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Moisture content and water absorption of medium density board

Moisture content and water absorption of medium density board

Relationship between moisture content and strength of density board

When the density and other conditions of density board remain unchanged, the change of moisture content has a significant impact on the strength, especially on the static bending strength. If the water content is 5% – 10%, the static bending strength will decrease by 3% – 5% whenever the water content increases by 1%. When the water content exceeds 20%, the static bending strength decreases by 30% – 40% for every 5% increase in water content. This variation is much greater than that of wood.

The influence of the change of the moisture content of the density board on the tensile strength is basically similar to the static bending strength, but the change rate is not as large as the static bending strength. When the moisture content increases by 1%, the tensile strength will decrease by 2% – 3%.

When the moisture content of density board is 3% – 5%, its static bending strength and tensile strength are the highest. At this time, the moisture in the density board plays a similar role to the binding force between fibers. However, the moisture content exceeding this value will weaken the adhesion between fibers.

Moisture content and hygroscopicity of density board

Under standard atmospheric conditions (temperature 20 ℃, relative humidity 65%), the average moisture content of the board varies with the density. The average moisture content of density board with high density is low. The equilibrium moisture content of hard density board and medium density fiberboard is 4% – 8%, and that of soft density board is 8% – 12%.

The equilibrium moisture content of density board is related to temperature and humidity. In the constant temperature state, when the humidity increases, the moisture content increases. In the constant humidity state, the moisture content will decrease with the increase of temperature.

Under the condition of constant temperature and humidity, the moisture content of density board in the dehumidification process is higher than that in the moisture absorption process. This hysteresis phenomenon is similar to that of wood.

Due to the change of humidity, the thickness of the density board and the linear expansion or contraction in the length and width directions, especially the thickness change is more obvious. However, in the process of dehumidification, when the humidity reaches zero, its thickness cannot be restored to its original state, which is called relaxation. Due to the long-term dry wet cycle, the thickness of the density board increases, which will also reduce the density of the board.

In the dehumidification process of density board, the change in length direction is smaller than that in thickness direction, and the length change rate is in a downward trend. This is because the long-term dry wet cycle leads to a tendency to reduce the length change. Because the length direction change value is small, it has little impact on the directional difference of the density board

Water absorption expansion and drying shrinkage of density board

Density of density board has certain influence on water absorption. Generally, fiberboard has high density, low water absorption and good water and moisture resistance. During the water absorption process, the density board basically reaches the equilibrium state after 24 ~ 48h, as shown in Figure 1-5. The water temperature is related to the water absorption rate and water absorption rate of the density board. When the water temperature is 15 ~ 60 ℃, the water absorption rate increases with the increase of water temperature, especially when the water temperature is 20 ~ 50 ℃.

The pH value of water will also affect the water absorption of the density board. The pH value of water is alkaline, and the water absorption of density board is high. When the pH value of the water is weakly acidic, the water absorption of the density board is low.

The thickness expansion rate of density board when absorbing water changes in proportion to the density. Because the number of fiber cells in the density board is in direct proportion to the density of the density board, when the moisture content reaches above the fiber saturation point, the fiber cell wall will no longer expand. Therefore, the expansion or contraction caused by the change of moisture occurs below the fiber saturation point. Therefore, the greater the density of the density board, the greater the thickness expansion rate.

The linear expansion rate of density board has little relationship with the density of board. Due to the expansion in the length direction of the density plate, it is mainly elongated or shortened along the long axis of the fiber cells, and its change is very small; At the same time, the longitudinal and transverse arrangement of the fibers of the density board itself is relatively uniform. When the density board absorbs water, the difference between the longitudinal and transverse expansion rates is very small.

The shrinkage and expansion of the density board will lead to the warpage and deformation of the density board. The drying shrinkage of density board has the same directionality as that of expansion, and the drying shrinkage rate along the thickness direction is the most obvious; The shrinkage rate in the horizontal and vertical directions is small, and the difference between longitudinal and horizontal shrinkage is also small.

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