Modified wood is a generic term that describes the application of chemical, physical, or biological methods that are used to alter the cellular structure of the wood. The U.S.D.A. ‘s National Forest Service Library defines modified wood as “wood that is processed by chemical treatment, compression, or other means, with or without heat, to impart permanent properties quite different from those of the original wood.”
The aim of modified wood is to overcome the shortcomings of standard timber. The modification process enhances the performance of the wood, resulting in improvements in the longevity of timbers – dimensional stability, decay resistance, reduced water absorption, less expansion/contraction, etc. In addition, since most advanced modification processes are environmentally friendly, the materials can still be disposed of properly at the end of the product life cycle.
CHEMICAL MODIFICATION OF WOOD – ACETYLATION
Typically, a process called acetylation is used to achieve chemically modified wood. It’s a chemical reaction that has been used for more than a century. In the simplest terms, the process subjects vinegar to a softwood, which turns it into a hardwood by preventing the cells in the wood from being able to absorb water.
The physical properties of any material are determined by its chemical structure. Wood contains an abundance of chemical groups called “free hydroxyls”. Free hydroxyl groups absorb and release water according to changes in the climatic conditions to which the wood is exposed. This is the main reason why wood swells and shrinks. It is also believed that the digestion of wood by enzymes initiates at the free hydroxyl sites – which is one of the principal reasons why wood is prone to decay.
Acetylation effectively changes the free hydroxyls within the wood into acetyl groups. This is done by the reaction wood has with acetic anhydride, which comes from acetic acid (vinegar when in diluted form). When the free hydroxyl group is transformed into an acetyl group, the ability of the wood to absorb water is greatly reduced, rendering the wood more dimensionally stable and extremely durable.
What is Accoya Wood®?
A great example of this chemical modification method in use is Accoya wood. Accoya is wood, but not as you know it. It is manufactured through the acetylation of plantation grown Radiata pine softwood.
Acetyl groups are already naturally present in all wood species. This means that the manufacturing process adds nothing to the wood that does not already naturally occur within it. 1https://www.accoya.com/why-accoya/benefits/
Accoya Modification Process
- Wood is sourced
Accoya sources sustainable, plantation-grown Radiata pine.
- Raw wood reacts with acetic anhydride.
Naturally occurring free hydroxyl groups are exposed to acetic anhydride (a byproduct of acetic acid) and the molecular structure is changed.
- Free hydroxyl groups transform into acetyl groups.
The ability of the wood to absorb water is greatly reduced, and the wood is now more dimensionally stable and durable.
- The resulting acetic anhydride is recycled.
Acetic anhydride comes from acetic acid (vinegar in a diluted form) and does not release toxins into the air or the surrounding environment.
CHEMICAL/THERMAL MODIFICATION OF WOOD – FURFURYLATION
Thermally modified wood is redefining the properties, features, and applications of natural wood in a wide spectrum of residential and commercial applications.
Thermal modification is a process where the dimensional composition of wood is modified by applying heat in the absence of oxygen, which creates a controlled chemical reaction. Due to the lack of oxygen, the wood does not burn at high temperatures. The modification process permanently modifies the cell walls of the wood and typically results in the wood having a rich, brown color.
What is Kebony Wood®?
A great example of the thermal modification method is Kebony wood. The Kebony technology modifies wood by forming stable, locked-in furan polymers in the wood cell walls. These increase the dimensional stability, as well as durability and hardness, of the wood. This process is based on impregnation with furfuryl alcohol, which is produced from agricultural crop waste. Kebony thus uses a plant-derived waste product to give enhanced strength and durability to another plant product – namely wood.
Impregnation is the process in which Radiata pine is soaked in a bio-based liquid. During impregnation, furfuryl alcohol produced from a bio-based liquid is imparted into the wood to make it dimensionally stable.
- Curing and Drying
After impregnation, the wood is heated – whereby the polymerization of the furfuryl alcohol occurs. This step is referred to as the curing step. The resulting polymer, which is now permanently locked into the wood cells, is stable and will not disintegrate or leak out of the wood.
- Kebony Wood is Born
After treatment, the wood cell walls are 50 percent thicker due to the stable, locked-in polymers. This permanent modification of the wood cell walls gives the final wood product outstanding stability, the maximum amount of hardness and guaranteed long life.
Kebony wood also provides a high level of safety as the wood does not splinter and contains no toxins or chemicals, nor does the wood get too hot in the summer. Furthermore, Kebony wood is resistant against rot and fungi and other wood-destroying microorganisms in above-ground context. Kebony is a low-maintenance material that does not require any additional treatment beyond normal cleaning, thus, no oils or other impregnation solutions are needed. 2https://kebony.com/en/content/technology
Since Kebony is a natural product there will be some swelling and shrinkage due to changes in relative humidity. Maximum swelling across the board, from completely dry to complete water saturation, could be up to six percent of the board width for Kebony character and four percent for Kebony clear grade wood. The swelling of untreated wood in the same conditions will be more – about eight percent of board width.
Kebony wood will be influenced by local environmental conditions over time. After exposure to sun and rain, the wood develops a natural, beautiful silver-grey patina. The rate of weathering to grey depends on the severity of exposure to exterior elements such as sun, rain and wind and the type of cladding application. If keeping the original color is desired, the wood can be maintained using UV protection oils.
ABODO wood is another wonderful option for thermally modified wood that is durable and rapidly renewable. The timber has exceptional weatherability and maintains its durability and beauty for a lifetime.
This wood is harvested from New Zealand’s FSC certified rapidly renewable Radiata pine plantation forests. They are ethically crafted with respect from beginning to end – which meets today’s increasing timber needs and green building practices without disadvantaging future generations.
ABODO Modification Process
- Temperature Increase
The temperature in the kiln is raised rapidly to around 212°F, affecting the natural composition of the wood. The use of steam prevents checking in the wood due to the high heat. The moisture content of the wood is reduced from 15 to 20% to nearly zero.
- Thermal modification
The temperature goes even higher to reach target color levels, around 415°F. The sugars and natural acids are modified from the high heat, which alters the physical structure of the wood. The wood’s ability to absorb moisture decreases, making it less prone to rot and decay.
- Conditioning and Re-Cooling
The temperature is reduced by spraying water onto the wood to cool it down. Conditioning and remoistening bring the wood moisture content to around 4 to 6%.
- Patented glue lamination is used for a modern, clean vertical grain orientation. This provides superior weathering characteristics, greater stability and less surface cracking – this means the wood ages with grace as it’s left to weather outdoors
- Typically used for exterior siding, decking, or difficult installations
- 15 and 30-year structural warranty packages against rot and decay
- FSC Certified
- For every tree milled, Abodo ensures it provides more than 30 years of service, which is as long as it takes to grow a replacement tree
CHARRED MODIFICATION PROCESS – SHOU SUGI BAN
The process of charring the surface of wood with a hot flame, namely sugi wood (Japanese cypress) as a preservation technique is thought to date back to the 1700s in Scandinavia and Japan. The technique entails burning enough of the surface to create a layer of char on the outside. The wood is then cooled and cleaned of loose soot and debris, then finished with oil.
The shou sugi ban finishing technique is perceived to solve a number of issues that associated with standard timber:
- It seals and preserves the wood
- It makes it significantly more fire-resistant
- It’s more resistant to insect and moisture damage
Today, the shou sugi ban technique is used in multiple applications, from commercial and high end residential projects to institutions and furniture. Not only is it aesthetically beautiful – the dark finish achieves a striking, standout curb appeal – it’s a natural, sustainable, non-toxic alternative to treated wood that is harmful to people and the environment.
reSAWN applies our own interpretation to the ancient Japanese shou sugi ban process. We take great pride in designing shou sugi ban products on high-performance modified woods. For exterior applications, we recommend shou sugi ban charred Accoya, Kebony and ABODO. Using the modified wood as a base increases the durability of the wood which makes it an excellent choice for a dimensionally stable option that will perform well when exposed to harsh exterior conditions like snow, sun and rain. Plus, the charcoal stays on the surface of modified wood much better than standard softwood species that will chip over time. Our modern manufacturing techniques allow for a higher production yield and many design variations. The designs and species used vary greatly in color and aesthetics from a fully charred black to light, subtle gray.
At reSAWN TIMBER co. we celebrate and visually leverage wood’s inherent tendency to weather over time by designing modern, high-performance products that are meant to age in place with grace. We work with woods natural tendency to grey, not against it. Our products look beautiful on day one and continue to as nature takes over – with little to no aesthetic maintenance needed. Each board works cohesively together yet each board has its own visual story. The result is modern originality & inherent textural beauty with each and every project.
If you’re interested to see how modified wood will perform on your project site, we offer a mockup program. Having a small amount of material on-site has many benefits:
- specify material confidently knowing that the product suits the exact exterior conditions where the structure is being built
- observe some amount of weathering
- understand product appearance over a larger field of material
- observe tolerance for wood movement in a project-specific climate
- monitor effectiveness in rear ventilation & drainage through assembly details (i.e. rainscreen)
- include all stakeholders in project to set realistic expectations for how wood will preform and weather
Samples of our standard products and modified wood offerings are available for architects and designers at no cost and shipped via UPS Ground. Click on the button below to explore reSAWN’s complete product offering – sort by species, color or application.