The moisture content of wood is an important indicator for investigating plywood properties. Qualified boards have strict regulations on moisture content, etc. Failure to meet the standards may cause the boards to warp and crack.
The moisture in wood can be divided into three categories: free water, adsorbed water and chemical water.
Free water refers to the water that exists in the cell cavity and interstitial space, also known as capillary water; sorption water refers to the water that exists between the microfibrils of the cell wall, also known as attached water, sorption water or bound water; combined water refers to the existence is the moisture in the chemical composition of wood.
Free water and absorbed water are the main moisture in wood. Free water is closely related to wood density, burning, drying, and penetration; absorbed water affects the properties of wood; combined water has little to do with wood properties.
wood moisture content is the ratio of the wood moisture content to the weight of the wood. The moisture content of wood is divided into fiber saturation point (moisture content), raw wood moisture content, wet wood moisture content, equilibrium moisture content, kiln dry moisture content, absolute dry wood moisture content, etc. Among them, fiber saturation point, raw material moisture content and equilibrium moisture content are more important.
Fiber saturation point
The moisture content of the wood cell wall is in the saturated state, the moisture content of the cell wall (adsorbed water) is in the saturated state, and the moisture content of the cell cavity without free water is called the fiber saturation point or fiber saturation. The fiber saturation point varies with different tree species, generally 23%-31%, usually 30% is the average value of the fiber saturation point of each tree species. The fiber saturation point of green poplar wood is 23%.
The change of fiber saturation point is the turning point of the change of wood properties. Above the fiber saturation point, the strength properties of wood remain unchanged; below this point, the strength of wood increases due to the decrease in moisture content, and vice versa, it decreases due to the increase in moisture content until it reaches the fiber saturation point. Similarly, when the moisture content of the wood is above the fiber saturation point, the wood does not shrink or expand; when the moisture content drops below the fiber saturation point, the wood shrinks with the decrease in the moisture content, and when it decreases to zero, the shrinkage reaches the maximum; On the contrary, it expands with the increase of the moisture content until the moisture content of the fiber saturation point is reached.
Moisture content of raw wood
The moisture content of freshly felled trees refers to the moisture content of freshly felled wood. The moisture content of raw wood has a certain significance for the transportation and storage of wood. The moisture in raw wood is mainly free water and absorbed water, and its moisture content varies with the seasons, but there is no certain rule for each tree species. Generally, there are more in winter and spring when the sap flows, which can reach more than 80%-100% of the weight of the whole tree.
Equilibrium moisture content
Wood stored in the atmosphere for a long time is called air-dried wood. When the water vapor pressure in the air is greater than the water vapor pressure on the surface of the wood, the wood absorbs water from the outside; on the contrary, the water in the wood is lost. If the water vapor pressure in the air is the same as the water vapor pressure on the wood surface, the wood neither absorbs nor loses moisture. At this time, the moisture content of the wood is called the equilibrium moisture content. The equilibrium moisture content of wood varies with the state of the surrounding air. If the actual moisture content of the wood is less than the equilibrium moisture content, the wood will exhibit moisture absorption; otherwise, the wood will exhibit evaporation.