KR102560059B1 - Polyimide-based varnish, method for producing a polyimide-based film using the same, and polyimide-based film - Google Patents

Abstract

An object of the present invention is to provide a polyimide-based varnish capable of forming a film having good appearance and good flexibility, and comprising a polyimide-based polymer, a solvent, and water. The polyimide-based polymer is a transparent polyimide-based polymer that, when a film with a thickness of 50 μm is formed with the polyimide-based polymer, has a total light transmittance of 85% or more and a yellowness of 5 or less, and a solvent, A polyimide-based varnish capable of dissolving the polyimide-based polymer and having a water content of 0.60 to 4.5% by mass based on the total mass of the polyimide-based varnish is provided.

Description

Polyimide-based varnish, method for producing a polyimide-based film using the same, and polyimide-based film

The present invention relates to a polyimide-based varnish, a method for producing a polyimide-based film using the same, and a polyimide-based film.

BACKGROUND ART Conventionally, glass has been used as a material for substrates of various display members such as solar cells and displays and transparent members such as front plates. However, glass has the drawback of being fragile and heavy. In addition, with respect to thinning and weight reduction of recent displays, and flexibility, they did not have sufficient materials. For this reason, a polyimide-type film is being studied as a transparent member of a flexible device instead of glass (see Patent Document 1, for example).

Specification of US Patent No. 8207256

However, conventional polyimide-based films are not necessarily flexible enough to be used as display members or front plates of flexible devices. In addition, it is also required that the film used for the display member or the front plate is free from defects such as fish eyes, agglomerates, and streaks and has a good appearance.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide a polyimide-based varnish capable of forming a film having good appearance and good flexibility, a method for producing a polyimide-based film using the same, and a polyimide-based film.

In order to achieve the above object, the present invention is a polyimide-based varnish comprising a polyimide-based polymer, a solvent, and water, wherein the polyimide-based polymer is a transparent polyimide-based polymer that, when a film having a thickness of 50 μm is formed from the polyimide-based polymer, has a total light transmittance of 85% or more and a yellowness of 5 or less, the solvent is capable of dissolving the polyimide-based polymer, and the water content is polyimide. A polyimide-based varnish of 0.60 to 4.5% by mass based on the total mass of the de-based varnish is provided.

According to the polyimide varnish, by containing water in the above specific ratio, the appearance and flexibility of the polyimide film formed using the polyimide varnish can be improved. The reason why the appearance and flexibility of the polyimide-based film can be improved by the presence of water is not clear, but it is thought that the presence of water appropriately during drying when forming the film suppresses aggregation of the polyimide-based polymer and forms a dense film with a uniform composition. This is considered to be able to suppress problems such as poor appearance due to defects such as fish eyes, clumps, and streaks, and tearing during bending.

Further, in the polyimide-based varnish, the polyimide-based polymer is a transparent polyimide-based polymer that, when a film having a thickness of 50 μm is formed from the polyimide-based polymer, has a total light transmittance of 85% or more and a yellowness of 5 or less. As for the total light transmittance of the said film|membrane, it is more preferable that it is 90 % or more. By using such a transparent polyimide-based polymer, a highly transparent polyimide-based film can be obtained.

In the polyimide-based varnish, the polyimide-based polymer preferably contains a halogen atom in its molecule, and the halogen atom is more preferably a fluorine atom. Introduction of a halogen atom, particularly a fluorine atom, into the polyimide polymer contributes to reducing the yellowness of the resulting polyimide film. Here, reduction of yellowness contributes to improvement of transparency and external appearance. In addition, since the polyimide-based polymer containing fluorine atoms has low hygroscopicity, it becomes possible to sufficiently remove water after forming a film, and it becomes possible to obtain a film with good appearance more easily.

The polyimide varnish may further contain silica particles. In this case, while being able to improve the intensity|strength of the polyimide-type film obtained, favorable transparency of a film can be obtained.

The polyimide-based varnish containing the silica particles may further contain an alkoxysilane having an amino group. In this case, the effect of improving the strength of the polyimide-based film by the silica particles and the effect of obtaining good transparency of the film tend to be higher.

The present invention also provides a polyimide-based film formed from the polyimide-based varnish of the present invention. Such a polyimide-based film is excellent in appearance and flexibility.

It is preferable that the said polyimide-type film has yellowness of 5 or less. Further, the polyimide-based film preferably has a total light transmittance of 85% or more. As for the total light transmittance of a polyimide-type film, it is more preferable that it is 90 % or more.

The present invention also provides a method for producing a polyimide-based film comprising a step of applying the polyimide-based varnish of the present invention on a substrate to form a coating film, and a step of drying the coating film. As a manufacturing method, you may further include the process of peeling the dried coating film from a base material. According to this manufacturing method, since a polyimide-based film is formed by using a polyimide-based varnish containing a specific amount of water through each step of application, drying, and peeling as necessary, as described above, especially during drying of the coating film, moisture has a favorable effect on the structure formation behavior of the polyimide-based polymer, and a polyimide-based film having excellent appearance and flexibility can be obtained.

The present invention also provides a polyimide-based film manufactured by the method for producing the polyimide-based film. Such a polyimide-based film is excellent in appearance and flexibility.

According to the present invention, a polyimide-based varnish capable of forming a film having good appearance and good flexibility, a method for producing a polyimide-based film using the same, and a polyimide-based film having good appearance and good flexibility can be provided.

Hereinafter, the present invention will be described in detail based on the preferred embodiments.

[Polyimide varnish]

The polyimide-based varnish according to the present embodiment includes a polyimide-based polymer, a solvent capable of dissolving the polyimide-based polymer, and water, and the water content is 0.60% by mass to 4.5% by mass based on the total mass of the polyimide-based varnish.

In the polyimide-based varnish, the content of the polyimide-based polymer can be 20% by mass or more, preferably 40% by mass or more, based on the total solid content in the varnish.

In this specification, a polyimide-type polymer means a polymer containing at least 1 sort(s) of repeating structural unit represented by Formula (PI), Formula (a), Formula (a'), or Formula (b). Especially, if the repeating structural unit represented by formula (PI) is the main structural unit of a polyimide-type polymer, it is preferable from a viewpoint of the intensity|strength and transparency of a film. The repeating structural unit represented by the formula (PI) is preferably 40 mol% or more, more preferably 50 mol% or more, still more preferably 70 mol% or more, particularly preferably 90 mol% or more, and particularly preferably 98 mol% or more, based on all repeating structural units of the polyimide polymer.

G in Formula (PI) represents a tetravalent organic group, and A represents a divalent organic group. G ² in formula (a) represents a trivalent organic group, and A ² represents a divalent organic group. G ³ in formula (a') represents a tetravalent organic group, and A ³ represents a divalent organic group. G ⁴ and A ⁴ in formula (b) each represent a divalent organic group.

In formula (PI), the organic group of the tetravalent organic group represented by G (hereinafter sometimes referred to as the organic group of G) is a group selected from the group consisting of an acyclic aliphatic group, a cyclic aliphatic group, and an aromatic group. From the viewpoint of transparency and flexibility of the polyimide film, G is preferably a tetravalent cyclic aliphatic group and a tetravalent aromatic group. Examples of the aromatic group include a monocyclic aromatic group, a condensed polycyclic aromatic group, and a non-condensed polycyclic aromatic group in which the aromatic groups are linked to each other directly or by a bonding group. From the viewpoint of transparency and coloration of the polyimide film, the organic group of G may be a cyclic aliphatic group, a cyclic aliphatic group having a fluorine-based substituent, a monocyclic aromatic group having a fluorine-based substituent, a condensed polycyclic aromatic group having a fluorine-based substituent, or a non-condensed polycyclic aromatic group having a fluorine-based substituent. In this specification, a fluorine-based substituent means a group containing a fluorine atom. The fluorine-based substituent is preferably a fluoro group (fluorine atom, -F) and a perfluoroalkyl group, and more preferably a fluoro group and a trifluoromethyl group.

More specifically, the organic group of G is selected from, for example, a saturated or unsaturated cycloalkyl group, a saturated or unsaturated heterocycloalkyl group, an aryl group, a heteroaryl group, an arylalkyl group, an alkylaryl group, a heteroalkylaryl group, and groups having any two of these groups (which may be the same) and which are linked to each other directly or by a bonding group. Examples of the bonding group include -O-, an alkylene group having 1 to 10 carbon atoms, -SO ₂ -, -CO-, or -CO-NR- (R represents an alkyl group having 1 to 3 carbon atoms, such as a methyl group, an ethyl group, or a propyl group, or a hydrogen atom).

The number of carbon atoms of the tetravalent organic group represented by G is usually 2 to 32, preferably 4 to 15, more preferably 5 to 10, still more preferably 6 to 8. When the organic group of G is a cyclic aliphatic group or an aromatic group, at least one of the carbon atoms constituting these groups may be substituted with a hetero atom. As a hetero atom, O, N or S is mentioned.

As a specific example of G, the group represented by the following formula (20), formula (21), formula (22), formula (23), formula (24), formula (25), or formula (26) is mentioned. * in the formula represents a bonding hand. Z in Formula (26) represents a single bond, -O-, -CH ₂ -, -C(CH ₃ ) ₂ -, -Ar-O-Ar-, -Ar-CH ₂ -Ar-, -Ar-C(CH ₃ ) ₂ -Ar- or -Ar-SO ₂ -Ar-. Ar represents an aryl group having 6 to 20 carbon atoms, and examples thereof include a phenylene group. At least one of the hydrogen atoms of these groups may be substituted with a fluorine-based substituent.

In formula (PI), the organic group of the divalent organic group represented by A (hereinafter sometimes referred to as the organic group of A) is a divalent organic group selected from the group consisting of an acyclic aliphatic group, a cyclic aliphatic group, and an aromatic group. The divalent organic group represented by A is preferably a divalent cyclic aliphatic group and a divalent aromatic group. Examples of the aromatic group include a monocyclic aromatic group, a condensed polycyclic aromatic group, and a non-condensed polycyclic aromatic group having two or more aromatic rings linked to each other directly or by a bonding group. It is preferable that a fluorine-based substituent is introduced into the organic group of A from the viewpoint of transparency of the polyimide film and suppression of coloring.

More specifically, the organic group of A is, for example, a saturated or unsaturated cycloalkyl group, a saturated or unsaturated heterocycloalkyl group, an aryl group, a heteroaryl group, an arylalkyl group, an alkylaryl group, a heteroalkylaryl group, and any of these It is selected from groups having two groups (which may be the same) of which are linked to each other directly or by a linking group. Examples of the heteroatom include O, N, and S. Examples of the bonding group include -O-, an alkylene group having 1 to 10 carbon atoms, -SO ₂ -, -CO-, or -CO-NR- (R is a methyl group, an ethyl group, representing an alkyl group having 1 to 3 carbon atoms such as a propyl group or a hydrogen atom).

The number of carbon atoms of the divalent organic group represented by A is usually 2 to 40, preferably 5 to 32, more preferably 12 to 28, still more preferably 24 to 27.

As a specific example of A, group represented by the following formula (30), formula (31), formula (32), formula (33) or formula (34) is mentioned. * in the formula represents a bonding hand. Each of Z ¹ to Z ³ independently represents a single bond, -O-, -CH ₂ -, -C(CH ₃ ) ₂ -, -SO ₂ -, -CO-, or -CO-NR- (R represents an alkyl group having 1 to 3 carbon atoms, such as a methyl group, an ethyl group, or a propyl group, or a hydrogen atom). In the group described below, it is preferable that Z ¹ and Z ² , and Z ² and Z ³ are each positioned at a meta position or a para position with respect to each ring. In addition, it is preferable that Z ¹ and terminal single bond, Z ² and terminal single bond, and Z ³ and terminal single bond are respectively in meta position or para position. An example of A is that Z ¹ and Z ³ are -O-, and Z ² is -CH ₂ -, -C(CH ₃ ) ₂ - or -SO ₂ -. One or two or more hydrogen atoms of these groups may be substituted with fluorine-based substituents.

In at least one of A and G, at least one hydrogen atom among the hydrogen atoms constituting them may be substituted with at least one functional group selected from the group consisting of a fluorine-based substituent, a hydroxyl group, a sulfone group, an alkyl group having 1 to 10 carbon atoms, and the like. Further, when the organic group of A and the organic group of G are each a cyclic aliphatic group or an aromatic group, it is preferable that at least one of A and G have a fluorine-based substituent, and both A and G have a fluorine-based substituent. It is more preferable.

G ² in formula (a) is a trivalent organic group. This organic group can be selected from the same group as the organic group of G in formula (PI) except that it is trivalent. Examples of G ² include groups in which any one of the four bonds of groups represented by formulas (20) to (26) given as specific examples of G is substituted with a hydrogen atom. A ² in formula (a) can be selected from the same group as A in formula (PI).

G ³ in formula (a') can be selected from the same group as G in formula (PI). A ³ in formula (a') can be selected from the same group as A in formula (PI).

G ⁴ in formula (b) is a divalent organic group. This organic group can be selected from the same group as the organic group of G in formula (PI) except that it is a divalent group. Examples of G ⁴ include groups in which any two of the four bonds of groups represented by formulas (20) to (26) given as specific examples of G are substituted with hydrogen atoms. A ⁴ in formula (b) can be selected from the same group as A in formula (PI).

The polyimide-based polymer contained in the polyimide-based film may be a condensation type polymer obtained by polycondensation of at least one of diamines and tetracarboxylic acid compounds (including analogs of tetracarboxylic acid compounds such as acid chloride compounds and tetracarboxylic dianhydride) or tricarboxylic acid compounds (including derivatives of tricarboxylic acid compounds such as acid chloride compounds and tricarboxylic acid anhydrides). Furthermore, you may polycondense a dicarboxylic acid compound (including analogs, such as an acid chloride compound). The repeating structural unit represented by formula (PI) or formula (a') is usually derived from diamines and tetracarboxylic acid compounds. The repeating structural unit represented by formula (a) is usually derived from diamines and tricarboxylic acid compounds. The repeating structural unit represented by formula (b) is usually derived from diamines and dicarboxylic acid compounds.

As a tetracarboxylic acid compound, an aromatic tetracarboxylic acid compound, an alicyclic tetracarboxylic acid compound, and an acyclic aliphatic tetracarboxylic acid compound are mentioned. A tetracarboxylic acid compound may use 2 or more types together. The tetracarboxylic acid compound is preferably tetracarboxylic dianhydride. As tetracarboxylic dianhydride, aromatic tetracarboxylic dianhydride, alicyclic tetracarboxylic dianhydride, and acyclic aliphatic tetracarboxylic dianhydride are mentioned.

From the viewpoints of the solubility of the polyimide-based polymer in a solvent and the transparency and flexibility when a polyimide-based film is formed, the tetracarboxylic acid compound is preferably an alicyclic tetracarboxylic acid compound or an aromatic tetracarboxylic acid compound. From the viewpoint of transparency of the polyimide film and suppression of coloring, the tetracarboxylic acid compound is preferably an alicyclic tetracarboxylic acid compound having a fluorine-based substituent and an aromatic tetracarboxylic acid compound having a fluorine-based substituent, and more preferably an alicyclic tetracarboxylic acid compound.

Examples of the tricarboxylic acid compound include aromatic tricarboxylic acid, alicyclic tricarboxylic acid, acyclic aliphatic tricarboxylic acid, and analogous acid chloride compounds and acid anhydrides thereof. The tricarboxylic acid compound is preferably an aromatic tricarboxylic acid, an alicyclic tricarboxylic acid, an acyclic aliphatic tricarboxylic acid, or a derivative acid chloride compound thereof. You may use together 2 or more types of tricarboxylic acid compounds.

From the viewpoints of the solubility of the polyimide-based polymer in a solvent and the transparency and flexibility when a polyimide-based film is formed, the tricarboxylic acid compound is preferably an alicyclic tricarboxylic acid compound or an aromatic tricarboxylic acid compound. From the viewpoint of transparency of the polyimide film and suppression of coloring, the tricarboxylic acid compound is preferably an alicyclic tricarboxylic acid compound having a fluorine-based substituent and an aromatic tricarboxylic acid compound having a fluorine-based substituent.

Examples of the dicarboxylic acid compound include aromatic dicarboxylic acids, alicyclic dicarboxylic acids, acyclic aliphatic dicarboxylic acids, and analogous acid chloride compounds and acid anhydrides thereof. The dicarboxylic acid compounds are preferably aromatic dicarboxylic acids, alicyclic dicarboxylic acids, acyclic aliphatic dicarboxylic acids and their derivative acid chloride compounds. A dicarboxylic acid compound may use 2 or more types together.

From the viewpoint of the solubility of the polyimide-based polymer in a solvent and the transparency and flexibility when a polyimide-based film is formed, the dicarboxylic acid compound is preferably an alicyclic dicarboxylic acid compound or an aromatic dicarboxylic acid compound. From the viewpoint of transparency of the polyimide film and suppression of coloring, the dicarboxylic acid compound is preferably an alicyclic dicarboxylic acid compound having a fluorine-based substituent and an aromatic dicarboxylic acid compound having a fluorine-based substituent.

As diamines, aromatic diamine, alicyclic diamine, and aliphatic diamine are mentioned. Diamines may use together 2 or more types. From the viewpoint of the solubility of the polyimide-based polymer in a solvent and the transparency and flexibility when a polyimide-based film is formed, the diamines are preferably an alicyclic diamine and an aromatic diamine having a fluorine-based substituent.

When such a polyimide-based polymer is used, it is easy to obtain a polyimide-based film having particularly excellent flexibility, high light transmittance (for example, 85% or more, preferably 88% or more with respect to light of 550 nm), and low yellowness (YI value, for example, 5 or less, preferably 3 or less) and low haze (for example, 1.5% or less, preferably 1.0% or less).

The polyimide-based polymer may be a copolymer containing a plurality of the above repeating units of different types. The weight average molecular weight of the polyimide-based polymer is usually 10,000 to 500,000. The weight average molecular weight of the polyimide polymer is preferably 50,000 to 500,000, more preferably 70,000 to 400,000. The weight average molecular weight is a molecular weight in terms of standard polystyrene measured by GPC. High flexibility tends to be obtained when the weight average molecular weight of the polyimide polymer is large, but when the weight average molecular weight of the polyimide polymer is too large, the viscosity of the varnish increases and processability tends to decrease.

The polyimide-based polymer may contain a halogen atom such as a fluorine atom that can be introduced by the above-described fluorine-based substituent or the like. When the polyimide-based polymer contains a halogen atom, the elastic modulus of the polyimide-based film can be improved and yellowness can be reduced. As a result, it is possible to suppress the occurrence of scratches and wrinkles on the polyimide-based film and to improve the transparency of the polyimide-based film. The halogen atom is preferably a fluorine atom. It is preferable that it is 1-40 mass %, and, as for content of the halogen atom in a polyimide-type polymer, based on the total mass of a polyimide-type polymer, it is more preferable that it is 1-30 mass %.

The polyimide-based polymer is preferably a transparent polyimide-based polymer that, when a film (layer) made of the polyimide-based polymer and having a thickness of 50 μm is formed, the total light transmittance of the polyimide-based polymer film is 85% or more and the yellowness (YI value) of the polyimide-type polymer film is 10 or less. It is preferable that the said total light transmittance is 90 % or more. It is preferable that the said yellowness degree is 5 or less. By using such a transparent polyimide-based polymer, a highly transparent polyimide-based film can be obtained. Further, the total light transmittance of the polyimide-based polymer film is more preferably 91% or higher, and still more preferably 92% or higher. As for yellowness, it is more preferable to be 3 or less, and it is particularly preferable that it is 2.5 or less. Here, the polyimide-based polymer film can be formed by applying and drying a polyimide-based polymer dissolved in a solvent. The total light transmittance of the polyimide-based polymer film can be determined in accordance with JIS K7105:1981. The yellowness YI of the polyimide-based polymer film can be obtained in accordance with JIS K7373:2006.

In the polyimide varnish, the solvent is not particularly limited as long as it dissolves the polyimide polymer, but examples include N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), dimethylsulfoxide (DMSO), γ-butyrolactone (GBL), N-methylpyrrolidone (NMP), ethyl acetate, methylethylketone (MEK), tetrahydrofuran, 1, 4-dioxane, acetone, cyclopentanone, dimethylsulfoxide, xylene, and combinations thereof.

A polyimide-type varnish contains water in addition to the said solvent. In the polyimide varnish, the water content (moisture content) is 0.60 to 4.5% by mass, preferably 0.7 to 4.0% by mass, and more preferably 1.0 to 4.0% by mass based on the total mass of the polyimide varnish. The lower limit of the water content is more preferably 1.5% by mass, particularly preferably 2.0% by mass, and still more preferably 2.5% by mass. The upper limit of the water content is more preferably 3.5% by mass.

When the polyimide varnish contains silica particles, the water content in the polyimide varnish is preferably 1.5 to 3.5% by mass, more preferably 2.5 to 3.5% by mass, and particularly preferably 2.5 to 3.0% by mass based on the total mass of the polyimide varnish. When a film is produced using such a polyimide-based varnish, a polyimide-based film having excellent flexibility tends to be easily obtained.

When the water content in the polyimide varnish is within the above range, the presence of water has a favorable effect on the structure formation behavior of the polyimide polymer, and the formed film can have good appearance and flexibility. The moisture content in the polyimide-based varnish can be measured by the Karl Fischer method. Measurement by the Karl Fischer method is performed based on JIS K0068:2001. As the titration reagent, one that does not cause side reactions with the solvent is used. Examples of combinations of anolyte and catholyte suitable for ketone solvents such as N,N-dimethylacetamide include a combination of Culomat AK manufactured by Sigma-Aldrich and Culomat CG-K manufactured by Sigma-Aldrich. As a measuring device, an 831KF coulometer manufactured by Metrom Co., Ltd., or the like can be used.

The polyimide-based varnish can further contain inorganic particles from the viewpoint of increasing the strength of the resulting polyimide-based film. Particles containing silicon atoms are exemplified as inorganic particles, and silica particles are exemplified as particles containing silicon atoms. Other examples of inorganic particles include titania particles, alumina particles, and zirconia particles.

The average primary particle size of the inorganic particles is usually 100 nm or less. When the average primary particle diameter of the inorganic particles is 100 nm or less, the transparency of the film tends to improve. The primary particle diameter of the inorganic particles can be set to a positive direction diameter by a transmission electron microscope (TEM). The average primary particle diameter can be obtained as an average value of 10 primary particle diameters measured by TEM observation.

In the polyimide-based varnish, the blending ratio of the polyimide-based polymer and the inorganic particles, in terms of mass ratio, is usually 1:9 to 10:0, preferably 3:7 to 10:0, more preferably 3:7 to 8:2, and still more preferably 3:7 to 7:3. When the blending ratio of the polyimide-based polymer and the inorganic particles is within the above range, the transparency and mechanical strength of the polyimide-based film tend to be improved.

When the polyimide-based varnish contains inorganic particles, the obtained polyimide-based film WHEREIN: The inorganic particles may be bonded to each other by molecules having siloxane bonds (-SiOSi-).

When the polyimide-based varnish contains inorganic particles, the varnish may contain a metal alkoxide such as alkoxysilane in order to improve solution stability. It is preferably an alkoxysilane having an amino group. In particular, when the polyimide-based varnish contains silica particles as inorganic particles, by further containing an alkoxysilane having an amino group, the dispersibility of the silica particles is improved, and the effect of improving the strength of the polyimide-based film and the effect of obtaining good transparency of the film tend to be higher.

The addition amount of the metal alkoxide can be 0.1 to 10 parts by mass, preferably 0.5 to 5 parts by mass, based on 100 parts by mass of the inorganic particles.

The polyimide-based varnish may contain other components as long as the transparency and flexibility of the obtained polyimide-based film are not impaired. Examples of other components include antioxidants, release agents, stabilizers, coloring agents such as bluing agents, flame retardants, lubricants, thickeners, and leveling agents. The content of the other components in the resulting polyimide-based film is preferably greater than 0% by mass and 20% by mass or less, and more preferably greater than 0% by mass and 10% by mass or less, based on the total mass of the polyimide-based film.

The polyimide-based varnish may contain organosilicon compounds such as silsesquioxane derivatives and quaternary alkoxysilanes such as tetraethyl orthosilicate (TEOS (Tetra Ethyl Ortho Silicate)).

It is preferable that it is 5-30 mass %, and, as for the solid content concentration of a polyimide-type varnish, it is more preferable that it is 10-25 mass % from a viewpoint of storage stability and coatability.

(Polyimide film)

The polyimide-based film of this embodiment is a film formed using the polyimide-based varnish described above.

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

The total light transmittance of this polyimide film based on JIS K7105:1981 is preferably 85% or more, and more preferably 90% or more. Moreover, it is preferable that the haze based on JISK7105:1981 is 1 or less, and, as for this polyimide-type film, it is more preferable that it is 0.9 or less. Moreover, as for this polyimide-type film, it is preferable that yellowness YI based on JISK7373:2006 is 5 or less, and it is more preferable that it is 3 or less. A polyimide-based film having such optical properties can be suitably used as an optical film for smartphones and tablet PCs requiring high visibility.

(Manufacturing method)

Next, an example of the manufacturing method of the polyimide-based varnish of this embodiment and the manufacturing method of the polyimide-type film of this embodiment is demonstrated.

Polyimide-based varnish is prepared by dissolving a solvent-soluble polyimide-based polymer polymerized using a known method for synthesizing polyimide-based polymers in a solvent, and adding water and, if necessary, the above-mentioned inorganic particles, metal alkoxides, and other components, and mixing them. In the case of adding inorganic particles to the polyimide varnish, the inorganic particles can be uniformly dispersed in the polyimide varnish by stirring and mixing by a known stirring method. Examples of solvents are as described above. The polyimide-based polymer may be a solvent-soluble polyimide-based polymer, and as described above, one or more of tetracarboxylic dianhydrides such as aromatic tetracarboxylic dianhydride, alicyclic tetracarboxylic dianhydride, and acyclic aliphatic tetracarboxylic dianhydride, and diamines such as aromatic diamines, alicyclic diamines, and acyclic aliphatic diamines. Or what was obtained by polycondensing 2 or more types can be used. Tetracarboxylic dianhydride and diamines preferably have a fluorine-based substituent introduced therein.

Preparation of the polyimide-based varnish does not necessarily require the addition of water. That is, when the polyimide-based polymer, solvent, inorganic particle, metal alkoxide, or other component to be blended contains moisture due to moisture absorption, the moisture content in the polyimide-based varnish is within the range specified in the present embodiment, so it is not necessary to add water when preparing the polyimide-based varnish. For example, when preparation of a polyimide-based varnish is performed in an environment with humidity to some extent, there are cases where water is appropriately introduced into the polyimide-based varnish even without adding water.

When the polyimide-based varnish contains inorganic particles, there is also an advantage that gelation of the varnish is suppressed because the varnish contains water. For this reason, the effect of properly containing water in the varnish occurs particularly remarkably when the polyimide-based varnish contains inorganic particles, and in the polyimide-based film formed, appearance defects due to gelation of the varnish are less likely to occur, and a film with high flexibility is obtained.

The prepared polyimide-based varnish is then applied onto a PET substrate, a SUS belt, or a glass substrate by a known roll-to-roll or batch method to form a coating film. This coating film is dried and becomes a polyimide-type film.

Drying of the coating film is performed by evaporating the solvent and water at a temperature of 50 to 350° C. under inert atmosphere or reduced pressure conditions as appropriate. Drying of the coating film may be performed in multiple stages by changing the temperature conditions. In that case, you may raise the temperature to some extent in the subsequent stage. By drying the coating film in multiple stages in this way, the speed at which the solvent and water evaporate can be controlled, the structure of the polyimide-based polymer can be made uniform, and aggregation of the polyimide-based polymer can be further suppressed, and the appearance and flexibility of the obtained film can be further improved.

In addition, you may further perform drying of a coating film after peeling from a base material. That is, after the coating film is dried on the substrate as the first drying, it can be separated from the substrate and further dried as the second drying. The second drying can be performed by attaching a metal frame to the coating film peeled off from the base material or by using a known tenter facility. 2nd drying can be performed at a higher temperature than 1st drying, and, for example, 1st drying can be performed at 50-190 degreeC, and 2nd drying can be performed at 190-350 degreeC. In addition, you may perform each of 1st drying and 2nd drying in multiple steps by changing temperature conditions.

(Usage)

Since such a polyimide-based film has excellent appearance and flexibility, it can be used as a component of a flexible display. For example, it can be used as a front plate (window film) for surface protection of a flexible display.

Furthermore, it is also possible to make a laminate in which various functional layers such as a UV absorbing layer, a hard coat layer, an adhesive layer, a hue adjusting layer, and a refractive index adjusting layer are added to this polyimide film.

Example

Hereinafter, the present invention will be described more specifically by way of Examples and Comparative Examples, but the present invention is not limited to the following Examples.

[Example 1]

(Synthesis of polyimide)

A compound represented by formula (1), a compound represented by formula (2), a compound represented by formula (3), a solvent (γ-butyrolactone and dimethylacetamide), and a catalyst were placed in a polymerization tank purged with nitrogen. The charged amount was 75.0 g of the compound represented by formula (1), 36.5 g of compound represented by formula (2), 76.4 g of compound represented by formula (3), 438.4 g of γ-butyrolactone, 313.1 g of dimethylacetamide, and 1.5 g of catalyst. The molar ratio between the compound represented by Formula (2) and the compound represented by Formula (3) was 3:7, the total of the compound represented by Formula (2) and the compound represented by Formula (3), and the molar ratio of the compound represented by Formula (1) was 1.00: 1.02.

After stirring the mixture in the polymerization tank to dissolve the raw materials in the solvent, the temperature of the mixture was raised to 100 ° C. while stirring, then the temperature was raised to 200 ° C. without stirring, and kept at 200 ° C. for 4 hours to polymerize polyimide. Also, during this heating, water in the liquid was removed. Thereafter, polyimide was obtained by purification and drying.

(Preparation of polyimide varnish)

Subsequently, a γ-butyrolactone solution of polyimide adjusted to a concentration of 20% by mass, a dispersion obtained by dispersing silica particles having a solid content concentration of 30% by mass in γ-butyrolactone, and a dimethylacetamide solution of an alkoxysilane having an amino group were mixed and stirred for 30 minutes to prepare a polyimide varnish.

Here, the mass ratio of silica and polyimide was 60:40, and the amount of alkoxysilane having an amino group was 1.67 parts by mass per 100 parts by mass in total of silica and polyimide. All of the amounts of these silica, polyimide, and alkoxysilane having an amino group are amounts of the solid content excluding the solvent (hereinafter, the same). It was 0.80 mass % when the moisture content of the obtained polyimide-type varnish was measured by the Karl Fischer method. The water content of the polyimide-based varnish was measured with an 831KF coulometer manufactured by Metrohm Corporation. The measurement was performed in accordance with JIS K0068:2001, and Culomat AK manufactured by Sigma-Aldrich was used as the positive electrolyte solution and Culomat CG-K manufactured by Sigma-Aldrich was used as the negative electrolyte.

(Manufacture of polyimide film)

The polyimide-based varnish was applied to a polyethylene terephthalate substrate (PET substrate), heated at 50 ° C. for 30 minutes and then at 140 ° C. for 10 minutes, then peeled off from the PET substrate, mounted on a metal frame, and further heated at 210 ° C. for 1 hour to obtain a polyimide-based film having a thickness of 50 μm.

[Example 2]

During preparation of the polyimide varnish, a polyimide varnish was prepared and a polyimide film using the same was prepared in the same manner as in Example 1 except that 10 parts by mass of water was added to a total of 100 parts by mass of silica and polyimide so that the water content of the polyimide varnish was 2.69% by mass.

[Example 3]

In preparing the polyimide varnish, a polyimide varnish was prepared and a polyimide film using the varnish was prepared in the same manner as in Example 2, except that dehydration solvents (dehydrated dimethylacetamide and dehydrated γ-butyrolactone) were used for dimethylacetamide and γ-butyrolactone in the silica particle dispersion. It was 2.45 mass % when the moisture content of the prepared polyimide-type varnish was measured by the Karl Fischer method.

[Example 4]

A solution (polyimide-based varnish) was prepared by dissolving polyimide "KPI-MX300F (100)" manufactured by Kawamura Sangyo Co., Ltd. in dimethylacetamide to have a concentration of 16% by mass. Further, when a water content was evaluated after adding a small amount of water, the water content of the polyimide-based varnish was 1.22% by mass. A polyimide film was produced in the same manner as in Example 1 except that this polyimide varnish was used.

[Example 5]

A polyimide varnish was prepared by mixing a 20% by mass γ-butyrolactone solution of "Neopurim C6A20", a polyimide varnish manufactured by Mitsubishi Gas Chemical Co., Ltd., a dispersion obtained by dispersing silica particles having a solid content concentration of 30% by mass in γ-butyrolactone, a dimethylacetamide solution of an alkoxysilane having an amino group, and water, and stirring for 30 minutes. The amount of water added was 10 parts by mass with respect to 100 parts by mass in total of silica and polyimide.

Here, the mass ratio of silica and polyimide was 55:45, and the amount of alkoxysilane having an amino group was 1.67 parts by mass per 100 parts by mass in total of silica and polyimide. The water content of the polyimide-based varnish was 2.56% by mass. A polyimide film was produced in the same manner as in Example 1 except that this polyimide varnish was used.

[Comparative Example 1]

In preparing the polyimide varnish, a polyimide varnish was prepared and a polyimide film using the varnish was prepared in the same manner as in Example 1 except that dehydration solvents (dehydrated dimethylacetamide and dehydrated γ-butyrolactone) were used for dimethylacetamide and γ-butyrolactone in the silica particle dispersion. When the water content of the prepared polyimide-based varnish was measured by the Karl Fischer method, it was 0.55% by mass. In addition, the obtained polyimide-based film had many agglomerates and was inferior in appearance compared to the film of Example 1.

[Comparative Example 2]

During preparation of the polyimide varnish, a polyimide varnish was prepared and a polyimide film using the same was prepared in the same manner as in Example 2, except that the mixing ratio of water was adjusted so that the water content of the polyimide varnish was 4.59% by mass. The obtained polyimide-based film had low transparency and became a cloudy film. Further, when the polyimide-based varnish left to stand for one day was observed, the liquid was separated into two phases.

[Comparative Example 3]

Preparation of a polyimide varnish and production of a polyimide film using the varnish were tested in the same manner as in Example 1 except that the water content of the polyimide varnish was adjusted to 6.48% by mass by adjusting the mixing ratio of water during preparation of the polyimide varnish.

[Comparative Example 4]

A solution having a concentration of 16% by mass was prepared by dissolving polyimide “KPI-MX300F (100)” manufactured by Kawamura Sangyo Co., Ltd. in dimethylacetamide, and then water was added to prepare a polyimide varnish having a moisture content of 10% by mass. Production of a polyimide film was tested in the same manner as in Example 1 except that this polyimide varnish was used, but solid content was precipitated during preparation of the polyimide varnish, making it difficult to prepare a uniform film.

[Comparative Example 5]

A polyimide varnish was prepared in the same manner as in Example 5, except that water was further added to make the moisture content 10% by mass at the time of preparation of the polyimide varnish. Production of a polyimide film was tested in the same manner as in Example 1 except that this polyimide varnish was used, but solid content was precipitated during preparation of the polyimide varnish, making it difficult to produce a uniform film.

[Evaluation of Film Appearance]

Each of the appearances of the polyimide-based films obtained in Examples and Comparative Examples was observed with the naked eye, and defects such as fish eyes, clumps, and streaks were not found as "A", and defects such as fish eyes, agglomerates and streaks were found. "B" was evaluated, and "C" was evaluated when a uniform film could not be formed or when a film could be formed but was uneven and blurry. The results are shown in Table 1. As for the film whose external appearance was evaluated as "C", yellowness and total light transmittance were not measured.

[Measurement of yellowness (YI value)]

The yellowness (Yellow Index: YI value) of each of the polyimide-based films obtained in Examples and Comparative Examples was measured using an ultraviolet, visible, and near-infrared spectrophotometer V-670 manufactured by Nippon Spectroscopy Co., Ltd. After performing the background measurement in the absence of the sample, the polyimide-based film was set in the sample holder, and the transmittance to light of 300 to 800 nm was measured to obtain tristimulus values (X, Y, Z). YI value was computed based on the following formula. The results are shown in Table 1.

YI=100×(1.2769X-1.0592Z)/Y

[Measurement of total light transmittance]

The total light transmittance of each of the polyimide-based films obtained in Examples and Comparative Examples was measured by a fully automatic direct reading haze computer HGM-2DP manufactured by Suga Test Instruments Co., Ltd. in accordance with JIS K7105:1981. The results are shown in Table 1.

[Evaluation of Flexibility]

The flexibility of the polyimide-based films obtained in Examples and Comparative Examples was evaluated according to the following criteria. When the film was bent by hand and a crease was applied, only a crease remained and no abnormality occurred around the crease, "A", and the periphery of the crease became white. "B" was evaluated for a change in appearance around the crease, and "C" was evaluated for a tear in the crease.

Claims (11)

A polyimide-based varnish containing a polyimide-based polymer, a solvent, and water,
The polyimide-based polymer is a transparent polyimide-based polymer that, when a film having a thickness of 50 μm is formed from the polyimide polymer, the film has a total light transmittance of 85% or more and a yellowness of 5 or less,
The solvent is capable of dissolving the polyimide polymer,
A polyimide varnish for forming a film for a front panel for surface protection of a flexible display, wherein the water content is 0.60 to 4.5% by mass based on the total mass of the polyimide varnish. According to claim 1,
A polyimide-based varnish for forming a film for a front panel for surface protection of a flexible display in which a polyimide-based polymer contains a halogen atom in a molecule. According to claim 2,
A polyimide-based varnish for forming a film for a front plate for surface protection of a flexible display in which a halogen atom is a fluorine atom. According to claim 1,
A polyimide-based varnish for forming a film for a front plate for surface protection of a flexible display further comprising silica particles. According to claim 4,
A polyimide-based varnish for forming a film for a front panel for surface protection of a flexible display, further comprising an alkoxysilane having an amino group. A polyimide-based film for a front plate for surface protection of a flexible display formed from the polyimide-based varnish according to any one of claims 1 to 5. According to claim 6,
A polyimide-based film for a front plate for surface protection of a flexible display having a yellowness of 5 or less. According to claim 6,
A polyimide-based film for a front plate for surface protection of a flexible display having a total light transmittance of 85% or more. A step of applying the polyimide-based varnish for forming a film for a front panel for surface protection of a flexible display according to any one of claims 1 to 5 on a substrate to form a coating film;
A method for producing a polyimide-based film for a front plate for surface protection of a flexible display, comprising a step of drying the coating film. According to claim 9,
A method for producing a polyimide-based film for a front plate for surface protection of a flexible display, further comprising a step of peeling the dried coating film from the substrate. A polyimide-based film for a front panel for protecting the surface of a flexible display manufactured by the method for manufacturing a polyimide-based film for a front panel for protecting the surface of a flexible display according to claim 9.