Preparation method of SiO2 mineralization delignification and hydrogel treatment poplar composite material

Abstract

The invention discloses a method for preparing a poplar composite material through SiO2 mineralization delignification and hydrogel treatment, which comprises the following steps of: by taking fast-growing poplar as a raw material, simulating a natural biological mineralization process, and removing lignin by adopting NaClO2 to expose a highly ordered wood cellulose skeleton of the poplar; the wood subjected to delignification treatment shows good arrangement property and nano porosity; the biocompatibility is improved through gelatin hydrogel impregnation, and a nucleation site is provided for subsequent SiO2 mineralization; a sol-gel method is utilized, tetraethoxysilane is used as a silicon source, in-situ mineralization of SiO2 in poplar cell walls and cell cavities is achieved under the assistance of pH value regulation and control and a low-voltage electrostatic field, and the poplar composite material subjected to SiO2 mineralization delignification and hydrogel treatment is prepared. The physical and chemical properties of the SiO2 mineralized delignification and hydrogel treated poplar composite material are remarkably improved, the absolute dry density is increased, the radial and chordwise saturated water swelling rate and the 24-hour water absorption rate are reduced, the dimensional stability is enhanced, and the flame retardance is good.

Description

Technical Field

[0001] The present invention belongs to the technical field of composite material preparation, and particularly relates to a preparation method of a poplar wood composite material treated by SiO mineralization delignification and hydrogel. 2 Background Art

[0002] Artificially grown fast-growing forest wood has some inherent defects, such as being light and soft in texture, poor in dimensional stability, poor in water resistance, and low in mechanical strength. These defects limit its application range. Therefore, the modification treatment of fast-growing wood becomes crucial. Fast-growing poplar wood in plantations is one of the most important renewable resources in northern China. The research on its modification has become an effective way for the wood processing and furniture industries to improve the added value and utilization rate of poplar wood products and maximize enterprise benefits.

[0003] Biomineralization is a common phenomenon in nature, such as the deposition of inorganic minerals in diatoms, shells, and animal bones. Under the control of biological genetic materials, these inorganic minerals nucleate, crystallize, and assemble orderly with bioactive substances as templates to form structures with excellent mechanical properties. Wood such as poplar is a structural nanocellulose material, in which cellulose accounts for about 40% to 50% of the wood mass. The inorganic nanomaterials obtained by the process of natural biomineralization of wood have the advantages of being non-toxic, low in energy consumption, and mild in synthesis conditions, but the natural mineralization time is long and it is difficult to meet the production requirements. Summary of the Invention

[0004] To solve the above problems, the present invention proposes a preparation method of a poplar wood composite material treated by SiO mineralization delignification and hydrogel. This method broadens the application of poplar wood in high-grade wood products and structural materials by improving the performance and added value of poplar wood. 2

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] A preparation method of a poplar wood composite material treated by SiO mineralization delignification and hydrogel, comprising the following steps: 2

[0007] S1. Pretreat poplar wood to obtain oven-dry poplar wood;

[0008] S2. Immerse the oven-dry poplar wood under pressure in a NaOH solution with a mass fraction of 2% to activate the hydroxyl groups on the surface of the poplar wood;

[0009] S3. Put the activated poplar wood into a sodium chlorite solution with a concentration of 4 wt% for delignification treatment to remove part of the lignin in the wood. Among them, the conditions for the delignification treatment are: temperature 100 °C, time 6 h, and the mass ratio of poplar wood to sodium chlorite solution is 1:30;

[0010] S4. Take out the delignified poplar wood, and successively wash it with deionized water and perform vacuum drying to obtain the delignified poplar wood after cleaning and drying;

[0011] S5. Prepare gelatin with a mass fraction of 4% and dissolve it in a citric acid solution of 4 g·L -1 to obtain a hydrogel precursor solution. Then place the delignified poplar wood after cleaning and drying in the hydrogel precursor solution for vacuum impregnation, and set up a low-voltage electrostatic field device 10 cm above the poplar wood to perform low-voltage electrostatic field treatment to achieve the hydrogel treatment of the poplar wood;

[0012] S6. Use tetraethyl orthosilicate and water with a mass ratio of 4:1, use hydrochloric acid as a catalyst, adjust the pH value of the solution to 2, and stir magnetically for 30 min. Through hydrolysis and polycondensation reactions under the condition of 30 °C, obtain a nano-SiO 2 precursor solution;

[0013] S7. Immerse the poplar wood after hydrogel treatment in the nano-SiO 2 precursor solution, and set up a low-voltage electrostatic field device 10 cm above the poplar wood, and perform low-voltage electrostatic field treatment simultaneously during the vacuum impregnation process;

[0014] S8. After the low-voltage electrostatic field treatment is completed, turn off and remove the low-voltage electrostatic field device, and perform pressure treatment on the poplar wood immersed in the nano-SiO 2 precursor solution to achieve in-situ mineralization of SiO 2 in the cell wall and cell cavity of the poplar wood, and obtain a SiO 2 mineralized delignified and hydrogel-treated poplar wood composite material.

[0015] Preferably, the specific process of step S1 is: Under normal temperature and pressure, immerse the poplar wood in deionized water for 12 h to remove impurities on the surface of the poplar wood, and then put the poplar wood into a blast drying oven. At temperature gradients of 40 °C, 60 °C, and 80 °C, dry for 2 h at each temperature gradient in turn. After drying to a bone-dry state, obtain bone-dry poplar wood.

[0016] Preferably, in step S2, the pressure for immersing the bone-dry poplar wood in the NaOH solution is 0.5 MPa, and the immersion time is 6 h.

[0017] Preferably, the specific process of step S4 is: Take out the delignified poplar wood, wash it with deionized water, perform vacuum drying under the condition of a negative pressure of 0.1 MPa, adjust the temperature of the vacuum drying oven to 30 °C, 50 °C, and 70 °C in turn, with drying times of 3 h respectively. Then adjust the temperature of the vacuum drying oven to 100 °C, with a drying time of 12 h. After taking out the poplar wood and weighing it, put it into the vacuum drying oven and dry at 70 °C for 1 h.

[0018] Preferably, in step S5, the negative pressure of the vacuum impregnation is 0.1 MPa, and the time of the vacuum impregnation is 3 h; the voltage of the low-voltage electrostatic field device is set to 600 V·cm -1 , and the treatment time of the low-voltage electrostatic field is 3 h.

[0019] Preferably, in step S7, the negative pressure of the vacuum impregnation for poplar is 0.1 MPa, and the time of the vacuum impregnation is 8 h; the voltage of the low-voltage electrostatic field device is set to 600 V·cm -1 , and the treatment time of the low-voltage electrostatic field is 8 h.

[0020] Preferably, in step S8, the pressure of the pressure treatment is 0.5 MPa, the impregnation temperature is 30 °C, and the impregnation time is 10 h.

[0021] A SiO 2 mineralized delignified and hydrogel-treated poplar wood composite material, the SiO 2 mineralized delignified and hydrogel-treated poplar wood composite material is prepared by using the preparation method of the SiO 2 mineralized delignified and hydrogel-treated poplar wood composite material.

[0022] After adopting the above technical solutions, the present invention has the following beneficial effects: The present invention conducts a series of modification treatments on poplar wood by means of biomimetic mineralization and chemical treatment to improve its material properties. First, through the delignification treatment, a highly ordered poplar wood cellulose skeleton is obtained, maintaining the structural hierarchy and oriented cell structure of the wood. Subsequently, the biocompatibility of the wood is improved by impregnating with gelatin hydrogel, providing abundant nucleation sites for subsequent mineralization reactions. By precisely controlling the pH value of the TEOS aqueous solution and adopting the "negative pressure - positive pressure" impregnation method combined with low-voltage electrostatic field treatment, the precursor solution is deeply penetrated into the wood. During the mineralization process, by adjusting the temperature and time, SiO 2 particles are successfully in-situ generated on the wood cell wall, between cell walls or in cell cavities. The finally obtained SiO 2 mineralized delignified and hydrogel-treated poplar wood composite material (SDP) has significant improvements in both physical and chemical properties: the weight gain rate reaches 12.56%, and the oven-dry density increases from 0.42 g·cm -3 to 0.63 g·cm -3 , and the saturated water swelling rate and water absorption rate are significantly reduced, indicating that the dimensional stability and water resistance of SDP are significantly improved. In addition, the surface color of SDP becomes darker, the surface hydrophilicity is enhanced, and the surface free energy is increased, which is beneficial to the adhesion of coatings and adhesives. FTIR analysis shows that the hydrogen bond interaction between gelatin and poplar wood is enhanced, and nano-SiO 2The successful introduction, along with the weakening of the C=O stretching vibration peak, reflects the formation of the organic-inorganic hybrid structure and enhances the overall performance of the composite material. In terms of thermal stability, SDP exhibits better thermal stability than the control group. During the pyrolysis process, a denser carbon layer structure is formed, effectively hindering heat transfer and the release of flammable gases, which can significantly improve the flame retardancy of poplar wood. It has strong fire resistance and can effectively improve the safety of use. Thermogravimetric analysis further confirms that SDP contains more inorganic substances that are difficult to pyrolyze and have more stable chemical properties. SEM analysis shows that the microstructure of SDP is denser, and the pores are effectively filled with SiO 2 particles, reducing the porosity and increasing the density of the material. The results show that the SiO2 mineralized delignification and hydrogel-treated poplar wood composite material of the present invention not only improves the physical and chemical properties of poplar wood, but also provides a new strategy for the development of safe, easily accessible, and environmentally friendly biomass composite materials, and is expected to promote the process of replacing traditional high-grade hardwoods with fast-growing poplar wood. Description of the Drawings

[0023] Figure 1 It is a diagram showing the test results of the differences in the saturated water swelling rates in the radial and tangential directions, the saturated water swelling rate of the volume, and the water absorption rate in 24 hours of CK and SDP of the present invention;

[0024] Figure 2 It is a diagram showing the test results of the Fourier transform infrared spectra of CK and SDP of the present invention;

[0025] Figure 3 It is a diagram showing the test results of the thermogravimetry of CK and SDP of the present invention;

[0026] Figure 4 It is a diagram showing the test results of the derivative thermogravimetry of CK and SDP of the present invention;

[0027] Figure 5 It is a cross-sectional scanning electron micrograph of CK of the present invention;

[0028] Figure 6 It is a cross-sectional scanning electron micrograph of SDP of the present invention;

[0029] Figure 7 It is a tangential cross-sectional scanning electron micrograph of CK of the present invention;

[0030] Figure 8 It is a tangential cross-sectional scanning electron micrograph of SDP of the present invention. Detailed Embodiments

[0031] In order to make the objectives, technical solutions, and advantages of the present invention clearer and more understandable, the following further describes the present invention in detail with reference to embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0032] As Figures 1 to 8 shown, a preparation method of SiO 2 mineralized delignified and hydrogel-treated poplar wood composite material includes the following steps:

[0033] S1. Pretreat poplar wood to obtain oven-dry poplar wood; in this embodiment, the poplar wood selected is the wood of 9-year-old fast-growing plantation poplar (P. euramevicana) in Siyang County, Suqian City, China;

[0034] The specific process of step S1 is: at normal temperature and pressure, immerse poplar wood in deionized water for 12 h to remove impurities on the surface of poplar wood, then put the poplar wood into a blast drying oven, and at temperature gradients of 40 °C, 60 °C and 80 °C, dry for 2 h at each temperature gradient in turn. After drying to oven-dry state, obtain oven-dry poplar wood;

[0035] S2. Pressurize and immerse the oven-dry poplar wood in a 2% NaOH solution to activate the hydroxyl groups on the surface of poplar wood;

[0036] In step S2, the pressure for immersing the oven-dry poplar wood in the NaOH solution is 0.5 MPa, and the immersion time is 6 h;

[0037] S3. Put the activated poplar wood into a 4 wt% sodium chlorite (NaClO 2 ) solution for delignification treatment to remove part of the lignin in the wood; among them, the conditions for delignification treatment are: temperature 100 °C, time 6 h, and the mass ratio of poplar wood to sodium chlorite solution is 1:30;

[0038] S4. Take out the delignified poplar wood after delignification treatment, and successively carry out deionized water washing and vacuum drying to obtain the washed and dried delignified poplar wood;

[0039] The specific process of step S4 is: take out the delignified poplar wood after delignification treatment, wash it with deionized water, carry out vacuum drying under the condition of a negative pressure of 0.1 MPa, adjust the temperature of the vacuum drying oven to 30 °C, 50 °C and 70 °C in turn, the drying time is 3 h respectively, then adjust the temperature of the vacuum drying oven to 100 °C, the drying time is 12 h, take out the poplar wood and weigh it, and then put it into the vacuum drying oven and dry at 70 °C for 1 h;

[0040] S5. Prepare a 4% gelatin dissolved in a 4 g·L -1 citric acid solution to obtain a hydrogel precursor solution, then place the washed and dried delignified poplar wood in the hydrogel precursor solution for vacuum impregnation, and set a low-voltage electrostatic field device 10 cm above the poplar wood for low-voltage electrostatic field treatment to realize the hydrogel treatment of poplar wood;

[0041] In step S5, the negative pressure of the vacuum impregnation is 0.1 MPa, and the time of the vacuum impregnation is 3 h; the voltage of the low-voltage electrostatic field device is set to 600 V·cm -1 , and the treatment time of the low-voltage electrostatic field is 3 h;

[0042] S6. Using tetraethyl orthosilicate and water with a mass ratio of 4:1, using hydrochloric acid as a catalyst, adjusting the pH value of the solution to 2, and magnetically stirring for 30 min. Through hydrolysis and polycondensation reactions under the condition of 30 °C, a nano-SiO 2 precursor solution is obtained;

[0043] S7. The poplar wood after hydrogel treatment is impregnated in the nano-SiO 2 precursor solution, and a low-voltage electrostatic field device is set 10 cm above the poplar wood, and the low-voltage electrostatic field treatment is carried out simultaneously during the vacuum impregnation process;

[0044] In step S7, the negative pressure of the vacuum impregnation of the poplar wood is 0.1 MPa, and the time of the vacuum impregnation is 8 h; the voltage of the low-voltage electrostatic field device is set to 600 V·cm -1 , and the treatment time of the low-voltage electrostatic field is 8 h;

[0045] S8. After the low-voltage electrostatic field treatment is completed, the low-voltage electrostatic field device is turned off and removed, and the poplar wood impregnated in the nano-SiO 2 precursor solution is subjected to pressure treatment to achieve in-situ mineralization of SiO 2 in the cell wall and cell cavity of the poplar wood, and a SiO 2 mineralized delignified and hydrogel-treated poplar wood composite material is obtained;

[0046] In step S8, the pressure of the pressure treatment is 0.5 MPa, the impregnation temperature is 30 °C, and the impregnation time is 10 h.

[0047] Performance test:

[0048] During the test, the above-prepared SiO 2 mineralized delignified and hydrogel-treated poplar wood composite material is used as the experimental group (SDP), and untreated poplar wood is used as the control group (CK). Due to the large variability of wood, the initial poplar wood of both the experimental group (GSP) and the control group (CK) is selected from the same 9-year-old fast-growing plantation poplar (P. euramevicana) tree in Siyang County, Suqian City, China.

[0049] 1. Water resistance property test and analysis:

[0050] 1.1. Weight gain rate test: The poplar wood specimen with the size specification of 20 mm×20 mm×20 mm (radial×tangential×longitudinal direction) (R×T×L) without treatment is dried to absolute dryness and weighed to obtain the absolute dry mass M of the poplar wood in the blank control group1 。The SiO of the present invention with a size specification of 20 mm × 20 mm × 20 mm (radial × tangential × longitudinal direction) (R × T × L) 2 The mineralized delignified and hydrogel-treated poplar wood composite sample was dried to absolute dryness and weighed to obtain SiO 2 The absolute dry mass M of the mineralized delignified and hydrogel-treated poplar wood composite 2 ; The weight gain rate of the sample was calculated according to the following formula:

[0051] WPG = (M 2 - M 1 ) / M 1 × 100%.

[0052] 1.2. Absolute dry density test: The absolute dry density of the SiO 2 mineralized delignified and hydrogel-treated poplar wood composite prepared by the present invention was tested according to the standard GB / T 1927.5-2021 "Test Methods for Physical and Mechanical Properties of Clear Wood Small Specimens - Part 5: Density Determination".

[0053] 1.3. Saturated water swelling property test: For the experimental group (SDP) and the control group (CK) respectively, according to the standard GB / T1927.8-2021 "Test Methods for Physical and Mechanical Properties of Clear Wood Small Specimens - Part 8: Swelling Property Determination", the specimen sizes in the absolute dry state and the saturated water state of the specimens with a size specification of 20 mm × 20 mm × 20 mm (radial × tangential × longitudinal direction) (R × T × L) were measured.

[0054] The radial saturated water swelling rate (a rs ) was calculated according to the following formula:

[0055] a rs = (L rs - L ro ) / L ro × 100%,

[0056] The tangential saturated water swelling rate (a ts ) was calculated according to the following formula:

[0057] a ts = (L ts - L to ) / L to × 100%,

[0058] In the formula, a rs , a ts are the radial and tangential saturated water swelling rates respectively; L rs , L ts are the radial and tangential dimensions of the specimen when the moisture content of the specimen is higher than the fiber saturation point (i.e., saturated water), in mm; Lro , L to are the radial and tangential dimensions of the specimen at oven-dry condition, in mm.

[0059] When considering the dimensional change in the longitudinal direction of the specimen from oven-dry to saturated water state, the volumetric swelling rate of saturated water (a vs ) is calculated according to the following formula:

[0060] a vs = [(L rs ×L ts ×L hs ) - (L r0 ×L t0 ×L h0 )] / L r0 ×L t0 ×L h0 × 100%,

[0061] In the formula, a vs is the volumetric swelling rate of saturated water of the specimen; 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; L r0 , L t0 and L h0 are the radial, tangential and longitudinal dimensions of the specimen at oven-dry condition, in mm.

[0062] 1.4. Water absorption test: For the experimental group (SDP) and the control group (CK) respectively, according to the standard GB / T1927.7 - 2021 "Test methods for physical and mechanical properties of wood with clear specimens - Part 7: Determination of water absorption", specimens with dimensions of 20 mm × 20 mm × 20 mm (radial × tangential × longitudinal) (R × T × L) are prepared. Weigh the mass m of each specimen after 24 h, and calculate the 24 h water absorption rate (X) according to the following formula:

[0063] X = (m - m 0 ) / m 0 × 100%,

[0064] In the formula, X is the 24 h water absorption rate of the specimen; m is the mass of the specimen after 24 h of water absorption; m 0 is the mass of the specimen at oven-dry condition.

[0065] The test results of water resistance properties show that a large amount of SiO is filled in the pores and cell cavities of the SiO₂ mineralized delignified and hydrogel-treated poplar wood composite prepared by the present invention. 2The particles led to an increase in the mass of the experimental group (SDP), with a weight gain rate reaching 12.56%, and changes in various performance indicators. The changes in the weight gain rate, density, and water absorption rate are the most intuitive reflections of the properties of the modified wood and are directly related to the mechanical properties of the wood. The oven-dry density increased from 0.42 g·cm -3 in the control group (CK) to 0.63 g·cm -3 in the experimental group (SDP).

[0066] As Figure 1 shown, a rs in the control group (CK) and the experimental group (SDP) decreased from 4.33% to 2.48%, a ts decreased from 7.11% to 3.36%, a vs decreased from 12.26% to 6.92%, and the 24-hour water absorption rate decreased from 90.53% to 49.39%. Compared with the control group (CK), the experimental group (SDP) had a significant decrease in the four indicators of a rs , a ts , a vs , and 24-hour water absorption rate, indicating that the experimental group (SDP) had good dimensional stability performance. The main reason is that during the process of treating poplar wood with sodium chlorite (NaClO 2 ) solution in this invention, part of the lignin was dissolved, and at the same time, channels such as wood pits were opened, enabling the tetraethyl orthosilicate (TEOS) hydrosol and the modifier solution to better penetrate into the interior of the wood during the mineralization process. In addition, after the delignification treatment, part of the lignin and hemicellulose were degraded, resulting in a decrease in the water absorption of SDP. This invention uses the sol-gel method to impregnate poplar wood specimens with SiO 2 precursor sol. During the hydrolysis of TEOS, unstable orthosilicic acid will be formed and undergo a dehydration condensation reaction with -OH on the wood surface, thereby in-situ generating SiO 2 particles in the wood cell wall or cell lumen, hindering the penetration of water. The electrostatic field equipment used in the preparation process of SDP accelerated the migration rate of water inside the poplar wood and the hydrolysis reaction of the inorganic precursor solution, improved the permeability of nano-SiO 2 , and accelerated its in-situ mineralization, thereby improving the dimensional stability of SDP. In addition, the hydrogen bonds and chemical crosslinks formed between gelatin and wood cellulose helped to further reduce the water absorption of SDP.

[0067] 2. Surface color and glossiness test and analysis:

[0068] 2.1 Surface color difference measurement: The experimental group (SDP) and the control group (CK) were respectively detected on the surface of the specimens using a color difference meter. Three specimens with uniform surface color were randomly selected from each group for measurement, and the results were averaged. The color difference parameters in this invention include lightness (L*), red-green chromaticity index (a*), yellow-blue chromaticity index (b*), chroma (C*), and total color difference (ΔE*). ΔE* (representing the magnitude of color difference) was calculated through the CIE L*, a*, b* uniform color space.

[0069] The chroma (C*) is calculated according to the following formula:

[0070] C* = (a* 2 + b* 2 )1 / 2,

[0071] The total color difference (ΔE*) is calculated according to the following formula:

[0072] ΔE* = [(ΔL*) 2 + (Δa*) 2 + (Δb*) 2 )1 / 2,

[0073] In the formula, ΔL* = L t * - L 1 *, Δa* = a t * - a 1 *, Δb* = b t * - b 1 *; L 1 *, a 1 *, b 1 * are respectively the lightness, red-green chromaticity index, and yellow-blue chromaticity index of the control group (CK); L t *, a t *, b t * are respectively the lightness, red-green chromaticity index, and yellow-blue chromaticity index of the experimental group (SDP).

[0074] The hue angle (H o ) is calculated according to the following formula:

[0075] H o = tan -1 (b* / a*).

[0076] 2.2. Measurement of surface glossiness: The glossiness shown on the wood surface is related to tree species, surface flatness, tyloses and inclusions, wood structural characteristics, and wood cut surfaces. In this invention, a glossmeter was used to measure the experimental group (SDP) and the control group (CK) respectively. The main research was on the light source incident direction parallel GZL (%) and perpendicular GZT (%) 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.

[0077] The test results of surface color and glossiness show that: As can be seen from Table 1, the L* of the control group (CK) and the experimental group (SDP) on the cross-section is lower than that of the corresponding tangential section, and on the same section, the L* of the control group (CK) is greater than that of the experimental group (SDP). The reason lies in the biomimetic mineralization technology adopted in this invention. This technology utilizes the physical and chemical properties of gelatin, such as its easy film-forming property and high reactivity of side chain groups, to combine gelatin, poplar wood, and SiO 2 The three are combined to form a film within the cell walls and cell spaces of poplar wood. The side chain active groups provide nucleation sites for inorganic particles, thereby inducing the orderly deposition and oriented crystallization of SiO 2 on the cell walls and pores of poplar wood. This method changes the porous structure of poplar wood into a dense structure, resulting in an increase in density and an increase in the absorption of light, so L* decreases. On the other hand, the a* and b* of the control group (CK) and the experimental group (SDP) on the cross-section are higher than those of the corresponding tangential section respectively. On the same section, the a* and b* of the experimental group (SDP) are both increased compared with the control group (CK), and the color of the experimental group (SDP) changes towards red and yellow. The reason is that gelatin and SiO 2 form an organic-inorganic hybrid structure through the sol-gel method. This structure not only enhances the mechanical properties and flame retardant properties of poplar wood but also changes its optical properties. Specifically, nano-SiO 2 particles are filled into the pores and cell cavities of poplar wood, changing the reflection and scattering characteristics of its surface, thus resulting in a change in color. Table 1 shows the change results of the surface color and glossiness of the control group (CK) and the experimental group (SDP).

[0078] Table 1: Change results of surface color and glossiness of the control group (CK) and the experimental group (SDP)

[0079]

[0080]

[0081] The chromaticity values (C*) and hue angles (H o) and glossiness (G*) showed significant differences, as shown in Table 1. On the same cross-section, the C* value of the experimental group (SDP) increased compared to the control group (CK), while H o and G* both decreased. Generally speaking, the total color difference (ΔE*) on the cross-section was greater than that on the tangential section, indicating that the color change of the specimens after mineralization treatment was more obvious on the cross-section. The treatment with gelatin and nano-SiO 2 changed the optical properties of poplar. Nano-SiO 2 changed the microstructure of poplar by filling the pores, thereby affecting the reflection and absorption of light, and finally resulting in the change of C*. In addition, there are a large number of chromogenic groups or auxochromic groups such as carbon-oxygen (C=O), carbon-carbon (C=C) conjugated double bond structures, -OH and methoxy (-OCH 3 ) in poplar. Under the external action conditions such as NaOH, O 2 , heat treatment, etc., the breaking and recombination of chemical bonds occurred, resulting in the change of color indexes of the experimental group (SDP). The dense microstructure formed by the coordination of gelatin and SiO 2 may reduce the intensity of surface reflected light, ultimately leading to the decrease of G*.

[0082] 3. Surface contact angle test analysis:

[0083] Surface contact angle measurement: The principle of contact angle measurement is based on the surface tension of the liquid and the properties of the solid surface. In the experiment, distilled water was used as the test liquid, and the surface contact angles of the experimental group (SDP) and the control group (CK) were measured respectively. The test results are shown in Table 2.

[0084] Table 2: Surface contact angle test results of the control group (CK) and the experimental group (SDP)

[0085]

[0086]

[0087] It can be seen from Table 2 that the initial contact angles of the experimental group (SDP) on the cross-section and tangential section were both smaller than those of the control group (CK). This is because the addition of gelatin and nano-SiO 2 introduced new functional groups, changed the surface chemical properties of poplar, increased the surface energy and hydrophilicity, and the wettability of the specimen surface increased, making the liquid easier to spread on the specimen surface, resulting in the decrease of the contact angle. The initial contact angles of the control group (CK) and the experimental group (SDP) on the cross-section were smaller than those on the tangential section. The level of surface free energy is an important indicator to measure the interaction ability between the specimen surface and other substances. The maximum surface free energy of the experimental group (SDP) on the cross-section was 70.17 mJ·m -2, the surface free energy of the control group (CK) on the tangential section was the lowest at 43.33 mJ·m -2 , on the same section, the surface free energy of the experimental group (SDP) was higher than that of the control group (CK), indicating that the surface of the experimental group (SDP) had stronger adhesion and wettability, which was helpful for the adhesion of substances such as coatings and adhesives. In summary, the change in the surface contact angle of the SiO 2 -mineralized delignified and hydrogel-treated poplar wood composites was the result of the combined action of the formation of new chemical bonds due to chemical reactions, the consumption of -OH groups, the physical change of the internal structure of poplar wood, and the change in surface hydrophilicity.

[0088] 4. Fourier transform infrared spectroscopy (FTIR) analysis:

[0089] Fourier transform infrared spectroscopy determination: All sample powders (particle size < 200 mesh) were dried in a forced-air drying device at 105 °C for 12 h and then subjected to the detection test. Accurately weigh 2.0 mg of the sample and mix it evenly with 200 mg of potassium bromide, press it into a transparent tablet with a thickness of about 1 mm, put it into the FTIR sample chamber for determination, and the spectrum was scanned in the range of 400 - 4000 cm -1 with a resolution of 4 cm -1 to analyze the chemical structures of the control group (CK) and the experimental group (SDP). The test results are as Figure 2 shown.

[0090] As Figure 2 shown, for the two samples of the control group (CK) and the experimental group (SDP), the absorption peaks corresponding to the stretching vibration of the -OH group, the C-H stretching vibration in the CH -1 and CH -1 groups, and the stretching vibration of C=O were near 3410 cm -1 , 2922 cm 2 and 1650 cm 3 respectively. Compared with the control group (CK), the experimental group (SDP) showed broadening and enhancement in the -OH stretching vibration band at 3000 - 3600 cm -1 , indicating that hydrogen bonds between the gelatin hydrogel and delignified poplar wood were dominant. Compared with the control group (CK), the experimental group (SDP) containing a large amount of -OH groups in the gelatin solution and nano-SiO 2 showed an increase in the intensity in the -OH absorption peak band. The main absorption peaks of the samples were concentrated at 465 cm -1 , 799 cm -1 , 1070 cm -1 , 1111 cm -1 , 1325 cm -1 , 1376 cm -1 , 1596 cm -1, 1650 cm -1 , 1737 cm -1 , 2922 cm -1 and 3410 cm -1 , where the absorption peak near 2922 cm -1 is related to the C-H vibrations of methylene (-CH 2 -) and methyl (-CH 3 ) in the sample. The absorption peak at 1737 cm -1 is attributed to the stretching vibration of the ester carbonyl (C=O). Here, the absorbance intensity of the control group (CK) is significantly greater than that of the experimental group (SDP). The absorption peak at 1650 cm -1 is related to the stretching vibration of C=O or unsaturated C=C. The medium absorption peak at 1376 cm -1 corresponds to the bending vibrations of -CH 3 - and -CH 2 - respectively. The stretching vibration absorption peak and bending vibration absorption peak of Si-O-Si are at 1070 cm -1 and 465 cm -1 respectively. The absorption peak near 1070 cm -1 in the experimental group (SDP) shows a very high-intensity and wide absorption band, mainly due to the Si-O-Si stretching vibration of SiO 2 in the experimental group (SDP). This may also indicate an increase in the bonding between SiO 2 and poplar. At the same time, the characteristic peak at 848 cm 3 representing Si-CH -1 does not appear in the experimental group (SDP), indicating that SiO 2 does not directly form a bond with the poplar cell wall, but reacts with -OH on the poplar surface to form an organic-inorganic hybrid structure of Si-O-C. The FTIR spectrum shows that TEOS generates unstable orthosilicic acid through hydrolysis and reacts with -OH and carboxyl (-COOH) on the surface of the poplar cell wall under weak acidic conditions, thus achieving the effect of mineralizing poplar.

[0091] 5. Thermogravimetric analysis (TGA):

[0092] Thermogravimetric analysis determination: All the sample powders (particle size < 200 mesh) of the thermogravimetric analyzer were dried in a blast drying device at 45 °C for 8 h and then tested. Under an N -1 atmosphere of 80 mL·min 2 , the samples were detected at a heating rate of 10 °C·min -1 and heated to 800 °C to analyze the thermal stability of the control group (CK) and the experimental group (SDP). The test results are shown in Figure 3 and Figure 4 .

[0093] As shown Figure 3 in the figure, the pyrolysis processes of the control group (CK) and the experimental group (SDP) protected by N 2 can be divided into four stages over the entire temperature range. At a heating rate of 10 °C·min -1 , when heated to 800 °C, the residual mass of the experimental group (SDP) is 29.48%, while that of the control group (CK) is 13.42%, with a difference of 16.06% between the two. This indicates that the synergistic effect of gelatin and SiO 2 can promote the formation of the carbon layer. The organic matter in the control group (CK) is almost completely pyrolyzed, leaving only ash. After pyrolysis of the experimental group (SDP), in addition to ash, a large amount of SiO 2 remains. The pyrolysis processes of the control group (CK) and the experimental group (SDP) are the result of the comprehensive pyrolysis of components such as hemicellulose, cellulose, and lignin. From Figure 4 the derivative thermogravimetry (DTG) analysis, it can be seen that the first stage is the drying stage (temperature < 100 °C). Near 55.4 °C and 61.0 °C, a peak appears in the drying stage of both samples, indicating the endothermic evaporation process of water in the samples. The second stage is the preheating stage before pyrolysis (temperature range 100 - 260 °C). When the temperature rises to about 200 °C, the pyrolysis reaction gradually starts and the pyrolysis process begins to release heat. The pyrolysis process of the experimental group (SDP) is relatively stable in this stage because SiO 2It can hinder the diffusion of combustible gases released during the pyrolysis of cellulose and hemicellulose in the specimen within poplar wood, thereby preventing heat transfer. When the temperature is in the range of 260 - 300 °C, the slope of the specimen weight loss curve and the peak value of the corresponding DTG vs. temperature (DTG-T) gradually increase, mainly because the main components in the control group (CK) and the experimental group (SDP) are gradually transformed into small molecule volatile substances. It can be seen from the DTG curve that the DTG of the control group (CK) specimen shows a violent pyrolysis reaction near 280 - 380 °C and there are two obvious downward peaks. This is mainly because the pyrolysis of poplar wood has two stages: the previous peak is mainly when the pyrolysis of hemicellulose is basically completed, while the pyrolysis reaction of cellulose has not reached the maximum value, and the latter peak is when the pyrolysis reaction of cellulose reaches the maximum value. In this temperature range, only one peak appears in the experimental group (SDP), which confirms that the proportion of organic matter that can be pyrolyzed in the experimental group (SDP) decreases, while the proportion of inorganic matter that is difficult to pyrolyze increases. Compared with the control group (CK), the pyrolysis in the third stage of the experimental group (SDP) is inhibited, indicating that when gelatin is introduced into the inorganic modification of poplar wood, gelatin can cooperate with SiO2 to form a carbon layer structure, effectively hindering heat transfer and the release of combustible gases, thereby improving the thermal stability. In the fourth stage (380 - 800 °C), as the temperature continues to rise, the weight loss rate rapidly decreases, and the pyrolysis reaction of lignin continues. During the coke oxidation process, the carbon layer structure formed by the experimental group (SDP) is denser than that of the control group (CK), which can slow down the penetration and diffusion of oxygen in the coke layer, hinder the further oxidation and decomposition of the experimental group (SDP), and improve its thermal oxidation stability.

[0094] 6. Scanning electron microscope (SEM) analysis:

[0095] The SEM is equipped with an energy dispersive X-ray analysis device (EDS). All samples are sputter-coated with gold to increase conductivity. The measurement voltage is 20 KV, and the microstructural morphology of the control group (CK) and the experimental group (SDP) is observed. The test results are as Figure 5 、 Figure 6 、 Figure 7 and Figure 8 shown.

[0096] Figure 5 and Figure 7 are the cross-sectional and tangential surface scanning electron micrographs of the control group (CK) respectively, which have a relatively high porosity; Figure 6 and Figure 8 are the cross-sectional and tangential surface scanning electron micrographs of the experimental group (SDP) respectively. Compared with the control group (CK), a large amount of SiO 2Poplar is a typical anisotropic structural material composed of cellulose, hemicellulose, and lignin, where lignin fills the cellulose framework to enhance the mechanical strength of the poplar cell wall. In the present invention, after the poplar is delignified, the internal connection of the cellulose fibrils is broken, and the poplar cells become loose and soft, forming a more compact structure. At the same time, the nano-pores of the poplar fiber bundles are opened, which is beneficial to the penetration of the gelatin hydrogel and SiO 2 The delignification treatment exposes a large number of -OH on the poplar surface. Under vacuum conditions, the low-concentration gelatin hydrogel fully penetrates into the poplar cellulose skeleton and forms hydrogen bonds with the -OH on the cellulose molecular chain. The gelatin hydrogel fills the voids between the fibers, and SiO generated by the hydrolysis and polycondensation of tetraethyl orthosilicate (TEOS) 2 The surface contains a large number of -OH, and at the same time, the active groups (-NH 2 , -COOH, -OH) of the gelatin hydrogel provide nucleation sites for the mineralization of SiO 2 , inducing the continuous deposition of SiO 2 in the poplar cell wall and poplar voids. At the same time, the nano-scale SiO 2 aggregates, fills the micron-scale poplar pores, and forms chemical bonds with the poplar components, forming an organic-inorganic hybrid structure of SiO 2 -cellulose, transforming the porous structure of the control group (CK) into the dense wall structure of the experimental group (SDP).

[0097] Figure 8 is the tangential section of the experimental group (SDP). It can be seen that the tracheids, pits, and pores of the poplar are filled with a large amount of SiO 2 , and due to the aggregation effect, these SiO 2 show no specific shape. When the poplar is impregnated by the sol-gel method, TEOS first hydrolyzes to generate Si-OH, and then a large number of Si-OH continuously undergo polycondensation reactions to form Si-O-Si, which results in the amorphous structure of SiO 2 and forms a SiO 2 film layer. At this time, the surface of the SiO 2 film layer carries -OH and -CH 3 , and the -OH undergoes dehydration polycondensation reaction with gelatin, thus eliminating the -OH on the surface of the SiO 2 film layer, reducing the aggregation phenomenon, and forming a dense SiO 2 film layer and filler on the inner and outer surfaces of the poplar.

[0098] As described above, it is only the preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.

Claims

1. A method for preparing a SiO2 mineralized delignified and hydrogel treated poplar composite material, characterized in that: The following steps are involved: S1, pre-treating the poplar wood to obtain absolutely dry poplar wood; S2, immersing the absolutely dried poplar wood in a 2% by mass NaOH solution under pressure to activate the hydroxyl groups on the surface of the poplar wood; S3, placing the activated poplar wood in a sodium chlorite solution with a concentration of 4wt% for delignification treatment to remove part of the lignin in the wood; wherein the conditions for the delignification treatment are: temperature 100°C, time 6h, and the mass ratio of the poplar wood to the sodium chlorite solution is 1:30; S4, taking out the delignified poplar wood, and washing it with deionized water and vacuum drying it in sequence to obtain washed and dried delignified poplar wood; S5. Prepare 4% gelatin and dissolve it in 4 g L -1 citric acid solution to obtain a hydrogel precursor solution, and then the delignified poplar wood after cleaning and drying is placed in the hydrogel precursor solution for vacuum impregnation, and a low-voltage electrostatic field device is set 10 cm above the poplar wood to perform low-voltage electrostatic field treatment to achieve hydrogel treatment of the poplar wood; S6, using ethyl orthosilicate and water in a mass ratio of 4:1, using hydrochloric acid as a catalyst, adjusting the solution pH to 2, magnetic stirring for 30 minutes, and obtaining a nano-SiO2 precursor solution through hydrolysis and polycondensation reaction at 30°C; S7, the poplar wood treated with the hydrogel is immersed in the nano-SiO2 precursor solution, and a low-voltage electrostatic field device is set 10 cm above the poplar wood, and the low-voltage electrostatic field treatment is performed simultaneously during the vacuum impregnation process; S8. After the low-voltage electrostatic field treatment is completed, the low-voltage electrostatic field equipment is turned off and withdrawn, and the poplar wood immersed in the nano-SiO2 precursor solution is subjected to pressure treatment to achieve in-situ mineralization of SiO2 in the poplar wood cell wall and cell cavity, and obtain a SiO2 mineralized delignified and hydrogel-treated poplar wood composite material.

2. The method for preparing a SiO2 mineralized delignified and hydrogel treated poplar composite material according to claim 1, characterized in that: The specific process of step S1 is: at normal temperature and pressure, the poplar wood is immersed in deionized water for 12 hours to remove impurities on the surface of the poplar wood, and then the poplar wood is placed in a forced air drying oven, and dried for 2 hours at each temperature gradient at 40°C, 60°C and 80°C, and dried to a completely dry state to obtain the completely dried poplar wood.

3. The method for preparing a SiO2 mineralized delignified and hydrogel treated poplar composite material according to claim 1, characterized in that: In step S2, the pressure of immersing the absolutely dry poplar wood in the NaOH solution is 0.5 MPa, and the immersion time is 6 hours.

4. The method for preparing a SiO2 mineralized delignified and hydrogel treated poplar composite material according to claim 1, characterized in that: The specific process of step S4 is: take out the delignified poplar wood, wash it with deionized water, and vacuum dry it under a negative pressure of 0.1 MPa. Adjust the temperature of the vacuum drying oven to 30°C, 50°C and 70°C respectively, and the drying time is 3 hours respectively. Then adjust the temperature of the vacuum drying oven to 100°C, and the drying time is 12 hours. After taking out the poplar wood and weighing it, put it into the vacuum drying oven and dry it at 70°C for 1 hour.

5. The method for preparing a SiO2 mineralized delignified and hydrogel treated poplar composite material according to claim 1, characterized in that: In step S5, the negative pressure of the vacuum impregnation is 0.1 MPa, and the vacuum impregnation time is 3 hours; the voltage of the low-voltage electrostatic field device is set to 600 V·cm -1 , the low-voltage electrostatic field treatment time is 3h.

6. The method for preparing a SiO2 mineralized delignified and hydrogel treated poplar composite material according to claim 1, characterized in that: In step S7, the negative pressure of the vacuum impregnation of the poplar wood is 0.1 MPa, and the vacuum impregnation time is 8 hours; the voltage of the low-voltage electrostatic field device is set to 600 V·cm -1 , the low-voltage electrostatic field treatment time is 8h.

7. The method for preparing a SiO2 mineralized delignified and hydrogel treated poplar composite material according to claim 1, characterized in that: In step S8, the pressure of the pressurization treatment is 0.5 MPa, the immersion temperature is 30° C., and the immersion time is 10 h.

8. A SiO2 mineralized delignified and hydrogel treated poplar composite material, characterized in that: The SiO2 mineralized delignified and hydrogel-treated poplar wood composite material is prepared by the preparation method of the SiO2 mineralized delignified and hydrogel-treated poplar wood composite material as described in any one of claims 1 to 7.

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