Preparation and characterization of urea–formaldehyde adhesives modified with glyoxalated tannin

Urea–formaldehyde (UF) resins are the most important type of adhesive for the preparation of wood-based panels, due to its good performance and the low price. However, because of poor water resistance and formaldehyde emission problem, some modification of the UF resin is essential. In the present research, glyoxal was used to activate bayberry tannin and obtain glyoxalated tannin (GT). Then, it was used to modify UF resins. Plywood was prepared to evaluate the bonding strength and formaldehyde emission. Fourier transform infrared (FTIR) and solid-state CP-MAS 13C Nuclear Magnetic Resonance (13CNMR) were used to characterize the chemical structures of the resins. Differential scanning calorimetry (DSC) was used to study the curing properties. The results show that the addition of glyoxalated tannin effectively decreases the formaldehyde emission. UF resin modified by glyoxalated tannin (UFGT) also has a good bonding performance, especially good wet strength, which meets the requirements of the Chinese National Standard GB/T 17657-2013. The structure of the hardened UFGT resins shows high contents of methylene bond and low content of ether bond, which are the probable reasons for the improvement of bonding strength and formaldehyde emission.