High-barrier multilayer metal-plated bottom shell and preparation method thereof

Abstract

The application discloses a high-barrier multilayer metal plating bottom shell and a preparation method thereof. The high-barrier multilayer metal plating bottom shell comprises a plastic bottom shell, wherein a multilayer metal plating layer with an odd-layer structure is plated on the surface of the plastic bottom shell. The multilayer metal plating layer comprises a bottom layer, a connecting layer and a protective layer. The bottom layer and the protective layer are both nickel plating layers. The thickness of the bottom layer is 10-20 nm, and the thickness of the protective layer is 10-20 nm. The connecting layer is a single-layer aluminum plating layer or a composite plating film structure layer with an alternating plating structure of an aluminum plating layer-a nickel plating layer-an aluminum plating layer and an odd number of layers. The thickness of the aluminum plating layer of the connecting layer is 50-80 nm, and the thickness of the nickel plating layer of the connecting layer is 1-10 nm. The application greatly improves the barrier effect of the plastic bottom shell. A thin and dense NiO oxide layer with a thickness of 1-3 nm is formed on the nickel plating layer of the protective layer through pre-oxidation treatment. The NiO oxide layer is covered on the outside of the nickel plating layer of the protective layer. After the pre-oxidation treatment, the barrier effect of the nickel plating layer of the protective layer is greatly improved.

Description

Technical Field

[0001] This invention relates to the field of bottom shell barrier technology for vacuum insulation panels, and particularly to a high-barrier multilayer metal-coated bottom shell and its preparation method. Background Technology

[0002] Vacuum insulation panels (VIP panels) are mainly composed of core materials, barrier bags, and getters. The barrier bags, primarily composed of a polymer film and a specially coated film or aluminum foil composite, are a key component of the VIP panel, preventing air, moisture, and other gases from entering the bag to achieve vacuum insulation. The barrier bags are formed by sealing the four sides of the bottom shell. The bottom shell of the barrier bag is usually made of plastics such as PP, PET, or PA. Due to the poor barrier properties of plastics, the testing method for barrier performance refers to patent application 200610122868.4, "Leakage Rate Testing Method for Barrier Films in Vacuum Insulation Panels." For example, for PET plastics, the leakage rate Q of the barrier film using helium as the medium is 1.83 × 10⁻⁶. -3 mbar·L / S; PA plastic material, leakage rate Q is 2.12×10 -3 mbar·L / S; PP plastic material, leakage rate Q is 3.8×10 -3 mbar·L / S; the leakage rate Q of the different plastic materials mentioned above is greater than 1.0×10 -6 The mbar·L / S ratio is insufficient to meet the requirements for the use of barrier bags on vacuum barrier plates.

[0003] Currently, there is no readily available technology to reference for the barrier properties of plastic base shells in the field of vacuum insulation panels (VIP panels), creating a market gap. Therefore, it is necessary to conduct in-depth research and improvement on the barrier properties of plastic base shells to obtain plastic base shells with high barrier performance, thereby meeting the application requirements of plastic base shells in vacuum barrier applications. To this end, the applicant has conducted preliminary research on plastic base shells, exploring various methods such as plating various metals (aluminum, copper, nickel) onto the plastic surface. For example, a 50nm-80nm aluminum layer was plated onto the plastic surface using a vacuum evaporation aluminum deposition method. Under high vacuum conditions, aluminum is melted and evaporated at high temperature, causing the aluminum vapor to precipitate and accumulate on the surface of the plastic film. After aluminum plating, the plastic base shell exhibits excellent flexural strength and good toughness, and its barrier properties are greatly improved. Figure 2 However, this preparation method can cause pinholes or cracks on the surface of the aluminum plating layer. Furthermore, simply increasing the thickness of the aluminum plating layer cannot cover these pinholes, nor can it effectively enhance the barrier properties of the aluminum plating layer. (Refer to...) Figure 1 After aluminum plating, as the film thickness increases from 0 nm to 80 nm, the barrier performance of the plastic base shell decreases as the leakage rate Q increases from 1.83 × 10⁻⁶. -3 mbar·L / S gradually increased to 1×10-5 The coating thickness is mbar·L / S, but when the coating thickness is further increased after 80nm on the plastic base shell, the barrier performance of the plastic base shell remains almost unchanged, resulting in little improvement in the barrier performance of the vacuum insulation panel. If a nickel layer is plated on the surface of the plastic, it is found that due to the magnetic properties of nickel, the thickness of the nickel layer is difficult to increase, and the barrier performance of the plastic base shell is not significantly improved. Therefore, it is not very meaningful to plate nickel separately on the plastic base shell. Thus, the above-mentioned aluminum plating, nickel plating, and other methods used in the vacuum insulation panel in the existing technology cannot meet the performance requirements of the vacuum insulation panel for product application. Summary of the Invention

[0004] Therefore, the present invention aims to provide a high-barrier multilayer metal-coated base shell for use on vacuum insulation panels (VIP panels) to effectively improve the barrier performance of vacuum insulation panels and solve the problem of poor barrier performance of existing aluminum-coated base shells.

[0005] Another objective of this invention is to provide a method for preparing a multilayer metal-coated substrate.

[0006] The technical solution of the present invention is: a high-barrier multilayer metal-coated base shell, comprising a plastic base shell, wherein the surface of the plastic base shell is coated with at least three odd-numbered layers of multilayer metal coating, wherein the multilayer metal coating comprises a bottom layer, a connecting layer, and a protective layer, wherein the bottom layer and the protective layer are both nickel-plated layers, the thickness of the bottom layer is 10nm to 20nm, the thickness of the protective layer is 10nm to 20nm, the connecting layer is a single-layer aluminum-plated layer or a composite coating structure layer with alternating aluminum-nickel-aluminum plating layers and an odd number of layers, the thickness of the aluminum plating layer of the connecting layer is 50nm to 80nm, and the thickness of the nickel plating layer of the connecting layer is 1nm to 10nm.

[0007] Preferably, in the connecting layer, the total thickness of the multiple aluminum plating layers is 100nm to 400nm, and in the bottom layer, the protective layer and the connecting layer, the total thickness of the multiple nickel plating layers is 15nm to 80nm, and the ratio of the total thickness of the multiple aluminum plating layers to the total thickness of the multiple nickel plating layers is 5:1 to 10:1.

[0008] Preferably, the connecting layer is a three-layer structure consisting of an aluminum plating layer, a nickel plating layer, and another aluminum plating layer.

[0009] Preferably, the connecting layer is a five-layer structure consisting of an aluminum plating layer, a nickel plating layer, an aluminum plating layer, a nickel plating layer, and an aluminum plating layer.

[0010] Preferably, the protective layer further includes a NiO oxide layer disposed outside the nickel plating layer, the thickness of the NiO oxide layer being 1–3 nm.

[0011] Preferably, the bottom layer has an adhesion level of less than 2 on the surface of the plastic base shell and a pencil hardness greater than 3B.

[0012] Preferably, the protective layer has a surface adhesion level of less than 2 and a pencil hardness greater than 3B.

[0013] Preferably, the bonding layer has an adhesion level of less than 2 on the surface of its aluminum plating layer and a pencil hardness greater than 3B, and its nickel plating layer has an adhesion level of less than 2 on the surface of its nickel plating layer and a pencil hardness greater than 3B.

[0014] Preferably, the plastic base shell is made of any one of PET, PA, or PP.

[0015] Another object of the present invention is to provide a method for preparing a high-barrier multilayer metal-coated bottom shell, comprising the following steps:

[0016] S1, Bottom shell cleaning: Place the plastic bottom shell into a plasma cleaner and treat it for 3 to 5 minutes in an argon atmosphere with a purity of 99.99% to obtain a clean plastic bottom shell.

[0017] S2, nickel plating on the bottom layer: The plastic bottom shell is placed in a high-vacuum magnetron sputtering coating machine, and pure nickel target material is used to sputter and coat the surface of the plastic shell under an argon atmosphere with a purity of 99.99% to obtain a bottom layer with a nickel plating thickness of 10nm to 20nm.

[0018] S3, Adhesive Layer Composite Coating Structure: Replace the target position of the plastic base shell, and use a pure aluminum target to sputter and coat the plastic shell surface under an argon atmosphere with a purity of 99.99% to obtain an aluminum coating layer with a thickness of 50-80nm for the adhesive layer; or continue to replace the target position of the plastic base shell, and repeat the sputtering and coating of the plastic shell surface using a pure nickel target to obtain a nickel coating layer with a thickness of 1nm-10nm for the adhesive layer.

[0019] S4, Nickel plating of protective layer: A 10-20nm nickel layer is plated on the outer bonding layer;

[0020] In step S4, the nickel plating layer of the protective layer also includes a pre-oxidation treatment. The plastic bottom shell is placed in a nitrogen-oxygen mixed gas and the nickel plating layer on the protective layer is circulated and purged at a temperature of 50-60°C for 1-8 hours to obtain a NiO oxide layer with a thickness of 1-3 nm after pre-oxidation treatment. After cooling to room temperature, a high-barrier multilayer metal coating bottom shell is obtained.

[0021] In the nitrogen-oxygen mixed gas, the nitrogen volume content is 93% to 95%, and the oxygen volume content is 5% to 7%.

[0022] This invention employs a composite coating structure layer with a nickel plating layer as the bottom layer and a nickel plating layer as the protective layer, and an odd number of aluminum plating layers, nickel plating layers, and aluminum plating layers in the connecting layer, which greatly improves the barrier effect of the plastic bottom shell.

[0023] This invention forms a thin and dense NiO oxide layer with a thickness of 1-3 nm by pre-oxidizing the nickel plating layer of the protective layer. The NiO oxide layer covers the outside of the nickel plating layer of the protective layer. After the pre-oxidation treatment, the barrier effect of the nickel plating layer of the protective layer is greatly improved. Attached Figure Description

[0024] Figure 1 A schematic diagram showing how the barrier properties of a single-layer aluminum film coated on a plastic base vary with thickness.

[0025] Figure 2 This is a schematic diagram of the appearance of Comparative Example 2.

[0026] Figure 3 This is a schematic diagram of the appearance of Comparative Example 4.

[0027] Figure 4 This is a schematic diagram of a multi-layer metal-coated bottom shell structure.

[0028] Among them: 1-plastic bottom shell, 2-bottom layer, 3-connecting layer, 4-protective layer, 5-NiO oxide layer. Detailed Implementation

[0029] The present invention will be further described in detail below with reference to embodiments. It should also be understood that the following embodiments are only for further illustration of the present invention and should not be construed as limiting the scope of protection of the present invention. The specific mass, reaction time, temperature, process parameters, etc., in the examples are merely examples within a suitable range. Any non-essential improvements and adjustments made by those skilled in the art based on the above description of the present invention are within the scope of protection of the present invention.

[0030] Example 1:

[0031] A high-barrier, multi-layered metal-coated bottom shell, as shown in the reference. Figure 4 The device includes a plastic base shell 1, the surface of which is coated with a multi-layer metal plating layer with at least three odd-numbered layers. The multi-layer metal plating layer includes a bottom layer 2, a connecting layer 3, and a protective layer 4. Both the bottom layer 2 and the protective layer 4 are nickel plating layers. The thickness of the bottom layer 2 is 10nm to 20nm, and the thickness of the protective layer 4 is 10nm to 20nm. The connecting layer 3 is a composite coating structure layer with an odd number of layers, consisting of a single aluminum plating layer or alternating aluminum plating layer, nickel plating layer, and aluminum plating layer. The thickness of the aluminum plating layer in the connecting layer 3 is 50nm to 80nm, and the thickness of the nickel plating layer in the connecting layer 3 is 1nm to 10nm.

[0032] In the connecting layer 3, the total thickness of the multi-layer aluminum plating layer is 100nm to 400nm. In the bottom layer 2, the protective layer 4 and the connecting layer 3, the total thickness of the multi-layer nickel plating layer is 15nm to 80nm, and the ratio of the total thickness of the multi-layer aluminum plating layer to the total thickness of the multi-layer nickel plating layer is 5:1 to 10:1.

[0033] The plastic base shell 1 is made of PET, and the connecting layer 3 adopts a single-layer aluminum-plated structure, with the thickness of the single-layer aluminum plating controlled between 50nm and 80nm.

[0034] The method for preparing a high-barrier multilayer metal-coated substrate includes the following steps:

[0035] S1, Bottom shell cleaning: Place the plastic bottom shell 1 into a plasma cleaner and treat it for 3 to 5 minutes in an argon atmosphere with a purity of 99.99% to obtain a clean plastic bottom shell 1.

[0036] S2, nickel plating of the bottom layer: The plastic bottom shell 1 is placed in a high vacuum magnetron sputtering coating machine. Under an argon atmosphere with a purity of 99.99%, a pure nickel target is used to sputter and coat the surface of the plastic shell to obtain a bottom layer 2 with a nickel plating layer thickness of 10nm to 20nm.

[0037] S3, the bonding layer is coated with a composite film structure: the target position of the plastic base shell 1 is replaced, and a pure aluminum target is used to sputter and coat the surface of the plastic shell under an argon atmosphere with a purity of 99.99% to obtain a single-layer aluminum coating layer of bonding layer 3 with a thickness of 50-80nm.

[0038] S4, Nickel plating for the protective layer: A 10-20 nm nickel layer is plated on the outer bonding layer; thus, a high-barrier multilayer metal-coated bottom shell is obtained.

[0039] Among them, the bottom layer 2 has an adhesion level of less than 2 on the surface of the plastic bottom shell 1 and a pencil hardness greater than 3B; the protective layer 4 has an adhesion level of less than 2 on the surface and a pencil hardness greater than 3B; the connecting layer 3 has an adhesion level of less than 2 on the surface of its aluminum plating layer and a pencil hardness greater than 3B, and an adhesion level of less than 2 on the surface of its nickel plating layer and a pencil hardness greater than 3B.

[0040] The high-barrier multilayer metal-coated bottom shell is shaped as 45cm×28cm×0.1cm. The barrier performance can be tested by referring to the leakage rate test method of barrier film for vacuum insulation board in patent application 200610122868.4. The characterization results are shown in Table 1.

[0041] The hardness, adhesion, and other properties of the coating on the plastic base shell 1 were measured according to the corresponding national standards, as follows: the hardness was measured according to national standard GB / T 6739-2022, and the adhesion was measured according to national standard GB / T1720-79. The results are shown in Table 1.

[0042] To assess the appearance of the coating, the product was placed in a dark room with an LED light source on one side. The LED light was set to a pure white light source with a color temperature of 6000-6500K to illuminate one side of the product. A 2x camera was used to take a picture of the other side of the product from above to observe the light transmission of the surface. The coating was tested for light leakage, which allowed for a good observation of the coating film. The results are shown in Table 1.

[0043] Table 1 Performance characteristics of the shell structure

[0044]

[0045] Example 2:

[0046] The plastic base shell 1 is made of PET, and the connecting layer 3 is a three-layer structure of aluminum-plated layer-nickel-plated layer-aluminum-plated layer, and the rest is the same as in Example 1.

[0047] The high-barrier multilayer metal-coated bottom shell is shaped as 45cm×28cm×0.1cm. The barrier performance can be tested by referring to the leakage rate test method of barrier film for vacuum insulation board in patent application 200610122868.4. The characterization results are shown in Table 1.

[0048] The hardness, adhesion, appearance, and other properties of the coating on the plastic base shell 1 were determined according to Example 1, and the results are shown in Table 1.

[0049] Example 3:

[0050] The plastic base shell 1 is made of PET, and the connecting layer 3 is a five-layer structure consisting of an aluminum-plated layer, a nickel-plated layer, an aluminum-plated layer, a nickel-plated layer, and an aluminum-plated layer. The rest is the same as in Example 1.

[0051] The high-barrier multilayer metal-coated bottom shell is shaped as 45cm×28cm×0.1cm. The barrier performance can be tested by referring to the leakage rate test method of barrier film for vacuum insulation board in patent application 200610122868.4. The characterization results are shown in Table 1.

[0052] The hardness, adhesion, appearance, and other properties of the coating on the plastic base shell 1 were determined according to Example 1, and the results are shown in Table 1.

[0053] Example 4:

[0054] The plastic base shell 1 is made of PET, and the connecting layer 3 has a seven-layer structure consisting of an aluminum-plated layer, a nickel-plated layer, an aluminum-plated layer, a nickel-plated layer, an aluminum-plated layer, a nickel-plated layer, and an aluminum-plated layer. The rest is the same as in Example 1.

[0055] The high-barrier multilayer metal-coated bottom shell is shaped as 45cm×28cm×0.1cm. The barrier performance can be tested by referring to the leakage rate test method of barrier film for vacuum insulation board in patent application 200610122868.4. The characterization results are shown in Table 1.

[0056] The hardness, adhesion, appearance, and other properties of the coating on the plastic base shell 1 were determined according to Example 1, and the results are shown in Table 1.

[0057] Example 5:

[0058] The plastic base shell 1 is made of PP, and the rest is the same as in Example 1.

[0059] The high-barrier multilayer metal-coated bottom shell is shaped as 45cm×28cm×0.1cm. The barrier performance can be tested by referring to the leakage rate test method of barrier film for vacuum insulation board in patent application 200610122868.4. The characterization results are shown in Table 1.

[0060] The hardness, adhesion, appearance, and other properties of the coating on the plastic base shell 1 were determined according to Example 1, and the results are shown in Table 1.

[0061] Example 6:

[0062] Similar to Example 3, the difference is that the nickel plating layer of the protective layer 4 also includes a pre-oxidation treatment. The plastic bottom shell 1 is placed in a nitrogen-oxygen mixed gas, and the nickel plating layer on the protective layer 4 is circulated and purged at a temperature of 50-60°C for 1-8 hours to obtain a NiO oxide layer 5 with a thickness of 1-3 nm after pre-oxidation treatment. After cooling to room temperature, a high-barrier multilayer metal coating bottom shell is obtained.

[0063] In the nitrogen-oxygen mixed gas, the nitrogen volume content is 93% to 95%, and the oxygen volume content is 5% to 7%.

[0064] Using a pre-oxidation treatment, the nickel plating layer of the protective layer 4 undergoes surface oxidation in a nitrogen-oxygen mixed atmosphere to form a 3nm thick NiO oxide layer 5. The film thickness of the NiO oxide layer 5 is measured by a film thickness gauge. Preferably, in order to achieve a better pre-oxidation treatment, before the pre-oxidation treatment, the nitrogen and oxygen are dried by molecular sieves so that the moisture content in the nitrogen-oxygen mixed gas is less than 5ppmv.

[0065] The high-barrier multilayer metal-coated bottom shell is shaped as 45cm×28cm×0.1cm. The barrier performance can be tested by referring to the leakage rate test method of barrier film for vacuum insulation board in patent application 200610122868.4. The characterization results are shown in Table 1.

[0066] The hardness, adhesion, appearance, and other properties of the coating on the plastic base shell 1 were determined according to Example 1, and the results are shown in Table 1.

[0067] Example 7:

[0068] Same as Example 3, except that the material of the plastic bottom shell 1 is PP, and the rest of the processing methods are the same. The performance index characterization results are shown in Table 1.

[0069] Comparative Example 1:

[0070] The plastic base shell 1, made of PET material with a shape of 45cm×28cm×0.1cm, was used directly for the test. The characterization results of various performance indicators are shown in Table 1.

[0071] Comparative Example 2:

[0072] Referring to Comparative Example 1, the difference is that an 80nm aluminum layer is deposited on the PET plastic base shell 1. The performance characteristics are shown in Table 1. Figure 2 After aluminum plating, defects such as pinholes and numerous bubbles may occur on the surface.

[0073] Comparative Example 3:

[0074] Referring to Comparative Example 2, the difference is that the coating of the plastic base shell 1 made of PET material is changed to a nickel plating layer, and the performance index characterization results are shown in Table 1.

[0075] Comparative Example 4:

[0076] Referring to Comparative Example 2, the difference lies in the coating of the PET plastic base shell: an aluminum plating layer - a nickel plating layer - an aluminum plating layer. That is, the protective layer of the plastic base shell 1's coating is an aluminum plating layer, and the bottom layer of the plastic base shell 1's coating is an aluminum plating layer. The performance indicators are shown in Table 1. Figure 3 With aluminum plating as the protective layer and the bottom layer, the surface still has pinholes and a small number of bubbles. However, compared to Comparative Example 2, the appearance is better due to the addition of a nickel plating layer, and the color of the irradiated appearance is also darker.

[0077] Table 1 shows that the barrier performance of PET plastic base shell 1 is better than that of PP. Under the same processing conditions, the barrier performance of PET plastic base shell 1 with multi-layer metal coating is better than that of PP. Moreover, both PET and PP plastic base shell 1 can meet the application requirements for vacuum insulation panels after being treated with multi-layer metal coating.

[0078] In Examples 1 to 4, the barrier performance of the plastic base shell 1 decreased by an order of 10 as the number of connecting layers 3 increased from 1 layer to 7 layers in an odd-numbered manner. However, when the number of connecting layers 3 reached 7 layers, the adhesion of the plastic shell 1 significantly decreased. Based on the above results, the optimal number of connecting layers 3 in the plastic base shell 1 is 5 layers.

[0079] Comparing Examples 3 and 6, and Examples 5 and 7, the nickel plating layer of the protective layer in Examples 6 and 7 underwent pre-oxidation treatment to obtain a 3nm thick NiO oxide layer. The leakage rate Q test results of the barrier performance of the plastic bottom shell 1 showed that the barrier performance of the plastic bottom shell 1 was significantly improved after the nickel plating layer underwent pre-oxidation treatment.

[0080] Comparative Examples 2 and 4 show that the bottom layer and protective layer of the plastic base shell 1 are aluminum-plated. The leakage rate Q of the barrier performance of the plastic base shell 1 indicates that although the barrier effect is improved, the aluminum plating layer has poor density due to defects such as pinholes, cracks, and bubbles on the surface. This affects the barrier effect of the product. In actual use, the vacuum inside the vacuum insulation board is easily leaked due to the above defects, making it difficult to meet the application requirements of the plastic base shell 1 in the vacuum insulation board.

[0081] The above embodiments merely illustrate implementation methods of the present invention, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention.

Claims

1. A high-barrier multilayer metal-coated bottom shell, comprising a plastic bottom shell, characterized in that: The surface of the plastic base shell is coated with a multilayer metal plating structure with at least three odd-numbered layers. The multilayer metal plating includes a bottom layer, a connecting layer, and a protective layer. The bottom layer and the protective layer are both nickel plating layers. The thickness of the bottom layer is 10nm~20nm, the thickness of the protective layer is 10nm~20nm, and the connecting layer is a single-layer aluminum plating layer or a composite coating structure layer with alternating aluminum plating layer-nickel plating layer-aluminum plating layer and an odd number of layers. The thickness of the aluminum plating layer of the connecting layer is 50nm~80nm, and the thickness of the nickel plating layer of the connecting layer is 1nm~10nm.

2. The high-barrier multilayer metal-coated bottom shell according to claim 1, characterized in that: The connecting layer is a three-layer structure consisting of an aluminum plating layer, a nickel plating layer, and another aluminum plating layer.

3. The high-barrier multilayer metal-coated bottom shell according to claim 1, characterized in that: The connecting layer is a five-layer structure consisting of an aluminum plating layer, a nickel plating layer, an aluminum plating layer, a nickel plating layer, and an aluminum plating layer.

4. The high-barrier multilayer metal-coated bottom shell according to claim 1, characterized in that: It also includes a NiO oxide layer disposed outside the protective layer, the thickness of which is 1~3nm.

5. The high-barrier multilayer metal-coated bottom shell according to claim 1, characterized in that: The plastic base shell is made of any one of PET, PA, or PP.

6. A method for preparing a high-barrier multilayer metal-coated bottom shell according to claim 4, characterized in that, Includes the following steps: S1, Bottom shell cleaning: Place the plastic bottom shell into a plasma cleaner and treat it for 3 to 5 minutes in an argon atmosphere with a purity of 99.99% to obtain a clean plastic bottom shell. S2, Bottom layer coating: The plastic bottom shell is placed in a high vacuum magnetron sputtering coating machine. Under an argon atmosphere with a purity of 99.99%, a pure nickel target is used to sputter and coat the surface of the plastic bottom shell to obtain a bottom layer with a nickel coating thickness of 10nm~20nm. S3, Interconnecting layer coating: Replace the target position of the plastic base shell, and use a pure aluminum target to sputter and coat the surface of the plastic base shell under an argon atmosphere with a purity of 99.99% to obtain an aluminum-plated interconnecting layer with a thickness of 50~80nm; or continue to replace the target position of the plastic base shell, and repeatedly use a pure nickel target to sputter and coat the surface of the plastic base shell to obtain a nickel-plated interconnecting layer with a thickness of 1nm~10nm. S4, Protective layer coating: A protective layer with a thickness of 10~20nm is coated on the outer bonding layer; S5, Pre-oxidation treatment: The plastic bottom shell is placed in a nitrogen-oxygen mixed gas and the protective layer is circulated and purged at 50~60℃ for 1~8h to obtain a NiO oxide layer with a thickness of 1~3nm after pre-oxidation treatment. After cooling to room temperature, a high-barrier multilayer metal coating bottom shell is obtained. In the nitrogen-oxygen mixture, the nitrogen volume content is 93% to 95%, and the oxygen volume content is 5% to 7%.

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