Thermally modified wood (TMW) refers to wood that has been subjected to high temperatures in a controlled environment to enhance its properties. The modification processes typically involve heating the wood to temperatures between 320°F (160°C) and 500°F (260°C ) in an oxygen-deprived environment, which alters the wood’s chemical and physical properties, resulting in improved durability, stability, and resistance to biological decay.
The thermal modification process begins by heating the wood in a kiln or specialized chamber under an inert atmosphere, often using nitrogen or steam to prevent combustion. The treatment can be divided into stages: drying, heat treatment, and cooling. Initially, the wood is dried to reduce moisture content. During heat treatment, the temperature is gradually increased, causing the breakdown of hemicelluloses[1] and reducing the wood’s hygroscopicity[2].
The final cooling stage brings the wood back to ambient temperature, often involving reconditioning to stabilize moisture content. The Westwood process is a proprietary thermal modification technique developed by the Finnish company Westwood Oy.
This process involves heating the wood in a closed system using superheated steam, which helps to create a uniform modification throughout the wood. The process enhances the wood’s dimensional stability and resistance to decay while maintaining its mechanical properties. The AlphaInterloc process is another thermal modification method
that uses a combination of heat and steam in a controlled environment to modify the wood. This process is designed to optimize the balance between enhancing the wood’s durability and maintaining its mechanical strength.
The AlphaInterloc process is particularly known for producing high-quality, thermally modified wood suitable for exterior applications. Retification, also known as rectification, is a French-developed process that modifies wood using high temperatures and an inert gas, typically nitrogen.
The wood is heated to temperatures around 392°F (200°C) to 482°F (250°C), which alters its chemical structure, enhancing its resistance to moisture and biological decay. Retified wood is often used in outdoor applications, including decking and cladding. ThermoWood is a widely recognized brand and process for thermally modified wood developed by the Finnish ThermoWood Association.
The process involves heating the wood to temperatures between 356°F (180°C) and 446°F (230°C) in the presence of steam. ThermoWood is known for its improved dimensional stability, reduced equilibrium moisture content, and enhanced resistance to decay. It is commonly used in exterior cladding, decking, and interior applications.
Thermally modified wood exhibits several enhanced properties compared to untreated wood. These include:
- Improved Durability: Increased resistance to fungi and insects.
- Dimensional Stability: Reduced swelling and shrinkage due to lower moisture absorption.
- Enhanced Aesthetics: Darker color and improved grain visibility.
- Reduced Thermal Conductivity: Making it more energy-efficient for building applications.
- Non-Toxic: The process does not involve harmful chemicals, making it environmentally friendly.
While thermally modified wood (TMW) offers enhanced durability, dimensional stability, and resistance to biological decay, it also has some drawbacks. One significant issue is the reduction in mechanical strength, particularly in bending and impact resistance, due to the degradation of hemicelluloses during the thermal modification process. Additionally, TMW can become more brittle and less flexible, which can limit its use in structural applications. The thermal modification process can also lead to a darkening of the wood color, which may not be desirable for all aesthetic applications. Furthermore, the initial cost of thermally modified wood is higher compared to untreated wood due to the energy-intensive modification process. These challenges necessitate careful consideration in its selection and application.
Thermal modification technology has reached a level of maturity where it is commercially viable and widely adopted in various industries. Continuous improvements in process control and understanding of wood behavior during treatment have enhanced the quality and consistency of thermally modified wood.
Several companies are leading the production of thermally modified wood. Notable ones include:
- Stora Enso: A major player in the forest products industry, offering a range of thermally modified wood products.
- Thermory: Specializes in high-quality thermally modified wood for decking, cladding, and interior use.
- Kebony: Uses a proprietary process combining heat and biological agents to modify wood.
- Lunawood: Offers ThermoWood products with a focus on sustainability and high performance.
- Americana Real Wood: a brand specializing in high-quality, thermally modified hardwoods designed for enhanced durability and stability, ideal for both exterior and interior applications.
Research in the field of thermally modified wood continues to focus on improving process efficiency, understanding the long-term performance of modified wood, and expanding the range of wood species that can be effectively treated. Studies are also exploring the environmental impacts of thermal modification and the potential for using modified wood in new applications such as structural components and advanced composites.
Footnotes
- Hemicelluloses are a group of complex carbohydrates found in plant cell walls alongside cellulose and lignin. Unlike cellulose, which is a linear polymer of glucose, hemicelluloses are branched polysaccharides composed of various sugar monomers, including xylose, mannose, galactose, rhamnose, and arabinose. They have a lower molecular weight compared to cellulose and are more amorphous and soluble in water. Hemicelluloses play a critical role in the structure and function of cell walls by providing flexibility and facilitating the interaction between cellulose fibers and lignin. They are also more susceptible to hydrolysis and thermal degradation, which is a significant factor in processes like thermal modification of wood. [Back]
- Hygroscopicity is the ability of a substance to absorb moisture from its environment. This property is particularly significant in materials like wood, which can absorb and desorb water vapor depending on the ambient humidity and temperature. Hygroscopicity affects the physical and mechanical properties of wood, such as its dimensional stability, strength, and durability. When wood absorbs moisture, it swells, and when it loses moisture, it shrinks, which can lead to warping and other forms of deformation. Understanding and controlling hygroscopicity is crucial in various applications, including construction, preservation, and material science, to ensure the reliability and longevity of wood and other hygroscopic materials. [Back]
Further Reading
Sources
- Wikipedia “Thermally modified wood” https://en.wikipedia.org/wiki/Thermally_modified_wood
- Wayback Machine “Westwood technology and Thermo-treatment of wood worldwide” https://web.archive.org/web/20190919220822/http://www.westwoodcorporation.com/Technology.html
- BMC “New alternatives for wood preservation based on thermal and chemical modification of wood— a review” https://annforsci.biomedcentral.com/articles/10.1007/s13595-015-0531-4
- International ThermoWood Assoc. “ThermoWood®
Handbook” https://asiakas.kotisivukone.com/files/en.thermowood.palvelee.fi/downloads/tw_handbook_080813.pdf - Academia “Treated Wood Quality : Durability and Mechanical 5 Properties of Thermally Modified Wood 6 7” https://www.academia.edu/65052466/Treated_Wood_Quality_Durability_and_Mechanical_5_Properties_of_Thermally_Modified_Wood_6_7
- ResearchGate “Wood modification by heat treatment: A review” https://www.researchgate.net/publication/279900105_Wood_modification_by_heat_treatment_A_review
