Sustainable Halogen-Free Flame-Retardant Flexible Polyurethane Foams via Lignin-Waterborne Polyurethane Pickering Foam Templates

This study presents a sustainable strategy to fabricate flame-retardant flexible polyurethane foam (FPUF) via a green aqueous Pickering foam templating method. Lignin was used as a biobased stabilizer and gelation agent, while 10-(2,5-dihydroxyphenyl)-10H-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO-HQ) served as a halogen-free flame-retardant and reinforcing component. Rheology and interfacial tension results confirmed that a lignin content above 4 wt % formed a strong physical network with waterborne polyurethane (WPU), effectively stabilizing the foam and preventing drainage. Eco-friendly epoxy cross-linkers (TGE or GPTMS) were used to replace toxic aziridine, enabling a safer and more sustainable fabrication process. The resulting foam monoliths showed a uniform open-cell morphology, good mechanical properties with compressive strength up to 231 kPa, and over 99% volume recovery. With the combined effects of lignin and DOPO-HQ, the foam achieved a limiting oxygen index (LOI) of 26.5% and UL-94 V-0 rating. Cone calorimetry showed the peak heat release rate was reduced from 468 to 280 kW/m2, and char yield reached 19.41%, indicating significantly improved thermal stability and flame retardancy. This work provides a green and environmentally friendly route for producing high-performance flame-retardant FPUF, showing great potential for insulation, transportation, and household furniture applications.