Effect of Thermal History on Mechanical Relaxations of Dry Wood Determined in Tensile Mode at Temperatures from −136°C to 120°C

Abstract The influence of thermal history on the viscoelastic behavior of Chinese fir wood ( Cunninghamia lanceolata ) was investigated in the temperature range between −136°C and 120°C. The storage modulus ( E ′), loss modulus ( E ′′), and loss factor (tan δ ) of the radial specimens dried over P 2 O 5 were determined in tensile mode at multiple frequencies ranging from 0.5 to 10 Hz. Results showed that the E ′ values of specimens were negatively correlated with the measured temperature. Moreover, δ-relaxation, γ-relaxation, and β-relaxation were detected in the E ′′ or tan δ spectrum. Wood specimens with thermal history exhibited δ-relaxation, with a corresponding temperature range of −129.3°C to −107.4°C, which has not been previously reported or discussed. The γ-relaxation was also influenced by thermal history; its peak temperature shifted to a higher range. Furthermore, clear differences in the viscoelastic behavior of wood were found between cooling and heating runs. A comparison demonstrated that β-relaxation at approximately 31.9°C to 37.6°C was only observed in the heating run. The β-relaxation peak temperature showed no frequency dependence, but the β-relaxation intensity significantly decreased with increasing frequency. In contrast, both the peak temperature and intensity of δ-relaxation and γ-relaxation showed significant frequency dependence. The apparent activation energy of δ-relaxation and γ-relaxation was 37.24 to 49.87 kJ/mol and 59.69 to 74.30 kJ/mol, respectively, which indicated that δ-relaxation was attributable to limited torsional vibration of groups in the amorphous wood cell walls.