Preparation method and use method of lignocellulose supramolecular adhesive

Abstract

The invention discloses a preparation method and a use method of a lignocellulose supramolecular adhesive, and belongs to the technical field of wood material deep processing / adhesives. The invention aims to solve the problem that the adhesive cannot be prepared by using all components of lignocellulose in the prior art. The preparation method comprises the following steps: mixing lignocellulose, sodium periodate and water, then heating and reacting in a dark place, standing and precipitating after the reaction, and removing supernate to obtain the lignocellulose supramolecular adhesive. The application method comprises the following steps: 1, gluing; (2) assembling; and 3, hot pressing. The method is used for preparing and using the lignocellulose supramolecular adhesive.

Description

Technical Field

[0001] The invention belongs to the technical field of deep processing of wood materials / adhesives. Background Art

[0002] Wood adhesives are the most widely used adhesive products in the world. However, existing commercial adhesives are mainly formaldehyde adhesives (such as urea-formaldehyde resin adhesives, phenolic resin adhesives, melamine-formaldehyde resin adhesives, etc.), which release free formaldehyde and endanger human health. In addition, most of these formaldehyde-based adhesives are derived from non-renewable fossil resources. Therefore, in order to reduce dependence on fossil resources, researchers from industry and academia have prepared bio-based wood adhesives from a variety of renewable resources, such as soybeans, starch, chitosan, etc., to replace petrochemical resources. However, the raw materials of some bio-based adhesives (such as soybeans, starch, etc.) conflict with food crops. Therefore, it is necessary to further expand the source of raw materials for bio-based wood adhesives.

[0003] Among many alternative raw materials, cellulose has become a very promising choice due to its abundant reserves, low cost, sustainability and degradability. However, due to the low activity, poor adhesion and complex structure of cellulose, most existing studies simply use cellulose as a filler instead of directly as an adhesive. Another method is to extract highly active cellulose and lignin from cellulose to make wood adhesives, but currently obtaining highly active cellulose and lignin from cellulose still faces the challenges of complex processes and high costs. In addition, the solution of extracting cellulose or lignin alone is not conducive to the utilization of all components of cellulose. Summary of the invention

[0004] The present invention aims to solve the problem that the prior art cannot use all components of lignocellulose to prepare adhesives, and further provides a preparation method of a lignocellulose supramolecular adhesive and a use method thereof.

[0005] A method for preparing a lignocellulose supramolecular adhesive is carried out according to the following steps:

[0006] The lignocellulose, sodium periodate and water are mixed, and then heated in the dark for reaction. After the reaction, the mixture is allowed to stand for precipitation and the supernatant is removed to obtain the lignocellulose supramolecular adhesive.

[0007] A method for using a lignocellulose supramolecular adhesive is carried out according to the following steps:

[0008] 1. Gluing:

[0009] coating the lignocellulose supramolecular adhesive on the surface of the veneer to obtain a veneer coated with the lignocellulose supramolecular adhesive;

[0010] 2. Assembly:

[0011] Assembling the veneers coated with the wood cellulose supramolecular adhesive in a direction perpendicular to the textures of adjacent veneers to obtain an assembly;

[0012] 3. Hot pressing:

[0013] The assembly is hot-pressed to obtain plywood, thus completing the method for using the wood cellulose supramolecular adhesive.

[0014] The beneficial effects of the present invention are:

[0015] In terms of raw materials, the raw material of lignocellulose supramolecular adhesive is lignocellulose, the world's largest renewable resource. Lignocellulose can be obtained from biomass resources such as agricultural and forestry residues, avoiding the use of food crops such as starch and soybeans to ensure food security;

[0016] In terms of preparation, the lignocellulose supramolecular adhesive can be prepared by a simple one-step selective activation method, avoiding the cumbersome extraction process of traditional adhesives made from lignocellulose.

[0017] In terms of performance, the dry and wet shear strength of the wood cellulose supramolecular adhesive meets the GB / T 9846-2015 standard (>0.7MPa), and has excellent high bonding performance and resistance to 63°C hot water.

[0018] In terms of the universality of the method, lignocellulose supramolecular adhesives can be prepared using lignocellulose from different sources such as beech, pine, and sugarcane bagasse. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 are optical photos and SEM electron microscope photos, a and c are the lignocellulose adhesive prepared in Comparative Example 1 after drying, b and d are the lignocellulose supramolecular adhesive prepared in Example 2 after drying;

[0020] Figure 2 FTIR spectra of the lignocellulose adhesive prepared in Comparative Example 1 after drying, the lignocellulose supramolecular adhesive prepared in Example 2 after drying, and the lignocellulose supramolecular adhesive in Example 2 after hot pressing using the method of Example 1;

[0021] Figure 3 : XPS C1s energy spectrum and its content diagram, a is the lignocellulose adhesive prepared in comparative example 1 after drying, b is the lignocellulose supramolecular adhesive prepared in example 2 after drying, c is the lignocellulose supramolecular adhesive in example 2 after hot pressing using the method in example 1, and d is the content diagram;

[0022] Figure 4The size distributions of the lignocellulose adhesive prepared in Comparative Example 1 after drying and the lignocellulose supramolecular adhesive prepared in Example 2 after drying. DETAILED DESCRIPTION

[0023] Specific implementation method 1: This implementation method is a method for preparing a lignocellulose supramolecular adhesive, which is carried out according to the following steps:

[0024] The lignocellulose, sodium periodate and water are mixed, and then heated in the dark for reaction. After the reaction, the mixture is allowed to stand for precipitation and the supernatant is removed to obtain the lignocellulose supramolecular adhesive.

[0025] Principle: The lignocellulose supramolecular adhesive prepared in this specific embodiment converts the hydroxyl groups of lignocellulose into active bonding groups such as aldehyde groups and quinone groups through the activation of sodium periodate, so that it produces bonding effect. In this process, the surface of the lignocellulose supramolecular adhesive is etched, the specific surface area increases, the particle size decreases, and a stronger activity is formed. At the same time, after hot pressing, the aldehyde groups of the lignocellulose supramolecular adhesive can be covalently cross-linked with the remaining hydroxyl groups through acetal reaction to form ether bonds, thereby having strong water resistance.

[0026] Although the prior art can convert lignocellulose into glue-free self-adhesive materials, the self-adhesive technology can only achieve the bonding of treated material surfaces, and cannot achieve the bonding of untreated material surfaces. The core of this technology is to enhance the cohesive force between self-adhesive units. In contrast, the lignocellulose supramolecular adhesive of this specific embodiment not only needs to achieve bonding with the treated material surface (i.e., the cohesive force of the adhesive is enhanced), but also needs to achieve bonding with the untreated material surface (i.e., the bonding force between the adhesive and the untreated wood surface is enhanced). To meet this requirement, the lignocellulose supramolecular adhesive of this specific embodiment has higher chemical activity, larger specific surface area and more active structures. These characteristics enable the lignocellulose supramolecular adhesive of this specific embodiment to become a highly efficient wood adhesive to meet a wider range of application needs.

[0027] The beneficial effects of this embodiment are:

[0028] In terms of raw materials, the raw material of lignocellulose supramolecular adhesive is lignocellulose, the world's largest renewable resource. Lignocellulose can be obtained from biomass resources such as agricultural and forestry residues, avoiding the use of food crops such as starch and soybeans to ensure food security;

[0029] In terms of preparation, the lignocellulose supramolecular adhesive can be prepared by a simple one-step selective activation method, avoiding the cumbersome extraction process of traditional adhesives made from lignocellulose.

[0030] In terms of performance, the dry and wet shear strength of the wood cellulose supramolecular adhesive meets the GB / T 9846-2015 standard (>0.7MPa), and has excellent high bonding performance and resistance to 63°C hot water.

[0031] In terms of the universality of the method, lignocellulose supramolecular adhesives can be prepared using lignocellulose from different sources such as beech, pine, and sugarcane bagasse.

[0032] Specific embodiment 2: This embodiment differs from the specific embodiment 1 in that 2 parts of lignocellulose, 0.6 to 4 parts of sodium periodate and 10 to 50 parts of water are weighed by mass. The rest is the same as the specific embodiment 1.

[0033] Specific implementation method 3: This implementation method is different from specific implementation method 1 or 2 in that the wood cellulose is pine powder, beech powder, birch powder, poplar powder, bamboo powder or bagasse powder. The rest is the same as specific implementation method 1 or 2.

[0034] Specific embodiment 4: This embodiment is different from specific embodiments 1 to 3 in that the particle size of the wood cellulose is 1.0 μm to 140.0 μm, and the moisture content is 2% to 10%. Other aspects are the same as specific embodiments 1 to 3.

[0035] Specific embodiment 5: This embodiment differs from specific embodiments 1 to 4 in that the light-proof heating reaction is specifically carried out under the conditions of light-proof and temperature of 30°C to 100°C for 2h to 10h. The rest is the same as specific embodiments 1 to 4.

[0036] Specific implementation method 6: This implementation method is different from specific implementation methods 1 to 5 in that the precipitation is allowed to stand for 1 hour to 3 hours. The rest is the same as specific implementation methods 1 to 5.

[0037] Specific embodiment 7: This embodiment differs from specific embodiments 1 to 6 in that the solid content of the lignocellulose supramolecular adhesive is 10% to 30% and the size of the solid particles in the lignocellulose supramolecular adhesive is 4.0 μm to 90.0 μm. Other aspects are the same as specific embodiments 1 to 6.

[0038] Specific embodiment eight: This embodiment is a method for using a lignocellulose supramolecular adhesive, which is carried out according to the following steps:

[0039] 1. Gluing:

[0040] coating the lignocellulose supramolecular adhesive on the surface of the veneer to obtain a veneer coated with the lignocellulose supramolecular adhesive;

[0041] 2. Assembly:

[0042] Assembling the veneers coated with the wood cellulose supramolecular adhesive in a direction perpendicular to the textures of adjacent veneers to obtain an assembly;

[0043] 3. Hot pressing:

[0044] The assembly is hot-pressed to obtain plywood, thus completing the method for using the wood cellulose supramolecular adhesive.

[0045] Specific embodiment 9: This embodiment is different from specific embodiment 8 in that: the veneer described in step 1 is a peeled wood veneer or bamboo veneer; the peeled wood veneer is made of poplar, birch, ash, oak, fir, pine or beech; the coating amount on one side in step 1 is 100g / m 2 ~250g / m 2 , coating the wood cellulose supramolecular adhesive on the surface of the veneer. The rest is the same as the eighth embodiment.

[0046] Specific embodiment 10: This embodiment differs from specific embodiment 8 or 9 in that the hot pressing in step 3 is performed at a temperature of 140°C to 220°C and a pressure of 0.7MPa to 2.5MPa for 5min to 20min. The rest is the same as specific embodiment 8 or 9.

[0047] The following examples are used to verify the beneficial effects of the present invention:

[0048] Embodiment 1:

[0049] A method for preparing a lignocellulose supramolecular adhesive is carried out according to the following steps:

[0050] Weigh 2 parts of lignocellulose, 0.6 parts of sodium periodate and 30 parts of water by weight, mix the weighed lignocellulose, sodium periodate and water, react for 3 hours under the condition of avoiding light and temperature of 70°C, then let it stand for precipitation for 3 hours and remove the supernatant to obtain a lignocellulose supramolecular adhesive;

[0051] The wood cellulose is pine wood powder;

[0052] The particle size of the wood cellulose is 2.4 μm to 117.6 μm, and the moisture content is 3% to 7%;

[0053] The solid content of the wood cellulose supramolecular adhesive is 22%; the size of the solid particles in the wood cellulose supramolecular adhesive is 4.2 μm to 85.5 μm.

[0054] Embodiment 2:

[0055] A method for preparing a lignocellulose supramolecular adhesive is carried out according to the following steps:

[0056] 2 parts of lignocellulose, 2 parts of sodium periodate and 12 parts of water were weighed by mass, the weighed lignocellulose, sodium periodate and water were mixed, reacted for 6 hours under the condition of light protection and temperature of 90°C, then allowed to stand and settle for 2 hours and the supernatant was removed to obtain a lignocellulose supramolecular adhesive.

[0057] The wood cellulose is beech wood powder.

[0058] The particle size of the wood cellulose is 3.3 μm to 114.5 μm, and the moisture content is 4% to 8%.

[0059] The solid content of the wood cellulose supramolecular adhesive is 20%; the size of the solid particles in the wood cellulose supramolecular adhesive is 5.3.μm-74.4μm.

[0060] Embodiment 3:

[0061] A method for preparing a lignocellulose supramolecular adhesive is carried out according to the following steps:

[0062] 2 parts of lignocellulose, 3 parts of sodium periodate and 40 parts of water were weighed by mass, the weighed lignocellulose, sodium periodate and water were mixed, reacted for 8 hours in a dark environment at a temperature of 60°C, and then allowed to stand for precipitation for 1 hour and the supernatant was removed to obtain a lignocellulose supramolecular adhesive.

[0063] The wood cellulose is birch powder.

[0064] The particle size of the wood cellulose is 5.6 μm to 128.7 μm, and the moisture content is 3% to 6%.

[0065] The solid content of the wood cellulose supramolecular adhesive is 17%; the size of the solid particles in the wood cellulose supramolecular adhesive is 4.5 μm to 81.2 μm.

[0066] Embodiment 4:

[0067] A method for preparing a lignocellulose supramolecular adhesive is carried out according to the following steps:

[0068] 2 parts of lignocellulose, 4 parts of sodium periodate and 20 parts of water were weighed by mass, the weighed lignocellulose, sodium periodate and water were mixed, reacted for 2 hours under the condition of avoiding light and temperature of 100°C, then allowed to stand and settle for 3 hours and the supernatant was removed to obtain a lignocellulose supramolecular adhesive.

[0069] The wood cellulose is poplar wood powder.

[0070] The particle size of the wood cellulose is 4.6 μm to 95.4 μm, and the moisture content is 2% to 6%.

[0071] The solid content of the wood cellulose supramolecular adhesive is 15%; the size of the solid particles in the wood cellulose supramolecular adhesive is 6.1 μm to 64.9 μm.

[0072] Embodiment 5:

[0073] A method for preparing a lignocellulose supramolecular adhesive is carried out according to the following steps:

[0074] 2 parts of lignocellulose, 1 part of sodium periodate and 50 parts of water were weighed by mass, the weighed lignocellulose, sodium periodate and water were mixed, reacted for 10 hours in a dark place at a temperature of 40°C, and then allowed to stand for 1 hour to settle and the supernatant was removed to obtain a lignocellulose supramolecular adhesive.

[0075] The wood cellulose is bamboo powder.

[0076] The particle size of the wood cellulose is 6.3 μm to 117.3 μm, and the moisture content is 2% to 5%.

[0077] The solid content of the wood cellulose supramolecular adhesive is 25%; the size of the solid particles in the wood cellulose supramolecular adhesive is 8.5 μm to 88.4 μm.

[0078] Embodiment 6:

[0079] A method for preparing a lignocellulose supramolecular adhesive is carried out according to the following steps:

[0080] 2 parts of lignocellulose, 1 part of sodium periodate and 25 parts of water were weighed by mass, the weighed lignocellulose, sodium periodate and water were mixed, reacted for 3 hours in a dark place at a temperature of 80°C, and then allowed to stand for precipitation for 2 hours and the supernatant was removed to obtain a lignocellulose supramolecular adhesive.

[0081] The wood cellulose is bagasse powder.

[0082] The particle size of the wood cellulose is 2.1 μm to 100.7 μm, and the moisture content is 4% to 7%.

[0083] The solid content of the wood cellulose supramolecular adhesive is 19%; the size of the solid particles in the wood cellulose supramolecular adhesive is 4.4 μm to 68.3 μm.

[0084] Comparative Example 1:

[0085] A method for preparing a lignocellulose adhesive is carried out according to the following steps:

[0086] 2 parts of lignocellulose and 40 parts of water were weighed by mass, the weighed lignocellulose and water were mixed, heated for 8 hours at 60° C. in a dark environment, and then allowed to settle for 1 hour and the supernatant was removed to obtain a lignocellulose adhesive.

[0087] The wood cellulose is beech wood powder.

[0088] The particle size of the wood cellulose is 3.4 μm to 107.9 μm, and the moisture content is 3% to 8%.

[0089] The solid content of the wood cellulose supramolecular adhesive is 20%; the size of the solid particles in the wood cellulose supramolecular adhesive is 3.4 μm to 107.9 μm.

[0090] Embodiment 1:

[0091] The method for using the adhesive prepared in Examples 1 to 6 and Comparative Example 1 is carried out according to the following steps:

[0092] 1. Gluing:

[0093] The coating amount on one side is 200g / m 2 , coating the lignocellulose supramolecular adhesive on the surface of the veneer to obtain a veneer coated with the lignocellulose supramolecular adhesive;

[0094] 2. Assembly:

[0095] Assembling the veneers coated with the wood cellulose supramolecular adhesive in a direction perpendicular to the textures of adjacent veneers to obtain an assembly;

[0096] 3. Hot pressing:

[0097] Under the conditions of hot pressing temperature of 170°C and hot pressing pressure of 1.5MPa, the assembly is hot pressed for 8 minutes to obtain a plywood, thus completing the method for using the wood cellulose supramolecular adhesive;

[0098] The veneer described in step 1 is a peeled poplar veneer.

[0099] Embodiment 2: This embodiment is different from Embodiment 1 in that the veneer described in step 1 is a peeled birch veneer. The rest is the same as Embodiment 1.

[0100] Embodiment 3: This embodiment is different from Embodiment 1 in that the veneer described in step 1 is a peeled beech veneer. The rest is the same as Embodiment 1.

[0101] Embodiment 4: This embodiment is different from Embodiment 1 in that the veneer described in step 1 is a peeled pine veneer. The rest is the same as Embodiment 1.

[0102] Glue shear strength test: The specimens were prepared according to the national standard GB / T 9846-2015. The wood grain direction of the core board between the glue layers to be tested should be perpendicular to the length direction of the specimens. The groove width and depth of the specimens were carried out in accordance with the national standard requirements. Dry shear strength: According to the GB / T 9846-2015 standard, the prepared standard specimens were stored in a desiccator for 2 days. The test was performed using a universal mechanical testing machine (AGS-X, Shimadzu, Japan), with a tensile speed of 5mm / min and a mechanical sensor of 10kN. Wet shear strength: According to the GB / T 9846-2015 standard, the prepared standard specimens were immersed in 63℃ hot water for 3h, and then the wet shear strength was tested.

[0103] Table 1. Dry and wet shear strength of lignocellulose supramolecular adhesive

[0104]

[0105]

[0106]

[0107] As can be seen from the table, the dry shear strength and wet shear strength of the wood cellulose supramolecular adhesive plywood prepared in Examples 1 to 6 meet the standard of GB / T 9846-2015 (>0.7 MPa), and have good bonding performance and water resistance, while the comparative example cannot meet the requirements.

[0108] Figure 1 The optical photos and SEM electron microscope photos are shown in Figures a and c, respectively, after the lignocellulose adhesive prepared in Comparative Example 1 is dried, and b and d, respectively, after the lignocellulose supramolecular adhesive prepared in Example 2 is dried. In Comparative Example 1, the lignocellulose surface is smooth, the particle size is large, and the specific surface area is small. The lignocellulose supramolecular adhesive prepared in Example 2 has a rough surface, a small particle size, and a large specific surface area. This indicates that the physical activity of the lignocellulose supramolecular adhesive is stronger.

[0109] Figure 2 The FTIR spectra of the lignocellulose adhesive prepared in comparative example 1 after drying, the lignocellulose supramolecular adhesive prepared in example 2 after drying, and the lignocellulose supramolecular adhesive in example 2 after hot pressing using the method of example 1 were analyzed using Fourier transform infrared spectroscopy (FTIR) to analyze the changes in chemical groups of the lignocellulose supramolecular adhesive before and after activation and before and after hot pressing. Before activation, the lignocellulose had a wavelength of 1730 cm -1 and 1110cm -1 There are absorption peaks, which belong to C=O bond and COC bond. After activation, the wood cellulose supramolecular adhesive has an absorption peak at 1730cm -1The absorption peak at 1630 cm -1 A new shoulder peak was generated at the oxidized lignin quinone group. After hot pressing, the intensity of the C=O bond absorption peak of the lignocellulose supramolecular adhesive decreased, while the intensity of the COC bond absorption peak increased, indicating that the aldehyde group participated in the thermochemical cross-linking and formed an ether bond.

[0110] Figure 3 : XPS C1s spectrum and its content diagram, a is the lignocellulose adhesive prepared in comparative example 1 after drying, b is the lignocellulose supramolecular adhesive prepared in example 2 after drying, c is the lignocellulose supramolecular adhesive in example 2 after hot pressing using the method of example 1, and d is the content diagram. X-ray photoelectron spectroscopy (XPS) was further used to analyze the changes in chemical groups of the lignocellulose supramolecular adhesive before and after activation and before and after hot pressing. Before activation, the XPS C1s spectrum of lignocellulose showed peaks at 284.8eV, 286.5eV, 287.9eV and 288.9eV, corresponding to CC / CH, CO, C=O and O=CO groups. After activation, the CO bond content of the lignocellulose supramolecular adhesive decreased, while the C=O bond content increased, which also indicated that the hydroxyl group was converted into an aldehyde group. After hot pressing, the CO bond content of the lignocellulose supramolecular adhesive increased, while the C=O bond content decreased, indicating that after hot pressing, the LCSA formed a CO bond through self-crosslinking.

[0111] Figure 4 The size distribution of the lignocellulose adhesive prepared in comparative example 1 after drying and the lignocellulose supramolecular adhesive prepared in example 2 after drying. As can be seen from the figure, the d(0.1), d(0.5) and d(0.9) of the lignocellulose in comparative example 1 are 3.4μm, 32.5μm and 107.9μm, indicating that 80% of the size distribution is between 3.4μm and 107.9μm; while 80% of the size distribution of the lignocellulose supramolecular adhesive is between 4.5μm and 74.4μm, which is significantly smaller than that of lignocellulose and has an increased specific surface area. Among them, d(0.1) is the diameter of the particle with a diameter in the 10th percentile; d(0.5) is the diameter of the particle with a diameter in the 50th percentile, that is, the median of the diameter distribution; d(0.9) is the diameter of the particle with a diameter in the 90th percentile.

Claims

1. A method for preparing a lignocellulose supramolecular adhesive, characterized in that It is carried out in the following steps: The lignocellulose, sodium periodate and water are mixed, and then heated in the dark to react. After the reaction, the mixture is allowed to stand and precipitate, and the supernatant is removed to obtain the lignocellulose supramolecular adhesive.

2. The method for preparing a lignocellulose supramolecular adhesive according to claim 1, characterized in that Weigh 2 parts of wood cellulose, 0.6 to 4 parts of sodium periodate and 10 to 50 parts of water by mass.

3. The method for preparing a lignocellulose supramolecular adhesive according to claim 1, characterized in that The wood cellulose is pine powder, beech powder, birch powder, poplar powder, bamboo powder or bagasse powder.

4. The method for preparing a lignocellulose supramolecular adhesive according to claim 3, characterized in that The particle size of the wood cellulose is 1.0 μm to 140.0 μm, and the moisture content is 2% to 10%.

5. The method for preparing a lignocellulose supramolecular adhesive according to claim 1, characterized in that The light-proof heating reaction is specifically carried out under the conditions of light-proof and temperature of 30° C. to 100° C. for 2 h to 10 h.

6. The method for preparing a lignocellulose supramolecular adhesive according to claim 1, characterized in that Let it settle for 1 to 3 hours.

7. The method for preparing a lignocellulose supramolecular adhesive according to claim 1, characterized in that The solid content of the wood cellulose supramolecular adhesive is 10% to 30%; the size of the solid particles in the wood cellulose supramolecular adhesive is 4.0 μm to 90.0 μm.

8. The method for using the lignocellulose supramolecular adhesive according to claim 1, characterized in that It is carried out in the following steps:

1. Gluing: coating the lignocellulose supramolecular adhesive on the surface of the veneer to obtain a veneer coated with the lignocellulose supramolecular adhesive; 2. Assembly: Assembling the veneers coated with the wood cellulose supramolecular adhesive in a direction perpendicular to the textures of adjacent veneers to obtain an assembly; 3. Hot pressing: The assembly is hot-pressed to obtain plywood, thus completing the method for using the wood cellulose supramolecular adhesive.

9. The method for using the lignocellulose supramolecular adhesive according to claim 8, characterized in that The veneer described in step 1 is a peeled wood veneer or bamboo veneer; the peeled wood veneer is made of poplar, birch, ash, oak, fir, pine or beech; the coating amount on one side in step 1 is 100g / m 2 ~250g / m 2 , the wood cellulose supramolecular adhesive is coated on the surface of the veneer.

10. The method for using the lignocellulose supramolecular adhesive according to claim 8, characterized in that The hot pressing described in step 3 is specifically performed at a hot pressing temperature of 140° C. to 220° C. and a hot pressing pressure of 0.7 MPa to 2.5 MPa for 5 min to 20 min.

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