Influence of habitat, density, lignin structure, and extraction treatment on thermal-softening properties of water-swollen wood: a study of 87 wood specimens

This study aims to reveal the diversity of thermal-softening temperatures and identify the factors that determine this temperature. To achieve this, the thermal-softening properties of the radial direction of wood were measured under water-saturated conditions for 15 softwood and 72 hardwood specimens. Wood samples were obtained from the xylarium of the Forestry and Forest Products Research Institute, Japan. A dynamic viscoelastic measurement was performed on samples with uniform heating and cooling history because the difference in cooling rate can alter in the mechanical properties of wood. The storage and loss elastic moduli increased linearly as wood density increased, regardless of the wood species. However, the thermal-softening temperature (defined in this study as the peak temperature of loss tangent) was unrelated to the density, anatomical features, species, latitude, and annual rainfall in the habitat. When the relationship between thermal-softening temperature and lignin structure was investigated, a negative correlation was observed between the thermal-softening temperature and the syringyl ratio (syringyl/(syringyl+guaiacyl)) of lignin aromatics. This indicates that the thermal-softening temperature is higher for wood species with denser lignin structures, supporting the prior research showed correlation between thermal-softening temperature and methoxyl group content of wood.