Method for modifying wood through high-voltage electrostatic field synergistic heat treatment and application of method

Abstract

The invention relates to a method for modifying wood through high-voltage electrostatic field synergistic heat treatment and application of the method, and belongs to the technical field of wood processing. The method for modifying the wood through high-voltage electrostatic field synergistic heat treatment comprises the following steps that a wood sample is placed in a high-voltage electrostatic field, the electric field direction is perpendicular to the fiber direction of the wood, heat treatment is conducted after high-voltage electrostatic field treatment is completed, and the wood modified through high-voltage electrostatic field synergistic heat treatment is obtained. According to the method, the microstructure of the wood is optimized through the directional polarization effect of the high-voltage electrostatic field HVEF, and a physical basis for pyrolytic reaction interface regulation and control is provided for the carbonization process; and meanwhile, the synergistic interaction of the high-voltage electrostatic field HVEF and heat treatment on wood modification is achieved, and therefore high-quality utilization of wood is promoted.

Description

Technical Field

[0001] The present application relates to the technical field of wood processing, and particularly to a method for modifying wood by synergistic heat treatment with a high-voltage electrostatic field and its application. Background Art

[0002] Populus spp., as one of the fast-growing tree species widely planted globally, has become an important part of plantation resources due to its short growth cycle and strong adaptability. According to statistics, poplar is the main tree species for fast-growing artificial afforestation in northern China, and its annual output accounts for more than 30% of the total industrial timber, making it a key material to alleviate the shortage of natural forest resources. However, the inherent defects of poplar wood, such as loose texture, low mechanical strength, poor toughness, easy moisture absorption and deformation, and poor decay resistance, severely limit its application in high-end furniture, construction and other fields. How to improve the physical and mechanical properties and functionality of poplar wood through green and efficient modification technologies has become an important topic for the sustainable development of the wood industry. To solve these problems, traditional wood modification technologies usually adopt methods such as heat treatment, chemical impregnation, and resin reinforcement. Among them, heat modification can significantly change the chemical composition and structural characteristics of wood. Usually, wood is heated to a high temperature (the temperature range is generally 150°C to 260°C) in a controlled atmosphere (such as N2) to reduce oxidation and degradation. Research shows that the increase in temperature during the heat treatment process usually leads to an increase in the crystallinity of cellulose. This phenomenon results from the degradation of the amorphous components, further solidifying the wood structure and reducing the moisture absorption of wood. In addition, heat treatment also causes hydrolysis and dehydration of wood, mainly affecting hemicellulose and cellulose, and forming new chemical components such as acetic acid and formic acid. These changes can significantly improve the decay resistance and insect prevention effect of wood, making it more suitable for outdoor applications. It is worth noting that heat-treated wood usually shows higher compressive strength, especially at lower temperatures (such as 160°C) and shorter treatment times, its compressive strength can be significantly improved. Liu explored the temperature distribution characteristics of wood-based composites during microwave heating and its impact on material properties, and found that by optimizing microwave heating parameters, the temperature distribution of wood-based composites can be effectively controlled, thereby improving their mechanical properties and thermal stability. At the same time, chemically modified poplar wood can enhance its physical, mechanical, and thermal properties. This modification can involve various methods and substances, usually targeting the cell wall composition and structure of wood. Jiang proposed a thermal synergistic silica sol mineralization modification technology. This technology reconstructs the porous structure of the wood cell wall through heat treatment, promoting the in-situ deposition of silica networks (Si-O-Si / Si-O-C), thereby synergistically improving the hydrophobicity, dimensional stability, and surface hardness of wood. The principle lies in the synergistic effect of the micro-nano channels formed by the degradation of cell wall components and the physical filling / barrier effect of silica. This research result provides a green solution for the development of high-performance bio-based outdoor building materials. In addition, research shows that heat treatment can induce chemical changes in wood and promote the penetration of silica sol, thereby improving its hydrophobicity, thermal stability, and surface hardness. Although chemical modification methods (such as phenolic resin impregnation) can improve the mechanical strength of wood, there are problems such as organic solvent pollution, high energy consumption, and resin migration during long-term use. Therefore, there is an urgent need to develop new modification strategies that are both environmentally friendly and synergistically optimize performance.Resin reinforcement using epoxy resin and other polymers can also significantly change the properties of wood. Luo et al. prepared wood-based composites with ultra-low density (0.51 g·cm³) and ultra-high specific bending strength within 4.5 h by using a trace amount of resin (weight gain rate of 11.45%) combined with stepwise temperature control (60 °C pre-curing / 120 °C curing). -3 ) The pre-curing process achieved a gradient enrichment of resin from the surface to the bottom (enhanced interfacial Si-O-C bonding) by inhibiting excessive resin penetration, significantly improving the mechanical properties (bending strength increase > 200%) and dimensional stability of the material while reducing raw material consumption.

[0003] In recent years, physical field-assisted modification technologies have received extensive attention due to their non-contact and pollution-free characteristics. As a typical physical field, the High Voltage Electrostatic Field (HVEF) can induce the molecular orientation rearrangement of wood cell wall components (cellulose, lignin) through polarization and trigger microstructural changes. Research shows that HVEF treatment can reconstruct the hydrogen bond network of wood cellulose, thereby increasing its elastic modulus. The electric field-induced cell wall pore expansion effect may provide a diffusion channel for subsequent chemical treatment. Modifying poplar wood using the high voltage electrostatic field and high temperature carbonization technology can enhance the physical and mechanical properties of wood. This modification technology can affect the structure of wood and improve its strength and durability. He et al. clarified that high voltage electrostatic field (HVEF) treatment is an effective way to improve the mechanical properties of wood-based composites. In addition, other scholars have studied HVEF treatment as a method to enhance the bonding properties of wood by increasing the degree of polarization of wood and improving the permeability of phenolic resin (PF) adhesives. However, existing research has mainly focused on the regulation of single properties of wood by high voltage electrostatic field (HVEF) or heat treatment, and its potential in multi-scale synergistic modification has not been fully explored. Summary of the Invention

[0004] In view of this, the present application provides a method for modifying wood by synergistic treatment of high voltage electrostatic field and heat treatment and its application. By using the directional polarization effect of the high voltage electrostatic field HVEF to optimize the microstructure of wood, it provides a physical basis for the regulation of pyrolysis reaction interfaces during the carbonization process; at the same time, it realizes the synergistic effect of high voltage electrostatic field HVEF and heat treatment on wood modification, thereby promoting the high-quality utilization of wood and effectively overcoming the defects existing in the above-mentioned prior art.

[0005] The first aspect of the present application provides a method for modifying wood by synergistic treatment of high voltage electrostatic field and heat treatment, including the following steps:

[0006] Place the wood sample in a high voltage electrostatic field, with the electric field direction perpendicular to the fiber direction of the wood. After the high voltage electrostatic field treatment is completed, perform heat treatment to obtain the wood modified by synergistic treatment of high voltage electrostatic field and heat treatment.

[0007] Preferably, the specific conditions of the heat treatment are as follows:

[0008] Place the wood sample in a tube furnace under nitrogen protection and heat it from room temperature to 100 °C at a rate of 5 °C·min -1 Then heat it to 180 °C at a rate of 2 °C·min -1 Keep the temperature for 1.5 h to achieve controllable degradation of hemicellulose while avoiding excessive carbonization of cellulose. After that, cool it down to 80 °C at a rate of 3 °C·min -1 and then naturally cool it to below 40 °C and take it out.

[0009] Preferably, the moisture content of the wood is 10-12%.

[0010] Preferably, the nitrogen flow rate is 20-30 mL·min -1 , and its oxygen concentration < 3%.

[0011] Preferably, the specific conditions of the high-voltage electrostatic field are as follows:

[0012] Place the wood sample in a high-voltage electrostatic field treatment device. During the treatment, the voltage is set to 60 kV, the treatment time is 12 h, and the ambient temperature is 28 °C;

[0013] Among them, the high-voltage electrostatic field treatment device consists of two electrode plates with dimensions of length × width × height of 150 mm × 60 mm × 10 mm. One electrode plate is used as the negative electrode and connected to the high-voltage generator, and the other is used as the positive electrode and grounded. The wood sample is placed between the two electrode plates. The conductive metal electrode plate connected to the negative electrode does not contact the wood sample, and the distance is 1 cm. The conductive metal electrode plate connected to the positive electrode contacts the wood sample.

[0014] The wood is a block with radial × transverse × tangential dimensions of 20 mm × 20 mm × 10 mm cut from the sapwood of a tree with a diameter at breast height of 45 cm.

[0015] The second aspect of this application also provides the application of the above method for modifying wood by combining high-voltage electrostatic field and heat treatment in the field of wood processing.

[0016] Compared with the prior art, this application has the following beneficial effects:

[0017] This application applies the synergistic use of high-voltage electrostatic field (HVEF) pretreatment and high-temperature carbonization to wood modification, and explores their synergistic effects on the microstructure, chemical components, and physical and mechanical properties of wood. The results show that this synergistic modification technology significantly reduces the water absorption and related swelling rates of wood, and the dimensional stability becomes stronger. Surface property analysis shows that the contact angle of HHP increases significantly, the surface free energy decreases, the hydrophilicity weakens significantly, and at the same time, there are also phenomena such as deepening of surface color and reduction of glossiness. This is mainly attributed to the restructuring effect of the high-voltage electrostatic field combined with heat treatment on the chemical components of wood, which promotes the degradation of hemicellulose and the cross-linking of lignin, reduces the number of hydrophilic hydroxyl groups, and some components such as hemicellulose undergo pyrolysis. Under the same decay conditions of white-rot fungi or brown-rot fungi, the mass loss rates of HHP are significantly lower than those of CK and HP. The brown-rot resistance level of HHP can reach the first-level strong decay resistance level, which benefits from the dense environment formed by carbonized particles and the large loss of organic matter during the treatment process, effectively inhibiting the growth of microorganisms, and the high-voltage electrostatic field enhances this effect. Fourier transform infrared spectroscopy (FTIR) analysis reveals significant changes in the chemical bonds of wood during the modification process, such as the breakage of hemicellulose ester bonds and the condensation of hydroxyl groups between cellulose molecular chains, further confirming the in-depth regulation of the chemical structure of wood by the synergistic modification. Combustibility tests show that the self-extinguishing time of HHP samples after moving away from the fire source is significantly shortened, and the residue mass is the highest, showing excellent flame retardant properties. This application is expected to realize the high-value utilization of wood in high-end wood application fields such as wood components, floors, and doors, and promote the sustainable development of the wood industry. Brief Description of the Drawings

[0018] In order to more clearly illustrate the technical solutions in this application or the prior art, the following will briefly introduce the drawings required for the description of this application or the prior art. Obviously, the drawings in the following description are some embodiments of this application. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0019] Figure 1 It is the flow chart of the study on the modification of poplar wood by high-voltage electrostatic field combined with heat treatment and its properties;

[0020] Figure 2 It is the change of relevant parameters of surface contact angle and surface free energy on the cross-section of poplar wood-like, heat-treated poplar wood, and poplar wood modified by high-voltage electrostatic field combined with heat treatment;

[0021] Figure 3 It is the change of relevant parameters of surface contact angle and surface free energy on the tangential section of poplar wood-like, heat-treated poplar wood, and poplar wood modified by high-voltage electrostatic field combined with heat treatment;

[0022] Figure 4 It is the mass loss rate of the three specimens after being decayed;

[0023] Figure 5 are the FTIR spectra of three specimens;

[0024] Figure 6 are the continuous combustion tests of poplar and modified poplar specimens, where A is the control sample (CK), B is heat-treated poplar (HP), and C is poplar treated by high-voltage electrostatic field combined with heat treatment (HHP). Detailed implementation manners

[0025] To make the objectives, technical solutions and advantages of this application clearer, the technical solutions in this application will be clearly and completely described below in conjunction with the accompanying drawings in this application. Obviously, the described embodiments are some, but not all, of the embodiments of this application. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in this application without creative efforts shall fall within the protection scope of this application.

[0026] The experimental methods used in the embodiments of this application are all conventional methods unless otherwise specified.

[0027] In the following embodiments, unless otherwise specified, all raw materials can be obtained through commercial purchase or conventional methods.

[0028] It should be noted that the types of poplar can include any one of poplar, eucalyptus, masson pine, paulownia, Chinese fir, etc., not limited to poplar.

[0029] 1. Materials and methods

[0030] 1.1 Materials and equipment

[0031] Poplar trees with a tree age of 10 years planted in Siyang County, Suqian City, China were selected. Wood blocks with dimensions of 20 mm × 20 mm × 10 mm (radial × tangential × chordal) were cut from the sapwood of poplar with a breast diameter of 45 cm. Brown rot fungus: Gloeophyllum trabeum was provided by the College of Life Sciences, Fujian Agriculture and Forestry University; white rot fungus: Coriolus versicolor was provided by the China Forestry Microbial Culture Collection Center.

[0032] HVEF generator (output range 0 - 10 kV·cm -1 ). Tube furnace: model TF1200 - 300, produced by Qimiao New Materials Technology (Shanghai) Co., Ltd., temperature control accuracy ±1 °C, tube diameter Automatic color difference meter, model NR10QC, 8° / d (8-degree illumination diffuse reflectance reception), produced by Shenzhen 3nh Technology Co., Ltd. Contact angle measuring instrument: CA200, produced by Kunshan Beidou Precision Instruments Co., Ltd.; biochemical incubator: model Herocell 180, produced by Shanghai Runde Biotechnology Co., Ltd.

[0033] 1.2 Experimental design

[0034] Control sample (CK), heat-treated poplar (HP), and high-voltage electrostatic field combined with heat-treated poplar (HHP).

[0035] The steps of high-voltage electrostatic field combined with heat-treated poplar (HHP) are as follows: Place the poplar sample in a high-voltage electrostatic field (HVEF), where the electric field direction is perpendicular to the fiber direction of the poplar. The high-voltage electrostatic field treatment device consists of two electrode plates with dimensions of 150 mm × 60 mm × 10 mm (length × width × height). One electrode plate is connected to the high-voltage generator as the negative electrode, and the other is grounded as the positive electrode. The poplar sample is placed between the two electrodes. The conductive metal plate connected to the negative electrode does not touch the poplar sample, with a spacing of 1 cm, and the conductive metal plate connected to the positive electrode touches the poplar sample. During the treatment, the voltage is set to 60 kV, the treatment time is 12 h, and the ambient temperature is 28 °C. After the high-voltage electrostatic field treatment, the steps of heat-treating the poplar are carried out.

[0036] The specific steps of heat-treated poplar (HP) are as follows: Place the poplar (with a moisture content of about 10%) in a tube furnace under nitrogen protection (flow rate 20 - 30 mL·min -1 , oxygen concentration < 3%), and heat it from room temperature to 100 °C at a rate of 5 °C·min -1 , then heat it to the target temperature of 180 °C at a rate of 2 °C·min -1 , hold for 1.5 h (to achieve controllable degradation of hemicellulose while avoiding excessive carbonization of cellulose), then cool it down to 80 °C at a rate of 3 °C·min -1 , and then naturally cool it to below 40 °C and take it out. This process can balance the treatment efficiency and the properties of poplar.

[0037] 1.3 Performance testing and characterization

[0038] 1.3.1 Determination of oven-dry density, saturated water swelling property, and water absorption

[0039] ① Refer to the standard GB / T 1927.5 - 2021 "Test methods for physical and mechanical properties of clear small specimens of poplar - Part 5: Determination of density". Dry the specimens with a size of 20 mm × 20 mm × 20 mm (radial × tangential × longitudinal direction) (R × T × L) to oven-dry weight and weigh them. The oven-dry density (ρ0) of the specimens is calculated according to formula (1):

[0040] ρ0 = m0 / (R × T × L) = m0 / V0 (1)

[0041] In the formula: ρ0 is the density of the specimen at oven-dry, g·cm -3 ;

[0042] m0 is the mass of the specimen at oven-dry, g;

[0043] R, T, and L are the dimensions of the oven-dry specimen in the radial, tangential, and longitudinal directions, respectively, in cm;

[0044] V0 is the volume of the specimen at oven-dry condition, in cm 3 .

[0045] ② Refer to the standard GB / T 1927.8-2021 "Test methods for physical and mechanical properties of defect-free small specimens of poplar - Part 8: Determination of swelling in the wet state", measure the dimensions of the specimen with the specification of 20 mm × 20 mm × 20 mm (radial × tangential × longitudinal direction) (R × T × L) in the oven-dry state and in the saturated water state. The radial saturated water swelling rate (a rs ) is calculated according to formula (2), and the tangential saturated water swelling rate (a ts ) is calculated according to formula (3).

[0046] a rs = (L rs - L ro ) / L ro × 100% (2)

[0047] a ts = (L ts - L to ) / L to × 100% (3)

[0048] In the formulas: a rs , a ts are the radial and tangential saturated water swelling rates, respectively;

[0049] L rs , L ts are the radial and tangential dimensions of the specimen when the moisture content is higher than the fiber saturation point (i.e., saturated water), in mm;

[0050] L ro , L to are the radial and tangential dimensions of the specimen at oven-dry condition, in mm.

[0051] When considering the dimension change in the longitudinal direction during the process of the specimen from oven-dry to saturated water state, the volume saturated water swelling rate (a vs ) is calculated according to formula (4).

[0052] a vs = [(L rs × L ts × L hs ) - (L r0 × L t0 × L h0 )] / L r0 × L t0 × L h0×100% (4)

[0053] Where: a vs is the saturated water swelling rate of the specimen volume;

[0054] L rs 、L ts and L hs are the radial, tangential and longitudinal dimensions of the specimen when the moisture content of the specimen is higher than the fiber saturation point (i.e., saturated water), in mm;

[0055] L r0 、L t0 and L h0 are the radial, tangential and longitudinal dimensions of the specimen at oven-dry condition, in mm.

[0056] ③ Refer to the standard GB / T 1927.7-2021 "Test methods for physical and mechanical properties of clear small specimens of poplar - Part 7: Determination of water absorption", prepare specimens with dimensions of 20mm×20mm×20mm (radial×tangential×longitudinal direction) (R×T×L). Weigh the mass m of each specimen after 48h. Calculate the water absorption rate (X) according to formula (5).

[0057] X = (m - m0) / m0×100% (5)

[0058] Where: X is the 48h water absorption rate of the specimen;

[0059] m is the mass of the specimen after 48h of water absorption, in g;

[0060] m0 is the mass of the specimen at oven-dry condition, in g.

[0061] 1.3.2 Determination of surface color difference and glossiness

[0062] ① Use a color difference meter to detect on the surface of the specimen. Randomly select 3 specimens with uniform surface color for each group for determination, and the results are averaged. The color difference parameters in this study include lightness (L*), red-green chromaticity index (a*), yellow-blue chromaticity index (b*), chroma (C*), and total color difference (ΔE*). Calculate ΔE* (representing the magnitude of color difference) through the CIE L*, a*, b* uniform color space. The chroma (C*) is calculated according to formula (6):

[0063] C* = (a* 2 + b* 2 ) 1 / 2 (6)

[0064] The total color difference (ΔE*) is calculated according to formula (7):

[0065] ΔE* = [(ΔL*) 2 + (Δa*)2 + (Δb*) 2 1 / 2 (7)

[0066] In the formula, ΔL* = L t *-L1*, Δa* = a t *-a1*, Δb* = b t *-b1*;

[0067] L1*, a1*, b1* are control specimens, and L t *, a t *, b t * are specimens after treatment.

[0068] The hue angle (H o ) is calculated according to formula (8):

[0069] H o = tan -1 (b* / a*) (8)

[0070] ② Measurement of surface glossiness (G*). The glossiness shown on the surface of poplar wood is related to tree species, surface flatness, tyloses and inclusions, poplar wood structural characteristics, and poplar wood cross-sections, etc. In this study, a glossmeter was used for measurement. The main research was on the parallel GZL(%) and perpendicular GZT(%) of the light source incident direction to the grain direction. Each specimen was measured multiple times at multiple points (3 points were measured, and each point was measured 3 times). The measurement angle was 60°, and the average value was taken as the measured value.

[0071] 1.3.3 Measurement of contact angle

[0072] The principle of contact angle measurement is based on the surface tension of the liquid and the properties of the solid surface. In the test, distilled water was used as the test liquid to measure the contact angle of the specimen surface. The specimen was placed on a stage with adjustable inclination and height. The focal length was adjusted to make the image clear. The grain direction of the specimen placement was perpendicular to the illumination system of the instrument. Different test areas were selected according to different grains of poplar wood. When distilled water was used as the test liquid, a microsyringe (10 μL) was used. The syringe was fixed on the optical contact angle instrument, and then the height was adjusted so that the test liquid was at the same height from the poplar wood surface, and it was vertically dropped to form a sessile drop. The state of the liquid drop changing with time on the poplar wood surface was photographed in real time through a video control and measurement system. Poplar wood is a porous material that can enable the liquid to continuously penetrate and spread, so that the contact angle changes continuously over time. The environmental temperature during detection: 23 °C, humidity: 55%. The initial contact angle was 1 s, the shooting interval time of the cross-sectional contact angle was 0.8 s, and the shooting interval time of the tangential contact angle was 2 s.

[0073] 1.3.4 Decay resistance test

[0074] ​According to the relevant requirements of the national standard GB / T 13942.1-2009, the corrosion resistance test was carried out on each specimen. The specimen was sterilized in an autoclave for 20 min, and then placed on the inoculated wood with mycelium fully grown in a culture bottle under sterile conditions (the grain direction was perpendicular to the mycelium growth direction). The culture bottle was placed in an incubator (temperature 28±2 °C, relative air humidity 75% - 85%) and infected by Populus wood decay fungi for 12 weeks. After the cultivation, the specimen was taken out from the culture bottle, the mycelium and impurities on the surface of the specimen were removed, and it was dried to absolute dryness at 40 °C and then weighed (accurate to 0.001 g). The mass loss rate of the specimen was calculated according to formula (9):

[0075] Specimen mass loss rate = [(m1 - m2) / m1]×100% (9)

[0076] In the formula, m1 is the oven-dry mass of the specimen before the test, and m2 is the oven-dry mass of the specimen after the test.

[0077] Table 1 Classification of corrosion resistance grades

[0078] Mass loss rate / % Corrosion resistance Corrosion resistance level 0～10 Strong corrosion resistance Ⅰ 11～24 Corrosion resistance Ⅱ 25～44 Slightly corrosion resistant Ⅲ ＞45 Not corrosion resistant Ⅳ

[0079] 1.3.5 Fourier transform infrared spectroscopy (FTIR) detection

[0080] In a dry environment, 2.0 mg of the sample powder was accurately weighed and mixed with 200 mg of potassium bromide, ground thoroughly for many times, and then pressed into a tablet (a transparent thin film) on a tablet press. When testing, the background was collected first, and then the infrared spectrum of the sample was collected, with a resolution of 4 cm -1 , the number of scans was 32 times, and the test wavenumber range was 400 - 4000 cm -1 .

[0081] 1.3.6 Flammability test

[0082] The flammability of natural Populus wood and treated Populus wood samples was tested by a self-built butane torch combustion device. The flame length of the flammability test device was 4 cm, and the combustion behavior of the sample was recorded.

[0083] 2. Results and discussion

[0084] 2.1 Dimensional stability analysis

[0085] The natural porous structure of poplar not only endows it with low density and light weight characteristics, but also provides abundant storage space for moisture. However, this structure also leads to the dimensional instability of poplar. A large number of studies on poplar modification focus on reducing its water absorption by decreasing the -OH groups on the poplar surface, while improving dimensional stability and anti-corrosion performance. As reactive groups, -OH is suitable as the reaction binding site for modifiers. Compared with the control sample (CK), the density (ρ) of heat treatment (HP) and heat treatment assisted by high-voltage electrostatic field (HHP) both decreased to varying degrees, which is mainly due to partial carbonization of poplar and the loss of internal extracts and other substances caused by heat treatment or heat treatment assisted by high-voltage electrostatic field. The a rs 、a ts 、a vs 、and water absorption rates of both significantly decreased, and the decrease rate of HHP was greater than that of HP (as shown in Table 1). It can be seen that poplar is a material with good water absorption. There are a large number of hydrophilic groups -OH on its surface, so the water absorption rate is the highest. During the long-term high-temperature and nitrogen treatment process, a series of reactions such as degradation and recombination of the main components of poplar occurred, mainly involving changes in cellulose crystallinity, a decrease in the number of hydrophilic hydroxyl groups in hemicellulose, hemicellulose degradation, and lignin cross-linking reactions. These reactions effectively improved the dimensional stability of the samples after heat treatment. The high-voltage electrostatic field can not only polarize the water molecules in poplar and cause directional migration, accelerating the diffusion rate of internal water in poplar to the surface, but also polarize the polar molecules in poplar, generating an electric field force, further promoting the pyrolysis reaction or changing the reaction path and degree, so that the chemical structure of poplar is more stable, further improving its hydrophobicity and dimensional stability.

[0086] Table 2 shows the differences in the dimensional stability related indicators of poplar lignin-like, heat-treated poplar, and poplar modified by heat treatment assisted by high-voltage electrostatic field.

[0087] Table 2 Differences in the dimensional stability related indicators of poplar lignin-like, heat-treated poplar, and poplar modified by heat treatment assisted by high-voltage electrostatic field

[0088]

[0089]

[0090] Note: Control sample (CK), heat-treated poplar (HP), poplar modified by heat treatment assisted by high-voltage electrostatic field (HHP).

[0091] 2.2 Differences in surface color and gloss

[0092] Table 3 shows the changes in the surface color and gloss of poplar lignin-like, heat-treated poplar, and poplar modified by heat treatment assisted by high-voltage electrostatic field.

[0093] Table 3 Changes in surface color and glossiness of poplar lignin-like, heat-treated poplar, and poplar modified by high-voltage electrostatic field combined with heat treatment

[0094]

[0095] Note: Control sample (CK), heat-treated poplar (HP), poplar modified by high-voltage electrostatic field combined with heat treatment (HHP).

[0096] In this application, the surface color and glossiness of three types of specimens were analyzed in detail to evaluate their changes in terms of poplar aesthetics and functionality. Figure 1 And Table 3 shows the specific color changes of each specimen. The surface colors of the CK, HP, and HHP specimens gradually changed from light to light brown to brown, presenting a uniform and soft tone, and the surface glossiness gradually decreased. In the cross-section and tangential section observations, the values of a*, b*, and C* all showed HHP > HP > CK, indicating that under the heat treatment conditions at 180 °C or the modification conditions of high-voltage electrostatic field combined with heat treatment, substances such as hemicellulose and extracts containing chromogenic groups and auxochromic groups in poplar underwent pyrolysis, resulting in a decrease in L* and H o values, and further making the colors of the HP and HHP specimens darker and duller. Therefore, heat treatment technology can be used to adjust the wood color of poplar, enabling common tree species to simulate the appearance of precious tree species, while increasing the added value of common tree species and alleviating the supply-demand contradiction of precious dark hardwoods. In addition, research shows that heat treatment can also improve the color stability of poplar to a certain extent. In the artificial aging test of heat-treated Albizia julibrissin heartwood, it was found that the color stability of heat-treated poplar was better than that of untreated poplar, which may be related to the formation of photooxidation products of unsaturated phenolic substances.

[0097] Under the same section conditions, the ΔE value of HHP is larger than that of HP, indicating that the combined treatment of high-voltage electrostatic field and heat treatment has a more significant effect on the color change of poplar. The main reason for the color change of poplar caused by thermal modification is the presence of a large number of chromophoric groups and auxochromic groups in poplar, such as carbon-oxygen (C=O), carbon-carbon (C=C) conjugated double bond structures, hydroxyl groups (-OH), and methoxy groups (-OCH3). During the heat treatment process, these chemical bonds are prone to breakage and recombination, resulting in a change in the tone of poplar. In addition, the high-voltage electrostatic field can polarize polar molecules in poplar, enhancing the intermolecular interaction, thereby promoting the reaction of poplar chemical components. During the combined treatment of high-voltage electrostatic field and heat treatment, the chemical reaction rate of components such as cellulose and hemicellulose increases, causing changes in the internal chemical structure of poplar. At the same time, the high-voltage electrostatic field can also change the heat conduction and heat diffusion characteristics of poplar during the heat treatment process, affecting the heat uniformity and reaction degree of poplar, thereby enhancing the modification effect.

[0098] 2.3 Influence of modified poplar on its surface wettability

[0099] In the actual production processes of poplar gluing, coating, anti-corrosion, etc., the hydrophilicity and surface free energy of the poplar surface are key factors affecting properties such as gluing strength, coating adhesion, and preservative penetration. As Figure 2 and Figure 3 shown, the surface contact angle of all specimens is negatively correlated with time, while the surface free energy is positively correlated with time. There are differences in the variation ranges of the contact angle and surface free energy between the cross-section and tangential section, which mainly result from the different anatomical structures of poplar. Different anatomical structures lead to differences in the surface chemical composition, microscopic morphology, and the way of interaction with water molecules between the two sections, thus making the cross-section and tangential section respond differently to the same treatment method. At the same section and the same time, the order of the surface contact angle size is HPP>HP>CK, and the order of the surface free energy size is CK>HP>HPP. Figure 2 It also shows that the contact angle of CK gradually decreases from the initial 102° to 28° at 6.4 s, and the surface free energy increases from 21.4 mJ·m -2 to 65.2 mJ·m -2 . The initial value of the contact angle of HP is slightly higher than that of CK, and the decreasing speed is relatively gentle, and finally it is about 75° at 6.4 s. The initial value of the contact angle of HPP is the lowest, and the decreasing speed is also the slowest. The contact angle drops to 107° at 6.4 s, and the surface free energy increases from 9 mJ·m -2 to 18.7 mJ·m -2 . This indicates that HPP can more effectively reduce the hydrophilicity of the poplar surface, increase the contact angle, and significantly reduce the surface free energy of poplar. This result is consistent with most conclusions about the decrease in the surface wettability of poplar after heat treatment. For example, the research by Liu et al. showed that when using polyurethane and isocyanate adhesives to explore the influence of heat treatment on the interface of heat-treated Scotch pine glued wood, high-temperature heat treatment significantly reduced the surface wettability of poplar, and there was a negative correlation between the heat treatment temperature and the surface wettability. The research by Chu found that the wettability of poplar decreased after heat treatment, while the surface brittleness increased. Deeply understanding the influence of different treatment methods on the surface contact angle and surface free energy of the cross-section and tangential section of poplar is of great significance for the subsequent processing and application of poplar. The main reasons for the decrease in the surface wettability and paint film adhesion of poplar after heat treatment include: during the heat treatment process, the number of hydrophilic groups (mainly hydroxyl and carbonyl groups) in poplar decreases, weakening the hydrogen bond interaction between the polar groups (-COOH, -COOR, N-C-O) in the coating and the substrate; the crystallization of amorphous cellulose reduces the distance between cellulose molecular chains and makes the molecular arrangement more dense; some hydrophobic extracts migrate to the poplar surface; the roughness affects the wetting performance, and a certain degree of roughness on the poplar surface can increase the actual contact area between poplar and the coating and enhance the paint film adhesion performance.

[0100] 2.4 Corrosion Resistance

[0101] White rot fungi have the ability to degrade lignin. They invade the lumen of wood cells and release enzymes that degrade lignin and other wood components such as cellulose, hemicellulose, and pectin, ultimately causing poplar wood to decay into a white spongy state. In contrast, brown rot fungi mainly damage the cellulose and hemicellulose of poplar wood, making the appearance of poplar wood reddish-brown or brownish-tan and the texture brittle. Figure 4 It shows that after 12 weeks of infection by Trametes versicolor and Gloeophyllum trabeum, the mass loss rate of the control sample is CK > HP > HHP for the three specimens. The mass loss rate of the same specimen after being decayed by Trametes versicolor is higher than that after being decayed by Gloeophyllum trabeum. This is mainly because white rot fungi have a better degradation effect on broad-leaved wood than on coniferous wood, while brown rot fungi have a better degradation effect on coniferous wood than on broad-leaved wood. CK and HP reach the III-level corrosion resistance level after being decayed by Trametes versicolor, while HHP reaches the II-level corrosion resistance level after being decayed by Trametes versicolor. CK and HP reach the II-level corrosion resistance level after being decayed by Gloeophyllum trabeum, while HHP reaches the I-level corrosion resistance level after being decayed by Trametes versicolor. Usually, when the heat treatment temperature reaches a certain level, hemicellulose (especially polysaccharide aldehydes) in poplar wood will undergo chemical changes, and at the same time, the hydroxyl groups in the cellulose molecular chain will combine with each other to form hydrogen bonds, thereby eliminating or alleviating the internal stress of poplar wood and improving its dimensional stability. The mass loss rate of HP heat-treated at 180°C after brown rot is lower than that of the untreated control sample CK, indicating that heat treatment at this temperature can degrade part of the hemicellulose and nutrients such as amino acids and fructose, thus producing a certain corrosion resistance effect. The corrosion resistance effect of HHP can reach the I-level strong corrosion resistance level, which indicates that the high-voltage electrostatic field and heat-treated poplar wood have a synergistic effect on its corrosion resistance. On the one hand, hard carbonized particles form a dense environment where microorganisms are difficult to survive; on the other hand, HHP loses a large amount of organic substances such as starch and sugar during the treatment process, thus significantly enhancing its corrosion resistance effect.

[0102] 2.5 FTIR Characteristic Analysis and Characterization

[0103] FTIR analysis can clearly identify chemical bonds and functional groups in biomass materials. As Figure 5 shown, the spectral characteristics of the modified poplar wood treated by heat treatment at 180°C and high-voltage electrostatic field synergistic heat treatment are basically similar to those of the untreated specimen. However, due to the pyrolysis of the chemical components of poplar wood caused by the synergistic treatment of high-voltage electrostatic field and high temperature, obvious changes occur in some local areas of the spectrogram.

[0104] At positions such as 3430, 2895, 1738, 1642, 1594, 1370, 1035 cm -1 etc., absorption peaks are clearly visible. Among them, 3430 cm -1The absorption band at [location] is attributed to the stretching vibration of O-H in hydroxyl and carboxyl groups. The characteristic peaks of cellulose at 1370 and 897 cm -1 correspond to the C-H bending vibration peaks of the sample, while the peaks at 1035, 1159, 1370, and 1426 cm -1 are the characteristic peaks of the bending vibrations of C-O, C-O-C, CH, and CH2, respectively. In addition, the peak at 1059 cm -1 also belongs to the characteristic peak of C-O bending vibration. Among the characteristic peaks of lignin, the peak at 1642 cm -1 is the characteristic peak of the stretching vibration of the carbonyl conjugated aryl ketone C=O, the peak at 1594 cm -1 is the characteristic peak of the C=C stretching vibration and the skeleton vibration of the syringyl structural unit, the peak at 1508 cm -1 is the characteristic peak of the skeleton vibration of the guaiacyl structural unit, the peak at 1460 cm -1 is the characteristic peak of C-H deformation and CH3 asymmetric bending vibration, the peak at 1426 cm -1 is the characteristic peak of the benzene ring skeleton vibration and the in-plane bending vibration of cellulose C-H, the peak at 1370 cm -1 is the stretching vibration peak of the bond between lignin and carbohydrate, the peak at 1335 cm -1 is the characteristic peak of the condensation vibration of the syringyl and guaiacyl structural unit skeletons and the bending vibration peak of the syringyl structural unit and CH2, the peak at 1268 cm -1 is the peak of the guaiacyl structural unit skeleton plus C=O stretching vibration, the peak at 1242 cm -1 is the C-O-C stretching vibration peak, the peak at 1059 cm -1 is the peak of C-O deformation, C=O stretching, and in-plane deformation vibration of C-H. Among the characteristic peaks of hemicellulose, the peak at 1738 cm -1 is the C=O stretching vibration peak of the acetyl group, the peak at 1242 cm -1 is the C-O stretching vibration peak in the acetyl group, the peak at 1160 cm -1 is the C-O-C stretching vibration peak, the peak at 1035 cm -1 is the C-O stretching vibration peak. The intensity of the peak at 1738 cm -1 shows CK > HP > HHP, indicating that during the process of high-voltage electrostatic field combined with high temperature treatment, the ester bond of hemicellulose in poplar wood breaks and deacetylation reaction occurs. The peak at 1370 cm -1 is the C-H bending vibration of cellulose and hemicellulose, and the intensity of this peak weakens during pyrolysis and the process of high-voltage electrostatic field combined with pyrolysis, indicating that the hemicellulose skeleton undergoes pyrolysis. At 1035 cm -1The absorption peak at [location] corresponds to the C-O stretching vibration in cellulose and hemicellulose, which is a characteristic peak of holocellulose. With the prolongation of the treatment time of the high-voltage electrostatic field combined with high temperature, the intensity of this absorption peak gradually weakens, indicating that the side chain structures of hemicellulose and partial cellulose are damaged. After modification by high temperature and the combination of high-voltage electrostatic field and high temperature, the characteristic peak at 2895 cm -1 strengthens, indicating that the C-H stretching vibration of methyl and methylene groups strengthens. The characteristic peaks of lignin at 1594, 1508, 1460, and 1059 cm -1 increase in peak value with the increase of the modification temperature during the high-temperature modification process. This is due to the increase in the relative content of lignin caused by high-temperature treatment. The absorption peak at 1426 cm -1 belongs to the characteristic peak of cellulose in carbohydrates and changes little before and after the high-temperature modification of the specimen. The main reason is that the chemical components of cellulose have high heat resistance. The intensity of the characteristic peak of C-O-C in cellulose (1160 cm -1 ) shows the phenomenon of CK < HP < HHP. This is because high temperature promotes the condensation of hydroxyl groups between molecular chains in the amorphous region of cellulose, removes water and generates glycosidic bonds, and the combination of high-voltage electrostatic field and high temperature further exacerbates this phenomenon. The difference in the intensity of the characteristic peak at 897 cm -1 is not obvious, indicating that the degradation effect of 180 °C high temperature or the combination of high-voltage electrostatic field and high temperature on poplar cellulose is not obvious.

[0105] 2.6 Combustibility Test

[0106] As Figure 6 shown, to explore the combustion characteristics of poplar and modified poplar specimens, the materials were directly exposed to an open flame for qualitative evaluation. A butane flame gun was used to simulate a fire in a real environment, and a stable fire source was provided for each sample to be ignited. The heat supply time of the fire source was set to 20 s. Under this fire source, the combustion data of each sample are shown in Table 4. The results show that the self-extinguishing time of HP and HHP after moving away from the fire source decreases significantly, decreasing by 11.25% and 20.42% respectively compared with CK. After moving away from the fire source, the residual masses of samples HP and HHP are as high as 20.2% and 23.35% respectively, both higher than CK, and HHP shows the best flame retardancy.

[0107] Table 4 Continuous combustion test data of poplar samples, heat-treated poplar, and poplar modified by the combination of high-voltage electrostatic field and heat treatment

[0108] Specimen Residue mass / % Duration of continuous ignition source / s Self-extinguishing duration after moving away from the ignition source / s CK 11.79 20 160 HP 20.2 20 142 HHP 23.35 20 131

[0109] Note: Control sample (CK), heat-treated poplar (HP), poplar treated by HVEF combined with heat treatment (HHP).

[0110] 3. Conclusion

[0111] In this application, 10-year-old poplar (Populus euramericana) was used as the experimental material. Through the synergistic modification technology of high-voltage electrostatic field (HVEF) pretreatment and high-temperature carbonization, the effects on poplar were systematically studied, providing a new green and efficient way to improve the comprehensive properties of poplar and obtaining the following conclusions: HHP is significantly superior to CK and HP in terms of dimensional stability, surface properties, decay resistance, chemical structure, and flame retardancy. Specifically, compared with CK, HHP has a lower volume saturation water swelling rate, lower water absorption rate, a larger surface contact angle, and significantly weakened hydrophilicity, with a darker color and lower gloss. At the same time, the decay resistance of HHP against white rot fungi (Coriolus versicolor) and brown rot fungi (Gloeophyllum trabeum) is enhanced, and the mass loss rate after decay is significantly reduced. Fourier transform infrared spectroscopy (FTIR) analysis shows that significant changes have occurred in the chemical bonds of HHP, such as the breakage of hemicellulose ester bonds and the condensation of hydroxyl groups between cellulose molecular chains. HHP also exhibits excellent flame retardancy, with a significantly shortened self-extinguishing time and the highest residue mass. This application provides a new green and efficient way to improve the comprehensive properties of poplar.

[0112] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application and are not intended to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for modifying wood by synergistic heat treatment with a high-voltage electrostatic field, characterized in that, It includes the following steps: Place the wood sample in a high-voltage electrostatic field, where the electric field direction is perpendicular to the fiber direction of the wood. After the high-voltage electrostatic field treatment is completed, heat treatment is carried out to obtain the wood modified by the synergistic effect of the high-voltage electrostatic field and heat treatment.

2. The method for modifying wood by synergistic heat treatment with a high-voltage electrostatic field according to claim 1, wherein, The specific conditions for the heat treatment are as follows: Place the wood sample in a nitrogen-protected tube furnace and heat it from room temperature to 100 °C at a rate of 5 °C·min -1 Then heat it to 180 °C at a rate of 2 °C·min -1 Keep it at this temperature for 1.5 h to achieve controllable degradation of hemicellulose while avoiding excessive carbonization of cellulose. Then cool it down to 80 °C at a rate of 3 °C·min -1 and then naturally cool it to below 40 °C and take it out.

3. The method for modifying wood by high-voltage electrostatic field synergistic heat treatment according to claim 2, characterized in that, The moisture content of the wood is 10-12%.

4. The method for modifying wood by synergistic heat treatment with a high-voltage electrostatic field according to claim 2, characterized in that, The nitrogen gas flow rate is 20-30 mL·min -1 , and its oxygen concentration < 3%.

5. The method for modifying wood by synergistic heat treatment with a high-voltage electrostatic field according to claim 1, characterized in that, The specific conditions for the high-voltage electrostatic field are as follows: Place the wood sample in a high-voltage electrostatic field treatment device. During the treatment, the voltage is set to 60 kV, the treatment time is 12 h, and the ambient temperature is 28 °C; Among them, the high-voltage electrostatic field treatment device is composed of two electrode plates with dimensions of length×width×height of 150 mm×60 mm×10 mm. One of the electrode plates is used as the negative electrode and connected to the high-voltage generator, and the other is used as the positive electrode and grounded. The wood sample is placed between the two electrode plates. The conductive metal electrode plate connected to the negative electrode does not contact the wood sample, and the distance is 1 cm. The conductive metal electrode plate connected to the positive electrode contacts the wood sample.

6. The method for modifying wood by synergistic heat treatment with a high-voltage electrostatic field according to claim 1, characterized in that, The wood is a block with radial×transverse×tangential dimensions of 20 mm×20 mm×10 mm cut from the sapwood of a tree with a breast diameter of 45 cm.

7. Application of the method for modifying wood by the synergistic effect of the high-voltage electrostatic field and heat treatment according to any one of claims 1 to 6 in the field of wood processing.

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