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 bacteria 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 ethanol amine or ammonia structures. They are also applied as supplemental inter­ may be used to solubilize the active ingredients. 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 creosote 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