Evaluation of Sweetgum, Yellow-Poplar, and Swamp Tupelo for Use in Structural Plywood

Three important medium-density southern hardwoods were investigated for use in structural plywood to determine compliance with U.S. Product Standard PS 1-74 (now PS 1-83), Percent wood failure and shear strength were determined from all face-core combinations of sweetgum, yellow-poplar, swamp tupelo, and southern pine. Gluebond durability was evaluated by the vacuum pressure soak and standard two cycle boil aging methods which are outlined in PS 1-74, and with an eight cycle boil aging method. The effect of core ply lathe check orientation of shear specimens on percent wood failure and shear strength among panel combinations was also determined. Panels were 24-inch square, 3/8-inch nominal thickness, and three-ply construction. Independent panel processing variables included in the study were two commercial phenolic adhesives and the following three assembly times: 10, 20, and 40 minutes. Although significant wood failure and shear strength differences were found among panel constructions, all hardwoods that were evaluated, either alone or in conjunction with southern pine, conformed with PS 1-74 exterior durability specifications when processed under conditions commonly used with southern pine. The data suggest that sweetgum may be a troublesome species. The vacuum-pressure soak and two-cycle boil aging methods were as effective in detecting nondurable gluebonds for the hardwood panels as they were for southern pine panels. For all panel constructions tested, higher wood failure and shear strength were produced when core ply lathe checks were pulled closed than when they were pulled open. The effects of lathe check orientation on the hardwoods did not significantly differ from those for pine in wood failure but did exhibit higher shear strength when used as core.

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