A Brief Analysis of the Process and Application of Carbonized Wood

Summary

In recent years, the popularity of carbonized wood in the domestic market has gradually increased, and more and more people have heard of this new term. However, most people are still at a vague stage of understanding based on hearsay and a lack of clear popularization of basic knowledge about carbonized wood within the industry. This article aims to provide a basic introduction to the process and application of carbonized wood.

What is carbonized wood?

According to textbook definitions, "heat-treated wood, commonly known domestically as 'carbonized wood,' is a product obtained by heating wood at 150℃~260℃ using gases such as steam, air (oxygen-free), nitrogen, or vegetable oil as a medium." However, during heating, only some components of the wood undergo thermal decomposition; the actual wood substance is not carbonized. Therefore, calling it "carbonized wood" is not entirely accurate. Internationally, it is generally called "Thermal Modified Timber," or TMT for short, which is more precise. TMT is typically used outdoors, with a lifespan of up to 25 years under extreme conditions, and contains no artificially added chemical additives, making it completely harmless to the environment and human health.

history

The history of carbonized wood dates back to the Tang Dynasty in China (7th-10th centuries AD). The formidable Vikings discovered that wood with a charred surface could resist seawater corrosion, and used it to build warships. This marked the earliest conscious application of carbonized wood by humankind. From a technological perspective, it wasn't until the last century that Europe developed a stable and controllable carbonization process for wood, and commercial production has only been underway for about 20 years. European and American countries primarily utilize its anti-corrosion properties, as carbonized wood is a greener, safer, and more environmentally friendly upgraded outdoor material, and they already have relatively mature application experience. In China, the market is just beginning to recognize this new product. The carbonized wood industry, still in its growth stage, has much to explore and develop in terms of production, development, design, and application.

Carbonized wood outdoor flooring

process

During the carbonization process, the product's performance changes depending on parameters such as temperature, humidity, pressure, and circulation. For example, increasing the temperature and extending the time deepens the product's color and improves its corrosion resistance, but also reduces the material's toughness and makes it more brittle; decreasing the temperature and shortening the time lightens the product's color and reduces its corrosion resistance and stability. Figure 5 illustrates the curves showing how product performance changes with the degree of carbonization.

The material changes and results caused by carbonization are as follows:

1. Irreversible and permanent changes occurred in the pore structure. Figures 6 and 7 clearly show the changes in the wood microstructure during carbonization. Control is before heat treatment, followed by the wood microstructure at different temperatures. As the temperature increases, the cell walls of the wood undergo significant deformation, becoming more brittle.

2. Anti-mildew and anti-corrosion

High-temperature heat treatment alters the chemical composition of wood, removing the conditions necessary for the survival of organisms that erode it. Hemicellulose in the wood undergoes pyrolysis, producing formic acid and acetic acid. Simultaneously, low-molecular-weight nutrients in the wood volatilize or are destroyed. These changes in the wood's pH, nutrients, and equilibrium moisture content disrupt the survival mechanisms of fungi and other microorganisms, significantly enhancing the anti-corrosion, anti-mildew, and anti-discoloration properties of carbonized wood.

3. Expansion upon drying and shrinkage upon drying

The hygroscopicity of wood is mainly due to the large number of hydroxyl groups it contains. During heat treatment, hemicellulose, which contains a large number of hydroxyl groups, has the worst heat resistance and is the first to decompose, significantly reducing the number of hydroxyl groups inside the wood. This greatly reduces the hygroscopicity of carbonized wood, minimizing shrinkage and swelling, and making the wood more dimensionally stable.

4. Avoid cracking and deformation

Growth stress (residual stress) in wood is one of the main causes of cracking, warping, and shrinkage. After high-temperature carbonization treatment, wood undergoes changes such as reduced density, removal of low-molecular-weight volatiles, degradation of hemicellulose, reduction of amorphous regions, and increase in crystallinity. This reduces the difference in tangential and radial shrinkage rates, allowing growth stress to be released.

5. Color stability

After carbonization, the wood darkens in color and becomes uniform throughout. Besides eliminating color variations in lighter-colored species, some species, such as boxwood and soft maple, become very similar in appearance to black walnut after carbonization, increasing the material's application value.

6. Density decreases

Similarly, due to the pyrolysis of hemicellulose, the density of wood decreases after carbonization. Generally, the density of hardwood decreases by 10% to 12% after carbonization, but the results vary depending on the wood species and carbonization process. In addition, due to the reduction of amorphous regions, the toughness of the wood decreases, making it more brittle than before.

7. Reduced hardness

The hardness of wood decreases by about 3% after carbonization. However, most hardwoods already have high hardness (see Table 1), so even after carbonization, their hardness can still ensure quality when used in ordinary flooring, furniture, wooden doors, stairs, and other products.

Carbonization defects

Because the carbonization process is completed at high temperatures close to the ignition point, and the moisture content must reach the limit of absolute dryness, the control of the carbonization process must be very precise. Slight deviations can cause quality problems such as cracking of the plate surface, bursting of knots, surface contamination, and color discoloration, or even lead to the explosion of the carbonization tank.

Compared with other preservative-treated wood

In Europe and America, the primary use of carbonized wood is in outdoor projects. It is a new type of all-natural, heavily rot-resistant material with irreplaceable advantages. Table 2 compares carbonized ash with common outdoor flooring materials.

Precautions during use

Due to the changes in wood properties caused by carbonization, the following issues should be noted when using carbonized wood, especially carbonized hardwood:

1. Due to the reduced number of hydroxyl groups and a more closed pore structure, carbonized wood exhibits decreased water, oil, and glue absorption capabilities. During gluing, a longer open time should be allowed after applying the glue to ensure sufficient penetration into the wood to form glue joints and guarantee the strength of the bonded surface. During painting, the amount of primer applied should be appropriately reduced to allow each coat sufficient time to penetrate.

2. Due to the increased brittleness of wood, all screws must be pre-drilled to avoid the use of self-tapping screws, which could cause cracking. Countersunk holes should be drilled in the screw head to prevent excessive pressure on the wood surface when screwing in, which could cause chipping. Similarly, during machining, the cutting tools must be sufficiently sharp to prevent edge chipping.

3. The color of carbonized wood will gradually lighten due to ultraviolet radiation, so it must be coated with UV-resistant paint, otherwise it will change color.

Conclusion

As a novel wood processing technology, carbonization perfectly combines the natural properties and performance characteristics of wood, representing a rare technological innovation in this traditional industry. Although the Chinese market is still in its initial stage, based on strong domestic demand for high-end products, it is expected that after one or two years of promotion and education, carbonized wood will enter a period of explosive growth. This will be a new sustainable growth point for our industry, something worth looking forward to and striving for.

Acknowledgments

I would like to thank Dr. Hu Jinbo from the School of Materials Science and Engineering at Central South University of Forestry and Technology. He is an expert in carbonization and provided materials for this article and made patient corrections.