Bark Boards from Four West Coast Softwood Species

Boards composed of bark constitute a new class of products with different properties from the types of wood particleboards presently being manufactured. Research was conducted on the effect of particle size and geometry, resin level, and board specific gravity on the properties of boards produced entirely from the barks of western larch, Douglas-fir, and ponderosa pine. Boards produced with particles passing through a 6-mesh screen and retained on a 14-mesh revealed relatively comparable bending and internal bond strengths for Douglas-fir and ponderosa pine. Identical boards produced from larch bark, however, showed lower internal bond strengths, but much higher bending strengths. All of these boards had very high linear expansions. The performance of the larch bark was of particular interest. Larch bark tends to break down into flaky particles during attrition or hammer-milling. This type of particle contributes to better bending strength. On the other hand, Douglas-fir and ponderosa pine barks break down into chunky particles which enhance internal bond values. This relationship of particle geometry to board properties is also characteristic of conventional wood particleboard. Other work on western redcedar bark yielded boards with suitable bending strengths but low internal bonds. Small amounts of impregnating resin added to bark particles increased their inherent strength and afforded boards with higher bending and internal bond values. Of greatest significance was that combinations of western redcedar bark on the faces and Douglas-fir bark in the core resulted in boards meeting low- and medium-density particleboard specifications in bending and internal bond. Additionally, these boards had very low linear expansion values close to those of particleboard. Thickness swell values of larch, ponderosa pine, and Douglas-fir boards in the 24-hour water-soak test were similar to those of wood particleboard with Douglas-fir bark board values being very close to those of Douglas-fir particleboard. The three-layer boards had much higher water absorption and slightly higher thickness swell values. Preliminary evaluations on insulation-type Douglas-fir bark panels showed thermal conductivity values similar to insulation-type fiberboard.

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