Influence of Surface Aging Prior to Gluing on Bond Strength of Douglas-Fir and Redwood

Changes in bond strength due to surface aging were found to take place in the absence of light, surface contamination due to the settling of dust and other atmospheric contaminants, and in the absence of surface damage due to gross changes in moisture content. Tensile bond strength was shown to decrease linearly about 1 to 2 pounds per square inch for each day stored. There was no leveling off up to the 5-months maximum tested. No measurable influence on wood failure was found. Four possible mechanisms influencing surface gluing properties are assumed 1) Migration to the surface of extraneous material detrimental to gluability, 2) chemical changes in the wood substance or in the extraneous material at the surface, 3) adsorption of airborne contaminants such as gases, vapors or particles on the surface, and chemical changes or molecular rearrangements decreasing the surface free energy, and 4) a decrease in the strength of the wood fibers at the surface, causing a corresponding decrease in the tensile strength of the bond.

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