Chapter 5
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Weathering and Photoprotection of Wood Philip D.
Evans
Centre for Advanced Wood Processing, University of British Columbia, Vancouver V6T 1Z4, Canada (
[email protected])
This Chapter describes the weathering of wood exposed outdoors, the environmental factors and mechanisms responsible for such deterioration, and the ability of chemical treatments to prevent the weathering of wood. Also included are the prospects for the development of more effective protection systems that restrict surface deterioration and protect the structural properties of wood.
© 2008 American Chemical Society In Development of Commercial Wood Preservatives; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.
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70 Preservative treated wood is used in a wide variety of industrial (poles, cross-arms, ties, highway posts), agricultural (fences, vineyard and orchard posts, barns) and residential (decks, windows, framing) applications. The market for treated wood has increased significantly in the last 30 years due, in part, to the increased use of wood in residential applications, particularly decks. The decking market alone consumes 2 billion board feet of Southern pine (Pinus sp.) lumber per annum, and 38 percent of all of the treated Southern pine in the U S A is used for outdoor decking (7). The performance requirements of treated decking differ from those needed for wood in industrial and agricultural applications. In particular, the appearance of the wood assumes greater importance because consumers regard decks as extensions of household living areas and expect decks to maintain their visual appeal over time. Evidence to support this assertion is the finding that surface deterioration of treated wood is a significant reason for the premature replacement of chromated-copper-arsenate (CCA) treated timber decks in the U S A (2). The inability of conventional wood preservatives to prevent the surface deterioration of wood is well known, and there has been longstanding interest in developing hydrophobic emulsion additives to increase the water repellency of treated wood and reduce surface checking (3, 4). In general, however, most of the wood preservatives currently on the market fall short of meeting the desire of consumers for a treatment that will maintain the visual appeal of wood exposed outdoors as well as protect its structural integrity. This has provided an opportunity for alternative decking products, for example woodplastic and plastic lumber, that are more resistant to surface deterioration, to capture an increasing share of the market for treated timber decking (5). These lumber substitutes are two to five times the cost of treated wood and their success indicates the price premium that consumers are willing to pay for an aesthetically pleasing, maintenance-free, deck (5). This has led to renewed interest in surface treatments and wood protection systems that are more effective at restricting the weathering of wood. This Chapter describes the weathering of wood exposed outdoors, the environmental factors and mechanisms responsible for such deterioration, and the ability of chemical treatments to prevent the weathering of wood. Also included are the prospects for the development of more effective protection systems that restrict surface deterioration and protect the structural properties of wood.
Weathering of Wood The surface degradation that occurs when wood is used outdoors and above ground is termed weathering (
