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

Abstract

Abstract 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.