Flame Retardancy and Thermal Decomposition of Waterborne Polyurethane Composites Comprising Bio-Based Phytic Acid/Melamine-Coated SiO2 and a DOPO Derivative

A novel P–N–Si coflame retardant comprising bio-based phytic acid/melamine–coated SiO 2 (PM@SiO 2 ) was designed. The effects of a combination of a 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO)-derived compound (PN-DOPO) with PM@SiO 2 on the flame retardancy and thermal decomposition of WPU were investigated. Compared with pure WPU, the WPU/PN-DOPO/PM@SiO 2 composites possessing different PM@SiO 2 addition amounts demonstrated enhanced flame retardancy. The addition of 5 wt% PM@SiO 2 and 9 wt% PN-DOPO increased the limiting oxygen index (LOI) of pure WPU from 19.0% to 29.4%, and the resulting material achieved a V-0 rating in the UL-94 test. Furthermore, the peak heat release rate (pHRR), total heat release (THR), and fire growth index of WPU/9%PN-DOPO/5%PM@SiO 2 considerably decreased by 36.1%, 29.1%, and 41.9%, respectively. The excellent flame retardancy is attributed to the ability of PM@SiO 2 to effectively repair defects in the char layer formed by the gas-phase PN-DOPO flame retardant via its silica-cross-linked network. Moreover, the initial thermal decomposition temperature of WPU/9%PN-DOPO/5%PM@SiO 2 increased from 229.6°C to 260.1°C. Moreover, the tensile strength and storage modulus were considerably enhanced. This study presents a new research paradigm for developing environmentally friendly flame-retardant WPUs based on green and synergistic multimechanistic strategies.