Wood and Fiber Science (E-Journal)
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Theoretical Thermal Conductivity Equation for Uniform Density Wood Cells
Get PDFThe anisotropy of wood creates a complex problem requiring that analyses be based on fundamental material properties and characteristics of the wood structure to solve heat transfer problems. A two-dimensional finite element model that evaluates the effective thermal conductivity of a wood cell over the full range of moisture contents and porosities was previously developed, but its dependence on software limits its use. A statistical curve-fit to finite-element results would provide a simplified expression of the model's results without the need for software to interpolate values. This paper develops an explicit equation for the values from the finite-element thermal conductivity analysis. The equation is derived from a fundamental equivalent resistive-circuit model for general thermal conductivity problems. Constants were added to the equation to improve the regression-fit for the resistive model. The equation determines thermal conductivity values for the full range of densities and moisture contents. This new equation provides thermal conductivity values for uniform-density wood material using inputs of only oven-dry density and moisture content. An explicit method for determining thermal conductivity of uniform density wood cells has potential uses for many wood applications
Improving Core Bond Strength of Particleboard Through Particle Size Redistribution
Get PDFNovel particleboard furnish mixtures were formulated to improve the core-bonding and screw-holding of industrial particleboard without increasing resin content or board density. Single-layer (uniform vertical density with core furnish only) and conventional 3-layer particleboards were manufactured at two density levels from four novel mixes plus control (unscreened industrial core furnish). Board mean and core density, internal bond strength, edge screw withdrawal resistance, and moduli of rupture and elasticity were measured.The core of commercial furniture-grade particleboard appears to contain too many fine particulates and insufficient coarser particles. Uniform density profile single-layer boards containing novel mixes with higher-coarse (>2 mm) and lower-fines
Improvement of Wood CT Images by Consideration of the Skewing of Ultrasound Caused by Growth Ring Angle
Get PDFFor the purpose of removing distortions in ultrasonic computerized tomographic (CT) images of wood, this study proposes a technique for taking into account the skewing effect in reconstructing the image. First, it was experimentally confirmed that an ultrasonic wave is refracted because of the anisotropic characteristics of wood. Transmission paths of an ultrasonic wave through a cross-section of wood were predicted by considering the change in wave velocity based on the annual ring angle and the presence of juvenile wood. Then, the methodology of the application of the predicted paths to CT image reconstruction was proposed and verified. The accuracy of defect detection in wood was significantly improved by the proposed technique
Improving the Color Stability of Wood-Plastic Composites Through Fiber Pre-Treatment
Get PDFWood-plastic composites (WPCs) continue to make inroads into the decking market. One of the main drawbacks from the consumer's perspective is that they can fade during weathering. It has been shown that WPCs fade more when exposed to a combination of light and water spray than when exposed only to light. This suggests that the loss of the color-imparting extractives from the wood could be partly to blame for color fade. Pigments are routinely added to only the plastic matrix to combat WPC color fade despite the fact that change in color could be due primarily to changes in the wood component of WPCs. In this study, we investigated two methods of fiber pretreatment in an attempt to improve the color stability of WPCs. The first was to remove water-soluble extractives from wood flour through a series of washes. Washed wood flour was then compounded with high-density polyethylene and injection-molded into test specimens. The second method involved coloring wood flour with either a water-based dye or an oil-based stain. Colored wood flour was also compounded with high-density polyethylene and injection-molded into test specimens. Pigments were added to half of the composites containing colored wood. The composites underwent accelerated weathering in a xenon-arc weathering apparatus, and were monitored for changes in color. Washing wood flour and coloring wood flour with a water-based dye were ineffective for improving composite color stability during weathering. However, coloring wood flour with an oil-based stain resulted in a more color stable WPC
A Percolation Model for Electrical Conduction in Wood with Implications for Wood-Water Relations
Get PDFThe first models used to describe electrical conduction in cellulosic materials involved conduction pathways through free water. These models were abandoned in the middle of the 20th century. This article re-evaluates the theory of conduction in wood by using a percolation model that describes electrical conduction in terms of overlapping paths of loosely bound or capillary water (Type II water). The model contains two physical parameters: wc, the critical moisture fraction, which is the amount of water required to form a continuous path of Type II water in wood; and σc, the conductivity of the aqueous pathways. The model gives a good fit to previously published data of the DC conductivity of wood when wc is equal to 16% moisture content and σc is equal to 0.88 S m-1. This analysis indicates that electrical conduction in wood can be explained by percolation theory and that there exists a continuous path of Type II water in wood at wc, which is below the traditional fiber saturation point
Mechanical Properties of Lumber from Partially CAD-Deficient Loblolly Pine (Pinus Taeda)
Get PDFFast-growing loblolly pine produced by the North Carolina State University-Industry Co-operative Tree Improvement Program is a natural carrier of a rare gene, cad-n1, an allele code for deficiency in cinnamyl alcohol dehydrogenase (CAD), an enzyme that catalyzes the last step in the biosynthesis of lignin precursors. Wood from totally CAD-deficient loblolly pine trees is known to have low mechanical properties, but not much is known about the mechanical performance of wood from partially CAD-deficient trees. The effect associated with this genetic modification of loblolly pine was evaluated by comparing the mechanical properties in bending, compression parallel and perpendicular to the grain, tension perpendicular to the grain, and shear parallel to the grain of small clear specimens of wood from partially CAD-deficient with those from wild-type loblolly pine trees. Results indicate that there is no significant difference between the two genotypes for all mechanical properties measured
Influence of Machining Parameters on the Tensile Strength of Finger-Jointed Sugar Maple Lumber
Get PDFPresently, finger-jointed softwood lumber is used in manufacturing of structural engineered wood products such as glued laminated (glulam) beams and prefabricated wood I-joists. However, the use of high-density hardwoods appears to be an attractive alternative material to achieve a higher performance of these products. Certain machining parameters have to be controlled in order to produce suitable gluing surfaces and to optimize the finger-jointing process. The main objective of this study was to evaluate the effect of machining parameters on the ultimate tensile strength parallel to grain (UTS) of finger-jointed sugar maple dimension lumber. Three different chip-loads and three cutting speeds were used as variables. Based on test results, the cutting speed appeared to influence the UTS the most. The best average UTS (47.1 MPa) of finger-jointed sugar maple specimens was achieved with a chip-load of 0.60 mm and a cutting speed of 2726 m/min (rotation speed of 3250 RPM and feed speed of 11.7 m/min)
Measuring and Simulating the Effects of the Pressing Schedule on the Density Profile Development in Wood-Based Composites
Get PDFThe cross-sectional density profile of wood-based panels has a strong impact on many of its end-use properties. In modern panel manufacture, not only press closure but also changes in mat thickness later in the pressing cycle are of importance for density profile development, particularly in the MDF process. A full-factorial analysis of the effects of the pressing cycle on the density profile is presented, with the factors considered being press-closing time, mat thickness level after first densification, and time of secondary densification. It was found that the mat thickness level after first densification dictates the density difference between surface and core regions. For pressing schedules that include a second densification step, intermediate density maxima appear, with magnitude and position of the intermediate density peaks being determined mainly by the first density level and the time span between press closure and final densification. Computer simulations are used to link the density variations to the conditions of local temperature and moisture content during pressing
Impact of Mountain Pine Beetle-Attacked Lodgepole Pine Logs on Veneer Processing
Get PDFPilot plant tests and mill trials were conducted to quantify the impact of using mountain pine beetle(MPB)-attacked lodgepole pine (Pinus contorta Dougl.) wood on green veneer processing, and determine if it makes economic sense to sort and process MPB logs separately from normal logs of white SPF (spruce-lodgepole pine-alpine fir) mix for plywood manufacturing. The results demonstrated that log dry-out, improper log conditioning, and veneer peeling contribute to the breakage of veneer ribbon, and in turn, loss of veneer recovery at the green end when processing MPB wood. Compared with the green SPF veneer controls, green MPB veneer has lower moisture content (MC) with smaller variation. The MPB veneer can be clipped narrower with an equivalent of 1% increase in recovery due to less width shrinkage, and be sorted more accurately requiring only two green sorts: heart and light-sap. The MPB veneer can also be dried faster with a reduction in drying time by about 25% for the heart veneer and 35% for the light-sap veneer. However, due to higher volume of narrower random sheets and increased waste from manual handling and composing, the net recovery of the MPB logs is about 8% lower than that of the control SPF logs. Furthermore, the color of the stained MPB veneer is lightened after drying, but it still causes interference with visual grading. Since MPB wood has unique MC and processing characteristics, it is recommended that it be sorted in the log yard when its proportion reaches about 10% of the total logs procured