Types of wood
Wood is divided, according to its botanical origin, into two kinds: Softwoods from coniferous trees and hardwoods from broadleaved trees. Structurally softwoods are generally simple in structure and lighter whereas hardwoods are generally complex in structure and harder. However, in Australia softwoods generally refer to rainforest trees and hardwoods refer to sclerophyllous species namely Eucalyptus spp.
Softwood (like pine wood) is much lighter and easier to process than the heavy hardwood (like fruit tree wood). The density of softwoods ranges between 350-700 kg/m, while hardwoods are 450-1250 kg/m. Both consist of approximately 12 % moisture (Desch and Dinwoodie, 1996). Due to the more dense and complex structure of hardwood, the permeability is very low in comparison to softwood, thus making it more difficult to dry. Even though there are about hundred times more species of hardwood trees than softwood trees, the ability to process and dry softwood faster and more easily makes it the main supply of commercial wood today.
Wood-water relationships
The timber of living trees and freshly felled logs contains a large amount of water, which often constitutes over 50% of the woods actual weight. Water has a significant influence on wood: wood continually exchanges moisture (water) with its surroundings, although the rate of exchange is strongly affected by the degree wood is sealed.
Water in wood may be present in two forms:
Free water: The bulk of water contained in the cell lumina is only held by capillary forces: it is not bound chemically and is termed free water. Free water is not in the same thermodynamic state as liquid water: energy is required to overcome the capillary forces. Furthermore, free water may contain chemicals, altering the drying characteristics.
Bound or hygroscopic water: Bound water is bound to the wood via hydrogen bonds. The attraction of wood for water arises from the presence of free hydroxyl (OH) groups in the cellulose, hemicelluloses and lignin molecules in the cell wall. The hydroxyl groups are negatively charged electrically. Water is a polar liquid. The free hydroxyl groups in cellulose attract and hold water by hydrogen bonding.
Water in cell lumina may be in the form of water vapour, but the total amount is normally negligible, at normal temperatures and moisture contents.[citation needed]
Moisture content of wood
The moisture content of wood is calculated by the formula (Siau, 1984):
(1.1)
Here, is the green mass of the wood, is its oven-dry mass (the attainment of constant mass generally after drying in an oven set at 103 +/- 2 C for 24 hours as mentioned by Walker et al., 1993). This can also be expressed as a fraction of the mass of the water and the mass of the oven-dry wood rather than a percentage, for example, 0.59 kg/kg (oven dry basis) expresses the same moisture content as 59% (oven dry basis).
Students in the United Kingdom would recognise this formula written as
x100%
Where the wet weight is the weight of the original ‘wet’ sample and the dry weight being the weight of the sample after drying in an oven. Moisture contents being expressed as a percentage.
Fibre saturation point
When green wood dries, the first water to go is the free water from the cell lumina. It is held only by the capillary forces. Most physical properties, such as strength and shrinkage, are unaffected by the removal of free water. The fibre saturation point (FSP) is defined as the moisture content at which free water should be completely gone, while the cell walls are saturated with bound water. In most woods, the fibre saturation point is at 25 to 30% moisture content. Siau (1984) reported that the fibre saturation point (kg/kg) is dependent on the temperature T (C) according to the following equation:
(1.2)
Keey et al. (2000) use a different definition of the fibre saturation point (equilibrium moisture content of wood in an environment of 99% relative humidity).
Many important properties of wood show a considerable change as the wood is dried below the fibre saturation point. These include:
Volume: ideally no shrinkage occurs until some bound water is lost, i.e. until the wood is dried below FSP.
Most strength properties show a consistent increase as the wood is dried below the FSP (Desch and Dinwoodie, 1996). An exception is impact bending strength and, in some cases toughness.
Electrical resistivity increases very rapidly with the loss of bound water when the wood dries below the FSP.
Equilibrium moisture content
Main article: Equilibrium moisture content
Wood is a hygroscopic substance. It has the ability to take in or give off moisture in the form of vapour. The water contained in wood exerts a vapour pressure of its own, which is determined by the maximum size of the capillaries filled with water at any time. If the water vapour pressure in the ambient space is lower than the vapour pressure within wood, desorption takes place. The largest sized capillaries, which are full of water at the time, empty first. The vapour pressure within the wood falls as water is successively contained in smaller and smaller sized capillaries. A stage is eventually reached when the vapour pressure within the wood equals the vapour pressure in the ambient space above the wood, and further desorption ceases. The amount of moisture that remains in the wood at this stage is in equilibrium with the water vapour pressure in the ambient space, and is termed the equilibrium moisture content or EMC (Siau, 1984). Because of its hygroscopicity, wood tends to reach a moisture content that is in equilibrium with the relative humidity and temperature of the surrounding air. The EMC of wood varies with the ambient relative humidity (a function of temperature) significantly, to a lesser degree with the temperature. Siau (1984) reported that the EMC also varies very slightly with species, mechanical stress, drying history of wood, density, extractives content and the direction of sorption in which the moisture change takes place (i.e. adsorption or desorption).
Moisture content of wood in service
Wood retains its hygroscopic characteristics after it is put into use. It is then subjected to fluctuating humidity, the dominant factor in determining its EMC. These fluctuations may be more or less cyclical, such as diurnal changes or annual seasonal changes. In order to minimise the changes in wood moisture content or the movement of wooden objects in service, wood is usually dried to a moisture content that is close to the average EMC conditions to which it will be exposed. These conditions vary for interior uses compared with exterior uses in a given geographic location. For example, according to the Australian Standard for Timber Drying Quality (AS/NZS 4787, 2001), the EMC is recommended to be 10-12% for the majority of Australian states, although extreme cases may be up to 15 to 18% for some places in Queensland, Northern Territory, Western Australia and Tasmania. However, the EMC may be as low as 6 to 7% in dry centrally heated houses and offices or in permanently air-conditioned buildings.
The primary reason for drying wood to a moisture content equivalent to its mean EMC under use conditions is to minimise the dimensional changes (or movement) in the final product.
Shrinkage and swelling
Shrinkage and swelling may occur in wood when the moisture content is changed (Stamm, 1964). Shrinkage occurs as moisture content decreases too much, while swelling takes place when it increases. Volume change is not equal in all directions. The greatest dimensional change occurs in a direction tangential to the growth rings. Shrinkage from the pith outwards, or radially, is usually considerably less than tangential shrinkage, while longitudinal (along the grain) shrinkage is so slight as to be usually neglected. The longitudinal shrinkage is 0.1 to 0.3%, in contrast to transverse shrinkages, which is 2-10%. Tangential shrinkage is often about twice as great as in the radial direction, although in some species it may be as much as five times as great. The shrinkage is about 5 to 10% in the tangential direction and about 2 to 6% in the radial direction (Walker et al., 1993).
Differential transverse shrinkage of wood is related to:
the alternation of late wood and early wood increments within the annual ring;
the influence of wood rays in the radial direction (Kollmann and Cote, 1968)
the features of the cell wall structure such as microfibril angle modifications and pits; and,
the chemical composition of the middle lamella.
Wood drying
Wood drying may be described as the art of ensuring that gross dimensional changes through shrinkage are confined to the drying process. Ideally, wood is dried to that equilibrium moisture content as will later (in service) be attained by the wood. Thus, further dimensional change will be kept to a minimum.
It is probably impossible to completely eliminate movement in wood, but this may be approximated by chemical modification. This is the treatment of wood with chemicals to replace the hydroxyl groups with other hydrophobic functional groups of modifying agents (Stamm, 1964). Among all the existing processes, wood modification with acetic anhydride has considerable promise due to the high anti-shrink or anti-swell efficiency (ASE) attainable without damaging the wood properties. However, acetylation of wood has been slow to be commercialised due to the cost, corrosion and the entrapment of the acetic acid in wood. There is extensive literature relating to the chemical modification of wood (Rowell, 1983, 1991; Kumar, 1994; Haque, 1997).
Drying timber is one approach for adding value to sawn products from the primary wood processing industries. According to the Australian Forest and Wood Products Research and Development Corporation (FWPRDC), green sawn hardwood, which is sold at about 0 per cubic metre or less, increases in value to ,000 per cubic metre or more with drying and processing. However, currently-used conventional drying processes often result in significant quality problems from cracks, both externally and internally, reducing the value of the product. As an example, in Queensland alone (Anon, 1997), assuming that 10% of the dried softwood is devalued by 0 per cubic metre because of drying defects, sawmillers are losing about million per year in that State alone. Australia wide this could be million per year for softwood and an equal or higher amount for hardwood. Thus proper drying under controlled conditions (prior to use) is of great importance in timber utilisation in any country, where climatic conditions vary considerably at different times of the year.
Drying, if carried out promptly after the felling of trees, also protects timber against primary decay, fungal stain and attack by certain kinds of insects. Organisms, which cause decay and stain, generally cannot thrive in timber with a moisture content below 20%. Several, though not all, insect pests can live only in green timber. Dried wood is less susceptible to decay than green wood (above 20% moisture content).
Apart from the above important advantages of drying timber, the following points are also significant (Walker et al., 1993; Desch and Dinwoodie, 1996):
Dried timber is lighter, and hence the transportation and handling costs are reduced.
Dried timber is stronger than green timber in most strength properties.
Timbers for impregnation with preservatives have to be properly dried if proper penetration is to be accomplished, particularly in the case of oil-type preservatives.
In the field of chemical modification of wood and wood products, the material should be dried to a certain moisture content for the appropriate reactions to occur.
Dry wood works, machines, finishes and glues better than green timber. Paints and finishes last longer on dry timber.
The electrical and thermal insulation properties of wood are improved by drying.
Prompt drying of wood immediately after felling therefore results in significant upgrading of, and value adding to, the raw timber. Drying enables substantial long term economy in timber utilisation by rationalising the utilisation of timber resources. The drying of wood is thus an area for research and development, which concerns many researchers and timber companies around the world.
How wood dries: the mechanisms of moisture movement
Water in wood normally moves from zones of higher to zones of lower moisture content (Walker et al., 1993). In simple terms, this means that drying starts from the outside and moves towards the centre, and it also means that drying at the outside is also necessary to expel moisture from the inner zones of the wood. Wood, after a period of time, attains a moisture content in equilibrium with the surrounding air (the EMC, as mentioned earlier).
Mechanisms for moisture movement
Moisture passageways
The basic driving force for moisture movement is chemical potential. However, it is not always straightforward to relate chemical potential in wood to commonly observable variables, such as temperature and moisture content (Keey et al., 2000). Moisture in wood moves within the wood as liquid or vapour through several types of passageways depending on the nature of the driving force, (e.g. pressure or moisture gradient), and variations in wood structure (Langrish and Walker, 1993), as explained in the next section on driving forces for moisture movement. These pathways consist of cavities of the vessels, fibres, ray cells, pit chambers and their pit membrane openings, intercellular spaces and transitory cell wall passageways. Movement of water takes place in these passageways in any direction, longitudinally in the cells, as well as laterally from cell to cell until it reaches the lateral drying surfaces of the wood. The higher longitudinal permeability of sapwood of hardwood is generally caused by the presence of vessels. The lateral permeability and transverse flow is often very low in hardwoods. The vessels in hardwoods are sometimes blocked by the presence of tyloses and/or by secreting gums and resins in some other species, as mentioned earlier. The presence of gum veins, the formation of which is often a result of natural protective response of trees to injury, is commonly observed on the surface of sawn boards of most eucalypts. Despite the generally higher volume fraction of rays in hardwoods (typically 15% of wood volume), the rays are not particularly effective in radial flow, nor are the pits on the radial surfaces of fibres effective in tangential flow (Langrish and Walker, 1993).
Moisture movement space
The available space for air and moisture in wood depends on the density and porosity of wood. Porosity is the volume fraction of void space in a solid. The porosity is reported to be 1.2 to 4.6% of dry volume of wood cell wall (Siau, 1984). On the other hand, permeability is a measure of the ease with which fluids are transported through a porous solid under the influence of some driving forces, e.g. capillary pressure gradient or moisture gradient. It is clear that solids must be porous to be permeable, but it does not necessarily follow that all porous bodies are permeable. Permeability can only exist if the void spaces are interconnected by openings. For example, a hardwood may be permeable because there is intervessel pitting with openings in the membranes (Keey et al., 2000). If these membranes are occluded or encrusted, or if the pits are aspirated, the wood assumes a closed-cell structure and may be virtually impermeable. The density is also important for impermeable hardwoods because more cell-wall material is traversed per unit distance, which offers increased resistance to diffusion (Keey et al., 2000). Hence lighter woods, in general, dry more rapidly than do the heavier woods. The transport of fluids is often bulk flow (momentum transfer) for permeable softwoods at high temperature while diffusion occurs for impermeable hardwoods (Siau, 1984). These mechanisms are discussed below.
Driving forces for moisture movement
Three main driving forces used in different version of diffusion models are moisture content, the partial pressure of water vapour, and the chemical potential (Skaar, 1988; Keey et al., 2000). These are discussed here, including capillary action, which is a mechanism for free water transport in permeable softwoods. Total pressure difference is the driving force during wood vacuum drying.
Capillary action
Capillary forces determine the movements (or absence of movement) of free water. It is due to both adhesion and cohesion. Adhesion is the attraction between water to other substances and cohesion is the attraction of the molecules in water to each other.
As wood dries, evaporation of water from the surface sets up capillary forces that exert a pull on the free water in the zones of wood beneath the surfaces. When there is no longer any free water in the wood capillary forces are no longer of importance.
Moisture content differences
The chemical potential is explained here since it is the true driving force for the transport of water in both liquid and vapour phases in wood (Siau, 1984). The Gibbs free energy per mole of substance is usually expressed as the chemical potential (Skaar, 1933). The chemical potential of unsaturated air or wood below the fibre saturation point influences the drying of wood. Equilibrium will occur at the equilibrium moisture content (as defined earlier) of wood when the chemical potential of the wood becomes equal to that of the surrounding air. The chemical potential of sorbed water is a function of wood moisture content. Therefore, a gradient of wood moisture content (between surface and centre), or more specifically of activity, is accompanied by a gradient of chemical potential under isothermal conditions. Moisture will redistribute itself throughout the wood until the chemical potential is uniform throughout, resulting in a zero potential gradient at equilibrium (Skaar, 1988). The flux of moisture attempting to achieve the equilibrium state is assumed to be proportional to the difference in chemical potential, and inversely proportional to the path length over which the potential difference acts (Keey et al., 2000).
The gradient in chemical potential is related to the moisture content gradient as explained in above equations (Keey et al., 2000). The diffusion model using moisture content gradient as a driving force was applied successfully by Wu (1989) and Doe et al. (1994). Though the agreement between the moisture-content profiles predicted by the diffusion model based on moisture-content gradients is better at l
Find More Freeze Dried Food Articles
^ Jump to Top ^
©Copyright 1997-2010, Vocus PRW Holdings, LLC.&#thirteen