Phytic acid-iron synergized straw cellulose-based polyurethane biocomposite for flame-retardant applications

In this study, a pilot-scale straw cellulose-based polyurethane biocomposite modified with phytic acid (PA) and Fe₂O₃ (SC/PA/Fe) was developed through an eco-oriented and scalable fabrication route, and its flame retardance, physical properties, colour characteristics, and application potential were systematically investigated. The composite was fabricated via sequential spray-assisted delignification, stepwise spray impregnation of PA and Fe₂O₃, high-shear reactive blending, and room-temperature press molding. The optimized SC/PA/Fe achieved a limiting oxygen index (LOI) of 27.9 and a total smoke release (TSR) of 48.89 m²/m², demonstrating markedly improved fire safety relative to untreated SC. Mechanistic analysis indicated that the synergistic effect of PA and Fe₂O₃ facilitated the formation of a stable phosphorus-rich char layer during combustion, which enhanced thermal stability and reduced smoke generation. In addition, interfacial cross-linking between PA and the -NCO groups of TDI helped preserve the structural integrity of the composite, resulting in a tensile strength close to 1.0 MPa and a bending strength of 5.2 ± 0.2 MPa. Based on Material Driven Design (MDD), the warm red-toned appearance together with the improved flame-retardant performance suggests that SC/PA/Fe is a promising candidate for non-load-bearing interior applications, such as furniture surfaces and decorative panels.