Improvement of the water resistance of soybean protein-based wood adhesive by a thermo-chemical treatment approach

Abstract In order to extend the applications of wood composites and products bonded by soybean protein adhesive from interior to exterior fields of application, this study proposes a novel approach for improving the water resistance of soybean protein-based wood adhesives using thermo-chemical treatment of soybean protein. The soybean protein formed stable three-dimensional networks due to repolymerization or self-crosslinking during thermo-chemical treatment, confirmed by both increases in the water-insoluble content of the treated soybean protein and the improved hydrothermal-aged wet bond strength of the resulting soybean protein adhesive. Thermo-chemical treatment in the presence of 1 wt% sodium sulfite (which cleaves disulfide bonds) and 1 wt% sodium dodecyl sulfate (which destroys the hydrophobic interactions of proteins) released active groups buried within the globular structure of soybean protein via unfolding. This release both promoted the repolymerization of the soybean protein molecules and exposed more active sites for effective crosslinking by the post-added crosslinker EMPA. Plywood bonded by the optimal soybean protein adhesive possessed a good hydrothermal-aged bond strength of 1.22 MPa, exceeding the value required for structural use according to the JIS K6806-2003 commercial standard.