JP7509132B2 - Manufacturing method of colorless transparent resin film - Google Patents

Description

The present invention relates to a method for producing a colorless and transparent resin film.

Polyimide resins have excellent mechanical properties and heat resistance, and various applications in the fields of electrical and electronic components are being considered. 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.

General-purpose polyimide films are generally produced by casting an organic solvent solution containing polyamic acid onto a belt or drum, and then subjecting the polyamic acid to an imidization reaction, such as by heating, to form a solution-cast film.

General-purpose polyimides are known to have high heat resistance. They are obtained from aromatic tetracarboxylic anhydrides and aromatic diamines, and because of their molecular rigidity, resonance stabilization, and strong chemical bonds, they have excellent heat resistance, chemical resistance, mechanical properties, and electrical characteristics, and are therefore widely used in fields such as molding materials, composite materials, and electrical or electronic components.

However, polyimides synthesized from the above aromatic raw materials are colored yellow or brown after film formation due to absorption resulting from intramolecular or intermolecular electron transfer complex formation, making them unsuitable for optical applications such as substrate materials for flat panel displays and mobile phones, optical fibers, optical waveguides, optical sensors, and optical adhesives.

For this reason, fluorinated polyimides in which perfluoroalkyl groups have been introduced into the repeating units to improve transparency while taking advantage of the high heat resistance characteristic of polyimides, and alicyclic polyimides made from alicyclic raw materials such as 1,2,4,5-cyclohexanetetracarboxylic anhydride have been developed.

Patent Document 1 discloses a method for producing a polyimide film with excellent peelability and appearance from a liquid containing a polyimide polymer and γ-butyrolactone and N,N-dimethylacetamide.

Patent Document 2 discloses a method for producing an optical film that uses a soluble polyimide resin as the main component and reduces scaly unevenness by adding an alcohol-based solvent to a liquid that uses dichloromethane as the solvent.

JP 2017-25204 A JP 2017-187617 A

However, in the method of Patent Document 1, when an organic solvent having a high boiling point such as γ-butyrolactone or N,N-dimethylacetamide is used, it is necessary to increase the temperature in the drying step to remove the solvent, which causes a problem of reduced transparency due to coloring.
Furthermore, in the method of Patent Document 2, when a low-boiling point solvent such as dichloromethane is used, the solvent evaporates immediately, so that the polyimide oriented by the casting method becomes fixed, causing optical anisotropy.

The problem that this invention aims to solve is to provide a method for producing a colorless, transparent resin film that does not discolor when dried at high temperatures and can achieve good optical isotropy.

The inventors discovered that when producing a film by the solution casting method, the above problem can be solved by having the organic solvent in the resin organic solvent solution contain a low-boiling point solvent as the main component and a high-boiling point solvent, and thus completed the present invention.

That is, the present invention relates to a method for producing a resin film by a solution casting method, which includes a step of casting a solution of a resin in an organic solvent onto a support and drying the solution, and relates to a method for producing a colorless and transparent resin film, in which the organic solvent contains one or more organic solvents (S1) having a boiling point of 80°C or less and one or more organic solvents (S2) having a boiling point of 130°C or more.

According to the method of the present invention, a colorless and transparent resin film having excellent transparency and optical isotropy can be produced. The colorless and transparent resin film obtained by the method of the present invention has good optical properties such as total light transmittance, yellow index (YI) value, and haze, and has low retardation indicating optical isotropy. Therefore, the colorless and transparent resin film obtained by the method of the present invention exhibits good performance when used as a transparent substrate for liquid crystal display elements and organic EL display elements, or as a base material for transparent conductive films for touch panels.

The method for producing a colorless and transparent resin film of the present invention is a method for producing a resin film by a solution casting method including a step of casting a solution of a resin in an organic solvent onto a support and drying the solution, wherein the organic solvent contains one or more organic solvents (S1) having a boiling point of 80°C or less and one or more organic solvents (S2) having a boiling point of 130°C or more.
In the present invention, "colorless and transparent" means that when a film having a thickness of 20 to 50 μm is formed, the total light transmittance of the film is preferably 85% or more, the yellow index (YI) is preferably 5 or less, and the haze is preferably 2% or less.

The resin that can be used in this embodiment is not particularly limited as long as it can be used to produce a resin film by a solution casting method including a process of casting an organic solvent solution of the resin onto a support and drying it, but it is preferable that the resin of the resin film is polyimide. The organic solvent solution of the resin is preferably an organic solvent solution of polyamic acid or an organic solvent solution of polyimide. Hereinafter, this embodiment will be described in detail using the case where the resin of the resin film is polyimide as a representative example.

As the resin for the resin film that can be used in this embodiment, from the viewpoints of transparency and optical isotropy, for example, a polyimide containing a repeating unit represented by the following formula [I] is more 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-, _-SO2- , -CO-, _-CH2- , -C( _CH3 ) _2- , -OSi( _CH3 ) _2- , _-C2H4O- 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%, even more preferably 70 to 100 mol%, and even more preferably 90 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, and 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, but 1,2,4,5-cyclohexanetetracarboxylic dianhydride is particularly preferred. In general, polyimides that contain aliphatic diamines 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 that contain 1,2,4,5-cyclohexanetetracarboxylic dianhydride and aliphatic diamines, the complex between the 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- Examples of the tetracarboxylic acid derivatives include at least one compound selected from the group consisting of 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 the linear or branched aliphatic tetracarboxylic acid is ethylene tetracarboxylic acid.

The diamine-based component constituting the nitrogen of the imide ring and Φ in formula [I] includes diamine, diisocyanate, diaminodisilanes, etc., but diamine is preferred. The diamine content in the diamine-based component is preferably 50 mol% or more, more preferably 70 mol% or more, and even more preferably 90 mol% or more (including 100 mol%).

The diamine used in the synthesis of polyimide may be any of aromatic diamines, aliphatic diamines, or a mixture thereof. In the present invention, "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. "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.

Examples of aromatic diamines used in the synthesis of polyimides 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-diamino Nonaphthalene, 4,4'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 4,4'-diaminodiphenylmethane, 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)hexafluoro Fluoropropane, 2,2-bis(4-(2-methyl-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-amino 1,4-bis(4-aminophenoxy)phenyl)sulfone, bis(4-(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, 1,4-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)benzene, 2,2-bis(4-aminophenyl)propane, 2,2-bis(2-methyl-4-aminophenyl)propane, 2,2-bis(2,6-dimethyl-4-aminophenyl)propane, 2,2-bis(4-aminophenyl)hexafluoropropane, 2,2-bis(2-methyl-4-aminophenyl)hexafluoropropane, 2,2-bis(2,6-dimethyl-4-aminophenyl)hexafluoropropane, α,α'-bis(4-aminophenyl) α,α'-bis(2-methyl-4-aminophenyl)-1,4-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 benzene, 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(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.

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 (varnish). In the method of the present invention, an organic solvent containing at least one organic solvent (S1) having a boiling point of 80°C or less and at least one organic solvent (S2) having a boiling point of 130°C or more is used as the organic solvent. In the present invention, by using an organic solvent (S1) having a relatively low boiling point and an organic solvent (S2) having a relatively high boiling point in combination, the organic solvent (S1) can be efficiently volatilized in the drying process to suppress yellowing of the film, and the organic solvent (S2) can be intentionally left in the film to obtain a film with excellent optical isotropy.

The boiling point of the organic solvent (S1) is 80°C or less, preferably 75°C or less, more preferably 70°C or less, even more preferably 65°C or less, and even more preferably 60°C or less. The lower limit of the boiling point of the organic solvent (S1) is not particularly limited, but is preferably 30°C or more from the viewpoint of work efficiency. By using an organic solvent (S1) having a relatively low boiling point of 80°C or less, the organic solvent (S1) can be volatilized at a relatively low drying temperature of about 120 to 150°C to obtain a resin film.

The organic solvent (S1) is not particularly limited, but examples thereof include dichloromethane (DCM, boiling point 39.6°C), 1,3-dioxolane (boiling point 75°C), tetrahydrofuran (THF, boiling point 66°C), acetone (boiling point 56°C), chloroform (boiling point 61°C), and ethyl acetate (boiling point 77°C), and two or more types may be used in combination. Among these, from the viewpoint of the performance of the polyimide varnish, at least one type selected from the group consisting of dichloromethane and 1,3-dioxolane is preferable as the organic solvent (S1).

The boiling point of the organic solvent (S2) is 130°C or higher, preferably 140°C or higher, more preferably 150°C or higher, and even more preferably 160°C or higher. The upper limit of the boiling point of the organic solvent (S2) is not particularly limited, but may be, for example, 300°C or lower, or 250°C or lower. By using an organic solvent (S2) having a relatively high boiling point of 130°C or higher, a resin film with low retardation and excellent optical isotropy can be obtained when the organic solvent solution of the resin is dried at a drying temperature of about 120 to 150°C. The reason for this is not yet clear, but it is presumed that by drying the organic solvent solution of the resin at a relatively low drying temperature of about 120 to 150°C, an amount of organic solvent (S2) that is not problematic in use remains in the resin film, which relaxes the orientation of the polymer chains and achieves low retardation.

The organic solvent (S2) is not particularly limited, but for example, cyclopentanone (boiling point 131°C), cyclohexanone (boiling point 156°C), N-methyl-2-pyrrolidone (boiling point 202°C), N,N-dimethylacetamide (DMAc, boiling point 165°C), N,N-dimethylformamide (boiling point 153°C), γ-butyrolactone (GBL, boiling point 204°C), dimethyl sulfoxide (boiling point 189°C), dimethylisobutyric acid amide (boiling point 179°C), etc. can be used, and two or more types may be used in combination. Among these, from the viewpoint of the performance of the polyimide varnish, at least one selected from the group consisting of N-methyl-2-pyrrolidone, N,N-dimethylacetamide, γ-butyrolactone, and dimethyl sulfoxide is preferable as the organic solvent (S2).

The mass ratio [(S1)/(S2)] of the organic solvent (S1) to the organic solvent (S2) contained in the organic solvent solution of polyimide is preferably 90/10 to 99/1, and more preferably 93/7 to 97/3, from the viewpoint of drying the organic solvent solution of the resin at a relatively low drying temperature and suppressing yellowing of the film.

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 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 these 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. Among these tertiary amine compounds, TEA is particularly preferable.

In addition, the concentration of the polyimide organic solvent solution used in the present invention is preferably 4 to 45% by mass, more preferably 10 to 40% by mass, of the polyimide component. Specifically, the resin organic solvent solution contains preferably 5 to 80 g, more preferably 10 to 70 g, of dissolved resin per 100 g of organic solvent. If the amount is within this range, the surface smoothness of the resulting 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 an organic solvent contained in the solution are removed.

One method for producing a colorless, transparent polyimide film is to cast a polyimide organic solvent solution onto a support and dry it to produce a film using a solution casting method. Specifically, after casting a polyimide organic solvent solution onto a support, the organic solvent is volatilized using a film-forming machine that preferably blows gas at 80°C to 250°C onto the cast material on the support, and the film is peeled off from the support to produce a self-supporting film. It is preferable to perform primary drying before blowing the gas. The conditions for primary drying are not particularly limited, but it is preferable to hold the film at a temperature of 80 to 120°C for 10 to 30 minutes, for example.

The gas to be blown can be air or nitrogen, with air being preferred from the viewpoint of cost, and nitrogen being preferred from the viewpoint of preventing discoloration of the film. The temperature of the gas to be blown is more preferably 80°C or higher and 250°C or lower, and even more preferably 100°C or higher and 220°C or lower. If the temperature of the gas to be blown is lower than 80°C, the organic solvent may not volatilize sufficiently, and the film may stick to the support when peeling it off from the support. If the temperature of the gas is higher than 250°C, the solvent may suddenly volatilize, causing foaming in the film, and the solvent may decompose and cause discoloration 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. It is also possible to provide multiple zones in which the temperature of the gas to be blown on the cast product is different.

In the method for producing a colorless and transparent polyimide film of the present invention, the step of casting a solution of a resin in an organic solvent on a support and drying the same preferably includes: (i) a step of removing a part of the solvent so that the weight loss rate at 120 to 300° C., as determined by simultaneous differential thermal and thermogravimetric measurements, is more than 1% and not more than 10%, and (ii) after the step (i) of removing a part of the solvent, a step of performing a heat treatment within a range of (Tg-50) to (Tg+100)° C., where Tg is the glass transition temperature of the resin (° C.).
The weight loss rate in the step (i) is preferably more than 1% and not more than 10%, more preferably 1% or more and not more than 5%, and even more preferably 1% or more and not more than 3%. Within this range, drying does not take too long, and a colorless and transparent resin film excellent in transparency and optical isotropy can be produced.
The heat treatment temperature in the step (ii) is preferably from (Tg-50)° C. to (Tg+100)° C., more preferably from (Tg-30)° C. to (Tg+80)° C., and even more preferably from (Tg-10)° C. to (Tg+60)° C. Within this range, a colorless and transparent resin film having excellent transparency and optical isotropy can be produced.

The polyimide film may further contain other components to the extent that the transparency and flexibility 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.

The thickness of the polyimide film obtained by this manufacturing method 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 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 50 nm or less, more preferably 40 nm or less, and even more preferably 30 nm or less.
The polyimide film preferably has an in-plane retardation (Re) of 20 nm or less, and more preferably 15 nm or less.

The polyimide film obtained by this manufacturing method may contain additives as other components. For example, the inclusion of titanium dioxide or the like improves the reflectance of white light. Furthermore, the inclusion of nanofillers or the like increases the apparent glass transition temperature of the resin composition molded product, improving heat resistance, and further increases the tensile modulus and mechanical strength.

The colorless and transparent resin film obtained by the method of the present invention is suitable for use as a film for various components such as touch sensors, color filters, flexible displays, semiconductor parts, and optical components. The colorless and transparent resin film obtained by the method of the present invention is also useful as a transparent substrate for liquid crystal display elements and organic EL display elements, and as a base material for transparent conductive films for touch panels.

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) Organic Solvent Content in Film Using a differential thermal/thermogravimetric simultaneous analyzer "TG/DTA6200" manufactured by Hitachi High-Tech Science Corporation, the measurement was carried out under conditions of a nitrogen gas flow and a temperature increase rate of 10°C/min. The temperature was increased from 120°C to 300°C and then maintained at 300°C for 30 minutes. The mass reduction during this period was recorded as the organic solvent content in the film.

(6) Glass transition temperature (Tg)
Using a thermomechanical analyzer "TMA/SS6100" manufactured by Hitachi High-Tech Science Corporation, the specimen was heated to a temperature sufficient to remove the residual stress under the conditions of a sample size of 2 mm x 20 mm, a load of 0.1 N, and a heating rate of 10°C/min in tensile mode, and then cooled to room temperature. Then, the test piece elongation was measured under the same conditions as the treatment for removing the residual stress, and the point at which an inflection point of elongation was observed was determined as the glass transition temperature.

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 lump, 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 uniform solution, and a uniform polyimide varnish (a) with a solid content concentration of 25% by mass was obtained.

The resulting polyimide varnish was then dropped into methyl alcohol to precipitate polyimide powder, and the solid was suction filtered using a Kiriyama funnel, washed with methyl alcohol, and dried at 200° C. for 30 minutes to remove the solvent, yielding 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, 80.75 g of dichloromethane (DCM), and 4.25 g of N,N-dimethylacetamide (DMAc) were added all at once, and the mixture was stirred at room temperature for 1 hour to obtain a homogeneous solution, thereby obtaining a homogeneous polyimide varnish (b) with a solid content concentration of 15 mass %.

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

Example 2
The polyimide film obtained in Example 1 was further dried in an air atmosphere for 20 minutes by blowing hot air at 250° C. onto the polyimide film to obtain a film having a thickness of 35 μm. The evaluation results of this polyimide film are shown in Table 1.

<Comparative Example 1>
The polyimide varnish (a) 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 a 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 cooling tube, a thermometer, and a glass end cap, 15 g of the obtained polyimide powder and 85 g of dichloromethane (DCM) 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 20 minutes to obtain a film with a thickness of 35 μm. The evaluation results of this polyimide film are shown in Table 1.

<Comparative Example 2>
The polyimide film obtained in Comparative Example 1 was further dried by blowing hot air at 250° C. for 20 minutes in an air atmosphere to obtain a film having a thickness of 35 μm. The evaluation results of this polyimide film are shown in Table 1.

As shown in Table 1, the polyimide films obtained in Examples 1 and 2 have good optical properties such as total light transmittance, haze, and YI, and have low Rth and excellent optical isotropy. In contrast, the polyimide films obtained in Comparative Examples 1 and 2 have high Rth and poor optical isotropy.

Claims (8)

A method for producing a resin film by a solution casting method, comprising a step of casting a solution of a resin in an organic solvent onto a support and drying the resulting solution, the organic solvent comprising at least one organic solvent (S1) having a boiling point of 80° C. or less and at least one organic solvent (S2) having a boiling point of 130° C. or more ;
A step of casting a solution of the resin in an organic solvent on a support and drying the same,
(i) removing a portion of the solvent so that the weight loss rate at 120 to 300° C., as determined by simultaneous differential thermal and thermogravimetric measurements, is greater than 1% and not greater than 10%; and
(ii) after the step (i) of removing a portion of the solvent, a step of performing a heat treatment within a range of (Tg-50)°C to (Tg+100)°C, where Tg is the glass transition temperature of the resin.
A method for producing a colorless and transparent resin film comprising the steps of : The method for producing a colorless and transparent resin film according to claim 1, wherein when a film having a thickness of 20 to 50 μm is formed from an organic solvent solution of the resin, the film has a total light transmittance of 85% or more, a yellow index (YI) of 5 or less, a haze of 2% or less, an in-plane retardation (Re) of 20 nm or less, and a thickness direction retardation (Rth) of 50 nm or less. The method for producing a colorless and transparent resin film according to claim 1 or 2, wherein the organic solvent solution of the resin is an organic solvent solution of a polyamic acid or an organic solvent solution of a polyimide. The method for producing a colorless and transparent resin film according to any one of claims 1 to 3, wherein the resin of the resin film is a polyimide having a repeating unit represented by the following formula [I]:

(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-, _-SO2- , -CO-, _-CH2- , -C( _CH3 ) _2- , -OSi( _CH3 ) _2- , _-C2H4O- and _-S- .) The method for producing a colorless and transparent resin film according to any one of claims 1 to 4, wherein the mass ratio [(S1)/(S2)] of the organic solvent (S1) to the organic solvent (S2) contained in the organic solvent solution of the resin is 90/10 to 99/1. The method for producing a colorless and transparent resin film according to any one of claims 1 to 5, wherein the organic solvent (S1) is at least one selected from the group consisting of dichloromethane and 1,3-dioxolane. The method for producing a colorless and transparent resin film according to any one of claims 1 to 6, wherein the organic solvent (S2) is at least one selected from the group consisting of N-methyl-2-pyrrolidone, N,N-dimethylacetamide, γ-butyrolactone, and dimethylsulfoxide. The method for producing a colorless and transparent resin film according to any one of claims 1 to 7, wherein the organic solvent solution of the resin contains 5 to 80 g of resin dissolved per 100 g of organic solvent.