An All-Natural Adhesive for Bonding Wood

A novel adhesive that is solely based on natural materials of defatted soy flour (SF) and magnesium oxide (MgO) has been investigated for preparation of five-ply plywood panels. The resulting plywood panels met the industrial water-resistant requirement for interior plywood. In this study, mechanisms by which an aqueous mixture of SF and MgO served as a strong and water-resistant adhesive for bonding wood were investigated. SF was first fractionated into soy protein isolates (SPI), a water-soluble fraction, and insoluble carbohydrates (ICs) that were mixed with MgO, respectively, for preparation of maple laminates. The water resistance of the resulting maple laminates was evaluated by a three-cycle water-soaking-and-drying (WSAD) test and a two-cycle boiling-water test (BWT). The mixture of MgO and the soluble fraction was not able to bond maple veneers together. The shear strengths of the resulting maple laminates before and after WSAD and BWT all had the following order: MgO-SPI > MgO-SF > SF only > MgO-IC. The water solubility of SF in the heat-cured SF-MgO mixture was much lower than that of the heat-cured SF. We believe that the low water solubility of SF-MgO and close interactions between MgO and soy proteins instead of soy carbohydrates were responsible for the superior strengths and high water resistance of the soy-MgO adhesive.