Influence of borate chemical treatment on adhesive bond performance of Douglas fir and grand fir laminates

Abstract

This study investigates borate treatments of Douglas fir (Pseudotsuga menziesii) and grand fir (Abies grandis) lamstock and their impact on adhesive bonding performance. Lamstock were treated using three different methods (dip, double vacuum, and vacuum/pressure) with the aim to achieve a target retention of 0.1% (w/w) B₂O₃. The degree to which B₂O₃ was sorbed by the lamstocks was evaluated using image analysis technique. Treated lamstocks were bonded with one component polyurethane adhesive, and bond performance was evaluated by a cyclic delamination and block shear test. Results revealed that the uptake of treating solution differs by treatment methods and species. Uptake by vacuum/pressure-treated lamstocks is highest while uptake in dip treated lamstocks is the lowest. Grand fir had higher uptake values than Douglas fir throughout all treatments. The level of retention of B₂O₃ was influenced by treating solution concentration and wood properties. Dip treated lamstock showed limited spread of retained B₂O₃ with bulk of the B₂O₃ retained close to the surface. All conditions of grand fir, planed or unplaned, were able to meet the requirement of ANSI A190.1-2022 for block shear wood failure and total delamination, while unplaned specimens of Douglas-fir did not meet the requirement of ANSI A190.1-2022. Borate treatments increased the hydrophilicity of the lamstocks, causing the potential formation of superficial bonds between the adhesive and the accumulated chemicals on the surface of the unplaned dip treated lamina. This study presents insightful information about maximizing the chemical treatment of B₂O₃ to have the least possible impact on adhesive bond performance.