Copper-Based Wood Preservative Systems Used ... - ACS Publications

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Chapter 12

Copper-Based Wood Preservative Systems Used for Residential Applications in North America and Europe Stefan Schmitt,1 Jun Zhang,2 Stephen Shields,3 and Tor Schultz*,4 1Rütgers

Organics, GMBH, Oppauer Strasse 43, Mannheim D-68305, Germany 2Osmose, Inc., 1016 Everee Inn Road, Griffin, Georgia 30224, U.S.A. 3Lonza Wood Protection, 5660 New Northside Drive NW, Atlanta, Georgia 30328, U.S.A. 4Silvaware, Inc., 303 Mangrove Palm, Starkville, Mississippi 39759, U.S.A. *E-mail: [email protected].

For many years chromated copper arsenate (CCA) provided excellent and economical protection for treated wood products used in residential and industrial applications. However, CCA was limited to non-residential applications in Europe and Japan in the 1990s and in North America in 2004, and replaced with other copper-based systems. This chapter reviews the major copper-based wood preservatives used commercially to pressure treat lumber and other wood products for the large residential markets in North America and Europe. Residential systems which currently are standardized but have only minor use in North America, and the likely short-term future trends in copper-based residential systems, are also discussed.

Introduction Waterborne chromated copper arsenate (CCA) was developed in the 1930s to treat utility poles. CCA treated wood proved very effective in above-ground and ground-contact applications and, being waterborne, had no residual petroleum odor or oily surface. Thus, although originally developed for industrial © 2014 American Chemical Society In Deterioration and Protection of Sustainable Biomaterials; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


applications, CCA also proved ideal for residential applications. Starting in the 1960s the use of CCA expanded rapidly with the growth of the residential treated wood market and over a 40-year period grew from 30% to 95% share in that market. Numerous studies found that CCA treated wood products posed negligible risk in residential applications except when improperly burned. However, public perceptions alleging possible arsenic exposure and concerns regarding disposal of CCA treated wood led to the replacement of CCA preservative for use in residential products with other copper-based, waterborne systems in the 1990s in Japan and Europe and in 2004 in North America. These new systems have all undergone many years of testing, including extensive field trials, prior to approval by a standard-setting or product evaluation organization and subsequent commercialization. Further, third-party inspection protocols for treated wood help to ensure that the products meet the established requirements. These treated wood products provide home owners with effective, economical and renewable building materials that will give many years of satisfactory service in applications such as outdoor decks, privacy fences, benches, and fresh water docks. This chapter discusses the major copper-based systems used for residential applications in North America in chronological order and briefly discusses other lesser-used copper systems which have been recently standardized, copper-based systems in Europe, and the likely near-term trends in preservative systems for residential uses. Many of the waterborne systems discussed below are also standardized for freshwater, industrial and agricultural applications.

Systems Employed in North America Dissolved Copper Systems Copper in the cupric or copper(II) oxidation state is effective against most wood destroying fungi and insects. As with all preservatives intended for pressure treatment, the system must be formulated in a liquid carrier as solubilized, emulsified, or as extremely small dispersed particles so that the biocide(s) deeply impregnate the porous wood matrix during the treatment process. All residential wood preservatives for pressure treatment are formulated with water as the carrier and are generally referred to as waterborne. Systems where the copper is in a soluble form are formulated using ammonia and/or monoethanolamine (MEA), an organic amine, which reacts with copper(II) to form water soluble copper-ammonia or copper-MEA complexes. MEA has become the dominant amine complexing agent as it eliminates or minimizes ammonia odor and reduces metal corrosion in treating plants from the treating solutions. Once impregnated into wood, the copper solution interacts with the wood lignin and cellulosic groups to form water insoluble copper compounds/complexes or precipitates, which render the copper leach resistant. While copper is effective against most wood-destroying fungi and insects, it is weak against some copper-tolerant fungi. To prevent infestation and subsequent deterioration by copper-tolerant fungi, a carbon-based co-biocide is added to all residential copper-based systems. 218 In Deterioration and Protection of Sustainable Biomaterials; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


Alkaline Copper Quat (ACQ)

The first major waterborne system to replace CCA was ACQ. The co-biocide in ACQ is a quaternary ammonium compound (quat). Quats are commonly used as disinfectants in a variety of household cleaning products due to their low mammalian toxicity. In addition, quats are water soluble and can be readily formulated with waterborne copper solutions to make a wood preservative treating solution. When impregnated into wood, quats fix onto the acidic wood groups by ion-exchange mechanisms to become more leach resistant. Quats are effective against a broad range of wood-destroying fungi and insects and also control wood-inhabiting molds and stains. Four different types of ACQ formulations are currently listed in the American Wood Protection Association (AWPA) 2013 Book of Standards (1). The classification of ACQ formulations is determined by the type of copper complexing agent, the type of quat and the ratio of copper to quat. All ACQ formulations, with the exception of ACQ-B, employ either monoethanolamine (MEA) or MEA/ammonia combination to solubilize copper. ACQ-A [AWPA P26] (1) consists of 50% copper as CuO and 50% quat as didecyldimethyl ammonium compound with chloride or carbonate/bicarbonate anions (DDAC). ACQ-B [AWPA P27] (1) is formulated with only an ammonia complexing agent and has a CuO:DDAC ratio of 2:1. Ammonia is used as the solvent as it facilitates the penetration of copper into refractory western species, especially Douglas fir. ACQ-C [AWPA P28] (1) is formulated with 66.7% CuO and 33.3% of alkylbenzyldimethyl ammonium compounds (BAC). ACQ-D [AWPA P29] (1) is formulated with 66.7% CuO and 33.3% DDAC. The Use Class 4A (UC4A) ground contact retention used for general residential applications in North America for all ACQ formulations is 6.4 kg/m3 (0.40 pcf). Lower retentions are specified for above-ground UC1-UC3B applications, due to the lower deterioration hazard for treated wood in above ground use.

Copper Azole (CA)

The second waterborne copper system introduced to replace CCA was copper azole. Copper azole (CA) is also based on copper solubilized in ethanolamine, but employs azoles as the co-biocide(s). Azoles, either tebuconazole or a mixture of tebuconazole and propiconazole, are extremely effective against basidiomycete brown and white-rot fungi, so relatively low levels are necessary to protect wood against fungi in comparison to quats. Azole fungicides are commonly used in agricultural applications and many home gardening sprays. Currently, two CA systems are standardized by the AWPA. CA-B [AWPA P32] (1) is formulated with 96.1% copper as metal and 3.9% tebuconazole. CA-C [AWPA P48] (1) has the same copper to total azole ratio, but has a 1:1 mixture of propiconazole:tebuconazole. 219 In Deterioration and Protection of Sustainable Biomaterials; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.

The Use Class 4A (UC4A) ground contact retention used for general residential applications in North America for CA formulations ranges from 2.4 kg/m3 (0.15 pcf) to 3.3 kg/m3 (0.21 pcf). Just as for ACQ, lower retentions are specified for above-ground UC1-UC3B applications due to the lower deterioration hazard for treated wood in above ground use.


Dispersed Particulate Copper Systems In 2006 an entirely new type of copper based wood preservative was introduced. Instead of the copper being solubilized and stabilized by ammonia or MEA, basic copper carbonate is milled into fine particles. A dispersant is added during the milling process to keep the copper particles from clumping together and to maintain the dispersion when diluted in water for pressure treatment. The size of the basic copper carbonate particles for the commercial dispersed copper systems used in the U.S. is greater than 100 nanometers, with a mean particle size of about 300 – 500 nanometers. These systems are referred to as dispersed, particulate or micronized copper preservatives. They offer several advantages compared to soluble copper systems which include reducing the copper leaching and eliminating the nitrogen containing amine or ammonia which reduces the formulation cost and may also reduce undesirable surface mold. In addition, the concentrates can be formulated at much higher copper concentrations compared to soluble copper formulations which reduces shipping costs, and wood treated with dispersed copper wood preservatives have reduced corrosion properties to metal fasteners (2–5).

Dispersed or Micronized Copper Quat (MCQ) The first commercial micronized copper system was MCQ. Much like ACQ-D and ACQ-A, this system is formulated with two CuO:quat ratios, 66.7% to 33.3% and 50% to 50% . Within a few years, this system was superseded by copper azole systems and MCQ is no longer commercially available in the U.S. market but may still be available in Canada. MCQ is listed in ICC Evaluation Service report (6) ESR-1980 with a ground contact retention of 5.44 kg/m3 (0.34 pcf).

Dispersed Copper Azole (DCA) There are two commercial dispersed, or micronized or particulate, copper azole systems available in the US market, known as MCA (6) [ICC ESR-2240] or µCA-C (6) [ICC ESR-1721]. The latter system is sometimes formulated with both micronized and soluble copper; these systems will be generally referred to as DCA in this chapter. DCA has a formulation similar to soluble copper 220 In Deterioration and Protection of Sustainable Biomaterials; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


azoles, with 96.1% copper and either 3.9% tebuconazole or with 3.9% of a 1:1 ratio of tebuconazole:propiconazole. There is one commercial DCA preservative available in the Canadian market, using tebuconazole as the co-biocide (7). The azole co-biocide(s) can be formulated as a particulate dispersion or an emulsion as commonly used in the solubilized formulations. At the present time DCA systems reportedly account for about 70% of the total volume of copper-based waterborne residential systems employed in North America (5), although this value may relate only to the relatively large eastern US production. The ground contact retention specified by the ICC evaluation reports (6) for MCA and µCA-C in the US market is 2.4 kg/m3 (0.15 pcf) and 2.2 kg/m3 (0.14 pcf), respectively, with lower retentions employed for above-ground residential applications.

Other Standardized Copper-Based Waterborne Systems Several other copper-based systems have been standardized by the AWPA (1) or listed in ICC Evaluation Service reports (6). Some of these systems, described below, have been recently or may be currently employed at a few treating facilities and their use may expand further. Copper HDO Type A (CX-A), AWPA P33 (1) contains 61.5% copper as CuO, 24.5% boron as H3BO3, and 14% N-cyclohexyldizeniumdioxide (HDO) which complexes with some of the copper. This system is formulated as a soluble copper system employing ethanolamine. It is standardized by AWPA only for aboveground applications, UC1-UC3B. Waterborne copper naphthenate (CuN-W), AWPA P34 (1), contains 5% copper as metal and 48% copper naphthenate formulated in ethanolamine. An oilborne formulation of copper naphthenate has a lengthy record of safe and effective use. Copper naphthenate has long been available at local hardware stores for homeowners to brush on the end-cuts of treated wood to give added protection to any untreated heartwood in the middle of the lumber. The waterborne system was recently standardized by AWPA as a pressure-treatment system. The UC4A ground contact residential retention is 1.76 kg/m3 (0.11 pcf). Another ethanolamine based, soluble copper system (ACD) contains the effective isothiazolone biocide 4,5-dichloro-2-n-octyl-4-isothiazolin-3-one (DCOI) and has been standardized as ACD, AWPA P54 (1), but retentions have not yet been standardized for either above ground or ground contact use categories. ACD is formulated with 95.8% copper as CuO and 4.2% DCOI. Two formulations of alkaline copper betaine (KDS, KDS-B) have been standardized in AWPA P55 and P56 (1). KDS is formulated with 47.2% copper as CuO, 30.2% borate as boric acid, and 22.6% betaine (polymeric betaine, dodecyl-bis, 2-hydroxethyl ammonium borate, didecylpolyoxethylammonium borate, DPAB). Betaine is an oligomer based on alternating quat and borate ether units. Both the quat and borate groups in this oligomer can fix to wood. KDS-B is formulated with 67.7% copper as CuO and 32.3% DPAB. The standardized AWPA UC4A ground contact retention for KDS is 7.5 kg/m3 (0.47 pcf). The ground contact retention specified by the ICC Evaluation Services report ICC ESR-2500 (6) for KDS-B is 4.3 kg/m3 (0.27 pcf). 221 In Deterioration and Protection of Sustainable Biomaterials; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


Non-Biocidal Additives As with all products, manufacturers of wood preservatives seek value-added improvements to their products which will result in greater commercial appeal. Most of these improvements involve non-biocidal additives. One type of commonly used additive is waterborne wax emulsion water repellents, which are formulated as part of the preservative system for above-ground applications. Water repellents reduce undesirable dimensional change and checking of lumber used for decking and other above-ground applications by reducing water absorption during rainstorms. Another common additive is colorants or pigments to give enhanced visual appeal and slow surface graying of treated wood. Researchers are also examining additives to reduce photodegradation of the lumber surface, such as nano zinc oxide which is colorless to visible light but blocks UV radiation (8).

Trends in North America In very general terms, treated wood in residential use is employed in either above-ground or ground-contact applications. We believe that for ground-contact applications, waterborne copper-based systems will continue to be the dominant preservatives employed in North America for the foreseeable future, for several reasons. First, several carbon-based waterborne systems are standardized and commercially available for above-ground use, such as EL2 and PTI (1). However, at this time no carbon-based or non-metallic system is effective for ground contact applications unless formulated in a heavy oil carrier but such formulations are not suitable for residential applications. Some nonbiocidal additives are available which enhance the efficacy of organic biocides, such as the antioxidant BHT or water repellents, but the high levels of these additives required to be effective for ground-contact applications apparently makes them uneconomical for waterborne systems. Wood-plastic composites (WPCs) are mainly used for above ground applications and this is likely to remain the case for the foreseeable future. Timbers cut from the heartwood of naturally-durable species, such as western red cedar, are available, but unless treated with a preservative these timbers do not perform satisfactorily in long-term ground-contact applications. Copper-based systems are also the dominant product used in above-ground residential applications; however, there is a wider range of available products. In recent years, two carbon-based preservatives have been introduced in the U.S. market. Systems currently standardized by the AWPA include propiconazole / tebuconazole / imidacloprid (PTI) [AWPA P45] and DCOI / imidacloprid (EL2) [AWPA P47] (1). Both systems use a water repellent to give treated lumber improved dimensional stability and reduced decay hazard in above-ground uses. Further, while WPCs have found only a very limited market for ground-contact, in the past decade they have enjoyed a steadily increasing share of the above-ground decking market although this trend has recently ended. Finally, several plants, one in the US and two in Europe, have recently started producing chemically-modified wood as a premium decking product. 222 In Deterioration and Protection of Sustainable Biomaterials; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


It might also be noted that a modified copper azole formulation was recently released which includes a third biocide, a quat, to provide greater protection against certain copper-tolerant fungi (9, 10). In this system the quat is used as an additive, that is, in addition to the required components in the standardized system. Another laboratory independently found that the combination of copper/azole and the quat DDAC was synergistic against all three copper-tolerant fungi tested (11). It is expected that evolution of copper-based products will continue with additional use of biocidal and non-biocidal additives to enhance the treatment and protection of wood. In 2013, soluble azole-based systems gained some market share at the expense of the quat-based preservatives, especially in the eastern U.S., and this trend may continue. Also, dispersed copper systems may gain some market share in ground-contact applications in Canada with the recent CSA listing (7) for dispersed copper tebuconazole, and if one or more DCA systems are standardized by the AWPA they may gain further market share in the eastern U.S. market. In the past decade WPC products have gained some market share from copper-based systems for decking applications, but recently WPCs apparently have lost some decking market share, which suggests that the use of WPCs may be relatively level or slightly decrease over the next few years. For treating the refractory western US and Canadian softwoods, it is likely that soluble copper ammonia/amine systems will continue to be dominant. For the foreseeable future, unless unexpected governmental policies or regulations restrict their use, waterborne copper-based systems will likely continue to be the dominant preservatives for ground-contact residential applications in North America. Copper-based systems currently are also the major preservative for above-ground applications but carbon-based systems have had a recent increase in market share.

Waterborne Copper-Based Systems in Europe The North American market is estimated to be about 60% of the world-wide residential market, while Europe is estimated to have about a 20% share (12). As discussed below, the environmental concerns which North America is just now facing concerning leaching and disposal of metallic-treated lumber has been an issue in Europe for a longer period. In 1998 the Biocidal Product Directive (BPD) (13) went into force and was superseded by the Biocidal Product Regulation (BPR) in 2013. This legislation focused on environmental and toxicological evaluations of biocidal products in a two-step process: first the active ingredients and, second, the products. Low critical values in the assessments has resulted in a limited use of some active ingredients such as boric acid in use class 3 or 4 due to its high leaching potential. The high cost associated with the development of a new biocide followed by the lengthy registration process has generally inhibited the development of new active ingredients for the relatively small wood protection market (14). As a result, during the past decade few new wood preservatives have become available while 223 In Deterioration and Protection of Sustainable Biomaterials; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


many of the traditional biocides have been restricted. The leaching potential of borates and their possible health effects (15) has also resulted in suppliers removing this biocide from their systems. Typical copper-based wood preservatives for use class 3, above ground, are products which contain azoles, quats, betaine or HDO as the co-biocide, much as in North America. However, as Europe generally has a lower decay hazard than many parts of North America, wood for above ground use can sometimes be effectively protected with copper-free wood preservatives, although weather degradation for these metal–free systems remains a drawback. Consequently, while copper-based systems remain the dominant above ground systems employed throughout Europe, effective waterborne carbon-based systems for above-ground applications are gaining market share, with Germany, France, Scandinavia and Switzerland already having registered products on the market. While this remains a limited trend in Europe the practice is somewhat ahead of what is currently occurring in North America where carbon-based systems have only been available in the mass market for the past one or two years. In use class 4, ground-contact, copper used with carbon-based co-biocides (azoles, quats, betaine or HDO) is still employed for ground-contact residential applications. Due to the milder climatic conditions in most of the European areas, the copper level is typically lower than in North America. The copper concentrations approved in Europe do however differ from country to country, depending on the different evaluation criteria, the effectiveness of the carbon co-biocide, severity of use and actual performance in use. While regions like Scandinavia have set retentions based on field test data for many years, there has been a clear recent trend in other countries to also set retentions based on long-term field performance as opposed to laboratory tests. France and the UK now require five or even ten years field data to support certain approvals, and both countries have more than one use class 4 retention to take into account severity of use and desired service life. In the last few years, micronized copper systems have been introduced to European market, and there are currently several UK and Scandinavian treating companies using a micronized copper system. Micronized pigment additives are often used in the preservative treating solutions to improve the esthetic appearance as well as enhance the UV stability of the treated wood. Even greater focus is now being given to preservative penetration. In addition to the ability of a preservative to fully penetrate permeable sapwood, high intensity incising is emerging as a technique that delivers improved penetration of heartwood and European refractory species. Besides the common vacuum-pressure treatment process, an oscillating pressure/vacuum process is employed in the Germanic regions of Europe. A continuous change between pressure and vacuum allows the treatment of wet wood with a moisture-content above 60 % without any negative impact on the preservative penetration. Even though unpublished studies have shown a better environmental footprint for preservative treated wood durable wood species are an alternative in Europe, mainly in use class 3. Further, to avoid the in-ground contact of treated or naturally-durable wood materials other than wood such as metal, plastics or stone are often used to build fences. 224 In Deterioration and Protection of Sustainable Biomaterials; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.

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225 In Deterioration and Protection of Sustainable Biomaterials; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.

Copper-Based Wood Preservative Systems Used ... - ACS Publications

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