Chunping Dai

Hot-pressing stress graded aspen veneer for laminated veneer lumber (LVL)

Abstract Aspen (Populus tremuloides) is emerging as an important species for laminated veneer lumber (LVL) products in North America. During LVL manufacturing, both veneer stress grades and hot-pressing schedules are vital to product performance. In this study, an experimental design with four three-level variables comprising veneer moisture content (MC), veneer stress grade, platen pressure, and glue spread level was employed to investigate their effects on pressing behavior and stiffness and strength properties of LVL panels. The results show that, within the ranges studied, glue spread level and platen pressure were the two most important variables affecting the hot-pressing time needed for the innermost glueline to reach a target temperature of 105°C. The MC from the glue (mainly in the glueline) affected the rise of the innermost temperature more significantly than the MC in the veneer. Among the four variables studied, veneer stress grade and veneer MC were the two dominant variables that affect LVL stiffness and strength properties. Flat-wise and edge-wise bending stiffness (MOE) and strength (MOR) of LVL panels made from higher stress grade veneers were higher compared to those made from lower stress grade veneers, but there was no direct correlation between LVL shear strength and veneer stress grade. In addition, LVL edge-wise bending stiffness had the highest veneer MC tolerance among all LVL stiffness and strength properties. Further, LVL stiffness enhancement (the ratio of LVL MOE over veneer MOE) was lower with higher stress grade veneers than with lower stress grade veneers. These findings are useful for manufacturing high-stiffness LVL for engineered applications.

Heat and mass transfer in wood composite panels during hot pressing: Part 3. Predicted variations and interactions of the pressing variables

Abstract As a continuation of previous publications on a physical-mathematical model of heat and mass transfer and a structural model of mat permeability, this paper presents typical prediction results for 15 pressing variables for strand mats. A case study and complete solutions to the governing equations are provided. The results show how the heat and mass transfer is controlled by heat conduction and convection involving gas flow and phase change. The model predictions provide a comprehensive illustration of the temporal and spatial variations of basic pressing variables, including mat temperature, gas pressure, moisture control and resin curing rate. The model offers a powerful tool for simulating the effects of mat structure, pressing schedule and initial mat conditions.

Air permeability of aspen veneer and glueline : Experimentation and implications

Abstract 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.