Novel Eco-Friendly Flame Retardants Based on Nitrogen–Silicone Schiff Base and Application in Cellulose

A novel schiff base (BD) containing nitrogen and silicone was prepared, and a phosphorus–nitrogen silicone flame retardant (BDD) was synthesized based on BD and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO). It was found that the structure of BD changed continuously during the heating process and formed a self-cross-linking network structure at about 400 °C, while the porous fluffy shape was formed by BDD as an intumescent flame retardant. The char yields of BD and BDD at 700 °C were 62% and 38% in nitrogen, respectively. The obtained BD and BDD were blended to enhance the flame retardancy of cellulose membranes via a cooperative effect, and the properties were significantly achieved, which can be proved by the limit oxygen index (LOI), cone calorimeter test, and thermogravimetric results. The results showed that the incorporation of 25 wt % BD brought a 29% LOI, and the peak heat release rate (PHRR) was reduced to 732.2 kW/m2, in contrast with an LOI of 18% and PHRR of 1050.1 kW/m2 for pure cellulose. The addition of 25% BDD increased the LOI to 27% and decreased the PHRR to 644.9 kW/m2. The improved flame-retardant performance for cellulose membranes was primarily assigned to the synergistic effect from the physical barriers of self-cross-linking properties in the condensed phase and capturing or dilution of combustible gases by free radicals. This suggested that wielding BD as a self-crossing monomer and applying BDD as an intumescent flame retardant to achieve effective results on fire suppression had great potential for fire-safe cellulose.