Undercure . . . Casehardening in Plywood

This is a study on Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and Engelmann spruce (Picea Engelmannii) in relation to bond quality considering the following factors: species, glues, sapwood and heartwood, veneer drying history, dry veneer storage time, veneer moisture content, adsorption and desorption cycles, sanded, and unsanded veneers, assembly time, and press time. The most significant result of this study is that it showed that undercured bonds are a major cause of low wood failure in softwood plywood. This undercure is influenced by such factors as veneer drying history, assembly time, veneer storage time, and adsorbed moisture. It has been shown that, for a particular glue, certain species are much more sensitive than others to the factors that tend to produce undercure bonds. Differences in the tendency for different glues to produce casehardening were also demonstrated. It is suggested that the particularly good tolerance of coast-type Douglas-fir to conditions that tend to produce undercured phenolic bonds may not be a species characteristic but may be characteristic that has been built into that particular glue through years of research to develop the best glue for exterior quality plywood which has, in fact, been Douglas-fir plywood. It may well be that an equivalent research effort will produce glues that will work equally well with each of the other species.

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