Wood drying

Wood drying refers to reducing the moisture content of wood prior to its use. The two most important issues are 1) the level of moisture desirable and 2) the means to achieve this.

With regard to the level of moisture, for some purposes wood is not dried at all, prior to its use. It is used as is, "green". For other purposes wood must be in equilibrium to the outside air, as in much construction wood. For use indoors, as in furniture, wood must be in equilibrium with the air indoors.

With regard to the means used in drying a distinction is made between air-dried wood (AD) and kiln-dried wood (KD). There are many different types of kilns, as time used for drying is an economic factor, and new types of kiln are constantly being developed to reduce drying time.

Usually wood is sawn prior to drying, but this is not always so. As drying in the log takes a long time, especially for big logs, sawing prior to drying is the rule. If wood is to be kiln dried it will always be sawn first.

Wood structure

Wood is composed of cells, and is an anisotropic material. Commercial timbers are broadly classified into two categories, namely softwoods and hardwoods.

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Softwoods are relatively simple in structure: more than 90% of the volume is composed of tracheids (Walker et al., 1993), axially elongate cells of 2 to 5 mm in length. Density will commonly be in the range of 350 to 700 kg/m3, basic density at 12% moisture content (Desch and Dinwoodie, 1996). Conversion and drying procedures for softwoods are better established than for hardwoods. A prime reason is that softwoods comprise the bulk of the wood used, and commercial plantings are mostly softwood. Research institutes in Europe and North America, also in New Zealand and Australia have devoted time to softwoods. The plantation area for softwoods is about 1 million ha in Australia according to the Australian Bureau of Agricultural and Resource Economics (ABARE, 2000) and about 1.7 million ha in New Zealand (source: New Zealand Forestry, 2002). Much of this is radiata pine (Pinus radiata)

The processing of hardwoods is more complex because of diversity. There are up to a hundred times as many species of hardwood trees as there are softwoods, and hardwood trees grow in a much wider range of ecological niches than softwoods. Density of commercial hardwoods commonly is in the range of 450 to 1250 kg/m3 basic density at 12% moisture content (Desch and Dinwoodie, 1996). Hardwood may have low lateral permeability, compared with softwoods. For example, the transverse permeability of green wood from Eucalyptus delegatensis is in the order of 4.6x10e-18 m2, whereas the permeability of green wood of Pinus radiata is 263 to 410x10e-18 m2 (Langrish and Walker, 1993). Blackbutt (Eucalyptus pilularis) is a difficult to dry hardwood species (Bootle, 1994). Also the structure of hardwood shows a much greater range of variation than for softwood.

Wood-water relationships

The timber of living trees and freshly felled logs contains a large amount of water, which often constitutes more weight than the actual wood. 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.

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Water in wood may be present in two forms:

Free water

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 vapor, but the total amount is normally negligible, at normal temperatures and moisture contents.

Wood is a hygroscopic substance. It has the ability to take in or give off moisture in the form of vapor. The water contained in wood exerts a vapor pressure of its own, which is determined by the maximum size of the capillaries filled with water at any time. If the water vapor pressure in the ambient space is lower than the vapor pressure within wood, desorption takes place. The largest sized capillaries, which are full of water at the time, empty first. The vapor 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).

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Wood drying (not to be confused with "seasoning") 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, acetylating of wood has been slow to be commercialized 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 $350 per cubic meter or less, increases in value to $2,000 per cubic meter 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 $200 per cubic meter because of drying defects, saw millers are losing about $5 million per year in that State alone. Australia wide this could be $40 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 utilization 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):

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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 utilization by rationalizing the utilization 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.

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