Relation Between Physical and Chemical Properties of Wood and Adhesion

This study used 22 hardwood species chosen on the basis of wide variation in pH and wettability over an extremely wide range of specific gravity. The study resulted in the following conclusions. In gluing with Perkins L-100 urea formaldehyde resin and catalyst S-120, closed assembly times as long as 52 minutes, almost three times the maximum recommended by the manufacturer, are not deleterious to glue-bond quality, and with some high-density woods actually increase the quality of bond. With Koppers Penacolite G-1260, a resorcinol-phenol resin, closed assembly times beyond that recommended by the manufacturer result in inferior glue-bonds. Closed assembly times of 6.5 to 52.0 minutes with urea resin and 1 to 8 hours with resorcinol-phenol resin have no effect on the basic relationships of pH, wettability, and specific gravity to glue-bond quality. Of the above three variables studied for their effect on quality of glue bonds, specific gravity is dominant in the low-density region with urea resin, and throughout the entire range of density tested in the case of the resorcinol phenol resin. Specific gravity was followed, in order of their effects, by wettability and pH over the same range. In the high-density region with urea glue-bonds, pH is dominant, followed in order of their effects on quality by wettability and specific gravity. In general, specific gravity is more important, and pH and wettability less important, with resorcinol-phenol glue bonds than with those glued with urea resin. The resorcinol-phenol adhesive is more consistently limited by solid wood shear strength, which is, in turn, dependent upon specific gravity. As long as wood failure is high, shear ratio may contradict shear strength as a measure of glue-bond quality, especially when the bulk of the data are collected between species rather than within. In all other respects, shear ratio, shear strength, and percentage of wood failure are in agreement as to glue-bond quality. Shear ratio is useful as a glue-bond evaluator, and may become more so with increased familiarity, especially in an experiment particularly designed for its use. A design of this sort would be most efficient if provision were made for the numerator and denominator of the ratio to be formed from results of tests on matched material, and if the bulk of the testing was within species rather than between. This study has positively demonstrated that pH, wettability, and specific gravity are closely related to glue-bond quality. Before this knowledge can be utilized, the exact nature of these relationships must be clarified by further investigation. It is hoped that the result of these investigations will be a more fundamental knowledge of the properties of woods that influence adhesion.

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