Modeling viscoelastic cellular materials for the pressing of wood composites

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Virginia Polytechnic Institute and State University

With the large number and diversity of materials available today, the ability of the manufacturer to control properties is critical for the success of a product in the market. Although we have little or no control over the engineering properties of solid wood, the potential for the design of material properties in composites is great. Large strides are presently being made in the design of non-veneer structural panels by using material science principles. However, a large gap in our knowledge of the composite system is in the understanding of how raw material properties and processing variables interact to influence the internal geometry and material properties of the components in situ. The ability to use production variables to control material properties of the composite is an extremely valuable tool.

The goal of this research is to provide an understanding of how the heat and mass transfer inside a flakeboard during pressing, interacts with the viscoelastic behavior of individual flakes to influence density gradient formation and in situ flake properties. The specific objectives:

l. To use observed changes in the temperature and gas pressure of the internal environment of panels during the pressing cycle to describe the composition of the gas phase.

  1. To use the calculated composition of the gas phase and measured temperature for the internal environment as boundary conditions for a fundamental heat and mass transfer model to access changes in the temperature and moisture content of the wood component during pressing.

  2. To use the temperature and moisture content relations above to qualitatively relate press conditions to the formation of density gradients through changes in the glass transition temperature of the amorphous polymers in wood.

  3. To utilize micromechanical models of cellular materials in conjunction with linear viscoelasticity of polymers to develop a nonlinear viscoelasticity model for wood in transverse compression.

The approach couples the viscoelastic behavior of the amorphous polymers in wood with the structure imposed by anatomy. These theories, if applicable to wood, can greatly simplify the study of many similar systems combining environmental conditions and mechanical properties.