An ultrahigh square resistance flexible base self-limiting temperature PTC carbon paste, a preparation method and application thereof

Abstract

This invention discloses an ultra-high sheet resistance flexible substrate self-regulating PTC carbon paste, its preparation method, and its applications, belonging to the technical field of electronic pastes and thick-film heating elements. The carbon paste, by weight, comprises: 47-53 parts of polymer resin, 4-10 parts of conductive filler, 35-40 parts of organic solvent, and 1-3 parts of additives. This invention achieves megaohm-level ultra-high sheet resistance (>1 MΩ / □) by controlling the proportion of conductive filler in the resin matrix to form a sparse and stable conductive network, while also exhibiting a strong PTC effect and a Curie temperature of 85-95℃. It can be directly used for mains-powered drive to achieve self-regulating temperature function. This carbon paste can be cured at low temperatures, is suitable for screen printing, has excellent adhesion to flexible substrates such as PI or PET, and is low in cost, meeting the special application requirements of high-pressure micro-heating and precision temperature control.

Description

Technical Field

[0001] This invention belongs to the technical field of electronic pastes and thick film heating elements, specifically relating to an ultra-high sheet resistance flexible substrate self-limiting temperature PTC carbon paste suitable for flexible substrates (such as PI, PET, PEN, etc.), its preparation method and application. Background Technology

[0002] With the deep integration of printed electronics and smart materials, electrothermal and circuit protection components are developing towards greater functionality, refinement, and high reliability. Carbon-based electronic pastes based on the positive temperature coefficient (PTC) effect, due to their advantages such as self-limiting heating, overcurrent protection, and printability and flexibility, are showing broad application prospects in fields such as thermal management of new energy vehicles, smart wearable devices, dehumidification and heating of 5G base stations, and flexible circuit protection.

[0003] Existing research on PTC carbon paste technology mainly focuses on increasing heating power and reducing driving voltage. To achieve higher heat output per unit area, the mainstream technical approach in the industry is to reduce the sheet resistance of the paste, typically setting the target sheet resistance in the range of several ohms to tens of kiloohms per square (Ω / □ - kΩ / □). By increasing the filling amount of conductive carbon black, carbon nanotubes, or graphene, a dense conductive network is constructed to achieve rapid low-temperature start-up and high-power output.

[0004] However, this technical approach that pursues low sheet resistance has the following inherent limitations: 1) Incomplete application scenario coverage. Many special operating conditions do not require high-intensity heating, but rather low-temperature insulation, precise temperature control, or overcurrent protection under high voltage. For example, in outdoor precision optical instruments, drone battery insulation, or dehumidification and anti-condensation scenarios for high-voltage power modules, the heating element needs to generate only milliwatt-level power consumption at hundreds of volts to achieve safe, long-lasting, and gentle heating. This requires the heating paste to have ultra-high sheet resistance characteristics (typically >1MΩ / □) to limit the current to the microampere level at the same voltage. Existing low sheet resistance pastes in such applications either overheat and burn out due to excessive power density or require complex pulse width modulation (PWM) circuitry for voltage reduction control, increasing system complexity and cost.

[0005] 2) The contradiction between PTC effect and high resistance. In the ultra-high sheet resistance region (>1 MΩ / □), the content of conductive filler is usually close to or below the percolation threshold. In this region, the conductivity mechanism changes from ohmic conduction to tunneling effect or field emission, and the material is extremely sensitive to temperature, electric field, and mechanical strain. While this provides a physical basis for obtaining a strong PTC effect, it also brings serious problems with process reproducibility and stability. Existing technologies struggle to achieve a uniform, stable conductive network with high PTC strength (i.e., a large rate of resistance change with temperature) at such high sheet resistance, resulting in severe resistance drift and loss of PTC characteristics after energized cycling or environmental aging.

[0006] 3) Blind spots in formulation design. Current research on electronic pastes mainly focuses on areas such as solar cell silver paste and low sheet resistance conductive adhesives. There are few reports on dedicated pastes with sheet resistance >1MΩ / □ that also possess flexibility and PTC characteristics. Conventional methods for achieving high resistance (such as significantly reducing the proportion of conductive phase) often lead to poor film continuity, decreased adhesion, and the disappearance of the PTC effect (exhibiting a negative temperature coefficient NTC), which cannot meet the stringent requirements of flexible substrates for flexibility and reliability.

[0007] Therefore, developing a novel carbon-based paste that combines ultra-high sheet resistance (>1MΩ / □), strong and stable PTC effect, and good flexibility and substrate adhesion to fill the gap in existing technologies for precision micro-heating and high-voltage insulation protection has become a key technical problem that urgently needs to be solved in this field. Summary of the Invention

[0008] The technical problem to be solved by this invention is to address the issues of insufficient sheet resistance, low withstand voltage, and inability to be directly used in high-voltage drives of mains power or new energy vehicles by providing a flexible self-limiting temperature PTC carbon paste with sheet resistance >1MΩ / □ (megohm level), low-temperature curing capability, and self-limiting temperature characteristics, as well as its preparation method and application. This invention meets the requirements of circuit design for high sheet resistance pastes in special application scenarios. It is an electronic paste with high PTC strength, low cost, low-temperature curing capability, and true self-limiting temperature capability.

[0009] To solve the above-mentioned technical problems, the technical solution proposed by this invention is as follows: A high sheet resistance flexible self-regulating PTC carbon paste, comprising the following components by weight: Polymer resin: 47-53 parts; Conductive filler: 4-10 parts; Organic solvent: 35-40 parts; Additives: 1-3 parts; The polymer resin is one or more of polyimide resin, thermoplastic polyurethane resin, and vinylidene fluoride-hexafluoropropylene copolymer; the conductive filler includes conductive carbon black and carbon nanotubes, wherein the carbon nanotubes are 1 to 2 parts by weight and the conductive carbon black is 3 to 8 parts by weight; the organic solvent includes divalent ester, ethylene glycol monobutyl ether acetate, and N-methylpyrrolidone; and the additives include dispersants, adhesion promoters, and defoamers.

[0010] Preferably, in the above-mentioned ultra-high sheet resistance flexible self-regulating PTC carbon paste, the thermoplastic polyurethane resin is a non-crystalline polyurethane.

[0011] Preferably, the carbon nanotubes are multi-walled carbon nanotube powder with a diameter of 10–25 nm.

[0012] Preferably, the mass ratio of divalent ester, ethylene glycol monobutyl ether acetate and N-methylpyrrolidone in the organic solvent is (30-60):(30-60):(10-40).

[0013] Preferably, the mass ratio of dispersant, adhesion promoter and defoamer in the additive is (40-60):(40-50):(10-30).

[0014] Preferably, the dispersant is BYK-2155; the adhesion promoter is a phosphate ester; and the defoamer is Deqian 4500.

[0015] Preferably, the ultra-high sheet resistance flexible self-regulating PTC carbon paste has a sheet resistance > 1 MΩ / □, a Curie temperature of 85℃~95℃, and a PTC strength ≥ 1, wherein the PTC strength is defined as Lg(R 95℃ / R 20℃ ), where R 95℃ R is the resistance value at 95℃. 20℃ The resistance value is given at 20℃.

[0016] Based on a general inventive concept, this invention also provides a method for preparing ultra-high sheet resistance flexible-based self-regulating temperature PTC carbon paste, comprising the following steps: (1) The polymer resin and organic solvent are mixed and heated under stirring conditions to obtain a homogeneous resin solution; (2) Add conductive filler and additives to the resin solution, disperse and grind until fineness ≤5μm to obtain the ultra-high sheet resistance flexible self-limiting temperature PTC carbon paste.

[0017] In the above preparation method, preferably, the dispersion is carried out by stirring at a speed of 500-1500 rpm; the grinding is carried out by a three-roll mill for mixing.

[0018] Based on a general inventive concept, this invention also provides an application of ultra-high sheet resistance flexible-based self-regulating PTC carbon paste. The ultra-high sheet resistance flexible-based self-regulating PTC carbon paste is used to screen print and cure on the surface of a flexible substrate, wherein the flexible substrate is PI, PET, or PEN, resulting in a heating layer with a dry film thickness of 10μm-15μm; the curing temperature is 120-135 ℃, and the curing time is 10-25 min. The obtained ultra-high sheet resistance flexible-based self-regulating PTC product is used in flexible heaters, industrial insulation jackets, or building underfloor heating, etc.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. The ultra-high sheet resistance flexible self-regulating PTC carbon paste of the present invention has megaohm-level ultra-high sheet resistance (>1MΩ / □), stable and strong self-regulating characteristics, excellent process adaptability and flexibility, meets the requirements of high voltage direct drive, and is suitable for high voltage application scenarios such as flexible heaters, industrial insulation sleeves, and building underfloor heating films that are directly driven by mains power or higher voltage.

[0020] 2. This invention achieves a breakthrough in sheet resistance >1MΩ / □ by precisely controlling the ratio of high-structure carbon black and carbon nanotubes in heat-resistant resin to form a sparse but stable conductive network, filling a market gap. This characteristic allows the slurry to be directly used for mains power or higher voltage drive without the need for complex step-down circuits, meeting the special application requirements of high-voltage direct drive.

[0021] 3. This invention utilizes the high temperature sensitivity of a conductive network close to the percolation threshold to achieve a strong PTC effect, with PTC intensity (Lg(R)) increasing significantly. 95℃ / R 20℃ With a resistance of ≥1, the Curie temperature is between 85℃ and 95℃. After reaching the Curie temperature, the resistance increases by more than 10 times, automatically limiting the current and power, thus achieving an intrinsically safe self-limiting temperature function and avoiding the risk of overheating and burning.

[0022] 4. This invention adopts a low-temperature (120℃~135℃) curing process, which has low energy consumption and good compatibility with various flexible substrates such as polyimide (PI) and polyester (PET), with adhesion reaching grade 5B; the rheological properties of the slurry have been optimized, making it suitable for large-scale screen printing; the cured film layer has good flexibility and can meet the requirements of complex curved surface bonding and shock resistance.

[0023] 5. This invention uses pure carbon-based resistive paste, eliminating the need for precious metals such as silver and gold, thus reducing raw material costs by more than 60% compared to metal-based pastes; at the same time, the paste formulation complies with RoHS and REACH environmental standards. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 This is a schematic diagram of the silver paste wire circuit used for testing in an embodiment of the present invention; Figure 2 This is a schematic diagram of a carbon paste resistor circuit used for testing in an embodiment of the present invention; Figure 3 This is a schematic diagram of a PTC heating film composed of a silver paste wire circuit and a carbon paste resistor circuit in an embodiment of the present invention. Figure 4 These are the resistance values ​​and fitting curves of the slurry obtained in Example 1 of the present invention at various temperatures; Figure 5 These are the temperature curves of the slurry obtained in Example 1 of this invention under different voltages; Figure 6 These are the resistance values ​​and fitting curves of the slurry obtained in Example 2 of the present invention at various temperatures; Figure 7 This is the temperature curve of the slurry obtained in Example 2 of the present invention under different voltages; Figure 8 These are the resistance values ​​and fitting curves of the slurry obtained in Example 3 of the present invention at various temperatures; Figure 9 This is the temperature curve of the slurry obtained in Example 3 of the present invention under different voltages. Detailed Implementation

[0026] To facilitate understanding of the present invention, the present invention will be described more fully and in detail below with reference to the accompanying drawings and preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0027] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0028] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0029] The performance testing method is as follows: Sheet resistance: The sheet resistance of the cured film was tested using a four-probe tester.

[0030] Curie temperature: Place the test sample in a constant temperature chamber and connect it with external wires. Set the temperature to a fixed value and wait for the temperature to remain constant for 20 minutes before testing the resistance value of the sample. Test the resistance value at different temperatures in sequence. The temperature point at which the resistance value changes sharply is the Curie temperature.

[0031] PTC strength: Calculate the resistance value (R) at 95℃ 95℃ ) and resistance value at 20℃ (R) 20℃ The commonly used logarithm Lg(R) 95℃ / R 20℃ ).

[0032] Example 1 This embodiment provides an ultra-high sheet resistance flexible self-regulating PTC carbon paste, the components of which, by weight, are: Thermoplastic polyurethane (non-crystalline): 52.5 parts; Conductive carbon black: 5 parts; Carbon nanotubes (multi-walled, 15 nm in diameter): 1.5 parts; Solvent (DBE:BCA:NMP=50:30:20): 38 parts; Dispersant (BYK-2155): 1.5 parts; Defoamer (Deqian 4500): 0.5 parts; Adhesion promoter (phosphate ester): 1.0 part; The preparation method of ultra-high sheet resistance flexible substrate self-regulating PTC carbon paste in this embodiment is as follows: (1) Mix thermoplastic polyurethane (non-crystalline) with a solvent and stir at 50°C to dissolve it, thereby obtaining a resin mixture solution; (2) Add conductive carbon black, carbon nanotubes, dispersant, defoamer and adhesion promoter to the above resin mixture solution. First disperse it with a high-speed disperser at a stirring rate of 800 rpm. Then transfer the dispersed slurry to a three-roll mill for grinding until the slurry fineness is ≤5μm.

[0033] The obtained PTC carbon paste is screen-printed onto a polyimide (PI) film using a 200-mesh screen and cured at 130°C for 20 minutes to obtain a heating layer with a dry film thickness of 10μm-15μm, which is the PTC heating film.

[0034] The diagrams of the silver paste wire circuit, carbon paste resistor circuit, and PTC heating film composed of the silver paste wire circuit and carbon paste resistor circuit used for testing are shown below. Figures 1 to 3 .

[0035] The performance of the slurry and the cured film was tested, and the results are shown in Table 1. Figure 4 and Figure 5 : Viscosity: 40.13 Pa·s (Brookfield DV2T SCE-14# rotor, 10 rpm, 25℃); Solid content (w%): 59%; Sheet resistance: 1.57 MΩ / □; PTC strength (Lg(R) 95℃ / R 20℃ = 1.429; Curie temperature: 89.4℃; Adhesion (100-cross test): 5B.

[0036]

[0037] Example 2 This embodiment provides an ultra-high sheet resistance flexible self-regulating PTC carbon paste, the components of which, by weight, are: Polyimide resin: 47 parts; Conductive carbon black: 8 parts; Carbon nanotubes (multi-walled, 15 nm in diameter): 2 parts; Solvent (DBE:BCA:NMP=30:30:40): 40 parts; Dispersant (BYK-2155): 1.5 parts; Defoamer (Deqian 4500): 0.5 parts; Adhesion promoter (phosphate ester): 1 part.

[0038] The preparation method of ultra-high sheet resistance flexible substrate self-regulating PTC carbon paste in this embodiment is as follows: (1) The polyimide resin and solvent are heated to 85°C and stirred to obtain a resin solution; (2) Add conductive carbon black, carbon nanotubes, dispersant, defoamer and adhesion promoter to the above resin solution. First disperse it with a high-speed disperser at a stirring rate of 800 rpm. Then transfer the dispersed slurry to a three-roll mill for grinding until the fineness of the slurry is ≤5 μm.

[0039] The testing method is the same as in Example 1, and the performance test results are shown in Table 2. Figure 6 and Figure 7 : Viscosity: 45.88 Pa·s; Solid content (w%): 57%; Sheet resistance: 1.03 MΩ / □; PTC strength (Lg(R) 95℃ / R 20℃ = 1.196; Curie temperature: 91.3℃; Adhesion (100-cross test): 5B.

[0040]

[0041] Example 3 This embodiment provides an ultra-high sheet resistance flexible self-regulating PTC carbon paste, the components of which, by weight, are: Vinylidene fluoride-hexafluoropropylene copolymer resin: 53 parts; Conductive carbon black: 3 parts; Carbon nanotubes (multi-walled, 15 nm in diameter): 1 part; Solvent (DBE:BCA:NMP=40:40:20): 40 parts; Dispersant (BYK-2155): 1.5 parts; Defoamer (Deqian 4500): 0.5 parts; Adhesion promoter (phosphate ester): 1 part.

[0042] The preparation and testing methods of the ultra-high sheet resistance flexible substrate self-regulating PTC carbon paste in this embodiment are the same as those in Example 2. The performance test results are shown in Table 3. Figure 8 and Figure 9 : Viscosity: 30.5 Pa·s; Solid content: 57%; Sheet resistance: 13.84 MΩ / □; PTC strength (Lg(R) 95℃ / R 20℃ = 2.235; Curie temperature: 88.4℃; Adhesion: 5B.

[0043]

[0044] Comparative Example 1 This comparative example provides a conventional low sheet resistance PTC carbon paste, the components of which, by weight parts, are: Siloxane-modified epoxy resin (molecular weight approximately 80,000, epoxy value 0.25 eq / 100g): 30 parts; Conductive carbon black: 20 parts; Solvent (DBE:BCA:NMP=50:40:10): 45 parts; Dispersant: 1.5 parts; Defoamer: 0.5 parts; Adhesion promoter (phosphate ester): 1 part.

[0045] Blocked isocyanate crosslinking agent (BI7963): 2 parts.

[0046] The preparation and testing methods are the same as in Example 1.

[0047] The performance test results are as follows: Viscosity: 35.3 Pa·s; Solid content: 50%; Sheet resistance: 300Ω / □; PTC strength (Lg(R) 95℃ / R 20℃ =0.1; Curie temperature: None; Adhesion: 5B.

[0048] The sheet resistance of the comparative sample is far lower than that required by this invention, and the PTC effect is very weak. When the temperature exceeds 100°C, the NTC effect even appears, making it completely impossible to achieve effective self-limiting temperature.

[0049] As can be seen from Examples 1-3 and Comparative Example 1, this invention successfully prepared megaohm-level ultra-high sheet resistance (>1MΩ / □) and strong PTC effect (intensity Lg(R) by controlling the total amount of conductive filler within a specific range and combining it with a specific ratio of conductive carbon black and carbon nanotubes. 95 ℃ / R 20 The carbon paste has a temperature ≥1℃ and a Curie temperature between 85℃ and 95℃. However, in Comparative Example 1, the conductive filler content is too high, the sheet resistance is low, and the PTC effect is almost non-existent, which cannot meet the requirements of high-pressure self-regulating temperature applications.

Claims

1. A self-regulating temperature-limiting PTC carbon paste with ultra-high sheet resistance flexible substrate, characterized in that, By weight, it includes the following components: Polymer resin: 47-53 parts; Conductive filler: 4-10 parts; Organic solvent: 35-40 parts; Additives: 1-3 parts; The polymer resin is one or more of polyimide resin, thermoplastic polyurethane resin, and vinylidene fluoride-hexafluoropropylene copolymer; the conductive filler is conductive carbon black and carbon nanotubes, wherein the carbon nanotubes are 1 to 2 parts by weight and the conductive carbon black is 3 to 8 parts by weight; the organic solvent includes divalent ester, ethylene glycol monobutyl ether acetate, and N-methylpyrrolidone; and the additives include dispersants, adhesion promoters, and defoamers.

2. The ultra-high sheet resistance flexible substrate self-regulating temperature PTC carbon paste according to claim 1, characterized in that, The thermoplastic polyurethane resin is a non-crystalline polyurethane.

3. The ultra-high sheet resistance flexible substrate self-regulating temperature PTC carbon paste according to claim 1, characterized in that, The carbon nanotubes are multi-walled carbon nanotube powders with a diameter of 10–25 nm.

4. The ultra-high sheet resistance flexible substrate self-regulating temperature PTC carbon paste according to claim 1, characterized in that, The mass ratio of divalent ester, ethylene glycol monobutyl ether acetate and N-methylpyrrolidone in the organic solvent is (30-60):(30-60):(10-40).

5. The ultra-high sheet resistance flexible substrate self-regulating PTC carbon paste according to claim 1, characterized in that, The mass ratio of dispersant, adhesion promoter and defoamer in the additive is (40-60):(40-50):(10-30).

6. The ultra-high sheet resistance flexible substrate self-regulating temperature PTC carbon paste according to claim 1, characterized in that, The dispersant is BYK-2155; the adhesion promoter is a phosphate ester; and the defoamer is Deqian 4500.

7. The ultra-high sheet resistance flexible substrate self-regulating PTC carbon paste according to any one of claims 1-6, characterized in that, The ultra-high square resistance flexible base self-limiting temperature PTC carbon paste has a square resistance of >1 MΩ / □, a Curie temperature of 85-95℃, and a PTC strength of ≥1, wherein the PTC strength is defined as Lg(R 95℃ / R 20℃ ), wherein R 95℃ is the resistance value at 95℃, and R 20℃ is the resistance value at 20℃.

8. A method for preparing ultra-high sheet resistance flexible self-regulating temperature PTC carbon paste according to any one of claims 1-7, characterized in that, Includes the following steps: (1) The polymer resin and organic solvent are mixed and heated under stirring conditions to obtain a homogeneous resin solution; (2) Add conductive filler and additives to the resin solution, disperse and grind until fineness ≤5μm to obtain the ultra-high sheet resistance flexible self-limiting temperature PTC carbon paste.

9. The preparation method according to claim 8, characterized in that, The dispersion method is stirring dispersion, with a stirring rate of 500-1500 rpm; the grinding is performed using a three-roll mill for mixing.

10. An application of a self-regulating PTC carbon paste with ultra-high sheet resistance flexible substrate according to any one of claims 1-7, characterized in that, The ultra-high sheet resistance flexible substrate self-limiting temperature PTC carbon paste is screen printed and cured on the surface of a flexible substrate, wherein the flexible substrate is PI, PET or PEN, to obtain a heating layer with a dry film thickness of 10μm-15μm; the curing temperature is 120-135℃ and the curing time is 10-25 min.

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