Potential Use of Pressure Treatments with Preservatives to Manage the Risk of Pests in Transported Wood

Jeffrey J. Morrell

Application of supplemental chemicals represents one method for reducing the risk of pest introduction on wood packing materials. Chemicals can be applied using pressure or nonpressure systems. Nonpressure systems brush, spray, or dip the wood in the treatment solution at atmospheric pressure to deliver a thin shell of protection to the wood surface.

Pressure treatment involves the application of a preservative using combinations of vacuum and pressure to “force” the chemical more deeply into the wood. Pressure treatment has been commercially available since the 1830s and is widely used to improve the durability of wood products exposed to adverse environmental conditions. For example, nearly 360 million cubic meters of wood are pressure-treated with preservatives each year. Pressure treatment is more commonly used to protect wood used in long-term applications, such as utility poles, railway ties, and decking lumber, but it may prove useful for packing materials subjected to regular reuse. One application of pressure treatment to wood is used for shipping fruit packing crates. Many larger packing bins used to transport and store harvested fruits are pressure-treated with copper-8-quinolinate, which is labeled for direct food contact.

Pressure treatment has several advantages over spray or dip treatments. First, the quality and uniformity of treatment is far better with pressure treatment, which reduces the risk of damage that exposes untreated wood and creates a thicker barrier that must be traversed before pests already established in the wood can exit the lumber. In addition, the use of elevated pressures may increase the potential for penetration of preservative along insect galleries exposed in sawn wood. Finally, the wood of some species is often kiln-dried prior to treatment. Kiln drying largely eliminates the risk of internally established pests; however, packing materials have relatively low value and may not support this initial drying cost.

Treatment Processes
There are two basic pressure processes used for wood impregnation, termed the full and empty cell processes. In both processes, wood is placed into a treatment vessel or retort that can be up to 48 m long. Attached to the retort are vacuum and pressure pumps, along with a storage vessel containing the treatment solution (called a Rueping tank). In the full cell process, a short vacuum is drawn over the wood to remove as much air as possible from the wood. The treatment solution is added to the retort while under vacuum, then the pressure is raised to 50 to 200 psi (depending on the wood species) and held until the amount of solution injected into the wood meets a minimal target level. Pressure is released, and the solution is drained. Depending on the process, a series of vacuums may be applied to the wood to increase solution recovery and relieve any internal pressure in the wood that might lead to bleeding or exudation of preservative at a later time. The full cell process is used to deliver a maximum amount of treatment solution to a maximum depth into the wood.

Empty cell processes eliminate the initial vacuum and leave air in the wood. The treatment solution is added, and the pressure is raised and maintained until the desired amount of chemical has been injected. The pressure is released, and the air trapped in the wood carries additional preservative out of the wood. This process reduces the amount of chemical left in the wood in comparison to the full cell process. The empty cell process can be taken a step further by adding additional initial air pressure at the start of the process to further increase the amount of preservative released at the end of the process.

Final stages of both the empty and full cell processes use heat and/or vacuum to immobilize the preservative in the wood. This has important implications for reducing the potential environmental effects of these products once they leave the treatment plant.

The full cell process is typically used to treat wood with creosote for marine exposures, where maximum amounts of chemical are required, or is used with water-based preservatives, where the solution concentration can be varied to deliver the desired amount of active ingredient to the wood. Empty cell processes are used for oil-based preservatives, where there is a need to reduce the amount of oil delivered into the wood (oil is expensive and too much of it in the wood can lead to bleeding in service).

Chemicals
The chemicals used for pressure treatment are either oil- or waterborne. Waterborne preservatives are primarily inorganic metal systems that include chromated copper arsenate (CCA), copper azole, ammoniacal copper quaternary, copper citrate, and ammoniacal copper zinc arsenate. CCA is by far the most commonly used waterborne system for wood treatment and tends to leave the wood a greenish color. Boron solutions are also used to pressure treat wood in some regions of the world. This chemical is the only system in this section that does not react with the wood or otherwise become immobilized to resist leaching.

Oil-based preservatives include creosote, pentachlorophenol, copper naphthenate, and copper-8-quinolinolate. In most applications, wood treated with these chemcials has an oily appearance that makes it less attractive. There are also some specialized systems used for high-value applications, such as 3-iodo-2-propynyl butylcarbamate plus chlorpyrifos, which is used to treat laminated timbers used in tropical environments where termite and fungal protection is required.

Treatment Results
The results of wood treatment are usually expressed on the basis of the depth of preservative penetration and the amount of chemical delivered to a specific assay zone within the wood (retention). For example, Douglas fir lumber treated with CCA for ground contact must have a minimum penetration of 10 mm and a minimum retention of 6.4 kg of CCA per cubic meter of wood in an assay zone 0 to 15 mm from the wood surface. The treatment requirements differ with wood species, treatment chemical, and the decay risk to which the wood will be exposed.

Factors that Affect Treatment
Wood treatment is a combination of art and engineering. Treatment results vary widely with wood species, treatment chemical, and process. One of the most important factors affecting treatment is the ratio of sapwood to heartwood. In general, sapwood of most species is relatively easy to pressure treat, while the heartwood of most species is virtually impossible to penetrate. As a result, species with high proportions of heartwood will tend to treat more poorly than those with large amounts of sapwood. In addition, moisture content at the time of treatment can affect results. Pressure treatment of very wet wood (>60% moisture content) will result in highly variable preservative distributions that lead to poor performance. Most wood treats best between 20 and 40% moisture content. Drying, however, adds cost to the treatment, a particular problem with low value materials such as packing wood.

Preservative formulation can also affect treatment. For example, CCA is an acidic formulation, while the other systems mentioned are ammoniacal. Ammonia-based systems tend to produce deeper treatment of a given species and are used in applications for the treatment of thin sapwood species such as Douglas fir. They are, however, more costly (approx 30% more expensive than CCA), placing them at a disadvantage for treatment of low-value packing wood.

Service Life
There is little or no data on the service life of pressure-treated wood used for packing or shipping. Given the nature of the chemicals (most are “fixed” or react with the wood), their activity against a range of wood-colonizing organisms, and the depth to which they are delivered, it is likely that the limiting factor in the service-life of these materials will be mechanical or physical rather than biological. Service-life limitations occur if the treated shell is damaged either during construction of the packing unit or during rough handling. Damage provides an avenue for invasion by wood-inhabiting pests. It is likely that most pressure-treated packing material will be removed from use far before any biological colonization occurs.

Disposal
One detrimental aspect of using pressure-treated packing material is that much of this material must eventually be disposed of. While disposal in a lined landfill with a leachate-collection system poses little problem, nontraditional disposal through burning can release arsenic gases (for CCA and ACZA) and produces a metal-laden ash that can pose health risks. Regulating disposal of pressure-treated packing material represents one of the major drawbacks to this approach to mitigation. The extensive use of pressure-treated wood for this purpose would necessitate the development of a recycling system for either reuse or reconstitution into other products. Neither of these approaches is feasible in the short term, particularly in developing countries where most waste wood is used as fuel for cooking.

Research Needs
In comparison with other mitigation methods, pressure treatment has far fewer research needs. Pressure-treated wood has been successfully used under a wide range of deterioration risks, and there is a wealth of data demonstrating its efficacy. As a result, the research needs relate more to the potential that many wood species that have not traditionally been pressure treated would enter this market. The development of treatability data on these species would be essential for ensuring that a given species could be adequately treated. This research might also evaluate the ability of various systems to penetrate through existing insect galleries. Additional research needs to identify the potential for using lower chemical loadings. The current specifications have been developed to provide long service life (20 to 30 or more years) under varying decay hazards. Given the physical damage experienced with packing and the potential for single-use materials, lower chemical loadings may be acceptable, provided they continue to prevent pest infestations. This approach might also allow the substitution of less broadly toxic preservative systems. A side benefit of reduced chemical loadings would be a lower risk of health-related problems arising from improper disposal.

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