STUDIES ON THE SOFTENING POINT OF WOOD POWDER AS A BASIS FOR UNDERSTANDING THE RELEASE OF RESIDUAL GROWTH STRESSES IN LOGS

In a previous report, log air heating by smoking was found to reduce t he residual stresses in fresh logs of larch, fir, Japanese cedar, oak, and ash. The residual stresses, evaluated by the released strain, wer e diminished in normal wood and reaction wood. To understand the mecha nism of stress releasing, changes in the structure and chemical compos ition related to the thermal softening point of wood powder of these s pecies were studied. The hemicellulose fraction was observed to be red uced because of the heat treatments done, as verified by the sugar ana lysis. The thermal softening point was determined, and differences in the temperature of softening were found in untreated wood powder, amon g the species studied. The temperature of wood softening was always in creased, after the wood powder received a heat treatment over 100 degr ees C. The results showed that hemicellulose was the main wood compone nt influenced by the conditions of treatment. The higher thermal softe ning point of wood observed can be attributable to the degradation of the hemicelluloses. The release of residual stresses in logs is trigge red by the heat treatment, which causes molecular motion. The hygrothe rmal treatment first affects the less stable elements in the wood (hem icellulose and lignin). The degradation of hemicellulose results in so ftening of the wood and has an important effect in the release of resi dual stresses.