Response Mechanism Of Temperature, Time And Frequency On Dynamic Viscoelastic Properties Of Wood

In order to investigate the dynamic viscoelasticities of wood, and provide theoretical basis and scientific instructions for drying technology, manufacture processing and high value-added materials of plantation wood, the dynamic viscoelasticity of Chinese fir (Cunninghamia lanceolata [Lamb.]Hook) wood under temperature - time - frequency coupling was systematically investigated in this current study.Firstly, the critical strain of wood linear viscoelastic region was determined at different temperatures, therefore, some valuable information concerning the changes of critical strain with temperature and measurement frequency could be obtained. Secondly, the dynamic viscoelastic properties in longitudinal, radial and tangential directions were also investigated, and the differences in wood anisotropic behaviour of viscoelasticity were discussed. Then, this dissertation systematically described and analysed the dynamic mechanical temperature/ time/frequency spectrum of wood. In the investigation of dynamic viscoelastic properties during temperature ramping process, the storage modulus, loss modulus and loss factor of specimens were measured. Thus, the changes of mechanical relaxation behavior during temperature ramping process can be clarified. Furthermore, the dynamic viscoelastic behavior of three kinds of dried wood was also investigated. As a result, the effects of heating/drying history on wood structure and properties were obtained. With respect to the dynamic viscoelastic properties under different constant temperatures, the dynamic mechanical behavior during isothermal processes at various constant temperatures was analyzed. Furthermore, the relationship between dynamic viscoelastic properties of wood over temperature and heating time were obtained. Moreover, the influence of frequency on wood viscoelasticity under two types of heating conditions was also investigated.In the investigation of the validity of wood TTSP (Time-Temperature Superposition Principle), a master curve was generated, and the shift factors were analyzed as a function of temperature to evaluate the fit of the data to the WLF equation and Arrhenius relation. As a result, the applicability of TTSP that predicts the stiffness and damping of wood over broad frequency scales were verificated and analyzed. The major achievements of this study were summarized as follows:1. Under constant temperatures ranged from -120 to 220oC, the critical strain value of linear viscoelastic region are between 0.03% and 0.19%, and it generally reduced with increasing temperature except temperatures of -80, -20, 40, 120 and 220oC. These five exceptions were reasoned by the occurrence of relaxation processes. With the increasing of applied frequency, the critical strain presented a smoothly decreasing tendency.2. The specimens oriented parallel to the grain presented the highest storage modulus, and the storage modulus was much lower in the tangential direction than that in the radial direction. Two relaxation processes were observed for all of longitudinal sample (L), radial sample (R) and tangential sample (T). While L, R and T samples differed in loss peak temperatures. The L sample showed a lower loss peak temperature than that for the R and T sample, and it was in conflict with polymer composites where the higher loss peak temperatures were found in the stiffer direction. The rheological properties of wood showed a dependence upon the mechanical modes used during experiments.3. During the temperature ramping process at temperature range from -120 to 280oC, the storage modulus decreased with the increase of temperature. Four relaxation processes were detected in this temperature range:â‘ the relaxation process at about -100 to -80oC, which was most probably attributed to the motions of methyl groups in amorphous region of wood cell wall (absolutely dry state) and the motions of absorbed water molecules in wood (water existence state);â‘¡the relaxation process at about 0 to 40oC, which was presumably assigned to glass transition of hemicellulose with low molecular weight;â‘¢the relaxation process at around 90oC, which was attributed to molecular motion of lignin, andâ‘£the relaxation process at around 240oC, which was assigned to micro-Brownian motion of cell-wall polymers in the non-crystalline region. With the increase of moisture content, the loss peak temperatures of relaxation processes shifted to lower temperature range. The apparent activation energy of relaxation process at higher temperature location showed a higher value than that at lower temperature location.4. Three kinds of dried wood and control wood differed in the dynamic stiffness properties, the loss peak temperatures, the loss peak density and the apparent activation energy of relaxation processes. 115oC dried wood displayed more stiffness than the other two kinds of dried specimens as well as the untreated wood. It was probably due to the crosslinking action or cellulose crystallization during drying process. Damage to the wood cell walls during the freeze vacuum drying process probably caused the lowest stiffness of vacuum-freeze dried wood. Furthermore, the moisture adsorption capability and adsorption state were altered by different heating/drying history. With increasing moisture content, the differnence in dynamic viscoelastic properties was reduced among dried and control wood.5. Under constant temperatures ranged from 25 to 220oC, the temperature and heating time mainly resulted in the reduction of wood stiffness, thermal softening and thermal degradation of wood. Elevation of temperature or prolongation of heating time has the equal effects on wood stiffness. Lignin softening was suggested to be the cause of the relaxation process that appeared at temperatures of 140, 160 and 180oC. The relaxation time for glass transition of lignin could be shortened by elevating temperature. At temperatures above 180oC, the loss of amorphous polysaccharides due to degradation was considered to be the main factor affecting wood viscoelasticity.6. Under constant temperatures ranged from 25 to 220oC, storage modulus exhibited lower values at higher temperatures and decreased gradually as frequencies decreased from 100 to 0.1 Hz for specimens acclimated at constant temperatures. The minimum value of the loss factor slightly shifted towards higher frequencies at higher temperatures. After suffered rapid heating process, the dynamic stiffness and damping of wood showed decreasing and increasing, respectively. It was suggested that the unstable structures were formed in the cell walls because of rapid heating and consequently increased the mobility of molecular chains.7. The time-temperarture superposition principle (TTSP) could be used effectively to demonstrate frequency-temperature equivalence for the dynamic stiffness property of wood with minor moisture content (about 0.6%) over a temperature range of 25 to 150oC. While it was failed in predicting the relaxation transition behavior of wood over broad frequency scales.