Abstract
To elucidate the origin of shrinkage anisotropy of wood during the drying process, wood from three tree species, Quercus sp., Juglans nigra, and Pometia pinnata, was analyzed using thin cryomicrotome sections and sequential drying on a micro-scale. The data on shrinkage, based on the transverse direction, were calculated using Image Pro Plus software to measure the thickness of the cell wall of fibers. The results showed that: (1) In the tangential direction, the shrinkage of wood fibers were all in the “smallest-bigger-smaller (-bigger-smaller)” pattern from A to C (A: The cells closest to the wood rays; C: The cells in the middle between the wood rays) and fibers next to the rays always have the minimum shrinkage at different moisture contents; (2) the width of the rays has no negative correlation with the shrinkage of wood fibers; and (3) the rays have the same effect on the shrinkage of wood fiber cells in both latewood and earlywood. In addition, the shrinkage of latewood is more severe than that of earlywood, which leads to tangential shrinkage.
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Effects of Wood Rays on the Shrinkage of Wood during the Drying Process
Qianwen Xue,a Weisheng Sun,a,b,* Kurt Fagerstedt,c Xi Guo,a,b Mingrui Dong,a Wenbin Wang,a and Huimin Cao a
To elucidate the origin of shrinkage anisotropy of wood during the drying process, wood from three tree species, Quercus sp., Juglans nigra, and Pometia pinnata, was analyzed using thin cryomicrotome sections and sequential drying on a micro-scale. The data on shrinkage, based on the transverse direction, were calculated using Image Pro Plus software to measure the thickness of the cell wall of fibers. The results showed that: (1) In the tangential direction, the shrinkage of wood fibers were all in the “smallest-bigger-smaller (-bigger-smaller)” pattern from A to C (A: The cells closest to the wood rays; C: The cells in the middle between the wood rays) and fibers next to the rays always have the minimum shrinkage at different moisture contents; (2) the width of the rays has no negative correlation with the shrinkage of wood fibers; and (3) the rays have the same effect on the shrinkage of wood fiber cells in both latewood and earlywood. In addition, the shrinkage of latewood is more severe than that of earlywood, which leads to tangential shrinkage.
Keywords: Shrinkage; Wood ray; Restraint; Anisotropy; Drying
Contact information: a: School of Engineering, Zhejiang A&F University, Lin’an 311300, China; b: Laboratory of Wood Science and Technology, Zhejiang A&F University, Lin’an 311300, China; c: Viikki Plant Science Centre VIPS, Faculty of Biological and Environmental Sciences, University of Helsinki, Helsinki 00014, Finland; *Corresponding author: 18268158266@163.com
INTRODUCTION
Dimensional stability, mechanical strength, and fire and rot resistance are all very important properties for wood when used in architecture, in interior decoration, and as furniture material. Among these, dimensional stability is the foundation and is greatly dependent on the quality of wood drying. Because wood is an irregular, heterogeneous, and anisotropic material (Thuvander et al. 2002), the longitudinal, tangential, and radial shrinkages of wood are different during the drying process, and they may produce drying defects such as cracks, warps, and splits.
Previous reports have clarified that in normal wood, the longitudinal shrinkage from the green to the dried condition is the smallest, while the transverse shrinkage has distinctly higher values. In addition, significant anisotropy has been verified and is manifested in the fact that tangential shrinkage is usually 1.5 to 2.5 times that of radial shrinkage (Spear and Walker 2006). Therefore, understanding the cause of anisotropic shrinkage of wood during the drying process is important for ensuring the dimensional stability of wood.
Many researchers have presented theories related to the anisotropic shrinkage of wood. Panshin and Zeeuw (1980) held the opinion that wood shrinkage happens only when moisture content is below the fiber saturation point (except for certain abnormal cases). Pambou et al. (2017) considered that wood is in elastic deformation when the moisture content is higher than the fiber saturation point while the plastic deformation of wood can not be recovered when the moisture content is lower than the fiber saturation point. In addition, Bonarski et al. (2015) considered that wood shrinkage is governed by its chemical composition, ultrastructure, and gross anatomy, and indicated that the transverse shrinkage of wood depends mostly on a specific ultrastructural arrangement of the moderately organized cell wall compounds. Therefore, studying the ultrastructures of wood is one of important ways to understand the shrinkage deeply to ensure the dimensional stability of wood.
There are many theories concerning the ultrastructures that influence the anisotropy behaviors of wood. All factors leading to anisotropic shrinkage of wood on the micro-scale can be divided into three groups: (1) the rays exert a restraining influence on the radial shrinkage of the fibers (Wijesinghe 1959); (2) the transverse shrinkage anisotropy of earlywood is more pronounced than that of the latewood, which increases the tangential shrinkage (Skaar 1988; Pentoney 1953); and (3) the microfibril angle in the S2 layer is an important factor affecting the degree of shrinking anisotropy of wood during the drying progress (Meylan 1968, 1972; Cave 1972; Barrett et al. 1972; Koponen et al. 1989; Watanabe and Norimoto 1996).
Skaar (1988) believed that both the ray restraint theory and latewood domination theory are most probable in explaining the anisotropy of wood during drying. When it came to latewood domination theory, Gu et al. (2001) observed, similarly, that the radial cell wall of Scots pine latewood is about 25% thicker than the tangential wall, while earlywood radial cell walls do not show such a difference. Dang et al. (2018) believed that the mean radial strain in latewood is higher than that reached in earlywood during tangential adsorption and tangential desorption. In the ray restraint theory, it is partly believed that rays play the most important role in restraining radial shrinkage, as the ray tissue shrinks less in the radial direction. However, Boutelje (1962) denied the importance of rays in this regard in some species. Wu et al. (2006) presented that ray parenchyma proportions determine total shrinkages. Taylor et al.(2013) reported that the shrinkages near the rays are smaller than the shrinkages distant from the rays by employing X-ray computed tomography, which directly confirms the “ray-restraint” on the micro-scale. Patera et al. (2018) revealed that the role of rays in the cellular structure in restraining the tangential swelling of thin-cell-walled earlywood.
The literature about ray restraint theory mentioned above were based on the research of effect of wood rays on a region of wood fibers, which considered total shrinkages rather than unit shrinkage. Therefore, in the view of the authors, there are good reasons to assume that rays also are responsible for the effect on unit shrinkage of wood fibers in this context.
To enrich the explanation of the inhibitory effect theory of xylem rays on radial shrinkage and to clarify how the structures affect shrinking of earlywood and latewood during the drying process, an experiment to observe the shrinkage of wood fiber cells between two wood rays was designed. The experiment also compared the difference between earlywood and latewood using thin cryomicrotome sections and Image Pro Plus to find a region of interest (ROI) to measure the thickness of the cell wall of fibers during sequential drying.
EXPERIMENTAL
Specimens
Three types of woods from Zhejiang Province in China were selected: ring porous wood (oak; Quercus sp.), semi ring porous wood (black walnut; Juglans nigra), and diffuse porous wood (pometia; Pometia pinnata). Every block was selected from mature tree sapwood with dimensions of 20 × 20 × 20 mm, cut with the use of a wheel saw. All wood blocks were soaked in distilled water for 5 days and then divided into four parts with dimensions of 10 × 10 × 10 mm. Finally, 25 µm sections were cut with a cryomicrotome at -12 ºC and placed in a dish with distilled water.
For measurements, one glass slide, one cover slip, and two pieces of tape were weighed in grams (wn, n = number of samples). Three pieces of wood sections were selected and placed on the slide together for precise weighing. The sections were covered with the cover slip and taped down. The slides were marked with the first letter of the wood species and number (e.g., J01, Q01, P01). No glue or other media that could possibly interfere with the wood samples were used during drying.
Classification of Wood Fiber Cells
In order to observe the ray restraint theory, the xylem cells were classified into three categories: A, B, and C, as shown in Fig. 1.
Fig. 1. The classification of cells for measurements in a cross section (take Pometia pinnata as an example). A: The cells closest to the wood rays; B: The cells close to A and between the wood rays (the smaller the number is, the closer it is to the rays); C: The cells in the middle between the two rays; R1: The rays on the left of the picture; R2: The rays on the right of the picture.
Measurement of the Shrinking Process
The specimen was dried gradually in a drying oven at 105 ºC. The weight in grams Wna (n = number of sample; a = 0, 15, 30 min, the period of drying) and the cell wall thickness D (µm) of the specimens were recorded every 15 min. Image Pro Plus (IPP) 6.0 software (Media Cybernetics, Rockville, USA) designed for cell measurements was used to measure the cell wall thickness. As is shown in Fig. 2, IPP was used to find a region of interest (ROI) according to color (a1) to generate the contours of the wood fibers (a2). The area of the cell wall was measured automatically by IPP through find ROI (white in a2). Then, the skeleton of the cells were generated by the “thinning function” of IPP. Also, they were measured automatically by IPP.