Durability of Fortified Urea-Resin Exposed to Exterior Weathering

A series of yellow birch (Betula alleghaniensis Britton), yellow-poplar, (Liriodendron tulipifera L.), and Douglas-fir (Pseudotsuga menziesii (Mirb) Franco) plywood panels were made with 10 laboratory-formulated, hot press urea-resin glues combining different amounts of melamine-resin or resorcinol and paraformaldehyde fortifiers. Duplicate sets of panels were exposed to weather over a 12-year period near Madison, Wis. Amounts of delamination were observed at intervals. Plywood shear specimens were cut from one panel of each set after 7 and after 12 years of weathering and tested under three sets of conditions. Results indicated that any fortification was better than the unfortified glue on exposure, but that only the two highest resorcinol fortifications closely approached the durability of the straight melamine-resin glue. Delamination tended to increase as the amount of each fortifier decreased. This effect was most apparent on yellow birch panels and least apparent on Douglas-fir panels. The effect was generally more noticeable on the 3-ply, 3/16-inch panels than on the 5-ply, 5/8-inch panels. Differences in shear test results were less conclusive and tended to be more erratic than were corresponding differences in amounts of delamination.

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