Effects of Seven Variables on Properties of Southern Pine Plywood: Part II. Maximizing Wet Shear Strength

The paper explores the interacting effects of seven variables upon the wet-shear strength of loblolly pine (Pinus taeda L.) plywood on the premise that wet-shear strength may be an indicator of exterior durability. The seven variables were: specific gravity, ring count per inch, quality of veneer peel, percent resin in mixed adhesive, secondary extension, glue spread rate, and assembly time. Three-ply 3/8-inch plywood specimens were cut from 576 hot-pressed panels. Half the specimens from each panel were cut so the lathe checks pulled closed during testing and half were cut so the lathe checks pulled open. Specimens were treated to the APA vacuum-pressure soak before testing wet on a Globe-type plywood quality control machine. Analysis of variance was used to evaluate the results. Wet shear-strength differed significantly with changes in the level of each primary variable except percent resin solids, tree specific gravity, and growth rate. Wet-shear strength was maximum under the following conditions: specific gravity greater than 0.5, growth rate slower than 6 rings per inch, tight-hot veneer peel, 26 percent resin solids, no secondary extender, 75-pound spread, and 13-minute assembly time. When all other variables were held optimum, the type of peel, i.e. cold and loose, caused the greatest reduction in strength from the maximum. The most important variables in terms of their effect upon the total variation of test results were, in order of decreasing importance: lathe-check depth, specific gravity, lathe-check frequency, and growth rate. An unexplained observation was that wet-shear specimens tested to pull the lathe checks open consistently yielded higher strengths than those tested to pull the checks closed. This is the second of a series of four studies exploring the effects of these variables on the properties of southern pine plywood.

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