What They Are and How They’re Used
Courtesy of The Forest Products Laboratory
Typical steps in pressure treating process: A, untreated wood is placed in cylinder; B, a vacuum is applied to pull air out of the wood; C, the wood is immersed in solution while still under vacuum; D, pressure is applied to force the preservative into the wood; E, preservative is pumped out, and a final vacuum is pulled to remove excess preservative; F, excess preservative is pumped away, and the wood is removed from the cylinder.
Courtesy USDA Forest Service, Forest Products Laboratory
Since many portable buildings are built on pressure-treated timbers, it’s a good idea to know what you’re handling on a day-to-day basis. This article explains three of the preservative treatments for making what’s commonly known as “pressure treated wood.” It’s excerpted from the document, Preservative-Treated Wood and Alternative Products in the Forest Service, authored by James “Scott” Groenier and Stan Lebow. The 44-page guide was published by the U.S. Department of Agriculture’s Forest Products Laboratory. [Full document available: http://bit.ly/WoodPres].
Included here are profiles of water-based wood preservatives commonly used in the construction trade.
Ammoniacal Copper Zinc Arsenate (ACZA)
Ammoniacal copper zinc arsenate (ACZA) contains copper oxide (50 percent), zinc oxide (25 percent), and arsenic pentoxide (25 percent). ACZA is a refinement of an earlier formulation, ACA, which is no longer available in the United States. The color of the treated wood varies from olive to bluish green. The wood may have a slight ammonia odor until it has dried thoroughly. ACZA is an established preservative that is used to protect wood from decay and insect attack in a wide range of exposures and applications. Exposure tests showed that it protected stakes and posts that contacted the ground.
ACZA is listed in the AWPA standards for treatment of a range of softwood and hardwood species and wood products. The minimum ACZA retention is 0.25 pounds per cubic foot (4 kilograms per cubic meter) for aboveground applications and 0.4 pounds per cubic foot (6.4 kilograms per cubic meter) for wood that contacts the ground. A slightly higher retention, 0.6 pounds per cubic foot (9.6 kilograms per cubic meter), is required for wood used in highway construction and for critical structural components that are exposed to high decay hazard. The ammonia in the treating solution, in combination with processing techniques such as steaming and extended pressure periods at elevated temperatures, allow ACZA do a better job of penetrating difficult-to-treat species of wood than many other water-based wood preservatives.
ACZA is used frequently in the Western United States to treat Douglas-fir lumber and timbers used to construct secondary highway bridges, trail bridges, and boardwalks. The ACZA treatment can accelerate corrosion in comparison to untreated wood, requiring the use of hot-dipped galvanized or stainless steel fasteners. Treatment facilities using ACZA are located in Western States, where many native tree species are difficult to treat with CCA.
ACZA contains inorganic arsenic and is classified as an RUP by the EPA. Producers of treated wood, in cooperation with the EPA, have created consumer information sheets that suggest appropriate handling precautions and precautions at sites where wood treated with inorganic arsenic (appendix B) will be used. These sheets should be available to all personnel who handle wood treated with ACZA.
Alkaline Copper Quaternary (ACQ) Compounds
Alkaline copper quat (ACQ) is one of several wood preservatives that have been developed in recent years to meet market demands for alternatives to CCA. The fungicides and insecticides in ACQ are copper oxide (67 percent) and a quaternary ammonium compound (quat).
Many variations of ACQ have been standardized or are being standardized. ACQ type B (ACQ–B) is an ammoniacal copper formulation, ACQ type D (ACQ–D) is an amine copper formulation, and ACQ type C (ACQ–C) is a combined ammoniacal-amine formulation with a slightly different quat compound.
Wood treated with ACQ–B is dark greenish brown and fades to a lighter brown. It may have a slight ammonia odor until the wood dries. Wood treated with ACQ–D has a lighter greenish-brown color and has little noticeable odor; wood treated with ACQ–C varies between the color of ACQ–B and that of ACQ–D, depending on the formulation. Stakes treated with these three formulations have demonstrated their effectiveness against decay fungi and insects when the stakes contacted the ground.
The ACQ formulations are listed in the AWPA standards for a range of applications and many softwood species.
The listings for ACQ–C are limited because it is the most recently standardized. The minimum ACQ retentions are 0.25 pounds per cubic foot (4 kilograms per cubic meter) for above ground applications, 0.4 pounds per cubic foot (6.4 kilograms per cubic meter) for applications involving ground contact, and 0.6 pounds per cubic foot (9.6 kilograms per cubic meter) for highway construction. The different formulations of ACQ allow some flexibility in achieving compatibility with a specific wood species and application. An ammonia carrier improves the ability of ACQ to penetrate into wood that is difficult to treat. For wood species that are easier to treat, such as southern pine, an amine carrier will provide a more uniform surface appearance.
All ACQ treatments accelerate corrosion of metal fasteners relative to untreated wood. Hot-dipped galvanized copper or stainless steel fasteners must be used. The number of pressure-treatment facilities using ACQ is increasing.
In the Western United States, the ACQ–B formulation is used because it will penetrate difficult-to-treat Western species better than other waterborne preservatives.
Treatment plants elsewhere generally use the ACQ–D formulation. Researchers at the USDA Forest Service’s Forest Products Laboratory in Madison, WI, are evaluating the performance of a secondary highway bridge constructed using Southern pine lumber treated with ACQ–D (Ritter and Duwadi 1998).
Copper Azoles
(CBA–A and CA–B)
Copper azole is another recently developed preservative formulation that relies primarily on amine copper, but with additional biocides, to protect wood from decay and insect attack. The first copper azole formulation developed was the copper azole type A (CBA–A), which contains 49-percent copper, 49-percent boric acid, and 2-percent tebuconazole. Type A is no longer used in the United States. The copper azole type B (CA–B) formulation was standardized recently. CA–B does not contain boric acid. It is comprised of 96-percent copper and 4-percent tebuconazole. Wood treated with either copper azole formulation has a greenish-brown color and little or no odor.
Tests showed that the copper azole formulations protected stakes in the ground from attack by decay fungi and insects. The formulations are listed in the AWPA standards for treatment of a range of softwood species.
Minimum CA–B retentions in the wood are 0.10, 0.21, or 0.31 pounds per cubic foot (1.6, 3.4, or 5 kilograms per cubic meter) for wood used aboveground, contacting the ground, or in critical structural components, respectively.
Copper azole is an amine formulation. Ammonia may be added at the treating plant when the copper azole is used on Western species that are difficult to treat. This formulation is often used to treat Douglas-fir. Formulations with ammonia slightly darken the surface appearance and initially affect the odor of the treated wood.
Copper azole treatments increase the rate of corrosion of metal fasteners relative to untreated wood. Appropriate hot-dipped galvanized steel, copper or stainless steel fasteners, connectors, and flashing are recommended.
Although copper azole was introduced to North America recently, almost 100 treating facilities now use this preservative.
Groenier, James Scott; LeBow, Stan. 2006. Preservative-Treated Wood and Alternative Products in the Forest Service. 0677 2809P. Missoula, MT: U.S. Department of Agriculture, Forest Service, National Technology and Development Program.
http://bit.ly/WoodPres