TWI779155B - Overcurrent protection device - Google Patents

Abstract

An overcurrent protection device includes a first electrode, a second electrode, and a positive temperature coefficient (PTC) multilayer structure arranged between the first electrode and the second electrode. The positive temperature coefficient multilayer structure includes a first polymer layer bonded to the first electrode, an intermediate layered unit bonded to the first polymer layer and including a second polymer layer, and a bonded And a third polymer layer is arranged between the middle layer unit and the second electrode. The first polymer layer, the second polymer layer and the third polymer layer respectively have a first volume resistivity, a second volume resistivity and a third volume resistivity, and the second volume resistivity higher than the first volume resistivity and the third volume resistivity.

Description

overcurrent protection device

The present invention relates to an overcurrent protection device, in particular to an overcurrent protection device comprising three polymer layers, wherein one polymer layer is sandwiched between the other two polymer layers, and has a structure higher than the other two layers. The volume resistivity of the volume resistivity of the polymer layer.

A positive temperature coefficient (PTC) overcurrent protection device exhibits a PTC effect, and the PTC effect makes the positive temperature coefficient overcurrent protection device useful as a circuit protection device. Referring to FIG. 1 , a conventional circuit protection device 1 includes a PTC polymer layer 11 and two electrodes 12 connected to two opposite surfaces of the PTC polymer layer 11 . The PTC polymer layer 11 comprises a polymer matrix including a crystalline region and an amorphous region, and a polymer matrix dispersed in the non-crystalline region of the polymer matrix and forming a continuous conductive path to electrically connect the electrodes 12 Granular conductive filler. The PTC effect is a phenomenon that when the temperature of a polymer matrix is raised to its melting point, the crystals in the crystalline region begin to melt and cause a new amorphous region to be created. As the new amorphous region increases to a certain extent and merges with the original amorphous region, the conductive path of the granular conductive filler will become discontinuous, and the resistance of the PTC polymer material will increase rapidly, resulting in the The electrodes are not electrically connected.

The circuit protection device 1 is used to protect electronic equipment, and the polymer matrix of the PTC polymer layer 11 is determined according to the operating current and operating voltage of the electronic equipment. choose. The polymer matrix of the PTC polymer layer 11 is usually made of polyethylene-based composition. However, due to poor adhesion between the PTC polymer layer 11 and the electrodes 12 , the circuit protection device 1 may not have desired conductivity.

US Patent No. 6238598 discloses a PTC polymer blend composition and a circuit protection device. The PTC polymer blend composition includes an ungrafted polyolefin, a grafted polyolefin and a particle conductive material. The circuit protection device includes a PTC element with the PTC polymer blend composition, and two electrodes respectively connected to two opposite sides of the PTC element. With the inclusion of grafted polyolefin in the PTC polymer blend composition, the circuit protection device has good electrical stability, and the adhesion between the PTC element and the electrodes is good.

However, some electrical characteristics (such as volume resistivity, withstand voltage, etc.) of the circuit protection device disclosed in the US patent can be further improved to meet industrial requirements.

Therefore, it is an object of the present invention to provide an overcurrent protection device that can overcome at least one of the disadvantages of the above-mentioned prior art.

Therefore, the overcurrent protection device of the present invention includes a first electrode, a second electrode, and a positive temperature coefficient (positive temperature coefficient, PTC for short) multilayer structure.

The positive temperature coefficient multilayer structure is disposed between the first electrode and the second electrode, and includes a first polymer layer, an intermediate layer unit, and a third polymer layer.

The first polymer layer joins the first electrode and includes a first polymer matrix and a first granular conductive filler dispersed in the first polymer matrix. The first polymer matrix is made from a first polymer mixture.

The intermediate layered unit joins the first polymer layer and includes a second polymer layer. The second polymer layer includes a second polymer matrix and a second granular conductive filler dispersed in the second polymer matrix. The second polymer matrix is made of a second polymer mixture.

The third polymer layer is bonded and disposed between the middle layer unit and the second electrode, and includes a third polymer matrix and a third granular conductive filler dispersed in the third polymer matrix. The third polymer matrix is made of a third polymer mixture.

The first polymer layer, the second polymer layer, and the third polymer layer respectively have a first volume resistivity, a second volume resistivity, and a third volume resistivity, and the second volume resistivity rate is higher than the first volume resistivity and the third volume resistivity.

The effect of the present invention is: by controlling the volume resistivity of the second polymer layer to be higher than the volume resistivity of the first polymer layer and the third polymer layer, the overcurrent protection device of the present invention exhibits good performance electrical properties.

2: The first electrode

3: Positive temperature coefficient multi-layer structure

30: Intermediate layered unit

31: first polymer layer

32: second polymer layer

33: third polymer layer

34: fourth polymer layer

35: fifth polymer layer

4: Second electrode

Other features and effects of the present invention will be clearly presented in the implementation manner with reference to the drawings, wherein: FIG. 1 is a perspective view of a conventional overcurrent protection device; FIG. 2 is a first embodiment of the overcurrent protection device of the present invention a stereogram of 3 is a perspective view of the second embodiment of the overcurrent protection device of the present invention; and FIG. 4 is a perspective view of a variation of the second embodiment of the overcurrent protection device of the present invention.

Before the present invention is described in detail, it should be noted that in the following description, similar elements are denoted by the same numerals.

Referring to Fig. 2, a first embodiment of the overcurrent protection device of the present invention comprises a first electrode 2, a second electrode 4, and a positive temperature coefficient ( positive temperature coefficient, referred to as PTC) multi-layer structure 3.

The PTC multilayer structure 3 includes a first polymer layer 31 , an intermediate layer unit 30 , and a third polymer layer 33 .

The first polymer layer 31 joins the first electrode 2 and includes a first polymer matrix and a first granular conductive filler dispersed in the first polymer matrix. The first polymer matrix is made of a first polymer mixture.

The middle lamellar unit 30 joins the first polymer layer 31 and includes a second polymer layer 32 . The second polymer layer 32 includes a second polymer matrix and a second particulate conductive filler dispersed in the second polymer matrix. The second polymer matrix is made of a second polymer mixture.

The third polymer layer 33 is bonded and disposed between the middle layer unit 30 and the second electrode 4 . The third polymer layer 33 includes a third polymer matrix and a third granular conductive filler dispersed in the third polymer matrix. The third polymer matrix is made of a third polymer mixture.

The first polymer layer 31, the second polymer layer 32, and the third polymer layer 33 respectively have a first volume resistivity, a second volume resistivity and a third volume resistivity, and the first The second volume resistivity is higher than the first volume resistivity and the third volume resistivity.

In some embodiments, the second volume resistivity is at least 1.4 times higher than the first volume resistivity and the third volume resistivity.

In the present invention, each of the first polymer mixture, the second polymer mixture and the third polymer mixture independently comprises an ungrafted olefinic polymer and a grafted olefinic polymer.

The ungrafted olefinic polymer of each of the first polymer mixture, the second polymer mixture, and the third polymer mixture is high density polyethylene. The grafted olefinic polymer of each of the first polymer mixture, the second polymer mixture, and the third polymer mixture includes an unsaturated carboxylic acid grafted polyolefin.

In some embodiments, based on the total amount of the first polymer mixture and the first particulate conductive filler, the content of the grafted olefin-based polymer in the first polymer mixture ranges from 19 wt % to 23 wt %. Based on the total amount of the third polymer mixture and the third granular conductive filler, the content of the grafted olefin polymer in the third polymer mixture ranges from 19wt% to 23wt%.

In some embodiments, based on the total amount of the second polymer mixture and the second particulate conductive filler, the content of the grafted olefin-based polymer in the second polymer mixture ranges from 22 wt % to 25 wt %.

Each of the first granular conductive filler, the second granular conductive filler and the third granular conductive filler is, for example but not limited to, carbon black, metal powder, conductive ceramic powder, or any combination thereof.

In some embodiments, the first granular conductive filler, the second granular conductive filler and the third granular conductive filler are carbon black.

Referring to FIG. 3 , a second embodiment of the overcurrent protection device of the present invention is illustrated. The second embodiment of the overcurrent protection device of the present invention has a similar structure to the first embodiment, except that, in the second embodiment, the intermediate laminar unit 30 also includes two additional polymer layers , that is, a fourth polymer layer 34 and a fifth polymer layer 35 .

In some embodiments, the second polymer layer 32 is disposed between the fourth polymer layer 34 and the fifth polymer layer 35 , but the arrangement of the polymer layers 32 , 34 , 35 is not limited thereto. In a variation of the second embodiment, the fourth polymer layer 34 is disposed between the second polymer layer 32 and the fifth polymer layer 35 (see FIG. 4 ). In another variation of the second embodiment, the fifth polymer layer 35 is disposed between the second polymer layer 32 and the fourth polymer layer 34 .

Each of the fourth polymer layer 34 and the fifth polymer layer 35 includes a polymer matrix made of a polymer mixture, and a particulate conductive filler dispersed within the polymer matrix. The suitable composition of the polymer mixture and the particulate conductive filler of the fourth polymer layer 34 and the fifth polymer layer 35 can refer to the first polymer mixture, the second polymer mixture and the third polymer mixture, As well as the definition of the first granular conductive filler, the second granular conductive filler and the third granular conductive filler, so for the sake of brevity, it will not be repeated.

The fourth polymer layer 34 and the fifth polymer layer 35 respectively have a fourth volume resistivity and a fifth volume resistivity. The fourth volume resistivity and the fifth volume resistivity may be higher or lower than the first volume resistivity, the second volume resistivity and the third volume resistivity, as long as the second volume resistivity is higher than the The first volume resistivity and the third volume resistivity are sufficient.

In some embodiments, the second volume resistivity is higher than at least one of the fourth volume resistivity and the fifth volume resistivity. In an exemplary embodiment, the second volume resistivity is at least 1.4 times higher than at least one of the fourth volume resistivity and the fifth volume resistivity.

In other embodiments, each of the fourth volume resistivity and the fifth volume resistivity is higher than the first volume resistivity and the third volume resistivity. In an exemplary embodiment, the second volume resistivity is at least 1.4 times higher than each of the fourth volume resistivity and the fifth volume resistivity, and the fourth volume resistivity and the fifth volume resistivity The resistivity is at least 1.4 times higher than the first volume resistivity and the third volume resistivity.

It should be noted that the number of polymer layers in the middle laminar unit 30 can vary according to actual needs. For example, in addition to the second polymer layer 32, the intermediate layered unit 30 may only further comprise a fourth polymer layer 34 disposed between the first polymer layer 31 and the second polymer layer 32, or It includes a fourth polymer layer 34 disposed between the second polymer layer 32 and the third polymer layer 33 .

The present invention will be further described with reference to the following examples, but it should be understood that these examples are for illustrative purposes only, and should not be construed as limitations on the implementation of the present invention.

Preparation of polymer blend compositions

The three polymer blend compositions (ie, RH, RM and RL) used in the following examples were prepared using carbon black (abbreviated as CB) as the particulate conductive filler and the polymer mixture. Wherein, the carbon black is purchased from Columbian Chemicals Company, the model is Raven 430UB, and has a DBP/D of 0.95 and a bulk density of ^0.53g /cm ; Ethylene (referred to as HDPE, purchased from Formosa Plastics Corp. and model is HDPE9002) and unsaturated carboxylic acid grafted polyolefin as a grafted olefin polymer. The unsaturated carboxylic acid grafted polyolefin is maleic anhydride grafted-high-density polyethylene (abbreviated as MA-G-HDPE) and purchased from Dupont, and its model is MB100D. The weight percentages of HDPE, MA-G-HDPE and CB in each of the three polymer blend compositions are shown in Table 1.

The carbon black and polymer mixture were compounded separately in a Brabender mixer. The compounding temperature was 200° C., the stirring speed was 30 rpm, and the compounding time was 10 minutes to obtain three polymer blend compositions (ie, RH, RM and RL). Each of the polymer blend compositions was placed in a mold, and then hot-pressed at 200° C. and a pressure of 80 kg/cm ² for 4 minutes to form a sheet. The sheet was removed and placed between two nickel-plated copper foils with a thickness of 0.43 mm. Then, the sheet and the nickel-plated copper foils were assembled together using the above hot pressing conditions to form a thin plate with a thickness of 0.5 mm. The thin plate was cut into a plurality of chips, and each chip had an area of 64 cm ² . Chips made from each polymeric blend composition were irradiated with cobalt-60 gamma rays generated by an irradiator with a total radiation dose of 150 kGy. The initial resistance of each chip at a temperature of 25 °C was measured using a micro-ohmmeter. The average value of electrical resistance and volume resistivity of chips made from each polymer blend composition are shown in Table 1. As shown in Table 1, the volume resistivity of the polymer blend composition RH is higher than that of the polymer blend composition RM, and the volume resistivity of the polymer blend composition RM is higher than that of the polymer blend composition RM. The volume resistivity of the blend composition RL.

Preparation of circuit protection device

The circuit protection devices of each of Examples 1-12 and Comparative Examples 1-18 were prepared using at least one of the above three polymer blend compositions R-H, R-M, and R-L to form a single-layer structure or a multi-layer structure (See Table 2). The detailed steps and conditions for preparing the circuit protection device of each embodiment are described below.

<Example 1>

First, the polymer blend composition RM is placed in a mold, and then the polymer blend composition RH and the polymer blend composition RM are stacked in sequence, that is, the polymer blend composition RH is sandwiched between two polymer blend compositions RM. After hot pressing at 200°C and 80kg/cm ² for 4 minutes, the two polymer blend compositions RM form a first polymer layer 31 and a third polymer layer 33, and the polymer blend composition RH forms an intermediate laminar unit 30 (i.e., second polymer layer 32), and the intermediate lamellar unit 30 is connected to the first polymer layer 31 and the third polymer layer 33, as shown in FIG. 2 . The thickness of the first polymer layer 31 , the second polymer layer 32 and the third polymer layer 33 is 0.64 mm.

Next, two nickel-plated copper foils (as the first electrode 2 and the second electrode 4) are respectively connected to the first polymer layer 31 and the third polymer layer opposite to the second polymer layer 32 33, and continue hot pressing at 200°C and 80kg/cm ² for 4 minutes to form a PTC polymer laminate with a thickness of 2mm. The PTC polymer laminated board was cut into a plurality of chips, and the area size of each chip was 8mm×8mm. The chips were irradiated with cobalt-60 gamma rays, and the cobalt-60 gamma rays were generated by an radiator with a total radiation dose of 150 kGy, thereby forming a plurality of test circuit protection devices of embodiment 1.

<Examples 2 to 4>

The steps and conditions for preparing the test circuit protection device of Examples 2 to 4 are similar to those of Example 1, but the difference is that the first polymer layer 31, the second polymer layer 32 and the third polymer layer 33 are used polymer blend compositions. It should be noted that in each of Examples 1 to 4, the volume resistivity of the second polymer layer 32 is higher than the volume resistivity of the first polymer layer 31 and the third polymer layer 33 Rate.

<Examples 5 to 12>

The steps and conditions of preparing the test circuit protection devices of Examples 5 to 12 are similar to those of Example 1, but the difference is that the middle laminar unit 30 of each of Examples 5 to 12 includes a fourth polymer layer 34 and a fifth polymer layer 35 . In addition, in the embodiment 5, the second polymer layer 32 is disposed between the fourth polymer layer 34 and the fifth polymer layer 35, and the fourth polymer layer 34 and the fifth polymer layer 35 respectively connects the first polymer layer 31 and the third polymer layer 33 (refer to FIG. 3 ). In the embodiment 6 to the embodiment 12, the fourth polymer layer 34 is arranged on the second polymer layer 32 and the fifth polymer layer 35, and the second polymer layer 32 and the fifth polymer layer Layer 35 connects the first polymer layer 31 and the third polymer layer 33 respectively (see FIG. 4 ).

Each of the first polymer layer 31, the second polymer layer 32, the third polymer layer 33, the fourth polymer layer 34, and the fifth polymer layer 35 is listed in Table 2 The resulting polymer blend composition is made and has a thickness of 0.39 mm.

<Comparative Example 1, Comparative Example 4 and Comparative Example 7)>

The steps and conditions for preparing the test circuit protection devices of Comparative Example 1, Comparative Example 4, and Comparative Example 7 are similar to those of Example 1, but the difference is that Comparative Example 1, Comparative Example 4, and Comparative Example 7 do not have the second polymer layer 32 and the third polymer layer 33 (that is, a single-layer structure), and two nickel-plated copper foils are respectively connected to two opposite surfaces of the first polymer layer 31 (see FIG. 1 ). In addition, the thickness of the first polymer layer 31 is 1.93mm, and it is made of the polymer blend composition listed in Table 2.

<Comparative Example 2, Comparative Example 5, Comparative Example 8, and Comparative Examples 10 to 12>

The steps and conditions of preparing the test circuit protection devices of Comparative Example 2, Comparative Example 5, Comparative Example 8 and Comparative Examples 10 to 12 are similar to those of Example 1, but the difference lies in Comparative Example 2, Comparative Example 5, Comparative Example 8 and Comparative Example The test circuit protection devices of Examples 10 to 12 did not have the second polymer layer 32 (that is, a bilayer structure), and each of the first polymer layer 31 and the third polymer layer 33 had a 0.97 mm in thickness and made from the polymer blend compositions listed in Table 2.

<Comparative Example 3, Comparative Example 6, Comparative Example 9, and Comparative Examples 13 to 16>

The steps and conditions for preparing the test circuit protection devices of Comparative Example 3, Comparative Example 6, Comparative Example 9 and Comparative Examples 13 to 16 are similar to those of Example 1, but the difference lies in Comparative Example 3, Comparative Example 6, Comparative Example 9 and Comparative Example The volume resistivity of the second polymer layer 32 of Examples 13 to 16 is lower than that of the first polymer layer 31 and the third polymer layer 33 . Each of the first polymer layer 31 and the third polymer layer 33 of the comparative examples was made of the polymer blend compositions listed in Table 2.

<Comparative Examples 17 to 18>

The steps and conditions for preparing the test circuit protection devices of Comparative Examples 17 to 18 are similar to those of Example 6, but the difference lies in the second polymer layer 32, the fourth polymer layer 34 and the fifth polymer layer of Comparative Examples 17 to 18 Each of the 35 has a volume resistivity lower than Volume resistivity of the first polymer layer 31 and the third polymer layer 33 . Each of the first polymer layer 31, the second polymer layer 32, the third polymer layer 33, the fourth polymer layer 34, and the fifth polymer layer 35 of Comparative Examples 17 to 18 Those were made from the polymer blend compositions listed in Table 2.

Performance Testing

[Initial resistance test] (resistance at room temperature)

A resistance test was performed on each of the test circuit protection devices in Examples 1 to 12 and Comparative Examples 1 to 18 using a micro-ohmmeter.

The initial resistance of each test circuit protection device in Examples 1 to 12 and Comparative Examples 1 to 18 was measured at 25°C. Table 3 shows the average value of initial resistance and volume resistivity of each tested circuit protection device in Examples 1 to 12 and Comparative Examples 1 to 18. The volume resistivity is calculated using the initial resistance and the law of resistance.

[Breakdown Test]

A breakdown test was performed on each test circuit protection device in Examples 1 to 12 and Comparative Examples 1 to 18, and the pass rate of the test circuit protection devices that passed the breakdown test without being burned was calculated. The breakdown test was performed in 10 cycles. Each cycle is to apply a certain DC voltage (100V, 150V and 200V) and a certain current (3A, 5A and 7A) to the test circuit protection device for 60 seconds, and then stop applying the constant DC voltage and the constant current for 60 seconds.

The results of the breakdown test (n/10×100%, n represents the number of test circuit protection devices that passed the breakdown test without being burned) are shown in Table 3.

It can be seen from Table 3 that the test circuit protection devices of Comparative Examples 1 to 3, Comparative Examples 4 to 6, and Comparative Examples 7 to 9 exhibit substantially the same initial resistance and the same volume resistivity, which means that using the same Blending the polymer composition to form a single layer structure or a multilayer structure does not change the electrical resistance of the resulting circuit protection device.

In addition, almost all of the test circuit protection devices of Examples 1 to 12 passed the breakdown test. The test circuit protection devices of Comparative Examples 1 to 18 had relatively low pass rates at a high voltage of 200Vdc (ranging from 0% to 80%), which means that the test circuit protection devices of Comparative Examples 1 to 18 were easily damaged under high voltage or burnt.

In detail, compared with the test circuit protection devices of Comparative Examples 1 to 3, the test circuit protection devices of Examples 1 to 3 (that is, the second polymer layer 32 is sandwiched between the first polymer layer 32 having a relatively low resistance material layer 31 and the third polymer layer 33) have relatively low volume resistivity, and relatively high pass rate of breakdown test, especially under high voltage. Similar results can be observed in the test circuit protection devices of Example 4 and Comparative Examples 4 to 9. In addition, compared with Examples 1 to 4, the test circuit protection devices of Comparative Examples 13 to 16 (including the second polymer layer interposed between the first polymer layer and the third polymer layer) are due to the second The volume resistivity of the first polymer layer 31 and the third polymer layer 33 is higher than that of the second polymer layer 32 , so they are easy to be damaged. However, by incorporating the second polymer layer 32 having a higher volume resistivity than the first polymer layer 31 and the third polymer layer 33, the overcurrent protection device can effectively withstand breakdown. Applicants of the present invention believe that when a polymer layer having a relatively high resistance is directly bonded to an electrode at high voltage and high current (eg, Comparative Examples 10 to 16), undesired electric arcs and flashovers may occur ( flashover), resulting in damage or burnout of the overcurrent protection device. However, in this context, since each polymer layer with a rather high resistance does not directly contact the electrodes, undesired arcing and flashover phenomena can be significantly avoided.

Regarding the test circuit protection devices having a five-layer structure (i.e., Embodiments 5 to 12 and Comparative Examples 17 to 18), the test circuit protection devices of Embodiments 5 to 12 have a relatively high passing rate at a high voltage of 200Vdc, while the comparison The test circuit of Examples 17 to 18 guarantees The protection device has only a pass rate of 10% to 50%, which means that when the volume resistivity of the second polymer layer 32 of the intermediate layered unit 30 is higher than that of the first polymer layer 31 and the third polymer layer 33 The volume resistivity, the passing rate of the breakdown test of the test circuit protection device of the embodiments 5 to 12 can be significantly increased.

In summary, by controlling the volume resistivity of the second polymer layer 32 to be higher than the volume resistivity of the first polymer layer 31 and the third polymer layer 33, the overcurrent protection device of the present invention exhibits Good electrical properties.

But what is described above is only an embodiment of the present invention, and should not limit the scope of the present invention. All simple equivalent changes and modifications made according to the patent scope of the present invention and the content of the patent specification are still within the scope of the present invention. Within the scope covered by the patent of the present invention.

2: The first electrode

3: Positive temperature coefficient multi-layer structure

30: Intermediate layered unit

31: first polymer layer

32: second polymer layer

33: third polymer layer

4: Second electrode

Claims (15)

An overcurrent protection device comprising: a first electrode; a second electrode; and a positive temperature coefficient multilayer structure disposed between the first electrode and the second electrode and including a first polymer layer bonded to the The first electrode includes a first polymer matrix and a first granular conductive filler dispersed in the first polymer matrix, the first polymer matrix is made of a first polymer mixture, an intermediate A layered unit joined to the first polymer layer and comprising a second polymer layer comprising a second polymer matrix and a second granular second polymer matrix dispersed within the second polymer matrix Conductive filler, the second polymer matrix is made of a second polymer mixture, and a third polymer layer is bonded and disposed between the intermediate layer unit and the second electrode, and includes a third polymer Material matrix and a third granular conductive filler dispersed in the third polymer matrix, the third polymer matrix is made of a third polymer mixture; wherein, the first polymer layer, the second The polymer layer, and the third polymer layer respectively have a first volume resistivity, a second volume resistivity and a third volume resistivity, and the second volume resistivity is higher than the first volume resistivity and the The third volume resistivity is at least 1.4 times. The overcurrent protection device according to claim 1, wherein each of the first polymer mixture, the second polymer mixture, and the third polymer mixture independently comprises an ungrafted olefin-based polymer and A grafted olefin polymer. The overcurrent protection device according to claim 2, wherein, based on the total amount of the first polymer mixture and the first granular conductive filler, the content range of the grafted olefin polymer in the first polymer mixture is It is 19wt% to 23wt%, and based on the total amount of the third polymer mixture and the third granular conductive filler, the content of the grafted olefin polymer in the third polymer mixture ranges from 19wt% to 23wt%. . The overcurrent protection device according to claim 2, wherein, based on the total amount of the second polymer mixture and the second granular conductive filler, the content range of the grafted olefin polymer in the second polymer mixture is It is 22wt% to 25wt%. The overcurrent protection device according to claim 2, wherein the ungrafted olefin-based polymer of each of the first polymer mixture, the second polymer mixture, and the third polymer mixture is a high-density polymer vinyl. The overcurrent protection device according to claim 2, wherein the grafted olefin-based polymer of each of the first polymer mixture, the second polymer mixture, and the third polymer mixture includes an unsaturated carboxylic acid Acid-grafted polyolefins. The overcurrent protection device according to claim 1, wherein the first granular conductive filler, the second granular conductive filler and the third granular conductive filler The material is independently selected from carbon black, metal powder, conductive ceramic powder, or any combination of the above. The overcurrent protection device as claimed in claim 7, wherein the first granular conductive filler, the second granular conductive filler and the third granular conductive filler are carbon black. The overcurrent protection device as claimed in claim 1, wherein the middle layered unit further comprises a fourth polymer layer. The overcurrent protection device as claimed in claim 9, wherein the intermediate layered unit further includes a fifth polymer layer, and the second polymer layer is disposed between the fourth polymer layer and the fifth polymer layer. The overcurrent protection device as claimed in claim 9, wherein the middle layered unit further includes a fifth polymer layer, and the fifth polymer layer is disposed between the second polymer layer and the fourth polymer layer. The overcurrent protection device as claimed in claim 9, wherein the middle layered unit further includes a fifth polymer layer, and the fourth polymer layer is disposed between the second polymer layer and the fifth polymer layer. The overcurrent protection device as claimed in claim 1, wherein the intermediate layered unit further comprises a fourth polymer layer having a fourth volume resistivity, and a fifth polymer layer having a fifth volume resistivity layer, the second volume resistivity is higher than at least one of the fourth volume resistivity and the fifth volume resistivity. The overcurrent protection device as claimed in claim 13, wherein the second volume resistivity is at least 1.4 times higher than at least one of the fourth volume resistivity and the fifth volume resistivity. The overcurrent protection device according to claim 14, wherein the second volume resistivity is at least 1.4 times higher than the fourth volume resistivity and the fifth volume resistivity, and the fourth volume resistivity and the fifth volume resistivity The volume resistivity is at least 1.4 times higher than the first volume resistivity and the third volume resistivity.

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