Drying
Kiln drying of timber has become more important to wood pallet and packaging users and makers, since over the last 10 years chemicals which hitherto kept wood in clean condition have been used less and less. Food and drink shippers and customers who store paper on wood pallets need dry pallets and the most economical way is to use kiln drying.
Timber drying kiln
Drying kilns use high temperature, humidity and forced airflow to dry wood, all of which change throughout the drying cycle which for pallets may last 48 hours or more. Through carefully changing humidity and temperature, the quality of timber is not degraded and the sequence of changes is called a kiln-schedule. Operators use different kiln-schedules for different end-uses and the sizes and species of wood also affect these.
Drying cycle times vary because living trees (standing timber) contain a large amount of water which varies between species and the moisture content will also change from season to season in standing timber and also vary between different parts of the tree such as the sapwood and the heartwood. The water is held in the cellular structure of the wood, which is made up of cell cavities and cell walls. The water held in the cell cavities is called free water whereas the water in the cell walls is called bound water.
Pallets undergoing ISPM 15
heat treatment.
The free water is lost before the bound water. This reduces the weight of the wood but does not affect its dimensions. The point at which the free water has been removed and the bound water remains is called the fibre saturation point. The moisture content of the timber when the fibre saturation point has been reached, is between 25 % and 30 % so for convenience is taken as 28%. Timber that is dried to moisture contents below 28% will exhibit shrinkage as the cell walls lose moisture. The fibre saturation point of 28% should not to be confused with the decay safety limit of 20%, (below 20% is the level at which timber is unlikely to be attacked by wood decaying fungi or bluestain/sapstain).
Note that ISPM 15 insect treatment lasting a mandatory 30 mins does not normally dry timber, drying can only be done by a substantially longer period in a drying kiln
The main reasons for drying pallet and packaging timber are:
- To meet specifications such as Europallets which require a maximum of 22% moisture content
- To prevent unacceptable shrinkage after delivery
- For food and drink shippers and customers who store paper on wood pallets
- To maximise strength, since mechanical properties of timber increase as it dries below 28% moisture content
- To make empty pallets easier to move by hand, since seasoned timber is much lighter than green, undried timber
- To prevent corrosion of nails and metal fixings and to prevent water damage to paper/cartons protecting the goods being shipped
- To reduce susceptibility to fungal decay.
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