Chapter 22
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Can Understanding the Mechanisms of Biodegradation Help Preservative Development? Alan F. Preston Chemical Specialties, Inc., 200 East Woodlawn Road, Suite 250, Charlotte, N C 28217
The development of preservative treatments for wood products has historically been a process of trial and error. Formulations considered to have potential, usually through previous use in related biocide applications, are tested against wood destroying organisms, either in laboratory culture challenge tests, or in treated wood specimens in laboratory or field tests. This is usually a very long and often unsuccessful process, where experimental science plays the predominant role. Understanding biodeterioration mechanisms can assist in the preservative development process. The scope of such understanding can be quite broad, including fungal mechanisms, insect attack modes, weathering effects, chemical degradation related to biological mechanisms, potential for preservative loss in service, preservative fixation mechanisms, etc. This paper addresses the various mechanisms that affect biodegradation of wood products, and illustrates these with examples of their use in furthering preservative development.
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© 2003 American Chemical Society Goodell et al.; Wood Deterioration and Preservation ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
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Biodeteriogens of Wood The primary biodeteriogens of wood are fungi and insects. As a general rule, with wood products exposed in a natural environment for an extended service life fungi present the primary hazard to the wood and are generally more difficult to control than insects. However, in specific environments, for example inside the framing of houses, insects can present the predominant hazard.
Insects The use of fundamental knowledge of biodeterioration mechanisms for vector control has lead to the development of toxicant baits for termites. This is a clear example of a biodeteriogen suppression strategy developed from knowledge of causal agent's mode of action. While this application is not directly within the realm of wood preservation in the sense of applying a treatment to wood, it is nevertheless an example of wood protection through control of the organism in the environment prior to its potential attack on wood.
Fungi Much of the research in wood preservation over the last half century has focused on developing advanced knowledge of mechanisms of action of wooddestroying fungi. In this regard, significant advances have been made in the understanding of specific brown rot fungi, for example Postia placenta. In some fields, for instance control of short life cycle crop diseases in agriculture, knowledge of specific fungal mechanisms can provide useful insights. In contrast, the multiplicity of organisms that wood can be subject to during its long service life makes knowledge of specific fungal mechanisms less useful in wood preservation than in other fields. One area where fundamental understanding has played an important role is in the understanding of the mode of action of copper tolerant fungi. With the change in the wood preservative market in some regions from copper arsenate preservatives to copper-based systems, the understanding of when, where and how such fimgi work has led to strategies for the development of secondary biocides to be used in copper-based preservative systems. Copper tolerant fungi are considered to be of little importance in above-ground exposures, especially when low moisture contents are maintained in the wood. However, they can
Goodell et al.; Wood Deterioration and Preservation ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
374 present a hazard to wood in ground contact, although their presence is highly sporadic and largely unpredictable.
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Degradative Processes While it can be argued that knowledge of specific mechanistic processes of particular fimgi is of limited value in wood protection when wood is subject to degradation by a plethora of organisms, depending of geographic, construction, treatment, meteorological and other factors, the key fungal degradation processes of wood products in service are oxidative. In a similar manner, wood weathering is also an oxidative process. Beyond the need for an oxidative atmosphere, most wood destroying fimgi also require the presence of appropriate moisture conditions for optimal growth. Increasing moisture content of wood generally increases the propensity for fungal attack, although this becomes limiting at very high moisture contents. Wood destroying fiingi utilize oxidative processes, and it has been found that anti-oxidants may synergize fungicide performance in wood preservative formulations. While anti-oxidants are unlikely to provide control as a sole wood protection treatment at economic levels, this ability to provide secondary protection may allow enhance the performance of protection concepts or chemicals that currently provide marginal performance, or allow the development of systems having acceptable cost effectiveness and protection. A t this time anti-oxidants do not appear to be significant as additives in metalbased preservatives, although scant data exists, as with copper-based systems the phenolic group of the anti-oxidant and the copper(II) will likely form a complex. This complex would negate the anti-oxidant properties of the phenolic compound. Thus it appears that anti-oxidants are more likely to be important with pending moves toward water-based organic preservative and non-biocide systems. As mentioned above, optimal moisture contents provide optimal conditions for fungal decay of wood products. The converse is also true, and moisture control strategies can play a significant role in the future of wood protection. Wood moisture content reduction strategies have received increased attention in recent years, although the concept dates back to antiquity. It could be argued that to some extent the oil-borne preservative pentachlorophenol utilizes a water controlling strategy in the synergism seen between oil carrier type and preservative performance in pole treatments. Similarly, millwork treatments have long been known to be enhanced by the presence of water repellents in the solvent treatment. For the water-based preservatives, water repellents were initially developed to enhance the long-term physical weathering aspects of
Goodell et al.; Wood Deterioration and Preservation ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
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375 chromated copper arsenate (CCA) treated wood and provide little, if any, benefit to C C A in enhancing preservative performance. However, there is evidence that water repellent additives can enhance the performance of copper-based preservatives, allowing lowering of preservative retentions in some situations. Similarly, water repellents appear to enhance the performance of organic preservatives in above ground applications, although further development is required to meet expectations for ground contact applications. Recent research in Europe has seen the commercialization of heat treatments for wood products. The concept of protecting wood by heating in a non-oxidative atmosphere has been known for many years, with the work of Stamm in the 1940's being particularly important in this regard. The heating process largely impacts the hemi-celluloses in wood and interferes with the wood's susceptibility to decay. Questions remain as to the broad applicability of the technology, but in today's changing environment this technology certainly merits further development. Issues that need to be addressed include strength losses, emissions during processing, weathering performance, insect protection, ground contact applications, etc, but heat treatments in many respects are not inconsistent in properties with the wood-plastic composites that are currently in vogue in some quarters. In a similar vein, interest in wood modification processes has seen something of a revival as the wood preservative market changes and diversifies. For solid wood products, the costs of an approximately 15-20% weight add-on of a modifying agent remain a deterrent to commercialization, but clearly wood modification is a technically sound approach to wood protection. The fundamental strategy behind wood modification is one of making the wood substrate inert to fungal attack through reaction of the active hydroxyl sites in wood to provide ester or ether functionalities that are essentially impervious to enzymatic or chemical attack. The strategies described above all have their weaknesses in providing cost effective protection, but combinations of such treatments may improve the commercial likelihood of success. A t the same time enhanced properties are commanding increased value in the marketplace and will likely allow for treatments that would not be commercially viable in today's market for wood preservatives.
Realities of Developments in Wood Preservation In many respects wood preservation appears to have highly attractive commercial potential for biocide applications, but there are a number of development constraints that impact this potential. The first of these is the long
Goodell et al.; Wood Deterioration and Preservation ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
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376 product service expectations for the products used. The second is the relatively small value ascribed to the wood protection agent relative to the value of the raw material (wood) used. Overall, the value of treated wood sold is large, but the economic value of the chemical treatment is a minor component of this treated wood cost. Furthermore, the implied liability of the failure of the treatment involves the entire value of the product. That is, the risk to reward profile is not as attractive as in other biocides businesses, and the time to failure, and hence total product risk, is long. In the world of biocides, wood preservation is a relatively low value sector. For new chemical biocide developments, specifically fungicides and insecticides, the wood protection industry is dependent on the agricultural and pharmaceutical businesses to provide new products. On top of that, newly developed fungicides in the agricultural and pharmaceutical areas are highly organism-specific, while wood protection requires broad-spectrum protection. This dependence is due to the fact that registration costs for active ingredients is nowadays very high, and the rapid returns necessary to sustain such costs are a large constraint to the development of specific biocides for use solely in the slow-to-change field of wood protection. The chances of success in wood protection are low, due to the multiplicity of degrading organisms. Furthermore, broad-spectrum biocides may display less than optimal properties in their toxicological and/or ecotoxicological profile. In other words, there is an unbalanced up-front risk for a hard-to-quantify long-term potential gain. In developing new protection agents for wood products, it is interesting to consider the amount, or retention, of various agents in protecting wood. Current wood preservatives are used at levels around 1% mass/mass of wood. A t these levels distribution in wood is generally acceptable although distribution gradients can be steep. To be competitive, newer fungicides will have to be used in the 0.01 - 0.1% mass/mass add-on range in wood. A t these retentions, macro distribution can become problematic, especially with biocides that react with the wood substrate. Conversely, of course, biocides that don't react with the wood substrate may be more leachable than desired. At the other end of the scale, investigation of wood extractives responsible for natural durability effects has had a rebirth in some quarters. Difficulties here include the fact that utilization of natural biocides as biocides will still require highly expensive toxicological testing with, at best, dubious patent protection, and that the effective use levels of such extractives in wood are often well in excess of 1% mass/mass. Thus a combinatorial strategy from those described is probably the most likely to provide a cost effective protection product at acceptable levels of financial risk. In some jurisdictions, and Europe in particular, performance criteria for product approvals are based on laboratory test protocols. Such laboratory tests often give results which significantly under-estimate the level of preservative
Goodell et al.; Wood Deterioration and Preservation ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
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377 required for sound protection of treated wood products in service. Over the last decade this has led to a rapid, and probably irreversible, decline in product performance and expectations. Most of the rest of the world believe that laboratory results provide valuable insight into performance against specific known organisms but that field evaluations are far more likely to provide assurance of performance in various environments where treated wood products are subject to attack by multiple organisms. This is especially important where the performance expectations are subject to litigation against the producers rather than being seen as being established by some governmental authority. In the former environment, it is prudent that one take into account expected worst case usages in designing test protocols. Also, of increasing importance is the need to take into account fit-for-purpose aspects of the products, rather than just protection from biodegradation.
Conclusions Fundamental scientific research into understanding termite behavior has led to the development of commercial baiting systems for protection of structures in a low environmental impact treatment. However, fungal mechanisms are more challenging as a broad-spectrum protection strategy is necessary for wood products with long-term service expectations. It is clear that stopping, modifying or slowing oxidative processes can provide a positive impact in enhancing the performance of some wood preservative chemicals and that reducing moisture content in wood also can provide positive benefits. Further developments may provide the key to organic or non-biocidal treatments for long term service life of wood products.
Goodell et al.; Wood Deterioration and Preservation ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
