1.1 Wood modification
As a natural renewable resource, wood is generally a non-toxic, easily accessible and inexpensive biomass-derived material. Since ancient times, wood has been used by mankind due to its inherent properties, where a specific part of a tree of a particular species that could be found locally was utilised to achieve the best performance when used in construction, for different types of tools or for purposes not included in the practical tasks of life. Apart from drying, modification of timber has been rare in historical terms. Nevertheless, since wood is a natural product that originates from different individual trees, limits are imposed on its use, and the material may need to be transformed to acquire the desired functionality. This has become increasingly evident in the modern and highly industrial era. Modification is thus applied to overcome weaknesses in points of the wood material that are mainly related to moisture sensitivity, low dimensional stability, low hardness and wear resistance, low resistance to bio-deterioration against fungi, termites, marine borers, and low resistance to UV radiation.
Nowadays, wood modification is defined as a process adopted to improve the physical, mechanical, or aesthetic properties of sawn timber, veneer or wood particles used in the production of wood composites. This process produces a material that can be disposed of at the end of a productâs life cycle without presenting any environmental hazards greater than those that are associated with the disposal of unmodified wood.
The wood modification industry is currently undergoing major developments, driven in part by environmental concerns regarding the use of wood treated with certain classes of preservatives. Several fairly new technologies, such as thermal modification, acetylation, furfurylation, and various impregnation processes, have been successfully introduced into the market and demonstrate the potential of these modern technologies.
The main reasons for the increased interest in wood modification during the last decades with regard to research, the industry, and society in general can be summarised as:
- a change in wood properties as a result of changes in silvicultural practices and the ways of using wood,
- awareness of the limited availability of rare species with outstanding properties for modern use, such as durability and appearance,
- awareness and restrictions by law of the use of environmental non-friendly chemicals for increasing the durability and reducing the maintenance of wood products,
- increased interest from the industry to add value to sustainably sourced local sawn timber and by-products from the sawmill and refining processes further along the value chain,
- EU policies supporting the development of a sustainable society, and
- the international concern for climate change and related activities mainly organised within the frame of the United Nations (UN), such as the Paris Agreement under the United Nations Framework Convention on Climate Change (UNFCCC).
What is wood modification?
Wood modification is an all-encompassing term describing the application of chemical, mechanical, physical, or biological methods to alter the properties of the material. This definition of wood modification includes almost everything that happens to the wood material after it has left the forest and is more or less useless for simplifying and giving an overview of an area of interest. For this reason, it is necessary to define the purpose and the area in which the modification is intended to be applied. Such a purpose may, of course, change over time.
In the field of wood technology, wood modification includes any method or process that tends towards a better performance of the wood, where the term wood refers to roundwood (e.g., round timber, logs or other log-like products such as pit props, pylons, etc.), hewn timber, sawn timber, veneer, strands, chips and other types of wood particles used in wood composite products. In modern wood technology, it has become desirable for the modified wood to be non-toxic in service and that disposal at the end of life does not result in the generation of any toxic residues. This means that chemical treatments of wood that reach the core of the cellulosic microfibrils, destroying the amorphous and crystalline structures and eliminating most of the composite structure of wood, are in most cases excluded. Such subversive treatments radically modify the chemical components of the wood, and the material produced consequently lacks practically all the intrinsic characteristics of untreated natural wood. An example of a subversive treatment is liquefaction of lignocelluloses (Yao et al., 1994), which is mainly adopted to produce oil from biomass under very severe conditions.
Hill (2006) has provided a well-accepted definition of wood modification: âWood 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 modified wood should itself be non toxic under service conditions, and furthermore, there should be no release of any toxic substances during service, or at end of life, following disposal or recycling of the modified wood. If the modification is intended for improved resistance to biological attack, then the mode of action should be non biocidalâ.
It should be noted that the above does not necessarily exclude the use of a hazardous chemical in the preparation of modified wood, provided that no hazardous residues remain in the wood when the modification process is complete.
In this book, the focus is on methods that have introduced a modified solid wood or veneer product or are introduced in large-scale wood preservation projects. This means that processes that are producing or are very near to being involved in the production of modified products are dealt with. The book does not focus on the following areas, however some are briefly described in Chapter 5, and readers are referred to other sources for an introduction to the field:
- treatments aiming to improve wood properties such as fire/flame stability (cf. Lowden and Hull, 2013; Visakh and Arao, 2015),
- preservation of ancient small artefacts (cf. Unger et al., 2001),
- wood particles or disintegrated wo...
