A model of anisotropic swelling and shrinking process of wood - Part 2. A simulation of shrinking wood

To elucidate the origin of the shrinking anisotropy of wood during the dryi ng process, as well as to begin to gain an understanding of the interaction between the moisture and the cell wall components, the shrinking process o f a single wood fiber regarding water desorption was simulated by using an analytical model which was developed in the previous report (Part 1). Resul ting data were compared with the experimental ones in this paper. The follo wing conclusions were obtained: (1) The matrix substance, as a skeleton in the secondary wall, tends to shrink isotropically. However, the cellulose m icrofibrils, as a rigid framework of the cell wall, almost did not shrink a t all due to the water desorption. As result, wood shrinks anisotropically during a drying process. The microfibril angle in the S2 layer is one of th e most important factors related to the degree of shrinking anisotropy of t he wood while drying. (2) According to the simulation, the expansive strain caused in the matrix skeleton by the water sorption increases by 15% (= 15 0,000 micro-strains) from the oven-dried condition to the green condition. Based on this value, the moisture content at the fiber saturation point is calculated to be about 35%, which is close to the experimentally obtained o ne. These results give quantitative evidences that the hygroexpansion of th e wood cell wall is controlled by the mechanism of the reinforced matrix hy pothesis.