An effective strategy to enhance the flame retardancy and mechanical properties of urea-formaldehyde resin by using an aromatic-triazine structure hyperbranched flame retardant

Modifications to wood adhesives to improve flame resistance are often accompanied by undesirable properties, such as reduced bonding strength, increased moisture absorption, color changes, and increased toxicity. Herein, a highly reactive hyperbranched flame retardant (HBFR) was synthesized via polymerization of biphenyl tetracarboxylic dianhydride (BPDA) and tris(2-hydroxyethyl)isocyanurate (THEIC). Consequently, HBFR was treated with urea-formaldehyde (UF) to form a covalently crosslinked HBFR–UF resin network. Compared with UF resin, the char residue of HBFR-UF increases from 8% to 25% at 800 °C, and the limiting oxygen index (LOI) improves to 28.8%. The peak heat release rate (HRR) of HBFR-plywood was 27.5% lower than that of plywood. Furthermore, the shear strength, flexural strength, and impact toughness of HBFR-plywood increase by 37.5%, 37.5%, and 36.1% respectively, compared to the plywood. This study elucidates the intrinsic reaction pathway between HBFR and resin molecules, revealing the synergistic mechanisms that enhance flame retardancy and mechanical strength. The findings provide a theoretical basis for advancing flame-retardant UF resins.