Organic Preservative Systems for the Protection of Wood Windows

Chapter 28

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Organic Preservative Systems for the Protection of Wood Windows and Doors Alan S. Ross Kop-Coat, Inc., 3020 William Pitt Way, Pittsburgh, P A 15238

For over 70 years, the manufacturers of wood windows and doors have utilized preservative systems based on organic and organometallic chemicals to provide their products with protection against decay, mold, termites and water damage. For the first 50 of those years, pentachlorophenol (PCP) was the basic preservative of the industry. This was replaced with products having improved safety and environmental attributes such as tri-nbutyltin oxide (TBTO) and 3-iodo-2-propynyl butyl carbamate (IPBC). In recent years, a new generation of organic preservatives which focus on long-term performance has been introduced. These materials are designed to further increase the service life of wood windows and doors while maintaining excellent health, safety and environmental properties. This chapter reviews the evolution of treatments and treating methods and also examines several of the new generation treatments for wood windows and doors.

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

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Introduction Wood has long been the favored building material for residential windows and doors. It imparts a warm, high quality look, is easy to fabricate, has excellent strength properties and is a renewable resource. In spite of these advantages, in recent years wood has been losing market share to plastic - particularly in the residential window market. Wood's growth, which was 4.6% per year in the 1999-2004 period, is expected to slow to just 1.1% per year between 2004 and 2009 (7). Plastic windows, however, are projected to grow at an annual rate of 7.9% over the same period (7). What accounts for this disparity? Much of the increased use of plastic is due to its lower cost, but there is also a perceived advantage of superior long-term durability. Wood, being a natural material, faces the stigma of ultimately being susceptible to attack from decay, mold and termites, while plastic is considered to be generally impervious to these challenges. Wood windows and doors have been protected from decay and other organisms through treatment with organic preservatives since the 1930's, and they have an excellent record of performance in service. The use of plastic, however, has raised the bar on long-term durability expectations. The industry is responding to this potential market threat by improving on window design and incorporating new higher-performance preservative systems in their products. This is not the first time the wood window and door industry responded to a threat from an alternate building material. Over 70 years ago, metal windows posed a similar challenge.

Historical Perspective Preservatives Originally, wood windows and doors contained no preservatives. However, by the 1930's it was recognized that these materials were susceptible to decay and insect attack, particularly in areas of high moisture contact. The metal industries of the 1930's, in search of new markets, were promoting metal windows and using as their principal sales argument the fact that wood rots, warps, shrinks and swells (2). In response to this threat, the manufacturers of wood windows and doors upgraded designs and began evaluating the use of preservative chemicals and water repellents to protect the wood. Early materials evaluated included zinc chloride, beta naphthol and several chlorinated phenol isomers (2). Dr. Ε. E. Hubert of the Western Pine Association evaluated over 25 candidate preservatives and in 1936 concluded that pentachlorophenol (PCP) provided the best performance in controlling wood decay and staining without imparting color to the wood (5). In 1938, pentachlorophenol was adopted as the standard preservative

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


472 treatment for wood windows and doors, and it remained die workhorse of the industry for nearly 50 years. In the mid 1980's, the U.S. EPA, acting in response to concerns over safety and environmental issues, placed restrictions on the use of PCP in many applications including the treatment of wood windows and doors. This necessitated the development of a second generation of preservatives. Tri-n-butyltin oxide (TBTO) had been utilized since the 1960's as a preservative for external joinery (millwork) in the U.K. and in the U.S. in antifouling paints and as a mildewcide for coatings (4). Similarly, 3-iodo-2-propynyl butyl carbamate (IPBC) was developed by Troy Corporation in the late 1970's as a paint mildewcide (5). During the mid 1980's, both of these preservatives were adopted for use in the treatment of wood windows and doors in the U.S., typically as 0.5% (active ingredient) solutions in a mineral spirits carrier. To a lesser extent, 2-(thiocyanomethylthio)benzothiazole (TCMTB) was also used for this application. Today, TBTO, IPBC and T C M T B are still used as preservatives for wood windows and doors. However, their use is being supplanted by a new generation of preservative systems, as will be discussed below.

Substrates Historically, white pine was the basic wood species utilized in the manufacture of wood windows and doors in the United States (5). In the late nineteenth and early twentieth centuries, window manufacturers located their facilities along the upper Mississippi River and its tributaries due to the abundant supply of white pine in that region. This area, encompassing the states of Minnesota, Iowa and Wisconsin, remains today as a major center for the production of wood windows and doors even though the white pine supply is long exhausted. As the supply of white pine began to dwindle in the 1920's, producers looked west for sources of sugar pine and ponderosa pine. These species were chosen for their uniformity, ease of treatment and relative abundance in the western states. A second concentration of manufacturers soon developed in close proximity to the supply of these species in the states of Oregon and Washington. By the turn of the twentieth century, supplies of high quality sugar pine and ponderosa pine began to diminish, as well. As a result, window and door manufacturers are now incorporating alternative wood species and wood composites in their products.

Treating Methods Most of the wood used in the manufacture of windows and doors in the United States is treated by immersion in the preservative solution. This has been the case since treating was started in the 1930's. Up until about 20 years ago, virtually all components were bundled and dipped en masse in an immersion tank for three



473· minutes. After being allowed to drain, the bundle was either air dried or placed in an oven or conditioning room to drive off the solvent. To improve production throughput, many manufacturers today employ in-line treating systems. In these processes, individual milled components are passed on a conveyer line through a trough or spray of treating solution and then into a drying oven. As the parts emerge from the drying oven, they are ready for painting or further processing. Typical immersion times on in-line treaters are on the order of 15-30 seconds. Because parts are treated individually rather than as components of large bundles, appropriate penetration and retention of the preservative solution can be attained using these shorter immersion times. Many manufacturers today employ solvent recovery systems to reduce air emissions and enhance cost efficiency. In Europe, the double vacuum process, which had been widely used to treat millwork for many years, has recently fallen out of favor due to environmental concerns over solvent emissions. In double vacuum treatments, the wood components are placed in a treating cylinder. After an initial vacuum is applied, the cylinder is flooded with the treating solution. The solution is then drained, and a final vacuum is applied. This process is only suitable for the application of low viscosity organicsolvent preservatives such as mineral spirits, and in situations where only limited penetration is necessary. It was most often used for the treatment of Scots pine as the sapwood of this species is readily treated while the heartwood possesses reasonable natural durability (6).

Standards Window and door manufacturers realized early on that standards would be essential in ensuring that proper treatments were utilized to preserve and protect their products. As early as 1936 a Preservative Standards Committee was established by the industry. With help from the chemical suppliers and public and private research laboratories, the committee developed a "Code of Minimum Standards for Millwork Preservatives" and set up a "Seal of Approval Program." The program's stated objectives were: To counteract die propaganda of metal competitors who were making capital of the fact that wood rots; to stimulate the preservative treatment of millwork; to provide a method of identifying treated millwork; to establish minimum standards for treating solutions; and to establish minimum standards for the method of treatment (7). Over the years, this organization evolved into what is today the Window & Door Manufacturers Association (WDMA). The basic standard governing the treatment of wood window and door components is W D M A I.S.4 2005: Water-Repellent Preservative Non-Pressure Treatment for Millwork (5). The American Wood-Preservers' Association (AWPA) has also developed a standard for the preservative treatment of millwork components, A W P A Ν1-04: A l l Millwork Products - Preservative Treatment by Nonpressure Processes (9).


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Special Protection Needs


In addition to decay resistance, wood windows and doors may require special protection needs depending upon their expected service life and exposure conditions. Three important protection considerations are water repellency, termite resistance and mold resistance. Each of these is discussed below.

Water Repellency Wood in the presence of moisture has a tendency to swell, warp and check. In addition, moisture contents of 20% and higher can promote growth of decay and mold fungi. For these reasons, water repellents are an integral component of most millwork treating systems. Both of the major treating standards, W D M A I.S.4 - 2005 and A W P A Ν1-04, include a requirement for water repellency of the treating solution. The standard test method cited by each is the W D M A Test Method TM-2 Swellometer Test, "Test Method to Determine the Water-Repellent Effectiveness of Treating Formulations"(/0). In this test, treated and untreated matched ponderosa pine end grain wafers are immersed in water. The swelling of each wafer is measured in the tangential direction, and the swelling of the treated wafer is compared to that of its matched untreated partner. Performance is expressed as water-repellent effectiveness. Typically, a water-repellent effectiveness of not less than 55% is required to meet the standard. Historically, water repellents were hydrocarbon waxes, although today a variety of waxes and polymers are employed. Since many parts are painted subsequent to treatment, it is important for the water repellents to not interfere with paint adhesion or drying properties of the paint. This can be particularly challenging when waterbased paints are coated over water-repellent preservatives.

Termite Resistance Wood windows and doors are susceptible to attack from termites. Pentachlorophenol functioned as both a fungicide and an insecticide when it was in use; however, the fungicides which replaced it in the mid-1980's were not effective insecticides. As a consequence, millwork treaters began, in the late 1980's, to incorporate chlorpyrifos as an insect control additive to their treating solutions. Chlorpyrifos is a broad spectrum insecticide which is widely used for many applications including the protection of crops. In recent years, its use has been restricted from some household and remedial applications by the U.S. EPA, although its use as a millwork treatment is still permitted. In the past five years, permethrin, a member of the synthetic pyrethroid family of insecticides, has begun to replace chlorpyrifos in millwork protection applications. Both chlorpyrifos and permethrin are used at a level of 0.1% as active in the treating solutions.


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Termite resistance is not a requirement of the W D M A or A W P A millwork treating standards; however, both standards recognize the need for a termiticidal additive, especially when windows are in service in areas of high termite hazard. Although termites have been found in all states except Alaska, the main regions of concern are Hawaii and the southern and Gulf Coast states. It is estimated that over 80% of wood windows manufactured in the U.S. are protected with an insecticide treatment.

Mold Resistance In just over five years, the topic of mold growth in indoor environments has evolved from a minor nuisance area to a major public health controversy (77). Reactions to mold concerns have been dramatic: regulatory agencies have made mold a top indoor air quality issue; thousands of mold-related lawsuits have been filed; and insurance companies have almost entirely eliminated homeowner coverage for mold-related claims. Most importantly, the public has made up its mind that mold growth indoors is no longer acceptable. This presents a special concern for suppliers of wood and wood composite-based building materials such as windows and doors. These substrates, if unprotected, provide excellent surfaces for mold growth. Fortunately, many of the same wood preservatives used to protect wood windows and doors from decay fungi are also effective against mold.

New Directions in Millwork Protection Preservatives A new generation of millwork preservatives has been introduced in recent years (see Table I for a list of biocides used in these formulas). Instead of relying upon one active ingredient, the new formulas contain multiple active ingredients. There are two advantages to this approach. First, the multicomponent systems offer a broader spectrum of protection against the wide variety of mold and decay fimgi present in the environment. Secondly, combinations of active fungicides sometimes provide synergistic effects enhancing efficacy at lower required concentrations. For example, one such product is based upon three fungicides: IPBC, propiconazole and tebuconazole. These three ingredients are present at 0.21% each for a total preservative content of 0.63%. They have been shown in laboratory testing to provide seven times the efficacy against brown rot fungi to a system based upon 0.5% IPBC alone (72). Although most millwork preservatives continue to rely upon solvent-based carriers such as mineral spirits and naphtha, there are several products which use



476 water as the preservative carrier. Advantages of using water-based preservatives are lack of flammability concerns, lower odor, and lower air emissions. In addition, with the price of oil continuing to rise, water is a much more cost efficient carrier than petroleum-based solvents. However, there are some disadvantages to using waterbased systems. Water requires much more energy to evaporate, increasing drying times and slowing production speeds. Water also has a tendency to cause wood to swell which can affect the dimensional stability of machined millwork components. In some cases, it can cause grain raising which requires an additional sanding step before cladding or painting. For these reasons, water-based treatments are not as widely used as solvent-based systems. However, as long as petroleum prices increase and air quality regulations become more stringent, there will be an incentive to develop a non-petroleum based carrier with low air emissions which can offset some of the disadvantages of water-based treatments.

Table I. Biocides Used in Millwork Protection in the U.S. - 2006

Common Name or Abbreviation IPBC TBTO ZB Propiconazole Tebuconazole TCMTB Chlorpyrifos Permethrin

Imidacloprid

Chemical Name

Fungicides 3-iodo-2-propynyl butyl carbamate Bis(tri-n-butyltin)oxide Zinc borate cis-trans-1 -[2-(2,4-dichloropheny l)-4-propyl1,3-dioxolan-2-ylmethyl]-1 H-1,2,4-triazole (RS)-1 -p-chlorophenyK4-dimethyl-3 -( 1H1,2,4-triazol-1 -ylmethyl)pentan-3-ol 2-(Thiocyanomethylthio)benzothiazole Insecticides Ο,Ο-diethyl 0-3,5,6-trichloro-2-pyridyl phosphorothioate 3-phenoxybenzyl (lRS)-cis,trans-3-(2,2 dichlorvinyl)-2,2dimethlycyclopropanecarboxylate l-{(6-chloro-3-pyridinyl)methyl}-N-nitro-2imidazolidinimine

Substrates As noted above, the supply of high quality ponderosa pine, which has been the basic component of wood windows for over 50 years, is diminishing. As a consequence, manufacturers are looking at two approaches for their substrates. The first is the use of alternate species such as poplar, maple and radiata pine. Poplar and maple are available domestically while most radiata pine is being imported from



477 Chile and New Zealand. These species do not have the inherent decay resistance of ponderosa pine. Therefore, the newer multicomponent preservative systems with their improved efficacies are better suited to protect them. The second approach involves the increasing use of wood composites in window and door manufacture. Wood composites are combinations of wood components, such as flakes, veneers and strands, with non-wood components such as resins and plastics. The advantages of wood composites are cost, uniformity and availability. However, if the composite contains more than about 40% of wood components, it is susceptible to attack from decay and mold fimgi as well as termites. As a consequence, it must still be treated with an appropriate preservative and insecticide system. One method for treating wood composites is to combine die preservative and water repellent with the wood furnish (flakes, strands or chips) prior to processing. This type of integral treatment has the advantage of providing protection throughout the thickness of the entire part. Of course, the preservative and water repellent must be able to withstand the processing conditions which can involve exposure to pressure and elevated temperatures. One such preservative used to treat wood composites in this manner is zinc borate. The zinc borate is introduced as a powder, or it is sprayed on to the wood flakes as a water-based emulsion prior to final processing (75). At the proper retentions zinc borate offers protection against both decay and termite attack; however, it is not particularly effective against surface mold.

Treating Methods As noted above, most window manufacturers today treat their wood components through immersion in a wood preservative solution. At least one window manufacturer uses pressure treatment. This process can result in more uniform and deeper penetration of the preservative into the treated part, but it is a much costlier and slower process than immersion treatment. Millwork manufacturers are thus faced with a dilemma: to date, greater preservative penetration can only be achieved with processes having slower line speeds and greater treating costs ( e.g., double vacuum or pressure treatment). In-line immersion treating systems are efficient, but they only result in an envelope or outer shell treatment of the preservative. With the right preservative, this can still result in many years of service life. However, in extreme exposure environments or in situations where the protective envelope is breached, a more complete penetration of the preservative may be desired. One recent development which addresses the penetration vs. line speed dilemma is the Penetrating Barrier System. In this system, an organic solvent-compatible borate ester is combined with a traditional mineral spirits-based water-repellent wood preservative. Parts can be treated in existing immersion equipment alleviating the need for costly treating cylinders. Initially after treatment, the borate ester is present in the outer envelope along with the traditional preservative. However, when die part is exposed to moisture or humidity, the borate ester hydrolyzes to boric acid which is


478 then capable of penetrating to the core of the treated part through diffusion. Testing has shown that leaching of the borate out of the treated part is minimal under A W P A Use Category 3A exposure conditions (14). The main advantage of the Penetrating Barrier System is that it can provide penetration to the core of treated parts without the need for special equipment such as pressure cylinders.


Special Needs Although water repellents present some challenges for paintability of treated parts, it is likely that they will continue to be incorporated in millwork preservative systems. In addition to providing efficacy against fungal attack by keeping moisture out, water repellents also help to maintain dimensional stability of the wood in case of exposure to moisture or high humidity. Insecticidal additives will also continue to be used in millwork treatment. Subterranean termites are a problem in many parts of the U.S., and Formosan termites present a special challenge in Hawaii, the Gulf Coast and the Southeast. In areas of termite activity, wood windows need to be protected since they are one of the first points of attack in the home. Several U.S. window manufacturers export products to Japan. Japanese standards require termite protection in all wood windows, and they do not allow the use of chlorpyrifos for this purpose. For this reason, permethrin is used to protect exported windows. In the future, other insecticides such as deltamethrin and imidacloprid will see broader use in millwork products. The need for mold protection of wood windows and doors continues to increase. The consuming public is firmly convinced that any mold growth in the home is unacceptable. Moreover, recent hurricanes (Ivan, Jeanne, Francis and Charlie in 2004 and Dennis, Katrina, Rita and Wilma in 2005) and associated flooding in the Southeast and Midwest have greatly compounded problems with mold growth. Interior windowsills and jambs and patio doors are often exposed to moisture and are potential targets for mold growth. Treatment of these components with millwork preservatives at the point of manufacture helps resist mold growth. Active ingredients with particularly good mold resistance include IPBC and isothiazolones. Additionally, primers and topcoats may contain these additives to help resist mold growth.

Conclusions The U.S. wood window and door industry has a long history of providing high value, long-lasting products. Although the use of plastic has increased in recent years, wood and wood-based composites will continue to be major substrates in window manufacture. As such, there will always be a need to protect these materials from


479 biological and environmental hazards such as decay, mold, termites and water damage. This need will be met by modern wood preservative treatments which offer the attributes of long-term efficacy, cost-effectiveness, ease of use, and favorable health, safety and environmental profiles.


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Anonymous, Door & Window Manufacturer Magazine September 2005, 6(6), 72. Hubert, E. Indus. Engineering Chem., Indus. Ed 1938,30(11),1241. Hubert, E. Indus. Engineering Chem., Analytical Ed 1940, 12(4), 197. Ross, A . S. Amer. Wood-Preservers' Assoc. Proc. 1988, 84, 309. Ross, A . S.; Cheeks, C. N.; Smith, L . L . Amer. Wood-Preservers' Assoc. Proc. 2001, 97, 79. Richardson, B . A . Wood Preservation; The Construction Press: Lancaster, England, 1978; ρ 98. Lance, O. C. History and Development of Wood Preservation for Millwork in the United States. National Woodwork Manufacturer's Association (now Window & Door Manufacturers Association): Des Plaines, IL, 1958. W D M A I.S. 4-2005; Industry Standard for Water-Repellent Preservative NonPressure Treatment for Millwork; Window & Door Manufacturers Association: Des Plaines, EL, 2005. Amer. Wood-Preservers' Assoc. Book of Standards; Standard N1-04; All Millwork Products-Preservative Treatment by Nonpressure Processes; 2005; pp 102-103. W D M A T M 2; Test Method to Determine the Water-Repellent Effectiveness of Treating Formulations; Window & Door Manufacturers Association: Des Plaines, IL, 2004. Ross, A . S. Canadian Wood Preservation Assoc. Proc. 2004, Twenty-Fifth Annual Meeting, Vancouver, B.C., Canada: October 19-20. Kop-Coat, Inc., Internal Communication, 2000. Knudson, R.M.,Gnatowski,M.J., U.S. Patent 4,879,083, 1989. Ross, A . S. New Directions in Millwork Protection; 8th Annual W D M A Technical Conference: Chicago, IL, May 4-6, 2004.


Organic Preservative Systems for the Protection of Wood Windows

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