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Overview
In recent years, carbonized wood has become increasingly popular in the domestic market, and more and more people have heard of this new term. However, most people still remain at a vague stage of understanding the meaning of the word and hearsay, and the industry lacks a clear understanding of the basic knowledge of carbonized wood. This article aims to provide a basic introduction to the process and applications of carbonized wood.
What is carbonized wood?
According to textbook definitions, "heat-treated wood, commonly known in China as 'carbonized wood,' is produced by heating wood at 150°C to 260°C using steam, air (oxygen-deficient), nitrogen, or vegetable oil as a medium." However, during heating, only some of the wood's components undergo pyrolysis and release; the wood's actual substance remains carbonized, making the term "carbonized wood" somewhat inaccurate. Internationally, it's generally referred to as "Thermal Modified Timber," or TMT, which is more accurately translated into Chinese as "thermally modified wood." TMT is typically used outdoors and can last up to 25 years under extreme conditions. It contains no artificially added chemical additives and is completely harmless to the environment and humans.
history
The history of carbonized wood can be traced back to China's Tang Dynasty (7th to 10th centuries AD). The sturdy Vikings discovered that wood with a charred surface could resist corrosion from seawater, and they used it to build warships. This marked the earliest conscious application of carbonized wood by humans. Technically, it wasn't until the last century that Europe developed a stable and controllable carbonized wood process, and commercial production has only been around for the past 20 years. Europe and the United States primarily utilize its corrosion-resistant properties, as carbonized wood is a greener, safer, and more environmentally friendly alternative to outdoor materials, and its application is relatively mature. In China, the market is just beginning to recognize this new product, and the carbonized wood industry, still in its growth phase, has much to explore and develop in terms of production, development, design, and application.
Carbonized wood outdoor flooring
Craftsmanship
During the carbonization process, product properties change by varying parameters such as temperature, humidity, pressure, and circulation. For example, increasing the temperature and extending the time darkens the product's color and improves its corrosion resistance, but also reduces its toughness and makes it more brittle. Lowering the temperature and shortening the time results in a lighter color and reduced corrosion resistance and stability. Figure 5 illustrates the curve of product properties changing with the degree of carbonization.
The material changes and results caused by carbonization are as follows:
Figures 6 and 7 clearly show the changes in the wood microstructure during carbonization. The control is the wood before heat treatment, followed by the wood microstructure at different temperatures. As the temperature increases, the wood cell walls undergo significant deformation, becoming more brittle.
2. Anti-mildew and anti-corrosion
High-temperature heat treatment alters the chemical composition of wood, eliminating the conditions necessary for the survival of wood-eating organisms. Pyrolysis of the hemicellulose in the wood produces formic and acetic acids. Simultaneously, low-molecular-weight nutrients in the wood are volatilized or destroyed. These changes in the wood's pH, nutrients, and equilibrium moisture content undermine the essential habitats for fungi and other microorganisms, significantly improving the carbonized wood's anti-corrosion, anti-mildew, and anti-discoloration properties.
3. Small swelling when wet and shrinkage when dry
Wood's moisture absorption is primarily due to the presence of a large number of hydroxyl groups within it. During heat treatment, hemicellulose, which contains a large number of hydroxyl groups, has the poorest heat resistance and is the first to decompose, significantly reducing the number of hydroxyl groups within the wood. This greatly reduces the hygroscopicity of carbonized wood, minimizing both shrinkage and swelling, and resulting in greater dimensional stability.
4. Avoid cracking and deformation
Growth stress (residual stress) in wood is one of the main causes of cracking, warping, and swelling and shrinkage. After carbonization and high-temperature treatment, wood undergoes changes such as reduced density, removal of low-molecular-weight volatiles, degradation of hemicellulose, reduction of amorphous regions, and increased crystallinity. This reduces the difference in tangential and radial shrinkage between the wood, releasing growth stress.
5. Color stability
After carbonization, the color of the wood becomes darker and more consistent inside and outside. In addition to eliminating the color difference of lighter-colored wood species, some wood species such as boxwood and soft maple can look very similar to black walnut after carbonization, increasing the application value of the material.
6. Density reduction
Due to the pyrolysis of hemicellulose, the density of carbonized wood decreases. Typically, the density of hardwood decreases by 10% to 12% after carbonization, with results varying depending on the wood type and carbonization process. Furthermore, due to the reduction of the amorphous region, the wood's toughness decreases, making it more brittle.
7. Reduced hardness
The hardness of wood decreases by about 3% after carbonization. However, most hardwoods are inherently very hard (see Table 1). Therefore, even after carbonization, their hardness can still ensure the quality of the wood when used in ordinary flooring, furniture, doors, stairs, etc.
Carbonization defects
Since the carbonization process is completed at a high temperature close to the ignition point, and the moisture content must reach the absolute dry limit state, the control of the carbonization process must be very precise. Any slight deviation may cause quality problems such as cracking of the board surface, bursting of scars, surface pollution, color change, etc., and in severe cases, it may cause the carbonization tank to explode.
Compared with other antiseptic wood
Carbonized wood is primarily used in outdoor projects in Europe and the United States. It is a new, all-natural, heavy-duty, corrosion-resistant material with irreplaceable advantages. For example, a comparison of carbonized white wax with common outdoor flooring materials is shown in Table 2.
Precautions during use
Because carbonization causes changes in wood properties, the following issues should be paid attention to during the use of carbonized wood, especially carbonized hardwood:
1. Due to the reduction in hydroxyl groups and the more closed pore structure, carbonized wood's ability to absorb water, oil, and glue is reduced. When gluing, allow a longer open time after applying the glue to allow the glue to penetrate the wood and form a glue nail, ensuring the strength of the bond surface. When painting, reduce the amount of primer sprayed to allow each coat of primer sufficient time to penetrate.
2. Due to the increased brittleness of wood, all screws must be pre-drilled. Avoid using self-tapping screws to prevent cracking. Countersink the screw cap to prevent excessive pressure on the wood surface when screwing in, which can cause chipping. Similarly, during machining, tools must be sufficiently sharp to avoid chipping.
3. The color of carbonized wood will gradually become lighter due to ultraviolet radiation, so anti-UV coating must be used, otherwise it will change color.
Conclusion
As a novel wood treatment process, carbonization perfectly combines wood's natural properties with its performance, representing a rare technological innovation in this traditional industry. While the Chinese market is still in its nascent stages, driven by strong domestic high-end demand, carbonized wood is expected to enter a period of explosive growth after a year or two of promotion and education. This will also be a new sustainable growth point for our industry, a development worth anticipating and striving for.
Acknowledgements
We would like to thank Dr. Hu Jinbo from the School of Materials Science and Engineering at Central South University of Forestry and Technology, an expert in carbonization, for providing materials and patiently correcting the results of this article.