Moisture-dependent orthotropic viscoelastic properties of Chinese fir wood in low temperature environment

Abstract

The influence of moisture content (MC) on the orthotropic viscoelasticity of Chinese fir wood (Cunninghamia lanceolata [Lamb.] Hook.) has been examined in low temperature environment. Storage modulus E′ and loss modulus E″ of wood with six different levels of MC ranging from 0.6 to 22.0% were determined from − 120 to 40 °C and at multi-frequency range of 0.5, 1, 2, 5, and 10 Hz using a TA instruments ® Dynamic Mechanical Analyzer (DMA 2980). The results showed that a distinct moisture dependency is exhibited by the orthotropic viscoelastic behaviour of Chinese fir wood. With the exception of some apparent activation energy (ΔE) for β-relaxation process, the E′ decreased and the E″ peak temperatures moved towards lower temperature and the ΔE for α-relaxation process became lower with MC increasing in all orthotropic directions, whereby individual decline of E′ and the E″ peak temperatures were affected by MC to different degrees. Besides, a little E″ peak at around 0 °C was only seen in L direction, which could be attributed to the melting of frozen water. Furthermore, the dynamic viscoelastic behavior of wood is also dependent on the measurement frequency. The findings suggest that the orthotropic structure and moisture content have an important influence on the viscoelastic performance in low temperature environment.

Effects of Moisture on Diffusion in Unmodified Wood Cell Walls: A Phenomenological Polymer Science Approach

“…Further complicating the hemicelluloses glass transition question are DMA experiments performed over temperature and moisture conditions in which the expected hemicelluloses glass transition is not observed. As previously mentioned, Li et al [217] observed hemicelluloses glass transitions when their specimens were oriented such that the tensile stresses were being applied in in radial and tangential wood orientations, but not the longitudinal orientation. Jakes [32] also performed nanoindentation in the transverse plane of the S2 in which the loads are applied parallel to the cellulose fibrils, but no thermal transition was observed in that orientation.…”

“…They did not report any water loss tests for higher temperatures. Both Jiang and Lu [216] and Li et al [217] report that up to 40 °C in their experiments the MC changed less than 0.5%. The specimen cross-section and heating rate are the two most likely experimental parameters that will affect the specimen drying during the temperature scans.…”

“…The specimen cross-section and heating rate are the two most likely experimental parameters that will affect the specimen drying during the temperature scans. Considering the other three DMA studies in Figure 10 all used wood specimens with similar sizes and heating rates that were either the same or faster than Jiang and Lu [216] and Li et al [217], it is reasonable to assume that the reported hemicelluloses glass transitions at or below 40 • C are not appreciably affected by the specimens drying during the experiments.…”

“…The long axis of their bending specimens was in the radial wood orientation, and their reported hemicelluloses glass transition temperatures were assessed at the peaks in loss modulus spectra. Also in 2018, Li et al [217] used tensile mode DMA to measure viscoelastic properties of Chinese fir from −120 • C to 40 • C over a range of frequencies form 0.5 Hz to 10 Hz. Specimens were prepared with the tensile direction in one of the three primary wood orientations.…”

Frequency-dependent viscoelastic properties of Chinese fir (Cunninghamia lanceolata) under hygrothermal conditions. Part 1: moisture adsorption

“…In addition, Backman and Lindberg (2001), and Li et al (2018b) reported a small peak at around room temperature or even below ambient temperatures when the wood MC ranged from around 3 to 14.1%. Li et al (2018b) assumed that this small peak was attributed to the melting of frozen water. Backman and Lindberg (2001) and assumed that the small peak was attributed to the glass transition of hemicellulose.…”