Delamination of Southern Pine Plywood During Three Years of Exterior Exposure

Plywood was made from eight loblolly pine trees selected to exhibit a range of specific gravity and growth rate. Three-eighths-inch, three-ply specimens were exposed outdoors for 3 years in central Louisiana, and percentage of delamination was measured annually. Rings per inch and tightness of peel had only minor effects on the amount and rate of delamination. low gluespread resulted in relatively rapid delamination, particularly if applied to high-density veneer or if assembly time was long. Of glues formulated with a low percentage of resin solids, those extended solely with wheat flour resisted delamination comparatively well. Glues extended with blood suffered more severe delamination, even when they contained a high percentage of resin solids. High-gravity wood delaminated more rapidly than low-gravity wood, particularly if gluespread was low or assembly time long. Plywood given a long assembly time tended to delaminate, and dense veneer or light glue-spread accelerated the effect. Rate of delamination decreased with each succeeding period of exposure after the first year. A multiple regression expression was developed for predicting percentage of delamination solely from wood properties and results of the standard vacuum-pressure-soak shear test; the 11-factor equation accounted for 35 percent of the variation observed after 3 years. By simple correlation, three factors had r values of 0.15 or more; percentage of wood failure was negatively correlated with percentage of delamination; wood specific gravity and modulus of elasticity of the dry plywood loaded parallel to the grain of face and back plies were positively correlated with percentage of delamination. General conclusions concerning the effects of primary variables on delamination after 36 months of exposure were the same as those reported after 6 and 12 months of observation.

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