Minimizing and Predicting Delamination of Southern Plywood in Exterior Exposures

This paper explores the interacting effects of wood specific gravity, rate of growth, tightness of peel, resin content of glue, type of secondary extender, gluespread, and assembly time on delamination of southern pine plywood. The plywood was made from eight loblolly pine trees selected to exhibit a range of specific, gravity and growth rate. Three-eighths-inch 3-ply specimens (1 by 3-1/4 inches) were exposed outdoors for 1 year at Alexandria, Louisiana. For samples of all specific gravities combined, delamination was minimized at 0.68 percent in gluelines made by: 1) using wheat flour as the secondary extender (as contrasted with no secondary extender or with blood plus-wheat flour extender); 2) increasing percent of phenolic resin solids in the wet glue mix to 26 percent (the lower level considered was 21 percent); 3) reducing assembly time to 13 minutes (other times considered were 24 and 32 minutes); and 4) increasing gluespread to 75 pounds per 1,000 square feet of core (as contrasted to 65 pounds). Delamination occurred at a rate nearly linear with time, i.e., percent of delamination at 12 months was almost twice that at 6 months. Plywood of high specific gravity delaminated 3 to 4 times more rapidly than samples of low density. The shortest assembly time (13 minutes), and heaviest gluespread (75 pounds) were necessary to minimize delamination in dense plywood. Among samples equal in percent of wood failure (as tested in thoroughly wetted shear specimens), high-density pieces delaminated faster than low-density pieces. By regression analysis, dense wood appeared to delaminate less if peeled tight than if peeled loose; the less sensitive variance analysis did not support this conclusion. For wood of low or average density, peel had no effect on delamination. For all densities, loose-peeled veneer typically displayed higher percent of wood failure than wood peeled tight– and required this higher percent for equal durability. Loose-to-loose bonds delaminated no faster than loose-to-tight bonds. The glueline nearest the sunny side of the specimens delaminated no faster than the line nearest the shady side. While percent of wood failure proved to be the best single indicator of tight possible predictors analyzed, it accounted for only 17 percent of the variation in delamination, and hence could not be called good in this experiment. The best multiple regression accounted tor only 22 percent of the variation, in order of importance the significant factors were: percent wood failure (-), wet rolling-shear strength (-) , strength gravity (0), and compression strength parallel to grain (-).

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