High-temperature-resistant hydrophobic sealing film for plain foil box and preparation method of high-temperature-resistant hydrophobic sealing film

Abstract

The invention provides a high-temperature-resistant hydrophobic sealing film for a plain foil box and a preparation method of the high-temperature-resistant hydrophobic sealing film, and relates to the field of packaging materials, the high-temperature-resistant hydrophobic sealing film comprises a PET layer, an aluminum foil layer and a heat sealing layer which are sequentially connected through an adhesive; the heat sealing layer comprises metallocene polyethylene, linear low-density polyethylene, an ethylene-vinyl acetate copolymer, maleic anhydride grafted polyethylene, a polyolefin elastomer, maleic anhydride grafted polyethylene wax, a composite antioxidant, an antibacterial agent and a water repellent agent; the hydrophobic agent is nano silicon dioxide with a core-shell structure. The sealing film has excellent high temperature resistance, heat sealability, hydrophobicity and antibacterial property.

Description

Technical Field

[0001] This invention relates to the field of packaging materials, and more specifically, to a high-temperature resistant hydrophobic sealing film for plain foil boxes and its preparation method. Background Technology

[0002] Aluminum foil boxes are packaging containers made primarily of aluminum foil or aluminum-based composite materials through processes such as stamping and composite processing. They are widely used in the food, pharmaceutical, cosmetic, and electronic component industries. Plain foil boxes are a type of aluminum foil box that are made directly from aluminum foil without any coating on the surface.

[0003] In the food packaging industry, aluminum foil boxes are commonly used as disposable tableware, containers, and food packaging. To ensure the hygiene, freshness, and flavor of food during transportation, and to provide some insulation, a sealing film is usually applied to the aluminum foil box for sealing. However, currently available sealing films still have significant shortcomings in handling complex scenarios involving high temperatures, high oil content, and high moisture. For example, when sealing freshly cooked, hot food (such as hot soup, rice, or dishes containing cheese), the heat-sealing layer of ordinary plastic sealing film is prone to softening and deformation, leading to seal failure or overall delamination. Simultaneously, starch, protein, and oil in food easily adhere to the inner wall of the film, causing food residue when it is opened, severely impacting the user experience. Summary of the Invention

[0004] The purpose of this invention is to provide a high-temperature resistant hydrophobic sealing film for plain foil boxes, which has excellent high-temperature resistance, heat-sealing performance, hydrophobicity and antibacterial properties.

[0005] Another objective of this invention is to provide a method for preparing a high-temperature resistant hydrophobic sealing film for aluminum foil boxes. The method involves plasma activation and surface grafting modification of the aluminum foil, which improves the bonding strength between the aluminum foil and the heat-sealing layer and the PET layer, ensuring that the sealing film does not delaminate during high-temperature cooking or long-term storage.

[0006] The invention solves its technical problem by employing the following technical solutions.

[0007] On one hand, embodiments of the present invention provide a high-temperature resistant hydrophobic sealing film for plain foil boxes, comprising a PET layer, an aluminum foil layer, and a heat-sealing layer connected in sequence by an adhesive; The heat-sealing layer comprises the following raw materials by weight: Metallocene polyethylene 30-35 parts, linear low-density polyethylene 25-30 parts, ethylene-vinyl acetate copolymer 8-12 parts, maleic anhydride grafted polyethylene 8-12 parts, polyolefin elastomer 5-8 parts, maleic anhydride grafted polyethylene wax 1.5-2.5 parts, composite antioxidant 1-1.5 parts, antibacterial agent 2-4 parts, hydrophobic agent 2-4 parts. The hydrophobic agent is a core-shell structured nano-silica.

[0008] In some embodiments of the present invention, the method for preparing the hydrophobic agent includes the following steps: Nano-silica was added to an ethanol aqueous solution, stirred until homogeneous, and the pH was adjusted to 9-10. Then, an ethanol solution of silane coupling agent was added dropwise, and the mixture was refluxed at 60-70℃ for 6-8 hours. After centrifugation, washing, and drying, hydrophobic silica was obtained. Hydrophobic silica was dispersed in ethyl acetate, and then hydroxyl-terminated polyethylene wax, diisocyanate and dibutyltin dilaurate were added. The mixture was refluxed at 80-85°C for 8-12 hours under a nitrogen atmosphere. After centrifugation, washing and drying, the hydrophobic agent was obtained.

[0009] In some embodiments of the present invention, the mass ratio of nano-silica, hydroxyl-terminated polyethylene wax and diisocyanate is 100:(8-10):(4-5).

[0010] In some embodiments of the present invention, the composite antioxidant includes antioxidant 1010 and antioxidant 168, with a mass ratio of (1-3):(1-3).

[0011] In some embodiments of the present invention, the method for preparing the antibacterial agent includes the following steps: ε-polylysine hydrochloride and triethylamine were dispersed in ethyl lactate and stirred until homogeneous to obtain an ε-polylysine suspension. Under nitrogen protection, maleic anhydride-grafted polyethylene and ethyl lactate are mixed, heated to 90-100°C, and stirred until the maleic anhydride-grafted polyethylene dissolves; the system is then cooled to 85-95°C and reacted for 3-4 hours. Under stirring conditions, ε-polylysine suspension was added dropwise to the system. After the addition was complete, the temperature was raised to 85-90℃ and the reaction was carried out for 4-6 hours. After the reaction is complete, the reaction solution is cooled to room temperature and poured into ice-cold methanol. The mixture is stirred vigorously to precipitate a solid. The solid is collected by filtration, washed 3-5 times, and dried to obtain the antibacterial agent.

[0012] In some embodiments of the present invention, the mass ratio of the maleic anhydride-grafted polyethylene to the ε-polylysine hydrochloride is 100:15-25.

[0013] On the other hand, embodiments of the present invention provide a method for preparing a high-temperature resistant hydrophobic sealing film for foil boxes, comprising the following steps: S1, hydrophobic agent, antibacterial agent, maleic anhydride grafted polyethylene wax, composite antioxidant and part of metallocene polyethylene are fully mixed in a high-speed mixer, and then melted, mixed, extruded and granulated in a twin-screw extruder at 170-180℃ to obtain masterbatch; S2, the masterbatch is thoroughly mixed with the remaining raw materials and fed into a blown film machine, where it is blown into a film at 170-185℃ to obtain a heat-sealing film; S3 uses a mixture of argon and oxygen as the working gas to perform plasma activation treatment on the cleaned aluminum foil; then, a hydroxy acrylic acid dispersion is coated on the surface and heated to cure, resulting in modified aluminum foil. S4, an adhesive is coated on both surfaces of the modified aluminum foil, and a heat-sealing layer film and a PET layer film are respectively covered on the two surfaces. The film is then hot-pressed to obtain a raw sealing film. S5. Place the raw sealing film in a curing chamber at 50-60°C and cure for 36-48 hours to obtain the sealing film.

[0014] In some embodiments of the present invention, during plasma treatment in step S3, the volume percentage of oxygen in the mixed gas is 1-5%; the aluminum foil moves at a speed of 20-50 m / min through the plasma spray gun array; and the power of the plasma spray gun is 50-100 W.

[0015] In some embodiments of the present invention, in step S3, the solid content of the hydroxyacrylic acid dispersion is 20-30%, and the heating and curing temperature is 90-110°C.

[0016] In some embodiments of the present invention, in step S4, the pressure of hot-pressing composite is 0.6-0.8 MPa and the temperature is 70-80°C.

[0017] Compared with the prior art, the embodiments of the present invention have at least the following advantages or beneficial effects: The sealing film provided by this invention has a heat-sealing layer primarily composed of metallocene polyethylene, offering excellent heat-sealing performance, such as low heat-sealing initiation temperature, high heat adhesion strength, high strength and rigidity, while ensuring film transparency. Linear low-density polyethylene, in synergy with metallocene polyethylene, improves processing flowability, reduces overall cost, and simultaneously enhances the film's toughness, puncture resistance, and tear resistance. A core-shell structured hydrophobic agent, with a core of nano-silica providing roughness and a shell of grafted low-surface-energy polyethylene wax, together constructs a microscopic hydrophobic structure, making it difficult for droplets to wet the surface. The heat-sealing layer surface has a large contact angle and good rollability, allowing condensation, liquids, etc., to form beads and roll off, preventing food from sticking to the film, maintaining packaging aesthetics, and reducing the risk of leakage. The chemical grafting structure enhances the bonding strength between the hydrophobic agent and the matrix.

[0018] The sealing film preparation method provided by this invention involves first melt-blending and granulating a small amount of base resin (metallocene polyethylene) with the remaining functional additives. The high shear force of a twin-screw extruder ensures thorough dispersion of the hydrophobic and antibacterial agents, forming uniform masterbatches. During subsequent blown film production, the raw materials are evenly distributed throughout the sealing film, guaranteeing uniform hydrophobicity and antibacterial properties across the entire surface of the sealing film.

[0019] Plasma activation treatment is applied to the aluminum foil layer. Plasma cleans and etches the aluminum foil surface at the nanoscale, while simultaneously introducing active groups such as carbonyl and hydroxyl groups. This transforms the aluminum foil surface from inert to active, forming chemical bonds and mechanical interlocks with the subsequently coated hydroxyl acrylic dispersion. This robust interfacial bonding ensures that the composite film will not delaminate or bubble during high-temperature cooking or long-term storage, improving product reliability and lifespan.

[0020] Example 1 Prepare the hydrophobic agent according to the following steps: Nano-silica was added to an ethanol aqueous solution, stirred until homogeneous, and the pH was adjusted to 9-10. Then, an ethanol solution containing silane coupling agent (KH-550) was added dropwise, and the mixture was refluxed at 70°C for 6 hours. After centrifugation, washing, and drying, hydrophobic silica was obtained. Hydrophobic silica was dispersed in ethyl acetate, and then hydroxyl-terminated polyethylene wax, diisocyanate and dibutyltin dilaurate were added. The mixture was refluxed at 85°C for 8 hours under a nitrogen atmosphere. After centrifugation, washing and drying, the hydrophobic agent was obtained.

[0021] Among them, hydroxyl-terminated polyethylene wax, also known as oxidized polyethylene wax, is manufactured by Wuhan Xindongyi Chemical Co., Ltd.

[0022] Based on the above steps, adjust the amount of raw materials to prepare different hydrophobic agents: Hydrophobic agent A: The mass ratio of nano silica, hydroxyl-terminated polyethylene wax, diisocyanate and silane coupling agent is 100:8:4:20; Hydrophobic agent B: The mass ratio of nano silica, hydroxyl-terminated polyethylene wax, diisocyanate and silane coupling agent is 100:9:5:10; Hydrophobic agent C: The mass ratio of nano silica, hydroxyl-terminated polyethylene wax, diisocyanate and silane coupling agent is 100:10:5:15.

[0023] Example 2 Prepare the antibacterial agent according to the following steps: ε-polylysine hydrochloride and triethylamine were dispersed in ethyl lactate and stirred until homogeneous to obtain an ε-polylysine suspension. Under nitrogen protection, maleic anhydride-grafted polyethylene and ethyl lactate were mixed, heated to 100°C, and stirred until the maleic anhydride-grafted polyethylene dissolved; the system was then cooled to 95°C and reacted for 4 hours. Under stirring conditions, ε-polylysine suspension was added dropwise to the system. After the addition was complete, the temperature was raised to 90°C and the reaction was carried out for 4 hours. After the reaction was completed, the reaction solution was cooled to room temperature and poured into ice-cold methanol. The mixture was stirred vigorously, and a solid was precipitated. The solid was collected by filtration, washed five times, and dried to obtain the antibacterial agent.

[0024] Based on the above steps, adjust the amount of each raw material to prepare different antibacterial agents: Antibacterial agent A: The mass ratio of maleic anhydride-grafted polyethylene and ε-polylysine hydrochloride is 100:15; Antibacterial agent B: The mass ratio of maleic anhydride-grafted polyethylene and ε-polylysine hydrochloride is 100:20; Antibacterial agent C: The mass ratio of maleic anhydride-grafted polyethylene and ε-polylysine hydrochloride is 100:25.

[0025] Example 3 Prepare the heat-sealing layer material according to the following proportions: Metallocene polyethylene 35 parts, linear low-density polyethylene 30 parts, ethylene-vinyl acetate copolymer 12 parts, maleic anhydride grafted polyethylene 12 parts, polyolefin elastomer 8 parts, maleic anhydride grafted polyethylene wax 2.5 parts, composite antioxidant 1.5 parts, antibacterial agent A 4 parts, hydrophobic agent A 4 parts.

[0026] The compound antioxidant is a mixture of antioxidant 1010 and antioxidant 168 in a mass ratio of 1:1.

[0027] Based on the above formula, prepare the sealing film according to the following steps: S1, hydrophobic agent, antibacterial agent, maleic anhydride grafted polyethylene wax, composite antioxidant and 20% metallocene polyethylene are fully mixed in a high-speed mixer, and then melted, mixed, extruded and granulated in a twin-screw extruder at 170-180℃ to obtain masterbatch; S2, mix the masterbatch with the remaining raw materials thoroughly, feed them into a blown film machine, and blown film at 170-185℃ to obtain a heat-sealing film (thickness of 60um). S3 uses a mixture of argon and oxygen as the working gas to perform plasma activation treatment on cleaned aluminum foil (7µm thick). Subsequently, a hydroxyl acrylic acid dispersion is coated onto the surface and cured by heating to obtain modified aluminum foil. The plasma equipment used is a plasma spray gun array. The aluminum foil, under the action of a transport device, passes through the emission port of the plasma spray gun array to achieve plasma treatment. The volume percentage of oxygen in the mixed gas is 3%; the aluminum foil moves through the plasma spray gun array at a speed of 30 m / min; and the power of the plasma spray gun is 80 W.

[0028] The solid content of the hydroxyacrylic acid dispersion is 30%, which is obtained by diluting a purchased hydroxyacrylic acid dispersion with a solid content of 40-50% with water; the curing temperature is 100℃.

[0029] S4. Coat the two surfaces of the modified aluminum foil with an adhesive (two-component polyurethane adhesive), and cover the two surfaces with a heat-sealing layer film and a PET layer film (12um thick), respectively. Hot-press the film (pressure 0.8MPa, temperature 70℃) to obtain a green sealing film. S5, the raw sealing film is placed in a curing chamber at 50°C and cured for 48 hours to obtain the sealing film.

[0030] Example 4 The difference from Example 3 is that the heat-sealing layer material ratio is as follows: 30 parts metallocene polyethylene, 25 parts linear low-density polyethylene, 8 parts ethylene-vinyl acetate copolymer, 8 parts maleic anhydride-grafted polyethylene, 5 parts polyolefin elastomer, 1.5 parts maleic anhydride-grafted polyethylene wax, 1 part composite antioxidant, 2 parts antibacterial agent A, and 2 parts hydrophobic agent A.

[0031] The remaining raw materials and preparation methods are the same as those in Example 3.

[0032] Example 5 The difference from Example 3 is that the heat-sealing layer material ratio is as follows: Metallocene polyethylene 32 parts, linear low-density polyethylene 28 parts, ethylene-vinyl acetate copolymer 10 parts, maleic anhydride grafted polyethylene 10 parts, polyolefin elastomer 7 parts, maleic anhydride grafted polyethylene wax 2 parts, composite antioxidant 1.2 parts, antibacterial agent A 3 parts, hydrophobic agent A 3 parts.

[0033] The remaining raw materials and preparation methods are the same as those in Example 3.

[0034] Example 6 The difference from Example 3 is that antibacterial agent B and hydrophobic agent B are used in the heat-sealing layer material. The remaining raw materials and preparation methods are the same as in Example 3.

[0035] Example 7 The difference from Example 3 is that antibacterial agent C and hydrophobic agent C are used in the heat-sealing layer material. The remaining raw materials and preparation methods are the same as in Example 3.

[0036] Example 8 The difference from Example 3 is that antibacterial agent A and hydrophobic agent B are used in the heat-sealing layer material. The remaining raw materials and preparation methods are the same as in Example 3.

[0037] Example 9 The difference from Example 3 is that antibacterial agent A and hydrophobic agent C are used in the heat-sealing layer material. The remaining raw materials and preparation methods are the same as in Example 3.

[0038] Example 10 The difference from Example 3 is that antibacterial agent C and hydrophobic agent A are used in the heat-sealing layer material. The remaining raw materials and preparation methods are the same as in Example 3.

[0039] Comparative Example 1 The difference from Example 3 is that no hydrophobic agent is added, but the other raw material ratios and preparation methods are the same as in Example 3.

[0040] Comparative Example 2 The difference from Example 1 is that when preparing hydrophobic agent A in Example 1, the hydrophobic silica obtained is used instead of the hydrophobic agent in Example 3, and the remaining raw material ratios and preparation methods are the same as in Example 3.

[0041] Comparative Example 3 The difference from Example 1 is that the nano-silica used in the preparation of hydrophobic agent A in Example 1 is used instead of the hydrophobic agent in Example 3, while the other raw material ratios and preparation methods are the same as in Example 3.

[0042] Comparative Example 4 The difference from Example 1 is that ε-polylysine is used directly instead of the antibacterial agent in Example 3, while the remaining raw material ratios and preparation methods are the same as in Example 3.

[0043] Experimental Example The high-temperature resistant hydrophobic sealing films for plain foil boxes of Examples 3-10 and Comparative Examples 1-4 were used as test objects, and tests were conducted according to the methods in Table 1.

[0044] Table 1

[0045] The test results are shown in Table 2-4.

[0046] Table 2

[0047] Table 3

[0048] Table 4

[0049] Wherein, the tensile strength retention rate (%) = (σ1 / σ0) × 100%, σ0 (initial tensile strength): the average tensile strength (unit: MPa) of the sample before heat treatment at 121℃. σ1 (tensile strength after treatment): the average tensile strength (unit: MPa) of the same sample after treatment at 121℃ for 30 minutes and cooling to room temperature.

[0050] Table 2 shows that Examples 3, 5, 6, and 7 exhibit high strength and high toughness. The mechanical properties of Comparative Examples 2 and 3 are significantly reduced (especially elongation at break), which is due to the agglomeration of incompletely modified nano-silica in the matrix of Comparative Examples 2 and 3, becoming stress concentration points and leading to material embrittlement. Regarding heat-sealing performance, all examples exhibit excellent heat-sealing strength, while Comparative Examples 2 and 3 show a slight decrease in heat-sealing strength due to poor compatibility.

[0051] Table 3 shows that Examples 3, 5, 6, and 7 all exhibit excellent hydrophobic properties. Comparative Example 1, without the addition of a hydrophobic agent, has a contact angle that is only at the level of the polymer itself. Comparative Example 2, modified only with KH-550, has insufficient hydrophobicity. Comparative Example 3 shows that the original SiO2 is hydrophilic, and the unmodified SiO2 reduced the contact angle of the sealing film. Comparative Example 4 shows a significant decrease in antibacterial rate; the physically blended ε-polylysine had already migrated and been lost in large quantities before testing, failing to provide a durable antibacterial effect.

[0052] The haze of Comparative Examples 2 and 3 increased significantly. The size of the aggregated nanoparticles was larger than the wavelength of visible light, which caused strong scattering of light, resulting in decreased film transparency and whitening.

[0053] Table 4 shows that the sealing films in the examples all exhibit excellent high-temperature cooking resistance. Example 3: The migration amount is far below the food safety limit (10 mg / dm² in the EU), and the antibacterial rate remains close to 100% after simulated immersion. Comparative Example 4: The migration amount seriously exceeds the standard, failing to meet the safety standards for food contact materials. Furthermore, due to the large-scale dissolution of its antibacterial components, the long-lasting antibacterial performance rapidly diminishes, with an antibacterial rate of only 80.2% after immersion, which cannot meet practical application requirements.

[0054] The embodiments described above are some, but not all, embodiments of the present invention. The detailed description of the embodiments of the present invention is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A high-temperature resistant, hydrophobic sealing film for plain foil boxes, characterized in that, It includes a PET layer, an aluminum foil layer, and a heat-sealing layer connected in sequence by an adhesive; The heat-sealing layer comprises the following raw materials by weight: Metallocene polyethylene 30-35 parts, linear low-density polyethylene 25-30 parts, ethylene-vinyl acetate copolymer 8-12 parts, maleic anhydride grafted polyethylene 8-12 parts, polyolefin elastomer 5-8 parts, maleic anhydride grafted polyethylene wax 1.5-2.5 parts, composite antioxidant 1-1.5 parts, antibacterial agent 2-4 parts, hydrophobic agent 2-4 parts. The hydrophobic agent is a core-shell structured nano-silica.

2. The high-temperature resistant hydrophobic sealing film for foil boxes according to claim 1, characterized in that, The preparation method of the hydrophobic agent includes the following steps: Nano-silica was added to an ethanol aqueous solution, stirred until homogeneous, and the pH was adjusted to 9-10. Then, an ethanol solution of silane coupling agent was added dropwise, and the mixture was refluxed at 60-70℃ for 6-8 hours. After centrifugation, washing, and drying, hydrophobic silica was obtained. Hydrophobic silica was dispersed in ethyl acetate, and then hydroxyl-terminated polyethylene wax, diisocyanate and dibutyltin dilaurate were added. The mixture was refluxed at 80-85°C for 8-12 hours under a nitrogen atmosphere. After centrifugation, washing and drying, the hydrophobic agent was obtained.

3. The high-temperature resistant hydrophobic sealing film for foil boxes according to claim 2, characterized in that, The mass ratio of nano-silica, hydroxyl-terminated polyethylene wax and diisocyanate is 100:(8-10):(4-5).

4. The high-temperature resistant hydrophobic sealing film for foil boxes according to claim 1, characterized in that, The composite antioxidant includes antioxidant 1010 and antioxidant 168, with a mass ratio of (1-3):(1-3).

5. The high-temperature resistant hydrophobic sealing film for foil boxes according to claim 1, characterized in that, The preparation method of the antibacterial agent includes the following steps: ε-polylysine hydrochloride and triethylamine were dispersed in ethyl lactate and stirred until homogeneous to obtain an ε-polylysine suspension. Under nitrogen protection, maleic anhydride-grafted polyethylene and ethyl lactate are mixed, heated to 90-100°C, and stirred until the maleic anhydride-grafted polyethylene dissolves; the system is then cooled to 85-95°C and reacted for 3-4 hours. Under stirring conditions, ε-polylysine suspension was added dropwise to the system. After the addition was complete, the temperature was raised to 85-90℃ and the reaction was carried out for 4-6 hours. After the reaction is complete, the reaction solution is cooled to room temperature and poured into ice-cold methanol. The mixture is stirred vigorously to precipitate a solid. The solid is collected by filtration, washed 3-5 times, and dried to obtain the antibacterial agent.

6. The high-temperature resistant hydrophobic sealing film for foil boxes according to claim 5, characterized in that, The mass ratio of the maleic anhydride-grafted polyethylene to the ε-polylysine hydrochloride is 100:15-25.

7. A method for preparing a high-temperature resistant hydrophobic sealing film for a foil box as described in any one of claims 1-6, characterized in that, Includes the following steps: S1, hydrophobic agent, antibacterial agent, maleic anhydride grafted polyethylene wax, composite antioxidant and part of metallocene polyethylene are fully mixed in a high-speed mixer, and then melted, mixed, extruded and granulated in a twin-screw extruder at 170-180℃ to obtain masterbatch; S2, the masterbatch is thoroughly mixed with the remaining raw materials and fed into a blown film machine, where it is blown into a film at 170-185℃ to obtain a heat-sealing film; S3 uses a mixture of argon and oxygen as the working gas to perform plasma activation treatment on the cleaned aluminum foil; then, a hydroxy acrylic acid dispersion is coated on the surface and heated to cure, resulting in modified aluminum foil. S4, an adhesive is coated on both surfaces of the modified aluminum foil, and a heat-sealing layer film and a PET layer film are respectively covered on the two surfaces. The film is then hot-pressed to obtain a raw sealing film. S5. Place the raw sealing film in a curing chamber at 50-60°C and cure for 36-48 hours to obtain the sealing film.

8. The method for preparing the high-temperature resistant hydrophobic sealing film for plain foil boxes according to claim 7, characterized in that, During plasma treatment in step S3, the volume percentage of oxygen in the mixed gas is 1-5%; the aluminum foil moves at a speed of 20-50 m / min through the plasma spray gun array; and the power of the plasma spray gun is 50-100 W.

9. The method for preparing the high-temperature resistant hydrophobic sealing film for plain foil boxes according to claim 7, characterized in that, In step S3, the solid content of the hydroxyacrylic acid dispersion is 20-30%, and the curing temperature is 90-110℃.

10. The method for preparing the high-temperature resistant hydrophobic sealing film for plain foil boxes according to claim 7, characterized in that, In step S4, the pressure of hot pressing is 0.6-0.8 MPa and the temperature is 70-80℃.

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