Effect of Moisture on Lumber-Core Panels

The objectives of the study were to 1) present visual evidence of shear stress development in lumber-core furniture panels caused by changes in moisture content of the core; 2) determine the approximate percentage of moisture added to a lumber core by cold-setting urea-formaldehyde and casein adhesive having similar curing times; and 3) show statistically that the extreme case of panel stress (as indirectly predicted by the data) would probably not be great enough to cause damage to the lumber-core panel, provided the moisture content of the core was maintained at an intermediate level during the bonding process. Twenty 8 by 15 inch lumber cores 3/4-inch thick were made of straight-grained yellow-poplar. Average moisture content was 8 percent. Poplar crossbands and yellow birch faces were applied; then the panels were divided into five groups to be conditioned to moisture content levels of 10.5, 8.5, 6.0, 5.4 and 3.1 percent. Eighty stress sections (40 for each adhesive) 8 inches across the grain of the core and 1 inch parallel to the grain were prepared. A saw cut along the core crossband glueline exposed the core so that changes in core moisture content resulted in measurable deviations. Deviations of the free end of each specimen were subsequently measured at 12, 10, 8, 6, and 4 percent moisture content. Flat panels occurred when the moisture content to which a stress section had been conditioned approached the moisture content of the panel core at the time the adhesive cured. lt was concluded that when using these cold-setting adhesives, lumber cores should be conditioned to moisture content values of 7 to 8 percent before bonding crossbands and face plies to them. A moisture content of 8 or 9 percent would then occur in the cores at the time of actual bond formation, representing an intermediate condition between the extreme of 4 and 13.5 percent found in the interior wood in buildings in the United States. If the panels are bonded at an intermediate moisture content, statistical analysis showed that deviations occurring at the moisture content extremes would not be serious.

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