Wood modification

Wood Modification Chemical, Thermal and Other Processes C. Hill 2006 John Wiley Sons, Ltd... 

The purpose of this chapter is to briefly consider the environmental credentials of timber utilization and the changes that are now affecting the way in which timber is used. It is not intended to comprehensively cover the topic, which would require an entire book, but it serves to outline the case for timber as a renewable material and to illustrate how environmental considerations are changing the way in which the material is being used. In particular, the use of wood preservatives, which have ensured that this renewable material has continued to remain competitive against nonrenewables, will be discussed. Finally, the importance of wood modification as an emerging technology will be briefly considered. 

Due to environmental concerns regarding the use of certain classes of preservatives, there has recently been a renewed interest in wood modification. Wood modification represents a process that is used to improve the material properties of wood, but produces a material that be disposed of at the end of a product life cycle without presenting an environmental hazard any greater than that associated with the disposal of unmodified wood. Although wood modification has been the subject of a great deal of study at an academic level for over 50 years, it is only comparatively recently that there has been significant commercial development. 

Since many of the properties of wood are ultimately determined by its chemical constim-ents, wood modification often seeks to make changes at this level, in order to produce a material that has the desired properties. However, there are also wood modifications that do not involve alteration of the chemical composition of the material. 

Wood modification is a means of altering the material to overcome or ameliorate one or more of its disadvantages. The aim of a modification may be to bring about an improvement in decay resistance or dimensional stability, to reduce water sorption, to improve weathering performance and so on. The term wood modification applies to the application of a process that alters the properties of the material such that during the lifetime of a product no loss of the enhanced performance of the wood should occur. 

Wood modification involves the action of a chemical, biological or physical agent upon the material, resulting in a desired property enhancement during the service life of the modified wood. The... 

Note that the above does not necessarily preclude the use of a hazardous chemical in the preparation of the modified wood, provided that no hazardous residues remain once the wood modification process is complete. 

Figure 2.1 A diagram illustrating different types of wood modification at the cellular level (derived from diagrams of Norimoto and Gril, 1993).

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

Many of the physical, chemical and biological properties of wood can be understood by referring to the polymeric chemical constituents. In many cases of wood modification, these polymeric components are modified to some extent. The three structural polymeric components of the wood cell wall are cellulose, hemicellulose and lignin. There are many excellent texts describing the structure and function of these components, and only a brief account is given here. 

It is worth considering the measurement of the ESP in some detail, since many wood modification techniques result in a reduction of EMC at a given RH and there have been smdies in which attempts have been made to correlate the ESP with other properties, such as decay resistance. 

The reduction in EMC as a result of wood modification is sometimes reported in terms of a parameter referred to as the Moisture Exclusion Efficiency (MEE), which is defined as follows ... 

The effect of a wood modification where stabilization is due to cell wall bulking is illustrated in Figure 2.8. 

Wood modification can improve the dimensional stabilization of wood by two mechanisms. Where the cell wall is filled in some way by the reagent (whether covalently bound or not), the cell wall is swollen. When the dimensional stability of the modified wood is subsequently determined, the wood can then only swell by an additional amount, which is dependent upon the bulking of the cell wall due to the volume occupied by the modifying agent. This is illustrated in Figures 2.8 and 2.9b. 

Figure 2.9 Changes in ASE due to different forms of wood modification, according to Ohmae etal. (2002). Modification due to bulking (b) results in an ASE of zero, and modification with cross-linking (c) yields a positive ASE, whereas cell wall damage produces a negative ASE (d).

In most cases, wood modification results in a change in the way in which the wood responds to moisture and, with a great many wood modifications, a reduction in EMC at a... 

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