Treatment Technologies: Past and Future - American Chemical Society

Chapter 11

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Treatment Technologies: Past and Future Adam Taylor1 and Jeffrey J. Morrell*,2 1Tennessee

Forest Products Center, University of Tennessee, Knoxville, Tennessee 37996 2Department of Wood Science & Engineering, Oregon State University, Corvallis, Oregon 97331 *E-mail: [email protected].

Impregnation with preservatives markedly extends the useful life of a variety of wood products used under adverse conditions. A variety of methods have been developed to accomplish this process. This chapter reviews the available technologies for delivering biocides into wood and outlines potential avenues for improving the treatment process.

Introduction Compared to other biological materials, wood has substantial resistance to biodeterioration (1), but it can be degraded under the proper conditions (2, 3). Humans have long sought to prolong the useful life of wood-based materials through practices that have included charring, daubing with various oils, soaking, and ultimately impregnation. These efforts have yielded varying results, but ultimately, successful protection can help us to make more efficient use of our resources. It was once estimated that 10 % of our forest harvest was used to replace timber that had failed in service for various reasons, but mostly due to biodeterioration (4). While this figure seems staggering, a trip to any wood recycling center highlights the level of decayed wood removed from service. Various methods of wood protection have the potential to reduce these losses. Wood protection entails a variety of approaches, the most effective and widely practiced being design to exclude moisture. In cases where extra protection is required, preservative treatments are commonly employed. For the purposes of © 2014 American Chemical Society In Deterioration and Protection of Sustainable Biomaterials; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


this chapter, we will consider treatment strategies that coat or impregnate wood with biocides. Although water repellant systems can also be employed (5, 6), they will not be considered here, although many of these treatments are delivered into the wood using the same processes used for biocides. Biocidal wood protection has been the predominant industrial practice for over a century. Wood modification techniques, including heat treatment and chemical modification, are the subject of much research and increasing commercial practice, especially in Europe; however, they will not be discussed here because they involve different principles and are only use to a limited extent. Wood treatment provides a barrier against biological agents. The extent of the barrier required depends on the biological hazard and the consequences of wood failure, and this will in turn be instrumental in determining the most appropriate process for delivering the preferred preservative system to the required depth at an effective loading. There are a variety of approaches for predicting the risk of decay (7, 8). The process of selecting a treatment involves considering the application, the environmental conditions, the treatment chemical, the solvent, and the wood species. For example, there is little value in placing large quantities of a chemical deep in the wood if it is primarily there as a surface protectant and is not expected to perform for long periods. Conversely, a shallow treatment that is easily compromised serves little purpose in an application where the product is expected to last for decades. The Wood Substrate We may view wood as a collection of parallel tubes running longitudinally between the roots and the foliage or perpendicular to this direction (radially) from the pith to the bark (ray tissue) (9). These tubes move fluids in the living tree and they differ among species in a number of important ways. The greatestr differences among woods arise between hardwood and softwoods (or angiosperms and gymnosperms). Softwoods have two cell types, tracheids and parenchyma, which can be oriented either longitudinally or radially. Longitudinal tracheids are most abundant and are dead, empty tubes that transport fluids from the roots to the needles. Parenchyma cells, which may remain alive in the wood for years, may contain materials such as starches, sugars, proteins and lipids for the tree. There are also epithelial cells in some species that produce resin, but these do not appreciably influence wood treatment. Wood cells are connected to one another through pits that can be simple, bordered or semi-bordered. Each pit has a semi-permeable membrane that restricts flow. The size of the openings in this membrane can be considered to be the limiting factor in movement of fluids through softwoods. Fluids with large particles or with high viscosity may plug the pits, limiting flow. In addition, the size and number of pits can affect flow. The pits in the sapwood tend to be open, allowing fluids to move between cells. As the sapwood ages and the parenchyma die, these pits can become encrusted with hydrophobic materials that block flow and/or the pit membranes can close to become aspirated. These processes result in sharply reduced permeability. The pits in the heartwood of most species are extremely resistant to fluid movement. Thus, preservative treatment of softwoods tends to occur primarily through the tracheids in the sapwood. 204 In Deterioration and Protection of Sustainable Biomaterials; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


Hardwoods are structurally more complex than softwoods. Hardwoods have parenchyma and tracheids but liquids are moved from the roots to the leaves mostly through vessels, long series of cells that are individually called vessel elements. These short cells are connected by sieve plates that generally allow for liquid movement; however, vessels in the non-conductive regions (esp. heartwood) of the wood can be blocked by gums or tyloses. The vessels are surrounded by thick walled fibers that provide structural support for the tree but do not function in conduction; liquid movement among fibers is extremely limited. As with softwoods, the limiting factor in liquid flow will be the smallest pore size, which will be in the pits. Thus, even though vessels in a given hardwood may be open and receptive to treatment, the surrounding fibers may be highly resistant to fluid movement, resulting in inconsistent treatments that allow decay to develop in seemingly well-treated wood. The overall ability of a liquid to move into wood can be described using the viscosity of the fluid, the length the fluid must move, the difference in pressure between the surface and the interior, and the pore size (10, 11). The effects of each of these components can be used to predict flow according to Poiseuille’s Law where:

Where Q = Flow R = radius of the capillary (in this case, the pits) P = Pressure η = viscosity L = length of the flow path

The most influential component of this flow equation is pore size, since it is raised to the fourth power, but one can see where increasing pressure or reducing viscosity can also improve treatment and these are the two primary factors addressed in preservative treatment. In general sapwood is relatively easily treated, but heartwood of some species poses a major challenge to impregnation (Table 1). Pore size is very difficult to alter, although incising and through boring/radial drilling are employed in an attempt to create longer flow paths. Incising involves driving sharp metal teeth into the wood (Figure 1). The process increases the amount of cross section exposed to preservative flow and, because fluids flow more easily in the longitudinal direction, incising improves preservative treatment to the depth of the incisions (12). Incising is required for treatment of many species in the Western U.S. and is also used to help accelerate drying on hardwood railway ties (sleepers). Through-boring involves drilling slightly angled holes perpendicular to the grain through a timber or pole in areas where deterioration is most likely to occur, such as the area about the groundline. Like incising, the process exposes end grain to preservative flow, producing deeper, more uniform treatment in the drilled area. 205 In Deterioration and Protection of Sustainable Biomaterials; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.

Treatability

Softwoods

Hardwoods

Less Difficult

Ponderosa pine (Pinus ponderosa)

Blackgum (Nyssa sylvatica)

Redwood (Sequoia sempervirens)

Red oak (Quercus spp.) White ash (Fraxinus americana)

Moderately difficult

Difficult

206


Table 1. Relative Difficulty of Impregnating the Heartwoods of Selected Wood Speciesa

Very Difficult

a

Coastal Douglas-fir (Pseudotsuga menziesii)

Maples (Acer spp.)

Loblolly pine (Pinus taeda)

Cottonwood (Populus spp.)

Sugar pine (Pinus lambertiana)

Mockernut hickory (Carya tomentosa)

Grand fir (Abies grandis)

Sycamore (Platanus occidentalis)

Sitka spruce (Picea sitchensis)

Hackberry (Celtis occidentalis)

White spruce (Picea glauca)

Yellow poplar (Liriodendron tulipifera)

Inter-Mountain Douglas-fir (P. menziesii)

Sweetgum(Liquidambar styraciflua)

Tamarack (Larix laricina)

Black locust (Robinia pseudoacacia)

Western redcedar (Thuja plicata)

White oaks (Quercus spp.)

Information taken from source (13).

In Deterioration and Protection of Sustainable Biomaterials; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


The other primary factor affecting treatability is the wood moisture content. Freshly cut wood has a moisture content ranging from 40 to over 100% depending on wood species (Table 2). Some of this moisture must be removed prior to treatment in order to create space for the treatment chemical. Moisture can be removed from wood by air-seasoning, kiln drying, steam conditioning, or Boulton seasoning (heating under vacuum). The amount of moisture that must be removed depends on the wood as well as the treatment chemical. In theory, an individual wood cell that is at or below the fiber saturation point (~25-30% moisture content) will be sufficiently free of liquid water and therefore accepting of treatment liquids. In reality, there is almost always a gradient of moisture between the wetter core and the drier surface of the wood. Generally, wood is most readily treated at bulk moisture contents between 20 and 40 %. Wood can become more difficult to treat as moisture contents decline below 20 % MC because the drying can be accompanied by pit aspiration.

Figure 1. Incisor teeth penetrate into the wood surface to increase the amount of end-grain exposed to potential preservative flow.

Some preservatives may bond chemically to the wood substrate (“fixed” biocides, eg. Copper, chromium or arsenic) and are relatively immobile in the wood in service. Others are non-reactive but also insoluble in water and thus, once deposited in the wood, mostly remain in place. Examples include the oil-borne preservatives such as pentachlorophenol or copper naphthenate. Creosote is similar in this regard, but the preservative is also the solvent. Still others, notably boron-based preservatives, are water-soluble and will diffuse into (and out of) wet wood. For diffusible preservatives, movement within the wood can be described according to Fick’s law (11): 207 In Deterioration and Protection of Sustainable Biomaterials; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


where •

J is the "diffusion flux" [(amount of substance) per unit area per unit

•

time], example . J measures the amount of substance that will flow through a small area during a small time interval. D is the diffusion coefficient or diffusivity in dimensions of [length2 time−1], example

• •

(for ideal mixtures) is the concentration in dimensions of [amount of substance per unit volume], example is the position [length], example m

In practice, the diffusivity of the treated wood is affected by temperature and, especially, by moisture content. Diffusible preservatives will not move appreciably in dry wood, but diffusion will continue as long as the wood is wet. Wood that is dip or spray treated with diffusible preservatives must be kept wet to achieve deep penetration in the wood, but moisture contact with treated wood in service will leach diffusible preservatives. The concentration gradient is also an important practical consideration in diffusible treatments. For example, borates can generally only be dissolved in water to concentrations of 15 to 20 %. Thus, dipping wood in this chemical can only deliver a limited amount to the surface and the resulting concentration in the wood (assuming complete diffusion) will be dependent on the wood dimensions. Surface application of high-concentration colloids and the insertion of borate solids (e.g. fused rods) into the wood are two approaches for achieving higher gradients. Choice of Treatment Process Treatment processes can be divided into non-pressure and pressure. Non-pressure processes include spraying, dipping and soaking, while pressure processes use combinations of vacuum and pressure to force the chemical more deeply into the wood.

Dip or Spray on Systems There is a long history of protecting wood by dipping or spraying with various concoctions and these techniques are still commonly used for providing temporary protection, e.g. sapstain prevention prior to lumber drying (12). This approach delivers a relatively small amount of chemical to the wood surface. The 208 In Deterioration and Protection of Sustainable Biomaterials; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


chemical can move into the wood via capillary flow; however, the depth to which fluids move into wood in this process is typically limited to a few mm, depending on the wood species, grain orientation (greater penetration on end-grain) and whether the wood is sapwood or heartwood. Dipping or soaking typically are used where minimal preservative penetration is required. These applications include temporary (

Treatment Technologies: Past and Future - American Chemical Society

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