Development of Commercial Wood Preservatives Efficacy

Chapter 13

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Wood Preservative Fungicides and the American Wood Preservers' Association Use Category System Peter E. Laks School of Forest Resources and Environmental Science, Michigan Technological University, Houghton, MI 49931

Fungi are the primary biological hazard for wood products. Thus, the fungicide component of a wood preservative system is typically the most important constituent. The type of fungal hazard and the nature of the fungicide most appropriate to control that hazard will depend on the application for the treated wood product. The biological hazard for a given application has been systemized by the American WoodPreservers' Association Use Category System. This is a useful tool in understanding how wood preservative fungicides are currently used. The chemical, physical, and biological characteristics of a fungicide determines its suitability for a given wood application. Important characteristics are efficacy/cost ratio, breadth of efficacy spectrum, activity against non-target organisms, stability, and leach resistance. Wood preservative fungicides are a very diverse group and are in the process of undergoing further change due to government regulations, environmental issues, and the demands of the marketplace.

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© 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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Fungi that Attack Wood-in-Use Fungi are a kingdom of organisms with a broad range of eco-niches. They are commonly defined as non-motile, filamentous or single-celled microbes, lacking chlorophyll, with chitinous cell walls, reproducing by spores, some of which produce large fleshy fruiting bodies (mushrooms) (7). With respect to wood-in-use, the most important general types of fiingi are the basidiomycetes, soft-rot fungi, and the mold and stain species. There is some overlap between these groups, as species that are commonly classified as molds can also cause soft-rot decay under the appropriate conditions. A thorough discussion of the fiingi that attack wood is beyond the scope of this paper, but some understanding of fiingi is useful when discussing fungicides. Fungi have a number of classic growth requirements, but three are especially important in determining whether a wood-based product will be prone to fungal attack. The first is wood moisture content. Under certain conditions, some fiingi can grow on wood when the moisture content is less than 20% (2), however, most authorities state that a moisture content at or above the fiber saturation point (about 30% for temperate-climate commercial wood species) is necessary for significant fungal growth (5). The second important requirement for fungal growth is available nutrients. A l l wood contains the polymeric structural carbohydrates that basidiomycetes and soft-rot fiingi utilize as a fixed-carbon source. Mold and stain fungi utilize low molecular weight carbohydrates, starches, fats, and fatty acids that have a more variable distribution in wood products. For example, the heartwood of many wood species contains little or none of these nutrients, making it quite resistant to mold and stain fiingi. The third factor is the presence/absence of fungitoxic chemicals that inhibit fungal growth. These chemicals can either be naturally occurring extractives present in the heartwood of some wood species, or synthetic wood preservatives introduced on or into the wood using a treating process. A n appropriate loading of an effective preservative can completely inhibit fungal activity. Basidiomycetes are fimgi that feed on the structural carbohydrate polymers in the wood (cellulose and the hemicelluloses). They include the mushroomforming fiingi. Some of the basidiomycetes, particularly the so-called white-rots, also produce enzymes that can degrade the third major wood structural polymer, lignin. Through their ability to attack these structural polymers, the basidiomycetes can rapidly reduce wood strength properties. Basidiomycete decay is characterized by softening and mass loss in a relatively large volume of the wood article. In untreated susceptible wood, the volume limitation is often the moisture content of the wood. The optimal moisture content for basidiomycete growth in wood depends on the fungal species and the investigator, but is typically in the 30-80% range (4). Some species have acquired the ability to transport water to drier portions of a wood article.

In Development of Commercial Wood Preservatives; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.


230 Basidiomycetes are the classic wood decay fimgi and cause the most damage to wood-in-use. Soft-rot fiingi are lignolytic ascomycetes and deuteromycetes. Generally, they require a longer time to degrade wood compared to the basidiomycetes. Soft rot decay tends to dominate in treated wood under high wood moisture content conditions where the growth of basidiomycetes may be inhibited. Mold fiingi are imperfect fiingi with colored mycelium and/or spores that grow on the surface of wood, utilizing low M W sugars and starch. Mold fiingi do not cause significant strength loss in the wood. Their damage is aesthetic. Molds are commonly a problem on unweathered wood surfaces. For example, freshly-cut sapwood lumber is very prone to mold growth. To maintain a clean surface under warm conditions, either the lumber has to be dried quite quickly or a moldicide applied to the wood surfaces. Sapwood that has been pressure treated with common waterborne wood preservatives is also often initially susceptible to mold. This susceptibility usually is reduced as the exposed wood surfaces weather (e.g. the treated lumber is used as exterior decking), although older wood surfaces can support mold growth i f nutrients such as pollen or other plant residues accumulate on the surfaces. A n understanding of the fimgi that attack wood is important when discussing the agents used to control them - fungicides. It has been said many times that the most common wood preservative method is drying the wood and keeping it dry. This comes from an understanding of the basic moisture requirements of fiingi. Similarly, there is no point in changing the availability of free sugars and starches i f basidiomycetes are being targeted for control since basidiomycetes typically utilize the cellulose and hemicellulose components of the wood. A n understanding of the basic biology of fiingi is important in their efficient control.

The Use Category System Wood products are used in a broad array of applications, ranging from furniture in a heated home where there is little or no biological deterioration hazard to marine exposure in a tropical climate. In the United States, this broad range of possible applications has been classified with the Use Category System (UCS) (5). This system was adopted by the American Wood-Preservers' Association in 1999. A n understanding of the U C S is important in any discussion of wood preservative fungicides because it provides a means to group applications together that have similar fungicide performance requirements. B y understanding the fungicide needs for a given Use Category, biocides can be specifically selected for that U C . Following is a summary of the U C S classifications:



231 UC1 - Interior Construction, Above Ground, Dry - These are applications protected from external and internal water sources. Typical examples are interior furniture and millwork, as well as studs, joists, subflooring, etc. Since, ideally, there should be no sources of liquid water contacting the wood, the moisture content of these materials should remain below the fiber saturation point (FSP) and, therefore, there should not be a fungal hazard in U C 1 . Termites, beetle larvae, and carpenter ants can attack relatively dry wood in the appropriate climates and settings, so UC1 applications should only experience an insect hazard. One of the most common insecticides used for pressure-treating UC1 commodities is disodium octaborate tetrahydrate (DOT, also abbreviated SBX). D O T is an effective fungicide as well as being an insecticide. Its use is an example of fungicide protection being provided when it is not really needed.

U C 2 - Interior Construction, Above Ground, Damp - UC2 applications have limited protection from interior and exterior water sources. The typical example is a sill plate. This is the horizontal lumber that sits atop the foundation and then contacts the studs, joists, or rimboards in conventional construction. Water can be wicked up through the foundation and come in contact with the sill plates resulting in a decay hazard. Usually there is some kind of moisture protection between the foundation and the sill plate, but it may not be 100% effective. Another U C 2 example is a window sash. During the winter in a cold climate, condensation is possible on the interior of the window leading to liquid water contact with the sash framing. Leachable preservative systems can be used in UC2 applications because there is no continual contact with liquid water that would provide the depletion route. Fungicides used in UC2 applications include DOT, organics such as 3-iodo-2-propynyl butyl carbamate (IPBC), and the common waterbornes such as the alkaline copper quat (ACQ) and copper azole (CA) systems.

U C 3 A - Exterior Construction, Above Ground, Coated and Rapid Water Runoff - The use category for above-ground exterior construction is split into two subgroups depending on whether the application has some protection from liquid water. The U C 3 A category is for commodities that have some limited protection from precipitation and are in applications where there is rapid draining of water from the surfaces. Painted siding is a good example of a U C 3 B commodity. The paint provides some protection from water absorption and the vertical orientation of the siding results in rapid runoff of any liquid water on the siding surfaces. Other UC3B products are exterior millwork and trim. A common preservative used in these applications is zinc borate, used as a preservative in a variety of exterior wood composite products (6).



232 U C 3 B - Exterior Construction, Above Ground, Uncoated and Poor Water Runoff - This is the other use category for above-ground exterior construction. In U C 3 B , the wood product has no coating and/or the commodity is oriented in a fashion that does not allow rapid water runoff. Examples are decking, railings, fence pickets, utility pole crossarms, and any uncoated wood used on the exterior of a building. This use category includes some of the most important applications for pressure-treated wood - most treated wood is used in exterior deck construction. A variety of wood preservatives are used in U C 3 B commodities, including the dominant A C Q and C A systems. Fungal decay and insect damage control is important for both UC3 subdivisions.

U C 4 A , B , and C - Ground/Fresh Water Contact - UC4 covers all groundcontact and fresh water applications. The three subcategories (UC4A, U C 4 B , and UC4C) increase in severity of exposure and/or criticality of the application in this sequence. U C 4 A is for non-critical treated wood products in contact with the ground or freshwater. Examples are fence/deck posts, landscaping timbers, utility poles in low decay hazard regions, and backyard dock pilings. U C 4 B covers ground contact applications in critical applications and/or when replacement of the treated wood is difficult. Examples are utility poles in moist temperate climates, wood foundations for houses, building poles, and sea walls subject to salt water splash. UC4C is for products in ground contact in severe climates and applications where replacement would be very difficult. Examples are utility poles in tropical and semitropical climates, and foundation pilings for commercial buildings. A broader range of wood preservative treatments are used in UC4 applications. Preservatives that are not allowed for residential use such as chromated copper arsenate (CCA), creosote, and pentachlorophenol are commonly used in non-residential UC4 applications. Railway crossties are another important commodity that fall within this general group. Similar to utility poles, the exact classification for the crosstie depends on the severity of the installation climate. Fungal decay control is critical for UC4 applications, although insect performance is important as well.

U C 5 A , B , and C - Salt/Brackish Water Contact - UC5 covers applications where the treated wood product is exposed to marine borer attack. The three subdivisions reflect the degree of exposure severity. UC5C is the most severe exposure where there is exposure to warm water organisms that are difficult to control with wood preservatives. The common preservatives used in UC5 applications are creosote, C C A , or a dual treatment of the two. Fungicide performance in U C 5 is relatively unimportant because of the difficulty in controlling the mariner borers. Retentions needed for marine borers are many times higher that those needed for fungal or insect control.


233 Figure 1 graphically illustrates how the various types of biological degradation vectors correlate with A W P A use category. Different fungicides have different efficacy spectrums. It is important that the fungicide cover the

Mold Basidiomycete Soft Rot Insects Marine Borers


UC1 UC2 UC3A UC3B UC4A UC4B UC4C UC5

I

Figure 1. Biological hazardfor wood products correlated with Use Category.

appropriate range of problem fungi when selecting a biocide for use in a product meant for a given use category. For example, in ground contact applications (UC4) efficacy against soft rots is necessary. If the fungicide used in a wood preservative system meant for ground contact does not have efficacy against soft rots, an additional fungicide with this activity should be included in the formulation. Conversely, i f a preservative system has been designed only for above ground use, efficacy against soft rots is not required. This may lower the total cost of the system.

Characteristics of Wood Preservative Fungicides Preservative-treated wood products typically have a long service life. As shown above in the discussion of the U C S , applications for treated wood are diverse, leading to a broad range of performance requirements. Following is a list and discussion of ideal wood preservative (WP) fungicide characteristics. High Efficacy/Cost Ratio - A fundamental aspect of most building products is that they are relatively inexpensive. This comes from the very large volumes produced and competition from different materials. Depending on the specific


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application, pressure-treated wood has competition from materials based on steel, concrete, thermoplastics and others. Since the treated wood product has to be inexpensive, the efficacy/cost ratio for the preservative treatment has to be high. Copper II has dominated as the wood preservative fungicide of choice because of its reasonable intrinsic antifungal activity and very low cost compared to organic fungicides. Broad UC Application Spectrum - As described above and depending on the application, WP fungicides have to control a broad range of target fungi. A single fungicide with an efficacy spectrum broad enough to cover all of the target fiingi would obviously be most desirable as long as the other performance requirements are met. This is often difficult to achieve, however, leading to the common practice of mixing complementary fungicides in a formulation to achieve control of the entire range of problem fiingi. The most obvious example of this practice is with the current generation of "copper plus cobiocide" formulations. Copper II has many desirable WP characteristics, but there are "copper-tolerant" basidiomycetes and soft rots that can attack wood treated with only copper (4). A quaternary ammonium salt or triazole is added to the A C Q or C A formulations, respectively, to control the copper-tolerants. Benign to Non-Target Organisms - By definition, fungicides are biocides. It is most desirable, of course, that a wood preservative fungicide not affect nontarget organisms, especially the human user of the treated wood. As a nonhuman example, birds perching or nesting on treated wood structures should not be negatively affected by contact with the treated wood. Chemical/Physical Stability - Pressure-treated wood products typically have a long service life. Sill plates must last for the lifetime of the house in which they are installed, and utility poles are commonly described as having lifetimes of 3040 years. Warranties for pressure-treated lumber are typically "Limited Lifetime", but since the average residential deck in the United States is replaced or modified 11 years after installation (7), the actual lifetime of use in this very important segment of the market is relatively short. Nevertheless, the fungicide has to be stable in the treated wood product over a long period of time. This stability takes a number of forms. The stability issue is more of a consideration for organic fungicides than inorganics. These must be stable to oxidation, heat, and interaction with other formulation components. Inorganic fungicides, such as copper (II) and borates, typically have fewer issues with stability, but there can still be loss of the active ingredient (a.i.) with time through water leaching. Leach Resistance - In UC3 through UC5, resistance to loss from water leaching is a very important characteristic. In general, the leach resistance requirement



235 increases in importance with increasing use category. UC1 and 2 are interior applications where the treated wood should never be exposed to significant amounts of liquid water, so leachable fungicides such as D O T can be used. There are two fundamental impacts of a.i. leaching from treated wood. First, the amount of a.i. in the wood is reduced over time. Eventually this will lead to reduction below a critical level and fungi will be able to attack the wood. Secondly, the a.i.(s) migrate into the surrounding environment and may cause significant contamination of surrounding soil or bodies of water. It is generally accepted, however, that some water mobility is necessary for the fiingicide(s) in a conventional wood preservative system to perform. M i n i m a l Negative Effects on Wood Properties - Incorporating a chemical formulation into wood can have effects on many of its nondurability-related properties. The most important are strength, corrosivity, and fire properties. A good discussion of the effects on strength properties can be found in the Wood Handbook (8). Obviously, it is best that the fungicide(s) and other components of a WP formulation have a minimal negative effect on these properties. Readily Available - A W P fungicide has to be registered with the U.S. E P A before it can be used commercially. It is also desirable from the points of view of the formulator, treater, and user that the fungicide be available from more than one chemical manufacturer. Not only does this typically reduce the cost of the a.i., it also means there are alternative suppliers of the chemistry in case one manufacturer goes out of business or simply decides not to manufacture that fungicide anymore. Easy to Formulate - Pressure treatment of wood involves penetration to a significant depth within the wood structure. The actual required depth depends on the wood species being treated and the nature of the commodity being manufactured. These parameters are detailed in the A W P A Book of Standards (J). The a.i.(s) in the formulation have to be in a form that allows this penetration to occur. A true solution is, by far, the most common formulation type, but emulsions and small particle suspensions may become more important in the future. A W P fungicide has to have chemical properties that allow it to be formulated in an appropriate carrier that results in adequate penetration, but does not contribute negative characteristics as described above in this list. A n obvious example is that the emulsifiers in an emulsion formulation should not promote leachability of the a.i.(s). In many cases, the relative importance of these fungicide characteristics depends on the use category (ies) in which the treated product will be used. As described above, resistance to leaching is only important in the higher UCs. As another example, a reduction in strength properties may not be important i f the treated wood is being used in a non-structural application.


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Common Wood Preservative Systems and Fungicides


A n excellent and detailed discussion of WP systems and biocides is given in Schultz and Nicholas (P). For more detail on individual fungicides and wood preservative systems, please see their paper. The purpose of this section is to provide more general information. Following is a list of WP systems in common use in the United States and Canada and their acronyms: •

Chromated Copper Arsenate (CCA)

•

Creosote (CR)

•

Pentachlorophenol in P9 Type A Oil (PCP-A)

•

Alkaline Copper Quat (ACQ)

•

Copper Azole (CA)

•

Disodium Octaborate Tetrahydrate (SBX)

•

Copper Naphthenate (CuN)

•

Ammoniacal Copper Zinc Arsenate ( A C Z A )

•

IPBC plus cobiocide(s) (IPBC+)

•

Zinc Borate (ZB)

The major wood preservatives are commonly divided into first generation systems (CCA, creosote, PCP, A C Z A ) and second generation systems (ACQ, C A ) . Some of the others are not so easy to classify. For example, S B X has only developed as an important preservative in the United States during the last 15 years, but has been used in other parts of the world since the 1950's (70). Others of these systems have been developed for specific application niches. A s mentioned above, zinc borate is commonly used as an in-process preservative for wood-based composites, while the IPBC+ systems are common as non-pressure applied treatments as used for manufacture of window parts or moldicides for OSB (6). The active ingredients in these basic WP types are listed in Table 1. Copper (II) is a common fungicide found in many of the major wood preservative systems. It possesses many of the desirable fungicide characteristics listed above. In particular, its efficacy/cost ratio, U C application spectrum, stability, leach resistance, effects on wood properties, formulation ease, and availability are all excellent. Two notable issues are copper's weakness against copper-tolerant fungi, which requires the addition of a co-fiingicide to copperbased formulations, and its possible effect on non-target marine organisms i f it is used in UC5 applications (77). The second generation systems require a particularly high loading of copper into the treated wood (Table 2). This has lead to speculation that the next generation of wood preservatives may be similar to the current systems in that they are based on copper plus a cobiocide, but may require a lower level of copper in the wood.


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Table 1. Fungicide and insecticide active ingredients found in commonly used wood preservative systems. Fungicides in parentheses may be present, depending on the exact formulation used.


Preservative Active Ingredient(s)

CCA CR PCP-A ACQ CA SBX CuN ACZA IPBC+ ZB

Cu(ll), As(V) oxides Phenols, pyridines, etc Pentachlorophenol, oil components Cu(ll), quaternary ammonium salts Cu(ll), tebuconazole (B0 - ) 3

3

B0 -3 3

Cu(ll), naphthenates Cu(ll), Zn(ll), As(V) oxides IPBC (propiconazole, tebuconazole, chlorpyrifos, permethrin, imidichloprid) BO3-3

Table 2. Copper content (as Cu)) of wood preservatives in southern pine lumber when treated according to AWPA Standard Ul-05 (in pounds per cubic feet, pcf) (5). Letter designations after the preservative acronym refer to the Type as defined in the AWPA Standards, e.g. C C A - C = C C A Type C.

Retention (pcf)

WP

Total

CuO

Cobiocide

CCA-C

0.4

0.074

0.14

ACQ-D

0.4

0.27

0.13

CA-B

0.26

0.25

0.0082


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S B X IPBC+ Z B T E B Quat Cu+ P C P C R UC1 UC2

I


UC3A UC3B UC4A UC4B UC4C UC5 Figure 2. Major wood preservative fungicides correlated to A WPA use category. TEB = tebuconazole, Quat = quaternary ammonium salt, see text for additional abbreviations used.

Such systems could be referred to as generation 2B wood preservatives. These 2B systems would need to use either a higher loading of the cobiocide used in existing systems, or use a mixture of cobiocides along with the lower loading of copper. The wood preservative fungicides can be classified in terms of the use categories in which they are used. Figure 2 illustrates this point for sodium borate, IPBC, zinc borate, tebuconazole, quaternary ammonium salts, copper II, pentachlorophenol, and creosote. Fungicide characteristics determine which use categories it is suitable for. S B X (disodium octaborate tetrahydrate) is readily leachable by liquid water. Hence, it is only suitable for use in UC1 and 2, where there is no significant leaching hazard. Creosote and pentachlorophenol are not suitable for use in or around the house because of odor and human toxicity issues, so they are restricted to heavy-duty applications found in UC3B through UC5. Examples of these applications are utility poles, railway ties, pilings, and commercial dock structures.

Conclusions Wood preservative fungicides comprise a diverse group of chemistries. Technologies to preserve wood from fungal attack are literally hundreds of years



239 old (12). At the present, many of these old technologies are used alongside newer W P systems that have been developed with modern environmental impact and human toxicity concerns in mind. The market for wood preservatives has also grown considerably over the last 25 years (13). A l l this has led to a very broad array of biocides that are available in the United States and Canada for wood preservative use. The A W P A Use Category System is a useful tool in classifying these preservatives and their component fungicide(s), and understanding how they are currently used. Suitability of a wood preservative fungicide for a given application depends on a large number of its characteristics. Some of the most important ones are efficacy/cost, breadth of efficacy spectrum, activity against non-target organisms, stability, and leach resistance. The marketplace for treated wood products continues to change due to competition from competing materials like steel and wood/thermoplastic composites, new environmental and human toxicity concerns, participation by chemical-producing companies that are new to this market, and the demands of the consumer. The fungicides used in wood preservative applications will also continue to change.

References 1.

2. 3.

4. 5. 6. 7. 8.

9.

Hawksworth, D . L . ; Kirk, P . M . ; Sutton, B.C.; Pegler, D . N . Ainsworth & Bisby's Dictionary of the Fungi; C A B Interational, University Press, Cambridge, U K , Eighth Edition, 1995. Morton, L . H . G . ; Eggins, H.O.W. Material und Organismen 1976, 11, 279294. Highley, T.L. Wood Handbook, Wood as an Engineering Material; Forest Products Laboratory, Forest Service, U.S. Dept. of Agriculture: Madison, WI, 2002, 283-297. Eaton, R.A.; Hale, M . D . C . Wood: Decay, Pests and Protection; Chapman and Hall, New York, N Y , 1993; 76-110. Book of Standards; Standard U1-05; American Wood-Preservers' Association: Selma, AL, 2005, 5-7. Laks, P.E. Proceedings of the American Wood-Preservers' Association; A W P A : Selma, AL, 2004, V 700, 78-82. Truini, J. Home Mechanix 1996, 92(805), 12. Green, D.W.; Winandy, J.E.; Kretschmann, D.E. Wood Handbook, Wood as an Engineering Material; Forest Products Laboratory, Forest Service, U.S. Dept. of Agriculture: Madison, WI, 2002, 103-105. Schultz, T.P.; Nicholas, D.D. In Wood Deterioration and Preservation, Advances in Our Changing World; Goodell, B ; Nicholas, D.D.; Schultz, T.P., Eds.; A C S Symposium Series 845; American Chemical Society: Washington DC , 2003; Chapter 26, pp 420-432.


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10. Murphy, R.J. Wood Preservation in the '90s and Beyond; Proceedings No. 7308; Forest Products Society: Madison, WI, 1995, 162-168. 11. Lebow, S.T.; Foster, D.O.; Lebow, P.K. Forest Products Journal 1999, 40(7/8), 80-89. 12. Freeman, M . H . ; Shupe, T.F.; Vlosky, R.P. Forest Products Journal 2003, 55(10), 8-15. 13. Preston, A . F . Forest Products Journal 2000, 50(9), 12-19.


Development of Commercial Wood Preservatives Efficacy

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