Effects of heat treatment on wet shear strength of plywood bonded with soybean meal-based adhesive

• Four heat processes were used to treat plywood bonded by soy protein based adhesive.
• We examine vertical density profile of plywood to explain the bond strength change.
• Heat treatment could improve the wet shear strength of plywood by 175% at maximum.
• The crosslink density of the adhesive increased after the heat treatment.

The aim of this study was to improve the water resistance of soybean meal-based adhesive with heat treatment after hot pressing. The effects of four different heat-treatment processes on plywood bonded with soybean meal-based adhesive were examined. Five-ply plywood specimens were fabricated to measure the adhesive's water resistance. Extending the hot press time allowed the adhesive to cure more completely and improve its water resistance. The surface and core layer wet shear strength of plywood at a 70 s/mm hot press time increased by 18.8% and 109%, respectively, compared with that of a 60 s/mm hot press time. The surface and core layer wet shear strength improved by 56.3% and 102.3%, respectively, with 4 min low pressure heat treatment, which makes it practical for use in industrial applications. Heat treatment could also improve the water resistance of the adhesive by improving the cross-linking density of the adhesive layer in plywood and releasing its interior force, according to a vertical density profile analysis. Therefore, 8 h oven drying heat treatment at 120 °C, the surface and core layer wet shear strength improved by 60.0% and 175.0%, respectively.

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