Permeability

Abstract

The transport of fluids through wood may be subdivided into two main classifications which will be described in this text. The first is the bulk flow of fluids through the interconnected voids of the wood structure under the influence of a static or capillary pressure gradient. This is sometimes designated as momentum transfer because it can be attributed to a momentum-concentration gradient. The second is diffusion consisting of two types: intergas diffusion, which includes the transfer of water vapor through the air in the lumens of the cells, and bound-water diffusion, which takes place within the cell walls of wood. Some practical applications of bulk flow are the pressure treatment of wood with liquid preservatives for protection against biological decay or fire, the impregnation of wood with monomers for subsequent in situ polymerization in the manufacture of wood-polymer composites, and the impregnation of wood chips with pulping chemicals. Diffusion, on the other hand, occurs during the air-drying or kiln-drying of wood, in the migration of moisture through the wood members of the exterior walls of a building, and in interior woodwork and furniture in response to seasonal changes in relative humidity. The magnitude of the bulk flow of a fluid through wood is determined by its permeability. Diffusion will be discussed in detail in Chapter 6. Another type of diffusion which occurs along with the bulk flow of gases, known as Knudsen diffusion, will be discussed in this chapter.