Responses Mechanism Of Wood Viscoelasticity During Moisture Adsorption And Desorption Processes

Moisture plays an important role in wood formation and utilization processes. Fully investigation of the relationship between wood and moisture is an essentially basic research filed in wood science. In this study, the changes of wood viscoelasticity during moisture adsorption and desorption processes were systematically investigated, and the response mechanism of wood viscoelastic properties to hygrothermal environment was investigated as well. This study provided theoretical basis and scientific instructions for thermos-hydro and thermos-hydro-mechanical technology of wood, and had important theoretical and practical values.In this study, the time-dependent and frequency dependent viscoelasticity of Chinese fir(Cunninghamia lanceolata [Lamb.] Hook.) were examined by dynamic mechanical analysis(DMA Q800), and the visualized network of wood viscoelastic properties under hygrothermal environmental were built. At six constant temperatures(30, 40, 50, 60, 70 and 80°C), the influecnes of RH ramping rate and isohume level on viscoelasicity were analyzed during moisture adsorption and desorption processes, respectively. In addition, the frequency-dependency viscoelasticity ranged from 1 to 50 Hz were tested during moisture adsorption and desorption processes. The mechano-sorptive(MS) effect under non-equilibrium moisture state was analyzed by the time dependent wood viscoelasicity. The time-moisture superposition principle(TMSP) of dynamic stiffness and the accelerated mechanism of relaxation behavior of wood polymers uner non-equilibrium moisture state were discussed by the frequency dependent wood viscoelasicity.The major achievements of this study were summarized as follows:1. The plasticization effect during moisture adsorption process decreased dynamic stiffness and increased damping. The reformed hydrogen bonds(RHB) effect during moisture desorption process increased dynamic stiffness and decreased damping. The MS effect duirng moisture adsorption and desorption processes and the heating effect casued the increasing damping.2. During moisture adsorption process, the lower relativei humidity(RH) ramping rate or higher isohume level, the higher the influence of plasticization effect, which caused more dynamic stiffness and less damping. When temperature was 30°C, RH ramping rate decreased from 2.0 to 1.0 or 0.5 % RH/min, the end value of normalized storage modulus(n E′) decreased from 0.93 to 0.91 and 0.90, respectively, the end value of normzalied loss modulus(n E′′) increased from 1.85 to 2.01 and 2.02, respectively. When isohume level increased from 30 to 60 or 90% RH, the end value of n E′ decreased from 0.97 to 0.96 and 0.89, respectively, and the end value of nE′′ increased from 1.05 to 1.07 and 1.17, respectively.3. During the moisture desorption process, the lower RH ramping rate or the low isohume level, the higher the influence of RHB effect, which caused less dynamic stiffness and more damping. When temperature was 30°C, RH ramping-down rate decreased from 2.0 to 1.0 or 0.5 % RH/min, the end value of n E′ increased from 1.05 to 1.10 and 1.17, respectively, the end value of nE′′ decreased from 0.93 to 0.82 and 0.80, respectively. When isohume level decreased from 60 to 30 or 0% RH, the end value of n E′ increased from 1.05 to 1.09 and 1.13, respectively, the end value of nE′′ decreased from 0.87 to 0.84 and 0.81, respectively.4. The ratio of loss modulus at 1 and 20 Hz could characterized the changes of MS effect during moisture adsorption and desorption processes. Under RHramp or RHramp-down periods, the MS effect increased gradually. Increasing RH ramping or ramping-down rate would aggravated the MS effect. Under RHisohume period, the MS effect decreased and decreasing isohume level would diminished the MS effect. With the analysis between the ratio of loss modulus at 1 and 20 Hz and the difference MC, the therortical residual instability of wood cell wall could be estimated. During moisture adsorption process, the higher the isohume level, the greater the therortical residaul instability, Duirng moisture desorption process, the less the isohume level, the greater the therortical residual instability.5. Elevating temperature would accelerate the moisture adsorption and desorption rate, thus enhance the plasticization effect and RHB effect. As for moisture adsorption process, during RHramping period from 0 to 90 % RH with a ramping rate of 2.0% RH/min, when temperature increased from 30 to 80°C, the end value of nE′ decreased from 0.93 to 0.75, and the end value of nE′′ increased from 1.85 to 2.49. During RHisohume period at 90 % RH, when temperature increased from 30 to 80°C, the end value of n E′ decreased from 0.89 to 0.68, and the end value of n E′′ increased from 1.17 to 1.46. As for moisture desorption process, during RHramping-down period from 85 to 0 % RH with a ramping rate of 2.0 % RH/min, when temperature increased from 30 to 80°C, the end value of n E′ increased from 1.05 to 1.25, and the end value of nE′′ decreased from 0.93 to 0.86. During RHisohume period at 0 % RH, when temperature increased from 30 to 80°C, the end value of n E′ increased from 1.13 to 1.17, and the end value of nE′′ decreased from 0.81 to 0.74.6. Under constant temperature ranged from 40 to 60°C, the softening of hemicellulose could be observed during moisture adsorption process. Under constant temperature at 70 or 80°C, the softening of lignin could be observed during both moisture adsorption or desorption processes. Elevating temperature or RH level would shortened the softening time of hemicellulose and lignin.7. The higher the testing frequency, the higher the storage modulus. Increasing moisture content(MC) and decreasing frequency had equivalence on changes of storage modulus. Under constant temperatures from 30 to 80°C, as for moisture adsorption process, the mater curves could be established at reference MC of 0.6% among MCs from 0.6 to 18.3%. The spanning frequency ranged from 1011~1017 Hz, as for moisture desorption process, the mater curves could be established at reference MC from 19.0 to 22.2% among MCs from 3.6 to 22.2%. The spanning frequency ranged from 1016~1024 Hz. During moisture adsorption and desorption processes, the relations between shift factor and MC at each isotherm temperature satisfied the re-written WLF equation, confirming the validity of TMSP on dynamic stiffness. In addition, the secondary master curve of storage modulus between 30 to 80°C during moisture adsorption process was established, confirming the validity of time-moisture- temperature superposition for dynamic stiffness.8. The loss storage modulus decreased as first and then increased with the increasing frequency. The character frequency according to the minimum value of loss modulus was attributed to the transition of the α-relaxation process(attributed to the glass transition of hemicellulose) and the β-relaxation process(assigned to the reorientation of the methylol groups in amorphous wood cell walls and the reorientation of adsorbed water molecules). The traisition of these two relaxation processes moved to higher frequency with increasing MC. MS effect would shorten the relaxation time of wood polymer.9. The visualized network composed by temperature, RH and viscoelastic parameters could be divided into four layers. The establishment of visualized network confirmed that temperature had equal effect to wood elastic and damping, and RH had prevailed effect on the later one.