What is the fiber saturation point?
What is the effect of the fiber saturation point on the shrinkage of wood in the different directions?
The correct answer and explanation is :
The fiber saturation point (FSP) refers to the moisture content at which the cell walls of wood are fully saturated with water, but the cavities inside the cells (the lumens) are empty. At this point, the wood has absorbed as much water as it can into the cell walls, and the wood is considered to be in a state of equilibrium with its surrounding environment in terms of moisture content. Typically, the FSP occurs at around 30-35% moisture content for most species of wood, though it can vary slightly depending on the type of wood.
Effect of Fiber Saturation Point on Shrinkage:
The fiber saturation point plays a crucial role in understanding how wood behaves as it dries. When wood loses moisture below the FSP, significant shrinkage occurs due to the loss of water from the cell walls. Shrinkage is the reduction in volume of the wood, which primarily happens as a result of the wood’s cell walls contracting when moisture is lost.
The shrinkage of wood can occur in three different directions:
- Radial direction (from the center of the tree to the bark): Shrinkage in this direction is the smallest and occurs when the moisture content decreases below the FSP. Radial shrinkage is typically around 4-6%.
- Tangential direction (around the circumference of the tree): Shrinkage in this direction is the largest because the tangential cell structure is more prone to changes in moisture content. The tangential shrinkage can range from 7-12%.
- Longitudinal direction (along the length of the tree): Shrinkage in this direction is minimal and usually less than 1%. This is because the wood fibers are oriented parallel to the length, and moisture loss does not significantly affect the volume along this axis.
In Summary:
- Below the fiber saturation point, wood shrinks as moisture is lost from the cell walls.
- The greatest shrinkage occurs in the tangential direction, followed by the radial direction, with little shrinkage in the longitudinal direction.
- The fiber saturation point marks the transition from a saturated state to one where the wood experiences dimensional changes due to moisture loss.