Inuence of Lignin On Plastic Flow Deformation of Wood

In this study, we claried the inuence of lignin in wood on its plastic ow deformation due to shear sliding of wood cells. Wood samples were subjected to delignication, where the lignin structure gradually changed, and characterized for their chemical and physicochemical properties, and deformability by free compression testing. The delignied wood deformed by ecient 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 ow and nal stretchability. The deformability of the delignied 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 delignication, the molecular mass of lignin in the compound middle lamella decreased, which reduced the initial resistance to plastic ow. However, during the early stages of delignication, the stretchability of delignied wood was scarcely affected by changes in lignin. As the amount of lignin was further reduced and delignication proceeded in the vicinity of the polysaccharides, the stretchability signicantly improved. The correlation between chemical and physicochemical properties and plastic ow deformability presented in this paper will be helpful for low-energy and highly productive forming of solid-state wood. 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 ow deformation of wood. Furthermore, the strategy of controlling the molecular mass and amount of lignin via delignication can yield more productive materials, as well as metal and plastic materials for achieving low-energy plastic ow deformation of wood. In this study, we examined delignied 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 delignied wood.


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 e caciously 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 ow forming (WFF), has been developed, which can form complex shapes with solid-state wood because it takes advantage of plastic ow deformation ( ). The wood is compressed in a heated mold, which enables it to plastically ow owing to the shear sliding between wood cells to produce a nal shaped product ). By applying the traditional plastic forming techniques used for metal and plastic materials, wood products can be e ciently 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 modi ed by impregnating the wood with resin monomers (Miki et al. 2014-2;Seki et al. 2016) and/or chemical modi cation (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 ow originates; therefore, lignin has a signi cant effect on plastic ow.
Deligni cation 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, deligni ed wood (DW) ( 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 signi cantly softening the wood (Nakajima et al. 2009). Therefore, it is expected that the changes in lignin due to deligni cation will promote the plastic ow 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 ow deformation of wood. DW and untreated-wood (UW) samples with different molecular masses and amounts of lignin were prepared by subjecting them to deligni cation and varying the deligni cation time. Free compression testing was used to evaluate the deformability of the samples based on the initial resistance to plastic ow and nal stretchability. The samples were characterized by attenuated total re ection 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 ow deformation of wood are also discussed.

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 deligni cation, 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-nished prior to subsequent deligni cation.

Preparation of deligni ed wood samples
The pre-treated samples were deligni ed 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 deligni cation was calculated using the following formula: 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 deligni cation of the dried samples was calculated using the following formula: Attenuated total re ection 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 Scienti c 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 nal 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 con ned 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 deligni cation (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 nal compression ratio (C d ) was calculated as follows: 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 magni cation (AM d ) was calculated as follows: 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 deligni ed samples The mass loss (ML) and the area increase rate (A RT ) 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 deligni cation. 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 deligni cation time. This indicates the reaction is initiated during the early stages of deligni cation, 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 deligni cation 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 ole nic 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 deligni cation (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 eld (172 ppm) increased for 30 min, then gradually decreased as deligni cation 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 rst step of  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 (Tg) in Fig. 6 corresponds to the glass transition temperature of lignin in the wood samples (Kojiro et al. 2008;Nakajima et al. 2009). As deligni cation progressed, the Tg 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 deligni ed by NaClO 2 and reported that the shift of Tg 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 deligni cation (t r : 10 min). 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 signi cantly increases at a constant rate, followed by an in ection 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 . Before the in ection point, the cell lumens were completely closed and the sample was consolidated, resulting in a rapid increase in σ. After the in ection point, the rate of increase in σ decreased because the wood sample plastically deformed in the unconstrained direction (L or R). The in ection point was also detected in the at 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 in ection 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 in ection point due to the plastic deformation overlapped with the region denoting the rapid increase in σ.

Deformability of untreated and deligni ed samples
The σ at the in ection point detected in Fig. 7 is the nominal compressive stress at the starting point of ow (σ y ), which indicates the initial resistance of the samples to plastic ow. Regardless of temperature, the longer the deligni cation time, the smaller the σ y . Therefore, the deligni cation process reduced the initial resistance to plastic ow. 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 ow deformation is consistent with the phenomenon observed in our previous report . At high T c , the sample that was deligni cation 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 con rmed. The plastic ow 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 deligni cation time (Fig. 7 (c, d)) displayed several slip surfaces, and plastic ow occurred in units with a smaller number of cells. The cells slightly protruding in the L direction were also observed, suggesting that deligni cation promoted slip deformation in the L direction.
In uence of lignin on plastic ow deformation of wood Figure 10 shows the relationship between the ML by deligni cation of wood samples and σ y , which indicates the initial resistance of wood samples to plastic ow. The higher the T c , the lower the initial resistance to plastic ow. The initial resistance to plastic ow was logarithmically reduced to a ML of 4%, which indicates that the improvement in plastic deformability was signi cant in the early stages of deligni cation.
The nal stretchability of the samples was evaluated using the AM d . Figure 11 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 deligni cation (ML: < 4%), but a signi cant increase was observed after a ML of 4% (t r : > 60 min). Deligni cation 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 deligni cation (t r > 60 min), which increases cell wall exibility. 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 deligni cation 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 deligni cation was likely due to the increased exibility of the cell wall rather than the cleavage of the lignin network in the CML that occurred during the early stages of deligni cation. This exibility of the cell wall generated many slip surfaces during plastic ow (Fig. 9 (c and d)).
A strong correlation is observed between the Tg (Fig. 6) and σ y (Fig. 10), as shown in Fig. 12. This indicates that the reduction in the molecular mass of lignin signi cantly contributed toward the improved deformability observed during the initial resistance to plastic ow. Because the plastic ow of wood was shear failure originating from the CML (Fig. 9), structural defects in the CML generated during the early stages of deligni cation considerably affect the initiation of plastic ow.
In compression testing, at T c > Tg ( lled circles) and T c < Tg (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 Tg and σ y . Therefore, the in uence of the molecular motility of lignin on the deformability was considered to be negligible.

Conclusion
We clari ed the in uence of lignin in wood on plastic ow deformation due to the shear sliding between wood cells. The ATR-IR and solid-state NMR spectroscopic analyses of deligni cation 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 deligni cation time was extended. Free compression testing was performed to evaluate the deformability (the initial resistance to plastic ow and nal stretchability) of the samples. The decrease in the molecular mass of lignin in the CML that occurs in the early stages of deligni cation reduces the initial resistance to plastic ow deformation. However, the effect of changes in the chemical structure of lignin that occurred during the early stages of deligni cation on the nal stretchability was relatively small. Furthermore, as the deligni cation 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 ow deformation of wood. Furthermore, the strategy of controlling the molecular mass and amount of lignin via deligni cation can yield more productive materials, as well as metal and plastic materials for achieving low-energy plastic ow deformation of wood. In this study, we examined deligni ed 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 deligni ed wood.

Declarations
Funding (information that explains whether and by whom the research was supported) This study was nancially supported by Asahi Kasei Corporation.

Con icts of interest/Competing interests (include appropriate disclosures)
The authors declare no con ict of interest.

Availability of data and material (data transparency)
The data that support the ndings of this study are available from the corresponding author, Masako Seki, upon reasonable request. 29. Xu E, Wang D, Lin L (2020) Chemical structure and mechanical properties of wood cell walls treated with acid and alkali solution.  Figure 1 Schematic drawing of the experimental apparatus  ATR-IR spectra of the untreated-wood (tr = 0 min) and deligni ed wood (tr = 10, 30, 60, 180 and 360 min) samples in the dry state  The SEM images of the radial-tangential surfaces of untreated wood (tr = 0 min) and deligni ed wood (tr = 360 min) samples after free compression testing