Wood preservatives protect wood from rot, insects, and water damage. Copper is the most common substance used, but other compounds like borate and silicate are also used. Chromated copper arsenate (CCA) is no longer used due to its toxicity, and alternatives like alkali copper quaternary (ACQ) and copper azole (CA-B) have been developed. Heat treatments are also used as an alternative.
A wood preservative is a process or substance that, when applied to wood, will protect it from hazards such as rot, insects, or water damage longer than if the wood had been left untreated. There are a large number of different chemical and mechanical processes used to preserve wood. The most commonly used substance in non-industrial wood is copper. As far as purely mechanical processes are concerned, heat and fire treatment are more common. In all cases, the goal of a wood preservative is to remove air and water from the wood without causing it to splinter or crack.
The first preservation of wood was done with pitch or tar. These substances are still used in modern wood preservation in some circumstances. Generally, the petrochemicals in pitch and tar are applied by themselves, removing some of the mess associated with these substances.
There are different compounds used based on the overall protection goal and use for the wood. The most common non-domestic substance is chromated copper arsenate (CCA). CCA wood preservative uses copper as a fungicide and arsenic as an insecticide, while chromium keeps both of them in the wood. This causes the wood to have a slight green color, a common sight on outdoor fences, decking material and electricity poles.
Because arsenic is so poisonous, many areas have moved away from new construction using CCA. In its place, most wood preservatives have switched to alkali copper quaternary (ACQ) or copper azole (CA-B). These preservatives work in much the same way as CCA, but with fewer negative side effects. On the downside, the high amount of copper in ACQ destroys iron and steel, resulting in much higher construction costs.
Outside of copper, many other chemicals are used. Two of the most common non-copper compounds are borate and silicate based. These provide adequate storage but very little by way of insecticide. Additionally, these chemicals leach from the wood when exposed to water, making them unusable in some areas.
Copper, borate and silicate compounds are all water-based wood preservatives. Oil-based compounds, such as petrochemicals and vegetable oils, are common in industrial treatments. Some of these chemicals are highly toxic to humans and have a foul odor. Consequently, they are only used in areas where the wood will be constantly wet and where there are not many people.
Heat treatments are common in some areas as a chemical alternative. Heat as a wood preservative has mixed results; it often depends on the method used and the type of wood. The basic idea is that heat changes the composition of wood fibers, making them more water-repellent and less attractive to insects.
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