In Situ Polymerization and Flame Retardant Mechanism of Bio-Based Nitrogen and Phosphorus Macromolecular Flame Retardant in Plywood

To improve the flame retardant performance of plywood and reduce the reagent loss and moisture absorption of the flame retardant, a bio-based supramolecular flame retardant is prepared by vacuum-pressure impregnation and high-temperature in situ polymerization in plywood. The best value of bonding strength appears at 170 degrees C, and the limiting oxygen index (LOI) of 170BF-B plywood is 42.3%. After hot pressing, the moisture absorption rate of the 170BF-B veneer is only 18.51%, while the loss resistance rate achieves 83.45%. Its residue at 700 degrees C is 91.36% higher than that of poplar veneer. In the combustion process, the peak value of heat release rate (PHRR) and heat release rate (HRR) of 170BF-B plywood are only 10.69% and 37.11% of that of untreated plywood. After combustion, an intumescent flame retardant layer exhibits a graphitization trend. In the flame retardant layer, there are not only functional groups, such as PO, PO43-, P-O-C decomposed by flame retardant but also characteristic functional groups of wood fiber, like CO, C-H, etc. The prepolymer BF-B, which is composed of phytic acid, urea, and dicyandiamide polymerized with chitosan or lignocellulose to form a supramolecular flame retardant connected with P-O-C and P-O-N functional groups, thus improving the flame retardant and anti-loss property by in situ polymerization.