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Preservative-Treated Wood and Alternative Products in the Forest Service

Treatment Processes

Methods that preserve wood generally are either:

  • Pressure processes, in which the wood is impregnated in closed vessels at pressures considerably higher than atmospheric pressure

  • Processes that do not involve pressure
Pressure Processes

In commercial practice, wood usually is treated by immersing it in preservative in an apparatus that applies high pressure, driving the preservative into the wood. Pressure processes differ in details, but the general principle is the same. The wood is carried on cars or trams into a long steel cylinder, which is closed and filled with preservative. Pressure forces the preservative into the wood until the desired amount has been absorbed and has penetrated relatively deeply. Commonly, three general pressure processes are used: full cell, modified full cell, and empty cell. Commercial treaters often use variations or combinations of these processes.

Full-Cell Processes

The full-cell (Bethel) process is used when the goal is for wood to retain as much of the preservative as possible. For instance, it is a standard procedure to treat timbers with creosote using the full-cell process to protect the timbers from marine borers. Waterborne preservatives sometimes are applied by the full-cell process. Preservative retention can be controlled by regulating the concentration of the treating solution. The steps in the full-cell process are:

  1. Wood is sealed in the treatment cylinder and a preliminary vacuum is applied for a half an hour or longer to remove the air from the cylinder and as much air as possible from the wood.

  2. The preservative (at ambient temperature or higher, depending on the system) is pumped into the cylinder without breaking the vacuum.

  3. After the cylinder is filled, pressure is applied until the wood will take no more preservative or until the required retention of preservative has been achieved.

  4. After pressure has been applied for the specified time, the preservative is pumped from the cylinder.

  5. A short final vacuum may be used to remove dripping preservative from the wood.

Modified Full-Cell Processes

The modified full-cell process is basically the same as the full-cell process except that it uses lower levels of initial vacuum and often uses an extended final vacuum. The amount of initial vacuum is determined by the wood species, material size, and retention desired. Residual air in the wood expands during the final vacuum to drive out part of the injected preservative solution. For this reason, modified full-cell schedules are sometimes called low-weight schedules. They are now the most common method of treating wood with waterborne preservatives.

Empty-Cell Processes

The empty-cell process is designed to obtain deep penetration with a relatively low net retention of preservative. The empty-cell process should always be used for treatment with oil preservatives if it provides the desired retention.

Two empty-cell processes, the Rueping and the Lowry, are commonly employed; both use the expansive force of compressed air to drive out part of the preservative absorbed during the pressure period. The Rueping empty-cell process, often called the empty-cell process with initial air, has been widely used for many years in Europe and the United States. The following general procedure is employed:

  1. Air under pressure is forced into the treatment cylinder, which contains the wood. The air penetrates some species easily, requiring just a few minutes of application pressure. In treating the more resistant species, the common practice is to maintain air pressure from half an hour to 1 hour before pumping in the preservative, although the need to maintain air pressure for longer than a few minutes does not seem to be fully established. The air pressures employed generally range between 25 to 100 pounds per square inch (172 to 689 kilopascals), depending on the net retention of preservative desired and the resistance of the wood.

  2. After the period of preliminary air pressure, preservative is forced into the cylinder. As the preservative is pumped in, air escapes from the treatment cylinder into an equalizing tank (also known as a Rueping tank) at a rate that keeps the pressure constant in the cylinder. When the treatment cylinder is filled with preservative, the treatment pressure is increased above the initial air pressure and is maintained until the wood absorbs no more preservative, or until enough preservative has been absorbed for the required retention of preservative.

  3. At the end of the pressure period, the preservative is drained from the cylinder, and surplus preservative is removed from the wood with vacuum. From 20 to 60 percent of the total preservative injected into the cylinder can be recovered after the vacuum has been applied.

Treating Pressures and Preservative Temperatures

The pressures used in treatments vary from about 50 to 250 pounds per square inch (345 to 1,723 kilopascals), depending on the species and the ease with which the wood takes the treatment; pressures commonly range from about 125 to 175 pounds per square inch (862 to 1,207 kilopascals). Many woods are sensitive to (and could be damaged by) high treatment pressures. Heated preservatives are used sometimes to improve penetration, but the elevated temperatures can affect the wood's properties and the stability of the treatment solution. The AWPA specifications require that the temperature of the preservative during the entire pressure period not exceed 120 degrees Fahrenheit (49 degrees Celsius) for ACC and CCA and 150 degrees Fahrenheit (60 degrees Celsius) for ACQ-B, ACQ-D, ACZA, CBA-A, CA-B, and CDDC. The maximum temperature for inorganic boron is 200 degrees Fahrenheit (93 degrees Celsius). Please refer to the Wood Handbook for more information on treating pressures and temperatures.

Penetration and Retention

Penetration and retention requirements are equally important in determining the quality of preservative treatment. Penetration levels vary widely, even in pressure-treated material. In most species, heartwood is more difficult to penetrate than sapwood. In addition, species differ greatly in the degree to which their heartwood may be penetrated. Incising (perforating the surface of the wood with small slits) tends to improve the penetration of preservative in many refractory species, but species that are highly resistant to penetration will not have deep or uniform penetration, even when they are incised. When the heart faces of these species are not incised, penetration may be as deep as ¼ inch (6 millimeters), but often is not more than 1/16 inch (1.6 millimeters).

Experience has shown that even slight penetration has some value, although deeper penetration is highly desirable to prevent untreated wood from being exposed when the wood checks, particularly for important members that are costly to replace. The heartwood of coastal Douglas-fir, southern pines, and various hardwoods, although resistant, will frequently show transverse penetrations of 6 to 12 millimeters (¼ to ½ inch) and sometimes penetrations that are considerably deeper.

Complete penetration of the sapwood should be the ideal in all pressure treatments. This penetration often can be accomplished in small-size timbers of various commercial woods. With skillful treatment, it may be obtained in pilings, ties, and structural timbers. However, the operator cannot always ensure complete penetration of sapwood in every large piece of round material with thick sapwood, such as poles and piles. Specifications permit some tolerance for less than complete penetration. Refer to the AWPA standards for required penetration and retention of various species and treatments.

Nonpressure Processes

The numerous nonpressure processes differ widely in the penetration and retention levels that may be achieved and in the degree of protection they provide. When similar retention and penetration levels are reached, the service life of wood treated by a nonpressure method should be comparable to that of wood treated by a process that uses pressure. Nevertheless, nonpressure treatments, particularly those involving surface applications, generally do not produce results as satisfactory as those produced by pressure treatments. The nonpressure processes do serve a useful purpose when more thorough treatments are impractical or when little protection is required.

In general, nonpressure methods consist of:

  • Surface application of preservatives by brushing or brief dipping

  • Soaking wood in preservative oils or steeping it in solutions of waterborne preservatives

  • Diffusion processes using waterborne preservatives

  • Vacuum treatment

  • Other miscellaneous processes

Surface Applications

The simplest treatment is to dip wood into preservative or to brush preservative on the wood. Preservatives that have low viscosity when cold should be used, unless the preservative can be heated. The preservative should be flooded over the wood rather than merely painted. Every check and depression in the wood should be thoroughly filled with the preservative. Any untreated wood that is left exposed will provide ready access for fungi.

Rough lumber may require as much as 10 gallons of oil per 1,000 square feet (40 liters of oil per 100 square meters) of surface. Surfaced lumber requires considerably less oil. The transverse penetration usually will be less than 1/10 inch (2.5 millimeters), although in easily penetrated species, end-grain (longitudinal) penetration will be considerably deeper. The additional life obtained by such treatments will be affected greatly by the conditions of service. For treated wood that contacts the ground, service life may be from 1 to 5 years.

Dipping wood for a few seconds to several minutes in a preservative provides more assurance that all surfaces and checks will be thoroughly coated with the preservative. In addition, dipping usually produces slightly deeper penetration. Window sashes, frames, and other millwork commonly are treated by dipping them in a water-repellent preservative, either before or after assembly. Transverse penetration of the preservative applied by brief dipping is very shallow, usually less than a few hundredths of an inch (a millimeter). The exposed end surfaces at joints are the most vulnerable to decay in millwork products. Good end-grain penetration is especially important.

Dip applications provide very limited protection to wood that contacts the ground or that is used in very moist conditions. They provide very limited protection against attack by termites. However, they do have value for exterior woodwork and millwork that is painted, that does not contact the ground, and that is exposed to moisture just for brief periods.

Cold Soaking and Steeping

Cold soaking well-seasoned wood for several hours or days in low-viscosity preservative oils or steeping green or seasoned wood for several days in waterborne preservatives has provided varying levels of success for fenceposts, lumber, and timbers.

Pine posts treated by cold soaking for 24 to 48 hours or longer in a solution containing 5 percent of pentachlorophenol in No. 2 fuel oil have had an average life of 16 to 20 years or longer.

The sapwood in these posts was well penetrated with retention levels of the preservative solution ranging from 2 to 6 pounds per cubic foot (32 to 96 kilograms per cubic meter). Posts of woods such as birch, aspen, and sweetgum treated by this method have deteriorated much more quickly than treated pine posts.

Preservative penetration and retention levels obtained by cold soaking lumber for several hours are considerably better than those obtained by brief dips. However, preservative retention levels seldom equal those obtained by pressure treatment, except in cases such as the sapwood of pines that has become highly absorbent because of infection by molds and stains.

Steeping wood in waterborne preservatives has had very limited use in the United States, but it has been used for many years in Europe. When seasoned wood is treated, both the water and the preservative salt soak into the wood. When green wood is treated, the preservative only enters the wood by diffusion. Preservative retention and penetration levels vary widely. The process generally is not recommended when more reliable treatments are practical.

Diffusion Processes

Diffusion processes may be used with green or wet wood. These processes employ waterborne preservatives that will diffuse out of the water in a treatment paste (or treatment solution) into water in the wood.

The double-diffusion process developed by the Forest Products Laboratory has produced good results in tests of fenceposts and standard 2- by 4-inch (38- by 89-millimeter) stakes, particularly for full-length immersion treatments. This process consists of steeping green or partially seasoned wood first in one chemical solution, then in another.

The two chemicals diffuse into the wood and react to precipitate an effective preservative with high resistance to leaching. The process has had commercial application in cooling towers and fenceposts where preservative protection is needed. The chemicals evaluated by the Forest Products Laboratory for the double-diffusion process are not registered by the EPA for this type of application.

Other diffusion processes involve applying preservatives to the butt or around the groundline of posts or poles. The preservative can be injected into standing poles at the groundline with a special tool, applied on the poles' surface as a paste or bandage, or poured into holes bored in the pole at the groundline. These treatments are valuable for untreated standing poles and for treated poles when preservative retention levels are inadequate.

Vacuum Processes

The vacuum process, or VAC-VAC as it is called in Europe, has been used to treat millwork with water-repellent preservatives and to treat construction lumber with waterborne and water-repellent preservatives.

In treating millwork, the objective is to use a limited quantity of water-repellent preservative and obtain retention and penetration levels similar to those obtained by dipping for 3 minutes. The vacuum process treatment is included in WDMA I.S. 4-05 for Water Repellent Preservative Non Pressure Treatment for Millwork.

After a quick, low initial vacuum, the cylinder is filled with preservative under vacuum, the vacuum is released and preservative soaks into the wood, followed by a final vacuum. The treatment is better than the 3-minute dip treatment. Penetration and retention are better, and the surface of the wood dries quickly, leaving it ready for glazing, priming, and painting. The vacuum treatment is also reported to be less likely than dip treatment to leave objectionably high levels of preservative in bacteria-infected wood, referred to as sinker stock.

When pressure treatment is not required, lumber intended for buildings may be treated by the vacuum process. The process uses a higher initial vacuum and a longer immersion or soaking period than used in treating millwork.

Preservative retention is harder to control in vacuum treatment than in empty-cell pressure treatment. However, the concentration of waterborne preservatives can be adjusted to provide good control over retention levels during vacuum treatment.

Other Nonpressure Processes

Several other nonpressure methods of various types have been used. Some involve applying waterborne preservatives to living trees. The Boucherie process for treating green, unpeeled poles has been used in Europe for many years. This process involves attaching liquid-tight caps to the butt ends of the poles. A waterborne preservative is forced under hydrostatic pressure into the cap and into the pole.

A tire-tube process is a simple adaptation of the Boucherie process used for treating green, unpeeled fenceposts. In this treatment, a section of used inner tube is fastened tight around the butt end of the post to make a bag that holds a solution of waterborne preservative. This process has limitations in the United States because preservative can leak into the soil at the treatment site.

Refer to the Wood Handbook for more information on treatment processes.