Hygroscopic Properties and Shrinkage of Southern Yellow Pine Plywood

Five constructions of southern yellow pine plywood, unidirectionally laminated veneer, and loblolly pine solid wood were used for investigation of hygroscopic properties at several RH conditions. EMC values of the southern yellow pine plywood between 20% RH and 90% RH were approximately 1% lower than corresponding values of loblolly pine solid wood. Most plywood panels shrank less along the face grain than across. From water-soaked condition to about 4.5% MC, thickness shrinkages of plywood varied from 7% to 9% while length shrinkages were always less than 1% except for 1/2-inch, 4-ply plywood. The ratio of number of glue lines to plywood thickness influenced thickness and volume shrinkage. Plywood with a higher ratio had a tendency to shrink less. FSP values were determined by two intersection point methods-one with volumetric shrinkage, the other with MOE. The respective approximate FSP values were 28.7% and 27.3% MC for plywood, and 31.0% and 27.5% for solid wood.