Polyimide resin, polyimide varnish, and polyimide film

Abstract

A polyimide resin containing a constituent unit A derived from a tetracarboxylic acid dianhydride and a constituent unit B derived from a diamine, wherein the constituent unit A includes a constituent unit (A1) derived from a tetracarboxylic acid dianhydride having a two-norbornane skeleton in each molecule, the constituent unit B includes a constituent unit (B1) derived from a compound represented by formula (b1), and the proportion of the constituent unit (B1) in the constituent unit B is 35-95 mol%.

Description

Technical Field

[0001] The present invention relates to polyimide resins, polyimide varnishes, and polyimide films.

Background Art

[0002] Polyimide resins are being studied for various applications in the fields of electric and electronic components, etc. For example, it is desired to replace the glass substrate used in image display devices such as liquid crystal displays and OLED displays with a plastic substrate for the purpose of reducing the weight and making the device flexible, and research on polyimide films suitable as the plastic substrate is underway. Various optical properties are required for films used in image display device applications. For example, when light emitted from a display element is emitted through a plastic substrate, the plastic substrate is required to have transparency.

[0003] In order to satisfy the above-mentioned performance, polyimide resins with various compositions have been developed. For example, Patent Document 1 discloses a polyimide film having a structure composed of a combination of an acid dianhydride having a norbornane skeleton and 9,9-bis(4-aminophenyl)fluorene as a diamine component for the purpose of obtaining a polyimide film excellent in transparency and high heat resistance.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] Polyimide films are required to replace glass substrates and are required to have transparency. In the manufacturing of image display devices, for example, heat treatment is performed with an inorganic film laminated onto a polyimide film. Outgassing from the polyimide film accumulates between the polyimide film and the inorganic film, which can cause discoloration such as yellowing of the polyimide film. Therefore, the polyimide film is required to have heat resistance that suppresses discoloration when exposed to high temperatures with the inorganic film laminated on top. Furthermore, since the manufacturing of image display devices can reach process temperatures exceeding 400°C, the polyimide film used as the substrate requires heat resistance capable of withstanding temperatures above 400°C. Polyimide films with high glass transition temperatures (Tg) and excellent heat resistance are susceptible to cracking and other defects in the inorganic films when exposed to high temperatures while laminated. Therefore, polyimide films require heat resistance that prevents cracking and other defects in the inorganic films when exposed to high temperatures while laminated. This invention has been made in view of the above circumstances, and the object of this invention is to provide a polyimide resin and a polyimide varnish that can produce a polyimide film with excellent transparency and heat resistance when an inorganic film is laminated, as well as a polyimide film with excellent transparency and heat resistance when an inorganic film is laminated. [Means for solving the problem]

[0006] The inventors have discovered that a polyimide resin containing a structural unit derived from a tetracarboxylic dianhydride having two norbornane skeletons in its molecule and a structural unit derived from a specific diamine can solve the above problem, and have completed the invention.

[0007] In other words, the present invention is as follows: <1> ~ <12> Regarding. <1> A polyimide resin comprising structural unit A derived from tetracarboxylic dianhydride and structural unit B derived from diamine, Constituent unit A includes a constituent unit (A1) derived from a tetracarboxylic dianhydride having two norbornane skeletons within the molecule. Constituent unit B includes constituent unit (B1) derived from a compound represented by the following formula (b1), A polyimide resin in which the proportion of constituent unit (B1) in constituent unit B is 35 mol% or more and 95 mol% or less. [ka] <2> The constituent unit (A1) includes at least one selected from the group consisting of constituent unit (A11) derived from a compound represented by the following formula (a11), constituent unit (A12) derived from a compound represented by the following formula (a12), constituent unit (A13) derived from a compound represented by the following formula (a13), and constituent unit (A14) derived from a compound represented by the following formula (a14). <1> The polyimide resin described above. [ka] <3> Constituent unit A further includes constituent unit (A2), and constituent unit (A2) includes at least one selected from the group consisting of constituent unit (A21) derived from a compound represented by the following formula (a21), and constituent unit (A22) derived from a compound represented by the following formula (a22). <1> or <2> The polyimide resin described above. [ka] <4> Constituent unit B further includes constituent unit (B2), and constituent unit (B2) includes at least one selected from the group consisting of constituent unit (B21) derived from a compound represented by the following formula (b21), constituent unit (B22) derived from a compound represented by the following formula (b22), constituent unit (B23) derived from a compound represented by the following formula (b23), and constituent unit (B24) derived from a compound represented by the following formula (b24). <1> ~ <3> A polyimide resin as described in any one of the following. [ka] [ka] [Chemical formula] [Chemical formula] <5> The polyimide resin according to <4>, wherein the structural unit (B2) contains a structural unit (B21) derived from a compound represented by the following formula (b21). [Chemical formula] <6> A polyimide varnish obtained by dissolving the polyimide resin according to any one of <1> to <5> in an organic solvent. <7> A polyimide film containing the polyimide resin according to any one of <1> to <5>. <8> The polyimide film according to <7>, having a tensile elongation at break of 9.5% or more, measured under the conditions of conforming to JIS K7127:1999, chuck distance: 50 mm, test piece size: 10 mm × 70 mm, tensile speed: 20 mm / min, and measurement temperature: 23°C. <9> The polyimide film according to <7> or <8>, having a total light transmittance of 80% or more, measured in accordance with JIS K7136:2000. <10> The polyimide film according to any one of <7> to <9>, used as a transparent substrate constituting a display device. <11> A method for producing a polyimide film, including a step of applying or molding the polyimide varnish according to <6> into a film shape and then removing the organic solvent. <12> An image display device including the polyimide film according to any one of <7> to <10> as a transparent substrate. [Advantages of the Invention]

[0008] According to the present invention, it is possible to provide a polyimide resin and a polyimide varnish that can be used to obtain a polyimide film with excellent transparency and heat resistance when an inorganic film is laminated, as well as a polyimide film with excellent transparency and heat resistance when an inorganic film is laminated. [Modes for carrying out the invention]

[0009] A description in detail will be given of an embodiment for carrying out the present invention (hereinafter simply referred to as "this embodiment"). The following embodiment is illustrative for explaining the present invention and does not limit the content of the present invention. The present invention can be carried out by modifying it as appropriate within the scope of its gist. In this embodiment, the provisions that are considered preferred can be adopted arbitrarily, and combinations of preferred provisions are considered more preferred. In this embodiment, the description "XX~YY" means "XX or more and YY or less".

[0010] [Polyimide resin] The polyimide resin of the present invention is a polyimide resin comprising a constituent unit A derived from a tetracarboxylic dianhydride and a constituent unit B derived from a diamine, wherein constituent unit A comprises a constituent unit (A1) derived from a tetracarboxylic dianhydride having two norbornane skeletons in the molecule, and constituent unit B comprises a constituent unit (B1) derived from a compound represented by the following formula (b1), and the ratio of constituent unit (B1) in constituent unit B is 35 mol% or more and 95 mol% or less.

[0011] [ka]

[0012] The reason why the polyimide resin of the present invention can be used to obtain a polyimide film with excellent transparency and heat resistance when an inorganic film is laminated is not entirely clear, but it is thought that because it has a norbornane skeleton and a nonlinear and small molecular weight constituent unit (B1) that has a high imide group concentration, it is possible to improve toughness and heat resistance while maintaining good transparency, and thus it is excellent in transparency and heat resistance when an inorganic film is laminated (suppression of discoloration and suppression of cracks in the inorganic film).

[0013] <Component Unit A> Constituent unit A is a constituent unit derived from tetracarboxylic dianhydride in the polyimide resin. Constituent unit A includes a constituent unit (A1) derived from a tetracarboxylic dianhydride having two norbornane skeletons within the molecule. From the viewpoint of heat resistance, transparency and optical isotropy, the constituent unit (A1) preferably includes at least one selected from the group consisting of constituent unit (A11) derived from a compound represented by the following formula (a11), constituent unit (A12) derived from a compound represented by the following formula (a12), constituent unit (A13) derived from a compound represented by the following formula (a13), and constituent unit (A14) derived from a compound represented by the following formula (a14). From the viewpoint of making the molecular skeleton more rigid and further improving heat resistance, the constituent unit derived from the compound represented by the following formula (a11) is preferable. (A11) is more preferably composed of at least one selected from the group consisting of a constituent unit (A12) derived from a compound represented by the following formula (a12) and a constituent unit (A14) derived from a compound represented by the following formula (a14), and it is even more preferably composed of at least one selected from the group consisting of a constituent unit (A11) derived from a compound represented by the following formula (a11) and a constituent unit (A14) derived from a compound represented by the following formula (a14), and it is even more preferably composed of a constituent unit (A11) derived from a compound represented by the following formula (a11).

[0014] [ka]

[0015] By including a constituent unit (A1) derived from a tetracarboxylic dianhydride having two norbornane skeletons within the molecule, the heat resistance, transparency, and optical isotropy of the resulting polyimide film can be improved.

[0016] Constituent unit A may include constituent unit (A2) in addition to constituent unit (A1). For example, constituent unit (A2) may be at least one selected from the group consisting of constituent unit (A21) derived from a compound represented by the following formula (a21) and constituent unit (A22) derived from a compound represented by the following formula (a22).

[0017] [ka]

[0018] The compound represented by formula (a21) is a biphenyltetracarboxylic dianhydride (BPDA), and specific examples include 3,3',4,4'-biphenyltetracarboxylic dianhydride (s-BPDA) represented by formula (a211s), 2,3,3',4'-biphenyltetracarboxylic dianhydride (a-BPDA) represented by formula (a211a), and 2,2',3,3'-biphenyltetracarboxylic dianhydride (i-BPDA) represented by formula (a211i). Among these, 3,3',4,4'-biphenyltetracarboxylic dianhydride (s-BPDA) represented by formula (a211s) is preferred.

[0019] [ka]

[0020] The ratio of constituent unit (A1) in constituent unit A is preferably 40 mol% or more, more preferably 60 mol% or more, even more preferably 70 mol% or more, even more preferably 80 mol% or more, even more preferably 85 mol% or more, even more preferably 90 mol% or more, even more preferably 95 mol% or more, and even more preferably 99 mol% or more, from the viewpoint of heat resistance, transparency and optical isotropy. The upper limit of this ratio is not particularly limited, but it is 100 mol% or less. Furthermore, if constituent unit A further contains constituent unit (A2), the ratio of constituent unit (A2) in constituent unit A is preferably 60 mol% or less, more preferably 40 mol% or less, even more preferably 30 mol% or less, even more preferably 20 mol% or less, even more preferably 15 mol% or less, even more preferably 10 mol% or less, even more preferably 5 mol% or less, and even more preferably 1 mol% or less, from the viewpoint of heat resistance, transparency, and optical isotropy. The lower limit of this ratio is not particularly limited, but it is 0.01 mol% or more.

[0021] Constituent unit A may include constituent units other than constituent units (A1) and (A2). Examples of tetracarboxylic dianhydrides that give such constituent units are not particularly limited, but include aromatic tetracarboxylic dianhydrides such as 4,4'-oxydiphthalic acid anhydride, pyromellitic acid dianhydride, and 4,4'-(hexafluoroisopropylidene)diphthalic acid anhydride (excluding compounds represented by formula (a21) or (a22)); alicyclic tetracarboxylic dianhydrides such as 1,2,4,5-cyclohexanetetracarboxylic dianhydride and 1,2,3,4-cyclobutanetetracarboxylic dianhydride (excluding compounds represented by any of formulas (a11) to (a14)); and aliphatic tetracarboxylic dianhydrides such as 1,2,3,4-butanetetracarboxylic dianhydride. In this specification, "aromatic tetracarboxylic dianhydride" means a tetracarboxylic dianhydride containing one or more aromatic rings, "alicyclic tetracarboxylic dianhydride" means a tetracarboxylic dianhydride containing one or more alicyclic rings but no aromatic rings, and "aliphatic tetracarboxylic dianhydride" means a tetracarboxylic dianhydride containing neither aromatic nor alicyclic rings. The constituent units that are optionally included in constituent unit A may be one type or two or more types.

[0022] <Component Unit B> Constituent unit B is a constituent unit derived from diamine in the polyimide resin. Constituent unit B includes constituent unit (B1) derived from the compound represented by the following formula (b1).

[0023] [ka]

[0024] The compound represented by formula (b1) is 1,3-phenylenediamine. By including structural unit (B1) in structural unit B, toughness can be improved while maintaining heat resistance.

[0025] The ratio of constituent unit (B1) in constituent unit B is 35 mol% or more, preferably 40 mol% or more, more preferably 45 mol% or more, even more preferably 50 mol% or more, and even more preferably 60 mol% or more, from the viewpoint of improving heat resistance when inorganic films are laminated. The ratio of constituent unit (B1) in constituent unit B is 95 mol% or less, preferably 90 mol% or less, more preferably 85 mol% or less, even more preferably 80 mol% or less, and even more preferably 75 mol% or less, from the viewpoint of improving polymerizability, solubility, transparency and optical isotropy.

[0026] Constituent unit B may include constituent units other than constituent unit (B1). It is preferable that constituent unit B further includes constituent unit (B2) in addition to constituent unit (B1). Constituent unit (B2) preferably includes at least one selected from the group consisting of constituent unit (B21) derived from a compound represented by the following formula (b21), constituent unit (B22) derived from a compound represented by the following formula (b22), constituent unit (B23) derived from a compound represented by the following formula (b23), and constituent unit (B24) derived from a compound represented by the following formula (b24). From the viewpoint of making the molecular skeleton more rigid and further improving heat resistance, it is more preferable that it includes at least one selected from the group consisting of constituent unit (B21) derived from a compound represented by the following formula (b21) and constituent unit (B23) derived from a compound represented by the following formula (b23), and even more preferable that it includes constituent unit (B21) derived from a compound represented by the following formula (b21). By including constituent unit (B2) in constituent unit B, heat resistance is particularly improved, and optical isotropy is also improved. Furthermore, the constituent unit (B2) improves the solubility of the imidized resin, allowing polymerization to proceed more effectively.

[0027] [ka]

[0028] [ka]

[0029] [ka]

[0030] [ka]

[0031] If constituent unit B further contains constituent unit (B2), the proportion of constituent unit (B2) in constituent unit B is preferably 5 mol% or more, more preferably 10 mol% or more, even more preferably 15 mol% or more, even more preferably 20 mol% or more, and even more preferably 25 mol% or more. The upper limit of this proportion is 65 mol% or less, preferably 60 mol% or less, more preferably 55 mol% or less, even more preferably 50 mol% or less, and even more preferably 40 mol% or less. If constituent unit B further contains constituent unit (B2), the total ratio of constituent units (B1) and (B2) in constituent unit B is preferably 50 mol% or more, more preferably 70 mol% or more, even more preferably 90 mol% or more, even more preferably 95 mol% or more, and even more preferably 99 mol% or more. The upper limit of the total ratio of constituent units (B1) and (B2) in constituent unit B is not particularly limited, for example, 100 mol% or less. Constituent unit B may consist only of constituent unit (B1) and constituent unit (B2). If constituent unit B further contains constituent unit (B2), the molar ratio of constituent unit (B1) to constituent unit (B2) in constituent unit B [(B1) / (B2)] is preferably 35 / 65 to 95 / 5, more preferably 40 / 60 to 90 / 10, even more preferably 45 / 55 to 85 / 15, even more preferably 50 / 50 to 80 / 20, and even more preferably 60 / 40 to 75 / 25, from the viewpoint of improving transparency, optical isotropy, toughness, and heat resistance.

[0032] Constituent unit B may include constituent units other than constituent units (B1) and (B2). The diamines that provide such constituent units are not particularly limited, but include 1,4-phenylenediamine, p-xylylenediamine, 1,5-diaminonaphthalene, 2,2'-dimethylbiphenyl-4,4'-diamine, 4,4'-diaminodiphenyl ether, 4,4'-diamino-2,2'-bistrifluoromethyldiphenyl ether, 4,4'-diaminodiphenylmethane, 4,4'-diaminobenzanilide, 1-(4-aminophenyl)-2,3-dihydro-1,3,3-trimethyl-1H-inden-5-amine, and α,α'-bis(4-aminophenyl)-1,4-diisopropylben Examples include aromatic diamines such as zen, N,N'-bis(4-aminophenyl)terephthalamide, 4,4'-bis(4-aminophenoxy)biphenyl, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, and 2,2-bis(4-(4-aminophenoxy)phenyl)hexafluoropropane (excluding compounds represented by any of formulas (b1), (b21) to (b24)); alicyclic diamines such as 1,3-bis(aminomethyl)cyclohexane and 1,4-bis(aminomethyl)cyclohexane; and aliphatic diamines such as ethylenediamine and hexamethylenediamine. In this specification, aromatic diamine means a diamine containing one or more aromatic rings, alicyclic diamine means a diamine containing one or more alicyclic rings but not aromatic rings, and aliphatic diamine means a diamine that does not contain either aromatic or alicyclic rings. The constituent units that are optionally included in constituent unit B may be one type or two or more types.

[0033] <Properties of polyimide resin> The weight-average molecular weight of the polyimide resin is preferably 5,000 to 300,000, from the viewpoint of the mechanical strength of the resulting polyimide film. The weight-average molecular weight of the polyimide resin can be determined, for example, from the value converted to standard polymethyl methacrylate (PMMA) by gel filtration chromatography.

[0034] Polyimide resins may contain structures other than polyimide chains (structures in which constituent unit A and constituent unit B are bonded by imide bonds). Examples of structures other than polyimide chains that may be included in polyimide resins include structures containing amide bonds. The polyimide resin preferably contains polyimide chains (a structure in which constituent unit A and constituent unit B are bonded together by imide) as its main structure. Therefore, the proportion of polyimide chains in the polyimide resin is preferably 50% by mass or more, more preferably 70% by mass or more, even more preferably 90% by mass or more, particularly preferably 99% by mass or more, and also 100% by mass or less. The polyimide resin may consist only of polyimide chains.

[0035] A polyimide resin composition containing the above-mentioned polyimide resin can form a polyimide film that maintains transparency and optical isotropy while exhibiting excellent toughness and heat resistance. The preferred physical properties of the polyimide film are as follows.

[0036] The total light transmittance, when the polyimide resin is used as a film with a thickness of 10 μm, is preferably 80% or more, more preferably 85% or more, even more preferably 88% or more, even more preferably 88.5% or more, and even more preferably 89% or more. The yellow index (YI) is preferably 10.0 or less, more preferably 5.0 or less, even more preferably 3.0 or less, even more preferably 2.5 or less, and still more preferably 2.0 or less, when the polyimide resin is used as a 10 μm thick film. The absolute value of the thickness phase difference (Rth) is preferably 200 nm or less, more preferably 180 nm or less, even more preferably 160 nm or less, and even more preferably 140 nm or less, when the polyimide resin is used as a 10 μm thick film.

[0037] Furthermore, the film that can be formed using the above-mentioned polyimide resin has good mechanical properties and heat resistance, and possesses the following desirable physical properties. The tensile strength is preferably 70 MPa or higher, more preferably 80 MPa or higher, and even more preferably 90 MPa or higher, when the polyimide resin is used as a film with a thickness of 10 μm. The tensile modulus of elasticity is preferably 1.0 GPa or higher, more preferably 1.5 GPa or higher, even more preferably 2.0 GPa or higher, and even more preferably 2.3 GPa or higher, when the polyimide resin is used as a film with a thickness of 10 μm. The tensile elongation at break is preferably 9.5% or more, more preferably 10.0% or more, and even more preferably 10.5% or more, when the polyimide resin is used as a 10 μm thick film. The upper limit of the tensile elongation at break is preferably 20.0% or less, more preferably 15.0% or less, and even more preferably 13.5% or less. The glass transition temperature (Tg) is preferably 380°C or higher, more preferably 400°C or higher, and even more preferably 410°C or higher. The temperature at which the weight loss of 5% (Td5%) occurs is preferably 460°C or higher, more preferably 470°C or higher, and even more preferably 480°C or higher. The heat resistance when inorganic films are laminated is such that, when a 300 nm thick SiO2 film is formed on a polyimide film by sputtering, and a 1230 nm thick ITO (indium tin oxide) film is formed on top of it to form a laminated film, preferably no defects such as cracks or yellowing occur in the laminated film after "annealing treatment at 400°C for 1 hour", and more preferably no defects such as cracks or yellowing occur in the laminated film after "annealing treatment at 420°C for 1 hour". The above-mentioned physical properties in this invention can be specifically measured by the methods described in the examples.

[0038] <Method for producing polyimide resin> The polyimide resin of the present invention can be produced by reacting a tetracarboxylic acid component containing a compound that gives the above-mentioned structural unit (A1) with a compound that gives the above-mentioned structural unit (B1).

[0039] Compounds that provide the constituent unit (A1) include, but are not limited to, compounds represented by any of formulas (a11) to (a14), and derivatives thereof within the range of providing the same constituent unit may also be used. Examples of such derivatives include tetracarboxylic acids corresponding to tetracarboxylic dianhydrides represented by any of formulas (a11) to (a14), and alkyl esters of said tetracarboxylic acids. Among these, the tetracarboxylic dianhydride represented by formula (a11) is preferred.

[0040] The tetracarboxylic acid component may include a compound that provides a further structural unit (A2) in addition to the compound that provides structural unit (A1). Examples of compounds that provide the constituent unit (A2) include compounds represented by formula (a21) and compounds represented by formula (a22), but are not limited to these; derivatives thereof may also be used as long as they provide the same constituent unit.

[0041] The tetracarboxylic acid component contains, preferably, 40 mol% or more, more preferably 60 mol% or more, even more preferably 70 mol% or more, even more preferably 80 mol% or more, even more preferably 85 mol% or more, even more preferably 90 mol% or more, even more preferably 95 mol% or more, and even more preferably 99 mol% or more of the compound that gives the constituent unit (A1). The upper limit of this ratio is not particularly limited, but it is 100 mol% or less.

[0042] The tetracarboxylic acid component may include any compound other than the compound that gives structural unit (A1) and the compound that gives structural unit (A2). Examples of such arbitrary compounds include the aromatic tetracarboxylic dianhydrides, alicyclic tetracarboxylic dianhydrides, and aliphatic tetracarboxylic dianhydrides mentioned above, as well as their derivatives (tetracarboxylic acids, alkyl esters of tetracarboxylic acids, etc.). The tetracarboxylic acid component may optionally contain one or more compounds.

[0043] Examples of compounds that provide the structural unit (B1) include, but are not limited to, compounds represented by formula (b1), and derivatives thereof that provide the same structural unit may also be used. Examples of such derivatives include diisocyanates corresponding to the compound represented by formula (b1). The compound that provides the structural unit (B1) is preferably the compound represented by formula (b1) (i.e., a diamine). The diamine component may include compounds that provide structural unit (B2) in addition to compounds that provide structural unit (B1). Examples of compounds that provide structural unit (B2) include, but are not limited to, compounds represented by formula (b21), formula (b22), formula (b23), and formula (b24), and derivatives thereof within the range of providing the same structural unit. Examples of such derivatives include diisocyanates corresponding to compounds represented by formula (b21), formula (b22), formula (b23), and formula (b24). As compounds that provide structural unit (B2), compounds represented by formula (b21), formula (b22), formula (b23), and formula (b24) (i.e., diamines) are preferred.

[0044] The diamine component contains 35 mol% or more, preferably 40 mol% or more, more preferably 45 mol% or more, even more preferably 50 mol% or more, and still more preferably 60 mol% or more, of the compound that gives the constituent unit (B1). The upper limit of this ratio is 95 mol% or less, preferably 90 mol% or less, more preferably 85 mol% or less, even more preferably 80 mol% or less, and still more preferably 75 mol% or less. If the diamine component contains a compound that provides the constituent unit (B2), it preferably contains 5 mol% or more, more preferably 10 mol% or more, even more preferably 15 mol% or more, even more preferably 20 mol% or more, and even more preferably 25 mol% or more of the compound that provides the constituent unit (B2). The upper limit of this ratio is 65 mol% or less, preferably 60 mol% or less, more preferably 55 mol% or less, even more preferably 50 mol% or less, and even more preferably 40 mol% or less. If the diamine component contains a compound that provides the structural unit (B2), it preferably contains 50 mol% or more, more preferably 70 mol% or more, even more preferably 90 mol% or more, even more preferably 95 mol% or more, and even more preferably 99 mol% or more in total the compound that provides the structural unit (B1) and the compound that provides the structural unit (B2). The upper limit is not particularly limited, and the diamine component contains, for example, 100 mol% or less in total the compound that provides the structural unit (B1) and the compound that provides the structural unit (B2). The diamine component may consist only of a compound that provides the structural unit (B1) and a compound that provides the structural unit (B2). The molar ratio [(B1) / (B2)] of the compound giving the constituent unit (B1) in the diamine component to the compound giving the constituent unit (B2) is preferably 35 / 65 to 95 / 5, more preferably 40 / 60 to 90 / 10, even more preferably 45 / 55 to 85 / 15, even more preferably 50 / 50 to 80 / 20, and even more preferably 60 / 40 to 75 / 25, from the viewpoint of improving polymerizability, solubility, transparency, optical isotropy, toughness, and heat resistance.

[0045] The diamine component may further contain any compound other than the compound that provides the structural unit (B1) and the compound that provides the structural unit (B2). Examples of such arbitrary compounds include the aromatic diamines, alicyclic diamines, and aliphatic diamines mentioned above, as well as their derivatives (such as diisocyanates). The diamine component may contain one or more compounds as desired.

[0046] In the production of the polyimide resin of the present invention, the preferred ratio of the tetracarboxylic acid component to the diamine component used in the production of the polyimide resin is 0.9 to 1.1 moles of the diamine component per mole of the tetracarboxylic acid component.

[0047] Furthermore, in the production of the polyimide resin of the present invention, in addition to the tetracarboxylic acid component and diamine component described above, an end-cap encapsulant may also be used in the production of the polyimide resin. Monoamines or dicarboxylic acids are preferred as the end-cap encapsulant. The amount of end-cap encapsulant to be introduced is preferably 0.0001 to 0.1 moles, and more preferably 0.001 to 0.06 moles, per mole of tetracarboxylic acid component. Examples of monoamine end-cap encapsulants include methylamine, ethylamine, propylamine, butylamine, benzylamine, 4-methylbenzylamine, 4-ethylbenzylamine, 4-dodecylbenzylamine, 3-methylbenzylamine, 3-ethylbenzylamine, aniline, 3-methylaniline, 4-methylaniline, etc., with benzylamine and aniline being preferred. Dicarboxylic acids are preferred as the end-cap encapsulant, and a portion of them may be ring-closed. Examples include phthalic acid, phthalic anhydride, 4-chlorophthalic acid, tetrafluorophthalic acid, 2,3-benzophenone dicarboxylic acid, 3,4-benzophenone dicarboxylic acid, cyclohexane-1,2-dicarboxylic acid, cyclopentane-1,2-dicarboxylic acid, and 4-cyclohexene-1,2-dicarboxylic acid, with phthalic acid and phthalic anhydride being preferred.

[0048] There are no particular restrictions on the method of reacting the aforementioned tetracarboxylic acid component with the diamine component; known methods can be used. Specific reaction methods include: (1) charging the tetracarboxylic acid component, diamine component, and reaction solvent into a reactor, stirring at 0-10°C for 0.5-30 hours, and then raising the temperature to carry out the imidation reaction; (2) charging the diamine component and reaction solvent into a reactor and dissolving them, then charging the tetracarboxylic acid component, stirring at room temperature (0-10°C) for 0.5-30 hours as needed, and then raising the temperature to carry out the imidation reaction; and (3) charging the tetracarboxylic acid component, diamine component, and reaction solvent into a reactor and immediately raising the temperature to carry out the imidation reaction.

[0049] The reaction solvent used in the production of polyimide resins should not inhibit the imidation reaction and should be able to dissolve the resulting polyimide. Examples include aprotic solvents, phenolic solvents, etheric solvents, and carbonate solvents.

[0050] Specific examples of aprotic solvents include amide solvents such as N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone (NMP), N-methylcaprolactam, 1,3-dimethylimidazolidinone, and tetramethylurea; lactone solvents such as γ-butyrolactone (GBL) and γ-valerolactone; phosphorus-containing amide solvents such as hexamethylphosphoricamide and hexamethylphosphinetriamide; sulfur-containing solvents such as dimethylsulfone, dimethyl sulfoxide, and sulfolane; ketone solvents such as acetone, cyclohexanone, and methylcyclohexanone; amine solvents such as picoline and pyridine; and ester solvents such as acetic acid (2-methoxy-1-methylethyl).

[0051] Specific examples of phenolic 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-based 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-based solvents include diethyl carbonate, methyl ethyl carbonate, ethylene carbonate, and propylene carbonate. Among the above reaction solvents, aprotic solvents are preferred, amide solvents and lactone solvents are more preferred, and lactone solvents are even more preferred. Furthermore, the above reaction solvents may be used individually or in combination of two or more.

[0052] In the imidation reaction, it is preferable to carry out the reaction while removing the water generated during production using a Dean-Stark apparatus or similar. By performing this operation, the degree of polymerization and the imidation rate can be further increased.

[0053] In the above imidation reaction, known imidation catalysts can be used. Examples of imidation catalysts include base catalysts and acid catalysts. Examples of base catalysts include organic base catalysts such as pyridine, quinoline, isoquinoline, α-picoline, β-picoline, 2,4-lutidine, 2,6-lutidine, trimethylamine, triethylamine (TEA), tripropylamine, tributylamine, triethylenediamine, imidazole, N,N-dimethylaniline, and N,N-diethylaniline, as well as inorganic base catalysts such as potassium hydroxide, sodium hydroxide, potassium carbonate, sodium carbonate, potassium bicarbonate, and sodium bicarbonate. Examples of acid catalysts include crotonic acid, acrylic acid, trans-3-hexenoic acid, cinnamic acid, benzoic acid, methylbenzoic acid, oxybenzoic acid, terephthalic acid, benzenesulfonic acid, p-toluenesulfonic acid, and naphthalenesulfonic acid. The above imidation catalysts may be used individually or in combination of two or more types. Of the above, from the viewpoint of ease of handling, it is preferable to use a base catalyst, more preferable to use an organic base catalyst, and even more preferable to use triethylamine or triethylenediamine.

[0054] The temperature of the imidation 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 generated water.

[0055] [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, wherein the polyimide resin is dissolved in the organic solvent. The organic solvent can be any solvent that dissolves polyimide resin and is not particularly limited, but it is preferable to use the above-mentioned compounds individually or in a mixture of two or more as the reaction solvent used in the production of polyimide resin. The polyimide varnish of the present invention may be the polyimide solution itself, obtained by dissolving a polyimide resin obtained by polymerization in a reaction solvent, or it may be a polyimide solution that has been further diluted by adding a solvent.

[0056] Since the polyimide resin of the present invention is solvent-soluble, it can be used to produce a stable, high-concentration varnish at room temperature. The polyimide varnish of the present invention preferably contains 5 to 40% by mass of the polyimide resin of the present invention, and more preferably 5 to 20% by mass. The viscosity of the polyimide varnish is preferably 1 to 200 Pa·s, and more preferably 1 to 100 Pa·s. The viscosity of the polyimide varnish is the value measured at 25°C using an E-type viscometer. Furthermore, the polyimide varnish of the present invention may contain various additives such as inorganic fillers, adhesion promoters, release agents, flame retardants, ultraviolet stabilizers, surfactants, leveling agents, defoamers, fluorescent whitening agents, crosslinking agents, polymerization initiators, and photosensitive agents, to the extent that they 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 known methods can be applied.

[0057] [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 has excellent heat resistance, transparency, toughness, optical isotropy, peelability, and chemical resistance. The preferred physical properties of the polyimide film of the present invention are as described above under <Properties of the Polyimide Resin>. There are no particular limitations on the method for producing the polyimide film of the present invention, and known methods can be used. For example, one method involves applying the polyimide varnish of the present invention in a film-like manner onto a smooth support such as a glass plate, metal plate, or plastic, or forming it into a film, and then removing organic solvents such as reaction solvents and diluent solvents contained in the varnish by heating. As a method for producing the polyimide film of the present invention, a method including the step of removing organic solvents after applying or forming the polyimide varnish into a film is preferred.

[0058] Known coating methods include spin coating, slit coating, and blade coating, with spin coating and slit coating being preferred. Among these, slit coating is more preferred from the viewpoint of controlling intermolecular orientation, improving chemical resistance, and ease of workability. A method for removing organic solvents contained in varnish by heating is to first evaporate the organic solvent at a temperature of 150°C or lower to make it tack-free, and then dry it at a temperature above the boiling point of the organic solvent used (not particularly limited, but preferably 200 to 500°C). It is also preferable to dry it in an air atmosphere or a nitrogen atmosphere. The pressure of the drying atmosphere may be reduced pressure, atmospheric pressure, or increased pressure. The method for peeling the polyimide film, which has been deposited on a support, from the support is not particularly limited, but methods such as the mechanical peel-off method and the laser lift-off method can be used.

[0059] Furthermore, the polyimide film of the present invention can also be manufactured using a polyamic acid varnish obtained by dissolving 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 the product of a polyaddition reaction between a tetracarboxylic acid component containing a compound that gives the above-mentioned structural unit (A1) and a diamine component containing a compound that gives the above-mentioned structural unit (B1). By imidizing (dehydrating and cyclizing) this polyamic acid, the final product, the polyimide resin of the present invention, is obtained. 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 production of the polyimide film of the present invention, the polyamic acid varnish may be the polyamic acid solution itself obtained by polyaddition reaction of a tetracarboxylic acid component and a diamine component in a reaction solvent, or it may be a polyamic acid solution that has been further diluted by adding a solvent.

[0060] There are no particular restrictions on the method for producing a polyimide film using polyamic acid varnish, and known methods can be used. For example, a polyimide film can be produced by coating or forming a film of polyamic acid varnish onto a smooth support such as a glass plate, metal plate, or plastic, removing organic solvents such as reaction solvents and diluent solvents contained in the varnish by heating to obtain a polyamic acid film, and then imidizing the polyamic acid in the polyamic acid film by heating. The heating temperature when drying the polyamic acid varnish to obtain a polyamic acid film is preferably 50 to 120°C. The heating temperature when imidizing the polyamic acid by heating is preferably 200 to 450°C. Furthermore, the imidation method is not limited to thermal imidation; chemical imidation can also be applied.

[0061] The thickness of the polyimide film of the present invention can be appropriately selected depending on the application, 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 even more preferably 10 to 80 μm. A thickness of 1 to 250 μm makes it possible to use it practically as a self-supporting film. The thickness of the polyimide film can be easily controlled by adjusting the solid content concentration and viscosity of the polyimide varnish.

[0062] In the polyimide film of the present invention, the tensile elongation at break, measured in accordance with JIS K7127:1999 under the conditions of a chuck distance of 50 mm, a test specimen size of 10 mm × 70 mm, a tensile speed of 20 mm / min, and a measurement temperature of 23°C, is preferably 9.5% or more, more preferably 10.0% or more, and even more preferably 10.5% or more, from the viewpoint of further suppressing cracking of the inorganic film when the inorganic film is exposed to high temperatures in a laminated state. The upper limit of the tensile elongation at break is preferably 20.0% or less, more preferably 15.0% or less, and even more preferably 13.5% or less.

[0063] In the polyimide film of the present invention, the total light transmittance, measured in accordance with JIS K7136:2000, is preferably 80% or higher, more preferably 85% or higher, even more preferably 88% or higher, even more preferably 88.5% or higher, and even more preferably 89% or higher, from the viewpoint of further improving transparency.

[0064] The polyimide film of the present invention is suitably used as a film for various components such as color filters, flexible displays, semiconductor components, optical components, solar cells, and image display devices, and is particularly suitably used as a transparent substrate constituting these devices. The polyimide film of the present invention is particularly suitably used as a transparent substrate constituting image display devices such as liquid crystal displays, OLED displays, and touch panels.

[0065] [Image display device] The image display device of the present invention comprises the polyimide film of the present invention as a transparent substrate. The image display device of the present invention comprises, for example, a transparent substrate made of the polyimide film of the present invention, and a display unit provided on the transparent substrate. The display unit is not particularly limited, but examples include TFT elements, organic EL elements, color filters, LEDs, transistors, electron emission elements, electronic ink, electrophoretic elements, GLV (grating light bulb), MEMS (micro-electromechanical system) display elements, DMD (digital micromirror device), DMS (digital micro shutter), IMOD (interferometric modulation) elements, electrowetting elements, piezoelectric ceramic displays, and carbon nanotube display elements. Examples of image display devices of the present invention include liquid crystal displays, OLED displays, and touch panels. The image display device of the present invention can be manufactured based on known information, except that the polyimide film of the present invention is used as a transparent substrate. The image display device of the present invention uses the polyimide film of the present invention, which has excellent heat resistance when an inorganic film is laminated, as a transparent substrate. Therefore, cracks in the inorganic film and discoloration of the transparent substrate are less likely to occur, resulting in excellent reliability. [Examples]

[0066] The present invention will be specifically described below with reference to examples. However, the present invention is not limited in any way by these examples.

[0067] <Film properties and evaluation> The physical properties of the films obtained in the examples and comparative examples were measured by the following methods.

[0068] (1) Film thickness The film thickness was measured using a micrometer manufactured by Mitutoyo Corporation.

[0069] (2) Tensile strength, tensile modulus, and tensile elongation at fracture Tensile strength, tensile modulus, and tensile elongation at break were measured in accordance with JIS K7127:1999 using a tensile testing machine "Strograph VG-1E" manufactured by Toyo Seiki Co., Ltd. The chuck distance was 50 mm, the test specimen size was 10 mm x 70 mm, the test speed (tensile speed) was 20 mm / min, and the measurement temperature was 23 °C.

[0070] (3) Glass transition temperature (Tg) Using a thermomechanical analyzer "TMA / SS6100" manufactured by Hitachi High-Tech Science Co., Ltd., the sample size was 3 mm x 20 mm, the load was 0.1 N, and the heating rate was 10 °C / min in tensile mode. The sample was heated to a temperature sufficient to remove residual stress, and then cooled to room temperature. Subsequently, the elongation of the test specimen was measured under the same conditions as the treatment to remove residual stress, and the inflection point of the elongation was extrapolated to determine the glass transition temperature.

[0071] (4) Total light transmittance and yellow index (YI) Total light transmittance was measured in accordance with JIS K7136:2000, and YI was measured in accordance with ASTM E313-05 (D light source, 65°) using the COH7700 color and turbidity simultaneous meter manufactured by Nippon Denshoku Industries Co., Ltd.

[0072] (5)5% weight loss temperature (Td5%) A differential thermogravimetric analyzer, "NEXTA STA200RV," manufactured by Hitachi High-Tech Science Corporation, was used. The sample was heated from 40 to 150°C at a heating rate of 10°C / min, held at 150°C for 30 minutes to remove moisture, and then heated to 510°C. The temperature at which the weight decreased by 5% compared to the weight after holding at 150°C for 30 minutes was defined as the 5% weight loss temperature. A higher weight loss temperature indicates better heat resistance.

[0073] (6) Thickness phase difference (Rth) (evaluation of optical isotropy) The thickness phase difference (Rth) was measured using an ellipsometer "M-220" manufactured by JASCO Corporation. The value of the thickness phase difference was measured at a measurement wavelength of 590 nm. Rth is expressed by the following formula, where nx is the maximum refractive index in the plane of the polyimide film, ny is the minimum refractive index, nz is the refractive index in the thickness direction, and d is the thickness of the film. Rth = [{(nx+ny) / 2}-nz] × d

[0074] (7) Evaluation of the heat resistance of the laminated film We fabricated a laminated film by simulating the manufacturing process of an image display device, and evaluated its heat resistance. The laminated film was fabricated as follows: Without peeling the polyimide films obtained in the examples and comparative examples from the glass plate, a 300 nm thick SiO2 film was formed on the polyimide film by sputtering, and a 1230 nm thick ITO (indium tin oxide) film was formed on top of it to obtain a laminated film. Next, the resulting multilayer film was subjected to annealing (heating) at 400°C for 1 hour. The presence or absence of defects (cracks, yellowing, etc.) in the laminated film before and after annealing was visually observed, and the heat resistance of the polyimide film (laminated film) with an inorganic film layered on top was evaluated according to the following criteria. ○: No defects such as cracks or yellowing occurred in the laminated film before or after annealing. × (Cracks or yellowing): Defects such as cracks or yellowing occurred in the laminated film before or after annealing. If there are no defects, polyimide film offers excellent heat resistance when layered with inorganic films.

[0075] <Abbreviations for ingredients, etc.> The tetracarboxylic acid and diamine components used in the examples and comparative examples, as well as their abbreviations, are as follows.

[0076] (Tetracarboxylic acid component) CpODA: norbornane-2-spiro-α-cyclopentanone-α'-spiro-2''-norbornane-5,5'',6,6''-tetracarboxylic dianhydride (manufactured by ENEOS Corporation; compound represented by formula (a11)) BPDA: 3,3',4,4'-biphenyltetracarboxylic dianhydride (manufactured by Mitsubishi Chemical Corporation, compound represented by formula (a211s) (s-BPDA)) BPAF: 9,9-Bis(3,4-dicarboxyphenyl)fluorendiohydride (manufactured by JFE Chemical Corporation; compound represented by formula (a22))

[0077] (Diamine component) MPD: 1,3-Phenylenediamine (manufactured by Tokyo Chemical Industry Co., Ltd.; compound represented by formula (b1)) BAFL: 9,9-Bis(4-aminophenyl)fluorene (manufactured by JFE Chemical Corporation; compound represented by formula (b21)) TFMB: 2,2'-Bis(trifluoromethyl)benzidine (manufactured by Seika Co., Ltd.; compound represented by formula (b22)) PPD: 1,4-phenylenediamine (manufactured by Tokyo Chemical Industry Co., Ltd.)

[0078] <Manufacturing of polyimide resin, varnish, and polyimide film> Example 1 In a 500 mL five-necked round-bottom flask equipped with a stainless steel crescent-shaped stirring blade, nitrogen inlet tube, condenser, Dean-Stark flask, thermometer, and glass end cap, 5.407 g (0.050 mol) of MPD, 17.423 g (0.050 mol) of BAFL, and 73.521 g of γ-butyrolactone (manufactured by Mitsubishi Chemical Corporation) were added, and the mixture was stirred at 200 rpm under a nitrogen atmosphere at a system temperature of 70°C to obtain a solution. To this solution, 38.438 g (0.100 mol) of CpODA and 18.380 g of γ-butyrolactone (manufactured by Mitsubishi Chemical Corporation) were added in one go. Then, 0.506 g of triethylamine (manufactured by Kanto Chemical Co., Ltd.) and 0.056 g of triethylenediamine (manufactured by Tokyo Chemical Industry Co., Ltd.) were added as imidation catalysts. The mixture was heated with a mantle heater, and the temperature inside the reaction system was raised to 190°C over approximately 20 minutes. The components that were removed by distillation were collected, and the reaction system temperature was maintained at 190°C and refluxed for 3 hours while adjusting the rotation speed according to the increase in viscosity. Subsequently, γ-butyrolactone (manufactured by Mitsubishi Chemical Corporation) was added to achieve a solid content concentration of 15% by mass. After cooling the reaction system temperature to 100°C, the mixture was stirred for approximately 1 hour to homogenize it, thereby obtaining polyimide varnish. Next, the obtained polyimide varnish was applied to a glass plate by spin coating, held at 80°C for 20 minutes on a hot plate, and then heated in a hot air dryer at 400°C for 30 minutes under a nitrogen atmosphere (heating rate 5°C / min) to evaporate the solvent and obtain a film.

[0079] Example 2 A polyimide varnish with a solid content of 15% by mass was obtained by the same method as in Example 1, except that the amount of MPD was changed from 5.407 g (0.050 mol) to 7.570 g (0.070 mol) and the amount of BAFL was changed from 17.423 g (0.050 mol) to 10.454 g (0.030 mol). Using the obtained polyimide varnish, a film was obtained in the same manner as in Example 1.

[0080] Example 3 A polyimide varnish with a solid content of 15% by mass was obtained by the same method as in Example 1, except that the amount of MPD was changed from 5.407 g (0.050 mol) to 8.651 g (0.080 mol) and the amount of BAFL was changed from 17.423 g (0.050 mol) to 6.969 g (0.020 mol). Using the obtained polyimide varnish, a film was obtained in the same manner as in Example 1.

[0081] Example 4 A polyimide varnish with a solid content of 15% by mass was obtained by the same method as in Example 1, except that the amount of CpODA was changed from 38.438 g (0.100 mol) to 30.750 g (0.080 mol), BPDA was used at 5.885 g (0.020 mol), the amount of MPD was changed from 5.407 g (0.050 mol) to 6.488 g (0.060 mol), and the amount of BAFL was changed from 17.423 g (0.050 mol) to 13.938 g (0.040 mol). Using the obtained polyimide varnish, a film was obtained in the same manner as in Example 1.

[0082] Example 5 A polyimide varnish with a solid content of 15% by mass was obtained by the same method as in Example 1, except that the amount of CpODA was changed from 38.438 g (0.100 mol) to 34.594 g (0.090 mol), BPAF was used at 4.584 g (0.010 mol), the amount of MPD was changed from 5.407 g (0.050 mol) to 6.488 g (0.060 mol), and the amount of BAFL was changed from 17.423 g (0.050 mol) to 13.938 g (0.040 mol). Using the obtained polyimide varnish, a film was obtained in the same manner as in Example 1.

[0083] Comparative Example 1 A polyimide varnish with a solid content of 15% by mass was obtained by the same method as in Example 1, except that the amount of BAFL was changed from 17.423 g (0.050 mol) to 34.845 g (0.100 mol) without using MPD. Using the obtained polyimide varnish, a film was obtained in the same manner as in Example 1.

[0084] Comparative Example 2 A polyimide varnish with a solid content of 15% by mass was obtained by the same method as in Example 1, except that the amount of CpODA was changed from 38.438 g (0.100 mol) to 23.063 g (0.060 mol), BPDA was used at 11.769 g (0.040 mol), MPD was not used, the amount of BAFL was changed from 17.423 g (0.050 mol) to 13.938 g (0.040 mol), and TFMB was used at 19.214 g (0.060 mol). Using the obtained polyimide varnish, a film was obtained in the same manner as in Example 1.

[0085] Comparative Example 3 A polyimide varnish with a solid content of 15% by mass was obtained by the same method as in Example 1, except that the amount of CpODA was changed from 38.438 g (0.100 mol) to 34.594 g (0.090 mol), BPAF was used at 4.584 g (0.010 mol), MPD was not used, the amount of BAFL was changed from 17.423 g (0.050 mol) to 15.680 g (0.045 mol), and TFMB was used at 17.613 g (0.055 mol). Using the obtained polyimide varnish, a film was obtained in the same manner as in Example 1.

[0086] Comparative Example 4 We attempted to synthesize the polyimide varnish using the same method as in Example 2, except that we changed the amount of MPD from 7.570 g (0.070 mol) to 7.570 g (0.070 mol). However, after adding the imidation catalyst, a solid precipitate formed during the heating process up to 190°C, and we were unable to obtain the polyimide varnish.

[0087] Comparative Example 5 In a 300 mL five-necked round-bottom flask equipped with a stainless steel crescent-shaped stirring blade, nitrogen inlet tube, condenser, thermometer, and glass end cap, 10.814 g (0.100 mol) of MPD and 182.403 g of NMP were added, and the mixture was stirred at 200 rpm under a nitrogen atmosphere at a system temperature of 25 °C to obtain the solution. To this solution, 29.422 g (0.100 mol) of BPDA and 45.601 g of NMP were added all at once, and the mixture was stirred for 3 hours to obtain a polyamic acid varnish with a solid content of 15.0% by mass. Next, the obtained polyamic acid varnish was applied to a glass plate by spin coating, held at 80°C for 20 minutes on a hot plate, and then heated in a hot air dryer at 430°C for 60 minutes under a nitrogen atmosphere (heating rate of 5°C / min) to evaporate the solvent and obtain a film.

[0088] Comparative Example 6 A polyimide varnish with a solid content of 15% by mass was obtained by the same method as in Example 1, except that 16.012 g (0.050 mol) of TFMB was used instead of MPD as the diamine. Using the obtained polyimide varnish, a film was obtained in the same manner as in Example 1.

[0089] Comparative Example 7 We attempted to synthesize the polyimide varnish using the same method as in Example 1, except that we changed the amount of MPD from 5.407 g (0.050 mol) to 10.814 g (0.100 mol) and did not use BAFL. However, after adding the imidation catalyst, a solid precipitate formed during the heating process up to 190°C along with the imidation, and we were unable to obtain the polyimide varnish.

[0090] The polyimide films obtained in the examples and comparative examples were subjected to the aforementioned physical property measurements and evaluations. The results are shown in Table 1.

[0091] [Table 1]

[0092] As shown in Table 1, the polyimide films of the examples exhibited excellent transparency and heat resistance when laminated with inorganic films. Furthermore, the polyimide films of the examples showed good optical isotropy. Comparative Examples 1 and 6 exhibited excellent optical isotropy, but the heat resistance of the laminated films was poor. Comparative Example 2 exhibited poor optical isotropy and poor heat resistance of the laminated films. Comparative Example 3 exhibited poor heat resistance of the laminated films. In Comparative Examples 4 and 7, the imidized resin had poor solubility and precipitated, preventing further polymerization and making it impossible to obtain polyimide varnish. Comparative Example 5 exhibited poor transparency, poor optical isotropy, and poor heat resistance of the laminated films. Therefore, a polyimide film produced using an acid dianhydride having two norbornane skeletons in the molecule as the tetracarboxylic acid component and MPD as the diamine component can be suitably used as a transparent substrate for display devices such as liquid crystal displays, OLED displays, and touch panels, as a film with excellent transparency, optical isotropy, and heat resistance of the laminated film.

Claims

1. A polyimide resin comprising structural unit A derived from tetracarboxylic dianhydride and structural unit B derived from diamine, The aforementioned structural unit A includes a structural unit (A1) derived from a tetracarboxylic dianhydride having two norbornane skeletons within the molecule, The aforementioned constituent unit B includes a constituent unit (B1) derived from a compound represented by the following formula (b1), The aforementioned constituent unit B further includes a constituent unit (B2), wherein the constituent unit (B2) includes at least one selected from the group consisting of a constituent unit (B21) derived from a compound represented by the following formula (b21), a constituent unit (B22) derived from a compound represented by the following formula (b22), a constituent unit (B23) derived from a compound represented by the following formula (b23), and a constituent unit (B24) derived from a compound represented by the following formula (b24). A polyimide resin in which the proportion of the constituent unit (B1) in the constituent unit B is 35 mol% or more and 95 mol% or less. 【Chemistry 1】 【Chemistry 2】 【Transformation 3】 【Chemistry 4】 【Transformation 5】

2. The polyimide resin according to claim 1, wherein the constituent unit (A1) includes at least one selected from the group consisting of a constituent unit (A11) derived from a compound represented by the following formula (a11), a constituent unit (A12) derived from a compound represented by the following formula (a12), a constituent unit (A13) derived from a compound represented by the following formula (a13), and a constituent unit (A14) derived from a compound represented by the following formula (a14). 【Transformation 6】

3. The polyimide resin according to claim 1 or 2, wherein the constituent unit A further comprises a constituent unit (A2), and the constituent unit (A2) comprises at least one selected from the group consisting of a constituent unit (A21) derived from a compound represented by the following formula (a21), and a constituent unit (A22) derived from a compound represented by the following formula (a22). 【Transformation 7】

4. The polyimide resin according to any one of claims 1 to 3, wherein the aforementioned constituent unit (B2) includes a constituent unit (B21) derived from a compound represented by the following formula (b21). 【Transformation 8】

5. A polyimide varnish comprising a polyimide resin according to any one of claims 1 to 4 dissolved in an organic solvent.

6. A polyimide film comprising the polyimide resin described in any one of claims 1 to 4.

7. The polyimide film according to claim 6, wherein the tensile elongation at break is 9.5% or more, as measured in accordance with JIS K7127:1999, under the conditions of a chuck distance of 50 mm, a test specimen size of 10 mm x 70 mm, a tensile speed of 20 mm / min, and a measurement temperature of 23°C.

8. The polyimide film according to claim 6 or 7, wherein the total light transmittance, as measured in accordance with JIS K7136:2000, is 80% or more.

9. A polyimide film according to any one of claims 6 to 8, used as a transparent substrate constituting a display device.

10. A method for producing a polyimide film, comprising the step of removing an organic solvent after applying or molding the polyimide varnish described in claim 5.

11. An image display device comprising a polyimide film according to any one of claims 6 to 9 as a transparent substrate.

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