Controlling thickness swell in oriented strand composites

Thickness swell in non-veneer wood composites can be much greater than swelling in solid wood of the same species and has been linked to panel degradation. For this reason, it is a topic of significant concern for structural composites such as oriented strandboard and oriented strand lumber. Strength loss due to thickness swell originates from two sources: 1) permanent density reductions due to non-recoverable thickness swell; and 2) fractures arising from swelling stresses. This study characterized the swelling behavior of an oriented strand composite and applied a model to estimate the damage in an oriented strand panel arising from swelling stresses. Thickness swell in the strand composite studied was shown to increase with increased density and moisture content and decrease with increased resin content. Damage due to differential swelling stresses followed similar trends. Extrapolation from empirical relations developed in this study indicate that an methyl di-isocyanate (MDI) resin level of approximately 12.5 percent should control thickness swelling in a strand composite, and a resin content near 10 percent should minimize strength loss due to differential swelling stresses. While these resin levels are significantly higher than those currently in use, the advantages (higher price) of a durable (value-added) strand composite may maintain profitability. Additionally, increased resin levels would allow for density reductions to maintain a given strength and further reduce swelling. This impact would decrease costs of wood, drying, pressing, and shipping, which could partially offset increased resin costs. Increasing resin and decreasing density is a starting point for creating a durable strand composite.