Treated wood is lumber, timbers or round poles that have been treated with a chemical preservative to prolong their service life. The treatment can reduce the impact of fungal decay (rot), weathering or wood-eating insects such as termites. Wood intended for structural applications (such as preserved wood foundations) or non-structural applications (such as utility poles or fence posts) can be treated with preservatives.
A range of chemicals are used as wood preservatives. Since the chemicals are meant to be toxic to fungus and insects, they can pose health and environmental risks. Some chemicals that were common 20 years ago [e.g., chromated copper arsenate (CCA)] have been replaced by less toxic (and therefore safer but often less effective) chemicals [e.g., ammoniacal copper quaternary (ACQ)].
Since the entire piece of wood, both the exterior and the core, needs to be protected, the preservatives are usually introduced into the wood during a pressure treatment step, creating a chemical envelope. It is sometimes necessary to cut tiny slits into the surface of the wood (called incising) before the chemical treatment to increase the uptake of the liquid preservative.
The service life of treated wood products will depend on the type of chemical used, its uptake in the wood and the severity of the exposure conditions. Because of the inherent toxic nature of the chemicals used, it is important to minimize any leaching into the environment over the life of the product.
The most common treatments can be divided into two groups: those used only on industrial products and those used primarily for residential products. Industrial uses of treated wood include telephone and power poles, landscaping ties, bridge parts and decking, and commercial decks and boardwalks. Residential products include primarily outdoor decking, landscaping ties and fence posts, with some treated products used in house foundations.
The use of treated wood in North America is declining as wood plastic composites continue to take market share in decking and stone takes market share in landscaping.
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