Theory of Evaporation of Free Moisture Inside Wood and Research on Drying Wood with Carbon Fiber Electric Heating Tubes

According to the way water combines with materials, drying science divides materials into three categories. The first category is capillary-porous bodies, where dimensional changes are minimal or nonexistent when moisture changes, such as coke, charcoal, soil, and gravel. The second category is colloids, where both size and volume change with moisture content, such as gelatin. The third category is capillary-porous colloids, which are more common and exhibit characteristics of both previous categories. Many objects we encounter daily, such as wood, leather, grains, food, and medicinal herbs, belong to this category.

When drying Rignan wood, it is prone to wrinkling. In the late 1950s, researchers believed this was caused by drying stress, where surface shrinkage was greater than internal shrinkage in the initial stages of drying, resulting in stress inside the wood causing wrinkling in the final product. However, further research revealed that wrinkling is not due to macroscopic drying stress but rather due to microscopic capillary tension. Wrinkling occurs when capillary tension exceeds the transverse compression strength. For example, the compressive strength limit of a certain wood species in its green state (wet state) is 3.59 MPa. Wrinkling may occur if the pore radius is less than 0.04 μm. Therefore, understanding the sudden appearance of capillary tension inside wood due to the expulsion of large capillary water during drying is paramount.

The model of evaporation of free moisture inside wood is quite tedious, so I will not delve into it here. Instead, let’s discuss the mechanism of wrinkling during wood drying. During the expulsion of free moisture from inside wood, it is necessary to use low temperatures in the initial stages of drying. This explains why we generally allow wood to air dry naturally to a certain extent before placing it in drying kilns. Wet wood should not be directly placed into drying kilns as it may lead to cracking and deformation, resulting in losses. The reason for cracking and deformation is that drainage from large capillaries occurs more rapidly at high temperatures, leading to sudden capillary tension inside the wood fibers exceeding the transverse compression strength, resulting in visible deformation and cracking of the wood.

According to the theory that capillary tension causes wrinkling in wood, could replacing water inside wood with liquids having lower surface tension reduce wood wrinkling during drying? Researchers attempted to replace water inside wood with methanol and ethanol, which have lower surface tensions, and observed significant improvement in the wrinkling phenomenon during drying. However, this approach is economically impractical for daily production. Nevertheless, this research result confirms that wood wrinkling is indeed caused by capillary tension inside the wood, providing theoretical basis for subsequent research efforts.

Experimental verification reveals the following pattern of moisture migration inside wood:

1. Free water on the wood surface and water inside cells evaporate completely.
2. Free water in inner cells migrates towards the surface along capillaries.
3. Adherent water on the surface, i.e., water in the pores, also evaporates, reducing surface moisture below the fiber saturation point.
4. Moisture inside wood is higher in inner layers than on the surface, creating a gradient of moisture content from inside to outside.
5. Due to the moisture content gradient, moisture inside wood migrates from higher to lower moisture content areas.

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