Modified Wood:

Technology Meets Nature

Wood has been a major focus of the sustainability space, due to its use as a fundamental building block of construction. Structurally and aesthetically, wood is pervasive in our homes, workplaces and where we socialize and interact. With this focus has come better stewardship of our forests, and certifications like SFI and FSC have helped to ensure the careful curation and protection of our forests and wood supply. That is not to imply there is no risk of deforestation in certain parts of the world, but there exist protections in place today such that wood is now considered a sustainable resource.

Photo by Nicola Pavan on Unsplash

Further, wood is a source of carbon capture, sequestering 7.6 billion metric tons of CO2 in managed forests. Wood products on the other hand, sequester between 300 and 400 million metric tons of carbon. This may seem small in comparison, but it makes the case for the use of wood products–which are carbon sinks–from responsibly managed forests in more applications.

There are, however, some drawbacks to wood. It can be susceptible to moisture, pests, and mold, meaning that its durability and life span are not as long as other, more resource intensive materials like plastics and stones. Tropical hardwoods like teak, ipe, and black locust can perform better, but they are a coveted, expensive resource, and such demand has decimated ecosystems. Tropical hardwoods are often extremely dense and difficult to cut.

Which then makes the case for modified wood, a carbon sink, but imparted with increased durability and performance, and better workability. Wood modification has been around for a couple of decades, and can be done in three primary ways: thermal modification, chemical modification, and impregnation. Thermal modification uses natural elements like heat and a controlled environment to modify the wood, whereas chemical and impregnation modification imparts non-natural factors like chemicals or resins into the process to enhance the wood performance.

Thermal Modification

Thermally modified wood decking from Arbor Wood co.

Because water is the major culprit in wood’s unfavorable performance in some applications, removing some of it from the wood itself has a major impact on the material’s behavior. Thermal modification uses heat via a kiln with temperatures up to 400F degrees in a nitrogen rich, low oxygen environment to remove water while not allowing the wood to burn. This heat modifies the carbohydrates, or hemicellulose structure of the wood cell walls, rendering it inedible to organisms and mitigating its ability to absorb new moisture. This process gives the wood a full, through body aesthetic with richer coloring and eliminates expansion or contraction of the wood, which limits its ability to bend and offers more dimensional stability.  

Thermal modification of wood is not limited to a single species; rather, it can be applied on up to 15 different species of wood. This allows suppliers of thermally modified wood to quickly adapt to supply chain changes and opens up a wide range of aesthetic options. Unlike chemically modified wood, thermally modified wood sports a much lower carbon footprint because the wood can be sourced within the country and modified within the country.  (On the other hand, many chemically modified wood products traverse much of the globe before they are installed on a project.) Thermally modified wood does dry out the cell walls, so while they are more dimensionally stable, they can be considered more brittle than some chemically modified products. Because there are no preserving agents which can leach out over time, these products can be recycled at the end of their use with no issues. Thermally modified wood products typically offer warranties up to 30 years. Suppliers include Thermory and Arbor Wood Co.

Chemical Modification – Acetylation

Products like Accoya use chemical modification to take responsibly managed Radiata Pine, and turn it into incredibly durable products. Accoya uses a process called acetylation, which takes the hydroxyl group of molecules in wood (which is what pests and microorganisms feed on, causing wood to rot) and converts them into an acetyl group of molecules, which are hydrophobic. This process is performed using heat, time, and acetic anhydride. The byproduct of the process, once the chemical reaction in the wood is complete (hydroxyl→ acetyl), is acetic acid, which is basically vinegar. The Accoya wood itself often has this residual smell. The result is a durable product with a 50 year warranty above ground and 25 year in ground. The Netherlands-based factory this product comes from has been relegated to Radiata Pine thus far, but is exploring other options. The Radiata Pine comes from responsibly managed forests in New Zealand, which means that the materials themselves have traversed hemispheres before they get to actual installation.

Impregnation (bio-based)

Kebony uses a process called furfurylation, which introduces an agricultural bioproduct (furfuryl alcohol), in a 100C environment to cause the cellular walls of the wood to permanently swell and thicken, increasing the durability of the wood. The stiffness is increased by 10-20% and the hardness by 30-50%. Kebony is made from FSC certified wood and offers a 30 year warranty.  The product comes as a dark brown only, which fades to a silvery gray over time, which should be understood by the customer as part of their selection process. The Kebony factories are in Belgium and Norway, but the wood is sourced from the US, New Zealand and Scandinavia.  One other drawback of Kebony is that it adds about 50% more additional weight to the material, versus thermal modification which actually makes it lighter.  

Resin Modification

Impregnation resins are often imparted into wood products under vacuum such that the resin permeates the cellular structure of the wood, making it more durable. These resins, while often organic, also often contain formaldehyde or other chemicals that take away from the natural, biodegradable aspects of the wood. Some resins include phenol formaldehyde, urea formaldehyde, and melamine formaldehyde, where the resin molecules are so small, they are able to penetrate and reinforce the wood. This process can also be used with paper fiber based products like PaperStone and Richlite.


Thermally modified wood siding from Arbor Wood Co.

With interest in natural, ecologically-conscious, and biophilic design reaching new heights, wood is making a major comeback. And, since the industry is now mature enough to manage its harvesting, use, and supply chain, we should see a resurgence of its popularity due to its carbon sequestering. Wood modification pushes the material further into the architectural mainstream for casework, millwork, decking, and cladding as the performance is dramatically increased. Plus, since thermally modified wood is now a US sourced and produced material, you can choose a truly sustainable, modified wood option for your projects.

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