Resin composition and insulated wire

Abstract

To provide a resin composition having high storage stability and capable of forming an insulated layer with its coating to elongate well.SOLUTION: A resin composition according to one embodiment contains a polyimide precursor, which is a reaction product between an aromatic tetracarboxylic acid dianhydride and an aromatic diamine, and an organic solvent. The aromatic diamine contains compounds represented by the following formulae (1)-(3). Relative to the aromatic diamine 100 mol%, the content of the compound is less than 20 mol%. (In the formulae (1)-(3), R is a C1-4 alkyl group, a C2-4 alkenyl group or a C1-4 alkoxy group. n is an integer of 1-4. If n is 2 or greater, a plurality of R's are the same or different).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to a resin composition and an insulated wire. [Background technology]

[0002] Patent Document 1 describes a resin composition used to form an insulating layer of an insulated wire, which contains a polyamic acid having a specific molecular structure and a solvent. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2020 / 255360 Summary of the Invention

[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, and an organic solvent, wherein the aromatic diamine includes compounds represented by the following formulas (1) to (3), and the content of the compounds relative to the total amount of the aromatic diamine is less than 20 mol %: [ka] (In formulas (1) to (3), R is an alkyl group having 1 to 4 carbon atoms, an alkenyl group having 2 to 4 carbon atoms, or an alkoxy group having 1 to 4 carbon atoms. n is an integer of 1 to 4. When n is 2 or greater, multiple Rs are the same or different.) DETAILED DESCRIPTION OF THE INVENTION

[0005] [Problem to be solved by this disclosure] One method for forming an insulating layer of an insulated wire using polyimide includes a coating step in which a resin composition (resin varnish) containing a polyimide precursor (polyamic acid) and a solvent is applied to the outer periphery of a conductor, and a heating step in which the resulting coating is heated. In the heating step, the polyimide precursor is imidized to form a polyimide. In this method, only a relatively thin coating can be formed in a single coating and heating step, so a coating of the desired thickness is usually formed by repeating the coating and heating steps. In order to increase the thickness of the coating formed in a single coating and heating step as much as possible and thereby improve the manufacturing efficiency of insulated wires, the resin varnish is made more concentrated.

[0006] Since the viscosity of a resin varnish may change over time when the concentration is increased, there is a demand for a resin varnish in which the change in viscosity over time is suppressed (hereinafter also referred to as "excellent storage stability"). Furthermore, there is a demand for a resin varnish that has excellent film elongation as a film property of the insulating layer of an insulated electric wire.

[0007] The present disclosure has been made in light of the above-mentioned circumstances, and an object of the present disclosure is to provide a resin composition that can form an insulating layer that has excellent storage stability and film elongation.

[0008] [Effects of this disclosure] The resin composition according to one embodiment of the present disclosure has excellent storage stability. Furthermore, the resin composition according to one embodiment of the present disclosure can form an insulating layer with excellent film elongation.

[0009] [Description of the embodiments of the present disclosure] First, embodiments of the present disclosure will be listed and described.

[0010] 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, and an organic solvent, wherein the aromatic diamine contains compounds represented by the following formulas (1) to (3), and the content of the compounds relative to 100 mol % of the aromatic diamine is less than 20 mol %: [ka] (In formulas (1) to (3), R is an alkyl group having 1 to 4 carbon atoms, an alkenyl group having 2 to 4 carbon atoms, or an alkoxy group having 1 to 4 carbon atoms. n is an integer of 1 to 4. When n is 2 or greater, multiple Rs are the same or different.)

[0011] By using a specific compound as the aromatic diamine and setting the content ratio of the specific compound to be equal to or less than the above upper limit, the resin composition can improve storage stability and the film elongation of the insulating layer formed can be improved.

[0012] The concentration of the polyimide precursor is preferably 25% by mass or more, which makes it possible to reduce the number of repetitions required to form the insulating layer of the insulated wire, thereby contributing to improved production efficiency.

[0013] Preferably, the aromatic diamine further contains 4,4'-diaminodiphenyl ether, which makes it possible to form an insulating layer having both good heat resistance and toughness.

[0014] The aromatic tetracarboxylic dianhydride may contain pyromellitic dianhydride, which allows the formation of an insulating layer having both good heat resistance and toughness.

[0015] An insulated wire according to another aspect of the present disclosure includes a conductor and an insulating layer covering the conductor, the insulating layer being formed from the resin composition according to the aspect of the present disclosure.

[0016] The insulated wire has an insulating layer formed from the resin composition described above, and therefore has excellent coating elongation.

[0017] [Details of the embodiments of the present disclosure] The resin composition and insulated wire according to one embodiment of the present disclosure will be described in detail below.

[0018] <Resin composition> The resin composition contains a polyimide precursor that is a reaction product of an aromatic tetracarboxylic dianhydride and an aromatic diamine, and an organic solvent. The resin composition may contain a pore-forming agent as a suitable component. The resin composition may contain components other than the polyimide precursor, organic solvent, and pore-forming agent.

[0019] The resin composition can be suitably used as a material for forming an insulating layer of an insulated wire.

[0020] Each component contained in the resin composition will be described below.

[0021] (Polyimide precursor) A polyimide precursor is a reaction product of an aromatic tetracarboxylic dianhydride and an aromatic diamine. Specifically, it is a compound also known as a polyamic acid (polyamic acid). The polyimide precursor undergoes a dehydration cyclization reaction to form a cyclic imide, which becomes a polyimide.

[0022] The molar ratio of the aromatic tetracarboxylic dianhydride to the aromatic diamine used as the raw material for the polyimide precursor (aromatic tetracarboxylic dianhydride:aromatic diamine) can be, for example, 95:105 or more and 105:95 or less, more preferably 97:103 or more and 103:97 or less, and even more preferably 99:101 or more and 101:99 or less, from the viewpoint of ease of synthesis of the polyimide precursor. Furthermore, it is preferable that the aromatic tetracarboxylic dianhydride and the aromatic diamine are substantially equimolar amounts. 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.

[0023] The aromatic tetracarboxylic dianhydride preferably contains pyromellitic dianhydride (PMDA). The aromatic tetracarboxylic dianhydride may contain an aromatic tetracarboxylic dianhydride other than PMDA (hereinafter also referred to as "other aromatic tetracarboxylic dianhydride").

[0024] 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 (BTDA), 4,4'-oxydiphthalic dianhydride, 2,2',3,3'-benzophenonetetracarboxylic dianhydride, 2,2-bis(3,4-dicarboxyphenyl)propane dianhydride, 2,2 Examples of suitable aromatic tetracarboxylic dianhydrides include 1,1-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. Among these, s-BPDA or BTDA is preferred from the viewpoint of imparting heat resistance, toughness, and hydrolysis resistance. The other aromatic tetracarboxylic dianhydrides may be used alone or in combination of two or more.

[0025] The lower limit of the content of PMDA relative to 100 mol% of the aromatic tetracarboxylic dianhydride is preferably 10 mol%, more preferably 15 mol%, and even more preferably 20 mol%. By setting the content of PMDA at or above the lower limit, good heat resistance can be imparted inexpensively. The upper limit of the content of PMDA is, for example, 100 mol%.

[0026] The aromatic diamine includes compounds represented by the following formulas (1) to (3). The resin composition has excellent storage stability due to the inclusion of the above compound as the aromatic diamine. The reason for this is not entirely clear, but is presumed to be as follows: The inclusion of structural units derived from the above compound in the molecular chain of the polyimide precursor makes it easier for the molecular structure of the polyimide precursor to assume a bent structure, making it more difficult for the molecular chain of the polyimide precursor to be oriented, thereby improving the fluidity of the resin composition. As a result, it is believed that the fluidity is less likely to decrease even when the resin composition is highly concentrated, and the viscosity is less likely to increase even as imidization progresses.

[0027] [ka]

[0028] In the above formulas (1) to (3), R is an alkyl group having 1 to 4 carbon atoms, an alkenyl group having 2 to 4 carbon atoms, or an alkoxy group having 1 to 4 carbon atoms. n is an integer of 1 to 4. When n is 2 or greater, multiple Rs may be the same or different.

[0029] The carbon number means the number of carbon atoms constituting the group.

[0030] Examples of the alkyl group having 1 to 4 carbon atoms include a methyl group, an ethyl group, an n-propyl group, an i-propyl group, an n-butyl group, a sec-butyl group, an isobutyl group, and a tert-butyl group.

[0031] Examples of the alkenyl group having 2 to 4 carbon atoms include a vinyl group, an allyl group, and a 3-butenyl group.

[0032] Examples of the alkoxy group having 1 to 4 carbon atoms include a methoxy group, an ethoxy group, an n-propoxy group, an isopropoxy group, an n-butoxy group, an isobutoxy group, a sec-butoxy group, and a tert-butoxy group.

[0033] R is preferably an alkyl group having 1 to 4 carbon atoms, more preferably a methyl group or an ethyl group.

[0034] n is preferably 1 to 3, more preferably 2 or 3, and even more preferably 3. For example, when n is 1, this means that two amino groups and one R are bonded to each of the three carbon atoms constituting the benzene ring, and hydrogen atoms are bonded to the remaining three carbon atoms.

[0035] The compound is preferably a compound (diethyldiaminotoluene) in which n is 3, one R is a methyl group, and two R are ethyl groups. In this case, the molecular chains of the polyimide precursor are less likely to be oriented, and the flowability of the resin composition is further improved.

[0036] From another viewpoint, the compound is preferably a compound represented by the formula (1), and more preferably 2,6-diamino-3,5-diethyltoluene, which makes it easier for the molecular structure of the polyimide precursor to have a more curved structure, thereby further improving the fluidity of the resin composition.

[0037] The content of the compound relative to 100 mol % of the aromatic diamine is less than 20 mol %. By setting the content of the compound to less than 20 mol %, the resin composition can achieve a balance between the storage stability of the resin composition and the film elongation of the insulating layer formed from the resin composition. The reason for this is not entirely clear, but it is presumed to be, for example, as follows. As described above, the polyimide precursor contains structural units derived from the compound, which imparts flexibility to the molecular chain of the polyimide precursor, improving its fluidity and, as a result, improving its storage stability in the polyamic acid state. Furthermore, the toughness, i.e., film elongation, achieved after polyimidization is largely dependent on interactions such as π-π stacking between polyimide molecular chains. If the proportion of bent structures in the molecular chains is too high, the inter-molecular chain interactions will be weakened, resulting in reduced film elongation. Therefore, it is believed that setting the content of the compound to less than 20 mol % can suppress the decrease in film elongation.

[0038] The upper limit of the content of the compound relative to 100 mol% of the aromatic diamine is preferably 18 mol%, more preferably 15 mol%, even more preferably 12 mol%, even more preferably 10 mol%, and particularly preferably 8 mol%. By setting the content of the compound at or below the upper limit, a better balance can be achieved between the storage stability of the resin composition and the film elongation of the insulating layer formed from the resin composition. More specifically, the storage stability of the resin composition can be improved and a decrease in the film elongation of the insulating layer formed from the resin composition can be suppressed, thereby achieving a better balance between the two.

[0039] The lower limit of the content of the compound 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, and can be, for example, 0.1 mol%, preferably 0.5 mol%, more preferably 1 mol%. By setting the content of the compound to be equal to or greater than the lower limit, the storage stability of the resin composition can be further improved.

[0040] The aromatic diamine may contain aromatic diamines other than the above compounds (hereinafter also referred to as "other aromatic diamines"). Examples of aromatic diamines other than the above compounds include diaminodiphenyl ethers (ODAs) such as 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), 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, and 4,4'-diaminodiphenylsulfonyl ether. Examples of suitable amino acids include sulfone, 3,4'-diaminodiphenyl sulfone, 3,3'-diaminodiphenyl sulfone, 2,4'-diaminodiphenyl sulfone, 2,2'-diaminodiphenyl sulfone, 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 thereof. As the other aromatic diamine, 4,4'-diaminodiphenyl ether is preferred.

[0041] The lower limit of the content of the other aromatic diamine relative to 100 mol% of the aromatic diamine is preferably 80 mol%, more preferably 82 mol%, even more preferably 85 mol%, still more preferably 88 mol%, particularly preferably 90 mol%, and most preferably 92 mol%. By setting the content of the other aromatic diamine to the above upper limit or less, a better balance can be achieved between the storage stability of the resin composition and the film elongation of the insulating layer formed from the resin composition.

[0042] The upper limit of 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, and can be, for example, 99.9 mol%, preferably 99.5 mol%, and more preferably 99 mol%. By setting the content of the other aromatic diamine to the upper limit or less, the storage stability of the resin composition can be further improved.

[0043] The lower limit of the concentration of the polyimide precursor in the resin composition is preferably 25% by mass, more preferably 27% by mass. The upper limit of the concentration is preferably 40% by mass, more preferably 35% by mass. By setting the concentration at or above the lower limit, it is possible to reduce the amount of resin composition required in the entire manufacturing process to obtain an insulating layer of the desired thickness when forming an insulating layer using the resin composition, and to reduce the number of coating and heating steps. By setting the concentration at or below the upper limit, it is possible to appropriately adjust the viscosity of the resin composition while maintaining good film properties, thereby improving coatability.

[0044] The lower limit of the weight-average molecular weight of the polyimide precursor is 15,000, preferably 16,000. The upper limit of the weight-average molecular weight of the polyimide precursor is preferably 100,000, more preferably 50,000. If the weight-average molecular weight is below the lower limit, the coating may not elongate sufficiently when forming an insulating layer for an insulated wire. On the other hand, if the weight-average molecular weight of the polyimide precursor exceeds the upper limit, the viscosity of the resin composition may become too high. The "weight-average molecular weight" of the polyimide precursor refers to the value measured in terms of polystyrene by gel permeation chromatography in accordance with JIS-K7252-1 (2008) "Plastics - Determination of average molecular weight and molecular weight distribution of polymers by size exclusion chromatography - Part 1: General rules."

[0045] (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. The polymerization condensation reaction can be carried out in the same manner as in conventional polyimide precursor synthesis. A specific example of the polymerization condensation reaction is mixing the aromatic tetracarboxylic dianhydride and the aromatic diamine in an organic solvent. This method polymerizes the aromatic tetracarboxylic dianhydride and the aromatic diamine, resulting in a solution of the polyimide precursor dissolved in the organic solvent. The polymerization condensation reaction can be carried out in the presence of an end-capping agent to control the degree of polymerization (weight-average molecular weight).

[0046] Examples of the end-capping agent include alcohols having 1 to 15 carbon atoms. Specific examples include monohydric alcohols such as ethanol, methanol, propanol, butanol, and pentanol, and polyhydric alcohols such as ethylene glycol, propylene glycol, and glycerin. Among these, methanol or ethanol is preferred from the viewpoints of reactivity and cost, and methanol is more preferred.

[0047] The reaction conditions for the polymerization may be appropriately set depending on the raw materials used, etc., but for example, the reaction temperature may be 10° C. or higher and 100° C. or lower, and the reaction time may be 0.5 hours or higher and 24 hours or lower.

[0048] Examples of the organic solvent used in the polymerization condensation reaction include the same organic solvents as those contained in the resin composition described below.

[0049] (organic solvent) Examples of organic solvents that can be used include aprotic polar organic solvents such as N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMAc), N,N-dimethylformamide, dimethyl sulfoxide, and γ-butyrolactone. These organic solvents may be used alone or in combination of two or more. "Aprotic polar organic solvent" refers to a polar organic solvent that does not have a group that releases a proton.

[0050] The amount of the organic solvent used is not particularly limited as long as it is an amount that can uniformly dissolve and disperse the aromatic tetracarboxylic dianhydride and the aromatic diamine, but if the amount is too large, a large amount of solvent needs to be volatilized when forming the insulating layer of the insulated wire, which may require a long time to form the insulating layer. Therefore, the amount of the organic solvent used 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 aromatic tetracarboxylic dianhydride and the aromatic diamine combined.

[0051] (pore-forming agent) Examples of the pore-forming agent include chemical foaming agents, thermally expandable microcapsules, hollow-forming particles with a core-shell structure, and high-boiling solvents.

[0052] By including a pore-forming agent in the resin composition, when the resin composition is used to form an insulating layer, pores can be formed in the insulating layer. This allows the insulating layer to have a low dielectric constant and an increased corona discharge inception voltage. This makes it possible to make the insulating layer less susceptible to dielectric breakdown.

[0053] As the chemical foaming agent, for example, a thermally decomposable substance such as azobisisobutyronitrile or azodicarbodiamide, which generates nitrogen gas (N2 gas) when heated, is preferably used.

[0054] The chemical foaming agent foams when heated during curing by baking when the resin composition is applied to a conductor and cured to form an insulating layer for an insulated wire, thereby generating pores in the insulating layer. By using a chemical foaming agent as the pore-forming agent, pores can be easily formed during curing of the resin composition by baking.

[0055] The lower limit of the foaming temperature of the chemical foaming agent is preferably 180°C, more preferably 210°C. On the other hand, the upper limit of the foaming temperature is preferably 300°C, more preferably 260°C. If the foaming temperature is below the lower limit, foaming is likely to occur before baking, which may make it difficult to adjust the thickness of the insulating layer. Conversely, if the foaming temperature exceeds the upper limit, the baking temperature may increase and the baking time may become longer, which may increase the manufacturing cost of the insulated wire. Here, the "foaming temperature" refers to the temperature at which the foaming agent begins to foam. Furthermore, the "baking time" refers to the time during which the conductor coated with the resin composition is held at the baking temperature.

[0056] The thermally expandable microcapsules have a core material containing a thermal expansion agent and an outer shell enclosing the core material. When heated during the baking process, the thermal expansion agent contained in the core material expands or foams, pushing the outer shell open to form pores. Therefore, by using the pore-forming agent as the thermally expandable microcapsules, the controllability of the pore size can be improved.

[0057] The thermal expansion agent may be any agent that expands or generates gas upon heating, regardless of the mechanism of action. Examples of the thermal expansion agent include low-boiling-point liquids, chemical foaming agents, and mixtures thereof. Examples of the low-boiling-point liquid include alkanes such as butane, i-butane, n-pentane, i-pentane, and neopentane, and freon compounds such as trichlorofluoromethane. Examples of the chemical foaming agent include thermally decomposable substances such as azobisisobutyronitrile and azodicarbodiamide, which generate N2 gas upon heating.

[0058] The core material preferably contains the thermal expansion agent as a main component, and among these, azobisisobutyronitrile and azodicarbodiamide are preferred as the main components of the core material. Azobisisobutyronitrile and azodicarbodiamide generate N2 gas when heated, allowing the thermally expandable microcapsules to be thermally expanded while maintaining their chemical stability.

[0059] The expansion initiation temperature of the thermal expansion agent, i.e., the boiling point of the low-boiling liquid or the thermal decomposition temperature of the chemical foaming agent, is set to be equal to or higher than the softening temperature of the shell of the thermally expandable microcapsules described below. More specifically, the lower limit of the expansion initiation temperature of the thermal expansion agent is preferably 60°C, more preferably 70°C. Meanwhile, the upper limit of the expansion initiation temperature of the thermal expansion agent is preferably 200°C, more preferably 150°C. If the expansion initiation temperature of the thermal expansion agent is below the lower limit, the thermally expandable microcapsules may unintentionally expand during the formation of the insulating layer, transportation, or storage. Conversely, if the expansion initiation temperature of the thermal expansion agent exceeds the upper limit, the energy cost required to expand the thermally expandable microcapsules may be excessive.

[0060] The shell is made of a material with sufficient stretchability to expand without rupture during expansion of the thermal expansion agent and to form microballoons that contain the generated gas. The main component of the shell is typically a resin composition such as a thermoplastic resin. Examples of the thermoplastic resin include polymers formed from monomers such as vinyl chloride, vinylidene chloride, acrylonitrile, acrylic acid, methacrylic acid, acrylate, methacrylate, and styrene, or copolymers formed from two or more monomers. Among these, vinylidene chloride-acrylonitrile copolymer is preferred as the main component of the shell. Vinylidene chloride-acrylonitrile copolymers have excellent stretchability, allowing them to expand without rupture during expansion of the thermally expandable microcapsules and easily form microballoons that contain the generated gas. When vinylidene chloride-acrylonitrile copolymer is used as the main component of the shell, the expansion initiation temperature of the thermal expansion agent is set to 80°C or higher and 150°C or lower.

[0061] The hollow particles having a core-shell structure are obtained by gasifying and removing the core through heating during the baking process. The hollow particles have pores and an outer shell that are formed by thermal decomposition of the core after the resin composition is cured, so that interconnection of the pores is suppressed even during pore formation. Therefore, when used as a resin composition for forming an insulating layer, the dielectric breakdown voltage of the insulating layer can be easily increased.

[0062] The core of the hollow-forming particles may be primarily composed of a thermally decomposable resin. Examples of thermally decomposable resins include, but are not limited to, 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 having 1 to 6 carbon atoms, such as polymethyl(meth)acrylate, polyethyl(meth)acrylate, polypropyl(meth)acrylate, and polybutyl(meth)acrylate; polymers of modified (meth)acrylates, such as urethane oligomers, urethane polymers, urethane(meth)acrylates, epoxy(meth)acrylates, and ε-caprolactone(meth)acrylates; poly(meth)acrylic acid; crosslinked products thereof; polystyrene, crosslinked polystyrene, and the like. Among these, polymers of (meth)acrylic acid esters having an alkyl group having 1 to 6 carbon atoms are preferred because they facilitate the formation of pores in the insulating layer. Examples of such (meth)acrylic acid ester polymers include polymethyl methacrylate (PMMA).

[0063] The core preferably has a spherical shape. To achieve a spherical core shape, for example, spherical thermally decomposable resin particles may be used as the core. When spherical thermally decomposable resin particles are used, the lower limit of the average particle diameter of the resin particles is not particularly limited, but is preferably 0.1 μm, more preferably 0.5 μm, and even more preferably 1 μm. On the other hand, the upper limit of the average particle diameter of the resin particles is preferably 15 μm, more preferably 10 μm. If the average particle diameter of the resin particles is less than the lower limit, it may be difficult to prepare hollow-forming particles having the resin particles as the core. Conversely, if the average particle diameter of the resin particles exceeds the upper limit, the hollow-forming particles having the resin particles as the core may become too large, making it difficult to achieve a uniform pore distribution in the insulating layer, and potentially resulting in biased dielectric constant distribution.

[0064] The main component of the shell is preferably one having a thermal decomposition temperature higher than that of the thermally decomposable resin. By constructing the hollow-forming particles in this manner, hollow particles consisting only of an outer shell with a hollow interior can be obtained by heating, thereby facilitating the formation of pores. Furthermore, the main component of the shell preferably has a low dielectric constant and high heat resistance. Examples of materials used as the main component of the shell include resins such as polystyrene, silicone, fluororesin, and polyimide. Here, "fluororesin" refers to a polymer in which at least one hydrogen atom bonded to a carbon atom constituting a repeating unit of the polymer chain is substituted with a fluorine atom or an organic group containing a fluorine atom (hereinafter also referred to as a "fluorine atom-containing group"). The fluorine atom-containing group is a linear or branched organic group in which at least one hydrogen atom is substituted with a fluorine atom, and examples thereof include a fluoroalkyl group, a fluoroalkoxy group, and a fluoropolyether group. The shell may also contain a metal as long as the insulating properties are not impaired.

[0065] Among these, silicone is preferred as the main component of the shell. As a result, the closed pores of the hollow-forming particles are more easily maintained. By using silicone as the main component of the shell of the hollow-forming particles, elasticity is imparted to the shell and insulating properties and heat resistance are easily improved, and as a result, the closed pores of the hollow-forming particles are more easily maintained.

[0066] The lower limit of the average thickness of the shell is not particularly limited, but is preferably 0.01 μm, more preferably 0.02 μm. On the other hand, the upper limit of the average thickness of the shell is preferably 0.5 μm, more preferably 0.4 μm. If the average thickness of the shell is below the lower limit, the effect of suppressing interconnection of pores may not be sufficiently obtained. Conversely, if the average thickness of the shell exceeds the upper limit, the pore volume may become too small, and the porosity of the insulating layer may not be increased to a predetermined level. The shell may be formed of one layer or multiple layers. If the shell is formed of multiple layers, the average total thickness of the multiple layers may be within the above range.

[0067] The upper limit of the CV value of the hollow-forming particles is preferably 30%, more preferably 20%. If the CV value of the hollow-forming particles exceeds the upper limit, the insulating layer will contain multiple pores of different sizes, which may lead to a bias in the dielectric constant distribution. There is no particular limit to the lower limit of the CV value of the hollow-forming particles, but a value of 1% is preferred, for example. If the CV value of the hollow-forming particles is below the lower limit, the manufacturing cost of the hollow-forming particles may be too high. Here, "CV value" refers to the variable defined in JIS-Z8825 (2013).

[0068] The hollow-forming particle may have a core formed of a single heat-decomposable resin particle, or may have a core formed of a plurality of heat-decomposable resin particles, with the shell covering these plurality of heat-decomposable resin particles. The surface of the hollow-forming particle may be smooth without irregularities, or may have irregularities.

[0069] The high-boiling solvent has a boiling point higher than that of the solvent of the resin composition and is used for forming bubbles. The lower limit of the boiling point of the high-boiling solvent is preferably 180°C, more preferably 210°C. On the other hand, the upper limit of the boiling point of the high-boiling solvent is preferably 300°C, more preferably 260°C. If the boiling point of the high-boiling solvent is below the lower limit, the amount of volatilization when volatilizing the solvent of the resin composition increases, and bubbles may not be formed sufficiently. Conversely, if the boiling point of the high-boiling solvent exceeds the upper limit, the high-boiling solvent may be difficult to volatilize, and bubbles may not be formed sufficiently.

[0070] Examples of the high-boiling solvent that can be used include diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, diethylene glycol dibutyl ether, tetraethylene glycol dimethyl ether, and tetraethylene glycol monomethyl ether. Triethylene glycol dimethyl ether is preferred because it has a small variation in bubble diameter. In addition to these, dipropylene glycol dimethyl ether, diethylene glycol ethyl methyl ether, dipropylene glycol monomethyl ether, diethylene glycol diethyl ether, diethylene glycol monomethyl ether, diethylene glycol butyl methyl ether, tripropylene glycol dimethyl ether, diethylene glycol monobutyl ether, ethylene glycol monophenyl ether, triethylene glycol monomethyl ether, triethylene glycol butyl methyl ether, polyethylene glycol dimethyl ether, polyethylene glycol monomethyl ether, and propylene glycol monomethyl ether can also be used.

[0071] The high-boiling point solvents may be used alone, but are preferably used in combination of two or more, since this provides the effect of generating bubbles over a wide temperature range. When two or more high-boiling point solvents are used in combination, preferred combinations include tetraethylene glycol dimethyl ether and diethylene glycol dibutyl ether, diethylene glycol dibutyl ether and triethylene glycol dimethyl ether, triethylene glycol monomethyl ether and tetraethylene glycol dimethyl ether, and triethylene glycol butyl methyl ether and tetraethylene glycol dimethyl ether, and more preferred combinations are diethylene glycol dibutyl ether and triethylene glycol dimethyl ether, and triethylene glycol monomethyl ether and tetraethylene glycol dimethyl ether.

[0072] As described above, the high-boiling solvent has a higher boiling point than the solvent of the resin composition. When one high-boiling solvent is used, the lower limit of the difference in boiling points is preferably 10°C. It is known that when one high-boiling solvent is used, the high-boiling solvent functions as both a bubble nucleating agent and a blowing agent. When two or more high-boiling solvents are used, the high-boiling solvent with the highest boiling point (hereinafter also referred to as the "highest boiling solvent") acts as a blowing agent, and the other high-boiling solvents act as bubble nucleating agents. In this case, the lower limit of the difference in boiling points between the highest boiling solvent and the solvent of the resin composition is preferably 20°C, more preferably 30°C. Meanwhile, the upper limit of the difference in boiling points is preferably 60°C. The lower limit of the difference in boiling points between the other high-boiling solvents and the solvent of the resin composition is preferably 10°C.

[0073] When two or more high-boiling solvents are used, the lower limit of the ratio of the highest boiling solvent to the other high-boiling solvents (the total when two or more solvents are used) is preferably 1:99 by mass, more preferably 1:10. On the other hand, the upper limit of this ratio is preferably 99:1, more preferably 10:1. By keeping the ratio within this range, bubbles can be easily generated.

[0074] In addition, it is preferable that the solubility of the polyimide precursor in the other high-boiling solvents be greater than the solubility of the polyimide precursor in the highest-boiling solvent. When the solubilities of the polyimide precursor satisfy the above-mentioned relationship, uniform bubbles can be easily formed.

[0075] <Insulated wire> The insulated wire includes a conductor and an insulating layer covering the conductor, and can be suitably used as a winding wire for a coil (magnet wire).

[0076] (conductor) The conductor generally contains a metal as its main component. While the metal is not particularly limited, copper, copper alloys, aluminum, or aluminum alloys are preferred. By using such a metal for the conductor, an insulated wire having excellent processability and electrical conductivity can be obtained. The conductor may contain other components, such as known additives, in addition to the metal as its main component.

[0077] 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.

[0078] (insulating layer) The insulating layer is laminated on the peripheral surface of the conductor so as to cover the conductor. The insulating layer is a layer formed from the resin composition described above. The insulating layer may cover the conductor directly or indirectly via another layer. 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 layer can be mentioned.

[0079] The average thickness of the insulating layer is not particularly limited, but is usually 2 μm or more and 200 μm or less.

[0080] The insulated wire may further have another layer laminated on the outer periphery of the insulating layer, such as a surface lubricating layer.

[0081] (Insulated Wire Manufacturing Method) The insulated wire can be manufactured by a method including, for example, a step of applying the resin composition to the outer periphery of a conductor (hereinafter also referred to as the "application step") and a step of heating the resin composition applied to the conductor (hereinafter also referred to as the "heating step").

[0082] In the coating step, the resin composition is applied to the outer peripheral side of the conductor. For example, a method for applying the resin composition to the outer peripheral side of the conductor may use a coating device equipped with a liquid composition tank that stores 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 side of the conductor, and then passes through the coating die, thereby applying the resin composition to a uniform thickness.

[0083] In the heating step, the resin composition applied to the conductor in the coating step is heated. This heating volatilizes the solvent in the resin composition and hardens the polyimide precursor to form polyimide. In this way, an insulating layer with excellent electrical, mechanical, and thermal properties is obtained.

[0084] The heating step may be performed using a cylindrical baking oven that is long in the direction of travel of the conductor, but is not limited to this. The heating method may be any conventional method, such as hot air heating, infrared heating, or high-frequency heating.

[0085] The heating temperature can be, for example, 300°C or higher and 800°C or lower, and the heating time can be 5 seconds or higher and 1 minute or lower. If the heating temperature or heating time is below the lower limit, the solvent may not volatilize sufficiently, or the insulating layer may not be formed sufficiently, which may result in poor appearance, electrical properties, mechanical properties, thermal properties, etc. of the insulated wire. Conversely, if the heating temperature exceeds the upper limit, excessive rapid heating may cause foaming of the insulating layer or a deterioration in mechanical properties. Furthermore, if the heating time exceeds the upper limit, the productivity of the insulated wire may decrease.

[0086] The coating step and the heating step are usually repeated multiple times. In this way, the thickness of the insulating layer can be increased. At this time, the hole diameter of the coating die is appropriately adjusted according to the number of repetitions. [Example]

[0087] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0088] <Preparation of Resin Composition> The abbreviations for the components used in preparing the resin compositions are shown below. PMDA: Pyromellitic dianhydride BTDA: 3,3',4,4'-benzophenonetetracarboxylic dianhydride BPDA: 3,3',4,4'-biphenyltetracarboxylic dianhydride ODA: 4,4'-diaminodiphenyl ether DETDA: 2,6-diamino-3,5-diethyltoluene BAPB: 4,4'-bis(4-aminophenoxy)biphenyl NMP: N-methyl-2-pyrrolidone

[0089] [No.1] NMP (aprotic polar solvent) and methanol (end-capping agent) were mixed in a 1-L flask equipped with a stirring blade, and ODA (aromatic diamine) was dissolved in the resulting solution. Then, while stirring the solution at 200 rpm, the mixture of aromatic tetracarboxylic dianhydrides (PMDA and BTDA) was divided into two equal parts. Each of the divided parts was added at 10-minute intervals, i.e., over a 10-minute addition time. The solution was left at room temperature for 2 hours to prepare Resin Composition No. 1. The molar ratio of the aromatic tetracarboxylic dianhydride to the aromatic diamine was 100:100, and the molar ratio of PMDA to BTDA was 30:70.

[0090] The conductor is a round copper wire with an average diameter of 1 mm. Wire Resin composition No. 1 was applied to the surface of the conductor, and the conductor coated with resin composition No. 1 was heated in a heating furnace at a set temperature of 500°C, and this process was repeated to form an insulating layer with an average thickness of 40 μm, thereby producing insulated wire No. 1.

[0091] [No.2~15] Resin compositions Nos. 2 to 15 were prepared in the same manner as in No. 1, except that the types and amounts of each component were used as shown in Table 1 below, and insulated wires Nos. 2 to 15 were produced. The concentrations of the polyimide precursor in the resulting resin compositions Nos. 2 to 15 are also shown in Table 1 below.

[0092] [Polyimide precursor concentration] For the resin compositions Nos. 1 to 15 prepared above, the resin compositions were dried at 250°C for 2 hours, and the mass before drying (W0) and the mass after drying (W1) were measured, and the concentration (unit: mass%) was calculated by W1 / W0 x 100. The results are shown in the "Concentration" row in Table 1 below.

[0093] [Evaluation of storage stability] The viscosity of each of the prepared resin compositions No. 1 to No. 15 at 30°C (initial viscosity η0) was measured using a Brookfield viscometer (RB-80L manufactured by Toki Sangyo Co., Ltd.). The resin compositions were then sealed and stored at 30°C for two weeks, and the viscosity at 30°C after two weeks had passed (post-storage viscosity η1) was measured. The ratio η1 / η0 of the post-storage viscosity η1 to the initial viscosity η0 was calculated. A ratio η1 / η0 of less than 2.2 was evaluated as having good storage stability. The results are shown in the "storage stability" row of Table 1 below.

[0094] [Evaluation of film elongation] For the insulated wires Nos. 1 to 15 prepared above, the conductors were removed from the insulated wires to form tubular insulating layers, which were then subjected to a tensile test using a tensile tester (Shimadzu Corporation's "Autograph AGS-X") at a chuck distance of 20 mm and a speed of 10 mm / min to measure the coating elongation (elongation at break) (unit: %). Coating elongation of 30% or more was considered to be good. The results are shown in the "Coating Elongation" row in Table 1 below.

[0095] In the columns of "aromatic tetracarboxylic dianhydride" and "aromatic diamine" in Table 1 below, the units of values ​​are mol %, and "-" indicates that the corresponding component was not used.

[0096] [Table 1]

[0097] The results in Table 1 show that Nos. 3 to 5, 8 to 10, and 13 to 15 have excellent storage stability and can form insulating layers with excellent film elongation.

Claims

1. a polyimide precursor which is a reaction product of an aromatic tetracarboxylic dianhydride and an aromatic diamine; an organic solvent; Contains The aromatic diamine contains a compound represented by the following formula (1): the content of the compound relative to 100 mol % of the aromatic diamine is less than 20 mol %, the aromatic diamine further comprises 4,4'-diaminodiphenyl ether; The resin composition wherein the aromatic tetracarboxylic dianhydride comprises pyromellitic dianhydride. 【Chemical 1】 (In formulas (1) to (3), R represents an alkyl group having 1 to 4 carbon atoms, an alkenyl group having 2 to 4 carbon atoms, or an alkoxy group having 1 to 4 carbon atoms. n represents an integer from 1 to 4. When n is 2 or greater, multiple Rs may be the same or different.)

2. 2. The resin composition according to claim 1, wherein the concentration of the polyimide precursor is 25% by mass or more.

3. A conductor; an insulating layer covering the conductor; Equipped with An insulated wire, wherein the insulating layer is formed from the resin composition according to claim 1 or 2.

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