Veneer and Assembly Condition Effects on Bond Quality in Southern Pine Plywood

The effects of veneer moisture content (average levels of 1.6, 3.1 and 7.7 percent), veneer temperature (78?, 110?, and 140?F), assembly temperature (78? and 100?F) and their interactions with assembly time (15, 30 and 60 minutes) on the bond quality of southern pine plywood were studied. Panels were for the most part 12 inches by 12 inches, 3-ply, 3/8 inch in size and construction (all from 1/8 inch thick veneer), although a few 5/8 inch thick panels were also made and tested. Evaluation of bond quality was based on percentage wood failure in the PS-1 standard vacuum-pressure test for exterior-grade gluelines. Analysis of variance for the data statistically confirmed the particular significance of veneer moisture content, assembly temperature and the interaction of assembly time with veneer moisture content. Bond dryout tendencies are shown to be greater at the low veneer moisture content and higher assembly time. Likewise, the bond quality-assembly, time relationship is markedly affected, by veneer moisture content. Shorter assembly times are better for very dry veneer, whereas longer assembly times improve bond quality for more moist veneer. The relationships noted in this study compare favorably with observations and theories from basic gluing studies. Results indicate that there is an important “moisture balance” factor which almost certainly affects matters of adhesive flow and penetration and may also be involved with thoroughness of cure.

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