Characterization of Pacific Northwest softwoods for wood composites production

Christopher E. Langum

Forest management practices and the composition of forests are changing. Old growth trees that were once relied upon for timber production are becoming less available. Consequently, the forest industry has begun harvesting higher volumes of fast grown, small diameter trees. Due to the age of these trees, a larger percentage of juvenile timber is present, which in turn reduces physical and mechanical properties. It is critical to know and understand the characteristics of available wood resources for process and product optimization The goal of this research is to characterize wood from small diameter trees to effectively utilize them in engineered wood composites. This research focused on evaluating and examining the variation in physical and mechanical properties of small diameter Douglas-fir and western hemlock from the Olympic Peninsula of Washington State relevant to the production of wood-based composites. Testing was conducted in two stages. The first stage consisted of analyzing density profiles and conducting flexure, tension, and compression tests on clear specimens to determine respective modulus and rupture stress values. These values were examined to determine the influence of location with respect to height and diameter. The second stage of testing involved further processing the material into furnish typically used in modern engineered wood composites. This furnish was then further evaluated to identify differences based on location; properties evaluated were wood flour particle size distribution, pH and buffering capacity, and tensile Young’s modulus and rupture stress of typical OSB strands. Results from testing of clear specimens indicated the highest values of strength and modulus in tension, compression, and flexure could be expected from the bottom or mid-height region with further property increases as distance from the pith increased. Although some variation was encountered between species, the general trends remained the same. As for particle size distribution, analysis by location yielded very little variation with respect to location in both species; however, western hemlock did produce a greater overall percent of larger particles based on one particular processing technique. Similarly to the particle size distribution, analysis of pH and buffering capacity showed very little variation with respect to location in both species. Douglas-fir however, was considerably more acidic. Finally, tensile testing of strands indicated density and grain angle of the specimen played a much larger role in the quality of the strand than location. Unlike clear specimen properties, when height was considered, little variation with respect to strength or modulus was encountered. However, average strength and modulus reductions of up to 50% should be expected when comparing strand properties to clear specimen properties due to processing induced damage.

Characterization of Pacific Northwest softwoods for wood composites production

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