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Thermally modified timber (TMT) reduces hygroscopicity, increases dimensional stability, and decay resistance. Although there is a substantial amount of research on heat-treated wood, investigations on the sensitivity of cell wall swelling behavior to water variation are limited. Therefore, this study focused on the swelling behavior of TMT from cellular and chemical perspectives. The effects of heat treatment on the hygroscopicity and swelling behavior of the wood cell walls were studied using dynamic vapor sorption and confocal laser scanning microscopy. Changes in the chemical composition and cellulose crystallinity were studied using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and X-ray diffraction. The results revealed that the hygroscopic swelling of latewood significantly decreased with increasing treatment temperature, reducing the anisotropy of the wood cell swelling. The hygroscopicity of wood was reduced by heat treatment, and a clear decrease in hysteresis was observed at elevated treatment temperatures. The FTIR spectra indicated that both hemicellulose and lignin were degraded. The crystallinity and lateral size of the cellulose increased after heat treatment. Similar changes in the chemical composition and crystallinity of cellulose were observed in the latewood and earlywood. [ABSTRACT FROM AUTHOR]

The surface checks or internal cracks from drying could significantly affect the processing and utilization of the wood. Therefore, it is of great significance to investigate the formation mechanism of cracks and reduce its occurrence. In this work, the development of cracking was quantified and its relationship with moisture content (MC) gradient and anisotropic shrinkage was analyzed. Results showed that the slight shrinkage occurred at the MC of 40%, and the shrinkage ratio at the end of drying was about 7.38% in the tangential (T) direction and 4.35% in the radial (R) direction. The shrinkage ratio of T/R in the MC range of 30%–15% was about 1.8, which was higher than that (1.65) in the MC range of 15%–5%. Cracks initiation was observed at average MC of 45%. The area and number of cracks reached a maximum at 15% MC. During the late stage of drying, the drying stress of the surface layer was reversed from tensile to compressive, which leads to the narrowing or closure of the cracks. Moreover, the fracture morphology of wood cells was observed at the microscopic level, indicating that the failure firstly occurred in the intercellular layer between the ray tissue and the tracheid in the latewood. The ray cells presented intact, and the lamellar structure between ray tissues and between the ray cells and the tracheids were the weak points of wood during drying. The results would contribute to keeping in minimum drying defects and improving the drying quality of wood. [ABSTRACT FROM AUTHOR]

Wood is widely considered to be a green and renewable building material. However, many of the challenges to using wood as an engineering material arise from the shrinkage and swelling behaviors during moisture content (MC) loss or gain. In this study, the shrinkage behavior of Queensland peppermint (Eucalyptus exserta F. V. Muell) wood was investigated at the macroscopic and cellular levels, and the interdependence between the two levels was explored. Results revealed that the tangential shrinkage of 2% and radial shrinkage of 1% was observed as MC decreased from about 60% to 32% in the macroscopic level. The most obvious anisotropic shrinkage was presented at the MC region of 32–22%, and the ratio of T/R was ranged from 1.6 to 1.75 below the fiber saturation point. At the cellular level, a considerable shrinkage was observed at MC of 42%. The cell wall was shrunk, but the lumen in radial (TR plane) was expanded with MC loss. The shrinkage of the total wood cell showed a linear relationship with MC. The shrinkage ratio of T/R was maintained around 1.5 below the fiber saturation point, indicating that the MC states have little effect on the anisotropic shrinkage. Besides, the shrinkage value at the macroscopic level was slightly smaller than the cellular level, but the anisotropic shrinkage showed an opposite trend. [ABSTRACT FROM AUTHOR]

Anisotropic shrinkage is a typically feature in wood, which is of critical importance in wood drying. In this study, the shrinkage strains over each growth ring were determined by a full-field strain measurement system during moisture content (MC) loss. Color maps were used to visualize the full-field distribution of displacement and shrinkage strain under different MC conditions. The variation of tangential and radial shrinkage strain from pith to bark, as well as the anisotropic shrinkage in heartwood and sapwood were studied. Both of the displacement and strain values increased as the MC decreased. From pith to bark, the tangential strains were higher at two poles as compared to the center, showing a parabolic distribution below fiber saturation point. While for radial shrinkage strain, a minor difference was observed except for the MC of 10%. An intersection between tangential and radial shrinkage ratio curve was observed at the MC of 28%. Both expansion and shrinkage in tangential direction were larger than radial counterparts, and the transformation from expansion to shrinkage occurred at the MC region of 32–28%. In addition, the shrinkage in heartwood was larger than sapwood, whereas anisotropic shrinkage in sapwood was more pronounced as compared to heartwood. [ABSTRACT FROM AUTHOR]

The knowledge of moisture content (MC) and drying stress are crucial parameters to control the drying process and maintain the quality of dried wood. Herein, we investigated the pattern of moisture transfer and stress development in Chinese fir during the high-temperature drying process. The MC in each layer of lumber was separated into bound water and free water via nuclear magnetic resonance (NMR), and the drying and residual stress were measured using prong test method. There was different MC in each layer along the thickness, resulting in an MC gradient that initially increased and then decreased, which is consistent with the trend of drying stress. The T21 peak indicating bound water shifted to the left especially when MC was below the fiber saturation point, signifying that the discharge of moisture became difficult with prolonged drying time. The ratio of bound water to free water was different in each layer, indicating that the moisture transfer was different along the thickness. Furthermore, the residual stress was greater than the corresponding drying stress though the disparity reduced gradually, which suggests that the MC gradient was the largest affecting factor for drying stress at high MC stages but decreased to some extent as the drying process continued. [ABSTRACT FROM AUTHOR]

Radio frequency heating combined with convection (RF/C) drying of larch boxed-heart square timber and its influence on drying kinetics such as rate, moisture content distribution, and stresses was explored. Results revealed that RF heating increased the drying rate and in RF/C drying was twice as high as in conventional drying. Below fiber saturation point, RF heating reduced internal moisture gradients, especially around moisture content of 20%. The effect of RF heating on moisture transfer was strongly associated with moisture content. Specifically, above the fiber saturation point, RF heating played a minor role in moisture transfer however, it reached maximum around fiber saturation point and thereafter, it largely decreased with moisture content. RF heating relieved some drying stresses during RF/C drying and reduced residual stresses in the timber surface layers. Furthermore, it changed the original development pattern of drying stresses in conventional drying. [ABSTRACT FROM AUTHOR]

A one-dimensional mathematical model is presented that accounts for a moving evaporation interface in simulating the coupled heat and mass transfer during convective drying of wood. In the model proposed, the only mechanism considered in water transport within wood is diffusion. Additionally, the transport of moisture is dominated by the gradient of the moisture content. The controlling equations were established from Whitaker’s volume averaging laws and solved numerically with the finite volume method. The simulation results for the density of vapor and the volume rate of evaporation indicate that the migrating moisture was mainly in the form of gas under conditions of lower moisture content. The evaporation interface moved at approximately constant speed and the evaporation rate of the interface decreased with time. Finally, the core temperature and average moisture content in wood were successfully simulated. [ABSTRACT FROM PUBLISHER]

The focus of this study is the variation of practical shrinkage strain, elastic strain, viscoelastic creep strain, and mechanosorptive creep characteristics of white birch ( Betula platyphylla Suk) disks caused by shrinkage anisotropy (ShrA) and its interaction with moisture content (MC) gradient. The rheological properties of white birch disks have been analyzed as a function of MC based on two different drying schedules at constant and increasing temperature. The drying conditions influence the strains remarkably. The stress characteristics and stress reversal mechanism under the conditions of increasing temperature are discussed based on stress analysis diagrams. Under both drying schedules, the wood disks are initially subjected to tangential tensile stress, and with decreasing MC, the tensile stress turns into compressive stress. Before stress reversal, the stresses of ShrA and MC gradient are aligned in the same direction and then point in opposite directions. [ABSTRACT FROM AUTHOR]