High-Strength, Dimensionally Stable, and Flame-Retardant Fast-Growing Poplar Prepared by Ammonium Polyphosphate–Waterborne Epoxy Impregnation

The simple, high-performance, and flame-retardant modification method is significantly important for the effective utilization of wood. In this work, ammonium polyphosphate (APP) and waterborne epoxy resin (WEP) were used for the treatment of fast-growing poplar by vacuum pressure impregnation to resolve the flammability, poor dimensional stability, and low mechanical strength problems. The results showed that 5A-E-Wood had strong self-extinguishing properties and passed the UL-94 V-0 combustion test, and the limiting oxygen index (LOI) value can reach 32.1%. Furthermore, the residue char weight of 5A-E-Wood reached 41.07%, which significantly promoted char-forming. Especially, the cone results showed a low heat release rate and an excellent smoke suppression effect. In addition, the incorporation of APP/WEP into wood can reduce water uptake (WU) and equilibrium moisture content (EMC), significantly improving dimensional stability and mechanical properties. Most important of all, the weight leaching ratio (WLR) of APP was extremely reduced. Therefore, this research presented that the mechanical properties and flame retardancy of the wood improved visibly due to the successful penetration of APP and WEP into the wood cell, which laid the foundation of its use as a high-performance wood-based material.