Flexible skin multi-directional stretchable temperature sensor and method of making

Abstract

The present application relates to the field of sensor, specifically relates to a kind of flexible skin multidirectional stretchable temperature sensor and preparation method.It includes flexible substrate and at least one temperature-sensitive unit on flexible substrate;The temperature-sensitive unit includes N connected shrinkage unit, and constraint point is arranged between adjacent shrinkage unit.The shrinkage unit of the present application is arranged as inverted U shape, and the contractible angle in shrinkage unit can provide multidirectional strain insensitive structure margin for sensor, adaptive deformation in the process of stretching in different directions of flexible skin, to realize multidirectional strain insensitive temperature decoupling measurement;Solve the problem that existing stretchable temperature sensor cannot realize decoupling of uniaxial tensile strain in different directions, while the strain insensitive range can be improved.

Description

A flexible skin multi-directional stretchable temperature sensor and its fabrication method Technical Field

[0001] This invention relates to the field of sensors, specifically to a flexible skin multidirectional stretchable temperature sensor and its fabrication method. Background Technology

[0002] Flexible skin is a key component for high-speed variable-structure aircraft, enabling smooth and continuous deformation and protecting against aerodynamic heat generated during high-speed flight. It plays a crucial role in the aerodynamic and safety performance of the aircraft. Active thermal protection flexible skin significantly improves the high-temperature resistance of flexible skin through active thermal protection methods such as convective heat transfer, phase change heat absorption, and film insulation generated by self-pumping cooling media. However, existing active thermal protection flexible skins cannot differentiate the cooling media pumped according to the surface temperature distribution. Each time, the cooling media demand at the highest temperature point is used to deliver the same amount of cooling media to all pipes of the entire skin at the same standard, resulting in low cooling media utilization and increasing the burden on the aircraft's payload. To improve the efficiency of active thermal protection, the flexible skin needs to have temperature self-sensing capabilities to monitor the temperature distribution on the flexible skin surface in real time. Current methods use stretchable temperature sensors made of temperature-sensitive materials and flexible substrate materials to achieve temperature measurement.

[0003] Currently, the strain-insensitive structure design of stretchable temperature sensors is mainly divided into three types: (1) Micro-geometric structure design mainly involves applying pre-stretch strain to a stretchable flexible substrate to form a micro-scale, periodic, wavy geometric structure on its surface, thereby achieving strain insensitivity in the direction of pre-strain application. The strain insensitivity range can be controlled by changing the magnitude of the pre-strain.

[0004] (2) Macroscopic serpentine structures mainly involve designing temperature-sensitive materials into S-shaped, serpentine, or horseshoe-shaped serpentine structures. During the stretching process, the change in curvature of the arc provides a strain-insensitive structural margin, thereby achieving strain decoupling. However, most current research in this area focuses on unidirectional structural design, achieving only uniaxial tensile strain insensitivity in the designed structural direction. A very small number of studies have arranged serpentine structures longitudinally and transversely, orthogonally integrating them to support strain decoupling in different stretching directions, but their stretchable range is very limited, with the maximum strain-insensitive stretching rate reaching only 30%.

[0005] (3) The macroscopic Kirigami structure integrates the temperature-sensitive material and the flexible substrate material in layers, and then cuts them into Kirigami structures to release strain through out-of-plane deformation. It also only supports uniaxial tensile strain decoupling in the arrangement direction of Kirigami structural units.

[0006] Due to the directional nature of the strain-insensitive structural design of stretchable temperature sensors, most can only achieve decoupled temperature measurement under unidirectional tensile strain, and cannot achieve decoupling of uniaxial tensile strain in different directions. A very small number of stretchable temperature sensors can achieve decoupling of uniaxial tensile strain in different directions through fractal structure design, but the strain insensitivity range is very limited. Therefore, there is a need to develop a stretchable temperature sensor with a large tensile range and capable of multi-directional strain insensitivity for flexible skins used in active thermal protection of aircraft. Summary of the Invention

[0007] To address the problems mentioned in existing technologies, and to improve the efficiency of active thermal protection of flexible skins, it is necessary to enable the active thermal protection flexible skins to possess temperature self-sensing capabilities and monitor the temperature distribution on the surface of the flexible skin in real time. This invention proposes a multi-directional strain-insensitive (stretchability >80%) stretchable temperature sensor that reliably integrates a temperature-sensitive material with a flexible substrate material and deforms with the flexible skin, thereby enabling real-time monitoring of the surface temperature of the flexible skin and giving the active thermal protection flexible skin temperature self-sensing capabilities.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] The present invention discloses a flexible skin multidirectional stretchable temperature sensor, comprising a flexible substrate and at least one temperature-sensitive unit disposed on the flexible substrate; the temperature-sensitive unit comprises N interconnected shrinkable units, with constraint points provided between adjacent shrinkable units, wherein the shrinkable unit comprises a polymer package and a temperature-sensitive electrode disposed within the polymer package.

[0010] Preferably, the shrinking unit is arranged in an inverted U-shape, and the included angle formed by the shrinking unit is an included angle that can shrink in any direction.

[0011] Preferably, the end of the contraction unit away from the included angle is provided with a constraint point.

[0012] Preferably, N is an even number and N≥4, wherein N temperature-sensitive units are connected in a ring in sequence.

[0013] Preferably, the rings are connected in sequence to form an axisymmetric geometric shape.

[0014] Preferably, the flexible substrate is any one of silicone, Ecoflex, PDMS, and rubber.

[0015] A method for fabricating a flexible skin multi-directional stretchable temperature sensor includes the following steps:

[0016] S1: Take the same mass of the two-component polymer precursor, mix and stir evenly to obtain the polymer prepolymer;

[0017] S2: Spray the release agent onto the surface of the temperature-sensitive unit mold so that the release agent forms a uniform coating on the mold surface;

[0018] S3: After the coating dries, drip the polymer prepolymer from step S1 into the temperature-sensitive unit mold and remove air bubbles, then perform the first heating curing.

[0019] S4: Adhere the temperature-sensitive electrode to the polymer prepolymer, drip the polymer prepolymer from step S1 into the temperature-sensitive unit mold again and remove air bubbles, perform a second heating and curing, and complete the polymer encapsulation.

[0020] S5: After cooling, the polymer is encapsulated and demolded to obtain the temperature-sensitive unit.

[0021] Preferably, the polymer package has protrusions that are matched and connected to recesses on a flexible substrate.

[0022] Preferably, the removal of bubbles in steps S3 and S4 is performed by vacuum degassing.

[0023] Preferably, in step S3, the first heating and curing is performed at 80–120°C for 0.5–3 hours; in step S4, the second heating and curing is performed at 80–120°C for 0.5–3 hours.

[0024] Compared with the prior art, the present invention achieves the following technical effects:

[0025] The shrinkage unit of this invention is set in an inverted U-shape. The shrinkable included angle in the shrinkage unit can provide the sensor with a multi-directional strain insensitive structural margin, which can adaptively deform during the stretching process of the flexible skin in different directions, thereby realizing multi-directional strain insensitive temperature decoupling measurement. This solves the problem that existing stretchable temperature sensors cannot achieve decoupling of uniaxial tensile strain in different directions, and can improve the strain insensitivity range.

[0026] This invention discloses a method for fabricating a stretchable temperature sensor. By completely encapsulating the metal electrode material with a flexible substrate material, the mechanical flexibility, process reliability, electrical stability, and applicability to the stretchable elastic substrate of the flexible temperature sensor are ensured. After repeated stretching, not only is the stability and repeatability of the stretchable temperature sensor guaranteed, but the relative position of the temperature-sensitive material and the flexible substrate material also remains unchanged. Attached Figure Description

[0027] Figure 1 is a schematic diagram of the structure of the present invention;

[0028] Figure 2 shows the experimental simulation effect of the temperature-sensitive unit of the present invention;

[0029] Figure 3 shows the experimental simulation effect of another temperature-sensitive unit of the present invention;

[0030] Figure 4 is a graph showing the strain of the temperature-sensitive unit with different stretching angle offsets as a function of substrate stretching rate.

[0031] Figure 5 shows the maximum stretching ratio of the temperature-sensitive unit under different stretching angle offsets of the present invention.

[0032] Figure 6 is a schematic diagram of the preparation method of the present invention;

[0033] Figure 7 is a physical image of the present invention;

[0034] Figure 8 shows the resistance-temperature curves of the sensor under different X-axis tensile strains of the present invention;

[0035] Figure 9 shows the strain-resistance change rate curves of the temperature sensor with different uniaxial tensile offset angles according to the present invention.

[0036] Reference numerals: 1. Flexible substrate; 2. Constraint point; 3. Included angle; 4. Temperature-sensitive unit; 5. Mold; 6. Temperature-sensitive electrode; 7. Polymer encapsulation; 8. Protrusion. Detailed Implementation

[0037] The embodiments of this application are described in detail below, examples of which are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain this application, and should not be construed as limiting this application. Rather, the embodiments of this application include all variations, modifications, and equivalents falling within the spirit and scope of the appended claims.

[0038] Examples of this application are described in detail below, with examples shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The examples described below with reference to the accompanying drawings are exemplary and intended to explain this application, and should not be construed as limiting this application.

[0039] As shown in Figure 1, this invention discloses a flexible skin multidirectional stretchable temperature sensor, comprising a flexible substrate and at least one temperature-sensitive unit disposed on the flexible substrate. The temperature-sensitive unit includes N interconnected contractile units, with constraint points between adjacent contractile units. Each contractile unit includes a polymer package and a temperature-sensitive electrode disposed within the polymer package. The contractile units are arranged in an inverted U-shape, forming a contractible angle. A constraint point is located at the end of the contractile unit away from the contractible angle. N ≥ 4, where the N temperature-sensitive units are connected in a ring-like sequence. The ring-like sequence forms an axially symmetric geometric shape.

[0040] As shown in Figure 1 in the embodiment, the temperature-sensitive unit is composed of N connected shrinking units, where N≥4 and N is an even number. The purpose of N≥4 is to ensure that the stretchable temperature sensor structure maintains symmetry in the horizontal and vertical directions to facilitate processing and manufacturing, while also ensuring a large deformation margin (80%) when stretched uniaxially in multiple directions.

[0041] The set constraint points can be connected to the flexible substrate, where the shrinkage unit can deform freely in any direction. The included angle formed by the shrinkage unit can provide a multi-directional strain-insensitive structural margin for the temperature-sensitive unit. During the stretching process of the flexible substrate, the entire temperature-sensitive unit can adaptively deform, thereby realizing multi-directional strain decoupled temperature measurement.

[0042] The flexible substrate provided in the embodiment can replace the flexible skin for subsequent tests. The flexible substrate is made of a polymer material, specifically any one of silicone, Ecoflex (Ecological Flexibility), PDMS (polydimethylsiloxane), and rubber, which can simulate the mechanical properties of the flexible skin.

[0043] As shown in Figure 1, the constraint points set in the embodiment are used to fix multiple shrinking units. Specifically, the constraint points are protrusions set on the shrinking units. When the protrusions are connected to the flexible substrate, the flexible substrate has a pit at the corresponding position of the protrusion, which facilitates the connection of the temperature-sensitive unit to the flexible substrate.

[0044] In this embodiment, the number of constraint points N=6 and the initial value of the pre-shrinkage angle θ0=30° were selected. In the finite element analysis software, uniaxial tensile strain with different offset angles was applied to the flexible substrate. The range of the offset angle γ was 0° to 80°. The strain distribution of the temperature-sensitive unit under different substrate stretching rates was analyzed and calculated, as shown in Figures 2 and 3. When the stretching angle offset γ=0°, the average strain of the temperature-sensitive unit was the largest. When the substrate stretching rate was 100%, the average strain of the temperature-sensitive unit was 3.1195%. When the stretching angle offset γ=80°, the strain of the temperature-sensitive unit was the smallest. When the substrate stretching rate was 100%, the strain of the temperature-sensitive unit was 2.0624%.

[0045] As shown in Figures 4 and 5, when the uniaxial tensile offset angle γ = 0°, the maximum strain-insensitive tensile rate of the temperature-sensitive unit is 180%. As the uniaxial tensile offset angle gradually increases, the maximum strain-insensitive tensile rate of the temperature-sensitive unit will drift slightly, with a fluctuation range of 170% to 180%, and a drift rate of <10%. The finite element analysis results show that the shrinkage unit of the present invention has multi-directional and tensile strain-insensitive properties.

[0046] As shown in Figure 8 of the embodiment, the stretchable temperature sensor proposed in this invention provides the resistance response of the stretchable temperature sensor under different ambient temperatures and different magnitudes of X-axis tensile strain. Specifically, a horizontal tensile testing machine is used to apply X-axis tensile strains of 0%, 20%, 40%, 60%, and 80% to the stretchable temperature sensor, respectively. In each tensile state, an ambient temperature of 20℃-200℃ is applied to the stretchable temperature sensor, with a temperature increment of 10℃. The resistance response data of the stretchable temperature sensor is collected using a digital source meter.

[0047] As shown in Figure 8, the temperature response of the stretchable temperature sensor of this invention under 20% and 40% tensile strain is consistent with its unstrained state, indicating that the stretchable temperature sensor has excellent strain decoupling effect under X-axis tensile strain of less than 40%. For tensile strains of 60% and 80%, the temperature response is relatively consistent with its unstrained state in the low-temperature range (<100℃); however, it deviates in the high-temperature range (>100℃), with its resistance being slightly lower than that in the unstrained state, and the resistance deviation increases with increasing temperature.

[0048] To verify the strain insensitivity characteristics of the stretchable temperature sensor of the present invention under different uniaxial tensile conditions, the uniaxial tensile direction of the tensile test platform was adjusted in the embodiment, and the resistance change rate of the stretchable temperature sensor under different uniaxial tensile offset angles and different tensile strains at room temperature was tested, as shown in Figure 9.

[0049] As shown in Figure 9, the resistance change rate of the stretchable temperature sensor fluctuates within the range of 0–1.95% under uniaxial tensile strain of 0–80% at different uniaxial tensile offset angles. For the experimental control group with different uniaxial tensile offset angles, the resistance change rate versus uniaxial tensile strain shows a similar trend, generally increasing. This means that as the uniaxial tensile strain increases, the resistance change rate of the stretchable temperature sensor increases, a phenomenon consistent with the finite element simulation analysis results. Therefore, this experiment demonstrates that the stretchable temperature sensor prepared in this invention possesses strain insensitivity characteristics in different directions, enabling decoupled temperature measurement under uniaxial tensile strain in different directions.

[0050] Example 1

[0051] As shown in Figure 6, the method for fabricating a flexible skin multi-directional stretchable temperature sensor according to the present invention includes the following steps:

[0052] S1: Take equal mass of the two-component polymer precursors, mix and stir evenly to obtain the polymer prepolymer; in the example, one set of polymer precursors is platinum-catalyzed silica gel Ecoflex 00-30 component A, and the other set of polymer precursors is Ecoflex 00-30 component B.

[0053] S2: Spray the release agent onto the surface of the temperature-sensitive unit mold to form a uniform coating on the mold surface; in this embodiment, the release agent is specifically a silicone rubber release agent.

[0054] S3: After the coating dries, the polymer prepolymer from step S1 is dripped into the temperature-sensitive unit mold and air bubbles are removed. In this embodiment, after the coating dries, the polymer prepolymer is dripped into the temperature-sensitive unit mold using a micro-syringe and placed in a vacuum drying oven to remove air bubbles from the polymer prepolymer using negative pressure vacuum. At this time, the polymer prepolymer is the lower substrate.

[0055] S4: At this point, place the temperature-sensitive unit mold of the polymer prepolymer on a constant temperature heating table and cure it at 80°C for 4 hours. At this point, the polymer prepolymer is in a semi-cured state.

[0056] S5: At this point, place the metal temperature-sensitive electrode on the surface of the polymer prepolymer and use the strong adhesion of the semi-cured surface of the polymer prepolymer to fix the position of the metal temperature-sensitive electrode.

[0057] S6: Use a micro-syringe to drip the polymer prepolymer into the temperature-sensitive unit mold and place it in a vacuum drying oven to remove air bubbles from the polymer prepolymer using negative pressure vacuum. Then place it on a constant temperature heating table and cure it at 80°C for 4 hours. At this time, the polymer prepolymer is the upper substrate.

[0058] S7: Move the temperature-sensitive unit mold to room temperature and wait for it to cool completely before demolding the temperature-sensitive unit to obtain a stretchable temperature sensor.

[0059] The temperature-sensitive electrode in the embodiments can be one of the resistivity and thermistor materials such as platinum, copper, and silver, or an oxide of one of them, or a composite or mixture of two or more of them, which can be adjusted according to different application requirements.

[0060] Figure 7 shows the actual product of the stretchable temperature sensor of the present invention in the embodiment.

[0061] The stretchable temperature sensor of the present invention has the ability to sense temperature insensitive to tensile strain in different directions. The included angle of the contraction unit in the present invention can provide the sensor with a multi-directional strain-insensitive structural margin, which can adaptively deform during the stretching process of the flexible skin in different directions, thereby realizing multi-directional strain-insensitive temperature decoupled measurement. The resistance change rate of the stretchable temperature sensor prepared by the present invention is less than 2% under 0-80% tensile strain in the X-axis at an ambient temperature of 20-200℃ and under 0-80% uniaxial tensile strain in different directions at room temperature.

[0062] Example 2

[0063] The fabrication method of the stretchable sensor in this embodiment is basically the same as that in Embodiment 1, except that...

[0064] S1: Take equal mass of the two-component polymer precursors, mix and stir evenly to obtain the polymer prepolymer; in the example, one set of polymer precursors is platinum-catalyzed silica gel Dragon Skin 30 component A, and the other set of polymer precursors is platinum-catalyzed silica gel Dragon Skin 30 component B.

[0065] S2: Spray the release agent onto the surface of the temperature-sensitive unit mold to form a uniform coating on the mold surface; in this embodiment, the release agent is specifically a silicone rubber release agent.

[0066] S3: After the coating dries, the polymer prepolymer from step S1 is dripped into the temperature-sensitive unit mold and air bubbles are removed. In this embodiment, after the coating dries, the polymer prepolymer is dripped into the temperature-sensitive unit mold using a micro-syringe and placed in a vacuum drying oven to remove air bubbles from the polymer prepolymer using negative pressure vacuum. At this time, the polymer prepolymer is the lower substrate.

[0067] S4: At this point, place the temperature-sensitive unit mold of the polymer prepolymer on the constant temperature heating table and cure it at 120°C for 1 hour. At this point, the polymer prepolymer is in a semi-cured state.

[0068] S5: At this point, place the metal temperature-sensitive electrode on the surface of the polymer prepolymer and use the strong adhesion of the semi-cured surface of the polymer prepolymer to fix the position of the metal temperature-sensitive electrode.

[0069] S6: Use a micro-syringe to drip the polymer prepolymer into the temperature-sensitive unit mold and place it in a vacuum drying oven to remove air bubbles from the polymer prepolymer using negative pressure vacuum. Then place it on a constant temperature heating table and cure it at 120°C for 1 hour. At this time, the polymer prepolymer is the upper substrate.

[0070] S7: Move the temperature-sensitive unit mold to room temperature and wait for it to cool completely before demolding the temperature-sensitive unit to obtain a stretchable temperature sensor.

[0071] Example 3

[0072] The preparation method of the stretchable sensor in this embodiment is basically the same as that in Embodiment 1 and Embodiment 2, except that, in S1: take two polymer precursors of the same mass, mix and stir evenly to obtain a polymer prepolymer; in the embodiment, one set of polymer precursors is platinum-catalyzed silica gel SILICONE 3030 A component, and the other set of polymer precursors is SILICONE 3030 B component.

[0073] S2: Spray the release agent onto the surface of the temperature-sensitive unit mold to form a uniform coating on the mold surface; in this embodiment, the release agent is specifically a silicone rubber release agent.

[0074] S3: After the coating dries, the polymer prepolymer from step S1 is dripped into the temperature-sensitive unit mold and air bubbles are removed. In this embodiment, after the coating dries, the polymer prepolymer is dripped into the temperature-sensitive unit mold using a micro-syringe and placed in a vacuum drying oven to remove air bubbles from the polymer prepolymer using negative pressure vacuum. At this time, the polymer prepolymer is the lower substrate.

[0075] S4: At this point, place the temperature-sensitive unit mold of the polymer prepolymer on the constant temperature heating table and cure it at 100°C for 2 hours. At this point, the polymer prepolymer is in a semi-cured state.

[0076] S5: At this point, place the metal temperature-sensitive electrode on the surface of the polymer prepolymer and use the strong adhesion of the semi-cured surface of the polymer prepolymer to fix the position of the metal temperature-sensitive electrode.

[0077] S6: Use a micro-syringe to drip the polymer prepolymer into the temperature-sensitive unit mold and place it in a vacuum drying oven to remove air bubbles from the polymer prepolymer using negative pressure vacuum. Then place it on a constant temperature heating table and cure it at 120°C for 2 hours. At this time, the polymer prepolymer is the upper substrate.

[0078] S7: Move the temperature-sensitive unit mold to room temperature and wait for it to cool completely before demolding the temperature-sensitive unit to obtain a stretchable temperature sensor.

[0079] It should be noted that in the description of this application, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance. Furthermore, in the description of this application, unless otherwise stated, "a plurality of" means two or more.

[0080] Any process or method described in the flowchart or otherwise herein can be understood as representing a module, segment, or portion of code comprising one or more executable instructions for implementing a particular logical function or process, and the scope of the preferred embodiments of this application includes additional implementations in which functions may be performed not in the order shown or discussed, including substantially simultaneously or in reverse order depending on the function involved, as will be understood by those skilled in the art to which embodiments of this application pertain.

[0081] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0082] Although embodiments of this application have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting this application. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of this application.

Claims

1. A flexible skin multi-directional stretchable temperature sensor, characterized in that, The device includes a flexible substrate and at least one temperature-sensitive unit disposed on the flexible substrate. The temperature-sensitive unit includes N interconnected shrinkable units, with constraint points between adjacent shrinkable units. Each shrinkable unit includes a polymer package and a temperature-sensitive electrode disposed within the polymer package. The shrinkable units are arranged in an inverted U-shape, and the included angle formed by the shrinkable units is an angle that can shrink in any direction. A constraint point is provided at the end of the shrinkable unit away from the included angle. N is an even number, and N≥4, wherein the N temperature-sensitive units are connected in a ring in sequence. The ring in sequence forms an axially symmetric geometric shape.

2. The flexible skin multi-directional stretchable temperature sensor according to claim 1, characterized in that, The flexible substrate is any one of silicone, Ecoflex, PDMS, and rubber.

3. A method for fabricating a flexible skin multi-directional stretchable temperature sensor according to any one of claims 1 or 2, characterized in that, Includes the following steps: S1: Take the same mass of the two-component polymer precursor, mix and stir evenly to obtain the polymer prepolymer; S2: Spray the release agent onto the surface of the temperature-sensitive unit mold to form a uniform coating on the mold surface. S3: After the coating dries, drip the polymer prepolymer from step S1 into the temperature-sensitive unit mold and remove air bubbles, then perform the first heating curing; S4: Adhere the temperature-sensitive electrode onto the polymer prepolymer, drip the polymer prepolymer from step S1 into the temperature-sensitive unit mold again and remove air bubbles, then perform the second heating curing to complete the polymer encapsulation; S5: Demold the cooled polymer encapsulation to obtain the temperature-sensitive unit.

4. The method for fabricating a flexible skin multi-directional stretchable temperature sensor according to claim 3, characterized in that, The polymer package has protrusions that are matched and connected to recesses on a flexible substrate.

5. The method for fabricating a flexible skin multi-directional stretchable temperature sensor according to claim 3, characterized in that, In steps S3 and S4, bubble removal is performed using vacuum degassing.

6. The method for fabricating a flexible skin multi-directional stretchable temperature sensor according to claim 3, characterized in that, In step S3, the first heating and curing is carried out at 80-120°C for 0.5-3 hours; in step S4, the second heating and curing is carried out at 80-120°C for 0.5-3 hours.

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