an improved stress transfer efficiency during compression. Compared with unmodified RPU foam, APP@AL-modified RPU showed a 10.3% increase in compressive strength and a 6.2% reduction in thermal conductivity. This work provided a novel strategy for thermal insulation, reinforcement, and smoke suppression of rigid polyurethane foams.