Accelerated Aging Methods for Exterior Plywood Faced with Southeast Asian Hardwood Veneers

This study compared five aging methods for detecting unsatisfactory gluebonds in exterior plywood faced with Southeast Asian hardwood veneers. The methods were: vacuum-pressure soak (PS 1-74); standard boil (PS 1-74) for 2, 5, 10, or 25 cycles; automatic boil (ASTM D-3434) for 20, 40, 100, or 200 cycles; weatherometer (ASTM G-23-69); or wet-dry cycling. Veneers from Southeast Asian logs were used to face plywood panels (4 by 4 ft., 3 ply, 3/8-in. thick) made with Douglas-fir cores. Three adhesives were used with assembly times of 5, 20, and 45 minutes and a press time of 5-1/2 minutes at 300?F. Matched samples were subjected to the aging methods. The best aging method was considered the one that caused the least amount of wood failure when samples were tested and correlated well with test performance after 12 months of outdoor exposure. When vacuum-pressure soaked and sheared after 12 months of outdoor exposure, many samples of red balau and kapur were low in wood failure–30 to 50 percent–and some samples of keruing were near or below 85 percent wood failure. Before outdoor exposure, these poor bonds were not detected consistently by shearing samples after vacuum-pressure soak or two cycles of standard boil. Delamination of samples aged by the weatherometer or wet-dry cycle was not sufficient to indicate poor bonds. Poor bonds appearing in panels after 12 months of outdoor exposure could be predicted by 5 or more cycles of standard boil, or by 20 or more cycles of automatic boil. The correlation coefficients, R, between these accelerated-aging methods and outdoor exposure were about 0.9. A 2-day manual boil cycle caused nearly as much bond degradation as the 40-cycle automatic boil. Keruing seems to respond to type of adhesive more than other species. An adhesive of low molecular weight resulted in consistently good bonds, but the performance with an adhesive of high molecular weight was sporadic. Neither of these adhesives bonded kapur satisfactorily.

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