JP7512902B2 - Colorless and transparent polyimide film - Google Patents

Description

The present invention relates to a polyimide film, more specifically to a colorless and transparent polyimide film.

Polyimide resins have excellent mechanical properties and heat resistance, and various applications are being considered in the fields of electrical and electronic components. For example, it is desirable to replace the glass substrates used in image display devices such as liquid crystal displays and organic electroluminescence displays with plastic substrates in order to make the devices lighter and more flexible, and research is also underway into polyimide resins suitable as such plastic materials.

For example, Patent Document 1 discloses that an optical film having improved bending resistance while maintaining optical properties such as transparency, total light transmittance, and YI value can be obtained from a composition containing silica microparticles having an average particle size within a specific range and an alkoxysilane compound having a reactive group in a polyimide polymer.

Patent Document 2 discloses that by forming a polyimide film using a polyimide varnish containing a polyimide polymer and a specific ratio of water, the presence of water can improve the appearance and flexibility of the polyimide film.

JP 2017-203159 A International Publication No. 2017/014286

However, the inclusion of inorganic silica particles in a resin is problematic in terms of the deterioration of transparency and bending resistance and stability due to poor dispersion of the silica particles. In addition, when water is included in a polyimide-based varnish, the varnish viscosity changes over time due to the influence of water, and this causes problems with the stabilization of quality, such as the stabilization of film thickness and appearance, during continuous film formation.
An object of the present invention is to provide a colorless and transparent polyimide film which improves bending resistance while maintaining optical properties such as transparency, total light transmittance, YI value, and haze of an optical film.

The inventors discovered that the above problems can be solved by including a specific amount of organic solvent in the polyimide film, and thus completed the present invention.

That is, the present invention relates to a colorless and transparent polyimide film containing polyimide and volatile matter, which is measured using a simultaneous differential thermal and thermogravimetric analyzer under a nitrogen gas flow at a heating rate of 10°C/min from 120°C to 300°C, and then held at 300°C for 30 minutes, and the volatile matter content in the film is defined as the mass reduced from the total mass of the film before measurement, and the volatile matter content is the ratio of the volatile matter content to the total mass of the film before measurement, and is 0.5 to 15 mass%.

The polyimide film of the present invention contains a specific amount of organic solvent, making it possible to provide a colorless and transparent polyimide film that improves bending resistance while maintaining optical properties such as transparency, total light transmittance, yellow index (YI) value, and haze of the optical film. The polyimide film of the present invention can be suitably used for substrates of various components of flexible displays.

The polyimide film of the present invention is a colorless and transparent polyimide film containing polyimide and volatile matter, and is measured using a simultaneous differential thermal and thermogravimetric analyzer under a nitrogen gas flow at a heating rate of 10°C/min from 120°C to 300°C, and then held at 300°C for 30 minutes. The volatile matter content in the film is the mass reduced from the total mass of the film before measurement, and is the ratio of the volatile matter content to the total mass of the film before measurement. The volatile matter content is 0.5 to 15 mass%, and is a colorless and transparent polyimide film.

（式中、Ｒは炭素数４～３９の４価の脂環基であり、Φは合計の炭素数が２～３９の２価の脂肪族基、脂環基、芳香族基又はこれらの組合せからなる基であって、結合基として－Ｏ－、－ＳＯ_２－、－ＣＯ－、－ＣＨ_２－、－Ｃ（ＣＨ_３）_２－、－ＯＳｉ（ＣＨ_３）_２－、－Ｃ_２Ｈ_４Ｏ－及び－Ｓ－からなる群から選ばれる少なくとも１つを有していてもよい。) As the polyimide resin according to the present embodiment, from the viewpoints of transparency and flexibility, for example, a polyimide containing a repeating unit represented by the following formula [I] is preferable.

(In the formula, R is a tetravalent alicyclic group having 4 to 39 carbon atoms, and Φ is a divalent aliphatic group, alicyclic group, aromatic group or a combination thereof having a total of 2 to 39 carbon atoms, which may have at least one bonding group selected from the group consisting of -O-, -SO ₂ -, -CO-, -CH ₂ -, -C(CH ₃ ) ₂ -, -OSi(CH ₃ ) ₂ -, -C ₂ H ₄ O-, and -S-.)

The content of the repeating unit of formula [I] in the polyimide is preferably 10 to 100 mol %, more preferably 50 to 100 mol %, based on 100 mol % of all repeating units in the polyimide. The number of repeating units of formula [I] in one polyimide molecule is preferably 10 to 2000, more preferably 20 to 200.

Polyimides are obtained by reacting alicyclic tetracarboxylic acid or its derivative with diamine or its derivative, with tetravalent alicyclic tetracarboxylic acid and divalent diamine as constituents. Examples of alicyclic tetracarboxylic acid or its derivative include alicyclic tetracarboxylic acid, alicyclic tetracarboxylic acid esters, alicyclic tetracarboxylic acid dianhydrides, etc., with alicyclic tetracarboxylic acid dianhydrides being preferred. Examples of diamine and its derivative include diamine, diisocyanate, diaminodisilane, etc., with diamine being preferred.

Examples of alicyclic tetracarboxylic dianhydrides used in the synthesis of polyimides include 1,2,3,4-cyclobutanetetracarboxylic dianhydride, 1,2,4,5-cyclopentanetetracarboxylic dianhydride, 1,2,4,5-cyclohexanetetracarboxylic dianhydride, and bicyclo[2,2,2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride. From the viewpoint of film transparency and suppression of coloration, 1,2,4,5-cyclohexanetetracarboxylic dianhydride is particularly preferred. In general, polyimides containing aliphatic diamines as their constituents are difficult to polymerize because the intermediate product polyamic acid and diamine form a strong complex, so it is necessary to use a solvent (e.g. cresol) that has a relatively high solubility for the complex. However, in polyimides containing 1,2,4,5-cyclohexanetetracarboxylic dianhydride and aliphatic diamines as their constituents, the complex between polyamic acid and diamine is bonded by a relatively weak bond, so it is easy to increase the molecular weight and to obtain a flexible film. The tetracarboxylic acid component includes isomers.

The above-mentioned tetracarboxylic acid components can be used in combination with tetracarboxylic acids other than alicyclic tetracarboxylic acids or their derivatives, particularly dianhydrides, to the extent that the solvent solubility of the polyimide, the flexibility of the film, and the transparency are not impaired.

Tetracarboxylic acids other than alicyclic tetracarboxylic acids include aromatic tetracarboxylic acids and linear or branched aliphatic tetracarboxylic acids. Specific examples of aromatic tetracarboxylic acids include pyromellitic acid, 3,3',4,4'-biphenyltetracarboxylic acid, 2,3,3',4'-biphenyltetracarboxylic acid, 2,2-bis(3,4-dicarboxyphenyl)propane, 2,2-bis(2,3-dicarboxyphenyl)propane, 2,2-bis(3,4-dicarboxyphenyl)-1,1,1,3,3,3-hexafluoropropane, 2,2-bis(2,3-dicarboxyphenyl)-1,1,1,3,3,3-hexafluoropropane, bis(3,4-dicarboxyphenyl)sulfone, bis(3,4- bis(2,3-dicarboxyphenyl)ether, bis(2,3-dicarboxyphenyl)ether, 3,3',4,4'-benzophenonetetracarboxylic acid, 2,2',3,3'-benzophenonetetracarboxylic acid, 4,4-(p-phenylenedioxy)diphthalic acid, 4,4-(m-phenylenedioxy)diphthalic acid, 1,1-bis(2,3-dicarboxyphenyl)ethane, bis(2,3-dicarboxyphenyl)methane, bis(3,4-dicarboxyphenyl)methane, and derivatives of these tetracarboxylic acids, particularly dianhydrides. A specific example of a linear or branched aliphatic tetracarboxylic acid is ethylene tetracarboxylic acid.

The diamine-based component constituting the nitrogen and Φ of the imide ring in formula [I] includes diamines, diisocyanates, diaminodisilanes, etc., but diamines are preferred. The diamine content in the diamine-based component is preferably 50 mol % or more (including 100 mol %).

The diamines used in the synthesis of the polyimides may be either aromatic diamines, aliphatic diamines or mixtures thereof.
Among these, aromatic diamines are preferred from the viewpoint of heat resistance.
In the present invention, the term "aromatic diamine" refers to a diamine in which an amino group is directly bonded to an aromatic ring, and may contain an aliphatic group, an alicyclic group, or other substituent as part of its structure. The term "aliphatic diamine" refers to a diamine in which an amino group is directly bonded to an aliphatic group or an alicyclic group, and may contain an aromatic group, or other substituent as part of its structure.

From the viewpoints of low birefringence and suppression of coloration, the aromatic diamine used in the synthesis of the polyimide is preferably a diamine having an ether group, a diamine having an isopropyl group, or a diamine having a fluorine-based substituent, more preferably a diamine having an ether group or a diamine having an isopropyl group.
Examples of aromatic diamines include p-phenylenediamine, m-phenylenediamine, 2,4-diaminotoluene, 2,6-diaminotoluene, benzidine, o-tolidine, m-tolidine, bis(trifluoromethyl)benzidine, octafluorobenzidine, 3,3'-dihydroxy-4,4'-diaminobiphenyl, 3,3'-dimethoxy-4,4'-diaminobiphenyl, 3,3'-dichloro-4,4'-diaminobiphenyl, 3,3'-difluoro-4,4'-diaminobiphenyl, 2,6-diaminonaphthalene, 1,5-diaminonaphthalene, and 4,4'-diaminodiphenyl. ether, 3,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl methane, 4,4'-diaminodiphenyl sulfone, 3,4'-diaminodiphenyl sulfone, 4,4'-diaminobenzophenone, 2,2-bis(4-(4-aminophenoxy)phenyl)propane, 2,2-bis(4-(2-methyl-4-aminophenoxy)phenyl)propane, 2,2-bis(4-(2,6-dimethyl-4-aminophenoxy)phenyl)propane, 2,2-bis(4-(4-aminophenoxy)phenyl)hexafluoropropane, 2,2-bis(4-(2-methyl-4-aminophenoxy)phenyl)hexafluoropropane, 2,2-bis(4-(2,6-dimethyl-4-aminophenoxy)phenyl)hexafluoropropane, 2,2-bis(4-(2,6-dimethyl-4-aminophenoxy)phenyl)hexafluoropropane, 4,4'-bis(4-aminophenoxy)biphenyl, 4,4'-bis(2-methyl-4-aminophenoxy)biphenyl, 4,4'-bis(2,6-dimethyl-4-aminophenoxy)biphenyl, 4,4'-bis(3-aminophenoxy)biphenyl, bis(4-(4-aminophenoxy)phenyl)sulfone, bis(4-(2-methyl-4-aminophenoxy)phenyl)sulfone, bis(4-(2,6-dimethyl-4-aminophenoxy)phenyl)sulfone, 1,4-bis(4-aminophenoxy)benzene, 1,4-bis(2-methyl-4-aminophenoxy)benzene, 1,4-bis(2,6-dimethyl-4-aminophenoxy)benzene, 1,3-bis(4-aminophenoxy)benzene, 1,3-bis(2-methyl-4-aminophenoxy)benzene, 1,3-bis(2,6-dimethyl-4-aminophenoxy)phenyl)sulfone, bis(4-(4-aminophenoxy)phenyl)ether, bis(4-(2-methyl-4-aminophenoxy)phenyl)ether, bis(4-(2,6-dimethyl-4-aminophenoxy)phenyl)ether, 1,4-bis(4-aminophenoxy)benzene, 1,4-bis(2-methyl-4-aminophenoxy)benzene, 1,3-bis(2,6-dimethyl-4-aminophenoxy)benzene, α,α'-bis(4-aminophenyl)-1,4-diisopropylbenzene, α,α'-bis(2-methyl-4-aminophenyl)-1,4-diisopropylbenzene, α,α'-bis(2-methyl-4-aminophenyl)-1,4-diisopropylbenzene, α,α'-bis(2-methyl-4-aminophenyl)-1,4-diisopropylbenzene, α,α'-bis(2-methyl-4-aminophenyl)-1,4-diisopropylbenzene, 4-diisopropylbenzene, α,α'-bis(2,6-dimethyl-4-aminophenyl)-1,4-diisopropylbenzene, α,α'-bis(3-aminophenyl)-1,4-diisopropylbenzene, α,α'-bis(4-aminophenyl)-1,3-diisopropylbenzene, α,α'-bis(2-methyl-4-aminophenyl)-1,3-diisopropylbenzene, α,α'-bis(2,6-dimethyl-4-aminophenyl)-1,3-diisopropylbenzene, α,α'-bis(3-aminophenyl)-1,3-diisopropylbenzene, 9,9-bis(4-aminophenyl )fluorene, 9,9-bis(2-methyl-4-aminophenyl)fluorene, 9,9-bis(2,6-dimethyl-4-aminophenyl)fluorene, 1,1-bis(4-aminophenyl)cyclopentane, 1,1-bis(2-methyl-4-aminophenyl)cyclopentane, 1,1-bis(2,6-dimethyl-4-aminophenyl)cyclopentane, 1,1-bis(4-aminophenyl)cyclohexane, 1,1-bis(2-methyl-4-aminophenyl)cyclohexane, 1,1-bis(2,6-dimethyl-4-aminophenyl)cyclohexane, 1,1-bis(4-aminophenyl ) 4-methyl-cyclohexane, 1,1-bis(4-aminophenyl)norbornane, 1,1-bis(2-methyl-4-aminophenyl)norbornane, 1,1-bis(2,6-dimethyl-4-aminophenyl)norbornane, 1,1-bis(4-aminophenyl)adamantane, 1,1-bis(2-methyl-4-aminophenyl)adamantane, 1,1-bis(2,6-dimethyl-4-aminophenyl)adamantane, and the like. Among these, 4,4'-diaminodiphenyl ether and α,α'-bis(4-aminophenyl)-1,3-diisopropylbenzene are preferred.

Furthermore, examples of aliphatic diamines used in the synthesis of polyimides include ethylene diamine, hexamethylene diamine, polyethylene glycol bis(3-aminopropyl) ether, polypropylene glycol bis(3-aminopropyl) ether, 1,3-bis(aminomethyl)cyclohexane, 1,4-bis(aminomethyl)cyclohexane, metaxylylene diamine, paraxylylene diamine, 1,4-bis(2-amino-isopropyl)benzene, 1,3-bis(2-amino-isopropyl)benzene, isophorone diamine, norbornane diamine, and siloxane diamines.

Polyimide is usually produced as an organic solvent solution. The organic solvent is not particularly limited, but examples include N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMAC), N,N-diethylacetamide, N,N-dimethylformamide, N,N-diethylformamide, N-methylcaprolactam, hexamethylphosphoramide, tetramethylene sulfone, dimethyl sulfoxide, m-cresol, phenol, p-chlorophenol, 2-chloro-4-hydroxytoluene, diglyme, triglyme, tetraglyme, dioxane, gamma-butyrolactone (GBL), dioxolane, cyclohexanone, cyclopentanone, 1,4-dioxane, epsilon caprolactam, dichloromethane, and chloroform, and two or more of them may be used in combination. However, in consideration of the performance of a polyimide varnish consisting of a polyimide and a solvent, it is preferable to use at least one selected from the group consisting of γ-butyrolactone, N,N-dimethylacetamide, and N-methyl-2-pyrrolidone, and it is more preferable to use at least one selected from the group consisting of γ-butyrolactone and N,N-dimethylacetamide. In addition, in the case of production by solution polymerization, poor solvents such as hexane, heptane, benzene, toluene, xylene, chlorobenzene, and o-dichlorobenzene can be used in addition to these solvents to the extent that the polymer does not precipitate.

Methods for producing an organic solvent solution of polyimide include, but are not limited to, the following methods (1) to (3).

(1) A tetracarboxylic acid component is added to an organic solvent solution of a diamine component, or a diamine component is added to an organic solvent solution of a tetracarboxylic acid component, and the mixture is maintained at a temperature of preferably 80°C or lower, particularly at or near room temperature, for 0.5 to 3 hours. An azeotropic dehydrating solvent such as toluene or xylene is added to the resulting polyamic acid solution, which is a reaction intermediate, to carry out a dehydration reaction while removing the generated water from the system by azeotropy, thereby obtaining an organic solvent solution of polyimide.

(2) A dehydrating agent such as acetic anhydride is added to the polyamic acid solution of the reaction intermediate obtained in the same manner as in (1) above to imidize it, and then a solvent that has poor solubility for polyimide, such as methanol, is added to precipitate the polyimide. After separating it as a solid by filtration, washing, and drying, it is dissolved in an organic solvent to obtain an organic solvent solution of polyimide.

(3) In the above (1), a polyamic acid solution is prepared using a high boiling point solvent such as cresol, and the solution is kept at 150-220°C for 3-12 hours to convert it to polyimide, after which a solvent that has poor solubility for polyimide, such as methanol, is added to precipitate the polyimide. After separating it as a solid by filtration, washing, and drying, the polyimide is dissolved in an organic solvent such as N,N-dimethylacetamide to obtain an organic solvent solution of polyimide.

In addition, when polyimide is produced by solution polymerization, it is preferable to use a tertiary amine compound as a catalyst. Examples of such a catalyst include trimethylamine, triethylamine (TEA), tripropylamine, tributylamine, triethanolamine, N,N-dimethylethanolamine, N,N-diethylethanolamine, triethylenediamine, N-methylpyrrolidine, N-ethylpyrrolidine, N-methylpiperidine, N-ethylpiperidine, imidazole, pyridine, quinoline, and isoquinoline. Of these tertiary amines, TEA is particularly preferable.

In addition, the concentration of the polyimide in the organic solvent solution used in the present invention is preferably 1 to 50% by mass, more preferably 10 to 40% by mass, of the polyimide component. If it is within this range, the surface smoothness of the obtained polyimide film is good.

From the viewpoint of the flexibility and mechanical strength of the resulting polyimide, the weight average molecular weight of the polyimide used in the present invention is preferably 10,000 or more, and more preferably 50,000 or more. The weight average molecular weight of the polyimide can be measured by a known method, for example, gel filtration chromatography. Another method is to measure the absolute molecular weight with a light scattering detector using N,N-dimethylformamide as the developing solvent.

A surfactant such as a fluorine-based or polysiloxane-based surfactant may be added to the polyimide organic solvent solution. Adding a surfactant makes it easier to obtain a film with good surface smoothness.

Antioxidants such as phenol-based, sulfur-based, phosphoric acid-based, and phosphorous acid-based antioxidants may be added to the organic solvent solution of polyimide.

The method for producing the polyimide film of the present invention is not particularly limited, and known methods can be used. For example, a solution containing the polyimide of the present invention, or a solution containing the polyimide of the present invention and the various additives described above, is applied to a smooth support such as a glass plate, a metal plate, or a plastic, or formed into a film, and then the solvent components such as organic solvents contained in the solution are removed.

An example of a method for producing a polyimide film is a method of forming a film by a solution casting method in which a polyimide organic solvent solution is cast on a support and dried. Specifically, after casting a polyimide organic solvent solution on a support, the organic solvent is volatilized using a film-forming machine of a type that preferably sprays gas at 50°C to 300°C onto the cast material on the support, and the film is peeled off from the support to obtain a self-supporting film. By volatilizing the organic solvent using a film-forming machine of a spraying type in this way, the drying property is improved. In addition, although the reason is not clear, it is thought that the in-plane retardation and thickness direction retardation of the film are reduced by using the spraying type, and the optical isotropy is improved.
It is preferable to carry out primary drying before blowing the gas. The conditions for the primary drying are not particularly limited, but it is preferable to hold the material at a temperature of 80 to 120° C. for 10 to 30 minutes, for example.
Examples of the gas to be blown include air or nitrogen, and air is preferred from the viewpoint of cost, and nitrogen is preferred from the viewpoint of preventing coloring of the film. The temperature of the gas to be blown is more preferably 50°C to 250°C, and even more preferably 100°C to 220°C. If the temperature of the gas to be blown is lower than 50°C, the organic solvent does not volatilize sufficiently, and the film may stick to the support when peeled off from the support. If the temperature of the gas is higher than 300°C, the solvent may suddenly volatilize, causing foaming in the film, and the solvent may decompose and cause coloring of the film. The time for blowing the gas varies depending on the temperature of the gas to be blown, but is preferably 15 to 30 minutes, more preferably 15 to 25 minutes. In addition, multiple zones with different temperatures of the gas to be blown on the cast product may be provided.

The polyimide film of the present invention contains the polyimide and a volatile component. The volatile component is preferably the organic solvent described above. Specifically, for example, N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMAC), N,N-diethylacetamide, N,N-dimethylformamide, N,N-diethylformamide, N-methylcaprolactam, hexamethylphosphoramide, tetramethylene sulfone, dimethyl sulfoxide, m-cresol, phenol, p-chlorophenol, 2-chloro-4-hydroxytoluene, diglyme, triglyme, tetraglyme, dioxane, γ-butyrolactone (GBL), dioxolane, cyclohexanone, cyclopentanone, 1,4-dioxane, epsilon caprolactam, dichloromethane, chloroform, and the like can be used, and two or more of them may be used in combination. From the viewpoint of surface smoothness of the film, it is preferable to contain at least one selected from the group consisting of γ-butyrolactone, N,N-dimethylacetamide, and N-methyl-2-pyrrolidone, and it is more preferable to contain at least one selected from the group consisting of γ-butyrolactone and N,N-dimethylacetamide.
In the present invention, the volatile content in the film is defined as the mass reduced from the total mass of the film before measurement, measured using a simultaneous differential thermal and thermogravimetric analyzer, under a nitrogen gas flow, at a temperature increase rate of 10°C/min from 120°C to 300°C, and then held at 300°C for 30 minutes. The polyimide film of the present invention has a volatile content, which is the ratio of the volatile content to the total mass of the film before measurement, of 0.5 to 15% by mass, preferably 0.7 to 10% by mass, and more preferably 1 to 5% by mass. When the volatile content in the film is 0.5 to 15% by mass, the film has excellent bending resistance and can be used practically as a free-standing film.

The polyimide film may further contain other components to the extent that the transparency and flex resistance are not impaired. Examples of other components include plasticizers, antioxidants, release agents, stabilizers, colorants such as bluing agents, flame retardants, lubricants, thickeners, and leveling agents. For example, the reflectance of white light can be improved by mixing an additive such as titanium dioxide for the purpose of coloring the film white, and the addition of nanofillers increases the apparent glass transition temperature of the resin composition molded product, improving heat resistance, and further increasing the tensile modulus and mechanical strength.

The thickness of the polyimide film is adjusted appropriately depending on the application, but is usually 10 to 500 μm, preferably 15 to 200 μm, and more preferably 20 to 100 μm.

This polyimide film preferably has a total light transmittance of 85% or more, and more preferably 90% or more, in accordance with JIS K7361-1, when it has a thickness of 20 to 50 μm.
Furthermore, this polyimide film preferably has a haze (cloudiness) of 2% or less, more preferably 1% or less, in accordance with JIS K7361-1, at a thickness of 20 to 50 μm.
Furthermore, this polyimide film preferably has a yellow index (YI) of 5 or less, more preferably 3 or less, in accordance with JIS K7361-1, at a thickness of 20 to 50 μm.

The polyimide film preferably has a retardation in the thickness direction (Rth) of 200 nm or less, more preferably 150 nm or less, and even more preferably 100 nm or less.
This polyimide film preferably has an in-plane retardation (Re) of 100 nm or less, more preferably 70 nm or less, and even more preferably 50 nm or less, within a thickness of 20 to 50 μm.

The polyimide film of the present invention has excellent flex resistance. Specifically, the number of times that the polyimide film can be bent 180° to R = 1 mm at a speed of 40 times/min before breaking is preferably 400,000 or more, more preferably 500,000 or more, and even more preferably 700,000 or more.

The polyimide film of the present invention is suitable for use as a film for various components such as touch sensors, color filters, flexible displays, semiconductor components, and optical components.

The present invention will be described in detail below with reference to examples, although the present invention is not limited to these examples in any way.
The methods for measuring the physical properties of the films obtained in the following examples are shown below.

(1) Film Thickness The film thickness was measured using a micrometer manufactured by Mitutoyo Corporation.

(2) Total light transmittance, haze, yellow index (YI)
The measurement was performed in accordance with JIS K7361-1 using a color and turbidity simultaneous measuring device "COH400" manufactured by Nippon Denshoku Industries Co., Ltd.

(3) In-plane retardation (Re)
The in-plane retardation (Re) was measured using an ellipsometer "M-220" manufactured by JASCO Corporation. The in-plane phase difference value was measured at a measurement wavelength of 590 nm.

(4) Thickness direction retardation (Rth)
The thickness retardation (Rth) was measured using an ellipsometer "M-220" manufactured by JASCO Corporation. The thickness retardation value was measured at a measurement wavelength of 590 nm. Rth is expressed by the following formula, where the maximum in-plane refractive index of the polyimide film is nx, the minimum is ny, the refractive index in the thickness direction is nz, and the thickness of the film is d.
Rth = [{(nx + ny) / 2} - nz] x d

(5) Bending Resistance: The polyimide film was bent 180° to R=1 mm at a rate of 40 times/min, and the number of times until breakage was recorded.

(6) Volatile content in film Using a differential thermal/thermogravimetric simultaneous analyzer (TG/DTA6200) manufactured by Hitachi High-Tech Science Corporation, measurements were performed under conditions of a nitrogen gas flow and a temperature rise rate of 10° C./min, the temperature was raised from 120° C. to 300° C., and the film was then held at 300° C. for 30 minutes, and the mass that was reduced from the total mass of the film before the measurement was taken as the volatile content in the film. The ratio of the volatile content to the total mass of the film before the measurement was taken as the volatile content.

Example 1
In a 2 L five-neck glass round-bottom flask equipped with a stainless steel half-moon stirring blade, a nitrogen inlet tube, a Dean-Stark equipped with a cooling tube, a thermometer, and a glass end cap, 239.772 g (0.696 mol) of α,α'-bis(4-aminophenyl)-1,3-diisopropylbenzene (manufactured by Mitsui Fine Chemicals, Inc.), 34.842 g (0.174 mol) of 4,4-diaminodiphenyl ether (manufactured by Wakayama Seika Kogyo Co., Ltd.), 376.453 g of γ-butyrolactone (manufactured by Mitsubishi Chemical Corporation), and 44.018 g of triethylamine (manufactured by Kanto Chemical Co., Ltd.) and 0.488 g of triethylenediamine (manufactured by Tokyo Chemical Industry Co., Ltd.) as catalysts were stirred at 200 rpm under a nitrogen atmosphere at a reaction system temperature of 70°C to obtain a solution. 195.028 g (0.870 mol) of 1,2,4,5-cyclohexanetetracarboxylic dianhydride (manufactured by Mitsubishi Gas Chemical Co., Ltd.) and 94.113 g of γ-butyrolactone (manufactured by Mitsubishi Chemical Co., Ltd.) were added to the mixture in one batch, and the mixture was heated with a mantle heater to raise the temperature in the reaction system to 200° C. over about 20 minutes. The components distilled off were collected, and the temperature in the reaction system was maintained at 200° C. for 5 hours while adjusting the stirring speed according to the increase in viscosity. After adding 847.067 g of N,N-dimethylacetamide, the mixture was stirred at about 100° C. for about 1 hour to obtain a homogeneous solution, and a homogeneous polyimide varnish with a solid content of 25% by mass was obtained.

The resulting polyimide varnish was then applied to a PET substrate and held at 100°C for 20 minutes to evaporate the solvent, yielding a colorless, transparent, self-supporting primary dried film. The film was then fixed to a stainless steel frame and dried in an air atmosphere for 20 minutes by blowing hot air at 210°C onto the film to obtain a film with a thickness of 35 μm. The evaluation results of this polyimide film are shown in Table 1.

<Comparative Example 1>
The polyimide varnish obtained in Example 1 was dropped into methyl alcohol to precipitate polyimide powder, and the solid was suction filtered using a Kiriyama funnel and further washed with methyl alcohol and dried at 200° C. for 30 minutes to remove the solvent, thereby obtaining polyimide powder.
In a 300 mL five-neck glass round-bottom flask equipped with a stainless steel half-moon shaped stirring blade, a nitrogen inlet tube, a Dean-Stark equipped with a cooling tube, a thermometer, and a glass end cap, 15 g of the obtained polyimide powder and 85 g of dichloromethane were added all at once, and then stirred at room temperature for 1 hour to obtain a homogeneous solution, thereby obtaining a homogeneous polyimide varnish with a solid content concentration of 15 mass %.

The resulting polyimide varnish was then applied to a PET substrate, held at room temperature for 5 minutes, held at 50°C in an air atmosphere for 5 minutes, and finally dried by blowing hot air at 150°C in an air atmosphere for 30 minutes to obtain a film with a thickness of 35 μm. The evaluation results of this polyimide film are shown in Table 1.

As shown in Table 1, the polyimide film of Example 1 has good optical properties such as total light transmittance, haze, and YI, and also has excellent flex resistance. In contrast, the polyimide film of Comparative Example 1 has significantly poor flex resistance.

Claims (7)

A polyimide film containing polyimide and a volatile content, the film being heated from 120°C to 300°C at a heating rate of 10°C/min in a nitrogen stream and then held at 300°C for 30 minutes using a simultaneous differential thermal and thermogravimetric analyzer, the mass of the film measured being determined as a mass reduction from the total mass of the film before the measurement, and the volatile content, which is the ratio of the volatile content to the total mass of the film before the measurement, being 0.5 to 15 mass%,
the polyimide is a reaction product of an alicyclic tetracarboxylic acid or a derivative thereof with a diamine or a derivative thereof, the alicyclic tetracarboxylic acid or a derivative thereof includes 1,2,4,5-cyclohexanetetracarboxylic dianhydride, and the diamine or a derivative thereof includes α,α'-bis(4-aminophenyl)-1,3-diisopropylbenzene;
A colorless and transparent polyimide film having a total light transmittance of 85% or more in a film having a thickness of 20 to 50 μm, a yellow index (YI) of 5 or less, and a haze of 2% or less . The colorless and transparent polyimide film according to claim 1, wherein the volatile component is an organic solvent. The colorless and transparent polyimide film according to claim 1 or 2, wherein the volatile matter contains at least one selected from the group consisting of γ-butyrolactone, N,N-dimethylacetamide, and N-methyl-2-pyrrolidone. 4. The colorless and transparent polyimide film according to claim 1 , wherein the in-plane retardation (Re) at a thickness of 20 to 50 μm is 50 nm or less, and the retardation in the thickness direction (Rth) is 100 nm or less. 5. The colorless and transparent polyimide film according to claim 1, wherein the polyimide has a weight average molecular weight of 50,000 or more . 6. The method for producing a colorless and transparent polyimide film according to claim 1 , wherein the film is formed by a solution casting method in which a solution of polyimide in an organic solvent is cast onto a support and dried. 7. The method for producing a colorless and transparent polyimide film according to claim 6, comprising the steps of: casting a solution of a polyimide in an organic solvent onto a support; volatilizing the organic solvent using a film-forming machine which blows a gas at 50° C. or more and 300 ° C. or less onto the cast product on the support; and peeling the film from the support as a self-supporting film.