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    Author(s): R. M. Rowell
    Date: 2004
    Source: Enclyclopedia of forest sciences : volume three. Oxford : Elsevier Academic Press, 2004: Pages 1269-1274.
    Publication Series: Miscellaneous Publication
    PDF: View PDF  (134 KB)

    Description

    Wood is a hygroscopic resource that was designed to perform, in nature, in a wet environment. Nature is programmed to recycle wood in a timely way through biological, thermal, aqueous, photochemical, chemical, and mechanical degradations. In simple terms, nature builds wood from carbon dioxide and water and has all the tools to recycle it back to the starting chemicals. We harvest a green tree and convert it into dry products, and nature. with its arsenal of degrading reactions, starts to reclaim it at its first opportunity. The properties of any resource are, in general, a result of the chemistry of the components of that resource. In the case of wood, the cell wall polymers (cellulose, hemicelluloses, and lignin) are the components that, if modified, would change the properties of the resource. If the properties of wood are modified, the performance of wood will be changed. This is the basis of chemical modification of wood to change properties and improve performance. Wood changes dimensions with changing moisture content because the cell wall polymers contain hydroxyl and other oxygen-containing groups that attract moisture through hydrogen bonding. The hemicelluloses are mainly responsible for moisture sorption, but the accessible cellulose, noncrystalline cellulose, lignin, and the surface of crystalline cellulose also play major roles. Moisture swells the cell wall, and the fiber expands until the cell wall is saturated with water (fiber saturation point, FSP). Beyond this saturation point, moisture exists as free water in the void structure and does not contribute to further expansion. This process is reversible, and the fiber shrinks as it loses moisture below the FSP. The swelling pressures exerted when wood swells due to the uptake of water are very large. Stamm estimated these forces to be approximately 24 000 psi (165 MPa) but could only measure a swelling force of 12 000 psi (82.7 MPa). The ancient Egyptians split their large granite stones using the swelling forces of wood. They would chip rectangular holes (approximately 7 × 15 cm and 10 cm deep) into the rock the desired distance from the face of the mountain. They would then drive dry wooden stakes into the holes and wet them with water. The swelling forces would then split the granite stone from the face of the mountain. Wood is degraded biologically because organisms recognize the carbohydrate polymers (mainly the hemicelluloses) in the cell wall and have very specific enzyme systems capable of hydrolyzing these polymers into digestible units. Biodegradation of the cell wall matrix and the high molecular weight cellulose weakens the fiber cell. Strength is lost as the cell wall polymers and matrix undergo degradation through oxidation, hydrolysis, and dehydration reactions. Wood exposed outdoors undergoes photochemical degradation caused by ultraviolet radiation. This degradation takes place primarily in the lignin component, which is responsible for the characteristic color changes. The lignin acts as an adhesive in the cell walls, holding the cellulose fibers together. The surface becomes richer in cellulose content as the lignin degrades. In comparison to lignin, cellulose is much less susceptible to ultraviolet light degradation. After the lignin has been degraded, the poorly bonded carbohydrate-rich fibers erode easily from the surface, which exposes new lignin to further degradative reactions. In time, this ‘weathering’ process causes the surface of the composite to become rough and can account for a significant loss in surface fibers. Wood burns because the cell wall polymers undergo reactions with increasing temperature to give off volatile, flammable gases. The hemicellulose and cellulose polymers are degraded by heat much before the lignin. The lignin component contributes to char formation, and the charred layer helps insulate the material from further thermal degradation. This article discusses t

    Publication Notes

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    • This article was written and prepared by U.S. Government employees on official time, and is therefore in the public domain.

    Citation

    Rowell, R. M. 2004. Chemical modification. Enclyclopedia of forest sciences : volume three. Oxford : Elsevier Academic Press, 2004: Pages 1269-1274.

    Keywords

    Wood chemistry, chemical modification of wood

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