Polyimide varnish, polyimide film, and methods for producing the same

Abstract

To provide a transparent polyimide varnish having improved film formation property.SOLUTION: A polyimide varnish contains (a) polyimide and (b) a mixed solvent containing two or more kinds of solvents, in which in each parameter represented by a dispersion term δd, a dipole term δp and a hydrogen bond term δh of a Hansen solubility parameter (HSP), the dispersion terms and the hydrogen bond terms of the polyimide and the mixed solvent satisfy the following expression 1. In expression 1, δdPI is the dispersion term of the polyimide, δhPI is the hydrogen bond term of the polyimide, δdsolv is the dispersion term of the mixed solvent, and δhsolv is the hydrogen bond term of the mixed solvent.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a polyimide varnish, a polyimide film, and methods for producing them.

Background Art

[0002] In recent years, the development of foldable flexible devices such as flexible displays and curved surface devices having a curved surface such as organic EL lighting or an organic EL display has been underway. In such devices, it has been considered to use a foldable film (plastic film) made of a polymer material as a substrate for forming a surface protective layer, a color filter, a touch panel, a thin film transistor (TFT), and the like. Among plastic films, polyimide films are particularly expected as substrate materials in the above fields because of their excellent heat resistance, chemical resistance, etc. Aromatic polyimide is colored by a charge transfer complex (CT complex) between a site derived from diamine near nitrogen of the imide ring and a site derived from acid anhydride near the carbonyl group of the imide ring, and is difficult to use for optical applications. Further, aromatic polyimide is hardly soluble in general-purpose organic solvents and has poor processability, and its application range is limited. Therefore, in particular, in the above fields, a polyimide that is colorless and transparent and highly soluble in organic solvents is desired.

[0003] Patent Document 1 aims to provide a polyimide that is colorless and transparent, highly soluble in organic solvents, and hardly colored even at high temperatures. Patent Document 1 describes that a polyimide film is obtained by applying a polyamic acid varnish containing a polyimide precursor synthesized from an aromatic diamine and a tetracarboxylic dianhydride as a polyimide precursor onto a substrate and then performing thermal imidization. In addition, Patent Document 2 aims to provide a polyimide film or the like that contains a polyimide having good solubility in a solvent, excellent processability, is colorless and transparent, and has excellent toughness. Patent Document 2 describes that a polyimide varnish containing a polyimide synthesized from an aromatic diamine and a tetracarboxylic dianhydride is applied onto a substrate, and then the solvent is dried to obtain a polyimide film.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, when applying a polyimide varnish (also simply referred to as "varnish") to produce a film, in order to perform the film production process in as short a time as possible to improve productivity, the solvent of the polyimide varnish is required to have quick-drying properties. Specifically, the solvent is required to have a low boiling point and a high vapor pressure. It has been confirmed that polyamic acid, which is a polyimide precursor, dissolves in solvents such as N-methyl-2-pyrrolidone, γ-butyrolactone, N,N-dimethylacetamide, and N,N-dimethylformamide. However, since these solvents all have a boiling point of 150°C or higher, the drying time of the solvent could not be sufficiently shortened. Therefore, one object of the present invention is to provide a transparent polyimide varnish having quick-drying properties and thus improved film-forming properties.

Means for Solving the Problems

[0006] As a result of intensive studies to solve the above problems, the inventors of the present application have found that a polyimide varnish containing polyimide and two or more solvents, in which each parameter represented by the dispersion term δd, dipole term δp, and hydrogen bond term δh of the Hansen solubility parameter (HSP) satisfies a specific mathematical formula, can solve the above problems, and have completed the present invention. Examples of embodiments of the present invention are listed in the following items [1] to

[19] . [1] (a) Polyimide, and (b) A mixed solvent containing two or more solvents, a polyimide varnish comprising: In each parameter represented by the dispersion term δd, dipole term δp, and hydrogen bond term δh of the Hansen solubility parameter (HSP), the dispersion term and hydrogen bond term of the above polyimide and the above mixed solvent satisfy the following mathematical formula 1:

Equation

Chemical formula

Chemical formula

Chemical formula

[10] The polyimide varnish according to any one of items 1 to 6, wherein the above-mentioned (a) polyimide contains at least one selected from the group consisting of a tetravalent organic group represented by the following formula (A1), and divalent organic groups represented by the following formulas (B3) and (B4).

Chemical formula

Chemical formula

[11] The polyimide varnish according to any one of items 1 to 6, wherein the above-mentioned (a) polyimide contains at least one selected from the group consisting of tetravalent organic groups represented by the following formulas (A2), (A3) and (A4), and divalent organic groups represented by the following formula (B5). [Chemical formula] [Chemical formula] [Chemical formula] [Chemical formula]

[12] A step of casting the polyimide varnish according to any one of items 1 to 11 onto a substrate, A step of drying the cast polyimide varnish to form a self-supporting film, A step of peeling the self-supporting film from the substrate and further heating it, A method for producing a polyimide film, comprising:

[13] A polyimide film obtained by drying the polyimide varnish according to any one of items 1 to 11.

[14] Containing 5 to 40 parts by mass of a solvent based on 100 parts by mass of the polyimide, The polyimide film according to item 13, wherein the above-mentioned solvent contains (b1) a cyclic ester and (b2) a cyclic ketone.

[15] The polyimide film according to item 14, wherein the above-mentioned (b1) cyclic ester is γ-butyrolactone and the above-mentioned (b2) cyclic ketone is cyclopentanone.

[16] A light-emitting device having a polyimide film obtained by drying the varnish according to any one of items 1 to 11.

[17] A curved display having a polyimide film obtained by drying the varnish according to any one of Items 1 to 11.

[18] A foldable display having a polyimide film obtained by drying the varnish according to any one of Items 1 to 11.

[19] A method for producing a polyimide varnish, comprising dissolving a diamine and a tetracarboxylic dianhydride in a mixed solvent containing two or more solvents to form a polyimide in the mixed solvent, wherein the two or more solvents are selected such that in each parameter represented by the dispersion term δd, dipole term δp, and hydrogen bond term δh of the Hansen solubility parameter (HSP), the dispersion term and hydrogen bond term of the polyimide and the mixed solvent satisfy the following formula 1: [Equation] {In Formula 1, δd PI is the dispersion term of the polyimide, δh PI is the hydrogen bond term of the polyimide, δd solv is the dispersion term of the mixed solvent, and δh solv is the hydrogen bond term of the mixed solvent.}, A method for producing a polyimide varnish. [Advantages of the Invention]

[0007] According to the present invention, a transparent polyimide varnish having improved film-forming properties can be provided. The polyimide varnish of the present invention can efficiently produce a polyimide film having high transparency and bending resistance. It should not be considered that the above description discloses all embodiments of the present invention and all advantages related to the present invention. [Modes for Carrying Out the Invention]

[0008] Hereinafter, embodiments of the present invention (hereinafter referred to as "the present embodiment") will be described in detail. Note that the present invention is not limited to the following embodiments, and various modifications can be made within the scope of the gist thereof. In the present specification, the upper and lower limit values of each numerical range can be arbitrarily combined.

[0009] 《Polyimide varnish》 〈(a): Polyimide〉 The polyimide varnish of the present embodiment contains (a) polyimide. Polyimide is preferable from the viewpoint of suppressing the decrease in molecular weight during varnish storage. In the present specification, "polyamic acid" means a polymer having an imidization rate of less than 20%, that is, a polymer in which 80% or more of the repeating units are represented by the following formula (P1). In the present embodiment, "polyimide" means a polymer having an imidization rate of 20% or more, that is, a polymer in which 20% or more of the repeating units are represented by the following formula (P2). From the viewpoint of suppressing the decrease in molecular weight during varnish storage, the imidization rate in polyimide is preferably 40% or more, more preferably 60% or more, and even more preferably 80% or more and 100% or less.

[0010]

Chemical formula

[0011]

Chemical formula

[0012] The polyimide of the present embodiment can be synthesized by a conventionally known method using a tetracarboxylic dianhydride (denoted as A) and a diamine (denoted as B) as raw materials. In this embodiment, the imidization rate is calculated by the following formula from the amount of water generated when the amino acid cyclizes to form an imide during imidization. The imidization rate (%) is the number obtained by multiplying the value obtained by dividing the number of moles of water by the number obtained by doubling the number of moles of the lesser of A or B charged, by 100. Imidization rate (%) = Generated water (mol) / [(Lesser of A or B (mol) × 2)] × 100

[0013] When a hydrocarbon solvent such as toluene that azeotropes with water is added to the synthesis solvent, the amount of water generated by imidization is determined by separating the aqueous layer from the recovered components during reflux and measuring the weight. Also, for example, when the recovered components do not separate, the amount of water in the recovered components is measured with a Karl Fischer moisture meter.

[0014] In this embodiment, the polyimide is not particularly limited, but from the viewpoint of enhancing the transparency when formed into a polyimide film, it is preferable that the tetravalent organic group X contains at least one selected from the tetravalent organic groups represented by the following formulas (A1) to (A7).

[0015]

Chemical formula

Chemical formula

Chemical formula

Chemical formula

Chemical formula

Chemical formula

Chemical formula

[0016] The divalent organic group Y in the polyimide may contain at least one of the divalent organic groups represented by the following formulas (B1) to (B5). The divalent organic groups represented by (B1) to (B5) in the polyimide may be present in the structure of X. For example, when the organic group represented by (A7) is in the polyimide, the divalent organic group of (B1) and / or (B2) may be contained in the organic group X.

[0017] From the viewpoint of enhancing transparency when formed into a film, the divalent organic group Y preferably contains at least one of the divalent organic groups represented by the following formulas (B1) to (B5).

Chemical formula

Chemical formula

Chemical formula

Chemical formula

Chemical formula

[0018] Among the divalent organic groups represented by (B1) to (B5), the polyimide in the present embodiment is preferably the following polyimide from the viewpoints of enhancing solubility and film transparency. For example, it is preferable that the divalent organic group Y in (P2) has the following formula (B1). The repeating unit of this polyimide can be represented by the following formula (P2-B1).

[0019]

Chemical formula

[0020] It is also preferable that the divalent organic group Y in (P2) has the following formula (B2). The repeating unit of this polyimide can be represented by the following formula (P2-B2). The polyimide containing the repeating unit represented by (P2-B1) preferably has a divalent organic group Y further having the following formula (B2) in addition to the above formula (B1). The polyimide having (B1) and (B2) is a copolymer of (P2-B1) and (P2-B2).

[0021]

Chemical formula

[0022] From the viewpoint of enhancing solubility, the copolymer of (P2-B1) and (P2-B2) more preferably has a tetravalent organic group X having a tetravalent organic group represented by (A6). This polyimide is a copolymer of the repeating units (P2-A6 / B1) and (P2-A6 / B2).

[0023]

Chemical formula

Chemical formula

[0024] It is preferable that (P2) has at least one selected from the group consisting of a tetravalent organic group represented by (A1) and divalent organic groups represented by (B3) and (B4). The repeating units of these polyimides can be represented by the following formulas (P2-A1), (P2-B3), and (P2-B4), respectively.

[0025]

Chemical formula

Chemical formula

Chemical formula

[0026] Although not wishing to be bound by theory, it is considered that since the polyimide in the present embodiment has a structure represented by the formula (A1) or (B1) to (B4), steric hindrance groups such as a sulfonyl group or a fluoroalkyl group suppress the formation of a CT (Charge Transfer) complex of the polyimide, making it easier to ensure transparency.

[0027] From the viewpoint of ensuring the transparency of the film, the polyimide in the present embodiment preferably has at least one selected from the group consisting of a tetravalent organic group represented by the formulas (A2), (A3), (A4) and a divalent organic group represented by the formula (B5). The repeating units of these polyimides can be represented by, for example, the following formulas (P2 - A2), (P2 - A3), (P2 - A4), (P2 - B5 - 1), (P2 - B5 - 2).

[0028] [Chemical formula] [Chemical formula] [Chemical formula] [Chemical formula] [Chemical formula]

[0029] Although not wishing to be bound by theory, since the polyimide has a structure represented by the alicyclic formulas (A2) to (A4) or (B5), the aromatic π electrons are reduced, so it is considered that the formation of a CT (Charge Transfer) complex of the polyimide is suppressed, making it easier to ensure transparency.

[0030] The polyimide of the present embodiment can be synthesized from a tetracarboxylic dianhydride and a diamine as raw materials by a conventionally known method. The polyimide of this embodiment can use conventionally known compounds. As raw materials for the polyimide, for example, the following Component A and Component B can be used. In that case, X corresponds to a tetravalent organic group represented by Formulae (A1) to (A6) and is derived from an acid dianhydride. Also, Y corresponds to a divalent organic group represented by Formulae (B1) to (B5) and is derived from a diamine.

[0031] (A) Tetracarboxylic dianhydride For example, the tetravalent organic group represented by Formula (A1) is derived from 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (abbreviation: 6FDA), the tetravalent organic group represented by Formula (A2) is derived from 1,2,4,5-cyclohexanetetracarboxylic dianhydride (abbreviation: HPMDA), the tetravalent organic group represented by Formula (A3) is derived from bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride (abbreviation: BTA), the tetravalent organic group represented by Formula (A4) is derived from bicyclo[2,2,1]heptane-2,3,5,6-tetracarboxylic dianhydride (abbreviation: NBDAn), the tetravalent organic group represented by Formula (A5) is derived from 1,3,3a,4,5,9b-hexahydro-5-(tetrahydro-2,5-dioxo-3-furanyl)naphtho[1,2-c]furan-1,3-dione (hereinafter also referred to as TDA), and the tetravalent organic group represented by Formula (A6) is derived from 4,4'-oxydiphthalic anhydride (abbreviation: ODPA). In addition to these, the following tetracarboxylic dianhydrides may be used in combination. For example, pyromellitic dianhydride, 3,3’,4,4’-biphenyltetracarboxylic dianhydride, 2,3,3’,4’-biphenyltetracarboxylic dianhydride, 3,3’,4,4’-benzophenonetetracarboxylic dianhydride, 2,3,3’,4’-benzophenonetetracarboxylic dianhydride, 3,3’,4,4’-biphenyl ether tetracarboxylic dianhydride, 2,3,3’,4’-biphenyl ether tetracarboxylic dianhydride, 3,3’,4,4’-biphenylsulfone tetracarboxylic dianhydride, 2,2’-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride, 2,2’-bis(3,4-dicarboxyphenyl)propane dianhydride, 1,4,5,8-naphthalenetetracarboxylic dianhydride, 2,3,6,7-naphthalenetetracarboxylic dianhydride, hydroquinone-bis(trimellitate anhydride), 1,2,3,4-cyclobutanetetracarboxylic dianhydride, 1,2,3,4-cyclopentanetetracarboxylic dianhydride, 1-carboxymethyl-2,3,5-cyclopentanetricarboxylic acid-2,6:3,5-dianhydride, cyclobutane-1,2,3,4-tetracarboxylic dianhydride, cyclohexane-1,2,3,4-tetracarboxylic dianhydride, and the like.

[0032] (B) Diamine For example, the divalent organic group represented by formula (B1) is derived from 3,3'-diaminodiphenyl sulfone (abbreviation: 3DAS), the divalent organic group represented by formula (B2) is derived from 4,4'-diaminodiphenyl sulfone (abbreviation: 4DAS), the divalent organic group represented by formula (B3) is derived from 2,2'-bis(trifluoromethyl)benzidine (abbreviation: TFMB or TFDB), the divalent organic group represented by formula (B4) is derived from 2,2-bis(4-aminophenyl)hexafluoropropane (abbreviation: 6FDAm), and the divalent organic group represented by formula (B5) is derived from 1,4-cyclohexanediamine (trans form, cis form, or cis-, trans-mixture) (abbreviation: CHDA) or 1,4-bisaminomethylcyclohexane (trans form, cis form, or cis-, trans-mixture) (abbreviation: 14BAC). In addition to these, the following diamines may be used in combination. For example, p-phenylenediamine, m-phenylenediamine, 2,4-diaminotoluene, 2,5-diaminotoluene, 2,4-diaminoxylene, 2,5-diaminoxylene, 2,5-diaminodurene, 2,5-dihydroxy-1,4-phenylenediamine, 4,4'-diaminobenzophenone, 3,3'-diaminobenzophenone, 4,4'-diaminobiphenyl, o-tolidine, m-tolidine, 3,3'-dihydroxy-4,4'-diaminobiphenyl, 3,3'-dimethoxy-4,4'-diaminobiphenyl, hexamethylenediamine, 1,3-diaminocyclohexane, 4,4'-diaminodicyclohexane, 3,3'-diaminodicyclohexane, 5-amino-2-(4-aminophenyl)-benzimidazole (abbreviation: ABI), 4,4'-diaminobenzanilide (abbreviation: DABA), 4,4'-bis(4-aminophenoxy)biphenyl (abbreviation: BAPB), α,α'-bis(4-aminophenyl)-1,4-diisopropylbenzene (abbreviation: BADB), 9,9-bis(4-aminophenyl)fluorene, 9,9-bis(4-aminophenoxyphenyl)fluorene, etc. may be mentioned.

[0033] Molecular weight of polyimide The molecular weight of the polyimide is 15,000 or more and less than 100,000 in terms of the number average molecular weight in terms of polystyrene (hereinafter also referred to as Mn). From the viewpoints of solubility in the varnish solvent and the toughness of the film when formed into a film, it is preferably 20,000 or more and less than 50,000. In order to adjust the Mn of the polyimide to 15,000 or more and less than 100,000, n in (P1) and (P2) is preferably 20 or more and less than 200, more preferably 40 or more and less than 100, although it depends on the molecular weight of the repeating unit.

[0034] The number average molecular weight (Mn) of the polyimide can be measured by gel permeation chromatography (GPC) together with the weight average molecular weight (hereinafter also referred to as Mw). As the GPC measurement method, N,N-dimethylformamide is used as the eluent, and 24.8 mol / L lithium bromide monohydrate and 63.2 mol / L phosphoric acid are added to the eluent to suppress the association of polymers during the measurement. In this specification, the calibration curve for calculating the number average molecular weight (Mn) and the weight average molecular weight (Mw) is prepared using standard polystyrene (manufactured by Agilent Technologies, product number EasiCal PS-1). By adjusting the Mn of the polyimide, the viscosity of the varnish can be adjusted to a range suitable for film formation. By controlling the viscosity of the varnish within the range of 3,000 mPa·s or more and 300,000 mPa·s or less, phenomena such as dripping, orange peel, and foaming can be suppressed, and a transparent film with a smooth surface can be obtained. The varnish viscosity is measured with an E-type viscometer at 23°C. From the viewpoint of viscosity suitable for coating, the number average molecular weight (Mn) of the polyimide is preferably 20,000 or more and less than 50,000. Since the viscosity of the varnish has a high correlation with the weight average molecular weight (Mw), the preferred Mw is 40,000 or more and less than 100,000.

[0035] 〈(b): Solvent〉 The polyimide varnish of this embodiment contains (b) a mixed solvent containing two or more kinds of solvents. As the (b) mixed solvent, two or more known solvents capable of dissolving polyimide may be used. Examples of the solvent include amide solvents, cyclic esters, solvents containing an ester group, an ether group, a ketone group, a hydroxyl group, a sulfone group, and a sulfinyl group. Examples of the amide solvent include N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), and the like.

[0036] Examples of the cyclic ester include lactone solvents such as γ-butyrolactone (abbreviation, GBL), δ-valerolactone, ε-caprolactone, γ-crotonolactone, γ-hexanolactone, α-methyl-γ-butyrolactone, γ-valerolactone, α-acetyl-γ-butyrolactone, δ-hexanolactone, and the like. Examples of the solvent having an ester group include ester solvents such as methyl acetate, ethyl acetate, butyl acetate, dimethyl carbonate, and the like. Examples of the solvent having a ketone group include ketone solvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, cyclopentanone, and the like. Examples of the solvent having a hydroxyl group include phenol solvents such as m-cresol and the like. Examples of the solvent having a sulfone group include methyl sulfone, ethyl phenyl sulfone, diethyl sulfone, diphenyl sulfone, sulfolane, bisphenol S, sorapson, dapsone, bisphenol A polysulfone, sulfolane, and the like. Examples of the solvent having a sulfinyl group include sulfoxide solvents such as N,N-dimethyl sulfoxide (DMSO) and the like.

[0037] Among these, from the viewpoints of suppressing yellowing of the polyimide film, imidization at a relatively low temperature of about 220°C or lower, imidization in air, etc., cyclic ester solvents are preferred, and among them, γ-butyrolactone (abbreviation, GBL), which can suppress yellowing even in film formation in air, is preferred.

[0038] The varnish of this embodiment is used by combining a high-boiling-point solvent with a low-boiling-point solvent having a high vapor pressure in a highly soluble solvent. Examples of the highly soluble solvent include cyclic esters and amide solvents. For example, γ-butyrolactone (abbreviation, GBL), N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc). Examples of the low-boiling-point solvent include ethyl acetate, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, cyclopentanone, tetrahydrofuran, dichloroethane, chloroform, etc., which have a boiling point of 160°C or lower and can maintain solubility. Among them, cyclic ketones are preferred from the viewpoint of the solubility of polyimide, and among them, cyclohexanone and cyclopentanone are particularly preferred. As a combination of a highly soluble solvent and a low-boiling-point solvent, for example, a combination of a cyclic ester and a cyclic ketone is preferred, and among them, a combination of γ-butyrolactone (abbreviation, GBL) and cyclohexanone is preferred from the viewpoints of the viscosity of the varnish and the solubility of the polyimide. Based on the total mass of (b1) cyclic ester and (b2) cyclic ketone, it is more preferable that the amount of (b1) cyclic ester is 20% by mass or more and 90% by mass or less from the viewpoint of ease of drying when forming a film.

[0039] 〈Solubility parameter〉 The Hansen solubility parameter (HSP) and the dispersion term δd, dipole term δp, and hydrogen bond term δh can be calculated using software for calculating the Hansen solubility parameter, HSPiP (Hansen Solubility Parameter in Practice) (manufactured by HSP Science). The parameters of the mixed solvent were calculated by the following calculation formula from the volume ratio of each solvent.

Number

[0040] Conventionally, when the dispersion term δd, dipole term δp, and hydrogen bond term δh of the Hansen solubility parameter (HSP) are plotted in three-dimensional space, it is well known that the polymer to be dissolved and the solvent are in close proximity to each other. As a result of intensive studies, the inventors have surprisingly found that in the polyimide and solvent of the present embodiment, among the three terms of the Hansen solubility parameter (HSP), namely the dispersion term δd, dipole term δp, and hydrogen bond term δh, when two of the dispersion term δd and hydrogen bond term δh are within a specific distance, a polyimide varnish having good solubility, rapid drying property, and improved film-forming property can be obtained. Being within a specific distance can be specifically represented by the following mathematical formula 1, and its value is 8.5 or less. More preferably, it can be represented by the following mathematical formula 2, and its value is 4.7 or less.

Number

Equation

[0041] Even more surprisingly, the (b) mixed solvent of this embodiment has a dispersion term (δd solv ), a dipole term (δp solv ), and a hydrogen bond term (δh solv ) that satisfy the following: Dispersion term (δd solv ) ≥ 16.8 Dipole term (δp solv ) ≥ 11.5 Hydrogen bond term (δh solv ) ≥ 6.2 and it is preferable that the Hansen solubility parameter (HSP) is 22.5 or more and 26.7 or less. As a result, a polyimide varnish having better solubility and more improved quick-drying property and film-forming property tends to be obtained.

[0042] 〈Method for producing polyimide varnish〉 The method for producing a polyimide varnish according to this embodiment includes dissolving the diamine and tetracarboxylic dianhydride described above in a mixed solvent containing two or more solvents, and (b) generating (a) polyimide in the mixed solvent. At this time, the two or more solvents are such that in each parameter represented by the dispersion term δd, dipole term δp, and hydrogen bond term δh of the Hansen solubility parameter (HSP), the dispersion term and hydrogen bond term of the polyimide and the mixed solvent satisfy the following mathematical formula 1:

Number

[0043] 〈Light transmittance of polyimide varnish〉 The light transmittance of the varnish of this embodiment is 55% or more. In the present specification, the "light transmittance" of the varnish is the light transmittance measured at a polyimide concentration of 20% by mass, an optical path length of 10 mm (1.0 cm), and a wavelength of 450 nm. More specifically, in ultraviolet-visible absorption spectrum measurement, a solvent with a light transmittance of 90% or more at a wavelength of 450 nm is used as the solvent, the polyimide concentration of the varnish is adjusted to 20% by mass, the optical path length is 10 mm (1.0 cm), and the measurement is performed at a measurement temperature of 23°C. The solvent with a light transmittance of 90% or more at a wavelength of 450 nm is not particularly limited, but γ-butyrolactone (abbreviation, GBL), N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), cyclohexanone, and cyclopentanone can be used. When diluting the varnish to adjust the solid content concentration to 20% by mass, it may contain the solvent constituting the varnish. When concentrating the varnish to adjust the solid content concentration to 20% by mass, the solvent constituting the varnish may be used as it is. A polyimide film obtained from a varnish having a light transmittance of 55% or more at a wavelength of 450 nm tends to have a low yellowness index (hereinafter also referred to as YI).

[0044] Although not wishing to be bound by theory, the light absorption derived from the solvent or the primary structure of the polyimide is mainly seen in the wavelength range of 300 nm or less. Considering this, the absorption in the visible light region that reduces the transparency of the film is presumably due to the red shift of the absorption wavelength due to the exciton interaction of the polyimide, the higher-order structure derived from the formation of the CT (Charge Transfer) complex of the polyimide, or the yellowing component due to side reactions or impurities. That is, the main cause of the coloring after film formation is considered to be latent in the state of the varnish diluted with the solvent. Therefore, when the light transmittance of the varnish at a wavelength of 450 nm is less than 55%, it is considered that the yellowness index (yellowness index, hereinafter also referred to as YI) of the film deteriorates. The higher the light transmittance of the varnish at a wavelength of 450 nm, the more preferable it is. For example, 55% or more is preferable, and 60% or more is more preferable. The upper limit of the light transmittance of the varnish at a wavelength of 450 nm is not particularly limited, but it is 100% or less, and it may be 99% or less, 98% or less, 95% or less, or 90% or less.

[0045] 〈Polyimide film〉 The polyimide film of this embodiment is a polyimide film obtained by drying the varnish of this embodiment. The polyimide film of this embodiment is preferably a colorless and transparent film. In the present specification, the "colorless and transparent film" refers to a film having a light transmittance of 80% or more at a wavelength of 450 nm, a haze of 2 or less, and a yellowness index (YI) of 5.0 or less. Therefore, the polyimide film of this embodiment can be suitably used for optical applications such as touch panels and displays.

[0046] The polyimide film according to this embodiment may be, for example, a polyimide film formed on the surface of a support, or a self-supporting film (self-standing film) without a support. From the perspective of applying it to the processing process as a roll film, it is preferably a self-supporting film. The polyimide film according to this embodiment can be used as a substitute for glass, similar to a PET film or a COP film, and can further be used for a foldable display or a display that follows a curved surface.

[0047] The yellowness index (YI) of the polyimide film according to this embodiment is preferably 5.0 or less, more preferably 3.0 or less, and even more preferably 2.0 or less. In that case, the haze of the polyimide film is preferably 2 or less, and more preferably 1 or less.

[0048] In this embodiment, the polyimide film is a film containing polyimide as a main component. In this specification, that the film "contains polyimide as a main component" means that it contains polyimide in an amount of 50% by mass or more based on the total mass of the film. The polyimide film preferably contains polyimide in an amount of 60% by mass or more, more preferably 70% by mass or more, and even more preferably 80% by mass or more based on the total mass of the film.

[0049] The film thickness of the polyimide film is preferably in the range of 1 μm to 100 μm, and more preferably in the range of 20 μm to 50 μm from the perspective of achieving both easy handling and flexibility of the film.

[0050] The polyimide film of this embodiment preferably contains 5 to 40 parts by mass of a solvent based on 100 parts by mass of the polyimide. As the solvent contained in the polyimide film, the solvents described in the column of <(b): Solvent> above can be used. The solvent preferably contains (b1) a cyclic ester and (b2) a cyclic ketone, and more preferably, (b1) the cyclic ester is γ-butyrolactone and (b2) the cyclic ketone is cyclopentanone. It is preferable that γ-butyrolactone is contained in the polyimide film in an amount of 0.01 to 10 parts by weight based on 100 parts by weight of the polyimide. The content of γ-butyrolactone is 0.01 part by weight or more from the viewpoint of suppressing the yellowing of the film and obtaining a colorless and transparent film, and is 10 parts by weight or less from the viewpoint of suppressing the stacking due to film roll or surface stickiness. The γ-butyrolactone in the polyimide film can be derived from the (b) solvent of the varnish according to this embodiment.

[0051] The varnish of this embodiment optionally contains additives. The additives are, for example, a leveling agent, a dispersant or a surfactant for improving the coatability of the film, a surfactant or an adhesion promoter for adjusting the peelability or adhesiveness from the film support, a flame retardant for imparting flame retardancy to the film, inorganic particles other than silica such as strontium carbonate, organic compounds such as polystyrene, polyvinyl naphthalene, polymethyl methacrylate, cellulose triacetate, fluorene derivatives, antioxidants, ultraviolet absorbers, light stabilizers, plasticizers, waxes, fillers, pigments, dyes, foaming agents, defoaming agents, dehydrating agents, antistatic agents, antibacterial agents, antifungal agents, etc. are preferable. Further, the polyimide film may contain silica particles and / or additives.

[0052] As the leveling agent, for example, a copolymer of ethylene oxide and polydimethylsiloxane is preferable from the viewpoint of the transparency of the film.

[0053] As an additive, for example, a single additive such as a phosphate ester compound having a plasticizing effect and a flame retardant effect may be used for multiple purposes. Although the uses are not limited, examples of the compound structure include phosphite compounds, phenolic compounds, thioether compounds, hydrazine compounds, amide compounds, benzotriazole compounds, triazine compounds, isocyanuric acid compounds, hindered amine compounds, hindered phenol compounds, phosphate ester compounds, phosphazene compounds, urethane compounds, acrylic compounds, methacrylic compounds, epoxy compounds, isocyanate compounds, and the like may be added. In addition, the polyimide varnish of the present embodiment preferably contains at least one compound selected from the group consisting of monovalent primary amines, monovalent alcohols, isocyanates, acid dianhydrides, and dicarboxylic acids for the purpose of stabilizing the viscosity of the varnish. The amount of these compounds is preferably 100 ppm or more and 4,000 ppm or less with respect to the mass of the polyimide, and more preferably 500 ppm or more and 2,000 ppm or less from the viewpoint of achieving both the storage stability of the varnish and the strength of the film after film formation. As the additive, from the viewpoint of having no absorption in visible light and being able to suppress the coloring of the varnish among monovalent primary amines, at least one selected from the group consisting of aniline, benzylamine, and cyclohexylamine is preferable. Note that the additive added to the varnish may remain in the polyimide film.

[0054] <Manufacturing method of polyimide film> The manufacturing method of the polyimide film according to the present embodiment is a step of casting the varnish of the present embodiment onto a substrate, and a step of drying the cast varnish to form a self-supporting film, and a step of peeling the self-supporting film from the support and further heating it, and can include.

[0055] The casting step can be carried out using various coating devices according to the use of the film. For example, known coating methods such as spin coating, slit coating, comma coating, die coating, and blade coating may be used.

[0056] Examples of the support used include a rigid substrate, a metal drum, a metal endless belt, a plastic film substrate, a metal foil, and flexible glass. Examples of the rigid substrate include a glass substrate such as an alkali glass substrate and a non-alkali glass substrate (Eagle XG (registered trademark), manufactured by Corning), and a metal substrate such as a copper substrate, an aluminum substrate, and a SUS substrate. Examples of the plastic film substrate include an Upilex (registered trademark) film (manufactured by Ube Industries), a Kapton (registered trademark) film (manufactured by Toray DuPont), a polycarbonate film, a PEN film, and a PET film. Examples of the metal foil include a copper foil, an aluminum foil, and a SUS foil.

[0057] The step of drying the cast varnish can be carried out in two stages: primary drying and secondary drying. Between the primary drying and the secondary drying, the self-supporting film can be peeled off from the support to obtain a self-supporting film. The peeled self-supporting film can be further heated as the secondary drying.

[0058] In the primary drying, for example, at a temperature of 50 to 150°C, the solvent is dried from the varnish containing polyimide to form a non-sticky and self-supporting film (self-supporting film). The amount of the solvent in the self-supporting film at that time is preferably 5% to 40%, more preferably 10% to 20% based on 100 parts by weight of polyimide.

[0059] After obtaining the self-supporting film, the self-supporting film can be peeled off from the support. The peeled self-supporting film is conveyed by a pin tenter, a clip tenter, etc., and further heated as a secondary drying to dry the solvent, thereby obtaining a polyimide film. The amount of the solvent in the finally obtained polyimide film is preferably from 0.01% to 10%, more preferably from 0.1% to 5%.

[0060] Although not wishing to be bound by theory, when the drying process is carried out in an inert gas atmosphere such as nitrogen gas, a colorless and transparent polyimide film tends to be obtained regardless of the type of (b) solvent contained in the varnish. On the other hand, when the (b) solvent contained in the varnish is γ-butyrolactone (GBL), it is preferable to carry out the drying process in air from the viewpoints of economy and the colorless transparency of the film.

[0061] In this embodiment, the obtained polyimide film can be subjected to a stretching process to thin the film. For example, in order to obtain a film having a thickness of less than 10 μm, it can be produced by stretching a polyimide film having a film thickness of 10 μm or more.

[0062] 〈Light-emitting device, curved display, and foldable display〉 Using the polyimide film (polyimide layer) obtained by drying and imidizing the varnish of this embodiment, devices such as a light-emitting device, a curved display, and / or a foldable display can be manufactured. The polyimide film of this embodiment can be used, for example, as at least one layer of a film substrate and, for example, as a part of a light-emitting portion, a curved surface, a display portion, etc. Examples of the foldable display include an organic EL display, for example, a top emission type organic EL display; and a flexible liquid crystal display, etc.

[0063] In another embodiment, a functional layer, for example, a transparent electrode layer can be provided on the surface of the polyimide film of this embodiment to manufacture a laminate. The laminate can be obtained by forming, for example, a transparent electrode layer on the surface of the polyimide film with a sputtering apparatus. The polyimide film may have a support or may be a single layer without a support. The laminate may have transparent electrode layers on both sides of the polyimide film. At this time, it is preferable that at least one or more transparent electrode layers are provided on each of the two sides of the polyimide film. Further, the laminate of the transparent electrode layer and the polyimide film may further have other layers, such as an undercoat layer for imparting smoothness, a hard coat layer for imparting surface hardness, an index matching layer for improving visibility, and a gas barrier layer for imparting gas barrier properties. The hard coat layer for imparting surface hardness, the index matching layer for improving visibility, etc. may be laminated on the transparent electrode layer and the polyimide film. The laminate of this embodiment is suitable for use in touch panel materials such as transparent electrode films.

Examples

[0064] Hereinafter, the present invention will be described in more detail based on examples, but these are described for the purpose of explanation and the scope of the present invention is not limited to the following examples. Various evaluations in the examples were conducted as follows.

[0065] 《Evaluation and Measurement Methods》 [Number average molecular weight (Mn) and weight average molecular weight (Mw)] (a) The number average molecular weight (Mn) and weight average molecular weight (Mw) of the polyimide were measured by gel permeation chromatography (GPC) under the following conditions. As the solvent, N,N-dimethylformamide (manufactured by Wako Pure Chemical Industries, Ltd., for high performance liquid chromatography) was used, and a solution obtained by adding 24.8 mol / L of lithium bromide monohydrate (manufactured by Wako Pure Chemical Industries, Ltd., purity 99.5%) and 63.2 mol / L of phosphoric acid (manufactured by Wako Pure Chemical Industries, Ltd., for high performance liquid chromatography) before measurement was used. Also, a calibration curve for calculating the number average molecular weight (Mn) and weight average molecular weight (Mw) was prepared using standard polystyrene (manufactured by Agilent Technologies, product number EasiCal PS-1). Column: TSK-GEL SUPER HM-H × 2 columns Flow rate: 0.5 mL / min Column temperature: 40 °C Pump: PU-2080 (manufactured by JASCO) Detector: RI-2031Plus (RI: differential refractometer, manufactured by JASCO) UV-2075Plus (UV-Vis: ultraviolet-visible spectrophotometer, manufactured by JASCO)

[0066] [Light transmittance] The light transmittance of the polyimide varnish was measured under the following conditions after adjusting the polyimide concentration to 20% by mass and then measuring the ultraviolet-visible absorption spectrum. As the apparatus, a UV / VIS SPECTROPHOTOMETER (V-550, manufactured by JASCO) was used. For the background, quartz cells with an optical path length of 10 mm filled with the same solvent as the varnish in both the reference chamber and the sample chamber were placed, and the measurement was carried out in a constant temperature chamber at 23 °C. From the obtained data, the light transmittance at an optical path length of 10 mm (1.0 cm) and a wavelength of 450 nm was obtained. Apparatus: UV / VIS SPECTROPHOTOMETER (V-550, manufactured by JASCO) Quartz cell size: Optical path length 10 mm × width 10 mm × height 400 mm Measurement wavelength: 300 nm - 800 nm Band width: 2.0 nm Scanning speed: 200 nm / min The obtained light transmittance was ranked according to the following criteria. (Light transmittance rank) 〇 (Good): The light transmittance at a wavelength of 450 nm is 55% or more. × (Bad): The light transmittance at a wavelength of 450 nm is less than 55% (including the decrease in light transmittance due to cloudiness caused by decreased solubility).

[0067] [Hansen solubility parameter] The Hansen solubility parameter (HSP), dispersion term δd, dipole term δp, and hydrogen bond term δh were calculated using software for calculating the Hansen solubility parameter, HSPiP (Hansen Solubility Parameter in Practice) (manufactured by HSP Science). The parameters of the mixed solvent were calculated from the volume ratio of each solvent. Requirement 1: 〇 if Equation 1 is satisfied, and × if not. The calculated values were also listed in the column of Requirement 1 in Table 2. Requirement 2: 〇 if the dispersion term (δd solv ) ≥ 16.8, the dipole term (δp solv ) ≥ 11.5, and the hydrogen bond term (δh solv ) ≥ 6.2 are all satisfied, and × if any one of them is not satisfied. Requirement 3: 〇 if the Hansen solubility parameter (HSP) satisfies 22.5 or more and 26.7 or less, and × if not.

[0068] [Preparation of polyimide film, and evaluation of solvent drying property and film self-supporting property] The polyimide varnish was cast on the untreated surface of a PET film (Cosmo Shine A4100 (registered trademark) (manufactured by Toyobo Co., Ltd.)) as a substrate using an applicator with a coating thickness of 200 μm. The cast varnish was dried at 80 °C for 10 minutes. Then the substrate was peeled off to obtain a self-supporting film. Here, "sticky on the surface" means the case where the drying rate of the solvent calculated from the following formula is 80% or less. Drying rate (%) = {(film weight before drying) - (film weight after drying)} / weight of the solvent contained in the film before drying × 100 The self-standing film was cut into a 10 cm square, and two opposite sides of the self-standing film were fixed to a metal frame having a 10 cm opening with Kapton (registered trademark) tape. The self-standing film fixed to the metal frame was dried at 270 °C for 20 minutes (secondary drying). The secondary drying was carried out under air or nitrogen as shown in Table 3. Then, it was cooled to room temperature and removed from the metal frame to obtain a polyimide film. (Solvent Drying Property Evaluation) 〇: No stickiness on the surface after drying at 80 °C for 10 minutes ×: Stickiness on the surface after drying at 80 °C for 10 minutes (Film Self-Supporting Property Evaluation) 〇: A self-standing film was obtained by peeling from the substrate ×: The film was broken when peeled from the substrate

[0069] [Yellowness Index (YI) and Haze] A self-standing film was prepared by the method described in "Preparation of Polyimide Film" above so that the film thickness after drying was 20 ± 1 μm. The yellowness index (YI value) and haze of the self-standing film were measured using a spectrophotometer (CM3600A) manufactured by Konica Minolta, Inc. and a D65 light source. The obtained yellowness index (YI) and haze were ranked according to the following criteria. (YI Rank) ◎(Very Good): The YI of the film is 3 or less. 〇(Good): The YI of the film is greater than 3 and 5 or less. ×(Poor): The YI of the film exceeds 5. (Haze Rank) 〇(Good): The haze is 1% or less. ×(Poor): The haze exceeds 1%.

[0070] 《Raw Materials》 〈Tetracarboxylic Dianhydride〉 6FDA: 4,4'-(Hexafluoroisopropylidene)diphthalic Anhydride (corresponding to A1) HPMDA: Cyclohexane-1,2,4,5-tetracarboxylic Dianhydride (corresponding to A2) BTA: Bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride (corresponding to A3) ODPA: Oxydiphthalic dianhydride (corresponding to A6) BPDA: Biphenyl-3,4,3’,4’-tetracarboxylic dianhydride

[0071] 〈Diamine〉 3DAS: 3,3’-Diaminodiphenyl sulfone (corresponding to B1) 4DAS: 4,4’-Diaminodiphenyl sulfone (corresponding to B2) TFMB: 2,2’-Bis(trifluoromethyl)benzidine (corresponding to B3) 14BAC: 1,4-Bisaminomethylcyclohexane (corresponding to B6) BAPB: 4,4’-Bis(4-aminophenoxy)biphenyl BADB: α,α’-Bis(4-aminophenyl)-1,4-diisopropylbenzene

[0072] 〈Solvent〉 GBL: γ-Butyrolactone MNP: N-Methyl-2-pyrrolidone CPN: Cyclopentanone 〈Imidization catalyst〉 Py: Pyridine The Py used in the synthesis was confirmed to have a light transmittance of 90% or more at an optical path length of 1 cm and a wavelength of 350 nm in advance and was used. Also, for ODPA, 3DAS, and 4DAS, they were dissolved in GBL in advance to form 1%, 10%, and 10% solutions respectively, and it was confirmed that their light transmittances at an optical path length of 1 cm and a wavelength of 450 nm were 97% or more, and they were used.

[0073] 《Preparation of polyimide varnish》 While introducing nitrogen gas into a 500 mL separable flask with a stir bar equipped with a Dean-Stark tube and a reflux tube at the top, as shown in Table 1, diamine was first dissolved in Solvent 1 at 60 °C. Next, dianhydride was added in powder form and stirred for 30 minutes. Then, Py (4 mol% based on the charged dianhydride) was added, and the temperature in the system was raised to 165 °C using an oil bath and stirred appropriately for 30 minutes to 3 hours. After the imidization rate calculated from the recovered amount of water reached 50% or more, the temperature was raised to 180 °C, nitrogen bubbling was performed, and the reaction was continued for 8 to 12 hours until the residual amount of Py was 0.01% or less and the imidization rate calculated from the recovered amount of water exceeded 90%. Then, the oil bath was removed and the temperature in the system was returned to room temperature. Solvent 1 and Solvent 2 were added as shown in Table 1 so that the solid content concentration became 20% by mass, and a polyimide varnish was obtained.

[0074] The composition of the polyimide and the varnish is shown in Table 1, and the varnish evaluation results and the film evaluation results are shown in Table 2.

[0075]

Table 1

[0076]

Table 2

Industrial Applicability

[0077] The varnish of this embodiment can be used in the production of polyimide films used as surface protection films, color filters, substrate films such as TFTs, and insulating protection films. The polyimide film obtained by drying the varnish in this embodiment can be suitably used in products such as flexible optical devices, optical devices having curved surfaces, foldable optical devices, for example, flexible displays, flexible solar cells, flexible touch panels, flexible lighting, curved displays, and foldable smartphones.

Claims

1. (a) a polyimide, and (b) a mixed solvent containing two or more solvents, a polyimide varnish, characterized in that In each parameter represented by the dispersion term δd, dipole term δp, and hydrogen bond term δh of the Hansen solubility parameter (HSP), the dispersion term and hydrogen bond term of the polyimide and the mixed solvent satisfy the following formula 1: 【Number 1】 {In Equation 1, δd ＰＩ is the dispersion term of the polyimide, δh ＰＩ is the hydrogen bonding term of the polyimide, δd ｓｏｌｖ is the dispersion term of the mixed solvent, and δh ｓｏｌｖ is the hydrogen bonding term of the mixed solvent.} satisfy, the (b) mixed solvent contains an amide solvent or a cyclic ester and a cyclic ketone, The dispersion term (δd ｓｏｌｖ ) of the mixed solvent, the dipole term (δp ｓｏｌｖ ) and the hydrogen bond term (δh ｓｏｌｖ ) are as follows: Dispersion term (δd ｓｏｌｖ ) ≥ 16.8 Dipole term (δp ｓｏｌｖ ) ≥ 11.5 Hydrogen bonding term (δh ｓｏｌｖ ) ≧ 6.2 satisfy, and the Hansen solubility parameter (HSP) is 22.5 or more and 26.7 or less, when the polyimide concentration of the polyimide varnish is 20% by mass and measured at an optical path length of 10 mm and a wavelength of 450 nm, the light transmittance of the polyimide varnish is 55% or more, however, the polyimide is a polyimide varnish excluding a polyimide having 2,2'-bis(trifluoromethyl)benzidine (TFMB) as a diamine and 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) as a tetracarboxylic dianhydride as constituent units.

2. The polyimide varnish according to claim 1, wherein the cyclic ester is γ-butyrolactone.

3. The polyimide varnish according to claim 1 or 2, wherein the (a) polyimide contains a divalent organic group represented by the following formula (B1). 【Chemical 1】

4. The polyimide varnish according to claim 3, wherein the (a) polyimide further contains a divalent organic group represented by the following formula (B2). 【Chemical Formula 2】

5. The polyimide varnish according to claim 3 or 4, wherein the (a) polyimide further contains a tetravalent organic group represented by the following formula (A6). [Chemical Formula 3]

6. The polyimide varnish according to claim 1 or 2, wherein the (a) polyimide contains at least one selected from the group consisting of a tetravalent organic group represented by the following formula (A1) and divalent organic groups represented by the following formulas (B3) and (B4). 【Chemical 4】 【Chemical Formula 5】 【Chemical Formula 6】

7. The polyimide varnish according to claim 1 or 2, wherein the (a) polyimide contains at least one selected from the group consisting of tetravalent organic groups represented by the following formulas (A2), (A3), and (A4) and a divalent organic group represented by the following formula (B5). [Chemical Formula 7] 【Chemical 8】 【Chemical Formula 9】 【Chemical 10】

8. A step of casting the polyimide varnish according to any one of claims 1 to 7 onto a substrate, a step of drying the cast polyimide varnish to form a self-supporting film, a step of peeling the self-supporting film from the substrate and further heating. A method for producing a polyimide film, comprising

9. A polyimide film obtained by drying the polyimide varnish according to any one of Claims 1 to 7.

10. Containing 5 to 40 parts by mass of a solvent based on 100 parts by mass of the polyimide, The polyimide film according to Claim 9, wherein the solvent contains the cyclic ester and the cyclic ketone.

11. The polyimide film according to Claim 10, wherein the cyclic ester is γ-butyrolactone and the cyclic ketone is cyclopentanone.

12. A light-emitting device having a polyimide film obtained by drying the varnish according to any one of Claims 1 to 7.

13. A curved display having a polyimide film obtained by drying the varnish according to any one of Claims 1 to 7.

14. A foldable display having a polyimide film obtained by drying the varnish according to any one of Claims 1 to 7.

15. A method for producing the polyimide varnish, comprising dissolving a diamine and a tetracarboxylic dianhydride in a mixed solvent containing two or more solvents to form a polyimide in the mixed solvent, and obtaining a polyimide varnish containing the polyimide and the mixed solvent, wherein in each parameter represented by the dispersion term δd, dipole term δp, and hydrogen bond term δh of the Hansen solubility parameter (HSP) of the two or more solvents, the dispersion term and hydrogen bond term of the polyimide and the mixed solvent satisfy the following formula 1: 【Number 2】 {In Equation 1, δd ＰＩ is the dispersion term of the polyimide, δh ＰＩ is the hydrogen bond term of the polyimide, δd ｓｏｌｖ is the dispersion term of the mixed solvent, and δh ｓｏｌｖ is the hydrogen bond term of the mixed solvent.} is selected so as to satisfy, The mixed solvent contains (b1) a cyclic ester and (b2) a cyclic ketone, and based on the total mass of the (b1) cyclic ester and the (b2) cyclic ketone, the amount of the (b1) cyclic ester is 20% by mass or more and 90% by mass or less, A method for producing a polyimide varnish, wherein when the polyimide concentration of the polyimide varnish is 20% by mass and measured at an optical path length of 10 mm and a wavelength of 450 nm, the light transmittance of the polyimide varnish is 55% or more.

16. A method for producing the polyimide varnish, comprising dissolving a diamine and a tetracarboxylic dianhydride in a mixed solvent containing two or more solvents to form a polyimide in the mixed solvent, and obtaining a polyimide varnish containing the polyimide and the mixed solvent, The above two or more solvents are such that, in each parameter represented by the dispersion term δd, dipole term δp, and hydrogen bonding term δh of the Hansen solubility parameter (HSP), the dispersion term and hydrogen bonding term of the polyimide and the mixed solvent satisfy the following mathematical formula 1: 【Number 3】 {In Formula 1, δd ＰＩ is the dispersion term of the polyimide, δh ＰＩ is the hydrogen bonding term of the polyimide, δd ｓｏｌｖ is the dispersion term of the mixed solvent, and δh ｓｏｌｖ is the hydrogen bonding term of the mixed solvent.} is selected so as to satisfy, The mixed solvent contains an amide-based solvent or a cyclic ester and a cyclic ketone, The dispersion term (δd ｓｏｌｖ ) of the mixed solvent, the dipole term (δp ｓｏｌｖ ) and the hydrogen bond term (δh ｓｏｌｖ ) are as follows: Dispersion term (δd ｓｏｌｖ ) ≥ 16.8 Dipole term (δp ｓｏｌｖ ) ≥ 11.5 Hydrogen bonding term (δh ｓｏｌｖ ) ≧ 6.2 and satisfy the Hansen solubility parameter (HSP) is 22.5 or more and 26.7 or less, A method for producing a polyimide varnish, wherein the polyimide concentration of the polyimide varnish is 20% by mass, and when measured at an optical path length of 10 mm and a wavelength of 450 nm, the light transmittance of the polyimide varnish is 55% or more.

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