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Extensive experiments were conducted to investigate the transverse (vertical) air permeability of trembling aspen ( Populus tremuloides ) veneer and phenol formaldehyde (PF) gluelines, as well as aspen plywood and strandboard. The theory for permeability through laminates was used to determine the relative contribution of the veneer and glueline to panel permeability. Based on the classic Carman-Kozeny theory for porous materials, a concept of effective porosity was proposed to explain the difference in panel permeability and the resulting hot-pressing behavior. The results show that, for veneer-based panels, the panel compression ratio (CR) was the most important factor affecting panel permeability, followed by the sapwood/heartwood composition and glueline. As a 3–7% CR was reached, the panel permeability substantially decreased by approximately 80%. On average, the permeability of sapwood veneer panels was approximately four-fold higher than that of heartwood veneer panels. The glueline permeability decreased during glue curing, but the reduction from the uncured to the cured state was only ∼15%. In addition, the glueline permeability also decreased with increasing glue spread. At the normal level of glue spread for plywood, the average permeability of cured gluelines was approximately 20% of that for non-compressed veneer, but was close to that for veneer with a 3–7% CR. Since the thickness of the glueline was only ∼5% of that for normally peeled veneer, the net contribution of the glueline to panel permeability was limited. During hot pressing, therefore, the small deformation of the veneer ply effectively acts as the main barrier to gas and moisture movement, rather than the curing glueline. The rate of convection is negligible. The effective porosity in veneer panels was only ∼0.05–0.5% compared to the total panel voids, which ranged from 50% to 70%. At the same panel density, veneer panels had much lower permeability than strandboards. Although these commercial panel types had almost the same average permeability, the veneer panels showed approximately two-fold less variation due to the formation of layered and uniform gas and moisture barriers. As a result, the temperature and gas pressure responses during hot pressing were essentially different between plywood and strandboard.

Air permeability of aspen veneer and glueline: Experimentation and implications