Effects of Seven Variables on Properties of Southern Pine Plywood: Part I. Maximizing Wood Failure

The durability of southern Pine plywood in exterior exposure is usually estimated on the basis of percentage of wood failure in wet shear specimens. Eight loblolly pines in the range from 14 to 20 inches in diameter were selected from natural stands in central Louisiana. A pair was chosen in each of four categories: 1) fast growth and low density, 2) fast growth and high density, 3) slow growth and low density, and 4) slow growth and high density. One 10-foot length was cut from each tree. Each green length was cut into a pair of bolts to yield a total of 16 bolts, each 51 inches long. One half were sent to the G-P plant at Fordyce, Arkansas, and the other half sent to U.S. Forest Products Lab., Madison, Wisconsin, for Manufacture into veneer. Wood failure, as determined on thoroughly soaked shear specimens, was maximized at 99 percent by using 1) veneer of low specific gravity cut from slow-growing trees; 2) peeling the veneer cold and loose; 3) increasing the percent of phenol-formaldehyde resin solids in the wet glue mix; 4) using only wheat flour as a secondary extender; and 5) increasing the amount of gluespread on the core to 75 lbs./M sq.ft. With low-density woods, assembly times of 32 or 24 minutes were preferable to 13 minutes. Veneer from fast-grown dense trees when peeled cold and loose yielded 91 percent wood failure. In the three-ply construction, 78 percent of the shear failures occurred in the loose-to loose interface. Frequency of lathe checks increased as depth decreased.

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