JP7434704B2 - Method for producing a retardation film, retardation film, display panel and image display device using the retardation film - Google Patents

Description

The present invention relates to a method for producing a retardation film, a retardation film, and a display panel and an image display device using the retardation film.

In recent years, flexible image display devices such as those with curved screens and those with foldable screens have been proposed as image display devices such as liquid crystal display devices and organic EL display devices. In particular, since organic EL display devices do not require the use of glass substrates, they can be easily adapted to flexible applications.
As an optical film used in a flexible image display device, an optical film based on polyimide, which has excellent flexibility, has been proposed.

Further, it is known that the phase difference of the optical film itself that is used in an image display device affects the visibility of the image. For example, Patent Documents 1 and 2 have been proposed as polyimide films for optical films.
The polyimide films of Patent Documents 1 and 2 are used as so-called optical compensation films, and are characterized by high birefringence in order to obtain a predetermined retardation value with as thin a thickness as possible.

However, although optical films having a predetermined retardation as disclosed in Patent Documents 1 and 2 can improve visibility for certain image display devices, they may actually reduce visibility for other image display devices. was there. This is because the optical characteristics such as the wavelength distribution and polarization characteristics of light are different for each display element constituting the image display device.

As an optical film that can be applied to any type of image display device, one with a small retardation can be considered. Patent Document 3 proposes a polyimide film for optical films with a small retardation in the thickness direction.

Japanese Patent Application Publication No. 2010-180350 JP2015-209487A Japanese Patent Application Publication No. 2016-204569

However, in the polyimide film of Patent Document 3, the retardation value in the thickness direction of Example 1 was only 101 nm, and the retardation could not be said to be sufficiently low.

An object of the present invention is to provide a method for easily manufacturing a polyimide-based retardation film having a small retardation. Another object of the present invention is to provide a polyimide-based retardation film, and a display panel and an image display device using the same.

The present invention provides the following [1] to [4].
[1] A method for producing a retardation film, comprising the following steps (1) and (2).
(1) A step of applying a coating solution for forming a retardation layer in which polyimide having a glass transition temperature of 180 to 300° C. dissolved in a solvent is applied onto a substrate and dried to form a coating film.
(2) A step of heating the coating film at a temperature equal to or higher than the glass transition temperature of polyimide.
[2] A retardation film, wherein the retardation film contains polyimide, the polyimide has a glass transition temperature of 180 to 300°C, and the in-plane retardation of the retardation film at a wavelength of 589 nm is Re(589). A retardation film, wherein Re (589) and Rth (589) are both 100 nm or less, where Rth (589) is the retardation in the thickness direction at a wavelength of 589 nm.
[3] A display panel in which the retardation film described in [2] above is disposed on the light-emitting surface side of a display element.
[4] An image display device comprising the display panel described in [3] above.

The method for producing a retardation film of the present invention can easily produce a polyimide-based retardation film having a sufficiently small retardation value. Further, the retardation film, display panel, and image display device of the present invention can provide stable visibility regardless of the type of display element.

Embodiments of the present invention will be described below.
[Method for manufacturing retardation film]
The method for producing a retardation film of the present invention includes the following steps (1) and (2).
(1) A step of applying a coating solution for forming a retardation layer in which polyimide having a glass transition temperature of 180 to 300° C. dissolved in a solvent is applied onto a substrate and dried to form a coating film.
(2) A step of heating the coating film at a temperature equal to or higher than the glass transition temperature of polyimide.

<Step (1)>
Step (1) is a step in which a coating solution for forming a retardation layer in which polyimide having a glass transition temperature of 180 to 300° C. is dissolved in a solvent is applied onto a substrate and dried to form a coating film.

<<Base material>>
Examples of the base material include various plastic films.
Even if the plastic film itself has a retardation, if the plastic film has releasability, by peeling the coating from the plastic film after step (1) or step (2), A retardation film with a small retardation can be obtained.
Moreover, if a plastic film with a small retardation, such as a triacetyl cellulose film, is used as the plastic film, a retardation film with a small retardation, which has a retardation layer on the plastic film, can be obtained.

Further, examples of the base material include base materials attached to manufacturing equipment such as a drum with a smooth surface (casting drum) and a stainless steel smooth belt.
When using a base material attached to such manufacturing equipment, it is a premise that the coating film is peeled off from the base material after step (1) or step (2), as in step (3) described below. .

The base material preferably has a smooth surface. Specifically, the arithmetic mean roughness Ra of the base material surface according to JIS B0601:2001 at a cutoff value of 0.8 mm is preferably 0.05 μm or less, preferably 0.03 μm or less, and 0.05 μm or less. More preferably, it is 0.01 μm or less.

When the coating film is peeled off from the substrate after step (1), the peeled coating film may be left in the form of a sheet or may be wound up into a roll.

<<Solvent>>
The solvent can be used without any particular restriction as long as it dissolves the polyimide and remains in a predetermined amount in the coating film at the start of step (2).
Such solvents include ethylene glycol monoethyl ether, ethylene glycol monoethyl ether acetate, ethylene glycol mono-normal-butyl ether, ethylene glycol monomethyl ether, propylene glycol monomethyl ether acetate, ortho-dichlorobenzene, xylene, cresol, chloro Benzene, methyl acetate, ethyl acetate, isobutyl acetate, isopentyl acetate, n-butyl acetate, n-propyl acetate, n-pentyl acetate, cyclohexanol, cyclohexanone, cyclopentanone, 1,4-dioxane, tetrachloroethylene, toluene, Examples include methyl ethyl ketone, methyl isobutyl ketone, methyl cyclohexanol, methyl cyclohexanone, methyl-n-butyl ketone, dichloromethane, dichloroethane, N,N-dimethylacetamide, N-methylpyrrolidone, γ-butyrolactone, and mixed solvents thereof. Among these, at least one selected from the group consisting of dichloromethane, ethyl acetate, n-butyl acetate, N,N-dimethylacetamide, and mixed solvents thereof is preferred.

The content of the solvent in the coating solution for forming a retardation layer is 50 to 91% based on the total amount of the coating solution, from the viewpoint of suitability for coating on the substrate, smoothness of the surface of the dried coating film, and reduction of drying time. It is preferably 75 to 85% by mass, more preferably 75 to 85% by mass.

<<Drying conditions>>
It is preferable to adjust the drying temperature and drying time appropriately, taking into consideration the type of solvent used and the thickness of the coating film, so that the proportion of the residual solvent in the peeled coating film falls within an appropriate range. Further, in order to prevent the surface shape of the retardation film from becoming rough and the physical properties from varying depending on the location of the retardation film, it is preferable that the coating liquid is not dried rapidly.
The drying temperature is preferably 80 to 150°C under normal pressure, and preferably 10 to 100°C under reduced pressure. The drying time is preferably 2 to 60 minutes under normal pressure and 1 to 60 minutes under reduced pressure.
Note that it is preferable to dry the solvent under an inert atmosphere.

<<Polyimide>>
The coating liquid for forming a retardation layer contains polyimide as a resin component. When polyimide is not included as a resin component, the bending resistance of the retardation film cannot be improved.
The resin component may contain resins other than polyimide as long as the effects of the present invention are not impaired, but the content of polyimide is preferably 80% by mass or more of the total resin of the retardation layer, and 90% by mass or more of the total resin of the retardation layer. It is more preferably at least 95% by mass, even more preferably at least 99% by mass, and most preferably 100% by mass.

Preferably, the polyimide is a reaction product of a tetracarboxylic acid and a diamine. That is, the polyimide preferably contains a tetracarboxylic acid residue and a diamine residue. Note that the term "tetracarboxylic acid residue" refers to a residue obtained by removing four carboxyl groups from a tetracarboxylic acid, and represents the same structure as a residue obtained by removing the acid dianhydride structure from a tetracarboxylic dianhydride. Moreover, a diamine residue refers to a residue obtained by removing two amino groups from a diamine.

The polyimide used in step (1) must have a glass transition temperature of 180 to 300°C.
When the glass transition temperature of polyimide is less than 180°C, the heat resistance of the retardation film cannot be improved. Further, when the glass transition temperature of polyimide exceeds 300° C., the retardation in the thickness direction cannot be reduced (the reason for this phenomenon will be described in step (2)). Moreover, when the glass transition temperature of polyimide exceeds 300° C., the heating temperature in step (2) becomes high, and the flatness of the retardation film tends to decrease or the yellowness index (YI) increases.
The polyimide used in step (1) preferably has a glass transition temperature of 185 to 250°C.

<<<Tetracarboxylic acid>>>
Examples of the tetracarboxylic acid that is a raw material for polyimide include tetracarboxylic dianhydride having an aromatic ring and tetracarboxylic dianhydride having an aliphatic ring.

Examples of the tetracarboxylic dianhydride having an aromatic ring include pyromellitic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 2,2',3,3' -Benzophenonetetracarboxylic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 2,2',3,3'-biphenyltetracarboxylic dianhydride, 2,2-bis (3,4-dicarboxyphenyl)propane dianhydride, 2,2-bis(2,3-dicarboxyphenyl)propane dianhydride, bis(3,4-dicarboxyphenyl)ether dianhydride, bis( 3,4-dicarboxyphenyl)sulfone dianhydride, 1,1-bis(2,3-dicarboxyphenyl)ethane dianhydride, bis(2,3-dicarboxyphenyl)methane dianhydride, bis(3 ,4-dicarboxyphenyl)methane dianhydride, 1,3-bis[(3,4-dicarboxy)benzoyl]benzene dianhydride, 1,4-bis[(3,4-dicarboxy)benzoyl]benzene Dianhydride, 2,2-bis{4-[4-(1,2-dicarboxy)phenoxy]phenyl}propane dianhydride, 2,2-bis{4-[3-(1,2-dicarboxy) ) phenoxy]phenyl}propane dianhydride, bis{4-[4-(1,2-dicarboxy)phenoxy]phenyl}ketone dianhydride, bis{4-[3-(1,2-dicarboxy)phenoxy) [phenyl}ketone dianhydride, 4,4'-bis[4-(1,2-dicarboxy)phenoxy]biphenyl dianhydride, 4,4'-bis[3-(1,2-dicarboxy)phenoxy] ] Biphenyl dianhydride, bis{4-[4-(1,2-dicarboxy)phenoxy]phenyl}ketone dianhydride, bis{4-[3-(1,2-dicarboxy)phenoxy]phenyl}ketone Dianhydride, bis{4-[4-(1,2-dicarboxy)phenoxy]phenyl}sulfone dianhydride, bis{4-[3-(1,2-dicarboxy)phenoxy]phenyl}sulfone dianhydride bis{4-[4-(1,2-dicarboxy)phenoxy]phenyl}sulfide dianhydride, bis{4-[3-(1,2-dicarboxy)phenoxy]phenyl}sulfide dianhydride, 4,4'-oxydiphthalic anhydride, 3,4'-oxydiphthalic anhydride, 2,3,6,7-naphthalenetetracarboxylic dianhydride, 1,4,5,8-naphthalenetetracarboxylic dianhydride 1,2,5,6-naphthalenetetracarboxylic dianhydride, 1,2,3,4-benzenetetracarboxylic dianhydride, 3,4,9,10-perylenetetracarboxylic dianhydride , 2,3,6,7-anthracenetetracarboxylic dianhydride, 1,2,7,8-phenanthrenetetracarboxylic dianhydride, and the like.
Examples of the tetracarboxylic dianhydride having an aliphatic ring include cyclohexanetetracarboxylic dianhydride, cyclopentanetetracarboxylic dianhydride, and dicyclohexane-3,4,3',4'-tetracarboxylic dianhydride. Examples include anhydride, cyclobutanetetracarboxylic dianhydride, and the like.

<<<Diamine>>>
Diamines that are raw materials for polyimide include various diamines such as diamines having an aromatic ring, diamines having an aliphatic ring, and diamines having no ring structure.
Among these various diamines, diamines having a silicon atom in the main chain are preferred. That is, it is preferable that the polyimide contains a diamine residue having a silicon atom in its main chain as a diamine residue.
When the polyimide contains a diamine residue having a silicon atom in its main chain, the bending resistance of the retardation film can be easily improved. Furthermore, since the polyimide contains a diamine residue having a silicon atom in its main chain, the glass transition temperature of the polyimide can be easily lowered to the above-mentioned range.

Examples of diamines having a silicon atom in the main chain include those represented by the following general formulas (i) and (ii). Diamines represented by the following general formulas (i) and (ii) are preferable because they easily reduce the birefringence inherent in polyimide, and furthermore, they easily reduce the retardation of the retardation film.

[In formula (i), n represents an integer of 1 to 5. Further, in formula (i), R ¹ and R ² may be the same or different, and each represents an alkylene group having 1 to 20 carbon atoms. Further, in formula (i), R ³ to R ⁶ may be the same or different, and each represents an alkyl group having 1 to 20 carbon atoms. ]

In formula (i), n is preferably 1 to 3, more preferably 1 to 2, and even more preferably 1.
Further, in formula (i), R ¹ and R ² are preferably an alkylene group having 1 to 10 carbon atoms, more preferably an alkylene group having 1 to 8 carbon atoms, and preferably an alkylene group having 2 to 5 carbon atoms. It is more preferably an alkylene group, even more preferably an alkylene group having 2 to 3 carbon atoms. Moreover, it is preferable that R ¹ and R ² are the same.
Further, in formula (i), R ³ to R ⁶ are preferably an alkyl group having 1 to 10 carbon atoms, more preferably an alkyl group having 1 to 8 carbon atoms, and R 3 to R 6 are preferably an alkyl group having 1 to 8 carbon atoms. It is more preferably an alkyl group, even more preferably an alkyl group having 1 to 3 carbon atoms. Further, R ³ to R ⁶ are preferably the same.

[In formula (ii), R ⁷ and R ⁸ may be the same or different, and each represents an alkylene group having 1 to 20 carbon atoms. Furthermore, in formula (ii), R ⁹ and R ¹⁰ may be the same or different, and each represents an alkyl group having 1 to 20 carbon atoms. ]

Furthermore, in formula (ii), R ⁷ and R ⁸ are preferably an alkylene group having 1 to 10 carbon atoms, more preferably an alkylene group having 1 to 8 carbon atoms, and preferably an alkylene group having 2 to 5 carbon atoms. It is more preferably an alkylene group, even more preferably an alkylene group having 2 to 3 carbon atoms. Moreover, it is preferable that R ⁷ and R ⁸ are the same.
Further, in formula (ii), R ⁹ and R ¹⁰ are preferably an alkyl group having 1 to 10 carbon atoms, more preferably an alkyl group having 1 to 8 carbon atoms, and R 9 and R 10 are preferably an alkyl group having 1 to 8 carbon atoms. It is more preferably an alkyl group, even more preferably an alkyl group having 1 to 3 carbon atoms. Moreover, it is preferable that R ⁹ and R ¹⁰ are the same.

Further, when using a diamine having a silicon atom in the main chain as the diamine which is a raw material for polyimide, it is preferable to use a diamine having an aromatic ring or an aliphatic ring. That is, when the polyimide contains a diamine residue having a silicon atom in its main chain, it is preferable that the polyimide further contains a diamine residue having an aromatic ring or an aliphatic ring.
In addition to the diamine residue having a silicon atom in the main chain, the polyimide contains a diamine residue having an aromatic ring or an aliphatic ring, thereby retaining the effects of the diamine having a silicon atom in the main chain. At the same time, the heat resistance of the retardation film can be easily improved.

Molar ratio of diamine residues having a silicon atom in the main chain to diamine residues having an aromatic ring or aliphatic ring in polyimide (diamine residue having a silicon atom in the main chain/aromatic ring or (residue of diamine having an aliphatic ring) is preferably 0.05 to 1.50, more preferably 0.08 to 1.20, and preferably 0.10 to 1.00. More preferred.
By setting the ratio to 0.10 or more, the bending resistance of the retardation film can be easily improved, and the glass transition temperature of the polyimide can be easily lowered to the above-mentioned range. Furthermore, when the diamine residue has an aromatic ring, the polyimide becomes rigid and tends to cause a phase difference. Therefore, by setting the ratio to 0.05 or more, it is possible to suppress the polyimide from becoming rigid and to easily reduce the retardation of the retardation film.
Further, by setting the ratio to 1.50 or less, it is possible to easily suppress the strength of the retardation film from decreasing.

Examples of diamines having an aromatic ring include p-phenylenediamine, m-phenylenediamine, o-phenylenediamine, 3,3'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl ether, 3, 3'-diaminodiphenylsulfide, 3,4'-diaminodiphenylsulfide, 4,4'-diaminodiphenylsulfide, 3,3'-diaminodiphenylsulfone, 3,4'-diaminodiphenylsulfone, 4,4'-diaminodiphenyl Sulfone, 3,3'-diaminobenzophenone, 4,4'-diaminobenzophenone, 3,4'-diaminobenzophenone, 4,4'-diaminobenzanilide, 3,3'-diaminodiphenylmethane, 4,4'-diaminodiphenylmethane , 3,4'-diaminodiphenylmethane, 2,2-di(3-aminophenyl)propane, 2,2-di(4-aminophenyl)propane, 2-(3-aminophenyl)-2-(4-amino phenyl)propane, 1,1-di(3-aminophenyl)-1-phenylethane, 1,1-di(4-aminophenyl)-1-phenylethane, 1-(3-aminophenyl)-1-( 4-aminophenyl)-1-phenylethane, 1,3-bis(3-aminophenoxy)benzene, 1,3-bis(4-aminophenoxy)benzene, 1,4-bis(3-aminophenoxy)benzene, 1,4-bis(4-aminophenoxy)benzene, 1,3-bis(3-aminobenzoyl)benzene, 1,3-bis(4-aminobenzoyl)benzene, 1,4-bis(3-aminobenzoyl) Benzene, 1,4-bis(4-aminobenzoyl)benzene, 1,3-bis(3-amino-α,α-dimethylbenzyl)benzene, 1,3-bis(4-amino-α,α-dimethylbenzyl) ) benzene, 1,4-bis(3-amino-α,α-dimethylbenzyl)benzene, 1,4-bis(4-amino-α,α-dimethylbenzyl)benzene, 2,6-bis(3-amino phenoxy)benzonitrile, 2,6-bis(3-aminophenoxy)pyridine, N,N'-bis(4-aminophenyl)terephthalamide, 9,9-bis(4-aminophenyl)fluorene, 2,2'-dimethyl-4,4'-diaminobiphenyl,3,3'-dichloro-4,4'-diaminobiphenyl,3,3'-dimethoxy-4,4'-diaminobiphenyl,3,3'-dimethyl-4,4'-diaminobiphenyl,4,4'-bis(3-aminophenoxy)biphenyl,4,4'-bis(4-aminophenoxy)biphenyl, bis[4-(3-aminophenoxy)phenyl]ketone, bis[ 4-(4-aminophenoxy)phenyl]ketone, bis[4-(3-aminophenoxy)phenyl]sulfide, bis[4-(4-aminophenoxy)phenyl]sulfide, bis[4-(3-aminophenoxy) phenyl] sulfone, bis[4-(4-aminophenoxy)phenyl]sulfone, bis[4-(3-aminophenoxy)phenyl]ether, bis[4-(4-aminophenoxy)phenyl]ether, 2,2- Bis[4-(3-aminophenoxy)phenyl]propane, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, 1,3-bis[4-(3-aminophenoxy)benzoyl]benzene, 1,3-bis[4-(4-aminophenoxy)benzoyl]benzene, 1,4-bis[4-(3-aminophenoxy)benzoyl]benzene, 1,4-bis[4-(4-aminophenoxy) benzoyl]benzene, 1,3-bis[4-(3-aminophenoxy)-α,α-dimethylbenzyl]benzene, 1,3-bis[4-(4-aminophenoxy)-α,α-dimethylbenzyl] Benzene, 1,4-bis[4-(3-aminophenoxy)-α,α-dimethylbenzyl]benzene, 1,4-bis[4-(4-aminophenoxy)-α,α-dimethylbenzyl]benzene, 4,4'-bis[4-(4-aminophenoxy)benzoyl]diphenyl ether, 4,4'-bis[4-(4-amino-α,α-dimethylbenzyl)phenoxy]benzophenone, 4,4'-bis [4-(4-amino-α,α-dimethylbenzyl)phenoxy]diphenylsulfone, 4,4'-bis[4-(4-aminophenoxy)phenoxy]diphenylsulfone, 3,3'-diamino-4,4 '-Diphenoxybenzophenone, 3,3'-diamino-4,4'-dibiphenoxybenzophenone, 3,3'-diamino-4-phenoxybenzophenone, 3,3'-diamino-4-biphenoxybenzophenone, 6,6 '-bis(3-aminophenoxy)-3,3,3',3'-tetramethyl-1,1'-spirobiindane, 6,6'-bis(4-aminophenoxy)-3,3,3', Examples include 3'-tetramethyl-1,1'-spirobiindane.
A diamine having an aromatic ring is suitable because it can improve the heat resistance of the retardation film. In addition, among diamines having an aromatic ring in the molecule, from the viewpoint of transparency, those having a fluorine atom in the molecule, and alkylene which may have aromatic rings substituted with a sulfonyl group or fluorine. It is preferable to use a group connected by a group.

Examples of diamines having an aliphatic ring include trans-cyclohexane diamine, trans-1,4-bismethylenecyclohexane diamine, 2,6-bis(aminomethyl)bicyclo[2,2,1]heptane, and 2,5-bis( (aminomethyl)bicyclo[2,2,1]heptane and the like.
Diamines having an aliphatic ring are suitable because they can improve the transparency of the retardation film.

From the viewpoint of improving the transparency of the retardation film, it is preferable that the tetracarboxylic acid residue and/or the diamine residue contain a fluorine atom.
As the tetracarboxylic acid that can contain a fluorine atom in the residue, 2,2-bis(3,4-dicarboxyphenyl)-1,1,1,3,3,3-hexafluoropropane dianhydride , 2,2-bis(2,3-dicarboxyphenyl)-1,1,1,3,3,3-hexafluoropropane dianhydride, 4,4'-(hexafluoroisopropylidene) diphthalic anhydride , 3,4'-(hexafluoroisopropylidene) diphthalic anhydride, and 3,3'-(hexafluoroisopropylidene) diphthalic anhydride.
Examples of diamines that can contain a fluorine atom in the residue include 2,2'-bis(trifluoromethyl)benzidine, 2,2-di(3-aminophenyl)-1,1,1,3,3, 3-hexafluoropropane, 2,2-di(4-aminophenyl)-1,1,1,3,3,3-hexafluoropropane, 2-(3-aminophenyl)-2-(4-aminophenyl) )-1,1,1,3,3,3-hexafluoropropane, 1,3-bis(3-amino-α,α-ditrifluoromethylbenzyl)benzene, 1,3-bis(4-amino-α , α-ditrifluoromethylbenzyl)benzene, 1,4-bis(3-amino-α,α-ditrifluoromethylbenzyl)benzene, 1,4-bis(4-amino-α,α-ditrifluoromethylbenzyl) Benzene, 2,2-bis[3-(3-aminophenoxy)phenyl]-1,1,1,3,3,3-hexafluoropropane, 2,2-bis[4-(4-aminophenoxy)phenyl ]-1,1,1,3,3,3-hexafluoropropane and the like. The diamines exemplified here that can contain a fluorine atom in their residues all have aromatic rings in their molecules.
Examples of diamines whose residues can include a structure in which aromatic rings are connected by a sulfonyl group or an alkylene group optionally substituted with fluorine include bis[4-(3-aminophenoxy)phenyl] Examples include sulfone and bis[4-(4-aminophenoxy)phenyl]sulfone.

The coating liquid for forming a retardation layer may contain other materials such as inorganic fine particles, ultraviolet absorbers, and light stabilizers within a range that does not impede the effects of the present invention. Examples of the inorganic fine particles include elastic modulus improvers such as silica and alumina, and refractive index adjusters such as zirconia and titanium oxide. Further, in order to suppress an increase in haze, the inorganic fine particles preferably have an average particle diameter of 200 nm or less. Further, it is preferable that the other materials do not exhibit an in-plane retardation or a retardation in the thickness direction.

<<<Manufacture of polyimide>>>
Polyimide can be produced, for example, by the following steps (A1) and (A2).
(A1) Preparation of polyimide precursor composition (A2) Imidization step

In step (A1), the polyimide precursor composition can be obtained by reacting a tetracarboxylic acid and a diamine in a solvent. The molar ratio of tetracarboxylic acid and diamine is preferably approximately 1:1.
The solvent may be selected from general-purpose solvents that can dissolve the tetracarboxylic acid and diamine. The content of the solvent is preferably 100 to 1900 parts by mass based on 100 parts by mass of the polyimide precursor. By controlling the content of the solvent to 100 to 1900 parts by mass, the molecular weight can be easily improved.

The weight average molecular weight of the polyimide precursor measured by gel permeation chromatography (GPC) is preferably 10,000 or more, more preferably 20,000 or more in terms of polystyrene, from the viewpoint of the strength of the retardation film. . On the other hand, if the weight average molecular weight is too large, the viscosity may become high and workability such as filtration may deteriorate, so it is preferably 10,000,000 or less, and more preferably 500,000 or less. .

Step (A2) is a step of imidizing the polyimide precursor. Examples of imidization methods include chemical imidization and thermal imidization. Among these, chemical imidization is preferred because it easily lowers the yellowness index (YI) of the retardation film. That is, the polyimide used in step (1) is preferably a chemically imidized polyimide.
Chemical imidization is a method of treatment with a dehydrating reagent such as a mixture of acetic anhydride and pyridine. The thermal imidization method is a method of heat treatment at 200 to 350°C. In either method, the polyimide precursor can be imidized in a state in which the polyimide precursor is dissolved in a solvent, that is, in the state of a polyimide precursor composition.

<Step (2)>
Step (2) is a step of heating the coating film at a temperature equal to or higher than the glass transition temperature of polyimide.

In step (2), by heating the coating film at a temperature higher than the glass transition temperature of polyimide, the polyimide molecules move more easily in the coating film, and the polyimide whose thickness direction in the coating film is approximately horizontal. The orientation of the molecules in the thickness direction is randomly disturbed, and the phase difference in the thickness direction can be made sufficiently small. However, if the glass transition temperature of polyimide exceeds 300°C, polyimide molecules will be difficult to move in the coating even if the coating is heated to a temperature higher than the glass transition temperature of the polyimide, so it is necessary to reduce the retardation in the thickness direction. (See Experimental Examples 4-2 to 4-4 in Examples). That is, in the present invention, by intentionally suppressing the level of heat resistance, it is possible to manufacture a retardation film having a sufficiently small retardation in the thickness direction.
Moreover, by performing the heating in step (2), it is possible to release the strain generated in the coating film in step (1) and suppress thermal shrinkage of the retardation film. Specifically, by performing the heating in step (2), the retardation film does not undergo thermal shrinkage even when heated at any temperature in the temperature range from 25 ° C. to the glass transition temperature of polyimide. can do.

When the glass transition temperature of polyimide is A (°C), the heating temperature in step (2) is preferably A+10°C or higher, more preferably A+20°C or higher.
Further, from the viewpoint of making it easier to randomize the orientation of the polyimide molecules in the thickness direction, the heating temperature in step (2) is preferably 230°C or higher, more preferably 240°C or higher.
In addition, in step (2), if the heating temperature is made too high, the flatness of the retardation film may decrease or the yellowness index (YI) of JIS K7373:2006 may increase. For this reason, the heating temperature in step (2) is preferably A+50°C or lower. From the same viewpoint, the heating temperature in step (2) is preferably 370°C or lower, more preferably 350°C or lower, even more preferably 330°C or lower, and even more preferably 300°C or lower. More preferred.

Step (2) may be performed under reduced pressure or under normal pressure.
Further, the heating time in step (2) is preferably 1 to 40 minutes, more preferably 5 to 35 minutes, and even more preferably 10 to 30 minutes.
Further, the heating in step (2) may be performed in the air or in an inert atmosphere.

When the total mass of the coating film at the start of step (2) is W, and the mass of the residual solvent contained in the coating film at the start of step (2) is W1, W1/W is 0.02 or less. It is preferably at most 0.01, more preferably at most 0.005, and even more preferably at most 0.005.
By setting W1/W to 0.02 or less, the amount of solvent volatilization between step (1) and step (2) is suppressed, so that curling and shrinkage of the coating film can be easily suppressed.

At the end of step (2), it is preferred that the coating film is substantially free of solvent. Substantially not containing means 0.1% by mass or less, preferably 0.02% by mass or less, more preferably 0.01% by mass or less based on the total amount of the coating film at the end of step (2). It is.

In step (1), when the coating film is peeled from the base material and wound up into a roll, in step (2), in order to suppress shrinkage of the coating film, while unrolling the coating film from the roll, It is preferable to do so.

In order to suppress shrinkage of the coating film, step (2) is preferably performed with the coating film fixed in four directions. For example, when performing step (2) while unwinding a coating film wound into a roll, in addition to both ends in the flow direction fixed by the unwinding device, both ends in the width direction are secured with clips, etc. Preferably, it is fixed. In particular, when performing step (3), which will be described later, after step (1) and before step (2), the coating film is in a state where it is susceptible to heat shrinkage, so as mentioned above, It is important to perform step (2) while fixing the four directions of the coating film.
In addition, in step (1), if the retardation layer forming coating liquid is applied within a frame such as a metal frame and dried to form a coating film, and then step (2) is performed, the coating liquid can be formed by using the frame. can be fixed in four directions.

After completing step (2), it is preferable to cool the coating film. By cooling the coating film, the polyimide molecules are fixed and become difficult to move, making it easier to maintain the effect obtained by the heating step in step (2).
Although the conditions for cooling the coating film are not particularly limited, it is preferable, for example, that the temperature is 15 to 40°C and the time is 1 to 40 minutes.

It is preferable that the method for producing a retardation film of the present invention further includes the following step (3). By performing step (3), the retardation film becomes base-free, and the thickness of the retardation film can be reduced.
<Step (3)>
(3) A step of peeling off the coating film from the substrate after step (1) or step (2).

If the above-mentioned coating film cooling step and step (3) are performed after the completion of step (2), from the viewpoint of workability, the coating film cooling step should be performed before step (3). is preferred.

The method for producing a retardation film of the present invention may include other steps as long as the effects of the present invention are not impaired. In addition, in order to prevent in-plane retardation from occurring as much as possible, it is preferable that the method for producing a retardation film of the present invention does not include a stretching step.

The thickness of the retardation film obtained by the production method of the present invention is not particularly limited, but from the viewpoint of improving handleability and mechanical strength, it is preferably 15 to 300 μm, more preferably 20 to 200 μm. The thickness is preferably 25 to 100 μm, more preferably 25 to 100 μm.
The thickness of the retardation film can be measured, for example, with a micrometer (trade name: Digimatic Micrometer, manufactured by Mitutoyo Corporation).

<Size, shape, etc.>
The retardation film obtained by the production method of the present invention may be in the form of a sheet or a roll.
Further, the size of the leaves is not particularly limited, but generally the size is about 2 to 500 inches diagonally. The width and length of the roll are not particularly limited, but generally the width is about 500 to 3000 mm and the length is about 500 to 5000 m.
Further, the shape of each leaf is not particularly limited, and may be, for example, a polygon (triangle, quadrangle, pentagon, etc.), a circle, or a random amorphous shape.
<Physical properties>
The retardation film obtained by the production method of the present invention has various physical properties (in-plane retardation, thickness direction retardation, yellowness index (YI), haze, total light transmittance, mandrel diameter, heat shrinkage) as described below. It is preferable that the retardation film of the present invention falls within the range of the retardation film of the present invention.

<Application>
Since the retardation film obtained by the production method of the present invention can reduce the retardation, it can be suitably used for various optical applications, and in particular, it can be used as a substrate for optical films for image display devices, and as a base material for optical films for image display devices. It can be suitably used for image display devices such as base materials for touch panels.
Furthermore, the retardation film contains polyimide and therefore has excellent bending resistance. Therefore, the retardation film is preferably applied to a foldable image display device. An example of a foldable image display device is an organic EL display device.
Further, since the retardation film contains polyimide and has excellent heat resistance, it can be suitably used as a member exposed to heat. Examples of members exposed to heat include members of image display devices (particularly organic EL display devices) and base materials of transparent conductive films. In particular, since the base material of the transparent conductive film is exposed to extremely high temperatures, the retardation film is preferably used.
From the above, the retardation film is useful as a base material for a transparent conductive film for a touch panel incorporated in an image display device, and especially as a base material for a transparent conductive film for a touch panel incorporated in an organic EL display device. It is extremely useful as a

[Retardation film]
The retardation film of the present invention includes polyimide, the polyimide has a glass transition temperature of 180 to 300°C, and the in-plane retardation at a wavelength of 589 nm of the retardation film is Re (589), and the wavelength of the retardation film is When the retardation in the thickness direction at 589 nm is Rth (589), Re (589) and Rth (589) are both 100 nm or less.

<Polyimide>
Polyimide has a glass transition temperature of 180 to 300°C.
When the glass transition temperature of polyimide is less than 180°C, the heat resistance of the retardation film cannot be improved. Furthermore, when the glass transition temperature of polyimide exceeds 300° C., even if a heating step above the glass transition temperature is performed to reduce the retardation, the retardation in the thickness direction cannot be reduced. Furthermore, when the glass transition temperature of polyimide exceeds 300°C, high heat is applied during heating to reduce the retardation, which tends to reduce the flatness of the retardation film and increase the yellowness index (YI). .
That is, in the present invention, the glass transition temperature of polyimide is set to 180 to 300°C, which is not unnecessarily high, and the heat resistance level is deliberately suppressed, thereby reducing the retardation of the retardation film and improving various aspects of the retardation film. Physical properties can be easily improved.
The polyimide preferably has a glass transition temperature of 185 to 250°C.

The embodiment of the polyimide contained in the retardation film is the same as the embodiment of the polyimide used in the method for producing a retardation film of the present invention described above.

In the retardation film of the present invention, when the in-plane retardation at a wavelength of 589 nm is Re (589) and the retardation in the thickness direction at a wavelength of 589 nm is Rth (589), Re (589) and Rth (589) are both It is also required that the thickness is 100 nm or less.
When Re (589) and Rth (589) exceed 100 nm, stable visibility cannot be provided regardless of the type of display element.
Re(589) and Rth(589) are preferably 75 nm or less, more preferably 50 nm or less.

In this specification, Re (589) and Rth (589) are "N _x ", the refractive index in the slow axis direction, which is the direction in which the retardation film has the largest refractive index, and the slow axis in the plane. When the refractive index in the fast axis direction which is perpendicular to the direction is "N _y ", the refractive index in the thickness direction of the retardation film is "N _z ", and the thickness of the retardation film is "d (nm)". can be expressed by the following formula.
In-plane phase difference (Re) = (N _x - N _y ) x d
Phase difference in thickness direction (Rth) = ((N _x + N _y )/2-N _z )×d

The retardation film may be used as a retardation film with a functional layer formed by laminating one or more functional layers on one or both surfaces of the retardation film. Examples of the functional layer include a hard coat layer, a transparent conductive layer, an adhesive layer, an optical adhesive layer (OCA), and an antistatic layer.
When these functional layers have substantially no in-plane retardation or thickness direction retardation, Re (589) and Rth (589) are applied with the functional layers laminated on the retardation film. May be measured. "Substantially no in-plane retardation and no thickness direction retardation" means that the retardation is 1 nm or less.

In addition, if one or more functional layers are laminated on one or both sides of the retardation film, the functional layers may be removed or pre-treated to make the thickness of the functional layers as thin as possible, and then the Re (589) and Rth (589) of the retardation film may be measured. However, it is important not to affect the thickness of the retardation film during the pretreatment. That is, if the retardation film is already clearly visible and the functional layer is presumed to be extremely thin, it is possible to measure it as is, so it is better not to forcibly peel off the entire film.
For example, when a conductive film comprising a patterned transparent conductive layer on a retardation film is used as a touch panel sensor, a cover film is placed on the conductive film via an adhesive layer. There is a cover glass. For this purpose, first, insert a cutter blade into the edge and peel off the cover film or cover glass. If it does not peel off easily, move on to the next step without forcing it off. Next, the process of immersing the sample in 40°C warm water for 10 seconds and taking it out is repeated three times. After that, use a cutter or the like to check the degree of peeling of the adhesive layer, and if necessary, repeat the process of immersing it in 40°C warm water for 10 seconds and taking it out three more times. The adhesive layer is then peeled off using a tool that does not damage the transparent conductive layer (for example, something thin and flat but without a blade). Note that even if the adhesive layer cannot be peeled off from the entire surface, it is sufficient if it can be peeled off at the part to be measured. By doing so, a conductive film including a patterned transparent conductive layer on a retardation film can be taken out from the touch panel. Since the transparent conductive layer usually does not have a retardation, the retardation of the retardation film can be measured.

<Physical properties>
The retardation film preferably has a yellowness YI of JIS K7373:2006 of 5.0 or less, more preferably 3.0 or less, even more preferably 2.5 or less, 2. It is even more preferable that it is 3 or less.

The retardation film preferably has a haze of 1.0% or less according to JIS K7136:2000, more preferably 0.9% or less, and even more preferably 0.8% or less.
The retardation film preferably has a total light transmittance of 80% or more according to JIS K7361-1:1997, more preferably 85% or more, and even more preferably 90% or more.
The above ranges of YI, haze and total light transmittance are, in principle, within the range of a single layer of retardation film. However, if the functional layer does not have a substantial effect on the values of YI, haze, and total light transmittance, the YI, haze, and total light transmittance may be changed with the functional layer laminated on the retardation film. May be measured.

In this specification, Re (589), Rth (589), YI, haze, and total light transmittance mean average values of measured values at 16 locations. The 16 measurement points are the 16 points of intersection when drawing a line that divides the area inside the margin into 5 equal parts in the vertical and horizontal directions, using a 1 cm area from the outer edge of the measurement sample as a margin. It is preferable to set it at the center of If the measurement sample is a rectangle, take a 1cm area from the outer edge of the rectangle as a margin, and perform measurements centered on 16 points of intersection of lines that divide the area inside the margin into 5 equal parts in the vertical and horizontal directions. , it is preferable to calculate the average value thereof. In addition, if the measurement sample has a shape other than a quadrilateral such as a circle, ellipse, triangle, or pentagon, draw a quadrilateral with the maximum area inscribed in these shapes and perform measurements at 16 locations using the above method regarding the quadrilateral. preferable. Note that depending on the relationship between the size of the measurement sample and the size of the measurement spot, the measurement areas may partially overlap, and in that case, it is sufficient to perform the measurement overlappingly.
Among the measured values at 16 locations measured as above, the variation in Re (589) and Rth (589) is preferably within ±10 nm, more preferably within ±5 nm with respect to the average value. , more preferably within ±3 nm, even more preferably within ±1 nm. Furthermore, among the measured values at 16 locations measured as above, the variation in YI, haze, and total light transmittance is preferably within ±15% of the average value, and preferably within ±10%. is more preferably within ±7%, even more preferably within ±5%.

It is preferable that the retardation film does not undergo thermal shrinkage even when heated at any temperature in the temperature range from 25°C to the glass transition temperature of polyimide (temperature range of 25°C or higher and lower than or equal to the glass transition temperature of polyimide). When the retardation film has this configuration, it is possible to suppress the retardation from increasing over time due to heat shrinkage. Note that the lower limit of 25° C. defines an average room temperature.

In the bending resistance test of the retardation film using the cylindrical mandrel method specified in JIS K5600-5-1:1999, it is preferable that the diameter of the mandrel in which cracks occur in the retardation film is 3 mm or less, and 2 mm or less. It is more preferable that there be.
By setting the diameter of the mandrel to 3 mm or less, better bending resistance can be achieved.

[Display panel]
The display panel of the present invention includes the above-described retardation film of the present invention disposed on the light exit surface side of the display element.
The retardation film disposed on the display element may be one in which one or more functional layers are laminated on one or both surfaces of the retardation film. Examples of the functional layer include a hard coat layer, a transparent conductive layer, an adhesive layer, an optical adhesive layer (OCA), and an antistatic layer.

<Display element>
Examples of display elements include liquid crystal display elements, organic EL display elements, inorganic EL display elements, plasma display elements, electronic paper display elements, LED display elements (such as micro LEDs), display elements using quantum dots, and the like. Note that the liquid crystal display element may be an in-cell touch panel liquid crystal display element that has a touch panel function within the element.
Among these display elements, organic EL display elements are preferred in that they can effectively exhibit the effects of the present invention.

Furthermore, the display panel of the present invention may include a touch panel. A touch panel is arranged, for example, between a display element and a retardation film. Further, the retardation film of the present invention described above may be used as a constituent member of a touch panel (for example, the retardation film of the present invention may be used as a base material of a transparent conductive film of a touch panel).
Examples of the touch panel include a resistive touch panel, a capacitive touch panel, an optical touch panel, an ultrasonic touch panel, and an electromagnetic induction touch panel. Note that these touch panels may be equipped with a pressure detection function.

[Image display device]
The image display device of the present invention is not particularly limited as long as it includes the display panel of the present invention, but the image display device accommodates the display panel of the present invention, a drive control section electrically connected to the display panel, and It is preferable to include a housing.

Next, the present invention will be explained in more detail with reference to examples, but the present invention is not limited to these examples in any way. Note that "parts" and "%" are based on mass unless otherwise specified.

1. Measurement and Evaluation The following measurements and evaluations were performed on the retardation films obtained in each experimental example. The results are shown in Table 1. The atmosphere during each measurement and evaluation was such that the temperature was 23°C ± 5°C and the humidity was 40 to 65%. Furthermore, before measurement and evaluation, the samples were exposed to the above atmosphere for 30 minutes or more. The results are shown in Table 1.

1-1. Measurement of Retardation A sample for measurement was prepared by cutting the retardation film obtained in each experimental example into a size of 50 mm in length x 50 mm in width. Re (589) and Rth (589) of the measurement sample were measured using a product named "RETS-100 (measurement spot: diameter 5 mm)" manufactured by Otsuka Electronics. The measurement locations were 16 as defined in the main text of the specification, and the average value of the values at the 16 locations was taken as Re (589) and Rth (589) for each experimental example.

1-2. Measurement of YI Using an ultraviolet-visible near-infrared spectrophotometer (JASCO Corporation, trade name "V-7100"), the YI of the sample prepared in 1-1 above was measured in accordance with JIS K7373:2006. Specifically, transmittance was measured using the ultraviolet-visible near-infrared spectrophotometer according to the spectrophotometric method specified in JIS Z8720, and YI was calculated based on the transmittance. The measurement locations were 16 as specified in the main text of the specification, and the average value of the values at the 16 locations was taken as the YI of each experimental example.

1-3. Measurement of total light transmittance and haze The samples were prepared in 1-1 above using a haze meter (Murakami Color Research Institute, trade name "HM150") in accordance with JIS K7361-1:1997 and JIS K7136:2000. The total light transmittance and haze of the sample were measured. The measurement locations were 16 as specified in the main text of the specification, and the average value of the values at the 16 locations was used as the total light transmittance and haze of each experimental example.

1-4. Flexibility (mandrel diameter)
The retardation film of each experimental example was subjected to a bending resistance test using the cylindrical mandrel method specified in JIS K5600-5-1:1999. The diameter of the mandrel was gradually reduced, and the diameter of the mandrel at which the retardation film first cracked is shown in Table 1.

1-5. Heat shrinkage Using a thermomechanical analyzer (manufactured by Shimadzu Corporation, product name "TMA-60"), the retardation film of each experimental example was heated from 25 °C to the glass transition temperature at a temperature increase of 10 °C/min. The dimensions of the sample at each temperature were measured. When the temperature was raised continuously from 25°C to the glass transition temperature, there was no temperature range in which the sample size contracted, and it was rated "A," and there was a temperature range in which the sample size contracted. ”.

1-6. Glass transition temperature (Tg)
The glass transition temperature (Tg) of the polyimide used in each experimental example was measured using a dynamic viscoelasticity measuring device (manufactured by TA Instruments, trade name "RSA-G2"). The measurement conditions were a measurement range of -150°C to 490°C, tension as the deformation mode, frequency of 1Hz, heating rate of 5°C/min, sample width of 5mm, and distance between chucks of 20mm to determine dynamic viscoelasticity. Measurements were performed to obtain a tan δ (tan δ = loss modulus (E'')/storage modulus (E')) curve, and the temperature at the top of the peak was determined. The measurement conditions of the device were set as follows. When there were multiple peaks in the tan δ curve, the temperature at the apex of the peak with the maximum peak value was defined as the glass transition temperature. When analyzing peaks and inflection points, the data was digitized and analyzed from the numerical values without visual evaluation.
<RSA-G2 measurement conditions>
(Initial value)
Axial force: 3.0g
Sensitivity: 1.0g
Proportional force Mode: Force Tracking
Axial Force > Dynamic Force: 1.5%
Minimum axial force: 2.0g
Programmed Extension Below: 0Pa
(Auto strain)
Mode: Enabled
Strain adjustment: 20.0%
Minimum strain: 0.01%
Maximum strain: 3.0%
Minimum force: 1.5g
Maximum force: 200.0g
(Test parameters)
Sampling rate: 10pts/s
Strain%: 0.1%
Frequency: Single point
Frequency 1Hz
In addition, as a sample for measuring the tan δ curve, a polyimide film that has been left for 24 hours in an environment of 23°C ± 2°C and RH 30-50% is sampled to a size of 10 cm or more, and the central part of the film is sampled with a razor or scalpel. Using a cutting jig with a 5 mm wide slit, a sample was cut into a size of 5 mm wide x 50 mm long (so that the sample length was 20 mm when chucked). The width was measured three times at different positions using calipers, and the average value was recorded. At this time, if part of the width measurement had a variation range of 3% or more of the average value, that sample was not used. For the thickness of the polyimide film, the value measured by the film thickness measurement method described above was used.

1-7. Residual solvent at the start of step (2) The total mass of the coating film at the start of step (2) is W, and the mass of the residual solvent contained in the coating film at the start of step (2) is W1, and each experimental example is The W1/W of the sample was calculated, and those with W1/W exceeding 0.02 were graded as "C", and those with W1/W of 0.02 or less were graded as "A". The residual solvent in the coating film was measured using a gas chromatography mass spectrometer.

1-8. Visibility of Image Display Devices Commercially available image display devices (three types each of liquid crystal display devices and organic EL display devices) were prepared and observed from various angles. Further, the retardation film of each experimental example was placed on the image display device and observed from various angles. Twenty adults evaluated the visibility of images with and without a retardation film and ranked them based on the following criteria.
AA: 18 or more people feel that the visibility of images does not change with or without a retardation film, regardless of the model.
A: Regardless of the model, between 16 and 17 people feel that the visibility of images does not change with or without a retardation film.
B: 10 to 15 people feel that the visibility of images does not change with or without a retardation film, regardless of the model.
C: Regardless of the model, 10 or fewer people feel that the visibility of images does not change with or without a retardation film.

2. Preparation of polyimide precursor composition

<Polyimide precursor composition 1>
A solution in which 100.0 g of dehydrated dimethylacetamide and 1.9 g (7.6 mmol) of 1,3-bis(3-aminopropyl)tetramethyldisiloxane (AprTMOS) were dissolved in a 500 ml separable flask. Add 1.7 g (3.8 mmol) of 4,4'-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) to a liquid temperature controlled at 30°C so that the temperature rise is 2°C or less. The mixture was gradually added and stirred using a mechanical stirrer for 1 hour. To this, 29.8 g (68.9 mmol) of bis[4-(3-aminophenoxy)phenyl]sulfone (BAPS-M) was added, and after confirming that it had completely dissolved, 4,4'-(hexafluoro 34.0 g (76.5 mmol) of isopropylidene) diphthalic anhydride (6FDA) was gradually added in several portions so that the temperature rise was 2° C. or less, and the polyimide precursor composition in which polyimide precursor 1 was dissolved was prepared. Product 1 (solid content 40% by weight) was obtained. The molar ratio of AprTMOS and BAPS-M used in polyimide precursor 1 was 10:90. The weight average molecular weight of polyimide precursor 1 measured by GPC was 230,000.

<Polyimide precursor composition 2>
Polyimide precursor composition 2 (solid content 40% by weight) was obtained in the same manner as polyimide precursor composition 1 except that the molar ratio of AprTMOS and BAPS-M was changed to 20:80. The weight average molecular weight of polyimide precursor 2 measured by GPC was 185,000.

<Polyimide precursor composition 3>
A solution in which 187.7 g of dehydrated dimethylacetamide and 12.5 g (50 mmol) of 1,3-bis(3-aminopropyl)tetramethyldisiloxane (AprTMOS) were dissolved was placed in a 500 ml separable flask. 11.1 g (25 mmol) of 4,4'-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) was gradually added to the temperature controlled at 30°C so that the temperature rise was 2°C or less. , and stirred for 1 hour using a mechanical stirrer. To this, 16.0 g (50 mmol) of 2,2'-bis(trifluoromethyl)benzidine (TFMB) was added, and after confirming that it had completely dissolved, 4,4'-(hexafluoroisopropylidene) diphthalic acid was added. 33.3 g (74.9 mmol) of anhydride (6FDA) was gradually added in several portions so that the temperature rise was 2°C or less, and polyimide precursor composition 3 (solid content) in which polyimide precursor 3 was dissolved was added. 28% by weight). The molar ratio of AprTMOS and TFMB used in polyimide precursor 3 was 50:50. The weight average molecular weight of polyimide precursor 3 measured by GPC was 79,000.

<Polyimide precursor composition 4>
Polyimide precursor composition 4 (solid content 28% by weight) was obtained in the same manner as polyimide precursor composition 3 except that the molar ratio of AprTMOS and TFMB was changed to 5:95. The weight average molecular weight of polyimide precursor 4 measured by GPC was 200,000.

3. Imidization (chemical imidization)
0.4 g (4.9 mmol) of pyridine as a catalyst and 38.0 g (372 mmol) of acetic anhydride were added to polyimide precursor composition 1 and stirred at room temperature for 24 hours to obtain polyimide 1. 250 g of the obtained polyimide solution was transferred to a 5 L separable flask, 500 g of DMAc was added, and the mixture was stirred until it became homogeneous. Next, 2-isopropanol was gradually added until a slight cloudiness was observed to obtain a cloudy solution. 2 kg of 2-isopropanol was added all at once to the cloudy solution to obtain a white slurry. The slurry was filtered and washed 10 times with 2-isopropanol to obtain polyimide 1.
0.7 g (8.9 mmol) of pyridine as a catalyst and 73.4 g (719 mmol) of acetic anhydride were added to polyimide precursor composition 2 and stirred at room temperature for 24 hours to obtain polyimide 2. 300 g of the obtained polyimide solution was transferred to a 5 L separable flask, 840 g of DMAc was added, and the mixture was stirred until it became homogeneous. Next, 2-isopropanol was gradually added until a slight cloudiness was observed to obtain a cloudy solution. 2 kg of 2-isopropanol was added all at once to the cloudy solution to obtain a white slurry. The slurry was filtered and washed 10 times with 2-isopropanol to obtain polyimide 2.
0.4 g (4.9 mmol) of pyridine as a catalyst and 38.0 g (372 mmol) of acetic anhydride were added to polyimide precursor composition 3 and stirred at room temperature for 24 hours to obtain a polyimide solution. 280 g of the obtained polyimide solution was transferred to a 5 L separable flask, 400 g of butyl acetate was added, and the mixture was stirred until it became homogeneous. Next, 2-isopropanol was gradually added until a slight cloudiness was observed to obtain a cloudy solution. 2 kg of 2-isopropanol was added all at once to the cloudy solution to obtain a white slurry. The slurry was filtered and washed 10 times with 2-isopropanol to obtain polyimide 3.
Similarly, 0.4 g (4.9 mmol) of pyridine as a catalyst and 39.6 g (388 mmol) of acetic anhydride were added to polyimide precursor composition 4 and stirred at room temperature for 24 hours to obtain a polyimide solution. 300 g of the obtained polyimide solution was transferred to a 5 L separable flask, 430 g of butyl acetate was added, and the mixture was stirred until it became homogeneous. Next, 2-isopropanol was gradually added until a slight cloudiness was observed to obtain a cloudy solution. 2 kg of 2-isopropanol was added all at once to the cloudy solution to obtain a white slurry. The slurry was filtered and washed 10 times with 2-isopropanol to obtain polyimide 4.

4. Preparation of coating liquid for forming retardation layer <Coating liquid 1 to 4 for forming retardation film>
Polyimide 1 was dissolved in dichloromethane to obtain a coating liquid 1 for forming a retardation film having a solid content of 15.4% by mass.
Polyimide 2 was dissolved in dichloromethane to obtain a coating liquid 2 for forming a retardation film having a solid content of 18.2% by mass.
Polyimide 3 was dissolved in dichloromethane to obtain a coating liquid 3 for forming a retardation film having a solid content of 28.6% by mass.
Polyimide 4 was dissolved in dichloromethane to obtain a retardation film forming coating liquid 4 having a solid content of 13.3% by mass.

5. Production of retardation film [Experiment example 1-4]
<Production of retardation film>
Coating liquid 1 for forming a retardation layer was applied onto a 250 μm thick polyethylene terephthalate (PET) film (product name “Lumirror T60”, manufactured by Toray Industries, Inc.), left at room temperature for 5 minutes, and then dried at 130° C. for 30 minutes. A coating film was formed (step (1)).
Next, the coating film (retardation layer) is peeled off from the PET film (step (3)), and the peeled coating film is held between two metal frames (outer dimensions 150 mm x 200 mm, inner dimensions 130 mm x 180 mm). The metal frame and the paint film were fixed using a fixing jig. The fixed coating film was heated in a circulation oven at a temperature (280° C.) above the glass transition temperature for 20 minutes (step (2)).
Next, the coating film (retardation layer) was removed from the metal frame to obtain the retardation film of Experimental Example 1-4. The thickness of the retardation film (retardation layer) was 29 μm.

[Experimental Examples 1-2 to 1-3, 2-2 to 2-4, 3-2 to 3-4, 4-1 to 4-4]
Experimental Example 1 was carried out in the same manner as in Experimental Example 1-4, except that the coating liquid for forming a retardation layer, the heating temperature in step (2), and the thickness of the retardation film (retardation layer) were set to the conditions shown in Table 1. Retardation films of -2 to 1-3, 2-2 to 2-4, 3-2 to 3-4, and 4-1 to 4-4 were obtained.

[Experimental Examples 1-1, 2-1, 3-1]
Experimental Example 1-4 was carried out in the same manner as Experimental Example 1-4, except that the coating liquid for forming a retardation layer and the thickness of the retardation film (retardation layer) were as shown in Table 1, and step (2) was not performed. Retardation films 1, 2-1 and 3-1 were obtained.

From the results in Table 1, the method for manufacturing retardation films of Experimental Examples 1-3, 1-4, 2-3, 2-4, and 3-2 to 3-4 that satisfies the conditions of steps (1) and (2) According to the above, it can be confirmed that a retardation film having a sufficiently small retardation value and capable of suppressing thermal shrinkage can be easily produced.

Claims (8)

It has the following steps (1) and (2), where W is the total mass of the coating film at the start of step (2), and W1 is the mass of the residual solvent contained in the coating film at the start of step (2). A method for producing a retardation film, wherein W1/W is 0.02 or less.
(1) A coating solution for forming a retardation layer in which polyimide with a glass transition temperature of 234 to 244°C is dissolved in a solvent is applied onto the base material, and dried to form a coating film. A step in which the tetracarboxylic acid is a tetracarboxylic dianhydride having an aromatic ring, and the diamine is a diamine having a silicon atom in its main chain .
(2) A step of heating the coating film at a temperature equal to or higher than the glass transition temperature of polyimide. The method for producing a retardation film according to claim 1, wherein the polyimide used in step (1) is a chemically imidized polyimide. The method for producing a retardation film according to claim 1 or 2, further comprising the following step (3).
(3) A step of peeling off the coating film from the substrate after step (1) or step (2). The retardation film includes polyimide, the polyimide is a reaction product of a tetracarboxylic acid and a diamine, and the tetracarboxylic acid is a tetracarboxylic dianhydride having an aromatic ring. , the diamine is a diamine having a silicon atom in the main chain , the glass transition temperature of the polyimide is 234 to 244°C, and the in-plane retardation of the retardation film at a wavelength of 589 nm is Re (589), When the retardation in the thickness direction at a wavelength of 589 nm of the retardation film is Rth (589), both Re (589) and Rth (589) are 100 nm or less, and YI exhibits the yellowness of JIS K7373:2006. is 5.0 or less, and the diameter of the mandrel in which cracks occurred in the retardation film in the bending resistance test by the cylindrical mandrel method specified in JIS K5600-5-1:1999 is 3 mm or less. . The retardation film according to claim 4, which does not cause thermal shrinkage even when heated at any temperature in the temperature range from 25° C. to the glass transition temperature of polyimide. A display panel comprising the retardation film according to claim 4 or 5 disposed on the light exit surface side of a display element. The display panel according to claim 6, wherein the display element is an organic EL display element. An image display device comprising the display panel according to claim 6 or 7.