Based on the research of Dinwoodie (2000), the existences of four constituents of wood are revealed by X-ray diffraction and chemical analysis. The data on their relative proportions is shown in the figure 1.
Figure 1. Chemical composition of timber
Cellulose occurs in the form of long slender filaments or chains, which are built up within the cell wall from the glucose monomer. In spite of the considerable variation of the number of units per cellulose molecule even within one cell wall, it is believed by Goring (1962) that a value of 8000-10000 is a realistic average for the secondary cell wall whereas the primary cell wall has a degree of polymerisation of only 2000-4000, which is proved by Simson (1978).
The hemicelluloses and lignin, which are two of the other constituents of wood additional to cellulose, are defined as cementing materials contributing to the structural integrity of wood and also to its high stiffness. The hemicelluloses are carbohydrates built up of sugar units that are similar to cellulose itself. But these are unlike cellulose in the type of units they comprise. The hemicelluloses of softwoods and hardwoods are different. Generally, the total percentage of the hemicelluloses present in hardwoods is greater than those in softwoods. As result, the hemicellulose fraction in the softwoods comprises from 15% to 20% of galactoglucomannans and approximately 10% of arabinoglucoronoxylan. Compared with softwoods, the hardwoods contain 20-30% of glucoronoxylan and 5% glucamannan. In general, both the degree of crystallization and the degree of polymerisation of the hemicelluloses are low and lack resistance to alkali solutions. In addition, the hemicelluloses are different from true cellulose according to Siau (1984).
Lignin is about equal proportions to the hemicelluloses. It is chemically dissimilar to hemicelluloses and to cellulose. In addition, lignin is a complex, three-dimensional, aromatic molecule composed of phenyl groups with a molecular weight of about 11000. The structure of lignin varies between wood from a conifer and from a broadleaved tree. It is found out that approximately 25% of the total lignin in timber is in the middle lamella, which is an intercellular layer consists of lignin and pectin, together with the primary cell wall. The concentration of lignin is correspondingly high (about 70%) due to the compound middle lamella is very thin. In this layer, deposition of the lignin is rapid. Most of the lignin (about 75%) is in the secondary cell wall, which is stored in the cellulosic framework. The lignin within the framework of timber is largely responsible for the stiffness of timber, especially in the dried condition.
Furthermore, the presence of extractives is one of the chemical compositions of wood. It is composed of a series of highly complex organic compounds present in certain timbers, which is in relatively small amounts. Some extractives are of considerable economic importance such as rubber and resin. While some of them such as waxes, fats and sugars have little economic significance. In addition, most of these compounds are toxic to both fungi and insects.
Cell wall layer
The middle lamella, which is a lignin-pectin complex, is lacking of cellulosic micro fibrils while in the primary wall the micro fibrils are loose packed and interweave at random, and no lamellation is present. In the secondary wall, the micro fibrils are closely arranged and parallel to each other. As shown in the figure 2, the outer layer is also thin and is characterized by having from four to six concentric lamellae. The middle layer of the secondary wall is thick and consists of 30-150 lamellae. As this layer occupies three-quarters of the cell wall, the infrastructure of this layer will have a significant influence on the behaviour of the timber.
Figure 2. Simplified structure of the cell wall
Wood modification
Suttie (2007) demonstrates that wood modification includes the action of a chemical, biological, or physical agent on the material, which results in a desired property enhancement during the service life of the modified wood. It is mentioned that the modified wood should itself be non-toxic under service conditions. Furthermore, there should be no release of any toxic substances during service, or disposal, or recycling of the modified wood. If the modification is aimed to improve resistance to biological attack, then the mode of action should be non-biocidal.
Wood modification intends to improve the properties of hardwood such as dimensional stability, hardness, durability and aesthetics. It also provides the possibilities for new improved wood-based panels and wood composites and improved coating adhesion.
It is introduced that chemical modification (acetylation), thermal modification (heat treatment) and densification are the most promising and widely exploited wood-modification techniques. Modified wood can provide a range of the properties, which can meet the demands of construction elements. Emerging alternative durability-enhancing technologies involves thermal modification and chemical modification
Water coating systems can provide a stable appearance to the product and protect against moisture ingress, wear and weathering. Water-borne technology is one of the modern exterior wood coating systems.
The durability of the product is the most important for the users of construction, such as windows, claddings, load-bearing structures, houses and fences. Durability properties usually depend on the product quality properties, design and use conditions.
Hill (2011) reveals that a chemical reaction with wood cell wall polymers is not necessarily required for the impregnation modification. However, it is necessary for the reagent to penetrate the cell wall, which is non-leachable in service. The furfurylation (wood modification with furfuryl alcohol) is the impregnation with the longest pedigree. But the problem of these earlier processes is that the zinc chloride used as a catalyst will result in serious degradation of the wood. The problem has been overcome by researchers in Sweden and Canada. Keywood, a new furfurlyated wood invented by Arch Timber protection, is proved to be good in commercial use. In spite of a good product, furfurlyated wood is dark in colour.
Thermal modification wood may create a permanent change in the polysaccharides of the wood. It is shown that the process of thermal modification is conducted in the environment in which oxygen is restricted or eliminated from the system. The processes to exclude oxygen are heating in nitrogen, heating green timber that can create a steam veil to protect the wood and heating in oil.
Suttie (2001) points out that chemical modification of wood can increase dimensional stability, resistance to biological attack and weathering, and improve acoustic properties. But it may reduce tensile strength and elasticity of wood. Chemical modification that can enhance the durability of wood is based on the theory that fungal enzymes must directly contact the wood cellulose to degrade it, and the substrate must have a particular configuration for this to occur. The dimensional stability of modified wood can be tested by repeated water soak and oven dry cycles.
Physical modification is based on the precondition that the moisture from the wood is excluded by blocking available pathways to prevent increases in moisture content. The most basic ways to achieve this are through a physical barrier such as pole sleeves, which can be wrapped around the end of a commodity such as a transmission pole before it is erected. Furthermore, brush applied end grain sealants can be applied to the end grains of timber joinery prior to assembling the joints. This offers protection against moisture ingress by providing a physical barrier.
All the processes of thermal modification are usually heating wood to temperatures in excess of 200??C for several hours. The formation of cohesive structures between the cell wall components and chemical alternation may be induced by heating wood in a restricted atmosphere, which will improve the dimensional stability and water repellency, abrasion and permeability to water vapour. In addition, heat treatment also darkens the timber while the density remains almost unchanged. The modified wood is more brittle than the starting substrate and needs very sharp blades for machining.
Thermal treatments of wood rely on the change of the polysaccharides. During heating, the molecular structure of the cell wall constituents changes, especially lignin and hemicellulose breakdown and start to form new water insoluble polymers. It is proved that the degree of polymerization will be reduced by acetic acid liberation from the hemicelluloses and the catalysis of carbohydrate cleavage. Then auto-condensation of lignin will result in an increase in cross-linking and consequent dimensional stability and decreases in hygroscopicity. If the heat treatments are conducted in the presence of oxygen then major damage can lead to breakdown of the cellulose as well. As result ‘shielding gases’ are necessary to prevent the cellulose from breaking down and rearranging. The correct balance is required for the heat treatment of timber to be stuck in delivering desirable durability enhancements with minimum loss in strength. This can guarantee end uses of the product under large structural loads, such as window joinery, outdoor furniture and garden fencing.
The extent of the properties change in timber during heat treatment depends on several factor that are listed below:
‘ The maximum temperature and the maximum length of the actual heat
treatment period
‘ The gradient of temperature
‘ The maximum duration of the whole heat treatment
‘ The use and amount of water vapour
‘ The kiln drying process before the actual heat treatment
‘ The wood species and its characteristic properties
The process of heat treatment developed in Finland in the 1990s uses water vapour originated from the timber moisture content as the shielding gas to prevent the wood structure from deleterious breakdown. The product, which is called Thermo Wood, is typically made by pine, birch and spruce species that can be treated green or kiln dried. The three stages including the temperature rise, the treatment stage and the cooling phase needs to be carefully controlled based on the core temperature of the timber. Temperatures between 180??C and 250??C can change the physical and chemical properties of timber permanently. The properties of timber that are changed by heat treatments are listed below:
‘ Colour, which changes to dark or mid brown, but is not UV stable
‘ The equilibrium moisture content, which is reduced by 50%
‘ The shrinking and swelling, which are also reduced by 50-90%
‘ The biological durability out of ground contact, which is improved
‘ The mechanical properties, which are reduced by up to 30%.
Although steaming or heating wood in a compressed state can enhance hardness, it also results in slight decreases in other mechanical properties and produce a slight darkening in colour. The compression of wood while heating (170??C) causes lignin flow, which may relieve the internal stresses. Lignin flow refers to a rearrangement of the cementing material between the cellulose fibres. This greatly reduces the tendency of wood to swell when wet and increases the strength. Heating wood under vacuum without compressing it may also causes lignin flow and increases in stability, but it may decrease the strength.
Steam bending
When the wood is bent, the two faces will not be equal in length. Because compressive force can lead fibres on the concave face to shorten and tensile force can lead fibres on the convex face to stretch. There will be no change of fibres in length and stressing only along the ‘neutral axis’. In the natural state, timber shows elastic properties over a limited stress range. If the limiting stress value is exceeded, permanent deformation of the timber will occur.
Dinwoodie (2000) shows that steam bending of timbers is a long-established process, which was used extensively. The mechanics of steam bending includes a presteaming operation to soften the lignin, swelling the timber and render of timber to make it less stiff. The timber is commonly bent round a former with the ends restrained. After bending, timber must be held in the restrained mode until it dries out and the bend is set. In mostly terms the deformation of timber is irreversible. But over a long period of time, in particularly with prominent alternations in humidity of the atmosphere, a certain degree of recovery will occur. Despite that most timbers can be bent slightly, only certain species, principally hardwood, can be bent to sharp radii without cracking.
The wood of beech is usually defined as heavy, hard, strong, high in resistance to shock. Thus, this kind of wood is highly suitable for steam bending. Good shock resistance of courbaril makes it also suitable for steam bending. It is widely used for steam bending parts such as flooring, furniture, railroad crossties and other specialty items.
Properties
According to Kretschmann (2010), generally the mechanical properties of wood decrease when heated and increase when cooled. It is proved that mechanical properties wood are approximately linearly related to temperature at a constant moisture content and below approximately 150??C. When wood is quickly heated or cooled, the change in properties occurs. This is termed an immediate effect. At temperature below 100 ??C, the immediate effect is originally reversible, which refers to the fact that the property will return to the value at the original temperature if the temperature change is rapid.
The strength of dry lumber, at approximately 12% moisture content, will change little with temperature increasing from -29??C to 38??C. When it comes to green lumber, strength generally decreases with increasing temperature. But the changes for temperature between 7??C and 38??C may not differ significantly from those at room temperature. The percentage change in bending properties of lumber with change in temperature is shown below in the figure 3.
Figure 3. Percentage change in bending properties of lumber with change in temperature
Except for the reversible effect of temperature on timber, there is the irreversible effect at increased temperature. This is a permanent effect on timber, which results in loss of weight and strength. Several factors may influent the loss such as moisture content, heating medium, temperature, exposure period, and to some extent, species and size of piece involved.
Based on the tests of clear pieces of Douglas fir and Sitka spruce, the permanent decrease of modulus of rupture caused by steam and water heating is shown in the figure 4. It is described as the relationship between a function of temperature and heating time. In the same cases, steam and water heating affected work to maximum load more than modulus of rupture, which is illustrated in the figure 5.
Figure 4. Permanent effect on modulus of rupture of clear, defect-free wood
Figure 5. Permanent effect on work to maximum load and
modulus of rupture of clear, defect free wood
Reference
Dinwoodie, J. M., 2000. Timber: Its nature and behaviour. 2nd ed. New York: E&FN Spon.
Goring, D. A. I. and Timmell, T. E., 1962. Molecular weight of native cellulose. TAPPI, 15, pp454’459.
Hill, C. A., 2011. Wood modification: An update. BioResources, 6(2), pp918-919.
Kretschmann, D. E., 2010. Wood handbook: Wood as an Engineering Material[Online]. Available from: http://www.fpl.fs.fed.us/documnts/fplgtr/fpl_gtr190.pdf [Accessed May 2014].
Simson, B. W. and Timell, T. E., 1978. Polysaccharides in cambial tissues of populustremuloides and tilia americana. Chem Technol, 12, pp51’62.
Siau, J., 1984. Transport processes in wood. Berlin: Springer-Verlag.
Suttie, E., 2007. Modified wood: An introduction to products in UK construction [Online]. Available from: http://www.bath.ac.uk/library/ [Accessed May 2014].
Suttie, E., 2001. Wood modification state of the art review[Online]. Available from: http://www.bath.ac.uk/library/ [Accessed May 2014].