Main content area

Influence of hygrothermal condition on dynamic viscoelasticity of Chinese fir (Cunninghamia lanceolata). Part 1: moisture adsorption

Zhan, Tianyi, Jiang, Jiali, Lu, Jianxiong, Zhang, Yaoli, Chang, Jianmin
Holzforschung 2018 v.72 no.7 pp. 567-578
Cunninghamia lanceolata, adsorption, cell walls, equilibrium moisture content, glass transition, lignin, relative humidity, temperature, viscoelasticity, wood, xylan
The dynamic viscoelasticity of Chinese fir was investigated by dynamic mechanical analysis (DMA), while the influence of the moisture adsorption (MAdₛ) was the focus of the study. The specimens were climatized at 30, 40, 50, 60, 70 and 80°C under two relative humidity (RH) modes. The RHᵣₐₘₚ mode included 0, 30, 60 and 90% RH with varied ramping rates (0.5, 1.0 and 2.0% RH min⁻¹), while the RHᵢₛₒₕᵤₘₑ method worked with constant RHs of 30, 60 and 90%, respectively, for 60, 120 or 240 min. During the MAdₛ process, a decrease in stiffness and an increase in damping of the wood were seen and were attributed to the combination of the plasticization, mechano-sorptive (MS) effects and heating. High temperatures accelerate the MAdₛ pace and intensify the plasticization and MS effects. In RHᵣₐₘₚ experiments, the glass transition RH (RHg) of xylan was visible, as determined by the maximum of damping. The values of RHg varied from 60 to 90% at 50°C or above. In the RHᵢₛₒₕᵤₘₑ mode, the thermal softening of hemicelluloses and lignin occurred for shorter time periods under more severe hygrothermal (HT) levels between 40–60°C and 70–80°C, respectively. During MAdₛ, the MS effect diminished as the wood approached a new equilibrium moisture content (EMC). The MAdₛ times to new EMCs became shorter at the expense of intensifying the unstable arrangement of the wood cell wall. Severe HT levels led to great residual instabilities in the wood cell wall, which could be evaluated by the relationship between the extent of the MS effect and the difference between the MC and EMC.