Method for producing polyimide film and method for preparing polyamic acid solution

Abstract

A polyimide film is produced by using a polyimide precursor solution obtained by adding a polyamic acid curing catalyst and a dehydrating agent to a polyamic acid solution containing an organic solvent and polyamic acid. The thickness distribution can be reduced by adjusting the solids concentration so that an evaluation solution obtained by adding a predetermined amount of 2,6-lutidine and acetic anhydride to a polyamic acid solution has a predetermined shear stress, and producing a polyimide film using a polyimide precursor solution having the same solids concentration as the evaluation solution.

Description

Method for producing polyimide film and method for preparing polyamic acid solution

[0001] The present invention relates to a method for producing a polyimide film, and a method for preparing a polyamic acid solution used in the production of a polyimide film.

[0002] Polyimide films have excellent heat resistance and electrical insulation properties and are widely used in the electronics field as substrate materials for flexible printed circuit boards (FPCs) and in electrical insulating tapes. Polyimide films are generally manufactured by coating a polyimide precursor solution, which is obtained by adding a dehydrating agent and a catalyst to a polyamic acid solution, onto a support, and proceeding with imidation while drying and removing the solvent by heating (for example, Patent Document 1). Fillers such as silica particles may be added to polyimide films to adjust properties such as sliding properties, electrical properties, and corona resistance (for example, Patent Document 2).

[0003] Japanese Patent Publication No. 11-198157 Japanese Patent Publication No. 2007-90770

[0004] As described in Patent Document 1, in the manufacture of polyimide films, it is generally believed that a lower viscosity polyimide precursor solution results in better surface properties of the film. However, when the polyimide acid solution (polyimide precursor solution) contains fillers such as silica particles, the viscoelastic properties of the solution change, and using a low-viscosity solution can result in a large thickness distribution in the film width direction, leading to a decrease in surface properties.

[0005] In view of the above, the present invention aims to provide a polyimide film with a small thickness distribution. More specifically, the present invention aims to provide a method for producing a polyimide film suitable for obtaining a polyimide film with a small thickness distribution, and a method for preparing a polyimide acid solution (polyimide precursor solution) used therein.

[0006] One aspect of the present invention relates to a method for producing a polyimide film, comprising: dispensing a polyimide precursor solution, obtained by adding a polyamic acid curing catalyst and a dehydrating agent to a polyamic acid solution containing an organic solvent and polyamic acid, from a die and coating it onto a support; drying off a portion of the organic solvent from the polyimide precursor solution to produce a gel film; and after peeling the gel film from the support, imidizing the polyamic acid by heating. Another aspect of the present invention relates to a method for preparing a polyamic acid solution used in the production of a polyimide film.

[0007] The polyamic acid solution used in the present invention may contain silica particles having an average primary particle diameter of 10 to 200 nm. The silica particle content in the polyamic acid solution may be 10 to 40 parts by weight per 100 parts by weight of the solid content of the polyamic acid.

[0008] Polyamic acids are addition polymers of tetracarboxylic dianhydrides and diamines. An example of a polyamic acid is one which contains pyromellitic dianhydride as the tetracarboxylic dianhydride and 4,4'-diaminodiphenyl ether as the diamine.

[0009] In this invention, the viscoelastic properties are evaluated using an evaluation solution prepared by adding 0.34 molar equivalents of 2,6-lutidine and 2.64 molar equivalents of acetic anhydride to a polyamic acid solution, relative to the amide units of the polyamic acid. Specifically, a rheometer is used to evaluate the properties at a temperature of 0°C and a shear rate of 141 s⁻¹. －１ ,353s －１ , and 706s －１ The shear stress of the evaluation solution is measured.

[0010] Shear rate 141 s －１ ,353s －１ , and 706s －１ Let the shear stresses of the evaluation solutions be A, B, and C, respectively, and calculate the shear stress ratio using the following formula: Shear stress ratio = (A - B) / C

[0011] The solid content concentration of the evaluation solution is adjusted such that this shear stress ratio satisfies 0.20 to 1.50 (the solid content concentration at which the shear stress ratio of the evaluation solution satisfies 0.20 to 1.50 is determined). A polyimide precursor solution adjusted to have the same solid content concentration as this is discharged from a die and coated onto a support to produce a polyimide film, whereby a polyimide film with a small thickness distribution can be obtained. By adjusting the amount of the dilution solvent to be added, the solid content concentrations of the polyamic acid solution and the polyimide precursor solution can be adjusted to desired values.

[0012] The evaluation solution has a shear rate of 141 s －１ −1, and the shear stress A therein may be 550 to 2000 poise. The evaluation solution has a shear rate of 353 s －１ −1, and the shear stress B therein may be 500 to 1200 poise. The evaluation solution has a shear rate of 706 s －１ −1, and the shear stress C therein may be 400 to 800 poise. A polyimide film may be produced using a polyimide precursor solution having the same solid content concentration as the evaluation solution having a shear stress falling within any of these ranges.

[0013] It should be noted that the statement that the polyimide precursor solution has "the same solid content concentration" as the evaluation solution means that the solid content concentration of the polyimide precursor solution is in the range of 0.95 to 1.05 times the solid content concentration of the evaluation solution. The solid content concentration of the polyimide precursor solution may be 0.97 to 1.03 times, 0.98 to 1.02 times, 0.99 to 1.01 times, or 0.995 to 1.005 times the solid content concentration of the evaluation solution. The solid content concentration of the polyimide precursor solution may be 11.5 to 16.0% by weight.

[0014] In the production of a polyimide film, the temperature of the die when discharging the polyimide precursor solution from the die may be 7°C or lower.

[0015] According to the present invention, a polyimide film with a small thickness distribution can be obtained.

[0016] This is the thickness profile in the width direction of the polyimide film of Example 1. This is the thickness profile in the width direction of the polyimide film of Example 2. This is the thickness profile in the width direction of the polyimide film of Example 3. This is the thickness profile in the width direction of the polyimide film of Comparative Example 1.

[0017] In a method for producing a polyimide film according to one embodiment of the present invention, a polyimide precursor solution is used, which is obtained by adding a polyamic acid curing catalyst and a dehydrating agent to a polyamic acid solution containing polyamic acid. The polyimide precursor solution is extruded from a die and coated onto a support, and a portion of the solvent is dried off from the polyimide precursor solution to produce a gel film. After peeling the gel film from the support, the polyimide film is obtained by heating to imide the polyamic acid.

[0018] [Polyimide Precursor Solution] The polyimide precursor solution used for the production of polyimide films contains polyamic acid, a curing catalyst, a dehydrating agent, and a solvent. The polyimide precursor solution may further contain additives such as fillers.

[0019] <Polyamic Acids> Polyamic acids are obtained by polymerizing approximately equimolar amounts of tetracarboxylic dianhydride and diamine in a solvent.

[0020] Tetracarboxylic acid dianhydrides are not particularly limited. Specific examples of tetracarboxylic acid dianhydrides include pyromellitic acid dianhydride, 2,3,6,7-naphthalenetetracarboxylic acid dianhydride, 3,3',4,4'-biphenyltetracarboxylic acid dianhydride, 1,2,5,6-naphthalenetetracarboxylic acid dianhydride, 2,2',3,3'-biphenyltetracarboxylic acid dianhydride, 3,3',4,4'-benzophenonetetracarboxylic acid dianhydride, 4,4'-oxyphthalic acid dianhydride, 2,2-bis(3,4-dicarboxyphenyl)propane dianhydride, 3,4,9,10-perylenetetracarboxylic acid dianhydride, and bis(3,4-dicarboxyphenyl) Examples include phenyl)propane dianhydride, 1,1-bis(2,3-dicarboxyphenyl)ethane dianhydride, 1,1-bis(3,4-dicarboxyphenyl)ethane dianhydride, bis(2,3-dicarboxyphenyl)methane dianhydride, bis(3,4-dicarboxyphenyl)ethane dianhydride, oxydiphthalic acid dianhydride, bis(3,4-dicarboxyphenyl)sulfone dianhydride, p-phenylenebis(trimellitic acid monoester anhydride), ethylenebis(trimellitic acid monoester anhydride), bisphenol A bis(trimellitic acid monoester anhydride), and the like.

[0021] Diamines are not particularly limited. Specific examples of diamines include 4,4'-diaminodiphenyl ether, 4,4'-diaminodiphenylpropane, 4,4'-diaminodiphenylmethane, benzidine, 3,3'-dichlorobenzidine, 3,3'-dimethylbenzidine, 2,2'-dimethylbenzidine, 3,3'-dimethoxybenzidine, 2,2'-dimethoxybenzidine, 4,4'-diaminodiphenyl sulfide, 3,3'-diaminodiphenyl sulfone, 4,4'-diaminodiphenyl sulfone, 4,4'-oxydianiline, 3,3'-oxydianiline, 3,4'-oxydianiline, 1,5-diaminonaphthalene, 4,4'-diaminodiphenyldiethylsilane, 4,4'-diaminodiphenylsilane, 4,4'-diaminodiphenylethylphosphine oxide, and 4,4'-diaminodiphenyl N Examples include methylamine, 4,4'-diaminodiphenyl N-phenylamine, 1,4-diaminobenzene (p-phenylenediamine), 1,3-diaminobenzene, 1,2-diaminobenzene, bis{4-(4-aminophenoxy)phenyl}sulfone, bis{4-(4-aminophenoxy)phenyl}propane, bis{4-(3-aminophenoxy)phenyl}sulfone, 4,4'-bis(4-aminophenoxy)biphenyl, 4,4'-bis(3-aminophenoxy)biphenyl, 1,3-bis(3-aminophenoxy)benzene, 1,3-bis(4-aminophenoxy)benzene, 1,3-bis(4-aminophenoxy)benzene, 1,3-bis(3-aminophenoxy)benzene, 3,3'-diaminobenzophenone, 4,4'-diaminobenzophenone, etc.

[0022] Tetracarboxylic acid dianhydride and diamine may be used individually, or two or more may be mixed in any proportion. Among the above, from the viewpoint of the mechanical strength and electrical properties of the polyimide film, the polyamic acid is preferably one which contains one or more tetracarboxylic acid dianhydride components selected from the group consisting of pyromellitic acid dianhydride, 3,3',4,4'-biphenyltetracarboxylic acid dianhydride, 3,3',4,4'-benzophenonetetracarboxylic acid dianhydride, p-phenylenebis(trimellitic acid monoester acid anhydride), and 4,4'-oxydiphthalic acid dianhydride, and one or more diamine components selected from the group consisting of 4,4'-diaminodiphenyl ether, 1,3-bis(4-aminophenoxy)benzene, 1,3-bis(3-aminophenoxy)benzene, 1,4-bis(4-aminophenoxy)benzene, 1,4-diaminobenzene, 4,4'-bis(4-aminophenoxy)biphenyl, and 2,2'-dimethyl-4,4'-diaminobiphenyl. In particular, those containing pyromellitic dianhydride as the tetracarboxylic dianhydride component and 4,4'-diaminodiphenyl ether as the diamine component are especially preferred. The polyamic acid may contain pyromellitic dianhydride in amounts of 30 mol% or more, 50 mol% or more, 70 mol% or more, 80 mol% or more, or 90 mol% or more relative to the total amount of the tetracarboxylic dianhydride component, and 4,4'-diaminodiphenyl ether in amounts of 30 mol% or more, 50 mol% or more, 70 mol% or more, 80 mol% or more, or 90 mol% or more relative to the total amount of the diamine component.

[0023] The polymerization method for polyamic acid is not particularly limited, and any known method can be applied. Typically, a polyamic acid solution is obtained by dissolving substantially equimolar amounts of tetracarboxylic dianhydride and diamine in an organic solvent and reacting them while stirring. The solid content concentration of the polyamic acid solution during polymerization is usually about 10 to 35% by weight.

[0024] <Organic solvents> Organic solvents are not particularly limited as long as they can dissolve polyamic acid. Examples of organic solvents capable of dissolving polyamic acid include polar solvents such as amide solvents, sulfoxide solvents, pyrrolidone solvents, phenolic solvents, alcoholic solvents, and cellosolves. Among polar organic solvents, amide solvents such as N,N-dimethylformamide, N,N-dimethylacetamide, and N-methyl-2-pyrrolidone are preferred, and among amide solvents, N,N-dimethylformamide and N,N-dimethylacetamide are particularly preferred.

[0025] <Additives> In addition to polyamic acid and organic solvents, the polyamic acid solution may also contain thermosetting resins such as epoxy resins and phenoxy resins, and thermoplastic resins such as polyether ketones and polyether ether ketones. The polyamic acid solution may also contain fillers to improve various properties of the film, such as sliding properties, thermal conductivity, electrical conductivity, corona resistance, and loop stiffness. Examples of fillers include silica, titanium dioxide, alumina, silicon nitride, boron nitride, calcium hydrogen phosphate, calcium phosphate, and mica. The particle size of the fillers is approximately 10 nm to 5 μm.

[0026] In one embodiment, the polyamic acid solution contains silica particles. The inclusion of silica particles in the polyamic acid solution tends to improve the electrical insulation and corona resistance of the polyimide film. The average primary particle size of the silica particles is preferably 10 to 200 nm, but may be 10 to 100 nm or 10 to 50 nm. It is preferable to add the silica particles to the polyamic acid solution as a dispersion sol of colloidal silica.

[0027] If the polyamic acid solution contains silica particles, the silica particle content is preferably 10 to 40 parts by weight, but may also be 15 to 35 parts by weight or 20 to 30 parts by weight, per 100 parts by weight of the solid content of the polyamic acid.

[0028] <Dehydrating Agent and Curing Catalyst> The polyimide precursor solution contains a dehydrating agent and a curing catalyst for polyamic acid, in addition to the polyamic acid solution (polyamic acid and solvent, as well as additives such as silica particles). By using a polyimide precursor solution in which a dehydrating agent and a curing catalyst are added as curing agents to the polyamic acid solution, the production efficiency of polyimide films tends to be increased, and the mechanical strength of the films tends to improve.

[0029] The dehydrating agent is a dehydrating cyclizing agent for polyamic acid, and examples include aliphatic acid anhydrides such as acetic anhydride, aromatic acid anhydrides, N,N'-dialkylcarbodiimides, lower aliphatic halides, halogenated lower aliphatic acid anhydrides, aryl sulfonic acid dihalides, thionyl halides, and other compounds. Among these, aliphatic acid anhydrides and aromatic acid anhydrides are preferred, and acetic anhydride is particularly preferred.

[0030] The curing catalyst is a component that promotes the dehydration and cyclization action of the dehydrating agent on polyamic acid, and tertiary amines such as aliphatic tertiary amines, aromatic tertiary amines, and heterocyclic tertiary amines are preferred. Among these, nitrogen-containing heterocyclic compounds such as isoquinoline, quinoline, imidazole, benzimidazole, β-picoline, γ-picoline, dimethylpyridine, and diethylpyridine are preferred, with isoquinoline being particularly preferred.

[0031] In the polyimide precursor solution, the amount of dehydrating agent is preferably 1.0 to 3.5 molar equivalents relative to the amide acid units of the polyamic acid, and the amount of curing catalyst is preferably 0.3 to 1.5 molar equivalents relative to the amide acid units of the polyamic acid.

[0032] <Preparation of Polyimide Precursor Solution> The method for preparing a polyimide precursor solution is not particularly limited, and it is sufficient to mix each of the above components. Generally, a polyimide precursor solution is prepared by adding a curing catalyst, a dehydrating agent, and a diluting solvent to the polyamic acid after polymerization. From the viewpoint of suppressing the progress of imidization in the stage before the polyimide precursor solution is used for producing a polyimide film (discharged from a die) (for example, during storage of the solution), it is preferable to keep the solution temperature low during and after the addition of the curing catalyst and the dehydrating agent. The solution temperature is preferably 10°C or lower, and may be 7°C or lower, 3°C or lower, or 1°C or lower.

[0033] When the polyamic acid solution or the polyimide precursor solution contains additives such as silica particles, the timing of adding the additives may be either before or after the addition of the curing catalyst and the dehydrating agent, or may be simultaneous with the addition of the curing catalyst and the dehydrating agent.

[0034] A polyimide precursor solution having a desired solid content concentration can be obtained by adjusting the amount of the organic solvent added as the diluting solvent. The timing of adding the diluting solvent may be either before or after the addition of the curing catalyst and the dehydrating agent, or may be simultaneous with the addition of the curing catalyst and the dehydrating agent. When the polyamic acid solution or the polyimide precursor solution contains additives such as silica particles, the timing of adding the diluting solvent may be either before or after the addition of the additives, or may be simultaneous with the addition of the additives. For example, it may be added to the polyamic acid solution as a solution in which silica particles (silica sol) as an additive are dispersed in a diluting solvent.

[0035] The solid content concentration of the polyimide precursor solution is the ratio of the weight of the solid content (non-volatile content) to the weight of the polyimide precursor solution. When the polyimide precursor solution contains non-volatile components such as silica particles as additives, the solid content concentration of the polyimide precursor solution is the total ratio of the solid content of polyamic acid and the solid content of the additives to the weight of the polyimide precursor solution.

[0036] In the present invention, adjusting the solid content concentration of a polyimide precursor solution can reduce the thickness distribution of a polyimide film produced using the polyimide precursor solution. Details of the concentration adjustment of the polyimide precursor solution will be described later.

[0037] [Production of Polyimide Film from Polyimide Precursor Solution] The above polyimide precursor solution is discharged from a die and coated onto a support, part of the solvent is removed by drying from the coating film to prepare a gel film, after peeling the gel film from the support, heating and imidization are performed to obtain a polyimide film.

[0038] As a die for discharging the polyimide precursor solution, a T-die is preferred. Stainless steel is preferred as a material for the T-die. The temperature of the die when discharging the polyimide precursor solution from the die is preferably 7°C or lower, more preferably 5°C or lower, and may be 3°C or lower. Lowering the die temperature can suppress the progress of imidization during discharge, and reduce the precipitation and adhesion of solids to the die. If the die temperature is excessively low, the viscosity of the polyimide precursor solution increases, the discharge pressure rises, and the film thickness may become non-uniform. Therefore, the temperature of the die is preferably -20°C or higher, and may be -15°C or higher, -10°C or higher, or -5°C or higher.

[0039] The support for coating the polyimide precursor solution discharged from the die is not particularly limited as long as it has smoothness capable of forming a liquid film and does not dissolve in the organic solvent contained in the polyimide precursor solution, and metal belts, metal rollers, resin belts, resin films, resin rollers and the like can be used. A metallic endless belt or a metallic roller is preferred as the support because of excellent peelability of the gel film.

[0040] A gel film is formed by drying and removing at least a portion of the solvent from the polyimide precursor solution formed on the support. Heating is preferable from the viewpoint of promoting the volatilization of the organic solvent. The heating conditions are not particularly limited as long as the coating can be gelled to a degree that allows it to be peeled off the support. The heating temperature is, for example, 60 to 200°C, and may be around 80 to 150°C. The heating time is, for example, around 1 to 600 seconds, and may be 10 to 500 seconds or 30 to 400 seconds.

[0041] A polyimide film is obtained by peeling the gel film from the support and then heating it to imidize it. During heating, it is preferable to fix both ends of the gel film with a pin tenter or clip tenter. The heating conditions are not particularly limited as long as the imidization reaction of the polyamic acid is completed and unreacted components such as water, residual solvent, catalyst, and dehydrating agent in the gel film can be removed. The heating temperature is, for example, about 150 to 500°C, but may be 200 to 450°C or 250 to 420°C. The heating time is about 5 to 1000 seconds, but may be 10 to 500 seconds or 30 to 400 seconds.

[0042] [Concentration adjustment of polyimide precursor solution using evaluation solution] As described above, in the present invention, a polyimide film with minimal thickness distribution can be obtained by adjusting the solid content concentration of the polyimide precursor solution. Specifically, by preparing an evaluation solution with a composition similar to the polyimide precursor solution used in the production of the polyimide film and evaluating its viscoelasticity, it is possible to determine the appropriate solid content concentration of the polyimide precursor solution suitable for the production of a polyimide film with minimal thickness distribution.

[0043] <Preparation of Evaluation Solution> The evaluation solution is prepared by adding 0.34 molar equivalents of 2,6-lutidine and 2.64 molar equivalents of acetic anhydride to a polyamic acid solution relative to the amide units of the polyamic acid. The composition of the polyamic acid solution used to prepare the evaluation solution is the same as the composition of the polyamic acid solution used in the polyimide precursor solution for the production of polyimide films.

[0044] If the polyimide precursor solution contains additives such as silica particles in addition to polyamic acid, organic solvent, curing catalyst, and dehydrating agent, the evaluation solution will also contain the same additives in the same amount. For example, if the polyimide precursor solution contains 20 parts by weight of silica particles per 100 parts by weight of solids of polyamic acid, the evaluation solution will also contain 20 parts by weight of silica particles per 100 parts by weight of solids of polyamic acid.

[0045] The polyimide precursor solution and the evaluation solution have the same composition of polyamic acid solution, but differ in the type and amount of curing catalyst and dehydrating agent. As mentioned above, in the polyimide precursor solution, the type of curing catalyst and dehydrating agent is not particularly limited, the amount of curing catalyst is in the range of 0.3 to 1.5 molar equivalents relative to the amide acid units of polyamic acid, and the amount of dehydrating agent is in the range of 1.0 to 3.5 molar equivalents relative to the amide acid units of polyamic acid. On the other hand, in the evaluation solution, 2,6-lutidine is added as a curing catalyst in an amount of 0.34 molar equivalents relative to the amide acid units of polyamic acid, and acetic anhydride is added as a dehydrating agent in an amount of 2.64 molar equivalents relative to the amide acid units of polyamic acid.

[0046] Thus, since the evaluation solution has a fixed type and amount of curing catalyst and dehydrating agent, it is suitable for evaluating the relationship between the solid content concentration and viscosity of the solution according to a consistent standard. Furthermore, 2,6-lutidine, used as the curing catalyst in the evaluation solution, has a lower catalytic activity compared to isoquinoline, etc., and in a low-temperature environment around 0°C, the imidization of polyamic acid hardly proceeds even after time has passed since solution preparation. Therefore, the reproducibility of viscosity measurement results is high.

[0047] The method for preparing the evaluation solution is not particularly limited and can be carried out using the same procedure as for preparing the polyimide precursor solution, except that the type and amount of curing catalyst and dehydrating agent differ. Specifically, the evaluation solution is prepared by preparing a polyamic acid solution and adding the above-mentioned amounts of curing catalyst (2,6-lutidine) and dehydrating agent (acetic anhydride). To suppress the progression of imidization, the curing catalyst and dehydrating agent are added at a solution temperature of 0°C.

[0048] If the polyamic acid solution and the polyimide precursor solution contain additives other than polyamic acid, such as silica particles, as solid components (non-volatile components), the evaluation solution will also contain the same additives. In preparing the evaluation solution, the timing of adding additives such as silica particles may be before or after adding the curing catalyst and dehydrating agent, or it may be done simultaneously with the addition of the curing catalyst and dehydrating agent.

[0049] By adjusting the amount of organic solvent added as a diluent, evaluation solutions with various solid content concentrations can be obtained. The diluent can be added before or after the addition of the curing catalyst and dehydrating agent, or simultaneously with the addition of the curing catalyst and dehydrating agent.

[0050] <Viscosity Measurement of Evaluation Solutions> Evaluation solutions with various solid content concentrations were prepared, and the viscosity of each was measured using a rotary rheometer at a temperature of 0°C. The measurement was started 120 seconds after adding the curing catalyst (2,6-lutidine) and dehydrating agent (acetic anhydride) to the polyamic acid solution, and the shear rate was 141 s. －１ Shear stress A and shear rate 353 s in this case. －１ Shear stress B and shear rate 706 s in this case. －１ Determine the shear stress C in this location.

[0051] From the obtained shear stresses A, B, and C, the following shear stress ratio is calculated: Shear stress ratio = (A - B) / C

[0052] The shear stress ratio of the evaluation solution is preferably 0.20 to 1.50. More preferably 0.30 to 1.00, and may be 0.35 to 0.95 or 0.40 to 0.90. When the shear stress ratio of the evaluation solution is within this range, the thickness distribution of polyimide films made using polyimide precursor solutions with the same solid content concentration tends to be smaller. When the shear stress ratio of the evaluation solution is excessively small or excessively large, the thickness distribution of polyimide films made using polyimide precursor solutions with the same solid content concentration tends to be larger.

[0053] By changing the amount of organic solvent in the polyamic acid solution (the amount of diluent added), evaluation solutions with various solid content concentrations are prepared, and viscosity measurements are performed to determine the range of solid content concentrations in which the evaluation solution has a shear stress ratio within the above range. If the amount of diluent is large and the solid content concentration is excessively small, the shear stress ratio tends to be less than 0.20. In that case, the shear stress ratio can be adjusted to within the above range by reducing the amount of diluent and increasing the solid content concentration.

[0054] By adjusting the amount of organic solvent in the polyamic acid solution (amount of diluent added) so that the polyimide precursor solution used in the production of the polyimide film has a solid content concentration within the specified range, a polyimide film with a small thickness distribution can be obtained.

[0055] Shear rate of the evaluation solution: 141 s －１ The shear stress A is preferably 550 to 2000 poise, more preferably 700 to 1600 poise, and may also be 800 to 1400 poise or 900 to 1300 poise.

[0056] Shear rate of the evaluation solution: 353 s －１ The shear stress B is preferably 500 to 1200 poise, more preferably 550 to 1100 poise, and may also be 600 to 1000 poise or 700 to 900 poise.

[0057] Shear rate of the evaluation solution: 706 s －１ The shear stress C is preferably 400 to 800 poise, more preferably 450 to 750 poise, and may also be 500 to 700 poise or 550 to 650 poise.

[0058] By using a polyimide precursor solution having the same solid content concentration as the evaluation solutions having the shear stresses A, B, and C described above, the thickness distribution of the polyimide film tends to become smaller.

[0059] The above shear stresses A, B, C and shear stress ratios evaluate the behavior of the polyimide precursor solution when it is extruded from the die, with a shear rate of 706 s. －１The shear stress C in this case corresponds to the behavior near the entrance of the T-die (just before the polyamic acid solution spreads in the width direction). Shear rate 353 s －１ The shear stress B in the T-die is near the center in the width direction, at a shear rate of 141 s. －１ The shear stress A in this case corresponds to the behavior near both ends in the width direction of the T-die.

[0060] The pressure is high and the shear rate is large near the entrance of the T-die. However, as the solution spreads in the width direction within the T-die, the pressure is released, the shear rate decreases, and consequently the shear stress increases. The shear rate is smaller and the shear stress is larger at both ends compared to the center in the width direction. (A-B) corresponds to the difference in shear stress between the center and both ends in the width direction of the T-die, and the shear stress ratio is obtained by dividing this by the shear stress C near the entrance of the T-die.

[0061] Generally, it is believed that a lower solid content concentration and lower solution viscosity result in better coating performance. However, if the solution viscosity is excessively low, the shear stress ratio tends to be small, and the film thickness distribution tends to be large, especially at the edges in the width direction. This invention utilizes this knowledge, and by determining the shear stress ratio of the evaluation solution, it is possible to determine the solid content concentration of a polyimide precursor solution suitable for obtaining a polyimide film with a small thickness distribution.

[0062] In particular, when fillers such as silica particles are added, the viscoelastic behavior of the solution changes significantly, making it difficult to predict how the film thickness distribution will be. In the method of the present invention, by measuring the viscosity of the evaluation solution, it is possible to predict the solid content concentration suitable for obtaining a polyimide film with a small thickness distribution. By adjusting the solid content concentration of the polyimide precursor solution based on this, a polyimide film with a small thickness distribution can be produced.

[0063] In other words, it is preferable that the polyimide precursor solution used in the production of the polyimide film has the same solid content concentration as the evaluation solution that satisfies the above-mentioned shear stress conditions. The solid content concentration of the polyimide precursor solution is preferably 0.95 to 1.05 times, more preferably 0.97 to 1.03 times, even more preferably 0.98 to 1.02 times, and may also be 0.99 to 1.01 times or 0.995 to 1.005 times the solid content concentration of the evaluation solution.

[0064] As mentioned above, in the evaluation solution, the amounts of 2,6-lutidine as a curing catalyst and acetic anhydride as a dehydrating agent added to the polyamic acid are constant. In contrast, in the polyimide precursor solution used to produce polyimide films, the amount of curing catalyst is 0.3 to 1.5 molar equivalents relative to the amide units of the polyamic acid, and the amount of dehydrating agent is 1.0 to 3.5 molar equivalents relative to the amide units of the polyamic acid. Within this range, the amounts are optimized according to the composition of the polyamic acid and the conditions for producing the polyimide film (heating conditions), etc.

[0065] Therefore, the amounts of curing catalyst and dehydrating agent added may differ between the evaluation solution and the polyimide precursor solution. Even in such cases, by adjusting the amount of diluent added to the polyamic acid solution, it is possible to obtain a polyimide precursor solution having the same solid content concentration (with an acceptable error within the above range) as the evaluation solution that satisfies the above shear stress conditions, and thus a polyimide film with a small thickness distribution can be produced.

[0066] As long as the evaluation solution satisfies the above shear stress conditions, the solid content concentration of the evaluation solution and the polyimide precursor solution is not particularly limited, and the optimal range will vary depending on the composition of the monomers constituting the polyamic acid (tetracarboxylic dianhydride and diamine), the molecular weight of the polyamic acid, the type of solvent, the type and content of additives, etc. As a guideline, the solid content concentration is approximately 11.5 to 16.0% by weight.

[0067] [Applications of Polyimide Film] The applications of the polyimide film of the present invention are not particularly limited. The polyimide film has excellent heat resistance and electrical insulation properties and can be suitably used in flexible printed circuit boards (FPCs), TAB tapes, electrical insulation tapes, wire insulation materials, etc.

[0068] [Preparation of Polyamic Acid Solution] 58.4 kg of 4,4'-diaminodiphenyl ether (ODA) was added to 500 kg of N,N-dimethylformamide (DMF), and 60.9 kg of pyromellitic dianhydride (PMDA) was added while stirring under a nitrogen atmosphere, and the mixture was stirred for 20 minutes. A PMDA solution was prepared by dissolving PMDA in DMF to a solid content concentration of 5%, and this solution was gradually added to the above reaction solution. When the viscosity at 23°C reached 3000 poise, the solution was added, stirring was stopped, and a polyamic acid solution was obtained.

[0069] [Preparation of evaluation solution] The above polyamic acid solution was diluted by adding a curing agent consisting of 2.64 molar equivalents of acetic anhydride (dehydrating agent) and 0.34 molar equivalents of 2,6-lutidine (imidization catalyst) relative to the amide acid units of the polyamic acid, and then by adding the amount of DMF described later.

[0070] Subsequently, dimethylacetamide-dispersed silica sol (Nissan Chemical's "DMAC-ST", solid content concentration 20% by weight) was added to 100 parts by weight of polyamic acid so that the solid content of the silica sol was 25 parts by weight, and the mixture was mixed in a mixer to obtain a mixed solution of polyamic acid / silica sol / curing agent (evaluation solution).

[0071] <Solid Content of Solution> The amount of DMF used for dilution was adjusted so that the solid content concentration of the mixed solution of polyamic acid / silica sol / curing agent was 14.1% by weight in Example 1, 12.1% by weight in Example 2, 11.6% by weight in Example 3, and 11.2% by weight in Comparative Example 1. The solid content concentration of the mixed solution is expressed by the following formula: Solid content concentration (%) = 100 × (Weight of solid content of polyamic acid + Weight of solid content of silica sol) / (Weight of polyamic acid solution + Weight of dimethylacetamide-dispersed silica sol + Weight of dehydrating agent + Weight of imidization catalyst + Weight of DMF for dilution)

[0072] [Preparation of Polyimide Film] <Preparation of Polyimide Precursor Solution> The imidation catalyst used in the preparation of the evaluation solution above was changed from 2,6-lutidine to isoquinoline to obtain a mixed solution of polyamic acid / silica sol / curing agent (polyimide precursor solution). Similar to the preparation of the evaluation solution, the amount of diluting DMF was adjusted so that the solid content concentration after adding the silica sol dispersion was 14.1% by weight in Example 1, 12.1% by weight in Example 2, 11.6% by weight in Example 3, and 11.2% by weight in Comparative Example 1.

[0073] <Formation of Polyimide Film> Using a 1700 mm wide T-die, the above polyimide precursor solution was cast onto a stainless steel endless belt and heated and dried at 130°C for 130 seconds to form a self-supporting gel film. The gel film was peeled from the support, and both ends were fixed with pin tenters. While being transported through a tenter furnace capable of continuous firing, it was heated in a first hot air furnace at 300°C for 19 seconds, in a second hot air furnace at 350°C for 21 seconds, and in a third hot air furnace at 395°C for 17 seconds to perform heat imidization.

[0074] The imidized film was peeled off the pins, and both ends were slit to obtain a polyimide film with a width of 1650 mm and an average thickness of approximately 28 μm.

[0075] [Evaluation] <Shear stress of the evaluation solution> Using a rotary rheometer (UBM "Rheosol-G5000NT"), at a temperature of 0°C, A: shear rate 141 s －１ Shear stress, B: shear rate 353 s －１ The shear stress and C: shear rate 706 s －１ The shear stress was measured, and the following shear stress ratio was calculated: Shear stress ratio = (A - B) / C

[0076] <Polyimide Film Thickness> The overall thickness of the polyimide film in the width direction was measured at 1 mm intervals using a continuous film thickness gauge (manufactured by Matsuo Sangyo). The average and standard deviation of the thickness were calculated from the thickness data in the range excluding 75 mm from both ends in the width direction (1500 mm width). In addition, the average thickness and standard deviation of the thickness were calculated for each of the following ranges: 50 to 150 mm from one end (E1), 50 to 150 mm from the other end (E2), and 100 mm in the center of the width direction (M). Furthermore, the difference between the average of the standard deviations of the thickness at both ends (E1, E2) and the standard deviation of the thickness at the center (M): (E1 + E2) / 2 - M was calculated.

[0077] [Evaluation Results] Table 1 shows the measured solid content concentration of the solution, the shear stress of the evaluation solution, and the thickness of the polyimide film for each of Examples 1-3 and Comparative Example 1. Figures 1-4 show the thickness profiles in the width direction of the polyimide films for Examples 1-3 and Comparative Example 1. In Figures 1-4, the horizontal axis represents the position in the width direction (mm), and the vertical axis represents the thickness (μm).

[0078]

[0079] The polyimide film of Comparative Example 1, which used a polyimide precursor solution with a solid content concentration of 11.2% by weight, exhibited large variations in thickness, particularly at both ends in the width direction, where the standard deviation of thickness was large. On the other hand, in Examples 1 to 3, which used polyimide precursor solutions with a higher solid content concentration than Comparative Example 1, the variations in thickness were smaller, and polyimide films with a good appearance were obtained.

[0080] From a comparison of Examples 1-3 and Comparative Example 1, it can be seen that by adjusting the solid content concentration (amount of diluent added) of the evaluation solution prepared using 2,6-lutidine as an imidation catalyst so that the shear stress ratio is within a predetermined range, a polyimide film with excellent thickness uniformity can be obtained.

Claims

1. A method for producing a polyimide film, comprising: discharging a polyimide precursor solution, obtained by adding a curing catalyst for polyamic acid and a dehydrating agent to a polyamic acid solution containing an organic solvent and polyamic acid, from a die onto a support; drying off a portion of the organic solvent from the polyimide precursor solution to produce a gel film; and after peeling the gel film from the support, heating to imide the polyamic acid, wherein the amount of curing catalyst added is 0.3 to 1.5 molar equivalents relative to the amide acid units of the polyamic acid, the amount of dehydrating agent added is 1.0 to 3.5 molar equivalents relative to the amide acid units of the polyamic acid, and the shear stress ratio of an evaluation solution obtained by adding 0.34 molar equivalents of 2,6-lutidine and 2.64 molar equivalents of acetic anhydride relative to the amide acid units of the polyamic acid to the polyamic acid solution is 0.20 to 1.

50. －１ ,353s －１ , and 706s －１ The shear stress at points A, B, and C are defined as the shear stress ratio = (A - B) / C.

2. The method for producing a polyimide film according to claim 1, wherein the polyamic acid solution contains silica particles having an average primary particle diameter of 10 to 200 nm.

3. The method for producing a polyimide film according to claim 2, wherein the content of the silica particles in the polyamic acid solution is 10 to 40 parts by weight per 100 parts by weight of the solid content of the polyamic acid.

4. The evaluation solution was tested at a shear rate of 141 s. －１ A method for producing a polyimide film according to any one of claims 1 to 3, wherein the shear stress A is 550 to 2000 poise.

5. The evaluation solution was tested at a shear rate of 353 s. －１ A method for producing a polyimide film according to any one of claims 1 to 3, wherein the shear stress B at is 500 to 1200 poise.

6. The evaluation solution was tested at a shear rate of 706 s. －１ A method for producing a polyimide film according to any one of claims 1 to 3, wherein the shear stress C at is 400 to 800 poise.

7. A method for producing a polyimide film according to any one of claims 1 to 3, wherein the solid content concentration of the polyimide precursor solution is 11.5 to 16.0% by weight.

8. A method for producing a polyimide film according to any one of claims 1 to 3, wherein the temperature of the die when the polyimide precursor solution is discharged from the die is 7°C or lower.

9. The method for producing a polyimide film according to any one of claims 1 to 3, wherein the polyamic acid is an addition polymer of a tetracarboxylic dianhydride and a diamine, the tetracarboxylic dianhydride comprises pyromellitic dianhydride, and the diamine comprises 4,4'-diaminodiphenyl ether.

10. A method for preparing a polyamic acid solution used for producing a polyimide film, wherein the polyamic acid solution contains an organic solvent and a polyamic acid, and wherein a shear stress ratio of an evaluation solution, which is obtained by adding 0.34 molar equivalents of 2,6-lutidine and 2.64 molar equivalents of acetic anhydride to the polyamic acid solution at a temperature of 0°C, based on the amic acid units of the polyamic acid, satisfies 0.20 to 1.50, the amount of the organic solvent in the polyamic acid solution is adjusted. A method for preparing a polyamic acid solution: provided that the shear stress ratio is measured by a rheometer at a temperature of 0°C at a shear rate of 141 s －１ , 353 s －１ , and 706 s －１ , wherein the shear stresses at are respectively A, B and C, and the shear stress ratio is a value defined by shear stress ratio = (A−B) / C.

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