Polyimide varnish and polyimide film, and method for producing them

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 the same. [Background technology]

[0002] In recent years, development of foldable flexible devices such as flexible displays and curved devices such as organic electroluminescence (EL) lighting and organic electroluminescence (EL) displays has progressed. In such devices, the use of foldable films (plastic films) made of polymeric materials as substrates for forming surface protective layers, color filters, touch panels, thin film transistors (TFTs), etc. has been considered. Among plastic films, polyimide films are particularly excellent in heat resistance, chemical resistance, and the like, and are therefore expected to be used as substrate materials in the aforementioned fields. Aromatic polyimides are colored by charge transfer complexes (CT complexes) between the diamine moiety near the nitrogen of the imide ring and the acid anhydride moiety near the carbonyl group of the imide ring, making them difficult to use for optical applications. In addition, aromatic polyimides are poorly soluble in general-purpose organic solvents and have poor processability, limiting their range of application. Therefore, there is a demand for polyimides that are colorless, transparent, and highly soluble in organic solvents, especially in the aforementioned fields.

[0003] Patent Document 1 aims to provide a polyimide that is colorless and transparent, highly soluble in organic solvents, and does not easily become discolored even at high temperatures. Patent Document 1 describes a method of obtaining a polyimide film 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 thermally imidizing the polyimide. Patent Document 2 aims to provide a polyimide film or the like that contains a polyimide that has good solubility in a solvent and excellent processability, is colorless and transparent, and has excellent toughness. Patent Document 2 describes a method of obtaining a polyimide film by applying a polyimide varnish containing a polyimide synthesized from an aromatic diamine and a tetracarboxylic dianhydride onto a substrate, and then drying the solvent. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2012-72118 A [Patent Document 2] International Publication No. 2016 / 158825 Summary of the Invention [Problem to be solved by the invention]

[0005] However, when a polyimide varnish (also simply called "varnish") is applied to produce a film, in order to complete the film production process in as short a time as possible to increase productivity, the solvent for the polyimide varnish is required to have a fast drying property. 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 all of these solvents have boiling points of 150°C or higher, it has not been possible to sufficiently shorten the drying time of the solvent. It is therefore one object of the present invention to provide a transparent polyimide varnish that has fast drying properties and therefore improved film-forming properties. [Means for solving the problem]

[0006] As a result of intensive research into solving the above problems, the inventors of the present application have found that the above problems can be solved by using a polyimide varnish containing polyimide and two or more solvents, in which each parameter represented by the dispersion term δd, the dipole term δp, and the hydrogen bond term δh of the Hansen Solubility Parameter (HSP) satisfies a specific formula, and a solvent composition, thereby completing the present invention. Examples of embodiments of the present invention are listed in the following items [1] to

[19] . [1] (a) a polyimide; and (b) a mixed solvent containing two or more solvents, In the Hansen solubility parameter (HSP) represented by the dispersion term δd, the dipole term δp, and the hydrogen bond term δh, the dispersion term and the hydrogen bond term of the polyimide and the mixed solvent are expressed by the following formula 1:

number

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

[11] The polyimide varnish according to any one of items 1 to 6, wherein the polyimide (a) 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). [ka] [ka] [ka] [ka]

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

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

[14] The solvent is contained in an amount of 5 parts by mass or more and 40 parts by mass or less based on 100 parts by mass of polyimide, Item 14. The polyimide film according to item 13, wherein the solvent comprises (b1) a cyclic ester and (b2) a cyclic ketone.

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

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

[17] 12. A curved display having a polyimide film on which the varnish according to any one of items 1 to 11 has been dried.

[18] 12. 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, and producing a polyimide in the mixed solvent, The above two or more solvents are such that, in the parameters represented by the dispersion term δd, the dipole term δp, and the hydrogen bond term δh of the Hansen solubility parameters (HSP), the dispersion term and the hydrogen bond term of the polyimide and the mixed solvent satisfy the following formula 1:

number

[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 be noted that the above description should not be considered as a disclosure of all embodiments of the present invention and all advantages related to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] Hereinafter, an embodiment 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 embodiment, and can be practiced with various modifications within the scope of the gist. In this specification, the upper and lower limits of each numerical range can be combined arbitrarily.

[0009] <Polyimide varnish> (a): Polyimide The polyimide varnish of the present embodiment contains (a) a polyimide. The polyimide is preferable from the viewpoint of suppressing a decrease in the molecular weight of the varnish during storage. In this specification, the term "polyamic acid" refers to 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" refers to a polymer having an imidization rate of 20% or more, i.e., a polymer in which 20% or more of the repeating units are represented by the following formula (P2). From the viewpoint of suppressing a decrease in molecular weight during storage of the varnish, the imidization rate in the polyimide is preferably 40% or more, more preferably 60% or more, and even more preferably 80% or more and 100% or less.

[0010] [ka] (In formula (P1), X is a tetravalent organic group, Y is a divalent organic group, and n is a positive integer.)

[0011] [ka] (In formula (P2), X is a tetravalent organic group, Y is a divalent organic group, and n is a positive integer.)

[0012] The polyimide of this embodiment can be synthesized by a conventionally known method using tetracarboxylic dianhydride (denoted as A) and diamine (denoted as B) as raw materials. In this embodiment, the imidization rate is calculated from the amount of water generated when the amic acid closes its ring to form an imide during imidization, using the following formula: The imidization rate (%) is calculated by dividing the number of moles of water by twice the number of moles of the charged A or B, whichever is smaller, and multiplying the result by 100. Imidization rate (%) = water generated (mol) / [A or B, whichever is less (mol) x 2] x 100

[0013] When a hydrocarbon solvent such as toluene that forms an azeotrope with water is added to the synthesis solvent, the amount of water generated during imidization is measured by separating the water layer from the recovered component during reflux and measuring its weight. Also, for example, when the recovered component is not separated, the amount of water in the recovered component is measured with a Karl Fischer moisture meter.

[0014] In the present embodiment, the polyimide is not particularly limited. However, from the viewpoint of improving the transparency when made 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] [ka] [ka] [ka] [ka] [ka] [ka] [ka]

[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 present in the polyimide, the divalent organic groups of (B1) and / or (B2) may be contained in the organic group X.

[0017] From the viewpoint of improving the transparency when made 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). [ka] [ka] [ka] [ka] [ka]

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

[0019] [ka]

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

[0021] [ka]

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

[0023] [ka] [ka]

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

[0025] [ka] [ka] [ka]

[0026] Without wishing to be bound by theory, it is believed that when the polyimide in the present embodiment has a structure represented by formula (A1) or (B1) to (B4), a steric hindrance group such as a sulfonyl group or a fluoroalkyl group suppresses 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 this embodiment preferably has at least one selected from the group consisting of tetravalent organic groups represented by formulae (A2), (A3), and (A4) and divalent organic groups represented by formula (B5). The repeating units of these polyimides can be represented by, for example, the following formulae (P2-A2), (P2-A3), (P2-A4), (P2-B5-1), and (P2-B5-2), respectively.

[0028] [ka] [ka] [ka] [ka] [ka]

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

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

[0031] (A) Tetracarboxylic acid 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), and the tetravalent organic group represented by formula (A4) is derived from 1,2,4,5-cyclohexanetetracarboxylic dianhydride (abbreviation: HPMDA). 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'-biphenyl tetracarboxylic dianhydride, 2,3,3',4'-biphenyl tetracarboxylic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 2,3,3',4'-benzophenone tetracarboxylic dianhydride, 3,3',4,4'-biphenyl ether tetracarboxylic dianhydride, 2,3,3',4'-biphenyl ether tetracarboxylic dianhydride, 3,3',4,4'-biphenyl sulfone tetracarboxylic dianhydride, 2,2'-bis(3,4-dicarboxyphenyl)hexafluoropropanoic dianhydride, 2,2' -bis(3,4-dicarboxyphenyl)propanoic 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-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 isomer, cis isomer, or cis-,trans-mixture) (abbreviation: CHDA) or 1,4-bisaminomethylcyclohexane (trans isomer, cis isomer, 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, and 9,9-bis(4-aminophenoxyphenyl)fluorene.

[0033] Polyimide molecular weight The molecular weight of the polyimide is, in terms of polystyrene equivalent number average molecular weight (hereinafter also referred to as Mn), 15,000 or more and less than 100,000. From the viewpoints of solubility in a varnish solvent and toughness of a film when made 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 this depends on the molecular weight of the repeating unit.

[0034] The number average molecular weight (Mn) of polyimide can be measured together with the weight average molecular weight (hereinafter also referred to as Mw) by gel permeation chromatography (GPC). In the measurement method of GPC, N,N-dimethylformamide is used as a developing solvent, and 24.8 mol / L lithium bromide monohydrate and 63.2 mol / L phosphoric acid are added to the developing solvent to suppress the association of polymers during the measurement. In the present specification, the calibration curve for calculating the number average molecular weight (Mn) and the weight average molecular weight (Mw) was 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 that allows easy film formation. By controlling the viscosity of the varnish within the range of 3,000 mPa·s to 300,000 mPa·s, phenomena such as dripping, orange peel, and foaming can be suppressed, and a transparent film with a smooth surface can be obtained. The viscosity of the varnish is measured at 23°C using an E-type viscometer. 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 is closely correlated 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 includes (b) a mixed solvent containing two or more kinds of solvents. As the (b) mixed solvent, two or more kinds of known solvents capable of dissolving polyimide may be used. Examples of the solvent include amide solvents, solvents containing cyclic esters, ester groups, ether groups, ketone groups, hydroxyl groups, sulfone groups, and sulfinyl groups. Examples of amide solvents include N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), and N,N-dimethylacetamide (DMAc).

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

[0037] Among these, from the viewpoints of suppressing yellowing of polyimide films, imidization at relatively low temperatures of about 220°C or less, and imidization in air, cyclic ester solvents are preferred, and among these, gamma-butyrolactone (abbreviated as GBL), which can suppress yellowing even in film formation in air, is preferred.

[0038] The varnish of this embodiment uses a highly soluble solvent in combination with a low boiling point solvent with a high vapor pressure. Examples of highly soluble solvents include cyclic ester and amide solvents. For example, γ-butyrolactone (abbreviated as GBL), N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), and N,N-dimethylacetamide (DMAc). Examples of low-boiling point solvents include those having a boiling point of 160° C. or less and capable of retaining solubility, such as ethyl acetate, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, cyclopentanone, tetrahydrofuran, dichloroethane, chloroform, etc. Among these, from the viewpoint of the solubility of polyimide, cyclic ketones are preferred, and cyclohexanone and cyclopentanone are particularly preferred. A preferred combination of a highly soluble solvent and a low boiling point solvent is, for example, a combination of a cyclic ester and a cyclic ketone, and among these, a combination of γ-butyrolactone (abbreviated as GBL) and cyclohexanone is preferred from the viewpoints of the viscosity of the varnish and the solubility of the polyimide. It is more preferable that the amount of the (b1) cyclic ester is 20% by mass or more and 90% by mass or less, based on the total mass of the (b1) cyclic ester and the (b2) cyclic ketone, from the viewpoint of ease of drying when made into a film.

[0039] <Solubility parameters> The Hansen solubility parameter (HSP), 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 from the volume ratio of each solvent using the following formula.

number

[0040] It is well known that when the dispersion term δd, dipole term δp, and hydrogen bond term δh of the Hansen solubility parameters (HSP) are plotted in three-dimensional space, the dissolved polymer and the solvent are close to each other in the space. As a result of intensive research, the present inventors have surprisingly found that, with respect to the polyimide and solvent in this embodiment, when two of the three Hansen solubility parameters (HSP), namely, the dispersion term δd, the dipole term δp, and the hydrogen bond term δh, are within a specific distance, a polyimide varnish having good solubility, rapid drying properties, and improved film-forming properties can be obtained. Specifically, being within a specific distance can be expressed by the following formula 1, the value of which is 8.5 or less, and more preferably, can be expressed by the following formula 2, the value of which is 4.7 or less.

number

number

[0041] Even more surprisingly, the (b) mixed solvent of this embodiment has a dispersion term (δd solv ), dipole term (δp solv ) and hydrogen bond term (δh solv ) but: Dispersion term (δd solv )≧16.8 Dipole term (δp solv )≧11.5 Hydrogen bond term (δh solv )≧6.2 and the Hansen solubility parameter (HSP) is preferably 22.5 or more and 26.7 or less. This tends to provide a polyimide varnish having better solubility and improved quick-drying properties and film-forming properties.

[0042] <Production method of polyimide varnish> The method for producing a polyimide varnish according to the present embodiment includes dissolving the above-described diamine and tetracarboxylic dianhydride in a mixed solvent containing two or more kinds of solvents (b) to produce a polyimide (a) in the mixed solvent (b). In this case, the two or more kinds of solvents are such that, in the parameters represented by the dispersion term δd, the dipole term δp, and the hydrogen bond term δh of the Hansen solubility parameter (HSP), the dispersion term and the hydrogen bond term of the polyimide and the mixed solvent satisfy the following formula 1:

number

[0043] <Light transmittance of polyimide varnish> The light transmittance of the varnish of this embodiment is 55% or more. In this 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 the ultraviolet-visible absorption spectrum measurement, a solvent having 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 set to 10 mm (1.0 cm), and the measurement temperature is 23°C. There are no particular limitations on the solvent having a light transmittance of 90% or more at a wavelength of 450 nm, but γ-butyrolactone (abbreviation: GBL), N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), cyclohexanone, cyclopentanone, and the like can be used. When the varnish is diluted to adjust the solid content to 20% by mass, the varnish may contain a solvent that constitutes the varnish. When the varnish is concentrated to adjust the solid content to 20% by mass, the varnish may contain a solvent that constitutes the varnish as is. Polyimide films obtained from varnishes having a light transmittance of 55% or more at a wavelength of 450 nm tend to have a low yellowness index (hereinafter also referred to as YI).

[0044] Although not wishing to be bound by theory, the absorption due to the solvent or the primary structure of polyimide is mainly seen in the wavelength range of 300 nm or less. Considering this, it is speculated that the absorption in the visible light region that reduces the transparency of the film is due to a red shift in the absorption wavelength due to the exciton interaction of polyimide, due to the higher-order structure due to the formation of a CT (Charge Transfer) complex of polyimide, or due to yellowing components due to side reactions or impurities. In other words, the main cause of coloring after film formation is thought to be latent in the varnish that is diluted with the solvent. Therefore, it is thought that if the light transmittance of the varnish at a wavelength of 450 nm is less than 55%, the yellowness of the film (yellowness index, hereinafter also referred to as YI) will deteriorate. The higher the light transmittance of the varnish at a wavelength of 450 nm, the more preferable. For example, the light transmittance is preferably 55% or more, and more preferably 60% or more. The upper limit of the light transmittance of the varnish at a wavelength of 450 nm is not particularly limited, but may be 100% or less, may be 99% or less, may be 98% or less, may be 95% or less, or may be 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 this specification, the term "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 the present embodiment may be, for example, a polyimide film formed on the surface of a support, or may be a supportable film (self-supporting film) without a support. The film is preferably a self-supporting film from the viewpoint of being applied to a processing process as a roll film. The polyimide film according to the present 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 folding display body or a display body that conforms to 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 this case, the haze of the polyimide film is preferably 2 or less, and even more preferably 1 or less.

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

[0049] The thickness of the polyimide film is preferably within a range of 1 μm to 100 μm, and more preferably within a range of 20 μm to 50 μm from the viewpoint of both ease of handling and flexibility of the film.

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

[0051] The varnish of the present embodiment optionally contains additives. The additives may be, for example, a leveling agent for improving the coating properties of the film, a dispersant or surfactant, a surfactant or adhesion aid for adjusting the peelability or adhesion of the film from the 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, polyvinylnaphthalene, polymethyl methacrylate, cellulose triacetate, and fluorene derivatives, antioxidants, ultraviolet inhibitors, light stabilizers, plasticizers, waxes, fillers, pigments, dyes, foaming agents, defoamers, dehydrating agents, antistatic agents, antibacterial agents, and antifungal agents. The polyimide film may also contain silica particles and / or additives.

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

[0053] As an additive, for example, a phosphoric acid ester-based compound having a plasticizing effect and a flame retardant effect may be used for multiple purposes. Although the use is not limited, examples of the compound structure that may be added include phosphite-based compounds, phenol-based compounds, thioether-based compounds, hydrazine-based compounds, amide-based compounds, benzotriazole-based compounds, triazine-based compounds, isocyanuric acid-based compounds, hindered amine-based compounds, hindered phenol-based compounds, phosphoric acid ester-based compounds, phosphazene-based compounds, urethane-based compounds, acrylic compounds, methacrylic compounds, epoxy compounds, and isocyanate-based compounds. In addition, the polyimide varnish of this 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 relative to the mass of (a) 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, among the monovalent primary amines, at least one selected from the group consisting of aniline, benzylamine, and cyclohexylamine is preferable from the viewpoint of not absorbing visible light and being able to suppress coloring of the varnish. The additives added to the varnish may remain in the polyimide film.

[0054] <Production method of polyimide film> The method for producing a polyimide film according to the present embodiment includes the steps of: A step of casting the varnish of the present embodiment onto a substrate; drying the cast varnish to form a free-standing film; A step of peeling the self-supporting film from the support and further heating it; may include.

[0055] The casting step can be carried out using various coating devices depending on the application 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] The supports used include, for example, a rigid substrate, a metal drum, a metal endless belt, a plastic film substrate, a metal foil, and flexible glass. Examples of hard substrates include glass substrates such as an alkali glass substrate and an alkali-free glass substrate (Eagle XG (registered trademark), manufactured by Corning Incorporated), and metal substrates such as a copper substrate, an aluminum substrate, and a SUS substrate. Examples of plastic film substrates include Upilex (registered trademark) film (manufactured by Ube Industries), Kapton (registered trademark) film (manufactured by Toray DuPont), polycarbonate film, PEN film, and PET film. Examples of the metal foil include copper foil, aluminum foil, and SUS foil.

[0057] The step of drying the cast varnish can be divided into primary drying and secondary drying. Between the primary drying and secondary drying, the free-standing film can be peeled off from the support to obtain a free-standing film. The peeled free-standing film can be further heated for secondary drying.

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

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

[0060] Without wishing to be bound by theory, when the drying step is carried out under an inert gas atmosphere such as nitrogen gas, a colorless and transparent polyimide film tends to be obtained regardless of the type of solvent (b) contained in the varnish. On the other hand, when the solvent (b) contained in the varnish is γ-butyrolactone (GBL), it is preferable to carry out the drying step in air from the viewpoints of economy and colorless transparency of the film.

[0061] In this embodiment, the obtained polyimide film can be subjected to a stretching process to reduce the thickness of the film. For example, to obtain a film having a thickness of less than 10 μm, a polyimide film having a thickness of 10 μm or more can be stretched.

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

[0063] In another embodiment, a laminate can be manufactured by providing a functional layer, for example, a transparent electrode layer, on the surface of the polyimide film of this embodiment. The laminate can be obtained by forming, for example, a transparent electrode layer on the surface of the polyimide film using a sputtering device. The polyimide film may have a support, or may be a single layer without a support. The laminate may have a transparent electrode layer on both sides of the polyimide film. In this case, it is preferable that at least one or more transparent electrode layers are provided on both sides of the polyimide film. In addition, 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] The present invention will be described in more detail below with reference to examples, but these are described for the purpose of illustration only and are not intended to limit the scope of the present invention. Various evaluations in the examples were carried out 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 polyimide were measured by gel permeation chromatography (GPC) under the following conditions. As a solvent, N,N-dimethylformamide (Wako Pure Chemical Industries, Ltd., for high performance liquid chromatography) was used, to which 24.8 mol / L of lithium bromide monohydrate (Wako Pure Chemical Industries, Ltd., purity 99.5%) and 63.2 mol / L of phosphoric acid (Wako Pure Chemical Industries, Ltd., for high performance liquid chromatography) were added before the measurement. In addition, a calibration curve for calculating the number average molecular weight (Mn) and weight average molecular weight (Mw) was prepared using standard polystyrene (Agilent Technologies, product number EasiCal PS-1). Column: TSK-GEL SUPER HM-H x 2 Flow rate: 0.5mL / min Column temperature: 40℃ Pump: PU-2080 (JASCO) Detector: RI-2031Plus (RI: differential refractometer, manufactured by JASCO) UV-2075Plus (UV-Vis: Ultraviolet-visible absorption spectrometer, manufactured by JASCO)

[0066] [Light transmittance] The light transmittance of the polyimide varnish was measured by adjusting the polyimide concentration to 20% by mass, and then measuring the ultraviolet-visible absorption spectrum under the following conditions. A UV / VIS SPECTROPHOTOMETER (V-550, manufactured by JASCO) was used as the device. The background was measured in a thermostatic chamber at 23°C, with quartz cells with an optical path length of 10 mm filled with the same solvent as the varnish placed in the reference and sample chambers. 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. Equipment: UV / VIS SPECTROPHOTOMETER (V-550, manufactured by JASCO) Quartz cell size: optical path length 10mm x width 10mm x height 400mm Measurement wavelength: 300nm-800nm Bandwidth: 2.0nm Scanning speed: 200nm / 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. × (Poor): 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 ratios of the respective solvents. Requirement 1: It was marked as 〇 if Formula 1 was satisfied, and × if not. The calculated values were also noted in the column of Requirement 1 in Table 2. Requirement 2: It was marked as 〇 if all of the following were satisfied: dispersion term (δd solv ) ≥ 16.8, dipole term (δp solv ) ≥ 11.5, and hydrogen bond term (δh solv ) ≥ 6.2; otherwise, it was marked as ×. Requirement 3: It was marked as 〇 if the Hansen solubility parameter (HSP) satisfied 22.5 or more and 26.7 or less; otherwise, it was marked as ×.

[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 solvent contained in the film before drying × 100 The free-standing film was cut into a 10 cm square and two opposing sides of the free-standing film were fixed to a metal frame with a 10 cm opening with Kapton (registered trademark) tape. The free-standing film fixed to the metal frame was dried at 270°C for 20 minutes (secondary drying). The secondary drying was performed in air or nitrogen as shown in Table 3. After that, it was cooled to room temperature and removed from the metal frame to obtain a polyimide film. (Solvent drying evaluation) 〇: No stickiness on the surface after drying for 10 minutes at 80℃ ×: The surface is sticky after drying at 80℃ for 10 minutes. (Film independence evaluation) ◯: A free-standing film was obtained after peeling from the substrate. ×: The film was broken when peeled off from the substrate.

[0069] [Yellowness index (YI) and haze] A free-standing film was prepared by the method described in "Preparation of polyimide film" above so that the film thickness after drying would be 20±1 μm. The yellowness index (YI value) and haze of the free-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. ◯ (Good): The YI of the film is greater than 3 and less than or equal to 5. × (Poor): The film's YI exceeds 5. (Hays Rank) ◯ (Good): Haze is 1% or less. × (poor): Haze exceeds 1%.

[0070] "raw materials" <Tetracarboxylic acid 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: gamma-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 a wavelength of 350 nm with an optical path length of 1 cm before use. ODPA, 3DAS, and 4DAS were dissolved in GBL in advance to make 1%, 10%, and 10% solutions, respectively, and it was confirmed that the light transmittance of the solutions was 97% or more at a wavelength of 450 nm with an optical path length of 1 cm before use.

[0073] <Preparation of polyimide varnish> While introducing nitrogen gas into a 500mL separable flask with a stirring rod equipped with a Dean-Stark tube and a reflux tube at the top, diamine was first dissolved in solvent 1 at 60°C as shown in Table 1. Next, acid dianhydride was added in powder form and stirred for 30 minutes, after which Py (4 mol% relative to the amount of acid dianhydride charged) was added, and the temperature in the system was raised to 165°C using an oil bath and stirred for 30 minutes to 3 hours as appropriate. After the imidization rate calculated from the amount of recovered 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 amount of residual Py was 0.01% or less and the imidization rate calculated from the amount of recovered water exceeded 90%, after which the oil bath was removed and the temperature in the system was returned to room temperature, and solvent 1 and solvent 2 were added as shown in Table 1 so that the solid concentration was 20% by mass, to obtain a polyimide varnish.

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

[0075] [Table 1]

[0076] [Table 2] [Industrial Applicability]

[0077] The varnish of this embodiment can be used to produce polyimide films used as surface protection films, color filters, substrate films for TFTs, etc., and insulating protective films. The polyimide film obtained by drying the varnish of this embodiment can be suitably used in products such as flexible optical devices, optical devices having curved surfaces, and 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, In the Hansen solubility parameter (HSP) represented by the dispersion term δd, the dipole term δp, and the hydrogen bond term δh, the dispersion term and the hydrogen bond term of the polyimide and the mixed solvent are expressed by the following formula 1: [0010] {In formula 1, δd ＰＩ is the dispersion term of the polyimide, and δh ＰＩ is the hydrogen bond term of the polyimide, and δd ｓｏｌｖ is the dispersion term of the mixed solvent, and δh ｓｏｌｖ is the hydrogen bond term of the mixed solvent. Meet the polyimide varnish.

2. 2. The polyimide varnish according to claim 1, wherein the polyimide varnish has a polyimide concentration of 20% by mass, and when measured at an optical path length of 10 mm and a wavelength of 450 nm, the polyimide varnish has a light transmittance of 55% or more.

3. The dispersion term of the mixed solvent (δd ｓｏｌｖ ), dipole term (δp ｓｏｌｖ ) and the hydrogen bond term (δh ｓｏｌｖ ) but the following: Dispersion term (δd ｓｏｌｖ )≧16.8 Dipole term (δp ｓｏｌｖ ) ≧11.5 Hydrogen bond term (δh ｓｏｌｖ ) ≧6.2 and 3. The polyimide varnish according to claim 1 or 2, having a Hansen solubility parameter (HSP) of 22.5 or more and 26.7 or less.

4. The polyimide varnish according to any one of claims 1 to 3, wherein the (b) mixed solvent contains (b1) a cyclic ester and (b2) a cyclic ketone.

5. 5. The polyimide varnish according to claim 4, wherein an amount of the cyclic ester (b1) is 20 mass % or more and 90 mass % or less, based on the total mass of the cyclic ester (b1) and the cyclic ketone (b2).

6. The polyimide varnish according to claim 5, wherein the cyclic ester (b1) is γ-butyrolactone.

7. The polyimide varnish according to any one of claims 1 to 6, wherein the polyimide (a) contains a divalent organic group represented by the following formula (B1): 【Chemistry 1】

8. The polyimide varnish according to claim 7 , wherein the polyimide (a) further contains a divalent organic group represented by the following formula (B2): 【Chemistry 2】

9. The polyimide varnish according to claim 7 or 8, wherein the polyimide (a) further contains a tetravalent organic group represented by the following formula (A6): 【Chemistry 3】

10. The polyimide varnish according to any one of claims 1 to 6, wherein the polyimide (a) contains at least one selected from the group consisting of a tetravalent organic group represented by the following formula (A1) and a divalent organic group represented by the following formulas (B3) and (B4). 【Chemistry 4】 【Chemistry 5】 【Chemistry 6】

11. The polyimide varnish according to any one of claims 1 to 6, wherein the polyimide (a) 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): 【Chemistry 7】 【Chemistry 8】 【Chemistry 9】 【Chemistry 10】

12. A step of casting the polyimide varnish according to any one of claims 1 to 11 onto a substrate; drying the cast polyimide varnish to form a free-standing film; peeling the free-standing 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 claims 1 to 11.

14. The solvent is contained in an amount of 5 parts by mass or more and 40 parts by mass or less based on 100 parts by mass of polyimide, The polyimide film according to claim 13 , wherein the solvent comprises (b1) a cyclic ester and (b2) a cyclic ketone.

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

16. A light-emitting device comprising a polyimide film on which the varnish according to any one of claims 1 to 11 has been dried.

17. A curved display having a polyimide film on which the varnish according to any one of claims 1 to 11 has been dried.

18. A foldable display having a polyimide film on which the varnish according to any one of claims 1 to 11 has been dried.

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 produce a polyimide in the mixed solvent, The two or more solvents are such that, in the parameters represented by the dispersion term δd, the dipole term δp, and the hydrogen bond term δh of the Hansen solubility parameters (HSP), the dispersion term and the hydrogen bond term of the polyimide and the mixed solvent satisfy the following formula 1: [0025] {In formula 1, δd ＰＩ is the dispersion term of the polyimide, and δh ＰＩ is the hydrogen bond term of the polyimide, and 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 polyimide varnish.

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