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Wood Preservatives

A great deal of research has been done on wood preservatives over the years. Here are some of the compounds that have been studied.


Borate-treated lumber

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Currently, zinc borate is available in some composite products, and disodium octaborate tetrahydrate (DOT) is popular for pretreatment of construction wood. With pressure treatment, borates can penetrate refractory wood species like Douglas-fir very well, which is an advantage over some other wood treatments. DOT solutions are also sometimes applied to the wood surface for remedial termite control, but these remedial treatments do not penetrate as deeply as pressure treatment or dip-diffusion.

Laboratory and field tests of borate-treated wood and composites have demonstrated the effectiveness of borates against termite attack. However, it is important to have the right concentration of borate in the wood to protect it against a particular termite species, and to not leave unpainted treated wood exposed to running water.


CCA-treated lumber

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CCA (chromated copper arsenate) is a popular preservative which is quite effective against termites and decay at relatively low retentions, so long as the wood is thoroughly penetrated by pressure treatment. However, refractory wood species like Hawaii's perennial favorite, Douglas-fir, do not attain the same degree of penetration beneath the surface as other more easily treated woods, such as pine.

Although it has been shown that even relatively low retentions of CCA are quite effective, it has been suggested that more thorough penetration of CCA would be necessary for longer-term exposure of refractory woods.


Copper naphthenate
Lumber end-treated with copper naphthenate

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Copper naphthenate (abbreviated Cu-Naph) is a fungicidal wood preservative that has drawn intrest as a treatment for utility poles. Until recently, information on termite resistance of wood pressure-treated with copper naphthenate has been somewhat scarce.

Results of our laboratory study of copper naphthenate suggest that even under high weathering conditions, substantial amounts of preservative can be expected to remain in wood treated to target retentions of 0.150 pcf (pounds per cubic foot). Weathered wood samples treated with this retention of copper naphthenate sustained less than 4% mass loss after laboratory exposure to Formosan subterranean termites. These findings indicate promise for copper naphthenate applications, so long as the retention is sufficiently high.


ACZA-treated lumber

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ACZA (ammoniacal copper zinc arsenate) is an arsenical, ammoniacal wood preservative which tends to achieve better penetration in difficult to treat (refractory) woods such as Douglas-fir. This penetration is aided by the practice of incising the wood. ACZA is not as popular in construction as some of the other wood preservatives due to its cosmetic appearance (incising marks and dark color).

Laboratory and field studies support the use of ACZA as a protective treatment against termite attack, even in refractory wood species, despite its relative lack of popularity for construction purposes.



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Chlorothalonil (tetrachloroisophthalonitrile, or CTL), is a versatile fungicide which is widely used in agriculture in the U.S. It is used as an anti-fungal agent in some wood coatings, and is under development for use with oil solvents as a preservative.

A Laboratory study involving different retentions and solvents indicates that CTL will discourage Formosan subterranean termites from feeding at appropriate retentions. Although it is not commercially available, research continues with CTL in an effort to narrow down threshold retentions and to assess the value of combining CTL with other preservative components, like insecticides.

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