Synergistic enhancement of flame retardancy in flexible polyurethane foams via phosphorous-modified luffa seed oil polyol and expandable graphite

Flexible polyurethane foam (FPUF) is commonly prepared from petroleum-based polyols and exhibits high flammability, which limits its sustainable and safe applications. In this study, a bio-based phenylphosphonic acid-grafted polyol derived from luffa seed oil (PPOA-LSO) was synthesized through the epoxidation–phosphorylation reaction of LSO. Expandable graphite (EG) and PPOA-LSO were coupled to create a phosphorus–carbon synergistic flame-retardant system for bio-based FPUF. A phosphorus–carbon synergistic flame-retardant solution for FPUF was created by combining EG and PPOA-LSO. The resulting foams were systematically characterized via scanning electron microscopy (SEM), thermogravimetric analysis (TGA), UL-94 horizontal burning tests, limiting oxygen index (LOI), and cone calorimetry (CONE). The results revealed that the modified foams achieved a LOI value of 22.7%, and the char residue increased by 4.4–6.0 times compared with neat FPUF. The burning rate decreased by 63.6%, while the total heat release (THR) and peak heat release rate (PHRR) decreased by 48.0% and 29.8%, respectively. The substantial phosphorus–carbon synergistic flame-retardant action of PPOA-LSO and EG was identified as the cause of these improvements. Additionally, the foams maintained good support performance and tear strength. This study offers a viable approach for the high-value utilization of non-edible vegetable oils and the production of environmentally friendly flame-retardant FPUFs.