Looking into the Future of Wood Preservation

Carol Clausen and her group are serious about wood as a sustainable and versatile building material. Here Amy Blodgett, Rachel Arango, and Bessie Woodword check soil block samples in their ongoing research.

Amy-B--Rachel-A--Bessie-W(1)

Members of the Durability and Wood Protection Research Unit check soil block samples.

The soil block decay test method determines the minimum amount of preservative that is effective in preventing decay of selected species of wood by selected fungi under optimum laboratory conditions. Conditioned blocks of wood are impregnated with solutions, emulsions, or dispersions of a preservative in water or suitable organic solvent to form one or more series of retentions of the preservative in the blocks. After periods of conditioning or weathering, the impregnated blocks are exposed to recognized destructive species of both brown-rot and white-rot wood-destroying fungi.

Speaking about her work, Clausen says that wood’s “increased use in construction minimizes life-cycle impacts of a structure while maximizing carbon storage for the life of the structure.” An upcoming report by Clausen, Frederick Green III, Grant Kirker, and Stan Lebow summarizes presentations and comments from the inaugural Wood Protection Research Council meeting, where research needs for the wood protection industry were identified and prioritized.

As a part of that conversation, Clausen states that chemicals used to protect wood from deterioration by fungi and insects have generally been broad-spectrum biocides that have been discovered by the traditional screening approach. However, a more logical approach is to develop selective and targeted biocides by defining the target first, characterizing that target, and then designing inhibitors based on the mechanism of action of the defined biotarget.

Despite substantial progress in explaining the biochemistry of wood degradation, biochemical targets for fungal inhibition have seldom been described. Discovering the mechanism of preservative tolerance(s) in economically important fungi and insects will enable researchers to design preservative systems that neutralize, block, prevent, and eliminate the preservative tolerance. Development of a genetic database of microbial activity during the process of wood deterioration will offer a better understanding of precisely how and when decay or insect attack begins. A molecular database would provide myriad of capabilities to the wood preservation community. For example, researchers could characterize decay risks for a particular location, define the prevalence of tolerant fungi on a national and international basis, and determine the influence of preservative exposure on the ecology at test plots.

The next generation of novel wood protection methods may incorporate nanotechnology for design or controlled delivery of biocides for improved durability of building materials, with an emphasis on engineered composites. Nanotechnology may also play a vital role in the development of water-resistant coatings and treatments for the prevention of fire. Results from basic research on genetic analysis, biochemical processes, nanotechnology, and characterization of biotargets will lead to technological developments that extend the service life of wood and wood-based materials in all major end uses emphasizing environmentally friendly methods.

 

Keeping Wood Dry Isn’t Always Enough: Beware of Dry Rot

Build Green: Wood Can Last for Centuries by Carol Clausen and Samuel Glass tells us that most wood-decay problems only occur when decay fungi grow in wet wood. But one kind of fungus is uniquely capable of transporting its own water from a source of moisture (usually soil) into wood that is typically too dry to decay. While decay by such water-conducting fungi is uncommon, when it occurs, it is devastating.

dry-rot

Dry rot starts when an infected piece of wood forms a bridge between soil and other wood in a house (Photo by Carol Clausen, FPL).

Large areas of flooring and walls can be destroyed each year unless the fungus is stopped. Ironically, it may be the easiest fungus to prevent or control. Unlike typical decay fungi that start growing from airborne spores, water-conducting fungi usually start growing from previously infected lumber that forms a bridge between the soil and other wood in the house. This can happen if old, discarded beams that have been lying on the ground are used in home repairs or additions; new wood has been improperly stored in contact with soil; or infected wood waste is used as fill under a porch or addition. This type of decay can be stopped by simply breaking the contact between susceptible wood and the source of moisture. Once the water supply is broken and the infected wood dries, the fungus will die.

Good news for the homeowner: a potentially destructive source of rot is easy to control with attention and care.

How do Wood Extractives Affect Metal Corrosion?

Did you know that metal fasteners corrode in wood? This week we will look further into the work of Sam Zelinka on this subject. Zelinka is the FPL corrosion expert, and our post today borrows from Corrosion of Fasteners in Wood Treated with Newer Wood Preservatives, a compilation of several papers Zelinka has written on the subject in recent years.

Wood may seem like a simple material, but the lumber you build with is actually chemically and physiologically complex. Research has shown that different wood species contain different extractives that may affect the corrosion of embedded metals. This week, we consider the chemical components of wood and the effect of tannins and pH on the corrosion process. Much of today’s post draws from a 2011 study on corrosion of steel in wood extracts.

Wood is comprised of polymers, natural and synthetic compounds of high molecular weight consisting of millions of repeated linked units. In addition to structural polymers (cellulose, hemicellulose, and lignin), wood contains a variety of additional chemical components.

Wood also consists of a variety of organic compounds that are low in molecular weight—extractives. These small molecules get their name because they can be extracted by rinsing with various solvents (including water). Generally, extractives are present in small amounts, and thousands of different extractives are present in wood.

The type and amount of extractives vary widely among wood species. In some naturally durable species (such as locust or white oak), extractives can protect the wood from decay.

Although a single piece of wood can contain over 700 different extractives, only three types have been thought to affect the corrosion of metals in contact with wood or the black liquors of wood pulp: small organic acids, tannins, and phenols.

According to Zelinka, “many researchers have found correlations between the acidity of wood and its corrosiveness, and pH is largely controlled by formation of acetic [essentially vinegar] and formic acid. However pH cannot be the only variable that affects corrosion.”

Zelinka tells us, “Although the pH of wood, a solid material, is not well defined, the water extracts of nearly all wood species are acidic. The reason for this acidity is that in the presence of water, acetyl and formyl groups in the hemicelluloses are hydrolyzed [decompose by reacting with water] to form different kinds of acid. Many researchers have found correlations between the acidity of wood and its corrosiveness, and pH is largely controlled by formation of acetic and formic acid.”

Research has shown that this process is chemical, rather than biological. Previous research on sawblade corrosion suggested that wood tannins accelerated the corrosion of sawblades; however, in general, tannins are known as a corrosion inhibitor. In the 2011 study, Zelinka and Stone showed that tannins in solid wood act as a corrosion inhibitor to the embedded fasteners. In addition to the corrosion rate data, Zelinka and Stone observed a blue-black patina forming on the steel, indicative of the formation of iron-tannate, a stable blue/black corrosion product.

Blue-black precipitate forming on the surface of the steel plug
exposed to synthetic oak extract.

Tannins and other extractives are often mentioned in the literature as compounds that affect corrosion in wood. The different behavior of tannins most likely depends on what application is being studied. For instance, Zelinka and Stone discovered that tannins act as a corrosion inhibitor in wood extracts, which contradicts the earlier sawblade corrosion findings. The difference is most likely due to the friction and heat produced during sawing.

By combining kinetic models in the literature, Zelinka and Stone created an isocorrosion map for wood extracts as a function of pH and tannins. An isocorrosion map is a kind of tool used to recognize high-corrosion situations during the design process of equipment.

“This map,” says Zelinka, “was based on limited data and it does not explain why synthetic extracts behave differently; nevertheless, in the future with additional data such maps may be able to assess the relative effects of these chemicals when developing a new, non-metallic preservative system.”

After all this work and explanation, Zelinka tells us that “the effect of tannins on the corrosion of metals in wood remains unclear.” As Zelinka and his colleagues continue to study corrosion, perhaps this question will have more answers.

 

Why and How Do Metals Corrode in Treated Wood? Chemistry and Design Mysteries Revealed

Scientist Sam Zelinka is the FPL expert on corrosion of fasteners in treated wood. This week we continue our discussion of this subject from Zelinka’s publication Guide for Materials Selection and Design for Metals Used in Contact with Copper-Treated Wood and a related publication, Corrosion of Fasteners in Wood Treated with Newer Wood Preservatives.

In these publications, Zelinka discusses the chemistry of waterborne wood preservatives. These preservatives contain cupric ions (copper molecules) that are thermodynamically unstable in the presence of steel or zinc galvanized fasteners. A chemical reaction, such as the one that will be discussed here, requires a lot of energy to be formed. To be thermodynamically unstable is to be unfavorable and means that the reaction is not spontaneous. It requires energy.

Let’s talk about the mechanism of corrosion in treated wood. This process, according to Zelinka, involves “the transport of cupric ions through the wood to the fastener surface, where the cupric ions are reduced and the fastener (zinc or iron) is oxidized.”

What does that mean?  In chemistry, oxidation is the loss of electrons or an increase in oxidation state by a molecule, atom, or ion. Reduction, on the other hand, is the gain of electrons or a decrease in oxidation state by a molecule, atom, or ion.

How does that work? The following graphic illustrates this point by showing the mechanism of corrosion in treated wood.  We can see how cupric ions migrate through the wood to the metal surface where they are reduced as the fastener is oxidized.

fig_04

Zelinka tells us, “For carbon steel and zinc-galvanized fasteners, the reduction of cupric ions is thermodynamically favorable and will occur. The corrosion of embedded metals is strongly dependent upon moisture content.” This, according to Zelinka, “is a war that Nature will eventually win.”

However, Zelinka also gives the homeowner numerous tips for prolonging the life of their outdoor projects through careful materials selection and design. By understanding the corrosion mechanism, “it is possible to develop strategies for maximizing the life of embedded fasteners.”

One strategy starts with understanding that “When wood is dry, embedded metals do not corrode.” Although your deck will obviously be exposed to rain and snow, Zelinka assures us that sound design principles can help, such as keeping rainwater from seeping in through the end grain and designing roofs and overhangs so they do not drain onto lower structures.

Another strategy Zelinka suggests for making sure that the fasteners holding that new deck might have a long life is using a metal noble to copper.  Noble metals are metallic chemical elements that have outstanding resistance to oxidation, the most common (and affordable) being stainless steel. This means that the nail is hot-dipped in a noble metal, and if these nails or other fasteners are not damaged in construction, the coating can go a long way in protecting against corrosion.

With this knowledge at hand, your outdoor structures can (and will) last a long time.

 

 

What Is Treated Wood? The Copperific Truth Behind Green Wood

Last week we introduced the subject of corrosion in the fasteners used in wood construction. Homeowners have an enormous choice of lumber to use in their projects. We all know the feeling of being in the big box store and looking at the seemingly endless choices of lumber stacked in huge piles. And why is it colored that weird green? Let’s take a few minutes to talk about treated wood.

Many Types of Wood Preservatives Are Now in Use

Going back again to Forest Products Laboratory Researcher Sam Zelinka’s Guide for Materials Selection and Design for Metals Used in Contact with Copper-Treated Wood, “Wood preservatives are chemicals that are injected into the wood to help the wood resist attack by decay fungi, mold, and/or termites. Waterborne wood preservatives are used in most cases where the wood may be in contact with humans or will be painted. While many different formulations of waterborne preservative treatments have been developed, only a few of these have been used commercially. Most of the commercial treatments contain cupric ions [copper molecules], which give treated wood its characteristic greenish-brown coloration.”

As we mentioned last week, 2004 ushered in major changes with treated wood when Environmental Protection Agency regulations restricted the use of chromated copper arsenate (CCA) in the United States. The European Union and Australasia made similar changes in their regulations at about the same time. This was a significant change, as CCA had dominated the U.S. preservative market for many years.

Zelinka tells us that “CCA can still be used in certain situations, specifically wood used in highway construction (excluding pedestrian bridges or hand railings).” Since the regulation change, alternatives to CCA have been introduced, and these alternatives now dominate the market.

FPL’s Stan Lebow has summarized alternatives to CCA in many publications, particularly Alternatives to chromated copper arsenate (CCA) for residential construction and the Wood Preservation chapter of the Wood Handbook.

Several alternatives with different formulas are now available. Zelinka says, “Although the formulations of . . . wood preservatives are different from each other, they all have a higher percentage of copper than CCA.” This is important, as the corrosion mechanism has to do with reducing cupric ions in the preservative. In 2007, Zelinka and others found in Direct current testing to measure corrosiveness of wood preservatives that chromates and arsenates in CCA act as corrosion inhibitors.

Many of the post-2004 preservatives have been standardized by the American Wood Protection Association. Additionally, several commercially important preservatives have been introduced to the market by ICC-ES (ICC Evaluation Services) evaluation reports.

According to Zelinka, “These preservatives include “micronized” formulations . . . which have various trade names. In these formulations, soluble copper is not injected into the wood; rather solid copper, copper oxide, or copper carbonate is ground into submicron particles or “micronized” and suspended in solution prior to injection. Several different formulations of these preservatives are covered by different ICC-ES evaluation reports. These formulations differ in the listed uses, required retentions, and have slight differences in the formulations, but in general require less copper than the nonmicronized counterparts.”

From left to right are examples of different treated wood: micronized copper quaternary (MCQ), didecyldimethylammonium carbonate (DDAC), and alkaline copper quaternary (ACQ-D). Cupric ions from the wood preservative causes the dark coloration of the wood. Excess copper has deposited on the MCQ (green splotches) and the ACQ (along the end grain).

From left to right are examples of different treated wood: micronized copper quaternary (MCQ), didecyldimethylammonium carbonate (DDAC), and alkaline copper quaternary (ACQ-D). Cupric ions from the wood preservative causes the dark coloration of the wood. Excess copper has deposited on the MCQ (green splotches) and the ACQ (along the end grain).

Behold the many choices available to the homeowner. Armed with knowledge, that deck you build next summer can be beautiful and will last a long time.

####