Preservative Treatments for Building Components

Stan Lebow

Abstract The wood species most commonly used in construc­ specific application: preservative formulations may con­ tion have little natural durability Thus, they are treated tain a combination of biocides to provide protection with preservatives when used in conditions that favor against a broad range of organisms. Although some pre­ biodeterioration. The type of preservative used varies servatives are effective in almost all applications, they with the type of wood product, exposure condition, and may be excessive for applications involving frequent hu­ specific agent of deterioration. This paper discusses the man contact or for exposures that present only low to characteristics of several preservative systems, which moderate biodeterioration hazards. Additional consider­ are grouped on the basis of their ability to protect wood ations include cost, potential odor, surface dryness, ad­ in a range of exposure environments. Copper remains hesive bonding, and ease of finish application (USDA the primary biocide component used to protect wood 1999). used in contact with the ground or fully exposed to the In an attempt to categorize the degree of deterioration weather, but preservatives containing boron or organic hazard for various applications, many countries have de­ biocides are gaining importance for more protected ap­ veloped‘hazard class’ or‘use category’ systems thatspec­ plications. The treated wood industry continues to un­ ify the preservative formulations that are suitable in par­ dergo a transition, moving from the use of broad- ticular situations (Aston 2002). These categories may spectrum preservative systems toward preservatives also specify the preservative retention (concentration in that are more closely matched to the building applica­ the wood) that is necessary for protection. For example, tion and exposure environment. direct contact with soil or water is considered a severe de­ terioration hazard, and preservatives used in these appli­ Introduction cations must have a high degree of leach resistance and The extent and type of protection needed for wood efficacy against a broad spectrum of organisms. These used in conditions that Favor biodeterioration varies, de­ same preservatives may also be used at lower retentions pending on the type of end use, exposure environment, to protect wood exposed in lower deterioration hazards, and wood-attacking organism(s). In some applications, such as aboveground. The exposure is less severe for wood species with natural durability. such as cedar and wood that is partially protected from the weather, and redwood, provide sufficient protection. In most cases, preservatives that lack the permanence or toxicity to however, protection is provided by the application of withstand continued exposure to precipitation may be ef­ wood preservative biocides. The type or types of biocide fective in those applications. Other formulations may applied is often dependent on the requirements of the leach so easily that they can only be used indoors. The de­ termination of the suitability of a preservative formula­ tion or retention for these deterioration hazard catego­ ries is not an exact science. Preservatives are often tested Lebow: under severe conditions to shorten the time needed for Research Scientist, USDA Forest Service, Forest Products evaluation, and it can be difficult to use these tests to pre­ Laboratory, Madison, Wisconsin, USA dict performance in less severe exposures.

Lebow 57 Copper is the primary biocide in many wood preserva­ They provide adequate protection as long as the wood is tive formulations used in ground contact because of its not sufficiently wetted to leach the preservative. excellent fungicidal properties and low mammalian tox­ icity. Because some types of fungi are copper tolerant, Borate compounds preservative formulations often include a co-biocide to Borate compounds are the most commonly used un­ provide further protection. There is continued interest in fixed water-based preservatives. They include formula­ the development of wood preservatives that contain no tions prepared from sodium tetraborate, sodium copper or other heavy metals. Such preservatives would pentaborate, and boric acid, but the most common form depend on combinations of organic fungicides and insec­ is disodium octaborate tetrahydrate (DOT). DOT has ticides ofrelatively low toxicity that are developed for ag­ higher water solubility than many other forms of borate, ricultural uses. Development of such systems presents allowing the use of higher solution concentrations and challenges because these organic compounds may be de­ increasing the mobility of the borate through the wood. graded by or other non-wood-attacking organ­ Glycol is also used to increase solubility in some formula­ isms. These challenges are particularly acute for wood tions. With the use of heated solutions, extended pressure used in ground-contact applications. periods, and diffusion periods after treatment, DOT is Historically, preservatives have also been classified by able to penetrate relatively refractory species such as their solubility in either water- or oil-type solvents. This spruce. classification is becoming less relevant, as some preser­ Borates are used For pressure treatment of framing vatives can be formulated with either type of solvent lumber used in areas of high termite hazard, such as Ha­ while others may be emulsions or suspensions. Biocides waii, and as surface treatments for a wide range of wood in water-based preservatives typically are not water solu­ products, such as log cabins and the interiors of wood ble, and co-solvents such as amine or ammonia structures. They are also applied as supplemental inter­ may be used to solubilize the active . Water- nal treatments via rods or pastes. At higher retentions, and oil-type solvents each have advantages and disadvan­ borates are used as fire-retardant treatments for wood. tages depending on the application. Oil-type preserva­ Boron has some important advantages, including low tives, such as and pentachlorophenol in heavy mammalian toxicity, activity against both fungi and in­ oil, are among the oldestwoodpreservatives (Lebow et al. sects, and low cost. Another advantage ofboron is its abil­ 2004). These systems usuallyleave the wood surface dark ity to move and diffuse with water into wood that nor­ brown, but they have the advantage of imparting some mally resists traditional pressure treatment. Wood water repellency. They are commonly used for “heavy treated with borates has no color or odor, is non-corro­ duty” applications such as utility poles, bridge timbers, sive, and can be finished. railroad ties, and piles. Concerns about odor and surface While boron has many potential applications in fram­ cleanliness may limit their use in applications that in­ ing, it is not suitable for applications where the wood is volve frequent human contact. exposed to frequent wetting unless the boron can some­ Wood treated with water-based preservatives typically how be protected from liquid water. In some countries, has a dry, paintable surface, and it may also have less such as New Zealand, boron can be used in applications odor than wood treated with some types of oil preserva­ where occasional wetting occurs (Anon. 2005), and there tives. Water-based treatments, however, do not improve is interest in the use of borates in slightly more exposed the dimensional stability of the treated wood unless they applications with coating requirements. There is also in­ are formulated with a water-repellent additive. Hard­ terest in dual treatments, in which borate treatment is woods treated with water-based preservatives that utilize followed by pressure treatment with a water-repellent copper as the primary fungicide may not be adequately oil-typepreservative. protected from soft-rot attack. In addition. some wa­ Research continues to develop borate formulations ter-based treatments may increase the susceptibility of that have increased resistance to leaching while main­ metal fasteners to corrosion. taining biocidal efficacy. Various combinations of silca and boron have been developed that appear to somewhat Preservatives Grouped by Exposure Hazard retard boron depletion, but the degree of permanence and applicability of the treated wood to outdoor expo­ Applications Protected from Liquid Water sures has not been well defined. An example of this type of application is framing lum­ her used in areas of high termite hazard. The primary Applications Used Aboveground with Partial threat in these applications is often insect attack. but pro­ Protection tection against mold fungi or decay fungi may also be de­ The preservatives listed in this section provide ade­ sirable. This section describes water-based preservatives quate protection for wood that is aboveground and occa­ that do not fix in the wood and thus are readily leachable. sionally exposed to wetting. Wood used in this manner

58 Wood Protection 2006 - Session I typically has some type of surface finish. The most com­ Isothiazolone mon example of this type of application is millwork. Isothiazolones are a class of organic compounds often Some aboveground applications that retain moisture used for mold control. They are sometimes added to and/or collect organic debris may present a more severe wood preservatives for this purpose and are also used as deterioration hazard, and a preservative from one of the additives to paints and coatings. One of these com­ following sections may then be more appropriate. Al­ pounds, 4,5-dichloro-2-N-octyl-4-isothiazolin-3-one though preservatives used for millwork treatments were (DCOI), has been evaluated fairly extensively and is cur­ traditionally carried in light solvents to prevent dimen­ rently used as a marine anti-fouling agent in paint films. sional changes in the wood, there is an increasing trend As with other oil-soluble preservatives, the properties of to move away from the use of light solvents because of wood treated with DCOI are somewhat dependent on the economic and environmental concerns. type of solvent used. The treatment may impart a light In this category, the distinction between oil- and wa­ brown color to the wood. DCOI has a noticeable odor and ter-based preservatives blurs, as many of these compo­ the treated wood may have some odor, depending on the nents can be delivered either with solvents or as micro- concentration of the treating solution. In some applica­ emulsions. The azole fungicides, such as tebuconazole tions, skin sensitization can be a concern. and propiconazole, are becoming more widely used. Other azoles, including cypraconazole and azaconazole, 3-iodo-2-propynyl butyl carbamate are used in limited quantities. IPBC (3-iodo-2-propynyl butyl carbamate) is com­ monly used as an in anti-sapstain formula­ Azoles: Propiconazole and tebuconazole tions or as a fungicide in water-repellent finishes for Propiconazole (PPZ) and tebuconazole (TEB) are tri­ decks or siding. It is also used to treat millwork, and it azole agricultural fungicides which are becoming widely may be combined with azoles to enhance efficacy against used as components of preservative formulations. Al­ mold fungi. It may be used as either a solvent- or wa­ though soluble in light organic solvents, they are com­ ter-based formulation. IPBC is colorless, and depending monly used in micro-emulsions. Wood treated with these on the solvent and formulation, the treated wood may be azoles has little color and is readily paintable. PPZ and paintable. Some formulations may have a noticeable TEB are sometimes combined to provide improved effi­ odor, but formulations with little or no odor are also pos­ cacy against a wider range of fungi. Neither is highly ef­ sible. IPBC is not an effective insecticide, and it is not fective in preventing growth by some types of stain fungi, used as a stand-alone treatment for critical structural and they may be combined with a preservative such as members. Some pressure-treating facilities are using a 3-iodo-2-propynyl butyl carbamate (IPBC) to improve mixture of IPBC and an insecticide to treat structural protection. Their efficacy against soft-rot fungi is also members that are to be used aboveground and largely protected from the weather. Combinations of IPBC and less documented, but they are not usually used as the pri­ mary biocide in applications where soft-rot is a concern. , for example, have been used for many years in Hawaii to pressure treat I-joists, laminated veneer Tebuconazole is used as a co-biocide component in a lumber, other engineered wood products. The advantage commercial ground contact copper azole wood preserva­ ofthis treatment is that it is colorless and allows the wood tive. Azoles do not protect against insect attack in situa­ to maintain its natural appearance. tions where insects present a hazard, azoles may be com­ bined with insecticides such as pyrethroids. 2-(thiocyanomethylthio) benzothiozole TCMTB(2-(thiocyanomethylthio) benzothiozole)has Quaternary ammonium compounds been used for many years as an anti-sapstain formulation Didecyldimethyl ammonium chloride (DDAC) and and millwork preservative. It has been formulated in alkylbenzyl ammonium chloride (BAC) are quaternary both solvent-based and water-based forms; the solvent- ammonium compounds that are widely used as based formulation is more prevalent for treatment of bactericides, antiseptics, and fungicides. Both are used millwork. as the “quat” component of the ACQ wood preservative formulations, and DDAC is used as a component of Tributyltin oxide anti-sapstain formulations. The quaternary ammonium Bis(tri-n-butyltin) oxide (TBTO) is a colorless to compounds are colorless, nearly odorless, and can be for­ slightly yellow liquid preservative that is soluble in or­ mulated for use with either water- or oil-based carriers, ganic solvents but not soluble in water. It has been used although solvency is diminished in lighter aliphatic hy­ extensively as an anti-fouling agent in marine paints, as a drocarbons such as mineral spirits. In Europe, BAC and preservative in finishes, and in dip treatments for wood DDAC are sometimes used in combination with azole used in millwork. Used alone, TBTO is not effective in preservatives and/or an insecticide, but they are not usu­ protecting wood placed in ground contact, but it is effec­ ally used as stand-alone preservatives. tive in protecting wood products that are used above­

Lebow 59 ground and partially exposed to the weather. The use of per solutions are somewhat heat sensitive, which limits TBTO has declined in some areas because of concerns the use of heat to increase preservative penetration. But, about the environmental and health effects of tin as well adequate penetration of difficult-to-treat species can still as the loss of preservative efficacy over time. he achieved, and oxine copper is sometimes used for treatment of the aboveground portions of wood bridges naphthenate and deck railings. Oil-borne oxine copper does not accel­ Zinc naphthenate is used extensively as a component erate corrosion of metal fasteners relative to corrosion on in over-the-counter wood preservative products. It can he untreated wood. formulated as either a solvent-borne or water-based pre­ servative. Unlike copper naphthenate, zinc napthenate Pentachlorophenol (light solvent) imparts little color to the wood and thus is more compati­ The performance of pentachlorophenol and the prop­ ble with transparent finishes. When zinc naphthenate is erties of the treated wood are influenced by the proper­ formulated in light solvent, the treated wood may also be ties of the solvent. Pentachlorophenol is most effective paintable. But, wood treated with zinc naphthenate may when applied with a heavy solvent, but it performs well in have a noticeable odor, limiting its indoor use. Zinc is not lighter solvents for aboveground applications. Lighter as effective a fungicide as copper, and zinc naphthenate is solvents also provide the advantage of a less oily surface, not typically used as a stand-alone preservative for ex­ lighter color, and improved paintability. Pentachloro­ posed structural members. Zinc naphthenate has some in light oil can he used to treat relatively refrac­ preservative efficacy, and it may be sufficient to protect tory wood species, and it does not accelerate corrosion. wood used aboveground and partially protected from the One disadvantage of the lighter oil is that less water weather. In Mississippi, zinc naphthenate pressure treat­ repellency is imparted to the wood. Although penta­ ments have been shown to extend the life of exposed chlorophenol in light oil provides a dryer surface, the stakes, and brush treatments of a water-based zinc naph­ same active ingredient is present, and this treatment may thenate significantly improved the performance of pine not be appropriate for applications where exposure to fully exposed to the weather (Barnes et al. 2004). Zinc humans is likely. naphthenate, however, was less effective in protecting hardwoods. The addition of a water-repellent component Applications in Direct Contact with the to the treating solution appears to increase the efficacy of Ground or Fresh Water zinc naphthenate treatments. These preservatives exhibit sufficient toxicity and leach resistance to protect wood in contact with the ground, in Applications Used Aboveground But Fully fresh water, or in other high moisture, high deterioration Exposed to Weather hazard applications. These preservatives are also effective The preservatives listed in this section generally may in preventing decay in other, less severe exposures, but not provide long-term protection for wood used in direct they may not be well-suited to those applications because contact with soil or standing water, hut they are effective of cost, color, toxicity, odor, or other characteristics. in preventing attack in wood exposed aboveground, even if it is directly exposed to rainfall. A typical example of Acid copper chromate this type of application is decking. The preservatives Acid copper chromate (ACC) is an acidic water-based listed in the following section also can be used at lower, preservative that has been used in Europe and the aboveground retention levels. Some aboveground appli­ United States since the 1920s. ACC contains 31.8 per­ cations that retain moisture and/or collect organic debris cent copper oxide and 68.2 percent chromium trioxide. may present a deterioration hazard similar to ground The treated wood is light greenish-brown and has little contact. Preservatives discussed in the following section noticeable odor. Tests on stakes and posts exposed to de­ may be more appropriate for such applications, espe­ cay and termite attack indicate that wood well-im­ cially in critical structural members. pregnated with ACC gives acceptable service. It may be susceptible to attack by some species of copper-tolerant Oxine copper (copper-8-quinolinolate) fungi, and consequently its use is sometimes limited to Oxine copper is an organometallic preservative com­ aboveground applications. It may be difficult to obtain prised of 10 percent copper-8-quinolinolate and 10 per­ adequate penetration of ACC in some of the more refrac­ cent nickel-2-ethylhexoate. It is characterized by low tory wood species such as white oak and Douglas-fir. mammalian toxicity and is permitted by the U.S. This is because ACC must be used at relatively low treat­ and Drug Administration for treatment of wood used in ing temperatures and because rapid reactions of chro­ direct contact with food (e.g., pallets). The treated wood mium in the wood can hinder further penetration dur­ is greenish-brown and has little or no odor. It can be dis­ ing longer pressure periods. The high chromium solved in a range of hydrocarbon solvents but provides content of ACC. however, has the benefit of preventing longer protectionwhendelivered in heavy oil. Oxine cop­ much corrosion that might otherwise occur with an

60 Wood Protection 2006 -Session I acidic copper preservative. The treatment solution uti­ percent of the sales of water-based wood preservatives in lizes hexavalent chromium: chromium is converted to the United States. It has since been withdrawn from most the more benign trivalent state during treatment and residential applications. There are several formulations subsequent storage of the wood. This process of chro­ with varying ratios of copper, chromium, and arsenic. mium reduction is the basis for fixation in ACC, and it is One common formulation is comprised of 47.5 percent dependent on time, temperature, and moisture. chromium trioxide, 18.5 percent copper oxide, and 34.0 percent arsenic pentoxide dissolved in water (CCA Type Alkaline copper quat C). CCA has decades ofproven performance in field trials Alkaline copper quat (ACQ) is one of several water- andin-service applications. In accelerated testing, CCAis based copper-based preservatives that were developed as still the reference preservative used to evaluate the per­ an alternative to chromated copper arsenate (CCA). The formance of other water-based wood preservatives. As fungicides and insecticides in ACQ are copper oxide with ACC. it may be difficult to obtain adequate penetra­ (67%) and a quaternary ammonium compound (quat). tion of CCA in some wood species that are difficult to Multiple variations ofACQ have been standardized or are treat. Temperature limitations during treatment and in theprocessofstandardization.ACQType B (ACQ-B) is rapid reaction of chromium within the wood structure an ammoniacal copper formulation; ACQ Type D (ACQ- can hinder penetration during longer pressure periods. D) is an amine copper formulation; and ACQ Type C But, chromium serves as a corrosion inhibitor, and cor­ (ACQ-C) is a combined ammoniacal-amineformulation rosion of fasteners in CCA-treated wood is not as much of with a slightly different quat compound. Wood treated a concern as it can he with some chromium-free alterna­ withACQ-B has a darkgreenish-brown colorthat fades to tivepreservatives. a lighter brown, and it may have a slight ammonia odor until the wood dries. Wood treated with ACQ-D is lighter Copper chromium boron brown and has little noticeable odor. The appearance of In copper chromium boron (CCB), the arsenic in CCA wood treated with ACQ-C lies between that of wood is replaced with boric acid. This preservative was devel­ treated with ACQ-B or ACQ-D, depending on the formu­ oped as a direct response to toxicity concerns with the lation. Stakes treated with these three formulations have arsenic in CCA. In addition to lower toxicity, boric acid demonstrated resistance against decay fungi and insects has the advantage of migrating more deeply into the when exposed in ground contact. The multiple formula­ wood after treatment. Because boric acid is leachable, it tions ofACQ allowsome flexibilityin achieving compati­ may be depleted from the surface of the wood, rendering bility with a specific wood species and application. When the wood vulnerable to attack by copper-tolerant fungi. ammonia is used as the carrier, ACQ has improved ability The chromium in CCB provides the benefit of reduced to penetrate into difficult-to-treat species. But, if the corrosion. wood species is readily treatable, such as southern pine sapwood, an amine carrier can be used to provide a more Copper azole uniform surface appearance. Copper azole is another recently developed water- based preservative formulation that relies primarily on Ammoniacal copper zinc arsenate amine copper, along with co-biocides, to protect wood Ammoniacal copper zinc arsenate (ACZA) is a wa­ from decay and insect attack. The first copper azole for­ ter-based Preservative that contains copper oxide (50%). mulation contained 49 percent copper, 49 percent boric zinc oxide (25%). and arsenic pentoxide (25%). It is a re­ acid, and 2 percenttebuconazole. More recently, a formu­ finement ofan earlier formulation (ACA) that is no longer lation containing 96 percent copper and 4 percent tebu­ in use. The color of the treated wood varies from brown to conazole has been used. Wood treated with either copper bluish-green. The wood may have a slight ammonia odor azole formulation is greenish-brown and has little or no until it is thoroughly dried after treatment. The ammonia odor. Although listed as an amine formulation, copper in the treating solution, in combination with processing azole may also be formulated with an amine-ammonia techniques such as steaming and extended pressure peri­ formulation. Ammonia may be included when the copper ods, allows ACZA to obtain better penetration of diffi­ azole formulations are used to treat refractory species, cult-to-treat wood species than do many other water­ and the ability of such a formulation to adequately treat basedwoodpreservatives.TreatingfacilitiesusingACZA Douglas-fir has been demonstrated. The inclusion of am­ are currently located in the western United States, where monia, however, is likely to have slight effects on the sur­ many native tree species are difficult to treat with other face appearance and initial odor of the treated wood. water-basedpreservatives. Copper HDO Chromated copper arsenate Copper HDO is an amine copper water-based preser­ CCA was once the most commonly used of all wood vative that has been used in Europe but is not yet used in preservatives and until very recently represented over 90 the United States. The active ingredients are copper ox-

Lebow 61 ide, boric acid, and copper-HDO (bis-(N-cyclohexyl-dia­ oil should not be used in applications where frequent zenium dioxy)copper). The appearance and handling contact with skin is likely (i.e., hand rails). Pentachloro­ characteristics of wood treated with copper HDO are phenol in heavy oil has long been a popular choice for similar to that of other amine copper-based treatments. treatment of utility poles, bridge timbers, glued-lami­ Copper HDO is also referred to as copper xyligen. nated beams, and foundation piling. Like creosote, it is effective in protecting both hardwoods and softwoods, Coal-tar creosote and it is often thought to improve the dimensional stabil­ Coar-tar creosote is the oldest wood preservative still ity of the treated wood. With the use of heated solutions in commercial use. It is made by distilling the tar ob­ and extended pressure periods, pentachlorophenol is tained after high-temperature carbonization of coal. Un­ fairly effective at penetrating difficult-to-treat species. It like other oil-type preservatives, creosote is not usually does not accelerate corrosion of metal fasteners relative dissolved in oil, but it has properties that make it look and feel oily. Creosote contains a chemically complex mixture to untreated wood, and the heavy oil solvent helps to im­ of organic molecules, most of which are polycyclic aro­ part some water repellency to the treated wood. matic hydrocarbons (PAHs). The composition of creo­ Copper naphthenate (heavy oil) sote depends on the method of distillation and is some­ The preservative efficacy of copper naphthenate has what variable. The small differences in composition within modem , however, do not significantly been known since the early 1900s. and various formula­ affect its performance as a wood preservative. Creo­ tions have been used commercially since the 1940s. Cop­ sote-treated wood is dark brown to black and has a no­ per naphthenate is an organometallic compound formed ticeable odor, which some people consider unpleasant. It as a reaction product of copper and petroleum- is very difficult to paint creosote-treated wood. Workers derived naphthenic acids. It is often recommended for sometimes object to creosote-treated wood because it field treatment of cut ends and drill holes made during soils clothes and photosensitizes the skin upon contact. construction with pressure-treated wood. Wood treated Because of these concerns, creosote-treated wood is of­ with copper naphthenate initially has a distinctive bright ten not the first choice for applications where there is a green color that weathers to light brown. The treated wood high probability of human contact. This is a serious con­ also has an odor that dissipates somewhat over time. De­ sideration for treated members that are readily accessi­ pending on the solvent used and treatment procedures, it ble to the public. But, creosote-treated wood has advan­ may be possible to paint copper naphthenate treated wood tages to offset concerns about its appearance and odor. It after it has been allowed to weather for a few weeks. Like has a lengthy record of satisfactory use in a wide range of pentachlorophenol, copper naphthenate can be dissolved applications and a relatively low cost. Creosote is also ef­ in a variety of solvents, but it has greater efficacy when dis­ fective in protecting both hardwoods and softwoods, and solved in heavy oil. Although not as widely used as creosote it is often thought to improve the dimensional stability of and pentachlorophenol treatments, copper naphthenate is the treated wood. With the use of heated solutions and increasingly used in the treatment of utility poles. Copper lengthy pressure periods, creosote can be fairly effective naphthenate has also been formulated as a water-based sys­ at penetrating even fairly difficult-to-treat wood species. tem and is sold commercially in this form for consumer Finally, creosote treatment does not accelerate, and may use. The water-based formulation helps to minimize con­ even inhibit, the rate of corrosion of metal fasteners rela­ cerns about odor and surface oils, but it is not currently tive to corrosion on untreated wood. used for pressure treatment. Pentachlorophenol (heavy oil) Chlorothalonil (heavy oil) Pentachlorophenol has been widely used as a pressure treatment since the 1940s. The active ingredients, chlori­ Chlorothalonil (CTL) has been used for decades as a nated , are clystalline solids that can be dissolved broad spectrum agricultural fungicide. More recently, it in different types of organic solvents. The performance of has been proposed for use as an oil-based wood preserva­ pentachlorophenol and the properties of the treated tive comprised of approximately 96 percent tetrachloro­ wood are influenced by the properties of the solvent. The isophthalonitrile. CTL is colorless, but the appearance of heavy oil solvent is preferable when the treated wood is to the treated wood depends on the solvent used. CTL has be used in ground contact because wood treated with low solubility in some organic solvents. When CTL is lighter solvents is not as durable in such exposures. Wood used in heavy oil solvent, the color of the treated wood is treated with pentachlorophenol in heavy oil is typically brown, and the wood may have a slightly oily surface that brown and may have a slightly oily surface that is difficult is difficult to paint. The low solubility of CTL limits the to paint. It also has some odor, which is associated with solution concentrations. Availability of wood treated the solvent. Like creosote, pentachlorophenol in heavy with CTL is currently limited.

62 Wood Protection 2006 -Session I Oligomeric alkylphenol polysulfide exposed outdoors but partially protected from the Oligomeric alkylphenol polysulfide (PXTS) is a re­ weather. Another preservative used to protect compos­ cently developed and somewhat unique preservative sys­ ites is ammoniacal copper acetate, which is applied by tem. It is an oligomer formed by the reaction of cresylic spraying onto oriented strandboard flakes before drying acid and sulfur chlorides in the presence of excess sulfur. (Laks 2004). PXTS is solid at room temperature, but it becomes liquid when heated to above approximately 58°C. It can also be Insecticidal Additives dissolved and diluted in some aromatic and organic chlo­ Many preservative formulations, such as those that rinated solvents. PXTS was initially developed as an al­ contain copper, have insecticidal activity. In other formu­ ternative to creosote, but it may have potential for other lations, insecticidal components may be incorporated for applications as well. The treated wood is light brown. certain applications. These insecticides may include syn­ thetic pyrethroids, such as permethrin and cyper­ Marine (Seawater) Applications methrin, or chloronicontinyl, neonicotinoids, pyroles, Marine borers present a severe challenge to preserva­ and insect growth regulators. The insecticidal additives tive treatments. Some preservatives that are very effec­ permethrin, chlorpyrifos, and imidachlorprid are used in tive against decay fungi and insects do not provide pro­ the United States. tection in seawater. The preservatives most commonly used to protect wood in marine environments are forms Specification and Quality Assurance of creosote, as well as the water-based preservatives that Most countries have some type of standard-setting contain copper and/or arsenic. Properly applied, creosote body that lists preservative systems that have shown effi­ effectively prevents attack by all marine borers except the cacy. In the United States, the American Wood-Pre­ gribble Limnoria tripunctata. Water-based preservatives servers’ Association (AWPA) is the primary standard- such as CCA or ACZA effectively protect against attack by setting body, but there is also overlap with standards de­ shipworms ( Teredo and Bankia spp.) and gribbles ( Lim­ veloped by the American Society for Testing and Mate­ noria ssp.) but do not protect against pholads ( Martesia rials (ASTM), the Window and Door Manufacturers Asso­ spp.). Because no single preservative is effective against ciation (WDMA), and other organizations. To guide all marine borers, dual treatments may be required in selection of the types of preservatives and loadings ap­ some locations. Dual treatments involve an initial treat­ propriate to a specific end-use, the AWPA recently devel­ ment with a water-based preservative followed by a con­ oped Use Category System (UCS) standards (AWPA ventional creosote treatment. Dual treatments are more 2005). The UCS standards simplify the process of finding expensive than single treatments. For both creosote and appropriate preservatives and preservative retentions for water-based treatments, much higher preservative re­ specific end-uses. They categorize treated wood applica­ tentions are required to protect against marine borers tions by the severity of the deterioration hazard. The low­ than are needed to protect wood in terrestrial or freshwa­ est category, Use Category 1 (UC1), is for wood that is ter applications. Physical barriers such as plastic sleeves used in interior construction and kept dry, whereas UC2 or wraps have also been used to protect piling, but these is for interior wood that is completely protected from the systems are vulnerable to breaches from mechanical weather but is occasionally damp. UC3 is for exterior damage. They are most effective when applied to piles wood used aboveground, and UC4 is for wood used in that have been pressure treated with preservatives. ground contact in exterior applications. UC5 includes ap­ plications that place treated wood in contact with seawa­ Treatments for Wood Composites ter and marine borers. Using the UCS standards only re­ Many structural composite wood products, such as quires knowledge about the intended end-use of the glued-laminated beams, plywood, parallel strand lumber, treated wood. A table in the UCS standards lists most and laminated veneer lumber, are typically pressure types of applications for treated wood and provides the treated with wood preservatives in a manner similar to appropriate Use Category and Commodity Specification. that used for lumber. By contrast, flake- or fiber-based The Commodity Specification lists all of the preserva­ composites are often protected by adding preservative to tives standardized for that Use Category, as well as the ap­ the wood furnish during manufacture. The most com­ propriate retention and penetration requirements. The monly used preservative for these types of composites is user needs only specify that the product be treated ac­ zinc borate (Laks 2004). Zinc borate is a white, odorless cording to the appropriate Use Category. powder with low water solubility that is added directly to As the treating industry adapts to the use of new wood the furnish or during panel manufacture. Zinc bo­ preservatives. it is more important than ever to ensure rate has greater leach resistance than the more soluble that wood is being treated to standard specifications forms of borate used For pressure treatment, and thus it (Lebow et al. 2004). Many countries have some type of in­ can be used to treat composite siding products that are dependent oversight or inspection program to help en­

Lebow 63 sure quality control. In the United States, the American treatments to appropriate end-uses. The wood preserva­ Lumber Standard Committee (ALSC) in the U.S. Depart­ tion industry continues to evolve, with emphasis on using ment of Commerce accredits third-party inspection low toxicity preservatives and more closely matching the agencies for treated-wood products. Quality control degree of protection to the intended end-use. As addi­ overview by ALSC-accredited agencies is preferable to tional types of preservative systems enter the market, it is simple certification by a treating plant or other claims of particularly important to use only those preservatives conformance made by the producer without inspection that have been evaluated and standardized by nationally by an independent agency. The ALSC Treated Wood Pro­ recognized standard-setting organizations. In addition, gram currently has six accredited independent third- it is important to select products that have been treated in party agencies headquartered throughout the United accordance with standard specifications. States and Canada. Updated lists of accredited agencies can be obtained from the ALSC website (www.alsc.org). Literature Cited The use of treated wood with such third-party certifica­ American Society of Testing and Materials (ASTM). 2005. Speci­ tion may be mandated by applicable building code regu­ fication for pressure treatment of timber products. D1760, vol. 04.09. ASTM, Philadelphia, PA. lations. Wood treated in accordance with quality assur­ American Wood-Preservers' Association (AWPA). 2005. Book of ance programs will have a quality mark or stamp of an Standards. AWPA, Selma, AL. accredited inspection agency. In addition to identifying Anonymous. 2005. Timber treatment requirements: Notes for information about the producer, the stamp indicates the builders. New Zealand Dept. of Building and Housing. 16 p. type of preservative, retention level, and intended expo­ www.Building.DBH.Govt.NZ/e/uploads/builders-notes2. sure conditions. Retention levels are specific to the type pdf. Aston, D. 2002. Wood preservatives, Chapter 8. In: The diocide of preservative, species, and intended exposure condi­ business: Regulation, safety and applications. D.J. Knight tions. Detailed specifications for different treatments can and M. Cooke, Eds. Wiley-VCH Verlag GmbH. Weinheim, be found in the applicable standards of AWPA (2005) and Germany. pp. 197-224. ASTM (2005). The ASTM specifications for pressure Barnes, H.M., T.L Amburgey, and M.G. Sanders. 2004. Perfor­ treatment of timber products are listed in ASTM D 1760. mance of zinc-based preservative systems in ground con­ tact. Wood Design Focus. 14(2): 13-17. Laks, P.E. 2004. Protection of wood-based composites. In: Proc. Conclusions of the AWPA annual meeting, Vancouver, BC. 100: 78-82. A wide range of preservative formulations are used to Lebow, S.T., J.E. Winandy, and D. Bender. 2004. Treated wood in protect wood. These formulations can be loosely grouped transition: A look at CCA and the candidates to replace it. by the severity of deterioration hazard in which they are Wood Design Focus. 14(2): 3-8. effective in protecting wood. Some preservative systems Morrell, J.J and S.T. Lebow. 2005. Initial preservative treatment of wood in covered bridges, Chapter 19. In: Covered Bridge can protect wood in almost all exposure environments, Manual. FHWA-HRT-04-098. U.S. Dept. of Transportation, but may not be optimum for all applications. Additional Federal Highway Administration, Research, Development factors such as cost, appearance, odor, volatility, and tox­ and Technology, Turner-Fairbank Highway Research Cen­ icity must be considered in matching a preservative to a ter, McLean, VA. pp. 219-239. particular end-use application. The United States and USDA Forest Service, Forest Products Lab. (USDA). 1999. Wood preservation, Chapter 14. In: Wood Handbook. Wood as an other countries have developed use-category-type stan­ Engineering Material. Forest Products Society, Madison, dards that provide guidance in matching preservative WI.

64 Wood Protection 2006 - Session I

Wood Protection 2006

Edited by H. Michael Barnes Professor; Department of Forest Products, Mississippi State University, Mississippi, USA

March 21-23, 2006 Omni Royal Orleans Hotel New Orleans, Louisiana, USA

Sponsored by Arch Wood Protection, Inc., Chemical Specialities, Inc., Copper Care Wood Preservatives, Inc., Forest Products Society, Osmose, Inc., and Timber Products Inspection, Inc.

Forest Products Society 2801 Marshall Ct. Madison, WI 53705-2295 phone: 608-231-1361 fax: 608-231-2152 www.forestprod.org The opinions expressed are those of the authors and do not necessarily represent those of the Forest Products Society. Copyright © 2007 by the Forest Products Society. ISBN 1-892529-48-3 Publication No. 7229 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, elec­ tronic, mechanical, photocopying, recording, or otherwise, without the written prior permission of the copyright owner. Individual readers and nonprofit libraries are permitted to make fair use of this material such as to copy an article for use in teaching or research. To reproduce single or multiple copies of figures, tables, excerpts, or entire articles requires per­ mission from the Forest Products Society and may require permission from one of the original authors. inted in the United States of America. 0703300