JP2024065610A - Polyimide film manufacturing method - Google Patents

Abstract

[Problem] To provide a method for producing a polyimide film capable of producing a polyimide film with little variation in b* value and high production efficiency. [Solution] The method for producing a polyimide film comprises the steps of applying a varnish containing polyamic acid or polyimide onto a substrate, and heating the applied varnish to obtain a polyimide film. The polyimide contains 60 mol % or more of aromatic monomers relative to all monomers constituting the polyimide. In the step of heating the applied varnish, when the glass transition temperature of the polyimide is Tg, the varnish is heated to a temperature higher than (Tg + 15) °C, and the heating rate in the temperature range from (Tg + 15) °C to (Tg + 30) °C is 1.0 °C/min or less. [Selected Figure] Figure 1

Description

This disclosure relates to a method for producing a polyimide film.

Conventionally, in displays such as liquid crystal display elements and organic EL display elements, inorganic glass, a transparent material, has been used for panel substrates, etc. However, inorganic glass has a high specific gravity (weight) and low flexibility and impact resistance. Therefore, the use of polyimide film, which is lightweight, impact-resistant, processable, and flexible, for the panel substrate of display devices is being considered.

Here, the panel substrate of the display device is required to have high transparency. Furthermore, the panel substrate may be exposed to heat during the process of forming elements such as thin-film transistors and transparent electrodes on the panel substrate. For this reason, the panel substrate is also required to have high heat resistance.

As a polyimide film to be used as such a panel substrate, for example, a film containing polyimide containing structural units derived from diamines including 2,2-bis(trifluoromethyl)benzidine (TFMB) and structural units derived from tetracarboxylic dianhydrides including 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) and 9,9'-bis(3,4'-dicarboxyphenyl)fluorene dianhydride (BPAF) has been proposed (for example, Patent Documents 1 and 2).

These polyimide films can be obtained by a method of applying a varnish containing polyamic acid onto a substrate, then heating it to remove the solvent while imidizing, or by a method of applying a varnish containing polyimide onto a substrate and then heating it to remove the solvent. For example, Patent Document 1 discloses that a varnish containing polyamic acid is applied onto a substrate, then heated to 350°C at a rate of 5°C/min and held for a predetermined time, and then further heated to 420°C at a rate of 5°C/min and held for a predetermined time to imidize and obtain a polyimide film. Patent Document 2 discloses that a varnish containing polyimide is applied onto a substrate, then heated to 80°C and held for a predetermined time, and then further heated to 400°C to evaporate the solvent, to obtain a polyimide film.

International Publication No. 2019/188265 International Publication No. 2019/188306

In the process of producing a polyimide film, it is desirable to increase the temperature rise rate when heating the applied varnish in order to improve production efficiency. However, according to the investigations of the present inventors, a high temperature rise rate can easily cause variation in the b ^* value of the obtained film.

Such variation in the b ^* value is particularly likely to occur when producing a polyimide film containing many structural units derived from aromatic monomers. In addition, in order to obtain a polyimide film that can be applied to high-temperature processes, it is desirable to achieve a high temperature during film formation, but when the achieved temperature is high, variation in the b ^* value is particularly likely to occur.

The present disclosure has been made in view of the above circumstances, and has an object to provide a method for producing a polyimide film that can provide a polyimide film with little variation in b ^* value.

（式（ａ１）において、
Ｒ^１及びＲ^２は、それぞれ独立して炭素数１～４のアルキル基であり、
ｍ及びｎは、それぞれ０～２の整数であり、かつｍ＋ｎは３以下である）

（式（ａ３）において、
Ｒ^３及びＲ^４は、それぞれ独立して炭素数１～４のアルキル基であり、
ｏ及びｐは、それぞれ０～３の整数であり、かつｏ＋ｐは３以下である）
前記ジアミンは、式（ｂ１）で表される化合物を含み、

前記式（ａ１）で表される化合物の含有量は、前記テトラカルボン酸二無水物の全量に対して１０モル％以上であり、前記式（ａ１）で表される化合物及び式（ａ２）で表される化合物の合計量は、前記テトラカルボン酸二無水物の全量に対して３０～８０モル％である、［１］～［３］のいずれかに記載のポリイミドフィルムの製造方法。
［５］ 前記テトラカルボン酸二無水物は、前記式（ａ３）で表される化合物を含む、［４］に記載のポリイミドフィルムの製造方法。
［６］ 前記式（ａ３）で表される化合物の含有量は、前記テトラカルボン酸二無水物の全量に対して１０～７０モル％である、［５］に記載のポリイミドフィルムの製造方法。
［７］ 前記式（ａ１）で表される化合物の含有量は、前記式（ａ２）で表される化合物の含有量よりも多い、［４］～［６］のいずれかに記載のポリイミドフィルムの製造方法。
［８］ 前記式（ａ１）で表される化合物の含有量は、前記テトラカルボン酸二無水物の全量に対して３０～８０モル％である、［４］～［７］のいずれかに記載のポリイミドフィルムの製造方法。
［９］ 前記式（ａ２）で表される化合物及び前記式（ａ３）で表される化合物の合計量は、前記テトラカルボン酸二無水物の全量に対して１０～７０モル％である、［４］～［８］のいずれかに記載のポリイミドフィルムの製造方法。 The present disclosure relates to the following method for producing a polyimide film.
[1] A method for producing a polyimide film comprising the steps of: applying a varnish containing a polyamic acid or a polyimide onto a substrate; and heating the applied varnish to obtain a polyimide film, wherein a monomer constituting the polyimide contained in the polyimide film contains 60 mol % or more of an aromatic monomer relative to the total monomer constituting the polyimide, and in the step of heating the varnish,
When the glass transition temperature of the polyimide measured by TMA is Tg, the polyimide is heated to a temperature higher than (Tg+15)° C., and the heating rate in the temperature range from (Tg+15)° C. to (Tg+30)° C. is set to 1.0° C./min or less.
A method for producing a polyimide film.
[2] The method for producing a polyimide film according to [1], wherein the glass transition temperature of the polyimide is 350° C. or higher.
[3] The method for producing a polyimide film according to [1] or [2], wherein the b ^* value of the polyimide film is 12 or less.
[4] The monomer constituting the polyimide includes a tetracarboxylic dianhydride and a diamine, and the tetracarboxylic dianhydride includes a compound represented by formula (a1), a compound represented by formula (a2), and/or a compound represented by formula (a3),

(In formula (a1),
R ¹ and R ² are each independently an alkyl group having 1 to 4 carbon atoms;
m and n are each an integer of 0 to 2, and m+n is 3 or less.

(In formula (a3),
^R3 and ^R4 are each independently an alkyl group having 1 to 4 carbon atoms;
Each of o and p is an integer of 0 to 3, and o+p is 3 or less.
The diamine includes a compound represented by formula (b1):

The method for producing a polyimide film according to any one of [1] to [3], wherein the content of the compound represented by formula (a1) is 10 mol % or more based on the total amount of the tetracarboxylic dianhydride, and the total amount of the compound represented by formula (a1) and the compound represented by formula (a2) is 30 to 80 mol % based on the total amount of the tetracarboxylic dianhydride.
[5] The method for producing a polyimide film according to [4], wherein the tetracarboxylic dianhydride contains a compound represented by formula (a3).
[6] The method for producing a polyimide film according to [5], wherein the content of the compound represented by formula (a3) is 10 to 70 mol % based on the total amount of the tetracarboxylic dianhydride.
[7] The method for producing a polyimide film according to any one of [4] to [6], wherein the content of the compound represented by formula (a1) is greater than the content of the compound represented by formula (a2).
[8] The method for producing a polyimide film according to any one of [4] to [7], wherein the content of the compound represented by formula (a1) is 30 to 80 mol % based on the total amount of the tetracarboxylic dianhydride.
[9] The method for producing a polyimide film according to any one of [4] to [8], wherein the total amount of the compound represented by formula (a2) and the compound represented by formula (a3) is 10 to 70 mol% based on the total amount of the tetracarboxylic dianhydride.

According to the present disclosure, it is possible to provide a method for producing a polyimide film that can provide a polyimide film with little variation in b ^* value.

1A and 1B are graphs showing an example of the relationship between the heating time and the temperature rise rate when heating an applied varnish. FIG. 2 is a graph showing another example of the relationship between the heating time and the temperature rise rate when the applied varnish is heated.

In this specification, a numerical range expressed using "~" means a range that includes the numerical values written before and after "~" as the lower and upper limits, respectively. In numerical ranges described in stages in this specification, the upper or lower limit value described in a certain numerical range may be replaced with the upper or lower limit value of another numerical range described in stages.

As described above, when the temperature increase rate is increased in the step of heating the applied varnish, there are cases where the b ^* value of the obtained film is likely to vary.

In response to this, the present inventors have found that, when the glass transition temperature of polyimide is Tg, the variation in the b ^* value of the resulting film can be reduced by heating the applied varnish to a temperature higher than (Tg+15)° C. and decreasing the rate of temperature increase in the temperature range from (Tg+15)° C. to (Tg+30)° C. The mechanism behind this is not clear, but is presumed to be as follows.

In the temperature range from (Tg+15)°C to (Tg+30)°C, the polymer molecular chains are easy to move and tend to adopt one of two stable structures with different orientation states. Here, when the heating rate is high, a large amount of energy is given all at once, so that some polymer molecular chains reach another stable structure B (high orientation, high coloration) stabilized with high energy before reaching the most stable structure A (low orientation, low coloration) stabilized with low energy due to changes in entanglement. As a result, the b ^* value of the obtained film is likely to vary. In particular, polyimides containing a large amount of aromatic monomers are prone to interactions (orientation) between molecules, so the variation in b ^* value is likely to be large.
In contrast, when the heating rate is low, the energy given is small, so that the film does not reach another stable structure B (high orientation, high coloration) that requires high energy, and the film tends to reach the most stable structure A (low orientation, low coloration) that is stabilized by low energy. As a result, the variation in the b ^* value of the obtained film is thought to be reduced. Furthermore, the variation in not only the b ^* value but also other physical properties such as the CTE value can be reduced.

That is, the method for producing a polyimide film according to the present disclosure includes the steps of 1) applying a varnish containing polyamic acid or polyimide onto a substrate, and 2) heating the applied varnish to obtain a polyimide film. The applied varnish is then heated to a temperature higher than (Tg+15)°C, and the heating rate in the temperature range from (Tg+15)°C to (Tg+30)°C is set to 1°C/min or less. The method for producing a polyimide film according to the present disclosure will be described in detail below.

1. Method for Producing Polyimide Film As described above, a method for producing a polyimide film according to one embodiment of the present invention includes the steps of 1) applying a varnish containing polyamic acid or polyimide onto a substrate, and 2) heating the applied varnish to obtain a polyimide film.

1-1. Step of applying varnish First, a varnish containing polyamic acid or polyimide is prepared.

1-1-1. Preparation of Varnish A varnish containing a polyamic acid or a polyimide can be obtained by reacting a monomer containing a tetracarboxylic dianhydride and a diamine in a solvent. Specifically, the tetracarboxylic dianhydride is reacted with the diamine in a solvent.

(monomer)
For example, when the obtained polyimide film is used as a panel substrate, the panel substrate may be subjected to heat in the process of forming elements such as thin film transistors and transparent electrodes on the panel substrate, so the panel substrate is required to have high heat resistance, i.e., high Tg. Therefore, the above-mentioned monomers constituting the polyimide preferably contain aromatic monomers in an amount of 60 mol % or more relative to the total monomers constituting the polyimide. The content of aromatic monomers in the monomers constituting the polyimide is preferably 90 mol % or more relative to the total monomers constituting the polyimide. The aromatic monomer may contain an aromatic tetracarboxylic dianhydride and an aromatic diamine.

The tetracarboxylic dianhydride and diamine used in the reaction are the same as the tetracarboxylic dianhydride and diamine that constitute the polyimide contained in the polyimide film described below. These will be described in detail later.

(solvent)
The solvent is not particularly limited as long as it can dissolve the tetracarboxylic dianhydride and the diamine. For example, aprotic polar solvents and alcohol-based solvents can be used.

Examples of aprotic polar solvents include amide solvents such as N,N-dimethylacetamide (DMAc), N-methyl-2-pyrrolidone (NMP), 1,3-dimethyl-2-imidazolidinone (DMI), N,N-dimethylformamide (DMF), N,N-diethylformamide (DEF), and hexamethylphosphoramide (HMPA); dimethyl sulfoxide; and 2-methoxyethanol, 2-ethoxyethanol, 2-(methoxymethoxy)ethoxyethanol, 2-isopropoxyethanol, 2-butoxyethanol, tetrahydrofurfuryl alcohol, diethylene glycol, diethylene glycol monomethyl ether, and diethylene glycol. These include ether-based solvents such as diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, triethylene glycol, triethylene glycol monoethyl ether, tetraethylene glycol, 1-methoxy-2-propanol, 1-ethoxy-2-propanol, dipropylene glycol, dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, tripropylene glycol monomethyl ether, polyethylene glycol, polypropylene glycol, tetrahydrofuran, dioxane, 1,2-dimethoxyethane, diethylene glycol dimethyl ether, and diethylene glycol diethyl ether.

Examples of alcohol-based solvents include methanol, ethanol, 1-propanol, 2-propanol, tert-butyl alcohol, ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 1,5-pentanediol, 2-butene-1,4-diol, 2-methyl-2,4-pentanediol, 1,2,6-hexanetriol, diacetone alcohol, etc.

These solvents may be used alone or in combination of two or more. Among them, N,N-dimethylacetamide, N-methyl-2-pyrrolidone, 1,3-dimethyl-2-imidazolidinone (DMI) or a mixed solvent thereof is preferred.

(reaction)
When carrying out the above reaction, the ratio (y/x) of the total molar amount x of the diamines to the total molar amount y of the tetracarboxylic dianhydrides can be, for example, 0.9 to 1.1, and preferably 0.95 to 1.05.

When preparing a varnish containing polyamic acid, the above reaction is preferably carried out by heating the tetracarboxylic dianhydride and the diamine in a solvent at a relatively low temperature (a temperature at which imidization does not occur). Specifically, the temperature at which imidization does not occur can be 5 to 120°C, more preferably 25 to 80°C. In addition, the reaction is preferably carried out in the substantial absence of an imidization catalyst (e.g., triethylamine, etc.).

When preparing a varnish containing polyimide, the above reaction is preferably carried out by heating the tetracarboxylic dianhydride and the diamine in a solvent at a relatively high temperature (the temperature at which imidization occurs). The temperature at which imidization occurs may be, specifically, 150 to 200°C. The reaction may also be carried out in the presence of an imidization catalyst.

The above reaction (polymerization reaction) can be carried out by a known method. For example, a container equipped with a stirrer and a nitrogen inlet tube is prepared, and the solvent is poured into the nitrogen-substituted container. Then, diamine is added so that the final concentration of polyamic acid or polyimide falls within the above-mentioned range, and the temperature is adjusted and stirring is performed. A predetermined amount of tetracarboxylic dianhydride is added to the solution. Then, the mixture is stirred for about 1 to 50 hours while adjusting the temperature.

The content of polyamic acid or polyimide is not particularly limited, but from the viewpoint of coatability, it can be 5 to 30% by mass, preferably 10 to 25% by mass, of the varnish.

(Physical Properties)
The intrinsic viscosity (η) of the polyamic acid or polyimide is not particularly limited, but is preferably 0.3 to 2.0 dL/g, and more preferably 0.6 to 1.6 dL/g. When the intrinsic viscosity (η) of the polyamic acid or polyimide is within the above range, it is easy to achieve both coatability and film-forming property.

The intrinsic viscosity (η) can be adjusted by the molar ratio of the tetracarboxylic dianhydride and diamine to be reacted. For example, when the dianhydride/diamine ratio is equimolar, the intrinsic viscosity (η) tends to be high. In addition, the intrinsic viscosity (η) is the value measured with an Ubbelohde viscometer at 25°C when the concentration of polyamic acid or polyimide in N-methyl-2-pyrrolidone (NMP) is 0.5 g/dL.

1-1-2. Coating of Varnish Next, the varnish prepared above is coated onto the substrate.

There are no particular limitations on the substrate, so long as it has solvent resistance and heat resistance. The substrate may be any substrate from which the resulting polyimide film can be easily peeled off, and is preferably a flexible substrate such as a glass plate, metal, or heat-resistant polymer film.

Examples of flexible substrates made of metal include metal foils made of copper, aluminum, stainless steel, iron, silver, palladium, nickel, chromium, molybdenum, tungsten, zirconium, gold, cobalt, titanium, tantalum, zinc, lead, tin, silicon, bismuth, indium, or alloys thereof. The surface of the metal foil may be coated with a release agent.

Examples of flexible substrates made of heat-resistant polymer films include polyimide films, aramid films, polyether ether ketone films, polyether sulfone films, etc. Flexible substrates made of heat-resistant polymer films may contain a release agent or an antistatic agent, and may be coated on the surface with a release agent or an antistatic agent. The substrate is preferably a polyimide film, since the film has good peelability and high heat and solvent resistance.

The shape of the substrate is appropriately selected according to the shape of the polyimide film to be produced, and may be a single sheet or a long piece. The thickness of the substrate is preferably 5 to 150 μm, and more preferably 10 to 70 μm. If the thickness of the substrate is 5 μm or more, the substrate is less likely to wrinkle or tear during application of the varnish.

There are no particular limitations on the method of applying the varnish, so long as it is a method that allows for application of a uniform thickness. Examples of application methods include methods using a die coater, comma coater, roll coater, gravure coater, curtain coater, spray coater, lip coater, etc.

1-2. Step of Obtaining Polyimide Film Next, the applied varnish is heated to obtain a polyimide film. Hereinafter, an example of a varnish containing polyamic acid will be described.

In the case of a varnish containing polyamic acid, the applied varnish is heated to imidize the polyamic acid and the solvent is evaporated to produce a polyimide film.

Specifically, the coating of the varnish is heated, for example from room temperature to 150-200°C, while removing the solvent in the coating, and imidizing (ring-closing) the polyamic acid. The coating is then heated further to a maximum temperature Tmax (for example, 500°C) that exceeds Tg, while further removing the solvent in the coating.

(Heating conditions)
As described above, the applied varnish is heated from room temperature to a maximum temperature Tmax (for example, about 500° C.) exceeding Tg. In the present disclosure, the rate of temperature increase is reduced in a predetermined temperature range exceeding Tg.

1A, 1B and 2 are graphs showing an example of the relationship between the heating time and the temperature rise rate when heating an applied varnish.
In this embodiment, as shown in these figures, when the glass transition temperature of polyimide measured by TMA is Tg, the film is heated to a temperature higher than (Tg+15)° C., and the heating rate in the temperature range from (Tg+15)° C. to (Tg+30)° C. is set to 1.0° C./min or less. This makes it possible to reduce the variation in the b ^* value of the obtained film.

That is, in the temperature range from (Tg+15)°C to (Tg+30)°C, the polymer molecular chain becomes easy to move, and the polymer molecular chain is likely to adopt one of two stable structures with different orientation states; that is, the most stable structure A (low orientation) stabilized with low energy, and another stable structure B (high orientation) that requires high energy for stabilization. Here, when the heating rate is high, a large amount of energy is given all at once, so that before reaching the most stable structure A (low orientation), another stable structure B (high orientation) is likely to appear. As a result, the variation in the b ^* value of the obtained film is likely to become large.
In contrast, when the heating rate is low, the energy given is small, so the film cannot reach another stable structure B (high orientation) that requires high energy, and it is likely to reach the most stable structure A (low orientation) that is stabilized by low energy. Once the most stable structure A (low orientation) is reached, the film is unlikely to move to another stable structure B (high orientation) because the energy given is small. As a result, it is thought that the variation in the b ^* value of the obtained film is small.

As described above, the heating rate in the temperature range from (Tg+15)°C to (Tg+30)°C is 1.0°C/min or less, and preferably 0.7°C/min or less. The lower limit of the heating rate is not particularly limited, but may be, for example, 0.3°C/min or more from the viewpoint of production efficiency. The heating temperature refers to the actual coating temperature. The coating temperature can be measured by contacting a thermocouple with the varnish coating.

The maximum temperature Tmax is, for example, the temperature at which the amount of solvent in the film is 0.5% by mass or less, and may be (Tg + 30)°C, or it may be higher or lower than that. For example, the maximum temperature Tmax is preferably, for example, (Tg + 20)°C or higher, and more preferably higher than (Tg + 30)°C. For example, the maximum temperature Tmax can be 400°C or higher, preferably 430°C or higher, and more preferably 450°C. After the maximum temperature is reached, heating may be performed at that temperature for a certain period of time.

When heating at the maximum temperature Tmax, the heating time can usually be about 0.5 to 2 hours. By increasing the maximum temperature, damage to the device caused by outgassing remaining in the film system and stress changes can be suppressed even when exposed to high temperatures in the subsequent device manufacturing process. For example, in the case of TFTs for display substrates, if the TFT device type is a low-temperature polysilicon (LTPS) type, a higher process temperature (e.g., 400°C or higher) may be required than for conventional amorphous silicon or IGZO types. Even after such high-temperature thermal history, damage to the device can be reduced.

On the other hand, the heating rate in other temperature ranges than the above temperature range, for example, in a temperature range higher than (Tg + 30) ° C. or lower than (Tg + 15) ° C., may be the same as the heating rate in the above temperature range, or may be higher or lower.

For example, in a temperature range higher than (Tg+30)°C, the temperature is unlikely to deviate from the most stable structure A or another stable structure B, so the heating rate may be higher than the heating rate in the above temperature range from the viewpoint of increasing production efficiency (see Figures 1B and 2).

In addition, in a temperature range lower than (Tg+15)° C., since polymer molecular chains are difficult to move in the first place, the heating rate may be higher than in the above-mentioned temperature range from the viewpoint of improving production efficiency (see FIGS. 1A, 1B, and 2). However, in this temperature range, from the viewpoint of further suppressing deterioration of the surface properties of the coating film due to bumping of the solvent, it is preferable that the heating rate is not too high.
For example, in a temperature range lower than the above-mentioned temperature range, at least in the vicinity of the imidization temperature (usually 150 to 200° C.), the temperature is preferably raised gradually. For example, the rate of temperature rise in the temperature range of 150 to 200° C. is preferably 1 to 50° C./min, more preferably 2 to 30° C./min, and may be 4 to 20° C./min. When the rate is 50° C./min or less, the temperature rise is appropriately gentle, so that imidization of the polyamic acid on the coating film surface is suppressed before the solvent volatilizes from the coating film, and it is easy to suppress the solvent remaining in the coating film from generating bubbles or causing unevenness on the coating film surface.

In each temperature region, the heating rate may be constant (see Figures 1A and 1B) or may not be constant (see Figure 2). For example, in the temperature region from (Tg+15)°C to (Tg+30)°C, as long as the heating rate is 1°C or less throughout the entire temperature region, the heating rate may be constant (see Figures 1A and 1B) or may vary (see Figure 2).

(Heating method)
The method of heating the applied varnish is not particularly limited, but for example, a method of heating a monolithic coating film while increasing its temperature includes a method of heating with a hot plate and a method of increasing the temperature inside an oven. In this case, the temperature increase rate is adjusted by the settings of the hot plate or oven. In addition, when a long coating film is heated while increasing its temperature, for example, a plurality of heating furnaces for heating the coating film are arranged along the conveying (moving) direction of the substrate; the temperature of each heating furnace is changed for each heating furnace. For example, the temperature of each heating furnace may be increased along the moving direction of the substrate. In this case, the temperature increase rate is adjusted by the conveying speed of the substrate.

(Heating atmosphere)
Polyimide is easily oxidized when heated at a temperature exceeding 200° C. If polyimide is oxidized, the resulting polyimide film may turn yellow. Therefore, in the temperature range exceeding 200° C., it is preferable to (i) use an inert gas atmosphere as the heating atmosphere or (ii) reduce the pressure of the heating atmosphere.

(i) If the heating atmosphere is an inert gas atmosphere, the oxidation reaction of the polyimide is suppressed. The type of inert gas is not particularly limited, and can be argon gas, nitrogen gas, or carbon dioxide gas. Furthermore, the oxygen concentration in the temperature range exceeding 200°C is preferably 1% by volume or less, more preferably 0.1% by volume (1000 ppm) or less, and even more preferably 0.01% by volume (100 ppm) or less. The oxygen concentration in the atmosphere is measured by a commercially available oxygen concentration meter (for example, a zirconia type oxygen concentration meter).

(ii) The oxidation reaction of polyimide can also be suppressed by reducing the pressure of the heating atmosphere. When reducing the pressure of the heating atmosphere, the pressure in the atmosphere is preferably 15 kPa or less, more preferably 5 kPa or less, and even more preferably 1 kPa or less. When reducing the pressure of the heating atmosphere, the coating film is heated in a reduced pressure oven or the like.

After the polyamic acid is imidized (ring-closed), the substrate is peeled off to obtain a polyimide film.

In the case of a varnish containing polyimide, the applied varnish is heated to evaporate the solvent to obtain a polyimide film. The heating conditions are the same as for varnishes containing polyamic acid.

1-3. Function According to the method for producing a polyimide film according to the above embodiment, the variation in the b ^* value of the obtained polyimide film can be reduced. Furthermore, the variation in not only the b ^* value of the obtained film but also other physical properties such as CTE can be reduced. Therefore, the production efficiency of the polyimide film can be improved.

2. Polyimide Film The polyimide film is obtained by the above-mentioned method for producing a polyimide film.

The polyimide contained in the polyimide film is obtained by reacting monomers containing tetracarboxylic dianhydride and diamine as described above. The monomers constituting the polyimide contain aromatic monomers in an amount of 60 mol % or more, preferably 90 mol % or more, based on the total monomers constituting the polyimide. The aromatic monomers contain aromatic tetracarboxylic dianhydride and aromatic diamine.

That is, the polyimide contains structural units derived from tetracarboxylic dianhydride and structural units derived from diamine. The tetracarboxylic dianhydride contains an aromatic tetracarboxylic dianhydride, and the diamine contains an aromatic diamine.

The types of aromatic tetracarboxylic dianhydride and aromatic diamine are not particularly limited, but from the viewpoint of obtaining a polyimide film that has little coloration and has both a low CTE and high bending resistance, it is preferable that the tetracarboxylic dianhydride contains, as the aromatic tetracarboxylic dianhydride, a compound represented by formula (a1), and a compound represented by formula (a2) and/or a compound represented by formula (a3); and the diamine contains, as the aromatic diamine, a specific diamine (TFMB).

The compound represented by formula (a1) can reduce the CTE while suppressing an increase in the b ^* value of the resulting polyimide film.
That is, the benzene ring contained in pyromellitic anhydride is likely to absorb light due to the formation of an intermolecular charge transfer complex associated with the donor-acceptor with two adjacent acid dianhydride moieties, and 1,4,5,8-naphthalenetetracarboxylic dianhydride and tetracarboxylic dianhydrides having a condensed ring in which three or more benzene rings are condensed (for example, 3,4,9,10-perylenetetracarboxylic dianhydride, etc.) are likely to absorb light due to the expansion of the π-electron conjugated system in the molecule, so both are likely to cause coloring. In contrast, the naphthalene ring contained in the acid dianhydride represented by formula (a1) is unlikely to form an intermolecular charge transfer complex associated with the donor-acceptor that causes light absorption, and the expansion of the π-electron conjugation in the molecule is also small, so it is unlikely to cause coloring. In addition, the naphthalene ring has a rigid structure and is likely to increase the orientation, so it is easy to reduce the CTE of the film.

In addition, the compounds represented by formula (a2) and formula (a3) are relatively flexible, and therefore can increase the bending resistance of the polyimide film.

The polyimide structure in this embodiment is described in detail below.

2-1. Composition [Tetracarboxylic acid dianhydride]
The tetracarboxylic dianhydride preferably contains a compound represented by formula (a1), and a compound represented by formula (a2) and/or a compound represented by formula (a3).

In formula (a1), ^R1 and ^R2 are each independently an alkyl group having 1 to 4 carbon atoms. The number of carbon atoms in the alkyl group is preferably 1 or 2. m and n are each an integer of 0 to 2, and m+n is 3 or less. From the viewpoint of easily lowering the CTE, it is preferable that the orientation (linearity of the molecule) is high, and it is preferable that m and n are 0.

The compound represented by formula (a1) and the compound represented by formula (a2) can lower the CTE of the polyimide film. In particular, the compound represented by formula (a2) can lower the CTE of the resulting polyimide film without coloring the polyimide film (without increasing the b ^* value) as compared with the compound represented by formula (a1).

Examples of the compound represented by formula (a1) include 2,3,6,7-naphthalenetetracarboxylic dianhydride (NTCDA) and those partially substituted with alkyl groups, with 2,3,6,7-naphthalenetetracarboxylic dianhydride (NTCDA) being preferred. Examples of the compound represented by formula (a2) include pyromellitic dianhydride (1,2,4,5-benzenetetracarboxylic acid, PMDA).

In formula (a3), ^R3 and ^R4 are each independently an alkyl group having 1 to 4 carbon atoms. The number of carbon atoms in the alkyl group is preferably 1 or 2. o and p are each an integer of 0 to 3, and o+p is 3 or less.

The compound represented by formula (a3) can increase the bending resistance of the polyimide film. Examples of the compound represented by formula (b1) include the compounds represented by formula (a3-1) and formula (a3-2).

An example of the compound represented by formula (a3-1) includes 3,3',4,4'-biphenyltetracarboxylic dianhydride (s-BPDA). An example of the compound represented by formula (a3-2) includes 2,3,3',4'-biphenyltetracarboxylic dianhydride (a-BPDA). Among them, from the viewpoint of increasing bending resistance, the compound represented by formula (a3-1) is preferable, and 3,3',4,4'-biphenyltetracarboxylic dianhydride (s-BPDA) is more preferable. Furthermore, from the viewpoint of decreasing the birefringence Δn, the compound represented by formula (a3-2) is preferable, and 2,3,3',4'-biphenyltetracarboxylic dianhydride (a-BPDA) is more preferable.

In particular, from the viewpoint of further lowering the CTE while increasing the bending resistance of the film, the tetracarboxylic dianhydride preferably contains a compound represented by formula (a1) and a compound represented by formula (a3), and may further contain a compound represented by formula (a2). In other words, the compound represented by formula (a2) may have intermediate properties between the compound represented by formula (a1) and the compound represented by formula (a3).

The content of the compound represented by formula (a1) is preferably 10 mol% or more based on the total amount of tetracarboxylic dianhydride. When the content of the compound represented by formula (a1) is 10 mol% or more, coloration of the polyimide film can be suppressed, and the b ^* value can be reduced while the CTE can be reduced. From the same viewpoint, the content of the compound represented by formula (a1) is more preferably 30 to 80 mol%, and even more preferably 40 to 65 mol%, based on the total amount of tetracarboxylic dianhydride.

From the viewpoint of further lowering the CTE while suppressing coloration of the film, the content of the compound represented by formula (a1) is preferably greater than that of the compound represented by formula (a2). Specifically, the ratio of the content of the compound represented by formula (a1) to the total amount of the compound represented by formula (a1) and the compound represented by formula (a2) (a1/(a1+a2)) is preferably 0.5 to 1.0, and more preferably 0.6 to 1.0.

The compound represented by formula (a3) is preferably 10 mol% or more relative to the total amount of tetracarboxylic dianhydride. That is, the compound represented by formula (a3) has a slightly non-planar structure. Therefore, when the compound represented by formula (a3) is further contained in an amount of 10 mol% or more in addition to the compound represented by formula (a1), the intermolecular interaction between the structural unit derived from the compound represented by formula (a1) and the structural unit derived from the compound represented by formula (a3) of the obtained polyimide is easily weakened to an appropriate degree. As a result, the coloration and b ^* value of the polyimide film can be further reduced, while the bending performance of the film can be further improved. From the same viewpoint, the compound represented by formula (a3) is preferably 10 to 70 mol% relative to the total amount of tetracarboxylic dianhydride, more preferably 20 to 65 mol%, and even more preferably 30 to 60 mol%.

The total amount (a1+a2) of the compound represented by formula (a1) and the compound represented by formula (a2) is preferably 30 to 80 mol% based on the total amount of tetracarboxylic dianhydride. If the total amount is 30 mol% or more, the CTE of the film can be sufficiently low, and if it is 80 mol% or less, the bending resistance of the film is unlikely to be impaired. From the same viewpoint, the total amount is more preferably 30 to 70 mol%, and even more preferably 40 to 65 mol% based on the total amount of tetracarboxylic dianhydride.

The total amount (a2+a3) of the compound represented by formula (a2) and the compound represented by formula (a3) is preferably 10 to 80 mol% based on the total amount of tetracarboxylic dianhydride. If the total amount is 10 mol% or more, the bending resistance of the film is easily increased, and if it is 80 mol% or less, an excessive increase in CTE can be suppressed. From the same viewpoint, the total amount is more preferably 10 to 70 mol%, even more preferably 23 to 60 mol%, and particularly preferably 40 to 60 mol% based on the total amount of tetracarboxylic dianhydride.

From the viewpoint of further increasing the bending resistance of the film, the ratio (a1+a2)/(a2+a3) of the total amount (a1+a2) of the compound represented by formula (a1) and the compound represented by formula (a2) to the total amount (a2+a3) of the compound represented by formula (a2) and the compound represented by formula (a3) is preferably 4 or less, more preferably 2 or less, more preferably 1 or less, and particularly preferably 0.9 or less. From the viewpoint of further lowering the CTE of the obtained film, (a1+a2)/(a2+a3) is preferably 0.4 or more, more preferably 0.6 or more, more preferably 0.8 or more, and even more preferably 1 or more. From these viewpoints, (a1+a2)/(a2+a3) is preferably 0.4 to 4, more preferably 0.4 to 2, more preferably 0.4 to 1, even more preferably 0.6 to 1, and particularly preferably 0.6 to 0.9.

It is preferable that the tetracarboxylic dianhydride further contains a compound represented by formula (a4).

In formula (a4), ^R5 and ^R6 each independently represent an optionally substituted alkyl group having 1 to 4 carbon atoms or a fluorine atom. The number of carbon atoms in the alkyl group is preferably 1 or 2. Examples of the substituent that the alkyl group may have include a fluorine atom. Each of q and r represents an integer of 0 to 3. However, q+r is 3 or less.

The compound represented by formula (a4) can impart high transparency and heat resistance when made into a film, and can also reduce the birefringence Δn of the film. The compound represented by formula (a4) is preferably 9,9'-bis(3,4-dicarboxyphenyl)fluorene dianhydride (BPAF).

The content of the compound represented by formula (a4) is preferably set so that the total amount of the tetracarboxylic dianhydride having a fluorene skeleton and the diamine (the total amount of the compound represented by formula (a4) and the compound represented by formula (b2)) falls within the range described below.

The tetracarboxylic dianhydride may further contain other tetracarboxylic dianhydrides other than those mentioned above, as long as the effects of the present disclosure are not impaired. Examples of other tetracarboxylic dianhydrides include aromatic tetracarboxylic dianhydrides, alicyclic tetracarboxylic dianhydrides, aliphatic tetracarboxylic dianhydrides, and the like other than those mentioned above. The content of the other tetracarboxylic dianhydrides may be 10 mol % or less based on the total amount of tetracarboxylic dianhydrides.

[Diamine]
The diamine preferably includes a compound represented by formula (b1).

The compound represented by formula (b1) is 2,2'-bis(trifluoromethyl)benzidine (TFMB), which can impart high transparency and a low b ^* value when made into a film.

The diamine preferably further contains a compound represented by formula (b2), for example from the viewpoint of increasing the heat resistance of the film or reducing the birefringence Δn.

In formula (b2), ^R7 and ^R8 each independently represent an optionally substituted alkyl group having 1 to 4 carbon atoms or a fluorine atom. The number of carbon atoms in the alkyl group is preferably 1 or 2. Examples of the substituent that the alkyl group may have include a fluorine atom. s and t each represent an integer of 0 to 3. However, s+t is 3 or less.

Examples of the compound represented by formula (b2) include 9,9-bis(4-aminophenyl)fluorene (BAFL), 9,9-bis(4-amino-3-methylphenyl)fluorene (FFDA), and 9,9-bis(aminofluorophenyl)fluorene.

The content of the compound represented by formula (b1) is preferably 80 mol% or more based on the total amount of diamine. When the content of the compound represented by formula (b1) is 80 mol% or more, coloration of the film is easily suppressed (b ^* value is easily reduced). From the same viewpoint, the content of the compound represented by formula (b1) is preferably 90 mol% or more, and may be 100 mol%.

The content of the compound represented by formula (b2) is set so that the total amount of the compound represented by formula (a4) and the compound represented by formula (b2) (the content of the component having a fluorene skeleton) is 0 to 15 mol %, preferably 0.5 to 15 mol %, relative to the total of the diamine and the tetracarboxylic dianhydride. If the total amount is 0.15 mol % or more, it is easy to reduce the birefringence Δn, and if it is 15 mol % or less, it is easy to suppress an increase in CTE. From the same viewpoint, it is more preferable that the total amount is 2 to 8 mol %.

The diamine may further include other diamines other than those mentioned above, as long as the effect of the present disclosure is not impaired. Examples of other diamines include aromatic diamines other than those mentioned above, such as bis(3-aminophenyl)sulfone (3,3-DAS), 4,4'-diaminodiphenylsulfone (4,4-DAS), 1,5-diaminonaphthalene (15DAN), 4,4'-diaminobenzanilide (DABA), and m-diaminobenzene (MDA), as well as alicyclic diamines such as 1,2-cyclohexanediamine (DACH), 1,3-bis(aminomethyl)cyclohexane, and 1,4-bis(aminomethyl)cyclohexane, and aliphatic diamines such as ethylenediamine and hexamethylenediamine. The content of other diamines may be 10 mol% or less based on the total amount of diamines.

2-2. Physical properties (b ^* value in the L ^* a ^* b ^* color system)
The b ^* value of the polyimide film in the L ^* a ^* b ^* color system is preferably 12 or less, more preferably 8 or less, further preferably 6 or less, and particularly preferably 4 or less. The lower limit of the b ^* value is usually about 1, and preferably 0. Polyimide films having a b ^* value that satisfies these ranges are less colored and have excellent transparency, and are therefore suitable for optical films, i.e., panel substrates for various display devices, and the like.

The b ^* value of the polyimide film is the value measured using a colorimeter (for example, a tristimulus value direct reading colorimeter (Colour Cute i CC-i type) manufactured by Suga Test Instruments Co., Ltd.) in a transmission mode (when the above-mentioned instrument is used, the transmission mode is set to 0°di).

The b ^* value can be adjusted by the monomer composition of the polyamic acid. For example, the b* value tends to ^decrease when the content ratio of the compound represented by formula (b1) (TFMB) in the diamine is increased or the content ratio of the compound represented by formula (a2) (PMDA) in the acid anhydride is decreased.

(Total light transmittance)
The total light transmittance (TT) of the polyimide film depends on the thermal imidization conditions (particularly the heating temperature), but is, for example, 80% or more, preferably 85% or more, more preferably 87% or more, and even more preferably 89% or more. The upper limit of the total light transmittance is preferably 100%, but is usually about 92% or 90%. Such a polyimide film with a high total light transmittance is suitable for optical films, that is, panel substrates (transparent substrates) for various display devices. The total light transmittance (TT) indicates the average transmittance in all wavelength ranges (300 to 830 nm) of the standard light source D65. In addition, from the viewpoint of further improving visibility, the transmittance at a wavelength of 450 nm (T@450 nm) is preferably 60% or more, more preferably 75% or more.

The total light transmittance of a polyimide film is measured in accordance with JIS-K7361-1 using a light source D65. The light transmittance at a wavelength of 450 nm can be determined by measuring the UV-visible spectrum in the wavelength range of 300 to 800 nm, and the transmittance of light at a wavelength of 450 nm can be calculated as T@450 nm.

The total light transmittance and transmittance at a wavelength of 450 nm of the polyimide film can be adjusted by the monomer composition of the polyimide. For example, if the content ratio of the compound represented by formula (b1) (TFMB) in the diamine or the content ratio of the compound represented by formula (a4) (e.g., BPAF) in the acid anhydride is high, the total light transmittance and transmittance at a wavelength of 450 nm tend to be high.

(Glass Transition Temperature (Tg))
The glass transition temperature (Tg) of the polyimide is preferably 350°C or higher, more preferably 370°C or higher, and even more preferably 400°C or higher. If the Tg of the polyimide film is 350°C or higher, the film can be applied to a substrate for a TFT array. Specifically, the Tg of the polyimide film is preferably 350°C or higher, more preferably 390°C or higher, when used for preparing a TFT array using IGZO (an oxide semiconductor composed of indium, gallium, zinc, and oxygen); it is preferably 400°C or higher when used for preparing a TFT array using low-temperature polysilicon. If the Tg is within this range, it can be used even under the working environment during the preparation of each TFT array, and a highly reliable display device can be easily obtained. The upper limit of the Tg of the polyimide film is not particularly limited, but from the viewpoint of moldability, it can be, for example, about 500°C.

The Tg of a polyimide can be measured by the following method.
After mounting the test piece on a thermomechanical analyzer (TMA), the test piece is heated once from room temperature to 300° C. at 5° C./min under a nitrogen atmosphere and a load of 100 mN (1st run), and then cooled to 50° C. or less to remove residual stress accumulated in the test piece. Next, the test piece is heated again in the same manner under a nitrogen atmosphere and a load of 100 mN at 5° C./min to 450° C. (2nd run), and the inflection point of elongation at this time is determined as the glass transition temperature.

(Coefficient of Linear Thermal Expansion (CTE))
The coefficient of linear thermal expansion (CTE) of the polyimide film varies depending on the thermal imidization conditions (particularly the heating temperature), but is preferably, for example, −10 to 50 ppm/K, more preferably −10 to 30 ppm/K, even more preferably −5 to 20 ppm/K, and particularly preferably 0 to 10 ppm/K. If the coefficient of linear thermal expansion is within the above range, the polyimide film is less likely to deform even at high temperatures when used as a panel substrate for various display devices, and displacement and destruction of electric elements due to thermal deformation such as warping caused by a high-temperature environment in the process of forming electric elements on the substrate can be easily suppressed, making it easy to laminate various elements.

The coefficient of linear thermal expansion (CTE) can be calculated by measuring the glass transition temperature by TMA, and taking the TMA elongation ratio in the temperature range of 100 to 350°C when the temperature is increased in the second run.

The coefficient of linear thermal expansion (CTE) can be adjusted by the monomer composition of the polyimide (or its precursor, polyamic acid), etc. For example, the CTE tends to be lower when the total amount (a1+a2) (or (a1+a2)/(a2+a3)) of the compound represented by formula (a1) and the compound represented by formula (a2) in the acid dianhydride is increased, or when the content of the compound represented by formula (a1) is increased.

(Tensile strength)
The polyimide film preferably has a tensile strength of, for example, 160 MPa or more, and a tensile elongation of, for example, 4 to 15%.

The tensile strength and tensile elongation can be measured by the following procedure.
A dumbbell-shaped punched test piece is prepared from the polyimide film, and measurements are performed using a tensile tester (Shimadzu Corporation, EZ-S) under the conditions of a gauge width of 5 mm, a sample length of 30 mm, and a tensile speed of 30 mm/min. From the obtained stress-strain curve, the strength and elongation at the point of break are taken as the tensile strength and tensile elongation, respectively, and the average values of five measured values are calculated as the tensile strength TS and the tensile elongation EL.

(Bending resistance)
The number of times the polyimide film can be folded in the MIT folding endurance test is preferably 10,000 times or more, more preferably 40,000 times or more, even more preferably 100,000 times or more, and even more preferably 200,000 times or more. A film having a number of times the polyimide film can be folded in the MIT folding endurance test within the above range has high flexibility and is therefore suitable as a flexible display substrate.

The MIT folding endurance test can be carried out in the following manner.
A polyimide film (thickness 10 μm) is cut into a shape of length 120 mm × width 15 mm to prepare a test piece. One end of this test piece is set in a Yasuda Seiki Seisakusho MIT type folding endurance tester (307 type), the other end is gripped, and the test piece is folded back and forth under the conditions of a curvature radius of 0.38 mm, a load of 0.5 kg, a folding accuracy of 270 degrees (135 degrees left and right), and a folding speed of 175 times/min, and the number of times until breakage (folding endurance number) is measured. The measurement conditions can be as described in the examples below. The thickness of the polyimide film can be set to 9 to 12 μm depending on the thickness of the film.

The bending resistance can be adjusted by the monomer composition of the polyimide, etc. For example, by increasing the total amount (a2+a3) of the compound represented by formula (a2) and the compound represented by formula (a3) among the acid dianhydrides (or decreasing (a1+a2)/(a2+a3)), or by increasing the content of the compound represented by formula (a3), the bending resistance tends to increase.

(Thickness)
The thickness of the polyimide film is not particularly limited and may be appropriately selected depending on the application of the film, etc. The thickness of the polyimide film is, for example, 1 to 100 μm, preferably 5 to 50 μm, and more preferably 5 to 20 μm.

2-3. Applications The polyimide film has a low CTE and high bending resistance while having high transparency. Therefore, it is suitable for use as a substrate for electronic devices. Examples of substrates for electronic devices include transparent substrates (display panel substrates) for display devices such as touch panels, liquid crystal displays, and organic EL displays; and substrates carrying sensors for fingerprint authentication and face authentication. For example, substrate applications for carrying sensors for fingerprint authentication and face authentication require flexibility, high transparency that does not obstruct the light to be measured, and little coloring, and heat resistance (low CTE) is required because heat is applied during the manufacturing process of the substrate carrying the sensor. The polyimide film is also suitable for such applications.

The present disclosure will be described in more detail below with reference to examples. However, the scope of the present disclosure is not limited thereby in any way.

1. Materials The tetracarboxylic dianhydrides and diamines used are as follows:

[Tetracarboxylic acid dianhydride]
NTCDA: 2,3,6,7-naphthalenetetracarboxylic dianhydride PMDA: pyromellitic dianhydride BPAF: fluorenylidene bisphthalic anhydride s-BPDA: 3,3',4,4'-biphenyltetracarboxylic dianhydride

[Diamine]
TFMB: 4,4'-diamino-2,2'-bis(trifluoromethyl)biphenyl BAFL: 9,9-bis(4-aminophenyl)fluorene t-DACH: trans-1,4-diaminocyclohexane

2. Tests [Examples 1 to 3, Comparative Examples 1 to 2]
(Preparation of Polyamic Acid Varnish 1)
A diamine and N-methyl-2-pyrrolidinone (NMP) (the weight of NMP was calculated in advance so that the concentration of the total monomers was 20% by mass) in Table 1 were added to a flask equipped with a thermometer, a condenser, a nitrogen inlet tube, and a stirring blade, and the mixture was stirred under a nitrogen atmosphere to obtain a homogeneous solution. A predetermined amount of the corresponding tetracarboxylic dianhydride in Table 1 was added as powder to the solution, and the mixture was stirred at room temperature for 30 minutes, and then the solution temperature was raised and the mixture was stirred at an internal temperature of 85 to 90° C. for 1 hour, resulting in a homogeneous solution. The mixture was then cooled to room temperature and stirred overnight at room temperature to obtain a pale yellow viscous polyamic acid varnish (polyamic acid concentration 20% by mass, aromatic monomer content 99.75 mol%).

The composition of the polyamic acid and the physical properties of the varnish are shown in Table 1.

The intrinsic viscosity η of the varnish was measured using the following method.

(Intrinsic Viscosity η)
The intrinsic viscosity η of the resulting varnish was measured using an Ubbelohde viscometer at a polymer concentration of 0.5 g/dL, NMP, and 25° C.

(Preparation of polyimide film)
The polyamic acid varnish prepared above was applied to a glass substrate with a doctor blade to form a varnish coating. A laminate consisting of the substrate and the varnish coating was placed in an inert oven. Thereafter, the oxygen concentration in the inert oven was controlled to 100 ppm or less, and the atmosphere in the oven was heated under the conditions shown in Table 2. After completion of heating, the sample was further cooled naturally under the inert, and then immersed in distilled water to peel off a polyimide film having a thickness of 9 to 12 μm from the substrate.

(evaluation)
The polyimide films thus produced were measured for (1) glass transition temperature, (2) coefficient of linear thermal expansion (CTE), and (3) b ^* value by the following methods.

(1) Glass transition temperature (Tg), (2) Coefficient of linear thermal expansion (CTE)
A test piece (width 5 mm, length 20 mm) was attached to a thermomechanical analyzer (TMA), and the test piece was heated once from room temperature to 300°C at 5°C/min under a nitrogen atmosphere and a load of 100 mN (1st run), then cooled to 50°C or less to remove residual stress accumulated in the test piece, and then heated again to 450°C at 5°C/min (2nd run). The inflection point of the elongation at this time was determined as the glass transition temperature. The TMA elongation ratio in the temperature range of 100 to 350°C was taken as the coefficient of linear thermal expansion (CTE).

(3) b ^* Value The b* value in the L ^* a ^* b ^* color system, which is an index of yellowness, of the obtained ^polyimide film was measured in a transmission mode at 0° di using a tristimulus value direct reading colorimeter (Colour Cute i CC-i type) manufactured by Suga Test Instruments Co., Ltd.

The evaluation results of the polyimide films of Examples 1 to 3 and Comparative Examples 1 and 2 are shown in Table 2.

As shown in Table 2, the standard deviation of the b ^{* values of the polyimide films of Examples 1 to 3, which were heated at a rate of 1°C/min or less in the temperature range of at least (Tg+15)°C to (Tg+30)°C, is smaller than the standard deviation of the b* values of the polyimide films of Comparative Examples 1 and 2, which were heated at a rate of more than 1°C/min. This shows that there is little variation in the b* values} .

Similarly, the standard deviation of the CTE values of the polyimide films of Examples 1 to 3 is smaller than the standard deviation of the CTE values of the polyimide films of Comparative Examples 1 and 2, and it can be seen that the CTE variation is also smaller.

3. Reference Tests [Reference Examples 1 to 3]
(Preparation of Polyamic Acid Varnish 2)
In a flask equipped with a thermometer, a condenser, a nitrogen inlet tube, and a stirring blade, t-DACH and N-methyl-2-pyrrolidinone (NMP) (the weight of NMP was calculated in advance so that the concentration of the total of all monomers was 15% by mass) were added as diamines, and the mixture was stirred under a nitrogen atmosphere to obtain a homogeneous solution. A predetermined amount of BPDA as a tetracarboxylic dianhydride was added as powder to the solution, and the mixture was stirred at room temperature for 30 minutes, and then the solution temperature was raised and the mixture was stirred at an internal temperature of 85 to 90°C for 1 hour, resulting in a homogeneous solution. The mixture was then cooled to room temperature, and stirring was continued overnight at room temperature to obtain a pale yellow viscous polyamic acid varnish (polyamic acid concentration 15% by mass, aromatic monomer content 50 mol%).

The intrinsic viscosity of the polyamic acid varnish was measured using the same method as above and was found to be 1.03 dL/g.

(Preparation of polyimide film)
The polyamic acid varnish prepared above was applied to a glass substrate with a doctor blade to form a varnish coating. A laminate consisting of the substrate and the varnish coating was placed in an inert oven. Thereafter, the oxygen concentration in the inert oven was controlled to 100 ppm or less, and the atmosphere in the oven was heated under the conditions shown in Table 3. After the heating was completed, the sample was further cooled naturally under the inert, and then immersed in distilled water to peel off the polyimide film having a thickness of 9 to 12 μm from the substrate.

(evaluation)
The glass transition temperature, b ^* value and CTE of the polyimide films prepared in Reference Examples 1 to 3 were measured in the same manner as above. The results are shown in Table 3.

As shown in Table 3, in the case of polyimides containing less than 60 mol% aromatic monomer, the standard deviation of the b ^* value is small at 0.4 even when the temperature is increased at a rate exceeding 1°C/min in the region of (Tg+17)°C or higher. In other words, it is understood that the variation of the b ^* value is unlikely to occur in the first place.

It is also clear that the reduction in the standard deviation of the b ^* value caused by slowing down the heating rate is significantly smaller than that of the polyimides of the Examples and Comparative Examples having an aromatic monomer content of 60 mol % or more. In other words, it is clear that the effect of slowing down the heating rate in suppressing the variation in the b ^* value is also significantly smaller.

The polyamic acid composition of the present disclosure can provide a method for producing a polyimide film that can provide a polyimide film with little variation in b ^* value and high production efficiency.

Claims (9)

A step of applying a varnish containing a polyamic acid or a polyimide onto a substrate;
and heating the applied varnish to obtain a polyimide film.
Monomers constituting the polyimide contained in the polyimide film contain aromatic monomers in an amount of 60 mol % or more based on the total monomers constituting the polyimide,
In the step of heating the varnish,
When the glass transition temperature of the polyimide measured by TMA is Tg, the polyimide is heated to a temperature higher than (Tg+15)° C., and the heating rate in the temperature range from (Tg+15)° C. to (Tg+30)° C. is set to 1.0° C./min or less.
A method for producing a polyimide film. The glass transition temperature of the polyimide is 350° C. or higher.
The method for producing the polyimide film according to claim 1 . The b ^* value of the polyimide film is 12 or less.
The method for producing the polyimide film according to claim 1 . The monomer constituting the polyimide includes a tetracarboxylic dianhydride and a diamine,
The tetracarboxylic dianhydride is
A compound represented by formula (a1),
A compound represented by formula (a2) and/or a compound represented by formula (a3),
Including,

(In formula (a1),
R ¹ and R ² are each independently an alkyl group having 1 to 4 carbon atoms;
m and n are each an integer of 0 to 2, and m+n is 3 or less.

(In formula (a3),
^R3 and ^R4 are each independently an alkyl group having 1 to 4 carbon atoms;
Each of o and p is an integer of 0 to 3, and o+p is 3 or less.
The diamine includes a compound represented by formula (b1):

The content of the compound represented by the formula (a1) is 10 mol % or more based on the total amount of the tetracarboxylic dianhydride,
the total amount of the compound represented by the formula (a1) and the compound represented by the formula (a2) is 30 to 80 mol % based on the total amount of the tetracarboxylic dianhydride;
The method for producing the polyimide film according to claim 1 . The tetracarboxylic dianhydride includes a compound represented by formula (a3).
The method for producing a polyimide film according to claim 4 . The content of the compound represented by the formula (a3) is 10 to 70 mol % based on the total amount of the tetracarboxylic dianhydride.
The method for producing a polyimide film according to claim 5 . The content of the compound represented by the formula (a1) is greater than the content of the compound represented by the formula (a2).
The method for producing a polyimide film according to claim 4 . The content of the compound represented by the formula (a1) is 30 to 80 mol % based on the total amount of the tetracarboxylic dianhydride.
The method for producing a polyimide film according to claim 4 . the total amount of the compound represented by the formula (a2) and the compound represented by the formula (a3) is 10 to 70 mol % based on the total amount of the tetracarboxylic dianhydride;
The method for producing a polyimide film according to claim 4 .

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