One-Step Preparation and Characterization of a Protein–Sucrose Wood Adhesive with Excellent Bonding Performance

In this study, a simple one-step blend of isolated soy protein and sucrose was used directly as a wood adhesive for plywood manufacturing. The bonding performance, water resistance, curing performance, and thermal stability of the adhesive were evaluated. The preparation process of the plywood was optimized and the curing mechanism was also investigated. The results demonstrate the following: (1) Sucrose was successfully converted into furan compounds, especially 5-hydroxymethylfurfural (5-HMF), which underwent a sufficient cross-linking reaction with the SPI, and this was the key during the curing of the adhesive. (2) The effect of hot-pressing temperature on the bonding performances was the most significant and played a key role in the success of the test, followed by hot-pressing time, solid content, and adhesive loading. (3) In this study, 200 °C was the critical point at which the adhesive obtained good wet bonding strength and was also the critical temperature at which the effective conversion of sucrose into 5-HMF occurred. (4) The optimum preparation parameters of plywood were a hot-pressing temperature of 216 °C, a hot-pressing time of 1 min/mm, a solid content of 50%, and adhesive consumption of 220 g/m2. Using this process, a bonding strength in warm water of 1.74 MPa, a bonding strength in boiling water of 1.50 MPa, and a wood failure rate of more than 80% were obtained for the plywood.