electrolytic capacitor

Abstract

Provided is an electrolytic capacitor which is equipped with a resin layer and in which temporal increase in ESR is suppressed. The electrolytic capacitor is provided with: a capacitor element including a positive electrode foil, a negative electrode foil, and an electrolytic solution; a case for housing the capacitor element; a sealing member for sealing the case; and a resin layer disposed near the sealing member. The resin layer disposed near the sealing member contains an epoxy resin composition including an ester bond. In the electrolytic solution included in the electrolytic capacitor, a compound having a hydroxy group is contained in a solvent in a range of 45 wt% or less, including 0 wt%.

Description

[Technical Field]

[0001] The present invention relates to an electrolytic capacitor having a resin layer. [Background technology]

[0002] Electrolytic capacitors using valve metals such as tantalum or aluminum can achieve a small size and large capacitance by enlarging the surface area of ​​the dielectric by forming the valve metal as a sintered body or etched foil as the anode-side counter electrode. This type of electrolytic capacitor is constructed by filling the gaps with electrolyte to ensure close contact between the anode's dielectric oxide film and the counter electrode. Specifically, electrolytic capacitors include liquid electrolytic capacitors containing only an electrolyte, hybrid electrolytic capacitors containing both an electrolyte and a solid electrolyte, and bipolar electrolytic capacitors with a dielectric film formed on both electrodes.

[0003] The electrolyte uses ethylene glycol or gamma-butyrolactone as a solvent and contains a carboxylic acid or its salt, such as 1,6-decanedicarboxylic acid, 1,7-octanedicarboxylic acid, or azelaic acid, as a solute. This electrolyte directly contacts the dielectric film, acting as a true cathode and repairing the dielectric film. However, over time, the electrolyte evaporates and escapes from the electrolytic capacitor. As a result, the electrolytic capacitor dries out, its capacitance decreases over time, its dissipation factor (tanδ) increases over time, and it eventually reaches the end of its life.

[0004] Therefore, the capacitor element is housed in a bottomed outer case, and the opening of the outer case is sealed with a sealing material to seal in the electrolyte impregnated in the capacitor element, suppress evaporation of the electrolyte, and extend the capacitor's lifespan. However, elastomers such as butyl rubber and ethylene propylene diene rubber (EPDM) are used for the sealing material. Therefore, the electrolyte is not completely contained within the case, but rather permeates the sealing material and gradually volatilizes to the outside of the electrolytic capacitor. Therefore, as in Patent Document 1, a proposal has been made to cover the sealing material with a resin layer to suppress evaporation of the electrolyte due to permeation through the sealing material.

[0005] Resin layers generally have excellent thermal insulation properties. As the thermal insulation properties of electrolytic capacitors increase, it becomes more difficult to dissipate heat generated within the case, making it easier for heat to accumulate. Therefore, Patent Document 2 proposes the use of ethylene glycol as a solvent for the electrolyte. This is because ethylene glycol has a thermal conductivity approximately twice as high as that of γ-butyrolactone or sulfolane. Patent Document 2 suggests that the higher the proportion of ethylene glycol in the solvent, the higher the thermal conductivity. However, it also suggests that the ethylene glycol content should be 90% by mass or less in order to allow for the inclusion of other types of solvents. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 60-245106 [Patent Document 2] WO2018 / 123525 Summary of the Invention [Problem to be solved by the invention]

[0007] The resin layer contains a curable resin composition. Examples of the curable resin composition include an epoxy resin composition containing an epoxy resin and an acid anhydride curing agent. The epoxy resin composition is a composition of epoxy resin cured with an acid anhydride curing agent. Epoxy resin compositions containing an acid anhydride curing agent have improved heat resistance, and because the reaction temperature between the epoxy resin and the acid anhydride curing agent is 100°C or higher, the curing reaction can be accelerated using temperature as a trigger. Therefore, epoxy resin compositions containing an epoxy resin and an acid anhydride curing agent are considered to be suitable for the resin layer.

[0008] The inventors coated a part of an electrolytic capacitor with epoxy resin cured with an acid anhydride curing agent, and then exposed the electrolytic capacitor to a temperature environment of 150°C. It was confirmed that the ESR of the electrolytic capacitor exposed to this temperature environment for 400 hours increased significantly.

[0009] The present invention has been proposed to solve the above-mentioned problems, and its purpose is to provide an electrolytic capacitor having a resin layer in which the increase in ESR over time is suppressed. [Means for solving the problem]

[0010] As a result of extensive research, the present inventors have discovered that the increase in ESR over time of an electrolytic capacitor is specific to when a resin layer is formed from an epoxy resin composition containing an epoxy resin and an acid anhydride curing agent (hereinafter referred to as acid anhydride-cured epoxy resin) and ethylene glycol is used as a solvent for the electrolyte.

[0011] Based on this finding, the inventors conducted extensive research and discovered that acid anhydride-cured epoxy resin reacts with water or ethylene glycol and dissolves in the electrolyte, leading to an increase in the ESR of electrolytic capacitors. Acid anhydride-cured epoxy resin contains ester bonds in its chemical structure. Even when water is not intentionally added to electrolytic capacitors, small amounts of water can be introduced into the capacitor during manufacturing. Ethylene glycol is a compound containing hydroxyl groups, and compounds containing hydroxyl groups have a structure that makes them easily accessible to water. Therefore, when compounds containing hydroxyl groups permeate the sealing material, water also permeates. The ester bonds in the acid anhydride-cured epoxy resin react with water, resulting in hydrolysis of the acid anhydride-cured epoxy resin. Furthermore, esterification reactions occur between the carbonyl groups generated by hydrolysis and compounds containing hydroxyl groups. It is believed that some of the acid anhydride-cured epoxy resin components reacting with water or compounds containing hydroxyl groups permeate the sealing material and dissolve into the electrolyte.

[0012] Furthermore, as a result of extensive research, the present inventors have found that the hydrolysis of the epoxy resin composition having an ester bond and the elution into the electrolyte can be solved by reviewing the amount of the compound having a hydroxy group present relative to the total amount of the solvent.

[0013] The present invention was made based on this finding, and provides an electrolytic capacitor comprising: a capacitor element having an anode foil, a cathode foil, and an electrolyte; a case that houses the capacitor element; a sealing member that seals the case; and a resin layer that is disposed in the vicinity of the sealing member, wherein the resin layer contains an epoxy resin composition that includes an ester bond; and the solvent of the electrolyte contains a compound having a hydroxy group in an amount of 45 wt % or less, including 0 wt %.

[0014] The epoxy resin composition may be made from an epoxy resin and an acid anhydride curing agent as raw materials, or may have a chemical structure containing an epoxy resin and an acid anhydride curing agent.

[0015] The solvent of the electrolytic solution may contain 10 wt % or more of a compound having a hydroxy group.

[0016] The compound having a hydroxy group may be ethylene glycol, diethylene glycol, or polyethylene glycol.

[0017] The electrolytic solution may contain sulfolane in an amount of 35 wt % or more of the total amount of solvent in the electrolytic solution.

[0018] The capacitor element may further include a solid electrolyte.

[0019] The electrolyte solution may contain 76 mmol or less of cationic components per 100 g of the electrolyte solution, or 25 mmol or less of cationic components per 100 g of the electrolyte solution. [Effects of the Invention]

[0020] According to the present invention, it is possible to suppress an increase in ESR over time in an electrolytic capacitor provided with a resin layer. [Brief explanation of the drawings]

[0021] [Figure 1] 1 is a graph showing the relationship between the ratio of ethylene glycol and ESR. [Figure 2] 10 shows fluoroscopic images taken from a plurality of directions in Comparative Example 1, Comparative Example 2, and Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0022] Hereinafter, electrolytic capacitors according to embodiments of the present invention will be described, but the present invention is not limited to the embodiments described below.

[0023] (Overall composition) Electrolytic capacitors are passive devices that store and discharge electric charge through capacitance. These include liquid electrolytic capacitors, which contain only an electrolyte solution, and hybrid electrolytic capacitors, which combine an electrolyte solution with a solid electrolyte such as a conductive polymer or gel. These also include electrolytic capacitors with a dielectric oxide film intentionally formed only on the anode side and bipolar electrolytic capacitors, which have a dielectric oxide film formed on both electrodes. Furthermore, these electrolytic capacitors include electrolytic capacitors with lead terminals, electrolytic capacitors that are surface-mounted on a board with a base, and electrolytic capacitors that are completely coated with resin along with other electronic circuit components. Hereinafter, these various combinations of electrolytic capacitors will be collectively referred to as "electrolytic capacitors."

[0024] An electrolytic capacitor has a capacitor element, a case, and a sealing member. The case houses the capacitor element. The sealing member is attached to the opening of the case by crimping, sealing the opening. The capacitor element includes an anode foil, a cathode foil, a separator, and an electrolyte. The anode foil and the cathode foil face each other via the separator. A dielectric oxide film is formed on the surface of the anode foil. A dielectric oxide film is also formed on the cathode foil, if necessary. The electrolyte includes an electrolyte. In addition to the electrolyte, a solid electrolyte may also be included. The solid electrolyte is interposed between the anode foil and the cathode foil and is in close contact with the dielectric oxide film. The electrolyte is filled in the gaps of the capacitor element. The electrolytic capacitor may also have a base, if necessary.

[0025] (Sealing member) The sealing member is attached to the case by crimping. The case is made of aluminum, an aluminum alloy containing aluminum or manganese, or stainless steel, and is, for example, a cylindrical body with a bottom and an open end. During crimping, the opening of the case is bent inward and crushed, thereby tightly sealing the case to the sealing member. The elastic sealing member is a plate made of an elastomer, or a plate made by laminating a synthetic resin plate or a metal plate to an elastomer. The elastomer adheres to the crimped case by its elasticity, maintaining the airtightness of the case. However, sealing members containing elastomers have a non-zero permeability to volatilized electrolyte. Examples of such elastomers include butyl rubber, ethylene propylene diene rubber (EPDM), silicone rubber, and butyl rubber.

[0026] (resin layer) The electrolytic capacitor further includes a resin layer. The resin layer suppresses evaporation of the electrolyte. That is, the resin layer may be provided anywhere in the electrolytic capacitor near the sealing member through which the electrolyte may pass. For example, the resin layer covers at least a portion of the surface of the sealing member. This resin layer is molded to cover the sealing member and reduce the permeability of the electrolyte through the sealing member. That is, "nearby" does not necessarily mean being spaced apart, and includes a contact arrangement. Furthermore, for example, if the electrolytic capacitor is a chip type with a base, the resin layer is provided near the sealing member, for example, at or around the contact point between the base and the case. The resin layer suppresses evaporation of the electrolyte that has passed through the sealing member. In other words, this resin layer may also react with water or ethylene glycol in the electrolyte that has passed through the sealing member and dissolve into the electrolyte through the sealing member.

[0027] This resin layer is made of an acid anhydride-cured epoxy resin. The resin layer may contain compounds other than the acid anhydride-cured epoxy resin. The acid anhydride-cured epoxy resin is an epoxy resin composition made from epoxy resin and an acid anhydride curing agent as raw materials, and contains the epoxy resin and the acid anhydride curing agent in its chemical structure, and is obtained by curing the epoxy resin with an acid anhydride-based curing agent. This acid anhydride-cured epoxy resin has an ester bond in its chemical structure.

[0028] Specifically, acid anhydride-cured epoxy resins are produced by the reaction of hydroxyl groups present in the epoxy resin with the acid anhydride groups of the acid anhydride curing agent to form monoesters, the resulting carboxyl groups then react with epoxy groups to form diesters, and simultaneously forming hydroxyl groups, which then react with the next acid anhydride group, through a series of successive reactions.As a result, this acid anhydride-cured epoxy resin has ester bonds in its chemical structure.

[0029] As can be seen from this reaction system, an epoxy resin composition containing an epoxy resin and an acid anhydride curing agent in its chemical structure does not mean an epoxy resin composition in which the epoxy resin and the acid anhydride curing agent are incorporated into the chemical structure in the same state as before the reaction. "Containing an epoxy resin and an acid anhydride curing agent in its chemical structure" means that the epoxy resin and the acid anhydride curing agent are incorporated into the chemical structure after substitution of each group for bonding has occurred.

[0030] The epoxy resin contained in acid anhydride-cured epoxy resins is an epoxy oligomer with two or more reactive epoxy groups at the terminals. This epoxy resin changes from a liquid to a solid resin by crosslinking the epoxy resins with each other through the addition reaction of the acid anhydride curing agent. Typical examples of epoxy resins include bisphenol A diglycidyl ether, a condensation product of bisphenol A and epichlorohydrin. Other examples of epoxy resins include other glycidyl-type epoxy resins and alicyclic epoxides such as 3,4-epoxycyclohexylmethyl-3',4'-epoxycyclohexanecarboxylate.

[0031] Glycidyl epoxy resins include bisphenol types obtained by glycidylating bisphenols, such as bisphenol A, bisphenol F, bisphenol AD, bisphenol S, tetramethylbisphenol A, tetramethylbisphenol F, tetramethylbisphenol AD, tetramethylbisphenol S, tetrabromobisphenol A, tetrachlorobisphenol A, and tetrafluorobisphenol A.

[0032] Glycidyl epoxy resins include epoxy resins obtained by glycidylating dihydric phenols, such as biphenol, dihydroxynaphthalene, and 9,9-bis(4-hydroxyphenyl)fluorene.

[0033] Glycidyl epoxy resins include epoxy resins obtained by glycidylating trisphenols. Triphenols include 1,1,1-tris(4-hydroxyphenyl)methane and 4,4-(1-(4-(1-(4-hydroxyphenyl)-1-methylethyl)phenyl)ethylidene)bisphenol.

[0034] Glycidyl epoxy resins include epoxy resins obtained by glycidylating tetrakisphenols, such as 1,1,2,2-tetrakis(4-hydroxyphenyl)ethane.

[0035] The glycidyl epoxy resins include novolac epoxy resins obtained by glycidylating novolacs, such as phenol novolac, cresol novolac, bisphenol A novolac, brominated phenol novolac, and brominated bisphenol A novolac.

[0036] Examples of glycidyl type epoxy resins include epoxy resins obtained by glycidylating polyhydric phenols, and aliphatic ether type epoxy resins obtained by glycidylating polyhydric alcohols such as glycerin and polyethylene glycol.

[0037] Examples of glycidyl epoxy resins include ether ester epoxy resins obtained by glycidylating hydroxycarboxylic acids, ester epoxy resins obtained by glycidylating polycarboxylic acids, glycidyl compounds of amine compounds, and amine epoxy resins. Examples of hydroxycarboxylic acids include p-oxybenzoic acid and β-oxynaphthoic acid. Examples of polycarboxylic acids include phthalic acid and terephthalic acid. Examples of amine compounds include 4,4-diaminodiphenylmethane and m-aminophenol. Examples of amine epoxy resins include triglycidyl isocyanurate.

[0038] Examples of the acid anhydride curing agent contained in the acid anhydride-cured epoxy resin include phthalic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, methyltetrahydrophthalic anhydride, methylhexahydrophthalic anhydride, methylnadic anhydride, succinic anhydride, dodecenylsuccinic anhydride, chloric anhydride, maleic anhydride, dichloromaleic anhydride, etc. Other examples of the acid anhydride curing agent include aromatic polycarboxylic acid anhydrides and styrene-maleic anhydride copolymers.

[0039] (Capacitor element) (electrolyte) The electrolyte to be impregnated into the capacitor element is a solution of an ion-dissociating salt that dissociates into an anion component and a cation component, or a solvent that does not contain an ion-dissociating salt. The proportion of the compound having a hydroxy group in the solvent is 45 wt% or less, including 0 wt%, based on the total solvent. It is also possible for the solvent to contain no compound having a hydroxy group, including 0 wt%. Preferably, the compound having a hydroxy group is contained in 10 wt% or more based on the total solvent.

[0040] Examples of compounds having a hydroxy group include protic organic polar solvents. Examples of protic organic solvents include monohydric alcohols, polyhydric alcohols, and oxyalcohol compounds. Examples of monohydric alcohols include ethanol, propanol, butanol, pentanol, hexanol, cyclobutanol, cyclopentanol, cyclohexanol, and benzyl alcohol. Examples of polyhydric alcohols and oxyalcohol compounds include ethylene glycol, diethylene glycol, propylene glycol, glycerin, methyl cellosolve, ethyl cellosolve, methoxypropylene glycol, dimethoxypropanol, and alkylene oxide adducts of polyhydric alcohols such as polyethylene glycol and polyoxyethylene glycerin.

[0041] When the proportion of the compound having a hydroxyl group is 45 wt% or less relative to the total solvent, the increase in ESR of the electrolytic capacitor over time is suppressed even if an epoxy resin having an ester bond, such as an acid anhydride-cured epoxy resin, is used in the resin layer.

[0042] The sudden change in ESR at the 45 wt% threshold is speculative and not limited to this mechanism, but it is thought to be as follows: First, the acid anhydride-cured epoxy resin reacts with water and undergoes hydrolysis. Furthermore, dehydration condensation occurs as a reverse reaction to hydrolysis. Hydrolysis and dehydration condensation are reversible reactions, so they maintain equilibrium. However, in addition to the hydrolysis and dehydration condensation reactions, an esterification reaction occurs between the carbonyl groups produced by hydrolysis and compounds containing hydroxyl groups. This esterification reaction disrupts the equilibrium between hydrolysis and dehydration condensation, accelerating the hydrolysis reaction. As a result, the epoxy resin decomposes and dissolves in the electrolyte, increasing the ESR of the electrolytic capacitor. However, when the proportion of compounds containing hydroxyl groups in the solvent is below 45 wt%, the esterification reaction does not occur so rapidly as to accelerate hydrolysis, and it is thought that the equilibrium between hydrolysis and dehydration condensation is less likely to be disrupted.

[0043] Furthermore, it is preferable for the proportion of the hydroxyl group-containing compound to be between 10 wt% and 45 wt% of the total solvent, as this further suppresses the increase in ESR of the electrolytic capacitor over time. When a solid electrolyte is used in combination with an electrolyte solution containing a hydroxyl group-containing compound such as ethylene glycol as the solvent, a proportion of 10 wt% or more of the total solvent is thought to improve the electrical conductivity of the conductive polymer by changing the higher-order structure of the conductive polymer and reorienting the crystalline structure of the polymer chain. However, this effect cannot be obtained with gamma-butyrolactone.

[0044] The solvent may be composed of a compound having a hydroxy group and an aprotic organic polar solvent. Representative examples of aprotic organic polar solvents include sulfones, amides, lactones, cyclic amides, nitriles, and sulfoxides. Examples of sulfones include dimethyl sulfone, ethyl methyl sulfone, diethyl sulfone, sulfolane, 3-methyl sulfolane, and 2,4-dimethyl sulfolane. Examples of amides include N-methylformamide, N,N-dimethylformamide, N-ethylformamide, N,N-diethylformamide, N-methylacetamide, N,N-dimethylacetamide, N-ethylacetamide, N,N-diethylacetamide, and hexamethylphosphoric amide. Examples of lactones and cyclic amides include γ-butyrolactone, γ-valerolactone, δ-valerolactone, N-methyl-2-pyrrolidone, ethylene carbonate, propylene carbonate, butylene carbonate, and isobutylene carbonate. Examples of nitriles include acetonitrile, 3-methoxypropionitrile, and glutaronitrile. Examples of sulfoxides include dimethyl sulfoxide.

[0045] Among these, sulfolane is particularly preferred. Sulfolane has a high boiling point, which makes it difficult for the solvent to evaporate, thereby enabling the electrolytic capacitor to have a longer life. Furthermore, sulfolane does not contain an ether bond. Compared to compounds having an ether bond, such as γ-butyrolactone, sulfolane has a structure that makes it difficult for water to approach. Therefore, compared to compounds having an ether bond, sulfolane does not attract water when passing through the sealing material, further suppressing the leaching of a portion of the acid anhydride-cured epoxy resin component through the sealing material into the electrolyte. Furthermore, when comparing compounds having an ether bond with compounds having a hydroxy group, compounds having a hydroxy group are far more easily accessible to water.

[0046] When sulfolane is mixed with a compound having a hydroxyl group such as ethylene glycol to form a two-component solvent, the proportion of sulfolane in the solvent is preferably 55 wt% or more but less than 100 wt%, and more preferably 55 wt% or more but 90 wt% or less. Furthermore, from the viewpoint of further suppressing partial elution of the acid anhydride-cured epoxy resin component into the electrolyte, the proportion of sulfolane in the solvent is preferably 35 wt% or more.

[0047] Examples of organic acids that serve as anionic solutes include carboxylic acids such as oxalic acid, succinic acid, glutaric acid, pimelic acid, suberic acid, sebacic acid, phthalic acid, isophthalic acid, terephthalic acid, maleic acid, adipic acid, benzoic acid, toluic acid, enanthic acid, malonic acid, 1,6-decanedicarboxylic acid, 1,7-octanedicarboxylic acid, azelaic acid, resorcylic acid, phloroglucinic acid, gallic acid, gentisic acid, protocatechuic acid, pyrocatechuic acid, trimellitic acid, and pyromellitic acid, as well as phenols and sulfonic acids. Examples of inorganic acids include boric acid, phosphoric acid, phosphorous acid, hypophosphorous acid, carbonic acid, and silicic acid. Examples of composite compounds of organic acids and inorganic acids include borodisalicylic acid, borodioxalic acid, borodiglycolic acid, borodimalonic acid, borodisuccinic acid, borodiadipic acid, borodiazelaic acid, borodibenzoic acid, borodimaleic acid, borodilactic acid, borodimalic acid, boroditartaric acid, borodicitric acid, borodiphthalic acid, borodi(2-hydroxy)isobutyric acid, borodiresorcylic acid, borodimethylsalicylic acid, borodinaphthoic acid, borodimandelic acid, and borodi(3-hydroxy)propionic acid.

[0048] Furthermore, examples of at least one salt of an organic acid, an inorganic acid, or a composite compound of an organic acid and an inorganic acid include ammonium salts, quaternary ammonium salts, quaternized amidinium salts, amine salts, sodium salts, and potassium salts. Examples of quaternary ammonium ions of quaternary ammonium salts include tetramethylammonium, triethylmethylammonium, and tetraethylammonium. Examples of quaternized amidinium salts include ethyldimethylimidazolinium and tetramethylimidazolinium. Examples of amine salts include salts of primary amines, secondary amines, and tertiary amines. Examples of primary amines include methylamine, ethylamine, and propylamine. Examples of secondary amines include dimethylamine, diethylamine, ethylmethylamine, and dibutylamine. Examples of tertiary amines include trimethylamine, triethylamine, tributylamine, ethyldimethylamine, and ethyldiisopropylamine.

[0049] The cationic component of the solute is preferably added in an amount of 76 mmol or less per 100 g of electrolyte, more preferably 51 mmol or less per 100 g of electrolyte, even more preferably 34 mmol or less per 100 g of electrolyte, particularly preferably 25 mmol or less per 100 g of electrolyte, and most preferably 17 mmol or less per 100 g of electrolyte. The ratio of the cationic component to the anionic component of the solute may be equimolar, or there may be an excess of the cationic component or the anionic component.

[0050] Cationic components easily accelerate the dedoping reaction of conductive polymers in the solid electrolyte under high-heat environments, such as during the reflow process when mounting electrolytic capacitors. When the dedoping reaction is accelerated by cationic components, the electrical conductivity of the solid electrolyte decreases, and the ESR of the electrolytic capacitor increases. On the other hand, when a resin layer is placed near the sealing material, the resin layer prevents the cationic components from evaporating through the sealing material, suppressing the reduction of cationic components in the electrolytic capacitor. Therefore, when a resin layer is placed near the sealing material, dedoping reactions caused by cationic components occur frequently, and the ESR of the electrolytic capacitor increases.

[0051] However, if the amount of the cationic solute in the electrolyte is 76 mmol or less per 100 g of electrolyte, the increase in ESR is suppressed compared to when the amount exceeds 76 mmol. Furthermore, if the amount of the cationic solute in the electrolyte is 51 mmol or less per 100 g of electrolyte, the increase in ESR is dramatically suppressed compared to when the amount is, for example, 76 mmol per 100 g of electrolyte. If the amount of the cationic solute in the electrolyte is 25 mmol or less per 100 g of electrolyte, the increase in ESR is even more dramatically suppressed compared to when the amount is, for example, 51 mmol per 100 g of electrolyte.

[0052] Furthermore, other additives can be added to the liquid. Examples of additives include complex compounds of boric acid and polysaccharides (mannitol, sorbitol, etc.), complex compounds of boric acid and polyhydric alcohols, boric acid esters, nitro compounds (o-nitrobenzoic acid, m-nitrobenzoic acid, p-nitrobenzoic acid, o-nitrophenol, m-nitrophenol, p-nitrophenol, p-nitrobenzyl alcohol, etc.), and phosphate esters. These may be used alone or in combination of two or more.

[0053] (electrode foil) The anode foil and cathode foil are long foils made of valve metals such as aluminum, tantalum, niobium, niobium oxide, titanium, hafnium, zirconium, zinc, tungsten, bismuth, and antimony. The purity of the anode foil is preferably 99.9% or higher, and that of the cathode foil is preferably about 99% or higher, but impurities such as silicon, iron, copper, magnesium, and zinc may be included.

[0054] The anode foil has an enlarged surface, either as a sintered body made by sintering valve metal powder or as an etched foil made by etching a drawn foil. The enlarged surface structure is formed by tunnel-like pits, spongy pits, or voids between densely packed powder particles. The enlarged surface structure is typically formed by direct current etching or alternating current etching, in which direct current or alternating current is applied in an acidic aqueous solution containing halogen ions, such as hydrochloric acid, or by vapor deposition or sintering metal particles or the like into the core. The cathode foil may also have an enlarged surface structure by vapor deposition, sintering, or etching.

[0055] The dielectric oxide film is typically an oxide film formed on the surface of the anode foil. If the anode foil is made of aluminum, it is aluminum oxide formed by oxidizing the porous structure region. This dielectric oxide film is formed by chemical conversion treatment in which a voltage is applied in an aqueous solution of adipic acid, boric acid, phosphoric acid, or the like. Alternatively, a thin dielectric oxide film (approximately 1 to 10 V) may be formed on the surface of the cathode foil by chemical conversion treatment, as needed. Furthermore, the dielectric oxide film may be formed by using a layer made of a metal nitride, metal carbide, or metal carbonitride formed by vapor deposition, or by using a material containing carbon on the surface.

[0056] (separator) Examples of materials for the separator include cellulose papers such as kraft, Manila hemp, esparto, hemp, and rayon, and mixtures thereof; polyester-based resins such as polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, and derivatives thereof; polytetrafluoroethylene-based resins, polyvinylidene fluoride-based resins, vinylon-based resins; polyamide-based resins such as aliphatic polyamides, semi-aromatic polyamides, and wholly aromatic polyamides; polyimide-based resins, polyethylene resins, polypropylene resins, trimethylpentene resins, polyphenylene sulfide resins, acrylic resins, and polyvinyl alcohol resins. These resins can be used alone or in combination.

[0057] (solid electrolyte) When a solid electrolyte is formed within a capacitor element, it includes a conductive polymer. Conductive polymers are conjugated polymers or doped conjugated polymers. Conjugated polymers are obtained by chemical oxidative polymerization or electrolytic oxidative polymerization of monomers or their derivatives that have π-conjugated double bonds. Conductivity is exhibited by adding a small amount of an acceptor that easily accepts electrons or a donor that easily donates electrons to a conjugated polymer. When an acceptor or donor is added to a conjugated polymer, the acceptor extracts π electrons from the conjugated polymer, creating negatively charged atoms (positive holes), while the donor supplies electrons, creating negatively charged carriers, thereby exhibiting conductivity.

[0058] As the conjugated polymer, any known polymer can be used without any particular limitation. Examples include polypyrrole, polythiophene, polyfuran, polyaniline, polyacetylene, polyphenylene, polyphenylene vinylene, polyacene, polythiophene vinylene, etc. These conjugated polymers may be used alone or in combination of two or more types, or may even be a copolymer of two or more types of monomers.

[0059] Among the above conjugated polymers, conjugated polymers obtained by polymerizing thiophene or its derivatives are preferred, and conjugated polymers obtained by polymerizing 3,4-ethylenedioxythiophene (i.e., 2,3-dihydrothieno[3,4-b][1,4]dioxin), 3-alkylthiophene, 3-alkoxythiophene, 3-alkyl-4-alkoxythiophene, 3,4-alkylthiophene, 3,4-alkoxythiophene, or derivatives thereof are preferred. Thiophene derivatives are preferably compounds selected from thiophenes having substituents at the 3rd and 4th positions, and the substituents at the 3rd and 4th positions of the thiophene ring may form a ring together with the carbon atoms at the 3rd and 4th positions. Alkyl or alkoxy groups having 1 to 16 carbon atoms are suitable, and a polymer of 3,4-ethylenedioxythiophene known as EDOT, i.e., poly(3,4-ethylenedioxythiophene) known as PEDOT, is particularly preferred. Alternatively, alkylated ethylenedioxythiophenes in which an alkyl group is added to 3,4-ethylenedioxythiophene may be used, such as methylated ethylenedioxythiophene (i.e., 2-methyl-2,3-dihydro-thieno[3,4-b][1,4]dioxine) and ethylated ethylenedioxythiophene (i.e., 2-ethyl-2,3-dihydro-thieno[3,4-b][1,4]dioxine).

[0060] Known dopants can be used without any particular limitation. Examples of dopants include inorganic acids such as boric acid, nitric acid, and phosphoric acid, and organic acids such as acetic acid, oxalic acid, citric acid, ascot acid, tartaric acid, squaric acid, rhodizonic acid, croconic acid, salicylic acid, p-toluenesulfonic acid, 1,2-dihydroxy-3,5-benzenedisulfonic acid, methanesulfonic acid, trifluoromethanesulfonic acid, borodisalicylic acid, bisoxalateborate acid, sulfonylimide acid, dodecylbenzenesulfonic acid, propylnaphthalenesulfonic acid, and butylnaphthalenesulfonic acid. Examples of polyanions include polyvinylsulfonic acid, polystyrenesulfonic acid, polyallylsulfonic acid, polyacrylicsulfonic acid, polymethacrylic acid, poly(2-acrylamido-2-methylpropanesulfonic acid), polyisoprenesulfonic acid, polyacrylic acid, polymethacrylic acid, and polymaleic acid.

[0061] These dopants may be used alone or in combination of two or more. Furthermore, these dopants may be polymers of a single monomer or copolymers of two or more monomers. Furthermore, the dopants may be polymers or monomers. [Example]

[0062] (Examples 1 to 11) The electrolytic capacitor of the present invention will be described in more detail below based on examples. It should be noted that the present invention is not limited to the following examples. Various electrolytic capacitors were fabricated by coating a sealing member with a resin layer containing an acid anhydride curing agent epoxy resin. The electrolytic capacitors of Comparative Examples 1 to 3 and Examples 1 to 11 have an electrolyte that combines an electrolytic solution and a solid electrolyte, and the ratio of ethylene glycol in the solvent of the electrolytic solution is different. The acid anhydride curing agent epoxy resin is an example of an epoxy resin having an ester bond, and ethylene glycol is an example of a compound having a hydroxy group.

[0063] The electrolytic capacitors of Comparative Examples 1 to 3 and Examples 1 to 11 have the following in common: The anode foil was aluminum foil, which was etched to enlarge its surface and then chemically treated to form a dielectric oxide film. The cathode foil was plain aluminum foil, i.e., unetched. The same anode and cathode foils were used in all electrolytic capacitors, each with a lead wire connected, and the anode and cathode foils were wound facing each other with a manila separator interposed between them. The capacitor elements were repaired by immersing them in an ammonium dihydrogen phosphate aqueous solution of the same concentration for 10 minutes.

[0064] Next, a dispersion of the same conductive polymer, polyethylenedioxythiophene doped with polystyrene sulfonic acid (PEDOT / PSS), was prepared at the same concentration. The capacitor elements were immersed in the dispersion, removed, and dried at 150°C for 30 minutes. The immersion and drying process was repeated the same number of times. This formed a solid electrolyte in the capacitor elements. Next, a different electrolyte solution was prepared for each electrolytic capacitor, and the capacitor elements with the solid electrolyte formed were immersed in the electrolyte solution. This capacitor element was inserted into a bottomed cylindrical outer case of the same metal type, dimensions, and shape. A sealing material was attached to the open end and the capacitors were sealed by crimping with the same pressure. A sealing material made of butadiene rubber was used for each electrolytic capacitor.

[0065] Each electrolytic capacitor was housed in an aluminum container of the same dimensions, and a resin layer of the same type of epoxy resin was formed so that it was completely embedded in the capacitor. However, the lead wires from the electrolytic capacitor were pulled out of the epoxy resin so that electricity could be passed through the electrolytic capacitor. The base of the epoxy resin poured into the container was a bisphenol A-type liquid epoxy resin, and the curing agent was tetrahydromethylphthalic anhydride, an acid anhydride. As a result, the outer surface of the sealing material of each electrolytic capacitor was completely covered with acid anhydride-cured epoxy resin.

[0066] The electrolyte solutions prepared for the electrolytic capacitors of Comparative Examples 1 to 3 and Examples 1 to 11 are as shown in Table 1 below. (Table 1) TIFF0007768122000001.tif107132

[0067] In Table 1 above, the composition ratio of the solvent indicates the weight ratio relative to the total amount of solvent. As shown in Table 1 above, each electrolytic capacitor has a different composition and composition ratio of the solvent in the electrolytic solution. The electrolytic solutions of the electrolytic capacitors of Comparative Examples 1 to 3 contain ethylene glycol at a concentration of 50 wt% or more relative to the total amount of solvent. The electrolytic solutions of the electrolytic capacitors of Examples 1 to 10 contain ethylene glycol in the range of 0 wt% to 45 wt% relative to the total amount of solvent, with the amount of ethylene glycol varying in 5 wt% increments.

[0068] In the electrolytic solutions of the electrolytic capacitors of Comparative Examples 2 and 3 and Examples 1 to 10, the remaining solvent other than ethylene glycol was sulfolane. In the electrolytic solution of the electrolytic capacitor of Example 11, ethylene glycol was not used, and γ-butyrolactone was used instead of sulfolane, which was the total solvent in Example 10. In the electrolytic capacitors of Comparative Examples 1 to 3 and Examples 1 to 11, no solute was added in order to clarify the influence of only the solvent in the electrolytic solution.

[0069] After filling each electrolytic capacitor with acid anhydride-cured epoxy resin, a constant voltage of 35 V was applied to each electrolytic capacitor, and they were then left in a 150°C temperature environment for 400 hours. The ESR was measured before and after this thermal stress. The ESR was measured at 100 kHz. In addition, the inside of each electrolytic capacitor after the thermal stress was photographed with an X-ray machine, and the internal condition was observed using a fluoroscopic image.

[0070] The ESR and internal condition of each electrolytic capacitor before and after thermal stress loading, as well as the results of fluoroscopic images, are shown in Table 2 below. (Table 2) TIFF0007768122000002.tif70161

[0071] A graph showing the relationship between the ratio of ethylene glycol and ESR is shown in Figure 1, based on the ESR after thermal stress in Table 2. Note that the ESR of Examples 1 to 11 and Comparative Examples 1 to 3 is different in magnitude, so only the data for the electrolytic capacitors of Examples 1 to 10 is plotted in Figure 1. As shown in Figure 1 and Table 2, the ESR of Examples 1 to 11 and Comparative Examples 1 to 3 is different in magnitude. The ESR of Examples 1 to 11 was suppressed to at least 1 / 35 and at most 1 / 94 of that of Comparative Examples 1 to 3.

[0072] As a result, it was confirmed that ESR can be suppressed by keeping the proportion of ethylene glycol in the solvent below 45 wt%. Furthermore, as shown in Figure 1, when the proportion of ethylene glycol in the solvent is less than 10 wt%, ESR tends to increase, although not as sharply as in the range of 45 wt% to 100 wt%. Therefore, it was confirmed that an ethylene glycol content of 10 wt% or more is preferable. In particular, when ethylene glycol is present in the solvent in combination with a solid electrolyte, changes in the higher-order structure of the conductive polymer and reorientation of the crystalline structure of the polymer chains improve the electrical conductivity of the conductive polymer. However, since this effect cannot be obtained with γ-butyrolactone, an ethylene glycol proportion of 10 wt% or more is preferable.

[0073] Furthermore, as shown in Table 2, fluoroscopic images taken with an X-ray device revealed cracks in the resin layer of acid anhydride-cured epoxy resin in which the electrolytic capacitors of Comparative Examples 1 to 3 were embedded. In contrast, no cracks were found in the resin layer of acid anhydride-cured epoxy resin in which the electrolytic capacitors of Examples 1 to 11 were embedded. Here, fluoroscopic images taken from multiple directions of Comparative Example 1, Comparative Example 2, and Example 1 are shown in FIG. 2. As shown in FIG. 2, cracks are visible in the areas surrounded by dotted lines in Comparative Examples 1 to 3. On the other hand, no cracks are found in the fluoroscopic images of Example 1. The area in which cracks are visible is between the butadiene rubber sealing member and the resin layer of acid anhydride-cured epoxy resin.

[0074] The results confirmed that the resin in the resin layer decomposed when the proportion of ethylene glycol in the solvent exceeded 45 wt%. The electrolytic capacitors of Comparative Example 1 and Example 11 were placed in aluminum containers, filled with epoxy resin, and left at 150°C for 400 hours. The epoxy resin base was a bisphenol A liquid epoxy resin, and the curing agent was tetrahydromethylphthalic anhydride, an acid anhydride. After this hot storage, the electrolytes from the electrolytic capacitors of Comparative Example 1 and Example 11 were extracted and filtered, and high-performance liquid chromatography was used to test whether substances derived from the resin layer were detected in the electrolyte. The results are shown in Table 3 below.

[0075] (Table 3) TIFF0007768122000003.tif39165

[0076] In Table 3, EG represents ethylene glycol and GBL represents γ-butyrolactone. An electrolyte solution was prepared and a reference chromatograph for the electrolyte solution alone was obtained by high-performance liquid chromatography. When a peak not seen in the reference chromatograph was present in the chromatograph obtained by analyzing Comparative Example 1 and Example 11, it was deemed that a resin-derived component was detected. As shown in Table 3, a substance derived from the resin layer was detected in Comparative Example 1. On the other hand, no substance derived from the resin layer was detected in Example 11.

[0077] From the above results, it can be understood that the resin in the resin layer is an acid anhydride-cured epoxy resin with an ester bond, and ethylene glycol is a compound with a hydroxy group. Therefore, the acid anhydride-cured epoxy resin decomposes due to hydrolysis, the decomposition products react with ethylene glycol, a compound with a hydroxy group, these reaction products dissolve into the electrolyte, and the components derived from the resin layer in the electrolyte increase the ESR.

[0078] When an epoxy resin having an ester bond is used as the resin layer, or at least when an acid anhydride-cured epoxy resin is used as the resin layer, the amount of the compound having a hydroxyl group is limited to 45 wt% or less of the total amount of solvent in the electrolyte solution, which prevents components from eluting into the electrolyte solution and suppresses the increase in ESR.

[0079] Examples 12 to 19 Next, electrolytic capacitors of Examples 12 to 19 were fabricated. In Examples 12 to 19, azelaic acid was added to the electrolyte as the anionic solute component, and ammonia was added to the electrolyte as the cationic solute component. The electrolytic capacitors of Examples 12 to 19 differed in the amount of cationic solute component in the electrolyte. The electrolytic capacitors of Examples 12 to 19 were housed in aluminum containers of the same dimensions as Examples 1 to 11 and completely embedded in the same acid anhydride-cured epoxy resin as Examples 1 to 11. Additionally, the electrolytic capacitors of Examples 12 to 19 were fabricated using the same method and under the same conditions as Examples 1 to 11.

[0080] The electrolytic capacitors of Examples 12 to 19 were filled with an acid anhydride-cured epoxy resin, and then left in a 150°C temperature environment for 800 hours while applying a constant voltage of 35 V to each electrolytic capacitor. The ESR was measured before and after the thermal stress load, and the rate of change in ESR after the thermal stress load was calculated. The ESR was measured at 100 kHz.

[0081] The compositions and amounts of the electrolytes of Examples 12 to 19, as well as the ESR before thermal stress (initial ESR), the ESR after thermal stress (post-test ESR), and the rate of change in ESR after thermal stress are shown in Table 4 below. The amount of solvent in Table 4 indicates the content relative to the total solvent, and the amount of solute in Table 4 is converted per 100 g of electrolyte. The electrolytic capacitors of Examples 12 to 19 were wound types with a diameter of 6 mm and a total length of 6 mm, with a rated voltage of 35 V and a rated capacitance of 47 μF. (Table 4) TIFF0007768122000004.tif60166

[0082] As shown in Table 4, the electrolytic capacitor of Example 18, which contained 76 mmol of ammonia as a cationic component, had an ESR change rate suppressed to about 55% compared to the electrolytic capacitor of Example 19, which contained 86 mmol of ammonia. Furthermore, the electrolytic capacitor of Example 17, which contained 51 mmol of ammonia, had an ESR change rate suppressed to about 24% compared to the electrolytic capacitor of Example 18, which contained 76 mmol of ammonia. Furthermore, the electrolytic capacitor of Example 14, which contained 25 mmol of ammonia, had an ESR change rate suppressed to about 18% compared to the electrolytic capacitor of Example 17, which contained 51 mmol of ammonia.

[0083] Thus, when an electrolytic capacitor has a resin layer, the cationic components tend to promote the undoping reaction and increase the ESR. However, it was confirmed that the increase in ESR can be suppressed by adjusting the cationic components to 76 mmol or less per 100 g of electrolyte, that the increase in ESR can be further suppressed by adjusting the cationic components to 51 mmol or less per 100 g of electrolyte, and that the ESR after thermal stress can be further suppressed by adjusting the cationic components to 25 mmol or less per 100 g of electrolyte.

[0084] Examples 20 to 23 Electrolytic capacitors of Examples 20 to 23 and Comparative Example 4 were fabricated. The electrolyte solutions of Examples 20 to 23 and Comparative Example 4 differed in the ratio of sulfolane in the solvent. Sulfolane is an example of a compound that lacks a hydroxy group and an ether bond. In order to clarify the influence of the electrolyte solvent alone, no solute was added to the electrolytic capacitors of Examples 20 to 23 and Comparative Example 4. Additionally, the electrolytic capacitors of Examples 20 to 23 and Comparative Example 4 were fabricated using the same method and under the same conditions as Examples 1 to 11, including being housed in aluminum containers of the same dimensions and completely embedded in acid anhydride-cured epoxy resin.

[0085] After filling with acid anhydride cured epoxy resin, each electrolytic capacitor was left in a temperature environment of 150°C for 5,500 hours. Tan δ was measured before and after this thermal stress load, and the rate of change in tan δ after the thermal stress load was calculated.

[0086] The compositions and amounts of the electrolytes added in Examples 20 to 23 and Comparative Example 4, as well as tan δ before thermal stress (initial tan δ), tan δ after thermal stress (post-test tan δ), and the rate of change in tan δ after thermal stress are shown in Table 5. The electrolytic capacitors in each Example and Comparative Example were wound type with a diameter of 10 mm and a total length of 10 mm, a rated voltage of 35 V, and a rated capacity of 270 μF. (Table 5) TIFF0007768122000005.tif42165

[0087] As shown in Table 5, the electrolyte of Comparative Example 4 was composed solely of ethylene glycol, which has a hydroxyl group, while the electrolytes of Examples 22 and 23 had a reduced ethylene glycol content and an increased sulfolane content to compensate for the reduced ethylene glycol content. The electrolyte of Example 23 was composed solely of sulfolane, which has neither a hydroxyl group nor an ether bond. Comparing Comparative Example 4, Example 22, and Example 23, it can be seen that reducing the ethylene glycol content and increasing the sulfolane content suppresses the increase in tan δ before and after thermal stress.

[0088] Furthermore, Example 20 and Example 21 have the same amount of ethylene glycol but different amounts of γ-butyrolactone, and Example 21 has a reduced amount of γ-butyrolactone compared to Example 20, but sulfolane was added instead. Comparing Example 20 and Example 21, it can be confirmed that when sulfolane is added in a proportion of 35 wt % or more relative to the total amount of solvent in the electrolyte solution, the increase in tan δ before and after thermal stress loading is suppressed.

[0089] In this way, it can be seen that by reducing the amount of compound having a hydroxy group and compound having an ether bond added to the electrolyte solution and instead adding sulfolane, which has neither a hydroxy group nor an ether bond, in an amount of 35 wt% or more relative to the total amount of solvent in the electrolyte solution, further elution of components derived from the resin layer into the electrolyte solution is prevented, and deterioration of the characteristics of the electrolytic capacitor is further suppressed.

Claims

1. a capacitor element having an anode foil, a cathode foil, and an electrolyte; a case that houses the capacitor element; a sealing member that seals the case; a resin layer disposed in the vicinity of the sealing member; Equipped with the resin layer comprises an epoxy resin composition containing an ester bond, the solvent of the electrolytic solution contains a compound having a hydroxy group in an amount of 10 wt % or more and 45 wt % or less; An electrolytic capacitor characterized by:

2. The epoxy resin composition is made from an epoxy resin and an acid anhydride curing agent as raw materials; 2. The electrolytic capacitor according to claim 1,

3. the epoxy resin composition has a chemical structure including an epoxy resin and an acid anhydride curing agent; 2. The electrolytic capacitor according to claim 1,

4. the compound having a hydroxy group is ethylene glycol, diethylene glycol, or polyethylene glycol; 4. The electrolytic capacitor according to claim 1, wherein:

5. The electrolyte solution contains sulfolane in an amount of 35 wt % or more of the total solvent content of the electrolyte solution; 5. The electrolytic capacitor according to claim 1, wherein:

6. the capacitor element further comprises a solid electrolyte; 6. The electrolytic capacitor according to claim 1, wherein:

7. The electrolyte solution contains 76 mmol or less of cationic components per 100 g of the electrolyte solution; 7. The electrolytic capacitor according to claim 6,

8. The electrolyte solution contains 25 mmol or less of cationic components per 100 g of the electrolyte solution; 7. The electrolytic capacitor according to claim 6,

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