Wood is categorized based on its botanical origin relative to its anatomical structure. Generally speaking, wood is classified as either a hardwood (angiosperm) or softwood (gymnosperm). Softwoods are comprised of about 90% long fiber cells or tracheids that serve as a means of both mechanical support and fluid transport. Approximately 10% of the cells in softwoods are ray parenchyma tissue that contribute to horizontal fluid conduction. Certain softwood species also contain resin canals which conduct fluids. Hardwoods are comprised of fibers, parenchyma, and vessel cellular elements. The fibers provide the mechanical support and the vessel elements or pores conduct fluids. During tree growth, the cellular structure changes over the growth season. The change in cellular structure contributes to growth rings or annual increments in the tree stem. In softwoods, the tracheids in springwood, or the early part of the growth increment, are rounded and thin-walled. Tracheids in the late part of the growth ring or summerwood are rectangular and thick-walled. In hardwoods, the vessels or pores can be evenly sized and distributed throughout the growth ring (diffuse porous) or the springwood vessel elements can be large and the summerwood pores can be small (ring porous). The cellular structure of wood results in a surface roughness across a range of scales including macroscopic and microscopic.
Wood is considered an anisotropic material and has three reference planes of orientation including transverse or cross-section, radial, and tangential. A cross-sectional surface is formed by cutting a log or piece of lumber to length, while radial and tangential surfaces result from cutting along the grain. A radial surface is made by cutting longitudinally along the radius of a round cross section. Tangential surfaces result from cutting perpendicular to a radius. Wood also contains knots which are tree branch bases embedded within the wood structure.
All woody material is composed of the primary polymeric components cellulose, hemicellulose and lignin. Cellulose makes-up 50% of wood weight. Hemicellulose makes-up 25 to 35% of the weight of wood and lignin makes-up 16 to 33% of the weight of wood. The amount of hemicellulose and lignin in wood differs mainly depending on whether it comes from an angiosperm or gymnosperm source. Extractives in wood also vary depending on the source, and can comprise from 1 to 10% of wood weight. Cellulose and hemicellulose are polymers comprised of monomeric carbohydrates including glucose, galactose, mannose, and xylose, etc. Lignin is a polymeric material based on phenyl-propanoid (C-9) units. The major difference in the chemistry of lignin is whether the source of lignin is based entirely on guaiacyl monomers (gymnosperm or softwood lignin) or a 50:50 mixture of guaiacyl and syringyl monomers (angiosperm or hardwood lignin).
The secondary components of the wood are referred to as extractives since they are easily separated from the primary wood components using either water or organic solvents. The extractives in lignocellulosics are low molecular weight by-products of physiological processes and include a vast array of chemical compounds such as fatty acids, waxes, and polynuclear aromatic compounds. Although extractives constitute a small weight percentage of the chemical composition of wood, extractives dominate the surface chemistry of wood. The primary reason for this is because extractives tend to concentrate at the wood surface when the wood is dried. As water is volatilized from the wood during drying, many extractives move tp the surface of the wood, and those that are not volatilized with the water will remain on the surface. In a tree, the extractive content is greater in the center portion of the stem (heartwood), and lower in the outer portion of the stem (sapwood). Thus, the wood produced from sapwood and heartwood will have a different chemical character.
In addition to the anatomical and chemical composition of hardwoods and softwoods, the physical properties such as density, moisture content, porosity, and surface roughness of wood also influences surface properties. The density of wood can vary from 0.10 g/cm3 for Balsa up to 1.1 g/cm3 for lignum vitae. Anatomical differences between hardwoods and softwoods, and among different woods, in general, contribute to differences in surface roughness. The different surface roughness among woods also contributes to different surface areas. For example, the surface area of Sugar Pine wood based on the adsorption of stearic acid is about 0.23 m 2 g -1. The moisture content of wood can vary from 6 to 8 % for most wood used indoors to 18 to 20 % for wood in protected exterior conditions. The density, porosity and moisture content of wood influence liquid adhesive penetration, and the formation of mechanical interlocks with cured adhesives. Penetration of liquids into springwood is usually easier than into denser and less porous summerwood. The penetration of liquids into wood depends on the molecular size and viscosity of the liquid. Penetration into wood decreases as the molecular size and viscosity of the adhesive increases.
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