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Experimentation and modeling of hot pressing behaviour of veneer-based composites Wang, Brad Jianhe

Abstract

To understand the unique hot pressing behaviour of veneer-based composites, the key objectives of this research were to: (1) characterize air permeabilities of veneers and gluelines; (2) examine fundamentals of veneer contacts for bonding; (3) explore mechanics of veneer compression; and (4) develop the first hot pressing simulation model to predict heat and mass transfer and panel densification in veneer-based composites. To determine the mechanism of heat transfer, transverse air permeabilities of aspen (Populus tremuloides) veneers, phenol formaldehyde (PF) gluelines and aspen plywood/strandboard were first investigated. The laminate permeability theory was adopted to determine the relative contribution of the veneers and gluelines to panel permeability. A concept of effective porosity was also proposed based on the classic Carman-Kozeny theory to explain the difference in panel permeability and resulting hot pressing behaviour. It was concluded that veneer compression rather than glueline curing serves as the main barrier to gas and moisture movement, resulting in a negligible rate of transverse heat convection during hot pressing. By examining veneer compression behaviour under various temperature and moisture content (MC) conditions, a revised wood transverse compression theory was proposed to include the first stage of "progressive contact" and define true yield displacement, and a novel method was developed for assessing veneer surface roughness/quality on an area basis. Through analyses of veneer contact area (glue coverage) under changing loads, the minimum compression required for achieving adequate veneer-to-veneer contacts was determined. Furthermore, the Greenwood and Tripp's contact theory was modified to explain how veneer surface roughness and compression affect panel bonding contacts. This led to the establishment of an optimum panel densification target for performance plywood/LVL products and shed a light on how to help prevent panel delamination. Finally, the revised compression theory was applied to characterize veneer compressive stress-strain relationship, creep and springback behaviour during hot pressing. By introducing a concept of wood-glue mix layers and applying theories of heat and mass transfer and solid mechanics, a one-dimensional hot pressing model was successfully developed and validated. The model can predict the changes of temperature, MC, gas pressure and vertical density profile during hot pressing. The outcome of this research provides insight into the plywood/LVL hot pressing processes.

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