The Effect of Dryer Temperatures Upon the Gluing Properties of Douglas- Fir Veneer

Drying Douglas fir veneer commercially at temperatures ranging up to 394?F. is believed to be the cause of problems in the gluing of plywood. Dryer temperatures of 330?, 444?, and 543?F. for drying times of ovendry, ovendry plus 10 minutes, ovendry plus 20 minutes, and ovendry plus 30 minutes were tested. Interior and Coast types of veneer were used. Drying methods provided a range of drying temperatures and times which could not be obtained in factory drier but which were essential to the study. Veneer specimens were conditioned at 70?F. and 32 percent R.H. and glued with a hot-press phenolic resin and with the tight faces together. Bond quality was tested by tension shear, using a 2-ply specimen; by tension normal to the glue line; and by a glue-line cleavage test. The highest wood failure (90-100 percent) occurred at the point where temperature and drying time were at a minimum, namely, 330?F. for time required to ovendry wood. Breaking loads for the Tension-Normal test ranged from 128 pounds, for specimens dried at 330?F. to zero (OD) moisture content, to 64 pounds for those dried at 543?F. to zero moisture. The same trend in breaking loads was obtained for all three methods of testing. It was concluded that: 1) The strength of Douglas-fir and glue-bond strength were reduced by drying veneers at excessively high temperatures for excessively long times. 2) Veneers dried at 444?F. developed a greater percentage of glue failure than those dried at 330?F. 3) Veneers cut from trees which grew in the Interior of British Columbia developed a greater percentage of glue failure when dried at high temperatures than did those from trees which grew in the coastal area. 4) Wood failure results did not give a very reliable estimate of bond quality. Two other series of tests were run and essentially the same conclusions as those presented above were reached. The low wood failure was attributed neither to any loss of adhesion nor to any decrease in the cohesive strength of the glue, but to a reduction in the strength of the wood. It was reasoned that the loss of wood strength was most severe in the fibers at the surface of the veneer, and that this tended to produce very shallow wood failure.

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