Properties of Soy-Based Wood Adhesives Enhanced by Waterborne Polyurethane Modification

To improve water resistance of soy-based adhesives, waterborne polyurethane (WPU) resin was applied to modify defatted soy flour. The influence of WPU addition level on the water resistance of soy-based adhesives was investigated by Fourier transform infrared spectroscopy, scanning electron microscopy, thermogravimetric analyzer analysis and tensile tests. Strong intermolecular interactions were found to exist between soy protein and WPU with WPU addition level increasing. In addition, the wet shear strength of the WPU modified soy-based adhesive was improved significantly from 0.65 MPa up to 1.1 MPa and the cohesive wood failure (CWF) dramatically increased from 0 to 71%. Therefore, this is a novel pathway to prepare modified soy-based adhesives that can meet the requirement for type-II plywood applications.