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Wood Preservatives (page 2 of 3)

by Alex Wilson

Paintable preservatives

Most home centers have a whole shelf of compounds used for treating outdoor wood surfaces. The most common active ingredients are copper naphthenate, zinc naphthenate, and 3- Iodo-2-propynyl butyl carbamate (IPBC). Of these three, copper naphthenate is usually the most effective (a 10 to 20 percent concentration is recommended). Zinc naphthenate is a little less effective in preventing decay and mildew, and IPBC is less effective still and is not recommended for ground contact uses.

These products can be brushed, rolled, or sprayed onto finished or unfinished wood to provide some level of protection against decay or degradation. Many of these surface treatments also stain the wood, protect it from degradation by ultraviolet light, or seal it against moisture penetration.

The effectiveness of surface treatments is limited both by the less toxic chemicals used and by the fact that the chemicals soak only a little way into the wood. How far they penetrate depends on the wood species, how dry the wood is, and the solvent used in the preservative, but they rarely penetrate the wood more than a quarter inch.

Pressure-treated wood

Generally, much more effective protection is achieved when waterborne preservatives are forced under pressure deep into wood. This type of treatment is essential to protect most types of wood in direct contact with soil. Wood intended for ground contact should be rated .40 pcf, or pounds of chemical per cubic foot of wood. Wood for above-ground use should be .25 pcf.

The most common pressure-treating chemical in use today is chromated copper arsenate (CCA). In the 1930s, researchers found that sodium dichromate would fix arsenic salts to wood. They then added copper sulfate to improve resistance to fungal decay, to form CCA. Unlike woods with oil-based creosote and penta, CCA-treated wood can be painted. It was approved for treating wood in the late 1940s, and came into widespread use during the 1960s, primarily for treating southern pine (including loblolly, longleaf, shortleaf, and slash pine). By 1995, 5.1 billion board-feet of CCA-treated lumber, timbers, and other wood products were being produced annually in the United States--roughly 17 percent of all softwood lumber. A similar compound, ammoniacal copper zinc arsenate (ACZA) is used in the West for treating more dense Douglas fir, for which CCA is not as effective. (Douglas fir requires incisement, or punctures, for preservatives to penetrate deeply.)

Because CCA becomes tightly fixed to the wood fibers, it is relatively safe. Exactly how safe and under what conditions, however, remain hotly contested. For instance, studies indicate that some leaching does occur and that the chemicals move several inches through the soil, a valid concern if you're building a playset, for instance.

But from a broader, environmental-impact point of view, the primary problem with CCA-treated wood comes at the disposal end of the wood's life cycle, especially if it is incinerated. Some of the CCA (particularly the arsenic) may become airborne, but most will end up in the incinerator ash, where it is highly leachable and the risk of groundwater contamination is significant. While incineration of municipal solid waste accounts for only 16 percent of total waste disposal nationwide, it accounts for much more in some states: 60 percent in Connecticut and 45 percent in Massachusetts, for example. Currently, more than 2.5 billion board-feet of CCA-treated wood are estimated to be reaching the waste stream annually, and that amount is expected to increase significantly over the next few decades. Because of this concern, the wood products industry and EPA have agreed to stop selling CCA-treated wood by the end of 2003.

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