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17.11.2023
This week we look at wood shrinkage and deformation during drying. These clarifications are presented in Technical data sheet number 3.
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This Friday, we're talking about reaction wood.
Wood is a material made up of three types of macromolecules (celluloses, hemicelluloses, lignins) whose combination gives wood a porous structure that allows water to pass between cells. Water is found in wood in 3 different forms: constitutive water (water molecules internal to the cells), bound water (attached to the cell wall by adsorption) and free water (water circulating in the wood's vessels).
When wood is dried, all the free water and some of the bound water must be removed to achieve the right moisture content for use. the first stages of drying will begin by evaporating the free water present in the vessels. Once the free water has evaporated from the vessels, the bound water will gradually leave the wall until the desired moisture content, or anhydrous state, is reached. During the drying process, the characteristic water content when the wood is free of free water and saturated with bound water is called the fiber saturation point (FSP). From this water content to its anhydrous state, wood is in its hygroscopic range. It undergoes dimensional variations and changes in its properties. As the water content decreases, the thickness of the cell wall decreases, causing the wood to shrink.
These deformations of wood during drying are due to the anisotropy of shrinkage. Shrinkage within wood depends on the direction in which it is viewed. In fact, depending on the wood's three directions (radial, tangential and longitudinal), it deforms differently and irregularly. Generally speaking, radial shrinkage is less important than tangential shrinkage (longitudinal shrinkage is not taken into account, as it is negligible), causing the wood to distort during drying. This difference between radial and tangential shrinkage also explains why cracks can appear radially in a wooden slice during drying, since the periphery of the section (tangential direction) tends to shrink more than the radius (radial direction).
Sheet number 3 on shrinkage presents mathematical formulas and the various deformations of cross-sections. To complete it, you'll also find a few bibliographical references at the end of this newsletter for further reading on the subject.
Le bois, un matériau hygroscopique - Delphine JULLIEN
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