Rapid Evaluation of Glue Joints in Laminated Timbers

Specimens of 6 by 6 inches in cross section and 1, 2, and 3 inches long along the grain were taken from laminated beams of Douglas-fir and white oak glued with urea-resin glue; from Douglas-fir, southern pine, and white oak glued with phenol-resorcinol; and from Douglas-fir and red oak glued with resorcinol resin adhesive. The specimens, submerged in water in an autoclave, were subjected to vacuum of 25 inches of mercury for 1 hour, then pressure of 75 ? 2 psi was applied for an hour, dried at 112?F. and 20 percent relative humidity for 20 hours. This cycle was repeated. In the third cycle, steam at 212?F. flowed through the autoclave for 1-1/2 hours, after which specimens were covered with water and pressure of 75 ?2 psi as described in the first cycle. Test results indicated well-cured resorcinol and phenol-resorcinol glue joints did not separate appreciably when subjected to test, whereas joints glued with urea delaminated severely in all specimens. Further tests used specimens of 3-inch-long sections from 6 by 6 inch beams of white oak, Douglas-fir, and southern pine. Four glues were used with each species: phenol-resorcinol resin, room temperature-setting urea resin, high-temperature-setting melamine, resin, and room-temperature-setting urea resin mixed equal parts by weight and thermosetting polyvinyl resin. Twelve 6 by 6 inches in cross-section beams, one for each glue-species combination, were glued up. Specimens from all 12 beams were also provided for block shear test. The first procedure was carried according to ASTM D 1101-59 except, at the beginning of the second cycle, steam at 212?F. was permitted to flow through the autoclave for 1-1/2 hours and a pressure of 150 ? 5 psi was applied for 2 hours. The second procedure is a 3-cycle test. it consists of impregnating the specimens with water using a 5-minute vacuum period followed by a 1-hour pressure period at 75 ?2 psi. This vacuum and pressure series is repeated. The specimens are dried at 150?F. and 15 percent relative humidity for 21-5/6 hours. This completes the first cycle and the two remaining cycles duplicate the first in the third procedure, specimens were placed in an autoclave, stickered and weighed, submerged in water of 65 to 80?F., vacuum of 25 inches of mercury was drawn and held for 5 minutes. Then pressure of 75 ?2 psi was applied for 1 hour. The autoclave was drained and steam at 2l2?F flowed through the autoclave for 1-1/2 hours, dried for 4 hours at 175?F., and with air-circulation of 500 fpm. Test results showed the specimens glued with phenol-resorcinol developed very little or no delamination regardless of species involved. Those glued with urea resin showed a great deal of delamination. The melamine-urea resin combination showed better performance than the thermosetting polyvinyl glue.

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