Moisture Transport and Changes in Mechanical Properties in Oriented Strand Board: Experimental and Modeling

• Author / Creator

  Araya Olguin, Rodrigo J

• The demand by the building industry for engineered wood composites is continuously increasing due to its importance as a structural building material. Even though the forest products sector is constantly working to find solutions to improve the design of material properties, research to predict the performance of wood-based composites in service (e.g., subject to moisture) for different manufacturing variables are limited. The objective of this thesis was to investigate and predict the moisture content and transport in unidirectional oriented strand board (OSB) based on various material and manufacturing parameters including strand grain direction, wax content, and panel density. In addition, the effect of moisture content on changes to mechanical properties including modulus of elasticity (MOE) and modulus of rupture (MOR) were also studied. A three-dimensional finite element model (FEM) was developed to predict the moisture distribution in unidirectional OSB panels, and was used to determine the effective diffusivity coefficients of various panel parameters based on a series of moisture absorption experiments. The FEM was based on Fick’s second law which was found to provide an accurate fit with the experimental data during the first stages of moisture ingress.
  The results showed that the moisture absorption and diffusivity coefficients of OSB panels were greater with decreasing panel densities, and were also greater for specimens without wax and in strand directions perpendicular to the grain. Specifically, the results show that a higher strand-strand contact or compact structure of the board reduced the void spaces, thus reducing the migration of water molecules to the board. Wax is another manufacturing variable which affects the moisture absorption and diffusivity coefficient. In the experiments, waxed and un-waxed OSB specimens were tested, and the results indicate that strands treated with the water-repellent additive reduced the water absorption rate in OSB but did not prevent overall moisture saturation (i.e. wax simply delayed moisture transport). The grain direction of the strands was another factor affecting the diffusivity coefficient and water transport behaviour in composite boards. The findings show that water uptake parallel to the strand grain direction was greater than in the perpendicular to grain direction due to the natural structure of the aligned wood cells (lumen).
  In addition, the findings show that board density, moisture content, and grain direction have a significant effect on bending properties. The MOE and MOR of panels both decreased as panel moisture content increased. At lower average moisture contents, the panel density had a much more significant effect on bending properties relative to higher moisture contents (>25%) where changes in board density did not significantly affect bending properties. Overall, wax content did not show any significant effects on MOE and MOR at a given average moisture content. The resulting MOE and MOR datasets for unidirectional OSB panels were also fit to a polynomial mathematical model with first order coefficient for density and second-order coefficients for moisture content.
  The results obtained from this research provides valuable information on further understanding and modeling of moisture absorption/distribution in OSB panels exposed to water. Tests performed on unidirectional panels also provide a unique set of basic property data at the ply level which can be used in subsequent studies to create more complex models of real OSB systems (e.g., commercial panels). These predictive tools could then be used to possibly improve OSB performance and durability in existing and new applications.

• Subjects / Keywords

  • OSB
  • Moisture Movement
  • Wood composites

• Graduation date

  Fall 2021

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-0p9f-b097

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.