Wood Drying - Wood Drying

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 dimensional change in wood, but elimination of change in size may be approximated by chemical modification. For example, wood can be treated 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 been noted for the high anti-shrink or anti-swell efficiency (ASE) attainable without damage to wood. 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 an extensive volume of literature relating to the chemical modification of wood (Rowell, 1983, 1991; Kumar, 1994; Haque, 1997).

Drying timber is one method of 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 metre or less, increases in value to $2,000 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. For example, in Queensland(Anon, 1997), on the assumption that 10% of the dried softwood is devalued by $200 per cubic metre because of drying defects, saw millers are losing about $5 million a year. In Australia, the loss could be $40 million a 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 use, in countries where climatic conditions vary considerably at different times of the year.

Drying, if carried out promptly after 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 as the moisture content in green wood is generally above 20%.

In addition to the above advantages of drying timber, the following points are also significant (Walker et al., 1993; Desch and Dinwoodie, 1996):

1. Dried timber is lighter, and the transportation and handling costs are reduced.
2. Dried timber is stronger than green timber in most strength properties.
3. 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.
4. 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.
5. Dry wood generally works, machines, finishes and glues better than green timber (although there are exceptions; in many ways green wood is easier to turn than dry wood). Paints and finishes last longer on dry timber.
6. The electrical and thermal insulation properties of wood are improved by drying.

Prompt drying of wood immediately after felling therefore significantly upgrades and adds value to raw timber. Drying enables substantial long-term economy by rationalizing the use of timber resources. The drying of wood is thus an area for research and development, which concern many researchers and timber companies around the world.