The moisture content and moisture absorption of MDF board
I. The relationship between water content and strength of MDF board Under the same density and other conditions, the change of water content has a significant effect on strength of MDF board, especially on static flexural strength. Such as moisture content in 5%-10%, whenever moisture content increases 1%, its static flexural strength will drop 3%-5% when moisture content exceeds 20%, moisture content increases 5%, its static flexural strength will drop 30%-40% this change characteristics is much larger than wood.
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II. The effect of water content change on tensile strength of MDF board is basically similar to that of static flexural strength, but the change rate is not as large as that of static flexural strength. When the water content increases by 1%, the tensile strength decreases by 2%-3%. When the moisture content of MDF board is 3%-5%, its static flexural strength and tensile strength are the highest. At this point, the water in the MDF board plays a similar role to the binding force between the fibers. However, moisture content exceeding this value will weaken the binding force between fibers.
MDF board moisture content and hygroscopicity Under standard atmospheric conditions (temperature 20℃, relative humidity 65%), the average moisture content of the board varies with density. The average moisture content of dense MDF board is low, and the equilibrium moisture content of hardboard and MDF is 4-8%, and that of soft MDF board is 8-12%. The equilibrium moisture content of MDF board is related to temperature and humidity. At constant temperature, moisture content increases when humidity increases. In constant humidity, its moisture content will decrease with the rise of temperature.
III. Under the condition of constant temperature and humidity, the moisture content of MDF board is higher in the process of dehumidification than in the process of moisture absorption, which is similar to that of wood. Due to the change of humidity, the thickness of MDF board and the length and width of the line expansion or contraction, especially the thickness change is more obvious. But in the process of dehumidification, when the humidity reaches zero, its thickness can not be restored to its original state, this phenomenon is called relaxation phenomenon. The thickness of MDF board increases due to the long drying and wetting cycle, and the density of MDF board decreases.
During the dehumidification of MDF board, the change of length direction is smaller than that of thickness direction, and the change rate of length is in a decreasing trend. This is because the length change tends to decrease due to the long-term dry and wet cycle. Because the length direction change value is small, it has little influence on the directional difference of the MDF board.
The water absorption of MDF board and the density of dry shrinkage MDF board have a certain effect on water absorption. Generally, MDF board has high density and low water absorption, good water resistance and moisture resistance. In the process of absorbing water, the MDF board basically reaches the equilibrium state after 24-48h.
The water temperature is related to the absorption rate of the MDF board. When the water temperature is 15-60 ℃, the absorption rate increases with the increase of the water temperature, especially when the water temperature is 20-50 ℃, the absorption rate of the MDF board is more obvious.
IV. The pH value of water also affects the absorption rate of MDF board. The pH value of the water is alkaline, and the absorption rate of the MDF board is high. When the pH of the water is weakly acidic, the absorption rate of the MDF board is low. When MDF board absorbs water, the thickness expansion rate is proportional to the density. Because the number of fiber cells in MDF board is directly proportional to the density of MDF board, when the water content reaches above the fiber saturation point, the fiber cell wall will no longer expand. Therefore, expansion or contraction due to changes in water content occurs below the fiber saturation point.
V. Therefore, the greater the density of MDF board, its thickness expansion rate also increases. The linear expansion rate of MDF board has little to do with board density. Because of the expansion of MDF board in length direction, it is mainly elongated or shortened along the long axis of fiber cells, and the change amount is very little. At the same time, the longitudinal and transverse directions of the fiber board itself are relatively uniform, and when the fiber board absorbs water, the difference in the longitudinal and transverse direction of the expansion rate is very small. Both dry shrinkage and expansion of MDF board can lead to warping and deformation of MDF board. The fiber board also has directivity when it dries and expands, and the dry shrinkage rate is most obvious along the thickness direction. The horizontal and horizontal direction of the drying shrinkage rate is small, the difference between vertical and horizontal drying shrinkage is also very small.