Covalent Grafting of the Phosphorus Element Endowsthe Tannin–Gelatin-Based Adhesive with Intrinsic Flame Retardancyand Water Resistance

Abstract Formaldehyde-free gelatin–tannin adhesives are promising for engineered wood products, but their use is constrained by moisture sensitivity and by the migration or phase separation that can accompany physically blended flame retardants. Here, tannic acid was maleated to form maleated tannic acid (MA@TA), which supplied carboxyl-containing functionalities for reaction with gelatin and residual C═C sites for subsequent phosphorus functionalization. FTIR, nuclear magnetic resonance, and X-ray photoelectron spectroscopy results collectively supported covalent network formation between MA@TA and gelatin and the incorporation of DOPO-derived groups through a proposed P–H addition to residual C═C bonds. The optimized D-MTG-4% adhesive gave hot-water and boiling-water shear strengths of 1.54 and 1.22 MPa, respectively. In Limiting Oxygen Index (LOI) and UL-94 tests, plywood bonded with D-MTG-4% and surface-coated with D-MTG-11% reached an LOI of 31.2% and a V-0 classification. Cone calorimetry, TG-FTIR, and residue morphology results indicate that phosphorus-promoted condensed-phase carbonization is the principal flame-retardant contribution, while gas-phase dilution or inhibition may provide a secondary contribution. This work presents a covalent design strategy for combining wet bonding with flame-retardant functionality in a biobased adhesive system.