| Most wood products can be attacked and degraded by many different organisms. These include decay fungi that degrade wood in ground contact or wood that is frequently wetted, termites and other insects, and marine borers that attack wood in salt or brackish waters. To prevent biodeterioration, non-durable wood products are treated with biocides. The heartwood of tree species such as redwood, cedar, and baldcypress are naturally durable and do not require preservative treatments for some above-ground applications but harvesting restrictions are expected to greatly limit their future availability.
Most solid wood products are treated by a pressure process where the wood product is first dried and impregnated with a preservative solution in a pressure-treating cylinder. Wood composites are either treated during or after production. The latter includes plywood and laminated veneer lumber that are treated much as solid lumber. For the former wood composites, such as fiber or flake products, a solid or liquid biocide such as zinc borate is added directly to the wood furnish prior to mat formation. Millwork is often treated by dipping, spraying or brushing with a biocide solution.
Southern pine (SP) sapwood is easily treated to obtain deep and uniform biocide penetration. Thus, about 70% of all wood pressure-treated in North America are SP products. Wood from some western softwood species (e.g. Douglas fir, spruce) and hardwoods (e.g. white oak) are more difficult to treat with the product often incised, where small numerous holes are punched into the wood surface prior to treatment to increase biocide penetration and retention.
Preservatives intended for industrial applications such as utility poles and railroad ties are formulated with the biocide dissolved in oil (oilborne) or water (waterborne). Preservative systems for residential applications are waterborne, since oilborne treated wood has an oily surface and odor and can be difficult to paint. In North America, about 70% of all treated wood is used for residential applications.
Wood products treated with the first-generation biocides were cost-effective, long-lasting and had broad efficacy against the many organisms that attack wood. The first preservative developed was creosote, a viscous black coal tar by-product. It is currently used to treat railroad ties, bridge timbers, utility poles and marine pilings, and recently accounted for about 15% by volume of treated wood. Oilborne pentachlorophenol is another industrial preservative and is mainly used to treat utility poles and crossarms and accounts for about 5% of all treated wood. The major wood preservative during the last half of the 20th century was the waterborne metallic arsenicals, principally chromated copper arsenate (CCA). Up to the end of 2003 CCA was used for both industrial and residential applications and accounted for about 80% of all treated wood and over 95% of wood for residential applications. However, public concerns with the heavy metals arsenic and chromium led suppliers to voluntarily restrict CCA to non-residential applications in 2004.
Biocides that have replaced CCA for residential applications are the second-generation waterborne copper-rich systems that contain uncomplexed copper and an organic co-biocide to control copper-tolerant fungi. The major systems are alkaline copper quat (ACQ) and copper azole (CA). These preservatives are about as effective as CCA but relatively high levels of copper are susceptible to leaching that can impact aquatic ecosystems, there is increased potential for metal fastener corrosion and surface mold growth, and concerns exist over the ultimate disposal of metallic-treated lumber. These problems have led three European countries to require third-generation totally-organic preservatives for residential applications, and a few localities in North America have recently enacted usage limitations on copper- and creosote-treated wood. Restrictions will undoubtedly increase in the future.
The third-generation systems will only employ organic biocides. Most organic biocides are not water soluble so a special formulation or treatment procedure is necessary. Problems with organic compared to metallic biocides include a relatively high cost per unit weight and the biotic and abiotic mechanisms that deplete organic biocides over the long service life expected from treated wood. Finally, many organic biocides are only effective against some of the many organisms that attack wood, so two or three biocides are necessary for broad efficacy. No totally-organic wood preservative for exterior residential applications is currently available but some systems are being developed. Another alternative to copper-rich preservatives might be copper-poor systems such as waterborne copper napthenate or oxine copper.
Another waterborne preservative for residential applications is sodium borate (SBX). Borates are low cost and have an extremely low mammalian toxicity, a broad range of activity against decay fungi and insects, are not corrosive to metal fasteners, and are colorless and odorless. However, borates can be easily leached by water so borate-treated lumber is limited to applications with no or minimal leaching potential such as home framing.
Some nonbiocidal methods exist to protect wood against biodegradation. Heat-treated lumber has had some commercial success in Europe where the treatment provides some protection against decay fungi and the wood has increased dimensional stability, but it is not suitable for ground-contact applications and has greatly reduced strength properties. Chemically-modified wood, such as obtained by acetylation, can provide good deterioration resistance and dimensional stability. An acetylation plant is being built in Europe. Water repellents, either alone or combined with a wood preservative, are also becoming more important.
Due to environmental considerations, public concerns over biocides and governmental regulations, wood preservation is expected to continue to undergo rapid and profound changes on a worldwide basis. This is especially true with treated wood products for the residential market.
Further Reading
Anonymous. Pressure-Treated Southern Pine. Available from the Southern Pine Council (2006), http://www.southernpine.com.
Barnes, H.M., Murphy, R.J. “Wood Preservation: The Classics and the New Age”. Forest Products Journal. 45, issue 9 (1995), 16-26.
Evans, P. “Emerging Technologies in Wood Protection”. Forest Products Journal 53, issue 1, (2003):14-22.
Goodell, B., Nicholas, D.D., Schultz, T.P., editors. Wood Deterioration and Preservation: Advances in Our Changing World. Washington, D.C.: American Chemical Society, 2003.
Townsend, T.G., Solo-Gabriele, H., editors. Environmental Impacts of Treated Wood. New York: CRC/Taylor and Francis, 2006.
Tor P. Schultz and Darrel D. Nicholas are Professors, Forest Products Laboratory, FWRC, Mississippi State University, Mississippi State, MS 39762-9820
Posted: 21 April 2007
Updated 23 August 2007
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