Protection of Wood Using Combinations of Biocides - ACS Symposium

Chapter 24

Protection of Wood Using Combinations of Biocides Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 7, 2016 | http://pubs.acs.org Publication Date: March 31, 2003 | doi: 10.1021/bk-2003-0845.ch024

Liam E. Leightley Rohm and Haas Company, 100 Independence M a l l West, Philadelphia, P A 19106-2399

Traditional wood preservatives provide a broad spectrum of activity but are being increasing regulated due to perceived environmental concerns. New organic biocides, developed for applications other than protecting wood, possess low toxicity and high selectivity against some organisms. However, the high selectivity also means that a particular biocide may be relatively inactive against a few of the many wood-destroying or staining/molding organisms. One possible solution is to develop new formulations based upon synergistic blends of biocides which are already registered for other non-wood applications. These formulated products can possess a broad spectrum of activity and low environmental impact. Examples of several synergistic combinations which are effective against stains and molds, or wood-destroying organisms, are given. This approach offers a good opportunity to develop new, relatively low cost and environmentally-benign wood protection systems.

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

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 7, 2016 | http://pubs.acs.org Publication Date: March 31, 2003 | doi: 10.1021/bk-2003-0845.ch024

391 Wood enjoys universal use as a versatile construction material. However, wood can be attacked and deteriorated through the action of a wide variety of fungi, insects and marine boring animals. This deterioration can be prevented i f construction practices are followed which create unsuitable conditions for the activities of these destructive agents, such as keeping wood dry. However, if the wood product cannot be kept dry, or is in ground contact or an aquatic environment, then wood must be protected by treatment with biocides. Treated wood is resistant to fungi and insect attack and helps homeowners protect their property while conserving our nation's forests. M o l d or stain fungi can also inhabit wood. While these microorganisms do not degrade the wood, a moldy wood surface is unsightly, can stain the surface of lumber, and will cause any stain or paint applied later to quickly fail. Furthermore, the surface of painted wood surface can sometimes quickly become moldy, especially in humid environments. The global market for wood protection chemicals is about US$120 million. Typical end uses for these types of chemicals are for treating wood/lumber intended for decking, fencing and utility poles. In addition to these end uses, biocides are applied to wood surfaces to inhibit stain and mold fimgi. Biocides can also be added to wood-coating products such as stains, clears, varnishes and primers to prevent molds and stains on the painted surface. The global value for these second type of biocide products is approximately US$80 million. Thus, the total value of biocides used to treat wood products, or added to stains and paints, is relatively modest compared to the market for agrochemicals or drug development. Consequently, the market is simply unable to support the extensive research and development costs associated with developing entirely new biocides specifically for wood products. Biocides used to protect wood are coming under increasingly stringent regulatory requirements. These requirements have increased the cost of supporting these chemicals. Furthermore, as discussed above it is unlikely that new biocides specifically for wood protection will be developed. Thus, an opportunity exists to develop new wood protection systems using blends of two or more biocides which are already developed and labeled for uses other than wood protection, such as agrochemicals used to protect plants against fungal or insect attack.

Requirements for the Ideal Wood Protection Biocide Wood protection biocides protect wood from attack and subsequent degradation by fungi or insects. Typically, to be considered satisfactory as a wood protection chemical the product should possess the following characteristics • Good efficacy at low cost against a wide variety of wood-destroying organisms • benign to humans and other non-target organisms

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

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• • • • • • •

permanent/stable for the guaranteed life of the treated wood good penetration easy and safe to use, and acceptable to regulatory agencies not deleterious to wood readily available capable of being formulated into a commercial system (see below) allow the treated wood to be easily disposed, or recycled, at the end of the treated wood's life

Few of the newer biocides possess all of the above characteristics, especially the broad activity against the wide variety of organisms that degrade wood. However, mixtures of two or more biocides might offer broader protection. A classic product in this regard would be copper-chromium-arsenic (CCA), the major wood protection system currently used in the U.S. The presence of two biocides (Cu and As), and the Cr which fixes the metal biocides in wood and minimizes corrosion of metal fasteners, offers an economical system with permanency and broad activity against fimgi and insects. To protect wood against mold and stain fimgi, a biocide needs many of the same characteristics as listed above but only has to be effective for a relatively short time. However, when used as a mold/stain inhibitor in coatings, a biocide must be safe and effective for a long time, and stable against photodegradation. Not only must a biocide have the above characteristics, it also must be capable of being formulated into an effective and economical wood protection system. Formulations used to protect wood, or prevent staining/molding, need the following properties: • have low volatility and flammability • penetrate the wood to sufficient depth • non-corrosive to metal fasteners • capable of being concentrated (liquid or solid) for shipment • capable of formulating into a solution, preferably water-based • be chemically stable during shipment and at a treating facility • not deleterious to wood • not hazardous at a plant or in the treated product • economical • allow wood to be painted after treatment Furthermore, formulations for stains/coatings must be resistant to water leaching or degradation by U V light, and have abrasion (for decks), checking and cracking resistance. New wood protection formulations need to satisfy the demands of both regulators and the market. Local, national or global regulations must be complied with, and the use of registered ingredients is mandatory. The



393 marketplace also demands manufacturers provide novel and economical solutions to the many requests of homeowners. For example, a wood protection system is expected to effectively protect wood exposed to a wide variety of different environments, such as the relatively benign conditions in the northeast U.S. and the high humidity and temperatures favorable to decay fungi and the introduced aggressive Formosan termites along the Gulf Coast. Furthermore, the homeowner not only expects the price of treated wood to remain competitive with other non-wood construction materials, but also demands the performance of any new environmentally-benign system to be equal or superior to the traditional wood preservatives. Finally, the durability of current exterior finishes, clears and stains on wood surfaces is viewed as inadequate by homeowners. However, at the same time V O C issues and regulations are requiring the use of water-borne instead of solvent-based formulations which generally has a negative impact on performance.

Biocides The 1. 2. 3.

classic wood protection biocides consist of three basic types: Tar oil preservatives, e.g. creosote Light organic solvent preservatives, e.g. pentachlorophenol Water-borne arsenicals, e.g. chromated copper arsenate

These historical biocides already have some restrictions and will undoubtedly face more restrictions in the future. As use of these very economical and efficacious biocides is further limited, biocide manufactures will need to offer the wood protection industry a variety of formulated products for many different wood-protection applications. Today there is an extensive list of agrochemicals that are already available and registered for non-wood use, and which could possibly be used to create new wood protection products. Table I lists a few organic chemicals that already are, or could be, used to protect wood against decay and insect attack. However, many of these compounds are only active against insects but not decay fimgi, such as the synthetic pyrethoids, or active against decay fungi but not molds and stain fungi such as the triazoles. Also, while the cost of didecyldimethyammonium chloride (DDAC) is an order of magnitude cheaper than the triazoles, for example, it is considerably less effective at controlling decay fimgi than the triazoles. Furthermore, these organic biocides are non-water soluble and, thus, cannot be formulated using a water-borne system like today's C C A system [except for D D A C , which is water soluble but then fixes in wood]. Thus, use of these


394 biocides will likely require the development of oil-in-water emulsion formulations. Many of the biocides listed in Table 1 are also capable of controlling stain and mold fungi. For example, a combination of D D A C and 3-iodo-2-propyl butyl carbamate (IPBC) is used commercially, as are 2,4,5,6tetrachloroisophthalonitrile, a mixture of isothiazolinones, or 2(thiocyanomethylthio) benzothiazole (Busan 30).


Table I. Potential Organic Biocides for Wood Protection Chemical 3-Iodo-2-propnylbutyl carbamate Didecyldimethylammonium chloride Triazoles Isothiazolones Synthetic Growth Inhibitors 2-(Thiocyanomethio)benzothiazole Synthetic Pyrethroids 2,4,5,6-Tetrachloroisophthalonitrile

Target Fungi Fungi and Insects Fungi Fungi Insects Fungi Insects Fimgi and Insects

Synergism Studies The benefits of combining two or more biocides has long been recognized and utilized. These benefits include activity against a broader range of organisms than is possible with only one biocide, and the possibility of synergistic action. Synergism is where a mixture of two or more bioactive compounds give greater activity than would be predicted from each components' individual activity (1,2), and is detected for a mixture of two biocides using the equation below: Ca + Cb = Synergy Index (SI) CA CB where Ca is the concentration of biocide A in a mixture of biocides' (A and B) necessary to give a specific response, Cb is the concentration of biocide B in a mixture of A and B necessary to obtain a specified response, and C A and C B are the concentrations of biocides A and B , respectively, which when used alone give the same specified biological response. A SI of less than 1 indicates a synergistic effect from combining biocides. It is also possible to obtain a value greater than 1, which indicates antagonism (an inhibitory effect) from combining biocides. Most commonly, however, an



395 additive effect is observed (SI = 1). While the above equation is for two biocides, a synergistic mixture of three or more biocides is also possible. A desired combination biocide formulation is apparent when synergism is observed. A synergistic combination allows less total biocidal material to be used than when a single biocide used alone. The commercial value here is that it is easier to comply with regulations while also offering a more cost-effective formulation to customers. In addition, the mixture may provide a product with broader spectrum activity. Furthermore, a synergistic combination can be patented to protect a company's development costs. Identification of synergistic mixtures is thus a key objective in designing new formulations. This means understanding the target organisms and their modes of action. Once this difficult task is accomplished, it might be thought that synergy could be easily predicted from understanding the mechanism(s) of action. However, such predictions have often not been demonstrated in practice. Basically, the antibiotic literature generally suggests only that synergy is most likely to occur between chemicals that have dissimilar modes of action and that an additive action is the most probable results of combining biocides having the same action. Thus, finding combinations that are synergistic usually requires extensive and tedious trial-and-error laboratory studies. As a cautionary note, observation of enhanced efficacy by combining two components does not always mean that the combination is synergistic. For example, combining copper(II) with a carboxylic acid or phenolic will form a copper(II)-organic ligand complex. This new compound might be more active than either of the two starting "reagents" but, since a new compound is formed by the addition of the copper and organic ligand, the observed increase in bioactivity is not strictly synergistic. One example is the large bioactivity increase observed against a wide variety of wood-destroying organisms using various laboratory and outdoor exposure tests with the combination of copper(II) with oxine copper [Cu-8] (5). However, the authors suggested that the reason for the greatly enhanced activity was not synergism but rather the formation of mono-Cu-8 by the combination of copper(II) with the bis-Cu-8, and prior literature indeed suggests that the mono form of Cu-8 has greater fungicidal activity than the bis form. In addition, it is possible to observe synergism by combining a biocide with a non-biocidal additive; for example, see Chp. by Green and Schultz in this book.

Synergistic Examples Combinations to Control Molds/Stains Using laboratory test methods, the combination of 4,5-dichloro-2-n-octyl-3isothiazole (DCOIT) and 3 -iodo-2-propyny lbuty 1 carbamate (IPBC) was evaluated for synergy (4). The following values were used to determine synergy, with M I C (minimal inhibitory concentration, ppm, or the minimal


396 biocide level necessary to completely inhibit growth of a particular fungus grown in an agar plate) used as the specified biological response:


C A - Concentration needed to achieve MIC when only DCOIT is present C B - Concentration needed to achieve M I C when only IPBC is present Ca - Concentration of DCOIT, in a mixture of IPBC and DCOIT. to obtain M I C Cb - Concentration of IPBC, in a mixture of IPBC and DCOIT. to obtain M I C A : B - Ratio ofDCOIT:IPBC in the mixture The results obtained from these evaluations are presented in Table II, and show that the combination is synergistic against a wide variety of stains and molds.

Table II. Results between DCOIT and IPBC Using Various Stain and Mold Fungi MICROBE A . niger A . niger A . pullulans C. albicans C. albicans E. coli

CA 16 16 16 32 32 8

CB 4 4 2 16 16 250

Ca 4 8 8 4 16 4

Cb 2 0.5 0.25 8 2 62

SI 0.75 0.62 0.62 0.62 0.62 0.75

A:B 2:1 16:1 32:1 1:2 8:1 1:16

The application of the synergistic combination of DCOIT and IPBC has been described by Tsunoda et al. (5). A n emulsified mixture of 2% IPBC and 1.5% DCOIT controlled stain fungi on wood better than the biocide trichlorophenol. The mixture is currently used in commercial anti-sapstain formulations, and could be used for other wood applications. Other examples of synergistic combinations to control stain/molds are the combination of D D A C and IPBC (6), a commercial product, and the combination of an oxathiazine and benzothiophene-carboxamide-S,S,-dioxide (7), a non-commercial system.

Synergistic Wood Preservative Mixtures The combination of copper(II) and the quat D D A C was found to be synergistic against various wood-destroying fungi, including a copper-tolerant fungus (8). This combination is now sold commercially as ammoniacal copper



397 quat (ACQ-B) or amine copper quat (ACQ-D), depending on the particular formulation (9). The combination of copper(II) with the triazole tebuconazole is also synergistic against wood-destroying fimgi, especially a copper-tolerant fungus (10). This latter system has been examined further (77-72), and the above systems are now commercial products, principally in Europe and Asia. Both systems are also listed in the American Wood Preservers' Association Standards (9) and will likely replace C C A for treating residential construction lumber as the use of arsenically-treated lumber is phased out by 2004. However, the presence of copper in treated wood products has several potential disadvantages (environmental concerns with copper, especially in aquatic systems, and questions on the ultimate dispose of treated wood products which contain a metal). Thus, several European countries are already moving towards totally organic wood preservative systems, a move that the U.S. may follow in the future. Other non-commercial examples of synergistic mixtures for protecting wood include 2,4,5,6-tetrachloroisophthalonitrile and chlorophyrifos (75), and a metal/carboxylate acid with an isothiazolone (14).

Conclusions Novel wood protection formulations, based upon mixtures of alreadydeveloped biocides, offer some real advantages to the wood protection industry. Through careful design, formulations can be developed relatively quickly and at a much lower cost than for developing entirely new biocides. The demonstration of synergy in mixtures of biocides can provide broader activity, lower overall biocide concentrations, make it easier to register the final formulation, and provide patent protection to the developing company. A s the current wood protection biocides are removed from the marketplace, the use of mixtures of commercially-available and labeled environmentally-benign chemicals presents excellent opportunities for manufacturers of wood protection formulations.

References 1. 2.

Hodges, N.A.; G.W. Hanlon. In: Mechanisms of Action of Chemical Biocides; Blackwell Science, 1990; 297-310. Schultz, T.P., D.D. Nicholas. Utilizing Synergism to Develop New Wood Preservatives; In: Wood Preservativation in the 90's and Beyond, FPS Symposium Proceedings, Forest Products Soc.: Madison, WI, 1994; 187191.


398 3.

4. 5.


6. 7. 8. 9. 10. 11. 12. 13. 14.

Schultz, T.P.; Nicholas, D.D. Enhanced Efficacy from the Combination of Copper(II) and Cu-8: Laboratory and Field Studies; In: Enhancing the Durability of Lumber and Engineered Wood Products, FPS Symposium Proceedings, in press. Forest Products Soc.: Madison, WI. Hsu, J.C. U.S. Patent 5,292,763 1994. Tsunoda, K . ; Kumagai, H . ; Sakurai, M. Internat. Res. Group Paper IRG/WP/94-30035 1994. Ward, H.A. U.S. Patent 4,950,685 1990. DeWitte, L . A . ; Valcke, A . R . A . ; Van der Flass, M . A . J . ; Willems, W . M . L . U.S. Patent 6,242,440 2001. Findlay, D . M . ; Richardson, N . G . U.S. Patent 4,929,454 1990. Anomyous. American Wood-Preserver's Association Standards 2001. Granbury, T X . Williams, G.; Cornfield, J.A.; Brown, J.; Ryan, N.P. U.S. Patent 5,527,384 1996. Williams, G.; Cornfield, J.A.; Borwn, J.; Ryan, N.P. U.S. Patent 5,634,967 1997. Williams, G.; Cornfield, J.A.; Borwn, J.; Ryan, N.P. U.S. Patent 5,916,356 1999. Woods, T.L. Wood Preservation in the '90s and Beyond; Forest Products Soc.: Madison, WI, 1995; 192-197. Grove, S.L. U.S. Patent 4,783,221 1998.


Protection of Wood Using Combinations of Biocides - ACS Symposium

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