WO2018151091A1 - 電着液及びこれを用いた絶縁皮膜付き導体の製造方法 - Google Patents

電着液及びこれを用いた絶縁皮膜付き導体の製造方法 Download PDF

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WO2018151091A1
WO2018151091A1 PCT/JP2018/004873 JP2018004873W WO2018151091A1 WO 2018151091 A1 WO2018151091 A1 WO 2018151091A1 JP 2018004873 W JP2018004873 W JP 2018004873W WO 2018151091 A1 WO2018151091 A1 WO 2018151091A1
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insulating film
electrodeposition
resin particles
particles
mass
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PCT/JP2018/004873
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English (en)
French (fr)
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慎太郎 飯田
桜井　英章
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三菱マテリアル株式会社
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Priority to KR1020197021240A priority Critical patent/KR102545341B1/ko
Priority to US16/464,847 priority patent/US11286576B2/en
Priority to EP18755015.7A priority patent/EP3584293B1/en
Priority to CN201880007709.0A priority patent/CN110234717A/zh
Publication of WO2018151091A1 publication Critical patent/WO2018151091A1/ja

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    • H01—ELECTRIC ELEMENTS
    • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
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Definitions

  • the present invention relates to an electrodeposition liquid for forming an insulating film having a low dielectric constant and a high dielectric breakdown voltage, and a method for producing a conductor having an insulating film using the electrodeposition liquid.
  • non-film forming fine particles having an average particle diameter of 1 ⁇ m or less and a relative dielectric constant of 3 or less and organic particles made of at least one of a polymerizable compound and a polymer are dispersed in an aqueous medium, and an insulating film is formed by electrodeposition.
  • An aqueous dispersion for forming a low dielectric constant insulating film that can be formed is disclosed (for example, see Patent Document 1).
  • the non-film forming fine particles are at least one selected from fluorine-containing fine particles and crosslinked organic fine particles.
  • the aqueous dispersion is prepared using an aqueous emulsion in which organic particles are dispersed in an aqueous medium.
  • the “aqueous medium” means a medium containing water as a main component, and the content of water in the aqueous medium is usually 40% by weight or more, preferably 50% by weight or more.
  • an insulating film having a low dielectric constant and good adhesion can be formed by electrodeposition using this dispersion as an electrodeposition liquid.
  • this low dielectric constant insulating film is produced by electrodeposition using the above aqueous dispersion, the film thickness controllability and the film formation position controllability are superior to those produced by coating.
  • a method for producing an insulated wire in which an insulating layer is formed by electrodeposition of a water-dispersed resin emulsion obtained by water-dispersing a polyimide resin, a fluororesin, and a charge imparting agent on a conductor, followed by drying and baking (for example, Patent Document 2).
  • a conductor is coated with a water-dispersed resin emulsion by electrodeposition.
  • This electrodeposition is performed by applying a voltage with the conductor immersed in the water-dispersed resin emulsion, for example, by passing the conductor through an electrodeposition bath filled with the water-dispersed resin emulsion.
  • an insulated wire having an insulation layer having a sufficient insulation performance, a small thickness, and excellent heat resistance can be produced.
  • the insulated wire can be suitably used for wiring inside or between electronic devices where heat resistance is required.
  • Japanese Patent Laid-Open No. 2001-294815 (Claims 1, 2 and paragraphs [0023] and [0068])
  • Japanese Patent Laid-Open No. 2002-298664 (A) (Claim 1, paragraphs [0020] and [0037], FIG. 1)
  • An object of the present invention is to use an electrodeposition liquid capable of thickening a composite film of a polyimide resin and a fluororesin, and to obtain an insulating film having a low dielectric constant and a high dielectric breakdown voltage, and the same. It is providing the manufacturing method of a conductor with an insulating film.
  • the first aspect of the present invention comprises a dispersion medium and a solid content
  • the solid content includes polyimide resin particles and fluororesin particles
  • the content ratio of the fluororesin particles in the solid content is 20 to 70% by mass.
  • the electrodeposition liquid is characterized in that the median diameter of the polyimide resin particles is 50 to 400 nm.
  • a second aspect of the present invention is the invention based on the first aspect, wherein the dispersion medium further contains a polar solvent, water and a base, the polar solvent has a higher boiling point than water, and the water in the dispersion medium The content ratio of is 15 to 40% by mass.
  • the third aspect of the present invention is an invention based on the first or second aspect, and is characterized in that the polyimide resin particles further have a median diameter smaller than that of the fluororesin particles.
  • a fourth aspect of the present invention is a method for manufacturing a conductor with an insulating film, in which an insulating film is formed on the surface of the conductor by electrodeposition using the electrodeposition liquid according to any one of the first to third aspects.
  • the fifth aspect of the present invention is an invention based on the fourth aspect, and is characterized in that the conductor is a metal wire.
  • the median diameter of the polyimide resin particles is as small as 50 to 400 nm, the amount of the dispersion medium confined between the polyimide resin particles and the fluorine resin particles during electrodeposition is reduced. It is possible to suppress foaming of the insulating film during heat treatment. As a result, the insulating film, which is a composite film of polyimide resin and fluororesin, can be thickened, so that the dielectric constant is low and partial discharge is difficult to occur, and the dielectric breakdown voltage of the insulating film can be increased. .
  • the polar solvent has a boiling point higher than that of water, and the content of water in the dispersion medium is as low as 15 to 40% by mass.
  • the foaming of the insulating film during heat treatment can be further suppressed.
  • foaming of the insulating film can be further suppressed even when it is left standing in a furnace finally set to a temperature required for heat treatment of the film.
  • the polyimide resin particles have a median diameter smaller than that of the fluororesin particles, the polyimide resin particles easily enter the gaps between the fluororesin particles. As a result, the amount of the dispersion medium confined can be reduced, and foaming of the insulating film during heat treatment can be further suppressed.
  • the insulating film is formed on the conductor surface by electrodeposition using the electrodeposition liquid, foaming of the insulating film during heat treatment can be suppressed.
  • the insulating film can be thickened, a conductor having an insulating film having a low dielectric constant, hardly causing partial discharge, and having a high dielectric breakdown voltage can be manufactured.
  • the insulating film is formed on the surface of the metal wire by using the electrodeposition liquid, foaming of the insulating film during heat treatment can be suppressed.
  • the insulating film can be thickened, a metal wire having an insulating film having a low dielectric constant, hardly causing partial discharge, and having a high dielectric breakdown voltage can be manufactured.
  • the electrodeposition liquid consists of a dispersion medium and a solid part.
  • the solid part includes polyimide resin particles and fluororesin particles.
  • the polyimide resin include polyamideimide resin, polyimide resin, polyesterimide resin, and polyamic acid resin.
  • the fluororesin include polytetrafluoroethylene (tetrafluoroethylene) resin, tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer resin, perfluoroethylene propene copolymer resin, and polyvinylidene fluoride resin.
  • the content ratio of the fluororesin particles in the solid content is 20 to 70% by mass, preferably 30 to 70% by mass.
  • the median diameter of the polyimide resin particles is 50 to 400 nm, preferably 50 to 200 nm.
  • the median diameter of the fluororesin particles is 50 to 500 nm, preferably 50 to 300 nm.
  • the polyimide resin particles preferably have a median diameter smaller than that of the fluororesin particles.
  • the content ratio of the fluororesin particles in the solid content is limited to the range of 20 to 70% by mass. If the content is less than 20% by mass, the dielectric constant of the insulating film cannot be lowered and exceeds 70% by mass. This is because the insulating film does not become a continuous film and cracks are likely to occur.
  • the median diameter of the polyimide resin particles is limited to the range of 50 to 400 nm because if the thickness is less than 50 nm, the dispersion medium existing between the resin particles is small and the resistance of the film is large, so that the electrodeposition rate is slow and the thick film It takes time to obtain, and when the thickness exceeds 400 nm, the dispersion stability of the electrodeposition liquid decreases, and when the electrodeposition liquid dries, the volatilization amount of water increases to form a thick insulating film, and the insulating film tends to foam. Because it becomes.
  • the preferable median diameter of the fluororesin particles is limited to the range of 50 to 500 nm because if the thickness is less than 50 nm, the dispersion medium existing between the resin particles is small and the resistance of the film is increased, so that the electrodeposition rate is slow and the thick film This is because it takes time to obtain, and when the thickness exceeds 500 nm, the electrodeposition liquid aggregates and precipitates, resulting in a decrease in dispersion stability.
  • the dispersion medium preferably contains a polar solvent, water and a base.
  • the polar solvent preferably has a boiling point higher than that of water.
  • the polar solvent include organic solvents such as N-methyl-2-pyrrolidone, 1,3-dimethyl-2-imidazolidinone, dimethyl sulfoxide, and N, N dimethylacetamide.
  • examples of the base include tri-n-propylamine, dibutylamine, piperidine, triethylamine and the like.
  • the content ratio of water in the dispersion medium is preferably 15 to 40% by mass, and more preferably 18 to 30% by mass.
  • the content of the polar solvent in the dispersion medium is preferably 60 to 70% by mass, and more preferably 65 to 70% by mass.
  • the base content in the dispersion medium is preferably 0.1 to 0.3% by mass, more preferably 0.1 to 0.2% by mass.
  • the solid content in the electrodeposition liquid is preferably 1 to 10% by mass, and more preferably 2
  • the preferable content ratio of water in the dispersion medium is limited to the range of 15 to 40% by mass. If the content is less than 15% by mass, the conductivity of the electrodeposition liquid is small and an insulating film cannot be formed by electrodeposition. If the amount exceeds 40% by mass, the volatilization rate of the dispersion medium increases when the electrodeposition liquid is dried, and if the insulating film is formed thick, the insulating film is liable to foam.
  • the preferable content ratio of the polar solvent in the dispersion medium is limited to the range of 60 to 70% by mass. If the content is less than 60% by mass, the ratio of water in the dispersion medium increases and the volatilization rate increases and foaming occurs.
  • the preferable content ratio of the base in the dispersion medium is limited to the range of 0.1 to 0.3% by mass. If the content is less than 0.1% by mass, the median diameter of the polyimide resin particles increases and the dispersion stability is increased. When the content exceeds 0.3% by mass, the median diameter of the polyimide resin particles decreases, the dispersion medium existing between the resin particles decreases, and the resistance of the film increases. This is because it takes time to obtain Furthermore, the preferable content ratio of the solid content in the electrodeposition liquid is limited to the range of 1 to 10% by mass.
  • the median diameter of the polyimide resin particles and the median diameter of the fluororesin particles are volume-based average particle diameters measured using a dynamic light scattering particle size distribution analyzer (LB-550 manufactured by Horiba, Ltd.).
  • diphenylmethane-4,4′-diisocyanate diphenylmethane-4,4′-diisocyanate (MDI), diphenylmethane-3,3′-diisocyanate, diphenylmethane-3,4′-diisocyanate, diphenylether-4,4′-diisocyanate, benzophenone-4 , 4′-diisocyanate, aromatic diisocyanate such as diphenylsulfone-4,4′-diisocyanate, etc.
  • the acid component includes trimellitic anhydride (TMA), 1,2,5-trimellitic acid (1, 2,5-ETM), biphenyltetracarboxylic dianhydride, benzophenonetetracarboxylic dianhydride, diphenylsulfonetetracarboxylic dianhydride, oxydiphthalic dianhydride (OPDA), pyromellitic dianhydride (PMDA
  • a commercially available fluororesin particle is diluted with water and then stirred to obtain a dispersion of fluororesin particles having a median diameter of 50 to 500 nm.
  • An electrodeposition liquid is obtained by mixing a dispersion of polyimide resin particles and a dispersion of fluororesin particles.
  • an electrodeposition film is formed on the surface of the conductor by an electrodeposition method using the electrodeposition liquid, and then the conductor is dried and baked. It is preferable to form an insulating film on the surface.
  • the thickness of this insulating film is preferably 10 to 70 ⁇ m, and more preferably 20 to 50 ⁇ m.
  • the dielectric breakdown voltage of the insulating film is preferably at least 5 kV when the thickness of the insulating film is 40 ⁇ m.
  • a metal wire, a metal plate, etc. are mentioned as said conductor.
  • the material of the metal wire is preferably copper, copper alloy, aluminum, aluminum alloy or the like having good conductivity.
  • the median diameter of the polyimide resin particles in the electrodeposition liquid for forming the insulating film is as small as 50 to 400 nm.
  • the amount of the dispersion medium confined between the two can be reduced, and foaming of the insulating film during heat treatment can be suppressed.
  • the median diameter of the polyimide resin particles 11a of the solid resin 11 and the fluorine resin particles 11b is as small as 50 to 400 nm, the polyimide on the surface of the conductor 12 is obtained. Since the resin particles 11a are closely packed between the fluororesin particles 11b and densely packed, and the dispersion medium 13 remaining between these particles 11a and 11b is reduced (FIG.
  • foam 24a is likely to occur in the insulating film 24 after the baking process (FIG. 2C).
  • the insulating film 14 that is a composite film of polyimide resin and fluororesin can be thickened, the dielectric breakdown voltage of the insulating film 14 of the conductor 10 with an insulating film can be increased.
  • the polar solvent has a boiling point higher than that of water, and the content rate of water in the dispersion medium is reduced to 15 to 40% by mass, so that the volatilization rate of the dispersion medium during the heat treatment is reduced.
  • foaming of the insulating film during heat treatment can be further suppressed.
  • the polyimide resin particles have a median diameter smaller than that of the fluororesin particles, the polyimide resin particles can easily enter the gaps between the fluororesin particles, thereby reducing the amount of dispersion medium trapped. And foaming of the insulating film during heat treatment can be further suppressed.
  • Example 1 A method for producing the electrodeposition liquid will be described.
  • Synthesis of polyimide resin varnish First, 747 g of N-methyl-2-pyrrolidone (polar solvent) and 298 g (1.19 mol) of a 2-liter four-necked flask equipped with a stirrer, a condenser tube, a nitrogen inlet tube and a thermometer were placed. Diphenylmethane-4,4′-diisocyanate (MDI, isocyanate component) and 227 g (1.18 mol) of trimellitic anhydride (TMA, acid component) were added and the temperature was raised to 130 ° C.
  • MDI isocyanate component
  • TMA trimellitic anhydride
  • polyamideimide resin polyamideimide resin having a number average molecular weight of 17000 was obtained. Thereafter, the synthesized polyamideimide resin is diluted with N-methyl-2-pyrrolidone (polar solvent) so that the concentration of the polyamideimide resin (non-volatile content) becomes 20% by mass, and the polyamideimide resin varnish (polyamide) is obtained.
  • Imide resin: N-methyl-2-pyrrolidone 20 mass%: 80 mass%) was obtained.
  • the polyamideimide resin particles and fluororesin particles are solids, and the N-methyl-2-pyrrolidone (polar solvent), water and tri-n-propylamine (base) are dispersion media.
  • An insulating film was prepared using the electrodeposition solution prepared above. Specifically, first, the electrodeposition liquid was stored in the electrodeposition tank, and the temperature of the electrodeposition liquid in the electrodeposition tank was set to 20 ° C. Next, a copper wire (conductor) having a diameter and length of 1 mm and 300 mm, respectively, is used as an anode, a cylindrical copper plate inserted into the electrodeposition liquid in the electrodeposition tank is used as a cathode, and a copper wire and a cylindrical copper plate are used. The copper wire and the cylindrical copper plate were held in the electrodeposition liquid in the electrodeposition tank for 30 seconds with a DC voltage of 100 V applied between them. As a result, an electrodeposition film was formed on the surface of the copper wire.
  • Example 1 the copper wire on which the electrodeposited film was formed was allowed to stand in a muffle furnace at 250 ° C. for 3 minutes and dried to obtain a copper wire having an insulating film having a thickness of 40 ⁇ m formed on the surface.
  • This copper wire with an insulating film was taken as Example 1.
  • the dielectric breakdown voltage was measured using a dielectric breakdown tester (manufactured by Tohoku Paint Co., Ltd.) after winding a metal foil around a conductive wire with an insulating film having a thickness of 40 ⁇ m. The results are shown in Table 1.
  • PI resin particles are polyimide resin particles
  • PAI polyamideimide resin particles
  • PI polyimide resin particles
  • PTFE polytetrafluoroethylene ( Tetrafluoride) resin particles (polyflon PTFE-D made by Daikin Industries, Ltd.)
  • PFA is tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer resin particles (neoflon PFA made by Daikin Industries).
  • PI / solid content is the content ratio of polyimide resin particles in solid content (total amount of fluororesin particles and polyimide resin particles)
  • F / solid content is solid content (fluororesin resin).
  • “dispersion medium” is the total amount of water, polar solvent and base.
  • Comparative Example 1 using only the polyamideimide resin particles without using the fluororesin particles, the appearance of the insulating film was good and the dielectric breakdown voltage was as high as 6.3 kV.
  • Comparative Example 2 where the dielectric constant is as high as 4.7 and the fluororesin particles are as low as 10% by mass, the appearance of the insulating film is good and the dielectric breakdown voltage is as high as 6.1 kV.
  • Comparative Example 3 where the amount of fluororesin particles is as high as 80% by mass, the relative dielectric constant of the insulating film was as low as 2.7. However, cracks occurred in the insulating film, resulting in dielectric breakdown. The voltage was as low as 0.4 kV.
  • Comparative Example 6 in which the median diameter of the polyamideimide resin particles is as large as 500 nm, although the dielectric constant of the insulating film was as low as 3.4, the insulating film was foamed and the dielectric breakdown voltage was 0.5 kV.
  • Comparative Example 7 in which the median diameter of the polyamideimide resin particles was as small as 20 nm, the dielectric constant of the insulating film was as low as 3.4.
  • the thickness of the insulating film did not reach 40 ⁇ m, and the thickness was 40 ⁇ m. The dielectric breakdown voltage of the insulating film could not be measured.
  • Example 8 in which PFA resin particles were mixed with polyamideimide resin particles, the appearance of the insulating film was good, the relative dielectric constant was as low as 3.4, and the dielectric breakdown voltage was as high as 5.8 kV.
  • Example 9 in which PTFE resin particles were mixed with polyimide resin particles, the appearance of the insulating film was good, the relative dielectric constant was as low as 3.0, and the dielectric breakdown voltage was as high as 5.5 kV.
  • Example 10 in which PFA resin particles were mixed with polyimide resin particles, the appearance of the insulating film was good, the relative dielectric constant was as low as 3.0, and the dielectric breakdown voltage was as high as 5.3 kV.
  • the electrodeposition liquid of the present invention is excellent in partial discharge resistance like an insulated wire and can be used for the manufacture of articles that require a coating with a high dielectric breakdown voltage.

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Abstract

本発明の電着液は、分散媒（１３）と固形分（１１）からなる。固形分（１１）は、ポリイミド系樹脂粒子（１１ａ）とフッ素樹脂粒子（１１ｂ）とを含む。また、固形分（１１）中のフッ素樹脂粒子（１１ｂ）の含有割合は２０～７０質量％である。更に、ポリイミド系樹脂粒子（１１ａ）のメジアン径は５０～４００ｎｍである。

Description

電着液及びこれを用いた絶縁皮膜付き導体の製造方法
　本発明は、誘電率が低くかつ絶縁破壊電圧が高い絶縁皮膜を形成するための電着液と、この電着液を用いて絶縁皮膜を有する導体を製造する方法に関するものである。
　本願は、２０１７年２月１６日に日本に出願された特願２０１７－０２７１００号について優先権を主張し、その内容をここに援用する。
　従来、水性媒体中に、平均粒径１μｍ以下かつ比誘電率３以下の非成膜性微粒子と、重合性化合物及び重合体の少なくとも一方からなる有機粒子とが分散し、電着により絶縁膜を形成可能な低誘電率絶縁膜形成用水性分散液が開示されている（例えば、特許文献１参照。）。この低誘電率絶縁膜形成用水性分散液では、非成膜性微粒子は、フッ素含有微粒子及び架橋有機微粒子から選択された少なくとも一種である。また、水性分散液は、有機粒子が水性媒体に分散した水性エマルジョンを用いて調製される。ここで「水性媒体」とは水を主成分とする媒体を意味し、この水性媒体中における水の含有率は通常４０重量％以上、好ましくは５０重量％以上である。
　このように構成された低誘電率絶縁膜形成用水性分散液では、この分散液を電着液に用いた電着により、低誘電率でかつ密着性の良い絶縁膜を形成することができる。また、この低誘電率の絶縁膜は、上記水性分散液を用いた電着により作製されるので、塗布により作製された場合等に比べて膜厚制御性および膜形成位置の制御性に優れる。
　また、ポリイミド樹脂とフッ素樹脂と電荷付与剤とを水分散してなる水分散型樹脂エマルジョンを導体上に電着した後に、乾燥し焼付けすることによって絶縁層を形成する絶縁電線の製造方法（例えば、特許文献２参照。）が開示されている。この絶縁電線の製造方法では、電着によって、導体が水分散型樹脂エマルジョンで被覆される。この電着は、水分散型樹脂エマルジョンで満たされた電着バス中に上記導体を通過させるなどして、導体を水分散型樹脂エマルジョン中に浸漬させた状態で電圧を印加することにより行われる。
　このように構成された絶縁電線の製造方法では、十分な絶縁性能を有し、かつ厚みが小さく、更に耐熱性にも優れる絶縁層を備える絶縁電線を製造できる。この結果、絶縁電線は、耐熱性が要求される電子機器内や電子機器間における配線に好適に使用できる。
日本国特開２００１－２９４８１５号公報（Ａ）（請求項１、請求項２、段落［００２３］、［００６８］） 日本国特開２００２－２９８６７４号公報（Ａ）（請求項１、段落［００２０］、［００３７］、図１）
　しかし、上記従来の特許文献１に示された低誘電率絶縁膜形成用水性分散液では、有機粒子の粒径が示されておらず、電着液中の溶媒に占める水の比率が高いため、乾燥時の溶媒の揮発速度が速く、発泡が生じ易くなって、厚膜を形成するのが難しい問題点もあった。一方、上記従来の特許文献２に示された絶縁電線の製造方法では、ポリイミド樹脂の粒径が示されておらず、水分散型樹脂エマルジョン（電着液）中の溶媒が全て水であるため、乾燥時の溶媒の揮発速度が速く、発泡が生じ易くなって、厚膜を形成するのが難しい問題点があった。
　本発明の目的は、ポリイミド系樹脂及びフッ素樹脂の複合膜を厚膜化することができ、誘電率が低くかつ絶縁破壊電圧が高い絶縁皮膜を得ることができる、電着液及びこれを用いた絶縁皮膜付き導体の製造方法を提供することにある。
　本発明の第１の態様は、分散媒と固形分からなり、固形分が、ポリイミド系樹脂粒子とフッ素樹脂粒子とを含み、固形分中のフッ素樹脂粒子の含有割合が、２０～７０質量％であり、ポリイミド系樹脂粒子のメジアン径が５０～４００ｎｍであることを特徴とする電着液である。
　本発明の第２の態様は、第１の態様に基づく発明であって、更に分散媒が、極性溶剤、水及び塩基を含み、極性溶剤が水より高い沸点を有し、分散媒中の水の含有割合が１５～４０質量％であることを特徴とする。
　本発明の第３の態様は、第１又は第２の態様に基づく発明であって、更にポリイミド系樹脂粒子がフッ素樹脂粒子より小さいメジアン径を有することを特徴とする。
　本発明の第４の態様は、第１ないし第３の態様のいずれかに記載の電着液を用いて、導体表面に電着により絶縁皮膜を形成する絶縁皮膜付き導体の製造方法である。
　本発明の第５の態様は、第４の態様に基づく発明であって、更に導体が金属線であることを特徴とする。
　本発明の第１の態様の電着液では、ポリイミド系樹脂粒子のメジアン径が５０～４００ｎｍと小さいので、電着時にポリイミド系樹脂粒子やフッ素樹脂粒子の間に閉じ込められる分散媒の量を減らすことができ、熱処理時における絶縁皮膜の発泡を抑制できる。この結果、ポリイミド系樹脂及びフッ素樹脂の複合膜である絶縁皮膜を厚膜化することができるので、誘電率が低く部分放電が発生し難く、かつ絶縁皮膜の絶縁破壊電圧を高くすることができる。
　本発明の第２の態様の電着液では、極性溶剤が水より高い沸点を有し、分散媒中の水の含有割合が１５～４０質量％と少ないので、熱処理時の分散媒の揮発速度が遅くなり、熱処理時における絶縁皮膜の発泡をより抑制できる。特に、最終的に皮膜の熱処理に必要な温度に設定した炉内に静置した場合でも絶縁皮膜の発泡をより抑制できる。
　本発明の第３の態様の電着液では、ポリイミド系樹脂粒子がフッ素樹脂粒子より小さいメジアン径を有するので、フッ素樹脂粒子間の隙間にポリイミド系樹脂粒子が入り込み易くなる。この結果、分散媒の閉じ込め量を減らすことができ、熱処理時における絶縁皮膜の発泡を更に抑制できる。
　本発明の第４の態様の絶縁皮膜付き導体の製造方法では、上記電着液を用いて絶縁皮膜を導体表面に電着により形成するので、熱処理時における絶縁皮膜の発泡を抑制できる。この結果、絶縁皮膜を厚膜化することができるので、誘電率が低く部分放電が発生し難く、かつ絶縁破壊電圧が高い絶縁皮膜を有する導体を製造できる。
　本発明の第５の態様の絶縁皮膜付き導体の製造方法では、上記電着液を用いて絶縁皮膜を金属線表面に電着により形成するので、熱処理時における絶縁皮膜の発泡を抑制できる。この結果、絶縁皮膜を厚膜化することができるので、誘電率が低く部分放電が発生し難く、かつ絶縁破壊電圧が高い絶縁皮膜を有する金属線を製造できる。
本発明実施形態のポリイミド系樹脂粒子のメジアン径が小さい電着液を用いて絶縁皮膜を形成したときに気泡が発生しないメカニズムを示す模式図であり、（ａ）は電着開始直後の電着液を示し、（ｂ）は電着完了後の電着膜を示し、（ｃ）は焼付処理後の絶縁皮膜を示す断面模式図である。 参考形態のポリイミド系樹脂粒子のメジアン径が大きい電着液を用いて絶縁皮膜を形成したときに気泡が発生するメカニズムを示す模式図であり、（ａ）は電着開始直後の電着液を示し、（ｂ）は電着完了後の電着膜を示し、（ｃ）は焼付処理後の絶縁皮膜を示す断面模式図である。
　次に本発明を実施するための形態を図面に基づいて説明する。電着液は、分散媒と固形部からなる。固形部は、ポリイミド系樹脂粒子とフッ素樹脂粒子とを含む。ポリイミド系樹脂としては、ポリアミドイミド樹脂、ポリイミド樹脂、ポリエステルイミド樹脂、ポリアミック酸樹脂等が挙げられる。また、フッ素樹脂としては、ポリテトラフルオロエチレン（４フッ化）樹脂、テトラフルオロエチレン・パーフルオロアルキルビニルエーテル共重合体樹脂、パーフルオロエチレンプロペンコポリマー樹脂、ポリフッ化ビニリデン樹脂等が挙げられる。
　固形分中のフッ素樹脂粒子の含有割合は、２０～７０質量％、好ましくは３０～７０質量％である。また、ポリイミド系樹脂粒子のメジアン径は、５０～４００ｎｍ、好ましくは５０～２００ｎｍである。また、フッ素樹脂粒子のメジアン径は、５０～５００ｎｍ、好ましくは５０～３００ｎｍである。更に、ポリイミド系樹脂粒子はフッ素樹脂粒子より小さいメジアン径を有することが好ましい。ここで、固形分中のフッ素樹脂粒子の含有割合を２０～７０質量％の範囲内に限定したのは、２０質量％未満では絶縁皮膜の誘電率を下げることができず、７０質量％を超えると絶縁皮膜が連続膜化せずクラックが発生し易いからである。また、ポリイミド系樹脂粒子のメジアン径を５０～４００ｎｍの範囲内に限定したのは、５０ｎｍ未満では樹脂粒子間に存在する分散媒が少なく膜の抵抗が大きくなるため電着速度が遅く厚膜を得るのに時間を要し、４００ｎｍを超えると電着液の分散安定性が低下するとともに、電着液の乾燥時に水の揮発量が増加して絶縁皮膜を厚く形成すると絶縁皮膜が発泡し易くなってしまうからである。更に、フッ素樹脂粒子の好ましいメジアン径を５０～５００ｎｍの範囲内に限定したのは、５０ｎｍ未満では樹脂粒子間に存在する分散媒が少なく膜の抵抗が大きくなるため電着速度が遅く厚膜を得るのに時間を要し、５００ｎｍを超えると電着液が凝集して沈殿が発生し分散安定性が低下してしまうからである。
　分散媒は、極性溶剤、水及び塩基を含むことが好ましい。また、極性溶剤は水より高い沸点を有することが好ましい。極性溶剤としては、Ｎ－メチル－２－ピロリドン、１，３－ジメチル－２－イミダゾリジノン、ジメチルスルホキシド、Ｎ，Ｎジメチルアセトアミド等の有機溶剤が挙げられる。更に、塩基としては、トリ－ｎ－プロピルアミン、ジブチルアミン、ピペリジン、トリエチルアミン等が挙げられる。分散媒中の水の含有割合は、１５～４０質量％であることが好ましく、１８～３０質量％であることが更に好ましい。また、分散媒中の極性溶剤の含有割合は６０～７０質量％であることが好ましく、６５～７０質量％であることが更に好ましい。分散媒中の塩基の含有割合は０．１～０．３質量％であることが好ましく、０．１～０．２質量％であることが更に好ましい。更に、電着液中の固形分の含有割合は１～１０質量％であることが好ましく、２～５質量％であることが更に好ましい。
　ここで、分散媒中の水の好ましい含有割合を１５～４０質量％の範囲内に限定したのは、１５質量％未満では電着液の導電率が小さく電着による絶縁皮膜を形成できず、４０質量％を超えると電着液の乾燥時に分散媒の揮発速度が速くなり絶縁皮膜を厚く形成すると絶縁皮膜が発泡し易くなってしまうからである。また、分散媒中の極性溶剤の好ましい含有割合を６０～７０質量％の範囲内に限定したのは、６０質量％未満では分散媒中における水の割合が多くなり揮発速度が速くなって発泡し易くなり、７０質量％を超えると分散媒中における水の割合が減り電着速度が遅くなって厚膜を得るのに時間を要するからである。また、分散媒中の塩基の好ましい含有割合を０．１～０．３質量％の範囲内に限定したのは、０．１質量％未満ではポリイミド系樹脂粒子のメジアン径が増加し分散安定性が悪化してしまい、０．３質量％を超えるとポリイミド系樹脂粒子のメジアン径が減少し樹脂粒子間に存在する分散媒が少なくなって膜の抵抗が大きくなるため電着速度が遅く厚膜を得るのに時間を要するからである。更に、電着液中の固形分の好ましい含有割合を１～１０質量％の範囲内に限定したのは、１質量％未満では電着速度が遅く厚膜を得るのに時間を要し、１０質量％を超えると分散安定性が悪化してしまうからである。なお、上記ポリイミド系樹脂粒子のメジアン径及びフッ素樹脂粒子のメジアン径は、動的光散乱粒径分布測定装置（堀場製作所製ＬＢ－５５０）を用いて測定した体積基準平均粒径である。
　次に電着液の製造方法を説明する。
　［ポリイミド系樹脂ワニスの合成］
　先ず、撹拌機、冷却管、窒素導入管及び温度計を備えた２リットルの四つ口フラスコ内に、極性溶剤と、イソシアネート成分と酸成分とを混合し、８０～１３０℃の温度に昇温してこの温度に２～８時間保持して反応させることにより、ポリイミド系樹脂を得る。ここで、イソシアネート成分としては、ジフェニルメタン－４，４’－ジイソシアネート（ＭＤＩ）、ジフェニルメタン－３，３’－ジイソシアネート、ジフェニルメタン－３，４’－ジイソシアネート、ジフェニルエーテル－４，４’－ジイソシアネート、ベンゾフェノン－４，４’－ジイソシアネート、ジフェニルスルホン－４，４’－ジイソシアネート等の芳香族ジイソシアネート等が挙げられ、酸成分としてはトリメリット酸無水物（ＴＭＡ）、１，２，５－トリメリット酸（１，２，５－ＥＴＭ）、ビフェニルテトラカルボン酸二無水物、ベンゾフェノンテトラカルボン酸二無水物、ジフェニルスルホンテトラカルボン酸二無水物、オキシジフタル酸二無水物（ＯＰＤＡ）、ピロメリット酸二無水物（ＰＭＤＡ）、４，４’－（２，２’－ヘキサフルオロイソプロピリデン）ジフタル酸二無水物等が挙げられる。その後、上記合成したポリイミド系樹脂を、極性溶剤により希釈してポリイミド系樹脂ワニスを調製する。
　［ポリイミド系樹脂粒子の分散液の調製］
　次いで、上記得られたポリイミド系樹脂ワニスを、有機溶剤で更に希釈し、塩基性化合物を加えた後、撹拌しつつ、室温下で水を添加する。これにより、メジアン径５０～４００ｎｍのポリイミド系樹脂粒子の分散液が得られる。
　［フッ素樹脂粒子の分散液の調製］
　市販のフッ素樹脂粒子を水で希釈した後、撹拌することにより、メジアン径５０～５００ｎｍのフッ素樹脂粒子の分散液が得られる。
　［電着液の調製］
　ポリイミド系樹脂粒子の分散液とフッ素樹脂粒子の分散液を混合することにより、電着液が得られる。
　上記電着液を用いて導体表面に絶縁皮膜を形成するには、電着液を用いて電着法により導体の表面に電着膜を形成した後、乾燥処理及び焼付処理することにより、導体の表面に絶縁皮膜を形成することが好ましい。この絶縁皮膜の厚さは１０～７０μｍであることが好ましく、２０～５０μｍであることが更に好ましい。絶縁皮膜の絶縁破壊電圧は、絶縁皮膜の厚さが４０μｍである場合、少なくとも５ｋＶであることが好ましい。また、上記導体としては、金属線、金属板等が挙げられる。導体が金属線である場合、金属線の材質としては、良好な導電性を有する銅、銅合金、アルミニウム、アルミニウム合金等であることが好ましい。
　このように構成された絶縁皮膜付き導体では、絶縁皮膜を形成するための電着液中のポリイミド系樹脂粒子のメジアン径が５０～４００ｎｍと小さいので、電着時にポリイミド系樹脂粒子やフッ素樹脂粒子の間に閉じ込められる分散媒の量を減らすことができ、熱処理時における絶縁皮膜の発泡を抑制できる。具体的には、図１に示すように、固形分１１であるポリイミド系樹脂粒子１１ａ及びフッ素樹脂粒子１１ｂのうちポリイミド系樹脂粒子１１ａのメジアン径が５０～４００ｎｍと小さい場合、導体１２表面においてポリイミド系樹脂粒子１１ａがフッ素樹脂粒子１１ｂの間に多く進入して密にパッキングされ、これらの粒子１１ａ，１１ｂ間に残留する分散媒１３が少なくなるので（図１（ｂ））、焼付処理後に絶縁皮膜１４に発泡が生じ難い（図１（ｃ））。これに対し、図２に示すように、固形分２１であるポリイミド系樹脂粒子２１ａ及びフッ素樹脂粒子２１ｂのうち、フッ素樹脂粒子２１ｂのメジアン径が上記フッ素樹脂粒子１１ｂと同じであり、ポリイミド系樹脂粒子２１ａのメジアン径が上記範囲より大きい場合、導体２２表面においてポリイミド系樹脂粒子２１ａがフッ素樹脂粒子２１ｂの間に多く進入できず粗くパッキングされ、これらの粒子２１ａ，２１ｂ間に残留する分散媒２３が多くなるので（図２（ｂ））、焼付処理後に絶縁皮膜２４に発泡２４ａが生じ易くなる（図２（ｃ））。この結果、本実施の形態では、ポリイミド系樹脂及びフッ素樹脂の複合膜である絶縁皮膜１４を厚膜化することができるので、絶縁皮膜付き導体１０の絶縁皮膜１４の絶縁破壊電圧を高くすることができる。また、本実施の形態では、極性溶剤が水より高い沸点を有し、分散媒中の水の含有割合を１５～４０質量％と少なくすることにより、熱処理時の分散媒の揮発速度が遅くなり、熱処理時における絶縁皮膜の発泡をより抑制できる。更に、本実施の形態では、ポリイミド系樹脂粒子がフッ素樹脂粒子より小さいメジアン径を有すれば、フッ素樹脂粒子間の隙間にポリイミド系樹脂粒子が入り込み易くなるので、分散媒の閉じ込め量を減らすことができ、熱処理時における絶縁皮膜の発泡を更に抑制できる。
　次に本発明の実施例を比較例とともに詳しく説明する。
　＜実施例１＞
　電着液の製造方法を説明する。
　［ポリイミド系樹脂ワニスの合成］
　先ず、撹拌機、冷却管、窒素導入管及び温度計を備えた２リットルの四つ口フラスコ内に、７４７ｇのＮ－メチル－２－ピロリドン（極性溶剤）と、２９８ｇ（１．１９モル）のジフェニルメタン－４，４’－ジイソシアネート（ＭＤＩ、イソシアネート成分）と、２２７ｇ（１．１８モル）のトリメリット酸無水物（ＴＭＡ、酸成分）とを投入して１３０℃まで昇温させた。この温度で約４時間反応させることにより、数平均分子量が１７０００のポリマー（ポリアミドイミド樹脂）を得た。その後、上記合成したポリアミドイミド樹脂を、Ｎ－メチル－２－ピロリドン（極性溶剤）で、ポリアミドイミド樹脂（不揮発分）の濃度が２０質量％となるように希釈して、ポリアミドイミド樹脂ワニス（ポリアミドイミド樹脂：Ｎ－メチル－２－ピロリドン＝２０質量％：８０質量％）を得た。
　［ポリイミド系樹脂粒子の分散液の調製］
　次いで、上記得られたポリアミドイミド樹脂ワニス６２．５ｇを、Ｎ－メチル－２－ピロリドン１４０ｇ（極性溶剤）で更に希釈し、トリ－ｎ－プロピルアミン（塩基性化合物（塩基））０．５ｇを加えた後、この液を回転速度１００００ｒｐｍの高速で撹拌しつつ、室温下（２５℃）で水を４７ｇ添加した。これにより、メジアン径１６０ｎｍのポリアミドイミド樹脂粒子の分散液（ポリアミドイミド樹脂粒子：Ｎ－メチル－２－ピロリドン：水：トリ－ｎ－プロピルアミン＝５質量％：７６質量％：１８．８質量％：０．２質量％）２５０ｇを得た。
　［フッ素樹脂粒子の分散液の調製］
　フッ素樹脂粒子：水が６０質量％：４０質量％であるポリフロンＰＴＦＥ－Ｄ（ダイキン工業社製）を水で希釈し、メジアン径２００ｎｍのフッ素樹脂粒子の分散液を得た（フッ素樹脂粒子：水＝３０質量％：７０質量％）。
　［電着液の調製］
　ポリイミド系樹脂粒子の分散液６０ｇとフッ素樹脂粒子の分散液１０ｇを混合し電着液を得た（ポリアミドイミド樹脂粒子：フッ素樹脂粒子：Ｎ－メチル－２－ピロリドン：水：トリ－ｎ－プロピルアミン＝４．３質量％：４．３質量％：６５質量％：２６．２質量％：０．２質量％）。なお、上記ポリアミドイミド樹脂粒子及びフッ素樹脂粒子が固形分であり、上記Ｎ－メチル－２－ピロリドン（極性溶剤）、水及びトリ－ｎ－プロピルアミン（塩基）が分散媒である。
　［絶縁皮膜の作製］
　上記調製した電着液を用いて絶縁皮膜を作製した。具体的には、先ず、電着液を電着槽内に貯留し、この電着槽内の電着液の温度を２０℃とした。次いで、直径及び長さがそれぞれ１ｍｍ及び３００ｍｍである銅線（導体）を陽極とし、上記電着槽内の電着液に挿入された円筒型の銅板を陰極とし、銅線と円筒型の銅板との間に直流電圧１００Ｖを印加した状態で、銅線及び円筒型の銅板を電着槽内の電着液中に３０秒間保持した。これにより銅線の表面に電着膜が形成された。次に、電着膜が形成された銅線を２５０℃のマッフル炉中に３分間静置し乾燥処理して、表面に厚さ４０μｍの絶縁皮膜が形成された銅線を得た。この絶縁皮膜付きの銅線を実施例１とした。
　＜実施例２～１０及び比較例１～８＞
　表１に示すように、ポリイミド系樹脂粒子及びフッ素系樹脂粒子の種類と、固形分中のポリイミド系樹脂粒子の含有割合と、固形分中のフッ素樹脂粒子の含有割合と、分散媒中の極性溶媒の含有割合と、分散媒中の水の含有割合と、分散媒中の塩基の含有割合と、ポリイミド系樹脂粒子のメジアン径とをそれぞれ変えて、実施例１と同様にして電着液を調製し、この電着液を用いて絶縁皮膜付き銅線を作製した。これらの絶縁皮膜付き銅線を実施例２～１０及び比較例１～５とした。
　＜比較試験１及び評価＞
　実施例１～１０及び比較例１～５の絶縁皮膜付き銅線の絶縁皮膜の外観を観察するとともに、比誘電率及び絶縁破壊電圧をそれぞれ測定した。具体的には、絶縁皮膜の外観は、目視により観察した。また、比誘電率は、ＬＣＲメーター（日置電機社製）を用いて、絶縁皮膜の静電容量と絶縁皮膜の膜厚から誘電率を算出し、この誘電率を真空の誘電率ε0（８．８５×１０-12Ｆ／ｍ）で除することにより求めた。なお、絶縁皮膜の膜厚はマイクロメーター（ミツトヨ社製）を用いて測定した。更に、絶縁破壊電圧は、厚さ４０μｍの絶縁皮膜付きの導線に金属箔を巻付け、絶縁破壊試験機（東特塗料社製）を用いて測定した。その結果を表１に示す。
　なお、表１の絶縁皮膜の外観において、「良好」は絶縁皮膜にクラックが発生せずに絶縁皮膜を電着できたことを示し、「クラック」は絶縁皮膜にクラックが発生したことを示し、「電着不可」は厚さ４０μｍの絶縁皮膜を形成できなかったことを示し、「発泡」は絶縁皮膜に発泡が生じたことを示す。また、表１において、「ＰＩ系樹脂粒子」はポリイミド系樹脂粒子であり、「ＰＡＩ」はポリアミドイミド樹脂粒子であり、「ＰＩ」はポリイミド樹脂粒子であり、「ＰＴＦＥ」はポリテトラフルオロエチレン（４フッ化）樹脂粒子（ダイキン工業社製のポリフロンＰＴＦＥ－Ｄ）であり、「ＰＦＡ」はテトラフルオロエチレン・パーフルオロアルキルビニルエーテル共重合体樹脂粒子（ダイキン工業社製のネオフロンＰＦＡ）である。また、表１において、ＰＩ／固形分は、固形分（フッ素樹脂粒子及びポリイミド系樹脂粒子の合計量）中のポリイミド系樹脂粒子の含有割合であり、Ｆ／固形分は、固形分（フッ素樹脂粒子及びポリイミド系樹脂粒子の合計量）中のフッ素樹脂粒子の含有割合であり、「分散媒」は水、極性溶剤及び塩基の合計量である。
Figure JPOXMLDOC01-appb-T000001
　表１から明らかなように、フッ素樹脂粒子を用いずにポリアミドイミド樹脂粒子のみを用いた比較例１では、絶縁皮膜の外観は良好であり、絶縁破壊電圧は６．３ｋＶと高かったけれども、比誘電率が４．７と高くなってしまい、フッ素樹脂粒子が１０質量％と少ない比較例２では、絶縁皮膜の外観は良好であり、絶縁破壊電圧は６．１ｋＶと高かったけれども、比誘電率が４．５と高くなってしまい、フッ素樹脂粒子が８０質量％と多い比較例３では、絶縁皮膜の比誘電率は２．７と低くなったけれども、絶縁皮膜にクラックが発生し、絶縁破壊電圧が０．４ｋＶと低くなった。これらに対し、フッ素樹脂粒子が２０～７０質量％と適切な範囲内である実施例１～３では、絶縁皮膜の外観は良好であり、比誘電率は２．８～４．３と低く、絶縁破壊電圧は５．０～６．０ｋＶと高くなった。
　また、ポリアミドイミド樹脂粒子のメジアン径が５００ｎｍと大きい比較例６では、絶縁皮膜の比誘電率が３．４と低くなったけれども、絶縁皮膜が発泡してしまい、絶縁破壊電圧が０．５ｋＶと低くなり、ポリアミドイミド樹脂粒子のメジアン径が２０ｎｍと小さい比較例７では、絶縁皮膜の比誘電率が３．４と低くなったけれども、絶縁皮膜の厚さが４０μｍまで達せず、厚さ４０μｍの絶縁皮膜の絶縁破壊電圧を測定できなかった。これらに対し、ポリアミドイミド樹脂粒子のメジアン径が５０～４００ｎｍと適切な範囲内である実施例６及び７では、絶縁皮膜の外観は良好であり、比誘電率はそれぞれ３．４と低く、絶縁破壊電圧はそれぞれ５．５ｋＶと高くなった。
　更に、水／分散媒が１０～４０質量％と好ましい範囲の下限値及び上限値である実施例４及び５では、絶縁皮膜の外観は良好であり、比誘電率はそれぞれ３．４と低く、絶縁破壊電圧は５．５～５．７ｋＶと高くなった。
　一方、ポリアミドイミド樹脂粒子にＰＦＡ樹脂粒子を混合した実施例８では、絶縁皮膜の外観は良好であり、比誘電率は３．４と低く、絶縁破壊電圧は５．８ｋＶと高くなった。また、ポリイミド樹脂粒子にＰＴＦＥ樹脂粒子を混合した実施例９では、絶縁皮膜の外観は良好であり、比誘電率は３．０と低く、絶縁破壊電圧は５．５ｋＶと高くなった。更に、ポリイミド樹脂粒子にＰＦＡ樹脂粒子を混合した実施例１０では、絶縁皮膜の外観は良好であり、比誘電率は３．０と低く、絶縁破壊電圧は５．３ｋＶと高くなった。
　本発明の電着液は、絶縁電線のように耐部分放電性に優れ、絶縁破壊電圧の高い被覆を必要とする物品の製造に利用できる。
　１０　絶縁皮膜付き導体
　１１　固形分
　１１ａ　ポリイミド系樹脂粒子
　１１ｂ　フッ素樹脂粒子
　１２　導体
　１３　分散媒
　１４　絶縁皮膜

Claims (5)

  1. 　分散媒と固形分からなり、
    　前記固形分が、ポリイミド系樹脂粒子とフッ素樹脂粒子とを含み、
    　前記固形分中の前記フッ素樹脂粒子の含有割合が、２０～７０質量％であり、
    　前記ポリイミド系樹脂粒子のメジアン径が５０～４００ｎｍである
    　ことを特徴とする電着液。
  2. 　前記分散媒が、極性溶剤、水及び塩基を含み、前記極性溶剤が前記水より高い沸点を有し、前記分散媒中の前記水の含有割合が１５～４０質量％である請求項１記載の電着液。
  3. 　前記ポリイミド系樹脂粒子が前記フッ素樹脂粒子より小さいメジアン径を有する請求項１又は２記載の電着液。
  4. 　請求項１ないし３いずれか１項に記載の電着液を用いて、導体表面に電着により絶縁皮膜を形成する絶縁皮膜付き導体の製造方法。
  5. 　前記導体が金属線である請求項４記載の絶縁皮膜付き導体の製造方法。
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