JP6920568B2 - Polyimide-based varnish, method for manufacturing polyimide-based film using it, and polyimide-based film - Google Patents

Description

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

Conventionally, glass has been used as a material for a base material of various display members such as a solar cell and a display and a transparent member such as a front plate. However, glass has drawbacks such as being fragile and heavy. Further, in recent years, the display has not had a sufficient material in terms of thinning and weight reduction and flexibility. Therefore, a polyimide-based film has been studied as a transparent member of a flexible device instead of glass (see, for example, Patent Document 1).

U.S. Pat. No. 8,207,256

However, the conventional polyimide-based film is not always sufficiently flexible to be used as a display member or a front plate of a flexible device. Further, the film used for the display member or the front plate is also required to have a good appearance without defects such as fish eyes, agglomerates, and streaks.

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

In order to achieve the above object, the present invention is a polyimide-based varnish containing a polyimide-based polymer, a solvent, and water, and the polyimide-based polymer forms a film having a thickness of 50 μm from the polyimide-based polymer. When this is done, the film is a transparent polyimide polymer having a total light transmittance of 85% or more and a yellowness of 5 or less, and the solvent can dissolve the polyimide polymer, and the content of water. However, the polyimide-based varnish having a total mass of the polyimide-based varnish of 0.60 to 4.5% by mass is provided.

According to the polyimide varnish, by containing water in the specific ratio, the appearance and flexibility of the polyimide film formed by 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 the presence of water in an appropriate amount during drying when forming the film suppresses the aggregation of the polyimide-based polymer. It is considered that this is because a film having a dense and uniform composition is formed. It is considered that this makes it possible to suppress problems such as poor appearance due to defects such as fish eyes, agglomerates, and streaks, and cracking at the time of bending.

Further, in the polyimide-based varnish, when a film having a thickness of 50 μm is formed from the polyimide-based polymer, the polyimide-based polymer is transparent so that the total light transmittance of the film is 85% or more and the yellowness is 5 or less. It is a polyimide-based polymer. The total light transmittance of the film is more preferably 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 the molecule, and the halogen atom is more preferably a fluorine atom. The introduction of halogen atoms, especially fluorine atoms, into the polyimide polymer contributes to the reduction of the yellowness of the obtained polyimide film. Here, the reduction of yellowness contributes to the improvement of transparency and appearance. Further, since the polyimide polymer containing a fluorine atom has low hygroscopicity, it is possible to sufficiently remove water after forming the film, and it is possible to more easily obtain a film having a good appearance.

The polyimide-based varnish may further contain silica particles. In this case, the strength of the obtained polyimide-based film can be improved, and good transparency of the 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 further enhanced.

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

The polyimide film preferably has a yellowness of 5 or less. Further, the polyimide-based film preferably has a total light transmittance of 85% or more. The total light transmittance of the polyimide film is more preferably 90% or more.

The present invention also provides a method for producing a polyimide film, which comprises 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. The manufacturing method may further include a step of peeling the dried coating film from the substrate. According to such a production method, a polyimide-based film containing a specific amount of water is used to form a polyimide-based film through each step of coating, drying, and peeling if necessary, so that a coating film is particularly coated as described above. Moisture has a good effect on the structure-forming behavior of the polyimide-based polymer at the time of drying, and a polyimide-based film having excellent appearance and flexibility can be obtained.

The present invention further provides a polyimide-based film produced by the above-mentioned method for producing a polyimide-based film. Such a polyimide-based film has excellent appearance and flexibility.

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

Hereinafter, the present invention will be described in detail according to the preferred embodiment thereof.

[Polyimide varnish]
The polyimide-based varnish according to the present embodiment contains a polyimide-based polymer, a solvent capable of dissolving the polyimide-based polymer, and water, and the water content is 0.60 based on the total mass of the polyimide-based varnish. It is from mass% to 4.5 mass%.

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 amount of solids in the varnish.

In the present specification, the polyimide-based polymer means a polymer containing at least one type of repeating structural unit represented by the formula (PI), the formula (a), the formula (a') or the formula (b). Among them, it is preferable that the repeating structural unit represented by the formula (PI) is the main structural unit of the polyimide-based polymer from the viewpoint of film strength and transparency. 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, based on all the repeating structural units of the polyimide-based polymer. % Or more, particularly preferably 90 mol% or more, and even more preferably 98 mol% or more.

G in the formula (PI) represents a tetravalent organic group, and A represents a divalent organic group. ^{G 2} in the formula (a) represents a trivalent organic group, and A ² represents a divalent organic group. ^{G 3} in the formula (a') represents a tetravalent organic group, and A ³ represents a divalent organic group. ^{G 4} and ^{A 4} in the formula (b) represents each a divalent organic group.

In the formula (PI), the organic group of the tetravalent organic group represented by G (hereinafter, may be referred to as the organic group of G) is an acyclic aliphatic group, a cyclic aliphatic group, and an aromatic group. Examples include groups selected from the group consisting of groups. 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 aromatic groups are directly linked to each other or by a bonding group. From the viewpoint of transparency and suppression of coloring of the polyimide film, the organic group of G is a cyclic aliphatic group, a cyclic aliphatic group having a fluorine-based substituent, or a monocyclic aromatic group having a fluorine-based substituent. , A fused polycyclic aromatic group having a fluorine-based substituent or a non-condensed polycyclic aromatic group having a fluorine-based substituent may be used. In the present specification, the 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, 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. It is selected from groups that have a group and any two of these (which may be the same) and which are linked to each other either directly or by a linking group. 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, a propyl group and the like having 1 to 1 carbon atoms. (Representing an alkyl group of 3 or a hydrogen atom).

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

Specific examples of G include groups represented by the following formulas (20), formulas (21), formulas (22), formulas (23), formulas (24), formulas (25) or formulas (26). Be done. * In the formula indicates a bond. Z in formula (26) is a single bond, -O-, -CH _2- , -C (CH ₃ ) _2- , -Ar-O-Ar-, -Ar _{-CH 2-Ar-, -Ar-} Represents 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 the formula (PI), the organic group of the divalent organic group represented by A (hereinafter, may be referred to as the organic group of A) is derived from an acyclic aliphatic group, a cyclic aliphatic group and an aromatic group. Divalent organic groups selected from the group of The divalent organic group represented by A is preferably a divalent cyclic aliphatic group and a divalent aromatic group. The aromatic group includes a monocyclic aromatic group, a fused polycyclic aromatic group, and a non-condensed polycyclic aromatic group having two or more aromatic rings and those connected to each other directly or by a bonding group. The group is mentioned. From the viewpoint of transparency and suppression of coloring of the polyimide film, it is preferable that a fluorine-based substituent is introduced into the organic group of A.

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. It is selected from groups that have a group and any two of these (which may be the same) and to which they are linked to each other either directly or by a binding group. Examples of the heteroatom include O, N or S, and examples of the bonding group include -O-, an alkylene group having 1 to 10 carbon atoms, -SO _2- , -CO- or -CO-NR- (R is methyl). It represents an alkyl group having 1 to 3 carbon atoms such as a group, an ethyl group, and a propyl group, or a hydrogen atom).

The divalent organic group represented by A has usually 2 to 40 carbon atoms, preferably 5 to 32 carbon atoms, more preferably 12 to 28 carbon atoms, and further preferably 24 to 27 carbon atoms.

Specific examples of A include a group represented by the following formula (30), formula (31), formula (32), formula (33) or formula (34). * In the formula indicates a bond. Z ^{1 to} Z ³ are independently single-bonded, -O-, -CH _2- , -C (CH ₃ ) _2- , -SO _2- , -CO- or -CO-NR- (R is Represents an alkyl group having 1 to 3 carbon atoms such as a methyl group, an ethyl group, and a propyl group, or a hydrogen atom). In the following groups, Z ¹ and Z ² and Z ² and Z ³ are preferably in the meta or para position with respect to each ring, respectively. Further, ^{it is preferable that the single bond between Z 1} and the terminal, the single bond between Z ² and the terminal, and ^{the single bond between Z 3} and the terminal are in the meta position or the para position, respectively. One 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 more of the hydrogen atoms of these groups may be substituted with fluorine-based substituents.

At least one of A and G has at least one hydrogen atom 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. It may be substituted with one kind of functional group. Further, when the organic group of A and the organic group of G are cyclic aliphatic groups or aromatic groups, respectively, it is preferable that at least one of A and G has a fluorine-based substituent, and both A and G have a fluorine-based substituent. It is more preferable to have a fluorine-based substituent.

^{G 2} in the formula (a) is a trivalent organic group. This organic group can be selected from the same groups as the organic group of G in the formula (PI) except that it is trivalent. As ^{an example of G 2} , a group in which any one of the four bonds of the groups represented by the formulas (20) to (26) given as a specific example of G is replaced with a hydrogen atom is mentioned. Can be done. ^{A 2} in formula (a) can be selected from the same groups as A in formula (PI).

^{G 3} in formula (a') can be selected from the same groups as G in formula (PI). ^{A 3} in formula (a') can be selected from the same groups as A in formula (PI).

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

The polyimide-based polymer contained in the polyimide-based film includes diamines and tetracarboxylic acid compounds (including acid chloride compounds and tetracarboxylic acid compound analogs such as tetracarboxylic acid dianhydride) or tricarboxylic acid compounds (acid chloride compounds). It may be a condensed polymer obtained by polycondensing with at least one of (including tricarboxylic acid compound analogs such as tricarboxylic acid anhydride). Further, a dicarboxylic acid compound (including an analog such as an acid chloride compound) may be polycondensed. The repeating structural unit represented by the formula (PI) or the formula (a') is usually derived from diamines and tetracarboxylic acid compounds. The repeating structural unit represented by the formula (a) is usually derived from diamines and tricarboxylic acid compounds. The repeating structural unit represented by the formula (b) is usually derived from diamines and dicarboxylic acid compounds.

Examples of the tetracarboxylic acid compound include an aromatic tetracarboxylic acid compound, an alicyclic tetracarboxylic acid compound, and an acyclic aliphatic tetracarboxylic acid compound. Two or more kinds of tetracarboxylic acid compounds may be used in combination. The tetracarboxylic acid compound is preferably a tetracarboxylic dianhydride. Examples of the tetracarboxylic dianhydride include aromatic tetracarboxylic dianhydride, alicyclic tetracarboxylic dianhydride and acyclic aliphatic tetracarboxylic dianhydride.

From the viewpoint of solubility of the polyimide polymer in a solvent, transparency and flexibility when a polyimide film is formed, the tetracarboxylic acid compound shall be an alicyclic tetracarboxylic acid compound and an aromatic tetracarboxylic acid compound. Is preferable. From the viewpoint of transparency and suppression of coloring of the polyimide-based film, the tetracarboxylic dian compound may be an alicyclic tetracarboxylic dian compound having a fluorine-based substituent and an aromatic tetracarboxylic dian compound having a fluorine-based substituent. It is preferable that it is an alicyclic tetracarboxylic acid compound.

Examples of the tricarboxylic acid compound include aromatic tricarboxylic acids, alicyclic tricarboxylic acids, acyclic aliphatic tricarboxylic acids, acid chloride compounds related thereto, acid anhydrides and the like. The tricarboxylic acid compound is preferably an aromatic tricarboxylic acid, an alicyclic tricarboxylic acid, an acyclic aliphatic tricarboxylic acid and an acid chloride compound related thereto. Two or more kinds of tricarboxylic acid compounds may be used in combination.

The tricarboxylic acid compound is preferably an alicyclic tricarboxylic acid compound or an aromatic tricarboxylic acid compound from the viewpoint of solubility of the polyimide polymer in a solvent, transparency when a polyimide film is formed, and flexibility. From the viewpoint of transparency and suppression of coloring of the polyimide film, 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, acid chloride compounds related thereto, acid anhydrides and the like. The dicarboxylic acid compound is preferably an aromatic dicarboxylic acid, an alicyclic dicarboxylic acid, an acyclic aliphatic dicarboxylic acid and an acid chloride compound related thereto. Two or more kinds of dicarboxylic acid compounds may be used in combination.

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

Examples of diamines include aromatic diamines, alicyclic diamines and aliphatic diamines. Two or more kinds of diamines may be used in combination. From the viewpoint of solubility of the polyimide polymer in a solvent, transparency and flexibility when a polyimide film is formed, the diamines are preferably an alicyclic diamine and an aromatic diamine having a fluorine-based substituent. ..

When such a polyimide polymer is used, it has 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. A polyimide film having a low haze (for example, 1.5% or less, preferably 1.0% or less) having a YI value (for example, 5 or less, preferably 3 or less) can be easily obtained.

The polyimide-based polymer may be a copolymer containing a plurality of the above-mentioned repeating units of different types. The weight average molecular weight of the polyimide 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 standard polystyrene-equivalent molecular weight measured by GPC. The larger the weight average molecular weight of the polyimide polymer, the easier it is to obtain high flexibility. However, if the weight average molecular weight of the polyimide polymer is too large, the viscosity of the varnish tends to increase and the processability tends to decrease. There is.

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

When a film (layer) having a thickness of 50 μm is formed of the polyimide polymer, the total light transmittance of the polyimide polymer film is 85% or more, and the polyimide polymer has a total light transmittance of 85% or more. A transparent polyimide-based polymer having a film yellowness (YI value) of 10 or less is preferable. The total light transmittance is preferably 90% or more. The yellowness is preferably 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 polymer film is more preferably 91% or more, and further preferably 92% or more. The yellowness is more preferably 3 or less, and particularly preferably 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 polymer film can be determined in accordance with JIS K7105: 1981. The yellowness YI of the polyimide polymer film can be determined in accordance with JIS K 7373: 2006.

In the polyimide-based varnish, the solvent is not particularly limited as long as it is a solvent that dissolves the polyimide-based polymer, and for example, N, N-dimethylacetamide (DMAc), N, N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO). ), Γ-Butyrolactone (GBL), N-methylpyrrolidone (NMP), ethyl acetate, methyl ethyl ketone (MEK), tetrahydrofuran, 1,4-dioxane, acetone, cyclopentanone, dimethyl sulfoxide, xylene and combinations thereof. ..

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

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

When the amount of water in the polyimide varnish is within the above range, the presence of water has a good influence on the structure forming behavior of the polyimide polymer, and the appearance and flexibility of the formed film can be improved. can. The amount of water in the polyimide varnish can be measured by the Karl Fischer method. The Karl Fischer method is measured according to JIS K0068: 2001. Use a reagent for titration that does not cause a side reaction with the solvent. Examples of the combination of the anode solution and the cathode solution suitable for a ketone solvent such as N, N-dimethylacetamide include a combination of Sigma-Aldrich Coulomat AK and Sigma-Aldrich Coulomat CG-K. As the measuring device, an 831KF coolometer manufactured by Metronome or the like can be used.

The polyimide-based varnish can further contain inorganic particles from the viewpoint of increasing the strength of the obtained polyimide-based film. Examples of the inorganic particles include particles containing a silicon atom, and examples of the particles containing a silicon atom include silica particles. Other examples of inorganic particles include titania particles, alumina particles, zirconia particles and the like.

The average primary particle size of the inorganic particles is usually 100 nm or less. When the average primary particle size of the inorganic particles is 100 nm or less, the transparency of the film tends to be improved. The primary particle size of the inorganic particles can be a directional diameter measured by a transmission electron microscope (TEM). The average primary particle size can be obtained by measuring 10 points of the primary particle size by TEM observation and as an average value thereof.

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

When the polyimide-based varnish contains inorganic particles, the inorganic particles may be bonded to each other by a molecule having a siloxane bond (−SiOSi−) in the obtained polyimide-based film.

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

The amount of the metal alkoxide added can be 0.1 to 10 parts by mass with respect to 100 parts by mass of the inorganic particles, and is preferably 0.5 to 5 parts by mass.

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

The polyimide-based varnish can also contain an organosilicon compound such as a quaternary alkoxysilane such as tetraethyl orthosilicate (TEOS) and a silsesquioxane derivative.

The solid content concentration of the polyimide varnish is preferably 5 to 30% by mass, more preferably 10 to 25% by mass, from the viewpoint of storage stability and coatability.

(Polyimide film)
The polyimide-based film of the present embodiment is a film formed by using the above-mentioned polyimide-based varnish.

The thickness of the polyimide-based film is appropriately adjusted according to the intended use, but is usually 10 to 500 μm, preferably 15 to 200 μm, and more preferably 20 to 100 μm.

This polyimide-based film preferably has a total light transmittance of 85% or more, more preferably 90% or more, in accordance with JIS K7105: 1981. Further, this polyimide-based film preferably has a Haze according to JIS K7105: 1981 of 1 or less, and more preferably 0.9 or less. Further, this polyimide-based film preferably has a yellowness YI of 5 or less, and more preferably 3 or less, in accordance with JIS K 7373: 2006. A polyimide-based film having such optical characteristics can be suitably used as an optical film for smartphones and tablet PCs that require high visibility.

(Production method)
Next, an example of the method for producing the polyimide-based varnish of the present embodiment and the method for producing the polyimide-based film of the present embodiment will be described.

The polyimide-based varnish is prepared by dissolving a solvent-soluble polyimide-based polymer polymerized using a known method for synthesizing a polyimide-based polymer in a solvent, and further adding water and, if necessary, the above-mentioned inorganic particles, metal alkoxides, and the above-mentioned inorganic particles and metal alkoxides. And other ingredients are added and mixed to prepare. When the inorganic particles are added 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 any solvent-soluble polyimide-based polymer, and as described above, aromatic tetracarboxylic dianhydride, alicyclic tetracarboxylic dianhydride, and acyclic aliphatic tetra. One or more tetracarboxylic dianhydrides such as carboxylic dianhydride and one or two diamines such as aromatic diamines, alicyclic diamines, and acyclic aliphatic diamines. Those obtained by polycondensing the above can be used. The tetracarboxylic dianhydride and diamines are preferably those having a fluorine-based substituent introduced therein.

The preparation of the polyimide varnish does not necessarily require the addition of water. That is, when the polyimide polymer, the solvent, the inorganic particles, the metal alkoxide, or other components to be blended contain water due to moisture absorption or the like, the amount of water in the polyimide varnish is defined by the water content in the present embodiment. If it is within the range, it is not necessary to add water when preparing the polyimide varnish. For example, when the polyimide varnish is prepared in an environment with a certain degree of humidity, water may be appropriately taken into the polyimide varnish without adding water.

When the polyimide-based varnish contains inorganic particles, the varnish contains water, which has an advantage that gelation of the varnish is suppressed. Therefore, the effect of the varnish containing water appropriately is particularly remarkable when the polyimide-based varnish contains inorganic particles, and the polyimide-based film formed is less likely to have an appearance defect due to gelation of the varnish. Moreover, a film having high flexibility can be 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 to become a polyimide-based film.

The coating film is dried at a temperature of 50 to 350 ° C. by evaporating the solvent and water under conditions of an inert atmosphere or reduced pressure. The coating film may be dried in multiple steps by changing the temperature conditions. In that case, the temperature may be raised toward the latter stage. By drying the coating film in multiple steps in this way, the rate at which the solvent and water evaporate can be controlled, the structure of the polyimide polymer can be made uniform, and the aggregation of the polyimide polymer can be further suppressed. The appearance and flexibility of the obtained film can be further improved.

Further, the coating film may be further dried after being peeled from the substrate. That is, the coating film can be dried on the base material as the first drying, then peeled off from the base material, and further dried as the second drying. The second drying can be performed by attaching a metal frame to the coating film peeled from the substrate, or using a known tenter facility. The second drying can be performed at a higher temperature than the first drying, for example, the first drying can be performed at 50 to 190 ° C., and the second drying can be performed at 190 to 350 ° C. Further, each of the first drying and the second drying may be performed in multiple stages by changing the temperature conditions.

(Use)
Such a polyimide-based film is excellent in appearance and flexibility, and can be used as a component of a flexible display. For example, it can be used as a front plate (window film) for protecting the surface of a flexible display.

Further, it is also possible to obtain a laminate in which various functional layers such as an ultraviolet 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.

Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to the following Examples.

[Example 1]
(Synthesis of polyimide)
The compound represented by the formula (1), the compound represented by the formula (2), the compound represented by the formula (3), the solvent (γ-butyrolactone and dimethylacetamide), and the catalyst were charged into the nitrogen-substituted polymerization tank. .. The amount charged was 75.0 g of the compound represented by the formula (1), 36.5 g of the compound represented by the formula (2), 76.4 g of the compound represented by the formula (3), 438.4 g of γ-butyrolactone, and dimethyl. The amount of acetamide was 313.1 g and the amount of catalyst was 1.5 g. The molar ratio of the compound represented by the formula (2) to the compound represented by the formula (3) is 3: 7, and the sum of the compound represented by the formula (2) and the compound represented by the formula (3). The molar ratio of the compound to the compound represented by the formula (1) was 1.00: 1.02.

After stirring the mixture in the polymerization tank to dissolve the raw material in the solvent, the mixture was heated to 100 ° C. with stirring, then heated to 200 ° C. without stirring, and kept warm at 200 ° C. for 4 hours. The polyimide was polymerized. During this heating, water in the liquid was removed. Then, it was purified and dried to obtain a polyimide.

(Preparation of polyimide varnish)
Next, a γ-butyrolactone solution of polyimide adjusted to a concentration of 20% by mass, a dispersion in which silica particles having a solid content concentration of 30% by mass are dispersed in γ-butyrolactone, and a dimethylacetamide solution of alkoxysilane having an amino group are mixed. A polyimide varnish was prepared by stirring for 30 minutes.

Here, the mass ratio of silica to polyimide was 60:40, and the amount of alkoxysilane having an amino group was 1.67 parts by mass with respect to a total of 100 parts by mass of silica and polyimide. The amounts of silica, polyimide, and alkoxysilane having an amino group are all the amounts of solids excluding the solvent (hereinafter, the same applies). The water content of the obtained polyimide varnish was measured by the Karl Fischer method and found to be 0.80% by mass. The water content of the polyimide varnish was measured with an 831KF coolometer manufactured by Metronome. The measurement was performed in accordance with JIS K0068: 2001, and Sigma-Aldrich Coulomat AK was used as the anode solution and Sigma-Aldrich Coulomat CG-K was used as the cathode solution.

(Preparation of polyimide film)
A polyimide terephthalate substrate (PET substrate) is coated with a polyimide varnish, heated at 50 ° C. for 30 minutes and then at 140 ° C. for 10 minutes, then peeled off from the PET substrate and attached to a metal frame, and further at 210 ° C. for 1 hour. The mixture was heated to obtain a polyimide film having a thickness of 50 μm.

[Example 2]
Example 1 and Example 1 except that when preparing the polyimide varnish, 10 parts by mass of water was added to 100 parts by mass of the total of silica and polyimide so that the water content of the polyimide varnish was 2.69% by mass. In the same manner, a polyimide-based varnish was prepared and a polyimide-based film was produced using the same.

[Example 3]
Preparation of polyimide varnish and its preparation in the same manner as in Example 2 except that a dehydration solvent (dehydrated dimethylacetamide and dehydrated γ-butyrolactone) was used for γ-butyrolactone of the dimethylacetamide and silica particle dispersion liquid when preparing the polyimide-based varnish. A polyimide-based film was produced using the above. The water content of the prepared polyimide varnish was measured by the Karl Fischer method and found to be 2.45% by mass.

[Example 4]
A solution (polyimide-based varnish) in which a polyimide "KPI-MX300F (100)" manufactured by Kawamura Sangyo Co., Ltd. was dissolved in dimethylacetamide to a concentration of 16% by mass was prepared. When the water content was evaluated after adding a small amount of water, the water content of the polyimide 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]
Dimethylacetamide of alkoxysilane having an amino group, a dispersion liquid in which silica particles having a solid content concentration of 30% by mass are dispersed in γ-butyrolactone in a 20% by mass γ-butyrolactone solution of “Neoprim C6A20”, a polyimide varnish manufactured by Mitsubishi Gas Chemicals. A polyimide varnish was prepared by mixing the solution 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 to polyimide was 55:45, and the amount of alkoxysilane having an amino group was 1.67 parts by mass with respect to a total of 100 parts by mass of silica and polyimide. The water content of the polyimide 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]
Preparation of polyimide varnish and its preparation in the same manner as in Example 1 except that a dehydration solvent (dehydrated dimethylacetamide and dehydrated γ-butyrolactone) was used for γ-butyrolactone of the dimethylacetamide and silica particle dispersion liquid when preparing the polyimide-based varnish. A polyimide-based film was produced using the above. The water content of the prepared polyimide varnish was measured by the Karl Fischer method and found to be 0.55% by mass. In addition, the obtained polyimide-based film had many agglomerates and was inferior in appearance to the film of Example 1.

[Comparative Example 2]
Preparation of polyimide varnish and its preparation in the same manner as in Example 2 except that the water content of the polyimide varnish was adjusted to 4.59% by mass by adjusting the mixing ratio of water when preparing the polyimide varnish. A polyimide-based film was produced using the above. The obtained polyimide-based film became a cloudy film with low transparency. Moreover, when observing the polyimide-based varnish left for one day, the liquid was separated into two phases.

[Comparative Example 3]
When preparing the polyimide varnish, the polyimide varnish was prepared and the same 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. However, it was difficult to prepare a uniform film because a large amount of solid content was precipitated during the preparation of the varnish.

[Comparative Example 4]
A solution of 16% by mass of the polyimide "KPI-MX300F (100)" manufactured by Kawamura Sangyo Co., Ltd. dissolved in dimethylacetamide was prepared, and water was further added to prepare a polyimide varnish having a water content of 10% by mass. bottom. An attempt was made to prepare a polyimide-based film in the same manner as in Example 1 except that this polyimide-based varnish was used, but it was difficult to prepare a uniform film due to precipitation of solid content during the preparation of the polyimide-based varnish.

[Comparative Example 5]
A polyimide varnish was prepared in the same manner as in Example 5 except that water was further added to adjust the water content to 10% by mass at the time of preparing the polyimide varnish. An attempt was made to prepare a polyimide-based film in the same manner as in Example 1 except that this polyimide-based varnish was used, but it was difficult to prepare a uniform film due to precipitation of solid content during the preparation of the polyimide-based varnish.

[Evaluation of film appearance]
The appearances of the polyimide films obtained in Examples and Comparative Examples were visually observed, and those in which no defects such as fish eyes, agglomerates, and streaks were found were "A", fish eyes, agglomerates, and the like. Those in which defects such as streaks were observed were evaluated as "B", and those in which a uniform film could not be formed, or those in which a film could be formed but were non-uniform and cloudy were evaluated as "C". The results are shown in Table 1. Films with an appearance rating of "C" were not measured for yellowness or total light transmittance.

[Measurement of yellowness (YI value)]
The yellowness (Yellow Index: YI value) of each of the polyimide films obtained in Examples and Comparative Examples was measured by an ultraviolet-visible near-infrared spectrophotometer V-670 manufactured by JASCO Corporation. After performing background measurement in the absence of a sample, a polyimide film was set in a sample holder, transmittance was measured for light of 300 to 800 nm, and tristimulus values (X, Y, Z) were determined. The YI value was calculated 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 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 films obtained in Examples and Comparative Examples was evaluated according to the following criteria. When the film is folded by hand and creases are made, the ones that only make creases and do not have any abnormalities around the creases are marked with "A", and the ones around the folds are whitened, and the appearance around the creases has changed. "B" and those with cracked creases were evaluated as "C".

Claims (11)

A polyimide varnish containing a polyimide polymer, a solvent, and water.
The polyimide-based polymer is a transparent polyimide-based polymer having a total light transmittance of 85% or more and a yellowness of 5 or less when a film having a thickness of 50 μm is formed from the polyimide-based polymer.
The solvent is capable of dissolving the polyimide polymer.
A polyimide varnish for forming a film for a front plate for protecting the surface 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. The polyimide varnish for forming a film for a front plate for protecting the surface of a flexible display according to claim 1, wherein the polyimide polymer contains a halogen atom in the molecule. The polyimide varnish for forming a film for a front plate for protecting the surface of a flexible display according to claim 2, wherein the halogen atom is a fluorine atom. The polyimide-based varnish for forming a film for a front plate for protecting the surface of a flexible display according to any one of claims 1 to 3, further comprising silica particles. The polyimide-based varnish for forming a film for a front plate for protecting the surface of a flexible display according to claim 4, further comprising an alkoxysilane having an amino group. A polyimide film for a front plate for surface protection of a flexible display formed from the polyimide varnish according to any one of claims 1 to 5. The polyimide film for a front plate for surface protection of a flexible display according to claim 6, wherein the yellowness is 5 or less. The polyimide film for a front plate for surface protection of a flexible display according to claim 6 or 7, wherein the total light transmittance is 85% or more. A step of applying the polyimide varnish for forming a film for a front plate for surface protection of a flexible display according to any one of claims 1 to 5 onto a base material to form a coating film.
The step of drying the coating film and
A method for manufacturing a polyimide film for a front plate for surface protection of a flexible display including. The method for producing a polyimide film for a front plate for protecting the surface of a flexible display according to claim 9, further comprising a step of peeling the dried coating film from the substrate. A polyimide film for a front plate for surface protection of a flexible display manufactured by the method for producing a polyimide film for a front plate for surface protection of a flexible display according to claim 9 or 10.