Abstract
The shrinking and swelling of wood due to moisture changes are intrinsic material properties that control and limit the use of wood in many applications. Herein, hygroscopic deformations of normal and compression wood of Chinese fir (Cunninghamia lanceolata [Lamb.] Hook.) were measured during desorption and absorption processes. The dimensional changes were observed in situ by an environmental scanning electron microscope and analyzed at different hierarchical levels (tissue, cell and cell wall). The relationship between moisture variation and hygroscopic deformation was measured. During initial desorption periods from 95 to 90 or 75% RH, an expansion of the lumen and a shrinkage of the cell wall were observed, revealing a non-uniform and directional deformation of single wood cells. The variation of shrinking or swelling at different hierarchical levels (tissue, cell and cell wall) indicates that the hygroscopic middle lamella plays a role in the deformation at the tissue level. Higher microfibril angles and helical cavities on the cell wall in compression wood correlate with a lower shrinking/swelling ratio. Normal wood showed a more pronounced swelling hysteresis than compression wood, while the sorption hysteresis was almost the same for both wood types. This finding is helpful to elucidate effects of micro- and ultrastructure on sorption. The present findings suggest that the sophisticated system of wood has the abilities to adjust the hygroscopic deformations by fine-tuning its hierarchical structures.
Highlights
In the living tree, wood is in its fully hydrated state
It is widely accepted that the swelling properties of wood depend both on density and on the cellulose microfibril angle
Compression wood, which is known for its larger density (Timell 1986), showed smaller differences between early- and latewood with a cell wall proportion of 24.8% in early- and 44.2% in latewood
Summary
Wood is in its fully hydrated state. After harvest, it undergoes drying and the intrinsically hygroscopic material is commonly subjected to variations in the environmental conditions, such as relative humidity (RH) during its use. It swells and with desorption it shrinks (Patera et al 2013). The dimensional change of cell walls is directional and depends on the orientation of the cellulose fibrils in the cell walls since their lengths do not undergo large changes upon water uptake. The rate of shrinking or swelling depends on the length scale, and it has been shown on micropillars of the S2 layer that cell wall swelling exceeds the swelling of the cell and tissue (Chen et al 2020; Rafsanjani et al 2014). Honeycomb cellular solids with homogenous cell walls deform isotropically, while honeycombs with a layered cell wall structure display anisotropic swelling behavior (Arzola-Villegas et al 2019; Rafsanjani et al 2013). In a tissue such a twist of a single fiber is constrained by adjacent fibers
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