Effect of Resin Treatment and Compression Upon the Weathering Properties of Veneer Laminates

Five-ply veneer laminates, both hardwood and softwood, gain improved weathering properties when impregnated with phenolic resin whether the resin is precured before assembly (impreg) or not (compreg). In a factorial experiment, 10-inch squares of 1/16-inch rotary-cut veneers from six hardwood and five softwood species were made into 24 panels for each species, or 264 panels in all. Variables for each species were three treatments (untreated, treated and precured, treated and uncured); two assemblies (crossbanded and parallel laminated); and four assembly pressures (250, 500, 750, 1000 psi). The hardwoods were sweetgum, sugar maple, yellow birch, yellow-poplar, cottonwood, and basswood; the softwoods were western hemlock, Sitka spruce, Douglas-fir, white fir, and redwood. Increases in specific gravity under compression were recorded. Wood with lowest initial specific gravity increased most. Major increases occurred in range of 250 to 500 psi, although for yellow-poplar and cottonwood at 750 to 1000 psi. At higher pressures, specific gravities approached limit of that of dry wood substance, 1.46. Panels were cycled 10 times for hot (200?F.) and cold (-20?F.) temperature extremes. Face checking and warping due to thermal changes were practically absent from the hardwood impreg and compreg. Softwood panels warped and checked, although less than untreated and press-cured compreg. No outstanding species difference was observed. Panels were cycled 10 times for wet (water soaked) and dry (ovendried @ 120?F.) humidity extremes. Hardwood specimens showed practically no degradation from the humidity cycles. No hardwood impreg and only one compreg showed any sign of checking. Degradation of softwood specimens was more severe. The humidity cycling was least drastic of the four treatments. After 8 months of outdoor exposure from June to February, only 41 percent of all specimens remained free from face checking. Check-free specimens were 5 percent untreated veneers, 50 percent impreg, and 45 percent compreg. After 27 months, all untreated panels face checked severely and all resin-treated specimens showed some. Higher pressure laminates showed more grain raising due to overstressed fibers.

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