Anatomical study of short-term thermo-mechanically densified alder wood veneer with low moisture content

Abstract

The effects of short-term thermo-mechanical (STTM) densification by varying temperature and pressure regimes on the changes in anatomical structure of alder wood (Alnus glutinosa Gaertn.) veneers with low moisture content (~5 %) were investigated. Scanning electron microscopy (SEM) images of oblique cross and radial sections of non-densified and densified alder veneer were analysed. Veneer sheets were densified using pressure levels of 4, 8 and 12 MPa at three temperatures: 100, 150 and 200 °C for a short time of 4 min. The obtained results show that STTM densification of veneers causes irreversible changes in their morphology. Significant deformation of vessels and fibres and appearance of small spherical-like droplets (condensing compounds of lignin and degradation products of hemicelluloses) on the surface of the cell wall as well as pronounced thermo-mechanical wrinkling accompanied by the formation of axial cracks and rippled surface of the cell wall of the vessels were observed at higher temperatures and pressures. SEM images showed that the cell lumens collapsed and a certain amount of fractures in cell walls developed with increasing densification temperature and pressure. Moreover, densification of veneer with low moisture content at high temperatures and pressures causes a considerable fragility and occurrence of thermal erosion of the cell wall of vessel elements and fibres.