The cell substance of wood is described chemically as a composite material consisting of three types of organic polymers: cellulose (40-50%), hemicelluloses (20-30%) and lignin (25-30%). These constituents serve as skeletal, matrix and encrusting substances, respectively. In addition, a minor amount of inorganic (ash) content (0.1-0.5%) is present in wood. Depending upon species, extractives (extraneous substances, 1-5%) may also be present, mainly in heartwood.
Cellulose, the major chemical constituent of wood, is in many respects the most important. It is also the most easily defined and described. Wood cellulose is chemically defined as (C6H10O5)n. The basic monomer units of glucose anhydride are alternately linked in forming long linear-chain polymeric cellulose with an average degree of polymerization (DP) of about 10,000. Figure 2.12 illustrates a representative portion of the cellulose molecule.
The hemicelluloses found in wood are linear polysaccharides of moderate size (DP averaging 150-200 or greater) of the types that are invariably associated with cellulose and lignin in plant cell walls. Predominant types include xylan (the principal hemicellulose in hardwoods), glucomannan, and galactoglucoman-nans (the major hemicellulose of softwoods). A small percentage of hemicellulose in all woods is acetylated and capable of releasing acetic acid on hydrolysis, especially under conditions of elevated temperature and relative humidity.
Lignin has complex three-dimensional polymeric structure comprising various phenyl propane units. Lignin apparently infiltrates and
Glucose anhydride CH2OH
Glucose anhydride CH2OH
Figure 2.12 A representative portion of a molecule of cellulose, the major chemical constituent of wood. The cellulose polymer is composed of glucose anhydride
Figure 2.12 A representative portion of a molecule of cellulose, the major chemical constituent of wood. The cellulose polymer is composed of glucose anhydride monomers significant decay resistance of cedars that results from the presence of tropolones.
The cellulose structure within cell encrusts the cell wall structure after the polysaccharides are in place. Although lignin contributes to the compressive strength of wood, tensile strength is provided principally by its cellulose.
Extractives are typically low-molecular weight compounds which are principally associated with heartwood formation and are located as much outside the cell wall as within. Extractives represented among the various species of wood fall within classifications such as tannins, terpenes, polyphenols, lignans, resin acids, fats, waxes and carbohydrates. In addition to changing appearance of the wood - mainly as colour - extractives may contribute to other properties of the wood, such as the
As the most important constituent of wood, the nature and orientation of cellulose determine the architecture of the cells. Insight into the configuration of the cellulose within cell walls provides an important key to understanding and anticipating many of the properties and the behaviour of wood. As a basis for discussion, Figure 2.13 depicts a conceptual model representing a typical longitudinal wood cell, such as a hardwood fibre or a softwood tracheid. The cell wall is layered. The outer layer, the primary wall, was the functional cell wall during cell division in the cambium and during subsequent enlargement/elongation of the developing daughter cell. Next, the secondary wall formed within, giving permanence to the cell dimensions and shape. The primary wall is very thin and lacks any apparent structural orientation; by contrast, the secondary wall occupies the dominant portion of the cell wall and has three layers, designated as S^ S2 and S3. When examined with an electron microscope, the substance comprising the secondary wall appears to have oriented striations. These striations indicate the general direction of cellulose mole-
Figure 2.13 Diagrammatic representation of the structure of a wood fibre. Fibres give wood its strength, have closed ends and thicker walls than vessels, which are used to conduct fluid and nutrients. The lumen is the void or hole in the centre of the cell. The cell wall has four layers and is composed of microfibrils. The microfibrils are arranged randomly in the primary wall. The direction of the microfibrils in the three secondary walls alternates. There are both amorphous and crystalline regions within the microfibrils, which are themselves composed largely of cellulose cules, the apparent groupings referred to as fibrils (sub-groupings sometimes termed micro fibrils). It is the orientation of fibrils which defines the layering of the secondary wall. Within the thinner S1 and S3 layers, the fibril orientation is nearly perpendicular to the cell axis, whereas fibrils within the dominant S2 layer are oriented more nearly parallel with the cell axis.
Experimental evidence provides a theoretical explanation of the arrangement of cellulose within fibrils. In random areas, called crystallites, cellulose molecules (or more likely, portions of cellulose molecules) are aligned into compact crystalline arrangement. Adjacent areas where cellulose is non-parallel are called amorphous regions. The hemicelluloses and lignin are also dispersed between crystallites and through the amorphous regions.
Within the fibrils, water molecules cannot penetrate or disarrange the crystallites. Water molecules can, however, be adsorbed by hydrogen bonding in one or more layers to the exposed surfaces of crystallites and components of amorphous regions, namely at the sites of available hydroxyl groups. Such polar groups of the polysaccharide fractions on exposed wall surfaces provide the principal active sites for bonding of adhesives and finishes and for other chemical reactions with wood.
Because the average length of cellulose molecules is far greater than the apparent length of the crystallites, it is concluded that an individual cellulose molecule may extend through more than one crystalline region, being incorporated in crystal arrangement at various points along its total length. Therefore, within the fibrillar network, the random end-wise connection of crystallites would appear to offer linear strength to the fibril. Since crystallites would be more readily displaced laterally from one another due to the intrusion or loss of water molecules (or other chemicals capable of entering the fibrils), dimensional response would be expected perpendicular to the fibril direction. In summary, the linear organization of cellulose within the fibrils, the dominance of the S2 layer, and the near-axial orientation of fibrils within the S2 layer, together provides a foundation of understanding of the greater strength and dimensional stability of the cell in its longitudinal direction. It follows that wood itself - as the composite of its countless cells -has oriented properties.
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