Enhancing adhesion strength of protein-based adhesives through plywood heat treatment

Abstract

Heat treatment of wood in the absence of oxygen is widely used in industry to enhance raw wood properties, but cannot be broadly applied to engineered wood like plywood. Most synthetic adhesives, particularly formaldehyde-based ones, degrade at high-temperatures and release toxic substances, harming the environment. This study explored protein-based adhesives for heat-treated plywood, inspired by high-temperature, controlled oxygen or non-oxygen conditions in baking. Three adhesive formulations—soy flour (SF), polyamide-epichlorohydrin modified soy flour (PAE/SF), and urea formaldehyde (UF)—were used with yellow pine wood and heat-treated at 190–200 °C for 1–4 h. Untreated UF plywood demonstrated good dry and wet strength, but significant strength loss and complete delamination after heat treatment. Conversely, heat-treated SF plywood showed substantial improvements in wet strength, reaching 1.31 MPa at 190 °C for 2 h and 1.48 MPa at 200 °C for 1.5 h. PAE/SF plywood maintained high wet strength, peaking at 1.74 MPa at 190 °C for 1 h. The enhanced water resistance resulted from protein denaturation, loss of hydrophilic groups, and crosslinking during heat treatment. Maillard reactions further improved water resistance by forming covalent bonds and melanoidins. Heat treatment at 200 °C for 2 h also improved the dimensional stability of the plywood while maintaining acceptable bending and tensile strengths. This study highlights the potential of protein-based adhesives as an eco-friendly alternative for improving heat-treated plywood properties.