Effects of Drying Time on Thermal and Dynamic Viscoelastic Behaviors of Wood at 105`180
Tsunehisa MIKI, Hiroyuki SUGIMOTO, Kozo KANAYAMA, Keisuke KOJIRO, Yuzo FURUTA and Makoto OHKOSHI
Abstract:To investigate the effect of drying time on the thermal behavior as well as the dynamic viscoelastic behavior of wood, thermal analyses such as modulated temperature differential scanning calorimetry (MT-DSC), thermal gravimetry (TG) and dynamic mechanical analysis (DMA) were performed under a similar temperature schedule in which three times cyclical heating and cooling processes between 105 and 180 C was conducted after drying at 105 C for from 15 to 960 min. Results obtained showed as follows. 1) From the TG measurements, mass losses were hardly recognized during drying and heating-cooling processes, whereas heat flows in the first heating process of the DSC measurements showed different behaviors. The heat flow got exothermic around 140 C compared with those of the second and third heating processes. The beginning of the exothermic behavior showed at a higher temperature when the drying time set longer. The release of some amounts of energy seen as an exothermic behavior during the first heating could be believed that finestructure of wood changed to a lower energy level, and it was thought that a longer drying time resulted in a lower energy level in the finestructure of wood which was likely observed in an enthalpy relaxation of a glassy polymer during annealing. 2) Dynamic heat capacities obtained from MT-DSC measurements showed different behaviors in the first heating process, and it was slightly larger than those of the second and third heating processes. For a longer dry time, the dynamic heat capacity was decreased at temperatures lower than about 150 C. This decreased heat capacity suggested a packing of finestructure occurred during the first heating. 3) tanΒ from DMA measurements showed a similar temperature dependency to the dynamic heat capacity, and this supported the finestructure change of wood deduced from the DSC results. Key Words:Thermal behavior, Dynamic heat capacity, Dynamic viscoelasticity, Drying time, DSC, Relaxation phenomena.