Determining Pore Size Distribution in Softwoods with a Mercury Porosimeter

In wood processing methods involving movement of substances including water into and through wood, pore size distribution is more important than knowledge of total pore volume. The principal purpose of this investigation was to compare softwood pore size distributions obtained with a high pressure mercury porosimeter with distributions obtained from measurements of microscopic projections of matched material. A second objective was to examine the possibility of obtaining a valid estimate of capillary surface area of wood by use of porosimeter data. A third objective was to determine the density of the cell wall from the porosimeter data. The pore size distributions of Pinus taeda L., Picea engelmannii Parry, and Chamaecyparis lawsoniana A. were determined with matched samples by mercury porosimeter using ovendry samples and by microscopic projection methods using, dehydrated micro-sections. Capillary surface areas were calculated from these distributions. Cell-wall density values were also calculated from porosimeter data. The median values of the porosimeter pore size distributions ranged from 2 to 9 microns less than the microscopic projection values. No fully satisfactory explanation for these differences was apparent. The porosimeter capillary surface area values were 50 to 100 percent larger than the projection values but were still somewhat below published estimates. The average cell-wall density values of pine – 1.445; spruce 1.440; and cedar – 1.445 were in excellent agreement with helium displacement values reported as 1.46 and gave no evidence of voids within the cell-wall structure in the ovendry state.

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