A sea-island structure conductive composite material and PTC element

Abstract

The present invention discloses a sea-island structure conductive composite material and a PTC element. The sea-island structure conductive composite material has a resistivity of no more than 10Ω.cm and comprises: a first conductive composite material composition and a second conductive composite material composition, wherein the first crystalline polymer in the first conductive composite material composition has a melting point higher than that of the second crystalline polymer in the second conductive composite material composition by more than 15°C; the first conductive composite material composition is distributed in the second conductive composite material composition in the form of particles, presenting a sea-island structure, with a particle size of 0.1-50um, and accounting for 8% to 60% of the volume fraction of the sea-island structure conductive composite material. The PTC element prepared from the sea-island structure conductive composite material has a PTC effect with a double-step protection feature in its RT curve, and has an excellent overcurrent protection effect. The shortcomings of overcurrent protection element burning and failure to trigger protection in time can be avoided.

Description

Technical Field

[0001] The present invention relates to the preparation of a sea-island structure conductive composite material and a PTC element, in particular to a PTC element with a "resistance-temperature" (RT) curve showing a double-step protection feature. Background Art

[0002] The resistance of polymer-based conductive composite materials obtained by blending polymers and conductive fillers usually has a nonlinear relationship with temperature. As the temperature rises, the thermal expansion of the polymer matrix causes the distance between the conductive particles to increase, which increases the volume resistivity of the material. When the external temperature is close to the melting point of the polymer matrix, the thermal expansion of the matrix is ​​most significant, and the distance between the conductive particles is sharply increased. Therefore, the volume resistivity of the conductive composite material will increase by several orders of magnitude, that is, the PTC (positive temperature coefficient of resistance) effect will occur. This type of polymer conductive composite material has many applications in the fields of electronic circuit protection components, heaters, sensors, etc.

[0003] For electronic circuit protection components, it is generally desired to have the lowest possible room temperature resistance and the highest possible positive temperature coefficient effect. However, for relatively low melting point polymers such as ethylene-vinyl acetate copolymer, ethylene-butene copolymer, ethylene-octene copolymer, low-density polyethylene and linear low-density polyethylene, it is difficult to obtain sufficiently high PTC strength at very low room temperature resistance; after being prepared into electronic circuit protection devices, they are prone to burning and other phenomena during the protection process. Relatively high melting point polymers such as high-density polyethylene, polypropylene and polyvinylidene fluoride also tend to have higher PTC strength at very low room temperature resistance; however, after being prepared into electronic circuit protection components, due to the high turning temperature of their "resistance-temperature" (RT) curve, it is difficult to trigger protection in time at low temperatures.

[0004] The present invention combines the characteristics of low-melting point and high-melting point polymers, blends and compounds the conductive filler with a relatively high-melting point polymer, grinds it into conductive composite material composition particles, and disperses it in the low-melting point polymer-based conductive composite material in the form of an island structure. When an overcurrent fault occurs in the electronic circuit, the PTC element prepared by the island-structure conductive composite material firstly expands the low-melting point polymer to produce a positive temperature coefficient effect, which plays a primary protection role. When the protection level is insufficient or even fails, the high-melting point polymer expands again to further enhance the positive temperature coefficient effect, which plays a secondary protection role, thereby effectively ensuring the safety of the electronic circuit. Summary of the invention

[0005] The object of the present invention is to provide a sea-island structure conductive composite material to meet the requirements of the lowest possible room temperature resistance and the highest possible positive temperature coefficient effect.

[0006] Another object of the present invention is to provide a PTC element using a sea-island structure conductive composite material.

[0007] The technical problem described in the present invention is achieved by the following technical solution: a sea-island structure conductive composite material, the resistivity of which is not more than 10Ω.cm at an ambient temperature of 25°C; comprising:

[0008] a) a first conductive composite material composition, consisting of a first crystalline polymer and a first conductive filler, wherein the first crystalline polymer accounts for 20 to 90% by volume of the first conductive composite material composition, and the rest is the first conductive filler;

[0009] b) a second conductive composite material composition, consisting of a second crystalline polymer and a second conductive filler, wherein the second crystalline polymer accounts for 20 to 90% by volume of the second conductive composite material composition, and the rest is the second conductive filler; wherein the first crystalline polymer has a melting point higher than that of the second crystalline polymer by more than 15°C;

[0010] The first conductive composite material composition is distributed in the second conductive composite material composition in the form of particles, showing a sea-island structure. The sea-island particle size is 0.1-50um, and the first conductive composite material composition accounts for 8% to 60% of the volume fraction of the sea-island structure conductive composite material.

[0011] Furthermore, the first crystalline polymer may be any one selected from ethylene-butene copolymer, ethylene-octene copolymer, low-density polyethylene, linear low-density polyethylene, high-density polyethylene, polypropylene and polyvinylidene fluoride.

[0012] Furthermore, the second crystalline polymer may be any one selected from ethylene-vinyl acetate copolymer, ethylene-butene copolymer, ethylene-octene copolymer, low-density polyethylene, linear low-density polyethylene, high-density polyethylene and polypropylene.

[0013] The conductive filler can be any one of carbon black, carbon fiber, ceramic powder, ceramic fiber, metal powder and metal fiber.

[0014] The present invention also provides a PTC element prepared using the sea-island structure conductive composite material, which is composed of two metal electrodes and a sea-island structure conductive composite material sheet, wherein the metal electrodes are tightly connected to the sea-island structure conductive composite material sheet, wherein the metal electrodes can be used to receive electrical energy to allow current to pass through the PTC element, and the sea-island structure conductive composite material sheet has a thickness of 0.01~3.00mm and is divided into individual elements with a planar shape, wherein the individual elements have two surfaces perpendicular to the direction of current flow, and the resistivity of the PTC element is not greater than 10Ω.cm at 25°C.

[0015] The PTC element prepared therefrom has a double-step protection characteristic in its "resistance-temperature" (RT) curve and has an excellent overcurrent protection effect.

[0016] Furthermore, the single element is a sheet structure in a square, triangle, circle, ring, polygon or other irregular shapes.

[0017] The method for preparing the PTC element of the present invention comprises the following steps:

[0018] 1) melt-blending a first crystalline polymer and a first conductive filler to obtain a first conductive composite material composition, and then grinding the first conductive composite material into particles with a particle diameter of 0.1 to 50 μm, preferably 0.5 to 35 μm;

[0019] 2) melt-mixing the first conductive composite material composition in particle form, the second crystalline polymer, and the second conductive filler, wherein the melt-mixing temperature is between that of the first crystalline polymer and the second crystalline polymer, and then processing the mixed sea-island structure conductive composite material into a sea-island structure conductive composite material sheet with a thickness of 0.01 to 3.0 mm by extrusion molding, compression molding, or calendaring molding, preferably 0.08 to 2.3 mm, and more preferably 0.1 to 1.8 mm for the convenience of processing;

[0020] 3) When the sea-island structure conductive composite material sheet is still in a molten state, the metal electrodes are tightly connected to the upper and lower surfaces of the sheet by hot pressing rollers or flat plates to obtain a composite sheet;

[0021] 4) The composite sheet is punched, cut or laser cut into individual components, wherein the individual components have a planar shape, i.e., have two surfaces perpendicular to the direction of current flow, and the distance between the two surfaces is not greater than 3.0 mm, preferably not greater than 2.0 mm, and more preferably not greater than 1.5 mm, to prepare a PTC component;

[0022] 5) Cross-linking and / or heat treatment of the PTC element. Usually, the characteristics of the PTC element can be stabilized by means of cross-linking and / or heat treatment. The cross-linking can be chemical or high-energy ray irradiation cross-linking. The irradiation cross-linking of the PTC element generally does not exceed 64 Mrads, and 6.4~32 Mrads is more preferably selected, and 9.6~24 Mrads is more preferably selected. The heat treatment can be annealing, thermal cycling, high and low temperature alternation, such as 85℃ / -40℃ high and low temperature alternation.

[0023] The advantages of the present invention are: the island-structure conductive composite material has good conductivity, and the PTC element prepared therefrom has a "resistance-temperature" (RT) curve showing a double-step protection feature, has an outstanding overcurrent protection effect, and can effectively avoid the shortcomings of overcurrent protection elements being burned out and unable to trigger protection in time.

[0024] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a structural schematic diagram of the PTC element of the present invention;

[0026] Figure 2 Another structural schematic diagram of the PTC element of the present invention;

[0027] Figure 3 ” are the “resistance-temperature” (RT) curves of Comparative Example 1 and Comparative Example 2;

[0028] Figure 4 ” are the “resistance-temperature” (RT) curves of Examples 1 and 2 of the present invention;

[0029] Description of the numbers in the figure:

[0030] 11 is a second conductive composite material composition

[0031] 12 is a first conductive composite material composition of a sea-island structure

[0032] 13, 14 are metal electrodes

[0033] 15 and 16 are metal conductive pins. DETAILED DESCRIPTION

[0034] Example 1

[0035] A PTC component using a sea-island structure conductive composite material is prepared according to the following steps:

[0036] The first crystalline polymer is high-density polyethylene, the melting temperature is 134°C, and the volume fraction of the first conductive composite material composition is 62%; the first conductive filler is carbon black, and the volume fraction of the first conductive composite material composition is 38%;

[0037] The second crystalline polymer is an ethylene-octene polymer, the melting point is 99°C, and the volume fraction of the second conductive composite material composition is 67%; the second conductive filler is carbon black, and the volume fraction of the second conductive composite material composition is 33%;

[0038] The volume fraction of the first conductive composite material composition in the sea-island structure conductive composite material is 27%, and the volume fraction of the second conductive composite material composition in the sea-island structure conductive composite material is 73%;

[0039] The preparation process is as follows:

[0040] The mixer temperature is 180° C., the speed is 30 rpm, firstly, a first crystalline polymer high-density polyethylene is added and mixed for 3 minutes, 1 / 2 weight of the conductive filler is added, and mixed for 5 minutes, and then another 1 / 2 weight of the conductive filler is added, and the mixing is continued for 10 minutes, and the material is discharged, and the obtained material is crushed and ground by a grinder to obtain first conductive composite material composition particles with a particle size range of 1 to 30 μm;

[0041] The temperature of the internal mixer is 120°C, the speed is 30 rpm, first add the second crystalline polymer ethylene-octene polymer and internally knead for 3 minutes, then add 1 / 2 weight of the first conductive composite material composition particles and the second conductive filler carbon black, internally knead for 5 minutes, then add another 1 / 2 weight of the first conductive composite material composition particles and the second conductive filler carbon black, continue to internally knead for 10 minutes, discharge the material, and use an open mixer to thinly pull the sheet to obtain a sheet with a thickness of 0.28-0.32 mm; finally, cover the upper and lower surfaces of the sheet with metal foil as a metal electrode, and use a vacuum press to hot press the sheet, the vacuum press temperature is 220°C, preheat for 5 minutes, the hot pressing pressure is 10 MPa, the hot pressing time is 8 minutes, the cold pressing pressure is 10 MPa, the cold pressing time is 10 minutes, and a mold is used to punch out a single component of 10×7 mm;

[0042] The above single components were cross-linked by 16M electron beam radiation, and then the two metal pins were connected to the two metal electrode surfaces by reflow soldering. Finally, they were subjected to 6 cold and hot shocks at 85℃ / 1h and -40℃ / 1h to obtain the final PTC component. The structural diagram of the PTC component is shown in Figure 1 The first conductive composite material composition 12 having a sea-island structure is evenly distributed in the second conductive composite material composition 11, and the two surfaces are covered with metal electrodes 13 and 14.

[0043] The electrical properties of the PTC element of this embodiment are shown in Table 1, and the "resistance-temperature" (RT) curve is shown in Table 1. Figure 4 , the curve shows a double-step protection feature.

[0044] Example 2

[0045] A PTC component using a sea-island structure conductive composite material is prepared according to the following steps:

[0046] The first crystalline polymer is polyvinylidene fluoride, the melting temperature is 173°C, and the volume fraction of the first conductive composite material composition is 73%; the first conductive filler is carbon black, and the volume fraction of the first conductive composite material composition is 27%;

[0047] The second crystalline polymer is linear low-density polyethylene, with a melting temperature of 125°C, and a volume fraction of 65% in the second conductive composite material composition; the second conductive filler is carbon black, and a volume fraction of 35% in the second conductive composite material composition; wherein,

[0048] The volume fraction of the first conductive composite material composition in the sea-island structure conductive composite material is 30%, and the volume fraction of the second conductive composite material composition in the sea-island structure conductive composite material is 70%;

[0049] The temperature of the internal mixer for preparing the first conductive composite material composition is 230°C; the temperature of the internal mixer for melt-blending the first conductive composite material composition in a granular state and the second conductive composite material composition is 160°C; the two metal pins are respectively connected to the surfaces of two metal electrodes by a reflow soldering method to obtain an element, and finally, the element is subjected to cold and hot shocks 6 times under the conditions of 135°C / 1h and -40°C / 1h to obtain the final PTC element.

[0050] See Figure 2 As shown, the first conductive composite material composition 12 with a sea-island structure is evenly distributed in the second conductive composite material composition 11, and two surfaces are covered with metal electrodes 13, 14, and two pins 15, 16 are connected to the surfaces of the metal electrodes 13, 14 respectively.

[0051] The electrical properties of the PTC element of this embodiment are shown in Table 1, and the "resistance-temperature" (RT) curve is shown in Table 1. Figure 4 , the curve shows a double-step protection feature.

[0052] Comparative Example 1

[0053] The composition of the conductive composite material for preparing the PTC element is:

[0054] The crystalline polymer is ethylene-octene polymer with a melting temperature of 99°C and a volume fraction of 67% in the conductive composite material composition; the conductive filler is carbon black, with a volume fraction of 33% in the conductive composite material composition.

[0055] The preparation process is as follows:

[0056] The mixer temperature is 120℃, the speed is 30 rpm, first add the crystalline polymer ethylene-octene polymer and mix for 3min, add 1 / 2 weight of conductive filler carbon black, mix for 5min, then add another 1 / 2 weight of conductive filler carbon black, continue mixing for 10min, discharge, and pull the sheet through the open mixer to obtain a sheet with a thickness of 0.28-0.32mm. Finally, the upper and lower surfaces of the sheet are covered with metal foil as metal electrodes, and a vacuum press is used for hot pressing. The vacuum press temperature is 220℃, preheating for 5min, hot pressing pressure of 10MPa, hot pressing time of 8min, cold pressing pressure of 10MPa, cold pressing time of 10min, and a mold is used to stamp into a single component of 10×7mm. The above single component is cross-linked by 16M electron beam radiation, and then the two metal pins are connected to the surfaces of the two metal electrodes by reflow soldering. Finally, the final PTC component is obtained by hot and cold shock 6 times under the conditions of 85℃ / 1h and -40℃ / 1h.

[0057] The electrical properties of the PTC element of this comparative example are shown in Table 1, and the "RT" curve is shown in Table 1. Figure 3 , no steps appear on the curve.

[0058] Comparative Example 2

[0059] The steps for preparing the conductive composite material and the PTC element are the same as those in Comparative Example 1, except that the crystalline polymer is changed from ethylene-octene polymer to linear low-density polyethylene with a melting temperature of 125°C and a volume fraction of 73% in the conductive composite material composition; the conductive filler is carbon black, and the volume fraction of the conductive composite material composition is 27%.

[0060] The temperature of the internal mixer for preparing the conductive composite material composition is 180°C; the two metal pins are respectively connected to the surfaces of two metal electrodes by the reflow soldering method to obtain the component, and finally the component is subjected to cold and hot shock 6 times under the conditions of 85°C / 1h and -40°C / 1h to obtain the final PTC component.

[0061] The electrical properties of the PTC element of this comparative example are shown in Table 1, and the "RT" curve is shown in Table 1. Figure 3 , no steps appear on the curve.

[0062] Table 1 shows the formula composition and electrical performance test results of the PTC element embodiments and comparative examples of the present invention, wherein:

[0063] R0 represents the finished resistance of the PTC element;

[0064] R 32V / 50A It is the resistance value of the PTC after it is powered at 32V / 50A and the withstand voltage is maintained for 24 hours and then placed in a 25℃ environment for at least 1 hour;

[0065] R 36V / 50A It is the resistance value of the PTC after it is kept at 36V / 50A for 24 hours and then placed in a 25℃ environment for at least 1 hour.

[0066] T is the starting transition temperature, which is the temperature corresponding to the resistance of the PTC element when it is 10 times R0 on the "resistance-temperature" curve;

[0067] N is the PTC strength, which is characterized by the ratio of the maximum resistance to R0 on the "resistance-temperature" curve of the PTC component.

[0068]

[0069] It can be seen from Table 1 that the performance of the PTC element of the present invention is much better than that of the comparative example.

Claims

1. A PTC element made of a sea-island structure conductive composite material, characterized in that: It is composed of two metal electrodes and a sea-island structure conductive composite material sheet, wherein the metal electrodes are closely connected to the sea-island structure conductive composite material sheet, the sea-island structure conductive composite material sheet has a thickness of 0.01-3.00 mm, and is divided into single elements with a planar shape, wherein the single element has two surfaces perpendicular to the direction of current flow, and the resistivity of the PTC element is not greater than 10Ω.cm at 25°C; wherein, The sea-island structure conductive composite material has a resistivity of no more than 10Ω.cm at an ambient temperature of 25°C, and comprises: a) a first conductive composite material composition, consisting of a first crystalline polymer and a first conductive filler, wherein the first crystalline polymer accounts for 20 to 90% by volume of the first conductive composite material composition, and the rest is the first conductive filler; b) a second conductive composite material composition, consisting of a second crystalline polymer and a second conductive filler, wherein the second crystalline polymer accounts for 20 to 90% by volume of the second conductive composite material composition, and the rest is the second conductive filler; the first crystalline polymer has a melting point higher than that of the second crystalline polymer by more than 15°C; The first conductive composite material composition is distributed in the second conductive composite material composition in the form of particles, presenting a sea-island structure, with a particle size of 0.1-50 μm, accounting for 8% to 60% of the volume fraction of the sea-island structure conductive composite material; The first crystalline polymer is selected from any one of ethylene-butene copolymer, ethylene-octene copolymer, low-density polyethylene, linear low-density polyethylene, high-density polyethylene, polypropylene and polyvinylidene fluoride; the second crystalline polymer is selected from any one of ethylene-vinyl acetate copolymer, ethylene-butene copolymer, ethylene-octene copolymer, low-density polyethylene, linear low-density polyethylene, high-density polyethylene and polypropylene.

2. The PTC element prepared from the sea-island structure conductive composite material according to claim 1, characterized in that: The first conductive composite material composition particle diameter is 0.5-35 μm.

3. The PTC element prepared from the sea-island structure conductive composite material according to claim 1, characterized in that: The conductive filler is any one of carbon black, carbon fiber, metal powder and metal fiber.

4. The PTC element prepared from the sea-island structure conductive composite material according to claim 1, characterized in that: The single element is a square, triangular, circular or annular sheet structure.

5. The method for preparing a PTC element according to any one of claims 1 to 4, comprising the following steps: 1) high-density polyethylene with a melting temperature of 134° C. as the first crystalline polymer and carbon black as the first conductive filler are melt-blended to obtain a first conductive composite material composition, and then the first conductive composition is processed into particles with a particle diameter of 1 to 30 μm by grinding, so that the first crystalline polymer accounts for 62% by volume in the first conductive composite material composition; the first conductive filler accounts for 38% by volume in the first conductive composite material composition; The second crystalline polymer is an ethylene-octene polymer, the melting point is 99°C, and the volume fraction of the second conductive composite material composition is 67%; the second conductive filler is carbon black, and the volume fraction of the second conductive composite material composition is 33%; The volume fraction of the first conductive composite material composition in the sea-island structure conductive composite material is 27%, and the volume fraction of the second conductive composite material composition in the sea-island structure conductive composite material is 73%; 2) melt-mixing the first conductive composite material composition in particle form, the second crystalline polymer, and the second conductive filler, wherein the melt-mixing temperature is between that of the first crystalline polymer and the second crystalline polymer, and then processing the mixed sea-island structure conductive composite material into a sea-island structure conductive composite material sheet having a thickness of 0.01 to 3.0 mm by extrusion molding, compression molding, or calendering molding; 3) When the sea-island structure conductive composite material sheet is still in a molten state, the metal electrodes are tightly connected to the upper and lower surfaces of the sheet by hot pressing rollers or flat plates to obtain a composite sheet; 4) The composite sheet is punched, cut or laser cut into individual components, wherein the individual components have a planar shape, that is, have two surfaces perpendicular to the direction of current flow, and the distance between the two surfaces is not greater than 3.0 mm to prepare a PTC component; 5) Cross-linking and / or heat treating the PTC element.

6. The method for preparing a PTC element according to any one of claims 1 to 4, comprising the following steps: 1) melt-blending polyvinylidene fluoride with a melting temperature of 173° C. as a first crystalline polymer and carbon black as a first conductive filler to obtain a first conductive composite material composition, and then grinding the first conductive composition into particles of a first conductive composite material composition with a particle diameter of 1 to 30 μm, so that the volume fraction of the first crystalline polymer in the first conductive composite material composition is 73%; the volume fraction of the first conductive filler in the first conductive composite material composition is 27%; The second crystalline polymer is linear low-density polyethylene, with a melting temperature of 125° C., and a volume fraction of 65% in the second conductive composite material composition; the second conductive filler is carbon black, and a volume fraction of 35% in the second conductive composite material composition; The volume fraction of the first conductive composite material composition in the sea-island structure conductive composite material is 30%, and the volume fraction of the second conductive composite material composition in the sea-island structure conductive composite material is 70%; 2) melt-mixing the first conductive composite material composition in particle form, the second crystalline polymer, and the second conductive filler, wherein the melt-mixing temperature is between that of the first crystalline polymer and the second crystalline polymer, and then processing the mixed sea-island structure conductive composite material into a sea-island structure conductive composite material sheet having a thickness of 0.01 to 3.0 mm by extrusion molding, compression molding, or calendering molding; 3) When the sea-island structure conductive composite material sheet is still in a molten state, the metal electrodes are tightly connected to the upper and lower surfaces of the sheet by hot pressing rollers or flat plates to obtain a composite sheet; 4) The composite sheet is punched, cut or laser cut into individual components, wherein the individual components have a planar shape, that is, have two surfaces perpendicular to the direction of current flow, and the distance between the two surfaces is not greater than 3.0 mm to prepare a PTC component; 5) Cross-linking and / or heat treating the PTC element.

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