WO2015198970A1

WO2015198970A1 - Polyimide film having pores and method for producing same

Info

Publication number: WO2015198970A1
Application number: PCT/JP2015/067656
Authority: WO
Inventors: 佳季宮本; 康史飯塚; 加藤　聡; 隆行金田
Original assignee: 旭化成イーマテリアルズ株式会社
Priority date: 2014-06-25
Filing date: 2015-06-18
Publication date: 2015-12-30
Also published as: CN106414575B; JP2018048344A; TW201605978A; KR102305617B1; CN112080026B; TW201835165A; CN106414575A; JP6254274B2; TWI689532B; JP7033171B2; KR20200091953A; KR20170010383A; JP7037534B2; JP6820827B2; JPWO2015198970A1; CN112080026A; TWI670327B; KR102139455B1; JP2020128522A; JP2020183539A

Abstract

　A polyimide film characterized by having pores of 100 nm or less and being used in the production of a flexible device.

Description

Polyimide film having voids and method for producing the same

The present invention relates to a polyimide film having voids, for example, used for a substrate for a flexible device, and a method for producing the same.
The polyimide film preferably has high transparency.

Generally, for applications requiring high heat resistance, a film made of polyimide (PI) is used as a resin film. A general polyimide is prepared by solution polymerization of an aromatic tetracarboxylic dianhydride and an aromatic diamine to produce a polyimide precursor (polyamic acid), followed by thermal imidization by dehydration at high temperature or using a catalyst. It is produced by chemical imidization by ring-closing dehydration.

Polyimide is an insoluble and infusible super heat resistant resin, and has excellent properties such as heat oxidation resistance, heat resistance, radiation resistance, low temperature resistance, and chemical resistance. For this reason, polyimide is used in a wide range of fields including electronic materials such as insulating coating agents, insulating films, etc .; semiconductor protective films; TFT-LCD electrode protective films. Recently, it has been studied to adopt a polyimide film as a flexible substrate utilizing its transparency, lightness, and flexibility in place of a glass substrate conventionally used as a substrate for display.
As for the polyimide film as the flexible substrate, for example, Patent Documents 1 and 2 have been reported.

JP 2011-74384 A International Publication No. 2012/11820 Pamphlet

However, the physical properties of known transparent polyimides are not sufficient for use as, for example, semiconductor insulating films, TFT-LCD insulating films, electrode protective films, ITO electrode substrates for touch panels, and heat-resistant substrates for flexible displays. .

For example, when a polyimide film is used as a flexible display substrate, the following steps are generally performed.
First, a polyimide film is formed on a support glass by applying polyamic acid, which is a polyimide precursor, on a glass substrate as a support substrate, and then thermally curing it. Next, an inorganic film is formed on the upper surface of the polyimide film. And after forming a display element on this inorganic film, a flexible display is obtained by finally peeling the polyimide film which has a TFT element and an inorganic film from the said support glass.
Here, when a polyimide film having low transparency is applied to a flexible display, color correction is required. In particular, when a film with extremely low transparency is used, correction becomes difficult. Therefore, the film applied to the flexible display needs to have high transparency.
Yellowness YI is widely used as an index of film transparency. As a polyimide with reduced yellowness, for example, there is a report of Patent Document 1. This publication discloses a polyimide with a very low yellowness. In general, a polyimide having a low yellowness tends to have a high residual stress. Moreover, a polyimide with low yellowness does not have absorption in the wavelength (308 nm and 355 nm) of the laser used when peeling a film from the said support glass. Therefore, when such a polyimide film is applied to a flexible display, the energy required for laser peeling increases, or wrinkles tend to occur during peeling.
By the way, Patent Document 2 discloses a technique for reducing the residual stress while maintaining the glass transition temperature and Young's modulus of polyimide. This patent document aims to reduce peeling marks when the polyimide film is mechanically peeled while maintaining the adhesion between the polyimide film and the glass substrate. Patent Document 2 describes that the above object is achieved by introducing a block having a structure derived from a flexible silicon-containing diamine into a polymer chain of polyimide. Paragraphs 55 and 151 of the patent document state that the residual stress is reduced by forming a microphase-separated structure in which silicone has a uniform structure with a size of about 1 nm to 1 μm. In paragraph 31, there is a description that the size of the silicone domain was confirmed by TEM measurement.
As a result of confirmation by the present inventors, a polyimide film having a silicone microphase-separated structure has a tendency to lower the glass transition temperature because a flexible skeleton is present in the film. Moreover, although the polyimide film of patent document 2 had high yellowness, when laser peeling was applied to this, when the irradiation energy of the laser was small, it turned out that this polyimide film cannot be peeled from a glass substrate. . Here, when peeling is attempted by increasing the laser irradiation energy, there arises a problem that the polyimide film burns and particles are generated.

The present invention has been made in view of the above-described problems.
That is, the present invention
Low residual stress generated between the glass substrate and the inorganic film;
Excellent adhesion to glass substrate;
Preferably highly transparent;
An object of the present invention is to provide a polyimide film which can be satisfactorily peeled even when the irradiation energy in the laser peeling step is low, and does not cause burning and particles, and a method for producing the same.

The inventors of the present invention have made extensive studies to solve the above problems. As a result, a polyimide film having a low YI and having a specific structure of voids has a high Tg, exhibits high adhesion between the glass substrate and the inorganic film, and further causes burns and particles in the laser peeling process. It has been found that the film has excellent peelability, and the present invention has been made based on this finding. That is, the present invention is as follows.

[1] A polyimide film characterized by having a gap of 100 nm or less and being used for production of a flexible device.
[2] The polyimide film according to [1], wherein the yellowness in a 20 μm film thickness is 7 or less.
[3] The polyimide film according to [1] or [2], which has a tensile elongation of 30% or more.
[4] The polyimide film according to any one of [1] to [3], which has a silicone residue.
[5] The polyimide film according to any one of [1] to [4], wherein the porosity is in the range of 3% to 15% by volume.
[6] The polyimide film according to any one of [1] to [5], wherein the shape of the void is a flat ellipsoidal sphere having an average major axis diameter of 30 nm to 60 nm.
[7] The polyimide film according to any one of [1] to [6], wherein the voids are present uniformly in the film thickness direction of the polyimide film.

[8] In the resin skeleton, unit 1 represented by the following general formula (1) and unit 2 represented by the following general formula (2):

{In the general formula (1) and the general formula (2), each R ₁ independently represents a hydrogen atom, a monovalent aliphatic hydrocarbon having 1 to 20 carbon atoms, or an aromatic having 6 to 10 carbon atoms. A group;
R ₂ and R ₃ are each independently a monovalent aliphatic hydrocarbon having 1 to 3 carbon atoms or an aromatic group having 6 to 10 carbon atoms;
X ₁ is a tetravalent organic group having 4 to 32 carbon atoms; and X ₂ is a divalent organic group having 4 to 32 carbon atoms. }
A resin precursor for producing a polyimide film according to any one of [1] to [7], characterized by comprising:

[9] tetracarboxylic dianhydride;
Diamine,
The following general formula (3):

{In the general formula (3), a plurality of R ₄ are each independently a single bond or a divalent organic group having 1 to 20 carbon atoms;
R ₅ and R ₆ are each independently a monovalent organic group having 1 to 20 carbon atoms;
R ₇ is each independently a monovalent organic group having 1 to 20 carbon atoms when a plurality of R ₇ are present; L ₁ , L ₂ , and L ₃ are each independently an amino group, an isocyanate group, a carboxyl group; A group, an acid anhydride group, an acid ester group, an acid halide group, a hydroxy group, an epoxy group, or a mercapto group;
j is an integer from 3 to 200; and k is an integer from 0 to 197. }
A compound represented by
The resin precursor according to [8], which is a copolymer of

[10] Tetracarboxylic dianhydride is
Pyromellitic dianhydride, 3,3 ′, 4,4′-benzophenonetetracarboxylic dianhydride, 3,3 ′, 4,4′-biphenyltetracarboxylic dianhydride, and 4,4′-biphenyl The resin precursor according to [9], which is at least one tetracarboxylic dianhydride selected from the group consisting of bis (trimellitic acid monoester anhydride).
[11] The mass of the compound represented by the general formula (3) used when synthesizing the resin precursor is the same as that of the compound represented by the tetracarboxylic dianhydride, the diamine, and the general formula (3). The resin precursor according to [9] or [10], which is 6% by mass to 25% by mass in total.
[12] A resin composition comprising the resin precursor according to any one of [8] to [11] and a solvent.

[13] On the surface of the support, the resin composition according to [12] is developed to form a coating film,
The support and the coating film are heated under conditions of an oxygen concentration of 23% by mass or less and a temperature of 250 ° C. or more to imidize the resin precursor in the coating film and form voids in the coating film. The polyimide film according to any one of claims 1 to 7, which is produced by
[14] The polyimide film according to [13], wherein an oxygen concentration during the heating is 2,000 ppm or less.

[15] On the surface of the support, a coating film forming step of forming the coating film by developing the resin composition according to [12];
The support and the coating film are heated under an oxygen concentration of 2,000 ppm or less and a temperature of 250 ° C. or more to imidize the resin precursor in the coating film and form voids in the coating film. Heating step to obtain a polyimide film having voids,
A peeling step of peeling the polyimide film having the voids from the support;
The manufacturing method of a polyimide film characterized by having.
[16] A flexible display comprising the polyimide film according to any one of [1] to [7], an inorganic film, and a TFT.

In addition, as a method for producing a polyimide film having voids, a method described in Non-Patent Document 1 is known.
Non-Patent Document 1 discloses a method of producing a polyimide film having voids by using a polyimide precursor in which polypropylene oxide is introduced into a main chain or a side chain. When a coating film of a polyimide precursor having a polypropylene oxide portion is formed, a film structure in which polypropylene oxide is microphase-separated is obtained. When this coating film is heat-treated, a polyimide film having voids is obtained by simultaneous imidization and thermal decomposition of polypropylene oxide. However, when polypropylene oxide is introduced into the main chain, film physical properties such as a decrease in transparency occur. Moreover, in order to introduce polypropylene oxide into the side chain, there is a problem of complicated synthesis.
The present invention provides a polyimide film that achieves the above-mentioned object and a method for producing the same by a simple method without causing deterioration of film properties.

According to the present invention, the residual stress generated between the glass substrate and the inorganic film is low, the adhesiveness with the glass substrate is excellent, preferably high transparency, and the irradiation energy is low in the laser peeling process. Even in such a case, it is possible to form a polyimide film that can be peeled off and does not cause burning of the polyimide film or generation of particles.

STEM image (left) and SEM image (right) of Example 1 ATR spectra of the films obtained in Examples 1 and 2 and Reference Example SEM image of Example 7

Hereinafter, an embodiment of the present invention (hereinafter abbreviated as “embodiment”) will be described in detail. In addition, this invention is not limited to the following embodiment, It can implement by changing variously within the range of the summary.

The polyimide film having voids according to the present embodiment is a film made of polyimide having a void structure with a size of 100 nm or less. The shape of the void can be a spherical structure, a flat elliptical sphere, or the like, and is preferably a flat elliptical sphere.
When the voids are oblate spheroids, the maximum major axis diameter is preferably 100 nm or less on average, more preferably 80 nm or less, more preferably in the range of 10 to 70 nm, and most preferably in the range of 30 to 60 nm. It is. If the gap is larger than 100 nm, haze is generated in the polyimide film. When the thickness is 1 nm or less, sufficient peelability cannot be ensured at the time of laser peeling, and the polyimide film is burnt by laser irradiation, resulting in generation of particles.

The porosity of the polyimide film having voids according to the present embodiment is preferably in the range of 3% by volume to 15% by volume, and more preferably in the range of 6% by volume to 12% by volume. When the porosity is 3% by volume or more, the easy peelability at the time of laser peeling is improved, the burning of the polyimide film is suppressed, and the generation of particles tends to be suppressed. If the volume is 15% or less, the film tends to exhibit excellent physical properties.
This porosity can be calculated by image analysis in scanning transmission electron microscope (STEM) or scanning electron microscope (SEM) observation.

The voids in the polyimide film are preferably present uniformly throughout the film. A polyimide film in which voids are present uniformly is preferable because it has a high tensile elongation and a low birefringence (Rth). In particular, the gap is preferably uniform in the film thickness direction of the polyimide film.
The uniformity in the film thickness direction of the voids can be known by image analysis in cross-sectional observation of the polyimide film performed using STEM or SEM. The details are as follows:
The obtained electron microscopic image is divided into regions of 2 μm in the film thickness direction, and the porosity is obtained for each region. For these void ratios, the difference between the maximum value and the minimum value is obtained. When the difference between the maximum value and the minimum value (Δ porosity (%) = maximum porosity (%) − minimum porosity (%)) is 5% or less, the film thickness of the void It can be evaluated that the uniformity in the direction is high, which is preferable. This value is more preferably 3% or less, still more preferably 1% or less, and particularly preferably 0.5% or less.

The polyimide film of the present invention preferably includes a part of a silicone structure because of excellent adhesion and adhesion between the glass substrate and the inorganic film. Examples of the inorganic film include CVD films such as silicon nitride and silicon oxide, and sputtered films.
The content (mass ratio) of the silicone residues contained in the polyimide film is preferably in the range of 3 to 15% by mass, and more preferably 6 to 12% by mass. When the content of the silicone residue exceeds 15% by mass, sufficient peelability cannot be ensured at the time of laser peeling, and the polyimide film may be burnt by laser irradiation, resulting in generation of particles. On the other hand, if this value is 3% by mass or less, sufficient adhesion to the glass substrate cannot be secured.

A method for specifically producing a polyimide film having a void structure according to this embodiment will be described below.
Specifically, on the resin skeleton, a unit 1 represented by the following general formula (1) and a unit 2 represented by the following general formula (2):

{In the general formula (1) and the general formula (2), each R ₁ independently represents a hydrogen atom, a monovalent aliphatic hydrocarbon having 1 to 20 carbon atoms, or an aromatic having 6 to 10 carbon atoms. A group;
R ₂ and R ₃ are each independently a monovalent aliphatic hydrocarbon having 1 to 3 carbon atoms or an aromatic group having 6 to 10 carbon atoms;
X ₁ is a tetravalent organic group having 4 to 32 carbon atoms; and X ₂ is a divalent organic group having 4 to 32 carbon atoms. }
A resin composition comprising a resin precursor (polyamic acid) having a solvent and a solvent is spread on a substrate to form a coating film,
By performing a heat treatment on the support and the coating film while controlling the oxygen concentration and the heating temperature, it is possible to form a polyimide film having voids having the structure as described above.

In the above resin precursor, the unit structure 1 shown in the general formula (1) is a structure obtained by reacting tetracarboxylic dianhydride and diamine. X ₁ is derived from tetracarboxylic dianhydride and X ₂ is derived from diamine.
The unit structure 2 shown in the general formula (2) is a structure derived from a silicone monomer.

In the resin precursor according to the present embodiment, X ₂ in the general formula (1) is 2,2′-bis (trifluoromethyl) benzidine, 4,4- (diaminodiphenyl) sulfone, 3,3- ( A residue derived from (diaminodiphenyl) sulfone is preferred.
It is preferable that a part of R ₂ and R ₃ in the general formula (2) is a phenyl group.
In the resin precursor of this invention, it is preferable that the total mass of the resin structure which consists of the said unit 1 and the said unit 2 is 30 mass% or more with respect to all the resin precursors.

<Tetracarboxylic dianhydride>
Next, a tetracarboxylic dianhydride that leads to a tetravalent organic group X ₁ contained in the unit 1 will be described.

Specific examples of the tetracarboxylic dianhydride include aromatic tetracarboxylic dianhydrides having 8 to 36 carbon atoms, aliphatic tetracarboxylic dianhydrides having 6 to 50 carbon atoms, and carbon numbers. Is preferably a compound selected from 6-36 alicyclic tetracarboxylic dianhydrides. The number of carbons herein includes the number of carbons contained in the carboxyl group.

More specifically, examples of the aromatic tetracarboxylic dianhydride having 8 to 36 carbon atoms include 4,4 ′-(hexafluoroisopropylidene) diphthalic anhydride (hereinafter also referred to as 6FDA), 5- ( 2,5-dioxotetrahydro-3-furanyl) -3-methyl-cyclohexene-1,2 dicarboxylic acid anhydride, pyromellitic dianhydride (hereinafter also referred to as PMDA), 1,2,3,4-benzene Tetracarboxylic dianhydride, 3,3 ′, 4,4′-benzophenone tetracarboxylic dianhydride (hereinafter also referred to as BTDA), 2,2 ′, 3,3′-benzophenone tetracarboxylic dianhydride, 3,3 ′, 4,4′-biphenyltetracarboxylic dianhydride (hereinafter also referred to as BPDA), 3,3 ′, 4,4′-diphenylsulfonetetracarboxylic dianhydride (Hereinafter also referred to as DSDA), 2,2 ′, 3,3′-biphenyltetracarboxylic dianhydride, methylene-4,4′-diphthalic dianhydride, 1,1-ethylidene-4,4′- Diphthalic dianhydride, 2,2-propylidene-4,4′-diphthalic dianhydride, 1,2-ethylene-4,4′-diphthalic dianhydride, 1,3-trimethylene-4,4 ′ -Diphthalic dianhydride, 1,4-tetramethylene-4,4'-diphthalic dianhydride, 1,5-pentamethylene-4,4'-diphthalic dianhydride, 4,4'-oxydiphthalic acid Dianhydride (hereinafter also referred to as ODPA), thio-4,4′-diphthalic dianhydride, sulfonyl-4,4′-diphthalic dianhydride, 1,3-bis (3,4-dicarboxyphenyl) ) Benzene dianhydride, 1,3-bis (3,4-dical) Xylphenoxy) benzene dianhydride, 1,4-bis (3,4-dicarboxyphenoxy) benzene dianhydride, 1,3-bis [2- (3,4-dicarboxyphenyl) -2-propyl] benzene Dianhydride, 1,4-bis [2- (3,4-dicarboxyphenyl) -2-propyl] benzene dianhydride, bis [3- (3,4-dicarboxyphenoxy) phenyl] methane dianhydride Bis [4- (3,4-dicarboxyphenoxy) phenyl] methane dianhydride, 2,2-bis [3- (3,4-dicarboxyphenoxy) phenyl] propane dianhydride, 2,2-bis [4- (3,4-dicarboxyphenoxy) phenyl] propane dianhydride (hereinafter also referred to as BPADA), bis (3,4-dicarboxyphenoxy) dimethylsilane dianhydride, 1,3-bi (3,4-dicarboxyphenyl) -1,1,3,3-tetramethyldisiloxane dianhydride, 2,3,6,7-naphthalenetetracarboxylic dianhydride, 1,4,5,8 -Naphthalenetetracarboxylic dianhydride, 1,2,5,6-naphthalenetetracarboxylic dianhydride, 3,4,9,10-perylenetetracarboxylic dianhydride, 2,3,6,7-anthracene Tetracarboxylic dianhydride, 1,2,7,8-phenanthrenetetracarboxylic dianhydride and the like;

Examples of the aliphatic tetracarboxylic dianhydride having 6 to 50 carbon atoms include ethylene tetracarboxylic dianhydride and 1,2,3,4-butanetetracarboxylic dianhydride;
Examples of the alicyclic tetracarboxylic dianhydride having 6 to 36 carbon atoms include 1,2,3,4-cyclobutanetetracarboxylic dianhydride (hereinafter also referred to as CBDA), cyclopentanetetracarboxylic dianhydride. , Cyclohexane-1,2,3,4-tetracarboxylic dianhydride, cyclohexane-1,2,4,5-tetracarboxylic dianhydride (hereinafter referred to as CHDA), 3,3 ′, 4,4 '-Bicyclohexyltetracarboxylic dianhydride, carbonyl-4,4'-bis (cyclohexane-1,2-dicarboxylic acid) dianhydride, methylene-4,4'-bis (cyclohexane-1,2-dicarboxylic acid ) Dianhydride, 1,2-ethylene-4,4′-bis (cyclohexane-1,2-dicarboxylic acid) dianhydride, 1,1-ethylidene-4,4′-bis (cyclohexane-1,2) Dicarboxylic acid) dianhydride, 2,2-propylidene-4,4′-bis (cyclohexane-1,2-dicarboxylic acid) dianhydride, oxy-4,4′-bis (cyclohexane-1,2-dicarboxylic acid) ) Dianhydride, thio-4,4′-bis (cyclohexane-1,2-dicarboxylic acid) dianhydride, sulfonyl-4,4′-bis (cyclohexane-1,2-dicarboxylic acid) dianhydride, bicyclo [2,2,2] oct-7-ene-2,3,5,6-tetracarboxylic dianhydride, rel- [1S, 5R, 6R] -3-oxabicyclo [3,2,1] octane -2,4-dione-6-spiro-3 '-(tetrahydrofuran-2', 5'-dione), 4- (2,5-dioxotetrahydrofuran-3-yl) -1,2,3,4 Tetrahydronaphthalene-1,2-dicarboxylic acid Water, ethylene glycol-bis- (3,4-dicarboxylic anhydride phenyl) ether, 4,4′-biphenylbis (trimellitic acid monoester anhydride) (hereinafter also referred to as TAHQ), etc. It is done.

Among them, the use of one or more selected from the group consisting of BTDA, PMDA, BPDA and TAHQ can reduce CTE, improve chemical resistance, improve glass transition temperature (Tg), and improve mechanical elongation. It is preferable from the viewpoint. In addition, when it is desired to obtain a film with higher transparency, it is possible to use one or more selected from the group consisting of 6FDA, ODPA and BPADA to reduce yellowness, birefringence, and mechanical elongation. It is preferable from the viewpoint of improvement. BPDA is preferable from the viewpoints of reducing residual stress, reducing yellowness, reducing birefringence, improving chemical resistance, improving Tg, and improving mechanical elongation. Further, CHDA is preferable from the viewpoints of reduction of residual stress and reduction of yellowness. Among these, at least one selected from the group consisting of PMDA and BPDA having a tough structure that exhibits high chemical resistance, high Tg and low CTE, and low yellowness and birefringence, from 6FDA, ODPA and CHDA It is preferable to use in combination with at least one selected from the group consisting of high chemical resistance, residual stress reduction, yellowness reduction, birefringence reduction, and total light transmittance improvement. .

In the resin precursor of the present invention, it is preferable that a component derived from biphenyltetracarboxylic acid (BPDA) is contained in an amount of 20 mol% or more of the total tetracarboxylic dianhydride-derived component of the resin precursor.

The resin precursor in this Embodiment is good also as a polyamideimide precursor by using dicarboxylic acid in addition to the above-mentioned tetracarboxylic dianhydride in the range which does not impair the performance. By using such a precursor, various performances such as improvement of mechanical elongation, improvement of glass transition temperature, reduction of yellowness, etc. can be adjusted in the obtained film. Examples of such dicarboxylic acids include dicarboxylic acids having an aromatic ring and alicyclic dicarboxylic acids. In particular, it is preferably at least one compound selected from the group consisting of aromatic dicarboxylic acids having 8 to 36 carbon atoms and alicyclic dicarboxylic acids having 6 to 34 carbon atoms. The number of carbons herein includes the number of carbons contained in the carboxyl group.
Of these, dicarboxylic acids having an aromatic ring are preferred.

Specifically, for example, isophthalic acid, terephthalic acid, 4,4′-biphenyldicarboxylic acid, 3,4′-biphenyldicarboxylic acid, 3,3′-biphenyldicarboxylic acid, 1,4-naphthalenedicarboxylic acid, 2,3 -Naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 4,4'-sulfonylbisbenzoic acid, 3,4'-sulfonylbisbenzoic acid, 3,3'-sulfonylbisbenzoic acid 4,4′-oxybisbenzoic acid, 3,4′-oxybisbenzoic acid, 3,3′-oxybisbenzoic acid, 2,2-bis (4-carboxyphenyl) propane, 2,2-bis (3-carboxy Phenyl) propane, 2,2′-dimethyl-4,4′-biphenyldicarboxylic acid, 3,3′-dimethyl-4,4′-biphenyldicarboxylic acid, 2,2′-di Til-3,3′-biphenyldicarboxylic acid, 9,9-bis (4- (4-carboxyphenoxy) phenyl) fluorene, 9,9-bis (4- (3-carboxyphenoxy) phenyl) fluorene, 4,4 '-Bis (4-carboxyphenoxy) biphenyl, 4,4'-bis (3-carboxyphenoxy) biphenyl, 3,4'-bis (4-carboxyphenoxy) biphenyl, 3,4'-bis (3-carboxyphenoxy) ) Biphenyl, 3,3′-bis (4-carboxyphenoxy) biphenyl, 3,3′-bis (3-carboxyphenoxy) biphenyl, 4,4′-bis (4-carboxyphenoxy) -p-terphenyl, 4 , 4′-bis (4-carboxyphenoxy) -m-terphenyl, 3,4′-bis (4-carboxyphenoxy) -p-turf Nyl, 3,3′-bis (4-carboxyphenoxy) -p-terphenyl, 3,4′-bis (4-carboxyphenoxy) -m-terphenyl, 3,3′-bis (4-carboxyphenoxy) -M-terphenyl, 4,4'-bis (3-carboxyphenoxy) -p-terphenyl, 4,4'-bis (3-carboxyphenoxy) -m-terphenyl, 3,4'-bis (3 -Carboxyphenoxy) -p-terphenyl, 3,3'-bis (3-carboxyphenoxy) -p-terphenyl, 3,4'-bis (3-carboxyphenoxy) -m-terphenyl, 3,3 ' -Bis (3-carboxyphenoxy) -m-terphenyl, 1,1-cyclobutanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, 1,2-cyclohexanedicarboxylic acid, 4,4 ′ Benzophenone dicarboxylic acid, 1,3-phenylene diacetic acid, 1,4-phenylene diacetic acid and the like; and 5-amino isophthalic acid derivative according to and WO 2005/068535 pamphlet can be mentioned. When these dicarboxylic acids are actually copolymerized with a polymer, they may be used in the form of an acid chloride form, an active ester form or the like derived from thionyl chloride or the like.

Among these, terephthalic acid is particularly preferable from the viewpoint of reducing the YI value and improving the Tg. When dicarboxylic acid is used together with tetracarboxylic dianhydride, it is obtained that the dicarboxylic acid is 50 mol% or less with respect to the total number of moles of the total of dicarboxylic acid and tetracarboxylic dianhydride. It is preferable from the viewpoint of chemical resistance in the film.

<Diamine>
The resin precursor according to the present embodiment is, for example, 4,4- (diaminodiphenyl) sulfone (hereinafter also referred to as 4,4-DAS), 3,4 as diamine for deriving X ₂ in unit 1. 4- (diaminodiphenyl) sulfone and 3,3- (diaminodiphenyl) sulfone (hereinafter also referred to as 3,3-DAS), 2,2′-bis (trifluoromethyl) benzidine (hereinafter also referred to as TFMB), 2 2,2'-dimethyl 4,4'-diaminobiphenyl (hereinafter also referred to as m-TB), 1,4-diaminobenzene (hereinafter also referred to as p-PD), 1,3-diaminobenzene (hereinafter also referred to as m-PD) ), 4-aminophenyl 4′-aminobenzoate (hereinafter also referred to as APAB), 4,4′-diaminobenzoate (hereinafter also referred to as DABA), 4,4 ′-(or 3) , 4'-, 3,3'-, 2,4 '-) diaminodiphenyl ether, 4,4'-(or 3,3 '-) diaminodiphenyl sulfone, 4,4'-(or 3,3'-) Diaminodiphenyl sulfide, 4,4′-benzophenonediamine, 3,3′-benzophenonediamine, 4,4′-di (4-aminophenoxy) phenylsulfone, 4,4′-di (3-aminophenoxy) phenylsulfone, 4,4′-bis (4-aminophenoxy) biphenyl, 1,4-bis (4-aminophenoxy) benzene, 1,3-bis (4-aminophenoxy) benzene, 2,2-bis {4- (4 -Aminophenoxy) phenyl} propane, 3,3 ′, 5,5′-tetramethyl-4,4′-diaminodiphenylmethane, 2,2′-bis (4-aminophenyl) propane, 2,2 ′, , 6'-tetramethyl-4,4'-diaminobiphenyl, 2,2 ', 6,6'-tetratrifluoromethyl-4,4'-diaminobiphenyl, bis {(4-aminophenyl) -2-propyl } 1,4-benzene, 9,9-bis (4-aminophenyl) fluorene, 9,9-bis (4-aminophenoxyphenyl) fluorene, 3,3′-dimethylbenzidine, 3,3′-dimethoxybench Gin and 3,5-diaminobenzoic acid, 2,6-diaminopyridine, 2,4-diaminopyridine, bis (4-aminophenyl-2-propyl) -1,4-benzene, 3,3′-bis (tri Fluoromethyl) -4,4′-diaminobiphenyl (3,3′-TFDB), 2,2′-bis [3 (3-aminophenoxy) phenyl] hexafluoropropane (3-BDAF), 2, '-Bis [4 (4-aminophenoxy) phenyl] hexafluoropropane (4-BDAF), 2,2'-bis (3-aminophenyl) hexafluoropropane (3,3'-6F), 2,2' An aromatic diamine such as -bis (4-aminophenyl) hexafluoropropane (4,4'-6F) can be mentioned. Of these, the use of one or more selected from the group consisting of 4,4-DAS, 3,3-DAS, 1,4-cyclohexanediamine, TFMB, and APAB reduces yellowness, CTE It is preferable from the viewpoint of reduction and high Tg.

<Introduction of silicon compounds>
The structure represented by the general formula (2) is derived from a silicone monomer. The amount of the silicone monomer used when synthesizing the resin precursor is preferably 6% by mass to 25% by mass based on the mass of the resin precursor. It is advantageous that the amount of the silicone monomer used is 6% by mass or more from the viewpoint of sufficiently obtaining the effect of reducing the stress generated between the resulting polyimide film and the inorganic film and the effect of reducing the yellowness. This value is more preferably 8% by mass or more, and further preferably 10% by mass or more. On the other hand, the amount of the silicone monomer used is 25% by mass or less, which is advantageous from the viewpoint of improving the transparency and obtaining good heat resistance without causing the resulting polyimide film to become cloudy. This value is more preferably 22% by mass or less, and further preferably 20% by mass or less. From the viewpoint of chemical resistance, total light transmittance, residual stress, adhesion to a glass substrate, and ease of laser peeling, the amount of silicone monomer used is particularly preferably 10% by mass or more and 20% by mass or less. . As will be described later, when the resin precursor coating is thermally cured under control of the oxygen concentration, a part of the silicone incorporated into the resin precursor is diluted in the form of cyclic trimer, cyclic tetramer, etc. It is thought to be scattered. It is preferable to adjust the introduction amount of the silicone monomer at the time of the resin precursor so that the mass ratio of the silicone remaining after the diffusion is in the range of 4 to 18% by mass with respect to the mass of the total polyimide film. .

Examples of the monovalent aliphatic hydrocarbon group having 1 to 20 carbon atoms in the general formula (2) include an alkyl group having 1 to 20 carbon atoms and a cycloalkyl group having 3 to 20 carbon atoms;
Examples of the aromatic group having 6 to 10 carbon atoms include an aryl group. The alkyl group having 1 to 20 carbon atoms is preferably an alkyl group having 1 to 10 carbon atoms from the viewpoint of heat resistance and residual stress. Specifically, for example, a methyl group, an ethyl group, a propyl group, an isopropyl group, Examples thereof include a butyl group, an isobutyl group, a t-butyl group, a pentyl group, and a hexyl group. The cycloalkyl group having 3 to 20 carbon atoms is preferably a cycloalkyl group having 3 to 10 carbon atoms from the above viewpoint, and specific examples thereof include a cyclopentyl group and a cyclohexyl group. Specific examples of the aryl group having 6 to 10 carbon atoms include a phenyl group, a tolyl group, and a naphthyl group from the above viewpoint.

As the silicone monomer that leads to the unit 2 as described above, for example, the following general formula (3):

{In the general formula (3), a plurality of R ₄ are each independently a single bond or a divalent organic group having 1 to 20 carbon atoms;
R ₅ and R ₆ are each independently a monovalent organic group having 1 to 20 carbon atoms;
R ₇ is each independently a monovalent organic group having 1 to 20 carbon atoms when a plurality of R ₇ are present;
L ₁ , L ₂ , and L ₃ are each independently an amino group, isocyanate group, carboxyl group, acid anhydride group, acid ester group, acid halide group, hydroxy group, epoxy group, or mercapto group;
j is an integer from 3 to 200, and k is an integer from 0 to 197. } Is preferably used.

Examples of the divalent organic group having 1 to 20 carbon atoms in R ₄ include a methylene group, an alkylene group having 2 to 20 carbon atoms, a cycloalkylene group having 3 to 20 carbon atoms, and an arylene group having 6 to 20 carbon atoms. Can be mentioned. The alkylene group having 2 to 20 carbon atoms is preferably an alkylene group having 2 to 10 carbon atoms from the viewpoint of heat resistance, residual stress and cost, and specifically, for example, dimethylene group, trimethylene group, tetramethylene group, pentamethylene group. Group, hexamethylene group and the like. The cycloalkylene group having 3 to 20 carbon atoms is preferably a cycloalkylene group having 3 to 10 carbon atoms from the above viewpoint. Specific examples include a cyclobutylene group, a cyclopentylene group, a cyclohexylene group, a cycloheptylene group, and the like. Of these, divalent aliphatic hydrocarbons having 3 to 20 carbon atoms are preferred from the above viewpoint. The arylene group having 6 to 20 carbon atoms is preferably an aromatic group having 3 to 20 carbon atoms from the above viewpoint, and specific examples thereof include a phenylene group and a naphthylene group.

In the general formula (3), R ₅ and R ₆ have the same meanings as R ₂ and R ₃ in the general formula (2), and a preferred embodiment is as described above for the general formula (2). The preferred embodiment of R ₇ is the same as R ₂ and R ₃ .

In the general formula (3), j is an integer of 3 to 200, preferably an integer of 10 to 200, more preferably an integer of 20 to 150, still more preferably an integer of 30 to 100, particularly preferably 35 to 80. Is an integer. In the general formula (3), k is an integer of 0 to 197, preferably 0 to 100, more preferably 0 to 50, and particularly preferably 0 to 25. When k exceeds 197, when a resin composition containing a resin precursor and a solvent is prepared, problems such as clouding of the composition may occur. When k is 0, it is preferable from the viewpoint of improving the molecular weight of the resin precursor and the heat resistance of the resulting polyimide. When k is 0, it is advantageous that j is 3 to 200 from the viewpoint of improving the molecular weight of the resin precursor and the heat resistance of the resulting polyimide.

In the general formula (3), L ₁ , L ₂ and L ₃ are each independently an amino group, an isocyanate group, a carboxyl group, an acid anhydride group, an acid ester group, an acid halide group, a hydroxy group, an epoxy group, Or a mercapto group.

The amino group may be substituted. Examples of the substituted amino group include a bis (trialkylsilyl) amino group. Specific examples of the compound in which L ₁ , L ₂ , and L ₃ in the general formula (3) are amino groups include amino end-modified methylphenyl silicone (for example, X22-1660B-3 (number average, manufactured by Shin-Etsu Chemical Co., Ltd.) Molecular weight 4,400) and X22-9409 (number average molecular weight 1,300)); both-end amino-modified dimethyl silicone (for example, X22-161A (number average molecular weight 1,600), X22-161B (number manufactured by Shin-Etsu Chemical Co., Ltd.)) Average molecular weight 3,000) and KF8012 (number average molecular weight 4,400); BY16-835U (number average molecular weight 900) manufactured by Toray Dow Corning; and Silaplane FM3311 (number average molecular weight 1000 manufactured by Chisso) It is done.
Specific examples of the compound in which L ₁ , L ₂ , and L ₃ are isocyanate groups include the above-mentioned isocyanate-modified silicones obtained by reacting both terminal amino-modified silicones with phosgene compounds.

Specific examples of the compound in which L ₁ , L ₂ , and L ₃ are carboxyl groups include, for example, X22-162C (number average molecular weight 4,600) manufactured by Shin-Etsu Chemical Co., Ltd. and BY16-880 (number average manufactured by Toray Dow Corning). Molecular weight 6,600) and the like.

Examples of the case where L ₁ , L ₂ and L ₃ are acid anhydride groups include, for example, the following formula group

And acyl compounds having at least one group represented by each of the above.

Specific examples of the compound in which L ₁ , L ₂ , and L ₃ are acid anhydride groups include, for example, X22-168AS (manufactured by Shin-Etsu Chemical, number average molecular weight 1,000), X22-168A (manufactured by Shin-Etsu Chemical, number average). Molecular weight 2,000), X22-168B (manufactured by Shin-Etsu Chemical, number average molecular weight 3,200), X22-168-P5-8 (manufactured by Shin-Etsu Chemical, number average molecular weight 4,200), DMS-Z21 (manufactured by Gerest, Number average molecular weight 600 to 800).

Specific examples of the compound in which L ₁ , L ₂ , and L ₃ are acid ester groups include a reaction of the compound in which L ₁ , L ₂ , and L ₃ are carboxyl groups or acid anhydride groups with an alcohol. And the like.

Examples of the case where L ₁ , L ₂ and L ₃ are acid halide groups include carboxylic acid chlorides, carboxylic acid fluorides, carboxylic acid bromides, carboxylic acid iodides and the like.

Specific examples of the compound in which L ₁ , L ₂ , and L ₃ are hydroxy groups include, for example, KF-6000 (manufactured by Shin-Etsu Chemical, number average molecular weight 900), KF-6001 (manufactured by Shin-Etsu Chemical, number average molecular weight 1,800) ), KF-6002 (manufactured by Shin-Etsu Chemical, number average molecular weight 3,200), KF-6003 (manufactured by Shin-Etsu Chemical, number average molecular weight 5,000), and the like. A compound having a hydroxy group is considered to react with a compound having a carboxyl group or an acid anhydride group.

Specific examples of the compound in which L ₁ , L ₂ , and L ₃ are epoxy groups include X22-163 (manufactured by Shin-Etsu Chemical, number average molecular weight 400), KF-105 (manufactured by Shin-Etsu Chemical, Number average molecular weight 980), X22-163A (manufactured by Shin-Etsu Chemical, number average molecular weight 2,000), X22-163B (manufactured by Shin-Etsu Chemical, number average molecular weight 3,500), X22-163C (manufactured by Shin-Etsu Chemical, number average molecular weight 5) , 400); both end alicyclic epoxy type, X22-169AS (manufactured by Shin-Etsu Chemical, number average molecular weight 1,000), X22-169B (manufactured by Shin-Etsu Chemical, number average molecular weight 3,400); X22-9002 (manufactured by Shin-Etsu Chemical Co., Ltd., functional group equivalent: 5,000 g / mol); The compound having an epoxy group is considered to react with diamine.

Specific examples of the compound in which L ₁ , L ₂ , and L ₃ are mercapto groups include, for example, X22-167B (manufactured by Shin-Etsu Chemical, number average molecular weight 3,400), X22-167C (manufactured by Shin-Etsu Chemical, number average molecular weight 4). , 600). A compound having a mercapto group is considered to react with a compound having a carboxyl group or an acid anhydride group.

L ₁ , L ₂ , and L ₃ are each independently preferably an amino group or an acid anhydride group from the viewpoint of improving the molecular weight of the resin precursor or the heat resistance of the resulting polyimide. From the viewpoint of avoiding white turbidity of the resin composition containing the precursor and the solvent, and from the viewpoint of cost,
It is preferable that all of L ₁ , L ₂ and L ₃ are amino groups; or L ₁ and L ₂ are each independently an amino group or an acid anhydride group, and k is 0. . In the latter case, it is more preferable that both L ₁ and L ₂ are amino groups.

The number average molecular weight of the resin precursor according to the present embodiment is preferably 3,000 to 1,000,000, more preferably 5,000 to 500,000, still more preferably 7,000 to 300,000. 000, particularly preferably 10,000 to 250,000. The molecular weight is preferably 3,000 or more from the viewpoint of obtaining good heat resistance and strength (for example, high elongation), and is 1,000,000 or less to obtain good solubility in a solvent. From the viewpoint, it is preferable from the viewpoint that coating can be performed without bleeding at a desired film thickness at the time of processing such as coating. From the viewpoint of obtaining a high mechanical elongation, the molecular weight is preferably 50,000 or more. In the present disclosure, the number average molecular weight is a value determined by standard polystyrene conversion using gel permeation chromatography.

The resin precursor according to the present embodiment may be partially imidized. The imidation of the resin precursor can be performed by known chemical amidation or thermal amidation. Of these, thermal imidization is preferred. As a specific method, it is preferable to prepare a resin composition by a method described later and then heat the solution at 130 to 200 ° C. for 5 minutes to 2 hours. By this method, a part of the polymer can be dehydrated and imidized to such an extent that the resin precursor does not precipitate. Here, the imidization rate can be controlled by controlling the heating temperature and the heating time. By performing partial imidization, the viscosity stability of the resin composition when stored at room temperature can be improved. The range of the imidization rate is preferably 5% to 70% from the viewpoint of solubility in a solution and storage stability.

Further, N, N-dimethylformamide dimethyl acetal, N, N-dimethylformamide diethyl acetal or the like may be added to the above resin precursor and heated to esterify a part or all of the carboxylic acid. By carrying out like this, the viscosity stability at the time of storage at room temperature of a resin composition can be improved.

<Resin composition>
The resin precursor according to the present embodiment as described above is preferably used as a resin composition (varnish) obtained by dissolving it in a solvent.
With this configuration, a transparent polyimide film can be produced without requiring a special solvent combination.

In a more preferred aspect, the resin composition according to the present embodiment is an aspect of a resin precursor obtained by reacting tetracarboxylic dianhydride, diamine, and silicone monomer by dissolving them in a solvent, for example, an organic solvent. It can be produced as a polyamic acid solution containing a polyamic acid and a solvent. Here, the conditions at the time of reaction are not particularly limited, and examples thereof include a reaction temperature of −20 to 150 ° C. and a reaction time of 2 to 48 hours. In order to sufficiently proceed the reaction with the silicone monomer, it is preferable to perform heating for about 30 minutes or more at a temperature of 120 ° C. or higher during the synthesis reaction. The reaction is preferably performed in an inert atmosphere such as argon or nitrogen.

The solvent is not particularly limited as long as it is a solvent that dissolves polyamic acid. Examples of known reaction solvents include dimethylene glycol dimethyl ether (DMDG), m-cresol, N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), acetone, One or more polar solvents selected from diethyl acetate, ecamide M100 (trade name: manufactured by Idemitsu Kosan Co., Ltd.), and ecamide B100 (trade name: manufactured by Idemitsu Kosan Co., Ltd.) are useful. Among these, Preferably, it is 1 or more types chosen from NMP, DMAc, Ecamide M100, and Ecamide B100. In addition, a low-boiling solution such as tetrahydrofuran (THF) or chloroform, or a low-absorbing solvent such as γ-butyrolactone may be used together with or in place of the above solvent.

In the resin composition according to the present embodiment, 0.01 to 2% by mass of an alkoxysilane compound is added to 100% by mass of the resin precursor in order to give the obtained polyimide film sufficient adhesion to the support. It may contain.

When the content of the alkoxysilane compound is 0.01% by mass or more with respect to 100% by mass of the resin precursor, good adhesion to the support can be obtained, and the content of the alkoxysilane compound is It is preferable that it is 2 mass% or less from a viewpoint of the storage stability of a resin composition. The content of the alkoxysilane compound is more preferably 0.02 to 2% by mass, still more preferably 0.05 to 1% by mass, and more preferably 0.05 to 0.5% with respect to the resin precursor. It is particularly preferable that the content is 1% by mass, and particularly preferable is 0.1 to 0.5% by mass.

Examples of alkoxysilane compounds include 3-ureidopropyltriethoxysilane, bis (2-hydroxyethyl) -3-aminopropyltriethoxysilane, 3-glycidoxypropyltrimethoxysilane, phenyltrimethoxysilane, and γ-aminopropyl. Triethoxysilane, γ-aminopropyltrimethoxysilane, γ-aminopropyltripropoxysilane, γ-aminopropyltributoxysilane, γ-aminoethyltriethoxysilane, γ-aminoethyltrimethoxysilane, γ-aminoethyltripropoxy Silane, γ-aminoethyltributoxysilane, γ-aminobutyltriethoxysilane, γ-aminobutyltrimethoxysilane, γ-aminobutyltripropoxysilane, γ-aminobutyltributoxysilane, etc. That. Two or more of these may be used in combination.

<Production of polyimide film having voids>
The polyimide resin film having a void structure according to the present embodiment forms the coating film by developing the above resin composition on the surface of the support,
It can be produced by heating the support and the coating film under conditions of an oxygen concentration of 23% by mass or less and a temperature of 250 ° C. or more.
In the present specification, the unit “mass%” relating to the oxygen concentration is a percentage based on volume, and the unit “ppm” relating to the oxygen concentration which will be described later is a percentage based on volume.

Here, the support is an inorganic substrate such as a glass substrate such as a non-alkali glass substrate, but is not particularly limited.
Examples of the method for spreading the polyimide precursor on the substrate include known coating methods such as spin coating, slit coating, and blade coating.
Next, the solvent is evaporated by heating to 80 ° C. to 200 ° C. using a hot plate, oven, or the like, and a coating film (pre-baked film) is produced. At this time, the silicone portion and the polyimide portion of the resin precursor form a film forming a microphase separation structure.

Next, the support and the coating film are put into an oven having an oxygen concentration of 23% by mass or less, and heated to 250 ° C. or more to dehydrate and imidize the resin precursor, and at the same time, the silicone part that is microphase-separated. A polyimide film according to the present embodiment can be created by disassembling and removing a part to form a void. By heating at 250 ° C. or higher, it is considered that the silicone portion in the resin precursor is thermally decomposed to form a cyclic trimer and / or a cyclic tetramer and is evaporated and removed. Without preparing the pre-baked film, the coated support may be put into an oven with controlled oxygen concentration as it is and heated to 250 ° C. or higher.

The size and porosity of the voids can be controlled, for example, by setting the silicone content in the polymer, the curing temperature, the curing time, the oxygen concentration, etc. within appropriate ranges.
Specifically, for example, when the introduction amount of the silicone moiety represented by the general formula (2) in the resin precursor is increased, the silicone domain size in the pre-baked film increases. The size of the silicone domain structure is one factor that controls the void structure. If the silicone part is completely pyrolyzed, the domain size in the prebaked film will be the maximum size of the voids in the resulting polyimide film. Therefore, by controlling the silicone domain size in the pre-baked film, the void size (major axis average) in the resulting polyimide film can be controlled. In order to control the silicone domain size in the pre-baked film to 100 nm or less, the mass ratio of the silicone portion represented by the general formula (2) in the resin precursor may be 25% by mass or less of the entire resin precursor. . Here, by controlling one or more factors of the curing temperature, the curing time, and the oxygen concentration during curing, the size relationship between the size of the void in the polyimide film and the domain size of the silicone in the pre-baked film is It can be adjusted to any degree.

The oxygen concentration during heating in the present embodiment is preferably 2,000 ppm or less. When the oxygen concentration at the time of heating is within this range, uniform voids tend to occur in the film. Therefore, the tensile elongation of the film is high and the birefringence (Rth) tends to be low, which is preferable. On the other hand, when heating is performed at an oxygen concentration exceeding 2,000 ppm and not more than 23% by mass, the uniformity of the voids in the film thickness direction tends to be slightly impaired.
This phenomenon is presumed to be caused by the fact that the thermal decomposition reaction of the silicone portion of the resin precursor hardly occurs when the oxygen concentration is 2,000 ppm or more. Although the cause is unknown, the present inventors, under conditions where a significant amount of oxygen exists, the organic group on the silicon atom of the silicone is oxidized by oxygen, for example, formaldehyde, formic acid, hydrogen, carbon dioxide, etc., It is presumed that this is because it is converted into a highly crosslinked gel-like heat-resistant polymer.

However, by controlling the oxygen concentration to 2,000 ppm or less, a uniform void structure starts to occur in the polyimide film. When compared at the same heating temperature, it was confirmed that the void size increases as the oxygen concentration decreases.
Further, when the oxygen concentration is 2,000 ppm or less, if the oxygen concentration is the same, the void size of the polyimide film can be increased as the heating temperature is increased.
As a result of confirmation by the present inventor, it is preferable from the viewpoint of controlling the size of the voids that the oxygen concentration during the heat treatment is suppressed to 1,000 ppm or less. The heating temperature is preferably in the range of 250 ° C. to 480 ° C., and more preferably in the range of 280 ° C. to 450 ° C. from the viewpoint of controlling the size of the voids.
Particularly preferably, the oxygen concentration is controlled to 100 ppm or less, and the heating temperature is controlled in the range of 280 ° C. to 450 ° C.
Examples of the inert gas used when controlling the oxygen concentration include nitrogen gas and Ar gas. Nitrogen gas is preferable from the economical viewpoint. In order to control the oxygen concentration, heating may be performed under reduced pressure using a vacuum oven or the like.

The thickness of the polyimide film according to the present embodiment is not particularly limited, and is preferably in the range of 1 to 200 μm, more preferably 5 to 50 μm.
Furthermore, the polyimide film according to the present embodiment preferably has a residual stress at a thickness of 10 μm of 25 MPa or less.
The polyimide film according to this embodiment preferably has a yellowness (YI) at a film thickness of 20 μm of 7 or less. A polyimide film having a YI value in this range can be used without color correction when applied to a flexible display substrate. The YI value of the polyimide film at a film thickness of 20 μm is more preferably 6 or less, and particularly preferably 5 or less.
In addition, when the thickness of a resin film is not 20 micrometers, the yellowness degree in thickness 20 micrometers can be known by performing thickness conversion with respect to the measured value of this film.

<Laminate>
The present invention also provides a laminate comprising a support and a polyimide film formed on the support. The laminate is formed by spreading the above resin composition on the surface of the support to form a coating film,
It can be obtained by heating the support and the coating film under conditions of an oxygen concentration of 23% by mass or less and a temperature of 250 ° C. or more.

This laminated body is used for manufacturing a flexible device, for example.
More specifically, a semiconductor device is formed on a polyimide film having a relative relationship, and then the support is peeled off to obtain a flexible device composed of the polyimide film and the semiconductor device formed thereon.

As described above, the polyimide film according to the present embodiment has a specific void structure, so that the residual stress generated between the glass substrate or the inorganic film is low and the adhesiveness to the glass substrate is excellent. Moreover, even when the irradiation energy is low in the laser peeling process, good peeling is possible, and no burning and particles are generated. Therefore, the polyimide film according to the present embodiment is extremely suitable for application as a substrate of a flexible display.

Hereinafter, a further preferable aspect when the polyimide film according to the present embodiment is applied as a substrate of a flexible display will be described.

When forming a flexible display, a polyimide film as a flexible substrate is formed thereon using a glass substrate as a support, and a TFT or the like is further formed thereon. The process of forming the TFT is typically performed at a wide range of temperatures from 150 to 650 ° C. In order to actually realize the desired performance, TFT-IGZO (InGaZnO) oxide semiconductors and TFTs (a-Si-TFTs, LTPS-TFTs) are mainly formed at around 250 ° C. to 450 ° C. .

At this time, if the residual stress generated between the flexible substrate and the polyimide film is high, the glass substrate warps and breaks when it shrinks during normal temperature cooling after expansion in the high temperature TFT formation process, from the glass substrate of the flexible substrate. This causes problems such as peeling. Generally, since the thermal expansion coefficient of a glass substrate is smaller than that of a resin, a residual stress is generated between the flexible substrate and the resin film. In consideration of this point, it is preferable that the residual stress generated between the polyimide film according to the present embodiment and the glass is 25 MPa or less on the basis of the film thickness of 10 μm.

In addition, the polyimide film according to the present embodiment has a tensile strength of 30% or more on the basis of a film thickness of 20 μm from the viewpoint of improving yield by being excellent in breaking strength when handled as a flexible substrate. It is preferable. In particular, when the tensile elongation is 33% or more, when an inorganic film on the polyimide film is provided, peeling or cracking of the film tends not to occur. Among these, 40% or more is particularly preferable.

The polyimide film according to this embodiment has at least one glass transition temperature in each of the −150 ° C. to 0 ° C. region and the 150 ° C. to 380 ° C. region, and is greater than 0 ° C. and less than 150 ° C. It is preferred not to have a glass transition temperature in the region.
In addition, the polyimide film according to the present embodiment preferably has a glass transition temperature of 250 ° C. or higher in the high temperature region so as not to be softened at the TFT element forming temperature.

Furthermore, it is preferable that the polyimide film according to the present embodiment has chemical resistance that can withstand a photoresist stripping solution in a photolithography process used when manufacturing a TFT element.

There are two known types of light extraction methods for flexible displays: a top emission method for extracting light from the front surface side of the TFT element and a bottom emission method for extracting light from the back surface side. The top emission method has a feature that it is easy to increase the aperture ratio because the TFT element does not get in the way. On the other hand, the bottom emission method is characterized by easy alignment and easy manufacture. If the TFT element is transparent, it is possible to improve the aperture ratio even in the bottom emission method. Therefore, it is expected that a bottom emission method that is easy to manufacture will be adopted as a large organic EL flexible display. ing. When a resin substrate is used for the colorless and transparent resin substrate used in the bottom emission method, the resin substrate is disposed on the side to be visually recognized. Therefore, the resin substrate is required to have particularly low yellowness (YI value) and high total light transmittance from the viewpoint of improving the image quality.

The polyimide film and laminate according to the present embodiment can be suitably used particularly as a substrate, for example, in the production of semiconductor insulating films, TFT-LCD insulating films, electrode protective films, flexible devices, and the like. Here, the flexible device is, for example, a flexible display, a flexible solar cell, a flexible touch panel electrode substrate, flexible lighting, a flexible battery, or the like. The polyimide film according to the present embodiment satisfying the above various physical properties can be used particularly for applications in which use is limited by the yellow color of existing polyimide films, particularly for colorless transparent substrates for flexible displays.

In addition to this, the polyimide film according to the present embodiment includes, for example, a protective film, a light-diffusing sheet and a coating film (for example, TFT-LCD interlayer, gate insulating film, liquid crystal alignment film, etc.) in TFT-LCD, It can be used in fields requiring colorless and transparent and low birefringence, such as ITO substrates for touch panels and cover glass substitute resin substrates for smartphones. When the polyimide according to this embodiment is applied as a liquid crystal alignment film, it contributes to an increase in the aperture ratio, and a TFT-LCD with a high contrast ratio can be manufactured.

Hereinafter, the present invention will be described in more detail based on examples. However, these are described for illustrative purposes, and the scope of the present invention is not limited to the following examples.

Various evaluations in Examples and Comparative Examples were performed as follows.
(Measurement of number average molecular weight)
The number average molecular weight (Mn) was measured using gel permeation chromatography (GPC) under the following conditions.
Solvent: 24.8 mmol / L lithium bromide monohydrate (manufactured by Wako Pure Chemical Industries, Ltd.) immediately before the measurement with respect to N, N-dimethylformamide (manufactured by Wako Pure Chemical Industries, Ltd., for high performance liquid chromatograph) Purity 99.5%) and 63.2 mmol / L phosphoric acid (manufactured by Wako Pure Chemical Industries, Ltd., for high performance liquid chromatography) Calibration curve: prepared using standard polystyrene (manufactured by Tosoh Corporation) Column: Shodex KD-806M (Showa Denko)
Flow rate: 1.0 mL / min Column temperature: 40 ° C
Pump: PU-2080 Plus (manufactured by JASCO)
Detector: RI-2031Plus (RI: differential refractometer, manufactured by JASCO) and UV-2075Plus (UV-VIS: UV-visible absorptiometer, manufactured by JASCO)

(Production of laminate and isolated film)
The resin precursor composition obtained in each synthesis example was applied to a non-alkali glass substrate (thickness 0.7 mm) with a bar coater, leveled at room temperature for 5 to 10 minutes, and then subjected to a vertical curing oven (Koyo). A polyimide film having a film thickness of 20 μm is formed on a glass substrate by heating (prebaking) at 140 ° C. for 60 minutes using a Lindberg company, model name VF-2000B), and further heating in a hot air oven under a nitrogen atmosphere for 60 minutes. The laminated body which has was produced.
Here, the oxygen concentration and the curing temperature in the hot air oven were set as shown in Table 1. As the oxygen concentration meter, a zirconia LC-750L manufactured by Toray Engineering Co., Ltd. was used. After the cured laminate was immersed in water and allowed to stand for 24 hours, the polyimide film was peeled from the glass and subjected to the following evaluations. However, the evaluation of laser peelability and the measurement of adhesive strength were carried out in a state where they were not peeled off from the glass substrate, and the polyimide film was formed separately for evaluation of residual stress and infrared measurement.

(Evaluation of tensile elongation)
The cured polyimide film was cut into a size of 5 mm × 50 mm, and was pulled at a speed of 100 mm / min using a tensile tester (manufactured by A & D Co., Ltd .: RTG-1210), and the tensile elongation was measured.

(Evaluation of glass transition temperature and linear expansion coefficient)
The glass transition temperature and linear expansion coefficient (CTE) in the region above room temperature were measured by thermomechanical analysis using a cured polyimide film cut to a size of 5 mm × 50 mm as a test piece. Using a thermomechanical analyzer (TMA-50) manufactured by Shimadzu Corporation as a measuring device, in a temperature range of 50 to 450 ° C. under conditions of a load of 5 g, a heating rate of 10 ° C./min and a nitrogen stream (flow rate of 20 ml / min). The test piece elongation was measured. The inflection point of the obtained chart was determined as the glass transition temperature, and the CTE of the polyimide film at 100 to 250 ° C. was determined.

(Evaluation of laser peelability)
With the third harmonic (355 nm) of the Nd: Yag laser, irradiation was performed while gradually increasing the irradiation energy from the glass substrate side of the laminate obtained above, and the polyimide was peeled off.
Here, the polyimide surface that was peeled with the minimum irradiation energy at which peeling was possible was observed with an optical microscope, and the presence or absence of scorching and particle generation on the polyimide surface was examined. When these occur on almost the entire surface of the film, the releasability is “bad”, when these occur only on a small part of the film, the releasability is “good”, and when these are not generated, the releasability is “good” As evaluated.

(Evaluation of residual stress)
Using a residual stress measuring device (model name FLX-2320, manufactured by Tencor Corporation), the “warping amount” of a 6-inch silicon wafer having a thickness of 625 μm ± 25 μm was measured. On this silicon wafer, the resin precursor composition obtained in each synthesis example was applied by a bar coater, pre-baked at 140 ° C. for 60 minutes, and then in a vertical curing furnace (manufactured by Koyo Lindberg Co., model name VF-2000B). Then, a heat treatment was performed at an oxygen concentration and a curing temperature shown in Table 1, and a silicon wafer having a polyimide film with a thickness of 10 μm was produced.
The amount of warpage of the wafer with polyimide was measured using the above-described residual stress measuring device, and the residual stress generated between the silicon wafer and the resin film was evaluated by comparison with the amount of warpage of the silicon wafer.

(Observation of voids with an electron microscope)
An ultra-thin section prepared by embedding a polyimide film in an epoxy resin and using a microtome (LEICA EM UC6) was used as a sample for microscopy. Using a transmission electron microscope (manufactured by Hitachi, Ltd .: S-5500), observation was performed from the film cross-sectional direction in SEM and STEM modes at an acceleration voltage of 30 kV.
From the state of the void structure observed by the STEM image, the average values of the void ratio and the maximum long axis length were obtained using image processing software.
Furthermore, the uniformity in the film thickness direction of the voids in the polyimide film was determined as follows. The electron microscopic image of each polyimide film was divided into regions of 2 μm in the film thickness direction, image processing was performed for each region, and the porosity was determined. Next, for these void ratios, the difference between the maximum value and the minimum value (Δ void ratio (%) = maximum void ratio (%) − minimum void ratio (%)) was determined. The value of Δ void ratio was used as an index of uniformity in the film thickness direction of the void.
When this value is 5% or less, it can be evaluated that the uniformity of the voids in the film thickness direction is high. This value is more preferably 3% or less, still more preferably 1% or less, and particularly preferably 0.5% or less.

(Measurement of inter-domain distance and electron density of void structure by small angle X-ray scattering measurement (SAXS))
Small angle X-ray scattering (SAXS) measurement was performed under the following conditions to estimate the inter-domain distance of the void structure and the electron density of the sea-island structure.
Device: Rigaku NanoViewer
Optical system: Point collimation (1st slit: 0.4 mmφ, 2nd slit: 0.2 mmφ, guard slit: 0.8 mmφ)
Incident X-ray wavelength λ: 0.154 nm
X-ray incident direction: perpendicular to the film surface (two view)
Detector: PILATUS100K
Camera length: 842mm
Measurement time: 900 seconds Sample: Measured with 10 layers of each film. For electron density, invariant Q is calculated by the following formula (1), the electron density difference Δρ is estimated, and is the island domain in the sea-island structure silicon? Whether it was a void was judged from the difference in electron density from polyimide.

{In the above formula (1), Q is an invariant;
q is the scattered wave vector;
I (q) is the scattering intensity;
V is the irradiation volume;
ρ is the electron density; and φ is the volume fraction of the island portion of the phase separation structure. }
Here, the scattering wave vector q was calculated in the range of 0.1 <q <2.0 (nm ⁻¹ ). Since the scattering intensity I (q) is subjected to absolute intensity correction, the volume V is not considered. The volume fraction was assumed to be φ = 0.1. Further, Q / 2π ² = 13,580 (0.1 <2θ <2.7 °) was calculated.

(Estimation of silicone content in polyimide film by infrared absorption spectroscopy (ATR))
The resin precursor composition was applied to a non-alkali glass substrate (thickness 0.7 mm) with a bar coater, leveled at room temperature for 5 to 10 minutes, and then a vertical curing oven (manufactured by Koyo Lindberg, model name) VF-2000B) was heated (prebaked) at 95 ° C. for 60 minutes. An ATR spectrum was obtained for this pre-baked film, the peak area at 1,500 cm ^−1, which is the absorption of the benzene ring, was normalized to 1, and the absorbance at 1,100 cm ⁻¹ , which was the absorption of SiO bond, was obtained.
The polyimide film after heating at the oxygen concentration and curing temperature shown in Table 1 was also measured in the same manner as described above, and the absorbance at 1,100 cm ^−1, which is the absorption of SiO bond, was obtained.
For the absorbance at 1,100 cm ⁻¹ , the residual ratio of silicone residues was estimated by comparing the value of the pre-baked film and the value of the cured polyimide film. And the silicone content in the obtained polyimide film was computed from the preparation amount of the silicone monomer at the time of synthesize | combining a polyimide precursor, and the residual rate of the silicone residue of the polyimide film after hardening.
As a measuring apparatus of ATR, “Nicolet Continium” manufactured by Thermo Fisher Scientific Co., Ltd. was used.
In FIG. 2, the ATR spectrum of the film obtained by Example 1, 2 and the reference example was shown. The chart of FIG. 2 is a spectrum of the films obtained in Reference Example 1, Example 2, and Example 1 in order from the top.

(Adhesive strength with glass substrate)
The polyimide film of the laminate obtained above is cut using a cutter knife with two cuts having a width of 10 mm and a length of 100 mm, the end is peeled off and sandwiched between chucks, and the tensile speed is 100 mm / min. 180 ° peel strength was measured.
As a tensile tester, RTG-1210 manufactured by A & D Corporation was used.

(Measurement of birefringence (Rth))
A polyimide film having a film thickness of 15 μm was used as a sample, and measurement was performed using a phase difference birefringence measuring apparatus (manufactured by Oji Scientific Instruments, KOBRA-WR). The wavelength of the measurement light was 589 nm.
(Measurement method of yellowness (YI))
Using a polyimide film having a thickness of 20 μm as a sample, measurement was performed using Nippon Denshoku Industries Co., Ltd. (Spectrophotometer: SE600). A D65 light source was used as the light source.

<Preparation and Evaluation of Resin Precursor Composition>
[Synthesis Example 1]
While introducing nitrogen gas into a 3 L separable flask equipped with a stir bar equipped with an oil bath, 1,000 g of NMP was charged, and 239.6 g (0.965 mol) of 4,4- (diaminodiphenyl) sulfone as a diamine was stirred. Then, 294.22 g (1.0 mol) of 3,3 ′, 4,4′-biphenyltetracarboxylic dianhydride was added as a tetracarboxylic dianhydride, and the mixture was stirred at room temperature for 30 minutes. This was heated to 50 ° C. and stirred for 12 hours. Thereafter, 109.3 g (17% by mass with respect to the entire resin precursor) of both-end amine-modified methylphenyl silicone oil (manufactured by Shin-Etsu Chemical Co., Ltd .: X22-1660B-3 (number average molecular weight 4,400)), which is a silicone monomer, was added. A silicone monomer solution obtained by dissolving in 298 g of NMP was added dropwise from a dropping funnel. Subsequently, after raising the temperature of the reaction system to 80 ° C. and stirring for 1 hour, the oil bath was removed and the temperature was returned to room temperature to obtain an NMP solution (resin precursor composition) of a transparent resin precursor (polyamic acid). It was. The number average molecular weight (Mn) of the polyamic acid obtained here was about 33,000.

[Synthesis Examples 2 to 6 and 9]
In the above Synthesis Example 1, the type and amount of diamine and tetracarboxylic dianhydride, and the content of the silicone monomer solution, respectively, were changed as described in Table 1, respectively. An NMP solution (resin precursor composition) of a resin precursor (polyamic acid) was obtained.
Table 1 shows the number average molecular weight (Mn) of the obtained polyamic acid.

[Synthesis Example 7]
While introducing nitrogen gas into a 10 L separable flask equipped with a stir bar equipped with an oil bath, NMP 5,502 g was charged, and 308.8 g (0.96 mol) of 2,2′-bis (trifluoromethyl) benzidine as a diamine. With stirring, followed by 185.4 g (0.85 mol) of pyromellitic dianhydride as tetracarboxylic dianhydride and 66.64 g of 4,4 ′-(hexafluoroisopropylidene) diphthalic anhydride ( 0.15 mol) was added sequentially. Further, while stirring this, a silicone monomer solution obtained by dissolving 113.64 g of silicone monomer X22-1660B-3 (17% by mass with respect to the whole resin precursor) in 568 g of NMP was dropped from a dropping funnel. After completion of dropping, the mixture was stirred at room temperature for 1 hour, heated to 80 ° C., stirred for 4 hours, and then returned to room temperature by removing the oil bath, thereby transparent NMP containing polyamic acid having an average molecular weight of 62,000. A solution (resin precursor composition) was obtained.

[Synthesis Example 8]
A transparent containing polyamic acid having a number average molecular weight of 58,000 was carried out in the same manner as in Synthesis Example 7 except that the amount of TFMB added was 317.02 g (0.99 mol) and no silicone monomer solution was added. NMP solution (resin precursor composition) was obtained.

Abbreviations of each component in Table 1 have the following meanings.
(Diamine)
4,4-DAS: 4,4- (diaminodiphenyl) sulfone TFMB: 2,2′-bis (trifluoromethyl) benzidine (tetracarboxylic dianhydride)
BPDA: 3,3 ′, 4,4′-biphenyltetracarboxylic dianhydride PMDA: pyromellitic dianhydride 6FDA: 4,4 ′-(hexafluoroisopropylidene) diphthalic anhydride (silicone monomer)
1660B: manufactured by Shin-Etsu Chemical Co., Ltd., product name “X22-1660B-3”, both-end amine-modified methyl phenyl silicone oil, number average molecular weight 4,400
FM3311: manufactured by Chisso Corporation, product Mie “Shiraplane FM3311”: amine-modified dimethyl silicone oil at both ends, number average molecular weight 1,000

[Examples 1 to 18 and Comparative Examples 1 to 3]
Using the resin precursor composition synthesized in the above synthesis example, a polyimide film was produced under the conditions of oxygen concentration and cure temperature described in Table 1 according to the above-described method, and various evaluations were performed.
The evaluation results are shown in Tables 2 and 3.
In FIG. 1, the STEM image (left) and SEM image (right) which image | photographed about the polyimide film obtained in Example 1;
In FIG. 3, the SEM image of the polyimide film obtained in Example 7;
Shown respectively.

Reference example 1
This reference example was carried out in order to verify that when the curing temperature was lowered, all of the silicone component remained in the film and no voids were formed.
A film was formed by the above-described method except that the resin precursor composition obtained in Synthesis Example 1 was used and the curing conditions were an oxygen concentration of 50 ppm and a curing temperature of 95 ° C., and ATR measurement and electron microscope observation were performed. It was.
The results are shown in Table 2.

The difference in electron density between domain structures in the sea-island structure obtained by SAXS observation is
In the examples, since the value was close to the difference in electron density between polyimide and air, voids were formed in the film;
In one comparative example, since the value was close to the difference in electron density between polyimide and silicone, no voids were formed;
Each was confirmed. Moreover, when the cross-sectional STEM image of the film thickness direction of Example 1 is referred, it can confirm that an island part is white. From this, it can be determined that the island portion is a void. Similarly, from the SEM image, it can be confirmed that the island portion is recessed, so that it can be determined that the portion is a void.
As shown in Table 2, it was confirmed that Examples 1 to 18 simultaneously satisfy the following conditions in film physical properties.
(1) The residual stress is 25 MPa or less,
(2) No burning of the polyimide film after laser peeling (3) No generation of particles after laser peeling,
(4) The glass transition temperature does not decrease compared to a polymer introduced with silicone,
(5) Tensile elongation is 30% or more, and (6) Excellent adhesion to a glass substrate.
From the results of Table 3, in Examples 1, 4, 5, and 6 in which the oxygen concentration during curing was 2,000 ppm or less, the uniformity of the formed voids in the film thickness direction was extremely high, and birefringence was observed. It was found that the value of (Rth) was extremely small.

Therefore, all of the polyimide films obtained in these examples satisfied the performance for application to a flexible display substrate.

In contrast, in the polyimide films obtained in Comparative Examples 1 to 3, the polyimide was burnt and colored during laser peeling, and as a result, particles were generated.

From the above results, the polyimide film obtained from the resin precursor according to the present invention has low residual stress generated between the glass substrate and the inorganic film, excellent adhesion to the glass substrate, and irradiation energy in the laser peeling process. It was confirmed that good peeling is possible even when the film thickness is low, and that no burning of the polyimide film or generation of particles occurs at the time of peeling.

It should be noted that the present invention is not limited to the above embodiment, and can be implemented with various modifications.

The polyimide film of the present invention can be suitably used for, for example, semiconductor insulating films, TFT-LCD insulating films, electrode protective films, flexible display substrates, touch panel ITO electrode substrates, and the like. It is particularly useful as various substrates.

Claims

A polyimide film characterized by having a void of 100 nm or less and being used for production of a flexible device.
The polyimide film according to claim 1, wherein the yellowness in a 20 μm film thickness is 7 or less.
The polyimide film according to claim 1 or 2, wherein the tensile elongation is 30% or more.
The polyimide film according to any one of claims 1 to 3, which has a silicone residue.
The polyimide film according to any one of claims 1 to 4, wherein the porosity is in the range of 3% to 15% by volume.
The polyimide film according to any one of claims 1 to 5, wherein the shape of the void is a flat ellipsoidal sphere having an average major axis diameter of 30 nm to 60 nm.
The polyimide film according to any one of claims 1 to 6, wherein the voids are present uniformly in the film thickness direction of the polyimide film.
In the resin skeleton, unit 1 represented by the following general formula (1) and unit 2 represented by the following general formula (2):

{In the general formula (1) and the general formula (2), each R 1 independently represents a hydrogen atom, a monovalent aliphatic hydrocarbon having 1 to 20 carbon atoms, or an aromatic having 6 to 10 carbon atoms. A group;
R 2 and R 3 are each independently a monovalent aliphatic hydrocarbon having 1 to 3 carbon atoms or an aromatic group having 6 to 10 carbon atoms;
X 1 is a tetravalent organic group having 4 to 32 carbon atoms; and X 2 is a divalent organic group having 4 to 32 carbon atoms. }
The resin precursor for producing a polyimide film according to any one of claims 1 to 7, characterized by comprising:
Tetracarboxylic dianhydride;
Diamine,
The following general formula (3):

{In the general formula (3), a plurality of R 4 are each independently a single bond or a divalent organic group having 1 to 20 carbon atoms;
R 5 and R 6 are each independently a monovalent organic group having 1 to 20 carbon atoms;
R 7, each independently in the presence of a plurality, a monovalent organic group of 1 to 20 carbon atoms; L 1, L 2, and L 3 each independently, an amino group, isocyanate group, carboxyl A group, an acid anhydride group, an acid ester group, an acid halide group, a hydroxy group, an epoxy group, or a mercapto group;
j is an integer from 3 to 200; and k is an integer from 0 to 197. }
A compound represented by
The resin precursor according to claim 8, which is a copolymer of
Tetracarboxylic dianhydride is
Pyromellitic dianhydride, 3,3 ′, 4,4′-benzophenonetetracarboxylic dianhydride, 3,3 ′, 4,4′-biphenyltetracarboxylic dianhydride, and 4,4′-biphenyl The resin precursor according to claim 9, wherein the resin precursor is one or more tetracarboxylic dianhydrides selected from the group consisting of bis (trimellitic acid monoester anhydride).
The mass of the compound represented by the general formula (3) used when synthesizing the resin precursor is 6 in total of the compounds represented by the tetracarboxylic dianhydride, the diamine, and the general formula (3). The resin precursor according to claim 9 or 10, wherein the content is from 25% by mass to 25% by mass.
A resin composition comprising the resin precursor according to any one of claims 8 to 11 and a solvent.
On the surface of the support, the resin composition according to claim 12 is developed to form a coating film,
The support and the coating film are heated under conditions of an oxygen concentration of 23% by mass or less and a temperature of 250 ° C. or more to imidize the resin precursor in the coating film and form voids in the coating film. The polyimide film according to any one of claims 1 to 7, which is produced by
The polyimide film according to claim 13, wherein the oxygen concentration during the heating is 2,000 ppm or less.
On the surface of the support, a coating film forming step of developing the resin composition according to claim 12 to form a coating film;
The support and the coating film are heated under an oxygen concentration of 2,000 ppm or less and a temperature of 250 ° C. or more to imidize the resin precursor in the coating film and form voids in the coating film. Heating step to obtain a polyimide film having voids,
A peeling step of peeling the polyimide film having the voids from the support;
The manufacturing method of a polyimide film characterized by having.
A flexible display comprising the polyimide film according to any one of claims 1 to 7, an inorganic film, and a TFT.