Girma Kifetew

Shape stability of modified engineering wood product subjected to moisture variation

Abstract A modified softwood product would enable the utilization of softwood in new areas. Densification is an old modification method to improve wood properties such as hardness and resistance to abrasion. A major problem with densified wood is, however, its ability to retain its original dimensions under the influence of moisture. Therefore, this study investigated the influence of surface to bottom layer thickness ratio on the shape stability of a modified and three-layered cross-laminated engineering wood product (EWP) subjected to moisture variations. The study describes a simple solid wood densification technique based on compressing a clear solid piece of softwood with vertical annual rings in the radial direction by restraining the tangential expansion. Three-layered cross-laminated EWP was manufactured with the densified wood as a surface layer. The recovery of the densified wood in the surface layer was then reduced to movements in the same level as the other two layers of unmodified wood. The EWP was subjected to climatic variations in order to investigate its shape stability. The results disclosed that an appreciable degree of shape stability was obtained by an increase in the surface to bottom layer thickness ratio of the EWP.

Effects of cell wall ultrastructure on the transverseshrinkage anisotropy of Scots pine wood

Abstract A hypothesis for explaining the differential anisotropic shrinkage behavior of wood has been proposed, and it was based on the differences in the cell wall ultrastructure. The starting point of the consideration is that wood shrinkage is governed by its chemical composition, ultrastructure, and gross anatomy. It is also well known that the transverse shrinkage anisotropy of earlywood (EW) is more pronounced than that of the latewood (LW). In the paper, the cell wall ultrastructure and shrinkage anisotropy has been related to each other, and to this purpose, a set of crystallographic texture descriptors was applied. The descriptors are based on X-ray diffraction (XRD) experiments conducted on matched EW samples from different growth rings of Scots pine. The range of the microfibril angle (MFA) was identified. The ratio of the maxima of inverse pole figures (IPFs) of both the tangential (T) and radial (R) directions was determined. The ratios quantify the inhomogeneity of the spatial arrangement of the ordered areas. The results of the study clearly indicate that the transverse shrinkage of wood is governed mostly by a specific ultrastructural organization of moderately organized cell wall compounds.