Reasons for different shrinkage of wood
The reasons for the difference between longitudinal and transverse shrinkage of wood
The difference between vertical and horizontal wood shrinkage is due to the different structure of wood. In wood, only wood ray cells are arranged horizontally, and most cells are arranged vertically.
The cell wall is dominated by the secondary wall, in which the middle layer is predominant. The drying shrinkage of wood depends on the arrangement direction of the microfibrils in the middle layer of the secondary wall.
The arrangement direction of the microfibrils in the middle layer of the secondary wall is entangled with the cell axis at an angle of 10° to 30°. The arrangement direction of the microfibrils in the middle layer of the secondary wall is basically parallel to the main axis. When the wood loses moisture, the microfibrils move closer and shrink laterally. It is larger than the vertical shrinkage.
Reasons for the difference between chordwise shrinkage and radial shrinkage of wood
1. The thickness of the cell wall in the early and late wood is inconsistent and the arrangement direction of the microfibrils in each layer of the secondary wall is different.
The latewood cell wall is thicker, and the proportion of S2 layer is larger, so the lateral shrinkage is larger.
The cell wall of earlywood is thinner and the proportion of S2 layer is less. Relatively speaking, the effect of S2 and S3 layers is greater, resulting in the lateral shrinkage of earlywood is much smaller than that of latewood.
2. The chordwise shrinkage is mainly affected by the change of late wood
The latewood has enough strength to force the entire growth ring to contract.
The radial shrinkage is different. The shrinkage of each growth ring is the sum of the earlywood shrinkage and the latewood shrinkage, that is, in addition to the latewood shrinkage, it also includes the shrinkage of the earlywood part with smaller lateral shrinkage, so the radial shrinkage is smaller. It also has a great influence on radial drying shrinkage.
3. Some people think that the drying shrinkage of various wood cells is uneven when drying.
The shrinkage of ducts and parenchyma cells was larger in the chord direction than in the radial direction.
The drying shrinkage of wood fibers is approximately the same in all directions.
The width (chordwise) of the wood ray is larger than its length (radial).
4. In the radial direction, due to the restraining effect of the wood ray cells, the radial contraction is restrained, but the chord direction has no such effect, so the chord direction shrinks greatly.
5. The number of pits on the chord wall of many cells is small and the shape is small, which has little effect on the orientation of microfibrils.