Durability of Fortified Urea-Resin Glues in Plywood Joints

Nine glues were formulated with urea resin fortified with either 0, 20, 40, 60, or 80 percent melamine resin or 10, 20, 40, or 60 percent resorcinol. These glues and a tenth glue consisting of 100 percent melamine were used to bond plywood for glue bond tests. Specimens were exposed up to 4 years to three controlled laboratory conditions and up to 4 years to exterior exposure. Incorporation of at least 20 Per cent melamine resin or 10 percent resorcinol with added para formaldehyde was shown to have distinct value in improving the resistance of the bonds in yellow birch plywood made with a conventional hot-press urea-resin glue to continuous exposure at 158?F. and 20 percent relative humidity, and under repeating cycles of high and low humidity at 80?F. Such fortification was least effective in a continuous exposure at 80?F. and 97 percent relative humidity. Generally, the improvement with either melamine resin or resorcinol fortifier was in direct proportion to the amount of fortifier added, with the most significant improvement being noted between the straight urea resin and the lowest amounts of fortifier of each type. Melamine-resin fortification seemed to have some advantage over resorcinol in the continuous high-temperature exposure, while resorcinol fortification had an advantage over melamine resin in the cyclic high-low humidity exposures. The straight melamine-resin glue was definitely more durable than any of the fortified urea-resin formulations in the high-temperature exposure. Higher proportions of both fortifiers tended to approach the performance of straight melamine resin quite closely in the continuous and cyclic high-humidity conditions.

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