ALTERATION OF TEXTURE AND PROPERTIES OF WOOD BY MECHANICAL AND THERMAL STRESSES

It is a well-known fact, that following the laws of mechanics in wood subjected to pressure load, inclined slip planes and fibrillar loosening occur at the moment of failure or shortly before; in literature this is generally accepted. It could now be shown that dynamic bending has a particularly marked effect in this respect and similar observations were made in dynamic crushing tests. The behaviour of wood under universal pressure at elevated temperatures was investigated and the differences in deformation of wooden beads in tangential and radial direction were charted. Microphotographs proved particularly revealing, showing that the major deformations always occur at the periphery and slowly decrease towards the core of the test specimen. Rays were noted to be especially affected. Several simultaneously occurring processes were to be expected in tests at temperatures between 20 and 180° C, e. g. the plastification of the polyoses had to be considered. The Munich Institute has done extensive research since 1963 on the subject of changes in the ultrastructure of wood caused by thermal stress. When heating wood chemical changes take place within the morphological structure which are visible macroscopically. Colour changes are typical of this. The shrinkage caused by a loss of substance on heating wood brings about mechanical changes in the texture and ultrastructure of cells. It was proved that a flowing of the amorphous pit substance occurs at ultra thin cross sections. Higher temperatures influencing the system of cellulose, polyoses and lignin equally affect the molecular and the supra-molecular structure. Under the prevailing test conditions 120°C proved a critical temperature, above which significant changes in the polysaccharides were noted. In the molecular range even lower temperatures were noted to effect changes in the cellulose. Changes in lignin were observed above treatment temperatures of 180° C. The influence of thermal treatment of wood on dry weight, dry dimensions and the sorptional behaviour was also studied for a temperature range up to 200° C. Heat at 70° C had none, at 100° C only very little influence on these wood properties. Rising temperatures peratures led to a decrease in dry weight and dry dimensions, the more so the higher the temperatures and the longer the time of heating. The decrease in dry length was found to be greater in tangential than in radial direction. Also, the sorptional capacity diminuishes in a temperature range between 100 and 180° C. Heat treatment for 6 and 24 hours at 180° C resulted in even greater reductions in sorptional capacity; this influence, however, was smaller after 48 hours' heating than after 24 hours. The stated changes in sorptional behaviour can be explained by the different thermal stabilities and the different sorptional capacities of the main chemical components of wood, i.e. of the polyoses, cellulose and lignin. © 1969 Springer-Verlag.