Study on the Effect of Lignin Removal Rate on the Dielectric Properties of Delignified Materials

Abstract

To investigate the relationship between the lignin removal rate change of wood and its dielectric properties, this study employed Mongolian Scotch Pine and Paulownia as the test materials. The acidic sodium chlorite method was used to delignify the treated material, and the lignin removal rate was determined at a specified reaction time interval to ascertain the dielectric constant and the tangent of the dielectric loss angle. The findings revealed that: As the delignification process progresses, the lignin content declines, accompanied by a reduction in the dielectric constant at elevated frequencies. This decline reaches a plateau near 10 MHz. The results demonstrated that the dielectric constant of the samples decreased with an increase in frequency and exhibited a stabilizing effect near 10 MHz. However, the dielectric constant of delignified wood was significantly higher than that of untreated wood. Additionally, the dielectric constant exhibited a linear relationship with the increase in lignin removal rate, while the tangent of the dielectric loss angle demonstrated a tendency to increase and then decrease. An investigation into the dielectric properties of delignified wood can yield valuable data and a theoretical foundation for the development of wood-based dielectric materials.

1. Introduction

The internal structure and chemical composition of wood are closely related to its dielectric properties. Consequently, non-destructive testing techniques based on dielectric properties can be used to determine the internal defects of wood, assess its material strength characteristics, and deduce other properties of wood. The study of the dielectric properties of wood represents a significant avenue of inquiry for elucidating its electrical properties and dipole dynamics. Concurrently, the advancement of synthetic polymer materials for the substrates of conventional electronic devices in the direction of recyclability, decarbonization, and biodegradability has positioned wood as a focal point in investigating novel basic materials for electrical properties.

The primary constituents of the wood cell wall are cellulose, hemicellulose, and lignin, which are intimately associated with and collectively represent over 90% of the cell wall system [1]. Cellulose exists in the cell wall in the form of microfilaments and constitutes the microskeletal structure [2]. It is generally accepted that cellulose and lignin are amorphous substances wrapped around and filling the interstices of the microskeletal structure [3].

Lignin is an aromatic polymer compound with a three-dimensional structure, comprising phenylpropane-like structural units linked by carbon-carbon and ether bonds [4]. It contains a variety of reactive functional groups, including phenolic hydroxyl groups, methoxy groups, and carbonyl groups. The cellulose polymer chains have a high proportion of hydroxyl groups, which are capable of undergoing orientational polarization. However, intermolecular and intramolecular hydrogen bonding limits the orientational polarization in the AC electric field [5]. The Gu team [6] found that polar functional groups promote the formation of covalent and hydrogen bonds, and lignin molecules produce clusters of aggregates, which generate structural blockage under the interaction force and prevent polar groups from polarization shift in the electric field. Therefore, the delignification of wood can reduce the structural blockage phenomenon to a certain extent and optimize the dielectrophoretic polarization phenomenon.

Wood is hygroscopic, whereas water molecules are polar, readily orientated, and polarized. The relationship between the dielectric properties of wood below the fiber saturation point (FSP) and its moisture content (MC) has been demonstrated to exhibit an approximate exponential increase in dielectric constant (ε′) with rising MC, as evidenced by [7]. Nevertheless, the impact of alterations in the internal chemical composition of delignified wood (DW) with regard to clustering and structural blocking on the alterations in the dielectric properties of DW across different moisture content ranges remains to be explored from a theoretical perspective. In particular, wood can be employed as a novel environmentally friendly substrate material for electronic products in the fields of electronics and nanotechnology. This is due to the fact that it is a renewable and biodegradable resource, which may contribute to its use in the direction of insulation or energy storage. It is therefore essential to gain an understanding of the coordination mechanisms of the internal chemical composition of wood and the influence of its own dielectric properties.

The objective of this study was to analyze the mechanism of change in the dielectric properties of delignified wood with varying lignin contents following the delignification treatment of the wood. The effect of the lignin removal rate on the dielectric properties was analyzed by measuring the dielectric loss factor (tanδ) and the complex permittivity (ε′) at different frequencies (F). The resulting data and theoretical basis will inform the preparation of wood-based dielectric materials.

2. Experimental Procedure

In this experiment, artificial Mongolian Scotch Pine and Paulownia are employed as the raw materials, and delignification is conducted by varying the reaction time. Subsequently, the dielectric properties of the treated specimens are evaluated. The specific test procedure is illustrated in Figure 1.

2.1. Materials and Reagents

The commercially purchased artificial Mongolian scotch pine (Pinus sylvestris Linn. var. mongolica Litv.) and paulownia (Paulownia fortunei (Seem.) Hemsl.) were processed into small pieces of wood measuring 20 mm × 20 mm × 2 mm, which were subsequently noted as Mongolian scotch pine (M) and paulownia (P).

The chemicals sodium chlorite (80%), barium chloride, and calcium nitrate were procured from the Shanghai Aladdin Biochemical Science and Technology Co., Ltd. (Shanghai, China). The chemicals glacial acetic acid, anhydrous ethanol, nitric acid, sulfuric acid, and hydrochloric acid were obtained from the Tianjin Windship Chemical Reagent Technology Co. (Tianjin, China). The aforementioned chemicals were of analytical grade, and the distilled water was prepared in the laboratory.

2.2. Instruments and Equipment

The Electrothermal Constant Temperature Triple Water Tank SHHW21-420, produced by the Shanghai Benting Instrument Co., Ltd. (Shanghai, China), is a device designed to maintain a constant temperature for water heating. The Universal High-Speed Pulveriser (DE-150 g), produced by the Zhejiang Hongjingtian Industry & Trade Co., Ltd. (Jinhua, China), is a pulverizing apparatus that employs high-speed technology for the pulverization of materials. The Electrothermal Constant Temperature Blast Drying Oven GZX-GF101-1-BS-II/H is produced by the Shanghai Yuejin Medical Instrument Co. (Shanghai, China) and is an apparatus designed for the drying of materials through the application of high temperatures. The Vacuum Drying Oven DZF-6020 is produced by the Changzhou Jintan Jingda Instrument Manufacturing Co. (Changzhou, China). The Circulating Water Vacuum Pump SHZ-D(III) is produced by the Shanghai Dongxi Refrigeration Instrument and Equipment Co., Ltd. (Shanghai, China). The analytical balance is the Sartorius BS224S, produced by Sartorius, Germany. The pH meter is the FE28-Standard, produced by the Qingdao Limi Biotechnology Co., Ltd. (Qingdao, China). The 4200A-SCS Electrical Characteristics Parameter Tester is produced by the Keithley Corporation (Cleveland, OH, USA).

2.3. Test Method

Lignin Removal Experiment

The wood specimens were subjected to a drying process in a drying oven maintained at a temperature of 105 °C for a period of 24 h. Subsequently, the wood was treated with an acidic sodium chlorite method for delignification in a water bath maintained at a temperature of 80 °C. The pH value of the 3 wt% sodium chlorite solution was adjusted to 4.5 using glacial acetic acid, and the wood was mixed with the reaction solution (mass ratio 1:40) to initiate the reaction. The reaction time was controlled at the set temperature and time, with six groups of experimental times (2, 4, 6, 8, 10, and 12 h) designed to control Mongolian Scotch Pine and Paulownia, respectively. The treatment solution was replaced at 6 h intervals. At the conclusion of the treatment, the samples were subjected to three rinses with a solution of anhydrous ethanol and water (mass ratio 1:1) [8], followed by a neutralization wash with distilled water. Thereafter, the samples were freeze-dried and processed to yield delignified samples (DW) at the designated reaction times, designated as M-DW-x and P-DW-x (x representing the reaction time).

2.4. Testing and Characterization

2.4.1. Determination of Lignin Content and Removal Rate

The determination of lignin content is conducted by the national standard GB/T 2677.8-1994, titled “Determination of acid-insoluble lignin content of paper raw materials” [9]. This is achieved through the application of Formulas (1) and (2), which are used to ascertain the lignin removal rate.

A_m is the lignin content of the sample (%); M_m1 is the mass of the adiabatic ash and filter paper (g); M_m0 is the mass of the adiabatic filter paper (g); and M₀ is the mass of the weighed adiabatic sample (g).

$${\mathrm{A}}_{\mathrm{m}}={\displaystyle \frac{{\mathrm{M}}_{\mathrm{m}1}-{\mathrm{M}}_{\mathrm{m}0}}{{\mathrm{M}}_0}}\times 100\%$$

(1)

$${\mathrm{A}}_{\mathrm{mT}}={\displaystyle \frac{{\mathrm{A}}_{\mathrm{m}0}-{\mathrm{A}}_{\mathrm{m}1}}{{\mathrm{A}}_{\mathrm{m}0}}}\times 100\%$$

(2)

where A_mT is the lignin removal rate (%) of the sample; A_m0 is the lignin content (%) of the unlignified sample; and A_m1 is the lignin content (%) of the delignified sample.

2.4.2. Determination of Dielectric Properties

The test employed the 4200A-SCS electrical characteristic parameter tester to assess the dielectric properties of DW samples at a frequency range of 10³–10⁷ Hz. This enabled the determination of the dielectric constant (ε′) and dielectric loss tangent (tanδ) of the samples, as well as the construction of electrical conductivity (σ) curves with varying frequencies.

By determining the capacitance value (C_p) and loss angle tangent (tanδ) of different DW materials, their dielectric constants (ε′) and dielectric loss angle tangent (tanδ) can be calculated using Equations (3) and (4). The conductivity is known to have a certain correlation with the relative dielectric loss of the medium [10]. Its value is obtained according to Equation (5), which is calculated by repeating the readings three times and taking the average value for plotting. Figure 1 shows the flow chart of one of the lignin removal and dielectric testing experiments.

$${\mathsf{\epsilon }}^{\prime }={\displaystyle \frac{{\mathrm{t}}_{\mathsf{\alpha }}\times {\mathrm{C}}_{\mathrm{p}}}{\mathrm{S}\times {\mathsf{\epsilon }}_0}}$$

(3)

where t_α average thickness of the material (m); C_p capacitance value (F); S electrode area (m²); and ε₀ vacuum capacitance (8.854 × 10⁻¹² F/m).

$$\tan \mathsf{\delta }={\displaystyle \frac{{\mathrm{C}}_{\mathrm{p}0}}{{\mathrm{C}}_{\mathrm{p}0}-{\mathrm{C}}_{\mathrm{p}1}}}\times \left({\tan \mathsf{\delta }}_1-{\tan \mathsf{\delta }}_0\right)$$

(4)

where capacitance value (F) before C_p0 is put into DW; capacitance value (F) after C_p1 is put into DW; tangent of loss angle after tanδ₁ is put into DW; and tangent of loss angle before tanδ₀ is put into DW.

$$\mathsf{\sigma }={\mathsf{\epsilon }}_0\times {\mathsf{\epsilon }}^{\prime }\times \mathsf{\omega }\times \tan \mathsf{\delta }$$

(5)

where σ is the conductivity, S/m; ω is the angular frequency (rad/s); ω = 2πf, f is the frequency (Hz); ε₀ vacuum capacitance (8.854 × 10⁻¹² F/m); and ε′ is the dielectric constant of the medium.

3. Results and Analysis

3.1. Influence of Different Reaction Times on the Morphology Analysis of DW

To confirm the effect of lignin removal time on the surface structure and hollow pore cavity size of the wood, samples were obtained from the physical and SEM tests, the results of which are presented in Figure 2, Figure 3 and Figure 4.

As illustrated in Figure 2, the oxidation of the carbonyl group and quinone-type structure of the lignin side chain [11] by chlorite during delignification results in the destruction of the chromophore group, leading to the bleaching of the DW samples. The color of the samples tends to become increasingly pure white with an extended delignification reaction time.

Despite the differing wood species, the same result is observed in Figure 3 and Figure 4. As the removal time increases, the lignin gradually decreases, and the thickness of the cell wall gradually becomes thinner, resulting in wrinkles. This indicates a reduction in the mechanical support capacity of the wood, which proves the success of the removal of lignin and other chemical constituents. However, when the reaction time was approximately 12 h, as illustrated in Figure 2 and Figure 4, it became evident that the M-DW-12 and P-DW-12 samples exhibited a phenomenon of sparsification, which resulted in the inability to maintain their original morphology.

3.2. Effects of Different Reaction Times on Changes in Lignin Content

To investigate the effect of different reaction times on the removal of lignin from DW samples, the lignin content and lignin removal rate were calculated according to the method described in Section 2.4.1, as illustrated in Figure 5, and analyzed in conjunction with Section 3.1.

The analysis of Figure 5 demonstrates that the lignin content of the samples exhibited a gradual decline with the extension of reaction time when comparing different tree species. Upon increasing the reaction time from 0 h to 10 h, it was observed that the lignin content of M decreased from 30.63% to 7.91% while that of P declined from 27.45% to 5.93%. Notably, the lignin content of M was consistently higher than that of P throughout the treatment period. As the reaction time approached 12 h, the difference in lignin content within the M-DW and P-DW samples was not significantly closer to approximately 4.89%, and the lignin removal rate was approximately 82.5%, which is in close alignment with the conclusions drawn in the study of transparent wood [12].

It can thus be concluded that when the reaction time of the removal process is 10 h, the lignin in M and P is essentially removed while maintaining the basic form. Consequently, the DW samples with a reaction time of 0–10 h were selected for the dielectric characterization experiments.

3.3. Effect of Different Reaction Times on the Molecular Structure of Wood

As indicated in Section 3.2, the lignin content within M and P is observed to decrease with the prolongation of removal time. To examine the alterations in chemical functional groups, FT-IR was employed to analyze the chemical composition of M and P before and after the removal treatment. The results are illustrated in Figure 6, and the corresponding attribution of their infrared absorption signals is presented in

Table 1. Infrared absorption signal attribution of Mongolian Scotch Pine and Paulownia spp.

Mongolian Scotch Pine Absorption Signal cm−1	Paulownia Absorption Signal cm−1	Functional Group (Chemistry)
3323	3323	Mainly -OH stretching vibrations in the free and bound water fractions
2871	2907	-CH3, -CH2 stretching vibrations of cellulose
1729	1730	-C=O stretching vibrational peaks of non-conjugated acetyl or acid groups
1593	1598	Stretching vibrations induced by the conjugate group-C=O of lignin
1508	1509	Lignin benzene ring backbone vibrations and -CH2 deformation vibrations
1450	1425	Stretching vibrations of the -C=O bond of lignin with the carbon skeleton of the benzene ring
1369, 1312	1367, 1313	C-H absorption peaks of aliphatic methyl and ether hydroxyl groups
1266	1244	Characteristic -C-O and -C-C peaks of hemicellulose acetyl, sugar carbonyl ester groups
1203	1202	Mainly includes -C-H in-plane bending vibration and -C-O telescopic vibration
1157	1158	Stretching vibrations of the -C-O bond of cellulose
1104	1106	Intra-lignin benzene ring -CH stretching vibrations
1056, 1029	1054, 1027	Vibrations induced by -C-O-C and -C=O of cellulose and hemicellulose
1418, 877	891	β-glycosidic bond vibrations of cellulose stretching vibrations of glycosidic bonds

.

As illustrated in Figure 6, an increase in removal time has been observed to result in a reduction in the intensity of characteristic peaks associated with certain lignin components in M and P. This phenomenon can be observed in Figure 6b in the case of M at 1450 cm⁻¹ and 1312 cm⁻¹, which correspond to the benzene ring carbon skeleton vibration [13] and aliphatic methyl vibration [14], respectively. Despite this reduction in intensity, the position of these peaks remains unaltered. Figure 6d demonstrates that the intensity of the benzene ring backbone vibration and aliphatic methyl group in P at 1509 and 1313 cm⁻¹ is slightly weakened, which corroborates the effective removal of lignin during the removal process. This finding aligns with the analysis results presented in Section 3.2. However, the intensity of the characteristic peaks of some lignin is observed to undergo an enhancement trend. This is exemplified by the C-H absorption peaks corresponding to the benzene ring conjugate group -C=O and the aliphatic upper ether hydroxyl group at 1593, 1369, and 1104 cm⁻¹ for M, which exhibit a slight increase in the intra-benzene ring -CH telescoping vibration [15,16,17].

Additionally, the characteristic peaks of 1593, 1367, and 1106 cm⁻¹ for P display a similar trend. Similarly, the stretching vibration caused by the -C=O [18] conjugate group at 1593 cm⁻¹ for P exhibited an initial increase, followed by a decline. This phenomenon may be attributed to the disruption of the covalent bond-bound connections between lignin during its removal, while the molecular weight of lignin [19] is negatively correlated with the content of polar functional groups. The intermolecular forces the strength of the conjugation, and the removal of the lignin results in a significant number of conjugated groups being exposed. However, a portion of the lignin remains in the DW-10 sample, resulting in an elevated trend in the intensity of the characteristic peak at 1730 cm⁻¹, which represents the non-conjugated C=O stretching vibrational peaks.

As the lignin content decreases, the cellulose characteristic peaks [20,21,22] in Figure 6a,b, such as M around 2883, 1418, and 877 cm⁻¹, become increasingly prominent. This indicates that the cellulose content within the DW sample is elevated. Furthermore, the peak strengths of cellulose characteristic peaks representing -OH, -C-O, and -C-O-C bonds [23,24,25] also demonstrate an enhanced trend, as evidenced by the elevated trend of P in Figure 6c,d.

3.4. Effect of Different Reaction Times on the Dielectric Properties of Wood

The presence of lignin polymers’ aromatic multifunctional groups, coupling systems, and highly reactive groups indicates that they have certain electrical conductivity. Meanwhile, lignin’s polar functional groups (phenolic and aliphatic hydroxyl, carbonyl, and carboxyl groups) [19] undergo dipole polarization under the action of the electric field. This results in the removal of a large amount of lignin and an increase in the relative content of cellulose in the DW samples. This is combined with Section 3.3, which indicates that, with the increase of the removal time, the lignin benzene ring skeleton vibration is weakened, the relative content of phenolic hydroxyl, phenylmethyl, and conjugated, non-conjugated carbonyl and other functional group changes, and the cellulose of the secondary hydroxyl and primary hydroxyl and other reactive groups is “exposed”. This is combined with Section 3.3. It can thus be concluded that the removal experiment has a discernible impact on the dielectric properties of the DW samples. The dielectric constant (ε′), the dielectric loss tangent (tanδ), and the conductivity (σ) of the M-DW and P-DW at varying frequencies (F) and under adiabatic conditions for different removal times were measured and calculated using the 4200A-SCS Electrical Characteristics Tester, as illustrated in Figure 7, Figure 8 and Figure 9.

Figure 7 illustrates the frequency dependence of ε′ (ω) and tanδ (ω) at varying removal times. As can be observed in Figure 7a,b, the dielectric constant ε′ demonstrates an increase with the progression of removal time within the frequency range of 10³–10⁷ Hz. Further analysis of the trend of the ε′ value versus the lignin removal rate at 10 MHz can be seen in Figure 9a, which demonstrates that the ε′ value varies roughly linearly with the lignin removal rate. The outcomes of this examination are in close alignment with the statistical data for natural wood and the internal chemical constituents (lignin and cellulose) presented in Table 2. This may be attributed to the fact that as the ‘package’ of lignin is disrupted, its covalent bonds are interwoven with each other and its condensed structure is compromised [26], and the relative number of lignin’s polar groups (phenolic and aliphatic hydroxyl, carbonyl, and carboxyl) increases, and its cellulosic surface is damaged. The relative number of lignin functional groups (phenolic and aliphatic hydroxyls, carbonyls, and carboxyls) increases, as does the proportion of its cellulose surface active groups (sec-hydroxyls and primary hydroxyls). Additionally, the number of polaritons increases, while the intermolecular hydrogen bonding network is weakened, resulting in an increase in the dipole moment of the molecules and the formation of a new polar region [27]. Consequently, ε′ also rises. The slope [d(lgσ)/d(lgω)] calculated from the frequency dependence of σ(ω) (Figure 8) indicates that the K value decreases with the increase of the removal time. This phenomenon can be attributed to the electric field dissociation of polar groups within the wood. The pKa values associated with the main types of phenolic structures, namely those with conjugated, non-conjugated carbonyls, and condensed structures, are 7.7, 9.9, and 12.0 units [28], respectively. Similarly, the pKa values pertaining to the main types of alkane structures, which include sec-hydroxyl and primary hydroxyl groups, are 15.5 and 15.9 units, respectively. Furthermore, it is demonstrated that in electric field polarization, the main source of polariton change is the dipole undergoing polarization in the early stage, which is mainly lignin. With the increase in the removal time, the dipole is mainly supplied by the hydroxyl group of cellulose.

Concurrently, the value of ε′ diminishes in conjunction with an increase in F. As the removal time and lignin removal degree increase, the trend of ε′ becomes more pronounced. This may be attributed to the alternating electric field, which primarily affects wood internally through orientation polarization. However, orientation polarization requires a period to reach its maximum value under the corresponding electric field. The orientation movement of the polarizers cannot keep pace with the changing electric field, leading to a reduction in polarization and, consequently, a decline in ε′.

The dipole of the wood undergoes orientational polarization, which has a time scale for the response of the alternating electric field. This is due to the internal friction and mutual interaction of the dipoles during their movement, which results in a loss effect on the energy of the electric field. This phenomenon is responsible for the production of chirality. Figure 7c,d illustrates that in the frequency range of 10³–10⁵ Hz, the value of tanδ decreases with the increase of F, and in the range of 10⁵–10⁷ Hz, the value of tanδ gradually flattens out. It may be postulated that as the frequency of the electric field increases [29], the orientation of the dielectric dipole moment lags behind the change in the electric field. Consequently, the dielectric’s orientation polarization is unable to keep pace with the electric field response, resulting in a stabilization of the dielectric’s orientation movement at high frequencies. Consequently, tanδ demonstrates a declining and subsequently stabilizing trajectory.

A comparison of the changes in tanδ following the removal of different quantities of material revealed a trend whereby the value of tanδ initially increased and then decreased with the prolongation of the removal time. A further analysis of the trend of the tanδ value and lignin removal rate at 10 MHz is provided in Figure 9b. About Mongolian scotch pine, the trend of tanδ was observed to increase in the range of 0%–15.98% with the increase of the lignin removal rate and to decrease in the range of 15.98%–74%. The tanδ value of the M-DW sample was larger than that of the untreated material of M-DW-0 until 61.64%. For Paulownia, tanδ tends to increase in the range of 0-13.5% and decrease in the range of 13.5-78.48%. However, it should be noted that this is only the case at P-DW-2, where the value of tanδ is observed to deviate from the norm. The delta value is greater than that of P-DW-0, with the test results being in close alignment with the statistical outcomes for the tanδ of medium natural wood with an internal chemical composition (cellulose) as presented in Table 2. However, only for P-DW-2 was the tanδ value greater than that of P-DW-0. This phenomenon may be attributed to the sodium chlorite method employed for the removal of lignin from wood, which destroys the condensation structure of lignin. Concurrently, the number of polar groups, comprising phenolic hydroxyl, carbonyl, and carboxyl, in lignin is increased. Additionally, the van der Waals and electrostatic forces of the condensation structure are weakened [30], thereby enhancing the freedom of the polar groups in the electric field polarization response. Consequently, during the initial stages of the removal experiments, tanδ exhibits an upward trend. However, with the prolongation of the removal process, the lignin content in the DW samples diminished, accompanied by a reduction in the number of polar groups in lignin. During this period, tanδ exhibited a downward trajectory, while the lignin content declined and the cellulose content increased, with the primary polarization remaining concentrated in the neutral hydroxyl and primary hydroxyl of cellulose, along with other polar groups. Consequently, the tanδ value remained elevated in comparison to that of the untreated material, DW-0. As the removal experiment continues, the cellulose amorphous zone decreases while DW crystallinity increases. The crystallinity of M-DW-10 (50.63%) is greater than that of DW-0 (32.85%), while that of P-DW-10 (52.43%) is greater than that of P-DW-0 (35.31%). The hydroxyl group at the crystalline zone is subjected to the action of hydrogen bonding, which restricts its polarization response and causes a decreasing trend in tanδ. Concurrently, the removal of substantial quantities of lignin from the sample results in a reduction in density. The correlation between tanδ and density, as demonstrated in reference [31], is positive. The removal of lignin leads to a decrease in DW density, which in turn causes a reduction in tanδ.

Table 2. Comparison of the results of tests on the dielectric parameters of the different components within the wood under different reports.

Measuring Materials	Test Condition	Dielectric Constant (Ε′)	Dielectric Loss (Ε″)	Dielectric Loss Angle Tangent (TanΔ)	Source
Natural wood Keranji NaOH-treated Keranji	Frequency (Hz)	Natural wood low frequency: 2.56 NaOH-treated LF: 4.63.	Natural wood low frequency: low frequency; 0.57 High frequency: 0.08 NaOH treated Low frequency: 0.74 High frequency: 0.092	-	[32]
Natural wood Bubinga wood	Different test directions 8.2–12.4 GHz	X:2.691 Y:2.767 Z:3.270	X:0.2958 Y:0.2173 Z:0.4941	-	[33]
Norway spruce, Juniper, Aspen	10−2–107 Hz	N:1.425–1.375 J:1.6–1.41 A:1.36–1.30	-	N:0.06–0.002 J:0.01–0.007 A:0.1–0.0001	[34]
cellulose	Frequency (Hz)	5.9–7.5		tanδ less than 0.1	[35]
Cellulose crystals Iα and Iβ	Different XC generalized computational fits	Iα: 0.6~0.75 Iβ: 0.7~0.95	-	-	[5]
lignin	10−1~107 Hz	2.2–2.9	1.5–0.6	-	[36]
CMC composite film CMC-lignin composite film	103–106 Hz	CMC:5.1–4.2 CMC-lingin:5.8–4.6	-	-	[37]
chemically modified kraft lignin	105 Hz 80–240 °C	0.31–0.12 4.2–2.1		Tanδ ML:0.4–0.15	[38]

Table 2. Comparison of the results of tests on the dielectric parameters of the different components within the wood under different reports.

Measuring Materials	Test Condition	Dielectric Constant (Ε′)	Dielectric Loss (Ε″)	Dielectric Loss Angle Tangent (TanΔ)	Source
Natural wood Keranji NaOH-treated Keranji	Frequency (Hz)	Natural wood low frequency: 2.56 NaOH-treated LF: 4.63.	Natural wood low frequency: low frequency; 0.57 High frequency: 0.08 NaOH treated Low frequency: 0.74 High frequency: 0.092	-	[32]
Natural wood Bubinga wood	Different test directions 8.2–12.4 GHz	X:2.691 Y:2.767 Z:3.270	X:0.2958 Y:0.2173 Z:0.4941	-	[33]
Norway spruce, Juniper, Aspen	10−2–107 Hz	N:1.425–1.375 J:1.6–1.41 A:1.36–1.30	-	N:0.06–0.002 J:0.01–0.007 A:0.1–0.0001	[34]
cellulose	Frequency (Hz)	5.9–7.5		tanδ less than 0.1	[35]
Cellulose crystals Iα and Iβ	Different XC generalized computational fits	Iα: 0.6~0.75 Iβ: 0.7~0.95	-	-	[5]
lignin	10−1~107 Hz	2.2–2.9	1.5–0.6	-	[36]
CMC composite film CMC-lignin composite film	103–106 Hz	CMC:5.1–4.2 CMC-lingin:5.8–4.6	-	-	[37]
chemically modified kraft lignin	105 Hz 80–240 °C	0.31–0.12 4.2–2.1		Tanδ ML:0.4–0.15	[38]

4. Conclusions and Prospects

The lignin content of the wood is the primary factor influencing its dielectric properties. This paper investigates the effect of post-delignification wood samples with varying degrees of delignification on the dielectric properties of wood, specifically the effect on the dielectric constant (ε′) and the dielectric loss factor (tanδ).

(1)

As the removal time is increased, the lignin content in the wood decreases, the lignin removal rate rises, reaching approximately 82.5%, and the intensity of the lignin benzene ring skeleton vibration characteristic peaks decreases, while the intensity of the phenol hydroxyl, hydroxyl, carbonyl, and carboxyl characteristic peaks is generally elevated.

(2)

As the rate of lignin removal increases, the value of ε′ varies in a roughly linear fashion with the lignin removal rate, with tanδ exhibiting an initial upward trend followed by a downward trend.

(3)

Further discussion is required regarding the relationship between the dielectric properties of electrical parameters and sample internal lignin content, as well as changes in the latter. It is yet unclear which ε′ and tanδ are affected by changes in chemical composition. While it has been proposed that the main polarization group number and degrees of freedom are restricted, the dielectric chirp mechanism of different polarity groups at different frequencies has not been sufficiently analyzed. To gain insight into the differing polarity groups in the AC electric field with a specific orientation, it is essential to consider the changes in ε′ and tanδ as a consequence of the combined influence of multiple factors. Further studies are required to elucidate the changing law of dielectric properties in Mongolian Scotch Pine and Paulownia.