Effect of synthesis parameters of cold-setting melamine-urea-formaldehyde resins on the adhesion in wood bonding

This work studied the effect of the synthesis parameters (i.e. synthesis method and melamine content) on the adhesion of cold-setting melamine-urea-formaldehyde (MUF) resins in wood bonding. Two types of cold-setting MUF resins were synthesized: MUF-A resins with three melamine contents (30, 40, and 50 wt%) prepared by the simultaneous reaction of urea, formaldehyde, and melamine, and MUF-B resins with the same melamine levels but synthesized by a sequential reaction process of melamine with formaldehyde and urea. The curing behaviors of both resins were characterized by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA), and their adhesion performance in the dry and wet states were compared in block-shear strength (BSS) and wood failure (WF) tests. As the melamine content increased, the viscosity and peak temperature (T_p) of the resins increased. However, MUF-B resins showed lower activation energy (E_a) than those of MUF-A resins. Dynamic and isothermal DMA scans also showed that MUF-B resins also had greater maximum storage modulus (E′_max) than those of MUF-A resins in their neat resins and resins/wood composites, indicating stronger interphase adhesion and cohesion. Lower T_p, lower E_a, and higher E′_max, of MUF-B resins than those of MUF-A resins resulted in higher BSS and WF of MUF-B resins in wood bonding. In particular, MUF-B resins with 40% melamine showed the highest BSS and WF. These results indicate that the MUF resin synthesis method is closely related to the adhesion performance in wood bonding.