Preservative-treated wood is wood which has been surface coated or impregnated by means of pressure with chemicals which improve resistance to damage from decay and insect attack.

Preservative-treatment processes do not alter the basic characteristics of wood but do provide much improved service life for wood building materials in severe service conditions.

For a wood preservative to function effectively it must be applied under controled conditions, to specifications known to ensure that the preserved wood will perform in service. The Canadian Standards Association (CSA) publishes a wood preserving standard containing these specifications. CSA Standard O80-M Wood Preservation gives detailed requirements which are particular to Canadian wood species and also adopts certain standards of the American Wood Protection Association (AWPA).

The ease by which a species can be treated is a function of the cell biology and the process used. Canada has a abundant variety of wood species. Some species are more difficult to pressure-treat than others.

Process

Most preservative-treatments are applied using the full-cell pressure method by which wood is placed in a pressure vessel and a vacuum is applied to draw air from the wood cells. The preservative solution is admitted to the cylinder and is first drawn into the cells by the vacuum. Then pressure of 690 to 1380 kPa (100 to 200 psi) is applied to force additional preservative into the wood as shown in Figure 9.8 below. A final vacuum is then applied to remove excess surface chemical.

For all types of preservative-treatment, the quantity of preservative which can be forced into the cells and be retained depends in part upon the moisture content of the wood. The amount of water present in the cell cavities at the time of treatment influences the influx of preservative.

For this reason, the moisture content of the raw wood material is an important aspect of quality control.

For waterborne preservatives, the moisture content of lumber to be treated should be in the low (15 to 25 percent) range to permit easy entry of preservative, and chemical reation with the cell wall. If there is excess cell water present, rebound pressure will have the effect of expelling preservative from the cells once the process pressure abates.

In order to enhance the penetration of wood preservative, the moisture content of the wood is reduced by a conditioning process. This can be achieved bu air-seasoning, kiln drying, or by a process carried out in the treatment cylinder for example by the application of steam and subsequent vacuum, or by boiling under a vacuum in the presence of the treating solution.

To avoid release of excess chemical after treatment, time should be allowed at the place of manufacture for the fixation of the chemical into the wood cell walls to take place. Once the set period has finished, the waterborne preservative is chemically affixed to the cell walls.

 Figure 9.8: Manufacture of Pressure-Treated Wood

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