JP7702249B2 - Polyimide resin, polyimide varnish and polyimide film - Google Patents

Description

The present invention relates to polyimide resins, polyimide varnishes and polyimide films.

Various applications of polyimide resins are being considered in the fields of electrical and electronic components. For example, it is desirable to replace the glass substrates used in image display devices such as liquid crystal displays and OLED displays with plastic substrates in order to make the devices lighter and more flexible, and research is being conducted into polyimide films suitable for such plastic substrates. Polyimide films for such applications are required to be colorless and transparent.

In image display devices such as liquid crystal displays and OLED displays, thin film transistors (TFTs) are used as pixel switching elements. Polycrystalline silicon (polysilicon), which has excellent crystallinity, has higher electron mobility than amorphous silicon, and therefore significantly improves TFT characteristics. One method for forming a polysilicon film is the excimer laser annealing (ELA) method. The dehydrogenation process of amorphous silicon in this method is a high-temperature process. Therefore, in order to form a polysilicon film on a polyimide film as a plastic substrate, the polyimide film is required to have high heat resistance (i.e., a high glass transition temperature). Furthermore, at high temperatures, organic compounds (outgassing) volatilized from the substrate material itself may have a serious adverse effect on the element. Therefore, the polyimide film is also required to have high thermal stability to suppress the generation of outgassing up to as high a temperature range as possible.

In addition, when a polyimide film is formed by heating and curing a varnish applied to a glass support or a silicon wafer, residual stress is generated in the polyimide film. If the residual stress in the polyimide film is large, the glass support or the silicon wafer may be warped, so it is necessary to reduce the residual stress in the polyimide film.
Patent Document 1 discloses a polyimide resin that provides a film with low residual stress, which is synthesized using 4,4'-oxydiphthalic dianhydride as a tetracarboxylic acid component and α,ω-aminopropylpolydimethylsiloxane having a number average molecular weight of 1000 and 4,4'-diaminodiphenyl ether as diamine components.

JP 2005-232383 A

As described above, various characteristics are required for a polyimide film, and it is not easy to satisfy all of these characteristics at the same time.
The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a polyimide resin which is excellent in colorless transparency, heat resistance, and thermal stability and which is capable of forming a film having low residual stress, a method for producing the same, and a polyimide varnish and polyimide film each containing the polyimide resin.

The inventors discovered that a polyimide resin containing a specific combination of structural units can solve the above problems, and thus completed the invention.

That is, the present invention relates to the following [1] to [9].
[1]
A polyimide resin having a structural unit A derived from a tetracarboxylic dianhydride and a structural unit B derived from a diamine,
The structural unit A includes a structural unit (A-1) derived from a compound represented by the following formula (a-1) and a structural unit (A-2) derived from a compound represented by the following formula (a-2):
A polyimide resin, wherein the structural unit B contains a structural unit (B-1) derived from a compound represented by the following formula (b-1):

[2]
the ratio of the structural unit (A-1) in the structural unit A is 40 to 95 mol %,
The polyimide resin according to the above-mentioned [1], wherein the ratio of the structural unit (A-2) in the structural unit A is 5 to 60 mol %.
[3]
The polyimide resin according to the above [1] or [2], wherein the ratio of the structural unit (B-1) in the structural unit B is 50 mol % or more.
[4]
The polyimide resin according to any one of the above [1] to [3], wherein the structural unit B further contains a structural unit derived from 9,9-bis(4-aminophenyl)fluorene.
[5]
The polyimide resin according to any one of the above [1] to [4], wherein the ratio of the structural unit (A-1) to the structural unit (A-2) [(A-1)/(A-2)] (mol/mol) is 25/75 to 95/5.
[6]
A method for producing a polyimide resin, comprising heating a tetracarboxylic acid component containing a compound represented by the above formula (a-1) and a compound represented by the above formula (a-2) and a diamine component containing a compound represented by the above formula (b-1) in the presence of a reaction solvent to carry out an imidization reaction.
[7]
The method for producing a polyimide resin according to the above [6], wherein the reaction solvent is at least one selected from the group consisting of amide-based solvents and lactone-based solvents.
[8]
A polyimide varnish obtained by dissolving the polyimide resin according to any one of the above [1] to [5] in an organic solvent.
[9]
A polyimide film comprising the polyimide resin according to any one of [1] to [5] above.

According to the present invention, it is possible to form a film that is colorless and transparent, has excellent heat resistance and thermal stability, and also has low residual stress.

[Polyimide resin]
The polyimide resin of the present invention has a structural unit A derived from a tetracarboxylic dianhydride and a structural unit B derived from a diamine, in which the structural unit A includes a structural unit (A-1) derived from a compound represented by the following formula (a-1) and a structural unit (A-2) derived from a compound represented by the following formula (a-2), and the structural unit B includes a structural unit (B-1) derived from a compound represented by the following formula (b-1):

<Structural Unit A>
The structural unit A is a structural unit derived from a tetracarboxylic dianhydride contained in a polyimide resin, and includes a structural unit (A-1) derived from a compound represented by the following formula (a-1) and a structural unit (A-2) derived from a compound represented by the following formula (a-2).

The compound represented by formula (a-1) is 9,9'-bis(3,4-dicarboxyphenyl)fluorene dianhydride.
When the structural unit A contains the structural unit (A-1), the colorless transparency, heat resistance, and thermal stability of the film are improved.

The compound represented by formula (a-2) is biphenyltetracarboxylic dianhydride (BPDA), and specific examples thereof include 3,3',4,4'-biphenyltetracarboxylic dianhydride (s-BPDA) represented by the following formula (a-2s), 2,3,3',4'-biphenyltetracarboxylic dianhydride (a-BPDA) represented by the following formula (a-2a), and 2,2',3,3'-biphenyltetracarboxylic dianhydride (i-BPDA) represented by the following formula (a-2i).

When structural unit A contains structural unit (A-2), the heat resistance and thermal stability of the film are improved and residual stress is reduced.

The proportion of the structural unit (A-1) in the structural unit A is preferably 25 to 95 mol%, more preferably 30 to 90 mol%, even more preferably 35 to 85 mol%, still more preferably 40 to 80 mol%, and particularly preferably 50 to 80 mol%.
The proportion of the structural unit (A-2) in the structural unit A is preferably 5 to 75 mol%, more preferably 10 to 70 mol%, even more preferably 15 to 65 mol%, still more preferably 20 to 60 mol%, and particularly preferably 20 to 50 mol%.
Furthermore, the ratio of the structural unit (A-1) to the structural unit (A-2) [(A-1)/(A-2)] (mol/mol) is preferably 25/75 to 95/5, more preferably 30/70 to 90/10, even more preferably 35/65 to 85/15, still more preferably 40/60 to 80/20, and particularly preferably 50/80 to 50/20.
The total ratio of the structural units (A-1) and (A-2) in the structural unit A is preferably 50 mol% or more, more preferably 70 mol% or more, even more preferably 90 mol% or more, and particularly preferably 99 mol% or more. There is no particular upper limit to the total ratio of the structural units (A-1) and (A-2), that is, it is 100 mol%. The structural unit A may be composed of only the structural unit (A-1) and the structural unit (A-2).

The structural unit A may contain a structural unit other than the structural units (A-1) and (A-2). The tetracarboxylic acid dianhydride that gives such a structural unit is not particularly limited, but examples thereof include aromatic tetracarboxylic acid dianhydrides such as pyromellitic dianhydride and 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (excluding the compounds represented by formula (a-1) and formula (a-2)); alicyclic tetracarboxylic acid dianhydrides such as 1,2,3,4-cyclobutane tetracarboxylic acid dianhydride, 1,2,4,5-cyclohexane tetracarboxylic acid dianhydride, and norbornane-2-spiro-α-cyclopentanone-α'-spiro-2"-norbornane-5,5",6,6"-tetracarboxylic acid dianhydride; and aliphatic tetracarboxylic acid dianhydrides such as 1,2,3,4-butane tetracarboxylic acid dianhydride.
In this specification, the term "aromatic tetracarboxylic acid dianhydride" refers to a tetracarboxylic acid dianhydride containing one or more aromatic rings, the term "alicyclic tetracarboxylic acid dianhydride" refers to a tetracarboxylic acid dianhydride containing one or more alicyclic rings but no aromatic rings, and the term "aliphatic tetracarboxylic acid dianhydride" refers to a tetracarboxylic acid dianhydride containing neither an aromatic ring nor an alicyclic ring.
The structural units other than the structural units (A-1) and (A-2) optionally contained in the structural unit A may be of one type or of two or more types.

<Structural Unit B>
The structural unit B is a structural unit derived from a diamine contained in a polyimide resin, and includes a structural unit (B-1) derived from a compound represented by the following formula (b-1).

The compound represented by formula (b-1) is 2,2'-bis(trifluoromethyl)benzidine.
When the structural unit B contains the structural unit (B-1), the colorless transparency, heat resistance, and thermal stability of the film are improved, and the residual stress is reduced.

The ratio of structural unit (B-1) in structural unit B is preferably 50 mol% or more, more preferably 70 mol% or more, even more preferably 80 mol% or more, even more preferably 90 mol% or more, and particularly preferably 99 mol% or more. There is no particular upper limit to the ratio of structural unit (B-1), that is, it is 100 mol%. Structural unit B may consist of only structural unit (B-1).

The structural unit B may contain a structural unit other than the structural unit (B-1). Diamines that provide such a structural unit are not particularly limited, but include 1,4-phenylenediamine, p-xylylenediamine, 3,5-diaminobenzoic acid, 1,5-diaminonaphthalene, 2,2'-dimethylbiphenyl-4,4'-diamine, 4,4'-diaminodiphenyl ether, 4,4'-diaminodiphenylmethane, 2,2-bis(4-aminophenyl)hexafluoropropane, 4,4'-diaminodiphenyl sulfone, 4,4'-diaminobenzanilide, 1-(4-aminophenyl)-2,3-dihydro-1,3,3-trimethyl-1H-inden-5-amine, α,α'-bis(4-aminophenyl)-1,4-diisopropylbenzene, N,N ... Examples of the diamines include aromatic diamines (excluding the compound represented by formula (b-1)) such as 4,4'-bis(4-aminophenoxy)biphenyl, 2,2-bis[4-(4-aminophenoxy)phenyl]terephthalamide, 4,4'-bis(4-aminophenoxy)biphenyl, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, 2,2-bis(4-(4-aminophenoxy)phenyl)hexafluoropropane, and 9,9-bis(4-aminophenyl)fluorene; alicyclic diamines such as 1,3-bis(aminomethyl)cyclohexane and 1,4-bis(aminomethyl)cyclohexane; and aliphatic diamines such as ethylenediamine and hexamethylenediamine. Of these, aromatic diamines are preferred, and 9,9-bis(4-aminophenyl)fluorene is more preferred.
In this specification, an aromatic diamine means a diamine containing one or more aromatic rings, an alicyclic diamine means a diamine containing one or more alicyclic rings but no aromatic rings, and an aliphatic diamine means a diamine containing neither an aromatic ring nor an alicyclic ring.
The structural unit other than the structural unit (B-1) optionally contained in the structural unit B may be of one type, or of two or more types.

The number average molecular weight of the polyimide resin of the present invention is preferably 5,000 to 300,000, more preferably 5,000 to 100,000, from the viewpoint of the mechanical strength of the resulting polyimide film. The number average molecular weight of the polyimide resin can be determined, for example, from a standard polymethyl methacrylate (PMMA) equivalent value measured by gel filtration chromatography.

The polyimide resin of the present invention may contain a structure other than a polyimide chain (a structure formed by imide bonding between the structural unit A and the structural unit B). Examples of structures other than a polyimide chain that can be contained in the polyimide resin include a structure containing an amide bond.
The polyimide resin of the present invention preferably contains, as a main structure, a polyimide chain (a structure formed by imide bonding between the structural unit A and the structural unit B). Therefore, the ratio of the polyimide chain in the polyimide resin of the present invention is preferably 50% by mass or more, more preferably 70% by mass or more, even more preferably 90% by mass or more, and particularly preferably 99% by mass or more.

By using the polyimide resin of the present invention, a film having excellent colorless transparency, heat resistance, and thermal stability and further low residual stress can be formed, and the preferable physical properties of the film are as follows.
The total light transmittance, when made into a film having a thickness of 10 μm, is preferably 85% or more, more preferably 87% or more, and further preferably 88% or more.
The yellow index (YI) of the resin when made into a film having a thickness of 10 μm is preferably 5.0 or less, more preferably 4.0 or less, and further preferably 3.5 or less.
The glass transition temperature (Tg) is preferably 370° C. or higher, more preferably 380° C. or higher, and even more preferably 400° C. or higher.
The weight loss rate at 450° C. is preferably 1.00% or less, more preferably 0.80% or less, and further preferably 0.50% or less.
The weight loss rate at 480° C. is preferably 3.00% or less, more preferably 2.50% or less, and further preferably 2.00% or less.
The residual stress is preferably 40.0 MPa or less, more preferably 35.0 MPa or less, and further preferably 30.0 MPa or less.

A film that can be formed by using the polyimide resin of the present invention has good mechanical properties and the following preferable physical properties.
The tensile modulus is preferably 2.5 GPa or more, more preferably 3.0 GPa or more, and even more preferably 3.5 GPa or more.
The tensile strength is preferably 70 MPa or more, more preferably 90 MPa or more, and further preferably 100 MPa or more.
The above-mentioned physical properties in the present invention can be specifically measured by the methods described in the Examples.

[Method of producing polyimide resin]
The polyimide resin of the present invention can be produced by reacting a tetracarboxylic acid component containing a compound which provides the above-mentioned structural unit (A-1) and a compound which provides the above-mentioned structural unit (A-2) with a diamine component which contains a compound which provides the above-mentioned structural unit (B-1).
More specifically, in the method for producing a polyimide resin of the present invention, a tetracarboxylic acid component containing a compound that provides the structural unit A and a diamine component containing a compound that provides the structural unit (B-1) are heated in the presence of a reaction solvent to carry out an imidization reaction.
That is, a tetracarboxylic acid component containing a compound represented by formula (a-1) and a compound represented by formula (a-2) and a diamine component containing a compound represented by formula (b-1) are heated in the presence of a reaction solvent to carry out an imidization reaction.

Compounds that provide the structural unit (A-1) include, but are not limited to, compounds represented by formula (a-1), and may be derivatives thereof as long as they provide the same structural unit. Examples of such derivatives include tetracarboxylic acids corresponding to the tetracarboxylic dianhydride represented by formula (a-1) and alkyl esters of the tetracarboxylic acid. Compounds that provide the structural unit (A-1) are preferably compounds represented by formula (a-1) (i.e., dianhydrides).
Similarly, examples of compounds that provide the structural unit (A-2) include, but are not limited to, compounds represented by formula (a-2), and may be derivatives thereof as long as they provide the same structural unit. Examples of such derivatives include tetracarboxylic acids corresponding to the tetracarboxylic dianhydride represented by formula (a-2) and alkyl esters of the tetracarboxylic acid. Compounds that provide the structural unit (A-2) are preferably compounds represented by formula (a-2) (i.e., dianhydrides).

The tetracarboxylic acid component contains a compound that provides the structural unit (A-1) in an amount of preferably 25 to 95 mol %, more preferably 30 to 90 mol %, even more preferably 35 to 85 mol %, still more preferably 40 to 80 mol %, and particularly preferably 50 to 80 mol %.
The tetracarboxylic acid component contains a compound that provides the structural unit (A-2) in an amount of preferably 5 to 75 mol %, more preferably 10 to 70 mol %, even more preferably 15 to 65 mol %, still more preferably 20 to 60 mol %, and particularly preferably 20 to 50 mol %.
The tetracarboxylic acid component preferably contains at least 50 mol %, more preferably at least 70 mol %, even more preferably at least 90 mol %, and particularly preferably at least 99 mol % of the compound that provides the structural unit (A-1) and the compound that provides the structural unit (A-2). There is no particular upper limit on the total content of the compound that provides the structural unit (A-1) and the compound that provides the structural unit (A-2), and this upper limit is 100 mol %. The tetracarboxylic acid component may consist only of the compound that provides the structural unit (A-1) and the compound that provides the structural unit (A-2).

The tetracarboxylic acid component may contain a compound other than the compound that provides the structural unit (A-1) and the compound that provides the structural unit (A-2). Examples of such compounds include the above-mentioned aromatic tetracarboxylic acid dianhydrides, alicyclic tetracarboxylic acid dianhydrides, and aliphatic tetracarboxylic acid dianhydrides, as well as derivatives thereof (tetracarboxylic acids, alkyl esters of tetracarboxylic acids, etc.).
The compounds optionally contained in the tetracarboxylic acid component other than the compound providing the structural unit (A-1) and the compound providing the structural unit (A-2) may be one type or two or more types.

Compounds that provide the structural unit (B-1) include, but are not limited to, compounds represented by formula (b-1), and may be derivatives thereof as long as they provide the same structural unit. Examples of such derivatives include diisocyanates that correspond to the diamines represented by formula (b-1). Compounds that provide the structural unit (B-1) are preferably compounds represented by formula (b-1) (i.e., diamines).

The diamine component preferably contains 50 mol% or more of the compound that gives the structural unit (B-1), more preferably 70 mol% or more, even more preferably 80 mol% or more, even more preferably 90 mol% or more, and particularly preferably 99 mol% or more. The upper limit of the content of the compound that gives the structural unit (B-1) is not particularly limited, that is, it is 100 mol%. The diamine component may consist only of the compound that gives the structural unit (B-1).

The diamine component may contain a compound other than the compound that provides the structural unit (B-1). Examples of such compounds include the above-mentioned aromatic diamines, alicyclic diamines, and aliphatic diamines, as well as derivatives thereof (diisocyanates, etc.).
The compound other than the compound that provides the structural unit (B-1) that is optionally contained in the diamine component may be one type or two or more types.

In the present invention, the ratio of the amount of the tetracarboxylic acid component and the diamine component used in the production of the polyimide resin is preferably 0.9 to 1.1 moles of the diamine component per mole of the tetracarboxylic acid component.

In addition, in the present invention, in addition to the above-mentioned tetracarboxylic acid component and diamine component, a terminal blocking agent may be used in the production of the polyimide resin. As the terminal blocking agent, monoamines or dicarboxylic acids are preferred. The amount of the terminal blocking agent to be introduced is preferably 0.0001 to 0.1 moles per mole of the tetracarboxylic acid component, and particularly preferably 0.001 to 0.06 moles. As the monoamine terminal blocking agent, for example, methylamine, ethylamine, propylamine, butylamine, benzylamine, 4-methylbenzylamine, 4-ethylbenzylamine, 4-dodecylbenzylamine, 3-methylbenzylamine, 3-ethylbenzylamine, aniline, 3-methylaniline, 4-methylaniline, etc. are recommended. Of these, benzylamine and aniline are preferably used. As the dicarboxylic acid terminal blocking agent, dicarboxylic acids are preferred, and a part of them may be ring-closed. For example, phthalic acid, phthalic anhydride, 4-chlorophthalic acid, tetrafluorophthalic acid, 2,3-benzophenonedicarboxylic acid, 3,4-benzophenonedicarboxylic acid, cyclopentane-1,2-dicarboxylic acid, 4-cyclohexene-1,2-dicarboxylic acid, etc. are recommended. Of these, phthalic acid and phthalic anhydride are preferably used.

The method for reacting the tetracarboxylic acid component and the diamine component is not particularly limited, and any known method can be used.
Specific reaction methods include: (1) a method in which a tetracarboxylic acid component, a diamine component, and a reaction solvent are charged into a reactor, and the mixture is stirred at room temperature to 80° C. for 0.5 to 30 hours, and then the temperature is raised to carry out the imidization reaction; (2) a method in which a diamine component and a reaction solvent are charged into a reactor and dissolved, and then a tetracarboxylic acid component is charged, and the mixture is stirred at room temperature to 80° C. for 0.5 to 30 hours as necessary, and then the temperature is raised to carry out the imidization reaction; and (3) a method in which a tetracarboxylic acid component, a diamine component, and a reaction solvent are charged into a reactor, and the temperature is immediately raised to carry out the imidization reaction.

The reaction solvent used in the production of polyimide resins may be any solvent that does not inhibit the imidization reaction and can dissolve the resulting polyimide. Examples include aprotic solvents, phenolic solvents, ether solvents, and carbonate solvents.

Specific examples of aprotic solvents include amide-based solvents such as N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone, N-methylcaprolactam, 1,3-dimethylimidazolidinone, and tetramethylurea; lactone-based solvents such as γ-butyrolactone and γ-valerolactone; phosphorus-containing amide-based solvents such as hexamethylphosphoric amide and hexamethylphosphine triamide; sulfur-containing solvents such as dimethyl sulfone, dimethyl sulfoxide, and sulfolane; ketone-based solvents such as acetone, cyclohexanone, and methylcyclohexanone; amine-based solvents such as picoline and pyridine; and ester-based solvents such as 2-methoxy-1-methylethyl acetate.

Specific examples of phenol-based solvents include phenol, o-cresol, m-cresol, p-cresol, 2,3-xylenol, 2,4-xylenol, 2,5-xylenol, 2,6-xylenol, 3,4-xylenol, and 3,5-xylenol.
Specific examples of ether solvents include 1,2-dimethoxyethane, bis(2-methoxyethyl)ether, 1,2-bis(2-methoxyethoxy)ethane, bis[2-(2-methoxyethoxy)ethyl]ether, tetrahydrofuran, and 1,4-dioxane.
Specific examples of carbonate solvents include diethyl carbonate, methyl ethyl carbonate, ethylene carbonate, and propylene carbonate.
Among the above reaction solvents, amide-based solvents and/or lactone-based solvents are preferred, and lactone-based solvents are more preferred. The above reaction solvents may be used alone or in combination of two or more. When two or more solvents are used in combination, it is particularly preferred to use an amide-based solvent and a lactone-based solvent in combination.

In the imidization reaction, it is preferable to carry out the reaction while removing the water generated during production using a Dean-Stark apparatus or the like. By carrying out such an operation, it is possible to further increase the degree of polymerization and the imidization rate.

In the above imidization reaction, a known imidization catalyst can be used, such as a base catalyst or an acid catalyst.
Examples of the base catalyst include organic base catalysts such as pyridine, quinoline, isoquinoline, α-picoline, β-picoline, 2,4-lutidine, 2,6-lutidine, trimethylamine, triethylamine, tripropylamine, tributylamine, triethylenediamine, imidazole, N,N-dimethylaniline, and N,N-diethylaniline; and inorganic base catalysts such as potassium hydroxide, sodium hydroxide, potassium carbonate, sodium carbonate, potassium hydrogencarbonate, and sodium hydrogencarbonate.
Examples of the acid catalyst include crotonic acid, acrylic acid, trans-3-hexenoic acid, cinnamic acid, benzoic acid, methylbenzoic acid, oxybenzoic acid, terephthalic acid, benzenesulfonic acid, paratoluenesulfonic acid, naphthalenesulfonic acid, etc. The above imidization catalysts may be used alone or in combination of two or more kinds.
Among the above, from the viewpoint of handleability, it is preferable to use a base catalyst, more preferably an organic base catalyst, further preferably triethylamine, and particularly preferably a combination of triethylamine and triethylenediamine.

The temperature of the imidization reaction is preferably 120 to 250°C, more preferably 160 to 200°C, from the viewpoint of the reaction rate and suppression of gelation, etc. The reaction time is preferably 0.5 to 10 hours after the start of distillation of the produced water.

[Polyimide varnish]
The polyimide varnish of the present invention is obtained by dissolving the polyimide resin of the present invention in an organic solvent. That is, the polyimide varnish of the present invention contains the polyimide resin of the present invention and an organic solvent, and the polyimide resin is dissolved in the organic solvent.
The organic solvent is not particularly limited as long as it dissolves the polyimide resin. However, it is preferable to use the above-mentioned compounds as reaction solvents used in the production of the polyimide resin, either alone or in combination of two or more kinds.
The polyimide varnish of the present invention may be a polyimide solution itself in which a polyimide resin obtained by polymerization is dissolved in a reaction solvent, or may be a polyimide solution to which a dilution solvent is further added.

Since the polyimide resin of the present invention has solvent solubility, it can be made into a highly concentrated varnish that is stable at room temperature. The polyimide varnish of the present invention preferably contains 5 to 40 mass %, more preferably 5 to 30 mass %, and even more preferably 10 to 30 mass % of the polyimide resin of the present invention. The viscosity of the polyimide varnish is preferably 1 to 200 Pa·s, more preferably 1 to 150 Pa·s, and even more preferably 5 to 150 Pa·s. The viscosity of the polyimide varnish is a value measured at 25°C using an E-type viscometer.
In addition, the polyimide varnish of the present invention may contain various additives such as inorganic fillers, adhesion promoters, release agents, flame retardants, UV stabilizers, surfactants, leveling agents, defoamers, fluorescent brightening agents, crosslinking agents, polymerization initiators, and photosensitizers, as long as the additives do not impair the required properties of the polyimide film.
The method for producing the polyimide varnish of the present invention is not particularly limited, and any known method can be applied.

[Polyimide film]
The polyimide film of the present invention contains the polyimide resin of the present invention. Therefore, the polyimide film of the present invention is excellent in colorless transparency, heat resistance, and thermal stability, and further has low residual stress. The preferable physical properties of the polyimide film of the present invention are as described above.
The method for producing the polyimide film of the present invention is not particularly limited, and known methods can be used. For example, the polyimide varnish of the present invention is applied to a smooth support such as a glass plate, a metal plate, or a plastic plate, or formed into a film, and then the organic solvents contained in the varnish, such as a reaction solvent or a dilution solvent, are removed by heating. If necessary, a release agent may be applied to the surface of the support in advance.
The following method is preferred as a method for removing the organic solvent contained in the varnish by heating. That is, it is preferred to evaporate the organic solvent at a temperature of 120° C. or less to form a self-supporting film, peel the self-supporting film from the support, fix the ends of the self-supporting film, and dry at a temperature equal to or higher than the boiling point of the organic solvent used to produce a polyimide film. It is also preferred to dry under a nitrogen atmosphere. The pressure of the drying atmosphere may be reduced pressure, normal pressure, or pressurized. The heating temperature when drying the self-supporting film to produce a polyimide film is not particularly limited, but is preferably 200 to 500° C., more preferably 200 to 400° C.

The polyimide film of the present invention can also be produced by using a polyamic acid varnish prepared by dissolving a polyamic acid in an organic solvent.
The polyamic acid contained in the polyamic acid varnish is a precursor of the polyimide resin of the present invention, and is a product of a polyaddition reaction between a tetracarboxylic acid component containing a compound that gives the above-mentioned structural unit (A-1) and a compound that gives the above-mentioned structural unit (A-2) and a diamine component containing a compound that gives the above-mentioned structural unit (B-1). The polyamic acid is imidized (dehydration ring closure) to obtain the final product, the polyimide resin of the present invention.
As the organic solvent contained in the polyamic acid varnish, the organic solvent contained in the polyimide varnish of the present invention can be used.
In the present invention, the polyamic acid varnish may be a polyamic acid solution itself obtained by subjecting a tetracarboxylic acid component containing a compound which provides the above-mentioned structural unit (A-1) and a compound which provides the above-mentioned structural unit (A-2) to a polyaddition reaction in a reaction solvent with a diamine component containing a compound which provides the above-mentioned structural unit (B-1), or may be a polyamic acid solution to which a dilution solvent has been further added.

The method for producing a polyimide film using the polyamic acid varnish is not particularly limited, and a known method can be used. For example, the polyamic acid varnish is applied to a smooth support such as a glass plate, a metal plate, or a plastic plate, or formed into a film, and the organic solvent contained in the varnish, such as a reaction solvent or a dilution solvent, is removed by heating to obtain a polyamic acid film, and the polyamic acid in the polyamic acid film is imidized by heating, thereby producing a polyimide film.
The heating temperature when the polyamic acid varnish is dried to obtain a polyamic acid film is preferably 50 to 120° C. The heating temperature when the polyamic acid is imidized by heating is preferably 200 to 500° C., more preferably 200 to 480° C., even more preferably 200 to 450° C., and still more preferably 200 to 400° C.
The imidization method is not limited to thermal imidization, and chemical imidization can also be applied.

The thickness of the polyimide film of the present invention can be appropriately selected depending on the application, etc., but is preferably in the range of 1 to 250 μm, more preferably 5 to 100 μm, even more preferably 8 to 80 μm, and still more preferably 10 to 80 μm. When the thickness is 1 to 250 μm, practical use as a free-standing film becomes possible.
The thickness of the polyimide film can be easily controlled by adjusting the solids concentration and viscosity of the polyimide varnish.

The polyimide film of the present invention is suitable for use as a film for various components such as color filters, flexible displays, semiconductor parts, and optical components. The polyimide film of the present invention is particularly suitable for use as a substrate for image display devices such as liquid crystal displays and OLED displays.

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 solids concentration of the varnishes and the physical properties of the films obtained in the examples and comparative examples were measured by the methods described below.

(1) Solid Content Concentration The solid content concentration of the varnish was measured by heating a sample at 320° C. for 120 min in a small electric furnace “MMF-1” manufactured by AS ONE Corporation, and calculating from the mass difference of the sample before and after heating.
(2) Film Thickness The film thickness was measured using a micrometer manufactured by Mitutoyo Corporation.
(3) Total light transmittance, yellow index (YI) (evaluation of colorless transparency)
The total light transmittance and YI were measured in accordance with JIS K7361-1:1997 using a color/turbidity simultaneous measuring instrument "COH400" manufactured by Nippon Denshoku Industries Co., Ltd. The closer the total light transmittance is to 100%, and the smaller the YI value is, the more excellent the colorless transparency is.
(4) Glass transition temperature (Tg) (evaluation of heat resistance)
Using a thermomechanical analyzer "TMA/SS6100" manufactured by Hitachi High-Tech Science Corporation, the specimen was heated to a temperature sufficient to remove 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 where an inflection point of elongation was observed was determined as the glass transition temperature. The higher the Tg value, the better the heat resistance.
(5) Weight loss rate at 450°C and 480°C (evaluation of thermal stability)
A differential thermal thermogravimetry simultaneous measurement device "TG/DTA6200" manufactured by Hitachi High-Tech Science Corporation was used. The sample was heated from 40°C to a predetermined temperature (450°C or 480°C) at a heating rate of 10°C/min and held at that temperature for 1 hour. The ratio of the weight lost during 1 hour at 450°C to the weight before 1 hour was defined as the 450°C weight loss rate, and the ratio of the weight lost during 1 hour at 480°C to the weight before 1 hour was defined as the 480°C weight loss rate. The higher the value of each weight loss temperature, the better the thermal stability.
(6) Residual Stress Polyimide varnish or polyamic acid varnish was applied using a spin coater onto a 4-inch silicon wafer with a thickness of 525 μm ± 25 μm, the "warpage" of which had been measured in advance using a residual stress measuring device "FLX-2320" manufactured by KLA-Tencor Corporation, and the wafer was pre-baked. After that, a heat curing treatment was performed at 400°C for 1 hour in a nitrogen atmosphere using a hot air dryer, and a silicon wafer with a polyimide film having a thickness of 8 to 20 μm after curing was produced. The warpage of this wafer was measured using the aforementioned residual stress measuring device, and the residual stress generated between the silicon wafer and the polyimide film was evaluated. The smaller the residual stress value, the better the result.
(7) Tensile Modulus and Tensile Strength The tensile modulus and tensile strength were measured in accordance with JIS K7127 using a tensile tester "Strograph VG-1E" manufactured by Toyo Seiki Co., Ltd.

The tetracarboxylic acid components and diamine components used in the examples and comparative examples, and their abbreviations, are as follows:
<Tetracarboxylic acid component>
BPAF: 9,9'-bis(3,4-dicarboxyphenyl)fluorene dianhydride (manufactured by JFE Chemical Corporation; compound represented by formula (a-1))
BPDA: 3,3',4,4'-biphenyltetracarboxylic dianhydride (manufactured by Mitsubishi Chemical Corporation; compound represented by formula (a-2))
HPMDA: 1,2,4,5-cyclohexanetetracarboxylic dianhydride (manufactured by Mitsubishi Gas Chemical Company, Inc.)
<Diamine component>
TFMB: 2,2'-bis(trifluoromethyl)benzidine (manufactured by Wakayama Seika Kogyo Co., Ltd.; compound represented by formula (b-1))
BAFL: 9,9-bis(4-aminophenyl)fluorene (manufactured by Taoka Chemical Co., Ltd.)

Example 1
32.024 g (0.100 mol) of TFMB and 89.499 g of N-methylpyrrolidone (manufactured by Mitsubishi Chemical Corporation) were placed in a 1 L five-neck 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, and the mixture was stirred at a rotation speed of 150 rpm under a nitrogen atmosphere at an internal temperature of 70° C. to obtain a solution.
To this solution, 36.674 g (0.080 mol) of BPAF, 5.884 g (0.020 mol) of BPDA, and 22.375 g of N-methylpyrrolidone (manufactured by Mitsubishi Chemical Corporation) were added all at once, and then 0.506 g of triethylamine (manufactured by Kanto Chemical Co., Ltd.) was added as an imidization catalyst, and the reaction system was heated with a mantle heater and the temperature in the reaction system was raised to 190° C. over about 20 minutes. The components distilled off were collected, and the temperature in the reaction system was maintained at 190° C. and refluxed for 3 hours while adjusting the rotation speed according to the increase in viscosity.
Thereafter, 526.935 g of γ-butyrolactone (manufactured by Mitsubishi Chemical Corporation) was added, the temperature inside the reaction system was cooled to 120° C., and the mixture was further stirred for about 3 hours to be homogenized, to obtain a polyimide varnish with a solid content concentration of 10.0% by mass.
The polyimide varnish was then applied to a glass plate and a silicon wafer, and held on a hot plate at 80° C. for 20 minutes, and then heated in a hot air dryer at 400° C. for 30 minutes in a nitrogen atmosphere to evaporate the solvent, yielding a film with a thickness of 8 μm. The results are shown in Table 1.

Example 2
A polyimide varnish was prepared in the same manner as in Example 1, except that the amount of BPAF was changed from 36.674 g (0.080 mol) to 27.506 g (0.060 mol) and the amount of BPDA was changed from 5.884 g (0.020 mol) to 11.769 g (0.040 mol), to obtain a polyimide varnish with a solids concentration of 10.0 mass%.
The obtained polyimide varnish was used to prepare a film having a thickness of 9 μm in the same manner as in Example 1. The results are shown in Table 1.

Example 3
A polyimide varnish having a solids concentration of 10.0 mass % was prepared in the same manner as in Example 1, except that the amount of BPAF was changed from 36.674 g (0.080 mol) to 18.337 g (0.040 mol), the amount of BPDA was changed from 5.884 g (0.020 mol) to 17.653 g (0.060 mol), and the dilution solvent after 3 hours of reaction was changed from γ-butyrolactone (manufactured by Mitsubishi Chemical Corporation) to N-methylpyrrolidone (manufactured by Mitsubishi Chemical Corporation).
The obtained polyimide varnish was used to prepare a film having a thickness of 10 μm in the same manner as in Example 1. The results are shown in Table 1.

Example 4
The amount of BPAF was changed from 36.674 g (0.080 mol) to 22.921 g (0.050 mol), the amount of BPDA was changed from 5.884 g (0.020 mol) to 14.711 g (0.050 mol), the amount of TFMB was changed from 32.024 g (0.100 mol) to 16.012 g (0.050 mol), 17.423 g (0.050 mol) of BAFL was added, and the synthesis solvent was changed from N-methylpyrrolidone (manufactured by Mitsubishi Chemical Corporation) to γ-butyrolactone (manufactured by Mitsubishi Chemical Corporation). Except for this, a polyimide varnish was prepared in the same manner as in Example 1, and a polyimide varnish with a solid content concentration of 10.0 mass% was obtained.
The obtained polyimide varnish was used to prepare a film having a thickness of 9.5 μm in the same manner as in Example 1. The results are shown in Table 1.

Example 5
The amount of BPAF was changed from 36.674 g (0.080 mol) to 22.921 g (0.050 mol), the amount of BPDA was changed from 5.884 g (0.020 mol) to 14.711 g (0.050 mol), the amount of TFMB was changed from 32.024 g (0.100 mol) to 25.619 g (0.080 mol), 6.969 g (0.020 mol) of BAFL was added, and the synthesis solvent was changed from N-methylpyrrolidone (manufactured by Mitsubishi Chemical Corporation) to γ-butyrolactone (manufactured by Mitsubishi Chemical Corporation). Except for this, a polyimide varnish was prepared in the same manner as in Example 1, and a polyimide varnish with a solid content concentration of 10.0 mass% was obtained.
The obtained polyimide varnish was used to prepare a film having a thickness of 9 μm in the same manner as in Example 1. The results are shown in Table 1.

<Comparative Example 1>
A polyimide varnish with a solids concentration of 10.0 mass % was obtained by the same method as in Example 1, except that the amount of BPAF was changed from 36.674 g (0.080 mol) to 45.843 g (0.100 mol) and BPDA was not added.
The obtained polyimide varnish was used to prepare a film having a thickness of 9 μm in the same manner as in Example 1. The results are shown in Table 1.

<Comparative Example 2>
A polyimide varnish was prepared in the same manner as in Example 1, except that the amount of BPAF (36.674 g, 0.080 mol) and the amount of BPDA (5.884 g, 0.020 mol) were changed to 22.417 g (0.100 mol) of HPMDA, and the synthesis solvent was changed from N-methylpyrrolidone (manufactured by Mitsubishi Chemical Corporation) to γ-butyrolactone (manufactured by Mitsubishi Chemical Corporation), to obtain a polyimide varnish having a solid content concentration of 10.0 mass%.
The obtained polyimide varnish was used to prepare a film having a thickness of 9 μm in the same manner as in Example 1. The results are shown in Table 1.

<Comparative Example 3>
A polyimide varnish was prepared in the same manner as in Example 1, except that the amount of BPAF was changed from 36.674 g (0.080 mol) to 22.922 g (0.050 mol), 11.209 g (0.050 mol) of HPMDA was added, no BPDA was added, and the synthesis solvent was changed from N-methylpyrrolidone (manufactured by Mitsubishi Chemical Corporation) to γ-butyrolactone (manufactured by Mitsubishi Chemical Corporation), and a polyimide varnish with a solids concentration of 10.0 mass% was obtained.
The obtained polyimide varnish was used to prepare a film having a thickness of 15 μm in the same manner as in Example 1. The results are shown in Table 1.

<Comparative Example 4>
32.024 g (0.100 mol) of TFMB and 196.627 g of N-methylpyrrolidone (manufactured by Mitsubishi Chemical Corporation) were placed in a 1 L five-neck 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, and the mixture was stirred at a system temperature of 50° C. under a nitrogen atmosphere at a rotation speed of 150 rpm to obtain a solution.
To this solution, 294.22 g (0.100 mol) of BPDA and 49.157 g of N-methylpyrrolidone (manufactured by Mitsubishi Chemical Corporation) were added all at once, and the mixture was stirred for 7 hours while being maintained at 50° C. by a mantle heater.
Thereafter, 307.230 g of N-methylpyrrolidone (manufactured by Mitsubishi Chemical Corporation) was added, and the mixture was further stirred for about 3 hours to be homogenized, to obtain a polyamic acid varnish having a solid content concentration of 10% by mass.
The polyamic acid varnish obtained was then applied onto a glass plate and a silicon wafer, and then held on a hot plate at 80° C. for 20 minutes, and then heated in a hot air dryer at 400° C. for 30 minutes in a nitrogen atmosphere to evaporate the solvent and further imidize the film to obtain a film having a thickness of 12 μm. The results are shown in Table 1.

As shown in Table 1, the polyimide films of Examples 1 to 5 produced using a specific tetracarboxylic acid component and a specific diamine component were excellent in colorless transparency, heat resistance, and thermal stability, and further had low residual stress.
On the other hand, the polyimide film of Comparative Example 1, which was produced using only BPAF as the tetracarboxylic acid component without using a combination of BPAF and BPDA, had a large residual stress as compared with the polyimide films of Examples 1-5.
The polyimide film of Comparative Example 4, which was produced using only BPDA as the tetracarboxylic acid component without using BPAF and BPDA in combination, had a large YI and therefore was inferior in colorless transparency, and had a low Tg and therefore was inferior in heat resistance, as compared with the polyimide films of Examples 1 to 5.
The polyimide film of Comparative Example 2 produced using only HPMDA as the tetracarboxylic acid component had poor heat resistance due to a low Tg, poor thermal stability due to large weight loss rates at 450°C and 480°C, and large residual stress, as compared with the polyimide films of Examples 1 to 5.
The polyimide film of Comparative Example 3 prepared by using BPAF and HPMDA in combination as the tetracarboxylic acid components had a large weight loss rate at 450° C. and 480° C. compared with the polyimide films of Examples 1 to 5, and thus had poor thermal stability and large residual stress.

Claims (7)

A polyimide resin having a structural unit A derived from a tetracarboxylic dianhydride and a structural unit B derived from a diamine,
The structural unit A includes a structural unit (A-1) derived from a compound represented by the following formula (a-1) and a structural unit (A-2) derived from a compound represented by the following formula (a-2):
The structural unit B is composed of a structural unit (B-1) derived from a compound represented by the following formula (b-1), or is composed of the structural unit (B-1) and a structural unit derived from 9,9-bis(4-aminophenyl)fluorene:
the ratio of the structural unit (A-1) in the structural unit A is 40 to 60 mol %,
the ratio of the structural unit (A-2) in the structural unit A is 40 to 60 mol %,
The polyimide resin is produced by mixing a tetracarboxylic acid component and a diamine component in a ratio of 1 to 1.1 moles of the diamine component per mole of the tetracarboxylic acid component.
2. The polyimide resin according to claim 1 , wherein the ratio of the structural unit (B-1) in the structural unit B is 50 mol % or more. 3. The polyimide resin according to claim 1 , wherein the ratio of the structural unit (A-1) to the structural unit (A-2) [(A-1)/(A-2)] (mol/mol) is 40/60 to 60/40 . A method for producing a polyimide resin, comprising heating a tetracarboxylic acid component including a compound represented by the following formula (a-1) and a compound represented by the following formula (a-2), and a diamine component consisting of a compound represented by the following formula (b-1), or consisting of a compound represented by the following formula (b-1) and 9,9-bis(4-aminophenyl)fluorene, in the presence of a reaction solvent, to carry out an imidization reaction, wherein the ratio of the compound represented by the following formula (a-1) in the tetracarboxylic acid component is 40 to 60 mol %, the ratio of the compound represented by the following formula (a-2) in the tetracarboxylic acid component is 40 to 60 mol %, and the ratio of the tetracarboxylic acid component to the diamine component is 1 to 1.1 mol of the diamine component per 1 mol of the tetracarboxylic acid component.
5. The method for producing a polyimide resin according to claim 4 , wherein the reaction solvent is at least one selected from the group consisting of amide-based solvents and lactone-based solvents. A polyimide varnish obtained by dissolving the polyimide resin according to any one of claims 1 to 3 in an organic solvent. A polyimide film comprising the polyimide resin according to any one of claims 1 to 3 .