The objective of this study is to use wheat flour (WF) and hydroxymethyl melamine prepolymer (HMP) to develop a low cost, highly water-resistant, starch-based bio-adhesive for plywood fabrication. Three-layer plywood was fabricated using the resultant adhesive, and the wet shear strength of the plywood samples was measured under various conditions. After determining that water resistance was significantly improved with the addition of HMP, we evaluated the physical characteristics of the starch-based adhesive and functional groups and analyzed the thermal stability and fracture surface of the cured adhesive samples. Results showed that by adding 20 wt.% HMP into WF adhesive, the sedimentation volume in the resultant adhesive decreased by 11.3%, indicating that the increase of crosslinking in the structure of the adhesives increased the bond strength, and the wet shear strength of the resultant plywood in 63 °C water improved by 375% when compared with the WF adhesive. After increasing the addition of HMP to 40 wt.%, the wet shear strength of the resultant plywood in 100 °C water changed from 0 MPa to 0.71 MPa, which meets the exterior use plywood requirement. This water resistance and bond strength improvement resulted from (1) HMP reacting with functions in WF and forming a crosslinking structure to prevent moisture intrusion; and (2) HMP self-crosslinking and combining with crosslinked WF to form a microphase separation crosslinking structure, which improved both the crosslinking density and the toughness of the adhesive, and subsequently, the adhesive’s bond performance. In addition, the microphase separation crosslinking structure had better thermostability and created a compact ductile fracture surface, which further improved the bond performance of the adhesive. Thus, using a prepolymer to form a microphase separation crosslinking structure within the adhesive improves the rigidity, toughness, and water resistance of the material in a practical and cost-effective manner.
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