Preparation method of flexible automatic temperature-limiting conductive material

Abstract

The application discloses a preparation method of a flexible automatic temperature-limiting conductive material and belongs to the technical field of functional material preparation. The application uses EVA as a supporting material, palmitic acid and stearic acid as phase change materials, and graphite as a conductive filler, adjusts the use proportion of each material, and prepares a normal-temperature flexible thermal management material, so that excellent temperature control performance and a lower conversion temperature point are achieved. Meanwhile, the material is combined with an interdigital electrode, the heat sensitivity of a device is improved, the resistance of the composite material is reduced, the use demand under different powers is met by adjusting the pattern of the interdigital electrode, so that the overall response speed of the composite material to temperature is improved, and the content of the conductive filler in the composite material is changed to achieve higher PTC strength and good room temperature resistivity. Under different proportions of components, the composite material has good electrothermal performance, and the temperature control condition is good under different voltages, cyclic pressurization and further pressurization.

Description

TECHNICAL FIELD

[0001] The application relates to a preparation method of a flexible automatic temperature-limiting conductive material and belongs to the technical field of functional material preparation. BACKGROUND

[0002] With the popularity of wearable devices, how to achieve effective thermal management in limited space becomes a challenge in design to ensure the comfort and performance of the device. In the existing thermal control system, a fixed resistor is usually used as a heating element, and the heating power of the resistor needs to be controlled by adjusting the input power of the heating circuit, but since the resistivity of the common resistor is almost constant, the thermal control system must monitor the temperature and current. If the common resistor in the system is replaced by a material that can adjust its resistivity with temperature change, the system can be greatly simplified.

[0003] A positive temperature coefficient (PTC) composite material can exhibit resistance change with temperature rise. The high molecular PTC material is mainly composed of a polymer matrix and a conductive filler, and the matrix material is usually a crystalline and semi-crystalline matrix (PP, HDPE, EVA) and the like. For the high molecular PTC composite material, when the temperature reaches the transition temperature, the crystalline polymer changes in volume due to phase transition, which increases the average distance between the conductive particles, thereby destroying the structure of the conductive network. In the polymer matrix, the distribution of the conductive filler changes with temperature, which eventually leads to a large increase in the resistivity of the PTC material, thereby achieving the effect of breaking the conductive network. The higher the strength of PTC, the more complete the destruction of the conductive network.

[0004] The Curie temperature point is a key parameter of the PTC material, which reveals the threshold temperature of the maximum resistance of the PTC material. Nowadays, the Curie temperature of most high molecular PTC material systems is mainly concentrated in the range of 90-160 DEG C, but such high Curie temperature cannot meet the requirements of room temperature thermal control devices such as household electronic devices, capacity batteries, wearable electronics, etc. For most electronic components, controlling the ambient temperature in the range of 40-60 DEG C can ensure the normal operation of the device.

[0005] Although the phase transition temperature of palmitic acid (PA) and stearic acid (SA) is in this range, the thermal conductivity of palmitic acid (PA) and stearic acid (SA) is low, which limits its use in applications requiring fast thermal response. The stability of the organic acid phase change material is poor at the phase transition temperature, and the volume may change during the phase change, resulting in leakage and reduction of the heat conduction area, causing the problem of increased thermal resistance.

[0006] The existing PTC materials mostly have too high or too low Curie temperature points, low thermal sensitivity, low flexibility of the composite material as a whole, low PTC strength, and low thermal stability as a whole. SUMMARY

[0007] In order to obtain a material with a Curie temperature in the range of 40-60 DEG C, meet the requirements of room temperature thermal control devices of household electronic devices, capacity batteries, wearable electronics, etc., and ensure the normal operation of the devices, the present application uses EVA as a supporting material, palmitic acid (PA) and stearic acid (SA) as phase change materials, to prepare a flexible thermal management material for household electronic appliances, and by adjusting the ratio of PA and SA, excellent temperature control performance and suitable Curie temperature points are achieved. At the same time, the PTC composite material is combined with the interdigital electrode, which greatly improves the thermal sensitivity of the device, reduces the resistance value of the composite material, and improves the overall temperature response speed of the PTC composite material. By changing the content of the conductive filler (GP) in the composite material, a higher PTC strength and a resistance value less than 1000 Ω at room temperature are achieved. Under the proportioning components of the present application, the PTC composite material has good electrothermal performance, and the temperature control is good under different voltages, cyclic pressurization and further pressurization. And during the pressurization process, the temperature is stable and the fluctuation is small.

[0008] The present application provides a kind of PTC composite material, the composite material includes: ethylene-vinyl acetate copolymer matrix material, phase change material, conductive filler, the phase change material is one or more of stearic acid, lauric acid, palmitic acid, pentadecanoic acid, eicosanoic acid, the conductive filler includes one or several of not limited to carbon black, carbon nanotube, graphite powder and graphene;

[0009] In the composite material, the mass fraction of ethylene-vinyl acetate copolymer is 4-30%, preferably 10-20%, and further preferably 20%; the mass fraction of conductive filler is 4-16%, preferably 6-10%; the mass fraction of phase change material is 4-30%, preferably 10-20%, and further preferably 20%. The mass ratio of the added phase change material and ethylene-vinyl acetate copolymer is 0.5-1.5:1, and the other is an organic solvent. The organic solvent includes but is not limited to toluene, tetrahydrofuran, N,N-dimethylformamide.

[0010] In an embodiment of the present application, the phase change material is palmitic acid and stearic acid; the added ratio of palmitic acid and stearic acid is 1-4:1-4.

[0011] In an embodiment of the present application, the mass fraction of VA in the ethylene-vinyl acetate copolymer matrix material is 10-30% in EVA

[0012] In one embodiment of the present application, the organic solvent includes, but is not limited to, toluene, tetrahydrofuran, N,N-dimethylformamide.

[0013] The present application also provides a preparation method of the PTC material, comprising the following steps:

[0014] (1) adding ethylene-vinyl acetate into an organic solvent, oscillating at 1000-2000 rpm for 1-5 min, homogenizing, and then stirring at 600-800 rpm at 25-40℃ for 6-10 h to obtain an EVA solution with a mass fraction of 4-30%, preferably 10-20%, and further preferably 20%;

[0015] (2) adding a phase change material into the EVA solution obtained in step (1), oscillating at 1000-2000 rpm for 1-5 min, homogenizing, and then stirring at 600-800 rpm at 25-40℃ for 6-10 h to obtain an EVA-phase change material mixed solution, wherein the phase change material is added in an amount of 4-30%, preferably 10-20%, and further preferably 20%; the phase change material is one or more of stearic acid, lauric acid, hexadecanoic acid, pentadecanoic acid, and eicosanoic acid; and the mass ratio of the added amount of the phase change material to the ethylene-vinyl acetate copolymer is 0.5-1.5:1;

[0016] (3) adding a conductive filler into the EVA-phase change material mixed solution obtained in step (2), wherein the mass fraction of the conductive filler in the mixed solution is 4-16%, preferably 6-10%, oscillating at 1000-2000 rpm for 1-5 min, stirring at 600-800 rpm at 25-40℃ for 6-10 h, and then performing vacuum dispersion to obtain a PTC composite material; the conductive filler includes, but is not limited to, one or more of carbon black, carbon nanotubes, graphite powder, and graphene.

[0017] In one embodiment of the present application, the mass fraction of VA in the ethylene-vinyl acetate copolymer matrix material is 10-30%.

[0018] In one embodiment of the present application, the conductive filler is spherical graphite GP; preferably, the spherical graphite GP is dried in an oven at 50-90℃ for 20-30 h, and then ball-milled in a ball mill at a rotation speed of 200-500 rpm for 4-8 h to make the grinding and dispersion more uniform.

[0019] In one embodiment of the present application, after the conductive filler is added into the EVA-phase change material mixed solution, oscillation, ultrasonic treatment for 1-10 min, and stirring at 400-800 rpm at 25-40℃ for 4-8 h are performed.

[0020] In an embodiment of the present application, the vacuum dispersion is mixing and dispersing in a vacuum debubbling machine at 1500-3000 rpm for 5-15 min.

[0021] In an embodiment of the present application, the phase change material is hexadecanoic acid and stearic acid; the added ratio of the hexadecanoic acid and stearic acid is 1-4:1-4.

[0022] In an embodiment of the present application, the ethylene-vinyl acetate copolymer matrix material is EVA260 and / or EVA180.

[0023] In an embodiment of the present application, the organic solvent includes but is not limited to toluene, tetrahydrofuran, N,N-dimethylformamide.

[0024] The present application provides a method for improving the response speed of PTC composite material to temperature and / or PTC strength, which comprises using ethylene-vinyl acetate copolymer as a matrix material, using hexadecanoic acid and stearic acid as a phase change material, and using one or more of carbon black, carbon nanotube, graphite powder and graphene as a conductive filler to prepare a PTC composite material.

[0025] The added ratio of the hexadecanoic acid and stearic acid is 1-4:1-4.

[0026] In an embodiment of the present application, the ethylene-vinyl acetate copolymer matrix material is EVA, and the mass fraction of VA in EVA is 10-30%.

[0027] In an embodiment of the present application, the method specifically comprises:

[0028] (1) adding ethylene-vinyl acetate into an organic solvent, oscillating at 1000-2000 rpm for 1-5 min, homogenizing, and then stirring at 600-800 rpm at 25-40℃ for 6-10 h to obtain an EVA solution with a mass fraction of 4-30%, preferably 10-20%, and further preferably 20%;

[0029] (2) adding a phase change material into the EVA solution obtained in step (1), oscillating at 1000-2000 rpm for 1-5 min, homogenizing, and then stirring at 600-800 rpm at 25-40℃ for 6-10 h to obtain an EVA-phase change material mixed solution, wherein the added amount of the phase change material is 4-30%, preferably 10-20%, and further preferably 20%; the phase change material is one or more of stearic acid, lauric acid, hexadecanoic acid, pentadecanoic acid and eicosanoic acid; and the mass ratio of the added amount of the phase change material to the added amount of ethylene-vinyl acetate copolymer is 0.5-1.5:1.

[0030] (3) adding conductive fillers into the EVA-phase change material mixed solution obtained in step (2), the mass fraction of the conductive fillers in the mixed solution is 4-16%, preferably 6-10%, oscillating at 1000-2000 rpm for 1-5 min, stirring at 600-800 rpm for 6-10 h at 25-40℃, and then performing vacuum dispersion to obtain a PTC composite material; the conductive fillers include, but are not limited to, one or more of carbon black, carbon nanotubes, graphite powder and graphene.

[0031] In an embodiment of the present application, the ethylene-vinyl acetate copolymer matrix material is EVA260 and / or EVA180.

[0032] In an embodiment of the present application, the organic solvent includes, but is not limited to, toluene, tetrahydrofuran and N,N-dimethylformamide.

[0033] The present application also provides a flexible automatic temperature-limiting electrothermal device, which is composed of the PTC composite material prepared by the above method, an electrode and a flexible substrate.

[0034] In an embodiment of the present application, the PTC composite material and the electrode are compounded by an additive manufacturing process to form a film.

[0035] In an embodiment of the present application, the additive manufacturing process includes, but is not limited to, one or more of screen printing, coating and spin coating.

[0036] In an embodiment of the present application, the additive manufacturing process further includes inkjet printing, screen printing and intaglio / letterpress printing.

[0037] In an embodiment of the present application, the thickness of the flexible automatic temperature-limiting electrothermal device is 30-200 μm.

[0038] In an embodiment of the present application, after the PTC composite material and the electrode are compounded, the sheet resistance of the dried PTC composite material film is 100-1000 Ω.

[0039] The present application also provides the use of the above PTC composite material or the above preparation method or the above method in the preparation of lithium batteries, self-regulating heating components, temperature sensors, detectors and instruments requiring temperature control.

[0040] The present application also provides the use of the above PTC composite material or the above preparation method or the above method in the preparation of electrodes.

[0041] Advantages

[0042] (1) The present application adjusts the range of the temperature control point by regulating the proportion of the phase change energy storage material, which is (40℃-52℃). The response time of the material system to temperature is about 5s, the overall temperature control accuracy is ±0.4℃, the overall PTC strength is improved by 4.5 orders of magnitude, and it is basically stable after three cold and hot cycles.

[0043] (2) The PTC material prepared by the present application has low percolation threshold, uniform analysis of conductive material, good flexibility, light weight, high voltage breakdown resistance, and can restore to the initial state after pressure, still having the ability to disconnect the circuit.

[0044] (3) The combination between the PTC material prepared by the present application and the silver electrode can greatly improve the thermosensitivity of the device, reduce the resistance of the composite material, and obtain different patterns by adjusting the interdigital spacing of the silver electrode to meet the demand of different power.

[0045] (4) The patterning process used in the present application (such as inkjet printing, screen printing, aerosol printing, etc.) can make the overall heating of the electronic components more uniform and the heat distribution more concentrated, reducing the phenomenon of hot spots on the surface. The process of the present application is simple, the temperature control point meets the normal temperature range of electronic components, and the experimental reproducibility is high, the energy consumption is low, safe, the use of materials is cheap, environmentally friendly, and there is no pollution, which can realize mass production of PTC ink. BRIEF DESCRIPTION OF DRAWINGS

[0046] Figure 1 It is a schematic diagram for the preparation of EVA / PA / SA / GP.

[0047] Figure 2 It is a process schematic diagram of EVA / PA / SA / interdigital electrode pattern, wherein (a) is a schematic diagram of interdigital electrode; (b) is a film layer structure of electrothermal film; (c) is a schematic diagram of electrothermal film.

[0048] Figure 3 The electrothermal performance diagram of the composite material under different proportions in the cycle ladder.

[0049] Figure 4 It is a temperature response diagram of the composite material with different GP contents under different voltages. DETAILED DESCRIPTION

[0050] The ethylene-vinyl acetate (EVA260) involved in the following examples is purchased from the American DuPont Company.

[0051] Example 1: Preparation of EVA / PA / SA / GP composite material

[0052] The specific preparation process is as follows (the preparation schematic diagram of the EVA / PA / SA / GP composite material is shown in Figure 1

[0053] (1) Put 50 nm spherical graphite GP into an oven and dry at 80°C for 24 h, then put the spherical GP into a ball mill at a speed of 400 rpm and mill for 6 h to obtain spherical graphite GP powder with more uniform grinding and dispersion;

[0054] (2) Preparation of EVA / PA / SA / GP composite material:

[0055] After ethylene-vinyl acetate (EVA) is added to toluene, oscillation is performed at 1500 rpm for 1 min, homogenization is performed by a homogenizer (2000 rpm, 10 min), and then magnetic stirring is performed at 40°C and 400 rpm for 6 h to obtain an EVA toluene solution with a mass fraction of 20%;

[0056] After palmitic acid (PA) and stearic acid (SA) are sequentially added to the above EVA toluene solution, oscillation is performed at 1500 rpm for 1 min, homogenization is performed by a homogenizer (2000 rpm, 10 min), and then magnetic stirring is performed at 25°C and 400 rpm for 6 h to obtain an EVA-PA-SA mixed toluene solution;

[0057] Among them, the addition amount of ethylene-vinyl acetate (EVA), palmitic acid (PA) and stearic acid (SA) is shown in Table 1:

[0058] Table 1: Concentration of each substance in the composite material

[0059]

[0060]

[0061] (3) 0.24 g of spherical graphite GP powder obtained in step (1) is added to the EVA-PA-SA mixed toluene solution to make the mass fraction of spherical graphite GP 7%, oscillation is performed at 1500 rpm for 1 min, ultrasonic treatment is performed at 40 KHz for 5 min, and then stirring is performed at 25°C and 600 rpm for 6 h. After stirring, a vacuum demolding and debubbling machine is used for mixing and dispersion at 2000 rpm under vacuum for 10 min to obtain an EVA / PA / SA (5:4:1) / GP-7% composite material, an EVA / PA / SA (5:1:4) / GP-7% composite material, an EVA / PA / SA (5:2:3) / GP-7% composite material, an EVA / PA / SA (5:3:2) / GP-7% composite material, an EVA / PA (1:1) / GP-7% composite material and an EVA / SA (1:1) / GP-7% composite material, respectively.​

[0062] Example 2: Preparation of EVA / PA / SA / GP composite material

[0063] The specific implementation is the same as that in Example 1, except that the mass fraction of GP is 0%, 6%, 8%, 9%, 10%, the mass fraction ratio of EVA / PA / SA is 5:4:1, and other conditions remain unchanged. The specific steps are as follows:

[0064] (1) Put 50 nm spherical graphite GP into an oven and dry at 80°C for 24 h. Then put the spherical GP into a ball mill at a speed of 400 rpm and mill for 6 h to obtain a more uniformly ground and dispersed spherical graphite GP powder;

[0065] (2) Preparation of EVA / PA / SA / GP composite material:

[0066] After adding ethylene-vinyl acetate (EVA) into toluene, oscillate for 1 min using a shaker at 1500 rpm, homogenize using a homogenizer (2000 rpm, 10 min), and then stir at 400 rpm and 40°C for 6 h to obtain an EVA toluene solution with a mass fraction of 20%;

[0067] After adding palmitic acid (PA) and stearic acid (SA) into the above EVA toluene solution, oscillate for 1 min using a shaker at 1500 rpm, homogenize using a homogenizer (2000 rpm, 10 min), and then stir at 400 rpm and 25°C for 6 h to obtain an EVA-PA-SA mixed toluene solution (in which the final mass fraction of palmitic acid (PA) in the solution is 16%, and the final mass fraction of stearic acid (SA) in the solution is 4%);

[0068] Add the spherical graphite GP powder obtained in step (1) to the EVA-PA-SA mixed toluene solution to obtain a mass fraction of 0%, 6%, 8%, 9%, and 10% of spherical graphite GP, respectively. After oscillating at 1500 rpm for 1 min, respectively, ultrasonic treatment at 40 KHz for 5 min, respectively, and stirring at 25°C and 600 rpm for 6 h, respectively, use a vacuum demolding and debubbling machine to mix and disperse at 2000 rpm under vacuum for 10 min, respectively, to obtain:

[0069] EVA / PA / SA (5:4:1) composite material, EVA / PA / SA (5:4:1) / GP-6% composite material, EVA / PA / SA (5:4:1) / GP-8% composite material, EVA / PA / SA (5:4:1) / GP-9% composite material, and EVA / PA / SA (5:4:1) / GP-10% composite material.

[0070] Example 3: Performance Testing of Composite Materials in EVA / PA / SA-GP Electrothermal Films

[0071] 1. Preparation of electrothermal film

[0072] The composite materials from Examples 1 and 2 were prepared into EVA / PA / SA-GP electrothermal films, and the various properties of the composite materials were further measured using the electrothermal films. The preparation method is as follows (the process schematic diagram of the EVA / PA / SA / interdigital electrode pattern is shown in the figure). Figure 2 As shown):

[0073] Silver paste interdigitated electrode patterns were printed using screen printing. The silver paste interdigitated electrodes were then coated onto polyethylene terephthalate (PET) as a flexible substrate. The prepared EVA / PA / SA / GP composite material was uniformly coated onto the surface of the silver paste interdigitated electrodes. The size was fixed at 25×30mm and the thickness was controlled at 120μm to obtain an electrothermal film. The obtained electrothermal film was vacuum dried at 45℃ for 24h to finally obtain the EVA / PA / SA / GP electrothermal film.

[0074] 2. Testing of PTC performance in electrothermal films

[0075] (1) PTC performance of composite materials in electrothermal films

[0076] Table 2 shows the PTC characteristics of the EVA / PA / SA composite material in the electrothermal film with different PA / SA ratios.

[0077] Table 2: Properties of Composite Materials in Electrothermal Films

[0078]

[0079] It can be seen that the resistance of the composite material in the electrothermal film increases geometrically with the increase of temperature under different PA / SA ratios, showing obvious PTC behavior. The Curie temperature of the composite material is roughly concentrated at 48℃, which is related to the structural transformation of the phase change material in the molten state and the conduction of temperature in the composite material.

[0080] The PTC strength of the composite material in the electrothermal film is shown in Table 3:

[0081] Table 3: PTC strength of composite materials in electrothermal films

[0082] Composite materials in electrothermal films PTC strength EVA / PA / SA (5:4:1) / GP-7% 3.66 EVA / PA / SA (5:1:4) / GP-7% 3.84 EVA / PA / SA (5:2:3) / GP-7% 2.78 EVA / PA / SA (5:3:2) / GP-7% 3.47 EVA / PA (1:1) / GP-7% 4.4 EVA / SA (1:1) / GP-7% 3.98 EVA / PA / SA (5:4:1) / GP-8% 4.31 EVA / PA / SA (5:4:1) / GP-9% 3.47 EVA / PA / SA (5:4:1) / GP-10% 2.86

[0083] The difference in PTC strength of composite materials with different proportions is related to their different conductive network structures and the distribution of conductive fillers in the composite materials.

[0084] When the GP mass fraction is 8%, the overall composite material in the electrothermal film responds to temperature quickly, indicating that the conductive content is near the critical percolation threshold, the resistance change rate is high, and thus the temperature sensitivity is good; when the conductive filler content exceeds 8%, too many contact points appear in the conductive network, which affects the PTC performance of the composite material in the electrothermal film.

[0085] (2) Electrothermal performance of the composite material in the composite film

[0086] The temperature response, maximum temperature and electrothermal cycle stability of the electrothermal film were tested, and the results are as follows:

[0087] (a) Thermal response under different voltages

[0088] It can be seen that: Figure 3

[0089] The composite material EVA / PA(1:1) / GP-7% and EVA:PA:SA(5:4:1) / GP-7% in the electrothermal film gradually increased in temperature under the step cycle voltage of 24V-72V, and basically no temperature overshoot phenomenon occurred. Under the step cycle voltage of 72V-120V, the stable temperature no longer increased further with the increase of voltage; other proportions also showed similar change law.

[0090] It can be seen that the composite material in the electrothermal film with different PA / SA ratios has different thermal effects under different voltages, and the temperature basically remains stable with the gradual increase of voltage. After power on, the composite material quickly converts electrical energy into heat energy, resulting in an increase in the surface temperature of the composite material, which conforms to Joule's law.

[0091] The obtained EVA / PA / SA-GP electrothermal film was tested at 48V using a thermal imager to capture the surface temperature and its distribution of the composite material in real time. The surface maximum temperature of the composite material in the electrothermal film with different PA / SA ratios is shown in Table 4:

[0092] Table 4: Surface maximum temperature of the composite material in the electrothermal film

[0093] Composite materials in electrothermal films Surface maximum temperature (°C) EVA / PA / SA (5:4:1) / GP-7% 48.3 EVA / PA / SA (5:1:4) / GP-7% 46.1 EVA / PA / SA (5:2:3) / GP-7% 40.9 EVA / PA / SA (5:3:2) / GP-7% 44.7 EVA / PA (1:1) / GP-7% 44.7 EVA / SA (1:1) / GP-7% 42.1

[0094] The composite material EVA / PA / SA(5:4:1) in the electrothermal film does not have conductivity, so its electrothermal performance is not studied.

[0095] The temperature response results of the composite material in the electrothermal film with different GP contents under different voltages are shown in Figure 4 and Table 5:

[0096] ​Table 5: Temperature response results at different voltages

[0097]

[0098] The temperature response results of the electrothermal film with different GP contents at different voltages are as shown in Figure 4 (a-b) shows that the overall response speed of the composite material in the electrothermal film prepared by different mass fractions of GP to temperature is fast, the temperature sensitivity of the composite material is high, and the temperature can be stabilized in a controllable range within 20s after pressurization.

[0099] (b) Temperature control under cyclic voltage

[0100] Figure 4 In c-d, the temperature control of the composite material in the electrothermal film prepared by different mass fractions of GP under cyclic voltage is shown, and the stability of the electrothermal film is reflected by the resistance change during cooling. The smaller the resistance change, the higher the stability of the cyclic heating and cooling. Specifically:

[0101] The composite material EVA / PA / SA(5:4:1) / GP-7% in the electrothermal film basically maintains the temperature unchanged under cyclic pressurization, but the resistance value changes greatly. When the initial pressurization is performed, the temperature overshoot phenomenon is obvious

[0102] The composite material EVA / PA / SA(5:4:1) / GP-9% in the electrothermal film has good stability and overall temperature control performance under cyclic pressurization.

[0103] It can be seen that the stable temperature of the composite material in the electrothermal film prepared by different mass fractions of GP under cyclic voltage is basically the same, indicating that the electrothermal film has good stability and cooling repeatability.

[0104] Example 4: Effect of phase change material and ethylene-vinyl acetate copolymer ratio on composite material

[0105] The specific implementation is the same as that in Example 1, except that the ratio of the phase change material and the ethylene-vinyl acetate copolymer is adjusted to 3:7, 4:6, and 7:3, i.e.

[0106] (1) The mass fraction of EVA in the solvent is 12%, the final mass fraction of palmitic acid (PA) in the solution is 14%, and the final mass fraction of stearic acid (SA) in the solution is 14%;

[0107] (2) The mass fraction of EVA in the solvent is 16%, the final mass fraction of palmitic acid (PA) in the solution is 12%, and the final mass fraction of stearic acid (SA) in the solution is 12%;

[0108] (3) the mass fraction of EVA in the solvent is 28%, the final mass fraction of palmitic acid (PA) in the solution is 6%, and the final mass fraction of stearic acid (SA) in the solution is 6%;

[0109] EVA:PA:SA (3:3.5:3.5) composite material, EVA:PA:SA (4:3:3) composite material, and EVA:PA:SA (7:1.5:1.5) composite material were prepared respectively.

[0110] The results show that:

[0111] The wrapping performance of the EVA:PA:SA (3:3.5:3.5) composite material and the EVA:PA:SA (4:3:3) composite material is poor, and there is a small amount of phase change material precipitated on the surface. The PTC performance of the EVA:PA:SA (7:1.5:1.5) composite material is poor, and the overall temperature control precision is not high.

[0112] Comparative Example 1: Effect of Different Support Materials on Composite Material

[0113] The specific implementation is the same as that of Example 1 (the mass fraction ratio of EVA / PA / SA is 5:4:1), except that the support material ethylene-vinyl acetate EVA is replaced by high-density polyethylene HDPE, polylactic acid PLA, and polyvinyl alcohol PVA to prepare the composite material.

[0114] The results show that: when using high-density polyethylene HDPE or polylactic acid PLA as the support material, the existing solvent is difficult to fully dissolve them due to their high melting temperature, and it is not possible to use the solution mixing method in the present application to prepare PTC materials.

[0115] When polyvinyl alcohol PVA is used as the support material, it exhibits excellent flexibility as a whole, but its PTC performance is poor, with a change in resistance of only about one order of magnitude, so it also does not have excellent PTC performance.

[0116] Comparative Example 2: Effect of Different Phase Change Materials on Composite Material

[0117] The specific implementation is the same as that of Example 1, except that the phase change material PA / SA is replaced by dodecanoic acid, tetradecanoic acid, paraffin, and wax emulsion to prepare the composite material; the specific steps are as follows:

[0118] (1) 50 nm spherical graphite GP was placed in an oven and dried at 80°C for 24 h, and then the spherical GP was placed in a ball mill at a speed of 400 rpm for 6 h to obtain spherical graphite GP powder with more uniform grinding and dispersion;

[0119] (2) EVA / phase change material / GP composite material preparation:

[0120] After adding ethylene-vinyl acetate (EVA) into toluene, oscillate for 1 min at 1500 rpm using an oscillator, homogenize for 10 min using a homogenizer, then stir for 6 h at 40℃ under 400 rpm magnetic stirring to obtain a 20% mass fraction of EVA toluene solution;

[0121] After adding the phase change material into the above EVA toluene solution, oscillate for 1 min at 1500 rpm using an oscillator, homogenize for 10 min using a homogenizer, then stir for 6 h at 25℃ under 400 rpm magnetic stirring to obtain an EVA-phase change material mixed toluene solution (in which the final mass fraction of the phase change material in the solution is 20%;

[0122] Add 0.24 g of spherical graphite GP powder obtained in step (1) to the EVA-phase change material mixed toluene solution to make the mass fraction of spherical graphite GP 7%, oscillate for 1 min at 1500 rpm, and then ultrasonic treat for 5 min at 40 KHz, then stir for 6 h at 25℃ under 600 rpm, after stirring, use a vacuum demolding and debubbling machine to mix and disperse for 10 min under vacuum at 2000 rpm to obtain an EVA / phase change material / GP-7% composite material.

[0123] The results show that: the wax emulsion part is difficult to fully dissolve by solution method, and the overall preparation process is complex; while using dodecanoic acid and tetradecanoic acid, the overall shows a lower Curie temperature point, and the resistivity of the composite material at room temperature is higher, the PTC performance is poorer, and the thermal weight repeatability is lower compared with PA / SA, which cannot meet the set requirements.

[0124] Comparative Example 3: Effect of Different Conductive Fillers on Composite Materials

[0125] The specific implementation is the same as Example 1 (the mass fraction ratio of EVA / PA / SA is 5:4:1), except that the conductive filler GP is replaced by carbon nanotubes, carbon black, and expandable graphite to prepare the composite material.

[0126] The results show that: when using expandable graphite as a conductive filler, it will produce certain agglomeration due to its specific worm structure, and it is difficult to disperse uniformly, and the overall basically has no PTC phenomenon; when using carbon nanotubes as a conductive filler, it is easy to form a high degree of entanglement between the tubular structures, and the agglomeration phenomenon is obvious, and the overall basically has no PTC phenomenon; when using carbon black as a conductive filler, the specific surface between the materials is large, but there is still certain agglomeration, and the PTC phenomenon is weak.

[0127] Although the present application has been disclosed in its preferred embodiments with reference to the accompanying drawings, it is not intended to limit the present application thereto, and various modifications and alterations can be made thereto by those skilled in the art without departing from the spirit and scope of the present application, and the scope of protection of the present application should be defined by the appended claims.

Claims

1. A PTC composite material, characterized by, The PTC composite material comprises: an ethylene-vinyl acetate copolymer base material, a phase change material, and a conductive filler, the phase change material is palmitic acid and stearic acid, the mass ratio of the palmitic acid and the stearic acid added is 1-4:1-4, the conductive filler is graphite powder and / or graphene, in the PTC composite material, the mass fraction of the ethylene-vinyl acetate copolymer is 4-30%, the mass fraction of the conductive filler is 4-16%, and the mass ratio of the phase change material and the ethylene-vinyl acetate copolymer added is 0.5-1.5:1, and the rest is an organic solvent.

2. The PTC composite material according to claim 1, characterized in that The organic solvent is toluene, tetrahydrofuran or N,N-dimethylformamide.

3. The PTC composite material according to claim 1, characterized in that, The mass fraction of VA in the ethylene-vinyl acetate copolymer base material EVA is 10-30%.

4. The PTC composite material according to any one of claims 1 to 3, characterized in that After the PTC composite material is compounded with an electrode, the sheet resistance of the PTC composite material dried into a film is 100-1000 Ω. The temperature control point of the PTC composite material ranges from 40°C to 52°C, the response time of the PTC composite material to temperature is 5 s, and the temperature control accuracy of the PTC composite material is ±0.4°C.

5. Process for the production of a PTC composite material according to any one of claims 1 to 4, characterized in that The method comprises the following steps: (1) ethylene-vinyl acetate copolymer is added to an organic solvent, stirred and homogenized to obtain an EVA solution with a mass fraction of 4-30%; (2) a phase change material is added to the EVA solution obtained in step (1), stirred and homogenized to obtain an EVA-phase change material mixed solution, wherein the mass ratio of the added amount of the phase change material to the ethylene-vinyl acetate copolymer is 0.5-1.5:1, the phase change material is palmitic acid and stearic acid, and the mass ratio of the palmitic acid and the stearic acid added is 1-4:1-4; (3) a conductive filler is added to the EVA-phase change material mixed solution obtained in step (2), the mass fraction of the conductive filler in the mixed solution is 4-16%, and the mixture is stirred and homogenized to obtain a PTC composite material, and the conductive filler is graphite powder and / or graphene.

6. The production method according to claim 5, wherein The organic solvent is toluene, tetrahydrofuran or N,N-dimethylformamide.

7. The preparation method according to claim 5, characterized in that, The mass fraction of VA in the ethylene-vinyl acetate copolymer base material EVA is 10-30%.

8. A method for improving the response speed to temperature and / or the PTC strength of a PTC composite material, characterized by, The method is to prepare a PTC composite material by taking ethylene-vinyl acetate copolymer as a base material, palmitic acid and stearic acid as a phase change material, and graphite powder and / or graphene as a conductive filler, the mass ratio of the palmitic acid and the stearic acid added is 1-4:1-4, and the mass ratio of the phase change material and the ethylene-vinyl acetate copolymer added is 0.5-1.5:

1.

9. The method of claim 8, wherein, The method further comprises compounding the prepared PTC composite material with an electrode by using an additive manufacturing process, the additive manufacturing process is one or more of screen printing, coating and spin coating, and the sheet resistance of the PTC composite material dried into a film is 100-1000 Ω.

10. The method of claim 9, wherein, The method specifically comprises: (1) ethylene-vinyl acetate copolymer is added to an organic solvent, stirred and homogenized to obtain an EVA solution with a mass fraction of 4-30%; (2) adding phase change material into the EVA solution obtained in step (1), stirring and homogenizing to obtain an EVA-phase change material mixed solution; (3) adding conductive filler into the EVA-phase change material mixed solution obtained in step (2), the mass fraction of the conductive filler in the mixed solution being 4-16%, stirring and homogenizing to obtain a PTC composite material.

11. The method of claim 10, wherein, The organic solvent is toluene, tetrahydrofuran or N,N-dimethylformamide.

12. The method of claim 8, wherein, The mass fraction of VA in the ethylene-vinyl acetate copolymer matrix material EVA is 10-30%.

13. Use of the PTC composite material according to any one of claims 1-4 in the preparation of electrodes, batteries, self-regulating heating components, temperature sensors, detectors, instruments requiring temperature control and / or wearable electronics.

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