Bonding Performance of Mildly Thermally Modified Petung Bamboo Using a Citric Acid-Based Adhesive

Thermal modification is an important treatment process for enhancing the dimensional stability and durability of laminated lignocellulosic products; however, it may adversely affect bonding performance depending on treatment severity and adhesive characteristics. In this study, a mild thermal modification condition (160°C for 3 h) was selected to achieve a balance between inducing sufficient chemical modification to improve dimensional stability and color uniformity while limiting excessive deterioration of mechanical and adhesion-related properties. This study investigated the effect of mild thermal modification under this condition on the adhesion properties of laminated bamboo. Two citric acid-based adhesives, namely citric acid-starch and citric acid-sucrose, were used at a ratio of 75/25 wt%. Urea formaldehyde (UF) was also used as a comparative treatment. Physical properties and chemical composition of bamboo were evaluated before and after thermal modification. Two-layer laminated bamboo panels were manufactured with citric acid-based adhesives and hot-pressed at 200°C for 20 min. Adhesion performance was assessed using the delamination rate, bending strength, shear strength, and failure percentage. The results showed that thermal modification at 160°C for 3 h significantly reduced the wettability of bamboo strips. This treatment negatively affected the shear strength of laminated bamboo bonded with citric acid-sucrose and UF, whereas a positive effect was observed for citric acid-starch-bonded samples. These findings indicated that the selected mild thermal modification condition provides a balanced modification level and is particularly suitable for laminated bamboo bonded with citric acid–starch adhesive.