Polyimide precursor varnish, method for producing polyimide film using same, and method for producing metal-clad laminate

Abstract

To provide a polyimide precursor varnish having good film-forming property and giving a polyimide film having excellent visibility and provide a method for producing a polyimide film by using the varnish.SOLUTION: A polyimide precursor varnish containing a polyimide precursor and an organic solvent and capable of producing a polyimide film having a total light transmittance of 70% or more, wherein the polyimide precursor varnish contains, as organic solvents, (A) an aprotic polar solvent and (B) water, wherein the content of the component A is not less than 50% by mass, the content of the component B is from 300 to 1000 ppm, the solids content of the varnish is from 10 to 18% by mass, the viscosity at 25 °C. is not more than 60,000 cP, and the light transmittance at 550 nm is not less than 90%. In addition, the polyimide precursor in the varnish has a weight average molecular weight (Mw) of 100,000 to 700,000, and a polyimide film is produced using the polyimide precursor varnish.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a polyimide precursor varnish, a method for producing a polyimide film using the same, and a method for producing a metal-clad laminate. More specifically, the present invention relates to a polyimide precursor varnish that has good film-forming properties when made into a polyimide film and can produce a polyimide film that has excellent visibility when mounting a COF or TFT, and a method for producing a polyimide film using the same, and a method for producing a metal-clad laminate. [Background technology]

[0002] Polyimide has excellent heat resistance, mechanical properties, and electrical properties. Polyimide films made from this polyimide are widely used in a variety of applications, including circuit wiring boards such as flexible printed circuits (FPCs).

[0003] Typical known methods for producing polyimide films are the tenter method and the casting method. The tenter method involves casting a solution of a polyimide precursor (polyamic acid) onto a rotating drum, peeling it off as a gel film from the rotating drum, and heating and curing it in a tenter furnace to produce a polyimide film. The casting method involves applying a solution of the polyimide precursor to a substrate such as copper foil, followed by drying and curing the solution by heat treatment to obtain a polyimide film.

[0004] Polyimide precursors obtained by reacting tetracarboxylic dianhydrides with diamines typically consist of polyamic acids and are soluble in organic solvents. However, when they are heat-treated at temperatures above 200°C, intramolecular dehydration (imidization) occurs, resulting in polyimides that are insoluble in organic solvents. Therefore, in both the tenter and casting methods, polyimides are prepared by using a polyimide precursor varnish containing an organic solvent and then heat-treating it to imidize it.

[0005] It is known that, for example, when the organic solvent of this polyimide precursor varnish contains water together with an aprotic polar solvent and the water content is controlled within a predetermined range, the imidization of the polyimide precursor occurs at a higher temperature, reducing the amount of solvent remaining after imidization, and as a result, the haze of a polyimide film produced can be suppressed (see Patent Document 1).

[0006] It is also known that, when a polyimide precursor varnish containing an organic solvent is heat-treated (first heat treatment) to form a polyamic acid resin film, and this resin film is then further heat-treated (imidized) to produce a polyimide film, the solvent vapor generated in the first heat treatment is brought into gas-liquid contact with water, and the resulting solvent recovery liquid is distilled to separate the solvent and water, thereby reusing the solvent in the raw material polyimide precursor varnish (see Patent Document 2).

[0007] Furthermore, a polyimide precursor varnish is known that exhibits little change in viscosity over time and excellent storage stability, achieved by using a specific diamine component to suppress the imidization rate, increasing the water content in the polyimide precursor varnish, and increasing the solids concentration (see Patent Document 3).

[0008] Polyimide is a resin that can be polymerized relatively easily at room temperature. However, because the reaction occurs easily, the viscosity can increase excessively during polymerization. Therefore, if the polyimide precursor varnish has a high viscosity or a high solids concentration, it may not be possible to form a polyimide film or the film may have streaks.

[0009] Furthermore, for example, when COF (Chip on Film) is used to mount liquid crystal driver ICs (chips) on FPCs, such as those used in liquid crystal monitors and televisions, or when thin film transistors (TFTs) are mounted on polyimide films to create flexible devices, visibility through the polyimide film is required. However, if the polyimide precursor varnish has a high viscosity and a high solid content, this visibility may be affected.

[0010] Therefore, in general, the solids concentration of the polyimide precursor varnish is controlled by adjusting the amount of solvent, and the monomer components such as the acid anhydride component and the diamine component that make up the polyimide precursor (polyamic acid) are selected, and the ratio of these components is adjusted to ensure the film-forming properties of the polyimide film.

[0011] However, increasing the amount of solvent in the polyimide precursor varnish to reduce the solid content makes it difficult to obtain a thick polyimide film. Furthermore, the use of a rigid monomer component constituting the polyimide precursor, such as one in which aromatic rings are directly linked by single bonds, tends to increase the viscosity of the varnish. Therefore, although the increase in viscosity can be suppressed by using a monomer having a flexible group linked thereto, this limits the types and properties of the polyimide film that can be obtained. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] Patent Publication No. 2021-134275 [Patent Document 2] Patent No. 7277208 [Patent Document 3] International Publication No. WO2023 / 058288 Brochure Summary of the Invention [Problem to be solved by the invention]

[0013] As mentioned above, polyimide films have become indispensable for electronic devices, as typified by flexible printed circuit boards (FPCs), and there is a real demand for the ability to arbitrarily produce desired polyimide films without being limited by film thickness or the monomer components that make up the polyimide.

[0014] Therefore, the present inventors have conducted extensive research to solve the above-mentioned conventional problems in obtaining polyimide films. As a result, they have found that by adjusting the water content in the solvent of the polyimide precursor varnish to fall within a predetermined range and by previously controlling the solids concentration and viscosity, polyimide films with excellent film-forming properties and visibility can be obtained regardless of the monomer components constituting the polyimide, and have completed the present invention.

[0015] Therefore, an object of the present invention is to provide a polyimide precursor varnish that has good film-forming properties when made into a polyimide film and can give a polyimide film that has excellent visibility when mounted on a COF or TFT.

[0016] Another object of the present invention is to provide a method for producing a polyimide film, which can provide a polyimide film having excellent film-forming properties and visibility.

[0017] Furthermore, another object of the present invention is to provide a method for producing a metal-clad laminate comprising an insulating resin layer consisting of a plurality of polyimide layers and a metal layer laminated on at least one surface of the insulating resin layer, by utilizing the above-mentioned method for producing a polyimide film. [Means for solving the problem]

[0018] That is, the gist of the present invention is as follows. (1) A polyimide precursor varnish containing a polyimide precursor and an organic solvent, which can be used to produce a polyimide film having a light transmittance of 70% or more, The organic solvent may include the following components A and B: A) aprotic polar solvents; B) water; wherein the content of component A measured by gas chromatography is 50% by mass or more, and the content of component B measured by Karl Fischer method is within a range of 300 to 1000 ppm, The polyimide precursor varnish is characterized by having a solid content of 10 to 18% by mass, a viscosity at 25°C of 60,000 cP or less, a weight-average molecular weight (Mw) of the polyimide precursor of 100,000 to 700,000, and a light transmittance of 90% or more at 550 nm. (2) A polyimide precursor varnish according to (1), which is used to form at least one polyimide layer among a plurality of polyimide layers in a metal-clad laminate having an insulating resin layer composed of a plurality of polyimide layers and a metal layer laminated on at least one surface of the insulating resin layer. (3) A method for producing a polyimide film, comprising the following steps a to c: a) preparing a mixed solution containing a tetracarboxylic acid anhydride component, a diamine component, and an organic solvent; b) reacting the tetracarboxylic acid anhydride component with the diamine component in the mixed solution to obtain a varnish containing a polyimide precursor; c) a step of heat-treating the varnish containing the polyimide precursor to imidize it, thereby obtaining a polyimide film; It is equipped with The organic solvent is a mixture of the following components A and B: A) aprotic polar solvents; B) water; wherein the content of component A measured by gas chromatography is 50% by mass or more, and the content of component B measured by Karl Fischer method is within a range of 300 to 1000 ppm, A method for producing a polyimide film, wherein the polyimide precursor varnish obtained in step b has a solid content of 10 to 18 mass %, a viscosity at 25°C of 60,000 cP or less, a weight-average molecular weight (Mw) of the polyimide precursor of 100,000 to 700,000, and a light transmittance of the varnish at 550 nm of 90% or more. (4) The method for producing a polyimide film according to (3), wherein the component A is at least one selected from the group consisting of N,N-dimethylformamide, N,N-dimethylacetamide, and dimethyl sulfoxide. (5) The method for producing a polyimide film according to (4), wherein in the step c, the varnish containing the polyimide precursor is heat-treated to generate solvent vapor, from which the organic solvent is recovered and reused as the organic solvent for the mixed solution. (6) The method for producing a polyimide film according to (5), wherein the organic solvent used in the step (a) is obtained via a step of heat-treating a varnish containing a polyimide precursor in the step (c) and recovering the generated solvent vapor. (7) A method for producing a metal-clad laminate comprising an insulating resin layer made of a plurality of polyimide layers and a metal layer laminated on at least one surface of the insulating resin layer, comprising: A method for producing a metal-clad laminate, characterized in that, to obtain a base polyimide layer having the greatest thickness among the plurality of polyimide layers forming the insulating resin layer, the method for producing a polyimide film described in (3) is used, in which the polyimide precursor varnish obtained in the step (b) is applied directly or indirectly to a metal layer, and dried to obtain a polyimide precursor resin film, and then the polyimide precursor resin film is imidized by heat treatment in the step (c), thereby forming the base polyimide layer. [Effects of the Invention]

[0019] According to the present invention, a polyimide film having good film-forming properties and excellent visibility can be obtained regardless of the monomer components constituting the polyimide. Therefore, for polyimide films that are indispensable for electronic devices, etc., it is possible to arbitrarily produce polyimide films having excellent film-forming properties and visibility without being limited by film thickness or monomer components. DETAILED DESCRIPTION OF THE INVENTION

[0020] Hereinafter, an embodiment of the present invention will be described.

[0021] [Polyimide precursor varnish] The present invention relates to a polyimide precursor varnish that contains a polyimide precursor and an organic solvent and can be used to produce a polyimide film having a total light transmittance of 70% or more, and that contains the following components A and B as the organic solvent. In this specification, the polyimide precursor varnish may also be simply referred to as a varnish. A) Aprotic polar solvent B) water

[0022] <Organic solvents> Here, the organic solvent contained in the polyimide precursor varnish of the present invention has a content of Component A of 50 mass% or more as measured by gas chromatography, and the aprotic polar solvent of Component A is the main component of the organic solvent. From the viewpoint of ease of control of the content of Component B, which will be described later, the content of Component A in the organic solvent is preferably 99% or more, more preferably 99.5% or more, and even more preferably 99.9% or more.

[0023] Component A is not particularly limited as long as it is an aprotic polar solvent. However, from the viewpoint of reducing the amount of aprotic polar solvent contained in the polyimide film after imidization, which is easily released outside the system during the imidization process of the polyimide precursor (polyamic acid), it is preferable for the aprotic polar solvent to have a boiling point of 210°C or less at 1 atmosphere, more preferably 200°C or less. The amount of remaining aprotic polar solvent is closely related to the imidization rate of the polyamic acid. If the aprotic polar solvent ratio is high relative to the imidization rate, the aprotic polar solvent functions as a plasticizer, causing molecular chain reconformation and increasing haze when the film is formed. Therefore, setting the boiling point of the aprotic polar solvent to 210°C or less is advantageous in that it can suppress the increase in haze.

[0024] Furthermore, from the viewpoint of compatibility with water (component B) and ease of control of the content of component B, it is preferable that component A have a boiling point of 120°C or higher at 1 atmosphere. In particular, if the boiling point of component B is lower than that of component A, and component B evaporates first, the polymer chains that form the film swell, creating an evaporation route that facilitates the evaporation of component A, making it easier to remove component A, which is advantageous.

[0025] Specific examples of the aprotic polar solvent as component A include N,N-dimethylformamide (boiling point: 153°C), N,N-dimethylacetamide (boiling point: 166°C), dimethyl sulfoxide (boiling point: 189°C), N,N-diethylacetamide (boiling point: 168°C), 2-butanone (boiling point: 79°C), N-methylcaprolactam (boiling point: 106°C), cyclohexanone (boiling point: 155°C), dioxane (boiling point: 101°C), tetrahydrofuran (boiling point: 66°C), diglyme (boiling point: 162°C), γ-butyrolactone (boiling point: 204°C), and N-methyl-2-pyrrolidone (boiling point: 202°C). Among these, from the viewpoint of ease of controlling the physical properties of the polyamic acid resin film, N,N-dimethylformamide, N,N-dimethylacetamide, and dimethyl sulfoxide are preferred, and N,N-dimethylacetamide is more preferred.

[0026] On the other hand, the content of Component B in the organic solvent, as measured by the Karl Fischer method, is within the range of 300 to 1000 ppm, preferably within the range of 500 to 800 ppm, and more preferably within the range of 550 to 700 ppm. By ensuring that the organic solvent in the polyimide precursor varnish of the present invention contains a certain amount of water within the above range, the progress of the polyimide polymerization reaction can be easily controlled, and an unexpected increase in viscosity can be prevented.

[0027] The organic solvent may contain solvents other than the above-mentioned Component A and Component B, provided that the effects of the present invention are not impaired. Examples of other solvents other than Component A and Component B include hexamethylphosphoramide, dimethyl sulfate, triglyme, cresol, etc. Two or more of these solvents may be mixed, and aromatic hydrocarbons such as xylene and toluene may also be mixed.

[0028] <Polyimide precursor> The varnish according to the present invention contains a polyimide precursor (polyamic acid) obtained by reacting a tetracarboxylic anhydride component with a diamine component, and can produce a polyimide film having a total light transmittance of 70% or more.

[0029] The tetracarboxylic acid anhydride component and diamine component that form such a polyimide precursor are not particularly limited, but from the viewpoint of achieving a total light transmittance of 70% or more when formed into a polyimide film, it is preferable that the polyimide precursor contains an aromatic tetracarboxylic acid residue containing a fluorine atom or an ether group, or an aromatic diamine residue containing a fluorine atom or an ether group, or that the polyimide precursor satisfies both of these requirements. Note that the tetracarboxylic acid residue refers to a tetravalent group derived from a tetracarboxylic acid anhydride, and the diamine residue refers to a divalent group derived from a diamine compound.

[0030] In particular, the diamine component preferably contains 50 mol % or more, more preferably 70 mol % or more, of a diamine compound represented by the following formula (1): [ka] [In formula (1), the substituents X independently represent a monovalent hydrocarbon group or alkoxy group having 1 to 3 carbon atoms which may be substituted with a hydrogen atom or a fluorine atom, m and n independently represent integers of 1 to 4, and the linking group Y represents a divalent group selected from a single bond, -CONH-, -O-, -O-Ph-O-, -O-Ph-SOO-Ph-O-, -COO, or -COO-]

[0031] Among the aromatic diamines represented by the formula (1), those having a biphenyl skeleton or two phenyl skeletons connected by -CONH- or -COO- have a rigid structure, which facilitates the formation of an ordered structure when made into a polyimide film, promotes in-plane orientation of molecular chains, and suppresses an increase in the thermal expansion coefficient of the polyimide film.

[0032] Specific preferred examples of such aromatic diamines include 4,4'-diamino-2,2'-bis(trifluoromethyl)biphenyl (TFMB), 2,2'-dimethyl-4,4'-diaminobiphenyl (m-TB), 2,2'-diethyl-4,4'-diaminobiphenyl (m-EB), 2,2'-diethoxy-4,4'-diaminobiphenyl (m-EOB), and 2,2'-dipropoxy-4,4'-diaminobiphenyl (m-EOB). Examples of the diaminobenzanilide include 2,2'-n-propyl-4,4'-diaminobiphenyl (m-POB), 2,2'-n-propyl-4,4'-diaminobiphenyl (m-NPB), 2,2'-divinyl-4,4'-diaminobiphenyl (VAB), 4,4'-diaminobiphenyl, 4-aminophenyl-4'-aminobenzoate (APAB), 2'-methoxy-4,4'-diaminobenzanilide (MABA), and 4,4'-diaminobenzanilide (DABA). Furthermore, examples of compounds in which two phenyl skeletons are linked by -O-, -O-Ph-O-, or -O-Ph-SOO-Ph-O- include 1,3-bis(4-aminophenoxy)benzene (TPE-R), 1,3-bis(4-aminophenoxy)benzene (APB), bis[4-(aminophenoxy)phenyl]sulfone (BAPS), and 2,2-bis(4-aminophenoxyphenyl)propane (BAPP).

[0033] Among these, from the viewpoint of simultaneously maintaining transparency and heat resistance, 4,4'-diamino-2,2'-bis(trifluoromethyl)biphenyl (TFMB), 2,2'-dimethyl-4,4'-diaminobiphenyl (m-TB), and 4,4'-diaminobenzanilide (DABA) are more preferred, and 4,4'-diamino-2,2'-bis(trifluoromethyl)biphenyl (TFMB), 1,3-bis(4-aminophenoxy)benzene (TPE-R), and 1,3-bis(4-aminophenoxy)benzene (APB) are even more preferred.

[0034] The tetracarboxylic acid anhydride component is preferably pyromellitic dianhydride and / or one represented by the following formula (2). [ka] [In formula (2), the linking group Z represents a divalent group selected from a single bond, -O-, -COO-, -C(CF3)2-, -COO-Ar-COO-, or an optionally substituted 9,9-fluorenediyl group, and Ar represents a divalent group selected from the following formulas:]

[0035] [ka]

[0036] In the above formula, R1 represents an alkyl group having 1 to 3 carbon atoms, and m' independently represents an integer of 0 to 4.

[0037] Among pyromellitic dianhydride and / or tetracarboxylic acid anhydride components represented by the above formula (2), pyromellitic dianhydride or one in which the linking group Z in the above formula (2) is a single bond has a rigid structure, and therefore, when made into a polyimide film, it has a low thermal expansion coefficient and can exhibit high heat resistance.

[0038] Here, preferred examples of the tetracarboxylic acid anhydride component include 2,2-bis(3,4-dicarboxyphenyl)-hexafluoropropane dianhydride (6FDA), pyromellitic dianhydride (PMDA), 4,4'-oxydiphthalic dianhydride (ODPA), 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA), and p-biphenylenebis(trimellitic acid monoester dihydrate) (BP-TME). One or more of these can be used. Preferably, the tetracarboxylic acid anhydride component contains at least one of these at 50 mol% or more, more preferably at 70 mol% or more.

[0039] Among these, the tetracarboxylic acid anhydride component is more preferably 2,2-bis(3,4-dicarboxyphenyl)-hexafluoropropane dianhydride (6FDA) or pyromellitic dianhydride (PMDA) from the viewpoint of simultaneously maintaining transparency and heat resistance.

[0040] In the present invention, as long as the purpose of the present invention is not impaired, the polyimide film may contain other diamine residues derived from diamine components other than those mentioned above. However, when other diamine residues are contained, the other diamine residues should account for less than 30 mol %, preferably less than 10 mol %, of the total diamine residues. As such other diamine residues, any of those known as diamine components for polyimides, particularly transparent polyimides, can be used. Specific examples include, but are not limited to, the following:

[0041] That is, examples of other diamine residues include 4,6-dimethyl-m-phenylenediamine, 2,5-dimethyl-p-phenylenediamine, 2,4-diaminomesitylene, 3,3'-dimethyl-4,4'-diaminodiphenylmethane, 3,5,3',5'-tetramethyl-4,4'-diaminodiphenylmethane, 2,4-toluenediamine, m-phenylenediamine, p-phenylenediamine, 4,4'-diaminodiphenylpropane, 3,3'-diaminodiphenylpropane, 4,4'-diaminodiphenylethane, and 3,3'-diaminodiphenylene. benzene, 4,4'-diaminodiphenylmethane, 3,3'-diaminodiphenylmethane, 2,2-bis(4-aminophenoxyphenyl)propane, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, 4,4'-diaminodiphenyl sulfide, 3,3'-diaminodiphenyl sulfide, 4,4'-diaminodiphenyl sulfone, 3,3'-diaminodiphenyl sulfone, 4,4'-diaminodiphenyl ether, 3,3'-diaminodiphenyl ether, 1,3-bis(3-aminophenoxy)benzene, 1,4-bis(4-aminophenoxy)phenyl p-bis(2-methyl-4-aminopentyl)benzene, p-bis(1,1-dimethyl-5-aminopentyl)benzene, 1,5-diaminonaphthalene, 4,4"-diamino-p-terphenyl, 3,3"-diamino-p-terphenyl, bis(p-aminocyclohexyl)methane, bis(p-β-amino-t-butylphenyl)ether, bis(p-β-methyl-δ-aminopentyl)benzene, p-bis(2-methyl-4-aminopentyl)benzene, p-bis(1,1-dimethyl-5-aminopentyl)benzene, 1,5-diaminonaphthalene, 2,6-diaminonaphthalene, 2,4-bis(β-amino-t-butyl)toluene, 2,4-diaminotoluene, m-xylene-2,5-diamine, p-xylene-2,5-diamine, m-xylylenediamine, p-xylylenediamine, 2,6-diaminopyridine, 2,5-diaminopyridine, 2,5-diamino-1,3,4-oxadiazole, piperazine, 4-(1H,1H,11H-eicosafluoroundecanoxy)-1,3-diaminobenzene, 4-(1H,1H-perfluoro-1-butanoxy)-1,3-diaminobenzene, 4-(1H,1H-perfluoro-1-heptanoxy)-1,3-diaminobenzene, 4-(1H,1H-perfluoro-1-octanoxy)-1,3-diaminobenzene, 4-pentafluorophenoxy-1,3-diaminobenzene, 4-(2,3,5,6-tetrafluorophenoxy)-1,3-diaminobenzene, 4-(4-fluorophenoxy)-1,3-diaminobenzene, 4-(1H,1H,2H,2H-perfluoro-1-hexanoxy)-1,3-diaminobenzene, 4-(1H,1H,2H,2H-perfluoro-1-dodecanoxy) )-1,3-Diaminobenzene, (2,5)-Diaminobenzotrifluoride, Diaminotetra(trifluoromethyl)benzene, Diamino(pentafluoroethyl)benzene, 2,5-Diamino(perfluorohexyl)benzene, 2,5-Diamino(perfluorobutyl)benzene, 2,2'-Bis(trifluoromethyl)-4,4'-diaminobiphenyl, 3,3'-Bis(trifluoromethyl)-4,4'-diaminobiphenyl, Octafluorobenzidine, 4,4'-Diaminodiphenyl ether, 2,2-Bis(4-amino phenyl)hexafluoropropane, 1,3-bis(anilino)hexafluoropropane, 1,4-bis(anilino)octafluorobutane, 1,5-bis(anilino)decafluoropentane, 1,7-bis(anilino)tetradecafluoroheptane, 2,2'-bis(trifluoromethyl)-4,4'-diaminodiphenyl ether, 3,3'-bis(trifluoromethyl)-4,4'-diaminodiphenyl ether, 3,3',5,5'-tetrakis(trifluoromethyl)-4,4'-diaminodiphenyl ether, 3,3'-bis (Trifluoromethyl)-4,4'-diaminobenzophenone, 4,4'-diamino-p-terphenyl, 1,4-bis(p-aminophenyl)benzene, p-(4-amino-2-trifluoromethylphenoxy)benzene, bis(aminophenoxy)bis(trifluoromethyl)benzene, bis(aminophenoxy)tetrakis(trifluoromethyl)benzene, 2,2-bis{4-(4-aminophenoxy)phenyl}hexafluoropropane, 2,2-bis{4-(3-aminophenoxy)phenyl}hexafluoropropane, 2,2-bis{4-(2-aminophenoxy)phenyl}hexafluoropropane, 2,2-bis{4-(4-aminophenoxy)-3,5-dimethylphenyl}hexafluoropropane, 2,2-bis{4-(4-aminophenoxy)-3.5-ditrifluoromethylphenyl}hexafluoropropane, 4,4'-bis(4-amino-2-trifluoromethylphenoxy)biphenyl, 4,4'-bis(4-amino-3-trifluoromethylphenoxy)biphenyl, 4,4'-bis(4-amino-2-trifluoromethylphenoxy)diphenyl sulfone, 4 Examples of diamine residues include those derived from 4'-bis(3-amino-5-trifluoromethylphenoxy)diphenyl sulfone, 2,2-bis{4-(4-amino-3-trifluoromethylphenoxy)phenyl}hexafluoropropane, bis{(trifluoromethyl)aminophenoxy}biphenyl, bis[{(trifluoromethyl)aminophenoxy}phenyl]hexafluoropropane, bis{2-[(aminophenoxy)phenyl]hexafluoroisopropyl}benzene, and 4,4'-bis(4-aminophenoxy)octafluorobiphenyl.

[0042] Similarly, the polyimide constituting the polyimide film of the present invention may contain tetracarboxylic acid residues derived from tetracarboxylic acid anhydrides other than those mentioned above, as long as the object of the present invention is not impaired. However, when other tetracarboxylic acid residues are contained, the content of these residues is less than 30 mol%, preferably less than 10 mol%, based on the total tetracarboxylic acid residues. As such other tetracarboxylic acid residues, any of those known as tetracarboxylic acid anhydride components of polyimides, particularly transparent polyimides, can be used. Specific examples include, but are not limited to, the following:

[0043] That is, examples of other tetracarboxylic acid residues include 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 2,2',3,3'-benzophenonetetracarboxylic dianhydride, 2,3,3',4'-benzophenonetetracarboxylic dianhydride, naphthalene-1,2,5,6-tetracarboxylic dianhydride, naphthalene-1,2,4,5-tetracarboxylic dianhydride, naphthalene-1,4,5,8-tetracarboxylic dianhydride, naphthalene-1,2,6,7-tetracarboxylic dianhydride, 4,8-dimethyl-1,2,3,5,6,7-hexacarboxylic acid dianhydride, and 4,8-dimethyl-1,2,3,5,6,7-hexacarboxylic acid dianhydride. hexahydronaphthalene-1,2,5,6-tetracarboxylic dianhydride, 4,8-dimethyl-1,2,3,5,6,7-hexahydronaphthalene-2,3,6,7-tetracarboxylic dianhydride, 2,6-dichloronaphthalene-1,4,5,8-tetracarboxylic dianhydride, 2,7-dichloronaphthalene-1,4,5,8-tetracarboxylic dianhydride, 2,3,6,7-tetrachloronaphthalene-1,4,5,8-tetracarboxylic dianhydride, 1,4,5,8-tetrachloronaphthalene-2,3,6,7-tetracarboxylic dianhydride, 2,2',3,3'- Biphenyltetracarboxylic dianhydride, 2,3,3',4'-biphenyltetracarboxylic dianhydride, 3,3'',4,4''-p-terphenyltetracarboxylic dianhydride, 2,2'',3,3''-p-terphenyltetracarboxylic dianhydride, 2,3,3'',4''-p-terphenyltetracarboxylic dianhydride, 2,2-bis(2,3-dicarboxyphenyl)-propane dianhydride, 2,2-bis(3,4-dicarboxyphenyl)-propane dianhydride, bis(2,3-dicarboxyphenyl)ether dianhydride, bis(2,3-dicarboxyphenyl)ether dianhydride Bis(3,4-dicarboxyphenyl)methane dianhydride, bis(2,3-dicarboxyphenyl)sulfone dianhydride, bis(3,4-dicarboxyphenyl)sulfone dianhydride, 1,1-bis(2,3-dicarboxyphenyl)ethane dianhydride, 1,1-bis(3,4-dicarboxyphenyl)ethane dianhydride, perylene-2,3,8,9-tetracarboxylic dianhydride, perylene-3,4,9,10-tetracarboxylic dianhydride, perylene-4,5,10,11-tetracarboxylic dianhydride, perylene-5,6,11,12-tetracarboxylic dianhydride, phenanthrene-1,2,7,8-tetracarboxylic dianhydride, phenanthrene-1,2,6,7-tetracarboxylic dianhydride, phenanthrene-1,2,9,10-tetracarboxylic dianhydride, cyclopentane-1,2,3,4-tetracarboxylic dianhydride, pyrazine-2,3,5,6-tetracarboxylic dianhydride, pyrrolidine-2,3,4,5-tetracarboxylic dianhydride, thiophene-2,3,4,5-tetracarboxylic dianhydride, (trifluoromethyl)pyromellitic dianhydride, di(trifluoromethyl)pyromellitic dianhydride, di(heptafluoropropyl)pyromellitic dianhydride, pentafluoromethyl Fluoroethyl pyromellitic dianhydride, Bis{3,5-di(trifluoromethyl)phenoxy}pyromellitic dianhydride, 5,5'-bis(trifluoromethyl)-3,3',4,4'-tetracarboxybiphenyl dianhydride, 2,2',5,5'-tetrakis(trifluoromethyl)-3,3',4,4'-tetracarboxybiphenyl dianhydride, 5,5'-bis(trifluoromethyl)-3,3',4,4'-tetracarboxydiphenyl ether dianhydride, 5,5'-bis(trifluoromethyl)-3,3',4,4' -Tetracarboxybenzophenone dianhydride, bis{(trifluoromethyl)dicarboxyphenoxy}benzene dianhydride, bis{(trifluoromethyl)dicarboxyphenoxy}trifluoromethylbenzene dianhydride, bis(dicarboxyphenoxy)trifluoromethylbenzene dianhydride, bis(dicarboxyphenoxy)bis(trifluoromethyl)benzene dianhydride, bis(dicarboxyphenoxy)tetrakis(trifluoromethyl)benzene dianhydride, 2,2-bis{(4-(3,4-dicarboxyphenoxy) bis(trifluoromethyl)dicarboxyphenoxy}biphenyl dianhydride, bis{(trifluoromethyl)dicarboxyphenoxy}bis(trifluoromethyl)biphenyl dianhydride, bis{(trifluoromethyl)dicarboxyphenoxy}diphenyl ether dianhydride, bis(dicarboxyphenoxy)bis(trifluoromethyl)biphenyl dianhydride, 1,2,3,4-cyclobutanetetracarboxylic acid dianhydride, fluorenylidenebisphthalic anhydride, 1,2,4,Examples include acid anhydride residues derived from 5-cyclohexanetetracarboxylic dianhydride.

[0044] In the present invention, the weight average molecular weight (Mw) of the polyimide precursor is 100,000 to 700,000, and preferably 120,000 to 500,000. In this way, polyimide precursors having weight average molecular weights in a relatively wide range can be included.

[0045] The polyimide precursor varnish of the present invention has a solids concentration of 10 to 18% by mass, preferably 12 to 16% by mass. When the solids content is within this range, it becomes possible to obtain a polyimide film having a wide range of thicknesses, for example, from a relatively thin polyimide film of about 2 to 10 μm to a relatively thick polyimide film of about 30 to 50 μm, during actual production of the polyimide film.

[0046] Furthermore, the polyimide precursor varnish of the present invention has a viscosity of 60,000 cP or less at 25°C. A viscosity within this range at 25°C ensures the film-forming properties of the polyimide film, suppresses thickness unevenness even in the thick polyimide film described above, and prevents the occurrence of poor appearance of the polyimide film, such as streaks and bubbles (voids). Considering the ease of adjusting the coating width and ensuring the coating thickness, it can be said that 800 cP is essentially the lower limit of this viscosity.

[0047] Furthermore, the polyimide precursor varnish of the present invention has a light transmittance at 550 nm of 90% or more, preferably 95% or more, in the varnish state. Having such a light transmittance is advantageous in terms of ensuring the visibility of the polyimide film. While this does not directly affect visual visibility, it is preferable that the light transmittance at 400 nm of the varnish be 0.01% or more, in terms of the influence on the optical properties of the film after film formation. Furthermore, from the viewpoint of visibility during mounting, it is preferable that the yellowness index (YI) of the varnish be 30 or less.

[0048] The polyimide film obtained from the polyimide precursor varnish of the present invention has a total light transmittance of 70% or more, preferably 80% or more, and more preferably 90% or more. Such a polyimide film ensures sufficient visibility through the polyimide film, for example, when mounting a liquid crystal driver IC on an FPC, or when mounting a TFT or the like to create a flexible device. This total light transmittance is a value exhibited by a polyimide film having a thickness of at least 20 μm, as shown in the examples described below.

[0049] Furthermore, the polyimide film obtained from the polyimide precursor varnish of the present invention preferably has a coefficient of thermal expansion (CTE) of 60 ppm / K or less, more preferably 40 ppm / K or less.

[0050] The polyimide film obtained from the polyimide precursor varnish of the present invention is not particularly limited in its applications, and can be used, for example, in circuit wiring substrates such as flexible printed wiring boards (FPCs). In particular, the polyimide film is suitable for forming at least one polyimide layer among multiple polyimide layers in a metal-clad laminate used to obtain a flexible printed wiring board, the metal-clad laminate comprising an insulating resin layer made of multiple polyimide layers and a metal layer laminated on at least one surface of the insulating resin layer.

[0051] That is, a metal-clad laminate generally comprises an insulating resin layer composed of multiple polyimide layers and a metal layer laminated on at least one surface of the insulating resin layer. The insulating resin layer composed of multiple polyimide layers is formed by laminating a polyimide layer composed of a non-thermoplastic polyimide as a base layer (base polyimide layer) on a metal layer such as copper foil via a thermoplastic polyimide that serves as an adhesive layer. Furthermore, a polyimide layer composed of a thermoplastic or non-thermoplastic polyimide that forms a top layer may be laminated on the base layer. While these multiple polyimide layers each have their own roles and functions, the polyimide precursor varnish according to the present invention can form a polyimide layer (polyimide film) of any thickness with excellent film-forming properties, making it extremely suitable for forming these polyimide layers.

[0052] [Polyimide film manufacturing method] The method for producing a polyimide film according to the present invention includes the following steps a to c: a) preparing a mixed solution containing a tetracarboxylic acid anhydride component, a diamine component, and an organic solvent; b) reacting the tetracarboxylic acid anhydride component with the diamine component in the mixed solution to obtain a varnish containing a polyimide precursor; c) a step of heat-treating the varnish containing the polyimide precursor to imidize it, thereby obtaining a polyimide film; It is equipped with the following.

[0053] <Process a> The organic solvent used in step a contains the following components A and B, and the content of component A measured by gas chromatography is 50 mass% or more. The content of component B measured by the Karl Fischer method is in the range of 300 to 1000 ppm. Details of components A and B are as described above in [Polyimide precursor varnish]. A) aprotic polar solvents; B) water;

[0054] This organic solvent is used in step a to obtain a mixed solution by mixing the tetracarboxylic anhydride component and the diamine component, as described above in the section on polyimide precursor varnish.

[0055] <Process b> In step b, when reacting the tetracarboxylic anhydride component and the diamine component in the mixed solution, although there are no particular limitations, preferably, the tetracarboxylic anhydride and the diamine compound are dissolved in an organic solvent in approximately equimolar amounts and stirred at a temperature in the range of 0 to 100°C for 30 minutes to 24 hours to cause a polymerization reaction, thereby obtaining a polyamic acid solution.

[0056] That is, what is obtained in this step b is the polyimide precursor varnish according to the present invention described above, which has a solids concentration of 10 to 18 mass %, a viscosity at 25°C of 60,000 cP or less, a weight-average molecular weight (Mw) of the polyimide precursor of 100,000 to 700,000, and a light transmittance of 90% or more at 550 nm. Details are as described in [Polyimide precursor varnish].

[0057] <Process c> In step c, the polyimide precursor varnish obtained in step b is heat-treated to imidize it, thereby obtaining a polyimide film. In this process, for example, a tenter method may be used in which the polyimide precursor varnish is cast onto a rotating drum or the like, peeled off from the rotating drum in the form of a gel film, and heated in a tenter furnace to imidize it, thereby obtaining a polyimide film. Alternatively, a casting method may be used in which the polyimide precursor varnish is applied to a substrate such as copper foil, and then dried and imidized by heat treatment, thereby obtaining a polyimide film.

[0058] The heat treatment is generally preferably performed by heating for 1 to 60 minutes at a temperature in the range of 80 to 400° C. In this case, in order to promote imidization of the polyamic acid, a treatment for evaporating the organic solvent that dissolves or mixes the polyamic acid and a treatment for releasing the organic solvent coordinated to the polyamic acid to the outside of the system are necessary, and these treatments are to be included.

[0059] In step c, a resin film of the polyimide precursor (polyamic acid) may be formed on a support such as a rotating drum or a substrate, and then imidized, or the resin film may be peeled off from the support and then imidized. When multiple polyimide films are stacked, for example, a polyimide precursor varnish may be applied to the support and dried multiple times before imidization. Separately from the tenter method or casting method described above, for example, multiple layers of polyimide precursor varnish may be simultaneously applied and dried by multilayer extrusion, and then imidization may be performed.

[0060] From the viewpoint of maintaining the thickness and dimensional accuracy of the polyimide film, it is desirable to complete the imidization of the polyimide precursor (polyamic acid) on a support, as in the tenter method or casting method. Since the polyimide precursor (polyamic acid) resin film is imidized while fixed on the support, it is possible to suppress changes in the expansion and contraction of the polyimide during the imidization process.

[0061] That is, the support is used for the purpose of reinforcing the polyimide film when it is obtained and for the purpose of suppressing changes in the expansion and contraction of the polyimide film and maintaining dimensional accuracy. Furthermore, the support is the target on which the polyimide precursor varnish is applied, and can be in the form of a cut sheet, a roll, an endless belt, or the like. To obtain productivity, it is efficient to use a roll or endless belt form that allows for continuous production. Furthermore, from the viewpoint of achieving a greater effect of improving the dimensional accuracy of the polyimide film, a support formed in a long roll, as in the casting method, is preferred.

[0062] The material of the support is not particularly limited, but examples thereof include heat-resistant materials such as metals, ceramics, resins, and carbon. Among these, metals are preferred from the viewpoints of thermal conductivity and flexibility. For example, in the case of a casting method, examples of the support include metal foils (metal films) such as copper foil, aluminum foil, stainless steel foil, iron foil, silver foil, gold foil, zinc foil, indium foil, tin foil, zirconium foil, tantalum foil, titanium foil, cobalt foil, and alloy foils thereof. When a polyimide film is used as an insulating layer of a metal-clad laminate (circuit wiring board) and the support is used as a metal layer (wiring layer) of the metal-clad laminate (circuit wiring board), copper foil or copper alloy foil is preferred as the support. Furthermore, when the polyimide film is used by peeling it from the support, a smooth stainless steel belt or a stainless steel drum is preferably used.

[0063] When the support is used as a metal layer (wiring layer) of a metal-clad laminate (circuit wiring board), the thickness of the metal foil constituting the support is, for example, preferably in the range of 5 to 35 μm, more preferably in the range of 9 to 18 μm. If the metal foil is thicker than 35 μm, the flexibility and foldability of the metal-clad laminate will be poor. On the other hand, if the metal foil is thinner than 5 μm, it will be difficult to adjust the tension and other factors during the manufacturing process of the metal-clad laminate, and defects such as wrinkles will be more likely to occur. Furthermore, these metal foils may be subjected to chemical or mechanical surface treatments on their surfaces to improve adhesive strength, etc., or chemical surface treatments for rust prevention.

[0064] <Recovery and reuse of organic solvents> In the present invention, it is desirable to recover the organic solvent from the solvent vapor generated by heat-treating the polyimide precursor varnish in step c and reuse it as the organic solvent for the mixed solution. For example, the organic solvent can be recovered from the solvent vapor generated by drying the polyimide precursor varnish after coating it on a support, but it may also be recovered from the solvent vapor generated by heat-treating a resin film of the polyimide precursor. The solvent recovered in this way can be reused as the organic solvent in step a.

[0065] The method for recovering and reusing the solvent vapor is not particularly limited, but examples thereof include a cooling method, an adsorption method using a solid adsorbent such as activated carbon or zeolite, an adsorption method using a liquid, hardly volatile solvent, an absorption method using water, etc. In addition, a general solvent recovery device can be used, such as a multi-effect steam solvent recovery device, a heat pump solvent recovery device, a heat pump multi-effect concentrator, an evaporation concentrator, etc.

[0066] The solvent vapor is hydrophilic and easily soluble in water, and because the boiling points of water and the organic solvent are different and the solvent vapor does not have an azeotropic point with water, the step of recovering the solvent from the solvent vapor preferably includes a step of dissolving water-soluble substances contained in the solvent vapor in water. The recovered liquid can be concentrated by evaporating water with high vapor pressure using thermal energy supplied from an external source. The thermal energy preferably uses the heat of the solvent vapor, but if the solvent vapor is at a low temperature, heat from a heater or the like may also be used. The concentrated recovered liquid may be treated with an acid, an alkali, activated carbon, or the like, as necessary. The concentrated recovered liquid can also be regenerated and reused as the raw organic solvent in step a by purification, for example, distillation.

[0067] When using an organic solvent containing a recycled solvent, it is preferable that the recycled solvent accounts for 50% by volume or more of the raw organic solvent. By making the ratio of recycled solvent to the raw organic solvent 50% by volume or more, cost benefits are increased.

[0068] The method for producing a polyimide film including these steps a to c can also be applied to the production of the metal-clad laminate described above. That is, to obtain the thickest base polyimide layer among the multiple polyimide layers that form the insulating resin layer in the metal-clad laminate, for example, the polyimide precursor varnish obtained in the previous step b may be applied to a copper foil directly or indirectly via an adhesive layer and dried to obtain a polyimide precursor resin film, and then the polyimide precursor may be imidized by heat treatment in step c to form the base polyimide layer.

[0069] By going through the above steps a to c, a polyimide film having a total light transmittance of 70% or more can be obtained. [Example]

[0070] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples. In the following examples, unless otherwise specified, various measurements and evaluations are as follows.

[0071] [Moisture content measurement] The moisture content of organic solvents was measured using a Karl Fischer moisture meter (coulometric titration method) <mkc-710m>Measurements were carried out using a meter (manufactured by Kyoto Electronics Manufacturing Co., Ltd.).

[0072] [Measurement by gas chromatography] The content of component A (DMAc, described later) in the organic solvent was measured using a JEOL JMS-Q1600GC. The percentage is the ratio of the main peak area of ​​the solvent to the total peak area obtained.

[0073] [Viscosity measurement] The viscosity of the polyamic acid solution obtained in the synthesis example was measured at 25°C using a cone-plate viscometer equipped with a thermostatic water bath (manufactured by Tokimec Co., Ltd.).

[0074] [Weight average molecular weight (Mw)] Measurement was performed by gel permeation chromatography (manufactured by Tosoh Corporation, product name: HLC-8220GPC). Polystyrene was used as a standard substance, and N,N-dimethylacetamide was used as a developing solvent.

[0075] [Calculation of total light transmittance (TT)] After removing the support (copper foil), a test piece of 50 mm in the machine direction (MD) × 50 mm in the width direction (TD) of the film was cut from the polyimide layer (hereinafter referred to as polyimide film). The total light transmittance (TT) was measured in accordance with JIS K 7136 using a HAZE METER NDH500 manufactured by Nippon Denshoku Industries Co., Ltd.

[0076] [Light transmittance of varnish at 550 nm and 400 nm wavelengths] Using a Shimadzu UV-3600 spectrophotometer and a standard cell (1 cm x 1 cm x 10 cm) with a 1 cm optical path length, the light transmittance (T550) of the varnish at 550 nm was measured, using pure water as a blank. Similarly, the light transmittance (T400) of the varnish at 400 nm was measured.

[0077] [Varnish Yellowness Index YI] The YI (yellowness index) of the varnish was calculated based on the calculation formula represented by the following formula (1). YI=100×(1.2879X-1.0592Z) / Y (1) (Here, X, Y, and Z represent the tristimulus values ​​defined in the spectrophotometric colorimetry method in JIS Z 8722:2009 "Methods for measuring color - Reflection and transmission object color.")

[0078] [Measurement of coefficient of thermal expansion (CTE)] A test piece of 3 mm MD x 15 mm TD was heated from 30°C to 200°C at a heating rate of 10°C / min while applying a load of 5.0 g in a thermomechanical analysis (TMA) device, and then cooled to 30°C. The thermal expansion coefficient was measured from the elongation (linear expansion) as the temperature was lowered from 180°C to 100°C.

[0079] [Possibility of controlling varnish polymerization] When reacting a polyimide precursor varnish to obtain a polyimide layer (polyimide film), the evaluation was performed as follows: if the viscosity of the varnish after polymerization did not exceed 60,000 cP, it was marked as "Good"; if the viscosity exceeded 60,000 cP, it was marked as "Poor."

[0080] [Polyimide film formability] When a polyimide layer (polyimide film) was obtained from the polyimide precursor varnish by heat treatment, the viscosity was too high to be uniformly applied, or the obtained polyimide layer (polyimide film) was brittle, which was rated as "X." When a polyimide layer (polyimide film) of uniform thickness was formed without these problems, it was rated as "O."

[0081] [Appearance evaluation of polyimide film] When a polyimide layer (polyimide film) was obtained from the polyimide precursor varnish by heat treatment, if the polyimide film had no streaks within it and a clean appearance, it was marked as "○", and if bubbles or streaks were visible on the surface, it was marked as "×".

[0082] [180° peel strength] Using a tension tester, the polyimide layer side of a test sample having a 1 mm-wide circuit obtained from a laminate of copper foil and a polyimide layer (hereinafter referred to as a "copper-clad laminate") serving as a support was fixed to an aluminum plate with double-sided tape, and the copper was peeled off in a 180° direction at a rate of 50 mm / min to determine the peel strength between the copper foil and the polyimide layer.

[0083] The abbreviations used in the examples represent the following compounds. TFMB: 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl BAPS: Bis[4-(aminophenoxy)phenyl]sulfone m-TB: 2,2'-dimethyl-4,4'-diaminobiphenyl TPE-R: 1,3-bis(4-aminophenoxy)benzene PMDA: Pyromellitic dianhydride 6FDA: 2,2-bis(3,4-dicarboxyphenyl)-hexafluoropropane dianhydride BPDA: 3,3',4,4'-biphenyltetracarboxylic dianhydride DMAc(I): N,N-dimethylacetamide (keep for 1 week after opening) DMAc(II): N,N-dimethylacetamide (kept for 3 weeks after opening) DMAc(III): N,N-dimethylacetamide recycled with superheated steam (regenerated product)

[0084] Of the above, the solvent DMAc (I) was used after opening Commercial Product 1 (purity 99.0% or higher) and storing it with the lid closed at 23°C for one week, and had the water content shown in Table 1. Similarly, Commercial Product 2 (purity 99.0% or higher) was opened and stored for three weeks, and was used as DMAc (II). DMAc (III) was a recycled product obtained from Reference Example 1 described later, and also had the water content shown in Table 1.

[0085] [Table 1]

[0086] Synthesis Example 1 To synthesize polyimide precursor varnish 1, the solvent DMAc (III) was added to a 200 ml separable flask under a nitrogen stream to a solids concentration of 15% by mass. The diamine component and acid anhydride component of resin composition C shown in Table 2 were added and dissolved at room temperature with stirring to achieve the desired molar ratio. The resulting mixture was then stirred at room temperature for 20 hours to carry out a polymerization reaction, yielding polyimide precursor varnish 1, a viscous solution of polyamic acid. The viscosity (25°C) of the resulting polyimide precursor varnish 1 was 11,271 cP. The molecular weight Mw of the polyimide precursor (polyamic acid) contained in polyimide precursor varnish 1 was 151,754. Furthermore, the light transmittance of polyimide precursor varnish 1 at 550 nm was 99.4%.

[0087] Synthesis Example 2 To synthesize polyimide precursor varnish 2, the solvent DMAc (III) was added to a 200 ml separable flask under a nitrogen stream to a solids concentration of 15% by mass, and the diamine component and acid anhydride component of resin composition B shown in Table 2 were added and dissolved at room temperature with stirring to achieve the desired molar ratio. The resulting mixture was then stirred at room temperature for 5 hours to carry out a polymerization reaction, yielding polyimide precursor varnish 2, a viscous solution of polyamic acid. The viscosity (25°C) of the resulting polyimide precursor varnish 2 was 33,973 cP. The molecular weight Mw of the polyimide precursor (polyamic acid) contained in polyimide precursor varnish 2 was 351,526. Furthermore, the light transmittance of polyimide precursor varnish 2 at 550 nm was 98.5%.

[0088] Synthesis Example 3 To synthesize polyimide precursor varnish 3, the solvent DMAc (III) was added to a 200 ml separable flask under a nitrogen stream to a solids concentration of 15% by mass. The diamine component and acid anhydride component of resin composition A shown in Table 2 were added and dissolved at room temperature with stirring to achieve the desired molar ratio. The resulting mixture was then stirred at room temperature for 5 hours to allow for polymerization, resulting in polyimide precursor varnish 3, a viscous solution of polyamic acid. The viscosity (25°C) of the resulting polyimide precursor varnish 3 was 6813 cP. The molecular weight Mw of the polyimide precursor (polyamic acid) contained in polyimide precursor varnish 3 was 102362. Furthermore, the light transmittance at 550 nm of polyimide precursor varnish 4 was 91.2%.

[0089] Synthesis Example 4 To synthesize polyimide precursor varnish 4, the solvent DMAc (II) was added to a 200 ml separable flask under a nitrogen stream to a solids concentration of 15% by mass. The diamine component and acid anhydride component of resin composition A shown in Table 2 were added and dissolved at room temperature with stirring to achieve the desired molar ratio. The resulting mixture was then stirred at room temperature for 5 hours to allow for polymerization, resulting in polyimide precursor varnish 4, a viscous solution of polyamic acid. The viscosity (25°C) of the resulting polyimide precursor varnish 4 was 63,025 cP. The molecular weight Mw of the polyimide precursor (polyamic acid) contained in polyimide precursor varnish 4 was 166,702. Furthermore, the light transmittance at 550 nm of polyimide precursor varnish 4 was 92.3%.

[0090] Synthesis Example 5 To synthesize Polyimide Precursor Varnish 5, the solvent DMAc (I) was added to a 200 ml separable flask under a nitrogen stream to a solids concentration of 15% by mass. The diamine component and acid anhydride component of Resin Composition A shown in Table 2 were added and dissolved at room temperature with stirring to achieve the desired molar ratio. The solution was then stirred at room temperature for 5 hours to allow for polymerization, resulting in Polyimide Precursor Varnish 5, a viscous solution of polyamic acid. The viscosity (25°C) of the resulting Polyimide Precursor Varnish 5 was 221,000 cP. The molecular weight Mw of the polyimide precursor (polyamic acid) contained in Polyimide Precursor Varnish 5 was 418,255. Furthermore, the light transmittance at 550 nm of Polyimide Precursor Varnish 5 was 96.7%.

[0091] Synthesis Example 6 To synthesize polyimide precursor varnish 56, the solvent DMAc (II) was added to a 200 ml separable flask under a nitrogen stream to a solids concentration of 15% by mass. The diamine component and acid anhydride component of resin composition B shown in Table 2 were added and dissolved at room temperature with stirring to the desired molar ratio. The solution was then stirred at room temperature for 5 hours to allow the polymerization reaction to occur, resulting in polyimide precursor varnish 6, a viscous solution of polyamic acid. The viscosity (25°C) of the resulting polyimide precursor varnish 6 was 244,000 cP. The molecular weight Mw of the polyimide precursor (polyamic acid) contained in polyimide precursor varnish 6 was 462,680. Furthermore, the light transmittance at 550 nm of polyimide precursor varnish 6 was 98.5%.

[0092] [Table 2]

[0093] [Reference example 1] Polyimide precursor varnish 5 prepared in Synthesis Example 5 was uniformly applied to one side of a long copper foil I (electrolytic copper foil, manufactured by Mitsui Mining & Smelting Co., Ltd., product name: CF-T49A-DS-HD2 foil, thickness: 12 μm) having a thickness of 12 μm, and a width of 12 μm, so that the thickness after curing would be 2.5 μm. The coating was then dried by heating at 120° C. to remove the solvent.

[0094] Next, the polyimide precursor varnish 4 prepared in Synthesis Example 4 was uniformly applied onto the dried polyimide precursor varnish 5 so that the thickness after curing would be 20 μm, and then the coating was dried by heating at 130° C. to remove the solvent.

[0095] Furthermore, polyimide precursor varnish 5 prepared in Synthesis Example 5 was uniformly applied onto the dried polyimide precursor varnish 4 so that the thickness after curing would be 2.5 μm, and then the coating was dried by heating at 130° C. to remove the solvent. In this case, the above-mentioned process used for applying and heating polyimide precursor varnish 5, polyimide precursor varnish 4, and polyimide precursor varnish 5 was defined as a first heat treatment process.

[0096] Thereafter, a second heat treatment step was carried out in which the temperature was raised from 130° C. to 360° C. to effect imidization, thereby obtaining a copper-clad laminate consisting of multiple polyimide resin layers with a thickness of 25 μm.

[0097] (Process for recovering and regenerating solvent) The solvent vapor generated in the first and second heat treatment steps was dissolved in water by gas-liquid contact to obtain a solvent recovery liquid, which was then separated into the solvent and water by distillation to obtain DMAc (III), which had a water content of 575 (ppm by mass) as described above.

[0098] Example 1 Polyimide precursor varnish 1 was uniformly applied to copper foil I (electrolytic copper foil, manufactured by Mitsui Mining & Smelting Co., Ltd., product name: CF-T49A-DS-HD2 foil, thickness: 12 μm) to a cured thickness of 20 μm, and then heated and dried at 70 to 120°C for 3 minutes to remove the solvent. Next, a stepwise heat treatment was performed from 130 to 360°C, completing imidization within 10 minutes. A 20 μm-thick insulating resin layer consisting of polyimide layer 1c was formed (film-formed), and metal-clad laminate 1C was obtained.

[0099] The copper foil of the metal-clad laminate 1C obtained above was etched away using an aqueous ferric chloride solution to prepare a polyimide film 1C. The total light transmittance (TT) and coefficient of thermal expansion (CTE) of the polyimide film 1C were measured. Table 3 summarizes the metal-clad laminate of Example 1, including the results of these measurements.

[0100] [Table 3]

[0101] Example 2, Comparative Examples 1 to 3 Metal-clad laminates 2B (Example 2), 3A (Example 3), 4A (Comparative Example 1), 5B (Comparative Example 2), and 6B (Comparative Example 3) were produced in the same manner as in Example 1, except that the organic solvents and polyimide precursor varnishes (simply referred to as "varnish" in the table) having the polyimide (PI) compositions shown in Table 3 were used, and polyimide films 2b (Example 2), 3a (Example 3), 4a (Comparative Example 1), 5b (Comparative Example 2), and 6b (Comparative Example 3) were obtained, respectively. The resulting polyimide films 2b, 3a, 4a, 5b, and 6b were evaluated in the same manner as in Example 1. The results are shown in Table 3.

[0102] As described above, according to the present invention, it is possible to produce a polyimide film that is excellent in film-forming properties and visibility, without being limited by the monomer component.

Claims

1. A polyimide precursor varnish containing a polyimide precursor and an organic solvent, capable of producing a polyimide film having a light transmittance of 70% or more, The organic solvent may be the following components A and B: A) aprotic polar solvent; B) water; and the content of component A measured by gas chromatography is 50 mass% or more, and the content of component B measured by Karl Fischer method is within the range of 300 to 1000 ppm, The polyimide precursor varnish is characterized by having a solids content of 10 to 18% by mass, a viscosity at 25°C of 60,000 cP or less, a weight-average molecular weight (Mw) of the polyimide precursor of 100,000 to 700,000, and a light transmittance of the varnish at 550 nm of 90% or more.

2. 2. The polyimide precursor varnish according to claim 1, which is used to form at least one polyimide layer among a plurality of polyimide layers in a metal-clad laminate having an insulating resin layer composed of a plurality of polyimide layers and a metal layer laminated on at least one surface of the insulating resin layer.

3. A method for producing a polyimide film, comprising the following steps a to c: a) preparing a mixed solution containing a tetracarboxylic acid anhydride component, a diamine component, and an organic solvent; b) reacting the tetracarboxylic acid anhydride component with the diamine component in the mixed solution to obtain a varnish containing a polyimide precursor; c) a step of heat-treating the varnish containing the polyimide precursor to imidize it, thereby obtaining a polyimide film; It is equipped with The organic solvent is selected from the group consisting of the following components A and B: A) aprotic polar solvent; B) water; and the content of component A measured by gas chromatography is 50 mass% or more, and the content of component B measured by Karl Fischer method is within the range of 300 to 1000 ppm, The polyimide precursor varnish obtained in step (b) has a solids content of 10 to 18 mass %, a viscosity at 25°C of 60,000 cP or less, a weight-average molecular weight (Mw) of the polyimide precursor of 100,000 to 700,000, and a light transmittance of 90% or more at 550 nm.

4. 4. The method for producing a polyimide film according to claim 3, wherein the component A is at least one selected from the group consisting of N,N-dimethylformamide, N,N-dimethylacetamide, and dimethyl sulfoxide.

5. 5. The method for producing a polyimide film according to claim 4, wherein in the step (c), the varnish containing the polyimide precursor is heat-treated to generate a solvent vapor, from which the organic solvent is recovered and reused as the organic solvent for the mixed solution.

6. 6. The method for producing a polyimide film according to claim 5, wherein the organic solvent used in step a is obtained by heat-treating a varnish containing a polyimide precursor in step c and recovering the generated solvent vapor.

7. A method for producing a metal-clad laminate including an insulating resin layer made of a plurality of polyimide layers and a metal layer laminated on at least one surface of the insulating resin layer, comprising: A method for producing a metal-clad laminate, characterized in that, in obtaining a base polyimide layer having the greatest thickness among the plurality of polyimide layers forming the insulating resin layer, the method for producing a polyimide film according to claim 3 is used, in which the polyimide precursor varnish obtained in step b is applied directly or indirectly to a metal layer and dried to obtain a polyimide precursor resin film, and then the polyimide precursor varnish is heat-treated in step c to be imidized, thereby forming the base polyimide layer.

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