Wood cannot be employed excessively in all mechanical applications because of its poor dimensional stability and limited natural durability. Hence, modification is required to improve these qualities, and thermal alteration is the most popular method used in industry. This article focuses on the process of heat treatment / thermal modification of wood, including an overview of the industrial process.
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Heat treatment, also known as thermal modification, is a technique in which wood is heated to temperatures between 170 and 260 degrees Celsius. The procedure modifies its chemical and physical characteristics, producing a much more resilient, stable, and robust structure. Due to its potential to increase the quality and durability of wood-based goods, heat treatment has gained considerable popularity.
Historical Background
Harry Tiemann, a kiln specialist at the Forest Products Laboratory in Madison, Wisconsin, discovered a reduction in hygroscopicity when he heated air-dry wood in superheated steam at 150 degrees Celsius in 1915. This was the first time that the thermal alteration of wood was reported.
Early attempts to use this technique in the United States involved heating wood immersed in molten metal at temperatures between 160 and 260 degrees Celsius.
In the 1990s, there was a revival of interest in thermal modification, and the commercial environment was finally perfect for its implementation. Environmental concerns and the implementation of regulations fueled this increased interest, notably in Europe.
Why Thermally Modify Wood?
Wood's dimensional instability and lack of durability restrict its application in comparison to plastic and synthetic materials.
In addition to being porous and hygroscopic, wood possesses cell walls. It is biodegradable and very sensitive to deterioration by termites, fungi, and other organisms. Due to the high carbohydrate content held in parenchymal cells and the absence of toxic extracts, timber is less durable. Its sensitivity to several wood-degrading agents, including Basidiomycota fungus, is also a major concern.
The presence of these sorts of degrading chemicals in a wooden structure can damage goods in a short amount of time, resulting in financial losses and harmful environmental effects.
Modification aids in reducing the hygroscopic character of wood, which is necessary for the enhancement of its dimensional stability and the prevention of assault by biological degradation agents.
Difference between “Dry” and “Wet” Thermal Modification
Early phases of dry thermal modification entail a temperature increase (ramp) during which the wood loses absorbed moisture. Instead, the heating can occur under temperatures optimized to retain the water in the cell wall of the wood in a wet process.
Wet conditions are defined as heat treatment processes in which the cell wall of the wood still retains water throughout the cycle, whereas dry conditions are defined as those in which the wood has been dehydrated to contain almost zero moisture content before the thermal annealing process.
In dry thermal alteration methods, wood can be treated under a nitrogen blanket, in a vacuum, with superheated steam, or with ambient air. Formally, wet circumstances can be split between techniques that employ saturated steam as the heat transfer medium and those in which the wood is submerged in water. The breakdown products produced by wet treatment procedures stay in the cell wall until a leaching phase is carried out.
Advantages of Thermal Modification
Wood absorbs or releases water in response to the relative humidity of its surrounding environment. This causes wood to expand and contract, which can lead to increased degradation or warping and breaking over time. Unheated wood is less stable than heat-treated wood. The procedure eliminates certain organic compounds from the cell walls of the wood and lowers the quantity of water the wood retains. This makes heat-treated wood more resilient to climatic and atmospheric changes.
The heating procedure eliminates all wood resins, thus there is no possibility of tannin seeping. The heat treatment procedure is chemical-free, rendering the goods organic and non-toxic. They will not have a detrimental influence on human health in indoor applications or on plant life in landscapes and natural environments. Moreover, thermally modified wood has superior insulating properties compared to untreated wood. So, thermal modification imparts advantageous features to the wood.
Limitations Associated with Thermal Modification
The thermal modification process is expensive, and the treatment process increases the overall cost of the product. One of the major drawbacks is a reduction in the strength of the wood, especially in its fracture resistance; the thermal modification process can make the wood more brittle.
What is Hydrothermal Modification?
As per the research article published in the journal Polymers, hydrothermal modification is carried out in the presence of pressurized steam or liquid water at varying temperatures. Normal treatment temperatures vary from 180 to 260 degrees Celsius.
In general, hydrothermal treatment of wood may be divided into three categories: supercritical water treatment, subcritical water treatment, and liquid water at ambient temperature.
The difference between these processes is the temperature at which they are administered. The treatment temperature for supercritical water is more than 374 ºC, whereas the treatment temperatures for subcritical and ambient liquid water are 100 to 374 ºC and 25 to 100 ºC, respectively.
Hydrothermal modification decreases the equilibrium moisture content (EMC) of treated wood specimens, leading to an increase in dimensional stability. The reduction in EMC is generated by heat-induced variables, namely the elimination of hydroxyl groups, and is considered an environmentally beneficial and cost-effective strategy because no chemicals are employed.
Challenges
Thermal modification technology faces several technical as well as economic challenges. As wood is a very complicated material, the first obstacle is the intricacy of the raw material and reaction circumstances. The testing approach is an additional obstacle. Such tests are essential for quality assurance and control. Nonetheless, they may be costly and time-consuming. Assuming good laboratory findings, the modification procedure must be scaled up for use on full-sized wood and industrial machinery.
Market Assessment
Market Watch has published a market assessment report that states that the worldwide Thermally Modified Wood market is predicted to be worth USD 534 million in 2022 and is projected to reach USD 828.7 million by 2028, expanding at a CAGR of 7.6% over the forecast period. Nowadays, there are several competitors in the Thermally Modified Wood market. Stora Enso, Thermory AS, Oy Lunawood Ltd, Oy SWM-Wood Ltd, Novawood, and Thermoarena OÃ are key companies when it comes to commercial thermally modified wood production.
To summarize, it is safe to assume that thermally modified wood plays a vital role in commercial and industrial applications.
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References and Further Reading
Adinata Furniture, 2023. Thermally Modified Wood: Properties, Pros, Cons, And Application. [Online]
Available at: https://adinatafurniture.com/thermowood-thermally-modified-wood-properties/
GWoodPro, 2023. Thermally Modified Wood Pros and Cons. [Online]
Available at: https://www.gwoodpro.com/thermally-modified-wood-pros-and-cons
Market Watch, 2023. Thermally Modified Wood Boards Market Research Report 2023-2028 Analysis by Growth Trends, Regional Developments, Increasing Demand Status. [Online]
Available at: https://www.marketwatch.com/
Ali R. et. al. (2021). "Hydrothermal Modification of Wood: A Review". Polymers. 13(16). 2612. Available at: https://doi.org/10.3390/polym13162612
Hill, C., Altgen, M., & Rautkari, L. (2021). Thermal modification of wood—A review: Chemical changes and hygroscopicity. Journal of Materials Science. 56. 6581-6614. Available at: https://doi.org/10.1007/s10853-020-05722-z
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