Wood Deterioration and Preservation - ACS Publications - American

Chapter 1

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Introduction to Wood Deterioration and Preservation 1

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Barry Goodell, Darrel D. Nicholas, and Tor P. Schultz 1

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Wood Science and Technology Department, University of Maine, Orono, ME 04469-5755 Department of Forest Products and Forest Products Laboratory, Mississippi State University, Mississippi State, M S 39762-9820 3

Wood-degrading fungi, insects, bacteria and marine borers cause damage resulting in billions of dollars being spent on repair and replacement of wooden structures every year. The equivalent of one-tenth of the forest products produced every year is estimated to be destroyed by these agents. Although wood degradation results in an enormous waste of resources, without wood degrading organisms our world would be buried under cellulose and lignin debris, as these organisms are among the few that efficiently recycle lignocellulosic carbon. Further, in recent years some of the mechanisms employed by microorganisms to degrade wood have been used in bioindustrial processes to benefit humans. For example, fungal-based oxidative reagents are being examined in biopulping and biobleaching processes, and microbial enzymes from wood degrading organisms have been used in systems ranging from wastewater cleanup to the production of fuels from biomass. Wood degrading agents therefore cause many problems, but also greatly benefit mankind. With greater knowledge of their capabilities and potential we will be able to find better controls for their unwanted actions and direct their biochemical mechanisms to desirable applications. The chapters in this text discuss not only the mechanisms that fungi, bacteria, termites and marine borers use to degrade wood but also ways to harness degradative processes for humankind's benefit. Additional chapters of the text are devoted to methods for detecting these organisms and developing environmentally benign methods to protect wood against the many organisms that can attack and degrade wood. This book is important because the field of wood protection is rapidly and dramatically changing. This text, and the associated Symposium from which most of the chapters come, attempts to provide some perspective on these changes. Historically, humans have battled wood deterioration agents through

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© 2003 American Chemical Society In Wood Deterioration and Preservation; Goodell, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.


3 the ages. The story of the 'little Dutch boy' plugging the dike in Holland relates back to the concerns over failures of wooden dikes that were often riddled with marine borers. The Spanish Armada was defeated off the coast of England in the 1580's in large part because their wooden ships were so badly deteriorated that they broke up in storms. We have relied on chemical treatments to protect wood from deterioration since the time of the earliest recorded history. Heating wood in an anoxic environment, and the application of cedar oil or copper salts to prevent decay and insect attack, have long been known. For the past 150 years we have used pressure systems to force preservative chemicals such as creosote or water-borne zinc chloride into wood. However, environmental concerns have recently dramatically changed the way we view wood preservative chemicals. This is particularly true because the market has changed in the past 30 years from mainly producing products for industry such as utility poles and railroad ties, to principally manufacturing products for residential construction (70% of the treated wood market) such as decking. However, arsenical-based preservatives were banned in Germany some 20 years ago, setting in motion a world-wide effort to ban or restrict the use of the three most commonly used, traditional preservatives, the arsenicals, pentachlorphenol, and creosote. Planned restrictions on the use of these chemicals, primarily for residential applications in Asia, Europe, and North America, has already caused the wood preservation industry to rush to find more environmentally acceptable methods to protect wood. In the short-term several coppenorganie mixtures are poised to fill the void in water-borne treatments. These chemicals, however, are also now being scrutinized by the public because of the potential leaching of copper, which in some cases can be 10-times that which occurs with arsenicalbased preservatives such as chromated copper arsenate (CCA). In addition, because metals cannot be broken down in the environment, the disposal of any wood treated with a metal-based preservative will be more expensive and difficult in the future. Thus, an active search is underway to develop environmentally acceptable organic preservatives that contain no metals. Currently, the driving forces in wood preservation include both regulatorybased and market/cost-based issues. In the short-term, regulatory pressure may drive the industry to use more environmentally acceptable chemicals that may not protect wood as well as traditional preservative chemicals. As knowledge of wood protection methods develops, and methods to improve treatment longevity using benign chemicals become better understood, both environmental acceptance and long-term effectiveness will improve. In the short-term, however, wood may be protected with less effective biocides. This may lead to failures with subsequent liability problems for specific manufacturers, and market loss for forest products companies in general. The wood products industry has already seen an influx in the use of metal studs for residential construction and plastic/wood as a substitute for wooden decking.

In Wood Deterioration and Preservation; Goodell, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.


4 Both water- and oil-borne wood preservatives are currently used for the treatment of wood. Water-borne chemicals have an advantage in that the water carrier is low-cost, safe, and the wood is normally left with a clean surface. Unfortunately, water swells wood and water-borne preservatives are often used for some wood products like large timbers, where dimensional change during treatment cannot be tolerated. For many industrial applications such as bridge timbers, utility poles and marine applications, the oil carriers used with some preservatives are preferable because of the properties imparted to the wood by the oil treatment, including water repellency and greater efficacy. With the restricted use of water-borne C C A , and increasing restrictions on traditional oilborne chemicals such as pentachlorophenol and creosote, new preservative and wood protection systems that can replace these older chemicals must be developed. In some cases, the addition of water repellents may allow waterborne chemicals to partially substitute for some of the traditional oil-borne treatments. The use of water repellants has been shown to improve the performance of preservatives, particularly in above-ground applications such as decking. Also, environmental concerns, disposal issues and public perceptions - even perceptions that may be erroneous - are directing researchers and the industry to develop environmentally acceptable methods to protect wood. The problem is to better understand how wood deteriorating organisms attack wood so that they may be safely controlled, yet allow the wood product to be safely disposed of or recycled at the end of the product's life. One of the greatest challenges to protecting wood from the variety of degradative agents found in the environment is the length of time that wood must be protected. Unlike the protection of crops or other commodities where protection against pathogens is normally needed only for a relatively short-term period of weeks to months, wood is expected to last for decades without supplemental protection. The use of long-lasting biocides as wood preservatives is often at odds with the development of environmentally friendly protection strategies. Biocides and other chemicals that persist in the environment without degrading proved excellent for protecting wood, but were later found to diffuse or move to non-target sites where they became long-term pollutants. The use of biocides that degrade rapidly, however, is not consistent with the need to protect wood for the long times expected by consumers. What is needed therefore, are targeted biocides that are resistant to degradation and that are specific only to the biochemical mechanisms used by wood degradation agents. The development of these 'site-specific' biocides, and/or wood-modification processes that are permanently fixed in the wood to prevent leaching of chemicals into the environment, is also critical. Development of site specific systems that target biochemical mechanisms specific to wood degradation agents is also consistent with consumers' apparent need for frequent changes and remodeling of structures. Today, the average life



5 of an outdoor deck in the U S A is only about 13 years. This relatively short life span is not the result of decay or other deterioration, but stems from the desire of new or existing owners to redesign homes and other structures including the replacement of decks. This has resulted in the disposal of large volumes of treated wood products in landfills, and a search for ways to degrade and detoxify these products that were treated specifically to resist this type of environmental breakdown. Alternately, methods to recycle these products are being explored. The development of targeted preservatives or systems that can undergo degradation or be easily recycled would benefit this effort. The public will also require any preservative system to be low cost and impregnated into wood using a non-polluting carrier. Furthermore, to be competitive with other construction materials, any new wood protection system must be economical. However, new organic-based systems will likely consist of biocides that are relatively expensive and, because of solubility problems, may only be compatible with hydrocarbon solvents. Development of co-solvent systems providing improved solubility with aqueous solutions may be one remedy to this problem. While the field of wood preservation has rapidly changed in the past decade, so too has our understanding of wood decay and deterioration processes. Recently uncovered biochemical mechanisms employed by wood decay microorganisms are reviewed in this text. These mechanisms help to explain the unique ability of fungi and bacteria to break down the very complex biopolymer that we know of as wood. New insights into marine-boring organisms and termites are also changing the way we think about their methods for attacking wood. In addition, new methods to detect and monitor decay and insect attack are being developed. These methods will help us to better understand how the various organisms attack wood and, to detect it in-situ to better utilize protection and remediation strategies. Our evolving understanding of wood degradation by bacteria, fungi, insects and marine organisms has allowed us to think about new methods to protect wood that are less harmful to the environment. For example, the use of relatively benign and low cost antioxidants for combating the free radicals produced by wood-degrading fungi has shown promise to economically enhance the efficacy of relatively-expensive organic biocides (see Chapter by Green and Schultz). In addition, synergistic systems for the protection of wood, where two or more chemicals may be used in relatively low dosages to protect wood because of the increased effectiveness of both when used together, has the potential to reduce preservative levels used in treated wood. This provides both economic and environmental benefits. Other methods to protect wood continue to be explored. The chemical modification of wood to circumvent the biochemical mechanisms that microorganisms, insects and marine borers have developed specifically to attack the holocellulose and lignin components has proven to be effective i f the component chemicals of wood are adequately disguised in this process. (See the chapter on Wood Composite Protection for a



6 discussion of chemical modification of wood). The recent resurrection of thermal treatments to protect wood in Europe also shows potential for limited applications. Although thermal treatments have shown some success in protecting wood in above-ground, low decay and insect hazard environments, the protection of wood in ground contact or moist conditions still requires chemical protectants. In our effort to define the contents of the Symposium as well as this text, we drew on the expertise of some of the leading scientists on wood deterioration and protection from around the world. To protect wood we must understand how organisms break wood down. For this reason we also looked beyond the borders of our own field. In this regard, this text begins with a description of free radical mechanisms, including a general overview of free radicals in the environment by Barry Halliwell followed by one specific to white rot degradation of wood from N . Scott Reading, Kevin Welsh and the laboratory of Steven Aust. A s discussed above, given our current state of knowledge free radical reactions clearly appear to be the key to understanding degradation mechanisms, and methods to control free radical reaction chemistry may well guide the development of future wood protection systems. A n excellent overview by Geoffrey Daniel on bacteria and fungal degradation patterns, visualized through microscopy and electron microscopy, opens the section of the text of Wood Deterioration Processes specifically related to microbial degradation. This is followed by chapters on White Rot and Brown Rot decay by Kurt Messner and co-authors, and Barry Goodell, respectively. This section continues with works by Timothy Filley on lignin degradation by decay fungi, and a chapter by Akio Enoki, Hiromi Tanaka, Shunji Itakura on a unique hypothesis relating fungal degradation by white rot, brown rot and soft rot fimgi. Jaime Rodriguez, Andre Ferraz and Maricilda de Mello's chapter focuses on metal based reactions that occur in the fungi, whereas William Henry looks at the chemistry of metals from the perspective of an organometallic chemist. The final chapters of this section by Kaichang L i , and William Kenealy and Thomas Jeffries, focus on fungi and fungal mechanisms that have been employed in biotechnological applications including the use of mediators, and fungal enzymes for bio-pulping and related processes. Three chapters on termites and marine borers complete the section on Wood Deterioration Processes. Shelton and Grace give a broad overview of the termites in their chapter. Daniel Distel and Simon Cragg provide chapters on the marine borers reviewing marine boring bivalves and arthropods, respectively, to provide a badly needed update on the modes of action of the marine boring organisms and their distribution. The third section of the book concerns methods for the detection and monitoring of the agents of wood deterioration. Darrel Nicholas' and Douglas Crawford's chapter starts the section with an overview on the early stage progress of wood decay, and the development of an accelerated method for



7 decay detection based on mechanical properties. Following this are two chapters by Susan Diehl and Carol Clausen that focus on chemical and immunological methods for decay detection. Barbara Illman follows with an update on the use of synchrotron technology as a research tool to detect both decay in wood as well the valence state of metals used in wood preservatives. Jody Jellison and Claudia Hussenender and their co-authors round out this section with chapters on D N A methods for the detection and monitoring of wood degrading fungi and termites, respectively. The final section of the text concerns the important topic of wood protection and wood preservation systems. The first chapter by Alan Preston provides a perspective and introduction to current problems and the long-term approach for wood preservative development from an industrial perspective. Rick Green and Tor Schultz then introduce information on the relatively new development of adding non-biocidal chelators and anti-oxidants to wood preservatives to synergistically enhance biocidal activity. This has the benefit of reducing the amount of the relatively expensive bioactive chemicals needed to protect wood. Liam Leightley then adds to this perspective, from an industrial standpoint, in looking at synergistic wood preservative combinations with near-term commercialization potential. Douglas Gardner, Cihat Tascioglu, and Magnus Walinder follow this with a much-needed overview on the state of wood protection in the wood composites field. The final chapter by Tor Schultz and Darrel Nicholas provides an encompassing summary of the pros and cons for many of the newer non-arsenical wood preservatives that have recently been developed, or that show potential. This final chapter provides guidance for the future in reviewing wood preservatives that are under development now. Some of these chemicals will become the next generation of wood preservatives. Others will fall by the wayside, perhaps to be looked at again when problems develop with existing protection methods. It is impossible to predict the future, but this book hopefully sheds light on recent developments in our understanding of wood deterioration mechanisms, new ways to detect the agents of deterioration, and trends in methods for wood protection. The co-editors look forward to your comments.


Wood Deterioration and Preservation - ACS Publications - American

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