Chemical Treatment

A range of different chemical treatments has been developed to enhance the durability and service life of timber. Such chemical treatments impregnate the wood cells, making them resistant to decay, insects, weather or fire.

While chemical treatments add to the cost of the timber, they can significantly increase its lifetime. When used in environments in which there are known biological hazards, it is cost-effective to specify treated timber and expect a longer service life or a lower cost maintenance schedule.

Pressure Treatment Process

Most common treatment processes are pressure treatments. The following points outline the basic steps involved in pressure treatment processes:

  • Wood is placed in a pressure vessel for treatment. The vessel is designed to be able to give a positive pressure and a negative pressure (suction).
  • The first step is generally a suction step and it literally sucks moisture out of the wood in the vessel. This takes some time to happen. (The suction also opens up pores in the wood that make it easier for the chemicals to be forced into the wood.)
  • A mixture of treatment chemicals and a solvent that will carry them into the wood is added to the vessel. The vessel is placed under pressure and the chemicals and solvent are forced into the wood. Pressure can be varied during this time to get as much penetration as needed.
  • The pressure is released, the chemicals are removed from the vessel, and can be used in the next treatment cycle. Once the chamber is drained, the wood is removed.
  • The wood is left to stand on a hard pad and any chemicals and solvent that drains from the pack are trapped and used again. Once the packs have drained, then the charge is sent for redrying.
  • The wood has a very high moisture content at the completion of the treatment, so the solvent must be removed in the same way that water was removed from newly milled green timber. The first steps are air-drying to bring the moisture content down to around the fibre saturation point. This product can then be sold as unseasoned timber, or further drying can be undertaken to produce a redried, seasoned product. Where redrying uses kilns, it has been found that lower temperatures must be used to prevent degrade of mechanical properties.
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