Influence of Lignin On Plastic Flow Deformation of Wood

Influence of Lignin On Plastic Flow Deformation of Wood | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Influence of Lignin On Plastic Flow Deformation of Wood Masako Seki, Mitsuru Abe, Tsunehisa Miki, Masakazu Nishida This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-893506/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 6 You are reading this latest preprint version Abstract In this study, we clarified the influence of lignin in wood on its plastic flow deformation due to shear sliding of wood cells. Wood samples were subjected to delignification, where the lignin structure gradually changed, and characterized for their chemical and physicochemical properties, and deformability by free compression testing. The delignified wood deformed by efficient stretching and maintained its cell structures at a lower pressure compared to the untreated wood. The deformability was evaluated from two viewpoints: the initial resistance to plastic flow and final stretchability. The deformability of the delignified and untreated wood increased with increasing compressive temperature, even though the changes in molecular motility associated with the glass transition of lignin contributed minimally to the improvement in deformability. In the early stages of delignification, the molecular mass of lignin in the compound middle lamella decreased, which reduced the initial resistance to plastic flow. However, during the early stages of delignification, the stretchability of delignified wood was scarcely affected by changes in lignin. As the amount of lignin was further reduced and delignification proceeded in the vicinity of the polysaccharides, the stretchability significantly improved. The correlation between chemical and physicochemical properties and plastic flow deformability presented in this paper will be helpful for low-energy and highly productive forming of solid-state wood. Polymer Science Wood Plastic deformation Deformability Delignification NMR Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Introduction The deformation processing of wood can effectively promote its use in various applications, such as furniture, building, construction, automotive, and other daily necessities. Some conventional methods of wood deformation include compression and bending processing using cell-wall deformations (Sandberg et al. 2012). These methods produce simple two-dimensional products while retaining the cell arrangement and structure of the wood. On the other hand, reducing the element size of wood can efficaciously enable the formation of more complex products. Wood-plastic composites (WPCs), which are a mixture of wood powder and plastic, are widely used as building materials (Spear et al. 2015). However, WPCs require an input of energy to miniaturize the wood, which results in the destruction of the original cell structure, such that the products do not retain the wood texture. Furthermore, in conventional processing, the element size of wood and the complexity of the product shape have a contradictory relationship. To date, the formation of a complex shape from solid-state wood has seen little success. Recently, a new technique, wood flow forming (WFF), has been developed, which can form complex shapes with solid-state wood because it takes advantage of plastic flow deformation (Abe et al. 2020; Abe et al. 2021; Miki et al. 2014-1; Miki et al 2014-2; Miki et al. 2017; Seki et al. 2016; Yamashita et al. 2009). The wood is compressed in a heated mold, which enables it to plastically flow owing to the shear sliding between wood cells to produce a final shaped product (Miki et al. 2017). By applying the traditional plastic forming techniques used for metal and plastic materials, wood products can be efficiently produced in a short time. These techniques can maintain the original cell structure of wood, providing a unique cell-derived texture to the product (Miki et al. 2014-1). However, to improve the deformability of wood during the forming process and the durability of the products, it is necessary to modify the wood by pretreatment before forming. The cell wall and compound middle lamella (CML) are modified by impregnating the wood with resin monomers (Miki et al. 2014-2; Seki et al. 2016) and/or chemical modification (Abe et al. 2020; Abe et al. 2021). WFF has great potential for various applications; however, it requires high temperatures (< 100°C) and pressures (< 50 MPa), making it energy-intensive and less productive. To solve these problems, this study focused on lignin, which acts as an adhesive and binds the polysaccharides (cellulose and hemicellulose) and cells together. In addition to the CML containing the highest amount of lignin, it is where plastic flow originates; therefore, lignin has a significant effect on plastic flow. Delignification is a process that can remove lignin without breaking the cellular structure of the wood, and it has primarily been studied for pulping wood. However, in recent years, delignified wood (DW) (Kumar et al. 2021) has attracted attention as a functional material for use in transparent wood (Li et al. 2016; Li et al. 2017; Li et al. 2020), high-strength structural materials (Frey et al. 2019; Jakob et al. 2020; Song et al. 2018), high-performance thermal insulators (Li et al. 2018), and thermal energy storage materials (Montanari et al. 2019). Deformation processing that takes advantage of the flexibility of DW has also been developed (Khakalo et al. 2020; Frey et al. 2018; Frey et al. 2019). Frey et al. (2019) reported that a completely delignified veneer in a water-swollen state exhibited significant deformability in the fiber direction. However, the effect of delignification on the plastic flow deformation associated WFF has not been reported. The thermal softening properties of water-swollen wood depend on the glass transition of lignin (Kojiro et al. 2008; Nakajima et al. 2009). When the molecular mass of lignin in wood decreases, its glass transition temperature also decreases, in turn significantly softening the wood (Nakajima et al. 2009). Therefore, it is expected that the changes in lignin due to delignification will promote the plastic flow deformation of wood and reduce the production energy required for WFF. The objective of this study was to clarify the effect of lignin on the plastic flow deformation of wood. DW and untreated-wood (UW) samples with different molecular masses and amounts of lignin were prepared by subjecting them to delignification and varying the delignification time. Free compression testing was used to evaluate the deformability of the samples based on the initial resistance to plastic flow and final stretchability. The samples were characterized by attenuated total reflection infrared (ATR-IR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and dynamic viscoelastic measurements. The effects of the chemical structure and glass transition temperature of lignin on the plastic flow deformation of wood are also discussed. Materials And Methods Materials Wood samples were successively cut in the longitudinal (L) direction from a block of sapwood of a Japanese cypress ( Chamaecyparis obtusa ) log collected from the Kiso region of Japan. The dimensions of the samples used for the dynamic viscoelastic measurements were 1 mm (L) × 30 mm (radial (R) direction) × 3 mm (tangential (T) direction). The samples for the other measurements (ATR-IR, NMR, and free compression testing) were 5 mm (L) × 5 mm (R) × 5 mm (T). Prior to delignification, the wood samples were pre-treated with approximately 100°C distilled water for 4 h, followed by methanol for 6 h to remove the low-molecular-weight components. The pre-treated samples were then dried at 50°C for 18 h and 105°C for 2 h to a relatively constant mass ( m 0 ). One side of the RT surface of some samples (5 mm × 5 mm × 5 mm) was microtome-finished prior to subsequent delignification. Preparation of delignified wood samples The pre-treated samples were delignified using 4 wt % sodium chlorite (NaClO 2 ) containing acetic acid solution (pH 3). The NaClO 2 solution was impregnated into the pre-treated samples under vacuum, and the impregnated samples were treated at 45°C for several reaction times ( t r ) of 10, 30, 60, 180, or 360 min. Then, the DW samples were washed several times with distilled water and stored in water at 20–25°C Note that the DW sample with a t r longer than 360 min was brittle and fragile; therefore, it was considered unsuitable for WFF and not evaluated in this study. In addition, the UW samples were impregnated with distilled water, instead of the NaClO 2 solution, at 20–25°C for 500 min or more ( t r : 0 min). The area in the RT cross section of the water-swollen DW and UW samples ( s d and s u , respectively) was measured, and the area increase rate ( A RT ) of the water-swollen samples due to delignification was calculated using the following formula: A RT = ( s d - s u ) / s u × 100 [%] Some of the water swollen samples were then dried at 35°C for 24 h, 50°C for 18 h, and 105°C for 3 h to a relatively constant mass ( m t ). The mass loss ( ML ) due to delignification of the dried samples was calculated using the following formula: ML = ( m o – m t ) / m o × 100 [%] Attenuated total reflection infrared (ATR-IR) spectroscopy The block-shaped sample (5 mm × 5 mm × 5 mm) in the dry state was cut in half in the L direction, and ATR-IR measurements were performed near the center of the cut surface (RT surface, that is, near the center of the wood sample). The ATR-IR spectra were measured on a Nicolet 6700 spectrometer (Thermo Scientific Inc., Waltham, MA, USA) at a 4 cm − 1 resolution in the standard ATR mode; 32 scans were performed in the range of 4000–700 cm − 1 . Nuclear magnetic resonance (NMR) spectroscopy Solid-state 13 C NMR spectra were measured on a Varian 400 NMR system spectrometer (Palo Alto, CA) with a Varian 4 mm double-resonance T3 solid probe. The dried samples were placed in a 4 mm ZrO 2 rotor spun at 15 kHz within a temperature range of 20–22°C. The 13 C cross-polarization and magic-angle spinning (CP-MAS) NMR spectra were collected with a 2.6 µs π/2 pulse at 100.56 MHz for the 13 C nuclei and a 40 ms acquisition period over a 30.7 kHz spectral width. Proton decoupling was performed with an 86 kHz 1 H decoupling radio frequency with a small phase incremental alteration (SPINAL) decoupling pulse sequence. The 1 H- 13 C cross-polarization for the spectrum acquisition was conducted with a 5.0 s recycle delay in 1024 transients using a ramped-amplitude pulse sequence with a 2 ms contact time and a 2.6 µs π/2 pulse for the 1 H nuclei. The amplitude of the 1 H nuclei was linearly ramped down from 92.6% of its final value during the CP contact time. The 1 H spin-lattice relaxation time in the laboratory frame ( T 1 H) was indirectly measured by detecting the 13 C resonance enhanced by the cross-polarization in the 13 C CP-MAS sequence, and was applied after a π pulse to 1 H nuclei using the inversion recovery method. The T 1 H analysis of the sample was carried out using the same solid-state probe used to obtain the 13 C CP-MAS NMR spectra of the sample at the same contact time and acquisition period. Dynamic viscoelastic measurement The temperature dependence of the loss tangent (tan δ ) was measured by the tensile forced oscillation method using a thermomechanical analysis apparatus (TMA SS6100; Hitachi High-Tech Science Corp., Tokyo, Japan). The water-swollen sample (1 mm (L) × 30 mm (R) × 3 mm (T)) immersed in water was subjected to a temperature increase from 30°C to 100°C at a rate of 0.5°C/min. The frequencies for the measurement were 0.01 Hz; the span was 18 mm in the R direction; and the load amplitude was 70 ± 20 mN. Viscoelastic behavior is sensitive to the drying and heat history of the samples before the measurement (Kojiro et al. 2008). To unify the histories of the samples, they were heated to 100°C, naturally cooled down to approximately 20°C, and then subjected to measurements in the water-swollen state. Free compression testing A uniaxial compression test was carried out using a material testing machine (CATY TC-2kN-NS; Yonekura Mfg. Co. Ltd., Osaka, Japan) (Fig. 1 ). The testing machine equipped with a container enables horizontal compression tests of samples in a water-swollen state under confined heating conditions while recording load-stroke curves during hydrothermal compression. The compressive temperature ( T c ) was controlled by heating the water with a cartridge heater immersed in a water pit. The contained was sealed and heated until the temperature detected by the thermocouple stabilized to the target temperature ( T c ). Then, the water-swollen sample was placed between the cylinders (diameter: 15 mm) under 2–3 N to ensure that the R was in the compression direction. The container was closed again to heat the sample to a constant temperature under saturated steam. Five minutes after achieving the temperature T c , compression testing was conducted at a constant speed of 1 mm/min up to 3000 N of the maximum compression load or until the maximum stroke of 5 mm of the testing machine. The T c was set to 40, 60, 80, and 100°C in consideration of the decrease in the glass transition temperature of lignin due to delignification (Nakajima et al. 2009). Compression testing was conducted using three or more samples under the same conditions. For one sample under each condition, the container was opened after compression testing, and the compressed sample was dried at room temperature to retain its shape while maintaining the stroke after testing, followed by further drying at 105°C for 1 h. Then, the mass ( m d ) and thickness ( r c ) in the compression direction were measured. The final compression ratio ( C d ) was calculated as follows: C d = ( r b – r c ) / r b × 100 [%], where r b is the dimension in the R direction of the water-swollen sample before the compression testing. The appearance of the compressed sample from the LT planes was observed using an optical microscope (VHX-970F; Keyence Corp., Osaka, Japan). The area of the LT planes before and after compression testing was calculated by binarizing the captured image, and the area magnification ( AM d ) was calculated as follows: AM d = A c / A b × 100 [%] where A b and A c are the areas of the LT planes of the water-swollen sample before compression and the dried sample after compression, respectively. Results And Discussion Characterization of untreated and delignified samples The mass loss (ML) and the area increase rate (A RT ) Figure 2 shows the variation in ML (left axis, closed circles) and A RT (right axis, open circles) as functions of delignification time ( t r ). ML linearly increased with increasing delignification time, i.e., the lignin content of the wood sample decreased with delignification. Based on the original lignin content of cypress of 33 % (Pettersen 1984), approximately 40 % of the lignin was removed after 360 min of t r . The A RT also increased with increasing delignification time. The increase in the dimensions of the water-swollen sample was caused by the swelling of the cell wall and CML. The total volume of the adsorbed water in the cell wall and CML exceeded the volume of lignin removed by delignification. ATR-IR spectroscopy Figure 3 shows the ATR-IR spectra of the UW and DW samples, which indicate that the chemical structure of the wood sample changes with delignification. The area of the peak at 1490–1530 cm − 1 , which corresponds to the skeletal vibrations of the benzene ring in lignin, decreased with increasing delignification time. This indicates the reaction is initiated during the early stages of delignification, as the benzene rings of lignin begin to disappear. The absorbance peak at 1725–1750 cm − 1 , which corresponds to the C = O stretch of lignin and hemicellulose, initially increased ( t r = 10 min), then decreased in the later stages ( t r = 360 min). The initial increase in this peak can be attributed to the reaction between NaClO 2 and lignin, which indicates that the aromatic ring was cleaved (Li et al. 2017). On the other hand, the decrease in the later stage was due to the decrease in the amount of lignin. The initial structural changes of lignin due to delignification consisted of the elimination and cleavage of the benzene ring, followed by the elimination of the benzene ring. Solid state NMR measurements Figure 4 shows the 13 C cross-polarization (CP)MAS NMR spectra for each substituent in the UW and DW samples. Signals corresponding to biomass constituents in the wood were assigned based on our previous report (Nishida et al. 2014). The carbohydrates appeared as relatively large and sharp signals in the range of 60–110 ppm. However, most of the signals for cellulose and hemicellulose, except cellulose C4, overlapped with each other, and the crystalline and amorphous signals for cellulose C4 and C6 could be separately observed. The aromatic and olefinic groups in lignin appeared as broader signals in the range of 110–160 ppm, while the methoxy groups in lignin appeared as isolated signals at 56 ppm. During the early stages of delignification ( t r = 10 min), the signal intensity of the OCH 3 (56 ppm) and aromatic (110–160 ppm) groups in lignin rapidly decreased was observed. Meanwhile, the intensity of the C = O signal at a lower magnetic field (172 ppm) increased for 30 min, then gradually decreased as delignification progressed. The trends of the signal intensities for the aromatic and C = O groups in the 13 C CP-MAS NMR spectra were similar to those in the ATR-IR spectra. Therefore, in the first step of delignification, oxidation of the benzene ring of the guaiacyl unit on the surface of the lignin unit afforded two carboxyl groups at C3 and C4 positions (Hamzeh et al. 2008). Next, the cleavage of C–C and C–O bonds that provide bridging with other aromatic ring units at the inner site of the lignin resulted in higher ML of the delignified wood, because the oxidized portion was removed (Tarvo et al. 2010 and Qu el al. 2020). However, the signal pattern of the carbohydrates in the 13 C CP-MAS NMR spectra barely changed as a result of delignification, indicating that the chemical structures of cellulose and hemicellulose remained unchanged as delignification progressed. Figure 5 shows the 1 H spin-lattice relaxation times in the laboratory frame ( T 1 H) of the dry samples. A minimal change was observed in the T 1 H values at 65 and 75 ppm for t r = 60 min, but tended to increase after 60 min. For wood in the dry state, the spin-lattice relaxation for each wood constituent occurs via lignin, which has the shortest T 1 H value among the biomass constituents (Nishida et al. 2017). The increase in T 1 H of the DW samples at t r > 60 min was caused by the loss of lignin units with the short T 1 H values. In the early stages ( t r 60 min), the reaction progressed toward the polysaccharide chain, resulting in an increase in the T 1 H value because of the reduced interactions between the polysaccharides and lignin. Dynamic viscoelastic measurement Figure 6 shows the temperature dependence of tan δ for the water-swollen samples. The tan δ peak is attributable to the glass transition of lignin, and the peak temperature ( T g) in Fig. 6 corresponds to the glass transition temperature of lignin in the wood samples (Kojiro et al. 2008; Nakajima et al. 2009). As delignification progressed, the T g gradually decreased and the shape of the peak broadened. These trends correspond well with those by Nakajima et al. (2009); they measured tan δ of Japanese cypress delignified by NaClO 2 and reported that the shift of T g and the broadening of the tan δ peak were due to the decrease in the molecular mass and amount of lignin, respectively. The results in Fig. 6 demonstrate that the molecular mass and amount of lignin begin to decrease in the early stages of delignification ( t r : 10 min). Deformability of untreated and delignified samples Figure 7 shows the relationship between the nominal compressive stress ( σ ) and the compression ratio ( C ) during compression testing at each T c . The σ slightly increased to a C of approximately 60% in all samples, during which the cell lumens in the wood samples gradually closed because of the buckling of the cell wall. As C increases, σ significantly increases at a constant rate, followed by an inflection point at which the rate of increase in σ decreases (indicated by the arrow in Fig. 7 ). These compressive behaviors were similar to those observed in our previous report (Miki et al. 2017). Before the inflection point, the cell lumens were completely closed and the sample was consolidated, resulting in a rapid increase in σ . After the inflection point, the rate of increase in σ decreased because the wood sample plastically deformed in the unconstrained direction (L or R). The inflection point was also detected in the flat region for t r = 360 min at 100°C (shown by (d) ▽); under this condition, plastic deformation occurred before the cell lumens were completely closed. The inflection points were not detected at 40°C, 60°C, and 80°C for t r = 360 min and at 100°C for t r = 180 min. This is because the inflection point due to the plastic deformation overlapped with the region denoting the rapid increase in σ . The σ at the inflection point detected in Fig. 7 is the nominal compressive stress at the starting point of flow ( σ y ), which indicates the initial resistance of the samples to plastic flow. Regardless of temperature, the longer the delignification time, the smaller the σ y . Therefore, the delignification process reduced the initial resistance to plastic flow. Figure 8 shows photographs of the samples that were dried after free compression testing to preserve their shape. After compression testing, all the samples were able to maintain their stretched state after drying by pressure; they only stretched in the T, and not in the L direction. Such anisotropy of plastic flow deformation is consistent with the phenomenon observed in our previous report (Miki et al. 2017). At high T c , the sample that was delignification for a longer time exhibited the highest C d of 91% (Fig. 8 ), indicating that a considerably thin product could be fabricated from solid-state wood. Figure 9 shows an SEM image of the RT surface of the samples. None of the samples displayed cell wall destruction. Furthermore, evidence of mutual positional change between the cells was confirmed. The plastic flow deformation was mainly caused by the shear sliding phenomenon between the cells and at the boundary of the CML, regardless of the difference in T c and lignin state (such as quantity, quality, and molecular motility). The sample subjected to a longer delignification time (Fig. 7 (c, d)) displayed several slip surfaces, and plastic flow occurred in units with a smaller number of cells. The cells slightly protruding in the L direction were also observed, suggesting that delignification promoted slip deformation in the L direction. Influence of lignin on plastic flow deformation of wood Figure 10 shows the relationship between the ML by delignification of wood samples and σ y , which indicates the initial resistance of wood samples to plastic flow. The higher the T c , the lower the initial resistance to plastic flow. The initial resistance to plastic flow was logarithmically reduced to a ML of 4%, which indicates that the improvement in plastic deformability was significant in the early stages of delignification. The final stretchability of the samples was evaluated using the AM d . Figure 11 (a) shows the relationship between ML and the AM d of the wood samples. The AM d increased with increasing T c . The value of AM d was almost unchanged during the early stages of delignification ( ML : < 4%); however, as delignification progressed, AM d remarkably increased and the sample was extensively stretched. The value of AM d reached a maximum of 2.7 times for the longest t r of 360 min at the highest T c of 100°C. Furthermore, delignification caused a change in the density of the sample; therefore, the area per unit mass ( A d / m d ), which is an index of stretchability, was calculated based on the area of the LT surface ( A d ) and the mass ( m d ) of the dried compressed samples. The relationship between ML and A d / m d of the wood samples is shown in Fig. 11 (b). The change in A d / m d depending on the T c and the tendency toward ML were similar to those of AM d (Fig. 11 (a)). In contrast to the tendency of σ y in Fig. 10 , the stretchability did not increase during the early stages of delignification ( ML : 60 min). Delignification with NaClO 2 initially proceeds from the lignin-rich CML during the early stages of the reaction and selectively softens the CML region. Then, the reaction and softening of the cell wall progresses during the later stages of the reaction (Xu et al. 2020). The results of T 1 H in this study (Fig. 5 ) suggest that the lignin removal in the vicinity of the polysaccharide chains that make up the cell wall proceeds during the later stages of delignification ( t r > 60 min), which increases cell wall flexibility. Although the T 1 H results of the dry state are represented, water was adsorbed between the constituents in the cell wall and CML during compression testing. Because the total volume of the adsorbed water on the cell wall increased as delignification progressed (Fig. 2 ), the adsorbed water acted as an intermolecular lubricant and contributed to the increase in stretchability. Therefore, the increase in stretchability that was observed during the later stages of delignification was likely due to the increased flexibility of the cell wall rather than the cleavage of the lignin network in the CML that occurred during the early stages of delignification. This flexibility of the cell wall generated many slip surfaces during plastic flow (Fig. 9 (c and d)). A strong correlation is observed between the T g (Fig. 6 ) and σ y (Fig. 10 ), as shown in Fig. 12 . This indicates that the reduction in the molecular mass of lignin significantly contributed toward the improved deformability observed during the initial resistance to plastic flow. Because the plastic flow of wood was shear failure originating from the CML (Fig. 9 ), structural defects in the CML generated during the early stages of delignification considerably affect the initiation of plastic flow. In compression testing, at T c > T g (filled circles) and T c < T g (open circles), the molecular motility of lignin greatly differed because the lignin was in the glass and rubber states, respectively. Nevertheless, the different molecular motilities have minimal impact on the relationship between T g and σ y . Therefore, the influence of the molecular motility of lignin on the deformability was considered to be negligible. Conclusion We clarified the influence of lignin in wood on plastic flow deformation due to the shear sliding between wood cells. The ATR-IR and solid-state NMR spectroscopic analyses of delignification showed that the oxidative opening of guaiacyl ring on the surface of the lignin unit occurred in the early stages and the oxidized portion was released from the inner site of the lignin unit when delignification time was extended. Free compression testing was performed to evaluate the deformability (the initial resistance to plastic flow and final stretchability) of the samples. The decrease in the molecular mass of lignin in the CML that occurs in the early stages of delignification reduces the initial resistance to plastic flow deformation. However, the effect of changes in the chemical structure of lignin that occurred during the early stages of delignification on the final stretchability was relatively small. Furthermore, as the delignification progressed, the amount of lignin in wood decreased and the reaction reached the vicinity of the polysaccharide chains, resulting in a remarkable increase in stretchability. We also observed that an increase in T c tends to improve the plastic deformability. However, the effect of the changes in molecular motility due to the glass transition of lignin on the deformability was very small. These results suggest that the molecular mass and amount of lignin in the CML and in the vicinity of the polysaccharide chains in the cell wall affect the plastic flow deformation of wood. Furthermore, the strategy of controlling the molecular mass and amount of lignin via delignification can yield more productive materials, as well as metal and plastic materials for achieving low-energy plastic flow deformation of wood. In this study, we examined delignified wood in the water-swollen state, but in the future, we plan to investigate the effect of other adsorbents, such as resin monomers, instead of water, on the plastic deformability of delignified wood.</p Declarations Funding (information that explains whether and by whom the research was supported) This study was financially supported by Asahi Kasei Corporation. Conflicts of interest/Competing interests (include appropriate disclosures) The authors declare no conflict of interest. Availability of data and material (data transparency) The data that support the findings of this study are available from the corresponding author, Masako Seki, upon reasonable request. Code availability (software application or custom code) Not applicable Additional declarations for articles in life science journals that report the results of studies involving humans and/or animals Not applicable Ethics approval (include appropriate approvals or waivers) Not applicable Consent to participate (include appropriate statements) We agreed. Consent for publication (include appropriate statements ) We agreed. Acknowledgements The authors like to thank the Asahi Kasei Corporation for financial support towards this study. References Abe M, Enomoto Y, Seki M, Miki T (2020) Esterification of solid wood for plastic forming. BioRes 15(3): 6282–6298. https://doi.org/ 10.15376/biores.15.3.6282-6298 Abe M, Seki M, Miki T, Nishida M (2021) Effect of the propionylation method on the deformability under thermal pressure of block-shaped wood. 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Int J Nanotechnol 11(5/6/7/8):509–519. https://doi.org/10.1504/IJNT.2014.060572 Miki T, Nakaya R, Seki M, Tanaka S, Sobue N, Shigematsu I, Kanayama K (2017) Large deformability derived from a cell–cell slip mechanism in intercellular regions of solid wood. Acta Mech 228(8):2751–2758. https://doi.org/10.1007/s00707-015-1523-z Montanari C, Li Y, Chen H, Yan M, Berglund L (2019) Transparent wood for thermal energy storage and reversible optical transmittance. Appl Mater Interfaces 11(22):20465–20472. https://doi.org/10.1021/acsami.9b05525 Nakajima M, Furuta Y, Ishimaru Y, Ohkoshi M (2009) The effect of lignin on the bending properties and fixation by cooling of wood. J Wood Sci 55:258–263. https://doi.org/10.1007/s10086-009-1019-3 Nishida M, Tanaka T, Miki T, Shigematsu I, Kanayama K, Kanematsu W (2014) Study of nanoscale structural changes in isolated bamboo constituents using multiscale instrumental analyses. J Appl Polym Sci 131:40243. https://doi.org/10.1002/app.40243 Nishida M, Tanaka T, Miki T, Hayakawa Y, Kanayama K (2017) Instrumental analyses of nanostructures and interactions with water molecules of biomass constituents of Japanese cypress. Cellulose 24:5295–5312. https://doi.org/10.1007/s10570-017-1507-3 Pettersen RC (1984) The chemical composition of wood. In: Rowell RM (ed.) The Chemistry of Solid Wood, Advances in Chemistry Series 207, American Chemical Society: Washington, DC, pp. 57–126 Qu Y, Yin W, Zhang RY, Zhao S, Liu L, Yu J (2020) Isolation and characterization of cellulosic fibers from ramie using organosolv degumming process. Cellulose 27:1225–1237. https://doi.org/10.1007/s10570-019-02835-w Sandberg D, Haller P, Navi P (2012) Thermo-hydro and thermo-hydro-mechanical wood processing: An opportunity for future environmentally friendly wood products. Wood Mater Sci & Eng 8:64–88. https://doi.org/10.1080/17480272.2012.751935 Seki M, Kiryu T, Miki T, Tanaka S, Shigematsu I, Kanayama K (2016). Extrusion of solid wood impregnated with phenol formaldehyde (PF) resin: Effect of resin content and moisture content on extrudability and mechanical properties of extrudate. BioRes 11(3):7697–7709. https://doi.org/10.15376/biores.11.3.7697-7709 Song J, Chen C, Zhu S, Zhu M, Dai J, Ray U, Li Y, Kuang Y, Li Y, Quispe N, Yao Y, Gong A, Leiste UH, Bruck HA, Zhu JY, Vellore A, Li H, Minus ML, Jia Z, Martini A, Li T, Hu L (2018) Processing bulk natural wood into a high-performance structural material. Nature 554, 224–228. https://doi.org/10.1038/nature25476 Spear MJ, Eder A, Carus M (2015) 10 - Wood polymer composites. In: Ansell MP (ed) Wood Composites, 1st edn. Woodhead Publishing Elsevier Ltd.: Cambridge, UK, pp 195–249. http://dx.doi.org/10.1016/B978-1-78242-454-3.00010-X Tarvo V, Lehtimaa T, Kuitunen S, Alopaeus V, Vuorinen T, Aittamaa J (2010) A model for chlorine dioxide delignification of chemical pulp. J Wood Chem Tech 30:230–268. http://dx.doi.org/10.1080/02773810903461476 Xu E, Wang D, Lin L (2020) Chemical structure and mechanical properties of wood cell walls treated with acid and alkali solution. Forests 11(1): 87. https://doi.org/10.3390/f11010087 Yamashita O, Yokochi H, Miki T, Kanayama K (2009) The pliability of wood and its application to molding. J Mater Proc Tech 209(12–13):5239–5244. https://doi.org/10.1016/j.jmatprotec.2008. 12.011 Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Major revisions 01 Nov, 2021 Reviews received at journal 28 Sep, 2021 Reviewers invited by journal 17 Sep, 2021 Editor invited by journal 12 Sep, 2021 Editor assigned by journal 09 Sep, 2021 First submitted to journal 08 Sep, 2021 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-893506","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":52882542,"identity":"37628b7b-ba96-438b-a5de-89b8e9850788","order_by":0,"name":"Masako Seki","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAxklEQVRIiWNgGAWjYJACxgYDGwY2KIeZWC1pQC3MJGlhOEy0YgYG3Qb2hw9nFJyX5+M/f4DxSwUDuzkhLWYHeIwNNxjcNmyTSGZgljnDwGzZQEjL/Tdskg8MbjO2STAzMEu2MTAbHCBoC/vznw8Mztm38R8mWguDGeMGgwOJbQzJDIwfidPCYyw5wyA5GegXg8MMZySI8MsB9ocfe/7Y2c7vP/jw4Y8Km2SCIYYCDvMwAO0iSQvjDwYGO9K0jIJRMApGwUgAANk1OeOjHE6JAAAAAElFTkSuQmCC","orcid":"https://orcid.org/0000-0001-5699-9376","institution":"National Institute of Advanced Industrial Science and Technology Chubu Center Multi-Materials Research Institute: Sangyo Gijutsu Sogo Kenkyujo Chubu Center Multi-Material Kenkyu Bumon","correspondingAuthor":true,"submittingAuthor":false,"prefix":"","firstName":"Masako","middleName":"","lastName":"Seki","suffix":""},{"id":52882543,"identity":"d7fde482-46c3-45a7-a49b-5f5e8aafa443","order_by":1,"name":"Mitsuru Abe","email":"","orcid":"","institution":"National Institute of Advanced Science and Technology","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Mitsuru","middleName":"","lastName":"Abe","suffix":""},{"id":52882544,"identity":"20a3bead-d4e4-44e8-bbe4-c1ff58dbaa61","order_by":2,"name":"Tsunehisa Miki","email":"","orcid":"","institution":"National Institute of Advanced Industrial Science and Technology","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Tsunehisa","middleName":"","lastName":"Miki","suffix":""},{"id":52882545,"identity":"97cd7dca-d05c-4ab5-b4a1-d458350180f0","order_by":3,"name":"Masakazu Nishida","email":"","orcid":"","institution":"National Institute of Advanced Industrial Science and Technology","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Masakazu","middleName":"","lastName":"Nishida","suffix":""}],"badges":[],"createdAt":"2021-09-10 14:55:13","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-893506/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-893506/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":13848065,"identity":"ff353566-8bc8-4bc8-917a-e6f252996884","added_by":"auto","created_at":"2021-09-21 20:51:28","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":44835,"visible":true,"origin":"","legend":"Schematic drawing of the experimental apparatus","description":"","filename":"fig1.png","url":"https://assets-eu.researchsquare.com/files/rs-893506/v1/699ea4c277f4c7b17f557fce.png"},{"id":13847782,"identity":"bddbe1a3-0bb5-4d16-9aa3-22ceaf57959b","added_by":"auto","created_at":"2021-09-21 20:48:28","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":18823,"visible":true,"origin":"","legend":"Variation of mass loss (ML) and area increase rate (ART) with delignification time (tr) ","description":"","filename":"fig2.png","url":"https://assets-eu.researchsquare.com/files/rs-893506/v1/1bf6f98cfb2e87a013904dff.png"},{"id":13847642,"identity":"37ed294d-1c0a-4373-b786-806302126acc","added_by":"auto","created_at":"2021-09-21 20:45:28","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":58899,"visible":true,"origin":"","legend":"ATR-IR spectra of the untreated-wood (tr = 0 min) and delignified wood (tr = 10, 30, 60, 180 and 360 min) samples in the dry state","description":"","filename":"fig3.png","url":"https://assets-eu.researchsquare.com/files/rs-893506/v1/ab7a808cc1465f23c517f300.png"},{"id":13847641,"identity":"7f0c2f0f-9e88-4148-8b3a-1068a3774fe7","added_by":"auto","created_at":"2021-09-21 20:45:28","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":35007,"visible":true,"origin":"","legend":"13C CP-MAS NMR spectra of untreated wood (tr = 0 min) and delignified wood (tr = 10, 30, 60, 180 and 360 min) samples in dry state","description":"","filename":"fig4.png","url":"https://assets-eu.researchsquare.com/files/rs-893506/v1/fd1f51534f39194f88e74c04.png"},{"id":13847638,"identity":"e87f301c-fce1-4aa2-999b-a5851a78adb4","added_by":"auto","created_at":"2021-09-21 20:45:28","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":12150,"visible":true,"origin":"","legend":"1H spin-lattice relaxation time in the laboratory frame (T1H) of the untreated wood (tr = 0 min) and delignified wood (tr = 10, 30, 60, 180 and 360 min) samples in the dry state at 65 and 75 ppm","description":"","filename":"fig5.png","url":"https://assets-eu.researchsquare.com/files/rs-893506/v1/d211de0b90bf64c0f2b81247.png"},{"id":13848066,"identity":"ac1691b5-bc5f-4e05-a8c0-966c8a9bd467","added_by":"auto","created_at":"2021-09-21 20:51:28","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":41690,"visible":true,"origin":"","legend":"Temperature dependence of tan δ of untreated wood (tr = 0 min) and delignified wood(tr = 10, 30, 60, 180 and 360 min) samples in the water-swollen state ","description":"","filename":"fig6.png","url":"https://assets-eu.researchsquare.com/files/rs-893506/v1/844d92a117dc872f3c1b02a1.png"},{"id":13847788,"identity":"35ad1895-1e5c-452f-a1c3-cbe5410fb68e","added_by":"auto","created_at":"2021-09-21 20:48:28","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":154184,"visible":true,"origin":"","legend":"Relationship between the nominal compressive stress (σ) and compression ratio (C) during compression testing at each compressive temperature (Tc)","description":"","filename":"fig7.png","url":"https://assets-eu.researchsquare.com/files/rs-893506/v1/66d7921b901ceeb6db05adf5.png"},{"id":13847646,"identity":"8c88a3b0-503d-4705-b9aa-594a12a6c7e6","added_by":"auto","created_at":"2021-09-21 20:45:28","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":373638,"visible":true,"origin":"","legend":"Photographs of the untreated wood (tr = 0 min) and delignified wood (tr = 360 min) samples whose shape was fixed by drying post-free compression testing. The final compression ratio (Cd) is shown in each photograph. The part surrounded by the dotted line of the circle shows the sample protruding from the cylinders of the testing machine","description":"","filename":"fig8.png","url":"https://assets-eu.researchsquare.com/files/rs-893506/v1/b600de2d534caa151a5decb0.png"},{"id":13847648,"identity":"26d7dc2d-2b4b-4571-9ee0-7b48d0663f69","added_by":"auto","created_at":"2021-09-21 20:45:28","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":507397,"visible":true,"origin":"","legend":"The SEM images of the radial-tangential surfaces of untreated wood (tr = 0 min) and delignified wood (tr = 360 min) samples after free compression testing","description":"","filename":"fig9.png","url":"https://assets-eu.researchsquare.com/files/rs-893506/v1/82f8f77dd46b94d83215f391.png"},{"id":13848292,"identity":"9c2cd85d-1d3b-427d-a684-0f09c9ab5d7a","added_by":"auto","created_at":"2021-09-21 20:54:28","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":30437,"visible":true,"origin":"","legend":"Relationship between the mass loss (ML) by delignification of wood samples and nominal compressive stress at starting point of flow (σy). The results of reaction time (tr) of 180 min at 100 °C and tr of 360 min were not described because the inflection point could not be detected after the consolidation of the sample in Fig. 7","description":"","filename":"fig10.png","url":"https://assets-eu.researchsquare.com/files/rs-893506/v1/843ac07ae2253270dad562c5.png"},{"id":13847784,"identity":"69bf9be6-bfcb-4cfd-bee8-37d52a0e6d91","added_by":"auto","created_at":"2021-09-21 20:48:28","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":39974,"visible":true,"origin":"","legend":"Relationship between mass loss (ML) by delignification and area magnification (AMd, a), and area per unit mass (Ad/md, b) of the wood samples","description":"","filename":"fig11.png","url":"https://assets-eu.researchsquare.com/files/rs-893506/v1/bc890cf10c3c838774b4fa7f.png"},{"id":13847787,"identity":"0b031a8e-4425-43ce-89b6-91e67c3fa9d5","added_by":"auto","created_at":"2021-09-21 20:48:28","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":34909,"visible":true,"origin":"","legend":"The relationship between the peak temperature of tan δ (Tg) from Figure 6 and the σy from Figure 10. Tg is the average of three measurements. Filled circles: Tc \u003c Tg; open circles: Tc \u003e Tg","description":"","filename":"fig12.png","url":"https://assets-eu.researchsquare.com/files/rs-893506/v1/1987bcc9cd40323c37ccb3c6.png"},{"id":13848293,"identity":"832a5e47-461d-4345-b988-4d2b8bdfe8ba","added_by":"auto","created_at":"2021-09-21 20:54:36","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1548268,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-893506/v1/ea212aa2-3ac5-48df-b4b3-4d523db52af2.pdf"}],"financialInterests":"","formattedTitle":"\u003cp\u003eInfluence of Lignin On Plastic Flow Deformation of Wood\u003c/p\u003e","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe deformation processing of wood can effectively promote its use in various applications, such as furniture, building, construction, automotive, and other daily necessities. Some conventional methods of wood deformation include compression and bending processing using cell-wall deformations (Sandberg et al. 2012). These methods produce simple two-dimensional products while retaining the cell arrangement and structure of the wood. On the other hand, reducing the element size of wood can efficaciously enable the formation of more complex products. Wood-plastic composites (WPCs), which are a mixture of wood powder and plastic, are widely used as building materials (Spear et al. 2015). However, WPCs require an input of energy to miniaturize the wood, which results in the destruction of the original cell structure, such that the products do not retain the wood texture. Furthermore, in conventional processing, the element size of wood and the complexity of the product shape have a contradictory relationship. To date, the formation of a complex shape from solid-state wood has seen little success.\u003c/p\u003e \u003cp\u003eRecently, a new technique, wood flow forming (WFF), has been developed, which can form complex shapes with solid-state wood because it takes advantage of plastic flow deformation (Abe et al. 2020; Abe et al. 2021; Miki et al. 2014-1; Miki et al 2014-2; Miki et al. 2017; Seki et al. 2016; Yamashita et al. 2009). The wood is compressed in a heated mold, which enables it to plastically flow owing to the shear sliding between wood cells to produce a final shaped product (Miki et al. 2017). By applying the traditional plastic forming techniques used for metal and plastic materials, wood products can be efficiently produced in a short time. These techniques can maintain the original cell structure of wood, providing a unique cell-derived texture to the product (Miki et al. 2014-1). However, to improve the deformability of wood during the forming process and the durability of the products, it is necessary to modify the wood by pretreatment before forming. The cell wall and compound middle lamella (CML) are modified by impregnating the wood with resin monomers (Miki et al. 2014-2; Seki et al. 2016) and/or chemical modification (Abe et al. 2020; Abe et al. 2021). WFF has great potential for various applications; however, it requires high temperatures (\u0026lt;\u0026thinsp;100\u0026deg;C) and pressures (\u0026lt;\u0026thinsp;50 MPa), making it energy-intensive and less productive. To solve these problems, this study focused on lignin, which acts as an adhesive and binds the polysaccharides (cellulose and hemicellulose) and cells together. In addition to the CML containing the highest amount of lignin, it is where plastic flow originates; therefore, lignin has a significant effect on plastic flow.\u003c/p\u003e \u003cp\u003eDelignification is a process that can remove lignin without breaking the cellular structure of the wood, and it has primarily been studied for pulping wood. However, in recent years, delignified wood (DW) (Kumar et al. 2021) has attracted attention as a functional material for use in transparent wood (Li et al. 2016; Li et al. 2017; Li et al. 2020), high-strength structural materials (Frey et al. 2019; Jakob et al. 2020; Song et al. 2018), high-performance thermal insulators (Li et al. 2018), and thermal energy storage materials (Montanari et al. 2019). Deformation processing that takes advantage of the flexibility of DW has also been developed (Khakalo et al. 2020; Frey et al. 2018; Frey et al. 2019). Frey et al. (2019) reported that a completely delignified veneer in a water-swollen state exhibited significant deformability in the fiber direction. However, the effect of delignification on the plastic flow deformation associated WFF has not been reported.\u003c/p\u003e \u003cp\u003eThe thermal softening properties of water-swollen wood depend on the glass transition of lignin (Kojiro et al. 2008; Nakajima et al. 2009). When the molecular mass of lignin in wood decreases, its glass transition temperature also decreases, in turn significantly softening the wood (Nakajima et al. 2009). Therefore, it is expected that the changes in lignin due to delignification will promote the plastic flow deformation of wood and reduce the production energy required for WFF.\u003c/p\u003e \u003cp\u003eThe objective of this study was to clarify the effect of lignin on the plastic flow deformation of wood. DW and untreated-wood (UW) samples with different molecular masses and amounts of lignin were prepared by subjecting them to delignification and varying the delignification time. Free compression testing was used to evaluate the deformability of the samples based on the initial resistance to plastic flow and final stretchability. The samples were characterized by attenuated total reflection infrared (ATR-IR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and dynamic viscoelastic measurements. The effects of the chemical structure and glass transition temperature of lignin on the plastic flow deformation of wood are also discussed.\u003c/p\u003e"},{"header":"Materials And Methods","content":"\u003cdiv class=\"Section2\" id=\"Sec3\"\u003e\n \u003ch2\u003eMaterials\u003c/h2\u003e\n \u003cp\u003eWood samples were successively cut in the longitudinal (L) direction from a block of sapwood of a Japanese cypress (\u003cem\u003eChamaecyparis obtusa\u003c/em\u003e) log collected from the Kiso region of Japan. The dimensions of the samples used for the dynamic viscoelastic measurements were 1 mm (L) \u0026times; 30 mm (radial (R) direction) \u0026times; 3 mm (tangential (T) direction). The samples for the other measurements (ATR-IR, NMR, and free compression testing) were 5 mm (L) \u0026times; 5 mm (R) \u0026times; 5 mm (T). Prior to delignification, the wood samples were pre-treated with approximately 100\u0026deg;C distilled water for 4 h, followed by methanol for 6 h to remove the low-molecular-weight components. The pre-treated samples were then dried at 50\u0026deg;C for 18 h and 105\u0026deg;C for 2 h to a relatively constant mass (\u003cem\u003em\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e). One side of the RT surface of some samples (5 mm \u0026times; 5 mm \u0026times; 5 mm) was microtome-finished prior to subsequent delignification.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv class=\"Section2\" id=\"Sec4\"\u003e\n \u003ch2\u003ePreparation of delignified wood samples\u003c/h2\u003e\n \u003cp\u003eThe pre-treated samples were delignified using 4 wt % sodium chlorite (NaClO\u003csub\u003e2\u003c/sub\u003e) containing acetic acid solution (pH 3). The NaClO\u003csub\u003e2\u003c/sub\u003e solution was impregnated into the pre-treated samples under vacuum, and the impregnated samples were treated at 45\u0026deg;C for several reaction times (\u003cem\u003et\u003c/em\u003e\u003csub\u003er\u003c/sub\u003e) of 10, 30, 60, 180, or 360 min. Then, the DW samples were washed several times with distilled water and stored in water at 20\u0026ndash;25\u0026deg;C Note that the DW sample with a \u003cem\u003et\u003c/em\u003e\u003csub\u003er\u003c/sub\u003e longer than 360 min was brittle and fragile; therefore, it was considered unsuitable for WFF and not evaluated in this study. In addition, the UW samples were impregnated with distilled water, instead of the NaClO\u003csub\u003e2\u003c/sub\u003e solution, at 20\u0026ndash;25\u0026deg;C for 500 min or more (\u003cem\u003et\u003c/em\u003e\u003csub\u003er\u003c/sub\u003e: 0 min).\u003c/p\u003e\n \u003cp\u003eThe area in the RT cross section of the water-swollen DW and UW samples (\u003cem\u003es\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e and \u003cem\u003es\u003c/em\u003e\u003csub\u003eu\u003c/sub\u003e, respectively) was measured, and the area increase rate (\u003cem\u003eA\u003c/em\u003e\u003csub\u003eRT\u003c/sub\u003e) of the water-swollen samples due to delignification was calculated using the following formula:\u003c/p\u003e\n \u003cp\u003e\u003cem\u003eA\u003c/em\u003e \u003csub\u003eRT\u003c/sub\u003e = (\u003cem\u003es\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e - \u003cem\u003es\u003c/em\u003e\u003csub\u003eu\u003c/sub\u003e) / \u003cem\u003es\u003c/em\u003e\u003csub\u003eu\u003c/sub\u003e \u0026times; 100 [%]\u003c/p\u003e\n \u003cp\u003eSome of the water swollen samples were then dried at 35\u0026deg;C for 24 h, 50\u0026deg;C for 18 h, and 105\u0026deg;C for 3 h to a relatively constant mass (\u003cem\u003em\u003c/em\u003e\u003csub\u003et\u003c/sub\u003e). The mass loss (\u003cem\u003eML\u003c/em\u003e) due to delignification of the dried samples was calculated using the following formula:\u003c/p\u003e\n \u003cp\u003e\u003cem\u003eML\u003c/em\u003e = (\u003cem\u003em\u003c/em\u003e\u003csub\u003eo\u003c/sub\u003e \u0026ndash; \u003cem\u003em\u003c/em\u003e\u003csub\u003et\u003c/sub\u003e) / \u003cem\u003em\u003c/em\u003e\u003csub\u003eo\u003c/sub\u003e \u0026times; 100 [%]\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv class=\"Section2\" id=\"Sec5\"\u003e\n \u003ch2\u003eAttenuated total reflection infrared (ATR-IR) spectroscopy\u003c/h2\u003e\n \u003cp\u003eThe block-shaped sample (5 mm \u0026times; 5 mm \u0026times; 5 mm) in the dry state was cut in half in the L direction, and ATR-IR measurements were performed near the center of the cut surface (RT surface, that is, near the center of the wood sample). The ATR-IR spectra were measured on a Nicolet 6700 spectrometer (Thermo Scientific Inc., Waltham, MA, USA) at a 4 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e resolution in the standard ATR mode; 32 scans were performed in the range of 4000\u0026ndash;700 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv class=\"Section2\" id=\"Sec6\"\u003e\n \u003ch2\u003eNuclear magnetic resonance (NMR) spectroscopy\u003c/h2\u003e\n \u003cp\u003eSolid-state \u003csup\u003e13\u003c/sup\u003eC NMR spectra were measured on a Varian 400 NMR system spectrometer (Palo Alto, CA) with a Varian 4 mm double-resonance T3 solid probe. The dried samples were placed in a 4 mm ZrO\u003csub\u003e2\u003c/sub\u003e rotor spun at 15 kHz within a temperature range of 20\u0026ndash;22\u0026deg;C. The \u003csup\u003e13\u003c/sup\u003eC cross-polarization and magic-angle spinning (CP-MAS) NMR spectra were collected with a 2.6 \u0026micro;s \u0026pi;/2 pulse at 100.56 MHz for the \u003csup\u003e13\u003c/sup\u003eC nuclei and a 40 ms acquisition period over a 30.7 kHz spectral width. Proton decoupling was performed with an 86 kHz \u003csup\u003e1\u003c/sup\u003eH decoupling radio frequency with a small phase incremental alteration (SPINAL) decoupling pulse sequence. The \u003csup\u003e1\u003c/sup\u003eH-\u003csup\u003e13\u003c/sup\u003eC cross-polarization for the spectrum acquisition was conducted with a 5.0 s recycle delay in 1024 transients using a ramped-amplitude pulse sequence with a 2 ms contact time and a 2.6 \u0026micro;s \u0026pi;/2 pulse for the \u003csup\u003e1\u003c/sup\u003eH nuclei. The amplitude of the \u003csup\u003e1\u003c/sup\u003eH nuclei was linearly ramped down from 92.6% of its final value during the CP contact time. The \u003csup\u003e1\u003c/sup\u003eH spin-lattice relaxation time in the laboratory frame (\u003cem\u003eT\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003eH) was indirectly measured by detecting the \u003csup\u003e13\u003c/sup\u003eC resonance enhanced by the cross-polarization in the \u003csup\u003e13\u003c/sup\u003eC CP-MAS sequence, and was applied after a \u0026pi; pulse to \u003csup\u003e1\u003c/sup\u003eH nuclei using the inversion recovery method. The \u003cem\u003eT\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003eH analysis of the sample was carried out using the same solid-state probe used to obtain the \u003csup\u003e13\u003c/sup\u003eC CP-MAS NMR spectra of the sample at the same contact time and acquisition period.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv class=\"Section2\" id=\"Sec7\"\u003e\n \u003ch2\u003eDynamic viscoelastic measurement\u003c/h2\u003e\n \u003cp\u003eThe temperature dependence of the loss tangent (tan \u003cem\u003e\u0026delta;\u003c/em\u003e) was measured by the tensile forced oscillation method using a thermomechanical analysis apparatus (TMA SS6100; Hitachi High-Tech Science Corp., Tokyo, Japan). The water-swollen sample (1 mm (L) \u0026times; 30 mm (R) \u0026times; 3 mm (T)) immersed in water was subjected to a temperature increase from 30\u0026deg;C to 100\u0026deg;C at a rate of 0.5\u0026deg;C/min. The frequencies for the measurement were 0.01 Hz; the span was 18 mm in the R direction; and the load amplitude was 70\u0026thinsp;\u0026plusmn;\u0026thinsp;20 mN. Viscoelastic behavior is sensitive to the drying and heat history of the samples before the measurement (Kojiro et al. 2008). To unify the histories of the samples, they were heated to 100\u0026deg;C, naturally cooled down to approximately 20\u0026deg;C, and then subjected to measurements in the water-swollen state.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv class=\"Section2\" id=\"Sec8\"\u003e\n \u003ch2\u003eFree compression testing\u003c/h2\u003e\n \u003cp\u003eA uniaxial compression test was carried out using a material testing machine (CATY TC-2kN-NS; Yonekura Mfg. Co. Ltd., Osaka, Japan) (Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e). The testing machine equipped with a container enables horizontal compression tests of samples in a water-swollen state under confined heating conditions while recording load-stroke curves during hydrothermal compression. The compressive temperature (\u003cem\u003eT\u003c/em\u003e\u003csub\u003ec\u003c/sub\u003e) was controlled by heating the water with a cartridge heater immersed in a water pit.\u003c/p\u003e\n \u003cp\u003eThe contained was sealed and heated until the temperature detected by the thermocouple stabilized to the target temperature (\u003cem\u003eT\u003c/em\u003e\u003csub\u003ec\u003c/sub\u003e). Then, the water-swollen sample was placed between the cylinders (diameter: 15 mm) under 2\u0026ndash;3 N to ensure that the R was in the compression direction. The container was closed again to heat the sample to a constant temperature under saturated steam. Five minutes after achieving the temperature \u003cem\u003eT\u003c/em\u003e\u003csub\u003ec\u003c/sub\u003e, compression testing was conducted at a constant speed of 1 mm/min up to 3000 N of the maximum compression load or until the maximum stroke of 5 mm of the testing machine. The \u003cem\u003eT\u003c/em\u003e\u003csub\u003ec\u003c/sub\u003e was set to 40, 60, 80, and 100\u0026deg;C in consideration of the decrease in the glass transition temperature of lignin due to delignification (Nakajima et al. 2009). Compression testing was conducted using three or more samples under the same conditions.\u003c/p\u003e\n \u003cp\u003eFor one sample under each condition, the container was opened after compression testing, and the compressed sample was dried at room temperature to retain its shape while maintaining the stroke after testing, followed by further drying at 105\u0026deg;C for 1 h. Then, the mass (\u003cem\u003em\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e) and thickness (\u003cem\u003er\u003c/em\u003e\u003csub\u003ec\u003c/sub\u003e) in the compression direction were measured. The final compression ratio (\u003cem\u003eC\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e) was calculated as follows:\u003c/p\u003e\n \u003cp\u003e\u003cem\u003eC\u003c/em\u003e \u003csub\u003ed\u003c/sub\u003e = (\u003cem\u003er\u003c/em\u003e\u003csub\u003eb\u003c/sub\u003e \u0026ndash; \u003cem\u003er\u003c/em\u003e\u003csub\u003ec\u003c/sub\u003e) / \u003cem\u003er\u003c/em\u003e\u003csub\u003eb\u003c/sub\u003e \u0026times; 100 [%],\u003c/p\u003e\n \u003cp\u003ewhere \u003cem\u003er\u003c/em\u003e\u003csub\u003eb\u003c/sub\u003e is the dimension in the R direction of the water-swollen sample before the compression testing. The appearance of the compressed sample from the LT planes was observed using an optical microscope (VHX-970F; Keyence Corp., Osaka, Japan). The area of the LT planes before and after compression testing was calculated by binarizing the captured image, and the area magnification (\u003cem\u003eAM\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e) was calculated as follows:\u003c/p\u003e\n \u003cp\u003e\u003cem\u003eAM\u003c/em\u003e \u003csub\u003ed\u003c/sub\u003e = \u003cem\u003eA\u003c/em\u003e\u003csub\u003ec\u003c/sub\u003e / \u003cem\u003eA\u003c/em\u003e\u003csub\u003eb\u003c/sub\u003e \u0026times; 100 [%]\u003c/p\u003e\n \u003cp\u003ewhere \u003cem\u003eA\u003c/em\u003e\u003csub\u003eb\u003c/sub\u003e and \u003cem\u003eA\u003c/em\u003e\u003csub\u003ec\u003c/sub\u003e are the areas of the LT planes of the water-swollen sample before compression and the dried sample after compression, respectively.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"Results And Discussion","content":"\u003cdiv class=\"Section2\" id=\"Sec10\"\u003e\n \u003ch2\u003eCharacterization of untreated and delignified samples\u003c/h2\u003e\n \u003cdiv class=\"Section3\" id=\"Sec11\"\u003e\n \u003ch2\u003eThe mass loss (ML) and the area increase rate (A\u003csub\u003eRT\u003c/sub\u003e)\u003c/h2\u003e\n \u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e shows the variation in \u003cem\u003eML\u003c/em\u003e (left axis, closed circles) and \u003cem\u003eA\u003c/em\u003e\u003csub\u003eRT\u003c/sub\u003e (right axis, open circles) as functions of delignification time (\u003cem\u003et\u003c/em\u003e\u003csub\u003er\u003c/sub\u003e). \u003cem\u003eML\u003c/em\u003e linearly increased with increasing delignification time, i.e., the lignin content of the wood sample decreased with delignification. Based on the original lignin content of cypress of 33 % (Pettersen 1984), approximately 40 % of the lignin was removed after 360 min of \u003cem\u003et\u003c/em\u003e\u003csub\u003er\u003c/sub\u003e. The \u003cem\u003eA\u003c/em\u003e\u003csub\u003eRT\u003c/sub\u003e also increased with increasing delignification time. The increase in the dimensions of the water-swollen sample was caused by the swelling of the cell wall and CML. The total volume of the adsorbed water in the cell wall and CML exceeded the volume of lignin removed by delignification.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv class=\"Section3\" id=\"Sec12\"\u003e\n \u003ch2\u003eATR-IR spectroscopy\u003c/h2\u003e\n \u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e shows the ATR-IR spectra of the UW and DW samples, which indicate that the chemical structure of the wood sample changes with delignification. The area of the peak at 1490\u0026ndash;1530 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which corresponds to the skeletal vibrations of the benzene ring in lignin, decreased with increasing delignification time. This indicates the reaction is initiated during the early stages of delignification, as the benzene rings of lignin begin to disappear. The absorbance peak at 1725\u0026ndash;1750 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which corresponds to the C\u0026thinsp;=\u0026thinsp;O stretch of lignin and hemicellulose, initially increased (\u003cem\u003et\u003c/em\u003e\u003csub\u003er\u003c/sub\u003e = 10 min), then decreased in the later stages (\u003cem\u003et\u003c/em\u003e\u003csub\u003er\u003c/sub\u003e = 360 min). The initial increase in this peak can be attributed to the reaction between NaClO\u003csub\u003e2\u003c/sub\u003e and lignin, which indicates that the aromatic ring was cleaved (Li et al. 2017). On the other hand, the decrease in the later stage was due to the decrease in the amount of lignin. The initial structural changes of lignin due to delignification consisted of the elimination and cleavage of the benzene ring, followed by the elimination of the benzene ring.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv class=\"Section3\" id=\"Sec13\"\u003e\n \u003ch2\u003eSolid state NMR measurements\u003c/h2\u003e\n \u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e shows the \u003csup\u003e13\u003c/sup\u003eC cross-polarization (CP)MAS NMR spectra for each substituent in the UW and DW samples. Signals corresponding to biomass constituents in the wood were assigned based on our previous report (Nishida et al. 2014). The carbohydrates appeared as relatively large and sharp signals in the range of 60\u0026ndash;110 ppm. However, most of the signals for cellulose and hemicellulose, except cellulose C4, overlapped with each other, and the crystalline and amorphous signals for cellulose C4 and C6 could be separately observed. The aromatic and olefinic groups in lignin appeared as broader signals in the range of 110\u0026ndash;160 ppm, while the methoxy groups in lignin appeared as isolated signals at 56 ppm.\u003c/p\u003e\n \u003cp\u003eDuring the early stages of delignification (\u003cem\u003et\u003c/em\u003e\u003csub\u003er\u003c/sub\u003e = 10 min), the signal intensity of the OCH\u003csub\u003e3\u003c/sub\u003e (56 ppm) and aromatic (110\u0026ndash;160 ppm) groups in lignin rapidly decreased was observed. Meanwhile, the intensity of the C\u0026thinsp;=\u0026thinsp;O signal at a lower magnetic field (172 ppm) increased for 30 min, then gradually decreased as delignification progressed. The trends of the signal intensities for the aromatic and C\u0026thinsp;=\u0026thinsp;O groups in the \u003csup\u003e13\u003c/sup\u003eC CP-MAS NMR spectra were similar to those in the ATR-IR spectra. Therefore, in the first step of delignification, oxidation of the benzene ring of the guaiacyl unit on the surface of the lignin unit afforded two carboxyl groups at C3 and C4 positions (Hamzeh et al. 2008). Next, the cleavage of C\u0026ndash;C and C\u0026ndash;O bonds that provide bridging with other aromatic ring units at the inner site of the lignin resulted in higher ML of the delignified wood, because the oxidized portion was removed (Tarvo et al. 2010 and Qu el al. 2020). However, the signal pattern of the carbohydrates in the \u003csup\u003e13\u003c/sup\u003eC CP-MAS NMR spectra barely changed as a result of delignification, indicating that the chemical structures of cellulose and hemicellulose remained unchanged as delignification progressed.\u003c/p\u003e\n \u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e shows the \u003csup\u003e1\u003c/sup\u003eH spin-lattice relaxation times in the laboratory frame (\u003cem\u003eT\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003eH) of the dry samples. A minimal change was observed in the \u003cem\u003eT\u003c/em\u003e \u003csub\u003e1\u003c/sub\u003eH values at 65 and 75 ppm for \u003cem\u003et\u003c/em\u003e\u003csub\u003er\u003c/sub\u003e = 60 min, but tended to increase after 60 min. For wood in the dry state, the spin-lattice relaxation for each wood constituent occurs via lignin, which has the shortest \u003cem\u003eT\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003eH value among the biomass constituents (Nishida et al. 2017). The increase in \u003cem\u003eT\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003eH of the DW samples at \u003cem\u003et\u003c/em\u003e\u003csub\u003e\u003cem\u003er\u003c/em\u003e\u003c/sub\u003e \u0026gt; 60 min was caused by the loss of lignin units with the short \u003cem\u003eT\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003eH values. In the early stages (\u003cem\u003et\u003c/em\u003e\u003csub\u003er\u003c/sub\u003e \u0026lt; 60 min), delignification occurred at the distal part of the polysaccharide chain (for example, CML), with minimal effect on the polysaccharides. With increasing \u003cem\u003et\u003c/em\u003e\u003csub\u003er\u003c/sub\u003e (\u0026gt;\u0026thinsp;60 min), the reaction progressed toward the polysaccharide chain, resulting in an increase in the \u003cem\u003eT\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003eH value because of the reduced interactions between the polysaccharides and lignin.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv class=\"Section3\" id=\"Sec14\"\u003e\n \u003ch2\u003eDynamic viscoelastic measurement\u003c/h2\u003e\n \u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e shows the temperature dependence of tan \u003cem\u003e\u0026delta;\u003c/em\u003e for the water-swollen samples. The tan \u003cem\u003e\u0026delta;\u003c/em\u003e peak is attributable to the glass transition of lignin, and the peak temperature (\u003cem\u003eT\u003c/em\u003eg) in Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e corresponds to the glass transition temperature of lignin in the wood samples (Kojiro et al. 2008; Nakajima et al. 2009). As delignification progressed, the \u003cem\u003eT\u003c/em\u003eg gradually decreased and the shape of the peak broadened. These trends correspond well with those by Nakajima et al. (2009); they measured tan \u003cem\u003e\u0026delta;\u003c/em\u003e of Japanese cypress delignified by NaClO\u003csub\u003e2\u003c/sub\u003e and reported that the shift of \u003cem\u003eT\u003c/em\u003eg and the broadening of the tan \u003cem\u003e\u0026delta;\u003c/em\u003e peak were due to the decrease in the molecular mass and amount of lignin, respectively. The results in Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e demonstrate that the molecular mass and amount of lignin begin to decrease in the early stages of delignification (\u003cem\u003et\u003c/em\u003e\u003csub\u003er\u003c/sub\u003e: 10 min).\u003c/p\u003e\n \u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv class=\"Section2\" id=\"Sec15\"\u003e\n \u003ch2\u003eDeformability of untreated and delignified samples\u003c/h2\u003e\n \u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e shows the relationship between the nominal compressive stress (\u003cem\u003e\u0026sigma;\u003c/em\u003e) and the compression ratio (\u003cem\u003eC\u003c/em\u003e) during compression testing at each \u003cem\u003eT\u003c/em\u003e\u003csub\u003ec\u003c/sub\u003e. The \u003cem\u003e\u0026sigma;\u003c/em\u003e slightly increased to a \u003cem\u003eC\u003c/em\u003e of approximately 60% in all samples, during which the cell lumens in the wood samples gradually closed because of the buckling of the cell wall. As \u003cem\u003eC\u003c/em\u003e increases, \u003cem\u003e\u0026sigma;\u003c/em\u003e significantly increases at a constant rate, followed by an inflection point at which the rate of increase in \u003cem\u003e\u0026sigma;\u003c/em\u003e decreases (indicated by the arrow in Fig. \u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e). These compressive behaviors were similar to those observed in our previous report (Miki et al. 2017). Before the inflection point, the cell lumens were completely closed and the sample was consolidated, resulting in a rapid increase in \u003cem\u003e\u0026sigma;\u003c/em\u003e. After the inflection point, the rate of increase in \u003cem\u003e\u0026sigma;\u003c/em\u003e decreased because the wood sample plastically deformed in the unconstrained direction (L or R). The inflection point was also detected in the flat region for \u003cem\u003et\u003c/em\u003e\u003csub\u003er\u003c/sub\u003e = 360 min at 100\u0026deg;C (shown by (d) ▽); under this condition, plastic deformation occurred before the cell lumens were completely closed. The inflection points were not detected at 40\u0026deg;C, 60\u0026deg;C, and 80\u0026deg;C for \u003cem\u003et\u003c/em\u003e\u003csub\u003er\u003c/sub\u003e = 360 min and at 100\u0026deg;C for \u003cem\u003et\u003c/em\u003e\u003csub\u003er\u003c/sub\u003e = 180 min. This is because the inflection point due to the plastic deformation overlapped with the region denoting the rapid increase in \u003cem\u003e\u0026sigma;\u003c/em\u003e.\u003c/p\u003e\n \u003cp\u003eThe \u003cem\u003e\u0026sigma;\u003c/em\u003e at the inflection point detected in Fig. \u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e is the nominal compressive stress at the starting point of flow (\u003cem\u003e\u0026sigma;\u003c/em\u003e\u003csub\u003e\u003cem\u003ey\u003c/em\u003e\u003c/sub\u003e), which indicates the initial resistance of the samples to plastic flow. Regardless of temperature, the longer the delignification time, the smaller the \u003cem\u003e\u0026sigma;\u003c/em\u003e\u003csub\u003e\u003cem\u003ey\u003c/em\u003e\u003c/sub\u003e. Therefore, the delignification process reduced the initial resistance to plastic flow.\u003c/p\u003e\n \u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003e shows photographs of the samples that were dried after free compression testing to preserve their shape. After compression testing, all the samples were able to maintain their stretched state after drying by pressure; they only stretched in the T, and not in the L direction. Such anisotropy of plastic flow deformation is consistent with the phenomenon observed in our previous report (Miki et al. 2017). At high \u003cem\u003eT\u003c/em\u003e\u003csub\u003ec\u003c/sub\u003e, the sample that was delignification for a longer time exhibited the highest \u003cem\u003eC\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e of 91% (Fig. \u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003e), indicating that a considerably thin product could be fabricated from solid-state wood.\u003c/p\u003e\n \u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003e shows an SEM image of the RT surface of the samples. None of the samples displayed cell wall destruction. Furthermore, evidence of mutual positional change between the cells was confirmed. The plastic flow deformation was mainly caused by the shear sliding phenomenon between the cells and at the boundary of the CML, regardless of the difference in \u003cem\u003eT\u003c/em\u003e\u003csub\u003ec\u003c/sub\u003e and lignin state (such as quantity, quality, and molecular motility). The sample subjected to a longer delignification time (Fig. \u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e (c, d)) displayed several slip surfaces, and plastic flow occurred in units with a smaller number of cells. The cells slightly protruding in the L direction were also observed, suggesting that delignification promoted slip deformation in the L direction.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv class=\"Section2\" id=\"Sec16\"\u003e\n \u003ch2\u003eInfluence of lignin on plastic flow deformation of wood\u003c/h2\u003e\n \u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e10\u003c/span\u003e shows the relationship between the \u003cem\u003eML\u003c/em\u003e by delignification of wood samples and \u003cem\u003e\u0026sigma;\u003c/em\u003e\u003csub\u003e\u003cem\u003ey\u003c/em\u003e\u003c/sub\u003e, which indicates the initial resistance of wood samples to plastic flow. The higher the \u003cem\u003eT\u003c/em\u003e\u003csub\u003ec\u003c/sub\u003e, the lower the initial resistance to plastic flow. The initial resistance to plastic flow was logarithmically reduced to a \u003cem\u003eML\u003c/em\u003e of 4%, which indicates that the improvement in plastic deformability was significant in the early stages of delignification.\u003c/p\u003e\n \u003cp\u003eThe final stretchability of the samples was evaluated using the \u003cem\u003eAM\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e. Figure \u003cspan class=\"InternalRef\"\u003e11\u003c/span\u003e(a) shows the relationship between \u003cem\u003eML\u003c/em\u003e and the \u003cem\u003eAM\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e of the wood samples. The \u003cem\u003eAM\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e increased with increasing \u003cem\u003eT\u003c/em\u003e\u003csub\u003ec\u003c/sub\u003e. The value of \u003cem\u003eAM\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e was almost unchanged during the early stages of delignification (\u003cem\u003eML\u003c/em\u003e: \u0026lt; 4%); however, as delignification progressed, \u003cem\u003eAM\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e remarkably increased and the sample was extensively stretched. The value of \u003cem\u003eAM\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e reached a maximum of 2.7 times for the longest \u003cem\u003et\u003c/em\u003e\u003csub\u003er\u003c/sub\u003e of 360 min at the highest \u003cem\u003eT\u003c/em\u003e\u003csub\u003ec\u003c/sub\u003e of 100\u0026deg;C. Furthermore, delignification caused a change in the density of the sample; therefore, the area per unit mass (\u003cem\u003eA\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e/\u003cem\u003em\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e), which is an index of stretchability, was calculated based on the area of the LT surface (\u003cem\u003eA\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e) and the mass (\u003cem\u003em\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e) of the dried compressed samples. The relationship between \u003cem\u003eML\u003c/em\u003e and \u003cem\u003eA\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e/\u003cem\u003em\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e of the wood samples is shown in Fig. \u003cspan class=\"InternalRef\"\u003e11\u003c/span\u003e(b). The change in \u003cem\u003eA\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e/\u003cem\u003em\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e depending on the \u003cem\u003eT\u003c/em\u003e\u003csub\u003e\u003cem\u003ec\u003c/em\u003e\u003c/sub\u003e and the tendency toward \u003cem\u003eML\u003c/em\u003e were similar to those of \u003cem\u003eAM\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e (Fig. \u003cspan class=\"InternalRef\"\u003e11\u003c/span\u003e(a)). In contrast to the tendency of \u003cem\u003e\u0026sigma;\u003c/em\u003e\u003csub\u003ey\u003c/sub\u003e in Fig. \u003cspan class=\"InternalRef\"\u003e10\u003c/span\u003e, the stretchability did not increase during the early stages of delignification (\u003cem\u003eML\u003c/em\u003e: \u0026lt; 4%), but a significant increase was observed after a \u003cem\u003eML\u003c/em\u003e of 4% (\u003cem\u003et\u003c/em\u003e\u003csub\u003er\u003c/sub\u003e: \u0026gt; 60 min). Delignification with NaClO\u003csub\u003e2\u003c/sub\u003e initially proceeds from the lignin-rich CML during the early stages of the reaction and selectively softens the CML region. Then, the reaction and softening of the cell wall progresses during the later stages of the reaction (Xu et al. 2020). The results of \u003cem\u003eT\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003eH in this study (Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e) suggest that the lignin removal in the vicinity of the polysaccharide chains that make up the cell wall proceeds during the later stages of delignification (\u003cem\u003et\u003c/em\u003e\u003csub\u003er\u003c/sub\u003e \u0026gt; 60 min), which increases cell wall flexibility. Although the \u003cem\u003eT\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003eH results of the dry state are represented, water was adsorbed between the constituents in the cell wall and CML during compression testing. Because the total volume of the adsorbed water on the cell wall increased as delignification progressed (Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e), the adsorbed water acted as an intermolecular lubricant and contributed to the increase in stretchability. Therefore, the increase in stretchability that was observed during the later stages of delignification was likely due to the increased flexibility of the cell wall rather than the cleavage of the lignin network in the CML that occurred during the early stages of delignification. This flexibility of the cell wall generated many slip surfaces during plastic flow (Fig. \u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003e (c and d)).\u003c/p\u003e\n \u003cp\u003eA strong correlation is observed between the \u003cem\u003eT\u003c/em\u003eg (Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e) and \u003cem\u003e\u0026sigma;\u003c/em\u003e\u003csub\u003e\u003cem\u003ey\u003c/em\u003e\u003c/sub\u003e (Fig. \u003cspan class=\"InternalRef\"\u003e10\u003c/span\u003e), as shown in Fig. \u003cspan class=\"InternalRef\"\u003e12\u003c/span\u003e. This indicates that the reduction in the molecular mass of lignin significantly contributed toward the improved deformability observed during the initial resistance to plastic flow. Because the plastic flow of wood was shear failure originating from the CML (Fig. \u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003e), structural defects in the CML generated during the early stages of delignification considerably affect the initiation of plastic flow.\u003c/p\u003e\n \u003cp\u003eIn compression testing, at \u003cem\u003eT\u003c/em\u003e\u003csub\u003ec\u003c/sub\u003e \u0026gt; \u003cem\u003eT\u003c/em\u003eg (filled circles) and \u003cem\u003eT\u003c/em\u003e\u003csub\u003ec\u003c/sub\u003e \u0026lt; \u003cem\u003eT\u003c/em\u003eg (open circles), the molecular motility of lignin greatly differed because the lignin was in the glass and rubber states, respectively. Nevertheless, the different molecular motilities have minimal impact on the relationship between \u003cem\u003eT\u003c/em\u003eg and \u003cem\u003e\u0026sigma;\u003c/em\u003e\u003csub\u003e\u003cem\u003ey\u003c/em\u003e\u003c/sub\u003e. Therefore, the influence of the molecular motility of lignin on the deformability was considered to be negligible.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"Conclusion","content":"\u003cp\u003eWe clarified the influence of lignin in wood on plastic flow deformation due to the shear sliding between wood cells. The ATR-IR and solid-state NMR spectroscopic analyses of delignification showed that the oxidative opening of guaiacyl ring on the surface of the lignin unit occurred in the early stages and the oxidized portion was released from the inner site of the lignin unit when delignification time was extended. Free compression testing was performed to evaluate the deformability (the initial resistance to plastic flow and final stretchability) of the samples. The decrease in the molecular mass of lignin in the CML that occurs in the early stages of delignification reduces the initial resistance to plastic flow deformation. However, the effect of changes in the chemical structure of lignin that occurred during the early stages of delignification on the final stretchability was relatively small. Furthermore, as the delignification progressed, the amount of lignin in wood decreased and the reaction reached the vicinity of the polysaccharide chains, resulting in a remarkable increase in stretchability. We also observed that an increase in \u003cem\u003eT\u003c/em\u003e\u003csub\u003ec\u003c/sub\u003e tends to improve the plastic deformability. However, the effect of the changes in molecular motility due to the glass transition of lignin on the deformability was very small. These results suggest that the molecular mass and amount of lignin in the CML and in the vicinity of the polysaccharide chains in the cell wall affect the plastic flow deformation of wood. Furthermore, the strategy of controlling the molecular mass and amount of lignin via delignification can yield more productive materials, as well as metal and plastic materials for achieving low-energy plastic flow deformation of wood. In this study, we examined delignified wood in the water-swollen state, but in the future, we plan to investigate the effect of other adsorbents, such as resin monomers, instead of water, on the plastic deformability of delignified wood.\u003c/p"},{"header":"Declarations","content":"\u003cp\u003eFunding\u0026nbsp;(information that explains whether and by whom the research was supported)\u003c/p\u003e\n\u003cp\u003eThis study was financially supported by Asahi Kasei Corporation.\u003c/p\u003e\n\u003cp\u003eConflicts of interest/Competing interests\u0026nbsp;(include appropriate disclosures)\u003c/p\u003e\n\u003cp\u003eThe authors declare no conflict of interest.\u003c/p\u003e\n\u003cp\u003eAvailability of data and material\u0026nbsp;(data transparency)\u003c/p\u003e\n\u003cp\u003eThe data that support the findings of this study are available from the corresponding author, Masako Seki, upon reasonable request.\u003c/p\u003e\n\u003cp\u003eCode availability\u0026nbsp;(software application or custom code)\u003c/p\u003e\n\u003cp\u003eNot applicable\u003c/p\u003e\n\u003cp\u003eAdditional declarations for articles in life science journals that report the results of studies involving humans and/or animals\u003c/p\u003e\n\u003cp\u003eNot applicable\u003c/p\u003e\n\u003cp\u003eEthics approval\u0026nbsp;(include appropriate approvals or waivers)\u003c/p\u003e\n\u003cp\u003eNot applicable\u003c/p\u003e\n\u003cp\u003eConsent to participate\u0026nbsp;(include appropriate statements)\u003c/p\u003e\n\u003cp\u003eWe agreed.\u003c/p\u003e\n\u003cp\u003eConsent for publication\u0026nbsp;(include appropriate statements\u003cem\u003e)\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eWe agreed.\u003c/p\u003e\n\u003cp\u003eAcknowledgements\u003c/p\u003e\n\u003cp\u003eThe authors like to thank the Asahi Kasei Corporation for financial support towards this study.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eAbe M, Enomoto Y, Seki M, Miki T (2020) Esterification of solid wood for plastic forming. 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J Wood Chem Tech 30:230\u0026ndash;268. http://dx.doi.org/10.1080/02773810903461476\u003c/li\u003e\n \u003cli\u003eXu E, Wang D, Lin L (2020) Chemical structure and mechanical properties of wood cell walls treated with acid and alkali solution. Forests 11(1): 87. https://doi.org/10.3390/f11010087\u003c/li\u003e\n \u003cli\u003eYamashita O, Yokochi H, Miki T, Kanayama K (2009) The pliability of wood and its application to molding. J Mater Proc Tech 209(12\u0026ndash;13):5239\u0026ndash;5244. https://doi.org/10.1016/j.jmatprotec.2008. 12.011\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"cellulose","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"cels","sideBox":"Learn more about [Cellulose](https://www.springer.com/journal/10570)","snPcode":"10570","submissionUrl":"https://submission.nature.com/new-submission/10570/3","title":"Cellulose","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Wood, Plastic deformation, Deformability, Delignification, NMR","lastPublishedDoi":"10.21203/rs.3.rs-893506/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-893506/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eIn this study, we clarified the influence of lignin in wood on its plastic flow deformation due to shear sliding of wood cells. Wood samples were subjected to delignification, where the lignin structure gradually changed, and characterized for their chemical and physicochemical properties, and deformability by free compression testing. The delignified wood deformed by efficient stretching and maintained its cell structures at a lower pressure compared to the untreated wood. The deformability was evaluated from two viewpoints: the initial resistance to plastic flow and final stretchability. The deformability of the delignified and untreated wood increased with increasing compressive temperature, even though the changes in molecular motility associated with the glass transition of lignin contributed minimally to the improvement in deformability. In the early stages of delignification, the molecular mass of lignin in the compound middle lamella decreased, which reduced the initial resistance to plastic flow. However, during the early stages of delignification, the stretchability of delignified wood was scarcely affected by changes in lignin. As the amount of lignin was further reduced and delignification proceeded in the vicinity of the polysaccharides, the stretchability significantly improved. The correlation between chemical and physicochemical properties and plastic flow deformability presented in this paper will be helpful for low-energy and highly productive forming of solid-state wood.\u003c/p\u003e","manuscriptTitle":"Influence of Lignin On Plastic Flow Deformation of Wood","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2021-09-21 20:45:26","doi":"10.21203/rs.3.rs-893506/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Major revisions","date":"2021-11-01T10:29:35+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2021-09-28T09:19:37+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2021-09-17T12:52:43+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"Cellulose","date":"2021-09-12T21:41:44+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2021-09-09T11:44:35+00:00","index":"","fulltext":""},{"type":"submitted","content":"Cellulose","date":"2021-09-09T03:35:44+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"cellulose","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"cels","sideBox":"Learn more about [Cellulose](https://www.springer.com/journal/10570)","snPcode":"10570","submissionUrl":"https://submission.nature.com/new-submission/10570/3","title":"Cellulose","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"6a46bc6e-1346-4af8-a5ac-d3dfead7afca","owner":[],"postedDate":"September 21st, 2021","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":7349097,"name":"Polymer Science"}],"tags":[],"updatedAt":"2022-03-22T07:32:23+00:00","versionOfRecord":[],"versionCreatedAt":"2021-09-21 20:45:26","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-893506","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-893506","identity":"rs-893506","version":["v1"]},"buildId":"rHA-KDH7Qsr4HCuvH75dn","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}