Untreated wood left exposed to outdoor conditions becomes susceptible to decay and degradation by natural forces. While some species of trees have some degree of natural resistance to decay most of the commonly used species do not. Therefore wood left exposed to the elements should be protected against decay and destructive insects by applying chemical preservatives. Several factors influence the degree of protection that can be achieved with chemical preservatives, including the species of wood being treated, the type of chemical being applied, the method of application, the degree of penetration of the preservative into the wood and the length of time the chemical is retained in the wood.
Wood preservatives are a type of pesticide, and as such their use is regulated by the Environmental Protection Agency. Certain preservatives are available for use by the general public while others, referred to as restricted use preservatives, are only available for use by certified applicators. Wood products treated with restricted use preservatives are not considered toxic by the EPA and their purchase and use is not regulated by the EPA. Consumers may obtain EPA approved Consumer Information Sheets from retailers for more information about different preservatives and the proper disposal of treated products.
There are three general types of exposure categories for wood preservatives, including ground contact, above-ground contact and marine exposure. Products exposed to ground contact conditions are highly susceptible to decay and require a heavy-duty preservative, usually applied through pressure-treatment. Products exposed to above-ground conditions are less susceptible to decay and are usually treated with non-pressure-treatment methods. Wood products exposed to under-water or marine environments are extremely susceptible to decay and require heavy-duty preservative treatments.
There are two general types of wood preservatives, including oilborne preservatives and waterborne preservatives.
Oilborne preservatives include, among others, coal-tar creosote, pentachlorophenol solutions, copper naphthenate, chlorothalonil, chlorothalonil/chlorpyrifos, oxine copper, zinc naphthenate, alkyl ammonium compound, propiconazole and tebuconazole. These chemicals are use in pressure-treatment methods and each has different uses, restrictions, benefits and hazards associated with them. Oilborne preservatives can also have an effect on the color, odor, paintability and fire protection attributes of the treated wood. While some of the volatile solvents may be removed after treatment doing so may adversely affect the degree of protection.
Waterborne preservatives have less of an objectionable effect on wood surfaces in terms of cleanliness, odors and paintability than oilborne preservatives. There are numerous variations of waterborne preservatives, including acid copper chromate, ammoniacal copper zinc arsenate, chromated copper arsenate, ammoniacal copper quat, ammoniacal copper citrate, copper azole and inorganic boron. As with oilborne preservatives the different waterborne preservatives each have different uses, restrictions, benefits and hazards associated with them.
The effectiveness of a preservative is dependent upon several factors, including the protective value of the chemical used, the method of application, the penetration and retention of the preservative, the proportion of heartwood to sapwood of the product and the moisture content at the time of application. The effectiveness of treatment is also very dependent upon the species of wood as the level of penetration into heartwood varies greatly by species. In general, the penetration into heartwood is greater with softwood species as opposed to hardwood species. Timbers should be properly prepared before treatment. This includes proper peeling, seasoning or conditioning and machined.
Preservatives are applied using either a pressure process or a non-pressure process. In pressure processes the wood is impregnated with the preservative chemical in closed vessels under very high pressure. In non-pressure processes the preservative is generally applied to the surface of the product in one of several different ways.
Pressure processes include the full-cell, or Bethel, process, the modified full-cell process and the empty-cell process. The full-cell process entails sealing the product in a treatment cylinder, inducing a vacuum, introducing the preservative and then applying pressure for a period of time until the desired penetration is achieved. The modified full-cell process is the same as the full-cell process with the exception that a final vacuum may be induced at the end of the process. The empty-cell process is one in which pressurized air is first forced into the treatment cylinder, followed by the introduction of the preservative. As the preservative is forced in the pressurized air is allowed to escape, keeping the pressure in the cylinder constant. After the cylinder is completely filled with preservative the pressure is increased until the wood will not absorb additional preservative. At this point the remaining preservative is drained from the cylinder and a vacuum applied.
Non-pressure processes generally consist of surface applications, soaking and steeping, diffusion, and vacuum processes. Surface applications, through brushing or dipping, are the simplest methods of applying preservatives. Soaking wood products for hours or days in preservative oils or steeping in waterborne preservatives is also widely used. Diffusion is a process in which the product is first steeped in one preservative chemical and then in another, allowing the two chemicals to react. In the vacuum process the product is placed in a treatment cylinder, an initial vacuum is induced, the preservative is introduce under vacuum, the vacuum is released and the product is allowed to soak for a period of time, after which another vacuum is induced to remove excess preservative.
Architects, engineers and construction contractors should be well versed in the types of wood preservatives, their uses and application methods. The study of wood preservatives is an excellent topic for the continuing education requirements of these professionals. More information is available from the U.S. Department of Agriculture’s Forest Service.
References: Wood Handbook Wood as an engineering material, Forest Products Laboratory, 1999.