Investigation of novel thermosetting amino resins based monomethylolurea, glyoxal and tannin

Amino resin is widely used in the preparation of wood-based panel adhesives due to its excellent performance and low cost. However, it has drawbacks such as poor water resistance and formaldehyde emission. In this study, we developed a new formaldehyde-free amino resin, glyoxal-monomethylolurea-tannin (GUT), which was synthesized from glyoxal, monomethylolurea, and tannin. The influence of tannin on the physical and mechanical properties of GUT plywood was assessed. The results indicated that the plywood made with GUT adhesive met the type III plywood requirements in accordance with the Chinese national standard GB/T 17657–2013, without the use of expensive melamine or phenolic.The wet shear strength in boiling water reached 0.85 ± 0.08 MPa, exceeding the national standard for exterior plywood use (≥ 0.70 MPa). The optimum hot-pressing time was 1 min per millimeter, the hot-pressing temperature was 180 degrees Celsius, the hot-pressing pressure was 1.6 MPa, and the dosage film was 200 g per square meter. The chemical structure of the resin and the adhesion mechanisms involved were analyzed and characterized using nuclear magnetic resonance (13C NMR), liquid chromatography-mass spectrometry (LC-MS), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). Curing properties were investigated using differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The findings indicated that GU reacted with tannin to form macromolecules, which reduced its curing temperature and enhanced the crystallinity of cured glyoxal-monomethylolurea (GU) resin. The increase in crystallinity could contribute to the improvement of mechanical characteristics and water resistance in glyoxal-monomethylolurea (GU) resins, primarily attributed to the presence of tannins.