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

Lignin,\nan abundant polyphenolic biomass, has excellent carbonization\npotential, making it highly promising for developing ecofriendly flame\nretardants. This study combined ammonium polyphosphate (APP) and alkaline\nlignin (AL) to create a novel core–shell flame retardant, APP@AL.\nAPP@AL was then applied to rigid polyurethane (RPU) foam to improve\nits flame-retardant performance and interfacial compatibility with\nthe matrix. Results showed that the mean heat release rate and total\nheat release of flame-retardant RPU foam decreased by 76.8% and 65.9%,\nreaching 58.61 kW/m<sup>2</sup> and 19.09 kW/m<sup>2</sup>, respectively,\nalong with a significant improvement in smoke suppression, and TSP\nwas decreased to 1.67 m<sup>2</sup>. The flame retardant content for\neach RPU was 25 wt % of the polymethylene polyphenyl isocyanate. The\nflame-retardant mechanism was further explained by analyzing the char\nlayer structure and the pyrolysis gas-phase products. Moreover, APP@AL\nenhanced the interfacial compatibility of RPU, as verified by digital\nimage correlation, which demonstrated an improved stress transfer\nefficiency during compression. Compared with unmodified RPU foam,\nAPP@AL-modified RPU showed a 10.3% increase in compressive strength\nand a 6.2% reduction in thermal conductivity. This work provided a\nnovel strategy for thermal insulation, reinforcement, and smoke suppression\nof rigid polyurethane foams.