Thermogravimetric investigation of anisotropy of dimensional shrinkage of softwood and hardwood during carbonization

Thermogravimetric analysis (TGA) was performed on five softwood and five hardwood thin wood samples in the longitudinal (L) and radial (R) directions. Dimensional changes were monitored using a charge-coupled device camera under a nitrogen flow. A comparison of the TG and derivative TG (DTG) curves revealed that shrinkage in the R direction began when the weight was reduced to 79–92% at 305–330 °C and 87–96% at 275–290 °C for softwoods and hardwoods, respectively. Hemicellulose is mainly degraded in this temperature range. In contrast, shrinkage in the L direction started at temperatures close to the DTG peaks, i.e., 360–380 °C and 345–370 °C, respectively, at which temperatures cellulose is mainly degraded. In general, the R/L shrinkage anisotropy was greater for hardwoods than for softwoods, but the species variation was large and the magnitude was directly related to the difference in the shrinkage onset temperatures between the R and L directions, regardless of the wood species. Therefore, shrinkage anisotropy can be attributed to the relative reactivity of hemicellulose and cellulose in wood cell walls. The shrinkage mechanism during carbonization is discussed in terms of the cell wall ultrastructure, in which cellulose microfibrils are covered by a hemicellulose–lignin matrix, and the orientation of the cells in the L and R directions.