JP5326253B2 - Method for producing polyimide film and aromatic polyimide - Google Patents

Description

  The present invention relates to a method for producing a polyimide film and an aromatic polyimide. This aromatic polyimide is a seamless belt used in electrophotographic image forming apparatuses such as photocopiers, laser beam printers, facsimiles and their combined devices, especially for fixing, for photoreceptor substrates, for intermediate transfer, for paper conveyance. And a seamless belt used for transfer fixing and the like.

  Aromatic polyimide has excellent properties such as heat resistance, chemical resistance, electrical properties, and mechanical strength, and thus is suitably used for electrical and electronic parts. Among them, as described in Patent Document 1, 3,3 ′, 4,4′-biphenyltetracarboxylic dianhydride (hereinafter sometimes abbreviated as s-BPDA) and paraphenylenediamine (hereinafter referred to as “s-BPDA”). Aromatic polyimides, which are mainly abbreviated as PPD), are particularly excellent in heat resistance, mechanical properties, dimensional stability, etc., so flexible substrates and photocopies that require these properties It is suitably used in applications such as seamless belts for machines.

Patent Document 2 discloses a polyimide film that is brittle by simply heat-treating a polyamic acid coating film in a method for producing a polyimide film mainly composed of pyromellitic dianhydride (hereinafter sometimes abbreviated as PMDA) and PPD. Since only a film can be obtained, a method has been proposed in which the film is immersed in a solution containing a dehydration condensing agent and an imidization catalyst and then heat-treated.
Patent Document 3 proposes an aromatic polyimide having improved long-term durability that can be suitably used for a seamless belt. This aromatic polyimide is a diamine component containing 3,4'-diaminodiphenyl ether.
In Patent Document 4, as a polyimide film for a flexible metal laminate having an adhesive layer and bonded to a metal foil, benzophenone tetracarboxylic dianhydride as a tetracarboxylic acid component and PMDA, 4,4′- as a diamine component are disclosed. A polyimide film using diaminodiphenyl ether and PPD is proposed.
Patent Document 5 proposes a seamless belt made of polyimide containing s-BPDA and PMDA as tetracarboxylic acid components and PPD and diaminodiphenyl sulfone as diamine components as main components.

Japanese Patent Laid-Open No. 55-7805 JP 2003-64196 A JP 2006-307114 A JP 2006-306972 A JP 2006-206778 A

An aromatic polyimide composed of s-BPDA and PPD is particularly excellent in heat resistance, mechanical properties, dimensional stability, and the like. However, when trying to mold a thick polyimide seamless belt using a polyamic acid solution composition composed of s-BPDA and PPD, the moldability (film forming property) such as foaming is likely to occur during the heat treatment. There was a problem, and it was not possible to easily obtain a polyimide seamless belt having good heat resistance, mechanical properties, dimensional stability, and the like.
Therefore, the object of the present invention is excellent in heat resistance, mechanical properties, dimensional stability, etc., and further, a method for producing a polyimide film with improved moldability, heat resistance, mechanical properties, dimensional stability, etc. Moreover, it is an object to provide an aromatic polyimide having improved moldability, particularly a thick polyimide film formed of the aromatic polyimide. This seamless belt made of polyimide film has excellent heat resistance, mechanical properties, dimensional stability, etc., and has improved moldability, so it can be used in electronic devices such as photocopiers, laser beam printers, facsimiles, and their combined devices. It can be suitably used as a seamless belt used in a photographic image forming apparatus, particularly as a seamless belt for fixing, a photoreceptor substrate, an intermediate transfer, a paper transport, and a transfer fixing.

  The present invention foams a polyimide film having a film thickness of more than 40 μm by heat-treating a coating film formed by applying a polyamic acid solution composition comprising a repeating unit of the following chemical formula (1) to a substrate. The present invention relates to a method for producing a polyimide film characterized by being obtained without any problems.

なお、前記化学式（１）の繰返し単位からなるポリアミック酸において、Ａ１００モル％中の２５〜９０モル％が下記化学式（２）で示される４価のユニットであり、７５〜１０モル％が下記化学式（３）及び／又は（４）で示される４価のユニットであり、Ｂ１００モル％中の２５〜９０モル％が下記化学式（５）で示される２価のユニットであり、７５〜１０モル％が下記化学式（６）で示される２価のユニットである。

In addition, in the polyamic acid composed of the repeating unit of the chemical formula (1), 25 to 90 mol% in A100 mol% is a tetravalent unit represented by the following chemical formula (2), and 75 to 10 mol% is represented by the following chemical formula. It is a tetravalent unit represented by (3) and / or (4), and 25 to 90 mol% in B100 mol% is a divalent unit represented by the following chemical formula (5), and is 75 to 10 mol%. Is a divalent unit represented by the following chemical formula (6).

（ここで、Ｒは、炭素数が１〜４のアルキル基又はヒドロキシル基である。）
また、本発明は、シームレスベルトを製造することを特徴とする前記ポリイミド膜の製造方法に関する。

(Here, R is an alkyl group having 1 to 4 carbon atoms or a hydroxyl group.)
The present invention also relates to a method for producing the polyimide film, wherein a seamless belt is produced.

  Moreover, this invention relates to the aromatic polyimide which consists of a repeating unit of following Chemical formula (7).

なお、前記化学式（７）の繰返し単位からなる芳香族ポリイミドにおいて、Ａ１００モル％中の２５〜９０モル％が前記化学式（２）で示される４価のユニットであり、７５〜１０モル％が前記化学式（３）及び／又は（４）で示される４価のユニットであり、Ｂ１００モル％中の２５〜９０モル％が前記化学式（５）で示される２価のユニットであり、７５〜１０モル％が前記化学式（６）で示される２価のユニットである。

In addition, in the aromatic polyimide comprising the repeating unit of the chemical formula (7), 25 to 90 mol% in A100 mol% is a tetravalent unit represented by the chemical formula (2), and 75 to 10 mol% is the above It is a tetravalent unit represented by the chemical formula (3) and / or (4), and 25 to 90 mol% in 100 mol% of B is a divalent unit represented by the chemical formula (5), and 75 to 10 mol. % Is a divalent unit represented by the chemical formula (6).

The present invention also relates to the aromatic polyimide, wherein the film has a tensile breaking strength of 300 MPa or more and a tensile elastic modulus of 5 GPa or more.
The present invention also relates to a polyimide film formed of the aromatic polyimide and having a thickness of more than 40 μm.

  Furthermore, this invention relates to the seamless belt formed with the said aromatic polyimide.

  According to the present invention, there is provided a method for producing a polyimide film having excellent heat resistance, mechanical properties, dimensional stability, and the like, and having good solution stability of a polyamic acid solution composition and improved moldability. It is possible to obtain an aromatic polyimide having excellent physical properties, dimensional stability and the like, and improved moldability, particularly a thick polyimide film formed by the aromatic polyimide film. This seamless belt made of polyimide film has excellent heat resistance, mechanical properties, dimensional stability, etc., and has improved moldability, so it can be used in electronic devices such as photocopiers, laser beam printers, facsimiles, and their combined devices. It can be suitably used as a seamless belt used in a photographic image forming apparatus, particularly as a seamless belt for fixing, a photoreceptor substrate, an intermediate transfer, a paper transport, and a transfer fixing.

  In the method for producing a polyimide film of the present invention, a coating film formed by applying a polyamic acid solution composition comprising the repeating unit of the chemical formula (1) to a substrate is heat-treated, whereby the film thickness exceeds 40 μm. The polyimide film is obtained without foaming.

  The tetravalent unit resulting from the tetracarboxylic acid component of the polyamic acid is a tetravalent unit based on the symmetrical biphenyl structure represented by the chemical formula (2) in an amount of 25 to 90 mol%, preferably 30 to 80 mol%. And 75 to 10 mol%, preferably 70 to 20 mol% is a tetravalent unit based on the asymmetric biphenyl structure represented by the chemical formula (3) and / or 4 based on the phenyl structure represented by the chemical formula (4). The divalent unit derived from the diamine component is a divalent unit based on the phenyl structure represented by 25 to 90 mol%, preferably 30 to 80 mol% of the chemical formula (5), 75 -10 mol%, preferably 80-20 mol% is a divalent unit based on a biphenyl structure having the specific substituent represented by the chemical formula (6). .

The tetracarboxylic acid component is a tetravalent unit based on the symmetrical biphenyl structure represented by the chemical formula (2) and is out of the range of 25 to 90 mol%, that is, in the chemical formula (3) and / or the chemical formula (4). When the tetravalent unit based on the phenyl structure shown is out of the range of 10 to 75 mol%, the foamability is likely to occur and the moldability is deteriorated, and it is difficult to obtain a polyimide film having excellent mechanical properties. .
Further, the diamine component has a biphenyl structure having a specific substituent represented by the chemical formula (6) in which the divalent unit based on the phenyl structure represented by the chemical formula (5) is outside the range of 25 to 90 mol%. If the divalent unit based is out of the range of 10 to 75 mol%, foaming is likely to occur and moldability is deteriorated, and it is difficult to obtain a polyimide film having excellent mechanical properties.

The polyamic acid is, for example, 25 to 90 mol% of 3,3 ′, 4,4′-biphenyltetracarboxylic acid and 75 to 10 mol% of 2,3,3 ′, 4′-biphenyl in an organic solvent. A tetracarboxylic acid component composed of tetracarboxylic acids and / or pyromellitic acids, 25 to 90 mol% PPD, and 75 to 10 mol% of a diamine represented by the following chemical formula (8), It can be suitably prepared by reacting the tetracarboxylic acid component and the diamine component at a ratio such that they are approximately equimolar.
Tetracarboxylic acids are those that can constitute the tetracarboxylic acid component of polyamic acid such as tetracarboxylic acid, acid dianhydride, and esterified alcohol thereof, and in particular tetracarboxylic dianhydride. is there.

（ここで、Ｒは、炭素数が１〜４のアルキル基（特にメチル基）又はヒドロキシル基である。）

(Here, R is an alkyl group having 1 to 4 carbon atoms (particularly a methyl group) or a hydroxyl group.)

  Specific examples of the diamine represented by the chemical formula (8) include 2,2′-dimethyl-4,4′-diaminobiphenyl, 3,3′-dimethyl-4,4′-diaminobiphenyl, and 2,2′-diethyl-4. , 4'-diaminobiphenyl, 3,3'-diethyl-4,4'-diaminobiphenyl, 2,2'-dipropyl-4,4'-diaminobiphenyl, 3,3'-dipropyl-4,4'-diamino Biphenyl, 3,3′-dihydroxy-4,4′-diaminobiphenyl, 2,2′-dihydroxy-4,4′-diaminobiphenyl and the like can be exemplified.

  For the preparation of the polyamic acid, known methods and conditions for preparing the polyamic acid can be suitably employed. Therefore, although not particularly limited, for example, amide bond of polyamic acid at a concentration such that tetracarboxylic dianhydride and diamine are about 5 to 50% by mass in terms of polyimide in an organic solvent. In order to suppress the imidation of the carboxylic acid group and the carboxyl group, a uniform polyamic acid solution is obtained by reacting with stirring at a temperature of preferably 100 ° C. or lower, more preferably 80 ° C. or lower for 0.1 to several tens of hours. It is preferable to obtain it. If the solid content concentration in terms of polyimide is less than 5% by mass, a large amount of solvent is used, which is not economical. If the solid content concentration exceeds 50% by mass, the viscosity tends to be high and handling tends to be difficult.

  A conventionally well-known organic solvent can be used suitably as an organic solvent used for preparation of polyamic acid. For example, organic compounds such as N-methyl-2-pyrrolidone, N, N-dimethylformamide, N, N-dimethylacetamide, cresol, N, N-dimethylsulfoxide, N-methylcaprolactam, methyltriglyme, methyldiglyme, sulfolane A polar solvent can be used suitably.

  The polyamic acid contained in the polyamic acid solution composition used in the present invention is not particularly limited, and may be a low molecular weight amic acid oligomer or a high molecular weight polyamic acid. That is, an amic acid oligomer having a weight average molecular weight of about 1000 to 10,000 or a polyamic acid having a weight average molecular weight of about 10,000 to 300,000 may be used. Preferably it is 10,000 to 300,000 polyamic acid. Although this polyamic acid has good solution stability, it does not cause precipitation of polymer components or gelation of the solution, and within a range where a uniform solution state can be maintained, the polyamic acid has an amide bond and a carboxyl group. A part may be imidized.

  The polyamic acid solution composition used in the present invention is a fine inorganic material such as finely divided silica, boron nitride, alumina, carbon black, in addition to the polyamic acid that is a polyimide precursor and the mixed solvent that dissolves the polyamic acid. Or an organic filler may be mix | blended and you may mix | blend another compounding component as needed. Other compounding ingredients are determined according to the application and required performance, but are colorants such as plasticizers, weathering agents, antioxidants, thermal stabilizers, lubricants, antistatic agents, brighteners, dyes and pigments. A conductive agent such as metal powder, a release agent, a surface treatment agent, a viscosity modifier, a coupling agent, a surfactant, and the like can be suitably blended. These blending components may be blended in advance with the solution composition, or may be added and blended when used.

If a fine filler is added to the polyamic acid solution composition, it is easy to suppress the formation of foam in the film-forming step, which may be advantageous in improving moldability (film-formability). In the method for producing a polyimide film of the present invention, when a polyamic acid solution composition containing a fine filler is used, the effect of suppressing foaming by the filler is also synergized, and the moldability may be further improved. Even if a fine filler is blended, if the polyamic acid composed of the repeating unit of the chemical formula (1) is not used, the moldability is not always improved sufficiently, and the film thickness and film forming conditions are limited. Depending on the thickness and film forming conditions, foaming may not be suppressed. Also, other problems such as cracking of the film are likely to occur during film formation.
The method for producing a polyimide film of the present invention is characterized in that a polyamic acid solution composition comprising the repeating unit of the chemical formula (1) is used. As a result, it has been explained that the moldability exemplified by foaming is improved as a representative example, and the effect is not necessarily limited only to the presence or absence of foaming. Film forming conditions are not limited, and a wide range of conditions can be adopted, problems such as cracks in the polyimide film are suppressed, and mechanical properties such as tear strength are improved.

The method for producing a polyimide film of the present invention is preferably a polyimide film having a film thickness of more than 40 μm by heat-treating a coating film formed by applying the polyamic acid solution composition on a substrate. It is one of the characteristics to obtain.
In the present invention, the base material can form a coating film by applying polyamic acid on the surface, and if it has a dense structure that does not substantially transmit liquid and gas, The material is not particularly limited. A circuit for forming a polyimide film as a protective film on the surface of a film-forming substrate such as a mold used for producing a known belt, roll, or seamless belt, which is used for producing a normal film. Preferable examples include parts and products in which a film is formed on the surface, such as a substrate, an electronic part, and a sliding part, and one film when a polyimide film is formed to form a multilayer film.

  As a coating method for forming a coating film on a substrate, for example, a method known per se such as a spray method, a roll coating method, a spin coating method, a bar coating method, an ink jet method, a screen printing method, a slit coating method, or the like is appropriately used. Can be adopted.

The coating film formed by coating on this substrate may be defoamed by a method of heating at a relatively low temperature under reduced pressure, for example.
The coating film made of the polyamic acid solution composition formed on the substrate is heat-treated to remove the solvent and imidized to form a polyimide film. The heat treatment is preferably a stepwise heat treatment in which the solvent is first removed at a relatively low temperature of 140 ° C. or lower and then the temperature is raised to the maximum heat treatment temperature to imidize rather than the heat treatment at a high temperature. Moreover, imidation is performed so that amidic acid groups do not substantially remain by performing a heat treatment at 140 ° C. or more for 0.01 to 30 hours, preferably 0.01 to 10 hours, more preferably 0.01 to 6 hours. Is preferred. The maximum heat treatment temperature is 300 to 600 ° C, preferably 350 to 550 ° C, more preferably 380 to 450 ° C. In this temperature range, 0.01 to 20 hours, preferably 0.01 to 6 hours, more preferably 0. It is preferable to perform heat treatment for 01 to 5 hours. As the heat treatment conditions for raising the temperature stepwise, for example, heat treatment is performed at 80 ° C. for 30 minutes, 130 ° C. for 10 minutes, 200 ° C. for 10 minutes, and finally 400 ° C. for 10 minutes (however, the following An example of the heat treatment condition is that the temperature is raised in 10 minutes to the stage).

  According to the method for producing a polyimide film of the present invention, extremely stable film formation is possible. That is, even when the film thickness exceeds 40 μm, a polyimide film can be obtained without causing problems during film formation such as foaming and cracking. That is, in the method for producing a polyimide film of the present invention, the thickness of the obtained polyimide film is more than 40 μm, preferably more than 45 μm, more preferably more than 50 μm, still more preferably more than 55 μm, and usually 200 μm or less, particularly 150 μm. It is as follows. With a polyimide film of 40 μm or less, the effect of improving moldability is not clear. On the other hand, when the film thickness of the polyimide film exceeds 200 μm, it is difficult to sufficiently improve the moldability.

  Furthermore, according to the method for producing a polyimide film of the present invention, the tensile breaking strength is 300 MPa or more, preferably 350 MPa or more, more preferably 400 MPa or more, the tensile breaking elongation is 30% or more, and the tensile elastic modulus is 5. A polyimide film having an extremely high mechanical strength of 0 GPa or more, preferably 6.0 GPa or more, more preferably 6.5 GPa or more, and particularly preferably 7.0 GPa or more can be suitably obtained.

  That is, the method for producing a polyimide film of the present invention is not particularly limited in molding conditions even when the polyimide film is excellent in heat resistance, mechanical properties, dimensional stability, etc., even when the film thickness is large. Efficient and easy to obtain in a good surface state. For this reason, it can employ | adopt suitably in a seamless belt.

  The seamless belt uses the inner peripheral surface or outer peripheral surface of a cylindrical mold as a base material, and the above-described film forming method and film forming conditions except that film formation (molding) is performed while rotating the mold. Can be suitably employed. However, the seamless belt usually has a larger film thickness (the normal film thickness is more than 40 μm, particularly about 50 to 150 μm) and often contains a relatively large amount of filler. It is preferable to select conditions such as performing the heat treatment time and the temperature raising rate longer and slowly.

Moreover, various fillers etc. are mix | blended with a seamless belt according to the use. The blending component is suitably performed by adding and blending into the polyamic acid solution. Moreover, when preparing a polyamic acid solution, you may mix | blend with the solution before reaction previously.
For example, when a polyimide endless tubular belt is used as a fixing belt of a copying machine, silica, boron nitride, alumina, or the like is preferably blended in order to improve thermal conductivity. Further, in order to prevent the toner adhering to the surface from being fused, the belt surface is made of a non-fluorine resin such as polytetrafluoroethylene, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer or tetrafluoroethylene-hexafluoropropylene copolymer. An adhesive layer may be laminated.
Further, when a polyimide endless tubular belt is used as a transfer belt of a copying machine, fine carbon black or the like is suitably blended in order to impart semiconductivity.

  The present invention will be further described below with reference to examples. In addition, this invention is not limited to a following example.

Abbreviations of the compounds used in the following examples are as follows.
s-BPDA: 3,3 ′, 4,4′-biphenyltetracarboxylic dianhydride a-BPDA: 2,3,3 ′, 4′-biphenyltetracarboxylic dianhydride PMDA: pyromellitic dianhydride PPD: paraphenylenediamine ODA: 4,4′-diaminodiphenyl ether m-TB: 2,2′-dimethyl-4,4′-diaminobiphenyl HAB: 3,3′-dihydroxy-4,4′-diaminobiphenyl

The measurement method used in the following examples will be described.
(Measurement method of tensile breaking strength)
It measured based on ASTM D882 using the tensile tester (Orientec RTC-1225A).

(Measurement method of tensile breaking elongation)
It measured based on ASTM D882 using the tensile tester (Orientec RTC-1225A).

(Measurement method of tensile modulus)
It measured based on ASTM D882 using the tensile tester (Orientec RTC-1225A).

(Log viscosity of solution composition)
The logarithmic viscosity (η _inh ) was prepared by uniformly dissolving a polyamic acid solution in N-methyl-2-pyrrolidone so that the polyamic acid concentration was 0.5 g / 100 ml of solvent. The solution viscosity was measured at 30 ° C. and calculated by the following formula.

(Solid content concentration)
The solid content concentration in terms of polyimide of the polyamic acid solution is a value obtained by drying the polyamic acid solution at 350 ° C. for 30 minutes, and obtaining the weight W1 before drying and the weight W2 after drying by the following formula.
Solid content concentration (% by weight) = {(W1-W2) / W1} × 100

(Solution viscosity)
The solution viscosity at 30 ° C. was measured using an E-type viscometer manufactured by Tokimec.

(Solution stability)
The solution stability of the polyamic acid solution was evaluated by measuring the change in the solution viscosity (rotational viscosity) at 30 ° C. with an E-type viscometer for the polyamic acid solution prepared at a monomer concentration of 20%. That is, the solution viscosity of the polyamic acid solution immediately after preparation is P1, and the solution viscosity measured after standing for 30 days in an atmosphere of 30 ° C. is P2. Was marked with ◯ and those with ± 10% or more were marked with ×.
Rate of change (%) = {(P2-P1) / P1} × 100

(Seamless belt condition observation)
The state of the seamless belt was judged visually. In the observation of the state of the seamless belt, “O” indicates that there is no foaming or cracking, “Δ” indicates that the foaming or cracking is about 30% of the total, and “X” indicates that the foaming or cracking is further.

[Example 1]
(Synthesis of polyamic acid solution composition)
A predetermined amount of N-methyl-2-pyrrolidone was added as a solvent to a glass reaction vessel having an internal volume of 500 ml equipped with a stirrer and a nitrogen gas introduction / discharge tube, and 20.45 g (0.189 mol) of PPD was added thereto. And m-TB (10.03 g, 0.047 mol), s-BPDA (55.62 g, 0.189 mol) and a-BPDA, 13.90 g (0.047 mol), and at 50 ° C. The mixture was stirred for 10 hours to obtain a polyamic acid solution having a solid concentration of 18.8% by weight, a solution viscosity of 4.9 Pa · s, and a logarithmic viscosity of 0.61. The solution stability of this polyamic acid solution composition was good.
The results of the properties of the polyamic acid solution composition are shown in Table 1.

(Manufacture of polyimide seamless belt)
The polyamic acid solution composition was uniformly applied to the inner surface of a stainless steel cylindrical mold having an inner diameter of 300 mm and a length of 150 mm while rotating at a rotation speed of 100 rpm, and then this coating film at a rotation speed of 200 rpm was applied at 120 ° C. Heat treatment was performed for 60 minutes, 150 ° C. for 30 minutes, 200 ° C. for 10 minutes, 250 ° C. for 10 minutes, and then 400 ° C. for 10 minutes to form a polyimide seamless belt having a thickness of 50 μm. No foaming was observed in this polyimide seamless belt.
The results of the characteristics and the like of this polyimide seamless belt are shown in Table 1.

[Examples 2 to 6]
A polyamic acid solution composition was prepared in the same manner as in Example 1 except that the composition of the polyamic acid was changed to that shown in Table 1.
The results of the properties of the polyamic acid solution composition are shown in Table 1.

A polyimide seamless belt was produced in the same manner as in Example 1 except that the polyamic acid solution composition was changed to that shown in Table 1.
The results of the obtained polyimide seamless belt are shown in Table 1.

[Comparative Examples 1-6]
A polyamic acid solution composition was prepared in the same manner as in Example 1 except that the composition of the polyamic acid was changed to that shown in Table 1.
The results of the characteristics of the polyamic acid solution composition are shown in Table 2.

A polyimide seamless belt was produced in the same manner as in Example 1 except that the polyamic acid solution composition was changed to that shown in Table 1.
The results of the obtained polyimide seamless belt are shown in Table 2.

  According to the present invention, a method for producing a polyimide film having excellent heat resistance, mechanical properties, dimensional stability and the like, and improved moldability, heat resistance, mechanical properties, dimensional stability, etc., and moldability are also provided. It is possible to obtain an improved aromatic polyimide, in particular, a thick polyimide film formed by the aromatic polyimide film. This seamless belt made of polyimide film has excellent heat resistance, mechanical properties, dimensional stability, etc., and has improved moldability, so it can be used in electronic devices such as photocopiers, laser beam printers, facsimiles, and their combined devices. It can be suitably used as a seamless belt used in a photographic image forming apparatus, particularly as a seamless belt for fixing, a photoreceptor substrate, an intermediate transfer, a paper transport, and a transfer fixing.

Claims (5)

Obtaining a polyimide film having a film thickness of more than 40 μm without foaming by heating a coating film formed by applying a polyamic acid solution composition consisting of repeating units of the following chemical formula (1) to a substrate. A method for producing a polyimide film characterized by the following.

In addition, in the polyamic acid composed of the repeating unit of the chemical formula (1), 25 to 90 mol% in A100 mol% is a tetravalent unit represented by the following chemical formula (2), and 75 to 10 mol% is represented by the following chemical formula. It is a tetravalent unit represented by (3) , 25 to 90 mol% in 100 mol% of B is a divalent unit represented by the following chemical formula (5), and 75 to 10 mol% is represented by the following chemical formula (6). Is a divalent unit.

(Here, R is an alkyl group having 1 to 4 carbon atoms or a hydroxyl group.)   The method for producing a polyimide film according to claim 1, wherein a seamless belt is produced. An aromatic polyimide comprising a repeating unit of the following chemical formula (7).

Note that in the aromatic polyimide consisting of repeating units of formula (7), a tetravalent unit 25-90 mol% in A100 mol% is represented by the following chemical formula (2), 75-10 mol% below is a tetravalent unit represented by the chemical formula (3), a divalent unit 25-90 mol% in B100 mol% is represented by the following chemical formula (5), 75-10 mol% following chemical formula (6 ) Is a divalent unit.

(Here, R is an alkyl group having 1 to 4 carbon atoms or a hydroxyl group.) A polyimide film having a film thickness of more than 40 μm formed by the aromatic polyimide according to claim 3 . A seamless belt comprising the polyimide film according to claim 4 .