Wood Fiber Modification Process Offers Non-Toxic Path to Preserve Wood
—Using acetic anhydride—a chemical as benign as vinegar—
wood acetylation can preserve, protect, and stabilize wood in
a very green approach.
Wood is popular because it’s versatile, affordable, and a regenerative natural resource. But it has three weaknesses: It absorbs water (becoming dimensionally unstable); it is food for insects and fungi; and 3) it can degrade when exposed to UV rays in sunlight.
The effort to preserve wood is as old as human history. (Tar or “pitch” is even mentioned in The Bible, when Noah received his instructions.) Since then, other approaches include natural oils—like linseed oil and sunflower oil. Tung oil was used centuries ago by Chinese ship builders. But there’s a reason that oils like these are not used today on a large scale. They perform just one aspect of wood preservation, and that is to repel water. The other elements that assault wood—like sunlight, insects, and fungi—cannot be prevented with these historical treatments. Additionally, natural seed or nut oils can allow the growth of unwanted mildew.
For wood treatments and wood modifications, we also have seen a number of approaches, so let’s take a look starting with creosote.
Creosote is an age-old wood treatment, still used in some industrial settings, like utilities poles and railroad ties. Creosote is actually a coal-tar-derived treatment that works by essentially coating the wood with poison, in the hopes of warding off insects and fungi. But creosote is toxic; it’s actually a possible human carcinogen according to the EPA.
Fungicides and Insecticides
Another common category of wood treatment involves impregnating wood with fungicide and insecticide. This is typically done by bathing the wood in an aqueous solution that is under pressure, a.k.a. pressure treating. As the science of wood treatment has become increasingly sophisticated, there has been a parade of innovative wood treatments designed to be used in the pressure treating process. However, these treatments depend on chemicals that are poisonous to insects and fungi at the same time they are poisonous to humans, because we share metabolic pathways.
Chromated Copper Arsenate: CCA
Let’s look at one instructive example of the pressure-treating wood treatment process. Chromated copper arsenate wood treatment, also called CCA, was extremely pervasive for many years. (The green lumber.) The copper component of CCA serves as the fungicide, because copper is toxic to fungus, but not to humans. A second component of CCA was arsenic, which comes in various forms, some of which as highly toxic to human. The arsenic in CCA serves as both a fungicide and an insecticide (sometimes collectively called biocides). Chromium, another toxin, is added to help “fix” the fungicide and insecticide in the wood fiber, and to offer ultraviolet (UV) light resistance.
As popular as CCA became, there were persistent reports of toxic leaching, and concerns that excess CCA chemical remained on the outside of the wood when it was shipped. CCA was largely banned in 2004, and it is unavailable in most applications today.
ACQ & CA
To replace CCA, today, under various brand names, you can find wood treated with alkaline copper quaternary (ACQ) or copper azole (CA). As their named imply, these treatments use copper as a fungicide to serve as an insecticide and antimicrobial agent.
Given copper’s unique ability to serve as a harmless anti-microbial, another preservation process has emerged that drives copper particles into the wood fiber after the copper has been ground down to microscopic sizes. When the particles are that small, they can be impregnated in the wood fiber just by using water, and there is no need to chemically dissolve the copper.
Boric acid, boric oxides, and boric salts are toxic to insects and fungus, yet they pose a low risk of toxicity to humans. However, borate is water-soluble and in exterior wood applications, most borate treatments will wash away over time. Consistent and continuous re-application of the borate solution is required to maintain protection against fungi and insects. (For interior applications of borate-treated wood, there is often no need to retreat the wood.)
Another class of wood treatments is called light organic solvent preservatives or LOSP, which is not that common in North America. With LOSP, spirits or oils act as a solvent to carry wood treatments into the wood. The treatments tend to be insecticides and fungicides, including synthetic pyrethroids—a natural insecticide that plants produce to protect themselves.
Thermally Modified Wood
With this approach, the wood is not treated but modified, rendering it as a non-food for many insects and fungus, and bringing it dimensional stability. As its name implies, the thermally modified process uses high heat and steam to change the very nature of the wood. The process removes the aspects of the wood that serve as food to insects and fungi by de-naturing the nutritious sugars in the wood. The drawback of thermally modified wood is that it requires an energy-intensive process, and potential discoloration.
Now, let’s look at a wood treatment that has seen a resurgence of interest wood acetylation. Wood acetylation makes changes to the cellular composition of wood, so it’s a wood modification, not wood treatment. Wood acetylation is not a new process—it’s nearly a century old, in fact. To understand how it works, and what promise it holds for wood modification, let’s take a look at the role played in wood by a chemical group called free hydroxyl groups.
Free hydroxyl groups are very prevalent in wood, yet they are undesirable in wood that is used in construction, where stability and resistance to fungi and insects are desired. That’s because free hydroxyl groups are the aspect of wood that absorbs and releases water as the weather changes, leading to dimensional instability of the wood.
An intriguing feature of free hydroxyl groups is that they can be converted, through an irreversible chemical process, to a new type of group, called acetyl groups. When you convert free hydroxyl groups to acetyl groups, you can achieve remarkable stability and durability in the wood. Plus, you get the added benefit of making the wood fiber non-digestible for fungi and less desirable to insects.
Wood acetylation: Non-toxic?
Can the free hydroxyl groups in wood be converted to acetyl groups in a safe, non-toxic way? It turns out that it is chemically simple to achieve this conversion, and it can be achieved through a wood modification process involving a common chemical called acetic anhydride. The name of the wood modification process—wood acetylation—actually takes its name from the acetic anhydride chemical. Acetic anhydride actually comes from acetic acid, an organic compound, which when mixed with water creates household vinegar.
The wood acetylation process offers other benefits beyond dimensional stability and the fact that it denies fungi and insects a desirable food source. It also does not leave behind a residual toxic chemical in the wood fiber. That’s because the acetylation process is not trying to poison the wood to keep it from being eaten by fungi and insects. Instead, the acetylation renders the wood as a dimensionally stable, non-food material to fungi and a food material of last resort to insects. It also makes the wood environmentally benign for all forms of disposal. Plus it maintains the look of natural wood.
Another interesting feature of the acetylation process is that it does not interfere with the performance of wood finishes, like paint and stain. In fact, the acetylation process may actually enhance the performance of paints and stains, because the substrate wood is so much more stable that non-modified wood. The acetylation process has consistently shown that the resulting wood product has significantly reduced cracking, cupping, shrinking, splitting, swelling, or warping, even after many years of exposure.
Finally, wood treatments and wood modifications fall into a few broad categories that can best be summarized in this way: Some wood treatments attempt to impregnate wood with poisons and biocides (CCA, ACQ, CA), while including additives to stabilize the wood. Some wood treatments attempt to use low-toxicity natural (borate) or synthetic natural insecticides, which can lose effectiveness due to the water-soluble nature of the treatment. And some wood preservation techniques—like TMW and acetylation—approach the process by modifying the wood to provide a barrier against insects and fungi and creating a more dimensionally stable product. When you consider the energy-intensive nature of the TMW process and risk TMW introduces by potentially weakening the wood, it’s clear that acetylated wood holds may hole the position as a low-impact approach to preserving wood without affecting its natural appearance.