- Chemistry of Wood Strength
The source of strength in solid wood and engineered wood composites is the wood ‹ber. Wood is basically a series of tubular ‹bers or cells cemented together. Each ‹ber wall is composed of various quantities of three polymers: cellulose, hemicelluloses, and lignin. Cellulose is the strongest polymer in wood and, thus, is highly responsible for strength in the wood ‹ber because of its high degree of polymerization and linear orientation. The hemicelluloses act as part of a matrix for the cellulose and increase the packing density of the cell wall; hemicelluloses and lignin are also closely associated and make up the cell wall matrix in which the cellulose is imbedded. The actual role of hemicelluloses relative to the strength of virgin wood has recently been shown to be far more critical toward the overall engineering performance of wood than had previously been assumed. It is now accepted that one important role of hemicelluloses is to act as highly speci‹c coupling agents capable of associating both with the more random areas (i.e., noncrystalline) of hydrophilic cellulose and the more amorphous hydrophobic lignin. Lignin not only holds wood ‹bers themselves together, but also helps bind carbohydrate molecules together within the cell wall of the wood ‹ber. The chemical components of wood that are responsible for mechanical properties can be viewed from three levels: macroscopic (cellular), microscopic (cell wall), and nano-molecular (polymeric) (Winandy and Rowell, 1984). Mechanical properties change with changes in the thermal, chemical, and/or biochemical environment. Changes in temperature, pressure, moisture, pH, chemical adsorption from the environment, UV radiation, ‹re, or biological degradation can have signi‹cant effects on the strength of wood.