Electrolytic capacitor

Abstract

Provided is an electrolytic capacitor which can maintain sealability preventing liquid leakage even when used in a low temperature range and achieve life characteristics similar to those of a conventional product. As a solution, an electrolytic capacitor (1) according to the present invention includes: a capacitor element (2) impregnated with an electrolytic solution (4); a bottomed exterior case (5) for accommodating the capacitor element (2); and a sealing body (3) for sealing an opening (5a) of the exterior case (5). The electrolytic capacitor (1) is characterized in that the sealing body (3) contains ethylene butene diene rubber.

Description

Electrolytic Capacitor

[0001] The present invention relates to an electrolytic capacitor.

[0002] As one form, an electrolytic capacitor comprises an anode foil subjected to surface enlargement treatment and chemical conversion treatment, a cathode foil, and lead terminals joined to the anode foil and the cathode foil respectively. The anode foil and the cathode foil are stacked with a separator interposed therebetween and wound to form a cylindrical element. After the element is impregnated with an electrolytic solution, it is accommodated in a bottomed outer case. Furthermore, the outer case has a configuration in which after a sealing member is fitted to the opening of the outer case, the opening is sealed by crimping.

[0003] Conventionally, in an electrolytic capacitor having a structure including a sealing member, butyl rubber (IIR), ethylene-propylene copolymer rubber (EPDM) or the like has been used as the sealing member from the viewpoint of material properties such as elasticity and airtightness.

[0004] In recent years, demands for reliability in a wide temperature range for electrolytic capacitors have increased, and N,N-dimethylformamide (DMF) and γ-butyrolactone (GBL) have come to be used as solvents for electrolytic solutions. However, DMF and GBL have high volatility, and vapor of the electrolytic solution permeates through conventional elastic sealing members, which causes a problem that the reliability of the electrolytic capacitor cannot be maintained. Therefore, elastic sealing members made of isobutylene isoprene rubber (IIR) with high airtightness have come to be used (see Patent Document 1: Japanese Unexamined Patent Publication No. 2000-173876).

[0005] However, due to the high airtightness of IIR, a new problem has arisen that failure is likely to occur due to swelling of the capacitor caused by generated hydrogen gas, so ethylene propylene rubber (EPDM) has come to be used in some capacitors (see Patent Document 2: Japanese Unexamined Patent Publication No. 2019-67961).

[0006] Japanese Unexamined Patent Publication No. 2000-173876 Japanese Unexamined Patent Publication No. 2019-67961

[0007] It has become clear that a capacitor using EPDM as a sealing member has a problem that the usable temperature range is narrow. Accordingly, the present inventor has conducted intensive research on the life characteristics and operating temperature of capacitors.

[0008] The research revealed that, because EPDM has a glass transition temperature of -45°C, when used as a capacitor seal under low temperature conditions below -40°C, it is unable to seal the electrolyte components, resulting in leakage. On the other hand, while using highly airtight IIR as a seal does not cause leakage, it is prone to failure due to bulging caused by gas generation inside the capacitor, resulting in poor lifespan characteristics.

[0009] The present invention has been made in view of the above circumstances, and aims to provide an electrolytic capacitor that maintains sealing performance so as not to leak even when used in a low-temperature range, and that can obtain life characteristics comparable to those of conventional products.

[0010] The present invention solves the above problem by the following means.

[0011] The electrolytic capacitor according to the present invention comprises a capacitor element impregnated with an electrolyte, a bottomed outer case housing the capacitor element, and a sealing body that seals the opening of the outer case, wherein the sealing body is mainly composed of ethylene butenediene rubber.

[0012] Furthermore, the ethylene butenediene rubber preferably contains 30 to 70% ethylene butenediene copolymer.

[0013] Furthermore, the ethylene-butenediene copolymer structure preferably contains 50-55% ethylene and 4-8% diene.

[0014] Furthermore, it is preferable that the sealing body contains a polymer rubber having a glass transition temperature of -45°C or lower.

[0015] Furthermore, it is preferable to include a polymer rubber having a glass transition temperature of -60°C or lower.

[0016] According to the present invention, it is possible to provide an electrolytic capacitor and a method for manufacturing an electrolytic capacitor that maintain sealing performance so as not to leak even when used in a low-temperature range, and that can obtain life characteristics comparable to those of conventional products.

[0017] Figure 1 is a partial cross-sectional view showing an example of an electrolytic capacitor according to this embodiment. Figure 2 is a schematic diagram showing a capacitor element according to this embodiment. Figure 3 is a schematic diagram showing the lead terminals according to this embodiment joined to the foil. Figure 4 is a partial cross-sectional view showing an example of an electrolytic capacitor processed for testing as described in the embodiment. Figure 5 is a photograph showing the top surface of the capacitor sealing body after the thermal shock test. Figures 6A and 6B are graphs recording the change over time of the dimension L when a voltage is applied to the capacitor according to this embodiment at a high temperature. Figure 6A is a graph showing a comparison when IIR and EPDM are used as the sealing body, and Figure 6B is a graph showing a comparison when EBT and EPDM are used as the sealing body. Figure 7 is a graph recording the change over time of the capacitance when a voltage is applied to the capacitor according to this embodiment at a high temperature. Figure 8 is a graph recording the change over time of the dielectric loss tangent when a voltage is applied to the capacitor according to this embodiment at a high temperature. Figure 9 is a graph recording the change over time of the ESR when a voltage is applied to the capacitor according to this embodiment at a high temperature.

[0018] Embodiments of the present invention will be described in detail below with reference to the drawings. Figure 1 is a schematic diagram showing an example of an electrolytic capacitor 1 of this embodiment, and is a partial cross-sectional view. The electrolytic capacitor 1 comprises a capacitor element 2 impregnated with an electrolyte 4, a bottomed outer case 5 housing the capacitor element 2, and a sealing body 3 that seals the open end 5a of the case 5.

[0019] In the example shown in Figure 1, a lateral constricted portion 5b is formed on the side of the case 5 that is open, and the open end 5a is bent. The sealing body 3 is supported and fixed by the lateral constricted portion 5b and the open end 5a of the case 5. The lead terminal 9 has a round bar portion 9a fitted into a through hole (not shown) in the sealing body 3 and is supported and fixed by the sealing body 3.

[0020] As shown in Figure 2, the capacitor element 2 comprises a metal foil 6 on which a dielectric layer is formed and lead terminals 9. The metal foil 6 has an anode foil 6a and a cathode foil 6b, and is wound together with a separator 8 disposed between the anode foil 6a and the cathode foil 6b to form a substantially cylindrical shape, thereby forming the capacitor element 2. Also, as shown in Figure 3, lead terminals 9 are attached to the metal foil 6.

[0021] Case 5 is a bottomed cylindrical shape and is made of a metal such as aluminum. The sealing body 3 is highly airtight to prevent moisture from entering and the electrolyte 4 from splashing, and is roughly cylindrical in shape to match the inner shape of Case 5.

[0022] The sealing body 3 in this embodiment is characterized by being composed mainly of ethylene butenediene rubber (EBT). The EBT can contain 30 to 70% ethylene butenediene copolymer, which contains 50 to 55% ethylene and 4 to 8% diene.

[0023] This section explains ethylene-butenediene copolymer (EBT polymer). Compared to ethylene-propylene-diene copolymer (EPDM polymer), which was conventionally used in capacitors, EBT polymer is a copolymer (polymer) in which butene is copolymerized with monomers (comonomers) instead of propylene.

[0024] As a result of using EBT as the sealing body 3 of the electrolytic capacitor 1, the electrolyte 4 leakage that was observed with the conventional EPDM sealing body 3 did not occur in the thermal shock test described in the example. The glass transition temperature of EPDM is -45°C, and it has been confirmed that electrolyte 4 leakage occurs when sealing performance tests are conducted in the low temperature range of -40°C or below. On the other hand, the glass transition temperature of EBT is -60°C, and it was confirmed that electrolyte 4 leakage does not occur even at -70°C in the thermal shock test.

[0025] The guaranteed temperature of electrolytic capacitors 1 is -40°C for those using conventional EPDM for the sealing body 3, and there are even lower guaranteed models using butyl rubber (IIR) with a lower limit of -55°C. Based on test results, it is estimated that the electrolytic capacitor 1 using EBT according to the present invention can have its guaranteed temperature set to -60°C or lower.

[0026] Furthermore, while using IIR as the sealing material 3 allows for maintaining sealing performance in low-temperature regions, hydrogen gas generated inside the case 5 of the electrolytic capacitor 1 can cause the case 5 to swell, potentially leading to failure. For this reason, EPDM has been used as the sealing material 3 in some electrolytic capacitors 1.

[0027] The results shown in this embodiment confirm that the dimension L from the sealing surface at the top of the electrolytic capacitor 1 to the bottom surface 5c of the case of the electrolytic capacitor 1 changes more significantly when IIR is used as the sealing material 3 compared to when EPDM is used as the sealing material 3. On the other hand, when comparing EPDM and EBT, the change in dimension L is about the same regardless of which rubber is used as the sealing material 3, and it was confirmed that there is less swelling compared to IIR.

[0028] Furthermore, when measuring the change in capacitor characteristics over time at 125°C, there was no difference in the amount of change for capacitance, dielectric loss tangent, and ESR up to 4,000 hours, comparing the case where conventional EPDM was used for the sealing body 3 with the case where the EBT of the present invention was used. Note that "capacitance" represents the amount of charge stored in the capacitor, "dielectric loss tangent" represents the degree of electrical energy loss within the dielectric, and "ESR" is an abbreviation for Equivalent Series Resistance, representing the equivalent series resistance.

[0029] This confirmed that the guaranteed temperature of the electrolytic capacitor 1 could be set lower than when IIR was used for the sealing body 3, while maintaining a product life equivalent to or better than when EPDM was used for the sealing body 3.

[0030] [Test 1] A capacitor 10 for thermal shock testing is created using a sealing body 3 made of conventional EPDM and EBT of the present invention, respectively. In a normal electrolytic capacitor 1, in order to prevent leakage of the internal electrolyte 4 and contamination of the inside of the capacitor, the capacitor element 2 is placed in the case 5 and then covered with the sealing body 3, and the opening end 5a of the case 5 is pressed against the surface of the sealing body 3 to curl and deform it, thereby sealing it. However, in this test, in order to make it easier to check whether or not electrolyte 4 leaks from between the sealing body 3 and the case 5 due to the contraction change of the sealing body 3 due to temperature changes, the opening end 5a of the case 5 is not curled against the surface of the sealing body 3, and it is sealed only by the lateral constriction portion 5b.

[0031] The condenser 10 prepared for thermal shock testing was subjected to a cycle of leaving it at -70°C for 3 hours, followed by leaving it at 85°C for 30 minutes, for a total of 26 cycles. The results of observing the surface of the sealing body 3 after 26 cycles are shown in Table 1.

[0032]

[0033] As shown in Figure 5, leakage (dashed line in Figure 5) was observed only in the sample using conventional EPDM as the sealing body 3. Furthermore, as shown in Table 1, in the sample using conventional EPDM as the sealing body 3, it was confirmed that electrolyte 4 leaked into the gap between the sealing body 3 and the case 5 in all 10 test samples. On the other hand, no leakage of electrolyte 4 was observed in any of the 10 test samples of the EBT of the present invention after 26 cycles.

[0034] Based on the above results, it was confirmed that the sealing body 3 using EBT maintains its sealing performance even at -70°C compared to the conventional sealing body 3 using EPDM, and that it can be used as a sealing body 3 in low temperature ranges below -40°C, where EPDM would leak and could not be used.

[0035] [Test 2] Electrolytic capacitors 1 with a rated voltage of 450V were manufactured using EBT, IIR, and EPDM, respectively, for the sealing body 3. The manufactured electrolytic capacitors 1 were used as test specimens and subjected to a test in which their characteristics were measured at predetermined intervals while being exposed to a high temperature of 125°C and continuously subjected to a voltage of 420V.

[0036] As shown in Figure 6A, the dimension L of the electrolytic capacitor 1 using IIR as the sealing body 3 increases over time, while it remains unchanged in the case of EPDM. This indicates that the sealing body 3 using IIR swells due to hydrogen gas generated inside the electrolytic capacitor 1. On the other hand, as shown in Figure 6B comparing EPDM and EBT, when EBT was used as the sealing body 3, no swelling of dimension L occurred, similar to EPDM.

[0037] As shown in Figures 7, 8, and 9, there was no difference in the capacitor characteristics (changes in capacitance, dielectric loss tangent, and ESR) when EPDM was used for the sealing body 3 compared to when EBT was used. This confirms that EBT can ensure equivalent or better capacitor characteristics when used for the sealing body 3, just like EPDM.

[0038] As a result, we have been able to realize an electrolytic capacitor that maintains sealing performance to prevent leakage even when used in low-temperature ranges, while also achieving lifespan characteristics comparable to conventional products.

Claims

1. An electrolytic capacitor comprising: a capacitor element impregnated with an electrolyte; a bottomed outer case housing the capacitor element; and a sealing body sealing the opening of the outer case, wherein the sealing body is mainly composed of ethylene butenediene rubber.

2. The electrolytic capacitor according to claim 1, characterized in that the ethylene butenediene rubber contains 30 to 70% ethylene butenediene copolymer.

3. The electrolytic capacitor according to claim 2, characterized in that the ethylene-butenediene copolymer contains 50-55% ethylene and 4-8% diene.

4. The electrolytic capacitor according to claim 1, characterized in that the sealing body contains a polymer rubber having a glass transition temperature of -45°C or lower.

5. The electrolytic capacitor according to claim 1, characterized in that the sealing body contains a polymer rubber having a glass transition temperature of -60°C or lower.

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