Determination of the Fiber Saturation Point of Wood By Centrifuging

Tests studied the feasibility of determining the fiber saturation point of wood by centrifuging green wood samples in closed tubes by the action of centrifugal force. It is believed the free water held in the microscopic capillaries (cell cavities and intercellular spaces) is removed by the action of centrifugal force, while the moisture which is absorbed in the submicroscopic capillaries of the cell wall is held there by forces greater than those produced by the action of the centrifuge. Samples of sapwood, of white spruce (Picea lauca) and of red pine (Pinus resinosa)were cut from green water logged material. The samples were cubes approximately 1/2 by 1/2 by 1 inch. The centrifuge tubes were held in rotors at angles of 26 or 55 degrees to the axis of rotation. The stainless steel tubes used were furnished with expansion type caps which could be hermetically tightened. Centrifuges between 130 to 36,000 times gravity were used; during the tests at higher acceleration, high speed centrifuges with angular rotors were used. The centrifuging period was 2 to 5 hours. Test results showed that a force of at least 10,000 times gravity was required to reduce the moisture content of wood samples to a characteristic reproducible value. In the range of centrifugal force of 400 to 500 times gravity, the amount of water removed from the samples was twice as much where the force acted in the direction of the grain as when the force acted across the grain. The difference between the radial and tangential directions was relatively small. Results showed that a) when centrifuging waterlogged wood samples in closed tubes at accelerations upwards of 10,000 times gravity, the moisture content will be reduced to an amount that appears to be characteristic of the specimen, b) when the relative centrifugal force is increased within certain limits in these experiments, this characteristic value of the moisture content value of wood will remain practically unchanged, and c) when the time of centrifuging is freely prolonged this characteristic value of moisture content will remain unchanged. The fiber saturation point values of normal wood of red pine averaged approximately 35.5 percent, varying in individual samples generally in the range of ? 0.5 percent. Test results showed also that because of lower permeability the free water could not be removed by centrifuging as easily from heartwood specimens as from those cut from sapwood of the species. The longest centrifuging period applied to heartwood samples was 5 hours at 10,000 to 35,000 times gravity, which proved to be inadequate to produce equilibrium conditions in the wood.

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