A building phase change material, a preparation method and application thereof

Abstract

The application relates to a building phase change material, a preparation method and application thereof. The building phase change material comprises 600-800 parts of a mature phase change material, 20-30 parts of a nucleating agent and 1-5 parts of a temperature adjusting agent according to weight percentage; wherein the mature phase change material is a mixture of anhydrous calcium chloride, sodium carbonate decahydrate and disodium hydrogen phosphate dodecahydrate; the nucleating agent is a mixture of strontium chloride hexahydrate, polyethylene glycol and carboxymethyl cellulose; and the temperature adjusting agent is urea. The application is used for preparing a phase change composite vacuum insulation board by using the building phase change material. The material has large phase change latent heat and high cycle stability, and is used as a building hydrated salt phase change material. The material overcomes the problem of low unit volume / mass enthalpy of traditional building phase change microcapsules, has the advantages of suitable phase change temperature, high cycle stability and the like. The material is used for preparing a vacuum insulation board, and the heat insulation effect is better.

Description

Technical Field

[0001] This invention relates to a phase change material for building, its preparation method and application, belonging to the field of materials technology. Background Technology

[0002] Building energy consumption accounts for over 30% of the nation's total energy consumption and shows a continuous upward trend. Accelerating building energy conservation efforts is crucial for achieving green and low-carbon development goals. Therefore, improving the thermal performance of building envelopes can improve the building's thermal environment, thereby reducing reliance on heating and air conditioning equipment. By integrating phase change materials (PCMs) into building envelopes, their latent heat can be used to store and release thermal energy, compensating for the shortcomings of current building insulation materials, which suffer from low heat capacity and thus cause large temperature fluctuations between indoors and outdoors, ultimately reducing building energy consumption. Compared to expensive and flammable organic PCMs, inorganic hydrated salt PCMs offer advantages such as low cost, wide availability, and non-flammability. However, the excessive supercooling, poor cycle stability, and leakage issues during the phase change process of current inorganic hydrated salt PCMs for buildings significantly limit their application.

[0003] To overcome the aforementioned drawbacks, the latest technological approach involves using porous materials to adsorb phase change materials (PCMs), such as expanded perlite and diatomaceous earth. However, to prevent leakage during the PCM transition, the amount of PCM adsorbed is limited, resulting in low latent heat. Another approach is microencapsulation, which uses micro- and nano-sized shells to encapsulate the PCM. However, this encapsulation technology is complex and costly, and the external encapsulation material lacks functionality. Furthermore, the latent heat of phase change per unit volume is lower than that of hydrated salt PCMs. Therefore, the encapsulation methods, low latent heat, and poor cycling stability of hydrated salt PCMs for building applications remain to be addressed. Summary of the Invention

[0004] (a) Technical problems to be solved

[0005] To address the aforementioned problems in the prior art, this invention provides a phase change material for building applications, its preparation method, and its application.

[0006] (II) Technical Solution

[0007] To achieve the above objectives, the main technical solutions adopted by the present invention include:

[0008] A phase change material for building applications comprises, by weight, 600-800 parts of phase change material, 20-30 parts of nucleating agent, and 1-5 parts of temperature regulator; wherein the phase change material is a mixture of anhydrous calcium chloride, sodium carbonate decahydrate, and disodium hydrogen phosphate dodecahydrate; the nucleating agent is a mixture of strontium chloride hexahydrate, polyethylene glycol, and carboxymethyl cellulose; and the temperature regulator is urea.

[0009] The phase change material for building as described above is a mixture of 600-650 parts anhydrous calcium chloride, 80-100 parts sodium carbonate decahydrate, and 50-80 parts disodium hydrogen phosphate dodecahydrate.

[0010] In the building phase change material described above, the nucleating agent is 20-23 parts of strontium chloride hexahydrate, 1-3 parts of polyethylene glycol, and 1-3 parts of carboxymethyl cellulose, wherein the molecular weight of the polyethylene glycol is 600-800.

[0011] A phase change composite vacuum insulation panel includes a heat insulation layer and a phase change layer. The heat insulation layer is one of aerogel ceramic fiber board, aerogel glass fiber board, fumed silica composite inorganic fiber board, ceramic fiber felt board, or glass fiber felt board. The phase change layer includes a matrix, a phase change material, and an encapsulation material. The phase change material covers the matrix, and the encapsulation material covers the phase change material to form the phase change layer. The matrix is ​​an aerogel ceramic fiber board or aerogel glass fiber board with five hydrophobic outer surfaces and one hydrophilic inner surface. The phase change material is one of hydrated salt phase change materials, composite hydrated salt phase change materials, etc. The encapsulation material is a hydrophobic aerogel ceramic fiber felt or a hydrophobic aerogel glass fiber felt. The phase change layer is placed on top of the heat insulation layer and encapsulated together in a vacuum barrier membrane to form a phase change composite vacuum insulation panel.

[0012] Furthermore, the composite hydrated salt phase change material is the building phase change material as described above.

[0013] A method for preparing a phase change composite vacuum insulation panel includes the following steps:

[0014] S1. Mix the above-mentioned phase change material for building with water; then add nucleating agent and mix; finally add temperature regulator and mix to obtain phase change material, and keep it in a liquid state in an environment of 50-60℃ for later use.

[0015] S2. Pour the liquid phase change material onto the phase change layer substrate and cool it. After the phase change material becomes solid, cover it with encapsulation material to obtain a complete phase change encapsulation board.

[0016] S3. Place the phase change encapsulation plate and the heat insulation layer together in the vacuum barrier film to obtain the phase change composite vacuum insulation plate to be vacuumed.

[0017] S4. Vacuuming is used to obtain the finished phase change composite vacuum insulation panel.

[0018] In the preparation method described above, preferably, in step S1, the stirring rate is 50-200 rpm and the stirring time is 30-60 min.

[0019] In the preparation method described above, preferably, the material of the phase change layer substrate is an aerogel ceramic fiber board or an aerogel glass fiber board; wherein, the five outer surfaces of the phase change layer substrate, excluding the top surface, are coated with a hydrophobic material;

[0020] The encapsulation material is hydrophobic aerogel ceramic fiber felt or hydrophobic aerogel glass fiber felt;

[0021] The insulation layer material is any one or two or more of aerogel ceramic fiberboard, aerogel glass fiberboard, fumed silica composite inorganic fiberboard, ceramic fiber mat or glass fiber mat.

[0022] Furthermore, aerogel ceramic fiberboard or aerogel glass fiberboard can be prepared by the following method: 1. Prepare 130 parts of commercially available silica sol, 10 parts of aluminum silicate, 4 parts of dimethyldimethoxysilane, 10 parts of hydrochloric acid, 4 parts of silane coupling agent KH550, and 150 parts of deionized water by mass.

[0023] 2. Add aluminum silicate to deionized water and stir thoroughly until the aluminum silicate dissolves and the mixture is homogeneous;

[0024] 3. Add silica sol, dimethyldimethoxysilane and silane coupling agent sequentially to aluminum silicate solution, stir evenly to obtain mixed sol;

[0025] 4. Adjust the pH of the mixed sol to 7;

[0026] 5. Completely impregnate the above solution into ceramic or glass fiber mat, and after gelation and aging, obtain fiber-reinforced aerogel;

[0027] 6. Place the above-mentioned fiber-reinforced aerogel in a dry place, and spray the hydrophobic agent methyltrimethoxysilane evenly on the five sides of the outer surface of the board except for the top, to obtain aerogel ceramic fiber board or aerogel glass fiber board.

[0028] In the preparation method described above, preferably, in step S2, the amount of phase change material used is determined according to the size of the phase change layer substrate, i.e., 1.5–1.7 kg / m². 2 The cooling process is carried out at a temperature of 4–8°C for 10–18 hours.

[0029] In the preparation method described above, preferably, the thickness of the encapsulation material is 1-2 mm, and the thickness of the heat insulation layer is 10-20 mm.

[0030] In the preparation method described above, preferably, in step S4, the vacuum degree of the vacuuming is 0.02 to 0.8 Pa.

[0031] (III) Beneficial Effects

[0032] The beneficial effects of this invention are:

[0033] This invention provides a phase change material for building applications. This material exhibits high latent heat of phase change and high cycle stability. As a hydrated salt phase change material for building applications, it overcomes the problem of low enthalpy per unit volume / mass of traditional phase change microcapsules, and possesses advantages such as suitable phase change temperature and high cycle stability. When used to prepare vacuum insulation panels, this material demonstrates excellent thermal insulation performance.

[0034] This invention also provides a method for preparing a phase change composite vacuum insulation panel. Specifically, a phase change material for building applications is encapsulated within a phase change layer matrix and an encapsulation material. Compared to traditional structures, this encapsulation structure offers better thermal insulation performance. It uses a five-sided hydrophobic fiberboard as the matrix for the phase change layer, ensuring that the phase change material will not leak when it is in a molten liquid state and guaranteeing the overall strength of the vacuum insulation panel. By using this structure to encapsulate the phase change layer of the phase change material, there is no need to use materials such as PET for encapsulation, and it perfectly solves the leakage problem of water-soluble salt phase change materials in buildings. Attached Figure Description

[0035] Figure 1 This is a schematic diagram of the phase change layer substrate.

[0036] Figure 2 This is a schematic diagram of phase change material encapsulation.

[0037] Figure 3 A schematic diagram of the phase change composite vacuum insulation panel;

[0038] Figure 4 The cooling-heating curve of PCM1 in Example 1 after 50 cycles of hot and cold cycling;

[0039] Figure 5 The cooling-heating curve of the PCM2 in Example 2 after 50 cycles of hot and cold cycling;

[0040] Figure 6 This is a comparison chart of the phase change layer test results for Example 1 and Comparative Example 2;

[0041] Figure 7 The results are shown in the comparison chart of the finished VIP boards in Example 1, Example 2 and Comparative Example 3.

[0042] [Explanation of Labels in the Attached Image]

[0043] 1: Hydrophilic surface;

[0044] 2: Hydrophobic surface;

[0045] 3: Phase change layer encapsulation material;

[0046] 4: Phase transition layer;

[0047] 5: Insulation layer;

[0048] 6: Packaging bag. Detailed Implementation

[0049] This invention addresses the problems of low enthalpy per unit volume and leakage in existing phase change materials (PCCs) for building applications. It provides a method for preparing a hydrated salt PCC with suitable phase change temperature, high cycle stability, and high enthalpy, along with its encapsulation method and applications. The hydrated salt PCC described in this invention overcomes the problem of low enthalpy per unit volume / mass of traditional PCC microcapsules used in building applications, offering advantages such as suitable phase change temperature and high cycle stability. Furthermore, to address the leakage problem of PCCs, this invention combines it with vacuum insulation panels, perfectly solving the leakage issue associated with hydrated salt PCCs in building applications.

[0050] To better explain and facilitate understanding of this invention, a detailed description of the invention is provided below with reference to the accompanying drawings and specific embodiments. The raw materials used in this invention can be commercially available products; for example, vacuum barrier membranes can be purchased from Sinoma Science & Technology Co., Ltd.

[0051] Example 1

[0052] This embodiment describes the preparation of a building hydrated salt phase change material with high latent heat of phase change and high cycle stability, and its encapsulation method, specifically including the following steps:

[0053] I. Weighing:

[0054] Weigh 625 parts of anhydrous calcium chloride, 90 parts of sodium carbonate decahydrate, 65 parts of disodium hydrogen phosphate dodecahydrate, 590 parts of deionized water, 22 parts of strontium chloride hexahydrate, 2 parts of polyethylene glycol 600, 3 parts of carboxymethyl cellulose, and 2 parts of urea.

[0055] II. Preparation of Phase Change Materials

[0056] 590 parts of deionized water were placed in a magnetic stirrer at a constant temperature of 60°C. Then, 625 parts of anhydrous calcium chloride, 90 parts of sodium carbonate decahydrate, and 65 parts of disodium hydrogen phosphate dodecahydrate were added sequentially. The mixture was stirred at 60 rpm for 60 min. Then, 22 parts of strontium chloride hexahydrate, 2 parts of polyethylene glycol 600, and 3 parts of carboxymethyl cellulose were added sequentially. The mixture was stirred at 80 rpm for 55 min. Finally, 2 parts of urea were added and the mixture was stirred at 60 rpm for 30 min. The resulting liquid phase change material was labeled PCM1 and placed in a 60°C oven to maintain its liquid state for later use.

[0057] III. Packaging

[0058] Remove the prepared liquid phase change material from the oven and pour it onto the hydrophilic surface of the phase change layer substrate (aerogel glass fiber plate, 300×300mm). A schematic diagram of the phase change layer substrate is shown below. Figure 1 As shown, the top surface of the phase change layer substrate is hydrophilic (1) on the inside (i.e., untreated), while the other five outer surfaces are hydrophobic (2), coated with a hydrophobic material. After the phase change material completely covers the phase change layer substrate (170g), it is placed in a low-temperature environment at 4°C for cooling. After cooling for 12 hours, the encapsulation material for the phase change layer (1mm hydrophobic aerogel fiberglass mat) is placed on the hydrophilic surface of the aerogel fiberglass board substrate. Figure 2 As shown, the phase change layer encapsulation material 3 is covered on the hydrophilic surface of the aerogel fiberglass substrate containing the phase change material to obtain a complete phase change encapsulation plate, which is the phase change layer. Then, the phase change layer is placed on top of a 15mm thick heat insulation layer (aerogel fiberglass board), as shown. Figure 3 As shown, the phase change layer 4 and the heat insulation layer 5 are placed together in the vacuum barrier membrane 6 to obtain the phase change composite vacuum insulation panel sample to be extracted.

[0059] IV. Finished Products

[0060] The phase change composite vacuum insulation board sample was vacuumed using a vacuum sealing machine with a vacuum degree of 0.02 Pa. After vacuum sealing, the finished phase change composite vacuum insulation board was obtained.

[0061] In this embodiment, the aerogel glass fiber board is prepared by the following method:

[0062] 1) Weigh out 130 parts of commercially available silica sol, 10 parts of aluminum silicate, 10 parts of 36% hydrochloric acid, 4 parts of dimethyldimethoxysilane, 4 parts of KH550 brand silane coupling agent, and 150 parts of deionized water by weight.

[0063] 2) Weigh the aluminum silicate according to the ratio and add it to deionized water. Stir thoroughly until the aluminum silicate dissolves and the mixture is uniform.

[0064] 3) Add silica sol, dimethyldimethoxysilane and silane coupling agent to the inorganic additive solution in sequence according to the ratio. After each addition, stir thoroughly to obtain a mixed sol.

[0065] 4) Adjust the pH of the mixed sol to 7 using hydrochloric acid;

[0066] 5) The above solution is completely impregnated into the glass fiber mat, gelled and aged at 45°C for 30 hours to obtain fiber-reinforced aerogel;

[0067] 6) Place the above fiber-reinforced aerogel in a constant temperature oven at 110°C and dry it under normal pressure for 30 hours. Then, spray the hydrophobic agent methyltrimethoxysilane evenly on the five outer surfaces of the board to obtain the aerogel glass fiber board.

[0068] Example 2

[0069] This embodiment describes the preparation of a building hydrated salt phase change material with high latent heat of phase change and high cycle stability, and its encapsulation method, specifically including the following steps:

[0070] I. Weighing

[0071] Weigh 640 parts of anhydrous calcium chloride, 100 parts of sodium carbonate decahydrate, 60 parts of disodium hydrogen phosphate dodecahydrate; 620 parts of deionized water; 23 parts of strontium chloride hexahydrate, 3 parts of polyethylene glycol 600, 2 parts of carboxymethyl cellulose, and 2 parts of urea.

[0072] II. Preparation of Phase Change Materials

[0073] 620 parts of deionized water were placed in a magnetic stirrer at a constant temperature of 60℃. Then, 640 parts of anhydrous calcium chloride, 100 parts of sodium carbonate decahydrate, and 60 parts of disodium hydrogen phosphate dodecahydrate were added sequentially. The mixture was stirred at 70 rpm for 50 min. Then, 23 parts of strontium chloride hexahydrate, 3 parts of polyethylene glycol 600, and 2 parts of carboxymethyl cellulose were added sequentially. The mixture was stirred at 70 rpm for 60 min. Finally, 2 parts of urea were added and the mixture was stirred at 60 rpm for 25 min. The resulting liquid phase change material sample was labeled PCM2 and placed in a 60℃ oven to maintain its liquid state for later use.

[0074] III. Packaging

[0075] The prepared liquid phase change material sample PCM2 was removed from the oven and poured onto the aerogel ceramic fiber substrate (300×300mm). The amount of phase change material used was 185g. After completion, it was placed in a low temperature of 4°C for cooling treatment. After cooling for 12 hours, the phase change material changed from liquid to solid and was preserved in the substrate. A 1mm hydrophobic aerogel ceramic fiber felt was placed on the hydrophilic surface of the aerogel ceramic fiber substrate to obtain a complete phase change encapsulation plate phase change layer. Then, the phase change encapsulation plate phase change layer and the heat insulation layer (15mm aerogel ceramic fiber board) were placed together in a vacuum barrier membrane to obtain the phase change composite vacuum insulation board sample to be extracted. In this embodiment, the preparation method of the aerogel ceramic fiber board is the same as in Example 1, except that the glass fiber felt is replaced with ceramic.

[0076] IV. Finished Products

[0077] The phase change composite vacuum insulation board sample was vacuumed using a vacuum sealing machine with a vacuum degree of 0.02 Pa. After vacuum sealing, the finished phase change composite vacuum insulation board was obtained.

[0078] Comparative Example 1

[0079] This comparative example is based on Example 1, except that the nucleating agent for the second step of preparing the phase change material is only strontium chloride hexahydrate, without polyethylene glycol 600 and carboxymethyl cellulose. The resulting phase change material is denoted as PCM3.

[0080] Comparative Example 2

[0081] This comparative example is based on Example 1, but differs in the third step of encapsulation. The structure of the phase change layer is altered; the phase change layer substrate is an aerogel fiberglass board, and the encapsulation material is a PET bag. The total thickness remains the same as the phase change layer thickness in Example 1. The phase change material is impregnated in the substrate and then encapsulated in a PET bag to obtain a phase change encapsulation plate, which is the phase change layer.

[0082] Comparative Example 3

[0083] This comparative example is based on the preparation of phase change material by the same method as in Example 1, but the phase change layer is replaced with the phase change layer of Comparative Example 2 prepared by a conventional encapsulation method, and then vacuum insulation board is formed, resulting in a finished vacuum insulation board with the same total thickness as in Example 1.

[0084] The phase change material and phase change composite vacuum insulation panel prepared by this invention have the following properties:

[0085] Enthalpy value:

[0086] Differential scanning calorimetry was performed on PCM1, PCM2, and PCM3 in Examples 1, 2, and Comparative Example 1, and the phase transition temperatures of PCM1, PCM2, and PCM3 were found to be 27.3℃, 27.1℃, and 21.8℃, respectively, and the enthalpy values ​​were 206.3 J / g, 200.5 J / g, and 135.5 J, respectively.

[0087] Because the phase change material developed in this invention is intended for use in summer air-conditioned environments, the optimal phase change temperature is 24–30°C. In Comparative Example 1, the phase change temperature and enthalpy of PCM3 are 21.8°C and 135.5 J / g, respectively. Compared to Examples 1 and 2, the phase change temperature of Comparative Example 1 is not suitable for the optimal temperature of buildings, and its enthalpy is also lower. This indicates that adding the nucleating agent polyethylene glycol 600 and carboxymethyl cellulose is necessary to adjust the phase change temperature of the phase change material and increase its enthalpy.

[0088] The phase change layers prepared in Examples 1 and 2 were subjected to cyclic testing on a heating stage with a hot surface temperature of 80°C and a cooling stage with a cold surface temperature of -10°C, respectively. Temperature data was collected on the back surface using thermocouples. The heating and cooling curves after 50 cycles are shown below. Figure 4 and Figure 5 As can be seen from the figure, after 50 cycles of hot and cold, the melting and solidification temperatures of Examples 1 and 2 are not much different. The supercooling of Example 1 is 1.1℃ and the supercooling of Example 2 is 0.85℃, which indicates that the phase change material prepared by the present invention has good cycle stability.

[0089] The product prepared in Comparative Example 2 was placed on a heating platform with a hot surface temperature of 80°C. The temperature of the cold surface was collected using a type K thermocouple. The temperature-time curve was collected as follows: Figure 6 The conventional structure shown in the figure is the product of Comparative Example 2, and the novel structure is the product of Example 1. From Figure 6 As can be seen, Example 1 and Comparative Example 2 each have a plateau period at around 27°C, which proves that a phase change occurs inside the phase change material at this temperature. At the same time, the plateau period of Example 1 is longer than that of Comparative Example 2, which indicates that the new structure is superior to the traditional structure in terms of thermal insulation performance.

[0090] The product prepared in Comparative Example 3 was placed on a heating platform with a hot surface temperature of 80°C. The temperature on the back surface was collected using a K-type thermocouple, and the temperature-time curve was collected as follows: Figure 7 As shown, compared with Embodiment 1 and Embodiment 2, it can be seen that the packaging structure of Embodiment 1 and Embodiment 2 of the present invention maintains its position for a longer period of time in the plateau phase, which indicates that the packaging structure of the present invention is better.

[0091] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any other way. Any person skilled in the art can make changes or modifications to the above-disclosed technical content to create equivalent embodiments. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A phase change material for building applications, characterized in that, It is made of 600-800 parts by weight of a mature phase change material, 20-30 parts by weight of a nucleating agent, and 1-5 parts by weight of a temperature regulator; wherein the mature phase change material is a mixture of 600-650 parts of anhydrous calcium chloride, 80-100 parts of sodium carbonate decahydrate, and 50-80 parts of disodium hydrogen phosphate dodecahydrate; the nucleating agent is 20-23 parts of strontium chloride hexahydrate, 1-3 parts of polyethylene glycol, and 1-3 parts of carboxymethyl cellulose; the temperature regulator is urea; and the polyethylene glycol has a molecular weight of 600-800.

2. A phase change composite vacuum insulation panel, characterized in that, It includes a thermal insulation layer and a phase change layer. The thermal insulation layer is any one of aerogel ceramic fiber board, aerogel glass fiber board, fumed silica composite inorganic fiber board, ceramic fiber felt board, and glass fiber felt board. The phase change layer includes a matrix, a phase change material, and an encapsulation material. The phase change material covers the matrix, and the encapsulation material covers the phase change material to form the phase change layer. The matrix is ​​an aerogel ceramic fiber board or aerogel glass fiber board with a five-sided hydrophobic outer surface. The phase change material is the building phase change material as described in claim 1. The encapsulation material is a hydrophobic aerogel ceramic fiber felt or a hydrophobic aerogel glass fiber felt. The phase change layer is placed on top of the thermal insulation layer and encapsulated together in a vacuum barrier membrane to form a phase change composite vacuum insulation panel.

3. A method for preparing a phase change composite vacuum insulation panel, characterized in that, It includes the following steps: S1. The mature phase change material of the building phase change material according to claim 1 is mixed with water; then a nucleating agent is added and mixed; finally, a temperature regulator is added and mixed to obtain the phase change material, which is then kept in a liquid state at 50-60°C for later use. S2. Pour the liquid phase change material onto the phase change layer substrate and cool it. After the phase change material becomes solid, cover it with encapsulation material to obtain a complete phase change encapsulation board. S3. Place the phase change encapsulation plate and the heat insulation layer together in the vacuum barrier film to obtain the phase change composite vacuum insulation plate to be vacuumed. S4. Vacuuming is performed to obtain the finished phase change composite vacuum insulation panel, wherein the vacuum degree of the vacuuming is 0.02 to 0.8 Pa; The phase change layer substrate is made of aerogel ceramic fiber board or aerogel glass fiber board; the outer surface of the phase change layer substrate, except for the top, is coated with a hydrophobic material on five sides; the encapsulation material is hydrophobic aerogel ceramic fiber felt or hydrophobic aerogel glass fiber felt. The insulation layer material is any one or two or more of aerogel ceramic fiber board, aerogel glass fiber board, fumed silica composite inorganic fiber board, ceramic fiber mat or glass fiber mat.

4. The preparation method according to claim 3, characterized in that, In step S1, the stirring rate is 50-200 rpm and the stirring time is 30-60 min.

5. The preparation method according to claim 3, characterized in that, In step S2, the amount of phase change material used is 1.5–1.7 kg / m³. 2 The cooling process is carried out at a temperature of 4–8°C for 1–18 hours.

6. The preparation method according to claim 3, characterized in that, The thickness of the encapsulation material is 1-2 mm, and the thickness of the heat insulation layer is 10-20 mm.

7. The preparation method according to claim 3, characterized in that, In step S4, the vacuum level of the vacuum pump is 0.02 Pa.

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