Resin composition, insulated wire, and method for producing insulated wire

Abstract

A resin composition containing: a polyimide precursor that is a reaction product of an aromatic tetracarboxylic dianhydride and an aromatic diamine; an organic solvent; and an aliphatic polyvalent carboxylic acid ester, wherein the total number of carbon atoms in the aliphatic polyvalent carboxylic acid ester is 9 or more excluding the carbon atom of the carbonyl group.

Description

Resin composition, insulated wire, and method for producing insulated wire

[0001] The present disclosure relates to a resin composition, an insulated wire, and a method for producing an insulated wire.

[0002] Patent Document 1 describes an insulating varnish containing a coating film-forming resin and a heat-decomposable resin that decomposes at a temperature lower than the baking temperature of the coating film-forming resin, and an insulated electric wire having a heat-cured film of the insulating varnish, in which pores are formed in the heat-cured film due to the thermal decomposition of the heat-decomposable resin.

[0003] JP 2012-224714 A

[0004] A resin composition according to one embodiment of the present disclosure contains a polyimide precursor that is a reaction product of an aromatic tetracarboxylic dianhydride and an aromatic diamine, an organic solvent, and an aliphatic polycarboxylic acid ester, wherein the total number of carbon atoms in the aliphatic polycarboxylic acid ester, excluding the carbon atoms of the carbonyl groups, is 9 or more.

[0005] [Problem to be Solved by the Present Disclosure] The problem to be solved by the present disclosure is to provide a resin composition that can form an insulating coating having voids.

[0006] Effect of the Present Disclosure According to the resin composition according to one aspect of the present disclosure, an insulating coating having pores can be formed.

[0007] [Description of Embodiments of the Present Disclosure] First, embodiments of the present disclosure will be listed and described. Item 1. A resin composition comprising: a polyimide precursor that is a reaction product of an aromatic tetracarboxylic dianhydride and an aromatic diamine; an organic solvent; and an aliphatic polycarboxylic acid ester, wherein the total number of carbon atoms in the aliphatic polycarboxylic acid ester, excluding the carbon atoms of the carbonyl groups, is 9 or more. Item 2. The resin composition according to Item 1, wherein the aliphatic polycarboxylic acid ester is an aliphatic dicarboxylic acid ester or an aliphatic tricarboxylic acid ester. Item 3. The resin composition according to Item 1 or 2, wherein the aliphatic polycarboxylic acid ester is derived from an aliphatic polycarboxylic acid having 2 or more and 8 or less carbon atoms, excluding the carbon atoms of the carbonyl groups. Item 4. The resin composition according to any one of Items 1 to 3, wherein the total number of carbon atoms in the aliphatic polycarboxylic acid ester, excluding the carbon atoms of the carbonyl groups, is 19 or less. Item 5. Item 6. The resin composition according to any one of items 1 to 4, wherein the content of the aliphatic polycarboxylic acid ester is 1 part by mass or more and 20 parts by mass or less per 100 parts by mass of the total of the polyimide precursor and the organic solvent. Item 7. The resin composition according to any one of items 1 to 5, wherein the boiling point of the organic solvent is 150°C or higher. Item 8. The resin composition according to any one of items 1 to 6, further comprising thermally decomposable resin-containing particles. Item 8. The resin composition according to any one of items 1 to 7, which is used to form an insulating coating for an insulated wire. Item 9. An insulated wire comprising a conductor and an insulating coating covering the conductor, wherein the insulating coating has a resin matrix and a plurality of pores, and the insulating coating is formed from the resin composition according to any one of items 1 to 8. Item 10. 10. A method for producing an insulated wire according to item 9, comprising: a step of applying the resin composition to an outer peripheral surface of the conductor; and a step of heating the resin composition applied in the application step.

[0008] [Details of Embodiment of the Present Disclosure] Hereinafter, a resin composition, an insulated wire, and a method for producing an insulated wire according to one aspect of the present disclosure will be described.

[0009] <Resin Composition> The resin composition contains a polyimide precursor which is a reaction product of an aromatic tetracarboxylic dianhydride and an aromatic diamine, an organic solvent, and an aliphatic polycarboxylic acid ester, wherein the total number of carbon atoms in the aliphatic polycarboxylic acid ester, excluding the carbon atoms of the carbonyl groups, is 9 or more.

[0010] The resin composition contains an aliphatic polycarboxylic acid ester having a total carbon number of 9 or more, excluding the carbon atoms of the carbonyl group, and is therefore capable of forming an insulating coating having voids. While not intending to be restrictive, it is believed that when an insulating coating is formed using the resin composition, voids are formed by phase separation between the polyimide precursor and the aliphatic polycarboxylic acid ester, and that the success or failure of void formation by phase separation is related to the balance between the hydrophobicity and hydrophilicity of the aliphatic polycarboxylic acid ester. Furthermore, it is believed that when the total carbon number of the aliphatic polycarboxylic acid ester is 9 or more, excluding the carbon atoms of the carbonyl group, an appropriate balance between hydrophobicity and hydrophilicity is achieved, enabling the formation of an insulating coating having voids.

[0011] The resin composition can be suitably used as a resin composition (resin varnish) for forming an insulating coating for an insulated wire.

[0012] Each component contained in the resin composition will be described below.

[0013] (Polyimide Precursor) A polyimide precursor is a reaction product obtained by a polymerization condensation reaction between an aromatic tetracarboxylic dianhydride and an aromatic diamine. The polyimide precursor is a compound also called a polyamic acid (polyamic acid). The polyimide precursor undergoes a dehydration cyclization reaction to form a cyclic imide, resulting in a polyimide.

[0014] When the aromatic tetracarboxylic dianhydride contains pyromellitic dianhydride (PMDA), the heat resistance of the insulating coating can be improved because PMDA has a rigid and linear molecular structure.

[0015] The lower limit of the content of PMDA relative to 100 mol% of the aromatic tetracarboxylic dianhydride may be 0 mol%, 10 mol%, 20 mol%, or 30 mol%, and the upper limit of the content of PMDA relative to 100 mol% of the aromatic tetracarboxylic dianhydride may be 100 mol%, 90 mol%, 80 mol%, or 70 mol%.

[0016] The aromatic tetracarboxylic dianhydride may contain an aromatic tetracarboxylic dianhydride other than PMDA (hereinafter also referred to as "other aromatic tetracarboxylic dianhydrides"). Examples of the other aromatic tetracarboxylic dianhydrides include 3,3',4,4'-biphenyltetracarboxylic dianhydride (s-BPDA), 2,3,3',4'-biphenyltetracarboxylic dianhydride (a-BPDA), 2,2',3,3'-biphenyltetracarboxylic dianhydride (i-BPDA), 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 4,4'-oxydiphthalic dianhydride, 2,2',3,3'-benzophenonetetracarboxylic dianhydride, 2,2-bis(3,4-dicarboxyphenyl)propane dianhydride, 2,2-bis(3,4-dicarboxyphenyl)propane dianhydride, and 2,2-bis(3,4-dicarboxyphenyl)propane dianhydride. Examples of the other aromatic tetracarboxylic dianhydrides include bis(2,3-dicarboxyphenyl)propane dianhydride, 1,1-bis(3,4-dicarboxyphenyl)ethane dianhydride, 1,1-bis(2,3-dicarboxyphenyl)ethane dianhydride, bis(3,4-dicarboxyphenyl)methane dianhydride, bis(2,3-dicarboxyphenyl)methane dianhydride, bis(3,4-dicarboxyphenyl)sulfone dianhydride, bis(3,4-dicarboxyphenyl)ether dianhydride, 1,2,5,6-naphthalenetetracarboxylic dianhydride, and 2,3,6,7-naphthalenetetracarboxylic dianhydride. These other aromatic tetracarboxylic dianhydrides may be used alone or in combination of two or more.

[0017] The content of the other aromatic tetracarboxylic dianhydride relative to 100 mol% of the aromatic tetracarboxylic dianhydride can be appropriately determined within a range that does not impair the effects of the present disclosure. The upper limit of the content may be 30 mol% or 20 mol%, and the lower limit of the content may be 0 mol% or 10 mol%.

[0018] When the aromatic diamine contains diaminodiphenyl ether (ODA), the heat resistance of the insulating coating can be improved. This is because ODA has a rigid and linear molecular structure. Examples of diaminodiphenyl ethers include 4,4'-diaminodiphenyl ether (4,4'-ODA), 3,4'-diaminodiphenyl ether (3,4'-ODA), 3,3'-diaminodiphenyl ether (3,3'-ODA), 2,4'-diaminodiphenyl ether (2,4'-ODA), and 2,2'-diaminodiphenyl ether (2,2'-ODA). 4,4'-diaminodiphenyl ether (4,4'-ODA) can improve the elongation of the insulating coating.

[0019] The lower limit of the ODA content relative to 100 mol% of the aromatic diamine may be 50 mol%, 60 mol%, or 70 mol%, and the upper limit of the ODA content relative to 100 mol% of the aromatic diamine may be 100 mol% or 90 mol%.

[0020] The aromatic diamine may further contain an aromatic diamine other than ODA (hereinafter also referred to as "other aromatic diamine"). Examples of the other aromatic diamine include 2,2-bis[4-(4-aminophenoxy)phenyl]propane (BAPP), 4,4'-bis(4-aminophenoxy)biphenyl (BAPB), 4,4'-diaminodiphenylmethane, 3,4'-diaminodiphenylmethane, 3,3'-diaminodiphenylmethane, 2,4'-diaminodiphenylmethane, 2,2'-diaminodiphenylmethane, 4,4'-diaminodiphenylsulfone, 3,4'-diaminodiphenylsulfone, 3,3'-diaminodiphenylsulfone, 2,4'-diaminodiphenylsulfone, 2,2'-diaminodiphenylsulfone, 4,4' Examples of the other aromatic diamines include 4,4'-diaminodiphenyl sulfide, 3,4'-diaminodiphenyl sulfide, 3,3'-diaminodiphenyl sulfide, 2,4'-diaminodiphenyl sulfide, 2,2'-diaminodiphenyl sulfide, paraphenylenediamine, metaphenylenediamine, p-xylylenediamine, m-xylylenediamine, 2,2'-dimethyl-4,4'-diaminobiphenyl (mTBHG), 1,5-diaminonaphthalene, 4,4'-benzophenonediamine, 3,3'-dimethyl-4,4'-diaminodiphenylmethane, and 3,3',5,5'-tetramethyl-4,4'-diaminodiphenylmethane. The other aromatic diamines may be used alone or in combination of two or more.

[0021] The content of the other aromatic diamine relative to 100 mol% of the aromatic diamine can be appropriately determined within a range that does not impair the effects of the present disclosure. The upper limit of the content may be 40 mol% or 30 mol%. The lower limit of the content may be 0 mol% or 10 mol%.

[0022] The polyimide precursor preferably has a glass transition temperature of 250°C or higher, as measured by the following method. The lower limit of the glass transition temperature may be 280°C or 300°C. While not intended to be restrictive, a glass transition temperature of more than 280°C can suppress softening of the polyimide during pore formation, which tends to facilitate an improvement in porosity. The upper limit of the glass transition temperature is not particularly limited and may be, for example, 400°C. The glass transition temperature is measured using a dynamic viscoelasticity measuring device under conditions of 1 Hz and a temperature increase rate of 10°C / min, after preparing a film-like test specimen by coating a thin film of the resin composition on a glass plate and heating it at 350°C for 1 hour. The glass transition temperature can be adjusted by, for example, changing the type of monomer constituting the polyimide precursor.

[0023] The lower limit of the concentration of the polyimide precursor in the resin composition may be 25% by mass or 27% by mass. The upper limit of the concentration may be 40% by mass or 35% by mass. By setting the concentration at or above the lower limit, the amount of resin composition required in the entire manufacturing process to obtain an insulating coating of a desired thickness when forming an insulating coating using the resin composition can be reduced, and the number of coating and heating steps can be reduced. By setting the concentration at or below the upper limit, the viscosity of the resin composition can be appropriately adjusted while maintaining good coating properties, thereby improving coatability.

[0024] The molar ratio of the aromatic tetracarboxylic dianhydride to the aromatic diamine used as raw materials for the polyimide precursor (aromatic tetracarboxylic dianhydride:aromatic diamine) may be, for example, 95:105 or more and 105:95 or less, 97:103 or more and 103:97 or less, or 99:101 or more and 101:99 or less, from the viewpoint of ease of synthesis of the polyimide precursor. The aromatic tetracarboxylic dianhydride and the aromatic diamine may be substantially equimolar. In this case, the molecular weight of the polyimide precursor can be easily increased. "Substantially equimolar amounts" refers to a molar ratio of the aromatic tetracarboxylic dianhydride to the aromatic diamine (aromatic tetracarboxylic dianhydride:aromatic diamine) in the range of 99:101 or more and 101:99 or less.

[0025] (Method for synthesizing polyimide precursor) The polyimide precursor can be obtained by a polymerization condensation reaction between the aromatic tetracarboxylic dianhydride and the aromatic diamine described above. The polymerization condensation reaction can be carried out in the same manner as in conventional synthesis of polyimide precursors. Specific methods for the polymerization condensation reaction include, for example, mixing the aromatic tetracarboxylic dianhydride and the aromatic diamine in an organic solvent. This method polymerizes the aromatic tetracarboxylic dianhydride and the aromatic diamine, thereby obtaining a solution in which the polyimide precursor is dissolved in the organic solvent. For example, the degree of polymerization (weight average molecular weight) can be controlled by carrying out the polymerization condensation reaction in the presence of a reaction inhibitor.

[0026] The reaction inhibitor may be, for example, water (H ２ and alcohols having 1 to 15 carbon atoms. Examples of the alcohols having 1 to 15 carbon atoms include monohydric alcohols such as ethanol, methanol, propanol, butanol, and pentanol; and polyhydric alcohols such as ethylene glycol, propylene glycol, and glycerin.

[0027] The reaction conditions for the polymerization can be appropriately set depending on the raw materials used, etc. For example, the reaction temperature can be set to 10° C. or higher and 100° C. or lower, and the reaction time can be set to 0.5 hours or higher and 24 hours or lower.

[0028] Examples of the organic solvent used in the above polymerization condensation reaction include the same organic solvents as those described below.

[0029] (Organic Solvent) Examples of the organic solvent include aprotic polar organic solvents such as N-methyl-2-pyrrolidone (NMP, boiling point: 202°C), N,N-dimethylacetamide (DMAc, boiling point: 165°C), N,N-dimethylformamide (boiling point: 153°C), dimethyl sulfoxide (boiling point: 189°C), and γ-butyrolactone (boiling point: 204°C). One type of organic solvent may be used alone, or two or more types may be used in combination. The term "aprotic polar organic solvent" refers to a polar organic solvent that does not have a group that releases a proton.

[0030] When the boiling point of the organic solvent is 150° C. or higher, unintended drying before baking can be suppressed in the process of forming the insulating coating.

[0031] The content of the organic solvent is not particularly limited as long as it is an amount that can uniformly dissolve or disperse the aromatic tetracarboxylic dianhydride and the aromatic diamine, but if the amount is too large, a large amount of organic solvent needs to be volatilized when forming the insulating coating of the insulated electric wire, which may require a long time to form the insulating coating. Therefore, the content of the organic solvent can be, for example, 100 parts by mass or more and 1,000 parts by mass or less per 100 parts by mass of the total of the aromatic tetracarboxylic dianhydride and the aromatic diamine.

[0032] (Aliphatic polycarboxylic acid ester) The aliphatic polycarboxylic acid ester has a total carbon number of 9 or more excluding the carbon atoms of the carbonyl groups. "Carbon number" refers to the number of carbon atoms constituting a compound or functional group. "Aliphatic polycarboxylic acid ester" refers to an ester derived from an aliphatic polycarboxylic acid. "Aliphatic polycarboxylic acid" refers to an aliphatic carboxylic acid having two or more carboxy groups. "Carboxylic acid" refers to a compound having a carboxy group, and is not limited to carboxylic acids in the narrow sense, but also includes carboxylic acids in the broad sense such as hydroxycarboxylic acids. "A total carbon number of 9 or more excluding the carbon atoms of the carbonyl groups" means that the carbon number obtained by subtracting the carbon atoms of the carbonyl groups from the total number of carbon atoms constituting the aliphatic polycarboxylic acid ester (hereinafter simply referred to as "total carbon number") is 9 or more.

[0033] When the total number of carbon atoms in the aliphatic polycarboxylic acid ester is 9 or more, an insulating coating having pores can be formed.

[0034] The lower limit of the total carbon number is 9, and may be 10, 11, or 12. The upper limit of the total carbon number may be 30, 20, 19, 18, 17, or 16. When the total carbon number is 19 or less, the balance between the hydrophobicity and hydrophilicity of the aliphatic polycarboxylic acid ester is further improved, and the stability of the resin composition can be increased.

[0035] The aliphatic polycarboxylic acid esters are classified according to the number of carboxy groups, and include, for example, aliphatic dicarboxylic acid esters, aliphatic tricarboxylic acid esters, and aliphatic tetracarboxylic acid esters. When the aliphatic polycarboxylic acid ester is an aliphatic dicarboxylic acid ester or an aliphatic tricarboxylic acid ester, the molecular weight is smaller and the boiling point and thermal decomposition temperature are lower than those of aliphatic polycarboxylic acids having four or more carboxy groups (e.g., aliphatic tetracarboxylic acid esters), and therefore the amount of aliphatic dicarboxylic acid ester remaining in the insulating coating can be reduced. By reducing the amount of aliphatic dicarboxylic acid ester remaining in the insulating coating, the dielectric constant of the insulating coating can be reduced.

[0036] When the aliphatic polycarboxylic acid ester is derived from an aliphatic polycarboxylic acid having 2 to 8 carbon atoms excluding the carbon atom of the carbonyl group, the hydrophobicity of the aliphatic polycarboxylic acid ester becomes appropriate, thereby improving dispersibility in the resin composition. Furthermore, compared with an aliphatic polycarboxylic acid ester derived from an aliphatic polycarboxylic acid having 9 or more carbon atoms excluding the carbon atom of the carbonyl group, the molecular weight is smaller, and the boiling point and thermal decomposition temperature are lower, so the amount of residue in the insulating coating can be reduced.

[0037] Examples of the aliphatic polycarboxylic acid ester include compounds represented by the following formula (1).

[0038]

[0039] In the above formula (1), R １ represents a substituted or unsubstituted n-valent aliphatic hydrocarbon group. ２ represents a substituted or unsubstituted monovalent aliphatic hydrocarbon group, and n is an integer of 2 or more. １ (number of carbon atoms in R ２ The number of carbon atoms in the alkyl group is 9 or more.

[0040] The "valence" of a group refers to the number of atoms to which the group is bonded. "Aliphatic hydrocarbon groups" include "chain hydrocarbon groups" and "alicyclic hydrocarbon groups". From another perspective, "aliphatic hydrocarbon groups" include "saturated hydrocarbon groups" and "unsaturated hydrocarbon groups". "Chain hydrocarbon groups" refer to hydrocarbon groups that do not contain a ring structure and are composed only of a chain structure, and include both straight-chain hydrocarbon groups and branched-chain hydrocarbon groups. "Alicyclic hydrocarbon groups" refer to hydrocarbon groups that contain only alicyclic rings as a ring structure and do not contain aromatic rings, and include both monocyclic alicyclic hydrocarbon groups and polycyclic alicyclic hydrocarbon groups. However, they do not have to be composed only of alicyclic rings, and may contain a chain structure as part of them.

[0041] In the above formula (1), R １ The number of carbon atoms in R ２ The sum of n times the number of carbon atoms ((R １ (number of carbon atoms in R ２The total number of carbon atoms corresponds to the number of carbon atoms. More specifically, for example, when the aliphatic polycarboxylic acid ester is dibutyl adipate (a compound represented by the following formula (2)), the total number of carbon atoms is 12. In the case of dibutyl adipate, the number of carbon atoms in R １ The structure corresponding to R has 4 carbon atoms. ２ The number of carbon atoms in the structure corresponding to the formula (1) is 4, and n is 2, so the total number of carbon atoms is 4 + 2 × 4 = 12. For example, diethyl adipate (a compound represented by the following formula (3)) has a total of 8 carbon atoms, and therefore does not fall under the category of the aliphatic polycarboxylic acid esters. In diethyl adipate, the R １ The carbon number of the structure corresponding to R ２ The number of carbon atoms in the structure corresponding to this is 2, and n is 2, so the total number of carbon atoms is 4 + 2 × 2 = 8.

[0042]

[0043] R in the above formula (1) １ As for R, which will be described later ２ Examples of such groups include groups in which (n-1) hydrogen atoms have been removed from a monovalent aliphatic hydrocarbon group.

[0044] R in the above formula (1) ２ Examples of the monovalent aliphatic hydrocarbon group having 1 to 20 carbon atoms include monovalent chain hydrocarbon groups having 1 to 20 carbon atoms and monovalent alicyclic hydrocarbon groups having 3 to 20 carbon atoms.

[0045] Examples of the monovalent chain hydrocarbon group having 1 to 20 carbon atoms include alkyl groups such as methyl, ethyl, n-propyl, i-propyl, n-butyl, sec-butyl, isobutyl, and tert-butyl; alkenyl groups such as ethenyl, propenyl, butenyl, and 2-methylprop-1-en-1-yl; and alkynyl groups such as ethynyl, propynyl, and butynyl.

[0046] Examples of the monovalent alicyclic hydrocarbon group having 3 to 20 carbon atoms include monocyclic alicyclic saturated hydrocarbon groups such as a cyclopentyl group and a cyclohexyl group; polycyclic alicyclic saturated hydrocarbon groups such as a norbornyl group, an adamantyl group, a tricyclodecyl group and a tetracyclododecyl group; monocyclic alicyclic unsaturated hydrocarbon groups such as a cyclopentenyl group and a cyclohexenyl group; and polycyclic alicyclic unsaturated hydrocarbon groups such as a norbornenyl group, a tricyclodecenyl group and a tetracyclododecenyl group.

[0047] R １ or R ２ The aliphatic hydrocarbon group giving the formula (I) may have a substituent, such as a halogen atom, a hydroxy group, a carboxy group, a cyano group, a nitro group, an alkoxy group, an alkoxycarbonyl group, an alkoxycarbonyloxy group, an acyl group, or an acyloxy group.

[0048] R １ R may be an n-valent aliphatic hydrocarbon group having 2 to 8 carbon atoms, or an n-valent chain hydrocarbon group having 2 to 8 carbon atoms. １ When the number of carbon atoms is 3 or more, the porosity of the insulating coating can be improved.

[0049] R ２ The alkyl group may be a monovalent aliphatic hydrocarbon group having 1 to 4 carbon atoms, a monovalent chain hydrocarbon group having 1 to 4 carbon atoms, or an alkyl group having 1 to 4 carbon atoms.

[0050] The number n may be 2 or more, and may be 2, 3, or 4.

[0051] (R １ (number of carbon atoms in R ２ The number of carbon atoms in the alkyl group is the same as the total number of carbon atoms.

[0052] Examples of the aliphatic dicarboxylic acid esters include aliphatic dicarboxylic acid esters such as dibutyl fumarate, dibutyl succinate, diethyl sebacate, diisobutyl adipate, dibutyl adipate, dibutyl sebacate, and di(2-butoxyethyl) adipate, and citric acid esters such as triethyl citrate, triethyl O-acetylcitrate, tributyl citrate, and tributyl O-acetylcitrate. When comparing aliphatic dicarboxylic acid esters and citric acid esters, if the total number of carbon atoms is the same, citric acid esters tend to be able to form insulating coatings with higher porosity.

[0053] The content of the aliphatic polycarboxylic acid ester in the resin composition can be appropriately set within a range that achieves the effects of the present disclosure. The upper limit of the content of the aliphatic polycarboxylic acid ester may be 20 parts by mass, 18 parts by mass, 15 parts by mass, 12 parts by mass, 10 parts by mass, or 9 parts by mass, relative to 100 parts by mass of the polyimide precursor and the organic solvent. When the content is 20 parts by mass or less, the insulating coating formed has a good shape with no foaming in appearance. Although not intended to be limiting, it is presumed that when the content is 20 parts by mass or less, the balance between the island and sea portions of the sea-island structure formed by phase separation between the polyimide precursor and the aliphatic polycarboxylic acid ester is appropriately adjusted, thereby suppressing the occurrence of poor appearance of the insulating coating, such as foaming, swelling, or peeling. The lower limit of the aliphatic polycarboxylic acid ester may be 1 part by mass, 2 parts by mass, 4 parts by mass, or 6 parts by mass, relative to 100 parts by mass of the polyimide precursor and the organic solvent.

[0054] (Thermal Decomposable Resin-Containing Particles) When the resin composition further contains thermally decomposable resin-containing particles, an insulating coating having a good appearance can be formed even when the porosity is high. The thermally decomposable resin-containing particles are gasified by thermal decomposition, and voids are formed in the insulating coating at the locations where the thermally decomposable resin-containing particles were present. In this case, the particles can be uniformly distributed as an island phase of fine particles in the sea phase of the resin matrix that forms the insulating coating, forming independent voids.

[0055] The thermally decomposable resin contained in the thermally decomposable resin-containing particles is preferably a resin that thermally decomposes at a temperature lower than the baking temperature of the polyimide that is the main component of the resin matrix of the insulating coating. The baking temperature is set appropriately depending on the type of polyimide, but is usually about 200°C or higher and 600°C or lower. The thermal decomposition temperature refers to the temperature at which the mass loss rate reaches 50% when the temperature is increased from room temperature at a rate of 10°C / min in an air atmosphere. The thermal decomposition temperature can be measured by measuring the thermogravimetry using a thermogravimetry-differential thermal analyzer ("TG / DTA" from SII NanoTechnology, Inc.).

[0056] Examples of thermally decomposable resins include compounds such as polyethylene glycol and polypropylene glycol in which one or both ends or a portion thereof are alkylated, (meth)acrylated, or epoxidized; polymers of (meth)acrylic acid esters having an alkyl group containing 1 to 6 carbon atoms, such as polymethyl(meth)acrylate, polyethyl(meth)acrylate, polypropyl(meth)acrylate, and polybutyl(meth)acrylate; urethane oligomers, urethane polymers, polymers of modified (meth)acrylates, such as urethane(meth)acrylate, epoxy(meth)acrylate, and ε-caprolactone(meth)acrylate; poly(meth)acrylic acid; crosslinked products thereof; polystyrene; and crosslinked polystyrene. Polymers of (meth)acrylic acid esters having an alkyl group containing 1 to 6 carbon atoms are prone to thermal decomposition at the baking temperatures described above, easily forming voids in the insulating coating. An example of such a polymer of (meth)acrylic acid ester is polymethyl methacrylate (PMMA). The term "(meth)acrylic acid" encompasses both "acrylic acid" and "methacrylic acid."

[0057] The thermally decomposable resin-containing particles may be particles consisting only of the thermally decomposable resin, or may be particles having a core-shell structure having a core mainly composed of the thermally decomposable resin and a shell mainly composed of a resin having a thermal decomposition temperature higher than that of the thermally decomposable resin. In the case of particles having a core-shell structure, interconnection of pores can be suppressed, and variation in pore size can be reduced.

[0058] The main component of the shell is not particularly limited as long as it has a thermal decomposition temperature higher than that of the core, and is preferably a synthetic resin with a low dielectric constant and high heat resistance. Examples include polystyrene, silicone, fluororesin, and polyimide. Silicone tends to increase elasticity, which in turn tends to improve the dispersion of pores in the insulating coating, resulting in excellent insulating properties and heat resistance.

[0059] (Other Components) The resin composition may contain other components in addition to the above-described components. The other components are not particularly limited as long as they are additives that can be blended into a resin varnish for forming an insulating coating for an insulated wire, and examples thereof include a filler, an antioxidant, a leveling agent, a curing agent, and an adhesion aid.

[0060] <Insulated Wire> The insulated wire includes a conductor and an insulating coating that covers the conductor. The insulated wire can be suitably used as a winding wire for a coil (magnet wire).

[0061] (Conductor) The conductor is usually composed mainly of a metal. The metal is not particularly limited, but if copper, a copper alloy, aluminum, or an aluminum alloy is used, an insulated electric wire having good processability, electrical conductivity, etc. can be obtained. The conductor may contain other components such as known additives in addition to the metal as the main component.

[0062] The cross-sectional shape of the conductor is not particularly limited, and various shapes such as a circle, a square, a rectangle, etc. The size of the cross section of the conductor is also not particularly limited, and the diameter (short side width) can be, for example, 0.2 mm or more and 8.0 mm or less.

[0063] (Insulating Coating) The insulating coating is laminated on the peripheral surface of the conductor so as to cover the conductor. The insulating coating may cover the conductor directly or indirectly. In the case of indirect coverage, for example, a multilayer structure in which the covering layer of the conductor includes a layer other than the insulating coating can be mentioned.

[0064] The insulating coating is formed from the resin composition described above, and therefore has a plurality of pores.

[0065] The average thickness of the insulating coating is not particularly limited, but is usually set to 2 μm or more and 200 μm or less.

[0066] The insulated wire may further have another layer laminated on the outer peripheral surface of the insulating coating, such as a surface lubricating layer.

[0067] The lower limit of the porosity of the insulating coating may be 10 vol%, 15 vol%, 20 vol%, 25 vol%, or 30 vol%. The upper limit of the porosity may be 70 vol%, 60 vol%, 50 vol%, or 40 vol%. "Porosity" refers to the percentage of the volume of pores relative to the volume of the insulating coating including the pores.

[0068] When the thermally decomposable resin-containing particles are particles with the core-shell structure, the pores have an outer shell at their periphery that originates from the shell of the particle with the core-shell structure.

[0069] The insulating coating may contain other components in addition to the above components. The other components are not particularly limited as long as they are additives that can be incorporated into the insulating coating of an insulated electric wire, and examples thereof include fillers, antioxidants, leveling agents, curing agents, and adhesion aids.

[0070] <Method for manufacturing insulated wire> The insulated wire can be manufactured by a method including, for example, a step of applying the resin composition to the outer peripheral surface of a conductor (hereinafter referred to as a “coating step”) and a step of heating the resin composition applied to the conductor (hereinafter referred to as a “heating step”).

[0071] In the coating step, the resin composition is applied to the outer peripheral surface of the conductor. For example, a method for applying the resin composition to the outer peripheral surface of the conductor may use a coating device equipped with a liquid composition tank containing the resin composition and a coating die. With this coating device, the conductor passes through the liquid composition tank, causing the resin composition to adhere to the outer peripheral surface of the conductor. The conductor then passes through the coating die, coating the resin composition to a uniform thickness.

[0072] In the heating step, the resin composition applied to the conductor in the application step is heated, which volatilizes the solvent in the resin composition and cures the polyimide precursor to form polyimide.

[0073] The heating step may be performed using any suitable device, such as a cylindrical baking oven that is long in the direction of travel of the conductor. The heating method may be any suitable method, such as hot air heating, infrared heating, or high-frequency heating.

[0074] The heating temperature can be, for example, 300° C. to 800° C. The heating time can be, for example, 5 seconds to 1 minute.

[0075] The coating step and the heating step are usually repeated multiple times. By repeating the steps multiple times, the thickness of the insulating coating can be increased. The hole diameter of the coating die can be adjusted appropriately depending on the number of repetitions.

[0076] [Other Embodiments] The embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. The scope of the present invention is not limited to the configurations of the above-described embodiments, but is defined by the claims, and is intended to include all modifications within the meaning and scope of the claims.

[0077] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0078] Test Example 1 In Test Example 1, the influence of the type of aliphatic polycarboxylic acid ester on pore formation was tested.

[0079] [No. 1] (Preparation of Resin Composition) 4,4'-diaminodiphenyl ether (ODA) as an aromatic diamine was dissolved in N-methyl-2-pyrrolidone (NMP). Pyromellitic dianhydride (PMDA) as an aromatic tetracarboxylic dianhydride was added so that the mixing ratio (molar ratio) of the aromatic tetracarboxylic dianhydride to the aromatic diamine was 100:100. The mixture was allowed to react at 30°C for 3 hours with stirring under a nitrogen atmosphere to synthesize a polyimide precursor, and a polyimide precursor solution containing NMP as the solvent (solids concentration: 28% by mass) was obtained. 7.97 parts by mass of diethyl adipate was added to 100 parts by mass of the polyimide precursor solution to prepare Resin Composition No. 1.

[0080] [No. 2 to No. 13] Resin compositions No. 2 to No. 13 were prepared in the same manner as No. 1, except that aliphatic polycarboxylic acid esters of the types and contents shown in Table 1 below were used.

[0081] <Evaluation> The resin compositions No. 1 to No. 13 prepared above were subjected to a film whitening test according to the method described below. Furthermore, for some of the resin compositions for which whitening of the film was confirmed in the film whitening test, insulated wires were prepared according to the method described below, and the porosity was measured. The results are shown in Table 1 below.

[0082] [Film Whitening Test] A film whitening test was performed on the resin compositions No. 1 to No. 13 prepared above. Specifically, each resin composition was applied to copper foil at intervals of 200 μm, and heated at 350° C. for 1 hour under conditions of an oxygen concentration of 1% or less to produce a film. The evaluation criteria for the film whitening test are as follows: A: Whitening was observed in the film. B: Whitening was not observed in the film.

[0083] The film whitening test is a simple test to determine whether or not voids have formed. If whitening is observed in the film (the film is opaque), it can be determined that voids have formed, and if whitening is not observed (the film is transparent), it can be determined that voids have not formed.

[0084] [Preparation of insulated wire and measurement of porosity] (Preparation of insulated wire) A round copper wire having an average diameter of 1 mm was used as a conductor. Resin compositions No. 4, No. 7, No. 8, and No. 10 to No. 13 were applied to the surface of the conductor, and the conductor coated with the resin composition was heated in a heating furnace at an inlet temperature of 400°C, an outlet temperature of 450°C, and a linear speed of 3 m / min. This process was repeated 10 times to form an insulating coating having an average thickness of 35 μm, thereby preparing an insulated wire.

[0085] (Measurement of porosity) For the insulated wires prepared above, the insulating coating was peeled off from the conductor into a tube-like shape, and the mass W2 of the cylindrical insulating coating was measured. The apparent volume V1 was determined from the outer shape of the cylindrical insulating coating, and the mass W1 without pores was calculated by multiplying the volume V1 by the density ρ1 of the material of the insulating coating. From these values ​​of W1 and W2, the porosity (unit: volume %) was calculated using the following formula 1. Formula 1: Porosity = (W1 - W2) × 100 / W1

[0086] In Table 1 below, "R １ "," "R ２ " and "n" represent the number of carbon atoms in the corresponding structure in the above formula (1). The "total number of carbon atoms" is the value obtained by subtracting the number of carbon atoms in the carbonyl group from the total number of carbon atoms in the aliphatic polycarboxylic acid ester (i.e., (R １ (number of carbon atoms in R ２ The "amount (parts by mass)" refers to the content of the aliphatic polycarboxylic acid ester relative to 100 parts by mass of the polyimide precursor solution in the resin composition (100 parts by mass of the polyimide precursor and the organic solvent combined). A "-" in the "Porosity (volume %)" column indicates that the insulated wire was not prepared and the porosity was not measured.

[0087]

[0088] It can be seen from Table 1 that whitening was observed in the films when resin compositions No. 3 to No. 13 were used, whereas whitening was not observed in the films when resin compositions No. 1 and No. 2 were used. This shows that pores can be formed when the total number of carbon atoms in the aliphatic polycarboxylic acid ester is 9 or more.

[0089] Furthermore, Table 1 shows that resin compositions No. 4, No. 7, No. 8, and No. 10 to No. 13 can actually form insulating coatings with porosities of 12 volume % or more.

Claims

1. a polyimide precursor which is a reaction product of an aromatic tetracarboxylic dianhydride and an aromatic diamine; an organic solvent; Aliphatic polycarboxylic acid ester Contains A resin composition in which the total number of carbon atoms in the aliphatic polycarboxylic acid ester is 9 or more, excluding the carbon atoms of the carbonyl groups.

2. 2. The resin composition according to claim 1, wherein the aliphatic polycarboxylic acid ester is an aliphatic dicarboxylic acid ester or an aliphatic tricarboxylic acid ester.

3. 2. The resin composition according to claim 1, wherein the aliphatic polycarboxylic acid ester is derived from an aliphatic polycarboxylic acid having 2 or more and 8 or less carbon atoms excluding the carbon atom of the carbonyl group.

4. 2. The resin composition according to claim 1, wherein the total number of carbon atoms in said aliphatic polycarboxylic acid ester is 19 or less, excluding the carbon atoms in the carbonyl group.

5. 2. The resin composition according to claim 1, wherein the content of the aliphatic polycarboxylic acid ester is 1 part by mass or more and 20 parts by mass or less per 100 parts by mass of the polyimide precursor and the organic solvent combined.

6. 2. The resin composition according to claim 1, wherein the organic solvent has a boiling point of 150°C or higher.

7. The resin composition according to claim 1, further comprising thermally decomposable resin-containing particles.

8. The resin composition according to claim 1, which is used to form an insulating coating for an insulated wire.

9. A conductor; an insulating coating that covers the conductor; Equipped with the insulating coating has a resin matrix and a plurality of pores, An insulated wire, wherein the insulating coating is formed from the resin composition according to any one of claims 1 to 8.

10. The method for producing an insulated wire according to claim 9, applying the resin composition to an outer peripheral surface of the conductor; a step of heating the resin composition applied in the application step; A method for manufacturing an insulated wire comprising: