Mechanical and physical properties of semi-isostatically densified wood

When wood is densified through semi-isostatical compression in a Quintus- press at pressures up to 140 MPa, the material properties change. The cells are flattened, size is decreased and shape is changed, as a consequence of this the density is increased. Most properties of native woods are strongly correlated to the density. This is also true for densified wood. To understand the compression mechanisms plastic and elastic strains were studied at different pressures. Strength, density, anatomy and swelling were studied. Some of the methods used were: image analysis, computer tomography scanning (CT), scanning electron microscopy (SEM) and mechanical testing. Data was statistically analysed by linear regression and multivariate statistical methods. A big advantage of using semi-isostatic pressure is that wood of all dimensions, with knots and anomalous wood can be compressed without major checking. As the pressure is mediated through a flexible diaphragm the density becomes homogenous. Plain-sawn wood with inside-face to press-table gets the most homogenous density and the most rectangular shape. Strength is improved by the densification, especially the hardness, the bending and the axial compression strength. At water-soaking densified wood, the cell-shape recovers almost completely. This indicates the non-destructive character of the process. The swelling pressure, that develops when densified wood is restrained from dimensional change and then water-soaked, is more than twice as high as for native wood. The swelling can be reduced by deep impregnation with oil in combination with a surface lacquer.Godkänd; 2006; 20061130 (pafi)