Catalyzed PVA Emulsions as Wood Adhesives

This paper deals with the testing of PVA emulsion adhesives. Two catalyzed PVA emulsion adhesives were used along with two resorcinol-formaldehyde adhesives and an uncatalyzed PVA adhesive for comparative purposes. Adhesive strength obtained by the shear block method showed that the two commercial catalyzed PVA adhesives developed glueline shear strength comparable to that of the resorcinol-formaldehyde adhesive, while the uncatalyzed did not. Another test involved the rate of development of dry strength at room temperature. The catalyzed PVA adhesive developed dry strength more slowly than did the conventional PVA systems. The resorcinol glue bond did not develop maximum strength and wood failure until 2 to 2 weeks had elapsed. Development of strength at room temperature showed that the catalyzed PVA adhesive developed a wet strength up to about half of the dry strength and in a sense is greatly superior to the conventional PVA. However, unlike resorcinol formaldehyde, the catalyzed PVA adhesives failed to develop any significant amount of wood failure. This inability to develop wood failure indicates a lack of durability of the adhesive to the cyclic wetting and drying conditions of outdoor exposure. Another test involved cure rate at elevated temperatures. Data revealed that the dry strength goes through a maximum as the curing temperature is increased, and then falls substantially. The wet-strength data show a similar maximum strength, although the maximum is reached at a higher temperature. On the basis of the available information, none of these new PVA adhesives can be classed as room-temperature curing, fully-durable adhesives suitable for structural wood products. When cured at room temperature they eventually develop an appreciable level of wet shear strength which may be adequate for exterior wood products in which the gluelines are not subjected to sustained loads or strong delaminating forces. It is entirely possible that these adhesives will prove satisfactory for light-duty applications, such as gluing a thin plywood door skin to a frame, although this is a question that can only be answered by actual experience with doors exposed to the weather.

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