Effects of Veneer Surface Roughness on Gluebond Quality in Southern Pine Plywood

Inconsistent gluebond quality continues to be a periodic problem for the southern pine plywood industry. There has been a reduction in size and quality of southern pine veneer logs due to cost, demand, and availability factors. This reduction has sometimes led to increases in the yield of rough veneer surfaces. This study investigated how veneer surface roughness and related material and processing factors can affect the quality of plywood gluebonds. Veneer surfaces were visually classified into three roughness classes: rough, intermediate, and smooth. Four glueline roughness combinations were assembled in three-ply (1/2-in.) test panels: rough/rough (R/R), intermediate/rough (I/R), rough/smooth (R/S), and smooth/smooth (S/S). Assembly time, veneer temperature, spread rate, press pressure, and glue mix resin solids were the other variables included in the study. Two levels of each of these variables yielded a 25 by 4 full factorial experiment. The entire experiment was replicated once for a total of 256 test panels. Plywood shear specimens from each panel were tested using the following: vacuum pressure, boil/dry test pretreatments of PS1-74, and simple dry shear test, Wet and dry shear strengths and wood failures were recorded and evaluated. The results indicate that average wood failure is significantly reduced by rougher veneer surfaces. The interaction of glueline roughness, assembly time, and veneer temperature resulted in an even greater reduction of wood failure than expected when conditions for overpenetration or wash-out were encountered. If further studies should show that glueline roughness also reduces wood failure when conditions for dry-out are present (as is the observation and belief in most southern pine plywood plants), then the overall effects of rough veneer are to reduce the maximum wood failure that can be achieved and reduce the allowable range of material and manufacturing conditions which produce maximum wood failure.

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