Flexible PPTC thermistor composite film with secondary protection function and preparation method thereof

Abstract

This invention discloses a flexible PPTC thermistor composite film with secondary protection function and its preparation method. The flexible PPTC thermistor includes a substrate layer, a PPTC layer, and an encapsulation layer stacked together. The substrate layer is a flexible polymer film, the encapsulation layer is made of an insulating polymer, and the PPTC layer is prepared from components comprising the following mass fractions: 50%–90% conductive particles, 8%–45% aqueous polymer, 1%–10% high-molecular polymer, and 1%–10% coupling agent. The aqueous polymer is selected from at least one of aqueous polyurethane, polyvinyl alcohol, and polyvinylpyrrolidone, and the high-molecular polymer is selected from at least one of low-density polyethylene, linear low-density polyethylene, high-density polyethylene, polypropylene, polystyrene, polytetrafluoroethylene, polyoxymethylene, and polyamide. The flexible PPTC thermistor composite film of this invention can undergo two resistance abrupt changes, possessing secondary protection function. Furthermore, the composite film exhibits small temperature differences and uniform temperature distribution across different regions, while also demonstrating excellent bending resistance.

Description

Technical Field

[0001] This invention relates to the field of electronic materials technology, specifically to a flexible PPTC thermistor composite film with secondary protection function and its preparation method. Background Technology

[0002] With the development of flexible electronics technology, traditional thermistors rely on external temperature control systems, resulting in problems such as large size, high energy consumption, and slow response. In contrast, self-controlled heaters based on positive temperature coefficient (PTC) thermistor materials can autonomously adjust their heating power according to ambient temperature, achieving passive temperature protection and significantly improving safety and energy efficiency. Among these, polymer-based positive temperature coefficient (PPTC) materials have become an ideal choice for flexible thermistors due to their tunable resistance-temperature characteristics and excellent processing performance.

[0003] However, existing PPTC-based thermistors still face some challenges. First, the limitation of single-point temperature protection: Most PPTC materials have a single resistance change temperature (i.e., protection temperature). For example, the operating temperature of low-density polyethylene (LDPE) is between 70~90℃; high-density polyethylene (HDPE) between 90~120℃; and polypropylene (PP) between 130~160℃. These materials can only trigger the protection mechanism at a specific temperature, making it difficult to meet the needs of complex applications requiring multi-level temperature protection. Second, uneven temperature distribution and localized overheating: Due to the uneven current distribution of the PPTC layer on the flexible substrate, large-area or irregularly shaped heaters are prone to regional temperature differences, causing some areas to trigger protection prematurely while other areas remain underprotected, or even leading to material aging or burnout due to localized overheating. Third, the balance between thermal conductivity and insulation of the substrate: While existing polyimide (PI) or polyacrylonitrile (PAN) substrates possess excellent insulation and mechanical strength, their thermal conductivity is low (typically <0.2 W / m·K), hindering rapid heat dissipation and exacerbating temperature inhomogeneity. This, in turn, affects the response speed and safety performance of the thermistors. Fourth, existing PPTC preparation methods include melt blending, solution blending, in-situ polymerization, and multilayer composite structure methods. However, these methods all have drawbacks. For example, melt blending makes it difficult to control the uniformity of filler dispersion, potentially leading to uneven conductive networks. Solution blending requires organic solvents, which can pollute the environment and are costly. In-situ polymerization is complex and requires controlled polymerization conditions. Multilayer composite structure methods are not only complex but also expensive.

[0004] Therefore, there is an urgent need for a flexible PPTC thermistor composite film with secondary protection function and its preparation method to overcome the shortcomings of existing technologies. Summary of the Invention

[0005] The purpose of this invention is to provide a flexible PPTC thermistor composite film with secondary protection function and its preparation method. The flexible PPTC thermistor composite film can undergo two resistance changes, thus having a secondary protection function. Moreover, the temperature difference between different regions of the composite film is small, the temperature distribution is uniform, and it also has excellent bending resistance.

[0006] To achieve the above objectives, the present invention provides a flexible PPTC thermistor composite film with secondary protection function, comprising a substrate layer, a PPTC layer, and an encapsulation layer stacked together; the substrate layer is a flexible polymer film, the encapsulation layer is made of an insulating polymer slurry, and the PPTC layer is made of components comprising the following mass fractions: 50%~90% conductive particles, 8%~45% aqueous polymer, 1%~10% high molecular weight polymer, and 1%~10% coupling agent; the aqueous polymer is selected from at least one of aqueous polyurethane, polyvinyl alcohol, and polyvinylpyrrolidone, and the high molecular weight polymer is selected from at least one of low-density polyethylene, linear low-density polyethylene, high-density polyethylene, polypropylene, polystyrene, polytetrafluoroethylene, polyoxymethylene, and polyamide.

[0007] Compared with the prior art, the flexible PPTC thermistor composite film (hereinafter sometimes referred to as "composite film") of the present invention has at least the following beneficial effects: (1) The raw materials for preparing the PPTC layer of the present invention include aqueous polymers and high molecular weight polymers. The aqueous polymers can serve as a dispersion carrier for conductive particles, allowing them to be uniformly dispersed in the material system of the PPTC layer, thereby providing better conductivity and more uniform thermal conductivity. On the other hand, the aqueous polymers of the present invention, such as aqueous polyurethane, polyvinyl alcohol, and polyvinylpyrrolidone, have relatively low operating temperatures, while the high molecular weight polymers, such as low-density polyethylene, linear low-density polyethylene, high-density polyethylene, polypropylene, polystyrene, polytetrafluoroethylene, polyoxymethylene, and polyamide, have relatively high operating temperatures. By combining the aforementioned specific aqueous polymers with high molecular weight polymers, the PPTC layer of the present invention can undergo a first resistance change at a lower temperature and a second resistance change at a higher temperature, resulting in a secondary protection effect and higher safety performance.

[0008] (2) The present invention uses a flexible polymer film as the substrate layer, which makes the PPTC thermistor of the present invention more suitable for the deformation requirements of flexible electronic devices. It can be closely attached to the flexible circuit to accurately monitor local temperature changes, without increasing the overall volume and weight of the device, thus meeting the design requirements of lightweight and miniaturized flexible electronic devices.

[0009] (3) The present invention uses insulating polymer slurry as the preparation material of the encapsulation layer and sets it on the surface of the PPTC layer. This not only extends the service life of the PPTC layer, but also improves its thermal conductivity and makes the temperature distribution more uniform, effectively avoiding the situation where some areas are prematurely triggered for protection due to local overheating while other areas are still underprotected.

[0010] In summary, the composite film provided by this invention largely solves the limitations of single-point temperature protection, uneven temperature distribution, and local overheating problems, thus exhibiting rapid thermal response, wide-range adaptive control, and excellent mechanical reliability.

[0011] As a preferred technical solution, the aqueous polymer of the present invention is selected from aqueous polyurethane, and the polymer is selected from polyamide or polypropylene. Specifically, the inventors further discovered during the research and development process that, in addition to providing a secondary protection effect, using aqueous polyurethane in combination with polyamide or polypropylene can effectively improve the bending resistance of the composite film of the present invention, so that its conductivity will not be significantly affected after multiple bends, making it more suitable for flexible electronic devices such as flexible displays, wearable devices, flexible sensors, and other fields.

[0012] As a preferred technical solution, the flexible polymer film of the present invention is selected from polyimide film, polyvinylidene fluoride film, thermoplastic polyurethane film, polyethylene terephthalate film, polyethylene naphthalate film, polycarbonate film, polyetheretherketone film, polypropylene film, or polystyrene film.

[0013] As a preferred technical solution, the flexible polymer film of the present invention is selected from polyimide film.

[0014] As a preferred technical solution, the insulating polymer slurry of the present invention includes an insulating polymer and a curing agent, wherein the insulating polymer is selected from polydimethylsiloxane, polyvinylidene fluoride, polyethylene naphthalate, polyethersulfone, thermoplastic polyurethane or polyethylene.

[0015] As a preferred technical solution, the insulating polymer of the present invention is selected from polydimethylsiloxane.

[0016] As a preferred technical solution, the conductive particles of the present invention are selected from at least one of metallic conductive particles and carbon-based conductive particles.

[0017] As a preferred technical solution, the conductive particles of the present invention are selected from carbon-based conductive particles.

[0018] As a preferred technical solution, the coupling agent of the present invention is selected from silane coupling agents, titanate coupling agents, aluminate coupling agents, or zirconate coupling agents.

[0019] As a preferred technical solution, the coupling agent of the present invention is selected from silane coupling agents.

[0020] As a preferred technical solution, the particle size Dv50 of the conductive particles of the present invention is 5nm~300μm.

[0021] As a preferred technical solution, the particle size Dv50 of the polymer of the present invention is 10μm~500μm.

[0022] As a preferred technical solution, the thickness of the substrate layer of the present invention is 50μm~2000μm.

[0023] As a preferred technical solution, the thickness of the PPTC layer in this invention is 50μm~1000μm.

[0024] As a preferred technical solution, the thickness of the encapsulation layer of the present invention is 50μm~700μm.

[0025] Another aspect of the present invention provides a method for preparing the aforementioned flexible PPTC thermistor composite film with secondary protection function, comprising the steps of: (1) Provide a base layer; mix the conductive particles, polymer, coupling agent and aqueous polymer in a mixer to obtain a resistive slurry; provide an insulating polymer slurry; (2) A resistive paste is coated on one surface of the substrate and dried to form a PPTC layer; (3) Coating the surface of the PPTC layer with an insulating polymer paste and drying it to form an encapsulation layer, thereby obtaining a flexible PPTC thermistor composite film.

[0026] As a preferred technical solution, the coating method of the present invention is scraping with a doctor blade.

[0027] Compared with existing technologies, the preparation process of this invention is simple, low-cost, and uses green and pollution-free raw materials. The resulting flexible thermistor composite film not only possesses excellent flexibility, conductivity, and PTC effect, but also exhibits excellent thermal conductivity on both the surface and interior of the film. Furthermore, it can undergo a resistance surge at low temperatures, eliminating the risk of fire and burns. At high temperatures, it can also activate secondary protection; in the event of an unexpected increase in current leading to a temperature rise, it will generate secondary protection, resulting in higher safety performance. In addition, adding an encapsulation layer to the thermistor layer not only extends the film's lifespan but also improves its thermal conductivity, effectively preventing localized overheating that could cause some areas to prematurely trigger protection while other areas remain underprotected. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the structure of the flexible PPTC thermistor composite film with secondary protection function of the present invention; Figure 2 The RT curve of the flexible PPTC thermistor composite film prepared in Example 1 of this invention; Figure 3 The ρ-T curve of the flexible PPTC thermistor composite film prepared in Example 1 of this invention; Figure 4 This is a scanning electron microscope (SEM) image of the cross-section of the flexible PPTC thermistor composite film prepared in Example 1 of the present invention. Detailed Implementation

[0029] To better illustrate the purpose, technical solution, and beneficial effects of this invention, the invention will be further described below with reference to specific embodiments and accompanying drawings. It should be noted that the methods described below are further explanations of this invention and should not be construed as limiting it.

[0030] It should be noted that the specific technical details provided in the following embodiments are only for the purpose of helping to understand the technical solutions of the present invention, but the scope of protection of the present invention is not limited to these specific embodiments. Based on the basic principles of the present invention, those skilled in the art can implement other feasible technical solutions through equivalent substitutions, reasonable modifications, etc., without departing from the core idea of ​​the present invention. Therefore, any modifications, equivalent substitutions, or improvements made within the scope of the technical concept of the present invention should be included within the scope of protection of the present invention.

[0031] To address the limitations of most current thermistors' single-point temperature protection and the potential for localized overheating, this invention provides a flexible PPTC thermistor composite film with secondary protection capabilities and its preparation method. For details, please refer to... Figure 1 The present invention provides a flexible PPTC thermistor composite film 100 with secondary protection function, comprising a substrate layer 11, a PPTC layer 12, and an encapsulation layer 13 stacked together. The substrate layer 11 is a flexible polymer film, the encapsulation layer 13 is made of an insulating polymer slurry, and the PPTC layer is made of components comprising the following mass fractions: 50%~90% conductive particles, 8%~45% aqueous polymer, 1%~10% high molecular weight polymer, and 1%~10% coupling agent.

[0032] Specifically, the thickness of the substrate layer 11 is 50 μm to 2000 μm, preferably 50 μm to 1500 μm, and more preferably 50 μm to 1200 μm. As an example, the thickness of the substrate layer 11 can be, but is not limited to, 50 μm, 100 μm, 300 μm, 500 μm, 800 μm, 1000 μm, 1200 μm, 1400 μm, 1500 μm, 1600 μm, 1800 μm, or 2000 μm. The flexible polymer film is selected from polyimide film, polyvinylidene fluoride film, thermoplastic polyurethane film, polyethylene terephthalate film, polyethylene naphthalate film, polycarbonate film, polyetheretherketone film, polypropylene film, or polystyrene film. More specifically, the flexible polymer film is selected from polyimide film. It should be noted that the flexible polymer films listed above can be obtained through conventional commercial means or prepared by conventional methods in the art.

[0033] Further, the thickness of the PPTC layer 12 is 50 μm to 1000 μm, preferably 100 μm to 700 μm, and more preferably 100 μm to 500 μm. As an example, the thickness of the PPTC layer can be, but is not limited to, 50 μm, 100 μm, 150 μm, 200 μm, 250 μm, 300 μm, 350 μm, 400 μm, 450 μm, 500 μm, 600 μm, 700 μm, 800 μm, 900 μm, or 1000 μm. The mass fraction of the conductive particles can be, but is not limited to, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, or 90%. The particle size Dv50 of the conductive particles is 5 nm to 300 μm, preferably 5 nm to 100 μm, and more preferably 5 nm to 500 nm. As an example, the particle size Dv50 of the conductive particles can be, but is not limited to, 5 nm, 50 nm, 100 nm, 300 nm, 500 nm, 800 nm, 1 μm, 50 μm, 100 μm, 150 μm, 200 μm, 250 μm, and 300 μm. The conductive particles are selected from at least one of metallic conductive particles and carbon-based conductive particles. Specifically, the metallic conductive particles are selected from at least one of silver, copper, gold, nickel, and aluminum; the carbon-based conductive particles are selected from at least one of carbon black, graphite, carbon fiber, graphene, graphene oxide, and carbon nanotubes. The mass fraction of the aqueous polymer can be, but is not limited to, 8%, 10%, 12%, 15%, 18%, 20%, 25%, 30%, 35%, 40%, and 45%. The aqueous polymer is selected from at least one of aqueous polyurethane, polyvinyl alcohol, and polyvinylpyrrolidone. The mass fraction of the polymer can be, but is not limited to, 1%, 2%, 4%, 6%, 8%, or 10%. The particle size Dv50 of the polymer is 10 μm to 500 μm, preferably 100 μm to 500 μm, more preferably 300 μm to 500 μm. As an example, the particle size Dv50 of the polymer can be, but is not limited to, 10 μm, 50 μm, 80 μm, 100 μm, 150 μm, 200 μm, 250 μm, 300 μm, 350 μm, 400 μm, 450 μm, or 500 μm. The polymer is selected from at least one of low-density polyethylene, linear low-density polyethylene, high-density polyethylene, polypropylene, polystyrene, polytetrafluoroethylene, polyoxymethylene, and polyamide. Furthermore, the conductive particles of the present invention are selected from carbon-based conductive particles, the waterborne polymer is selected from waterborne polyurethane, and the polymer is selected from polyamide or polypropylene. The solid content of the waterborne polyurethane can be 40% to 60%, and as an example, the solid content of the waterborne polyurethane can be, but is not limited to, 40%, 45%, 50%, 55%, or 60%.This invention combines waterborne polyurethane with polyamide or polypropylene, ensuring a change in resistance in the PPTC layer at two temperatures. It also allows for more uniform dispersion of conductive particles, particularly carbon-based conductive particles, within the PPTC layer material system, resulting in better conductivity and more uniform heat transfer. Furthermore, it exhibits good bending resistance, maintaining excellent conductivity even after repeated bending. The mass fraction of the coupling agent can be, but is not limited to, 1%, 2%, 4%, 6%, 8%, or 10%. The coupling agent is selected from silane coupling agents, titanate coupling agents, aluminate coupling agents, or zirconate coupling agents. Specifically, the coupling agent is selected from silane coupling agents. More specifically, the silane coupling agent is selected from at least one of γ-aminopropyltriethoxysilane (KH-550), γ-aminopropyltrimethoxysilane (KH-540), N-(β-aminoethyl)-γ-aminopropylmethyldimethoxysilane (KH-602), γ-mercaptopropyltrimethoxysilane (KH-580), γ-glycidoxypropyltrimethoxysilane (KH-560), γ-glycidoxypropyltriethoxysilane (KH-560E), vinyltriethoxysilane (A-151), vinyltrimethoxysilane (A-171), vinyltri(β-methoxyethoxy)silane (A-172), and γ-methacryloyloxypropyltrimethoxysilane (KH-570). Silane coupling agents can effectively improve the dispersibility of conductive fillers in polymer systems and enhance their bonding force with the polymer, thereby effectively improving conductivity and mechanical properties. It should be noted that the substances listed above in the PPTC layer of this invention can be obtained commercially available materials or prepared using conventional methods in the art.

[0034] The thickness of the encapsulation layer 13 is 50 μm to 700 μm, preferably 50 μm to 500 μm, more preferably 50 μm to 400 μm. As an example, the thickness of the encapsulation layer 13 may be, but is not limited to, 50 μm, 100 μm, 200 μm, 300 μm, 400 μm, 500 μm, 600 μm, or 700 μm. The insulating polymer slurry includes an insulating polymer and a curing agent. The insulating polymer is selected from polydimethylsiloxane, polyvinylidene fluoride, polyethylene naphthalate, polyethersulfone, thermoplastic polyurethane, or polyethylene. Preferably, the insulating polymer is selected from polydimethylsiloxane. Polydimethylsiloxane has hydrophobic, chemically inert, and elastomer properties, which can form a dense protective film to isolate corrosive media and buffer mechanical damage. It also withstands a wide temperature range, thereby extending the service life of the flexible PPTC thermistor. Furthermore, its uniform thermal conductivity and tight adhesion can quickly dissipate local hot spots in the thermistor, ensuring uniform temperature distribution in the composite film and preventing premature protection in some areas while other areas are underprotected. The curing agent is a compound compatible with each insulating polymer, sufficient to cure the insulating polymer slurry; details are not provided here. The mass ratio of insulating polymer to curing agent can be 10:0.5 to 1.5. As an example, the mass ratio of insulating polymer to curing agent can be, but is not limited to, 10:0.5, 10:0.8, 10:1, 10:1.2, 10:1.4, or 10:1.5. It should be noted that the insulating polymer slurry described in the encapsulation layer of this invention can be obtained from commercially available sources or prepared using conventional methods in the art, such as mixing the insulating polymer and curing agent uniformly in a solvent.

[0035] The present invention discloses a method for preparing a flexible PPTC thermistor composite film with secondary protection function, comprising the following steps: (1) Provide a base layer; mix the conductive particles, polymer, coupling agent and aqueous polymer in a mixer to obtain a resistive slurry; provide an insulating polymer slurry; (2) The resistive paste is coated onto one surface of the substrate and dried to form a PPTC layer; (3) Coating the surface of the PPTC layer with an insulating polymer paste and drying it to form an encapsulation layer, thereby obtaining a flexible PPTC thermistor composite film.

[0036] Specifically, in step (1), the mixing time is 1h to 12h, preferably 1h to 8h, more preferably 1h to 6h. As an example, the mixing time can be, but is not limited to, 1h, 2h, 4h, 6h, 8h, 10h, or 12h; the solid content of the resistive slurry is less than 90%.

[0037] In step (2), the coating is applied by scraping with a doctor blade. The drying temperature is room temperature, and the drying time is 24h to 360h, preferably 24 to 168h, more preferably 24 to 120h. As an example, the drying time can be, but is not limited to, 24h, 48h, 72h, 100h, 120h, 150h, 180h, 200h, 250h, 300h, 350h, or 360h.

[0038] In step (3), the coating is applied by scraping with a scraper. The drying temperature is room temperature, and the drying time is 24h to 360h, preferably 24 to 168h, more preferably 24 to 120h. As an example, the drying time can be, but is not limited to, 24h, 48h, 72h, 100h, 120h, 150h, 180h, 200h, 250h, 300h, 350h, or 360h.

[0039] The composite film of the present invention will be further described below with reference to specific embodiments.

[0040] In the embodiments and comparative examples of this invention, the polyimide film was 0.05 mm thick; the waterborne polyurethane was Yoshida waterborne polyurethane emulsion F0402 / 1926; the polypropylene was Lyon Basel RP348N; the polyamide was PA66 Asahi Kasei FR370; and the polydimethylsiloxane slurry was Dow Corning DC184PDMS184 potting compound. All other raw materials not described herein were obtained from commercially available sources or prepared using conventional methods in the art.

[0041] Example 1 This embodiment provides a flexible PPTC thermistor composite film with secondary protection function, comprising a substrate layer, a PPTC layer, and an encapsulation layer stacked together. The substrate layer is a polyimide film, the encapsulation layer is made of polydimethylsiloxane slurry, and the PPTC layer is prepared from components comprising the following by mass: 5g carbon black, 1.5g aqueous polyurethane, 0.6g polypropylene, and 0.3g γ-aminopropyltriethoxysilane. The thickness of the substrate layer is 50μm, the thickness of the PPTC layer is 100μm, and the thickness of the encapsulation layer is 300μm. The particle size Dv50 of the carbon black is 350nm, and the particle size Dv50 of the polypropylene is 400μm.

[0042] The preparation method of this flexible PPTC thermistor composite film with secondary protection function includes the following steps: (1) Provide a polyimide film with a thickness of 1000 μm; place 5.0 g of carbon black powder with a particle size of 350 nm and 0.6 g of polypropylene powder with a particle size of 400 μm and 0.3 g of γ-aminopropyltriethoxysilane in 1.5 g of waterborne polyurethane, put them into a new type of mixer and stir and mix for 3 h to obtain a resistive slurry, and take it out for use; provide polydimethylsiloxane slurry; (2) A resistive paste is coated on one surface of a polyimide film using a blade coating process and dried at room temperature for 72 hours to form a PPTC layer with a thickness of 100 μm. (3) Polydimethylsiloxane slurry was coated on the surface of the PPTC layer using a scraper coating process and dried at room temperature for 72 hours to form an encapsulation layer with a thickness of 300 μm, thus obtaining a flexible PPTC thermistor composite film.

[0043] Figure 2 The RT curve of the flexible PPTC thermistor composite film prepared in Example 1 is shown below. Figure 3 The ρ-T curve of the flexible PPTC thermistor composite film prepared in Example 1 is shown below. Figure 2 and Figure 3 It is known that the flexible PPTC thermistor composite film prepared by the present invention has excellent PTC effect, and the resistance can change by 5 orders of magnitude. It also has a secondary transition, which provides secondary protection for the thermistor. The safety performance during use is higher, and it can generate protection at low temperature, avoiding the risk of fire and burns.

[0044] Figure 4 This is a scanning electron microscope (SEM) image of a cross-section of the flexible PPTC thermistor composite film prepared in Example 1. As shown in the image, the surface of the carbide particles is coated with a polymer, and the polymer coating is uniform with small spacing between the conductive particles, indicating good dispersion, which is crucial for the electrical performance of the PPTC thermistor layer. When the temperature rises, the polymer particles expand, leading to an increase in the spacing between the conductive particles, breakage of the conductive network, and a sharp increase in resistance, thereby achieving overcurrent protection for the circuit.

[0045] Example 2 The difference between this embodiment and Embodiment 1 is that the polymer used is low-density polyethylene, while all other aspects are the same as in Embodiment 1.

[0046] Example 3 The difference between this embodiment and Embodiment 1 is that the polymer used is polyamide, while the rest is the same as in Embodiment 1.

[0047] Example 4 The difference between this embodiment and Embodiment 1 is that the polymer used is polystyrene, while all other aspects are the same as in Embodiment 1.

[0048] Example 5 The difference between this embodiment and Embodiment 1 is that the aqueous polymer is polyvinyl alcohol, while the rest are the same as in Embodiment 1.

[0049] Example 6 The difference between this embodiment and Embodiment 1 is that the aqueous polymer is polyvinylpyrrolidone, while the rest are the same as in Embodiment 1.

[0050] Example 7 The difference between this embodiment and Embodiment 1 is that the conductive particles are silver, while the rest are the same as in Embodiment 1.

[0051] Example 8 The difference between this embodiment and Embodiment 1 is that the base layer is a polyvinylidene fluoride film, while the rest are the same as in Embodiment 1.

[0052] Example 9 The difference between this embodiment and Embodiment 1 is that the base layer is a polyethylene terephthalate film, while the rest are the same as in Embodiment 1.

[0053] Example 10 The difference between this embodiment and Embodiment 1 is that the base layer is a thermoplastic polyurethane film, while the rest are the same as in Embodiment 1.

[0054] Example 11 The difference between this embodiment and Embodiment 1 is that the encapsulation layer is made of thermoplastic polyurethane slurry, while the rest is the same as in Embodiment 1.

[0055] Example 12 The difference between this embodiment and Embodiment 1 is that the encapsulation layer is made of polyvinylidene fluoride slurry, while the rest is the same as in Embodiment 1.

[0056] Example 13 The difference between this embodiment and Embodiment 1 is that the thickness of the PPTC layer is 200 μm, while the rest are the same as in Embodiment 1.

[0057] Example 14 The difference between this embodiment and Embodiment 1 is that the thickness of the PPTC layer is 400 μm, while the rest are the same as in Embodiment 1.

[0058] Example 15 The difference between this embodiment and Embodiment 1 is that the thickness of the PPTC layer is 500 μm, while the rest are the same as in Embodiment 1.

[0059] Example 16 The difference between this embodiment and Embodiment 1 is that the thickness of the PPTC layer is 700 μm, while the rest are the same as in Embodiment 1.

[0060] Example 17 The difference between this embodiment and Embodiment 1 is that the thickness of the PPTC layer is 300 μm and the carbon black content is 50% (that is, the mass of carbon black is 2.4 g), while the rest are the same as in Embodiment 1.

[0061] Example 18 The difference between this embodiment and Embodiment 1 is that the thickness of the PPTC layer is 300 μm and the carbon black content is 60% (3.6 g by mass), while the rest are the same as in Embodiment 1.

[0062] Example 19 The difference between this embodiment and Embodiment 1 is that the thickness of the PPTC layer is 300 μm and the carbon black content is 70% (5.6 g by mass), while the rest are the same as in Embodiment 1.

[0063] Example 20 The difference between this embodiment and Embodiment 1 is that the thickness of the PPTC layer is 300 μm and the carbon black content is 80% (9.6 g by mass), while the rest are the same as in Embodiment 1.

[0064] Example 21 The difference between this embodiment and Example 1 is that the thickness of the PPTC layer is 300 μm, the carbon black content is 80% (8.84 g by mass), and the coupling agent content is 1% (0.111 g by mass). All other aspects are the same as in Example 1.

[0065] Example 22 The difference between this embodiment and Example 1 is that the thickness of the PPTC layer is 300 μm, the carbon black content is 80% (9.882 g by mass), and the coupling agent content is 3% (0.371 g by mass). All other aspects are the same as in Example 1.

[0066] Comparative Example 1 The difference between this comparative example and Example 1 is that the PPTC layer does not contain a polymer, while the rest is the same as in Example 1.

[0067] Comparative Example 2 The difference between this comparative example and Example 1 is that it does not contain an encapsulation layer; otherwise, it is the same as Example 1.

[0068] The flexible PPTC thermistor composite films prepared in Examples 1-22 and Comparative Examples 1-2 were subjected to temperature and resistance magnitude tests when resistance changes abruptly, bending resistance tests, and temperature difference tests in various regions. The test methods are as follows, and the test results are shown in Table 1.

[0069] Initial resistance value: obtained using an RT tester.

[0070] Bending resistance test: The bending resistance performance was tested using a YN-MIT135G bending resistance tester with a bending radius of 1mm and 10,000 test cycles. The coating was observed to see if there were any irreversible creases, peeling, or cracks. After the bending resistance test, the resistance value was tested using the aforementioned method.

[0071] Resistance change temperature and resistance magnitude: The composite films prepared in the examples and comparative examples were placed in an oven, and the temperature at which the thermistor undergoes a sudden change was tested using an RT tester.

[0072] Temperature difference: obtained using an infrared thermal imager.

[0073] Table 1 Performance test results of the examples and comparative examples

[0074] As shown in Table 1, the composite films prepared in Examples 1-22 of this invention are heated uniformly overall with small temperature differences in different regions. They can also undergo a single resistance change at a lower temperature and a second resistance change at a higher temperature, providing two levels of protection. This effectively avoids the limitations of single-point temperature protection, uneven temperature distribution, and local overheating of traditional thermistors. It is significantly superior to the PPTC thermistor layer without doped polymer powder (Comparative Example 1) or the flexible thermistor device without encapsulation protection layer (Comparative Example 2).

[0075] Further, as can be seen from Examples 1 and 2-7, when the waterborne polymer is waterborne polyurethane, the polymer is polypropylene or polyamide, and the conductive particles are carbon-based conductive particles, it has better bending resistance, and the resistance after bending is less than the initial resistance, indicating that it still has good conductivity after multiple bends.

[0076] Further comparison of Examples 1 and 8-10 shows that using polyimide film as the base layer of the present invention results in a relatively small increase in resistance after multiple bending, indicating superior conductivity. Comparison of Examples 1 and 11-12 shows that using polydimethylsiloxane as the encapsulation layer material results in a smaller temperature difference and more uniform temperature distribution in the composite film.

[0077] Further, as shown in Examples 1 and 13-16, as the thickness of the PPTC layer increases, the mechanical properties of the composite film decrease, i.e., the bending resistance decreases, the surface coating peels off, and the initial resistance decreases relatively. As shown in Examples 17-20, as the carbon black content increases, the PTC effect decreases relatively, and the initial resistance decreases. At the same time, due to the reduction in polymer content, the coating is more prone to peeling off.

[0078] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, it is not limited to those listed in the embodiments. Those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A flexible PPTC thermistor composite film with secondary protection function, characterized in that, The device comprises a substrate layer, a PPTC layer, and an encapsulation layer stacked together. The substrate layer is a flexible polymer film, the encapsulation layer is made of an insulating polymer slurry, and the PPTC layer is made of components comprising the following mass fractions: 50%~90% conductive particles, 8%~45% aqueous polymer, 1%~10% high molecular weight polymer, and 1%~10% coupling agent. The aqueous polymer is selected from at least one of aqueous polyurethane, polyvinyl alcohol, and polyvinylpyrrolidone, and the high molecular weight polymer is selected from at least one of low-density polyethylene, linear low-density polyethylene, high-density polyethylene, polypropylene, polystyrene, polytetrafluoroethylene, polyoxymethylene, and polyamide.

2. The flexible PPTC thermistor composite film with secondary protection function according to claim 1, characterized in that, The aqueous polymer is selected from aqueous polyurethane, and the polymer is selected from polyamide or polypropylene.

3. The flexible PPTC thermistor composite film with secondary protection function according to claim 1, characterized in that, The flexible polymer film is selected from polyimide film, polyvinylidene fluoride film, thermoplastic polyurethane film, polyethylene terephthalate film, polyethylene naphthalate film, polycarbonate film, polyetheretherketone film, polypropylene film, or polystyrene film. The insulating polymer slurry includes an insulating polymer and a curing agent. The insulating polymer is selected from polydimethylsiloxane, polyvinylidene fluoride, polyethylene naphthalate, polyethersulfone, thermoplastic polyurethane, or polyethylene.

4. The flexible PPTC thermistor composite film with secondary protection function according to claim 3, characterized in that, The flexible polymer film is selected from polyimide film, and the insulating polymer is selected from polydimethylsiloxane.

5. The flexible PPTC thermistor composite film with secondary protection function according to claim 1, characterized in that, The conductive particles are selected from at least one of metallic conductive particles and carbon-based conductive particles, and the coupling agent is selected from silane coupling agents, titanate coupling agents, aluminate coupling agents, or zirconate coupling agents.

6. The flexible PPTC thermistor composite film with secondary protection function according to claim 5, characterized in that, The conductive particles are selected from carbon-based conductive particles, and the coupling agent is selected from silane coupling agents.

7. The flexible PPTC thermistor composite film with secondary protection function according to claim 1, characterized in that, The conductive particles have a particle size Dv50 of 5 nm to 300 μm, and the polymer has a particle size Dv50 of 10 μm to 500 μm.

8. The flexible PPTC thermistor composite film with secondary protection function according to claim 1, characterized in that, The thickness of the substrate layer is 50μm~2000μm, the thickness of the PPTC layer is 50μm~1000μm, and the thickness of the encapsulation layer is 50μm~700μm.

9. The method for preparing a flexible PPTC thermistor composite film with secondary protection function according to any one of claims 1 to 8, characterized in that, Including the following steps: (1) Provide a base layer; mix the conductive particles, the polymer, the coupling agent and the aqueous polymer in a mixer in the specified amounts to obtain a resistive slurry; provide an insulating polymer slurry; (2) The resistive paste is coated onto one surface of the substrate and dried to form the PPTC layer; (3) The insulating polymer slurry is coated on the surface of the PPTC layer and dried to form an encapsulation layer, thereby obtaining a flexible PPTC thermistor composite film.

10. The method for preparing a flexible PPTC thermistor composite film with secondary protection function according to claim 9, characterized in that, The coating is applied by scraping with a scraper.

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