Sulfonaphthalenecarboxylic acid compound production method, organic conductor containing disulfonaphthalic acid compound, and electrolytic capacitor

Abstract

This sulfonaphthalenecarboxylic acid compound production method comprises: a first step for sulfonating a naphthalenecarboxylic acid compound to obtain a first mixture containing a sulfonaphthalenecarboxylic acid compound; a second step for adding a basic aqueous solution to the first mixture to neutralize the first mixture; and a third step for purifying the second mixture obtained in the second step to obtain a purified sulfonaphthalenecarboxylic acid compound having a chlorine content of less than 50 ppm in terms of mass.

Description

Method for producing sulfonaphthalenecarboxylic acid compound, organic conductor containing disulfonaphthalic acid compound, and electrolytic capacitor

[0001] The present disclosure relates to a method for producing a sulfonaphthalene carboxylic acid compound, an organic conductor containing a disulfonaphthalic acid compound, and an electrolytic capacitor.

[0002] Conjugated polymers such as polythiophene and polypyrrole exhibit electrical conductivity when doped. Conjugated polymers with dopants are known as conductive polymers, organic conductors, or solid electrolytes. Self-doping organic conductors have also been developed in recent years. Organic conductors are inexpensive and lightweight, allowing their performance to be controlled to some extent by selecting the type of conjugated polymer or additive (e.g., dopant). Therefore, they are used in a variety of electronic components. For example, an electrolytic capacitor containing an organic conductor includes an anode body with a dielectric layer on its surface and a solid electrolyte containing an organic conductor that covers a portion of the dielectric layer.

[0003] Proton-addition compounds and electron-oxidation compounds have been used as additive dopants. For example, it has been proposed to add organic sulfonic acids to the solid electrolyte layer of solid electrolytic capacitors. Sulfonaphthalene carboxylic acid compounds such as sulfonaphthoic acid, sulfonaphthalic acid, and disulfonaphthoic acid may be used (Patent Documents 1 and 2).

[0004] Sulfonaphthalenecarboxylic acid compounds are produced through various steps, such as a reaction step, a step of converting an acid group into a salt, and a step of reconverting the salt into an acid group. For example, in Non-Patent Document 1, 1,4-naphthalenecarboxylic acid and fuming sulfuric acid are stirred at 130°C for 10 hours, cooled, and poured into cold water to recover the precipitate. Then, this precipitate is dissolved in 1M Na ２ CO ３ The filtrate was acidified with 1M HCl to give 5,7-disulfo-1,4-naphthalenedicarboxylic acid, which can be obtained in a high yield of 79%.

[0005] International Publication No. 2019 / 131476 International Publication No. 2021 / 230013

[0006] Inorganic Chemistry, 2020, 59, pp7265-7273

[0007] When the product obtained by the manufacturing method of Non-Patent Document 1 is used as an organic conductor in an electrolytic capacitor, the equivalent series resistance (ESR) increases significantly after a load test is carried out in a high humidity environment.

[0008] A first aspect of the present disclosure relates to a method for producing a sulfonaphthalenecarboxylic acid compound, the method including: a first step of sulfonating a naphthalenecarboxylic acid compound to obtain a first mixture containing the sulfonaphthalenecarboxylic acid compound; a second step of adding a basic aqueous solution to the first mixture to neutralize it; and a third step of purifying the second mixture obtained in the second step to obtain a purified sulfonaphthalenecarboxylic acid compound having a chlorine content of less than 50 ppm by mass.

[0009] A second aspect of the present disclosure relates to an organic conductor including a conjugated polymer and a dopant, wherein the dopant blended with the conjugated polymer includes a disulfonaphthalic acid compound having a chlorine content of less than 50 ppm by mass, and the disulfonaphthalic acid compound has a naphthalene ring, two sulfo groups bonded to the naphthalene ring, and two carboxy groups or acid anhydride groups thereof bonded to the naphthalene ring.

[0010] A third aspect of the present disclosure relates to an electrolytic capacitor including an anode body having a dielectric layer on a surface thereof, and a solid electrolyte covering a portion of the dielectric layer, wherein the solid electrolyte includes the organic conductor described above.

[0011] The present invention provides a method for producing a purified sulfonaphthalenecarboxylic acid compound, which can suppress an increase in ESR after a load test under a high-humidity environment in an electrolytic capacitor, an organic conductor containing the purified disulfonaphthalic acid compound, and an electrolytic capacitor containing the organic conductor.

[0012] FIG. 1 is a cross-sectional schematic view of an electrolytic capacitor according to an embodiment of the present disclosure.

[0013] The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

[0014] Sulfonaphthalene carboxylic acid compounds have a naphthalene ring and two types of acid groups, a sulfo group and a carboxy group. The two types of acid groups are thought to facilitate interaction with conjugated polymers. Furthermore, the naphthalene ring provides relatively high heat resistance. Therefore, sulfonaphthalene carboxylic acid compounds are expected to be used, for example, as dopants for organic conductors.

[0015] The manufacturing method described in Non-Patent Document 1 produces 5,7-disulfo-1,4-naphthalenedicarboxylic acid in high yield. However, when the sulfonaphthalenecarboxylic acid compound obtained by this manufacturing method is used in an organic conductor, the ESR significantly increases after a load test in a high-humidity environment. One possible cause of this significant increase in ESR is corrosion of the dielectric layer. After various investigations into the causes of dielectric layer corrosion, it was revealed that the corrosion is due to the influence of chlorine remaining in the product, 5,7-disulfo-1,4-naphthalenedicarboxylic acid. This chlorine originates from the HCl used in converting the salt of 5,7-disulfo-1,4-naphthalenedicarboxylic acid to the free acid. Approximately 1% by mass of chlorine remains in the resulting product, and this small amount of chlorine is the cause of corrosion of the dielectric layer. HCl is a relatively harmless inorganic acid that is frequently used in converting salts to the free acid, among other things. Even if the small amount of chlorine remains in the product, such as the above, it does not pose a problem when used as a precursor for pharmaceuticals or dyes.

[0016] (Technology 1) The method for producing a sulfonaphthalenecarboxylic acid compound of the present disclosure includes: a first step of sulfonating a naphthalenecarboxylic acid compound to obtain a first mixture containing the sulfonaphthalenecarboxylic acid compound; a second step of adding a basic aqueous solution to the first mixture to neutralize it; and a third step of purifying the second mixture obtained in the second step to obtain a purified sulfonaphthalenecarboxylic acid compound having a chlorine content of less than 50 ppm by mass. Note that, in this specification, ppm by mass may be simply referred to as "ppm by mass."

[0017] In the present disclosure, the second mixture obtained in the second step is purified in the third step so that the chlorine content is less than 50 ppm by mass. Therefore, when the resulting purified sulfonaphthalene carboxylic acid compound is used in an organic conductor and this organic conductor is further used in an electrolytic capacitor, the increase in ESR after a load test under a high-humidity environment (hereinafter, sometimes simply referred to as "after the load test") can be significantly suppressed. More specifically, in the present disclosure, the increase in ESR in an electrolytic capacitor after a load test under a high-humidity environment can be significantly suppressed (for example, to less than 10 times the initial ESR (particularly, to 2.0 times or less or 1.5 times or less)) compared to when a product obtained and purified by the manufacturing method described in Non-Patent Document 1 is used. This is thought to be due in part to the suppression of corrosion of the dielectric layer. Therefore, the sulfonaphthalene carboxylic acid compound (purified sulfonaphthalene carboxylic acid compound) obtained by the manufacturing method of the present disclosure is useful for obtaining highly reliable electrolytic capacitors.

[0018] A sulfonaphthalene carboxylic acid compound has a naphthalene ring, one or more sulfo groups bonded to the naphthalene ring, and one or more carboxy groups bonded to the naphthalene ring. In sulfonaphthalene carboxylic acid compounds (such as disulfonaphthalic acid compounds) used in organic conductors and in organic conductors, the sulfo groups are free (-SO ３ H) or anion (-SO ３ －) or in the form of a salt. In organic conductors, sulfo groups may be contained in a form bonded to or interacting with a conjugated polymer. In electrolytic capacitors, sulfo groups may be contained in a form bonded to or interacting with other components contained in the electrolytic capacitor. In this specification, sulfo groups in all of these forms may be simply referred to as "sulfo groups." Similarly, in sulfonaphthalenecarboxylic acid compounds (such as disulfonaphthalic acid compounds) used in organic conductors and organic conductors, carboxy groups may be in the form of free (-COOH) or anions (-COO － ) or in the form of a salt. Furthermore, when a sulfonaphthalene carboxylic acid compound (e.g., a disulfonaphthalic acid compound) has two or more carboxy groups, for example, two carboxy groups may be contained in the form of a single acid anhydride group, or four carboxy groups may be contained in the form in which two adjacent carboxy groups each form one acid anhydride group (in other words, a total of two acid anhydride groups). In organic conductors, carboxy groups (including carboxy groups formed by hydrolysis of acid anhydride groups) may be contained in a form bonded to or interacting with a conjugated polymer. In electrolytic capacitors, carboxy groups (including carboxy groups formed by hydrolysis of acid anhydride groups) may be contained in a form bonded to or interacting with other components contained in the electrolytic capacitor. In this specification, all of these forms of carboxy groups may be simply referred to as "carboxy groups." For example, a naphthalene compound having at least one sulfo group and at least one acid anhydride group formed by two carboxy groups is also included in the sulfonaphthalene carboxylic acid compound of the present disclosure. The salt may be a salt of a sulfonic acid or carboxylic acid with either an organic base (organic amine, organic ammonium, etc.) or an inorganic base (metal hydroxide, ammonia, etc.).

[0019] In this specification, a high humidity environment may have a relative humidity of 70% RH or higher, 80% RH or higher, or 85% RH or higher. The relative humidity of a high humidity environment is 100% RH or lower. In this specification, a load test is performed in a high temperature and high humidity environment, which is a more severe environment. A high temperature in a high temperature and high humidity environment may be 40°C or higher, 60°C or higher, 80°C or higher, or 85°C or higher. A high temperature may be 280°C or lower. For example, in a reflow treatment in a substrate mounting process, an electrolytic capacitor is exposed to a temperature of 220°C or higher and 280°C or lower. Electrolytic capacitors are also sometimes used in high temperature environments. In this specification, a load test is performed in a high humidity environment of 85% RH at a high temperature of 85°C, for example. Then, the increase in ESR (initial ESR(Z ０ ) to the ratio of ESR (Z) after the load test (= Z / Z ０ The presence or absence of corrosion of the dielectric layer is determined based on the results of the load test. The load test is performed, for example, by leaving the electrolytic capacitor in a high-temperature, high-humidity environment while applying a predetermined voltage (for example, a voltage of 10 V or more and 20 V or less).

[0020] (Technology 2) In the above (Technology 1), the purified sulfonaphthalene carboxylic acid compound may have a metal cation content of less than 100 ppm by mass. When the basic aqueous solution used in the second step contains a metal hydroxide or the like, the resulting second mixture contains metal salts and metal cations derived from the metal hydroxide. The sulfonaphthalene carboxylic acid compound purified in the third step also has a low content of such metal components. Using such a highly purified sulfonaphthalene carboxylic acid compound as an organic conductor in an electrolytic capacitor is advantageous in further suppressing an increase in ESR after a load test.

[0021] The metal cation content is the mass-based content (unit: ppm) of the metal cations calculated by adding together the metals present as metal cations and the metal salts of the sulfonaphthalene carboxylic acid compound in the purified sulfonaphthalene carboxylic acid compound. In other words, it is the mass-based content of the metals contained in the purified sulfonaphthalene carboxylic acid compound converted into metal cations.

[0022] (Technology 3) In the above (Technology 1) or (Technology 2), the second mixture may be purified by ion exchange and reprecipitation in the third step. Purifying the second mixture by such a method suppresses chlorine contamination in the purified sulfonaphthalenecarboxylic acid compound, thereby further reducing the chlorine content. In addition, metal contamination can also be further reduced.

[0023] (Technology 4) In any one of the above (Technology 1) to (Technology 3), in the second step, the basic aqueous solution may contain an alkali metal hydroxide. In this case, because the second mixture contains an alkali metal salt of a sulfonaphthalene carboxylic acid compound, alkali metal cations are likely to be mixed into the sulfonaphthalene carboxylic acid compound. In the present disclosure, when the chlorine content is reduced in the third step, the alkali metal cation content can also be reduced. Therefore, the impurity content in the resulting purified sulfonaphthalene carboxylic acid compound can be reduced, and when used in an organic conductor for an electrolytic capacitor, an increase in ESR after a load test can be further suppressed.

[0024] (Technology 5) In any one of the above (Technology 1) to (Technology 4), the naphthalenecarboxylic acid compound may be a naphthalic acid compound or its acid anhydride. Because naphthalic acid has two carboxy groups, it is more susceptible to contamination with impurities such as metal cations in steps 1 to 3 than naphthoic acid. In the present disclosure, when a sulfo group is introduced into a naphthalic acid compound to produce a sulfonaphthalenecarboxylic acid compound (also referred to as a sulfonaphthalic acid compound), contamination with impurities such as chlorine and metal cations is more easily suppressed. In this specification, an acid anhydride of a naphthalic acid compound (more specifically, an acid anhydride in which two carboxy groups of a naphthalic acid compound form an acid anhydride group) may also be referred to as a naphthalic acid compound.

[0025] (Technology 6) In the above (Technology 5), the purified sulfonaphthalene carboxylic acid compound may include a polysulfonaphthalic acid compound or its acid anhydride. A polysulfonaphthalic acid compound has two or more sulfo groups and two carboxy groups. Compared to naphthalene compounds having one sulfo group or one carboxy group (such as sulfonaphthoic acid, sulfonaphthalic acid, and disulfonaphthoic acid), impurities such as metal cations are more likely to be mixed in during the first to third steps. In the present disclosure, when producing a purified polysulfonaphthalene carboxylic acid compound through the second and third steps, it is easier to further suppress the mixing of impurities such as chlorine and metal cations. In this specification, the acid anhydride of a polysulfonaphthalic acid compound (more specifically, an acid anhydride in which two carboxy groups of a polysulfonaphthalic acid compound form an acid anhydride group) may also be referred to as a polysulfonaphthalic acid compound.

[0026] (Technology 7) In the above (Technology 5) or (Technology 6), the purified sulfonaphthalene carboxylic acid compound may include a disulfonaphthalic acid compound or its acid anhydride having a naphthalene ring, two sulfo groups bonded to the naphthalene ring, and two carboxy groups or acid anhydride groups thereof bonded to the naphthalene ring. Disulfonaphthalic acid compounds are prone to contamination with impurities such as metal cations in steps 1 to 3. In the present disclosure, when producing a purified disulfonaphthalic acid compound through steps 2 and 3, contamination with impurities such as chlorine and metal cations is more easily suppressed. In this specification, acid anhydrides of disulfonaphthalic acid compounds (more specifically, acid anhydrides in which two carboxy groups of a disulfonaphthalic acid compound form acid anhydride groups) may also be referred to as disulfonaphthalic acid compounds.

[0027] Furthermore, when a disulfonaphthalic acid compound is used as a dopant in an organic conductor, the two sulfo groups and two carboxy groups on the naphthalene ring are thought to easily interact with conjugated polymers. Therefore, using a disulfonaphthalic acid compound as a dopant in an organic conductor (solid electrolyte) of an electrolytic capacitor is thought to reduce de-doping, in which the dopant is detached from the organic conductor, during a load test under a high-humidity environment. From this perspective, the increase in ESR after a load test under a high-humidity environment can be reduced. Disulfonaphthalic acid compounds are more effective in reducing the increase in ESR than other sulfonaphthalene carboxylic acid compounds (e.g., sulfonaphthoic acid, sulfonaphthalic acid, disulfonaphthoic acid, etc.). Therefore, disulfonaphthalic acid compounds are thought to interact more easily with conjugated polymers than other sulfonaphthalene carboxylic acid compounds. The same effect can be obtained with the acid anhydride of a disulfonaphthalic acid compound.

[0028] (Technology 8) The present disclosure also encompasses organic conductors. The organic conductor includes a conjugated polymer and a dopant. The dopant blended with the conjugated polymer includes a disulfonaphthalic acid compound having a chlorine content of less than 50 ppm by mass. The disulfonaphthalic acid compound has a naphthalene ring, two sulfo groups bonded to the naphthalene ring, and two carboxy groups or acid anhydride groups thereof bonded to the naphthalene ring. When the dopant of the organic conductor includes a disulfonaphthalic acid compound (including acid anhydrides) with a low chlorine content as described above, corrosion of the dielectric layer can be suppressed when the organic conductor is used in an electrolytic capacitor. Therefore, the present disclosure significantly reduces the increase in ESR after a load test compared to when a disulfonaphthalic acid compound obtained by the manufacturing method described in Non-Patent Document 1 is used. Furthermore, using a disulfonaphthalic acid compound (including acid anhydrides) as a dopant reduces undoping in high-humidity environments. From this perspective, the increase in ESR after the load test can be reduced.

[0029] (Technology 9) In the above (Technology 8), the conjugated polymer may contain a monomer unit corresponding to a pyrrole compound. The disulfonaphthalic acid compound exhibits a relatively high bonding strength with such conjugated polymers, and therefore is likely to suppress undoping. Therefore, the increase in ESR after a load test can be further reduced.

[0030] (Technology 10) The present disclosure also encompasses an electrolytic capacitor. The electrolytic capacitor includes an anode body having a dielectric layer on its surface and a solid electrolyte covering a portion of the dielectric layer. The solid electrolyte includes the organic conductor described in (Technology 8) or (Technology 9) above. The dopant contained in the solid electrolyte includes a disulfonaphthalic acid compound (including acid anhydride) having a chlorine content of less than 50 ppm by mass, thereby suppressing corrosion of the dielectric layer and significantly suppressing an increase in ESR after a load test. Furthermore, because disulfonaphthalic acid compounds have a high interaction or bonding strength with conjugated polymers, from this perspective, an increase in ESR after a load test can be suppressed compared to when other sulfonaphthalene carboxylic acid compounds are used as dopants.

[0031] The organic conductor and electrolytic capacitor of the present disclosure will be described in more detail below, including the above (Technology 1) to (Technology 10), with reference to the drawings as necessary. At least one of the above (Technology 1) to (Technology 10) may be combined with at least one of the elements described below, provided that there is no technical contradiction. Note that the drawings are schematic illustrations, and the ratios of the dimensions (e.g., thickness, length, width, height) of each component may differ from the actual ratios.

[0032] [Method for Producing Sulfonaphthalene Carboxylic Acid Compound] The method for producing a sulfonaphthalene carboxylic acid compound of the present disclosure provides a purified sulfonaphthalene carboxylic acid compound having a chlorine content of less than 50 ppm by mass. This production method includes at least a step of sulfonating a naphthalene carboxylic acid compound (Step 1), a step of neutralizing the mixture (first mixture) obtained in Step 1, and a step of purifying the mixture (second mixture) obtained in Step 2 (Step 3). Each step is described in more detail below.

[0033] (First Step) In the first step, a first mixture containing a sulfonaphthalenecarboxylic acid compound is obtained from a naphthalenecarboxylic acid compound. By sulfonating the naphthalenecarboxylic acid compound, a sulfo group is introduced into the naphthalene ring, thereby producing the sulfonaphthalenecarboxylic acid compound.

[0034] The sulfonation is carried out using, for example, fuming sulfuric acid. More specifically, a mixture of fuming sulfuric acid and a naphthalenecarboxylic acid compound is stirred under heating to obtain a first mixture containing the sulfonaphthalenecarboxylic acid compound.

[0035] Oleum is made by adding excess sulfur trioxide (SO ) to concentrated sulfuric acid. ３ The concentration (content) of sulfur trioxide in fuming sulfuric acid may be 10% by mass or more and 80% by mass or less, 15% by mass or more and 70% by mass or less, or 15% by mass or more and 45% by mass or less.

[0036] The molar ratio of sulfur trioxide to the naphthalenecarboxylic acid compound is determined, for example, depending on the number of sulfo groups to be introduced into the naphthalene skeleton. For example, when two sulfo groups are introduced into the naphthalenecarboxylic acid compound, the sulfur trioxide may be adjusted to be in excess of 2 moles (for example, 2 moles or more and 3 moles or less) per mole of the naphthalenecarboxylic acid compound. For example, the molar ratio of sulfur trioxide to the naphthalenecarboxylic acid compound may be adjusted by adjusting the concentration (mass%) of sulfur trioxide in fuming sulfuric acid or by adjusting the mass of fuming sulfuric acid.

[0037] The temperature at which the mixture containing the naphthalenecarboxylic acid compound and fuming sulfuric acid is heated may be 100° C. or higher and 180° C. or lower, or 110° C. or higher and 150° C. or lower. By carrying out the reaction at such a temperature, the sulfo group is smoothly introduced into the naphthalene ring. The reaction is carried out, for example, under heating and stirring.

[0038] The reaction (or heating) time may be from 1 hour to 24 hours, or from 2 hours to 15 hours. The reaction time may be adjusted while checking the degree of progress of the reaction.

[0039] After the reaction, the resulting reaction mixture is cooled. The cooling may be air-cooling by leaving the mixture in the atmosphere, or water-cooling using a water bath or running water. From the viewpoint of mitigating the heat generated during dilution after cooling, it is preferable to sufficiently cool the reaction mixture using ice-cooling using an ice bath or the like, a cooling device, or dry ice. The reaction mixture is preferably cooled to a temperature of 5°C or lower, or 0°C or lower (e.g., -10°C or lower). The cooling temperature of the reaction mixture is, for example, -50°C or higher. The cooling is carried out before the neutralization in the second step. The cooling may be carried out, for example, in the first step, or as a separate step from the first step, or may be carried out in the second step before the neutralization.

[0040] The cooled reaction mixture is diluted. Dilution is performed, for example, by adding cold water or ice, or both, to the reaction mixture. The cold water or ice may be, for example, ion-exchanged water or distilled water. The mass ratio of the reaction mixture to the cold water and ice (calculated as water) (= reaction mixture / water) may be 0.1 or more and 0.3 or less. The diluted reaction mixture is performed after cooling the reaction mixture obtained in the first step and before neutralization in the second step. Dilution may be performed, for example, in the first step, as a separate step from the first step, or in the second step before neutralization. Furthermore, a cooling step including a substep of cooling the reaction mixture and a substep of diluting the cooled reaction mixture may be performed between the first step and the second step.

[0041] Although a chlorine-containing component may be used in each of the first step, the cooling step, the cooling of the reaction mixture, and the dilution of the cooled reaction mixture, it is preferable not to use such a component. Furthermore, when a chlorine-containing component is used, it is preferable that the chlorine content in the component be low (for example, 1 mass % or less).

[0042] (Naphthalene Carboxylic Acid Compound) A naphthalene carboxylic acid compound contains a naphthalene ring and at least one carboxy group bonded to the naphthalene ring. The carbon positions constituting the naphthalene ring are numbered as shown in formula (A) below. The 1st to 8th, 4a-, and 8a-positions of the naphthalene ring correspond to the numbers on the naphthalene ring below.

[0043]

[0044] The number of carboxy groups bonded to the naphthalene ring may be 1 to 7, 1 to 4, 1 to 3, or 2 to 3. When the naphthalene carboxylic acid compound has an acid anhydride group, the number of acid anhydride groups may be 1 or 2, preferably 1. In a naphthalic acid compound having two carboxy groups, two adjacent carboxy groups may form an acid anhydride group. Naphthalic acid compounds also include such acid anhydrides. Naphthalic acid compounds are considered to be more susceptible to impurity contamination than naphthoic acid compounds. However, in the present disclosure, even when a naphthalic acid compound is used as a raw material, the second and third steps further suppress the contamination of impurities such as chlorine and metal cations. Furthermore, the resulting sulfonaphthalic acid compound is considered to easily interact or bond with conjugated polymers when used as a dopant in an organic conductor. Therefore, electrolytic capacitors containing such organic conductors can further suppress the increase in ESR after load testing under high humidity conditions.

[0045] In the naphthalene carboxylic acid compound, the naphthalene ring may have a substituent (first substituent) other than a carboxy group or an acid anhydride group. Naphthalene carboxylic acid compounds having a first substituent are also included in the naphthalene carboxylic acid compounds used as raw materials. The first substituent may be an electron-donating group, an electron-withdrawing group other than a carboxy group or an acid anhydride group, or the like. From the viewpoint of easily exhibiting a higher electron acceptor function and easily coordinating to a conjugated polymer depending on the balance of the sulfo group and the carboxy group, sulfo groups, hydroxy groups, aliphatic hydrocarbon groups, and the like are preferred as the first substituent. The number of carbon atoms in the aliphatic hydrocarbon group may be, for example, 1 to 10, 1 to 6, or 1 to 4. The aliphatic hydrocarbon group may be saturated or unsaturated. Examples of the aliphatic hydrocarbon group include alkyl groups, alkenyl groups, alkynyl groups, and dienyl groups. Of these, alkyl groups are preferred. Examples of the alkyl group include a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, a hexyl group, an octyl group, and a 2-ethylhexyl group. The naphthalene carboxylic acid compound may have one first substituent, or may have two or more first substituents. When the naphthalene carboxylic acid compound has two or more first substituents, at least two of the first substituents may be the same, or all of the first substituents may be different.

[0046] In the sulfonaphthalenecarboxylic acid compound, from the viewpoint that a higher electron acceptor function is likely to be exhibited due to the balance between the sulfo group and the carboxy group, it is also preferable that the sulfonaphthalenecarboxylic acid compound and the starting naphthalenecarboxylic acid compound do not have a first substituent.

[0047] Among the first substituents, a substituent other than a sulfo group can also be introduced at an appropriate stage after the introduction of a sulfo group in Step 1. From the viewpoint of further suppressing the incorporation of impurities into the sulfonaphthalenecarboxylic acid compound, when the sulfonaphthalenecarboxylic acid compound has a first substituent (specifically, a substituent other than a sulfo group), it is preferable that the starting naphthalenecarboxylic acid compound has the first substituent.

[0048] From the viewpoint of reducing the inclusion of impurities, the carboxy group in the raw material naphthalenecarboxylic acid compound is preferably in the free form or in the salt form, more preferably in the free form. Also when the raw material naphthalenecarboxylic acid compound has a sulfo group as the first substituent, from the same viewpoint as above, the sulfo group is preferably in the free form or in the salt form, more preferably in the free form.

[0049] (Sulfonaphthalene Carboxylic Acid Compound) In the product sulfonaphthalene carboxylic acid compound, the number of sulfo groups may be 1 or more and 7 or less, 2 or more and 4 or less, or 2 or more and 3 or less. The sulfonaphthalene carboxylic acid compound may contain a polysulfonaphthalene compound (including acid anhydride) having two or more sulfo groups. Among them, polysulfonaphthalic acid compounds (including acid anhydride) are preferred.

[0050] The sulfonaphthalene carboxylic acid compound may include a disulfonaphthalic acid compound (including an acid anhydride) having a naphthalene ring and two sulfo groups and two carboxy groups or acid anhydride groups thereof bonded to the naphthalene ring.

[0051] Polysulfonaphthalic acid compounds and disulfonaphthalic acid compounds have many acid groups, making them relatively susceptible to impurity contamination. However, in the present disclosure, the second and third steps make it easy to suppress impurity contamination even when the purified sulfonaphthalene carboxylic acid compound contains a polysulfonaphthalic acid compound or a disulfonaphthalic acid compound. In particular, disulfonaphthalic acid compounds are thought to have a strong interaction or bonding strength with conjugated polymers when used as a dopant in an organic conductor. Therefore, an increase in ESR after a load test can be further suppressed when an electrolytic capacitor containing such an organic conductor is used.

[0052] (Disulfonaphthalic Acid Compound) The naphthalene ring has a structure in which two benzene rings are fused. In this specification, the two benzene rings constituting the naphthalene ring may be referred to as the first benzene ring and the second benzene ring, respectively. The benzene ring having the carbon atoms at positions 1 to 4 of the naphthalene ring may be referred to as the first benzene ring, and the benzene ring having the carbon atoms at positions 5 to 8 of the naphthalene ring may be referred to as the second benzene ring.

[0053] The positions of the sulfo and carboxy groups are thought to be likely to affect the ESR of an electrolytic capacitor during a load test under a high-humidity environment. From this perspective, the disulfonaphthalic acid compound preferably includes disulfonaphthalic acid compound I, in which two of the two sulfo groups and two carboxy groups are bonded to the first benzene ring and the remaining two are bonded to the second benzene ring. Disulfonaphthalic acid compound I is represented by the following formula (I). Disulfonaphthalic acid compound I also includes compounds in which the two carboxy groups form an acid anhydride group.

[0054] (In the formula, Y １ ~Y ４ Each of Y is a sulfo group or a carboxy group. １ ~Y ４ Two of them are sulfo groups and the remaining two are carboxy groups.)

[0055] The disulfonaphthalic acid compound I preferably contains a disulfonaphthalic acid compound IA (including an acid anhydride) having the above two groups at the 1- and 3-positions, the 1- and 4-positions, or the 2- and 3-positions, respectively, of the naphthalene ring, and the remaining two groups at the 5- and 7-positions, or the 6- and 8-positions, respectively, of the naphthalene ring. Furthermore, the disulfonaphthalic acid compound I preferably has two of the two sulfo groups and two carboxy groups at the m-positions on at least one of the first and second benzene rings. In these cases, the increase in ESR of the electrolytic capacitor after a load test under a high-humidity environment can be further suppressed.

[0056] The disulfonaphthalic acid compound IA is represented, for example, by the following formula:

[0057] (In the formula, Y １ ~Y ４ Each of Y is a sulfo group or a carboxy group. １ ~Y ４ Two of the groups are sulfo groups and the remaining two are carboxy groups. １ ~Y ４ Depending on the type of compound (IA-c), compound (IA-d) may be the same as compound (IA-e), and compound (IA-f) may be the same as compound (IA-f).

[0058] In the disulfonaphthalic acid compound IA, when two carboxy groups are adjacent to each other, they may form one acid anhydride group. For example, when a disulfonaphthalic acid compound has carboxy groups at the 2- and 3-positions, the acid anhydride formed by these carboxy groups is included in the disulfonaphthalic acid compound IA. Furthermore, when a disulfonaphthalic acid compound has carboxy groups at the 1- and 8-positions, the acid anhydride formed by these carboxy groups is included in the disulfonaphthalic acid compound IA.

[0059] The disulfonaphthalic acid compound IA preferably includes a disulfonaphthalic acid compound Ia (including acid anhydride) having two carboxy groups at the 1st and 3rd positions, the 1st and 4th positions, the 2nd and 3rd positions, the 1st and 7th positions, or the 1st and 8th positions of the naphthalene ring. The use of the disulfonaphthalic acid compound Ia can further suppress an increase in the ESR of the electrolytic capacitor after a load test. Furthermore, since the purified disulfonaphthalic acid compound has a chlorine content of less than 50 ppm by mass, it is believed that corrosion of the dielectric layer after a load test is suppressed.

[0060] Specific examples of disulfonaphthalic acid compounds Ia include compounds represented by the following formula:

[0061]

[0062] The disulfonaphthalic acid compound also includes compounds having the above-mentioned first substituent in these formulas. Compounds having no first substituent and represented by these formulas are also preferred.

[0063] (Second Step) In the second step, the first mixture obtained in the first step is neutralized. In the second step, the reaction mixture obtained in the first step may be neutralized. The reaction mixture may be cooled before neutralization. As described above, the cooling of the reaction mixture may be carried out in the first step, may be carried out before neutralization in the second step, or may be carried out between the first step and the second step.

[0064] The first mixture to be subjected to neutralization may be a mixture obtained by diluting the cooled reaction mixture. As described above, dilution of the cooled reaction mixture may be carried out in the first step, before neutralization in the second step, or between the first and second steps. In the present disclosure, this diluted mixture is also included in the first mixture.

[0065] The neutralization is carried out by mixing the first mixture with a basic aqueous solution. The base used in the basic aqueous solution is at least one selected from the group consisting of inorganic bases and organic bases. Examples of inorganic bases include metal hydroxides and ammonia. Examples of organic bases include various amines. From the viewpoint of easy removal of impurities, it is preferable to use a metal hydroxide as the base.

[0066] As the metal hydroxide, an alkali metal hydroxide is preferred. When an alkali metal hydroxide is used, the second mixture obtained in the second step also contains impurities such as alkali metal cations. However, since the alkali metal cations are also removed in the third step, the contamination of the purified sulfonaphthalenecarboxylic acid compound with impurities can be reduced. From the viewpoint of being a strong base and being cost-effective, the alkali metal hydroxide is preferably, but is not limited to, sodium hydroxide, potassium hydroxide, etc.

[0067] The basic aqueous solution may contain one type of base (such as an alkali metal hydroxide) or a combination of two or more types.

[0068] The concentration of the base in the basic aqueous solution may be 1 mol / L or more and 10 mol / L or less, or 3 mol / L or more and 7 mol / L or less, in which case the neutralization reaction tends to proceed gently.

[0069] The basic aqueous solution is mixed with the first mixture until the pH of the second mixture obtained by neutralization becomes, for example, from 7 to 9 or from 8 to 9. As a result, the acid groups or acids of the components contained in the first mixture form salts with the base contained in the basic aqueous solution.

[0070] In the second step, a component containing chlorine may be used, but it is preferable not to use such a component. Furthermore, when a component containing chlorine is used, it is preferable that the chlorine content in the component be low (for example, 1 mass % or less).

[0071] (Step 3) In Step 3, the second mixture obtained in Step 2 is purified. This results in a purified sulfonaphthalenecarboxylic acid compound having a chlorine content of less than 50 ppm by mass. In Step 3, organic impurities such as unreacted raw materials, sulfates derived from fuming sulfuric acid, unreacted bases, and the like are also removed. Therefore, in the purified sulfonaphthalenecarboxylic acid compound obtained in Step 3, the carboxy group and sulfo group may be in a free form. Therefore, the sulfonaphthalenecarboxylic acid compound obtained in Step 3 also has a low content of metal cations derived from salts and bases.

[0072] The chlorine content of the sulfonaphthalenecarboxylic acid compound obtained in the third step is less than 50 ppm by mass, and may be 10 ppm or less, or 1 ppm or less. The chlorine content of the sulfonaphthalenecarboxylic acid compound may be below the detection limit.

[0073] The metal cation content in the sulfonaphthalenecarboxylic acid compound obtained in the third step may be less than 100 ppm, 10 ppm or less, or 1 ppm or less by mass.

[0074] The sulfonaphthalene carboxylic acid compound obtained in the third step has a high degree of purification and a low impurity content. In an electrolytic capacitor containing an organic conductor using such a sulfonaphthalene carboxylic acid compound as a dopant in a solid electrolyte, corrosion of the dielectric layer after a load test is suppressed, and an increase in ESR is suppressed.

[0075] The chlorine content and metal cation content in a sulfonaphthalene carboxylic acid compound can be determined, for example, using ion chromatography. A sample for analysis is prepared according to the following procedure. First, a predetermined amount of the sulfonaphthalene carboxylic acid compound is vacuum-dried for 3 hours, and the mass after drying is weighed. Distilled water is added to the weighed sulfonaphthalene carboxylic acid compound to prepare an aqueous solution with a pH of 3 or higher, and this aqueous solution is used as the sample for analysis. A predetermined amount of the sample for analysis is injected into an ion chromatograph and analyzed. During analysis, the salt concentration of the eluent may be increased or an organic solvent may be added to the eluent so that the peak of the sulfonaphthalene carboxylic acid compound can be detected.

[0076] The sulfonaphthalene carboxylic acid compound contained in the second mixture is purified, for example, by ion exchange and reprecipitation. This removes impurities and yields a purified sulfonaphthalene carboxylic acid compound with a low chlorine content (preferably also a low metal cation content). Reprecipitation also includes recrystallization (including crystallization). The second step may further include at least one selected from known purification methods such as filtration, extraction, washing, chromatography (e.g., column chromatography), and decantation. The order of these purification methods and ion exchange and reprecipitation is not limited. For example, the second mixture may be reprecipitated and then ion exchanged, or the second mixture may be ion exchanged and then reprecipitated. Alternatively, the second mixture may be reprecipitated, the product may be extracted from the resulting precipitate using a solvent, the liquid mixture containing the extract may be filtered, the filtrate may be dried, the resulting dried product may be ion exchanged, and then reprecipitated. The second mixture contains a salt of the sulfonaphthalene carboxylic acid compound (e.g., a metal salt such as an alkali metal salt), a sulfate derived from fuming sulfuric acid, a base, organic impurities, and the like. Ion exchange and reprecipitation can also sufficiently reduce the chlorine content and metal cation content. Other purification methods can further reduce the contents of sulfates, bases, organic impurities, etc., so the degree of purification of the resulting sulfonaphthalenecarboxylic acid compound can be further increased by combining other purification methods with ion exchange and reprecipitation.

[0077] (Ion Exchange) The second mixture contains many cations (metal cations, etc.), including cations that constitute salts derived from bases (metal salts, etc.). Therefore, in the ion exchange, a cation exchange resin is used to exchange these cations for protons. In addition, a chelating resin or Cl 2 ion exchange resin may be used as needed. － Alternatively, a selective resin that selectively removes the above may be used.

[0078] For ion exchange, the second mixture may be used, or an aqueous solution obtained by diluting the second mixture with ion-exchanged water or distilled water may be used. Alternatively, an aqueous solution containing a sulfonaphthalene carboxylic acid compound obtained by appropriately purifying (including reprecipitation) the second mixture may be used for ion exchange. The concentration of the sulfonaphthalene carboxylic acid compound in the solution subjected to ion exchange can be selected depending on the type of ion exchange resin, and may be 1% by mass or more and 20% by mass or less, or 1% by mass or more and 10% by mass or less.

[0079] Ion exchange may be performed once or twice or more times. For example, after ion exchange, other purification treatment (including reprecipitation) may be performed, followed by another ion exchange. When ion exchange is performed twice or more times, the ion exchange resin used may be the same in at least two ion exchange treatments, or may be different in all ion exchange treatments.

[0080] (Reprecipitation) The second mixture contains a large amount of sulfate and base derived from fuming sulfuric acid. By performing reprecipitation, the content of sulfate and base can be reduced. For reprecipitation, the second mixture may be used, or a mixture obtained by concentrating the second mixture may be used. Furthermore, a mixture containing a sulfonaphthalenecarboxylic acid compound that has been appropriately purified (including ion exchange) from the second mixture may be used for reprecipitation.

[0081] In the reprecipitation, a high-concentration solution such as a saturated solution may be cooled to reduce the solubility, and the resulting precipitate may be collected (cooling method). Alternatively, a poor solvent (such as a nonpolar organic solvent) may be added to a mixture containing the sulfonaphthalenecarboxylic acid compound (e.g., a high-concentration solution such as a saturated aqueous solution), and the resulting precipitate may be collected (poor solvent method). In the reprecipitation, a precipitate of the sulfate and base may be generated and removed, or a precipitate of the sulfonaphthalenecarboxylic acid compound may be generated and collected.

[0082] The reprecipitation may be performed once or two or more times. Furthermore, the reprecipitation may be repeated two or more times consecutively. By performing the reprecipitation two or more times or by repeating the reprecipitation, the sulfate and base contents in the sulfonaphthalenecarboxylic acid compound can be further reduced. Furthermore, after the reprecipitation, ion exchange may be performed, followed by further reprecipitation. For example, impurities may be precipitated and removed in the first reprecipitation, and the sulfonaphthalenecarboxylic acid compound may be precipitated in the reprecipitation after the ion exchange, and the sulfonaphthalenecarboxylic acid compound with reduced impurities may be recovered.

[0083] When reprecipitation is used in the early stage of the third step, for example, a cooling method may be used. For example, first, volatile components such as water are removed from a mixture containing a sulfonaphthalene carboxylic acid compound, such as the second mixture, to recover a solid. A polar solvent (e.g., ion-exchanged water, distilled water, or a polar organic solvent) is added to the obtained solid and heated to prepare a saturated solution. The saturated solution is then cooled using an ice bath or the like to precipitate impurities such as sulfates and bases, and the filtrate is recovered by filtration. The volatile components are removed from the obtained filtrate to recover a solid, thereby obtaining a sulfonaphthalene carboxylic acid compound with reduced impurities. Alternatively, a saturated aqueous solution is prepared using this solid, followed by cooling, and the filtrate is recovered again, as described above. This process may be repeated continuously to obtain a sulfonaphthalene carboxylic acid compound with further reduced impurities. The filtered impurities may be washed with a cooled polar solvent or the like, and then combined with the filtrate to remove the volatile components.

[0084] Examples of polar organic solvents include alcohols such as methanol and ethanol (e.g., alcohols having from 1 to 4 carbon atoms), ketones such as acetone (e.g., aliphatic ketones having from 3 to 6 carbon atoms), nitriles such as acetonitrile (e.g., aliphatic nitriles having from 2 to 6 carbon atoms), esters such as ethyl acetate (e.g., aliphatic esters having from 3 to 8 carbon atoms), cyclic ethers (e.g., tetrahydrofuran), sulfoxides (e.g., dimethyl sulfoxide), etc. One type of solvent may be used alone, or two or more types may be used in combination.

[0085] Heating may be carried out at a temperature of 50°C or higher and 90°C or lower, or 55°C or higher and 75°C or lower, so long as the sulfonaphthalenecarboxylic acid compound is dissolved.

[0086] When reprecipitation is carried out after ion exchange, volatile components such as water are removed from a mixture containing the sulfonaphthalenecarboxylic acid compound obtained by ion exchange, and the recovered solid is used. More specifically, this solid is dissolved in a solvent, and the resulting solution is subjected to reprecipitation. The solvent may be the polar organic solvent exemplified above.

[0087] For reprecipitation after ion exchange, either a cooling method or a poor solvent method may be used. In the cooling method, the solid component is dissolved in the solvent under heating to prepare a saturated solution, and the resulting sulfonaphthalene carboxylic acid compound can be precipitated and recovered by cooling. In the poor solvent method, the solid component is dissolved in the solvent to prepare a saturated solution, and a poor solvent for the sulfonaphthalene carboxylic acid compound is added to precipitate and recover the sulfonaphthalene carboxylic acid compound. Examples of poor solvents include hydrocarbons such as hexane and ethers (acyclic ethers such as diethyl ether). One poor solvent may be used alone, or two or more poor solvents may be used in combination.

[0088] (Other) In a preferred embodiment, the solid content obtained by removing the solvent from the second mixture may be subjected to reprecipitation to remove the sulfate and base to some extent, and the resulting mixture containing the sulfonaphthalenecarboxylic acid compound may then be subjected to ion exchange, and a purified sulfonaphthalenecarboxylic acid compound may be obtained using further reprecipitation.

[0089] After reprecipitation and before ion exchange, other purification treatments may be performed to further reduce impurities. For example, an extraction treatment is performed by adding a solvent to the solid fraction obtained by removing the solvent from the filtrate containing the sulfonaphthalene carboxylic acid compound obtained by reprecipitation. Through the extraction treatment, the sulfonaphthalene carboxylic acid compound dissolves in the solvent, and the sulfate remains as a solid fraction. Therefore, the solid fraction is removed by filtration, and the filtrate is recovered. The liquid used to wash the solid fraction is also recovered. The obtained filtrate and the washing liquid are combined, and volatile components such as the solvent are removed from the resulting liquid mixture to obtain a solid fraction. This solid fraction may be dissolved in ion-exchanged water or distilled water, and the resulting aqueous solution may be subjected to ion exchange. If necessary, extraction and filtration may be repeated two or more times.

[0090] The extraction treatment may be carried out under heating. The heating temperature may be, for example, 40°C or higher and 60°C or lower. As the solvent used for the extraction, for example, a polar organic solvent is used. Examples of the polar organic solvent include the polar organic solvents exemplified for reprecipitation. The solvent may be used alone or in combination of two or more.

[0091] The solid matter separated by filtration after extraction may be washed with the solvent used in the extraction treatment.

[0092] The sulfonaphthalene carboxylic acid compound precipitated by reprecipitation after ion exchange is recovered, for example, by filtration. The filtered sulfonaphthalene carboxylic acid compound may be further washed. For example, a liquid containing a large amount of a poor solvent for the sulfonaphthalene carboxylic acid compound is used for washing. Such a liquid may be, for example, a poor solvent alone, or a mixed solvent of a poor solvent and a good solvent (such as the above-mentioned polar organic solvent). The mass ratio of the poor solvent in the mixed solvent may be more than 50 mass%, may be 75 mass% or more, or may be 90 mass% or more.

[0093] The washed sulfonaphthalenecarboxylic acid compound is dried. The drying may be performed under heating, under reduced pressure, or under a combination of these conditions. The drying conditions are determined, for example, taking into consideration the type of solvent used and its effect on the sulfonaphthalenecarboxylic acid compound.

[0094] In the third step, a component containing chlorine may be used, but it is preferable not to use such a component. Furthermore, when a component containing chlorine is used, it is preferable that the chlorine content in the component be low (for example, 1 mass % or less).

[0095] In this way, in the third step, a sulfonaphthalene carboxylic acid compound is purified from the second mixture to obtain a sulfonaphthalene carboxylic acid compound having a chlorine content of less than 50 ppm by mass. Furthermore, the resulting purified sulfonaphthalene carboxylic acid compound also has a low metal cation content and a high degree of purity. Therefore, when such a sulfonaphthalene carboxylic acid compound is used as a dopant in an organic conductor and the organic conductor is further used in an electrolytic capacitor, the increase in ESR after a load test can be significantly suppressed, and corrosion of the dielectric layer can be suppressed. In particular, when the sulfonaphthalene carboxylic acid compound contains a disulfonaphthalic acid compound, this is advantageous because the increase in ESR after a load test can be further suppressed.

[0096] [Organic Conductor] The organic conductor of the present disclosure includes a conjugated polymer and a dopant.

[0097] (Dopant) In the present disclosure, the dopant includes a disulfonaphthalic acid compound having a chlorine content of less than 50 ppm. Such a disulfonaphthalic acid compound can be obtained, for example, by the above-described production method of the present disclosure. The chlorine content of the disulfonaphthalic acid compound may be selected from the range described above for the sulfonaphthalene carboxylic acid compound.

[0098] The dopant may include a dopant other than the disulfonaphthalic acid compound (first dopant) (second dopant). The second dopant may be a sulfonaphthalene carboxylic acid compound other than the first dopant, or may be other relatively low molecular weight anions, polymeric anions, etc. The organic conductor may include one type of second dopant or a combination of two or more types.

[0099] Examples of anions include sulfate ions, nitrate ions, phosphate ions, borate ions, organic sulfonate ions, and carboxylate ions. Compounds that generate these anions are used as the second dopant. Examples of dopants that generate sulfonate ions include aromatic sulfonic acid compounds (such as paratoluenesulfonic acid and naphthalenesulfonic acid).

[0100] Examples of polymeric anions include polyvinyl sulfonic acid, polystyrene sulfonic acid (PSS), polyallylsulfonic acid, polyacrylic sulfonic acid, polymethacrylic sulfonic acid, poly(2-acrylamido-2-methylpropanesulfonic acid), polyisoprene sulfonic acid, polyester sulfonic acid (such as aromatic polyester sulfonic acid), phenolsulfonic acid novolac resin, and polyacrylic acid. The polymeric anion may be a polymer of a single monomer, a copolymer of two or more types of monomers, or a substituted product having a substituent.

[0101] However, these second dopants are merely examples and are not limited to these. The second dopants may be used alone or in combination of two or more.

[0102] In the organic conductor, the mass ratio of the first dopant to the entire dopant is preferably 50 mass% or more, more preferably 75 mass% or more, and even more preferably 90 mass% or more. This is because the incorporation of impurities is reduced and a higher effect of suppressing corrosion of the dielectric layer is obtained. The mass ratio of the first dopant to the entire dopant is 100 mass% or less. The dopant may be composed of only the first dopant.

[0103] (Conjugated Polymer) The conjugated polymer may be any polymer that becomes a good conductor by the action of a dopant, and examples thereof include π-conjugated polymers and σ-conjugated polymers. The organic conductor may contain one type of conjugated polymer or two or more types of conjugated polymers.

[0104] Conjugated polymers include polymers having a basic skeleton such as polypyrrole, polythiophene, polyaniline, polyfuran, polyacetylene, polyphenylene, polyphenylene vinylene, polyacene, or polythiophene vinylene. These polymers include homopolymers, copolymers of two or more monomers, and derivatives thereof (e.g., substituted products having substituent groups). For example, polythiophenes include poly(3,4-ethylenedioxythiophene).

[0105] Among conjugated polymers, preferred are those containing monomer units corresponding to pyrrole compounds. Combining such conjugated polymers with the above-mentioned dopants (such as disulfonaphthalic acid compounds) tends to achieve higher bonding strength. Examples of pyrrole compounds include pyrrole, compounds in which an aliphatic ring or heterocycle is condensed with pyrrole, and substituted derivatives thereof (e.g., compounds having substituents). Examples of substituents include alkyl groups (including aminoalkyl groups and hydroxyalkyl groups), amino groups, substituted amino groups, alkoxy groups, hydroxy groups, mercapto groups, and halogen atoms. Pyrrole or condensed compounds may contain one or more of these substituents. The conjugated polymer preferably has a repeating structure of monomer units corresponding to the pyrrole compound.

[0106] The weight average molecular weight (Mw) of the conjugated polymer is not particularly limited, but is, for example, 1,000 or more and 1,000,000 or less.

[0107] In this specification, the weight average molecular weight (Mw) is a value calculated as polystyrene as measured by gel permeation chromatography (GPC). GPC is usually performed using a polystyrene gel column and water / methanol (volume ratio 8 / 2) as the mobile phase.

[0108] The amount of the dopant is, for example, 0.1 parts by mass or more and 400 parts by mass or less, or may be 1 part by mass or more and 350 parts by mass or less, or may be 10 parts by mass or more and 300 parts by mass or less, relative to 100 parts by mass of the conjugated polymer.

[0109] Such organic conductors exhibit excellent reliability because they suppress an increase in ESR after a load test under a high-humidity environment, and are therefore suitable for use in a variety of electronic devices, particularly as solid electrolytes in electrolytic capacitors.

[0110] [Electrolytic Capacitor] The electrolytic capacitor includes an anode body having a dielectric layer on its surface and a solid electrolyte covering a portion of the dielectric layer. The solid electrolyte includes the organic conductor described above. The solid electrolyte constitutes a cathode portion of the electrolytic capacitor.

[0111] (Anode Body) The anode body can contain a valve metal, an alloy containing a valve metal, a compound containing a valve metal, or the like. These materials can be used alone or in combination of two or more. Examples of preferred valve metals include aluminum, tantalum, niobium, and titanium. Anode bodies with porous surfaces can be obtained by roughening the surface of a substrate (such as a foil-shaped or plate-shaped substrate) containing a valve metal, for example, by etching. The anode body can also be a compact of particles containing a valve metal, or a sintered body thereof. The sintered body has a porous structure.

[0112] (Dielectric Layer) The dielectric layer is formed by anodizing the valve metal on the surface of the anode body by chemical conversion treatment or the like. The dielectric layer is formed, for example, so as to cover at least a portion of the anode body. The dielectric layer is usually formed on the surface of the anode body. Since the dielectric layer is formed on the surface of a porous anode body, it is formed along the inner wall surfaces of holes and depressions (pits) on the surface of the anode body (including the inner wall surfaces of the holes).

[0113] The dielectric layer contains an oxide of the valve metal. For example, when tantalum is used as the valve metal, the dielectric layer contains Ta. ２ O ５ When aluminum is used as the valve metal, the dielectric layer contains Al ２ O ３ The dielectric layer is not limited to this, and may be a layer that functions as a dielectric.

[0114] (Cathode Section) The cathode section includes at least a solid electrolyte covering at least a part of the dielectric layer. The cathode section usually includes a solid electrolyte and a cathode extraction layer covering at least a part of the solid electrolyte.

[0115] (Solid Electrolyte) The solid electrolyte contains the organic conductor and is formed to cover the dielectric layer. The solid electrolyte does not necessarily have to cover the entire dielectric layer (entire surface). The solid electrolyte layer may be formed to cover at least a portion of the dielectric layer. In the electrolytic capacitor, the solid electrolyte may form a solid electrolyte layer.

[0116] The solid electrolyte may contain other additives, such as known additives other than dopants and known conductive materials other than organic conductors (e.g., conductive inorganic materials such as manganese dioxide).

[0117] The solid electrolyte can be formed, for example, by performing at least one of chemical polymerization and electrolytic polymerization of constituent monomers of a conjugated polymer on a dielectric layer in the presence of the additive. Alternatively, a solution in which the conjugated polymer and the additive are dissolved, or a dispersion in which the conjugated polymer and the additive are dispersed, can be brought into contact with the dielectric layer to form a solid electrolyte covering the dielectric layer. After the solution or dispersion is brought into contact with the dielectric layer, drying or heat treatment may be performed as necessary.

[0118] The solid electrolyte layer may be a single layer or may be composed of multiple layers. When the solid electrolyte layer is composed of multiple layers, the compositions of the layers (e.g., the type of conjugated polymer, the type of dopant or additive, and the ratio of each component) may be the same or different. If necessary, a layer for improving adhesion may be interposed between the dielectric layer and the solid electrolyte.

[0119] (Cathode Extraction Layer) The cathode extraction layer may include, for example, one or more conductive particle-containing layers, or may include a metal foil (cathode foil). The conductive particle-containing layer may include, for example, conductive particles (metal particles, conductive carbon particles, etc.) and a binder resin. The binder resin may be a thermoplastic resin or composition, or a curable resin composition. For example, the cathode extraction layer may be formed by a layer containing conductive carbon particles (e.g., a carbon layer) and a metal particle-containing layer covering the layer. The metal particles contained in the metal particle-containing layer may be formed of, for example, silver, copper, or an alloy thereof. The cathode extraction layer may be a metal foil coated with particles (e.g., non-metal particles such as carbon particles, metal particles, etc.). The configuration of the cathode extraction layer is not limited to these, and may also have a current-collecting function.

[0120] (Separator) When a metal foil is used for the cathode extraction layer, a separator may be disposed between the metal foil (cathode foil) and the anode body (e.g., anode foil). The separator is not particularly limited, and may be, for example, a nonwoven fabric containing fibers of cellulose, polyethylene terephthalate, vinylon, or polyamide (e.g., aliphatic polyamide, aromatic polyamide such as aramid). In this case, the separator may be impregnated with an organic conductor. In this case, the organic conductor is interposed between the anode foil having a dielectric layer and the cathode foil, and is in contact with each of the anode foil and the cathode foil.

[0121] (Other) For example, an electrolytic capacitor can be obtained by housing a capacitor element including an anode body and a cathode portion in a container or sealing it with an exterior body (such as a resin exterior body). The electrolytic capacitor may be either a chip type or a stacked type, or may be a wound type. The electrolytic capacitor may include at least one capacitor element, or may include two or more. The configuration of the capacitor element may be selected depending on the type of electrolytic capacitor. For example, the electrolytic capacitor may include a stack of two or more capacitor elements, or may include two or more wound-type capacitor elements. The configuration or number of capacitor elements may be selected depending on the type or application of the electrolytic capacitor.

[0122] In the capacitor element, one end of the cathode lead is electrically connected to the cathode extraction layer. One end of the anode lead is electrically connected to the anode body. The other end of the anode lead and the other end of the cathode lead are each drawn out from the exterior body or the container. The other end of each lead exposed from the exterior body or the container is used for soldering to a substrate on which the electrolytic capacitor is to be mounted. Each lead may be a lead wire or a lead frame. The container may have any shape as long as it can accommodate the capacitor element inside. The opening of the container is sealed, for example, with a sealing body while the capacitor element is housed inside.

[0123] FIG. 1 is a cross-sectional view schematically illustrating the structure of an electrolytic capacitor according to an embodiment of the present disclosure. As shown in FIG. 1 , the electrolytic capacitor 1 includes a capacitor element 2, a resin encapsulant 3 that encapsulates the capacitor element 2, and an anode terminal 4 and a cathode terminal 5, at least a portion of which is exposed outside the resin encapsulant 3. The anode terminal 4 and the cathode terminal 5 may be made of a metal such as copper or a copper alloy. The resin encapsulant 3 has a substantially rectangular parallelepiped outer shape, and the electrolytic capacitor 1 also has a substantially rectangular parallelepiped outer shape. The resin encapsulant 3 may be made of, for example, epoxy resin.

[0124] Capacitor element 2 includes an anode body 6, a dielectric layer 7 covering anode body 6, and a cathode portion 8 covering dielectric layer 7. Cathode portion 8 includes a solid electrolyte layer 9 covering dielectric layer 7, and a cathode extraction layer 10 covering solid electrolyte layer 9. Cathode extraction layer 10 has a carbon layer 11 and a metal particle-containing layer 12.

[0125] The anode body 6 includes a region facing the cathode portion 8 and a region not facing the cathode portion 8. Of the region of the anode body 6 not facing the cathode portion 8, an insulating separation layer 13 is formed in a strip-like shape on the surface of the anode body 6 in a portion adjacent to the cathode portion 8, thereby restricting contact between the cathode portion 8 and the anode body 6. Of the region of the anode body 6 not facing the cathode portion 8, another part is electrically connected to the anode terminal 4 by welding. The cathode terminal 5 is electrically connected to the cathode portion 8 via an adhesive layer 14 formed of a conductive adhesive.

[0126] The main surfaces 4S and 5S of the anode terminal 4 and the cathode terminal 5 are exposed from the same surface of the resin sealing material 3. These exposed surfaces are used for soldering to a substrate (not shown) on which the electrolytic capacitor 1 is to be mounted.

[0127] EXAMPLES The present invention will be specifically described below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0128] Example 1 (1) Synthesis and Purification of Sulfonaphthalenecarboxylic Acid Compound Purified 5,7-disulfo-1,4-naphthalic acid was produced from 1,4-naphthalic acid according to the following procedure, as shown in the reaction scheme below.

[0129]

[0130] (a) First Step: 0.17 mol of 1,4-naphthalic acid was added to 100 mL of fuming sulfuric acid (sulfur trioxide content: 30% by mass), and the resulting mixture was stirred at 130°C for 9 hours for sulfonation. The resulting reaction mixture was cooled to about -20°C using an ice bath. The cooled reaction mixture was diluted by pouring it onto ice made with ion-exchanged water.

[0131] (b) Second Step: A 5 mol / L aqueous sodium hydroxide solution was added to the diluted mixture (first mixture) obtained in the first step to neutralize the mixture. The aqueous sodium hydroxide solution was added until the pH of the resulting mixture reached 9.

[0132] (c) Third Step (c-1) Drying The mixture (second mixture) obtained in the second step (c-2) was vacuum dried at 80° C. to remove volatile components such as water. In this way, a solid content containing a sulfonaphthalenecarboxylic acid compound was obtained.

[0133] (c-2) Reprecipitation: Ion-exchanged water was added to the solid obtained in (c-1) above, and the mixture was heated to 60°C to prepare a saturated aqueous solution. This saturated aqueous solution was cooled to 0°C to precipitate sodium sulfate and sodium hydroxide, which were then removed by filtration. The precipitate was washed with cold water. The washed liquid and the filtrate were combined, and the resulting mixture was vacuum-dried at 80°C to remove volatile components such as water, and the solid was recovered.

[0134] The above procedure was repeated a total of eight times using the solid matter recovered above, thereby removing some of the sodium sulfate and sodium hydroxide remaining in the sulfonaphthalenecarboxylic acid compound.

[0135] (c-3) Extraction The solid content containing the sulfonaphthalenecarboxylic acid compound finally obtained in (c-2) above was subjected to an extraction treatment using methanol. More specifically, 2 L of methanol was added to the solid content. The resulting mixture was heated to 45°C and then stirred for 10 minutes. In the mixture, sodium sulfate precipitated, while sodium hydroxide and the sulfonaphthalenecarboxylic acid compound remained dissolved. Therefore, the mixture was filtered to remove the sodium sulfate. A liquid obtained by washing the sodium sulfate with methanol was also recovered. This liquid and the filtrate were combined and vacuum-dried to remove volatile components such as methanol, and the solid content was recovered.

[0136] (c-4) Ion Exchange: 10 g of the solid obtained in (c-3) above was dissolved in 350 mL of ion-exchanged water. The resulting aqueous solution was purified by ion exchange. Ion exchange was performed by filling a glass tube with a cation exchange resin that converts metal ions into protons and passing the aqueous solution through the cation exchange resin. By ion exchange, sodium sulfate and sodium hydroxide were converted to sulfuric acid and water, respectively, and the sulfonaphthalenecarboxylic acid compound in the form of a sodium salt was converted to organic sulfonaphthalenecarboxylic acid. The aqueous solution obtained by ion exchange was vacuum dried to obtain a solid. This solid contained traces of sulfuric acid, organic impurities, and sulfonaphthalenecarboxylic acid (mainly 5,7-disulfo-1,4-naphthalic acid).

[0137] (c-5) Reprecipitation The solid obtained in (c-4) above was dissolved in ethyl acetate at 25°C to prepare a saturated solution. Approximately 10 times the mass of n-hexane was added to this saturated solution. The addition of n-hexane caused a precipitate to form in the mixture. The precipitate was removed by filtering the mixture and washed with a mixed solvent of ethyl acetate and n-hexane (ethyl acetate:n-hexane=1:10 (mass ratio)). This removed organic impurities and sulfuric acid, yielding a precipitate containing the target 5,7-disulfo-1,4-naphthalic acid. The recovered precipitate was vacuum dried at 80°C to remove the organic solvent and other components contained in the precipitate, yielding dried 5,7-disulfo-1,4-naphthalic acid. The yield of 5,7-disulfo-1,4-naphthalic acid based on 1,4-naphthalic acid was 80%. The chlorine content in the obtained 5,7-disulfo-1,4-naphthalic acid (A1) was less than 1 ppm by mass, and the Na content was about 5 ppm by mass. 5,7-Disulfo-1,4-naphthalic acid (A1) is the compound of the formula (Ia-d1) above.

[0138] (2) Fabrication of Electrolytic Capacitor Electrolytic capacitor 1 shown in Fig. 1 was fabricated in the following manner, and its characteristics were evaluated. (a) Step of Preparing Anode Body 6 Anode body 6 was fabricated by roughening the surface of aluminum foil (thickness: 100 µm) as a substrate by etching.

[0139] (b) Step of forming dielectric layer 7 Anode body 6 was immersed in a phosphoric acid solution with a concentration of 0.3 mass % at a temperature of 70°C, and a direct current voltage of 70 V was applied for 20 minutes, thereby forming dielectric layer 7 containing aluminum oxide.

[0140] (c) Step of forming solid electrolyte layer 9: A solid electrolyte layer 9 containing polypyrrole and a dopant was formed on the dielectric layer 7 by electrolytic polymerization according to the following procedure. The dopant used was 5,7-disulfo-1,4-naphthalic acid synthesized in (1) above.

[0141] First, an aqueous solution containing a pyrrole monomer and a dopant was prepared. The concentration of the pyrrole monomer in this aqueous solution was 0.5 mol / L, and the concentration of the dopant in this aqueous solution was 0.3 mol / L. Sulfuric acid was added to the aqueous solution to adjust the pH to 3.0.

[0142] Anode body 6 on which dielectric layer 7 was formed in (b) above and a counter electrode were immersed in the obtained aqueous solution, and electrolytic polymerization was performed at 25° C. and a polymerization voltage of 3 V, thereby forming solid electrolyte layer 9.

[0143] (d) Step of forming cathode extraction layer 10: A dispersion of graphite particles dispersed in water was applied to the surface of solid electrolyte layer 9 obtained in (c) above, and the applied dispersion was dried in the air to form carbon layer 11. Next, a silver paste containing silver particles and an epoxy resin was applied to the surface of carbon layer 11, and heated to form metal particle-containing layer 12. In this way, cathode extraction layer 10 composed of carbon layer 11 and metal particle-containing layer 12 was formed. In this way, capacitor element 2 was produced.

[0144] (e) Assembling the Electrolytic Capacitor The cathode extraction layer 10 of the capacitor element 2 obtained in (d) above was joined to one end of the cathode terminal 5 with a conductive adhesive. One end of the anode body 6 protruding from the capacitor element 2 was joined to one end of the anode terminal 4 by laser welding. Next, a resin sealing material 3 made of an insulating resin was formed around the periphery of the capacitor element 2. At this time, the other end of the anode terminal 4 and the other end of the cathode terminal 5 were extended from the resin sealing material 3. In this way, an electrolytic capacitor 1 with a rated voltage of 2 V and a capacitance of 30 μF was completed.

[0145] (3) The ESR of the evaluated electrolytic capacitors was evaluated before and after the load test using the following procedure. In an environment of 20°C, the initial ESR (=Z ０ A load test was performed by applying a voltage of 16 V to the electrolytic capacitor for 1000 hours in an environment of 85°C and 85% RH. After the load test, the ESR (= Z) (mΩ) of the electrolytic capacitor was measured in an environment of 20°C using the same procedure as for the initial ESR. Z / Z ０ The ratio Z / Z was calculated and the corrosion of the dielectric layer was judged based on this value. ０When the ratio is 10 or more, the load test reveals that the ESR has increased significantly, and it can be determined that the main cause of such a significant increase is corrosion of the dielectric layer.

[0146] Example 2: 5,7-disulfo-2,3-naphthalic acid (A2) was synthesized and purified according to the following reaction scheme in the same manner as in Example 1, except that 2,3-naphthalic acid was used instead of 1,4-naphthalic acid. The purified 5,7-disulfo-2,3-naphthalic acid had a yield of 65% based on the 2,3-naphthalic acid, a chlorine content of less than 1 ppm by mass, and a sodium content of approximately 5 ppm by mass. An electrolytic capacitor was then fabricated and evaluated in the same manner as in Example 1, except that the resulting 5,7-disulfo-2,3-naphthalic acid was used as a dopant. 5,7-disulfo-2,3-naphthalic acid (A2) is a compound represented by the formula (Ia-e) above.

[0147]

[0148] Example 3: 3,6-disulfo-1,8-naphthalic anhydride was synthesized and purified according to the following reaction scheme in the same manner as in Example 1, except that 1,8-naphthalic anhydride was used instead of 1,4-naphthalic acid. The purified 3,6-disulfo-1,8-naphthalic anhydride had a yield of 71% based on 1,8-naphthalic anhydride, a chlorine content of less than 1 ppm by mass, and a sodium content of approximately 5 ppm by mass. An electrolytic capacitor was then fabricated and evaluated in the same manner as in Example 1, except that the resulting 3,6-disulfo-1,8-naphthalic anhydride was used as a dopant. 3,6-disulfo-1,8-naphthalic anhydride (A3) is a compound represented by the formula (Ia-b2).

[0149]

[0150] Example 4: 3,5-disulfo-1,7-naphthalic acid was synthesized and purified according to the following reaction scheme in the same manner as in Example 1, except that 1,7-naphthalic acid was used instead of 1,4-naphthalic acid. The purified 3,5-disulfo-1,7-naphthalic acid had a yield of 54% based on 1,7-naphthalic acid, a chlorine content of less than 1 ppm by mass, and a sodium content of approximately 1 ppm by mass. An electrolytic capacitor was then fabricated and evaluated in the same manner as in Example 1, except that the resulting 3,5-disulfo-1,7-naphthalic anhydride was used as a dopant. 3,5-disulfo-1,7-naphthalic acid (A4) is a compound represented by the formula (Ia-a1).

[0151]

[0152] Example 5: 5,7-disulfo-1,3-naphthalic acid was synthesized and purified according to the following reaction scheme in the same manner as in Example 1, except that 1,3-naphthalic acid was used instead of 1,4-naphthalic acid. The purified 5,7-disulfo-1,3-naphthalic acid had a yield of 71% based on 1,3-naphthalic acid, a chlorine content of less than 1 ppm by mass, and a sodium content of approximately 5 ppm by mass. An electrolytic capacitor was then fabricated and evaluated in the same manner as in Example 1, except that the resulting 5,7-disulfo-1,3-naphthalic acid was used as a dopant. 5,7-disulfo-1,3-naphthalic anhydride (A5) is a compound represented by the formula (Ia-a2).

[0153]

[0154] Comparative Examples 1 to 5 The same disulfonaphthalic acid compounds as those obtained in Examples 1 to 5 were produced and purified by the method described in Non-Patent Document 1. Electrolytic capacitors were produced and evaluated in the same manner as in Example 1, except that each of the disulfonaphthalic acids thus obtained was used as a dopant. The chlorine content of each of the obtained disulfonaphthalic acid compounds was approximately 1% by mass, and the sodium content was 1% by mass or more and 10% by mass or less.

[0155] The synthesis and purification of disulfonaphthalic acid were carried out according to the following procedure. First, the same naphthalic acid as the compound used in each of Examples 1 to 5 was used as the raw material naphthalic acid. 0.17 mol of naphthalic acid was added to 100 mL of fuming sulfuric acid (sulfur trioxide content: 30% by mass), and the resulting mixture was stirred at 130°C for 10 hours. The resulting reaction mixture was cooled to 25°C. Next, the cooled reaction mixture was poured into cooled ion-exchanged water to produce a precipitate. The precipitate was separated by filtration and diluted with NaCl at a concentration of 1 mol / L. ２ CO ３ The resulting solution was washed with an aqueous solution. The washed liquid and the filtrate were combined and acidified with an aqueous solution of hydrochloric acid with a concentration of 1 mol / LM. The resulting precipitate was then filtered and washed to obtain disulfonaphthalic acid corresponding to the raw material.

[0156] The evaluation results are shown in Table 1.

[0157]

[0158] As shown in Table 1, in the examples in which the chlorine content of the disulfonaphthalic acid was less than 1 ppm by mass, when any of the disulfonaphthalic acids A1 to A5 was used, Z / Z ０ was 1.0 or more and 1.1 or less, less than 10, and the increase in ESR after the load test was greatly reduced. This means that corrosion of the dielectric layer was suppressed. In contrast, in the comparative example in which the chlorine content of disulfonaphthalic acid was about 1 mass %, Z / Z ０ The ESR after the load test was 10 or more, and the increase in ESR after the load test was significant. This significant increase in ESR is thought to be mainly due to corrosion of the dielectric layer.

[0159] While the present invention has been described in terms of presently preferred embodiments, such disclosure is not to be interpreted as limiting. Various changes and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. It is therefore intended that the appended claims be interpreted to cover all changes and modifications that do not depart from the true spirit and scope of the invention.

[0160] The present disclosure provides a method for producing a sulfophthalic acid compound useful as a dopant for organic conductors, which can reduce the increase in ESR (or inhibit corrosion of dielectric layers) after a load test. By using the organic conductor in various electronic devices, such as electrolytic capacitors, the increase in ESR is reduced and corrosion of dielectric layers is inhibited, even in high-humidity environments. This stabilizes the quality of the electronic devices, resulting in high reliability.

[0161] 1: Electrolytic capacitor 2: Capacitor element 3: Resin sealant 4: Anode terminal 4S: Main surface of anode terminal 5: Cathode terminal 5S: Main surface of cathode terminal 6: Anode body 7: Dielectric layer 8: Cathode portion 9: Solid electrolyte layer 10: Cathode extraction layer 11: Carbon layer 12: Metal particle-containing layer 13: Separation layer 14: Adhesive layer

Claims

1. A method for producing a sulfonaphthalenecarboxylic acid compound, comprising: a first step of sulfonating a naphthalenecarboxylic acid compound to obtain a first mixture containing the sulfonaphthalenecarboxylic acid compound; a second step of adding a basic aqueous solution to the first mixture to neutralize it; and a third step of purifying the second mixture obtained in the second step to obtain a purified sulfonaphthalenecarboxylic acid compound having a chlorine content of less than 50 ppm by mass.

2. The method for producing a sulfonaphthalenecarboxylic acid compound according to claim 1, wherein the purified sulfonaphthalenecarboxylic acid compound has a metal cation content of less than 100 ppm by mass.

3. The method for producing a sulfonaphthalenecarboxylic acid compound according to claim 1 or 2, wherein in the third step, the second mixture is purified by ion exchange and reprecipitation.

4. The method for producing a sulfonaphthalenecarboxylic acid compound according to claim 1 or 2, wherein in the second step, the basic aqueous solution contains an alkali metal hydroxide.

5. The method for producing a sulfonaphthalenecarboxylic acid compound according to claim 1 or 2, wherein the naphthalenecarboxylic acid compound is a naphthalic acid compound or an acid anhydride thereof.

6. The method for producing a sulfonaphthalenecarboxylic acid compound according to claim 5, wherein the purified sulfonaphthalenecarboxylic acid compound comprises a polysulfonaphthalic acid compound or its acid anhydride.

7. The method for producing a sulfonaphthalene carboxylic acid compound according to claim 5, wherein the purified sulfonaphthalene carboxylic acid compound comprises a disulfonaphthalic acid compound or an acid anhydride thereof having a naphthalene ring, two sulfo groups bonded to the naphthalene ring, and two carboxy groups or acid anhydride groups thereof bonded to the naphthalene ring.

8. An organic conductor comprising a conjugated polymer and a dopant, wherein the dopant blended with the conjugated polymer comprises a disulfonaphthalic acid compound having a chlorine content of less than 50 ppm by mass, and the disulfonaphthalic acid compound has a naphthalene ring, two sulfo groups bonded to the naphthalene ring, and two carboxy groups or acid anhydride groups thereof bonded to the naphthalene ring.

9. The organic conductor according to claim 8, wherein the conjugated polymer contains a monomer unit corresponding to a pyrrole compound.

10. An electrolytic capacitor comprising an anode body having a dielectric layer on its surface, and a solid electrolyte covering a portion of the dielectric layer, wherein the solid electrolyte comprises the organic conductor according to claim 8 or 9.

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