Preparation and Performance Optimization of Tannin-modified Cold-Setting MUF Resin and Glued Wood

Abstract

The modification of melamine-urea-formaldehyde (MUF) resin was studied using plant tannins, focusing on the effects of tannin addition stage and dosage on resin properties. Second-stage addition failed due to system gelation caused by the acidic polycondensation environment. Although third-stage addition further reduced free formaldehyde content, the resulting viscosity surge led to difficulties in glue spreading, accompanied by significant deterioration in bond strength and water resistance. First-stage addition of bayberry tannin yielded optimal results, with 6% identified as the critical optimal dosage. At this level, the resin exhibited a viscosity of 206 mPa·s, free formaldehyde content of 0.09%, dry and wet shear strengths of 7.2 and 5.8 MPa, and impact toughness of 6.8 kJ/m². Beyond 6% addition, viscosity increased exponentially, free formaldehyde rebounded, and water resistance deteriorated sharply. Differential scanning calorimetry (DSC) analysis revealed that tannin modification reduced the curing peak temperature (by 2.7°C for bayberry tannin) and accelerated crosslinking reactions. The first-stage addition of 6% bayberry tannin was determined as the optimal technical solution, achieving a balanced compromise among impact toughness, shear strength, formaldehyde reduction efficacy, and process feasibility.