Effect of kaolin content on the performances of kaolin-hybridized soybean meal-based adhesives for wood composites

The development of an environmentally safe soybean-based adhesive with desired adhesion properties and technological applicabilities, but low cost, still remains a challenge. In this study, a facile and clean strategy was developed to improve the adhesion properties and technological applicabilities and to reduce the cost of a soybean meal flour (SMF)-based adhesive through organic-inorganic hybridization with kaolin powder. The effects of kaolin hybridization and kaolin content on the structure and properties of the SMF-based adhesive were revealed. The improved water resistance of the adhesion was attributed to the effective organic-inorganic hybridization of the SMF adhesive with a uniform distribution of kaolin, the hydrogen bonding interactions between the intercalated SMF with the partial kaolin interlayers, and the chemical reaction between the crosslinker and kaolin. In addition, kaolin significantly decreased the viscosity and alleviated the shear thinning characteristics of the SMF adhesive because the kaolin-kaolin intermolecular interactions were weaker than the interactions between the SMF chains. Hydrogen bonding interactions between the SMF chains and the kaolin particles were observed, and the water absorption and bonded water by kaolin were greatly reduced. The most preferable kaolin content in the SMF adhesive was 30%, which decreased the adhesive cost by 11.4%. Thus, this novel strategy could effectively expand the applications of SMF-based adhesives in environmentally friendly wood composites by replacing formaldehyde-based synthetic resins that release free formaldehyde.