Analysis of Thermally Modified Wood Strands and Thin Strand Veneers for Low-Value Balsam Fir

Joseph Roberson

Balsam fir is a species of wood located in the north-eastern regions of the US and around the Great lakesthat are typically small-diameter (less than 9-in), due to competition in the Great Lakes forests. Therefore, it acts as fuel for forest fires and is systematically logged to improve forest health by the United States Forest Service (USFS). Due to this, the USFS would like to determine whether there is a market for balsam-fir along the value chain for structural applications in strand-based composites. Thermal modification (TM) of balsam fir strands could improve its moisture and rot resistance which are key concerns when producing mass timber composites. The feedstock for mass timber products and for thin strand veneers are examined to determine the suitability of balsam fir strands. In the first part of the study, balsam fir wood strands were thermally treated in a closed inert environment with nitrogen gas at temperatures of 135°C, 150°C, and 165°C, with constant pressure of 1.4 MPa, and constant dwell time of 180 minutes. It was found that a higher intensity of thermal modification on the wood strands improved on the extraction of hemicellulose within the wood fibers, resulting in a 51.7% reduction in equilibrium moisture content (EMC). In the second part of the study, balsam fir strands were used to manufacture thin wood strand composites, which were then evaluated for their mechanical and physical characteristics. It was found that a higher intensity of thermal modification on the wood strands improved on the dimensional stability of the composites, resulting in a 71.8% reduction in thickness swell from the control strand composites. Prediction equations for cross-laminated strand veneer lumber (CLSVL) from laminated beam theory were used to predict the apparent stiffness of the mass timber composites that could be developed from the already manufactured feedstock. In the last part of the study, vacuum assisted thermal modification of balsam fir strands were examined along various temperature, pressure and dwell time parameters to determine the mechanical and physical properties of the wood strands under such conditions. Response surfaces were developed to reproduce the results given any change in parameter. Much like in the first part of the study, it was found that a higher intensity of thermal modification resulted in a 52.2% reduction in equilibrium moisture content (EMC).

Analysis of Thermally Modified Wood Strands and Thin Strand Veneers for Low-Value Balsam Fir

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