Wood Modification Methods

Chemical modification of wood is defined as the reaction of a chemical reagent with the wood polymeric constituents, resulting in the formation of a covalent bond between the reagent and the wood substrate. 

The application of heat to wood results in degradation associated with chemical changes in the material. If carefully controlled, the property changes that are obtained due to thermal modification can be used for certain applications. There has been a great deal of commercial activity in this sector, mainly in Finland, France and The Netherlands. Thermal modification is discussed in Chapter 5. 

The thermal modification of wood is defined as the application of heat to wood in order to bring about a desired improvement in the performance of the material. 

Impregnation modification of wood is darned as any method that results in the fdling of the wood substance with an inert material (impregnant) in order to bring about a desired petformance change. 

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Table 2.1 A classification of wood modification methods (see Figure 2.1)...

Impregnation modification is an area of research that is relatively unexplored compared to other wood modification methods and there are undoubtedly many other systems that remain to be studied in the future. 

There are a variety of thermal modification methods that can be applied to wood, and the exact method of treatment can have a significant effect upon the properties of the thermally modified wood. Important process variables include the following ... 

An important chemical modification method is the chemical coupling method, which improves the interfacial adhesion. The fibre surface is treated with a compound, that forms a bridge of chemical bonds between fibre and matrix. The increase in the mechanical properties of the fibreboards due to chemical modification is an indication of improved interaction and stress transfer between the components. Some authors have reported that softening and increased thermo-plasticity of wood fibre surface facilitates contact and dispersion of the fibre with thermoplastics [61, 62]. 

Its appeal Hes in the fact that synthesis gas can be produced from trash, municipal sewage, scrap wood, sawdust, newsprint, or other waste. The early work of Fischer and Tropsch on methanol synthesis showed that ethanol could be obtained in the process (165) and that by certain modifications the proportion of ethanol in the product could be increased (166). The Hterature concerning this method is extensive (167—176). The conditions that favor ethanol formation are 125—175°C and 1.42 MPa (14 atm) in the presence of reduction catalysts such as powdered iron. 

The method described above is a modification of that described by Wood, Colburn, Cox, and Garland. ... 

The surface energy of fibers is closely related to the hydrophilicity of the fiber [38]. Some investigations are concerned with methods to decrease hydrophilicity. The modification, of wood cellulose fibers with stearic acid [43] hydrophobizes those fibers and improves their dispersion in polypropylene. As can be observed in jute-reinforced unsaturated polyester resin composites, treatment with polyvinylacetate increases the mechanical properties [24] and moisture repellency. 

When dimensional stability is achieved due to cell wall bulking, the dimensional stabilization achieved is equal to the volume of the water-saturated sample minus the volume of the modified wood. Another class of modification reaction is due to cross-linking between the cell wall polymeric components. In this case, dimensional stability is imparted to the modified wood because movement of the cell wall is restrained, although the volume of the cell wall occupied by the modifying agent may still have an influence (Figure 2.9c). Ohmae etal. (2002) have suggested a method by which the various mechanisms can be... 

Four different methods (vapour-phase acetylation using acetic anhydride, acetylation using ketene gas, liquid phase acetylation using acetic anhydride/xylene, or neat acetic anhydride) were used to acetylate pine wood chips to a variety of WPGs for the production of MUF-bonded particleboards (Nilsson etal., 1988). Composite boards were exposed to unsterile soil in fungal cellar tests. Boards made from ketene acetylated chips were not found to be resistant to decay at the maximum WPG level achieved (17 %) with a liquid acetic anhydride modification, no decay was recorded at a WPG level of c. 18 % after 12 months exposure, whereas with a vapour-phase treatment at the same WPG, evidence for decay was found. 

It has been reported that thermoplastic properties can be imparted to wood by modification of wood particles with fatty acid chlorides in a dinitrogen tetroxide -dimethylformamide - pyridine mixture (Funakoshi etal., 1979 Shiraishi etal, 1979, 1983). A method has also been developed for the modification of wood sawdust without the addition of organic solvents (Thiebaud and Borredon, 1995), and the thermal properties of such modified wood determined (Thiebaud etal, 1997). 

The properties of thermally modified wood are highly dependent upon the thermal treatment employed, and it is very important to take these into account when comparing the various treatment methods employed. This chapter examines the effect of the process variables upon the properties of thermally modified wood, and then considers the chemistry of thermal modification. Studies of physical changes are discussed, followed by an overview of the biological properties of thermally modified wood. A short description of some recent literature on the use of thermal treatment combined with compression and on hot oil treatments is also included. 

The inherent heterogeneity of the material leads to variations in the responses of wood to thermal modification. The rate of transfer of heat into the interior of the wood is of paramount importance in order to ensure that there is a constant temperature throughout the sample. The thermal conductivity of dry wood is low and the heating method employed must ensure that the treatment is as even as possible. Heat transfer into the interior may be improved by the use of steam-heating. Heat transfer is a very significant factor in the treatment of timber of larger dimensions. 

The Royal (or Royale) process was originally developed as a method for drying timber, in which the wood is heated in oil under vacuum. The temperatures used are low (60-90 °C) compared to other thermal oil treatments, and although sufficient to lead to some curing of the oil itself, there is no direct modification of the wood as a result of this process. The oil does not penetrate the cell wall. In this process, wet timber is placed in a treatment vessel and oil is then introduced, which is heated to the desired temperature, whilst a vacuum is applied. Water is removed from the timber and the vapour is transported away by the vacuum system. When the wood has reached the desired MC, the oil is removed from the treatment vessel. After this, a vacuum is applied to removed excess oil from the wood. Some dimensional stability is imparted to the timber due to the water repellency of the oil. This treatment is marketed by Osmose as the Royale process. 

Kiguchi, M. (1993). Chemical modification of wood surfaces by etherification. IV. Benzylation with solvent-dilution and vapor-phase methods. Mokuzai Gakkaishi, 39(1), 80-85. 

Human neutrophils are isolated from acid citrate dextrose solution-A anticoagulated human Wood hy the method described by Boyum (11) with modifications (12) as described in detail in Chapter 36, Subheading 3.1. of this text. 

Explosion Unit Operation (See also "Explosive Disintegration"). The explosion technique has been applied successfully for the modification of substances and for separation of components, as well as for size reduction of materials. This method has been used in rubber reclaiming, wood pulping, manufacture of fiber board, pulverization of minerals and coal and finally for the prepn of puffed cereals... 

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